CYP3A4 is the major P450 present in human liver and small bowel? epithelial cells (enterocytes). The contribution of enterocyte CYP3A4? to drug elimination is believed to be substantial. However,? distinguishing the contributions of the intestinal vs. the liver enzyme,? as well as the role of the transporter P-glycoprotein, has been? problematic. We have developed experimental techniques that have? enabled us to safely study liver and enterocyte CYP3A4 in living people.? We have shown that there exists marked inter- and intraindividual? variation in the activity of enterocyte CYP3A4. We have also shown that? there is generally not a good intraindividual correlation between the? relative activities of CYP3A4 in liver and intestine. To further? investigate the mechanisms underlying these variations, we have chosen? to intensively pursue our observation that some fruit juices (grapefruit? juice and Seville orange juice) cause a marked and relatively rapid loss? of enterocyte CYP3A4 by a mechanism that does not appear to involve loss? of CYP3A4 mRNA. We have shown that 2 juice-derived furanocoumarins (FC? s), 6',7'-dihydroxybergamottin (DHB) and a related dimer (FC726), cause? selective loss of CYP3A4 in a novel human intestinal (Caco-2) cell? monolayer culture system, and that DHB is a mechanism-based inactivator? of CYP3A4. However, DHB and FC726 can not fully account for the effects? of whole juice, and their effects may not be CYP3A4 selective. We will? test the hypothesis that multiple FC s, which are quite ubiquitous in? fruits and vegetables, cause mechanism-based inactivation and? accelerated intracellular degradation of CYP3A4. To this end, we will? characterize the time course of the effects of selected FC s on CYP3A4? and on other major enterocyte P450s in human intestinal microsomes, in? our Caco-2 cell system, and in healthy volunteers. We will also? determine the effects of selected FC s on rates of synthesis and? degradation of CYP3A4 in our cultured cells. Finally, we will test the? hypothesis that some oral medication regimens that result in clinically? important induction of CYP3A4 in liver do not induce the enzyme in? enterocytes. The data obtained from the proposed studies should provide? substantial insight into the basis for variations in CYP3A4 activity? between people, within a person over time, and between liver and? intestine. In addition, the identification of orally administered FC? s that specifically ablate CYP3A4 activity in enterocytes (but not? liver), or drugs that selectively induce CYP3A4 in liver (but not in? enterocytes) would provide powerful research tools to assess the? relative contributions of enterocyte and hepatic CYP3A4 to the? disposition of xenobiotics in living people.
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