Utility of hepatocytes to model species differences in the metabolism of loxtidine and to predict pharmacokinetic parameters in rat, dog and man

1. The metabolism of loxtidine (1-methyl-5-[3-[3-[(1-piperidinyl) methyl] p henoxy] propyl] amino-1H-1,2,4-triazole-3-methanol) was studied in freshly isolated rat, dog and human hepatocytes. Metabolism in vitro was comparable with previously available in vivo data in all three species with the marke d species differences observed in vivo being reproduced in the hepatocyte m odel. 2. The major route for the metabolism of loxtidine by rat hepatocytes was N -dealkylation to form the propionic acid and hydroxymethyl triazole metabol ites. A minor metabolic route was the oxidation of loxtidine to a carboxyli c acid metabolite. The major route of metabolism for loxtidine in dog hepat ocytes was glucuronidation with oxidation to the carboxylic acid metabolite being of minor importance. Incubation of loxtidine with human hepatocytes resulted in the drug remaining largely unchanged but with the carboxylic ac id metabolite being produced in minor amounts. 3. In vitro studies were undertaken with rat, dog and human hepatocytes to determine the Michaelis-Menten parameters V-max and K-m for the sum of all the metabolic pathways. These kinetic parameters were used to calculate the intrinsic clearance of loxtidine. Using appropriate scaling factors, the p redicted in vivo hepatic clearance was then calculated. The predicted intri nsic clearances were 51.4+/-12.4, 8.0+/-0.8 and 1.0+/-0.6 ml/min/kg for rat , dog and human hepatocytes respectively. These data were then used to calc ulate hepatic clearances of 24.5, 3.1 and 0.2 ml/min/kg for rat, dog and ma n respectively. 4. In vivo hepatic and intrinsic clearances for loxtidine were determined i n rat, dog and human volunteers. The hepatic clearances of loxtidine were 2 6.6, 6.6 and 0.4 ml/min/kg in rat, dog and man respectively and intrinsic c learances were 58.5, 18.5 and 2.0 ml/min/kg in rat, dog and man respectivel y. 5. The present studies demonstrate that the hepatocyte model map be a valua ble in vitro tool for predicting both qualitative and quantitative aspects of the metabolism of a drug in animals and man at an early stage of the dru g development process.