The Company is authorized to issue 100,000,000 shares of common stock with a par value of $0.000001 as of June 30, 2021 and December 31, 2020. As of June 30, 2021 and December 31, 2020, the total number of shares of common stock issued and outstanding was 42,419,129 and 42,196,296, respectively.

As of June 30, 2021, the Company also has pre-funded warrants outstanding to purchase an aggregate of 12,663,010 shares of common stock. The pre-funded warrants are exercisable at any time, with 61 days' notice, for an exercise price of $0.000001, except that the pre-funded warrants cannot be exercised by the holders if, after giving effect thereto, the holders would beneficially own more than 9.99% of the outstanding common stock, subject to certain exceptions. The holders of the pre-funded warrants will not have the right to vote on any matter except to the extent required by Delaware law.


(in thousands)	Three Months Ended June 30,				Six Months Ended June 30,
	2021		2020		2021		2020
Research and development expenses	$	1,409	$	346	$	2,427	$	629
General and administrative expenses	1,509		676		2,911		1,076
Total stock-based compensation expense	$	2,918	$	1,022	$	5,338	$	1,705

Total unrecognized compensation expense for all stock-based compensation plans was $29.5 million as of June 30, 2021. This expense is expected to be recognized over a weighted average remaining vesting period of 2.91.

The fair values of stock options granted are estimated using the Black-Scholes option pricing model with the following weighted average assumptions:


	Three Months Ended June 30,				Six Months Ended June 30,
	2021		2020		2021		2020
Expected volatility	89.12	%	93.61	%	89.19	%	92.09	%
Expected dividends	0	%	0	%	0	%	0	%
Expected terms (years)	5.94		5.82		5.95		5.91
Risk free rate	1.07	%	0.41	%	1.00	%	0.56	%

A summary of the Company’s stock option activity and related information for the six months ended June 30, 2021 is as follows:


	Number of Shares	Weighted Average Exercise Price		Weighted Average Remaining Contractual Life (in Years)	Aggregate Intrinsic Value (in Thousands)
Outstanding at December 31, 2020	6,846,289	$	6.42	8.71	$	53,127
Options granted	1,050,300	$	12.29
Options exercised	(116,861)	$	3.21
Options cancelled/forfeited	(73,375)	$	11.25
Outstanding at June 30, 2021	7,706,353	$	7.22	8.43	$	26,289
Exercisable as of June 30, 2021	2,487,966	$	5.14	7.57	$	12,422

During the six months ended June 30, 2021, 116,861 shares of common stock were issued upon exercise of options with an aggregate intrinsic value of $1.2 million. During the six months ended June 30, 2020, 475,193 shares of common stock were issued upon exercise of options with an aggregate intrinsic value of $2.4 million. The weighted-average grant date fair value of options granted during the six months ended June 30, 2021 and June 30, 2020 was $8.99 and $7.84 per share, respectively.


	Number of Shares	Weighted Average Grant Date Fair Value
Non-vested at December 31, 2020	186,500	$	8.19
Restricted stock units granted	70,000	$	12.94
Restricted stock units vested	(83,000)	$	7.27
Restricted stock units forfeited	—	$	—
Non-vested at June 30, 2021	173,500	$	10.54

During the six months ended June 30, 2021, the Company granted 50,000 RSUs which vest based on certain performance conditions in two equal tranches. Each RSU entitles the holder to receive one share of the Company's common stock when the RSU vests. Twenty-five thousand of such performance-based RSUs vested during the six months ended June 30, 2021 as the related performance condition was met. Related stock-based compensation expense recognized was $0.3 million. The remaining 25,000 RSUs are not currently considered probable to vest. As such, expense of $0.3 million related to this tranche remained unrecognized as of June 30, 2021.

The Company’s 2020 Employee Stock Purchase Plan (“2020 ESPP”) was adopted by the Company’s Board of Directors in March 2020 and approved by the Company’s stockholders in May 2020. A total of 759,936 shares of common stock have been reserved for issuance under the 2020 ESPP.

Subject to share and dollar limits as described in the plan, the 2020 ESPP allows eligible employees to contribute, through payroll deductions, up to 15% of their earnings for the purchase of shares of the Company’s common stock at the lower of 85% of the closing price of the Company’s common stock on the first trading day of the offering period or 85% of the closing price of the Company’s common stock on the last trading day of the offering period. There are two six-month offering periods during each fiscal year, ending on May 15 and November 15.

For the six months ended June 30, 2021, the Company issued 22,972 shares of common stock at a purchase price per share of $9.53 under the 2020 ESPP. Cash received from the purchases under the 2020 ESPP for the six months ended June 30, 2021 was $0.2 million.