Assessment of the Metabolism and Intrinsic Reactivity of a Novel Catechol Metabolite

PH-302 ( 1) demonstrates potent inhibitory activity against the inducible form of nitric oxide synthase (iNOS). The primary metabolite of PH-302 is a catechol ( 2) resulting from oxidative demethylenation of the methylenedioxyphenyl moiety by cytochrome P450 3A4. Concerns regarding subsequent two-electron oxidation of 2 to an electrophilic quinone species and the potential for resulting toxicity prompted additional studies to examine the reactivity and metabolic fate of this metabolite. Contrary to literature reports of catechol reactivity, 2 appeared to be resistant to quinone formation in human liver microsomal incubations, as determined by the lack of detectable glutathione (GSH) adducts and no covalent binding to microsomal proteins. In addition, 2 showed no evidence of depletion of intracellular glutathione or cytotoxicity at concentrations up to 1 mM in primary human and rat hepatocytes. In the presence of tyrosinase, spectral evidence indicated that 2 was oxidized to the ortho-quinone, and upon incubation in the presence of GSH, two conjugates were detected and characterized by LC/MS/MS. Lastly, the metabolic pathways of 2 were investigated in rat and human hepatocytes and found to be similar, proceeding via glucuronidation, sulfation, and methylation of the catechol. Collectively, these studies demonstrate that 2 appears to be resistant to further oxidation to quinone in liver microsomes, as well as spontaneous redox cycling, while the formation of phase II metabolites in hepatocytes suggests that multiple detoxication pathways may provide added protection against toxicity in the liver.