Pharmacodynamics

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Pharmacodynamics
Mechanisms of drug action:
Drugs may act by:
* A) Receptor mechanism:
* Most drugs produce their action by binding to specific receptors located on the cell membrane (as adrenoceptors and cholinoceptors) or inside the cell (as steroid receptors). * This is the most important mechanism of drug action.

* Drugs produce their effects by interacting with these receptors. * These drugs may be:
a) Agonists:
* Drugs have the affinity to bind with their specific receptors and Mimic (produce an action similar to the action of) the endogenous regulatory substance are called agonists. * Example:
* salbutamol which is used in the treatment of bronchial asthma acts by binding to beta adrenergic receptors located on the smooth muscle of the lung producing bronchodilatation.
b) Antagonists:
* Drugs that bind to the receptors without producing effects but block the function of endogenous agonists. * Example:
* prazosin which is used in the treatment of hypertension acts by blocking the action of endogenous catecholamines on alpha adrenergic receptors by occupation of these receptors * B) Non-receptor mechanisms:

1- Chemical interaction as the antacids which neutralize gastric acidity and the chelating agents which bind different metal cations.
2- Physical alterations of cellular environment:
* i-Alteration of osmosis: e.g., I.V. mannitol to reduce cerebral edema and oral magnesium sulphate as cathartic. * ii-Lubrication: e.g., mineral oil administered by mouth to facilitate passage of stool. * iii-Adsorption: e.g., activated charcoal given orally to reduce the absorption of harmful toxins in case of poisoning. Also, Kaolin and pectin can adsorb toxins that cause diarrhea.

3- Enzymatic alteration
* Drugs altering enzyme activity and this alter processes catalyzed by these enzymes. * Examples:
Cholinesterase inhibitors
Monoamine oxidase inhibitors
4- Anti-metabolites:
* - Some drugs resemble the structure of normal biological cell constituents and so can enter biochemical reactions in place of these constituents resulting in formation of inactive products. * Example:

* Some anti-neoplastics.
* Some anti-infectives as sulphonamides.
TYPES OF DRUG ACTIONS
* 1) Desired : Therapeutic action or intended action.

* 2) Undesired: untoward, adverse drug reactions.
* Almost every drug exerts both the main (desired) or wanted effects and the (undesired), unwanted, or adverse effects. Adverse drug reactions (A.D.R.) are divided into: * a) Side effects: Such unwanted effects resulting from normal pharmacological action at clinical doses e.g., drowsiness caused by antihistaminic drugs. * b) Toxic effects: resulting from overdose i.e., dose-dependent. They are usually the amplification of normal pharmacological actions e.g., severe hypotension by antihypertensive drugs. All drugs are capable of producing toxic effects at high doses.

These toxic reactions may be:
* a) Local toxicity: occurs at the site of first contact between the biological system and the drug e.g., skin, mucous membranes, GIT, etc. * b) Systemic toxicity: occurs after systemic absorption and distribution of the drug e.g., hepatic injury, kidney damage, aplastic anemia, neuritis, severe hypotension, severe hypoglycemia, brain damage, coma, heart block, shock, etc. * Most of adverse drug reactions (which are dose-dependent) are predictable.

some of adverse drug reactions are unpredictable and dose-independent such as: * a) Allergic reactions: as anaphylactic shock following the administration of certain drug in sensitive patients. * b) Idiosyncratic reactions: abnormal response that occur in some individuals due to genetic abnormalities. * For example: succinylcholine apnea in patients with deficiency in pseudocholinesterase enzyme.

Therapeutic Index

The dose of...
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