Passage Bio (PASG) 10-Q 13 Nov 23 2023 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2023	Nov. 08, 2023
Document and Entity Information
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2023
Document Transition Report	false
Entity 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		54,819,638
Entity Central Index Key	0001787297
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	Q3
Amendment Flag	false

Balance Sheets

Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Current assets:
Cash and cash equivalents	$ 33,579	$ 34,601
Marketable securities	99,201	155,009
Prepaid expenses and other current assets	1,282	926
Prepaid research and development	1,649	6,508
Total current assets	135,711	197,044
Property and equipment, net	16,067	22,515
Right of use assets - operating leases	17,045	19,723
Other assets	433	4,267
Total assets	169,256	243,549
Current liabilities:
Accounts payable	2,749	4,065
Accrued expenses and other current liabilities	13,713	11,011
Operating lease liabilities	3,349	3,275
Total current liabilities	19,811	18,351
Operating lease liabilities - noncurrent	23,169	23,832
Total liabilities	42,980	42,183
Commitments and contingencies (note 10)
Stockholders' equity:
Common stock, $0.0001 par value: 300,000,000 shares authorized; 54,812,972 shares issued and outstanding at September 30, 2023 and 54,614,690 shares issued and outstanding at December 31, 2022	5	5
Additional paid-in capital	704,173	694,733
Accumulated other comprehensive income (loss)	(192)	(966)
Accumulated deficit	(577,710)	(492,406)
Total stockholders' equity	126,276	201,366
Total liabilities and stockholders' equity	$ 169,256	$ 243,549

Balance Sheets (Parenthetical)

Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2023	Dec. 31, 2022
Balance Sheets
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	54,812,972	54,614,690
Common stock, shares outstanding	54,812,972	54,614,690

Statements of Operations and Co

Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Operating expenses:
Research and development	$ 15,098	$ 15,362	$ 49,258	$ 68,396
Acquired in-process research and development		1,500		3,000
General and administrative	8,184	10,664	35,295	38,754
Impairment of long-lived assets	5,390		5,390
Loss from operations	(28,672)	(27,526)	(89,943)	(110,150)
Other income (expense), net	1,562	825	4,639	1,096
Net loss	$ (27,110)	$ (26,701)	$ (85,304)	$ (109,054)
Per share information:
Net loss per share of common stock, basic	$ (0.49)	$ (0.49)	$ (1.56)	$ (2.01)
Net loss per share of common stock, diluted	$ (0.49)	$ (0.49)	$ (1.56)	$ (2.01)
Weighted average common shares outstanding, basic	54,789,410	54,473,945	54,697,967	54,379,397
Weighted average common shares outstanding, diluted	54,789,410	54,473,945	54,697,967	54,379,397
Comprehensive loss:
Net loss	$ (27,110)	$ (26,701)	$ (85,304)	$ (109,054)
Unrealized gain (loss) on marketable securities	148	110	774	(1,055)
Comprehensive loss	$ (26,962)	$ (26,591)	$ (84,530)	$ (110,109)

Statements of Stockholders' Equ

Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Common stock	Additional paid-in capital	Accumulated other comprehensive income (loss)	Accumulated deficit	Total
Balance at Beginning of period at Dec. 31, 2021	$ 5	$ 675,346	$ (413)	$ (356,281)	$ 318,657
Balance at Beginning of period (in shares) at Dec. 31, 2021	54,244,996
Stockholders' equity
Exercise of stock options and vesting of restricted stock units		128			128
Exercise of stock options and vesting of restricted stock units (in shares)	146,056
Issuance of shares in connection with employee stock purchase plan		243			243
Issuance of shares in connection with employee stock purchase plan (in shares)	155,544
Unrealized gain (loss) on marketable securities			(1,055)		(1,055)
Share-based compensation expense		15,477			15,477
Net loss				(109,054)	(109,054)
Balance at end of period at Sep. 30, 2022	$ 5	691,194	(1,468)	(465,335)	224,396
Balance at end of period (in shares) at Sep. 30, 2022	54,546,596
Balance at Beginning of period at Jun. 30, 2022	$ 5	688,253	(1,578)	(438,634)	248,046
Balance at Beginning of period (in shares) at Jun. 30, 2022	54,463,235
Stockholders' equity
Issuance of shares in connection with employee stock purchase plan		79			79
Issuance of shares in connection with employee stock purchase plan (in shares)	83,361
Unrealized gain (loss) on marketable securities			110		110
Share-based compensation expense		2,862			2,862
Net loss				(26,701)	(26,701)
Balance at end of period at Sep. 30, 2022	$ 5	691,194	(1,468)	(465,335)	224,396
Balance at end of period (in shares) at Sep. 30, 2022	54,546,596
Balance at Beginning of period at Dec. 31, 2022	$ 5	694,733	(966)	(492,406)	201,366
Balance at Beginning of period (in shares) at Dec. 31, 2022	54,614,690
Stockholders' equity
Exercise of stock options and vesting of restricted stock units (in shares)	98,042
Issuance of shares in connection with employee stock purchase plan		89			89
Issuance of shares in connection with employee stock purchase plan (in shares)	100,240
Unrealized gain (loss) on marketable securities			774		774
Share-based compensation expense		9,351			9,351
Net loss				(85,304)	(85,304)
Balance at end of period at Sep. 30, 2023	$ 5	704,173	(192)	(577,710)	126,276
Balance at end of period (in shares) at Sep. 30, 2023	54,812,972
Balance at Beginning of period at Jun. 30, 2023	$ 5	701,349	(340)	(550,600)	150,414
Balance at Beginning of period (in shares) at Jun. 30, 2023	54,745,680
Stockholders' equity
Exercise of stock options and vesting of restricted stock units (in shares)	67,292
Unrealized gain (loss) on marketable securities			148		148
Share-based compensation expense		2,824			2,824
Net loss				(27,110)	(27,110)
Balance at end of period at Sep. 30, 2023	$ 5	$ 704,173	$ (192)	$ (577,710)	$ 126,276
Balance at end of period (in shares) at Sep. 30, 2023	54,812,972

Statements of Cash Flows (Unaud

Statements of Cash Flows (Unaudited) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Cash flows used in operating activities:
Net loss	$ (27,110,000)	$ (26,701,000)	$ (85,304,000)	$ (109,054,000)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Acquired in-process research and development		1,500,000		3,000,000
Depreciation and amortization	1,000,000	900,000	2,914,000	2,733,000
Share-based compensation			9,351,000	15,477,000
Amortization of premium and discount on marketable securities, net			(1,188,000)	1,206,000
Loss on disposal of property and equipment	0	0	463,000	0
Impairment of long-lived assets	5,390,000		5,390,000
Changes in operating assets and liabilities:
Prepaid expenses and other current assets, and other assets			3,478,000	1,701,000
Prepaid research and development			4,859,000	310,000
Right of use assets and operating lease liabilities			(97,000)	478,000
Accounts payable			(1,316,000)	(4,979,000)
Accrued expenses and other current and noncurrent liabilities			2,702,000	(5,061,000)
Net cash provided by (used in) operating activities			(58,748,000)	(94,189,000)
Cash flows provided by (used in) investing activities:
Purchases of marketable securities			(101,640,000)	(116,304,000)
Sales or maturities of marketable securities			159,410,000	149,661,000
Purchases of technology licenses				(3,000,000)
Purchases of property and equipment			(133,000)	(1,800,000)
Net cash provided by (used in) investing activities			57,637,000	28,557,000
Cash flows provided by (used in) financing activities:
Proceeds from the exercise of stock options				128,000
Proceeds from the issuance of common stock under employee stock purchase plan			89,000	243,000
Payments for insurance premium financing				(1,136,000)
Net cash provided by (used in) financing activities			89,000	(765,000)
Net increase (decrease) in cash and cash equivalents			(1,022,000)	(66,397,000)
Cash and cash equivalents at beginning of period			34,601,000	128,965,000
Cash and cash equivalents at end of period	$ 33,579,000	$ 62,568,000	33,579,000	62,568,000
Supplemental disclosure of non-cash investing and financing activities:
Unrealized gain (loss) on marketable securities			$ 774,000	(1,055,000)
Property and equipment in accounts payable and accrued expenses and other current liabilities				45,000
Right of use assets recognized upon the adoption of Topic 842				(20,375,000)
Operating lease liabilities recognized upon the adoption of Topic 842				$ 27,296,000

Nature of Operations

Nature of Operations	9 Months Ended
	Sep. 30, 2023
Nature of Operations
Nature of Operations	1. Nature of Operations Passage Bio, Inc., or the Company, a Delaware corporation incorporated in July 2017, is a clinical stage genetic medicines company focused on developing transformative therapies for central nervous system, or CNS, disorders, with limited or no approved treatment options. The Company has a strategic research collaboration with the Trustees of the University of Pennsylvania’s, or Penn, Gene Therapy Program, or GTP. Under this collaboration, GTP conducts discovery and preclinical activities enabling Investigational New Drug, or IND, applications and the Company conducts all clinical development, manufacturing, regulatory strategy, and commercialization activities under the agreement. Through this collaboration, the Company has assembled a strong portfolio of genetic medicine product candidates, including two lead clinical product candidates: PBGM01 for the treatment of GM1 gangliosidosis, or GM1, and PBFT02 for the treatment of frontotemporal dementia, or FTD. The Company also has a collaboration agreement and a development services and clinical supply agreement with Catalent Maryland, Inc., or Catalent,

Risks and Liquidity

Risks and Liquidity	9 Months Ended
	Sep. 30, 2023
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 $577.7 million as of September 30, 2023. 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 manufacturing 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 and establishing 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 research and development 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. 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

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2023
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, 2022. 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. 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 United States Securities and Exchange Commission, or 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 2022 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 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, and marketable securities. The Company maintains a deposit account in a federally insured financial institution in excess of federally insured limits. The Company maintains a money market account in a federally insured financial institution in excess of federally insured limits. 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. The Company maintains a portfolio of marketable debt securities, 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 marketable debt securities. The Company has not experienced any material losses in such portfolio. 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 September 30, 2023 consisted of only money market funds and commercial paper. Cash consists of cash deposits at banking institutions. Marketable securities The Company classifies its marketable securities as available-for-sale, which include commercial paper, certificates of deposit, corporate debt securities, United States, or U.S., government debt securities and U.S. government agency securities with original maturities of greater than three months. 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. Gains or losses on marketable securities sold are recognized as a component of other income, 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. Property and Equipment, net Property and equipment consists of laboratory equipment, office equipment, computer hardware and software, furniture and leasehold improvements and are 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 is retired or otherwise disposed of, the costs and accumulated depreciation are removed from the respective accounts, with any resulting gain or loss recognized concurrently. In the three and nine months ended September 30, 2023, the Company recognized losses on disposals of property and equipment of $0 and $0.5 million, respectively, within research and development expenses, compared to none in both the three and nine months ended September 30, 2022. 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. During both the three and nine months ended September 30, 2023, the Company recognized impairment expenses for property and equipment of $3.2 million, compared to none in both the three and nine months ended September 30, 2022. These impairment expenses represent the proportional allocation of total impairments recognized for the asset groups subject to impairment testing in connection with the Company’s sublease agreements, as further described in Note 9. Leasing The Company evaluates leases at their inception to determine if they are an operating lease or a finance lease. As of September 30, 2023, 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 twelve months or less. The Company reviews long-lived assets, such as right of use assets, for impairment when events or changes indicate the carrying amount of the right of use assets may not be recoverable. In both the three and nine months ended September 30, 2023, the Company recognized impairment expenses for right of use assets of $2.2 million, compared to none in both the three and nine months ended September 30, 2022. These impairment expenses represent the proportional allocation of total impairments recognized for the asset groups subject to impairment testing in connection with the Company’s sublease agreements, as further described in Note 9. 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 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. Research and Development Research and development costs are expensed as incurred and consist primarily of expenses incurred with Penn, 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. 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: ​ ​ ​ ​ ​ ​ ​ ​ Nine Months Ended September 30, ​ ​ 2023 2022 Stock options ​ 10,240,051 11,738,877 Unvested restricted stock units ​ 1,146,499 ​ 409,500 Employee stock purchase plan ​ 118,059 ​ 62,488 ​ ​ 11,504,609 12,210,865 ​ Recently Adopted Accounting Pronouncements In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses: Measurement of Credit Losses on Financial Instruments , or ASU 2016-13, which replaces the incurred loss impairment methodology under current U.S. GAAP with a methodology that reflects expected credit losses and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates. ASU 2016-13 was subsequently updated by ASU No. 2019-04, Codification Improvements to Topic 326, Financial Instruments—Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments , to clarify that entities should include recoveries when estimating the allowance for credit losses. This guidance was effective for the Company starting in fiscal year 2023. The Company adopted ASU 2016-13 as of January 1, 2023, which did not have a material impact on its financial statements.

Cash, Cash Equivalents and Mark

Cash, Cash Equivalents and Marketable Securities	9 Months Ended
	Sep. 30, 2023
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 September 30, 2023: ​ ​ ​ ​ Cash accounts in banking institutions ​ $ 5,234 ​ $ - ​ $ - ​ $ 5,234 Money market funds ​ ​ 27,347 ​ ​ - ​ ​ - ​ ​ 27,347 Commercial paper ​ ​ 998 ​ ​ - ​ ​ - ​ ​ 998 Total ​ $ 33,579 ​ $ - ​ $ - ​ $ 33,579 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2022: ​ ​ ​ ​ Cash accounts in banking institutions ​ $ 7,532 ​ $ - ​ $ - ​ $ 7,532 Money market funds ​ ​ 24,578 ​ ​ - ​ ​ - ​ ​ 24,578 Commercial paper ​ ​ 2,491 ​ ​ - ​ ​ - ​ ​ 2,491 Total ​ $ 34,601 ​ $ - ​ $ - ​ $ 34,601 ​ The following table provides details regarding the Company’s portfolio of marketable securities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ (in thousands) Amortized cost Unrealized gains Unrealized losses Fair value September 30, 2023: ​ ​ ​ ​ Certificates of deposit ​ $ 8,914 ​ $ 2 ​ $ (4) ​ $ 8,912 Commercial paper ​ ​ 44,836 ​ ​ - ​ ​ (18) ​ ​ 44,818 Corporate debt securities ​ ​ 20,064 ​ ​ 1 ​ ​ (31) ​ ​ 20,034 U.S. government securities ​ ​ 17,589 ​ ​ - ​ ​ (126) ​ ​ 17,463 U.S. government agency securities ​ ​ 7,990 ​ ​ - ​ ​ (16) ​ ​ 7,974 Total ​ $ 99,393 ​ $ 3 ​ $ (195) ​ $ 99,201 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2022: ​ ​ ​ ​ Certificates of deposit ​ $ 28,197 ​ $ 6 ​ $ (92) ​ $ 28,111 Commercial paper ​ ​ 58,572 ​ ​ 12 ​ ​ (72) ​ ​ 58,512 Corporate debt securities ​ ​ 67,206 ​ ​ 1 ​ ​ (786) ​ ​ 66,421 U.S. government securities ​ ​ 2,000 ​ ​ - ​ ​ (35) ​ ​ 1,965 Total ​ $ 155,975 ​ $ 19 ​ $ (985) ​ $ 155,009 ​ ​ ​ The contractual maturities of the Company’s marketable securities as of September 30, 2023, are as follows: ​ ​ ​ ​ ​ ​ ​ ​ (in thousands) ​ Amortized Cost ​ Fair Value Due within one year ​ $ 92,072 ​ $ 91,982 Due after one year through five years ​ ​ 7,321 ​ ​ 7,219 Total ​ $ 99,393 ​ $ 99,201 ​

Fair Value of Financial Instrum

Fair Value of Financial Instruments and Non-Financial	9 Months Ended
	Sep. 30, 2023
Fair Value of Financial Instruments and Non-Financial
Fair Value of Financial Instruments and Non-Financial	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 FASB ASC Topic 820, Fair Value Measurement ● 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) September 30, 2023: ​ ​ ​ Assets ​ ​ ​ Cash equivalents: ​ ​ ​ ​ ​ ​ ​ ​ ​ Money market funds ​ $ 27,347 ​ $ - ​ $ - Commercial paper ​ ​ - ​ ​ 998 ​ ​ - Total cash equivalents ​ ​ 27,347 ​ ​ 998 ​ ​ - ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Marketable securities: ​ ​ ​ ​ ​ ​ ​ ​ ​ Certificates of deposit ​ ​ - ​ ​ 8,912 ​ ​ - Commercial paper ​ ​ - ​ ​ 44,818 ​ ​ - Corporate debt securities ​ ​ - ​ ​ 20,034 ​ ​ - U.S. government securities ​ ​ - ​ ​ 17,463 ​ ​ - U.S. government agency securities ​ ​ - ​ ​ 7,974 ​ ​ - Total marketable securities ​ ​ - ​ ​ 99,201 ​ ​ - Total financial assets ​ $ 27,347 ​ $ 100,199 ​ $ - ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2022: ​ ​ ​ ​ ​ ​ ​ ​ ​ Assets ​ ​ ​ Cash equivalents: ​ ​ ​ ​ ​ ​ ​ ​ ​ Money market funds ​ $ 24,578 ​ $ - ​ $ - Commercial paper ​ ​ - ​ ​ 2,491 ​ ​ - Total cash equivalents ​ ​ 24,578 ​ ​ 2,491 ​ ​ - ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Marketable securities: ​ ​ ​ ​ ​ ​ ​ ​ ​ Certificates of deposit ​ ​ - ​ ​ 28,111 ​ ​ - Commercial paper ​ ​ - ​ ​ 58,512 ​ ​ - Corporate debt securities ​ ​ - ​ ​ 66,421 ​ ​ - U.S. government securities ​ ​ - ​ ​ 1,965 ​ ​ - Total marketable securities ​ ​ - ​ ​ 155,009 ​ ​ - Total financial assets ​ $ 24,578 ​ $ 157,500 ​ $ - ​ 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. The following non-financial instruments were measured at fair value, on a nonrecurring basis, during the period ended September 30, 2023. The significant assumptions utilized, which relate to future net cash flows, are further described in Note 9: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ (in thousands) ​ Level 1 ​ Level 2 ​ Level 3 ​ Impairment Losses Property and equipment, net $ - $ - $ 1,367 $ 3,205 Right of use assets ​ - ​ - ​ 933 ​ 2,185 Total $ - $ - $ 2,300 $ 5,390 ​

Property and Equipment, net

Property and Equipment, net	9 Months Ended
	Sep. 30, 2023
Property and Equipment, net
Property and Equipment, net	6. Property and Equipment, net ​ Property and equipment, net, consist of the following: ​ ​ ​ ​ ​ ​ ​ (in thousands) ​ September 30, 2023 ​ December 31, 2022 Laboratory equipment $ 10,042 ​ $ 9,972 Office equipment ​ 472 ​ ​ 601 Computer hardware and software ​ 1,077 ​ ​ 1,090 Furniture and fixtures ​ 419 ​ ​ 1,208 Leasehold improvements ​ 9,842 ​ ​ 13,506 Construction in progress ​ 643 ​ ​ 1,291 Total property and equipment ​ 22,495 ​ ​ 27,668 Accumulated depreciation and amortization ​ (6,428) ​ ​ (5,153) ​ $ 16,067 ​ $ 22,515 ​ ​ ​ ​ Depreciation expense was $1.0 million and $0.9 million for the three months ended September 30, 2023 and 2022, respectively. ​ Depreciation expense was $2.9 million and $2.7 million for the nine months ended September 30, 2023 and 2022, respectively.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	9 Months Ended
	Sep. 30, 2023
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) September 30, 2023 December 31, 2022 Professional fees ​ $ 418 ​ $ 602 Compensation and related benefits ​ 6,232 ​ 8,446 Research and development ​ 3,063 ​ 1,878 Property and equipment ​ ​ — ​ ​ 85 Amount due to Catalent in connection with Letter Agreements ​ 4,000 ​ — ​ ​ $ 13,713 ​ $ 11,011 ​

Severance

Severance	9 Months Ended
	Sep. 30, 2023
Severance
Severance	8. Severance ​ In July 2023, 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 26%. ​ In accordance with ASC 420, Exit and Disposal Activities, the Company recorded severance and termination-related costs of $1.0 million in general and administrative expenses and $1.4 million in research and development expenses for both the three and nine months ended September 30, 2023. ​ During the three months ended September 30, 2022, the Company recorded no severance and termination-related costs in general and administrative expenses or in research and development expenses. During the nine months ended September 30, 2022, the Company recorded severance and termination-related costs of $1.8 million in general and administrative expenses and $0.9 million in research and development expenses. ​ As of September 30, 2023, $0.4 million of severance and termination-related costs were unpaid and recognized in the balance sheet within accrued expenses and other current liabilities. ​

Leases

Leases	9 Months Ended
	Sep. 30, 2023
Leases
Leases	9. 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 right of use, or ROU, asset and operating lease liability as of September 30, 2023. ​ 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, Sublessee A has an option to extend the sublease agreement through November 30, 2031. The base sublease rent is $12,426 per month 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 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. ​ As a result of Sublease Agreement A, the Company determined an impairment indicator was present. The Company compared the estimated undiscounted cash flows to the carrying value of the asset group, which includes right of use assets, leasehold improvements, and other property and equipment allocable to Sublease Agreement A. The Company concluded the carrying value of the asset group was not recoverable as it exceeded the estimated undiscounted cash flows. The Company calculated the amount of impairment 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 Sublease Agreement A, including an estimate for cash flows in the residual period, and an estimated borrowing rate of a market participant subtenant. ​ ​ 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 begins 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 $75,000 per month for the entire term of the sublease. Additionally, Sublease 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. ​ As a result of Sublease Agreement B, the Company determined an impairment indicator was present. The Company compared the estimated undiscounted cash flows to the carrying value of the asset group, which includes right of use assets, leasehold improvements, and other property and equipment allocable to Sublease Agreement B. The Company concluded the carrying value of the asset group was not recoverable as it exceeded the estimated undiscounted cash flows. The Company calculated the amount of impairment 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 Sublease Agreement B, including an estimate for cash flows in the residual period, and an estimated borrowing rate of a market participant subtenant. ​ Based on the analyses for Sublease Agreement A and Sublease Agreement B, the Company recognized impairment expense of $5.4 million, including $2.2 million for the right of use assets and $3.2 million for the property and equipment during the three and nine months ended September 30, 2023. ​ 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 February 2036. The Company has an option to extend the term of the Laboratory Lease Agreement by up to two five-year terms. This option to extend was not recognized as part of the Company's measurement of the ROU asset and operating lease liability as of September 30, 2023. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ The following table summarizes future minimum lease payments for the Company’s lessee operating leases, which comprises of the 2005 Market Street Lease Agreement and the Laboratory Lease Agreement. The below table does not include expected cash inflows related to Sublease Agreement A and Sublease Agreement B, as the Company was not relieved of its primarily obligation under the 2005 Market Street Lease Agreement: ​ ​ ​ ​ ​ (in thousands) ​ 2023 ​ $ 868 2024 ​ 3,553 2025 ​ 3,654 2026 ​ 3,757 2027 ​ 3,863 Thereafter ​ 29,679 Total undiscounted lease payments ​ ​ 45,374 Less: imputed interest ​ ​ (18,856) Total lease liabilities ​ $ 26,518 ​ ​ Operating lease expense was $0.8 million for both the three months ended September 30, 2023 and 2022. ​ Operating lease expense was $2.5 million for both the nine months ended September 30, 2023 and 2022.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2023
Commitments and Contingencies.
Commitments and Contingencies	10. Commitments and Contingencies Amended and Restated Research, Collaboration and License Arrangement with Penn The Company has a research, collaboration and licensing agreement with Penn, as amended, or the Penn Agreement, for research and development collaborations and exclusive license rights to patents for certain products and technologies. Under the Penn Agreement, the Company has obligations to fund certain research relating to the preclinical development of selected products in research programs as well as exploratory research programs in non-rare and/or non-monogenic, or large CNS indications. In addition, the Company will fund discovery research conducted by Penn through August 3, 2026 and will receive exclusive rights, subject to certain limitations, to technologies resulting from the discovery research for the Company’s products developed with GTP, such as novel capsids, toxicity reduction technologies and delivery and formulation improvements. This funding commitment for the discovery research is $5.0 million annually, paid in quarterly increments of $1.3 million through June, 2026. The Penn Agreement includes an exploratory research program focused on discovering targets and novel gene therapy candidates for certain large CNS indications and can be expanded to other large CNS diseases upon mutual agreement. The initial term of the exploratory research program is until August 2024, which term can be extended by mutual agreement. Under the exploratory research program, the Company will have the right to further develop and commercialize any gene therapy product candidates specific for those selected targets (and any future large CNS diseases that are mutually agreed upon) that may arise from the exploratory research programs on substantially the same terms of the current Penn Agreement. Under the Penn Agreement, the Company has eight remaining options available to commence additional licensed programs for CNS indications and has until August 3, 2026, to exercise these options. If the Company were to exercise any of these options, it would owe Penn a non-refundable upfront 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 the obligation to fund certain research relating to the preclinical development of each licensed program. The Penn Agreement requires that the Company make payments of up to (i) $16.5 million per product candidate for rare, monogenic disorders in the aggregate and (ii) $39.0 million per product candidate in the aggregate arising from the exploratory program for large CNS indications. 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 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 Penn, on a licensed product-by-licensed product and country-by-country basis, tiered royalties (subject to customary reductions) in the mid-single digits on annual worldwide net sales of such licensed product. In addition, 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 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. In addition, the Company will pay a tiered transaction fee of 1-2% of the net proceeds upon certain change of control events. During the three months ended September 30, 2023, the Company did not make any payments for acquired in-process research and development. During the three months ended September 30, 2022, the Company made payments under the Penn Agreement for acquired in-process research and development of $1.5 million related to the achievement of a development milestone. During the nine months ended September 30, 2023, the Company did not make any payments for acquired in-process research and development. During the nine months ended September 30, 2022, the Company made payments under the Penn Agreement for acquired in-process research and development of $3.0 Catalent Agreements In June 2019, the Company entered into a collaboration agreement, or the Collaboration Agreement, with Catalent. As part of the Collaboration Agreement, the Company was required to pay an annual fee for five years ending in 2025 for the exclusive use of a dedicated clean room suite, or the Clean Room Suite. In April 2020, the Company entered into a development services and clinical supply agreement, or the Manufacturing and Supply Agreement, with Catalent to secure clinical scale manufacturing capacity for batches of active pharmaceutical ingredients for the Company’s gene therapy product candidates. The Manufacturing and Supply Agreement confirms the terms contemplated by the Collaboration Agreement and the Collaboration Agreement continues to be in effect pursuant to its terms. Under the terms of the Manufacturing and Supply Agreement, Catalent agreed to manufacture batches of drug product for the Company’s gene therapy product candidates at the Clean Room Suite at a Catalent facility provided for in the Collaboration Agreement. The Manufacturing and Supply Agreement provided for a term of five years. The Manufacturing and Supply Agreement also included minimum annual purchase commitments. The Company has the right to terminate the Manufacturing and Supply Agreement for convenience or other reasons specified in the Manufacturing and Supply Agreement upon prior written notice. If the Company terminates the Manufacturing and Supply Agreement, it will be obligated to pay an early termination fee to Catalent. Under both the Collaboration Agreement and the Manufacturing and Supply Agreement, the Company had an annual minimum commitment of $10.6 million per year owed to Catalent for five years from the validation of the Clean Room Suite, subject to certain inflationary adjustments. On March 31, 2023, the Company entered into certain letter agreements amending each of (i) the Collaboration Agreement and (ii) the Manufacturing and Supply Agreement, together with the Collaboration Agreement, the Existing Agreements. Letter agreement I, or Agreement I, eliminated the minimum annual purchase obligation and the obligation to pay an annual fee for use of the Clean Room Suite, thereby eliminating the annual minimum commitment of $10.6 million per year owed to Catalent through November 2025 under the Existing Agreements. In consideration of Agreement I, the Company will make aggregate payments to Catalent of $6.0 million between June 30, 2023 and May 1, 2024. Letter agreement II, or Agreement II, and together with Agreement I, the Letter Agreements, extended the term of the Existing Agreements until November 6, 2030, and established 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 Agreement II 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. In addition, in the event of certain transactions, the Company may terminate the Existing Agreements for convenience with respect to such products, in which case, the Company would pay to Catalent a certain termination fee. Immediately prior to the execution of the Letter Agreements, the Company had a $5.3 million prepaid asset related to upfront payments made to secure the Clean Room Suite. In connection with the Letter Agreements, the Company no longer has exclusive access to the Clean Room Suite at Catalent and, as a result, the Company recognized an expense of $5.3 million related to the elimination of the prepaid asset during the nine months ended September 30, 2023. The Company classified the $11.3 million of expenses, which comprises of $6.0 million in aggregate payments due to Catalent and the $5.3 million elimination of the prepaid asset, as general and administrative expense within the statement of operations for the nine months ended September 30, 2023, as both amounts do not directly relate to the future advancement of the Company’s research and development programs. As of September 30, 2023, the Company made payments of $2.0 million under the Letter Agreements. The remaining $4.0 million of aggregate payments due to Catalent under the Letter Agreements are included in accrued expenses and other current liabilities. Employment Agreements The Company has entered into employment agreements with key personnel providing for compensation and, in certain circumstances, severance and acceleration of vesting in stock-based compensation awards, as described in the respective employment agreements.

Share-Based Compensation

Share-Based Compensation	9 Months Ended
	Sep. 30, 2023
Share-Based Compensation
Share-Based Compensation	11. Share-Based Compensation 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, as amended, 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 September 30, 2023 was 13,101,661. Additionally, any awards previously issued under the 2018 Plan which are forfeited become available for issuance under the Incentive Plan. As of September 30, 2023, 7,644,421 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 or compensation committee 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 2,730,735 and 2,712,249 shares in January 2023 and 2022, 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 and subsequently amended in February 2022 and February 2023. The total number of shares authorized under the Inducement Plan as of September 30, 2023 was 2,500,000. Of this amount, 1,006,655 shares were available for future grants as of September 30, 2023. 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 for the period presented: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ Nine Months Ended September 30, (in thousands) 2023 2022 ​ 2023 2022 Research and development ​ $ 1,429 ​ $ 568 ​ $ 4,788 ​ $ 6,528 General and administrative ​ 1,395 ​ 2,294 ​ 4,563 ​ 8,949 ​ ​ $ 2,824 ​ $ 2,862 ​ $ 9,351 ​ $ 15,477 ​ The following table summarizes stock option activity for the nine months ended September 30, 2023: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted ​ ​ ​ ​ ​ Weighted ​ average ​ ​ ​ ​ ​ average ​ remaining ​ ​ ​ Number of ​ exercise price ​ contractual ​ ​ ​ shares ​ per share ​ term (years) ​ Outstanding at January 1, 2023 11,411,390 $ 9.01 7.1 ​ Granted 3,348,982 ​ ​ 1.06 ​ Exercised — ​ ​ — ​ Forfeited (2,256,315) ​ ​ 8.58 ​ Expired (2,264,006) ​ ​ 12.50 ​ Outstanding at September 30, 2023 10,240,051 ​ $ 5.73 8.2 ​ Vested and exercisable at September 30, 2023 4,935,214 ​ $ 8.43 7.4 ​ Vested or expected to vest at September 30, 2023 10,240,051 ​ $ 5.73 8.2 ​ ​ The weighted average grant date fair value of options granted was $0.82 and $2.52 for the nine months ended September 30, 2023 and 2022, respectively. As of September 30, 2023, the total unrecognized compensation expense related to unvested stock option awards was $12.4 million, which the Company expects to recognize over a weighted average period of 1.9 years. The 2018 Plan and the Incentive Plan provide certain holders of stock options an election to early exercise prior to vesting. The Company has the right to repurchase early exercised options without transferring any appreciation in the value of the underlying shares to the employee if the employee terminates employment before the end of the original vesting period. The repurchase price is the lesser of the original exercise price or the then fair value of the Company’s common stock. As of September 30, 2023, 3,426 options to purchase common stock are unvested, but exercisable, under early exercise provisions. The fair value of each option was estimated on the date of grant using the weighted average assumptions in the table below: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Nine Months Ended September 30, ​ ​ 2023 ​ 2022 Expected volatility ​ 92.6 % ​ 92.3 % Risk‑free interest rate ​ 3.6 % ​ 2.6 % Expected term ​ 6.0 years ​ 5.8 years Expected dividend yield ​ — ​ ​ — ​ ​ Restricted Stock Units The Company issues restricted stock units, or RSUs, to employees that vest over periods as determined by the board of directors. Any unvested shares are forfeited upon termination of services. The fair value of the RSUs is equal to the fair market value price 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 nine months ended September 30, 2023: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted average ​ ​ Number of shares ​ grant date fair value Unvested balance at January 1, 2022 ​ 1,229,166 $ 2.98 Granted ​ 195,000 $ 1.01 Vested ​ ​ (98,042) ​ ​ 5.82 Forfeited ​ (179,625) $ 2.13 Unvested balance at September 30, 2023 ​ 1,146,499 $ 2.58 ​ As of , the total unrecognized expense related to all RSUs was $1.5 million, which the Company expects to recognize over a weighted average period of 1.1 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 1,981,766 shares of the Company’s common stock. Of this amount, 1,565,821 were available for future grants as of September 30, 2023. 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 or compensation committee to determine a lesser number of shares shall be added for such year. As a result, on January 1, 2023, the number of shares reserved for issuance under the ESPP increased by 546,147 shares, resulting in a total of 1,981,766 shares authorized for issuance. Under the ESPP, eligible employees can purchase the Company’s common stock through accumulated payroll deductions at such times as are established by the 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 4,000 shares of the Company’s common stock in any single offering period. In accordance with the guidance in ASC 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. The Company did not recognize any share-based compensation expense during both the three months ended September 30, 2023 and 2022. The Company recognized expense related to the ESPP of $0 and $0.2 million during the nine months ended September 30, 2023 and 2022, respectively.

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2023
Subsequent Events.
Subsequent Events	12. Subsequent Events None.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2023
Summary of Significant Accounting Policies
Basis of Presentation	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.
Interim Financial Statements	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 United States Securities and Exchange Commission, or 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 2022 Annual Report filed on Form 10-K.
Use of Estimates	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 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	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	Concentration of credit risk Financial instruments that potentially subject the Company to significant concentrations of credit risk consist primarily of cash and cash equivalents, and marketable securities. The Company maintains a deposit account in a federally insured financial institution in excess of federally insured limits. The Company maintains a money market account in a federally insured financial institution in excess of federally insured limits. 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. The Company maintains a portfolio of marketable debt securities, 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 marketable debt securities. The Company has not experienced any material losses in such portfolio.
Cash and cash equivalents	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 September 30, 2023 consisted of only money market funds and commercial paper. Cash consists of cash deposits at banking institutions.
Marketable securities	Marketable securities The Company classifies its marketable securities as available-for-sale, which include commercial paper, certificates of deposit, corporate debt securities, United States, or U.S., government debt securities and U.S. government agency securities with original maturities of greater than three months. 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. Gains or losses on marketable securities sold are recognized as a component of other income, 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.
Property and Equipment, net	Property and Equipment, net Property and equipment consists of laboratory equipment, office equipment, computer hardware and software, furniture and leasehold improvements and are 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 is retired or otherwise disposed of, the costs and accumulated depreciation are removed from the respective accounts, with any resulting gain or loss recognized concurrently. In the three and nine months ended September 30, 2023, the Company recognized losses on disposals of property and equipment of $0 and $0.5 million, respectively, within research and development expenses, compared to none in both the three and nine months ended September 30, 2022. 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. During both the three and nine months ended September 30, 2023, the Company recognized impairment expenses for property and equipment of $3.2 million, compared to none in both the three and nine months ended September 30, 2022. These impairment expenses represent the proportional allocation of total impairments recognized for the asset groups subject to impairment testing in connection with the Company’s sublease agreements, as further described in Note 9.
Leasing	Leasing The Company evaluates leases at their inception to determine if they are an operating lease or a finance lease. As of September 30, 2023, 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 twelve months or less. The Company reviews long-lived assets, such as right of use assets, for impairment when events or changes indicate the carrying amount of the right of use assets may not be recoverable. In both the three and nine months ended September 30, 2023, the Company recognized impairment expenses for right of use assets of $2.2 million, compared to none in both the three and nine months ended September 30, 2022. These impairment expenses represent the proportional allocation of total impairments recognized for the asset groups subject to impairment testing in connection with the Company’s sublease agreements, as further described in Note 9.
Share-based compensation	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 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.
Research and Development	Research and Development Research and development costs are expensed as incurred and consist primarily of expenses incurred with Penn, 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.
Net Loss Per Share	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: ​ ​ ​ ​ ​ ​ ​ ​ Nine Months Ended September 30, ​ ​ 2023 2022 Stock options ​ 10,240,051 11,738,877 Unvested restricted stock units ​ 1,146,499 ​ 409,500 Employee stock purchase plan ​ 118,059 ​ 62,488 ​ ​ 11,504,609 12,210,865
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses: Measurement of Credit Losses on Financial Instruments , or ASU 2016-13, which replaces the incurred loss impairment methodology under current U.S. GAAP with a methodology that reflects expected credit losses and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates. ASU 2016-13 was subsequently updated by ASU No. 2019-04, Codification Improvements to Topic 326, Financial Instruments—Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments , to clarify that entities should include recoveries when estimating the allowance for credit losses. This guidance was effective for the Company starting in fiscal year 2023. The Company adopted ASU 2016-13 as of January 1, 2023, which did not have a material impact on its financial statements.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2023
Summary of Significant Accounting Policies
Schedule of antidilutive securities excluded from computation of earnings per share	​ ​ ​ ​ ​ ​ ​ ​ Nine Months Ended September 30, ​ ​ 2023 2022 Stock options ​ 10,240,051 11,738,877 Unvested restricted stock units ​ 1,146,499 ​ 409,500 Employee stock purchase plan ​ 118,059 ​ 62,488 ​ ​ 11,504,609 12,210,865

Cash, Cash Equivalents and Ma_2

Cash, Cash Equivalents and Marketable Securities (Tables)	9 Months Ended
	Sep. 30, 2023
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 September 30, 2023: ​ ​ ​ ​ Cash accounts in banking institutions ​ $ 5,234 ​ $ - ​ $ - ​ $ 5,234 Money market funds ​ ​ 27,347 ​ ​ - ​ ​ - ​ ​ 27,347 Commercial paper ​ ​ 998 ​ ​ - ​ ​ - ​ ​ 998 Total ​ $ 33,579 ​ $ - ​ $ - ​ $ 33,579 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2022: ​ ​ ​ ​ Cash accounts in banking institutions ​ $ 7,532 ​ $ - ​ $ - ​ $ 7,532 Money market funds ​ ​ 24,578 ​ ​ - ​ ​ - ​ ​ 24,578 Commercial paper ​ ​ 2,491 ​ ​ - ​ ​ - ​ ​ 2,491 Total ​ $ 34,601 ​ $ - ​ $ - ​ $ 34,601
Schedule of details regarding the Company's portfolio of marketable securities	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ (in thousands) Amortized cost Unrealized gains Unrealized losses Fair value September 30, 2023: ​ ​ ​ ​ Certificates of deposit ​ $ 8,914 ​ $ 2 ​ $ (4) ​ $ 8,912 Commercial paper ​ ​ 44,836 ​ ​ - ​ ​ (18) ​ ​ 44,818 Corporate debt securities ​ ​ 20,064 ​ ​ 1 ​ ​ (31) ​ ​ 20,034 U.S. government securities ​ ​ 17,589 ​ ​ - ​ ​ (126) ​ ​ 17,463 U.S. government agency securities ​ ​ 7,990 ​ ​ - ​ ​ (16) ​ ​ 7,974 Total ​ $ 99,393 ​ $ 3 ​ $ (195) ​ $ 99,201 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2022: ​ ​ ​ ​ Certificates of deposit ​ $ 28,197 ​ $ 6 ​ $ (92) ​ $ 28,111 Commercial paper ​ ​ 58,572 ​ ​ 12 ​ ​ (72) ​ ​ 58,512 Corporate debt securities ​ ​ 67,206 ​ ​ 1 ​ ​ (786) ​ ​ 66,421 U.S. government securities ​ ​ 2,000 ​ ​ - ​ ​ (35) ​ ​ 1,965 Total ​ $ 155,975 ​ $ 19 ​ $ (985) ​ $ 155,009
Contractual Final Maturities of Marketable Securities	​ ​ ​ ​ ​ ​ ​ ​ (in thousands) ​ Amortized Cost ​ Fair Value Due within one year ​ $ 92,072 ​ $ 91,982 Due after one year through five years ​ ​ 7,321 ​ ​ 7,219 Total ​ $ 99,393 ​ $ 99,201 ​

Fair Value of Financial Instr_2

Fair Value of Financial Instruments and Non-Financial (Tables)	9 Months Ended
	Sep. 30, 2023
Fair Value of Financial Instruments and Non-Financial
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) September 30, 2023: ​ ​ ​ Assets ​ ​ ​ Cash equivalents: ​ ​ ​ ​ ​ ​ ​ ​ ​ Money market funds ​ $ 27,347 ​ $ - ​ $ - Commercial paper ​ ​ - ​ ​ 998 ​ ​ - Total cash equivalents ​ ​ 27,347 ​ ​ 998 ​ ​ - ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Marketable securities: ​ ​ ​ ​ ​ ​ ​ ​ ​ Certificates of deposit ​ ​ - ​ ​ 8,912 ​ ​ - Commercial paper ​ ​ - ​ ​ 44,818 ​ ​ - Corporate debt securities ​ ​ - ​ ​ 20,034 ​ ​ - U.S. government securities ​ ​ - ​ ​ 17,463 ​ ​ - U.S. government agency securities ​ ​ - ​ ​ 7,974 ​ ​ - Total marketable securities ​ ​ - ​ ​ 99,201 ​ ​ - Total financial assets ​ $ 27,347 ​ $ 100,199 ​ $ - ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2022: ​ ​ ​ ​ ​ ​ ​ ​ ​ Assets ​ ​ ​ Cash equivalents: ​ ​ ​ ​ ​ ​ ​ ​ ​ Money market funds ​ $ 24,578 ​ $ - ​ $ - Commercial paper ​ ​ - ​ ​ 2,491 ​ ​ - Total cash equivalents ​ ​ 24,578 ​ ​ 2,491 ​ ​ - ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Marketable securities: ​ ​ ​ ​ ​ ​ ​ ​ ​ Certificates of deposit ​ ​ - ​ ​ 28,111 ​ ​ - Commercial paper ​ ​ - ​ ​ 58,512 ​ ​ - Corporate debt securities ​ ​ - ​ ​ 66,421 ​ ​ - U.S. government securities ​ ​ - ​ ​ 1,965 ​ ​ - Total marketable securities ​ ​ - ​ ​ 155,009 ​ ​ - Total financial assets ​ $ 24,578 ​ $ 157,500 ​ $ -
Summary of non-financial instruments measured at fair value, on a nonrecurring basis	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ (in thousands) ​ Level 1 ​ Level 2 ​ Level 3 ​ Impairment Losses Property and equipment, net $ - $ - $ 1,367 $ 3,205 Right of use assets ​ - ​ - ​ 933 ​ 2,185 Total $ - $ - $ 2,300 $ 5,390 ​

Property and Equipment, net (Ta

Property and Equipment, net (Tables)	9 Months Ended
	Sep. 30, 2023
Property and Equipment, net
Schedule of property and equipment, net	​ ​ ​ ​ ​ ​ ​ (in thousands) ​ September 30, 2023 ​ December 31, 2022 Laboratory equipment $ 10,042 ​ $ 9,972 Office equipment ​ 472 ​ ​ 601 Computer hardware and software ​ 1,077 ​ ​ 1,090 Furniture and fixtures ​ 419 ​ ​ 1,208 Leasehold improvements ​ 9,842 ​ ​ 13,506 Construction in progress ​ 643 ​ ​ 1,291 Total property and equipment ​ 22,495 ​ ​ 27,668 Accumulated depreciation and amortization ​ (6,428) ​ ​ (5,153) ​ $ 16,067 ​ $ 22,515

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	9 Months Ended
	Sep. 30, 2023
Accrued Expenses and Other Current Liabilities
Schedule of accrued liabilities and other current liabilities	​ ​ ​ ​ ​ ​ ​ ​ (in thousands) September 30, 2023 December 31, 2022 Professional fees ​ $ 418 ​ $ 602 Compensation and related benefits ​ 6,232 ​ 8,446 Research and development ​ 3,063 ​ 1,878 Property and equipment ​ ​ — ​ ​ 85 Amount due to Catalent in connection with Letter Agreements ​ 4,000 ​ — ​ ​ $ 13,713 ​ $ 11,011 ​

Leases (Tables)

Leases (Tables)	9 Months Ended
	Sep. 30, 2023
Leases
Schedule of future minimum rental payments for operating leases	​ ​ ​ ​ ​ (in thousands) ​ 2023 ​ $ 868 2024 ​ 3,553 2025 ​ 3,654 2026 ​ 3,757 2027 ​ 3,863 Thereafter ​ 29,679 Total undiscounted lease payments ​ ​ 45,374 Less: imputed interest ​ ​ (18,856) Total lease liabilities ​ $ 26,518

Share-Based Compensation (Table

Share-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2023
Share-Based Compensation
Schedule of share-based compensation expense category	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ Nine Months Ended September 30, (in thousands) 2023 2022 ​ 2023 2022 Research and development ​ $ 1,429 ​ $ 568 ​ $ 4,788 ​ $ 6,528 General and administrative ​ 1,395 ​ 2,294 ​ 4,563 ​ 8,949 ​ ​ $ 2,824 ​ $ 2,862 ​ $ 9,351 ​ $ 15,477
Summary of stock option activity plan	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted ​ ​ ​ ​ ​ Weighted ​ average ​ ​ ​ ​ ​ average ​ remaining ​ ​ ​ Number of ​ exercise price ​ contractual ​ ​ ​ shares ​ per share ​ term (years) ​ Outstanding at January 1, 2023 11,411,390 $ 9.01 7.1 ​ Granted 3,348,982 ​ ​ 1.06 ​ Exercised — ​ ​ — ​ Forfeited (2,256,315) ​ ​ 8.58 ​ Expired (2,264,006) ​ ​ 12.50 ​ Outstanding at September 30, 2023 10,240,051 ​ $ 5.73 8.2 ​ Vested and exercisable at September 30, 2023 4,935,214 ​ $ 8.43 7.4 ​ Vested or expected to vest at September 30, 2023 10,240,051 ​ $ 5.73 8.2 ​
Schedule of weighted average assumptions applied to options	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Nine Months Ended September 30, ​ ​ 2023 ​ 2022 Expected volatility ​ 92.6 % ​ 92.3 % Risk‑free interest rate ​ 3.6 % ​ 2.6 % Expected term ​ 6.0 years ​ 5.8 years Expected dividend yield ​ — ​ ​ — ​
Summary of activity related to restricted stock unit	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted average ​ ​ Number of shares ​ grant date fair value Unvested balance at January 1, 2022 ​ 1,229,166 $ 2.98 Granted ​ 195,000 $ 1.01 Vested ​ ​ (98,042) ​ ​ 5.82 Forfeited ​ (179,625) $ 2.13 Unvested balance at September 30, 2023 ​ 1,146,499 $ 2.58

Nature of Operations (Details)

Nature of Operations (Details)	9 Months Ended
	Sep. 30, 2023 item
Nature of Operations
Number of lead clinical product candidates	2

Risks and Liquidity (Details)

Risks and Liquidity (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Risks and Liquidity
Accumulated deficit	$ (577,710)	$ (492,406)

Summary of Significant Accoun_4

Summary of Significant Accounting Policies (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Asset impairment charges	$ 5,390,000		$ 5,390,000
Amount of antidilutive securities excluded from computation of earnings per share			11,504,609	12,210,865
Loss on disposal of property and equipment	0	$ 0	$ (463,000)	$ 0
Impairment expenses for property and equipment	3,200,000	0	3,205,000	0
Impairment expenses for right of use assets	$ 2,200,000	$ 0	$ 2,185,000	$ 0
Stock options (including shares subject to repurchase)
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Amount of antidilutive securities excluded from computation of earnings per share			10,240,051	11,738,877
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			1,146,499	409,500
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			118,059	62,488
Computer hardware and software
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Useful life			3 years
Office equipment
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Useful life			5 years
Laboratory equipment
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Useful life			5 years
Furniture and fixtures
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Useful life			7 years

Cash, cash equivalents and ma_3

Cash, cash equivalents and marketable securities - Portfolio of Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Cash and cash equivalents
Cost or Amortized cost	$ 33,579	$ 34,601
Fair value	33,579	34,601
Cash accounts in banking institutions
Cash and cash equivalents
Cost or Amortized cost	5,234	7,532
Fair value	5,234	7,532
Money market funds
Cash and cash equivalents
Cost or Amortized cost	27,347	24,578
Fair value	27,347	24,578
Commercial paper
Cash and cash equivalents
Cost or Amortized cost	998	2,491
Fair value	$ 998	$ 2,491

Cash, cash equivalents and ma_4

Cash, cash equivalents and marketable securities - Portfolio of Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	$ 99,393	$ 155,975
Unrealized gains	3	19
Unrealized losses	(195)	(985)
Fair value	99,201	155,009
Certificates of deposit
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	8,914	28,197
Unrealized gains	2	6
Unrealized losses	(4)	(92)
Fair value	8,912	28,111
Commercial paper
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	44,836	58,572
Unrealized gains		12
Unrealized losses	(18)	(72)
Fair value	44,818	58,512
Corporate debt securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	20,064	67,206
Unrealized gains	1	1
Unrealized losses	(31)	(786)
Fair value	20,034	66,421
U.S. government securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	17,589	2,000
Unrealized losses	(126)	(35)
Fair value	17,463	$ 1,965
U.S. government agency securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized cost	7,990
Unrealized losses	(16)
Fair value	$ 7,974

Cash, cash equivalents and ma_5

Cash, cash equivalents and marketable securities (Contractual Final Maturities of Marketable Securities) (Details) $ in Thousands	Sep. 30, 2023 USD ($)
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Amortized Cost [Abstract]
Due within one year, Amortized Cost	$ 92,072
Due after one year through five years, Amortized Cost	7,321
Total, Amortized Cost	99,393
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Fair Value [Abstract]
Due within one year, Fair Value	91,982
Due after one year through five years, Fair Value	7,219
Total, Fair Value	$ 99,201

Fair Value of Financial Instr_3

Fair Value of Financial Instruments and Non-Financial (Recurring basis) (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 33,579	$ 34,601
Marketable securities	99,201	155,009
Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	8,912	28,111
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	44,818	58,512
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	20,034	66,421
U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	17,463	1,965
U.S. government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	7,974
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	27,347	24,578
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	998	2,491
Level 1 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	27,347	24,578
Total financial assets	27,347	24,578
Level 1 | Recurring | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	27,347	24,578
Level 2 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	998	2,491
Marketable securities	99,201	155,009
Total financial assets	100,199	157,500
Level 2 | Recurring | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	8,912	28,111
Level 2 | Recurring | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		2,491
Marketable securities	44,818	58,512
Level 2 | Recurring | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	20,034	66,421
Level 2 | Recurring | U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	17,463	$ 1,965
Level 2 | Recurring | U.S. government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	7,974
Level 2 | Recurring | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 998

Fair Value of Financial Instr_4

Fair Value of Financial Instruments and Non-Financial (Nonrecurring Basis) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property and equipment, net, impairment losses	$ 3,200	$ 0	$ 3,205	$ 0
Right of use assets, impairment losses	2,200	$ 0	2,185	$ 0
Total impairment losses	5,390		5,390
Level 3 | Nonrecurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property and equipment, net	1,367		1,367
Right of use assets	933		933
Total financial assets	$ 2,300		$ 2,300

Property and Equipment, net (De

Property and Equipment, net (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 22,495		$ 22,495		$ 27,668
Accumulated depreciation	(6,428)		(6,428)		(5,153)
Total, property plant and equipment net	16,067		16,067		22,515
Depreciation expense	1,000	$ 900	2,914	$ 2,733
Laboratory equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	10,042		10,042		9,972
Office equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	472		472		601
Computer hardware and software.
Property, Plant and Equipment [Line Items]
Total property and equipment	1,077		1,077		1,090
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Total property and equipment	419		419		1,208
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property and equipment	9,842		9,842		13,506
Construction in progress
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 643		$ 643		$ 1,291

Accrued Expenses and Other Cu_3

Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Accrued Expenses and Other Current Liabilities
Professional fees	$ 418	$ 602
Compensation and related benefits	6,232	8,446
Research and development	3,063	1,878
Property and equipment		85
Amount due to Catalent in connection with Letter Agreements	4,000
Accrued expenses and other current liabilities	$ 13,713	$ 11,011

Severance (Details)

Severance (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Jul. 31, 2023
Severance
Percentage of reduction in headcount					26%
Accrued expenses and other current liabilities | Employee Severance [Member]
Severance
Unpaid severance and termination-related expenses recognized	$ 0.4		$ 0.4
General and administrative | Employee Severance [Member]
Severance
Severance and termination-related expenses	1	$ 0	1	$ 1.8
Research and development | Employee Severance [Member]
Severance
Severance and termination-related expenses	$ 1.4		$ 1.4	$ 0.9

Leases (Details)

Leases (Details)			3 Months Ended		9 Months Ended
	Sep. 29, 2023 USD ($) ft²	Aug. 07, 2023 USD ($) ft²	Sep. 30, 2023 USD ($) ft² item lease	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($) ft² item lease	Sep. 30, 2022 USD ($)
Lessee, Lease, Description [Line Items]
Rent expense			$ 800,000	$ 800,000	$ 2,500,000	$ 2,500,000
Impairment of long-lived assets			5,390,000		5,390,000
Impairment expenses for right of use assets			2,200,000	0	2,185,000	0
Impairment expenses for property and equipment			3,200,000	$ 0	3,205,000	$ 0
Future minimum lease payments
2023			868,000		868,000
2024			3,553,000		3,553,000
2025			3,654,000		3,654,000
2026			3,757,000		3,757,000
2027			3,863,000		3,863,000
Thereafter			29,679,000		29,679,000
Total Undiscounted lease payments			45,374,000		45,374,000
Less: imputed interest			(18,856,000)		(18,856,000)
Total Lease Liabilities			$ 26,518,000		$ 26,518,000
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 | lease			2		2
Extension term			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		$ 12,426
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	$ 75,000
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

Commitments and Contingencies -

Commitments and Contingencies - Collaboration Agreements (Details) $ in Millions		1 Months Ended			3 Months Ended	9 Months Ended	10 Months Ended
	Aug. 03, 2020 USD ($) Option	Aug. 31, 2021	Apr. 30, 2020 USD ($)	Jun. 30, 2019	Sep. 30, 2023 USD ($)	Sep. 30, 2023 USD ($)	May 01, 2024 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Development milestone payment					$ 0	$ 0
Catalent Maryland, Inc. (Catalent)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Annual fee payable, term				5 years
Period for which annual fee is payable			5 years
Term of manufacturing and supply agreement			5 years
Minimum amount agreed to purchase in batches of drug product			$ 10.6
Accrued payments on collaboration					4	4
Prepaid assets related to upfront payments					5.3	5.3
Payments made						2
Catalent Maryland, Inc. (Catalent) | General and administrative
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Expenses on elimination of prepaid asset						5.3
Expense on write off						11.3
Expense on write off of aggregate payments						6
Catalent Maryland, Inc. (Catalent) | Subsequent Events
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Aggregate payments on collaboration							$ 6
Amended and restated agreement with Penn
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Annual fee payable	$ 5
Quarterly fee payable	$ 1.3
Number of CNS indications | Option	8
Upfront payments	$ 1
Upfront fee payable upon clinical candidate designation	0.5
Upfront fee due	$ 0.5
Payment made on option exercise					1.5	3
Amount payable per product candidate arising from the exploratory program for large CNS indications					39	39
Sales milestone payments					55	55
Amended and restated agreement with Penn | Minimum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, tiered transaction fee percent		1%
Amended and restated agreement with Penn | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration agreement, tiered transaction fee percent		2%
Amount payable per product candidate for rare, monogenic disorders					$ 16.5	$ 16.5

Share-Based Compensation - Equi

Share-Based Compensation - Equity Incentive Plan & ESPP (Details)				3 Months Ended		9 Months Ended
	Jan. 01, 2023 shares	Jan. 01, 2022 shares	Feb. 28, 2020 USD ($) shares	Sep. 30, 2023 USD ($) shares	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($) plan shares	Sep. 30, 2022 USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity incentive plans | plan						3
Number of shares purchased			4,000
Share-based compensation expense | $				$ 2,824,000	$ 2,862,000	$ 9,351,000	$ 15,477,000
Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized				13,101,661		13,101,661
Shares available for future grant				7,644,421		7,644,421
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%
Term of award						10 years
Vesting period						4 years
Additional number of common shares reserved for future issuance	2,730,735	2,712,249
Equity Inducement Plan 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized				2,500,000		2,500,000
Shares available for future grant				1,006,655		1,006,655
Term of award						10 years
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized	1,981,766		1,981,766
Shares available for future grant				1,565,821		1,565,821
Additional number of common shares reserved for future issuance	546,147
Period till which there is automatic increase in rights granted under ESPP			10 years
Percentage of common stock outstanding			1%
Percentage of stock purchase price			85%
Percentage of maximum employee contribution			15%
Maximum amount of common stock that can be purchased | $			$ 25,000
Share-based compensation expense | $						$ 0	$ 200,000

Share-Based Compensation - Shar

Share-Based Compensation - Share-based compensation expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 2,824	$ 2,862	$ 9,351	$ 15,477
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	1,429	568	4,788	6,528
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 1,395	$ 2,294	$ 4,563	$ 8,949

Share-Based Compensation - Stoc

Share-Based Compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Number of shares
Outstanding, Beginning of period	11,411,390
Granted	3,348,982
Forfeited	(2,256,315)
Expired	(2,264,006)
Outstanding, Ending of period	10,240,051		11,411,390
Vested and Exercisable at December 31, 2022	4,935,214
Vested or expected to vest at December 31, 2022	10,240,051
Weighted average exercise price per share
Outstanding, Beginning of period	$ 9.01
Granted	1.06
Forfeited	8.58
Expired	12.50
Outstanding, Ending of period	5.73		$ 9.01
Vested and Exercisable at December 31, 2022	8.43
Vested or expected to vest at December 31, 2022	$ 5.73
Weighted average remaining contractual term (years)
Outstanding	8 years 2 months 12 days		7 years 1 month 6 days
Vested and Exercisable at December 31, 2022	7 years 4 months 24 days
Vested or expected to vest at December 31, 2022	8 years 2 months 12 days
Number of share options unvested and exercisable during the period	3,426
Weighted average grant date fair value of options granted	$ 0.82	$ 2.52
Unrecognized compensation expense related to unvested stock option awards	$ 12.4
Stock option
Weighted average remaining contractual term (years)
Weighted average period for recognition	1 year 10 months 24 days

Share-Based Compensation - Assu

Share-Based Compensation - Assumptions Used in Determining Fair Value & Early exercise of Stock Options (Details) - Stock option	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Expected volatility	92.60%	92.30%
Risk-free interest rate	3.60%	2.60%
Expected term	6 years	5 years 9 months 18 days

Share-Based Compensation - Rest

Share-Based Compensation - Restricted Stock Units (Details) - Unvested restricted stock units - USD ($) $ / shares in Units, $ in Millions	9 Months Ended
	Sep. 30, 2023	Dec. 31, 2022
Number of shares
Unvested balance, Beginning	1,229,166
Granted	195,000
Vested	(98,042)
Forfeited	(179,625)
Unvested balance, Ending	1,146,499
Weighted average grant date fair value
Unvested balance, Weighted average grant date fair value, Beginning	$ 2.58	$ 2.98
Granted, Weighted average grant date fair value	1.01
Vested, Weighted average grant date fair value	5.82
Forfeited, Weighted average grant date fair value	2.13
Unvested balance, Weighted average grant date fair value, Ending	$ 2.58
Total unrecognized expense	$ 1.5
Weighted average period for recognition	1 year 1 month 6 days