Ex vivo preparations of human tissue for drug metabolism, toxicity and transport

Before new drugs are allowed on the market, their safety and metabolite profile should be extensively tested, as often reactive metabolites are the ultimate toxicant. The exposure of the target cell to the drug and its metabolites is determined by the expression levels of the transporters and the metabolic enzymes in the target cells. Thus to predict drug-induced toxicity not only the intrinsic biological effects of the drug but also the exposure to the ultimate toxicant needs to be predicted. In the past decades, in vitro techniques were increasingly used to predict drug-induced toxicity, not only to reduce the use of experimental animals and to obtain insight in the mechanism, but also to be able to obtain human-specific information by using human cells. Although cellular and subcellular models have been proven to be very useful, they do not fully represent the complex intact tissues, and very often the tissue-specific differentiation is lost in cell lines. In the liver the presence of the Kupffer, stellate, biliary epithelial and endothelial cells are of importance for the toxic effects of drugs. In the intestine the situation is even more complex as the expression levels of the relevant transporters and metabolic enzymes show significant gradients along the length of the intestine. Therefore ex vivo tissue models have proven to be useful to fill the gap between in vitro cell models and the in vivo situation. In this lecture the use of ex vivo models of liver and intestinal tissue are presented. In precision-cut tissue slices, prepared from liver or intestine, all the cell types are present in their original conformation and cell-cell and cell-matrix interactions are intact. Their viability is maintained for up to several days and metabolism and toxic effects can be studied using innovative analytical methods including omics technologies. The Ussing Chamber technique allows to study the vectorial transport of drugs from the lumen to the serosal side and vice versa in addition to their metabolism and toxicity. These techniques can be applied to animal and human tissue and allow to study interspecies differences under identical experimental circumstances. Recent improvements in the technology and applications in prediction of toxicity will be discussed