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The extents to which small intestinal (SI) cytochrome P450 (P450) enzymes control the bioavailability of oral drugs are not well defined, particularly for drugs that are substrates for both P450 and the P-glycoprotein (P-gp). In this study, we have determined the role of SI P450 in the clearance of orally administered lovastatin (LVS), an anti-hypercholesterolemia drug, using an intestinal epithelium (IE)-specific P450 reductase knockout (IE-Cpr-null) mouse model. In the IE-Cpr-null mouse, which has little P450 activities in the IE, the oral bioavailability of LVS was substantially higher than that in wild-type (WT) mice (15 and 5%, respectively). In control experiments, the clearance rates were not different between the two strains, either for intraperitoneally dosed LVS, which bypasses SI metabolism, or for orally administered pravastatin, which is known to be poorly metabolized by P450. Thus, our results demonstrate a predominant role of SI P450 enzymes in the first-pass clearance of oral LVS. The absence of IE P450 activities in the IE-Cpr-null mice also facilitated the identification of the molecular targets for orally administered grapefruit juice (GFJ), which is known to inhibit LVS clearance in humans. We found that pretreatment of mice with oral GFJ enhanced the systemic exposure of LVS in WT, but not in IE-Cpr-null mice, a result suggesting that the main target of GFJ action in the small intestine is P450, but not P-gp.

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Role of Intestinal Cytochrome P450 (P450) in Modulating the Bioavailability of Oral Lovastatin: Insights from Studies on the Intestinal Epithelium-Specific P450 Reductase Knockout Mouse