Unlike food, drugs are not usually the cellular constituents. They are therefore, eliminated from the body mainly through the kidney. Drug metabolism means the necessary changes in the drug molecules which are essential for the easy excretion from the body. These changes cause the drugs to be more water soluble and / or more ionizable. The metabolic changes make the lipid soluble drugs to be accumulated in the extra-cellular fluid and so more filterable into the renal tubule and less reabsorbable from there and hence readily excretable from the kidney, The metabolism occurs more or less in every tissue but mainly in the liver.
It is obvious that careful dosing is essential to maintain adequate but nontoxic amounts of drug in the body, a task facilitated by understanding drug pharmacokinetics. One must make a quantitative assessment of drug dose-concentration factors, including drug absorption, distribution, metabolism and excretion. Alterations in the rate of any of these processes can account for significant changes in the drug's effect on the patient. Likewise, changes in the functional status of any of the organs alter dose requirements in a given patient.
Stated simply, metabolism is the process by which your body converts food into energy. During this biochemical process, calories — from carbohydrates, fats and proteins — are combined with oxygen to release the energy your body needs to function.
The number of calories your body burns each day is called your total energy expenditure.Metabolism is the engine that burns these calories and is the scale that regulates your energy needs.
Drug metabolism can result in toxication or detoxication - the activation or deactivation of the chemical. While both occur, the major metabolites of most drugs are detoxication products.
Drugs are almost all xenobiotics. Other commonly used organic chemicals are also drugs, and are metabolized by the same enzymes as drugs. This provides the opportunity for drug-drug and drug-chemical interactions or reactions.
The following mechanisms help in drug metabolism in the body:
1. Oxidation: It is done by the microsomal and mitochondrial oxidases, examples; Barbiturates (to hydroxy barbituric acid), alcohol (to aldehyde), adrenaline (to adrenochrome), phenacetine (to Paracetamol), Salicylate (to Gentisic .acid) parathione (to paraxon), quinine (to haemotinic acid), ephedrine (to nor-ephedrine), morphine (to nor-morphine), methyl phenobarbitone (to Phenobarbitone ).
2.Reduction: The drugs like chloral hydrate (to Trichlorethanol), Folic acid (to folinic acid), Chloramphenicol, halothane, pentavalent arsenic compounds (to Trivalent arsenoxide) etc. are reduced by the microsomaJ enzymes.
3.Hydrolysis : The microsomal esterase hydrolyses acetylcholine (to choline), glycoside (to genin), atropine, pethidine ; Penicillin (to Pencilloic acid ), aspirin (to Salicylic acid), procaine (to PABA) and heroine (to morphine).
4. Conjugation or Synthesis: With glucuronic acid such as morphine, salicylate, sulphonamides, steroids, adrenaline, phenols glucuronides of these compounds occur.
With glycine, salicylic acid, benzoic acid, nicotinic acid etc. glycine conjugates of them are formed. With sulphate,Phenolic, alcoholic and aromatic amines forms their corresponding sulphates. With acetic acid, sulphonamide and nicotinic acid make their acetylated products.
Facts: It is important to recognize that childhood depression differs from adult depression and that children may respond differently than adults to antidepressant medication. These variances are due to childhood brain development processes as well as age-related differences in drug metabolism.
"CYP3A4 and CYP3A5 play important roles in the metabolism of over half of the drugs used clinically, not just drugs used in treating cancer, while GSTM1 detoxifies and removes drugs that get inside cells," Researchers said. "Whether the variations are 'good' or 'bad' would depend on the drug being given."
Drug response among patients is multifactorial, including environmental, genetic, and disease determinants that affect the disposition (absorption, distribution, metabolism, and excretion) of a given drug. The interplay of these factors determines the profile of the plasma concentration over time for a drug and, therefore, its elicited pharmacologic effect at the site of interaction with targets (such as receptors and enzymes). Too little exposure leads to an ineffective drug regimen, and too much creates the potential for adverse effects
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