The following table summarizes warrant activity as of December 31, 2022, 2021, and 2020:


	Warrants			Weighted average exercise price
Outstanding at December 31, 2019		69,656		$	7.21
Issued		15,541			80.43
Exercised		(69,656	)		7.21
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

In connection with the entry into the Second Amendment, Hercules received warrants to purchase an aggregate 35,255 shares of the Company’s common stock at an exercise price of $31.91 per share ("2022 warrants") and in connection with the first advance of the 2020 Term Loan, Hercules received warrants to purchase an aggregate 15,541 shares of the Company’s common stock at an exercise price of $80.43 per share ("2020 warrants").

Both the 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 $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 outstanding warrants of 69,656 as of December 31, 2019 were fully exercised by Life Sciences Loans and WestRiver Innovation Lending during year ended December 31, 2020. These warrants were issued during 2016, 2018 and 2019 in connection with the Company's previous debt facility with SVB.

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


Black-Scholes option valuation assumptions	2022 warrants			2020 warrants			2019 warrants			2018 warrants			2016 warrants
Risk-free interest rate		3.1	%		0.5	%		2.8	%		3.0	%		1.8	%
Dividend yield		—			—			—			—			—
Volatility		94	%		88	%		97	%		85	%		73	%
Weighted average contractual term	7 years			7 years			7 years			7 years			7 years