The Company has three stock compensation plans, the 2020 Stock Incentive Plan (the “2020 Plan”), the 2014 Stock Incentive Plan (the “2014 Plan”) and the 2007 Stock Incentive Plan (the “2007 Plan”). The 2020 Plan permits the Company to make grants of incentive stock options, non-statutory stock options, restricted stock, restricted stock units, stock appreciation rights and other stock-based awards to the Company’s employees, officers, directors, consultants and advisors; however, incentive stock options may only be granted to the Company’s employees. The number of shares initially reserved for issuance under the 2020 Plan was 4,860,000 shares of common stock and may be increased by the number of shares under the 2014 Plan that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by the Company. Options remain outstanding under the 2007, 2014 and the 2020 Plan. The number of shares subject to and the exercise prices applicable to these outstanding options were adjusted in connection with the one-for-eighteen reverse stock-split. As of December 31, 2020, there were 1,151 and 126,974 options outstanding under the 2007 Plan and 2014 Plan, respectively. Options granted under the 2007, 2014 and 2020 Plans generally expire ten years from the date of grant. The Company intends for the 2020 Plan to be its primary stock compensation plan in the future. As of December 31, 2020, there were 692,956 options outstanding and 4,167,044 shares available to issue under the 2020 Plan.

As of December 31, 2020, a total of 4,167,044 stock awards were available for grant under the 2020 Plan. The 2014 Plan was closed to new grants following the approval of the 2020 Plan, and therefore, there were no shares reserved for issuance under the 2014 Plan as of December 31, 2020.

	Shares Issuable Under Options			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(In years)
Outstanding as of January 1, 2019		49,798		$	233.45		8.1	$	—
Granted		42,244			28.87
Exercised		—			—
Forfeited / Canceled		(639	)		79.38
Outstanding as of December 31, 2019		91,403			139.96		8.2		—
Granted		2,191,462			9.15
Options assumed in connection with acquisition		1,346,398			7.58
Exercised		—			—
Forfeited / Canceled		(7,784	)		586.16
Outstanding as of December 31, 2020		3,621,479		$	10.63		8.9	$	23,938,701
Options vested and expected to vest as of December 31, 2020		3,621,479		$	15.21		8.9	$	23,938,701
Options exercisable as of December 31, 2020		878,033		$	16.00		8.2	$	6,297,510

	Shares Issuable Under PRSUs			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(In years)
Outstanding as of January 1, 2019		4,358		$	86.94		9.2	$	—
Granted		3,056			66.96
PRSUs Vested		(4,358	)		86.94
Forfeited / Canceled		—			—
Outstanding as of December 31, 2019		3,056			66.96		9.2		—
Granted		—			—
PRSUs Vested		(3,056	)		66.96
Forfeited / Canceled		—			—
Outstanding as of December 31, 2020		—		$	—			$	—
PRSUs vested and expected to vest as of December 31, 2020		—		$	—			$	—
Options exercisable as of December 31, 2020		—		$	—			$	—

As of December 31, 2020, the range of exercise prices was between $0.96 and $2,147 for options outstanding.

Intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of the common stock for the options that had exercise prices that were lower than the fair value per share of the common stock on the date of exercise. There was no aggregate intrinsic value of options exercised during the years ended December 31, 2020 and 2019.

As of December 31, 2020, total unrecognized stock-based compensation expense related to non-vested equity awards was $17.1 million, which is expected to be recognized over an estimated weighted-average period of 2.7 years.

Total compensation expense related to all of the Company’s stock-based awards for the years ended December 31, 2020 and 2019 was comprised of the following (in thousands):

	Year Ended December 31,
	2020		2019
Stock-based compensation classified as:
Research and development expense	$	1,254	$	343
General and administrative expense		1,917		965
Total stock-based compensation expense	$	3,171	$	1,308

Stock-based compensation expense for the year ended December 31, 2020 included no stock-based compensation expense related to performance-based options granted during 2020.

During the year ended December 31, 2020, PRSUs awarded to employees totaling 3,059 shares vested and resulted in the recognition of $205,000 in stock-based compensation expense.

The following table presents the assumptions used in the Black-Scholes option pricing model to determine the fair value of stock options granted in the periods presented, as follows: