Equity Incentive Plan

9 Months Ended

Sep. 30, 2023

11. Equity Incentive Plan

Option Plan Activity

The following table summarizes the stock option activity for the nine months ended September 30, 2023:


	Shares			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)
Outstanding at December 31, 2022		20,749,349		$	14.61		6.44
Granted		16,116,979		$	1.70
Exercised		(369	)	$	1.32
Expired and forfeited		(5,548,978	)	$	9.47
Outstanding at September 30, 2023		31,316,981		$	8.88		6.53

The following table summarizes the restricted stock unit activity (“RSUs”) for the nine months ended September 30, 2023:


	Shares			Weighted-Average Grant Date Fair Value
Outstanding at December 31, 2022		3,167,397		$	6.46
Granted		1,757,506		$	2.39
Released		(1,351,303	)	$	6.48
Expired and forfeited		(1,787,579	)	$	4.21
Outstanding at September 30, 2023		1,786,021		$	4.68

Stock-based Compensation

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,
	2023		2022		2023		2022
Research and development	$	2,308	$	4,921	$	9,092	$	13,906
General and administrative		2,874		2,932		10,977		8,717
Sales and marketing		1,501		3,345		8,461		9,843
Total stock-based compensation expense	$	6,683	$	11,198	$	28,530	$	32,466

As of September 30, 2023, there was $30.4 million of total unrecognized compensation cost related to non-vested, stock-based payment awards granted under all of our equity compensation plans and all non-plan option grants. Total unrecognized compensation cost will be adjusted for future changes in estimated forfeitures. We expect to recognize this compensation cost over a weighted-average period of 3 years.

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 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,
	2023			2022
Risk-free interest rate		3.7	%		3.1	%
Dividend yield		0.0	%		0.0	%
Volatility		68.1	%		61.1	%
Expected life (years)	6 to 10				6

We estimated 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, 2023.