Metabolism of drugs

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Metabolism of drugs is a complex and major process within the body, occurring primarily in the liver. The aim of metabolism is to make the drug more polar to enable excretion via the kidneys. The basic understanding of drug metabolism is paramount to ensure drug optimisation, maximum therapeutic benefits and a reduction in adverse effects.

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Essentially drug metabolism is broken down into two phases, Phase I and Phase II. Phase I is concerned with the biotransformation of compounds, and then transferred to Phase II. However, for some drugs this is the end of their metabolic journey in the body, as they produce more polar compounds which are readily excreted. Phase II reactions are where compounds are conjugated to produce more water soluble compounds for easy excretion. Phase I reactions are dominated by the Cytochrome-450 enzyme superfamily. These enzymes are found predominantly in the liver, which is the major site of drug metabolism. However, drug metabolism is not localised merely to the liver, there are other major sites at which this process occurs. Some of these sites include the skin, lungs, gastro-intestinal tract and the kidneys; close to all tissues have the ability to metabolise drugs due to the presence of metabolising enzymes. The most important enzymes are the cytomchrome-450 superfamily, which are abundant in most tissues.

Inactive drugs with the ability to reconvert to the active parent drug once metabolised to exert their therapeutic actions are defined as prodrugs. They are classified depending on the site of conversion and actions (gastrio-intestinal fluids, intracellular tissues or blood). This report gives different study examples of such prodrugs and how their metabolism differs within the body, compared to their active metabolites. Individual drug metabolism may be affected by variant factors, such as, age or sex. Drug metabolism can cause an increase in toxcity. The bioactivation of a parent compound can form electrophiles that bind to proteins and DNA. Some of this toxicity can occur in Phase I metabolism e.g. acetaminophen. However, in some circumstances toxicity occurs in Phase II e.g. zomepirac, polymorphism can also cause idiosyncracity of certain drugs to be toxic.

1.1 Phase I

Phase one, otherwise known as drug biotransformation pathway is generally broken into oxidation, reduction and hydrolysis. A reaction under this phase involves an addition of oxygen molecule aiming to improve the water solubility of drugs. As the result some metabolites from this phase can be extracted immediately if they are polar enough however at times a single addition of oxygen is not sufficient enough to overcome the lipophilicity of certain drugs and hence their metabolite from this phase has to be carried onto phase II for further reactions.

Major example of Oxidation:

Accounting for roughly 20 complex reactions the most important oxidative metabolic pathway dominating phase I is the cytochrome-P450 (CYP450) monooxygenase system processed by C-P450.

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