Common Stock and Stock Option Plan (Tables)
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12 Months Ended |
Dec. 31, 2015 |
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| Assumptions Used in the Black-Scholes Option-Pricing Model for Stock Options Granted |
The assumptions used in the Black-Scholes option-pricing model for
stock options granted during the years ended December 31,
2015, 2014 and 2013 are as follows:
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DECEMBER 31, |
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2015 |
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2014 |
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2013 |
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| Expected option term (in years) |
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5.5 – 6.3 |
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5.5 – 6.3 |
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6.0 |
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| Expected volatility |
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67% – 71 |
% |
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64% – 65 |
% |
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64% – 69 |
% |
| Risk-free interest rate |
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1.5% – 1.9 |
% |
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1.7% – 2.0 |
% |
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1.2% – 1.9 |
% |
| Expected dividend yield |
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0.0 |
% |
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0.0 |
% |
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0.0 |
% |
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| Stock Option Activity for Employee and Nonemployee |
A summary of stock option activity for employee and non-employee
awards under the Plan are presented below:
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NUMBER
OF
OPTIONS |
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WEIGHTED-
AVERAGE
PRICE
PER SHARE |
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WEIGHTED-
AVERAGE
REMAINING
CONTRACTUAL
TERM (YEARS) |
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OUTSTANDING—January 1, 2015
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3,604,713 |
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$ |
11.28 |
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9.1 |
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Granted
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1,160,626 |
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15.39 |
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Exercised
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(29,506 |
) |
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4.98 |
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Forfeited/Canceled
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(336,408 |
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13.46 |
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OUTSTANDING—December 31, 2015
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4,399,425 |
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12.24 |
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8.1 |
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EXERCISABLE—December 31, 2015
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2,058,772 |
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11.05 |
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7.5 |
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Vested and expected to vest as of December 31, 2015
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4,281,737 |
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$ |
12.22 |
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8.1 |
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| Summary of Company's Restricted Common Stock |
A summary of the Company’s restricted common stock is
presented below:
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SHARES |
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WEIGHTED-
AVERAGE
GRANT DATE
FAIR VALUE
PER SHARE |
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Nonvested—January 1, 2014
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— |
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$ |
— |
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Issued
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44,000 |
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16.30 |
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Vested
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(4,000 |
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14.34 |
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Nonvested—December 31, 2014
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40,000 |
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16.30 |
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Issued
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— |
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— |
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Vested
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(10,000 |
) |
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16.30 |
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Nonvested—December 31, 2015
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30,000 |
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$ |
16.30 |
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| Stock-Based Compensation Expense |
Stock-based compensation expense is classified in the statements of
operations as follows:
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YEAR ENDED DECEMBER 31, |
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2015 |
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2014 |
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2013 |
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Research and development
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$ |
4,202 |
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$ |
4,183 |
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$ |
124 |
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General and administrative
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5,530 |
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4,054 |
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371 |
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Total
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$ |
9,732 |
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$ |
8,237 |
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$ |
495 |
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| Stock Options Granted to Nonemployees [Member] |
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| Assumptions Used in the Black-Scholes Option-Pricing Model for Stock Options Granted |
The assumptions used to estimate fair value were as follows:
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DECEMBER 31, |
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2015 |
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2014 |
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2013 |
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Stock price
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$ |
8.21 – $24.43 |
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$ |
9.37 – $41.12 |
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$ |
7.42 |
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Expected option term (in years)
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4.37 – 5.29 |
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0.25 – 6.86 |
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7.0 |
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Expected volatility
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66% – 73 |
% |
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56% – 68 |
% |
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66 |
% |
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Risk-free interest rate
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1.2% – 1.6 |
% |
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0.1% – 2.3 |
% |
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2.8 |
% |
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Expected dividend yield
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0.0 |
% |
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0.0 |
% |
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0.0 |
% |
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