Metabolic and inhibitory differences between cytochromes P450 3A4 and 3A5

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Metabolic and inhibitory differences between cytochromes P450 3A4 and 3A5

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Title: Metabolic and inhibitory differences between cytochromes P450 3A4 and 3A5
Author: McConn, Donavon J., 1970-
Abstract: Cytochrome P450 (CYP) 3A4 and 3A5 constitute the dominant drug metabolizing enzymes in the liver and small intestine, and the content and activity of these enzymes is widely variable. Accurate knowledge of in vivo CYP3A activity is vital to predicting potential drug-drug interactions that may occur. Furthermore, any inhibitory differences between the two CYP3A isoforms would further complicate the accurate prediction of in vivo interactions. The purpose of this dissertation was to evaluate the excretory profile and regioselective metabolism of midazolam, an in vivo probe of CYP3A activity, in order to better understand its disposition and metabolic characteristics. Moreover, the inhibitory capability of a series of compounds was tested with CYP3A4 and CYP3A5.Initial work focused on the variable urinary recovery and potential biliary secretion of midazolam in vivo. Biliary samples were analyzed to determine if secretion into bile could explain the variable urinary recovery of midazolam. Although midazolam metabolites were detectable in bile at relatively high levels, fecal analysis suggested the biliary compounds were reabsorbed, and that biliary secretion is an unlikely mechanism to describe variable urinary recovery.Midazolam 4-hydroxylation is a secondary metabolic pathway mediated by CYP3A, and its chemical instability in vivo may also contribute to variable urinary recovery. To address this, midazolam metabolism was examined in a panel of 60 human liver microsomes, and the relative importance of 1 '- and 4-hydroxylation was determined at three different substrate concentrations.Midazolam metabolism was also examined in intestinal samples acquired from cirrhotic patients. We examined the effects of liver cirrhosis on intestinal enzyme expression and activity. Our findings suggest that although cirrhosis has deleterious effects on hepatic CYP3A-dependent metabolism, no apparent effect was observed in the intestine.Once metabolic differences were studied, our goal was to determine if a series of inhibitors behaved differently towards CYP3A4 and CYP3A5. For erythromycin, diltiazem, ketoconazole and nicardipine, affinities were higher towards CYP3A4 compared to CYP3A5. CYP3A5 also showed resistance to mechanism-based inactivation compared to CYP3A4.In summary, no single mechanism seems responsible for CYP3A-dependent urinary recovery of midazolam. However, inhibitory differences do exist in time-dependent inhibition between CYP3A4 and CYP3A5.
Description: Thesis (Ph. D.)--University of Washington, 2001

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