5. STOCK-BASED COMPENSATION

Total stock-based compensation expense related to all of our stock-based awards was as follows (in thousands):

​

​

In November 2021, we announced a reduction of workforce, primarily in the research organization. As a result, we entered in severance agreements with the affected employees, which provided, among others, extension of the date through which certain affected employees can exercise their vested options. As a result of these modifications, we recorded an incremental stock-based compensation expense of approximately $0.4 million for the year ended December 31, 2021. The incremental compensation expenses were computed based on the fair values of the modified awards on the modification date.  

​

Equity Incentive Plans

​

On May 16, 2018, our stockholders approved the adoption of the Company’s 2018 Equity Incentive Plan (2018 Plan). The 2018 Plan is the successor plan to the 2011 Equity Incentive Plan, the 2000 Equity Incentive Plan, and the 2000 Non-Employee Directors' Stock Option Plan. We have two active equity plans, our 2018 Plan and the Rigel’s Inducement Plan, as amended (Inducement Plan, and together with 2018 Plan, the Equity Incentive Plans). The 2018 Plan provides for granting of stock awards to our officers, directors, all other employees and consultants. The Inducement Plan, which is a non-stockholder approved stock plan, is intended mainly to provide an inducement material by granting awards for certain individuals to enter into employment with us. Awards granted under our Equity Incentive Plans expire no later than 10 years from the date of grant. Awards may be granted with different vesting terms from time to time. To date, we granted stock options and restricted stock units (RSUs) under our Equity Incentive Plans.

​

In January 2021, our Board of Directors approved the 825,000 shares increase in available number of shares for future grant under our 2018 Plan, which became effective upon approval by our stockholders during the stockholders annual meeting in May 2021. In September 2021 and January 2022, our Board of Directors approved the increase of 469,000 shares and 610,000 shares, respectively, in common stock reserved for issuance under the Inducement Plan. As of December 31, 2021, a total of 41,739,675 shares and 1,752,333 shares are authorized for issuance under the 2018 Plan and Inducement Plan, respectively.

Stock Options and RSUs

The following table summarizes stock option and RSU activity, and shares available for grant under our Equity Incentive Plans for the periods presented:

​

​

Of the total stock options outstanding as of December 31, 2021 above, 1,991,250 shares outstanding are performance-based stock options wherein the achievement of the corresponding corporate-based milestones were not considered as probable. Accordingly, the related grant date fair value for these performance-based stock options of $4.0 million has not been recognized as stock-based compensation expense as of December 31, 2021.

During the year ended December 31, 2021, 2020 and 2019, stock options vested were 4,765,814 shares, 4,386,910 shares and 4,442,936 shares, respectively, with weighted-average exercise price of $2.67 per share, $2.41 per share, and $3.26 per share, respectively.

The aggregate intrinsic values of stock options outstanding, vested and expected to vest, and exercisable represents the difference between the exercise price of the underlying awards and the quoted price of our common stock for the options that were in-the-money as of December 31, 2021. During the year ended December 31, 2021, 2020 and 2019, aggregate intrinsic values of stock option exercises was approximately $2.1 million, $0.5 million and $0.01 million, respectively, representing the difference between the fair value of our common stock at the date of exercise and the exercise price paid.

​

During the year ended December 31, 2021, 2020 and 2019, we granted options to purchase 6,997,981 shares, 8,462,090 shares and 7,457,575 shares of common stock, respectively, with weighted-average grant date fair value of $2.34 per share, $1.42 per share and $1.27 per share, respectively.

​

The following table summarizes the weighted-average assumptions relating to stock options granted during the periods presented:

​

​

As of December 31, 2021, there were approximately $12.9 million of unrecognized stock-based compensation cost which is expected to be recognized over the remaining weighted-average period of 2.77 years, related to time-based stock options, performance-based stock options wherein achievement of the corresponding corporate-based milestones was considered as probable, and RSUs.

Details of our stock options by exercise price are as follows as of December 31, 2021:

​

​

Employee Stock Purchase Plan

Our Purchase Plan permits eligible employees to purchase common stock at a discount through payroll deductions during defined offering periods. Under our Purchase Plan, for the year ended December 31, 2021, 2020 and 2019, we issued 932,018 shares, 567,391 shares and 747,691 shares of common stock, respectively, at an average price of $1.51 per share, $1.54 per share and $1.92 per share, respectively.

​

In January 2021, our Board of Directors approved the 5,500,000 shares increase in the maximum number of shares authorized for issuance under the Purchase Plan, which became effective upon approval by our stockholders during the annual stockholders meeting in May 2021. As of December 31, 2021, there were 4,584,484 shares reserved for future issuance under the Purchase Plan.

​

The fair value of awards granted under our Purchase Plan is estimated on the date of grant using the Black-Scholes option pricing model, which uses weighted-average assumptions. Our Purchase Plan provides for a twenty-four-month offering period comprised of four six-month purchase periods with a look-back option. A look-back option is a provision in our Purchase Plan under which eligible employees can purchase shares of our common stock at a price per share equal to the lesser of 85% of the fair market value on the first day of the offering period or 85% of the fair market value on the purchase date. Our Purchase Plan also includes a feature that provides for a new offering period to begin when the fair market value of our common stock on any purchase date during an offering period falls below the fair market value of our common stock on the first day of such offering period. This feature is called a “reset.” Participants are automatically enrolled in the new offering period.

We had a “reset” on January 2, 2020 because the fair market value of our stock on December 31, 2019 was lower than the fair market value of our stock on January 1, 2019, the first day of the offering period. Following the “reset” in January 2020, January 1, 2020 was the new first day of the two-year offering period of our Purchase Plan. We applied modification accounting in accordance with the relevant accounting guidance. The total incremental fair value associated with this “reset” was approximately $0.8 million was recognized as expense from January 1, 2020 to December 31, 2021. In July 2020, we had another “reset” because the fair market value of our stock on June 30, 2020 was lower than the fair market value of our stock on January 1, 2020. Following the “reset” in July 2020, July 1, 2020 was the new start date of our two-year offering period of our Purchase Plan. We applied modification accounting in accordance with the relevant accounting guidance. The total incremental fair value associated with this “reset” was approximately $0.5 million and is being amortized to expenses from July 1, 2020 to June 30, 2022.

The table below summarizes the weighted-average assumptions related to our Purchase Plan for periods presented. Expected volatilities for our Purchase Plan are based on the two-year historical volatility of our stock. Expected term represents the weighted- average of the purchase periods within the offering period. The risk-free interest rate for periods within the expected term is based on US Treasury constant maturity rates.

​

* Not a measurement period since the last two-year offering period started on January 1, 2020, and there was no reset in 2021.

​

The weighted average fair value of awards under our Purchase Plan at the measurement period during the year ended December 31, 2020 and 2019 was $0.87 per share and $1.07 per share, respectively. As of December 31, 2021, unrecognized stock-based compensation cost related to our Purchase Plan amounted to $0.1 million, which is expected to be recognized over the remaining weighted average period of 0.17 years.