Share-Based Payments (Tables)

6 Months Ended

Jun. 30, 2015

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

Summary of Stock Option Activity and Related Information

A summary of the Company’s stock option activity and related information follows:

	Shares		Weighted-Average Exercise Price		Remaining Contractual Life (in years)
Outstanding at December 31, 2014	1,438,528		$	4.93	7.86
Granted	744,328		9.05
Exercised	(67,914	)	0.80
Cancelled or forfeited	(54,608	)	7.77
Outstanding at June 30, 2015	2,060,334		$	6.48	8.17
Exercisable at June 30, 2015	740,977		$	4.68	7.34
Vested and expected to vest at June 30, 2015(1)	1,733,902		$	6.90	8.36

(1) Represents the number of vested options, plus the number of unvested options expected to vest.

Summary of Status and Changes of Unvested Restricted Stock

A summary of the unvested restricted stock is presented below:

	Restricted Stock		Weighted-Average Grant Date Fair Value
Unvested at December 31, 2014	125,027		$	5.39
Granted	1,300		10.08
Vested	(55,017	)	2.26
Unvested at June 30, 2015	71,310		$	7.89

Schedule of Share-based Compensation, Restricted Stock and Restricted Stock Units Activity [Table Text Block]

From time to time, upon approval by the Board of Directors, certain employees have been granted restricted stock units. A summary of the restricted stock units is presented below:

	Restricted Stock Units	Weighted-Average Grant Date Fair Value
Unvested at December 31, 2014	—	$	—
Granted	211,400	2.76
Vested	—	—
Unvested at June 30, 2015	211,400	$	2.76

Estimation of Fair Value of Stock Option Granted using Black-Scholes Option-Pricing Model Assumptions

The fair value of each stock option granted to employees and directors was estimated on the date of grant using the Black-Scholes option-pricing model based on the assumptions noted in the following table:

	Three Months Ended June 30,		Six Months Ended June 30,
	2015	2014	2015	2014
Risk-free interest rate	1.52–1.82%	1.81-1.93%	1.42–1.82%	1.80-2.02%
Expected dividend yield	—	—	—	—
Expected term (in years)	5.75-6	6	5.75-6	5.75-6
Expected volatility	69.27–71.37%	69.18-69.52%	69.27–74.11%	69.18-69.58%

Summary of Stock-based Compensation Expense

During the three and six months ended June 30, 2015 and 2014, the Company recorded stock-based compensation expense for employee and non-employee stock options, restricted stock, and restricted stock units which was allocated as follows in the statements of operations (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2015		2014		2015		2014
Research and development expense	$	219	$	144	$	498	$	499
General and administrative expense	383		357		768		638
	$	602	$	501	$	1,266	$	1,137