Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2022

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Stock Option Activity

The following is a summary of option activity under the Stock Incentive Plans and Inducement Plan (in thousands, except share and per share amounts):

	Number of Shares			Weighted-Average Exercise Price Per Share		Weighted-Average Remaining Contractual Term (In years)		Aggregate Intrinsic Value
Outstanding at December 31, 2021		9,868,947		$	2.88		7.09	$	51
Granted		578,500			0.41
Exercised		—			—		—		—
Forfeited		(81,466	)		1.01
Cancelled		(426,205	)		2.29
Outstanding at March 31, 2022		9,939,776		$	2.77		7.25	$	118
Exercisable at March 31, 2022		5,688,369		$	4.07		6.16	$	28
Vested or expected to vest at March 31, 2022		9,321,200		$	2.89		7.14	$	102

Schedule of Estimated Assumptions Used to Calculate Weighted-Average Grant Date Fair Value of Options Granted

There were no options exercised in the three months ended March 31, 2022 and 42,626 options exercised in the three months ended March 31, 2021. The weighted-average grant date fair values of stock options granted in the three month periods ended March 31, 2022 and 2021 were $0.33 per share and $1.33 per share, respectively, and were calculated using the following estimated assumptions:

	Three Months Ended
	March 31,
	2022			2021
Weighted-average risk-free interest rate		1.67	%		0.95	%
Expected dividend yield		—	%		—	%
Expected volatility		105	%		104	%
Expected terms	6.0 years			6.0 years

Schedule of Nonvested Restricted Stock Units Activity

The following is a summary of restricted stock unit activity under the 2014 Plan:

	Number of Shares			Weighted-Average Grant Date Fair Value Per Share
Nonvested at December 31, 2021		7,120,475		$	1.84
Granted		7,883,807			0.46
Vested		(2,002,048	)		2.02
Forfeited		(106,849	)		1.50
Nonvested at March 31, 2022		12,895,385		$	0.97

Summary of Stock-Based Compensation Expense for Stock Options Granted that was Recorded in the Company's Results of Operations

The following table summarizes the stock-based compensation expense resulting from awards granted under Stock Incentive Plans, the Inducement Plan and the 2014 ESPP, that was recorded in the Company’s results of operations for the periods presented (in thousands):

	Three Months Ended March 31,
	2022		2021
Cost of product revenue	$	129	$	47
Research and development		422		178
Selling, general and administrative		1,994		1,068
Total stock-based compensation expense	$	2,545	$	1,293