A synthetic photosensitizer agent containing a large macrocyclic ring chelated with silicon. Silicon phthalocyanine 4 localizes primarily in mitochondrial cytosolic membranes and, after photoexcitation, forms reactive oxygen species that induce apoptosis. Check foractive clinical trials using this agent. or A substance being studied in the treatment of cancer. When absorbed by cancer cells and exposed to light, it becomes active and kills the cancer cells. It is a type of photodynamic therapy agent.