The following table summarizes warrant activity for the years ended December 31, 2023, 2022, and 2021:


	Warrants		Weighted average exercise price
Outstanding at December 31, 2020		15,541	$	80.43
Issued		—		—
Exercised		—		—
Outstanding at December 31, 2021		15,541		80.43
Issued		35,255		31.91
Exercised		—		—
Outstanding at December 31, 2022		50,796		46.75
Issued		28,424		74.75
Exercised		—		—
Outstanding at December 31, 2023		79,220	$	56.80

In connection with the entry into the Third Amendment, Hercules received warrants to purchase an aggregate of 18,724 shares of the Company’s common stock at an exercise price of $55.01 per share, and in connection with the draw down of the tranche 1C advance, Hercules received warrants to purchase 9,700 shares of the Company's common stock at an exercise price of $77.31 per share (collectively, the “2023 warrants”). In connection with the entry into the Second Amendment, Hercules received warrants to purchase an aggregate of 35,255 shares of the Company’s common stock at an exercise price of $31.91 per share (the “2022 warrants”), and in connection with the first advance of the 2020 Term Loan, Hercules received warrants to purchase an aggregate of 15,541 shares of the Company’s common stock at an exercise price of $80.43 per share (the “2020 warrants”).

The 2023 warrants, 2022 warrants and 2020 warrants were priced using the volume weighted average price of the Company’s common stock over the ten-day trading period immediately preceding the initial closing, subject to certain limited adjustments as specified in the warrant. The warrants are exercisable for seven years from the date of issuance. The warrants were classified as a component of stockholders’ equity. The relative fair value of the warrants of approximately $1.6 million for the 2023 warrants, $0.8 million for the 2022 warrants and $0.9 million for the 2020 warrants at the time of issuance, which was determined using the Black-Scholes option-pricing model, was recorded as additional paid-in capital and reduced the carrying value of the debt. The discount on the debt is being amortized to interest expense over the term of the debt utilizing the effective interest rate method.

The initial fair value of warrants outstanding was estimated using the Black‑Scholes option pricing model with the following assumptions:


Black-Scholes option valuation assumptions	2023 warrants		2022 warrants			2020 warrants
Risk-free interest rate	3.6 - 3.9%			3.1	%		0.5	%
Dividend yield		—		—			—
Volatility	92 - 95%			94	%		88	%
Weighted average contractual term	7 years		7 years			7 years