Dianthus Therapeutics (DNTH) 10-Q 13 Nov 19 2019 Q3 Quarterly report Financial data

Cover Page

Cover Page - shares	9 Months Ended
	Sep. 30, 2019	Oct. 31, 2019
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Trading Symbol	MGTA
Entity Registrant Name	Magenta Therapeutics, Inc.
Entity Central Index Key	0001690585
Entity Filer Category	Non-accelerated Filer
Entity Shell Company	false
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Common Stock, Shares Outstanding		38,989,517
Entity Ex Transition Period	false
Title of 12(b) Security	Common Stock
Security Exchange Name	NASDAQ
Entity Address, State or Province	MA

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 85,507	$ 58,345
Marketable securities	75,063	84,225
Prepaid expenses and other current assets	4,660	2,751
Total current assets	165,230	145,321
Restricted cash	1,780	1,780
Property and equipment, net	10,223	10,212
Total assets	177,233	157,313
Current liabilities:
Accounts payable	2,038	3,492
Accrued expenses and other current liabilities	7,591	6,927
Total current liabilities	9,629	10,419
Deferred rent	6,075	1,246
Total liabilities	15,704	11,665
Commitments and contingencies (Note 7)
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; 39,370,576 shares and 34,327,554 shares issued and 38,942,719 shares and 33,305,033 shares outstanding as of September 30, 2019 and December 31, 2018, respectively	39	33
Additional paid-in capital	317,665	248,349
Accumulated other comprehensive income (loss)	83	(8)
Accumulated deficit	(156,258)	(102,726)
Total stockholders' equity	161,529	145,648
Total liabilities and stockholders' equity	$ 177,233	$ 157,313

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2019	Dec. 31, 2018
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	39,370,576	34,327,554
Common stock, Shares outstanding	38,942,719	33,305,033

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenue	$ 0	$ 0	$ 0	$ 0
Operating expenses:
Research and development	16,524	11,418	40,494	28,950
General and administrative	6,120	5,284	17,838	13,083
Total operating expenses	22,644	16,702	58,332	42,033
Loss from operations	(22,644)	(16,702)	(58,332)	(42,033)
Interest and other income, net	1,654	687	4,800	1,197
Net loss	(20,990)	(16,015)	(53,532)	(40,836)
Accretion of redeemable convertible preferred stock to redemption value				(88)
Net loss attributable to common stockholders	$ (20,990)	$ (16,015)	$ (53,532)	$ (40,924)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.54)	$ (0.49)	$ (1.47)	$ (3.05)
Weighted average common shares outstanding, basic and diluted	38,824,209	32,997,346	36,322,804	13,396,856
Comprehensive loss:
Net loss	$ (20,990)	$ (16,015)	$ (53,532)	$ (40,836)
Other comprehensive income (loss):
Unrealized gains (losses) on marketable securities	(56)		91
Total other comprehensive income (loss)	(56)		91
Total comprehensive loss	$ (21,046)	$ (16,015)	$ (53,441)	$ (40,836)

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Series A, B and C Redeemable Convertible Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2017		$ 92,439
Beginning balance at Dec. 31, 2017	$ (42,118)		$ 2	$ 3,091		$ (45,211)
Beginning balance, shares at Dec. 31, 2017		49,178,527
Beginning balance, shares at Dec. 31, 2017			2,351,247
Issuance of Series C redeemable convertible preferred stock, net of issuance cost of $88		$ 52,212
Issuance of Series C redeemable convertible preferred stock, net of issuance cost of $88, shares		11,223,102
Accretion of Series C redeemable convertible preferred stock to redemption value	(88)	$ 88		(88)
Conversion of preferred stock to common stock	144,739	$ (144,739)	$ 23	144,716
Conversion of preferred stock to common stock, shares		(60,401,629)	23,375,405
Issuance of common stock upon initial public offering net of underwriting discounts, commissions and offering costs	89,906		$ 7	89,899
Issuance of common stock upon initial public offering net of underwriting discounts, commissions and offering costs, shares			6,666,667
Vesting of restricted stock			$ 1	(1)
Vesting of restricted stock, shares			704,696
Issuance of common stock upon exercise of stock options	29			29
Issuance of common stock upon exercise of stock options, shares			4,152
Stock-based compensation expense	8,152			8,152
Net loss	(40,836)					(40,836)
Ending balance at Sep. 30, 2018	159,784		$ 33	245,798		(86,047)
Ending balance, shares at Sep. 30, 2018			33,102,167
Beginning balance at Dec. 31, 2017		$ 92,439
Beginning balance at Dec. 31, 2017	(42,118)		$ 2	3,091		(45,211)
Beginning balance, shares at Dec. 31, 2017		49,178,527
Beginning balance, shares at Dec. 31, 2017			2,351,247
Net loss	(57,500)
Ending balance at Dec. 31, 2018	145,648		$ 33	248,349	$ (8)	(102,726)
Ending balance, shares at Dec. 31, 2018			33,305,033
Beginning balance at Jun. 30, 2018	172,217		$ 33	242,216		(70,032)
Beginning balance, shares at Jun. 30, 2018			32,889,056
Initial public offering costs	(123)			(123)
Vesting of restricted stock, shares			208,959
Issuance of common stock upon exercise of stock options	29			29
Issuance of common stock upon exercise of stock options, shares			4,152
Stock-based compensation expense	3,676			3,676
Net loss	(16,015)					(16,015)
Ending balance at Sep. 30, 2018	159,784		$ 33	245,798		(86,047)
Ending balance, shares at Sep. 30, 2018			33,102,167
Beginning balance at Dec. 31, 2018	145,648		$ 33	248,349	(8)	(102,726)
Beginning balance, shares at Dec. 31, 2018			33,305,033
Issuance of common stock upon initial public offering net of underwriting discounts, commissions and offering costs	60,276		$ 5	60,271
Issuance of common stock upon initial public offering net of underwriting discounts, commissions and offering costs, shares			4,887,500
Vesting of restricted stock			$ 1	(1)
Vesting of restricted stock, shares			592,849
Issuance of common stock upon exercise of stock options	1,107			1,107
Issuance of common stock upon exercise of stock options, shares			157,337
Stock-based compensation expense	7,939			7,939
Unrealized gains (losses) on marketable securities	91				91
Net loss	(53,532)					(53,532)
Ending balance at Sep. 30, 2019	161,529		$ 39	317,665	83	(156,258)
Ending balance, shares at Sep. 30, 2019			38,942,719
Beginning balance at Jun. 30, 2019	179,556		$ 39	314,646	139	(135,268)
Beginning balance, shares at Jun. 30, 2019			38,710,858
Vesting of restricted stock, shares			197,570
Issuance of common stock upon exercise of stock options	257			257
Issuance of common stock upon exercise of stock options, shares			34,291
Stock-based compensation expense	2,762			2,762
Unrealized gains (losses) on marketable securities	(56)				(56)
Net loss	(20,990)					(20,990)
Ending balance at Sep. 30, 2019	$ 161,529		$ 39	$ 317,665	$ 83	$ (156,258)
Ending balance, shares at Sep. 30, 2019			38,942,719

Consolidated Statements of St_2

Consolidated Statements of Stockholders' Equity (Deficit) (Parenthetical) $ in Thousands	9 Months Ended
	Sep. 30, 2018USD ($)
Payments of redeemable convertible preferred stock issuance costs	$ 2,618
Series C Preferred Stock [Member]
Payments of redeemable convertible preferred stock issuance costs	$ 88

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities:
Net loss	$ (53,532)	$ (40,836)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	7,939	8,152
Depreciation and amortization expense	1,358	516
Loss on disposal of property and equipment		67
Net accretion of discounts on marketable securities	(909)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,909)	(2,085)
Accounts payable	(629)	2,104
Accrued expenses and other current liabilities	1,377	1,740
Deferred rent	4,829	1,080
Net cash used in operating activities	(41,476)	(29,262)
Cash flows from investing activities:
Purchases of property and equipment	(2,907)	(3,321)
Proceeds from sale of property and equipment		12
Purchases of marketable securities	(94,176)
Maturities of marketable securities	104,338
Net cash provided by (used in) investing activities	7,255	(3,309)
Cash flows from financing activities:
Proceeds from public offerings, net of underwriting discounts and commissions	60,874	93,000
Proceeds from issuance of redeemable convertible preferred stock		52,300
Payments of public offering costs	(598)	(2,618)
Payments of redeemable convertible preferred stock issuance costs		(88)
Proceeds from exercise of common stock options	1,107	29
Net cash provided by financing activities	61,383	142,623
Net increase in cash, cash equivalents and restricted cash	27,162	110,052
Cash, cash equivalents and restricted cash at beginning of period	60,125	51,567
Cash, cash equivalents and restricted cash at end of period	$ 87,287	161,619
Supplemental disclosure of non-cash investing and financing activities:
Purchase of property and equipment included in accounts payable and accrued expenses		3,467
Offering costs included in accounts payable and accrued expenses		476
Conversion of redeemable convertible preferred stock to common stock		144,739
Accretion of redeemable convertible preferred stock to redemption value		$ 88

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	9 Months Ended
	Sep. 30, 2019
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 biopharmaceutical company developing novel medicines to bring the curative power of stem cell transplant , gene therapy, genome editing and cell therapy to more patients. The Company was incorporated under the laws of the State of Delaware 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 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 drug development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. In June 2018, the Company completed its initial public offering (“IPO”) pursuant to which it issued and sold 6,666,667 shares of common stock at a public offering price of $15.00 per share, resulting in net proceeds of $89.9 million, after deducting underwriting discounts and commissions and other offering expenses. Upon the closing of the IPO, the Company’s outstanding redeemable convertible preferred stock automatically converted into shares of common stock. In May 2019, the Company issued and sold 4,887,500 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 On August 8, 2019, the Company filed a shelf registration statement on Form S-3 350.0 100.0 “at-the-market” The Company has incurred recurring losses since inception, including net losses of $53.5 million and $57.5 million for the nine months ended September 30, 2019 and the year ended December 31, 2018, respectively. As of September 30, 2019, the Company had an accumulated deficit of $156.3 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 Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2019
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, 2018 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, 2019 and for the three and nine months ended September 30, 2019 and 2018 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, 2018 included in the Company’s Annual Report on Form 10-K, 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 more-likely-than-not 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, 2019, the Company’s only element of other comprehensive loss was unrealized gains (losses) on marketable securities. For the three and nine months ended September 30, 2018, there was no difference between net loss and comprehensive loss. Net Loss per Share Prior to the closing of its IPO, the Company followed the two-class two-class two-class In June 2018, upon the closing of the IPO, all outstanding shares of the Company’s redeemable convertible preferred stock automatically converted into 23,375,405 shares of the Company’s common stock. Subsequent to the IPO, basic net income (loss) per common 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 common 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 attributable to common stockholders for the three and nine months ended September 30, 2019 and 2018. Classification and Accretion of Redeemable Convertible Preferred Stock Prior to the conversion of the redeemable convertible preferred stock to common stock in connection with the IPO, the Company had classified redeemable convertible preferred stock outside of stockholders’ equity (deficit) because the shares contained certain redemption features that were not solely within the control of the Company. The Company immediately accreted the carrying value of its redeemable convertible preferred stock to redemption value at the end of each reporting period as if the end of the reporting period were the redemption date. Recently Adopted Accounting Pronouncements In June 2018, the Financing Accounting Standards Board (the “FASB”) issued Accounting Standards Update (“ASU”) No. 2018-07, Compensation – Stock Compensation (Topic 718), Improvements to Nonemployee Share-Based Payment Accounting 2018-07”). 2018-07 non-public 2018-07 No. 2014-09, Revenue from Contracts with Customers (Topic 606) 2018-07 non-employees 2018-07 non-employee non-employee Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases 2016-02”). 2016-02 right-of-use non-public 2016-02 2018-11, Leases (Topic 842) Targeted Improvements 2016-02 right-of-use

Fair Value of Financial Assets

Fair Value of Financial Assets	9 Months Ended
	Sep. 30, 2019
Fair Value of Financial Assets	3. Fair Value of Financial Assets As of September 30, 2019, marketable securities by security type consisted of (in thousands): Gross Gross Estimated Amortized Unrealized Unrealized Fair Cost Gains Losses Value United States Treasury Notes (due within one year) $ 65,871 $ 75 $ — $ 65,946 Agency Bonds (due within one year) 9,109 8 — 9,117 Total $ 74,980 $ 83 $ — $ 75,063 As of December 31, 2018, marketable securities by security type consisted of (in thousands): Gross Gross Estimated Amortized Unrealized Unrealized Fair Cost Gains Losses Value United States Treasury Notes (due within one year) $ 71,944 $ 5 $ (9 ) $ 71,940 Agency Bonds (due within one year) 12,289 1 (5 ) 12,285 Total $ 84,233 $ 6 $ (14 ) $ 84,225 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, 2019 Using: Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 86,180 $ — $ — $ 86,180 Marketable securities: United States Treasury Notes — 65,946 — 65,946 Agency Bonds — 9,117 — 9,117 Total $ 86,180 $ 75,063 $ — $ 161,243 Fair Value Measurements at December 31, 2018 Using: Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 45,725 $ — $ — $ 45,725 United States Treasury Notes — 12,488 — 12,488 Marketable securities: United States Treasury Notes — 71,940 — 71,940 Agency Bonds — 12,285 — 12,285 Total $ 45,725 $ 96,713 $ — $ 142,438 During the three and nine months ended September 30, 2019 and 2018, there were no transfers between Level 1, Level 2 and Level 3.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	9 Months Ended
	Sep. 30, 2019
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, 2019 December 31, 2018 Accrued external research and development expenses $ 3,894 $ 2,619 Accrued payroll and related expenses 2,428 2,526 Deferred rent, current portion 601 71 Accrued professional fees 446 787 Accrued property and equipment — 713 Accrued other 222 211 $ 7,591 $ 6,927

Stock-Based Awards

Stock-Based Awards	9 Months Ended
	Sep. 30, 2019
Stock-Based Awards	5. Stock-Based Awards 2018 Stock Option and Incentive Plan The Company grants stock-based awards under its 2018 Stock Option and Incentive Plan (the “2018 Plan”). The Company also has outstanding stock options under its 2016 Stock Option and Grant Plan (the “2016 Plan”) but is no longer granting awards under this plan. As of September 30, 2019, 2,477,390 shares of common stock were available for issuance under the 2018 Plan. 2019 Employee Stock Purchase Plan The Magenta Therapeutics, Inc. 2019 Employee Stock Purchase Plan (the “ESPP”) was adopted by the Company on April 18, 2019, subject to the approval of stockholders, and became effective following stockholder approval on June 7, 2019. An aggregate of 166,525 shares were reserved for issuance under the ESPP. In addition, on January 1, 2020 and each January 1 thereafter through January 1, 2029, the number of shares available for issuance shall be cumulatively increased by the least of (i) one percent (1​​​​​​​%) of the number of shares issued and outstanding on the immediately preceding December 31, (ii) 1,000,000 shares and (iii) such number of shares as determined by the Compensation Committee. The offering periods begin in December and June of each year, with the initial offering period commencing on December 1, 2019. As of September 30, 2019, no shares have been issued under the ESPP and 166,525 shares remained available for issuance. Grant of Stock Options During the nine months ended September 30, 2019, the Company granted options to certain employees and directors with service-based vesting for the purchase of 1,650,066 shares of common stock with a weighted average grant date fair value of $5.98 per share. 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, 2019 2018 2019 2018 Research and development expenses $ 1,293 $ 2,022 $ 3,817 $ 4,612 General and administrative expenses 1,469 1,654 4,122 3,540 $ 2,762 $ 3,676 $ 7,939 $ 8,152 As of September 30, 2019, total unrecognized compensation cost related to unvested share-based awards was $18.0 million, which is expected to be recognized over a weighted average period of 2.3 years. Prior to the adoption of ASU 2018-07 non-employees non-employee 2018-07, non-employees

Net Loss per Share

Net Loss per Share	9 Months Ended
	Sep. 30, 2019
Net Loss per Share	6. Net Loss per Share Basic and diluted net loss per share attributable to common stockholders was calculated as follows (in thousands, except share and per share amounts): Three Months Ended September 30, Nine Months Ended September 30, 2019 2018 2019 2018 Numerator: Net loss $ (20,990 ) $ (16,015 ) $ (53,532 ) $ (40,836 ) Accretion of redeemable convertible preferred stock to redemption value — — — (88 ) Net loss attributable to common stockholders $ (20,990 ) $ (16,015 ) $ (53,532 ) $ (40,924 ) Denominator: Weighted average common shares outstanding, basic and diluted 38,824,209 32,997,346 36,322,804 13,396,856 Net loss per share attributable to common stockholders, basic and diluted $ (0.54 ) $ (0.49 ) $ (1.47 ) $ (3.05 ) 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, 2019 2018 Stock options to purchase common stock 4,903,047 3,558,282 Unvested restricted common stock 427,857 1,225,387 5,330,904 4,783,669

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2019
Commitments and Contingencies	7. Commitments and Contingencies Leases In May 2018, the Company entered into a sublease for up to approximately 69,000 June 2018 February 2028 5.2 s The tenant improvement allowance and payment escalations specified in the sublease agreement are accrued or deferred as appropriate such that rent expense per square foot is recognized on a straight-line basis over the terms of occupancy. As of September 30, 2019 and December 31, 2018, the Company had long-term deferred rent of $6.1 million and $1.2 million, respectively, related to lease incentives and payment escalations. As of September 30, 2019 and December 31, 2018, the short-term portion of deferred rent of $0.6 million and $0.1 million, respectively, was included in accrued expenses and other current liabilities. The Company recorded rent expense of $1.5 million and $1.8 million during the three months ended September 30, 2019 and 2018, respectively, and $4.6 million and $2.9 million during the nine months ended September 30, 2019 and 2018, respectively. In the fourth quarter of 2018, with the landlord’s consent, the Company entered into two two-year sub-subleases sub-sublease As of September 30, 2019, the future minimum lease payments due under the noncancelable operating lease is as follows (in thousands): Remainder of 2019 (three months) $ 1,405 2020 5,782 2021 6,072 2022 6,375 2023 6,734 Thereafter 32,000 $ 58,368 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, 2019 and 2018, the Company recorded $0.5 million and less than $0.1 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, 2019 and 2018, the Company recorded $0.9 million and $0.8 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 In November 2016, the Company entered into a license agreement with the President and Fellows of Harvard College (“Harvard”) for an exclusive, worldwide, royalty-bearing license for certain technologies related to conditioning and mobilization. In consideration for these rights the Company paid Harvard an upfront fee of $0.1 million and reimbursed Harvard $0.3 million for costs incurred by Harvard related to the patented technology, which amounts were recorded as research and development expense during the year ended December 31, 2016. The Company also issued 385,063 shares of common stock in connection with the license agreement. The fair value of the shares of $0.3 million as of the issuance date was recorded as research and development expense during the year ended December 31, 2016. The Company is obligated to pay Harvard maintenance fees of less than $0.1 million annually through 2019 and $0.1 million annually thereafter 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 country-by-country In April 2017, the Company entered into a license agreement with Novartis International Pharmaceutical Ltd. (“Novartis”) 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 product-by-product country-by-country 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 months and nine months ended September 30, 2019 and 2018, the Company did not exercise its rights to the license of certain developed technologies under these agreements. 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, 2019 and December 31, 2018. 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

401(k) Savings Plan	9 Months Ended
	Sep. 30, 2019
401(k) Savings Plan	8. 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. To date, the Company has not made any contributions to the plan.

Related Parties

Related Parties	9 Months Ended
	Sep. 30, 2019
Related Parties	9. Related Parties National Marrow Donor Program (as successor in interest to Be The Match BioTherapies, LLC) Effective March 2018, the President of Be The Match BioTherapies, LLC became a member of the Company’s board of directors. The Company has a collaboration agreement with the National Marrow Donor Program (as successor in interest to Be The Match BioTherapies, LLC) and a research agreement with an affiliated organization, Center for International Blood and Marrow Transplant Research. During the three months ended September 30, 2019 and 2018, the Company recorded less than $0.1 million and $0.1 million, respectively, to research and development expenses and during each of the three months ended September 30, 2019 and 2018, recorded less than $0.1 million to general and administrative expenses. During each of the three months ended September 30, 2019 and 2018 , , respectively,

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2019
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of 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	Unaudited Interim Financial Information The consolidated balance sheet at December 31, 2018 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, 2019 and for the three and nine months ended September 30, 2019 and 2018 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, 2018 included in the Company’s Annual Report on Form 10-K,
Fair Value Measurements	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	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 more-likely-than-not
Comprehensive Loss	Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity that result from transactions and economic events other than those with stockholders. For the three and nine months ended September 30, 2019, the Company’s only element of other comprehensive loss was unrealized gains (losses) on marketable securities. For the three and nine months ended September 30, 2018, there was no difference between net loss and comprehensive loss.
Net Loss per Share	Net Loss per Share Prior to the closing of its IPO, the Company followed the two-class two-class two-class In June 2018, upon the closing of the IPO, all outstanding shares of the Company’s redeemable convertible preferred stock automatically converted into 23,375,405 shares of the Company’s common stock. Subsequent to the IPO, basic net income (loss) per common 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 common 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 attributable to common stockholders for the three and nine months ended September 30, 2019 and 2018.
Classification and Accretion of Redeemable Convertible Preferred Stock	Classification and Accretion of Redeemable Convertible Preferred Stock Prior to the conversion of the redeemable convertible preferred stock to common stock in connection with the IPO, the Company had classified redeemable convertible preferred stock outside of stockholders’ equity (deficit) because the shares contained certain redemption features that were not solely within the control of the Company. The Company immediately accreted the carrying value of its redeemable convertible preferred stock to redemption value at the end of each reporting period as if the end of the reporting period were the redemption date.
Recently Adopted Accounting Pronouncements and Recently Issued Accounting Pronouncements	Recently Adopted Accounting Pronouncements In June 2018, the Financing Accounting Standards Board (the “FASB”) issued Accounting Standards Update (“ASU”) No. 2018-07, Compensation – Stock Compensation (Topic 718), Improvements to Nonemployee Share-Based Payment Accounting 2018-07”). 2018-07 non-public 2018-07 No. 2014-09, Revenue from Contracts with Customers (Topic 606) 2018-07 non-employees 2018-07 non-employee non-employee Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases 2016-02”). 2016-02 right-of-use non-public 2016-02 2018-11, Leases (Topic 842) Targeted Improvements 2016-02 right-of-use

Fair Value of Financial Assets

Fair Value of Financial Assets (Tables)	9 Months Ended
	Sep. 30, 2019
Schedule of Marketable Securities by Security Type	As of September 30, 2019, marketable securities by security type consisted of (in thousands): Gross Gross Estimated Amortized Unrealized Unrealized Fair Cost Gains Losses Value United States Treasury Notes (due within one year) $ 65,871 $ 75 $ — $ 65,946 Agency Bonds (due within one year) 9,109 8 — 9,117 Total $ 74,980 $ 83 $ — $ 75,063 As of December 31, 2018, marketable securities by security type consisted of (in thousands): Gross Gross Estimated Amortized Unrealized Unrealized Fair Cost Gains Losses Value United States Treasury Notes (due within one year) $ 71,944 $ 5 $ (9 ) $ 71,940 Agency Bonds (due within one year) 12,289 1 (5 ) 12,285 Total $ 84,233 $ 6 $ (14 ) $ 84,225
Schedule of Financial Assets Measured at Fair Value on Recurring Basis	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, 2019 Using: Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 86,180 $ — $ — $ 86,180 Marketable securities: United States Treasury Notes — 65,946 — 65,946 Agency Bonds — 9,117 — 9,117 Total $ 86,180 $ 75,063 $ — $ 161,243 Fair Value Measurements at December 31, 2018 Using: Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 45,725 $ — $ — $ 45,725 United States Treasury Notes — 12,488 — 12,488 Marketable securities: United States Treasury Notes — 71,940 — 71,940 Agency Bonds — 12,285 — 12,285 Total $ 45,725 $ 96,713 $ — $ 142,438

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	9 Months Ended
	Sep. 30, 2019
Schedule of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of the following (in thousands): September 30, 2019 December 31, 2018 Accrued external research and development expenses $ 3,894 $ 2,619 Accrued payroll and related expenses 2,428 2,526 Deferred rent, current portion 601 71 Accrued professional fees 446 787 Accrued property and equipment — 713 Accrued other 222 211 $ 7,591 $ 6,927

Stock-Based Awards (Tables)

Stock-Based Awards (Tables)	9 Months Ended
	Sep. 30, 2019
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, 2019 2018 2019 2018 Research and development expenses $ 1,293 $ 2,022 $ 3,817 $ 4,612 General and administrative expenses 1,469 1,654 4,122 3,540 $ 2,762 $ 3,676 $ 7,939 $ 8,152

Net Loss per Share (Tables)

Net Loss per Share (Tables)	9 Months Ended
	Sep. 30, 2019
Schedule of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders	Basic and diluted net loss per share attributable to common stockholders was calculated as follows (in thousands, except share and per share amounts): Three Months Ended September 30, Nine Months Ended September 30, 2019 2018 2019 2018 Numerator: Net loss $ (20,990 ) $ (16,015 ) $ (53,532 ) $ (40,836 ) Accretion of redeemable convertible preferred stock to redemption value — — — (88 ) Net loss attributable to common stockholders $ (20,990 ) $ (16,015 ) $ (53,532 ) $ (40,924 ) Denominator: Weighted average common shares outstanding, basic and diluted 38,824,209 32,997,346 36,322,804 13,396,856 Net loss per share attributable to common stockholders, basic and diluted $ (0.54 ) $ (0.49 ) $ (1.47 ) $ (3.05 )
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, 2019 2018 Stock options to purchase common stock 4,903,047 3,558,282 Unvested restricted common stock 427,857 1,225,387 5,330,904 4,783,669

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2019
Schedule of Future Minimum Lease Payments Due Under Noncancelable Operating Lease	As of September 30, 2019, the future minimum lease payments due under the noncancelable operating lease is as follows (in thousands): Remainder of 2019 (three months) $ 1,405 2020 5,782 2021 6,072 2022 6,375 2023 6,734 Thereafter 32,000 $ 58,368

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Aug. 08, 2019	May 31, 2019	Jun. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Entity incorporation date						Jun. 30, 2015
Entity incorporation state code						DE
Net loss				$ (20,990)	$ (16,015)	$ (53,532)	$ (40,836)	$ (57,500)
Accumulated deficit				$ (156,258)		(156,258)		$ (102,726)
Net proceeds from common stock		$ 60,300				$ 60,874	$ 93,000
Maximum [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Shelf offering value	$ 350,000
Common Stock [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Stock issued, shares		4,887,500				4,887,500	6,666,667
Stock issued, price per shares		$ 13.25
Common Stock [Member] | IPO [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Stock issued, shares			6,666,667
Stock issued, price per shares			$ 15
Net proceeds from initial public offering			$ 89,900
Cowen and Company LLC [Member] | Maximum [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Common stock reserved for future issuance	$ 100,000

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Additional Information (Detail)	Jun. 25, 2018shares
Summary Of Significant Accounting Policies [Line Items]
Redeemable convertible preferred stock converted into common stock shares	23,375,405

Fair Value of Financial Asset_2

Fair Value of Financial Assets - Schedule of Marketable Securities by Security Type (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Marketable Securities [Line Items]
Amortized Cost	$ 74,980	$ 84,233
Gross Unrealized Gains	83	6
Gross Unrealized Losses		(14)
Estimated Fair Value	75,063	84,225
United States Treasury Notes [Member]
Marketable Securities [Line Items]
Amortized Cost	65,871	71,944
Gross Unrealized Gains	75	5
Gross Unrealized Losses		(9)
Estimated Fair Value	65,946	71,940
Agency Bonds [Member]
Marketable Securities [Line Items]
Amortized Cost	9,109	12,289
Gross Unrealized Gains	8	1
Gross Unrealized Losses		(5)
Estimated Fair Value	$ 9,117	$ 12,285

Fair Value of Financial Asset_3

Fair Value of Financial Assets - Schedule of Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	$ 75,063	$ 84,225
Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	161,243	142,438
Level 1 [Member] | Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	86,180	45,725
Level 2 [Member] | Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	75,063	96,713
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	86,180	45,725
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	86,180	45,725
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		12,488
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		12,488
United States Treasury Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	65,946	71,940
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	65,946	71,940
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	65,946	71,940
Agency Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	9,117	12,285
Agency Bonds [Member] | Fair Value on Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities	9,117	12,285
Agency Bonds [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	$ 9,117	$ 12,285

Fair Value of Financial Asset_4

Fair Value of Financial Assets - Additional Information (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, assets, level 1 to level 2 transfers, amount	$ 0	$ 0
Fair value, assets, level 2 to level 1 transfers, amount	0	0
Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset transfers into level 3	0	0
Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset, transfers out of level 3	$ 0	$ 0

Accrued Expenses and Other Cu_3

Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Accrued external research and development expenses	$ 3,894	$ 2,619
Accrued payroll and related expenses	2,428	2,526
Deferred rent, current portion	601	71
Accrued professional fees	446	787
Accrued property and equipment		713
Accrued other	222	211
Total	$ 7,591	$ 6,927

Stock-Based Awards - Additional

Stock-Based Awards - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	Apr. 18, 2019	Sep. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized stock-based compensation expense		$ 18
Period for recognition of unrecognized expense		2 years 3 months 18 days
Employees [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		1,650,066
Weighted average fair value of stock options granted		$ 5.98
2018 Stock Option and Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock available for issuance		2,477,390
2019 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock available for issuance	166,525	166,525
Annual Increase In Aggregate Number Of Shares Reserved For Future Issuance	1,000,000
Minimum percentage of shares issued and outstanding	1.00%

Stock-Based Awards - Schedule o

Stock-Based Awards - Schedule of Stock Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock based compensation expense	$ 2,762	$ 3,676	$ 7,939	$ 8,152
Research and Development Expenses [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock based compensation expense	1,293	2,022	3,817	4,612
General and Administrative Expenses [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock based compensation expense	$ 1,469	$ 1,654	$ 4,122	$ 3,540

Net Loss per Share - Schedule o

Net Loss per Share - Schedule of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Numerator:
Net loss	$ (20,990)	$ (16,015)	$ (53,532)	$ (40,836)	$ (57,500)
Accretion of redeemable convertible preferred stock to redemption value				(88)
Net loss attributable to common stockholders	$ (20,990)	$ (16,015)	$ (53,532)	$ (40,924)
Denominator:
Weighted average common shares outstanding, basic and diluted	38,824,209	32,997,346	36,322,804	13,396,856
Net loss per share attributable to common stockholders, basic and diluted	$ (0.54)	$ (0.49)	$ (1.47)	$ (3.05)

Net Loss Per Share - Schedule_2

Net Loss Per Share - Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share (Detail) - shares	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share, amount	5,330,904	4,783,669
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	4,903,047	3,558,282
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	427,857	1,225,387

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended			3 Months Ended			9 Months Ended		12 Months Ended
	May 31, 2018USD ($)ft²	Apr. 30, 2017USD ($)shares	Nov. 30, 2016USD ($)shares	Sep. 30, 2019USD ($)shares	Dec. 31, 2018USD ($)ft²Subleaseshares	Sep. 30, 2018USD ($)	Sep. 30, 2019USD ($)Milestoneshares	Sep. 30, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
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 for up to approximately 69,000 square feet of office and laboratory space in Cambridge, Massachusetts. The sublease was amended effective March 2019 to increase the lease payments over the lease term and to increase the sublandlord-funded tenant improvements. 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				$ 6,075,000	$ 1,246,000		$ 6,075,000
Deferred rent, current portion				601,000	$ 71,000		601,000
Rent expense				1,500,000		$ 1,800,000	4,600,000	$ 2,900,000
Research and development expense				$ 16,524,000		11,418,000	$ 40,494,000	28,950,000
Common stock issued | shares				39,370,576	34,327,554		39,370,576
Redeemable convertible preferred stock issued | shares				0	0		0
Office and Lab Sublease [Member]
Other Commitments [Line Items]
Proceeds from sublease facility							$ 5,200,000
Maximum [Member] | Office and Lab Sublease [Member]
Other Commitments [Line Items]
Square feet of property subject to sublease | ft²	69,000
Tenant improvement allowance	$ 5,500,000
Sub-Sublease [Member]
Other Commitments [Line Items]
Square feet of property subject to sublease | ft²					27,000
Rental payments receivable					$ 4,500,000
Lease agreement maturity period					2 years
Number of Sub-sublease | Sublease					2
License Agreement with Novartis International Pharmaceutical Ltd. [Member]
Other Commitments [Line Items]
Fair value of shares as of issuance date									$ 9,300,000
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		$ 177,000,000
License Agreement with Novartis International Pharmaceutical Ltd. [Member] | Achievement of Commercial Milestones [Member] | Maximum [Member]
Other Commitments [Line Items]
Payment obligation		$ 125,000,000
License Agreement with Novartis International Pharmaceutical Ltd. [Member] | Series A- Redeemable Convertible Preferred Stock [Member]
Other Commitments [Line Items]
Redeemable convertible preferred stock issued | shares		2,500,000
License Agreement with Novartis International Pharmaceutical Ltd. [Member] | Series B Redeemable Convertible Preferred Stock [Member]
Other Commitments [Line Items]
Redeemable convertible preferred stock issued | shares		643,550
Collaboration Agreement [Member]
Other Commitments [Line Items]
Research and development expense				$ 500,000			$ 900,000	$ 800,000
Collaboration Agreement [Member] | Maximum [Member]
Other Commitments [Line Items]
Research and development expense						$ 100,000
Collaboration Agreement [Member] | Development, Regulatory and Commercial Milestone [Member] | Per Target [Member] | Maximum [Member]
Other Commitments [Line Items]
Payment obligation				$ 83,500,000			83,500,000
License Agreement With Harvard [Member]
Other Commitments [Line Items]
Payment of upfront fee			$ 100,000
Common stock issued | shares			385,063
Fair value of shares as of issuance date										$ 300,000
License Agreement With Harvard [Member] | Maintenance [Member] | Annually Thereafter [Member]
Other Commitments [Line Items]
Payment obligation			$ 100,000
License Agreement With Harvard [Member] | Maintenance [Member] | Annually Through 2019 [Member] | Maximum [Member]
Other Commitments [Line Items]
Payment obligation			100,000
License Agreement With Harvard [Member] | Development and Regulatory Milestones [Member]
Other Commitments [Line Items]
Research and development expense							$ 100,000
License Agreement With Harvard [Member] | Achievement of Development and Regulatory Milestones [Member] | Maximum [Member]
Other Commitments [Line Items]
Payment obligation			7,400,000
License Agreement With Harvard [Member] | Patents [Member]
Other Commitments [Line Items]
Reimbursed costs			$ 300,000
Letter of Credit [Member] | Office and Lab Sublease [Member]
Other Commitments [Line Items]
Cash balance to secure letter of credit associated with sublease	$ 1,800,000

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Future Minimum Lease Payments Due Under Noncancelable Operating Lease (Detail) $ in Thousands	Sep. 30, 2019USD ($)
Operating Leased Assets [Line Items]
Remainder of 2019 (three months)	$ 1,405
2020	5,782
2021	6,072
2022	6,375
2023	6,734
Thereafter	32,000
Operating leases future minimum payments due	$ 58,368

Related Parties - Additional In

Related Parties - Additional Information (Detail) - Match Bio Therapies [Member] - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Related Party Transaction [Line Items]
Administrative expenses related party			$ 100,000	$ 100,000
Expenses paid to related party	$ 100,000	$ 100,000	500,000	600,000
Accounts Payable, Accrued Expenses and Other Current Liabilities [Member]
Related Party Transaction [Line Items]
Amounts owed to related party	400,000		400,000		$ 400,000
Research and Development Expenses [Member]
Related Party Transaction [Line Items]
Research and development expense		100,000		$ 300,000
Maximum [Member]
Related Party Transaction [Line Items]
Administrative expenses related party	100,000	$ 100,000
Maximum [Member] | Prepaid Expenses and Other Current Assets [Member]
Related Party Transaction [Line Items]
Amounts owed from related party	100,000		100,000		$ 100,000
Maximum [Member] | Research and Development Expenses [Member]
Related Party Transaction [Line Items]
Research and development expense	$ 100,000		$ 100,000