0001628280-22-004101 | 10-K | XBRL Viewer

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Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Cash flows from operating activities
Net loss	$ (170,060)	$ (88,045)	$ (28,208)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	855	1,010	1,245
Net amortization of premiums and discounts on investments	0	(86)	(222)
Stock-based compensation	18,409	7,172	4,872
Other non-cash items	723	148	103
Changes in operating assets and liabilities:
Accounts receivable	0	0	459
Prepaid expenses and other current assets	(2,734)	(1,950)	2,179
Other assets	(718)	(700)	0
Accounts payable	483	942	(3,110)
Accrued expenses	12,570	7,280	(3,569)
Operating lease assets	1,829	1,642	1,771
Operating lease liabilities	(1,827)	(1,281)	(1,883)
Deferred revenue	(43)	(828)	(41,381)
Other liabilities	525	0	0
Net cash used in operating activities	(139,988)	(74,696)	(67,744)
Cash flows from investing activities
Maturities of marketable securities	0	37,500	27,000
Purchase of marketable securities	0	0	(53,688)
Purchase of property and equipment	(648)	(473)	(605)
Net cash (used in) provided by investing activities	(648)	37,027	(27,293)
Cash flows from financing activities
Proceeds from exercise of stock options	1,837	3,138	175
Proceeds from purchases of common stock under ESPP	640	561	489
Payment of employee tax obligations related to vesting of restricted stock units	(259)	0	0
Proceeds from issuance of debt, net of issuance costs	24,042	(197)	4,965
Repayment of debt	(5,486)	0	0
Payments under finance lease obligations	(215)	(116)	(87)
Net cash provided by financing activities	63,646	230,412	97,704
Increase (decrease) in cash, cash equivalents and restricted cash	(76,990)	192,743	2,667
Cash, cash equivalents and restricted cash, beginning of period	255,415	62,672	60,005
Cash, cash equivalents and restricted cash, end of period	178,425	255,415	62,672
Supplemental disclosures of non-cash activities:
Fair value of common stock retired in exchange for issuance of common stock warrant	0	0	8,986
Purchases of property and equipment in accounts payable and accrued expenses	0	102	0
Debt financing costs in accrued expenses	0	0	180
Cash paid for interest	429	234	132
Right-of-use assets obtained in exchange for operating lease liabilities	3,783	9,980	4,369
Right-of-use assets obtained in exchange for financing lease liabilities	609	0	429
Public Stock Offering
Cash flows from financing activities
Net proceeds from issuance of common stock	0	163,990	92,162
At Market Equity Offering Program
Cash flows from financing activities
Net proceeds from issuance of common stock	$ 43,087	$ 63,036	$ 0

Commitments - License agreements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
License Agreement, Upfront Payments
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Research and development expense	$ 3,075	$ 250	$ 750
License Agreement, Milestone Payments
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Research and development expense	$ 2,125	$ 750	$ 600

Consolidated Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Issuance costs			$ 5,587
Exercise of stock options (in shares)	421,381
Exercise of stock options	$ 1,837	$ 3,138
Common Stock
Exercise of stock options (in shares)	421,381	697,428
Additional Paid-in Capital
Exercise of stock options	$ 1,837	$ 3,138
At market equity offering program (2018 ATM)
Issuance costs		2,176
Public Stock Offering
Issuance costs		$ 10,809
At Market Equity Offering Program (2020 ATM)
Issuance costs	$ 988

Accrued expenses (Tables)

12 Months Ended

Dec. 31, 2021

Accrued Liabilities, Current [Abstract]

Schedule of accrued expenses

Accrued expenses consist of the following as of December 31, 2021 and 2020:


	December 31, 2021		December 31, 2020
Accrued manufacturing expenses	$	8,476	$	4,157
Accrued clinical expenses	7,879		5,126
Accrued payroll and related expenses	7,319		5,412
Accrued preclinical expenses	3,848		619
Accrued professional fees	909		757
Accrued other	285		75
	$	28,716	$	16,146

Stock Options - Activity Under Stock Option Plan (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Number of Shares
Outstanding at beginning of period (in shares)	6,112,948
Options granted (in shares)	3,308,220
Exercised (in shares)	(421,381)
Cancelled (in shares)	(657,358)
Outstanding at end of period (in shares)	8,342,429	6,112,948
Vested and expected to vest (in shares)	8,342,429
Options exercisable, at end of period (in shares)	3,997,529
Weighted- Average Exercise Price
Outstanding at beginning of period (in dollars per share)	$ 7.84
Granted (in dollars per share)	16.78
Exercised (in dollars per share)	4.36
Cancelled (in dollars per share)	11.76
Outstanding at end of period (in dollars per share)	11.25	$ 7.84
Vested and expected to vest (in dollars per share)	11.25
Options exercisable, at end of period (in dollars per share)	$ 7.63
Additional Disclosures
Weighted Average Remaining Contractual Term, Options outstanding	7 years 2 months 12 days	7 years 3 months 18 days
Weighted Average Remaining Contractual Term, Options, Vested and expected to vest	7 years 2 months 12 days
Weighted Average Remaining Contractual Term, Options exercisable	5 years 6 months
Aggregate Intrinsic Value, Options outstanding	$ 8,458	$ 114,729
Aggregate Intrinsic Value, Options, Vested and expected to vest	8,458
Aggregate Intrinsic Value, Options exercisable	$ 7,539

Stock Options (Tables)

12 Months Ended

Dec. 31, 2021

Share-based Payment Arrangement [Abstract]

Schedule of stock option activity

A summary of the activity is as follows:


	Number of Shares	Weighted- Average Exercise Price		Weighted Average Remaining Contractual Term	Aggregate Intrinsic Value
Outstanding at January 1, 2021	6,112,948	$	7.84	7.3	$	114,729
Granted	3,308,220	$	16.78
Exercised	(421,381)	$	4.36
Cancelled	(657,358)	$	11.76
Outstanding at December 31, 2021	8,342,429	$	11.25	7.2	$	8,458
Vested and expected to vest at December 31, 2021	8,342,429	$	11.25	7.2	$	8,458
Exercisable at December 31, 2021	3,997,529	$	7.63	5.5	$	7,539

Schedule of restricted stock unit activity

A summary of the RSU activity under the 2017 Plan is as follow:


	Number of Shares	Weighted-Average Remaining Contractual Term	Aggregate Intrinsic Value		Weighted-Average Grant Date Fair Value
Unvested at January 1, 2021	716,767	1.0	$	19,073	$	6.00
Granted	715,716	—			$	18.39
Vested	(419,336)	—			$	5.33
Forfeited	(195,538)	—			$	12.34
Unvested at December 31, 2021	817,609	1.5	$	5,086	$	15.68

Schedule of stock-based compensation expense by award type

The following table presents stock-based compensation expense by award type included within the Company’s consolidated statement of operations and comprehensive loss:


	Year ended December 31,
	2021		2020		2019
Stock options	$	14,528	$	5,725	$	4,230
Restricted stock units	3,522		1,187		410
Employee stock purchase plan	359		260		232
Stock-based compensation expense included in Total operating expenses	$	18,409	$	7,172	$	4,872

Schedule of stock-based compensation expense as reflected in the Company’s condensed consolidated statements of operations and comprehensive loss

The following table presents stock-based compensation expense as reflected in the Company’s consolidated statements of operations and comprehensive loss:


	Year ended December 31,
	2021		2020		2019
Research and development	$	9,984	$	3,841	$	2,245
General and administrative	8,425		3,331		2,627
Stock-based compensation expense included in Total operating expenses	$	18,409	$	7,172	$	4,872

Schedule of weighted average assumptions for estimating fair value of option awards

The fair value of each option award is estimated on the date of grant using the Black–Scholes option pricing model with the following weighted average assumptions:


	December 31,
	2021		2020		2019
Risk-free interest rate	0.9	%	1.2	%	2.3	%
Expected dividend yield	—	%	—	%	—	%
Expected term (years)	6.06		6.05		5.99
Expected stock price volatility	82	%	74	%	74	%

Collaboration Agreements - Narrative (Detail)		1 Months Ended	3 Months Ended	12 Months Ended			60 Months Ended
Collaboration Agreements - Narrative (Detail)	Jan. 02, 2019	Jun. 30, 2014 USD ($) target milestone	Mar. 31, 2019 USD ($)	Dec. 31, 2021 USD ($) target	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($) agreement	Dec. 31, 2019 USD ($) agreement	Jan. 01, 2018 USD ($)	Jan. 31, 2016 target
Collaboration agreements
Deferred revenue				$ 3,944,000	$ 3,987,000	$ 4,815,000	$ 4,815,000
Amounts included in the contract liability at the beginning of the period				43,000	828,000
Collaboration revenue				43,000	828,000	42,123,000
Research and development				132,013,000	67,036,000	55,040,000
Merck KGaA
Collaboration agreements
Upfront payment received		$ 12,000,000
Number of targets \| target		6
Amount of additional fees receivable when target is designated and commercial license to target is granted		$ 0
Aggregate milestones		780,000,000
Development milestones		84,000,000
Regulatory milestones		264,000,000
Sales milestones		$ 432,000,000
Number of individual development milestones per target \| milestone		6
Number of individual regulatory milestones per target \| milestone		5
Number of individual commercial milestones \| milestone		3
Amount of development milestone payment received				0	0		$ 3,000,000
Next potential development milestone payment eligible to receive				$ 500,000
Prior written notice for termination period				60 days
Number of targets designated \| target				6
Revenue, transaction price, amount				$ 21,325,000	21,325,000
Number of service obligations completed of designated targets \| target				4
Collaboration revenue				$ 43,000	828,000	853,000
Milestone Payment Not Received, Included in Estimated Transaction Price								$ 0
Merck KGaA \| License and corresponding research and development services, first and second targets
Collaboration agreements
Revenue, transaction price, amount				3,941,000
Merck KGaA \| License and corresponding research and development services, third and sixth target
Collaboration agreements
Revenue, transaction price, amount				3,439,000
Merck KGaA \| License and corresponding research and development services, fourth target
Collaboration agreements
Revenue, transaction price, amount				3,152,000
Merck KGaA \| License and corresponding research and development services, fifth target
Collaboration agreements
Revenue, transaction price, amount				2,746,000
Merck KGaA \| Rights to future technological improvements
Collaboration agreements
Revenue, transaction price, amount				$ 425,000
Merck KGaA \| Rights to future technological improvements \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Collaboration agreements
Expected recognition period				10 years
Merck KGaA \| Joint research committee services
Collaboration agreements
Revenue, transaction price, amount				$ 242,000
Merck KGaA \| Joint research committee services \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Collaboration agreements
Expected recognition period				5 years
Merck KGaA Agreement and Merck KGaA supply agreement
Collaboration agreements
Accounts receivable				$ 0	0
Deferred revenue				3,944,000	3,987,000
XMT-1522 agreement
Collaboration agreements
Period of time until agreement terminated	30 days
Period of shared development costs following effective termination date	30 days
Research and development						200,000
ASC 808 costs billed to Takeda						$ 195,000
2016 Restated Tokeda agreement
Collaboration agreements
Number of targets \| target									7
Period of time until agreement terminated	45 days
2016 Restated Tokeda agreement and XMT-1522 agreement
Collaboration agreements
Amounts included in the contract liability at the beginning of the period			$ 39,965,000
Number of performance obligations \| agreement						14	14
Collaboration revenue				0	0	$ 39,965,000
2016 Restated Tokeda agreement and XMT-1522 agreement \| License and related research and development services, including phase 1 development and transfer certain materials and know how
Collaboration agreements
Number of performance obligations \| agreement						1	1
Asana BioSciences
Collaboration agreements
Next potential milestone payment eligible to receive				2,500,000
Asana BioSciences \| Limited services
Collaboration agreements
Collaboration revenue				0	0	$ 25,000
Merck KGaA supply agreement
Collaboration agreements
Collaboration revenue				$ 0	$ 0	$ 1,280,000

Employee Benefit Plan (Details) - USD ($) $ in Thousands	7 Months Ended	12 Months Ended			17 Months Ended
Employee Benefit Plan (Details) - USD ($) $ in Thousands	Jul. 31, 2019	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2020
Retirement Benefits [Abstract]
Eligible age to participate		21 years
Maximum contribution by employee (as a percent)		95.00%
Percentage employer matches of the employee's percentage contribution matched	50.00%	100.00%			100.00%
Employer matching contribution (as a percent)	6.00%	4.00%			4.00%
Maximum annual amount employer contributes per employee	$ 6	$ 7			$ 7
Employee benefit expenses		$ 764	$ 486	$ 404

Collaboration Agreements (Tables)

12 Months Ended

Dec. 31, 2021

Collaboration agreements

Schedule of changes in the balances of our contract assets and liabilities and recognized revenue

The following table presents changes in the balances of our contract assets and liabilities during the years ended December 31, 2021 and December 31, 2020:


	Balance at Beginning of Period		Additions		Deductions		Balance at End of Period
Year ended December 31, 2021
Contract assets	$	—	$	—	$	—	$	—
Contract liabilities:
Deferred revenue	$	3,987	$	—	$	43	$	3,944


	Balance at Beginning of Period		Additions		Deductions		Balance at End of Period
Year ended December 31, 2020
Contract assets	$	—	$	—	$	—	$	—
Contract liabilities:
Deferred revenue	$	4,815	$	—	$	828	$	3,987

During the year ended December 31, 2021, the Company recognized the following revenues as a result of changes in the contract asset and the contract liability balances in the respective periods.


	Year ended December 31,
	2021		2020
Revenue recognized in the period from:
Amounts included in the contract liability at the beginning of the period	$	43	$	828
Performance obligations satisfied in previous periods	$	—	$	—

Summary of Significant Accounting Policies - Property and Equipment (Details) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Property, equipment and software, net
Asset impairment charges	$ 0	$ 0	$ 0
Computer equipment and office equipment
Property, equipment and software, net
Estimated useful life	3 years
Laboratory equipment
Property, equipment and software, net
Estimated useful life	5 years
Leasehold improvements
Property, equipment and software, net
Leasehold improvements, estimated useful lives	Shorter of useful life or life of lease

Summary of Significant Accounting Policies - Other Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Other Assets
Other assets	$ 2,356	$ 2,153
Held by service provider	1,418	1,832
Letter of credit for security deposit	478	$ 321
Debt
Deferred financing costs	410
Line of Credit \| New Credit Facility
Debt
Deferred financing costs	$ 460

Collaboration Agreements

12 Months Ended

Dec. 31, 2021

Collaboration agreements

Collaboration Agreements

Merck KGaA

In June 2014, the Company entered into a Collaboration and Commercial License Agreement with Merck KGaA (the Merck KGaA Agreement). Upon the execution of the Merck KGaA Agreement, Merck KGaA paid the Company a non-refundable technology access fee of $12,000 for the right to develop ADCs directed to six exclusive targets over a specified period of time. No additional fees are due when a target is designated and the commercial license to the target is granted. Merck KGaA will be responsible for the product development and marketing of any products resulting from this collaboration.

Under the terms of the Merck KGaA Agreement, the Company and Merck KGaA develop research plans to evaluate Merck KGaA's antibodies as ADCs incorporating the Company's technology. The Company receives reimbursement for its efforts under the research plans. The goal of the research plans is to provide Merck KGaA with sufficient information to formally nominate a development candidate and begin IND-enabling studies or cease development on the designated target.

In addition to the payments received for research and development activities performed on behalf of Merck KGaA, the Company could be eligible to receive up to a total of $780,000 in future milestones related to all targets under the Merck KGaA Agreement, plus low to mid-single digit royalties on the commercial sales of any resulting products during the applicable royalty term. The total milestones are categorized as follows: development milestones $84,000; regulatory milestones $264,000; and sales milestones $432,000. There are six individual development milestones per target, payable upon the completion of various activities, from the delivery of ADCs meeting defined specifications, through the dosing in a Phase 3 clinical trial. There are five regulatory milestones, which are payable upon regulatory approvals for a first indication in each of the U.S., European Union and Japanese markets and regulatory approvals for both a second and a third indication in the United States. There are three individual commercial milestones, which are payable upon the attainment of certain defined thresholds for annual net sales.

Prior to 2020, the Company had received $3,000 related to development milestones under the Merck KGaA Agreement. There have been no additional milestone payments in the years ended December 31, 2021 or 2020. The next potential milestone payment the Company will be eligible to receive will be a development milestone of $500 on Merck KGaA's designation of a preclinical development candidate for any target. Revenue will be recognized when achievement of the milestone is considered probable.

Unless earlier terminated, the Merck KGaA Agreement will expire upon the expiration of the last royalty term for a product under the Merck KGaA Agreement, after which time, Merck KGaA will have a perpetual, royalty-free license, or if Merck KGaA does not designate any ADC product candidates produced by the Company under the Merck KGaA Agreement as preclinical development candidates, upon the expiration of the last to expire research program. Merck KGaA may terminate the Merck KGaA Agreement in its entirety or with respect to any target for convenience upon 60 days' prior written notice. Each party may terminate the Merck KGaA Agreement in its entirety upon bankruptcy or similar proceedings of the other party or upon an uncured material breach of the Merck KGaA Agreement by the other party. However, if such breach only relates to one target, the agreement may only be terminated with respect to such target.

In May 2018, the Company entered into a Supply Agreement with Merck KGaA (the Merck KGaA Supply Agreement). Under the terms of the Merck KGaA Supply Agreement, the Company will provide Merck KGaA preclinical non-GMP ADC drug substance and clinical GMP drug substance for use in clinical trials associated with one of the antibodies designated under the Merck KGaA Agreement. The Company receives fees for its efforts under the Merck KGaA Supply Agreement and reimbursement equal to the supply cost. The Company may also enter into future supply agreements to provide clinical supply material should Merck KGaA pursue clinical development of any other candidates nominated under the Merck KGaA Agreement.

Accounting Analysis

The Company identified the following performance obligations under the Merck KGaA Agreement: (i) exclusive license and research services for six designated targets, (ii) rights to future technological improvements and (iii) participation of project team leaders and providing joint research committee services.

The Company has concluded that each license for a designated target is not distinct from the research services performed related to the designated target as Merck KGaA cannot obtain the benefit of the license without the related research services. Each license for a designated target and the related services performance obligation is considered distinct from every other license for a designated target and related services performance obligation as each research plan is pursued independent of every other research plan for other designated targets.

The Company utilizes the expected value approach to estimate the amount of consideration related to the payment of fees associated with development and research services. The Company utilizes the most likely amount approach to estimate any development and regulatory milestone payments to be received. As of the date of initial application of Topic 606, there were no milestones payments that had not already been received, included in the estimated transaction price. The Company considered the stage of development and the remaining risks associated with the remaining development required to achieve the milestone, as well as whether the achievement of the milestone is outside the control of the Company or Merck KGaA. The milestone payment amounts were fully constrained, as a result of the uncertainty whether any of the associated milestones would be achieved. The Company has determined that any commercial milestones and sales based royalties will be recognized when the related sales occur as they were determined to relate predominantly to the license granted and therefore have also been excluded from the transaction price.

The transaction price was allocated to the performance obligations based on the relative estimated standalone selling prices of each performance obligation or in the case of certain variable consideration to one or more performance obligations. The estimated standalone selling prices for performance obligations, that include a license and research services, were developed using the estimated selling price of the license and an estimate of the overall effort to perform the research service and an estimated market rate for research services. The estimated standalone selling price of the licenses was established based on comparable transactions. The estimated standalone selling price for the rights to future technological improvements was developed based on the estimated selling prices of a license or rights received, as well as considering the probability that additional technology would be made available or the probability the counterpart would utilize the technology. The estimated standalone selling price for the joint research committee services was developed using an estimate of the time and costs incurred to participate in the committees.

The Company re-evaluates the transaction price in each reporting period and as uncertain events are resolved or other changes in circumstances occur. As of December 31, 2021 and 2020, the total estimated transaction price for the Merck KGaA Agreement was $21,325. The transaction price of $21,325 was allocated to the performance obligations as follows: approximately $3,941 for each of the license and corresponding research and development services units of account for the first and second designated targets; $3,439 for each of the license and corresponding research and development services units of account for the third and sixth designated target; $3,152 for the license and corresponding research and development services unit of account for the fourth designated target; $2,746 for the license and corresponding research and development services unit of account for the fifth designated target; $425 for rights to future technological improvements; and $242 for joint research committee services.

The Company is recognizing revenue related to the exclusive license and research and development services performance obligation over the estimated period of the research and development services using a proportional performance model. The Company measures proportional performance based on the costs incurred relative to the total costs expected to be incurred. To

the extent that the Company receives fees for the research services as they are performed, these amounts are recorded as deferred revenue. Revenue related to future technological improvements and joint research committee services will be recognized ratably over the performance period (which in the case of the joint research committee services approximate the time and cost incurred each period), which are 10 and 5 years, respectively. The Company is continuing to reassess the estimated remaining term at each subsequent reporting period.

As of December 31, 2021, the Company has completed its research service obligations associated with four of the six designated targets. During the years ended December 31, 2021, 2020 and 2019, the Company recorded collaboration revenue of $43, $828 and $853, respectively, related to its efforts under the Merck KGaA Agreement. During the year ended December 31, 2019, the Company recognized collaboration revenue and corresponding research and development expense of $1,280 related to the Merck KGaA Supply Agreement. There were no amounts recognized during the years ended December 31, 2021 and 2020 related to the Merck KGaA Supply Agreement. There was no balance in accounts receivable related to the Merck KGaA Agreement and Merck KGaA Supply Agreement as of either December 31, 2021 or December 31, 2020.

As of December 31, 2021 and 2020, the Company had recorded $3,944 and $3,987, respectively, in deferred revenue related to the Merck KGaA Agreement and Merck KGaA Supply Agreement that will be recognized over the remaining performance period.

Takeda XMT-1522 Strategic Partnership

In January 2016, the Company entered into a Development Collaboration and Commercial License Agreement with Takeda Pharmaceutical, Inc.'s wholly owned subsidiary, Millennium Pharmaceuticals, Inc. for the development and commercialization of XMT-1522 (the XMT-1522 Agreement). Under the XMT-1522 Agreement, Takeda was granted the exclusive right to commercialize XMT-1522 outside of the United States and Canada. Under the XMT-1522 Agreement, the Company was responsible for conducting certain Phase 1 development activities for XMT-1522, including the ongoing Phase 1 clinical trial, at its own expense. The parties agreed to collaborate on the further development of XMT-1522 in accordance with a global development plan (Post-Phase 1 Development). On January 2, 2019, the Company received notice from Takeda stating that Takeda was exercising its right to terminate the XMT-1522 Agreement upon 30 days’ prior written notice. The XMT-1522 Agreement terminated in accordance with its provisions, and the Company and Takeda wound down activities related to the XMT-1522 Agreement as of March 31, 2019. Under the XMT-1522 Agreement, the Company and Takeda shared equally all agreed Post-Phase 1 Development costs through the date of termination and for a period of 30 days after the effective termination date.

Takeda Strategic Research and Development Partnership

In March 2014, the Company entered into a Research Collaboration and Commercial License Agreement with Takeda’s wholly owned subsidiary, Millennium Pharmaceuticals, Inc. (the 2014 Agreement). The 2014 Agreement was amended in January 2015 and amended and restated in January 2016 (the 2016 Restated Agreement). The agreements provided Takeda with the right to develop ADCs directed to a total of seven exclusive targets, designated by Takeda, over a specified period of time. On January 2, 2019, the Company received notice from Takeda stating that Takeda was exercising its right to terminate the 2016 Restated Agreement upon 45 days’ prior written notice. The 2016 Restated Agreement terminated in accordance with its provisions, and the Company and Takeda wound down activities related to the 2016 Restated Agreement as of March 31, 2019.

Accounting Analysis

The Company’s collaboration agreements with Takeda were terminated following receipt of written notices during the first quarter of 2019. As there are no further performance obligations, the Company recognized the remaining deferred revenue of $39,965 related to the termination of the Takeda agreements in the first quarter of 2019.

Prior to the termination of the agreements, the Company had identified 14 performance obligations in the agreements. The Company concluded that the license related to each of the designated targets was not distinct from the research services performed related to each of the designated targets as Takeda could not have obtain the benefit of the license without the related research services. Each license to a designated target and the related service performance obligation was considered distinct from every other license to a designated target and related services performance obligation as each research plan was pursued independent of the any other research plans for other designated targets. Further, the material rights provided were determined

to be distinct from the other performance obligations in the arrangement as they were options in the contract Takeda agreements and not required for Takeda to obtain the benefit of the other promised goods or services in the arrangement. Similarly, the Company concluded that the XMT-1522 license and the related research and development services, including the Phase 1 development and the transfer of certain materials and know-how related to the Company's manufacturing processes, were one performance obligation. The license to the Company's intellectual property was not determined to be distinct from the research and related development services that the Company was obligated to perform. For the year ended December 31, 2019, the Company recorded total revenue of $39,965 related to its efforts under the 2016 Restated Agreement and the XMT-1522 Agreement. The Company did not record any revenue under the 2016 Restated Agreement and the XMT-1522 Agreement in the years ended December 31, 2021 and 2020.

The Company concluded that the Post-Phase 1 Development activities under the XMT-1522 Agreement represented joint operating activities in which both parties were active participants and of which both parties were exposed to significant risks and rewards that are dependent on the commercial success of the activities. Accordingly, the Company accounted for the Post-Phase 1 Development activities in accordance with ASC 808. For the year ended December 31, 2019, the Company was billed approximately $200 from Takeda representing Post-Phase 1 Development costs incurred by Takeda. These amounts have been reflected as research and development costs in the consolidated statement of operations. During the year ended December 31, 2019, the Company billed Takeda $195 related to ASC 808 costs.

Summary of Contract Assets and Liabilities

The following table presents changes in the balances of our contract assets and liabilities during the years ended December 31, 2021 and December 31, 2020:


	Balance at Beginning of Period		Additions		Deductions		Balance at End of Period
Year ended December 31, 2021
Contract assets	$	—	$	—	$	—	$	—
Contract liabilities:
Deferred revenue	$	3,987	$	—	$	43	$	3,944


	Balance at Beginning of Period		Additions		Deductions		Balance at End of Period
Year ended December 31, 2020
Contract assets	$	—	$	—	$	—	$	—
Contract liabilities:
Deferred revenue	$	4,815	$	—	$	828	$	3,987

During the year ended December 31, 2021, the Company recognized the following revenues as a result of changes in the contract asset and the contract liability balances in the respective periods.


	Year ended December 31,
	2021		2020
Revenue recognized in the period from:
Amounts included in the contract liability at the beginning of the period	$	43	$	828
Performance obligations satisfied in previous periods	$	—	$	—

Other Revenue

The Company has provided limited services for a collaboration partner, Asana BioSciences. For the years ended December 31, 2021, 2020 and 2019, the Company recorded revenue of $0, $0 and $25, respectively, related to these services. The next

potential milestone the Company is eligible to receive is $2,500 upon dosing the fifth patient in a Phase 1 clinical trial by Asana BioSciences. As of December 31, 2021, the Company considered this next milestone to be fully constrained as there is considerable judgment involved in determining whether it is probable that a significant revenue reversal would occur. As part of its evaluation of the constraint, the Company considered numerous factors, including the fact that achievement of the milestone is outside the control of the Company and there is a high level of uncertainty in achieving this milestone, as this would require initiation of clinical trials by the collaboration partner. The Company reevaluates the probability of achievement of a milestone subject to constraint at each reporting period and as uncertain events are resolved or other changes in circumstances occur.

Stockholders' Equity - Narrative (Details)			1 Months Ended	12 Months Ended				20 Months Ended
Stockholders' Equity - Narrative (Details)	Mar. 02, 2020 shares	Nov. 26, 2019 $ / shares shares	Jun. 30, 2020 USD ($) $ / shares shares	Dec. 31, 2021 USD ($) vote shares	Dec. 31, 2020 USD ($) shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2013 $ / shares shares	Dec. 31, 2021 USD ($) shares	May 31, 2020 USD ($)	Jul. 31, 2018 USD ($)
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares)				25,000,000	25,000,000			25,000,000
Preferred stock, shares issued (in shares)				0	0			0
Number of shares into which warrant issued during period may be converted							129,491
Warrant exercise price per share (in dollars per share) \| $ / shares							$ 0.05
Contractual life of warrants							10 years
Warrants outstanding (in shares)				39,474	39,474			39,474
Common stock issued upon exercise of warrant (in shares)				0
Number of shares reserved for future issuance (in shares)				9,199,512	6,869,189			9,199,512
Common Stock
Class of Stock [Line Items]
Issuance of common upon exercise of warrant (in shares)	2,574,971
Number of votes per common share \| vote				1
Exchange Warrants
Class of Stock [Line Items]
Warrant exercise price per share (in dollars per share) \| $ / shares		$ 0.0001
Warrants outstanding (in shares)				2,575,000				2,575,000
Number of common shares exchanged for warrants (in shares)		2,575,000
Number of shares of common stock into which warrants may be converted (in shares)		2,575,000
Warrants and share-based payment awards
Class of Stock [Line Items]
Number of shares reserved for future issuance (in shares)				9,199,512	6,869,189			9,199,512
At market equity offering program (2018 ATM)
Class of Stock [Line Items]
Amount of shares authorized to be offered and sold \| $										$ 75,000,000
Proceeds from issuance of common stock \| $					$ 62,976,000
At market equity offering program (2018 ATM) \| Common Stock
Class of Stock [Line Items]
Number of shares sold (in shares)					10,900,599
At market equity offering program (2018 ATM) \| Additional Paid-in Capital
Class of Stock [Line Items]
Proceeds from issuance of common stock \| $					$ 62,976,000
At Market Equity Offering Program (2020 ATM)
Class of Stock [Line Items]
Amount of shares authorized to be offered and sold \| $									$ 100,000,000
Proceeds from issuance of common stock \| $								$ 43,087,000
At Market Equity Offering Program (2020 ATM) \| Common Stock
Class of Stock [Line Items]
Number of shares sold (in shares)								3,961,074
Public Stock Offering
Class of Stock [Line Items]
Number of shares sold (in shares)			9,200,000
Proceeds from issuance of common stock \| $				$ 0	$ 163,990,000	$ 92,162,000
Shares sold, price per share (in dollars per share) \| $ / shares			$ 19.00
Proceeds from stock issuance, net of issuance costs \| $			$ 163,990,000
Public Stock Offering \| Common Stock
Class of Stock [Line Items]
Number of shares sold (in shares)					9,200,000	24,437,500

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	At market equity offering program (2018 ATM)	Public Stock Offering	At Market Equity Offering Program (2020 ATM)	Common Stock	Common Stock At market equity offering program (2018 ATM)	Common Stock Public Stock Offering	Additional Paid-in Capital	Additional Paid-in Capital Public Stock Offering	Additional Paid-in Capital At Market Equity Offering Program (2020 ATM)	Accumulated Other Comprehensive Loss	Accumulated Deficit
Balance at beginning of period (in shares) at Dec. 31, 2018					23,234,472
Balance at beginning of period at Dec. 31, 2018	$ 8,795				$ 3			$ 172,966			$ (8)	$ (164,166)
Increase (Decrease) in Stockholders' Equity
Issuance of stock (in shares)							24,437,500
Issuance of stock			$ 92,162				$ 2		$ 92,160
Exercise of stock options and warrants (in shares)					150,978
Exercise of stock options and warrants	175							175
Purchase of common stock under ESPP (in shares)					140,073
Purchase of common stock under ESPP	489							489
Retirement of common stock in exchange for common stock warrant (in shares)					(2,575,000)
Retirement of common stock in exchange for common stock warrant	(8,986)							(8,986)
Issuance of common stock warrant in exchange for retirement of common stock	8,986							8,986
Stock-based compensation expense	4,872							4,872
Other comprehensive income (loss)	33										33
Net loss	(28,208)											(28,208)
Balance at end of period (in shares) at Dec. 31, 2019					45,388,023
Balance at end of period at Dec. 31, 2019	78,318				$ 5			270,662			25	(192,374)
Increase (Decrease) in Stockholders' Equity
Issuance of stock (in shares)						10,900,599	9,200,000
Issuance of stock		$ 62,977	$ 163,991			$ 1	$ 1		$ 163,990
Purchase of common stock under ESPP (in shares)					80,267
Purchase of common stock under ESPP	561							561
Exercise of common stock warrant in exchange for common stock (in shares)					2,574,971
Exercise of stock options (in shares)					697,428
Exercise of stock options	3,138							3,138
Stock-based compensation expense	7,172							7,172
Other comprehensive income (loss)	(25)										(25)
Net loss	(88,045)											(88,045)
Balance at end of period (in shares) at Dec. 31, 2020					68,841,288
Balance at end of period at Dec. 31, 2020	228,087				$ 7			508,499			0	(280,419)
Increase (Decrease) in Stockholders' Equity
Issuance of stock				$ 43,087						$ 43,087
Purchase of common stock under ESPP (in shares)					78,253
Purchase of common stock under ESPP	$ 640							640
Exercise of stock options (in shares)	421,381				421,381
Exercise of stock options	$ 1,837							1,837
Stock-based compensation expense	18,409							18,409
Vesting of restricted stock units, net of employee tax obligation (in shares)					407,060
Vesting of restricted stock units, net of employee tax obligation	(259)							(259)
Net loss	(170,060)											(170,060)
Balance at end of period (in shares) at Dec. 31, 2021					73,709,056
Balance at end of period at Dec. 31, 2021	$ 121,741				$ 7			$ 572,213			$ 0	$ (450,479)

Summary of Significant Accounting Policies - Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash, cash equivalents and restricted cash
Cash and cash equivalents	$ 177,947	$ 255,094	$ 62,351
Restricted cash included in other assets, noncurrent	478	321	321
Total cash, cash equivalents and restricted cash per statement of cash flows	$ 178,425	$ 255,415	$ 62,672	$ 60,005

Leases - Components of Lease Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating lease cost	$ 3,502	$ 2,755	$ 2,160
Finance lease cost:
Amortization of right-of-use assets	169	101	75
Interest on lease liabilities	28	21	20
Finance lease cost	$ 197	$ 122	$ 95

Debt (Tables)

12 Months Ended

Dec. 31, 2021

Debt Disclosure [Abstract]

Schedule of debt

As of December 31, 2021, there was $25,000 outstanding under the New Credit Facility and the debt consisted of the following:


	December 31, 2021
Total debt	$	25,000
Less: Current portion of long-term-debt	—
Total debt, net of current portion	25,000
Debt financing costs, net of accretion	(410)
Accretion related to final payment	36
Long-term debt, net	$	24,626

Schedule of estimated future payments

As of December 31, 2021, the estimated future principal payments due are as follows:


2022	$	—
2023	—
2024	2,083
2025	12,500
2026	10,417
Total debt	$	25,000

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2021

Accounting Policies [Abstract]

Schedule of cash and cash equivalents


	Year ended December 31, 2021				Year ended December 31, 2020
	Beginning of period		End of period		Beginning of period		End of period
Cash and cash equivalents	$	255,094	$	177,947	$	62,351	$	255,094
Restricted cash included in other assets, noncurrent	321		478		321		321
Total cash, cash equivalents and restricted cash per statement of cash flows	$	255,415	$	178,425	$	62,672	$	255,415

Schedule of restricted cash


	Year ended December 31, 2021				Year ended December 31, 2020
	Beginning of period		End of period		Beginning of period		End of period
Cash and cash equivalents	$	255,094	$	177,947	$	62,351	$	255,094
Restricted cash included in other assets, noncurrent	321		478		321		321
Total cash, cash equivalents and restricted cash per statement of cash flows	$	255,415	$	178,425	$	62,672	$	255,415

Schedule of outstanding potentially dilutive securities excluded from calculation of diluted net loss per share

The following table sets forth the outstanding potentially dilutive securities that have been excluded from the calculation of diluted net loss per share because to include them would be anti-dilutive (in common stock equivalent shares):


	Year ended December 31,
	2021	2020	2019
Stock options	8,342,429	6,112,948	4,720,772
Unvested restricted stock units	817,609	716,767	447,336
Warrants	39,474	39,474	39,474
	9,199,512	6,869,189	5,207,582

Schedule of estimated useful life of property and equipment

Depreciation is computed using the straight-line method over the estimated useful life of each asset as follows:


Computer equipment, office equipment and software			3 years
Laboratory equipment			5 years
Leasehold improvements			Shorter of useful life or life of lease

Debt - Narrative (Details) - USD ($)				12 Months Ended
Debt - Narrative (Details) - USD ($)	Oct. 29, 2021	Aug. 28, 2020	May 08, 2019	Dec. 31, 2021	Dec. 31, 2020
Debt
Debt outstanding				$ 25,000,000.0
Interest expense				797,000	$ 340,000
Existing Credit Facility \| Line of Credit
Debt
Proceeds from line of credit			$ 5,000,000
Existing Credit Facility \| Minimum \| Line of Credit
Debt
Debt interest rate percentage			4.00%
Existing Credit Facility \| Prime rate \| Minimum \| Line of Credit
Debt
Spread (as a percent)			(1.50%)
Amended Credit Facility \| Line of Credit
Debt
Proceeds from line of credit		$ 5,200,000
Repayments of lines of credit	$ 5,500,000
Loss on extinguishment of debt				$ 398,000
Amended Credit Facility \| Minimum \| Line of Credit
Debt
Debt interest rate percentage		4.25%
Amended Credit Facility \| Prime rate \| Minimum \| Line of Credit
Debt
Spread (as a percent)		1.00%
New Credit Facility \| Line of Credit
Debt
Proceeds from line of credit	25,000,000	$ 25,000,000
Maximum borrowing capacity	$ 100,000,000
Final payment as percentage of original principal amount	4.25%
Additional rate in event of default	5.00%
Payments of debt issuance costs	$ 958,000
New Credit Facility \| Line of Credit \| Capitalized Other Assets
Debt
Payments of debt issuance costs	533,000
New Credit Facility \| Line of Credit \| Lenders
Debt
Payments of debt issuance costs	210,000
New Credit Facility \| Line of Credit \| Lenders \| Amortized Interest Expense
Debt
Payments of debt issuance costs	151,000
New Credit Facility \| Line of Credit \| Third Parties
Debt
Payments of debt issuance costs	748,000
New Credit Facility \| Line of Credit \| Third Parties \| Amortized Interest Expense
Debt
Payments of debt issuance costs	$ 273,000
New Credit Facility \| Line of Credit \| Prepayment Occurs Prior To October 29, 2022
Debt
Prepayment fee percentage	3.00%
New Credit Facility \| Line of Credit \| Prepayment Occurs Between October 29, 2022 and October 29, 2023
Debt
Prepayment fee percentage	2.00%
New Credit Facility \| Line of Credit \| Prepayment Occurs After October 29, 2023 but Before October 1, 2026
Debt
Prepayment fee percentage	1.00%
New Credit Facility \| Minimum \| Line of Credit
Debt
Debt interest rate percentage	8.50%
New Credit Facility \| Prime rate \| Minimum \| Line of Credit
Debt
Spread (as a percent)	5.25%
New Credit Facility, Tranche One \| Line of Credit
Debt
Maximum borrowing capacity	$ 60,000,000
New Credit Facility, Tranche Two \| Line of Credit
Debt
Maximum borrowing capacity	60,000,000
New Credit Facility, Tranche Three \| Line of Credit
Debt
Maximum borrowing capacity	60,000,000
New Credit Facility, Tranche Four \| Line of Credit
Debt
Maximum borrowing capacity	10,000,000
New Credit Facility, Tranche Five \| Line of Credit
Debt
Maximum borrowing capacity	10,000,000
New Credit Facility, Tranche Six \| Line of Credit
Debt
Maximum borrowing capacity	$ 20,000,000

Summary of Significant Accounting Policies - Net loss per share (Details) - shares	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Net Loss per Share
Warrants outstanding (in shares)	39,474	39,474
Anti-dilutive securities	9,199,512	6,869,189	5,207,582
Stock options
Net Loss per Share
Anti-dilutive securities	8,342,429	6,112,948	4,720,772
Unvested restricted stock units
Net Loss per Share
Anti-dilutive securities	817,609	716,767	447,336
Warrants
Net Loss per Share
Anti-dilutive securities	39,474	39,474	39,474
Exchange Warrants
Net Loss per Share
Warrants outstanding (in shares)	2,575,000

Employee Benefit Plan	12 Months Ended
Employee Benefit Plan	Dec. 31, 2021
Retirement Benefits [Abstract]
Employee Benefit Plan	Employee Benefit PlanThe Company has a defined contribution plan established under Section 401(k) of the Internal Revenue Code (401(k) Plan), which covers substantially all employees. Employees who have attained the age of 21 are eligible to participate in the 401(k) Plan with no service requirement. Employees may contribute up to 95% of eligible pay on a pre–tax basis up to the federal annual limits. For the period from January 1, 2019 to July 31, 2019, the Company matched the employees’ contributions at 50% on the first 6% up to $6. For the period from August 1, 2019 to December 31, 2020 and for the year ended December 31, 2021, the Company matched the employees’ contributions at 100% on the first 4% up to $7. For the years ended December 31, 2021, 2020 and 2019, the Company recorded expense of $764, $486 and $404, respectively, related to its contribution to its 401(k) Plan.

- Definition

The entire disclosure for retirement benefits.

+ References

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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

Income Taxes - Narrative (Details) - USD ($)	12 Months Ended
Income Taxes - Narrative (Details) - USD ($)	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income tax benefit	$ 0	$ 0	$ 0

- Definition

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

+ References + Details

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2021

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

Principles of Consolidation

The accompanying consolidated financial statements include those of the Company and its wholly owned subsidiary, Mersana Securities Corp. All intercompany balances and transactions have been eliminated.

Use of Estimates

The preparation of the Company's consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, equity, revenue, expenses and related disclosure of contingent assets and liabilities at the date of the financial statements and reported amounts of revenue and expenses during the reporting period. On an ongoing basis, the Company's management evaluates its estimates which include, but are not limited to, management's judgments with respect to the identification of performance obligations and standalone selling prices of those performance obligations within its revenue arrangements, accrued preclinical, manufacturing and clinical expenses, valuation of stock-based awards and income taxes. Actual results could differ from those estimates.

Segment Information

Operating segments are defined as components of an enterprise about which separate discrete information is available for evaluation by the chief operating decision-maker, or decision making group, in deciding how to allocate resources and assess performance. The Company views its operations and manages its business as a single operating segment, which is the business of discovering and developing ADCs.

Research and Development

Research and development costs are expensed as incurred and include:

•employee-related expenses, including salaries, bonuses, benefits, travel and stock-based compensation expense;

•fees and expenses incurred under agreements with contract research organizations, investigative sites and other entities in connection with the conduct of preclinical studies, clinical trials and related services;

•the cost of acquiring, developing and manufacturing ADC product candidates, clinical trial materials and other research and development materials;

•fees and costs related to regulatory filings and activities;

•costs associated with collaboration agreements and license fees and milestone payments related to license agreements;

•costs associated with creating and obtaining approval for the NaPi2b companion or complementary diagnostic biomarker;

•facilities, depreciation and other expenses, which include direct and allocated expenses for rent, utilities, maintenance of facilities, insurance and other supplies; and

•other costs associated with clinical, preclinical, discovery and other research activities.

Costs for certain development activities, such as preclinical studies, clinical trials and manufacturing development activities, are recognized based on an evaluation of the progress to completion of specific tasks using data such as patient enrollment, clinical site activations, and information provided to the Company by its vendors on their actual costs incurred or level of effort

expended. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected on the consolidated balance sheets as prepaid or accrued preclinical, manufacturing and clinical expenses.

Revenue Recognition

The Company enters into collaboration agreements which are within the scope of Accounting Standards Update (ASU) No. 2014-9, Revenue from Contracts with Customers (Topic 606), under which the Company licenses rights to its technology and certain of the Company’s product candidates and performs research and development services for third parties. The terms of these arrangements typically include payment of one or more of the following: non-refundable, up-front fees; reimbursement of research and development costs; development, regulatory and commercial milestone payments; and royalties on net sales of licensed products.

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

The promised good or services in the Company’s arrangement typically consist of license rights to the Company’s intellectual property and research and development services. The Company also has optional additional items in contracts, which are considered marketing offers and are accounted for as separate contracts with the customer if such option is elected by the customer, unless the option provides a material right which would not be provided without entering into the contract. Performance obligations are promised goods or services in a contract to transfer a distinct good or service to the customer. Promised goods or services are considered distinct when (i) the customer can benefit from the good or service on its own or together with other readily available resources or (ii) the promised good or service is separately identifiable from other promises in the contract. In assessing whether promised goods or services are distinct, the Company considers factors such as the stage of development of the underlying intellectual property, the capabilities of the customer to develop the intellectual property on their own or whether the required expertise is readily available.

The Company estimates the transaction price based on the amount expected to be received for transferring the promised goods or services in the contract. The consideration may include both fixed consideration and variable consideration. At the inception of each arrangement that includes variable consideration and at each reporting period, the Company evaluates the amount of potential payment and the likelihood that the payments will be received. The Company utilizes either the most likely amount method or expected value method to estimate the amount expected to be received based on which method better predicts the amount of consideration to which the Company will be entitled. If it is probable that a significant revenue reversal would not occur, the variable consideration is included in the transaction price. We assessed each of our revenue generating arrangements in order to determine whether a significant financing component exists and concluded that a significant financing component does not exist in any of our arrangements because: (a) the promised consideration approximates the cash selling price of the promised goods and services; and (b) timing of payment approximates the transfer of goods and services and performance is over a relatively short period of time within the context of the entire term of the contract.

The Company’s contracts often include development and regulatory milestone payments. At contract inception and at each reporting period, the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the Company’s control or the licensee’s control, such as regulatory approvals, are not included in the transaction price. At the end of each subsequent reporting period, the Company re-evaluates the probability of achievement of such development milestones and any related constraint, and if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect license, collaboration and other revenues and earnings in the period of adjustment.

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

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

For performance obligations consisting of licenses and other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue from non-refundable, up-front fees. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. If the license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company will recognize revenue from non-refundable, up-front fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license.

The Company receives payments from its customers based on billing schedules established in each contract. Such billings generally have 30-day terms. Up-front payments and fees are recorded as deferred revenue upon receipt or when due until the Company performs its obligations under these arrangements. Amounts are recorded as accounts receivable when the right to consideration is unconditional.

Collaborative Arrangements

The Company records the elements of its collaboration agreements that represent joint operating activities in accordance with ASC Topic 808, Collaborative Arrangements (ASC 808). Accordingly, the elements of the collaboration agreements that represent activities in which both parties are active participants and to which both parties are exposed to the significant risks and rewards that are dependent on the commercial success of the activities, are recorded as collaborative arrangements. The Company also considers the guidance in ASC Topic 606 by analogy in determining the appropriate treatment for the transactions between the Company and its collaborative partners and the transactions between the Company and third parties. Generally, the classification of transactions under the collaborative arrangements is determined based on the nature and contractual terms of the arrangement along with the nature of the operations of the participants. To the extent revenue is generated from a collaboration, the Company will recognize its share of the net sales on a gross basis if it is deemed to be the principal in the transactions with customers, or on a net basis if it is instead deemed to be the agent in the transactions with customers, consistent with the guidance in Topic 606.

Fair Value Measurements

Fair value is defined as the price that would be received upon sale of an asset or paid to transfer a liability between market participants at measurement dates. ASC Topic 820, Fair Value Measurement (ASC 820), establishes a three-level valuation

hierarchy for instruments measured at fair value. The hierarchy is based on the transparency of inputs to the valuation of an asset or liability as of the measurement date. The three levels are defined as follows:

Level 1—Inputs to the valuation methodology are quoted prices (unadjusted) for identical assets or liabilities in active markets.

Level 2—Inputs to the valuation methodology include quoted prices for similar assets and liabilities in active markets, and inputs that are observable for the asset or liability, either directly or indirectly, for substantially the full term of the financial instrument.

Level 3—Inputs to the valuation methodology are unobservable and significant to the fair value measurement.

Cash and Cash Equivalents

The Company considers all highly liquid investments with an original maturity, or a remaining maturity at the time of purchase, of three months or less to be cash equivalents. The Company invests excess cash primarily in money market funds, commercial paper and government agency securities, which are highly liquid and have strong credit ratings. These investments are subject to minimal credit and market risks. Cash and cash equivalents are stated at cost, which approximates market value.


	Year ended December 31, 2021				Year ended December 31, 2020
	Beginning of period		End of period		Beginning of period		End of period
Cash and cash equivalents	$	255,094	$	177,947	$	62,351	$	255,094
Restricted cash included in other assets, noncurrent	321		478		321		321
Total cash, cash equivalents and restricted cash per statement of cash flows	$	255,415	$	178,425	$	62,672	$	255,415

Other Assets

The Company recorded other assets of $2,356 and $2,153 as of December 31, 2021 and 2020, respectively, comprised of $1,418 and $1,832, respectively, held by a service provider, restricted cash of $478 and $321, respectively, held as a security deposit for a standby letter of credit related to a facility lease, and $460 as of December 31, 2021 of deferred financing costs related to the New Credit Facility (as defined below) with Silicon Valley Bank (SVB) and Oxford Financial LLC (Oxford). For additional information regarding the New Credit Facility, please refer to Note 7, Debt, to these consolidated financial statements.

Accounting for Stock-based Compensation

The Company accounts for its stock-based compensation in accordance with ASC Topic 718 Compensation—Stock Compensation (ASC 718). ASC 718 requires all stock-based payments to employees, directors and non-employees to be recognized as expense in the statements of operations based on their grant date fair values. The Company estimates the fair value of options granted using the Black-Scholes option pricing model. The Black-Scholes option pricing model requires inputs based on certain subjective assumptions, including (a) the expected stock price volatility, (b) the calculation of expected term of the award, (c) the risk-free interest rate and (d) expected dividends. Due to the lack of a public market for the Company's common stock prior to completion of the initial public offering and a lack of company-specific historical and implied volatility data, the Company has based its estimate of expected volatility on the historical volatility of a group of similar companies that are publicly traded. The historical volatility is calculated based on a period of time commensurate with the expected term assumption. The computation of expected volatility is based on the historical volatility of a representative group of companies with similar characteristics to the Company, including stage of product development and life science industry focus. The Company uses the simplified method as prescribed by the SEC Staff Accounting Bulletin No. 107, Share-Based Payment, to calculate the expected term for options granted to employees as it does not have sufficient historical exercise data to provide a reasonable basis upon which to estimate the expected term. For options granted to non-employees, the Company utilizes the contractual term of the arrangement as the basis for the expected term assumption. The risk-free interest rate is based on a

treasury instrument whose term is consistent with the expected term of the stock options. The expected dividend yield is assumed to be zero as the Company has never paid dividends and has no current plans to do so.

The Company determines the fair value of each restricted stock unit (RSU), at its grant date based on the closing market price of the Company’s common stock on that date. For stock-based compensation subject to service-based vesting conditions, the Company recognizes stock-based compensation expense equal to the grant date fair value of stock-based compensation on a straight-line basis over the requisite service period.

The Company records forfeitures as a cumulative adjustment in the period in which they occur.

Net Loss per Share

Basic net loss per share of common stock is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of shares outstanding and, for the year ended December 31, 2019, 2,575,000 Exchange Warrants (as defined in Note 8, Stockholders' Equity) outstanding during the period, without further consideration for potentially dilutive securities. In accordance with ASC Topic 260, Earnings Per Share, the Exchange Warrants are included in the computation of basic net loss per share because the exercise price is negligible and they are fully vested and exercisable at any time after the original issuance date. Diluted net loss per share is computed by dividing the net loss attributable to common stockholders by the weighted-average number of common shares and potentially dilutive securities outstanding for the period determined using the treasury stock method. For purposes of the diluted net loss per share calculation, stock options, unvested RSUs and warrants to purchase common stock are considered to be potentially dilutive securities, but are excluded from the calculation of diluted net loss per share because their effect would be anti-dilutive and therefore, basic and diluted net loss per share were the same for all periods presented.


	Year ended December 31,
	2021	2020	2019
Stock options	8,342,429	6,112,948	4,720,772
Unvested restricted stock units	817,609	716,767	447,336
Warrants	39,474	39,474	39,474
	9,199,512	6,869,189	5,207,582

Property and Equipment

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


Computer equipment, office equipment and software			3 years
Laboratory equipment			5 years
Leasehold improvements			Shorter of useful life or life of lease

Upon retirement or sale, the cost of the assets disposed of and the related accumulated depreciation are eliminated from the balance sheet and related gains or losses are reflected in the statement of operations. There were no material sales of assets during the years ended December 31, 2021, 2020 and 2019.

The Company reviews its long-lived assets for impairment whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. If an impairment review is performed to evaluate an asset for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the asset to its carrying value. If the carrying amount of the asset exceeds its estimated undiscounted future net cash flows, an impairment charge is recognized in the amount by which the carrying amount of the asset exceeds the fair value of the asset. The Company did not recognize impairment charges during the years ended December 31, 2021, 2020 and 2019.

Leases

Consistent with ASC Topic 842, Leases, the Company determines if an arrangement is a lease at inception. Operating leases are included in right-of-use lease assets (ROU assets), current portion of lease obligations and long-term lease obligations on the Company’s consolidated balance sheets. Assets subject to finance leases are included in property and equipment, and the related lease obligation is included in other current liabilities and other long-term liabilities on the Company’s consolidated balance sheets. Lease assets are tested for impairment in the same manner as long-lived assets used in operations. Lease expense for operating leases is recognized on a straight-line basis over the lease term as an operating expense while expense for financing leases is recognized as depreciation expense and interest expense using the effective interest method. The Company has elected the short-term lease recognition exemption for short-term leases, which allows the Company not to recognize lease liabilities and ROU assets on the consolidated balance sheets for leases with an original term of twelve months or less.

ROU 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 lease liabilities and their corresponding ROU assets are initially recorded based on the present value of lease payments over the expected remaining lease term. When determining the lease term, the Company includes options to extend or terminate the lease when it is reasonably certain that the option will be exercised. Certain adjustments to the ROU asset may be required for items such as incentives received. The interest rate implicit in lease contracts is typically not readily determinable. As a result, the Company utilizes its incremental borrowing rate to discount lease payments. The incremental borrowing rate reflects the fixed rate at which the Company could borrow, on a collateralized basis, the amount of the lease payments in the same currency, for a similar term, in a similar economic environment. Prospectively, the Company will adjust the ROU assets for straight-line rent expense, or any incentives received and remeasure the lease liability at the net present value using the same incremental borrowing rate that was in effect as of the lease commencement or transition date.

The Company has lease agreements with lease and non-lease components, which are generally accounted for separately.

Patent Costs

The Company expenses patent application and related legal costs as incurred and classifies such costs as general and administrative expenses in the accompanying consolidated statements of operations.

Income Taxes

The Company accounts for income taxes in accordance with ASC Topic 740, Accounting for Income Taxes, which provides for deferred taxes using an asset and liability approach. The Company recognizes deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements or tax returns. The Company determines its deferred tax assets and liabilities based on differences between financial reporting and tax bases of assets and liabilities, which are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. Valuation allowances are provided if, based upon the weight of available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized.

The Company recognizes the effect of income tax positions only if those positions are more likely than not to be sustained. Recognized income tax positions are measured at the largest amount that is more likely than not to be realized. Changes in recognition or measurement are reflected in the period in which the change in judgment occurs.

Comprehensive Income (Loss)

Comprehensive income (loss) comprises net loss and other comprehensive loss. For the year ended December 31, 2021, comprehensive loss equaled net loss. For the years ended December 31, 2020 and 2019, other comprehensive income (loss) consisted of changes in unrealized income and loss on marketable securities.

Concentration of Credit Risk and Off-balance Sheet Risk

The Company has no financial instruments with off-balance sheet risk, such as foreign exchange contracts, option contracts, or other foreign hedging arrangements. Financial instruments that potentially subject the Company to concentrations of credit risk

primarily consist of cash equivalents and marketable securities. Under its investment policy, the Company limits amounts invested in such securities by credit rating, maturity, industry group, investment type and issuer, except for securities issued by the U.S. government. The Company has not experienced any credit losses and does not believe that it is subject to any significant concentrations of credit risk from these financial instruments.

Recently Adopted Accounting Pronouncements

In December 2019, the FASB issued ASU No. 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes, as part of its initiative to reduce complexity in the accounting standards. The amendments in ASU 2019-12 eliminate certain exceptions related to the approach for intraperiod tax allocation, the methodology for calculating income taxes in an interim period and the recognition of deferred tax liabilities for outside basis differences. ASU 2019-12 also clarifies and simplifies other aspects of the accounting for income taxes. The amendments in ASU 2019-12 are effective for the fiscal years beginning after December 15, 2020. The adoption of this standard did not have a material impact on the Company’s consolidated financial statements and related disclosures.

Property and Equipment (Tables)

12 Months Ended

Dec. 31, 2021

Property, Plant and Equipment [Abstract]

Schedule of property and equipment

Property and equipment consists of the following as of December 31, 2021 and 2020:


	December 31, 2021		December 31, 2020
Laboratory equipment	$	6,725	$	6,520
Leasehold improvements	1,906		1,886
Computer equipment and office equipment	1,019		959
Total property and equipment at cost	9,650		9,365
Less: Accumulated depreciation	(7,682)		(7,635)
	$	1,968	$	1,730

Stock Options - Fair Value Assumptions (Details)	12 Months Ended
Stock Options - Fair Value Assumptions (Details)	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Share-based Payment Arrangement [Abstract]
Risk-free interest rate	0.90%	1.20%	2.30%
Expected dividend yield	0.00%	0.00%	0.00%
Expected term (years)	6 years 21 days	6 years 18 days	5 years 11 months 26 days
Expected stock price volatility	82.00%	74.00%	74.00%

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2021

Income Tax Disclosure [Abstract]

Schedule of reconciliation of income tax rates

A reconciliation of income taxes computed using the U.S. federal statutory rate to that reflected in operations as of December 31, 2021, 2020 and 2019 are as follows:


	2021		2020		2019
Income tax computed at federal statutory tax rate	21.0	%	21.0	%	21.0	%
State taxes, net of federal benefit	6.3	%	6.7	%	6.1	%
Permanent differences	(0.2)	%	1.2	%	(2.0)	%

General business credits	3.8	%	3.4	%	10.3	%
Stock compensation	0.1	%	—	%	—	%
Impact of ownership shift	—	%	—	%	(53.3)	%
Change in valuation allowance	(31.0)	%	(32.3)	%	17.9	%
	—	%	—	%	—	%

Schedule of significant components of net deferred tax assets

Significant components of the Company’s net deferred tax assets as of December 31, 2021 and 2020 are as follows:


	2021		2020
Deferred tax assets:
Net operating losses	$	106,055	$	64,259
Tax credit carryforwards	12,424		5,670
Accrued expenses	6,892		4,058
Lease liabilities	3,698		3,166
Licensed technology	2,775		1,534
Deferred revenue	1,076		1,088
Depreciation	493		502

Other	71		84
Total gross deferred tax assets	133,484		80,361
Valuation allowance	(130,051)		(77,375)
Net deferred tax assets less valuation allowance	3,433		2,986
Deferred tax liabilities
Right-of-use assets	(3,433)		(2,986)
Total gross deferred tax liabilities	(3,433)		(2,986)
Net deferred taxes	$	—	$	—

Stock Options - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		12 Months Ended
	Jan. 31, 2021	Jun. 30, 2017	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2017
Share-based awards
Number of shares outstanding (in shares)			8,342,429	6,112,948
Options granted (in shares)			3,308,220
Weighted-average grant date fair value of options granted (in dollars per share)			$ 11.71	$ 7.99	$ 2.47
Intrinsic value of options exercised			$ 4,299	$ 11,147	$ 202
Cash received from the exercise of stock options (in dollars)			$ 1,837	$ 3,138	$ 175
Expected dividend yield			0.00%	0.00%	0.00%
Number of shares reserved for future issuance (in shares)			9,199,512	6,869,189
Employee stock purchase plan
Share-based awards
Number of shares available for future issuance (in shares)			566,565
Number of shares reserved for future issuance (in shares)						225,000
Purchase of common stock under ESPP (in shares)			78,253	80,267
Stock options
Share-based awards
Unrecognized stock compensation cost			$ 38,958
Weighted-average amortization period of unrecognized stock compensation cost			2 years 7 months 6 days
Unvested restricted stock units
Share-based awards
Fair value of RSUs vested			$ 5,790	$ 0	$ 0
Unrecognized stock compensation cost			$ 9,929
Weighted-average amortization period of unrecognized stock compensation cost			2 years 9 months 18 days
Unvested restricted stock units \| Maximum
Share-based awards
Vesting period			4 years
Unvested restricted stock units \| Minimum
Share-based awards
Vesting period			1 year
2007 Plan
Share-based awards
Number of shares outstanding (in shares)		3,141,625
2017 Plan
Share-based awards
Number of shares authorized (in shares)		2,255,000
Cumulative annual increase in number of shares issuable, percentage		4.00%
Additional shares authorized (in shares)	2,753,651
Number of shares available for future issuance (in shares)			1,308,183
Options granted (in shares)			2,870,720
2017 Plan \| Maximum
Share-based awards
Expiration period			10 years
2017 Plan \| Stock options
Share-based awards
Vesting period			4 years
2017 Plan \| Unvested restricted stock units
Share-based awards
RSUs granted (in shares)			715,716
Inducement Award Program
Share-based awards
Number of shares outstanding (in shares)			757,500
2017 ESPP \| Employee stock purchase plan
Share-based awards
Cumulative annual increase in number of shares issuable, percentage						1.00%
2017 ESPP \| Employee stock purchase plan \| Common Stock
Share-based awards
Maximum annual increase (in shares)						450,000
2017 ESPP \| Employee stock purchase plan \| Maximum
Share-based awards
Number of shares authorized (in shares)						4,725,000

Debt

12 Months Ended

Dec. 31, 2021

Debt Disclosure [Abstract]

Debt

On May 8, 2019, the Company entered into a loan and security agreement (the Original Agreement) with SVB pursuant to which the Company borrowed $5,000. The Original Agreement accrued interest at a floating per annum rate equal to the greater of (i) 4.0% and (ii) 1.50% below the Prime Rate. The Original Agreement had an interest-only period through August 31, 2020.

On August 28, 2020, the Company entered into a second amendment (the Second Amendment) to the Original Agreement with SVB (the Prior Credit Facility). Pursuant to the Second Amendment, the Company drew $5,200 upon execution of the Second Amendment, the proceeds of which were used to repay the Company’s existing balance under the Original Agreement and satisfy its obligations to SVB. The Amended Credit Facility accrued interest at a floating per annum rate equal to the greater of (i) 4.25% and (ii) 1.00% above the Prime Rate.

On October 29, 2021, the Company entered into a loan and security agreement (the New Credit Facility) with SVB and Oxford (Oxford and SVB, together the Lenders). Pursuant to the New Credit Facility, the Company can borrow term loans in an aggregate amount of $100,000, which includes (i) $60,000 in up to three principal advances through December 31, 2022, (ii) an additional $10,000 in one principal advance, if the Company reaches certain development milestone events through December 31, 2022, (iii) an additional $10,000 in one principal advance, if the Company reaches additional development milestone events through June 30, 2023 and (iv) an additional tranche of $20,000, subject to conditional approval from the Lenders. The Company drew $25,000 upon execution of the New Credit Facility, of which $5,500 of the proceeds was used to repay the existing balance under the Prior Credit Facility and satisfy its obligations to SVB, resulting in the recording of a $398 loss on extinguishment, which is presented within interest expense for the year ended December 31, 2021 on the Consolidated Statements of Operations and Comprehensive Loss. Upon entering into the New Credit Facility, the Company terminated all commitments by SVB to extend further credit under the Prior Credit Facility and all guarantees and security interests granted by the Company to SVB under the Prior Credit Facility.

The New Credit Facility bears interest at a floating per annum rate equal to the greater of (i) 8.50% and (ii) 5.25% above the Prime Rate. Interest is payable monthly in arrears on the first day of each month. The Company is obligated to make interest-only payments through November 1, 2024, followed by equal monthly principal payments and applicable interest through the maturity date of October 1, 2026 (the Maturity Date). If certain development milestones are met, then the interest-only period will be extended to November 1, 2025.

The Company is also required to make a final payment to the Lenders equal to 4.25% of the principal amount of the term loans then extended to the Company. This final payment is accreted under the effective interest method over the life of each term loan. The term loans are secured by substantially all of the Company’s assets, except for its intellectual property which is subject to a negative pledge, and certain other customary exclusions.

At the Company’s option, it may prepay the outstanding principal balance of any term loans in whole but not in part, subject to a prepayment fee of: (a) 3.0% of the term loans then extended to the Company if the prepayment occurs on or prior to the first anniversary of the funding date of such term loan, (b) 2.0% of the term loans then extended to the Company if the prepayment occurs after the first anniversary of the funding date of such term loan but on or prior to the second anniversary of the funding date of such term loan, or (c) 1.0% of the term loans then extended to the Company if the prepayment occurs after the second anniversary of the funding date of such term loan but before the Maturity Date. The New Credit Facility includes customary affirmative and restrictive covenants applicable to the Company. Affirmative covenants include, among others, covenants requiring the Company to maintain its corporate existence and governmental approvals, deliver certain financial reports, maintain insurance coverage and satisfy certain requirements regarding deposit accounts. The restrictive covenants include, among others, requirements relating to the Company’s ability to transfer collateral, incur additional indebtedness, engage in mergers or acquisitions, pay dividends or make other distributions, make investments, create liens, sell assets and agree to a change in control, in each case subject to certain customary exceptions.

The Company’s payment obligations under the New Credit Facility are subject to acceleration upon the occurrence of specified events of default, which include, but are not limited to, the occurrence of a material adverse change in the Company’s business, operations, or financial or other condition. Amounts outstanding upon the occurrence of an event of default are payable upon the Lenders' demand and shall accrue interest at an additional rate of 5.0% per annum of the past due amount outstanding. As of December 31, 2021, the Company was in compliance with all covenants under the New Credit Facility. As such, as of

December 31, 2021, the classification of the loan balance as stated on the balance sheet was based on the timing of defined future payment obligations.

In connection with entering into the New Credit Facility, the Company paid $958 in costs, of which $210 was paid to Lenders and $748 was paid to third parties. Certain costs were recorded as a reduction of the carrying amount on the term loan and amortized as interest expense using the effective-interest method, which was comprised of $151 of the costs paid to the Lenders and $273 of the costs paid to third parties. The remaining costs of $533 were capitalized in other assets related to the Company's right to borrow additional amounts from the Lenders in the future and amortized to interest expense over the relevant draw period on a straight-line basis.

As of December 31, 2021, there was $25,000 outstanding under the New Credit Facility and the debt consisted of the following:


	December 31, 2021
Total debt	$	25,000
Less: Current portion of long-term-debt	—
Total debt, net of current portion	25,000
Debt financing costs, net of accretion	(410)
Accretion related to final payment	36
Long-term debt, net	$	24,626

As of December 31, 2021, the estimated future principal payments due are as follows:


2022	$	—
2023	—
2024	2,083
2025	12,500
2026	10,417
Total debt	$	25,000

During the year ended December 31, 2021 and 2020, the Company recognized $797 and $340, respectively, of interest expense related to the Prior Credit Facility and New Credit Facility, as applicable.

- Definition

The entire disclosure for information about short-term and long-term debt arrangements, which includes amounts of borrowings under each line of credit, note payable, commercial paper issue, bonds indenture, debenture issue, own-share lending arrangements and any other contractual agreement to repay funds, and about the underlying arrangements, rationale for a classification as long-term, including repayment terms, interest rates, collateral provided, restrictions on use of assets and activities, whether or not in compliance with debt covenants, and other matters important to users of the financial statements, such as the effects of refinancing and noncompliance with debt covenants.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
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-Paragraph 1B
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
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-Section 50
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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-Paragraph 1I
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Reference 5: http://www.xbrl.org/2003/role/disclosureRef
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-SubTopic 20
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-Paragraph 1I
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=123466505&loc=SL123495371-112611

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(c))
-URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 20
-Section 50
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Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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-URI http://asc.fasb.org/extlink&oid=123466505&loc=SL123495334-112611

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
-Paragraph 1E
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123466505&loc=SL123495348-112611

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
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-URI http://asc.fasb.org/extlink&oid=123466505&loc=SL123495323-112611

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
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Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=123466505&loc=SL123495323-112611

+ Details

Leases

12 Months Ended

Dec. 31, 2021

Leases [Abstract]

Leases

The Company has an operating lease for its office and lab space in Cambridge, MA and operating and finance leases for certain equipment. In March 2020, the Company entered into the Seventh Amendment to the office and lab space lease (the Office Lease) to extend the term of the lease through March 2026. The Company has an option to extend the lease term of the Office Lease for an additional five years.

On April 5, 2021, the Company entered into an Eighth Amendment to the Office Lease, which granted the Company additional office space in its existing building for five years, beginning July 1, 2021, and committed the Company to lease payments of $4,983 over that period (the Expansion Lease). In connection with the Expansion Lease, the Company increased the balance of the security deposit by increasing the standby letter of credit for the benefit of its landlord by $157. The Expansion Lease also provided the Company with a tenant improvement allowance of $51. Independent from the option under the Office Lease, the Company has an option to extend the lease term of the Expansion Lease for an additional five years. The Company’s exercise of the options to extend the lease terms of both the Office Lease and Expansion Lease were not considered reasonably certain as of December 31, 2021.

The Expansion Agreement is a lease modification accounted for as a separate contract, because it expands the scope of the Office Lease and the additional lease payments are commensurate with market rents. The Company assessed the lease classification of the Expansion Lease as of the date of signing and determined that the Expansion Lease should be accounted for as an operating lease. The right-of-use asset and corresponding operating lease liability have been calculated based on the present value of lease payments over the lease term. The Company determined the appropriate incremental borrowing rate to utilize as a discount rate by using a synthetic credit rating which was estimated based on an analysis of outstanding debt of companies with similar credit and financial profiles. Since the operating lease is a net lease, as the non-lease components (i.e., common area maintenance) are paid separately from rent based on actual costs incurred, such non-lease components were not included in the right-of-use (ROU) asset and liability and are reflected as an expense in the period incurred.

As a result of the signing of the Expansion Lease in April 2021, the Company recorded an increase of $3,783 to its ROU asset and lease liabilities in the second quarter of 2021.

The Company had a standby letter of credit agreement for the benefit of its landlord in the amount of $478 in connection with the Office Lease and Expansion Lease as of December 31, 2021 and $321 in connection with the Office Lease as of December 31, 2020, collateralized by a money market account.

The Company has remaining finance lease terms of one year to five years for certain equipment, some of which include options to purchase at fair value. For the year ended December 31, 2021, the Company recorded assets under finance leases of $609 as property and equipment.

The components of lease expense were as follows:


	Years ended December 31,
	2021		2020		2019
Operating lease cost	$	3,502	$	2,755	$	2,160

Finance lease cost:
Amortization of right-of-use assets	$	169	$	101	$	75
Interest on lease liabilities	28		21		20
	$	197	$	122	$	95

Supplemental balance sheet information related to leases was as follows:


	Year ended December 31,
	2021			2020
Operating leases:
Operating lease right-of-use assets	$	12,889		$	10,936
Operating lease liabilities, current	$	2,303		$	1,437
Operating lease liabilities	$	11,247		$	10,158

Finance leases:
Property and equipment, gross	$	1,038		$	429
Property and equipment, accumulated depreciation	$	(345)		$	(176)
Other liabilities, current	$	239		$	93
Other liabilities	$	449		$	174

Weighted-average remaining lease term:
Operating leases	4.3 years			5.2 years
Finance leases	4.0 years			2.9 years

Weighted-average discount rate:
Operating leases	10.8		%	10.8		%
Finance leases	5.4		%	6.9		%

Supplemental cash flow information related to leases was as follows:


	Year ended December 31,
	2021		2020
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	$	3,241	$	2,394
Operating cash flows from finance leases	$	28	$	21
Financing cash flows from finance leases	$	215	$	116

Rent expense was $3,390, $2,644 and $2,160 for the years ended December 31, 2021, 2020 and 2019, respectively.

Future minimum lease payments under non-cancellable leases as of December 31, 2021 were as follows:


	Operating leases		Finance leases
2022	$	3,795	$	269
2023	3,909		262
2024	4,027		141
2025	4,147		48
2026 and thereafter	1,310		8
Total lease payments	17,188		728
Present value adjustment	(3,638)		(40)
Present value of lease liabilities	$	13,550	$	688

Leases

As a result of the signing of the Expansion Lease in April 2021, the Company recorded an increase of $3,783 to its ROU asset and lease liabilities in the second quarter of 2021.

The components of lease expense were as follows:


	Years ended December 31,
	2021		2020		2019
Operating lease cost	$	3,502	$	2,755	$	2,160

Finance lease cost:
Amortization of right-of-use assets	$	169	$	101	$	75
Interest on lease liabilities	28		21		20
	$	197	$	122	$	95

Supplemental balance sheet information related to leases was as follows:


	Year ended December 31,
	2021			2020
Operating leases:
Operating lease right-of-use assets	$	12,889		$	10,936
Operating lease liabilities, current	$	2,303		$	1,437
Operating lease liabilities	$	11,247		$	10,158

Finance leases:
Property and equipment, gross	$	1,038		$	429
Property and equipment, accumulated depreciation	$	(345)		$	(176)
Other liabilities, current	$	239		$	93
Other liabilities	$	449		$	174

Weighted-average remaining lease term:
Operating leases	4.3 years			5.2 years
Finance leases	4.0 years			2.9 years

Weighted-average discount rate:
Operating leases	10.8		%	10.8		%
Finance leases	5.4		%	6.9		%

Supplemental cash flow information related to leases was as follows:


	Year ended December 31,
	2021		2020
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	$	3,241	$	2,394
Operating cash flows from finance leases	$	28	$	21
Financing cash flows from finance leases	$	215	$	116

Rent expense was $3,390, $2,644 and $2,160 for the years ended December 31, 2021, 2020 and 2019, respectively.

Future minimum lease payments under non-cancellable leases as of December 31, 2021 were as follows:


	Operating leases		Finance leases
2022	$	3,795	$	269
2023	3,909		262
2024	4,027		141
2025	4,147		48
2026 and thereafter	1,310		8
Total lease payments	17,188		728
Present value adjustment	(3,638)		(40)
Present value of lease liabilities	$	13,550	$	688

Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2021

Stockholders' Equity Note [Abstract]

Schedule for number of common stock reserved for exercise of outstanding stock options and warrants


	December 31, 2021	December 31, 2020
Stock options	8,342,429	6,112,948
Restricted stock units	817,609	716,767
Warrants	39,474	39,474
	9,199,512	6,869,189

- Definition

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

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+ Details

Property and Equipment

12 Months Ended

Dec. 31, 2021

Property, Plant and Equipment [Abstract]

Property and Equipment

Property and equipment consists of the following as of December 31, 2021 and 2020:


	December 31, 2021		December 31, 2020
Laboratory equipment	$	6,725	$	6,520
Leasehold improvements	1,906		1,886
Computer equipment and office equipment	1,019		959
Total property and equipment at cost	9,650		9,365
Less: Accumulated depreciation	(7,682)		(7,635)
	$	1,968	$	1,730

The Company recorded assets under finance leases of $609, $0, and $429 as property and equipment during the years ended December 31, 2021, 2020 and 2019, respectively. Financing leases are discussed in more detail in Note 10, Leases. Depreciation expense for the years ended December 31, 2021, 2020 and 2019 was $855, $1,010 and $1,245, respectively.

Accrued Expenses

12 Months Ended

Dec. 31, 2021

Accrued Liabilities, Current [Abstract]

Accrued Expenses

Accrued expenses consist of the following as of December 31, 2021 and 2020:


	December 31, 2021		December 31, 2020
Accrued manufacturing expenses	$	8,476	$	4,157
Accrued clinical expenses	7,879		5,126
Accrued payroll and related expenses	7,319		5,412
Accrued preclinical expenses	3,848		619
Accrued professional fees	909		757
Accrued other	285		75
	$	28,716	$	16,146

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 177,947	$ 255,094
Prepaid expenses and other current assets	10,951	3,486
Total current assets	188,898	258,580
Property and equipment, net	1,968	1,730
Operating lease right-of-use assets	12,889	10,936
Other assets	2,356	2,153
Total assets	206,111	273,399
Current liabilities:
Accounts payable	12,321	8,340
Accrued expenses	28,716	16,146
Deferred revenue	3,944	3,987
Operating lease liabilities	2,303	1,437
Other liabilities	239	93
Total current liabilities	47,523	30,003
Operating lease liabilities	11,247	10,158
Long-term debt, net	24,626	4,977
Other liabilities	974	174
Total liabilities	84,370	45,312
Commitments (Note 13)
Stockholders' equity
Preferred stock, $0.0001 par value; 25,000,000 shares authorized; 0 shares issued and outstanding at December 31, 2021 and December 31, 2020, respectively	0	0
Common stock, $0.0001 par value; 175,000,000 shares authorized; 73,709,056 and 68,841,288 shares issued and outstanding at December 31, 2021 and December 31, 2020, respectively	7	7
Additional paid-in capital	572,213	508,499
Accumulated deficit	(450,479)	(280,419)
Total stockholders’ equity	121,741	228,087
Total liabilities and stockholders’ equity	$ 206,111	$ 273,399

Stock Options - Stock Based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Stock options
Stock-based compensation expense included in Total operating expenses	$ 18,409	$ 7,172	$ 4,872
Stock options
Stock options
Stock-based compensation expense included in Total operating expenses	14,528	5,725	4,230
Unvested restricted stock units
Stock options
Stock-based compensation expense included in Total operating expenses	3,522	1,187	410
Employee stock purchase plan
Stock options
Stock-based compensation expense included in Total operating expenses	359	260	232
Research and development
Stock options
Stock-based compensation expense included in Total operating expenses	9,984	3,841	2,245
General and administrative
Stock options
Stock-based compensation expense included in Total operating expenses	$ 8,425	$ 3,331	$ 2,627

Leases - Narrative (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
Leases - Narrative (Details) - USD ($) $ in Thousands	Apr. 05, 2021	Jun. 30, 2021	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Leases
Lease liability			$ 13,550
Letter of credit for security deposit			478	$ 321
Increase in ROU asset		$ 3,783
Right-of-use assets obtained in exchange for financing lease liabilities			609	0	$ 429
Rent expense			$ 3,390	$ 2,644	$ 2,160
Operating lease liabilities		$ 3,783
Minimum
Leases
Finance lease, remaining lease term			1 year
Maximum
Leases
Finance lease, remaining lease term			5 years
Office space, Cambridge, MA, Leased
Leases
Renewal term			5 years
Office Space Expansion Cambridge Ma
Leases
Renewal term	5 years
Operating lease term of contract	5 years
Lease liability	$ 4,983
Letter of credit for security deposit	157
Tenant improvement allowance	$ 51

Income Taxes - Reconciliation (Details)	12 Months Ended
Income Taxes - Reconciliation (Details)	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Income tax computed at federal statutory tax rate	21.00%	21.00%	21.00%
State taxes, net of federal benefit	6.30%	6.70%	6.10%
Permanent differences	(0.20%)	1.20%	(2.00%)
General business credits	3.80%	3.40%	10.30%
Stock compensation	0.10%	0.00%	0.00%
Impact of ownership shift	0.00%	0.00%	(53.30%)
Change in valuation allowance	(31.00%)	(32.30%)	17.90%
Income tax benefit	0.00%	0.00%	0.00%

Income Taxes - Valuation allowance (Details)	12 Months Ended
Income Taxes - Valuation allowance (Details)	Dec. 31, 2021 USD ($) jurisdiction audit	Dec. 31, 2020 USD ($)
Valuation allowance
Valuation allowance	$ 130,051,000	$ 77,375,000
Increase (decrease) in valuation allowance	52,676,000	28,468,000
Unrecognized tax benefits	0	0
Accrued interest or penalties related to uncertain tax position	$ 0	$ 0
Number of state jurisdictions in which entity files income tax returns \| jurisdiction	4
Number of federal or state audits in progress \| audit	0

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2021	Dec. 31, 2020
Stockholders' equity
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, shares authorized (in shares)	25,000,000	25,000,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized (in shares)	175,000,000	175,000,000
Common stock, shares issued (in shares)	73,709,056	68,841,288
Common stock, shares outstanding (in shares)	73,709,056	68,841,288

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Statement [Abstract]
Collaboration revenue	$ 43	$ 828	$ 42,123
Operating expenses:
Research and development	132,013	67,036	55,040
General and administrative	36,888	21,902	17,283
Total operating expenses	168,901	88,938	72,323
Other income (expense):
Interest income	65	424	2,226
Interest expense	(1,267)	(359)	(234)
Total other income (expense), net	(1,202)	65	1,992
Net loss	(170,060)	(88,045)	(28,208)
Other comprehensive loss:
Unrealized gain (loss) on marketable securities	0	(25)	33
Comprehensive loss	(170,060)	(88,070)	(28,175)
Net loss attributable to common stockholders - basic	(170,060)	(88,045)	(28,208)
Net loss attributable to common stockholders - diluted	$ (170,060)	$ (88,045)	$ (28,208)
Net loss per share attributable to common stockholders — basic (in dollars per share)	$ (2.41)	$ (1.43)	$ (0.65)
Net loss per share attributable to common stockholders — diluted (in dollars per share)	$ (2.41)	$ (1.43)	$ (0.65)
Weighted-average number of shares of common stock used in net loss per share attributable to common stockholders — basic (in shares)	70,580,949	61,485,205	43,492,113
Weighted-average number of shares of common stock used in net loss per share attributable to common stockholders — diluted (in shares)	70,580,949	61,485,205	43,492,113

Income Taxes

12 Months Ended

Dec. 31, 2021

Income Tax Disclosure [Abstract]

Income Taxes

For the years ended December 31, 2021, 2020 and 2019, the Company recorded no income tax benefit for the net operating losses incurred in each year, due to its uncertainty of realizing a benefit from those items.

A reconciliation of income taxes computed using the U.S. federal statutory rate to that reflected in operations as of December 31, 2021, 2020 and 2019 are as follows:


	2021		2020		2019
Income tax computed at federal statutory tax rate	21.0	%	21.0	%	21.0	%
State taxes, net of federal benefit	6.3	%	6.7	%	6.1	%
Permanent differences	(0.2)	%	1.2	%	(2.0)	%

General business credits	3.8	%	3.4	%	10.3	%
Stock compensation	0.1	%	—	%	—	%
Impact of ownership shift	—	%	—	%	(53.3)	%
Change in valuation allowance	(31.0)	%	(32.3)	%	17.9	%
	—	%	—	%	—	%

Deferred income taxes reflect the net tax effects of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes. Significant components of the Company’s net deferred tax assets as of December 31, 2021 and 2020 are as follows:


	2021		2020
Deferred tax assets:
Net operating losses	$	106,055	$	64,259
Tax credit carryforwards	12,424		5,670
Accrued expenses	6,892		4,058
Lease liabilities	3,698		3,166
Licensed technology	2,775		1,534
Deferred revenue	1,076		1,088
Depreciation	493		502

Other	71		84
Total gross deferred tax assets	133,484		80,361
Valuation allowance	(130,051)		(77,375)
Net deferred tax assets less valuation allowance	3,433		2,986
Deferred tax liabilities
Right-of-use assets	(3,433)		(2,986)
Total gross deferred tax liabilities	(3,433)		(2,986)
Net deferred taxes	$	—	$	—

The Company has incurred net operating losses (NOL) since inception. At December 31, 2021, the Company had Federal and State net operating loss carryforwards of approximately $403,579 and $337,057, respectively. Of the $403,579 of Federal net operating loss carryforwards, $34,149 expire at various dates through 2037. The remaining $369,430 of Federal net operating loss carryforwards do not expire. The State net operating loss carryforwards expire at various dates through 2041. At December 31, 2021, the Company had Federal and State research and development tax credit carryforwards of approximately $10,077 and $3,061, respectively, which expire at various dates through 2041.

As required by ASC 740, management of the Company has evaluated the evidence bearing upon the reliability of its deferred tax assets. Based on the weight of available evidence, both positive and negative, management has determined that it is more likely than not that the Company will not realize the benefits of all of these assets. Accordingly, the Company recorded a valuation allowance of $130,051 and $77,375 at December 31, 2021 and December 31, 2020, respectively. The valuation allowance increased by $52,676 and $28,468 during the years ended December 31, 2021 and 2020, respectively, primarily as a result of the Company’s net operating losses generated during the periods, respectively.

Utilization of the NOLs and research and development tax credit carryforwards may be subject to a substantial annual limitation under Section 382 due to ownership change limitations that have occurred previously or that could occur in the future in accordance with Section 382, as well as similar state provisions. These ownership changes may limit the amount of NOLs and research and development tax credit carryforwards that can be utilized annually to offset future taxable income and tax, respectively. If a change in control as defined by Section 382 has occurred at any time since the Company’s formation, utilization of its NOLs or research and development tax credit carryforwards would be subject to an annual limitation under Section 382, which is determined by first multiplying the value of the Company’s stock at the time of the ownership change by the applicable long-term tax-exempt rate, which could then be subject to additional adjustments, as required. Any limitation may result in expiration of a portion of the NOLs or research and development tax carryforwards before their utilization. The Company has determined that ownership changes have occurred through November 4, 2019 and that certain NOLs and research and development tax credit carryforwards will be subject to limitation. The amounts presented do not include NOLs or research and development tax credit carryforwards that will expire unused due to ownership changes.

The Company applies the accounting guidance in ASC 740 related to accounting for uncertainty in income taxes. The Company’s reserves related to taxes 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 potential contingencies present related to the tax benefit. As of December 31, 2021 and 2020, the Company had no unrecognized tax benefits.

The Company has not conducted a study of its research and development credit carryforwards. This study may result in an adjustment to research and development credit carryforwards; however, until a study is completed and any adjustment is known, no amounts are being presented as an uncertain tax position. A full valuation allowance has been provided against the Company’s research and development credits and, if an adjustment is required, this adjustment would be offset by an adjustment to the valuation allowance. Thus, there would be no impact to the balance sheets or statements of operations if an adjustment were required.

Interest and penalties related to uncertain tax positions would be classified as income tax expense in the accompanying statements of operations. As of December 31, 2021 and 2020, the Company had no accrued interest or penalties related to uncertain tax positions.

The Company files income tax returns in the United States federal tax jurisdiction and four state jurisdictions. The Company did not have any foreign operations during the years ended December 31, 2021, 2020 and 2019. The statute of limitations for assessment by the Internal Revenue Service and state tax authorities is closed for tax years prior to 2017, although carryforward attributes that were generated prior to tax year 2017 may still be adjusted upon examination to the extent utilized in a future period. There are no federal or state audits currently in progress.

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section 50
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-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 6.I.5.Q1)
-URI http://asc.fasb.org/extlink&oid=122134291&loc=d3e330036-122817

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 14
-URI http://asc.fasb.org/extlink&oid=121826272&loc=d3e32705-109319

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 270
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6424409&loc=d3e44925-109338

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-URI http://asc.fasb.org/topic&trid=2144680

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6424122&loc=d3e41874-109331

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(h)(2))
-URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690

+ Details

Property and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Property, equipment and software, net
Total property and equipment at cost	$ 9,650	$ 9,365
Less: Accumulated depreciation	(7,682)	(7,635)
Property and equipment	1,968	1,730
Depreciation expense	855	1,010	$ 1,245
Laboratory equipment
Property, equipment and software, net
Total property and equipment at cost	6,725	6,520
Leasehold improvements
Property, equipment and software, net
Total property and equipment at cost	1,906	1,886
Computer equipment and office equipment
Property, equipment and software, net
Total property and equipment at cost	1,019	959
Finance leases
Property, equipment and software, net
Total property and equipment at cost	$ 609	$ 0	$ 429

Commitments	12 Months Ended
Commitments	Dec. 31, 2021
Commitments and Contingencies Disclosure [Abstract]
Commitments	Commitments License Agreements During the years ended December 31, 2021, 2020 and 2019, the Company recorded research and development expense related to non-refundable license payments of $3,075, $250, and $750, respectively. Further development milestones of $2,125, $750 and $600, respectively, were also recorded as research and development expense during the years ended December 31, 2021, 2020 and 2019. See Note 10, Leases, to these consolidated financial statements for the Company’s future obligations related to leases as of December 31, 2021.

Summary of Significant Accounting Policies - Revenue Recognition (Details)	12 Months Ended
	Dec. 31, 2021
Accounting Policies [Abstract]
Billing term	30 days

Summary of Significant Accounting Policies - Risks (Details)	12 Months Ended
	Dec. 31, 2021 USD ($)
Concentration of Credit Risk and Off-balance Sheet Risk
Financial instruments with off-balance sheet risk	$ 0

Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Accrued Liabilities, Current [Abstract]
Accrued manufacturing expenses	$ 8,476	$ 4,157
Accrued clinical expenses	7,879	5,126
Accrued preclinical expenses	3,848	619
Accrued payroll and related expenses	7,319	5,412
Accrued professional fees	909	757
Accrued other	285	75
Total	$ 28,716	$ 16,146

Stockholders' Equity - Common Stock (Details) - shares	Dec. 31, 2021	Dec. 31, 2020
Class of Stock [Line Items]
Number of shares reserved for future issuance (in shares)	9,199,512	6,869,189
Warrants and share-based payment awards
Class of Stock [Line Items]
Number of shares reserved for future issuance (in shares)	9,199,512	6,869,189
Share-based payments \| Stock options
Class of Stock [Line Items]
Number of shares reserved for future issuance (in shares)	8,342,429	6,112,948
Share-based payments \| Restricted stock units
Class of Stock [Line Items]
Number of shares reserved for future issuance (in shares)	817,609	716,767
Warrants
Class of Stock [Line Items]
Number of shares reserved for future issuance (in shares)	39,474	39,474

Subsequent Events	12 Months Ended
Subsequent Events	Dec. 31, 2021
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events Janssen Research Collaboration and License Agreement In February 2022, the Company entered into the Janssen Collaboration pursuant to which the Company granted Janssen an exclusive license to use the Company's proprietary Dolasynthen platform and other technology to develop, manufacture and commercialize ADCs directed to up to three targets selected by Janssen. The Company is responsible for performing bioconjugation activities to create ADCs for Janssen based on antibodies provided by Janssen. The Company will also perform certain chemistry, manufacturing and controls development and early stage manufacturing activities for ADCs that Janssen progresses through development, up to and including the manufacturing of clinical drug substance, at Janssen’s cost. Except with respect to this limited manufacturing, Janssen will be responsible for the further development, manufacturing and commercialization of the ADCs developed under the Janssen Collaboration, including obtaining any necessary regulatory approvals, at Janssen’s cost. The Company received an upfront payment of $40,000 in February 2022. The Company is eligible to receive development and regulatory milestones with an aggregate total of $501,000, if licensed products directed to all three Targets are advanced by Janssen. The Company is also eligible to receive commercial milestones with an aggregate total of $530,000 in the event of commercialization of three Targets by Janssen and tiered royalties based on mid-single digits to low-double digits on future net sales of licensed ADCs Unless earlier terminated, the Janssen Collaboration will continue in effect until the date on which the royalty term and all payment obligations with respect to all licensed ADCs in all countries have expired. Other Events On February 2, 2022, the Company amended the Commercial License and Option Agreement with Synaffix B.V. (Synaffix) in connection with the Janssen Collaboration, and agreed to pay Synaffix a non-refundable execution fee of $1,500 which will be applied against future target license fees or development milestones. Further in connection with the Janssen Collaboration, on February 17, 2022, the Company amended the New Credit Facility described in Note 7, Debt, to these consolidated financial statements. Pursuant to this amendment the Company has agreed to replace the tranche B term loan and the tranche C term loan with a single combined term loan tranche in an aggregate principal amount of $20,000. Also, the combined term loan tranche will now be available any time on or prior to June 30, 2023, within 90 days of the Company achieving both of the prior tranche B and tranche C term loan milestones. Subsequent to December 31, 2021 and through February 25, 2022, the Company sold 9,493,776 shares of common stock resulting in net proceeds of $45,579 from ATM offerings, with substantial participation from existing long-term investors. Approximately $9,414 remains unsold and available for sale under the 2020 ATM.

Leases - Supplemental balance sheet information (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Operating leases:
Operating lease right-of-use assets	$ 12,889	$ 10,936
Operating lease liabilities, current	2,303	1,437
Operating lease liabilities	11,247	10,158
Finance leases:
Property and equipment, gross	1,038	429
Less: Accumulated depreciation	(345)	(176)
Other liabilities, current	239	93
Other liabilities	$ 449	$ 174
Weighted-average remaining lease term:
Operating leases	4 years 3 months 18 days	5 years 2 months 12 days
Finance leases	4 years	2 years 10 months 24 days
Weighted-average discount rate:
Operating leases	10.80%	10.80%
Finance leases	5.40%	6.90%
Finance lease, liability, current, statement of financial position	Other liabilities	Other liabilities
Finance lease, liability, noncurrent, statement of financial position	Other liabilities	Other liabilities

Summary of Significant Accounting Policies (Policies)