The use of isolated enterocytes to study Phase I intestinal drug metabolism: validation with rat and pig intestine

An important step in the development of new drugs is to evaluate the extent of their metabolism during absorption in the small intestine. Reliable in vitro systems to do this can expediate the development process, but the current systems are often unsuitable because they lack the appropriate metabolic enzymes (e.g. Caco-2 cell monolayers) or are not representative of the physiological conditions present in the intact intestinal cells (e.g. isolated microsomes). The aim of this study was to validate the use of isolated intestinal epithelial cells (enterocytes), equivalent to hepatocytes, to evaluate Phase I drug metabolism. A method was developed to prepare enterocytes from rat and pig (as metabolically closer to man) that maintained good viability and activity for up to 90 min as judged by trypan blue exclusion and the release of the cytosolic enzyme lactate dehydrogenase. The Phase I metabolism of the established marker drugs: midazolam, bupropion and dextromethorphan were measured by LC-MS and confirmed the activities of the 3A, 2B and 2D families of CYP isoforms, respectively. The kinetic parameters, K(m) and V(max), were compared between isolated cells and isolated intestinal microsomes from the rat. The use of isolated intestinal cells is a simple and practical method to study the Phase I metabolism of drugs during their absorption and the potential for drug-drug interactions. The method could eventually be modified and usefully applied to human studies.