Passage Bio (PASG) 10-Q 13 Aug 20 2020 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2020	Aug. 11, 2020
Document and Entity Information
Document Type	10-Q
Document Period End Date	Jun. 30, 2020
Entity Registrant Name	Passage BIO, Inc.
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		45,486,601
Entity Central Index Key	0001787297
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q2
Amendment Flag	false

Balance Sheets (unaudited)

Balance Sheets (unaudited) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 353,423	$ 158,874
Prepaid expenses	2,176	156
Prepaid research and development	12,631	6,745
Total current assets	368,230	165,775
Property and equipment, net	1,096	1,087
Other assets	8,771	11,751
Total assets	378,097	178,613
Current liabilities:
Accounts payable	7,214	629
Accrued expenses and other current liabilities	6,920	3,052
Total current liabilities	14,134	3,681
Deferred rent	524	504
Other liabilities	43	76
Total liabilities	14,701	4,261
Convertible preferred stock, $0.0001 par value:
Total convertible preferred stock		230,605
Commitments (note 7)
Stockholders’ equity (deficit):
Common stock, $0.0001 par value: 300,000,000 shares authorized; 45,797,195 shares issued and 45,350,687 shares outstanding at March 31, 2020 and 5,194,518 shares issued and 4,293,039 shares outstanding at December 31, 2019	4
Additional paid‑in capital	466,812	2,410
Accumulated deficit	(103,420)	(58,663)
Total stockholders’ equity (deficit)	363,396	(56,253)
Total liabilities, convertible preferred stock and stockholders’ equity (deficit)	$ 378,097	178,613
Series A-1 Convertible Preferred Stock
Convertible preferred stock, $0.0001 par value:
Total convertible preferred stock		74,397
Series A-2 Convertible Preferred Stock
Convertible preferred stock, $0.0001 par value:
Total convertible preferred stock		46,311
Series B Convertible Preferred Stock
Convertible preferred stock, $0.0001 par value:
Total convertible preferred stock		$ 109,897

Balance Sheets (unaudited) (Par

Balance Sheets (unaudited) (Parenthetical) - $ / shares	Jun. 30, 2020	Dec. 31, 2019
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	45,841,971	5,194,518
Common stock, shares outstanding	45,443,541	4,293,039
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Series A-1 Convertible Preferred Stock
Convertible preferred stock, shares authorized		63,023,258
Convertible preferred stock, shares, issued		63,023,258
Convertible preferred stock, shares, outstanding		63,023,258
Series A-2 Convertible Preferred Stock
Convertible preferred stock, shares authorized		22,209,301
Convertible preferred stock, shares, issued		22,209,301
Convertible preferred stock, shares, outstanding		22,209,301
Series B Convertible Preferred Stock
Convertible preferred stock, shares authorized		33,592,907
Convertible preferred stock, shares, issued		33,592,907
Convertible preferred stock, shares, outstanding		33,592,907

Statements of Operations (Unaud

Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Operating expenses:
Research and development	$ 19,902	$ 6,299	$ 33,019	$ 9,332
Acquired in‑process research and development		500		500
General and administrative	7,402	968	12,197	2,122
Loss from operations	(27,304)	(7,767)	(45,216)	(11,954)
Change in fair value of future tranche right liability		(5,659)		(9,141)
Interest income	132		459
Net loss	$ (27,172)	$ (13,426)	$ (44,757)	$ (21,095)
Per share information:
Net loss per share of common stock, basic and diluted	$ (0.60)	$ (3.19)	$ (1.42)	$ (5.02)
Weighted average common shares outstanding, basic and diluted	45,386,308	4,209,716	31,581,851	4,203,694

Statements of Changes in Conver

Statements of Changes in Convertible Preferred Stock and Stockholders’ Equity (Deficit) - USD ($) $ in Thousands	Preferred Stock [Member]Series A-1 Convertible Preferred Stock	Preferred Stock [Member]Series A-2 Convertible Preferred Stock	Preferred Stock [Member]Series B Convertible Preferred Stock	Common stock	Additional paid‑in capital	Accumulated Deficit	Series A-1 Convertible Preferred Stock	Series A-2 Convertible Preferred Stock	Series B Convertible Preferred Stock	Total
Balance at Beginning of period at Dec. 31, 2018	$ 43,118
Balance at Beginning of period (in shares) at Dec. 31, 2018	44,418,606
Temporary equity
Sale of convertible preferred stock, net of issuance costs	$ 19,981	$ 46,311
Sale of convertible preferred stock, net of issuance costs (in shares)	18,604,652	22,209,301
Reclassification of future tranche right upon exercise	$ 11,298									$ 11,298
Balance at end of period at Jun. 30, 2019	$ 74,397	$ 46,311
Balance at end of period (in shares) at Jun. 30, 2019	63,023,258	22,209,301
Balance at Beginning of period at Dec. 31, 2018					$ 856	$ (13,029)				(12,173)
Balance at Beginning of period (in shares) at Dec. 31, 2018				4,195,649
Stockholders' equity
Vesting of early exercise option awards					14					14
Vesting of early exercise option awards (in shares)				14,666
Share‑based compensation expense					323					323
Net loss						(21,095)				(21,095)
Balance at end of period at Jun. 30, 2019					1,193	(34,124)				(32,931)
Balance at end of period (in shares) at Jun. 30, 2019				4,210,315
Balance at Beginning of period at Mar. 31, 2019	$ 63,099
Balance at Beginning of period (in shares) at Mar. 31, 2019	63,023,258
Temporary equity
Sale of convertible preferred stock, net of issuance costs		$ 46,311
Sale of convertible preferred stock, net of issuance costs (in shares)		22,209,301
Reclassification of future tranche right upon exercise	$ 11,298
Balance at end of period at Jun. 30, 2019	$ 74,397	$ 46,311
Balance at end of period (in shares) at Jun. 30, 2019	63,023,258	22,209,301
Balance at Beginning of period at Mar. 31, 2019					1,100	(20,698)				(19,598)
Balance at Beginning of period (in shares) at Mar. 31, 2019				4,209,187
Stockholders' equity
Vesting of early exercise option awards					1					1
Vesting of early exercise option awards (in shares)				1,128
Share‑based compensation expense					92					92
Net loss						(13,426)				(13,426)
Balance at end of period at Jun. 30, 2019					1,193	(34,124)				(32,931)
Balance at end of period (in shares) at Jun. 30, 2019				4,210,315
Balance at Beginning of period at Dec. 31, 2019	$ 74,397	$ 46,311	$ 109,897				$ 74,397	$ 46,311	$ 109,897	230,605
Balance at Beginning of period (in shares) at Dec. 31, 2019	63,023,258	22,209,301	33,592,907				63,023,258	22,209,301	33,592,907
Temporary equity
Conversion of convertible preferred stock upon initial public offering	$ (74,397)	$ (46,311)	$ (109,897)
Conversion of convertible preferred stock upon initial public offering (in shares)	(63,023,258)	(22,209,301)	(33,592,907)
Balance at Beginning of period at Dec. 31, 2019					2,410	(58,663)				(56,253)
Balance at Beginning of period (in shares) at Dec. 31, 2019				4,293,039
Stockholders' equity
Vesting of early exercise option awards					33					33
Vesting of early exercise option awards (in shares)				503,049
Exercise of stock options					46					$ 46
Exercise of stock options (in shares)				44,776						44,776
Sale of common stock upon consummation of initial public offering, net of issuance costs				$ 1	227,498					$ 227,499
Sale of common stock upon consummation of initial public offering, net of issuance costs (in shares)				13,798,900
Conversion of convertible preferred stock upon initial public offering				$ 3	230,602					230,605
Conversion of convertible preferred stock upon initial public offering (in shares)				26,803,777
Share‑based compensation expense					6,223					6,223
Net loss						(44,757)				(44,757)
Balance at end of period at Jun. 30, 2020				$ 4	466,812	(103,420)				363,396
Balance at end of period (in shares) at Jun. 30, 2020				45,443,541
Balance at Beginning of period at Mar. 31, 2020				$ 4	462,910	(76,248)				386,666
Balance at Beginning of period (in shares) at Mar. 31, 2020				45,350,687
Stockholders' equity
Vesting of early exercise option awards					1					1
Vesting of early exercise option awards (in shares)				48,078
Exercise of stock options					46					46
Exercise of stock options (in shares)				44,776
Share‑based compensation expense					3,855					3,855
Net loss						(27,172)				(27,172)
Balance at end of period at Jun. 30, 2020				$ 4	$ 466,812	$ (103,420)				$ 363,396
Balance at end of period (in shares) at Jun. 30, 2020				45,443,541

Statements of Changes in Conv_2

Statements of Changes in Convertible Preferred Stock and Stockholders’ Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Issuance cost		$ 3,495
Series A-1 Convertible Preferred Stock
Issuance cost			$ 19
Series A-2 Convertible Preferred Stock
Issuance cost	$ 1,439		$ 1,439

Statements of Cash Flows (Unaud

Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Cash flows used in operating activities:
Net loss	$ (44,757)	$ (21,095)
Adjustments to reconcile net loss to net cash used in operating activities:
Change in fair value of future tranche right liability		9,141
Acquired in‑process research and development		500
Depreciation and amortization	247	48
Share‑based compensation	6,223	323
Deferred rent	20	481
Changes in operating assets and liabilities:
Prepaid expenses and other assets	197	46
Prepaid research and development	(5,886)	(1,513)
Accounts payable	6,522	(104)
Accrued expenses	3,868	1,179
Net cash used in operating activities	(33,566)	(10,994)
Cash flows used in investing activities:
Purchase of technology licenses		(500)
Purchases of property and equipment	(193)	(1,041)
Net cash used in investing activities	(193)	(1,541)
Cash flows provided by financing activities:
Proceeds from issuance of common stock upon initial public offering	228,262
Deferred offering costs		(5)
Proceeds from early exercise stock options	46	176
Net cash provided by financing activities	228,308	66,463
Net increase in cash and cash equivalents	194,549	53,928
Cash and cash equivalents at beginning of year	158,874	24,861
Cash and cash equivalents at end of year	353,423	78,789
Supplemental disclosure of non‑cash investing and financing activities:
Reclassification of deferred offering costs paid in a prior period	763
Reclassification of the future tranche right liability upon exercise		11,298
Property and equipment in accounts payable	63	15
Vesting of early exercise option awards	$ 33	14
Series A-1 Convertible Preferred Stock
Cash flows provided by financing activities:
Proceeds from Issuance of Convertible Preferred Stock		19,981
Series A-2 Convertible Preferred Stock
Cash flows provided by financing activities:
Proceeds from Issuance of Convertible Preferred Stock		$ 46,311

Nature of Operations

Nature of Operations	6 Months Ended
	Jun. 30, 2020
Nature of Operations
Nature of Operations	1. Nature of Operations Passage Bio, Inc. (the Company), a Delaware corporation incorporated in July 2017, is a genetic medicines company focused on advancing transformative therapies for rare monogenic central nervous system diseases. The Company has a strategic research collaboration with the Trustees of the University of Pennsylvania’s (Penn’s), Gene Therapy Program (GTP) that provides the Company with access to one of the premier research institutions in the world for the discovery and preclinical development of genetic medicine product candidates and exclusive rights to certain rare, monogenic central nervous system (CNS) indications. Under this collaboration, GTP conducts discovery and investigation new drug enabling preclinical activities and the Company conducts all clinical development, regulatory strategy, and commercialization activities under the agreement. The Company also has a collaboration agreement and a development services and clinical supply agreement with Catalent Maryland, Inc. (Catalent) (formerly Paragon Bioservices, Inc.) for clinical scale manufacturing requirements.

Risks and Liquidity

Risks and Liquidity	6 Months Ended
	Jun. 30, 2020
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 $103.4 million as of June 30, 2020. 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. In March 2020, the Company closed its initial public offering (IPO) in which the Company issued and sold 13,798,900 shares of its common stock, which included shares sold pursuant to an option granted to the underwriters to purchase additional shares, at a public offering price of $18.00 per share for net proceeds of $227.5 million after deducting underwriting discounts, commissions and other offering expenses. The Company’s operations have consisted primarily of organizing the Company, securing financing, developing licensed technology, performing research and conducting preclinical studies. 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, including extensive preclinical and clinical testing 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 significant 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, the Company could be required to delay, reduce or eliminate research and development programs, product portfolio expansion or future commercialization efforts, which could adversely affect its business prospects.

Summary of Significant Accounti

Summary of Significant Accounting Policies	6 Months Ended
	Jun. 30, 2020
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 prospectus filed with the SEC on February 27, 2020. Basis of Presentation The accompanying financial statements have been prepared in accordance with accounting principles generally accepted in the United States (GAAP). Any reference in these notes to applicable guidance is meant to refer to GAAP as found in the Accounting Standards Codification (ASC) and Accounting Standards Updates (ASU) promulgated by the Financial Accounting Standards Board (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 Securities and Exchange Commission (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, convertible preferred stock and stockholders’ equity (deficit), 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 December 31, 2019 financial statements and footnotes included in the prospectus as filed with the SEC on February 27, 2020. 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. Prior to the IPO, significant areas that required management’s estimates included the fair value of the Company’s future tranche right liability and its common stock. After the IPO, the most significant judgements are used in estimates to determine the fair value of stock options issued. 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. The future tranche right liability was recorded at its estimated fair value. Share-based compensation 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. Estimating the fair value of share-based awards requires the input of subjective assumptions, including, prior to the IPO, the estimated fair value of the Company's common stock, and, for stock options, the expected life of the options and stock price volatility. The Company accounts for forfeitures for stock option awards as they occur. The Company uses the Black-Scholes option pricing model to value its stock option awards. 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 life 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 comparable public companies as a basis for its expected volatility to calculate the fair value of option grants. The risk-free rate is based on the U.S. Treasury yield curve commensurate with the expected life of the option. 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 convertible preferred stock, which would result in the issuance of incremental shares of common stock. For diluted net loss per share, the weighted‑average number of shares of common stock is the same for basic net loss per share due to the fact that when a net loss exists, dilutive securities are not included in the calculation as the impact is anti‑dilutive. The following potentially dilutive securities have been excluded from the computation of diluted weighted‑average shares of common stock outstanding, as they would be anti‑dilutive: Six Months Ended June 30, 2020 2019 Series A‑1 convertible preferred stock — 14,216,333 Series A‑2 convertible preferred stock — 5,009,808 Stock options (including shares subject to repurchase) 6,759,369 1,380,969 Stock options vested and exercised, but subject to settlement of nonrecourse promissory notes — 279,863 Employee stock purchase plan 7,248 — 6,766,617 20,886,973 Amounts in the above table reflect the common stock equivalents. Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016‑02, Leases , which requires a lessee to record a right‑of‑use asset and a corresponding lease liability on the balance sheet for all leases with terms longer than 12 months. A modified retrospective transition approach is required for lessees for capital and operating leases existing at, or entered into after, the beginning of the earliest comparative period presented in the financial statements, with certain practical expedients available. As the Company has elected to use the extended transition period for complying with new or revised accounting standards as available under the Jobs Act, the standard is effective for the Company beginning January 1, 2022, with early adoption permitted. The Company is currently evaluating the expected impact that the standard could have on its financial statements and related disclosures.

Fair Value of Financial Instrum

Fair Value of Financial Instruments	6 Months Ended
	Jun. 30, 2020
Fair Value of Financial Instruments
Fair Value of Financial Instruments	4. Fair Value of 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 , for financial assets and liabilities measured on a recurring basis. The guidance requires fair value measurements be classified and disclosed in one of the following three categories: · Level 1: Unadjusted quoted prices in active markets that are accessible at the measurement date for identical, unrestricted assets or liabilities. · Level 2: Quoted prices in markets that are not active, or inputs which are observable, either directly or indirectly, for substantially the full term of the asset or liabilities. · Level 3: Prices or valuation techniques that require inputs that are both significant to the fair value measurement and unobservable (i.e., supported by little or no market activity). The following fair value hierarchy table presents information about the Company’s assets measured at fair value on a recurring basis: Fair value measurement at reporting date using Quoted prices in active Significant markets for other Significant identical observable unobservable assets inputs inputs (in thousands) (Level 1) (Level 2) (Level 3) June 30, 2020: Assets: Cash equivalents (Money Market Fund) $ 284,305 $ — $ — December 31, 2019: Assets: Cash equivalents (Money Market Fund) $ 155,846 $ — $ — The Company evaluated the future tranche right feature within the Series A-1 convertible preferred stock issued in 2018 and determined that the future tranche right was a freestanding financial instrument that was classified as a liability and was re-measured at each reporting period until the redemption feature was exercised in connection with the sale and issuance of the Series A-2 convertible preferred stock in May 2019.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	6 Months Ended
	Jun. 30, 2020
Accrued Expenses and Other Current Liabilities
Accrued Expenses and Other Current Liabilities	5. Accrued Expenses and Other Current Liabilities Accrued expenses and other current liabilities consisted of the following: (in thousands) June 30, 2020 December 31, 2019 Professional fees $ 196 $ 997 Compensation and related benefits 2,103 1,502 Research and development 4,556 507 Other 65 46 $ 6,920 $ 3,052

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
	Jun. 30, 2020
Commitments and Contingencies.
Commitments and Contingencies	6. Commitments and Contingencies Amended and Restated Research, Collaboration and License Arrangement with Penn In May 2020, the Company amended and restated its research, collaboration and licensing agreement with Penn (the Penn Agreement) for research and development collaborations and exclusive license rights to patents for certain products and technologies, which superseded the Company’s existing sponsored research, collaboration and licensing agreement with Penn, dated September 18, 2018, as amended. Under the Penn Agreement, the Company will fund discovery research conducted by Penn for five years, beginning in May 2020, 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 is $5.0 million annually, with quarterly payments of $1.3 million beginning in the third quarter of 2020. The Penn Agreement also increased the number of remaining options available to the Company to commence additional licensed programs for rare, monogenic CNS indications from six to eleven, and extended the option exercise window by three years. Accordingly, the window to exercise all eleven remaining options extends to May 2025. 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. Catalent Agreements In June 2019, the Company entered into a collaboration agreement (the Catalent Collaboration Agreement) with Catalent . As part of the Catalent Collaboration Agreement, the Company paid Catalent an upfront fee for the commissioning, qualification, validation and equipping of a clean room suite (the Clean Room Suite). Subject to validation of the Clean Room Suite, which is expected to occur by the end of 2020, the Company will pay an annual fee for five years for the use of the Clean Room Suite and is also committed to minimum annual purchase commitments. In April 2020, the Company entered into a development services and clinical supply agreement (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 Catalent Collaboration Agreement. The Catalent Collaboration Agreement continues to be in effect pursuant to its terms. Under the terms of the Manufacturing and Supply Agreement, Catalent has 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 Catalent Collaboration Agreement. The Manufacturing and Supply Agreement provides for a term of five years which period may be extended once, at the Company’s option, for an additional five year-period. 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 has an annual minimum commitment of $10.6 million per year owed to Catalent for five years from the validation of the Clean Room, subject to certain inflationary adjustments. Operating Leases The Company leases office space in Philadelphia, Pennsylvania under a noncancelable lease (Existing Lease Agreement), as amended. The lease is classified as an operating lease and the Company recognizes rent expense on a straight‑line basis over the lease term. In April 2020, the Company entered into a new lease agreement (New Lease Agreement) for larger office space in Philadelphia to accommodate the Company’s continued growth and serve as the new corporate headquarters. The New Lease Agreement is expected to commence in March 2021 and is expected to expire in January 2031. The Company has an option to extend the term of the New Lease Agreement by up to two five-year terms. Upon signing the New Lease Agreement, the Company amended the Existing Lease Agreement such that the Existing Lease Agreement will terminate five days after the commencement of the New Lease Agreement with no further payments due under the Existing Lease Agreement. The landlord also will provide the Company with a tenant improvement allowance of up to $2.8 million. The future minimum lease payments under the Company’s New Lease Agreement are as follows: (in thousands) 2021 $ — 2022 889 2023 1,098 2024 1,131 2025 1,165 Thereafter 7,763 $ 12,046 The estimated future minimum lease payments under the Company’s Existing Lease Agreement as of June 30, 2020 is $0.1 million. The Company recognized rent expense of $0.1 million and $36,000 during the three months ended June 30, 2020 and 2019, respectively, and $0.1 million and $87,000 during the six months ended June 30, 2020 and 2019, respectively, related to its operating leases. Employment Agreements The Company has entered into employment agreements with key personnel providing for compensation and severance in certain circumstances, as described in the respective employment agreements. Patent Infringement Claim On February 18, 2020, the Company received a letter from Regenxbio Inc. (Regenx), which stated its view that the use of the Company’s AAVhu68 capsid infringes patent claims to which Regenx has an exclusive license and which expire in 2024. Regenx also stated that it has exclusive licenses to various pending patent applications regarding the use of AAV vectors administered via instar-cisterna magna injection, and that these applications may lead to issued claims that Regenx believes may, if issued, cover the Company’s planned method of administration for the Company’s lead product candidates. The Company believes it has valid defenses to the issued claims set forth by Regenx relating to AAVhu68. Further, the prosecution of pending patent applications is highly uncertain, and it is unclear whether any patents will be issued from these pending Regenx patent applications at all, much less with claims that are relevant to the administration of the Company’s product candidates. Regenx also asked for information regarding the Company’s relationship with Dr. Wilson while he was serving as an advisor to Regenx. Regenx's letter also offers to discuss licensing the applicable patent portfolios from them. In April 2020, the Company responded to Regenx indicating that it does not believe it requires a license to any of the specified Regenx patents or patent applications at this time, and that it found that Dr. Wilson’s relationship with the Company was consistent with his obligations to Regenx. The Company will continue to monitor the situation and, if necessary, take appropriate actions, which may include responding to further correspondence from Regenx, and engaging in discussions with Regenx regarding their claims. If any such patents were enforceable and such claims were ultimately successful, the Company might require a license to continue to use and sell any product candidates using such AAV vector.

Convertible Preferred Stock and

Convertible Preferred Stock and Common Stock	6 Months Ended
	Jun. 30, 2020
Convertible Preferred Stock and Common Stock
Convertible Preferred Stock and Common Stock	7. Convertible Preferred Stock and Common Stock Initial Public Offering In March 2020, the Company completed its IPO in which the Company sold 13,798,900 shares of its common stock, which included shares sold pursuant to an option granted to the underwriters to purchase additional shares, at a public offering price of $18.00 per share. The Company received net proceeds of $227.5 million after deducting underwriting discounts, commissions, and other offering expenses paid by the Company. In addition, immediately prior to the initial closing of the IPO on March 3, 2020, (i) all of the Company’s outstanding shares of convertible preferred stock converted into an aggregate of 26,803,777 shares of common stock and (ii) the Company filed an amended and restated certificate of incorporation to, among other things, increase the number of authorized shares of common stock to 300.0 million.

Share_Based Compensation

Share‑Based Compensation	6 Months Ended
	Jun. 30, 2020
Share‑Based Compensation
Share‑Based Compensation	8. Share‑Based Compensation Equity Incentive Plan The Company has two equity incentive plans: the 2018 Equity Incentive Plan, as amended, and the 2020 Equity Incentive Plan. New awards can only be granted under the 2020 Equity Incentive Plan (the Plan). The total number of shares authorized under the Plan as of June 30, 2020 was 5,362,823. Of this amount, 3,942,905 shares were available for future grants as of June 30, 2020. The number of shares of the Company’s common stock that may be issued pursuant to rights granted under the 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. The 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 vest based on the terms in each award agreement, generally over four‑year periods, and have a term of ten years. 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 June 30, Six Months Ended June 30, (in thousands) 2020 2019 2020 2019 Research and development $ 1,309 $ 22 $ 2,356 $ 26 General and administrative 2,546 70 3,867 297 $ 3,855 $ 92 $ 6,223 $ 323 During the six months ended June 30, 2020, the Company modified certain awards and recognized an additional $0.7 million related to the modifications, $0.6 million of which was recognized in research and development expense and $0.1 million was recognized in general and administrative expense. The following table summarizes stock option activity for the six months ended June 30, 2020: Weighted Weighted average average remaining Number of exercise price contractual shares per share term (years) Outstanding at January 1, 2020 2,577,719 $ 5.90 9.3 Granted 3,962,327 14.61 Exercised (44,776) 1.02 Forfeited (134,331) 1.02 Outstanding at June 30, 2020 6,360,939 $ 11.46 9.5 Exercisable at June 30, 2020 397,421 $ 5.26 9.0 Vested or expected to vest at June 30, 2020 6,360,939 $ 11.46 9.5 The weighted‑average grant date fair value of options granted was $11.09 and $0.76 for the six months ended June 30, 2020 and 2019, respectively. As of June 30, 2020, the total unrecognized compensation expense related to unvested stock option awards was $47.4 million, which the Company expects to recognize over a weighted‑average period of 3.4 years. The fair value of each option was estimated on the date of grant using the weighted average assumptions in the table below: Six Months Ended June 30, 2020 2019 Expected volatility 95.2% 87.1% Risk‑free interest rate 1.3% 2.4% Expected term 6.06 years 5.57 years Expected dividend yield — — The Plan provides 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. At June 30, 2020, $43,000 of proceeds from unvested early exercised options were recognized as a non‑current liability in other liabilities in the accompanying balance sheet. The 2018 Equity Incentive Plan allowed for the exercise of options to be financed with nonrecourse notes. For accounting purposes, payment of principal and interest are viewed as the exercise price of the option. Therefore, no interest income was recognized. The following table summarizes activity relating to early exercises of stock options during the six months ended June 30, 2020: Number of shares Unvested balance at January 1, 2020 494,603 Vested (96,173) Unvested balance at June 30, 2020 398,430 Nonrecourse Promissory Notes with Related Parties In February 2019, the Company’s interim chief executive officer and chief operating officer elected to early exercise 688,875 and 309,994 stock options, respectively, in exchange for cash proceeds of $0.2 million and nonrecourse promissory notes (the Notes) of $0.8 million. The Notes bore interest at 2.91% and were secured by the underlying shares of common stock that were issued. In January 2020, the Company forgave the Notes and associated interest related to the early exercise of stock options by the interim chief executive officer and chief operating officer. An aggregate of 406,894 shares that were previously not considered outstanding for accounting purposes due to being secured by the Notes became outstanding upon the forgiveness of the Notes in January 2020. Employee Stock Purchase Plan The Company’s 2020 Employee Stock Purchase Plan (the ESPP) became effective on February 28, 2020. The ESPP authorizes the issuance of up to 434,000 shares of the Company’s common stock. 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, commencing on January 1, 2021 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. 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. 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. Effective March 3, 2020, employees who elected to participate in the ESPP commenced payroll withholdings that accumulate through November 15, 2020. 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 recognized share-based compensation expense of $62,000 and $85,000 during the three and six months ended June 30, 2020 related to the ESPP.

Related Party Transactions

Related Party Transactions	6 Months Ended
	Jun. 30, 2020
Related Party Transactions
Related Party Transactions	9. Related-Party Transactions Penn Agreement Penn is a stockholder of the Company. Research and development expenses with Penn during the three months ended June 30, 2020 and 2019 were $10.2 million and $6.2 million, respectively. Research and development expenses with Penn during the six months ended June 30, 2020 and 2019 were $17.9 million and $9.1 million, respectively. The Company made $16.3 million in cash payments to Penn during the six months ended June 30, 2020, and had a prepaid research and development asset of $10.5 million and $5.7 million as of June 30, 2020 and December 31, 2019, respectively, in the accompanying balance sheets. Consulting Agreement James M. Wilson, M.D., Ph.D., an employee of Penn and a stockholder of the Company, serves as the Company’s chief scientific advisor pursuant to a consulting agreement. The Company recognized $31,000 and $62,000 of expense related to these services during the three and six months ended June 30, 2020, including $6,000 and $12,000, respectively, of share‑based compensation expense.

Subsequent Events

Subsequent Events	6 Months Ended
	Jun. 30, 2020
Subsequent Events [Abstract]
Subsequent Events	10. Subsequent Events None.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2020
Summary of Significant Accounting Policies
Basis of Presentation	Basis of Presentation The accompanying financial statements have been prepared in accordance with accounting principles generally accepted in the United States (GAAP). Any reference in these notes to applicable guidance is meant to refer to GAAP as found in the Accounting Standards Codification (ASC) and Accounting Standards Updates (ASU) promulgated by the Financial Accounting Standards Board (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 Securities and Exchange Commission (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, convertible preferred stock and stockholders’ equity (deficit), 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 December 31, 2019 financial statements and footnotes included in the prospectus as filed with the SEC on February 27, 2020.
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. Prior to the IPO, significant areas that required management’s estimates included the fair value of the Company’s future tranche right liability and its common stock. After the IPO, the most significant judgements are used in estimates to determine the fair value of stock options issued.
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. The future tranche right liability was recorded at its estimated fair value.
Share-based compensation	Share-based compensation 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. Estimating the fair value of share-based awards requires the input of subjective assumptions, including, prior to the IPO, the estimated fair value of the Company's common stock, and, for stock options, the expected life of the options and stock price volatility. The Company accounts for forfeitures for stock option awards as they occur. The Company uses the Black-Scholes option pricing model to value its stock option awards. 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 life 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 comparable public companies as a basis for its expected volatility to calculate the fair value of option grants. The risk-free rate is based on the U.S. Treasury yield curve commensurate with the expected life of the option.
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 convertible preferred stock, which would result in the issuance of incremental shares of common stock. For diluted net loss per share, the weighted‑average number of shares of common stock is the same for basic net loss per share due to the fact that when a net loss exists, dilutive securities are not included in the calculation as the impact is anti‑dilutive. The following potentially dilutive securities have been excluded from the computation of diluted weighted‑average shares of common stock outstanding, as they would be anti‑dilutive: Six Months Ended June 30, 2020 2019 Series A‑1 convertible preferred stock — 14,216,333 Series A‑2 convertible preferred stock — 5,009,808 Stock options (including shares subject to repurchase) 6,759,369 1,380,969 Stock options vested and exercised, but subject to settlement of nonrecourse promissory notes — 279,863 Employee stock purchase plan 7,248 — 6,766,617 20,886,973 Amounts in the above table reflect the common stock equivalents.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016‑02, Leases , which requires a lessee to record a right‑of‑use asset and a corresponding lease liability on the balance sheet for all leases with terms longer than 12 months. A modified retrospective transition approach is required for lessees for capital and operating leases existing at, or entered into after, the beginning of the earliest comparative period presented in the financial statements, with certain practical expedients available. As the Company has elected to use the extended transition period for complying with new or revised accounting standards as available under the Jobs Act, the standard is effective for the Company beginning January 1, 2022, with early adoption permitted. The Company is currently evaluating the expected impact that the standard could have on its financial statements and related disclosures.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	6 Months Ended
	Jun. 30, 2020
Summary of Significant Accounting Policies
Schedule of antidilutive securities excluded from computation of earnings per share	Six Months Ended June 30, 2020 2019 Series A‑1 convertible preferred stock — 14,216,333 Series A‑2 convertible preferred stock — 5,009,808 Stock options (including shares subject to repurchase) 6,759,369 1,380,969 Stock options vested and exercised, but subject to settlement of nonrecourse promissory notes — 279,863 Employee stock purchase plan 7,248 — 6,766,617 20,886,973

Fair Value of Financial Instr_2

Fair Value of Financial Instruments (Tables)	6 Months Ended
	Jun. 30, 2020
Fair Value of Financial Instruments
Summary of assets measured at fair value on a recurring basis	Fair value measurement at reporting date using Quoted prices in active Significant markets for other Significant identical observable unobservable assets inputs inputs (in thousands) (Level 1) (Level 2) (Level 3) June 30, 2020: Assets: Cash equivalents (Money Market Fund) $ 284,305 $ — $ — December 31, 2019: Assets: Cash equivalents (Money Market Fund) $ 155,846 $ — $ —

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	6 Months Ended
	Jun. 30, 2020
Accrued Expenses and Other Current Liabilities
Schedule of accrued liabilities and other current liabilities	(in thousands) June 30, 2020 December 31, 2019 Professional fees $ 196 $ 997 Compensation and related benefits 2,103 1,502 Research and development 4,556 507 Other 65 46 $ 6,920 $ 3,052

Commitments and Contingencies (

Commitments and Contingencies (Tables)	6 Months Ended
	Jun. 30, 2020
Commitments and Contingencies.
Schedule of future minimum rental payments for operating leases	(in thousands) 2021 $ — 2022 889 2023 1,098 2024 1,131 2025 1,165 Thereafter 7,763 $ 12,046

Share_Based Compensation (Table

Share‑Based Compensation (Tables)	6 Months Ended
	Jun. 30, 2020
Share‑Based Compensation
Schedule of share-based compensation expense category	Three Months Ended June 30, Six Months Ended June 30, (in thousands) 2020 2019 2020 2019 Research and development $ 1,309 $ 22 $ 2,356 $ 26 General and administrative 2,546 70 3,867 297 $ 3,855 $ 92 $ 6,223 $ 323
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, 2020 2,577,719 $ 5.90 9.3 Granted 3,962,327 14.61 Exercised (44,776) 1.02 Forfeited (134,331) 1.02 Outstanding at June 30, 2020 6,360,939 $ 11.46 9.5 Exercisable at June 30, 2020 397,421 $ 5.26 9.0 Vested or expected to vest at June 30, 2020 6,360,939 $ 11.46 9.5
Schedule of weighted average assumptions applied to options	Six Months Ended June 30, 2020 2019 Expected volatility 95.2% 87.1% Risk‑free interest rate 1.3% 2.4% Expected term 6.06 years 5.57 years Expected dividend yield — —
Summary of activity relating to early exercises of stock options	Number of shares Unvested balance at January 1, 2020 494,603 Vested (96,173) Unvested balance at June 30, 2020 398,430

Risks and Liquidity (Details)

Risks and Liquidity (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended
	Mar. 31, 2020	Jun. 30, 2020	Dec. 31, 2019
Risks And Liquidity [Line Items]
Accumulated deficit		$ (103,420)	$ (58,663)
IPO
Risks And Liquidity [Line Items]
Shares issued	13,798,900
Share price	$ 18
Net proceeds	$ 227,500

Summary of Significant Accoun_4

Summary of Significant Accounting Policies (Details) - shares	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Amount of antidilutive securities excluded from computation of earnings per share	6,766,617	20,886,973
Series A-1 Convertible Preferred Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Amount of antidilutive securities excluded from computation of earnings per share		14,216,333
Series A-2 Convertible Preferred Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Amount of antidilutive securities excluded from computation of earnings per share		5,009,808
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	6,759,369	1,380,969
Stock options vested and exercised, but subject to settlement of nonrecourse promissory notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Amount of antidilutive securities excluded from computation of earnings per share		279,863
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	7,248

Fair Value of Financial Instr_3

Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Level 1 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 284,305	$ 155,846

Accrued Expenses and Other Cu_3

Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Accrued Expenses and Other Current Liabilities
Professional fees	$ 196	$ 997
Compensation and related benefits	2,103	1,502
Research and development	4,556	507
Other	65	46
Accrued Expenses and Other Current Liabilities	$ 6,920	$ 3,052

Commitments and Contingencies -

Commitments and Contingencies - Collaboration Agreements (Details) - USD ($) $ in Millions	1 Months Ended		6 Months Ended
	May 31, 2020	Apr. 30, 2020	Jun. 30, 2020
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Minimum amount agreed to purchase in batches of drug product		$ 10.6
Catalent
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Period for which annual fee is payable			5 years
Term of manufacturing and supply agreement		5 years
Additional extension term of manufacturing and supply agreement		5 years
Amended and restated agreement with Penn
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Upfront payments	$ 1
Annual fee payable	5
Quarterly fee payable	$ 1.3
Extended term of option exercise window	3 years
Period for which annual fee is payable	5 years

Commitments and Contingencies_2

Commitments and Contingencies - Operating Lease (Details) - USD ($)	1 Months Ended	3 Months Ended		6 Months Ended
	Apr. 30, 2020	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Rent expense		$ 100,000	$ 36,000	$ 100,000	$ 87,000
Future minimum lease payments for the remainder of 2020		$ 100,000		$ 100,000
Termination period of existing lease agreement	5 days
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2022	$ 889,000
2023	1,098,000
2024	1,131,000
2025	1,165,000
Thereafter	7,763,000
Total	12,046,000
Maximum
Tenant improvement allowance	$ 2,800,000

Convertible Preferred Stock a_2

Convertible Preferred Stock and Common Stock (Details) - USD ($) $ / shares in Units, $ in Millions	Feb. 28, 2020	Mar. 31, 2020	Jun. 30, 2020	Dec. 31, 2019
Common stock, par value (in dollars per share)			$ 0.0001	$ 0.0001
Common stock, shares authorized			300,000,000	300,000,000
Preferred stock, par value (in dollars per share)			$ 0.0001	$ 0.0001
IPO
Shares issued		13,798,900
Share price		$ 18
Net proceeds		$ 227.5
Shares issued on stock conversion	26,803,777
Common stock, shares authorized	300,000,000

Share_Based Compensation - Equi

Share‑Based Compensation - Equity Incentive Plan & ESPP (Details)	Mar. 03, 2020USD ($)shares	Jun. 30, 2020USD ($)shares	Jun. 30, 2019USD ($)	Jun. 30, 2020USD ($)planshares	Jun. 30, 2019USD ($)	Dec. 31, 2019shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity incentive plans | plan				2
Share-based compensation expense | $		$ 3,855,000	$ 92,000	$ 6,223,000	$ 323,000
Equity Incentive Plan 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized | shares						5,362,823
Period till which there is automatic increase in rights granted under the Plan				10 years
Percent of the total number of shares of the Company’s common stock outstanding				5.00%
Shares available for grant | shares		3,942,905		3,942,905
Vesting period				4 years
Term of award				10 years
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized | shares	434,000
Period till which there is automatic increase in rights granted under ESPP	10 years
Percentage of common stock outstanding	1.00%
Percentage of stock purchase price	85.00%
Percentage of maximum employee contribution	15.00%
Maximum amount of common stock that can be purchased | $	$ 25,000
Share-based compensation expense | $		$ 62,000		$ 85,000

Share_Based Compensation - Shar

Share‑Based Compensation - Share-based compensation expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 3,855	$ 92	$ 6,223	$ 323
Additional share-based compensation expense due to modifications			700
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	1,309	22	2,356	26
Additional share-based compensation expense due to modifications			600
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 2,546	$ 70	3,867	$ 297
Additional share-based compensation expense due to modifications			$ 100

Share_Based Compensation - Stoc

Share‑Based Compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Millions	6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Number of shares
Outstanding at January 1, 2020	2,577,719
Granted	3,962,327
Exercised	(44,776)
Forfeited	(134,331)
Outstanding at June 30, 2020	6,360,939		2,577,719
Exercisable at June 30, 2020	397,421
Vested or expected to vest at June 30, 2020	6,360,939
Weighted average exercise price per share
Outstanding at January 1, 2020	$ 5.90
Granted	14.61
Exercised	1.02
Forfeited	1.02
Outstanding at June 30, 2020	11.46		$ 5.90
Exercisable at June 30, 2020	5.26
Vested or expected to vest at June 30, 2020	$ 11.46
Weighted average remaining contractual term (years)
Outstanding	9 years 6 months		9 years 3 months 18 days
Exercisable at June 30, 2020	9 years
Vested or expected to vest at June 30, 2020	9 years 6 months
Weighted average grant date fair value of options granted	$ 11.09	$ 0.76
Unrecognized compensation expense related to unvested stock option awards	$ 47.4
Weighted average period for recognition	3 years 4 months 24 days

Share_Based Compensation - Assu

Share‑Based Compensation - Assumptions Used in Determining Fair Value & Early exercise of Stock Options (Details) - USD ($)	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Expected volatility	95.20%	87.10%
Risk‑free interest rate	1.30%	2.40%
Expected term	6 years 22 days	5 years 6 months 26 days
Proceeds from unvested early exercised options recognized	$ 43,000
Interest income	$ 0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares [Roll Forward]
Unvested balance at January 1, 2020	494,603
Vested	(96,173)
Unvested balance at June 30, 2020	398,430

Share-Based Compensation - Nonr

Share-Based Compensation - Nonrecourse Promissory Notes with related parties (Details) - USD ($) $ in Millions	1 Months Ended	6 Months Ended
	Feb. 28, 2019	Jun. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares that becomes outstanding		406,894
Interim Chief Executive Officer
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares early exercised	688,875
Interim Chief Operating Officer
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares early exercised	309,994
Promissory Note With Related Parties
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Interest rate of notes receivable	2.91%
Promissory Note With Related Parties | Interim Chief Executive Officer
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Notes receivable in exchange of stock options exercised	$ 0.2
Promissory Note With Related Parties | Interim Chief Operating Officer
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Notes receivable in exchange of stock options exercised	$ 0.8

Related Party Transactions (Det

Related Party Transactions (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Penn Agreement
Related Party Transaction [Line Items]
Research and development expenses	$ 10,200,000	$ 6,200,000	$ 17,900,000	$ 9,100,000
Payments made to related party			16,300,000
Prepaid research and development asset	10,500,000		10,500,000		$ 5,700,000
Consulting Agreement
Related Party Transaction [Line Items]
Related party transaction expenses	31,000		62,000
Share-based compensation expense	$ 6,000		$ 12,000