The pharmacokinetics and metabolic disposition of sirolimus (rapamycin, Rapamune), a macrocyclic immunosuppressive agent for the prevention of allograft rejection in organ transplantation, were investigated in 6 healthy male volunteers after a single nominal 40-mg oral dose of the 14 C-radiolabeled drug, with the added aim of assessing the potential role of sirolimus metabolites in the clinical pharmacology of the parent drug. The absorption of parent drug and derived materials was rapid (t max 1.3±0.5 hours, mean±SD), and the elimination of sirolimus was slow (t ½ 60±10 hours, mean±SD) in whole blood. The high whole blood to plasma (B/P) concentration ratio of sirolimus (142±39) was consistent with its extensive partitioning into formed blood elements. The markedly lower B/P value based on radioactivity (2.7±0.4) suggested that drug-derived products partitioned into formed blood elements to a much lesser extent. Based on AUC 0-144h values, unchanged sirolimus represented an average 35% of total radioactivity in whole blood. Drug-derived products in whole blood were characterized by HPLC, LC/MS, and LC/MS/MS as 41-O-demethyl, 7-O-demethyl, and several hydroxy, dihydroxy, hydroxy-demethyl and didemethyl sirolimus metabolites. The percentage distribution of sirolimus metabolites in whole blood ranged from 3%-10% at 1 hour to 6%-17% at 24 hours after drug administration. Based on their low immunosuppressive activities and relative abundance in whole blood of humans after sirolimus administration, metabolites of sirolimus do not appear to play a major role in the clinical pharmacology of the parent drug. A majority of the administered radioactivity (91.0±8.0%) was recovered from feces, and only 2.2%±0.9% was renally excreted.