Species-specific, P450-and sulfotransferase-mediated novel ring contraction of a naphthyridine-N-oxide compound in cynomolgus monkey

BMS-A78277 (1) is a 5,10-dihydrobenzo[beta][1,8]naphthyridine-N-oxide compound that resides in a class of novel non-nucleoside reverse transcriptase inhibitors (NNRTIs), displaying improved activity against clinically relevant mutants of HIV-1 and possessing pharmacokinetic profiles amenable to once-daily dosing. In the course of investigating the nonclinical metabolism of 1, a circulating metabolite specific to the cynomolgus monkey was identified and subsequently characterized as the carboxyindole metabolite 2. The present investigation describes the biotransformation of this NNRTI in cynomolgus monkey, one which results in a net ring contraction of 1. The use of mass spectrometry and high field NMR analysis aided in the structural characterization of metabolite 2, the source of which originated from the urine and bile of cynomolgus monkeys receiving oral doses of 1. Preparation of a synthetic standard of 2 not only provided ultimate structural confirmation but also afforded ample material for biological testing. The metabolism of 1 was investigated in monkey hepatocytes and hepatic subcellular fractions. While microsomes were incapable of generating metabolite 2, incubation of I in monkey S9 fractions as well as hepatocytes resulted in measurable levels of the carboxyindole metabolite. Consequently, incubation of I in monkey hepatocytes, which were suspended in media containing O-18-labeled water, resulted in the incorporation of O-18 into the carboxyindole metabolite, 2. These data implicate a mechanism involving the bioactivation of 1 to an electrophilic intermediate that upon hydrolysis undergoes a concerted ring contraction, resulting in the formation of 2. Previously confined to discussions regarding the metabolism of natural products and select aliphatic heterocycles, the present investigation extends the discussion of metabolism- mediated ring contraction to aromatics such as the present naphthyridine compound, 1.