Stockholders' Equity

6 Months Ended

Jun. 30, 2016

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Stockholders' Equity

Stockholders’ Equity

Shares Reserved for Future Issuance

The following shares of common stock were reserved for future issuance as of June 30, 2016:

Common stock options outstanding	6,937,798
Common stock available for future grant under 2012 Equity Incentive Plan	2,164,263
Common stock available for future grant under 2015 Inducement Plan	581,806
Employee Stock Purchase Plan	1,697,431
Total common shares reserved for future issuance	11,381,298

The following table summarizes our stock option activity under all equity incentive plans for the six months endedJune 30, 2016 (shares in thousands):

	Number of options		Weighted average exercise price
Options outstanding at December 31, 2015	5,126		$	8.91
Granted	2,375		$	6.62
Exercised	(56	)	$	4.76
Canceled/forfeited/expired	(507	)	$	10.47
Options outstanding at June 30, 2016	6,938		$	8.04

Stock-Based Compensation

The following table summarizes the weighted average assumptions used to estimate the fair value of stock options and performance stock awards granted to employees under our 2012 Equity Incentive Plan and 2015 Inducement Plan and the shares purchasable under our Employee Stock Purchase Plan during the periods presented:

	Three months ended June 30,				Six months ended June 30,
	2016		2015		2016		2015
Stock options
Risk-free interest rate	1.5	%	1.8	%	1.4	%	1.8	%
Volatility	79.1	%	79.1	%	79.7	%	78.8	%
Dividend yield	—		—		—		—
Expected term (years)	5.8		6.1		5.9		6.1
Performance stock options
Risk-free interest rate	0.4	%	—		1.4	%	1.8	%
Volatility	78.5	%	—		79.3	%	76.4	%
Dividend yield	—		—		—		—
Expected term (years)	5.5		0		6.0		6.0
Employee stock purchase plan shares
Risk-free interest rate	0.5	%	0.1	%	0.4	%	0.1	%
Volatility	82.6	%	76.9	%	81.0	%	74.9	%
Dividend yield	—		—		—		—
Expected term (years)	0.5		0.5		0.5		0.5

The following table summarizes the allocation of our stock-based compensation expense for all stock awards during the periods presented (in thousands):

	Three months ended June 30,				Six months ended June 30,
	2016		2015		2016		2015
Research and development	$	1,299	$	3,496	$	2,731	$	5,452
General and administrative	939		2,373		3,255		3,520
Total	$	2,238	$	5,869	$	5,986	$	8,972