Intestinal MDR transport proteins and P-450 enzymes as barriers to oral drug delivery

Abstract

Cytochrome P-450 3A4 (CYP3A4), the major phase I drug metabolizing enzyme in humans, and the multidrug efflux pump, MDR or P-glycoprotein (P-gp), are present at high levels in the villus tip enterocytes of the small intestine, the primary site of absorption for orally administered drugs. These proteins are induced or inhibited 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. A series of studies from our laboratory of cyclosporine and tacrolimus in humans and a novel cysteine protease inhibitor in rats, dosed concomitantly with inhibitors and inducers of CYP3A4 and P-gp, suggest that gut extraction can be modeled using measures of intestinal metabolism and absorption rate, the latter reflecting changes in P-gp. Results evaluating a preliminary model applied to the CYP3A substrate drugs midazolam, indinavir, saquinavir, and rifabutin suggest that the model may be useful for predicting in vivo intestinal metabolism from in vitro data.

Introduction

Intestinal phase I metabolism and active extrusion of absorbed drug have recently been recognized as major determinants of oral drug bioavailability (e.g., Ref. [1]). Many factors are involved in oral drug delivery, yet, the measured oral bioavailability of a particular drug can be broken down into components that reflect delivery to the intestine (gastric emptying, pH, food), absorption from the lumen (dissolution, lipophilicity, particle size, active uptake), intestinal metabolism (phase I and/or phase II enzymes), active extrusion (drug efflux pumps) and finally first-pass hepatic extraction. Major advances have been implemented by pharmaceutical scientists to address the delivery and absorption issues as exemplified by the majority of the excellent papers presented at the June, 1998 symposium honoring Professor Tsuneji Nagai and published in this issue. The importance of the hepatic first pass metabolism and the ability to quantitate the hepatic extraction have been recognized since the mid-1970s [2]. Here, we will concentrate on intestinal phase I metabolism and intestinal active drug efflux.

Both cytochrome P-450 3A4 (CYP3A4), the major phase I drug metabolizing enzyme in humans, and the multidrug efflux pump, MDR or P-glycoprotein (P-gp), are present at high levels in the villus tip enterocytes of the small intestine, the primary site of absorption for orally administered drugs. These proteins are induced or inhibited 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 [3]. Clinical studies from our laboratory have demonstrated that the bioavailability of three immunosuppressive agents, cyclosporine [4], tacrolimus [5], and sirolimus [in preparation], can be increased by concomitant administration of ketoconazole, a potent CYP3A inhibitor (K_i approximately 1 μM) and an intermediate inhibitor of P-gp (K_i approximately 120 μM).

Conversely, concomitant administration of rifampin, a potent inducer of CYP3A and P-gp, markedly decreased the bioavailability of cyclosporine [6]. A recent clinical study in kidney transplant patients has indicated that variability of intestinal expression of P-gp in humans may be a more important determinant of cyclosporine bioavailability than the variability of intestinal CYP3A [7]. However, the presence of CYP3A is believed to be responsible for the decreased cyclosporine bioavailability. A series of studies in animals have indicated that inhibition of intestinal P-gp has marked effects on the bioavailability of paclitaxel [8], digoxin [9] and HIV-1 protease inhibitors [10], [11]. We have also recently demonstrated the marked increase in bioavailability of an investigational cysteine protease inhibitor when the drug was dosed concomitantly with ketoconazole to rats [12]. Most recently, we have begun to model the effects of CYP3A and P-gp on intestinal absorption and bioavailability, with the final goal of being able to use in vitro measures of drug metabolism by human intestinal CYP3A and bidirectional flux by human MDR transfected cell lines to predict in vivo the extent of gut first-pass extraction.