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Share-Based Compensation (Tables)
6 Months Ended
Jun. 30, 2018
Summary of Share-Based Compensation Expense
Share-based compensation expense recorded in these Consolidated Financial Statements for the three and six months ended June 30, 2018 and 2017 was based on awards granted under the LTIP. The following table summarizes share-based compensation expense for the periods presented (in thousands):
 
Three Months Ended
June 30,
 
Six Months Ended
June 30,
 
2018
 
2017
 
2018
 
2017
Research and development
$
2,553

 
$
2,735

 
$
4,810

 
$
5,043

General and administrative
3,756

 
3,918

 
8,401

 
7,212

Restructuring costs (1)
2,512

 

 
2,512

 

Total share-based compensation expense
$
8,821

 
$
6,653

 
$
15,723

 
$
12,255

Fair Value of Options Granted
The fair value of the options granted to employees and non-employee directors during the three months ended June 30, 2018 and 2017 was estimated as of the grant date using the Black-Scholes option-pricing model assuming the weighted-average assumptions listed in the following table:
 
Three Months Ended
June 30,
 
Six Months Ended
June 30,
 
2018
 
2017
 
2018
 
2017
Expected volatility
83.9%
 
72.2%
 
79.4%
 
72.8%
Risk-free interest rate
2.8%
 
2.0%
 
2.8%
 
2.0%
Expected dividend yield
—%
 
—%
 
—%
 
—%
Expected life (in years)
6.0
 
6.0
 
6.0
 
6.0
Weighted average grant date fair value
$11.28
 
$35.37
 
$13.82
 
$35.32
Summary of Company's Share Option Activity
The following table summarizes the Company’s stock option activity during the six months ended June 30, 2018:
 
Options
 
Weighted
Average
Exercise
Price
 
Weighted
Average
Remaining
Contractual
Term (years)
 
Aggregate
Intrinsic
Value
(in thousands)
Outstanding at December 31, 2017
4,406,752

 
$
38.93

 
7.60
 
$
30,455

Granted
4,046,300

 
20.39

 
 
 
 
Exercised
(174,536
)
 
25.63

 
 
 
 
Canceled
(559,092
)
 
49.80

 
 
 
 
Outstanding at June 30, 2018
7,719,424

 
$
28.73

 
7.78
 
$
4,882

Vested and expected to vest at June 30, 2018
7,271,257

 
$
28.99

 
7.70
 
$
4,880

Vested at June 30, 2018
2,505,501

 
$
31.82

 
5.67
 
$
4,869