Stockholders' Equity (Tables)

3 Months Ended

Mar. 31, 2018

Summary of Common Stock Reserved for Future Issuance

Common stock reserved for future issuance is as follows:

	March 31, 2018
Class X Preferred Stock (if-converted to common stock)		11,429,760
Common stock warrants		6,682,708
Common stock options and awards outstanding		5,611,880
Shares available under the 2015 Plan		971,911
Shares available under the 2015 ESPP		881,710
		25,577,969

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, 2018:

	Number of Outstanding Options			Weighted Average Exercise Price
Outstanding as of December 31, 2017		4,617,059		$	5.52
Granted		1,250,761		$	3.31
Exercised		(3,593	)	$	2.02
Canceled/forfeited/expired		(259,013	)	$	6.21
Outstanding as of March 31, 2018		5,605,214		$	5.00

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, 2018:

	Number of Outstanding Restricted Stock Units			Weighted Average Grant Date Fair Value
Balance as of December 31, 2017		49,300		$	4.28
Granted		—		$	—
Released		(35,968	)	$	4.64
Forfeited		(6,666	)	$	3.30
Balance as of March 31, 2018		6,666		$	3.30

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, including performance options with a market condition, 2015 ESPP and restricted stock units is as follows (in thousands):

	Three Months Ended March 31,
	2018		2017
Research and development	$	324	$	444
General and administrative		604		833
	$	928	$	1,277

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:

	Three Months Ended March 31,
	2018			2017
Expected term (in years)	5.77 – 6.08			6.02 – 6.06
Risk-free interest rate	2.3% – 2.7%			2.0% – 2.1%
Expected volatility	89.2% – 98.4%			104.0% – 105.2%
Expected dividend yield		0.0	%		0.0	%