In February 2023, we completed an underwritten follow-on public offering of 23,125,000 shares of our common stock, including the partial exercise of the underwriters’ option to purchase additional shares, at a price to the public of $2.25 per share. The total net proceeds from the offering were approximately $48.1 million, after deducting underwriting discounts, commissions and offering expenses payable by us.

In April 2022, we entered into an Open Market Sale AgreementSM with Jefferies LLC (Jefferies) implementing an “at-the-market” offering program (the Jefferies ATM Offering Program), pursuant to which we may offer and sell, from time to time and at our option, up to an aggregate of $65.0 million of shares of our common stock through Jefferies, acting as sales agent. Jefferies is entitled to a fixed commission rate of up to 3.0% of the gross sales proceeds of shares sold under the Jefferies ATM Offering Program. During the year ended December 31, 2023, we sold an aggregate of 10,530,795 shares of common stock at a weighted-average price of $1.82 per share for net proceeds of approximately $18.4 million under the Jefferies ATM Offering Program. During the three months ended March 31, 2024, we sold an aggregate of 5,045,518 shares of common stock at a weighted-average price of $1.79 per share for net proceeds of approximately $8.6 million under the Jefferies ATM Offering Program of which $0.8 million was included in other receivables as of March 31, 2024.

Common Stock Reserved for Future Issuance

Common stock reserved for future issuance was as follows:


	March 31, 2024
Common stock warrants		5,864
Common stock options and restricted stock units		5,712,725
Shares available under the 2015 equity incentive plan		2,238,757
Shares available under the 2022 inducement plan		106,483
Shares available under the employee stock purchase plan		727,311
		8,791,140

The following table summarizes our stock option activity under all equity incentive plans for the three months ended March 31, 2024:


	Number of Outstanding Stock Options			Weighted- Average Exercise Price
Outstanding as of December 31, 2023		3,956,827		$	6.00
Granted		1,750,098		$	1.50
Canceled/forfeited/expired		(35,416	)	$	4.64
Outstanding as of March 31, 2024		5,671,509		$	4.62

The assumptions used in the Black-Scholes option pricing model to determine the fair value of the employee stock option grants were as follows:


	Three Months Ended March 31,
	2024			2023
Expected term (in years)	5.94 – 6.02			$	6.02
Risk-free interest rate	3.8% – 4.0%				4.0	%
Expected volatility	80.3% – 80.5%				82.0	%
Expected dividend yield		0.0	%		0.0	%

The following table summarizes our restricted stock unit activity under all equity incentive plans for the three months ended March 31, 2024:


	Number of Outstanding Restricted Stock Units			Weighted-Average Grant Date Fair Value
Balance as of December 31, 2023		63,327		$	5.45
Released		(22,111	)	$	5.39
Balance as of March 31, 2024		41,216		$	5.48

The allocation of stock-based compensation for all options and restricted stock units and stock issued pursuant to our employee stock purchase plan is as follows (in thousands):


	Three Months Ended March 31,
	2024		2023
Research and development	$	169	$	129
General and administrative		571		490
Total stock-based compensation expense	$	740	$	619