Stock and employee benefit plans

The Company issues stock options and restricted stock units (“RSUs”) to employees, which generally vest ratably over a four year service period. The Company measures the fair value of stock options on the date of grant using the Black-Scholes option pricing model. The Company measures the fair value of RSUs on the date of grant using the underlying common stock fair value.

Stock-based compensation expense

Total stock-based compensation expense recognized for stock options and RSUs is as follows (in thousands):



	Three Months Ended March 31,
	2020		2019
Research and development	160		182
General and administrative	$	224	$	247
Total	$	384	$	429

Stock options

The following table summarizes stock option activity (shares in thousands):



	Shares		Weighted-Average Exercise Price		Weighted-Average Remaining Contractual Term (years)	Aggregate Intrinsic Value
Outstanding at December 31, 2019	1,323		$	11.65		$	—
Granted	66		$	1.99
Exercised	—		$	—
Cancelled	(19	)	$	4.85
Outstanding at March 31, 2020	1,370		$	11.28	7.90	$	—
Exercisable at March 31, 2020	585		$	18.90	6.72	$	—

RSUs

The following table summarizes RSU activity (shares in thousands):



	Shares	Weighted-Average Grant Date Fair Value
Outstanding as of December 31, 2019	—	$	—
Granted	45	$	1.98
Vested	—	$	—
Forfeited/cancelled	—	$	—
Outstanding as of March 31, 2020	45	$	1.98

Employee stock purchase plan

In February 2014, the Company’s board of directors adopted the 2014 Employee Stock Purchase Plan and subsequently amended the plan in June 2018 (the “ESPP”). The ESPP authorizes the issuance of up to 337,597 shares of common stock to participating eligible employees and provides for two six-month offering periods. As of March 31, 2020, there were 217,985 shares remaining for future issuance under the plan.