Activities for the Company’s stock option plans for the six months ended June 30, 2022 were as follows:


	Options Outstanding
	Number of Options			Weighted- Average Exercise Price Per Share
Balances at December 31, 2021		12,192,216		$	9.42
Options granted		3,416,918			3.55
Options exercised		(100,990	)		0.98
Option forfeited/expired		(1,077,318	)		8.93
Balances at June 30, 2022		14,430,826		$	8.13

The Company recorded $2.8 million and $3.3 million of stock-based compensation expense related to the stock options for the three months ended June 30, 2022 and 2021, respectively.

The Company recorded $5.7 million and $6.3 million of stock-based compensation expense related to the stock options for the six months ended June 30, 2022 and 2021, respectively.

Activities for the Company’s TRSUs for the six months ended June 30, 2022 were as follows:


	Number of Shares			Weighted Average Grant Date Fair Value Per Share

Balance at December 31, 2021		433,250		$	5.34
RSU's awarded		655,410			3.89
RSU's vested		—			—
RSU's cancelled		(97,044	)		4.75
Balance at June 30, 2022		991,616		$	4.44

The Company recorded $0.4 million and $0.7 million of stock-based compensation expense related to the TRSUs for the three and six months ended June 30, 2022, respectively. The Company began granting TRSUs in the fourth quarter of 2021.

In October 2021, the Company granted 435,000 PSUs as recognition awards to executive employees with an aggregated grant date fair value of $2.3 million. 50% of the PSUs granted will vest within one year of the grant date upon achievement of certain specific milestones ("Tranche 1") and the remaining 50% will vest within two years of the grant date upon achievement of additional company objectives ("Tranche 2"). The Company determined that it is not probable that the performance conditions will be satisfied for each of these Tranches and hence no compensation cost was recorded for these awards for the year ended December 31, 2021.

As of June 30, 2022, the Company determined that the achievement of the milestones for Tranche 1 was probable and hence recorded $1.0 million of stock-based compensation expense for the three and six months ended June 30, 2022. As the achievement of the milestones for Tranche 2 was not considered probable, no compensation cost was recorded for these awards as of and for the three and six months ended June 30, 2022.

Activities for the Company’s PSUs for the six months ended June 30, 2022 were as follows:


	Number of Shares			Weighted Average Grant Date Fair Value Per Share

Balance at December 31, 2021		435,000		$	5.34
PSU's awarded		—			—
PSU's vested		—			—
PSU's cancelled		(47,500	)		5.34
Balance at June 30, 2022		387,500		$	5.34

Total stock-based compensation recorded related to options, TRSUs, PSUs and the ESPP was as follows:


	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2022		2021		2022		2021
	(in thousands)
Stock-based compensation expense:
Research and development	$	2,091	$	1,548	$	3,696	$	2,885
General and administrative		2,399		1,877		4,164		3,574
Total stock-based compensation expense	$	4,490	$	3,425	$	7,860	$	6,459