Immunosuppressive drug interactions and resistance in mononuclear cells from renal transplant patients
Existing anti-rejection drug regimes are inadequate since patients receive drugs despite serious side effects and poor response. New drugs are being developed which ultimately may allow for prescribing of rational, patient-specific immunosuppressive drug protocols. During this thesis the investigation of lymphocyte responses from renal transplant recipients to the immunosuppressant drugs Cyclosporin A (Cy A), FK506 and SDZ RAD were explored to understand the variation in sensitivity of lymphocytes to Cy A and FK506, the development of drug resistance, including resistance mechanisms, and the interactions between FK506 and SDZ RAD. Cy A and FK506 are substrates for P-glycoprotein (P-gp), the product of the multidrug-resistance (MDR1) gene in man. A hypothesis established during this thesis was that P-gp dependent mechanisms explain variations in lymphocyte sensitivities to Cy A and FK50. Lymphocytes from renal transplant recipients were assessed for their sensitivity to Cy A and FK506 and subsequently for P-gp expression and functional activity by flow cytometry. In further lymphocyte cultures the effect of the specific P-gp inhibitor, PSC 833 on sensitivity was investigated. Finally, the effects of the combination of FK506 and SDZ RAD in lymphocyte cultures were analysed. Results demonstrate a wide range in lymphocyte sensitivity to both Cy A and FK506, with the development of selective resistance to the drug used for treatment. All patients demonstrated P-gp functional activity but P-gp expression was not demonstrable. P-gp function did not account for the variation in lymphocyte sensitivity. There was no evidence of antagonism of effect of SDZ RAD in combination with FK506. In conclusion, these results suggest that non-P-gp mechanisms account for variations in lymphocyte sensitivity to Cy A and FK506. Combination therapy with SDZ RAD and FK506 is unlikely to be antagonistic in future treatment protocols.