Metabolism of dimethyl-4,4 '-dimethoxy-5,6,5 ', 6 '- dimethylene dioxybiphenyl-2,2 '-dicarboxylate (DDB) by human liver microsomes: Characterization of metabolic pathways and of cytochrome P450 isoforms involved

Metabolic fate of DDB and identification of P450 isozymes involved in the m etabolism of DDB were investigated in human liver microsomes. DDB was rapid ly metabolized to five different metabolites, and the structures of each me tabolite were characterized based on UV, mass, and NMR spectral analyses. T he major metabolic pathways of DDB in human liver microsomes were identifie d as O-demethylation of the carboxymethyl moiety (M4) and demethylenation o f the methylenedioxyphenyl group (M2). The intramolecular lactonization bet ween the hydroxyl group at the C6 and carboxymethyl group at the C2' of M2 resulted in the generation of M5, which was either hydrolyzed to its hydrol yzed derivative (M1) or further metabolized to the O-demethylated derivativ e (M3). The interconversion of M1, M2, and M5 took place nonenzymatically d epending on the solvent condition. M5 was predominantly detected at the aci dic condition, whereas M1 was preferentially detected at the basic environm ent. Cytochrome P450 (P450) isoform( s) involved in the metabolism of DDB w as identified using several in vitro approaches. Chemical inhibition using isoform-selective P450 inhibitors, correlation of DDB metabolites formation with several isoform-specific P450 activities in a panel of liver microsom es, metabolism by microsomes derived from P450 cDNA-expressed B-lymphoblast oid cells, and immunoinhibition by isoform-specific anti-P450 antibodies co llectively indicated that CYP1A2, CYP2C9, and CYP3A4 are responsible for th e metabolism of DDB. O-Dealkylation of the carboxymethyl group was preferen tially catalyzed by CYP1A2, whereas demethylenation of the methylenedioxyph enyl moiety was catalyzed by CYP3A4 and CYP2C9.