"Even though they have elevated levels of procaspase-3, cancer cells never turn the enzyme on. So they keep growing and become tumors," he said. "PAC-1 restores the activity of procaspase-3 and, because the enzyme is elevated in cancer cells, it targets cancer cells over non-cancerous cells." 
A new drug that prompts cancer cells to self-destruct while sparing healthy cells is now entering phase I clinical trials in humans. The drug, called PAC-1, first showed promise in the treatment of pet dogs with spontaneously occurring cancers, and is still in clinical trials in dogs with osteosarcoma.
The compound was discovered and is being developed based on the hypothesis that most cancers have elevated levels of an enzyme called procaspase-3," said University of Illinois chemistry professor Paul Hergenrother, who discovered the anti-cancer effects of PAC-1 more than a decade ago. "Procaspase-3 is an enzyme that, when turned on, kills cells."
Cancer cells, however, override this normal cell-recycling pathway, he said.
Early tests of the drug's effectiveness came when Hergenrother collaborated with U. of I. veterinary clinical sciences professor Timothy Fan, who tested PAC-1 in his canine cancer patients. These clinical trials helped the researchers find the best way to deliver the drug - it is now in pill form for both human and canine patients - and led to new insights into the drug's activity and potential, Fan said.
