Stock Plans and Stock Based Compensation (Tables)

12 Months Ended

Dec. 31, 2015

Summary of Stock Option Activity

A summary of stock option activity under the Amended and Restated 2010 Plan, the 2000 Stock Incentive Plan and the 2000 Director Stock Option Plan is summarized as follows:

	Number of Shares			Weighted Average Exercise Price per Share
Outstanding, December 31, 2014 (7,534,372 exercisable at weighted average price of $2.45 per share)		11,319,619		$	2.66
Granted		3,228,250			2.33
Exercised		(695,338	)		1.63
Cancelled		(561,907	)		3.63
Outstanding, December 31, 2015 (7,946,442 exercisable at weighted average price of $2.58 per share)		13,290,624		$	2.60
Vested and unvested expected to vest		12,697,446		$	2.60

Summary of Compensation Expense Related to ESPP

For the years ended December 31, 2015, 2014 and 2013, the Company recorded compensation expense related to its ESPP and calculated the fair value of shares expected to be purchased under the ESPP using the Black-Scholes models with the following assumptions:

	For the Year Ended December 31,
	2015			2014			2013
Compensation expense recognized under ESPP	$	29,924		$	25,604		$	40,008
Expected term		6 months			6 months			6 months
Risk-free interest rate		0.06-0.5	%		0.06-0.09	%		0.08-0.1	%
Volatility		55-78	%		55-66	%		42-66	%
Dividends		None			None			None

Employee and Non-Employee Share Based Compensation Expense Allocation

For the years ended December 31, 2015, 2014 and 2013, the Company recorded employee and non-employee stock-based compensation expense to the following line items in its Costs and Expenses section of the Consolidated Statements of Operations and Comprehensive Loss:

	For the Year Ended December 31,
	2015		2014		2013
Research and development expenses	$	1,023,269	$	671,063	$	879,052
General and administrative expenses		2,851,977		2,388,926		2,123,189
Total stock-based compensation expense	$	3,875,246	$	3,059,989	$	3,002,241

Vesting Tied to Service Conditions [Member]

Valuation Assumptions Used to Calculate Fair Value of Employee Options Awarded

The Black-Scholes option pricing model employs the following key assumptions for employee and director options awarded during each of the following years:

	For the Year Ended December 31,
	2015			2014			2013
Expected term (years)—Employees		6.0			6.0			6.0
Expected term (years)—Officers and Directors		7.0			7.0			7.0
Risk-free interest rate		1.5-1.9	%		1.9	%		1.0-2.1	%
Expected volatility		68-70	%		70-71	%		70-72	%
Expected dividend yield		None			None			None

Vesting Tied to Market Conditions [Member]

Valuation Assumptions Used to Calculate Fair Value of Employee Options Awarded

The key assumptions used in these Monte Carlo simulation models and resulting valuations are noted in the following table:

	Market Condition Options Granted February 18, 2014	Market Condition Options Granted June 2, 2014
Expected life (years)—officers	6	6
Risk-free interest rate	1.9%	2.1%
Volatility	70%	65%
Dividends	None	None
Number of options granted	640,000	400,000
Fair value per share	$1.20	$0.34