In May 2019, we entered into a sales agreement with H.C. Wainwright & Co., LLC (Wainwright) with respect to an at-the-market offering (ATM Offering Program) under which we could offer and sell shares of our common stock having an aggregate offering price of up to $10.0 million. In November 2020, we amended our sales agreement with Wainwright to increase the amount of the ATM Offering Program up to $20.0 million. Wainwright was entitled to a commission at a fixed rate equal to 3% of the gross proceeds. In March 2021, the ATM Offering Program with Wainwright automatically terminated upon the issuance and sale of all of the shares of common stock having an aggregate offering price of $20.0 million. For the three months ended March 31, 2021, we sold an aggregate of 1,988,254 shares of common stock at an average price of $4.99 per share for net proceeds of $9.6 million under the ATM Offering Program.

In March 2021, we entered into a Capital on Demand^TM Sales Agreement with JonesTrading Institutional Services LLC (JonesTrading) for a new ATM Offering Program, pursuant to which we can sell from time to time, at our option, up to an aggregate of $25.0 million of shares of our common stock through JonesTrading, as sales agent or principal. JonesTrading is entitled to a commission at a fixed rate equal of up to 3% of the gross proceeds. For the three months ended March 31, 2021, we did not issue any shares under this ATM Offering Program.

In February 2020, we completed an underwritten follow-on public offering of 4,235,294 shares of our common stock at a price to the public of $4.25 per share. In March 2020, the underwriters fully exercised their option to purchase additional shares resulting in the issuance of an additional 635,294 shares of common stock. The total gross proceeds from the underwritten follow-on public offering, including the underwriters’ option to purchase additional shares, was approximately $20.7 million, before deducting underwriting discounts, commissions and offering expenses payable by us.

In September 2020, we entered into a common stock purchase agreement (Purchase Agreement) with Aspire Capital Fund, LLC (Aspire Capital), which provides that, upon the terms and subject to the conditions and limitations set forth therein, Aspire Capital is committed to purchase up to an aggregate of $20.0 million of shares of our common stock at our request from time to time during the 30 month term of the Purchase Agreement. Concurrently with entering into the Purchase Agreement, we also entered into a registration rights agreement with Aspire Capital, in which we agreed to file one or more registration statements, as permissible and necessary to register under the Securities Act of 1933, as amended, for the resale of the shares of our common stock that have been and may be issued to Aspire Capital under the Purchase Agreement. For the three months ended March 31, 2021, we sold an aggregate of 3,000,000 shares of common stock at an average price of $5.09 per share for net proceeds of $15.2 million under this Purchase Agreement.

	March 31, 2021
Common stock warrants		13,760
Common stock options and restricted stock units		898,306
Shares available under the 2015 equity incentive plan		63,391
Shares available under the employee stock purchase plan		75,315
		1,050,772

The following table summarizes our stock option activity under all equity incentive plans for the three months ended March 31, 2021:


	Number of Outstanding Stock Options			Weighted Average Exercise Price
Outstanding as of December 31, 2020		576,534		$	23.33
Granted		326,456		$	4.52
Canceled/forfeited/expired		(12,184	)	$	25.47
Outstanding as of March 31, 2021		890,806		$	16.49

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

	March 31,
	2021		2020
Expected term (in years)	5.99 – 6.02			6.02
Risk-free interest rate	0.6% – 0.9%			1.5	%
Expected volatility	104.0% – 104.8%			102.2	%
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, 2021:

	Number of Outstanding Restricted Stock Units			Weighted Average Grant Date Fair Value
Balance as of December 31, 2020		7,677		$	5.32
Granted		4,000		$	3.89
Released		(4,177	)	$	6.18
Balance as of March 31, 2021		7,500		$	4.08

The allocation of stock-based compensation for all options, 2015 Employee Stock Purchase Plan and restricted stock units is as follows (in thousands):

	March 31,
	2021		2020
Research and development	$	63	$	62
General and administrative		297		361
Total stock-based compensation expense	$	360	$	423