PURPOSE: Camptothecin (CPT) is a potent topoisomerase I inhibitor that has recently been undergoing phase I clinical trials. Though CPT shows high activity against various tumor cells, its biotransformation is still unknown. To investigate the metabolism and biliary excretion of CPT, an isolated perfused rat liver system was used. METHODS: CPT was added to the perfusion medium at a concentration of 20 microM, and bile and perfusate samples were collected for 90 min. CPT (lacton and carboxylate) and three novel metabolites were identified by mass spectroscopy and quantified by reversed-phase high-performance liquid chromatography (HPLC). Kinetic parameters of CPT and its biotransformation products were then estimated in bile and effluent perfusate. RESULTS: Biliary secretion of CPT and its three metabolites reached a peak secretion of 37.6 +/- 16.3, 0.94 +/- 0.29, 0.19 +/- 0.023 and 0.302 +/- 0.076 nmol/g liver/min, respectively, after 20 min. The total amount of CPT and M1-M3 excreted into bile during 90 min of perfusion was 63.5 +/- 15.4%, 1.8 +/- 0.37%, 0.43 +/- 0.06%, and 0.72 +/- 0.15% of CPT cleared from the perfusate during 90 min, respectively. In the perfusate, only one metabolite (M3) could be detected (cumulative release into the perfusion medium: 0.37 +/- 0.026 micromol/liver). Analysis of the biliary metabolites by mass spectroscopy supported the existence of dihydroxy-CPT derivatives (M1 and M2), whereas M3 appears to be a monohydroxy-analog. CONCLUSION: CPT is biotransformed to three novel metabolites, mainly excreted into bile. The possible pharmacological effects of these new metabolites need to be considered.