Prior to June 20, 2023, the Company had authorized the issuance of 5,900,000 shares of the Company's common stock under the 2016 Long-Term Incentive Plan (the 2016 Plan), of which 184,904 shares remained available for future grants.

At the Company’s Annual Meeting of Stockholders held on June 20, 2023, the Company’s stockholders approved the adoption of the 2023 Long Term Incentive Plan (the 2023 Plan) and authorized up to 3,500,000 shares of common stock reserved for issuance to participating employees plus the 184,904 shares that remained available for grant under the 2016 Plan upon adoption of the 2023 Plan plus any shares that would have otherwise have become available for grant under the Company's 2008 Plan or the 2016 Plan as a result of termination or forfeiture of awards under such plan. The 2023 Plan replaced the 2008 Plan and the 2016 Plan. At December 31, 2023, a total of approximately 2,274,000 shares were available for new awards under the 2023 Plan.

Starting March 2022, the Company also granted non-statutory stock options to new employees as inducement awards to enter into employment with the Company. The grants were approved by the Compensation Committee of the Board of Directors and awarded in accordance with Nasdaq Listing Rule 5635(c)(4). Although not awarded under the 2023 Plan or the 2016 Plan, the grants are subject to and governed by the terms and conditions of the plan in effect at the time of the grant.

The following table provides a reconciliation of stock option activity under the Company’s equity incentive plans and for inducement awards for the year ended December 31, 2023:


	Number of Options			Weighted Average Exercise Price		Weighted Average Remaining Contractual Life		Aggregate Intrinsic Value
						(in years)		(in thousands)
Outstanding at January 1, 2023		4,082,555		$	13.79
Granted		2,923,861			4.76
Exercised		(260,321	)		11.33
Forfeited		(385,075	)		7.38
Expired		(56,253	)		26.71
Outstanding at December 31, 2023		6,304,767		$	9.98		7.89	$	85,536
Exercisable at December 31, 2023		2,325,480		$	15.61		6.31	$	20,178

The Company has granted stock options with 25% of the option vesting after one year followed by ratable monthly vesting over the remaining three years. Nonemployee awards are granted similar to the Company’s employee awards. All option grants have a 10-year term. Options to purchase a total of 1,128,000 shares of the Company’s common stock vested during the year ended December 31, 2023.

In determining the grant date fair value of option awards during the years ended December 31, 2023 and 2022, the Company applied the Black-Scholes option pricing model based on the following key assumptions:


	Year Ended	Year Ended
	December 31,	December 31,
	2023	2022
Option life (in years)	5.27 - 6.08	5.50 - 6.09
Stock volatility	78% - 97%	76% - 78%
Risk-free interest rate	3.44% - 4.68%	1.46% - 4.15%
Expected dividends	0.0%	0.0%

The following table summarizes information about employee, non-executive director and external consultant stock options for the years ended December 31, 2023 and 2022 (in thousands except per share amounts):


	Year Ended		Year Ended
	December 31,		December 31,
	2023		2022
Weighted average grant date fair value per share	$	3.46	$	6.79
Total cash received from exercise of stock options		2,955		41
Total intrinsic value of stock options exercised		1,970		14

Time-vested restricted stock units (RSUs) issued to date under the 2016 Plan and the 2023 Plan generally vest on a ratable annual basis over three years. The related stock-based compensation expense is recorded over the requisite service period, which is the vesting period. The fair value of all time-vested RSUs is based on the closing share price of the Company’s common stock on the date of grant.

The following table provides a reconciliation of RSU activity under the 2016 Plan and the 2023 Plan for the year ended December 31, 2023:


	Number of Restricted Stock Units			Weighted Average Grant Date Fair Value
Nonvested at January 1, 2023		509,170		$	10.81
Granted		1,071,354			3.92
Vested		(201,414	)		11.04
Exercised		—			—
Forfeited		(45,918	)		8.60
Nonvested at December 31, 2023		1,333,192		$	5.31

At December 31, 2023, the weighted average remaining vesting term of the RSUs was 1.46 years.

The Company’s Employee Stock Purchase Plan (the ESPP) allows qualified participants to purchase the Company’s common stock twice a year at 85% of the lesser of the average of the high and low sales price of the Company’s common stock on (i) the first trading day of the relevant offering period and (ii) the last trading day of the relevant offering period. The number of shares of the Company’s common stock each employee may purchase under this plan, when combined with all other employee stock purchase plans, is limited to the lower of an aggregate fair market value of $25,000 during each calendar year, or 5,000 shares of the Company’s common stock in any one offering period. The Company has maintained consecutive six-month offering periods since August 1, 2019. During the year ended December 31, 2023, 107,056 shares of the Company’s common stock were issued pursuant to the ESPP.

The Company estimated the fair value of the option component of the ESPP shares at the date of grant using a Black-Scholes valuation model. For the years ended December 31, 2023 and 2022, the compensation expense from ESPP shares was $0.2 million and $0.2 million, respectively.

The Company’s consolidated statements of comprehensive loss included total compensation expense from stock-based payment awards for the years ended December 31, 2023 and 2022, respectively, as follows (in thousands):


	Year Ended		Year Ended
	December 31,		December 31,
	2023		2022
Research and development	$	4,650	$	6,130
Sales and marketing		289		1,650
General and administrative		7,118		6,397
Total stock-based compensation expense	$	12,057	$	14,177

At December 31, 2023, there was approximately $11.1 million of unrecognized compensation expense related to outstanding equity awards under the 2023 Plan, the 2016 Plan, the inducement awards and the ESPP that is expected to be recognized as expense over a weighted average period of approximately 1.62 years.