Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2020

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 condensed statements of operations and comprehensive loss as follows:

	Three Months Ended June 30,				Six Months Ended June 30,
	2020		2019		2020		2019
Research and development	$	289	$	499	$	689	$	776
General and administrative		470		444		1,045		831
Total stock-based compensation expense	$	759	$	943	$	1,734	$	1,607

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, 2019		215,043			2,562,800		$	7.85		8.84
Shares reserved for issuance		1,457,789
Options granted		(86,484	)		86,484			2.23
Options cancelled		509,272			(509,272	)		7.44
Restricted stock units cancelled		165,036
Outstanding, June 30, 2020		2,260,656			2,140,012			7.54		8.59	$	567
Exercisable, June 30, 2020					771,444			10.02		8.04		91
Vested and expected to vest, June 30, 2020					2,140,012			7.54		8.59		567

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,
	2020		2019	2020		2019
Employees:
Fair value of common stock		$2.23	$6.97 - $8.08		$2.23	$6.97 - $8.90
Expected term (in years)		5.5	5.5 - 6.1		5.5	5.5 - 6.1
Expected volatility	110.2%		94.5% - 104.8%	110.2%		93.7% - 104.8%
Risk-free interest rate	0.4%		1.9% - 2.4%	0.4%		1.9% - 2.6%
Expected dividend yield	0.0%		0.0%	0.0%		0.0%
Non-employees:
Fair value of common stock	$2.09 - $2.18		$8.90 - $10.20	$0.96 - $2.18		$6.82 - $10.20
Expected term (in years)	7.0 - 8.8		8.0 - 9.7	7.0 - 9.0		8.0 - 10.0
Expected volatility	100.8% - 103.5%		93.4% - 94.2%	99.6% - 103.5%		90.0% - 94.9%
Risk-free interest rate	0.5% - 0.6%		1.9% - 2.1%	0.5% - 0.9%		1.9% - 2.6%
Expected dividend yield	0.0%		0.0%	0.0%		0.0%

Summary of Restricted Stock Units Activity

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

	Number of Restricted Stock Units Outstanding
Unvested shares — December 31, 2019		828,935
Vested, net of shares withheld for taxes		(2,121	)
Cancelled		(165,036	)
Unvested shares — June 30, 2020		661,778