Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2023

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Summary of Shares of Common Stock Reserved for Issuance

Shares of common stock reserved for issuance under the Company’s equity incentive plans were as follows:

	March 31, 2023
Outstanding stock awards		9,971,919
Reserved for future award grants		2,748,592
Reserved for future ESPP		1,082,950
Total common stock reserved for stock awards		13,803,461

Summary of Stock Option Activity

A summary of the Company’s stock option activity (excluding performance-based stock option activity summarized further below) under the 2011 Plan, 2019 Plan, and Inducement Plan for the three months ended March 31, 2023 is as follows:

	Outstanding Options
(in thousands, except share and per share data)	Number of Shares			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value
Balance—December 31, 2022		5,451,132		$	9.90		5.31	$	7
Options granted		2,076,000			2.76
Options exercised		—			—
Options forfeited or expired		(173,221	)		12.67
Balance—March 31, 2023		7,353,911		$	7.82		6.26	$	202
Options vested and exercisable as of March 31, 2023		3,871,448		$	9.19		3.37	$	177

Summary of Weighted-average Assumptions Used in Determination of Fair Value of Stock Options

The fair value of stock options was estimated using the following weighted-average assumptions:

	Three Months Ended March 31,
	2023	2022
Expected term (in years)	5.77	6.00 - 6.08
Volatility	78.98%	68.37 - 69.10%
Risk-free interest rate	3.58%	1.62 - 2.13%
Dividend yield	–%	–%

Summary of Restricted Stock Units Activity

A summary of the Company’s RSU activity under the 2019 Plan and Inducement Plan for the three months ended March 31, 2023 is as follows:

	Unvested Restricted Stock Units
(in thousands, except share and per share data)	Number of Shares			Weighted- Average Grant Date Fair Value		Aggregate Fair Value
Balance—December 31, 2022		2,621,482		$	9.33	$	5,191
RSUs granted		—			—
RSUs vested		(67,406	)		18.77		184
RSUs forfeited		(357,068	)		8.13
Balance—March 31, 2023		2,197,008		$	9.23	$	6,064

Stock Based Compensation Expense by Award Type and Function

The following is a summary of stock-based compensation expense by award type (in thousands):

	Three Months Ended March 31,
	2023		2022
Stock options	$	1,343	$	2,396
RSUs		2,123		2,168
ESPP		229		252
Total stock-based compensation expense	$	3,695	$	4,816

The following is a summary of stock-based compensation expense by function (in thousands):

	Three Months Ended March 31,
	2023		2022
Cost of revenue	$	490	$	458
Research and development		1,245		1,341
Selling, general and administrative		1,960		3,017
Total stock-based compensation expense	$	3,695	$	4,816