15] Use of tolbutamide as a substrate probe for human hepatic cytochrome P450 2C9

Abstract

Publisher Summary This chapter describes an established high-performance liquid chromatographic (HPLC) assay for the measurement of hydroxytolbutamide formation, the rate-limiting step in tolbutamide metabolism, by human liver microsomes and CYP isoforms expressed in cell culture. The assay demonstrates that CYP2C9 is essentially solely responsible for the human hepatic hydroxylation of tolbutamide. Xenobiotic metabolizing isoforms of cytochrome P450 (CYP) exhibit distinct, but sometimes overlapping, patterns of substrate and inhibitor specificity and differ in terms of regulation. The availability of such compounds is an important component of strategies developed to link the metabolism of newly developed drugs to an individual CYP isoform(s). In humans the elimination of the oral hypoglycemic agent tolbutamide (1-butyl-3-p-tolylsulfonylurea) occurs along a single pathway, with the initial and rate-limiting step being methylhydroxylation to form hydroxytolbutamide. Hydroxytolbutamide is oxidized further in vivo by alcohol and aldehyde dehydrogenases producing carboxytolbutamide.