Enzyme kinetics in drug metabolism: fundamentals and applications.

Enzymes are protein catalysts that lower the energy barrier for a reaction and speed the rate of a chemical change. The kinetics of reactions catalyzed by enzymes, as well as several mechanisms underlying the kinetics, have been comprehensively studied and written in textbooks (1, 2). The importance of quantitative evaluation of enzymatic processes has been recognized in many fields of study, including biochemistry, molecular biology, and pharmaceutical sciences to name a few. In pharmaceutical sciences, the applications of enzyme kinetics range from hit finding efforts for new chemical entities on a pharmacological target to concentration effect relationships to large-scale biosynthesis. The study of the science of drug metabolism has two principal concepts-rate and extent. While understanding disposition pathways and identification of metabolites provides an insight into the extent of metabolism, kinetics of depletion of substrates (endogenous or exogenous) and formation of metabolites deals with the rate of metabolism. The current textbook specifically focuses on kinetics of drug-metabolizing enzymes, detailing specific enzyme classes, and discusses kinetics as they apply to drug transporters. This textbook also outlines additional factors that contribute to the kinetics of reactions catalyzed by these proteins such as variability in isoforms (pharmacogenomics) and experimental factors including key concepts such as alterations of substrate concentrations due to binding. Applications of these approaches in predicting kinetic parameters and alternative approaches for enzymes (systems biology) and transporters are also discussed. The final section focuses on real-life examples (case studies) to try and exemplify the applications of enzyme kinetic principles. This chapter provides a brief overview outlining some key concepts within each of the sections and the chapters within this textbook.