Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2020

Share-based Payment Arrangement [Abstract]

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

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

	Three Months Ended March 31,
	2020		2019
Research and development	$	546	$	1,184
General and administrative		372		515
Total stock-based compensation	$	918	$	1,699

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

The estimated grant-date fair values of all of the Company’s stock-based awards were calculated using the Black-Scholes option pricing model, based on assumptions as follows:

	Three Months Ended March 31,
	2020	2019
Expected term (in years)	5.7 – 6.7	5.3 – 6.9
Expected volatility	88 – 90%	91 – 94%
Risk-free interest rate	0.6 – 1.2%	2.5 – 2.6%
Expected dividend rate	—  %	—  %

Summary of Stock-Based Compensation Activity

A summary of activity under the 2008 Plan, 2015 Plan and 2018 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, 2019		51,514			326,023		$	102.56		7.45	$	11
Awards authorized		4,387,187
Options granted		(1,435,857	)		1,435,857			13.31
Options forfeited/cancelled		23,053			(23,053	)		52.35
Outstanding - March 31, 2020		3,025,897			1,738,827		$	29.52		9.37	$	—
Exercisable - March 31, 2020					220,647		$	114.63		6.23	$	—
Vested and expected to vest - March 31, 2020					1,738,827		$	29.52		9.37	$	—