In Vitro Enzyme Kinetics Applied to Drug-Metabolizing Enzymes
Most new drugs enter clinical trials with varying amounts of information on the human enzymes that may be involved in their metabolism. Most of this information is obtained from (1) animal studies, (2) human tissue preparations in conjunction with chemical inhibitors or antibodies, and (3) expressed enzymes. This chapter will focus on the techniques used to characterize the in vitro metabolism of drugs. Although many enzymes may play some role in drug metabolism, this chapter will focus on the cytochrome P450 (P450) enzymes. The P450 superfamily of enzymes represents the most important enzymes in the metabolism of hydrophobic drugs and other foreign compounds, and many drugdrug interactions result from altering the activities of these enzymes (1). Although not studied as extensively as the P450 enzymes, other drug-metabolizing enzymes, transporters, and xenobiotic receptors share a characteristics that is relatively unique in biochemistry: broad substrate selectivity. This versatility has a profound influence on the enzymology and kinetics of these proteins. Therefore, many of the techniques described for the P450s may apply to other drug-metabolizing enzymes, transporters, and xenobiotic receptors as well.