Turning Point Therapeutics (TPTX) S-1 3 Sep 19 IPO registration Financial data

Document and Entity Information

Document and Entity Information	6 Months Ended
	Jun. 30, 2019
Document And Entity Information [Abstract]
Document Type	S-1
Amendment Flag	false
Document Period End Date	Jun. 30, 2019
Entity Registrant Name	Turning Point Therapeutics, Inc.
Entity Central Index Key	0001595893
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	true

Condensed Balance Sheets

Condensed Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 78,781	$ 101,029	$ 45,033
Marketable securities, available for sale	171,389
Prepaid expenses and other current assets	4,998	494	581
Total current assets	255,168	101,523	45,614
Property and equipment, net	954	1,000	256
Right-of-use assets from operating leases	1,250
Security deposits	73	73	38
Deferred financing costs		684
Total assets	257,445	103,280	45,908
Current liabilities:
Accounts payable	4,086	1,494	1,339
Accrued expenses and other current liabilities	1,992	2,415	2,750
Accrued compensation	1,672	1,413	436
Current portion of operating lease liabilities	138
Total current liabilities	7,888	5,322	4,525
Other long-term liabilities	260
Operating lease liabilities, net of current portion	1,382	448	66
Commitments and contingencies
Convertible preferred stock		145,916	66,161
Stockholders' equity (deficit):
Common stock	34	1	1
Additional paid-in capital	328,978	2,346	1,123
Accumulated other comprehensive income	345
Accumulated deficit	(81,442)	(50,753)	(25,968)
Total stockholders' equity (deficit)	247,915	(48,406)	(24,844)
Total liabilities, convertible preferred stock, and stockholders' equity (deficit)	$ 257,445	$ 103,280	$ 45,908

Condensed Balance Sheets (Paren

Condensed Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Temporary equity, par value	$ 0.0001	$ 0.0001	$ 0.0001
Temporary equity, shares authorized		65,423,901	39,135,778
Temporary equity, shares issued	0	65,423,901	39,135,778
Temporary equity, shares outstanding	0	65,423,901	39,135,778
Temporary equity, liquidation preference	$ 0	$ 146,460	$ 66,460
Common stock, par value	$ 0.0001	$ 0.0001	$ 0.0001
Common stock, shares authorized	200,000,000	104,000,000	60,000,000
Common stock, shares issued	31,297,423	3,411,516	3,367,742
Common stock, shares outstanding	31,297,423	3,411,516	3,367,742

Condensed Statements of Operati

Condensed Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Operating expenses:
Research and development	$ 13,711	$ 4,369	$ 24,162	$ 8,713	$ 21,062	$ 15,241
General and administrative	4,743	803	8,357	1,319	4,578	1,487
Total operating expenses	18,454	5,172	32,519	10,032	25,640	16,728
Loss from operations	(18,454)	(5,172)	(32,519)	(10,032)	(25,640)	(16,728)
Interest income	1,312	135	1,830	262	856	136
Income taxes					(1)	(1)
Net loss	(17,142)	(5,037)	(30,689)	(9,770)	(24,785)	(16,593)
Other comprehensive income:
Unrealized gain on marketable securities, net of tax	345		345
Comprehensive loss	$ (16,797)	$ (5,037)	$ (30,344)	$ (9,770)	$ (24,785)	$ (16,593)
Net loss per share, basic and diluted	$ (0.70)	$ (1.49)	$ (2.19)	$ (2.89)	$ (7.31)	$ (4.97)
Weighted-average common shares outstanding, basic and diluted	24,479,767	3,380,899	14,004,957	3,374,787	3,388,586	3,337,640

Condensed Statements of Convert

Condensed Statements of Convertible Preferred Stock and Stockholders' Equity Deficit - USD ($) $ in Thousands	Total	IPO [Member]	Convertible Preferred Stock [Member]	Common Stock [Member]	Common Stock [Member]IPO [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member]IPO [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings (Accumulated Deficit) [Member]
Balance at Dec. 31, 2016	$ (8,658)			$ 1		$ 716			$ (9,375)
Balance at Dec. 31, 2016			$ 21,373
Balance, shares at Dec. 31, 2016				3,324,673
Balance, shares at Dec. 31, 2016			19,719,133
Issuance of Series C and Series D convertible preferred stock, net of issuance cost			$ 44,788
Option exercises	63					63
Issuance of stock, net of issuance cost			19,416,645
Option exercises, Shares				43,069
Stock-based compensation expense	344					344
Net loss	(16,593)								(16,593)
Balance at Dec. 31, 2017	$ 66,161		$ 66,161
Balance, shares at Dec. 31, 2017	39,135,778		39,135,778
Balance at Dec. 31, 2017	$ (24,844)			$ 1		1,123			(25,968)
Issuance of Series C and Series D convertible preferred stock, net of issuance cost			$ 44,788
Balance, shares at Dec. 31, 2017	3,367,742			3,367,742
Option exercises	$ 6					6
Option exercises, Shares				2,597
Stock-based compensation expense	85					85
Net loss	(4,733)								(4,733)
Balance at Mar. 31, 2018			$ 66,161
Balance, shares at Mar. 31, 2018			39,135,778
Balance at Mar. 31, 2018	(29,486)			$ 1		1,214			(30,701)
Balance, shares at Mar. 31, 2018				3,370,339
Balance at Dec. 31, 2017	(24,844)			$ 1		1,123			(25,968)
Balance at Dec. 31, 2017	$ 66,161		$ 66,161
Balance, shares at Dec. 31, 2017	3,367,742			3,367,742
Balance, shares at Dec. 31, 2017	39,135,778		39,135,778
Net loss	$ (9,770)
Balance at Jun. 30, 2018			$ 66,161
Balance, shares at Jun. 30, 2018			39,135,778
Balance at Jun. 30, 2018	(34,347)			$ 1		1,390			(35,738)
Balance, shares at Jun. 30, 2018				3,391,116
Balance at Dec. 31, 2017	(24,844)			$ 1		1,123			(25,968)
Balance at Dec. 31, 2017	$ 66,161		$ 66,161
Balance, shares at Dec. 31, 2017	3,367,742			3,367,742
Balance, shares at Dec. 31, 2017	39,135,778		39,135,778
Issuance of Series C and Series D convertible preferred stock, net of issuance cost			$ 79,755
Option exercises	$ 76					76
Issuance of stock, net of issuance cost			26,288,123
Option exercises, Shares	43,774			43,774
Stock-based compensation expense	$ 1,147					1,147
Net loss	(24,785)								(24,785)
Balance at Dec. 31, 2018	$ 145,916		$ 145,916
Balance, shares at Dec. 31, 2018	65,423,901		65,423,901
Balance at Dec. 31, 2018	$ (48,406)			$ 1		2,346			(50,753)
Issuance of Series C and Series D convertible preferred stock, net of issuance cost			$ 79,755
Balance, shares at Dec. 31, 2018	3,411,516			3,411,516
Balance at Mar. 31, 2018	$ (29,486)			$ 1		1,214			(30,701)
Balance at Mar. 31, 2018			$ 66,161
Balance, shares at Mar. 31, 2018				3,370,339
Balance, shares at Mar. 31, 2018			39,135,778
Option exercises	39					39
Option exercises, Shares				20,777
Stock-based compensation expense	137					137
Net loss	(5,037)								(5,037)
Balance at Jun. 30, 2018			$ 66,161
Balance, shares at Jun. 30, 2018			39,135,778
Balance at Jun. 30, 2018	(34,347)			$ 1		1,390			(35,738)
Balance, shares at Jun. 30, 2018				3,391,116
Balance at Dec. 31, 2018	(48,406)			$ 1		2,346			(50,753)
Balance at Dec. 31, 2018	$ 145,916		$ 145,916
Balance, shares at Dec. 31, 2018	3,411,516			3,411,516
Balance, shares at Dec. 31, 2018	65,423,901		65,423,901
Option exercises	$ 22					22
Option exercises, Shares				12,337
Stock-based compensation expense	1,926					1,926
Net loss	(13,547)								(13,547)
Balance at Mar. 31, 2019			$ 145,616
Balance, shares at Mar. 31, 2019			65,423,901
Balance at Mar. 31, 2019	(60,005)			$ 1		4,294			(64,300)
Balance, shares at Mar. 31, 2019				2,423,853
Balance at Dec. 31, 2018	(48,406)			$ 1		2,346			(50,753)
Balance at Dec. 31, 2018	$ 145,916		$ 145,916
Balance, shares at Dec. 31, 2018	3,411,516			3,411,516
Balance, shares at Dec. 31, 2018	65,423,901		65,423,901
Option exercises, Shares	255,213
Net loss	$ (30,689)
Balance, shares at Jun. 30, 2019	0
Balance at Jun. 30, 2019	$ 247,915			$ 34		328,978		$ 345	(81,442)
Balance, shares at Jun. 30, 2019	31,297,423			32,974,423
Balance at Mar. 31, 2019	$ (60,005)			$ 1		4,294			(64,300)
Balance at Mar. 31, 2019			$ 145,616
Conversion of convertible preferred stock into common stock	145,916		$ (145,916)	$ 15		145,901
Issuance of common stock in connection with a public offering, net of underwriting discounts, commissions, and offering costs		$ 175,151			$ 18		$ 175,133
Balance, shares at Mar. 31, 2019				2,423,853
Balance, shares at Mar. 31, 2019			65,423,901
Conversion of convertible preferred stock into common stock, shares			(65,423,901)	16,993,194
Option exercises	591					591
Issuance of stock, net of issuance cost					10,637,500
Option exercises, Shares				242,876
Stock-based compensation expense	3,059					3,059
Net loss	(17,142)								(17,142)
Other comprehensive income	$ 345							345
Balance, shares at Jun. 30, 2019	0
Balance at Jun. 30, 2019	$ 247,915			$ 34		$ 328,978		$ 345	$ (81,442)
Balance, shares at Jun. 30, 2019	31,297,423			32,974,423

Condensed Statement Of Cash Flo

Condensed Statement Of Cash Flows - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Operating activities
Net loss	$ (30,689)	$ (9,770)	$ (24,785)	$ (16,593)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	4,985	222	1,147	344
Depreciation	233	39	138	52
Deferred rent			(40)	(13)
Accretion of discount on marketable securities, available for sale	(359)
Loss on disposal of assets				11
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(3,821)	(454)	88	(179)
Security deposits			(35)	7
Accounts payable	2,595	266	(119)	1,286
Accrued expenses and other current liabilities	(246)	(912)	(905)	2,090
Accrued compensation	258	63	978	355
Operating lease right-of-use assets and liabilities, net	(95)	(7)
Net cash used in operating activities	(27,139)	(10,553)	(23,533)	(12,640)
Investing activities
Purchases of marketable securities, available for sale	(170,685)
Purchase of property and equipment	(188)	(138)	(302)	(88)
Net cash used in investing activities	(170,873)	(138)	(302)	(88)
Financing activities
Proceeds from issuance of convertible preferred stock, net of issuance costs			79,755	44,788
Proceeds from issuance of common stock in a public offering, net	175,151
Proceeds from issuance of common stock from stock option exercises	613	45	76	63
Net cash provided by financing activities	175,764	45	79,831	44,851
Net increase in cash and cash equivalents	(22,248)	(10,646)	55,996	32,123
Cash and cash equivalents at the beginning of period	101,029	45,033	45,033	12,910
Cash and cash equivalents at the end of period	78,781	34,387	101,029	45,033
Supplemental disclosure of cash flow information:
Cash paid for income taxes	1	$ 1	1	$ 1
Supplemental disclosure of non-cash investing and financing information:
Operating lease liabilities arising from obtaining right-of-use assets	$ 1,520
Costs incurred in connection with initial public offering included in accounts payable and accrued expenses			684
Capitalized value of tenant improvement allowance			$ 583

Condensed Statements of Conve_2

Condensed Statements of Convertible Preferred Stock and Stockholders' Equity Deficit (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Convertible Preferred Stock [Member]
Stock issuance cost	$ 245	$ 212

Formation and Business of the C

Formation and Business of the Company	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Formation and Business of the Company	Turning Point Therapeutics, Inc. Notes to Unaudited Condensed Financial Statements 1. Formation and Business of the Company; Basis of Presentation Organization Turning Point Therapeutics, Inc. (the Company) was organized in 2013 and commenced operations in 2014. The Company is a clinical stage biopharmaceutical company designing and developing novel small molecule, targeted oncology therapies. The Company’s principal operations are in the United States and the Company operates in one segment, with its headquarters in San Diego, California. The Company’s primary activities since inception have been to build infrastructure, conduct research and development, including clinical trials, perform business and financial planning, and raise capital. The accompanying unaudited condensed financial statements have been prepared in accordance with U.S. generally accepted accounting principles (GAAP) for interim financial information and pursuant to Form 10-Q S-X S-1, Initial Public Offering On April 22, 2019, the Company completed an initial public offering (IPO) of its common stock. In connection with its IPO, the Company issued and sold 10,637,500 shares of its common stock at a price to the public of $18.00 per share. The proceeds from the IPO were approximately $175.2 million after deducting underwriting discounts and commissions of $13.4 million and offering expenses of approximately $2.9 million payable by the Company. At the closing of the IPO, 65,423,901 shares of outstanding convertible preferred stock were automatically converted into 16,993,194 shares of common stock. Following the IPO, there were no shares of preferred stock outstanding. Reverse Stock Split On April 5, 2019, the Company effected a 1-for-3.85 Liquidity Substantial doubt about an entity’s ability to continue as a going concern exists when relevant conditions and events, considered in the aggregate, indicate that it is probable that the entity will be unable to meet its obligations as they become due within one year from the financial statement issuance date. The Company determined that there are no conditions or events that raise substantial doubt about its ability to continue as a going concern within one year after the date that the condensed financial statements for the quarter ended June 30, 2019 are issued.	1. Formation and Business of the Company Organization Turning Point Therapeutics, Inc. (the Company) was organized on October 8, 2013, and commenced operations in 2014. The Company is a clinical-stage biopharmaceutical company designing and developing novel small molecule, targeted oncology therapies. The Company’s principal operations are in the United States and operates in one segment, with its headquarters in San Diego, California. The Company’s primary activities since inception have been to build infrastructure, conduct research and development, including clinical trials, perform business and financial planning, and raise capital. Liquidity Management evaluates whether there are relevant conditions and events that in aggregate raise substantial doubt about the entity’s ability to continue as a going concern and to meet its obligations as they become due within one year from the date that the financial statements are issued. The Company’s activities are subject to significant risks and uncertainties, including concentration on the Company’s lead development program, which has significant competition from cancer therapies in development by other companies or already approved for sale by the U.S. Food and Drug Administration. 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. Drug 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.

Summary of Significant Accounti

Summary of Significant Accounting Policies	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that impact the reported amounts of assets, liabilities and expenses and the disclosure of contingent assets and liabilities in the Company’s financial statements and accompanying notes. The most significant estimates in the Company’s financial statements relate to preclinical and clinical study accruals and stock-based compensation costs. Management bases its estimates on historical experience and on various other market-specific and relevant assumptions that management believes to be reasonable under the circumstances. Although these estimates are based on the Company’s knowledge of current events and actions it may undertake in the future, actual results may ultimately materially differ from these estimates and assumptions. Marketable securities, Available-for-Sale The Company classifies all marketable securities as available-for-sale, available-for-sale Concentration of Credit Risk Substantially all the Company’s cash and money market funds are held with a single financial institution. Due to its size, the Company believes this financial institution represents a minimal credit risk. Cash amounts held at financial institutions are insured by the Federal Deposit Insurance Corporation (“FDIC”) up to $250,000. At June 30, 2019, the Company had $249.9 million in excess of the FDIC insured limit. At June 30, 2019, the Company’s money market funds and marketable securities, available for sale are not subject to FDIC insurance. The Company’s money market funds and short-term investments are invested in short term, high grade securities. As a result, the Company believes its money market and marketable securities, available for sale represent a minimal credit risk. Property and Equipment Property and equipment are stated at cost less accumulated depreciation. Depreciation is calculated using the straight-line method over the estimated useful life of the assets, which ranges between three to seven years. Tenant improvements are stated at cost and depreciated over the shorter of the estimated useful life or the remaining life of the lease at the time the asset is placed into service. Intellectual Property The legal and professional costs incurred by the Company to maintain its patent rights have been expensed as part of general and administrative expenses since inception. As of June 30, 2019, and 2018, the Company has determined that these expenses have not met the criteria to be capitalized. Intellectual property related expenses were $0.1 million for the three months ended June 30, 2019 and 2018, respectively and were $0.3 million for the six months ended June 30, 2019 and 2018, respectively. Research and Development Expenses Research and development costs are expensed as incurred. These costs consist primarily of salaries and other personnel-related expenses, including stock-based compensation; facility-related expenses; depreciation of facilities and equipment; laboratory consumables; and services performed by clinical research organizations, clinical manufacturing services, research institutions, and other outside service providers. Income Taxes Deferred tax assets and liabilities are determined based on the differences between the financial reporting and tax basis of assets and liabilities using enacted tax rates which will be in effect when the differences reverse. The Company provides a valuation allowance against net deferred tax assets unless, based upon the available evidence, it is more likely than not that the deferred tax asset will be realized. The Company follows the provisions of the Income Taxes Topic of the Financial Accounting Standards Board (FASB) Accounting Standards Codification that defines a recognition threshold and measurement attributes for the financial statement recognition and measurement of a tax position taken or expected to be taken in a tax return. Under the Income Taxes Topic, the impact of an uncertain income tax position on the income tax return must be recognized at the largest amount that is more-likely-than-not to Stock-Based Compensation Stock-based compensation expense represents the cost of the grant date fair value of employee and non-employee Net Loss Per Share The Company computes basic loss per share by dividing the net loss available to common stockholders by the weighted average number of common shares outstanding for the period, without consideration for common stock equivalents. Diluted net loss assumes the conversion, exercise or issuance of all potential common stock equivalents, unless the effect of inclusion would be anti-dilutive. For purposes of this calculation, common stock equivalents include the Company’s stock options and convertible preferred stock, which is convertible into shares of the Company’s common stock. No shares related to the convertible preferred stock were included in the diluted net loss calculation for the three and six months ended June 30, 2019 or 2018 because the inclusion of such shares would have had an anti-dilutive effect. The shares to be issued upon exercise of certain outstanding stock options were also excluded from the diluted net loss calculation for the three and six months ended June 30, 2019 and 2018 because such shares are anti-dilutive. Historical outstanding anti-dilutive securities not included in the diluted net loss per share calculation include the following: Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 Convertible preferred stock (as converted) — 10,165,120 — 10,165,120 Common stock options 4,590,804 1,055,445 4,590,804 1,055,445 Total 4,590,804 11,220,565 4,590,804 11,220,565 Recent Accounting Pronouncements Not Yet Adopted In June 2016, the FASB issued Accounting Standards Update (ASU) No. 2016-13, Financial Instruments—Credit Losses available-for-sale In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement: Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement Recently Adopted Accounting Standards Updates In February 2016, the FASB issued ASU No. 2016-02, Leases 2016-02 right-of-use 2016-02 non-lease The Company currently has one operating lease for office and laboratory spaces in San Diego, California. The operating lease was impacted by the new accounting standard and resulted in the present values of the future lease payments being presented as a right-to-use	2. Summary of Significant Accounting Policies Use of Estimates The preparation of financial statements in conformity with accounting principles generally accepted in the United States (U.S. GAAP) requires management to make estimates and assumptions that impact the reported amounts of assets, liabilities and expenses and the disclosure of contingent liabilities in the Company’s financial statements and accompanying notes. The most significant estimates in the Company’s financial statements relate to the valuation of equity awards, preclinical and clinical study accruals, fair value of assets and liabilities, income taxes, and stock-based compensation. Management bases its estimates on historical experience and on various other market-specific and relevant assumptions that management believes to be reasonable under the circumstances. Although these estimates are based on the Company’s knowledge of current events and actions it may undertake in the future, actual results may ultimately materially differ from these estimates and assumptions. Cash and Cash Equivalents The Company considers all highly liquid investments with an original maturity of three months or less to be cash equivalents. As of December 31, 2017 and 2018, cash equivalents consisted of checking, savings, and money-market balances. The Company places its cash with high-credit-quality financial institutions. All of the Company’s cash and cash equivalent balances are maintained at two financial institutions domiciled in the United States. Fair Value of Financial Instruments The carrying amounts of certain of the Company’s financial instruments, including cash and cash equivalents, prepaid expenses and other current assets, accounts payable, and accrued liabilities, approximate fair value due to the short-term nature of these items. Concentration of Credit Risk Substantially all of the Company’s cash and money market funds are held with a single financial institution. Due to its size, the Company believes this financial institution represents a minimal credit risk. Cash amounts held at financial institutions are insured by the Federal Deposit Insurance Corporation (“FDIC”) up to $250. At December 31, 2018, the Company had $2,261 in excess of the FDIC insured limit. At December 31, 2018, the Company’s money market funds totaling $98,268 are not subject to FDIC insurance. The Company’s money market funds are invested in short term, high grade U.S. Treasuries. As a result, the Company believes its money market funds represent a minimal credit risk. Property and Equipment Property and equipment are stated at cost less accumulated depreciation. Depreciation is calculated using the straight-line method over the estimated useful life of the assets, which ranges between three to seven years. Tenant improvements are stated at cost and depreciated over the shorter of the estimated useful life or the remaining life of the lease at the time the asset is placed into service. Impairment of Long-Lived Assets The Company reviews long-lived assets, including property and equipment, for impairment whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset over its respective fair value. The Company has not recognized any impairment losses during the years ended December 31, 2017 and 2018. Intellectual Property The legal and professional costs incurred by the Company to maintain its patent rights have been expensed as part of general and administrative expenses since inception. As of December 31, 2017 and 2018, the Company has determined that these expenses have not met the criteria to be capitalized. Intellectual property-related expenses for the years ended December 31, 2017 and 2018 were $203 and $453, respectively. Deferred Rent Deferred rent consists of the difference between cash payments and the recognition of rent expense on a straight-line basis for the facility the Company occupies. The Company’s lease for its facility provides for periods of discounted rent and fixed increases in minimum annual rental payments. The total amount of rental payments due over the lease term is being charged to rent expense ratably over the term of the lease. General and Administrative Expenses General and administrative expenses consist primarily of salaries and related costs, including stock-based compensation, for personnel in executive, finance and administrative functions. General and administrative expenses also include direct and allocated facility-related costs as well as professional fees for legal, consulting, accounting and audit services. Research and Development Expenses Research and development costs are expensed as incurred. These costs consist primarily of salaries and other personnel-related expenses, including stock-based compensation; facility-related expenses; depreciation of facilities and equipment; laboratory consumables; and services performed by clinical research organizations, research institutions, and other outside service providers. The Company records accrued liabilities for estimated costs of research and development activities conducted by third-party service providers, which include the conduct of preclinical studies and clinical trials, and contract manufacturing activities. The Company records the estimated costs of research and development activities based upon the estimated amount of services provided but not yet invoiced, and include these costs in accrued liabilities in the balance sheet and within research and development expense in the statement of operations. These costs are a significant component of the Company’s research and development expense. The Company accrues for these costs based on factors such as estimates of the work completed and in accordance with agreements established with its third-party service providers. The Company makes significant judgments and estimates in determining the accrued liabilities balance in each reporting period. As actual costs become known, the Company adjusts its accrued liabilities. The Company has not experienced any material differences between accrued costs and actual costs incurred. Income Taxes Deferred tax assets and liabilities are determined based on the differences between the financial reporting and tax basis of assets and liabilities using enacted tax rates which will be in effect when the differences reverse. The Company provides a valuation allowance against net deferred tax assets unless, based upon the available evidence, it is more likely than not that the deferred tax asset will be realized. The Company follows the provisions of the Income Taxes Topic of the Financial Accounting Standards Board (FASB) Accounting Standards Codification that defines a recognition threshold and measurement attributes for the financial statement recognition and measurement of a tax position taken or expected to be taken in a tax return. Under the Income Taxes Topic, the impact of an uncertain income tax position on the income tax return must be recognized at the largest amount that is more-likely-than-not Stock-Based Compensation For purposes of calculating stock-based compensation, the Company estimates the fair value of stock options issued using a Black-Scholes option-pricing model. The determination of the fair value of share-based payment awards utilizing the Black-Scholes model is affected by the Company’s stock price and a number of assumptions, including expected volatility, expected life, risk-free interest rate and expected dividends. Expected Term Expected Volatility Risk-Free Interest Rate Expected Dividend The estimated fair value of stock options granted to employees and non-employee Net Loss Per Share The Company computes basic loss per share by dividing the net loss available to common stockholders by the weighted average number of common shares outstanding for the period, without consideration for common stock equivalents. Diluted net loss assumes the conversion, exercise or issuance of all potential common stock equivalents, unless the effect of inclusion would be anti-dilutive. For purposes of this calculation, common stock equivalents include the Company’s stock options and convertible preferred stock, which is convertible into shares of the Company’s common stock. No shares related to the convertible preferred stock were included in the diluted net loss calculation for the years ended December 31, 2017 or 2018 because the inclusion of such shares would have had an anti-dilutive effect. The shares to be issued upon exercise of certain outstanding stock options were also excluded from the diluted net loss calculation for the years ended December 31, 2017 and 2018 because such shares are anti-dilutive. Historical outstanding anti-dilutive securities not included in the diluted net loss per share calculation include the following: Year Ended December 31, 2017 2018 Convertible preferred stock (as converted) 10,165,120 16,993,194 Common stock options 584,019 3,597,638 Total 10,749,139 20,590,832 Recent Accounting Pronouncements In February 2016, the FASB issued Accounting Standards Update (ASU) No. 2016-02, “ Leases 2016-02 right-of-use right-to-use 2016-02 2016-02 2016-02 2016-02 Recently Adopted Accounting Standards Updates In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers 2014-09 In June 2018, the FASB issued Accounting Standards Update No. 2018-07, Compensation—Stock Compensation non-employees non-employee 2018-07

Marketable Securities, Availabl

Marketable Securities, Available-for-Sale	6 Months Ended
	Jun. 30, 2019
Investments, Debt and Equity Securities [Abstract]
Marketable Securities, Available-for-Sale	3. Marketable Securities, Available-for-Sale The Company invests its excess cash in marketable securities, including debt instruments of financial institutions, corporations with investment grade credit ratings, commercial paper and government agencies. Marketable securities, available-for-sale, June 30, 2019 Maturity in Years Amortized Cost Unrealized Fair Value Gains Losses U.S. government agency securities Less than 1 $ 2,750 $ 1 $ — $ 2,751 Corporate debt securities Less than 1 97,756 194 — 97,950 Commercial Paper Less than 1 70,538 150 — 70,688 Total marketable securities $ 171,044 $ 345 $ — $ 171,389 The Company did not hold marketable securities as of December 31, 2018. Gross realized gains and losses on available-for-sale None of the investments have been in a gross unrealized loss for a period greater than 12 months. At each reporting date, we perform an evaluation of impairment to determine if any unrealized losses are other-than-temporary. Factors considered in determining whether a loss is other-than-temporary include the length of time and extent to which fair value has been less than the cost basis, the financial condition of the issuer, and our intent and ability to hold the investment until recovery of the amortized cost basis. We intend and have the ability to hold our investments in unrealized loss positions until their amortized cost basis has been recovered. Further, based on our evaluation, there were no unrealized losses at June 30, 2019.

Fair Value Measurements

Fair Value Measurements	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Fair Value Measurements	4. Fair Value Measurements 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 at the measurement date. Fair value should maximize the use of observable inputs and minimize the use of unobservable inputs. The Company determines the fair value of financial assets and liabilities using three levels of inputs as follows: Level 1 – Inputs which include quoted prices in active markets for identical assets or liabilities at the measurement date. Level 2 – Inputs (other than quoted market prices included in Level 1) that are either directly or indirectly observable, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the instrument’s anticipated life. Level 3 – Unobservable inputs for assets or liabilities and include little or no market activity. A financial instrument’s categorization within the valuation hierarchy is based upon the lowest level of input that is significant to the fair value measurement. During the three and six months ended June 30, 2019, the Company had no Level 3 financial assets or liabilities that were subject to fair value measurements on a recurring basis. During the three and six months ended June 30, 2018, the Company had no Level 2 or 3 financial assets or liabilities that were subject to fair value measurements on a recurring basis. The Company’s financial assets subject to fair value measurements on a recurring basis and the level of inputs used for such measurements were as follows (in thousands): Fair Value Measurements at June 30, 2019 Using: Level 1 Level 2 Level 3 Total Money market funds included in cash and cash equivalents $ 73,794 $ — $ — $ 73,794 U.S. government agency securities — 2,751 — 2,751 Corporate debt securities — 97,950 — 97,950 Commercial Paper — 70,688 — 70,688 Total $ 73,794 $ 171,389 $ — $ 245,183 Fair Value Measurements at December 31, 2018 Using: Level 1 Level 2 Level 3 Total Money market funds included in cash and cash equivalents $ 98,268 $ — $ — $ 98,268	3. Fair Value Measurements 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 at the measurement date. Fair value should maximize the use of observable inputs and minimize the use of unobservable inputs. The Company determines the fair value of financial assets and liabilities using three levels of inputs as follows: Level 1—Inputs which include quoted prices in active markets for identical assets or liabilities at the measurement date. Level 2—Inputs (other than quoted market prices included in Level 1) that are either directly or indirectly observable, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the instrument’s anticipated life. Level 3—Unobservable inputs for assets or liabilities and include little or no market activity. A financial instrument’s categorization within the valuation hierarchy is based upon the lowest level of input that is significant to the fair value measurement. During the years ended December 31, 2017 and 2018, the Company had no Level 2 or Level 3 financial assets or liabilities that were subject to fair value measurements on a recurring basis. The Company’s financial assets subject to fair value measurements on a recurring basis and the level of inputs used for such measurements were as follows: Fair Value Measurements at December 31, 2017 Using: Level 1 Level 2 Level 3 Total Money market funds included in cash and cash equivalents $ 40,921 $ — $ — $ 40,921 Fair Value Measurements at December 31, 2018 Using: Level 1 Level 2 Level 3 Total Money market funds included in cash and cash equivalents $ 98,268 $ — $ — $ 98,268

Property and Equipment, Net

Property and Equipment, Net	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
Property and Equipment, Net	5. Property and Equipment, Net Property and equipment, net consisted of the following (in thousands): June 30, December 31, Laboratory equipment $ 508 $ 388 Computer equipment and software 69 138 Tenant improvements 677 679 Furniture and fixtures 32 66 1,286 1,271 Less: accumulated depreciation (332 ) (271 ) $ 954 $ 1,000 Depreciation expense was $0.1 million and $19,000 for the three months ended June 30, 2019 and 2018, respectively and depreciation expense was $0.2 million and $39,000, six months ended June 30, 2019 and 2018, respectively.	4. Property and Equipment, Net Property and equipment, net consisted of the following: December 31 2017 2018 Laboratory equipment $ 260 $ 388 Computer equipment and software 57 138 Tenant improvements 25 679 Furniture and fixtures 50 66 392 1,271 Less: accumulated depreciation (136 ) (271 ) $ 256 $ 1,000 Depreciation expense for the years ended December 31, 2017 and 2018 was $52 and $138, respectively.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Accrued Expenses and Other Current Liabilities	6. Accrued Expenses and Other Current Liabilities Accrued expenses and other current liabilities consist of the following (in thousands): June 30, December 31, Accrued research and development expenses $ 1,739 $ 1,677 Accrued general and administrative expenses 208 548 Other current liabilities 45 190 Total $ 1,992 $ 2,415	5. Accrued Expenses and Other Current Liabilities Accrued expenses and other current liabilities consisted of the following: December 31, 2017 2018 Accrued research and development expenses $ 2,615 $ 1,677 Accrued general and administrative expenses 110 548 Other current liabilities 25 190 Total $ 2,750 $ 2,415

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	7. Commitments and Contingencies Operating Leases The Company entered into a lease agreement during January 2016, which commenced in July 2016, for its current office and primary research facility located in San Diego, California. In June 2018, the Company amended its existing lease agreement to expand its office and laboratory space within the same building, which the Company occupied commencing September 2018. The amended lease term for all leased premises had an expiration date of December 31, 2021, and an option to extend the lease term on all leased space for one additional five-year term. As of the date of adoption of ASC 842, the Company was not reasonably certain that it would exercise the extension option, and as such, did not include this option in the determination of the total lease term. The lease includes both discounts of certain base rents during 2016 and 2019, and escalating lease payments over the term. On January 1, 2019, in conjunction with the adoption of the guidance in ASU 2016-02—“ Leases”, right-of-use right-of-use right-of-use In June 2019, the Company amended the terms of its existing facility lease and in conjunction with entering into a lease for additional office and laboratory space, agreed to surrender a portion of its current laboratory and office space and extend the lease term for its remaining laboratory and office space to June 30, 2023. The execution of the new lease and the amendment to the Company’s existing facility lease were accounted for as a single contract for accounting purposes, as the counterparty to both contracts is the Company’s existing landlord and both agreements were negotiated contemporaneously as a package to achieve the same commercial objective. In June 2019, the Company accounted for the partial surrender of office and laboratory space as a reduction to its existing right-of-use In addition, in June 2019, and in connection with the extension of the lease term of our previously existing office and laboratory space, the Company recognized an incremental increase to its existing right of use asset and lease liability of $0.5 million, respectively. The adjustment was computed assuming a lease term ending in June 2023 and an estimated incremental borrowing rate of 8.5%. This right-of-use The commencement date for the new lease is July 2019 and it has a lease expiration date of June 30, 2023. In addition to base rental payments under this lease, which escalate over the term of the lease, the Company will also be responsible for the payment of its share of the estimated annual operating expenses, property tax expenses, and utilities costs related to this lease of additional space. The lease also contains an option to extend the lease term on all leased space for one additional five-year term. As of June 30, 2019, the Company was not reasonably certain that it would exercise the extension option, and as such, will not include this option in the determination of the total lease term for accounting purposes. The Company expects to record a right-to-use right-of-use right-of-use Future minimum payments under the amended lease as of June 30, 2019 are as follows (in thousands): 2019 (Six months remaining) $ 155 2020 457 2021 471 2022 486 2023 247 Total future minimum lease payments 1,816 Less: Amounts representing interest (296 ) Total lease liability $ 1,520 Remaining lease term 4.0 years Rent expense was $0.1 million for the three months ended June 30, 2019 and 2018, respectively and was $0.3 million and $0.2 million for each of the six months ended June 30, 2019 and 2018, respectively. The Company made cash payments related to its operating lease agreement of $0.2 million and $0.1 million for the three months ended June 30, 2019 and 2018, respectively and $0.4 million and $0.2 million for the six months ended June 30, 2019 and 2018, respectively. The Company expects to pay $0.7 million in 2019, $1.6 million in 2020, $1.7 million in 2021, $1.7 million in 2022, and $0.9 million in 2023, which includes lease payments for the new office space.	6. Commitments and Contingencies Operating Leases The Company entered into a lease agreement during January 2016, which commenced in July 2016, for its current office and primary research facility located in San Diego, California. The lease includes both discounts of certain base rents during 2016 and 2019, and escalating lease payments over the term, which are being expensed ratably over the extended lease term. In June 2018, the Company amended its existing lease agreement to include an additional 9,302 square feet of office and laboratory space within the same building as its current office and laboratory space. The additional space has the same base rental rate per square foot and expiration date of December 31, 2021 as the original facility per the existing lease with an option to extend the lease for the entire premises through the expiration of the initial terms of the master lease. The Company was obligated to, and has provided, a letter of credit of $73 for the lease which has been recorded to security deposits on the December 31, 2018 balance sheet. The Company occupied the additional space in September 2018. Rental expense, for the years ended December 31, 2017 and 2018 was $389 and $489, respectively. As of December 31, 2018, future minimum lease payments under the operating lease (assuming the Company does not elect the early cancellation option on this lease), are as follows: 2019 $ 792 2020 887 2021 914 $ 2,593

Related Party Transactions

Related Party Transactions	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Related Party Transactions [Abstract]
Related Party Transactions	8. Related Party Transactions The Company has entered into several contracts for drug product development and manufacturing with a vendor for which one of the Company’s former directors is a co-founder	7. Related Party Transactions The Company entered into several contracts for drug product development and manufacturing with a vendor for which one of the Company’s former directors is a co-founder On November 14, 2017, the Company entered into a consulting agreement with a member of the board of directors. The term of the agreement commenced on November 14, 2017 and continues for a period of four years. As compensation for the board member’s services, the Company granted the board member an option to purchase 112,861 shares of common stock, with such option grant vesting monthly over a four-year period, and agreed to reimburse the board member for certain preapproved expenses at cost.

Equity

Equity	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Equity [Abstract]
Equity	9. Equity The Company’s 2019 Equity Incentive Plan as amended (the Plan), provides for the grant of stock options, restricted stock and other equity awards of the Company’s common stock to employees, officers, consultants, and directors. As of June 30, 2019, the Plan had a maximum of 3,404,401 total shares available for issuance. Options expire within a period of not more than ten years from the date of grant. Initial option grants to employees typically vest 25% after one year and monthly thereafter over a three-year period and expire between one and three months after employee termination. Subsequent option grants to employees and grants to non-employees The following summarizes option activity under the Plan: Outstanding Options Outstanding Options Weighted Average Exercise Price Per Share Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (Years) (in thousands) Balances as of December 31, 2018 3,597,638 $ 4.40 9.5 $ 15,056 Options granted 1,472,323 $ 14.47 Options exercised (255,213 ) $ 2.40 Options cancelled (223,944 ) $ 4.72 Balances as of June 30, 2019 4,590,804 $ 7.73 9.1 151,371 Options vested and exercisable as of June 30, 2019 578,836 $ 3.80 7.2 $ 21,360 The weighted-average grant-date fair value of options granted to employees was $15.62 and $3.50 for the three months ended June 30, 2019 and 2018, respectively and was $14.65 and $3.64, for the six months ended June 30, 2019 and 2018, respectively. Stock-Based Compensation Expense The weighted-average assumptions used in the Black-Scholes option pricing model to determine the fair value of the employee and nonemployee stock option grants were as follows: Expected Term Expected Volatility Risk-Free Interest Rate Expected Dividend The fair values of the employee stock options granted under the Plan were estimated using the following assumptions: Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 Risk-free interest rate 2.29 % 2.79 % 2.42 % 2.79 % Volatility 83.1 % 82.5 % 80.6 % 82.5 % Expected term (in years) 6.00 6.07 6.04 6.04 Dividend yield — % — % — % — % Stock-based compensation expense, net of forfeitures, is reflected in the statements of operations and comprehensive loss as follows (in thousands): Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 Research and development $ 1,292 $ 76 $ 2,281 $ 117 General and administrative 1,767 61 2,704 105 Total stock-based compensation $ 3,059 $ 137 $ 4,985 $ 222 As of June 30, 2019, there was $34.0 million in total unrecognized compensation expense to be recognized over a weighted average period of 3.22 years. Common Stock Reserved for Future Issuance Common stock reserved for future issuance consists of the following: June 30, December 31, Conversion of preferred stock outstanding — 16,993,194 Common stock options outstanding 4,590,804 3,597,638 Shares available for issuance under the Plan 3,404,401 1,816,266 Total 7,995,205 22,407,098	9. Stockholders’ Equity Stock Option Plan The Company’s 2013 Equity Incentive Plan, as amended (the Plan), provides for the grant of stock options, restricted stock and other equity awards of the Company’s common stock to employees, officers, consultants, and directors. The Plan originally included an aggregate amount of up to a maximum of 389,610 shares. During 2017, the Plan was amended twice to increase the aggregate maximum to 1,558,442 total shares available for issuance. Concurrent with the Series D convertible preferred financing in October 2018, the plan was increased to a maximum 5,500,788 total shares available for issuance. Options expire within a period of not more than ten years from the date of grant. Initial option grants to employees typically vest 25% after one year and monthly thereafter over a three-year period and expire between one and six months after employee termination. Subsequent option grants to employees and grants to non-employees Outstanding Options Shares Weighted Weighted Aggregate (Years) Balances as of December 31, 2017 584,019 $ 1.92 9.0 $ 743 Options granted 3,099,984 $ 4.81 Options exercised (43,774 ) $ 1.74 Options cancelled (42,591 ) $ 3.20 Balances as of December 31, 2018 3,597,638 $ 4.40 9.5 $ 15,056 Options vested and exercisable as of December 31, 2018 428,481 $ 2.53 8.1 $ 2,596 The aggregate intrinsic values of options outstanding, vested and exercisable were calculated as the difference between the exercise price of the options and the estimated fair value of the Company’s common stock of $8.59 per share as of December 31, 2018. The weighted-average grant-date fair value of options granted to employees during the years ended December 31, 2017 and 2018 was $2.08 and $6.23, respectively. As of December 31, 2018, the total compensation expense related to non-vested The fair values of the employee stock options granted under the Plan during 2017 and 2018 were estimated at the date of grant using the Black-Scholes option-pricing model with the following assumptions: Year Ended December 31, 2017 2018 Risk-free interest rate 1.94 - 2.22 % 2.61 - 3.10 % Volatility 91.3 % 80.4 - 82.5 % Expected term (in years) 5.00 -6.08 5.77 - 6.08 Dividend yield — % — % The Company computed volatility to be used based on an index of publicly traded comparable companies. The expected term of the options is based on the average period the stock options are expected to remain outstanding calculated as the midpoint between the weighted-average vesting term and the contractual expiration period, as the Company does not have sufficient historical information to develop reasonable expectations about future exercise patterns and post-vesting employment termination behavior. The risk-free interest rate is based on U.S. Treasury instruments with terms consistent with the expected terms of the stock options, as determined at the time of grant. Stock-based compensation expense, net of forfeitures, is reflected in the statements of operations and comprehensive loss as follows: Year Ended December 31, 2017 2018 Research and development $ 160 $ 556 General and administrative 184 591 Total stock-based compensation $ 344 $ 1,147 Stock-based compensation expense for non-employees Common Stock Reserved for Future Issuance: Common stock reserved for future issuance consists of the following: December 31, 2017 2018 Conversion of preferred stock outstanding 10,165,120 16,993,194 Common stock options outstanding 584,019 3,597,638 Shares available for issuance under the Plan 931,336 1,816,266 Total 11,680,475 22,407,098

Convertible Preferred Stock

Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Convertible Preferred Stock	8. Convertible Preferred Stock In May 2017, the Company entered into a Series C convertible preferred stock purchase agreement, pursuant to which the Company sold 19,416,645 shares of Series C convertible preferred stock at a price of $2.3176 per share for proceeds of $44,788, net of issuance costs of $212. In October 2018, the Company entered into a Series D convertible preferred stock purchase agreement, pursuant to which the Company sold 26,288,123 shares of Series D convertible preferred stock at a price of $3.0432 per share for proceeds of $79,755, net of issuance costs of $245. The authorized, issued, and outstanding shares of convertible preferred stock at December 31, 2017 and December 31, 2018 were as follows: December 31, 2017 Series Shares Aggregate Carrying Series A convertible preferred stock 7,404,248 $ 3,480 $ 3,455 Series B convertible preferred stock 12,314,885 17,980 17,918 Series C convertible preferred stock 19,416,645 45,000 44,788 Total 39,135,778 $ 66,460 $ 66,161 December 31, 2018 Series Shares Aggregate Carrying Series A convertible preferred stock 7,404,248 $ 3,480 $ 3,455 Series B convertible preferred stock 12,314,885 17,980 17,918 Series C convertible preferred stock 19,416,645 45,000 44,788 Series D convertible preferred stock 26,288,123 80,000 79,755 Total 65,423,901 $ 146,460 $ 145,916 Dividends Holders of Series D and Series C convertible preferred stock, on a pari passu basis and in preference to the holders of Series B convertible preferred stock, Series A convertible preferred stock, and common stock, are entitled to receive, but only out of funds legally available therefore, cash dividends at a rate of 5% of the original issuance price of $3.0432 and $2.3176 of the Series D and Series C convertible preferred stock, respectively, per year on each outstanding share of Series D and Series C convertible preferred stock, as applicable, only when and if declared by the Board of Directors of the Company. After payment of declared dividends on the Series D and Series C convertible preferred stock, holders of the Series B convertible preferred stock, in preference to the holders of the Series A convertible preferred stock and common stock, are entitled to receive cash dividends at a rate of 5% of the Series B convertible preferred stock original issuance price of $1.46 per year on each outstanding share of Series B convertible stock, only when and if declared by the Board of Directors of the Company. After payment of declared dividends on the Series B convertible preferred stock, the holders of the Series A convertible preferred stock in preference to the holders of common stock, are entitled to receive cash dividends at a rate of 5% of the Series A convertible preferred stock original issuance price of $0.47 per year on each outstanding share of Series A convertible preferred stock, only when and if declared by the Board of Directors of the Company. All such dividends are non-cumulative. Voting Each share of convertible preferred stock has voting rights equal to the number of shares of common stock into which it is convertible. Election of Board of Directors The holders of the Series A and Series B convertible preferred stock, voting together as a single class, are entitled to elect one member of the Board of Directors of the Company. The holders of the Series C convertible preferred stock, voting as a separate class, are entitled to elect three members to the Board of Directors of the Company. The holders of the Series D convertible preferred stock, voting as a separate class, are entitled to elect two members to the Board of Directors of the Company. Liquidation Preference In the event of a liquidation, dissolution, or winding up of the Company, whether voluntary or involuntary, and including a merger, acquisition or sale of assets where there is a change in control, before any distribution or payment is made to the holders of any Series B convertible preferred stock, Series A convertible preferred stock or common stock, the holders of Series C convertible preferred stock and Series D convertible preferred stock, on a pari passu basis, shall be entitled to be paid out of the assets of the Company legally available for distribution (or the consideration received by the Company or its stockholders in an Acquisition) for each share of Series C convertible preferred stock or Series D convertible preferred stock held by them, as applicable, an amount per share of Series C convertible preferred stock equal to $2.3176 plus all declared and unpaid dividends on the Series C convertible preferred stock, if any, and an amount per share of Series D convertible preferred stock equal to $3.0432 plus all declared and unpaid dividends on the Series D convertible preferred stock, if any. If, upon any such Liquidation Event, the assets of the Company (or the consideration received in such transaction) shall be insufficient to make payment in full to all holders of Series C convertible preferred stock and Series D convertible preferred stock of the liquidation preference, then such assets (or consideration) shall be distributed among the holders of Series C convertible preferred stock and Series D convertible preferred stock at the time outstanding, ratably in proportion to the full amounts to which they would otherwise be respectively entitled. After the Series D and Series C liquidation preferences have been paid, the holders Series of B convertible preferred stock shall be entitled to receive, prior and in preference to any distribution of any assets of the Company to the holders of the Series A convertible preferred stock or common stock, an amount per share equal to $1.46 per share plus all declared and unpaid dividends. If, upon a liquidation event, the assets of the Company or consideration received in such a transaction, are insufficient to make payment in full to all holders of Series B convertible preferred stock of the Series B liquidation preference, then such assets or consideration shall be distributed among the holders of the Series B convertible preferred stock ratably in proportion to the full amounts to which they would have otherwise be entitled. After the Series D, Series C, and Series B liquidation preferences have been paid, the holders of Series A convertible preferred stock shall be entitled to receive, prior and in preference to any distribution of any assets of the Company to the holders of common stock, an amount per share equal to $0.47 per share plus all declared and unpaid dividends. If, upon a liquidation event, the assets of the Company or consideration received in such a transaction, are insufficient to make payment in full to all holders of Series A convertible preferred stock of the Series A liquidation preference, then such assets or consideration shall be distributed among the holders of the Series A preferred stock ratably in proportion to the full amounts to which they would have otherwise be entitled. Thereafter, the remaining assets of the Company legally available for distribution in such liquidation event (or the consideration received by the Company or its stockholders in an acquisition), if any, shall be distributed among the holders of Series B convertible preferred stock and common stock on an if-converted In the event of a liquidation event (including such events as an acquisition, asset transfer, or insolvency), if the Company does not effect a dissolution within 90 days after the liquidation event, the Company shall advise each holder of its convertible preferred stock of their right to require redemption of such shares. Upon a majority vote by each class of preferred stock (which majority, in the case of the Series D convertible preferred stock must include holders of at least 65% of the Series D convertible stock, in the case of the Series C convertible preferred stock must include holders of at least 60% of the Series C convertible preferred stock, and in the case of the Series B convertible preferred stock must include holders of at least a majority of the Series B convertible preferred stock who do not also hold Series A convertible preferred stock), the holders can compel the redemption of each class of preferred stock at the respective liquidation preference for each class of preferred stock. Conversion Each share of convertible preferred stock is immediately convertible at the holder’s option into shares of common stock based on the original issuance price of $0.47 for the Series A convertible preferred stock, $1.46 for the Series B convertible preferred stock, $2.3176 for the Series C convertible preferred stock, and $3.0432 for the Series D convertible preferred stock divided by the Conversion Price. The Conversion Price is $1.8095 for shares of Series A convertible preferred stock, $5.621 for shares of Series B convertible preferred stock, $8.9228 for shares of Series C convertible preferred stock, and $11.7163 for shares of Series D convertible preferred stock subject to proportional adjustments for certain dilutive issuances, splits, combinations, and other recapitalizations or reorganizations. Conversion of the preferred stock is automatic in the event of a public offering of the Company’s common stock meeting certain specified criteria (pursuant to which the pre-money as-converted If the Company issues any additional stock at a price per share that is less than the Conversion Price then in effect, the Conversion Price for the Series A, Series B, Series C, and Series D convertible preferred stock shall be reduced based on a weighted average formula. The Company recorded all convertible preferred stock issuances at fair value on the dates of issuance. The Company classifies the convertible preferred stock outside of stockholders’ deficit in temporary equity because the shares contain contingent liquidation features that are not solely within its control. During the years ended December 31, 2017 and 2018, the Company did not adjust the carrying values of the convertible preferred stock to the deemed redemption values of such shares since a liquidation event was not probable. Subsequent adjustments to increase the carrying values to the ultimate redemption values will be made only when it becomes probable that such a liquidation event will occur.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	11. Income Taxes No provision for federal or state income taxes has been recorded for the years ended December 31, 2017 and 2018 other than the $1 annual tax for C corporations paid to the state of California. A reconciliation of the federal statutory income tax rate and the Company’s effective income tax rate is as follows: December 31, 2017 2018 Tax computed at federal statutory rate 34.00 % 21.00 % Permanent items and other (9.49 ) (2.36 ) Stock based compensation (0.27 ) (0.78 ) Research and development tax credits (1.19 ) 1.07 Orphan Drug tax credits 13.13 6.47 Tax Cuts and Jobs Act (16.41 ) 0.00 Valuation allowance (19.77 ) (25.40 ) Effective income tax rate — % — % Deferred tax assets and liabilities consist of the following: December 31, 2017 2018 Deferred tax assets: Net operating loss carryforwards $ 4,303 $ 8,474 Research and development credits 86 353 Orphan Drug Credit 2,178 3,781 Accrued liabilities 82 287 Other, net 71 115 Total deferred tax assets 6,720 13,010 Deferred tax liabilities Fixed assets (13 ) (7 ) (13 ) (7 ) Total 6,707 13,003 Less: valuation allowance (6,707 ) (13,003 ) Net deferred tax assets $ — $ — The valuation allowance increased by $6,296 during the year ended December 31, 2018. Due to the uncertainties surrounding the realization of deferred tax assets, the Company has provided a full valuation allowance and, therefore, no benefit has been recognized for the net operating loss carryforwards and other deferred tax assets. At December 31, 2018, the Company has federal and state net operating loss carryforwards of approximately $40,350 and $47,187, respectively. The federal and state tax loss carryforwards will begin to expire in 2033 if not utilized. At December 31, 2018, the Company has federal and state research and development tax credits of approximately $257 and $578, respectively. At December 31, 2018, the Company has federal Orphan Drug tax credits of approximately $6,682. If not utilized, the federal research tax credit will begin to expire in 2035 and the Orphan Drug credit will begin to expire in 2037. The California research tax credit can be carried forward indefinitely. The Company files income tax returns in the United States federal jurisdiction and in California, and the tax returns filed for the years 2014 through 2017 have not been examined and the applicable statutes of limitation have not expired with respect to those returns. Because of net operating loss and research credit carryovers, substantially all of the Company’s tax years remain open to examination. Total unrecognized income tax benefits related to California net operating losses, federal and California research and development and federal orphan drug tax credit carryforwards were approximately $7,488 at December 31, 2018. The Company’s policy is to include interest and penalties related to unrecognized tax benefits within the Company’s provision for income taxes. As of December 31, 2018, the Company has no accrual for interest and penalties related to unrecognized tax benefits. There are no unrecognized tax benefits that, if recognized, would impact the Company’s effective tax rate due to valuation allowances. Pursuant to Internal Revenue Code sections 382 and 383, use of the Company’s net operating loss and credit carryforwards may be limited if a cumulative change in ownership of more than 50% occurs within a three-year period. The Company has not completed an IRC Section 382/383 analysis regarding the limitation of net operating loss and research and development credit carryforwards, and these financial statements do not contain any adjustment relating to such potential limitations. The Tax Cuts and Jobs Act was enacted on December 22, 2017. The Act reduces the US federal corporate tax rate from 34% to 21%. The reduction in the rate caused and resulted in a remeasurement of the Company’s deferred tax assets and liabilities at December 31, 2017. The remeasurement resulted in an income tax expense of $2,723, which was offset by a corresponding reduction in the valuation allowance. As of December 31, 2018, the Company has completed its accounting of the tax effects from the enactment of the Act and no changes to provisional amounts were recorded. The Company recognizes a tax benefit from an uncertain tax position when it is more likely than not that the position will be sustained upon examination, including resolutions of any related appeals or litigation processes, based on the technical merits. Income tax positions must meet a more likely than not recognition at the effective date to be recognized. At December 31, 2017 and 2018, the unrecognized tax benefits recorded were approximately $4,733 and $7,488, respectively. Approximately $6,557 of the unrecognized tax benefits would reduce the annual effective tax rate, if recognized, subject to the valuation allowance. It is not anticipated that there will be significant change in the unrecognized tax benefits over the next 12 months. A reconciliation of the beginning and ending amount of gross unrecognized tax benefits is as follows: December 31, 2017 2018 Beginning balance $ 954 $ 4,733 Reductions for tax positions taken in prior years (34 ) (118 ) Additions for tax positions taken in current year 3,813 2,873 Ending balance $ 4,733 $ 7,488 The Company files tax returns as prescribed by the tax laws of the jurisdictions in which it operates. In the normal course of business, the Company is subject to examination by the United States and state jurisdictions where applicable. There are currently no pending income tax examinations. The Company’s tax years from inception in 2013 are subject to examination by the federal and state tax authorities due to the carryforward of unutilized net operating losses and research and development credits. The Company’s practice is to recognize interest and penalties related to income tax matters in income tax expense. The Company has not recognized interest or penalties since inception.

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2018
Subsequent Events [Abstract]
Subsequent Events	12. Subsequent Events For purposes of the financial statements as of and for the year ended December 31, 2018, the Company evaluated subsequent events for recognition and measurement purposes through March 21, 2019, the date the financial statements were issued. The Company further evaluated subsequent events for disclosure purposes in these financial statements as of and for the year ended December 31, 2018, through April 16, 2019. Except as described below, the Company has concluded that no events or transactions have occurred that require disclosure. February 2019 Stock Option Grants On February 20, 2019, the Company granted options to purchase 757,031 shares of common stock to employees of the Company. One-fourth one-year Reverse Stock Split On April 5, 2019, the Company effected a 1-for-3.85

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Marketable securities, Available-for-Sale	Marketable securities, Available-for-Sale The Company classifies all marketable securities as available-for-sale, available-for-sale
Concentration of Credit Risk	Concentration of Credit Risk Substantially all the Company’s cash and money market funds are held with a single financial institution. Due to its size, the Company believes this financial institution represents a minimal credit risk. Cash amounts held at financial institutions are insured by the Federal Deposit Insurance Corporation (“FDIC”) up to $250,000. At June 30, 2019, the Company had $249.9 million in excess of the FDIC insured limit. At June 30, 2019, the Company’s money market funds and marketable securities, available for sale are not subject to FDIC insurance. The Company’s money market funds and short-term investments are invested in short term, high grade securities. As a result, the Company believes its money market and marketable securities, available for sale represent a minimal credit risk.	Concentration of Credit Risk Substantially all of the Company’s cash and money market funds are held with a single financial institution. Due to its size, the Company believes this financial institution represents a minimal credit risk. Cash amounts held at financial institutions are insured by the Federal Deposit Insurance Corporation (“FDIC”) up to $250. At December 31, 2018, the Company had $2,261 in excess of the FDIC insured limit. At December 31, 2018, the Company’s money market funds totaling $98,268 are not subject to FDIC insurance. The Company’s money market funds are invested in short term, high grade U.S. Treasuries. As a result, the Company believes its money market funds represent a minimal credit risk.
Property and Equipment	Property and Equipment Property and equipment are stated at cost less accumulated depreciation. Depreciation is calculated using the straight-line method over the estimated useful life of the assets, which ranges between three to seven years. Tenant improvements are stated at cost and depreciated over the shorter of the estimated useful life or the remaining life of the lease at the time the asset is placed into service.	Property and Equipment Property and equipment are stated at cost less accumulated depreciation. Depreciation is calculated using the straight-line method over the estimated useful life of the assets, which ranges between three to seven years. Tenant improvements are stated at cost and depreciated over the shorter of the estimated useful life or the remaining life of the lease at the time the asset is placed into service.
Intellectual Property	Intellectual Property The legal and professional costs incurred by the Company to maintain its patent rights have been expensed as part of general and administrative expenses since inception. As of June 30, 2019, and 2018, the Company has determined that these expenses have not met the criteria to be capitalized. Intellectual property related expenses were $0.1 million for the three months ended June 30, 2019 and 2018, respectively and were $0.3 million for the six months ended June 30, 2019 and 2018, respectively.	Intellectual Property The legal and professional costs incurred by the Company to maintain its patent rights have been expensed as part of general and administrative expenses since inception. As of December 31, 2017 and 2018, the Company has determined that these expenses have not met the criteria to be capitalized. Intellectual property-related expenses for the years ended December 31, 2017 and 2018 were $203 and $453, respectively.
Research and Development Expenses	Research and Development Expenses Research and development costs are expensed as incurred. These costs consist primarily of salaries and other personnel-related expenses, including stock-based compensation; facility-related expenses; depreciation of facilities and equipment; laboratory consumables; and services performed by clinical research organizations, clinical manufacturing services, research institutions, and other outside service providers	Research and Development Expenses Research and development costs are expensed as incurred. These costs consist primarily of salaries and other personnel-related expenses, including stock-based compensation; facility-related expenses; depreciation of facilities and equipment; laboratory consumables; and services performed by clinical research organizations, research institutions, and other outside service providers. The Company records accrued liabilities for estimated costs of research and development activities conducted by third-party service providers, which include the conduct of preclinical studies and clinical trials, and contract manufacturing activities. The Company records the estimated costs of research and development activities based upon the estimated amount of services provided but not yet invoiced, and include these costs in accrued liabilities in the balance sheet and within research and development expense in the statement of operations. These costs are a significant component of the Company’s research and development expense. The Company accrues for these costs based on factors such as estimates of the work completed and in accordance with agreements established with its third-party service providers. The Company makes significant judgments and estimates in determining the accrued liabilities balance in each reporting period. As actual costs become known, the Company adjusts its accrued liabilities. The Company has not experienced any material differences between accrued costs and actual costs incurred.
Income Taxes	Income Taxes Deferred tax assets and liabilities are determined based on the differences between the financial reporting and tax basis of assets and liabilities using enacted tax rates which will be in effect when the differences reverse. The Company provides a valuation allowance against net deferred tax assets unless, based upon the available evidence, it is more likely than not that the deferred tax asset will be realized. The Company follows the provisions of the Income Taxes Topic of the Financial Accounting Standards Board (FASB) Accounting Standards Codification that defines a recognition threshold and measurement attributes for the financial statement recognition and measurement of a tax position taken or expected to be taken in a tax return. Under the Income Taxes Topic, the impact of an uncertain income tax position on the income tax return must be recognized at the largest amount that is more-likely-than-not to	Income Taxes Deferred tax assets and liabilities are determined based on the differences between the financial reporting and tax basis of assets and liabilities using enacted tax rates which will be in effect when the differences reverse. The Company provides a valuation allowance against net deferred tax assets unless, based upon the available evidence, it is more likely than not that the deferred tax asset will be realized. The Company follows the provisions of the Income Taxes Topic of the Financial Accounting Standards Board (FASB) Accounting Standards Codification that defines a recognition threshold and measurement attributes for the financial statement recognition and measurement of a tax position taken or expected to be taken in a tax return. Under the Income Taxes Topic, the impact of an uncertain income tax position on the income tax return must be recognized at the largest amount that is more-likely-than-not
Stock-Based Compensation	Stock-Based Compensation Stock-based compensation expense represents the cost of the grant date fair value of employee and non-employee
Net Loss Per Share	Net Loss Per Share The Company computes basic loss per share by dividing the net loss available to common stockholders by the weighted average number of common shares outstanding for the period, without consideration for common stock equivalents. Diluted net loss assumes the conversion, exercise or issuance of all potential common stock equivalents, unless the effect of inclusion would be anti-dilutive. For purposes of this calculation, common stock equivalents include the Company’s stock options and convertible preferred stock, which is convertible into shares of the Company’s common stock. No shares related to the convertible preferred stock were included in the diluted net loss calculation for the three and six months ended June 30, 2019 or 2018 because the inclusion of such shares would have had an anti-dilutive effect. The shares to be issued upon exercise of certain outstanding stock options were also excluded from the diluted net loss calculation for the three and six months ended June 30, 2019 and 2018 because such shares are anti-dilutive.	Net Loss Per Share The Company computes basic loss per share by dividing the net loss available to common stockholders by the weighted average number of common shares outstanding for the period, without consideration for common stock equivalents. Diluted net loss assumes the conversion, exercise or issuance of all potential common stock equivalents, unless the effect of inclusion would be anti-dilutive. For purposes of this calculation, common stock equivalents include the Company’s stock options and convertible preferred stock, which is convertible into shares of the Company’s common stock. No shares related to the convertible preferred stock were included in the diluted net loss calculation for the years ended December 31, 2017 or 2018 because the inclusion of such shares would have had an anti-dilutive effect. The shares to be issued upon exercise of certain outstanding stock options were also excluded from the diluted net loss calculation for the years ended December 31, 2017 and 2018 because such shares are anti-dilutive.
Recent Accounting Pronouncements and Recently Adopted Accounting Standards Updates	Recent Accounting Pronouncements Not Yet Adopted In June 2016, the FASB issued Accounting Standards Update (ASU) No. 2016-13, Financial Instruments—Credit Losses available-for-sale In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement: Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement Recently Adopted Accounting Standards Updates In February 2016, the FASB issued ASU No. 2016-02, Leases 2016-02 right-of-use 2016-02 non-lease The Company currently has one operating lease for office and laboratory spaces in San Diego, California. The operating lease was impacted by the new accounting standard and resulted in the present values of the future lease payments being presented as a right-to-use	Recent Accounting Pronouncements In February 2016, the FASB issued Accounting Standards Update (ASU) No. 2016-02, “ Leases 2016-02 right-of-use right-to-use 2016-02 2016-02 2016-02 2016-02 Recently Adopted Accounting Standards Updates In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers 2014-09 In June 2018, the FASB issued Accounting Standards Update No. 2018-07, Compensation—Stock Compensation non-employees non-employee 2018-07
Use of Estimates		Use of Estimates The preparation of financial statements in conformity with accounting principles generally accepted in the United States (U.S. GAAP) requires management to make estimates and assumptions that impact the reported amounts of assets, liabilities and expenses and the disclosure of contingent liabilities in the Company’s financial statements and accompanying notes. The most significant estimates in the Company’s financial statements relate to the valuation of equity awards, preclinical and clinical study accruals, fair value of assets and liabilities, income taxes, and stock-based compensation. Management bases its estimates on historical experience and on various other market-specific and relevant assumptions that management believes to be reasonable under the circumstances. Although these estimates are based on the Company’s knowledge of current events and actions it may undertake in the future, actual results may ultimately materially differ from these estimates and assumptions.
Cash and Cash Equivalents		Cash and Cash Equivalents The Company considers all highly liquid investments with an original maturity of three months or less to be cash equivalents. As of December 31, 2017 and 2018, cash equivalents consisted of checking, savings, and money-market balances. The Company places its cash with high-credit-quality financial institutions. All of the Company’s cash and cash equivalent balances are maintained at two financial institutions domiciled in the United States.
Fair Value of Financial Instruments		Fair Value of Financial Instruments The carrying amounts of certain of the Company’s financial instruments, including cash and cash equivalents, prepaid expenses and other current assets, accounts payable, and accrued liabilities, approximate fair value due to the short-term nature of these items.
Impairment of Long-Lived Assets		Impairment of Long-Lived Assets The Company reviews long-lived assets, including property and equipment, for impairment whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset over its respective fair value. The Company has not recognized any impairment losses during the years ended December 31, 2017 and 2018.
Deferred Rent		Deferred Rent Deferred rent consists of the difference between cash payments and the recognition of rent expense on a straight-line basis for the facility the Company occupies. The Company’s lease for its facility provides for periods of discounted rent and fixed increases in minimum annual rental payments. The total amount of rental payments due over the lease term is being charged to rent expense ratably over the term of the lease.
General and Administrative Expenses		General and Administrative Expenses General and administrative expenses consist primarily of salaries and related costs, including stock-based compensation, for personnel in executive, finance and administrative functions. General and administrative expenses also include direct and allocated facility-related costs as well as professional fees for legal, consulting, accounting and audit services.
Stock-Based Compensation		Stock-Based Compensation For purposes of calculating stock-based compensation, the Company estimates the fair value of stock options issued using a Black-Scholes option-pricing model. The determination of the fair value of share-based payment awards utilizing the Black-Scholes model is affected by the Company’s stock price and a number of assumptions, including expected volatility, expected life, risk-free interest rate and expected dividends. Expected Term Expected Volatility Risk-Free Interest Rate Expected Dividend The estimated fair value of stock options granted to employees and non-employee

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Summary of Outstanding Anti-dilutive Securities Not Included in the Diluted Net Loss Per Share	Historical outstanding anti-dilutive securities not included in the diluted net loss per share calculation include the following: Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 Convertible preferred stock (as converted) — 10,165,120 — 10,165,120 Common stock options 4,590,804 1,055,445 4,590,804 1,055,445 Total 4,590,804 11,220,565 4,590,804 11,220,565	Historical outstanding anti-dilutive securities not included in the diluted net loss per share calculation include the following: Year Ended December 31, 2017 2018 Convertible preferred stock (as converted) 10,165,120 16,993,194 Common stock options 584,019 3,597,638 Total 10,749,139 20,590,832

Marketable Securities, Availa_2

Marketable Securities, Available-for-Sale (Tables)	6 Months Ended
	Jun. 30, 2019
Investments, Debt and Equity Securities [Abstract]
Summary of Marketable Securities Available for Sale	Marketable securities, available-for-sale, June 30, 2019 Maturity in Years Amortized Cost Unrealized Fair Value Gains Losses U.S. government agency securities Less than 1 $ 2,750 $ 1 $ — $ 2,751 Corporate debt securities Less than 1 97,756 194 — 97,950 Commercial Paper Less than 1 70,538 150 — 70,688 Total marketable securities $ 171,044 $ 345 $ — $ 171,389

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Schedule of Financial Assets Subject to Fair Value Measurements on a Recurring Basis and the Level of Inputs	The Company’s financial assets subject to fair value measurements on a recurring basis and the level of inputs used for such measurements were as follows (in thousands): Fair Value Measurements at June 30, 2019 Using: Level 1 Level 2 Level 3 Total Money market funds included in cash and cash equivalents $ 73,794 $ — $ — $ 73,794 U.S. government agency securities — 2,751 — 2,751 Corporate debt securities — 97,950 — 97,950 Commercial Paper — 70,688 — 70,688 Total $ 73,794 $ 171,389 $ — $ 245,183 Fair Value Measurements at December 31, 2018 Using: Level 1 Level 2 Level 3 Total Money market funds included in cash and cash equivalents $ 98,268 $ — $ — $ 98,268	The Company’s financial assets subject to fair value measurements on a recurring basis and the level of inputs used for such measurements were as follows: Fair Value Measurements at December 31, 2017 Using: Level 1 Level 2 Level 3 Total Money market funds included in cash and cash equivalents $ 40,921 $ — $ — $ 40,921 Fair Value Measurements at December 31, 2018 Using: Level 1 Level 2 Level 3 Total Money market funds included in cash and cash equivalents $ 98,268 $ — $ — $ 98,268

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
Summary of Property and Equipment, Net	Property and equipment, net consisted of the following (in thousands): June 30, December 31, Laboratory equipment $ 508 $ 388 Computer equipment and software 69 138 Tenant improvements 677 679 Furniture and fixtures 32 66 1,286 1,271 Less: accumulated depreciation (332 ) (271 ) $ 954 $ 1,000	Property and equipment, net consisted of the following: December 31 2017 2018 Laboratory equipment $ 260 $ 388 Computer equipment and software 57 138 Tenant improvements 25 679 Furniture and fixtures 50 66 392 1,271 Less: accumulated depreciation (136 ) (271 ) $ 256 $ 1,000

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities (Tables)	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Summary of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consist of the following (in thousands): June 30, December 31, Accrued research and development expenses $ 1,739 $ 1,677 Accrued general and administrative expenses 208 548 Other current liabilities 45 190 Total $ 1,992 $ 2,415	Accrued expenses and other current liabilities consisted of the following: December 31, 2017 2018 Accrued research and development expenses $ 2,615 $ 1,677 Accrued general and administrative expenses 110 548 Other current liabilities 25 190 Total $ 2,750 $ 2,415

Commitments and Contingencies (

Commitments and Contingencies (Tables)	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Summary of Operating Leases Future Minimum Payments	Future minimum payments under the amended lease as of June 30, 2019 are as follows (in thousands): 2019 (Six months remaining) $ 155 2020 457 2021 471 2022 486 2023 247 Total future minimum lease payments 1,816 Less: Amounts representing interest (296 ) Total lease liability $ 1,520 Remaining lease term 4.0 years	As of December 31, 2018, future minimum lease payments under the operating lease (assuming the Company does not elect the early cancellation option on this lease), are as follows: 2019 $ 792 2020 887 2021 914 $ 2,593

Equity (Tables)

Equity (Tables)	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Equity [Abstract]
Schedule of Option Activity Under the Plan	The following summarizes option activity under the Plan: Outstanding Options Outstanding Options Weighted Average Exercise Price Per Share Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (Years) (in thousands) Balances as of December 31, 2018 3,597,638 $ 4.40 9.5 $ 15,056 Options granted 1,472,323 $ 14.47 Options exercised (255,213 ) $ 2.40 Options cancelled (223,944 ) $ 4.72 Balances as of June 30, 2019 4,590,804 $ 7.73 9.1 151,371 Options vested and exercisable as of June 30, 2019 578,836 $ 3.80 7.2 $ 21,360	The majority of options outstanding at December 31, 2018 had vesting periods of four years. Outstanding Options Shares Weighted Weighted Aggregate (Years) Balances as of December 31, 2017 584,019 $ 1.92 9.0 $ 743 Options granted 3,099,984 $ 4.81 Options exercised (43,774 ) $ 1.74 Options cancelled (42,591 ) $ 3.20 Balances as of December 31, 2018 3,597,638 $ 4.40 9.5 $ 15,056 Options vested and exercisable as of December 31, 2018 428,481 $ 2.53 8.1 $ 2,596
Schedule of Fair Values of Employee Stock Options Granted	The fair values of the employee stock options granted under the Plan were estimated using the following assumptions: Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 Risk-free interest rate 2.29 % 2.79 % 2.42 % 2.79 % Volatility 83.1 % 82.5 % 80.6 % 82.5 % Expected term (in years) 6.00 6.07 6.04 6.04 Dividend yield — % — % — % — %	The fair values of the employee stock options granted under the Plan during 2017 and 2018 were estimated at the date of grant using the Black-Scholes option-pricing model with the following assumptions: Year Ended December 31, 2017 2018 Risk-free interest rate 1.94 - 2.22 % 2.61 - 3.10 % Volatility 91.3 % 80.4 - 82.5 % Expected term (in years) 5.00 -6.08 5.77 - 6.08 Dividend yield — % — %
Schedule of Stock Based Compensation Expense	Stock-based compensation expense, net of forfeitures, is reflected in the statements of operations and comprehensive loss as follows (in thousands): Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 Research and development $ 1,292 $ 76 $ 2,281 $ 117 General and administrative 1,767 61 2,704 105 Total stock-based compensation $ 3,059 $ 137 $ 4,985 $ 222	Stock-based compensation expense, net of forfeitures, is reflected in the statements of operations and comprehensive loss as follows: Year Ended December 31, 2017 2018 Research and development $ 160 $ 556 General and administrative 184 591 Total stock-based compensation $ 344 $ 1,147
Schedule Schedule of Common Stock Reserved for Future	Common stock reserved for future issuance consists of the following: June 30, December 31, Conversion of preferred stock outstanding — 16,993,194 Common stock options outstanding 4,590,804 3,597,638 Shares available for issuance under the Plan 3,404,401 1,816,266 Total 7,995,205 22,407,098	Common stock reserved for future issuance consists of the following: December 31, 2017 2018 Conversion of preferred stock outstanding 10,165,120 16,993,194 Common stock options outstanding 584,019 3,597,638 Shares available for issuance under the Plan 931,336 1,816,266 Total 11,680,475 22,407,098

Convertible Preferred Stock (Ta

Convertible Preferred Stock (Tables)	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Authorized, Issued, and Outstanding Shares of Convertible Preferred Stock	The authorized, issued, and outstanding shares of convertible preferred stock at December 31, 2017 and December 31, 2018 were as follows: December 31, 2017 Series Shares Aggregate Carrying Series A convertible preferred stock 7,404,248 $ 3,480 $ 3,455 Series B convertible preferred stock 12,314,885 17,980 17,918 Series C convertible preferred stock 19,416,645 45,000 44,788 Total 39,135,778 $ 66,460 $ 66,161 December 31, 2018 Series Shares Aggregate Carrying Series A convertible preferred stock 7,404,248 $ 3,480 $ 3,455 Series B convertible preferred stock 12,314,885 17,980 17,918 Series C convertible preferred stock 19,416,645 45,000 44,788 Series D convertible preferred stock 26,288,123 80,000 79,755 Total 65,423,901 $ 146,460 $ 145,916

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Reconciliation of Federal Statutory Income Tax Rate and Effective Income Tax Rate	A reconciliation of the federal statutory income tax rate and the Company’s effective income tax rate is as follows: December 31, 2017 2018 Tax computed at federal statutory rate 34.00 % 21.00 % Permanent items and other (9.49 ) (2.36 ) Stock based compensation (0.27 ) (0.78 ) Research and development tax credits (1.19 ) 1.07 Orphan Drug tax credits 13.13 6.47 Tax Cuts and Jobs Act (16.41 ) 0.00 Valuation allowance (19.77 ) (25.40 ) Effective income tax rate — % — %
Summary of Deferred Tax Assets and Liabilities	Deferred tax assets and liabilities consist of the following: December 31, 2017 2018 Deferred tax assets: Net operating loss carryforwards $ 4,303 $ 8,474 Research and development credits 86 353 Orphan Drug Credit 2,178 3,781 Accrued liabilities 82 287 Other, net 71 115 Total deferred tax assets 6,720 13,010 Deferred tax liabilities Fixed assets (13 ) (7 ) (13 ) (7 ) Total 6,707 13,003 Less: valuation allowance (6,707 ) (13,003 ) Net deferred tax assets $ — $ —
Reconciliation of Beginning and Ending Amount of Gross Unrecognized Tax Benefits	A reconciliation of the beginning and ending amount of gross unrecognized tax benefits is as follows: December 31, 2017 2018 Beginning balance $ 954 $ 4,733 Reductions for tax positions taken in prior years (34 ) (118 ) Additions for tax positions taken in current year 3,813 2,873 Ending balance $ 4,733 $ 7,488

Formation and Business of the_2

Formation and Business of the Company; Basis of Presentation - Additional Information (Details) (Detail) $ / shares in Units, $ in Millions	Apr. 22, 2019USD ($)$ / sharesshares	Apr. 05, 2019	Oct. 08, 2013Segment	Jun. 30, 2019shares	Dec. 31, 2018shares	Dec. 31, 2017shares
Basis Of Presentation [Line Items]
Number of operating segments | Segment			1
Convertible preferred stock, outstanding				0	65,423,901	39,135,778
Reverse stock split ratio		0.259
IPO [Member]
Basis Of Presentation [Line Items]
Issuance of common stock in connection with a public offering, net of underwriting discounts, commissions, and offering costs, Shares	10,637,500
Shares issued, price per share | $ / shares	$ 18
Proceeds from issuance of shares | $	$ 175.2
Underwriting discounts and commissions | $	13.4
Offering expenses | $	$ 2.9
Convertible preferred stock, outstanding	65,423,901
Number of convertible preferred stock converted into common stock	16,993,194
Preferred stock, shares outstanding	0

Summary of Significant Accoun_4

Summary of Significant Accounting Policies- Additional Information (Detail)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019USD ($)shares	Jun. 30, 2018USD ($)shares	Jun. 30, 2019USD ($)OperatingLeaseshares	Jun. 30, 2018USD ($)shares	Dec. 31, 2018USD ($)OperatingLeaseshares	Dec. 31, 2017USD ($)
Significant Accounting Policies [Line Items]
Excess of federal deposits insurance limit			$ 249,900,000		$ 2,261,000
Intellectual property related expenses	$ 100,000	$ 100,000	$ 300,000	$ 300,000	$ 453,000	$ 203,000
Number of shares related to convertible preferred stock included in diluted net loss calculation | shares	0	0	0	0	0
Number of operating lease | OperatingLease			1		1
Money market funds					$ 98,268,000
Dividend yield					0
Maximum
Significant Accounting Policies [Line Items]
Cash FDIC insured amount	$ 250,000		$ 250,000		$ 250,000
Estimated useful life			7 years		7 years
Minimum
Significant Accounting Policies [Line Items]
Estimated useful life			3 years		3 years

Summary of Significant Accoun_5

Summary of Significant Accounting Policies-Summary of Outstanding Anti-dilutive Securities Not Included in the Diluted Net Loss Per Share (Detail) - shares	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities not included in the diluted net loss per share	4,590,804	11,220,565	4,590,804	11,220,565	20,590,832	10,749,139
Convertible Preferred Stock [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities not included in the diluted net loss per share		10,165,120		10,165,120	16,993,194	10,165,120
Common Stock Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities not included in the diluted net loss per share	4,590,804	1,055,445	4,590,804	1,055,445	3,597,638	584,019

Marketable Securities, Availa_3

Marketable Securities, Available-for-Sale - Summary of Marketable Securities Available for Sale (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2019USD ($)
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 171,044
Unrealized Gains	345
Fair Value	$ 171,389
U.S. Government Agency Securities
Schedule Of Available For Sale Securities [Line Items]
Maturity in Years	Less than 1
Amortized Cost	$ 2,750
Unrealized Gains	1
Fair Value	$ 2,751
Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Maturity in Years	Less than 1
Amortized Cost	$ 97,756
Unrealized Gains	194
Fair Value	$ 97,950
Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Maturity in Years	Less than 1
Amortized Cost	$ 70,538
Unrealized Gains	150
Fair Value	$ 70,688

Marketable Securities, Availa_4

Marketable Securities, Available-for-Sale - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2019USD ($)
Investments Debt And Equity Securities [Abstract]
Unrealized losses recognized	$ 0
Maximum period in which investment is in gross unrealized loss	12 months

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Financial Assets Subject to Fair Value Measurements on a Recurring Basis and the Level of Inputs (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 245,183
Money Market Funds
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	73,794	$ 98,268	$ 40,921
U.S Government Agency Securities
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	2,751
Corporate Debt Securities
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	97,950
Commercial Paper
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	70,688
Level 1
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	73,794
Level 1 | Money Market Funds
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	73,794	$ 98,268	$ 40,921
Level 2
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	171,389
Level 2 | U.S Government Agency Securities
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	2,751
Level 2 | Corporate Debt Securities
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	97,950
Level 2 | Commercial Paper
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 70,688

Property and Equipment, Net - S

Property and Equipment, Net - Summary of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 1,286	$ 1,271	$ 392
Less: accumulated depreciation	(332)	(271)	(136)
Property and Equipment, net	954	1,000	256
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	508	388	260
Computer Equipment And Software
Property Plant And Equipment [Line Items]
Property and equipment, gross	69	138	57
Tenant Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	677	679	25
Furniture And Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 32	$ 66	$ 50

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Depreciation	$ 100	$ 19	$ 233	$ 39	$ 138	$ 52

Accrued Expenses and Other Cu_3

Accrued Expenses and Other Current Liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Accrued research and development expenses	$ 1,739	$ 1,677	$ 2,615
Accrued general and administrative expenses	208	548	110
Other current liabilities	45	190	25
Total	$ 1,992	$ 2,415	$ 2,750

Commitment and Contingencies -

Commitment and Contingencies - Additional Information (Detail) ft² in Thousands, $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018ft²	Jun. 30, 2019USD ($)	Jun. 30, 2018USD ($)ft²	Jun. 30, 2019USD ($)	Jun. 30, 2018USD ($)ft²	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Jan. 01, 2019USD ($)
Operating Leased Assets [Line Items]
Operating lease expiration date	Dec. 31, 2021
Operating lease, option to extend	An option to extend the lease term on all leased space for one additional five-year term.
Lessee, Operating Lease, Existence of Option to Extend	true
Operating lease, option to extend lease term	5 years
Operating lease, remaining lease term		4 years		4 years				36 months
Operating lease, estimated discount rate								8.50%
Operating lease right-to-use liability		$ 1,520		$ 1,520				$ 2,300
Operating lease, right-of-use asset		1,250		1,250				1,700
Previously capitalized tenant improvement allowance and deferred rent								$ 600
Rent expense		100	$ 100	300	$ 200	$ 489	$ 389
Operating lease, cash payments		200	$ 100	400	$ 200
2019(Six months remaining)		700		700
2020		1,600		1,600		887
2021		1,700		1,700		914
2022		1,700		1,700
2023		900		900
Security deposits		$ 73		$ 73		$ 73	$ 38
Additional office and laboratory space included in amendment of existing lease | ft²	9,302		9,302		9,302
Lease Maturity Extension
Operating Leased Assets [Line Items]
Operating lease, estimated discount rate		8.50%		8.50%
Operating lease right-to-use liability		$ 500		$ 500
Operating lease, right-of-use asset		500		500
Write-off of lease liabilities		900		900
Write-off of right of use assets		600		600
Deffered gain		300		300
Lease Maturity Extension | Unamortized Capitalized Tenant Improvement Allowance and Deferred Rent
Operating Leased Assets [Line Items]
Operating lease, right-of-use asset		$ 300		$ 300

Commitment and Contingencies _2

Commitment and Contingencies - Summary of Operating Leases Future Minimum Payments (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Jan. 01, 2019
Commitments and Contingencies Disclosure [Abstract]
2019 (Six months remaining)	$ 155
2020	457
2021	471
2022	486
2023	247
Total future minimum lease payments	1,816
Less: Amounts representing interest	(296)
Total lease liability	$ 1,520	$ 2,300
Remaining lease term	4 years	36 months

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - USD ($) $ in Thousands	Nov. 14, 2017	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Payments made for related party transactions		$ 100	$ 100	$ 100	$ 100
Related party transaction due from (to) related party		$ 0	$ 0	$ 0	$ 0
Option granted				1,472,323		3,099,984
Former Director
Related Party Transaction [Line Items]
Payments made for related party transactions						$ 69	$ 145
Director
Related Party Transaction [Line Items]
Option granted	112,861
Option vesting period	4 years

Equity - Additional Information

Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Oct. 31, 2018	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average grant-date fair value of options granted to employees	$ 15.62	$ 3.50	$ 14.65	$ 3.64	$ 6.23	$ 2.08
Expected term			10 years
Dividend yield	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%
Unrecognized compensation expense	$ 34,000		$ 34,000		$ 18,214
Unrecognized compensation cost to be recognized over weighted average period			3 years 2 months 19 days		3 years 6 months
Estimated fair value common stock					$ 8.59
Stock-based compensation expense	$ 3,059	$ 137	$ 4,985	$ 222	$ 1,147	$ 344
Non Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense					$ 115	$ 46
2019 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum number of shares available for issuance	3,404,401		3,404,401
2019 Equity Incentive Plan | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options expiration period			10 years
2019 Equity Incentive Plan | After 2nd year | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options expiration period			3 months
2019 Equity Incentive Plan | After 2nd year | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options expiration period			1 month
2019 Equity Incentive Plan | Employees and non - employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights			Subsequent option grants to employees and grants to non-employees typically vest monthly over a four-year period. The majority of options outstanding at June 30, 2019, had vesting periods of four years.
Options vesting period			4 years
2019 Equity Incentive Plan | Employees | After 2nd year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options vesting period			3 years
2019 Equity Incentive Plan | Employees | After one year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights			Initial option grants to employees typically vest 25% after one year and monthly thereafter over a three-year period and expire between one and three months after employee termination.
Exercise price of options as percentage			25.00%
2013 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum number of shares available for issuance						1,558,442	5,500,788	389,610
2013 Equity Incentive Plan | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options expiration period					10 years
2013 Equity Incentive Plan | After 2nd year | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options expiration period					6 months
2013 Equity Incentive Plan | After 2nd year | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options expiration period					1 month
2013 Equity Incentive Plan | Employees | After 2nd year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options vesting period					3 years
2013 Equity Incentive Plan | Employees | After one year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights					Initial option grants to employees typically vest 25% after one year and monthly thereafter over a three-year period and expire between one and six months after employee termination.
Exercise price of options as percentage					25.00%
2013 Equity Incentive Plan | Employees and non - employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights					Subsequent option grants to employees and grants to non-employees typically vest monthly over a four-year period. The majority of options outstanding at December 31, 2018 had vesting periods of four years.
Options vesting period					4 years

Equity - Schedule of Option Act

Equity - Schedule of Option Activity Under the Plan (Detail) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Outstanding Options
Balances	3,597,638	584,019
Options granted	1,472,323	3,099,984
Options exercised	(255,213)	(43,774)
Options cancelled	(223,944)	(42,591)
Balances	4,590,804	3,597,638	584,019
Options vested and exercisable	578,836	428,481
Weighted Average Exercise Price Per Share
Balances	$ 4.40	$ 1.92
Options granted	14.47	4.81
Options exercised	2.40	1.74
Options cancelled	4.72	3.20
Balances	7.73	4.40	$ 1.92
Options vested and exercisable	$ 3.80	$ 2.53
Weighted average remaining contractual term
Weighted average remaining contractual term	9 years 1 month 6 days	9 years 6 months	9 years
Options vested and exercisable	7 years 2 months 12 days	8 years 1 month 6 days
Aggregate Value Intrinsic
Balances	$ 151,371	$ 15,056	$ 743
Options vested and exercisable	$ 21,360	$ 2,596

Equity - Schedule of Fair Value

Equity - Schedule of Fair Values of Employee Stock Options Granted (Details) (Detail)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate, minimum					2.61%	1.94%
Risk-free interest rate	2.29%	2.79%	2.42%	2.79%
Risk-free interest rate, maximum					3.10%	2.22%
Volatility	83.10%	82.50%	80.60%	82.50%		91.30%
Volatility, minimum					80.40%
Expected term (in years)	6 years	6 years 25 days	6 years 14 days	6 years 14 days
Volatility, maximum					82.50%
Dividend yield	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)					5 years 9 months 7 days	5 years
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)					6 years 29 days	6 years 29 days

Equity - Schedule of Stock Base

Equity - Schedule of Stock Based Compensation Expense (Details) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	$ 3,059	$ 137	$ 4,985	$ 222	$ 1,147	$ 344
Research and Development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	1,292	76	2,281	117	556	160
General and Administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation	$ 1,767	$ 61	$ 2,704	$ 105	$ 591	$ 184

Equity - Schedule of Common Sto

Equity - Schedule of Common Stock Reserved for Future Issuance (Details) (Detail) - shares	Jun. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Equity [Abstract]
Conversion of preferred stock outstanding		16,993,194	10,165,120
Common stock options outstanding	4,590,804	3,597,638	584,019
Shares available for issuance under the Plan	3,404,401	1,816,266	931,336
Total	7,995,205	22,407,098	11,680,475

Commitment and Contingencies _3

Commitment and Contingencies - Future Minimum Payments under Operating Lease (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
2019		$ 792
2020	$ 1,600	887
2021	$ 1,700	914
Operating Leases, Future Minimum Payments Due, Total		$ 2,593

Convertible Preferred Stock - A

Convertible Preferred Stock - Additional Information (Detail) - USD ($)	1 Months Ended		12 Months Ended
	Oct. 31, 2018	May 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Number of shares sold, price per share			$ 8.59
Proceeds from sale of convertible preferred stock			$ 79,755,000	$ 44,788,000
Dividends declared			0
Minimum pre-money valuation of the Company in the event of public offering of common stock that triggers automatic conversion			350,000,000
Gross cash proceeds from public offering, before underwriting discounts, commissions and fees, that triggers automatic conversion			$ 50,000,000
Series C Preferred Stock
Temporary Equity [Line Items]
Number of shares sold		19,416,645
Number of shares sold, price per share		$ 2.3176
Proceeds from sale of convertible preferred stock		$ 44,788,000
Issuance cost		$ 212,000
Cash dividends rate			5.00%
Liquidation preference, per share, in addition to all declared and unpaid dividends			$ 2.3176
Majority vote by each class of preferred stock after liquidation event, percentage			60.00%
Conversion Price			$ 8.9228
Series D Preferred Stock
Temporary Equity [Line Items]
Number of shares sold	26,288,123
Number of shares sold, price per share	$ 3.0432
Proceeds from sale of convertible preferred stock	$ 79,755,000
Issuance cost	$ 245,000
Cash dividends rate			5.00%
Liquidation preference, per share, in addition to all declared and unpaid dividends			$ 3.0432
Majority vote by each class of preferred stock after liquidation event, percentage			65.00%
Conversion Price			$ 11.7163
Series A Preferred Stock
Temporary Equity [Line Items]
Number of shares sold, price per share			$ 0.47
Cash dividends rate			5.00%
Liquidation preference, per share, in addition to all declared and unpaid dividends			$ 0.47
Conversion Price			1.8095
Series B Preferred Stock
Temporary Equity [Line Items]
Number of shares sold, price per share			$ 1.46
Cash dividends rate			5.00%
Liquidation preference, per share, in addition to all declared and unpaid dividends			$ 1.46
Conversion Price			5.6210
Series B Preferred Stock | Maximum
Temporary Equity [Line Items]
Liquidation preference, per share, in addition to all declared and unpaid dividends			$ 4.38

Convertible Preferred Stock -_2

Convertible Preferred Stock - Authorized, Issued, and Outstanding Shares of Convertible Preferred Stock (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Shares Authorized		65,423,901	39,135,778
Shares Issued	0	65,423,901	39,135,778
Shares Outstanding	0	65,423,901	39,135,778
Aggregate Liquidation Preference	$ 0	$ 146,460	$ 66,460
Carrying Value		$ 145,916	$ 66,161
Series A Preferred Stock
Temporary Equity [Line Items]
Shares Authorized		7,404,248	7,404,248
Shares Issued		7,404,248	7,404,248
Shares Outstanding		7,404,248	7,404,248
Aggregate Liquidation Preference		$ 3,480	$ 3,480
Carrying Value		$ 3,455	$ 3,455
Series B Preferred Stock
Temporary Equity [Line Items]
Shares Authorized		12,314,885	12,314,885
Shares Issued		12,314,885	12,314,885
Shares Outstanding		12,314,885	12,314,885
Aggregate Liquidation Preference		$ 17,980	$ 17,980
Carrying Value		$ 17,918	$ 17,918
Series C Preferred Stock
Temporary Equity [Line Items]
Shares Authorized		19,416,645	19,416,645
Shares Issued		19,416,645	19,416,645
Shares Outstanding		19,416,645	19,416,645
Aggregate Liquidation Preference		$ 45,000	$ 45,000
Carrying Value		$ 44,788	$ 44,788
Series D Preferred Stock
Temporary Equity [Line Items]
Shares Authorized		26,288,123
Shares Issued		26,288,123
Shares Outstanding		26,288,123
Aggregate Liquidation Preference		$ 80,000
Carrying Value		$ 79,755

Defined Contribution Benefit Pl

Defined Contribution Benefit Plan - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Retirement Benefits [Abstract]
Matching contributions	$ 152	$ 21

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Provision for federal or state income taxes	$ 1	$ 1
Increase in valuation allowance	6,296
Federal net operating loss carryforwards	40,350
State net operating loss carryforwards	$ 47,187
Federal and state tax loss carrforwards expiration year	2033
Federal research and development tax credits	$ 257
State research and development tax credits	578
Federal Orphan Drug tax credits	$ 6,682
Federal research tax credit carry forwards expiration year start	2035
Orphan drug credit carryforwards expiration year	2037
Unrecognized tax benefits	$ 7,488	4,733	$ 954
Accrual for interest and penalties related to unrecognized tax benefits	$ 0
Remeasurement in an income tax expense		$ 2,723
Remeasurement resulted in an income tax expense	21.00%	34.00%
Unrecognized tax benefits would reduce annual effective tax rate, if recognized	$ 6,557

Reconciliation of Federal Statu

Reconciliation of Federal Statutory Income Tax Rate and Effective Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Tax computed at federal statutory rate	21.00%	34.00%
Permanent items and other	(2.36%)	(9.49%)
Stock based compensation	(0.78%)	(0.27%)
Research and development tax credits	1.07%	(1.19%)
Orphan Drug tax credits	6.47%	13.13%
Tax Cuts and Jobs Act	0.00%	(16.41%)
Valuation allowance	(25.40%)	(19.77%)
Effective income tax rate	0.00%	0.00%

Deferred Tax Assets and Liabili

Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 8,474	$ 4,303
Research and development credits	353	86
Orphan Drug Credit	3,781	2,178
Accrued liabilities	287	82
Other, net	115	71
Total deferred tax assets	13,010	6,720
Deferred tax liabilities
Fixed assets	(7)	(13)
Deferred Tax Liabilities, Gross	(7)	(13)
Total	13,003	6,707
Less: valuation allowance	(13,003)	(6,707)
Net deferred tax assets	$ 0	$ 0

Reconciliation of Beginning and

Reconciliation of Beginning and Ending Amount of Gross Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Beginning balance	$ 4,733	$ 954
Reductions for tax positions taken in prior years	(118)	(34)
Additions for tax positions taken in current year	2,873	3,813
Ending balance	$ 7,488	$ 4,733

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) $ in Millions	Apr. 15, 2019	Apr. 05, 2019	Feb. 20, 2019USD ($)shares	Jun. 30, 2019shares	Dec. 31, 2018shares
Subsequent Event [Line Items]
Options granted				1,472,323	3,099,984
Reverse stock split ratio		0.259
Subsequent Event
Subsequent Event [Line Items]
Options granted			757,031
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights			One-fourth of the shares subject to each option grant shall vest on the one-year anniversary of the date of grant with the remainder vesting in equal monthly installments for 36 months thereafter.
Unrecognized compensation expense | $			$ 10.4
Unrecognized compensation cost to be recognized over weighted average period			4 years
Reverse stock split ratio	0.259