Equity Incentive Plan (Tables)

9 Months Ended

Sep. 30, 2024

Equity [Abstract]

Summary of Stock Option Activity

The following table summarizes the stock option activity for the nine months ended September 30, 2024:

	Shares (in thousands)			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)
Outstanding at December 31, 2023		24,575		$	7.06		5.60
Granted		7,697		$	2.23
Exercised		(446	)	$	2.57
Expired and forfeited		(5,670	)	$	12.19
Outstanding at September 30, 2024		26,156		$	4.61		8.15

Summary of Fair Value of Option Grant on Grant Date and Employee Stock Purchase Plan Using Black-Scholes Option Pricing Model and Market Based Stock Option Grants Monte Carlo Simulation Model

We estimated the fair value of each option grant on the grant date using the Black-Scholes option pricing model and for market-based stock option grants using the Monte Carlo simulation model. The following are the weighted-average assumptions:

	For the Nine Months Ended
	September 30,
	2024			2023
Risk-free interest rate		3.7	%		3.7	%
Dividend yield		0.0	%		0.0	%
Volatility		79.8	%		68.1	%
Expected life (years)	6 to 10			6 to 10

Summary of Restricted Stock Unit Activity

The following table summarizes the restricted stock unit (“RSU”) activity for the nine months ended September 30, 2024:

	Shares (in thousands)			Weighted-Average Grant Date Fair Value
Outstanding at December 31, 2023		1,405		$	4.43
Granted		1,533		$	2.29
Released		(928	)	$	2.66
Expired and forfeited		(463	)	$	3.69
Outstanding at September 30, 2024		1,547		$	2.74

The fair value of RSUs is estimated based on the closing market price of our common stock on the date of the grant. RSUs generally vest quarterly over a four-year period.

We estimate the fair value of each purchase right granted under our 1997 Employee Stock Purchase Plan, as amended, at the beginning of each new offering period using the Black-Scholes option pricing model. There were no new offering periods during the three months ended September 30, 2024.

Summary of Stock-Based Compensation Expense related to Stock-Based Payment Awards Granted

The following table summarizes stock-based compensation expense related to stock-based payment awards granted pursuant to all of our equity compensation arrangements (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2024		2023		2024		2023
Research and development	$	361	$	1,193	$	1,615	$	5,624
General and administrative		1,671		3,989		5,609		14,445
Sales and marketing		690		1,501		3,443		8,461
Total stock-based compensation expense	$	2,722	$	6,683	$	10,667	$	28,530