Recombinant DNA technology as an investigative tool in drug metabolism research

Drug metabolism influences the pharmaco-toxicological properties of a vast array of compounds and is controlled by a complex system of drug metabolizing enzymes. A thorough understanding of this system allows the more effective development of therapeutic drugs, as well as a significant improvement of risk assessment, particularly in the field of chemical carcinogenesis. The early identification of potential therapeutic problems relating to drug metabolism could reduce the development costs for pharmaceuticals. Recently, techniques using recombinant DNA have become available for this purpose. In these approaches the genetic information for the enzyme under investigation is expressed in vitro or in vivo, following gene transfer. This approach is called heterologous expression. In addition it is possible to inactivate genes in cells and animals by homologous recombination (gene-targeting, -knock out). Heterologous expression and gene knock outs can be used to define the catalytic parameters as well as the biological role of xenobiotic metabolizing enzymes. Some heterologous expression systems supply sufficient amounts of these enzymes for structure/function analysis, thus immensely improving the prospects of rational drug design. In addition, these systems provide the basis for rapidly generating immunological tools for the selective quantitation of xenobiotic metabolizing enzymes in human tissues. This combined with the knowledge about the catalytic parameters of a particular enzyme, allows predictions on the exact role of enzymes in drug metabolism as well as drug-drug interactions to be made. However in this regard an important and unfortunately often neglected issue is the appropriate validation of the different heterologous expression systems. Transgenes have also been used to study the regulation of drug metabolizing enzymes by endogenous and exogenous substances using reporter constructs. These studies may also lead to a thorough understanding of the mechanisms underlying interindividual differences in the level of xenobiotic metabolizing enzymes. This article surveys and critically examines the applicability of the different mammalian, yeast, insect and bacterial systems for evaluating the structure, the enzymatic function, the biological role and the regulation of drug metabolizing enzymes in vitro and in vivo.