In May 2020, several existing accredited investors exercised the June 2018 Warrants (the “May 2020 Warrant Exercise Transaction”) to purchase up to an aggregate of 1,392,345 shares of the Company’s common stock at a reduced exercise price of $4.90 per share for gross proceeds of $6.8 million. As consideration for the exercise of the June 2018 Warrants, the Company issued new unregistered warrants to purchase up to 1,392,345 shares of common stock (the “May 2020 Warrants”) at an exercise price of $4.775 per share with an exercise period of five years from the date of issuance. The May 2020 Warrants were valued at $4.4 million, which was calculated using the Black-Scholes Model with the following assumptions: volatility of 128 percent, stock price of $3.81, and risk-free rate of 0.38%. In connection with the May 2020 Warrant Exercise Transaction, the Company granted to a placement agent warrants to purchase a total of 69,617 shares of common stock (the “May 2020 Placement Agent Warrants”) that have an exercise price per share equal to $6.125 and a term of five years. The value of the May 2020 Placement Agent Warrants was determined to be $215,000 using the Black-Scholes Model. The Company recognized a gain on extinguishment of warrant liability in the amount of $47,000 associated with this transaction, during the quarter ended June 30, 2020.

In July 2020, an existing accredited investor exercised its March 2018 Warrants (the “July 2020 Warrant Exercise Transaction”) to purchase up to an aggregate of 641,416 shares of the Company’s common stock at a reduced exercise price of $3.975 per share for gross proceeds of $2.6 million. As consideration for the exercise of these March 2018 Warrants, the Company issued new unregistered warrants to purchase up to 641,416 shares of common stock (the “July 2020 Warrants”) at an exercise price of $3.85 per share with an exercise period of 5.5 years from the date of issuance. The July 2020 Warrants were valued at $2.1 million, which was calculated using the Black-Scholes Model with the following assumptions: volatility of 126 percent, stock price of $3.73, and risk-free rate of 0.35%. In connection with the July 2020 Warrant Exercise Transaction, the Company granted to a placement agent warrants to purchase a total of 32,071 shares of common stock (the “July 2020 Placement Agent Warrants”) that have an exercise price per share equal to $4.969 and a term of 5.5 years. The value of the July 2020 Placement Agent Warrants was determined to be $101,000 using the Black-Scholes Model. The Company recognized a loss on extinguishment of warrant liability in the amount of $682,000 associated with this transaction, during the quarter ended September 30, 2020.

In connection with professional services agreements with non-affiliated third parties, during the years ended December 31, 2020 and 2019, the Company issued service and performance warrants (“Service and Performance Warrants”).

As of December 31, 2020, the Company issued the following warrants to purchase shares of its common stock. These warrants are exercisable any time at the option of the holder until their expiration date.

	Issuance Date	Term	Exercise Price Per Share		Warrants Exercised during the Year Ended December 31, 2019		Warrants Outstanding at December 31, 2019		Warrants Exercised during the Year Ended December 31, 2020		Warrants Outstanding at December 31, 2020
December 2020 Warrants	December 2020	5.5 years	$	3.00		—		—		—		2,618,658
December 2020 Placement Agent Warrants	December 2020	5 years	$	3.82		—		—		—		130,933
July 2020 Warrants	July 2020	5.5 years	$	3.85		—		—		—		641,416
July 2020 Placement Agent Warrants	July 2020	5.5 years	$	4.97		—		—		—		32,071
May 2020 Warrants	May 2020	5 years	$	4.78		—		—		—		1,392,345
May 2020 Placement Agent Warrants	May 2020	5 years	$	6.13		—		—		—		69,617
March 2020 Service and Performance Warrants	March 2020	3 years	$	2.50		—		—		—		18,350
February 12, 2020 Service and Performance Warrants	February 2020	2 years	$	4.71		—		—		—		150,000
February 3, 2020 Service and Performance Warrants	February 2020	2 years	$	4.91		—		—		—		10,000
September 2019 Placement Agent Warrants	September 2019	5 years	$	9.48		—		65,942		—		65,942
August 2019 Service and Performance Warrants	August 2019	2 years	$	1.92		—		20,000		—		20,000
July 2019 Service and Performance Warrants	July 2019	2 years	$	2.19		—		10,000		—		10,000
June 2019 Placement Agent Warrants	June 2019	5 years	$	6.29		—		74,479		—		74,479
April 2019 Service and Performance Warrants	April 2019	5 years	$	6.18		—		145,154		—		145,154
June 2018 Placement Agent Warrants	June 2018	5 years	$	12.57		—		69,617		—		69,617
March 2018 Placement Agent Warrants	March 2018	5 years	$	41.56		—		15,038		—		15,038
Total						—		400,230		—		5,463,620

Certain warrants contain a contingent cash payment feature and therefore were accounted for as a liability at the date of issuance and are adjusted to fair value at each balance sheet date. The change in fair value of the warrant liability is recorded as change in fair value of common stock warrant liabilities in the consolidated statements of operations and comprehensive loss. The key terms and activity of the liability classified common stock warrants are summarized as follows:

	Issuance Date	Term	Exercise Price Per Share		Warrants Exercised during the Year Ended December 31, 2019		Warrants Outstanding at December 31, 2019		Warrants Exercised during the Year Ended December 31, 2020		Warrants Outstanding at December 31, 2020
September 2019 Warrants	September 2019	5.5 years	$	7.52		—		659,414		—		659,414
June 2019 Warrants	June 2019	5.5 years	$	5.00		1,053,745		435,830		—		435,830
June 2018 Warrants	June 2018	5.5 years	$	9.94		—		1,392,345		1,392,345		—
March 2018 Warrants	March 2018	5 years	$	10.73		—		1,282,832		641,416		641,416
Total						1,053,745		3,770,421		2,033,761		1,736,660

See Note 6 for the Black-Scholes option-pricing model and weighted-average assumptions used to estimate the fair value of the warrant liabilities.