Effective February 12, 2021, the Board of Directors approved the adoption of the 2021 Incentive Award Plan (the "2021 Plan") as a successor to the 2014 Equity Incentive Plan (the "2014 Plan"), which permits the Company to grant options, stock appreciation rights, restricted stock, restricted stock units, performance bonus, performance stock unit, dividend equivalents, or other stock or cash based awards to employees, directors, or consultants. Shares remaining for issuance, forfeited, expired, or other manner available to issue under terms of the 2014 Plan roll over to and become available for awards under the 2021 Incentive Award Plan. As of December 31, 2023, 2,592,830 shares were authorized for issuance under the 2021 Plan. In addition, the shares authorized for the 2021 Plan may be increased on an annual basis beginning January 1, 2022, in an amount equal to 5% of the outstanding common stock on the last day of the immediately preceding fiscal year for a period of 10 years.

On January 1, 2022, the Company began offering an Employee Stock Purchase Plan ("ESPP"). The ESPP allows eligible employees to contribute a portion of their eligible earnings toward the semi-annual purchase of our shares of common stock at a discounted price, subject to an annual maximum dollar amount. Employees can purchase stock at a 15% discount applied to the lower closing stock price on the first or last day of the six-month purchase period.


	2023			2022
	Number of Options	Weighted Average Exercise Price		Number of Options	Weighted Average Exercise Price
Outstanding at January 1	598,155	$	24.35	739,762	$	26.15
Granted(1)	136,326	4.21		5,707	25.62
Exercised	(18,054)	3.12		(58,337)	4.42
Canceled(1)	(253,010)	30.26		(79,732)	52.46
Expired	(249)	4.06		(9,245)	60.76
Outstanding at December 31	463,168	16.03		598,155	24.35
Exercisable at December 31	440,324	$	15.19	524,974	$	20.34

The grant date fair value of each option was estimated on the date of grant using the Black-Scholes option pricing model. The key assumptions for grants presented on a weighted average basis are as follows:

Stock-based compensation expense related to options was $1,199 and $3,448 during the years ended December 31, 2023 and December 31, 2022, respectively. Generally, employees are subject to four year vesting terms of 25% after one year and monthly thereafter, with a maximum term of 10 years.

The intrinsic value of a stock option is the amount by which the current market value of the underlying stock exceeds the exercise price of the option. The total intrinsic value of options exercised was $29 during 2023 and $1,416 during 2022. At December 31, 2023, options outstanding had an intrinsic value of $763 with a weighted average remaining life of 4.76 years. At December 31, 2023, options vested and exercisable had an intrinsic value of $746 with a weighted average remaining life of 4.63 years. At December 31, 2022, options outstanding had an intrinsic value of $1,489 with a weighted average remaining life of 5.82 years. At December 31, 2022, options vested and exercisable had an intrinsic value of $1,489 with a weighted average remaining life of 5.52 years.

The total grant date fair value of options vested during 2023 and 2022 was $1,153 and $3,958, respectively. The grant date fair value of options granted during 2023 and 2022 was $171 and $91, respectively. Weighted average grant date fair value of options granted during 2023 and 2022 was $1.26 and $15.97, respectively. Stock options vested and expected to vest at December 31, 2023 totaled 463,168 shares, with an intrinsic value of $763, weighted average exercise price of $16.03, and weighted average remaining life of 4.76 years. Cash received from stock options exercised during 2023 and 2022 was immaterial.


	Year Ended December 31,
	2023			2022
	Shares	Weighted Average Grant Date Fair Value		Shares	Weighted Average Grant Date Fair Value
Unvested at January 1	494,671	$	37.77	290,220	$	57.15
Granted	1,331,899	3.96		545,692	34.33
Vested	(286,601)	35.17		(94,552)	54.51
Forfeited	(75,896)	35.57		(246,689)	46.55
Unvested at December 31	1,464,073	$	7.64	494,671	$	37.77

RSUs are valued at the market value on the date of grant and compensation expense for employees is expensed over the vesting period. Generally, employees are subject to either a four year vesting term with 25% vesting after one year and quarterly thereafter, or on a 2 year vesting term with 50% after one year and the remaining after the second year, depending on grant reason. Grants to directors vest after one year.

Stock-based compensation expense related to RSU grants was $7,510 and $8,206 during the years ended December 31, 2023 and December 31, 2022, respectively. The aggregated fair value of RSUs granted during the years ended December 31, 2023 and 2022 was $5,274 and $18,650, respectively. The aggregated fair value of RSUs vested during the years ended December 31, 2023 and 2022 was $10,080 and $5,239, respectively.


	Year Ended December 31,
	2023			2022
	Shares	Weighted Average Grant Date Fair Value		Shares	Weighted Average Grant Date Fair Value
Unvested at January 1	87,321	$	36.36	—	$	—
Granted	—	—		132,433	36.37
Vested	(7,518)	36.40		—	—
Forfeited	(8,375)	35.97		(45,112)	36.40
Unvested at December 31	71,428	$	36.40	87,321	$	36.36

The PRSU awards function in the same manner as restricted stock units except that vesting terms are based on achievement of performance measures, such as the achievement of net revenue targets and obtaining certain FDA regulatory results. PRSUs are recognized as expense following a graded vesting schedule with their performance re-assessed and updated on a quarterly basis, or more frequently as changes in facts and circumstances warrant.

Stock-based compensation related to PRSU grants was $663 and $428 for the years ended December 31, 2023 and 2022, respectively. No PRSUs were granted during the year ended December 31, 2023. The aggregated fair value of PRSUs granted during the year ended December 31, 2022 was $4,817. The aggregated fair value of PRSUs vested during the year ended December 31, 2023 was $274. No PRSUs vested during the year ended December 31, 2022.

Employees purchased 43,274 shares at an average price of $4.96 during the year ended December 31, 2023, and 17,937 shares at an average price of $23.80 during the year ended December 31, 2022. The intrinsic value of shares purchased was $38 and $64, respectively, for the years ended December 31, 2023 and 2022. The intrinsic value is calculated as the difference between the market value on the date of purchase and the purchase price of the shares.

During the years ended December 31, 2023 and 2022, the rollover provision of our ESPP was triggered and resulted in incremental expense to be recognized over the new twenty-four-month offering period, which did not have a material impact on our consolidated statements of operations and comprehensive loss.

As of December 31, 2023, the Company had $1,070 of unrecognized stock-based compensation costs related to non-vested options that will be recognized over a weighted average period of 0.96 years, $9,506 of unrecognized stock-based compensation costs related to unvested RSUs that will be recognized over a weighted average period of 1.92 years, and $482 of unrecognized stock-based compensation costs related to unvested PRSUs that will be recognized over a weighted average period of 1.56 years.

During the year ended December 31, 2022, the Company capitalized $36 of stock-based compensation attributable to internally developed software. No stock-based compensation was capitalized during the year ended December 31, 2023.