Our net interest income was $6.0 million for the six months ended June 30, 2019, compared to $1.9 million in the corresponding period in 2018. The increase in interest income was due primarily to a higher average principal balance in our investment account in 2019, and increased interest rates.
Liquidity and Capital Resources
Since inception, we have incurred significant net losses and we have funded our operations primarily through the issuance of convertible debt, the issuance of shares of our common stock and shares of our preferred stock, and the proceeds from the Merger. Our most significant use of capital pertains to salaries and benefits for our employees, including clinical, scientific, operational, financial and management personnel, and external research and development expenses, such as clinical trials and preclinical activity related to our product candidates.
As of June 30, 2019, we had cash, cash equivalents and marketable securities totaling $466.4 million compared to $483.7 million as of December 31, 2018, with the decrease primarily attributable to our net operating loss. Our cash and investment balances are held in a variety of interest bearing instruments, including obligations of U.S. government agencies, U.S. Treasury debt securities, corporate debt securities and money market funds. Cash in excess of immediate requirements is invested in accordance with our investment policy with a view toward capital preservation and liquidity.
We anticipate continuing to incur operating losses for the foreseeable future. While our rate of cash usage may increase in the future, in particular to support our product development and clinical trial efforts, we believe our available cash resources as of June 30, 2019 will be sufficient to fund our operations past one year from the issuance of the financial statements contained herein. Future capital requirements will be substantial and will depend on many factors. To meet future capital requirements, we will need to raise additional capital to fund our operations through equity or debt financing. We regularly consider fundraising opportunities and may decide, from time to time, to raise capital based on various factors, including market conditions and our plans of operation. Additional capital may not be available on terms acceptable to us, or at all. If adequate funds are not available, or if the terms of potential funding sources are unfavorable, our business and our ability to develop our product candidates could be harmed. Furthermore, any sales of additional equity securities may result in dilution to our stockholders, and any debt financing may include covenants that restrict our business.
The following table provides a summary of our net cash flow activity (in thousands):
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Six Months Ended June 30,
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Net cash used in operating activities
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$
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(18,018
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)
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$
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(14,004
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)
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Net cash (used in) provided by investing activities
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32,947
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(183,036
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)
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Net cash provided by financing activities
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200
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313,274
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Net increase in cash and cash equivalents
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$
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15,129
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|
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$
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116,234
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|
Net cash used in operating activities was $18.0 million for the six months ended June 30, 2019, compared to $14.0 million for the corresponding period in 2018. The use of cash in these periods resulted primarily from our losses from operations, as adjusted for non-cash charges for stock-based compensation, and changes in our working capital accounts.
Net cash provided by investing activities was $32.9 million for the six months ended June 30, 2019, compared to $183.0 million used in investing activities for the corresponding period in 2018. Net cash provided by investing activities for the six months ended June 30, 2019 consisted of $299.1 million from sales and maturities of marketable securities, partially offset by $266.1 million of purchases of marketable securities for our investment portfolio. Net cash used by investing activities for the six months ended June 30, 2018 consisted of $251.9 million of purchases of marketable securities for our investment portfolio, partially offset by $68.8 million from sales and maturities of marketable securities.
Net cash provided by financing activities was $0.2 million for the six months ended June 30, 2019, compared to $313.3 million for the corresponding period in 2018. Net cash provided by financing activities for the six months ended June 30, 2019 consisted of proceeds from exercise of stock options. Net cash provided by financing activities for the six months ended June 30, 2018 consisted of net proceeds from the issuance of common stock pursuant to the June 2018 Offering and the exercise of stock options.