5. STOCK-BASED COMPENSATION

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

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Stock-based compensation expense for the years ended December 31, 2022 and 2021 include incremental charges of $1.4 million and $0.4 million, respectively, related to stock option modifications. In November 2021, we announced a reduction of workforce, primarily in the research organization and entered in severance agreements with certain affected employees, which provided, among others, extension of the date through which certain affected employees can exercise their vested options. In March 2022, our Board of Directors approved to extend the exercise period of stock option grants made to our two former Board of Directors whose terms expired in May 2022. In October 2022, our Board of Directors approved to accelerate the vesting and extend the exercise period of certain stock option grants made to a former officer who retired in October 2022. The incremental compensation expenses were computed based on the fair values of the modified awards on the modification date. As a result of these stock option modifications, we recorded the incremental stock-based compensation expense for each of the respective periods. The incremental stock-based compensation expense related to the modification of stock options of our two former Board of Directors and former officer were recorded within sales, general and administrative expense, and the modification of stock options of certain affected employees due to workforce reduction were recorded within restructuring charges.

Equity Incentive Plans

On May 16, 2018, our stockholders approved the adoption of our 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 RSUs under our Equity Incentive Plans.

In March 2022 and August 2022, we filed Registration Statements to register additional 1,709,000 shares and 626,000 shares, respectively, of common stock reserved for issuance under our Inducement Plan. In May 2022, our stockholders approved to amend our 2018 Plan, among other items, added an additional 5,000,000 shares to the number of shares of common stock authorized for issuance under our 2018 Plan, and such shares were registered in the Registration Statement filed in August 2022. Further, in December 2022, our Board of Directors approved to increase the number of common stock reserved for issuance under our Inducement Plan by 375,000 shares.

Stock Options and RSUs

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

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Of the total stock options outstanding as of December 31, 2022, 2,222,500 shares outstanding are performance-based stock options wherein the achievements of the corresponding corporate-based milestones were not probable. Accordingly, the related grant date fair value for these performance-based stock options of $4.3 million has not been recognized as stock-based compensation expense as of December 31, 2022.

For the years ended December 31, 2022, 2021 and 2020, stock options vested were 5,280,235 shares, 4,765,814 shares and 4,386,910 shares, respectively, with weighted-average exercise price of $2.60 per share, $2.67 per share, and $2.41 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, 2022. For the years ended December 31, 2022, 2021 and 2020, the aggregate intrinsic values of stock option exercises were approximately $0.2 million, $2.1 million and $0.5 million, respectively, representing the difference between the fair value of our common stock at the date of exercise and the exercise price paid.

For the years ended December 31, 2022, 2021 and 2020, we granted options to purchase 8,179,113 shares, 6,997,981 shares and 8,462,090 shares, respectively, of common stock, with weighted-average grant date fair value of $1.29 per share, $2.34 per share and $1.42 per share, respectively.

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

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As of December 31, 2022, there was approximately $14.1 million of unrecognized stock-based compensation cost which is expected to be recognized over the remaining weighted-average period of 2.55 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, 2022:

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Employee Stock Purchase Plan

Our Purchase Plan permits eligible employees to purchase common stock at a discount through payroll deductions during defined offering periods. Our Purchase Plan provides for a 24-month offering period comprised 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.

Our previous 24-month offering period under our Purchase Plan ended on June 30, 2022, and a new 24-month offering period started on July 1, 2022. The fair value of awards under our Purchase Plan is estimated on the date of our new offering period using the Black-Scholes option pricing model, which is being amortized over the requisite service periods.

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.

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

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The weighted average fair value of awards under our Purchase Plan at the measurement period during the year ended December 31, 2022 and 2020 was $0.78 per share and $0.87 per share, respectively. As of December 31, 2022, unrecognized stock-based compensation cost related to our Purchase Plan amounted to $0.9 million, which is expected to be recognized over the remaining weighted average period of 0.99 years.

For the years ended December 31, 2022, 2021 and 2020, there were 1,146,851 shares, 932,018 shares and 567,391 shares, respectively, of common stock purchased under the Purchase Plan, at an average price of $1.01 per share, $1.51 per share and $1.54 per share, respectively. As of December 31, 2022, there were 3,437,633 shares reserved for future issuance under the Purchase Plan.