CAR T cells are a breakthrough class of effective but often toxic cancer therapies To prevent overactivation, switchable CAR T cells were engineered that can be turned on and off with an approved, widely used cancer drug BOSTON - Scientists at Dana-Farber Cancer Institute and Mass General Cancer Center have created molecular ON-OFF switches to regulate the activity of CAR T cells, a potent form of cell-based immunotherapy that has had dramatic success in treating some advanced cancers, but which pose a significant risk of toxic side effects. CAR T cells are immune cells genetically modified to recognize and attack tumors cells. Once given, these "living drugs" proliferate and kill tumor cells over weeks to months, in some cases causing life-threatening inflammatory reactions that are difficult to control. In this way, CAR T cells are unlike more established forms of cancer therapy - chemotherapy or radiotherapy for instance - whose dose can be precisely tuned up or down over time.