Liquidity and Capital Resources
Since our inception in 2007, we have devoted most of our cash resources to research and development and general and administrative activities. We have financed our operations primarily with the proceeds from the sale of equity securities (most notably our initial public offering, our follow-on public offerings and sales under our “at-the-market” offering) and convertible promissory notes, state and federal grants and research services.
To date, we have not generated any revenue from the sale of products, and we do not anticipate generating any revenue from the sales of products for the foreseeable future. We have incurred losses and generated negative cash flows from operations since inception. As of June 30, 2020, our principal sources of liquidity were our cash and cash equivalents of $77.0 million. Our working capital was $72.3 million as of June 30, 2020.
Management believes that current cash and cash equivalents are sufficient to fund operations and capital requirements into the fourth quarter of 2021. However, the economic effects of the COVID-19 pandemic remain fluid and management will continue to closely monitor the situation to ensure our cash and cash equivalents will help us manage the impact of the COVID-19 pandemic on our business and related liquidity needs. Substantial additional financings will be needed to fund our operations and to complete clinical development of and to commercially develop our product candidates. There is no assurance that such financing will be available when needed or on acceptable terms. Our ability to access the capital markets or otherwise raise such capital may be adversely impacted by potential worsening global economic conditions and the recent disruptions to, and volatility in, financial markets in the United States and worldwide resulting from the ongoing COVID-19 pandemic.
Equity Financings
In August 2019, we entered into a Controlled Equity Offering Sales AgreementSM, or the 2019 Sales Agreement, with Cantor Fitzgerald & Co., Canaccord Genuity, LLC, H.C. Wainwright & Co. LLC and Ladenburg Thalmann & Co. Inc., as sales agents pursuant to which we may sell, from time to time, up to $75.0 million of our common stock. In 2019, we sold and issued 13,381 shares of our common stock in the open market at a weighted-average selling price of $7.00, for gross and net proceeds of $0.1 million. In the first quarter of 2020, we sold and issued 356,000 shares of our common stock in the open market at a weighted-average selling price of $5.10 per share, for gross proceeds of $1.8 million and net proceeds, after deducting commissions and offering expenses, of $1.6 million. In the second quarter of 2020, we sold and issued 5,682,784 shares of our common stock in the open market at a weighted-average selling price of $4.92 per share, for gross proceeds of $27.9 million and net proceeds, after deducting commissions and offering expenses, of $27.2 million.
In June 2017, we entered into an Open Market Sales Agreement, or the 2017 Sales Agreement, with Jefferies LLC, or Jefferies, pursuant to which we sold $50.0 million of our common stock. In the first quarter of 2019, we sold and issued 3,439,523 shares of common stock under the 2017 Sales Agreement with Jefferies in the open market at a weighted average selling price of $5.44 per share, resulting in gross proceeds of $18.7 million. Net proceeds received after deducting commissions and offering expenses were $18.1 million. In the second quarter of 2019, we sold and issued 2,082,031 shares of common stock under the 2017 Sales Agreement with Jefferies in the open market at a weighted average selling price of $13.50 per share, resulting in gross proceeds of $28.1 million. Net proceeds received after deducting commissions and offering expenses were $27.0 million. From June 2017 through May 16, 2019, we had cumulative gross proceeds of $50.0 million from shares sold in the open market under the 2017 Sales Agreement. The last sale under the 2017 Sales Agreement was made on May 16, 2019, following which the 2017 Sales Agreement terminated pursuant to its terms.
Debt
We had no debt outstanding as of June 30, 2020 or December 31, 2019.
Future Capital Requirements
During the six months ended June 30, 2020, net cash used in operating activities was $21.7 million, and our accumulated deficit as of June 30, 2020 was $183.5 million. Our expectations regarding future cash requirements do not reflect the potential impact of any future acquisitions, mergers, dispositions, joint ventures or investments that we may make in the