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Equity Plans | 3 Months Ended
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Mar. 31, 2022
Equity Plans |
Equity Plans | Note 11. Equity Plans Equity Incentive Plan In July 2016, the Company’s board of directors and stockholders approved the Company’s 2016 Equity Incentive Plan (the “2016 Plan”) to replace the 2007 Stock Option Plan. The 2016 Plan is administered by the board of directors, or a committee appointed by the board of directors, which determines the types of awards to be granted, including the number of shares subject to the awards, the exercise price and the vesting schedule. Awards granted under the 2016 Plan expire no later than ten years from the date of grant. As of March 31, 2022, 1,095,340 shares were available for issuance under the 2016 Plan. Inducement Plan In May 2018, the Company’s board of directors approved the 2018 Inducement Plan, as subsequently amended. The 2018 Inducement Plan is a non-stockholder approved stock plan, under which the Company awards options and restricted stock unit awards to persons that were not previously employees or directors of the Company, or following a bona fide period of non-employment, as an inducement material to such persons entering into employment with the Company, within the meaning of Rule 5635(c)(4) of the Nasdaq Listing Rules. The 2018 Inducement Plan is administered by the board of directors or the Compensation Committee of the board, which determines the types of awards to be granted, including the number of shares subject to the awards, the exercise price and the vesting schedule. Awards granted under the 2018 Inducement Plan expire no later than ten years from the date of grant. As of March 31, 2022, 743,125 shares were available for issuance under the Amended and Restated 2018 Inducement Plan. Stock Options Stock option activity under the Company’s equity incentive and inducement plans is set forth below: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Weighted- | ​ | Weighted- | ​ | ​
​ | ​ | ​ | ​ | Average | ​ | Average | ​ | ​
​ | ​ | ​ | ​ | Exercise | ​ | Remaining | ​ | Aggregate
​ | ​ | Options | ​ | Price Per | ​ | Contractual | ​ | Intrinsic
​ |  | Outstanding |  | Share |  | Life (years) |  | Value (1)
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | (in millions)
Balances at December 31, 2021 |  | 5,890,540 |  | $ | 17.66 |  | 7.47 |  | $ | 102.7
Options granted |  | 1,122,100 | ​ | ​ | 28.85 |  | ​ |  | ​ |
Options exercised |  | (165,488) | ​ | ​ | 11.68 |  | ​ |  | ​ | ​
Options forfeited | ​ | (37,542) | ​ | ​ | 39.04 | ​ | ​ | ​ | ​ | ​
Balances at March 31, 2022 |  | 6,809,610 |  | $ | 19.53 |  | 7.37 | ​ | $ | 46.0
Options exercisable – March 31, 2022 | ​ | 3,349,241 |  | $ | 13.59 |  | 6.05 | ​ | $ | 34.5
Options vested and expected to vest – March 31, 2022 | ​ | 6,809,610 | ​ | $ | 19.53 |  | 7.37 | ​ | $ | 46.0
(1) | The aggregate intrinsic values were calculated as the difference between the exercise price of the options and the closing price of the Company’s common stock on March 31, 2022. The calculation excludes options with an exercise price higher than the closing price of the Company’s common stock on March 31, 2022.
The estimated weighted-average grant-date fair value of common stock underlying options granted to employees during the three months ended March 31, 2022 was $22.56 per share. Stock Options Valuation Assumptions The fair value of employee stock option awards was estimated at the date of grant using a Black-Scholes option-pricing model with the following assumptions: ​
​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended March 31,
​ | ​ | 2022 | ​ | 2021
Expected term (in years) |  | 5.27- 6.08 |  | 5.27- 6.08
Expected volatility |  | 97.1% - 98.2% | ​ | 89.8% - 90.2%
Risk-free interest rate |  | 1.64% - 2.13% | ​ | 0.11% - 0.97%
Dividend yield |  | — |  | —
​ In determining the fair value of the options granted, the Company uses the Black-Scholes option-pricing model and assumptions discussed below. Each of these inputs is subjective and generally requires judgment to determine. Expected Term—The Company’s expected term represents the period that the Company’s options granted are expected to be outstanding and is determined using the simplified method (based on the mid-point between the vesting date and the end of the contractual term). The Company has limited historical exercise information to develop reasonable expectations about future exercise patterns and post-vesting employment termination behavior for its stock option grants. Expected Volatility—For the year ended December 31, 2021, the Company’s expected volatility was estimated based upon a mix of 50% of the average volatility for comparable publicly traded biopharmaceutical companies over a period equal to the expected term of the stock option grants and 50% of the volatility of the Company’s stock price since its initial public offering in August 2016. Beginning January 1, 2022, the Company’s expected volatility is estimated based upon a mix of 25% of the average volatility for comparable publicly traded biopharmaceutical companies over a period equal to the expected term of the stock option grants and 75% of the volatility of the Company’s stock price since its initial public offering in August 2016. Risk-Free Interest Rate—The risk-free interest rate is based on the U.S. Treasury zero coupon issues in effect at the time of grant for periods corresponding with the expected term of option. Expected Dividend—The Company has never paid dividends on its common stock and has no plans to pay dividends on its common stock. Therefore, the Company used an expected dividend yield of zero. Restricted Stock Units Restricted stock unit activity under the Company’s equity incentive plans is set forth below: ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Weighted
​ | ​ | ​ | ​ | Average
​ | ​ | Number of | ​ | Grant Date
​ |  | Shares |  | Fair Value
Unvested RSUs at December 31, 2021 | ​ | 405,972 | ​ | $ | 20.13
Granted | ​ | 312,550 | ​ | ​ | 28.73
Vested | ​ | (104,712) | ​ | ​ | 14.73
Forfeited | ​ | (7,000) | ​ | ​ | 28.73
Unvested RSUs at March 31, 2022 | ​ | 606,810 | ​ | $ | 24.61
​ Performance Stock Units Performance stock unit (“PSU”) activity under the Company’s equity incentive plans is set forth below: ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Weighted
​ | ​ | ​ | ​ | Average
​ | ​ | Number of | ​ | Grant Date
​ |  | Shares |  | Fair Value
Unvested PSUs at December 31, 2021 | ​ | 105,500 | ​ | $ | 23.57
Granted |  | — | ​ | ​ | —
Vested | ​ | — | ​ | ​ | —
Forfeited | ​ | — | ​ | ​ | —
Unvested PSUs at March 31, 2022 | ​ | 105,500 | ​ | $ | 23.57
​ The terms of the unvested PSUs provide for 100% of shares to be earned based on the achievement of certain pre-determined performance objectives, subject to the participant’s continued employment. The PSUs will expire on February 28, 2026 if the performance objectives are not achieved. The PSUs will vest, if at all, upon certification by the Compensation Committee of the Company’s Board of Directors of the actual achievement of the performance objectives, subject to specified change of control exceptions. Stock-based compensation expense associated with PSUs is based on the fair value of the Company’s common stock on the grant date, which equals the closing price of the Company’s common stock on the grant date. The Company recognizes compensation expense over the vesting period of the awards that are ultimately expected to vest when the achievement of the related performance objective becomes probable. The total fair value of outstanding PSUs as of March 31, 2022 was $2.5 million. As of March 31, 2022, the achievement of the related performance objective was deemed not probable and, accordingly, no stock-based compensation for the PSUs has been recognized as expense as of March 31, 2022. Employee Stock Purchase Plan ​ The 2016 Employee Stock Purchase Plan (“2016 ESPP”) allows eligible employees to purchase shares of the Company’s common stock at a discount through payroll deductions of up to 15% of their eligible compensation. At the end of each offering period, eligible employees are able to purchase shares at 85% of the lower of the fair market value of the Company’s common stock at the beginning of the offering period or at the end of each applicable purchase period. During the three months ended March 31, 2022, a total of 28,931 shares of common stock were issued under the 2016 ESPP, and 1,285,068 shares remain available for issuance as of March 31, 2022. Stock-Based Compensation ​ Total stock-based compensation expense was as follows (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended March 31,
​ |  | 2022 |  | 2021
Research and development | ​ | $ | 3,326 | ​ | $ | 1,475
General and administrative | ​ |  | 2,609 | ​ |  | 1,185
Total stock-based compensation expense | ​ | $ | 5,935 | ​ | $ | 2,660
​ As of March 31, 2022, total unrecognized stock-based compensation expense was approximately $73.6 million, which the Company expects to recognize over a weighted-average period of approximately 2.9 years.

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