Liver dihydropyrimidine dehydrogenase activity in human, cynomolgus monkey, rhesus monkey, dog, rat and mouse.

Interspecies differences in dihydropyrimidine dehydrogenase (DPD), the initial and rate-limiting enzyme in pyrimidine degradation, were assessed in cytosol from livers isolated from human, monkey, dog, rat, and mouse. Hepatic DPD activity was measured by an HPLC assay with on-line radioactivity detection, using 14C-5-fluorouracil as a substrate. Activity was highly variable within each species and significant interspecies differences in liver DPD activity were observed. The order of activity was mouse > rat > human > dog > or = cynomolgus monkey > rhesus monkey. These data suggest that careful selection must be made when choosing in vivo models of human DPD for the preclinical development of novel fluoropyrimidine anticancer agents and DPD inhibitors.

Comparisons of phase I and phase II in vitro hepatic enzyme activities of human, dog, rhesus monkey, and cynomolgus monkey.

The metabolism of probe substrates of phase I and phase II enzymes in vitro were compared in hepatic subcellular fractions from humans, cynomolgus monkeys, rhesus monkeys, and beagle dogs. These studies were undertaken to compare the suitability of these species as models of metabolism in drug development. Eight cytochrome P450-dependent activities were measured in microsomal incubations: ethoxyresorufin O-deethylase, coumarin 7-hydroxylase, tolbutamide 4-hydroxylase, S-mephenytoin 4'-hydroxylase, bufuralol 1'-hydroxylase, N-nitrosodimethylamine N-demethylase, midazolam 1'-hydroxylase, and erythromycin N-demethylase. Seven phase II activities were determined in the appropriate subcellular fractions:acetaminophen UDP-glucurono-syltransferase, acetaminophen sulfotransferase, 17 alpha-ethinylestradiol UDP-glucuronosyltransferase, 17 alpha-ethinylestradiol sulfotransferase, 6-mercaptopurine methylase, dichloronitrobenzene (DCNB) glutathione S-transferase, and isoniazid N-acetylase. Hepatic subcellular fractions from cynomolgus and rhesus monkeys showed significantly higher activities than those from humans for ethoxyresorufin O-deethylase, bufuralol 1'-hydroxylase, midazolam 1'-hydroxylase, erythromycin N-demethylase, acetaminophen UDP-glucuronosyltransferase, acetaminophen sulfotransferase, and tolbutamide 4-hydroxylase. Cynomolgus monkey had higher activity than humans and rhesus monkeys for S-mephenytoin 4'-hydroxylase erythromycin N-demethylase. Rhesus monkey and human cytosol displayed an apparent genetic polymorphism in the N-acetylation of isoniazid, whereas cynomolgus monkey cytosol did not. All other monkey activities were not significantly different than human. Dog subcellular fractions showed higher activity than humans for midazolam 1'-hydroxylase, erythromycin N-demethylase, acetaminophen UDP-glucuronosyltransferase, acetaminophen sulfotransferase, 17 alpha-ethinylestradiol sulfotransferase, and DCNB glutathione S-transferase. Furthermore, dog samples had significantly lower activity for coumarin 7-hydroxylase and 6-mercaptopurine methylase, and no detectable activity for tolbutamide 4-hydroxylase or isoniazid N-acetylase. All other activities were not significantly different from human. These results reveal minor differences between the cynomolgus and rhesus monkey in drug metabolism capacities in vitro, but both species are generally more metabolically active than humans in both phase I and phase II metabolism, whereas dogs had more diverse deviations from humans.