scPharmaceuticals (SCPH) 10-K 22 Mar 22 2021 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2021	Mar. 21, 2022	Jun. 30, 2021
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	FY
Trading Symbol	SCPH
Entity Registrant Name	SCPHARMACEUTICALS INC.
Entity Central Index Key	0001604950
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
ICFR Auditor Attestation Flag	false
Entity Shell Company	false
Entity Interactive Data Current	Yes
Title of 12(b) Security	Common stock, par value $0.0001
Security Exchange Name	NASDAQ
Entity File Number	001-38293
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	46-5184075
Entity Address, Address Line One	2400 District Avenue
Entity Address, Address Line Two	Suite 310
Entity Address, City or Town	Burlington
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	01803
City Area Code	617
Local Phone Number	517-0730
Document Annual Report	true
Document Transition Report	false
Auditor Name	RSM US LLP
Auditor Location	Boston, Massachusetts
Auditor Firm ID	49
Entity Common Stock, Shares Outstanding		27,371,488
Entity Public Float			$ 94,066,126
Documents Incorporated by Reference	DOCUMENTS INCORPORATED BY REFERENCE The following documents (or parts thereof) are incorporated by reference into the following parts of this Form 10-K: Certain information required in Part III of this Annual Report on Form 10-K is incorporated from the registrant’s Definitive Proxy Statement for the 2022 Annual Meeting of Shareholders.

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Current assets
Cash and cash equivalents	$ 74,268	$ 71,819
Restricted cash	182
Short-term investments	1,010	33,276
Prepaid expenses	2,791	2,610
Other current assets	24	121
Total current assets	78,275	107,826
Restricted cash		182
Property and equipment, net	69	93
Right-of-use lease assets - operating, net	410	815
Deposits and other assets	283	132
Total assets	79,037	109,048
Current liabilities
Accounts payable	544	1,666
Accrued expenses	3,995	4,787
Term loan, short-term	9,805	2,408
Lease obligation - operating, short-term	476	460
Other current liabilities	26
Total current liabilities	14,846	9,321
Term loan, long-term	7,354	16,858
Lease obligation - operating, long-term		480
Other liabilities	367	219
Total liabilities	22,567	26,878
Commitments and contingencies (Note 12)
Stockholders’ Equity
Preferred stock, $0.0001 par value; 10,000,000 shares authorized and no shares issued and outstanding
Common stock; $0.0001 par value; 150,000,000 shares authorized at December 31, 2020; 27,325,959 and 27,366,707 shares issued and outstanding at December 31, 2020 and December 31, 2021, respectively	3	3
Additional paid-in capital	246,166	243,830
Accumulated deficit	(189,698)	(161,664)
Accumulated other comprehensive gain (loss)	(1)	1
Total stockholders’ equity	56,470	82,170
Total liabilities and stockholders’ equity	$ 79,037	$ 109,048

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2021	Dec. 31, 2020
Statement of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares issued	27,366,707	27,325,959
Common stock, shares outstanding	27,366,707	27,325,959

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Operating expenses:
Research and development	$ 16,039	$ 18,149
General and administrative	9,784	11,784
Total operating expenses	25,823	29,933
Loss from operations	(25,823)	(29,933)
Other (expense) income	315	(4)
Interest income	49	315
Interest expense	(2,575)	(2,587)
Net loss	$ (28,034)	$ (32,209)
Net loss per share, basic and diluted	$ (1.02)	$ (1.31)
Weighted—average common shares outstanding, basic and diluted	27,351,730	24,568,897
Other comprehensive loss:
Unrealized gain (loss) on short-term investments	$ (2)	$ 1
Comprehensive loss	$ (28,036)	$ (32,208)

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	At The Market Offering	Common Stock	Common StockAt The Market Offering	Additional Paid-in Capital	Additional Paid-in CapitalAt The Market Offering	Accumulated Deficit	Other Comprehensive Income (Loss)
Beginning Balance at Dec. 31, 2019	$ 51,365		$ 2		$ 180,818		$ (129,455)
Beginning Balance, Shares at Dec. 31, 2019			19,418,955
Net loss	(32,209)						(32,209)
Issuance of common stock, net of commissions and issuance costs		$ 10,253				$ 10,253
Issuance of common stock net of commissions and issuance costs, Shares				1,502,892
Common stock offering, net of commissions and issuance costs	50,188		$ 1		50,187
Common stock offering, net of commissions and issuance costs, Shares			6,220,589
Issuance of common stock upon exercise of stock options	$ 736				736
Issuance of common stock upon exercise of stock options, Shares	121,671		121,671
Vesting of restricted stock	$ (164)				(164)
Vesting of restricted stock, Shares			61,852
Stock-based compensation	2,000				2,000
Unrealized gain (loss) on short term investments	1	$ 1						$ 1
Ending Balance at Dec. 31, 2020	82,170		$ 3		243,830		(161,664)	1
Ending Balance, Shares at Dec. 31, 2020			27,325,959
Net loss	(28,034)						(28,034)
Issuance of common stock upon exercise of stock options	$ 9				9
Issuance of common stock upon exercise of stock options, Shares	2,501		2,501
Issuance of common stock purchased through employee stock purchase plan	$ 42				42
Issuance of common stock purchased through employee stock purchase plan, Shares			11,253
Vesting of restricted stock	(81)				(81)
Vesting of restricted stock, Shares			26,994
Stock-based compensation	2,366				2,366
Unrealized gain (loss) on short term investments	(2)							(2)
Ending Balance at Dec. 31, 2021	$ 56,470		$ 3		$ 246,166		$ (189,698)	$ 1
Ending Balance, Shares at Dec. 31, 2021			27,366,707

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Cash flows from operating activities
Net loss	$ (28,034,000)	$ (32,209,000)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation expense	34,000	34,000
Amortization expense - right-of-use leased assets - operating	404,000	365,000
Accretion expense	124,000	122,000
Stock-based compensation	2,366,000	2,000,000
Non-cash interest expense	554,000	517,000
Fair value adjustment to derivative liability		30,000
Changes in operating assets and liabilities
Prepaid expenses and other assets	(234,000)	236,000
Accounts payable, accrued expenses and other liabilities	(2,365,000)	1,212,000
Net cash flows used in operating activities	(27,151,000)	(27,693,000)
Cash flows from investing activities
Purchases of property and equipment	(9,000)
Maturities of short-term investments	41,150,000	38,700,000
Purchases of short-term investments	(9,011,000)	(72,160,000)
Net cash flows (used in) provided by investing activities	32,130,000	(33,460,000)
Cash flows from financing activities
Proceeds from common stock offering, net of underwriter discounts and offering costs		50,188,000
Proceeds from at-the-market offering, net		10,388,000
Proceeds from the exercise of stock options	9,000	736,000
Proceeds from employee stock purchase plan	42,000
Principal payments on term loan	(2,500,000)
Payment of term loan exit fee		(800,000)
Settlement of restricted stock units for tax withholding obligations	(81,000)	(164,000)
Net cash flows provided by (used in) financing activities	(2,530,000)	60,348,000
Net (decrease) increase in cash	2,449,000	(805,000)
Cash, cash equivalents and restricted cash, beginning of year	72,001,000	72,806,000
Cash, cash equivalents and restricted cash, end of year	74,450,000	72,001,000
Supplemental cash flow information
Interest paid	2,043,000	1,986,000
Taxes paid	$ 206,000	226,000
Supplemental disclosure of non-cash activities
Transfer of issuance costs from other noncurrent assets to equity		$ 135,000

Description of Business and Bas

Description of Business and Basis of Presentation	12 Months Ended
	Dec. 31, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Description of Business and Basis of Presentation	1. Description of Business and Basis of Presentation Description of Business scPharmaceuticals LLC was formed as a Limited Liability Company under the laws of the State of Delaware on February 19, 2013. On March 24, 2014, scPharmaceuticals LLC was converted to a Delaware Corporation and changed its name to scPharmaceuticals Inc. (“the Company”). The Company is a pharmaceutical company focused on developing and commercializing products that have the potential to optimize the delivery of infused therapies, advance patient care and reduce healthcare costs. The Company’s strategy is designed to enable the subcutaneous administration of therapies that have previously been limited to intravenous (“IV”) delivery. The Company’s headquarters and primary place of business is Burlington, Massachusetts. Basis of Presentation The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States (“U.S. GAAP”) and have been prepared on a basis which assumes that the Company will continue as a going concern, which contemplates the realization of assets and the satisfaction of liabilities and commitments in the normal course of business. The consolidated financial statements reflect the operations of the Company and its wholly-owned subsidiary, scPharmaceuticals Securities Corporation. All significant intercompany balances and transactions have been eliminated in consolidation. At December 31, 2021 , the Company had cash, cash equivalents, restricted cash and investments of $ 75.5 million and working capital of $ 63.4 million. During the year ended December 31, 2021 , the Company incurred a net loss totaling $ 28.0 million and used cash in operating activities totaling $ 27.2 million. The Company expects to continue to incur losses and use cash in operating activities in 2022. During the year ended December 31, 2020, the Company received net proceeds of $ 50.2 million from the completion of a public offering of 6,220,589 shares of common stock and $ 10.4 million from an at-the-market offering of 1,502,892 shares of common stock (Note 10). The Company currently has an at-the-market offering program with Cowen and Company, LLC pursuant to which it may offer and sell up to $ 50.0 million of shares of its common stock (Note 10). The Company believes that, based on its current development plans and activities, its resources will be sufficient to satisfy its liquidity requirements for more than one year from the issuance date of these consolidated financial statements.

Significant Accounting Policies

Significant Accounting Policies	12 Months Ended
	Dec. 31, 2021
Accounting Policies [Abstract]
Significant Accounting Policies	2. Significant Accounting Policies Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting periods. Significant items subject to such estimates and assumptions include the determination of fair value of financial instruments, accruals related to development costs and clinical activities, and the establishment of the tax valuation allowance. Actual results could differ from those estimates. Foreign Currency Transactions The functional currency of the Company is the U.S. dollar. Accordingly, gains and losses resulting from translating transactions denominated in currencies and balances of assets and liabilities outstanding at the balance sheet date, other than U.S. dollars, are included in net loss in the Statements of Operations and Comprehensive Loss. Cash, Cash Equivalents and Restricted Cash Cash, cash equivalents and restricted cash consists of bank deposits, certificates of deposit and money market accounts with financial institutions. Cash equivalents are carried at cost which approximates fair value due to their short-term nature and which the Company believes do not have a material exposure to credit risk. The Company considers all highly liquid investments with maturities of three months or less from the date of purchase to be cash equivalents. The Company places its cash and cash equivalents with institutions with high credit quality. However, at certain times such cash and cash equivalents may be in excess of Federal Deposit Insurance Corporation and Securities Investor Protection Corporation insurance limits. The Company has not experienced any losses with respect to these accounts. As of December 31, 2020 and 2021 , the Company classified $ 182,000 as restricted cash related to a letter of credit issued as a security deposit in connection with the Company’s lease of its corporate office facilities (Note 12). Cash, cash equivalents and restricted cash consists of the following (in thousands): December 31, December 31, Cash and cash equivalents $ 71,819 $ 74,268 Restricted cash 182 182 Cash, cash equivalents and restricted cash $ 72,001 $ 74,450 Concentration of Credit Risk Financial instruments that subject the Company to credit risk primarily consist of cash and cash equivalents and short-term investments. The Company maintains its cash and cash equivalent balances with high-quality financial institutions and, consequently, the Company believes that such funds are subject to minimal credit risk. The Company’s short-term investments consist of corporate debt securities. The Company has adopted an investment policy that limits the amounts the Company may invest in any one type of investment and requires all investments held by the Company to hold a minimum rating, thereby reducing credit risk exposure. Investments The Company invests excess cash balances in available-for-sale debt securities. The Company determines the appropriate classification of these securities at the time they are acquired and evaluates the appropriateness of such classifications at each balance sheet date. The Company reports available-for-sale investments at fair value at each balance sheet date and includes any unrealized gains and losses in accumulated other comprehensive income (loss), a component of stockholders’ equity. Realized gains and losses are determined using the specific identification method and are included in other income (expense). If any adjustment to fair value reflects a decline in the value of the investment, the Company considers all available evidence to evaluate the extent to which the decline is “other than temporary,” including the intention to sell and, if so, marks the investment to market through a charge to the Company’s consolidated statements of operations and comprehensive loss. Leases The Company determines if an arrangement is a lease at inception. Operating leases are included in right-of-use (“ROU”) lease assets, current portion of lease obligations, and long term lease obligations on the Company’s balance sheets. ROU lease assets represent the Company’s right to use an underlying asset for the lease term and lease obligations represent the Company’s obligation to make lease payments arising from the lease. Operating ROU lease assets and obligations are recognized at the commencement date based on the present value of lease payments over the lease term. As most of the Company’s leases do not provide an implicit rate, the Company uses its incremental borrowing rate based on the information available at the commencement date in determining the present value of lease payments. The ROU lease asset excludes lease incentives. The Company’s lease terms include options to extend or terminate the lease when it is reasonably certain that the Company will exercise that option. Lease expense for lease payments is recognized on a straight-line basis over the lease term. Research and Development Costs Research and development costs are expensed as incurred. Nonrefundable advance payments, if any, for goods or services used in research and development are initially recorded as an asset and then recognized as an expense as the related goods are delivered or services are performed. Research and development expenses include contract services, consulting, salaries, materials and supplies and overhead. Income Taxes The Company accounts for income taxes in accordance with Accounting Standard Codification (“ASC”) 740 Income Taxes (“ASC 740”) . Deferred tax assets and liabilities are recorded to reflect the impact of temporary differences between amounts of assets and liabilities for financial reporting purposes and such amounts as measured under enacted tax laws. A valuation allowance is required to offset any net deferred tax assets if, based upon the available evidence, it is more likely than not that some or all of the deferred tax asset will not be realized. The Company provides reserves for potential payments of tax to various tax authorities related to uncertain tax positions. The tax benefits recorded are based on a determination of whether and how much of a tax benefit taken by the Company in its tax filings or positions is “more likely than not” to be realized following resolution of any uncertainty related to the tax benefit, assuming that the matter in question will be raised by the tax authorities. Potential interest and penalties associated with such uncertain tax positions are recorded as a component of income tax expense. At December 31, 2020 and 2021 , the Company had no such accruals. Stock-Based Compensation Stock-based compensation expense for stock options is recognized based on the grant-date fair value using the Black-Scholes valuation model. Restricted stock units are valued at the fair market value per share of the Company’s common stock on the date of grant. The Company recognizes compensation expense only for those stock-based awards expected to vest after considering expected forfeitures. Cumulative compensation expense is at least equal to the compensation expense for vested awards. Stock-based compensation is recognized on a straight-line basis over the service period of each award. Segments 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 (“CODM”) in making decisions regarding resource allocation and assessing performance. The Company’s chief executive officer is the CODM, and he uses consolidated financial information in determining how to allocate resources and assess performance. The Company has determined that it operates in one segment. All of the Company’s assets are located in the United States. Recently Issued Accounting Standards The Company has reviewed recently issued accounting standards to determine if any of those standards will have a significant impact on its financial reporting. None were noted as of December 31, 2021.

Net Loss per Share

Net Loss per Share	12 Months Ended
	Dec. 31, 2021
Earnings Per Share [Abstract]
Net Loss per Share	3. Net Loss per Share Basic net loss per share is calculated by dividing the net loss by the weighted-average number of shares of common stock outstanding during the period without consideration of dilutive common stock equivalents. Diluted net loss per share is the same as basic net loss per common share, since the effects of potentially dilutive securities are anti-dilutive. Dilutive common stock equivalents are comprised of unexercised stock options outstanding under the Company’s equity plan and unvested restricted stock. The following table sets forth the computation of basic and diluted net loss per share of common stock (in thousands, except shares and per share data): FOR THE YEAR ENDED DECEMBER 31, DECEMBER 31, Net loss $ ( 32,209 ) $ ( 28,034 ) Weighted—average common shares 24,568,897 27,351,730 Net loss per share, basic and diluted $ ( 1.31 ) $ ( 1.02 ) The following table sets forth the outstanding potentially dilutive securities that have been excluded in the calculation of diluted net loss per share because their inclusion would be anti-dilutive (in common stock equivalent shares): FOR THE YEAR ENDED DECEMBER 31, DECEMBER 31, Stock options to purchase common stock 2,224,913 2,662,752 Unvested restricted stock 80,450 42,250 2,305,363 2,705,002

Investments

Investments	12 Months Ended
	Dec. 31, 2021
Investments, Debt and Equity Securities [Abstract]
Investments	4. Investments Cash in excess of the Company’s immediate requirements is invested in accordance with the Company’s investment policy that primarily seeks to maintain adequate liquidity and preserve capital. A summary of the Company’s available-for-sale classified investments as of December 31, 2020 and 2021 consisted of the following (in thousands): At December 31, 2020 Investments - Current: Cost Basis Accumulated Unrealized Gains Accumulated Unrealized Losses Fair Value United States Treasury securities $ 8,213 $ - $ - $ 8,213 Corporate debt securities 11,972 1 - 11,973 Commercial paper 13,090 - - 13,090 Total $ 33,275 $ 1 $ - $ 33,276 At December 31, 2021 Investments - Current: Cost Basis Accumulated Unrealized Gains Accumulated Unrealized Losses Fair Value Corporate debt securities $ 1,011 $ - $ ( 1 ) $ 1,010 Total $ 1,011 $ - $ ( 1 ) $ 1,010 The amortized cost and fair value of the Company’s available-for-sale investments, by contract maturity, as of December 31, 2021 consisted of the following (in thousands): Amortized Cost Due in one year or less $ 1,011 Total $ 1,011

Property and Equipment

Property and Equipment	12 Months Ended
	Dec. 31, 2021
Property, Plant and Equipment [Abstract]
Property and Equipment	5. Property and Equipment Purchased property and equipment consist of the following as of December 31, 2020 and 2021 (in thousands): ESTIMATED 2020 2021 Office equipment 5 years $ 10 $ 10 Office furniture 7 years 116 126 Computer equipment 3 years 8 8 Leasehold improvements Life of lease 95 95 229 239 Less: Accumulated depreciation ( 136 ) ( 170 ) Property and equipment, net $ 93 $ 69 Depreciation expense for the years ended December 31, 2020 and 2021 was $ 34,000 and $ 34,000 , respectively.

Accrued Expenses

Accrued Expenses	12 Months Ended
	Dec. 31, 2021
Accrued Liabilities, Current [Abstract]
Accrued Expenses	6. Accrued Expenses Accrued expenses at December 31, 2020 and 2021 consist of (in thousands): 2020 2021 Contract research and development $ 2,274 $ 2,350 Employee compensation and related costs 1,978 1,152 Consulting and professional service fees 364 265 Interest 88 154 Financing related costs 43 60 State taxes 40 5 Other - 9 Total accrued expenses $ 4,787 $ 3,995

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2021
Income Tax Disclosure [Abstract]
Income Taxes	7. Income Taxes The Company accounts for income taxes in accordance with ASC 740, which requires an asset and liability approach for measuring deferred taxes based on temporary differences between the financial statement and tax bases of assets and liabilities existing at each balance sheet date using enacted tax rates for the years in which taxes are expected to be paid or recovered. The tax benefit arising from the Company’s net loss has been offset by an increase in the valuation allowance. Accordingly, the Company had no net income tax provision or benefit during the years ended December 31, 2020 and 2021 . Components of the net deferred tax assets at December 31, 2020 and 2021 are as follows (in thousands): 2020 2021 Deferred tax assets: Federal net operating loss carryforwards $ 10,474 $ 12,813 State net operating loss carryforwards 2,938 3,630 Research and development tax credits 2,963 3,719 Accrued liabilities 588 327 Stock-based compensation 679 1,087 Depreciation and amortization 1,545 247 Capitalized research and development costs 23,977 29,529 Lease liabilities 253 129 Other — 29 Total deferred tax assets $ 43,417 $ 51,510 Deferred tax liabilities: Right-of-use lease assets ( 219 ) ( 111 ) Other ( 119 ) — Total deferred tax liabilities $ ( 338 ) $ ( 111 ) Valuation allowance $ ( 43,079 ) $ ( 51,399 ) Net deferred tax assets $ — $ — At December 31, 2021 , the Company had available federal net operating loss carryforwards of $ 17.5 million, which expire at various dates through 2037 , and $ 43.5 million, which may be carried forward indefinitely. At December 31, 2021, the Company had available state net operating loss carryforwards of $ 57.5 million, which expire at various dates through 2041 , and $ 200,000 , which may be carried forward indefinitely. In assessing the realizability of net deferred tax assets, management considers whether it is more likely than not that the net deferred tax assets will be realized. The ultimate realization of net deferred tax assets is dependent upon the generation of future taxable income during the periods in which temporary differences representing future deductible amounts become deductible. Management has established a full valuation allowance against the net deferred tax assets at December 31, 2020 and 2021 since it is more likely than not that these future tax benefits will not be realized. During 2021 , the valuation allowance increased by $ 8.3 million. At December 31, 2021 , the Company had federal and state research and development credit carryforwards of $ 3.0 million and $ 0.9 million, respectively. The net credit carryforwards may be used to offset future income taxes and expire at various dates through 2041 . Changes in the Company’s ownership, as defined in the U.S. Internal Revenue Code, may limit the Company’s ability to utilize the tax credit and net operating loss carryforwards. A reconciliation of income tax (expense) benefit at the statutory federal income tax rate and income taxes as reflected in the consolidated financial statements at December 31, 2020 and 2021 are as follows: 2020 2021 Federal income tax at statutory rate 21.00 % 21.00 % State income tax, net of federal benefit 5.65 % 5.97 % Research and development credits 2.03 % 3.16 % Book compensation related to stock options ( 0.91 )% 0.03 % Change in income tax rate 0.73 % 0.45 % Other ( 0.30 )% ( 0.93 )% Increase in valuation allowance ( 28.20 )% ( 29.68 )% Effective tax rate — % — % The Company files tax returns in the United States, Massachusetts and other states. The tax years 2017 through 2021 remain open to examination by major taxing jurisdictions to which the Company is subject, which are primarily the United States federal and Massachusetts. Carryforward attributes generated in years past may still be adjusted upon examination by the Internal Revenue Service or state tax authorities if they have or will be used in a future period. The Company is currently not under examination by the Internal Revenue Service or any other jurisdictions for any tax years. The Company recognizes both accrued interest and penalties related to unrecognized benefits in income tax expense. The Company has not recorded any interest or penalties on any unrecognized tax benefits since its inception. A reconciliation of the beginning and ending amount of uncertain tax benefits is as follows (in thousands): 2020 2021 Beginning uncertain tax benefits $ 628 $ 774 Prior year - increases 1 — Current year - decreases — — Current year - increases 145 203 Ending uncertain tax benefits $ 774 $ 977

Fair Value of Financial Instrum

Fair Value of Financial Instruments	12 Months Ended
	Dec. 31, 2021
Fair Value Disclosures [Abstract]
Fair Value of Financial Instruments	8. Fair Value of Financial Instruments The Financial Accounting Standards Board (“FASB”) ASC Topic, Fair Value Measurements and Disclosures ( “ASC 820” ), provides a fair value hierarchy, which classifies fair value measurements based on the inputs used in measuring fair value. Observable inputs are inputs that market participants would use in pricing the asset or liability based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company’s assumptions about the inputs that market participants would use in pricing the asset or liability and are developed based on the best information available in the circumstances. The fair value hierarchy applies only to the valuation inputs used in determining the reported fair value of the investments and is not a measure of the investment credit quality. The three levels of the fair value hierarchy are described below: Level 1—Valuations based on unadjusted quoted prices in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date. Level 2—Valuations based on quoted prices for similar assets or liabilities in markets that are not active or for which all significant inputs are observable, either directly or indirectly. Level 3—Valuations that require inputs that reflect the Company’s own assumptions that are both significant to the fair value measurement and observable. To the extent that valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The carrying values of the Company’s cash and restricted cash, prepaid expenses and deposits approximate their fair values due to their short-term nature. The carrying value of the Company’s loan payable was considered a reasonable estimate of fair value because the Company’s interest rate is near current market rates for instruments with similar characteristics. The following tables summarize the Company’s assets and liabilities that are measured at fair value on a recurring basis and indicates the level of the fair value hierarchy utilized to determine such fair values (in thousands): As of December 31, 2020 TOTAL Quoted Prices Significant Significant Assets: Cash equivalents $ 70,797 $ 68,798 $ 1,999 $ — Total cash equivalents 70,797 68,798 1,999 — United States Treasury securities 8,213 8,213 — — Corporate debt securities 11,973 — 11,973 — Commercial paper 13,090 — 13,090 — Investments 33,276 8,213 25,063 — Total $ 104,073 $ 77,011 $ 27,062 $ — As of December 31, 2021 TOTAL Quoted Prices Significant Significant Assets: Cash equivalents $ 72,449 $ 72,449 $ — $ — Total cash equivalents 72,449 72,449 — — Corporate debt securities 1,010 — 1,010 — Investments 1,010 — 1,010 — Total $ 73,459 $ 72,449 $ 1,010 $ — Changes in the fair value of the Company’s Level 3 derivative liability for the year ended December 31, 2020 are as follows: At December 31, 2019 $ 765 Change in fair value of derivative liability 30 Allocation of debt discount to term loan 5 Payment of derivative liability ( 800 ) At December 31, 2020 $ —

Term Loan

Term Loan	12 Months Ended
	Dec. 31, 2021
Debt Disclosure [Abstract]
Term Loan	9. Term Loan In May 2017, the Company entered into a loan and security agreement (the “2017 Loan Agreement”), with SLR Investment Corp. (f/k/a Solar Capital Ltd.) and Silicon Valley Bank, (together, the “Lenders”) for $ 10.0 million. The 2017 Loan Agreement had a maturity date of May 1, 2021 . Debt issuance costs for the 2017 Loan Agreement were to be amortized to interest expense over the remaining term of the 2017 Loan Agreement using the effective-interest method. In September 2019, the Company replaced the 2017 Loan Agreement with a new $ 20.0 million term loan with the Lenders (the "2019 Loan Agreement"). The restructured four-year term loan facility allows for an expansion of the 2017 Loan Agreement. Some of the proceeds from the 2019 Loan Agreement were used to pay off the 2017 Loan Agreement including the final fee of $ 325,000 . The 2019 Loan Agreement extends the term of the credit facility until September 17, 2023 . The payoff of the 2017 Loan Agreement was treated as a modification of the debt. Debt issuance costs for the 2019 Loan Agreement, including unamortized issuance costs for the 2017 Loan Agreement, will be amortized to interest expense over the remaining term of the 2019 Loan Agreement using the effective-interest method. The interest rate under the 2019 Loan Agreement is the higher of (i) LIBOR plus 7.95 % or (ii) 10.18 % and there is an interest-only period until September 30, 2021. The rate at December 31, 2021 was 10.18 %. Pursuant to the 2019 Loan Agreement, the Company provided a first priority security interest in substantially all of the Company’s assets, including intellectual property, subject to certain exceptions. The Company entered into the Exit Agreement in connection with the 2019 Loan Agreement which provides for an aggregate payment of 4 % of the loan commitment, or $ 800,000 , to the Lenders upon the occurrence of an exit event (the "Exit Fee"). The Company concluded that the exit payment obligation met the definition of a derivative that was required to be accounted for as a separate unit of accounting. The Company recorded the issuance-date fair value of the derivative liability of $ 763,000 as a debt discount and as a derivative liability in the Company’s balance sheet. The derivative liability is re-measured at each balance sheet date and any changes in estimated fair value is recorded as other income (expense). The Company paid the Exit Fee during the year ended December 31, 2020 in conjunction with the Company’s public offering, which was deemed to be an exit event pursuant to the Exit Agreement. Prior to its public offering in 2020, the Company recorded $ 30,000 in non-cash expense as a fair value adjustment to the derivative liability. As of December 31, 2021 , unpaid borrowings under the 2019 Loan Agreement totaled $ 17.5 million. For the years ended December 31, 2020 and 2021 , the Company recorded $ 357,000 and $ 392,000 , respectively, related to the amortization of the debt discount associated with the 2019 Loan Agreement. The 2019 Loan Agreement allows the Company to voluntarily prepay all (but not less than all) of the outstanding principal at any time. A prepayment premium of 3 % or 1 % through the one-year anniversary and the two-year anniversary, respectively, would be assessed on the outstanding principal. After the two-year anniversary, a 0.5 % prepayment premium would be assessed on the outstanding principal. A final payment fee of $ 500,000 is due upon the earlier to occur of the maturity date or prepayment of such borrowings. For the years ended December 31, 2020 and 2021 , the Company recorded $ 160,000 and $ 162,000 , respectively, related to the amortization of the final payment fee associated with the 2019 Loan Agreement. In an event of default under the 2019 Loan Agreement, the interest rate will be increased by 5 % and the balance under the loan may become immediately due and payable at the option of the lenders. The 2019 Loan Agreement includes restrictions on, among other things, the Company’s ability to incur additional indebtedness, change the name or location of the Company’s business, merge with or acquire other entities, pay dividends or make other distributions to holders of its capital stock, make certain investments, engage in transactions with affiliates, create liens, sell assets or pay subordinated debt. Total term loan and unamortized debt discount balances are as follows (in thousands): DECEMBER 31, DECEMBER 31, Face value $ 20,000 $ 17,500 Less: debt discount ( 734 ) ( 341 ) Total $ 19,266 $ 17,159 Less: current portion ( 2,408 ) ( 9,805 ) Long-term portion $ 16,858 $ 7,354 As of December 31, 2021, future principal payments due under the 2019 Loan Agreement are as follows (in thousands): Year ended: December 31, 2022 $ 10,000 December 31, 2023 7,500 Total minimum principal payments $ 17,500

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2021
Equity [Abstract]
Stockholders' Equity	10. Stockholders’ Equity Common Stock At December 31, 2020 and 2021 , the Company had 150,000,000 shares of common stock authorized with a par value of $ 0.0001 . There were 27,325,959 and 27,366,707 shares issued and outstanding at December 31, 2020 and 2021, respectively. Voting, dividend and liquidation rights of the holders of the common stock are subject to the Company’s articles of incorporation, corporate bylaws and underlying shareholder agreements. Reserved Shares The Company has reserved 2,662,752 and 42,250 shares of common stock for the exercise of outstanding options to purchase common stock and for the vesting of restricted stock units ("RSUs"), respectively. 2019 At-the-Market Issuance Sales Agreement On August 23, 2019, the Company entered into an Open Market Sale Agreement SM (“the 2019 ATM Agreement”), with Jefferies LLC (“Jefferies”) with respect to an at-the-market offering program (the "2019 ATM Program") under which the Company could offer and sell shares of its common stock (the “2019 ATM Shares”) having an aggregate offering price of up to $ 15.0 million through Jefferies as its sales agent. The offering and sale of 2019 ATM Shares were made pursuant to the Company’s shelf registration statement on Form S-3, which was declared effective by the SEC on February 11, 2019 (the “Registration Statement”). The Company agreed to pay Jefferies a commission equal to 3.0 % of the gross sales proceeds of such 2019 ATM Shares. The Company incurred $ 189,000 of legal, accounting and other costs to establish and activate the 2019 ATM Program. During the year ended December 31, 2020, the Company sold a total of 1,502,892 2019 ATM Shares under the 2019 ATM Agreement, in the open market, at a weighted average gross selling price of $ 7.13 per share for net proceeds of $ 10.4 million, which completed the program. The Company charged $ 135,000 in costs related to establishing and activating the program against additional paid in capital upon issuance of shares in 2020. 2021 At-the-Market Issuance Sales Agreement On March 23, 2021, the Company entered into an Open Market Sale Agreement SM (the “2021 ATM Agreement”) with Cowen and Company, LLC (“Cowen”) with respect to an at-the-market offering program (the “2021 ATM Program”) under which the Company could offer and sell shares of its common stock (the “2021 ATM Shares”), having an aggregate offering price of up to $ 50.0 million through Cowen as its sales agent. The Company agreed to pay Cowen a commission up to 3.0 % of the gross sales proceeds of such 2021 ATM Shares. As of December 31, 2021, the Company had received no proceeds from the sale of shares of common stock pursuant to the 2021 ATM Agreement. Sale of Common Stock In May 2020, the Company completed an underwritten public offering of 5,780,347 shares of its common stock (the “2020 Offering Shares”), pursuant to the Registration Statement. The 2020 Offering Shares were sold at an offering price of $ 8.65 per share, resulting in net proceeds of $ 46.6 million, after deducting underwriting discounts, commissions and offering expenses. In addition, the underwriters of the offering were granted the option for a period of 30 days to purchase up to an additional 867,052 shares of common stock at $ 8.65 per share. In June 2020, the underwriters exercised their option and purchased an additional 440,242 shares of common stock at $ 8.65 per share, resulting in additional net proceeds to the Company of $ 3.6 million, after deducting underwriting discounts, commissions and offering expenses. Preferred Stock At December 31, 2020 and 2021 , the Company had 10,000,000 shares of preferred stock authorized with a par value of $ 0.0001 and no shares of preferred stock were issued or outstanding.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2021
Share-based Payment Arrangement [Abstract]
Stock-Based Compensation	11. Stock-Based Compensation Stock Options In October 2017, the board of directors approved the 2017 Stock Option and Incentive Plan (the “2017 Stock Plan”) which became effective in November 2017 , upon the closing of the Company’s IPO. The 2017 Stock Plan will expire in October 2027 . Under the 2017 Stock Plan, the Company may grant incentive stock options, non-statutory stock options, restricted stock awards, RSUs and other stock-based awards. The Company’s 2014 Stock Incentive Plan (the “2014 Stock Plan”) terminated in November 2017 effective upon the completion of the Company’s IPO. No additional options will be granted under the 2014 Stock Plan. At December 31, 2021 , there were 600,688 options outstanding under the 2014 Stock Plan. At December 31, 2021 , there were 5,051,920 shares of the Company’s common stock authorized for issuance under the 2017 Stock Plan, including 359,651 options that have been forfeited from the 2014 Stock Plan. At December 31, 2021 , there were 2,917,802 options available for issuance, 2,062,064 options outstanding and 42,250 restricted stock units outstanding under the 2017 Stock Plan. Awards granted under the 2017 Stock Plan have a term of ten years . Vesting of awards under the 2017 Stock Plan is determined by the compensation committee of the board of directors but is generally over one to four-year terms. The fair value of options at date of grant was estimated using the Black-Scholes option-pricing model with the following assumptions: 2020 2021 Risk-free interest rate 0.33 %— 1.71 % 0.50 %— 1.25 % Expected dividend yield 0 % 0 % Expected life 5.5 — 6.6 years 5.5 — 6.7 years Expected volatility 72 %— 75 % 72 %— 74 % Weighted-average grant date $ 4.35 $ 4.18 Due to the lack of a public market for the trading of the Company’s common stock prior to its initial public offering and the lack of company-specific historical volatility, volatility was estimated using historical volatilities of similar companies. The expected life of the awards is estimated based on the simplified method, which calculates the expected life based upon the midpoint of the term of the award and the vesting period. The Company uses the simplified method because it does not have sufficient option exercise data to provide a reasonable basis upon which to estimate the expected term. The Company has no history of paying dividends nor does management expect to pay dividends over the contractual terms of these options. The risk-free interest rates are based on the United States Treasury yield curve in effect at the time of grant, with maturities approximating the expected life of the stock options. The following table summarizes information about stock option activity during 2020 and 2021 (in thousands, except share and per share data): NUMBER OF WEIGHTED- WEIGHTED- AGGREGATE Outstanding, December 31, 2019 1,439,518 $ 6.09 Granted 1,021,705 6.79 Exercised ( 121,671 ) 6.07 Forfeited ( 114,639 ) 9.01 Outstanding, December 31, 2020 2,224,913 $ 6.26 Granted 953,506 6.57 Exercised ( 2,501 ) 3.81 Forfeited ( 513,166 ) 6.72 Outstanding, December 31, 2021 2,662,752 $ 6.28 7.47 $ 993 Vested and exercisable, December 31, 2021 1,368,937 $ 6.08 6.22 $ 877 Vested and expected to vest, December 31, 2021 2,238,516 $ 6.24 7.22 $ 940 Of the options granted during the year ended December 31, 2021 , 112,050 were performance-based options. Vesting of these performance-based options is contingent on the occurrence of certain regulatory and commercial milestones. The Company is recognizing the expense straight-line over the expected performance achievement term. The following table summarizes information about RSU activity during 2020 and 2021: RSUs AVERAGE GRANT DATE FAIR VALUE (IN DOLLARS PER SHARE) RSUs outstanding, December 31, 2019 160,900 $ 3.25 Granted — — Vested ( 80,450 ) 3.25 Forfeited — — RSUs outstanding, December 31, 2020 80,450 3.25 Granted — — Vested ( 38,200 ) 3.25 Forfeited — — RSUs outstanding, December 31, 2021 42,250 $ 3.25 The number of RSUs vested includes shares of common stock withheld on behalf of employees to satisfy the minimum statutory tax withholding requirements. During 2020 and 2021 , the Company received $ 736,000 and $ 9,000 , respectively, upon exercise of stock options. The intrinsic value of the options exercised in 2020 and 2021 was $ 388,000 and $ 7,000 , respectively. Unrecognized compensation expense related to unvested options as of December 31, 2021 was $ 2.7 million and will be recognized over the remaining vesting periods of the underlying awards. The weighted-average period over which such compensation is expected to be recognized is 2.3 years. Unrecognized compensation expense related to unvested RSUs as of December 31, 2021 was $ 22,000 and will be recognized over the remaining vesting periods of the underlying awards. The weighted-average period over which such compensation is expected to be recognized is 0.7 years. Employee Stock Purchase Plan In October 2017, the board of directors approved the 2017 Employee Stock Purchase Plan (“the ESPP”) which became effective in November 2017, upon the closing of the Company’s IPO. As part of the ESPP, eligible employees may acquire an ownership interest in the Company by purchasing common stock, at a discount, through payroll deductions. Eligible employees who elected to participate were able to participate in the ESPP beginning September 1, 2021. During 2021, 11,253 shares of common stock were issued under the ESPP. As of December 31, 2021, there were 983,629 shares of common stock available for issuance under the ESPP. The Company recorded stock-based compensation expense in the following expense categories of its accompanying condensed consolidated statements of operations and comprehensive loss for employees, directors and non-employees during the years ended December 31, 2020 and 2021 as follows (in thousands): 2020 2021 Research and development $ 619 $ 947 General and administrative 1,381 1,419 Total $ 2,000 $ 2,366

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2021
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	12. Commitments and Contingencies Operating Leases The Company entered into noncancelable operating leases for office facilities located in Lexington, Massachusetts and Burlington, Massachusetts through December 31, 2022 and November 30, 2022 , respectively. Rent expense under the operating leases totaled $ 0.5 million and $ 0.5 million for the years ended December 31, 2020 and 2021, respectively. Certain leases provide for increases in future minimum annual rental payments as defined in the lease agreements. The leases generally also include real estate taxes and common area maintenance charges in the annual rental payments. Pursuant to the terms of its lease agreement for the Company’s headquarters in Burlington, Massachusetts, the Company obtained a letter of credit in the amount of approximately $ 182,000 as security on the lease obligation. The letter of credit is listed as restricted cash on the Company’s consolidated balance sheets. Short-term leases are leases having a term of twelve months or less. The Company recognizes short-term leases on a straight-line basis and does no t record a related lease asset or liability for such leases. The following is a maturity analysis of the annual undiscounted cash flows reconciled to the carrying value of the operating lease liabilities as of December 31, 2021 (in thousands): Year ended: December 31, 2022 $ 496 Total minimum lease payments 496 Less imputed interest ( 20 ) Total $ 476 2020 2021 Lease cost: Operating lease cost $ 491 $ 482 Short-term lease cost - 20 Sublease income ( 51 ) ( 51 ) Total lease cost $ 440 $ 451 Other information Cash paid for amounts included in the $ 529 $ 530 Operating cash flows from operating leases $ ( 45 ) $ ( 60 ) Weighted-average remaining lease term - 1.9 years 0.9 years Weighted-average discount rate - 10.1 % 10.1 % In July 2021, the Company signed a lease agreement for a new office facility located in Salem, New Hampshire. The lease commenced on September 1, 2021 and has an initial term of 12 months with an optional extension term through August 2023. The lease is considered short-term and is being recognized on a straight-line basis over the initial term as the Company does not currently intend to exercise the option. In February 2018, the Company signed a sublease agreement for its facility located in Lexington, Massachusetts. The sublease commenced on April 1, 2018 and has an initial term of three years with an extension term through December 2022. In February 2020, the sublease was extended until December 31, 2022. Research and Development Agreements As part of the Company’s research and development efforts, the Company enters into research and development agreements with unrelated companies. These agreements contain varying terms and provisions which include fees and milestones to be paid by the Company. Some of these agreements also contain provisions which require the Company to make payments for exclusivity in the development of products in the area of loop diuretics. Contingencies The Company follows subtopic 450-20 of the FASB ASC to report accounting for contingencies. Certain conditions may exist as of the date the financial statements are issued, which may result in a loss to the Company but which will only be resolved when one or more future events occur or fail to occur. The Company assesses such contingent liabilities, and such assessment inherently involves an exercise of judgment. If the assessment of a contingency indicates that it is probable that a material loss has been incurred and the amount of the liability can be estimated, then the estimated liability would be accrued in the Company’s financial statements. If the assessment indicates that a potential material loss contingency is not probable but is reasonably possible, or is probable but cannot be estimated, then the nature of the contingent liability, and an estimate of the range of possible losses, if determinable and material, would be disclosed. Loss contingencies considered remote are generally not disclosed unless they involve guarantees, in which case the guarantees would be disclosed. Due to the discontinuation of use of the first generation device in 2019, the Company has received notice of termination costs related to the program. Certain of the Company’s vendors have claimed or billed for additional costs for which the Company believes it is not obligated. The Company has evaluated this contingent liability in accordance with ASC 450, Contingencies, and determined that the possibility of additional costs is remote and no longer probable.

401(k) Savings Plan

401(k) Savings Plan	12 Months Ended
	Dec. 31, 2021
Retirement Benefits [Abstract]
401(k) Savings Plan	13. 401(k) Savings Plan In July 2014, the Company established a defined contribution savings plan under Section 401(k) of the Internal Revenue Code covering all of its employees. Employees may make contributions by withholding a percentage of their salary. The plan includes an employer match equal to 100 % on the first 3 % of deferred compensation and an additional 50 % on the next 2 % of deferred compensation. During the years ended December 31, 2020 and 2021 , the Company has recognized compensation expense of $ 151,000 and $ 198,000 , respectively, for the employer match contribution.

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2021
Subsequent Events [Abstract]
Subsequent Events	14. Subsequent Events The Company considers events or transactions that occur after the balance sheet date but prior to the issuance of the financial statements to provide additional evidence relative to certain estimates or to identify matters that require additional disclosure. During this period, the Company did not have any material subsequent events that impacted its consolidated financial statements or disclosures.

Significant Accounting Polici_2

Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2021
Accounting Policies [Abstract]
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting periods. Significant items subject to such estimates and assumptions include the determination of fair value of financial instruments, accruals related to development costs and clinical activities, and the establishment of the tax valuation allowance. Actual results could differ from those estimates.
Foreign Currency Transactions	Foreign Currency Transactions The functional currency of the Company is the U.S. dollar. Accordingly, gains and losses resulting from translating transactions denominated in currencies and balances of assets and liabilities outstanding at the balance sheet date, other than U.S. dollars, are included in net loss in the Statements of Operations and Comprehensive Loss.
Cash Cash Equivalents and Restricted Cash	Cash, Cash Equivalents and Restricted Cash Cash, cash equivalents and restricted cash consists of bank deposits, certificates of deposit and money market accounts with financial institutions. Cash equivalents are carried at cost which approximates fair value due to their short-term nature and which the Company believes do not have a material exposure to credit risk. The Company considers all highly liquid investments with maturities of three months or less from the date of purchase to be cash equivalents. The Company places its cash and cash equivalents with institutions with high credit quality. However, at certain times such cash and cash equivalents may be in excess of Federal Deposit Insurance Corporation and Securities Investor Protection Corporation insurance limits. The Company has not experienced any losses with respect to these accounts. As of December 31, 2020 and 2021 , the Company classified $ 182,000 as restricted cash related to a letter of credit issued as a security deposit in connection with the Company’s lease of its corporate office facilities (Note 12). Cash, cash equivalents and restricted cash consists of the following (in thousands): December 31, December 31, Cash and cash equivalents $ 71,819 $ 74,268 Restricted cash 182 182 Cash, cash equivalents and restricted cash $ 72,001 $ 74,450
Concentration of Credit Risk	Concentration of Credit Risk Financial instruments that subject the Company to credit risk primarily consist of cash and cash equivalents and short-term investments. The Company maintains its cash and cash equivalent balances with high-quality financial institutions and, consequently, the Company believes that such funds are subject to minimal credit risk. The Company’s short-term investments consist of corporate debt securities. The Company has adopted an investment policy that limits the amounts the Company may invest in any one type of investment and requires all investments held by the Company to hold a minimum rating, thereby reducing credit risk exposure.
Investments	Investments The Company invests excess cash balances in available-for-sale debt securities. The Company determines the appropriate classification of these securities at the time they are acquired and evaluates the appropriateness of such classifications at each balance sheet date. The Company reports available-for-sale investments at fair value at each balance sheet date and includes any unrealized gains and losses in accumulated other comprehensive income (loss), a component of stockholders’ equity. Realized gains and losses are determined using the specific identification method and are included in other income (expense). If any adjustment to fair value reflects a decline in the value of the investment, the Company considers all available evidence to evaluate the extent to which the decline is “other than temporary,” including the intention to sell and, if so, marks the investment to market through a charge to the Company’s consolidated statements of operations and comprehensive loss.
Leases	Leases The Company determines if an arrangement is a lease at inception. Operating leases are included in right-of-use (“ROU”) lease assets, current portion of lease obligations, and long term lease obligations on the Company’s balance sheets. ROU lease assets represent the Company’s right to use an underlying asset for the lease term and lease obligations represent the Company’s obligation to make lease payments arising from the lease. Operating ROU lease assets and obligations are recognized at the commencement date based on the present value of lease payments over the lease term. As most of the Company’s leases do not provide an implicit rate, the Company uses its incremental borrowing rate based on the information available at the commencement date in determining the present value of lease payments. The ROU lease asset excludes lease incentives. The Company’s lease terms include options to extend or terminate the lease when it is reasonably certain that the Company will exercise that option. Lease expense for lease payments is recognized on a straight-line basis over the lease term.
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred. Nonrefundable advance payments, if any, for goods or services used in research and development are initially recorded as an asset and then recognized as an expense as the related goods are delivered or services are performed. Research and development expenses include contract services, consulting, salaries, materials and supplies and overhead.
Income Taxes	Income Taxes The Company accounts for income taxes in accordance with Accounting Standard Codification (“ASC”) 740 Income Taxes (“ASC 740”) . Deferred tax assets and liabilities are recorded to reflect the impact of temporary differences between amounts of assets and liabilities for financial reporting purposes and such amounts as measured under enacted tax laws. A valuation allowance is required to offset any net deferred tax assets if, based upon the available evidence, it is more likely than not that some or all of the deferred tax asset will not be realized. The Company provides reserves for potential payments of tax to various tax authorities related to uncertain tax positions. The tax benefits recorded are based on a determination of whether and how much of a tax benefit taken by the Company in its tax filings or positions is “more likely than not” to be realized following resolution of any uncertainty related to the tax benefit, assuming that the matter in question will be raised by the tax authorities. Potential interest and penalties associated with such uncertain tax positions are recorded as a component of income tax expense. At December 31, 2020 and 2021 , the Company had no such accruals.
Stock-Based Compensation	Stock-Based Compensation Stock-based compensation expense for stock options is recognized based on the grant-date fair value using the Black-Scholes valuation model. Restricted stock units are valued at the fair market value per share of the Company’s common stock on the date of grant. The Company recognizes compensation expense only for those stock-based awards expected to vest after considering expected forfeitures. Cumulative compensation expense is at least equal to the compensation expense for vested awards. Stock-based compensation is recognized on a straight-line basis over the service period of each award.
Segments	Segments 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 (“CODM”) in making decisions regarding resource allocation and assessing performance. The Company’s chief executive officer is the CODM, and he uses consolidated financial information in determining how to allocate resources and assess performance. The Company has determined that it operates in one segment. All of the Company’s assets are located in the United States.
Recently Issued Accounting Standards	Recently Issued Accounting Standards The Company has reviewed recently issued accounting standards to determine if any of those standards will have a significant impact on its financial reporting. None were noted as of December 31, 2021.

Significant Accounting Polici_3

Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2021
Accounting Policies [Abstract]
Schedule of Cash, Cash Equivalents and Restricted Cash	As of December 31, 2020 and 2021 , the Company classified $ 182,000 as restricted cash related to a letter of credit issued as a security deposit in connection with the Company’s lease of its corporate office facilities (Note 12). Cash, cash equivalents and restricted cash consists of the following (in thousands): December 31, December 31, Cash and cash equivalents $ 71,819 $ 74,268 Restricted cash 182 182 Cash, cash equivalents and restricted cash $ 72,001 $ 74,450

Net Loss per Share (Tables)

Net Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2021
Earnings Per Share [Abstract]
Computation of Basic and Diluted Net Loss Per Share of Common Stock	The following table sets forth the computation of basic and diluted net loss per share of common stock (in thousands, except shares and per share data): FOR THE YEAR ENDED DECEMBER 31, DECEMBER 31, Net loss $ ( 32,209 ) $ ( 28,034 ) Weighted—average common shares 24,568,897 27,351,730 Net loss per share, basic and diluted $ ( 1.31 ) $ ( 1.02 )
Schedule of Antidilutive Securities Excluded from Computation of Net Loss Per Share	The following table sets forth the outstanding potentially dilutive securities that have been excluded in the calculation of diluted net loss per share because their inclusion would be anti-dilutive (in common stock equivalent shares): FOR THE YEAR ENDED DECEMBER 31, DECEMBER 31, Stock options to purchase common stock 2,224,913 2,662,752 Unvested restricted stock 80,450 42,250 2,305,363 2,705,002

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2021
Investments, Debt and Equity Securities [Abstract]
Summary of Available-for-Sale Classified Investments	A summary of the Company’s available-for-sale classified investments as of December 31, 2020 and 2021 consisted of the following (in thousands): At December 31, 2020 Investments - Current: Cost Basis Accumulated Unrealized Gains Accumulated Unrealized Losses Fair Value United States Treasury securities $ 8,213 $ - $ - $ 8,213 Corporate debt securities 11,972 1 - 11,973 Commercial paper 13,090 - - 13,090 Total $ 33,275 $ 1 $ - $ 33,276 At December 31, 2021 Investments - Current: Cost Basis Accumulated Unrealized Gains Accumulated Unrealized Losses Fair Value Corporate debt securities $ 1,011 $ - $ ( 1 ) $ 1,010 Total $ 1,011 $ - $ ( 1 ) $ 1,010
Summary of Amortized Cost and Fair Value of Available-for-Sale Investments by Contract Maturity	The amortized cost and fair value of the Company’s available-for-sale investments, by contract maturity, as of December 31, 2021 consisted of the following (in thousands): Amortized Cost Due in one year or less $ 1,011 Total $ 1,011

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2021
Property, Plant and Equipment [Abstract]
Schedule of Purchased Property and Equipment	Purchased property and equipment consist of the following as of December 31, 2020 and 2021 (in thousands): ESTIMATED 2020 2021 Office equipment 5 years $ 10 $ 10 Office furniture 7 years 116 126 Computer equipment 3 years 8 8 Leasehold improvements Life of lease 95 95 229 239 Less: Accumulated depreciation ( 136 ) ( 170 ) Property and equipment, net $ 93 $ 69

Accrued Expenses (Tables)

Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2021
Accrued Liabilities, Current [Abstract]
Schedule of Accrued Expenses	Accrued expenses at December 31, 2020 and 2021 consist of (in thousands): 2020 2021 Contract research and development $ 2,274 $ 2,350 Employee compensation and related costs 1,978 1,152 Consulting and professional service fees 364 265 Interest 88 154 Financing related costs 43 60 State taxes 40 5 Other - 9 Total accrued expenses $ 4,787 $ 3,995

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2021
Income Tax Disclosure [Abstract]
Schedule of Components of Net Deferred Tax Assets	Components of the net deferred tax assets at December 31, 2020 and 2021 are as follows (in thousands): 2020 2021 Deferred tax assets: Federal net operating loss carryforwards $ 10,474 $ 12,813 State net operating loss carryforwards 2,938 3,630 Research and development tax credits 2,963 3,719 Accrued liabilities 588 327 Stock-based compensation 679 1,087 Depreciation and amortization 1,545 247 Capitalized research and development costs 23,977 29,529 Lease liabilities 253 129 Other — 29 Total deferred tax assets $ 43,417 $ 51,510 Deferred tax liabilities: Right-of-use lease assets ( 219 ) ( 111 ) Other ( 119 ) — Total deferred tax liabilities $ ( 338 ) $ ( 111 ) Valuation allowance $ ( 43,079 ) $ ( 51,399 ) Net deferred tax assets $ — $ —
Schedule of Reconciliation of Income Tax (Expense) Benefit	A reconciliation of income tax (expense) benefit at the statutory federal income tax rate and income taxes as reflected in the consolidated financial statements at December 31, 2020 and 2021 are as follows: 2020 2021 Federal income tax at statutory rate 21.00 % 21.00 % State income tax, net of federal benefit 5.65 % 5.97 % Research and development credits 2.03 % 3.16 % Book compensation related to stock options ( 0.91 )% 0.03 % Change in income tax rate 0.73 % 0.45 % Other ( 0.30 )% ( 0.93 )% Increase in valuation allowance ( 28.20 )% ( 29.68 )% Effective tax rate — % — %
Schedule of Reconciliation of Uncertain Tax Benefits	A reconciliation of the beginning and ending amount of uncertain tax benefits is as follows (in thousands): 2020 2021 Beginning uncertain tax benefits $ 628 $ 774 Prior year - increases 1 — Current year - decreases — — Current year - increases 145 203 Ending uncertain tax benefits $ 774 $ 977

Fair Value of Financial Instr_2

Fair Value of Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2021
Fair Value Disclosures [Abstract]
Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis	The following tables summarize the Company’s assets and liabilities that are measured at fair value on a recurring basis and indicates the level of the fair value hierarchy utilized to determine such fair values (in thousands): As of December 31, 2020 TOTAL Quoted Prices Significant Significant Assets: Cash equivalents $ 70,797 $ 68,798 $ 1,999 $ — Total cash equivalents 70,797 68,798 1,999 — United States Treasury securities 8,213 8,213 — — Corporate debt securities 11,973 — 11,973 — Commercial paper 13,090 — 13,090 — Investments 33,276 8,213 25,063 — Total $ 104,073 $ 77,011 $ 27,062 $ — As of December 31, 2021 TOTAL Quoted Prices Significant Significant Assets: Cash equivalents $ 72,449 $ 72,449 $ — $ — Total cash equivalents 72,449 72,449 — — Corporate debt securities 1,010 — 1,010 — Investments 1,010 — 1,010 — Total $ 73,459 $ 72,449 $ 1,010 $ —
Schedule of Changes in Fair Value of Level 3 Derivative Liability	Changes in the fair value of the Company’s Level 3 derivative liability for the year ended December 31, 2020 are as follows: At December 31, 2019 $ 765 Change in fair value of derivative liability 30 Allocation of debt discount to term loan 5 Payment of derivative liability ( 800 ) At December 31, 2020 $ —

Term Loan (Tables)

Term Loan (Tables)	12 Months Ended
	Dec. 31, 2021
Debt Disclosure [Abstract]
Schedule of Total Term Loan and Unamortized Debt Discount	Total term loan and unamortized debt discount balances are as follows (in thousands): DECEMBER 31, DECEMBER 31, Face value $ 20,000 $ 17,500 Less: debt discount ( 734 ) ( 341 ) Total $ 19,266 $ 17,159 Less: current portion ( 2,408 ) ( 9,805 ) Long-term portion $ 16,858 $ 7,354
Schedule of Future Principal Payments Due	As of December 31, 2021, future principal payments due under the 2019 Loan Agreement are as follows (in thousands): Year ended: December 31, 2022 $ 10,000 December 31, 2023 7,500 Total minimum principal payments $ 17,500

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2021
Share-based Payment Arrangement [Abstract]
Schedule of Estimated Fair Value of Options Valuation Assumptions	The fair value of options at date of grant was estimated using the Black-Scholes option-pricing model with the following assumptions: 2020 2021 Risk-free interest rate 0.33 %— 1.71 % 0.50 %— 1.25 % Expected dividend yield 0 % 0 % Expected life 5.5 — 6.6 years 5.5 — 6.7 years Expected volatility 72 %— 75 % 72 %— 74 % Weighted-average grant date $ 4.35 $ 4.18
Summary of Information about Stock Option Activity	The following table summarizes information about stock option activity during 2020 and 2021 (in thousands, except share and per share data): NUMBER OF WEIGHTED- WEIGHTED- AGGREGATE Outstanding, December 31, 2019 1,439,518 $ 6.09 Granted 1,021,705 6.79 Exercised ( 121,671 ) 6.07 Forfeited ( 114,639 ) 9.01 Outstanding, December 31, 2020 2,224,913 $ 6.26 Granted 953,506 6.57 Exercised ( 2,501 ) 3.81 Forfeited ( 513,166 ) 6.72 Outstanding, December 31, 2021 2,662,752 $ 6.28 7.47 $ 993 Vested and exercisable, December 31, 2021 1,368,937 $ 6.08 6.22 $ 877 Vested and expected to vest, December 31, 2021 2,238,516 $ 6.24 7.22 $ 940
Schedule of RSU Activity	The following table summarizes information about RSU activity during 2020 and 2021: RSUs AVERAGE GRANT DATE FAIR VALUE (IN DOLLARS PER SHARE) RSUs outstanding, December 31, 2019 160,900 $ 3.25 Granted — — Vested ( 80,450 ) 3.25 Forfeited — — RSUs outstanding, December 31, 2020 80,450 3.25 Granted — — Vested ( 38,200 ) 3.25 Forfeited — — RSUs outstanding, December 31, 2021 42,250 $ 3.25
Summary of Stock-Based Compensation Expense	The Company recorded stock-based compensation expense in the following expense categories of its accompanying condensed consolidated statements of operations and comprehensive loss for employees, directors and non-employees during the years ended December 31, 2020 and 2021 as follows (in thousands): 2020 2021 Research and development $ 619 $ 947 General and administrative 1,381 1,419 Total $ 2,000 $ 2,366

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2021
Commitments and Contingencies Disclosure [Abstract]
Schedule of Maturity Analysis of Annual Undiscounted Cash Flows of Operating Lease Liabilities	The following is a maturity analysis of the annual undiscounted cash flows reconciled to the carrying value of the operating lease liabilities as of December 31, 2021 (in thousands): Year ended: December 31, 2022 $ 496 Total minimum lease payments 496 Less imputed interest ( 20 ) Total $ 476
Schedule of Lease Cost and Other Information of Operating Lease Liabilities	2020 2021 Lease cost: Operating lease cost $ 491 $ 482 Short-term lease cost - 20 Sublease income ( 51 ) ( 51 ) Total lease cost $ 440 $ 451 Other information Cash paid for amounts included in the $ 529 $ 530 Operating cash flows from operating leases $ ( 45 ) $ ( 60 ) Weighted-average remaining lease term - 1.9 years 0.9 years Weighted-average discount rate - 10.1 % 10.1 %

Description of Business and B_2

Description of Business and Basis of Presentation - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Description Of Business And Basis Of Presentation [Line Items]
Cash, cash equivalents, restricted cash and investments	$ 75,500,000
Working capital	63,400,000
Net loss	28,034,000	$ 32,209,000
Net cash flows used in operating activities	27,151,000	27,693,000
Common stock offer and sell		$ 10,388,000
Common Stock | Maximum | Cowen
Description Of Business And Basis Of Presentation [Line Items]
Aggregate offering price	$ 50,000,000
IPO | Common Stock
Description Of Business And Basis Of Presentation [Line Items]
Number of shares sold		6,220,589
Proceeds from sale of shares, Net		$ 50,200,000
At The Market Offering | Common Stock
Description Of Business And Basis Of Presentation [Line Items]
Number of shares sold		1,502,892
Proceeds from sale of shares, Net		$ 10,400,000

Significant Accounting Polici_4

Significant Accounting Policies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2021USD ($)Segment	Dec. 31, 2020USD ($)
Schedule Of Accounting Policies [Line Items]
Restricted cash, noncurrent		$ 182,000
Restricted cash, current	$ 182,000
Potential interest and penalties associated with uncertain tax positions, accruals	$ 0	0
Number of operating segments | Segment	1
Letter of Credit
Schedule Of Accounting Policies [Line Items]
Restricted cash, noncurrent		$ 182,000
Restricted cash, current	$ 182,000

Significant Accounting Polici_5

Significant Accounting Policies - Schedule of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	$ 74,268	$ 71,819
Restricted cash	182
Restricted cash		182
Cash, cash equivalents and restricted cash	$ 74,450	$ 72,001	$ 72,806

Net Loss per Share - Computatio

Net Loss per Share - Computation of Basic and Diluted Net Loss Per Share of Common Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Earnings Per Share [Abstract]
Net loss	$ (28,034)	$ (32,209)
Weighted—average common shares outstanding, basic and diluted	27,351,730	24,568,897
Net loss per share, basic and diluted	$ (1.02)	$ (1.31)

Net Loss per Share - Schedule o

Net Loss per Share - Schedule of Antidilutive Securities Excluded from Computation of Net Loss Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from calculation of diluted net loss per share	2,705,002	2,305,363
Stock Options to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from calculation of diluted net loss per share	2,662,752	2,224,913
Unvested Restricted Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from calculation of diluted net loss per share	42,250	80,450

Investments - Summary of Availa

Investments - Summary of Available-for-Sale Classified Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Marketable Securities [Line Items]
Cost Basis	$ 1,011	$ 33,275
Accumulated Unrealized Gains		1
Accumulated Unrealized Losses	(1)
Fair Value	1,010	33,276
United States Treasury securities
Marketable Securities [Line Items]
Cost Basis		8,213
Fair Value		8,213
Corporate Debt Securities
Marketable Securities [Line Items]
Cost Basis	1,011	11,972
Accumulated Unrealized Gains		1
Accumulated Unrealized Losses	(1)
Fair Value	$ 1,010	11,973
Commercial Paper
Marketable Securities [Line Items]
Cost Basis		13,090
Fair Value		$ 13,090

Investments - Summary of Amorti

Investments - Summary of Amortized Cost and Fair Value of Available-for-Sale Investments by Contract Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Amortized Cost, Due in one year or less	$ 1,011
Amortized Cost, Total	$ 1,011	$ 33,275

Property and Equipment - Schedu

Property and Equipment - Schedule of Purchased Property and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 239	$ 229
Less: Accumulated depreciation	(170)	(136)
Property and equipment, net	$ 69	93
Office Equipment
Property Plant And Equipment [Line Items]
ESTIMATED USEFUL LIFE	5 years
Property and equipment, gross	$ 10	10
Office Furniture
Property Plant And Equipment [Line Items]
ESTIMATED USEFUL LIFE	7 years
Property and equipment, gross	$ 126	116
Computer Equipment
Property Plant And Equipment [Line Items]
ESTIMATED USEFUL LIFE	3 years
Property and equipment, gross	$ 8	8
Leasehold Improvements
Property Plant And Equipment [Line Items]
ESTIMATED USEFUL LIFE	Life of lease
Property and equipment, gross	$ 95	$ 95

Property and Equipment - Additi

Property and Equipment - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 34,000	$ 34,000

Accrued Expenses - Schedule of

Accrued Expenses - Schedule of Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Accrued Liabilities, Current [Abstract]
Contract research and development	$ 2,350	$ 2,274
Employee compensation and related costs	1,152	1,978
Consulting and professional service fees	265	364
Interest	154	88
Financing related costs	60	43
State taxes	5	40
Other	9
Total accrued expenses	$ 3,995	$ 4,787

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Income Taxes [Line Items]
Net income tax provision or benefit	$ 0	$ 0
Federal net operating loss carryforwards expiration year	2037
State net operating loss carryforwards expiration year	2041
Increase in valuation allowance	$ 8,300,000
Research and development credit carryforwards	$ 3,719,000	$ 2,963,000
Federal and state research and development credit carryforwards expiration year	2041
Federal
Income Taxes [Line Items]
Net operating loss carryforwards	$ 43,500,000
Research and development credit carryforwards	3,000,000
Federal | 2037
Income Taxes [Line Items]
Net operating loss carryforwards	17,500,000
State
Income Taxes [Line Items]
Net operating loss carryforwards	200,000
Research and development credit carryforwards	900,000
State | 2041
Income Taxes [Line Items]
Net operating loss carryforwards	$ 57,500,000

Income Taxes - Schedule of Comp

Income Taxes - Schedule of Components of Net Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Deferred tax assets:
Federal net operating loss carryforwards	$ 12,813	$ 10,474
State net operating loss carryforwards	3,630	2,938
Research and development tax credits	3,719	2,963
Accrued liabilities	327	588
Stock-based compensation	1,087	679
Depreciation and amortization	247	1,545
Capitalized research and development costs	29,529	23,977
Lease liabilities	129	253
Other	29
Total deferred tax assets	51,510	43,417
Deferred tax liabilities:
Right-of-use lease assets	(111)	(219)
Other		(119)
Total deferred tax liabilities	(111)	(338)
Valuation allowance	(51,399)	(43,079)
Net deferred tax assets	$ 0	$ 0

Income Taxes - Schedule of Reco

Income Taxes - Schedule of Reconciliation of Income Tax (Expense) Benefit (Details)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Federal income tax at statutory rate	21.00%	21.00%
State income tax, net of federal benefit	5.97%	5.65%
Research and development credits	3.16%	2.03%
Book compensation related to stock options	0.03%	(0.91%)
Change in income tax rate	0.45%	0.73%
Other	(0.93%)	(0.30%)
Increase in valuation allowance	(29.68%)	(28.20%)
Effective tax rate	0.00%	0.00%

Income Taxes - Schedule of Re_2

Income Taxes - Schedule of Reconciliation of Uncertain Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Beginning uncertain tax benefits	$ 774	$ 628
Prior year - increases		1
Current year - decreases	0	0
Current year - increases	203	145
Ending uncertain tax benefits	$ 977	$ 774

Fair Value of Financial Instr_3

Fair Value of Financial Instruments - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Assets:
Cash equivalents	$ 72,449	$ 70,797
Investments	1,010	33,276
Total	73,459	104,073
United States Treasury Securities
Assets:
Investments		8,213
Corporate Debt Securities
Assets:
Investments	1,010	11,973
Commercial Paper
Assets:
Investments		13,090
Quoted Prices in Active Markets (Level 1) [Member]
Assets:
Cash equivalents	72,449	68,798
Investments		8,213
Total	72,449	77,011
Quoted Prices in Active Markets (Level 1) [Member] | United States Treasury Securities
Assets:
Investments		8,213
Significant Other Observable Inputs (Level 2) [Member]
Assets:
Cash equivalents		1,999
Investments	1,010	25,063
Total	1,010	27,062
Significant Other Observable Inputs (Level 2) [Member] | Corporate Debt Securities
Assets:
Investments	$ 1,010	11,973
Significant Other Observable Inputs (Level 2) [Member] | Commercial Paper
Assets:
Investments		$ 13,090

Fair Value of Financial Instr_4

Fair Value of Financial Instruments - Schedule of Changes in Fair Value of Level 3 Derivative Liability (Details) - Significant Unobservable Inputs (Level 3) [Member] $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 765
Change in fair value of derivative liability	30
Allocation of debt discount to term loan	5
Payment of derivative liability	(800)
Ending balance	$ 0

Term Loan - Additional Informat

Term Loan - Additional Information (Details) - USD ($)	1 Months Ended		12 Months Ended
	Sep. 30, 2019	May 31, 2017	Dec. 31, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Fair value adjustment to derivative liability				$ 30,000
SLR Investment Corp. and Silicon Valley Bank | 2017 Loan Agreement
Debt Instrument [Line Items]
Loan and security agreement, amount		$ 10,000,000
Loan agreement, maturity date		May 1, 2021
Payment of final fee	$ 325,000
SLR Investment Corp. and Silicon Valley Bank | 2019 Loan Agreement
Debt Instrument [Line Items]
Loan and security agreement, amount	$ 20,000,000		$ 17,500,000	20,000,000
Loan agreement, maturity date	Sep. 17, 2023
Amortization of debt discount			392,000	357,000
Final payment fee			500,000
Amortization of final payment fee			$ 162,000	$ 160,000
Loan agreement, term	4 years
Debt instrument, interest rate terms			The interest rate under the 2019 Loan Agreement is the higher of (i) LIBOR plus 7.95% or (ii) 10.18%
Debt instrument, effective interest rate			10.18%
Exit fee, percentage			4.00%
Exit fee			$ 800,000
Derivative liability recorded against exit fee obligation			763,000
Fair value adjustment to derivative liability			$ 30,000
Prepayment premium percentage after year two			0.50%
Increase in interest rate on default			5.00%
SLR Investment Corp. and Silicon Valley Bank | 2019 Loan Agreement | Minimum
Debt Instrument [Line Items]
Interest rate, minimum			10.18%
Prepayment premium percentage two			0.01
SLR Investment Corp. and Silicon Valley Bank | 2019 Loan Agreement | Maximum
Debt Instrument [Line Items]
Prepayment premium percentage one			0.03
SLR Investment Corp. and Silicon Valley Bank | 2019 Loan Agreement | Libor
Debt Instrument [Line Items]
Basis spread on variable base rate			7.95%

Term Loan - Schedule of Total T

Term Loan - Schedule of Total Term Loan and Unamortized Debt Discount (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020	Sep. 30, 2019
Debt Instrument [Line Items]
Less: current portion	$ (9,805)	$ (2,408)
Long-term portion	7,354	16,858
2019 Loan Agreement | SLR Investment Corp. and Silicon Valley Bank
Debt Instrument [Line Items]
Face value	17,500	20,000	$ 20,000
Less: debt discount	(341)	(734)
Total	17,159	19,266
Less: current portion	(9,805)	(2,408)
Long-term portion	$ 7,354	$ 16,858

Term Loan - Schedule of Future

Term Loan - Schedule of Future Principal Payments Due (Details) - 2019 Loan Agreement - SLR Investment Corp. and Silicon Valley Bank - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020	Sep. 30, 2019
Debt Instrument [Line Items]
December 31, 2022	$ 10,000
December 31, 2023	7,500
Total minimum principal payments	$ 17,500	$ 20,000	$ 20,000

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Details) - USD ($)	Mar. 23, 2021	Aug. 23, 2019	Jun. 30, 2020	May 31, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Stockholders Equity Note [Line Items]
Common stock, shares authorized					150,000,000	150,000,000
Common stock, par value					$ 0.0001	$ 0.0001
Common stock, shares issued					27,366,707	27,325,959
Common stock, shares outstanding					27,366,707	27,325,959
Reserved shares of common stock for exercise of outstanding options to purchase common stock					2,662,752	2,224,913	1,439,518
Preferred stock, shares authorized					10,000,000	10,000,000
Preferred stock, par value					$ 0.0001	$ 0.0001
Preferred stock, shares issued					0	0
Preferred stock, shares outstanding					0	0
Cowen | Common Stock | Maximum
Stockholders Equity Note [Line Items]
Aggregate offering price					$ 50,000,000
2019 ATM Agreement | Legal Accounting and Other Costs
Stockholders Equity Note [Line Items]
Stock issuance cost		$ 189,000
2019 ATM Agreement | Additional Paid-in Capital | Legal Accounting and Other Costs
Stockholders Equity Note [Line Items]
Stock issuance cost						$ 135,000
2019 ATM Agreement | Jefferies | Common Stock
Stockholders Equity Note [Line Items]
Percentage of commission on gross sales proceeds		3.00%
Number of shares sold						1,502,892
Proceeds from sale of shares, Net						$ 10,400,000
2019 ATM Agreement | Jefferies | Common Stock | Weighted Average
Stockholders Equity Note [Line Items]
Selling price per share						$ 7.13
2019 ATM Agreement | Jefferies | Common Stock | Maximum
Stockholders Equity Note [Line Items]
Aggregate offering price		$ 15,000,000
2021 ATM Agreement | Cowen | Common Stock
Stockholders Equity Note [Line Items]
Proceeds from sale of shares, Net					$ 0
2021 ATM Agreement | Cowen | Common Stock | Maximum
Stockholders Equity Note [Line Items]
Aggregate offering price	$ 50,000,000
Percentage of commission on gross sales proceeds	3.00%
Underwritten Public Offering | Common Stock
Stockholders Equity Note [Line Items]
Number of shares sold			440,242	5,780,347
Selling price per share			$ 8.65	$ 8.65
Proceeds from sale of shares, Net			$ 3,600,000	$ 46,600,000
Underwritten Public Offering | Common Stock | Maximum
Stockholders Equity Note [Line Items]
Option granted to purchase additional shares				867,052
Restricted Stock Units (RSUs)
Stockholders Equity Note [Line Items]
Number of reserved shares of common stock for vesting of RSUs					42,250

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Additional options granted	953,506	1,021,705
Stock option, number of shares outstanding	2,662,752	2,224,913	1,439,518
Cash received upon exercise of stock options	$ 9,000	$ 736,000
Intrinsic value of options exercised	7,000	$ 388,000
Unrecognized compensation expense	$ 2,700,000
Recognition period (in years)	2 years 3 months 18 days
Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of restricted stock units outstanding	42,250
Unrecognized compensation expense	$ 22,000
Recognition period (in years)	8 months 12 days
Performance Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Additional options granted	112,050
2017 Stock Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option and incentive plan effective date	Nov. 30, 2017
Stock option and incentive plan expiration date	Oct. 31, 2027
Stock option, number of shares outstanding	2,062,064
Number of common stock initially reserved for issuance	5,051,920
Stock options, available for issuance	2,917,802
Term of option granted	10 years
2017 Stock Plan | Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option vesting term	1 year
2017 Stock Plan | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option vesting term	4 years
2017 Stock Plan | Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of restricted stock units outstanding	42,250
2014 Stock Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option and incentive plan expiration date	Nov. 30, 2017
Additional options granted	0
Stock option, number of shares outstanding	600,688
Number of options forfeited	359,651
2017 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of common stock shares issued	11,253
Number of common stock shares available for issuance	983,629

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Estimated Fair Value of Options Valuation Assumptions (Details) - $ / shares	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate, minimum	0.50%	0.33%
Risk-free interest rate, maximum	1.25%	1.71%
Expected dividend yield	0.00%	0.00%
Expected volatility, minimum	72.00%	72.00%
Expected volatility, maximum	74.00%	75.00%
Weighted-average grant date fair value	$ 4.18	$ 4.35
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected life	5 years 6 months	5 years 6 months
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected life	6 years 8 months 12 days	6 years 7 months 6 days

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Information about Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
NUMBER OF SHARES
Number of shares outstanding, beginning balance	2,224,913	1,439,518
Number of shares granted	953,506	1,021,705
Number of shares exercised	(2,501)	(121,671)
Number of shares forfeited	(513,166)	(114,639)
Number of shares outstanding, ending balance	2,662,752	2,224,913
Vested and exercisable, ending balance	1,368,937
Vested and expected to vest, ending balance	2,238,516
WEIGHTED-AVERAGE EXERCISE PRICE
Weighted average exercise prices, outstanding beginning balance	$ 6.26	$ 6.09
Weighted average exercise prices, granted	6.57	6.79
Weighted average exercise prices, exercised	3.81	6.07
Weighted average exercise prices, forfeited	6.72	9.01
Weighted average exercise prices, outstanding ending balance	6.28	$ 6.26
Weighted average exercise prices, vested and exercisable ending balance	6.08
Weighted average exercise prices, vested and expected to vest ending balance	$ 6.24
WEIGHTED-AVERAGE REMAINING CONTRACTUAL TERM
Weighted average remaining contractual term, outstanding	7 years 5 months 19 days
Weighted average remaining contractual term, vested and exercisable	6 years 2 months 19 days
Weighted average remaining contractual term, vested and expected to vest	7 years 2 months 19 days
AGGREGATE INTRINSIC VALUE
Aggregate intrinsic value, outstanding ending balance	$ 993
Aggregate intrinsic value, vested and exercisable ending balance	877
Aggregate intrinsic value, vested and expected to vest ending balance	$ 940

Stock-Based Compensation - Sc_2

Stock-Based Compensation - Schedule of RSU Activity (Details) - Restricted Stock Units (RSUs) - $ / shares	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
RSUs outstanding	80,450	160,900
RSUs Granted	0	0
RSUs Vested	(38,200)	(80,450)
RSUs Forfeited	0	0
RSUs outstanding	42,250	80,450
AVERAGE GRANT DATE FAIR VALUE, Outstanding	$ 3.25	$ 3.25
AVERAGE GRANT DATE FAIR VALUE, Granted	0	0
AVERAGE GRANT DATE FAIR VALUE, Vested	3.25	3.25
AVERAGE GRANT DATE FAIR VALUE, Forfeited	0	0
AVERAGE GRANT DATE FAIR VALUE, RSUs outstanding	$ 3.25	$ 3.25

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Stock Based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 2,366	$ 2,000
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	947	619
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 1,419	$ 1,381

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details) - USD ($)	1 Months Ended		12 Months Ended
	Jul. 30, 2021	Feb. 28, 2018	Dec. 31, 2021	Dec. 31, 2020
Commitments And Contingencies [Line Items]
Operating leases rent expense			$ 500,000	$ 500,000
Restricted cash			182,000
Short-term lease asset			410,000	815,000
Short-term lease liability			476,000	$ 460,000
Operating sublease term	12 months	3 years
Short-term Leases Recognized on Straight-line Basis
Commitments And Contingencies [Line Items]
Short-term lease asset			0
Short-term lease liability			0
Letter of Credit
Commitments And Contingencies [Line Items]
Restricted cash			$ 182,000
Lexington, Massachusetts
Commitments And Contingencies [Line Items]
Operating leases expiration date			Dec. 31, 2022
Burlington, Massachusetts
Commitments And Contingencies [Line Items]
Operating leases expiration date			Nov. 30, 2022
Burlington, Massachusetts | Letter of Credit | Lease Agreement Require Standby Letter of Credit
Commitments And Contingencies [Line Items]
Restricted cash			$ 182,000

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Maturity Analysis of Annual Undiscounted Cash Flows of Operating Lease Liabilities (Details) $ in Thousands	Dec. 31, 2021USD ($)
Year ended:
December 31, 2022	$ 496
Total minimum lease payments	496
Less imputed interest	(20)
Total	$ 476

Commitments and Contingencies_3

Commitments and Contingencies - Schedule of Lease Cost and Other Information of Operating Lease Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Lease cost:
Operating lease cost	$ 482	$ 491
Short-term lease cost	20
Sublease income	(51)	(51)
Total lease cost	451	440
Other information
Cash paid for amounts included in the measurement of liabilities	530	529
Operating cash flows from operating leases	$ (60)	$ (45)
Weighted-average remaining lease term - operating leases	10 months 24 days	1 year 10 months 24 days
Weighted-average discount rate - operating leases	10.10%	10.10%

401(k) Savings Plan - Additiona

401(k) Savings Plan - Additional Information (Details) - USD ($)	Jul. 31, 2014	Dec. 31, 2021	Dec. 31, 2020
Defined Contribution Plan Disclosure [Line Items]
Defined Contribution Plan, Tax Status [Extensible List]		us-gaap:QualifiedPlanMember	us-gaap:QualifiedPlanMember
Compensation expense for employer match contribution		$ 198,000	$ 151,000
First 3% of Deferred Compensation
Defined Contribution Plan Disclosure [Line Items]
Employer matching contribution percentage	100.00%
Employer matching contribution percentage of employees contributions of deferred compensation	3.00%
Defined Contribution Plan, Tax Status [Extensible List]	us-gaap:QualifiedPlanMember
Next 2% of Deferred Compensation
Defined Contribution Plan Disclosure [Line Items]
Employer matching contribution percentage	50.00%
Employer matching contribution percentage of employees contributions of deferred compensation	2.00%
Defined Contribution Plan, Tax Status [Extensible List]	us-gaap:QualifiedPlanMember