Keros Therapeutics (KROS) 10-Q 4 Nov 21 2021 Q3 Quarterly report Financial data

Cover

Cover - shares	9 Months Ended
	Sep. 30, 2021	Oct. 28, 2021
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2021
Document Transition Report	false
Entity File Number	001-39264
Entity Registrant Name	KEROS THERAPEUTICS, INC.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	81-1173868
Entity Address, Address Line One	99 Hayden Avenue
Entity Address, Address Line Two	Suite 120
Entity Address, Address Line Three	Building E
Entity Address, City or Town	Lexington
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02421
City Area Code	617
Local Phone Number	314-6297
Title of 12(b) Security	Common Stock, $0.0001 par value per share
Trading Symbol	KROS
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding (in shares)		23,398,657
Amendment Flag	false
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Current Fiscal Year End Date	--12-31
Entity Central Index Key	0001664710

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
CURRENT ASSETS:
Cash and cash equivalents	$ 221,349	$ 265,876
Prepaid expenses and other current assets	4,725	1,850
Total current assets	226,074	267,726
Operating lease right-of-use assets	1,265	878
Property and equipment, net	1,295	724
Restricted cash	1,328	115
TOTAL ASSETS	229,962	269,443
CURRENT LIABILITIES:
Accounts payable	2,811	2,149
Current portion of operating lease liabilities	843	423
Accrued expenses and other current liabilities	7,361	4,612
Total current liabilities	11,015	7,184
Operating lease liabilities, net of current portion	453	476
Other liabilities	16	62
Total liabilities	11,484	7,722
STOCKHOLDERS' EQUITY:
Common stock, par value of $0.0001 per share; 200,000,000 shares authorized as of September 30, 2021 and December 31, 2020, respectively; 23,396,793 and 23,192,866 shares issued and outstanding as of September 30, 2021 and December 31, 2020, respectively	2	2
Additional paid-in capital	335,291	326,730
Accumulated deficit	(116,815)	(65,011)
Total stockholders' equity	218,478	261,721
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	$ 229,962	$ 269,443

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2021	Dec. 31, 2020
Statement of Financial Position [Abstract]
Common stock, par value (in USD per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized (in shares)	200,000,000	200,000,000
Common stock, shares issued (in shares)	23,396,793	23,192,866
Common stock, shares outstanding (in shares)	23,396,793	23,192,866

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
REVENUE:
Total revenue	$ 0	$ 0	$ 100	$ 0
OPERATING EXPENSES:
Research and development	(14,832)	(8,395)	(36,310)	(24,186)
General and administrative	(5,365)	(3,553)	(15,297)	(9,180)
Total operating expenses	(20,197)	(11,948)	(51,607)	(33,366)
LOSS FROM OPERATIONS	(20,197)	(11,948)	(51,507)	(33,366)
OTHER INCOME (EXPENSE), NET
Interest expense, net	(1)	(2)	(3)	(5)
Change in fair value of preferred stock tranche obligation	0	0	0	(1,490)
Other income (expense), net	(137)	(86)	(282)	4
Total other expense, net	(138)	(88)	(285)	(1,491)
Loss before income taxes	(20,335)	(12,036)	(51,792)	(34,857)
Income tax (provision) benefit	38	0	(12)	172
Net loss	(20,297)	(12,036)	(51,804)	(34,685)
Net loss attributable to common stockholders—basic	(20,297)	(12,036)	(51,804)	(35,697)
Net loss attributable to common stockholders— diluted	$ (20,297)	$ (12,036)	$ (51,804)	$ (35,697)
Net loss per share attributable to common stockholders - basic (in USD per share)	$ (0.87)	$ (0.60)	$ (2.22)	$ (2.65)
Net loss per share attributable to common stockholders - diluted (in USD per share)	$ (0.87)	$ (0.60)	$ (2.22)	$ (2.65)
Weighted-average common stock outstanding - basic (in shares)	23,362,237	20,175,883	23,299,720	13,452,606
Weighted-average common stock outstanding - diluted (in shares)	23,362,237	20,175,883	23,299,720	13,452,606
Revenue, Product and Service [Extensible Enumeration]	License	License	License	License

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Convertible Preferred Stock and Stockholders’ Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock	ADDITIONAL PAID-IN CAPITAL	ACCUMULATED DEFICIT	Series A preferred stock	Series A-1 preferred stock	Series B-1 preferred stock	Series C preferred stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Ending balance (in shares)		2,429,705
Ending balance	$ (19,446)	$ 1	$ 203	$ (19,650)
Beginning balance (in shares) at Dec. 31, 2019					4,607,652	368,612	1,579,043	0
Beginning balance at Dec. 31, 2019					$ 9,891	$ 944	$ 9,106	$ 0
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of series C convertible preferred stock, net of issuance costs of $219 (in shares)								4,169,822
Issuance of Series C convertible preferred stock, net of issuance costs of $219	0							$ 55,781
Ending balance (in shares) at Mar. 31, 2020					4,607,652	368,612	1,579,043	4,169,822
Ending balance at Mar. 31, 2020					$ 9,891	$ 944	$ 9,106	$ 55,781
Beginning balance (in shares) at Dec. 31, 2019		2,429,705
Beginning balance at Dec. 31, 2019	(19,446)	$ 1	203	(19,650)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of common stock options (in shares)		44,686
Exercise of common stock options	13		13
Vesting of restricted stock (in shares)		17,279
Vesting of restricted stock	0
Stock-based compensation	12		12
Settlement of preferred stock tranche liability	6,446		6,446
Net loss	(11,892)			(11,892)
Beginning balance (in shares) at Dec. 31, 2019					4,607,652	368,612	1,579,043	0
Beginning balance at Dec. 31, 2019					$ 9,891	$ 944	$ 9,106	$ 0
Ending balance (in shares) at Sep. 30, 2020					0	0	0	0
Ending balance at Sep. 30, 2020					$ 0	$ 0	$ 0	$ 0
Beginning balance (in shares) at Dec. 31, 2019		2,429,705
Beginning balance at Dec. 31, 2019	(19,446)	$ 1	203	(19,650)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(34,685)
Ending balance (in shares)		2,491,670
Ending balance	(24,867)	$ 1	6,674	(31,542)
Beginning balance (in shares) at Mar. 31, 2020					4,607,652	368,612	1,579,043	4,169,822
Beginning balance at Mar. 31, 2020					$ 9,891	$ 944	$ 9,106	$ 55,781
Increase (Decrease) in Temporary Equity [Roll Forward]
Conversion of convertible preferred stock upon initial public offering (in shares)		10,725,129			(4,607,652)	(368,612)	(1,579,043)	(4,169,822)
Conversion of convertible preferred stock upon initial public offering	75,722	$ 1	75,721		$ (9,891)	$ (944)	$ (9,106)	$ (55,781)
Ending balance (in shares) at Jun. 30, 2020					0	0	0	0
Ending balance at Jun. 30, 2020					$ 0	$ 0	$ 0	$ 0
Beginning balance (in shares) at Mar. 31, 2020		2,491,670
Beginning balance at Mar. 31, 2020	(24,867)	$ 1	6,674	(31,542)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Offering expenses associated with direct offering	(8)		(8)
Initial public offering, net of underwriting discounts, commissions and offering costs (in shares)		6,900,000
Initial public offering, net of underwriting discounts, commissions and offering costs of $9,390	100,123	$ 0	100,123
Exercise of common stock options (in shares)		24,003
Exercise of common stock options	9		9
Vesting of restricted stock (in shares)		17,278
Vesting of restricted stock	0
Stock-based compensation	1,139		1,139
Net loss	(10,757)			(10,757)
Ending balance (in shares)		20,158,080
Ending balance	141,361	$ 2	183,658	(42,299)
Ending balance (in shares) at Sep. 30, 2020					0	0	0	0
Ending balance at Sep. 30, 2020					$ 0	$ 0	$ 0	$ 0
Beginning balance (in shares) at Jun. 30, 2020		20,158,080
Beginning balance at Jun. 30, 2020	141,361	$ 2	183,658	(42,299)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of common stock options (in shares)		27,650
Exercise of common stock options	9		9
Stock-based compensation	1,424		1,424
Net loss	(12,036)			(12,036)
Ending balance (in shares)		20,185,730
Ending balance	130,758	$ 2	185,091	(54,335)
Ending balance (in shares)		23,192,866
Ending balance	261,721	$ 2	326,730	(65,011)
Beginning balance (in shares) at Dec. 31, 2020					0	0	0	0
Beginning balance at Dec. 31, 2020					$ 0	$ 0	$ 0	$ 0
Ending balance (in shares) at Mar. 31, 2021					0	0	0	0
Ending balance at Mar. 31, 2021					$ 0	$ 0	$ 0	$ 0
Beginning balance (in shares) at Dec. 31, 2020		23,192,866
Beginning balance at Dec. 31, 2020	261,721	$ 2	326,730	(65,011)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of common stock options (in shares)		78,628
Exercise of common stock options	42		42
Stock-based compensation	2,494		2,494
Net loss	(15,885)			(15,885)
Beginning balance (in shares) at Dec. 31, 2020					0	0	0	0
Beginning balance at Dec. 31, 2020					$ 0	$ 0	$ 0	$ 0
Ending balance (in shares) at Sep. 30, 2021					0	0	0	0
Ending balance at Sep. 30, 2021					$ 0	$ 0	$ 0	$ 0
Beginning balance (in shares) at Dec. 31, 2020		23,192,866
Beginning balance at Dec. 31, 2020	$ 261,721	$ 2	326,730	(65,011)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of common stock options (in shares)	203,927
Net loss	$ (51,804)
Ending balance (in shares)		23,271,494
Ending balance	248,372	$ 2	329,266	(80,896)
Beginning balance (in shares) at Mar. 31, 2021					0	0	0	0
Beginning balance at Mar. 31, 2021					$ 0	$ 0	$ 0	$ 0
Ending balance (in shares) at Jun. 30, 2021					0	0	0	0
Ending balance at Jun. 30, 2021					$ 0	$ 0	$ 0	$ 0
Beginning balance (in shares) at Mar. 31, 2021		23,271,494
Beginning balance at Mar. 31, 2021	248,372	$ 2	329,266	(80,896)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of common stock options (in shares)		57,277
Exercise of common stock options	57		57
Stock-based compensation	2,849		2,849
Net loss	(15,622)			(15,622)
Ending balance (in shares)		23,328,771
Ending balance	235,656	$ 2	332,172	(96,518)
Ending balance (in shares) at Sep. 30, 2021					0	0	0	0
Ending balance at Sep. 30, 2021					$ 0	$ 0	$ 0	$ 0
Beginning balance (in shares) at Jun. 30, 2021		23,328,771
Beginning balance at Jun. 30, 2021	235,656	$ 2	332,172	(96,518)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of common stock options (in shares)		68,022
Exercise of common stock options	35		35
Stock-based compensation	3,084		3,084
Net loss	(20,297)			(20,297)
Ending balance (in shares)		23,396,793
Ending balance	$ 218,478	$ 2	$ 335,291	$ (116,815)

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Convertible Preferred Stock and Stockholders’ Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Common stock, par value (in USD per share)	$ 0.0001	$ 0.0001	$ 0.0001	$ 0.0001
Issuance costs				$ 7,728
Net loss	$ (20,297)	$ (12,036)	$ (51,804)	$ (34,685)
ACCUMULATED DEFICIT
Net loss	$ (20,297)	$ (12,036)
Series A preferred stock
Par value (in USD per share)	$ 0.0001	$ 0.0001	$ 0.0001	$ 0.0001
Series A-1 preferred stock
Par value (in USD per share)	0.0001	0.0001	0.0001	0.0001
Series B-1 preferred stock
Par value (in USD per share)	0.0001	0.0001	0.0001	0.0001
Series C preferred stock
Par value (in USD per share)	$ 0.0001	$ 0.0001	$ 0.0001	$ 0.0001

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (51,804)	$ (34,685)
Adjustments to reconcile net loss to net cash provided by operating activities:
Depreciation expense	274	203
Loss on disposal of fixed asset	20	0
Stock-based compensation expense	8,427	2,575
Change in right to use asset	(709)	0
Non-cash lease expense	322	229
Changes in fair value of preferred stock tranche obligation	0	1,490
Changes in operating assets and liabilities:
Research and development incentive receivable	0	922
Prepaid expenses and other current assets	(2,875)	(2,225)
Deferred IPO costs	0	604
Accounts payable	662	(372)
Operating lease liabilities	397	(277)
Accrued expenses and other current liabilities	2,749	2,675
Other liabilities	(46)	(42)
Net cash used in operating activities	(42,583)	(28,903)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchase of property and equipment	(865)	(234)
Net cash used in investing activities	(865)	(234)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of Series C preferred stock	0	56,000
Payment of Series C preferred stock issuance costs	0	(227)
Proceeds from issuance of common stock, from the initial public offering, net of offering costs of $7,728	0	102,672
Payment of initial public offering costs	0	(2,549)
Proceeds from exercise of stock options	134	31
Net cash provided by financing activities	134	155,927
NET (DECREASE) INCREASE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	(43,314)	126,790
Cash, cash equivalents and restricted cash at beginning of period	265,991	7,135
Cash, cash equivalents and restricted cash at end of period	222,677	133,925
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION:
Right-of-use assets obtained in exchange for operating lease obligation	709	44
Property and equipment purchases still in accounts payable	135	0
Settlement of preferred stock tranche obligation	0	6,446
Conversion of preferred stock into common stock upon closing of initial public offering	0	75,714
Reconciliation of cash, cash equivalents and restricted cash
Cash and cash equivalents	221,349	133,810
Restricted cash	1,328	115
Total cash, cash equivalents and restricted cash	$ 222,677	$ 133,925

Consolidated Statements of Cash

Consolidated Statements of Cash Flows (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2020
Statement of Cash Flows [Abstract]
Issuance costs	$ 9,390	$ 219	$ 7,728

Nature of Business and Basis of

Nature of Business and Basis of Presentation	9 Months Ended
	Sep. 30, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of Business and Basis of Presentation	NATURE OF BUSINESS AND BASIS OF PRESENTATION Keros Therapeutics, Inc. (“Keros” or the “Company”) was incorporated in 2015 as a Delaware corporation. Its principal offices are in Lexington, Massachusetts. The Company is a clinical-stage biopharmaceutical company focused on the discovery, development and commercialization of novel treatments for patients suffering from hematological and musculoskeletal disorders with high unmet medical need. The accompanying unaudited interim condensed consolidated financial statements are prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and include the accounts of the Company and its wholly owned subsidiaries, Keros Therapeutics Australia Pty Ltd (“Keros Australia”) and Keros Security Corporation, a Massachusetts securities corporation. All significant intercompany transactions and accounts have been eliminated in consolidation. Since its inception in 2015, the Company has devoted the majority of its resources on business planning, research and development of its product candidates, including by conducting clinical trials and preclinical studies, raising capital and recruiting management and technical staff to support these operations. To date, the Company has not generated any revenue from product sales as none of its product candidates have been approved for commercialization. On April 13, 2020, the Company completed an initial public offering (“IPO”) in which the Company issued and sold 6,900,000 shares of its common stock, which includes 900,000 shares issued and sold pursuant to the full exercise of the underwriters’ option to purchase additional shares, at a public offering price of $16.00 per share, for aggregate gross proceeds of $110.4 million. The Company received approximately $100.1 million in net proceeds after deducting underwriting discounts and commissions and offering costs. On November 17, 2020, the Company completed an underwritten public offering in which the Company issued and sold 2,990,000 shares of common stock at a public offering price of $50.00 per share, which included 390,000 shares of common stock issued pursuant to the exercise in full of the underwriters' option to purchase additional shares. The aggregate gross proceeds to the Company from the public offering were approximately $149.5 million. The Company received approximately $140.1 million in net proceeds after deducting underwriting discounts and commissions and offering costs. In May 2021, the Company filed a registration statement on Form S-3, which was automatically effective upon filing. Pursuant to this registration statement, the Company may issue up to $150.0 million in common stock in sales deemed to be an “at the market offering,” as defined by the Securities Act, and, so long as the Company qualifies as a “well-known seasoned issuer” as defined in Rule 405 of the Securities Act, an unspecified amount of shares of our common stock, preferred stock, debt securities and warrants. The Company’s condensed consolidated financial statements have been prepared on the basis of the Company continuing as a going concern for the next 12 months. Management believes that the Company’s existing cash and cash equivalents, will allow the Company to continue its operations for at least the next 12 months. In the absence of a significant source of recurring revenue, the continued viability of the Company is dependent on its ability to continue to raise additional capital to finance its operations. If the Company is unable to obtain additional funding, the Company may be forced to delay, reduce or eliminate some or all of its research and development programs, product portfolio expansion or commercialization efforts, which could adversely affect its business prospects, or the Company may be unable to continue operations. The accompanying unaudited interim condensed consolidated financial statements as of September 30, 2021 and for the three and nine months ended September 30, 2021 and 2020 have been prepared by the Company in conformity with generally accepted accounting principles in the United States of America (“U.S. GAAP”) and, pursuant to the rules and regulations of Article 10 of Regulation S-X of the Securities Act published by the Securities and Exchange Commission (“SEC”) for interim financial statements. Certain information and footnote disclosures normally included in financial statements prepared in accordance with U.S. GAAP have been condensed or omitted pursuant to such rules and regulations. However, the Company believes the disclosures are adequate. These unaudited interim condensed consolidated financial statements should be read in conjunction with the Company’s audited financial statements and notes thereto for the year ended December 31, 2020 included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2020, filed with the SEC on March 25, 2021 (the “Annual Report”). The unaudited interim condensed consolidated financial statements have been prepared on the same basis as the audited financial statements. In the opinion of management, the accompanying unaudited interim condensed consolidated financial

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Significant Accounting Policies The significant accounting policies and estimates used in preparation of the unaudited interim condensed consolidated financial statements are described in the Company’s audited consolidated financial statements as of and for the year ended December 31, 2020, and the notes thereto, which are included in the Annual Report. Except as detailed below, there have been no material changes to the Company’s significant accounting policies during the nine months ended September 30, 2021. Risks and Uncertainties With the global COVID-19 pandemic continuing throughout 2021, the Company has implemented business continuity plans designed to address and mitigate the impact of the COVID-19 pandemic on its employees and its business operations, including its preclinical studies and clinical trials, supply chains and third-party providers. Additionally, in response to the spread of COVID-19, the Company closed its principal executive office in March 2020, with its administrative employees continuing their work outside of the office, and limited the number of staff in any given research laboratory. The Company further requested that its employees work from home if they are able to perform their duties remotely and limited the number of on-site employees to allow for proper social distancing in its offices and laboratories. For those employees on-site, the Company has implemented stringent safety measures designed to comply with applicable federal, state and local guidelines instituted in response to the COVID-19 pandemic. In July 2021, the Company implemented a plan to reopen its principal executive office to allow employees to return on-site to the office, which is based on a phased approach that is principles-based and local in design, with a focus on continuity of preclinical studies and clinical trial activities, employee safety and optimal work environment. The Company anticipates that the COVID-19 pandemic will have an impact on the development timelines for several of its preclinical and clinical programs. The extent to which the COVID-19 pandemic impacts the Company’s business, its clinical development and regulatory efforts, its corporate development objectives and the value of and market for its common stock will depend on future developments which are highly uncertain and cannot be predicted with confidence at this time, such as the ultimate duration of the pandemic, particularly in light of variant strains of the COVID-19 virus, travel restrictions, quarantines, social distancing and business closure requirements in the United States, Australia, New Zealand and other countries and the effectiveness of actions taken globally to contain and treat the disease. The global economic slowdown, the overall disruption of global healthcare systems and the other risks and uncertainties associated with the pandemic could have a material adverse effect on the Company’s business, financial condition, results of operations and growth prospects. As of the date of issuance of these financial statements, the Company is not aware of any specific event or circumstance that would require the Company to update its estimates, assumptions and judgments or revise the carrying value of its assets or liabilities. Actual results could differ from those estimates, and any such differences may be material to the Company’s financial statements. In addition, the Company is subject to other challenges and risks specific to its business and its ability to execute on its business plan and strategy, as well as risks and uncertainties common to companies in the biopharmaceutical industry with research and development operations, including, without limitation, risks and uncertainties associated with: obtaining regulatory approval of its product candidates; delays or problems in obtaining clinical supply, loss of single source suppliers or failure to comply with manufacturing regulations; product development and the inherent uncertainty of clinical success; the challenges of protecting and enhancing its intellectual property rights; the challenges of complying with applicable regulatory requirements; and identifying, acquiring or in-licensing additional products or product candidates. In addition, to the extent the ongoing COVID-19 pandemic adversely affects the Company’s business and results of operations, it may also have the effect of heightening many of the other risks and uncertainties discussed above. Recently Adopted Accounting Pronouncements On January 1, 2021, the Company adopted Financial Accounting Standards Board Accounting Standards Update No. 2019-12, Income Taxes-Simplifying the Accounting for Income Taxes ("ASU No. 2019-12"). ASU No. 2019-12 eliminates certain exceptions related to the approach for intraperiod tax allocation, the methodology for calculating income taxes in an interim period and the recognition of deferred tax liabilities for outside basis differences. ASU No. 2019-12 also simplifies aspects of the accounting for franchise taxes, enacted changes in tax laws or rates and clarifies the accounting for transactions that result in a step-up in the tax basis of goodwill. The adoption of this standard did not have an impact on the Company’s condensed consolidated financial statements and related disclosures.

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2021
Fair Value Disclosures [Abstract]
Fair Value Measurements	FAIR VALUE MEASUREMENTS The following table presents information about the Company’s financial assets and liabilities measured at fair value on a recurring basis and indicates the level of the fair value hierarchy utilized to determine such fair values (in thousands): DESCRIPTION SEPTEMBER 30, 2021 QUOTED PRICES ACTIVE MARKETS FOR IDENTICAL ASSETS SIGNIFICANT OTHER OBSERVABLE INPUTS SIGNIFICANT OTHER OBSERVABLE INPUTS Assets Money market funds $ 221,390 $ 221,390 $ — $ — Total financial assets $ 221,390 $ 221,390 $ — $ — DESCRIPTION DECEMBER 31, 2020 QUOTED PRICES ACTIVE MARKETS FOR IDENTICAL ASSETS (LEVEL 1) SIGNIFICANT OTHER OBSERVABLE INPUTS (LEVEL 2) SIGNIFICANT OTHER OBSERVABLE INPUTS (LEVEL 3) Assets Money market funds $ 262,043 $ 262,043 $ — $ — Total financial assets $ 262,043 $ 262,043 $ — $ —

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets	9 Months Ended
	Sep. 30, 2021
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid Expenses and Other Current Assets	PREPAID EXPENSES AND OTHER CURRENT ASSETS Prepaid expenses and other current assets consisted of the following (in thousands): SEPTEMBER 30, DECEMBER 31, Prepaid service contracts $ 2,231 $ 501 Income tax credit receivable 24 172 Prepaid sales tax 92 188 Prepaid rent 66 — R&D payroll tax credit 145 44 Prepaid insurance 1,854 785 Other 313 160 Total prepaid expenses and other current assets $ 4,725 $ 1,850

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	9 Months Ended
	Sep. 30, 2021
Payables and Accruals [Abstract]
Accrued Expenses and Other Current Liabilities	ACCRUED EXPENSES AND OTHER CURRENT LIABILITIES Accrued expenses and other current liabilities consisted of the following (in thousands): SEPTEMBER 30, DECEMBER 31, Accrued external R&D costs $ 1,604 $ 169 Accrued external manufacturing costs 3,050 2,265 Accrued compensation and benefits 1,765 1,510 Accrued tax 83 185 Accrued professional fees 657 265 Other 202 218 Total accrued expenses and other current liabilities $ 7,361 $ 4,612

Common Stock

Common Stock	9 Months Ended
	Sep. 30, 2021
Equity [Abstract]
Common Stock	COMMON STOCK As of September 30, 2021, the Company’s amended and restated certificate of incorporation authorized the Company to issue 200,000,000 shares of common stock at a par value of $0.0001 per share. In conjunction with the closing of the Company's initial public offering ("IPO") in April 2020, the Company issued and sold 6,900,000 shares of its common stock, including 900,000 shares pursuant to the full exercise of the underwriters' option to purchase additional shares, at a public offering price of $16.00 per share, for aggregate net proceeds of $100.1 million after deducting underwriting discounts and commissions and offering costs. In connection with the IPO, all outstanding shares of Preferred Stock converted into 10,725,129 shares of common stock. In conjunction with the Company's November 2020 public offering closing, the Company issued and sold 2,990,000 shares of its common stock, which included 390,000 shares pursuant to the full exercise of the underwriters' option to purchase additional shares, at a public offering price of $50.00 per share, for aggregate net proceeds of $140.1 million after deducting underwriting discounts and commissions and offering costs. In May 2021, the Company filed a registration statement on Form S-3, which was automatically effective upon filing. Pursuant to this registration statement, the Company may issue up to $150.0 million in common stock in sales deemed to be an “at the market offering,” as defined by the Securities Act, and, so long as the Company qualifies as a “well-known seasoned issuer” as defined in Rule 405 of the Securities Act, an unspecified amount of shares of our common stock, preferred stock, debt securities and warrants. See Note 8, Stockholders’ Equity , for further detail. As of September 30, 2021 and December 31, 2020, the Company had an aggregate of the following options to purchase shares of common stock: SEPTEMBER 30, DECEMBER 31, Options to purchase common stock 2,724,800 2,499,603 On May 3, 2021, the Company entered into a Sales Agreement (the “ATM Sales Agreement”) with SVB Leerink LLC (“SVB Leerink”), as agent, pursuant to which the Company may offer and sell, from time to time, shares of its common stock having an aggregate offering price of up to $150.0 million (the “ATM Shares”) from time to time through SVB Leerink (the “ATM Offering”). Under the ATM Sales Agreement, SVB Leerink may sell the ATM Shares by methods deemed to be an “at the market offering” as defined in Rule 415(a)(4) promulgated under the Securities Exchange Act of 1934, as amended. The Company may sell the ATM Shares in amounts and at times to be determined by the Company from time to time subject to the terms and conditions of the ATM Sales Agreement, but it has no obligation to sell any of the ATM Shares in the ATM Offering. ATM Shares sold under the ATM Sales Agreement will be issued pursuant to a prospectus supplement filed on May 3, 2021, and related prospectus filed with the Securities and Exchange Commission, or the SEC, pursuant to our automatically effective shelf registration statement on Form S-3 (Registration No. 333-255724), filed with the SEC on May 3, 2021. The Company or SVB Leerink may suspend or terminate the offering of ATM Shares upon notice to the other party and subject to other conditions. The Company has agreed to pay SVB Leerink commissions for its services in acting as agent in the sale of the ATM Shares in the amount of up to 3.0% of gross proceeds from the sale of the ATM Shares pursuant to the ATM Sales Agreement. The Company has also agreed to provide SVB Leerink with customary indemnification and contribution rights.

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2021
Share-based Payment Arrangement [Abstract]
Stock-Based Compensation	STOCK-BASED COMPENSATION 2017 Stock Incentive Plan The Board adopted the 2017 Stock Incentive Plan (the "2017 Plan") in February 2017, and the stockholders approved the 2017 Plan in March 2017. The 2017 Plan was most recently amended in March 2020. As of September 30, 2021, there was an aggregate of 777,476 shares of common stock issuable upon the exercise of outstanding options under the 2017 Plan. Any options or awards outstanding under the 2017 Plan remain outstanding and effective. 2020 Equity Incentive Plan In April 2020, the 2020 Equity Incentive Plan (the "2020 Plan") became effective, and, as a result, no further awards will be made under the 2017 Plan. The 2020 Plan provides for the grant of stock options qualifying as incentive stock options ("ISOs"), to employees and for the grant of nonstatutory stock options ("NSOs"), restricted stock awards, restricted stock unit awards, stock appreciation rights, performance stock awards and other forms of stock compensation to employees, consultants and directors. The 2020 Plan also provides for the grant of performance cash awards to employees, consultants and directors. Any previously granted awards under the 2017 Plan will remain outstanding in accordance with their respective terms. Under the 2020 Plan, there is an annual increase on January 1 of each year from January 1, 2021 continuing through January 1, 2030, by 4.0% of the total number of shares of common stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares as may be determined by the Board. On January 1, 2021, the Company increased the number of shares available for future grant under the 2020 Plan by 927,714 shares. As of September 30, 2021, there was an aggregate of 1,947,324 shares of common stock issuable upon the exercise of outstanding options under the 2020 Plan. Additionally, there were an aggregate of 1,038,029 shares reserved for future issuance under the 2020 Plan, including shares forfeited from the 2017 Plan. Stock Options A summary of option activity during the nine months ended September 30, 2021 is as follows (in thousands except share and per share data): NUMBER OF OPTIONS WEIGHTED-AVERAGE EXERCISE PRICE WEIGHTED-AVERAGE REMAINING CONTRACTUAL TERM (IN YEARS) AGGREGATE INTRINSIC VALUE Outstanding as of December 31, 2020 2,499,603 $ 11.77 8.62 $ 147,103 Granted 605,108 61.41 Exercised (203,927) 0.66 $ 10,288 Expired (175,984) 28.67 Outstanding as of September 30, 2021 2,724,800 $ 22.53 8.21 $ 59,493 Options exercisable as of December 31, 2020 723,130 $ 0.57 7.38 $ 50,599 Options exercisable as of September 30, 2021 1,152,714 $ 9.13 7.44 $ 35,472 The weighted-average grant date fair value price per share of options granted during the nine months ended September 30, 2021 and 2020 was $42.92 and $14.78, respectively. As of September 30, 2021, there was $32.9 million of unrecognized stock-based compensation expense related to unvested stock options. The unrecognized stock-based compensation expense is estimated to be recognized over a period of 2.68 years. The total fair value of options vested during the nine months ended September 30, 2021 was $8.1 million. Stock-Based Compensation Expense Total stock-based compensation expense recorded for employees, directors and non-employees during the three and nine months ended September 30, 2021 and 2020 was as follows (in thousands): THREE MONTHS ENDED SEPTEMBER 30, NINE MONTHS ENDED SEPTEMBER 30, 2021 2020 2021 2020 Research and development $ 1,018 $ 465 $ 3,032 $ 786 General and administrative 2,066 959 5,395 1,789 Total stock-based compensation expense $ 3,084 $ 1,424 $ 8,427 $ 2,575

Stockholders' Equity

Stockholders' Equity	9 Months Ended
	Sep. 30, 2021
Equity [Abstract]
Stockholders' Equity	COMMON STOCK As of September 30, 2021, the Company’s amended and restated certificate of incorporation authorized the Company to issue 200,000,000 shares of common stock at a par value of $0.0001 per share. In conjunction with the closing of the Company's initial public offering ("IPO") in April 2020, the Company issued and sold 6,900,000 shares of its common stock, including 900,000 shares pursuant to the full exercise of the underwriters' option to purchase additional shares, at a public offering price of $16.00 per share, for aggregate net proceeds of $100.1 million after deducting underwriting discounts and commissions and offering costs. In connection with the IPO, all outstanding shares of Preferred Stock converted into 10,725,129 shares of common stock. In conjunction with the Company's November 2020 public offering closing, the Company issued and sold 2,990,000 shares of its common stock, which included 390,000 shares pursuant to the full exercise of the underwriters' option to purchase additional shares, at a public offering price of $50.00 per share, for aggregate net proceeds of $140.1 million after deducting underwriting discounts and commissions and offering costs. In May 2021, the Company filed a registration statement on Form S-3, which was automatically effective upon filing. Pursuant to this registration statement, the Company may issue up to $150.0 million in common stock in sales deemed to be an “at the market offering,” as defined by the Securities Act, and, so long as the Company qualifies as a “well-known seasoned issuer” as defined in Rule 405 of the Securities Act, an unspecified amount of shares of our common stock, preferred stock, debt securities and warrants. See Note 8, Stockholders’ Equity , for further detail. As of September 30, 2021 and December 31, 2020, the Company had an aggregate of the following options to purchase shares of common stock: SEPTEMBER 30, DECEMBER 31, Options to purchase common stock 2,724,800 2,499,603 On May 3, 2021, the Company entered into a Sales Agreement (the “ATM Sales Agreement”) with SVB Leerink LLC (“SVB Leerink”), as agent, pursuant to which the Company may offer and sell, from time to time, shares of its common stock having an aggregate offering price of up to $150.0 million (the “ATM Shares”) from time to time through SVB Leerink (the “ATM Offering”). Under the ATM Sales Agreement, SVB Leerink may sell the ATM Shares by methods deemed to be an “at the market offering” as defined in Rule 415(a)(4) promulgated under the Securities Exchange Act of 1934, as amended. The Company may sell the ATM Shares in amounts and at times to be determined by the Company from time to time subject to the terms and conditions of the ATM Sales Agreement, but it has no obligation to sell any of the ATM Shares in the ATM Offering. ATM Shares sold under the ATM Sales Agreement will be issued pursuant to a prospectus supplement filed on May 3, 2021, and related prospectus filed with the Securities and Exchange Commission, or the SEC, pursuant to our automatically effective shelf registration statement on Form S-3 (Registration No. 333-255724), filed with the SEC on May 3, 2021. The Company or SVB Leerink may suspend or terminate the offering of ATM Shares upon notice to the other party and subject to other conditions. The Company has agreed to pay SVB Leerink commissions for its services in acting as agent in the sale of the ATM Shares in the amount of up to 3.0% of gross proceeds from the sale of the ATM Shares pursuant to the ATM Sales Agreement. The Company has also agreed to provide SVB Leerink with customary indemnification and contribution rights.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2021
Income Tax Disclosure [Abstract]
Income Taxes	INCOME TAXESOn March 27, 2020, the Coronavirus Aid, Relief, and Economic Security Act (“CARES Act”) was signed into law in the United States. The CARES Act provides numerous tax provisions and other stimulus measures, including temporary changes regarding the prior and future utilization of net operating losses and technical corrections from prior tax legislation for tax depreciation of certain qualified improvement property. The Company evaluated the provisions of the CARES Act and as a result, received approximately $0.2 million in February 2021 related to the carryback of our 2019 net operating loss to claim a refund for prior federal tax liabilities.

Loss Per Share

Loss Per Share	9 Months Ended
	Sep. 30, 2021
Earnings Per Share [Abstract]
Loss Per Share	LOSS PER SHARE Basic and diluted loss per share is computed by dividing net loss attributable to common stockholders by the weighted-average common shares outstanding (in thousands, except share and per share data): THREE MONTHS ENDED SEPTEMBER 30, NINE MONTHS ENDED SEPTEMBER 30, 2021 2020 2021 2020 Numerator: Net loss $ (20,297) $ (12,036) $ (51,804) $ (34,685) Less: Accruals of dividends of preferred stock — 0 — (1,012) Net loss attributable to common stockholders - basic and diluted $ (20,297) $ (12,036) $ (51,804) $ (35,697) Denominator: Weighted-average common stock outstanding - basic and diluted 23,362,237 20,175,883 23,299,720 13,452,606 Net loss per share attributable to common stockholders - basic and diluted $ (0.87) $ (0.60) $ (2.22) $ (2.65) The Company’s potentially dilutive securities, which includes stock options, have been excluded from the computation of diluted net loss per share as the effect would be to reduce the net loss per share. Therefore, the weighted-average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same. The Company excluded the following from the computation of diluted net loss per share attributable to common stockholders at September 30, 2021 and 2020 because including them would have had an anti-dilutive effect: SEPTEMBER 30, SEPTEMBER 30, Options to purchase common stock 2,724,800 2,484,152

Revenue from Contracts with Cus

Revenue from Contracts with Customers	9 Months Ended
	Sep. 30, 2021
Revenue from Contract with Customer [Abstract]
Revenue from Contracts with Customers	REVENUE FROM CONTRACTS WITH CUSTOMERS Neurona Therapeutics, Inc. License Agreement On June 22, 2021, the Company entered into a license agreement (the "Neurona Agreement") with Neurona Therapeutics, Inc. (“Neurona”). Under the Neurona Agreement, the Company granted Neurona a non-exclusive license to use LDN-193189, an early-stage research compound, which the Company licenses from a third party, solely as a reagent in connection with the manufacturing of diagnostic and/or therapeutic products to make, have made, use, import, offer for sale and sell products and services arising therefrom, and to make, have made, acquire, transfer, import and export the compound for such use. The license excludes Neurona from any rights to use, sell or distribute the compound for any therapeutic or diagnostic purpose. Unless terminated by either party for breach of contract or insolvency, the Neurona Agreement, which commenced on the execution date, will continue in perpetuity until the last patent expires. Under the Neurona Agreement, the Company was paid a one-time, upfront license payment of $0.1 million from Neurona as of September 30, 2021.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2021
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	COMMITMENTS AND CONTINGENCIES Leases Operating Leases In March 2017, the Company entered into a lease agreement (the “Lexington Lease”) for its current headquarters located in Lexington, Massachusetts. In July and August 2019, the Company entered into a first and second amendment to its Lexington Lease, respectively, to expand the rental space to 10,417 square feet. In August 2021, the Company entered into a third amendment to its Lexington Lease (the "Third Amendment") to extend the lease term through March 31, 2023 and to further expand the rental space to 15,622 square feet. The lease modification from the Third Amendment resulted in a non-cash increase to the Company's operating lease liabilities and right-of-use assets of $0.7 million in the quarter ended September 30, 2021. As required under the terms of the Lexington Lease, as amended to date, as collateral for the Lexington Lease, as amended to date, the Company has restricted cash of $0.1 million in the form of a certificate of deposit as of September 30, 2021 and 2020. The Lexington Lease, as amended to date, provides for scheduled annual rent increases throughout the lease term and does not include termination or purchase options. From time to time, leases may include options to renew the lease after the expiration of the initial lease term. A renewal period is included in the lease term only when it is reasonably certain that the Company will exercise such renewal options based on economic factors present. As of September 30, 2021, no renewal options existed that the Company believed were reasonably certain of being exercised. The following table contains a summary of the lease costs recognized (in thousands): THREE MONTHS ENDED SEPTEMBER 30, NINE MONTHS ENDED SEPTEMBER 30, 2021 2020 2021 2020 Lease cost Operating lease cost $ 120 $ 116 $ 360 $ 349 Variable payments — — — — Total lease cost $ 120 $ 116 $ 360 $ 349 The following table summarizes other information related to the Company’s operating leases (in thousands): NINE MONTHS ENDED SEPTEMBER 30, 2021 2020 Other information Operating lease payments $ 360 $ 349 Remaining lease term 1.5 years 2.2 years Discount rate 7.27 % 8.02 % Remaining maturities of the Company’s operating leases, excluding short-term leases, included in operating lease liabilities in the Company's condensed consolidated balance sheets as of September 30, 2021 are as follows (in thousands): 2021 $ 226 2022 913 2023 233 Total lease payments 1,372 Less: imputed interest (76) Total operating lease liabilities $ 1,296 Included in the consolidated balance sheet: Current portion of lease liabilities $ 843 Lease liabilities 453 Total operating lease liabilities $ 1,296 On September 7, 2021, the Company entered into an indenture of lease (the “1050 Waltham Lease”) with Revolution Labs Owner, LLC (the “Landlord”), pursuant to which the Company will lease approximately 35,662 square feet of office and laboratory space located at 1050 Waltham Street, Lexington, Massachusetts for its new principal executive office. The 1050 Waltham Lease has not yet commenced as of September 30, 2021 and is therefore not included in the maturity table above. The term of the 1050 Waltham Lease is currently expected to commence in the fourth quarter of 2022, on the date that the Landlord delivers the premises to the Company, which shall be after the substantial completion of agreed upon improvements to be performed by the Landlord (such date, the "Commencement Date"). This work is not expected to begin until the second quarter of 2022. The Company’s obligation for the payment of base rent for the premises begins four months after the Commencement Date (the “Rent Commencement Date”) and will initially be fixed at $0.2 million per month, which will increase by approximately 3% per annum. The Company will be obligated to reimburse the Landlord for certain variable costs, including its proportional share (approximately 20% of the rentable area of the building) of taxes and operating expenses, as specified in the 1050 Waltham Lease. The 1050 Waltham Lease has a term of eight years and four months, measured from the Commencement Date. The Company has the option to extend the term of the 1050 Waltham Lease for a period of an additional five years. In connection with its entry into the 1050 Waltham Lease and as a security deposit, the Company has provided the Landlord a letter of credit in the amount of approximately $1.2 million. The Landlord has the right to terminate the 1050 Waltham Lease upon customary events of default. The Company has the right to terminate the 1050 Waltham Lease if (i) the Landlord work on the premises has not commenced on or prior to July 1, 2022 or (ii) the premises are not ready for occupancy within a specified time period after October 21, 2022.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Basis of Accounting	The accompanying unaudited interim condensed consolidated financial statements are prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and include the accounts of the Company and its wholly owned subsidiaries, Keros Therapeutics Australia Pty Ltd (“Keros Australia”) and Keros Security Corporation, a Massachusetts securities corporation. All significant intercompany transactions and accounts have been eliminated in consolidation.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements On January 1, 2021, the Company adopted Financial Accounting Standards Board Accounting Standards Update No. 2019-12, Income Taxes-Simplifying the Accounting for Income Taxes ("ASU No. 2019-12"). ASU No. 2019-12 eliminates certain exceptions related to the approach for intraperiod tax allocation, the methodology for calculating income taxes in an interim period and the recognition of deferred tax liabilities for outside basis differences. ASU No. 2019-12 also simplifies aspects of the accounting for franchise taxes, enacted changes in tax laws or rates and clarifies the accounting for transactions that result in a step-up in the tax basis of goodwill. The adoption of this standard did not have an impact on the Company’s condensed consolidated financial statements and related disclosures.

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2021
Fair Value Disclosures [Abstract]
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis	The following table presents information about the Company’s financial assets and liabilities measured at fair value on a recurring basis and indicates the level of the fair value hierarchy utilized to determine such fair values (in thousands): DESCRIPTION SEPTEMBER 30, 2021 QUOTED PRICES ACTIVE MARKETS FOR IDENTICAL ASSETS SIGNIFICANT OTHER OBSERVABLE INPUTS SIGNIFICANT OTHER OBSERVABLE INPUTS Assets Money market funds $ 221,390 $ 221,390 $ — $ — Total financial assets $ 221,390 $ 221,390 $ — $ — DESCRIPTION DECEMBER 31, 2020 QUOTED PRICES ACTIVE MARKETS FOR IDENTICAL ASSETS (LEVEL 1) SIGNIFICANT OTHER OBSERVABLE INPUTS (LEVEL 2) SIGNIFICANT OTHER OBSERVABLE INPUTS (LEVEL 3) Assets Money market funds $ 262,043 $ 262,043 $ — $ — Total financial assets $ 262,043 $ 262,043 $ — $ —

Prepaid Expenses and Other Cu_2

Prepaid Expenses and Other Current Assets (Tables)	9 Months Ended
	Sep. 30, 2021
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Summary of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets consisted of the following (in thousands): SEPTEMBER 30, DECEMBER 31, Prepaid service contracts $ 2,231 $ 501 Income tax credit receivable 24 172 Prepaid sales tax 92 188 Prepaid rent 66 — R&D payroll tax credit 145 44 Prepaid insurance 1,854 785 Other 313 160 Total prepaid expenses and other current assets $ 4,725 $ 1,850

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	9 Months Ended
	Sep. 30, 2021
Payables and Accruals [Abstract]
Summary of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of the following (in thousands): SEPTEMBER 30, DECEMBER 31, Accrued external R&D costs $ 1,604 $ 169 Accrued external manufacturing costs 3,050 2,265 Accrued compensation and benefits 1,765 1,510 Accrued tax 83 185 Accrued professional fees 657 265 Other 202 218 Total accrued expenses and other current liabilities $ 7,361 $ 4,612

Common Stock (Tables)

Common Stock (Tables)	9 Months Ended
	Sep. 30, 2021
Equity [Abstract]
Summary of Preferred Stock	As of September 30, 2021 and December 31, 2020, the Company had an aggregate of the following options to purchase shares of common stock: SEPTEMBER 30, DECEMBER 31, Options to purchase common stock 2,724,800 2,499,603

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2021
Share-based Payment Arrangement [Abstract]
Summary of Option Activity	A summary of option activity during the nine months ended September 30, 2021 is as follows (in thousands except share and per share data): NUMBER OF OPTIONS WEIGHTED-AVERAGE EXERCISE PRICE WEIGHTED-AVERAGE REMAINING CONTRACTUAL TERM (IN YEARS) AGGREGATE INTRINSIC VALUE Outstanding as of December 31, 2020 2,499,603 $ 11.77 8.62 $ 147,103 Granted 605,108 61.41 Exercised (203,927) 0.66 $ 10,288 Expired (175,984) 28.67 Outstanding as of September 30, 2021 2,724,800 $ 22.53 8.21 $ 59,493 Options exercisable as of December 31, 2020 723,130 $ 0.57 7.38 $ 50,599 Options exercisable as of September 30, 2021 1,152,714 $ 9.13 7.44 $ 35,472
Schedule of Stock-based Compensation Expense	Total stock-based compensation expense recorded for employees, directors and non-employees during the three and nine months ended September 30, 2021 and 2020 was as follows (in thousands): THREE MONTHS ENDED SEPTEMBER 30, NINE MONTHS ENDED SEPTEMBER 30, 2021 2020 2021 2020 Research and development $ 1,018 $ 465 $ 3,032 $ 786 General and administrative 2,066 959 5,395 1,789 Total stock-based compensation expense $ 3,084 $ 1,424 $ 8,427 $ 2,575

Loss Per Share (Tables)

Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2021
Earnings Per Share [Abstract]
Basic and Diluted Loss Per Share Calculation	Basic and diluted loss per share is computed by dividing net loss attributable to common stockholders by the weighted-average common shares outstanding (in thousands, except share and per share data): THREE MONTHS ENDED SEPTEMBER 30, NINE MONTHS ENDED SEPTEMBER 30, 2021 2020 2021 2020 Numerator: Net loss $ (20,297) $ (12,036) $ (51,804) $ (34,685) Less: Accruals of dividends of preferred stock — 0 — (1,012) Net loss attributable to common stockholders - basic and diluted $ (20,297) $ (12,036) $ (51,804) $ (35,697) Denominator: Weighted-average common stock outstanding - basic and diluted 23,362,237 20,175,883 23,299,720 13,452,606 Net loss per share attributable to common stockholders - basic and diluted $ (0.87) $ (0.60) $ (2.22) $ (2.65)
Schedule of Antidilutive Securities Excluded From Computation of Diluted Net Loss Per Share	The Company excluded the following from the computation of diluted net loss per share attributable to common stockholders at September 30, 2021 and 2020 because including them would have had an anti-dilutive effect: SEPTEMBER 30, SEPTEMBER 30, Options to purchase common stock 2,724,800 2,484,152

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2021
Commitments and Contingencies Disclosure [Abstract]
Summary of Lease Costs	The following table contains a summary of the lease costs recognized (in thousands): THREE MONTHS ENDED SEPTEMBER 30, NINE MONTHS ENDED SEPTEMBER 30, 2021 2020 2021 2020 Lease cost Operating lease cost $ 120 $ 116 $ 360 $ 349 Variable payments — — — — Total lease cost $ 120 $ 116 $ 360 $ 349 The following table summarizes other information related to the Company’s operating leases (in thousands): NINE MONTHS ENDED SEPTEMBER 30, 2021 2020 Other information Operating lease payments $ 360 $ 349 Remaining lease term 1.5 years 2.2 years Discount rate 7.27 % 8.02 %
Schedule of Maturity of Lease Liability	Remaining maturities of the Company’s operating leases, excluding short-term leases, included in operating lease liabilities in the Company's condensed consolidated balance sheets as of September 30, 2021 are as follows (in thousands): 2021 $ 226 2022 913 2023 233 Total lease payments 1,372 Less: imputed interest (76) Total operating lease liabilities $ 1,296 Included in the consolidated balance sheet: Current portion of lease liabilities $ 843 Lease liabilities 453 Total operating lease liabilities $ 1,296

Nature of Business and Basis _2

Nature of Business and Basis of Presentation (Details) - USD ($)	May 03, 2021	Nov. 17, 2020	Apr. 13, 2020	May 31, 2021	Sep. 30, 2021	Sep. 30, 2020
Class of Stock [Line Items]
Issuance of initial public offering		$ 149,500,000	$ 110,400,000		$ 0	$ 102,672,000
Proceeds from initial public offering, net of discounts and offering costs		$ 140,100,000	$ 100,100,000
IPO
Class of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)			6,900,000
Sale of stock, price per share (in USD per share)			$ 16
Over-Allotment Option
Class of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)		390,000	900,000
Public Offering
Class of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)		2,990,000
Sale of stock, price per share (in USD per share)		$ 50
At the Market Offering
Class of Stock [Line Items]
Stock sales agreement, maximum amount authorized to be issued	$ 150,000,000			$ 150,000,000

Fair Value Measurements - Asset

Fair Value Measurements - Assets and Liabilities Measured at Fair Value (Details) - Fair Value, Recurring - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Assets
Money market funds	$ 221,390	$ 262,043
Total financial assets	221,390	262,043
QUOTED PRICES ACTIVE MARKETS FOR IDENTICAL ASSETS (LEVEL 1)
Assets
Money market funds	221,390	262,043
Total financial assets	221,390	262,043
SIGNIFICANT OTHER OBSERVABLE INPUTS (LEVEL 2)
Assets
Money market funds	0	0
Total financial assets	0	0
SIGNIFICANT OTHER OBSERVABLE INPUTS (LEVEL 3)
Assets
Money market funds	0	0
Total financial assets	$ 0	$ 0

Prepaid Expenses and Other Cu_3

Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid service contracts	$ 2,231	$ 501
Income tax credit receivable	24	172
Prepaid sales tax	92	188
Prepaid rent	66	0
R&D payroll tax credit	145	44
Prepaid insurance	1,854	785
Other	313	160
Total prepaid expenses and other current assets	$ 4,725	$ 1,850

Accrued Expenses and Other Cu_3

Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Payables and Accruals [Abstract]
Accrued external R&D costs	$ 1,604	$ 169
Accrued external manufacturing costs	3,050	2,265
Accrued compensation and benefits	1,765	1,510
Accrued tax	83	185
Accrued professional fees	657	265
Other	202	218
Total accrued expenses and other current liabilities	$ 7,361	$ 4,612

Common Stock - Narrative (Detai

Common Stock - Narrative (Details) - USD ($)	May 03, 2021	Nov. 17, 2020	Apr. 13, 2020	May 31, 2021	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Class of Stock [Line Items]
Common stock, shares authorized (in shares)					200,000,000	200,000,000
Common stock, par value (in USD per share)					$ 0.0001	$ 0.0001	$ 0.0001
Proceeds from initial public offering, net of discounts and offering costs		$ 140,100,000	$ 100,100,000
IPO
Class of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)			6,900,000
Sale of stock, price per share (in USD per share)			$ 16
Over-Allotment Option
Class of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)		390,000	900,000
Public Offering
Class of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)		2,990,000
Sale of stock, price per share (in USD per share)		$ 50
At the Market Offering
Class of Stock [Line Items]
Stock sales agreement, maximum amount authorized to be issued	$ 150,000,000			$ 150,000,000
Common Stock
Class of Stock [Line Items]
Common stock issuable upon conversion (in shares)			10,725,129

Common Stock - Summary of Commo

Common Stock - Summary of Common Stock Reserved for Future Issuance (Details) - shares	Sep. 30, 2021	Dec. 31, 2020
Options to purchase common stock
Class of Stock [Line Items]
Common stock, reserved for future issuance (in shares)	2,724,800	2,499,603

Stock-Based Compensation - Narr

Stock-Based Compensation - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	Jan. 01, 2021	Sep. 30, 2021	Sep. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)		605,108
Weighted-average grant date fair value of options granted during the period (in dollars per share)		$ 42.92	$ 14.78
Unrecognized stock-based compensation expense related to unvested stock options		$ 32.9
Total fair value of options vested during period		$ 8.1
Employee Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested award, period for recognition		2 years 8 months 4 days
2017 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (in shares)		777,476
2020 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual percentage increase in shares authorized		4.00%
Number of additional shares authorized (in shares)	927,714
Granted (in shares)		1,947,324
Number of shares available for future grants (in shares)		1,038,029

Stock-Based Compensation - Stoc

Stock-Based Compensation - Stock Option Activity (Details) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021USD ($)$ / sharesshares	Dec. 31, 2020USD ($)$ / sharesshares
NUMBER OF OPTIONS
Outstanding as of beginning of period (in shares) | shares	2,499,603
Granted (in shares) | shares	605,108
Exercised (in shares) | shares	(203,927)
Expired (in shares) | shares	(175,984)
Outstanding as of end of period (in shares) | shares	2,724,800	2,499,603
Exercisable (in shares) | shares	1,152,714	723,130
WEIGHTED-AVERAGE EXERCISE PRICE
Outstanding as of beginning of period (in USD per share) | $ / shares	$ 11.77
Granted (in USD per share) | $ / shares	61.41
Exercised (in USD per share) | $ / shares	0.66
Expired (in USD per share) | $ / shares	28.67
Outstanding as of end of period (in USD per share) | $ / shares	22.53	$ 11.77
Exercisable (in USD per share) | $ / shares	$ 9.13	$ 0.57
WEIGHTED-AVERAGE REMAINING CONTRACTUAL TERM (IN YEARS)
Outstanding	8 years 2 months 15 days	8 years 7 months 13 days
Exercisable	7 years 5 months 8 days	7 years 4 months 17 days
AGGREGATE INTRINSIC VALUE
Outstanding | $	$ 59,493	$ 147,103
Exercised | $	10,288
Exercisable | $	$ 35,472	$ 50,599

Stock-Based Compensation - St_2

Stock-Based Compensation - Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 3,084	$ 1,424	$ 8,427	$ 2,575
Research and development
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	1,018	465	3,032	786
General and administrative
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 2,066	$ 959	$ 5,395	$ 1,789

Stockholders' Equity (Details)

Stockholders' Equity (Details) - At the Market Offering - USD ($)	May 03, 2021	May 31, 2021
Class of Stock [Line Items]
Stock sales agreement, maximum amount authorized to be issued	$ 150,000,000	$ 150,000,000
Commission payment, percentage of gross on sale of ATM shares	3.00%

Income Taxes (Details)

Income Taxes (Details) $ in Millions	1 Months Ended
	Feb. 28, 2021USD ($)
Income Tax Disclosure [Abstract]
Proceeds from income tax refunds	$ 0.2

Loss Per Share - Basic and Dilu

Loss Per Share - Basic and Diluted Loss Per Share Calculation (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Numerator:
Net loss	$ (20,297)	$ (15,622)	$ (15,885)	$ (12,036)	$ (10,757)	$ (11,892)	$ (51,804)	$ (34,685)
Less: Accruals of dividends of preferred stock	0			0			0	(1,012)
Net loss attributable to common stockholders—basic	(20,297)			(12,036)			(51,804)	(35,697)
Net loss attributable to common stockholders— diluted	$ (20,297)			$ (12,036)			$ (51,804)	$ (35,697)
Denominator:
Weighted-average common stock outstanding - basic (in shares)	23,362,237			20,175,883			23,299,720	13,452,606
Weighted-average common stock outstanding - diluted (in shares)	23,362,237			20,175,883			23,299,720	13,452,606
Net loss per share attributable to common stockholders - basic (in USD per share)	$ (0.87)			$ (0.60)			$ (2.22)	$ (2.65)
Net loss per share attributable to common stockholders - diluted (in USD per share)	$ (0.87)			$ (0.60)			$ (2.22)	$ (2.65)

Loss Per Share - Antidilutive S

Loss Per Share - Antidilutive Securities (Details) - shares	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	2,724,800	2,484,152

Revenue from Contracts with C_2

Revenue from Contracts with Customers (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Total revenue	$ 0	$ 0	$ 100	$ 0
Neurona Therapeutics, Inc.
Disaggregation of Revenue [Line Items]
Total revenue			100
License | Neurona Therapeutics, Inc.
Disaggregation of Revenue [Line Items]
Total revenue			$ 100

Commitments and Contingencies -

Commitments and Contingencies - Narrative (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2021USD ($)vote	Sep. 07, 2021ft²	Aug. 31, 2021ft²	Dec. 31, 2020USD ($)	Sep. 30, 2020USD ($)	Mar. 31, 2017ft²
Lessee, Lease, Description [Line Items]
Operating lease, liability	$ 1,296
Operating lease right-of-use assets	1,265			$ 878
Restricted cash	$ 1,328			$ 115
Number of renewal options | vote	0
Lexington Lease
Lessee, Lease, Description [Line Items]
Area of real estate property | ft²			15,622			10,417
Operating lease, liability	$ 700
Operating lease right-of-use assets	700
Restricted cash	$ 100				$ 100
Waltham Lease
Lessee, Lease, Description [Line Items]
Area of real estate property | ft²		35,662
Lessee, operating lease, period after commencement date for which base rent is due	4 months
Lessee, operating lease, monthly base rent	$ 200
Percentage of annual increase in operating lease	3.00%
Proportional share of taxes and operating expenses	20.00%
Lessee, operating lease, term of contract	8 years 4 months
Renewal term	5 years
Letters of credit outstanding, amount	$ 1,200

Commitments and Contingencies_2

Commitments and Contingencies - Summary of Lease Costs (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Lease cost
Operating lease cost	$ 120	$ 116	$ 360	$ 349
Variable payments	0	0	0	0
Total lease cost	$ 120	$ 116	360	349
Other information
Operating lease liabilities			$ 360	$ 349
Remaining lease term	1 year 6 months	2 years 2 months 12 days	1 year 6 months	2 years 2 months 12 days
Discount rate	7.27%	8.02%	7.27%	8.02%

Commitments and Contingencies_3

Commitments and Contingencies - Maturity of Lease Liability (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
2021	$ 226
2022	913
2023	233
Total lease payments	1,372
Less: imputed interest	(76)
Total operating lease liabilities	1,296
Included in the consolidated balance sheet:
Current portion of lease liabilities	843	$ 423
Lease liabilities	453	$ 476
Total operating lease liabilities	$ 1,296