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Stock-based compensation for the periods presented was as follows (in thousands):

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Stock-based compensation expense within research and development in the six months ended June 30, 2023 include an incremental charge of approximately $0.5 million from stock option modifications recorded in the first quarter of 2023 related to the acceleration of vesting and extension of exercise period of vested stock option grants made to a former officer whose employment ended in March 2023. Stock-based compensation expense within selling, general and administrative in the six months ended June 30, 2022 include an incremental charge of approximately $0.8 million from stock option modifications recorded in the first quarter of 2022 related to the extension of the exercise period of the stock option grants made to our two former Board of Directors whose terms expired in May 2022.

During the six months ended June 30, 2023, we granted stock options to purchase 2,938,600 shares of common stock with weighted-average grant-date fair value of $1.37 per share, and 15,557 stock options were exercised. As of June 30, 2023, there were 35,244,121 stock options outstanding, of which, 2,870,000 are outstanding performance-based stock options wherein the achievement of the corresponding corporate-based milestones were not considered probable as of June 30, 2023. Accordingly, none of the $5.3 million grant date fair value for these awards has been recognized as stock-based compensation expense as of June 30, 2023.

The fair value of each option award is estimated on the date of grant using the Black-Scholes option pricing model. The following table summarizes the weighted-average assumptions relating to options granted pursuant to our Equity Incentive Plans (2018 Equity Incentive Plan and Inducement Plan) for the periods presented:

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During the six months ended June 30, 2023, we granted 1,387,600 RSUs with a grant-date weighted-average fair value of $1.80 per share, and 435,006 RSUs were released. The RSUs granted generally vest over 4 years. As of June 30, 2023, there were 1,963,487 RSUs outstanding.

As of June 30, 2023, there was approximately $14.6 million of unrecognized stock-based compensation cost which is expected to be recognized over a remaining weighted-average period of 2.68 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.

In April 2023, our Board of Directors approved additional 108,600 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. As of June 30, 2023, there were 12,186,098 shares of common stock available for future grant under our Equity Incentive Plans.

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Employee Stock Purchase Plan

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Our Purchase Plan provides for a 24-month offering period comprises 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 twenty-four-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. As of June 30, 2023, unrecognized stock-based compensation cost related to our Purchase Plan amounted to $0.4 million, which is expected to be recognized over the remaining weighted average period of 0.74 years.

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During the six months ended June 30, 2023, there were 509,190 shares purchased under the Purchase Plan. As of June 30, 2023, there were 2,928,443 shares reserved for future issuance under the Purchase Plan.