title: Cytochrome P450 3A-dependent metabolism of a potent and selective gamma-aminobutyric acid Aalpha2/3 receptor agonist in vitro: involvement of cytochrome P450 3A5 displaying biphasic kinetics.
abstract: In vitro metabolism studies were conducted to determine the human cytochrome P450 enzyme(s) involved in the biotransformation of 7-(1,1-dimethylethyl)-6-(2-ethyl-2H-1,2,4-triazol-3-ylmethoxy)-3-(2-fluorophenyl)-1,2,4-triazolo[4,3b]pyridazine (TPA023), a selective agonist of human gamma-aminobutyric acid(A) receptor alpha2 and alpha3 subunits. Incubation of TPA023 with NADPH-fortified human liver microsomes resulted in the formation of t-butyl hydroxy TPA023, N-desethyl TPA023, and three minor metabolites. Both t-butyl hydroxylation and N-deethylation reactions were greatly inhibited (>85%) in the presence of CYP3A-selective inhibitory antibodies and chemical inhibitors, indicating that members of the CYP3A subfamily play an important role in TPA023 metabolism. Eadie-Hofstee plots of t-butyl hydroxylation and N-deethylation in pooled CYP3A5-rich human liver microsomes revealed a low K(m) (3.4 and 4.5 microM, respectively) and a high K(m) (12.7 and 40.0 microM, respectively) component. For both metabolites, the high K(m) component was not observed with a pool of microsomal preparations containing minimal levels of CYP3A5. Preincubation of liver microsomes with mifepristone (selectivity for CYP3A4 > CYP3A5) greatly inhibited both t-butyl hydroxylation and N-deethylation (>75%); however, the residual activities were significantly higher in the pooled CYP3A5-rich liver microsomes (p < 0.0005). In addition, elevated levels of residual t-butyl hydroxylase and N-deethylase activities were observed in the presence of both CYP3A5-rich and CYP3A5-deficient preparations when the substrate concentration increased from 4 to 40 microM. In agreement, metabolite formation catalyzed by recombinant CYP3A5 was described by a biphasic model. It is concluded that CYP3A4 plays a major role in TPA023 metabolism, and CYP3A5 may also contribute at higher concentrations of the compound.
authors: Department of Drug Metabolism, Merck Research Laboratories, West Point, PA 19486, USA. bennett_ma@merck.com
authors: Bennett
authors: B
authors: Ma
authors: Stacey L
authors: SL
authors: Polsky-Fisher
authors: Stanley
authors: S
authors: Vickers
authors: Donghui
authors: D
authors: Cui
authors: A David
authors: AD
authors: Rodrigues
mesh_terms: D000818:Animals; D000911:Antibodies, Monoclonal / Q000494:pharmacology; D051544:Cytochrome P-450 CYP3A / Q000276:immunology / Q000378:metabolism*; D065692:Cytochrome P-450 CYP3A Inhibitors; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000235:genetics / Q000378:metabolism*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D058785:GABA-A Receptor Agonists; D006801:Humans; D006900:Hydroxylation; D007700:Kinetics; D008861:Microsomes / Q000187:drug effects / Q000378:metabolism; D008862:Microsomes, Liver / Q000187:drug effects / Q000378:metabolism; D015735:Mifepristone / Q000494:pharmacology; D011724:Pyridazines / Q000378:metabolism*; D011994:Recombinant Proteins / Q000378:metabolism; D014230:Triazoles / Q000378:metabolism*
chemical_list: C507018:7-(1,1-dimethylethyl)-6-(2-ethyl-2H-1,2,4-triazol-3-ylmethoxy)-3-(2-fluorophenyl)-1,2,4-triazolo(4,3-b)pyridazine; D000911:Antibodies, Monoclonal; D065692:Cytochrome P-450 CYP3A Inhibitors; D065607:Cytochrome P-450 Enzyme Inhibitors; D004791:Enzyme Inhibitors; D058785:GABA-A Receptor Agonists; D011724:Pyridazines; D011994:Recombinant Proteins; D014230:Triazoles; D015735:Mifepristone; D003577:Cytochrome P-450 Enzyme System; C510164:CYP3A5 protein, human; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human