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, 2023, 2022 and 2021 include incremental stock-based compensation charges of approximately $0.5 million, $1.4 million and $0.4 million, respectively, as a result of modifications of certain stock option grants. The stock option modifications during the years ended December 31, 2023 and 2022 were related to acceleration of vesting of certain stock option grants and extension of exercise period of vested stock options granted to former officers and board of directors. Such amounts were included within selling, general and administrative expenses and research and development expenses. The stock option modification in 2021 was

related to the reduction of workforce wherein certain affected employees were provided extension to exercise certain vested stock options. Such amount was 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 April 2023, June 2023, October 2023 and December 2023, our Board of Directors approved additional 1,019,700 shares of common stock reserved for issuance under our Inducement Plan. In May 2023, our stockholders approved an amendment to our 2018 Plan, to, among other items, add an additional 4,000,000 shares to the number of shares of common stock authorized for issuance under our 2018 Plan.

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, 2023, 2,745,000 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 $5.1 million has not been recognized as stock-based compensation expense as of December 31, 2023.

For the years ended December 31, 2023, 2022 and 2021, stock options vested were 4,346,977 shares, 5,280,235 shares and 4,765,814 shares, respectively, with weighted-average exercise price of $2.34 per share, $2.60 per share, and $2.67 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, 2023. For the years ended December 31, 2023, 2022 and 2021, the aggregate intrinsic values of stock option exercises were approximately $0.02 million, $0.2 million and $2.1 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, 2023, 2022 and 2021, we granted options to purchase 3,671,800 shares, 8,179,113 shares and 6,997,981 shares, respectively, of common stock, with weighted-average grant date fair value of $1.28 per share, $1.29 per share and $2.34 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, 2023, there was approximately $10.6 million of unrecognized stock-based compensation cost which is expected to be recognized over the remaining weighted-average period of 2.45 years, related to time-based stock options, performance-based stock options wherein achievement of the corresponding corporate-based milestones were considered as probable, and RSUs.

Details of our stock options by exercise price are as follows as of December 31, 2023:

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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.

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As of December 31, 2023, 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 0.49 years.

For the years ended December 31, 2023, 2022 and 2021, there were 941,798 shares, 1,146,851 shares and 932,018 shares, respectively, of common stock purchased under the Purchase Plan, at an average price of $1.06 per share, $1.01 per share and $1.51 per share, respectively. As of December 31, 2023, there were 2,495,835 shares reserved for future issuance under the Purchase Plan.