Effect of ultraviolet irradiation on free radical scavenging activity of immunosuppressants used in lung transplantation and comparative electron paramagnetic resonance study of kinetics of their interactions with model free radicals

Summary What is known and objective The immunosuppressive drugs used in solid organ transplantation or autoimmunological processes were studied by electron paramagnetic resonance (EPR) spectroscopy to estimate their free radical scavenging activity. The interactions of immunosuppressants with free radicals were examined by an X‐band (9.3 GHz) EPR spectroscopy and a model of DPPH free radicals. Methods The EPR spectra of DPPH and DPPH interacting with individual drugs were compared. Kinetic studies were performed, and the effect of ultraviolet (UV) irradiation on the free radical scavenging activity of the tested drugs was determined. Results and discussion The free radical scavenging activity of non‐irradiated drugs decreased in the order: rapamycin > mycophenolate mofetil > ciclosporin > tacrolimus. UV irradiation increased the free radical scavenging activity of all the tested immunosuppressive drugs, and the effect was highest for tacrolimus. For the non‐irradiated samples, the speed of free radical interactions decreased in the order: ciclosporin > tacrolimus > mycophenolate mofetil > rapamycin. UV irradiation only slightly affected the speed of interactions of the immunosuppressive drugs with the model DPPH free radicals. What is new and conclusion Electron paramagnetic resonance spectroscopy is useful for obtaining information on interactions of immunosuppressive drugs with free radicals. We hypothesized that the long‐term immunosuppressive effects of these drugs after transplantation or during autoimmune disorders may be mediated by anti‐inflammatory action in addition to the known receptor/cell cycle inhibition. EPR spectroscopy is a useful method to characterise the interactions of immunosuppressive drugs with free radicals. The kinetic studies indicated that the immunosuppressive drugs differed in the speed of interaction with DPPH free radicals. Cycl, ciclosporin; Myc, mycophenolate mofetil; Rap, rapamycin; Tac‐ tacrolimus.