Note 5 - Equity 

In August 2020, the Company entered into the Capital on Demand™ Sales Agreement with JonesTrading Institutional Services LLC (“JonesTrading”), pursuant to which the Company may sell, from time to time, through or to JonesTrading, up to an aggregate of $200 million of its common stock. Shares of common stock are offered pursuant to the Company’s shelf registration statement on Form S-3 filed with the SEC on August 7, 2020. As of December 31, 2020, the Company had sold 2.1 million shares of common stock, resulting in gross proceeds of $22.6 million and net proceeds of $21.7 million. For the three months ended March 31, 2021, the Company sold 1.7 million shares of common stock, resulting in gross proceeds of $14.8 million and net proceeds of $14.4 million.

In December 2018, the Company entered into the Amended and Restated At Market Issuance Sales Agreement with B. Riley FBR, Inc. and JonesTrading, pursuant to which the Company conducted its at-the market program. During the three months ended March 31, 2020, the Company sold 0.3 million shares of common stock through its at-the-market program, resulting in net proceeds of $2.5 million.

In October 2018, the Company and Lincoln Park Capital Fund, LLC (“Lincoln Park”) entered into a purchase agreement and a registration rights agreement, pursuant to which the Company has the right to sell to Lincoln Park shares of the Company’s common stock having an aggregate value of up to $32.5 million, subject to certain limitations and conditions set forth in the agreement. During the three months ended March 31, 2020, the Company elected to sell to Lincoln Park 27 thousand shares and received $0.2 million.

Stock Options

The following is a summary of stock option activity for the three months ended March 31, 2021:

The fair values of all options issued and outstanding are being amortized over their respective vesting periods. The unrecognized compensation expense at March 31, 2021 was $3.8 million related to unvested options, which is expected to be expensed over a weighted average of 3.2 years. During the three months ended March 31, 2021 and 2020, the Company recorded compensation expense related to stock options of $0.4 million and $0.4 million, respectively.

Warrants

Following is a summary of warrant activity for the three months ended March 31, 2021: