Pharmacodynamics is the study of what drugs do to the body and how they do it.

Dose-Response Relationships

• Basic Features of the Dose-Response Relationship:  The dose-response relationship is graded instead of all-or-nothing (as dose increases, response becomes progressively larger).

• Maximal Efficacy and Relative Potency

  • Maximal Efficacy: the largest effects that a drug can produce

  • Relative Potency:  Potency refers to the amount of drug that must be given to elicit an effect.

    • Potency is rarely an important characteristic of a drug.

    • Potency of a drug implies nothing about its maximal efficacy.

Drug-Receptor Interactions

• Introduction to Drug Receptors

  • Drug Receptor:  any functional macromolecule in a cell to which a drug binds to produce its effects.  Term is generally reserved for the group of macromolecules through which drugs act:  the body's own receptors for hormones, neurotransmitters, and other regulatory molecules.

  • At receptors, drugs mimic or block the action of the body's own regulatory molecules.  Drugs cannot give cells new functions.

• Receptors and Selectivity of Drug Action

  • If a drug interacts with only one kind of receptor, and if that receptor regulates just a few processes, then the effects of the drug will be limited.

  • Even though a drug is selective for one type of receptor, it can still produce a variety of effects.

  • Selectivity does not guarantee safety.

• Theories of Drug-Receptor Interaction

  • Simple Occupancy Theory:  Two factors - The intensity of the response to a drug is proportional to the number of receptors occupied by that drug, and the maximal response will occur when all available receptors have been occupied.

  • Modified Occupancy Theory:  Assumes that all drugs acting at a particular receptor are identical with respect to the ability to bind to the receptor and the ability to influence receptor function once binding has taken place.

    • Affinity:  The strength of the attraction between a drug and its receptor.  Affinity is reflected in potency.  (Drugs with high affinity are very potent).

    • Intrinsic Activity:  The ability of a drug to activate a receptor following binding.  Reflected in the maximal efficacy (drugs with high intrinsic activity have high maximal efficacy).

• Agonist, Antagonist, and Partial Agonists

  • Agonists:  molecules that activate receptors.  A drug that mimics the body's own regulatory processes.

  • Antagonists:  produce their effects by preventing receptors activation by endogenous regulatory molecules and drugs.  Block activation of receptors by agonists.

    • Noncompetive Antagonist:  Bind irreversibly to receptors, and reduce the maximal response that an agonist can elicit.

    •  Competitive Antagonist:  Bind reversibly to receptors, competing with agonists for binding sites.

  • Partial Agonists:  Have moderate intrinsic activity, the maximal effect that a partial agonist can produce is lower than that of a full agonist.  Act as antagonists as well as agonists.

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