Effect of cimetidine, probenecid, and ketoconazole on the distribution, biliary secretion, and metabolism of [³H]taxol in the Sprague-Dawley rat

Male Sprague-Dawley rats had their bile ducts cannulated and were dosed with [³H]taxol (2 mg/kg, 68-77 μCi/mg) as a continuous intravenous infusion for 6 hr so that the plasma concentrations, tissue distribution, metabolism, and biliary secretion of taxol could be studied. Defining potential drug- drug interactions of taxol with cimetidine (90 mg/kg), probenecid (360 mg/kg), and ketoconazole (50 mg/kg) was motivated by frequent concomitant clinical use or the potential to reduce clearance of taxol so that lower doses could be used. At 6 hr, rats were killed. Samples of blood (plasma), lung, spleen, liver, kidney, heart, skeletal muscle, brain, testes, and fat were obtained. Taxol and metabolites were measured by total radioactivity counting and HPLC separation using on-line radioactivity detection. Concentrations of taxol in plasma increased to 0.19 μM in the control rats and did not reach steady-state by 6 hr. Lung, spleen, liver, and kidneys had the greatest tissue taxol concentrations [4.7-5.7 nmol/g (μM)] and were >25- fold higher than the simultaneous 6-hr plasma taxol concentration. Taxol concentrations in brain and testes were negligible, 0.06 and 0.07 nmol/g, respectively. Radioactive metabolites were not found in plasma or most tissues. Only liver had appreciable concentrations of taxol and metabolites; however, >80% of hepatic radioactivity was parent taxol. Through 6 hr of collection, 24% of the dose was secreted in the bile ~38% of which was as parent taxol. Cimetidine had no effect on the distribution, metabolism, or elimination of [³H]taxol. Probenecid did not effect tissue distribution or plasma concentrations of taxol. Probenecid decreased the biliary secretion of taxol and metabolites by 50%. Rats pretreated with ketoconazole had biliary secretion of taxol, and metabolites decreased to 25% of control with a concomitant increase in plasma taxol concentration.