PRECLINICAL CHARACTERIZATION OF 2-[3-[3-[(5-ETHYL-4′-FLUORO-2-HYDROXY[1,1′-BIPHENYL]-4-YL)OXY]PROPOXY]-2-PROPYLPHENOXY]BENZOIC ACID METABOLISM: IN VITRO SPECIES COMPARISON AND IN VIVO DISPOSITION IN RATS

Assessment of the pharmacokinetics of [14C]2-[3-[3-[(5-ethyl-4'-fluoro-2-hydroxy[1,1'-biphenyl]-4-yl)oxy]propoxy]-2-propylphenoxy-]benzoic acid ([14C]LY293111), an experimental anti-cancer agent, suggested long-lived circulating metabolites in rats. In vivo metabolites of LY293111 were examined in plasma, bile, urine, and feces of Fischer 344 (F344) rats after oral administration of [14C]LY293111. Metabolites were profiled by high-performance liquid chromatography-radiochromatography, and identified by liquid chromatography (LC)/mass spectrometry and LC/NMR. The major in vivo metabolites of LY293111 identified in rats were phenolic (ether), acyl, and bisglucuronides of LY293111. Measurement of radioactivity in rat plasma confirmed that a fraction of LY293111-derived material was irreversibly bound to plasma protein and that this bound fraction increased over time. This was consistent with the observed disparity in half-lives between LY293111 and total radioactivity in rats and monkeys, and is likely due to covalent modification of proteins by the acyl glucuronide. In vitro metabolism of [14C]LY293111 in liver slices from CD-1 mice, F344 rats, rhesus and cynomolgus monkeys, and humans indicates that glucuronidation was the primary metabolic pathway in all species. The acyl glucuronide was the most prevalent radioactive peak (16% of total 14C) produced by F344 rat slices, whereas the ether glucuronide was the major metabolite in all other species (26-36% of total 14C). Several minor hydroxylated metabolites were detected in F344 rat slice extracts but were not observed in other species. The data presented suggest that covalent modification of proteins by LY293111 acyl glucuronide is possible in multiple species, although the relative reactivity of this metabolite appears to be low compared with those known to cause adverse drug reactions.

Characterization of 2-[[4-[[2-(1H-tetrazol-5-ylmethyl)phenyl]methoxy]methyl]quinoline N-glucuronidation by in vitro and in vivo approaches

RG 12525 is a new chemical entity recently evaluated for the treatment of type II diabetes. Clinical studies have previously identified the tetrazole N2-glucuronide conjugate of RG 12525 as the predominant metabolite in plasma following oral administration of RG 12525. Species differences in RG 12525 glucuronidation were first investigated with incubations of RG 12525 with rat, monkey, and human hepatocytes. The results showed the N2-glucuronide to be the major metabolite in human and monkey samples, with only low levels observed for the rat. The formation of this glucuronide by human liver microsomes was subsequently characterized. RG 12525 N2-glucuronidation was found to have a pH optimum of 7.0 to 7.5 and demonstrated a high affinity with a K(m) range of 16.6 to 21.1 microM RG 12525 (n = 3). The rate of N2-glucuronide formation ranged from 2.5 to 15.4 nmol of RG 12525 N2-glucuronide formed/min/mg of protein ( approximately 6-fold) in the 21 samples assayed. The reaction was inhibited by known substrates for glucuronidation, with imipramine (62%), naringenin (44%), and scopoletin (38%) producing the largest degree of inhibition at equimolar concentrations of substrate and inhibitor. Of the eight expressed UDP-glucuronosyltransferase (UGT) forms assayed, UGT1A1 and 1A3 displayed the highest rate of RG 12525 N2-glucuronidation (0.109 and 0.125 nmol/min/mg, respectively). Finally, low levels of N2-glucuronidation of RG 12525 by human jejunum microsomes were demonstrated, suggesting that presystemic clearance via glucuronidation may constitute a barrier to bioavailability.

Quantitation of N-(3,5-dichloropyrid-4-yl)-3-cyclopentyloxy-4-methoxybenzamide and 4-amino-3,5-dichloropyridine in rat and mouse plasma by LC/MS/MS

The metabolism of N-(3,5-dichloropyrid-4-yl)-3-cyclopentyloxy-4-methoxybenzamide (RP73401), a phosphodiesterase IV (PDE IV) inhibitor is extensive (unpublished); however, until recently, studies for this compound did not report 4-amino-3,5-dichloropyridine (ADCP) as a metabolite either in vitro or in vivo. This prompted a reinvestigation into the metabolism of RP73401 in rats and mice using mass spectrometry. The results of the reinvestigation confirmed that 4-amino-3,5-dichloropyridine was formed via the metabolism of RP73401 both in vitro and in vivo. In order to further investigate RP73401 hydrolysis in vivo, a liquid chromatography/mass spectrometry assay was developed and validated for the simultaneous determination of RP73401 and ADCP in rat and mouse plasma. The method used Waters Oasis HLB brand solid phase extraction cartridges to isolate the analytes (RP73401 and ADCP) and internal standard from the plasma. HPLC chromatographic separation was achieved using a Zorbax SB C18 HPLC column and detection was accomplished using positive ion atmospheric pressure chemical ionization tandem mass spectroscopy in multiple reaction monitoring (MRM) mode. The assay was developed and validated over the range of 0.5-100 ng ml(-1) for RP73401 and 5-500 ng ml(-1) for ADCP using 0.050 ml of plasma. The assay proved to be sensitive, accurate, precise and specific for RP73401 and ADCP. Intraday and interday quality control results routinely showed accuracy and precision to be within +/- 20%. This LC/MS/MS method was subsequently employed to investigate the hydrolysis of RP73401 in the rat and mouse, and determine the effects of tri-o-tolyl phosphate (TOTP, a carboxylesterase inhibitor) preadministration on the hydrolysis reaction in the rat.