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Stock Based Compensation (Tables)
9 Months Ended
Sep. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]  
Summary of Option Activity Under Company's Equity Incentive Plans

The following table summarizes stock option activity under the Company’s equity incentive plans and related information: 

 

 

Number of Shares

Underlying

Outstanding Options

 

 

Weighted-

Average Exercise

Price

 

 

Weighted-Average

Remaining

Contractual Term

(Years)

 

Outstanding — December 31, 2017

 

 

821,741

 

 

$

13.69

 

 

 

9.17

 

Options granted

 

 

564,050

 

 

$

13.43

 

 

 

 

 

Options exercised

 

 

(7,707

)

 

$

4.63

 

 

 

 

 

Options expired

 

 

(15,667

)

 

$

158.94

 

 

 

 

 

Options canceled/forfeited

 

 

(43,315

)

 

$

7.76

 

 

 

 

 

Outstanding — September 30, 2018

 

 

1,319,102

 

 

$

12.10

 

 

 

8.92

 

Exercisable — September 30, 2018

 

 

344,273

 

 

$

19.48

 

 

 

8.06

 

 

Summary of Weighted Average Valuation Assumptions Used to Estimate Fair Value of Employee Stock Options

The fair value of employee stock options was estimated using the following weighted-average assumptions for the nine months ended September 30, 2018 and 2017:

 

 

Nine Months Ended September 30,

 

 

 

2018

 

 

2017

 

Employee Stock Options:

 

 

 

 

 

 

 

 

Risk-free interest rate

 

 

2.66

%

 

 

2.02

%

Expected term (in years)

 

 

5.97

 

 

 

5.99

 

Dividend yield

 

 

 

 

 

 

Volatility

 

 

93.60

%

 

 

110.42

%

Weighted-average fair value of stock options granted

 

$

10.28

 

 

$

3.79

 

 

Summary of Stock-Based Compensation Recognized

Total stock-based compensation recognized was as follows (in thousands):

 

 

Three Months Ended September 30,

 

 

Nine Months Ended September 30,

 

 

 

2018

 

 

2017

 

 

2018

 

 

2017

 

Research and development

 

$

161

 

 

$

62

 

 

$

413

 

 

$

97

 

General and administrative(1)

 

 

541

 

 

 

263

 

 

 

1,445

 

 

 

459

 

Total stock-based compensation

 

$

702

 

 

$

325

 

 

$

1,858

 

 

$

556

 

 

 

(1)

2018 includes $0.1 million in modification stock-based compensation expense related to a Board member’s departure.