Forty Seven (FTSV) 10-Q 13 Aug 19 2019 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2019	Aug. 06, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q2
Trading Symbol	FTSV
Entity Registrant Name	Forty Seven, Inc.
Entity Central Index Key	0001667633
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Common Stock, Shares Outstanding		42,215,143
Entity Shell Company	false
Entity Current Reporting Status	Yes
Entity File Number	001-38554
Entity Tax Identification Number	474065674
Entity Address, Address Line One	1490 O’Brien Drive
Entity Address, Address Line Two	Suite A
Entity Address, City or Town	Menlo Park
Entity Address, State or Province	California
Entity Address, Postal Zip Code	94025
City Area Code	650
Local Phone Number	352-4150

Condensed Balance Sheets

Condensed Balance Sheets - USD ($) $ in Thousands		Jun. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents		$ 17,849	$ 10,837	[1]
Short-term investments		81,158	128,186	[1]
Prepaid expenses and other current assets		11,861	6,835	[1]
Total current assets		110,868	145,858	[1]
Property and equipment, net		1,288	1,360	[1]
Operating lease right-of-use assets		2,579
Other assets		1,067	2,219	[1]
Total assets		115,802	149,437	[1]
Current liabilities:
Accounts payable		8,499	4,621	[1]
Accrued liabilities		9,719	9,044	[1]
Lease liabilities, current		1,397
Deferred grant funding, current		2,742	1,744	[1]
Total current liabilities		22,357	15,409	[1]
Lease liabilities, noncurrent		1,600
Deferred rent, noncurrent	[1]		331
Other long-term liabilities		361	476	[1]
Total liabilities		24,318	16,216	[1]
Commitments and Contingencies				[1]
Stockholders’ equity:
Common stock		3	3	[1]
Additional paid-in capital		277,413	273,069	[1]
Accumulated other comprehensive income (loss)		59	(82)	[1]
Accumulated deficit		(185,991)	(139,769)	[1]
Total stockholders’ equity		91,484	133,221	[1]
Total liabilities and stockholders’ equity		$ 115,802	$ 149,437	[1]
[1]	The balance sheet as of December 31, 2018 is derived from the audited financial statements as of that date.

Condensed Statements of Operati

Condensed Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Operating expenses:
Research and development	$ 18,829	$ 13,596	$ 37,955	$ 24,749
General and administrative	5,057	3,362	9,641	7,205
Total operating expenses	23,886	16,958	47,596	31,954
Loss from operations	(23,886)	(16,958)	(47,596)	(31,954)
Interest and other income, net	680	236	1,374	457
Net loss	(23,206)	(16,722)	(46,222)	(31,497)
Unrealized gains on available-for-sale securities	41	43	141	16
Comprehensive loss	$ (23,165)	$ (16,679)	$ (46,081)	$ (31,481)
Net loss per share, basic and diluted	$ (0.74)	$ (2.52)	$ (1.48)	$ (4.76)
Shares used in computing net loss per share, basic and diluted	31,355,135	6,636,862	31,261,182	6,618,736

Condensed Statements of Stockho

Condensed Statements of Stockholders' Equity - USD ($) $ in Thousands	Total		Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance at Dec. 31, 2017	$ 83,462		$ 149,397	$ 1	$ 3,507	$ (44)	$ (69,399)
Beginning balance, shares at Dec. 31, 2017			16,215,896	6,751,157
Settlement of fractional shares from reverse stock split, shares				(15)
Vesting of early exercised stock options	13				13
Stock-based compensation	431				431
Repurchase of common stock, shares				(41,935)
Net loss	(14,775)						(14,775)
Other comprehensive income (loss)	(27)					(27)
Ending balance at Mar. 31, 2018	69,104		$ 149,397	$ 1	3,951	(71)	(84,174)
Ending balance, shares at Mar. 31, 2018			16,215,896	6,709,207
Beginning balance at Dec. 31, 2017	83,462		$ 149,397	$ 1	3,507	(44)	(69,399)
Beginning balance, shares at Dec. 31, 2017			16,215,896	6,751,157
Net loss	(31,497)
Other comprehensive income (loss)	16
Ending balance at Jun. 30, 2018	53,267		$ 149,397	$ 1	4,793	(28)	(100,896)
Ending balance, shares at Jun. 30, 2018			16,215,896	6,737,837
Beginning balance at Mar. 31, 2018	69,104		$ 149,397	$ 1	3,951	(71)	(84,174)
Beginning balance, shares at Mar. 31, 2018			16,215,896	6,709,207
Issuance of common stock for exercise of stock options	55				55
Issuance of common stock for exercise of stock options, shares				28,630
Vesting of early exercised stock options	171				171
Stock-based compensation	616				616
Net loss	(16,722)						(16,722)
Other comprehensive income (loss)	43					43
Ending balance at Jun. 30, 2018	53,267		$ 149,397	$ 1	4,793	(28)	(100,896)
Ending balance, shares at Jun. 30, 2018			16,215,896	6,737,837
Beginning balance at Dec. 31, 2018	133,221	[1]		$ 3	273,069	(82)	(139,769)
Beginning balance, shares at Dec. 31, 2018				31,079,150
Issuance of common stock for exercise of stock options	789				789
Issuance of common stock for exercise of stock options, shares				174,793
Issuance of common stock pursuant to the ESPP	588				588
Issuance of common stock pursuant to the ESPP, shares				44,656
Vesting of early exercised stock options	113				113
Stock-based compensation	1,104				1,104
Net loss	(23,016)						(23,016)
Other comprehensive income (loss)	100					100
Ending balance at Mar. 31, 2019	112,899			$ 3	275,663	18	(162,785)
Ending balance, shares at Mar. 31, 2019				31,298,599
Beginning balance at Dec. 31, 2018	133,221	[1]		$ 3	273,069	(82)	(139,769)
Beginning balance, shares at Dec. 31, 2018				31,079,150
Net loss	(46,222)
Other comprehensive income (loss)	141
Ending balance at Jun. 30, 2019	91,484			$ 3	277,413	59	(185,991)
Ending balance, shares at Jun. 30, 2019				31,432,322
Beginning balance at Mar. 31, 2019	112,899			$ 3	275,663	18	(162,785)
Beginning balance, shares at Mar. 31, 2019				31,298,599
Issuance of common stock for exercise of stock options	612				612
Issuance of common stock for exercise of stock options, shares				133,723
Vesting of early exercised stock options	13				13
Stock-based compensation	1,125				1,125
Net loss	(23,206)						(23,206)
Other comprehensive income (loss)	41					41
Ending balance at Jun. 30, 2019	$ 91,484			$ 3	$ 277,413	$ 59	$ (185,991)
Ending balance, shares at Jun. 30, 2019				31,432,322
[1]	The balance sheet as of December 31, 2018 is derived from the audited financial statements as of that date.

Condensed Statements of Cash Fl

Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flows from operating activities:
Net loss	$ (46,222)	$ (31,497)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	2,229	1,047
Depreciation and amortization	233	193
Amortization of right-of-use assets	461	0
Accretion of discounts on marketable securities	(730)	(180)
Realized gain on sale of available-for-sale securities	(4)	0
Change in fair value of embedded derivative	12	0
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(5,026)	853
Other assets	1,620	(1,412)
Accounts payable	3,720	(603)
Accrued liabilities	571	762
Deferred grant funding	997	2,806
Lease related liabilities	(530)	(56)
Net cash used in operating activities	(42,669)	(28,087)
Cash flows from investing activities:
Purchases of property and equipment	(70)	0
Purchases of available-for-sale securities	(68,819)	(16,368)
Proceeds from sales of available-for-sale securities	3,996	0
Proceeds from maturities of available-for-sale securities	112,727	35,365
Net cash provided by investing activities	47,834	18,997
Cash flows from financing activities:
Payments of deferred offering costs	(143)	(2,368)
Proceeds from issuance of common stock upon ESPP purchase	588	0
Proceeds from issuance of common stock upon exercise of stock options	1,402	155
Net cash provided by (used in) financing activities	1,847	(2,213)
Net increase (decrease) in cash and cash equivalents	7,012	(11,303)
Cash and cash equivalents — beginning of period	10,837	24,417
Cash and cash equivalents — end of period	17,849	13,114
Noncash investing and financing activities:
Purchase of property and equipment through accounts payable and accruals	91	0
Deferred offering costs included in accounts payable and accrued liabilities	326	1,697
Lease liability obtained in exchange for right-of-use asset	$ 712	$ 0

Description of Business

Description of Business	6 Months Ended
	Jun. 30, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Basis of Presentation	1. Description of Business The Company is a clinical-stage immuno-oncology company focused on developing novel checkpoint therapies to activate macrophages in the fight against cancer. Forty Seven was founded based on the insight that blocking CD47, a key signaling molecule that is over-expressed on cancer cells, renders tumors susceptible to macrophages and the innate immune system. By harnessing macrophages, the Company believes that its lead product candidate, 5F9, dosed as a monotherapy and in combination with marketed cancer therapies, can transform the treatment of cancer. Liquidity In the course of its development activities, the Company has sustained operating losses and expects to continue to generate operating losses for the foreseeable future. The Company’s ultimate success depends on the outcome of its research and development activities. The Company had cash, cash equivalents and short-term investments of $99.0 million as of June 30, 2019. Since inception through June 30, 2019, the Company has incurred cumulative net losses of $186.0 million. Management expects to incur additional losses in the future to conduct product research and development and recognizes the need to raise additional capital to fully implement its business plan. The Company intends to raise such capital through the issuance of additional equity financing and/or third-party collaboration funding. However, if such financing is not available at adequate levels, the Company will need to reevaluate its operating plan and may be required to delay the development of its products. The Company expects that its cash, cash equivalents and short-term investments will be sufficient to fund operating expenses and capital expenditure requirements for a period of at least one year from the date these interim condensed financial statements are filed with the Securities and Exchange Commission (“SEC”).

Summary of Significant Accounti

Summary of Significant Accounting Policies	6 Months Ended
	Jun. 30, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The interim condensed financial statements are unaudited. The unaudited interim condensed financial statements have been prepared on the same basis as the annual financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary to present fairly the Company’s financial position, results of operations and cash flows for the periods presented. The results of operations for the three and six months ended June 30, 2019 are not necessarily indicative of the results to be expected for the year ending December 31, 2019 or for any other future annual or interim period. The condensed balance sheet as of December 31, 2018 included herein was derived from the audited financial statements as of that date. These condensed financial statements should be read in conjunction with the Company's audited financial statements and related notes as set forth in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2018, as filed with the SEC on March 28, 2019. Use of Estimates The preparation of financial statements in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”) requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of expenses during the reporting period. Significant estimates and assumptions made in the accompanying financial statements include but are not limited to the fair value of common stock, the fair value of stock options, the fair value of investments, income tax uncertainties, lease liability, and certain accruals. The Company evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors and adjusts those estimates and assumptions when facts and circumstances dictate. Actual results could differ from those estimates. Fair Value Measurement Assets and liabilities recorded at fair value on a recurring basis in the balance sheets are categorized based upon the level of judgment associated with the inputs used to measure their fair values. Fair value is defined as the exchange price that would be received for an asset or an exit price that would be paid to transfer a liability 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. The authoritative guidance on fair value measurements establishes a three-tier fair value hierarchy for disclosure of fair value measurements as follows: Level 1—Observable inputs such as unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date; Level 2—Inputs (other than quoted prices included in Level 1) are either directly or indirectly observable for the asset or liability. These include quoted prices for similar assets or liabilities in active markets and quoted prices for identical or similar assets or liabilities in markets that are not active; Level 3— Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. Leases The Company adopted Accounting Standards Update (ASU) No. 2016-02, Leases Rent expense for the operating leases is recognized on a straight-line basis over the lease term and is included in operating expenses on the statements of operations and comprehensive loss. Variable lease payments include lease operating expenses. The accompanying condensed financial statements as of and for the three and six months ended June 30, 2019 are presented under Topic 842. The prior periods continue to be reported in accordance with previous lease guidance, ASC Topic 840, Leases The impact of the adoption of Topic 842 on the accompanying condensed balance sheet as of January 1, 2019 was as follows: December 31, 2018 Adjustments due to the adoption of Topic 842 January 1, 2019 (In thousands) Assets Operating lease right-of-use asset $ — $ 2,328 $ 2,328 Liabilities and stockholders' equity Deferred rent classified as accrued liabilities $ 155 $ (155 ) $ — Lease liability, current $ — $ 968 $ 968 Lease liability, noncurrent $ — $ 1,847 $ 1,847 Deferred rent, noncurrent $ 331 $ (331 ) $ — The adjustments due to the adoption of Topic 842 related to the recognition of an operating lease right-of-use asset and lease liability for the Company's existing property operating lease and the derecognition of the deferred rent recognized under Topic 840. There was no impact on the Company’s statement of operations and comprehensive loss from the adoption and no cumulative-effect adjustment to the beginning accumulated deficit.

Fair Value Measurements

Fair Value Measurements	6 Months Ended
	Jun. 30, 2019
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements The Company measures and records its cash equivalents and short-term investments at fair value. Money market funds are measured at fair value on a recurring basis using quoted prices and are classified as a Level 1 input. Short-term investments are measured at fair value based on inputs other than quoted prices that are derived from observable market data and are classified as Level 2 inputs. There were no transfers between Levels 1, 2 or 3 for any of the periods presented. All of the investments held as of June 30, 2019 and December 31, 2018 had maturities of less than one year. There were no significant realized gains or losses on investments for the three and six months ended June 30, 2019 and 2018. The fair value and amortized cost of cash equivalents and available-for-sale securities by major security type as of June 30, 2019 and December 31, 2018 are presented in the following tables: As of June 30, 2019 Fair Value Hierarchy Amortized Cost Unrealized Gains Unrealized Losses Market Value (In thousands) Money market funds Level 1 $ 15,670 $ — $ — $ 15,670 Commercial paper Level 2 24,271 — — 24,271 Corporate debt securities Level 2 20,453 24 — 20,477 Asset-backed securities Level 2 16,470 11 — 16,481 US government debt securities Level 2 19,905 24 — 19,929 Total cash equivalents and available-for-sale securities $ 96,769 $ 59 $ — $ 96,828 As of December 31, 2018 Fair Value Hierarchy Amortized Cost Unrealized Gains Unrealized Losses Market Value (In thousands) Money market funds Level 1 $ 7,959 $ — $ — $ 7,959 Commercial paper Level 2 43,277 — — 43,277 Corporate debt securities Level 2 46,186 — (54 ) 46,132 Asset-backed securities Level 2 22,842 — (27 ) 22,815 US government debt securities Level 2 15,963 — (1 ) 15,962 Total cash equivalents and available-for-sale securities $ 136,227 $ — $ (82 ) $ 136,145 The Company’s contingent milestone payments in its agreement with the Leukemia & Lymphoma Society, Inc. (“LLS”) were concluded to be an embedded derivative. The embedded derivative contains unobservable inputs that are supported by little or no market activity at the measurement date. Accordingly, the Company’s embedded derivative is measured at fair value on a recurring basis using unobservable inputs that are classified as Level 3 inputs. The Company recorded a liability for the derivative of approximately $0.3 million, as part of other long-term liabilities as of June 30, 2019 and December 31, 2018. Refer to Note 5 for the valuation techniques and assumptions used in estimating the fair value of the embedded derivative. The change in fair value of the embedded derivative is presented in the following table: Six Months Ended June 30, 2019 (In thousands) Beginning balance $ 331 Change in fair value of embedded derivative 12 Ending balance $ 343

Balance Sheet Components

Balance Sheet Components	6 Months Ended
	Jun. 30, 2019
Balance Sheet Related Disclosures [Abstract]
Balance Sheet Components	4. Balance Sheet Components Accrued Liabilities Accrued liabilities consist of the following: June 30, 2019 December 31, 2018 (In thousands) Accrued research and development expenses $ 6,338 $ 5,870 Accrued bonuses 1,379 1,602 Deferred rent, current — 155 Other 2,002 1,417 Total accrued liabilities $ 9,719 $ 9,044

Research and License Agreements

Research and License Agreements	6 Months Ended
	Jun. 30, 2019
Research And License Agreements [Abstract]
Research and License Agreements	5. Research and License Agreements California Institute of Regenerative Medicine Grants In J u r 1 t m n a w d a research grant I T R s t u t v m l s e - b s a y m n w t h t a w 0 ov r f y rs As of June 30, 2019 and December 31, 2018, the Company had received $9.2 million and $7.2 million under the award, respectively In November 2017, the Company was awarded a second research grant from CIRM for a separate clinical trial study. The total amount of the research grant awarded was $5.0 million in various milestone-based payments over a period of five years. During 2018, the award was amended to $3.2 million in various-milestone payments over a period of five years, as was provided for under the terms of the original award because the Company opted not to expand the patient population participating in the study. During the three and six months ended June 30, 2019, the Company received $0.7 million related to this grant. As of June 30, 2019 and December 31, 2018, the Company had received $3.1 million and $2.4 million under the award, respectively t C m a n is obligated t a o y a l i n li c e s i f e a s o e s a l RM - f n e product candidates I M - f n e e c n l g y Leukemia & Lymphoma Society Grant In In July 2019, the Company entered into an amendment of its agreement with LLS to further advance the treatment of myelodysplastic syndromes (“MDS”). Under the collaboration, the Company is eligible for up to $3.0 million in additional milestone payments from LLS upon the achievement of certain clinical or regulatory milestones in addition to the $4.2 million award that the Company is eligible for pursuant to the March 2017 agreement. Pursuant to the amendment, potential future payments by the Company to LLS upon reaching certain development and regulatory approval milestones on the additional funding could be up to $6.0 million in total. The Company recognizes research grants as a reduction of research and development expense when the eligible costs are incurred. Under both CIRM and LLS grants, the Company recognized a total of $0.9 million and $1.9 million as a reduction to research and development expenses for research grants for the three months ended June 30, 2019 and 2018, respectively. The Company recognized a total of $2.0 million and $3.4 million as a reduction to research and development expenses for research grants for the six months ended June 30, 2019 and 2018, respectively. Merck Collaboration Agreement In

Leases

Leases	6 Months Ended
	Jun. 30, 2019
Leases [Abstract]
Leases	6. Leases The Company leases office property and laboratory space at its headquarters in Menlo Park (the “Menlo Park Lease”) through August 2021. The lease requires monthly lease payments subject to annual increases throughout the lease term and includes a renewal option at the election of the Company to extend the lease for an additional five years. The landlord provided the Company with a tenant improvement allowance of $646,000. In April 2019, the Company entered into a sublease (the “Sublease”) to obtain 6,230 rentable square feet to expand its current headquarters through February 2021. The lease requires monthly lease payments subject to annual increases throughout the lease term. In April 2019, the Company also entered into an amendment of Menlo Park Lease to add additional space to its headquarter lease upon the expiration of the Sublease in February 2021. Under the terms of the lease amendment, the lease for the additional space will commence on March 1, 2021 and the base rent for the additional space will be approximately $36,000 per month. As of June 30, 2019, the weighted average remaining lease term was 2.1 years and the weighted average incremental borrowing rate used to determine the operating lease liabilities was 7.0%. The undiscounted future non-cancellable lease payments under the Company's operating leases are as follows: June 30, 2019 (In thousands) Years Remaining 2019 $ 777 2020 1,584 2021 864 Thereafter — Total undiscounted lease payments 3,225 Present value adjustment for minimum lease commitments (228 ) Lease liabilities $ 2,997 As of June 30, 2019, the current and noncurrent portion of the lease liabilities was $1.4 million and $1.6 million. Rent expense for the operating leases was $0.3 million for the three months ended June 30, 2019 and 2018, and $0.6 million for the six months ended June 30, 2019 and 2018. Variable lease payments for operating expenses were $0.1 million for the three months ended June 30, 2019 and 2018. Variable lease payments for operating expenses were $0.3 million and $0.2 million for the six months ended June 30, 2019 and 2018, respectively.

Stockholders' Equity

Stockholders' Equity	6 Months Ended
	Jun. 30, 2019
Equity [Abstract]
Stockholders' Equity	7 . Stockholders’ Equity 2015 and 2018 Equity Incentive Plans The following summarizes option activity for the six months ended June 30, 2019: Shares Issuable Under Options Weighted- Average Exercise Price Weighted- Average Remaining Contract Term Aggregate Intrinsic Value (In years) (In thousands) Balance, December 31, 2018 3,404,847 $ 6.55 8.82 $ 31,388 Options granted 186,652 14.57 Options exercised (308,516 ) 4.54 Options forfeited (101,845 ) 7.02 Balance outstanding June 30, 2019 3,181,138 $ 7.20 8.45 $ 12,331 Exercisable, June 30, 2019 2,071,836 $ 6.46 8.32 $ 8,906 Vested and expected to vest, June 30, 2019 3,181,138 $ 7.20 8.45 $ 12,331 Total stock-based compensation was as follows: Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 (In thousands) (In thousands) Research and development $ 431 $ 162 $ 886 $ 297 General and administrative 694 454 1,343 750 Total $ 1,125 $ 616 $ 2,229 $ 1,047 Restricted Stock As of June 30, 2019 and December 31, 2018, there was $18,000 and $144,000, respectively, recorded in other long-term liabilities related to shares held by employees and directors that were subject to repurchase. A summary of restricted stock activity follows: Number of Restricted Shares Outstanding Unvested shares—December 31, 2018 49,052 Restricted shares vested (33,999 ) Unvested shares—June 30, 2019 15,053 Employee Share Purchase Plan (ESPP) In June 2018, the Company adopted the 2018 Employee Stock Purchase Plan (“ESPP”), which became effective upon the execution of the underwriting agreement related to the IPO. The initial offering period began June 27, 2018 and will end on August 15, 2020 with purchase dates of February 15, 2019, August 15, 2019, February 15, 2020, and August 15, 2020. Each subsequent offering will be approximately 24 months long and will consist of four purchase periods with purchase dates occurring on February 15 and August 15 of each year. On each purchase date, ESPP participants will purchase shares of common stock at a price per share equal to 85% of the lesser of (1) the fair market value per share of the common stock on the offering date or (2) the fair market value of the common stock on the purchase date. Total stock-based compensation related to the ESPP for the three and six months ended June 30, 2019 was $187,000 and $415,000, respectively. Total stock-based compensation related to the ESPP for the three and six months ended June 30, 2018 was immaterial to the condensed financial statements. A total of 44,656 shares of common stock were purchased pursuant to the ESPP during the six months ended June 30, 2019 for total proceeds of $588,000. Convertible preferred stock The convertible preferred stock was temporarily reclassified to mezzanine equity as of June 30, 2018 before converting into common stock and permanent equity upon the closing of Company’s initial public offering in July 2018.

Net Loss Per Share

Net Loss Per Share	6 Months Ended
	Jun. 30, 2019
Earnings Per Share [Abstract]
Net Loss Per Share	8. Net Loss Per Share The following outstanding potentially dilutive shares have been excluded from the calculation of diluted net loss per share for the periods presented due to their anti-dilutive effect: As of June 30, 2019 2018 Convertible preferred stock — 16,215,896 Stock options to purchase common stock 3,181,138 3,374,318 Restricted stock subject to future vesting 15,053 71,633 Shares committed under ESPP 41,500 — Total 3,237,691 19,661,847

Subsequent Events

Subsequent Events	6 Months Ended
	Jun. 30, 2019
Subsequent Events [Abstract]
Subsequent Events	9 . Subsequent Events In July 2019, the Company filed a Registration Statement on Form S-3, as amended (file no. 333-232498), declared effective by the SEC on July 12, 2019 (the “Shelf Registration Statement”), covering the offering of up to $250 million of common stock, preferred stock, debt securities and warrants. The Company may use the Shelf Registration Statement at any time or from time to time to offer, in one or more offerings, . SM Cantor Fitzgerald & Co. Ono License and Collaboration Agreement In July 2019, the Company entered into an exclusive license and collaboration agreement with Ono Pharmaceutical Co., Ltd., (“Ono”). Under the agreement, the Company granted Ono an exclusive license to develop, manufacture and commercialize 5F9, the Company’s monoclonal antibody against CD47, as well as other anti-CD47 antibodies controlled by the Company in Japan, South Korea, Taiwan and the ASEAN countries(the “Ono Territory”). The Company retains all rights to 5F9 and other licensed antibodies outside of the Ono Territory. Under the agreement, the parties will collaborate on the development, manufacturing and commercialization of 5F9 and other licensed antibodies. Each party will be responsible for conducting development and commercialization of licensed antibodies in its respective territory at its own cost. Further, each party will have the right to participate, at its cost, in global clinical studies of 5F9 and other licensed antibodies conducted by the other party. The Company will receive a one-time upfront payment from Ono of 1.7 billion Japanese Yen (approximately $15.7 million US Dollar based on the exchange rate at July 10, 2019) and will be eligible to receive up to an additional 11.2 billion Japanese Yen (approximately $103.3 million US Dollar based on the exchange rate at July 10, 2019) if specified future development and commercial milestones are achieved by Ono. The Company is also eligible to receive tiered percentage royalties spanning from the mid-teens to the low-twenties on future net sales of 5F9 and other licensed antibodies in the Ono Territory, subject to certain offsets. Public Offering In July 2019, pursuant to the Shelf Registration Statement the Company completed an underwritten public offering of 10,781,250 shares of its common stock, including 1,406,250 shares sold pursuant to the underwriters’ exercise of their option to purchase additional shares at a public offering price of $8.00 per share. The gross proceeds from the offering to the Company, before underwriting discounts and commissions and offering expenses, were approximately $86.3 million.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The interim condensed financial statements are unaudited. The unaudited interim condensed financial statements have been prepared on the same basis as the annual financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary to present fairly the Company’s financial position, results of operations and cash flows for the periods presented. The results of operations for the three and six months ended June 30, 2019 are not necessarily indicative of the results to be expected for the year ending December 31, 2019 or for any other future annual or interim period. The condensed balance sheet as of December 31, 2018 included herein was derived from the audited financial statements as of that date. These condensed financial statements should be read in conjunction with the Company's audited financial statements and related notes as set forth in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2018, as filed with the SEC on March 28, 2019.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”) requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of expenses during the reporting period. Significant estimates and assumptions made in the accompanying financial statements include but are not limited to the fair value of common stock, the fair value of stock options, the fair value of investments, income tax uncertainties, lease liability, and certain accruals. The Company evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors and adjusts those estimates and assumptions when facts and circumstances dictate. Actual results could differ from those estimates.
Fair Value Measurement	Fair Value Measurement Assets and liabilities recorded at fair value on a recurring basis in the balance sheets are categorized based upon the level of judgment associated with the inputs used to measure their fair values. Fair value is defined as the exchange price that would be received for an asset or an exit price that would be paid to transfer a liability 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. The authoritative guidance on fair value measurements establishes a three-tier fair value hierarchy for disclosure of fair value measurements as follows: Level 1—Observable inputs such as unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date; Level 2—Inputs (other than quoted prices included in Level 1) are either directly or indirectly observable for the asset or liability. These include quoted prices for similar assets or liabilities in active markets and quoted prices for identical or similar assets or liabilities in markets that are not active; Level 3— Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities.
Leases	Leases The Company adopted Accounting Standards Update (ASU) No. 2016-02, Leases Rent expense for the operating leases is recognized on a straight-line basis over the lease term and is included in operating expenses on the statements of operations and comprehensive loss. Variable lease payments include lease operating expenses. The accompanying condensed financial statements as of and for the three and six months ended June 30, 2019 are presented under Topic 842. The prior periods continue to be reported in accordance with previous lease guidance, ASC Topic 840, Leases The impact of the adoption of Topic 842 on the accompanying condensed balance sheet as of January 1, 2019 was as follows: December 31, 2018 Adjustments due to the adoption of Topic 842 January 1, 2019 (In thousands) Assets Operating lease right-of-use asset $ — $ 2,328 $ 2,328 Liabilities and stockholders' equity Deferred rent classified as accrued liabilities $ 155 $ (155 ) $ — Lease liability, current $ — $ 968 $ 968 Lease liability, noncurrent $ — $ 1,847 $ 1,847 Deferred rent, noncurrent $ 331 $ (331 ) $ — The adjustments due to the adoption of Topic 842 related to the recognition of an operating lease right-of-use asset and lease liability for the Company's existing property operating lease and the derecognition of the deferred rent recognized under Topic 840. There was no impact on the Company’s statement of operations and comprehensive loss from the adoption and no cumulative-effect adjustment to the beginning accumulated deficit.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	6 Months Ended
	Jun. 30, 2019
Accounting Policies [Abstract]
Schedule of Impact of Adoption of Topic 842 on Accompanying Condensed Balance Sheet	The impact of the adoption of Topic 842 on the accompanying condensed balance sheet as of January 1, 2019 was as follows: December 31, 2018 Adjustments due to the adoption of Topic 842 January 1, 2019 (In thousands) Assets Operating lease right-of-use asset $ — $ 2,328 $ 2,328 Liabilities and stockholders' equity Deferred rent classified as accrued liabilities $ 155 $ (155 ) $ — Lease liability, current $ — $ 968 $ 968 Lease liability, noncurrent $ — $ 1,847 $ 1,847 Deferred rent, noncurrent $ 331 $ (331 ) $ —

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	6 Months Ended
	Jun. 30, 2019
Fair Value Disclosures [Abstract]
Fair Value, Assets Measured on Recurring and Nonrecurring Basis	The fair value and amortized cost of cash equivalents and available-for-sale securities by major security type as of June 30, 2019 and December 31, 2018 are presented in the following tables: As of June 30, 2019 Fair Value Hierarchy Amortized Cost Unrealized Gains Unrealized Losses Market Value (In thousands) Money market funds Level 1 $ 15,670 $ — $ — $ 15,670 Commercial paper Level 2 24,271 — — 24,271 Corporate debt securities Level 2 20,453 24 — 20,477 Asset-backed securities Level 2 16,470 11 — 16,481 US government debt securities Level 2 19,905 24 — 19,929 Total cash equivalents and available-for-sale securities $ 96,769 $ 59 $ — $ 96,828 As of December 31, 2018 Fair Value Hierarchy Amortized Cost Unrealized Gains Unrealized Losses Market Value (In thousands) Money market funds Level 1 $ 7,959 $ — $ — $ 7,959 Commercial paper Level 2 43,277 — — 43,277 Corporate debt securities Level 2 46,186 — (54 ) 46,132 Asset-backed securities Level 2 22,842 — (27 ) 22,815 US government debt securities Level 2 15,963 — (1 ) 15,962 Total cash equivalents and available-for-sale securities $ 136,227 $ — $ (82 ) $ 136,145
Schedule of Change in Fair Value of Embedded Derivative	The change in fair value of the embedded derivative is presented in the following table: Six Months Ended June 30, 2019 (In thousands) Beginning balance $ 331 Change in fair value of embedded derivative 12 Ending balance $ 343

Balance Sheet Components (Table

Balance Sheet Components (Tables)	6 Months Ended
	Jun. 30, 2019
Balance Sheet Related Disclosures [Abstract]
Schedule of Accrued Liabilities	Accrued liabilities consist of the following: June 30, 2019 December 31, 2018 (In thousands) Accrued research and development expenses $ 6,338 $ 5,870 Accrued bonuses 1,379 1,602 Deferred rent, current — 155 Other 2,002 1,417 Total accrued liabilities $ 9,719 $ 9,044

Leases (Tables)

Leases (Tables)	6 Months Ended
	Jun. 30, 2019
Leases [Abstract]
Schedule of Undiscounted Future Non-Cancellable Lease Payments under Company's Operating Leases	The undiscounted future non-cancellable lease payments under the Company's operating leases are as follows: June 30, 2019 (In thousands) Years Remaining 2019 $ 777 2020 1,584 2021 864 Thereafter — Total undiscounted lease payments 3,225 Present value adjustment for minimum lease commitments (228 ) Lease liabilities $ 2,997

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	6 Months Ended
	Jun. 30, 2019
Equity [Abstract]
Summary of Option Activity	The following summarizes option activity for the six months ended June 30, 2019: Shares Issuable Under Options Weighted- Average Exercise Price Weighted- Average Remaining Contract Term Aggregate Intrinsic Value (In years) (In thousands) Balance, December 31, 2018 3,404,847 $ 6.55 8.82 $ 31,388 Options granted 186,652 14.57 Options exercised (308,516 ) 4.54 Options forfeited (101,845 ) 7.02 Balance outstanding June 30, 2019 3,181,138 $ 7.20 8.45 $ 12,331 Exercisable, June 30, 2019 2,071,836 $ 6.46 8.32 $ 8,906 Vested and expected to vest, June 30, 2019 3,181,138 $ 7.20 8.45 $ 12,331
Schedule of Total Stock-Based Compensation	Total stock-based compensation was as follows: Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 (In thousands) (In thousands) Research and development $ 431 $ 162 $ 886 $ 297 General and administrative 694 454 1,343 750 Total $ 1,125 $ 616 $ 2,229 $ 1,047
Summary of Restricted Stock Activity	A summary of restricted stock activity follows: Number of Restricted Shares Outstanding Unvested shares—December 31, 2018 49,052 Restricted shares vested (33,999 ) Unvested shares—June 30, 2019 15,053

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	6 Months Ended
	Jun. 30, 2019
Earnings Per Share [Abstract]
Summary of Outstanding Potentially Dilutive Shares Excluded from Calculation of Diluted Net Loss Per Share	The following outstanding potentially dilutive shares have been excluded from the calculation of diluted net loss per share for the periods presented due to their anti-dilutive effect: As of June 30, 2019 2018 Convertible preferred stock — 16,215,896 Stock options to purchase common stock 3,181,138 3,374,318 Restricted stock subject to future vesting 15,053 71,633 Shares committed under ESPP 41,500 — Total 3,237,691 19,661,847

Description of Business - Addit

Description of Business - Additional Information (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	[1]
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Cash, cash equivalents and short-term investments	$ 99,000
Cumulative net losses	$ 185,991	$ 139,769
[1]	The balance sheet as of December 31, 2018 is derived from the audited financial statements as of that date.

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Schedule of Impact of Adoption of Topic 842 on Accompanying Condensed Balance Sheet (Details) - USD ($) $ in Thousands		Jun. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
Assets
Operating lease right-of-use asset		$ 2,579
Liabilities and stockholders' equity
Deferred rent classified as accrued liabilities				$ 155
Lease liability, current		1,397
Lease liability, noncurrent		$ 1,600
Deferred rent, noncurrent	[1]			$ 331
Accounting Standards Update 2016-02
Assets
Operating lease right-of-use asset			$ 2,328
Liabilities and stockholders' equity
Lease liability, current			968
Lease liability, noncurrent			1,847
Accounting Standards Update 2016-02 | Adjustments due to the adoption of Topic 842
Assets
Operating lease right-of-use asset			2,328
Liabilities and stockholders' equity
Deferred rent classified as accrued liabilities			(155)
Lease liability, current			968
Lease liability, noncurrent			1,847
Deferred rent, noncurrent			$ (331)
[1]	The balance sheet as of December 31, 2018 is derived from the audited financial statements as of that date.

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers between level 1 to Level 2 and Level 3 amount	$ 0		$ 0
Realized gains (losses) on investments	0	$ 0	0	$ 0
Other Long-Term Liabilities | Leukemia & Lymphoma Society, Inc.
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	$ 300,000		$ 300,000		$ 300,000
Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Maturity of investments			1 year		1 year

Fair Value Measurements - Fair

Fair Value Measurements - Fair Value and Amortized Cost of Cash Equivalents and Available for Sale Securities by Major Security Type (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 96,769	$ 136,227
Unrealized Gains	59
Unrealized Losses		(82)
Market Value	96,828	136,145
Level 1
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	15,670	7,959
Market Value	15,670	7,959
Commercial Paper | Level 2
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	24,271	43,277
Market Value	24,271	43,277
Corporate Debt Securities | Level 2
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	20,453	46,186
Unrealized Gains	24
Unrealized Losses		(54)
Market Value	20,477	46,132
Asset-backed Securities | Level 2
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	16,470	22,842
Unrealized Gains	11
Unrealized Losses		(27)
Market Value	16,481	22,815
US Government Debt Securities | Level 2
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	19,905	15,963
Unrealized Gains	24
Unrealized Losses		(1)
Market Value	$ 19,929	$ 15,962

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Change in Fair Value of Embedded Derivative (Details) - Embedded Derivative $ in Thousands	6 Months Ended
	Jun. 30, 2019USD ($)
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value of embedded derivative, Beginning balance	$ 331
Change in fair value of embedded derivative	12
Fair value of embedded derivative, Ending balance	$ 343

Balance Sheet Components - Sche

Balance Sheet Components - Schedule of Accrued Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Accrued Liabilities Current [Abstract]
Accrued research and development expenses	$ 6,338	$ 5,870
Accrued bonuses	1,379	1,602
Deferred rent, current		155
Other	2,002	1,417
Total accrued liabilities	$ 9,719	$ 9,044	[1]
[1]	The balance sheet as of December 31, 2018 is derived from the audited financial statements as of that date.

Research and License Agreemen_2

Research and License Agreements - Additional Information (Details) - USD ($)	1 Months Ended			3 Months Ended		6 Months Ended		12 Months Ended
	Jul. 31, 2019	Nov. 30, 2017	Jan. 31, 2017	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Mar. 31, 2017
Leukemia & Lymphoma Society, Inc. ("LLS")
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional milestone payments to be paid upon achievement of certain clinical or regulatory milestones	$ 4,200,000
Leukemia & Lymphoma Society, Inc. ("LLS") | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional milestone payments	3,000,000
Potential future milestone payments related to certain development and regulatory approval milestones	$ 6,000,000
Leukemia & Lymphoma Society, Inc. ("LLS") | Other Long-Term Liabilities
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Derivative Liability				$ 300,000		$ 300,000		$ 300,000
Research Grant | California Institute of Regenerative Medicine (CIRM)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaborative arrangement research grant receivable			$ 10,200,000	2,000,000		2,000,000
Collaborative arrangement milestone payments receivable period			4 years
Proceeds from collaborators						9,200,000		7,200,000
Research Grant | Leukemia & Lymphoma Society, Inc. ("LLS")
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaborative arrangement research grant receivable				0		0			$ 4,200,000
Proceeds from collaborators						3,900,000		3,900,000
Research Grant | Leukemia & Lymphoma Society, Inc. ("LLS") | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments to be paid upon achievement of development and regulatory approval milestones				15,000,000		15,000,000
Research Grant | Leukemia & Lymphoma Society, Inc. ("LLS") | Other Long-Term Liabilities
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Derivative Liability				300,000		300,000		300,000
Research Grant | California Institute of Regenerative Medicine (CIRM) and Leukemia & Lymphoma Society, Inc. ("LLS")
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Reduction to research and development expenses for research grants				900,000	$ 1,900,000	2,000,000	$ 3,400,000
Second Research Grant | California Institute of Regenerative Medicine (CIRM)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaborative arrangement research grant receivable		$ 5,000,000		700,000		700,000
Collaborative arrangement milestone payments receivable period		5 years
Proceeds from collaborators						3,100,000		2,400,000
Collaborative arrangement, amended research grant receivable								$ 3,200,000
Collaborative arrangement, amended milestone payments receivable period								5 years
Merck Collaboration Agreement | Ares Trading S.A
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Reduction to research and development expenses for research grants				300,000	$ 400,000	900,000	$ 600,000
Receivable for reimbursement of research and development costs incurred				$ 900,000		$ 900,000

Leases - Additional Information

Leases - Additional Information (Details)	1 Months Ended	3 Months Ended		6 Months Ended
	Apr. 30, 2019USD ($)ft²	Jun. 30, 2019USD ($)	Jun. 30, 2018USD ($)	Jun. 30, 2019USD ($)	Jun. 30, 2018USD ($)
Leases [Abstract]
Lease expiration date				Aug. 31, 2021
Renewal term		5 years		5 years
Tenant improvement allowance		$ 646,000		$ 646,000
Sublease agreement date	2019-04
Additional area rentable | ft²	6,230
Sublease expiration date	2021-02
Sublease amendment agreement date	2019-04
Additional space sublease commencement date	Mar. 1, 2021
Base rent per month	$ 36,000
Weighted average remaining lease term		2 years 1 month 6 days		2 years 1 month 6 days
Operating lease liabilities, weighted average incremental borrowing rate		7.00%		7.00%
Operating lease, liabilities, current		$ 1,397,000		$ 1,397,000
Operating lease, liabilities, noncurrent		1,600,000		1,600,000
Rent expense for operating leases		300,000	$ 300,000	600,000	$ 600,000
Variable lease payments for operating expenses		$ 100,000	$ 100,000	$ 300,000	$ 200,000

Leases - Schedule of Undiscount

Leases - Schedule of Undiscounted Future Non-Cancellable Lease Payments under Company's Operating Leases (Details) $ in Thousands	Jun. 30, 2019USD ($)
Operating Leases
Remaining 2019	$ 777
2020	1,584
2021	864
Total undiscounted lease payments	3,225
Present value adjustment for minimum lease commitments	(228)
Lease liabilities	$ 2,997

Stockholders' Equity - Summary

Stockholders' Equity - Summary of Option Activity (Details) - 2015 and 2018 Stock Plan $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2019USD ($)$ / sharesshares	Dec. 31, 2018USD ($)$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares Issuable Under Options, Beginning balance | shares	3,404,847
Shares Issuable Under Options, Options granted | shares	186,652
Shares Issuable Under Options, Options exercised | shares	(308,516)
Shares Issuable Under Options, Options forfeited | shares	(101,845)
Shares Issuable Under Options, Ending balance | shares	3,181,138	3,404,847
Shares Issuable Under Options, Exercisable | shares	2,071,836
Shares Issuable Under Options, Vested and expected to vest | shares	3,181,138
Weighted-Average Exercise Price, Beginning balance | $ / shares	$ 6.55
Weighted-Average Exercise Price, Options granted | $ / shares	14.57
Weighted-Average Exercise Price, Options exercised | $ / shares	4.54
Weighted-Average Exercise Price, Options forfeited | $ / shares	7.02
Weighted-Average Exercise Price, Ending balance | $ / shares	7.20	$ 6.55
Weighted-Average Exercise Price, Exercisable | $ / shares	6.46
Weighted-Average Exercise Price, Vested and expected to vest | $ / shares	$ 7.20
Weighted-Average Remaining Contract Term	8 years 5 months 12 days	8 years 9 months 25 days
Weighted-Average Remaining Contract Term Exercisable	8 years 3 months 25 days
Weighted-Average Remaining Contract Term Vested and expected to vest	8 years 5 months 12 days
Aggregate Intrinsic Value | $	$ 12,331	$ 31,388
Aggregate Intrinsic Value Exercisable | $	8,906
Aggregate Intrinsic Value Vested and expected to vest | $	$ 12,331

Stockholders' Equity - Schedule

Stockholders' Equity - Schedule of Total Stock-Based Compensation (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 1,125	$ 616	$ 2,229	$ 1,047
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	431	162	886	297
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 694	$ 454	$ 1,343	$ 750

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	$ 1,125,000	$ 616,000	$ 2,229,000	$ 1,047,000
Total proceeds			$ 1,402,000	$ 155,000
Employee Share Purchase Plan (ESPP)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee share purchase plan description			The initial offering period began June 27, 2018 and will end on August 15, 2020 with purchase dates of February 15, 2019, August 15, 2019, February 15, 2020, and August 15, 2020. Each subsequent offering will be approximately 24 months long and will consist of four purchase periods with purchase dates occurring on February 15 and August 15 of each year. On each purchase date, ESPP participants will purchase shares of common stock at a price per share equal to 85% of the lesser of (1) the fair market value per share of the common stock on the offering date or (2) the fair market value of the common stock on the purchase date.
Employee share purchase plan purchase price percentage	85.00%		85.00%
Total stock-based compensation	$ 187,000		$ 415,000
Number of common stock purchase to ESPP			44,656
Total proceeds			$ 588,000
Restricted Stock | Other Long-term Liabilities
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares held by employees and directors subject to repurchase	$ 18,000		$ 18,000		$ 144,000

Stockholders' Equity - Summar_2

Stockholders' Equity - Summary of Restricted Stock Activity (Details)	6 Months Ended
	Jun. 30, 2019shares
Equity [Abstract]
Number of Restricted Shares Outstanding, Beginning balance	49,052
Number of Restricted Shares Outstanding, Restricted shares vested	(33,999)
Number of Restricted Shares Outstanding, Ending balance	15,053

Net Loss Per Share - Summary of

Net Loss Per Share - Summary of Outstanding Potentially Dilutive Shares Excluded from Calculation of Diluted Net Loss Per Share (Details) - shares	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Total	3,237,691	19,661,847
Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Total		16,215,896
Stock Options to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Total	3,181,138	3,374,318
Restricted Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Total	15,053	71,633
Shares Committed under ESPP
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Total	41,500

Subsequent Events - Additional

Subsequent Events - Additional Information (Details) $ / shares in Units, ¥ in Billions	Jul. 12, 2019USD ($)	Jul. 31, 2019USD ($)$ / sharesshares	Jun. 30, 2019USD ($)	Jun. 30, 2018USD ($)	Jul. 10, 2019USD ($)	Jul. 10, 2019JPY (¥)
Subsequent Event [Line Items]
Issuance of common stock			$ 588,000	$ 0
Subsequent Event | License and Collaboration Agreement | Ono Pharmaceutical Co., Ltd.
Subsequent Event [Line Items]
One time upfront payment receivable					$ 15,700,000	¥ 1.7
Additional upfront payment receivable upon achievement of specified future development and commercial milestones					$ 103,300,000	¥ 11.2
Subsequent Event | Maximum
Subsequent Event [Line Items]
Securities available for issuance under shelf registration statement	$ 250,000,000
At-The-Market Offerings | Subsequent Event | Maximum
Subsequent Event [Line Items]
Issuance of common stock	$ 60,000,000
Underwritten Public Offering | Subsequent Event
Subsequent Event [Line Items]
Common stock shares, issued | shares		10,781,250
Shares issued pursuant to exercise of underwriters' option to purchase | shares		1,406,250
Common stock issued price per share | $ / shares		$ 8
Gross proceeds from offering		$ 86,300,000