Stock Based Awards (Tables)

12 Months Ended

Dec. 31, 2020

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Stock Option Activity

Activity under the Company’s stock option plans is set forth below:

						Weighted
						Average
				Weighted		Remaining		Aggregate
				Average		Contractual		Intrinsic
	Number of			Exercise		Life		Value
	Shares			Price		(in years)		(in thousands)
Balances, January 1, 2019		1,515,923		$	18.65
Options granted		483,328			5.55
Options exercised		(39,686	)		2.40
Options cancelled		(139,405	)		81.33
Balances, December 31, 2019		1,820,160			10.70
Options granted		799,460			9.19
Options exercised		(114,515	)		2.76
Options cancelled		(331,329	)		17.07
Balances, December 31, 2020		2,173,776		$	9.59		5.8	$	5,905
Vested and expected to vest as of December 31, 2020		2,074,651		$	9.68		5.6	$	5,891
Exercisable as of December 31, 2020		1,593,995		$	10.32		4.6	$	5,816

Summary of Fair Value of Employee Stock Options

The fair value of employee stock options was estimated using the Black-Scholes model with the following weighted-average assumptions:

	Year Ended December 31,
	2020			2019
Expected volatility		112.0	%		111.0	%
Risk-free interest rate		1.0	%		2.4	%
Dividend yield		0.0	%		0.0	%
Expected life (in years)		6.0			6.0

Schedule of Estimated Stock-Based Compensation Expense

Stock-based compensation expense, net of estimated forfeitures, is reflected in the statements of operations as follows (in thousands):

	Year Ended December 31,
	2020		2019
Operating Expenses
Research and development	$	536	$	347
General and administrative		1,430		3,052
Total	$	1,966	$	3,399