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Pharmacodynamics deals with the effects of drugs on biologic systems, whereas pharmacokinetics (Chapter 3) deals with actions of the biologic system on the drug. The principles of pharmacodynamics apply to all biologic systems, from isolated receptors in the test tube to patients with specific diseases.

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ReceptorA molecule to which a drug binds to bring about a change in function of the biologic system
Inert binding molecule or siteA molecule to which a drug may bind without changing any function
Receptor siteSpecific region of the receptor molecule to which the drug binds
Spare receptorReceptor that does not bind drug when the drug concentration is sufficient to produce maximal effect; present when Kd > EC50
EffectorComponent of a system that accomplishes the biologic effect after the receptor is activated by an agonist; often a channel or enzyme molecule, may be part of the receptor molecule
AgonistA drug that activates its receptor upon binding
Pharmacologic antagonistA drug that binds without activating its receptor and thereby prevents activation by an agonist
Competitive antagonistA pharmacologic antagonist that can be overcome by increasing the concentration of agonist
Irreversible antagonistA pharmacologic antagonist that cannot be overcome by increasing agonist concentration
Physiologic antagonistA drug that counters the effects of another by binding to a different receptor and causing opposing effects
Chemical antagonistA drug that counters the effects of another by binding the agonist drug (not the receptor)
Allosteric agonist, antagonistA drug that binds to a receptor molecule without interfering with normal agonist binding but alters the response to the normal agonist
Partial agonistA drug that binds to its receptor but produces a smaller effect at full dosage than a full agonist
Inverse agonistA drug that binds to the inactive state of receptor molecules and decreases constitutive activity (see text)
Graded dose-response curveA graph of increasing response to increasing drug concentration or dose
Quantal dose-response curveA graph of the fraction of a population that shows a specified response at progressively increasing doses
EC50, ED50, TD50, etcIn graded dose-response curves, the concentration or dose that causes 50% of the maximal effect or toxicity. In quantal dose-response curves, the concentration or dose that causes a specified response in 50% of the population under study
KdThe concentration of drug that binds 50% of the receptors in the system
Efficacy, maximal efficacyThe maximal effect that can be achieved with a particular drug, regardless of dose
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Receptors are the specific molecules in a biologic system with which drugs interact to produce changes in the function of the system. Receptors must be selective in their ligand-binding characteristics (so as to respond to the proper chemical signal and not to meaningless ones). Receptors must also be modifiable when they bind a drug molecule (so as ...

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