Motexafin lutetium is a photosensitizer that accumulates in atherosclerotic plaque and, after activation by far-red light, produces cytotoxic singlet oxygen. The combination of photosensitizer and illumination, known as photodynamic therapy (PDT), has been shown to reduce atheroma formation in animal models and is under clinical investigation. However, the effects of PDT with motexafin lutetium on isolated vascular cells are unknown. This study was designed to characterize the effects of PDT on vascular cell viability and to define the cell-death pathway for this agent. Fluorescence microscopy of RAW macrophages and human vascular smooth muscle cells revealed time-dependent uptake of motexafin lutetium. Illumination of motexafin lutetium-loaded cells with 732-nm light (2 J/cm(2)) impaired cellular viability and growth (IC(50) 5 to 20 micromol/L). Depletion of intracellular glutathione potentiated (P=0.035) and the addition of antioxidant N-acetylcysteine attenuated (P=0.002) cell death, suggesting that the intracellular redox state influences motexafin lutetium action. PDT was associated with the loss of mitochondrial membrane potential, mitochondrial release of cytochrome c, and caspase activation. PDT promoted phosphatidylserine externalization and induced apoptotic DNA fragmentation, with the number of apoptotic cells increasing from 7+/-2% to 34+/-3% of total cells. Reducing plaque cellularity by the induction of apoptosis may be one mechanism by which PDT reduces plaque burden, possibly modulates plaque vulnerability, and inhibits restenosis in vivo.