Cancer cells generally carry higher levels of reactive oxygen species, or ROS, than healthy cells. RK-251 is designed so that these elevated ROS levels act as the chemical trigger: in the tumour microenvironment the molecule breaks apart and releases NBDHEX, a compound that blocks proteins cancer cells depend on for survival and for resisting treatment. In healthy tissue, where ROS levels are lower, the drug is meant to stay locked. The design also carries a fluorescent element, so activation can be confirmed by the near-infrared fluorescence it produces, and fluorescence-based tests did show activation in high-ROS environments. In laboratory work the compound acted against aggressive triple-negative breast cancer cells while largely sparing healthy ones.

Conventional chemotherapy does not distinguish between cancerous and healthy dividing cells, which is the source of most of its side effects and of the limits on how much can be given. Triple-negative breast cancer is a particularly hard case because it lacks the oestrogen receptor, the progesterone receptor and HER2, the three targets that hormonal and targeted therapies act on, leaving chemotherapy as the mainstay. A drug that activates only where a tumour-specific chemical signature exists is an attempt to build selectivity into the molecule itself. The results so far are from cell cultures and zebrafish embryos; the compound has not entered human trials, and preclinical activity does not establish either safety or efficacy in people.