Kinetics of drug metabolism in rat liver slices. II. Comparison of clearance by liver slices and freshly isolated hepatocytes.

The kinetics of metabolism of diazepam, phenytoin, and caffeine have been determined in rat liver slices of 260 microns thickness. The formation of 4'-hydroxy metabolites of diazepam and phenytoin are described by one- and two-site Michaelis-Menten equations, respectively. The other oxidative pathways for diazepam are less readily saturable than the 4'-hydroxylation. This kinetic behavior is consistent with that previously reported for other in vitro systems. In contrast, the metabolism of caffeine, assessed by total metabolism, showed differences from that observed with other systems, and this is believed to result from the incubation conditions used. By determining the hepatocellularity of the standard slice used, CLint (Vmax/KM) data were expressed per million cells and compared with the same parameters derived from incubations with freshly isolated hepatocyte suspensions. Data on diazepam, phenytoin, and caffeine were combined with previously published data on tolbutamide, ethoxycoumarin, and ondansetron to give a total of nine CLint values representing different pathways. CLint values in slices are consistently less than those in hepatocytes (ratio differing from 0.4 to 0.05). The CLint ratio decreased in a regular fashion as the hepatocyte CLint increased from 1.4 microliters/min (caffeine) to 105 microliters/min (ondansetron). It was also observed that the KM value for a particular pathway in slices always exceeded the corresponding value in isolated hepatocytes (ratios differing from 1.09 to 7.59). The KM ratio was positively related to hepatocyte CLint. These observations are consistent with the delayed accessibility of substrate to all the cells within a slice. It is proposed that the parallel processes of drug transport and metabolism within the slice do not allow a distribution equilibrium to be achieved between all the cells within a slice and the incubation media.