In connection with the Separation, the Company adopted its own share‑based compensation plan, the Baudax Bio, Inc. 2019 Equity Incentive Plan, or the 2019 Plan, which allows for the grant of stock options, stock appreciation rights and stock awards for a total of 3,000,000 shares of common stock. On December 1^st of each year, pursuant to the “Evergreen” provision of the 2019 Plan, the number of shares available under the plan shall be increased by an amount equal to 5% of the outstanding common stock on December 1^st of that year or such lower amount as determined by the Board of Directors. In December 2019, the number of shares available for issuance under the 2019 Plan was increased by 467,535. The total number of shares authorized for issuance under the 2019 plan as of December 31, 2019 is 3,467,535.

Stock options are exercisable generally for a period of 10 years from the date of grant and generally vest over four years. As of December 31, 2019, 1,443,626 shares are available for future grants under the 2019 Plan.

The weighted average grant-date fair value of the Baudax Bio options awarded to employees during the year ended December 31, 2019 was $4.29. Under the 2019 Plan, the fair value of the Baudax  Bio options was estimated on the date of grant using a Black-Scholes option pricing model with the following assumptions:

Certain employees of the Company participated in Recro’s stock-based compensation plan, which provides for the grants of stock options and RSUs. The combined financial statements prior to the Separation reflect stock-based compensation expense related to Recro stock options and RSUs issued to the Company’s employees as well an allocation of a portion of Recro share-based compensation issued to corporate employees and members of the Board of Directors until the Separation date. The weighted average grant-date fair value of the options awarded to employees under the Recro plan during the years ended December 31, 2019 (prior to the Separation date) and 2018 was $5.53 and $6.07, respectively.

Under the Recro equity incentive plan for the year ended December 31, 2018, the fair value of the options granted to employees of the Company was estimated on the date of grant using a Black-Scholes option pricing model with the following assumptions:

The following table summarizes the Baudax Bio stock option activity during the year ended December 31, 2019:

The following table summarizes the Baudax Bio restricted stock units activity during the year ended December 31, 2019:

Stock-based compensation expense for the twelve months ended December 31, 2019 and 2018 was $5,463 and $4,574, respectively. This represents the allocated portion of Recro stock-based compensation expense through the Separation date in addition to the Company’s expense post-separation.

As of December 31, 2019, there was $10,996 of unrecognized compensation expense related to unvested options and time-based RSUs that are expected to vest and will be expensed over a weighted average period of 2.5 years.

The aggregate intrinsic value represents the total amount by which the fair value of the common stock subject to options exceeds the exercise price of the related options. As of December 31, 2019, the aggregate intrinsic value of the unvested Baudax Bio options was $380. There are no vested options as of December 31, 2019.