The use of adult human hepatocytes in primary culture and other in vitro systems to investigate drug metabolism in man

Among the numerous enzyme systems involved in the metabolism of xenobiotics, cytochromes P450 (CYP) from families CYP1, 2 and 3 play a prominent role. These cytochromes are monoxygenases mainly expressed in the liver. They are able to oxidize an apparently unlimited number of compounds and, on some occasions, generate cytotoxic or genotoxic metabolites responsible for various pathologies including hepatitis and chemical carcinogenesis. The expression and function of these cytochromes might be affected by a number of factors including, physiological (hormones, growth factors, cytokines, etc.), pathological (infections, inflammation, hepatectomy, etc.), genetic (polymorphism of expression or function) and environmental (drugs, diet compounds, pollutants) factors. These various properties account for the wide interindividual variability exhibited by the human populations in response to drugs and environmental pollutants in terms of metabolism and toxicity. From the analysis of a number of clinical reports focusing on adverse drug effects and from the above considerations, it appears that answering the following questions is absolutely required before a new drug is administered to man with maximum safety: (1) What is the metabolic pathway of the drug and what are the main metabolites? (2) Which enzyme system is involved in the metabolism of the drug? (3) Is the drug an inducer or inhibitor of drug metabolizing enzymes? (4) What are the possible drug interactions? (5) Can the drug be activated to cytotoxic or genotoxic metabolites? In this chapter, I shall describe the various human hepatocyte culture systems used in this and other laboratories, and show how the use of these cultures, in combination with the other in vitro systems including human liver microsomes, may help to answer the above questions concerning several drugs including: diazepam, ketotifen, zolpidem, omeprazole, lansoprazole, cyclosporin A, clometacin and cyproterone acetate. Emphasis will be placed on the comparison between the results obtained in vitro and the situation in man in vivo, as well as on the prediction, confirmation and/or a posteriori explanation of clinical observations.