USMLE1 Pharmacodynamics-1

Pharmacodynamics  described as what a drug does to the body, involves….

•       receptor binding

•      postreceptor effects

•      chemical interactions

Receptor: is the component of a cell or organism that interacts with a drug and initiates the chain of biochemical events leading to the drug's observed effects.

Drug  +  Receptor à Drug-receptor complex-àResponse

The magnitude of drug effect depends on its concentration at the receptor site, which in turn is determined by the dose of drug administered and by factors characteristic of the drug such as

▪Rate of absorption

▪Distribution and

▪Metabolism.

Pharmacodynamic characteristics of drug:

Agonist: Is defined as an agent that can bind to a receptor and elicit a response.

2 types:

·         Full agonist

·         Partial agonist

Full agonists: produce a maximal response, they have maximal efficacy.Partial agonists are incapable of eliciting a maximal response and are less effective than full agonists.

Antagonist: they interfere with the ability of an agonist to activate the receptor and blocking their biologic actions.

Efficacy: Is the maximum response produced by the drug.It depends on the number of drug-receptor. Efficacy is a Measure of how well a Drug produces a response (Effectiveness), shown by the Maximum Height reached by the Curve on X Axis.complexes formed.

Potency: potency is measured of how much drug is required to elicit a given response.The lower the dose is required for a given response, the more potent the drug.

The primary determinant of potency  is the affinity of the drug for the receptor.

Dose-response relationship:

Dose:  the amount of drug required to elicit a biologic response.

Dose-response relationship:  the intensity of the response elicited by a drug is proportional to the dose administered.

Two types :

Graded dose-response curve : for maximum response.

Quantal dose-response curve : for specific responses, e.g.ED50, LD50, TD50

Affinity: Ability of the drug to bind to the receptor, shown by the  proximity of  the curve to

the y axis; the nearer  the y axis, the greater  the affinity.

When two drugs interact with the same receptor, the D-R curves will have parallel slopes.

Affinity (ability of the drug to bind to the receptor) can be compared only when two drugs bind to the same receptor.

Duality of Partial Agonists:

In the presence of full agonist , partial agonist will act as antagonist. As the partial agonist displaces the full agonist from the receptor,the response is reduced-the partial agonist is acting as an Antagonist.