Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2023

Equity [Abstract]

Schedule of Shares of Common Stock Reserved for Issuance

The Company had reserved the following shares of common stock for issuance as follows (in thousands):

	December 31,
	2023	2022
Common stock options & RSUs outstanding	13,612	14,215
Reserve balance for Sales Agreement	10,937	10,937
Common stock available for grant	9,189	5,661
ESPP shares available for purchase	540	264
401(k) matching plan	65	192
Total	34,343	31,269

Summary of Outstanding Stock Options

A summary of the activity under the 2008 Plan and the 2018 Plan is as follows:

	Outstanding Options			Weighted Average Remaining Contractual Life (In Years)	Aggregate Intrinsic Value (In Thousands)
	Number of Options (In Thousands)	Weighted Average Exercise Price
Balances at December 31, 2022	14,215	$	14.74	6.89	$	1,749
Options granted	5,662	3.95
Options exercised	(351)	1.90
Options forfeited	(3,275)	10.89
Options expired	(3,261)	13.30
Balances at December 31, 2023	12,990	$	11.73	6.92	$	42
Vested and expected to vest at December 31, 2023	12,457	$	11.92	6.84	$	37
Exercisable at December 31, 2023	7,803	$	14.15	5.62	$	—

Summary of Stock Based Compensation Expense

Stock-based compensation expense was allocated as follows (in thousands):

	Year Ended December 31,
	2023		2022		2021
Research and development	$	14,258	$	17,875	$	14,271
General and administrative	14,461		14,508		11,971
Total stock-based compensation expense	$	28,719	$	32,383	$	26,242

Fair Value Assumptions

The fair value of stock option awards granted to employees and directors was estimated at the date of grant using a Black-Scholes option-pricing model with the following weighted average valuation assumptions:

	Year Ended December 31,
	2023		2022		2021
Volatility	84	%	78	%	72	%
Expected term (years)	5.89		5.93		5.98
Risk-free interest rate	4.16	%	2.52	%	0.95	%
Expected dividend yield	—		—		—

The fair value of the rights granted to employees under the ESPP was estimated at the date of offer using a Black-Scholes option-pricing model with the following weighted average valuation assumptions:

	Year Ended December 31,
	2023		2022		2021
Volatility	102	%	110	%	72	%
Expected term (years)	1.52		1.63		1.27
Risk-free interest rate	4.64	%	3.76	%	0.27	%
Expected dividend yield	—		—		—