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Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2018	Mar. 01, 2019	Jun. 29, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Trading Symbol	KURA
Entity Registrant Name	Kura Oncology, Inc.
Entity Central Index Key	0001422143
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Public Float			$ 525.4
Entity Common Stock, Shares Outstanding		38,169,041

BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 16,119	$ 11,433
Short-term investments	162,866	81,712
Accounts receivable, related party	224	216
Prepaid expenses and other current assets	1,988	1,280
Total current assets	181,197	94,641
Property and equipment, net		10
Other long-term assets	1,182	1,200
Total assets	182,379	95,851
Current liabilities:
Accounts payable and accrued expenses	13,385	8,284
Accounts payable and accrued expenses, related party	230	216
Current portion of long-term debt, net		1,531
Total current liabilities	13,615	10,031
Long-term debt, net	7,500	5,567
Other long-term liabilities	279	388
Total liabilities	21,394	15,986
Commitments and contingencies (Note 10)
Stockholders' equity:
Preferred stock, $0.0001 par value; 10,000 shares authorized; no shares issued and outstanding
Common stock, $0.0001 par value; 200,000 shares authorized; 38,148 and 30,217 shares issued as of December 31, 2018 and 2017, respectively; and 38,148 and 29,424 shares outstanding as of December 31, 2018 and 2017, respectively, excluding zero and 793 shares subject to repurchase as of December 31, 2018 and 2017, respectively	4	3
Additional paid-in capital	310,849	169,201
Accumulated other comprehensive loss	(131)	(49)
Accumulated deficit	(149,737)	(89,290)
Total stockholders' equity	160,985	79,865
Total liabilities and stockholders' equity	$ 182,379	$ 95,851

BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	38,148,000	30,217,000
Common stock, shares outstanding	38,148,000	29,424,000
Number of shares, subject to repurchase	0	793,000

STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Operating Expenses:
Research and development	$ 45,766	$ 25,114
Research and development, related party	1,021	1,312
General and administrative	15,823	9,404
General and administrative, related party	273	247
Total operating expenses	62,883	36,077
Other Income (Expense):
Management fee income, related party	735	780
Interest income	3,169	751
Interest expense	(970)	(888)
Loss from extinguishment of debt	(498)
Total other income	2,436	643
Net Loss	$ (60,447)	$ (35,434)
Net loss per share, basic and diluted	$ (1.72)	$ (1.52)
Weighted average number of shares used in computing net loss per share, basic and diluted	35,191	23,237
Comprehensive Loss:
Net loss	$ (60,447)	$ (35,434)
Other comprehensive loss	(82)	(31)
Comprehensive loss	$ (60,529)	$ (35,465)

STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid- In Capital	Unrealized Loss	Accumulated Deficit
Beginning balance at Dec. 31, 2016	$ 56,876	$ 2	$ 110,748	$ (18)	$ (53,856)
Beginning balance (in shares) at Dec. 31, 2016		19,348
Restricted stock awards vested	3		3
Restricted stock awards vested (in shares)		1,193
Share-based compensation expense	4,545		4,545
Issuance of common stock from exercise of options	232		232
Issuance of common stock from exercise of options (in shares)		47
Issuance of common stock, net of offering costs	53,674	$ 1	53,673
Issuance of common stock, net of offering costs, ( in shares)		8,836
Unrealized loss on marketable securities	(31)			(31)
Net loss	(35,434)				(35,434)
Ending balance at Dec. 31, 2017	79,865	$ 3	169,201	(49)	(89,290)
Ending balance (in shares) at Dec. 31, 2017		29,424
Restricted stock awards vested	2		2
Restricted stock awards vested (in shares)		793
Share-based compensation expense	8,654		8,654
Issuance of common stock from exercise of options and employee stock purchase plan	1,092		1,092
Issuance of common stock from exercise of options and employee stock purchase plan (in shares)		194
Issuance of common stock, net of offering costs	131,901	$ 1	131,900
Issuance of common stock, net of offering costs, ( in shares)		7,737
Unrealized loss on marketable securities	(82)			(82)
Net loss	(60,447)				(60,447)
Ending balance at Dec. 31, 2018	$ 160,985	$ 4	$ 310,849	$ (131)	$ (149,737)
Ending balance (in shares) at Dec. 31, 2018		38,148

STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Operating Activities
Net loss	$ (60,447)	$ (35,434)
Adjustments to reconcile net loss to net cash used in operating activities:
Share-based compensation expense	8,654	4,545
Non-cash interest expense	184	118
Loss on debt extinguishment	498
Depreciation expense	10	30
Amortization of premium and accretion of discounts on marketable securities, net	(1,935)	(260)
Changes in operating assets and liabilities:
Accounts receivable, related party	(8)	79
Prepaid expenses and other current assets	(457)	(196)
Other long-term assets	(504)	(593)
Accounts payable and accrued expenses	5,102	3,606
Accounts payable and accrued expenses, related party	14	(554)
Other long-term liabilities	234	218
Net cash used in operating activities	(48,655)	(28,441)
Investing Activities
Purchases of marketable securities	(237,443)	(100,635)
Maturities of marketable securities	158,143	77,217
Net cash used in investing activities	(79,300)	(23,418)
Financing Activities
Proceeds from issuances of common stock, net	132,172	53,679
Proceeds from exercise of stock options and purchases under employee stock purchase plan	1,092	232
Proceeds from issuance of long-term debt, net	627
Repayment of long-term debt	(1,250)
Payment of fee related to long-term debt		(344)
Net cash provided by financing activities	132,641	53,567
Net increase in cash and cash equivalents	4,686	1,708
Cash and cash equivalents at beginning of period	11,433	9,725
Cash and cash equivalents at end of period	16,119	11,433
Supplemental disclosure of cash flow information:
Interest paid	$ 641	$ 627

Organization and Basis of Presentation

12 Months Ended

Dec. 31, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Organization and Basis of Presentation

1. Organization and Basis of Presentation

Kura Oncology, Inc., is a clinical-stage biopharmaceutical company committed to realizing the promise of precision medicines for the treatment of cancer. Our pipeline consists of small molecule product candidates that target cancer signaling pathways where there is a strong scientific and clinical rationale to improve outcomes, and we intend to pair them with molecular or cellular diagnostics to identify those patients most likely to respond to treatment. We plan to advance our product candidates through a combination of internal development and strategic partnerships and maintain significant development and commercial rights.

References in these Notes to Financial Statements to “Kura Oncology, Inc.,” “we,” “our” or “us,” refer to Kura Oncology, Inc., a private Delaware corporation incorporated in the State of Delaware in August 2014, for the periods prior to our reverse merger transaction which took place on March 6, 2015, or the Merger, and Kura Oncology, Inc., a Delaware corporation incorporated in November 2007 and formerly known as Zeta Acquisition Corp. III, for the periods following the Merger.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Use of Estimates

Our financial statements are prepared in accordance with accounting principles generally accepted in the United States. The preparation of our financial statements requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting period.

Reported amounts and note disclosures reflect the overall economic conditions that are most likely to occur and anticipated measures management intends to take. Actual results could differ materially from those estimates. All revisions to accounting estimates are recognized in the period in which the estimates are revised and in any future periods affected.

Segment Reporting

Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by our president and chief executive officer, the chief operating decision-maker, in making decisions regarding resource allocation and assessing performance. We operate in a single industry segment which is the discovery and development of precision medicines for the treatment of cancer. Our chief operating decision-maker reviews the operating results on an aggregate basis and manages the operations as a single operating segment.

Cash and Cash Equivalents

Cash and cash equivalents consist of checking, money market and highly liquid investments that are readily convertible to cash and that have an original maturity of three months or less. The carrying amounts approximate fair value due to the short maturities of these instruments.

Short-Term Investments

Short-term investments are marketable securities with maturities greater than three months from date of purchase that are specifically identified to fund current operations. These investments are classified as current assets, even though the stated maturity date may be one year or more beyond the current balance sheet date, which reflects management’s intention to use the proceeds from sales of these securities to fund our operations, as necessary. The cost of short-term investments is adjusted for amortization of premiums or accretion of discounts to maturity, and such amortization or accretion is included in interest income. Dividend and interest income is recognized as interest income on the statements of operations and comprehensive loss when earned. Short-term investments are classified as available-for-sale securities and carried at fair value with unrealized gains and losses recorded in other comprehensive loss and included as a separate component of stockholders' equity. Realized gains and losses from the sale of available-for-sale securities and declines in value judged to be other than temporary on short-term investments, if any, are determined on a specific identification basis and are reclassified out of comprehensive loss and included in interest income on the statements of operations and comprehensive loss.

Fair Value Measurements

Fair value is defined as the exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or liability. As a basis for considering such assumptions, the guidance establishes a three-tier fair value hierarchy, which prioritizes the inputs used in measuring fair value as follows:

•

Level 1 - Quoted prices (unadjusted) in active markets for identical assets or liabilities;

•

Level 2 - Inputs other than quoted prices included within Level 1 that are either directly or indirectly observable;

•

Level 3 - Unobservable inputs in which little or no market activity exists, therefore requiring an entity to develop its own assumptions about the assumptions that market participants would use in pricing.

Concentration of Credit Risk

Financial instruments that potentially subject us to significant concentrations of credit risk consist primarily of cash, cash equivalents and short-term investments. We maintain deposits in federally insured financial institutions in excess of federally insured limits. We have established guidelines to limit our exposure to credit risk by placing investments with high credit quality financial institutions, diversifying our investment portfolio and placing investments with maturities that maintain safety and liquidity.

Property and Equipment

Property and equipment are stated at cost and depreciated using the straight-line method over the estimated useful lives of the assets. Computer software and equipment are depreciated over their estimated useful lives of three years.

Impairment of Long-Lived Assets

We review our long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying value of an asset may not be recoverable. If such circumstances are determined to exist, an estimate of undiscounted future cash flows produced by the long-lived asset, including its eventual residual value, is compared to the carrying value to determine whether impairment exists. In the event that such cash flows are not expected to be sufficient to recover the carrying amount of the assets, the assets are written-down to their estimated fair values. While our current and historical operating losses and negative cash flows are indicators of impairment, we believe that future cash flows to be received support the carrying value of our long-lived assets and, accordingly, have not recognized any impairment losses through December 31, 2018.

Research and Development Expenses

Research and development expenses consist of costs associated with our research and development activities including salaries, benefits, share-based compensation and other personnel costs, clinical trial costs, manufacturing costs for non-commercial products, contract services and research supply, equipment and facility costs. All such costs are charged to research and development expense as incurred when these expenditures have no alternative future uses. We are obligated to make upfront payments upon execution of certain research and development agreements. Advance payments, including nonrefundable amounts, for goods or services that will be used or rendered for future research and development activities are deferred. Such amounts are recognized as expense as the related goods are delivered or the related services are performed or such time when we do not expect the goods to be delivered or services to be performed. Payments that we make in connection with in-licensed technology for a particular research and development project that have no alternative future uses, in other research and development projects or otherwise, and therefore no separate economic values are expensed as research and development costs at the time such costs are incurred. As of December 31, 2018, we have no in-licensed technologies that have alternative future uses in research and development projects or otherwise.

Clinical Trial Costs and Accruals

A significant portion of our clinical trial costs relate to contracts with contract research organizations, or CROs. The financial terms of our CRO contracts may result in payment flows that do not match the periods over which materials or services are provided to us under such contracts. Our objective is to reflect the appropriate clinical trial expenses in our financial statements by matching those expenses with the period in which services and efforts are expended. As part of the process of preparing our financial statements, we rely on cost information provided by our CROs, concerning monthly expenses as well as reimbursement for pass through costs. We are also required to estimate certain of our expenses resulting from our obligations under our CRO contracts. Accordingly, our clinical trial accrual is dependent upon the timely and accurate reporting of CROs and other third-party vendors. If the contracted amounts are modified, for instance, as a result of changes in the clinical trial protocol or scope of work to be performed, we modify our accruals accordingly on a prospective basis. Revisions in the scope of a contract are charged to expense in the period in which the facts that give rise to the revision become reasonably certain. Historically, we have had no material changes in clinical trial expense that had a material impact on our results of operations or financial position.

Patent Costs

We expense all costs as incurred in connection with patent applications, including direct application fees, and the legal and consulting expenses related to making such applications, and such costs are included in general and administrative expenses on the statements of operations and comprehensive loss.

Share-Based Payments

Our share-based awards are measured at fair value on the date of grant based upon the estimated fair value of common stock. The fair value of awards expected to vest are recognized and amortized on a straight-line basis over the requisite service period of the award less actual forfeitures. The fair value of each stock option is estimated on the date of grant using the Black-Scholes option pricing model, or Black-Scholes model, that requires the use of subjective assumptions including volatility, expected term, risk-free rate and the fair value of the underlying common stock.

Awards granted to non-employees are subject to periodic revaluation over their vesting terms. The fair value of non-employee awards is remeasured at each reporting period as the underlying awards vest unless the instruments are fully vested, immediately exercisable and nonforfeitable on the date of grant. We record the expense for stock option grants to non-employees based on the estimated fair value of the stock options using the Black-Scholes model. Estimated fair value of the restricted stock awards granted to non-employees is recorded on the earlier of the performance commitment date or the date the services required are completed and are remeasured at fair value during the service period. As non-employee restricted stock awards vest, they are remeasured at fair value and expensed based on the intrinsic value method which is measured as the difference between the exercise price paid for the restricted stock award and the fair value of the shares as the right of the repurchase lapses each vesting period.

Income Taxes

Income taxes are accounted for using the asset and liability method. Under the asset and liability method, deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial carrying amounts of existing assets and liabilities and their respective tax bases and operating loss and tax credit carryforwards. Deferred tax assets and liabilities are measured using enacted tax rates applicable to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date. A valuation allowance against deferred tax assets is recorded if, based upon the weight of all available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized. For uncertain tax positions that meet “a more likely than not” threshold, we recognize the benefit of uncertain tax positions in the financial statements.

Comprehensive Loss

Comprehensive loss is defined as the change in equity during the period from transactions and other events and non-owner sources. For the periods presented, accumulated other comprehensive loss consists of unrealized losses on marketable securities and foreign currency translation adjustments.

Net Loss per Share

Net loss per common share is calculated by dividing the net loss by the weighted-average number of common shares outstanding for the period, without consideration for common stock equivalents. Diluted net loss per share is calculated by dividing the net loss by the weighted-average number of common shares and common stock equivalents outstanding for the period determined using the treasury-stock method. Dilutive common stock equivalents are comprised of unvested restricted stock awards, outstanding stock options, outstanding warrants and employee stock purchase plan rights.

For all periods presented, there is no difference in the number of shares used to calculate basic and diluted shares outstanding due to the antidilutive effect of the securities. Because of our net loss, unvested restricted stock awards, outstanding stock options, outstanding warrants and employee stock purchase plan rights are excluded from the calculation of diluted net loss per common share for the periods presented, due to the anti-dilutive effect of the securities.

The following table summarizes the number of potentially dilutive securities that were excluded from our calculation of diluted net loss per share, in thousands:

	Years Ended December 31,
	2018		2017
Stock options		3,186		2,233
Unvested restricted stock awards		—		793
Warrants		34		34
Employee stock purchase plan rights		5		—
Total		3,225		3,060

Recent Accounting Pronouncements

12 Months Ended

Dec. 31, 2018

Accounting Changes And Error Corrections [Abstract]

Recent Accounting Pronouncements

3. Recent Accounting Pronouncements

In February 2016, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, 2016-02, Leases (Topic 842), which provides principles for the recognition, measurement, presentation and disclosure of leases for both lessees and lessors. ASU 2016-02 was required to be adopted at the earliest period presented using a modified retrospective approach. Subsequently, the FASB issued practical expedients providing an alternative modified transition method for adoption of the new lease standard by recognizing a cumulative-effect adjustment to the opening balance sheet of retained earnings in the period of adoption and allowing issuers to carry forward their historical assessment of whether existing agreements are or contain a lease and the classification of existing lease arrangements. We adopted the new standard along with the available practical expedients on January 1, 2019 using the alternative modified transition method and will recognize a right-to-use asset and lease liability for all leases with terms greater than 12 months. We are still in the process of finalizing our assessment of the impact of ASC 842 on our financial statements and related disclosures.

Investments

12 Months Ended

Dec. 31, 2018

Investments All Other Investments [Abstract]

Investments

4. Investments

We invest in available-for-sale securities consisting of money market funds, U.S. Treasury securities, corporate debt securities and commercial paper. Available-for-sale securities are classified as part of either cash and cash equivalents or short-term investments on the balance sheets.

The following tables summarize, by major security type, our investments that are measured at fair value on a recurring basis, in thousands:

		As of December 31, 2018
	Maturities (years)	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
Cash equivalents:
Money market funds	1 or less	$	8,508	$	—	$	—		$	8,508

Short-term investments:
Commercial paper	1 or less		66,435		—		—			66,435
Corporate debt securities	1 or less		56,779		6		(77	)		56,708
U.S. Treasury securities	1 or less		39,780		—		(57	)		39,723
Total short-term investments			162,994		6		(134	)		162,866
Total		$	171,502	$	6	$	(134	)	$	171,374

		As of December 31, 2017
	Maturities (years)	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
Cash equivalents:
Money market funds	1 or less	$	5,848	$	—	$	—		$	5,848
Commercial paper	1 or less		2,993		—		—			2,993
Total cash equivalents			8,841		—		—			8,841

Short-term investments:
Commercial paper	1 or less		50,929		—		—			50,929
Corporate debt securities	1 or less		7,903		—		(7	)		7,896
U.S. Treasury securities	2 or less		22,929		—		(42	)		22,887
Total short-term investments			81,761		—		(49	)		81,712
Total		$	90,602	$	—	$	(49	)	$	90,553

The available-for-sale investments are classified as current assets, even though the stated maturity date may be one year or more beyond the current balance sheet date, which reflects management’s intention to use the proceeds from sales of these securities to fund our operations, as necessary. As of December 31, 2018, all of our short-term investments had maturities less than one year. As of December 31, 2017, $76.7 million of our short-term investments had maturities less than one year and $5.0 million had maturities between one to two years. There were no realized gains or losses for the years ended December 31, 2018 and 2017. As of December 31, 2018, $93.5 million of our marketable securities were in gross unrealized loss positions, of which $5.0 million of U.S. Treasury securities had been in such position for greater than 12 months and subsequently matured in January 2019. As of December 31, 2017, $30.8 million of our marketable securities were in gross unrealized loss position, all of which had been in such position for less than 12 months.

At each reporting date, we perform an evaluation of our marketable securities to determine if any unrealized losses are other-than-temporary. Factors considered in determining whether a loss is other-than-temporary include (i) the financial strength of the issuing institution, (ii) the length of time and extent for which fair value has been less than the cost basis and (iii) our intent and ability to hold our investments in unrealized loss positions until their amortized cost basis has been recovered. Based on our evaluation, we determined that our unrealized losses were not other-than-temporary at December 31, 2018 and 2017.

Fair Value Measurements

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Fair Value Measurements

5. Fair Value Measurements

As of December 31, 2018 and 2017, we had cash equivalents and short-term investments measured at fair value on a recurring basis. The carrying amounts of our financial instruments, which include cash equivalents, prepaid expenses, accounts payable, accrued expenses and all related party amounts approximate their fair values as of December 31, 2018 and 2017, primarily due to their short-term nature. Fair value estimates of these instruments are made at a specific point in time, based on relevant market information. These estimates may be subjective in nature and involve uncertainties and matters of significant judgment and therefore cannot be determined with precision.

Available-for-sale marketable securities consist of U.S. Treasury securities, which were measured at fair value using Level 1 inputs, and corporate debt securities and commercial paper, which were measured at fair value using Level 2 inputs. We determine the fair value of Level 2 related securities with the aid of valuations provided by third parties using proprietary valuation models and analytical tools. These valuation models and analytical tools use market pricing or prices for similar instruments that are both objective and publicly available, including matrix pricing or reported trades, benchmark yields, broker/dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids and/or offers. We validate the fair values of Level 2 financial instruments by comparing these fair values to a third-party pricing source. No transfers between levels have occurred during the periods presented.

The following tables summarize, by major security type, our cash equivalents and short-term investments that are measured at fair value on a recurring basis and are categorized using the fair value hierarchy, in thousands:

	As of December 31, 2018
	Balance		Level 1		Level 2
Cash equivalents:
Money market funds	$	8,508	$	8,508	$	—

Short-term investments:
Commercial paper		66,435		—		66,435
Corporate debt securities		56,708		—		56,708
U.S. Treasury securities		39,723		39,723		—
Total short-term investments		162,866		39,723		123,143
Total	$	171,374	$	48,231	$	123,143

	As of December 31, 2017
	Balance		Level 1		Level 2
Cash equivalents:
Money market funds	$	5,848	$	5,848	$	—
Commercial paper		2,993		—		2,993
Total cash equivalents		8,841		5,848		2,993

Short-term investments:
Commercial paper		50,929		—		50,929
Corporate debt securities		7,896		—		7,896
U.S. Treasury securities		22,887		22,887		—
Total short-term investments		81,712		22,887		58,825
Total	$	90,553	$	28,735	$	61,818

We believe that our term loan facility bears interest at a rate that approximates prevailing market rates for instruments with similar characteristics and, accordingly, the carrying value of the term loan facility approximates fair value. The fair value of our term loan facility is determined using Level 2 inputs in the fair value hierarchy. See Note 8, Long-Term Debt, for further discussion of our term loan facility.

Property and Equipment, Net

12 Months Ended

Dec. 31, 2018

Property Plant And Equipment [Abstract]

Property and Equipment, Net

6. Property and Equipment, Net

Property and equipment consisted of the following, in thousands:

	December 31,
	2018			2017
Computer software and equipment	$	92		$	92
Less: accumulated depreciation		(92	)		(82	)
Property and equipment, net	$	—		$	10

Depreciation expense was $10,000 and $30,000 for the years ended December 31, 2018 and 2017, respectively.

Accounts Payable and Accrued Liabilities

12 Months Ended

Dec. 31, 2018

Payables And Accruals [Abstract]

Accounts Payable and Accrued Liabilities

7. Accounts Payable and Accrued Liabilities

Accounts payable and accrued liabilities consisted of the following, in thousands:

	December 31,
	2018		2017
Accounts payable	$	3,890	$	1,248
Accrued compensation and benefits		3,437		2,345
Accrued research and development expenses		5,550		3,852
Other accrued expenses		508		839
Total accounts payable and accrued expenses	$	13,385	$	8,284

Long-Term Debt

12 Months Ended

Dec. 31, 2018

Debt Disclosure [Abstract]

Long-Term Debt

8. Long-Term Debt

In April 2016, we entered into a loan and security agreement with Oxford Finance LLC, or Oxford, and Silicon Valley Bank, or SVB, which was amended in May 2017 and October 2017, pursuant to which we borrowed $7.5 million, or SVB-Oxford Term Loan. We could, at our sole discretion, borrow up to an additional $12.5 million at a certain specified time. On October 31, 2017, the draw period for the additional loan expired without us drawing down the additional loan; therefore, we paid an unused fee of approximately $0.3 million on November 1, 2017. The unused fee was recorded as a debt discount in the year ended December 31, 2017 and was amortized to interest expense using the effective interest method.

The SVB-Oxford Term Loan would have matured on November 1, 2020. Repayment of the SVB-Oxford Term Loan was interest only through May 1, 2018, followed by 30 equal monthly payments of principal plus accrued interest commencing on June 1, 2018. The per annum interest rate for the SVB-Oxford Term Loan was the greater of (i) 7.75% and (ii) the sum of (a) the prime rate reported in The Wall Street Journal on the last business day of the month that immediately precedes the month in which the interest will accrue, plus (b) 4.25%. In addition, a final payment of 7.50% of the Term Loan, or approximately $0.6 million, was due upon prepayment of the SVB-Oxford Term Loan, as discussed below. The final payment was being accrued through interest expense using the effective interest method. In connection with the SVB-Oxford Term Loan, we issued warrants to purchase shares of our common stock. As of December 31, 2018, the warrant issued to Oxford to purchase up to 33,988 shares of our common stock at an exercise price of $3.31 per share remains outstanding.

On November 1, 2018, we entered into a loan and security agreement, or the SVB Loan Agreement, with SVB, or the Lender, providing for up to $20.0 million in a series of term loans. Upon entering into the SVB Loan Agreement, we borrowed $7.5 million, or Term A Loan. We used approximately $6.9 million of the proceeds from the Term A Loan to repay all amounts owed under the SVB-Oxford Loan Agreement, which included a prepayment charge of $0.1 million. The SVB Loan Agreement has substantially different terms than the SVB-Oxford Loan Agreement. In accordance with ASC 405, Extinguishment of Liabilities and ASC 470-50, Debt Modifications and Extinguishments, we accounted for the transaction as a debt extinguishment. Accordingly, we recorded a loss of approximately $0.5 million for the year ended December 31, 2018.

Under the terms of the SVB Loan Agreement, we may, at our sole discretion, borrow from the Lender up to an additional $12.5 million at any time between November 1, 2018 and May 1, 2020, or Term B Loan, and together with Term A Loan, the Term Loans. In addition, each Term B Loan must be in an amount equal to the lesser of $5.0 million or the amount that is remaining under the Term B Loan.

All of the Term Loans will be due on the scheduled maturity date of May 1, 2023, or Maturity Date. Repayment of the Term Loans will be interest only through November 30, 2020, followed by 30 equal monthly payments of principal plus accrued interest commencing on December 1, 2020. The per annum interest rate for any outstanding Term Loans is the greater of (i) 5.50% and (ii) the sum of (a) the prime rate reported in The Wall Street Journal plus (b) 0.25%. The interest rate as of December 31, 2018 was 5.75%. In addition, a final payment of 7.75% of the amounts of the Term Loans drawn will be due on the earlier of the Maturity Date, acceleration of any Term Loans, or prepayment of the Term Loans. If we elect to prepay the Term Loans, a prepayment fee equal to 1%, 2% or 3% of the then outstanding principal balance will also be due, depending upon when the prepayment occurs.

We are subject to customary affirmative and restrictive covenants under the term loan facility. Our obligations under the SVB Loan Agreement are secured by a first priority security interest in substantially all of our current and future assets, other than our intellectual property. We have also agreed not to encumber our intellectual property assets, except as permitted by the SVB Loan Agreement.

The SVB Loan Agreement also contains customary indemnification obligations and customary events of default, including, among other things, our failure to fulfill certain obligations under the SVB Loan Agreement and the occurrence of a material adverse change in our business, operations, or condition (financial or otherwise), a material impairment of the prospect of repayment of any portion of the loan, or a material impairment in the perfection or priority of Lender’s lien in the collateral or in the value of such collateral. In the event of default by us under the SVB Loan Agreement, the Lender would be entitled to exercise their remedies thereunder, including the right to accelerate the debt, upon which we may be required to repay all amounts then outstanding under the SVB Loan Agreement. The conditional exercisable call option related to the event of default is considered to be an embedded derivative which is required to be bifurcated and accounted for as a separate financial instrument. In the periods presented, the value of the embedded derivative is not material, but could become material in future periods if an event of default became more probable than is currently estimated. As of December 31, 2018, we were in compliance with all financial covenants under the SVB Loan Agreement and there had been no material adverse change.

The following table summarizes future minimum payments under the term loan facility as of December 31, 2018, in thousands:

Year Ending December 31,
2019	$	436
2020		689
2021		3,342
2022		3,168
2023		1,849
Total future minimum payments		9,484
Less: interest payments		(1,984	)
Principal amount of long-term debt		7,500
Current portion of long-term debt		—
Long-term debt, net	$	7,500

License Agreements

12 Months Ended

Dec. 31, 2018

License Agreements [Abstract]

License Agreements

9. License Agreements

Janssen License Agreement

In December 2014, we entered into a license agreement with Janssen Pharmaceutica NV, or Janssen, which was amended in June 2016, under which we received certain intellectual property rights related to tipifarnib in all indications other than virology for a non-refundable $1.0 million upfront license fee and payments upon achievement of certain development and sales-based milestones. Tipifarnib is a clinical stage compound and all ongoing development, regulatory and commercial work will be completed fully and at our sole expense. Under the license agreement, Janssen has a first right to negotiate for an exclusive license back from us to develop and commercialize tipifarnib on terms to be negotiated in good faith. Janssen could exercise this right of first negotiation during a 60-day period following delivery of clinical data as specified in the agreement. In June 2018, Janssen declined to exercise this first right to negotiate.

The agreement will terminate upon the last-to-expire patent rights or last-to-expire royalty term, or may be terminated by us with 180 days written notice of termination. Either party may terminate the agreement in the event of material breach of the agreement that is not cured within 45 days. Janssen may also terminate the agreement due to our lack of diligence that is not cured within a three-month period.

The University of Michigan License Agreement

In December 2014, we entered into a license agreement with the Regents of the University of Michigan, or the University of Michigan, which was amended in March 2015, July 2015, September 2016, February 2017, May 2017 and August 2017, under which we received certain license rights for a non-refundable upfront license, annual maintenance fees and payments upon achievement of certain development and sales-based milestones. The licensed asset consists of several compounds, including our development candidate KO-539. All future development, regulatory and commercial work on the asset will be completed fully and at our sole expense. The University of Michigan retains the right to use the asset for non-commercial research, internal and/or educational purposes, with the right to grant the same limited rights to other non-profit research institutions.

The agreement will terminate upon the last-to-expire patent rights, or may be terminated by us at any time with 90 days written notice of termination or terminated by the University of Michigan upon a bankruptcy by us, payment failure by us that is not cured within 30 days or a material breach of the agreement by us that is not cured within 60 days.

Future Milestone Payments under License Agreements

Collectively, all of our license agreements provide for specified development, regulatory and sales-based milestone payments up to a total of $80.0 million payable upon occurrence of each stated event, of which $0.5 million relates to the initiation of certain development activities, $28.7 million relates to the achievement of specified regulatory approvals for the first indication and up to $50.8 million for the achievement of specified levels of product sales. Additional payments will be due for each subsequent indication if specified regulatory approvals are achieved. All milestone payments under the agreements will be recognized as research and development expense upon completion of the required events because the triggering events are not considered to be probable until they are achieved. As of December 31, 2018, we have not achieved any milestones under the agreements. Furthermore, if all the programs are successfully commercialized, we will be required to pay tiered royalties on annual net product sales ranging from the low single digits to the low teens, depending on the volume of sales and the respective agreement.

Araxes Asset Purchase Agreement

In December 2014, we entered into an asset purchase agreement with Araxes Pharma LLC, or Araxes, which was amended and restated in February 2015, under which we purchased certain early stage patent rights related to compounds in the field of oncology for a purchase price of $0.5 million payable under a convertible promissory note. All ongoing development, regulatory and commercial work will be completed fully and at our sole expense. The agreement allows for contingent milestone payments of $9.7 million throughout development and commercialization of the asset, of which $1.2 million relates to the initiation of certain development activities, and $8.5 million relates to the submission of certain regulatory filings and receipt of certain regulatory approvals. Additional payments will be due for each subsequent indication if specified regulatory approvals are achieved. We will recognize the milestones as expense when each event occurs. Furthermore, if the program is successfully commercialized, we will be required to pay tiered royalties on annual net product sales ranging in the low single digits, depending on the volume of sales. All milestone payments under the agreement will be recognized upon completion of the required events because the triggering events will not be considered to be probable until they are achieved. For the year ended December 31, 2017, we paid to Araxes milestone payments of $0.2 million upon the dosing of the first patient in the first KO-947 Phase 1 clinical trial in the second quarter of 2017. There were no milestone payments to Araxes in 2018.

Commitments and Contingencies

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

10. Commitments and Contingencies

Sponsored Research Agreement with the University of Michigan

In February 2015, we entered into a sponsored research agreement with the University of Michigan, as amended in May 2017, under which we agreed to sponsor up to $2.1 million of research at the University of Michigan over a three-year period. We received a non-exclusive right to any technology developed under the agreement and had an option right for an exclusive license to invention made under the agreement. The sponsored research agreement expired by its terms in February 2018. Costs incurred for the sponsored research agreement were expensed as incurred. For the years ended December 31, 2018 and 2017, we recorded approximately $0.1 million and $0.2 million, respectively, in research and development expense under this sponsored research agreement.

Operating Leases

In August 2014, we entered into a sublease agreement, or the Sublease, with Wellspring Biosciences, Inc., or Wellspring, a wholly owned subsidiary of Araxes, for office space located on North Torrey Pines Road in La Jolla, California. The Sublease was amended effective September 1, 2014 to provide for a monthly rent of $4,820 per month. The Sublease included rent escalation of 3.0% per year. In addition to the base monthly rent, we were obligated to pay for operating expenses, taxes, insurance and utilities applicable to the subleased property. Pursuant to the terms of the Sublease, as amended again in June 2016, the Sublease would have expired on October 31, 2019. In December 2016, we entered into a third amendment to Sublease pursuant to which the Sublease expired in June 2017.

In December 2016, we entered into a sublease agreement, or the New Sublease, with Wellspring for 5,216 square feet of office space located on Science Park Road in San Diego, California for a monthly rent of approximately $16,000 per month and security deposit of approximately $16,000. The New Sublease includes rent escalation of 3.0% per year. In addition to the base monthly rent, we will be obligated to pay for operating costs, amenities fees and all other costs applicable to the subleased property. The terms of the New Sublease commenced in June 2017 and would have expired on October 31, 2019. In March 2019, the New Sublease was amended to extend until April 30, 2020 with the monthly rent increased to approximately $24,000 per month effective November 1, 2019.

In August 2015, we entered into a lease agreement for approximately 3,766 square feet of office space located in Cambridge, Massachusetts. We paid a security deposit of approximately $44,000. The lease is subject to a 60 month term expiring on August 1, 2020, with initial monthly rent of approximately $21,000 per month, and subject to a 1.4% annual rent increase. Total base rent payable over the lease period is $1.3 million. In addition to base monthly rent, we are obligated to pay for taxes, insurance and utilities applicable to the leased property.

Future minimum payments required under the facility leases as of December 31, 2018 are summarized as follows, in thousands:

Year Ending December 31,
2019	$	485
2020		256
Total future minimum lease payments	$	741

Rent expense for the years ended December 31, 2018 and 2017 was approximately $0.5 million and $0.4 million, respectively.

Litigation

From time to time, we may be involved in disputes, including litigation, relating to claims arising out of operations in the normal course of our business. Any of these claims could subject us to costly legal expenses and, while we generally believe that we have adequate insurance to cover many different types of liabilities, our insurance carriers may deny coverage or our policy limits may be inadequate to fully satisfy any damage awards or settlements. If this were to happen, the payment of any such awards could have a material adverse effect on our results of operations and financial position. Additionally, any such claims, whether or not successful, could damage our reputation and business. We currently are not a party to any legal proceedings, the adverse outcome of which, in management’s opinion, individually or in the aggregate, would have a material adverse effect on our results of operations or financial position.

Stockholders' Equity

12 Months Ended

Dec. 31, 2018

Equity [Abstract]

Stockholders' Equity

11. Stockholders’ Equity

In August 2017, we completed a public offering in which we sold an aggregate of 8,805,000 shares of common stock at a price of $6.50 per share. Net proceeds from the public offering, after deducting underwriting discounts, commissions and offering expenses, were approximately $53.5 million.

In January 2017, we entered into an at-the-market issuance sales agreement with Cowen and Company, LLC, or Cowen, which was amended in November 2017 and March 2018, under which we may offer and sell, from time to time, in our sole discretion, shares of our common stock having an aggregate offering price of up to $160.0 million through Cowen as our sales agent, or the ATM facility. In January 2018, we sold an aggregate of 3,136,722 shares of our common stock under the ATM facility at a weighted-average price per share of $18.85, for net proceeds of approximately $57.4 million, after deducting commissions and offering expenses. In July 2018, we terminated the ATM facility.

In July 2018, we completed a public offering in which we sold an aggregate of 4,600,000 shares of common stock at a price of $16.75 per share. Net proceeds from the public offering, after deducting underwriting discounts, commissions and offering expenses, were approximately $74.5 million.

Share-Based Compensation

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Share-Based Compensation

12. Share-Based Compensation

Equity Incentive Plan

As of December 31, 2018, under our Amended and Restated 2014 Equity Incentive Plan, or 2014 Plan, a total of up to 8,893,214 shares of common stock have been reserved for issuance under incentive stock options, non-statutory stock options, restricted stock awards, restricted stock unit awards, performance-based stock awards and other forms of equity compensation to our employees and other service providers. As of December 31, 2018, there were 586,559 shares of common stock reserved for future equity awards under the 2014 Plan. The number of shares available for future grant under the 2014 Plan will automatically increase on January 1 of each year through January 1, 2025 by 4% of the total number of shares of our common stock outstanding on December 31 of the preceding calendar year, subject to the ability of our board of directors to take action to reduce the size of the increase in any given year. On January 1, 2019, an automatic increase pursuant to the 2014 Plan occurred, resulting in 1,525,906 additional shares available for future grant under the 2014 Plan. We issue shares of common stock upon the exercise of options with the source of those shares of common stock being newly issued shares.

Stock Options

The exercise price of all stock options granted was equal to no less than the estimated fair market value of such stock on the date of grant. Stock options generally vest over a three to four-year period. The maximum contractual term for all options is ten years. The following is a summary of stock option activity for the year ended December 31, 2018, in thousands (except per share and years data):

	Number of Shares			Weighted- Average Exercise Price per Share		Weighted- Average Remaining Contractual Term (years)		Aggregate Intrinsic Value
Outstanding at December 31, 2017		2,233		$	6.77
Granted		1,492		$	18.91
Exercised		(190	)	$	5.65
Canceled		(349	)	$	12.19
Outstanding at December 31, 2018		3,186		$	11.93		8.2	$	13,421
Vested and expected to vest at December 31, 2018		3,186		$	11.93		8.2	$	13,421
Exercisable at December 31, 2018		1,396		$	8.59		7.3	$	8,740

The aggregate intrinsic value in the above table is calculated as the difference between the closing price of our common stock at December 31, 2018 of $14.04 per share and the exercise price of stock options that had strike prices below the closing price. For the years ended December 31, 2018 and 2017, the total intrinsic value of stock options exercised was approximately $1.8 million and $0.2 million, respectively, and cash received from stock option exercises was approximately $1.1 million and $0.2 million, respectively.

The assumptions used to estimate the fair value of stock options granted to employees using the Black-Scholes model were as follows:

	Years Ended December 31,
	2018		2017
Weighted average grant date fair value per share	$	12.95	$	4.76
Expected volatility	76.5% — 79.4%		75.4% — 77.7%
Expected term (in years)	5.50 — 6.08		5.50 — 6.08
Risk free interest rate	2.1% — 2.8%		1.4% — 2.0%
Expected dividend yield		—		—

In estimating fair value for stock options issued under the 2014 Plan, expected volatility was based, in part, on our historical volatility and the historical volatility of comparable publicly-traded companies because our common stock has only been publicly traded since September 16, 2015. Due to the lack of historical option exercise data, we estimated the expected term using the simplified method. The risk-free interest rates are based on the U.S. Treasury zero-coupon bonds with maturities similar to those of the expected term of the award being valued. The expected dividend yield of zero reflects that we have not paid cash dividends since inception and do not intend to pay cash dividends in the foreseeable future. Actual forfeitures are applied as they occur, and any compensation cost previously recognized for awards for which the requisite service has not been completed is reversed in the period that the award is forfeited.

For the years ended December 31, 2018 and 2017, we recognized $5.9 million and $2.3 million expense related to options, respectively. As of December 31, 2018, unrecognized estimated compensation expense related to options was $16.7 million, which is expected to be recognized over the weighted-average remaining requisite service period of approximately 2.6 years.

Restricted Stock Awards

Restricted stock awards were granted at a price equal to the estimated fair market value on the date of grant. The restricted stock awards generally vest over four years from the original vesting date, with certain grants subject to one-year cliff vesting. The vesting provisions of individual awards may vary as approved by our board of directors. In connection with the issuance of restricted common stock, we maintain a repurchase right where shares of restricted common stock are released from such repurchase right over a period of time of continued service by the recipient. The repurchase price for unvested stock awards will be the lower of (i) the fair market value of the shares of common stock on the date of repurchase or (ii) their original purchase price. As of December 31, 2018, there were no shares subject to repurchase.

The following is a summary of restricted stock awards activity for the year ended December 31, 2018, in thousands (except weighted-average grant date fair value data):

	Number of Shares			Employee			Non-employee			Weighted-Average Grant Date Fair Value of Employee Awards
Unvested at December 31, 2017		793			663			130		$	0.002
Granted		—			—			—		$	—
Vested		(793	)		(663	)		(130	)	$	0.002
Canceled		—			—			—		$	—
Unvested at December 31, 2018		—			—			—		$	—
Vested at December 31, 2018		4,885			4,038			847		$	0.002

The total fair value of restricted stock awards vested during the years ended December 31, 2018 and 2017 were $14.8 million and $12.7 million, respectively. As of December 31, 2018, there was no unrecognized compensation expense related to employee restricted stock awards.

Employee Stock Purchase Plan

In March 2015, our board of directors adopted the 2015 Employee Stock Purchase Plan, or ESPP. The ESPP permits eligible employees to purchase our common stock at a discount through payroll deductions during defined offering periods. Eligible employees may have up to 15% of their base earnings to purchase up to $25,000 of our common stock during each fiscal year. The price at which stock is purchased under the ESPP is equal to 85% of the fair market value of the common stock on the first day of the offering period or purchase date, whichever is lower. Successive six-month offering periods under the ESPP began on May 21, 2018. For the year ended December 31, 2018, cash received from the exercise of purchase rights was approximately $0.1 million. As of December 31, 2018, we issued 5,611 shares under the ESPP.

As of December 31, 2018, 233,094 shares of common stock are reserved for future issuance. The number of shares of our common stock reserved for issuance under the ESPP will automatically increase on January 1 of each calendar year through January 1, 2025 by the lesser of 1% of the total number of shares of our common stock outstanding on December 31 of the preceding calendar year and 2,000,000 shares, subject to the ability of our board of directors to take action to reduce the size of the increase in any given year. In November 2018, our board of directors elected not to increase the total number of shares of our common stock reserved for issuance under the ESPP for 2019.

The assumptions used to estimate the fair value of ESPP stock purchase rights using the Black-Scholes model were as follows:

	Year Ended
	December 31, 2018
Weighted average grant date fair value per share	$	3.87
Weighted average exercise price per share	$	10.15
Expected volatility	53.8% — 54.4%
Expected term (in years)		0.50
Risk free interest rate	1.8% — 2.3%
Expected dividend yield		—

In estimating fair value for ESPP purchase rights issued, expected volatility was based on our historical volatility. The expected term is six months, which represents the length of each purchase period. The risk-free interest rates are based on the U.S. Treasury zero-coupon bonds with maturities similar to those of the expected term. The expected dividend yield of zero reflects that we have not paid cash dividends since inception and do not intend to pay cash dividends in the foreseeable future.

The following table summarizes share-based compensation expense for all share-based compensation arrangements, in thousands:

	Years Ended December 31,
	2018		2017
Research and development	$	4,623	$	3,048
General and administrative		4,031		1,497
Total share-based compensation expense	$	8,654	$	4,545

Restricted stock awards:
Employee	$	95	$	132
Nonemployee		2,627		2,077
Total	$	2,722	$	2,209

Stock options:
Employee	$	5,581	$	2,190
Nonemployee		308		146
Total	$	5,889	$	2,336

Employee stock purchase plan:
Employee	$	43	$	—
Total	$	43	$	—

Related Party Transactions

12 Months Ended

Dec. 31, 2018

Related Party Transactions

13. Related Party Transactions

Our president and chief executive officer is also the sole managing member of Araxes, and is a significant stockholder of each of us and Araxes. The following is a summary of transactions with Araxes for the years ended December 31, 2018 and 2017:

•

Asset Purchase Agreement

Under the asset purchase agreement with Araxes, for the year ended December 31, 2017, we paid to Araxes a milestone payment of $0.2 million upon dosing of the first patient in the first KO-947 Phase 1 clinical trial in April 2017. There was no payment to Araxes under the asset purchase agreement for the year ended December 31, 2018.

•

Facility Sublease

We sublease office space in San Diego, California from Wellspring, a wholly owned subsidiary of Araxes. Rent expense, including operating costs, related to the Sublease and the New Sublease, as applicable, for each of the years ended December 31, 2018 and 2017 was approximately $0.3 million and $0.2 million, respectively.

Pursuant to the terms of the Sublease, as amended in June 2016, the Sublease would have expired on October 31, 2019. In December 2016, we entered into a third amendment to Sublease pursuant to which the Sublease expired in June 2017.

In December 2016, we entered into the New Sublease with Wellspring for office space in San Diego, California. The New Sublease commenced in June 2017 and would have expired on October 31, 2019. In March 2019, the New Sublease was amended to extend until April 30, 2020, and the monthly rent increased to approximately $24,000 per month effective November 1, 2019. See Note 10, Commitments and Contingencies, for further details of the terms of the Sublease and New Sublease.

•

Management Fees

We have a management services agreement with Araxes pursuant to which Araxes pays us monthly fees for management services calculated based on costs incurred by us in the provision of services to Araxes, plus a reasonable mark-up. For the years ended December 31, 2018 and 2017, we recorded approximately $0.7 million and $0.8 million, respectively, of management fee income. In addition, the agreement allows for Araxes to reimburse us an amount equal to the number of full-time equivalents, or FTE, performing research and development services for Araxes, at an annual FTE rate of approximately $367,000, plus actual expenses as reasonably incurred. The initial term of this agreement expired on December 31, 2015 but, pursuant to the terms of the agreement, renews automatically for additional consecutive one-year periods. The agreement may be terminated by either party with a notice of at least 30 days prior to the expiration of the then-renewal term. For the years ended December 31, 2018 and 2017, we recorded reimbursements of approximately $0.2 million and $0.3 million, respectively, for research and development services provided to Araxes, which was recorded as a reduction to research and development expenses on the statements of operations and comprehensive loss. As of December 31, 2018 and 2017, approximately $0.2 million in both periods related to management fees and reimbursements of research and development services are included in accounts receivable, related party on the balance sheets.

•

Services Agreement

We have a services agreement with Wellspring pursuant to which we pay Wellspring for research and development services provided to us in an amount equal to the number of FTE’s performing the services, at an annual FTE rate of $400,000, plus actual expenses as reasonably incurred. The initial term of this services agreement expired on December 31, 2015 but, pursuant to the terms of the agreement, renews automatically for additional consecutive one-year periods. The agreement may be terminated by either party with a notice of at least 30 days prior to the expiration of the then-renewal term. For the years ended December 31, 2018 and 2017, we recognized approximately $1.0 million and $1.2 million, respectively, from research and development services provided to us under this agreement as research and development expense, related party on the statements of operations and comprehensive loss. As of December 31, 2018 and 2017, approximately $0.2 million in both periods related to research and development services under this agreement are included in accounts payable and accrued expenses, related party on the balance sheets.

Employee Benefit Plan

12 Months Ended

Dec. 31, 2018

Postemployment Benefits [Abstract]

Employee Benefit Plan

14. Employee Benefit Plan

We have a defined contribution 401(k) plan for all employees. Under the terms of the plan, employees may make voluntary contributions as a percentage or defined amount of compensation. We provide a safe harbor contribution of 3.0% of the employee’s compensation, not to exceed eligible limits. For the years ended December 31, 2018 and 2017, we incurred approximately $0.2 million in expenses in each year related to the safe harbor contribution.

Income Taxes

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Income Taxes

15. Income Taxes

We file tax returns as prescribed by the tax laws of the jurisdictions in which we operate. In the normal course of business, our 2014 through 2018 tax years will be subject to examination by the federal and state jurisdictions where applicable. We have not been, nor are we currently, under examination by the federal or any state tax authority.

Our effective income tax rate differs from the statutory federal rate of 21% and 34% for the years ended December 31, 2018 and 2017, respectively, due to the following, in thousands:

	Years Ended December 31,
	2018			2017
Income taxes at statutory federal rate	$	(12,694	)	$	(12,048	)
State income tax, net of federal benefit		(4,447	)		(2,226	)
Research and development tax credits		(1,469	)		(476	)
Share-based compensation		870			1,046
Other		(8	)		(209	)
Impact of Tax Act		—			9,517
Valuation allowance		17,748			4,396
Income tax expense	$	—		$	—

Significant components of our deferred tax assets are shown below, in thousands:

	December 31,
	2018			2017
Deferred tax assets
Net operating loss carryforwards	$	37,329		$	22,050
Research and development tax credit carryforwards		3,140			1,690
Share-based compensation		1,426			601
Accruals		931			692
Intangibles		642			708
Other		131			110
Total deferred tax assets		43,599			25,851
Less valuation allowance		(43,599	)		(25,851	)
Net deferred tax assets	$	—		$	—

In accordance with the Tax Cut and Jobs Act, or the Tax Act, that was enacted on December 22, 2017 we provisionally recorded a $9.5 million reduction related to the remeasurement of our deferred tax balance. As of December 22, 2018, our accounting for the remeasurement of deferred tax balances was complete and there were no changes to the amount previously recorded.

As of December 31, 2018, we had federal net operating loss, or NOL, carryforwards of $128.3 million, of which $52.9 million were generated in fiscal year 2018 and can be carried forward indefinitely under the Tax Act. The remaining federal net operating loss carryforwards of $75.4 million, which were generated prior to 2018, will begin to expire in 2034, unless previously utilized. We had state loss carryforwards of $151.3 million, of which $150.8 million begin to expire in 2034 and $0.5 million begin to expire in 2030, unless previously utilized. We also have federal and state research and development credit carryforwards of $2.9 million (net of $0.8 million utilized as a payroll tax offset) and $1.6 million, respectively. The federal research and development credits will begin to expire in 2034, unless previously utilized. Of the state research and development credits, $1.2 million will carryforward indefinitely and approximately $0.4 million will begin to expire in 2032, unless previously utilized. In 2018, 2017 and 2016, pursuant to Section 41(h) of the Internal Revenue Code of 1986, as amended, or IRC, which was added as part of the Protecting Americans from Tax Hikes Act of 2015, we qualified to elect approximately $0.3 million federal research and development credits to be utilized as an offset against future payroll taxes. Accordingly, in 2018, 2017 and 2016, we have recognized benefit of approximately $0.3 million in each year as an offset to research and development expenses.

Management assesses the available positive and negative evidence to estimate if sufficient future taxable income will be generated to use existing deferred tax assets. Based on the weight of the evidence, including our limited existence and losses since inception, management has determined that it is more likely than not that the deferred tax assets will not be realized. The valuation allowance increased by $17.7 million and $4.4 million from December 31, 2017 to December 31, 2018 and from December 31, 2016 to December 31, 2017, respectively.

Pursuant to Sections 382 and 383 of the IRC annual use of our NOL or research and development credit carryforwards may be limited in the event a cumulative change in ownership of more than 50% occurs within a three-year period. We previously completed a study to assess whether an ownership change, as defined by IRC Section 382, had occurred from our formation through March 31, 2016. Based upon this study, we determined that an ownership change occurred but concluded the annual utilization limitation would be sufficient to utilize our pre-ownership change NOLs and research and development credits prior to expiration. We completed additional studies and concluded no further ownership changes occurred through December 31, 2018. Future ownership changes may limit our ability to utilize remaining tax attributes. Any carryforwards that will expire prior to utilization as a result of such additional limitations will be removed from deferred tax assets, with a corresponding reduction of the valuation allowance.

In accordance with authoritative guidance, the impact of an uncertain income tax position is recognized at the largest amount that is “more likely than not” to be sustained upon audit by the relevant taxing authority. An uncertain tax position will not be recognized if it has less than a 50% likelihood of being sustained.

The following table summarizes the activity related to our unrecognized tax benefits, in thousands:

	December 31,
	2018		2017
Gross unrecognized tax benefits at the beginning of the year	$	615	$	358
Increases related to prior year tax positions		—		42
Increases from tax positions taken in the current year		448		215
Gross unrecognized tax benefits at the end of the year	$	1,063	$	615

Our practice is to recognize interest and penalties related to income tax matters in income tax expense. We did not have any accrued interest or penalties included on the balance sheets and have not recognized interest and penalties on the statements of operations and comprehensive loss for the years ended December 31, 2018 or 2017.

We do not expect that there will be a significant change in the unrecognized tax benefits over the next 12 months. Due to the existence of the valuation allowance, future changes in our unrecognized tax benefits will not impact our effective tax rate.

Subsequent Event

12 Months Ended

Dec. 31, 2018

Subsequent Events [Abstract]

Subsequent Event

16. Subsequent Event

In March 2019, we entered into an amendment to the New Sublease pursuant to which the New Sublease will expire on April 30, 2020, with monthly rent increased to approximately $24,000 effective November 1, 2019.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Use of Estimates

Segment Reporting

Cash and Cash Equivalents

Short-Term Investments

Fair Value Measurements

•

Level 1 - Quoted prices (unadjusted) in active markets for identical assets or liabilities;

•

Level 2 - Inputs other than quoted prices included within Level 1 that are either directly or indirectly observable;

•

Concentration of Credit Risk

Property and Equipment

Impairment of Long-Lived Assets

Research and Development Expenses

Clinical Trial Costs and Accruals

The following table summarizes the number of potentially dilutive securities that were excluded from our calculation of diluted net loss per share, in thousands:

	Years Ended December 31,
	2018		2017
Stock options		3,186		2,233
Unvested restricted stock awards		—		793
Warrants		34		34
Employee stock purchase plan rights		5		—
Total		3,225		3,060

Recent Accounting Pronouncements

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Summary of Number of Potentially Dilutive Securities Excluded from Calculation of Diluted Net Loss Per Share

The following table summarizes the number of potentially dilutive securities that were excluded from our calculation of diluted net loss per share, in thousands:

	Years Ended December 31,
	2018		2017
Stock options		3,186		2,233
Unvested restricted stock awards		—		793
Warrants		34		34
Employee stock purchase plan rights		5		—
Total		3,225		3,060

Investments (Tables)

12 Months Ended

Dec. 31, 2018

Investments All Other Investments [Abstract]

Investments Measured at Fair Value on Recurring Basis

The following tables summarize, by major security type, our investments that are measured at fair value on a recurring basis, in thousands:

		As of December 31, 2018
	Maturities (years)	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
Cash equivalents:
Money market funds	1 or less	$	8,508	$	—	$	—		$	8,508

Short-term investments:
Commercial paper	1 or less		66,435		—		—			66,435
Corporate debt securities	1 or less		56,779		6		(77	)		56,708
U.S. Treasury securities	1 or less		39,780		—		(57	)		39,723
Total short-term investments			162,994		6		(134	)		162,866
Total		$	171,502	$	6	$	(134	)	$	171,374

		As of December 31, 2017
	Maturities (years)	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
Cash equivalents:
Money market funds	1 or less	$	5,848	$	—	$	—		$	5,848
Commercial paper	1 or less		2,993		—		—			2,993
Total cash equivalents			8,841		—		—			8,841

Short-term investments:
Commercial paper	1 or less		50,929		—		—			50,929
Corporate debt securities	1 or less		7,903		—		(7	)		7,896
U.S. Treasury securities	2 or less		22,929		—		(42	)		22,887
Total short-term investments			81,761		—		(49	)		81,712
Total		$	90,602	$	—	$	(49	)	$	90,553

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Schedule Of Fair Value Measurements, By Major Security Type

	As of December 31, 2018
	Balance		Level 1		Level 2
Cash equivalents:
Money market funds	$	8,508	$	8,508	$	—

Short-term investments:
Commercial paper		66,435		—		66,435
Corporate debt securities		56,708		—		56,708
U.S. Treasury securities		39,723		39,723		—
Total short-term investments		162,866		39,723		123,143
Total	$	171,374	$	48,231	$	123,143

	As of December 31, 2017
	Balance		Level 1		Level 2
Cash equivalents:
Money market funds	$	5,848	$	5,848	$	—
Commercial paper		2,993		—		2,993
Total cash equivalents		8,841		5,848		2,993

Short-term investments:
Commercial paper		50,929		—		50,929
Corporate debt securities		7,896		—		7,896
U.S. Treasury securities		22,887		22,887		—
Total short-term investments		81,712		22,887		58,825
Total	$	90,553	$	28,735	$	61,818

Property and Equipment, Net (Tables)

12 Months Ended

Dec. 31, 2018

Property Plant And Equipment [Abstract]

Property and Equipment, Net

Property and equipment consisted of the following, in thousands:

	December 31,
	2018			2017
Computer software and equipment	$	92		$	92
Less: accumulated depreciation		(92	)		(82	)
Property and equipment, net	$	—		$	10

Accounts Payable and Accrued Liabilities (Tables)

12 Months Ended

Dec. 31, 2018

Payables And Accruals [Abstract]

Accounts Payable and Accrued Liabilities

Accounts payable and accrued liabilities consisted of the following, in thousands:

	December 31,
	2018		2017
Accounts payable	$	3,890	$	1,248
Accrued compensation and benefits		3,437		2,345
Accrued research and development expenses		5,550		3,852
Other accrued expenses		508		839
Total accounts payable and accrued expenses	$	13,385	$	8,284

Long-Term Debt (Tables)

12 Months Ended

Dec. 31, 2018

Debt Disclosure [Abstract]

Future Minimum Payments under Term Loan Facility

The following table summarizes future minimum payments under the term loan facility as of December 31, 2018, in thousands:

Year Ending December 31,
2019	$	436
2020		689
2021		3,342
2022		3,168
2023		1,849
Total future minimum payments		9,484
Less: interest payments		(1,984	)
Principal amount of long-term debt		7,500
Current portion of long-term debt		—
Long-term debt, net	$	7,500

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Future Minimum Payments Required under Facility Leases

Future minimum payments required under the facility leases as of December 31, 2018 are summarized as follows, in thousands:

Year Ending December 31,
2019	$	485
2020		256
Total future minimum lease payments	$	741

Share-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

Summary of Stock Option Activities

	Number of Shares			Weighted- Average Exercise Price per Share		Weighted- Average Remaining Contractual Term (years)		Aggregate Intrinsic Value
Outstanding at December 31, 2017		2,233		$	6.77
Granted		1,492		$	18.91
Exercised		(190	)	$	5.65
Canceled		(349	)	$	12.19
Outstanding at December 31, 2018		3,186		$	11.93		8.2	$	13,421
Vested and expected to vest at December 31, 2018		3,186		$	11.93		8.2	$	13,421
Exercisable at December 31, 2018		1,396		$	8.59		7.3	$	8,740

Summary of Assumptions Used in Black-Scholes Model

The assumptions used to estimate the fair value of stock options granted to employees using the Black-Scholes model were as follows:

	Years Ended December 31,
	2018		2017
Weighted average grant date fair value per share	$	12.95	$	4.76
Expected volatility	76.5% — 79.4%		75.4% — 77.7%
Expected term (in years)	5.50 — 6.08		5.50 — 6.08
Risk free interest rate	2.1% — 2.8%		1.4% — 2.0%
Expected dividend yield		—		—

Summary of Restricted Stock Awards Activity

The following is a summary of restricted stock awards activity for the year ended December 31, 2018, in thousands (except weighted-average grant date fair value data):

	Number of Shares			Employee			Non-employee			Weighted-Average Grant Date Fair Value of Employee Awards
Unvested at December 31, 2017		793			663			130		$	0.002
Granted		—			—			—		$	—
Vested		(793	)		(663	)		(130	)	$	0.002
Canceled		—			—			—		$	—
Unvested at December 31, 2018		—			—			—		$	—
Vested at December 31, 2018		4,885			4,038			847		$	0.002

Summary of Share-based Compensation Expense

The following table summarizes share-based compensation expense for all share-based compensation arrangements, in thousands:

	Years Ended December 31,
	2018		2017
Research and development	$	4,623	$	3,048
General and administrative		4,031		1,497
Total share-based compensation expense	$	8,654	$	4,545

Restricted stock awards:
Employee	$	95	$	132
Nonemployee		2,627		2,077
Total	$	2,722	$	2,209

Stock options:
Employee	$	5,581	$	2,190
Nonemployee		308		146
Total	$	5,889	$	2,336

Employee stock purchase plan:
Employee	$	43	$	—
Total	$	43	$	—

2015 Employee Stock Purchase Plan

Summary of Assumptions Used in Black-Scholes Model

The assumptions used to estimate the fair value of ESPP stock purchase rights using the Black-Scholes model were as follows:

	Year Ended
	December 31, 2018
Weighted average grant date fair value per share	$	3.87
Weighted average exercise price per share	$	10.15
Expected volatility	53.8% — 54.4%
Expected term (in years)		0.50
Risk free interest rate	1.8% — 2.3%
Expected dividend yield		—

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Schedule of Effective Income Tax Rate Reconciliation

Our effective income tax rate differs from the statutory federal rate of 21% and 34% for the years ended December 31, 2018 and 2017, respectively, due to the following, in thousands:

	Years Ended December 31,
	2018			2017
Income taxes at statutory federal rate	$	(12,694	)	$	(12,048	)
State income tax, net of federal benefit		(4,447	)		(2,226	)
Research and development tax credits		(1,469	)		(476	)
Share-based compensation		870			1,046
Other		(8	)		(209	)
Impact of Tax Act		—			9,517
Valuation allowance		17,748			4,396
Income tax expense	$	—		$	—

Components of Deferred Tax Assets

Significant components of our deferred tax assets are shown below, in thousands:

	December 31,
	2018			2017
Deferred tax assets
Net operating loss carryforwards	$	37,329		$	22,050
Research and development tax credit carryforwards		3,140			1,690
Share-based compensation		1,426			601
Accruals		931			692
Intangibles		642			708
Other		131			110
Total deferred tax assets		43,599			25,851
Less valuation allowance		(43,599	)		(25,851	)
Net deferred tax assets	$	—		$	—

Schedule of Activity Related to Unrecognized Tax Benefits

The following table summarizes the activity related to our unrecognized tax benefits, in thousands:

	December 31,
	2018		2017
Gross unrecognized tax benefits at the beginning of the year	$	615	$	358
Increases related to prior year tax positions		—		42
Increases from tax positions taken in the current year		448		215
Gross unrecognized tax benefits at the end of the year	$	1,063	$	615

Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018 USD ($) Segment
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments \| Segment	1
Impairment losses on long-lived assets \| $	$ 0
Computer Software and Equipment
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life	3 years

Summary of Significant Accounting Policies - Summary of Number of Potentially Dilutive Securities Excluded from Calculation of Diluted Net Loss Per Share (Detail) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities	3,225	3,060
Stock Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities	3,186	2,233
Unvested Restricted Stock Awards
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities		793
Warrants
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities	34	34
Employee Stock Purchase Plan Rights
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities	5

Investments - Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Short-term investments, Fair Value	$ 162,866	$ 81,712
Fair Value Measurements on Recurring Basis
Schedule Of Available For Sale Securities [Line Items]
Cash equivalents, Amortized Cost		8,841
Cash equivalents, Fair Value		8,841
Short-term investments, Amortized Cost	162,994	81,761
Short-term investments, Unrealized Gains	6
Short-term investments, Unrealized Losses	(134)	(49)
Short-term investments, Fair Value	162,866	81,712
Cash equivalents and Short-term investments, Amortized Cost	171,502	90,602
Cash equivalents and Short-term investments, Unrealized Gains	6	0
Cash equivalents and Short-term investments, Unrealized Losses	(134)	(49)
Cash equivalents and Short-term investments, Fair Value	$ 171,374	$ 90,553
Fair Value Measurements on Recurring Basis \| Money Market Funds
Schedule Of Available For Sale Securities [Line Items]
Maturities (years)	1 or less	1 or less
Cash equivalents, Amortized Cost	$ 8,508	$ 5,848
Cash equivalents, Fair Value	$ 8,508	$ 5,848
Fair Value Measurements on Recurring Basis \| U.S. Treasury Securities
Schedule Of Available For Sale Securities [Line Items]
Maturities (years)	1 or less	2 or less
Short-term investments, Amortized Cost	$ 39,780	$ 22,929
Short-term investments, Unrealized Losses	(57)	(42)
Short-term investments, Fair Value	$ 39,723	$ 22,887
Fair Value Measurements on Recurring Basis \| Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Maturities (years)		1 or less
Cash equivalents, Amortized Cost		$ 2,993
Cash equivalents, Fair Value		$ 2,993
Maturities (years)	1 or less	1 or less
Short-term investments, Amortized Cost	$ 66,435	$ 50,929
Short-term investments, Fair Value	$ 66,435	$ 50,929
Fair Value Measurements on Recurring Basis \| Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Maturities (years)	1 or less	1 or less
Short-term investments, Amortized Cost	$ 56,779	$ 7,903
Short-term investments, Unrealized Gains	6
Short-term investments, Unrealized Losses	(77)	(7)
Short-term investments, Fair Value	$ 56,708	$ 7,896

Investments - Additional Information (Detail) - USD ($)	12 Months Ended
Investments - Additional Information (Detail) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Short-term investments with maturities less than one year		$ 76,700,000
Short-term investments with maturities between one to two years		5,000,000
Available-for-sale securities, realized gains or losses	$ 0	0
Marketable securities gross unrealized loss positions for less than twelve months	$ 93,500,000	30,800,000
Available for sale securities, debt maturities period	2019-01
Short-term investments maturities, description	As of December 31, 2018, all of our short-term investments had maturities less than one year. As of December 31, 2017, $76.7 million of our short-term investments had maturities less than one year and $5.0 million had maturities between one to two years.
Unrealized losses other-than-temporary	$ 0	$ 0
U.S. Treasury Securities
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value	$ 5,000,000

Fair Value Measurements - Fair Values of Investment Securities (Detail) - Fair Value Measurements on Recurring Basis - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents		$ 8,841
Short-term investments	$ 162,866	81,712
Cash equivalents and short-term investments	171,374	90,553
Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents		5,848
Short-term investments	39,723	22,887
Cash equivalents and short-term investments	48,231	28,735
Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents		2,993
Short-term investments	123,143	58,825
Cash equivalents and short-term investments	123,143	61,818
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	8,508	5,848
Money Market Funds \| Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	8,508	5,848
U.S. Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	39,723	22,887
U.S. Treasury Securities \| Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	39,723	22,887
Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents		2,993
Short-term investments	66,435	50,929
Commercial Paper \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents		2,993
Short-term investments	66,435	50,929
Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	56,708	7,896
Corporate Debt Securities \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	$ 56,708	$ 7,896

Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Less: accumulated depreciation	$ (92)	$ (82)
Property and equipment, net		10
Computer Software and Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 92	$ 92

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Depreciation expense	$ 10	$ 30

Accounts Payable and Accrued Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Accounts payable	$ 3,890	$ 1,248
Accrued compensation and benefits	3,437	2,345
Accrued research and development expenses	5,550	3,852
Other accrued expenses	508	839
Total accounts payable and accrued expenses	$ 13,385	$ 8,284

Long-Term Debt - Additional Information (Detail) $ / shares in Units, $ in Thousands			1 Months Ended	12 Months Ended
	Nov. 01, 2018 USD ($)	Nov. 01, 2017 USD ($)	Apr. 30, 2016 USD ($) MonthlyPayment $ / shares shares	Dec. 31, 2018 USD ($) MonthlyPayment
Debt Instrument [Line Items]
Principal amount outstanding on loan				$ 7,500
Loss from extinguishment of debt				$ (498)
Oxford Finance LLC and Silicon Valley Bank \| Term Loan
Debt Instrument [Line Items]
Principal amount outstanding on loan			$ 7,500
Term loan unused facility			$ 12,500
Commitment fees for unused borrowing capacity		$ 300
Additional borrowing description				We could, at our sole discretion, borrow up to an additional $12.5 million at a certain specified time. On October 31, 2017, the draw period for the additional loan expired without us drawing down the additional loan; therefore, we paid an unused fee of approximately $0.3 million on November 1, 2017. The unused fee was recorded as a debt discount in the year ended December 31, 2017 and is being amortized to interest expense using the effective interest method through the scheduled maturity date.
Term loan facility maturity date			Nov. 01, 2020
Number of equal monthly payments of principal and accrued interest \| MonthlyPayment			30
Frequency of periodic payment			monthly
Principal and interest payments commencement date			Jun. 01, 2018
Description of term loan payment terms				Term Loan would have matured on November 1, 2020. Repayment of the SVB-Oxford Term Loan was interest only through May 1, 2018, followed by 30 equal monthly payments of principal plus accrued interest commencing on June 1, 2018.
Percentage of prime rate included in effective interest rate			4.25%
Final payment as a percentage of amounts borrowed			7.50%
Final payment due on the maturity date			$ 600
Term loan interest rate description				The per annum interest rate for the SVB-Oxford Term Loan was the greater of (i) 7.75% and (ii) the sum of (a) the prime rate reported in The Wall Street Journal on the last business day of the month that immediately precedes the month in which the interest will accrue, plus (b) 4.25%.
Warrants exercisable exercise price \| $ / shares			$ 3.31
Oxford Finance LLC and Silicon Valley Bank \| Term Loan \| Maximum
Debt Instrument [Line Items]
Warrants issued to purchase of common stock \| shares			33,988
Oxford Finance LLC and Silicon Valley Bank \| Term Loan \| Base Rate \| Minimum
Debt Instrument [Line Items]
Minimum percentage of interest rate on borrowings			7.75%
Silicon Valley Bank \| Term Loan
Debt Instrument [Line Items]
Additional borrowing description				Under the terms of the New Debt, we may, at our sole discretion, borrow from the Lender up to an additional $12.5 million at any time between November 1, 2018 and May 1, 2020, or Term B Loan, and together with Term A Loan, the Term Loans. In addition, each Term B Loan must be in an amount equal to the lesser of $5.0 million or the amount that is remaining under the Term B Loan.
Term loan facility maturity date				May 01, 2023
Number of equal monthly payments of principal and accrued interest \| MonthlyPayment				30
Frequency of periodic payment				monthly
Principal and interest payments commencement date				Dec. 01, 2020
Description of term loan payment terms				All of the Term Loans will be due on the scheduled maturity date of May 1, 2023, or Maturity Date. Repayment of the Term Loans will be interest only through November 30, 2020, followed by 30 equal monthly payments of principal plus accrued interest commencing on December 1, 2020.
Minimum percentage of interest rate on borrowings				5.75%
Percentage of prime rate included in effective interest rate				0.25%
Final payment as a percentage of amounts borrowed				7.75%
Term loan interest rate description				The per annum interest rate for any outstanding Term Loans is the greater of (i) 5.50% and (ii) the sum of (a) the prime rate reported in The Wall Street Journal plus (b) 0.25%.
Prepayment charge	$ 100
Maximum borrowing capacity under term loans	20,000
Loss from extinguishment of debt				$ 500
Silicon Valley Bank \| Term Loan \| Transaction Occurring Assumption One
Debt Instrument [Line Items]
Prepayment fee equal to outstanding principal amount				1.00%
Silicon Valley Bank \| Term Loan \| Transaction Occurring Assumption Two
Debt Instrument [Line Items]
Prepayment fee equal to outstanding principal amount				2.00%
Silicon Valley Bank \| Term Loan \| Transaction Occurring Assumption Three
Debt Instrument [Line Items]
Prepayment fee equal to outstanding principal amount				3.00%
Silicon Valley Bank \| Term Loan \| Base Rate \| Minimum
Debt Instrument [Line Items]
Minimum percentage of interest rate on borrowings				5.50%
Silicon Valley Bank \| Term A Loan
Debt Instrument [Line Items]
Principal amount outstanding on loan	7,500
Repayment of Oxford Finance LLC and Silicon Valley Bank debt	6,900
Silicon Valley Bank \| Term B Loan
Debt Instrument [Line Items]
Term loan unused facility	12,500
Minimum remaining debt amount	$ 5,000

Long-Term Debt - Future Minimum Payments under Term Loan Facility (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Disclosure [Abstract]
2019	$ 436
2020	689
2021	3,342
2022	3,168
2023	1,849
Total future minimum payments	9,484
Less: interest payments	(1,984)
Principal amount of long-term debt	7,500
Current portion of long-term debt		$ (1,531)
Long-term debt, net	$ 7,500	$ 5,567

License Agreements - Additional Information (Detail) - USD ($)			1 Months Ended	3 Months Ended	12 Months Ended
	Dec. 22, 2014	Dec. 18, 2014	Dec. 31, 2014	Jun. 30, 2017	Dec. 31, 2018	Dec. 23, 2014
License Agreements [Line Items]
Total future milestone payments under license agreements with non-related parties	$ 80,000,000
Araxes Asset Purchase Agreement
License Agreements [Line Items]
Convertible notes payable						$ 500,000
Contingent Milestone Payables						$ 9,700,000
Milestone payment paid				$ 200,000	$ 0
The University of Michigan License Agreement
License Agreements [Line Items]
Agreement termination written notice	90 days
Agreement termination notice period in event of material breach	60 days
License agreement date	Dec. 22, 2014
Agreement termination period due to payment failure	30 days
Janssen License Agreement
License Agreements [Line Items]
Agreement termination written notice		180 days
Agreement termination notice period in event of material breach		45 days
Initiation of Certain Development Activities
License Agreements [Line Items]
Total future milestone payments under license agreements with non-related parties	$ 500,000
Specified Regulatory Approvals
License Agreements [Line Items]
Total future milestone payments under license agreements with non-related parties	28,700,000
Specified Levels of Product Sales
License Agreements [Line Items]
Total future milestone payments under license agreements with non-related parties	$ 50,800,000
Certain Regulatory Filings and Approvals \| Araxes Asset Purchase Agreement
License Agreements [Line Items]
Contingent milestone payments			$ 8,500,000
Initiation of Certain Development Activities \| Araxes Asset Purchase Agreement
License Agreements [Line Items]
Contingent milestone payments			$ 1,200,000
Up front Payment Arrangement \| Janssen License Agreement
License Agreements [Line Items]
Upfront license fee		$ 1,000,000

Commitments and Contingencies - Additional Information (Detail)				1 Months Ended		12 Months Ended
	Mar. 05, 2019 USD ($)	May 31, 2017 USD ($)	Aug. 01, 2015 USD ($) ft²	Dec. 31, 2016 USD ($) ft²	Aug. 31, 2014 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Loss Contingencies [Line Items]
Rent expense						$ 500,000	$ 400,000
Cambridge, Massachusetts
Loss Contingencies [Line Items]
Operating lease, area of space \| ft²			3,766
Security deposit			$ 44,000
Lease term period			60 months
Lease expiration date			Aug. 01, 2020
Office space lease monthly rent			$ 21,000
Operating lease rate of increase in annual rent			1.40%
Total base rent payable over the lease period			$ 1,300,000
Wellspring Biosciences Inc \| La Jolla, California
Loss Contingencies [Line Items]
Percentage of rent escalation					3.00%
Sublease expiration date					Oct. 31, 2019
Sublease expiration period					2017-06
Wellspring Biosciences Inc \| La Jolla, California \| Monthly
Loss Contingencies [Line Items]
Rent expense					$ 4,820
Wellspring Biosciences Inc \| San Diego, California
Loss Contingencies [Line Items]
Percentage of rent escalation				3.00%
Sublease expiration date				Oct. 31, 2019
Operating lease, area of space \| ft²				5,216
Security deposit				$ 16,000
Sublease term description				The terms of the New Sublease commenced in June 2017
Wellspring Biosciences Inc \| San Diego, California \| Subsequent Event
Loss Contingencies [Line Items]
Sublease extended date	Apr. 30, 2020
Wellspring Biosciences Inc \| San Diego, California \| Monthly
Loss Contingencies [Line Items]
Rent expense				$ 16,000
Wellspring Biosciences Inc \| San Diego, California \| Monthly \| Subsequent Event
Loss Contingencies [Line Items]
Rent expense	$ 24,000
The University of Michigan Sponsored Research Agreement
Loss Contingencies [Line Items]
Total sponsored research agreement amount		$ 2,100,000
Sponsored research agreement period		3 years
Research and development						$ 100,000	$ 200,000

Commitments and Contingencies - Future Minimum Payments Required under Facility Leases (Detail) $ in Thousands	Dec. 31, 2018 USD ($)
Leases [Abstract]
2019	$ 485
2020	256
Total future minimum lease payments	$ 741

Stockholder's Equity - Additional Information (Detail) - USD ($)			1 Months Ended		12 Months Ended
	Mar. 01, 2018	Jan. 01, 2018	Jul. 31, 2018	Aug. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Stockholders' Equity Note [Abstract]
Common stock, shares issued		3,136,722	4,600,000	8,805,000
Common stock, issued price per share			$ 16.75	$ 6.50
Proceeds from issuance of common stock, net		$ 57,400,000	$ 74,500,000	$ 53,500,000	$ 132,172,000	$ 53,679,000
Common stock weighted-average per share		$ 18.85
Cowen and Company, LLC \| At-The-Market Facility \| Maximum
Stockholders' Equity Note [Abstract]
Aggregate offering price	$ 160,000,000

Share-Based Compensation - Additional Information (Detail) - USD ($)		12 Months Ended
	May 21, 2018	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019	Mar. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expense		$ 8,654,000	$ 4,545,000
Number of shares, subject to repurchase		0	793,000
Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Closing price of common stock		$ 14.04
Total intrinsic value of stock option exercised		$ 1,800,000	$ 200,000
Cash received from stock options exercised		1,100,000	200,000
Share-based compensation expense		5,900,000	2,300,000
Unrecognized estimated stock option compensation expenses		$ 16,700,000
Weighted average service period		2 years 7 months 6 days
Restricted Stock Awards
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expense		$ 2,722,000	2,209,000
Fair value of awards vested		14,800,000	$ 12,700,000
Unrecognized Expense		$ 0
Maximum \| Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period		4 years
Term of options to be granted under the Plan		10 years
Minimum \| Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period		3 years
2014 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common shares reserved for future issuance pursuant to future option grants		586,559
Percentage of increase in common shares reserved for future issuance pursuant to future option grants		4.00%
2014 Plan \| Restricted Stock Awards
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period		4 years
2014 Plan \| Restricted Stock With Cliff Vesting
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period		1 year
2014 Plan \| Common Stock \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares reserved for issuance		8,893,214
2015 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common shares reserved for future issuance pursuant to future option grants		233,094
Additional common shares reserved for future issuance pursuant to future option grants		2,000,000
Percentage of maximum earnings withhold by employee to purchase shares					15.00%
Percentage of common stock purchase	85.00%
Cash received from exercise of ESPP purchase rights		$ 100,000
Shares issued under ESPP		5,611
2015 Employee Stock Purchase Plan \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of increase in common shares reserved for future issuance pursuant to future option grants		1.00%
Employee Stock Purchase Plan, Shares authorized for purchase					$ 25,000
Subsequent Event \| 2014 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Additional common shares reserved for future issuance pursuant to future option grants				1,525,906

Share-Based Compensation - Summary of Stock Option Activities (Detail) - Stock Option $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock Options Outstanding, Beginning Balance, Number of Shares \| shares	2,233
Granted, Number of Shares \| shares	1,492
Exercised, Number of Shares \| shares	(190)
Canceled, Number of Shares \| shares	(349)
Stock Options Outstanding, Ending Balance, Number of Shares \| shares	3,186
Vested and expected to vest, Number of Shares \| shares	3,186
Exercisable, Number of Shares \| shares	1,396
Outstanding, Beginning Balance, Weighted-Average Exercise Price per Share \| $ / shares	$ 6.77
Granted, Weighted-Average Exercise Price per Share \| $ / shares	18.91
Exercised, Weighted-Average Exercise Price per Share \| $ / shares	5.65
Canceled, Weighted-Average Exercise Price per Share \| $ / shares	12.19
Outstanding, Ending Balance, Weighted-Average Exercise Price per Share \| $ / shares	11.93
Vested and expected to vest, Weighted-Average Exercise Price per Share \| $ / shares	11.93
Exercisable, Weighted-Average Exercise Price per Share \| $ / shares	$ 8.59
Outstanding, Weighted-Average Remaining Contractual Term (years)	8 years 2 months 12 days
Vested and expected to vest, Weighted-Average Remaining Contractual Term (years)	8 years 2 months 12 days
Exercisable, Weighted-Average Remaining Contractual Term (years)	7 years 3 months 18 days
Outstanding, Aggregate Intrinsic Value \| $	$ 13,421
Vested and expected to vest, Aggregate Intrinsic Value \| $	13,421
Exercisable, Aggregate Intrinsic Value \| $	$ 8,740

Share-Based Compensation - Summary of Assumptions Used in Black-Scholes Model (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted average grant date fair value per share	$ 12.95	$ 4.76
2015 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted average grant date fair value per share	$ 3.87
Expected term (in years)	6 months
Weighted average exercise price per share	$ 10.15
Minimum \| Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility	76.50%	75.40%
Expected term (in years)	5 years 6 months	5 years 6 months
Risk free interest rate	2.10%	1.40%
Minimum \| 2015 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility	53.80%
Risk free interest rate	1.80%
Maximum \| Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility	79.40%	77.70%
Expected term (in years)	6 years 29 days	6 years 29 days
Risk free interest rate	2.80%	2.00%
Maximum \| 2015 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility	54.40%
Risk free interest rate	2.30%

Share-Based Compensation - Summary of Restricted Stock Awards Activity (Detail) - Restricted Stock Awards shares in Thousands	12 Months Ended
	Dec. 31, 2018 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Beginning Balance, Number of Shares	793
Vested, Number of Shares	(793)
Ending Balance, Vested, Number of Shares	4,885
Beginning Balance, Weighted-Average Grant Date Fair Value \| $ / shares	$ 0.002
Vested, Weighted-Average Grant Date Fair Value \| $ / shares	0.002
Ending Balance, Vested, Weighted-Average Grant Date Fair Value \| $ / shares	$ 0.002
Employee
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Beginning Balance, Number of Shares	663
Vested, Number of Shares	(663)
Ending Balance, Vested, Number of Shares	4,038
Non-employee
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Beginning Balance, Number of Shares	130
Vested, Number of Shares	(130)
Ending Balance, Vested, Number of Shares	847

Share-Based Compensation - Summary of Share-based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expense	$ 8,654	$ 4,545
Research and development
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expense	4,623	3,048
General and administrative
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expense	4,031	1,497
Restricted Stock Awards
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expense	2,722	2,209
Employee	95	132
Nonemployee	2,627	2,077
Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expense	5,889	2,336
Employee	5,581	2,190
Nonemployee	308	$ 146
Employee Stock Purchase Plan Rights
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expense	43
Employee	$ 43

Related Party Transactions - Additional Information (Detail) - USD ($)				1 Months Ended		12 Months Ended
	Mar. 05, 2019	Apr. 30, 2016	Oct. 01, 2014	Dec. 31, 2016	Aug. 31, 2014	Dec. 31, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Management fee income						$ 735,000	$ 780,000
Accounts receivable, related party						224,000	216,000
Research and development, related party						1,021,000	1,312,000
Accounts payable and accrued expenses, related party						230,000	216,000
Araxes Pharma LLC
Related Party Transaction [Line Items]
Milestone payment paid						0	$ 200,000
Araxes Pharma LLC \| Management Fees
Related Party Transaction [Line Items]
Research and development services reimbursement, description							In addition, the agreement allows for Araxes to reimburse us an amount equal to the number of full-time equivalents, or FTE, performing research and development services for Araxes, at an annual FTE rate of approximately $367,000, plus actual expenses as reasonably incurred. The initial term of this agreement expired on December 31, 2015 but, pursuant to the terms of the agreement, renews automatically for additional consecutive one-year periods. The agreement may be terminated by either party with a notice of at least 30 days prior to the expiration of the then-renewal term.
Management fee income						700,000	$ 800,000
Service Agreement for research and development		$ 367,000
Agreement expiration date			Dec. 31, 2015
Agreement termination notice period			30 days
Amount received on research and development services						200,000	300,000
Accounts receivable, related party						200,000	$ 200,000
Wellspring Biosciences Inc \| Service Agreements
Related Party Transaction [Line Items]
Research and development expense payment, description							We have a services agreement with Wellspring pursuant to which we pay Wellspring for research and development services provided to us in an amount equal to the number of FTE’s performing the services, at an annual FTE rate of $400,000, plus actual expenses as reasonably incurred. The initial term of this services agreement expired on December 31, 2015 but, pursuant to the terms of the agreement, renews automatically for additional consecutive one-year periods. The agreement may be terminated by either party with a notice of at least 30 days prior to the expiration of the then-renewal term.
Agreement expiration date			Dec. 31, 2015
Agreement termination notice period			30 days
Research and development services expense at full time equivalents rate			$ 400,000
Research and development, related party						1,000,000	$ 1,200,000
Accounts payable and accrued expenses, related party						$ 200,000	200,000
San Diego, California \| Wellspring Biosciences Inc
Related Party Transaction [Line Items]
Sublease expiration date				Oct. 31, 2019
Sublease commencement date						Jun. 30, 2017
San Diego, California \| Wellspring Biosciences Inc \| Subsequent Event
Related Party Transaction [Line Items]
Sublease extended date	Apr. 30, 2020
Sublease effective date	Nov. 01, 2019
San Diego, California \| Sublease
Related Party Transaction [Line Items]
Rent expense related to office space sublease						$ 300,000	$ 200,000
San Diego, California \| Monthly \| Wellspring Biosciences Inc \| Subsequent Event
Related Party Transaction [Line Items]
Rent expense related to office space sublease	$ 24,000
La Jolla, California \| Wellspring Biosciences Inc
Related Party Transaction [Line Items]
Sublease expiration date					Oct. 31, 2019
La Jolla, California \| Sublease
Related Party Transaction [Line Items]
Sublease agreement description						Pursuant to the terms of the Sublease, as amended in June 2016, the Sublease would have expired on October 31, 2019. In December 2016, we entered into a third amendment to Sublease pursuant to which the Sublease expired in June 2017.

Employee Benefit Plan - Additional Information (Detail) - Safe Harbor 401(k) Plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Defined Contribution Plan Disclosure [Line Items]
Expenses related to contribution plan	$ 0.2	$ 0.2
Maximum
Defined Contribution Plan Disclosure [Line Items]
Percent of employees compensation	3.00%

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Significant Components Of Deferred Tax Assets And Liabilities [Line Items]
Effective federal statutory income tax rate, percent	21.00%	34.00%
Remeasurement of net deferred tax balance reduction	$ 0	$ 9,517,000
Increase in valuation allowance	$ 17,700,000	4,400,000
Income tax examination, likelihood of unfavorable settlement	An uncertain tax position will not be recognized if it has less than a 50% likelihood of being sustained.
Unrecognized interest or penalties	$ 0	0
Domestic Tax Authority
Significant Components Of Deferred Tax Assets And Liabilities [Line Items]
Net operating loss carryforrwards	$ 128,300,000
Loss carryforwards begin to expire	2034
Research and development credit carryforwards	$ 2,900,000
Research and development credit carryforwards, payroll tax offset utilized	$ 800,000
Credit carry forward begin to expire	2034
Domestic Tax Authority \| Research and Development Credits
Significant Components Of Deferred Tax Assets And Liabilities [Line Items]
Research and development credit carryforwards payroll tax offset qualifying amount	$ 300,000
Deferred tax assets tax credit carryforwards recognized benefit	300,000	$ 300,000	$ 300,000
Domestic Tax Authority \| Fiscal Year 2018
Significant Components Of Deferred Tax Assets And Liabilities [Line Items]
Net operating loss carryforrwards	52,900,000
Domestic Tax Authority \| Expire in 2034
Significant Components Of Deferred Tax Assets And Liabilities [Line Items]
Net operating loss carryforrwards	75,400,000
State and Local Jurisdiction
Significant Components Of Deferred Tax Assets And Liabilities [Line Items]
Net operating loss carryforrwards	151,300,000
Research and development credit carryforwards	1,600,000
State and Local Jurisdiction \| Research and Development Credits
Significant Components Of Deferred Tax Assets And Liabilities [Line Items]
Credit carryforwards not subject to expiration	1,200,000
State and Local Jurisdiction \| Expire in 2034
Significant Components Of Deferred Tax Assets And Liabilities [Line Items]
Net operating loss carryforrwards	$ 150,800,000
Loss carryforwards begin to expire	2034
State and Local Jurisdiction \| Expire in 2030
Significant Components Of Deferred Tax Assets And Liabilities [Line Items]
Net operating loss carryforrwards	$ 500,000
Loss carryforwards begin to expire	2030
State and Local Jurisdiction \| Expire in 2032 \| Research and Development Credits
Significant Components Of Deferred Tax Assets And Liabilities [Line Items]
Credit carry forward begin to expire	2032
Credit carryforwards subject to expiration	$ 400,000

Income Taxes - Schedule of Effective Income Tax Rate Reconciliation (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income taxes at statutory federal rate	$ (12,694)	$ (12,048)
State income tax, net of federal benefit	(4,447)	(2,226)
Research and development tax credits	(1,469)	(476)
Share-based compensation	870	1,046
Other	(8)	(209)
Impact of Tax Act	0	9,517
Valuation allowance	17,748	4,396
Income tax expense	$ 0	$ 0

Income Taxes - Components of Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets
Net operating loss carryforwards	$ 37,329	$ 22,050
Research and development tax credit carryforwards	3,140	1,690
Share-based compensation	1,426	601
Accruals	931	692
Intangibles	642	708
Other	131	110
Total deferred tax assets	43,599	25,851
Less valuation allowance	(43,599)	(25,851)
Net deferred tax assets	$ 0	$ 0

Income Taxes - Schedule of Activity Related to Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Gross unrecognized tax benefits at the beginning of the year	$ 615	$ 358
Increases related to prior year tax positions		42
Increases from tax positions taken in the current year	448	215
Gross unrecognized tax benefits at the end of the year	$ 1,063	$ 615

Subsequent Event - Additional Information (Detail) - Wellspring Biosciences Inc - San Diego, California - USD ($)		1 Months Ended
	Mar. 05, 2019	Dec. 31, 2016
Subsequent Event
Subsequent Event [Line Items]
Sublease extended date	Apr. 30, 2020
Monthly
Subsequent Event [Line Items]
Rent expense		$ 16,000
Monthly \| Subsequent Event
Subsequent Event [Line Items]
Rent expense	$ 24,000