Prior to our IPO, we issued warrants to certain employees, board members and advisors ("Pre-IPO Plan"). Each vested warrant entitled the warrant holder to a single class A ordinary share. Holders of stock warrants were entitled to exercise the vested portion of the stock warrant. Stock warrants generally vest over a three-year period and expire five years from the vest date.

In November 2021, our board of directors adopted, and our stockholders approved, the 2021 Equity Incentive Plan (or the “2021 Plan”), which became effective on November 4, 2021. The 2021 Plan provides for the grant of incentive stock options, nonstatutory stock options, stock appreciation rights, awards of restricted stock, restricted stock units and other stock-based awards. The number of shares of our common stock reserved for issuance under the 2021 Plan is equal to 2,465,150, subject to an annual increase, to be added on the first day of each fiscal year, beginning with the fiscal year ending December 31, 2022 and continuing until, and including, the fiscal year ending December 31, 2031, equal to the lesser of (i) 4% of the number of shares of common stock outstanding on the first day of such fiscal year or (ii) such other amount determined by our board of directors. The 2,396,413 outstanding warrants granted under the Pre-IPO Plan were transferred to the 2021 Equity Plan and no further warrants were available to be issued under the Pre-IPO Plan. As of December 31, 2021, we had 1,789,950 options available for future grant under the 2021 Equity Plan.

There were no grants, exercises, or forfeitures of any stock warrants during the year ended December 31, 2020. We issued 2,306,478 warrants with a weighted average exercise price of $14.74 to certain employees, board members and advisors during the year ended December 31, 2021. These warrants, and all previously issued warrants, were transferred to the 2021 Equity Plan in November 2021. We also issued 675,200 options with an exercise price of $14.00 under our 2021 Equity Plan during the year ended December 31, 2021. We did not recognize any tax benefits for stock-based compensation during the years ended December 31, 2021 and 2020.

The following table summarizes our stock warrants for the years ended December 31, 2021 and 2020:


	Number of Warrants		Weighted- average exercise price per share		Weighted- average remaining contractual term (in years)		Aggregate intrinsic value
Outstanding December 31, 2019		89,935	$	15.00		5.6	$	—
Outstanding December 31, 2020		89,935	$	15.00		4.6	$	—
Granted		2,981,678	$	14.58
Outstanding, December 31, 2021		3,071,613	$	13.12		8.5	$	—
Exercisable at December 31, 2021		220,575	$	14.48		6.5	$	—

In November 2021, our board of directors adopted and our stockholders approved the 2021 Employee Stock Purchase Plan (or the “2021 ESPP”), which became effective on November 4, 2021. The number of shares of our common stock reserved for issuance under the 2021 ESPP is equal to 257,272, subject to an annual increase, to be added on the first day of each fiscal year, beginning January 1, 2023, equal to the lesser of (i) 1% of the number of shares of common stock outstanding on the first day of such fiscal year, (ii) 257,272 shares of our common stock or (iii) such other amount determined by our board of directors.

In October 2021, our board of directors approved the amendment of all nonvested warrant awards issued in July and August 2021 with an exercise price of $19.62 per share to reduce the exercise price of such warrants to $12.64 per share. Warrants to purchase an aggregate of 670,849 shares of our class A ordinary shares were modified. The vesting schedule of such awards was not modified. The modification resulted in a $0.6 million charge which will be recognized over the remaining vesting periods of each award averaging 3.75 years.

We recorded equity-based compensation expense of $1.8 million and $64,000 related to the issuance of stock options and warrants during the years ended December 31, 2021 and 2020, respectively. As of December 31, 2021, there was $22.6 million of unrecognized compensation cost related to unvested stock-based compensation arrangements that is expected to be recognized over a weighted average period of 3.7 years.

The fair values of the options granted were estimated based on the Black-Scholes model, using the following assumptions:


		Year Ended
		December 31, 2021
Expected volatility		72.9% - 82.6%
Risk-free interest rate		0.79% - 1.45%
Expected term (in years)		5.0 - 9.0
Expected dividend yield		0%

Equity-based compensation expense recorded as research and development and general and administrative expenses is as follows (in thousands):


	December 31,
	2021		2020
Research and development	$	600	$	22
General and administrative		1,191		42
Total equity-based compensation	$	1,791	$	64