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 t he more effective development of therapeutic drugs, as well as a signi ficant improvement of risk assessment, particularly in the field of ch emical carcinogenesis. The early identification of potential therapeut ic problems relating to drug metabolism could reduce the development c osts for pharmaceuticals. Recently, techniques using recombinant DNA h ave 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 heterolog ous expression. In addition it is possible to inactivate genes in cell s and animals by homologous recombination (gene-targeting, -knock out) . Heterologous expression and gene knock outs can be used to define th e catalytic parameters as well as the biological role of xenobiotic me tabolizing enzymes. Some heterologous expression systems supply suffic ient amounts of these enzymes for structure/function analysis, thus im mensely improving the prospects of rational drug design. In addition, these systems provide the basis for rapidly generating immunological t ools for the selective quantitation of xenobiotic metabolizing enzymes in human tissues. This combined with the knowledge about the catalyti c parameters of a particular enzyme, allows predictions on the exact r ole 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 heterol ogous expression systems. Transgenes have also been used to study the regulation of drug metabolizing enzymes by endogenous and exogenous su bstances using reporter constructs. These studies may also lead to a t horough understanding of the mechanisms underlying interindividual dif ferences in the level of xenobiotic metabolizing enzymes. This article surveys and critically examines the applicability of the different ma mmalian, yeast, insect and bacterial systems for evaluating the struct ure, the enzymatic function, the biological role and the regulation of drug metabolizing enzymes in vitro and in vivo.