Researchers are optimistic about the potential for the new cancer drug larotrectinib—which promises to help patients, young and old, who are suffering from a wide range of diagnoses. The medication selectively inhibits tropomyosin receptor kinase (TRK) fusion proteins, which are produced by a genetic abnormality when a TRK gene in a cancer cell fuses with another gene. Although rare in many common carcinomas, they occur in more than 90% of some rare cancers. Clinical trials involving patients with 17 various kinds of advanced disease have shown a response in about three-quarters of them, with nearly 80% remaining progression-free 1 year after initiating treatment. Most participants are still responding to therapy, with the longest duration so far at 25 months and counting. Unlike chemotherapy and multi-targeted therapy, fatigue and mild dizziness are the most common adverse effects of larotrectinib. Lead study author David Hyman, MD, of New York's Sloan Kettering Cancer Center attributes this to the fact that larotrectinib is designed to target only TRK. "If approved, " he says, "larotrectinib could become the first therapy of any kind to be developed and approved simultaneously in adults and children, and the first targeted therapy to be indicated for a molecular definition of cancer than spans all traditionally-defined types of tumors."