Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2017

Equity [Abstract]

Schedule of Shares of Common Stock Reserved for Future Issuance

Shares of common stock reserved for future issuance were as follows:

	AS OF DECEMBER 31,
	2017	2016
Shares available for issuance under the employee stock purchase plan	475,010	503,620
Options granted and outstanding	4,094,532	3,590,393
Unvested restricted stock units	195,172	107,250
Shares available for future option grants and restricted stock awards	947,199	724,723
Shares reserved for future issuance under equity incentive plans	5,711,913	4,925,986

Schedule of Restricted Stock Units

Summary RSU information is as follows:

	Shares		Weighted Average Grant Date Fair Value
Outstanding at January 1, 2016	—		$	—
Granted	118,000		$	19.39
Vested	—		$	—
Canceled/Forfeited	(10,750	)	$	19.39
Outstanding at December 31, 2016	107,250		$	19.39
Granted	224,540		$	5.60
Vested	(26,802	)	$	19.39
Canceled/Forfeited	(109,816	)	$	6.13
Outstanding at December 31, 2017	195,172		$	10.99

Summary of Stock Options

Summary stock option information is as follows:

	OPTIONS OUTSTANDING		WEIGHTED- AVERAGE EXERCISE PRICE		WEIGHTED- AVERAGE REMAINING CONTRACT TERM (IN YEARS)	AGGREGATE INTRINSIC VALUE (IN THOUSANDS)
Outstanding at January 1, 2016	2,832,467		$	11.48
Granted	991,110		$	15.49
Exercised	(20,782	)	$	3.12
Forfeited	(182,605	)	$	19.01
Expired	(29,797	)	$	25.65
Outstanding at December 31, 2016	3,590,393		$	12.13	7.33	$	5,062
Granted	954,301		$	6.46
Exercised	(175,183	)	$	2.56
Forfeited	(117,124	)	$	14.35
Expired	(157,855	)	$	17.19
Outstanding at December 31, 2017	4,094,532		$	10.96	7.21	$	2,771
Vested and expected to vest after December 31, 2017	4,094,532		$	10.96	7.21	$	2,771
Exercisable at December 31, 2017	2,464,086		$	10.98	6.29	$	2,771

Schedule of Stock-Based Compensation Expense Recognized

Total stock-based compensation expense recognized in our statements of operations is as follows (in thousands):

	YEARS ENDED DECEMBER 31,
	2017		2016		2015
Employee:
Research and development	$	3,613	$	3,923	$	2,034
General and administrative	4,879		5,029		3,810
Non-Employee:
Research and development	85		268		205
General and administrative	56		63		248
Total stock-based compensation expense	$	8,633	$	9,283	$	6,297

Schedule of Fair Value of Stock Options Granted to Employees

The fair values of stock options granted to employees were calculated using the following assumptions:

	YEARS ENDED DECEMBER 31,
	2017		2016		2015
Weighted-average estimated fair value	$	4.41	$	10.07	$	15.65
Risk-free interest rate (1)	1.9% - 2.2%		1.1% - 2.4%		1.5% - 1.9%
Expected term of options (in years) (2)	5.50 - 6.08		5.50 - 9.46		5.50 - 6.08
Expected stock price volatility (3)	80% - 91%		77% - 93%		77% - 91%
Expected dividend yield (4)	—%		—%		—%

(1) The risk-free interest rate assumption was based on zero-coupon U.S. Treasury instruments that had terms consistent with the expected term of our stock option grants.

(2) We used the “simplified method” for options to determine the expected term of our stock option grants. Under this approach, the weighted-average expected life is presumed to be the average of the vesting term and the contractual term of the option.

(3) Volatility is a measure of the amount by which a financial variable, such as share price, has fluctuated or is expected to fluctuate during a period. We analyzed the stock price volatility of companies at a similar stage of development to estimate expected volatility of our stock price.

(4) We have never declared or paid cash dividends and do not presently plan to pay cash dividends in the foreseeable future.