Inhibitory effect of nicotine and its metabolites on tolbutamide hydroxylation in rat liver microsomes.

A simple HPLC/fluorescence method to detect hydroxytolbutamide (a major metabolite of the anti-diabetic drug tolbutamide) has been developed. The effects of nicotine and some of its metabolites on tolbutamide hydroxylation is described. An extraction procedure with diethyl ether was followed by isocratic HPLC analysis of tolbutamide hydroxylation with a binary mobile phase composed of 10 mM monobasic sodium phosphate in methanol (45:55, v/v, apparent pH 2.28). A detection limit of sub-nanogram amounts (0.353 ng) of hydroxytolbutamide was obtained with fluorescence detection at 226 nm for excitation and 318 nm for emission. Overall precision values for hydroxytolbutamide was determined with coefficients of variation of 1.4-4.6% when nanogram levels of the metabolite were analyzed. Differential inhibitory responses were demonstrated for tolbutamide hydroxylation to nicotine and its metabolites. Tolbutamide hydroxylation was apparently inhibited by cotinine and relatively less inhibited by nicotine. Nornicotine, however, caused very little inhibition of tolbutamide hydroxylation. The implication is that nornicotine may not share similar affinity for the substrate binding site for tolbutamide. The results also suggest that heavy smokers may experience reduction in tolbutamide metabolism. The assay system itself will be useful for future studies of tolbutamide, and possibly related sulfonylureas.