Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2023

Share-Based Payment Arrangement [Abstract]

Summary of RSU Activity

The following is a summary of RSU activity under our 2014 EIP and Inducement Plan:

	RSUs
	Shares			Weighted Average Grant Date Fair Value
Balance as of December 31, 2022		6,708,608		$	10.61
Granted		5,635,916		$	3.57
Forfeited		(2,388,808	)	$	7.91
Vested		(3,688,003	)	$	8.89
Balance as of December 31, 2023		6,267,713		$	6.32

Summary of Stock Option Activity

The following is a summary of stock option activity under our 2014 EIP and Inducement Plan:

	Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value (in thousands)
Balance as of December 31, 2022		10,645,555		$	16.88		6.4	$	42
Granted		4,693,897			3.64
Exercised		—			—
Forfeited or expired		(4,619,801	)		15.60
Balance as of December 31, 2023		10,719,651		$	11.63		6.5	$	—
Vested and expected to vest as of    December 31, 2023		10,719,651		$	11.63		6.5	$	—
Exercisable as of December 31, 2023		6,389,752		$	15.40		5.1	$	—

Summary of Options Estimated with Weighted Average

The fair value of each option issued was estimated at the date of grant using the Black-Scholes valuation model. The following table summarizes the weighted-average assumptions used as inputs to the Black-Scholes model and resulting weighted-average grant date fair values of stock options granted during the periods indicated:

	Year ended December 31,
	2023			2022
Assumptions:
Expected term (years)		5.80			6.0
Expected volatility		83.9	%		73.2	%
Risk-free interest rate		4.2	%		2.1	%
Expected dividend yield		0.0	%		0.0	%
Fair Value:
Weighted-average estimated grant date fair value per share	$	2.62		$	5.88
Options granted		4,693,897			3,615,971
Total estimated grant date fair value	$	12,298,010		$	21,261,909

Schedule of Common Stock Reserved for Future Issuance Under Equity Incentive Plans

The following shares of common stock were reserved for future issuance under our equity incentive plans as of December 31, 2023:

	Total Shares Reserved
2014 Equity Incentive Plan		20,326,356
2018 Inducement Plan		4,773,147
2014 Employee Stock Purchase Plan		10,593
Total reserved shares of common stock		25,110,096

Schedule of Stock-based Compensation Related to Stock Awards

The following is a summary of stock-based compensation expense for the periods presented:

	Year Ended December 31,
	2023		2022
	(in thousands)
Research and development	$	26,529	$	31,363
General and administrative		18,857		22,475
Total stock-based compensation expense	$	45,386	$	53,838