title: Elucidating the Mechanisms of Formation for Two Unusual Cytochrome P450-Mediated Fused Ring Metabolites of GDC-0623, a MAPK/ERK Kinase Inhibitor.
abstract: Two isomeric metabolites of GDC-0623 [5-((2-fluoro-4-iodophenyl)amino)-N-(2-hydroxyethoxy)imidazo[1,5-a]pyridine-6-carboxamide], a mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) kinase inhibitor, were identified in radiolabeled mass balance studies in rats and dogs (approximately 5% in excreta) and were also observed in human circulation (nonradiolabeled). Mass spectrometric data indicated that both metabolites had formed a new ring structure fused to the imidazopyridine core. Given their unusual structures, we conducted experiments to elucidate their chemical structures and understand the mechanisms for their formation. For the first metabolite, M14, a pyrazol-3-ol ring was generated by N-N bond formation between the aniline and hydroxamate. For the second metabolite, M13, an imidazol-2-one was generated by a Hofmann-type rearrangement that involved C-C bond cleavage and C-N bond formation. Both reactions were catalyzed by CYP2C9 and CYP2C19. M14 was generated directly from GDC-0623 and we speculate that its formation was via oxidative activation of the hydroxamic ester by cytochrome P450 (P450) and intramolecular nucleophilic displacement of the ester side chain. M13 (the rearranged metabolite) formed from the N-reduced hydroxamate (amide) and not from GDC-0623 directly. We propose for M13 that a P450-mediated reaction formed a cationic amide intermediate, which enabled the molecular rearrangement of the imidazopyridine core migrating from the amide carbon to the nitrogen and subsequent cyclization reaction. Each of these metabolic pathways constitutes a novel biotransformation mediated by P450 enzymes.
authors.affiliation: Departments of Drug Metabolism and Pharmacokinetics (R.H.T., S.M., Q.Y., E.F.C., S.C.K.) and Small Molecule Pharmaceutical Sciences (S.J.R.), Genentech, Inc., South San Francisco, California takahashi.ryan@gene.com.
authors.forename: Ryan H
authors.initials: RH
authors.lastname: Takahashi
authors.affiliation: Departments of Drug Metabolism and Pharmacokinetics (R.H.T., S.M., Q.Y., E.F.C., S.C.K.) and Small Molecule Pharmaceutical Sciences (S.J.R.), Genentech, Inc., South San Francisco, California.
authors.forename: Shuguang
authors.initials: S
authors.lastname: Ma
authors.affiliation: Departments of Drug Metabolism and Pharmacokinetics (R.H.T., S.M., Q.Y., E.F.C., S.C.K.) and Small Molecule Pharmaceutical Sciences (S.J.R.), Genentech, Inc., South San Francisco, California.
authors.forename: Sarah J
authors.initials: SJ
authors.lastname: Robinson
authors.affiliation: Departments of Drug Metabolism and Pharmacokinetics (R.H.T., S.M., Q.Y., E.F.C., S.C.K.) and Small Molecule Pharmaceutical Sciences (S.J.R.), Genentech, Inc., South San Francisco, California.
authors.forename: Qin
authors.initials: Q
authors.lastname: Yue
authors.affiliation: Departments of Drug Metabolism and Pharmacokinetics (R.H.T., S.M., Q.Y., E.F.C., S.C.K.) and Small Molecule Pharmaceutical Sciences (S.J.R.), Genentech, Inc., South San Francisco, California.
authors.forename: Edna F
authors.initials: EF
authors.lastname: Choo
authors.affiliation: Departments of Drug Metabolism and Pharmacokinetics (R.H.T., S.M., Q.Y., E.F.C., S.C.K.) and Small Molecule Pharmaceutical Sciences (S.J.R.), Genentech, Inc., South San Francisco, California.
authors.forename: S Cyrus
authors.initials: SC
authors.lastname: Khojasteh
mesh_terms: D000818:Animals; D003577:Cytochrome P-450 Enzyme System / Q000502:physiology*; D004285:Dogs; D005260:Female; D006801:Humans; D020935:MAP Kinase Signaling System / Q000187:drug effects / Q000502:physiology*; D008297:Male; D008862:Microsomes, Liver / Q000187:drug effects / Q000378:metabolism; D020929:Mitogen-Activated Protein Kinase Kinases / Q000037:antagonists & inhibitors* / Q000378:metabolism*; D047428:Protein Kinase Inhibitors / Q000737:chemistry* / Q000378:metabolism* / Q000494:pharmacology; D051381:Rats
chemical_list: D047428:Protein Kinase Inhibitors; D003577:Cytochrome P-450 Enzyme System; D020929:Mitogen-Activated Protein Kinase Kinases