introduction

Introduction to Pharmacology

At the end of this lecture, the students will be able to:

1.Define receptors
2.Define agonists
3.List types of agonists
4. Define antagonists
5. List types of antagonists

Objectives:

Define as any functional molecule in a cell to which a drug binds to produce its effect (target molecule for drug action).

Drug Receptors

Affinity: The term affinity refers to the strength of the attraction between a drug and its receptor.


Intrinsic Activity: The term intrinsic activity refers to the ability of a drug to activate a receptor following binding.

Agonists:

Agonists are molecules that binds to receptors and activate it.
They initiates biochemical and physiological changes inside the cells that leads to drug's effect or response.

Examples: Cholinergic agonists such as carbachol

Agonists, Antagonists, Partial Agonists:

Antagonist bind to receptors but do not activate it. i.e. they have no effects of their own on receptor function.
They have affinity for a receptor but with no intrinsic activity.

Examples: Antihistamines: block histamine H1 receptors.

Antagonists:

Partial agonist is an agonist that has only moderate intrinsic activity. As a result the maximal effect that a partial agonist can produce is lower than that of a full agonist.
Pentazocin is an example of a partial agonist.
Partial Agonists:


Down regulation:
When the receptors of a cell are continuously exposed to an agonist, the cell usually becomes less responsive.

Up regulation:

Continuous exposure to antagonists has the opposite effect, causing the cell to become hypersensitive (supersensitive).
Down regulation and up regulation:

The effect of one drug or an endogenous substance is diminished or completely abolished in the presence of another drug.

Types:

Chemical antagonism
Pharmacokinetic antagonism
Antagonism by receptor block
Physiological antagonism
Drug Antagonism:

Examples: Antacids antagonize the action of Hcl in the stomach.

Chemical antagonism:

Pharmacokinetic antagonism describes the situation in which a drug reduces the concentration of the other drug at its site of action.


Warfarin degradation is increased by drugs that accelerate its hepatic metabolism such as phenobarbitone
Pharmacokinetic antagonism:

When one drug block a receptor preventing other drug or endogenous substance from binding to this receptor.

Examples: Drugs that block adrenergic receptors such as propranolol

Antagonism by receptor block:

Physiological antagonism occur when one drug antagonize the physiological action of another drug or endogenous substance.

Examples: Histamine acts on receptors of the parietal cells of the gastric mucosa to stimulate acid secretion, while omeprazole blocks this effect
Physiological Antagonism:

MEC is defined as the plasma drug level below which therapeutic effects will not occur.

Minimum effective concentration (MEC):

Therapeutic range is a plasma drug levels falling between the MEC and the toxic concentration
Therapeutic Range

The therapeutic index is a measure of a drug's safety. It is defined as the ratio of a drug's LD 50 to ED 50.


A large therapeutic index indicates that a drug is relatively safe.

Conversely, a small therapeutic index indicates that a drug is relatively unsafe.

The Therapeutic index(TI):