Xenobiotic metabolism in rat, dog, and human precision-cut liver slices, freshly isolated hepatocytes, and vitrified precision-cut liver slices.

Human, rat, and dog phase I and phase II xenobiotic metabolism in precision-cut liver slices and freshly isolated hepatocytes was compared using a range of substrates. Carbamazepine (50 microM) and styrene (2 mM) were used as probes to study the maintenance of cytochrome P450 and epoxide hydrolase-mediated metabolism in male Sprague-Dawley rat, precision-cut liver slices and hepatocytes. Carbamazepine metabolism in both models resulted in the formation of the bioactive 10,11-epoxide (KM = 766 microM and Vmax = 2.5 pmol/min/mg protein in precision-cut slices). Epoxide formation was higher (2.4-fold) in hepatocytes than slices. Styrene was deactivated to styrene diol at a higher rate in hepatocytes (9.7-fold) than slices. The lower rate of metabolism in slices compared with hepatocytes confirms our previous observations using testosterone, 7-ethoxycoumarin, 1-chloro-2,4-dinitrobenzene and 2-(5'-chloro-2'-phosphoryloxyphenyl)-6-chloro-4-(3H)-quinazolinone in the rat. Testosterone 6 beta-hydroxylation in human liver slices was similar to cultured hepatocytes, but lower than in freshly isolated hepatocytes. 7-Ethoxycoumarin O-deethylation was higher in freshly isolated human hepatocytes, as was the ratio of glucuronide to 7-hydroxycoumarin. Testosterone hydroxylations, 7-ethoxycoumarin O-deethylation, and 1-chloro-2,4-dinitrobenzene conjugation were also lower in male beagle dog slices, compared with freshly isolated hepatocytes. Attempts at long-term preservation of dog liver slices using vitrification and storage for up to 9 days at -196 degrees C resulted in the retention of phase I and phase II metabolism, although conjugation was lower than in freshly prepared slices. Xenobiotic metabolism in short-term incubations is consistently lower in dog and rat precision-cut slices than in freshly isolated hepatocytes; whereas, in humans, this quantitative difference is partly hidden by the large interindividual variation.

Quantitative differences in phase I and II metabolism between rat precision-cut liver slices and isolated hepatocytes.

Testosterone (250 microM), 7-ethoxycoumarin (25 microM), and 1-chloro-2,4-dinitrobenzene (CDNB, 50 microM) were used as substrates to compare phase I and II metabolism in rat precision-cut liver slices and rat hepatocytes. Overall clearance to metabolites was significantly greater in hepatocytes for testosterone (1.9- to 16.9-fold), 7-ethoxycoumarin (O-deethylation, 14.8-fold; glucuronidation, 3.1-fold), and CDNB (8.7-fold). The same metabolites for each of the substrates were detected in slices and hepatocytes. However, the ratio of sulfate to glucuronide conjugation was higher in slices, in line with the markedly slower rate of formation of 7-hydroxycoumarin. The slower rate of CDNB conjugation could not be explained by differences in the intracellular concentration of glutathione. These data suggest that metabolism in precision-cut slices is limited by diffusion of the substrate. This was illustrated by the use of 2-(5'-chloro-2'-phosphoryloxyphenyl)-6-chloro-4-(3H)-quinazolinone as a fluorescent probe to investigate diffusion in slices and hepatocytes.

Comparative biodisposition and metabolism of 14C-(+/-)-fenfluramine in mouse, rat, dog and man.

1. The comparative metabolism of fenfluramine was investigated in mouse, rat, dog and man following a single oral dose of 14C-(+/-)-fenfluramine hydrochloride (1 mg/kg), and also in rat after eight consecutive 12-h subcutaneous doses (24 mg/kg). 2. Main route of excretion of radioactivity in all species and at all doses was into urine (> 80%), with only minor amounts of radioactivity found in faeces. 3. From all species examined a total of 11 metabolites were observed in urine and plasma by t.l.c. and h.p.l.c. analysis and no metabolite was present in the plasma which was not present in urine. 4. All species dealkylate fenfluramine to the active metabolite norfenfluramine, to a relative greater or lesser extent, with plasma metabolic ratios (norfenfluramine/fenfluramine) showing inter-animal variation (rat >> dog >> mouse = man). 5. These differences are due to the efficient deamination of both compounds to polar inactive metabolites in man, with less dealkylation and lower plasma levels of norfenfluramine compared with the other species studied. 6. In conclusion, major species differences in the metabolism of (+/-)-fenfluramine, both qualitative and quantitative were observed, and no one species had a similar metabolic profile to that found in man.