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Stock award plan | 6 Months Ended
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Jun. 30, 2021
Share-based Payment Arrangement [Abstract] |
Stock award plan | 8. Stock award plan In May 2013, the Company’s Board of Directors and stockholders approved the 2013 Long Term Incentive Plan, which became effective upon the closing of the Company’s initial public offering. The 2013 Long Term Incentive Plan provides for the grant of incentive stock options, nonstatutory stock options, restricted stock awards and other stock-based awards. The number of shares of common stock reserved for issuance under the 2013 Long Term Incentive Plan is the sum of (1) 122,296 shares of common stock available for issuance under the Company’s 2009 Equity and Long Term Incentive Plan and 2013 Stock Incentive Plan, (2) the number of shares (up to 3,040,444 shares) equal to the sum of the number of shares of common stock subject to outstanding awards under the Company’s 1998 Employee, Director and Consultant Stock Option Plan, 2009 Equity and Long Term Incentive Plan and 2013 Stock Incentive Plan that expire, terminate or are otherwise surrendered, canceled, forfeited or repurchased by the Company at their original issuance price pursuant to a contractual repurchase right plus (3) an annual increase, to be added on the first day of each fiscal year until the expiration of the 2013 Long Term Incentive Plan, equal to the lowest of 2,500,000 shares of common stock, 4% of the number of shares of common stock outstanding on the first day of the fiscal year and an amount determined by the Company’s Board of Directors. As of June 30, 2021, awards for 849,277 shares of common stock are available for issuance under the 2013 Long Term Incentive Plan. There are no additional shares of common stock available for issuance under the Company’s 1998 Employee, Director and Consultant Stock Option Plan, 2009 Equity and Long Term Incentive Plan or 2013 Stock Incentive Plan. In January 2020, the Company’s Board of Directors approved the 2020 Inducement Stock Incentive Plan. The 2020 Inducement Stock Incentive Plan provides for the grant of incentive stock options, nonstatutory stock options, restricted stock awards and other stock-based awards for up to an aggregate of 1,000,000 shares of common stock. Any grants made under the 2020 Inducement Stock Incentive Plan must be made pursuant to the Nasdaq Listing Rule 5635(c)(4) inducement grant exception as a material component of the Company’s new hires’ employment compensation. In December 2020, the Company’s Board of Directors approved an additional 1,000,000 shares of common stock that may be issued under the 2020 Inducement Stock Incentive Plan. As of June 30, 2021, awards for 960,033 shares of common stock are available for issuance under the 2020 Inducement Stock Incentive Plan. From January 1, 2021 through June 30, 2021, the Company issued a total of 2,282,914 stock options to various employees. Of those, 206,025 were inducement grants for non-statutory stock options, all of which were made pursuant to the 2020 Inducement Stock Incentive Plan. A summary of stock option activity is as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ |  | ​ | ​ |  | Weighted- |  |  | ​ | ​
​ | ​ | ​ | ​ | Weighted- | ​ | average | ​ | ​ | Aggregate
​ | ​ | ​ | ​ | average | ​ | remaining | ​ | ​ | intrinsic
​ | ​ | Number of | ​ | exercise | ​ | contractual | ​ | ​ | value(in
​ | ​ | options | ​ | price | ​ | term | ​ | ​ | thousands)
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ |  | ​ | ​ | ​
Outstanding at December 31, 2020 |  | 9,663,677 | ​ | $ | 38.72 |  |  |  | ​ | ​ |
Granted |  | 2,282,914 | ​ | ​ | 63.38 |  |  |  | ​ | ​ |
Exercised |  | (475,942) | ​ | ​ | 27.66 |  |  |  | ​ | ​ |
Forfeited/Cancelled |  | (439,189) | ​ | ​ | 53.85 |  |  |  | ​ | ​ |
Outstanding at June 30, 2021 |  | 11,031,460 | ​ | $ | 43.70 |  | 7.34 | years | ​ | $ | 64,268
Vested or Expected to vest at June 30, 2021 |  | 5,192,055 | ​ | $ | 49.23 |  | 8.62 | years | ​ | $ | 16,236
Exercisable at June 30, 2021 |  | 5,255,302 | ​ | $ | 37.17 |  | 5.89 | years | ​ | $ | 47,129
​ The fair value of grants made in the six months ended June 30, 2021 was contemporaneously estimated on the date of grant using the following assumptions: ​
​ | ​ | ​
​ |  | Six months ended
​ |  | June 30, 2021
Risk-free interest rate |  | 0.51% - 0.96%
Expected volatility |  | 85.94% - 88.61%
Expected term |  | 5.5 years
​ The Company assumed no expected dividends for all grants. The weighted average grant date fair value of options granted during the six month period ended June 30, 2021 was $44.66 per share. The expected term of options was estimated based on the Company’s historical exercise data and the expected volatility of options was estimated based on the Company’s historical stock volatility. The risk-free rate of the options was based on U.S. Government Securities Treasury Constant Maturities yields at the date of grant for a term similar to the expected term of the option. Restricted Stock Awards and Restricted Stock Units—Restricted stock awards and restricted stock units are granted subject to certain restrictions, including in some cases service or time conditions (restricted stock). The grant-date fair value of restricted stock awards and restricted stock units, which have been determined based upon the market value of the Company’s shares on the grant date, are expensed over the vesting period. From January 1, 2021 through June 30, 2021, the Company issued a total of 951,558 restricted stock units to various employees. Of those, 68,540 were inducement grants for restricted stock units, all of which were made pursuant to the 2020 Inducement Stock Incentive Plan. ​ The following table summarizes information on the Company’s restricted stock awards and units: ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Restricted Stock Awards and Units
​ | ​ | ​ | ​ | Weighted
​ | ​ | ​ | ​ | Average
​ | ​ | ​ | ​ | Grant
​ | ​ | Number of | ​ | Date
​ |  | Shares |  | Fair Value
Unvested at December 31, 2020 | ​ | 982,058 | ​ | $ | 41.78
Granted |  | 951,558 | ​ | ​ | 64.21
Vested |  | (319,127) | ​ | ​ | 35.21
Forfeited |  | (82,059) | ​ | ​ | 57.75
Unvested at June 30, 2021 |  | 1,532,430 | ​ | $ | 56.23
​ Employee Stock Purchase Plan—In June 2016, the Company established an Employee Stock Purchase Plan (as amended, “ESPP” or the "Plan”), for certain eligible employees. The Plan is administered by the Company’s Board of Directors or a committee appointed by the Company’s Board of Directors. In June 2021, the Plan was amended to increase the total number of shares available for purchase under the Plan from one million shares to two million shares of the Company’s common stock. Employees may participate over a six month period through payroll withholdings and may purchase, at the end of the six month period, the Company’s common stock at a purchase price of at least 85% of the closing price of a share of the Company’s common stock on the first business day of the offering period or the closing price of a share of the Company’s common stock on the last business day of the offering period, whichever is lower. No participant will be granted a right to purchase the Company’s common stock under the Plan if such participant would own more than 5% of the total combined voting power of the Company or any subsidiary of the Company after such purchase. For the three and six month periods ended June 30, 2021, the Company recorded $0.7 million and $1.2 million, respectively, in compensation expense related to the ESPP. The Company recorded share-based compensation expense in the statement of operations related to incentive stock options, nonstatutory stock options, restricted stock awards, restricted stock units and the ESPP as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended June 30, | ​ | Six Months Ended June 30, | ​
​ |  | 2021 |  | 2020 |  | 2021 |  | 2020 | ​
Research and development | ​ | $ | 13,443 | ​ | $ | 8,562 | ​ | $ | 27,168 | ​ | $ | 16,741 | ​
Selling, general and administrative | ​ |  | 12,256 | ​ |  | 8,348 | ​ |  | 24,238 | ​ |  | 15,389 | ​
Total | ​ | $ | 25,699 | ​ | $ | 16,910 | ​ | $ | 51,406 | ​ | $ | 32,130 | ​
​ As of June 30, 2021, there was approximately $252.5 million of total unrecognized compensation cost related to unvested share-based compensation arrangements granted under the 2009 Equity and Long Term Incentive Plan, the 2013 Long Term Incentive Plan and equity awards made pursuant to the Nasdaq Listing Rule 5635(c)(4) inducement grant exception for new hires. This cost is expected to be recognized as share-based compensation expense over the weighted average remaining service period of approximately 2.88 years.

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