PHARMACODYNAMICS

Pharmacodynamics is a branch of pharmacology that deals with the study of the physiological & biochemical effects of the drug in the living Organisms, their reaction & ability to bind with the living cells and produce therapeutic effects.

DRUG ACTION

• The drugs that enter the human tend to stimulate certain receptors, ion channels, act on enzymes or transporter proteins. As a result, they cause the human body to react in a specific way. The types of drug action are as follows:
• Stimulation
• Depression
• Stabilization
• Replacement
• Scavenging Action
• Irritation
• Modification

Stimulation: These are agonists action that stimulate & activate the receptors.

·         E.g.. Adrenaline stimulates the heart

Depression: They are Antagonists that stop the agonists from stimulating the receptors to prevent hyper stimulation of the receptors.

·         E.g.. Quinidine depresses the heart & Barbiturates depress the CNS

Stabilization: These drugs are neither a stimulant or an depressant but some drugs possess receptor activity that allows them to stabilize general receptor activation.

·         E.g.. Buprenorphine in opioid dependent persons

Replacement / Exchange: These substances are accumulated to form a reserve or storage.

·         E.g.. Glycogen stores

Scavenging Action: These are compounds which have direct beneficial action by producing free radicals that forms chain reaction which may damage the cells of the organism.

·         Eg. Antioxidants

Irritation/ Destruction: They have direct harmful chemical reaction which might result in damage or destruction of the cells, through induced toxic or lethal damage.

·         Eg. Anti-microbial Action

Modification: These act by modifying the Immune Status either by enhancing & depressing the Immune Status.

·         Eg. Vaccines, Sera & Steroids.

Therapeutic Effects

• The therapeutic effect of the drug refers to the initial consequence of a drug-receptor interaction, and the subsequent drug effects.
• Potency
• Efficacy
• Selectivity
• Specificity
• Affinity
• Therapeutic Index

Potency: Potency refers to the concentration or dose of a drug required to produce 50% of the drug’s maximal effect. Potency depends on both the affinity of a drug for its receptor and the efficiency with which drug-receptor interacts to produce the response.

Efficacy: Efficacy (also referred to as intrinsic activity) of a drug is the ability of the drug to elicit a response when it binds to the receptor. The changes in receptors as a result of drug occupancy initiate biochemical and physiologic events that produce the response.

Selectivity: Selectivity refers to a drug’s ability to preferably produce a particular effect and is related to the structural specificity of drug binding to receptors.

·         E.g.. Cycloxygenase-2 produces selective action at the sites of Inflammation by inducing enzymes.

Specificity: Specificity of drug action relates to the number of different mechanisms involved. Egs. of specific drugs include atropine (a muscarinic receptor antagonist), salbutamol (a β2-adrenoceptor agonist).

·         By contrast, nonspecific drugs result in drug effects through several mechanisms of action.  E.g., phenothiazine causes blockade of D2-dopamine receptors, α-adrenergic receptors, and muscarinic receptors.

Affinity:  The affinity of a drug for a receptor describes how avidly the drug binds to the receptor.

·         E.g.. fluoroquinolones acting on bacteria through covalent binding of the drug to receptor.

Therapeutic Index: The therapeutic index of a drug is the ratio of the dose that results in an undesired effect to the dose that results in a desired effect.