Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2017

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Schedule of Stock-based Compensation Expense for Employees and Non-employees

In the accompanying consolidated statement of operations, the Company recognized stock-based compensation expense for its employees and non-employees as follows:

	Year Ended December 31,
	2017		2016		2015
	(in thousands)
Research and development	$	3,966	$	3,635	$	1,846
General and administrative		1,939		1,875		1,340
Total stock-based compensation	$	5,905	$	5,510	$	3,186

Schedule of Estimated Grant-date Fair Value of Stock-based Awards Using Black-Scholes Option Pricing Model Assumptions

The estimated grant-date fair value of all the Company’s stock-based awards was calculated using the Black-Scholes option pricing model, based on the following assumptions:

	Year Ended December 31,
	2017			2016			2015
Expected term (in years)		5.3 – 7.0			5.1 – 9.9			5.2 – 10.0
Expected volatility		86 – 96	%		77 – 87	%		75 – 84	%
Risk-free interest rate		1.8 – 2.3	%		1.1 – 2.4	%		1.5 – 2.4	%
Expected dividend rate		—	%		—	%		—	%

Summary of Stock-Based Compensation Activity

A summary of activity under the 2008 Plan and 2015 Plan and related information is as follows:

				Options Outstanding
	Shares Available for Grant			Number of Shares Outstanding			Weighted- Average Exercise Price Per Share		Weighted- Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value of Outstanding Options (in thousands)
Outstanding—December 31, 2014		1,550,101			2,138,096			$0.91		9.29	$	3,153
Awards authorized		3,400,000
Options granted		(1,471,664	)		1,471,664			8.06
Options exercised		—			(41,505	)		0.76
Options cancelled		45,442			(45,442	)		7.76
Outstanding—December 31, 2015		3,523,879			3,522,813			3.81		8.78		40,425
Awards authorized		1,202,324
Options granted		(3,710,980	)		3,710,980			3.22
Options exercised		—			(312,841	)		0.67
Options cancelled		377,298			(377,298	)		7.04
Outstanding—December 31, 2016		1,392,521			6,543,654			3.44		8.82		888
Awards authorized		1,214,837
Options granted		(1,502,025	)		1,502,025			1.45
Options exercised		—			(176,641	)		0.94
Options cancelled		398,437			(398,437	)		3.55
Outstanding—December 31, 2017		1,503,770			7,470,601			$3.09		8.12	$	12,363
Exercisable—December 31, 2017					3,995,823			$3.10		7.61	$	6,749
Vested and expected to vest—December 31, 2017					7,470,601			$3.09		8.12	$	12,363