Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2018

Summary of Stock Option Activity

Stock option activity is summarized as follows:

	Number of Outstanding Options			Weighted Average Exercise Price		Weighted Remaining Contractual Term		Aggregate Intrinsic Value
Outstanding as of December 31, 2017		4,617,059		$	5.52
Granted		2,091,361		$	2.35
Exercised		(12,141	)	$	1.10
Canceled/forfeited/expired		(1,705,605	)	$	4.72
Outstanding as of December 31, 2018		4,990,674		$	4.47		6.97	$	427
Options vested and expected to vest as of December 31, 2018		4,990,674		$	4.47		6.97	$	427
Options exercisable as of December 31, 2018		2,856,255		$	5.52		5.62	$	427

Summary of Assumptions Used to Determine Fair Value of Employee Stock Purchase Plan

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

	Years Ended December 31,
	2018			2017			2016
Expected term (in years)		0.50			0.50			0.50
Risk-free interest rate	1.4% – 2.1%			0.6% – 1.0%			0.4% – 0.6%
Expected volatility	71.5% – 99.7%			74.5% – 115.2%			75.5% – 80.8%
Expected dividend yield		0.0	%		0.0	%		0.0	%

Schedule of Restricted Stock Units Activity

Occasionally, we grant restricted stock units to employees. Restricted stock unit activity is summarized as follows:

	Number of Outstanding Restricted Stock Units			Weighted Average Grant Date Fair Value
Balance as of December 31, 2017		49,300		$	4.28
Granted		270,300		$	0.85
Released		(39,301	)	$	4.53
Forfeited		(63,599	)	$	1.24
Balance as of December 31, 2018		216,700		$	0.85

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

The allocation of stock-based compensation for all options, including performance options with market condition and restricted stock units is as follows (in thousands):

	Twelve Months Ended December 31,
	2018		2017		2016
Research and development	$	1,216	$	1,399	$	1,876
General and administrative		2,215		5,385	$	3,153
Total share-based compensation expense	$	3,431	$	6,784	$	5,029

Summary of Warrants Outstanding

Warrants outstanding as of December 31, 2018 are as follows:

Number		Exercise Price		Expiration
Outstanding		Per Share		Date
	6,488,205	$	4.64	December 2019
	2,006	$	7.48	March 2021
	14,913	$	20.12	July 2023
	95,542	$	3.14	November 2023
	41,666	$	3.60	June 2024
	40,376	$	3.72	December 2024
	6,682,708

Summary of Common Stock Reserved for Future Issuance

Common stock reserved for future issuance is as follows:

	Years Ended December 31,
	2018		2017
Class X Preferred Stock (if-converted to common stock)		11,429,760		11,429,760
Common stock warrants		6,682,708		6,682,708
Common stock options and awards outstanding		5,207,374		4,666,359
Shares available under the 2015 Plan		1,501,099		765,427
Shares available under the 2015 ESPP		839,305		583,819
		25,660,246		24,128,073

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:

	Years Ended December 31,
	2018			2017			2016
Expected term (in years)	5.00 – 6.08			5.50 – 6.08			5.50 – 6.08
Risk-free interest rate	2.3% – 3.0%			1.9% – 2.1%			1.2% – 2.1%
Expected volatility	87.9% – 98.4%			99.1% – 124.4%			80.7% – 84.5%
Expected dividend yield		0.0	%		0.0	%		0.0	%