Pharmacokinetics, sometimes described as what the body does to a drug, refers to the movement of drug into, through, and out of the body—the time course of its absorption Drug Absorption Drug absorption is determined by the drug’s physicochemical properties, formulation, and route of administration. Dosage forms (eg, tablets, capsules, solutions), consisting of the drug plus... read more , bioavailability Drug Bioavailability Bioavailability refers to the extent and rate at which the active moiety (drug or metabolite) enters systemic circulation, thereby accessing the site of action. Bioavailability of a drug is... read more , distribution Drug Distribution to Tissues After a drug enters the systemic circulation, it is distributed to the body’s tissues. Distribution is generally uneven because of differences in blood perfusion, tissue binding (eg, because... read more , metabolism Drug Metabolism The liver is the principal site of drug metabolism (for review, see ). Although metabolism typically inactivates drugs, some drug metabolites are pharmacologically active—sometimes even more... read more , and excretion Drug Excretion The kidneys are the principal organs for excreting water-soluble substances. The biliary system contributes to excretion to the degree that drug is not reabsorbed from the gastrointestinal ... read more .
Pharmacodynamics Overview of Pharmacodynamics Pharmacodynamics (sometimes described as what a drug does to the body) is the study of the biochemical, physiologic, and molecular effects of drugs on the body and involves receptor binding... read more , described as what a drug does to the body, involves receptor binding, postreceptor effects, and chemical interactions. Drug pharmacokinetics determines the onset, duration, and intensity of a drug’s effect. Formulas relating these processes summarize the pharmacokinetic behavior of most drugs (see table Formulas Defining Basic Pharmacokinetic Parameters Formulas Defining Basic Pharmacokinetic Parameters ).
Pharmacokinetics of a drug depends on patient-related factors as well as on the drug’s chemical properties. Some patient-related factors (eg, renal function, genetic makeup, sex, age) can be used to predict the pharmacokinetic parameters in populations. For example, the half-life of some drugs, especially those that require both metabolism and excretion, may be remarkably long in older people (see figure Comparison of pharmacokinetic outcomes for diazepam in a younger man [A] and an older man [B] Comparison of pharmacokinetic outcomes for diazepam in a younger man (A) and an older man (B) ). In fact, physiologic changes with aging affect many aspects of pharmacokinetics (see Pharmacokinetics in Older Adults Pharmacokinetics in Older Adults Pharmacokinetics is best defined as what the body does to the drug; it includes Absorption Distribution across body compartments Metabolism Excretion read more and Pharmacokinetics in Children Pharmacokinetics in Children Pharmacokinetics refers to the processes of drug absorption, distribution, metabolism, and elimination. There are important age-related variations in pharmacokinetics. Absorption from the gastrointestinal... read more ).
Other factors are related to individual physiology. The effects of some individual factors (eg, renal failure, obesity, hepatic failure, dehydration) can be reasonably predicted, but other factors are idiosyncratic and thus have unpredictable effects. Because of individual differences, drug administration must be based on each patient’s needs—traditionally, by empirically adjusting dosage until the therapeutic objective is met. This approach is frequently inadequate because it can delay optimal response or result in adverse effects.
Knowledge of pharmacokinetic principles helps prescribers adjust dosage more accurately and rapidly. Application of pharmacokinetic principles to individualize pharmacotherapy is termed therapeutic drug monitoring.
Comparison of pharmacokinetic outcomes for diazepam in a younger man (A) and an older man (B)
Diazepam is metabolized in the liver to desmethyldiazepam through P-450 enzymes. Desmethyldiazepam is an active sedative, which is excreted by the kidneys. 0 = time of dosing. (Adapted from Greenblatt DJ, Allen MD, Harmatz JS, Shader RI: Diazepam disposition determinants. 27:301–312, 1980.)