P-Glycoprotein Increases Portal Bioavailability of Loperamide in Mouse by Reducing First-Pass Intestinal Metabolism

Abstract

P-glycoprotein (P-gp) and CYP3A (cytochrome P450 3A, generally; Cyp3a, rodent enzyme) in the intestine can attenuate absorption of orally administered drugs. While some suggest that P-gp enhances intestinal metabolism by CYP3A/Cyp3a during absorption of a dual substrate, others suggest that P-gp reduces the metabolism in the intestine when substrates are at subsaturating concentrations. Hence, to elucidate the cellular mechanisms that can address these divergent reports, we studied intestinal absorption of the dual substrate loperamide in portal vein-cannulated P-gp-competent and P-gp-deficient mice. These studies showed that at low doses of loperamide, which produced intestinal concentrations near the apparent K m for oxidative metabolism, the bioavailability across the intestine (F G ) was 6-fold greater in the P-gp-competent mice than in P-gp-deficient mice. The higher F G of loperamide in the presence of P-gp was attributed to lower loperamide intestinal metabolism. However, at high doses of loperamide, the sparing of first-pass metabolism by P-gp was balanced against the attenuation of absorption by apical efflux, resulting in no net effect on F G . In vitro studies with intestinal tissue from P-gp-competent and -deficient mice confirmed that P-gp reduced the metabolic rate of loperamide during absorptive flux at concentrations near K m but had little effect on metabolism at higher (saturating) concentrations. Further, studies in which Cyp3a was chemically inactivated by aminobenzotriazole in P-gp-competent and -deficient mice, showed that P-gp and Cyp3a individually attenuated F G by 8-fold and 70-fold, respectively. These results confirmed that P-gp effectively protects loperamide at low doses from intestinal first-pass metabolism during intestinal absorption.

Intestinal First-Pass Metabolism by Cytochrome P450 and Not P-Glycoprotein Is the Major Barrier to Amprenavir Absorption

“…Our recent studies provided support for this hypothesis (Dufek et al, 2013). Furthermore, it was shown that the effect of P-gp on intestinal first-pass metabolism was dose dependent, such that P-gp-mediated efflux caused the greatest increase in the F G at doses that produced intestinal concentrations near the apparent K m for Cyp3a-mediated metabolism (Dufek et al, 2013). This was the first report of P-gp in the intestinal epithelium contributing to an increase, instead of a decrease, in the F G of any drug.…”

“…Hence, the following hypothesis was tested: P-gp-mediated efflux should decrease intestinal first-pass metabolism of loperamide, thus increasing the absorption of intact loperamide into the portal circulation [i.e., gut bioavailability (F G )] and attenuating or reversing the negative impact of AP efflux of the drug. Our recent studies provided support for this hypothesis (Dufek et al, 2013). Furthermore, it was shown that the effect of P-gp on intestinal first-pass metabolism was dose dependent, such that P-gp-mediated efflux caused the greatest increase in the F G at doses that produced intestinal concentrations near the apparent K m for Cyp3a-mediated metabolism (Dufek et al, 2013).…”

“…Cytochrome P450 3A (CYP3A) and P-glycoprotein (P-gp), which are both highly expressed in the intestinal epithelium, contribute to its barrier properties via oxidative first-pass metabolism and apical (AP) efflux, respectively. Dufek et al (2013) recently addressed the controversial question of whether P-gp works synergistically with CYP3A to increase oxidative metabolism within the intestinal epithelium (Gan et al, 1996;Cummins et al, 2002Cummins et al, , 2004Benet et al, 2004) or to decrease the intestinal firstpass metabolism by reducing intracellular concentrations through AP efflux (Tam et al, 2003;Knight et al, 2006;Knight, 2007). Dufek et al (2013) chose loperamide as a probe substrate to investigate the interaction of P-gp and Cyp3a during oral absorption in mice because it undergoes extensive first-pass metabolism (Kim et al, 2004), and P-gp-mediated AP efflux of loperamide is highly efficient in the mouse intestine with an absorption quotient (AQ) = 0.90, i.e., an approximately 90% reduction in the AP to basolateral (BL) permeability (see Eq.…”

“…However, it remains to be seen what impact P-gp would have on the intestinal first-pass metabolism and the F G of a drug if the efficiency of efflux by P-gp is not as high as it is for loperamide. To address this question, a second dual substrate of P-gp and Cyp3a, amprenavir, was investigated using the same experimental models and designs that were used for the loperamide studies (Dufek et al, 2013). Amprenavir (Agenerase; GlaxoSmithKline, Research Triangle Park, NC), a human immunodeficiency virus type 1 protease inhibitor that is primarily metabolized by CYP3A4 (Polk et al, 1999;Tréluyer et al, 2003), was selected for the present study because P-gp is less efficient in reducing the absorptive permeability of amprenavir (approximately 50% in Caco-2 cell model) (Polli et al, 1999) than of loperamide.…”

The Consequence of Drug-Drug Interactions Influencing the Interplay between P-Glycoprotein and Cytochrome P450 3a: An Ex Vivo Study with Rat Precision-Cut Intestinal Slices

“…Inhibition of P-gp would therefore decrease the total intestinal metabolism. On the other hand, when P-gp limits absorption in the proximal small intestine, the absorption is shifted to more distal, less catalytically efficient segments that contain lower amounts of CYP3A (Watkins, 1997;Dufek et al, 2013), thus P-gp inhibition will increase metabolism. In addition, Pang et al (2009) stated that efflux by P-gp limits metabolism due to the competition between CYP3A and P-gp within the cell and that their interplay is independent of the mean residence time of drug in the system.…”