Stockholders' Equity (Tables)

3 Months Ended

Mar. 31, 2021

Summary of Common Stock Reserved for Future Issuance

Common stock reserved for future issuance was as follows:

	March 31, 2021
Common stock warrants		13,760
Common stock options and restricted stock units		898,306
Shares available under the 2015 equity incentive plan		63,391
Shares available under the employee stock purchase plan		75,315
		1,050,772

Summary of Stock Option Activity

The following table summarizes our stock option activity under all equity incentive plans for the three months ended March 31, 2021:

	Number of Outstanding Stock Options			Weighted Average Exercise Price
Outstanding as of December 31, 2020		576,534		$	23.33
Granted		326,456		$	4.52
Canceled/forfeited/expired		(12,184	)	$	25.47
Outstanding as of March 31, 2021		890,806		$	16.49

Schedule of Restricted Stock Unit Activity

The following table summarizes our restricted stock unit activity under all equity incentive plans for the three months ended March 31, 2021:

	Number of Outstanding Restricted Stock Units			Weighted Average Grant Date Fair Value
Balance as of December 31, 2020		7,677		$	5.32
Granted		4,000		$	3.89
Released		(4,177	)	$	6.18
Balance as of March 31, 2021		7,500		$	4.08

Schedule of Allocation of Stock-Based Compensation for All Options Including Performance Options with Market Condition, 2015 ESPP and Restricted Stock Units

The allocation of stock-based compensation for all options, 2015 Employee Stock Purchase Plan and restricted stock units is as follows (in thousands):

	March 31,
	2021		2020
Research and development	$	63	$	62
General and administrative		297		361
Total stock-based compensation expense	$	360	$	423

Employee Stock Option [Member]

Summary of Assumptions Used to Determine Fair Value of Employee Stock Option Grants and Performance Options with Market Condition

The assumptions used in the Black-Scholes option pricing model to determine the fair value of the employee stock option grants were as follows:

	March 31,
	2021		2020
Expected term (in years)	5.99 – 6.02			6.02
Risk-free interest rate	0.6% – 0.9%			1.5	%
Expected volatility	104.0% – 104.8%			102.2	%
Expected dividend yield		0.0%		0.0	%