Cyclosporin A‐induced free radical generation is not mediated by cytochrome P‐450

Reactive oxygen species (ROS) have been proposed to play a role in the side effects of the immunosuppressive drug cyclosporin A (CsA). The aim of this study was to investigate whether cytochrome P‐450 (CYP) dependent metabolism of CsA could be responsible for ROS generation since it has been suggested that CsA may influence the CYP system to produce ROS. We show that CsA (1 – 10 μM) generated antioxidant‐inhibitable ROS in rat aortic smooth muscle cells (RASMC) using the fluorescent probe 2,7‐dichlorofluorescin diacetate. Using cytochrome c as substrate, we show that CsA (10 μM) did not inhibit NADPH cytochrome P‐450 reductase in microsomes prepared from rat liver, kidney or RASMC. CsA (10 μM) did not uncouple the electron flow from NADPH via NADPH cytochrome P‐450 reductase to the CYP enzymes because CsA did not inhibit the metabolism of substrates selective for several CYP enzymes that do not metabolize CsA in rat liver microsomes. CsA (10 μM) did not generate more radicals in CYP 3A4 expressing immortalized human liver epithelial cells (T5‐3A4 cells) than in control cells that do not express CYP 3A4. Neither diphenylene iodonium nor the CYP 3A inhibitor ketoconazole were able to block ROS formation in rat aortic smooth muscle or T5‐3A4 cells. These results demonstrate that CYP enzymes do not contribute to CsA‐induced ROS formation and that CsA neither inhibits NADPH cytochrome P‐450 reductase nor the electron transfer to the CYP enzymes. British Journal of Pharmacology (2002) 135, 977–986; doi:10.1038/sj.bjp.0704544