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Stockholders' Equity | 9 Months Ended
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Sep. 30, 2021
Stockholders' Equity |
Stockholders' Equity | Note 7 - Stockholders’ Equity Registration Statements On October 23, 2020, the Company filed a shelf registration statement No. 333-249657 on Form S-3 (the “2020 S-3”), which was declared effective on December 4, 2020. Under the 2020 S-3, the Company may sell up to a total of $100.0 million of its securities. As of September 30, 2021, approximately $19.3 million of the 2020 S-3 remains available for sales of securities. On April 23, 2021, the Company filed a shelf registration statement No. 333-255476 on Form S-3 (the “2021 S-3”), which was declared effective on May 24, 2021. Under the 2021 S-3, the Company may sell up to a total of $200.0 million of its securities. As of September 30, 2021, there have been no sales of securities under the 2021 S-3. Common Stock At-the-Market Offering On July 13, 2018, the Company filed a shelf registration statement No. 333-226175 on Form S-3, as amended on July 20, 2018 (the “2018 Mustang S-3”), which was declared effective in August 2018. Under the 2018 Mustang S-3, the Company may sell up to a total of $75.0 million of its securities. In connection with the 2018 Mustang S-3, the Company entered into an At-the-Market Issuance Sales Agreement (the “Mustang ATM”) with B. Riley Securities, Inc. (formerly B. Riley FBR, Inc.), Cantor Fitzgerald & Co., National Securities Corporation, and Oppenheimer & Co. Inc. (each an “Agent” and collectively, the “Agents”), relating to the sale of shares of common stock. Under the Mustang ATM, the Company pays the Agents a commission rate of up to 3.0% of the gross proceeds from the sale of any shares of common stock. On December 31, 2020, the Mustang ATM was amended to add H.C. Wainwright & Co., LLC as an Agent. During the nine months ended September 30, 2021, the Company issued approximately 17.3 million shares of common stock at an average price of $3.87 per share for gross proceeds of $66.9 million under the Mustang ATM. In connection with these sales, the Company paid aggregate fees of approximately $1.3 million. During the nine months ended September 30, 2020, the Company issued approximately 7.2 million shares of common stock at an average price of $3.56 per share for gross proceeds of $25.6 million under the Mustang ATM. In connection with these sales, the Company paid aggregate fees of approximately $0.5 million. ​ Pursuant to the Founders Agreement, the Company issued 517,304 shares of common stock to Fortress at a weighted average price of $3.84 per share for the nine months ended September 30, 2021, and recorded 52,019 shares issuable to Fortress in connection with the shares issued under the Mustang ATM. Pursuant to the Founders Agreement, Mustang issued 117,405 shares of common stock to Fortress at a weighted average price of $3.56 per share for the nine months ended September 30, 2020, in connection with the shares issued under the Mustang ATM. ​ Equity Incentive Plan The Company has in effect the 2016 Incentive Plan (the “Incentive Plan”). The Incentive Plan was adopted in 2016 by our stockholders and the compensation committee of the Company’s board of directors and is authorized to grant stock-based awards to directors, officers, employees and consultants. The plan initially authorized grants to issue up to 2,000,000 shares of authorized but unissued common stock, expires 10 years from adoption, and limits the term of each option to no more than 10 years from the date of grant. In June 2018, the Company’s stockholders approved an amendment to the Incentive Plan to increase the number of authorized shares issuable by 3,000,000 shares, for a total of 5,000,000 shares. In June 2021, the Company’s stockholders approved an amendment to the Incentive Plan to increase the number of authorized shares issuable by 3,000,000 shares, for a total of 8,000,000 shares. As of September 30, 2021, 3,447,642 shares are available for issuance under the Incentive Plan. Stock Options The following table summarizes stock option activities for the nine months ended September 30, 2021: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ |  | ​ | ​ |  | Weighted Average
​ | ​ | ​ | ​ | ​ | ​ | ​ | Remaining
​ | ​ | ​ | ​ | Weighted Average | ​ | Contractual Life (in
​ | ​ | Stock Options | ​ | Exercise Price | ​ | years)
Outstanding at December 31, 2020 |  | 1,141,675 | ​ | $ | 5.73 |  | 6.31
Outstanding at September 30, 2021 |  | 1,141,675 | ​ |  | 5.73 |  | 5.56
Options vested and exercisable at September 30, 2021 |  | 713,547 | ​ | $ | 5.73 |  | 5.56
​ As of September 30, 2021, the Company had unrecognized stock-based compensation expense related to options of approximately $12,000, which is expected to be recognized over the remaining weighted average vesting period of approximately 0.1 years. ​ Restricted Stock The following table summarizes restricted stock award activities for the nine months ended September 30, 2021: ​
​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ |  | Weighted Average
​ | ​ | ​ | ​ | Grant Date Fair
​ | ​ | Number of Shares | ​ | Value
Nonvested at December 31, 2020 |  | 302,114 | ​ | $ | 4.93
Granted |  | 68,870 | ​ |  | 3.63
Vested | ​ | (90,001) | ​ | ​ | 6.69
Nonvested at September 30, 2021 |  | 280,983 | ​ | $ | 4.05
​ As of September 30, 2021, the Company had unrecognized stock-based compensation expense related to restricted stock of $0.5 million, which is expected to be recognized over the remaining weighted average vesting period of approximately 2.0 years. Restricted Stock Units Certain employees and consultants have been awarded restricted stock units with time-based vesting. The following table summarizes restricted stock units’ activities for the nine months ended September 30, 2021: ​
​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ |  | Weighted Average
​ | ​ | ​ | ​ | Grant Date Fair
​ | ​ | Number of Units | ​ | Value
Nonvested at December 31, 2020 |  | 1,468,559 | ​ | $ | 3.87
Granted |  | 1,100,000 | ​ |  | 3.45
Forfeited | ​ | (298,687) | ​ | ​ | 3.64
Vested |  | (298,690) | ​ |  | 4.29
Nonvested at September 30, 2021 |  | 1,971,182 | ​ | $ | 3.60
​ As of September 30, 2021, the Company had unrecognized stock-based compensation expense related to restricted stock units of approximately $3.8 million, which is expected to be recognized over the remaining weighted average vesting period of approximately 1.9 years. The following table summarizes stock-based compensation expense for the three and nine months ended September 30, 2021 and 2020 (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | For the three months ended September 30, | ​ | For the nine months ended September 30, | ​
​ |  | 2021 |  | 2020 |  | 2021 |  | 2020 |
General and administrative | ​ | $ | 229 | ​ | $ | 348 | ​ | $ | 793 | ​ | $ | 1,213 | ​
Research and development | ​ |  | 655 | ​ |  | 258 | ​ |  | 1,634 | ​ |  | 1,155 | ​
Total stock-based compensation expense | ​ | $ | 884 | ​ | $ | 606 | ​ | $ | 2,427 | ​ | $ | 2,368 | ​
​ Employee Stock Purchase Plan ​ Eligible employees can purchase the Company’s Common Stock at the end of a predetermined offering period at 85% of the lower of the fair market value at the beginning or end of the offering period. The Employee Stock Purchase Plan (“ESPP”) is compensatory and results in stock-based compensation expense. The ESPP was initially authorized in 2019 to sell up to 400,000 shares of authorized but unissued common stock. In June 2021, the Company’s stockholders approved an amendment to the ESPP to increase the number of authorized shares issuable by 600,000 shares, for a total of 1,000,000 shares. ​ For the nine months ended September 30, 2021 and 2020, 114,321 and 140,856 shares, respectively, have been purchased and 744,823 shares are available for future sale under the Company’s ESPP. The Company received proceeds of $0.3 million and $0.3 million for the nine months ended September 30, 2021 and 2020, respectively. Share-based compensation expense recorded was approximately $0.4 million for the nine months ended September 30, 2021. Warrants A summary of warrant activities for the nine months ended September 30, 2021, is presented below: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ |  | ​ | ​ |  | Weighted Average
​ | ​ | ​ | ​ | ​ | ​ | ​ | Remaining
​ | ​ | ​ | ​ | Weighted Average | ​ | Contractual Life (in
​ | ​ | Warrants | ​ | Exercise Price | ​ | years)
Outstanding as of December 31, 2020 |  | 5,402,670 | ​ | $ | 8.21 |  | 1.39
Expired |  | (666,366) | ​ |  | 8.50 |  | —
Cashless exercised |  | (138) | ​ |  | — |  | —
Outstanding as of September 30, 2021 |  | 4,736,166 | ​ | $ | 8.16 |  | 0.73
​ Upon the cashless exercise of warrants, the Company will issue new shares of common stock.

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- Definition The entire disclosure for shareholders' equity comprised of portions attributable to the parent entity and noncontrolling interest, including other comprehensive income. Includes, but is not limited to, balances of common stock, preferred stock, additional paid-in capital, other capital and retained earnings, accumulated balance for each classification of other comprehensive income and amount of comprehensive income. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08(e)(1)) -URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=120397183&loc=d3e187085-122770 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -URI http://asc.fasb.org/topic&trid=2208762 + Details
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