Metabolism of the new anxiolytic agent, a pyrido[1,2-]benzimidazole (PBI) analog (RWJ-53050), in rat and human hepatic S9 fractions, and in dog; identification of cytochrome p450 isoforms mediated in the human microsomal metabolism

Summary: The in vitro and in vivo metabolism of RWJ-53050, an anxiolytic agent, was investigated after incubation with rat and human hepatic S9 fractions, and human microsomes and 7 microsomes containing individual human CYP isoforms, CYP1A2, CYP2A6, CYP2C9, CYP2C19, CYP2D6, CYP2E1 and CYP3A4 in the presence of NADPH-generating system, and a single oral dose administration to dogs (30 mg/kg). Unchanged RWJ-53050 (≥74% of the sample in vitro; ⪯13% in vivo) plus 16 metabolites were profiled, quantified and tentatively identified based on the API-MS and MS/MS data. The formation of RWJ-53050 metabolites are via the 5 pathways: 1.N/O-demethylation, 2. phenylhydroxylation, 3. pyrido-oxidation, 4. dehydration, and 5. conjugation. Pathway 1 formedO-desmethyl-phenyl-RWJ-53050 (M1, <1–12% in vitro & in vivo), O-desmethyl-benzimidazole-RWJ-53050 (M2), and N-desmethyl-RWJ-53050 (M3) (M2 & M3, ⪯3% in vitro & in vivo). Pathway 2 generated hydroxy-benzimidazole-RWJ-53050 (M4), hydroxy-phenyl-RWJ-53050 (M5), and hydroxy-phenyl-M4 (M9) (⪯3% in vitro & in vivo). Pathway 3 formed 2 trace oxidized metabolites, hydroxy-pyrido-RWJ-53050 (M6, ⪯1% in vitro) and oxo-pyrido-RWJ-53050 (M8, <1% in vitro) and in conjunction with pathway 1 produced 2 trace dioxidized metabolites, OH-benzimidazole-M6 (M10) and OH-benzimidazole-M8 (M11) (in vitro). Pathway 4 formed a minor dehydrated metabolite of M6 (M7, 3%, in vitro). Pathway 5 produced 3 in vivo conjugates, Ml-glucuronide (M14,17%), M5-glucuronide (M15,50%), and M5-sulfate (M16,10%). RWJ-53050 is substantially metabolized in vitro in the rat and human, and extensively metabolized in vivo in the dog. formation of oxidized metabolites, Ml, M2, M4, M5 and M9.