The intestinal epithelium is a critically important and highly complex barrier that comprises several physical as well as biochemical features that limit intestinal absorption of intact drug, thereby affecting systemic bioavailability. Cytochrome P450 3A (CYP3A) and P-glycoprotein (P-gp) are two important biochemical barriers present in the intestinal epithelium that contribute to the barrier properties via different mechanisms, i.e. first-pass metabolism and apical efflux, respectively. To date, mutually contradicting conclusions have been reached on whether P-gp would enhance or attenuate the intestinal metabolism of dual P-gp/CYP3A substrates during oral absorption. These conclusions are mostly based on findings from either static in vitro models or pharmacokinetic (PK) modeling. Systematic studies on the effect of P-gp on CYP3A-mediated intestinal first-pass metabolism of dual P-gp/ CYP3A substrates in vivo, where the interactions between these two proteins are likely to be more complex, are lacking. Here, the effect of the interplay of P-gp and CYP3A in mediating intestinal first-pass metabolism and absorption of the dual P-gp and CYP3A substrates, loperamide and amprenavir were investigated in vivo, using intact portal vein cannulated mice that were competent or deficient in P-gp expression. Additionally, mechanistic studies on the effect of P-gp efflux on intestinal first-pass metabolism of loperamide and amprenavir were conducted using fresh intestinal tissues from P-gp deficient and P-gp competent mice and in human intestinal epithelium, where P-gp efflux was functional (control) or inhibited with elacridar (GF120918), using an Ussing-type diffusion chamber.
These studies demonstrated that P-gp decreases intestinal Cyp3a-mediated metabolism of loperamide at intestinal lumen concentrations that are near the apparent Km for oxidative Cyp3a-mediated metabolism. As a result, P-gp shows a surprising effect of increasing, rather than decreasing, loperamide absorption into the portal circulation by sparing the drug from first-pass metabolism. Additional studies on the interplay of P-gp and Cyp3a in mice using the dual P-gp/Cyp3a substrate amprenavir showed that intestinal first-pass metabolism of amprenavir in intact P-gp competent and P-gp deficient mice was similar regardless of the presence or absence of P-gp. Additionally, these studies showed that the reduction in portal bioavailability (FG) observed in P-gp competent and P-gp deficient mice following an oral dose of amprenavir was solely due to extensive intestinal first-pass metabolism.
Studies on how P-gp efflux affects loperamide FG and intestinal first-pass metabolism in humans was studied in fresh intestinal epithelial tissue from humans, which showed that, similar to the findings in mouse, P-gp efflux reduced the rate and extent of metabolism at concentrations near the apparent Km for metabolism. Using the ex vivo data from human and mice intestinal tissue and in vivo data from mice, semi-physiologically based pharmacokinetic (PBPK) modeling was attempted to predict the effect of the interplay of P-gp and CYP3A during intestinal absorption of loperamide in humans. The modeling suggested that in the presence of P-gp the FG of loperamide in humans is expected to increase at doses that produce intestinal concentrations that are within the linear range of metabolism by CYP3A. Cumulatively, these studies highlight that the dynamic interaction of P-gp and CYP3A during intestinal absorption is highly complex and is expected to be significantly different for dual substrates that elicit different efficiencies of intestinal P-gp-mediated AP efflux and/or CYP3A-mediated metabolism.
|Advisor:||Smith, Philip C.|
|Commitee:||Cho, Moo J., Paine, Mary F., Polli, Joseph W., Thakker, Dhiren R., Troutman, Matthew M.|
|School:||The University of North Carolina at Chapel Hill|
|School Location:||United States -- North Carolina|
|Source:||DAI-B 72/10, Dissertation Abstracts International|
|Subjects:||Pharmacology, Pharmacy sciences|
|Keywords:||Bioavailability, Cytochrome P450, Intestine, Metabolism, P-glycoprotein|
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