New Delhi, July 6 -- LOS ANGELES - For decades, the drugs designed to fight small cell neuroendocrine cancers of the lung, prostate, and ovary have worked for a while and then stopped working. The tumors outrun chemotherapy. They develop resistance to immunotherapy. For patients diagnosed with these cancers, survival figures have barely moved in four decades. A study published this week in the Proceedings of the National Academy of Sciences suggests that the answer to this long failure may already be sitting on a pharmacy shelf, prescribed for something else entirely.

Leflunomide, which rheumatologists prescribe for rheumatoid arthritis, and teriflunomide, which neurologists use to slow multiple sclerosis, are both classified as autoimmu...