Note 12 - Stock-based compensation

The Company and its stockholders adopted an equity incentive plan (the “2013 Plan”) in 2013, which reserved shares of the Company’s common stock for the granting of incentive and nonqualified stock options to employees, directors, and consultants. On October 14, 2021, the Company replaced the 2013 Plan with the 2021 Plan in connection with its initial public offering. Under the 2021 Plan, 37,504,358 shares of common stock are reserved. The Company may grant options to purchase common stock, stock appreciation rights, restricted stock awards and other forms of stock-based compensation. Stock options generally vest over four years and expire no later than 10 years from the date of grant. The Board of Directors has the authority to select the employees to whom options are granted and determine the terms of each option, including: i) the number of shares of common stock subject to the option; ii) when the option becomes exercisable; iii) the option exercise price, which must be at least 100% of the fair market value of the common stock as of the date of grant; and iv) the duration of the option, which may not exceed 10 years.

As of March 31, 2025, options to purchase a total of 22,377,618 shares of common stock remained outstanding and 15,126,740 shares remain available for grant under the 2021 Plan and 709,633 remained outstanding outside of the 2021 Plan. As of March 31, 2025, options to purchase a total of 17,278,905 shares of common stock remained outstanding under the 2013 Plan. As of March 31, 2025, no shares of common stock remain available for grant under the 2013 Plan.

Stock option activity for the three-month period ended March 31, 2025, was as follows:

The fair value of the options granted to employees is estimated on the grant date using the Black-Scholes option valuation model. This valuation model for stock-based compensation expense requires the Company to make assumptions and judgments about the variables used in the calculation, including the expected term (weighted-average period of time that the options granted are expected to be outstanding), the volatility of the Company’s common stock, an assumed risk-free interest rate and expected dividends. The Company uses the simplified calculation of expected life and volatility is based on an average of the historical volatilities of the common stock of several publicly traded entities with characteristics similar to those of the Company. The risk-free rate is based on the U.S. Treasury yield curve in effect at the time of grant for periods corresponding with the expected life of the option. The Company uses the straight-line method for expense attribution. The weighted-average grant-date fair values of stock options granted during the three months ended March 31, 2025 and 2024, was $0.64 per share and $0.37 per share, respectively.

The following assumptions were used to calculate the grant-date fair value of employee stock options granted during the three months ended March 31, 2025 and 2024:

The following table presents classification of stock-based compensation expense within the accompanying unaudited condensed consolidated statements of operations for the three months ended March 31, 2025 and 2024:

At March 31, 2025, there was $5,573,573 of unamortized stock-based compensation cost, related to unvested stock options which is expected to be recognized over a weighted-average period of 2.94 years.