Introduction to Pharmacokinetics

Pharmacokinetics (drug kinetics) deals with the in vivo fate or disposition processes of the drug (i.e., how the body does to the drug) that determine drug exposure in the body and therefore pharmacological effects. In general, drug molecules undergo four main disposition processes in the body, namely absorption, distribution, metabolism, and excretion (ADME, the so-called four components of pharmacokinetics). Pharmacokinetic study is an integral part of drug research and development. Poor pharmacokinetic property (e.g., poor absorption, fast clearance, and toxic metabolite) is reported to be one of the main causes of drug attrition. Pharmacokinetic behaviors are influenced by various physiological factors such as blood flow, gastric motility, hepatic enzyme activity, and renal function. The time of administration is an additional variable influencing the pharmacokinetics (dosing time-dependent pharmacokinetics is defined as chronopharmacokinetics). In this chapter, we introduce the ADME processes and discuss the factors affecting the pharmacokinetics.