Document and Entity Information - shares	6 Months Ended
Document and Entity Information - shares	Jun. 30, 2025	Aug. 07, 2025
Document and Entity Information
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Jun. 30, 2025
Document Transition Report	false
Securities Act File Number	001-39231
Entity Registrant Name	Passage BIO, Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	82-2729751
Entity Address, Address Line One	One Commerce Square
Entity Address, Address Line Two	2005 Market Street, 39th Floor
Entity Address, City or Town	Philadelphia
Entity Address, State or Province	PA
Entity Address, Postal Zip Code	19103
City Area Code	267
Local Phone Number	866-0311
Title of 12(b) Security	Common Stock
Trading Symbol	PASG
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		3,178,710
Entity Central Index Key	0001787297
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2025
Document Fiscal Period Focus	Q2
Amendment Flag	false

Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2025	Dec. 31, 2024
Current assets:
Cash and cash equivalents	$ 57,626	$ 37,573
Marketable securities	0	39,183
Prepaid expenses and other current assets	1,402	838
Prepaid research and development	1,287	1,221
Total current assets	60,315	78,815
Property and equipment, net	5,340	9,331
Right of use assets - operating leases	13,273	13,803
Other assets	270	463
Total assets	79,198	102,412
Current liabilities:
Accounts payable	2,395	742
Accrued expenses and other current liabilities	4,073	6,707
Non-refundable sublicense and transition services payments received	9,741	8,226
Operating lease liabilities	3,592	3,688
Total current liabilities	19,801	19,363
Operating lease liabilities - noncurrent	21,139	21,788
Total liabilities	40,940	41,151
Commitments and contingencies (note 11)
Stockholders' equity:
Preferred stock, $0.0001 par value: 10,000,000 shares authorized; no shares issued and outstanding at both June 30, 2025 and December 31, 2024
Additional paidin capital	722,283	720,488
Accumulated other comprehensive income (loss)		8
Accumulated deficit	(684,025)	(659,235)
Total stockholders' equity	38,258	61,261
Total liabilities and stockholders' equity	$ 79,198	$ 102,412

Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 2025	Dec. 31, 2024
Balance Sheets
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized	300,000,000	300,000,000
Common stock, shares issued	3,178,710	3,161,503
Common stock, shares outstanding	3,178,710	3,161,503

Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Operating expenses:
Research and development	$ 5,814	$ 10,430	$ 13,551	$ 21,965
General and administrative	4,520	6,510	10,605	13,025
Impairment of long-lived assets		438	2,637	438
Loss from operations	(10,334)	(17,378)	(26,793)	(35,428)
Other income (expense), net	949	1,387	2,003	2,726
Net loss	$ (9,385)	$ (15,991)	$ (24,790)	$ (32,702)
Per share information:
Net loss per share of common stock, basic	$ (2.96)	$ (5.09)	$ (7.83)	$ (10.87)
Net loss per share of common stock, diluted	$ (2.96)	$ (5.09)	$ (7.83)	$ (10.87)
Weighted average common shares outstanding, basic	3,168,933	3,142,537	3,166,437	3,007,863
Weighted average common shares outstanding, diluted	3,168,933	3,142,537	3,166,437	3,007,863
Comprehensive loss:
Net loss	$ (9,385)	$ (15,991)	$ (24,790)	$ (32,702)
Unrealized gain (loss) on marketable securities		2	(8)	(24)
Comprehensive loss	$ (9,385)	$ (15,989)	$ (24,798)	$ (32,726)

Statements of Stockholders' Equity - USD ($) $ in Thousands	Common stock	Additional paidin capital	Accumulated other comprehensive income (loss)	Accumulated deficit	Total
Balance at Beginning of period at Dec. 31, 2023		$ 705,794	$ (43)	$ (594,468)	$ 111,283
Balance at Beginning of period (in shares) at Dec. 31, 2023	2,805,618
Balance at end of period at Mar. 31, 2024		716,131	(69)	(611,179)	104,883
Balance at end of period (in shares) at Mar. 31, 2024	3,139,002
Balance at Beginning of period at Dec. 31, 2023		705,794	(43)	(594,468)	111,283
Balance at Beginning of period (in shares) at Dec. 31, 2023	2,805,618
Stockholders' equity
Issuance of common stock under the ATM Facility, net of offering costs		8,742			8,742
Issuance of common stock under the ATM Facility, net of offering costs (in shares)	300,000
Exercise of stock options and vesting of restricted stock units		35			35
Exercise of stock options and vesting of restricted stock units (in shares)	35,607
Issuance of shares in connection with employee stock purchase plan		50			50
Issuance of shares in connection with employee stock purchase plan (in shares)	4,929
Unrealized gain (loss) on marketable securities			(24)		(24)
Sharebased compensation expense		3,173			3,173
Net loss				(32,702)	(32,702)
Balance at end of period at Jun. 30, 2024		717,794	(67)	(627,170)	90,557
Balance at end of period (in shares) at Jun. 30, 2024	3,146,154
Balance at Beginning of period at Mar. 31, 2024		716,131	(69)	(611,179)	104,883
Balance at Beginning of period (in shares) at Mar. 31, 2024	3,139,002
Stockholders' equity
Exercise of stock options and vesting of restricted stock units		35			35
Exercise of stock options and vesting of restricted stock units (in shares)	2,223
Issuance of shares in connection with employee stock purchase plan		50			50
Issuance of shares in connection with employee stock purchase plan (in shares)	4,929
Unrealized gain (loss) on marketable securities			2		2
Sharebased compensation expense		1,578			1,578
Net loss				(15,991)	(15,991)
Balance at end of period at Jun. 30, 2024		717,794	(67)	(627,170)	90,557
Balance at end of period (in shares) at Jun. 30, 2024	3,146,154
Balance at Beginning of period at Dec. 31, 2024		720,488	8	(659,235)	61,261
Balance at Beginning of period (in shares) at Dec. 31, 2024	3,161,503
Stockholders' equity
Exercise of stock options and vesting of restricted stock units (in shares)	14,325
Issuance of shares in connection with employee stock purchase plan		14			14
Issuance of shares in connection with employee stock purchase plan (in shares)	2,882
Unrealized gain (loss) on marketable securities			$ (8)		(8)
Sharebased compensation expense		1,781			1,781
Net loss				(24,790)	(24,790)
Balance at end of period at Jun. 30, 2025		722,283		(684,025)	38,258
Balance at end of period (in shares) at Jun. 30, 2025	3,178,710
Balance at Beginning of period at Mar. 31, 2025		721,346		(674,640)	46,706
Balance at Beginning of period (in shares) at Mar. 31, 2025	3,165,828
Stockholders' equity
Exercise of stock options and vesting of restricted stock units (in shares)	10,000
Issuance of shares in connection with employee stock purchase plan		14			14
Issuance of shares in connection with employee stock purchase plan (in shares)	2,882
Sharebased compensation expense		923			923
Net loss				(9,385)	(9,385)
Balance at end of period at Jun. 30, 2025		$ 722,283		$ (684,025)	$ 38,258
Balance at end of period (in shares) at Jun. 30, 2025	3,178,710

Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		18 Months Ended
Statements of Cash Flows - USD ($) $ in Thousands	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025
Cash flows used in operating activities:
Net loss	$ (9,385)	$ (15,991)	$ (24,790)	$ (32,702)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Depreciation and amortization	200	800	370	1,603
Sharebased compensation			1,781	3,173
Amortization of premium and discount on marketable securities, net			129	(741)
Impairment of long-lived assets		438	2,637	438
Other non-cash items			14
Changes in operating assets and liabilities:
Prepaid expenses and other current assets, and other assets			(571)	(357)
Prepaid research and development			(66)	912
Non-refundable sublicense and transition services payments received			1,515
Right of use assets and operating lease liabilities			(215)	(107)
Accounts payable			1,653	(677)
Accrued expenses and other current liabilities			(2,634)	(3,607)
Net cash provided by (used in) operating activities			(20,177)	(32,065)
Cash flows provided by (used in) investing activities:
Purchases of marketable securities				(50,825)
Sales or maturities of marketable securities			39,046	77,124
Sale of property and equipment and other assets			1,170
Net cash provided by (used in) investing activities			40,216	26,299
Cash flows provided by (used in) financing activities:
Proceeds from issuance of common stock under the ATM Facility, net of offering costs				8,742
Proceeds from the exercise of stock options				35
Proceeds from the issuance of common stock under employee stock purchase plan			14	50
Net cash provided by (used in) financing activities			14	8,827
Net increase (decrease) in cash and cash equivalents			20,053	3,061
Cash and cash equivalents at beginning of period			37,573	21,709	$ 21,709
Cash and cash equivalents at end of period	$ 57,626	$ 24,770	57,626	24,770	$ 57,626
Supplemental disclosure of noncash investing and financing activities:
Unrealized gain (loss) on marketable securities			$ (8)	(24)
Right of use assets recognized upon the commencement of sublease				(422)
Operating lease liabilities recognized upon the commencement of sublease				$ 422

Nature of Operations

6 Months Ended

Jun. 30, 2025

Nature of Operations

Nature of Operations

Passage Bio, Inc.

Notes to Unaudited Interim Financial Statements

1. Nature of Operations

Passage Bio, Inc., or the Company, a Delaware corporation incorporated in July 2017, is a clinical stage genetic medicines company on a mission to improve the lives of patients with neurodegenerative diseases. The Company’s primary focus is the development and advancement of cutting-edge, one-time therapies designed to target critical underlying pathology in these conditions. The Company’s lead clinical product candidate is PBFT02 for the treatment of frontotemporal dementia, or FTD, caused by progranulin deficiency, or FTD-GRN, which seeks to elevate progranulin levels to restore lysosomal function and slow disease progression.

Risks and Liquidity

6 Months Ended

Jun. 30, 2025

Risks and Liquidity

Risks and Liquidity

2. Risks and Liquidity

The Company has incurred recurring losses and negative cash flows from operations since inception and had an accumulated deficit of $684.0 million as of June 30, 2025. The Company anticipates incurring additional losses until such time, if ever, that it can generate significant sales of its product candidates currently in development. Substantial additional capital will be needed by the Company to fund its operations and to develop its product candidates.

The Company’s operations have consisted primarily of conducting preclinical studies, developing licensed technology, conducting clinical trials, and the development and manufacturing of clinical supply to support clinical trials. The Company faces risks associated with early-stage biotechnology companies whose product candidates are in development. Product candidates currently under development will require significant additional research and development efforts, the establishment of manufacturing capacity and regulatory approval prior to commercialization. These efforts require significant amounts of additional capital for the Company to complete its research and development, achieve its regulatory objectives, defend its intellectual property rights, and recruit and retain skilled personnel, and key members of management. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will realize revenue from product sales.

On March 5, 2021, the Company entered into a Sales Agreement, or the Sales Agreement, with Cowen and Company, LLC, or Cowen, relating to the applicable terms of at-the-market equity offerings, or the ATM Facility, pursuant to which the Company may, but is not obligated to, offer and sell, from time to time, shares of its common stock with an aggregate offering price up to $125.0 million through Cowen, as sales agent in the ATM Facility. The Company issued 6,000,000 shares of common stock (300,000 shares adjusted for the 1-for-20 reverse stock split of its common stock effected in 2025) under the ATM Facility resulting in net proceeds of $8.7 million, after deducting offering costs of $0.3 million in March 2024. The Company is limited in its capacity to offer and sell shares of its common stock under this sales agreement pursuant to the prospectus supplement to its shelf registration statement on Form S-3, filed on March 5, 2025. As of June 30, 2025, $15.8 million of capacity remains available to be sold under the ATM Facility.

The Company plans to seek additional funding through public or private equity offerings, debt financings, other collaborations, strategic alliances and licensing arrangements. The Company may not be able to obtain financing on acceptable terms, or at all, and the Company may not be able to enter into strategic alliances or other arrangements on favorable terms, or at all. The terms of any financing may adversely affect the holdings or the rights of the Company’s stockholders. If the Company is unable to obtain funding or prospects of funding are unfavorable, the Company could be required to further delay, reduce or eliminate research and development programs, product portfolio expansion or future commercialization efforts, which could adversely affect its business prospects.

In accordance with Accounting Standards Update, or ASU, No. 2014-15, Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern, the Company has evaluated whether there are certain conditions and events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date that the financial statements are issued. As of the issuance date of these financial statements, the Company expects that its cash and cash equivalents will be sufficient to fund its forecasted operating expenses and capital expenditure requirements for at least the next 12 months from the issuance date of these financial statements.

Summary of Significant Accounting Policies

6 Months Ended

Jun. 30, 2025

Summary of Significant Accounting Policies

Summary of Significant Accounting Policies

3. Summary of Significant Accounting Policies

The Company’s complete summary of significant accounting policies can be found in “Note 3. Summary of Significant Accounting Policies” in the audited financial statements included in the Company’s Annual Report filed on Form 10-K for the year ended December 31, 2024.

Basis of Presentation

The accompanying unaudited financial statements have been prepared in accordance with generally accepted accounting principles in the United States, or GAAP. Any reference in these notes to applicable guidance is meant to refer to GAAP as found in the Accounting Standards Codification, or ASC, and Accounting Standards Updates promulgated by the Financial Accounting Standards Board, or FASB.

On May 28, 2025, the Company’s stockholders provided authorization for the Company’s Board of Directors to effect a reverse stock split to regain compliance with Nasdaq’s listing requirements. On July 14, 2025, the Company effected a 1-for-20 reverse stock split of its common stock, or the Reverse Stock Split. No fractional shares were issued in connection with the Reverse Stock Split. Stockholders who were otherwise entitled to receive fractional shares received the number of shares of Common Stock as rounded up to the nearest whole share. All share and per share amounts in these financial statements and notes thereto, including the stock options, restricted stock units, and employee stock purchase plan activity, have been adjusted retroactively to reflect the Reverse Stock Split for all periods presented.

Interim Financial Statements

The accompanying unaudited interim financial statements have been prepared from the books and records of the Company in accordance with GAAP for interim financial information and Rule 10-01 of Regulation S-X promulgated by the SEC, which permits reduced disclosures for interim periods. All adjustments, consisting only of normal recurring adjustments, necessary for a fair presentation of the accompanying balance sheets, statements of operations and comprehensive loss, stockholders’ equity, and cash flows have been made. Although these interim financial statements do not include all of the information and footnotes required for complete annual financial statements, management believes the disclosures are adequate to make the information presented not misleading. Unaudited interim results of operations and cash flows are not necessarily indicative of the results that may be expected for the full year. Unaudited interim financial statements and footnotes should be read in conjunction with the audited financial statements and footnotes included in the Company’s 2024 Annual Report filed on Form 10-K.

Use of Estimates

The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and contingent liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.

Estimates and assumptions are periodically reviewed, and the effects of the revisions are reflected in the accompanying financial statements in the period they are determined to be necessary.

Fair Value of Financial Instruments

Management believes that the carrying amounts of the Company’s financial instruments, including cash equivalents, prepaid expenses, and accounts payable, approximate fair value due to the short-term nature of those instruments.

Concentration of Credit Risk

Financial instruments that potentially subject the Company to significant concentrations of credit risk consist primarily of cash and cash equivalents. The Company maintains a deposit account in a federally insured financial institution in excess of federally insured limits. The Company also maintains a portfolio of money market funds, which is diversified

to limit exposure related to counterparty risk, industry risk, and security type risk. The Company maintains an investment policy which dictates the allocation of funds within its portfolio of money market funds. The Company has not experienced any losses in such accounts and believes it is not exposed to significant risk on its cash and cash equivalents beyond the normal credit risk associated with commercial banking relationships and money market funds.

Cash and Cash Equivalents

The Company considers all highly-liquid investments that have maturities of three months or less when acquired to be cash equivalents. Cash equivalents as of June 30, 2025 consisted of money market funds as described in Note 4. Cash consists of cash deposits at banking institutions.

Marketable Securities

The Company classifies its marketable securities with original maturities of greater than three months as available-for-sale. These securities are carried at fair market value, with unrealized gains and losses reported in comprehensive loss and accumulated other comprehensive income (loss) within stockholders’ equity. Any premium or discount arising at purchase of debt securities is amortized and/or accreted over the term of the security to other income (expense), net. Gains or losses on marketable securities sold are recognized as a component of other income (expense), net in the statement of operations and comprehensive loss on the specific identification method. All marketable securities are available for use, as needed, to fund operations and therefore, the Company classifies all marketable securities as current assets within the balance sheet. As of June 30, 2025, the Company had no marketable securities.

Property and Equipment, Net

Property and equipment, net consists of laboratory equipment, office equipment, computer hardware and software, furniture and fixtures, and leasehold improvements and is recorded at cost. Maintenance and repairs that do not improve or extend the lives of the respective assets are expensed as incurred. Property and equipment are depreciated on a straight-line basis over their estimated useful lives. The Company estimates useful life on an asset-by-asset basis, which generally consists of three years for computer hardware and software, five years for office equipment, five years for laboratory equipment, and seven years for furniture and fixtures. Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the asset.

When property and equipment are retired or otherwise disposed of, the costs and accumulated depreciation and amortization are removed from the respective accounts, with any resulting gain or loss recognized concurrently. The Company recognized de minimis losses on disposals of property and equipment for the six months ended June 30, 2025. The Company did not recognize any losses on disposals of property and equipment for the three months ended June 30, 2025 or the three and six months ended June 30, 2024.

The Company reviews long-lived assets, such as property and equipment, for impairment when events or changes in circumstances indicate the carrying amount of the assets may not be recoverable. The Company recognized impairment expenses for laboratory equipment and certain other assets of $2.6 million for the six months ended June 30, 2025. The Company did not recognize any impairment expenses for long-lived assets for the three months ended June 30, 2025. The Company recognized impairment expenses for property and equipment of $0.4 million for both the three and six months ended June 30, 2024.

The impairment expenses for the six months ended June 30, 2025, relate to the Company’s January 2025 announcement to reduce its overall workforce by 55% and cease its lab operations in Hopewell, New Jersey. As a result, the Company reassessed asset groups at its lab in Hopewell, New Jersey, and evaluated such asset groups for impairment under FASB ASC Topic 360, Long-lived assets: Impairment or disposal of long-lived assets. The Company determined the laboratory equipment was a separate asset group based on management’s implemented plans to sell the laboratory equipment and estimated the fair value of the laboratory equipment based on the estimated future cash flows from the sale of such equipment. Subsequent to recording the impairment, the Company entered into a sales agreement for $1.2 million for substantially all the laboratory equipment and certain other assets.

Leasing

The Company evaluates leases at their inception to determine if they are an operating lease or a finance lease. As of June 30, 2025, the Company has classified all leases with terms greater than one year, as operating leases.

The Company recognizes assets and liabilities for operating leases at their inception, based on the present value of all payments due under the lease agreement. The Company uses its incremental borrowing rate to determine the present value of operating leases, which is determined by referencing collateralized borrowing rates for debt instruments with terms similar to the respective lease. The Company utilizes the accounting policy election to not separate lease and non-lease components and the accounting policy election to not apply the recognition requirement to leases with a term of 12 months or less.

The Company reviews long-lived assets, such as right of use assets, or ROU assets, for impairment when events or changes indicate the carrying amount of the ROU assets may not be recoverable. The Company did not recognize any impairment expenses for ROU assets for the three and six months ended June 30, 2025 and 2024.

Research and Development

Research and development costs are expensed as incurred and consist primarily of expenses incurred with the Trustees of the University of Pennsylvania’s, or Penn’s, Gene Therapy Program, or GTP, Gemma Biotherapeutics, Inc., or Gemma, contract research organizations, contract manufacturing organizations, internal analytical and testing activities, and employee-related expenses, including salaries, benefits, and share-based compensation. Management makes estimates of the Company’s external accrued research and development expenses, which primarily relates to contract research organizations and contract manufacturing organizations, as of each balance sheet date in the Company’s financial statements based on an estimate of progress to completion of specific tasks using facts and circumstances known to the Company at that time. The Company determines the estimates by reviewing contracts, vendor agreements, change orders, and through discussions with the Company’s internal clinical personnel and external service providers as to the progress to completion of services and the agreed-upon fee to be paid for such services. If the actual timing of the performance of services or the level of effort varies from the estimate, the Company will adjust the accrual and related expenses accordingly.

Other Income (Expense), Net

Other income (expense), net consists of interest earned on cash equivalents and marketable securities, amortization of premium and discount on marketable securities, and income from subleases.

The Company recorded $1.0 million to other income (expense), net for the three months ended June 30, 2025, which consisted of $0.6 million attributable to interest income and the amortization of premium and discount on the Company’s marketable securities, and $0.4 million related to income from subleases.

The Company recorded $1.4 million to other income (expense), net for the three months ended June 30, 2024, which consisted of $1.2 million attributable to interest income and the amortization of premium and discount on the Company’s marketable securities, and $0.2 million related to income from subleases.

The Company recorded $2.0 million to other income (expense), net for the six months ended June 30, 2025, which consisted of $1.3 million attributable to interest income and the amortization of premium and discount on the Company’s marketable securities, and $0.7 million related to income from subleases.

The Company recorded $2.7 million to other income (expense), net for the six months ended June 30, 2024, which consisted of $2.4 million attributable to interest income and the amortization of premium and discount on the Company’s marketable securities, and $0.3 million related to income from subleases.

Share-Based Compensation

The Company measures share-based awards at grant-date fair value and records compensation expense on a straight-line basis over the vesting period of the awards. The Company’s share-based compensation consists of restricted stock units, or RSUs, and options to purchase common stock, or stock option awards.

The Company uses the Black-Scholes option pricing model to value its stock option awards.

Estimating the fair value of stock option awards requires the input of assumptions, including, the expected term of stock options and stock price volatility. The assumptions used in estimating the fair value of share-based awards represent management’s estimate and involve inherent uncertainties and the application of management’s judgment. As a result, if factors change and management uses different assumptions, share-based compensation expense could be materially different for future awards.

The expected term of the stock options is estimated using the “simplified method,” as the Company has limited historical information from which to develop reasonable expectations about future exercise patterns and post-vesting employment termination behavior for its stock option grants. The simplified method is the midpoint between the vesting period and the contractual term of the option. For stock price volatility, the Company uses a composite of comparable public company data as a basis for its expected volatility and considers the historic volatility of its common stock from its initial public offering to date to calculate the fair value of option grants. The selection of comparable public company data requires the application of management’s judgement.

The Company accounts for forfeitures of RSUs and stock option awards as they occur.

License and Other Revenue

The Company may enter into license agreements and transition services agreements (see Note 8) under which it may license rights to research, develop, manufacture, and commercialize its product candidates to third parties, and provide transition services for such licenses. Payments under these arrangements may include non-refundable, upfront fees, reimbursement of certain costs, payments upon the achievement of certain milestones, and royalties on product sales.

The Company applies ASC Topic 606, Revenue from Contracts with Customers, or ASC 606, when all of the following criteria are met, to determine a valid contract exists: (i) the parties have approved the contract and are committed to perform their respective obligations; (ii) the Company can identify each party’s rights regarding the goods or services to be transferred; (iii) the Company can identify the payment terms for the goods or services to be transferred; (iv) the contract has commercial substance; and (v) the Company will collect substantially all of the consideration to which it will be entitled in exchange for the goods or services that will be transferred to the customer. Once it is determined that a valid contract exists, the Company performs the following steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including consideration of the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations on a relative stand-alone selling price basis; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. As part of the accounting for these arrangements, the Company must use its judgment to determine the number of performance obligations, the transaction price, the stand-alone selling price for each performance obligation identified in the contract for the allocation of transaction price, the contract term and pattern of satisfaction of the performance obligations. The Company uses judgment to determine whether milestones or other variable consideration, except for certain sales-based milestone payments and royalties, should be included in the transaction price as described further below.

At the inception of each arrangement that includes milestone payments, the Company evaluates whether the milestones are considered probable of being achieved and estimates the amount to be included in the transaction price using the most likely amount method set forth in ASC 606. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the control of the Company or the licensee, such as those subject to regulatory approvals, are not considered probable of being achieved

until those approvals are received. The Company evaluates factors such as the scientific, clinical, regulatory, commercial and other risks that must be overcome to achieve the particular milestone in making this assessment. There is considerable judgment involved in determining whether it is probable that a significant revenue reversal would not occur. At the end of each subsequent reporting period, the Company reevaluates the probability of achievement of all milestones subject to constraint and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis in the statements of operations and comprehensive loss in the period of adjustment.

For customer contracts in the scope of ASC 606, amounts due to the Company are recorded as accounts receivable on the Company’s balance sheet when the Company’s right to consideration is unconditional. Amounts received prior to satisfying the related performance obligations are classified on the Company’s balance sheet as current deferred revenue if expected to be recognized as revenue within 12 months following the balance sheet date and as deferred revenue, net of current portion, if amounts are not expected to be recognized as revenue within the 12 months following the balance sheet date. The Company does not evaluate a contract for a significant financing component if payment is expected within one year or less from the transfer of promised items to the customer.

Net Loss Per Share

Basic net loss per share of common stock is computed by dividing net loss by the weighted average number of shares of common stock outstanding during each period. Diluted loss per share of common stock includes the effect, if any, from the potential exercise or conversion of securities, such as stock options, which would result in the issuance of incremental shares of common stock. For diluted net loss per share, the weighted-average number of shares of common stock is the same for basic net loss per share due to the fact that when a net loss exists, dilutive securities are not included in the calculation as the impact is anti-dilutive.

The following potentially dilutive securities have been excluded from the computation of diluted weighted-average shares of common stock outstanding, as they would be anti-dilutive:


	Six Months Ended June 30,
	2025	2024
Stock options	726,590	622,283
Unvested restricted stock units	52,500	17,635
Employee stock purchase plan	1,357	1,839
	780,447	641,757

Recently Issued Accounting Pronouncements

In December 2023, the FASB issued ASU No. 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures, or ASU 2023-09, which requires that an entity, on an annual basis, disclose additional income tax information, primarily related to the rate reconciliation and income taxes paid. The amendments in ASU 2023-09 are intended to enhance the transparency and decision usefulness of income tax disclosures. The amendments in this ASU are effective for annual periods beginning after December 15, 2024 with early adoption permitted. The Company is currently evaluating the impact of this guidance on its disclosures.

In November 2024, the FASB issued ASU No. 2024-03, Income Statement – Reporting Comprehensive Income – Expense Disaggregation Disclosures (Subtopic 220-40): Disaggregation of Income Statement Expenses, or ASU 2024-03, which requires entities to provide disclosures to disaggregate operating expenses into specific categories, such as salaries and wages, depreciation, and amortization, to provide enhanced transparency into the nature and function of expenses. ASU 2024-03 is effective for the Company’s first fiscal year beginning after December 15, 2026, and for interim periods within the Company’s first fiscal year beginning after December 15, 2027, with early adoption permitted. ASU 2024-03 may be applied retrospectively or prospectively. The Company is currently evaluating the impact of this guidance on its disclosures.

Cash, Cash Equivalents and Marketable Securities

6 Months Ended

Jun. 30, 2025

Cash, Cash Equivalents and Marketable Securities

Cash, Cash Equivalents and Marketable Securities

4. Cash, Cash Equivalents and Marketable Securities

The following table provides details regarding the Company’s portfolio of cash and cash equivalents:


	Cost or
(in thousands)	Amortized cost		Unrealized gains		Unrealized losses		Fair value
June 30, 2025:
Cash accounts in banking institutions	$	2,500	$	—	$	—	$	2,500
Money market funds		55,126		—		—		55,126
Total	$	57,626	$	—	$	—	$	57,626

December 31, 2024:
Cash accounts in banking institutions	$	3,527	$	—	$	—	$	3,527
Money market funds		29,058		—		—		29,058
Commercial paper		4,988		—		—		4,988
Total	$	37,573	$	—	$	—	$	37,573

As of June 30, 2025, all of the Company’s marketable securities matured and the proceeds were invested into money market funds, which are included in cash and cash equivalents on the Company’s balance sheet.

Fair Value of Financial Instruments and Non-Financial Instruments

6 Months Ended

Jun. 30, 2025

Fair Value of Financial Instruments and Non-Financial Instruments

Fair Value of Financial Instruments and Non-Financial Instruments

5. Fair Value of Financial Instruments and Non-Financial Instruments

Financial Instruments

Fair value is the price that could be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. Fair value determination in accordance with applicable accounting guidance requires that a number of significant judgments be made. Additionally, fair value is used on a nonrecurring basis to evaluate assets for impairment or as required for disclosure purposes by applicable accounting guidance on disclosures about fair value of financial instruments. Depending on the nature of the assets and liabilities, various valuation techniques and assumptions are used when estimating fair value. The carrying amounts of certain of the Company’s financial instruments, including prepaid expense and accounts payable are shown at cost, which approximates fair value due to the short-term nature of these instruments. The Company follows the provisions of ASC Topic 820, Fair Value Measurement, for financial assets and liabilities measured on a recurring basis. The guidance requires fair value measurements be classified and disclosed in one of the following three categories:

●

Level 1: Unadjusted quoted prices in active markets that are accessible at the measurement date for identical, unrestricted assets or liabilities.

●

Level 2: Quoted prices in markets that are not active, or inputs which are observable, either directly or indirectly, for substantially the full term of the asset or liabilities.

●

Level 3: Prices or valuation techniques that require inputs that are both significant to the fair value measurement and unobservable (i.e., supported by little or no market activity).

The following fair value hierarchy table presents information about the Company’s assets measured at fair value on a recurring basis. Included within cash and cash equivalents on the balance sheet, but excluded from the fair value hierarchy table, are cash deposits held at financial institutions:


	Fair value measurement at
	reporting date using
	Quoted prices
	in active		Significant
		markets for	other		Significant
	identical		observable			unobservable
	assets		inputs		inputs
(in thousands)	(Level 1)		(Level 2)		(Level 3)
June 30, 2025:
Assets
Cash equivalents:
Money market funds	$	55,126	$	—	$	—
Total cash equivalents		55,126		—		—
Total financial assets	$	55,126	$	—	$	—

December 31, 2024:
Assets
Cash equivalents:
Money market funds	$	29,058	$	—	$	—
Commercial paper		—		4,988		—
Total cash equivalents		29,058		4,988		—

Marketable securities:
Certificates of deposit		—		5,971		—
Commercial paper		—		25,439		—
Corporate debt securities		—		1,865		—
U.S. government securities		—		5,908		—
Total marketable securities		—		39,183		—
Total financial assets	$	29,058	$	44,171	$	—

All of the marketable securities had contractual maturities less than one year as of December 31, 2024 and unrealized gains and losses on the marketable securities were de minimis as of December 31, 2024.

Non-Financial Instruments

Long-lived non-financial assets are measured at fair value on a nonrecurring basis for purposes of calculating impairment using Level 3 inputs as defined in the fair value hierarchy. The fair value of long-lived assets using Level 3 inputs is determined by estimating the amount and timing of net future cash flows (which are unobservable inputs) and discounting them using a risk-adjusted rate of interest. Significant increases or decreases in actual cash flows may result in valuation changes. Assets remeasured in the six months ended June 30, 2025 include the laboratory equipment and certain other assets which were sold prior to June 30, 2025 and are not included in the balance sheet as of June 30, 2025. The impairment related to the remeasurement of $2.6 million is included in the statement of operations for the six months ended June 30, 2025. There were no assets remeasured in the three months ended June 30, 2025.

- 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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (bbb)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 107
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482078/820-10-55-107

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 100
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482078/820-10-55-100

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)(3)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6A
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-6A

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2E
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2E

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6A
-Subparagraph (h)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-6A

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6A
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-6A

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6A
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-6A

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6A
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-6A

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (h)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (g)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Topic 940
-SubTopic 820
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478119/940-820-50-1

+ Details

Property and Equipment, Net

6 Months Ended

Jun. 30, 2025

Property and Equipment, Net

Property and Equipment, Net

6. Property and Equipment, Net

Property and equipment, net, consists of the following:


(in thousands)		June 30, 2025	December 31, 2024
Laboratory equipment	$	—	$	10,020
Office equipment		107		119
Computer hardware and software		988		1,111
Furniture and fixtures		419		419
Leasehold improvements		7,386		7,386
Total property and equipment		8,900		19,055
Accumulated depreciation and amortization		(3,560)		(9,724)
	$	5,340	$	9,331

In connection with the Company’s January 2025 announcement to reduce its overall workforce by 55% and cease its lab operations in Hopewell, New Jersey, management implemented plans to sell substantially all the laboratory equipment and certain other assets and estimated the fair value of the assets based on the estimated future cash flows from the sale of such assets. Subsequent to recording the impairment of $2.6 million, the Company entered into a sales agreement for $1.2 million for substantially all of the laboratory equipment and certain other assets. As a result, the Company did not record any depreciation on the impaired and disposed laboratory equipment during the three and six months ended June 30, 2025 as the equipment was considered held-for-sale in January 2025. Neither laboratory equipment nor accumulated depreciation related to such equipment are recorded on the balance sheet as of June 30, 2025.

Depreciation and amortization expense was $0.2 million and $0.8 million for the three months ended June 30, 2025 and 2024, respectively.

Depreciation and amortization expense was $0.4 million and $1.6 million for the six months ended June 30, 2025 and 2024, respectively.

Accrued Expenses and Other Current Liabilities

6 Months Ended

Jun. 30, 2025

Accrued Expenses and Other Current Liabilities

Accrued Expenses and Other Current Liabilities

7. Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following:


(in thousands)	June 30, 2025		December 31, 2024
Professional fees	$	426	$	406
Compensation and related benefits		2,494		4,405
Research and development		1,153		1,896
	$	4,073	$	6,707

Gemma License Agreement

6 Months Ended

Jun. 30, 2025

Gemma License Agreement

Gemma License Agreement

8. Gemma License Agreement

On July 31, 2024, the Company entered into a series of sublicense agreements with Gemma in connection with the outlicense of PBGM01 for the treatment of GM1 gangliosidosis, or GM1, PBKR03 for the treatment of Krabbe disease, and PBML04 for the treatment of metachromatic leukodystrophy, or MLD, collectively the Outlicensed Programs, and such agreements, the Gemma Sublicenses. On May 7, 2025, the Company agreed to amend each of the Gemma Sublicenses to revise certain financial terms related to the Outlicensed Programs, or the Amended Gemma Sublicenses. Pursuant to the Amended Gemma Sublicenses, the Company is entitled to receive (i) an aggregate total of $15.0 million in initial payments for licenses and clinical product supply, of which $5.0 million was previously received and $5.0 million of which was due in May 2025; (ii) an additional $5.0 million contingent on Gemma completing certain business milestones; (iii) up to an additional $114.0 million in development and commercial milestone payments; and (iv) single digit royalties as a percentage of annual worldwide net sales, in exchange for sublicenses to relevant intellectual property, transfer of regulatory dossiers and transfer of clinical trial materials and product supply related to the Outlicensed Programs. Gemma will be responsible for all payments due to Penn under the Company’s research, collaboration and licensing agreement with Penn, or the Penn License Agreement, related to the Outlicensed Programs.

On July 31, 2024 the Company also entered into a transition services agreement with Gemma, or the Transition Services Agreement, as amended by the First Amendment to the Transition Services Agreement, dated January 31, 2025, pursuant to which, the Company provided transitional services at cost to Gemma through May 31, 2025, and is entitled to reimbursement for transitional services performed retroactively from March 1, 2024, related to the transfer of the Outlicensed Programs. As of June 30, 2025, the Company has collected $5.0 million in initial payments and $4.7 million in transition services payments under these agreements.

As Gemma is a newly-formed company with a limited history of operations, the Company will not recognize revenue under ASC 606 until the Company either (i) has received payment and there are no remaining obligations to transfer goods and services under the Amended Gemma Sublicenses and Transition Services Agreement (as payments received by Gemma are nonrefundable), or (ii) concludes that substantially all of the transaction price is collectible. As of June 30, 2025, the Company has received an initial payment of $5.0 million associated with the aggregate $15.0 million of initial payments to be made under the Amended Gemma Sublicenses for licenses and clinical product supply and $4.7 million associated with the Transition Services Agreement. The Company recorded the $9.7 million received as non-refundable sublicense and transition services payments received on the balance sheet as of June 30, 2025, as the criteria set forth above has not yet been met.

Severance

6 Months Ended

Jun. 30, 2025

Severance

Severance

9. Severance

In January 2025, the Company announced a workforce reduction to reduce operating expenses and to extend its cash runway. In connection with the announcement, the Company reduced headcount by approximately 55%.

In accordance with ASC 420, Exit and Disposal Activities, the Company recorded severance and termination-related costs of $0.4 million in general and administrative expenses and $1.3 million in research and development expenses for the six months ended June 30, 2025. During the three months ended June 30, 2025, and the three and six months ended June 30, 2024, the Company recorded no severance and termination-related costs in general and administrative expenses or in research and development expenses. As of June 30, 2025, there were no unpaid severance and termination-related costs.

Leases

6 Months Ended

Jun. 30, 2025

Leases

Leases

10. Leases

2005 Market Street Lease Agreement

The Company is party to a lease agreement for office space, or the 2005 Market Street Lease Agreement, in Philadelphia, Pennsylvania. Under the 2005 Market Street Lease Agreement, the Company leased approximately 37,000 square feet. The 2005 Market Street Lease Agreement commenced in February 2021 and is expected to expire in December 2031. The Company has an option to extend the term of the 2005 Market Street Lease Agreement by two additional terms of five years each. The Company has an option to early terminate the 2005 Market Street Lease Agreement as of April 2029, given notice is provided to the landlord no less than fifteen months prior to April 2029. The optional extension and termination terms were not recognized as part of the Company’s measurement of the ROU asset and operating lease liability as of June 30, 2025. In 2023, the Company subleased all of the space at 2005 Market Street as further described in Sublease Agreement A and Sublease Agreement B below.

Sublease Agreement A

On August 7, 2023, the Company entered into a sublease agreement with a counterparty, or Sublessee A, to sublease approximately 8,000 square feet of the 2005 Market Street Lease Agreement, or Sublease Agreement A. This sublease term began on November 1, 2023, and continues through March 31, 2029. In the event the Company does not elect its early termination option under the 2005 Market Street Lease Agreement, Sublessee A has an option to extend the sublease agreement through November 30, 2031. The base sublease rent is $0.1 million per year and increases by 2.75% annually through the expiration of the agreement. Additionally, Sublessee A is required to pay the portion of the common area maintenance expenses, operating expenses and use and occupancy taxes which the Company is required to pay under the 2005 Market Street Lease Agreement.

Pursuant to ASC Topic 842, Leases, or ASC 842, the Company concluded the sublease is a separate lease, as the Company was not relieved of the primary obligation under the 2005 Market Street Lease Agreement. The Company continues to account for the 2005 Market Street Lease Agreement as a lessee and in the same manner as prior to the execution of Sublease Agreement A. The Company accounted for Sublease Agreement A as the lessor, and concluded the lease qualified as an operating lease, as it did not meet the criteria of a sales-type or direct financing lease.

Sublease Agreement B

On September 29, 2023, the Company entered into a sublease agreement with a counterparty, or Sublessee B, to sublease approximately 29,000 square feet of the 2005 Market Street Lease Agreement, or Sublease Agreement B. This sublease term began on March 1, 2024, and continues through August 2026. Sublessee B has an option to extend the term of the sublease agreement through March 31, 2029. The base sublease rent is $0.9 million per year for the entire term of the sublease. Additionally, Sublessee B is required to pay applicable use and occupancy taxes but is not obligated to make payments for operating expenses and common area maintenance expenses which the Company is required to pay under the 2005 Market Street Lease Agreement.

Pursuant to ASC 842, the Company concluded the sublease is a separate lease, as the Company was not relieved of the primary obligation under the 2005 Market Street Lease Agreement. The Company continues to account for the 2005 Market Street Lease Agreement as a lessee and in the same manner as prior to the execution of the Sublease Agreement B. The Company accounted for Sublease Agreement B as the lessor, and concluded the lease qualified as an operating lease, as it did not meet the criteria of a sales-type or direct financing lease.

1835 Market Street Sublease Agreement

On February 20, 2024, the Company entered into a sublease agreement with a counterparty, or the 1835 Market Street Sublease Agreement. Under the 1835 Market Street Sublease Agreement, the Company subleased approximately 16,000 square feet of office space in Philadelphia, Pennsylvania. The sublease term began on March 26, 2024 and expires on September 30, 2025. The Company has the option to extend the term of the sublease agreement through February 28, 2029. The base sublease rent is $0.3 million per year for the original 18-month term of the sublease. Additionally, the Company is required to pay utility costs associated with the subleased premises. The optional extension was not recognized as part of the Company’s measurement of the ROU asset and operating lease liability as of June 30, 2025.

Laboratory Lease Agreement

The Company is also party to a lease agreement for laboratory space, or the Laboratory Lease Agreement, in Hopewell, New Jersey. The laboratory is focused on state-of-the-art analytical capabilities, assay development and validation, and clinical product testing to support both viral vector manufacturing and clinical development. The Laboratory Lease Agreement commenced in March 2021 and is expected to expire in March 2036. The Company has an option to early terminate the Laboratory Lease Agreement as of March 2032 given notice is provided to the landlord no less than twelve months prior to March 2032. The Company has an option to extend the term of the Laboratory Lease Agreement by up to two five-year terms. These options were not recognized as part of the Company's measurement of the ROU asset and operating lease liability as of June 30, 2025.

Hopewell Sublease Agreement

On September 4, 2024, the Company entered into a sublease agreement with a counterparty, or Sublessee C, to sublease approximately 3,200 square feet, or 5% of its approximately 62,000 square feet of leased laboratory space under the Laboratory Lease Agreement, or Hopewell Sublease Agreement. This sublease term began on September 11, 2024 and expires on December 31, 2029. Sublessee C has the option to extend the term of the sublease through December 2032. The base sublease rent is $0.1 million per year and increases by 2.5% annually through the expiration of the Hopewell Sublease Agreement. Additionally, Sublessee C is required to pay the portion of the common area maintenance expenses, operating expenses, and use and occupancy taxes that the Company is required to pay under the Laboratory Lease Agreement.

Pursuant to ASC 842, the Company concluded the sublease is a separate lease, as the Company was not relieved of the primary obligation under the Laboratory Lease Agreement. The Company continues to account for the Laboratory Lease Agreement as a lessee and in the same manner as prior to the execution of the Hopewell Sublease Agreement. The Company accounted for the Hopewell Sublease Agreement as the lessor, and concluded the lease qualified as an operating lease, as it did not meet the criteria of a sales-type or direct financing lease.

In 2024, the Company determined triggering events were present and reassessed the asset groups related to its laboratory space under the Laboratory Lease Agreement, which resulted in changes to the Company’s identified asset groups. The Company determined whether an impairment indicator was present for each of the new asset groups. Where an impairment indicator was present, the Company compared the estimated undiscounted cash flows to the carrying value, which includes ROU assets, leasehold improvements, and other property and equipment allocable to the laboratory space for those asset groups. The Company concluded the carrying values of certain asset groups were not recoverable as it exceeded the estimated undiscounted cash flows. The Company calculated the amount of impairment on those asset groups using a discounted cash flow model to calculate the fair value of the asset group which incorporated the net identifiable cash flows for the term of the Hopewell Sublease Agreement, including an estimate for cash flows in the residual period, and an estimated borrowing rate of a market participant subtenant. The impairment charge was recorded as of the sublease execution date.

In connection with the January 2025 announcement to reduce its overall workforce by 55% and cease its lab operations in Hopewell, New Jersey, the Company determined triggering events were present and reassessed its asset groups related to its laboratory space under the Laboratory Lease Agreement. Laboratory equipment was separated from the ROU assets and leasehold improvements allocable to the laboratory space as the equipment was no longer being used in operations and the Company had implemented a plan to sell those assets. For the ROU assets and allocable leasehold improvements, the Company compared the estimated undiscounted cash flows from subleasing to the carrying value and determined there was no impairment. The Company is pursuing additional opportunities to sublease additional space to further offset portions of its financial obligations under the Laboratory Lease and additional impairments to ROU assets and leasehold improvements could occur based on the economic terms included in such agreements.

The following table summarizes future minimum lease payments for the Company’s lessee operating leases, which comprises of the 2005 Market Street Lease Agreement, 1835 Market Street Sublease Agreement, and the Laboratory Lease Agreement. The below table does not include expected cash inflows related to Sublease Agreement A, Sublease Agreement B, and the Hopewell Sublease Agreement as the Company was not relieved of its primary obligation under the 2005 Market Street Lease Agreement and Laboratory Lease Agreement:


(in thousands)
2025	$	1,911
2026		3,757
2027		3,863
2028		3,973
2029		4,085
Thereafter		21,621
Total undiscounted lease payments		39,210
Less: imputed interest		(14,479)
Total lease liabilities	$	24,731

The following table summarizes lease expense by lease type that was recognized during the three and six months ended June 30, 2025 and 2024:


	Three Months Ended				Six Months Ended
($ in thousands)	June 30, 2025		June 30, 2024		June 30, 2025		June 30, 2024
Operating lease cost	$	877	$	909	$	1,757	$	1,740
Variable lease cost		543		394		1,081		1,114
	$	1,420	$	1,303	$	2,838	$	2,854

The following table shows the weighted average discount rate and weighted average remaining lease term of the operating leases:


	Six Months Ended
($ in thousands)	June 30, 2025		June 30, 2024
Weighted-average discount rate		9.7%		9.7%
Weighted-average remaining lease term (years)		9.8		10.7

The cash paid for amounts included in the measurement of the Company’s operating lease liabilities for the six months ended June 30, 2025 and 2024 were $2.0 million and $1.8 million, respectively, recorded in operating cash flows.

The following table summarizes sublease income that was recognized in other income (expense), net during the three and six months ended June 30, 2025 and 2024:


	Three Months Ended				Six Months Ended
($ in thousands)	June 30, 2025		June 30, 2024		June 30, 2025		June 30, 2024
Sublease rental income	$	363	$	243	$	730	$	300

Commitments and Contingencies

6 Months Ended

Jun. 30, 2025

Commitments and Contingencies.

Commitments and Contingencies

11. Commitments and Contingencies

Amended and Restated Research, Collaboration and License Arrangement with Penn

In connection with the transfer of the Outlicensed Programs (GM1, Krabbe, and MLD), the Company restructured its research, collaboration and license agreement with Penn, as amended, previously the Penn Agreement and now referred to as the Penn License Agreement. Pursuant to the Penn License Agreement, as of July 31, 2024, the Company (i) terminated the funding of discovery research programs; (ii) terminated the research and exploratory research programs; (iii) terminated the remaining eight options it had for future central nervous system, or CNS, indications; (iv) terminated the transaction fee payable to Penn in the event of certain corporate transactions; and (v) retained its current exclusive and non-exclusive licenses to its programs in FTD, GM1, Krabbe and MLD and certain platform technologies resulting from the discovery programs that it funded.

For the Company’s licensed programs in FTD, GM1, Krabbe and MLD, the Penn License Agreement requires that it make payments of up to $16.5 million per product candidate. Each payment will be due upon the achievement of specific development milestone events by such licensed product for a first indication, reduced development milestone payments for the second and third indications and no development milestone payments for subsequent indications. In addition, on a product-by-product basis, the Company is obligated to make up to $55.0 million in sales milestone payments on each licensed product based on annual worldwide net sales of the licensed product in excess of defined thresholds. Pursuant to the Amended Gemma Sublicenses, Gemma is responsible for the payments to Penn related to the Outlicensed Programs.

Upon successful commercialization of a product using the licensed technology, the Company is obligated to pay to Penn, on a licensed product-by-licensed product and country-by-country basis, tiered royalties (subject to customary reductions) in the mid-single digits percentage on annual worldwide net sales of such licensed product. In addition, other than the Amended Gemma Sublicenses, the Company is obligated to pay to Penn a percentage of sublicensing income, ranging from the mid-single digits to low double digits, for sublicenses under the Penn License Agreement. The agreement will expire on a licensed product-by-licensed product and country-by-country basis upon the later of (i) the

expiration of the last valid claim of the licensed patent rights that covers the exploitation of such licensed product in such country, and (ii) the expiration of the royalty period. Pursuant to the Amended Gemma Sublicenses, Gemma is responsible for the payments to Penn related to the Outlicensed Programs.

Gemma - Research, Collaboration and License Agreement

In connection with the transfer of the Outlicensed Programs, on July 31, 2024, the Company entered into a research, collaboration and license agreement with Gemma, or the Gemma Collaboration Agreement. Pursuant to the Gemma Collaboration Agreement, (i) Gemma will conduct certain preclinical and IND-enabling work for the Company’s active research program in Huntington’s disease and a currently paused research program in Temporal Lobe Epilepsy, or TLE, which were previously being conducted by Penn under the Penn Agreement and (ii) Gemma will grant the Company options to conduct mutually-agreed research programs in four new CNS indications.

The Gemma Collaboration Agreement requires the Company to make payments of up to (i) $16.5 million per product candidate in the aggregate for Huntington’s disease and any future CNS indications available to the Company under its four options and (ii) $39.0 million per product candidate in the aggregate arising from the research program for TLE. Each payment will be due upon the achievement of specific development milestone events by such licensed product for a first indication, reduced development milestone payments for the second and third indications and no development milestone payments for subsequent indications. In addition, on a product-by-product basis, the Company is obligated to make up to $55.0 million in sales milestone payments on each licensed product based on annual worldwide net sales of the licensed product in excess of defined thresholds.

Upon successful commercialization of a product using the licensed technology, the Company is obligated to pay to Gemma, on a licensed product-by-licensed product and country-by-country basis, tiered royalties (subject to customary reductions) in the mid-single digits percentage on annual worldwide net sales of such licensed product. In addition, the Company is obligated to pay to Gemma a percentage of sublicensing income, ranging from the mid-single digits to low double digits, for sublicenses under the Gemma Collaboration Agreement. The agreement will expire on a licensed product-by-licensed product and country-by-country basis upon the later of (i) the expiration of the last valid claim of the licensed patent rights that covers the exploitation of such licensed product in such country, and (ii) the expiration of the royalty period.

If the Company was to exercise any of the four options under the Gemma Collaboration Agreement, it would owe Gemma a non-refundable aggregate fee of $1.0 million per product indication, with $0.5 million due upfront and another $0.5 million fee owed upon a further developmental milestone.

The Company has also entered into the Amended Gemma Sublicenses and Transition Services Agreement as described in Note 8.

The Amended Gemma Sublicenses, the Transition Services Agreement, and the Gemma Collaboration Agreement are collectively referred to as the Outlicense Transaction Agreements.

Catalent Agreements

The Company has entered into a collaboration agreement, and a development services and clinical supply agreement, or the Amended Catalent Agreements, with Catalent Maryland, a unit of Catalent, Inc. acquired by Novo Holdings A/S, or Catalent, to secure clinical scale manufacturing capacity for batches of active pharmaceutical ingredients for the Company’s gene therapy product candidates. Under the terms of the Amended Catalent Agreements, Catalent agreed to manufacture batches of drug product for the Company’s gene therapy product candidates.

The Amended Catalent Agreements remain in effect until November 6, 2030, and establish a limited exclusive relationship between the Company and Catalent for the manufacture of bulk drug substance and drug product for the Company’s adeno-associated virus delivery therapeutic product candidates for the treatment of FTD and GM1. The limited exclusive relationship under the Amended Catalent Agreements converts to a non-exclusive relationship (i) in the event Catalent fails to meet certain performance standards and (ii) following certain conditional events related to the

divestiture by the Company of either FTD or GM1, in which case, if such events occur, the Company would pay Catalent certain fees. The outlicense of GM1 to Gemma under the Outlicense Transaction Agreements, and subsequent business decisions implemented by Gemma in their sole discretion, could be considered an event related to the divesture of GM1 under the Amended Catalent Agreements and require the Company to make payment of certain fees to Catalent, which fees are immaterial.

Litigation

In the normal course of business, the Company from time to time is named as a party to legal claims and actions. The Company records a loss contingency reserve for a legal proceeding when the potential loss is considered probable and can be reasonably estimated. The Company has not recorded any amounts for loss contingencies as of June 30, 2025.

The Company is a defendant in litigation with a former employee, who filed a lawsuit in the Court of Common Pleas of Philadelphia County asserting claims for breach of contract and violation of the Pennsylvania Wage Payment and Collection Law. The plaintiff, who was terminated from his employment in 2019, contended that the Company entered into a binding settlement agreement in February 2020 under which he was to receive shares of company stock and additional compensation. Specifically, he contended that before the announcement of the Company’s initial public offering in February 2020, he was promised 150,000 shares of stock as part of the settlement, and that those shares were not subject to the reverse stock split that was implemented for all shareholders. The Company responded that the shares offered in settlement negotiations in 2020 were to be subject to the reverse split, and that had the settlement been finalized, the plaintiff would have been entitled to 33,836 shares (1,692 shares adjusted for the Reverse Stock Split effected in 2025). A trial in this case was held in October 2024. The jury found that an agreement was reached, but it agreed with the Company that any shares to be awarded to the plaintiff were subject to the reverse split. The jury awarded damages in an amount that was roughly equal to what the Company contended had been offered to the plaintiff before the initial public offering. Both sides then challenged the verdict, and on December 12, 2024, the judge who presided over the trial delivered a judgment in the Company’s favor, finding that no binding agreement was reached and that the plaintiff was not entitled to recover any damages. On December 23, 2024, the plaintiff filed an appeal with the Superior Court of Pennsylvania, which is currently pending. The Company intends to continue to defend against this claim.

Other than the above, we are not presently a party to any legal proceedings that, in the opinion of management, would, if decided against us, have a material adverse effect on our business. Regardless of outcome, litigation can have an adverse impact on us due to defense and settlement costs, diversion of management resources, negative publicity and reputational harm, and other factors.

Employment Agreements

The Company has employment agreements with certain key personnel providing for up to 18 months of salary continuation, up to 150% of target annual bonus amounts, and acceleration of vesting in stock-based compensation awards in certain circumstances.

Stockholder's Equity and Share-Based Compensation

6 Months Ended

Jun. 30, 2025

Stockholder's Equity and Share-Based Compensation

Stockholder's Equity and Share-Based Compensation

12. Stockholder’s Equity and Share-Based Compensation

On July 14, 2025, the Company effected the Reverse Stock Split. The Reverse Stock Split did not reduce the number of authorized shares of the common stock and did not change the par value of the common stock. In addition, proportionate adjustments were made to the number of shares of common stock available for issuance under the Company’s equity inducement and incentive plans; the number of shares underlying, and the exercise prices of outstanding equity awards under such plans. All share information in these financial statements has been adjusted for this Reverse Stock Split.

Equity Incentive Plan

The Company has three equity incentive plans: the 2018 Equity Incentive Plan, as amended, or the 2018 Plan, the 2020 Equity Incentive Plan, or the Incentive Plan, and the 2021 Equity Inducement Plan, or the Inducement Plan. New awards can only be granted under the Incentive Plan and the Inducement Plan.

The total number of shares authorized under the Incentive Plan as of June 30, 2025 was 947,598. Additionally, 182,340 shares previously issued under the 2018 Plan which were forfeited are available for issuance under the Incentive Plan. As of June 30, 2025, 366,007 shares were available for future grants under the Incentive Plan. The number of shares of the Company’s common stock that may be issued pursuant to rights granted under the Incentive Plan shall automatically increase on January 1st of each year, commencing on January 1, 2021 and continuing for ten years, in an amount equal to five percent of the total number of shares of the Company’s common stock outstanding on December 31st of the preceding calendar year, subject to the discretion of the board of directors to determine a lesser number of shares shall be added for such year. As a result, the number of shares reserved for issuance under the Incentive Plan increased by 155,155 and 137,361 shares in January 2025 and 2024, respectively.

The Incentive Plan provides for the granting of common stock, incentive stock options, nonqualified stock options, restricted stock awards, and/or stock appreciation rights to employees, directors, and other persons, as determined by the Company’s board of directors. The Company’s stock options awarded to date under the Incentive Plan vest based on a requisite service period, generally over four-year periods, and have a term of ten years.

The Inducement Plan was approved by the Company’s board of directors in July 2021. The total number of shares authorized under the Inducement Plan as of June 30, 2025 was 125,000. Of this amount, 87,166 shares were available for future grants as of June 30, 2025. The Inducement Plan provides for the granting of nonqualified stock options and restricted stock awards to employees hired by the Company, as determined by the Company’s board of directors. The Company’s stock options awarded to date under the Inducement Plan vest based on requisite service period and have a term of ten years. The Company’s restricted stock units awarded to date under the Inducement Plan vest based on requisite service period and have a term based on each award agreement.

The Company measures share-based awards at their grant-date fair value and records compensation expense on a straight-line basis over the vesting period of the awards. The Company recorded share-based compensation expense in the following expense categories in its accompanying statements of operations and comprehensive loss for the period presented:


	Three Months Ended June 30,				Six Months Ended June 30,
(in thousands)	2025		2024		2025		2024
Research and development	$	248	$	725	$	492	$	1,543
General and administrative		675		853		1,289		1,630
	$	923	$	1,578	$	1,781	$	3,173

The following table summarizes stock option activity for the six months ended June 30, 2025:


				Weighted
		Weighted		average
		average		remaining
	Number of	exercise price		contractual
	shares	per share		term (years)
Outstanding at January 1, 2025	577,581	$	77.56	7.5
Granted	247,083		7.78
Exercised	—		—
Forfeited	(98,074)		51.09
Expired	—		—
Outstanding at June 30, 2025	726,590	$	57.40	7.8
Vested and exercisable at June 30, 2025	363,689	$	99.39	6.4
Vested or expected to vest at June 30, 2025	726,590	$	57.40	7.8

The weighted average grant date fair value of options granted was $6.02 and $21.60 for the six months ended June 30, 2025 and 2024, respectively. As of June 30, 2025, the total unrecognized compensation expense related to unvested stock option awards was $4.1 million, which the Company expects to recognize over a weighted average period of 2.3 years.

The aggregate intrinsic value of options exercised, options outstanding, and options exercisable were each de minimis during the three and six months ended June 30, 2025 and 2024.

The fair value of each option was estimated on the date of grant using the weighted average assumptions in the table below:


	Six Months Ended June 30,
	2025		2024
Expected volatility	93.7	%	88.4	%
Risk‑free interest rate	4.1	%	4.3	%
Expected term	5.9	years	6.0	years
Expected dividend yield	—		—

Restricted Stock Units

The Company issues RSUs to employees that vest over periods of time as determined by the board of directors. Any unvested shares are forfeited upon termination of services. The fair value price of the RSUs is equal to the fair market value of the Company’s common stock on the date of grant. Compensation expense is recognized on a straight-line basis over the vesting period of the RSUs.

The following table summarizes activity related to RSU awards during the six months ended June 30, 2025:


			Weighted average
	Number of shares		grant date fair value
Unvested balance at January 1, 2025	7,093	$	44.80
Granted	60,000		11.70
Vested	(14,325)		27.56
Forfeited	(268)		90.40
Unvested balance at June 30, 2025	52,500	$	11.44

As of June 30, 2025, the total unrecognized expense related to all RSUs was $0.5 million, which the Company expects to recognize over a weighted-average period of 1.5 years.

Employee Stock Purchase Plan

The Company’s 2020 Employee Stock Purchase Plan, or the ESPP, became effective on February 28, 2020. The ESPP authorizes the issuance of up to 99,088 shares of the Company’s common stock. Of this amount, 60,703 were available for future grants as of June 30, 2025. The number of shares of the Company’s common stock that may be issued pursuant to rights granted under the ESPP shall automatically increase on January 1st of each year and continuing for ten years, in an amount equal to one percent of the total number of shares of the Company’s common stock outstanding on December 31st of the preceding calendar year, subject to the discretion of the board of directors to determine a lesser number of shares shall be added for such year. As a result, on January 1, 2025, subject to the discretion of the board of directors, the shares authorized for issuance under the ESPP was not increased.

Under the ESPP, eligible employees can purchase the Company’s common stock through accumulated payroll deductions at such times as are established by the board of director’s Compensation Committee. Eligible employees may purchase the Company’s common stock at 85% of the lower of the fair market value of the Company’s common stock on the first day of the offering period or on the last day of the offering period. The offering periods under the ESPP have a duration of six months, with periods ending in May and November of each calendar year. Eligible employees may contribute up to 15% of their eligible compensation. Under the ESPP, a participant may not accrue rights to purchase more than $25,000 worth of the Company’s common stock for each calendar year in which such right is outstanding or purchase more than 200 shares of the Company’s common stock in any single offering period.

In accordance with the guidance in ASC Topic 718-50, Compensation – Stock Compensation, the ability to purchase shares of the Company’s common stock at 85% of the lower of the price on the first day of the offering period or the last day of the offering period (i.e. the purchase date) represents an option and, therefore, the ESPP is a compensatory plan under this guidance. Accordingly, share-based compensation expense is determined based on the option’s grant-date fair value as estimated by applying the Black-Scholes option-pricing model and is recognized over the withholding period. No share-based compensation expense related to the ESPP was recorded during the three and six months ended June 30, 2025 and 2024.

Segment Reporting

6 Months Ended

Jun. 30, 2025

Segment Reporting

Segment Reporting

13. Segment Reporting

Operating segments are defined as components of an enterprise which engages in business activities from which it may recognize revenues and incur expenses about which separate discrete information is available for evaluation by the chief operating decision maker, or CODM, in deciding how to allocate resources and in assessing performance. The Company operates in a single reportable segment, developing and advancing genetic medicines designed to target critical underlying pathology of neurodegenerative diseases.

The accounting policies of the single segment are the same as those described in the summary of significant accounting policies in the Company’s 2024 Annual Report filed on Form 10-K. The Company’s CODM is its chief executive officer.

The measure of segment assets is reported on the balance sheet as total assets. All assets are located within the United States.

The CODM uses net loss as reported on the Company’s statement of operations to assess the Company’s performance. The CODM also uses cash forecast in deciding where to invest or expand operations within the business. In these cash forecasts, research and development expenses and general and administrative expenses exclude certain non-cash items such as share based compensation and depreciation and amortization expenses.

The following table summarizes significant segment expenses:


	Three Months Ended June 30,				Six Months Ended June 30,
(in thousands)	2025		2024		2025		2024
Research and development
Wages, benefits and other payroll	$	1,302	$	2,944	$	4,891	$	6,355
Third-party costs		4,154		6,067		7,948		12,679
Share-based compensation		248		725		492		1,543
Depreciation and amortization		110		694		220		1,388
Total research and development expenses		5,814		10,430		13,551		21,965
General and administrative
Wages, benefits and other payroll		1,798		2,240		4,588		4,540
Third-party costs		1,974		3,314		4,577		6,640
Share-based compensation		675		853		1,289		1,630
Depreciation and amortization		73		103		151		215
Total general and administrative expenses		4,520		6,510		10,605		13,025
Impairment of long-lived assets		—		438		2,637		438
Loss from operations		10,334		17,378		26,793		35,428
Other (income) expense, net		(949)		(1,387)		(2,003)		(2,726)
Net loss	$	9,385	$	15,991	$	24,790	$	32,702

- Definition

The entire disclosure for reporting segments including data and tables. Reportable segments include those that meet any of the following quantitative thresholds a) it's reported revenue, including sales to external customers and intersegment sales or transfers is 10 percent or more of the combined revenue, internal and external, of all operating segments b) the absolute amount of its reported profit or loss is 10 percent or more of the greater, in absolute amount of 1) the combined reported profit of all operating segments that did not report a loss or 2) the combined reported loss of all operating segments that did report a loss c) its assets are 10 percent or more of the combined assets of all operating segments.

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+ Details

Subsequent Events

6 Months Ended

Jun. 30, 2025

Subsequent Events.

Subsequent Events

14. Subsequent Events

On July 14, 2025, as authorized by the Company’s stockholders on May 28, 2025, the Company effected a 1-for-20 reverse stock split of its common stock. Please refer to Note 12 for further information.

Pay vs Performance Disclosure - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
Pay vs Performance Disclosure - USD ($) $ in Thousands	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Pay vs Performance Disclosure
Net Income (Loss)	$ (9,385)	$ (15,991)	$ (24,790)	$ (32,702)

- Definition

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

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+ Details

Insider Trading Arrangements	3 Months Ended
Insider Trading Arrangements	Jun. 30, 2025
Trading Arrangements, by Individual
Rule 10b5-1 Arrangement Adopted	false
Non-Rule 10b5-1 Arrangement Adopted	false
Rule 10b5-1 Arrangement Terminated	false
Non-Rule 10b5-1 Arrangement Terminated	false

Summary of Significant Accounting Policies (Policies)

6 Months Ended

Jun. 30, 2025

Summary of Significant Accounting Policies

Basis of Presentation

The accompanying unaudited financial statements have been prepared in accordance with generally accepted accounting principles in the United States, or GAAP. Any reference in these notes to applicable guidance is meant to refer to GAAP as found in the Accounting Standards Codification, or ASC, and Accounting Standards Updates promulgated by the Financial Accounting Standards Board, or FASB.

On May 28, 2025, the Company’s stockholders provided authorization for the Company’s Board of Directors to effect a reverse stock split to regain compliance with Nasdaq’s listing requirements. On July 14, 2025, the Company effected a 1-for-20 reverse stock split of its common stock, or the Reverse Stock Split. No fractional shares were issued in connection with the Reverse Stock Split. Stockholders who were otherwise entitled to receive fractional shares received the number of shares of Common Stock as rounded up to the nearest whole share. All share and per share amounts in these financial statements and notes thereto, including the stock options, restricted stock units, and employee stock purchase plan activity, have been adjusted retroactively to reflect the Reverse Stock Split for all periods presented.

Interim Financial Statements

The accompanying unaudited interim financial statements have been prepared from the books and records of the Company in accordance with GAAP for interim financial information and Rule 10-01 of Regulation S-X promulgated by the SEC, which permits reduced disclosures for interim periods. All adjustments, consisting only of normal recurring adjustments, necessary for a fair presentation of the accompanying balance sheets, statements of operations and comprehensive loss, stockholders’ equity, and cash flows have been made. Although these interim financial statements do not include all of the information and footnotes required for complete annual financial statements, management believes the disclosures are adequate to make the information presented not misleading. Unaudited interim results of operations and cash flows are not necessarily indicative of the results that may be expected for the full year. Unaudited interim financial statements and footnotes should be read in conjunction with the audited financial statements and footnotes included in the Company’s 2024 Annual Report filed on Form 10-K.

Use of Estimates

The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and contingent liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.

Estimates and assumptions are periodically reviewed, and the effects of the revisions are reflected in the accompanying financial statements in the period they are determined to be necessary.

Fair Value of Financial Instruments

Management believes that the carrying amounts of the Company’s financial instruments, including cash equivalents, prepaid expenses, and accounts payable, approximate fair value due to the short-term nature of those instruments.

Concentration of Credit Risk

Financial instruments that potentially subject the Company to significant concentrations of credit risk consist primarily of cash and cash equivalents. The Company maintains a deposit account in a federally insured financial institution in excess of federally insured limits. The Company also maintains a portfolio of money market funds, which is diversified

to limit exposure related to counterparty risk, industry risk, and security type risk. The Company maintains an investment policy which dictates the allocation of funds within its portfolio of money market funds. The Company has not experienced any losses in such accounts and believes it is not exposed to significant risk on its cash and cash equivalents beyond the normal credit risk associated with commercial banking relationships and money market funds.

Cash and Cash Equivalents

The Company considers all highly-liquid investments that have maturities of three months or less when acquired to be cash equivalents. Cash equivalents as of June 30, 2025 consisted of money market funds as described in Note 4. Cash consists of cash deposits at banking institutions.

Marketable Securities

The Company classifies its marketable securities with original maturities of greater than three months as available-for-sale. These securities are carried at fair market value, with unrealized gains and losses reported in comprehensive loss and accumulated other comprehensive income (loss) within stockholders’ equity. Any premium or discount arising at purchase of debt securities is amortized and/or accreted over the term of the security to other income (expense), net. Gains or losses on marketable securities sold are recognized as a component of other income (expense), net in the statement of operations and comprehensive loss on the specific identification method. All marketable securities are available for use, as needed, to fund operations and therefore, the Company classifies all marketable securities as current assets within the balance sheet. As of June 30, 2025, the Company had no marketable securities.

Property and Equipment, Net

Property and equipment, net consists of laboratory equipment, office equipment, computer hardware and software, furniture and fixtures, and leasehold improvements and is recorded at cost. Maintenance and repairs that do not improve or extend the lives of the respective assets are expensed as incurred. Property and equipment are depreciated on a straight-line basis over their estimated useful lives. The Company estimates useful life on an asset-by-asset basis, which generally consists of three years for computer hardware and software, five years for office equipment, five years for laboratory equipment, and seven years for furniture and fixtures. Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the asset.

When property and equipment are retired or otherwise disposed of, the costs and accumulated depreciation and amortization are removed from the respective accounts, with any resulting gain or loss recognized concurrently. The Company recognized de minimis losses on disposals of property and equipment for the six months ended June 30, 2025. The Company did not recognize any losses on disposals of property and equipment for the three months ended June 30, 2025 or the three and six months ended June 30, 2024.

The Company reviews long-lived assets, such as property and equipment, for impairment when events or changes in circumstances indicate the carrying amount of the assets may not be recoverable. The Company recognized impairment expenses for laboratory equipment and certain other assets of $2.6 million for the six months ended June 30, 2025. The Company did not recognize any impairment expenses for long-lived assets for the three months ended June 30, 2025. The Company recognized impairment expenses for property and equipment of $0.4 million for both the three and six months ended June 30, 2024.

The impairment expenses for the six months ended June 30, 2025, relate to the Company’s January 2025 announcement to reduce its overall workforce by 55% and cease its lab operations in Hopewell, New Jersey. As a result, the Company reassessed asset groups at its lab in Hopewell, New Jersey, and evaluated such asset groups for impairment under FASB ASC Topic 360, Long-lived assets: Impairment or disposal of long-lived assets. The Company determined the laboratory equipment was a separate asset group based on management’s implemented plans to sell the laboratory equipment and estimated the fair value of the laboratory equipment based on the estimated future cash flows from the sale of such equipment. Subsequent to recording the impairment, the Company entered into a sales agreement for $1.2 million for substantially all the laboratory equipment and certain other assets.

Leasing

The Company evaluates leases at their inception to determine if they are an operating lease or a finance lease. As of June 30, 2025, the Company has classified all leases with terms greater than one year, as operating leases.

The Company recognizes assets and liabilities for operating leases at their inception, based on the present value of all payments due under the lease agreement. The Company uses its incremental borrowing rate to determine the present value of operating leases, which is determined by referencing collateralized borrowing rates for debt instruments with terms similar to the respective lease. The Company utilizes the accounting policy election to not separate lease and non-lease components and the accounting policy election to not apply the recognition requirement to leases with a term of 12 months or less.

The Company reviews long-lived assets, such as right of use assets, or ROU assets, for impairment when events or changes indicate the carrying amount of the ROU assets may not be recoverable. The Company did not recognize any impairment expenses for ROU assets for the three and six months ended June 30, 2025 and 2024.

Research and Development

Research and development costs are expensed as incurred and consist primarily of expenses incurred with the Trustees of the University of Pennsylvania’s, or Penn’s, Gene Therapy Program, or GTP, Gemma Biotherapeutics, Inc., or Gemma, contract research organizations, contract manufacturing organizations, internal analytical and testing activities, and employee-related expenses, including salaries, benefits, and share-based compensation. Management makes estimates of the Company’s external accrued research and development expenses, which primarily relates to contract research organizations and contract manufacturing organizations, as of each balance sheet date in the Company’s financial statements based on an estimate of progress to completion of specific tasks using facts and circumstances known to the Company at that time. The Company determines the estimates by reviewing contracts, vendor agreements, change orders, and through discussions with the Company’s internal clinical personnel and external service providers as to the progress to completion of services and the agreed-upon fee to be paid for such services. If the actual timing of the performance of services or the level of effort varies from the estimate, the Company will adjust the accrual and related expenses accordingly.

Other Income (Expense), Net

Other income (expense), net consists of interest earned on cash equivalents and marketable securities, amortization of premium and discount on marketable securities, and income from subleases.

The Company recorded $1.0 million to other income (expense), net for the three months ended June 30, 2025, which consisted of $0.6 million attributable to interest income and the amortization of premium and discount on the Company’s marketable securities, and $0.4 million related to income from subleases.

The Company recorded $1.4 million to other income (expense), net for the three months ended June 30, 2024, which consisted of $1.2 million attributable to interest income and the amortization of premium and discount on the Company’s marketable securities, and $0.2 million related to income from subleases.

The Company recorded $2.0 million to other income (expense), net for the six months ended June 30, 2025, which consisted of $1.3 million attributable to interest income and the amortization of premium and discount on the Company’s marketable securities, and $0.7 million related to income from subleases.

The Company recorded $2.7 million to other income (expense), net for the six months ended June 30, 2024, which consisted of $2.4 million attributable to interest income and the amortization of premium and discount on the Company’s marketable securities, and $0.3 million related to income from subleases.

Share-Based Compensation

The Company measures share-based awards at grant-date fair value and records compensation expense on a straight-line basis over the vesting period of the awards. The Company’s share-based compensation consists of restricted stock units, or RSUs, and options to purchase common stock, or stock option awards.

The Company uses the Black-Scholes option pricing model to value its stock option awards.

Estimating the fair value of stock option awards requires the input of assumptions, including, the expected term of stock options and stock price volatility. The assumptions used in estimating the fair value of share-based awards represent management’s estimate and involve inherent uncertainties and the application of management’s judgment. As a result, if factors change and management uses different assumptions, share-based compensation expense could be materially different for future awards.

The expected term of the stock options is estimated using the “simplified method,” as the Company has limited historical information from which to develop reasonable expectations about future exercise patterns and post-vesting employment termination behavior for its stock option grants. The simplified method is the midpoint between the vesting period and the contractual term of the option. For stock price volatility, the Company uses a composite of comparable public company data as a basis for its expected volatility and considers the historic volatility of its common stock from its initial public offering to date to calculate the fair value of option grants. The selection of comparable public company data requires the application of management’s judgement.

The Company accounts for forfeitures of RSUs and stock option awards as they occur.

License and Other Revenue

The Company may enter into license agreements and transition services agreements (see Note 8) under which it may license rights to research, develop, manufacture, and commercialize its product candidates to third parties, and provide transition services for such licenses. Payments under these arrangements may include non-refundable, upfront fees, reimbursement of certain costs, payments upon the achievement of certain milestones, and royalties on product sales.

The Company applies ASC Topic 606, Revenue from Contracts with Customers, or ASC 606, when all of the following criteria are met, to determine a valid contract exists: (i) the parties have approved the contract and are committed to perform their respective obligations; (ii) the Company can identify each party’s rights regarding the goods or services to be transferred; (iii) the Company can identify the payment terms for the goods or services to be transferred; (iv) the contract has commercial substance; and (v) the Company will collect substantially all of the consideration to which it will be entitled in exchange for the goods or services that will be transferred to the customer. Once it is determined that a valid contract exists, the Company performs the following steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including consideration of the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations on a relative stand-alone selling price basis; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. As part of the accounting for these arrangements, the Company must use its judgment to determine the number of performance obligations, the transaction price, the stand-alone selling price for each performance obligation identified in the contract for the allocation of transaction price, the contract term and pattern of satisfaction of the performance obligations. The Company uses judgment to determine whether milestones or other variable consideration, except for certain sales-based milestone payments and royalties, should be included in the transaction price as described further below.

At the inception of each arrangement that includes milestone payments, the Company evaluates whether the milestones are considered probable of being achieved and estimates the amount to be included in the transaction price using the most likely amount method set forth in ASC 606. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the control of the Company or the licensee, such as those subject to regulatory approvals, are not considered probable of being achieved

until those approvals are received. The Company evaluates factors such as the scientific, clinical, regulatory, commercial and other risks that must be overcome to achieve the particular milestone in making this assessment. There is considerable judgment involved in determining whether it is probable that a significant revenue reversal would not occur. At the end of each subsequent reporting period, the Company reevaluates the probability of achievement of all milestones subject to constraint and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis in the statements of operations and comprehensive loss in the period of adjustment.

For customer contracts in the scope of ASC 606, amounts due to the Company are recorded as accounts receivable on the Company’s balance sheet when the Company’s right to consideration is unconditional. Amounts received prior to satisfying the related performance obligations are classified on the Company’s balance sheet as current deferred revenue if expected to be recognized as revenue within 12 months following the balance sheet date and as deferred revenue, net of current portion, if amounts are not expected to be recognized as revenue within the 12 months following the balance sheet date. The Company does not evaluate a contract for a significant financing component if payment is expected within one year or less from the transfer of promised items to the customer.

Net Loss Per Share

Basic net loss per share of common stock is computed by dividing net loss by the weighted average number of shares of common stock outstanding during each period. Diluted loss per share of common stock includes the effect, if any, from the potential exercise or conversion of securities, such as stock options, which would result in the issuance of incremental shares of common stock. For diluted net loss per share, the weighted-average number of shares of common stock is the same for basic net loss per share due to the fact that when a net loss exists, dilutive securities are not included in the calculation as the impact is anti-dilutive.

The following potentially dilutive securities have been excluded from the computation of diluted weighted-average shares of common stock outstanding, as they would be anti-dilutive:


	Six Months Ended June 30,
	2025	2024
Stock options	726,590	622,283
Unvested restricted stock units	52,500	17,635
Employee stock purchase plan	1,357	1,839
	780,447	641,757

Recently Issued Accounting Pronouncements

In December 2023, the FASB issued ASU No. 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures, or ASU 2023-09, which requires that an entity, on an annual basis, disclose additional income tax information, primarily related to the rate reconciliation and income taxes paid. The amendments in ASU 2023-09 are intended to enhance the transparency and decision usefulness of income tax disclosures. The amendments in this ASU are effective for annual periods beginning after December 15, 2024 with early adoption permitted. The Company is currently evaluating the impact of this guidance on its disclosures.

In November 2024, the FASB issued ASU No. 2024-03, Income Statement – Reporting Comprehensive Income – Expense Disaggregation Disclosures (Subtopic 220-40): Disaggregation of Income Statement Expenses, or ASU 2024-03, which requires entities to provide disclosures to disaggregate operating expenses into specific categories, such as salaries and wages, depreciation, and amortization, to provide enhanced transparency into the nature and function of expenses. ASU 2024-03 is effective for the Company’s first fiscal year beginning after December 15, 2026, and for interim periods within the Company’s first fiscal year beginning after December 15, 2027, with early adoption permitted. ASU 2024-03 may be applied retrospectively or prospectively. The Company is currently evaluating the impact of this guidance on its disclosures.

Summary of Significant Accounting Policies (Tables)

6 Months Ended

Jun. 30, 2025

Summary of Significant Accounting Policies

Schedule of antidilutive securities excluded from computation of earnings per share


	Six Months Ended June 30,
	2025	2024
Stock options	726,590	622,283
Unvested restricted stock units	52,500	17,635
Employee stock purchase plan	1,357	1,839
	780,447	641,757

Cash, Cash Equivalents and Marketable Securities (Tables)

6 Months Ended

Jun. 30, 2025

Cash, Cash Equivalents and Marketable Securities

Schedule of details regarding the Company's portfolio of cash and cash equivalents


	Cost or
(in thousands)	Amortized cost		Unrealized gains		Unrealized losses		Fair value
June 30, 2025:
Cash accounts in banking institutions	$	2,500	$	—	$	—	$	2,500
Money market funds		55,126		—		—		55,126
Total	$	57,626	$	—	$	—	$	57,626

December 31, 2024:
Cash accounts in banking institutions	$	3,527	$	—	$	—	$	3,527
Money market funds		29,058		—		—		29,058
Commercial paper		4,988		—		—		4,988
Total	$	37,573	$	—	$	—	$	37,573

Fair Value of Financial Instruments and Non-Financial Instruments (Tables)

6 Months Ended

Jun. 30, 2025

Fair Value of Financial Instruments and Non-Financial Instruments

Summary of assets measured at fair value on a recurring basis


	Fair value measurement at
	reporting date using
	Quoted prices
	in active		Significant
		markets for	other		Significant
	identical		observable			unobservable
	assets		inputs		inputs
(in thousands)	(Level 1)		(Level 2)		(Level 3)
June 30, 2025:
Assets
Cash equivalents:
Money market funds	$	55,126	$	—	$	—
Total cash equivalents		55,126		—		—
Total financial assets	$	55,126	$	—	$	—

December 31, 2024:
Assets
Cash equivalents:
Money market funds	$	29,058	$	—	$	—
Commercial paper		—		4,988		—
Total cash equivalents		29,058		4,988		—

Marketable securities:
Certificates of deposit		—		5,971		—
Commercial paper		—		25,439		—
Corporate debt securities		—		1,865		—
U.S. government securities		—		5,908		—
Total marketable securities		—		39,183		—
Total financial assets	$	29,058	$	44,171	$	—

Property and Equipment, Net (Tables)

6 Months Ended

Jun. 30, 2025

Property and Equipment, Net

Schedule of property and equipment, net


(in thousands)		June 30, 2025	December 31, 2024
Laboratory equipment	$	—	$	10,020
Office equipment		107		119
Computer hardware and software		988		1,111
Furniture and fixtures		419		419
Leasehold improvements		7,386		7,386
Total property and equipment		8,900		19,055
Accumulated depreciation and amortization		(3,560)		(9,724)
	$	5,340	$	9,331

Accrued Expenses and Other Current Liabilities (Tables)

6 Months Ended

Jun. 30, 2025

Accrued Expenses and Other Current Liabilities

Schedule of accrued liabilities and other current liabilities


(in thousands)	June 30, 2025		December 31, 2024
Professional fees	$	426	$	406
Compensation and related benefits		2,494		4,405
Research and development		1,153		1,896
	$	4,073	$	6,707

Leases (Tables)

6 Months Ended

Jun. 30, 2025

Leases

Schedule of future minimum rental payments for operating leases


(in thousands)
2025	$	1,911
2026		3,757
2027		3,863
2028		3,973
2029		4,085
Thereafter		21,621
Total undiscounted lease payments		39,210
Less: imputed interest		(14,479)
Total lease liabilities	$	24,731

Summary of operating leases


	Three Months Ended				Six Months Ended
($ in thousands)	June 30, 2025		June 30, 2024		June 30, 2025		June 30, 2024
Operating lease cost	$	877	$	909	$	1,757	$	1,740
Variable lease cost		543		394		1,081		1,114
	$	1,420	$	1,303	$	2,838	$	2,854


	Six Months Ended
($ in thousands)	June 30, 2025		June 30, 2024
Weighted-average discount rate		9.7%		9.7%
Weighted-average remaining lease term (years)		9.8		10.7


	Three Months Ended				Six Months Ended
($ in thousands)	June 30, 2025		June 30, 2024		June 30, 2025		June 30, 2024
Sublease rental income	$	363	$	243	$	730	$	300

Stockholder's Equity and Share-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2025

Stockholder's Equity and Share-Based Compensation

Schedule of share-based compensation expense category


	Three Months Ended June 30,				Six Months Ended June 30,
(in thousands)	2025		2024		2025		2024
Research and development	$	248	$	725	$	492	$	1,543
General and administrative		675		853		1,289		1,630
	$	923	$	1,578	$	1,781	$	3,173

Schedule of stock option activity plan


				Weighted
		Weighted		average
		average		remaining
	Number of	exercise price		contractual
	shares	per share		term (years)
Outstanding at January 1, 2025	577,581	$	77.56	7.5
Granted	247,083		7.78
Exercised	—		—
Forfeited	(98,074)		51.09
Expired	—		—
Outstanding at June 30, 2025	726,590	$	57.40	7.8
Vested and exercisable at June 30, 2025	363,689	$	99.39	6.4
Vested or expected to vest at June 30, 2025	726,590	$	57.40	7.8

Schedule of weighted average assumptions applied to options


	Six Months Ended June 30,
	2025		2024
Expected volatility	93.7	%	88.4	%
Risk‑free interest rate	4.1	%	4.3	%
Expected term	5.9	years	6.0	years
Expected dividend yield	—		—

Schedule of activity related to restricted stock unit


			Weighted average
	Number of shares		grant date fair value
Unvested balance at January 1, 2025	7,093	$	44.80
Granted	60,000		11.70
Vested	(14,325)		27.56
Forfeited	(268)		90.40
Unvested balance at June 30, 2025	52,500	$	11.44

Segment Reporting (Tables)

6 Months Ended

Jun. 30, 2025

Segment Reporting

Schedule of reconciliation of reported segment net loss to the statement of operations


	Three Months Ended June 30,				Six Months Ended June 30,
(in thousands)	2025		2024		2025		2024
Research and development
Wages, benefits and other payroll	$	1,302	$	2,944	$	4,891	$	6,355
Third-party costs		4,154		6,067		7,948		12,679
Share-based compensation		248		725		492		1,543
Depreciation and amortization		110		694		220		1,388
Total research and development expenses		5,814		10,430		13,551		21,965
General and administrative
Wages, benefits and other payroll		1,798		2,240		4,588		4,540
Third-party costs		1,974		3,314		4,577		6,640
Share-based compensation		675		853		1,289		1,630
Depreciation and amortization		73		103		151		215
Total general and administrative expenses		4,520		6,510		10,605		13,025
Impairment of long-lived assets		—		438		2,637		438
Loss from operations		10,334		17,378		26,793		35,428
Other (income) expense, net		(949)		(1,387)		(2,003)		(2,726)
Net loss	$	9,385	$	15,991	$	24,790	$	32,702

Risks and Liquidity (Details) $ in Thousands		3 Months Ended	6 Months Ended
Risks and Liquidity (Details) $ in Thousands	Mar. 05, 2021 USD ($)	Mar. 31, 2024 USD ($) shares	Jun. 30, 2025 USD ($)	Jun. 30, 2024 USD ($) shares	Dec. 31, 2024 USD ($)
Risks And Liquidity [Line Items]
Accumulated deficit			$ (684,025)		$ (659,235)
Sale of common stock, net of issuance costs				$ 8,742
Stock split			0.05
ATM Facility
Risks And Liquidity [Line Items]
Shares issued \| shares		6,000,000		300,000
Proceeds from issuance of common stock, net of offering costs		$ 8,700
Issuance cost		$ 300
Remaining capacity available to be sold			$ 15,800
ATM Facility \| Maximum
Risks And Liquidity [Line Items]
Sale of common stock, net of issuance costs	$ 125,000

Summary of Significant Accounting Policies (Details) $ in Thousands			1 Months Ended	3 Months Ended		6 Months Ended
	Jul. 14, 2025	Jun. 26, 2025 shares	Jan. 31, 2025	Jun. 30, 2025 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2025 USD ($) shares	Jun. 30, 2024 USD ($) shares	Dec. 31, 2024 USD ($)
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Other income (expense), net				$ 949	$ 1,387	$ 2,003	$ 2,726
Amortization of premium and discount				600	1,200	1,300	2,400
Sublease Income				363	243	$ 730	$ 300
Amount of antidilutive securities excluded from computation of earnings per share \| shares						780,447	641,757
Impairment of property and equipment					$ 438	$ 2,637	$ 438
Stock split						0.05
Stock issued during period, Reverse stock splits \| shares		0
Marketable securities				$ 0		$ 0		$ 39,183
Disposal group classified as held for sale or to be disposed of by sale \| Laboratory equipment
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Sales agreement						$ 1,200
Employee Severance
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Workforce reduction			55.00%			55.00%
Minimum [Member] \| Subsequent event
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Stock split	0.05
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Amount of antidilutive securities excluded from computation of earnings per share \| shares						726,590	622,283
Unvested restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Amount of antidilutive securities excluded from computation of earnings per share \| shares						52,500	17,635
Employee stock purchase plan
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Amount of antidilutive securities excluded from computation of earnings per share \| shares						1,357	1,839
Computer hardware
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Useful life				3 years		3 years
Office equipment
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Useful life				5 years		5 years
Laboratory equipment
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Useful life				5 years		5 years
Furniture and fixtures
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Useful life				7 years		7 years

Cash, Cash Equivalents and Marketable Securities - Portfolio of Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Dec. 31, 2024
Cash and cash equivalents
Cost or Amortized cost	$ 57,626	$ 37,573
Fair value	57,626	37,573
Cash accounts in banking institutions
Cash and cash equivalents
Cost or Amortized cost	2,500	3,527
Fair value	2,500	3,527
Money market funds
Cash and cash equivalents
Cost or Amortized cost	55,126	29,058
Fair value	$ 55,126	29,058
Commercial paper
Cash and cash equivalents
Cost or Amortized cost		4,988
Fair value		$ 4,988

Fair Value of Financial Instruments and Non-Financial Instruments (Recurring basis) (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Dec. 31, 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 57,626	$ 37,573
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	55,126	29,058
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		4,988
Level 1 \| Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	55,126	29,058
Total financial assets	55,126	29,058
Level 1 \| Recurring \| Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 55,126	29,058
Level 2 \| Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		4,988
Marketable securities		39,183
Total financial assets		44,171
Level 2 \| Recurring \| Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities		5,971
Level 2 \| Recurring \| Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		4,988
Marketable securities		25,439
Level 2 \| Recurring \| Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities		1,865
Level 2 \| Recurring \| U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities		$ 5,908

Fair Value of Financial Instruments and Non-Financial Instruments (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended		18 Months Ended
	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of long-lived assets	$ 438	$ 2,637	$ 438
Level 3 \| Nonrecurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of long-lived assets		$ 2,600		$ 0

Property and Equipment, Net (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024	Dec. 31, 2024
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 8,900		$ 8,900		$ 19,055
Accumulated depreciation and amortization	(3,560)		(3,560)		(9,724)
Total, property plant and equipment net	5,340		5,340		9,331
Depreciation and amortization expense	200	$ 800	370	$ 1,603
Laboratory equipment
Property, Plant and Equipment [Line Items]
Total property and equipment					10,020
Office equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	107		107		119
Computer hardware and software.
Property, Plant and Equipment [Line Items]
Total property and equipment	988		988		1,111
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Total property and equipment	419		419		419
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 7,386		$ 7,386		$ 7,386

Property and Equipment, Net - Narrative (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended	6 Months Ended
	Jan. 31, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Property, Plant and Equipment [Line Items]
Impairment of long-lived assets		$ 438	$ 2,637	$ 438
Laboratory equipment \| Disposal group classified as held for sale or to be disposed of by sale
Property, Plant and Equipment [Line Items]
Sales agreement			1,200
Depreciation on equipment			$ 0
Employee Severance
Property, Plant and Equipment [Line Items]
Workforce reduction	55.00%		55.00%

Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Dec. 31, 2024
Accrued Expenses and Other Current Liabilities
Professional fees	$ 426	$ 406
Compensation and related benefits	2,494	4,405
Research and development	1,153	1,896
Accrued expenses and other current liabilities	$ 4,073	$ 6,707

Gemma License Agreement (Details) - USD ($) $ in Thousands		6 Months Ended
Gemma License Agreement (Details) - USD ($) $ in Thousands	Jul. 31, 2024	Jun. 30, 2025	Dec. 31, 2024
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Upfront license fee receivable	$ 15,000
Non-refundable sublicense payments received		$ 9,741	$ 8,226
Gemma License Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Upfront license fee receivable		15,000
Payment received	5,000	5,000
Maximum additional amount in development and commercial milestone payments receivable	114,000
Amount of transition services payments		4,700
Non-refundable sublicense payments received		$ 9,700
Contingent proceeds upon completing certain business milestones	5,000
Amount of license fee due and receivables	$ 5,000

Severance (Details) - Workforce reduction - USD ($) $ in Millions	1 Months Ended	6 Months Ended
	Jan. 31, 2025	Jun. 30, 2025	Jun. 30, 2024
Severance
Workforce reduction	55.00%	55.00%
Unpaid severance		$ 0.0
General and administrative expense
Severance
Severance and termination-related expenses		0.4
Research and development
Severance
Severance and termination-related expenses		$ 1.3	$ 0.0

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References + Details

Leases (Details) $ in Thousands					1 Months Ended	3 Months Ended		6 Months Ended
Leases (Details) $ in Thousands	Sep. 04, 2024 USD ($) ft²	Feb. 20, 2024 USD ($) ft²	Sep. 29, 2023 USD ($) ft²	Aug. 07, 2023 USD ($) ft²	Jan. 31, 2025	Jun. 30, 2025 USD ($) ft² item	Jun. 30, 2024 USD ($)	Jun. 30, 2025 USD ($) ft² item	Jun. 30, 2024 USD ($)
Lessee, Lease, Description [Line Items]
Sublease Income						$ 363	$ 243	$ 730	$ 300
Future minimum lease payments
2025						1,911		1,911
2026						3,757		3,757
2027						3,863		3,863
2028						3,973		3,973
2029						4,085		4,085
Thereafter						21,621		21,621
Total undiscounted lease payments						39,210		39,210
Less: imputed interest						(14,479)		(14,479)
Total lease liabilities						$ 24,731		$ 24,731
Employee Severance
Lessee, Lease, Description [Line Items]
Number of positions eliminated, period percent					55.00%			55.00%
Operating leases for office space
Lessee, Lease, Description [Line Items]
Area leased \| ft²						37,000		37,000
Option to extend lease								true
Number of extension terms \| item						2		2
Renewal term of lease						5 years		5 years
Option to terminate lease								true
Notice Period for termination lease								15 months
Sublease Agreement A
Lessee, Lease, Description [Line Items]
Area leased \| ft²				8,000
Option to extend lease				true
Sublease Income				$ 100
Annual increase in sublease rent (as a percent)				2.75%
Sublease Agreement B
Lessee, Lease, Description [Line Items]
Area leased \| ft²			29,000
Option to extend lease			true
Sublease Income			$ 900
1835 market street sublease agreement
Lessee, Lease, Description [Line Items]
Area leased \| ft²		16,000
Option to extend lease		true
Sublease Income		$ 300
Term of Contract		18 months
Sublease Agreement C
Lessee, Lease, Description [Line Items]
Area Subleased \| ft²	3,200
Subleased area of land	5.00%
Area leased \| ft²	62,000
Sublease Income	$ 100
Annual increase in sublease rent (as a percent)	2.50%
Maximum \| Operating leases for laboratory space
Lessee, Lease, Description [Line Items]
Number of extension terms \| item						2		2
Extended term of new lease agreement								5 years

Leases - Operating lease cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Leases
Operating lease cost	$ 877	$ 909	$ 1,757	$ 1,740
Variable lease cost	543	394	1,081	1,114
Lease cost	$ 1,420	$ 1,303	$ 2,838	$ 2,854
Weighted-average discount rate	9.70%	9.70%	9.70%	9.70%
Weighted-average remaining lease term (years)	9 years 9 months 18 days	10 years 8 months 12 days	9 years 9 months 18 days	10 years 8 months 12 days
Operating lease payments			$ 2,000	$ 1,800

Leases - Sublease income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
Leases - Sublease income (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Leases
Sublease rental income	$ 363	$ 243	$ 730	$ 300

Commitments and Contingencies - Collaboration Agreements (Details) - USD ($) $ in Millions	1 Months Ended	6 Months Ended
	Feb. 29, 2020	Jun. 30, 2025
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Non - Refundable aggregate fees payable		$ 1.0
Non - Refundable upfront fees payable		0.5
Non - Refundable fees payable		0.5
Amount of loss contingency		$ 0.0
Common stock	150,000
Loss contingency, shares agreed by defendant	33,836	1,692
Target annual bonus		150.00%
Amended and restated agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Amount payable per product candidate for rare, monogenic disorders		$ 16.5
Sales milestone payments		55.0
Gemma License Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Amount payable per product candidate for rare, monogenic disorders		16.5
Amount payable per product candidate upon achievement of specific development milestone		39.0
Sales milestone payments		$ 55.0

Stockholder's Equity and Share-Based Compensation - Equity Incentive Plan & ESPP (Details)		1 Months Ended		3 Months Ended		6 Months Ended
	Feb. 28, 2020 USD ($) shares	Jan. 31, 2025 shares	Jan. 31, 2024 shares	Jun. 30, 2025 USD ($) shares	Jun. 30, 2024 USD ($)	Jun. 30, 2025 USD ($) plan shares	Jun. 30, 2024 USD ($)
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of equity incentive plans \| plan						3
Share-based compensation expense \| $				$ 923,000	$ 1,578,000	$ 1,781,000	$ 3,173,000
Equity Incentive Plan
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Shares authorized				947,598		947,598
Shares available for future issuance				182,340		182,340
Shares available for future grant				366,007		366,007
Period till which there is automatic increase in rights granted under the Plan						10 years
Percent of the total number of shares of the Company's common stock outstanding						5.00%
Term of award						10 years
Vesting period						4 years
Additional number of common shares reserved for future issuance		155,155	137,361
Equity inducement plan 2021
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Shares authorized				125,000		125,000
Shares available for future issuance				87,166		87,166
Term of award						10 years
Employee stock purchase plan
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Shares authorized	99,088
Shares available for future grant				60,703		60,703
Period till which there is automatic increase in rights granted under ESPP	10 years
Percentage of common stock outstanding	1.00%
Percentage of stock purchase price	85.00%
Percentage of maximum employee contribution	15.00%
Maximum amount of common stock that can be purchased \| $	$ 25,000
Share-based compensation expense \| $				$ 0	$ 0	$ 0	$ 0

Stockholder's Equity and Share-Based Compensation - Share-based compensation expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Share-based compensation expense	$ 923	$ 1,578	$ 1,781	$ 3,173
Research and development
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Share-based compensation expense	248	725	492	1,543
General and administrative
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Share-based compensation expense	$ 675	$ 853	$ 1,289	$ 1,630

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (f)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480429/718-10-50-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(v)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480429/718-10-50-2

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(i)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480429/718-10-50-2

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(iii)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480429/718-10-50-2

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+ Details

Stockholder's Equity and Share-Based Compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Millions	6 Months Ended		12 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Dec. 31, 2024
Number of shares
Outstanding, Beginning of period	577,581
Granted	247,083
Forfeited	(98,074)
Outstanding, Ending of period	726,590		577,581
Vested and exercisable at June 30, 2025	363,689
Vested or expected to vest at June 30, 2025	726,590
Weighted average exercise price per share
Outstanding, Beginning of period	$ 77.56
Granted	7.78
Forfeited	51.09
Outstanding, Ending of period	57.4		$ 77.56
Vested and exercisable at June 30, 2025	99.39
Vested or expected to vest at June 30, 2025	$ 57.4
Weighted average remaining contractual term (years)
Outstanding	7 years 9 months 18 days		7 years 6 months
Vested and exercisable at June 30, 2025	6 years 4 months 24 days
Vested or expected to vest at June 30, 2025	7 years 9 months 18 days
Weighted average grant date fair value of options granted	$ 6.02	$ 21.6
Unrecognized compensation expense related to unvested stock option awards	$ 4.1
Weighted average period for recognition	2 years 3 months 18 days

Stockholder's Equity and Share-Based Compensation - Assumptions Used in Determining Fair Value & Early exercise of Stock Options (Details) - Employee Stock Option	6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024
Share-Based Compensation Arrangement by Share-Based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Expected volatility	93.70%	88.40%
Risk-free interest rate	4.10%	4.30%
Expected term	5 years 10 months 24 days	6 years

Stockholder's Equity and Share-Based Compensation - Restricted Stock Units (Details) - USD ($) $ / shares in Units, $ in Millions	6 Months Ended
	Jun. 30, 2025	Dec. 31, 2024
Weighted average grant date fair value
Weighted average period for recognition	2 years 3 months 18 days
Unvested restricted stock units
Number of shares
Unvested balance, Beginning	7,093
Granted	60,000
Vested	(14,325)
Forfeited	(268)
Unvested balance, Ending	52,500
Weighted average grant date fair value
Unvested balance, Weighted average grant date fair value, Beginning	$ 11.44	$ 44.8
Granted, Weighted average grant date fair value	11.7
Vested, Weighted average grant date fair value	27.56
Forfeited, Weighted average grant date fair value	90.4
Unvested balance, Weighted average grant date fair value, Ending	$ 11.44
Total unrecognized expense	$ 0.5
Weighted average period for recognition	1 year 6 months

Segment Reporting (Details) $ in Thousands	3 Months Ended		6 Months Ended
Segment Reporting (Details) $ in Thousands	Jun. 30, 2025 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2025 USD ($) segment	Jun. 30, 2024 USD ($)
Segment Reporting
Number of reportable segments \| segment			1
Research and development
Total research and development expenses	$ 5,814	$ 10,430	$ 13,551	$ 21,965
General and administrative
Total general and administrative expenses	4,520	6,510	10,605	13,025
Impairment of long-lived assets		438	2,637	438
Loss from operations	10,334	17,378	26,793	35,428
Other (income) expense, net	(949)	(1,387)	(2,003)	(2,726)
Net loss	9,385	15,991	24,790	32,702
Genetic Medicines Segment
Research and development
Wages, benefits and other payroll	1,302	2,944	4,891	6,355
Third-party costs	4,154	6,067	7,948	12,679
Share-based compensation	248	725	492	1,543
Depreciation and amortization	110	694	220	1,388
Total research and development expenses	5,814	10,430	13,551	21,965
General and administrative
Wages, benefits and other payroll	1,798	2,240	4,588	4,540
Third-party costs	1,974	3,314	4,577	6,640
Share-based compensation	675	853	1,289	1,630
Depreciation and amortization	73	103	151	215
Total general and administrative expenses	4,520	6,510	10,605	13,025
Impairment of long-lived assets		438	2,637	438
Loss from operations	10,334	17,378	26,793	35,428
Other (income) expense, net	(949)	(1,387)	(2,003)	(2,726)
Net loss	$ 9,385	$ 15,991	$ 24,790	$ 32,702

Subsequent Events (Details)		6 Months Ended
Subsequent Events (Details)	Jul. 14, 2025	Jun. 30, 2025
Subsequent Events
Stock split		0.05
Subsequent event \| Minimum
Subsequent Events
Stock split	0.05