Pharmacokinetics

Abstract

Pharmacokinetics, derived from the Greek words “pharmakon” (drug) and “kinesis” (movement), is the study of the time course of drugs in a biological system. This chapter discusses the major aspects of pharmacokinetics as it applies to drugs in humans. Absorption is the process of drugs entering the bloodstream. This may occur by diffusion, facilitated diffusion, or active transport. There are multiple routes of drug administration including oral, inhaled, intravenous, intramuscular, rectal, oral mucosal, intrathecal, dermal, ocular, and intranasal. Once in the bloodstream, drugs are distributed throughout the body. Distribution is dependent on plasma protein binding, tissue perfusion, and pH characteristics. Elimination of drugs occurs either through metabolism or excretion. Metabolism is the process of changing the chemical structure of the drug to better enable removal from the body. Metabolism may be Phase I where changes in functional groups on the drug are made, or Phase II where a drug or Phase I metabolite is conjugated with an endogenous substance. Excretion is the final removal of the drugs and/or metabolic products from the body, either into the bile, urine, or other means. Mathematical models that describe the pharmacokinetics of drugs are also included.