Prelude Therapeutics (PRLD) S-1 4 Jan 21 IPO registration Financial data

Cover

Cover	9 Months Ended
	Sep. 30, 2020
Cover [Abstract]
Document Type	S-1
Amendment Flag	false
Entity Registrant Name	PRELUDE THERAPEUTICS INCORPORATED
Entity Central Index Key	0001678660
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false

STATEMENT OF BALANCE SHEETS

STATEMENT OF BALANCE SHEETS - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 234,792	$ 18,879	$ 15,595
Prepaid expenses and other current assets	3,269	1,345
Total current assets	238,061	20,224	15,595
Property and equipment, net	1,620	1,647	811
Total assets	239,681	21,871	16,406
Current liabilities:
Capital lease obligation		258
Accounts payable	5,074	1,974	1,362
Accrued expenses and other current liabilities	6,285	2,603	1,085
Total current liabilities	11,359	4,835	2,447
Other liabilities	15	5	57
Total liabilities	11,374	4,840	2,504
Total convertible preferred stock		66,443	36,595
Commitments
Stockholders' equity (deficit):
Additional paid-in capital	316,479	1,085	234
Accumulated deficit	(88,176)	(50,497)	(22,927)
Total stockholders' equity (deficit)	228,307	(49,412)	(22,693)
Total liabilities, convertible preferred stock and stockholders' equity (deficit)	239,681	21,871	16,406
Series A Convertible Preferred Stock
Current liabilities:
Total convertible preferred stock		36,595	$ 36,595
Series B Convertible Preferred Stock
Current liabilities:
Total convertible preferred stock		$ 29,848
Voting Common Stock
Stockholders' equity (deficit):
Common stock	3
Total stockholders' equity (deficit)	3
Non-voting Common Stock
Stockholders' equity (deficit):
Common stock	1
Total stockholders' equity (deficit)	$ 1

STATEMENT OF BALANCE SHEETS (Pa

STATEMENT OF BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Convertible preferred stock, par value	$ 0.0001	$ 0.0001	$ 0.0001
Convertible preferred stock, shares outstanding	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized		42,000,000
Common stock, shares, issued		3,161,653	2,380,270
Common stock, shares, outstanding		3,161,653	2,380,270
Series A Convertible Preferred Stock
Convertible preferred stock, shares authorized	0	13,574,008
Convertible preferred stock, shares issued	0	11,736,119	11,736,119
Convertible preferred stock, shares outstanding	0	11,736,119	11,736,119
Convertible preferred stock, liquidation value		$ 36,878
Series B Convertible Preferred Stock
Convertible preferred stock, shares authorized	0	18,500,000
Convertible preferred stock, shares issued	0	7,628,846	0
Convertible preferred stock, shares outstanding	0	7,628,846	0
Convertible preferred stock, liquidation value		$ 30,000
Series C Convertible Preferred Stock
Convertible preferred stock, shares authorized	0	0
Convertible preferred stock, shares issued	0	0
Convertible preferred stock, shares outstanding	0	0
Voting Common Stock
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	487,149,741	42,000,000
Common stock, shares, issued	32,593,010	3,161,653
Common stock, shares, outstanding	32,593,010	3,161,653
Non-voting Common Stock
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	12,850,259	0
Common stock, shares, issued	11,110,371	0
Common stock, shares, outstanding	11,110,371	0

STATEMENTS OF OPERATIONS

STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Operating expenses:
Research and development	$ 15,293	$ 5,490	$ 33,605	$ 15,430	$ 24,279	$ 12,621
General and administrative	2,851	1,394	5,712	2,584	3,830	2,354
Total operating expenses	18,144	6,884	39,317	18,014	28,109	14,975
Loss from operations	(18,144)	(6,884)	(39,317)	(18,014)	(28,109)	(14,975)
Other income, net	1,384	150	1,638	483	539	295
Net loss	$ (16,760)	$ (6,734)	$ (37,679)	$ (17,531)	$ (27,570)	$ (14,680)
Per share information:
Net loss per share of common stock, basic and diluted	$ (5.25)	$ (3.93)	$ (15.83)	$ (10.80)	$ (16.52)	$ (10.46)
Weighted average common shares outstanding, basic and diluted	3,194,471	1,713,371	2,380,100	1,622,546	1,668,549	1,402,928
Pro forma net loss per share of common stock, basic and diluted (unaudited)					$ (0.86)
Pro forma weighted average shares outstanding, basic and diluted (unaudited).					32,105,972

STATEMENTS OF CHANGES IN CONVER

STATEMENTS OF CHANGES IN CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Series A Convertible Preferred Stock	Series B Convertible Preferred Stock	Series C Convertible Preferred Stock	Voting Common Stock	Non-voting Common Stock	Additional Paid-in Capital	Accumulated Deficit	Common Stock
Convertible preferred stock, Beginning balance at Dec. 31, 2017		$ 18,658
Convertible preferred stock, Beginning balance (shares) at Dec. 31, 2017		6,007,743
Beginning balance at Dec. 31, 2017	$ (8,193)						$ 54	$ (8,247)
Beginning balance (shares) at Dec. 31, 2017									1,866,367
Sale of convertible preferred stock, net of issuance costs		$ 17,937
Sale of convertible preferred stock, net of issuance costs (shares)		5,728,376
Stock-based compensation expense, including issuance of restricted stock awards	180						180
Stock-based compensation expense, including issuance of restricted stock awards (shares)									513,903
Net loss	(14,680)							(14,680)
Convertible preferred stock, Ending balance at Dec. 31, 2018	36,595	$ 36,595
Convertible preferred stock, Ending balance (shares) at Dec. 31, 2018		11,736,119	0
Ending balance at Dec. 31, 2018	(22,693)						234	(22,927)
Ending balance (shares) at Dec. 31, 2018									2,380,270
Stock-based compensation expense, including issuance of restricted stock awards	82						82
Net loss	(5,241)							(5,241)
Convertible preferred stock, Ending balance at Mar. 31, 2019		$ 36,595
Convertible preferred stock, Ending balance (shares) at Mar. 31, 2019		11,736,119
Ending balance at Mar. 31, 2019	(27,852)						316	(28,168)
Ending balance (shares) at Mar. 31, 2019									2,380,270
Convertible preferred stock, Beginning balance at Dec. 31, 2018	36,595	$ 36,595
Convertible preferred stock, Beginning balance (shares) at Dec. 31, 2018		11,736,119	0
Beginning balance at Dec. 31, 2018	(22,693)						234	(22,927)
Beginning balance (shares) at Dec. 31, 2018									2,380,270
Net loss	(17,531)
Convertible preferred stock, Ending balance at Sep. 30, 2019		$ 36,595	$ 29,848
Convertible preferred stock, Ending balance (shares) at Sep. 30, 2019		11,736,119	7,628,846
Ending balance at Sep. 30, 2019	(39,699)						759	(40,458)
Ending balance (shares) at Sep. 30, 2019									3,161,653
Convertible preferred stock, Beginning balance at Dec. 31, 2018	36,595	$ 36,595
Convertible preferred stock, Beginning balance (shares) at Dec. 31, 2018		11,736,119	0
Beginning balance at Dec. 31, 2018	(22,693)						234	(22,927)
Beginning balance (shares) at Dec. 31, 2018									2,380,270
Exercise of stock options	$ 5						5
Exercise of stock options (shares)	3,242								3,242
Sale of convertible preferred stock, net of issuance costs			$ 29,848
Sale of convertible preferred stock, net of issuance costs (shares)			7,628,846
Stock-based compensation expense, including issuance of restricted stock awards	$ 846						846
Stock-based compensation expense, including issuance of restricted stock awards (shares)									778,141
Net loss	(27,570)							(27,570)
Convertible preferred stock, Ending balance at Dec. 31, 2019	66,443	$ 36,595	$ 29,848
Convertible preferred stock, Ending balance (shares) at Dec. 31, 2019		11,736,119	7,628,846	0
Ending balance at Dec. 31, 2019	(49,412)						1,085	(50,497)
Ending balance (shares) at Dec. 31, 2019					3,161,653				3,161,653
Convertible preferred stock, Beginning balance at Mar. 31, 2019		$ 36,595
Convertible preferred stock, Beginning balance (shares) at Mar. 31, 2019		11,736,119
Beginning balance at Mar. 31, 2019	(27,852)						316	(28,168)
Beginning balance (shares) at Mar. 31, 2019									2,380,270
Exercise of stock options	5						5
Exercise of stock options (shares)									3,242
Sale of convertible preferred stock, net of issuance costs			$ 29,848
Sale of convertible preferred stock, net of issuance costs (shares)			7,628,846
Stock-based compensation expense, including issuance of restricted stock awards	141						141
Stock-based compensation expense, including issuance of restricted stock awards (shares)									345,840
Net loss	(5,556)							(5,556)
Convertible preferred stock, Ending balance at Jun. 30, 2019		$ 36,595	$ 29,848
Convertible preferred stock, Ending balance (shares) at Jun. 30, 2019		11,736,119	7,628,846
Ending balance at Jun. 30, 2019	(33,262)						462	(33,724)
Ending balance (shares) at Jun. 30, 2019									2,729,352
Stock-based compensation expense, including issuance of restricted stock awards	297						297
Stock-based compensation expense, including issuance of restricted stock awards (shares)									432,301
Net loss	(6,734)							(6,734)
Convertible preferred stock, Ending balance at Sep. 30, 2019		$ 36,595	$ 29,848
Convertible preferred stock, Ending balance (shares) at Sep. 30, 2019		11,736,119	7,628,846
Ending balance at Sep. 30, 2019	(39,699)						759	(40,458)
Ending balance (shares) at Sep. 30, 2019									3,161,653
Convertible preferred stock, Beginning balance at Dec. 31, 2019	66,443	$ 36,595	$ 29,848
Convertible preferred stock, Beginning balance (shares) at Dec. 31, 2019		11,736,119	7,628,846	0
Beginning balance at Dec. 31, 2019	(49,412)						1,085	(50,497)
Beginning balance (shares) at Dec. 31, 2019					3,161,653				3,161,653
Sale of convertible preferred stock, net of issuance costs			$ 29,942
Sale of convertible preferred stock, net of issuance costs (shares)			7,628,846
Stock-based compensation expense, including issuance of restricted stock awards	362						362
Stock-based compensation expense, including issuance of restricted stock awards (shares)					432,301
Net loss	(9,511)							(9,511)
Convertible preferred stock, Ending balance at Mar. 31, 2020		$ 36,595	$ 59,790
Convertible preferred stock, Ending balance (shares) at Mar. 31, 2020		11,736,119	15,257,692
Ending balance at Mar. 31, 2020	(58,561)						1,447	(60,008)
Ending balance (shares) at Mar. 31, 2020					3,593,954
Convertible preferred stock, Beginning balance at Dec. 31, 2019	66,443	$ 36,595	$ 29,848
Convertible preferred stock, Beginning balance (shares) at Dec. 31, 2019		11,736,119	7,628,846	0
Beginning balance at Dec. 31, 2019	$ (49,412)						1,085	(50,497)
Beginning balance (shares) at Dec. 31, 2019					3,161,653				3,161,653
Exercise of stock options (shares)	98,254
Net loss	$ (37,679)
Convertible preferred stock, Ending balance (shares) at Sep. 30, 2020	0	0	0	0
Ending balance at Sep. 30, 2020	$ 228,307				$ 3	$ 1	316,479	(88,176)
Ending balance (shares) at Sep. 30, 2020					32,593,010	11,110,371
Convertible preferred stock, Beginning balance at Mar. 31, 2020		$ 36,595	$ 59,790
Convertible preferred stock, Beginning balance (shares) at Mar. 31, 2020		11,736,119	15,257,692
Beginning balance at Mar. 31, 2020	(58,561)						1,447	(60,008)
Beginning balance (shares) at Mar. 31, 2020					3,593,954
Exercise of stock options	31						31
Exercise of stock options (shares)					36,745
Stock-based compensation expense, including issuance of restricted stock awards	627						627
Net loss	(11,408)							(11,408)
Convertible preferred stock, Ending balance at Jun. 30, 2020		$ 36,595	$ 59,790
Convertible preferred stock, Ending balance (shares) at Jun. 30, 2020		11,736,119	15,257,692
Ending balance at Jun. 30, 2020	(69,311)						2,105	(71,416)
Ending balance (shares) at Jun. 30, 2020					3,630,699
Exercise of stock options	65						65
Exercise of stock options (shares)					61,509
Sale of convertible preferred stock, net of issuance costs				$ 49,826
Sale of convertible preferred stock, net of issuance costs (shares)				3,443,612
Conversion of convertible preferred stock into voting and non-voting common stock upon initial public offering	146,211				$ 2	$ 1	146,208
Conversion of convertible preferred stock into common stock upon initial public offering (shares)		(11,736,119)	(15,257,692)	(3,443,612)
Conversion of convertible preferred stock into common stock upon initial public offering		$ (36,595)	$ (59,790)	$ (49,826)
Conversion of convertible preferred stock into common stock upon initial public offering (shares)					19,327,052	11,110,371
Sale of common stock in initial public offering, net of offering costs of $2,538	166,630				$ 1		166,629
Sale of common stock in initial public offering, net of offering costs of $2,538, (shares)					9,573,750
Stock-based compensation expense, including issuance of restricted stock awards	1,472						1,472
Net loss	$ (16,760)							(16,760)
Convertible preferred stock, Ending balance (shares) at Sep. 30, 2020	0	0	0	0
Ending balance at Sep. 30, 2020	$ 228,307				$ 3	$ 1	$ 316,479	$ (88,176)
Ending balance (shares) at Sep. 30, 2020					32,593,010	11,110,371

STATEMENTS OF CHANGES IN CONV_2

STATEMENTS OF CHANGES IN CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended			12 Months Ended
	Sep. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Sale of common stock, offering costs	$ 2,538
Series A Convertible Preferred Stock
Sale of convertible preferred stock, issuance costs					$ 63
Series B Convertible Preferred Stock
Sale of convertible preferred stock, issuance costs		$ 58	$ 152	$ 152
Series C Convertible Preferred Stock
Sale of convertible preferred stock, issuance costs	$ 174

STATEMENTS OF CASH FLOWS

STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Cash flows used in operating activities:
Net loss	$ (37,679)	$ (17,531)	$ (27,570)	$ (14,680)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	409	254	382	149
Loss on disposal of property and equipment			10	15
Stock-based compensation	2,461	520	846	180
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,924)	(344)	(1,345)	66
Accounts payable	1,931	(76)	546	721
Accrued expenses and other liabilities	3,472	(19)	1,466	595
Net cash used in operating activities	(31,330)	(17,196)	(25,665)	(12,954)
Cash flows used in investing activities:
Purchases of property and equipment	(292)	(581)	(780)	(529)
Net cash used in investing activities	(292)	(581)	(780)	(529)
Cash flows provided by financing activities:
Proceeds from the issuance of common stock upon initial public offering, net of offering costs	167,929
Payment of capital lease obligation	(258)	(31)	(124)
Proceeds from the exercise of stock options	96	5	5
Net cash provided by financing activities	247,535	29,822	29,729	17,937
Net increase in cash and cash equivalents	215,913	12,045	3,284	4,454
Cash and cash equivalents at beginning of period	18,879	15,595	15,595	11,141
Cash and cash equivalents at end of period	234,792	27,640	18,879	15,595
Supplemental disclosures:
Issuance of capital lease obligation in connection with purchase of property and equipment			382
Property and equipment in accounts payable	90	108	66	6
Offering costs in accrued expenses and other current liabilities	220
Offering costs in accounts payable	1,079
Series C Convertible Preferred Stock
Cash flows provided by financing activities:
Proceeds from the sale of convertible preferred stock, net of offering costs	49,826
Series B Convertible Preferred Stock
Cash flows provided by financing activities:
Proceeds from the sale of convertible preferred stock, net of offering costs	$ 29,942	$ 29,848	$ 29,848
Series A Convertible Preferred Stock
Cash flows provided by financing activities:
Proceeds from the sale of convertible preferred stock, net of offering costs				$ 17,937

Nature of Operations

Nature of Operations	12 Months Ended
	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements Abstract
Nature of Operations	1. Nature of Operations Prelude Therapeutics Incorporated (the “Company”) was incorporated in Delaware on February 5, 2016 and is a clinical-stage biotechnology company focused on discovering and developing new medicines targeting chromatin function to treat cancer and rare diseases. Since beginning operations, the Company has devoted substantially all its efforts to research and development, conducting preclinical and clinical studies, recruiting management and technical staff, administration, and raising capital.

Risks and Liquidity

Risks and Liquidity	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Risks And Liquidity [Abstract]
Risks and Liquidity	2. Risks and liquidity The Company is subject to a number of risks common to early-stage companies in the biotechnology industry. Principal among these risks are the uncertainties in the development process, development of the same or similar technological innovations by competitors, protection of proprietary technology, dependence on key personnel, compliance with government regulations and approval requirements, and the need to obtain additional financing to fund operations. Product candidates currently under development will require significant additional research and development efforts, including extensive pre-clinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure, and extensive compliance-reporting capabilities. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s technology will be obtained, that any products developed will obtain necessary government regulatory approval, or that any approved products will be commercially viable. The Company operates in an environment of rapid change in technology and substantial competition from pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees, consultants and contractors. Since its inception, the Company has incurred operating losses and has an accumulated deficit of $88.2 million at September 30, 2020. The Company has no revenue to date and devotes its efforts to research and development. The Company anticipates incurring additional losses until such time, if ever, that it can generate significant sales of its product candidates currently in development. The Company believes that its cash and cash equivalents as of September 30, 2020, which includes the net proceeds of approximately $166.6 million from the IPO of the Company’s common stock completed in September 2020, will be sufficient to fund its operating expenses and capital expenditure requirements into the second half of 2022. To fund its operating expenses and capital expenditure requirements after that date, the Company plans to seek additional funding through public or private equity offerings, debt financings, 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. On March 10, 2020, the World Health Organization characterized the novel COVID-19 virus as a global pandemic. There is significant uncertainty as to the likely effects of this disease which may, among other things, materially impact the Company’s planned clinical trials. This pandemic or outbreak could result in difficulty securing clinical trial site locations, CROs, and/or trial monitors and other critical vendors and consultants supporting the trial. In addition, outbreaks or the perception of an outbreak near a clinical trial site location could impact the Company’s ability to enroll patients. These situations, or others associated with COVID-19, could cause delays in the Company’s clinical trial plans and could increase expected costs, all of which could have a material adverse effect on the Company’s business and its financial condition. At the current time, the Company is unable to quantify the potential effects of this pandemic on its future financial statements.	2. Risks and Liquidity The Company is subject to a number of risks common to early-stage companies in the biotechnology industry. Principal among these risks are the uncertainties in the development process, development of the same or similar technological innovations by competitors, protection of proprietary technology, dependence on key personnel, compliance with government regulations and approval requirements, and the need to obtain additional financing to fund operations. Product candidates currently under development will require significant additional research and development efforts, including extensive pre-clinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure, and extensive compliance-reporting capabilities. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s technology will be obtained, that any products developed will obtain necessary government regulatory approval, or that any approved products will be commercially viable. The Company operates in an environment of rapid change in technology and substantial competition from pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees, consultants and contractors. The Company follows the provisions of Financial Accounting Standards Board (“FASB”) Accounting Standards Codification (“ASC”) Topic 205-40, Presentation of Financial Statements—Going Concern, which requires management to assess the Company’s ability to continue as a going concern within one year after the date the financial statements are issued. Since its inception, the Company has incurred operating losses and has an accumulated deficit of $50.5 million at December 31, 2019. The Company has no revenue to date and devotes its efforts to research and development. The Company anticipates incurring additional losses until such time, if ever, that it can generate significant sales of its product candidates currently in development. The Company’s activities have been primarily funded by the sale of Convertible Preferred Stock (Note 8). These factors and the Company’s recurring losses from operations, negative cash from operations, and accumulated deficit since inception raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date these financial statements are issued. The Company’s cash and cash equivalents at December 31, 2019 were $18.9 million. This, combined with approximately $29.9 million raised through a Series B Preferred stock financing in March 2020 (Note 11) is expected to enable the Company to fund its operating expenses and capital expenditure requirements into December 2020, at which time the Company will need to secure additional funding in the future, from one or more equity or debt financings, collaborations, or other sources, in order to carry out all of its planned research and development and commercialization activities. However, there is no assurance that the Company will be able to obtain additional equity under acceptable terms, if at all. If the Company is unable to obtain additional financing, the lack of liquidity could have a material adverse effect on the Company’s future prospects. As a result of these factors, there is substantial doubt about the Company’s ability to continue as a going concern within one year after the date that these financial statements are issued. The accompanying financial statements have been prepared on a going-concern basis, which contemplates the realization of assets and satisfaction of liabilities in the normal course of business. The financial statements do not include any adjustments related to the recoverability and classification of recorded asset amounts or the amounts and classification of liabilities that might result from the outcome of this uncertainty. Management is currently evaluating different strategies to obtain the required funding of future operations. These strategies may include, but are not limited to, additional funding from current investors, funding from new investors including strategic corporate investors, and an initial public offering of the Company’s common stock. There can be no assurance these future funding efforts will be successful.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	3. Summary of significant accounting policies The summary of significant accounting policies included in the Company’s financial statements for the year ended December 31, 2019 can be found in “Note 3. Summary of significant accounting policies” of the Company’s prospectus filed with the Securities and Exchange Commission (the “SEC”) on September 25, 2020 (the “Prospectus”). Those policies have not materially changed, except as set forth below. Basis of presentation The accompanying unaudited interim financial statements have been prepared in accordance with generally accepted accounting principles (“GAAP”) for interim financial information, the instructions to Form 10-Q and Article 10 of Regulation S-X. They do not include all of the information and notes required by GAAP for complete financial statements. In the opinion of management, all adjustments (consisting of normal recurring accruals) considered necessary for a fair presentation have been included. Operating results for the three and nine month periods ended September 30, 2020 are not necessarily indicative of the results that may be expected for the year ended December 31, 2020. The accompanying unaudited interim financial statements should be read in conjunction with the annual audited financial statements and related notes as of and for the year ended December 31, 2019 found in the Prospectus filed with the SEC on September 25, 2020. 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”) of the Financial Accounting Standards Board (“FASB”). On September 18, 2020, the Company’s board of directors and stockholders approved a 1.1566-to-one reverse stock split of the Company’s issued and outstanding shares of common stock and convertible preferred stock. All share and per share amounts in the unaudited interim financial statements and notes hereto have been retrospectively adjusted for all periods presented to give effect to the reverse stock split. The completion of the IPO, as described above, impacts the comparability of certain amounts to the corresponding prior year period, including earnings per share. Use of estimates The preparation of the unaudited interim financial statements in conformity with U.S. 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 unaudited interim 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 unaudited interim 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 common stock, stock-based compensation assumptions and accrued clinical trial expenses. Subsequent to the IPO, the most significant estimate relates to accrued clinical trial expenses. Income taxes Based upon the historical and anticipated future losses, management has determined that the deferred tax assets generated by net operating losses and research and development credits do not meet the more likely than not threshold for realizability. Accordingly, a full valuation allowance has been recorded against the Company’s net deferred tax assets as of December 31, 2019 and September 30, 2020. 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. The weighted-average number of shares of common stock outstanding used in the basic net loss per share calculation does not include unvested restricted stock awards as these instruments are considered contingently issuable shares until they vest. Diluted net loss per share of common stock includes the effect, if any, from the potential exercise or conversion of securities, such as convertible preferred stock and 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 Company’s convertible preferred stock and unvested restricted stock entitles the holder to participate in dividends and earnings of the Company, and, if the Company were to recognize net income, it would have to use the two-class method to calculate earnings per share. The two-class method is not applicable during periods with a net loss, as the holders of the convertible preferred stock and unvested restricted stock have no obligation to fund losses. 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: September 30, 2019 2020 Series A convertible preferred stock 11,736,119 — Series B convertible preferred stock 7,628,846 — Unvested restricted stock awards 1,394,056 1,312,024 Stock options 2,189,333 6,649,882 22,948,354 7,961,906 Amounts in the above table reflect the common stock equivalents. Recently Issued Accounting Pronouncements Emerging Growth Company Status The Company is an emerging growth company, as defined in the Jumpstart Our Business Startups Act of 2012 (the “JOBS Act”). Under the JOBS Act, emerging growth companies can delay adopting new or revised accounting standards issued subsequent to the enactment of the JOBS Act, until such time as those standards apply to private companies. The Company has elected to use this extended transition period for complying with new or revised accounting standards that have different effective dates for public and private companies until the earlier of the date that it (i) is no longer an emerging growth company or (ii) affirmatively and irrevocably opts out of the extended transition period provided in the JOBS Act. As a result, these unaudited interim financial statements may not be comparable to companies that comply with the new or revised accounting pronouncements as of public company effective dates. Recent Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases In August 2018, the FASB issued ASU 2018-13, Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurements, Fair Value Measurement	3. Summary of Significant Accounting Policies 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”). 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. Significant areas that require management’s estimates include the fair value of the Company’s common stock, stock-based compensation assumptions, and accrued clinical trial expenses. Concentration of Credit Risk Financial instruments that potentially subject the Company to significant concentrations of credit risk consist of cash and cash equivalents. The Company maintains deposits in federally insured financial institutions in excess of federally insured limits. The Company has not experienced any losses in such accounts and believes it is not exposed to significant credit risk beyond the normal credit risk associated with commercial banking relationships. Segment Information Operating segments are defined as components of an enterprise about which separate discrete information is available for evaluation by the chief operating decision maker, or decision-making group, in deciding how to allocate resources and in assessing performance. The Company views and manages its operations as a single operating segment. Fair Value of Financial Instruments Management believes that the carrying amounts of the Company’s financial instruments, including cash, accounts payable, accrued expenses and a capital lease obligation, approximate fair value due to the short-term nature of these instruments. Cash Equivalents The Company’s cash equivalents include short-term highly liquid investments with an original maturity of 90 days or less when purchased and are carried at fair value in the accompanying balance sheets. Property and Equipment Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation expense is recognized using the straight-line method over the estimated useful life of the asset, ranging from 5-7 years Fixed Asset Type Estimated useful life Lab equipment 5 years Computers 5 years Furniture and fixtures 7 years Leasehold improvements are amortized over the shorter of the estimated useful life of the assets or the remaining lease term. Assets under capital leases are recorded in property and equipment, net on the balance sheets and depreciated in a manner similar to other property and equipment. Expenditures for repairs and maintenance of assets are charged to expense as incurred, while major betterments are capitalized. Upon retirement or sale, the cost and related accumulated depreciation of assets disposed of are removed from the accounts and any resulting gain or loss is included in the statements of operations. 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. If indicators of impairment are present, the assets are tested for recoverability by comparing the carrying amount of the assets to the related estimated future undiscounted cash flows that the assets are expected to generate. If the expected cash flows are less than the carrying value of the asset, then the asset is considered to be impaired and its carrying value is written down to fair value, based on the related estimated discounted future cash flows. Stock-Based Compensation The Company measures the cost of employee services received in exchange for stock-based awards based on the grant-date fair value of the award. The Company recognizes compensation expense using the straight-line method over the vesting period of the awards. The Company accounts for forfeitures as they occur. Estimating the fair value of stock-based awards requires the input of subjective assumptions, including 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 uses the Black-Scholes option pricing model to value its stock option awards. As the Company’s common stock has not been publicly traded, its board of directors periodically estimated the fair value of the Company’s common stock considering, among other things, contemporaneous valuations of its common stock prepared by an unrelated third-party valuation firm in accordance with the guidance provided by the American Institute of Certified Public Accountants 2013 Practice Aid, Valuation of Privately-Held-Company Equity Securities Issued as Compensation post-vesting The assumptions used in estimating the fair value of stock-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, stock-based compensation expense could be materially different for future awards. Grant Income and Research and Development Tax Credits The Company recognizes grant income and Delaware research and development tax credits, which are refundable irrespective of taxable income, in other income, net in the statements of operations when it is probable that the amounts will be received and the necessary qualifying conditions, as stated in the agreements, are met. Research and Development Research and development costs are expensed as incurred. Research and development expenses consist principally of personnel costs, including salaries, stock-based compensation, and benefits of employees, and other operational costs related to the Company’s research and development activities, including allocated facility-related Management makes estimates of the Company’s accrued research and development expenses as of each balance sheet date in the Company’s financial statements based on facts and circumstances known to the Company at that time. If the actual timing of the performance of services or the level of effort varies from the estimate, the Company will adjust the accrual accordingly. Nonrefundable advance payments for goods and services, including fees for process development or manufacturing and distribution of clinical supplies that will be used in future research and development activities, are deferred and recognized as expense in the period that the related goods are consumed or services are performed. Income Taxes Income taxes are accounted for under the asset-and-liability method as required by FASB ASC Topic 740, Income Taxes FASB ASC Subtopic 740-10, Accounting for Uncertainty of Income Taxes Net Loss Per Share and Unaudited Pro Forma 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. The weighted-average number of shares of common stock outstanding used in the basic net loss per share calculation does not include unvested restricted stock awards as these instruments are considered contingently issuable shares until they vest. Diluted net loss per share of common stock includes the effect, if any, from the potential exercise or conversion of securities, such as convertible preferred stock and 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 Company’s convertible preferred stock and unvested restricted stock entitles the holder to participate in dividends and earnings of the Company, and, if the Company were to recognize net income, it would have to use the two-class method to calculate earnings per share. The two-class method is not applicable during periods with a net loss, as the holders of the convertible preferred stock and unvested restricted stock have no obligation to fund losses. 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: December 31, 2018 2019 Series A convertible preferred stock 11,736,119 11,736,119 Series B convertible preferred stock — 7,628,846 Unvested restricted stock awards 856,438 1,335,349 Stock options 588,961 2,269,742 13,181,518 22,970,056 Amounts in the above table reflect the common stock equivalents. The unaudited pro forma net loss per share is computed using the weighted average number of shares of common stock outstanding after giving effect to the automatic conversion of all convertible preferred stock, inclusive of the 7,628,846 shares of Series B convertible preferred stock issued in March 2020 and the 3,443,612 shares of Series C convertible preferred stock issued in August 2020, into shares of common stock upon the closing of a qualified initial public offering, as if the qualified initial public offering had occurred at the beginning of the period. The following table summarizes the calculation of unaudited pro forma basic and diluted net loss per share of common stock for the year ended December 31, 2019: (in thousands, except share and per share data) Numerator: Net loss $ (27,570) Denominator: Weighted average shares of common stock outstanding 1,668,549 Conversion of convertible preferred stock 30,437,423 Shares issued in computing unaudited pro forma weighted average basic and diluted shares of common stock outstanding 32,105,972 Pro forma net loss per common share, basic and diluted $ (0.86) Recently Issued Accounting Pronouncements Emerging Growth Company Status The Company is an emerging growth company, as defined in the Jumpstart Our Business Startups Act of 2012 (the “JOBS Act”). Under the JOBS Act, emerging growth companies can delay adopting new or revised accounting standards issued subsequent to the enactment of the JOBS Act, until such time as those standards apply to private companies. The Company has elected to use this extended transition period for complying with new or revised accounting standards that have different effective dates for public and private companies until the earlier of the date that it (i) is no longer an emerging growth company or (ii) affirmatively and irrevocably opts out of the extended transition period provided in the JOBS Act. As a result, these financial statements may not be comparable to companies that comply with the new or revised accounting pronouncements as of public company effective dates. Recent Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases In June 2018, the FASB issued ASU 2018-07, Compensation-Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting Equity-Based Payments to Non-Employees In August 2018, the FASB issued ASU 2018-13, Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurements, Fair Value Measurement

Fair Value of Financial Instrum

Fair Value of Financial Instruments	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Fair Value Disclosures [Abstract]
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 Company follows the provisions of ASC 820, 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: • Level 2: • Level 3: The following fair value hierarchy table presents information about the Company’s assets and liabilities measured at fair value on a recurring basis: Fair value measurement at reporting date using (in thousands) Quoted prices in active markets for identical assets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) December 31, 2019: Assets: Cash equivalents (Money Market Funds) $ 18,779 $ — $ — September 30, 2020: Assets: Cash equivalents (Money Market Funds) $ 233,364 $ — $ —	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 Company follows the provisions of ASC 820, 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: • Level 2: • Level 3: The following fair value hierarchy table presents information about the Company’s assets and liabilities measured at fair value on a recurring basis: Fair value measurement at (in thousands) Quoted prices Significant Significant December 31, 2018: Assets: Cash equivalents (Money Market Funds) $ 15,400 $ — $ — December 31, 2019: Assets: Cash equivalents (Money Market Funds) $ 18,779 $ — $ —

Property and Equipment

Property and Equipment	12 Months Ended
	Dec. 31, 2019
Property, Plant and Equipment [Abstract]
Property and Equipment	5. Property and Equipment Property and equipment consisted of the following: December 31, 2018 2019 (in thousands) Lab equipment $ 674 $ 1,842 Leasehold improvements 312 312 Computers 13 10 Furniture and fixtures 3 39 1,002 2,203 Less accumulated depreciation (191) (556) Property and equipment, net $ 811 $ 1,647 Depreciation and amortization expense was $0.1 million and $0.4 million for the years ended December 31, 2018 and 2019, respectively. In September 2019, the Company signed a 12-month capital lease for $0.4 million of lab equipment with an effective interest rate of 9.67%. At December 31, 2019, the Company had $0.1 million of accumulated amortization related to the capital lease. At December 31, 2019, the Company owed $0.3 million in future minimum lease payments under the capital lease that are expected to be paid in 2020.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Payables and Accruals [Abstract]
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) December 31, 2019 September 30, 2020 Compensation and related benefits $ 1,631 $ 2,099 Research and development 658 3,763 Offering costs — 220 Professional services and other 314 203 $ 2,603 $ 6,285	6. Accrued Expenses Accrued expenses consisted of the following: December 31, 2018 2019 (in thousands) Compensation and related benefits $ 873 $ 1,631 Research and development 153 658 Other 59 314 $ 1,085 $ 2,603

Commitments

Commitments	12 Months Ended
	Dec. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments	7. Commitments Operating Leases The Company leases office and laboratory space in Wilmington, Delaware under two separate noncancelable leases, which expire in November 2020 and March 2021, respectively. The Company has an option to renew both leases for additional 1-year and 6-months periods, respectively. The leases are classified as operating leases and the Company recognizes rent expense on a straight-line basis over the lease terms. The Company recognized rent expense of $0.7 and $0.9 million during the years ended December 31, 2018 and 2019, respectively, related to these leases. The future minimum lease payments under the Company’s operating lease agreements as of December 31, 2019 are $1.0 million for 2020 and $21,000 for 2021, with no commitments thereafter. In June 2020, the Company extended the lease that was set to expire in November 2020 until December 2021. Employment Agreements The Company entered into employment agreements with key personnel providing for compensation and severance in certain circumstances, as defined in the respective employment agreements. Other Research and Development Arrangements The Company enters into agreements with contract research organizations (“CROs”) to assist in the performance of research and development activities. Expenditures to CROs will represent a significant cost in clinical development for the Company.

Convertible Preferred Stock and

Convertible Preferred Stock and Common Stock	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Equity [Abstract]
Convertible Preferred Stock and Common Stock	6. Convertible Preferred Stock and Common Stock Preferred Stock Financings In August 2020, the Company’s existing Convertible Preferred Stock investors as well as a new investor purchased 3,443,612 shares of Series C at a price of $14.5197 per share for net proceeds of approximately $49.8 million. In March 2020, the Company’s Series B investors exercised their Future Tranche Right and purchased 7,628,846 shares of Series B for net proceeds of approximately $29.9 million. Initial Public Offering In September 2020, the Company completed its IPO in which the Company sold 9,573,750 shares of its common stock at a public offering price of $19.00 per share. The Company received net proceeds of $166.6 million after deducting underwriting discounts, commissions, and other offering expenses paid by the Company. In addition, immediately prior to the closing of the IPO on September 29, 2020, (i) all of the Company’s outstanding shares of convertible preferred stock converted into an aggregate of 30,437,423 shares of common stock (of which, 11,110,371 shares are non-voting 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 500,000,000. Common Stock The Company has two classes of common stock; “voting common stock” and “non-voting common stock.” The holders of the voting common stock are entitled to one vote for each share of voting common stock held at all meetings of stockholders. Except as otherwise required by law, the holders of non-voting common stock shall not be entitled to vote at any meetings of stockholders (or written actions in lieu of meetings) and the shares of non-voting common stock shall not be included in determining the number of shares voting or entitled to vote on any matter. Unless required by law, there shall be no cumulative voting. Any holder of non-voting common stock may elect to convert each share of non-voting common stock into one fully paid and non-assessable share of voting common stock at any time by providing written notice to the Company; provided that as a result of such conversion, such holder, together with its affiliates and any members of a Schedule 13(d) group with such holder, would not beneficially own in excess of 9.99% of the Company’s common stock immediately prior to and following such conversion, unless otherwise as expressly provided for in the Company’s restated certificate of incorporation. However, this ownership limitation may be increased (not to exceed 19.99%) or decreased to any other percentage designated by such holder of non-voting common stock upon 61 days’ notice to the Company.	8. Convertible Preferred Stock and Common Stock Convertible Preferred Stock In August 2018, the Company issued an aggregate of 5,728,376 shares of Series A convertible preferred stock (“Series A”) for $3.1423 per share for aggregate net proceeds of $17.9 million. In May 2019, the Company issued an aggregate of 7,628,846 shares of Series B convertible preferred stock (“Series B”) to existing investors at $3.9325 per share for aggregate net proceeds of $29.8 million. The following is a summary of the rights, preferences, and terms of the Series A and Series B (collectively, “Convertible Preferred Stock”): Dividends The holders of the Convertible Preferred Stock are also entitled to receive dividends payable when, as and if declared by the Board of Directors of the Company, with the holders of common stock, paid out of any assets or on the common stock of the Company, on an as-converted to common stock basis. Dividends are non-cumulative and no dividends on common stock were declared or paid from inception through December 31, 2019. Voting Holders of the Convertible Preferred Stock are entitled to one vote for each share of common stock into which their shares may be converted and, subject to certain Convertible Preferred Stock class votes specified in the Company’s certificate of incorporation or as required by law, holders of the Convertible Preferred Stock and common stock vote together on an as-converted basis. Liquidation Preference In the event of a liquidation, dissolution or winding up of the Company, either voluntary or involuntary, or in the event of a deemed liquidation event, which includes a sale of the Company as defined in the Company’s articles of incorporation, holders of the Convertible Preferred Stock are entitled to receive, in preference to all other stockholders, an amount equal to the greater of their original investment amount plus any declared but unpaid dividends or the fair value of the common stock on a fully converted basis prior to the deemed liquidation event. If, upon the occurrence of such event, the assets and funds available for distribution are insufficient to pay such holders the full amount to which they are entitled, then the entire assets and funds legally available for distribution shall be distributed ratably among the holders of the Convertible Preferred Stock in proportion to the full amounts to which they would otherwise be entitled. Conversion Each share of Convertible Preferred Stock is convertible into one share of common stock at any time at the option of the holder at a conversion price then in effect. As of December 31, 2019, the conversion price of the Series A is $3.1423 per share and the conversion price of the Series B is $3.9325 per share. All outstanding Convertible Preferred Stock will automatically convert into common stock at the conversion price then in effect upon (i) a qualified initial public offering of common stock with a public offering price of at least $11.80 per share and aggregate gross proceeds of at least $50.0 million or (ii) the affirmative election of 66% of the holders of the outstanding shares of Convertible Preferred Stock. Redemption The Convertible Preferred Stock is subject to redemption under certain deemed liquidation events not solely within the control of the Company, as defined, and as such is considered contingently redeemable for accounting purposes and is classified as temporary equity in the Company’s balance sheets. Future Tranche Right Feature Pursuant to the October 2017 Series A Stock Purchase Agreement, the Series A investors could elect to purchase up to an aggregate of 5,728,376 additional shares of the Company’s Series A at a fixed purchase price of $3.1423 per share (the “Series A Future Tranche Right”). Additionally, upon the successful filing of an investigational new drug application (“IND”), the Series A investors were obligated to purchase the additional shares of Series A. In the event the holders did not purchase additional shares of Series A, their initial shares of Series A would have automatically converted into shares of the Company’s common stock at a conversion ratio of 10 shares of Series A for 1 share of common stock. In August 2018, the Series A Future Tranche Right was exercised in full by the Series A investors. Pursuant to the May 2019 Series B Stock Purchase Agreement, the Series B investors could elect to purchase an aggregate of 7,628,846 additional shares of the Company’s Series B at a fixed purchase price of $3.9325 per share (the “Series B Future Tranche Right”). Additionally, upon the successful demonstration of certain pharmacokinetic and safety profile milestones by the Company, the holders were obligated to purchase the additional shares of Series B. In the event the holders did not purchase additional shares of Series B, their initial shares of Series B would have automatically converted into shares of the Company’s common stock at a conversion ratio of 10 shares of Series B for 1 share of common stock. The Company determined that the Series A Future Tranche Right and Series B Future Tranche Right (Collectively, “Future Tranche Rights”) did not meet the definition of a freestanding financial instrument as they were not legally detachable. The Future Tranche Rights were also evaluated as an embedded derivative and the Company determined they did not meet the definition of a derivative instrument for which bifurcation would be required. In March 2020, the Series B Future Tranche Right was exercised in full by the Series B investors (Note 11). Common Stock The holders of the common stock are entitled to one vote for each share of common stock held at all meetings of stockholders. Unless required by law, there shall be no cumulative voting. In the event of any voluntary or involuntary liquidation, dissolution or winding up of the Company, after the payment of all preferential amounts required to be paid to the holders of shares of Convertible Preferred Stock, the remaining funds and assets available for distribution to the stockholders of the Company will be distributed among the holders of shares of common stock, pro rata based on the number of shares of common stock held by each such holder.

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	7. Stock-Based Compensation The Company has two equity incentive plans: the 2016 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 initially authorized under the Plan was 4,680,000. Of this amount, 4,677,407 shares were available for future grants as of September 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 with 25% of options vesting after 1 year and then monthly thereafter, and have a term of ten years. In September 2020, the Company also adopted the 2020 Employee Stock Purchase Plan (the “ESPP”), which includes 520,000 shares of common stock reserved for future issuance. No shares have been issued from the ESPP as of September 30, 2020. The Company measures stock-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 stock-based compensation expense in the following expense categories in its accompanying statements of operations: Three Months Ended September 30, Nine Months September 30, (in thousands) 2019 2020 2019 2020 Research and development $ 135 $ 630 $ 250 $ 1,047 General and administrative 162 842 270 1,414 $ 297 $ 1,472 $ 520 $ 2,461 Stock Options The following table summarizes stock option activity for the periods indicated: Number of shares Weighted average exercise price per share Weighted average remaining contractual term (years) Outstanding at January 1, 2020 2,269,742 $ 1.66 9.20 Granted 4,594,130 $ 9.38 Exercised (98,254) $ 0.98 Forfeited (115,736) $ 1.37 Outstanding at September 30, 2020 6,649,882 $ 7.00 9.40 Exercisable at September 30, 2020 783,236 $ 1.64 8.40 At September 30, 2020, the aggregate intrinsic value of outstanding options and exercisable options was $153.7 million and $22.3 million, respectively. The following table summarizes information about stock options outstanding at September 30, 2020 under the Plan: Options Outstanding Options Exercisable Range of Exercise Prices Number Outstanding Weighted Average Remaining Contractual Life (in years) Weighted Average Exercise Price Number Exercisable Weighted Average Exercise Price $0.31 - $1.02 253,931 7.43 $ 0.98 162,037 $ 0.96 $1.43 - $1.89 3,259,095 9.05 1.85 619,998 1.79 $12.85 - $19.00 3,136,856 9.92 12.86 1,201 12.85 6,649,882 783,236 The weighted-average grant date fair value of options granted was $1.40 and $8.44 per share for the nine months ended September 30, 2019 and 2020, respectively. The aggregate intrinsic value of options exercised was $2.9 million for the nine months ended September 30, 2020. The Company recorded stock-based compensation expense of $0.2 million and $1.2 million for the three months ended September 30, 2019 and 2020, respectively, related to stock options. The Company recorded stock-based compensation expense of $0.3 million and $1.9 million for the nine months ended September 30, 2019 and 2020, respectively, related to stock options. As of September 30, 2020, the total unrecognized compensation expense related to unvested stock option awards was $39.1 million, which the Company expects to recognize over a weighted-average period of 3.74 years. 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, 2019 2020 Expected volatility 91.60 % 115.09 % Risk-free interest rate 1.88 % 0.42 % Expected life (in years) 6.25 6.25 Expected dividend yield — — Restricted Stock Awards The Company issues restricted stock awards (“RSA”) to employees that generally vest over a four-year period with 25% of awards vesting after one year and then monthly thereafter. Any unvested shares will be forfeited upon termination of services. The fair value of an RSA is equal to the fair market value price of the Company’s common stock on the date of grant. RSA expense is amortized straight-line over the vesting period. The following table summarizes activity related to RSA stock-based payment awards: Number of shares Weighted-average grant date fair value Unvested balance at January 1, 2020 1,335,349 $ 1.42 Granted 432,301 $ 3.26 Vested (455,626) $ 1.40 Unvested balance at September 30, 2020 1,312,024 $ 2.03 The Company recorded stock-based compensation expense of $0.1 million and $0.3 million for the three months ended September 30, 2019 and 2020, respectively, related to RSAs. The Company recorded stock-based compensation expense of $0.3 million and $0.6 million for the nine months ended September 30, 2019 and 2020, respectively, related to RSAs. As of September 30, 2020, the total unrecognized expense related to all RSAs was $2.4 million, which the Company expects to recognize over a weighted-average period of 2.94 years.	9. Stock-Based Compensation Under the Prelude Therapeutics Incorporated 2016 Stock Incentive Plan, as amended (the “Plan”), the Company may grant stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards, performance stock awards and other forms of equity compensation to Company employees, directors and consultants. As of December 31, 2019, there were 364,853 shares of common stock available for issuance under the Plan. The Company’s stock options vest based on the terms in each award agreement, generally over four-year The Company measures stock-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 stock-based compensation expense in the following expense categories in its accompanying statements of operations: Year Ended 2018 2019 (in thousands) Research and development $ 178 $ 437 General and administrative 2 409 $ 180 $ 846 Stock Options The following table summarizes stock option activity for the Plan in the years indicated: Number Weighted Weighted Outstanding at January 1, 2018 25,938 $ 0.31 9.14 Granted 563,023 $ 1.15 Outstanding at December 31, 2018 588,961 $ 1.11 9.35 Granted 1,703,692 $ 1.84 Forfeited (19,669) $ 1.16 Exercised (3,242) $ 1.02 Outstanding at December 31, 2019 2,269,742 $ 1.66 9.20 Exercisable at December 31, 2019 245,794 $ 1.04 8.24 At December 31, 2019, the aggregate intrinsic value of outstanding options and exercisable options was $0.5 million and $0.2 million, respectively. The following table summarizes information about stock options outstanding at December 31, 2019 under the Plan: Options Outstanding Options Exercisable Range of Exercise Prices Number Weighted Average Weighted Number Weighted $0.31—$1.02 396,590 8.13 $ 0.97 189,233 $ 0.92 $1.43—$1.89 1,873,152 9.43 1.81 56,561 1.43 2,269,742 245,794 The weighted-average grant date fair value of options granted was $0.86 and $1.41 per share for the years ended December 31, 2018 and 2019, respectively. The aggregate intrinsic value of options exercised was $3,000 for the year ended December 31, 2019. The Company recorded stock-based compensation expense of $70,000 and $0.4 million for the years ended December 31, 2018 and 2019, respectively, related to stock options. As of December 31, 2019, the total unrecognized compensation expense related to unvested stock option awards was $2.3 million, which the Company expects to recognize over a weighted-average period of 3.47 years. The fair value of each option was estimated on the date of grant using the weighted average assumptions in the table below: Year Ended 2018 2019 Expected volatility 87.01 % 91.60 % Risk-free interest rate 2.85 % 1.87 % Expected life (in years) 6.25 6.25 Expected dividend yield — — Fair value of common stock $ 1.15 $ 1.84 Restricted Stock Awards The Company issues restricted stock awards (“RSA”) to employees that generally vest over a four-year period with 25% of awards vesting after 1 year and then monthly thereafter. Any unvested shares will be forfeited upon termination of services. The fair value of an RSA is equal to the fair market value price of the Company’s common stock on the date of grant. RSA expense is amortized straight-line over the vesting period. The following table summarizes activity related to RSA stock-based payment awards: Number of shares Weighted-average Unvested balance at January 1, 2019 856,438 $ 0.74 Granted 778,141 1.89 Vested (299,230) 0.68 Unvested balance at December 31, 2019 1,335,349 $ 1.42 The Company recorded stock-based compensation expense of $0.1 million and $0.4 million for the years ended December 31, 2018 and 2019, respectively, related to RSAs. As of December 31, 2019, the total unrecognized expense related to all RSAs was $1.6 million, which the Company expects to recognize over a weighted-average period of 3.14 years.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income Taxes	10. Income Taxes The tax effects of temporary differences that gave rise to significant portions of the deferred tax assets and liabilities were as follows: December 31, (in thousands) 2018 2019 Deferred tax assets: Net operating loss carryforwards $ 5,461 $ 13,659 Research and development credits 739 2,216 Gross deferred tax assets 6,200 15,875 Less: valuation allowance (6,164) (15,409) Total deferred tax asset 36 466 Deferred tax liability Stock-based compensation — (399) Depreciation (36) (67) Total deferred tax liabilities (36) (466) $ — $ — In assessing the need for a valuation allowance, management must determine that there will be sufficient taxable income to allow for the realization of deferred tax assets. Based upon the historical and anticipated future losses, management has determined that the deferred tax assets do not meet the more likely than not threshold for realizability. Accordingly, a full valuation allowance has been recorded against the Company’s net deferred tax assets as of December 31, 2018 and 2019. The valuation allowance increased by $4.6 million and $9.2 million during the years ended December 31, 2018 and 2019, respectively. A reconciliation of the federal income tax rate to the Company’s effective tax rate is as follows: Year ended 2018 2019 Federal tax benefit at statutory rate (21.0) % (21.0) % State tax, net of federal benefit (6.9) (6.9) Return to provision — (0.7) Permanent differences 1.4 0.4 Research and development (5.0) (5.4) Change in valuation allowance 31.5 33.6 — % — % The following table summarizes carryforwards of federal, state and local net operating losses (“NOL”) and research tax credits: December 31, (in thousands) 2018 2019 NOL carryforwards—Federal $ 20,298 $ 49,005 NOL carryforwards—State 20,298 49,005 Research tax credits—Federal 706 2,182 Research tax credits—State 43 43 The NOL carryforwards begin expiring in 2036 for federal and Delaware state income tax purposes, however; all federal and Delaware state NOL carryforwards generated subsequent to January 1, 2018, are able to be carried forward indefinitely. As of December 31, 2019, the Company also had federal and Delaware research and development tax credit carryforwards of $2.2 million and $43,000, respectively that will begin to expire in 2036, unless previously utilized. The NOL and tax credit carryforwards are subject to review and possible adjustment by the Internal Revenue Service and state tax authorities. NOL and tax credit carryforwards may become subject to an annual limitation in the event of certain cumulative changes in the ownership interest of significant stockholders over a three year period in excess of 50%, as defined under Sections 382 and 383 of the Internal Revenue Code, respectively, as well as similar state provisions. This could limit the amount of tax attributes that can be utilized annually to offset future taxable income or tax liabilities. The amount of the annual limitation is determined based on the value of the Company immediately prior to the ownership change. Subsequent ownership changes may further affect the limitation in future years. To date, the Company has not performed an analysis to determine whether or not ownership changes have occurred since inception. Delaware state NOLs may also be limited. As of December 31, 2019, the Company had no accrued interest or penalties related to uncertain tax positions and no amounts have been recognized in the Company’s statement of operations. Due to NOL and tax credit carry forwards that remain unutilized, income tax returns for tax years from 2016, 2017 and 2018 remain subject to examination by the taxing jurisdictions. The NOL carryforwards remain subject to review until utilized.

Subsequent Events

Subsequent Events	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Subsequent Events [Abstract]
Subsequent Events	8. Subsequent Events On November 1, 2020, the Company entered into a Second Amended and Restated Entrepreneur Client License Agreement (the “Amendment”) with Delaware Innovation Space, Inc. (“DISI”) to amend the Amended and Restated Entrepreneur Client License Agreement dated June 1, 2020 (the “Original Agreement”) relating to the Company’s office and laboratory space in Wilmington, Delaware. The Amendment provides for the expansion of the premises by approximately 8,800 rentable square feet (the “Expansion Space”). Upon inclusion of the Expansion Space, the Company will license approximately 19,800 rentable square feet of office space (the “Premises”) from DISI (the “License”). Pursuant to the Amendment, the initial term of the License is twenty-six months commencing on November 1, 2020 and terminating on December 31, 2022 (the “Initial Term”). The total base license fee for the Premises through the Initial Term is expected to be $3.0 million (the “License Fee”). The Company has an option to renew the License for twelve months commencing on January 1, 2023 at a monthly renewal license fee increased by 5.0% for a total of $1.4 million payable during the renewal term. Pursuant to the Amendment, the Company was granted a right of first offer (the “ROFO”) in connection with certain licensable additional space on the Premises that becomes vacant prior to October 31, 2021 (“Available ROFO Space”). If the Company exercises its ROFO, the Company will pay to DISI an increased monthly license fee not to exceed $0.2 million per month at any time during the Initial Term. As consideration for the ROFO, the Company paid DISI a one-time payment of $0.2 million.	11. Subsequent Events In preparing the financial statements as of and for the year ended December 31, 2019, the Company evaluated subsequent events for recognition and measurement purposes through July 23, 2020, the date the financial statements were originally issued, and through September 21, 2020, the date the revised financial statements were issued. Equity Transactions In March 2020, the Company’s Series B investors exercised their Future Tranche Right and purchased 7,628,846 shares of Series B for net proceeds of approximately $29.9 million. In August 2020, the Company’s existing Convertible Preferred Stock investors as well as a new investor purchased 3,443,612 shares of Series C convertible preferred stock at a price of $14.5197 per share for net proceeds of approximately $49.9 million. Reverse Stock Split In September 2020, the Company effected a one-for-1.1566 reverse stock split of its common stock and convertible preferred stock. No fractional shares were issued in connection with the reverse stock split. Any fractional share resulting from the reverse stock split was rounded down to the nearest whole share, and in lieu of any fractional shares, the Company will pay in cash to the holders of such fractional shares an amount equal to the fair value, as determined by the board of directors, of such fractional shares. All common stock, convertible preferred stock, per share and related information presented in the financial statements and accompanying notes have been retroactively adjusted to reflect the reverse stock split. Increase in Authorized Shares of Common Stock and Preferred Stock; Creation of Non-Voting Common Stock In August 2020, the Company effected an increase in the number of authorized shares of its common stock from 42,000,000 shares to 58,850,259 shares, including 12,850,259 shares of the common stock that are non-voting The holders of the voting common stock are entitled to one vote for each share of voting common stock held at all meetings of stockholders. Except as otherwise required by law, the holders of non-voting common stock shall not be entitled to vote at any meetings of stockholders (or written actions in lieu of meetings) and the shares of non-voting common stock shall not be included in determining the number of shares voting or entitled to vote on any matter. Unless required by law, there shall be no cumulative voting. Any holder of non-voting common stock may elect to convert each share of non-voting common stock into one fully paid and non-assessable share of voting common stock at any time by providing written notice to the Company; provided, however, that such shares of non-voting common stock may only be converted by each investors into shares of voting common stock during such time or times as immediately prior to or as a result of such conversion would not result in the holder(s) thereof being a beneficial owner (for purposes of Section 13(d) of the Securities Exchange Act of 1934, as amended, and the rules and regulations promulgated thereunder. Coronavirus Pandemic On March 10, 2020, the World Health Organization characterized the novel COVID-19 virus as a global pandemic. There is significant uncertainty as to the likely effects of this disease which may, among other things, materially impact the Company’s planned clinical trials. This pandemic or outbreak could result in difficulty securing clinical trial site locations, CROs, and/or trial monitors and other critical vendors and consultants supporting the trial. In addition, outbreaks or the perception of an outbreak near a clinical trial site location could impact the Company’s ability to enroll patients. These situations, or others associated with COVID-19, could cause delays in the Company’s clinical trial plans and could increase expected costs, all of which could have a material adverse effect on the Company’s business and its financial condition. At the current time, the Company is unable to quantify the potential effects of this pandemic on its future financial statements.

Background

Background	9 Months Ended
	Sep. 30, 2020
Background [Abstract]
Background	1. Background Prelude Therapeutics Incorporated (the “Company”) was incorporated in Delaware on February 5, 2016 and is a clinical-stage precision oncology company focused on discovering and developing small molecule therapies optimized to target the key driver mechanisms in cancers with high unmet need. Since beginning operations, the Company has devoted substantially all its efforts to research and development, conducting preclinical and clinical studies, recruiting management and technical staff, administration, and raising capital. In September 2020, the Company closed its initial public offering (“IPO”) in which the Company issued and sold 9,573,750 shares of its common stock at a public offering price of $19.00 per share for net proceeds of $166.6 million after deducting underwriting discounts, commissions and other offering expenses paid or to be paid. Subsequent to the closing of the IPO, there were no shares of convertible preferred stock outstanding. In connection with the closing of the IPO, the Company amended and restated its Fifth Amended and Restated Certificate of Incorporation to change the authorized capital stock to 500,000,000 shares designated as common stock (of which 12,850,259 shares are non-voting common stock) and 10,000,000 shares designated as preferred stock, all with a par value of $0.0001 per share.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Basis of presentation	Basis of presentation The accompanying unaudited interim financial statements have been prepared in accordance with generally accepted accounting principles (“GAAP”) for interim financial information, the instructions to Form 10-Q and Article 10 of Regulation S-X. They do not include all of the information and notes required by GAAP for complete financial statements. In the opinion of management, all adjustments (consisting of normal recurring accruals) considered necessary for a fair presentation have been included. Operating results for the three and nine month periods ended September 30, 2020 are not necessarily indicative of the results that may be expected for the year ended December 31, 2020. The accompanying unaudited interim financial statements should be read in conjunction with the annual audited financial statements and related notes as of and for the year ended December 31, 2019 found in the Prospectus filed with the SEC on September 25, 2020. 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”) of the Financial Accounting Standards Board (“FASB”). On September 18, 2020, the Company’s board of directors and stockholders approved a 1.1566-to-one reverse stock split of the Company’s issued and outstanding shares of common stock and convertible preferred stock. All share and per share amounts in the unaudited interim financial statements and notes hereto have been retrospectively adjusted for all periods presented to give effect to the reverse stock split. The completion of the IPO, as described above, impacts the comparability of certain amounts to the corresponding prior year period, including earnings per share.	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”).
Use of estimates	Use of estimates The preparation of the unaudited interim financial statements in conformity with U.S. 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 unaudited interim 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 unaudited interim 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 common stock, stock-based compensation assumptions and accrued clinical trial expenses. Subsequent to the IPO, the most significant estimate relates to accrued clinical trial expenses.	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. Significant areas that require management’s estimates include the fair value of the Company’s common stock, stock-based compensation assumptions, and accrued clinical trial expenses.
Concentration of Credit Risk		Concentration of Credit Risk Financial instruments that potentially subject the Company to significant concentrations of credit risk consist of cash and cash equivalents. The Company maintains deposits in federally insured financial institutions in excess of federally insured limits. The Company has not experienced any losses in such accounts and believes it is not exposed to significant credit risk beyond the normal credit risk associated with commercial banking relationships.
Segment Information		Segment Information Operating segments are defined as components of an enterprise about which separate discrete information is available for evaluation by the chief operating decision maker, or decision-making group, in deciding how to allocate resources and in assessing performance. The Company views and manages its operations as a single operating segment.
Fair Value of Financial Instruments		Fair Value of Financial Instruments Management believes that the carrying amounts of the Company’s financial instruments, including cash, accounts payable, accrued expenses and a capital lease obligation, approximate fair value due to the short-term nature of these instruments.
Cash Equivalents		Cash Equivalents The Company’s cash equivalents include short-term highly liquid investments with an original maturity of 90 days or less when purchased and are carried at fair value in the accompanying balance sheets.
Property and Equipment		Property and Equipment Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation expense is recognized using the straight-line method over the estimated useful life of the asset, ranging from 5-7 years Fixed Asset Type Estimated useful life Lab equipment 5 years Computers 5 years Furniture and fixtures 7 years Leasehold improvements are amortized over the shorter of the estimated useful life of the assets or the remaining lease term. Assets under capital leases are recorded in property and equipment, net on the balance sheets and depreciated in a manner similar to other property and equipment. Expenditures for repairs and maintenance of assets are charged to expense as incurred, while major betterments are capitalized. Upon retirement or sale, the cost and related accumulated depreciation of assets disposed of are removed from the accounts and any resulting gain or loss is included in the statements of operations. 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. If indicators of impairment are present, the assets are tested for recoverability by comparing the carrying amount of the assets to the related estimated future undiscounted cash flows that the assets are expected to generate. If the expected cash flows are less than the carrying value of the asset, then the asset is considered to be impaired and its carrying value is written down to fair value, based on the related estimated discounted future cash flows.
Stock-Based Compensation		Stock-Based Compensation The Company measures the cost of employee services received in exchange for stock-based awards based on the grant-date fair value of the award. The Company recognizes compensation expense using the straight-line method over the vesting period of the awards. The Company accounts for forfeitures as they occur. Estimating the fair value of stock-based awards requires the input of subjective assumptions, including 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 uses the Black-Scholes option pricing model to value its stock option awards. As the Company’s common stock has not been publicly traded, its board of directors periodically estimated the fair value of the Company’s common stock considering, among other things, contemporaneous valuations of its common stock prepared by an unrelated third-party valuation firm in accordance with the guidance provided by the American Institute of Certified Public Accountants 2013 Practice Aid, Valuation of Privately-Held-Company Equity Securities Issued as Compensation post-vesting The assumptions used in estimating the fair value of stock-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, stock-based compensation expense could be materially different for future awards.
Grant Income and Research and Development Tax Credits		Grant Income and Research and Development Tax Credits The Company recognizes grant income and Delaware research and development tax credits, which are refundable irrespective of taxable income, in other income, net in the statements of operations when it is probable that the amounts will be received and the necessary qualifying conditions, as stated in the agreements, are met.
Research and Development		Research and Development Research and development costs are expensed as incurred. Research and development expenses consist principally of personnel costs, including salaries, stock-based compensation, and benefits of employees, and other operational costs related to the Company’s research and development activities, including allocated facility-related Management makes estimates of the Company’s accrued research and development expenses as of each balance sheet date in the Company’s financial statements based on facts and circumstances known to the Company at that time. If the actual timing of the performance of services or the level of effort varies from the estimate, the Company will adjust the accrual accordingly. Nonrefundable advance payments for goods and services, including fees for process development or manufacturing and distribution of clinical supplies that will be used in future research and development activities, are deferred and recognized as expense in the period that the related goods are consumed or services are performed.
Income taxes	Income taxes Based upon the historical and anticipated future losses, management has determined that the deferred tax assets generated by net operating losses and research and development credits do not meet the more likely than not threshold for realizability. Accordingly, a full valuation allowance has been recorded against the Company’s net deferred tax assets as of December 31, 2019 and September 30, 2020.	Income Taxes Income taxes are accounted for under the asset-and-liability method as required by FASB ASC Topic 740, Income Taxes FASB ASC Subtopic 740-10, Accounting for Uncertainty of Income Taxes
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. The weighted-average number of shares of common stock outstanding used in the basic net loss per share calculation does not include unvested restricted stock awards as these instruments are considered contingently issuable shares until they vest. Diluted net loss per share of common stock includes the effect, if any, from the potential exercise or conversion of securities, such as convertible preferred stock and 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 Company’s convertible preferred stock and unvested restricted stock entitles the holder to participate in dividends and earnings of the Company, and, if the Company were to recognize net income, it would have to use the two-class method to calculate earnings per share. The two-class method is not applicable during periods with a net loss, as the holders of the convertible preferred stock and unvested restricted stock have no obligation to fund losses. 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: September 30, 2019 2020 Series A convertible preferred stock 11,736,119 — Series B convertible preferred stock 7,628,846 — Unvested restricted stock awards 1,394,056 1,312,024 Stock options 2,189,333 6,649,882 22,948,354 7,961,906 Amounts in the above table reflect the common stock equivalents.	Net Loss Per Share and Unaudited Pro Forma 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. The weighted-average number of shares of common stock outstanding used in the basic net loss per share calculation does not include unvested restricted stock awards as these instruments are considered contingently issuable shares until they vest. Diluted net loss per share of common stock includes the effect, if any, from the potential exercise or conversion of securities, such as convertible preferred stock and 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 Company’s convertible preferred stock and unvested restricted stock entitles the holder to participate in dividends and earnings of the Company, and, if the Company were to recognize net income, it would have to use the two-class method to calculate earnings per share. The two-class method is not applicable during periods with a net loss, as the holders of the convertible preferred stock and unvested restricted stock have no obligation to fund losses. 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: December 31, 2018 2019 Series A convertible preferred stock 11,736,119 11,736,119 Series B convertible preferred stock — 7,628,846 Unvested restricted stock awards 856,438 1,335,349 Stock options 588,961 2,269,742 13,181,518 22,970,056 Amounts in the above table reflect the common stock equivalents. The unaudited pro forma net loss per share is computed using the weighted average number of shares of common stock outstanding after giving effect to the automatic conversion of all convertible preferred stock, inclusive of the 7,628,846 shares of Series B convertible preferred stock issued in March 2020 and the 3,443,612 shares of Series C convertible preferred stock issued in August 2020, into shares of common stock upon the closing of a qualified initial public offering, as if the qualified initial public offering had occurred at the beginning of the period. The following table summarizes the calculation of unaudited pro forma basic and diluted net loss per share of common stock for the year ended December 31, 2019: (in thousands, except share and per share data) Numerator: Net loss $ (27,570) Denominator: Weighted average shares of common stock outstanding 1,668,549 Conversion of convertible preferred stock 30,437,423 Shares issued in computing unaudited pro forma weighted average basic and diluted shares of common stock outstanding 32,105,972 Pro forma net loss per common share, basic and diluted $ (0.86)
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements Emerging Growth Company Status The Company is an emerging growth company, as defined in the Jumpstart Our Business Startups Act of 2012 (the “JOBS Act”). Under the JOBS Act, emerging growth companies can delay adopting new or revised accounting standards issued subsequent to the enactment of the JOBS Act, until such time as those standards apply to private companies. The Company has elected to use this extended transition period for complying with new or revised accounting standards that have different effective dates for public and private companies until the earlier of the date that it (i) is no longer an emerging growth company or (ii) affirmatively and irrevocably opts out of the extended transition period provided in the JOBS Act. As a result, these unaudited interim financial statements may not be comparable to companies that comply with the new or revised accounting pronouncements as of public company effective dates. Recent Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases In August 2018, the FASB issued ASU 2018-13, Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurements, Fair Value Measurement	Recently Issued Accounting Pronouncements Emerging Growth Company Status The Company is an emerging growth company, as defined in the Jumpstart Our Business Startups Act of 2012 (the “JOBS Act”). Under the JOBS Act, emerging growth companies can delay adopting new or revised accounting standards issued subsequent to the enactment of the JOBS Act, until such time as those standards apply to private companies. The Company has elected to use this extended transition period for complying with new or revised accounting standards that have different effective dates for public and private companies until the earlier of the date that it (i) is no longer an emerging growth company or (ii) affirmatively and irrevocably opts out of the extended transition period provided in the JOBS Act. As a result, these financial statements may not be comparable to companies that comply with the new or revised accounting pronouncements as of public company effective dates. Recent Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases In June 2018, the FASB issued ASU 2018-07, Compensation-Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting Equity-Based Payments to Non-Employees In August 2018, the FASB issued ASU 2018-13, Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurements, Fair Value Measurement

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Schedule of Property Plant and Equipment Useful Life		Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation expense is recognized using the straight-line method over the estimated useful life of the asset, ranging from 5-7 years Fixed Asset Type Estimated useful life Lab equipment 5 years Computers 5 years Furniture and fixtures 7 years
Schedule of Potentially Dilutive Securities Excluded from Computation of Diluted Weighted-average Shares of Common Stock Outstanding	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: September 30, 2019 2020 Series A convertible preferred stock 11,736,119 — Series B convertible preferred stock 7,628,846 — Unvested restricted stock awards 1,394,056 1,312,024 Stock options 2,189,333 6,649,882 22,948,354 7,961,906	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: December 31, 2018 2019 Series A convertible preferred stock 11,736,119 11,736,119 Series B convertible preferred stock — 7,628,846 Unvested restricted stock awards 856,438 1,335,349 Stock options 588,961 2,269,742 13,181,518 22,970,056
Schedule of Unaudited Pro Forma Basic and Diluted Net Loss Per Share of Common Stock		The following table summarizes the calculation of unaudited pro forma basic and diluted net loss per share of common stock for the year ended December 31, 2019: (in thousands, except share and per share data) Numerator: Net loss $ (27,570) Denominator: Weighted average shares of common stock outstanding 1,668,549 Conversion of convertible preferred stock 30,437,423 Shares issued in computing unaudited pro forma weighted average basic and diluted shares of common stock outstanding 32,105,972 Pro forma net loss per common share, basic and diluted $ (0.86)

Fair Value of Financial Instr_2

Fair Value of Financial Instruments (Tables)	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis	The following fair value hierarchy table presents information about the Company’s assets and liabilities measured at fair value on a recurring basis: Fair value measurement at reporting date using (in thousands) Quoted prices in active markets for identical assets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) December 31, 2019: Assets: Cash equivalents (Money Market Funds) $ 18,779 $ — $ — September 30, 2020: Assets: Cash equivalents (Money Market Funds) $ 233,364 $ — $ —	The following fair value hierarchy table presents information about the Company’s assets and liabilities measured at fair value on a recurring basis: Fair value measurement at (in thousands) Quoted prices Significant Significant December 31, 2018: Assets: Cash equivalents (Money Market Funds) $ 15,400 $ — $ — December 31, 2019: Assets: Cash equivalents (Money Market Funds) $ 18,779 $ — $ —

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2019
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment consisted of the following: December 31, 2018 2019 (in thousands) Lab equipment $ 674 $ 1,842 Leasehold improvements 312 312 Computers 13 10 Furniture and fixtures 3 39 1,002 2,203 Less accumulated depreciation (191) (556) Property and equipment, net $ 811 $ 1,647

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Payables and Accruals [Abstract]
Summary of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of the following: (in thousands) December 31, 2019 September 30, 2020 Compensation and related benefits $ 1,631 $ 2,099 Research and development 658 3,763 Offering costs — 220 Professional services and other 314 203 $ 2,603 $ 6,285	Accrued expenses consisted of the following: December 31, 2018 2019 (in thousands) Compensation and related benefits $ 873 $ 1,631 Research and development 153 658 Other 59 314 $ 1,085 $ 2,603

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Stock-Based Compensation Expense Recorded in Statements of Operations	The Company recorded stock-based compensation expense in the following expense categories in its accompanying statements of operations: Three Months Ended September 30, Nine Months September 30, (in thousands) 2019 2020 2019 2020 Research and development $ 135 $ 630 $ 250 $ 1,047 General and administrative 162 842 270 1,414 $ 297 $ 1,472 $ 520 $ 2,461	The Company measures stock-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 stock-based compensation expense in the following expense categories in its accompanying statements of operations: Year Ended 2018 2019 (in thousands) Research and development $ 178 $ 437 General and administrative 2 409 $ 180 $ 846
Summary of Stock Option Activity	The following table summarizes stock option activity for the periods indicated: Number of shares Weighted average exercise price per share Weighted average remaining contractual term (years) Outstanding at January 1, 2020 2,269,742 $ 1.66 9.20 Granted 4,594,130 $ 9.38 Exercised (98,254) $ 0.98 Forfeited (115,736) $ 1.37 Outstanding at September 30, 2020 6,649,882 $ 7.00 9.40 Exercisable at September 30, 2020 783,236 $ 1.64 8.40	The following table summarizes stock option activity for the Plan in the years indicated: Number Weighted Weighted Outstanding at January 1, 2018 25,938 $ 0.31 9.14 Granted 563,023 $ 1.15 Outstanding at December 31, 2018 588,961 $ 1.11 9.35 Granted 1,703,692 $ 1.84 Forfeited (19,669) $ 1.16 Exercised (3,242) $ 1.02 Outstanding at December 31, 2019 2,269,742 $ 1.66 9.20 Exercisable at December 31, 2019 245,794 $ 1.04 8.24
Summary of Information about Stock Options Outstanding	The following table summarizes information about stock options outstanding at September 30, 2020 under the Plan: Options Outstanding Options Exercisable Range of Exercise Prices Number Outstanding Weighted Average Remaining Contractual Life (in years) Weighted Average Exercise Price Number Exercisable Weighted Average Exercise Price $0.31 - $1.02 253,931 7.43 $ 0.98 162,037 $ 0.96 $1.43 - $1.89 3,259,095 9.05 1.85 619,998 1.79 $12.85 - $19.00 3,136,856 9.92 12.86 1,201 12.85 6,649,882 783,236	The following table summarizes information about stock options outstanding at December 31, 2019 under the Plan: Options Outstanding Options Exercisable Range of Exercise Prices Number Weighted Average Weighted Number Weighted $0.31—$1.02 396,590 8.13 $ 0.97 189,233 $ 0.92 $1.43—$1.89 1,873,152 9.43 1.81 56,561 1.43 2,269,742 245,794
Schedule of Fair Value of Each Option Estimated on Date of Grant Using Weighted Average Assumptions	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, 2019 2020 Expected volatility 91.60 % 115.09 % Risk-free interest rate 1.88 % 0.42 % Expected life (in years) 6.25 6.25 Expected dividend yield — —	The fair value of each option was estimated on the date of grant using the weighted average assumptions in the table below: Year Ended 2018 2019 Expected volatility 87.01 % 91.60 % Risk-free interest rate 2.85 % 1.87 % Expected life (in years) 6.25 6.25 Expected dividend yield — — Fair value of common stock $ 1.15 $ 1.84
Summary of Activity related to RSA Stock-Based Payment Awards	The following table summarizes activity related to RSA stock-based payment awards: Number of shares Weighted-average grant date fair value Unvested balance at January 1, 2020 1,335,349 $ 1.42 Granted 432,301 $ 3.26 Vested (455,626) $ 1.40 Unvested balance at September 30, 2020 1,312,024 $ 2.03	The following table summarizes activity related to RSA stock-based payment awards: Number of shares Weighted-average Unvested balance at January 1, 2019 856,438 $ 0.74 Granted 778,141 1.89 Vested (299,230) 0.68 Unvested balance at December 31, 2019 1,335,349 $ 1.42

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Schedule of Deferred Tax Assets and Liabilities	The tax effects of temporary differences that gave rise to significant portions of the deferred tax assets and liabilities were as follows: December 31, (in thousands) 2018 2019 Deferred tax assets: Net operating loss carryforwards $ 5,461 $ 13,659 Research and development credits 739 2,216 Gross deferred tax assets 6,200 15,875 Less: valuation allowance (6,164) (15,409) Total deferred tax asset 36 466 Deferred tax liability Stock-based compensation — (399) Depreciation (36) (67) Total deferred tax liabilities (36) (466) $ — $ —
Schedule of Reconciliation of the Federal Income tax Rate to Effrective Tax Rate	A reconciliation of the federal income tax rate to the Company’s effective tax rate is as follows: Year ended 2018 2019 Federal tax benefit at statutory rate (21.0) % (21.0) % State tax, net of federal benefit (6.9) (6.9) Return to provision — (0.7) Permanent differences 1.4 0.4 Research and development (5.0) (5.4) Change in valuation allowance 31.5 33.6 — % — %
Schedule of Federal State and Local Net Operating Losses and Research Tax Credits	The following table summarizes carryforwards of federal, state and local net operating losses (“NOL”) and research tax credits: December 31, (in thousands) 2018 2019 NOL carryforwards—Federal $ 20,298 $ 49,005 NOL carryforwards—State 20,298 49,005 Research tax credits—Federal 706 2,182 Research tax credits—State 43 43

Risks and Liquidity - Additiona

Risks and Liquidity - Additional Information (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Risks And Liquidity [Line Items]
Accumulated deficit	$ (88,176)	$ (50,497)	$ (22,927)
Cash and cash equivalents	234,792	18,879	$ 15,595
IPO
Risks And Liquidity [Line Items]
Net proceeds from issuance of common stock	$ 166,600
Series B Preferred Stock
Risks And Liquidity [Line Items]
Cash financed from preferred stock		$ 29,900

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Detail)	Sep. 18, 2020	Dec. 31, 2019shares	Sep. 30, 2020
Summary Of Significant Accounting Policies [Line Items]
Depreciation expense description		Depreciation expense is recognized using the straight-line method over the estimated useful life of the asset, ranging from 5-7 years
Reverse stock split	0.8646
Series B Convertible Preferred Stock
Summary Of Significant Accounting Policies [Line Items]
Convertible preferred stock issued		7,628,846
Series C Convertible Preferred Stock
Summary Of Significant Accounting Policies [Line Items]
Convertible preferred stock issued		3,443,612
ASU 2018-13
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, ASU, adopted			true
Change in accounting principle, ASU, adoption date			Jan. 1, 2020
Change in accounting principle, ASU, immaterial effect			true
Maximum
Summary Of Significant Accounting Policies [Line Items]
Percentage of tax benefit recognized upon settlement		50.00%

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Schedule of Property Plant and Equipment Useful Life (Detail)	12 Months Ended
	Dec. 31, 2019
Lab equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	5 years
Computers
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	5 years
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	7 years

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Schedule of Potentially Dilutive Securities Excluded From Computation of Diluted Weighted-average Shares of Common Stock Outstanding (Detail) - shares	9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	7,961,906	22,948,354	22,970,056	13,181,518
Series A Convertible Preferred Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share		11,736,119	11,736,119	11,736,119
Series B Convertible Preferred Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share		7,628,846	7,628,846
Restricted Stock Awards
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	1,312,024	1,394,056	1,335,349	856,438
Stock Options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	6,649,882	2,189,333	2,269,742	588,961

Summary of Significant Accoun_7

Summary of Significant Accounting Policies - Schedule of Unaudited Pro Forma Basic and Diluted Net Loss Per Share of Common Stock (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended		12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Numerator:
Net loss	$ (16,760)	$ (11,408)	$ (9,511)	$ (6,734)	$ (5,556)	$ (5,241)	$ (37,679)	$ (17,531)	$ (27,570)	$ (14,680)
Denominator:
Weighted average shares of common stock outstanding	3,194,471			1,713,371			2,380,100	1,622,546	1,668,549	1,402,928
Conversion of convertible preferred stock									30,437,423
Shares issued in computing unaudited pro forma weighted average basic and diluted shares of common stock outstanding									32,105,972
Pro forma net loss per common share, basic and diluted									$ (0.86)

Fair Value of Financial Instr_3

Fair Value of Financial Instruments - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Quoted Prices in Active Markets for Identical Assets (Level 1) | Money Market Funds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, recurring basis, assets	$ 233,364	$ 18,779	$ 15,400

Property and Equipment - Schedu

Property and Equipment - Schedule of Property and Equipment (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		$ 2,203	$ 1,002
Less accumulated depreciation		(556)	(191)
Property and equipment, net	$ 1,620	1,647	811
Lab equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		1,842	674
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		312	312
Computers
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		10	13
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross		$ 39	$ 3

Property and Equipment - Additi

Property and Equipment - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Depreciation and amortization expense		$ 0.4	$ 0.1
Lab equipment
Property, Plant and Equipment [Line Items]
Lease term	12 months
Capital lease	$ 0.4
Effective interest percentage	9.67%
Accumulated amortization related to the capital lease		0.1
Future minimum lease payments to be paid in 2020		$ 0.3

Accrued Expenses and Other Cu_3

Accrued Expenses and Other Current Liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Compensation and related benefits	$ 2,099	$ 1,631	$ 873
Research and development	3,763	658	153
Offering costs	220
Professional services and other	203	314	59
Accrued expenses and other current liabilities	$ 6,285	$ 2,603	$ 1,085

Commitments - Additional Inform

Commitments - Additional Information (Detail)	1 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Other Commitments [Line Items]
Number of noncancelable leases		2	2
Rent expense		$ 900,000	$ 700,000
Future minimum lease payments for 2020		1,000,000
Future minimum lease payments for 2021		21,000
Future minimum lease payments thereafter		$ 0
Lease extension description	The Company extended the lease that was set to expire in November 2020 until December 2021.
Operating Lease Expiring November 2020
Other Commitments [Line Items]
Lease renewal period		1 year
Operating Lease Expiring March 2021
Other Commitments [Line Items]
Lease renewal period		6 months

Convertible Preferred Stock a_2

Convertible Preferred Stock and Common Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Aug. 31, 2020	Sep. 30, 2020	Mar. 31, 2020	May 31, 2019	Aug. 31, 2018	Oct. 31, 2017	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Class of Stock [Line Items]
Proceeds from the issuance of common stock upon initial public offering, net of offering costs							$ 167,929
Common stock, shares authorized									42,000,000
Common Stock
Class of Stock [Line Items]
Common stock, voting rights							The holders of the voting common stock are entitled to one vote for each share of voting common stock held at all meetings of stockholders.		The holders of the common stock are entitled to one vote for each share of common stock held at all meetings of stockholders.
Minimum | Common Stock
Class of Stock [Line Items]
Beneficial ownership percentage							9.99%
Maximum | Common Stock
Class of Stock [Line Items]
Beneficial ownership percentage							19.99%
IPO
Class of Stock [Line Items]
Proceeds from the issuance of common stock upon initial public offering, net of offering costs		$ 166,600							$ 50,000
Sale of common stock in initial public offering, net of offering costs of $2,538, (shares)		9,573,750
Convertible preferred stock converted into common stock		30,437,423					30,437,423
Common stock, shares authorized		500,000,000					500,000,000
IPO | Common Stock
Class of Stock [Line Items]
Public offering price		$ 19					$ 19		$ 11.80
Series C Convertible Preferred Stock
Class of Stock [Line Items]
Convertible preferred stock, shares issued	3,443,612	0					0		0
Convertible preferred stock, price per share	$ 14.5197
Net proceeds from issuance of preferred stock	$ 49,800						$ 49,826
Temporary equity, shares authorized		0					0		0
Convertible preferred stock converted into common stock									3,443,612
Series B Convertible Preferred Stock
Class of Stock [Line Items]
Convertible preferred stock, shares issued		0	7,628,846	7,628,846			0		7,628,846	0
Convertible preferred stock, price per share				$ 3.9325					$ 3.9325
Net proceeds from issuance of preferred stock			$ 29,900	$ 29,800			$ 29,942	$ 29,848	$ 29,848
Temporary equity, shares authorized		0		7,628,846			0		18,500,000
Convertible preferred stock, price per share				$ 3.9325
Temporary equity, contract terms				In the event the holders did not purchase additional shares of Series B, their initial shares of Series B would have automatically converted into shares of the Company's common stock at a conversion ratio of 10 shares of Series B for 1 share of common stock.
Convertible preferred stock converted into common stock									7,628,846
Non-voting Common Stock
Class of Stock [Line Items]
Common stock, shares authorized		12,850,259					12,850,259		0
Non-voting Common Stock | IPO
Class of Stock [Line Items]
Convertible preferred stock converted into common stock		11,110,371					11,110,371
Series A Convertible Preferred Stock
Class of Stock [Line Items]
Convertible preferred stock, shares issued		0			5,728,376		0		11,736,119	11,736,119
Convertible preferred stock, price per share					$ 3.1423				$ 3.1423
Net proceeds from issuance of preferred stock					$ 17,900					$ 17,937
Temporary equity, shares authorized		0				5,728,376	0		13,574,008
Convertible preferred stock, price per share						$ 3.1423
Temporary equity, contract terms						In the event the holders did not purchase additional shares of Series A, their initial shares of Series A would have automatically converted into shares of the Company's common stock at a conversion ratio of 10 shares of Series A for 1 share of common stock. In August 2018, the Series A Future Tranche Right was exercised in full by the Series A investors.
Convertible Preferred Stock
Class of Stock [Line Items]
Common stock, voting rights									Holders of the Convertible Preferred Stock are entitled to one vote for each share of common stock into which their shares may be converted and, subject to certain Convertible Preferred Stock class votes specified in the Company's certificate of incorporation or as required by law, holders of the Convertible Preferred Stock and common stock vote together on an as-converted basis.
Percentage of share holders elected for affirmative election									66.00%

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020USD ($)shares	Sep. 30, 2019USD ($)	Sep. 30, 2020USD ($)Unit$ / sharesshares	Sep. 30, 2019USD ($)$ / shares	Dec. 31, 2019USD ($)$ / sharesshares	Dec. 31, 2018USD ($)$ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate intrinsic value of outstanding options	$ 153,700,000		$ 153,700,000		$ 500,000
Aggregate intrinsic value of exercisable options	22,300,000		22,300,000		200,000
Stock-based compensation expense	1,472,000	$ 297,000	$ 2,461,000	$ 520,000	$ 846,000	$ 180,000
Number of equity incentive plans | Unit			2
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average grant date fair value of options granted | $ / shares			$ 8.44	$ 1.40	$ 1.41	$ 0.86
Aggregate intrinsic value of options exercised			$ 2,900,000		$ 3,000
Stock-based compensation expense	1,200,000	200,000	1,900,000	$ 300,000	400,000	$ 70,000
Total unrecognized compensation expense related to unvested stock option awards	39,100,000		$ 39,100,000		$ 2,300,000
Total unrecognized compensation expense, weighted-average period			3 years 8 months 26 days		3 years 5 months 19 days
Restricted Stock Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period			4 years		4 years
Stock-based compensation expense	300,000	$ 100,000	$ 600,000	$ 300,000	$ 400,000	$ 100,000
Total unrecognized compensation expense, weighted-average period			2 years 11 months 8 days		3 years 1 month 20 days
Share-based payment arrangement, nonvested award, unrecognized expense	$ 2,400,000		$ 2,400,000		$ 1,600,000
2020 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for future grants | shares	4,677,407		4,677,407
Award term			10 years
Number of shares authorized | shares	4,680,000		4,680,000
Terms of award			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.
Percentage of shares outstanding used to calculate annual increase in number of shares that can be issued			5.00%
2020 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for future grants | shares	520,000		520,000
Shares issued | shares			0
2016 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for future grants | shares					364,853
Award term					10 years
Maximum | 2020 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period			4 years
Maximum | 2016 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period					4 years
Minimum | 2020 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period			1 year
Minimum | 2016 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period					1 year
Vesting After One Year | Restricted Stock Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of stock options vested			25.00%		25.00%
Vesting After One Year | 2020 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of stock options vested			25.00%
Vesting After One Year | 2016 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of stock options vested					25.00%

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Stock-Based Compensation Expense Recorded in Statements of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 1,472	$ 297	$ 2,461	$ 520	$ 846	$ 180
Research and Development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	630	135	1,047	250	437	178
General and Administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 842	$ 162	$ 1,414	$ 270	$ 409	$ 2

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Detail) - $ / shares	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of shares, Outstanding at beginning of period	2,269,742	588,961	25,938
Number of shares, Granted	4,594,130	1,703,692	563,023
Number of shares, Exercised	(98,254)	(3,242)
Number of shares, Forfeited	(115,736)	(19,669)
Number of shares, Outstanding at end of period	6,649,882	2,269,742	588,961	25,938
Number of shares, Exercisable	783,236	245,794
Weighted average exercise price per share, Outstanding at beginning of period	$ 1.66	$ 1.11	$ 0.31
Weighted average exercise price per share, Granted	9.38	1.84	1.15
Weighted average exercise price per share, Exercised	0.98	1.16
Weighted average exercise price per share, Forfeited	1.37	1.02
Weighted average exercise price per share, Outstanding at end of period	7	1.66	$ 1.11	$ 0.31
Weighted average exercise price per share, Exercisable	$ 1.64	$ 1.04
Weighted average remaining contractual term (years), Outstanding	9 years 4 months 24 days	9 years 2 months 12 days	9 years 4 months 6 days	9 years 1 month 20 days
Weighted average remaining contractual term (years), Exercisable	8 years 4 months 24 days	8 years 2 months 26 days

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Information about Stock Options Outstanding (Detail) - $ / shares	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options Outstanding, Number	6,649,882	2,269,742
Options Exercisable, Number Exercisable	783,236	245,794
$0.31 - $1.02
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Range of Exercise Prices, Minimum	$ 0.31	$ 0.31
Range of Exercise Prices, Maximum	$ 1.02	$ 1.02
Options Outstanding, Number	253,931	396,590
Options Outstanding, Weighted Average Remaining Contractual Life (in years)	7 years 5 months 4 days	8 years 1 month 17 days
Options Outstanding, Weighted Average Exercise Price	$ 0.98	$ 0.97
Options Exercisable, Number Exercisable	162,037	189,233
Options Exercisable, Weighted Average Exercise Price	$ 0.96	$ 0.92
$1.43 - $1.89
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Range of Exercise Prices, Minimum	1.43	1.43
Range of Exercise Prices, Maximum	$ 1.89	$ 1.89
Options Outstanding, Number	3,259,095	1,873,152
Options Outstanding, Weighted Average Remaining Contractual Life (in years)	9 years 18 days	9 years 5 months 4 days
Options Outstanding, Weighted Average Exercise Price	$ 1.85	$ 1.81
Options Exercisable, Number Exercisable	619,998	56,561
Options Exercisable, Weighted Average Exercise Price	$ 1.79	$ 1.43
$12.85 - $19.00
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Range of Exercise Prices, Minimum	12.85
Range of Exercise Prices, Maximum	$ 19
Options Outstanding, Number	3,136,856
Options Outstanding, Weighted Average Remaining Contractual Life (in years)	9 years 11 months 1 day
Options Outstanding, Weighted Average Exercise Price	$ 12.86
Options Exercisable, Number Exercisable	1,201
Options Exercisable, Weighted Average Exercise Price	$ 12.85

Stock-Based Compensation - Sc_2

Stock-Based Compensation - Schedule of Fair Value of Each Option Estimated on Date of Grant Using Weighted Average Assumptions (Detail) - Stock Options - $ / shares	9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility	115.09%	91.60%	91.60%	87.01%
Risk-free interest rate	0.42%	1.88%	1.87%	2.85%
Expected life (in years)	6 years 3 months	6 years 3 months	6 years 3 months	6 years 3 months
Fair value of common stock			$ 1.84	$ 1.15

Stock-Based Compensation - Su_3

Stock-Based Compensation - Summary of Activity related to RSA Stock-Based Payment Awards (Detail) - Restricted Stock Awards - $ / shares	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares, Unvested balance at beginning of period	1,335,349	856,438
Number of shares, Granted	432,301	778,141
Number of shares, Vested	(455,626)	(299,230)
Number of shares, Unvested balance at end of period	1,312,024	1,335,349
Weighted-average grant date fair value, Unvested balance	$ 1.42	$ 0.74
Weighted-average grant date fair value, Granted	3.26	1.89
Weighted-average grant date fair value, Vested	1.40	0.68
Weighted-average grant date fair value, Unvested balance	$ 2.03	$ 1.42

Income Taxes - Schedule of Defe

Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Net operating loss carryforwards	$ 13,659	$ 5,461
Research and development credits	2,216	739
Gross deferred tax assets	15,875	6,200
Less: valuation allowance	(15,409)	(6,164)
Total deferred tax asset	466	36
Deferred tax liability
Stock-based compensation	(399)
Depreciation	(67)	(36)
Total deferred tax liabilities	(466)	(36)
Net deferred income tax assets liabilities	$ 0	$ 0

Income Tax - Additional Informa

Income Tax - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Line Items]
Deferred tax assets, valuation allowance increase	$ 9,200,000	$ 4,600,000
Research and development tax credits	$ 2,216,000	$ 739,000
NOL and tax credit carryforwards	The NOL and tax credit carryforwards are subject to review and possible adjustment by the Internal Revenue Service and state tax authorities. NOL and tax credit carryforwards may become subject to an annual limitation in the event of certain cumulative changes in the ownership interest of significant stockholders over a three year period in excess of 50%, as defined under Sections 382 and 383 of the Internal Revenue Code, respectively, as well as similar state provisions.
Internal Revenue Service (IRS)
Income Tax Disclosure [Line Items]
Research and development tax credits	$ 2,200,000
Tax credits expiration year	2036
Delaware
Income Tax Disclosure [Line Items]
Research and development tax credits	$ 43,000
Tax credits expiration year	2036

Income Taxes - Schedule of Reco

Income Taxes - Schedule of Reconciliation of The Effrective Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal tax benefit at statutory rate	(21.00%)	(21.00%)
State tax, net of federal benefit	(6.90%)	(6.90%)
Return to provision	(0.70%)
Permanent differences	0.40%	1.40%
Research and development	(5.40%)	(5.00%)
Change in valuation allowance	33.60%	31.50%
Effective tax rate	0.00%	0.00%

Income Tax - Schedule Of Federa

Income Tax - Schedule Of Federal State And Local Net Operating Losses And Research Tax Credits (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Federal
Operating Loss Carryforwards [Line Items]
NOL carryforwards	$ 49,005	$ 20,298
Research tax credits	2,182	706
State
Operating Loss Carryforwards [Line Items]
NOL carryforwards	49,005	20,298
Research tax credits	$ 43	$ 43

Subsequent Event - Additional I

Subsequent Event - Additional Information (Detail) $ / shares in Units, $ in Thousands	Sep. 18, 2020	Aug. 31, 2020USD ($)$ / sharesshares	Nov. 01, 2020USD ($)ft²	Sep. 30, 2020shares	Aug. 31, 2020USD ($)$ / sharesshares	Mar. 31, 2020USD ($)shares	May 31, 2019USD ($)$ / sharesshares	Sep. 30, 2020USD ($)shares	Sep. 30, 2019USD ($)	Dec. 31, 2019USD ($)shares	Dec. 31, 2018shares
Subsequent Event [Line Items]
Reverse stock split	0.8646
Common stock, shares authorized										42,000,000
IPO
Subsequent Event [Line Items]
Common stock, shares authorized				500,000,000				500,000,000
Common Stock
Subsequent Event [Line Items]
Common stock, voting rights								The holders of the voting common stock are entitled to one vote for each share of voting common stock held at all meetings of stockholders.		The holders of the common stock are entitled to one vote for each share of common stock held at all meetings of stockholders.
Series B Convertible Preferred Stock
Subsequent Event [Line Items]
Convertible preferred stock, shares issued				0		7,628,846	7,628,846	0		7,628,846	0
Net proceeds from issuance of preferred stock | $						$ 29,900	$ 29,800	$ 29,942	$ 29,848	$ 29,848
Convertible preferred stock, price per share | $ / shares							$ 3.9325
Series C Convertible Preferred Stock
Subsequent Event [Line Items]
Convertible preferred stock, shares issued		3,443,612		0	3,443,612			0		0
Net proceeds from issuance of preferred stock | $		$ 49,800						$ 49,826
Non-voting Common Stock
Subsequent Event [Line Items]
Common stock, shares authorized				12,850,259				12,850,259		0
Voting Common Stock
Subsequent Event [Line Items]
Common stock, shares authorized				487,149,741				487,149,741		42,000,000
Convertible Preferred Stock
Subsequent Event [Line Items]
Common stock, voting rights										Holders of the Convertible Preferred Stock are entitled to one vote for each share of common stock into which their shares may be converted and, subject to certain Convertible Preferred Stock class votes specified in the Company's certificate of incorporation or as required by law, holders of the Convertible Preferred Stock and common stock vote together on an as-converted basis.
Subsequent Event
Subsequent Event [Line Items]
Reverse stock split				0.8646
Area of premises | ft²			19,800
Subsequent Event | Common Stock
Subsequent Event [Line Items]
Common stock, shares authorized		42,000,000			42,000,000
Common stock, shares authorized increase		58,850,259			58,850,259
Common stock, voting rights										The holders of the voting common stock are entitled to one vote for each share of voting common stock held at all meetings of stockholders.
Subsequent Event | Series B Convertible Preferred Stock
Subsequent Event [Line Items]
Convertible preferred stock, shares issued						7,628,846
Net proceeds from issuance of preferred stock | $						$ 29,900
Subsequent Event | Series C Convertible Preferred Stock
Subsequent Event [Line Items]
Convertible preferred stock, shares issued		3,443,612			3,443,612
Net proceeds from issuance of preferred stock | $					$ 49,900
Convertible preferred stock, price per share | $ / shares		$ 14.5197			$ 14.5197
Common stock, shares authorized		4,000,000			4,000,000
Subsequent Event | Non-voting Common Stock
Subsequent Event [Line Items]
Common stock, shares authorized		12,850,259			12,850,259
Subsequent Event | Voting Common Stock
Subsequent Event [Line Items]
Common stock, shares authorized		46,000,000			46,000,000
Subsequent Event | Voting Common Stock | IPO
Subsequent Event [Line Items]
Common stock, shares authorized				46,000,000				46,000,000
Common stock, shares authorized increase				487,149,741				487,149,741
Subsequent Event | Convertible Preferred Stock
Subsequent Event [Line Items]
Common stock, shares authorized		32,074,008			32,074,008
Common stock, shares authorized increase		35,221,066			35,221,066
Subsequent Event | Second Amended and Restated Entrepreneur Client License Agreement
Subsequent Event [Line Items]
Area of premises | ft²			8,800
Initial term of license			26 months
License agreement term description			Pursuant to the Amendment, the initial term of the License is twenty-six months commencing on November 1, 2020 and terminating on December 31, 2022 (the "Initial Term").
License fee | $			$ 3,000
License renewal term			12 months
Description of license renewal option			The Company has an option to renew the License for twelve months commencing on January 1, 2023 at a monthly renewal license fee increased by 5.0% for a total of $1.4 million payable during the renewal term.
Percentage increase in monthly renewal license fee			5.00%
Monthly renewal license fee payable | $			$ 1,400
Increase in monthly license fee upon exercise of ROFO | $			200
One-time payment made as consideration for ROFO | $			$ 200

Background - Additional Informa

Background - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2020	Dec. 31, 2019
Subsidiary, Sale of Stock [Line Items]
Convertible preferred stock, shares outstanding	0	0
Common stock, shares authorized			42,000,000
Preferred stock, shares authorized	10,000,000	10,000,000
Common stock, par value	$ 0.0001	$ 0.0001	$ 0.0001
Preferred stock, par value	$ 0.0001	$ 0.0001
Non-voting Common Stock
Subsidiary, Sale of Stock [Line Items]
Common stock, shares authorized	12,850,259	12,850,259	0
Common stock, par value	$ 0.0001	$ 0.0001	$ 0.0001
IPO
Subsidiary, Sale of Stock [Line Items]
Net proceeds from issuance of common stock		$ 166.6
Common stock, shares authorized	500,000,000	500,000,000
IPO | Common Stock
Subsidiary, Sale of Stock [Line Items]
Number of shares issued and sold	9,573,750
Public offering price	$ 19	$ 19
Net proceeds from issuance of common stock	$ 166.6