The 2014 Equity Incentive Plan (the 2014 Plan) permits the grant of incentive stock options to the Company’s employees and the grant of nonstatutory stock options, restricted stock, restricted stock units, stock appreciation rights, performance units and performance shares to the Company’s employees, directors and consultants. Options granted under the 2014 Plan are generally scheduled to vest over four years, subject to continued service, and subject to certain acceleration of vesting provisions, expire no later than 10 years from the date of grant. Options granted under the 2014 Plan must have a per share exercise price equal to at least 100% of the fair market value of a share of the common stock as of the date of grant. Restricted stock units granted under the 2014 Plan are generally scheduled to vest over two to three years. The Company accounts for stock-based compensation expense related to stock options and employee stock purchase plan (ESPP) rights by estimating the fair value on the date of grant using the Black-Scholes-Merton option pricing model, while market price of the Company’s common stock at the date of grant is used for restricted stock unit awards. Forfeitures are recognized as incurred. For awards subject to time-based vesting conditions, stock-based compensation expense is recognized using the straight-line method.

The following table summarizes stock option activity for the six months ended June 30, 2022 (share amounts in thousands):


	Options			Weighted- Average Exercise Price
Outstanding as of December 31, 2021		11,708		$	4.15
Granted		1,327		$	2.02
Exercised		(23	)	$	1.76
Forfeited		(471	)	$	4.27
Outstanding as of June 30, 2022		12,541		$	3.92

The following table summarizes restricted stock unit activity for the six months ended June 30, 2022 (share amounts in thousands):


	Restricted Stock Units			Weighted-Average Grant-Date Fair Value
Unvested as of December 31, 2021		1,650		$	2.16
Granted		390		$	2.03
Vested		(189	)	$	2.21
Forfeited		(64	)	$	2.16
Unvested as of June 30, 2022		1,787		$	2.12

Total non-cash stock-based compensation expense recognized in the accompanying condensed statements of operations is as follows (in thousands):


	Three Months Ended June 30,				Six Months Ended June 30,
	2022		2021		2022		2021
Cost of product sales	$	—	$	—	$	—	$	3
Research and development		854		817		1,778		1,617
Selling, general and administrative		940		994		1,910		2,154
Total stock-based compensation	$	1,794	$	1,811	$	3,688	$	3,774