A Baeyer-Villiger oxidation specifically catalyzed by human flavin-containing monooxygenase 5

10-((4-Hydroxypiperidin-1-yl)methyl)chromeno[4,3,2-de]phthalazin-3(2H)-one (E7016), an inhibitor of poly(ADP-ribose) polymerase, is being developed for anticancer therapy. One of the major metabolites identified in preclinical animal studies was the product of an apparent oxidation and ring opening of the 4-hydroxypiperidine. In vitro, this oxidized metabolite could not be generated by incubating E7016 with animal or human liver microsomes. Further studies revealed the formation of this unique metabolite in hepatocytes. In a NAD(P)(+)-dependent manner, this metabolite was also generated by liver S9 fractions and recombinant human flavin-containing monooxygenase (FMO) 5 that was fortified with liver cytosol fractions. In animal and human liver S9, this metabolic pathway could be inhibited by 4-methylpyrazole, bis-p-nitrophenylphosphate (BNPP), or a brief heat treatment at 50°C. Based on these results, the overall metabolic pathway was believed to involve a two-step oxidation process: dehydrogenation of the secondary alcohol in liver cytosol followed by an FMO5-mediated Baeyer-Villiger oxidation in liver microsomes. The two oxidation steps were coupled via regeneration of NAD(P)(+) and NAD(P)H. To further confirm this mechanism, the proposed ketone intermediate was independently synthesized. In an NAD(P)H-dependent manner, the synthetic ketone intermediate was metabolized to the same ring-opened metabolite in animal and human liver microsomes. This metabolic reaction was also inhibited by BNPP or a brief heat treatment at 50°C. Methimazole, the substrate/inhibitor of FMO1 and FMO3, did not inhibit this reaction. The specificity of FMO5 toward catalyzing this Baeyer-Villiger oxidation was further demonstrated by incubating the synthetic ketone intermediate in recombinant enzymes.