In 2018, our stockholders approved the adoption of the SenesTech, Inc. 2018 Equity Incentive Plan (the “2018 Plan”). The 2018 Plan has since been amended and restated on certain occasions, most recently on October 12, 2022, when our stockholders approved an increase to the total number of authorized shares to 348,614 shares.

Stock options are generally issued with a per share exercise price equal to the fair market value of our common stock at the date of grant. Options granted generally vest immediately, or ratably over a 12- to 36-month period coinciding with their respective service periods, with terms of generally five years. Certain stock option awards provide for accelerated vesting upon a change in control.

As of March 31, 2023, we had 164,522 shares of common stock available for issuance under the 2018 Plan.

The following table summarizes the stock option activity for the periods indicated as follows:


	Number of Options		Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Term (years)
Outstanding as of December 31, 2022	280,810		$	17.00	3.9
Granted	—		—		—
Exercised	—		—		—
Forfeited	(326)		—		—
Expired	(36)		—		—
Outstanding as of March 31, 2023	280,448	(1)	17.11		4.4
Exercisable as of March 31, 2023	98,406		32.22		—

(1) - Includes options related to 99,000 shares that are inducement awards and not granted under the 2018 Plan.

The following table summarizes restricted stock unit activity for the three months ended March 31, 2023:


	Number of Units	Weighted Average Grant-Date Fair Value Per Unit
Outstanding as of December 31, 2022	18,799	$	2.71
Granted	—	—
Vested	(18,799)	2.71
Forfeited	—	—
Outstanding as of March 31, 2023	—	—


	Three Months Ended March 31,
	2023		2022

Research and development	$	4	$	1
Selling, general and administrative	162		223
Total stock-based compensation expense	$	166	$	224

The allocation between research and development and selling, general and administrative expense was based on the department and services performed by the employee or non-employee.

At March 31, 2023, the total compensation cost related to unvested options not yet recognized was $403,000, which will be recognized over a weighted average period of 24 months, assuming the employees and non-employees complete their service period required for vesting.