Formation of unusual glutamate conjugates of 1-[3-(aminomethyl)phenyl]-N-[3-fluoro-2'-(methylsulfonyl)-[1,1'-biphenyl]-4-yl]-3-(trifluoromethyl)-1H-pyrazole-5-carboxamide (DPC 423) and its analogs: the role of gamma-glutamyltranspeptidase in the biotransformation of benzylamines

The role of gamma-glutamyltranspeptidase (GGT) in transferring glutamate from endogenous glutathione (GSH) to the benzylamine moiety of a compound, such as 1-[3-(aminomethyl)phenyl]-N-[3-fluoro-2'-(methylsulfonyl)-[1,1'-biphenyl]-4-yl]-3-(trifluoromethyl)-1H-pyrazole-5-carboxamide (DPC 423), is described. Studies were performed with structurally related analogs of DPC 423 to demonstrate that this type of reaction was common to compounds possessing a benzylamine group. Synthesizing appropriate standards and confirming by liquid chromatography (LC)/mass spectroscopy and LC/NMR made unambiguous assignments of the structures of glutamate conjugates of DPC 423. The use of stable isotope-labeled GSH for metabolism studies has not been described before. In the present study, we report the novel use of deuterated GSH in conjunction with mass spectral analysis to demonstrate the glutamate transfer to the benzylamines in the presence of GGT. To further demonstrate that the alpha protons on the benzylamines and glutamate (as part of glutathione) were unaffected during the transpeptidation, these protons were replaced with deuterium. Acivicin (AT-125), a potent and selective inhibitor of GGT, was used to abolish the formation of the glutamate conjugates of DPC 423 in vitro and in vivo. This provided further evidence of the role of GGT in forming the glutamate conjugates of benzylamines. This study demonstrated conclusively that GGT was responsible for mediating the transfer of glutamic acid from GSH to the benzylamine moiety of a series of structurally related compounds.

Disposition of glutathione conjugates in rats by a novel glutamic acid pathway: characterization of unique peptide conjugates by liquid chromatography/mass spectrometry and liquid chromatography/NMR

With the advent of liquid chromatography/mass spectrometry and liquid chromatography/NMR, it has become easier to characterize metabolites that were once difficult to isolate and identify. These techniques have enabled us to uncover the existence of an alternate pathway for the disposition of glutathione adducts of several structurally diverse compounds. Studies were carried out using acetaminophen as a model compound to investigate the role of the glutamic acid pathway in disposition of the glutathione adducts. Although the mercapturic acid pathway was the major route of degradation of the glutathione adducts, it was found that the conjugation of the glutathione, cysteinylglycine, and cysteine adducts of acetaminophen with the gamma-carboxylic acid of the glutamic acid was both interesting and novel. The coupling of the glutathione adduct and the products from the mercapturic acid pathway with the glutamic acid led to unusual peptide conjugates. The natures of these adducts were confirmed unequivocally by comparisons with synthetic standards. This pathway (addition of glutamic acids) led to larger peptides, in contrast to the mercapturic acid pathway, in which the glutathione adducts are broken down to smaller molecules. The enzyme responsible for the addition of glutamic acid to the different elements of the mercapturic acid pathway is currently unknown. It is postulated that the gamma-carboxylic acid is activated (perhaps by ATP) before enzymatic addition to the alpha-amino group of cysteine or glutamate takes place. The discovery of these peptide conjugates of acetaminophen represents a novel disposition of glutathione adducts of compounds. The formation of such conjugates may represent yet another pathway by which drugs could produce covalent binding via their reactive intermediates.

Characterization of novel glutathione adducts of a non-nucleoside reverse transcriptase inhibitor, (S)-6-chloro-4-(cyclopropylethynyl)-4-(trifluoromethyl)-3, 4-dihydro-2(1H)-quinazolinone (DPC 961), in rats. Possible formation of an oxirene metabolic intermediate from a disubstituted alkyne

The postulated formation of oxirene-derived metabolites from rats treated with a disubstituted alkyne, (S)-6-chloro-4-(cyclopropylethynyl)-4-(trifluoromethyl)-3, 4-dihydro-2(1H)-quinazolinone (DPC 961), is described. The reactivity of this postulated oxirene intermediate led to the formation of novel glutathione adducts whose structures were confirmed by LC/MS and by two-dimensional NMR experiments. These metabolites were either excreted in rat bile or degraded to mercapturic acid conjugates and eliminated in urine. To demonstrate the oxidation of the triple bond, an analogue of DPC 961 was synthesized, whereby the two carbons of the alkyne moiety were replaced with (13)C stable isotope labels. Rats were orally administered [(13)C]DPC 961 and glutathione adducts isolated from bile. The presence of an oxygen atom on one of the (13)C labels of the alkyne was demonstrated unequivocally by NMR experiments. Administration of (14)C-labeled DPC 961 showed that biliary elimination was the major route of excretion with the 8-OH glucuronide conjugate (M1) accounting for greater than 90% of the eliminated radioactivity. On the basis of radiochemical profiling, the glutathione-derived metabolites were minor in comparison to the glucuronide conjugate. Studies with cDNA-expressed rat enzymes, polyclonal antibodies, and chemical inhibitors pointed to the involvement of P450 3A1 and P450 1A2 in the formation of the postulated oxirene intermediate. The proposed mechanism shown in Scheme 1 begins with P450-catalyzed formation of an oxirene, rearrangement to a reactive cyclobutenyl ketone, and a 1,4-Michael addition with endogenous glutathione to produce two isomeric adducts, GS-1 and GS-2. The glutathione adducts were subsequently catabolized via the mercapturic acid pathway to cysteinylglycine, cysteine, and N-acetylcysteine adducts. The transient existence of the alpha,beta-unsaturated cyclobutenyl ketone was demonstrated by incubating the glutathione adduct in the presence of N-acetylcysteine and monitoring the formation of N-acetylcysteine adducts by LC/MS. Epimerization of GS-1 to GS-2 was also observed when N-acetylcysteine was omitted from the incubation.