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Pharmacology is the body of knowledge concerned with the action of chemicals on biologic systems. Medical pharmacology is the area of pharmacology concerned with the use of chemicals in the prevention, diagnosis, and treatment of disease, especially in humans. Toxicology is the area of pharmacology concerned with the undesirable effects of chemicals on biologic systems. Pharmacokinetics describes the effects of the body on drugs, eg, absorption, excretion, etc. Pharmacodynamics denotes the actions of the drug on the body, such as mechanism of action and therapeutic and toxic effects. This chapter introduces the basic principles of pharmacokinetics and pharmacodynamics that will be applied in subsequent chapters.

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The Nature of Drugs

Drugs in common use include inorganic ions, nonpeptide organic molecules, small peptides and proteins, nucleic acids, lipids, and carbohydrates. Some are found in plants or animals, but many are partially or completely synthetic. Many biologically important endogenous molecules and exogenous drugs are optically active; that is, they contain one or more asymmetric centers and can exist as enantiomers. The enantiomers of optically active drugs usually differ, sometimes more than 1000-fold, in their affinity for their biologic receptor sites. Furthermore, such enantiomers may be metabolized at different rates in the body, with important clinical consequences.

Size and Molecular Weight

Drugs vary in size from molecular weight (MW) 7 (lithium) to over MW 50,000 (thrombolytic enzymes, other proteins). Most drugs, however, have molecular weights between 100 and 1000. Drugs smaller than MW 100 are rarely sufficiently selective in their actions, whereas drugs much larger than MW 1000 are often poorly absorbed and poorly distributed in the body.

Drug-Receptor Bonds

Drugs bind to receptors with a variety of chemical bonds. These include very strong covalent bonds (which usually result in irreversible action), somewhat weaker electrostatic bonds (eg, between a cation and an anion), and much weaker interactions (eg, hydrogen, van der Waals, and hydrophobic bonds).

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DrugsSubstances that act on biologic systems at the chemical (molecular) level and alter their functions
Drug receptorsThe molecular components of the body with which drugs interact to bring about their effects
Distribution phaseThe phase of drug movement from the site of administration into the tissues
Elimination phaseThe phase of drug inactivation or removal from the body by metabolism or excretion
Endocytosis, exocytosisEndocytosis: Absorption of material across a cell membrane by enclosing it in cell membrane material and pulling it into the cell, where it can be released. Exocytosis: Expulsion of material from vesicles in the cell into the extracellular space
PermeationMovement of a molecule (eg, drug) through the biologic medium
PharmacodynamicsThe actions of a drug on the body, including receptor interactions, dose-response phenomena, and mechanisms of therapeutic and toxic actions
PharmacokineticsThe actions of the body on the drug, including ...

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