Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2019	Feb. 21, 2020	Jun. 30, 2019
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Interactive Data Current	Yes
Trading Symbol	TOCA
Entity Current Reporting Status	Yes
Entity Registrant Name	Tocagen Inc
Entity Central Index Key	0001419041
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Shell Company	false
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Title of 12(b) Security	Common Stock, par value $0.001 per share
Security Exchange Name	NASDAQ
Entity File Number	001-38052
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	26-1243872
Entity Address, Address Line One	4242 Campus Point Court
Entity Address, Address Line Two	Suite 600
Entity Address, City or Town	San Diego
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	92121
City Area Code	858
Local Phone Number	412-8400
Document Annual Report	true
Document Transition Report	false
Entity Common Stock, Shares Outstanding		23,899,261
Entity Public Float			$ 223
Documents Incorporated by Reference	DOCUMENTS INCORPORATED BY REFERENCE Portions of the registrant’s definitive proxy statement to be filed with the Securities and Exchange Commission, or SEC, subsequent to the date hereof pursuant to Regulation 14A in connection with the registrant’s 2019 Annual Meeting of Stockholders, are incorporated by reference into Part III of this Annual Report on Form 10-K. Such proxy statement will be filed with the SEC not later than 120 days after the conclusion of the registrant’s fiscal year ended December 31, 2019.

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 8,986	$ 40,813
Marketable securities	12,835	55,273
Prepaid expenses and other current assets	1,135	1,662
Total current assets	22,956	97,748
Property and equipment, net	1,689	3,973
Operating lease right-of-use asset	3,515
Other assets		1,360
Total assets	28,160	103,081
Current liabilities:
Accounts payable	3,218	3,404
Accrued liabilities	5,242	13,094
Notes payable, current portion	4,744
Deferred license revenue		36
Total current liabilities	13,204	16,534
Operating lease liability, net of current portion	4,027
Notes payable, net of current portion		26,201
Deferred rent, net of current portion		2,201
Other long term liabilities	81
Total liabilities	17,312	44,936
Commitments and contingencies (Note 12)
Stockholders’ equity
Common stock, $0.001 par value; 200,000,000 shares authorized at December 31, 2019 and 2018, respectively; 23,899,261 and 23,000,151 shares issued and outstanding at December 31, 2019 and December 31, 2018, respectively	23	23
Additional paid-in capital	290,215	274,029
Accumulated deficit	(279,400)	(215,884)
Accumulated other comprehensive income (loss)	10	(23)
Total stockholders’ equity	10,848	58,145
Total liabilities and stockholders’ equity	$ 28,160	$ 103,081

Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	23,899,261	23,000,151
Common stock, shares outstanding	23,899,261	23,000,151

Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
License revenue	$ 36	$ 18,036	$ 41
Type of Revenue [Extensible List]	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember
Operating expenses:
Research and development	$ 45,299	$ 51,080	$ 29,113
General and administrative	16,248	12,809	8,556
Total operating expenses	61,547	63,889	37,669
Loss from operations	(61,511)	(45,853)	(37,628)
Other income (expense), net:
Interest income	1,564	1,534	595
Interest expense	(3,820)	(2,930)	(1,932)
Loss on disposal of assets	(1,187)	(7)
Gain on lease modification	1,439
Change in fair value of preferred stock warrants			37
Loss before income taxes	(63,515)	(47,256)	(38,928)
Income tax expense	1	1,699	1
Net loss	(63,516)	(48,955)	(38,929)
Other comprehensive income (loss):
Net unrealized gain (loss) on investments	33	11	(34)
Comprehensive loss	$ (63,483)	$ (48,944)	$ (38,963)
Net loss per common share, basic and diluted	$ (2.69)	$ (2.44)	$ (2.66)
Weighted-average number of common shares outstanding, basic and diluted	23,630,422	20,059,541	14,607,609

Statements of Changes in Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)
Beginning balance at Dec. 31, 2016	$ (124,417)		$ 2	$ 3,581	$ (128,000)
Beginning balance, shares at Dec. 31, 2016		46,163,605
Beginning balance at Dec. 31, 2016		$ 131,413
Beginning balance, shares at Dec. 31, 2016			2,202,517
Exercise of stock options	81			81
Exercise of stock options, shares			55,669
Issuance of common stock pursuant to employee stock purchase plan	426			426
Issuance of common stock pursuant to employee stock purchase plan, shares			50,121
Stock-based compensation	4,451			4,451
Fractional shares adjustment upon reverse stock split, shares			2
Preferred stock converted into shares of common stock	131,410		$ 7	131,403
Preferred stock converted into shares of common stock, shares		(46,163,605)
Preferred stock converted into shares of common stock		$ (131,413)
Preferred stock converted into shares of common stock, shares			6,690,066
Initial public offering of common shares,net of issuance costs/offering costs	86,948		$ 10	86,938
Initial public offering of common shares,net of issuance costs/offering costs, shares			9,775,000
Convertible promissory notes converted into shares of common stock, net of issuance costs	11,057		$ 1	11,056
Convertible promissory notes converted into shares of common stock, net of issuance costs, shares			1,109,176
Preferred stock warrant liabilities converted into warrants to purchase shares of common stock	89			89
Other comprehensive loss	(34)					$ (34)
Net Loss	(38,929)				(38,929)
Ending balance at Dec. 31, 2017	71,082		$ 20	238,025	(166,929)	(34)
Ending balance, shares at Dec. 31, 2017			19,882,551
Exercise of stock options	67			67
Exercise of stock options, shares			45,073
Issuance of common stock pursuant to employee stock purchase plan	594			594
Issuance of common stock pursuant to employee stock purchase plan, shares			72,527
Stock-based compensation	6,870			6,870
Initial public offering of common shares,net of issuance costs/offering costs	27,997		$ 3	27,994
Initial public offering of common shares,net of issuance costs/offering costs, shares			3,000,000
Issuance of common stock warrants	479			479
Other comprehensive loss	11					11
Net Loss	(48,955)				(48,955)
Ending balance at Dec. 31, 2018	58,145		$ 23	274,029	(215,884)	(23)
Ending balance, shares at Dec. 31, 2018			23,000,151
Exercise of stock options	252			252
Exercise of stock options, shares			69,697
Issuance of common stock pursuant to employee stock purchase plan	$ 299			299
Issuance of common stock pursuant to employee stock purchase plan, shares	191,972		69,324
Stock-based compensation	$ 8,060			8,060
Initial public offering of common shares,net of issuance costs/offering costs	7,575			7,575
Initial public offering of common shares,net of issuance costs/offering costs, shares			760,089
Other comprehensive loss	33					33
Net Loss	(63,516)				(63,516)
Ending balance at Dec. 31, 2019	$ 10,848		$ 23	$ 290,215	$ (279,400)	$ 10
Ending balance, shares at Dec. 31, 2019			23,899,261

Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
Statements of Cash Flows - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
OPERATING ACTIVITIES
Net loss	$ (63,516)	$ (48,955)	$ (38,929)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	8,060	6,870	4,451
Depreciation	843	625	292
Noncash interest expense	1,698	1,161	590
Change in fair value of preferred stock warrants			(37)
Accretion of discount on investments, net	161	(248)	(19)
Loss on disposal of property and equipment	1,212
Gain on lease modification	(1,439)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	2,502	(208)	(679)
Accounts payable	(58)	1,260	304
Accrued liabilities	(8,911)	4,127	2,946
Deferred license revenue	(36)	(36)	(41)
Other	246	935	(11)
Net cash used in operating activities	(59,238)	(34,469)	(31,133)
INVESTING ACTIVITIES
Proceeds from the sale/maturity of marketable securities	93,473	68,524	43,725
Purchases of marketable securities	(51,163)	(70,746)	(70,797)
Purchases of property and equipment	(428)	(1,968)	(655)
Proceeds from sale of property and equipment	437		20
Net cash provided by (used in) investing activities	42,319	(4,190)	(27,707)
FINANCING ACTIVITIES
Proceeds from issuance of notes payable, net of issuance costs		26,325
Cash paid on extinguishment of debt		(8,631)
Principal payments on notes payable	(23,155)	(3,000)	(7,200)
Proceeds from issuance of common stock	551	661	507
Proceeds from offering of common stock, net of issuance costs	7,696	28,200	88,618
Proceeds from issuance of convertible promissory notes, net of issuance costs			7,338
Cash paid for deferred equity issuance costs		(16)
Net cash (used in) provided by financing activities	(14,908)	43,539	89,263
Net increase (decrease) in cash and cash equivalents	(31,827)	4,880	30,423
Cash and cash equivalents, beginning of period	40,813	35,933	5,510
Cash and cash equivalents, end of period	8,986	40,813	35,933
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION
Cash paid for interest	$ 2,290	1,522	1,210
Allowance for tenant improvements included in deferred rent		1,243
Property and equipment purchases included in accounts payable and accrued liabilities		178	111
Fair value of common stock warrants issued in connection with notes payable		479
Convertible preferred stock converted into shares of common stock			131,410
Convertible promissory notes principal and accrued interest converted into shares of common stock			11,057
Preferred stock warrant liabilities converted into warrants to purchase shares of common stock			89
Deferred equity issuance costs paid in previous periods reclassified to equity on effective date of initial public offering			1,574
Deferred equity issuance costs in accounts payable and accrued liabilities		$ 310	$ 96

Organization and Basis of Presentation

12 Months Ended

Dec. 31, 2019

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Organization and Basis of Presentation

1. Organization and Basis of Presentation

Tocagen Inc. (Tocagen or the Company) is a clinical-stage, cancer-selective gene therapy company focused on developing first-in-class, broadly-applicable product candidates designed to activate a patient’s immune system against their own cancer. The Company’s cancer-selective gene therapy platform is built on retroviral replicating vectors which are designed to selectively deliver therapeutic genes into the DNA of cancer cells. Tocagen’s gene therapy approach is designed to fight cancer through immunotherapeutic mechanisms of action without the autoimmune toxicities commonly experienced with other immunotherapies. The Company views its operations and manages its business in one operating segment.

From inception through December 31, 2019, the Company has devoted substantially all of its efforts to developing its gene therapy platform and its lead product candidate, Toca 511 & Toca FC, as well as raising capital and building its infrastructure. The Company has not generated revenues from its principal operations.

On October 1, 2019, the Company commenced a corporate restructuring plan that included reducing its workforce by approximately 65% in order to preserve the Company’s resources. The restructuring was approved by the Company’s Board of Directors on October 1, 2019, and affected employees were informed on October 2, 2019. The Company incurred a personnel-related restructuring charge of $0.9 million for employee severance and other related termination benefits. As of December 31, 2019, $0.2 million of severance related payments remained unpaid.

ATM Facility

In November 2018, the Company entered into an Equity Distribution Agreement with Citigroup Global Markets Inc. (“Citigroup”), pursuant to which the Company may sell and issue shares of its common stock having an aggregate offering price of up to $30,000,000 from time to time through Citigroup, as its sales agent (the “ATM facility”). As of December 31, 2019, the Company has sold 760,089 shares of common stock and received net proceeds of $7.7 million under the ATM facility.

Public Offering

In December 2018, the Company completed a public offering in which it sold an aggregate of 3,000,000 shares of common stock at a price of $10.00 per share. Net proceeds from the public offering, after deducting underwriting discounts, commissions and offering expenses, were approximately $28.0 million.

Liquidity

The accompanying financial statements have been prepared on a basis which assumes the Company is a going concern, and do not include any adjustments to reflect the possible future effects on the recoverability and classification of assets or the amounts and classifications of liabilities that may result from any uncertainty related to the Company’s ability to continue as a going concern. The Company has experienced net losses and negative cash flows from operating activities since its inception. As of December 31, 2019, the Company had an accumulated deficit of $279.4 million and working capital of $9.8 million available to fund future operations.

Based on the Company’s operating plans, cash, cash equivalents and marketable securities may not be sufficient to fund operations for the next 12 months. As a result, there is substantial doubt about the Company’s ability to continue as a going concern. All amounts due under the Term Loans (see note 5) have been classified as a current liability as of December 31, 2019 due to the considerations discussed above and the assessment that a material adverse change clause under the Term Loans is not within the Company’s control. On October 31, 2019, the Company entered into an amendment (the Second Amendment) to its Amended and Restated Loan and Security Agreement with the two lenders, dated May 18, 2018, which was further amended on August 3, 2018 (the Loan Agreement), and made a prepayment of $23.3 million, which amount was used to prepay i) a portion equal to $21.5 million of the outstanding principal of the Term Loans plus all accrued and unpaid interest thereon through the prepayment date, ii) prorated portion of the final payment with respect to the portion of such Term Loans being prepaid, plus iii) all outstanding lenders’ expenses as of the date of the Second Amendment.

The Company may seek to fund its losses from operations and capital needs through debt and equity financing, or through collaborations or partnerships with other entities, such funding may not be available on a timely basis on terms acceptable to the Company, or at all. If the Company is unable to raise additional capital when required or on acceptable terms, the Company may be required to further scale back or discontinue the advancement of product candidates, further reduce headcount, reorganize, merge with another entity, file for bankruptcy, or cease operations.

As of December 31, 2019, the Company had cash, cash equivalents and marketable securities of $21.8 million.

Use of Estimates

The Company’s financial statements are prepared in accordance with accounting principles generally accepted in the United States, which requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenue and expenses and the disclosure of contingent assets and liabilities in the financial statements accompanying notes. Significant estimates in the Company’s financial statements relate to clinical trial accruals, the valuation of equity awards and the development period used for license revenue recognition. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results may differ from these estimates under different assumptions or conditions.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2019

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Cash, Cash Equivalents and Marketable Securities

Cash consists of the balance in a readily available checking account. Cash equivalents consist of money market funds, corporate debt securities and certificates of deposit with remaining maturities of three months or less at the time of purchase, and are considered highly liquid investments. Marketable securities consist of corporate debt securities, commercial paper, U.S. treasury securities and asset-backed securities that have original maturities greater than three months at the time of purchase.

The Company classifies its investments as available-for-sale and records such assets at fair value in the balance sheet, with unrealized gains and losses, if any, reported in stockholders’ equity. Realized gains and losses are calculated on the specific identification method and recorded to interest income.

A decline in the market value of any marketable security below cost that is determined to be other-than-temporary results in a revaluation of its carrying amount to fair value and a new cost basis for the security. Impairment losses are recognized in other expense in the statement of operations.

Concentration of Credit Risk and Off-Balance Sheet Risk

Financial instruments that potentially subject the Company to significant concentration of credit risk consist primarily of cash equivalents and marketable securities. The Company’s investment policy includes guidelines for the quality of the related institutions and financial instruments, and defines allowable investments that the Company may invest in, which the Company believes minimizes the exposure to concentration of credit risk.

Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets primarily represent amounts related to insurance, clinical trial and manufacturing agreements, and investment interest receivable.

Property and Equipment

Property and equipment is stated at cost and depreciated over the estimated useful lives of the assets (three to five years) using the straight-line method. Leasehold improvements are depreciated using the straight-line method over the lesser of the remaining lease term or an estimated useful life .

Costs of major additions and betterments are capitalized and depreciated on a straight-line basis over their useful lives. Repairs and maintenance costs are expensed as incurred. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation are removed from the accounts, and any resulting gain or loss is credited or charged to expense.

Deferred Equity Issuance Costs

Specific incremental costs directly attributable to an offering of securities are deferred and charged against the gross proceeds of the offering through additional paid-in capital.

Impairment of Long-Lived Assets

Long-lived assets consist of property and equipment. An impairment loss is recorded if and when events and circumstances indicate that assets might be impaired and the undiscounted cash flows estimated to be generated by those assets are less than the carrying amount of those assets. No impairment loss has been recognized for the years ended December 31, 2019 and 2018.

Fair Value of Financial Instruments

The Company’s financial instruments consist principally of cash, cash equivalents, marketable securities, prepaid expenses, other current assets, accounts payable and notes payable. The carrying amounts of these financial instruments approximate the related fair values due to the short-term maturities of these instruments.

The authoritative accounting guidance defines fair value, establishes a consistent framework for measuring fair value and expands disclosure for each major asset and liability category measured at fair value on either a recurring or nonrecurring basis. Fair value is defined as an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or liability. As a basis for considering such assumptions, the authoritative accounting guidance establishes a three-tier fair value hierarchy that prioritizes the inputs used in measuring fair value as follows:

Level 1:Observable inputs such as quoted prices in active markets;

Level 2:Inputs, other than the quoted prices in active markets, that are observable either directly or indirectly such as quoted prices for similar assets or liabilities, quoted prices in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities; and

Level 3:Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities.

Clinical Trial Accruals

Expenses related to clinical studies are based on estimates of the services received and efforts expended pursuant to the Company’s contract arrangements. The financial terms of these agreements are subject to negotiation, vary from contract to contract and may result in uneven payment flows. There may be instances in which payments made to the Company’s service providers will temporarily exceed the level of services provided and result in a prepayment of the clinical expense. Payments under some of these contracts depend on factors such as the successful enrollment of patients, site initiation and the completion of clinical milestones. The Company makes estimates of its accrued expenses as of each balance sheet date in its financial statements based on facts and circumstances known at that time. In accruing service fees, the Company estimates the time period over which services will be performed and the level of effort to be expended in each period. If the actual timing of the performance of services or the level of effort varies from its estimate, the Company adjusts the accrual or prepaid expense balance accordingly. Historically, the Company’s estimated accrued liabilities have materially approximated actual expense incurred.

Revenue Recognition

Revenue generally consists of license revenue with upfront payments and development milestones considered probable of achievement.

Revenue is recognized when control of the promised goods or services is transferred to the Company’s customers in an amount that reflects the consideration the Company expects to receive from its customers in exchange for those goods and services. This process involves identifying the contract with a customer, determining the performance obligations in the contract, determining the transaction price, allocating the contract price to the distinct performance obligations in the contract, and recognizing revenue when or as the Company satisfies the performance obligation(s).

At contract inception, the Company assesses the goods and services promised within each contract and assesses whether each promised good or service is distinct and determines that those are performance obligations. A performance obligation is considered distinct from other obligations in a contract when it provides a benefit to the customer either on its own or together with other resources that are readily available to the customer and is separately identified in the contract. The Company considers factors such as the research, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. The Company considers a performance obligation satisfied once the Company has transferred control of a good or service to the customer, meaning the customer has the ability to use and obtain the benefit of the good or service. The Company recognizes revenue for satisfied performance obligations only when the Company determines there are no uncertainties regarding payment terms or transfer of control.

Collaborative Arrangements

The Company enters into collaborative arrangements with partners that may include payment to the Company of one or more of the following: (i) license fees; (ii) payments related to the achievement of developmental, regulatory, or commercial milestones; and (iii) royalties on net sales of licensed products. Where a portion of non‑refundable upfront fees or other payments received are allocated to continuing performance obligations under the terms of a collaborative arrangement, they are recorded as contract liabilities and recognized as revenue when (or as) the underlying performance obligation is satisfied.

As part of the accounting for these arrangements, the Company must develop estimates and assumptions that require judgment to determine the underlying stand-alone selling price for each performance obligation which determines how the transaction price is allocated among the performance obligation(s). The stand-alone selling price may include items such as forecasted revenues, development timelines, discount rates, and probabilities of technical and regulatory success. The Company evaluates each performance obligation to determine if it can be satisfied at a point in time or over time. In addition, variable consideration must be evaluated to determine if it is constrained and, therefore, excluded from the transaction price.

License Fees

If a license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenues from non-refundable, upfront fees allocated to the license when the license is transferred to the licensee and the licensee is able to use and benefit from the license. For licenses that are bundled with other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Any such adjustments are recorded on a cumulative catch-up basis, which would affect license, collaboration or other revenues and earnings in the period of adjustment.

Milestone Payments

At the inception of each arrangement that includes milestone payments (variable consideration), the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price. If it is probable that a milestone event would occur at the inception of the arrangement, the associated milestone value is included in the transaction price. Milestone payments that are not within the Company’s control, such as regulatory approvals, are generally not considered probable of being achieved until those approvals are received. The transaction price is then allocated to each performance obligation on a relative stand-alone selling price basis, for which the Company recognizes revenue as or when the performance obligations under the contract are satisfied. At the end of each reporting period, the Company evaluates the probability of achievement of such milestones and any related constraint(s), and if necessary, may adjust the Company’s estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect license, collaboration or other revenues and earnings in the period of adjustment.

Royalties

For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and for which the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any royalty revenue resulting from its collaborative arrangements.

Research and Development Costs

Research and development expenses consist primarily of salaries and other personnel related expenses including non-cash stock-based compensation costs, preclinical costs, clinical trial costs, costs related to acquiring and manufacturing clinical trial materials, contract services, facilities costs, overhead costs, and depreciation. All research and development costs are expensed as incurred.

Patent Costs

Costs related to filing and pursuing patent applications are recorded as general and administrative expense and expensed as incurred because recoverability of such expenditures is uncertain.

Grant Funding

The Company receives certain research and development funding through grants from nonprofit organizations that serve the brain cancer community. The Company evaluates the terms of each grant to assess the Company’s obligations, and such funding is recognized in the statement of operations as a reduction to research and development expense as the related costs are incurred to meet those obligations over the grant period. Certain grants contain repayment provisions contingent on future events, such as future revenue milestones related to the Company’s lead product candidate under development. For each repayment provision, the Company assesses if it is obligated to repay the funds provided by the other parties regardless of the outcome of the funded research and development. For each arrangement, the Company also reviews the repayment provisions to determine the likelihood of repayment at the execution of each grant and on an ongoing basis. If the likelihood of repayment of a grant is determined to be remote and the Company is not obligated to repay the funds regardless of the outcome of the funded research and development, the grant is recognized as a reduction to research and development expense as related costs are incurred over the grant period. The Company subsequently reviews the repayment provisions of each grant at each reporting date and will record a related grant repayment liability if and when such repayment obligation is determined to be probable. If, at the execution of a grant with repayment provisions, the probability of repayment is probable, the Company will record the grant as a liability until such time as the grant requirements have been satisfied and the repayment provisions have lapsed.

Debt Issuance Costs

Debt issuance costs incurred to obtain debt financing are deferred and are amortized over the term of the debt using the effective interest method. The costs are recorded as a reduction to the carrying value of the debt and the amortization expense is included in interest expense in the statement of operations.

Warrants for Shares of Common Stock

The Company accounts for warrants for shares of common stock as equity instruments in the accompanying balance sheets at their fair value on the date of issuance because such warrants are indexed to the Company’s common stock and no cash settlement is required except for (i) liquidation of the Company, or (ii) a change in control in which the common stockholders also receive cash.

Income Taxes

The Company accounts for income taxes under the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences that have been included in the financial statements. Under this method, deferred tax assets and liabilities are determined on the basis of the differences between the financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. The Company provides a valuation allowance against net deferred tax assets unless, based upon the available evidence, it is more likely than not that the deferred tax assets will be realized.

The Company records uncertain tax positions on the basis of a two-step process whereby (i) management determines whether it is more likely than not that the tax positions will be sustained on the basis of the technical merits of the position and (ii) for those tax positions that meet the more likely-than-not recognition threshold, management recognizes the largest amount of tax benefit that is more than 50 percent likely to be realized upon ultimate settlement with the related tax authority. The Company will recognize interest and penalties in income tax expense if and when incurred.

Comprehensive Income (Loss)

All components of comprehensive income (loss) are reported in the financial statements in the period in which they are recognized. Other comprehensive income (loss) is defined as the change in equity during a period from transactions and other events and circumstances from non-owner sources, including unrealized gains and losses on investments. The Company’s only component of other comprehensive loss is unrealized gains (losses) on investments. Comprehensive gains (losses) have been reflected in the statements of operations and comprehensive loss for all periods presented.

Stock-Based Compensation

Stock-based compensation expense represents the cost of the grant date fair value of stock awards, including stock options, and stock purchase rights granted to employees and members of the Company’s board of directors. For awards with time-based vesting provisions, the Company estimates the fair value of stock options on the date of grant using the Black-Scholes option pricing model and recognizes the expense over the requisite service period of the awards, which is generally the vesting period, on a straight-line basis. For awards with performance-based vesting provisions, the Company estimates the fair value of stock option grants on the date of grant, or the date when all of the terms of the grant have been agreed to, if later, and recognizes the expense based on the probability of the occurrence of the individual milestones at each reporting period. The expense is recognized over the implicit service period that commences once management believes the performance criteria are probable of being met. For purchase rights, the Company estimates the fair value of the purchase as of the plan enrollment date and recognizes expense on a straight-line basis over the applicable offering period. The Company accounts for forfeitures when they occur, and reverses any compensation cost previously recognized for awards for which the requisite service has not been completed, in the period that the award is forfeited.

Net Loss Per Share

Basic and diluted net loss per common share for the periods presented is computed by dividing net loss by the weighted-average number of common shares outstanding during the respective periods, without consideration of common stock equivalents as they are anti-dilutive. Common stock equivalents that could potentially dilute earnings in the future are comprised of shares issuable upon the conversion of options to purchase shares of common stock outstanding under the Company’s equity incentive plan and warrants for the purchase of shares of common. For all periods presented, there is no difference in the number of shares used to calculate basic and diluted shares outstanding due to the Company’s net loss position.

Common stock equivalents from potentially dilutive securities that are not included in the calculation of diluted net loss per share, because to do so would be anti-dilutive, are as follows:

	Years Ended December 31,
	2019		2018		2017
Common stock options		4,334,935		3,476,847		2,589,348
Common stock warrants		66,514		67,238		10,660
Total		4,401,449		3,544,085		2,600,008

Recent Accounting Pronouncements

In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842). The new standard is aimed at making leasing activities more transparent and comparable. Under the new guidance, lessees are required to recognize substantially all leases on their balance sheet as a lease liability, which is a lessee’s obligation to make lease payments arising from a lease, measured on a discounted basis and a right-of-use asset, which is an asset that represents the lessee’s right to use, or control the use of, a specified asset for the lease term. The Company adopted Topic 842 on January 1, 2019 using the modified retrospective approach with a cumulative-effect adjustment as of January 1, 2019. The Company recognized a right-of-use asset and a lease liability on the balance sheet for the discounted value of future lease payments from the date of adoption. The impact on the balance sheet as of the date of adoption was as follows (in thousands):

	ASC 840		ASC 842		Impact of
	January 1, 2019		January 1, 2019		Adoption
Balance Sheet
Accrued liabilities	$	154	$	645	$	491
Deferred rent, net of current portion		2,201		—		(2,201	)
Operating lease right-of-use asset		—		8,060		8,060
Operating lease liability, net of current portion		—		9,770		9,770

In June 2018, the FASB issued ASU 2018-07, Compensation- Stock Compensation (Topic 718), Improvements to Nonemployee Share-Based Payment Accounting. This new standard is intended to simplify aspects of share-based compensation issued to non-employees by aligning the accounting for share-based payment awards issued to employees and non-employees as it relates to the measurement date and impact of performance conditions. The new standard became effective January 1, 2019 and did not have a material impact to the overall financial statements of the Company.

Fair Value of Financial Instruments

12 Months Ended

Dec. 31, 2019

Fair Value Disclosures [Abstract]

Fair Value of Financial Instruments

3. Fair Value of Financial Instruments

Fair Values of Assets Measured on a Recurring Basis

The following tables summarize the Company’s assets that require fair value measurements on a recurring basis and their respective input levels based on the fair value hierarchy (in thousands):

			Fair Value Measurements at End of Period Using:
	Total		Quoted Market Prices for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
December 31, 2019
Marketable securities:
Corporate debt securities	$	3,700	$	—	$	3,700	$	—
Commercial paper		5,485		—		5,485		—
Asset-backed securities		3,650		—		3,650		—
	$	12,835	$	—	$	12,835	$	—

			Fair Value Measurements at End of Period Using:
	Total		Quoted Market Prices for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
December 31, 2018
Cash equivalents:
Corporate debt securities	$	4,783	$	—	$	4,783	$	—
Commercial paper		1,987		—		1,987		—
	$	6,770	$	—	$	6,770	$	—
Marketable securities:
Corporate debt securities	$	16,301	$	—	$	16,301	$	—
Commercial paper		24,576		—		24,576		—
U.S. treasury securities		1,997		1,997		—		—
Asset-backed securities		12,399		—		12,399		—
	$	55,273	$	1,997	$	53,276	$	—

Marketable Securities. For fair values determined by Level 1 inputs, which utilize quoted prices in active markets for identical assets, the level of judgment required to estimate fair value is relatively low. The fair values of investments in U.S. treasury securities were determined using Level 1 inputs.

Fair values determined by Level 2 inputs, which utilize data points that are observable such as quoted prices, interest rates and yield curves, require the exercise of judgment and use of estimates, that if changed, could significantly affect the Company’s financial position and results of operations. Investments in corporate debt securities, certificates of deposit, commercial paper, repurchase agreements and asset-backed securities are valued using Level 2 inputs. Level 2 securities are initially valued at the transaction price and subsequently valued and reported utilizing inputs other than quoted prices that are observable either directly or indirectly, such as quotes from third-party pricing vendors.

There were no transfers in or out of Level 1 or Level 2 investments during the years ended December 31, 2019 or 2018.

At December 31, 2019 and 2018, the Company had investments in money market funds of $6.3 million and $30.9 million, respectively, that were measured at fair value using the net asset value per share (or its equivalent) that have not been classified in the fair value hierarchy. The funds invest primarily in U.S. government securities.

Fair Values of Other Financial Instruments

The carrying amounts of certain of the Company’s financial instruments, including cash and accounts payable, approximate their respective fair values due to their short-term nature. The carrying amount of the Company’s notes payable of $4.7 million at December 31, 2019 approximated their fair value as the terms of the notes are consistent with the market terms of transactions with similar profiles (Level 2 inputs).

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Certain Financial Statement Caption Information

12 Months Ended

Dec. 31, 2019

Balance Sheet Related Disclosures [Abstract]

Certain Financial Statement Caption Information

4. Certain Financial Statement Caption Information

Marketable Securities

The following is a summary of the Company’s marketable securities (in thousands):

	Maturity (in years)	Amortized Cost		Unrealized Gain		Unrealized Loss			Fair Value
December 31, 2019
Corporate debt securities	1 or less	$	2,699	$	1	$	—		$	2,700
Corporate debt securities	>1 and <5		999		1		—			1,000
Commercial paper	1 or less		5,481		4		—			5,485
Asset-backed securities	1 or less		3,646		4		—			3,650
		$	12,825	$	10	$	—		$	12,835
December 31, 2018
Corporate debt securities	1 or less	$	10,013	$	1	$	(4	)	$	10,010
Corporate debt securities	>1 and <5		6,293		2		(4	)		6,291
Commercial paper	1 or less		24,584		—		(8	)		24,576
U.S. treasury securities	1 or less		1,997		—		—			1,997
Asset-backed securities	1 or less		10,612		—		(8	)		10,604
Asset-backed securities	>1 and <5		1,797				(2	)		1,795
		$	55,296	$	3	$	(26	)	$	55,273

The Company has classified all of its available-for-sale investment securities, including those with maturity greater than one year, as current assets on the balance sheet based on the highly liquid nature of these investment securities and because these investment securities are considered available for use in current operations.

There were no impairments considered other-than-temporary during the periods presented, as it is management’s intention and ability to hold the securities until a recovery of the cost basis or recovery of fair value. Gross realized gains and losses on sales of marketable securities were immaterial for all periods presented.

Property and Equipment

Property and equipment is comprised of (in thousands):

	December 31,
	2019			2018
Laboratory equipment	$	2,184		$	4,445
Computers, software and office equipment		252			322
Furniture and fixtures		216			610
Leasehold improvements		636			1,927
		3,288			7,304
Less: accumulated depreciation		(1,599	)		(3,331	)
	$	1,689		$	3,973

Depreciation expense was $0.8 million and $0.6 million for the years ended December 31, 2019 and 2018, respectively.

Accrued Liabilities

Accrued liabilities are comprised of (in thousands):

	December 31,
	2019		2018
Clinical trial expenses	$	2,404	$	4,535
Payroll and other employee-related expenses		1,116		2,840
Contract manufacturing services		387		3,411
Lease liability		416		—
Professional fees		330		474
Interest payable		37		205
Other		552		1,629
Total accrued liabilities	$	5,242	$	13,094

Notes Payable

12 Months Ended

Dec. 31, 2019

Debt Disclosure [Abstract]

Notes Payable

5. Notes Payable

Loan Agreement

On October 30, 2015, the Company entered into a Loan and Security Agreement (Prior Agreement) with two lenders whereby it borrowed $18.0 million (the Initial Loans). Balances under the Prior Agreement were due in monthly principal and interest payments, with final maturity of the Initial Loans in May 2019. Each Initial Loan included a final payment fee of 7.95% of the original principal amount due upon maturity.

On May 18, 2018, the Company entered into an Amended and Restated Loan and Security Agreement, which was further amended on August 3, 2018, pursuant to which the lenders agreed to lend the Company $26.5 million as term loans (the Term Loans). Of the total proceeds, $8.6 million was applied to the repayment of outstanding principal, interest and final payment owed pursuant to the Initial Loans.

The Company evaluated the May 2018 Amended and Restated Loan and Security Agreement in accordance with ASC Topic 470, which requires assessment of whether the modification is considered a substantial modification, in which case the modification would be accounted for as a debt extinguishment. Based on the Company’s evaluation, the May 2018 Amended and Restated Loan and Security Agreement was considered substantial and therefore the unamortized discount associated with the Prior Agreement was written off through interest expense and the principal balance of the Prior Agreement was written off.

The Term Loans will mature on December 1, 2022 (the Maturity Date) and the Company makes interest-only payments through January 1, 2020, followed by 36 equal monthly payments of principal and interest.

The Term Loans bear interest at a floating per annum rate equal to the greater of (i) 8.50% and (ii) the sum of (a) the prime rate reported in the Wall Street Journal on the last business day of the month that immediately proceeds the month in which the interest will accrue, plus (b) 3.75%. The Company will be required to make a final payment of 7.95% of the principal amount of the Term Loans payable on the earlier of (i) the Maturity Date, (ii) the acceleration of any Term Loans, or (iii) the prepayment of the Term Loans. On October 31, 2019, the Company entered into the Second Amendment and made a prepayment of $23.3 million, which amount was used to prepay i) a portion equal to $21.5 million of the outstanding principal of the Term Loans plus all accrued and unpaid interest thereon through the prepayment date, ii) prorated portion of the final payment with respect to the portion of such Term Loans being prepaid, plus iii) all outstanding lenders’ expenses as of the date of the Second Amendment.

In connection with the Second Amendment, the lenders (i) agreed to waive any prepayment fee otherwise applicable to a prepayment of the Term Loans in connection with any prepayment of the Term Loans on or after the date of the Second Amendment, (ii) consent to the sale of certain specified equipment, so long as the net cash proceeds from the sale of such assets are used to repay the Term Loans, and (iii) release their lien on the specified equipment upon the closing of any such sale. Under the Second Amendment, the Company has also agreed to grant a security interest in the Company’s intellectual property as additional collateral to secure the Term Loans for the ratable benefit of the lenders.

In conjunction with the Loan Agreement, the Company issued the lenders warrants exercisable for 56,578 shares of common stock (the Warrants). The Warrants are exercisable in whole or in part, immediately, and have a per share exercise price of $9.35. The Warrants will terminate on the earlier of May 18, 2028 or the closing of a certain merger or consolidation transaction. The Company recorded the Warrants as a debt discount, which is a contra-liability against debt. The offset to the contra-liability is recorded as additional paid in capital in the Company’s balance sheet as the Warrants were determined to be an equity instrument. The Company determined the fair value of the Warrants at the date of issuance was $0.5 million using the Black-Scholes option pricing model based on significant unobservable inputs (Level 3) with an expected term of 10 years, volatility of 85.6%, risk free rate of 3.1% and expected dividend of 0%.

The costs incurred to issue the Term Loans of $0.1 million were deferred and are included in the discount to the carrying value of the Term Loans in the accompanying balance sheet. The deferred costs and the final payment fee are amortized to interest expense over the expected term of the Term Loans using the effective interest method with an effective interest rate of 10.7%.

The aggregate carrying amounts of the Term Loans and Initial Loans are comprised of the following, as applicable (in thousands):

	December 31,
	2019			2018
Principal	$	5,000		$	26,450
Add: accreted liability for final payment fee	138			276
Less: unamortized discount		(394	)		(525	)
	$	4,744		$	26,201

The Term Loans are secured by all of the Company’s assets, including intellectual property. The Company is also required to maintain its primary operating accounts at all times with one of the lenders. The Loan Agreement contains customary conditions of borrowing, events of default and covenants, including covenants that restrict the Company’s ability to dispose of assets, merge with or acquire other entities, incur indebtedness and make distributions to holders of its capital stock. Should an event of default occur, including the occurrence of a material adverse change, the Company could be liable for immediate repayment of all obligations under the Loan Agreement. As of December 31, 2019, the Company was in compliance with the covenants contained in the Loan Agreement.

Based on the Company’s operating plans, cash, cash equivalents and marketable securities may not be sufficient to fund operations for the next 12 months. As a result, there is substantial doubt about the Company’s ability to continue as a going concern. All amounts due under the Term Loans have been classified as a current liability on the balance sheet.

Future maturities of the Term Loans, including the final payment fee, as of December 31, 2019 are as follows (in thousands):

	December 31, 2019
Year ending December 31, 2020		1,667
Year ending December 31, 2021		1,667
Year ending December 31, 2022		2,064
Notes payable, current portion		5,398
Unaccreted balance for final payment fee on Loans		(260	)
Unamortized discounts		(394	)
		4,744

Stockholders' Equity

12 Months Ended

Dec. 31, 2019

Equity [Abstract]

Stockholders' Equity

6. Stockholders’ Equity

Upon completion of the Company’s IPO, all of the Company’s outstanding shares of convertible preferred stock were converted into an aggregate of 6,690,066 shares of the Company’s common stock. As of December 31, 2019, the Company’s authorized capital stock consists of 200,000,000 shares of common stock, par value $0.001 per share, and 10,000,000 shares of preferred stock, par value $0.001 per share.

In December 2018, the Company completed a public offering in which it sold an aggregate of 3,000,000 shares of common stock at a price of $10.00 per share. Net proceeds from the public offering, after deducting underwriting discounts, commissions and offering expenses, were approximately $28.0 million.

During the year ended December 31, 2019, the Company sold 760,089 shares of common stock under the Sales Agreement, respectively. The sales were made at a weighted average price of $10.41. The Company received net proceeds of $7.7 million during the year ended December 31, 2019 and may sell up to an additional $22.1 million in shares of the Company’s common stock under the Sales Agreement.

Common Stock Reserved for Future Issuance

Common stock reserved for future issuance as of December 31, 2019 and 2018 is as follows:

	December 31,
	2019		2018
Issued and Outstanding:
Stock options		4,334,935		3,476,847
Warrants for common stock		66,514		67,238
Shares reserved for issuance under the ESPP		486,855		326,178
Shares reserved for future award grants		543,284		451,063

Equity Incentive Plans and Stock-Based Compensation

12 Months Ended

Dec. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Equity Incentive Plans and Stock-Based Compensation

7. Equity Incentive Plans and Stock-Based Compensation

2017 Equity Incentive Plan

In March 2017, the Company’s board of directors and stockholders approved and adopted the Company’s 2017 Equity Incentive Plan, which became effective on April 12, 2017 and was subsequently amended September 30, 2018 and further amended February 12, 2019 (the 2017 Plan). The 2017 Plan provides for the grant of incentive stock options (ISOs), nonstatutory stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards, performance-based stock awards, other forms of equity compensation and performance cash awards. ISOs may be granted only to employees. All other awards may be granted to employees, including officers, and to non-employee directors and consultants of the Company and its affiliates.

Initially, 1,600,000 new shares of common stock were approved for issuance under the 2017 Plan and, on April 12, 2017, 75,517 shares of common stock reserved for issuance under the Company’s 2009 Equity Incentive Plan, as amended (the 2009 Plan), were added to the shares initially reserved under the 2017 Plan. No further grants will be made under the 2009 Plan and any shares subject to outstanding stock options under the 2009 Plan that would otherwise be returned to the 2009 Plan will instead be added to the shares reserved under the 2017 Plan. Additionally, the number of shares of common stock reserved for issuance under the 2017 Plan will automatically increase on January 1 of each calendar year through January 1, 2027, by 4% of the total number of shares of the Company’s capital stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares determined by the Company’s board of directors.

All grants of options to purchase common stock under the 2017 Plan expire in 10 years. Grants with time-based vesting provisions are generally subject to a four-year vesting schedule with 25% vesting after the first year, and the balance vesting monthly over the remaining 36 months. Grants with performance-based vesting provisions vest upon the achievement of three separate development and regulatory milestones, with one-third of the options vesting upon the achievement of each milestone.

The following table summarizes stock option activity under the Company’s equity incentive plans for the year ended December 31, 2019:

	Shares Subject to Options			Weighted- Average Exercise Price per Share		Weighted- Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value (in thousands)
Options Outstanding at December 31, 2018		3,476,847		$	12.67
Granted		1,841,380		$	7.60
Exercised		(69,697	)	$	3.61
Forfeitures and cancellations		(913,595	)	$	11.67
Options Outstanding at December 31, 2019		4,334,935		$	10.87	7.2
Options Exercisable at December 31, 2019		2,128,758		$	11.58	5.8	$	-

The following table summarizes certain information regarding stock options (in thousands):

	Years Ended December 31,
	2019		2018		2017
Fair value of options vested during the period	$	6,336	$	6,318	$	2,194
Cash received from options exercised during the period	$	252	$	67	$	81
Intrinsic value of options exercised during the period	$	491	$	508	$	677

The aggregate intrinsic value of options is calculated as the difference between the exercise price of the options and the fair value of the Company’s common stock for those options that had exercise prices lower than the fair value of the Company’s common stock.

2017 Employee Stock Purchase Plan

In March 2017, the Company’s board of directors and stockholders approved and adopted the Company’s 2017 Employee Stock Purchase Plan (ESPP) whereby eligible employees may elect to withhold up to 15% of their earnings to purchase shares of the Company’s common stock at a price per share equal to the lower of (i) 85% of the fair market value of a share of the Company’s common stock on the first date of an offering or (ii) 85% of the fair market value of a share of the Company’s common stock on the date of purchase (purchase right). The ESPP became effective on April 12, 2017. Initially, 250,000 shares of the Company’s common stock were approved for issuance under the ESPP pursuant to purchase rights granted to the Company’s employees or to employees of any of the Company’s designated affiliates. The number of shares of common stock reserved for issuance will automatically increase on January 1 of each calendar year through January 1, 2027, by the lesser of (a) 1% of the total number of shares of the Company’s common stock outstanding on December 31 of the preceding calendar year, (b) 300,000 shares, or (c) a number determined by the Company’s board of directors that is less than (a) and (b).

As of December 31, 2019, the Company had issued 191,972 shares of common stock under the ESPP, with 69,324 of such shares of common stock being issued during the year ended December 31, 2019. The Company had 486,855 shares available for future issuance under the ESPP as of December 31, 2019. Effective December 11, 2019, the Company suspended the ESPP for any additional offering periods after December 10, 2019. Any enrollments that have been submitted to date for the next enrollment period that was scheduled to begin on December 11, 2019 have been cancelled.

Stock-Based Compensation Expense

The weighted average assumptions used in the Black-Scholes option pricing model to determine the fair value of the employee stock option grants with both time-based and performance-based vesting provisions and stock purchase rights were as follows:

	Years Ended December 31,
	2019				2018				2017
Time based stock options
Risk-free interest rate			2.2	%			2.7	%			2.0	%
Volatility			82.6	%			85.9	%			82.2	%
Dividend yield			0	%		0%				0%
Expected term (in years)		5.9				6.1				6.1
Grant date fair value per share	$	5.31			$	8.12			$	10.39
Performance based stock options
Risk-free interest rate			—				—				2.1	%
Volatility			—				—				75.9	%
Dividend yield			—				—				0	%
Expected term (in years)			—				—			6.3
Grant date fair value per share	$		—		$		—		$	6.73
Employee stock purchase plan
Risk-free interest rate			2.6	%			2.5	%			1.2	%
Volatility			71.6	%			78.1	%			69.8	%
Dividend yield			0	%			0	%			0	%
Expected term (in years)		0.5				1.1				1.3
Grant date fair value per share	$	4.44			$	5.02			$	4.61

Risk-free interest rate. The Company bases the risk-free interest rate assumption on U.S. Treasury constant maturities with maturities similar to those of the expected term of the award being valued.

Expected volatility. Due to the Company’s limited trading of its common stock and lack of company-specific historical or implied volatility data, the Company has based its estimate of expected volatility on the historical volatility of a group of similar companies in the life sciences industry whose shares are publicly traded. The Company selects the peer group based on comparable characteristics, including development stage, product pipeline, and enterprise value. The Company computes historical volatility data using the daily closing prices for the selected companies’ shares during the equivalent period of the calculated expected term of the stock-based awards. The Company will continue to apply this process until sufficient amount of historical information regarding the volatility of its own stock price become available.

Expected term. The expected term of employee stock options granted with time-based vesting provisions was calculated using the simplified method which utilizes the midpoint between the weighted average time of vesting and the end of the contractual term. The expected term of employee stock options granted with performance-based vesting provisions was calculated using the midpoint between the estimated service period and the contractual term of the option. These methods were utilized due to a lack of historical exercise behavior by the Company's employees. The expected term for stock purchase rights is the term from the date of grant to the date of purchase.

Expected dividend yield. The Company bases the expected dividend yield assumption on the fact that it has never paid, and does not expect to pay, dividends in the foreseeable future.

The Company has not recognized non-cash stock-based compensation expense for outstanding options to purchase 188,651 shares of common stock with performance-based vesting provisions after its evaluation that the occurrence of the individual milestones is not probable as of December 31, 2019.

Total non-cash stock-based compensation expense for all stock awards and purchase rights, net of forfeitures recognized as they occur, that was recognized in the statements of operations is as follows (in thousands):

	Years Ended December 31,
	2019		2018		2017
Research and development	$	3,269	$	3,023	$	1,783
General and administrative		4,791		3,847		2,668
Total	$	8,060	$	6,870	$	4,451

Unrecognized compensation expense for stock options at December 31, 2019 was $12.1 million which is expected to be recognized over a weighted-average period of 2.5 years.

License and Collaboration Agreements

12 Months Ended

Dec. 31, 2019

Collaborative Arrangements [Abstract]

License and Collaboration Agreements

8. License and Collaboration Agreements

ApolloBio License

On April 18, 2018, the Company entered into a License Agreement (the License Agreement) with Beijing Apollo Venus Biomedical Technology Limited and ApolloBio Corp. (collectively, ApolloBio), which became effective in July 2018, pursuant to which the Company granted to ApolloBio an exclusive license to develop and commercialize Toca 511 & Toca FC within the greater China region, including mainland China, Hong Kong, Macao and Taiwan (the Licensed Territory).

Under the License Agreement, the Company has received net proceeds of $15.2 million which is comprised of a $16.0 million up-front payment and a $2.0 million development milestone payment less $1.7 million in foreign income taxes and $1.1 million in certain foreign non-income taxes. The foreign income taxes were recorded as income tax expense and the foreign non-income taxes were recorded as a general and administrative expense, on the statement of operations during the year ended December 31, 2018.

The Company is eligible to receive up to an aggregate $111.0 million, less withholding and other taxes, upon the achievement of specified development and commercial milestones. The Company completed its planned enrollment of 380 patients in the Toca 5 clinical trial in 2018 and earned a $2.0 million development milestone payment. The Company is also eligible for low double-digit tiered royalty payments based on annual net sales of licensed products in the Licensed Territory, subject to reduction under specified circumstances. ApolloBio will be responsible for all development and commercialization costs in the Licensed Territory. Future payments by ApolloBio are subject to the People’s Republic of China (PRC) currency exchange approval and may be subject to other approvals by PRC authorities.

Unless earlier terminated, the License Agreement will expire upon the expiration of the last-to-expire royalty term for any and all licensed products, which royalty term is, with respect to a licensed product in a particular region (i.e., mainland China, Hong Kong, Macao and Taiwan) of the Licensed Territory (each, a Region), the latest of (i) 10 years after the first commercial sale of such licensed product in such Region, (ii) the expiration of all regulatory exclusivity as to such licensed product in such Region and (iii) the date of expiration of the last valid patent claim covering such licensed product in such Region. Either party may terminate the License Agreement upon a material breach by the other party that remains uncured following 60 days (or, with respect to any payment breach, 10 days) after the date of written notice of such breach. ApolloBio may terminate the License Agreement at any time by providing 90 days’ prior written notice to the Company. In addition, the Company may terminate the License Agreement upon written notice to ApolloBio under specified circumstances if ApolloBio challenges the licensed patent rights.

Under Topic 606, the Company evaluated the terms of the License Agreement and the transfer of intellectual property rights (the “license”) was identified as the only performance obligation as of the inception of the License Agreement. The Company determined that the transaction price under the License Agreement was comprised solely of the $16.0 million upfront payment. The future potential development and commercial milestone payments were not included in the transaction price as they were determined to be fully constrained. As part of the evaluation of the development and commercial milestone constraint, the Company determined that the achievement of such milestones is contingent upon success in future clinical trials and regulatory approvals, each of which was uncertain at the inception of the License Agreement. The Company will re-evaluate the transaction price each quarter or as uncertain events are resolved or other changes in circumstances occur. Future potential development and commercial milestone amounts would be recognized as revenue, if unconstrained. Any reimbursable program costs are recognized proportionately with the performance of the underlying services and are accounted for as a reduction to research and development expense and are excluded from the transaction price.

The entire $16.0 million transaction price was allocated to the license performance obligation. The license was delivered in connection with the execution of the License Agreement and the performance obligation was fully satisfied (transfer of intellectual property). Additionally, the Company earned a $2.0 million development milestone payment upon completion of the planned enrollment of 380 patients in the Toca 5 clinical trial.

Grant Agreements

12 Months Ended

Dec. 31, 2019

Grant Agreement [Abstract]

Grant Agreements

9. Grant Agreements

In August 2017, the Company was awarded a $2.0 million grant by the U.S. Food and Drug Administration Office of Orphan Products Development to support its Phase 3 clinical trial (OOPD Grant). Under the grant agreement, the Company will be reimbursed for qualifying expenses over a four-year period subject to the availability of funds and satisfactory progress of the trial. The Company received reimbursable amounts of $0.5 million for each of the years ended December 31, 2019, 2018 and 2017 relating to the OOPD Grant as an offset against research and development costs incurred during the period.

Income Taxes

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

Income Taxes

10. Income Taxes

Significant components of the income tax expense are as follows (in thousands):

	Year ended December 31,
	2019		2018		2017
Current
Federal	$	—	$	—	$	—
State		1		1		1
Foreign		—		1,698		—
Total current provision		1		1,699		1
Deferred
Federal		—		—		—
State		—		—		—
Total Deferred		—		—		—
Income Tax Expense	$	1	$	1,699	$	1

The (benefit) provision for income taxes differs from the amount of income tax determined by applying the applicable U.S. statutory federal income tax rate to pretax income as a result of the following differences:

	Years Ended December 31,
	2019			2018			2017
Federal statutory rate		21.0	%		21.0	%		34.0	%
Adjustments for tax effects of:
State taxes, net		—	%		6.1	%		5.7	%
Withholding Tax		—	%		(3.6	)%
Permanent adjustments		(1.4	)%		(1.0	)%		(4.5	)%
Tax Cuts and Jobs Act		—	%		—	%		(3.7	)%
Net operating loss carryovers not recognized		(18.8	)%		(22.1	)%		(30.4	)%
Valuation allowance		1.1	%		(5.9	)%		(1.0	)%
Other		(1.9	)%		1.9	%		(0.1	)%
Effective income tax rate		—	%		(3.7	)%		—	%

Deferred taxes are recognized for temporary differences between the basis of assets and liabilities for financial statement and income tax purposes. Significant components of the Company’s deferred taxes are as follows (in thousands):

	December 31,
	2019			2018
Deferred tax assets:
Depreciation and amortization	$	32		$	—
Deferred license revenue		—			10
Share-based compensation		3,793			3,432
Debt discount		79			137
Lease liability		935			644
Accrued liabilities and other		288			1,134
Total deferred tax assets		5,127			5,357
Less valuation allowance		(4,386	)		(5,107	)
Net deferred tax assets	$	741		$	250

Deferred tax liabilities:
Right of use asset	$	(739	)	$	—
Depreciation and amortization		—			(250	)
Other		(2	)		—
Total Deferred tax liabilities	$	(741	)	$	(250	)
Net deferred taxes	$	—		$	—

The Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets. Based upon the Company’s history of operating losses, the Company has concluded that it is more likely than not that the benefit of its deferred tax assets will not be realized. Accordingly, the Company has provided a full valuation allowance for deferred tax assets as of December 31, 2019 and 2018. During 2019 and 2018, the valuation allowance decreased by $0.7 million and increased by $2.6 million, respectively.

The Company has federal and California net operating loss carryforwards which may be available to offset future income tax liabilities. As of December 31, 2019, the Company has federal net operating losses of $231.0 million, of which, $136.6 million begin expiring in 2028 unless previously utilized and $94.4 million that do not expire but are limited to 80% of taxable income in a given year. The Company has state net operating loss carryforwards of $76.0 million that begin to expire in 2028 unless previously utilized as of December 31, 2018. Excluded from the California net operating loss carryforward are net operating losses for the years ended December 31, 2013 through 2017 and 2019 which are impacted by a California Supreme Court ruling issued on December 31, 2015. This ruling held that taxpayers must use the single sales factor market based sourcing method in determining their California apportionment. As a result of the ruling, the Company has completed an analysis to determine the re-apportionment of its losses to California using the required single sales factor market sourcing method for 2013 through 2017 and 2019. In doing so, the Company treated its passive interest income as California-source income which results in a 100% apportionment percentage to California. While this portion may not reach the more-likely-than-not recognition threshold, the Company has excluded a cumulative net operating loss of $167.9 million from its California net operating loss carryforward.

As of December 31, 2019, the Company has federal and California research and development tax credit carryforwards of $29.2 million and $7.1 million, respectively. The federal research and development tax credits begin to expire in 2028 unless previously utilized. The California credits do not expire.

Pursuant to Internal Revenue Code (IRC) Sections 382 and 383, annual use of a company’s net operating loss and tax credit carryforwards may be limited if there is a cumulative change in ownership of greater than 50% (by value) within a three-year period. The amount of the annual limitation is determined based on the value of the Company immediately prior to the ownership change. Subsequent ownership changes may further affect the limitation in future years. The Company has completed several equity offerings since its inception which may have resulted in a change in control as defined by Sections 382 and 383 of the IRC, or could result in a change in control in the future. The Company has not completed an IRC Section 382 and 383 analysis regarding the limitation of net operating loss and research and development credit carryforwards. Until such an analysis has been completed, the Company has removed the deferred tax assets for net operating losses of $53.8 million and federal and California research and development credits of approximately $34.8 million from its deferred tax asset schedule, and has recorded a corresponding decrease to its valuation allowance. When this analysis is finalized, the Company plans to update its unrecognized tax benefits accordingly. The Company does not expect this analysis to be completed within the next 12 months and, as a result, the Company does not expect that the unrecognized tax benefits will change within 12 months of this reporting date. Due to the existence of the valuation allowance, future changes in the Company’s unrecognized tax benefits will not impact the Company’s effective tax rate.

The Company’s policy is to record interest and penalties relating to uncertain tax positions as a component of income tax expense. As of December 31, 2019 and 2018, there was no accrued interest or penalties for uncertain tax positions.

The Company is subject to taxation in the U.S. and various state jurisdictions. As of December 31, 2019, the Company’s tax years beginning 2007 to date are subject to examination by federal and California taxing authorities due to the carry forward of unutilized net operating losses and research and development tax credits. To the extent the Company has tax attribute carryforwards, the tax years in which the attribute was generated may still be adjusted upon examination by the Internal Revenue Service or state tax authorities to the extent utilized in a future period.

Retirement Plan

12 Months Ended

Dec. 31, 2019

Compensation And Retirement Disclosure [Abstract]

Retirement Plan

11. Retirement Plan

The Company sponsors an employee savings plan that qualifies as a deferred salary arrangement under Section 401(k) of the IRC. Participating employees may defer up to the Internal Revenue Service annual contribution limit. The Company has elected to match 50% of an employee’s contributions up to 6% of the employees’ eligible salary beginning January 1, 2019. The Company made contributions of $0.3 million for the year ended December 31, 2019.

Commitments and Contingencies

12 Months Ended

Dec. 31, 2019

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

12. Commitments and Contingencies

Leases and Other Commitments

The Company leases its office and laboratory space located in San Diego, California, under an operating lease agreement (the Lease). The Lease commenced in March 2018. The term of the Lease is eight years and the Company has one option to extend the Lease for a period of five additional years.

In connection with the inception of the Lease, the Company was provided and fully utilized a tenant improvement allowance of $1.2 million. The Lease provides for an abatement of a portion of the lease payments for the first nine months of the lease term and includes escalation clauses in the future.

On December 16, 2019, the Company entered into a First Amendment (the “Lease Amendment”) to the Lease. Under the terms of the Lease Amendment, the termination date of a portion of the premises containing approximately 21,180 rentable square feet was accelerated from June 30, 2026 to December 31, 2019. The Lease Amendment eliminated further rents due for the terminated rentable square feet, including aggregate base rent over the remaining term of approximately $7.6 million.

As of December 31, 2019, the Company continues to lease approximately 17,669 rentable square feet from the Landlord.

As of December 31, 2019, future annual minimum rental payments payable under the Lease are as follows (shown in thousands):

Years ended December 31:
2020		882
2021		913
2022		945
2023		978
2024		1,012
Thereafter		1,580
Total	$	6,310

The Company enters into service agreements with indemnification clauses in the ordinary course of business. Pursuant to such clauses, the Company indemnifies, defends, holds harmless, and agrees to reimburse the indemnified party for losses suffered or incurred by third party claims arising out of the indemnified party’s performance of service. The Company has not incurred costs to defend lawsuits pursuant to these indemnification clauses.

Legal Proceedings

From time to time, the Company may be involved in various claims and legal proceedings relating to claims arising out of the Company’s operations. The Company is not currently a party to any legal proceedings that, in the opinion of management, are likely to have a material adverse effect on the Company’s business. Regardless of outcome, litigation can have an adverse impact on the Company because of defense and settlement costs, diversion of management resources and other factors.

Selected Quarterly Financial Data (unaudited)

12 Months Ended

Dec. 31, 2019

Quarterly Financial Information Disclosure [Abstract]

Selected Quarterly Financial Data (unaudited)

13. Selected Quarterly Financial Data (unaudited)

The following table contains unaudited quarterly financial information for the years ended December 31, 2019 and 2018. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair statement of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period.

	First Quarter			Second Quarter			Third Quarter			Fourth Quarter
Year Ended December 31, 2019
Operating expenses	$	16,880		$	16,824		$	17,132		$	10,711
Net loss		(17,084	)		(17,114	)		(18,735	)		(10,583	)
Net loss per common share, basic and diluted	$	(0.74	)	$	(0.72	)	$	(0.78	)		(0.45	)
Year Ended December 31, 2018
Operating expenses	$	12,855		$	15,336		$	16,582		$	19,116
Net loss		(12,880	)		(16,089	)		(383	)		(19,603	)
Net loss per common share, basic and diluted	$	(0.65	)	$	(0.81	)	$	(0.02	)		(0.96	)

Subsequent Events

12 Months Ended

Dec. 31, 2019

Subsequent Events [Abstract]

Subsequent Events

14. Subsequent Events

Signed merger agreement

On February 19, 2020, the Company and Forte Biosciences, Inc. (“Forte”) signed an Agreement and Plan of Merger and Reorganization (“Merger Agreement”). Upon the terms and subject to the satisfaction of the conditions described in the Merger Agreement, including approval of the transaction by the Company’s stockholders, a wholly-owned subsidiary of the Company will be merged with and into Forte, with Forte surviving the Merger as a wholly-owned subsidiary of the Company.

The proposed Merger is structured as a stock-for-stock transaction whereby all of Forte's outstanding shares of common stock and securities convertible into or exercisable for Forte’s common stock will be converted into the right to receive Tocagen common stock and securities convertible into or exercisable for Tocagen common stock. Under the exchange ratio formula in the Merger Agreement, the former Forte equityholders immediately before the Merger are expected to own approximately 74.5% of the outstanding capital stock of Tocagen, and the equityholders of Tocagen immediately before the Merger are expected to own approximately 25.5% of the outstanding capital stock of Tocagen, on a fully diluted basis using the treasury stock method subject to certain assumptions. We anticipate that the Merger will close in the second quarter of 2020.

Additional restructuring

The Company further reduced its workforce in 2020 as a result of the proposed Merger. The Company expects that it will incur personnel-related restructuring charges of approximately $3.0 million for employee severance and other related termination benefits. Severance payments are expected to be paid in full in the first half 2020. The estimates of costs that the Company expects to incur and the timing of payments thereof are subject to a number of assumptions and actual results may differ.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2019

Accounting Policies [Abstract]

Use of Estimates

The Company’s financial statements are prepared in accordance with accounting principles generally accepted in the United States, which requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenue and expenses and the disclosure of contingent assets and liabilities in the financial statements accompanying notes. Significant estimates in the Company’s financial statements relate to clinical trial accruals, the valuation of equity awards and the development period used for license revenue recognition. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results may differ from these estimates under different assumptions or conditions.

Cash, Cash Equivalents and Marketable Securities

Cash consists of the balance in a readily available checking account. Cash equivalents consist of money market funds, corporate debt securities and certificates of deposit with remaining maturities of three months or less at the time of purchase, and are considered highly liquid investments. Marketable securities consist of corporate debt securities, commercial paper, U.S. treasury securities and asset-backed securities that have original maturities greater than three months at the time of purchase.

The Company classifies its investments as available-for-sale and records such assets at fair value in the balance sheet, with unrealized gains and losses, if any, reported in stockholders’ equity. Realized gains and losses are calculated on the specific identification method and recorded to interest income.

A decline in the market value of any marketable security below cost that is determined to be other-than-temporary results in a revaluation of its carrying amount to fair value and a new cost basis for the security. Impairment losses are recognized in other expense in the statement of operations.

Concentration of Credit Risk and Off-Balance Sheet Risk

Financial instruments that potentially subject the Company to significant concentration of credit risk consist primarily of cash equivalents and marketable securities. The Company’s investment policy includes guidelines for the quality of the related institutions and financial instruments, and defines allowable investments that the Company may invest in, which the Company believes minimizes the exposure to concentration of credit risk.

Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets primarily represent amounts related to insurance, clinical trial and manufacturing agreements, and investment interest receivable.

Property and Equipment

Property and equipment is stated at cost and depreciated over the estimated useful lives of the assets (three to five years) using the straight-line method. Leasehold improvements are depreciated using the straight-line method over the lesser of the remaining lease term or an estimated useful life .

Costs of major additions and betterments are capitalized and depreciated on a straight-line basis over their useful lives. Repairs and maintenance costs are expensed as incurred. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation are removed from the accounts, and any resulting gain or loss is credited or charged to expense.

Deferred Equity Issuance Costs

Specific incremental costs directly attributable to an offering of securities are deferred and charged against the gross proceeds of the offering through additional paid-in capital.

Impairment of Long-Lived Assets

Long-lived assets consist of property and equipment. An impairment loss is recorded if and when events and circumstances indicate that assets might be impaired and the undiscounted cash flows estimated to be generated by those assets are less than the carrying amount of those assets. No impairment loss has been recognized for the years ended December 31, 2019 and 2018.

Fair Value of Financial Instruments

The Company’s financial instruments consist principally of cash, cash equivalents, marketable securities, prepaid expenses, other current assets, accounts payable and notes payable. The carrying amounts of these financial instruments approximate the related fair values due to the short-term maturities of these instruments.

The authoritative accounting guidance defines fair value, establishes a consistent framework for measuring fair value and expands disclosure for each major asset and liability category measured at fair value on either a recurring or nonrecurring basis. Fair value is defined as an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or liability. As a basis for considering such assumptions, the authoritative accounting guidance establishes a three-tier fair value hierarchy that prioritizes the inputs used in measuring fair value as follows:

Level 1:Observable inputs such as quoted prices in active markets;

Level 2:Inputs, other than the quoted prices in active markets, that are observable either directly or indirectly such as quoted prices for similar assets or liabilities, quoted prices in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities; and

Level 3:Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities.

Clinical Trial Accruals

Expenses related to clinical studies are based on estimates of the services received and efforts expended pursuant to the Company’s contract arrangements. The financial terms of these agreements are subject to negotiation, vary from contract to contract and may result in uneven payment flows. There may be instances in which payments made to the Company’s service providers will temporarily exceed the level of services provided and result in a prepayment of the clinical expense. Payments under some of these contracts depend on factors such as the successful enrollment of patients, site initiation and the completion of clinical milestones. The Company makes estimates of its accrued expenses as of each balance sheet date in its financial statements based on facts and circumstances known at that time. In accruing service fees, the Company estimates the time period over which services will be performed and the level of effort to be expended in each period. If the actual timing of the performance of services or the level of effort varies from its estimate, the Company adjusts the accrual or prepaid expense balance accordingly. Historically, the Company’s estimated accrued liabilities have materially approximated actual expense incurred.

Revenue Recognition

Revenue generally consists of license revenue with upfront payments and development milestones considered probable of achievement.

Revenue is recognized when control of the promised goods or services is transferred to the Company’s customers in an amount that reflects the consideration the Company expects to receive from its customers in exchange for those goods and services. This process involves identifying the contract with a customer, determining the performance obligations in the contract, determining the transaction price, allocating the contract price to the distinct performance obligations in the contract, and recognizing revenue when or as the Company satisfies the performance obligation(s).

At contract inception, the Company assesses the goods and services promised within each contract and assesses whether each promised good or service is distinct and determines that those are performance obligations. A performance obligation is considered distinct from other obligations in a contract when it provides a benefit to the customer either on its own or together with other resources that are readily available to the customer and is separately identified in the contract. The Company considers factors such as the research, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. The Company considers a performance obligation satisfied once the Company has transferred control of a good or service to the customer, meaning the customer has the ability to use and obtain the benefit of the good or service. The Company recognizes revenue for satisfied performance obligations only when the Company determines there are no uncertainties regarding payment terms or transfer of control.

Collaborative Arrangements

The Company enters into collaborative arrangements with partners that may include payment to the Company of one or more of the following: (i) license fees; (ii) payments related to the achievement of developmental, regulatory, or commercial milestones; and (iii) royalties on net sales of licensed products. Where a portion of non‑refundable upfront fees or other payments received are allocated to continuing performance obligations under the terms of a collaborative arrangement, they are recorded as contract liabilities and recognized as revenue when (or as) the underlying performance obligation is satisfied.

As part of the accounting for these arrangements, the Company must develop estimates and assumptions that require judgment to determine the underlying stand-alone selling price for each performance obligation which determines how the transaction price is allocated among the performance obligation(s). The stand-alone selling price may include items such as forecasted revenues, development timelines, discount rates, and probabilities of technical and regulatory success. The Company evaluates each performance obligation to determine if it can be satisfied at a point in time or over time. In addition, variable consideration must be evaluated to determine if it is constrained and, therefore, excluded from the transaction price.

License Fees

If a license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenues from non-refundable, upfront fees allocated to the license when the license is transferred to the licensee and the licensee is able to use and benefit from the license. For licenses that are bundled with other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Any such adjustments are recorded on a cumulative catch-up basis, which would affect license, collaboration or other revenues and earnings in the period of adjustment.

Milestone Payments

At the inception of each arrangement that includes milestone payments (variable consideration), the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price. If it is probable that a milestone event would occur at the inception of the arrangement, the associated milestone value is included in the transaction price. Milestone payments that are not within the Company’s control, such as regulatory approvals, are generally not considered probable of being achieved until those approvals are received. The transaction price is then allocated to each performance obligation on a relative stand-alone selling price basis, for which the Company recognizes revenue as or when the performance obligations under the contract are satisfied. At the end of each reporting period, the Company evaluates the probability of achievement of such milestones and any related constraint(s), and if necessary, may adjust the Company’s estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect license, collaboration or other revenues and earnings in the period of adjustment.

Royalties

For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and for which the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any royalty revenue resulting from its collaborative arrangements.

Research and Development Costs

Research and development expenses consist primarily of salaries and other personnel related expenses including non-cash stock-based compensation costs, preclinical costs, clinical trial costs, costs related to acquiring and manufacturing clinical trial materials, contract services, facilities costs, overhead costs, and depreciation. All research and development costs are expensed as incurred.

Patent Costs

Costs related to filing and pursuing patent applications are recorded as general and administrative expense and expensed as incurred because recoverability of such expenditures is uncertain.

Grant Funding

The Company receives certain research and development funding through grants from nonprofit organizations that serve the brain cancer community. The Company evaluates the terms of each grant to assess the Company’s obligations, and such funding is recognized in the statement of operations as a reduction to research and development expense as the related costs are incurred to meet those obligations over the grant period. Certain grants contain repayment provisions contingent on future events, such as future revenue milestones related to the Company’s lead product candidate under development. For each repayment provision, the Company assesses if it is obligated to repay the funds provided by the other parties regardless of the outcome of the funded research and development. For each arrangement, the Company also reviews the repayment provisions to determine the likelihood of repayment at the execution of each grant and on an ongoing basis. If the likelihood of repayment of a grant is determined to be remote and the Company is not obligated to repay the funds regardless of the outcome of the funded research and development, the grant is recognized as a reduction to research and development expense as related costs are incurred over the grant period. The Company subsequently reviews the repayment provisions of each grant at each reporting date and will record a related grant repayment liability if and when such repayment obligation is determined to be probable. If, at the execution of a grant with repayment provisions, the probability of repayment is probable, the Company will record the grant as a liability until such time as the grant requirements have been satisfied and the repayment provisions have lapsed.

Debt Issuance Costs

Debt issuance costs incurred to obtain debt financing are deferred and are amortized over the term of the debt using the effective interest method. The costs are recorded as a reduction to the carrying value of the debt and the amortization expense is included in interest expense in the statement of operations.

Warrants for Shares of Common Stock

The Company accounts for warrants for shares of common stock as equity instruments in the accompanying balance sheets at their fair value on the date of issuance because such warrants are indexed to the Company’s common stock and no cash settlement is required except for (i) liquidation of the Company, or (ii) a change in control in which the common stockholders also receive cash.

Income Taxes

The Company accounts for income taxes under the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences that have been included in the financial statements. Under this method, deferred tax assets and liabilities are determined on the basis of the differences between the financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. The Company provides a valuation allowance against net deferred tax assets unless, based upon the available evidence, it is more likely than not that the deferred tax assets will be realized.

The Company records uncertain tax positions on the basis of a two-step process whereby (i) management determines whether it is more likely than not that the tax positions will be sustained on the basis of the technical merits of the position and (ii) for those tax positions that meet the more likely-than-not recognition threshold, management recognizes the largest amount of tax benefit that is more than 50 percent likely to be realized upon ultimate settlement with the related tax authority. The Company will recognize interest and penalties in income tax expense if and when incurred.

Comprehensive Income (Loss)

All components of comprehensive income (loss) are reported in the financial statements in the period in which they are recognized. Other comprehensive income (loss) is defined as the change in equity during a period from transactions and other events and circumstances from non-owner sources, including unrealized gains and losses on investments. The Company’s only component of other comprehensive loss is unrealized gains (losses) on investments. Comprehensive gains (losses) have been reflected in the statements of operations and comprehensive loss for all periods presented.

Stock-Based Compensation

Stock-based compensation expense represents the cost of the grant date fair value of stock awards, including stock options, and stock purchase rights granted to employees and members of the Company’s board of directors. For awards with time-based vesting provisions, the Company estimates the fair value of stock options on the date of grant using the Black-Scholes option pricing model and recognizes the expense over the requisite service period of the awards, which is generally the vesting period, on a straight-line basis. For awards with performance-based vesting provisions, the Company estimates the fair value of stock option grants on the date of grant, or the date when all of the terms of the grant have been agreed to, if later, and recognizes the expense based on the probability of the occurrence of the individual milestones at each reporting period. The expense is recognized over the implicit service period that commences once management believes the performance criteria are probable of being met. For purchase rights, the Company estimates the fair value of the purchase as of the plan enrollment date and recognizes expense on a straight-line basis over the applicable offering period. The Company accounts for forfeitures when they occur, and reverses any compensation cost previously recognized for awards for which the requisite service has not been completed, in the period that the award is forfeited.

Net Loss Per Share

Basic and diluted net loss per common share for the periods presented is computed by dividing net loss by the weighted-average number of common shares outstanding during the respective periods, without consideration of common stock equivalents as they are anti-dilutive. Common stock equivalents that could potentially dilute earnings in the future are comprised of shares issuable upon the conversion of options to purchase shares of common stock outstanding under the Company’s equity incentive plan and warrants for the purchase of shares of common. For all periods presented, there is no difference in the number of shares used to calculate basic and diluted shares outstanding due to the Company’s net loss position.

Common stock equivalents from potentially dilutive securities that are not included in the calculation of diluted net loss per share, because to do so would be anti-dilutive, are as follows:

	Years Ended December 31,
	2019		2018		2017
Common stock options		4,334,935		3,476,847		2,589,348
Common stock warrants		66,514		67,238		10,660
Total		4,401,449		3,544,085		2,600,008

Recent Accounting Pronouncements

In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842). The new standard is aimed at making leasing activities more transparent and comparable. Under the new guidance, lessees are required to recognize substantially all leases on their balance sheet as a lease liability, which is a lessee’s obligation to make lease payments arising from a lease, measured on a discounted basis and a right-of-use asset, which is an asset that represents the lessee’s right to use, or control the use of, a specified asset for the lease term. The Company adopted Topic 842 on January 1, 2019 using the modified retrospective approach with a cumulative-effect adjustment as of January 1, 2019. The Company recognized a right-of-use asset and a lease liability on the balance sheet for the discounted value of future lease payments from the date of adoption. The impact on the balance sheet as of the date of adoption was as follows (in thousands):

	ASC 840		ASC 842		Impact of
	January 1, 2019		January 1, 2019		Adoption
Balance Sheet
Accrued liabilities	$	154	$	645	$	491
Deferred rent, net of current portion		2,201		—		(2,201	)
Operating lease right-of-use asset		—		8,060		8,060
Operating lease liability, net of current portion		—		9,770		9,770

In June 2018, the FASB issued ASU 2018-07, Compensation- Stock Compensation (Topic 718), Improvements to Nonemployee Share-Based Payment Accounting. This new standard is intended to simplify aspects of share-based compensation issued to non-employees by aligning the accounting for share-based payment awards issued to employees and non-employees as it relates to the measurement date and impact of performance conditions. The new standard became effective January 1, 2019 and did not have a material impact to the overall financial statements of the Company.

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2019

Accounting Policies [Abstract]

Schedule of Common Stock Equivalents from Potentially Dilutive Securities Not Included in Calculation of Diluted Net Loss Per Share

Common stock equivalents from potentially dilutive securities that are not included in the calculation of diluted net loss per share, because to do so would be anti-dilutive, are as follows:

	Years Ended December 31,
	2019		2018		2017
Common stock options		4,334,935		3,476,847		2,589,348
Common stock warrants		66,514		67,238		10,660
Total		4,401,449		3,544,085		2,600,008

Schedule of Impact of Adoption of New Accounting Pronouncements on Balance Sheet

The impact on the balance sheet as of the date of adoption was as follows (in thousands):

	ASC 840		ASC 842		Impact of
	January 1, 2019		January 1, 2019		Adoption
Balance Sheet
Accrued liabilities	$	154	$	645	$	491
Deferred rent, net of current portion		2,201		—		(2,201	)
Operating lease right-of-use asset		—		8,060		8,060
Operating lease liability, net of current portion		—		9,770		9,770

Fair Value of Financial Instruments (Tables)

12 Months Ended

Dec. 31, 2019

Fair Value Disclosures [Abstract]

Summary of Fair Values of Assets Measured on a Recurring Basis

The following tables summarize the Company’s assets that require fair value measurements on a recurring basis and their respective input levels based on the fair value hierarchy (in thousands):

			Fair Value Measurements at End of Period Using:
	Total		Quoted Market Prices for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
December 31, 2019
Marketable securities:
Corporate debt securities	$	3,700	$	—	$	3,700	$	—
Commercial paper		5,485		—		5,485		—
Asset-backed securities		3,650		—		3,650		—
	$	12,835	$	—	$	12,835	$	—

			Fair Value Measurements at End of Period Using:
	Total		Quoted Market Prices for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
December 31, 2018
Cash equivalents:
Corporate debt securities	$	4,783	$	—	$	4,783	$	—
Commercial paper		1,987		—		1,987		—
	$	6,770	$	—	$	6,770	$	—
Marketable securities:
Corporate debt securities	$	16,301	$	—	$	16,301	$	—
Commercial paper		24,576		—		24,576		—
U.S. treasury securities		1,997		1,997		—		—
Asset-backed securities		12,399		—		12,399		—
	$	55,273	$	1,997	$	53,276	$	—

Certain Financial Statement Caption Information (Tables)

12 Months Ended

Dec. 31, 2019

Balance Sheet Related Disclosures [Abstract]

Summary of Marketable Securities

The following is a summary of the Company’s marketable securities (in thousands):

	Maturity (in years)	Amortized Cost		Unrealized Gain		Unrealized Loss			Fair Value
December 31, 2019
Corporate debt securities	1 or less	$	2,699	$	1	$	—		$	2,700
Corporate debt securities	>1 and <5		999		1		—			1,000
Commercial paper	1 or less		5,481		4		—			5,485
Asset-backed securities	1 or less		3,646		4		—			3,650
		$	12,825	$	10	$	—		$	12,835
December 31, 2018
Corporate debt securities	1 or less	$	10,013	$	1	$	(4	)	$	10,010
Corporate debt securities	>1 and <5		6,293		2		(4	)		6,291
Commercial paper	1 or less		24,584		—		(8	)		24,576
U.S. treasury securities	1 or less		1,997		—		—			1,997
Asset-backed securities	1 or less		10,612		—		(8	)		10,604
Asset-backed securities	>1 and <5		1,797				(2	)		1,795
		$	55,296	$	3	$	(26	)	$	55,273

Summary of Property and Equipment

Property and equipment is comprised of (in thousands):

	December 31,
	2019			2018
Laboratory equipment	$	2,184		$	4,445
Computers, software and office equipment		252			322
Furniture and fixtures		216			610
Leasehold improvements		636			1,927
		3,288			7,304
Less: accumulated depreciation		(1,599	)		(3,331	)
	$	1,689		$	3,973

Components of Accrued Liabilities

Accrued liabilities are comprised of (in thousands):

	December 31,
	2019		2018
Clinical trial expenses	$	2,404	$	4,535
Payroll and other employee-related expenses		1,116		2,840
Contract manufacturing services		387		3,411
Lease liability		416		—
Professional fees		330		474
Interest payable		37		205
Other		552		1,629
Total accrued liabilities	$	5,242	$	13,094

Notes Payable (Tables)

12 Months Ended

Dec. 31, 2019

Debt Disclosure [Abstract]

Schedule of Aggregate Carrying Amounts of Term and Initial Loans

The aggregate carrying amounts of the Term Loans and Initial Loans are comprised of the following, as applicable (in thousands):

	December 31,
	2019			2018
Principal	$	5,000		$	26,450
Add: accreted liability for final payment fee	138			276
Less: unamortized discount		(394	)		(525	)
	$	4,744		$	26,201

Schedule of Future Maturities of Term Loans Including Final Payment Fee

Future maturities of the Term Loans, including the final payment fee, as of December 31, 2019 are as follows (in thousands):

	December 31, 2019
Year ending December 31, 2020		1,667
Year ending December 31, 2021		1,667
Year ending December 31, 2022		2,064
Notes payable, current portion		5,398
Unaccreted balance for final payment fee on Loans		(260	)
Unamortized discounts		(394	)
		4,744

Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2019

Equity [Abstract]

Schedule of Common Stock Reserved for Future Issuance

Common stock reserved for future issuance as of December 31, 2019 and 2018 is as follows:

	December 31,
	2019		2018
Issued and Outstanding:
Stock options		4,334,935		3,476,847
Warrants for common stock		66,514		67,238
Shares reserved for issuance under the ESPP		486,855		326,178
Shares reserved for future award grants		543,284		451,063

Equity Incentive Plan and Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Stock Option Activity

The following table summarizes stock option activity under the Company’s equity incentive plans for the year ended December 31, 2019:

	Shares Subject to Options			Weighted- Average Exercise Price per Share		Weighted- Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value (in thousands)
Options Outstanding at December 31, 2018		3,476,847		$	12.67
Granted		1,841,380		$	7.60
Exercised		(69,697	)	$	3.61
Forfeitures and cancellations		(913,595	)	$	11.67
Options Outstanding at December 31, 2019		4,334,935		$	10.87	7.2
Options Exercisable at December 31, 2019		2,128,758		$	11.58	5.8	$	-

Summary of Information Regarding Stock Options

The following table summarizes certain information regarding stock options (in thousands):

	Years Ended December 31,
	2019		2018		2017
Fair value of options vested during the period	$	6,336	$	6,318	$	2,194
Cash received from options exercised during the period	$	252	$	67	$	81
Intrinsic value of options exercised during the period	$	491	$	508	$	677

Summary of Fair Value of Employee Stock Option Grants with Both Time-Based and Performance-Based Vesting Provisions and Stock Purchase Rights

The weighted average assumptions used in the Black-Scholes option pricing model to determine the fair value of the employee stock option grants with both time-based and performance-based vesting provisions and stock purchase rights were as follows:

	Years Ended December 31,
	2019				2018				2017
Time based stock options
Risk-free interest rate			2.2	%			2.7	%			2.0	%
Volatility			82.6	%			85.9	%			82.2	%
Dividend yield			0	%		0%				0%
Expected term (in years)		5.9				6.1				6.1
Grant date fair value per share	$	5.31			$	8.12			$	10.39
Performance based stock options
Risk-free interest rate			—				—				2.1	%
Volatility			—				—				75.9	%
Dividend yield			—				—				0	%
Expected term (in years)			—				—			6.3
Grant date fair value per share	$		—		$		—		$	6.73
Employee stock purchase plan
Risk-free interest rate			2.6	%			2.5	%			1.2	%
Volatility			71.6	%			78.1	%			69.8	%
Dividend yield			0	%			0	%			0	%
Expected term (in years)		0.5				1.1				1.3
Grant date fair value per share	$	4.44			$	5.02			$	4.61

Summary of Total Non-Cash Stock-Based Compensation Expense for All Stock Awards and Purchase Rights, Net of Forfeitures Recognized

Total non-cash stock-based compensation expense for all stock awards and purchase rights, net of forfeitures recognized as they occur, that was recognized in the statements of operations is as follows (in thousands):

	Years Ended December 31,
	2019		2018		2017
Research and development	$	3,269	$	3,023	$	1,783
General and administrative		4,791		3,847		2,668
Total	$	8,060	$	6,870	$	4,451

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

Significant Components of Income Tax Expense

Significant components of the income tax expense are as follows (in thousands):

	Year ended December 31,
	2019		2018		2017
Current
Federal	$	—	$	—	$	—
State		1		1		1
Foreign		—		1,698		—
Total current provision		1		1,699		1
Deferred
Federal		—		—		—
State		—		—		—
Total Deferred		—		—		—
Income Tax Expense	$	1	$	1,699	$	1

Reconciliation of U.S. Statutory Federal Income Tax Rate

The (benefit) provision for income taxes differs from the amount of income tax determined by applying the applicable U.S. statutory federal income tax rate to pretax income as a result of the following differences:

	Years Ended December 31,
	2019			2018			2017
Federal statutory rate		21.0	%		21.0	%		34.0	%
Adjustments for tax effects of:
State taxes, net		—	%		6.1	%		5.7	%
Withholding Tax		—	%		(3.6	)%
Permanent adjustments		(1.4	)%		(1.0	)%		(4.5	)%
Tax Cuts and Jobs Act		—	%		—	%		(3.7	)%
Net operating loss carryovers not recognized		(18.8	)%		(22.1	)%		(30.4	)%
Valuation allowance		1.1	%		(5.9	)%		(1.0	)%
Other		(1.9	)%		1.9	%		(0.1	)%
Effective income tax rate		—	%		(3.7	)%		—	%

Components of Deferred Taxes

Deferred taxes are recognized for temporary differences between the basis of assets and liabilities for financial statement and income tax purposes. Significant components of the Company’s deferred taxes are as follows (in thousands):

	December 31,
	2019			2018
Deferred tax assets:
Depreciation and amortization	$	32		$	—
Deferred license revenue		—			10
Share-based compensation		3,793			3,432
Debt discount		79			137
Lease liability		935			644
Accrued liabilities and other		288			1,134
Total deferred tax assets		5,127			5,357
Less valuation allowance		(4,386	)		(5,107	)
Net deferred tax assets	$	741		$	250

Deferred tax liabilities:
Right of use asset	$	(739	)	$	—
Depreciation and amortization		—			(250	)
Other		(2	)		—
Total Deferred tax liabilities	$	(741	)	$	(250	)
Net deferred taxes	$	—		$	—

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2019

Commitments And Contingencies Disclosure [Abstract]

Schedule of Future Annual Minimum Rental Payments Payable Under New Lease

As of December 31, 2019, future annual minimum rental payments payable under the Lease are as follows (shown in thousands):

Years ended December 31:
2020		882
2021		913
2022		945
2023		978
2024		1,012
Thereafter		1,580
Total	$	6,310

Selected Quarterly Financial Data (unaudited) (Tables)

12 Months Ended

Dec. 31, 2019

Quarterly Financial Information Disclosure [Abstract]

Unaudited Quarterly Financial Information

The following table contains unaudited quarterly financial information for the years ended December 31, 2019 and 2018. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair statement of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period.

	First Quarter			Second Quarter			Third Quarter			Fourth Quarter
Year Ended December 31, 2019
Operating expenses	$	16,880		$	16,824		$	17,132		$	10,711
Net loss		(17,084	)		(17,114	)		(18,735	)		(10,583	)
Net loss per common share, basic and diluted	$	(0.74	)	$	(0.72	)	$	(0.78	)		(0.45	)
Year Ended December 31, 2018
Operating expenses	$	12,855		$	15,336		$	16,582		$	19,116
Net loss		(12,880	)		(16,089	)		(383	)		(19,603	)
Net loss per common share, basic and diluted	$	(0.65	)	$	(0.81	)	$	(0.02	)		(0.96	)

Organization and Basis of Presentation - Additional Information (Details)				1 Months Ended	12 Months Ended
	Oct. 31, 2019 USD ($)	Oct. 01, 2019 USD ($)	May 18, 2018 Lender	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) Segment shares	Dec. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($)	Nov. 30, 2018 USD ($)
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of operating segment \| Segment					1
Percentage of reduction in total workforce		65.00%
Restructuring charge		$ 900,000
Severance costs remained unpaid					$ 200,000
Net proceeds from issuance of common stock					551,000	$ 661,000	$ 507,000
Accumulated deficit				$ 215,884,000	279,400,000	$ 215,884,000
Working capital					9,800,000
Cash, cash equivalents and marketable securities					$ 21,800,000
Second Amendment Loan Agreement
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Prepayment of term loans	$ 23,300,000
Term Loans \| Loan Agreement
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of Lenders \| Lender			2
Term Loans \| Second Amendment Loan Agreement
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Prepayment of term loans	$ 21,500,000
ATM Facility
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Common stock shares aggregate offering price								$ 30,000,000
Number of shares issued on private placement \| shares					760,089
Net proceeds from issuance of common stock					$ 7,700,000
Common stock issued, shares \| shares					760,089
Public Offering
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Net proceeds from issuance of common stock				$ 28,000,000
Common stock issued, shares \| shares				3,000,000
Shares price, per share \| $ / shares				$ 10.00		$ 10.00

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Significant Accounting Policies [Line Items]
Impairment loss on long lived assets	$ 0	$ 0
Income tax position likely of being realized upon ultimate settlement	more than 50 percent
Minimum
Schedule Of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life	3 years
Maximum
Schedule Of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life	5 years
Leasehold Improvements
Schedule Of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life description	Leasehold improvements are depreciated using the straight-line method over the lesser of the remaining lease term or an estimated useful life .

Summary of Significant Accounting Policies - Schedule of Common Stock Equivalents from Potentially Dilutive Securities Not Included in Calculation of Diluted Net Loss Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potential dilutive securities not included in calculation of diluted net loss per share	4,401,449	3,544,085	2,600,008
Common Stock Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potential dilutive securities not included in calculation of diluted net loss per share	4,334,935	3,476,847	2,589,348
Common Stock Warrants
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potential dilutive securities not included in calculation of diluted net loss per share	66,514	67,238	10,660

Summary of Significant Accounting Policies - Schedule of Impact of Adoption of New Accounting Pronouncements on Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Accrued liabilities	$ 5,242		$ 13,094
Deferred rent, net of current portion			$ 2,201
Operating lease right-of-use asset	3,515
Operating lease liability, net of current portion	$ 4,027
ASC 840
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Accrued liabilities		$ 154
Deferred rent, net of current portion		2,201
ASC 842
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Accrued liabilities		645
Operating lease right-of-use asset		8,060
Operating lease liability, net of current portion		9,770
ASC 842 \| Impact of Adoption
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Accrued liabilities		491
Deferred rent, net of current portion		(2,201)
Operating lease right-of-use asset		8,060
Operating lease liability, net of current portion		$ 9,770

Fair Value of Financial Instruments - Summary of Fair Value of Assets Measured on a Recurring Basis (Details) - Fair Value Measured on Recurring Basis - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents		$ 6,770
Marketable securities	$ 12,835	55,273
Quoted Market Prices for Identical Assets (Level 1)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities		1,997
Significant Other Observable Inputs (Level 2)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents		6,770
Marketable securities	12,835	53,276
Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents		4,783
Marketable securities	3,700	16,301
Corporate Debt Securities \| Significant Other Observable Inputs (Level 2)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents		4,783
Marketable securities	3,700	16,301
Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents		1,987
Marketable securities	5,485	24,576
Commercial Paper \| Significant Other Observable Inputs (Level 2)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents		1,987
Marketable securities	5,485	24,576
Asset-backed Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	3,650	12,399
Asset-backed Securities \| Significant Other Observable Inputs (Level 2)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	$ 3,650	12,399
U.S. Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities		1,997
U.S. Treasury Securities \| Quoted Market Prices for Identical Assets (Level 1)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities		$ 1,997

Fair Value of Financial Instruments - Additional Information (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investments, transfer of Level 1 to Level 2	$ 0	$ 0
Carrying Amount \| Significant Unobservable Inputs (Level 3)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Notes payable	4,700,000
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investments in money market funds measured at fair value using net asset value per share	$ 6,300,000	$ 30,900,000

Certain Financial Statement Caption Information - Summary of Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 12,825	$ 55,296
Unrealized Gain	10	3
Unrealized Loss		(26)
Fair Value	12,835	55,273
Corporate Debt Securities \| Maturity (in years) 1 or Less
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,699	10,013
Unrealized Gain	1	1
Unrealized Loss		(4)
Fair Value	2,700	10,010
Corporate Debt Securities \| Maturity More Than 1 Year and Less Than 5 Years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	999	6,293
Unrealized Gain	1	2
Unrealized Loss		(4)
Fair Value	1,000	6,291
Commercial Paper \| Maturity (in years) 1 or Less
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,481	24,584
Unrealized Gain	4
Unrealized Loss		(8)
Fair Value	5,485	24,576
U.S. Treasury Securities \| Maturity (in years) 1 or Less
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		1,997
Fair Value		1,997
Asset-backed Securities \| Maturity (in years) 1 or Less
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,646	10,612
Unrealized Gain	4
Unrealized Loss		(8)
Fair Value	$ 3,650	10,604
Asset-backed Securities \| Maturity More Than 1 Year and Less Than 5 Years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		1,797
Unrealized Loss		(2)
Fair Value		$ 1,795

Certain Financial Statement Caption Information - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Balance Sheet Related Disclosures [Abstract]
Other-than-temporary impairments during period	$ 0
Depreciation	$ 843,000	$ 625,000	$ 292,000

Certain Financial Statement Caption Information - Summary of Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 3,288	$ 7,304
Less: accumulated depreciation	(1,599)	(3,331)
Property and equipment, net	1,689	3,973
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	2,184	4,445
Computers, Software and Office Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	252	322
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	216	610
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 636	$ 1,927

Certain Financial Statement Caption Information - Components of Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Balance Sheet Related Disclosures [Abstract]
Clinical trial expenses	$ 2,404	$ 4,535
Payroll and other employee-related expenses	1,116	2,840
Contract manufacturing services	387	3,411
Lease liability	416
Professional fees	330	474
Interest payable	37	205
Other	552	1,629
Total accrued liabilities	$ 5,242	$ 13,094

Notes Payable - Additional Information (Details) $ / shares in Units, $ in Thousands				12 Months Ended
	Oct. 31, 2019 USD ($)	May 18, 2018 USD ($) $ / shares shares	Oct. 30, 2015 USD ($) Lender	Dec. 31, 2019	Dec. 31, 2018 USD ($)
Debt Instrument [Line Items]
Proceeds from term loans					$ 8,631
Prior Agreement
Debt Instrument [Line Items]
Number of Lenders \| Lender			2
Loans			$ 18,000
Debt instrument, frequency of periodic payment				due in monthly principal and interest payments, with final maturity of the Initial Loans in May 2019.
Debt instrument maturity			May 31, 2019
Debt instrument, final payment fee, percentage			7.95%
Loan Agreement \| Term Loans
Debt Instrument [Line Items]
Loans		$ 26,500
Debt instrument maturity		Dec. 01, 2022
Debt instrument, final payment fee, percentage		7.95%
Proceeds from term loans		$ 8,600
Number of equal monthly payments of principal and interest		36 months
Debt instrument payment terms				the Company makes interest-only payments through January 1, 2020, followed by 36 equal monthly payments of principal and interest
Debt instrument, interest rate terms				The Term Loans bear interest at a floating per annum rate equal to the greater of (i) 8.50% and (ii) the sum of (a) the prime rate reported in the Wall Street Journal on the last business day of the month that immediately proceeds the month in which the interest will accrue, plus (b) 3.75%.
Warrants to purchase shares of common stock \| shares		56,578
Warrant to purchase common stock, exercise price \| $ / shares		$ 9.35
Fair value of warrants at the date of issuance		$ 500
Deferred debt issuance cost		$ 100
Debt instrument effective interest rate		10.70%
Loan Agreement \| Term Loans \| Expected Term \| Significant Unobservable Inputs (Level 3)
Debt Instrument [Line Items]
Expected term in years		10 years
Loan Agreement \| Term Loans \| Volatility \| Significant Unobservable Inputs (Level 3)
Debt Instrument [Line Items]
Warrant rate		0.856
Loan Agreement \| Term Loans \| Risk Free \| Significant Unobservable Inputs (Level 3)
Debt Instrument [Line Items]
Warrant rate		0.031
Loan Agreement \| Term Loans \| Expected Dividend \| Significant Unobservable Inputs (Level 3)
Debt Instrument [Line Items]
Warrant rate		0.00
Loan Agreement \| Term Loans \| Prime Rate
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		3.75%
Loan Agreement \| Term Loans \| Minimum
Debt Instrument [Line Items]
Floating rate of interest		8.50%
Second Amendment Loan Agreement
Debt Instrument [Line Items]
Prepayment of term loans	$ 23,300
Second Amendment Loan Agreement \| Term Loans
Debt Instrument [Line Items]
Prepayment of term loans	$ 21,500

Notes Payable - Schedule of Aggregate Carrying Amounts of Term and Initial Loans (Details) - Loan Agreement - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Debt Instrument, Type [Extensible List]	toca:TermLoanMember	toca:TermLoanMember
Principal	$ 5,000	$ 26,450
Add: accreted liability for final payment fee	138	276
Less: unamortized discount	(394)	(525)
Loans, aggregate carrying amount	$ 4,744	$ 26,201

Notes Payable - Schedule of Future Maturities of Term Loans Including Final Payment Fee (Details) - Loan Agreement $ in Thousands	Dec. 31, 2019 USD ($)
Debt Instrument [Line Items]
Debt Instrument, Type [Extensible List]	toca:TermLoanMember
Year ending December 31, 2020	$ 1,667
Year ending December 31, 2021	1,667
Year ending December 31, 2022	2,064
Notes payable, current portion	5,398
Unaccreted balance for final payment fee on Loans	(260)
Less: unamortized discount	(394)
Loans, aggregate carrying amount	$ 4,744

Stockholders' Equity - Additional Information (Details) - USD ($)	1 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Apr. 19, 2017
Class Of Stock [Line Items]
Common stock, shares authorized	200,000,000	200,000,000	200,000,000
Common stock, par value	$ 0.001	$ 0.001	$ 0.001
Proceeds from issuance of common stock		$ 551,000	$ 661,000	$ 507,000
Maximum shares to be sold under Sales Agreement		$ 7,575,000	$ 27,997,000	$ 86,948,000
IPO
Class Of Stock [Line Items]
Outstanding shares of convertible preferred stock converted into common stock, shares					6,690,066
Common stock, shares authorized		200,000,000
Common stock, par value		$ 0.001
Preferred stock, shares authorized		10,000,000
Preferred stock, par value		$ 0.001
Public Offering
Class Of Stock [Line Items]
Common stock issued, shares	3,000,000
Shares price, per share	$ 10.00		$ 10.00
Proceeds from issuance of common stock	$ 28,000,000
ATM Facility
Class Of Stock [Line Items]
Common stock issued, shares		760,089
Proceeds from issuance of common stock		$ 7,700,000
Weighted average sales price, per share		$ 10.41
ATM Facility \| Maximum
Class Of Stock [Line Items]
Maximum shares to be sold under Sales Agreement		$ 22,100,000

Stockholders' Equity (Deficit) - Schedule of Common Stock Reserved for Future Issuance (Details) - shares	Dec. 31, 2019	Dec. 31, 2018
Shares Reserved for Issuance Under the ESPP
Class Of Stock [Line Items]
Common stock reserved for future issuance	486,855	326,178
Warrants
Class Of Stock [Line Items]
Common stock reserved for future issuance	66,514	67,238
Stock Options
Class Of Stock [Line Items]
Common stock reserved for future issuance	4,334,935	3,476,847
Shares Reserved for Future Award Grants
Class Of Stock [Line Items]
Common stock reserved for future issuance	543,284	451,063

Equity Incentive Plan and Stock-Based Compensation - Additional Information (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Apr. 12, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock shares issued under ESPP		191,972
2017 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares available for future grants		486,855	326,178
Common stock issued, shares		69,324
Performance-Based Vesting Provisions
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options outstanding		188,651
Stock Purchase Rights \| 2017 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of maximum earnings withhold by employee to purchase shares	15.00%
Discounted stock price percentage on the date of offering	85.00%
Discounted stock price percentage on the date of purchase	85.00%
Shares of common stock reserved under the plan	250,000
Annual increase in maximum percentage of number of shares of common stock reserved for issuance	1.00%
Annual increase in maximum number of shares of common stock reserved for issuance	300,000
Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares available for future grants		4,334,935	3,476,847
Unrecognized compensation expense		$ 12.1
Weighted-average period over which unrecognized compensation expense is expected to be recognized		2 years 6 months
2017 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares available for future grants	1,600,000
Annual increase in percentage of number of shares of common stock reserved for issuance	4.00%
Options to purchase common stock expiration term		10 years
Stock options outstanding		4,334,935	3,476,847
2017 Equity Incentive Plan \| Time-Based Vesting Provisions
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options vesting period		4 years
2017 Equity Incentive Plan \| Performance-Based Vesting Provisions
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options vesting description		Grants with performance-based vesting provisions vest upon the achievement of three separate development and regulatory milestones, with one-third of the options vesting upon the achievement of each milestone.
2017 Equity Incentive Plan \| Vesting After First Year \| Time-Based Vesting Provisions
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options vesting percentage		25.00%
2017 Equity Incentive Plan \| Balance Vesting Monthly Over the Remaining Terms \| Time-Based Vesting Provisions
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options vesting period		36 months
2009 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares available for future grants				75,517

Equity Incentive Plan and Stock-Based Compensation - Summary of Stock Option Activity (Details) - 2017 Equity Incentive Plan	12 Months Ended
	Dec. 31, 2019 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares Subject to Options, Outstanding at December 31, 2018 \| shares	3,476,847
Shares Subject to Options, Granted \| shares	1,841,380
Shares Subject to Options, Exercised \| shares	(69,697)
Shares Subject to Options, Forfeitures and cancellations \| shares	(913,595)
Shares Subject to Options, Outstanding at December 31, 2019 \| shares	4,334,935
Shares Subject to Options, Exercisable at December 31,2019 \| shares	2,128,758
Weighted-Average Exercise Price per Share, Outstanding at December 31, 2018 \| $ / shares	$ 12.67
Weighted-Average Exercise Price per Share, Granted \| $ / shares	7.60
Weighted-Average Exercise Price per Share, Exercised \| $ / shares	3.61
Weighted-Average Exercise Price per Share, Forfeitures and cancellations \| $ / shares	11.67
Weighted-Average Exercise Price per Share, Outstanding at December 31, 2019 \| $ / shares	10.87
Weighted-Average Exercise Price per Share, Exercisable at December 31, 2019 \| $ / shares	$ 11.58
Weighted-Average Remaining Contractual Term, Outstanding at December 31, 2019	7 years 2 months 12 days
Weighted-Average Remaining Contractual Term, Exercisable at December 31, 2019	5 years 9 months 18 days

Equity Incentive Plan and Stock-Based Compensation - Summary of Information Regarding Stock Options (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Fair value of options vested during the period	$ 6,336	$ 6,318	$ 2,194
Cash received from options exercised during the period	252	67	81
Intrinsic value of options exercised during the period	$ 491	$ 508	$ 677

Equity Incentive Plan and Stock-Based Compensation - Summary of Fair Value of Employee Stock Option Grants with Both Time-Based and Performance-Based Vesting Provisions and Stock Purchase Rights (Details) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Time-Based Vesting Provisions
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate	2.20%	2.70%	2.00%
Volatility	82.60%	85.90%	82.20%
Dividend yield	0.00%	0.00%	0.00%
Expected term (in years)	5 years 10 months 24 days	6 years 1 month 6 days	6 years 1 month 6 days
Grant date fair value per share	$ 5.31	$ 8.12	$ 10.39
Performance-Based Vesting Provisions
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate			2.10%
Volatility			75.90%
Dividend yield			0.00%
Expected term (in years)			6 years 3 months 18 days
Grant date fair value per share			$ 6.73
Shares Reserved for Issuance Under the ESPP
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate	2.60%	2.50%	1.20%
Volatility	71.60%	78.10%	69.80%
Dividend yield	0.00%	0.00%	0.00%
Expected term (in years)	6 months	1 year 1 month 6 days	1 year 3 months 18 days
Grant date fair value per share	$ 4.44	$ 5.02	$ 4.61

Equity Incentive Plan and Stock-Based Compensation - Summary of Total Non-Cash Stock-Based Compensation Expense for All Stock Awards and Purchase Rights, Net of Forfeitures Recognized (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total non-cash stock-based compensation expense	$ 8,060	$ 6,870	$ 4,451
Research and Development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total non-cash stock-based compensation expense	3,269	3,023	1,783
General and Administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total non-cash stock-based compensation expense	$ 4,791	$ 3,847	$ 2,668

License and Collaboration Agreements - Additional Information (Details) - License Agreement With Beijing Apollo Venus Biomedical Technology Limited and Apollo Bio Corp	Apr. 18, 2018 USD ($) Patient
Collaborative Arrangements [Line Items]
Net proceeds from collaborative agreement	$ 15,200,000
Upfront payment to be received	16,000,000
Development milestone payment	2,000,000
Foreign income taxes expense	1,700,000
Development milestone earned	$ 2,000,000
Future payment description	Future payments by ApolloBio are subject to the People’s Republic of China (PRC) currency exchange approval and may be subject to other approvals by PRC authorities.
Agreement termination description	the License Agreement will expire upon the expiration of the last-to-expire royalty term for any and all licensed products, which royalty term is, with respect to a licensed product in a particular region (i.e., mainland China, Hong Kong, Macao and Taiwan) of the Licensed Territory (each, a Region), the latest of (i) 10 years after the first commercial sale of such licensed product in such Region, (ii) the expiration of all regulatory exclusivity as to such licensed product in such Region and (iii) the date of expiration of the last valid patent claim covering such licensed product in such Region. Either party may terminate the License Agreement upon a material breach by the other party that remains uncured following 60 days (or, with respect to any payment breach, 10 days) after the date of written notice of such breach. ApolloBio may terminate the License Agreement at any time by providing 90 days’ prior written notice to the Company. In addition, the Company may terminate the License Agreement upon written notice to ApolloBio under specified circumstances if ApolloBio challenges the licensed patent rights.
Transaction price allocated to license performance obligation	$ 16,000,000
Health Care, Patient Service
Collaborative Arrangements [Line Items]
Number of patients \| Patient	380
Maximum
Collaborative Arrangements [Line Items]
Development and commercial milestone payment receivable	$ 111,000,000
General and Administrative
Collaborative Arrangements [Line Items]
Foreign non-income taxes expense	$ 1,100,000

Grant Agreements - Additional Information (Details) - OOPD Grant - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Aug. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Grant Agreements [Line Items]
Award granted by government	$ 2.0
Reimbursement of qualifying expenses period	4 years
Proceed from reimbursable amounts relating to grant as offset against research and development costs incurred		$ 0.5	$ 0.5	$ 0.5

Income Taxes - Significant Components of Income Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current
State	$ 1	$ 1	$ 1
Foreign		1,698
Total current provision	1	1,699	1
Deferred
Income Tax Expense	$ 1	$ 1,699	$ 1

Income Taxes - Reconciliation of U.S. Statutory Federal Income Tax Rate (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal statutory rate	21.00%	21.00%	34.00%
Adjustments for tax effects of:
State taxes, net		6.10%	5.70%
Withholding Tax		(3.60%)
Permanent adjustments	(1.40%)	(1.00%)	(4.50%)
Tax Cuts and Jobs Act			(3.70%)
Net operating loss carryovers not recognized	(18.80%)	(22.10%)	(30.40%)
Valuation allowance	1.10%	(5.90%)	(1.00%)
Other	(1.90%)	1.90%	(0.10%)
Effective income tax rate		(3.70%)

Income Taxes - Components of Deferred Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Depreciation and amortization	$ 32
Deferred license revenue		$ 10
Share-based compensation	3,793	3,432
Debt discount	79	137
Lease liability	935	644
Accrued liabilities and other	288	1,134
Total deferred tax assets	5,127	5,357
Less valuation allowance	(4,386)	(5,107)
Net deferred tax assets	741	250
Deferred tax liabilities:
Right of use asset	(739)
Depreciation and amortization		(250)
Other	(2)
Total Deferred tax liabilities	$ (741)	$ (250)

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Line Items]
Valuation allowance increase (decrease), amount	$ (700,000)	$ 2,600,000
Cumulative change in ownership percentage	50.00%
Period for cumulative change in ownership	3 years
Deferred tax assets for net operating losses	$ 53,800,000
Deferred tax assets for federal and California research and development credits	34,800,000
Accruals interest for uncertain tax position	0	0
Penalties for uncertain tax positions	0	$ 0
Federal
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 231,000,000
Net operating loss carryforwards expiration year	2028
Net operating loss deduction percentage	80.00%
Federal \| Research and Development
Income Tax Disclosure [Line Items]
Tax credit carryforward	$ 29,200,000
Tax credit carryforward expiration year	2028
Federal \| Expiry in 2028 Onwards
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 136,600,000
Federal \| Unlimited Expiration Period
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	94,400,000
California
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 76,000,000
Net operating loss carryforwards expiration year	2028
Percentage of apportionment of losses	100.00%
Cumulative net operating loss carryforwards	$ 167,900,000
California \| Research and Development
Income Tax Disclosure [Line Items]
Tax credit carryforward	$ 7,100,000

Retirement Plan - Additional Information (Details) - USD ($) $ in Millions		12 Months Ended
	Jan. 01, 2019	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Defined Contribution Plan, Description		The Company sponsors an employee savings plan that qualifies as a deferred salary arrangement under Section 401(k) of the IRC. Participating employees may defer up to the Internal Revenue Service annual contribution limit. The Company has elected to match 50% of an employee’s contributions up to 6% of the employees’ eligible salary beginning January 1, 2019. The Company made contributions of $0.3 million for the year ended December 31, 2019.
Percentage of company's matching contribution to employee contributions under retirement plan	50.00%
Defined benefit plan, contributions by employer		$ 0.3
Maximum
Defined Benefit Plan Disclosure [Line Items]
Employer matching contribution, percent of employees' eligible salary	6.00%

Commitments and Contingencies - Additional Information (Details) $ in Millions		12 Months Ended
	Dec. 16, 2019 USD ($) ft²	Dec. 31, 2019 USD ($) ft² LeaseOption
Commitments And Contingencies [Line Items]
Operating lease, description		The Company leases its office and laboratory space located in San Diego, California, under an operating lease agreement (the Lease). The Lease commenced in March 2018.
Operating lease, commenced period		2018-03
Operating lease, term of contract		8 years
Operating lease, existence of option to extend		true
Operating lease, option to extend		the Company has one option to extend the Lease for a period of five additional years.
Number of options to extend lease \| LeaseOption		1
Operating lease, additional term of contract		5 years
Operating lease, term of terminate		9 months
Operating lease, tenant improvement allowance		$ 1.2
Lease Amendment
Commitments And Contingencies [Line Items]
Net rentable area \| ft²	21,180	17,669
Remaining term of base rent	$ 7.6

Commitments and Contingencies - Schedule of Future Annual Minimum Rental Payments Payable Under New Lease (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Commitments And Contingencies Disclosure [Abstract]
2020	$ 882
2021	913
2022	945
2023	978
2024	1,012
Thereafter	1,580
Total	$ 6,310

Selected Quarterly Financial Data (unaudited) - Unaudited Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Operating expenses	$ 10,711	$ 17,132	$ 16,824	$ 16,880	$ 19,116	$ 16,582	$ 15,336	$ 12,855	$ 61,547	$ 63,889	$ 37,669
Net loss	$ (10,583)	$ (18,735)	$ (17,114)	$ (17,084)	$ (19,603)	$ (383)	$ (16,089)	$ (12,880)	$ (63,516)	$ (48,955)	$ (38,929)
Net loss per common share, basic and diluted	$ (0.45)	$ (0.78)	$ (0.72)	$ (0.74)	$ (0.96)	$ (0.02)	$ (0.81)	$ (0.65)	$ (2.69)	$ (2.44)	$ (2.66)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Subsequent Events - Additional Information (Details) - USD ($) $ in Millions	Feb. 19, 2020	Oct. 01, 2019
Subsequent Event [Line Items]
Restructuring charge		$ 0.9
Subsequent Event \| Employee Severance
Subsequent Event [Line Items]
Restructuring charge	$ 3.0
Tocagen Inc. \| Subsequent Event
Subsequent Event [Line Items]
Business merger percentage	25.50%
Forte Biosciences, Inc. \| Tocagen Inc. \| Subsequent Event
Subsequent Event [Line Items]
Business merger percentage	74.50%