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Accounting for Shared-based Payments (Tables)
 3 Months Ended
Mar. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Schedule of stock-based compensation expense
Stock-based compensation has been recognized in operating results as follows:
 
Three Months Ended
March 31,
 
Three Months Ended
March 31,
($ in thousands)
2017
 
2016
Included in research and development
$
1,873

 
$
810

Included in general and administrative
1,024

 
392

Total stock-based compensation expense
$
2,897

 
$
1,202
Schedule of weighted-average assumptions used to estimate fair value of stock option awards using the Black-Scholes option valuation model
The estimated fair value of stock option awards was determined on the date of grant using the Black-Scholes option valuation model with the following weighted-average assumptions during the periods indicated.
 
Three Months Ended
March 31, 2017
 
Three Months Ended
March 31, 2016
Risk-free interest rate
1.96%-2.26%

 
1.47%-1.74%

Dividend yield
—

 
—

Expected volatility
50
%
 
60
%
Expected term (in years)
6 years

 
6 years
Summary of stock option activity and of changes in stock options outstanding under the Plan
A summary of stock option activity and of changes in stock options outstanding under the Plan is presented below:
 
Number of
Options
 
Exercise Price
Per Share
 
Weighted Average
Exercise Price
Per Share
 
Intrinsic
Value
(in thousands)
 
Weighted Average
Remaining
Contractual Term
Balance outstanding, December 31, 2016 (1)
27,867,171

 
$0.44-9.12
 
$
3.78

 
$
65,618

 
7.1 years
Granted
843,500

 
$4.57-6.77
 
6.06

 
—

 

Exercised (2)
(89,978
)
 
$1.90-5.34
 
1.90

 
203,957

 

Forfeited
(836,145
)
 
$2.83-7.91
 
4.50

 
—

 

Balance outstanding, March 31, 2017 (1)
27,784,548

 
$0.44-9.12
 
$
3.91

 
$
31,558

 
6.9 years
 
 
 
 
 
 
 
 
 
 
Exercisable, at March 31, 2017
13,913,788

 
$0.44-9.12
 
$
3.70

 
$
17,227

 
5.7 years
__________________________
(1)  Number of options represented above includes 2,159,500 options vesting upon a change of control, granted between November 20, 2009 and June 22, 2010. The fair value at the date of grant was approximately $28.6 million. Because the probability of a change of control transaction is not predictable no stock based compensation expense associated with these options has been recognized since the grant date.
(2)
The Company received proceeds of approximately $0.3 million from the exercise of stock options during the three months ended March 31, 2017.