Equity Incentive Plans (Tables)

12 Months Ended

Dec. 31, 2021

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Summary of Stock Option Activity

The following summarizes option activity (in thousands, except price per option data):

	Options Outstanding
	Number of Options			Weighted-Average Exercise Price per Option		Aggregate Intrinsic Value
Outstanding — December 31, 2020		8,637		$	6.91
Options granted		2,043		$	2.86
Options exercised		(19	)	$	0.81
Options cancelled		(2,337	)	$	6.02
Outstanding — December 31, 2021		8,324		$	6.18	$	—
Exercisable — December 31, 2021		5,550		$	7.10	$	—
Vested and expected to vest — December 31, 2021		8,324		$	6.18	$	—

Summary of Restricted Stock Unit Activity

A summary of restricted stock unit activity was as follows (in thousands, except weighted-average grant-date fair value and contractual term amounts):

	Stock Awards (PSUs and RSUs)
	Shares			Weighted- Average Grant-Date Fair Value		Weighted- Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value
Outstanding — December 31, 2020		—		$	—
PSUs and RSUs — Awarded		2,190		$	2.94
PSUs and RSUs — Vested		(1,069	)	$	2.98
PSUs and RSUs — Cancelled		(405	)	$	2.98
Outstanding — December 31, 2021		716		$	2.86		1.10	$	477

Estimated Fair Value of Stock Option Awards to Employees Using a Black-Scholes Option Pricing Model

The fair value of stock option awards was estimated using a Black-Scholes option-pricing model with the following assumptions:

	Year Ended December 31,
	2021	2020	2019
Expected term	5.3 ─ 6.1 years	5.3 ─ 6.1 years	5.3 ─ 6.1 years
Volatility	81.9% ─ 88.0%	85.2% ─ 89.8%	84.6% ─ 85.7%
Risk-free interest rate	0.63% ─ 1.36%	0.31% ─ 1.69%	1.42% ─ 2.60%
Expected dividend rate	─%	─%	─%

Employee Stock Purchase Plan

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Summary of Stock-based Compensation Expense Recognized

Total stock-based compensation expense recognized related to the ESPP was as follows (in thousands):

	Year Ended December 31,
	2021		2020		2019
Research and development	$	115	$	343	$	269
General and administrative		44		108		88
Total stock-based compensation	$	159	$	451	$	357

Estimated Fair Value of Stock Offered

The Company used the following assumptions to estimate the fair value of stock offered under the ESPP for the years ended December 31, 2021, 2020, and 2019:

	Year Ended December 31,
	2021	2020	2019
Expected term	0.50 years	0.24 ─ 0.50 years	0.24 ─ 0.50 years
Volatility	103.1% ─ 126.6%	54.3% ─ 125.5%	52.9% ─ 82.0%
Risk-free interest rate	0.04% ─ 0.11%	0.08% ─ 1.59%	1.58% ─ 2.55%
Expected dividend rate	─%	─%	─%

2010 Plan, 2014 Plan and 2018 Inducement Plan

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Summary of Stock-based Compensation Expense Recognized

Total stock-based compensation recognized related to the 2010 Plan, 2014 Plan and 2018 Inducement Plan was as follows (in thousands):

	Year Ended December, 31
	2021		2020		2019
Research and development	$	5,462	$	3,742	$	3,223
General and administrative		4,643		3,867		2,886
Total stock-based compensation	$	10,105	$	7,609	$	6,109