0001193125-21-010863 | S-1 | XBRL Viewer

Cover	9 Months Ended
Sep. 30, 2020
Cover [Abstract]
Document Type	S-1
Amendment Flag	false
Entity Registrant Name	Dyne Therapeutics, Inc.
Entity Central Index Key	0001818794
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false

Condensed Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 379,606	$ 14,632	$ 8,124
Prepaid expenses and other current assets	234	127	34
Total current assets	379,840	14,759	8,158
Property and equipment, net	1,441	1,486	110
Other assets	191	191
Total assets	381,472	16,436	8,268
Current liabilities:
Accounts payable	1,488	1,256	518
Accrued expenses and other current liabilities	3,190	1,102	193
Total current liabilities	4,678	2,358	711
Long-term debt-net of unamortized debt discount	9,936		3,375
Deferred rent	13	42
Total liabilities	14,627	2,400	4,086
Redeemable convertible preferred stock		27,429	9,061
Commitments and contingencies
Stockholders' equity
Common stock, value	5	1	1
Additional paid-in capital	417,374	6,352	7
Accumulated deficit	(50,534)	(19,746)	(4,887)
Total stockholders' equity	366,845	14,036	(4,879)
Total liabilities and stockholders' equity	$ 381,472	16,436	$ 8,268
Convertible Preferred Stock
Stockholders' equity
Convertible preferred stock		$ 27,429

Condensed Statements of Operations - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
Condensed Statements of Operations - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Operating expenses:
Research and development	$ 9,679	$ 2,982	$ 23,102	$ 6,781	$ 11,040	$ 4,278
General and administrative	3,841	647	6,945	1,575	2,786	517
Total operating expenses	13,520	3,629	30,047	8,356	13,826	4,795
Loss from operations	(13,520)	(3,629)	(30,047)	(8,356)	(13,826)	(4,795)
Other (expense) income:
Interest income		110	24	223	290	5
Interest expense	(130)		(315)
Change in fair value of preferred stock tranche obligations		(2,352)		(1,323)	(1,323)	(21)
Change in success fee obligation	(270)		(450)
Total other (expense) income, net	(400)	(2,242)	(741)	(1,100)	(1,033)	(16)
Net loss	$ (13,920)	$ (5,871)	$ (30,788)	$ (9,456)	$ (14,859)	$ (4,811)
Net loss per share-basic and diluted	$ (2.01)	$ (2.36)	$ (7.51)	$ (3.94)	$ (6.08)	$ (10.15)
Weighted-average common shares outstanding used in net loss per share-basic and diluted	6,920,008	2,491,487	4,100,504	2,401,039	2,442,872	474,118

Condensed Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Series A Redeemable Convertible Preferred Stock	Series A Convertible Preferred Stock	Series B Convertible Preferred Stock	Redeemable Convertible Preferred Stock	Convertible Preferred Stock	Convertible Preferred Stock Series A Redeemable Convertible Preferred Stock	Convertible Preferred Stock Series A Convertible Preferred Stock	Convertible Preferred Stock Series B Convertible Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Deficit
Beginning Balance at Dec. 31, 2017					$ 0
Beginning Balance, Shares at Dec. 31, 2017					0
Beginning Balance at Dec. 31, 2017	$ (76)											$ (76)
Beginning Balance, Shares at Dec. 31, 2017										0
Issuance of stock, net of issuance costs	7									$ 1	$ 6
Issuance of stock, net of issuance costs, Shares										2,110,404
Issuance of Series A redeemable convertible preferred stock, net of issuance costs of $0.1 million					$ 9,061
Issuance of Series A redeemable convertible preferred stock, net of issuance costs of $0.1 million, shares					12,500,000
Stock-based compensation	1										1
Net loss	(4,811)											(4,811)
Ending Balance at Dec. 31, 2018	(4,879)									$ 1	7	(4,887)
Ending Balance, Shares at Dec. 31, 2018										2,110,404
Ending Balance at Dec. 31, 2018	$ 9,061		$ 9,061		$ 9,061
Ending Balance, Shares at Dec. 31, 2018	12,500,000		12,500,000		12,500,000
Vesting of restricted shares										239,503
Stock-based compensation	$ 2										2
Net loss	(2,087)											(2,087)
Ending Balance at Mar. 31, 2019	(6,964)									$ 1	9	(6,974)
Ending Balance, Shares at Mar. 31, 2019										2,349,907
Ending Balance at Mar. 31, 2019					$ 9,061
Ending Balance, Shares at Mar. 31, 2019					12,500,000
Beginning Balance at Dec. 31, 2018	$ 9,061		$ 9,061		$ 9,061
Beginning Balance, Shares at Dec. 31, 2018	12,500,000		12,500,000		12,500,000
Beginning Balance at Dec. 31, 2018	$ (4,879)									$ 1	7	(4,887)
Beginning Balance, Shares at Dec. 31, 2018										2,110,404
Settlement of Tranche Right I	(1,621)
Net loss	(9,456)
Ending Balance at Sep. 30, 2019	19,427					$ 27,429				$ 1	6,339	(14,342)
Ending Balance, Shares at Sep. 30, 2019						32,500,000				2,508,614
Beginning Balance at Dec. 31, 2018	$ 9,061		$ 9,061		$ 9,061
Beginning Balance, Shares at Dec. 31, 2018	12,500,000		12,500,000		12,500,000
Beginning Balance at Dec. 31, 2018	$ (4,879)									$ 1	7	(4,887)
Beginning Balance, Shares at Dec. 31, 2018										2,110,404
Issuance of Series A redeemable convertible preferred stock, net of issuance costs of $0.1 million					$ 19,989
Issuance of Series A redeemable convertible preferred stock, net of issuance costs of $0.1 million, shares					20,000,000
Settlement of Tranche Right I	(1,621)				$ (1,621)
Reclassification of redeemable convertible preferred stock and preferred stock tranche obligations	33,748				$ (27,429)	$ 27,429					6,319
Reclassification of redeemable convertible preferred stock and preferred stock tranche obligations, Shares					(32,500,000)	32,500,000
Vesting of restricted shares										476,131
Stock-based compensation	26										26
Net loss	(14,859)											(14,859)
Ending Balance at Dec. 31, 2019	14,036					$ 27,429				$ 1	6,352	(19,746)
Ending Balance, Shares at Dec. 31, 2019						32,500,000				2,586,535
Ending Balance at Dec. 31, 2019	$ 27,429		$ 27,429
Ending Balance, Shares at Dec. 31, 2019	32,500,000		32,500,000
Beginning Balance at Mar. 31, 2019					$ 9,061
Beginning Balance, Shares at Mar. 31, 2019					12,500,000
Beginning Balance at Mar. 31, 2019	$ (6,964)									$ 1	9	(6,974)
Beginning Balance, Shares at Mar. 31, 2019										2,349,907
Issuance of Series A redeemable convertible preferred stock, net of issuance costs of $0.1 million					$ 19,989
Issuance of Series A redeemable convertible preferred stock, net of issuance costs of $0.1 million, shares					20,000,000
Settlement of Tranche Right I					$ (1,621)
Vesting of restricted shares										84,478
Stock-based compensation	4										4
Net loss	(1,497)											(1,497)
Ending Balance at Jun. 30, 2019	(8,457)									$ 1	13	(8,471)
Ending Balance, Shares at Jun. 30, 2019										2,434,385
Ending Balance at Jun. 30, 2019					$ 27,429
Ending Balance, Shares at Jun. 30, 2019					32,500,000
Reclassification of redeemable convertible preferred stock and preferred stock tranche obligations	33,748				$ (27,429)	$ 27,429					6,319
Reclassification of redeemable convertible preferred stock and preferred stock tranche obligations, Shares					(32,500,000)	32,500,000
Vesting of restricted shares										74,229
Stock-based compensation	7										7
Net loss	(5,871)											(5,871)
Ending Balance at Sep. 30, 2019	19,427					$ 27,429				$ 1	6,339	(14,342)
Ending Balance, Shares at Sep. 30, 2019						32,500,000				2,508,614
Beginning Balance at Dec. 31, 2019	$ 27,429		$ 27,429
Beginning Balance, Shares at Dec. 31, 2019	32,500,000		32,500,000
Beginning Balance at Dec. 31, 2019	$ 14,036					$ 27,429				$ 1	6,352	(19,746)
Beginning Balance, Shares at Dec. 31, 2019						32,500,000				2,586,535
Issuance of stock, net of issuance costs		$ 1,972					$ 1,972
Issuance of stock, net of issuance costs, Shares							2,000,000
Vesting of restricted shares										70,538
Stock-based compensation	66										66
Net loss	(7,886)											(7,886)
Ending Balance at Mar. 31, 2020	8,188					$ 29,401				$ 1	6,418	(27,632)
Ending Balance, Shares at Mar. 31, 2020						34,500,000				2,657,073
Beginning Balance at Dec. 31, 2019	$ 27,429		$ 27,429
Beginning Balance, Shares at Dec. 31, 2019	32,500,000		32,500,000
Beginning Balance at Dec. 31, 2019	$ 14,036					$ 27,429				$ 1	6,352	(19,746)
Beginning Balance, Shares at Dec. 31, 2019						32,500,000				2,586,535
Exercise of stock options, Shares	30,409
Net loss	$ (30,788)
Ending Balance at Sep. 30, 2020	366,845									$ 5	417,374	(50,534)
Ending Balance, Shares at Sep. 30, 2020										45,004,247
Beginning Balance at Mar. 31, 2020	8,188					$ 29,401				$ 1	6,418	(27,632)
Beginning Balance, Shares at Mar. 31, 2020						34,500,000				2,657,073
Exercise of stock options, Shares										753
Vesting of restricted shares										70,539
Stock-based compensation	75										75
Net loss	(8,982)											(8,982)
Ending Balance at Jun. 30, 2020	(719)					$ 29,401				$ 1	6,493	(36,614)
Ending Balance, Shares at Jun. 30, 2020						34,500,000				2,728,365
Conversion of convertible preferred stock into common stock upon initial public offering						$ (162,215)				$ 3	162,212
Conversion of convertible preferred stock into common stock upon initial public offering, Shares						(93,159,724)				28,086,375
Issuance of stock, net of issuance costs	246,411		$ 17,495	$ 115,319				$ 17,495	$ 115,319	$ 1	246,410
Issuance of stock, net of issuance costs, Shares								17,500,000	41,159,724	14,089,314
Exercise of stock options	24										24
Exercise of stock options, Shares										29,656
Vesting of restricted shares										70,537
Stock-based compensation	2,235										2,235
Net loss	(13,920)											(13,920)
Ending Balance at Sep. 30, 2020	$ 366,845									$ 5	$ 417,374	$ (50,534)
Ending Balance, Shares at Sep. 30, 2020										45,004,247

Condensed Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
Condensed Statements of Cash Flows - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net loss	$ (30,788)	$ (9,456)	$ (14,859)	$ (4,811)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	2,376	12	26	1
Depreciation and amortization expense	483	125	271	24
Changes in fair value of preferred stock tranche obligations		1,323	1,323	21
Changes in success fee obligation	450
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(107)	(329)	(284)	(34)
Accounts payable and other liabilities	1,114	505	1,689	635
Net cash used in operating activities	(26,472)	(7,820)	(11,834)	(4,164)
Cash flows from investing activities:
Purchases of property and equipment	(438)	(910)	(1,647)	(134)
Net cash used in investing activities	(438)	(910)	(1,647)	(134)
Cash flows from financing activities:
Proceeds from initial public offering of common stock, net of issuance costs	247,454
Proceeds from issuance of common stock				7
Proceeds from issuance of debt, net of issuance costs	9,936
Proceeds from SAFEs				5,000
Settlement of success fee obligation	(450)
Proceeds from issuance of convertible preferred stock, net of issuance costs	134,920	19,989	19,989	7,415
Proceeds from exercise of stock options	24
Net cash provided by financing activities	391,884	19,989	19,989	12,422
Net increase in cash and cash equivalents	364,974	11,259	6,508	8,124
Cash and cash equivalents at beginning of period	14,632	6,508	6,508
Cash and cash equivalents at end of period	379,606	17,767	14,632	6,508
Supplemental cash flow information:
Cash paid for interest and taxes	311
Supplemental disclosure of non-cash investing and financing information:
Reclassification of preferred stock tranche obligation liability to permanent equity		6,319	6,319
Settlement of Tranche Right I		1,621	1,621
Conversion of convertible preferred stock to common stock	162,215
Purchase of property and equipment in accounts payable		370
Issuance costs from convertible preferred stock included in accounts payable or accrued expenses	134
Public offering costs included in accounts payable or accrued expenses	$ 1,043
Settlement of SAFEs with the issuance of Series A Preferred Stock				5,000
Previously Reported [Member]
Cash flows from financing activities:
Cash and cash equivalents at beginning of period		$ 8,124	$ 8,124
Cash and cash equivalents at end of period				$ 8,124

Nature of Business and Basis of Presentation	9 Months Ended	12 Months Ended
Nature of Business and Basis of Presentation	Sep. 30, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Nature of Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Dyne Therapeutics, Inc. (the “Company”) is building a leading muscle disease company focused on advancing innovative life-transforming therapeutics for people living with genetically driven diseases that was incorporated in Delaware on December 1, 2017 and has a principal place of business in Waltham, Massachusetts. The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, dependence on key personnel, protection of proprietary technology, risks of failure of preclinical studies and clinical trials, the need to obtain marketing approval for its product candidates, fluctuations in operating results, compliance with government regulations, the ability to establish clinical- and commercial-scale manufacturing processes, the impact of the COVID-19 pandemic and the ability to secure additional capital to fund operations. Programs currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval prior to commercialization of a product. These efforts require significant amounts of additional capital, adequate personnel and infrastructure and extensive compliance-reporting capabilities. Even if the Company’s development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. On September 21, 2020, the Company completed its initial public offering (“IPO”) pursuant to which it issued and sold 14,089,314 shares of its common stock, including 1,837,736 shares pursuant to the full exercise of the underwriters’ option to purchase additional shares, at a public offering price of $19.00 per share, resulting in net proceeds of $246.4 million, after deducting underwriting discounts and commissions and offering expenses. Upon the closing of the IPO, all of the Company’s then outstanding convertible preferred stock automatically converted into shares of common stock. The accompanying financial statements have been prepared on the basis of continuity of operations, realization of assets and the satisfaction of liabilities and commitments in the ordinary course of business. Since inception, the Company has funded its operations with proceeds from the sales of instruments convertible into convertible preferred stock (which converted into convertible preferred stock in 2018), the sale of convertible preferred stock, borrowings under a loan and security agreement (the “Loan Agreement”) and most recently, the sale of common stock in the IPO completed in September 2020. The Company has incurred recurring losses, including net losses of $30.8 million for the nine months ended September 30, 2020 and $14.9 million for the year ended December 31, 2019. As of September 30, 2020, the Company had an accumulated deficit of $50.5 million. The Company expects to continue to generate operating losses for the foreseeable future. As of November 5, 2020, the issuance date of these interim condensed financial statements, the Company expects that its cash and cash equivalents will be sufficient to fund its operating expenses and capital expenditure requirements for at least 12 months from the issuance of these interim condensed financial statements. To continue its development efforts, the Company will need to obtain substantial additional funding through public or private equity offerings, debt financings, collaborations, strategic alliances and/or licensing arrangements in order to fund its research and development and ongoing operating expenses. The Company may not be able to obtain financing on acceptable terms, when needed or at all, and the Company may not be able to enter into collaborations, strategic alliances or licensing arrangements. The terms of any financing may adversely affect the holdings or the rights of the Company’s stockholders. Any collaborations, strategic alliances or licensing arrangements may require the Company to relinquish rights to certain of its technologies, future revenue streams, research programs or product candidates or grant licenses on terms that may not be favorable to the Company. If the Company is unable to obtain funding, the Company could be forced to delay, limit, reduce or eliminate some or all of its research and development programs, pipeline expansion or future commercialization efforts or grant rights to develop and market product candidates, which could adversely affect its business prospects. Although management will continue to pursue these plans, there is no assurance that the Company will be successful in obtaining sufficient funding on terms acceptable to the Company to fund continuing operations when needed or at all. To date, the Company has not experienced material business disruptions, including with its vendors, as a result of the COVID-19 pandemic. In March 2020, the Company implemented a remote working policy for many of its employees and began restricting non-essential travel, and on May 18, 2020, when Massachusetts began its staged reopening plan, the Company began implementing a return-to-work plan, in accordance with the guidance and requirements of federal and state authorities. The Company expects to continue to take actions as may be required or recommended by government authorities or as it determines are in the best interests of its employees and other business partners. The Company is continuing to monitor the potential impact of the pandemic, but cannot be certain what the overall impact will be on its business, financial condition, results of operations and prospects.	1. Nature of business and basis of presentation *Nature of business* Dyne Therapeutics, Inc. (the “Company”) is a healthcare company that was incorporated in Delaware on December 1, 2017 and has a principal place of business in Waltham, Massachusetts. The Company’s focus is on advancing innovative life-transforming therapies for genetically driven muscle diseases. *Risks, uncertainties and going concern* The Company is subject to risks common to companies in the biotechnology industry, including, but not limited to, successful development of technology, obtaining additional funding, protection of proprietary technology, compliance with government regulations, risks of failure of preclinical studies and clinical trials, the need to obtain marketing approval for its product candidates, fluctuations in operating results, dependence on key personnel, risks associated with changes in technologies and development by competitors of technological innovations. To date, the Company has principally raised capital through the private placement of convertible preferred stock. The Company has incurred recurring losses since its inception, including net losses of $14.9 million for the year ended December 31, 2019. In addition, as of December 31, 2019, the Company had an accumulated deficit of $19.7 million. The Company expects to incur additional losses and negative operating cash flows at least for the development period and possibly into the commercialization stage. Based on its recurring losses from operations incurred since inception, expectation of continuing operating losses for the foreseeable future, and need to raise additional capital to finance its future operations, the Company has concluded that there is substantial doubt regarding the Company’s ability to continue as a going concern within one year after the date that these financial statements are issued. The Company will require substantial additional capital to fund its research and development and ongoing operating expenses. These capital requirements are expected to be funded through debt and equity offerings as well as possible strategic collaborations with other companies. If the Company is unable to raise additional funds when needed, it may be required to delay, reduce or eliminate its product development or future commercialization efforts, or grant rights to develop and market product candidates that the Company would otherwise prefer to develop and market itself. While there can be no assurance the Company will be able to reduce operating expenses or raise additional capital, management believes its historical success in managing cash flows and obtaining capital will continue in the foreseeable future. The Company is seeking to complete an initial public offering (“IPO”) of its common stock. Upon the completion of an IPO on specified terms, the Company’s outstanding convertible preferred stock will automatically convert into shares of common stock (see Note 8). In the event the Company does not complete an IPO, the Company expects to seek additional funding through private equity financings, debt financings or other capital sources, including collaborations with other companies or other strategic transactions. The Company may not be able to obtain funding on acceptable terms, or at all. The terms of any future financing may adversely affect the holdings or the rights of the Company’s existing stockholders. The accompanying financial statements have been prepared on a going concern basis, which contemplates the realization of assets and the satisfaction of liabilities in the normal course of business. These financial statements do not include any adjustments that might result from the outcome of this uncertainty. Table of Contents *COVID-19* In March 2020, the spread of the novel coronavirus began to cause business disruptions for the Company and many of the Company’s vendors. In addition, the Company has taken a series of actions aimed at safeguarding the Company’s employees and business associates, including implementing a work-at-home policy. These disruptions could result in increased costs of execution of development plans or may negatively impact the quality, quantity, timing and regulatory usability of data that the Company would otherwise be able to collect. While these disruptions are currently expected to be temporary, there is considerable uncertainty around the duration of these disruptions. Therefore, the related financial impact and duration cannot be reasonably estimated at this time. *Basis of presentation* The accompanying financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative U.S. GAAP as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Updates (“ASUs”) issued by the Financial Accounting Standards Board (“FASB”).

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

2. Summary of Significant Accounting Policies

The accompanying financial statements have been prepared in conformity with generally accepted accounting principles (“GAAP”) in the United States of America. Any reference in these notes to applicable guidance is meant to refer to the authoritative GAAP as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”).

Unaudited interim financial information

The financial statements of the Company included herein have been prepared, without audit, pursuant to the rules and regulations of the Securities and Exchange Commission (the “SEC”). The unaudited interim financial statements have been prepared on the same basis as audited annual financial statements, except certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted from this report, as is permitted by such rules and regulations. In the opinion of management, the information furnished reflects all adjustments, all of which are of a normal and recurring nature, necessary for a fair representation of the results for the reported periods. Accordingly, these financial statements should be read in conjunction with the financial statements and notes thereto included in the Company’s Prospectus that forms a part of the Company’s Registration Statement on Form S-1 (File No. 333-248414), which was filed with the SEC pursuant to Rule 424(b)(4) on September 17, 2020 (the “Prospectus”). The results for the nine months ended September 30, 2020 are not necessarily indicative of results to be expected for the year ending December 31, 2020, any other interim periods, or any future year or period.

Use of estimates

The preparation of the financial statements in accordance with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and 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.

Summary of significant accounting policies

The Company’s significant accounting policies are described in Note 2, “Summary of Significant Accounting Policies,” to the Company’s financial statements included in the Prospectus. There have been no material changes to the significant accounting policies during the three months ended September 30, 2020.

Accounting pronouncements issued and not adopted

In February 2016, the FASB issued ASU No. 2016-02, Leases (“ASU 2016-02”), which applies to all leases and will require lessees to record most leases on the balance sheet but recognize expense in a manner similar to the previous standard. The Company will use a modified retrospective approach of adoption for its leases. The Company plans to adopt this standard on January 1, 2022 and is currently evaluating the impact that the adoption will have on its financial statements. The Company expects to recognize a lease obligation and right to use asset upon adoption of ASU 2016-02.

2. Summary of significant accounting policies

Use of estimates

The preparation of the financial statements in accordance 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 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.

Cash and cash equivalents

Cash includes cash in readily available checking accounts and cash equivalents include all highly liquid investments maturing within 90 days from the date of purchase. Financial instruments that subject the Company to significant concentrations of credit risk consist primarily of cash and cash equivalents as the Company’s cash deposits on hand at one financial institution often exceed federally insured limits. The Company places its cash in a financial institution that management believes to be of high credit quality.

Property and equipment

Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense is recognized using the straight-line method over the estimated useful life of each asset as follows:



	Estimated useful life
Laboratory equipment		3 years
Furnitures and fixtures		5 years
Computer equipment		3 years
Leasehold improvements		Shorter of life of lease or 10 years

Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation and amortization are removed from the accounts and any resulting gain or loss is included in loss from operations. Expenditures for repairs and maintenance are charged to expense as incurred.

Impairment of long-lived assets

Long-lived assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of the asset may not be recoverable. When such events occur, the Company compares the carrying amounts of the assets to their undiscounted expected future cash flows. If this comparison indicates that there is an impairment, the amount of the impairment is calculated as the difference between the carrying value and the fair value. The Company has not recorded any impairment charges in the periods presented in these financial statements.

Segment information

Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker, or decision-making group, in making decisions on how to allocate resources and assess performance. The Company’s chief operating decision maker is the chief executive officer (“CEO”). The CEO and other members of senior management of the Company view the Company’s operations and manage its business as one operating segment.

Research and development costs

Research and development costs are expensed as incurred. Research and development costs that are paid in advance of performance (if any) are capitalized as a prepaid expense and amortized over the service period as the services are provided.

Income taxes

Deferred tax assets and liabilities are recognized for the expected future tax consequences of events that have been included in the financial statements or tax returns. Under this method, deferred tax assets and liabilities are determined based on the difference between the financial statement and tax bases of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. A valuation allowance is provided to reduce the deferred tax asset to an amount, which, more likely than not, will be realized.

The Company recognizes the tax benefit from any uncertain tax positions only if it is more likely than not that the tax position will be sustained on examination by the taxing authorities, based on the technical merits of the position. The tax benefits recognized in the financial statements from such a position are measured based on the largest benefit that has a greater than fifty percent likelihood of being realized upon ultimate settlement.

Fair value measurements

Certain assets and liabilities are carried at fair value. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable:

•

Level 1—Unadjusted quoted prices in active markets that are accessible to the reporting entity at the measurement date for identical assets and liabilities.

•

Level 2—Inputs other than quoted prices in active markets for identical assets and liabilities that are observable either directly or indirectly for substantially the full term of the asset or liability. Level 2 inputs include the following:

•

quoted prices for similar assets and liabilities in active markets;

•

quoted prices for identical or similar assets or liabilities in markets that are not active;

•

observable inputs other than quoted prices that are used in the valuation of the asset or liabilities (e.g., interest rate and yield curve quotes at commonly quoted intervals); and

•

inputs that are derived principally from or corroborated by observable market data by correlation or other means.

•

Level 3—Unobservable inputs for the assets or liability (i.e., supported by little or no market activity). Level 3 inputs include management’s own assumptions about the assumptions that market participants would use in pricing the asset or liability (including assumptions about risk).

Net loss per share

The Company follows the two-class method when computing net income (loss) per share as the Company has issued shares that meet the definition of participating securities. The two-class method determines net income (loss) per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to receive dividends as if all income for the period had been distributed. During periods of loss, the Company does not allocate loss to participating securities because they have no contractual obligation to share in the losses of the Company.

Basic net loss per share is computed by dividing the net income loss by the weighted average number of shares of common stock outstanding for the period. Diluted net loss is computed by adjusting net loss to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net loss per share is computed by dividing the diluted net loss by the weighted average number of shares of common stock outstanding for the period, including potential dilutive common shares assuming the dilutive effect of common stock equivalents.

Redeemable convertible preferred stock

The Company recorded redeemable convertible preferred stock at fair value upon issuance, net of any issuance costs. The Company’s redeemable convertible preferred stock was subject to a dividend when, as and if declared by the Company’s board of directors (the “Board”). Since the issuance of the Company’s outstanding redeemable convertible preferred stock, no dividends have been declared on any shares of redeemable convertible preferred stock. The Company classified stock that was redeemable in circumstances outside of the Company’s control outside of permanent equity. No accretion was recognized as the contingent events that could give rise to redemption were not deemed probable.

Stock-based compensation

The Company accounts for stock-based awards at fair value, and measures fair value using the Black-Scholes option-pricing model. Stock-based compensation costs are recognized as expense over the requisite service period, which is generally the vesting period, on a straight-line basis for all time-vested awards. Forfeitures are recognized as they occur.

Success fee obligation

The Company’s loan and security agreement (the “Loan Agreement”) with Pacific Western Bank (“PWB”) entered into in 2020 requires the Company to pay a success fee of $0.5 million upon the occurrence of a specified liquidity event, as described in the Loan Agreement, which includes the proposed IPO. The Company classifies this contingent obligation to pay a success fee as a liability on its balance sheet. The liability was initially deemed immaterial upon entering into the Loan Agreement and is subsequently remeasured to fair value at each reporting date. Changes in the success fee obligation will continue to be recognized until the liability is settled.

Comprehensive loss

Comprehensive loss includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. The Company’s comprehensive net loss equals the reported net loss for all periods presented.

Accounting pronouncements issued and not adopted

Fair Value Measurements

9 Months Ended

12 Months Ended

Sep. 30, 2020

Dec. 31, 2019

Fair Value Disclosures [Abstract]

Fair Value Measurements

3. Fair Value Measurements

The following tables set forth by level, within the fair value hierarchy, the assets and liabilities carried at fair value on a recurring basis for the periods presented:



	Fair Value Measurements as of December 31, 2019
(in thousands)	Level 1		Level 2		Level 3		Total

Assets:
Cash equivalents—money market funds	$	14,420		—		—	$	14,420

Total	$	14,420		—		—	$	14,420

Money market funds were valued by the Company based on quoted market prices. There were no transfers among Level 1, Level 2, or Level 3 categories during the periods presented.

The fair value of the liability recognized in connection with the contingent success fee associated with the Loan Agreement (see Note 6) was determined based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. The fair value of the liability was determined using the probability-weighted expected return method (“PWERM”), which considered as inputs the probability of occurrence of a specified liquidity event, the expected timing of a liquidity event, the amount of the success fee and a risk-adjusted discount rate. The initial fair value of the liability was de minimis due to the low probability of the events probable of triggering payment. As of September 21, 2020, the closing date of the IPO, the event occurred and the probability increased to 100% and the discount rate was assessed to be 0%. Based on these inputs, the Company determined that the fair value of the derivative liability was $0.5 million as of September 21, 2020. Following the completion of the IPO, the success fee of $0.5 million was paid in September 2020.

The following table presents a roll-forward of the aggregate fair value of the success fee obligation for which fair value is determined by Level 3 inputs:



(in thousands)	Success Fee Obligation
Balance—January 1, 2020	$	—
Initial fair value of success fee obligation		—
Change in fair value		450
Settlement of derivative instrument		(450	)

Balance—September 30, 2020	$	—

Financial instruments not recorded at fair value

The carrying values of cash, cash equivalents, accounts payable and accrued expenses that are reported on the balance sheets approximate their fair value due to the short-term nature of these assets and liabilities. The carrying value of the long-term debt at September 30, 2020 approximates fair value given that the debt was issued in February 2020 and settled in October 2020 at amounts that approximate the carrying value.

3. Fair value measurements

The following tables set forth by level, within the fair value hierarchy (see Note 2), the assets and liabilities carried at fair value on a recurring basis for the periods presented:



(in thousands)	Fair value measurements as of December 31, 2018
(in thousands)	Level 1		Level 2		Level 3		Total

Assets:
Cash equivalents—money market funds	$	7,005		—		—	$	7,005

Total	$	7,005		—		—	$	7,005


Liabilities:
Preferred stock tranche obligations	$	—		—	$	3,375	$	3,375

Total	$	—		—	$	3,375	$	3,375



(in thousands)	Fair value measurements as of December 31, 2019
(in thousands)	Level 1		Level 2		Level 3		Total

Assets:
Cash equivalents—money market funds	$	14,420		—		—	$	14,420

Total	$	14,420		—		—	$	14,420

Money market funds were valued by the Company based on quoted market prices. There were no transfers among Level 1, Level 2, or Level 3 categories during the periods presented.

The fair value of the preferred stock tranche obligations in connection with the unfunded tranches associated with the Series A Preferred Stock Purchase Agreement (see Note 8) was determined based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. The fair value of the preferred stock tranche obligations was determined by considering as inputs the future value of the Series A Preferred Stock, the difference between the future value of the Series A Preferred Stock and the contract price, the number of shares underlying the contract, the probability of achieving the funding of the tranches, expected timing of the tranche closing and the discount rate. The future value of Series A Preferred Stock was estimated by calibrating, or backsolving, to the price of the Series A Preferred Stock as set forth in the Series A Preferred Stock Purchase Agreement. The contractual price of the shares to be issued in addition to the number of shares underlying the contract are both provided for by the Series A Preferred Stock Purchase Agreement. The probability of achieving the funding of the tranches and the expected timing of those closings were estimates made by the Company. The discount rate was equal to the risk-free rate for the estimated timing of each tranche closing.

The following table presents a roll-forward of the aggregate fair values of the Company’s liabilities for which fair value is determined by Level 3 inputs:



(in thousands)	Preferred stock tranche obligations			Success fee obligation
Balance—January 1, 2018	$	—		$	—
Initial fair value of preferred stock tranche obligations		3,354			—
Change in fair value		21			—

Balance—December 31, 2018		3,375			—
Settlement of Tranche Right I		1,621			—
Change in fair value		1,323			—
Reclassification of Tranche Right II to equity		(6,319	)		—

Balance—December 31, 2019		—			—

Financial instruments not recorded at fair value

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Convertible Preferred Stock

9 Months Ended

12 Months Ended

Sep. 30, 2020

Dec. 31, 2019

Equity [Abstract]

Convertible Preferred Stock

7. Convertible Preferred Stock

The Company had issued Series A convertible preferred stock (the “Series A Preferred Stock”) and Series B convertible preferred stock (the “Series B Preferred Stock”). Collectively the Series A Preferred Stock and the Series B Preferred Stock are referred to as the Preferred Stock.

In July 2020, the Company issued and sold 17,500,000 shares of Series A Preferred Stock at a price of $1.00 per share, for gross proceeds of $17.5 million.

In August 2020, the Company issued and sold 41,159,724 shares of Series B Preferred Stock at a price of $2.81 per share for gross proceeds of $115.7 million.

Upon the closing of the IPO in September 2020, the Company’s Preferred Stock automatically converted into 28,086,375 shares of common stock.

8. Redeemable convertible preferred stock

Series A preferred stock

On November 29, 2018, the Company entered into the Series A Preferred Stock Purchase Agreement with its initial investors committing to purchase an aggregate of $50.0 million in shares of Series A Preferred Stock. At the initial closing, 12,500,000 shares of Series A Preferred Stock were issued by the Company at a purchase price of $1.00 per share, for gross cash proceeds of $7.5 million. Of the 12,500,000 shares issued, 5,000,000 shares were issued upon conversion of the then outstanding SAFEs, which had a fair value of $5.0 million (See Note 7). In April 2019, the Company issued an additional 20,000,000 shares of Series A Preferred Stock under the terms of the Series A Preferred Stock Purchase Agreement at a purchase price of $1.00 per share for total gross proceeds of $20.0 million. Issuance costs associated with each closing of the Series A Preferred Stock financing were $0.1 million. On March 18, 2020, the Company entered into a separate Series A Preferred Stock Purchase Agreement with an additional investor and issued 2,000,000 shares of Series A Preferred Stock at a purchase price $1.00 per share for gross cash proceeds of $2.0 million.

Included in the terms of the November 2018 Series A Preferred Stock Purchase Agreement were certain rights (“Tranche Rights”) granted to the investors who purchased the Series A Preferred Stock purchased in November 2018. The Tranche Rights contingently obligated the investors to purchase, and the Company to sell, up to an aggregate of 37,500,000 shares of Series A Preferred Stock at $1.00 per share upon the satisfaction of specified research and development milestones by the Company. The Tranche Rights are also exercisable at the option of the holders of the Series A Preferred Stock. The Tranche Rights were divided into separate rights and obligations to purchase 20,000,000 shares (“Tranche Right I”) and 17,500,000 shares (“Tranche Right II”) based on the achievement of specified milestones for each tranche.

The Company concluded that the Tranche Rights met the definition of a freestanding financial instrument, as the Tranche Rights were legally detachable and separately exercisable from the Series A Preferred Stock. Therefore, the Company allocated the proceeds from the November 2018 issuance between the Tranche Rights and the Series A Preferred Stock. As the Series A Preferred Stock was redeemable upon a deemed liquidation event at the election of the holder-controlled Board, and therefore outside of the control of the Company, the Tranche Rights were initially classified as a liability and were initially recorded at their fair value of $3.4 million. The Tranche Rights were then remeasured at fair value at each reporting period, with changes in fair value recorded in the statement of operations. The estimated fair value of the Tranche Rights was determined at each reporting date using a probability-weighted present value model that considers the probability of closing a tranche, the estimated future value of the Series A Preferred Stock to be issued at each closing, and the investment required at each closing. Future values were converted to present value using a discount rate appropriate for probability-adjusted cash flows.

In April 2019, Tranche Right I was settled when the Company closed on the issuance of 20,000,000 shares Series A Preferred Stock. The fair value of the Tranche Right I as of the closing date was reclassified from a liability to Series A Preferred Stock within equity.

In September 2019, the size of the Board was increased from five to six members, including three independent members, which allowed the Company to conclude that the redemption rights of the holders of the Series A Preferred Stock was within the control of the Company. As a result, the Company concluded that Tranche Right II qualified for equity classification as of that date. The preferred stock tranche obligation liability, with a fair value of $6.3 million, was reclassified into permanent equity and was no longer remeasured at fair value at each reporting period after that date.

As of each balance sheet date, convertible preferred stock consisted of the following:



(in thousands, except share data)	As of December 31, 2018
(in thousands, except share data)	Preferred stock authorized		Preferred stock issued and outstanding		Carrying value		Liquidation preference		Common shares issuable upon conversion
Series A Preferred Stock		50,000,000		12,500,000	$	9,061	$	12,500		3,768,575

Total		50,000,000		12,500,000	$	9,061	$	12,500		3,768,575



(in thousands, except share data)	As of December 31, 2019
(in thousands, except share data)	Preferred stock authorized		Preferred stock issued and outstanding		Carrying value		Liquidation preference		Common shares issuable upon conversion
Series A Preferred Stock		50,000,000		32,500,000	$	27,429	$	32,500		9,798,298

Total		50,000,000		32,500,000	$	27,429	$	32,500		9,798,298

The Series A Preferred Stock has the followings rights and privileges:

Dividends

Holders of the Series A Preferred Stock are entitled to receive non-cumulative dividends when, as and if declared by the Board at a rate of 6% of the Original Issue Price (as defined below) per share (the “Dividend Rate”), subject to adjustment. Holders of Series A Preferred Stock will have preference to any dividends being declared or paid on common stock. The Company has not, and has no plans to, declare dividends on any class of preferred or common stock.

Liquidation

In the event of any liquidation, dissolution, or winding-up of the Company, which would include the sale of the Company, the Series A Preferred Stock is senior to common stock. The holders of the Series A Preferred Stock would be entitled to preferential payment in the amount of $1.00 per share (the “Original Issue Price”). In the event that there are additional assets to be distributed, the holders of the Series A Preferred Stock will share in the distribution along with common stockholders as if the shares of Series A Preferred Stock had converted to common stock immediately prior to the distribution. If the amounts to be distributed to the holders of the Series A Preferred Stock exceeds $2.00 per share, subject to certain adjustments, then the holders of the Series A Preferred Stock will receive the greater of $2.00 or the amount such holder would have received if all shares of Series A Preferred Stock had converted to common stock immediately prior to the distribution.

Voting

The holders of the Series A Preferred Stock are entitled to the number of votes equal to the number of shares of common stock into which the shares of Series A Preferred Stock held by each holder are then convertible.

Conversion

The holders of the Series A Preferred Stock may convert, at any time, each share of Series A Preferred Stock into shares of common stock at the Series A Conversion Price. The Series A Conversion Price is initially equal to the Original Issue Price, subject to adjustment. Upon either (a) the closing of the sale of shares of common stock to the public at a price of at least $3.00 per share (subject to adjustment) in an initial public offering with net proceeds to the Company of at least $50.0 million (“Qualified IPO”) or (b) the written consent of the holders of a majority of the outstanding shares of Series A Preferred Stock, the Series A Preferred Stock will automatically convert into common stock at the Series A Conversion Price then in effect.

In the event that a holder of Series A Preferred Stock does not purchase the shares of Series A Preferred Stock that the holder is obligated to purchase pursuant to Tranche Right II, that holder will have its outstanding shares of Series A Preferred Stock automatically converted into common stock at a conversion price of $2.00 per share, meaning that the holder would receive 50% fewer shares than would be received upon an optional or mandatory conversion as set forth in the preceding paragraph.

Stock-Based Awards

9 Months Ended

12 Months Ended

Sep. 30, 2020

Dec. 31, 2019

Share-based Payment Arrangement [Abstract]

Stock-Based Awards

9. Stock-Based Awards

2018 Stock Incentive Plan

The Company’s 2018 Stock Incentive Plan (the “2018 Plan”) provided for the Company to sell or issue incentive stock options or nonqualified stock options, restricted stock, and other equity awards to employees, directors and consultants of the Company. The 2018 Plan was administered by the board of directors or, at the discretion of the board of directors, by a committee of the board of directors. The exercise prices, vesting and other restrictions were determined at the discretion of the board of directors, or its committee if so delegated.

The total number of shares of common stock authorized under the 2018 Plan was 8,267,252 shares, of which 1,928,487 shares remained available for future issuance prior to the effectiveness of the Company’s 2020 Stock Option and Incentive Plan (the “2020 Plan”). Upon the effectiveness of the 2020 Plan, the Company ceased granting awards under the 2018 Plan, and the 1,928,487 shares of common stock remaining under the 2018 Plan became available for future issuance under the 2020 Plan.

2020 Stock Incentive Plan

In August 2020 the Company’s board of directors adopted and the Company’s stockholders approved the 2020 Plan (together with the 2018 Plan, the “Plans”), which became effective on September 16, 2020. The 2020 Plan provides for the grant of incentive stock options, nonstatutory stock options, stock appreciation rights, restricted stock awards, restricted stock units and other stock-based awards to employees, directors, consultants and advisors of the Company. The 2020 Plan is administered by the Company’s board of directors or by a committee appointed by the board of directors. Upon the effectiveness of the 2020 Plan, the Company ceased granting awards under the 2018 Plan. The number of shares initially reserved for issuance under the 2020 Plan was 4,884,233. The number of shares of common stock reserved for issuance under the 2020 Plan will automatically increase on the first day of each fiscal year, beginning with the fiscal year commencing on January 1, 2021 and continuing for each fiscal year until, and including the fiscal year commencing on, January 1, 2030, in an amount equal to the lower of (1) 5% of the shares of common stock outstanding on such date and (2) an amount determined by the Company’s board of directors.

As of September 30, 2020, 3,061,794 shares remained available for future issuance under the 2020 Plan. Shares that are expired, terminated, surrendered or canceled without having been fully exercised under the 2018 Plan and the 2020 Plan will be available for future awards under the 2020 Plan.

2020 Employee Stock Purchase Plan

In August 2020 the Company’s board of directors adopted and the Company’s stockholders approved the 2020 Employee Stock Purchase Plan (the “2020 ESPP”), which became effective September 16, 2020. The 2020 ESPP is administered by the Company’s board of directors or by a committee appointed by the board of directors. The 2020 ESPP initially provides participating employees with the opportunity to purchase up to an aggregate of 488,414 shares of common stock. The number of shares of common stock reserved for issuance under the 2020 ESPP will automatically increase on the first day of each fiscal year, beginning with the fiscal year commencing on January 1, 2021 and continuing for each fiscal year until, and including the fiscal year commencing on, January 1, 2030, in an amount equal to the lowest of (1) 1,953,656 shares of common stock, (2) 1% of the shares of common stock outstanding on such date, and (3) an amount determined by the board of directors. As of September 30, 2020, no offering periods have commenced under the 2020 ESPP and 488,414 shares remain available for issuance.

Stock option valuation

The Company typically grants stock options at exercise prices deemed by the Board to be equal to the fair value of the common stock at the time of grant. In the periods prior to the IPO, the fair value of the common stock was determined by the Board at each measurement date based on a variety of different factors, including the results obtained from independent third-party appraisals, the Company’s financial position and historical financial performance, the status of development of the Company’s programs, the current climate in the marketplace, the illiquid nature of the common stock, the effect of the rights and preferences of the preferred stockholders, and the prospects of a liquidity event, among others. In the periods following the IPO, the fair value is determined based upon the quoted price of the Company’s common stock on Nasdaq.

The fair value of stock option grants is estimated using the Black-Scholes option-pricing model. The Company historically has been a private company and lacks company-specific historical and implied volatility information. Therefore, it estimates its expected stock volatility based on the historical volatility of a publicly traded set of peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. For options with service-based vesting conditions, the expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future.

The assumptions that the Company used to determine the grant-date fair value of options granted were as follows:



	Nine Months Ended September 30,
	2020		2019
Expected volatility		75%		74%
Risk-free interest rate		0.31% —1.67%		1.46% —2.50%
Expected term (in years)		6		6
Expected dividend yield		—		—

Stock option activity

A summary of the Company’s stock option activity and related information for the nine months ended September 30, 2020 is as follows:



(in thousands, except share and per share data)	Options			Weighted Average Exercise Price		Weighted Average Remaining Life (in years)		Aggregate Intrinsic Value
Outstanding as of January 1, 2020		311,303		$	0.93		9.5	$	30
Granted		6,167,369			6.94
Exercised		(30,409	)		0.82				589
Canceled		(110,484	)		1.04

Outstanding as of September 30, 2020		6,337,779		$	6.78		9.7	$	84,984

Options exercisable as of September 30, 2020		82,768		$	0.97		8.9	$	1,591
Options vested or expected to vest as of September 30, 2020		6,337,779		$	6.78		9.7	$	84,984

The weighted-average grant date fair value of the options granted during the nine months ended September 30, 2020 was $4.27 per share. As of September 30, 2020 there was $25.0 million of unrecognized compensation expense, which the Company expects to recognize over a weighted-average period of 3.67 years.

Restricted stock units

A restricted stock unit (“RSU”) represents the right to receive one share of common stock upon vesting of the RSU. The Company grants RSUs with service conditions that vest in four equal annual installments provided that the employee remains employed with the Company. The Company also grants RSUs with performance conditions that vest upon the achievement of specified milestones. The fair value of each RSU is based on the closing price of the Company’s common stock on the date of grant. A summary of the Company’s RSU activity and related information for the 2020 Plan for the nine months ended September 30, 2020 is as follows:



	Number of Shares Underlying RSUs		Weighted Average Grant Date Fair Value
Unvested as of January 1, 2020		—	$	—
Granted		375,137		23.90
Vested		—		—
Forfeited		—		—

Unvested as of September 30, 2020		375,137	$	23.90

As of September 30, 2020, there was $8.7 million of unrecognized compensation costs related to unvested RSUs, which are expected to be recognized over a weighted-average period of 3.96 years.

Restricted common stock

During the year ended December 31, 2018, the Company granted restricted common stock with service-based vesting conditions. Shares of unvested restricted common stock may not be sold or transferred by the holder. These restrictions lapse according to the time-based vesting conditions of each award. The aggregate grant date fair value of these awards was immaterial. The following table summarizes the Company’s restricted common stock award activity for the nine months ended September 30, 2020:



	Number of Restricted Shares
Issued and unvested as of January 1, 2020		652,482
Vested		(211,614	)

Issued and unvested as of September 30, 2020		440,868

The Company recorded stock-based compensation expense in the following expense categories of its statements of operations:



	Three Months Ended September 30,				Nine Months Ended September 30,
(in thousands)	2020		2019		2020		2019
Research and development	$	929	$	3	$	984	$	4
General and administrative		1,306		4		1,392		8

Total	$	2,235	$	7	$	2,376	$	12

10. Stock-based compensation

2018 Stock incentive plan

During 2018, the Company adopted the 2018 Stock Incentive Plan (the “2018 Plan”). The 2018 Plan, as amended, provided for the issuance of up to 1,968,528 shares of common stock as of December 31, 2019 to employees, officers, directors, consultants, and advisors in the form of nonqualified and incentive stock options, restricted stock awards, and other stock-based awards. Options typically vest over four years and have a maximum term of 10 years. There were no awards granted during the year ended December 31, 2018. At December 31, 2019, there were 528,593 shares of common stock available for issuance under the 2018 Plan.

Stock option valuation

The Company typically grants stock options to employees and nonemployees at exercise prices deemed to be equal to the fair value of the common stock at the time of grant. The fair value of the common stock has been determined by the Board at each measurement date based on a variety of different factors, including the results obtained from independent third-party appraisals, the Company’s financial position and historical financial performance, the status of development of the Company’s programs, the current climate in the marketplace, the illiquid nature of the common stock, the effect of the rights and preferences of the holders of Series A Preferred Stock, and the prospects of a liquidity event, among others.

The Company utilized the Black-Scholes option-pricing model to estimate the fair value of stock options awarded. The Black-Scholes option-pricing model requires several key assumptions. The assumptions that the Company used to determine the grant date fair value of options granted were as follows:



		Year ended December 31, 2019
Expected volatility		74%
Risk-free interest rate		1.46% — 2.50%
Expected term (in years)		6
Expected dividend yield		—

The risk-free interest rates are based on rates associated with U.S. Treasury issues approximating the expected life of the stock options. The expected term of options granted to employees was determined using the simplified method, which represents the midpoint of the contractual term of the option and the weighted-average vesting period of the option. The Company uses the simplified method because it does not have sufficient historical option exercise data to provide a reasonable basis upon which to estimate the expected term. The expected dividend-yield assumption was based on the Company’s expectation of no future dividend payments. The expected volatility of the underlying stock was based on the average historical volatility of comparable publicly traded companies based on weekly price returns as reported by a pricing service, as the Company does not have a trading history for its stock.

The weighted-average grant date fair value of the options granted during the year ended December 31, 2019 was $0.62 per share. As of December 31, 2019, there was $0.2 million of unrecognized compensation expense which will be recognized over a weighted-average period of 3.4 years.

The following table summarizes the option activity under the 2018 Plan for the periods presented:



(in thousands, except share and per share data)	Options			Weighted average exercise price		Weighted average remaining life (in years)		Aggregate intrinsic value
Outstanding January 1, 2019		—		$	—		—	$	—
Granted		314,317			0.93
Exercised		—			—
Canceled		(3,014	)		0.73

Outstanding December 31, 2019		311,303		$	0.93		9.5	$	30

Options exercisable—December 31, 2019		—		$	—		—	$	—
Options vested or expected to vest—December 31, 2019		311,303		$	0.93		9.5	$	30

Restricted common stock

During the year ended December 31, 2018, the Company granted restricted common stock with service-based vesting conditions. Shares of unvested restricted common stock may not be sold or transferred by the holder. These restrictions lapse according to the time-based vesting conditions of each award. The grant date fair value of these shares was immaterial. The following table summarizes the Company’s restricted common stock award activity for the year ended December 31, 2019:



	Number of restricted shares
Issued and unvested as of January 1, 2019		1,141,736
Vested		(476,131	)
Forfeited		(13,123	)

Issued and unvested as of December 31, 2019		652,482

The Company recorded stock-based compensation expense in the following expense categories of its statements of operations:



(in thousands)	Year ended December 31,
(in thousands)	2018		2019

Research and development	$	—	$	13
General and administrative		1		13

Total	$	1	$	26

Income taxes

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

Income taxes

11. Income taxes

There is no provision for income taxes because the Company has historically incurred net operating losses and maintains a full valuation allowance against its deferred tax assets. The reported amount of income tax expense/benefit for the years ended December 31, 2018 and 2019 differs from the amount that would result from applying domestic federal statutory rates to pretax losses primarily because of changes in the valuation allowance.

A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows:



	Year ended December 31,
	2018			2019
Federal income tax expense at statutory rate		21%			21%
State taxes—net of federal benefit		6			6
Change in fair value of preferred stock tranche obligations		—			(2	)
Other permanent differences		—			(1	)
Federal & state R&D credits		2			4
Increase in valuation allowance		(29	)		(28	)

Effective income tax rate		0%			0%

Significant components of the Company’s net deferred tax assets at December 31, 2018 and 2019 are as follows:



(in thousands)	Year ended December 31,
(in thousands)	2018			2019

Deferred tax assets:
Stock-based compensation	$	—		$	1
Net operating loss carryforwards		1,264			4,848
Credit carryforwards		66			612
Intangible assets		4			12
Accrued expenses		50			161

Total deferred tax assets		1,384			5,634
Valuation allowance		(1,382	)		(5,607	)

Total net deferred tax assets		2			27


Deferred tax liabilities:
Fixed assets		(2	)		(27	)

Total deferred tax liability		(2	)		(27	)

Total deferred tax assets (liabilities)	$	—		$	—

Since its inception in 2017, the Company has not recorded any U.S. federal or state income tax benefits for the net losses it has incurred in any year or for its earned research and development tax credits, due to the uncertainty of realizing a benefit from those items. The valuation allowance increased by $1.4 million and $4.2 million during the years ended December 31, 2018 and 2019, respectively. As of December 31, 2019, the Company had federal net operating loss carryforwards of $17.7 million and state net operating loss carryforwards of $17.7 million. The federal net operating loss carryforwards are indefinite lived, and the state net operating loss carryforwards begin to expire in 2038. As of December 31, 2019, the Company also had federal and state research and development tax credit carryforwards of $0.4 million and $0.2 million, respectively, which begin to expire in 2038 and 2033, respectively.

Utilization of the net operating loss carryforwards and research and development tax credit carryforwards may be subject to a substantial annual limitation under Section 382 of the Internal Revenue Code of 1986, as amended, due to ownership changes that have occurred previously or that could occur in the future. These ownership changes may limit the amount of carryforwards that can be utilized annually to offset future taxable income. In general, an ownership change, as defined by Section 382, results from transactions increasing the ownership of certain stockholders or public groups in the stock of a corporation by more than 50% over a three-year period. The Company has not conducted a study to assess whether a change of control has occurred or whether there have been multiple changes of control since inception due to the significant complexity and cost associated with such a study. If the Company has experienced a change of control, as defined by Section 382, at any time since inception, utilization of the net operating loss carryforwards or research and development tax credit carryforwards would be subject to an annual limitation under Section 382, which is determined by first multiplying the value of the Company’s stock at the time of the ownership change by the applicable long-term tax-exempt rate, and then could be subject to additional adjustments, as required. Any limitation may result in expiration of a portion of the net operating loss carryforwards or research and development tax credit carryforwards before utilization. Further, until a study is completed and any limitation is known, no amounts are being presented as an uncertain tax position.

The Company’s policy is to record estimated interest and penalties related to uncertain tax positions in income tax expense. The Company has no amounts recorded for any unrecognized tax positions, accrued interest or penalties as of December 31, 2018 and 2019.

The Company files tax returns as prescribed by the tax laws of the jurisdictions in which it operates, including the United States and the Commonwealth of Massachusetts. In the normal course of business, the Company is subject to examination by federal and state jurisdictions, where applicable. There are currently no pending tax examinations. The Company’s tax returns are open under statute from 2018 to the present.

- Definition

The entire disclosure for income taxes. Disclosures may include net deferred tax liability or asset recognized in an enterprise's statement of financial position, net change during the year in the total valuation allowance, approximate tax effect of each type of temporary difference and carryforward that gives rise to a significant portion of deferred tax liabilities and deferred tax assets, utilization of a tax carryback, and tax uncertainties information.

+ References + Details

Commitments and Contingencies

9 Months Ended

12 Months Ended

Sep. 30, 2020

Dec. 31, 2019

Commitments and Contingencies Disclosure [Abstract]

Commitments and Contingencies

11. Commitments and Contingencies

Operating leases

In May 2019, the Company entered into a sublease agreement for a portion of the laboratory and office space in Waltham, Massachusetts. The term of the sublease commenced on July 1, 2019 and expires on December 31, 2021.

Rent expense for the three months ended September 30, 2020 and 2019 totaled $0.2 million and $0.4 million, respectively. Rent expense for the nine months ended September 30, 2020 and 2019 totaled $0.2 million and $0.4 million, respectively.

Future minimum lease payments under the non-cancelable operating leases consisted of the following as of September 30, 2020:



Year Ending December 31,	(in thousands)
2020 (remaining 3 months)	$	197
2021		800

Total	$	997

Legal proceedings

The Company is not currently party to any material legal proceedings. At each reporting date, the Company evaluates whether a potential loss amount or a potential range of loss is probable and reasonably estimable under the provisions of the authoritative guidance that addresses accounting for contingencies. The Company expenses as incurred the costs related to any such legal proceedings.

Other contractual obligations

The Company enters into contracts in the normal course of business with third parties for preclinical research studies, clinical trials and testing and manufacturing services. These contracts typically do not contain minimum purchase commitments and are generally cancelable by the Company upon written notice. Payments due upon cancellation consist of payments for services provided or expenses incurred, including noncancelable obligations of the service providers, up to the date of cancellation and in the case of certain arrangements may include non-cancelable fees.

13. Commitments and contingencies

Operating leases

Rent expense for the years ended December 31, 2018 and 2019 totaled $0.3 million and $0.7 million, respectively.

Future minimum lease payments under the non-cancelable operating leases consisted of the following as of December 31, 2019:



Year ending December 31,	(in thousands)
2020	$	776
2021		800

Total	$	1,576

Legal proceedings

Other contractual obligations

The Company has also entered into a license agreement (the “UMONS Agreement”) with the University of Mons (“UMONS”) in April 2020, pursuant to which UMONS granted to the Company an exclusive, worldwide license to certain patents and patent applications and a non-exclusive, worldwide license to existing, related know-how. Each of the issued patents licensed to the Company under the UMONS Agreement is scheduled to expire in 2031. The licenses under the UMONS Agreement confer on the Company the right to research, develop and commercialize products (“licensed products”), and to practice processes, in each case, covered by the licensed patents and existing, related know-how.

Under the UMONS Agreement, the Company is obligated to use commercially reasonable efforts to develop at least one licensed product and, to the extent regulatory approval is obtained in such jurisdictions, to commercialize at least one licensed product in the United States and the United Kingdom or a member country of the European Union. Unless terminated earlier, the UMONS Agreement will remain in effect until the last to expire of the licensed patent rights on a licensed product-by-licensed product and country-by-country basis. UMONS may terminate the UMONS Agreement in the event of a material breach by the Company and the Company’s failure to cure such breach within a specified time period. The Company may voluntarily terminate the UMONS Agreement with prior notice to UMONS.

In connection with the Company’s entry into the UMONS Agreement, the Company paid UMONS an upfront payment of €0.1 million. The Company also agreed to make milestone payments to UMONS upon the achievement of specified development and regulatory milestones up to a maximum aggregate total of €0.4 million for the first licensed product to achieve such milestones and up to a maximum aggregate total of €0.2 million for each subsequent licensed product to achieve each such milestones, as well as a low single-digit percentage royalty on net sales of licensed products by the Company, its affiliates and sublicensees. These royalty obligations continue on a licensed product-by-licensed product and country-by-country basis until the expiration of the last licensed patent rights covering such licensed product in such country. In addition, if the Company sublicenses rights under the UMONS Agreement, the Company is required to pay a low double-digit percentage of the sublicense revenue to UMONS. Additionally, if the Company chooses to file, prosecute or maintain any patents included in the licensed patent rights under the UMONS Agreement, it will be required to bear the full cost and expenses of preparing, filing, prosecuting and maintaining any such patents.

No amounts have been accrued under this arrangement with respect to potential milestone payment obligations or royalty obligations given the uncertainty of achieving the specified milestones and commercial sales of any licensed products.

Summary of Significant Accounting Policies (Policies)

9 Months Ended

12 Months Ended

Sep. 30, 2020

Dec. 31, 2019

Accounting Policies [Abstract]

Use of Estimates

Use of estimates

Cash and Cash Equivalents

Cash and cash equivalents

Property and Equipment

Property and equipment



	Estimated useful life
Laboratory equipment		3 years
Furnitures and fixtures		5 years
Computer equipment		3 years
Leasehold improvements		Shorter of life of lease or 10 years

Impairment of Long-Lived Assets

Impairment of long-lived assets

Segment Information

Segment information

Research and Development Costs

Research and development costs

Income Taxes

Income taxes

Fair Value Measurements

Fair value measurements

•

Level 1—Unadjusted quoted prices in active markets that are accessible to the reporting entity at the measurement date for identical assets and liabilities.

•

quoted prices for similar assets and liabilities in active markets;

•

quoted prices for identical or similar assets or liabilities in markets that are not active;

•

observable inputs other than quoted prices that are used in the valuation of the asset or liabilities (e.g., interest rate and yield curve quotes at commonly quoted intervals); and

•

inputs that are derived principally from or corroborated by observable market data by correlation or other means.

•

Net Loss Per Share

Net loss per share

Redeemable Convertible Preferred Stock

Redeemable convertible preferred stock

Stock-Based Compensation

Stock-based compensation

Success Fee Obligation

Success fee obligation

Comprehensive Loss

Comprehensive loss

Accounting Pronouncements Issued and Not Adopted

Accounting pronouncements issued and not adopted

Unaudited Interim Financial Information

Unaudited interim financial information

Summary of Significant Accounting Policies

Summary of significant accounting policies

Fair Value Measurements (Tables)

9 Months Ended

12 Months Ended

Sep. 30, 2020

Dec. 31, 2019

Fair Value Disclosures [Abstract]

Summary of Assets and Liabilities Carried at Fair Value on Recurring Basis

The following tables set forth by level, within the fair value hierarchy, the assets and liabilities carried at fair value on a recurring basis for the periods presented:



	Fair Value Measurements as of December 31, 2019
(in thousands)	Level 1		Level 2		Level 3		Total

Assets:
Cash equivalents—money market funds	$	14,420		—		—	$	14,420

Total	$	14,420		—		—	$	14,420

The following tables set forth by level, within the fair value hierarchy (see Note 2), the assets and liabilities carried at fair value on a recurring basis for the periods presented:



(in thousands)	Fair value measurements as of December 31, 2018
(in thousands)	Level 1		Level 2		Level 3		Total

Assets:
Cash equivalents—money market funds	$	7,005		—		—	$	7,005

Total	$	7,005		—		—	$	7,005


Liabilities:
Preferred stock tranche obligations	$	—		—	$	3,375	$	3,375

Total	$	—		—	$	3,375	$	3,375



(in thousands)	Fair value measurements as of December 31, 2019
(in thousands)	Level 1		Level 2		Level 3		Total

Assets:
Cash equivalents—money market funds	$	14,420		—		—	$	14,420

Total	$	14,420		—		—	$	14,420

Roll-forward of Aggregate Fair Values of Liabilities Determined by Level 3 Inputs

The following table presents a roll-forward of the aggregate fair value of the success fee obligation for which fair value is determined by Level 3 inputs:



(in thousands)	Success Fee Obligation
Balance—January 1, 2020	$	—
Initial fair value of success fee obligation		—
Change in fair value		450
Settlement of derivative instrument		(450	)

Balance—September 30, 2020	$	—

The following table presents a roll-forward of the aggregate fair values of the Company’s liabilities for which fair value is determined by Level 3 inputs:



(in thousands)	Preferred stock tranche obligations			Success fee obligation
Balance—January 1, 2018	$	—		$	—
Initial fair value of preferred stock tranche obligations		3,354			—
Change in fair value		21			—

Balance—December 31, 2018		3,375			—
Settlement of Tranche Right I		1,621			—
Change in fair value		1,323			—
Reclassification of Tranche Right II to equity		(6,319	)		—

Balance—December 31, 2019		—			—

Stock-Based Awards (Tables)

9 Months Ended

12 Months Ended

Sep. 30, 2020

Dec. 31, 2019

Share-based Payment Arrangement [Abstract]

Summary of Assumptions Used to Determine Grant-Date Fair Value of Options Granted

The assumptions that the Company used to determine the grant-date fair value of options granted were as follows:



	Nine Months Ended September 30,
	2020		2019
Expected volatility		75%		74%
Risk-free interest rate		0.31% —1.67%		1.46% —2.50%
Expected term (in years)		6		6
Expected dividend yield		—		—

The assumptions that the Company used to determine the grant date fair value of options granted were as follows:



		Year ended December 31, 2019
Expected volatility		74%
Risk-free interest rate		1.46% — 2.50%
Expected term (in years)		6
Expected dividend yield		—

Summary of Stock Option Activity and Related Information

A summary of the Company’s stock option activity and related information for the nine months ended September 30, 2020 is as follows:



(in thousands, except share and per share data)	Options			Weighted Average Exercise Price		Weighted Average Remaining Life (in years)		Aggregate Intrinsic Value
Outstanding as of January 1, 2020		311,303		$	0.93		9.5	$	30
Granted		6,167,369			6.94
Exercised		(30,409	)		0.82				589
Canceled		(110,484	)		1.04

Outstanding as of September 30, 2020		6,337,779		$	6.78		9.7	$	84,984

Options exercisable as of September 30, 2020		82,768		$	0.97		8.9	$	1,591
Options vested or expected to vest as of September 30, 2020		6,337,779		$	6.78		9.7	$	84,984

The following table summarizes the option activity under the 2018 Plan for the periods presented:



(in thousands, except share and per share data)	Options			Weighted average exercise price		Weighted average remaining life (in years)		Aggregate intrinsic value
Outstanding January 1, 2019		—		$	—		—	$	—
Granted		314,317			0.93
Exercised		—			—
Canceled		(3,014	)		0.73

Outstanding December 31, 2019		311,303		$	0.93		9.5	$	30

Options exercisable—December 31, 2019		—		$	—		—	$	—
Options vested or expected to vest—December 31, 2019		311,303		$	0.93		9.5	$	30

Summary of Restricted Common Stock Award Activity

The following table summarizes the Company’s restricted common stock award activity for the nine months ended September 30, 2020:



	Number of Restricted Shares
Issued and unvested as of January 1, 2020		652,482
Vested		(211,614	)

Issued and unvested as of September 30, 2020		440,868

The following table summarizes the Company’s restricted common stock award activity for the year ended December 31, 2019:



	Number of restricted shares
Issued and unvested as of January 1, 2019		1,141,736
Vested		(476,131	)
Forfeited		(13,123	)

Issued and unvested as of December 31, 2019		652,482

Summary of Stock-based Compensation Expense

The Company recorded stock-based compensation expense in the following expense categories of its statements of operations:



	Three Months Ended September 30,				Nine Months Ended September 30,
(in thousands)	2020		2019		2020		2019
Research and development	$	929	$	3	$	984	$	4
General and administrative		1,306		4		1,392		8

Total	$	2,235	$	7	$	2,376	$	12

The Company recorded stock-based compensation expense in the following expense categories of its statements of operations:



(in thousands)	Year ended December 31,
(in thousands)	2018		2019

Research and development	$	—	$	13
General and administrative		1		13

Total	$	1	$	26

Summary of RSU Activity

A summary of the Company’s RSU activity and related information for the 2020 Plan for the nine months ended September 30, 2020 is as follows:



	Number of Shares Underlying RSUs		Weighted Average Grant Date Fair Value
Unvested as of January 1, 2020		—	$	—
Granted		375,137		23.90
Vested		—		—
Forfeited		—		—

Unvested as of September 30, 2020		375,137	$	23.90

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended						9 Months Ended		12 Months Ended
	Sep. 21, 2020	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
Nature Of Business And Basis Of Presentation [Line Items]
Entity incorporation date										Dec. 01, 2017
Net loss		$ (13,920)	$ (8,982)	$ (7,886)	$ (5,871)	$ (1,497)	$ (2,087)	$ (30,788)	$ (9,456)	$ (14,859)	$ (4,811)
Accumulated deficit		$ (50,534)						$ (50,534)		$ (19,746)	(4,887)
Net proceeds from issuance of common stock											$ 7
Initial Public Offering \| Common Stock
Nature Of Business And Basis Of Presentation [Line Items]
Number of shares issued and sold	14,089,314
Underwriters option to purchase additional shares	1,837,736
Public offering price	$ 19.00
Net proceeds from issuance of common stock	$ 246,400

Debt Financing - Additional Information (Detail) - USD ($)		1 Months Ended	9 Months Ended	12 Months Ended
	Feb. 20, 2020	Sep. 30, 2020	Sep. 30, 2020	Dec. 31, 2019
Line Of Credit Facility [Line Items]
Long-term debt, maturities, repayments of principal in 2021				$ 2,800,000
Long-term debt, maturities, repayments of principal in 2022				3,300,000
Long-term debt, maturities, repayments of principal in 2023				3,300,000
Long-term debt, maturities, repayments of principal in 2024				600,000
Series A Preferred Stock
Line Of Credit Facility [Line Items]
Cash proceeds from closing of third tranche				$ 17,500,000
Term Loan
Line Of Credit Facility [Line Items]
Credit facility, maximum borrowing capacity under loan agreement	$ 10,000,000
Credit facility, interest rate description			Interest on the outstanding loan balance accrued at a variable annual rate equal to the greater of (i) the lender's prime rate plus 0.25% and (ii) 5.00%, and the Company was required to make interest-only payments on the loans on a monthly basis through August 2021.	Interest on the outstanding loan balance will accrue at a variable annual rate equal to the greater of (i) PWB's prime rate plus 0.25% and (ii) 5.00%. The Company is required to make interest-only payments on the loans on a monthly basis through February 2021.
Debt instrument, frequency of periodic payment of interest			Interest-only payments on the loans on a monthly basis	Interest-only payments on the loans on a monthly basis
Credit facility, variable interest rate spread	0.25%
Credit facility, interest rate during period	5.00%
Loan maturity date				2024-02
Facility establishment fees				$ 100,000
Success fee required to pay in the event of liquidation, including IPO	$ 500,000			$ 500,000
Credit facility, interest rate at period end		5.00%	5.00%
Success fee paid after IPO		$ 500,000

Convertible Preferred Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands				1 Months Ended		3 Months Ended	9 Months Ended		12 Months Ended
	Mar. 18, 2020	Apr. 30, 2019	Nov. 29, 2018	Aug. 31, 2020	Jul. 31, 2020	Sep. 30, 2020	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Temporary Equity [Line Items]
Proceeds from issuance of convertible preferred stock							$ 134,920	$ 19,989	$ 19,989	$ 7,415
Convertible preferred stock, shares issued upon conversion									9,798,298	3,768,575
Preferred stock dividend rate									6.00%
Temporary equity liquidation preference per share									$ 1.00
Liquidation preference description									If the amounts to be distributed to the holders of the Series A Preferred Stock exceeds $2.00 per share, subject to certain adjustments, then the holders of the Series A Preferred Stock will receive the greater of $2.00 or the amount such holder would have received if all shares of Series A Preferred Stock had converted to common stock immediately prior to the distribution.
Conversion of stock, description									(a) the closing of the sale of shares of common stock to the public at a price of at least $3.00 per share (subject to adjustment) in an initial public offering with net proceeds to the Company of at least $50.0 million ("Qualified IPO") or (b) the written consent of the holders of a majority of the outstanding shares of Series A Preferred Stock, the Series A Preferred Stock will automatically convert into common stock at the Series A Conversion Price then in effect.
Common Stock
Temporary Equity [Line Items]
Shares issued						14,089,314				2,110,404
Convertible preferred stock, shares issued upon conversion						28,086,375	28,086,375
Series A Convertible Preferred Stock
Temporary Equity [Line Items]
Purchase of share			$ 50,000
Shares issued	2,000,000	20,000,000	12,500,000		17,500,000
Convertible preferred stock, price per share	$ 1.00	$ 1.00	$ 1.00		$ 1.00
Proceeds from issuance of convertible preferred stock	$ 2,000	$ 20,000	$ 7,500		$ 17,500
Convertible preferred stock, shares issued upon conversion			5,000,000						9,798,298	3,768,575
Fair value			$ 5,000					$ 6,300
Issuance costs		$ 100
Series A Convertible Preferred Stock \| Tranche Right
Temporary Equity [Line Items]
Fair value			$ 3,400
Series A Convertible Preferred Stock \| Tranche Right \| Maximum [Member]
Temporary Equity [Line Items]
Sale of stock			37,500,000
Series A Convertible Preferred Stock \| Tranche Right I
Temporary Equity [Line Items]
Shares issued		20,000,000
Sale of stock			20,000,000
Series A Convertible Preferred Stock \| Tranche Right II
Temporary Equity [Line Items]
Sale of stock			17,500,000
Series B Convertible Preferred Stock
Temporary Equity [Line Items]
Shares issued				41,159,724
Convertible preferred stock, price per share				$ 2.81
Proceeds from issuance of convertible preferred stock				$ 115,700

Redeemable convertible preferred stock - Schedule of Convertible Preferred Stock (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Nov. 29, 2018
Temporary Equity [Line Items]
Temporary equity, shares authorized	50,000,000	50,000,000
Temporary equity, shares issued	32,500,000	12,500,000
Temporary equity, shares outstanding	32,500,000	12,500,000
Temporary equity, value	$ 27,429	$ 9,061
Temporary equity, liquidation preference	$ 32,500	$ 12,500
Common shares issuable upon conversion	9,798,298	3,768,575
Series A Convertible Preferred Stock
Temporary Equity [Line Items]
Temporary equity, shares authorized	50,000,000	50,000,000
Temporary equity, shares issued	32,500,000	12,500,000
Temporary equity, shares outstanding	32,500,000	12,500,000
Temporary equity, value	$ 27,429	$ 9,061
Temporary equity, liquidation preference	$ 32,500	$ 12,500
Common shares issuable upon conversion	9,798,298	3,768,575	5,000,000

Stock-Based Awards - Additional Information (Detail)		9 Months Ended	12 Months Ended
Stock-Based Awards - Additional Information (Detail)	Sep. 16, 2020 shares	Sep. 30, 2020 USD ($) Installment $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares	Sep. 15, 2020 shares	Dec. 31, 2018 shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted-average grant date fair value of options granted \| $ / shares		$ 4.27	$ 0.62
Number of shares initially reserved for issuance			11,290,676		5,737,103
2018 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total number of shares of common stock authorized			1,968,528	8,267,252
Share based payments, vesting period			4 years
Share based payments, maximum term			10 years
Number of shares available for future issuance			528,593	1,928,487
2020 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares available for future issuance		3,061,794
Number of shares transferred from 2018 plan, available for future issuance	1,928,487
Number of shares initially reserved for issuance	4,884,233
Percentage of outstanding shares increased annually under the plan	5.00%
2020 ESPP
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total number of shares of common stock authorized	488,414
Number of shares available for future issuance	488,414
Percentage of outstanding shares increased annually under the plan	1.00%
Increase in common stock reserved for future issuance	1,953,656
Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation expense \| $		$ 25,000,000	$ 200,000
Unrecognized share based compensation expense, recognition period		3 years 8 months 1 day	3 years 4 months 24 days
Restricted Stock Units ("RSUs")
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation expense \| $		$ 8.7
Unrecognized share based compensation expense, recognition period		3 years 11 months 15 days
Number of share of common stock receivable upon vesting		1
Restricted Stock Units ("RSUs") \| 2020 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
'Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights		The Company grants RSUs with service conditions that vest in four equal annual installments provided that the employee remains employed with the Company. The Company also grants RSUs with performance conditions that vest upon the achievement of specified milestones.
Share-based Compensation Arrangement By Share-based Payment Award Grant Service Conditions to Vest Number of Annual Installments \| Installment		4

Stock-Based Awards - Summary of Stock Option Activity and Related Information (Detail) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares
Options
Outstanding, beginning of period \| shares	311,303
Granted \| shares	6,167,369	314,317
Exercised \| shares	(30,409)
Canceled \| shares	(110,484)	(3,014)
Outstanding, end of period \| shares	6,337,779	311,303
Options exercisable end of period \| shares	82,768
Options vested or expected to vest end of period \| shares	6,337,779	311,303
Weighted Average Exercise Price
Outstanding, beginning of period \| $ / shares	$ 0.93
Granted \| $ / shares	6.94	$ 0.93
Exercised \| $ / shares	0.82
Canceled \| $ / shares	1.04	0.73
Outstanding, end of period \| $ / shares	6.78	0.93
Options exercisable end of period \| $ / shares	0.97
Options vested or expected to vest end of period \| $ / shares	$ 6.78	$ 0.93
Aggregate Intrinsic Value
Exercised \| $	$ 589
Outstanding \| $	84,984	$ 30
Options exercisable end of period \| $	1,591
Options vested or expected to vest end of period \| $	$ 84,984	$ 30
Weighted Average Remaining Life (in years)
Outstanding	9 years 8 months 12 days	9 years 6 months
Options exercisable end of period	8 years 10 months 24 days
Options vested or expected to vest end of period	9 years 8 months 12 days	9 years 6 months

Income taxes - Significant Components of Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Stock-based compensation	$ 1
Net operating loss carryforwards	4,848	$ 1,264
Credit carryforwards	612	66
Intangible assets	12	4
Accrued expenses	161	50
Total deferred tax assets	5,634	1,384
Valuation allowance	(5,607)	(1,382)
Total net deferred tax assets	27	2
Deferred tax liabilities:
Fixed assets	(27)	(2)
Total deferred tax liability	(27)	(2)
Total deferred tax assets (liabilities)	$ 0	$ 0

Net Loss Per Share - Calculation of Basic and Diluted Net Loss per Share (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	$ (13,920)	$ (8,982)	$ (7,886)	$ (5,871)	$ (1,497)	$ (2,087)	$ (30,788)	$ (9,456)	$ (14,859)	$ (4,811)
Denominator:
Weighted-average common shares outstanding-basic and diluted	6,920,008			2,491,487			4,100,504	2,401,039	2,442,872	474,118
Net loss per share-basic and diluted	$ (2.01)			$ (2.36)			$ (7.51)	$ (3.94)	$ (6.08)	$ (10.15)

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Reference 2: http://www.xbrl.org/2003/role/exampleRef
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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Reference 5: http://www.xbrl.org/2003/role/disclosureRef
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Reference 6: http://www.xbrl.org/2003/role/disclosureRef
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Reference 7: http://www.xbrl.org/2003/role/disclosureRef
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Reference 8: http://www.xbrl.org/2003/role/disclosureRef
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Reference 9: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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Reference 10: http://www.xbrl.org/2003/role/disclosureRef
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-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8736-108599

Reference 11: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 12: http://fasb.org/us-gaap/role/ref/legacyRef
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-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

Reference 13: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 14: http://www.xbrl.org/2003/role/disclosureRef
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Reference 15: http://www.xbrl.org/2003/role/disclosureRef
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Reference 16: http://www.xbrl.org/2003/role/disclosureRef
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Reference 17: http://www.xbrl.org/2003/role/disclosureRef
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Reference 18: http://www.xbrl.org/2003/role/disclosureRef
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Reference 19: http://www.xbrl.org/2003/role/disclosureRef
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Reference 20: http://www.xbrl.org/2003/role/disclosureRef
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Reference 21: http://www.xbrl.org/2003/role/disclosureRef
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Reference 22: http://www.xbrl.org/2003/role/disclosureRef
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Reference 23: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

Condensed Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Common stock, par value	$ 0.0001	$ 0.0001	$ 0.0001
Common stock, shares authorized	200,000,000	68,000,000	68,000,000
Common stock, shares issued	45,445,115	3,239,017	3,252,140
Common stock, shares outstanding	45,004,247	2,586,535	2,110,404

Simple agreement for future equity	12 Months Ended
Simple agreement for future equity	Dec. 31, 2019
Text Block [Abstract]
Simple agreement for future equity	7. Simple agreement for future equity In January 2018, June 2018 and October 2018, the Company entered into simple agreements for future equity (the “SAFEs”) with an investor, receiving $5.0 million of gross proceeds in aggregate in exchange for the investor’s right to participate in a future equity financing. The SAFEs contained a number of conversion and redemption provisions, including settlement upon liquidity or dissolution events. In November 2018, the investor exercised its right to convert the SAFEs in connection with the Company’s equity financing (See Note 8) and exchanged the SAFEs for an aggregate of 5,000,000 shares of Series A Redeemable Convertible Preferred Stock, with a fair value of $5.0 million at issuance.

Related parties	12 Months Ended
Related parties	Dec. 31, 2019
Related Party Transactions [Abstract]
Related parties	14. Related parties The Company has received professional services from a major stockholder, Atlas Venture Life Science Advisors, LLC. The Company recorded expenses totaling $0.2 million and $0.1 million related to these services for the years ended December 31, 2018 and 2019, respectively. As of December 31, 2018, the Company had $0.1 million in accounts payable representing amounts owed to Atlas Venture Life Science Advisors, LLC. There were no amounts owed to Atlas Venture Life Science Advisors, LLC as of December 31, 2019.

Subsequent events	9 Months Ended	12 Months Ended
Subsequent events	Sep. 30, 2020	Dec. 31, 2019
Subsequent Events [Abstract]
Subsequent events	12. Subsequent events For the nine months ended September 30, 2020 (unaudited), subsequent events were evaluated through November 5, 2020, the date on which the unaudited interim financial statements were first issued, and January 11, 2021 as to the effects of the lease discussion below. The Company has concluded that no events have occurred subsequent to September 30, 2020 that require disclosure, except for those referenced below. *Lease agreement* On December 4, 2020, the Company entered into a lease agreement (the “Lease”), pursuant to which the Company will lease approximately 68,187 square feet of office and laboratory space located in Waltham, Massachusetts (the “Premises”). The Lease will have a term of eight years and six months that will commence when the Premises are ready for occupancy. The Company’s obligation for the payment of base rent for the Premises begins six months after the commencement date and will be $0.4 million per month, increasing up to $0.5 million during the term of the Lease. The Company has two options to extend the term of the Lease, each for a period of an additional five years. The Company expects the Premises to be ready for occupancy in the fourth quarter of 2021.	15. Subsequent events For the year ended December 31, 2019, subsequent events were evaluated through July 23, 2020, the date on which the audited financial statements were issued, and September 9, 2020 as to the effects of the reverse stock split discussion below. *Reverse stock split* The Company’s board of directors and stockholders approved a one-for-3.3169 reverse split of the Company’s issued and outstanding common stock and a proportional adjustment to the existing conversion ratios for the outstanding shares of convertible preferred stock, which became effective on September 9, 2020. Accordingly, all share and per share amounts for all periods presented in the accompanying financial statements and notes thereto have been retroactively adjusted, where applicable, to reflect the reverse stock split.

Summary of Significant Accounting Policies - Schedule of Estimated Useful Lives of Property, Plant and Equipment (Detail)	12 Months Ended
	Dec. 31, 2019
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property, plant and equipment, useful life	3 years
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property, plant and equipment, useful life	5 years
Computer Equipment
Property Plant And Equipment [Line Items]
Property, plant and equipment, useful life	3 years
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property, plant and equipment, useful life	Shorter of life of lease or 10 years

Debt Financing	9 Months Ended	12 Months Ended
Debt Financing	Sep. 30, 2020	Dec. 31, 2019
Debt Disclosure [Abstract]
Debt Financing	6. Debt Financing On February 20, 2020, the Company entered into a Loan Agreement pursuant to which the lender made term loans to the Company in an aggregate principal amount of $10.0 million. Borrowings under the Loan Agreement were collateralized by substantially all of the Company’s assets, excluding intellectual property. Interest on the outstanding loan balance accrued at a variable annual rate equal to the greater of (i) the lender’s prime rate plus 0.25% and (ii) 5.00%, and the Company was required to make interest-only payments on the loans on a monthly basis through August 2021. The interest rate was 5.00% at September 30, 2020. The outstanding borrowings were repaid in October 2020. As part of the Loan Agreement, a success fee of $0.5 million was required in the event of a liquidation event, including an IPO. The success fee represented an embedded derivative which the Company bifurcated from the debt arrangement and carried at fair value. In September 2020, the Company completed its IPO and paid the success fee of $0.5 million.	6. Debt financing On February 20, 2020, the Company entered into the Loan Agreement with PWB pursuant to which PWB made term loans to the Company in an aggregate principal amount of $10.0 million. Borrowings under the Loan Agreement are collateralized by substantially all of the Company’s assets, excluding intellectual property. Interest on the outstanding loan balance will accrue at a variable annual rate equal to the greater of (i) PWB’s prime rate plus 0.25% and (ii) 5.00%. The Company is required to make interest-only payments on the loans on a monthly basis through February 2021. Upon receipt of at least $17.5 million of gross cash proceeds from the closing of the third tranche of the Series A Preferred Stock, interest-only payments will be extended until August 2021. Subsequent to the interest-only periods, the Company will be required to make equal monthly payments of principal plus interest until the loan matures in February 2024. The Company incurred fees associated with establishing the facility of $0.1 million. The Company has an option to prepay the loan in full without a fee. In the event of a specified liquidity event, including the proposed IPO, the Company will be required to pay a success fee of $0.5 million. The fair value of the success fee and the transaction fees created a debt discount, which will be amortized over the facility term. The Loan Agreement contains customary representations, warranties and covenants and also includes customary events of default, including payment defaults, breaches of covenants, change of control and occurrence of a material adverse effect. There are no financial covenants associated with the Loan Agreement. The scheduled principal maturity of the amounts borrowed under the Loan Agreement is $2.8 million in 2021, $3.3 million in 2022, $3.3 million in 2023 and $0.6 million in 2024.

Equity	9 Months Ended	12 Months Ended
Equity	Sep. 30, 2020	Dec. 31, 2019
Equity [Abstract]
Equity	8. Equity As of September 30, 2020, the Company’s amended and restated certificate of incorporation authorized the Company to issue 200,000,000 shares of common stock, par value $0.0001 per share, and 10,000,000 shares of preferred stock, par value $0.0001 per share, all of which preferred stock is undesignated.	9. Common stock As of December 31, 2018 and 2019, the Company had authorized 68,000,000 shares of common stock. The voting, dividend and liquidation rights of the holders of the Company’s common stock is subject to and qualified by the rights, powers and preferences of the holders of the Series A Preferred Stock as set forth above. As of December 31, 2018 and 2019, the Company had reserved an aggregate of 5,737,103 and 11,290,676 shares of common stock, respectively, for the conversion of outstanding shares of Series A Preferred Stock, the exercise of outstanding stock options, the vesting of restricted shares of common stock and the potential issuance of shares available for grant under the Company’s 2018 Stock Incentive Plan (see Note 10).

Summary of Significant Accounting Policies - Additional Information (Detail)	1 Months Ended	12 Months Ended
	Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($) Segment
Summary Of Significant Accounting Policies [Line Items]
Cash and cash equivalent maturity period		90 days
Impairment Charges		$ 0
Number of operating segment \| Segment		1
Success fee paid	$ 500,000
Loan Agreement \| Pacific Western Bank
Summary Of Significant Accounting Policies [Line Items]
Success fee paid		$ 500,000

Fair Value Measurements - Summary of Assets and Liabilities Carried at Fair Value on Recurring Basis (Detail) - Fair Value, Recurring - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Assets:
Total	$ 14,420	$ 7,005
Liabilities:
Total		3,375
Preferred Stock Tranche Obligations
Liabilities:
Total		3,375
Level 1
Assets:
Total	14,420	7,005
Level 3
Liabilities:
Total		3,375
Level 3 \| Preferred Stock Tranche Obligations
Liabilities:
Total		3,375
Cash Equivalents - Money Market Funds
Assets:
Total	14,420	7,005
Cash Equivalents - Money Market Funds \| Level 1
Assets:
Total	$ 14,420	$ 7,005

Fair Value Measurements - Additional Information (Detail)		1 Months Ended
	Sep. 21, 2020 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value assets, transfers among level 1, level 2 or level 3			$ 0
Fair value liabilities, transfers among level 1, level 2 or level 3			$ 0
Fair value liability, payment percentage	100.00%
Fair value of derivative liability	$ 500,000
Success fee paid		$ 500,000
Discount Rate
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Measurement input	0.00

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 and equipment-at cost	$ 2,219	$ 1,781	$ 134
Less accumulated depreciation and amortization	(778)	(295)	(24)
Property and equipment-net	1,441	1,486	110
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and equipment-at cost	1,993	1,705	$ 134
Office and Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment-at cost	79	62
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment-at cost	14	14
Construction in Process
Property Plant And Equipment [Line Items]
Property and equipment-at cost	$ 133
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment-at cost		41
Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment-at cost		$ 20

Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Payroll and benefits	$ 1,318	$ 540	$ 184
Consulting services	23	60
Legal services	458
Research and development	818	484	8
Facility costs	36	18
Other	537		1
Total	$ 3,190	$ 1,102	$ 193

Stock-Based Awards - Summary of Restricted Common Stock Award Activity (Detail) - Restricted Common Stock - shares	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Number of Restricted Shares
Issued and unvested beginning of period	652,482	1,141,736
Vested	(211,614)	(476,131)
Forfeited		(13,123)
Issued and unvested end of period	440,868	652,482

Income taxes - Reconciliation of U.S. Federal Statutory Income Tax Rate to Effective Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Effective income tax rate reconciliation
Federal income tax expense at statutory rate	21.00%	21.00%
State taxes-net of federal benefit	6.00%	6.00%
Change in fair value of preferred stock tranche obligations	(2.00%)
Other permanent differences	(1.00%)
Federal & state R&D credits	4.00%	2.00%
Increase in valuation allowance	(28.00%)	(29.00%)
Effective income tax rate	0.00%	0.00%

Income taxes - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Line Items]
Increase in valuation allowance	$ 4.2	$ 1.4
U.S. Federal
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	17.7
U.S. Federal \| Research and Developments Tax Credit Carryforwards
Income Tax Disclosure [Line Items]
Research and development tax credit carryforwards	$ 0.4
Research and development tax credit carryforwards expiration year	2038
State and Local Jurisdiction
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 17.7
Net operating loss carryforwards expiration year	2038
State and Local Jurisdiction \| Research and Developments Tax Credit Carryforwards
Income Tax Disclosure [Line Items]
Research and development tax credit carryforwards	$ 0.2
Research and development tax credit carryforwards expiration year	2033

Commitments and Contingencies - Additional Information (Detail) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2019 EUR (€)	Dec. 31, 2018 USD ($)
Leases [Abstract]
Lease executed date			Jul. 01, 2019		Jul. 01, 2019	Jul. 01, 2019
Lease expiration date			Dec. 31, 2021		Dec. 31, 2021	Dec. 31, 2021
Rent expense \| $	$ 0.2	$ 0.4	$ 0.2	$ 0.4	$ 0.7		$ 0.3
Upfront payment						€ 0.1
Development and regulatory milestones payment						0.4
Development and regulatory possible second milestones payment						€ 0.2

Commitments and Contingencies - Schedule of Future Minimum Lease Payments under Non-Cancelable Operating Leases (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Operating Leases Future Minimum Payments Due [Abstract]
Operating lease liability to be paid, remainder of fiscal year	$ 197
Operating lease liability to be paid, year one	800	$ 776
Operating lease liability to be paid, year two		800
Total	$ 997	$ 1,576

Related parties - Additional Information (Detail) - Atlas Venture Life Science Advisors, LLC - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Related Party Transaction [Line Items]
Related party transaction expenses	$ 0.1	$ 0.2
Related party account payable	$ 0.0	$ 0.1

Subsequent Events (Detail)	Dec. 04, 2020 ft²	Sep. 09, 2020
Subsequent Event [Line Items]
Stockholders' equity, reverse stock split, description		One-for-3.3169
Stockholders' equity, reverse stock split ratio		3.3169
Subsequent Event [Member]
Subsequent Event [Line Items]
Leased office space	68,187
:ease term	8 years 6 months
Lease, description	The Company's obligation for the payment of base rent for the Premises begins six months after the commencement date and will be $0.4 million per month, increasing up to $0.5 million during the term of the Lease. The Company has two options to extend the term of the Lease, each for a period of an additional five years. The Company expects the Premises to be ready for occupancy in the fourth quarter of 2021.

Stock-Based Awards - Summary of RSU Activity (Detail) - Restricted Stock Units ("RSUs") - 2020 Plan	9 Months Ended
	Sep. 30, 2020 $ / shares shares
Number of Shares Underlying RSUs
Granted \| shares	375,137
Issued and unvested end of period \| shares	375,137
Weighted Average Grant Date Fair Value
Granted	$ 23.90
Vested	0
Forfeited	0
Unvested as of September 30, 2020	$ 23.90

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