METABOLISM OF INTRAVENOUS METHYLNALTREXONE IN MICE, RATS, DOGS AND HUMANS

were observed in rats. Dogs produced only one metabolite, MNTX-3-glucuronide (M9). In conclusion, MNTX was not extensively metabolized in humans. Conversion to methyl-6-naltrexol isomers (M4 and M5) and MNTX-3-sulfate (M2) were the primary pathways of metabolism in humans. MNTX was metabolized to a higher extent in mice than in rats, dogs, and humans. Glucuronidation was a major metabolic pathway in mice, rats and dogs, but not in humans. Overall, the data suggested species differences in the metabolism of MNTX

Metabolism and Toxicity of 2-Aminofluorene in a Genetic Mouse Model.

Previous work in our laboratory has shown that liver cytosolic N-acetyltransferase (NAT) from C57BL/6J (B6) mice N-acetylated 2-aminofluorene (AF) 5-10 times faster than NAT from A/J (A) mice. This thesis describes experiments which characterize further the mouse as a genetic model to study the importance of N-acetylation and deacetylation in AF metabolism and toxicity. With respect to deacetylation, a strain survey of nineteen inbred mouse strains showed that differences in AAF and N-OH-AFF deacetylase activities were under genetic control. AAF deacetylation was 2 times faster in B6 than in A mice and was analyzed further to determine the hereditary basis for the difference and kinetic properties of the enzyme(s) responsible. In analysis of AF N-acetylation, a correlation was established between differences in AF NAT activity in B6 and A mice and differences in AF elimination rates in the whole animal. Studies in intact B6 and A mice injected with AF showed that B6 mice eliminated AF from blood 3-4 times faster than A mice. B6AF(,1)/J mice had intermediate elimination rates consistent with intermediate inheritance of the NAT gene as did the congenic mouse, A.B6-NAT('r), suggesting a modifying influence of the background genes on AF elimination by the rapid NAT gene product. Differences in AF NAT activity between B6 and A mice also correlated with differences in rates of AF disappearance and AAF appearance in primary hepatocyte cultures. In the presence of paraoxon, an AAF deacetylase inhibitor, AF disappeared and AAF appeared 2 times faster in B6 hepatocytes than A hepatocytes. Preliminary studies indicated that differences in N-acetylating capacity between B6 and A mice may influence susceptibility to AF-induced DNA damage since hepatocytes from the rapid acetylator B6 mouse showed more unscheduled DNA synthesis than hepatocytes from the slow acetylator A mouse. Studies with reversible inhibitors of NAT were also carried out. Liver cytosolic NAT activity was inhibited competitively by para-aminobenzoic acid (PABA) and non-competitively by folic acid and methotrexate. PABA inhibited AAF appearance and AF disappearance completely in B6 hepatocytes but inhibited AF disappearance only partially in A hepatocytes suggesting the presence of an alternate route of AF metabolism in A hepatocytes.Ph.D.PharmacologyUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/159753/1/8402297.pd

In Vitro Metabolism, Permeability, and Efflux of Bazedoxifene in Humans

ABSTRACT: Bazedoxifene (BZA) acetate, a novel estrogen receptor modulator being developed for the prevention and treatment of postmenopausal osteoporosis, undergoes extensive metabolism in women after oral administration. In this study, the in vitro metabolism of [ 14 C]BZA was determined in human hepatocytes and hepatic and intestinal microsomes, and the UDP glucuronosyltransferase (UGT) isozymes involved in the glucuronidation of BZA were identified. In addition, BZA was evaluated for its potential as a substrate of P-glycoprotein (P-gp) transporter in Caco-2 cell monolayers. BZA was metabolized to two monoglucuronides, BZA-4-glucuronide and BZA-5-glucuronide, in hepatocytes and in liver and intestinal microsomes including jejunum, duodenum, and ileum. Both BZA-4-glucuronide and BZA-5-glucuronide were major metabolites in the intestinal microsomes, whereas BZA-4-glucuronide was the predominant metabolite in liver microsomes and hepatocytes. The kinetic parameters of BZA-4-glucuronide formation were determined in liver, duodenum, and jejunum microsomes and with UGT1A1, 1A8, and 1A10, the most active UGT isoforms involved in the glucuronidation of BZA, whereas those of BZA-5-glucuronide were determined with all the enzyme systems except in liver microsomes and in UGT1A1 because the formation of the BZA-5-glucuronide was too low. K m values in liver, duodenum, and jejunum microsomes and UGT1A1, 1A8, and 1A10, were similar and ranged from 5.1 to 33.1 M for BZA-4-glucuronide formation and from 2.5 to 11.1 M for BZA-5-glucuronide formation. V max values ranged from 0.8 to 2.9 nmol/(min ⅐ mg) protein for BZA-4-glucuronide and from 0.1 to 1.2 nmol/(min ⅐ mg) protein for BZA-5-glucuronide. In Caco-2 cells, BZA appeared to be a P-gp substrate