Metabolic Aromatization ofN-Alkyl-1,2,3,4-Tetrahydroquinoline Substructures to Quinolinium by Human Liver Microsomes and Horseradish Peroxidase

Abstract

ABSTRACT:Metabolic aromatization of xenobiotics is an unusual reaction with some documented examples. For instance, the oxidation of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine to the neurotoxic pyridinium ion metabolite 1-methyl-4-phenylpyridinium by monoamine oxidase (MAO) B in the brain has been of interest to a number of investigators. It has also been reported that although the aromatization of N-methyl-tetrahydroisoquinoline occurs with MAO B, the metabolism does not proceed for its isomer, N-methyltetrahydroquinoline, by the same enzyme. The aromatization of an N-alkyl-tetrahydroquinoline substructure was identified during in vitro metabolite profiling of compound A, which was designed as a potent renin inhibitor for the treatment of hypertension. The Nalkylquinolinium metabolite of compound A was identified by liquid chromatography-tandem mass spectrometry of human liver microsomal incubates and proton NMR of the isolated metabolite.Further in vitro metabolism studies with a commercially available chemical (compound B), containing the same substructure, also generated an N-alkylquinolinium metabolite. In vitro cytochrome P450 (P450) reaction phenotyping of compound A revealed that the metabolism was catalyzed exclusively by CYP3A4. Although compound B was a substrate for several P450 isoforms, its quinolinium metabolite was also generated predominantly by CYP3A4. Neither compound A nor compound B was a substrate of MAOs. The quinolinium metabolites were readily produced by horseradish peroxidase, suggesting that aromatization of the N-alkyltetrahydroquinoline could occur via a mechanism involving single electron transfer from nitrogen. Although dihydro intermediates from the tetrahydroquinoline substrates were not observed in the formation of quinolinium metabolites, cyanide trapping results indicated the occurrence of iminium intermediates.Compound A (Scheme 1) was a lead compound, consisting of a novel nonpeptidic ketopiperazine-based scaffold, that was designed as a potent renin inhibitor for the treatment of hypertension (Holsworth et al., 2005(Holsworth et al., , 2006. The renin-angiotensin system (RAS) plays a key role in blood pressure regulation. Pharmacological blockade of the RAS cascade is usually achieved with angiotensin-converting enzyme inhibitors or angiotensin receptor blockers. Renin inhibition has been sought for decades, as it would inhibit the initial and rate-limiting step of the RAS and hence might offer a therapeutic profile distinct from angiotensin-converting enzyme inhibitors and angiotensin receptor blockers. Recent progress shows new potential for the treatment of hypertension by renin inhibition (Fisher and Hollenberg, 2005;Wood et al., 2005).During in vitro metabolite profile studies on compound A, in support of lead optimization, an unusual metabolite was observed. It appeared to be derived from aromatization of the N-alkyltetrahydroquinoline ring, yielding an N-alkylquinolinium ion as a major metabolite in human liver microsomes. Similar metabolic oxidation of 1-methyl-4...

Metabolism of a G Protein-Coupled Receptor Modulator, Including Two Major 1,2,4-Oxadiazole Ring-Opened Metabolites and a Rearranged Cysteine-Piperazine Adduct

“…This difference could possibly be attributable to steric hindrance of a bulky substituent group at a piperazine carbon ring in MB243 (Doss et al, 2005;Doss and Baillie, 2006), which might have prevented the dehydrogenation step from occurring. The absence of a dihydro intermediate from metabolic aromatization such as the formation of M13 and M14 is not new in the literature, because two articles previously described the absence of dihydro intermediates from the formation of N-alkylquinolinium metabolites (Shaffer et al, 2001;Gu et al, 2006). In summary, it seems that the metabolic pathway leading to a rearranged Cys-piperazine adduct originally discovered by Doss et al (2005) is a common GSH detoxification mechanism for piperazine bioactivation products.…”

Further Optimization and Evaluation of Bioavailable, Mixed-Efficacy μ-Opioid Receptor (MOR) Agonists/δ-Opioid Receptor (DOR) Antagonists: Balancing MOR and DOR Affinities

“…20–24 Furthermore, the amine in the THQ core is also part of an aniline system, which is susceptible to aromatization, not only in vivo , 25 but potentially in ambient atmosphere with trace acid present. 26 As described below, we observed that N -acetylation of our parent compound 14i to form 14e improved DOR affinity, without altering MOR affinity, resulting in a better balance of MOR and DOR binding.…”

Dehydrogenation of Indoline by Cytochrome P450 Enzymes: A Novel “Aromatase” Process

“…There are no studies to date that have described the aromatization of indolines by P450s, but the aromatization of 9,10-dihydrobenzo[e]pyrene, 1,2-dihydronaphthalene, 1,2-dihydroanthracene, 1,4-dihydropyridines, as well as N-alkyl-1,2,3,4-tetrahydroquinoline by rat liver microsomes and/or P450s was previously observed (Wood et al, 1979;Guengerich and Bocker, 1988;Boyd et al, 1993;Gu et al, 2006). A dehydrogenation mechanism was proposed for the formation of naphthalene and anthracene from their dihydro compounds, rather than a mechanism of hydroxylation followed by dehydration (Boyd et al, 1993).…”