Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Stock-Based Compensation Expense

Total stock-based compensation expense is recognized for stock options granted to employees and non-employees and has been reported in the Company’s condensed consolidated statements of operations as follows:

	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Research and development	$	499	$	101	$	776	$	726
General and administrative		444		301		830		857
Total stock-based compensation expense	$	943	$	402	$	1,606	$	1,583

Summary of Stock Option Activity

The following table summarizes stock option activity under the Plans:

	Shares Available for Grant			Number of Options Outstanding			Weighted- Average Exercise Price per Option		Weighted- Average Remaining Contract Term		Aggregate Intrinsic Value
											(In thousands)
Outstanding, December 31, 2018		1,350,582			1,122,677		$	11.63		9.22
Shares reserved for issuance		1,122,213
Options granted		(1,049,524	)		1,049,524			8.30
Options exercised		—			(7,029	)		0.79
Options cancelled		77,900			(77,900	)		10.52
Outstanding, June 30, 2019		1,501,171			2,087,272			10.04		9.23	$	3,167
Exercisable, June 30, 2019					274,954			11.19		8.51		457
Vested and expected to vest, June 30, 2019					2,087,272			10.04		9.23		3,167

Schedule of Assumptions to Estimate Fair Value of Stock Options for Employees and Non-Employees Using Black-Scholes Option Pricing Model

The fair value of stock options for employees and non-employees was estimated using a Black-Scholes option pricing model with the following assumptions:

	Three Months Ended June 30,			Six Months Ended June 30,
	2019	2018		2019	2018
Employees and directors:
Fair value of common stock	$6.97 - $8.08	$8.57 - $9.91		$6.97 - $8.90	$8.57 - $15.45
Expected term (in years)	5.5 - 6.1		6.6	5.5 - 6.1	5.8 - 6.6
Expected volatility	94.5% - 104.8%	77.4% - 77.7%		93.7% - 104.8%	75.9% - 90.6%
Risk-free interest rate	1.9% - 2.4%	2.6% - 2.8%		1.9% - 2.6%	2.4% - 2.8%
Expected dividend yield	0.0%	0.0%		0.0%	0.0%
Non-employees:
Fair value of common stock	$8.90 - $10.20		—	$6.82 - $10.20	$15.45
Expected term (in years)	8.0 - 9.7		—	8.0 - 10.0		10.0
Expected volatility	93.4% - 94.2%		—	90.0% - 94.9%	77.9%
Risk-free interest rate	1.9% - 2.1%		—	1.9% - 2.6%	2.7%
Expected dividend yield	0.0%		—	0.0%	0.0%

Summary of Restricted Stock Activity

The summary of restricted stock activity and related information follows:

	Number of Restricted Shares Outstanding
Unvested shares — December 31, 2018		1,000
Vested		(1,000	)
Unvested shares — June 30, 2019		—

Summary of Restricted Stock Units Activity

The summary of restricted stock unit activity and related information follows:

	Number of Restricted Stock Units Outstanding
Unvested shares — December 31, 2018		24,960
Granted		—
Vested		(6,240	)
Unvested shares — June 30, 2019		18,720