Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2015

Stock-Based Compensation Expense Related to Common Stock Options, Common Stock, Restricted Common Stock and Common Stock Warrants

Following is the stock-based compensation expense related to common stock options, common stock, restricted common stock and common stock warrants:

	Three Months Ended June 30,				Six Months Ended June 30,
	2015		2014		2015		2014
	(in thousands)
Research and development	$	226	$	221	$	543	$	860
General and administrative		574		524		1,191		1,534
Total stock-based compensation expense	$	800	$	745	$	1,734	$	2,394

Summary of Stock Option Activity

The following table summarizes the stock option activity in the Company’s equity incentive plans from December 31, 2014 through June 30, 2015:

	Shares			Weighted Average Exercise Price
Outstanding, December 31, 2014		3,332,617		$	5.79
Granted		304,000			3.45
Exercised		(95,574	)		1.80
Options forfeited/cancelled		(348,718	)		4.42
Outstanding, June 30, 2015		3,192,325		$	5.83

Weighted Average Assumptions Used to Determine Fair Value of Options Granted

The fair value of all other options granted is determined using the Black-Scholes option-pricing model. The following weighted average assumptions were used:

	Six Months Ended June 30,	Six Months Ended June 30,
	2015	2014
Risk-free interest rate	1.64%	1.58%
Expected life of the options	6.0 years	6.0 years
Expected volatility of the underlying stock	104%	114%
Expected dividend rate	0%	0%

Summary of Restricted Stock Grant Activity

The following table summarizes the restricted stock grant activity in the Company’s equity incentive plans from December 31, 2014 through June 30, 2015:

	Shares
Outstanding, December 31, 2014		416,670
Granted		337,935
Exercised		—
Options forfeited/cancelled		—
Outstanding, March 31, 2015		754,605