SYNERGISTIC MECHANISMS BY WHICH SIROLIMUS AND CYCLOSPORINE INHIBIT RAT-HEART AND KIDNEY ALLOGRAFT-REJECTION

The studies presented herein examined the mechanism(s) whereby sirolim us (SRL) and cyclosporin (CsA) act synergistically to block allograft rejection. Combination index (CI = 1 reflects additive, CI > 1 antagon istic, and CI < 1 synergistic, effects) analysis documented potent syn ergism between SRL and CsA to block allograft rejection. Combinations of the two drugs produced synergistic prolongation of heart (CI = 0.00 1-0.2) or kidney (CI = 0.03-0.5) allograft survival at SRL/CsA ratios ranging from 1:12.5 to 1:200. Pharmacokinetic analysis of the individu al drugs showed that CsA does not affect the blood levels of SRL, and SRL mildly increases the levels of CsA in SRL/CsA-treated rats. Quanti tative polymerase chain reaction analysis was used to document that bo th subtherapeutic (1.0 mg/kg) and therapeutic (2.0 or 4.0 mg/kg) CsA d oses inhibited the expression of interferon-gamma (IFN-gamma) (P < 0.0 3) and IL-2 (P < 0.003) mRNA produced by T helper (Th) 1 cells, as wel l as IL-10 (P < 0.001), but not IL-4 (NS) mRNA produced by Th2 cells. Contrariwise, all tested SRL doses (0.02, 0.04 or 0.08 mg/kg) did not affect cytokine mRNA expression. However, heart allografts from rat re cipients treated with synergistic SRL/CsA doses displayed reduced leve ls of IFN-gamma (P < 0.01), IL-2 (P < 0.001) and IL-10 (P < 0.001) mRN A. Thus, because subtherapeutic doses of CsA reduce Th1/Th2 activity, thereby facilitating the inhibition of signal transduction by low does of SRL, the two agents act synergistically to inhibit allograft rejec tion.