Stockholders’ Equity

Shares Reserved for Future Issuance

The following shares of common stock were reserved for future issuance as of June 30, 2017:



Common stock options outstanding	10,819,036
Common stock available for future grant under 2012 Equity Incentive Plan	917,027
Common stock available for future grant under 2015 Inducement Plan	23,126
Employee Stock Purchase Plan	1,844,527
Total common shares reserved for future issuance	13,603,716

The following table summarizes our stock option activity under all equity incentive plans for the six months ended June 30, 2017 (shares in thousands):



	Number of options		Weighted average exercise price
Options outstanding at December 31, 2016	8,931		$	6.70
Granted	4,255		$	1.44
Exercised	(8	)	$	0.38
Canceled/forfeited/expired	(2,359	)	$	5.86
Options outstanding at June 30, 2017	10,819		$	4.82

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,
	2017		2016		2017		2016
Stock options
Risk-free interest rate	1.9	%	1.5	%	2.0	%	1.4	%
Volatility	89.5	%	79.1	%	89.4	%	79.7	%
Dividend yield	—		—		—		—
Expected term (years)	6.1		5.8		6.1		5.9
Performance stock options
Risk-free interest rate	—		0.4	%	2.1	%	1.4	%
Volatility	—		78.5	%	89.9	%	79.3	%
Dividend yield	—		—		—		—
Expected term (years)	0		5.5		5.6		6.0
Employee stock purchase plan shares
Risk-free interest rate	0.9	%	0.5	%	0.8	%	0.4	%
Volatility	111.5	%	82.6	%	113.8	%	81.0	%
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, including the adjustments to stock-based compensation expense associated with our May 2017 corporate restructuring (in thousands):



	Three months ended June 30,					Six months ended June 30,
	2017			2016		2017			2016
Research and development	$	650		$	1,299	$	1,758		$	2,731
Research and development-restructuring related adjustments	(1,399		)	—		(1,399		)	—
General and administrative	935			939		2,209			3,255
General and administrative-restructuring related adjustments	2,679			—		2,679			—
Total	$	2,865		$	2,238	$	5,247		$	5,986

In connection with our May 2017 corporate restructuring, we recorded a reversal of stock-based compensation in research and development expenses of $1.4 million for each of the three and six months ended June 30, 2017, as a result of the cancellation of unvested stock options. We recorded additional stock-based compensation in general and administrative expenses of $2.7 million for each of the three and six months ended June 30, 2017, as a result of termination provisions within certain employment agreements.