🔴Competitive antagonism 🔴

📌Reversible competitive antagonism is the commonest and most important type of antagonism ; it has two main characteristics:

–in the presence of the antagonist, the agonist log concentration–effect curve is shifted to the right without change in slope or maximum, the extent of the shift being a measure of thedose ratio

–the dose ratio increases linearly with antagonist concentration

📌•Antagonist affinity, measured in this way, is widely used as a basis for receptor classification.

🔴Agonists, antagonists and efficacy🔴

📌•Drugs acting on receptors may be agonists or antagonists.

📌•Agonists initiate changes in cell function, producing effects of various types; antagonists bind to receptors without initiating such changes.

📌•Agonist potency depends on two parameters: affinity (i.e.tendency to bind to receptors) and efficacy (i.e. ability, once bound, to initiate changes that lead to effects).

📌•For antagonists, efficacy is zero.

📌•Full agonists (which can produce maximal effects) have high efficacy; partial agonists (which can produce only submaximal effects) have intermediate efficacy.

📌•According to the two-state model, efficacy reflects the relative affinity of the compound for the resting and activated states of the receptor. Agonists show selectivity for the activated state; antagonists show no selectivity. This model, although helpful, fails to account for the complexity of agonist action. •Inverse agonists show selectivity for the resting state of the receptor, this being of significance only in situations where the receptors show constitutive activity.

📌•Allosteric modulators bind to sites on the receptor other than the agonist binding site and can modify agonist activity.

🔴Types of drug antagonism🔴
Drug antagonism occurs by various mechanisms:

📌 •Chemical antagonism (interaction in solution)

📌 •Pharmacokinetic antagonism (one drug affecting the absorption, metabolism or excretion of the other)

📌 •Competitive antagonism (both drugs binding to the same receptors) ; the antagonism may be reversible or irreversible

📌•Interruption of receptor–response linkage

📌•Physiological antagonism (two agents producing opposing physiological effects).

🔴Targets for drug action🔴

📌•A drug is a chemical applied to a physiological system that affects its function in a specific way.

📌 •With few exceptions, drugs acton target proteins, namely:
–receptors
–enzymes
–carriers
–ion channels.

📌 •The term receptor is used in differen tways. In pharmacology, it describes protein molecules whose function is to recognise and respond to endogenous chemical signals. Other macromolecules with which drugs interact to produce their effects arek nown as drug targets.

📌 •Specificity is reciprocal: individual classes of drug bind only to certain targets, and individual targets recognise only certain classes of drug.

📌•No drugs are completely specific in their actions. In many cases, increasing the dose of a drug will cause it to affect targets other than the principal one, and this can lead to side effects.

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