The barrier function of CYP3A4 and P-glycoprotein in the small bowel
Introduction
There is now overwhelming evidence that some orally administered drugs that are substrates for cytochrome P450 3A4 (CYP3A4) can undergo significant first-pass metabolism within small intestinal epithelial cells (enterocytes). These data are reviewed elsewhere in this volume and will not be discussed extensively here. CYP3A4 appears to be the most abundant cytochrome P450 present in human small bowel 1, 2; it is found only in the mature enterocytes lining the villus and is not present in crypt cells (Fig. 1). CYP3A4 present in human enterocytes appears to be functionally and structurally identical to CYP3A4 present in human liver. The complete coding region of intestinal CYP3A4 cDNA has been sequenced in our laboratory and determined to be identical to its liver counterpart (unpublished observations). In addition, the pattern of metabolites produced from several CYP3A4 substrates, and the kinetic properties of the reactions, appear to be essentially identical in human small bowel and liver 3, 4, 5, 6. It therefore appears that catalytic properties of CYP3A4 determined in human liver microsomes, or in recombinant systems, can generally be extrapolated to the small bowel.
The related enzyme, CYP3A5, is also detectable in enterocytes in about 70% of adults, and is not co-regulated with CYP3A4 7, 8, 9, 10. Its structure, function and contribution to first-pass metabolism has not been established. However, CYP3A5 appears to be a minor enterocyte enzyme in most individuals [7].
The potential role of P-glycoprotein (Pgp, the MDR-1 gene product) in determining oral availability of some drugs has only recently been appreciated. Pgp is a versatile xenobiotic pump that was first discovered in cancer cells (reviewed in [11]). Pgp functions to make certain cancer cells resistant to several chemotherapeutic agents by pumping them out of the cells, maintaining intracellular concentrations at sublethal levels (MDR stands for multiple drug resistance). Pgp is expressed in a variety of normal (non-cancerous) human tissues, including liver, brain, adrenal gland, kidney and intestinal tract epithelia [12]. In the small bowel, Pgp is present on the apical membrane of the mature epithelial cells and is not detectable in the crypt cells (Fig. 2). Pgp is oriented in the apical membrane to pump xenobiotics from inside the cells back into the lumen of the intestine (i.e., a “countertransport” function) [13]. In humans (as opposed to rodents), there is only one MDR1 gene, and it is generally assumed that intestinal Pgp is functionally identical to Pgp present in other epithelial cells and in cancer cells. Pgp appears to have approximately the same molecular weight in human intestinal cells as in other cell types (as judged by mobility on polyacrylamide gels) 14, 15, 16. However, the complete coding region of human intestinal Pgp cDNA has not been sequenced to the author's knowledge. Kinetic data regarding chemical inhibition of enterocyte Pgp vs. Pgp in other cell types is sparse and somewhat conflicting 16, 17, 18, 19, 20, 21, 22, 23. It can therefore not be assumed at present that characteristics of Pgp transport determined in cells other than enterocytes will be directly applicable to oral drug absorption.
Section snippets
Effect on drug absorption
The demonstration of Pgp in enterocytes provides a mechanism to account for prior observations concerning intestinal (non-biliary) excretion for certain xenobiotics. For example, erythromycin [24], digoxin [24]and some β-blockers [25]and antibiotics 26, 27, 28, 29undergo active secretion from blood into the small bowel intestinal lumen, and appear to be substrates for Pgp 5, 21, 30. Direct evidence that Pgp inhibits the absorption of orally administered drugs comes from several sources. First,
Effect on cyclosporine bioavailability
The simplest explanation for the observed in vivo findings with cyclosporin A is that Pgp functions to prevent the complete absorption of the drug and that the vast majority of metabolism of cyclosporin occurs in the liver. Although this may be the case, several studies have suggested that the intestine can be a very substantial site for metabolism of orally administered cyclosporin A 6, 39, 40, 41. An alternate possibility is that Pgp may not prevent complete absorption of cyclosporin A, but
Future directions
The potential interactions and synergy between CYP3A4 and P-glycoprotein remain largely speculative, mainly because methods available to study such interactions are limited at present. Mouse knock-out models are likely to provide some insight, but it should be remembered that rodents have two expressed Mdr1 genes and may not be a good model for the human situation. Relatively selective chemical inhibitors of Pgp or CYP3A4 that do not inhibit both protein functions are not generally available.
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