Note 4 - Equity 

In December 2018, the Company entered into the Amended and Restated At Market Issuance Sales Agreement with B. Riley FBR, Inc. and JonesTrading Institutional Services LLC (the "ATM Sales Agreement"), pursuant to which the Company conducted its at-the market program. During the three months ended March 31, 2020, the Company sold 9.3 million common shares through its at-the-market program, resulting in net proceeds of $2.5 million. During the three months ended March 31, 2019, the Company sold 0.9 million common shares through its at-the-market program, resulting in net proceeds of $0.4 million.

In October 2018, the Company and Lincoln Park Capital Fund, LLC ("Lincoln Park") entered into a purchase agreement (the "Lincoln Park 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 0.8 million shares and received $0.2 million.

For the three months ended March 2019, holders of March 2018 series A warrants exercised 2.5 million shares, resulting in the Company receiving $1.5 million. The remaining March 2018 series A warrants expired in March 2019.

Authorization for Reverse Stock Split

At the Company's Annual Meeting of Stockholders held on December 18, 2019, its stockholders approved an amendment to the Company's certificate of incorporation to effect a reverse stock split of its outstanding common stock by combining outstanding shares of common stock into a lesser number of outstanding shares of common stock by a ratio of not more than 1-for-75 prior to December 18, 2020, with the exact ratio to be set within this range by the Company's Board of Directors, or Board, at its sole discretion. The Board may alternatively elect to abandon such proposed amendment and not effect the reverse stock split authorized by stockholders, in its sole discretion. For more information on the potential reverse stock split, please see Note 1—Description of Business and Summary of Significant Accounting Policies herein.

Stock Options

The following is a summary of stock option activity for the three months ended March 31, 2020:

During the three months ended March 31, 2020, options to purchase 0.3 million shares were cancelled upon the termination of employment for several employees.

The fair values of all options issued and outstanding are being amortized over their respective vesting periods. The unrecognized compensation expense at March 31, 2020 was $1.7 million related to unvested options, which is expected to be expensed over a weighted average of 3.0 years. During the three months ended March 31, 2020 and 2019, the Company recorded compensation expense related to stock options of $0.4 million and $0.3 million, respectively.

Warrants

Following is a summary of warrant activity for the three months ended March 31, 2020:

Subsequent Events

In April 2020, the Company sold 171 thousand shares of common stock through its at-the-market program and realized net proceeds of $35 thousand.

On April 24, 2020, the Company issued and sold 128.3 million shares of common stock and 82.5 million pre-funded warrants to purchase shares of common stock. The price to the public in this offering for each share of common stock was $0.15 and for each pre-funded warrant was $0.1499. Each pre-funded warrant has an exercise price of $0.0001 per share and is exercisable immediately upon issuance. The warrants are subject to certain limitations on beneficial ownership. Gross proceeds from this offering to Actinium were $31.6 million, before deducting underwriting discounts and commissions and other offering expenses payable by the Company. Net proceeds from this offering were $29.1 million.

In June 2020, holders of 36.0 million pre-funded warrants exercised their warrants at $0.0001 per share and received 36.0 million shares of common stock.

In June 2020, the Company issued 157 thousand shares of restricted common stock for consulting services.

On June 19, 2020, the Company issued and sold 55.7 million shares of common stock and 21.3 million pre-funded warrants to purchase shares of common stock. The price to the public in this offering for each share of common stock was $0.325 and for each pre-funded warrant was $0.3249. Each pre-funded warrant has an exercise price of $0.0001 per share and is exercisable immediately upon issuance. The warrants are subject to certain limitations on beneficial ownership. Gross proceeds from this offering to Actinium were $25.0 million, before deducting underwriting discounts and commissions and other offering expenses payable by the Company. Net proceeds from this offering were $22.8 million.

In connection with the Company's June 2020 public offering, the Company suspended, and during the duration of the June 2020 public offering did not offer, any securities pursuant to the Lincoln Park Agreement and the ATM Sales Agreement. The Company will not make any sales of securities pursuant to the Lincoln Park Agreement and the ATM Sales Agreement unless and until a new prospectus supplement is filed with the SEC; however, the Lincoln Park Agreement and the ATM Sales Agreement remain in full force and effect.