0001193125-21-319502 | 10-Q | XBRL Viewer

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Cover Page - shares	9 Months Ended
Cover Page - shares	Sep. 30, 2021	Oct. 31, 2021
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Sep. 30, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Trading Symbol	MGTA
Entity Registrant Name	Magenta Therapeutics, Inc.
Entity Central Index Key	0001690585
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Ex Transition Period	false
Entity Emerging Growth Company	true
Entity Small Business	true
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		58,777,271
Entity Interactive Data Current	Yes
Title of 12(b) Security	Common Stock
Security Exchange Name	NASDAQ
Entity Incorporation, State or Country Code	DE
Entity Address, Address Line One	100 Technology Square
Entity Address, City or Town	Cambridge
Entity Address, Postal Zip Code	02139
Entity Address, State or Province	MA
City Area Code	(857
Entity File Number	001-38541
Entity Tax Identification Number	81-0724163
Local Phone Number	242-0170

Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 157,539	$ 58,152
Marketable securities	35,077	90,683
Prepaid expenses and other current assets	3,859	2,692
Total current assets	196,475	151,527
Restricted cash	1,780	1,780
Property and equipment, net	7,981	8,312
Total assets	206,236	161,619
Current liabilities:
Accounts payable	1,886	3,760
Accrued expenses and other current liabilities	8,261	7,670
Total current liabilities	10,147	11,430
Deferred rent	6,401	6,283
Total liabilities	16,548	17,713
Commitments and contingencies (Note 6)
Stockholders' Equity:
Preferred stock, $0.001 par value; 10,000,000 shares authorized; no shares issued or outstanding
Common stock, $0.001 par value; 150,000,000 shares authorized; 58,777,271 shares issued and outstanding as of September 30, 2021 and 48,541,601 shares issued and 48,533,135 shares outstanding as of December 31, 2020	59	49
Additional paid-in capital	495,903	398,311
Accumulated other comprehensive income (loss)	1	(23)
Accumulated deficit	(306,275)	(254,431)
Total stockholders' equity	189,688	143,906
Total liabilities and stockholders' equity	$ 206,236	$ 161,619

Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2021	Dec. 31, 2020
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, Shares authorized	10,000,000	10,000,000
Preferred stock, Shares issued	0	0
Preferred stock, Shares outstanding	0	0
Common stock, Par value	$ 0.001	$ 0.001
Common stock, shares authorized	150,000,000	150,000,000
Common stock, Shares issued	58,777,271	48,541,601
Common stock, Shares outstanding	58,777,271	48,533,135

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Operating expenses:
Research and development	$ 10,795	$ 11,786	$ 33,652	$ 38,359
General and administrative	7,450	6,595	20,900	21,278
Total operating expenses	18,245	18,381	54,552	59,637
Loss from operations	(18,245)	(18,381)	(54,552)	(59,637)
Interest and other income, net	818	703	2,708	2,869
Net loss	$ (17,427)	$ (17,678)	$ (51,844)	$ (56,768)
Net loss per share, basic and diluted	$ (0.30)	$ (0.37)	$ (0.97)	$ (1.34)
Weighted average common shares outstanding, basic and diluted	58,583,476	48,255,353	53,655,314	42,431,874
Comprehensive loss:
Net loss	$ (17,427)	$ (17,678)	$ (51,844)	$ (56,768)
Other comprehensive income (loss):
Unrealized gains (losses) on marketable securities	(1)	(114)	24	31
Total other comprehensive income (loss)	(1)	(114)	24	31
Total comprehensive loss	$ (17,428)	$ (17,792)	$ (51,820)	$ (56,737)

Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Private Placement [Member]	Common Stock [Member]	Common Stock [Member] Private Placement [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] Private Placement [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2019	$ 141,193		$ 39		$ 320,641		$ 8	$ (179,495)
Beginning balance, shares at Dec. 31, 2019			39,260,532
Issuance of common stock upon public offering net of underwriting discounts, commissions and offering costs	64,563		$ 9		64,554
Issuance of common stock upon public offering net of underwriting discounts, commissions and offering costs , shares			8,625,000
Vesting of restricted stock, shares			162,195
Issuance of common stock upon exercise of stock options	1,895				1,895
Issuance of common stock upon exercise of stock options, shares			240,089
Issuance of common stock under Employee Stock Purchase Plan, shares			5,653
Issuance of common stock under Employee Stock Purchase Plan	42				42
Stock-based compensation expense	7,610				7,610
Unrealized gains on marketable securities	31						31
Net loss	(56,768)							(56,768)
Ending balance at Sep. 30, 2020	158,566		$ 48		394,742		39	(236,263)
Ending balance, shares at Sep. 30, 2020			48,293,469
Beginning balance at Dec. 31, 2019	141,193		$ 39		320,641		8	(179,495)
Beginning balance, shares at Dec. 31, 2019			39,260,532
Net loss	(74,900)
Ending balance at Dec. 31, 2020	143,906		$ 49		398,311		(23)	(254,431)
Ending balance, shares at Dec. 31, 2020			48,533,135
Beginning balance at Jun. 30, 2020	173,914		$ 48		392,298		153	(218,585)
Beginning balance, shares at Jun. 30, 2020			48,241,212
Vesting of restricted stock, shares			36,522
Issuance of common stock upon exercise of stock options	78				78
Issuance of common stock upon exercise of stock options, shares			15,735
Stock-based compensation expense	2,366				2,366
Unrealized gains on marketable securities	(114)						(114)
Net loss	(17,678)							(17,678)
Ending balance at Sep. 30, 2020	158,566		$ 48		394,742		39	(236,263)
Ending balance, shares at Sep. 30, 2020			48,293,469
Beginning balance at Dec. 31, 2020	143,906		$ 49		398,311		(23)	(254,431)
Beginning balance, shares at Dec. 31, 2020			48,533,135
Issuance of common stock upon public offering net of underwriting discounts, commissions and offering costs		$ 86,097		$ 10		$ 86,087
Issuance of common stock upon public offering net of underwriting discounts, commissions and offering costs , shares				9,599,998
Vesting of restricted stock, shares			218,464
Issuance of common stock upon exercise of stock options	3,294				3,294
Issuance of common stock upon exercise of stock options, shares			411,685
Issuance of common stock under Employee Stock Purchase Plan, shares			13,989
Issuance of common stock under Employee Stock Purchase Plan	86				86
Stock-based compensation expense	8,125				8,125
Unrealized gains on marketable securities	24						24
Net loss	(51,844)							(51,844)
Ending balance at Sep. 30, 2021	189,688		$ 59		495,903		1	(306,275)
Ending balance, shares at Sep. 30, 2021			58,777,271
Beginning balance at Jun. 30, 2021	202,892		$ 59		491,679		2	(288,848)
Beginning balance, shares at Jun. 30, 2021			58,508,735
Vesting of restricted stock, shares			209,998
Issuance of common stock upon exercise of stock options	425				425
Issuance of common stock upon exercise of stock options, shares			58,538
Stock-based compensation expense	3,799				3,799
Unrealized gains on marketable securities	(1)						(1)
Net loss	(17,427)							(17,427)
Ending balance at Sep. 30, 2021	$ 189,688		$ 59		$ 495,903		$ 1	$ (306,275)
Ending balance, shares at Sep. 30, 2021			58,777,271

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Cash flows from operating activities:
Net loss	$ (51,844)	$ (56,768)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	8,125	7,610
Depreciation and amortization expense	1,485	1,483
Loss on disposal of property and equipment	95	1
Net amortization of premiums on marketable securities	630	111
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,167)	1,579
Accounts payable	(1,874)	(423)
Accrued expenses and other current liabilities	591	(3,888)
Deferred rent	118	5
Net cash used in operating activities	(43,841)	(50,290)
Cash flows from investing activities:
Purchases of property and equipment	(1,249)	(187)
Purchases of marketable securities		(4,974)
Maturities of marketable securities	55,000	65,500
Net cash provided by investing activities	53,751	60,339
Cash flows from financing activities:
Proceeds from private investment	86,400
Proceeds from public offerings, net of underwriting discounts and commissions		64,860
Payments of offering costs	(303)	(237)
Proceeds from exercise of common stock options	3,294	1,895
Proceeds from issuance of common stock under Employee Stock Purchase Plan	86	42
Net cash provided by financing activities	89,477	66,560
Net increase in cash, cash equivalents and restricted cash	99,387	76,609
Cash, cash equivalents and restricted cash at beginning of period	59,932	66,851
Cash, cash equivalents and restricted cash at end of period	$ 159,319	143,460
Supplemental disclosure of non-cash investing and financing activities:
Offering costs included in accounts payable and accrued expenses		$ 60

Nature of the Business and Basis of Presentation

9 Months Ended

Sep. 30, 2021

Nature of the Business and Basis of Presentation

1. Nature of the Business and Basis of Presentation

Magenta Therapeutics, Inc. (the “Company”) is a clinical-stage biotechnology company developing novel medicines

designed

to bring the curative power of stem cell transplants to more patients with blood cancers, genetic diseases and autoimmune diseases. The Company was incorporated under the laws of the State of Delaware in

June 2015

as HSCTCo Therapeutics, Inc. In February 2016, the Company changed its name to Magenta Therapeutics, Inc. and in June 2018 the Company completed its initial public offering.

The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, dependence on key personnel, protection of proprietary technology, compliance with government regulations, the continuing impact of the novel coronavirus

(“COVID-19”)

pandemic and the ability to secure additional capital to fund operations. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel and infrastructure and extensive compliance-reporting capabilities. Even if the Company’s development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales.

In May 2021, the Company issued and sold 9,599,998 shares of its common stock in a private placement at a purchase price of $9.00 per share, resulting in net proceeds of $86.1 million, after deducting offering expenses. In connection with the private placement, the Company filed a resale registration statement with the Securities and Exchange Commission (the “SEC”) in June 2021 to register the resale of these shares by the purchasers in the private placement.

In June 2020, the Company issued and sold 8,625,000 shares of its common stock, including the underwriters’ exercise in full of their option to purchase additional shares of common stock, in a

follow-on

public offering at a public offering price of $8.00 per share, resulting in net proceeds of $64.6 million after deducting underwriting discounts and commissions and other offering expenses.

The Company has a shelf registration statement on Form

S-3

(the “Shelf”) on file with the SEC, which covers the offering, issuance and sale of up to an aggregate of $350.0 million of common stock, preferred stock, debt securities, warrants and/or units of any combination thereof. The Company simultaneously entered into a sales agreement with Cowen and Company, LLC, as sales agent, to provide for the issuance and sale by the Company of up to $100.0 million of common stock from time to time in

“at-the-market”

offerings under the Shelf (the “ATM Program”). The Shelf was declared effective by the SEC on August 19, 2019. As of September 30, 2021, no sales have been made pursuant to the ATM Program.

The Company has incurred recurring losses since inception, including net losses of $51.8 million and $74.9 million for the nine months ended September 30, 2021 and the year ended December 31, 2020, respectively. As of September 30, 2021, the Company had an accumulated deficit of $306.3 million. The Company expects to continue to generate operating losses for the foreseeable future. The Company expects that its cash, cash equivalents and marketable securities will be sufficient to fund its operating expenses and capital expenditure requirements through at least 12 months from the issuance date of these consolidated financial statements. The future viability of the Company beyond that point is dependent on its ability to raise additional capital to fund its operations.

The Company expects its expenses to increase substantially in connection with ongoing activities, particularly as the Company advances its preclinical activities and clinical trials for its product candidates in development. Accordingly, the Company will need to obtain substantial additional funding in connection with continuing operations. If the Company is unable to raise capital when needed, or on attractive terms, it could be forced to delay, reduce or eliminate its research or drug development programs or any future commercialization efforts. Although management continues to pursue these plans, there is no assurance that the Company will be successful in obtaining sufficient funding on terms acceptable to the Company to fund continuing operations, if at all.

The consolidated financial statements include the accounts of the Company and its wholly owned subsidiary. All intercompany balances and transactions have been eliminated.

The consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”).

Summary of Significant Accounting Policies

9 Months Ended

Sep. 30, 2021

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Use of Estimates

The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting periods. Significant estimates and assumptions reflected in these financial statements include, but are not limited to, the accrual for research and development expenses and the valuation of stock-based awards. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results could differ from those estimates.

Unaudited Interim Financial Information

The consolidated balance sheet at December 31, 2020 was derived from audited financial statements but does not include all disclosures required by GAAP. The accompanying unaudited consolidated financial statements as of September 30, 2021 and for the three and nine months ended September 30, 2021 and 2020 have been prepared by the Company pursuant to the rules and regulations of the SEC for interim financial statements. Certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. The Company believes, however, that the disclosures are adequate to make the information presented not misleading. These consolidated financial statements should be read in conjunction with the Company’s audited financial statements for the year ended December 31, 2020 included in the Company’s Annual Report on Form

10-K,

on file with SEC. In the opinion of management, all adjustments, consisting only of normal recurring adjustments necessary for a fair statement of the Company’s consolidated financial position as of September 30, 2021 and consolidated results of operations for the three and nine months ended September 30, 2021 and 2020 and consolidated cash flows for the nine months ended September 30, 2021 and 2020 have been made. The results of operations for the nine months ended September 30, 2021 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2021 or any other interim period.

Fair Value Measurements

Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable:

•

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

•

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

•

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

The Company’s cash equivalents and marketable securities are carried at fair value, determined according to the fair value hierarchy described above (see Note 3). The carrying values of the Company’s accounts payable and accrued expenses approximate their fair values due to the short-term nature of these assets and liabilities.

Income Taxes

The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the consolidated financial statements or in the Company’s tax returns. Deferred taxes are determined based on the difference between the financial statement and tax basis of assets and liabilities using enacted tax rates in effect in the years in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies.

The Company accounts for uncertainty in income taxes recognized in its consolidated financial statements by applying a

two-step

process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed

more-likely-than-not

to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the consolidated financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties.

Comprehensive Loss

Comprehensive loss includes net loss as well as other changes in stockholders’ equity that result from transactions and economic events other than those with stockholders. For the three and nine months ended September 30, 2021 and 2020, the Company’s only element of other comprehensive income (loss) was unrealized gains (losses) on marketable securities.

Net Loss per Share

Basic net income (loss) per share is computed by dividing the net income (loss) by the weighted average number of shares of common stock outstanding for the period. Diluted net income (loss) per share is computed by dividing net income (loss) by the weighted average number of common shares outstanding for the period, including potential dilutive common shares assuming the dilutive effect of outstanding stock options. For periods in which the Company has reported net losses, diluted net loss per common share is the same as basic net loss per common share, since dilutive common shares are not assumed to have been issued if their affect is anti-dilutive.

The Company reported a net loss for the three and nine months ended September 30, 2021 and 2020. The following potential dilutive securities, presented based on amounts outstanding at each period end, have been excluded from the calculation of diluted net loss per share because including them would have had an anti-dilutive impact:


	As of September 30,

	2021		2020
Stock options to purchase common stock		6,091,220		5,782,911
Unvested restricted common stock and units		497,418		251,723
Shares of common stock issuable under Employee Stock Purchase Plan		8,843		8,213

		6,597,481		6,042,847

Recently Issued Accounting Pronouncements

In February 2016, the FASB issued ASU No.

2016-02,

Leases

(“ASU

2016-02”).

ASU

2016-02

will require lessees to recognize most leases on their balance sheet as a

right-of-use

asset and a lease liability. Leases will be classified as either operating or finance, and classification will be based on criteria similar to current lease accounting, but without explicit bright lines. For public entities, the guidance was effective for annual reporting periods beginning after December 15, 2018 and for interim periods within those fiscal years. For nonpublic entities and emerging growth companies that choose to take advantage of the extended transition period, the guidance was effective for annual reporting periods beginning after December 15, 2019. In June 2020, the FASB issued ASU

No. 2020-05,

which further deferred the effective date for nonpublic entities to annual reporting periods beginning after December 15, 2021, and interim periods within fiscal years beginning after December 15, 2022. Early adoption is permitted for all entities. ASU

2016-02

initially required adoption using a modified retrospective approach, under which all years presented in the financial statements would be prepared under the revised guidance. In July 2018, the FASB issued ASU No.

2018-11,

Leases (Topic 842) Targeted Improvements

, which added an optional transition method to the existing requirements whereby an entity could adopt the provisions of ASU

2016-02

by recognizing a cumulative-effective adjustment to the opening balance of retained earnings in the period of adoption without adjustment to the financial statements for periods prior to adoption. The Company expects that the adoption of the new leasing standards will result in the recognition of material

right-of-use

assets and lease liabilities on the consolidated balance sheets but does not expect it to have a material impact on its results of operations or cash flows.

In June 2016, the FASB issued ASU

No. 2016-13,

Financial Instruments – Credit Losses (Topic 326)

. The new standard adjusts the accounting for assets held at amortized costs basis, including marketable securities accounted for as available for sale. The standard eliminates the probable initial recognition threshold and requires an entity to reflect its current estimate of all expected credit losses. The allowance for credit losses is a valuation account that is deducted from the amortized cost basis of the financial assets to present the net amount expected to be collected. For public entities, the guidance was effective for annual reporting periods beginning after December 15, 2019 and for interim periods within those fiscal years. For nonpublic entities and emerging growth companies that choose to take advantage of the extended transition period, the guidance is effective for annual reporting periods beginning after December 15, 2020. Early adoption is permitted for all entities. In November 2019, the FASB issued ASU

No. 2019-10,

which deferred the effective date for nonpublic entities to annual reporting periods beginning after December 15, 2022, including interim periods within those fiscal years. The Company does not believe the guidance will have a material impact on its consolidated financial statements.

Fair Value of Financial Assets

9 Months Ended

Sep. 30, 2021

Fair Value of Financial Assets

3. Fair Value of Financial Assets

As of September 30, 2021, marketable securities by security type consisted of (in thousands):


			Gross		Gross		Estimated
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value

U.S. treasury notes (due within one year)	$	35,076	$	1	$	—	$	35,077

Total	$	35,076	$	1	$	—	$	35,077

As of December 31, 2020, marketable securities by security type consisted of (in thousands):


			Gross		Gross			Estimated
	Amortized		Unrealized		Unrealized			Fair
	Cost		Gains		Losses			Value

U.S. treasury notes (due within one year)	$	90,706	$	—	$	(23	)	$	90,683

Total	$	90,706	$	—	$	(23	)	$	90,683

The following tables present information about the Company’s financial assets measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values (in thousands):


	Fair Value Measurements at September 30, 2021 Using:
	Level 1		Level 2		Level 3		Total

Cash equivalents:
Money market funds	$	157,781	$	—	$	—	$	157,781
Marketable securities:
U.S. treasury notes		—		35,077		—		35,077

Total	$	157,781	$	35,077	$	—	$	192,858


	Fair Value Measurements at December 31, 2020 Using:
	Level 1		Level 2		Level 3		Total

Cash equivalents:
Money market funds	$	43,182	$	—	$	—	$	43,182
U.S. treasury notes		—		14,999		—		14,999
Marketable securities:
U.S. treasury notes		—		90,683		—		90,683

Total	$	43,182	$	105,682	$	—	$	148,864

Accrued Expenses and Other Current Liabilities

9 Months Ended

Sep. 30, 2021

Accrued Expenses and Other Current Liabilities

4. Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following (in thousands):


	September 30, 2021		December 31, 2020

Accrued external research and development expenses	$	3,294	$	2,662
Accrued payroll and related expenses		2,983		3,107
Accrued professional fees		932		693
Deferred rent, current portion		555		555
Accrued other		497		653

	$	8,261	$	7,670

Stock-Based Awards

9 Months Ended

Sep. 30, 2021

Stock-Based Awards

5. Stock-Based Awards

2018 Stock Option and Incentive Plan

The Company grants stock-based awards under the Magenta Therapeutics, Inc. 2018 Stock Option and Incentive Plan (the “2018 Plan”). The Company also has outstanding stock options under the Magenta Therapeutics, Inc. 2016 Stock Option and Grant Plan, as amended (the “2016 Plan”), but is no longer granting awards under the 2016 Plan. As of September 30, 2021, 3,151,538 shares of common stock were available for issuance under the 2018 Plan.

2019 Employee Stock Purchase Plan

Employees may elect to participate in The Magenta Therapeutics, Inc. 2019 Employee Stock Purchase Plan (the “ESPP”). The purchase price of common stock under the ESPP is equal to 85% of the lower of the fair market value of the common stock on the offering date or the exercise date. The

six-month offering

periods begin in December and June of each year. During the nine months ended September 30, 2021, 13,989 shares of common stock were purchased under the ESPP at a purchase price per share of $6.15. During the nine months ended September 30, 2020, 5,653 shares of common stock were purchased under the ESPP at a purchase price per share of $7.51. As of September 30, 2021, 136,835 shares remained available for issuance under the ESPP.

Grant of Stock Options

During the nine months ended September 30, 2021, the Company granted options to certain employees and directors with service-based vesting conditions for the purchase of 2,526,457 shares of common stock with a weighted average grant date fair value of $6.68 per share. Stock-based compensation expense is being recognized over the requisite service period of four years.

Grant of Restricted Stock Units

During the nine months ended September 30, 2021, the Company granted 275,625 restricted stock units to certain employees with a weighted average grant date fair value of $8.07 per share. Stock-based compensation expense is being recognized over the requisite service period of three years.

Grant of Performance Restricted Stock Units

During the nine months ended September 30, 2021, the Company granted 300,000 performance-based restricted stock units to certain employees with a weighted average grant date fair value of $10.65 per share. These performance-based restricted stock units vest upon the occurrence of certain operational and financial events. At the achievement of the performance-based vesting criteria, each performance-based restricted stock unit entitles the holder to a specified number of shares of common stock.

During the three and nine months ended September 30, 2021, the Company recorded stock compensation expense of $1.7 million related to the vesting of 170,000 performance-based restricted stock units.

Stock-Based Compensation

Stock-based compensation expense was classified in the statements of operations and comprehensive loss as follows (in thousands):


	Three Months Ended September 30,				Nine Months Ended September 30,

	2021		2020		2021		2020

Research and development expenses	$	1,335	$	878	$	3,276	$	2,680
General and administrative expenses		2,464		1,488		4,849		4,930

	$	3,799	$	2,366	$	8,125	$	7,610

As of September 30, 2021, unrecognized compensation expense related to unvested share-based awards with service-based vesting conditions was $21.8 million, which is expected to be recognized over a weighted average period of 2.9 years. Additionally, the Company had unrecognized compensation cost of $1.7 million related to the unvested performance restricted stock units for which the performance conditions were not considered probable of achievement as of September 30, 2021.

Commitments and Contingencies

9 Months Ended

Sep. 30, 2021

Commitments and Contingencies

6. Commitments and Contingencies

Leases

In May 2018, the Company entered into a sublease, as amended, for up to approximately 69,000 square feet of office and laboratory space in Cambridge, Massachusetts. The sublease is subject and subordinate to a prime lease between the sublandlord and the prime landlord. The term of the sublease commenced in June 2018 and expires in February 2028. The sublandlord has the right to terminate the sublease after five years. The Company is obligated to pay real estate taxes and other costs related to the premises, including costs of operations and management of the leased premises. In connection with the sublease, as amended, the sublandlord funded $5.2 million in tenant improvements to the leased facility during 2019. The Company is required to maintain a cash balance of $1.8 million to secure a letter of credit associated with the sublease. This amount was classified as noncurrent restricted cash in the consolidated balance sheets at September 30, 2021 and December 31, 2020.

As of September 30, 2021 and December 31, 2020, the Company had long-term deferred rent of $6.4 million and $6.3 million, respectively, related to lease incentives and payment escalations. As of September 30, 2021 and December 31, 2020, the short-term portion of deferred rent of $0.6 million for each period was included in accrued expenses and other current liabilities. The Company recorded rent expense of $1.5 million during each of the three months ended September 30, 2021 and 2020. The Company recorded rent expense of $4.6 million during each of the nine months ended September 30, 2021 and 2020.

As of September 30, 2021, the future minimum lease payments due under the noncancelable operating lease was as follows (in thousands):


Remainder of 2021 (three months)	$	1,549
2022		6,375
2023		6,734
2024		7,100
2025		7,455
Thereafter		17,444

	$	46,657

In 2018, the Company entered into two

two-year

sub-subleases

of approximately 27,000 square feet of office space in Cambridge, Massachusetts which were both set to expire in the fourth quarter of 2020. In the third quarter of 2020, the Company amended each of the

sub-subleases

to extend their expirations to July 2021 and April 2022, respectively. In the second quarter of 2021, the

sub-sublease

expiring in July 2021 was further amended to extend the expiration to December 2021. As of September 30, 2021, the remaining base rent payments due to the Company under the amended

sub-subleases

was $1.0 million. The Company recorded other income of $0.8 million and $0.6 million during the three months ended September 30, 2021 and 2020, respectively, related to these

sub-subleases.

The Company recorded other income of $2.7 million and $2.0 million during the nine months ended September 30, 2021 and 2020, respectively, related to these

sub-subleases.

Collaboration Agreement

In March 2018, the Company entered into a collaboration agreement with Heidelberg Pharma Research GmbH (“HDPR”) whereby the parties agreed to combine the Company’s stem cell platform with proprietary antibodies across up to four exclusive targets with HDPR’s proprietary Antibody Targeted Amanitin Conjugates platform. Under the agreement, the Company may pay upfront technology access fees, research exclusivity fees and payment for research support. Additionally, upon the exercise of certain license rights, the Company may be obligated to pay HDPR development, regulatory and commercial milestone payments of up to

$83.5 million per target as well as royalties on net sales of products licensed under the agreement. During the three months ended September 30, 2021 and 2020, the Company recorded less than $0.1 million and $0.3 million, respectively, of research and development expense related to this agreement for upfront technology access fees, research exclusivity fees and research support. During the nine months ended September 30, 2021 and 2020, the Company recorded $0.4 million and $0.6 million, respectively, of research and development expense related to this agreement for upfront technology access fees, research exclusivity fees and research support.

Intellectual Property Licenses

The Company has a license agreement with the President and Fellows of Harvard College (“Harvard”), entered into in November 2016, for an exclusive, worldwide, royalty-bearing license for certain technologies related to conditioning and mobilization. The Company is obligated to pay Harvard maintenance fees of

$0.1 million annually and to reimburse qualified expenses related to the patents. The Company is also obligated to pay milestone payments of up to $7.4 million for the first two licensed products upon the achievement of certain development and regulatory milestones and to pay royalties on a

product-by-product

and

country-by-country

basis on net sales of products licensed under the agreement. During the three months ended September 30, 2021 and 2020, the Company did not incur any expense related to the achievement of these milestones. During the nine months ended September 30, 2021, the Company recorded $0.1 million of expense related to the achievement of one of these milestones. During the nine months ended September 30, 2020, the Company did not incur any expense related to the achievement of these milestones.

The Company has a license agreement with Novartis International Pharmaceutical Ltd. (“Novartis”), entered into in April 2017, to use and develop certain patent rights (the “Novartis License”). Under the Novartis License, the Company was granted an exclusive, worldwide, sublicensable license to research, develop and commercialize certain licensed products that contain Novartis compounds for the expansion of cord blood derived

non-gene-edited/-modified

hematopoietic stem cells. The Company is obligated to make payments of up to

$177.0 million upon the achievement of specified clinical and regulatory milestones and up to $125.0 million upon the achievement of specified commercial milestones and to pay tiered royalties, on a

product-by-product

and

country-by-country

basis, up to a maximum of 20% on net sales of products licensed under the agreement. As of September 30, 2021, no milestones related to the Novartis License have been met.

The Company has agreements with third

parties in the normal course of business, under which it can license certain developed technologies. If the Company exercises its rights to license the respective technologies, it may be subject to additional fees and milestone payments. During the three and nine months ended September 30, 2021 and 2020, the Company did not incur any expense related to these licenses.

Indemnification Agreements

In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors and senior management that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnifications. The Company is not aware of any claims under indemnification arrangements, and it has not accrued any liabilities related to such obligations in its consolidated financial statements as of September 30, 2021.

Legal Proceedings

The Company is not currently a party to any material legal proceedings. At each reporting date, the Company evaluates whether or not a potential loss amount or a potential range of loss is probable and reasonably estimable under the provisions of the authoritative guidance that addresses accounting for contingencies. The Company expenses the costs related to its legal proceedings as they are incurred.

401(k) Savings Plan

9 Months Ended

Sep. 30, 2021

401(k) Savings Plan

7. 401(k) Savings Plan

The Company has a 401(k) available for participating employees who meet certain eligibility requirements. Eligible employees may defer a portion of their salary as defined by the plan. Company contributions to the plan may be made at the discretion of the board of directors of the Company. Effective August 2021, the Company began making matching contributions of up to 2% of eligible wages. During the three and nine months ended September 30, 2021, the Company recorded less than $0.1 million of expense related to this matching contribution.

Related Parties

9 Months Ended

Sep. 30, 2021

Related Parties

8. Related Parties

National Marrow Donor Program (as successor in interest to Be The Match BioTherapies Collection Services, LLC (formerly known as Be The Match BioTherapies, LLC))

Effective March 2018, Amy Lynn Ronneberg, the then serving President of Be The Match BioTherapies, LLC, became a member of the Company’s board of directors and subsequently was appointed Chief Executive Officer of the National Marrow Donor Program/Be The Match, or NMDP/Be The Match, organization in June 2020. The Company has collaboration agreements with the National Marrow Donor Program (as successor in interest to Be The Match BioTherapies Collection Services, LLC (formerly known as Be The Match BioTherapies, LLC)) and a research agreement with an affiliated organization, Center for International Blood and Marrow Transplant Research. In addition, in June 2020, the Company entered into a clinical collaboration agreement with NMDP/Be The Match to evaluate the potential utility of

MGTA-145

for mobilizing and collecting hematopoietic stem cells from donors in a single day and then using them for allogeneic transplants in patients. Under the terms of this agreement, the Company shall fund up to fifty percent of NMDP/Be The Match clinical trial costs and provide the trial drugs which will be included in research and development expense.

For the three months ended September 30, 2021 and 2020, the Company recorded expense of $0.2 million and $0.1 million, respectively, related to these agreements. For the nine months ended September 30, 2021 and 2020, the Company recorded expense of $0.5 million and $0.3 million, respectively, related to these agreements. As of September 30, 2021 and December 31, 2020,

amounts on the consolidated balance sheets related to these agreements were less than $

0.1 million, which amounts were included in accounts payable and accrued expenses and other current liabilities and less than $0.1 million, which amounts were included in prepaid expenses and other current assets.

Summary of Significant Accounting Policies (Policies)

9 Months Ended

Sep. 30, 2021

Use of Estimates

Unaudited Interim Financial Information

10-K,

Fair Value Measurements

•

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

•

Income Taxes

The Company accounts for uncertainty in income taxes recognized in its consolidated financial statements by applying a

two-step

more-likely-than-not

Comprehensive Loss

Net Loss per Share


	As of September 30,

	2021		2020
Stock options to purchase common stock		6,091,220		5,782,911
Unvested restricted common stock and units		497,418		251,723
Shares of common stock issuable under Employee Stock Purchase Plan		8,843		8,213

		6,597,481		6,042,847

Recently Issued Accounting Pronouncements

In February 2016, the FASB issued ASU No.

2016-02,

Leases

(“ASU

2016-02”).

ASU

2016-02

will require lessees to recognize most leases on their balance sheet as a

right-of-use

No. 2020-05,

2016-02

2018-11,

Leases (Topic 842) Targeted Improvements

, which added an optional transition method to the existing requirements whereby an entity could adopt the provisions of ASU

2016-02

right-of-use

assets and lease liabilities on the consolidated balance sheets but does not expect it to have a material impact on its results of operations or cash flows.

In June 2016, the FASB issued ASU

No. 2016-13,

Financial Instruments – Credit Losses (Topic 326)

No. 2019-10,

- Definition

Disclosure of accounting policy for income taxes, which may include its accounting policies for recognizing and measuring deferred tax assets and liabilities and related valuation allowances, recognizing investment tax credits, operating loss carryforwards, tax credit carryforwards, and other carryforwards, methodologies for determining its effective income tax rate and the characterization of interest and penalties in the financial statements.

+ References + Details

Summary of Significant Accounting Policies (Tables)

9 Months Ended

Sep. 30, 2021

Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share

The following potential dilutive securities, presented based on amounts outstanding at each period end, have been excluded from the calculation of diluted net loss per share because including them would have had an anti-dilutive impact:


	As of September 30,

	2021		2020
Stock options to purchase common stock		6,091,220		5,782,911
Unvested restricted common stock and units		497,418		251,723
Shares of common stock issuable under Employee Stock Purchase Plan		8,843		8,213

		6,597,481		6,042,847

Fair Value of Financial Assets (Tables)

9 Months Ended

Sep. 30, 2021

Schedule of Marketable Securities by Security Type

As of September 30, 2021, marketable securities by security type consisted of (in thousands):


			Gross		Gross		Estimated
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value

U.S. treasury notes (due within one year)	$	35,076	$	1	$	—	$	35,077

Total	$	35,076	$	1	$	—	$	35,077

As of December 31, 2020, marketable securities by security type consisted of (in thousands):


			Gross		Gross			Estimated
	Amortized		Unrealized		Unrealized			Fair
	Cost		Gains		Losses			Value

U.S. treasury notes (due within one year)	$	90,706	$	—	$	(23	)	$	90,683

Total	$	90,706	$	—	$	(23	)	$	90,683

Schedule of Financial Assets Measured at Fair Value on Recurring Basis


	Fair Value Measurements at September 30, 2021 Using:
	Level 1		Level 2		Level 3		Total

Cash equivalents:
Money market funds	$	157,781	$	—	$	—	$	157,781
Marketable securities:
U.S. treasury notes		—		35,077		—		35,077

Total	$	157,781	$	35,077	$	—	$	192,858


	Fair Value Measurements at December 31, 2020 Using:
	Level 1		Level 2		Level 3		Total

Cash equivalents:
Money market funds	$	43,182	$	—	$	—	$	43,182
U.S. treasury notes		—		14,999		—		14,999
Marketable securities:
U.S. treasury notes		—		90,683		—		90,683

Total	$	43,182	$	105,682	$	—	$	148,864

Accrued Expenses and Other Current Liabilities (Tables)

9 Months Ended

Sep. 30, 2021

Schedule of Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consisted of the following (in thousands):


	September 30, 2021		December 31, 2020

Accrued external research and development expenses	$	3,294	$	2,662
Accrued payroll and related expenses		2,983		3,107
Accrued professional fees		932		693
Deferred rent, current portion		555		555
Accrued other		497		653

	$	8,261	$	7,670

Stock-Based Awards (Tables)

9 Months Ended

Sep. 30, 2021

Schedule of Stock Based Compensation Expense

Stock-based compensation expense was classified in the statements of operations and comprehensive loss as follows (in thousands):


	Three Months Ended September 30,				Nine Months Ended September 30,

	2021		2020		2021		2020

Research and development expenses	$	1,335	$	878	$	3,276	$	2,680
General and administrative expenses		2,464		1,488		4,849		4,930

	$	3,799	$	2,366	$	8,125	$	7,610

Commitments and Contingencies (Tables)

9 Months Ended

Sep. 30, 2021

Schedule of Future Minimum Lease Payments Due Under Noncancelable Operating Lease

As of September 30, 2021, the future minimum lease payments due under the noncancelable operating lease was as follows (in thousands):


Remainder of 2021 (three months)	$	1,549
2022		6,375
2023		6,734
2024		7,100
2025		7,455
Thereafter		17,444

	$	46,657

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Jun. 30, 2020	Aug. 08, 2019	May 31, 2021	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Entity incorporation date						Jun. 01, 2015
Entity incorporation state code						DE
Net loss				$ (17,427)	$ (17,678)	$ (51,844)	$ (56,768)	$ (74,900)
Accumulated deficit				$ (306,275)		$ (306,275)		$ (254,431)
Net proceeds from common stock							$ 64,860
Maximum [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Shelf offering value		$ 350,000
Private Placement [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Stock issued, shares			9,599,998
Stock issued, price per shares			$ 9.00
Sale of stock net consideration received on the transaction			$ 86,100
Common Stock [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Stock issued, shares	8,625,000						8,625,000
Stock issued, price per shares	$ 8.00
Net proceeds from common stock	$ 64,600
Common Stock [Member] \| Private Placement [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Stock issued, shares						9,599,998
Cowen and Company LLC [Member] \| Maximum [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
ATM offering value		$ 100,000

Summary of Significant Accounting Policies - Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share (Detail) - shares	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share, amount	6,597,481	6,042,847
Stock Option [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share, amount	6,091,220	5,782,911
Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share, amount	497,418	251,723
Shares of common stock issuable under the ESPP [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share, amount	8,843	8,213

Fair Value of Financial Assets - Schedule of Marketable Securities by Security Type (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Marketable Securities [Line Items]
Amortized Cost	$ 35,076	$ 90,706
Gross Unrealized Gains	1
Gross Unrealized Losses		(23)
Estimated Fair Value	35,077	90,683
United States Treasury Notes [Member]
Marketable Securities [Line Items]
Amortized Cost	35,076	90,706
Gross Unrealized Gains	1
Gross Unrealized Losses		(23)
Estimated Fair Value	$ 35,077	$ 90,683

Fair Value of Financial Assets - Schedule of Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	$ 35,077	$ 90,683
Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	192,858	148,864
Level 1 [Member] \| Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	157,781	43,182
Level 2 [Member] \| Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	35,077	105,682
Money Market Funds [Member] \| Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	157,781	43,182
Money Market Funds [Member] \| Level 1 [Member] \| Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	157,781	43,182
United States Treasury Notes [Member] \| Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		14,999
Marketable securities	35,077
United States Treasury Notes [Member] \| Level 2 [Member] \| Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		14,999
Marketable securities	35,077
United States Treasury Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	$ 35,077	90,683
United States Treasury Notes [Member] \| Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities		90,683
United States Treasury Notes [Member] \| Level 2 [Member] \| Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities		$ 90,683

Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Payables and Accruals [Abstract]
Accrued external research and development expenses	$ 3,294	$ 2,662
Accrued payroll and related expenses	2,983	3,107
Accrued professional fees	932	693
Deferred rent, current portion	555	555
Accrued other	497	653
Total	$ 8,261	$ 7,670

Stock-Based Awards - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized stock-based compensation expense	$ 21,800		$ 21,800
Period for recognition of unrecognized expense			2 years 10 months 24 days
Stock compensation expense	3,799	$ 2,366	$ 8,125	$ 7,610
Employees And Directors [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, options, grants in period, gross			2,526,457
Weighted average fair value of stock options granted			$ 6.68
Share based compensation arrangement , requisite service period			4 years
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized stock-based compensation expense	1,700		$ 1,700
Performance Shares [member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation expense	$ 1,700		$ 1,700
Grant of restricted stock			300,000
Grant weighted average grant date fair value per share			$ 10.65
Stock compensation expense related to the vesting of performance based restricted stock units	170,000		170,000
Restricted Stock Units (RSUs) [Member] \| Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant of restricted stock			275,625
Grant weighted average grant date fair value per share			$ 8.07
Share based compensation arrangement , requisite service period			3 years
2018 Stock Option and Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock available for issuance	3,151,538		3,151,538
2019 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock available for issuance	136,835		136,835
Percentage of purchase price of common stock under the ESPP			85.00%
Common stock purchased			13,989	5,653
Stock issued, price per shares	$ 6.15	$ 7.51	$ 6.15	$ 7.51

Stock-Based Awards - Schedule of Stock Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock based compensation expense	$ 3,799	$ 2,366	$ 8,125	$ 7,610
Research and Development Expenses [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock based compensation expense	1,335	878	3,276	2,680
General and Administrative Expenses [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock based compensation expense	$ 2,464	$ 1,488	$ 4,849	$ 4,930

Commitments and Contingencies - Additional Information (Detail) $ in Thousands	1 Months Ended	3 Months Ended			9 Months Ended
	Apr. 30, 2017 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2018 ft² Sublease	Sep. 30, 2021 USD ($) Milestone	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)	May 31, 2018 USD ($) ft²	Nov. 30, 2016 USD ($)
Other Commitments [Line Items]
Sublease commencement period					2018-06
Sublease expiration period					2028-02
Property sublease, description					In May 2018, the Company entered into a sublease, as amended, for up to approximately 69,000 square feet of office and laboratory space in Cambridge, Massachusetts. The sublease is subject and subordinate to a prime lease between the sublandlord and the prime landlord. The term of the sublease commenced in June 2018 and expires in February 2028. The sublandlord has the right to terminate the sublease after five years.
Sublease termination period					5 years
Long-term deferred rent		$ 6,401			$ 6,401		$ 6,283
Deferred rent, current portion		555			555		555
Rent expense		1,500	$ 1,500		4,600	$ 4,600
Rental payments receivable		1,000			1,000
Research and development expense		10,795	11,786		33,652	38,359
Research and Development Expenses [Member]
Other Commitments [Line Items]
Research and development expense					100
Long Term Deferred Rent [Member]
Other Commitments [Line Items]
Long-term deferred rent		6,400			6,400		6,300
Short Term Deferred Rent [Member]
Other Commitments [Line Items]
Deferred rent, current portion		600			600		$ 600
Maximum [Member] \| Office and Lab Sublease [Member]
Other Commitments [Line Items]
Square feet of property subject to sublease \| ft²								69,000
Tenant improvement allowance								$ 5,200
Sub-Sublease [Member]
Other Commitments [Line Items]
Square feet of property subject to sublease \| ft²				27,000
Lease agreement maturity period				2 years
Number of Sub-sublease \| Sublease				2
Other income from sub-sublease		800	600		$ 2,700	2,000
License Agreement with Novartis International Pharmaceutical Ltd. [Member]
Other Commitments [Line Items]
Number of milestones met \| Milestone					0
License Agreement with Novartis International Pharmaceutical Ltd. [Member] \| Maximum [Member]
Other Commitments [Line Items]
License agreement royalty percentage	20.00%
License Agreement with Novartis International Pharmaceutical Ltd. [Member] \| Achievement of Clinical and Regulatory Milestones [Member] \| Maximum [Member]
Other Commitments [Line Items]
Payment obligation	$ 177,000
License Agreement with Novartis International Pharmaceutical Ltd. [Member] \| Development and Regulatory Milestones [Member] \| Maximum [Member]
Other Commitments [Line Items]
Payment obligation	$ 125,000
Collaboration Agreement [Member] \| Maximum [Member]
Other Commitments [Line Items]
Research and development expense		100	300		$ 400	$ 600
Collaboration Agreement [Member] \| Development, Regulatory and Commercial Milestone [Member] \| Per Target [Member] \| Maximum [Member]
Other Commitments [Line Items]
Payment obligation		83,500			$ 83,500
License Agreement With Harvard [Member] \| Maintenance [Member] \| Annually Through 2019 [Member] \| Maximum [Member]
Other Commitments [Line Items]
Payment obligation									$ 100
License Agreement With Harvard [Member] \| Achievement of Development and Regulatory Milestones [Member] \| Maximum [Member]
Other Commitments [Line Items]
Payment obligation									$ 7,400
License Agreement With Third Parties [Member] \| License [Member]
Other Commitments [Line Items]
Research and development expense		$ 0	$ 0
Letter of Credit [Member] \| Office and Lab Sublease [Member]
Other Commitments [Line Items]
Cash balance to secure letter of credit associated with sublease								$ 1,800

Commitments and Contingencies - Schedule of Future Minimum Lease Payments Due Under Noncancelable Operating Lease (Detail) $ in Thousands	Sep. 30, 2021 USD ($)
Operating Leased Assets [Line Items]
Remainder of 2021 (three months)	$ 1,549
2022	6,375
2023	6,734
2024	7,100
2025	7,455
Thereafter	17,444
Operating leases future minimum payments due	$ 46,657

401(k) Savings Plan - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2021
Retirement Benefits [Abstract]
Defined contribution plan,Employers matching contributions subject to time based vesting requirements		2.00%
Defined contribution plan, Employer discretionary contribution amount	$ 0.1	$ 0.1

Related Parties - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Related Party Transaction [Line Items]
Amounts owed to related party	$ 0.1		$ 0.1		$ 0.1
Match Bio Therapies [Member]
Related Party Transaction [Line Items]
Expenses paid to related party	0.2	$ 0.1	0.5	$ 0.3
Match Bio Therapies [Member] \| Prepaid Expenses and Other Current Assets [Member]
Related Party Transaction [Line Items]
Amounts owed to related party	$ 0.1		$ 0.1		$ 0.1

- Definition

Carrying amount as of the balance sheet date of obligations due all related parties. For classified balance sheets, represents the current portion of such liabilities (due within one year or within the normal operating cycle if longer).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 10
-Section S99
-Paragraph 1B
-Subparagraph (SX 210.13-02(a)(4)(iii)(C))
-URI http://asc.fasb.org/extlink&oid=124359900&loc=SL124442552-122756

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 10
-Section S99
-Paragraph 1B
-Subparagraph (SX 210.13-02(a)(5))
-URI http://asc.fasb.org/extlink&oid=124359900&loc=SL124442552-122756

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 10
-Section S99
-Paragraph 1A
-Subparagraph (SX 210.13-01(a)(4)(iv))
-URI http://asc.fasb.org/extlink&oid=124359900&loc=SL124442526-122756

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 10
-Section S99
-Paragraph 1A
-Subparagraph (SX 210.13-01(a)(4)(iii)(B))
-URI http://asc.fasb.org/extlink&oid=124359900&loc=SL124442526-122756

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 10
-Section S99
-Paragraph 1A
-Subparagraph (SX 210.13-01(a)(5))
-URI http://asc.fasb.org/extlink&oid=124359900&loc=SL124442526-122756

Reference 7: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(a))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 8: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(k)(1))
-URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 10
-Section S99
-Paragraph 1B
-Subparagraph (SX 210.13-02(a)(4)(iv))
-URI http://asc.fasb.org/extlink&oid=124359900&loc=SL124442552-122756

+ Details