Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2018	Feb. 22, 2019	Jun. 30, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Trading Symbol	TOCA
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
Entity Common Stock, Shares Outstanding		23,019,097
Entity Public Float			$ 186

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 40,813	$ 35,933
Marketable securities	55,273	52,792
Prepaid expenses and other current assets	1,662	1,904
Total current assets	97,748	90,629
Property and equipment, net	3,973	1,217
Other assets	1,360	227
Total assets	103,081	92,073
Current liabilities:
Accounts payable	3,404	1,951
Accrued liabilities	13,094	8,120
Notes payable, current portion		7,200
Deferred license revenue	36	36
Deferred grant funding		23
Total current liabilities	16,534	17,330
Notes payable, net of current portion	26,201	3,625
Deferred license revenue, net of current portion		36
Deferred rent, net of current portion	2,201
Total liabilities	44,936	20,991
Commitments and contingencies
Stockholders’ equity
Common stock, $0.001 par value; 200,000,000 shares authorized at December 31, 2018 and 2017, respectively; 23,000,151 and 19,882,551 shares issued and outstanding at December 31, 2018 and December 31, 2017, respectively	23	20
Additional paid-in capital	274,029	238,025
Accumulated deficit	(215,884)	(166,929)
Accumulated other comprehensive loss	(23)	(34)
Total stockholders’ equity	58,145	71,082
Total liabilities and stockholders’ equity	$ 103,081	$ 92,073

Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
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	23,000,151	19,882,551
Common stock, shares outstanding	23,000,151	19,882,551

Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
License revenue	$ 18,036	$ 41	$ 49
Type of Revenue [Extensible List]	us-gaap:LicenseMember	us-gaap:LicenseMember	us-gaap:LicenseMember
Operating expenses:
Research and development	$ 51,080	$ 29,113	$ 27,218
General and administrative	12,809	8,556	4,521
Total operating expenses	63,889	37,669	31,739
Loss from operations	(45,853)	(37,628)	(31,690)
Other income (expense), net:
Interest income	1,534	595	215
Interest expense	(2,937)	(1,932)	(2,052)
Change in fair value of preferred stock warrants		37	50
Loss before income taxes	(47,256)	(38,928)	(33,477)
Income tax expense	1,699	1	1
Net loss	(48,955)	(38,929)	(33,478)
Other comprehensive income (loss):
Net unrealized gain (loss) on investments	11	(34)	58
Comprehensive loss	$ (48,944)	$ (38,963)	$ (33,420)
Net loss per common share, basic and diluted	$ (2.44)	$ (2.66)	$ (15.22)
Weighted-average number of common shares outstanding, basic and diluted	20,059,541	14,607,609	2,199,964

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)
Cumulative effect of accounting change				$ 10	$ (10)
Balance at January 1, 2016	$ (92,330)		$ 2	2,248	(94,522)	$ (58)
Balance, shares at January 1, 2016		46,163,605
Balance at January 1, 2016		$ 131,413
Balance, shares at January 1, 2016			2,197,852
Beginning balance at Dec. 31, 2015	(92,330)		$ 2	2,238	(94,512)	(58)
Beginning balance, shares at Dec. 31, 2015		46,163,605
Beginning balance at Dec. 31, 2015		$ 131,413
Beginning balance, shares at Dec. 31, 2015			2,197,852
Exercise of stock options	10			10
Exercise of stock options, shares			4,665
Stock-based compensation	1,323			1,323
Other comprehensive income (loss)	58					58
Net Loss	(33,478)				(33,478)
Ending balance at Dec. 31, 2016	(124,417)		$ 2	3,581	(128,000)
Ending balance, shares at Dec. 31, 2016		46,163,605
Ending Balance at Dec. 31, 2016		$ 131,413
Ending 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 intoshares of common stock, net of issuance costs	11,057		$ 1	11,056
Convertible promissory notes converted intoshares 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 income (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	122,648		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 income (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

Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
Statements of Cash Flows - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
OPERATING ACTIVITIES
Net Loss	$ (48,955)	$ (38,929)	$ (33,478)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	6,870	4,451	1,323
Depreciation	625	292	255
Noncash interest expense	1,161	590	570
Change in fair value of preferred stock warrants		(37)	(50)
Accretion of discount on investments, net	(248)	(19)	(9)
Gain on disposal of property and equipment			(20)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(208)	(679)	(157)
Accounts payable	1,260	304	586
Accrued liabilities	4,127	2,946	1,547
Deferred license revenue	(36)	(41)	(49)
Deferred rent	958
Deferred grant funding	(23)	(11)	(57)
Net cash used in operating activities	(34,469)	(31,133)	(29,539)
INVESTING ACTIVITIES
Proceeds from the sale/maturity of marketable securities	68,524	43,725	48,095
Purchases of marketable securities	(70,746)	(70,797)	(23,003)
Purchases of property and equipment	(1,968)	(655)	(525)
Proceeds from sale of property and equipment		20
Net cash (used in) provided by investing activities	(4,190)	(27,707)	24,567
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	(3,000)	(7,200)	(600)
Proceeds from issuance of common stock	661	507	10
Proceeds from public offering of common stock, net of issuance costs	28,200	88,618
Proceeds from issuance of convertible promissory notes, net of issuance costs		7,338	3,374
Proceeds from convertible promissory note subscriptions			140
Cash paid for deferred equity issuance costs	(16)		(592)
Net cash provided by financing activities	43,539	89,263	2,332
Net increase (decrease) in cash and cash equivalents	4,880	30,423	(2,640)
Cash and cash equivalents, beginning of period	35,933	5,510	8,150
Cash and cash equivalents, end of period	40,813	35,933	5,510
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION
Cash paid for interest	1,522	1,210	1,460
Allowance for tenant improvements included in deferred rent	1,243
Property and equipment purchases included in accounts payable and accrued liabilities	178	111	28
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	$ 226

Organization and Basis of Presentation

12 Months Ended

Dec. 31, 2018

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, 2018, 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.

Initial Public Offering

On April 19, 2017, the Company completed its initial public offering (IPO), whereby the Company sold an aggregate of 9,775,000 shares of its common stock, at $10.00 per share, resulting in net proceeds of $86.9 million after underwriting discounts, commissions and offering costs of $10.8 million.

In addition, in connection with the 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, warrants to purchase up to 68,572 shares of the Company’s Series H convertible preferred stock were converted into warrants to purchase up to 9,936 shares of the Company’s common stock, each at an exercise price of $36.23 per share, and $11.1 million of aggregate principal and accrued interest underlying convertible promissory notes were automatically converted into an aggregate of 1,109,176 shares of the Company’s common stock at the IPO price of $10.00 per share.

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, 2018, the Company has not sold any shares of its common stock 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 Company has a limited operating history and the sales and income potential of the Company’s business and patient markets are unproven. The Company has experienced net losses and negative cash flows from operating activities since its inception. As of December 31, 2018, the Company had an accumulated deficit of $215.9 million and working capital of $81.2 million available to fund future operations. As the Company continues to incur net losses, its transition to profitability is dependent upon the successful development, approval, and commercialization of its product candidates and achieving a level of revenues adequate to support the Company’s cost structure. The Company may never achieve profitability, and unless and until it does, the Company will continue to need to raise additional capital. The Company plans to continue to fund its losses from operations and capital funding needs through debt and equity financing, or through collaborations or partnerships with other entities. Debt or equity financing, or collaborations and partnerships with other entities may not be available on a timely basis on terms acceptable to the Company, or at all.

As of December 31, 2018, the Company had cash, cash equivalents and marketable securities of $96.1 million. The Company has evaluated and concluded that there are no conditions or events, considered individually or in the aggregate, that raises substantial doubt about its ability to continue as a going concern for a period of one year following the date that these financial statements are issued.

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, 2018

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 of five years.

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, 2018 and 2017.

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

On January 1, 2018, the Company adopted Accounting Standards Update (ASU) 2014-09, Revenue from Contract with Customers (Topic 606), using the modified retrospective transition method. There was no impact to opening retained earnings or revenue as of January 1, 2018 related to the adoption of Topic 606. 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 all outstanding principal and accrued interest related to convertible promissory notes payable, shares issuable upon the conversion of convertible preferred stock, options to purchase shares of common stock outstanding under the Company’s equity incentive plan and warrants for the purchase of shares of common and preferred stock. 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, excluding shares issuable upon the conversion of all outstanding principal and accrued interest related to convertible promissory notes, 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,
	2018		2017		2016
Common stock options		3,476,847		2,589,348		1,385,855
Common stock warrants		67,238		10,660		724
Convertible preferred stock (as-converted)		—		—		6,690,066
Convertible preferred stock warrants (as-converted)		—		—		9,936
Total		3,544,085		2,600,008		8,086,581

Recent Accounting Pronouncements

In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842), which requires lessees to recognize most leases on their balance sheet as lease liabilities with corresponding right-of-use assets and disclose key information about leasing arrangements. The new standard is effective for the Company beginning in the first quarter of 2019 and requires a modified retrospective approach for all leases existing at, or entered into after, the date of initial application, with an option to elect to use certain transition relief. The Company has completed its identification of leases which is comprised of one building lease. The Company is in the process of quantifying the impact to the balance sheet while the impact to the statement of operations is expected to be immaterial.

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 will become effective January 1, 2019 and does not have a material impact to the overall financial statements of the Company.

Fair Value of Financial Instruments

12 Months Ended

Dec. 31, 2018

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, 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	$	—

			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, 2017
Cash equivalents:
Corporate debt securities	$	8,274	$	—	$	8,274	$	—
Repurchase agreements		5,000		—		5,000		—
	$	13,274	$	—	$	13,274	$	—
Marketable securities:
Corporate debt securities	$	24,713	$	—	$	24,713	$	—
Certificates of deposit		13,651		—		13,651		—
Commercial paper		12,329		—		12,329		—
Asset-backed securities		2,099		—		2,099		—
	$	52,792	$	—	$	52,792	$	—

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, 2018 or 2017.

At December 31, 2018 and 2017, the Company had investments in money market funds of $30.9 million and $20.2 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.

Warrant Liabilities. The Company had preferred stock warrants that were treated as liabilities and measured at fair value on a recurring basis. All preferred stock warrants outstanding converted into warrants to purchase shares of common stock in the connection with the Company’s IPO. The Company recorded a gain on warrant valuation, which was included in other expense, of $37,000 for the year ended December 31, 2017. Upon conversion of the preferred stock warrants, the Company recorded $0.1 million to additional paid in capital and reduced the preferred stock warrant liability to zero.

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 $26.2 million at December 31, 2018 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, 2018

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, 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
December 31, 2017
Corporate debt securities	1 or less	$	21,097	$	—	$	(16	)	$	21,081
Corporate debt securities	>1 and <5		3,636		—		(4	)		3,632
Certificates of deposit	1 or less		13,658		—		(7	)		13,651
Commercial paper	1 or less		12,333		—		(4	)		12,329
Asset-backed securities	1 or less		2,099		—		—			2,099
		$	52,823	$	—	$	(31	)	$	52,792

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,
	2018			2017
Laboratory equipment	$	4,445		$	3,553
Computers, software and office equipment		322			232
Furniture and fixtures		610			21
Leasehold improvements		1,927			117
		7,304			3,923
Less: accumulated depreciation		(3,331	)		(2,706	)
	$	3,973		$	1,217

Depreciation expense was $0.6 million and $0.3 million for the years ended December 31, 2018 and 2017, respectively.

Accrued Liabilities

Accrued liabilities are comprised of (in thousands):

	December 31,
	2018		2017
Clinical trial expenses	$	4,535	$	2,809
Payroll and other employee-related expenses		2,840		2,489
Contract manufacturing services		3,411		1,536
Professional fees		474		276
Interest payable		205		77
Other		1,629		933
Total accrued liabilities	$	13,094	$	8,120

Notes Payable

12 Months Ended

Dec. 31, 2018

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 with the two lenders, which was further amended on August 3, 2018 (the Loan Agreement), 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 will have interest-only payments through January 1, 2020, followed by 36 equal monthly payments of principal and interest; provided that the Term Loans will be interest-only (and the number of principal and interest payments will be correspondingly reduced) through (i) July 1, 2020 if the Company submits a Biologics License Application (BLA) for the Company’s product candidate, Toca 511 & Toca FC, to the United States Food and Drug Administration (FDA) prior to January 1, 2020, but not yet received FDA approval of such BLA prior to July 1, 2020 and (ii) January 1, 2021 if following such BLA submission to the FDA prior to January 1, 2020, the Company receives FDA approval of such BLA prior to July 1, 2020.

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. The Company may prepay all, but not less than all, of the Term Loans upon 10 days written notice provided the Company will be obligated to pay a prepayment fee equal to (i) 3.00% of the principal amount of the applicable Term Loan prepaid on or before the first anniversary of the effective date of the Loan Agreement, (ii) 2.00% of the principal amount of the applicable Term Loan prepaid on or before the second anniversary of the effective date of the Loan Agreement, and (iii) 1.00% of the principal amount of the applicable Term Loan prepaid thereafter, but prior to the Maturity Date.

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, and is amortizing the balance over the life of the underlying 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,
	2018			2017
Principal	$	26,450		$	10,200
Add: accreted liability for final payment fee	276			869
Less: unamortized discount		(525	)		(244	)
	$	26,201		$	10,825

The Term Loans are secured by substantially all of the Company’s assets other than its intellectual property, except rights to payment from the sale, licensing or disposition of such 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, 2018, the Company was in compliance with the covenants contained in the Loan Agreement.

Future maturities of the Term Loans, including the final payment fee, as of December 31, 2018 are as follows (in thousands):

	December 31, 2018
Year ending December 31, 2019	$	—
Year ending December 31, 2020		8,817
Year ending December 31, 2021		8,817
Year ending December 31, 2022		10,919
		28,553
Unaccreted balance for final payment fee on Loans		(1,827	)
Unamortized discounts		(525	)
Noncurrent portion	$	26,201

Convertible Promissory Notes

Upon completion of the Company’s IPO in April 2017, $11.1 million of aggregate principal and accrued interest underlying convertible promissory notes were automatically converted into an aggregate of 1,109,176 shares of the Company’s common stock at the IPO price of $10.00 per share.

Stockholders' Equity

12 Months Ended

Dec. 31, 2018

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, 2018, 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.

Common Stock Reserved for Future Issuance

Common stock reserved for future issuance as of December 31, 2018 and 2017 is as follows:

	December 31,
	2018		2017
Issued and Outstanding:
Stock options		3,476,847		2,589,348
Warrants for common stock		67,238		10,660
Shares reserved for issuance under the ESPP		326,178		199,879
Shares reserved for future award grants		451,063		513,333

Equity Incentive Plans and Stock-Based Compensation

12 Months Ended

Dec. 31, 2018

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 (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 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, 2018:

	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, 2017		2,589,348		$	13.33
Granted		1,222,150		$	11.08
Exercised		(45,073	)	$	1.49
Forfeitures and cancellations		(289,578	)	$	13.60
Options Outstanding at December 31, 2018		3,476,847		$	12.67	7.7	$	1,481
Options Exercisable at December 31, 2018		1,358,986		$	11.77	6.1	$	1,481

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

	Years Ended December 31,
	2018		2017		2016
Fair value of options vested during the period	$	6,318	$	2,194	$	1,087
Cash received from options exercised during the period	$	67	$	81	$	10
Intrinsic value of options exercised during the period	$	508	$	677	$	62

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, 2018, the Company had issued 122,648 shares of common stock under the ESPP, with 72,527 of such shares of common stock being issued during the year ended December 31, 2018. The Company had 326,178 shares available for future issuance under the ESPP as of December 31, 2018.

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,
	2018				2017				2016
Time based stock options
Risk-free interest rate			2.7	%			2.0	%			1.63	%
Volatility			85.9	%			82.2	%			73.4	%
Dividend yield		0%				0%					0	%
Expected term (in years)		6.1				6.1				6.1
Grant date fair value per share	$	8.12			$	10.39			$	9.69
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			2.5	%			1.2	%			—
Volatility			78.1	%			69.8	%			—
Dividend yield			0	%			0	%			—
Expected term (in years)		1.1				1.3					—
Grant date fair value per share	$	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, 2018.

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,
	2018		2017		2016
Research and development	$	3,023	$	1,783	$	562
General and administrative		3,847		2,668		761
Total	$	6,870	$	4,451	$	1,323

Unrecognized compensation expense for stock options at December 31, 2018 was $17.1 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, 2018

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. The Company has recorded total revenue of $18.0 million for the year ended December 31, 2018 related to the License Agreement.

Grant Agreements

12 Months Ended

Dec. 31, 2018

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, 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, 2018

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,
	2018		2017		2016
Current
Federal	$	—	$	—	$	—
State		1		1		1
Foreign		1,698		—		—
Total current provision		1,699		1		1
Deferred
Federal		—		—		—
State		—		—		—
Total Deferred		—		—		—
Income Tax Expense	$	1,699	$	1	$	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,
	2018			2017			2016
Federal statutory rate		21.0	%		34.0	%		34.0	%
Adjustments for tax effects of:
State taxes, net		6.1	%		5.7	%		5.6	%
Withholding Tax		(3.6	)%		—	%		—	%
Permanent adjustments		(1.0	)%		(4.5	)%		(5.4	)%
Tax Cuts and Jobs Act		—	%		(3.7	)%		—	%
Net operating loss carryovers not recognized		(22.1	)%		(30.4	)%		(32.8	)%
Valuation allowance		(5.9	)%		(1.0	)%		(1.1	)%
Other		1.9	%		(0.1	)%		(0.3	)%
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,
	2018			2017
Deferred tax assets:
Depreciation and amortization	$	—		$	52
Deferred license revenue		10			15
Share-based compensation		3,432			1,459
Debt discount		137			—
Accrued liabilities and other		1,778			808
Total deferred tax assets		5,357			2,334
Less valuation allowance		(5,107	)		(2,334	)
Net deferred tax assets	$	250		$	—

Deferred tax liabilities:
Depreciation and amortization	$	(250	)	$	—
Total Deferred tax liabilities	$	(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, 2018 and 2017. During 2018 and 2017, the valuation allowance increased by $2.6 million and $0.4 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, 2018, the Company has federal net operating losses of $174.1 million, of which, $136.6 million begin expiring in 2028 unless previously utilized and $37.6 million that do not expire but are limited to 80% of taxable income in a given year. The Company has California net operating loss carryforwards of $76.1 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, 2014, 2015, 2016 and 2017 which were impacted by a California Supreme Court ruling on December 31, 2015. This ruling clarified how companies are allowed to apportion income or losses in the state. 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 by treating 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 $109.3 million from its California net operating loss carryforward.

As of December 31, 2018, the Company has federal and California research and development tax credit carryforwards of $25.6 million and $6.1 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% 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 $42.0 million and federal and California research and development credits of approximately $30.4 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, 2018 and 2017, there was no accrued interest or penalties for uncertain tax positions.

The Company is subject to taxation in the U.S. and state jurisdictions. As of December 31, 2018, 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.

Pursuant to the federal tax legislation that was enacted on December 22, 2017 (the Tax Act), the Company re-measured its existing deferred tax assets and liabilities based on 21%, the current rate at which they are expected to reverse in the future. In 2017, the Company recorded provisional amounts for certain enactment-date effects of the Act by applying the guidance in SAB 118 because it had not yet completed the enactment-date accounting for these effects. In 2018 and 2017, the Company did not record tax expense related to the enactment-date effects of the Act as the Company maintained a full valuation allowance. Upon completion of the Company’s analysis of certain aspects of the Act and refinement of the calculations during the year ended December 31, 2018, the Company found no other adjustments were necessary.

Retirement Plan

12 Months Ended

Dec. 31, 2018

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 has not made any contributions for the years ended December 31, 2018, 2017 and 2016.

Commitments and Contingencies

12 Months Ended

Dec. 31, 2018

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. As of December 31, 2018 the Company has used the full allowance, which is classified as tenant improvements and deferred rent on the Company’s balance sheet and will be amortized against rent expense on a straight line basis over the term of the Lease. 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.

Future annual minimum rental payments payable under the Lease are as follows (shown in thousands):

Years ended December 31:
2019	$	1,759
2020		1,939
2021		2,007
2022		2,077
2023		2,150
Thereafter		5,700
Total	$	15,632

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, 2018

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, 2018 and 2017. 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, 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	)
Year Ended December 31, 2017
Operating expenses	$	8,564		$	8,662		$	9,747		$	10,696
Net loss		(9,073	)		(9,066	)		(9,953	)		(10,837	)
Net loss per common share, basic and diluted	$	(4.11	)	$	(0.56	)	$	(0.50	)	$	(0.55	)

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2018

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 of five years.

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, 2018 and 2017.

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

On January 1, 2018, the Company adopted Accounting Standards Update (ASU) 2014-09, Revenue from Contract with Customers (Topic 606), using the modified retrospective transition method. There was no impact to opening retained earnings or revenue as of January 1, 2018 related to the adoption of Topic 606. 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 Preferred Stock

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 all outstanding principal and accrued interest related to convertible promissory notes payable, shares issuable upon the conversion of convertible preferred stock, options to purchase shares of common stock outstanding under the Company’s equity incentive plan and warrants for the purchase of shares of common and preferred stock. 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, excluding shares issuable upon the conversion of all outstanding principal and accrued interest related to convertible promissory notes, 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,
	2018		2017		2016
Common stock options		3,476,847		2,589,348		1,385,855
Common stock warrants		67,238		10,660		724
Convertible preferred stock (as-converted)		—		—		6,690,066
Convertible preferred stock warrants (as-converted)		—		—		9,936
Total		3,544,085		2,600,008		8,086,581

Recent Accounting Pronouncements

In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842), which requires lessees to recognize most leases on their balance sheet as lease liabilities with corresponding right-of-use assets and disclose key information about leasing arrangements. The new standard is effective for the Company beginning in the first quarter of 2019 and requires a modified retrospective approach for all leases existing at, or entered into after, the date of initial application, with an option to elect to use certain transition relief. The Company has completed its identification of leases which is comprised of one building lease. The Company is in the process of quantifying the impact to the balance sheet while the impact to the statement of operations is expected to be immaterial.

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 will become effective January 1, 2019 and does not have a material impact to the overall financial statements of the Company.

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2018

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, excluding shares issuable upon the conversion of all outstanding principal and accrued interest related to convertible promissory notes, 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,
	2018		2017		2016
Common stock options		3,476,847		2,589,348		1,385,855
Common stock warrants		67,238		10,660		724
Convertible preferred stock (as-converted)		—		—		6,690,066
Convertible preferred stock warrants (as-converted)		—		—		9,936
Total		3,544,085		2,600,008		8,086,581

Fair Value of Financial Instruments (Tables)

12 Months Ended

Dec. 31, 2018

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, 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	$	—

			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, 2017
Cash equivalents:
Corporate debt securities	$	8,274	$	—	$	8,274	$	—
Repurchase agreements		5,000		—		5,000		—
	$	13,274	$	—	$	13,274	$	—
Marketable securities:
Corporate debt securities	$	24,713	$	—	$	24,713	$	—
Certificates of deposit		13,651		—		13,651		—
Commercial paper		12,329		—		12,329		—
Asset-backed securities		2,099		—		2,099		—
	$	52,792	$	—	$	52,792	$	—

Certain Financial Statement Caption Information (Tables)

12 Months Ended

Dec. 31, 2018

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, 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
December 31, 2017
Corporate debt securities	1 or less	$	21,097	$	—	$	(16	)	$	21,081
Corporate debt securities	>1 and <5		3,636		—		(4	)		3,632
Certificates of deposit	1 or less		13,658		—		(7	)		13,651
Commercial paper	1 or less		12,333		—		(4	)		12,329
Asset-backed securities	1 or less		2,099		—		—			2,099
		$	52,823	$	—	$	(31	)	$	52,792

Summary of Property and Equipment

Property and equipment is comprised of (in thousands):

	December 31,
	2018			2017
Laboratory equipment	$	4,445		$	3,553
Computers, software and office equipment		322			232
Furniture and fixtures		610			21
Leasehold improvements		1,927			117
		7,304			3,923
Less: accumulated depreciation		(3,331	)		(2,706	)
	$	3,973		$	1,217

Components of Accrued Liabilities

Accrued liabilities are comprised of (in thousands):

	December 31,
	2018		2017
Clinical trial expenses	$	4,535	$	2,809
Payroll and other employee-related expenses		2,840		2,489
Contract manufacturing services		3,411		1,536
Professional fees		474		276
Interest payable		205		77
Other		1,629		933
Total accrued liabilities	$	13,094	$	8,120

Notes Payable (Tables)

12 Months Ended

Dec. 31, 2018

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,
	2018			2017
Principal	$	26,450		$	10,200
Add: accreted liability for final payment fee	276			869
Less: unamortized discount		(525	)		(244	)
	$	26,201		$	10,825

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, 2018 are as follows (in thousands):

	December 31, 2018
Year ending December 31, 2019	$	—
Year ending December 31, 2020		8,817
Year ending December 31, 2021		8,817
Year ending December 31, 2022		10,919
		28,553
Unaccreted balance for final payment fee on Loans		(1,827	)
Unamortized discounts		(525	)
Noncurrent portion	$	26,201

Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2018

Equity [Abstract]

Schedule of Common Stock Reserved for Future Issuance

Common stock reserved for future issuance as of December 31, 2018 and 2017 is as follows:

	December 31,
	2018		2017
Issued and Outstanding:
Stock options		3,476,847		2,589,348
Warrants for common stock		67,238		10,660
Shares reserved for issuance under the ESPP		326,178		199,879
Shares reserved for future award grants		451,063		513,333

Equity Incentive Plan and Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

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, 2018:

	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, 2017		2,589,348		$	13.33
Granted		1,222,150		$	11.08
Exercised		(45,073	)	$	1.49
Forfeitures and cancellations		(289,578	)	$	13.60
Options Outstanding at December 31, 2018		3,476,847		$	12.67	7.7	$	1,481
Options Exercisable at December 31, 2018		1,358,986		$	11.77	6.1	$	1,481

Summary of Information Regarding Stock Options

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

	Years Ended December 31,
	2018		2017		2016
Fair value of options vested during the period	$	6,318	$	2,194	$	1,087
Cash received from options exercised during the period	$	67	$	81	$	10
Intrinsic value of options exercised during the period	$	508	$	677	$	62

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,
	2018				2017				2016
Time based stock options
Risk-free interest rate			2.7	%			2.0	%			1.63	%
Volatility			85.9	%			82.2	%			73.4	%
Dividend yield		0%				0%					0	%
Expected term (in years)		6.1				6.1				6.1
Grant date fair value per share	$	8.12			$	10.39			$	9.69
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			2.5	%			1.2	%			—
Volatility			78.1	%			69.8	%			—
Dividend yield			0	%			0	%			—
Expected term (in years)		1.1				1.3					—
Grant date fair value per share	$	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,
	2018		2017		2016
Research and development	$	3,023	$	1,783	$	562
General and administrative		3,847		2,668		761
Total	$	6,870	$	4,451	$	1,323

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

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,
	2018		2017		2016
Current
Federal	$	—	$	—	$	—
State		1		1		1
Foreign		1,698		—		—
Total current provision		1,699		1		1
Deferred
Federal		—		—		—
State		—		—		—
Total Deferred		—		—		—
Income Tax Expense	$	1,699	$	1	$	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,
	2018			2017			2016
Federal statutory rate		21.0	%		34.0	%		34.0	%
Adjustments for tax effects of:
State taxes, net		6.1	%		5.7	%		5.6	%
Withholding Tax		(3.6	)%		—	%		—	%
Permanent adjustments		(1.0	)%		(4.5	)%		(5.4	)%
Tax Cuts and Jobs Act		—	%		(3.7	)%		—	%
Net operating loss carryovers not recognized		(22.1	)%		(30.4	)%		(32.8	)%
Valuation allowance		(5.9	)%		(1.0	)%		(1.1	)%
Other		1.9	%		(0.1	)%		(0.3	)%
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,
	2018			2017
Deferred tax assets:
Depreciation and amortization	$	—		$	52
Deferred license revenue		10			15
Share-based compensation		3,432			1,459
Debt discount		137			—
Accrued liabilities and other		1,778			808
Total deferred tax assets		5,357			2,334
Less valuation allowance		(5,107	)		(2,334	)
Net deferred tax assets	$	250		$	—

Deferred tax liabilities:
Depreciation and amortization	$	(250	)	$	—
Total Deferred tax liabilities	$	(250	)	$	—
Net deferred taxes	$	—		$	—

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Schedule of Future Annual Minimum Rental Payments Payable Under New Lease

Future annual minimum rental payments payable under the Lease are as follows (shown in thousands):

Years ended December 31:
2019	$	1,759
2020		1,939
2021		2,007
2022		2,077
2023		2,150
Thereafter		5,700
Total	$	15,632

Selected Quarterly Financial Data (unaudited) (Tables)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Unaudited Quarterly Financial Information

The following table contains unaudited quarterly financial information for the years ended December 31, 2018 and 2017. 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, 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	)
Year Ended December 31, 2017
Operating expenses	$	8,564		$	8,662		$	9,747		$	10,696
Net loss		(9,073	)		(9,066	)		(9,953	)		(10,837	)
Net loss per common share, basic and diluted	$	(4.11	)	$	(0.56	)	$	(0.50	)	$	(0.55	)

Organization and Basis of Presentation - Additional Information (Details)		1 Months Ended		12 Months Ended
	Apr. 19, 2017 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Apr. 30, 2017 $ / shares shares	Dec. 31, 2018 USD ($) Segment $ / shares shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Nov. 30, 2018 USD ($)
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of operating segment \| Segment				1
Completion date of initial public offering				April 19, 2017
Net proceeds from issuance of initial public offering	$ 86,900,000
Conversion of convertible promissory notes into common stock \| shares			1,109,176
Conversion of convertible promissory notes price, per share \| $ / shares			$ 10.00
Net proceeds from issuance of public offering				$ 661,000	$ 507,000	$ 10,000
Accumulated deficit		$ 215,884,000		215,884,000	$ 166,929,000
Working capital		81,200,000		81,200,000
Cash, cash equivalents and marketable securities		$ 96,100,000		$ 96,100,000
IPO
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Common stock issued, shares \| shares	9,775,000
Shares price, per share \| $ / shares	$ 10.00
Stock issuance costs incurred for underwriting discounts, commissions and estimated offering costs	$ 10,800,000
Outstanding shares of convertible preferred stock converted into common stock, shares \| shares	6,690,066
Warrant to purchase common stock, exercise price \| $ / shares	$ 36.23
Aggregate principal and accrued interest, convertible promissory notes amount	$ 11,100,000
Conversion of convertible promissory notes into common stock \| shares	1,109,176
Conversion of convertible promissory notes price, per share \| $ / shares	$ 10.00
IPO \| Maximum
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Warrants to purchase shares of common stock \| shares	9,936
IPO \| Maximum \| Series H Convertible Preferred Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Warrants to purchase shares of common stock \| shares	68,572
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				0
Public Offering
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Common stock issued, shares \| shares		3,000,000
Shares price, per share \| $ / shares		$ 10.00		$ 10.00
Net proceeds from issuance of public offering		$ 28,000,000

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
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	5 years
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 of five years.

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, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potential dilutive securities not included in calculation of diluted net loss per share	3,544,085	2,600,008	8,086,581
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	3,476,847	2,589,348	1,385,855
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	67,238	10,660	724
Convertible Preferred Stock Warrants (as-converted)
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potential dilutive securities not included in calculation of diluted net loss per share			9,936
Convertible Preferred Stock (as-converted)
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potential dilutive securities not included in calculation of diluted net loss per share			6,690,066

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, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	$ 6,770	$ 13,274
Marketable securities	55,273	52,792
Repurchase Agreements
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents		5,000
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	13,274
Marketable securities	53,276	52,792
Significant Other Observable Inputs (Level 2) \| Repurchase Agreements
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents		5,000
Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	4,783	8,274
Marketable securities	16,301	24,713
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	8,274
Marketable securities	16,301	24,713
Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	1,987
Marketable securities	24,576	12,329
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	24,576	12,329
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
Asset-backed Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	12,399	2,099
Asset-backed Securities \| Significant Other Observable Inputs (Level 2)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	$ 12,399	2,099
Certificates of Deposit
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities		13,651
Certificates of Deposit \| Significant Other Observable Inputs (Level 2)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities		$ 13,651

Fair Value of Financial Instruments - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	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
Increase in additional paid in capital	100,000
Warrant liability	0
Carrying Amount \| Significant Unobservable Inputs (Level 3)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Notes payable		26,200,000
Other Expense
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Gain on warrant valuation	37,000
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	$ 20,200,000	$ 30,900,000

Certain Financial Statement Caption Information - Summary of Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 55,296	$ 52,823
Unrealized Gain	3
Unrealized Loss	(26)	(31)
Fair Value	55,273	52,792
Corporate Debt Securities \| Maturity (in years) 1 or Less
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,013	21,097
Unrealized Gain	1
Unrealized Loss	(4)	(16)
Fair Value	10,010	21,081
Corporate Debt Securities \| Maturity More Than 1 Year and Less Than 5 Years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,293	3,636
Unrealized Gain	2
Unrealized Loss	(4)	(4)
Fair Value	6,291	3,632
Commercial Paper \| Maturity (in years) 1 or Less
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	24,584	12,333
Unrealized Loss	(8)	(4)
Fair Value	24,576	12,329
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	10,612	2,099
Unrealized Loss	(8)
Fair Value	10,604	2,099
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
Certificates of Deposit \| Maturity (in years) 1 or Less
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		13,658
Unrealized Loss		(7)
Fair Value		$ 13,651

Certain Financial Statement Caption Information - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Balance Sheet Related Disclosures [Abstract]
Other-than-temporary impairments during period	$ 0
Depreciation	$ 625,000	$ 292,000	$ 255,000

Certain Financial Statement Caption Information - Summary of Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 7,304	$ 3,923
Less: accumulated depreciation	(3,331)	(2,706)
Property and equipment, net	3,973	1,217
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	4,445	3,553
Computers, Software and Office Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	322	232
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	610	21
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 1,927	$ 117

Certain Financial Statement Caption Information - Components of Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Balance Sheet Related Disclosures [Abstract]
Clinical trial expenses	$ 4,535	$ 2,809
Payroll and other employee-related expenses	2,840	2,489
Contract manufacturing services	3,411	1,536
Professional fees	474	276
Interest payable	205	77
Other	1,629	933
Total accrued liabilities	$ 13,094	$ 8,120

Notes Payable - Additional Information (Details) $ / shares in Units, $ in Thousands			1 Months Ended	12 Months Ended
	May 18, 2018 USD ($) Lender $ / shares shares	Oct. 30, 2015 USD ($) Lender	Apr. 30, 2017 USD ($) $ / shares shares	Dec. 31, 2018 USD ($)
Debt Instrument [Line Items]
Proceeds from term loans				$ 8,631
Aggregate carrying amounts of the convertible promissory notes			$ 11,100
Debt conversion, converted instrument, shares issued \| shares			1,109,176
Debt instrument, conversion price \| $ / shares			$ 10.00
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]
Number of Lenders \| Lender	2
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 will have 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%.
Period of prior written notice to lender	10 days
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 \| On or Before First Anniversary
Debt Instrument [Line Items]
Percentage of prepayment fee on principal amount	3.00%
Loan Agreement \| Term Loans \| On or Before Second Anniversary
Debt Instrument [Line Items]
Percentage of prepayment fee on principal amount	2.00%
Loan Agreement \| Term Loans \| Thereafter but Prior to Maturity Date
Debt Instrument [Line Items]
Percentage of prepayment fee on principal amount	1.00%
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%

Notes Payable - Schedule of Aggregate Carrying Amounts of Term and Initial Loans (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Loan Agreement
Debt Instrument [Line Items]
Debt Instrument, Type [Extensible List]	toca:TermLoanMember
Principal	$ 26,450
Add: accreted liability for final payment fee	276
Less: unamortized discount	(525)
Loans, aggregate carrying amount	$ 26,201
Prior Agreement
Debt Instrument [Line Items]
Debt Instrument, Type [Extensible List]		toca:InitialLoanMember
Principal		$ 10,200
Add: accreted liability for final payment fee		869
Less: unamortized discount		(244)
Loans, aggregate carrying amount		$ 10,825

Notes Payable - Schedule of Future Maturities of Term Loans Including Final Payment Fee (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Noncurrent portion	$ 26,201	$ 3,625
Loan Agreement
Debt Instrument [Line Items]
Debt Instrument, Type [Extensible List]	toca:TermLoanMember
Year ending December 31, 2020	$ 8,817
Year ending December 31, 2021	8,817
Year ending December 31, 2022	10,919
Long-term debt at maturity	28,553
Unaccreted balance for final payment fee on Loans	(1,827)
Less: unamortized discount	(525)
Noncurrent portion	$ 26,201

Stockholders' Equity - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	12 Months Ended
	Apr. 19, 2017	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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
Net proceeds from issuance of public offering			$ 661	$ 507	$ 10
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	200,000,000
Common stock, par value		$ 0.001	$ 0.001
Preferred stock, shares authorized		10,000,000	10,000,000
Preferred stock, par value		$ 0.001	$ 0.001
Common stock issued, shares	9,775,000
Shares price, per share	$ 10.00
Public Offering
Class Of Stock [Line Items]
Common stock issued, shares		3,000,000
Shares price, per share		$ 10.00	$ 10.00
Net proceeds from issuance of public offering		$ 28,000

Stockholders' Equity (Deficit) - Schedule of Common Stock Reserved for Future Issuance (Details) - shares	Dec. 31, 2018	Dec. 31, 2017
Shares Reserved for Issuance Under the ESPP
Class Of Stock [Line Items]
Common stock reserved for future issuance	326,178	199,879
Warrants
Class Of Stock [Line Items]
Common stock reserved for future issuance	67,238	10,660
Stock Options
Class Of Stock [Line Items]
Common stock reserved for future issuance	3,476,847	2,589,348
Shares Reserved for Future Award Grants
Class Of Stock [Line Items]
Common stock reserved for future issuance	451,063	513,333

Equity Incentive Plan and Stock-Based Compensation - Additional Information (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Apr. 12, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock shares issued under ESPP		122,648
2017 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares available for future grants		326,178	199,879
Common stock issued, shares		72,527
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		3,476,847	2,589,348
Unrecognized compensation expense		$ 17.1
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		3,476,847	2,589,348
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 $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares Subject to Options, Outstanding at December 31, 2017 \| shares	2,589,348
Shares Subject to Options, Granted \| shares	1,222,150
Shares Subject to Options, Exercised \| shares	(45,073)
Shares Subject to Options, Forfeitures and cancellations \| shares	(289,578)
Shares Subject to Options, Outstanding at December 31, 2018 \| shares	3,476,847
Shares Subject to Options, Exercisable at December 31,2018 \| shares	1,358,986
Weighted-Average Exercise Price per Share, Outstanding at December 31, 2018 \| $ / shares	$ 13.33
Weighted-Average Exercise Price per Share, Granted \| $ / shares	11.08
Weighted-Average Exercise Price per Share, Exercised \| $ / shares	1.49
Weighted-Average Exercise Price per Share, Forfeitures and cancellations \| $ / shares	13.60
Weighted-Average Exercise Price per Share, Outstanding at December 31, 2018 \| $ / shares	12.67
Weighted-Average Exercise Price per Share, Exercisable at December 31, 2018 \| $ / shares	$ 11.77
Weighted-Average Remaining Contractual Term, Outstanding at December 31, 2018	7 years 8 months 12 days
Weighted-Average Remaining Contractual Term, Exercisable at December 31, 2018	6 years 1 month 6 days
Aggregate Intrinsic Value, Outstanding at December 31, 2018 \| $	$ 1,481
Aggregate Intrinsic Value, Exercisable at December 31, 2018 \| $	$ 1,481

Equity Incentive Plan and Stock-Based Compensation - Summary of Information Regarding Stock Options (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Fair value of options vested during the period	$ 6,318	$ 2,194	$ 1,087
Cash received from options exercised during the period	67	81	10
Intrinsic value of options exercised during the period	$ 508	$ 677	$ 62

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, 2018	Dec. 31, 2017	Dec. 31, 2016
Time-Based Vesting Provisions
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate	2.70%	2.00%	1.63%
Volatility	85.90%	82.20%	73.40%
Dividend yield	0.00%	0.00%	0.00%
Expected term (in years)	6 years 1 month 6 days	6 years 1 month 6 days	6 years 1 month 6 days
Grant date fair value per share	$ 8.12	$ 10.39	$ 9.69
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	2.50%	1.20%
Volatility	78.10%	69.80%
Dividend yield	0.00%	0.00%
Expected term (in years)	1 year 1 month 6 days	1 year 3 months 18 days
Grant date fair value per share	$ 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, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total non-cash stock-based compensation expense	$ 6,870	$ 4,451	$ 1,323
Research and Development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total non-cash stock-based compensation expense	3,023	1,783	562
General and Administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total non-cash stock-based compensation expense	$ 3,847	$ 2,668	$ 761

License and Collaboration Agreements - Additional Information (Details)		12 Months Ended
	Apr. 18, 2018 USD ($) Patient	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Collaborative Arrangements [Line Items]
License revenue		$ 18,036,000	$ 41,000	$ 49,000
License Agreement With Beijing Apollo Venus Biomedical Technology Limited and Apollo Bio Corp
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
License revenue		$ 18,000,000
License Agreement With Beijing Apollo Venus Biomedical Technology Limited and Apollo Bio Corp \| Health Care, Patient Service
Collaborative Arrangements [Line Items]
Number of patients	380
License Agreement With Beijing Apollo Venus Biomedical Technology Limited and Apollo Bio Corp \| Maximum
Collaborative Arrangements [Line Items]
Development and commercial milestone payment receivable	$ 111,000,000
License Agreement With Beijing Apollo Venus Biomedical Technology Limited and Apollo Bio Corp \| 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, 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

Income Taxes - Significant Components of Income Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current
State	$ 1	$ 1	$ 1
Foreign	1,698
Total current provision	1,699	1	1
Deferred
Income Tax Expense	$ 1,699	$ 1	$ 1

Income Taxes - Reconciliation of U.S. Statutory Federal Income Tax Rate (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Federal statutory rate	21.00%	34.00%	34.00%
Adjustments for tax effects of:
State taxes, net	6.10%	5.70%	5.60%
Withholding Tax	(3.60%)
Permanent adjustments	(1.00%)	(4.50%)	(5.40%)
Tax Cuts and Jobs Act		(3.70%)
Net operating loss carryovers not recognized	(22.10%)	(30.40%)	(32.80%)
Valuation allowance	(5.90%)	(1.00%)	(1.10%)
Other	1.90%	(0.10%)	(0.30%)
Effective income tax rate	(3.70%)

Income Taxes - Components of Deferred Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Depreciation and amortization		$ 52
Deferred license revenue	$ 10	15
Share-based compensation	3,432	1,459
Debt discount	137
Accrued liabilities and other	1,778	808
Total deferred tax assets	5,357	2,334
Less valuation allowance	(5,107)	$ (2,334)
Net deferred tax assets	250
Deferred tax liabilities:
Depreciation and amortization	(250)
Total Deferred tax liabilities	$ (250)

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Line Items]
Valuation allowance increased, amount	$ 2,600,000	$ 400,000
Cumulative change in ownership percentage	50.00%
Period for cumulative change in ownership	3 years
Deferred tax assets for net operating losses	$ 42,000,000
Deferred tax assets for federal and California research and development credits	30,400,000
Accruals interest for uncertain tax position	0	0
Penalties for uncertain tax positions	$ 0	$ 0
Federal statutory rate	21.00%	34.00%	34.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount	$ 0	$ 0
Federal
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 174,100,000
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	$ 25,600,000
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	37,600,000
California
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 76,100,000
Net operating loss carryforwards expiration year	2028
Percentage of apportionment of losses	100.00%
Cumulative net operating loss carryforwards	$ 109,300,000
California \| Research and Development
Income Tax Disclosure [Line Items]
Tax credit carryforward	$ 6,100,000

Retirement Plan - Additional Information (Details) - USD ($)		12 Months Ended
	Jan. 01, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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 has not made any contributions for the years ended December 31, 2018, 2017 and 2016.
Defined benefit plan, contributions by employer		$ 0	$ 0	$ 0
Subsequent Event
Defined Benefit Plan Disclosure [Line Items]
Percentage of company's matching contribution to employee contributions under retirement plan	50.00%
Subsequent Event \| Maximum
Defined Benefit Plan Disclosure [Line Items]
Employer matching contribution, percent of employees' eligible salary	6.00%

Commitments and Contingencies - Additional Information (Details) - Laboratory and Office Space - CALIFORNIA $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) LeaseOption
Commitments And Contingencies [Line Items]
Initial lease term	8 years
Number of options to extend lease \| LeaseOption	1
Additional lease term	5 years
Tenant improvement allowance \| $	$ 1.2

Commitments and Contingencies - Schedule of Future Annual Minimum Rental Payments Payable Under New Lease (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Commitments And Contingencies Disclosure [Abstract]
2019	$ 1,759
2020	1,939
2021	2,007
2022	2,077
2023	2,150
Thereafter	5,700
Total	$ 15,632

Selected Quarterly Financial Data (unaudited) - Unaudited Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Operating expenses	$ 19,116	$ 16,582	$ 15,336	$ 12,855	$ 10,696	$ 9,747	$ 8,662	$ 8,564	$ 63,889	$ 37,669	$ 31,739
Net Loss	$ (19,603)	$ (383)	$ (16,089)	$ (12,880)	$ (10,837)	$ (9,953)	$ (9,066)	$ (9,073)	$ (48,955)	$ (38,929)	$ (33,478)
Net loss per common share, basic and diluted	$ (0.96)	$ (0.02)	$ (0.81)	$ (0.65)	$ (0.55)	$ (0.50)	$ (0.56)	$ (4.11)	$ (2.44)	$ (2.66)	$ (15.22)

- Definition

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

+ References + Details