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Stock Incentive Plans | 12 Months Ended
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Dec. 31, 2021
Stock Incentive Plans |
Stock Incentive Plans | 9. Stock Incentive Plans ​ Prior to the Company’s initial public offering in June 2015 (the “IPO”), the Company granted awards to eligible participants under its 2008 Equity Incentive Plan. In May 2015, the Company’s board of directors adopted and, in June 2015, the Company’s stockholders approved the 2015 Stock Incentive Plan, as amended and amended and restated since the IPO (“2015 Plan”), which became effective immediately prior to the effectiveness of the IPO. Subsequent to the IPO, option grants have been awarded to eligible participants only under the 2015 Plan. ​ The 2015 Plan provides for the grant of incentive stock options, non-statutory stock options, restricted stock awards, restricted stock units, stock appreciation rights and other stock-based awards. The Company’s employees, officers, directors and consultants and advisors are eligible to receive awards under the 2015 Plan. ​ Terms of stock option agreements, including vesting requirements, are determined by the Company’s board of directors, subject to the provisions of the applicable stock incentive plan. Options granted by the Company generally vest ratably over four years, with a one-year cliff, and options are exercisable from the date of grant for a period of ten years. For options granted through December 31, 2021, the exercise price or purchase price, as applicable, equaled the estimated fair value of the common stock as determined by the Company’s board of directors on the date of grant. ​ A summary of the Company’s stock option activity and related information for employees and non-employees follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | Weighted | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | Average | ​ | ​ | ​
​ | ​ | ​ | ​ | Weighted- | ​ | Remaining | ​ | Aggregate
​ | ​ | ​ | ​ | Average Exercise | ​ | Contractual | ​ | Intrinsic Value
​ |  | Shares |  | Price |  | Term (years) |  | (in thousands)
Outstanding at December 31, 2020 | ​ | 227,846 | ​ | $ | 68.25 | ​ | 8.13 | ​ | $ | —
Granted |  | 1,156,646 | ​ | $ | 15.91 | ​ | ​ | ​ | ​ | ​
Assumed in Quellis Acquisition | ​ | 55,414 | ​ | $ | 1.73 | ​ | ​ | ​ | ​ | ​
Exercised | ​ | (10,470) | ​ | $ | 2.10 | ​ | ​ | ​ | ​ | ​
Cancelled or forfeited |  | (81,196) | ​ | $ | 48.13 | ​ | ​ | ​ | ​ | ​
Expired | ​ | (1,507) | ​ | $ | 100.85 | ​ | ​ | ​ | ​ | ​
Outstanding at December 31, 2021 |  | 1,346,733 | ​ | $ | 22.25 |  | 9.02 | ​ | $ | 168,181
Vested and exercisable at December 31, 2021 |  | 135,412 | ​ | $ | 72.76 |  | 7.19 | ​ | $ | 77,798
​ The total intrinsic value of options exercised in the years ended December 31, 2021 and 2020 was $45 thousand and $29 thousand, respectively. The weighted-average grant date fair value of options granted to employees and non-employees for the years ended December 31, 2021 and 2020 was $9.82 and $3.81, respectively. ​ At December 31, 2021, the total unrecognized compensation expense related to unvested stock option awards was $10.8 million. The Company expects to recognize that cost over a weighted-average period of approximately 2.9 years. ​ Stock-Based Compensation Expense ​ The fair value of stock options granted to employees and non-employees was estimated using the Black-Scholes option-pricing model based on the following assumptions: ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Year Ended December 31, | ​
​ |  | 2021 |  | 2020 | ​
Weighted-average expected volatility |  | 69.28-71.32% | ​ | 75.51-82.08% | ​
Expected term (in years) |  | 5-6.25 | ​ | 5.50-6.25 | ​
Risk-free interest rate |  | 0.42-1.36% | ​ | 0.37-1.51% | ​
Expected dividend yield |  | 0% | ​ | 0% | ​
​ Volatility ​ Due to the lack of company-specific historical and implied volatility data of its common stock, the Company does not have relevant historical data to support its expected volatility. As such, the Company has used a weighted average of expected volatility based on the volatilities of a representative group of publicly traded biopharmaceutical companies. For purposes of identifying representative companies, the Company considered characteristics such as number of product candidates in early stages of product development, area of therapeutic focus, and length of trading history. The expected volatility was determined using an average of the historical volatilities of the representative group of companies for a period equal to the expected term of the option grant. The Company intends to continue to consistently apply this process using the same representative companies until a sufficient amount of historical information regarding the volatility of the Company’s own share price becomes available or until circumstances change, such that the identified entities are no longer representative companies. In the latter case, more suitable, similar entities whose share prices are publicly available would be utilized in the calculation. ​ Expected Term ​ The Company uses the “simplified method” to estimate the expected term of stock option grants. Under this approach, the weighted-average expected life is presumed to be the average of the contractual term (ten years) and the vesting term (generally four years) of the Company’s stock options, taking into consideration multiple vesting tranches. The Company utilizes this method due to lack of historical exercise data and the plain-vanilla nature of the Company’s share-based awards. ​ Risk-Free Rate ​ The risk-free rate was based on the yield curve of United States Treasury securities with periods commensurate with the expected term of the options being valued.

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