Eastman Kodak Co. warned investors that “substantial doubt” exists about its ability to remain in business after acknowledging it lacks committed financing or sufficient liquidity to repay roughly $500 million of upcoming debt. The disclosure appeared in the 133-year-old company’s second-quarter earnings filing released late Monday. The Rochester, New York-based firm swung to a net loss of $26 million in the quarter, compared with a $26 million profit a year earlier. Revenue slipped 1% to $263 million. Kodak ended June with about $155 million in cash, a $46 million decline since December, and said it may halt pension contributions while it seeks new funding or asset sales. The warning sent the stock tumbling: Kodak shares fell more than 6% in pre-market trading Tuesday and were down about 25% during the session. The latest slide deepens the volatility that has dogged the one-time photography icon since it emerged from bankruptcy in 2013. Chief Executive Officer Jim Continenza said Kodak is pressing ahead with cost cuts and investment in advanced materials and chemicals, adding that recently enacted U.S. tariffs on Chinese goods should have little impact because most of the company’s production is domestic. Management hopes to outline a financing plan before the end of the year.