Pharmacokinetics of rapamycin: single-dose studies in the rabbit

The pharmacokinetics of rapamycin was investigated in five New Zealand white rabbits following intravenous administration of 0.05 and 0.5 mg/kg rapamycin in a randomized crossover fashion. Whole blood concentrations of rapamycin were analyzed by high-performance liquid chromatography (HPLC). Model-dependent and -independent parameters were calculated. The volume of distribution at steady state and total body clearance increased significantly as the dose increased. Rapamycin pharmacokinetics appear to be nonlinear. The whole blood volume of distribution, especially at the higher dose, indicated distribution out of the blood component. The drug is not cleared rapidly, with a terminal half-life of > 13 hours as calculated by model-independent parameters. The 24-h whole blood trough concentrations of the drug are well within the analytical range of the HPLC procedure. This should permit trough level monitoring for therapeutic range studies involving the drug.