Active secretion and enterocytic drug metabolism barriers to drug absorption1

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Abstract

Intestinal phase I metabolism and active extrusion of absorbed drug have only recently been recognized as major determinants of oral drug bioavailability. Both CYP3A4, the major phase I drug metabolizing enzyme in humans, and the multidrug efflux pump, P-glycoprotein (P-gp), are present at high levels in the villus enterocytes of the small intestine, the primary site of absorption for orally administered drugs. Moreover, these proteins are induced by many of the same compounds and demonstrate a broad overlap in substrate and inhibitor specificities, suggesting that they act as a concerted barrier to drug absorption. Clinical studies have demonstrated that inhibition of CYP3A4-mediated intestinal metabolism can significantly improve the oral bioavailability of a wide range of drugs. Intestinal P-gp is a major route of elimination for both orally and intravenously administered anticancer drugs in animal models, and experiements with the Caco-2 cell line have provided strong evidence that inhibition of intestinal P-gp is another means by which oral drug bioavailability could be enhanced.

Introduction

Systemic bioavailability of orally administered drugs has, until recently, been considered primarily a function of physical drug absorption and subsequent phase I metabolism by the liver [1]. Degradation of drugs in the intestine, by luminal fluids, gut microflora, or hydrolytic and conjugative (phase II) enzymes in the gut wall, have presented significant challenges to drug delivery, but have only been addressed for a restricted number of compounds 2, 3, 4. Phase I metabolism by intestinal cytochromes P450 (CYPs) has been considered a relatively minor determinant of oral drug bioavailability because concentrations of individual CYPs normalized for the entire intestine are estimated to be approximately 20- to 200-fold lower than those found in the liver 5, 6.

This traditional view of intestinal metabolism has recently been reexamined in light of the finding that enzymes of the CYP3A sub-family — considered the major phase I drug-metabolizing enzymes in humans 7, 8— are expressed at high levels in the mature villus tip enterocytes of the small intestine 6, 9, 10, 11, 12, 13, 14. While CYP3A constitutes only 30% of total human hepatic CYP content [15], it accounts for approximately 70% of the CYP content in human enterocytes 9, 12. Furthermore, immunohistochemical studies [6], protein measurements and enzyme activity determinations [9]have found that concentrations of CYP3A in the small intestine equal or exceed the concentrations in the liver. It is worth noting that, although the small intestinal villi receive a much lower blood flow than the liver [16], the concentration of CYP3A in the villus tip enterocytes provides a much larger surface area over which the enzyme can interact with absorbed drug, facilitating substantial first-pass metabolism.

In addition to phase I metabolism, active secretion of absorbed drug is now becoming recognized as a significant factor in oral drug bioavailability 7, 17. Of particular interest is the MDR1 gene product P-glycoprotein (P-gp), a multidrug efflux pump, first recognized in tumor resistance [18]. As for CYP3A, P-gp is located in the jejunal villus enterocytes [19]and a recent review from our laboratory found that CYP3A and P-gp share a remarkable number of substrates and inhibitors (Table 1) [7]. These findings suggest that CYP3A and P-gp may form a concerted barrier to drug absorption and that intestinal drug metabolism and counter-transport processes are a major determinant of oral drug bioavailability and variability [17].

Section snippets

Principal drug metabolizing enzymes in the intestine

CYPs identified in human intestine include CYP1A1 20, 21, CYP2C8-10, CYP2D6, CYP2E1 (trace amounts), CYP3A4 6, 12, 13, 14, 22and, more recently, CYP3A5 13, 22. By far the most important of these enzymes to drug metabolism is CYP3A4. Immunohistochemical studies have shown that small intestinal concentrations of CYP3A4 are approximately 80–100% of the CYP3A4 concentration in the liver, while CYP2C8–10 concentrations are only 5–10% and CYP2D6 concentrations are around 20% of their respective liver

Clinical studies of intestinal drug metabolism and bioavailability

Clinical evidence of significant phase-I metabolism by the small intestine was available as early as 1977 in studies by Mahon et al. [36]who instilled 14C-labelled flurazepam into the stomach of one patient and the duodenum of two others. Thin-layer chromatographic analysis of portal venous blood demonstrated the rapid appearance of significant levels of flurazepam metabolites, consistent with metabolism by the small intestinal mucosa.

Much of the recent work establishing intestinal metabolism

Dietary effects on intestinal metabolism

Recent pharmacokinetic studies have also established inhibition of intestinal metabolism by dietary components as an important determinant of oral drug bioavailability. In a study following the same general plan as those decribed above, Gupta et al. [43]administered both i.v. and oral cyclosporine to healthy volunteers receiving low-fat and high-fat meals. The observed increase in Fmeas with high-fat meals was initially attributed to increased absorption (Fabs), however, the mean absorption

Variability, intestinal metabolism and drug bioavailability

Examination of the data in Table 2, Table 3 shows that coefficients of variation (C.V.) for Fmeas and AUC were unaffected by inhibition of intestinal metabolism. This lack of an effect on inter-subject variability is not unexpected, given the demonstrated heterogeneity of intestinal CYP3A expression described above. Since it is unlikely that 100% enzyme inhibition was achieved in these studies, differing inter-subject levels of intestinal CYP3A would result in inter-subject variations in oral

P-glycoprotein

While Fabs has so far been considered a measure of physical drug absorption, it is also expected to incorporate active extrusion of absorbed drug, in particular by intestinal P-gp [7]. The MDR1 gene product, P-gp, is a transmembrane protein associated with the multidrug resistance phenotype. It was first described in tumor cell lines displaying cross resistance to various anticancer agents (colchicine, anthracyclines, epipodophyllotoxines and vinca alkaloids) [71]and has been shown to act as an

Acknowledgements

Studies in the authors' laboratories at the University of California, San Francisco were supported in part by National Institutes of Health Grant GM 26691.

References (89)

  • Aungst, B.J. (1993) Novel formulation strategies for improving oral bioavailability of drugs with poor membrane...
  • Ilett, K.F., Tee, L.B.G., Reeves, P.T. and Minchin, R.F. (1990) Metabolism of drugs and other xenobiotics in the gut...
  • Tam, Y.K. (1993) Individual variation in first-pass metabolism. Clin. Pharmacokinet. 25,...
  • Krishna, D.R. and Klotz, U. (1994) Extrahepatic metabolism of drugs in humans. Clin. Pharmacokinet. 26,...
  • Back, D.J. and Rogers, S.M. (1987) First-pass metabolism by gastrointestinal mucosa. Aliment. Pharmacol. Ther. 1,...
  • de Waziers, P.H., Cugnenc, P.H., Yang, C.S., Leroux, J.-P. and Beaune, P.H. (1990) Cytochrome P450 isoenzymes, epoxide...
  • Wacher, V.J., Wu, C.-Y. and Benet L.Z. (1995) Overlapping substrate specificities and tissue distribution of cytochrome...
  • Watkins, P.B. (1994) Non-invasive tests of CYP3A enzymes. Pharmacogenetics 4,...
  • Watkins, P.B., Wrighton, S.A., Schuetz, E.G. and Guzelian, P.S. (1987) Identification of glucocorticoid-inducible...
  • Murray, G.I., Barnes, T.S., Sewell, H.F., Ewen, S.W.B., Melvin, W.T. and Burke, M.D. (1988) The immunocytochemical...
  • Peters, W.H.M. and Kremers, P.G. (1989) Cytochromes P-450 in the intestinal mucosa of man. Biochem. Pharmacol. 38,...
  • Kolars, J.C., Schmiedlin-Ren, P., Schuetz, J.D., Fang, C. and Watkins, P.B. (1992) Identification of rifampin-inducible...
  • Kolars, J.C., Lown, K.S., Schmiedlin-Ren, P., Ghosh, M., Fang, C., Wrighton, S.A., Merion, R.M. and Watkins, P.B....
  • McKinnon, R.A., Burgess, W.M., Hall, P. de la M., Roberts-Thomson, S.J., Gonzalez, F.J. and McManus, M.E. (1995)...
  • Shimada, T., Yamazaki, H., Mimura, M., Inui, Y. and Guengerich, F.P. (1994) Interindividual variations in human liver...
  • Forsyth, R.P., Nies, A.S., Wyler, F., Neutze, J. and Melmon, K.L. (1968) Normal distribution of cardiac output in the...
  • Benet, L.Z., Wu, C.-Y., Hebert, M.F. and Wacher, V.J. (1996) Intestinal drug metabolism and antitransport processes: A...
  • Levêque, D. and Jehl, F. (1995) P-glycoprotein and pharmacokinetics. Anticancer Res. 15,...
  • Thiebaut, F., Tsuruo, T., Hamada, H., Gottesman, M.M., Pastan, I. and Willingham, M.C. (1987) Cellular localization of...
  • McDonnell, W.M., Scheiman, J.M. and Traber, P.G. (1992) Induction of cytochrome P4501A genes by omeprazole in the human...
  • Buchthal, J., Grund, K.E., Buchmann, A., Schrenk, D., Beaune, P. and Bock, K.W. (1995) Induction of cytochrome P4501A...
  • Lown, K.S., Kolars, J.C., Thummel, K.E., Barnett, J.L., Kunze, K.L., Wrighton, S.A. and Watkins, P.B. (1994)...
  • Wrighton, S.A., Ring, B.J., Watkins, P.B. and Vandenbranden, M. (1989) Identification of a polymorphically expressed...
  • Wrighton, S.A., Brian, W.R., Sari, M.-A., Iwasaki, M., Guengerich, F.P., Raucy, J.L., Molowa, D.T. and Vandenbranden,...
  • Schuetz, E.G., Schuetz, J.D., Strom, S.C., Thompson, M.T., Fisher, R.A., Molowa, D.T., Li, D. and Guzelian, P.S. (1993)...
  • Aoyama, T., Yamano, S., Waxman, D.J., Lapenson, D.P., Meyer, U.A., Fischer, V., Tyndale, R., Inaba, T., Kalow, W.,...
  • Gorski, J.C., Hall, S.D., Jones, D.R., Vandenbranden, M. and Wrighton, S.A. (1994) Regioselective biotransformation of...
  • Thummel, K.E., Shen, D.D., Podoll, T.D., Kunze, K.L., Trager, W.F., Hartwell, P.S., Raisys, V.A., Marsh, C.L., McVicar,...
  • Thummel, K.E., Shen, D.D., Podoll, T.D., Kunze, K.L., Trager, W.F., Bacchi, C.E., Marsh, C.L., McVicar, J.P., Barr,...
  • Watkins, P.B., Murray, S.A., Winkelman, L.G., Heuman, D.M., Wrighton, S.A. and Guzelian, P.S. (1989). Erythromycin...
  • Schuetz, E.G., Schuetz, J.D., Grogan, W.M., Naray-Fejes-Toth, A., Fejes-Toth, G., Raucy, J., Guzelian, P., Gionela, K....
  • Guengerich, F.P. (1995) Influence of nutrients and other dietary materials on cytochrome P-450 enzymes. Am. J. Clin....
  • Warner, P.E., Brouwer, K.L.R., Hussey, E.K., Dukes, G.E., Heizer, W.D., Donn, K.H., Davis, I.M. and Powell J.R. (1995)...
  • Lacey, L.F., Hussey, E.K. and Fowler, P.A. (1995) Single dose pharmacokinetics of sumatriptan in healthy volunteers....
  • Kanfer, I., Dowse, R. and Vuma, V. (1993) Pharmacokinetics of oral decongestants. Pharmacotherapy 13,...
  • Mahon, W.A., Inaba, T. and Stone, R.M. (1977) Metabolism of flurazepam by the small intestine. Clin. Pharmacol. Ther....
  • Kolars, J.C., Awni, W.M., Merion, R.M. and Watkins, P.B. (1991) First-pass metabolism of cyclosporine by the gut....
  • Thummel, K.E. (1995) Modeling hepatic and intestinal metabolism of the CYP3A probe midazolam from in vitro data. AAPS...
  • Hebert, M.F., Roberts, J.P., Prueksaritanont, T. and Benet, L.Z. (1992) Bioavailability of cyclosporine with...
  • Gomez, D.Y., Wacher, V.J., Tomlanovich, S.J., Hebert, M.F. and Benet, L.Z. (1995) The effects of ketoconazole on the...
  • Gupta, S.K., Bakran, A., Johnson, R.W.G. and Rowland, M. (1988) Erythromycin enhances the absorption of cyclosporine....
  • Wu, C.-Y., Benet, L.Z., Hebert, M.F., Gupta, S.K., Rowland, M., Gomez, D.Y. and Wacher, V.J. (1995) Differentiation of...
  • Gupta, S.K., Manfro, R.C., Tomlanovich, S.J., Gambertoglio, J.G., Garovoy, M.R. and Benet, L.Z. (1990) Effect of food...
  • Barnwell, S.G., Laudanski, T., Story, M.J., Mallinson, C.B., Harris, R.J., Cole, S.K., Keating, M. and Attwood, D....
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    PII of original article: S0169-409X(96)003304. The article was originally published in Advanced Drug Delivery Reviews 20 (1996) 99–112.

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