0001664710-24-000017 | 10-K | XBRL Viewer

ACCRUED EXPENSES AND OTHER CURRENT LIABILITIES (Tables)

12 Months Ended

Dec. 31, 2023

Schedule of Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities as of December 31, 2023 and 2022 consisted of the following (in thousands):



	DECEMBER 31,
	2023		2022
Accrued external R&D costs	$	3,904	$	2,429
Accrued external manufacturing costs	5,288		4,240
Accrued compensation and benefits	7,542		4,811

Accrued legal and consultants	519		523
Other	665		750
Total accrued expenses and other current liabilities	$	17,918	$	12,753

FAIR VALUE MEASUREMENTS

12 Months Ended

Dec. 31, 2023

Fair Value Disclosures [Abstract]

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	DECEMBER 31, 2023		QUOTED PRICES ACTIVE MARKETS FOR IDENTICAL ASSETS (LEVEL 1)		SIGNIFICANT OTHER OBSERVABLE INPUTS (LEVEL 2)		SIGNIFICANT OTHER OBSERVABLE INPUTS (LEVEL 3)
Asset
Money market funds	$	325,898	$	325,898	$	—	$	—
Total financial assets	$	325,898	$	325,898	$	—	$	—




DESCRIPTION	DECEMBER 31, 2022		QUOTED PRICES ACTIVE MARKETS FOR IDENTICAL ASSETS (LEVEL 1)		SIGNIFICANT OTHER OBSERVABLE INPUTS (LEVEL 2)		SIGNIFICANT OTHER OBSERVABLE INPUTS (LEVEL 3)
Asset
Money market funds	$	271,734	$	271,734	$	—	$	—
Total financial assets	$	271,734	$	271,734	$	—	$	—

There have been no transfers between fair value levels during the years ended December 31, 2023 and 2022. The carrying values of prepaid expenses, other current assets, accounts payable, and accrued expenses approximate their fair values due to the short-term nature of these assets and liabilities.

- 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

Cover - USD ($) $ in Millions	12 Months Ended
Cover - USD ($) $ in Millions	Dec. 31, 2023	Feb. 21, 2024	Jun. 30, 2023
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2023
Current Fiscal Year End Date	--12-31
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	1050 Waltham Street,
Entity Address, Address Line Two	Suite 302
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 Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Document Financial Statement Error Correction [Flag]	false
Entity Shell Company	false
Entity Public Float			$ 905.3
Entity Common Stock, Shares Outstanding (in shares)		35,982,628
Documents Incorporated by Reference	DOCUMENTS INCORPORATED BY REFERENCE Portions of the registrant’s proxy statement to be filed with the Securities and Exchange Commission pursuant to Regulation 14A in connection with the registrant’s 2024 Annual Meeting of Stockholders, which will be filed subsequent to the date hereof, are incorporated by reference into Part III of this Form 10-K. Such proxy statement will be filed with the Securities and Exchange Commission not later than 120 days following the end of the registrant’s fiscal year ended December 31, 2023.
Entity Central Index Key	0001664710
Amendment Flag	false
Document Fiscal Year Period	FY
Document Fiscal Year Focus	2023

LICENSE AGREEMENTS (Details) $ in Thousands		12 Months Ended
LICENSE AGREEMENTS (Details) $ in Thousands	Apr. 05, 2016 USD ($)	Dec. 31, 2023 product	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Payment for achievement of clinical and regulatory milestones			$ 300	$ 50
MGH \| License Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of products \| product		3
Clinical and regulatory milestone payments	$ 8,600
Commercial milestone payments	$ 18,000
Royalty rate, maximum reduction	50.00%
Expiration period	10 years	10 years

ACCRUED EXPENSES AND OTHER CURRENT LIABILITIES (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Payables and Accruals [Abstract]
Accrued external R&D costs	$ 3,904	$ 2,429
Accrued external manufacturing costs	5,288	4,240
Accrued compensation and benefits	7,542	4,811
Accrued legal and consultants	519	523
Other	665	750
Total accrued expenses and other current liabilities	$ 17,918	$ 12,753

PROPERTY AND EQUIPMENT, NET

12 Months Ended

Dec. 31, 2023

Property, Plant and Equipment [Abstract]

PROPERTY AND EQUIPMENT, NET

Property and equipment, net as of December 31, 2023 and 2022 consisted of the following (in thousands):



	DECEMBER 31,
	2023		2022
Computer equipment and software	$	35	$	35
Laboratory equipment	5,279		2,652
Office furniture	540		284
Leasehold improvements	—		213
Total	5,854		3,184
Less: accumulated depreciation	(1,720)		(1,163)
Property and equipment, net	$	4,134	$	2,021

Depreciation expense was $0.8 million, $0.7 million, and $0.4 million for each of the years ended December 31, 2023, 2022, and 2021, respectively.

COMMON STOCK

12 Months Ended

Dec. 31, 2023

Equity [Abstract]

COMMON STOCK

As of December 31, 2023, 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.

On May 3, 2021, the Company entered into a Sales Agreement (the “ATM Sales Agreement”) with Leerink Partners LLC (“Leerink”), as agent, pursuant to which the Company may offer and sell, from time to time, shares of its common stock through Leerink (the “ATM Offering”). On May 3, 2021, the Company filed the Shelf Registration Statement, including a base prospectus and sales agreement prospectus, with the Securities and Exchange Commission (the “SEC”), which became effective immediately upon filing, for the issuance and sale of up to $150.0 million of shares of the Company’s common stock under the ATM Sales Agreement. On December 12, 2022, the Company filed a prospectus supplement to the Shelf Registration Statement for the issuance and sale, if any, of up to an additional $250.0 million of shares of its common stock under the ATM Sales Agreement.

For the year ended December 31, 2022, the Company raised gross proceeds of $121.4 million pursuant to the ATM Offering through the sale of 3,410,384 shares of common stock at a weighted average price of $35.60 per share. The net proceeds from the ATM Offering for the year ended December 31, 2022 were approximately $119.4 million after deducting sales agent commissions of $1.8 million and offering expenses of $0.2 million.

For the year ended December 31, 2023, the Company raised gross proceeds of $178.5 million pursuant to the ATM Offering through the sale of 4,061,606 shares of common stock at a weighted average price of $43.95 per share. The net proceeds from the ATM Offering for the year ended December 31, 2023 were approximately $175.8 million after deducting sales agent commissions of $2.7 million.

As of December 31, 2023, the Company was eligible to offer and sell, from time to time, shares of its common stock for an aggregate offering amount of up to $71.5 million available under the ATM Offering.

On January 8, 2024, the Company closed an underwritten public offering in which 4,025,000 shares of common stock were issued and sold, which included 525,000 shares of common stock issued and sold pursuant to the exercise in full of the underwriters’ option to purchase additional shares, at a public offering price of $40.00 per share. The aggregate net proceeds from the public offering were approximately $151.0 million, after deducting underwriting discounts and commissions and estimated offering expenses.

The following is a summary of the rights and privileges of the holders of common stock as of December 31, 2023:

Liquidation Preference: In the event of liquidation, dissolution or winding up, holders of common stock will be entitled to share ratably in the net assets legally available for distribution to stockholders after the payment of all debts and other liabilities and the satisfaction of any liquidation preference granted to the holders of any then-outstanding shares of preferred stock.

Dividends: Subject to preferences that may be applicable to any then-outstanding preferred stock, holders of common stock are entitled to receive ratably those dividends, if any, as may be declared from time to time by the Board out of legally available funds. As of December 31, 2023, no cash dividends have been declared or paid.

Voting Rights: Each holder of common stock is entitled to one vote for each share on all matters submitted to a vote of the stockholders, including the election of directors. Under the Company's amended and restated certificate of incorporation and amended and restated bylaws, stockholders will not have cumulative voting rights. Because of this, the holders of a majority of the shares of common stock entitled to vote in any election of directors can elect all of the directors standing for election, if they should so choose.

Rights and Preferences: Holders of common stock have no preemptive, conversion or subscription rights and there are no redemption or sinking fund provisions applicable to the common stock. The rights, preferences and privileges of the holders of common stock are subject to, and may be adversely affected by, the rights of the holders of shares of any series of preferred stock that the Company may designate in the future.

As of December 31, 2023, 2022, and 2021, the Company has reserved the following shares of common stock for the potential exercise of stock options:



	DECEMBER 31,
	2023	2022	2021


Options to purchase common stock	4,394,807	3,533,169	2,810,684
Total	4,394,807	3,533,169	2,810,684

- Definition

The entire disclosure for equity.

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+ Details

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

12 Months Ended

Dec. 31, 2023

Accounting Policies [Abstract]

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

Principles of Consolidation

The accompanying consolidated financial statements include the accounts of the Company, Keros Australia and Keros Security Corporation. All intercompany accounts and transactions have been eliminated in consolidation.

Use of Estimates

The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenue, and expenses, and the disclosure of contingent assets and liabilities as of and during the reporting period. The Company bases its estimates and assumptions on historical experience when available and on various factors that it believes to be reasonable under the circumstances. This process may result in actual results differing materially from those estimated amounts used in the preparation of the financial statements if these results differ from historical experience, or other assumptions do not turn out to be substantially accurate, even if such assumptions are reasonable when made. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, useful lives assigned to property and equipment and accrued and prepaid research and development expenses. The Company assesses estimates on an ongoing basis; however, actual results could materially differ from those estimates.

Fair Value Measurements

Certain assets and liabilities are reported on a recurring basis 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—Quoted prices in active markets for identical assets or liabilities.

Level 2—Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data.

Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques.

To the extent that the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The Company had no financial instruments measured at level 3 as of December 31, 2023.

Cash, Cash Equivalents, and Restricted Cash

Cash and cash equivalents consist of standard checking accounts and money market funds. The Company considers all highly liquid investments with an original maturity of 90 days or less at the date of purchase to be cash equivalents.

The Company had restricted cash in the form of a certificate of deposit related to its operating lease in Lexington, Massachusetts of $1.2 million and $1.3 million as of December 31, 2023 and December 31, 2022, respectively. The certificate of deposit related to the Company’s operating lease at 99 Hayden Avenue in Lexington, Massachusetts was returned in 2023 upon the lease termination.

Concentrations of Credit Risk

Financial instruments that potentially subject the Company to significant concentration of credit risk consist primarily of cash and cash equivalents. The Company may maintain deposits in financial institutions in excess of government insured limits. The Company believes that it is not exposed to significant credit risk as its deposits are held at financial institutions that management believes to be of high credit quality and the Company has not experienced any losses on these deposits. As of December 31, 2023 and 2022, the Company’s cash and cash equivalents were held with three financial institutions. The Company believes that the market risk arising from its holdings of these financial instruments is mitigated based on the fact that many of these securities are either government-backed or of high credit rating.

Property and Equipment

Property and equipment are recorded at cost. Expenditures for repairs and maintenance are expensed as incurred. When assets are retired or disposed of, the assets and related accumulated depreciation are written off from the accounts, and any resulting gain or loss is included in the determination of net loss. Depreciation is calculated using the straight-line method over the estimated useful lives of the related assets as follows:



			ESTIMATED USEFUL LIFE
Computer equipment and software			3 years
Laboratory equipment			5 years
Office furniture			5 years
Leasehold improvements			lesser of useful life or remaining lease term

Impairment of Long-Lived Assets

The Company evaluates its long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability of assets to be held and used is measured by a comparison of the carrying amount of an asset to future undiscounted net cash flows expected to be generated by the asset. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the carrying amount of the assets exceed the fair value of the assets. Assets to be disposed of are reported at the lower of the carrying amount or fair value less costs to sell. To date, no impairments have been recognized for these assets.

Leases

The Company accounts for its leases under Accounting Standards Codification (“ASC”) Topic 842, Leases (“ASC 842”). At the inception of an arrangement, the Company determines whether the arrangement is or contains a lease based on the unique facts and circumstances present in the arrangement. Leases with a term greater than 12 months are recognized on the balance sheet as right-of-use, or ROU, assets and current and non-current lease liabilities, as applicable. The Company has elected not to recognize on the balance sheet leases with terms of 12 months or less. The Company typically only includes an initial lease term in its assessment of a lease arrangement. Options to renew a lease are not included in the Company’s assessment unless there is reasonable certainty that the Company will renew.

Operating lease liabilities and their corresponding ROU assets are recorded based on the present value of future lease payments over the expected remaining lease term. Lease cost for operating leases is recognized on a straight-line basis over the lease term as an operating expense. Certain adjustments to the ROU asset may be required for items such as lease prepayments or incentives received. The interest rate implicit in lease contracts is typically not readily determinable. As a result, the Company utilizes its incremental borrowing rate, which reflects the fixed rate at which the Company could borrow on a collateralized basis the amount of the lease payments in the same currency, for a similar term, in a similar economic environment.

For all asset classes of its leases, the Company has elected to account for the lease and non-lease components together for existing classes of underlying assets.

Guarantees and Indemnifications

As permitted under Delaware law, the Company indemnifies its officers, directors, consultants and employees for certain events or occurrences that happen by reason of the relationship with, or position held at, the Company. Through December 31, 2023, the Company had not experienced any losses related to these indemnification obligations, and no claims were outstanding. The Company does not expect significant claims related to these indemnification obligations and, consequently, concluded that the fair value of these obligations is negligible, and no related liabilities have been established.

Research and Development Costs

Research and development costs are charged to expense as incurred. Research and development costs consist of expenses incurred in performing research and development activities, including salaries and benefits, materials and supplies, preclinical expenses, stock-based compensation expense, depreciation of equipment, contract services with contract research organizations that conduct research, preclinical and clinical activities on the Company’s behalf, as well as contract manufacturing organizations that manufacture drug product for use in the Company’s preclinical studies and clinical trials, facilities, and other outside expenses. Costs for external development activities are recognized based on an evaluation of the progress to completion of specific tasks using information provided to the Company by its vendors. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected in the consolidated financial statements as prepaid expense or accrued research and development expense.

Nonrefundable advance payments for goods or services to be received in the future for use in research and development activities are recorded as prepaid expenses and expensed as the related goods are delivered or the services are performed.

Research and Development Incentive

Research and development incentive income includes payments under the Research and Development Tax Incentive (the “R&D Incentive”) from the Australian government. The R&D Incentive is one of the key elements of the Australian government’s support for Australia’s innovation system and was developed to assist businesses recover some of the costs of undertaking research and development. Since July 1, 2021, the R&D Incentive has provided eligible companies that engage in research and development activities with either a refundable or non-refundable tax offset depending on a company’s aggregated revenue as follows:

•Refundable tax offset of up to 18.5% above a company’s underlying tax rate where aggregated revenue is less than AUD$20.0 million per annum, or

•Non-refundable tax offset of up to 16.5% above a company’s underlying tax rate where aggregated revenue is equal to or greater than AUD$20.0 million per annum.

The Company has assessed its research and development (“R&D”) activities and expenditures to determine which activities and expenditures in Australia are likely to be eligible under the R&D Incentive. The Company estimates the refundable or non-refundable tax offset available to it based on available information at the time. This estimate is also reviewed by the Company’s external tax advisors on an annual basis.

The Company’s estimate of the cash refund it expects to receive related to the R&D Incentive is included in other assets in the accompanying consolidated balance sheet and such amounts are recorded as research and development incentive income in the statement of operations. The Company recognizes research and development incentive income when there is reasonable assurance that the income will be received, the relevant expenditure has been incurred, and the consideration

can be reliably measured. The Company has received payments and has recorded other income from the R&D Incentive of $2.4 million and $7.1 million for the year ended December 31, 2023 and December 31, 2022, respectively, related to the R&D Incentive as defined above. The Company had no income from the R&D Incentive for the year ended December 31, 2021.

Revenue Recognition

The Company recognizes revenue in accordance with ASC Topic 606, Revenue from Contracts with Customers (“ASC 606”). The Company enters into certain agreements that are within the scope of ASC 606, under which the Company licenses, may license or grants an option to license rights to certain of the Company’s product candidates and performs research and development services in connection with such arrangements. The terms of these arrangements typically include payment of one or more of the following: non-refundable, upfront fees; reimbursement of research and development costs; development, clinical, regulatory and commercial sales milestone payments, and royalties on net sales of licensed products.

Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine the appropriate amount of revenue to be recognized for arrangements determined to be within the scope of ASC 606, the Company performs the following five steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect consideration it is entitled to in exchange for the goods or services it transfers to the customer.

The promised goods or services in the Company’s arrangements typically consist of a license, or option to license, rights to the Company’s intellectual property or research and development services. The Company provides options to additional items in such arrangements, which are accounted for as separate contracts when the customer elects to exercise such options, unless the option provides a material right to the customer. Performance obligations are promised goods or services in a contract to transfer a distinct good or service to the customer and are considered distinct when (i) the customer can benefit from the good or service on its own or together with other readily available resources and (ii) the promised good or service is separately identifiable from other promises in the contract. In assessing whether promised goods or services are distinct, the Company considers factors such as the stage of development of the underlying intellectual property, the capabilities of the customer to develop the intellectual property on its own or whether the required expertise is readily available and whether the goods or services are integral or dependent to other goods or services in the contract.

The Company estimates the transaction price based on the amount expected to be received for transferring the promised goods or services in the contract. The consideration may include fixed consideration and variable consideration. At the inception of each arrangement that includes variable consideration, the Company evaluates the amount of potential payment and the likelihood that the payments will be received. The Company utilizes either the most likely amount method or expected value method to estimate the amount expected to be received based on which method best predicts the amount expected to be received. The amount of variable consideration that is included in the transaction price may be constrained and is included in the transaction price only to the extent that it is probable that a significant reversal in the amount of the cumulative revenue recognized will not occur in a future period.

The Company’s contracts often include development and regulatory milestone payments that are assessed under the most likely amount method and constrained if it is probable that a significant revenue reversal would occur. Milestone payments that are not within the Company’s control or the licensee’s control, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. At the end of each reporting period, the Company re-evaluates the probability of achievement of such development and clinical milestones and any related constraint, and if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect revenue in the period of adjustment.

For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any royalty revenue resulting from any of the Company’s collaboration or strategic alliance arrangements.

The Company allocates the transaction price based on the estimated standalone selling price. The Company must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. The Company utilizes key assumptions to determine the stand-alone selling price, which may include other comparable transactions, pricing considered in negotiating the transaction and the estimated costs. Variable consideration is allocated specifically to one or more performance obligations in a contract when the terms of the variable consideration relate to the satisfaction of the performance obligation and the resulting amounts allocated are consistent with the amounts the Company would expect to receive for the satisfaction of each performance obligation.

The consideration allocated to each performance obligation is recognized as revenue when control is transferred for the related goods or services. For performance obligations which consist of licenses and other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition.

The Company receives payments from its customers based on billing schedules established in each contract. Upfront payments and fees are recorded as deferred revenue upon receipt or when due until the Company performs its obligations under these arrangements. Amounts are recorded as accounts receivable when the Company’s right to consideration is unconditional.

Foreign Currency Transactions

The functional currency for the Company’s wholly owned foreign subsidiary, Keros Australia, is the United States dollar. All foreign currency transaction gains and losses are recognized in the consolidated statement of operations.

Segment Information

Operating segments are identified as components of an enterprise about which separate discrete financial information is made available for evaluation by the chief operating decision maker (“CODM”) in making decisions regarding resource allocation and assessing performance. The CODM is the Company’s chief executive officer. The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company’s singular concentration is focused on the discovery and development of breakthrough therapeutics for hematological, pulmonary and cardiovascular disorders with high unmet medical need.

Stock-Based Compensation

The Company accounts for all stock-based awards granted to employees and non-employees as stock-based compensation expense at fair value. The Company’s stock-based awards include stock options. The measurement date for employee and non-employee awards is the date of grant, and stock-based compensation costs are recognized as expense over the requisite service period, which is the vesting period, on a straight-line basis. Stock-based compensation expense is classified in the accompanying consolidated statement of operations based on the function to which the related services are provided. The Company recognizes stock-based compensation expense for the portion of awards that have vested. Forfeitures are recorded as they occur.

The fair value of each stock option grant is estimated on the date of grant using the Black-Scholes option-pricing model. The Company 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. 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 on common stock and does not expect to pay any cash dividends in the foreseeable future.

Income Taxes

The Company accounts for income taxes under the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the consolidated financial statements. Under this method, the Company determines deferred tax assets and liabilities on the basis of the differences between the financial statement and tax bases of assets and liabilities by using enacted tax rates in effect for the year in which the differences are expected to reverse. The effect of a change in tax rates on deferred tax assets and liabilities is recognized in income in the period that includes the enactment date.

The Company recognizes deferred tax assets to the extent that these assets are more likely than not to be realized. In making such a determination, the Company considers all available positive and negative evidence, including future reversals of existing taxable temporary differences, projected future taxable income, tax-planning strategies and results of recent operations. If the Company determines that it would be able to realize its deferred tax assets in the future in excess of its net recorded amount, an adjustment would be made to the deferred tax asset valuation allowance, which would reduce the provision for income taxes.

The Company records uncertain tax positions in accordance with ASC 740 on the basis of a two-step process in which (1) the Company determines whether it is more likely than not that the tax positions will be sustained on the basis of the technical merits of the position and (2) for those tax positions that meet the more-likely-than-not recognition threshold, the Company recognizes the largest amount of tax benefit that is more than 50 percent likely to be realized upon ultimate settlement with the related tax authority.

The Company accounts for interest and penalties related to uncertain tax positions within the provision for income taxes. As of December 31, 2023, there have been no interest or penalties recorded.

Comprehensive Loss

Comprehensive loss is defined as the change in equity of a business enterprise during a period from transactions and other events and circumstances from non-owner sources. Comprehensive loss is equal to net loss for all periods presented.

Net Loss Per Share

Basic net loss per share is computed by dividing the net loss by the weighted-average number of common shares outstanding during the period, without consideration of potential dilutive securities. Diluted net loss per share is computed using the weighted-average number of common shares outstanding during the period, and if dilutive, the weighted-average number of potential shares of common shares.The Company’s potentially dilutive securities, which include 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 as of December 31, 2023, 2022, and 2021 because including them would have had an anti-dilutive effect:



	DECEMBER 31,
	2023	2022	2021


Options to purchase common stock	4,394,807	3,533,169	2,810,684
Total	4,394,807	3,533,169	2,810,684

Recently Adopted Accounting Pronouncements

The Company has not recently adopted any accounting pronouncements.

Recently Issued Accounting Pronouncements

In December 2023, the Financial Accounting Standards Board issued Accounting Standards Update No. 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures (“ASU No. 2023-09”). ASU No. 2023-09 requires a public business entity to disclose, on an annual basis, a tabular rate reconciliation using both percentages and currency amounts, broken out into specified categories with certain reconciling items further broken out by nature and jurisdiction to the extent those items exceed a specified threshold. In addition, all entities are required to disclose income taxes paid, net of refunds received disaggregated by federal, state/local, and foreign and by jurisdiction if the amount is at least 5% of total income tax payments, net of refunds received. The new standard is effective for annual periods beginning after December 15, 2024, with early adoption permitted. An entity may apply the amendments in this ASU prospectively by providing the revised disclosures for the period ending December 31, 2025 and continuing to provide the pre-ASU disclosures for the prior periods, or may apply the amendments retrospectively by providing the revised disclosures for all period presented. The Company is currently evaluating the impact of this ASU on its consolidated financial statements and related disclosures.

Risks and Uncertainties

There have been significant disruptions to global financial markets that have contributed to a general global economic slowdown. While recent trends towards rising inflation have eased, prices continue to rise, which may materially affect the Company's business and corresponding financial position and cash flows. Inflationary factors, such as increases in the cost of materials and supplies relating to the Company's preclinical studies and clinical trials, interest rates and overhead costs may adversely affect its operating results. Rising interest rates present a recent challenge impacting the U.S. economy and could make it more difficult for the Company to obtain traditional financing on acceptable terms, if at all, in the future. Although the Company does not believe that inflation or higher interest rates have had a material impact on its financial position or results of operations to date, the Company may experience increases in the near future (especially if inflation rates rise more quickly) on its operating costs, including its labor costs and research and development costs, due to supply chain constraints, consequences associated with public health crises and global geopolitical tensions, such as the ongoing war between Russia and Ukraine and the war in Israel, worsening global macroeconomic conditions and employee availability and wage increases, which may result in additional stress on the Company's working capital resources. 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.

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)

12 Months Ended

Dec. 31, 2023

Accounting Policies [Abstract]

Basis of Accounting

The accompanying 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.

Principles of Consolidation

Use of Estimates

Fair Value Measurements

Level 1—Quoted prices in active markets for identical assets or liabilities.

Cash, Cash Equivalents, and Restricted Cash

Concentrations of Credit Risk

Property and Equipment



			ESTIMATED USEFUL LIFE
Computer equipment and software			3 years
Laboratory equipment			5 years
Office furniture			5 years
Leasehold improvements			lesser of useful life or remaining lease term

Impairment of Long-Lived Assets

Leases

For all asset classes of its leases, the Company has elected to account for the lease and non-lease components together for existing classes of underlying assets.

Guarantees and Indemnifications

Research and Development Costs

Research and Development Incentive and Income Taxes

•Refundable tax offset of up to 18.5% above a company’s underlying tax rate where aggregated revenue is less than AUD$20.0 million per annum, or

•Non-refundable tax offset of up to 16.5% above a company’s underlying tax rate where aggregated revenue is equal to or greater than AUD$20.0 million per annum.

can be reliably measured.

Income Taxes

The Company accounts for interest and penalties related to uncertain tax positions within the provision for income taxes. As of December 31, 2023, there have been no interest or penalties recorded.

Revenue Recognition

Foreign Currency Transactions

Segment Information

Stock-Based Compensation

Comprehensive Loss

Net Loss Per Share



	DECEMBER 31,
	2023	2022	2021


Options to purchase common stock	4,394,807	3,533,169	2,810,684
Total	4,394,807	3,533,169	2,810,684

Recently Adopted Accounting Pronouncements and Recently Issued Accounting Pronouncements

The Company has not recently adopted any accounting pronouncements.

COMMITMENTS AND CONTINGENCIES - Schedule of Lease Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Commitments and Contingencies Disclosure [Abstract]
Operating lease cost	$ 2,950	$ 1,089	$ 587
Variable payments	1,317	555	295
Total lease cost	4,267	1,644	882
Lessee, Lease, Other Information [Abstract]
Cash paid for amounts included in the measurement of lease liabilities Operating cash flows from operating leases	1,122	913	587
Right-of-use assets obtained in exchange for operating lease obligation	$ 1,338	$ 15,423	$ 0
Weighted-average remaining lease term — operating leases	7 years 10 months 24 days	4 years
Weighted-average discount rate — operating leases	10.30%	10.00%

PROPERTY AND EQUIPMENT, NET - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 5,854	$ 3,184
Less: accumulated depreciation	(1,720)	(1,163)
Property and equipment, net	4,134	2,021
Computer equipment and software
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	35	35
Laboratory equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	5,279	2,652
Office furniture
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	540	284
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 0	$ 213

Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
REVENUE:
Total revenue	$ 151	$ 0	$ 20,100
OPERATING EXPENSES:
Research and development	(135,258)	(87,265)	(55,143)
General and administrative	(34,834)	(27,525)	(21,330)
Total operating expenses	(170,092)	(114,790)	(76,473)
LOSS FROM OPERATIONS	(169,941)	(114,790)	(56,373)
OTHER INCOME (EXPENSE), NET:
Interest expense, net	0	(1)	(4)
Research and development incentive income	2,400	7,081	0
Dividend income	14,755	3,644	27
Other income (expense), net	(206)	(613)	(383)
Total other income (expense), net	16,949	10,111	(360)
Loss before income taxes	(152,992)	(104,679)	(56,733)
Income tax provision	0	0	(2,011)
Net loss	(152,992)	(104,679)	(58,744)
Net loss attributable to common stockholders - basic	(152,992)	(104,679)	(58,744)
Net loss attributable to common stockholders - diluted	$ (152,992)	$ (104,679)	$ (58,744)
Net loss per share attributable to common stockholders - basic (in dollars per share)	$ (5.20)	$ (4.15)	$ (2.52)
Net loss per share attributable to common stockholders - diluted (in dollars per share)	$ (5.20)	$ (4.15)	$ (2.52)
Weighted-average common stock outstanding - basic (in shares)	29,447,119	25,241,030	23,333,914
Weighted-average common stock outstanding - diluted (in shares)	29,447,119	25,241,030	23,333,914
Service and other revenue
REVENUE:
Total revenue	$ 151	$ 0	$ 0
License revenue
REVENUE:
Total revenue	$ 0	$ 0	$ 20,100

COMMITMENTS AND CONTINGENCIES (Tables)

12 Months Ended

Dec. 31, 2023

Commitments and Contingencies Disclosure [Abstract]

Schedule of Lease Costs

The components of the lease cost as of December 31, 2023, 2022, and 2021 consisted of the following (in thousands):


	YEAR ENDED DECEMBER 31,
	2023		2022		2021
Operating lease cost	$	2,950	$	1,089	$	587

Variable payments	1,317		555		295
Total lease cost	$4,267		$1,644		$882

Other information as of December 31, 2023, 2022 and 2021 (in thousands):


	DECEMBER 31,
	2023	2022	2021
Cash paid for amounts included in the measurement of lease liabilities Operating cash flows from operating leases	$1,122	$913	$587
Right-of-use assets obtained in exchange for new operating lease liabilities	$1,338	$15,423	$—

The weighted-average remaining lease term and discount rate for the leases as of December 31, 2023 and 2022 were as follows:


	DECEMBER 31,
	2023		2022
Weighted-average remaining lease term — operating leases	7.9		4.0
Weighted-average discount rate — operating leases	10.3	%	10.0	%

Schedule of Maturity of Lease Liability

Maturities of operating lease liabilities at December 31, 2023 are as follows (in thousands):



MATURITY OF LEASE LIABILITY
2024	$	2,431
2025	2,504
2026	2,578
2027	2,657
2028	2,736
2029	2,818
2030	2,903
2031	2,734
Total lease payments	21,361
Less: imputed interest	(6,917)
Total operating lease liabilities	$	14,444
Included in the consolidated balance sheet:
Current portion of lease liabilities	$	1,005
Lease liabilities	13,439
Total operating lease liabilities	$	14,444

PROPERTY AND EQUIPMENT, NET - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 0.8	$ 0.7	$ 0.4

STOCK-BASED COMPENSATION - Schedule of Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 28,763	$ 18,682	$ 11,722
Research and development
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	14,082	8,205	4,258
General and administrative
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 14,681	$ 10,477	$ 7,464

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (152,992)	$ (104,679)	$ (58,744)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	815	674	378
Loss on disposal of fixed asset	4	12	46
Stock-based compensation expense	28,763	18,682	11,722
Non-cash lease expense	1,552	941	520
Changes in operating assets and liabilities:
Accounts receivable	(143)	18,000	(18,000)
Prepaid expenses and other current assets	(9,284)	(3,321)	(1,548)
Other assets	(478)	(1,574)	0
Accounts payable	1,614	(408)	1,485
Right-of-use assets and operating lease liabilities	384	(3,711)	(597)
Accrued expenses and other current liabilities	5,257	5,322	2,652
Other liabilities	0	0	(62)
Net cash used in operating activities	(124,508)	(70,062)	(62,148)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchase of property and equipment	(2,464)	(1,241)	(1,024)
Net cash used in investing activities	(2,464)	(1,241)	(1,024)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from exercise of stock options	3,225	818	379
Net cash provided by financing activities	178,956	120,309	28,550
NET INCREASE (DECREASE) IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	51,984	49,006	(34,622)
Cash, cash equivalents and restricted cash at beginning of year	280,375	231,369	265,991
Cash, cash equivalents and restricted cash at end of year	332,359	280,375	231,369
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION:
Property and equipment purchases still in accounts payable	599	75	11
Issuance cost still in accrued and accounts payable	0	63	75
Right-of-use assets obtained in exchange for operating lease obligation	1,338	15,423	0
Reconciliation of cash, cash equivalents and restricted cash
Cash and cash equivalents	331,147	279,048	230,042
Restricted cash	1,212	1,327	1,327
Total cash, cash equivalents and restricted cash	332,359	280,375	231,369
At The Market Offering
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock, net of offering cost	175,825	119,578	28,171
Payment of issuance costs	$ (94)	$ (87)	$ 0

COMMITMENTS AND CONTINGENCIES - Narrative (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2023 USD ($) ft²	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($) ft²	Sep. 07, 2021 USD ($) ft²
Lessee, Lease, Description [Line Items]
Operating lease right-of-use assets		$ 15,334	$ 15,548	$ 15,400
Total operating lease liabilities		$ 14,444		$ 12,900
Waltham lease for additional period		5 years
Waltham Lease
Lessee, Lease, Description [Line Items]
Area of rental space (sqft) \| ft²			31,991	35,662
Lessee, operating lease, monthly base rent		$ 200
Percentage of annual increase in operating lease		3.00%
Letters of credit outstanding, amount		$ 1,200
Operating lease, period for improvement		8 years 4 months
Phase B Premises
Lessee, Lease, Description [Line Items]
Area of rental space (sqft) \| ft²	3,671
Operating lease right-of-use assets	$ 1,900
Total operating lease liabilities	1,400
Phase A Premises
Lessee, Lease, Description [Line Items]
Right-of-use assets and operating lease liabilities	$ (500)

INCOME TAXES

12 Months Ended

Dec. 31, 2023

Income Tax Disclosure [Abstract]

INCOME TAXES

Loss before income taxes for the years ended December 31, 2023, 2022, and 2021 consisted of the following (in thousands):



	YEAR ENDED DECEMBER 31,
	2023		2022		2021
United States	$	(156,571)	$	(112,240)	$	(55,961)
Foreign	3,579		7,561		(772)
Loss before income taxes	$	(152,992)	$	(104,679)	$	(56,733)

The components of the provision for income taxes for the years ended December 31, 2023, 2022, and 2021 consisted of the following (in thousands):



	YEAR ENDED DECEMBER 31,
	2023		2022		2021
Current income tax provision:
Federal	$	—	$	—	$	—
State	—		—		11
Foreign	—		—		2,000
Total current income tax provision	$	—	$	—	$	2,011
Total deferred income tax provision	$	—	$	—	$	—
Total income tax provision	$	—	$	—	$	2,011

A reconciliation of the Company’s statutory income tax rate to the Company’s effective income tax rate for the years ended December 31, 2023, 2022, and 2021 was as follows:



	YEAR ENDED DECEMBER 31,
	2023		2022		2021
Income at U.S. statutory rate	21.0	%	21.0	%	21.0	%
State taxes	1.2		5.2		6.7
Stock compensation	(0.6)		(2.8)		(1.5)
Other permanent differences	(3.0)		(1.4)		—

Research and development credits	3.0		2.5		4.8
Impact of foreign operations	(0.1)		(0.3)		(0.6)
Foreign withholding taxes	—		—		(2.8)
Other	3.0		(3.3)		(0.3)
Tax credits	0.8		3.2		—
Change in valuation allowance	(25.3)		(24.1)		(30.9)
Effective tax rate	—	%	—	%	(3.5)	%

The net deferred income tax asset balance as of December 31, 2023 and 2022 related to the following (in thousands):



	YEAR ENDED DECEMBER 31,
	2023		2022
Net operating loss carryforwards	$	36,245	$	37,687
Research and development tax credits	14,134		10,459
Capitalized research and development expenses	36,363		10,822
Stock-based compensation	10,794		—
Operating lease liability	3,457		3,602
Accrued expenses	1,774		1,275


Intangibles	—		115
Other	$	285	$	908
Total deferred tax assets	$	103,052	$	64,868
Valuation allowance	(99,390)		(60,650)
Net deferred tax assets	$	3,662	$	4,218
Deferred tax liability
Operating lease right-of-use asset	(3,662)		(4,218)

Net deferred tax assets	$	—	$	—

As of December 31, 2023, the Company had U.S. federal and state net operating loss (“NOL”) carryforwards of $135.2 million and $124.4 million, respectively. Of the $135.2 million in federal NOLs, $0.2 million will expire in 2035 while the remaining $135.0 million can be carried forward indefinitely. The state NOL carryforwards begin to expire in 2039.

As of December 31, 2023, the Company had U.S. federal and state research and development tax credit carryforwards of $10.4 million and $4.8 million, respectively. The tax credits begin to expire in 2034.

Under the provisions of the Internal Revenue Code of 1986, as amended (the “Code”), the NOL and tax credit carryforwards are subject to review and potential adjustments by the Internal Revenue Services and state tax authorities. Under Section 382 of the Code (“Section 382”), certain substantial changes in the Company’s ownership, including the sale of the Company or significant changes in ownership due to sales of equity, may have limited, or may limit in the future, the amount of NOL carryforwards or tax credits which could be used annually to offset future taxable income. The Company completed analyses under Section 382 through December 31, 2022, and determined that on April 15, 2016 and April 13, 2020, ownership changes had occurred. Based on the Company’s analysis, the Company has determined that $0.3 million and $0.3 million of its federal and state NOL carryforwards, respectively, are limited by Section 382 and have been written off. The remaining unused carryforwards remain available for future periods. The Company may also experience ownership changes in the future as a result of subsequent shifts in the Company’s stock ownership, some of which may be outside the Company’s control. As a result, its ability to use its pre-change NOLs or tax credits to offset U.S. federal taxable income may be subject to limitations, which could potentially result in increased future tax liability. In addition, at the state level, there may be periods during which the use of NOLs is suspended or otherwise limited, which could accelerate or permanently increase state taxes owed.

Management of the Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets, which are comprised principally of NOL, research and development tax credit carryforwards, and capitalized R&D. Under the applicable accounting standards, management has considered the Company’s history of losses and concluded that it is more likely than not that the Company will not recognize the benefits of federal and state deferred tax assets. Accordingly, a full valuation allowance was maintained as of December 31, 2023, 2022 and 2021. The Company’s valuation allowance has increased by $38.7 million, $25.2 million and $17.5 million for the years ended December 31, 2023, 2022, and 2021, respectively, due primarily to the generation of additional NOLs and the associated gross deferred tax assets.

The Tax Cuts and Jobs Act resulted in significant changes to the treatment of research and development (“R&D”) expenditures under Section 174. For tax years beginning after December 31, 2021, taxpayers are required to capitalize and amortize all R&D expenditures that are paid or incurred in connection with their trade or business. Specifically, costs for U.S.-based R&D activities must be amortized over five years and costs for foreign R&D activities must be amortized over 15 years – both using a midyear convention.

The calculation of the Company's tax liabilities involves dealing with uncertainties in the application of complex tax laws and regulations for both federal taxes and in the various states in which it operates or does business in. ASC 740 states that a tax benefit from an uncertain tax position may be recognized when it is more likely than not that the position will be sustained upon examination, including resolutions of any related appeals or litigation processes, on the basis of the technical merits.

The Company records uncertain tax positions as liabilities in accordance with ASC 740 and adjusts these liabilities when its judgment changes as a result of the evaluation of new information not previously available. Because of the complexity of some of these uncertainties, the ultimate resolution may result in a payment that is materially different from the Company’s current estimate of the unrecognized tax benefit liabilities. These differences will be reflected as increases or decreases to income tax expense in the period in which new information is available. As of December 31, 2023, 2022 and 2021, the Company has not recorded any uncertain tax positions in its consolidated financial statements.

The Company files income tax returns as prescribed by the tax laws of the jurisdictions in which it operates. In the normal course of business, the Company is subject to income tax examination by federal, state and foreign jurisdictions, where applicable. There are currently no pending income tax examinations. The Company’s tax years are still open under statute for income tax examination by the Internal Revenue Service from December 31, 2020 to the present. There are currently no pending income tax examinations. To the extent the Company has tax attribute carryforwards, the tax years in which the tax attribute was generated may still be adjusted upon examination by the Internal Revenue Service, state and foreign tax authorities to the extent utilized in a future period.

- 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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480990/946-20-50-13

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(h)(2))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480678/235-10-S99-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-Name Accounting Standards Codification
-Publisher FASB
-URI https://asc.fasb.org//740/tableOfContent

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 14
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482685/740-10-50-14

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
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-Section 50
-Paragraph 21
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-URI https://asc.fasb.org//1943274/2147482685/740-10-50-21

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
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-Name Accounting Standards Codification
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-Paragraph 1
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-URI https://asc.fasb.org//1943274/2147482526/740-270-50-1

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
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-Paragraph 17
-Subparagraph (b)
-Publisher FASB
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Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
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-Section S99
-Paragraph 1
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Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
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-Name Accounting Standards Codification
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-Paragraph 2
-Subparagraph (SAB Topic 11.C)
-Publisher FASB
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Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482603/740-30-50-2

+ Details

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2023	Dec. 31, 2022
Statement of Financial Position [Abstract]
Preferred stock, par value (in USD per share)	$ 0.0001	$ 0.0001
Preferred stock, shares authorized (in shares)	10,000,000	10,000,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
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)	31,841,084	27,543,453
Common stock, shares outstanding (in shares)	31,841,084	27,543,453

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Property and Equipment (Details)	Dec. 31, 2023
Computer equipment and software
Property, Plant and Equipment [Line Items]
Estimated useful life	3 years
Laboratory equipment
Property, Plant and Equipment [Line Items]
Estimated useful life	5 years
Office furniture
Property, Plant and Equipment [Line Items]
Estimated useful life	5 years

ACCRUED EXPENSES AND OTHER CURRENT LIABILITIES

12 Months Ended

Dec. 31, 2023

Payables and Accruals [Abstract]

ACCRUED EXPENSES AND OTHER CURRENT LIABILITIES

Accrued expenses and other current liabilities as of December 31, 2023 and 2022 consisted of the following (in thousands):



	DECEMBER 31,
	2023		2022
Accrued external R&D costs	$	3,904	$	2,429
Accrued external manufacturing costs	5,288		4,240
Accrued compensation and benefits	7,542		4,811

Accrued legal and consultants	519		523
Other	665		750
Total accrued expenses and other current liabilities	$	17,918	$	12,753

STOCK-BASED COMPENSATION

12 Months Ended

Dec. 31, 2023

Share-Based Payment Arrangement [Abstract]

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 December 31, 2023, there were an aggregate of 507,973 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, 2022, the Company increased the number of shares available for future grant under the 2020 Plan by 958,993 shares. On January 1, 2023, the Company increased the number of shares available for future grant under the 2020 Plan by 1,101,738 shares. On January 1, 2024, the Company increased the number of shares available for future grant under the 2020 Plan by 1,273,643 shares. The awards granted under the 2020 Plan typically vest over a four-year period and have a 10-year contractual term.

As of December 31, 2023, there were an aggregate of 3,886,834 shares of common stock issuable upon the exercise of outstanding options under the 2020 Plan. Additionally, there were an aggregate of 976,452 shares reserved for future issuance under the 2020 Plan, including shares forfeited from the 2017 Plan.

Stock Option Valuation

The weighted-average assumptions that the Company used in the Black-Scholes option-pricing model to determine the grant-date fair value of stock options granted for the years ended December 31, 2023, 2022, and 2021 were as follows:



	YEAR ENDED DECEMBER 31,
	2023		2022		2021
Risk-free interest rate	3.96	%	2.24	%	0.88	%
Expected term (in years)	6.05		6.06		6.05
Expected volatility	81.34	%	83.03	%	83.00	%
Expected dividend yield	0.00	%	0.00	%	0.00	%

A summary of option activity during the year ended December 31, 2023 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 January 1, 2023	3,533,169	$	30.26	7.77	$	72,434
Granted	1,386,295	49.06
Exercised	(236,025)	13.66			$	6,369
Cancelled or forfeited	(258,211)	44.46
Expired	(30,421)	53.97
Outstanding as of December 31, 2023	4,394,807	$	36.08	7.31	$	46,319
Options exercisable as of December 31, 2023	2,375,136	$	26.40	6.11	$	43,205

The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the Company’s common stock.

The weighted-average grant date fair value per share of options granted during each of the years ended December 31, 2023, 2022, and 2021 was $35.24, $31.18, and $42.29, respectively. As of December 31, 2023, there was $57.8 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 2.57 years.

Stock-Based Compensation Expense

Total stock-based compensation expense recorded as research and development and general and administrative expenses, respectively, for employees, directors and non-employees during the years ended December 31, 2023, 2022, and 2021 is as follows (in thousands):


11722
	YEAR ENDED DECEMBER 31,
	2023		2022		2021
Research and development	$	14,082	$	8,205	$	4,258
General and administrative	14,681		10,477		7,464
Total stock-based compensation expense	$	28,763	$	18,682	$	11,722

STOCK-BASED COMPENSATION (Tables)

12 Months Ended

Dec. 31, 2023

Share-Based Payment Arrangement [Abstract]

Schedule of Stock Option Valuation Assumptions



	YEAR ENDED DECEMBER 31,
	2023		2022		2021
Risk-free interest rate	3.96	%	2.24	%	0.88	%
Expected term (in years)	6.05		6.06		6.05
Expected volatility	81.34	%	83.03	%	83.00	%
Expected dividend yield	0.00	%	0.00	%	0.00	%

Schedule of Option Activity

A summary of option activity during the year ended December 31, 2023 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 January 1, 2023	3,533,169	$	30.26	7.77	$	72,434
Granted	1,386,295	49.06
Exercised	(236,025)	13.66			$	6,369
Cancelled or forfeited	(258,211)	44.46
Expired	(30,421)	53.97
Outstanding as of December 31, 2023	4,394,807	$	36.08	7.31	$	46,319
Options exercisable as of December 31, 2023	2,375,136	$	26.40	6.11	$	43,205

Schedule of Stock-based Compensation Expense


11722
	YEAR ENDED DECEMBER 31,
	2023		2022		2021
Research and development	$	14,082	$	8,205	$	4,258
General and administrative	14,681		10,477		7,464
Total stock-based compensation expense	$	28,763	$	18,682	$	11,722

FAIR VALUE MEASUREMENTS (Tables)

12 Months Ended

Dec. 31, 2023

Fair Value Disclosures [Abstract]

Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis




DESCRIPTION	DECEMBER 31, 2023		QUOTED PRICES ACTIVE MARKETS FOR IDENTICAL ASSETS (LEVEL 1)		SIGNIFICANT OTHER OBSERVABLE INPUTS (LEVEL 2)		SIGNIFICANT OTHER OBSERVABLE INPUTS (LEVEL 3)
Asset
Money market funds	$	325,898	$	325,898	$	—	$	—
Total financial assets	$	325,898	$	325,898	$	—	$	—




DESCRIPTION	DECEMBER 31, 2022		QUOTED PRICES ACTIVE MARKETS FOR IDENTICAL ASSETS (LEVEL 1)		SIGNIFICANT OTHER OBSERVABLE INPUTS (LEVEL 2)		SIGNIFICANT OTHER OBSERVABLE INPUTS (LEVEL 3)
Asset
Money market funds	$	271,734	$	271,734	$	—	$	—
Total financial assets	$	271,734	$	271,734	$	—	$	—

INCOME TAXES - Schedule of Net Deferred Income Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 36,245	$ 37,687
Research and development tax credits	14,134	10,459
Capitalized research and development expenses	36,363	10,822
Stock-based compensation	10,794	0
Operating lease liability	3,457	3,602
Accrued expenses	1,774	1,275
Intangibles	0	115
Other	285	908
Total deferred tax assets	103,052	64,868
Valuation allowance	(99,390)	(60,650)
Net deferred tax assets	3,662	4,218
Deferred tax liability
Operating lease right-of-use asset	(3,662)	(4,218)
Net deferred tax assets	$ 0	$ 0

COMMON STOCK - Narrative (Details)				1 Months Ended	12 Months Ended
COMMON STOCK - Narrative (Details)	Jan. 08, 2024 USD ($) $ / shares shares	Dec. 12, 2022 shares	May 03, 2021 USD ($)	May 31, 2021 USD ($)	Dec. 31, 2023 USD ($) vote $ / shares shares	Dec. 31, 2022 USD ($) $ / shares shares	Dec. 31, 2021 USD ($)
Class of Stock [Line Items]
Common stock, shares authorized (in shares) \| shares					200,000,000	200,000,000
Common stock, par value (in USD per share) \| $ / shares					$ 0.0001	$ 0.0001
Number of votes per common share held \| vote					1
At The Market Offering
Class of Stock [Line Items]
Stock sales agreement, maximum amount authorized to be issued			$ 150,000,000	$ 150,000,000
Stock sales agreement, additional (in shares) \| shares		250,000,000
Issuance of common stock					$ 175,825,000	$ 119,578,000	$ 28,171,000
Common Stock \| Subsequent Event
Class of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares) \| shares	4,025,000
Sale of stock, price per share (in USD per share) \| $ / shares	$ 40.00
Issuance of common stock	$ 151,000,000.0
Common Stock \| At The Market Offering
Class of Stock [Line Items]
Proceeds from initial public offering, net of discounts and offering costs					$ 178,500,000	$ 121,400,000
Sale of stock, number of shares issued in transaction (in shares) \| shares					4,061,606	3,410,384
Sale of stock, price per share (in USD per share) \| $ / shares					$ 43.95	$ 35.60
Issuance of common stock					$ 175,800,000	$ 119,400,000
Sales agent commissions					2,700,000	1,800,000
Issuance costs						$ 200,000
Sale of stock, remaining authorized amount					71,500,000
Common Stock \| Over-Allotment Option \| Subsequent Event
Class of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares) \| shares	525,000
Dividend Declared \| Common Stock
Class of Stock [Line Items]
Dividends payable					$ 0

FAIR VALUE MEASUREMENTS (Details) - Fair Value, Recurring - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Asset
Money market funds	$ 325,898	$ 271,734
Total financial assets	325,898	271,734
QUOTED PRICES ACTIVE MARKETS FOR IDENTICAL ASSETS (LEVEL 1)
Asset
Money market funds	325,898	271,734
Total financial assets	325,898	271,734
SIGNIFICANT OTHER OBSERVABLE INPUTS (LEVEL 2)
Asset
Money market funds	0	0
Total financial assets	0	0
SIGNIFICANT OTHER OBSERVABLE INPUTS (LEVEL 3)
Asset
Money market funds	0	0
Total financial assets	$ 0	$ 0

INCOME TAXES - Schedule of Reconciliation of Statutory Income Tax Rate to Effective Income Tax Rate (Details)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Tax Disclosure [Abstract]
Income at U.S. statutory rate	21.00%	21.00%	21.00%
State taxes	1.20%	5.20%	6.70%
Stock compensation	(0.60%)	(2.80%)	(1.50%)
Other permanent differences	(3.00%)	(1.40%)	0.00%
Research and development credits	3.00%	2.50%	4.80%
Impact of foreign operations	(0.10%)	(0.30%)	(0.60%)
Foreign withholding taxes	0.00%	0.00%	(2.80%)
Other	3.00%	(3.30%)	(0.30%)
Tax credits	0.80%	3.20%	0.00%
Change in valuation allowance	(25.30%)	(24.10%)	(30.90%)
Effective tax rate	0.00%	0.00%	(3.50%)

PREPAID EXPENSES AND OTHER CURRENT ASSETS (Tables)

12 Months Ended

Dec. 31, 2023

Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]

Schedule of Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets as of December 31, 2023 and 2022 consisted of the following (in thousands):



	DECEMBER 31,
	2023		2022
Prepaid external R&D costs	$	10,116	$	3,303
Prepaid external manufacturing costs	2,303		16

Prepaid sales tax	429		872


Prepaid insurance	638		941
Prepaid subscriptions	568		470
Interest and dividend receivable	948		841
Other	1,001		276
Total prepaid expenses and other current assets	$	16,003	$	6,719

INCOME TAXES - Schedule of Loss Before Provision For (Benefit From) Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Tax Disclosure [Abstract]
United States	$ (156,571)	$ (112,240)	$ (55,961)
Foreign	3,579	7,561	(772)
Loss before income taxes	$ (152,992)	$ (104,679)	$ (56,733)

- Definition

Amount of income (loss) from continuing operations, including income (loss) from equity method investments, before deduction of income tax expense (benefit), and income (loss) attributable to noncontrolling interest.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(11))
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

COMMON STOCK (Tables)

12 Months Ended

Dec. 31, 2023

Equity [Abstract]

Schedule of Preferred Stock

As of December 31, 2023, 2022, and 2021, the Company has reserved the following shares of common stock for the potential exercise of stock options:



	DECEMBER 31,
	2023	2022	2021


Options to purchase common stock	4,394,807	3,533,169	2,810,684
Total	4,394,807	3,533,169	2,810,684

- Definition

Tabular disclosure of an entity's stock, including par or stated value per share, number and dollar amount of share subscriptions, shares authorized, shares issued, shares outstanding, number and dollar amount of shares held in an employee trust, dividend per share, total dividends, share conversion features, par value plus additional paid in capital, the value of treasury stock and other information necessary to a fair presentation, and EPS information. Stock by class includes common, convertible, and preferred stocks which are not redeemable or redeemable solely at the option of the issuer. Includes preferred stock with redemption features that are solely within the control of the issuer and mandatorily redeemable stock if redemption is required to occur only upon liquidation or termination of the reporting entity. If more than one issue is outstanding, state the title of each issue and the corresponding dollar amount; dollar amount of any shares subscribed but unissued and the deduction of subscriptions receivable there from; number of shares authorized, issued, and outstanding.

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+ Details

NATURE OF BUSINESS AND BASIS OF PRESENTATION (Details) - At The Market Offering - USD ($) $ in Thousands, shares in Millions		1 Months Ended		12 Months Ended
	May 03, 2021	Dec. 31, 2022	May 31, 2021	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Class of Stock [Line Items]
Stock sales agreement, maximum amount authorized to be issued	$ 150,000		$ 150,000
Additional common shares		250.0
Issuance of common stock				$ 175,825	$ 119,578	$ 28,171

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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+ Details

PROPERTY AND EQUIPMENT, NET (Tables)

12 Months Ended

Dec. 31, 2023

Property, Plant and Equipment [Abstract]

Schedule of Property and Equipment, Net

Depreciation is calculated using the straight-line method over the estimated useful lives of the related assets as follows:



			ESTIMATED USEFUL LIFE
Computer equipment and software			3 years
Laboratory equipment			5 years
Office furniture			5 years
Leasehold improvements			lesser of useful life or remaining lease term

Property and equipment, net as of December 31, 2023 and 2022 consisted of the following (in thousands):



	DECEMBER 31,
	2023		2022
Computer equipment and software	$	35	$	35
Laboratory equipment	5,279		2,652
Office furniture	540		284
Leasehold improvements	—		213
Total	5,854		3,184
Less: accumulated depreciation	(1,720)		(1,163)
Property and equipment, net	$	4,134	$	2,021

Insider Trading Arrangements	3 Months Ended	12 Months Ended
Insider Trading Arrangements	Dec. 31, 2023	Dec. 31, 2023
Trading Arrangements, by Individual
Material Terms of Trading Arrangement		During the three months ended December 31, 2023, no director or officer of the Company adopted or terminated a “Rule 10b5-1 trading arrangement” or “non-Rule 10b5-1 trading arrangement,” as each term is defined in Item 408(a) of Regulation S-K
Rule 10b5-1 Arrangement Adopted	false
Non-Rule 10b5-1 Arrangement Adopted	false
Rule 10b5-1 Arrangement Terminated	false
Non-Rule 10b5-1 Arrangement Terminated	false

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)

12 Months Ended

Dec. 31, 2023

Accounting Policies [Abstract]

Schedule of Estimated Useful Lives

Depreciation is calculated using the straight-line method over the estimated useful lives of the related assets as follows:



			ESTIMATED USEFUL LIFE
Computer equipment and software			3 years
Laboratory equipment			5 years
Office furniture			5 years
Leasehold improvements			lesser of useful life or remaining lease term

Property and equipment, net as of December 31, 2023 and 2022 consisted of the following (in thousands):



	DECEMBER 31,
	2023		2022
Computer equipment and software	$	35	$	35
Laboratory equipment	5,279		2,652
Office furniture	540		284
Leasehold improvements	—		213
Total	5,854		3,184
Less: accumulated depreciation	(1,720)		(1,163)
Property and equipment, net	$	4,134	$	2,021

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 as of December 31, 2023, 2022, and 2021 because including them would have had an anti-dilutive effect:



	DECEMBER 31,
	2023	2022	2021


Options to purchase common stock	4,394,807	3,533,169	2,810,684
Total	4,394,807	3,533,169	2,810,684

Consolidated Statements of Stockholders’ Equity - USD ($) $ in Thousands	Total	COMMON STOCK	ADDITIONAL PAID-IN CAPITAL	ACCUMULATED DEFICIT	At The Market Offering	At The Market Offering COMMON STOCK	At The Market Offering ADDITIONAL PAID-IN CAPITAL
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Ending balance (in shares)		23,192,866
Ending balance	$ 261,721	$ 2	$ 326,730	$ (65,011)
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]
Issuance of common stock under the ATM agreement, net of commissions and offering cost (in shares)						520,000
Issuance of common stock under the ATM agreement, net of commissions and offering cost					$ 28,096		$ 28,096
Exercise of common stock options (in shares)		261,968
Exercise of common stock options	379		379
Stock-based compensation	11,722		11,722
Net loss	(58,744)			(58,744)
Ending balance (in shares)		23,974,834
Ending balance	243,174	$ 2	366,927	(123,755)
Beginning balance (in shares) at Dec. 31, 2021		23,974,834
Beginning balance at Dec. 31, 2021	243,174	$ 2	366,927	(123,755)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under the ATM agreement, net of commissions and offering cost (in shares)						3,410,384
Issuance of common stock under the ATM agreement, net of commissions and offering cost					119,428		119,428
Exercise of common stock options (in shares)		158,235
Exercise of common stock options	818		818
Stock-based compensation	18,682		18,682
Net loss	(104,679)			(104,679)
Ending balance (in shares)		27,543,453
Ending balance	277,423	$ 2	505,855	(228,434)
Beginning balance (in shares) at Dec. 31, 2022		27,543,453
Beginning balance at Dec. 31, 2022	$ 277,423	$ 2	505,855	(228,434)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under the ATM agreement, net of commissions and offering cost (in shares)						4,061,606
Issuance of common stock under the ATM agreement, net of commissions and offering cost		$ 1			$ 175,794		$ 175,793
Exercise of common stock options (in shares)	236,025	236,025
Exercise of common stock options	$ 3,225		3,225
Stock-based compensation	28,763		28,763
Net loss	(152,992)			(152,992)
Ending balance (in shares)		31,841,084
Ending balance	$ 332,213	$ 3	$ 713,636	$ (381,426)

- Definition

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

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+ Details

- Definition

Equity impact of the value of new stock issued during the period. Includes shares issued in an initial public offering or a secondary public offering.

+ References + Details

- Definition

Amount of equity (deficit) attributable to parent. Excludes temporary equity and equity attributable to noncontrolling interest.

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+ Details

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
CURRENT ASSETS:
Cash and cash equivalents	$ 331,147	$ 279,048
Accounts receivable	143	0
Prepaid expenses and other current assets	16,003	6,719
Total current assets	347,293	285,767
Operating lease right-of-use assets	15,334	15,548
Property and equipment, net	4,134	2,021
Restricted cash	1,212	1,327
Other long term assets	2,052	2,118
TOTAL ASSETS	370,025	306,781
CURRENT LIABILITIES:
Accounts payable	5,450	3,339
Current portion of operating lease liabilities	1,005	455
Accrued expenses and other current liabilities	17,918	12,753
Total current liabilities	24,373	16,547
Operating lease liabilities, net of current portion	13,439	12,811
Total liabilities	37,812	29,358
COMMITMENTS AND CONTINGENCIES (Note 11)
STOCKHOLDERS’ EQUITY:
Preferred stock, par value of $0.0001 per share; 10,000,000 shares authorized as of December 31, 2023 and December 31, 2022, respectively; no shares issued and outstanding	0	0
Common stock, par value of $0.0001 per share; 200,000,000 shares authorized as of December 31, 2023 and December 31, 2022; 31,841,084 and 27,543,453 shares issued and outstanding as of December 31, 2023 and December 31, 2022, respectively	3	2
Additional paid-in capital	713,636	505,855
Accumulated deficit	(381,426)	(228,434)
Total stockholders’ equity	332,213	277,423
TOTAL LIABILITIES AND STOCKHOLDERS’ EQUITY	$ 370,025	$ 306,781

COMMITMENTS AND CONTINGENCIES

12 Months Ended

Dec. 31, 2023

Commitments and Contingencies Disclosure [Abstract]

COMMITMENTS AND CONTINGENCIES

Operating Leases

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 is leasing approximately 35,662 square feet of office, laboratory and vivarium space located at 1050 Waltham Street, Lexington, Massachusetts (the “Premises”) for its new principal executive office. In December 2022, the Company received access to approximately 31,991 square feet of the Premises (the “Phase A Premises”) pertaining to the office and laboratory space in order to prepare for use. Accordingly, the 1050 Waltham Lease for the Phase A Premises, which is considered a distinct lease component, was determined to be classified as an operating lease and the Company recorded an ROU asset of $15.4 million and a lease liability of $12.9 million on the consolidated balance sheet as of December 31, 2022. The difference between the ROU asset and lease liability pertains to payments made prior to the Phase A Premises commencement date for leasehold improvements that were deemed to be lessor owned.

In January 2023, the Company entered into a first amendment to the 1050 Waltham Lease (the “Lease Amendment”). Under the terms of the Lease Amendment, a phased delivery of the Premises by the Landlord was established as follows: (i) the delivery of the Phase A Premises in January 2023 and (ii) the delivery of the additional approximately 3,671 rentable square feet of vivarium space (the “Phase B Premises”) in March 2023.

As a result of the Lease Amendment, the lease liability for the Phase A Premises was remeasured, resulting in a non-cash decrease to the Company’s operating lease liabilities and ROU assets of $0.5 million in the quarter ended March 31, 2023. Upon delivery of the Phase B Premises in March 2023, the Phase B Premises was determined to be classified as an operating lease and the Company recorded an ROU asset of $1.9 million, inclusive of lease payments made prior to commencement, and a lease liability of $1.4 million on the consolidated balance sheets. Upon delivery of the Phase B Premises, the rent commencement date of the Premises was determined to begin in November 2023, with base rent will initially be fixed at $0.2 million per month and will increase by approximately 3% per annum until the 1050 Waltham Lease expires on November 30, 2031. The Company is obligated to reimburse the Landlord for certain variable costs, including its proportional share of taxes and operating expenses which are not included in the measurement of the 1050 Waltham Lease liability. In connection with its entry into the 1050 Waltham Lease, the Company has provided the Landlord a letter of credit in the amount of approximately $1.2 million, which is recognized as restricted cash within other assets on the consolidated

balance sheets. The Company has the option to extend the term of the 1050 Waltham Lease for a period of an additional 5 years. As of December 31, 2023, the Company has no reasonable certainty that this option to extend will be exercised.

The components of the lease cost as of December 31, 2023, 2022, and 2021 consisted of the following (in thousands):


	YEAR ENDED DECEMBER 31,
	2023		2022		2021
Operating lease cost	$	2,950	$	1,089	$	587

Variable payments	1,317		555		295
Total lease cost	$4,267		$1,644		$882

Other information as of December 31, 2023, 2022 and 2021 (in thousands):


	DECEMBER 31,
	2023	2022	2021
Cash paid for amounts included in the measurement of lease liabilities Operating cash flows from operating leases	$1,122	$913	$587
Right-of-use assets obtained in exchange for new operating lease liabilities	$1,338	$15,423	$—

The weighted-average remaining lease term and discount rate for the leases as of December 31, 2023 and 2022 were as follows:


	DECEMBER 31,
	2023		2022
Weighted-average remaining lease term — operating leases	7.9		4.0
Weighted-average discount rate — operating leases	10.3	%	10.0	%

Maturities of operating lease liabilities at December 31, 2023 are as follows (in thousands):



MATURITY OF LEASE LIABILITY
2024	$	2,431
2025	2,504
2026	2,578
2027	2,657
2028	2,736
2029	2,818
2030	2,903
2031	2,734
Total lease payments	21,361
Less: imputed interest	(6,917)
Total operating lease liabilities	$	14,444
Included in the consolidated balance sheet:
Current portion of lease liabilities	$	1,005
Lease liabilities	13,439
Total operating lease liabilities	$	14,444

Legal Proceedings

The Company is not a party to any litigation and does not have contingency reserves established for any litigation liabilities.

LICENSE AGREEMENTS	12 Months Ended
LICENSE AGREEMENTS	Dec. 31, 2023
Research and Development [Abstract]
LICENSE AGREEMENTS	LICENSE AGREEMENTS Massachusetts General Hospital On April 5, 2016, the Company entered into an exclusive patent license agreement with The General Hospital Corporation d/b/a Massachusetts General Hospital (“MGH”). Under the license agreement with MGH (as amended in May 2017 and February 2018, the “MGH Agreement”), the Company obtained an exclusive, worldwide license, with the right to sublicense, under certain patents and technical information of MGH, to make, have made, use, have used, sell, have sold, lease, have leased, import, have imported or otherwise transfer licensed products and processes for use in the treatment, diagnosis, palliation and prevention of diseases and disorders in humans and animals. The Company is required to use commercially reasonable efforts to develop and commercialize licensed products and processes and must achieve certain required diligence milestones. Under the terms of the MGH Agreement, the Company is required to pay a low-five digit to mid-five digit annual maintenance fee prior to the first commercial sale of its first product or process, a mid-five digit annual maintenance fee after the first commercial sale of its first product or process that is creditable against royalties, certain clinical and regulatory milestone payments for the first three products or indications to achieve such milestones, which milestone payments are $8.6 million in the aggregate, and certain commercial milestone payments for the first three products or indications to achieve such milestones, which milestone payments are $18.0 million in the aggregate. The Company made payments of $50,000 and $300,000 in 2020 and 2021, respectively, for the achievement of the clinical and regulatory milestones of (i) filing of an IND in the first country and (ii) the completion of a Phase 1 clinical trial, respectively. The Company is also obligated to pay tiered royalties on net sales of licensed products ranging in the low-single digits to mid-single digits. The royalty rates are subject to up to a maximum 50% reduction for lack of a valid claim, in the event that it is necessary for the Company to obtain a license to any third-party intellectual property related to the licensed products, and generic competition. The obligation to pay royalties under the MGH Agreement expires on a licensed product-by-licensed product and country-by-country basis upon the later of expiry of the last valid claim of the licensed patents that cover such licensed product in such country or ten years from the first commercial sale of such product in such country. The Company is also obligated to pay a percentage of non-royalty-related payments received by it from sublicensees ranging in the low-double digits and a change of control fee equal to a low-single digit percentage of the payments received as part of any completed transaction up to a low seven-digit amount. The MGH Agreement expires upon expiry of the last remaining royalty obligation for a licensed product or process. Under the MGH Agreement, MGH may terminate the agreement upon the Company’s uncured material breach or insolvency, a challenge by the Company of the licensed patents and certain other specified breaches of the MGH Agreement. The Company may terminate the agreement for any reason upon specified prior written notice to MGH. Hansoh Agreement On December 12, 2021, the Company entered into a license agreement with Hansoh. Refer to Note 12, Revenue from Contracts with Customers, for more information regarding this agreement.

Audit Information	12 Months Ended
Audit Information	Dec. 31, 2023
Audit Information [Abstract]
Auditor Firm ID	34
Auditor Name	Deloitte & Touche LLP
Auditor Location	Boston, Massachusetts

INCOME TAXES - Schedule of Components of Income Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Current income tax provision:
Federal	$ 0	$ 0	$ 0
State	0	0	11
Foreign	0	0	2,000
Total current income tax provision	0	0	2,011
Total deferred income tax provision	0	0	0
Total income tax provision	$ 0	$ 0	$ 2,011

- Definition

Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

+ References + Details

COMMITMENTS AND CONTINGENCIES - Schedule of Maturity of Lease Liability (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Sep. 07, 2021
MATURITY OF LEASE LIABILITY
2024	$ 2,431
2025	2,504
2026	2,578
2027	2,657
2028	2,736
2029	2,818
2030	2,903
2031	2,734
Total lease payments	21,361
Less: imputed interest	(6,917)
Total operating lease liabilities	14,444		$ 12,900
Included in the consolidated balance sheet:
Current portion of lease liabilities	1,005	$ 455
Lease liabilities	13,439	$ 12,811
Total operating lease liabilities	$ 14,444		$ 12,900

STOCK-BASED COMPENSATION - Schedule of Stock Option Valuation (Details) - Stock Options	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	3.96%	2.24%	0.88%
Expected term (in years)	6 years 18 days	6 years 21 days	6 years 18 days
Expected volatility	81.34%	83.03%	83.00%
Expected dividend yield	0.00%	0.00%	0.00%

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

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+ Details

PREPAID EXPENSES AND OTHER CURRENT ASSETS

12 Months Ended

Dec. 31, 2023

Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]

PREPAID EXPENSES AND OTHER CURRENT ASSETS

Prepaid expenses and other current assets as of December 31, 2023 and 2022 consisted of the following (in thousands):



	DECEMBER 31,
	2023		2022
Prepaid external R&D costs	$	10,116	$	3,303
Prepaid external manufacturing costs	2,303		16

Prepaid sales tax	429		872


Prepaid insurance	638		941
Prepaid subscriptions	568		470
Interest and dividend receivable	948		841
Other	1,001		276
Total prepaid expenses and other current assets	$	16,003	$	6,719

NATURE OF BUSINESS AND BASIS OF PRESENTATION	12 Months Ended
NATURE OF BUSINESS AND BASIS OF PRESENTATION	Dec. 31, 2023
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 developing and commercializing novel therapeutics to treat a wide range of patients with disorders that are linked to dysfunctional signaling of the transforming growth factor-beta (“TGF-ß”) family of proteins. The Company’s lead product candidate, KER-050 (elritercept), is being developed for the treatment of low blood cell counts (“cytopenias”), including anemia and thrombocytopenia, in patients with myelodysplastic syndromes (“MDS”) and in patients with myelofibrosis. The Company’s second product candidate, KER-012, is being developed for the treatment of pulmonary arterial hypertension (“PAH”) and for the treatment of cardiovascular disorders. The Company's third product candidate, KER-065, is being developed for the treatment of obesity and for the treatment of neuromuscular diseases. Since its inception in 2015, the Company has devoted the majority of its resources to business planning, research and development of its product candidates, including 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. In May 2021, the Company filed a registration statement on Form S-3, which was automatically effective upon filing (the “Shelf Registration Statement”). Pursuant to the Shelf Registration Statement, the Company could 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 the Company common stock, preferred stock, debt securities and warrants. In December 2022, the Company filed a prospectus supplement to the Shelf Registration Statement for the issuance and sale, if any, of up to an additional $250.0 million in common stock in sales deemed to be an “at the market offering,” as defined by the Securities Act. Liquidity and Capital Resources The Company’s 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, which include the net proceeds of approximately $151.0 million from its public offering of common stock in January 2024, 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 beyond that point 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 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.

Pay vs Performance Disclosure - USD ($) $ in Thousands	12 Months Ended
Pay vs Performance Disclosure - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Pay vs Performance Disclosure
Net loss	$ (152,992)	$ (104,679)	$ (58,744)

- Definition

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

+ References

+ Details

COMMON STOCK- Schedule of Preferred Stock (Details) - shares	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Class of Stock [Line Items]
Total (in shares)	4,394,807	3,533,169	2,810,684
Options to purchase common stock
Class of Stock [Line Items]
Total (in shares)	4,394,807	3,533,169	2,810,684

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

+ Details

STOCK-BASED COMPENSATION - Schedule of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
NUMBER OF OPTIONS
Outstanding as of beginning of period (in shares)	3,533,169
Granted (in shares)	1,386,295
Exercised (in shares)	(236,025)
Cancelled or forfeited (in shares)	(258,211)
Expired (in USD per share)	(30,421)
Outstanding as of end of period (in shares)	4,394,807	3,533,169
Options exercisable (in shares)		2,375,136
WEIGHTED- AVERAGE EXERCISE PRICE
Outstanding as of beginning of period (in USD per share)	$ 30.26
Granted (in USD per share)	49.06
Exercised (in USD per share)	13.66
Cancelled or forfeited (in USD per share)	44.46
Expired (in USD per share)	53.97
Outstanding as of end of period (in USD per share)	$ 36.08	$ 30.26
Options exercisable (in USD per share)		$ 26.40
WEIGHTED-AVERAGE REMAINING CONTRACTUAL TERM (IN YEARS)
Outstanding	7 years 3 months 21 days	7 years 9 months 7 days
Options exercisable		6 years 1 month 9 days
AGGREGATE INTRINSIC VALUE
Outstanding	$ 46,319	$ 72,434
Exercised	$ 6,369
Options exercisable		$ 43,205

Consolidated Statements of Stockholders’ Equity (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Common stock, par value (in USD per share)	$ 0.0001	$ 0.0001
Common Stock
Issuance of common stock under the ATM agreement, net of commissions and offering cost	$ 1
At The Market Offering
Commissions and offering cost	2,708	$ 1,971	$ 475
Issuance of common stock under the ATM agreement, net of commissions and offering cost	175,794	119,428	28,096
At The Market Offering \| ADDITIONAL PAID-IN CAPITAL
Issuance of common stock under the ATM agreement, net of commissions and offering cost	$ 175,793	$ 119,428	$ 28,096

- Definition

Equity impact of the value of new stock issued during the period. Includes shares issued in an initial public offering or a secondary public offering.

+ References + Details

INCOME TAXES (Tables)

12 Months Ended

Dec. 31, 2023

Income Tax Disclosure [Abstract]

Schedule of Loss Before Provision For (Benefit From) Income Taxes

Loss before income taxes for the years ended December 31, 2023, 2022, and 2021 consisted of the following (in thousands):



	YEAR ENDED DECEMBER 31,
	2023		2022		2021
United States	$	(156,571)	$	(112,240)	$	(55,961)
Foreign	3,579		7,561		(772)
Loss before income taxes	$	(152,992)	$	(104,679)	$	(56,733)

Schedule of Components of Income Tax Expense (Benefit)

The components of the provision for income taxes for the years ended December 31, 2023, 2022, and 2021 consisted of the following (in thousands):



	YEAR ENDED DECEMBER 31,
	2023		2022		2021
Current income tax provision:
Federal	$	—	$	—	$	—
State	—		—		11
Foreign	—		—		2,000
Total current income tax provision	$	—	$	—	$	2,011
Total deferred income tax provision	$	—	$	—	$	—
Total income tax provision	$	—	$	—	$	2,011

Schedule of Reconciliation of U.S. Federal Statutory Income Tax Rate to Effective Income Tax Rate

A reconciliation of the Company’s statutory income tax rate to the Company’s effective income tax rate for the years ended December 31, 2023, 2022, and 2021 was as follows:



	YEAR ENDED DECEMBER 31,
	2023		2022		2021
Income at U.S. statutory rate	21.0	%	21.0	%	21.0	%
State taxes	1.2		5.2		6.7
Stock compensation	(0.6)		(2.8)		(1.5)
Other permanent differences	(3.0)		(1.4)		—

Research and development credits	3.0		2.5		4.8
Impact of foreign operations	(0.1)		(0.3)		(0.6)
Foreign withholding taxes	—		—		(2.8)
Other	3.0		(3.3)		(0.3)
Tax credits	0.8		3.2		—
Change in valuation allowance	(25.3)		(24.1)		(30.9)
Effective tax rate	—	%	—	%	(3.5)	%

Schedule of Net Deferred Tax Assets

The net deferred income tax asset balance as of December 31, 2023 and 2022 related to the following (in thousands):



	YEAR ENDED DECEMBER 31,
	2023		2022
Net operating loss carryforwards	$	36,245	$	37,687
Research and development tax credits	14,134		10,459
Capitalized research and development expenses	36,363		10,822
Stock-based compensation	10,794		—
Operating lease liability	3,457		3,602
Accrued expenses	1,774		1,275


Intangibles	—		115
Other	$	285	$	908
Total deferred tax assets	$	103,052	$	64,868
Valuation allowance	(99,390)		(60,650)
Net deferred tax assets	$	3,662	$	4,218
Deferred tax liability
Operating lease right-of-use asset	(3,662)		(4,218)

Net deferred tax assets	$	—	$	—

REVENUE FROM CONTRACTS WITH CUSTOMERS	12 Months Ended
REVENUE FROM CONTRACTS WITH CUSTOMERS	Dec. 31, 2023
Revenue from Contract with Customer [Abstract]
REVENUE FROM CONTRACTS WITH CUSTOMERS	REVENUE FROM CONTRACTS WITH CUSTOMERS Hansoh License Agreement In December 2021, the Company entered into a license agreement with Hansoh (the “Hansoh Agreement”). Under the Hansoh Agreement, the Company granted to Hansoh the exclusive right to develop, manufacture and commercialize KER-050 and licensed products containing KER-050 within the territories of mainland China, Hong Kong and Macau (the “Territory”). In connection with the Hansoh Agreement, Hansoh will purchase clinical trial supply of KER-050 from the Company, and the parties will also negotiate in good faith to enter into an agreement for commercial supply prior to any anticipated commercialization in the Territory. In addition, Hansoh will use commercially reasonable efforts to develop, obtain regulatory approval for, and commercialize licensed products in any region in the Territory. Pursuant to the Hansoh Agreement, the Company received a net one-time $18.0 million upfront license payment and will also be eligible to receive up to an aggregate of (i) $26.5 million upon the achievement of specified development milestones and (ii) $144.0 million upon the achievement of specified net sales thresholds for all licensed products in the Territory. If a licensed product is approved for marketing in the Territory, the Company will be entitled to receive royalty payments based on a tiered percentage of annual net sales in each region within the Territory, with such percentage ranging from the low double digit to high teens, subject to specified potential royalty reductions. Hansoh’s obligation to pay royalties for a given licensed product in a given region in the Territory will begin on the date of the first commercial sale for such licensed product in such region and continue until the latest of (i) 10 years from the date of the first commercial sale for such licensed product in such region, (ii) the expiration of the last valid claim of certain licensed patents or joint patents, and (iii) expiration of regulatory exclusivity in such region. During the royalty term, neither party will directly or indirectly commercialize a competing product in the Territory. The Hansoh Agreement will continue in force on a region-by-region basis until the expiration of the royalty term. Hansoh may terminate the Agreement in its entirety for convenience, with notice. The Company may terminate the Hansoh Agreement in its entirety for a patent challenge brought by Hansoh or its affiliates or their sublicensees. Either party may terminate the Hansoh Agreement in its entirety (i) if the other party materially breaches the Hansoh Agreement and fails to cure such breach or (ii) upon the bankruptcy of the other party. The Company evaluated the Hansoh Agreement and concluded that it was subject to ASC 606, as the Company viewed the Hansoh Agreement as a contract with a customer. As such, the Company assessed the terms of the Hansoh Agreement and identified a single performance obligation for the Company to provide Hansoh an exclusive license to develop, manufacture and commercialize KER-050 and licensed products containing KER-050 in the Territory, including the underlying know-how related to such licenses. All other promised goods/services were deemed immaterial in the context of the Hansoh Agreement. Under the Hansoh Agreement, Hansoh was obligated to pay a one-time, net $18.0 million payment to the Company, which was paid in the first quarter of 2022. The Company recognized a gross upfront fee of $20.0 million as revenue and $2.0 million in withholding tax on its consolidated statement of operations for the year ended December 31, 2021, and a receivable, net of withholding tax on its consolidated balance sheet as of December 31, 2021. The Company will recognize development milestone payments as revenue at the point in time when it is determined that it is probable such milestones will be achieved as all performance obligations will have been satisfied at the point which a milestone might occur (i.e., Hansoh will have assumed all responsibility for the activities under the Hansoh Agreement). The Company will recognize royalty payments and commercial milestone payments as the associated sales of licensed products are recorded by Hansoh, as they predominantly relate to the license granted with the Hansoh Agreement. No milestones have been achieved as of December 31, 2023. In connection with the Hansoh Agreement, the Company entered into a manufacturing technology transfer agreement (the “Tech Transfer Agreement”) with Hansoh, effective in June 2023. The Tech Transfer Agreement governs the transfer to Hansoh, by the Company of all documents and information required to complete the manufacturing technology transfer. Under the Tech Transfer Agreement, Hansoh is obligated to make certain payments to the Company, at the rates set forth in the Tech Transfer Agreement, as manufacturing technology transfer services are provided over the term of the Tech Transfer Agreement. The Tech Transfer Agreement is set to expire on December 30, 2024, unless extended pursuant to its terms. As of December 31, 2023, work under the Tech Transfer Agreement has begun. The Company recognized $0.2 million of service and other revenue for the year ended December 31, 2023.

- Definition

The entire disclosure of revenue from contract with customer to transfer good or service and to transfer nonfinancial asset. Includes, but is not limited to, disaggregation of revenue, credit loss recognized from contract with customer, judgment and change in judgment related to contract with customer, and asset recognized from cost incurred to obtain or fulfill contract with customer. Excludes insurance and lease contracts.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 9
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479806/606-10-50-9

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 10
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479806/606-10-50-10

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 15
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479806/606-10-50-15

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 12
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479806/606-10-50-12

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 12
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479806/606-10-50-12

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 12
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479806/606-10-50-12

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 12
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479806/606-10-50-12

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 12
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479806/606-10-50-12

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)(2)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479806/606-10-50-13

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Name Accounting Standards Codification
-Topic 606
-Publisher FASB
-URI https://asc.fasb.org//606/tableOfContent

+ Details

REVENUE FROM CONTRACTS WITH CUSTOMERS (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
	Apr. 05, 2016	Mar. 31, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Disaggregation of Revenue [Line Items]
License revenue			$ 151	$ 0	$ 20,100
Service and other revenue			$ 200
MGH \| License Agreement
Disaggregation of Revenue [Line Items]
Expiration period	10 years		10 years
License revenue
Disaggregation of Revenue [Line Items]
License revenue			$ 0	$ 0	20,100
License revenue \| Hansoh Shanghai Healthtech Co Ltd
Disaggregation of Revenue [Line Items]
License revenue		$ 18,000
Revenue, remaining performance obligation, development milestones, variable consideration			26,500
Revenue, remaining performance obligation, sales milestones, variable consideration amount			$ 144,000
Gross upfront fee					20,000
Withholding taxes from upfront fee					$ 2,000

PREPAID EXPENSES AND OTHER CURRENT ASSETS (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid external R&D costs	$ 10,116	$ 3,303
Prepaid external manufacturing costs	2,303	16
Prepaid sales tax	429	872
Prepaid insurance	638	941
Prepaid subscriptions	568	470
Interest and dividend receivable	948	841
Other	1,001	276
Total prepaid expenses and other current assets	$ 16,003	$ 6,719

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Antidilutive Securities (Details) - shares	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total (in shares)	4,394,807	3,533,169	2,810,684
Options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total (in shares)	4,394,807	3,533,169	2,810,684

INCOME TAXES - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
INCOME TAXES - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Tax Credit Carryforward [Line Items]
Change in valuation allowance	$ 38.7	$ 25.2	$ 17.5
Federal
Tax Credit Carryforward [Line Items]
Net operating loss carryforwards	135.2
Net operating loss carryforwards written off in the current period	0.2
Operating loss carryforwards not subject to limitation	135.0
Operating loss carryforward, written off	0.3
Federal \| Research and Development Tax Credit
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	10.4
State
Tax Credit Carryforward [Line Items]
Net operating loss carryforwards	124.4
Operating loss carryforward, written off	0.3
State \| Research and Development Tax Credit
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	$ 4.8

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($) financialInstitution segment	Dec. 31, 2022 USD ($) financialInstitution	Dec. 31, 2021 USD ($)
Accounting Policies [Abstract]
Restricted cash	$ 1,212	$ 1,327
Number of financial institutions \| financialInstitution	3	3
Research and development incentive income	$ 2,400	$ 7,081	$ 0
Number of operating segments \| segment	1

STOCK-BASED COMPENSATION - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions				12 Months Ended
	Jan. 01, 2024	Jan. 01, 2023	Jan. 01, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of underlying common stock				$ 35.24	$ 31.18
Unrecognized stock-based compensation expense related to unvested stock options				$ 57.8
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of underlying common stock (in dollars per share)						$ 42.29
Unvested award, period for recognition				2 years 6 months 25 days
2017 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (in shares)				507,973
2020 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (in shares)				3,886,834
Annual percentage increase in shares authorized				4.00%
Number of additional shares authorized (in shares)		1,101,738	958,993
Vesting period				4 years
Contractual term				10 years
Number of shares available for future grants (in shares)				976,452
2020 Plan \| Subsequent Event
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of additional shares authorized (in shares)	1,273,643