Kaleido Biosciences (KLDO) 10-K 3 Mar 21 2020 FY Annual report Financial data

Document And Entity Information

Document And Entity Information - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Feb. 26, 2021	Jun. 30, 2020
Cover [Abstract]
Entity Registrant Name	KALEIDO BIOSCIENCES, INC.
Document Type	10-K
Amendment Flag	false
Trading Symbol	KLDO
Current Fiscal Year End Date	--12-31
Entity Common Stock, Shares Outstanding		42,466,791
Entity Central Index Key	0001751299
Entity Filer Category	Non-accelerated Filer
Document Period End Date	Dec. 31, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Shell Company	false
Entity Small Business	true
Entity Emerging Growth Company	true
Security Exchange Name	NASDAQ
Entity File Number	001-38822
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	47-3048279
Entity Address, Address Line One	65 Hayden Avenue
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	674-9000
Document Transition Report	false
Document Annual Report	true
Title of 12(b) Security	Common Stock, $0.001 Par Value
Entity Ex Transition Period	false
Entity Public Float			$ 250,730
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
ICFR Auditor Attestation Flag	false

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 46,222	$ 71,241
Prepaid expenses and other current assets	2,499	2,038
Total current assets	48,721	73,279
Property and equipment, net	8,462	6,742
Restricted cash	2,161	2,285
Total assets	59,344	82,306
Current liabilities:
Accounts payable	5,389	2,016
Accrued expenses and other current liabilities	8,636	8,361
Current term debt, net of unamortized debt discount	2,634
Total current liabilities	16,659	10,377
Long term debt, net of unamortized debt discount	18,375	20,391
Other liabilities	3,814	2,655
Total liabilities	38,848	33,423
Commitments and contingencies (Note 7)
Stockholders’ equity:
Preferred shares, $0.001 par value, 10,000,000 authorized; no shares issued or outstanding
Common shares, $0.001 par value, 150,000,000 shares authorized; 36,022,811 and 30,129,096 shares issued; 36,022,811 and 30,127,846 shares outstanding at December 31, 2020 and 2019, respectively	36	30
Additional paid-in capital	294,639	241,412
Accumulated deficit	(274,179)	(192,559)
Total stockholders' equity	20,496	48,883
Total liabilities and stockholders’ equity	$ 59,344	$ 82,306

Consolidated Balance Sheets - P

Consolidated Balance Sheets - Parenthetical - $ / shares	Dec. 31, 2020	Dec. 31, 2019
Preferred Stock
Preferred shares par value (in dollars per share)	$ 0.001	$ 0.001
Preferred shares authorized (in shares)	10,000,000	10,000,000
Preferred shares issued (in shares)	0	0
Preferred shares outstanding (in shares)	0	0
Common Stock
Common shares par value (in dollars per share)	$ 0.001	$ 0.001
Common shares authorized (in shares)	150,000,000	150,000,000
Common shares issued (in shares)	36,022,811	30,129,096
Common shares outstanding (in shares)	36,022,811	30,127,846

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Revenue:
Revenue from Contract with Customer, Product and Service [Extensible List]	kldo:CollaborationRevenueMember	kldo:CollaborationRevenueMember
Collaboration revenue	$ 975
Operating expenses:
Research and development	55,967	$ 64,232
General and administrative	23,882	22,428
Total operating expenses	79,849	86,660
Loss from operations	(78,874)	(86,660)
Other (expense) income:
Interest income	249	1,693
Interest expense	(2,802)	(977)
Change in fair value of warrant liability		252
Loss on extinguishment of debt	0	(580)
Other expense	(193)	(59)
Total other income (expense), net	(2,746)	329
Net loss	$ (81,620)	$ (86,331)
Net loss per share —basic and diluted	$ (2.44)	$ (3.36)
Weighted-average common shares outstanding —basic and diluted	33,450,213	25,703,269

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Deficit
Beginning balance at Dec. 31, 2018	$ (96,244)	$ 6	$ 9,978	$ (106,228)
Beginning balance (in shares) at Dec. 31, 2018		5,786,911
Conversion of redeemable convertible preferred stock into common stock	153,226	$ 19	153,207
Conversion of redeemable convertible preferred stock into common stock (in shares)		18,517,386
Conversion of preferred stock warrant to common stock warrant upon closing of initial public offering	871		871
Issuance of common stock, net of issuance costs	65,946	$ 5	65,941
Issuance of common stock, net of issuance costs (in shares)		5,000,000
Exercise of common stock warrant (in shares)		51,015
Exercise of stock options	630		630
Exercise of stock options (in shares)		445,160
Stock-based compensation	10,068		10,068
Vesting of restricted shares	717		717
Vesting of restricted shares (in shares)		327,374
Net loss	(86,331)			(86,331)
Ending balance at Dec. 31, 2019	48,883	$ 30	241,412	(192,559)
Ending balance (in shares) at Dec. 31, 2019		30,127,846
Conversion of preferred stock warrant to common stock warrant upon closing of initial public offering	0
Issuance of common stock, net of issuance costs	37,756	$ 5	37,751
Issuance of common stock, net of issuance costs (in shares)		5,296,299
Exercise of stock options	$ 2,790	$ 1	2,789
Exercise of stock options (in shares)	597,416	597,416
Stock-based compensation	$ 12,684		12,684
Vesting of restricted shares	3		3
Vesting of restricted shares (in shares)		1,250
Net loss	(81,620)			(81,620)
Ending balance at Dec. 31, 2020	$ 20,496	$ 36	$ 294,639	$ (274,179)
Ending balance (in shares) at Dec. 31, 2020		36,022,811

Consolidated Statements of St_2

Consolidated Statements of Stockholders' Equity - Parenthetical - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Statement Of Stockholders Equity [Abstract]
Common stock, issuance costs	$ 2,731	$ 9,055

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Operating activities:
Net loss	$ (81,620)	$ (86,331)
Reconciliation of net loss to net cash used in operating activities:
Depreciation and amortization	1,823	1,318
Loss on extinguishment of debt	0	580
Stock-based compensation	12,684	10,068
Amortization of debt discount	696	37
Non-cash interest expense	181	0
Loss on disposal of fixed asset	167	0
Change in fair value of warrant liability	0	(342)
Change in fair value of derivative	0	90
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(459)	(1,881)
Accounts payable	3,301	(684)
Accrued expense and other liabilities	1,709	1,349
Net cash used in operating activities	(61,518)	(75,796)
Investing activities:
Purchase of property and equipment	(4,024)	(3,586)
Net cash used in investing activities	(4,024)	(3,586)
Financing activities:
Proceeds from issuance of debt	0	37,500
Repayments on debt	0	(30,000)
Payments of issuance and extinguishment costs related to debt	(79)	(858)
Proceeds from exercise of stock options	2,790	630
Payments related to capital lease	(68)	(91)
Issuance of common stock, net of issuance costs	37,756	67,761
Settlement of derivative liability	0	(300)
Net cash provided by financing activities	40,399	74,642
Net decrease in cash, cash equivalents, and restricted cash	(25,143)	(4,740)
Cash, cash equivalents, and restricted cash, beginning of year	73,526	78,266
Cash, cash equivalents, and restricted cash, end of year	48,383	73,526
Supplemental cash flow information
Interest paid	1,925	881
Supplemental disclosure of non-cash investing and financing activities
Vesting of restricted stock	3	717
Reclassification of warrants to additional paid-in capital	0	871
Conversion of preferred stock to common stock upon closing of the initial public offering	0	153,226
Purchase of property and equipment in accounts payable and accrued expenses	$ 71	$ 385

Nature of the Business, Basis o

Nature of the Business, Basis of Presentation, and Going Concern	12 Months Ended
	Dec. 31, 2020
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of the Business, Basis of Presentation, and Going Concern	1. Nature of the Business, Basis of Presentation, and Going Concern Kaleido Biosciences, Inc. and its wholly owned subsidiaries (the “Company”) is a clinical-stage healthcare company that was incorporated in Delaware on January 27, 2015 and has a principal place of business in Lexington, Massachusetts. The Company was formed to use its differentiated, chemistry-driven approach to leverage the potential of the microbiome to treat disease and improve human health. The Company is subject to risks common to companies in the biotechnology industry, including, but not limited to, successful development of technology, obtaining additional funding, protection of proprietary technology, compliance with government regulations, risks of failure of preclinical studies (including ex vivo On March 4, 2019, the Company completed its initial public offering (the "IPO"), pursuant to which it issued and sold 5,000,000 shares of common stock. The aggregate net proceeds received by the Company from the IPO were $69.8 million, after deducting underwriting discounts and commissions, but before deducting offering costs payable by the Company, which totaled $3.8 million. Upon the closing of the IPO, all outstanding shares of convertible preferred stock converted into 18,517,386 shares of common stock. On June 4, 2020, the Company completed a public offering (the “Offering”), pursuant to which it issued and sold 4,750,000 shares of the common stock. The aggregate net proceeds received by the Company from the Offering were $33.2 million. On July 1, 2020, 185,000 shares were exercised under the Underwriter’s overallotment option for net proceeds of $1.3 million. On August 4, 2020, the Company entered into an Equity Distribution Agreement (the “Sales Agreement”) with a sales agent for the sale of up to $50.0 million of the Company’s shares of common stock, from time to time in an at-the-market public offering (the “ATM”). The sales agent is entitled to compensation at a fixed commission rate of 3.0% of the gross proceeds from the sale of the Company’s common stock pursuant to the Sales Agreement. During the year ended December 31, 2020, the Company sold 361,299 shares of its common stock under the ATM which resulted in aggregate net proceeds of $3.4 million after payment of related commissions. As of December 31, 2020, there was $46.5 million available under the ATM. In January and February 2021, the Company has sold 309,656 shares of its common stock under the ATM which resulted in aggregate net proceeds of $4.9 million. On February 8, 2021, the Company completed a public offering (the “2021 Offering”) including the Underwriters’ overallotment option, pursuant to which the Company issued and sold 6,037,500 shares of common stock for net proceeds of $65.3 million. During the years ended December 31, 2020 and 2019, the Company incurred net losses of $81.6 million and $86.3 million, respectively, and reported cash used in operations totaling $61.5 million and $75.8 million, respectively. As of December 31, 2020, the Company had cash and cash equivalents of $46.2 million. With the net proceeds from the January and February 2021 sale of shares under the ATM and from the 2021 Offering, management believes the Company has sufficient cash and cash equivalents or borrowing capacity to fund operating expenses and capital expenditures into the first quarter of 2022. Based on its recurring losses from operations incurred since inception, expectation of continuing operating losses for the foreseeable future, and need to raise additional capital to finance its future operations, the Company has concluded that there is substantial doubt regarding the Company’s ability to continue as a going concern within one year after the date that these consolidated financial statements are issued. These financial statements do not include any adjustments that might result from the outcome of this uncertainty. The Company will require substantial additional capital to fund its research and development and ongoing operating expenses. These capital requirements are expected to be funded through debt and equity offerings as well as possible strategic collaborations with other companies. If the Company is unable to raise additional funds when needed, it may be required to delay, reduce or eliminate its product development or future commercialization efforts, or grant rights to develop and market product candidates that the Company would otherwise prefer to develop and market itself. While there can be no assurance the Company will be able to successfully reduce operating expenses or raise additional capital, management believes the historical success in managing cash flows and obtaining capital will continue in the foreseeable future. A novel strain of coronavirus (COVID-19) was first identified in late 2019, and subsequently declared a global pandemic by the World Health Organization on March 11, 2020. As a result of the outbreak, many companies have experienced disruptions in their operations and in markets served. The Company has instituted some and may take additional temporary precautionary measures intended to help minimize the risk of the virus to its employees, including implementing a work-at-home policy, providing flexibility for working parents and suspending all business-related travel. The full extent of the future impacts of COVID-19 on our operations, including the timing and ability of the Company to complete certain clinical trials and other efforts to advance the development of our MMTs, is uncertain.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”). Principles of Consolidation The accompanying consolidated financial statements reflect the operations of the Company and its wholly owned subsidiaries. All intercompany accounts and transactions have been eliminated. Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenue and expenses during the reporting periods. Actual results may differ from those estimates. Cash and Cash Equivalents Cash includes cash in readily available checking accounts. The Company’s cash deposits on hand at one financial institution often exceed federally insured limits. Cash equivalents include all highly liquid investments maturing within 90 days from the date of purchase. Restricted Cash Restricted cash is cash that is restricted as to withdrawal or use under the terms of certain contractual agreements. The restricted cash consists of cash collateral for secured letters of credit for the security deposit on the Company’s leased laboratory and office facilities. Concentrations of Credit Risk and of Significant Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash and cash equivalents. The Company’s cash equivalents as of December 31, 2020 consisted only of money market funds. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company relies, and expects to continue to rely, on a small number of vendors to manufacture supplies and raw materials for its development programs. These programs could be adversely affected by a significant interruption in these manufacturing services or the availability of raw materials. Property and Equipment Property and equipment are stated at cost and depreciated using the straight-line method over the estimated useful lives of the respective assets. Laboratory and office equipment, computer equipment and furniture and fixtures are depreciated over a period of five years, and leasehold improvements are amortized over the lesser of the asset’s estimated useful life or the remaining lease term. Major additions and betterments are capitalized; maintenance and repairs, which do not improve or extend the life of the respective assets, are charged to operating expenses as incurred. Impairment of Long-Lived Assets Long-lived assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of the asset may not be recoverable. When such events occur, the Company compares the carrying amounts of the assets to their undiscounted expected future cash flows. If this comparison indicates that there is an impairment, the amount of the impairment is calculated as the difference between the carrying value and the fair value. The Company has not recorded any impairment charges in the periods presented. Segment Information Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker, or decision-making group, in making decisions on how to allocate resources and assess performance. The Company’s chief operating decision maker is the chief executive officer (“CEO”). The Company and CEO view the Company’s operations and manage its business as one operating segment. Collaboration Revenue The Company analyzes its collaboration arrangements to assess whether they are within the scope of ASC Topic 808, Collaborative Arrangements Revenue from Contracts with Customers The Company recognizes revenue after applying the following five steps: 1) Identification of the contract, or contracts, with a customer, 2) Identification of the performance obligations in the contract, including whether they are distinct within the context of the contract 3) Determination of the transaction price, including the constraint on variable consideration 4) Allocation of the transaction price to the performance obligations in the contract 5) Recognition of revenue when, or as, performance obligations are satisfied If a contract is determined to be within the scope of ASC 606 at inception, the Company assesses the goods or services promised within such contract, determines which of those goods and services are performance obligations, and assesses whether each promised good or service is distinct. Promised goods and services are considered distinct provided that: (i) the customer can benefit from the good or service either on its own or together with other resources that are readily available to the customer (that is, the good or service is capable of being distinct) and (ii) the entity’s promise to transfer the good or service to the customer is separately identifiable from other promises in the contract (that is, the promise to transfer the good or service is distinct within the context of the contract). In assessing whether a promised good or service is distinct, the Company considers factors such as the research, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. The Company also considers the intended benefit of the contract in assessing whether a promised good or service is separately identifiable from other promises in the contract. If a promised good or service is not distinct, an entity is required to combine that good or service with other promised goods or services until it identifies a bundle of goods or services that is distinct. The Company may provide options to additional goods or services in such arrangements exercisable at a customer’s discretion. The Company assesses if these options provide a material right to the customer and if so, they are considered performance obligations. The identification of material rights requires judgments related to the determination of the value of the underlying good and services to the option exercise price. The Company determines transaction price based on the amount of consideration the Company expects to receive for transferring the promised goods or services in the contract. Consideration may be fixed, variable, or a combination of both. The Company then allocates the transaction price to each performance obligation based on the relative standalone selling prices (“SSP”). SSP is determined at contract inception and is not updated to reflect changes between contract inception and when the performance obligations are satisfied. In developing the SSP for a performance obligation, the Company considers applicable market conditions and relevant entity-specific factors, including factors that were contemplated in negotiating the agreement with the customer and estimated costs. If the consideration promised in a contract includes a variable amount, the Company estimates the amount of consideration to which it will be entitled by using the expected value method or the most likely amount method. The Company includes the unconstrained amount of estimated variable consideration in the transaction price. The amount included in the transaction price is constrained to the amount for which it is probable that a significant reversal of cumulative revenue recognized will not occur. At the end of each subsequent reporting period, the Company re-evaluates the estimated variable consideration included in the transaction price and any related constraint, and if necessary, adjusts its estimate of the overall transaction price. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied, either at a point in time or over time, and if over time recognition is based on the use of an output or input method. Amounts received prior to satisfying the revenue recognition criteria are recorded as deferred revenue in the Company’s consolidated balance sheets. If the related performance obligation is expected to be satisfied within the next twelve months this will be classified in current liabilities. Amounts recognized as revenue prior to receipt are recorded as other current assets in the Company's balance sheets. If the Company expects to have an unconditional right to receive the consideration in the next twelve months, this will be classified in other current assets. Research and Development Costs Research and development costs are expensed as incurred. Research and development expenses consist of costs incurred in performing research and development activities, including salaries and bonuses, stock‑based compensation, employee benefits, facilities costs, laboratory supplies, depreciation, manufacturing expenses and external costs of vendors engaged to conduct preclinical development activities and clinical trials, as well as the cost of licensing technology. Upfront payments and milestone payments made for the licensing of technology are expensed as research and development in the period in which they are incurred. Advance payments for goods or services to be received in the future for use in research and development activities are recorded as prepaid expenses. The prepaid amounts are expensed as the related goods are delivered or the services are performed. Research and Manufacturing Contract Costs and Accruals The Company has entered into various research and development and manufacturing contracts. These agreements are generally cancelable, and related payments are recorded as the corresponding expenses are incurred. The Company records accruals for estimated ongoing costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the research studies or clinical trials and manufacturing activities, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates may be required in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs. Patent Costs All patent‑related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses. Income Taxes Deferred tax assets and liabilities are recognized for the expected future tax consequences of events that have been included in the consolidated financial statements or tax returns. Under this method, deferred tax assets and liabilities are determined based on the difference between the financial statement and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. A valuation allowance is provided to reduce the deferred tax asset to an amount, which, more likely than not, will be realized. The Company recognizes the tax benefit from any uncertain tax positions only if it is more likely than not that the tax position will be sustained on examination by the taxing authorities, based on the technical merits of the position. The tax benefits recognized in the consolidated financial statements from such a position are measured based on the largest benefit that has a greater than fifty percent likelihood of being realized upon ultimate settlement. Fair Value Measurements Certain assets and liabilities are carried at fair value. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1 – Unadjusted quoted prices in active markets that are accessible to the reporting entity at the measurement date for identical assets and liabilities. • Level 2 – Inputs other than quoted prices in active markets for identical assets and liabilities that are observable either directly or indirectly for substantially the full term of the asset or liability. Level 2 inputs include the following: • quoted prices for similar assets and liabilities in active markets • quoted prices for identical or similar assets or liabilities in markets that are not active • observable inputs other than quoted prices that are used in the valuation of the asset or liabilities (e.g., interest rate and yield curve quotes at commonly quoted intervals) • inputs that are derived principally from or corroborated by observable market data by correlation or other means • Level 3 – Unobservable inputs for the assets or liability (i.e., supported by little or no market activity). Level 3 inputs include management’s own assumptions about the assumptions that market participants would use in pricing the asset or liability (including assumptions about risk). The carrying amount of the Company’s other financial assets and liabilities including cash, accounts payable and long-term debt approximate fair value because of the relatively short period of time between origination and expected realization or settlement. Net Loss Per Share Basic net loss per share is computed by dividing the net loss by the weighted average number of shares of common stock outstanding for the period. Diluted net loss is computed by adjusting net loss to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net loss per share is computed by dividing the diluted net loss by the weighted average number of shares of common stock outstanding for the period, including potential dilutive common shares assuming the dilutive effect of common stock equivalents. The following table presents securities that have been excluded from the computations of diluted weighted-average shares outstanding as they would be anti-dilutive due to the net losses: As of December 31, 2020 2019 Options to purchase common stock 6,978,447 7,235,581 Unvested restricted stock units 316,249 50,000 Unvested restricted common stock — 1,250 7,294,696 7,286,831 Stock-Based Compensation For stock-based awards, the Company measures the estimated fair value of the stock-based award on the date of grant and recognizes compensation expense for those awards over the requisite service period, which is generally the vesting period of the respective award. For stock-based awards with service-based vesting conditions, the Company records the expense for these awards using the straight-line method. For stock options with performance-based vesting conditions, the Company records the expense for these awards over the requisite service period using an accelerated attribution method to the extent the achievement of the performance condition is probable. The Company accounts for forfeitures as they occur. The Company classifies stock-based compensation expense in its consolidated statements of operations in the same manner in which the award recipient’s cash compensation costs are classified. Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity that result from transactions and economic events other than those with stockholders. The Company’s comprehensive net loss equals the reported net loss for all periods presented. Subsequent events The Company evaluates events and/or transactions occurring after the balance sheet date and before the issue date of the financial statements to determine if any of those events and/or transactions require adjustment to or disclosure in the financial statements. Accounting Pronouncements Issued and Not Adopted In February 2016, the FASB issued ASU No. 2016-02, Leases

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements The following tables set forth by level, within the fair value hierarchy, the assets and liabilities carried at fair value on a recurring basis (in thousands): Fair Value Measurement as of December 31, 2020 Level 1 Level 2 Level 3 Total Assets: Money market funds included within cash and cash equivalents $ 45,410 — — $ 45,410 Total $ 45,410 — — $ 45,410 Fair Value Measurement as of December 31, 2019 Level 1 Level 2 Level 3 Assets: Money market funds included within cash and cash equivalents $ 25,304 — — Total $ 25,304 — — The fair value of money market funds was measured by the Company based on quoted market prices. There were no transfers among Level 1, Level 2, or Level 3 categories in the periods presented. Financial Instruments Not Recorded at Fair Value The carrying value of cash, cash equivalents, restricted cash, accounts payable and accrued expenses that are reported on the consolidated balance sheets approximate their fair value due to the short-term nature of these assets and liabilities. The carrying value of the long-term debt approximates fair value as evidenced by the recent refinancings.

Property and Equipment, net

Property and Equipment, net	12 Months Ended
	Dec. 31, 2020
Property Plant And Equipment [Abstract]
Property and Equipment, net	4. Property and Equipment, net Property and equipment consist of the following (in thousands): As of December 31, 2020 2019 Laboratory equipment $ 6,843 $ 4,526 Office and computer equipment 1,590 1,418 Leasehold improvements 1,910 687 Construction in process 1,612 2,650 Property and equipment – at cost 11,955 9,281 Less accumulated depreciation and amortization (3,493 ) (2,539 ) Property and equipment – net $ 8,462 $ 6,742 Depreciation and amortization expense was $1.8 million and $1.3 million for the years ended December 31, 2020 and 2019, respectively. During the year ended December 31, 2020, the Company recorded gross fixed asset disposal of $1.1 million and related accumulated depreciation of $0.9 million. There were no disposals for the year ended December 31, 2019.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	12 Months Ended
	Dec. 31, 2020
Payables And Accruals [Abstract]
Accrued Expenses and Other Current Liabilities	5. Accrued Expenses and Other Current Liabilities Accrued expenses and other current liabilities consisted of the following (in thousands): As of December 31, 2020 2019 Payroll and benefits $ 2,943 $ 2,426 Consulting service 1,049 230 Legal service 173 171 Research and development 3,010 4,259 Deferred revenue 498 — Interest 181 — Capital lease payable – short term — 68 Other 782 1,207 $ 8,636 $ 8,361

Debt Financing

Debt Financing	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Debt Financing	6. Debt Financing 2019 Credit Facility In December 2019, the Company repaid all borrowings under the 2019 Credit Facility. The aggregate principal amount of the loan outstanding at the time of repayment was $15.0 million. The Company also paid the prepayment fee of 2% totaling $0.3 million. The Company recognized a loss on the extinguishment of the 2019 Credit Facility of $0.4 million related to the unamortized debt discount and prepayment fee at the time of repayment. 2019 Credit Agreement On December 31, 2019, the Company entered into a Credit Agreement (the “Credit Agreement”) with Hercules Capital, Inc. (the “Lender”). Under the Credit Agreement, the Company borrowed $22.5 million, and the Company has the option to draw down an additional $12.5 million if certain milestones and conditions are met. The Company incurred fees of $0.3 million, which was paid to the lender on the closing date. These amounts were recorded as a debt discount and are being amortized as interest expense using the effective interest method over the life of the Credit Agreement. The Credit Agreement also includes an end of term charge equal to 7.55% of the aggregate principal amount of all advances. The end of term charge, totaling $1.7 million at December 31, 2019, was recognized as a debt discount and is reflected as a reduction in the carrying value of the debt and recorded in other long-term liabilities. The debt discount created by the end of term charge is being accreted and will be recognized as additional interest expense over the term of the Credit Agreement using the effective interest method. The Credit Agreement contains customary representations and warranties, events of default and affirmative and negative covenants, including, among others, covenants that limit or restrict the Company’s ability to, among other things, incur additional indebtedness, merge or consolidate, make acquisitions, pay dividends or other distributions or repurchase equity, make investments, dispose of assets and enter into certain transactions with affiliates, in each case subject to certain exceptions. As security for its obligations under the Credit Agreement, the Company granted the Lender a first priority security interest on substantially all of the Company’s assets (other than intellectual property), and subject to certain exceptions. The outstanding principal under the Credit Agreement has a 48-month term with interest only payments for the first 15 months, which period can be extended to up to 24 months, depending on the achievement of certain performance milestones. The principal bears an interest rate of equal to the greater of (i) 8.95% plus the prime rate minus 4.75% and (ii) 8.95%. The Credit Agreement includes mandatory prepayment provisions that require prepayment upon the occurrence of a change in control event. On June 15, 2020, the Company entered into a Second Amendment to Loan and Security Agreement (the “Amendment”). The Amendment was entered into for the primary purpose of amending the Credit Agreement as follows: (i) the second tranche of the term loan (the “Term Loan”) is terminated; (ii) amounts available under Tranche 3 of the Term Loan is increased to $12.5 million from the previous $7.5 million availability amount and its availability period is extended through December 15, 2021, subject to future approval by the Agent’s investment committee; (iii) the interest-only period is extended through July 31, 2021; (iv) the interest rate on borrowings is increased by 0.4%, such that the per annum interest rate on outstanding borrowings will be the greater of (a) 9.35% and (b) 9.35% plus the Wall Street Journal prime rate minus 3.25%; (v) the variable amount and duration of a minimum cash covenant in the Agreement are amended. The interest rate at December 31, 2020 is 9.35%. The Amendment has been accounted for as a modification and the Company incurred fees of $79,000, which was paid to the lender on the closing date and were recorded as a debt discount, which will be amortized as interest expense using the effective interest method. Future principal payments under the Credit Agreement as of December 31, 2020 are as follows (in thousands): 2021 3,379 2022 8,686 2023 9,547 2024 888 Total future principal payments 22,500 Less unamortized debt discount 1,491 Total balance $ 21,009

Commitments and contingencies

Commitments and contingencies	12 Months Ended
	Dec. 31, 2020
Commitments And Contingencies Disclosure [Abstract]
Commitments and contingencies	7. Commitments and contingencies In March 2018, the Company entered into a non-cancelable ten-year Rent expense for the years ended December 31, 2020 and 2019 was $6.9 million and $4.5 million, respectively. Future minimum lease payments under the non-cancelable operating leases consisted of the following as of December 31, 2020 (in thousands): Year Ending December 31, 2021 6,026 2022 6,207 2023 6,393 2024 6,584 2025 6,782 Thereafter 25,502 $ 57,494

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2020
Stockholders Equity Note [Abstract]
Stockholders' Equity	8. Stockholders’ Equity Stock-based compensation 2015 Stock Incentive Plan The Company’s 2015 Stock Incentive Plan (the “2015 Plan”) provided for the Company to sell or issue incentive stock options or nonqualified stock options, restricted stock, and other equity awards to employees, directors and consultants of the Company. The 2019 Stock Option and Incentive Plan (the “2019 Plan”) became effective in February 27, 2019. Upon effectiveness of the 2019 Plan, the remaining shares available under the 2015 Plan ceased to be available for issuance and no future issuances will be made under the 2015 Plan. The 2019 Plan provides for the grant of incentive stock options, nonqualified stock options, stock appreciation rights, restricted stock units, restricted stock awards, unrestricted stock awards, cash-based awards and dividend equivalent rights to the Company’s officers, employees, directors and consultants. Awards generally vest over a period up to 4 years and have a maximum term of 10 years. The number of shares initially reserved for issuance under the 2019 Plan is 2,168,976, has increased on January 1, 2020 and will continue to increase each January 1 thereafter by 4% of the number of shares of the Company’s common stock outstanding on the immediately preceding December 31 or such lesser number of shares determined by the Company’s board of directors or compensation committee of the board of directors. There were 2,923,603 shares available for future issuance at December 31, 2020. 2019 Employee Stock Purchase Plan The 2019 Employee Stock Purchase Plan (the “2019 ESPP”) became effective on February 27, 2019. A total of 180,748 shares of common stock were reserved for issuance under this plan. In addition, the number of shares of common stock that may be issued under the ESPP automatically increased on January 1, 2020, and will continue to increase each January 1 thereafter, by the lesser of (i) 542,244 shares of common stock, (ii) 1% of the number of shares of the Company’s common stock outstanding on the immediately preceding December 31 or (iii) such lesser number of shares as determined by the administrator of the 2019 ESPP. No shares were issued under the 2019 ESPP in 2019. There were 482,025 shares available for future issuance at December 31, 2020. Stock Option Valuation The fair value of stock option grants is estimated using the Black-Scholes option-pricing model. The Company historically has been a private company and lacks company-specific historical and implied volatility information. Therefore, it estimates its expected stock volatility based on the historical volatility of a publicly traded set of peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded stock price. For options with service-based vesting conditions, the expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. The expected term of stock options granted to non-employees is equal to the contractual term of the option award. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future. The Company typically grants stock options at exercise prices deemed by the Board to be equal to the fair value of the common stock at the time of grant. In the periods prior to the IPO, the fair value of the common stock has been determined by the Board at each measurement date based on a variety of different factors, including the results obtained from independent third-party appraisals, the Company’s financial position and historical financial performance, the status of development of the Company’s programs, the current climate in the marketplace, the illiquid nature of the common stock, the effect of the rights and preferences of the preferred stockholders, and the prospects of a liquidity event, among others. In the periods following the IPO, the fair value is determined based upon the quoted price of the Company’s common stock. The assumptions that the Company used to determine the grant-date fair value of options granted were as follows: Years Ended December 31, 2020 2019 Expected volatility 83.5% - 105.5% 66.1% - 84% Risk-free interest rate 0.15% - 1.41% 1.42% - 2.54% Expected term (in years) 1.39-6.32 5.50-6.25 Expected dividend yield — % — % Stock Options Activity A summary of the Company’s stock option activity and related information is as follows: Options Weighted Average Exercise Price Weighted Average Remaining Life (in Years) Aggregate Intrinsic Value Outstanding as of January 1, 2020 7,235,581 $ 8.21 8.7 $ 5,075 Granted 1,781,932 7.66 Exercised (597,416 ) 4.45 Canceled (1,441,650 ) 9.06 Outstanding as of December 31, 2020 6,978,447 $ 8.19 7.45 $ 15,565 Options exercisable as of December 31, 2020 3,154,929 7.76 5.84 $ 9,318 Options vested or expected to vest as of December 31, 2020 6,978,447 8.19 7.45 $ 15,565 The weighted-average grant date fair value of the options granted during the years ended December 31, 2020 and 2019 was $5.51 and $5.96 per share, respectively. As of December 31, 2020, there was $19.4 million of unrecognized compensation expense for stock options, which the Company expects to recognize over the weighted-average remaining term of 2.64 years. Restricted Stock Unit Activity A summary of the Company’s restricted stock unit activity and related information is as follows: Options Weighted Average Grant Date Fair Value Per Share Outstanding as of January 1, 2020 50,000 $ 6.56 Granted 270,753 6.79 Canceled (4,504 ) 6.00 Outstanding as of December 31, 2020 316,249 $ 6.76 As of December 31, 2020, there was $1.4 million of unrecognized compensation expense for the restricted stock units, which the Company expects to recognize over the weighted-average remaining term of 2.24 years. Stock- Based Compensation Expense The Company recorded stock-based compensation expense in the following expense categories of its consolidated statements of operations: Year Ended December 31, 2020 2019 Research and development $ 3,820 $ 3,245 General and administrative 8,864 6,823 $ 12,684 $ 10,068 On July 20, 2020, the Company entered into an amendment to the employment agreement with the former CEO, which amended an existing employment agreement and provided for changes in the term of service and compensation under the agreement. The outstanding options and restricted stock units held by the former CEO were modified to accelerate certain vesting provisions and extended the period of exercisability. As a result, the Company recorded stock-based compensation expense of $1.9 million related to the incremental fair value of the modified awards during the third quarter of 2020.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Income Taxes	9. Income Taxes There is no provision for income taxes because the Company has historically incurred net operating losses and maintains a full valuation allowance against its deferred tax assets. Significant components of the Company’s net deferred tax assets at December 31, 2020 and 2019 are as follows: Years ended December 31, 2020 2019 Deferred tax assets Stock-based compensation $ 5,484 $ 3,754 Net operating loss carryforwards 64,939 45,374 Credit carryforwards 7,988 6,519 Intangible assets 164 180 Charitable contributions 1 1 Accrued expenses 1,314 1,125 Total deferred tax assets 79,890 56,953 Valuation allowance (79,610 ) (56,817 ) Total net deferred tax assets 280 136 Deferred tax liabilities: Fixed assets (280 ) (136 ) Total net deferred tax liability (280 ) (136 ) Total deferred tax assets (liability) $ — $ — A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows: Years ended December 31, 2020 2019 Federal income tax expense at statutory rate 21.0 % 21.0 % Stock compensation expense (1.1 ) (0.5 ) Fair value change in warrant liability — 0.1 Permanent differences — (0.1 ) Federal research and development credit 1.2 1.7 State research and development credit 0.6 0.9 State income tax, net of federal benefit 6.0 6.3 Other — 0.7 Change in valuation allowance (27.7 ) (30.1 ) Effective income tax rate — % — % As of December 31, 2020, the Company had net operating loss (NOL) carryforwards for U.S. federal and state tax purposes of $238.5 million and $235 million, respectively. Federal NOLs of $38.8 million, generated before 2018, will begin expiring in varying amounts in 2035 unless utilized and the remaining NOL of $199.7 million, generated after 2018 will be carried forward indefinitely and could be used up to 80% of taxable income of each future tax year. The Commonwealth of Massachusetts does not follow federal on NOL carryforwards and as such the Company’s Massachusetts NOLs of $235 million will expire in at various times starting in 2035. As of December 31, 2020, the Company also has federal research and development tax credit carryforwards of approximately $ million, and state research and development tax credit carryforwards of approximately $ 3.2 million, which may be available to reduce future tax liabilities, and which expire at various dates through 20 40 . 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 net operating loss carryforwards research and development tax credit carryforwards and capitalized expenditures. Under the applicable accounting standards, management has considered the Company’s history of cumulative net losses incurred since inception and its lack of commercialization of any products or generation of any revenue from product sales since inception and concluded that it is more likely than not that the Company will not recognize the benefits of its federal and state deferred tax assets. Accordingly, a full valuation allowance has been established, and the valuation allowance increased $22.8 million and $26.0 million in the years ended December 31, 2020 and 2019, respectively. Utilization of the net operating loss and research and development credit carryforwards may be subject to a substantial annual limitation under Section 382 of the Code due to ownership change limitations that have occurred previously or that could occur in the future. These ownership changes may limit the amount of net operating loss and research and development credit carryforwards that can be utilized annually to offset future taxable income and tax, respectively. The Company has not completed a study to assess whether a change of ownership has occurred, or whether there have been multiple ownership changes since its formation, due to the significant cost and complexity associated with a study. There could also be additional ownership changes in the future which may result in additional limitations on the utilization of net operating loss carryforwards and tax credits. The Company files 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 examination by federal and state jurisdictions, where applicable. There are currently no pending tax examinations. Since the Company is in a loss carryforward position, it is generally subject to examination by the U.S. federal, state, and local income tax authorities for all tax years in which a loss carryforward is available. The Company’s policy is to record estimated interest and penalties related to uncertain tax positions in income tax expense. The Company has no amounts recorded for any unrecognized tax positions, accrued interest or penalties as of December 31, 2020 and 2019.

Revenue

Revenue	12 Months Ended
	Dec. 31, 2020
Revenue From Contract With Customer [Abstract]
Revenue	10. Revenue In December 2019, the Company entered into a research collaboration agreement with Janssen’s World Without Disease Accelerator (“Janssen”), part of the Janssen Pharmaceutical Companies of Johnson & Johnson, to explore the potential for a MMT to prevent the onset of childhood allergy and other atopic, immune and metabolic conditions. The collaboration includes three milestones of research and development with results of those efforts due to Janssen at end of the milestone at which point Janssen will have 60 days to decide to proceed with the next milestone. The Company assessed the Janssen collaboration agreement in accordance with ASC 808 and ASC 606 and concluded that the arrangement represents a contract with a customer and is within the scope of ASC 606. The promised goods and services represent one combined performance obligation and the entire transaction price will be allocated to that single combined performance obligation. In addition, the Company concluded the right to proceed with the following milestones does not provide any discounts to Janssen if it decides to proceed with the next milestones. As such, the Company concluded the milestone is not considered to be a material right. Each milestone is considered a separate contract and reflects applicable standalone selling prices. Under the Janssen collaboration agreement, Janssen is obligated to reimburse the Company for the costs incurred under an agreed upon research plan. Costs incurred are billed by the Company to Janssen at completion of each milestone. The Company recognizes revenue over the research period and as the performance obligation is satisfied. using total estimated hours to be incurred throughout each milestone. For the year ended December 31, 2020, the Company recognized $1.0 million as collaboration revenue under the agreement. Further, as of December 31, 2020, the Company recorded $ 498 as deferred revenue within other current liabilities on the Company’s consolidated balance sheets related to the Janssen collaboration agreement. The expected research term of this arrangement is expected to be completed during 2021 , and the aggregate consideration is not expected to be material .

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”).
Principles of Consolidation	Principles of Consolidation The accompanying consolidated financial statements reflect the operations of the Company and its wholly owned subsidiaries. All intercompany accounts and transactions have been eliminated.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenue and expenses during the reporting periods. Actual results may differ from those estimates.
Cash and Cash Equivalents	Cash and Cash Equivalents Cash includes cash in readily available checking accounts. The Company’s cash deposits on hand at one financial institution often exceed federally insured limits. Cash equivalents include all highly liquid investments maturing within 90 days from the date of purchase.
Restricted Cash	Restricted Cash Restricted cash is cash that is restricted as to withdrawal or use under the terms of certain contractual agreements. The restricted cash consists of cash collateral for secured letters of credit for the security deposit on the Company’s leased laboratory and office facilities.
Concentrations of Credit Risk and of Significant Suppliers	Concentrations of Credit Risk and of Significant Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash and cash equivalents. The Company’s cash equivalents as of December 31, 2020 consisted only of money market funds. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company relies, and expects to continue to rely, on a small number of vendors to manufacture supplies and raw materials for its development programs. These programs could be adversely affected by a significant interruption in these manufacturing services or the availability of raw materials.
Property and Equipment	Property and Equipment Property and equipment are stated at cost and depreciated using the straight-line method over the estimated useful lives of the respective assets. Laboratory and office equipment, computer equipment and furniture and fixtures are depreciated over a period of five years, and leasehold improvements are amortized over the lesser of the asset’s estimated useful life or the remaining lease term. Major additions and betterments are capitalized; maintenance and repairs, which do not improve or extend the life of the respective assets, are charged to operating expenses as incurred.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets Long-lived assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of the asset may not be recoverable. When such events occur, the Company compares the carrying amounts of the assets to their undiscounted expected future cash flows. If this comparison indicates that there is an impairment, the amount of the impairment is calculated as the difference between the carrying value and the fair value. The Company has not recorded any impairment charges in the periods presented.
Segment Information	Segment Information Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker, or decision-making group, in making decisions on how to allocate resources and assess performance. The Company’s chief operating decision maker is the chief executive officer (“CEO”). The Company and CEO view the Company’s operations and manage its business as one operating segment.
Collaboration Revenue	Collaboration Revenue The Company analyzes its collaboration arrangements to assess whether they are within the scope of ASC Topic 808, Collaborative Arrangements Revenue from Contracts with Customers The Company recognizes revenue after applying the following five steps: 1) Identification of the contract, or contracts, with a customer, 2) Identification of the performance obligations in the contract, including whether they are distinct within the context of the contract 3) Determination of the transaction price, including the constraint on variable consideration 4) Allocation of the transaction price to the performance obligations in the contract 5) Recognition of revenue when, or as, performance obligations are satisfied If a contract is determined to be within the scope of ASC 606 at inception, the Company assesses the goods or services promised within such contract, determines which of those goods and services are performance obligations, and assesses whether each promised good or service is distinct. Promised goods and services are considered distinct provided that: (i) the customer can benefit from the good or service either on its own or together with other resources that are readily available to the customer (that is, the good or service is capable of being distinct) and (ii) the entity’s promise to transfer the good or service to the customer is separately identifiable from other promises in the contract (that is, the promise to transfer the good or service is distinct within the context of the contract). In assessing whether a promised good or service is distinct, the Company considers factors such as the research, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. The Company also considers the intended benefit of the contract in assessing whether a promised good or service is separately identifiable from other promises in the contract. If a promised good or service is not distinct, an entity is required to combine that good or service with other promised goods or services until it identifies a bundle of goods or services that is distinct. The Company may provide options to additional goods or services in such arrangements exercisable at a customer’s discretion. The Company assesses if these options provide a material right to the customer and if so, they are considered performance obligations. The identification of material rights requires judgments related to the determination of the value of the underlying good and services to the option exercise price. The Company determines transaction price based on the amount of consideration the Company expects to receive for transferring the promised goods or services in the contract. Consideration may be fixed, variable, or a combination of both. The Company then allocates the transaction price to each performance obligation based on the relative standalone selling prices (“SSP”). SSP is determined at contract inception and is not updated to reflect changes between contract inception and when the performance obligations are satisfied. In developing the SSP for a performance obligation, the Company considers applicable market conditions and relevant entity-specific factors, including factors that were contemplated in negotiating the agreement with the customer and estimated costs. If the consideration promised in a contract includes a variable amount, the Company estimates the amount of consideration to which it will be entitled by using the expected value method or the most likely amount method. The Company includes the unconstrained amount of estimated variable consideration in the transaction price. The amount included in the transaction price is constrained to the amount for which it is probable that a significant reversal of cumulative revenue recognized will not occur. At the end of each subsequent reporting period, the Company re-evaluates the estimated variable consideration included in the transaction price and any related constraint, and if necessary, adjusts its estimate of the overall transaction price. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied, either at a point in time or over time, and if over time recognition is based on the use of an output or input method. Amounts received prior to satisfying the revenue recognition criteria are recorded as deferred revenue in the Company’s consolidated balance sheets. If the related performance obligation is expected to be satisfied within the next twelve months this will be classified in current liabilities. Amounts recognized as revenue prior to receipt are recorded as other current assets in the Company's balance sheets. If the Company expects to have an unconditional right to receive the consideration in the next twelve months, this will be classified in other current assets.
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred. Research and development expenses consist of costs incurred in performing research and development activities, including salaries and bonuses, stock‑based compensation, employee benefits, facilities costs, laboratory supplies, depreciation, manufacturing expenses and external costs of vendors engaged to conduct preclinical development activities and clinical trials, as well as the cost of licensing technology. Upfront payments and milestone payments made for the licensing of technology are expensed as research and development in the period in which they are incurred. Advance payments for goods or services to be received in the future for use in research and development activities are recorded as prepaid expenses. The prepaid amounts are expensed as the related goods are delivered or the services are performed.
Research and Manufacturing Contract Costs and Accruals	Research and Manufacturing Contract Costs and Accruals The Company has entered into various research and development and manufacturing contracts. These agreements are generally cancelable, and related payments are recorded as the corresponding expenses are incurred. The Company records accruals for estimated ongoing costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the research studies or clinical trials and manufacturing activities, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates may be required in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs.
Patent Costs	Patent Costs All patent‑related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses.
Income Taxes	Income Taxes Deferred tax assets and liabilities are recognized for the expected future tax consequences of events that have been included in the consolidated financial statements or tax returns. Under this method, deferred tax assets and liabilities are determined based on the difference between the financial statement and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. A valuation allowance is provided to reduce the deferred tax asset to an amount, which, more likely than not, will be realized. The Company recognizes the tax benefit from any uncertain tax positions only if it is more likely than not that the tax position will be sustained on examination by the taxing authorities, based on the technical merits of the position. The tax benefits recognized in the consolidated financial statements from such a position are measured based on the largest benefit that has a greater than fifty percent likelihood of being realized upon ultimate settlement.
Fair Value Measurements	Fair Value Measurements Certain assets and liabilities are carried at fair value. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable: • Level 1 – Unadjusted quoted prices in active markets that are accessible to the reporting entity at the measurement date for identical assets and liabilities. • Level 2 – Inputs other than quoted prices in active markets for identical assets and liabilities that are observable either directly or indirectly for substantially the full term of the asset or liability. Level 2 inputs include the following: • quoted prices for similar assets and liabilities in active markets • quoted prices for identical or similar assets or liabilities in markets that are not active • observable inputs other than quoted prices that are used in the valuation of the asset or liabilities (e.g., interest rate and yield curve quotes at commonly quoted intervals) • inputs that are derived principally from or corroborated by observable market data by correlation or other means • Level 3 – Unobservable inputs for the assets or liability (i.e., supported by little or no market activity). Level 3 inputs include management’s own assumptions about the assumptions that market participants would use in pricing the asset or liability (including assumptions about risk). The carrying amount of the Company’s other financial assets and liabilities including cash, accounts payable and long-term debt approximate fair value because of the relatively short period of time between origination and expected realization or settlement.
Net Loss Per Share	Net Loss Per Share Basic net loss per share is computed by dividing the net loss by the weighted average number of shares of common stock outstanding for the period. Diluted net loss is computed by adjusting net loss to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net loss per share is computed by dividing the diluted net loss by the weighted average number of shares of common stock outstanding for the period, including potential dilutive common shares assuming the dilutive effect of common stock equivalents. The following table presents securities that have been excluded from the computations of diluted weighted-average shares outstanding as they would be anti-dilutive due to the net losses: As of December 31, 2020 2019 Options to purchase common stock 6,978,447 7,235,581 Unvested restricted stock units 316,249 50,000 Unvested restricted common stock — 1,250 7,294,696 7,286,831
Stock-Based Compensation	Stock-Based Compensation For stock-based awards, the Company measures the estimated fair value of the stock-based award on the date of grant and recognizes compensation expense for those awards over the requisite service period, which is generally the vesting period of the respective award. For stock-based awards with service-based vesting conditions, the Company records the expense for these awards using the straight-line method. For stock options with performance-based vesting conditions, the Company records the expense for these awards over the requisite service period using an accelerated attribution method to the extent the achievement of the performance condition is probable. The Company accounts for forfeitures as they occur. The Company classifies stock-based compensation expense in its consolidated statements of operations in the same manner in which the award recipient’s cash compensation costs are classified.
Comprehensive Loss	Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity that result from transactions and economic events other than those with stockholders. The Company’s comprehensive net loss equals the reported net loss for all periods presented.
Subsequent Events	Subsequent events The Company evaluates events and/or transactions occurring after the balance sheet date and before the issue date of the financial statements to determine if any of those events and/or transactions require adjustment to or disclosure in the financial statements.
Accounting Pronouncements Issued and Not Adopted	Accounting Pronouncements Issued and Not Adopted In February 2016, the FASB issued ASU No. 2016-02, Leases

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Schedule of Securities Excluded from Computation of Diluted Weighted-Average Shares Outstanding	The following table presents securities that have been excluded from the computations of diluted weighted-average shares outstanding as they would be anti-dilutive due to the net losses: As of December 31, 2020 2019 Options to purchase common stock 6,978,447 7,235,581 Unvested restricted stock units 316,249 50,000 Unvested restricted common stock — 1,250 7,294,696 7,286,831

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Schedule of Assets and Liabilities Carried at Fair Value on a Recurring Basis	The following tables set forth by level, within the fair value hierarchy, the assets and liabilities carried at fair value on a recurring basis (in thousands): Fair Value Measurement as of December 31, 2020 Level 1 Level 2 Level 3 Total Assets: Money market funds included within cash and cash equivalents $ 45,410 — — $ 45,410 Total $ 45,410 — — $ 45,410 Fair Value Measurement as of December 31, 2019 Level 1 Level 2 Level 3 Assets: Money market funds included within cash and cash equivalents $ 25,304 — — Total $ 25,304 — —

Property and Equipment, net (Ta

Property and Equipment, net (Tables)	12 Months Ended
	Dec. 31, 2020
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment consist of the following (in thousands): As of December 31, 2020 2019 Laboratory equipment $ 6,843 $ 4,526 Office and computer equipment 1,590 1,418 Leasehold improvements 1,910 687 Construction in process 1,612 2,650 Property and equipment – at cost 11,955 9,281 Less accumulated depreciation and amortization (3,493 ) (2,539 ) Property and equipment – net $ 8,462 $ 6,742

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	12 Months Ended
	Dec. 31, 2020
Payables And Accruals [Abstract]
Schedule of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of the following (in thousands): As of December 31, 2020 2019 Payroll and benefits $ 2,943 $ 2,426 Consulting service 1,049 230 Legal service 173 171 Research and development 3,010 4,259 Deferred revenue 498 — Interest 181 — Capital lease payable – short term — 68 Other 782 1,207 $ 8,636 $ 8,361

Debt Financing (Tables)

Debt Financing (Tables)	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Summary of Future Principal Payments	Future principal payments under the Credit Agreement as of December 31, 2020 are as follows (in thousands): 2021 3,379 2022 8,686 2023 9,547 2024 888 Total future principal payments 22,500 Less unamortized debt discount 1,491 Total balance $ 21,009

Commitments and contingencies (

Commitments and contingencies (Tables)	12 Months Ended
	Dec. 31, 2020
Commitments And Contingencies Disclosure [Abstract]
Future Minimum Lease Payments	Future minimum lease payments under the non-cancelable operating leases consisted of the following as of December 31, 2020 (in thousands): Year Ending December 31, 2021 6,026 2022 6,207 2023 6,393 2024 6,584 2025 6,782 Thereafter 25,502 $ 57,494

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2020
Stockholders Equity Note [Abstract]
Assumptions to Determine Grant-Date Fair Value of Options	The assumptions that the Company used to determine the grant-date fair value of options granted were as follows: Years Ended December 31, 2020 2019 Expected volatility 83.5% - 105.5% 66.1% - 84% Risk-free interest rate 0.15% - 1.41% 1.42% - 2.54% Expected term (in years) 1.39-6.32 5.50-6.25 Expected dividend yield — % — %
Stock Option Activity	A summary of the Company’s stock option activity and related information is as follows: Options Weighted Average Exercise Price Weighted Average Remaining Life (in Years) Aggregate Intrinsic Value Outstanding as of January 1, 2020 7,235,581 $ 8.21 8.7 $ 5,075 Granted 1,781,932 7.66 Exercised (597,416 ) 4.45 Canceled (1,441,650 ) 9.06 Outstanding as of December 31, 2020 6,978,447 $ 8.19 7.45 $ 15,565 Options exercisable as of December 31, 2020 3,154,929 7.76 5.84 $ 9,318 Options vested or expected to vest as of December 31, 2020 6,978,447 8.19 7.45 $ 15,565
Restricted Stock Unit Activity and Related Information	A summary of the Company’s restricted stock unit activity and related information is as follows: Options Weighted Average Grant Date Fair Value Per Share Outstanding as of January 1, 2020 50,000 $ 6.56 Granted 270,753 6.79 Canceled (4,504 ) 6.00 Outstanding as of December 31, 2020 316,249 $ 6.76
Allocated Stock-based Compensation Expense	The Company recorded stock-based compensation expense in the following expense categories of its consolidated statements of operations: Year Ended December 31, 2020 2019 Research and development $ 3,820 $ 3,245 General and administrative 8,864 6,823 $ 12,684 $ 10,068

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Schedule of Net Deferred Tax Assets and Liabilities	Significant components of the Company’s net deferred tax assets at December 31, 2020 and 2019 are as follows: Years ended December 31, 2020 2019 Deferred tax assets Stock-based compensation $ 5,484 $ 3,754 Net operating loss carryforwards 64,939 45,374 Credit carryforwards 7,988 6,519 Intangible assets 164 180 Charitable contributions 1 1 Accrued expenses 1,314 1,125 Total deferred tax assets 79,890 56,953 Valuation allowance (79,610 ) (56,817 ) Total net deferred tax assets 280 136 Deferred tax liabilities: Fixed assets (280 ) (136 ) Total net deferred tax liability (280 ) (136 ) Total deferred tax assets (liability) $ — $ —
Schedule of Effective Income Tax Rate Reconciliation	A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows: Years ended December 31, 2020 2019 Federal income tax expense at statutory rate 21.0 % 21.0 % Stock compensation expense (1.1 ) (0.5 ) Fair value change in warrant liability — 0.1 Permanent differences — (0.1 ) Federal research and development credit 1.2 1.7 State research and development credit 0.6 0.9 State income tax, net of federal benefit 6.0 6.3 Other — 0.7 Change in valuation allowance (27.7 ) (30.1 ) Effective income tax rate — % — %

Nature of the Business, Basis_2

Nature of the Business, Basis of Presentation, and Going Concern (Details) - USD ($)	Feb. 08, 2021	Aug. 04, 2020	Jul. 01, 2020	Jun. 04, 2020	Mar. 04, 2019	Feb. 28, 2021	Jan. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Conversion of Stock [Line Items]
Proceeds from initial public offering								$ 37,756,000	$ 67,761,000
Number of shares exercised								597,416
Net proceeds from exercise of shares								$ 2,790,000	630,000
Common stock, value, issued under the ATM sales agreement								36,000	30,000
Common shares, $0.001 par value, 150,000,000 shares authorized; 36,022,811 and 30,129,096 shares issued; 36,022,811 and 30,127,846 shares outstanding at December 31, 2020 and 2019, respectively								36,000	30,000
Net loss								81,620,000	86,331,000
Net cash used in operating activities								61,518,000	75,796,000
Cash and cash equivalents								46,222,000	$ 71,241,000
IPO
Conversion of Stock [Line Items]
Issuance of common stock (in shares)					5,000,000
Proceeds from initial public offering					$ 69,800,000
Offering costs payable					$ 3,800,000
Public Offering
Conversion of Stock [Line Items]
Issuance of common stock (in shares)				4,750,000
Net proceeds from offering				$ 33,200,000
Over-Allotment Option
Conversion of Stock [Line Items]
Number of shares exercised			185,000
Net proceeds from exercise of shares			$ 1,300,000
At-The-Market Sales Agreement
Conversion of Stock [Line Items]
Common stock, value, issued under the ATM sales agreement								361,299
Percentage of gross proceeds payable as commission		3.00%
Common shares, $0.001 par value, 150,000,000 shares authorized; 36,022,811 and 30,129,096 shares issued; 36,022,811 and 30,127,846 shares outstanding at December 31, 2020 and 2019, respectively								361,299
Net proceeds from the sale of common stock								3,400,000
Common stock, value, available under the ATM sales agreement								$ 46,500,000
At-The-Market Sales Agreement | Subsequent Event
Conversion of Stock [Line Items]
Common stock, value, issued under the ATM sales agreement						$ 309,656	$ 309,656
Common shares, $0.001 par value, 150,000,000 shares authorized; 36,022,811 and 30,129,096 shares issued; 36,022,811 and 30,127,846 shares outstanding at December 31, 2020 and 2019, respectively						309,656	309,656
Net proceeds from the sale of common stock						$ 4,900,000	$ 4,900,000
Public Offering and Over Allotment Option | Subsequent Event
Conversion of Stock [Line Items]
Issuance of common stock (in shares)	6,037,500
Net proceeds from offering	$ 65,300,000
Common Stock
Conversion of Stock [Line Items]
Conversion of stock, shares converted (in shares)					18,517,386				18,517,386
Number of shares exercised								597,416	445,160
Maximum | At-The-Market Sales Agreement
Conversion of Stock [Line Items]
Common stock, value, issued under the ATM sales agreement		$ 50,000,000
Common shares, $0.001 par value, 150,000,000 shares authorized; 36,022,811 and 30,129,096 shares issued; 36,022,811 and 30,127,846 shares outstanding at December 31, 2020 and 2019, respectively		$ 50,000,000

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Narrative (Details)	12 Months Ended
	Dec. 31, 2020
Laboratory and office equipment
Summary Of Significant Accounting Policies [Line Items]
Property, plant and equipment, useful life	5 years
Computer equipment
Summary Of Significant Accounting Policies [Line Items]
Property, plant and equipment, useful life	5 years
Furniture and fixtures
Summary Of Significant Accounting Policies [Line Items]
Property, plant and equipment, useful life	5 years

Summary of Significant Accoun_5

Summary of Significant Accounting Policies (Details) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities	7,294,696	7,286,831
Options to purchase common stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities	6,978,447	7,235,581
Unvested restricted stock units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities	316,249	50,000
Unvested restricted common stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities	0	1,250

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Assets and Liabilities Carried at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets, Fair Value Disclosure [Abstract]
Assets, fair value disclosure	$ 45,410
Level 1
Assets, Fair Value Disclosure [Abstract]
Assets, fair value disclosure	45,410	$ 25,304
Level 2
Assets, Fair Value Disclosure [Abstract]
Assets, fair value disclosure	0	0
Level 3
Assets, Fair Value Disclosure [Abstract]
Assets, fair value disclosure	0	0
Money market funds included within cash and cash equivalents
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents fair value disclosure	45,410
Money market funds included within cash and cash equivalents | Level 1
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents fair value disclosure	45,410	25,304
Money market funds included within cash and cash equivalents | Level 2
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents fair value disclosure	0	0
Money market funds included within cash and cash equivalents | Level 3
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents fair value disclosure	$ 0	$ 0

Fair Value Measurements - Narra

Fair Value Measurements - Narrative (Details)	12 Months Ended
	Dec. 31, 2020USD ($)
Fair Value Inputs Quantitative Information [Abstract]
Asset transfers Level 1 to Level 2	$ 0
Asset transfers Level 2 to Level 1	0
Asset transfers into Level 3	0
Asset transfers out of Level 3	0
Liabilities transfers Level 1 to Level 2	0
Liabilities transfers Level 2 to Level 1	0
Liability transfers into Level 3	0
Liability transfers out of Level 3	$ 0

Property and Equipment, net (De

Property and Equipment, net (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Property Plant And Equipment [Line Items]
Property and equipment – at cost	$ 11,955,000	$ 9,281,000
Less accumulated depreciation and amortization	(3,493,000)	(2,539,000)
Property and equipment – net	8,462,000	6,742,000
Depreciation and amortization	1,800,000	1,300,000
Gross fixed asset disposal	1,100,000	0
Accumulated depreciation related to fixed asset disposal	900,000
Laboratory equipment
Property Plant And Equipment [Line Items]
Property and equipment – at cost	6,843,000	4,526,000
Office and computer equipment
Property Plant And Equipment [Line Items]
Property and equipment – at cost	1,590,000	1,418,000
Leasehold improvements
Property Plant And Equipment [Line Items]
Property and equipment – at cost	1,910,000	687,000
Construction in process
Property Plant And Equipment [Line Items]
Property and equipment – at cost	$ 1,612,000	$ 2,650,000

Accrued Expenses and Other Cu_3

Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Payables And Accruals [Abstract]
Payroll and benefits	$ 2,943	$ 2,426
Consulting service	1,049	230
Legal service	173	171
Research and development	3,010	4,259
Deferred revenue	498
Interest	181
Capital lease payable – short term		68
Other	782	1,207
Accrued expenses and other current liabilities	$ 8,636	$ 8,361

Debt Financing - Narrative (Det

Debt Financing - Narrative (Details) - USD ($)	Jun. 15, 2020	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Line Of Credit Facility [Line Items]
Loss on extinguishment of debt			$ 0	$ (580,000)
2019 Credit Facility
Line Of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity		$ 15,000,000		15,000,000
Prepayment fee		2.00%
Prepayment fees, amount		$ 300,000
2019 Credit Facility | Line of Credit
Line Of Credit Facility [Line Items]
Loss on extinguishment of debt		(400,000)
2019 Credit Agreement
Line Of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity		22,500,000		$ 22,500,000
Option to draw down an additional amount		12,500,000
Line of credit facility fees amount payable		$ 300,000
Credit agreement end of term charge percentage		7.55%
Debt discount		$ 1,700,000
Credit agreement, expiration period		48 months
Credit agreement interest payment period		15 months
Credit agreement interest payment extension period		24 months
2019 Credit Agreement | Line of Credit
Line Of Credit Facility [Line Items]
Debt instrument effective percentage interest rate		8.95%		8.95%
Debt instrument, basis spread on variable rate		4.75%
Debt instrument, description of variable rate basis			The principal bears an interest rate of equal to the greater of (i) 8.95% plus the prime rate minus 4.75% and (ii) 8.95%.
Tranche 3 | Term Loan
Line Of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 12,500,000
Current borrowing capacity	$ 7,500,000
Debt instrument, maturity date	Dec. 15, 2021
Debt instrument, interest maturity date	Jul. 31, 2021
2020 Amendment | Term Loan
Line Of Credit Facility [Line Items]
Line of credit facility fees amount payable	$ 79,000
Debt instrument, basis spread on variable rate	9.35%
Debt instrument, description of variable rate basis			9.35% plus the Wall Street Journal prime rate minus 3.25%
2020 Amendment | Term Loan | Maximum
Line Of Credit Facility [Line Items]
Debt instrument effective percentage interest rate	0.40%

Debt Financing - Summary of Fut

Debt Financing - Summary of Future Principal Payments (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Line Of Credit Facility [Line Items]
Long term debt, net of unamortized debt discount	$ 18,375	$ 20,391
2019 Credit Agreement
Line Of Credit Facility [Line Items]
2021	3,379
2022	8,686
2023	9,547
2024	888
Total future principal payments	22,500
Less unamortized debt discount	1,491
Long term debt, net of unamortized debt discount	$ 21,009

Commitments and contingencies -

Commitments and contingencies - Facilities Leases (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Mar. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Operating lease, term of contract	10 years			10 years
Lease expiration	2029
Rent expense		$ 6.9	$ 4.5

Commitments and contingencies_2

Commitments and contingencies - Future Minimum Lease Payments (Details) $ in Thousands	Dec. 31, 2020USD ($)
Commitments And Contingencies Disclosure [Abstract]
2021	$ 6,026
2022	6,207
2023	6,393
2024	6,584
2025	6,782
Thereafter	25,502
Total future minimum lease payment due	$ 57,494

Stockholders' Equity - Stock-ba

Stockholders' Equity - Stock-based compensation (Details) - shares	Feb. 27, 2019	Dec. 31, 2020	Dec. 31, 2019
2019 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued (in shares)	2,168,976
Percent of outstanding shares	4.00%
Number of shares available for future issuance (in shares)		2,923,603
2019 Plan | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards vesting period		4 years
2019 Plan | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards vesting period		10 years
2019 Employee Stock Purchase Plan | Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent of outstanding shares	1.00%
Number of shares available for future issuance (in shares)	180,748	482,025
Number of additional shares allowable under the plan (in shares)	542,244
Shares issued during period (in shares)			0

Stockholders' Equity - Assumpti

Stockholders' Equity - Assumptions to Determine Grant-date Fair Value of Options (Details) - Stock Option	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility, minimum	83.50%	66.10%
Expected volatility, maximum	105.50%	84.00%
Risk-free interest rate, minimum	0.15%	1.42%
Risk-free interest rate, maximum	1.41%	2.54%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	1 year 4 months 20 days	5 years 6 months
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 3 months 25 days	6 years 3 months

Stockholders' Equity - Stock Op

Stockholders' Equity - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Options
Outstanding at beginning of period (in shares)	7,235,581
Granted (in shares)	1,781,932
Exercised (in shares)	(597,416)
Canceled (in shares)	(1,441,650)
Outstanding at end of period (in shares)	6,978,447	7,235,581
Options exercisable (in shares)	3,154,929
Options vested or expected to vest (in shares)	6,978,447
Weighted Average Exercise Price
Outstanding at beginning of period (in dollars per share)	$ 8.21
Granted (in dollars per share)	7.66
Exercised (in dollars per share)	4.45
Canceled (in dollars per share)	9.06
Outstanding at end of period (in dollars per share)	8.19	$ 8.21
Options exercisable (in dollars per share)	7.76
Options vested or expected to vest (in dollars per share)	$ 8.19
Weighted Average Remaining Life and Aggregate Intrinsic Value
Options outstanding, weighted average remaining life (in years)	7 years 5 months 12 days	8 years 8 months 12 days
Options exercisable, weighted average remaining life (in years)	5 years 10 months 2 days
Options vested or expected to vest, weighted average remaining life (in years)	7 years 5 months 12 days
Options outstanding, aggregate intrinsic value beginning balance	$ 5,075
Options outstanding, aggregate intrinsic value ending balance	15,565	$ 5,075
Options exercisable, aggregate intrinsic value	9,318
Options vested or expected to vest, aggregate intrinsic value	$ 15,565
Fair value of options granted during period (in dollars per share)	$ 5.51	$ 5.96
Unrecognized compensation expense	$ 19,400
Stock Option
Weighted Average Remaining Life and Aggregate Intrinsic Value
Unrecognized compensation expense, weighted average remaining term	2 years 7 months 20 days

Stockholders' Equity - Restrict

Stockholders' Equity - Restricted Stock Unit Activity and Related Information (Details) - Restricted Stock Unit $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)$ / sharesshares
Options
Outstanding at beginning of period (in shares) | shares	50,000
Granted (in shares) | shares	270,753
Canceled (in shares) | shares	(4,504)
Outstanding at end of period (in shares) | shares	316,249
Weighted Average Grant Date fair value
Outstanding at beginning of period (in dollars per share) | $ / shares	$ 6.56
Granted (in dollars per share) | $ / shares	6.79
Canceled (in dollars per share) | $ / shares	6
Outstanding at end of period (in dollars per share) | $ / shares	$ 6.76
Unrecognized compensation expense | $	$ 1.4
Unrecognized compensation expense, weighted average remaining term	2 years 2 months 26 days

Stockholders' Equity - Stock-_2

Stockholders' Equity - Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense		$ 12,684	$ 10,068
Stock-based compensation expense	$ 1,900
Research and development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense		3,820	3,245
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense		$ 8,864	$ 6,823

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2017
Income Tax Disclosure [Line Items]
Provision for income taxes	$ 0
Net operating loss carryforwards	$ 199,700,000
Net operating loss carryforwards, expire date	Dec. 31, 2035
Tax credit carryforwards, expire date	Dec. 31, 2040
Increase in valuation allowance	$ 22,800,000	$ 26,000,000
Unrecognized tax positions	0	0
Accrued interest or penalties	0	$ 0
U.S. Federal
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	238,500,000		$ 38,800,000
State
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	235,000,000
Commonwealth of Massachusetts | State
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 235,000,000
Net operating loss carryforwards, expire date	Dec. 31, 2035
Research and Development | U.S. Federal
Income Tax Disclosure [Line Items]
Tax credit carryforwards	$ 5,500,000
Research and Development | State
Income Tax Disclosure [Line Items]
Tax credit carryforwards	$ 3,200,000

Income Taxes - Schedule of Net

Income Taxes - Schedule of Net Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets
Stock-based compensation	$ 5,484	$ 3,754
Net operating loss carryforwards	64,939	45,374
Credit carryforwards	7,988	6,519
Intangible assets	164	180
Charitable contributions	1	1
Accrued expenses	1,314	1,125
Total deferred tax assets	79,890	56,953
Valuation allowance	(79,610)	(56,817)
Total net deferred tax assets	280	136
Deferred tax liabilities:
Fixed assets	(280)	(136)
Total net deferred tax liability	$ (280)	$ (136)

Income Taxes - Schedule of Effe

Income Taxes - Schedule of Effective Income Tax Rate Reconciliation (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Federal income tax expense at statutory rate	21.00%	21.00%
Stock compensation expense	(1.10%)	(0.50%)
Fair value change in warrant liability		0.10%
Permanent differences		(0.10%)
Federal research and development credit	1.20%	1.70%
State research and development credit	0.60%	0.90%
State income tax, net of federal benefit	6.00%	6.30%
Other		0.70%
Change in valuation allowance	(27.70%)	(30.10%)

Revenue (Details)

Revenue (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Dec. 31, 2019Milestone	Dec. 31, 2020USD ($)	Dec. 31, 2019
Disaggregation Of Revenue [Line Items]
Revenue from Contract with Customer, Product and Service [Extensible List]		kldo:CollaborationRevenueMember	kldo:CollaborationRevenueMember
Collaboration revenue		$ 975
Deferred revenue		$ 498
Expected research term of arrangement completion year		2021
Janssen | Collaboration Agreement
Disaggregation Of Revenue [Line Items]
Number of collaboration milestones | Milestone	3
Number of days which decide to next milestone	60 days