What is ADME? - QS Study
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ADME – Absorption Distribution, Metabolism and Excretion.

Drugs are handled by the body in three stages: Absorption, distribution and elimination.

Absorption

Absorption is the process by which a drug moves from its site of application into the blood stream. This process arises in all forms of drug use apart from:

  • Intravenous injection: Here the drug is administered directly into the blood and consequently this route has several special pharmacokinetic properties, including this lack of any absorption stage.
  • Topical application: Here the drug is applied directly to the intended site of action and absorption into the blood stream is unnecessary. However, it is as well to remember that while absorption may not be intended, in many cases it does still arise and the drug may have effects throughout the body; corticosteroids in asthma inhalers are targeted for a topical effect within the lungs, but absorption into the rest of the body will still arise to some extent.

It might be thought that no absorption stage is required with intramuscular (i.m.) or subcutaneous (s.c.) injection as the drug has already been deposited into the body. However, we do not consider a drug to have been absorbed until it reaches the blood, so even an i.m. or s.c. dose needs to undergo absorption into the blood.

 

Distribution

The next phase of drug handling is distribution. This refers to the movement of drug backwards and forwards between blood and the various tissues of the body. This is illustrated in below figure. Three tissues have been indicated as examples, but the real situation is much more complex, with many different tissues involved.

Notice that we assume all movement from one tissue to another occurs exclusively via the blood. So, if drug is to move from muscle to the liver, then it will do so by moving from the muscle into the bloodstream, then the blood travels to the liver and discharges the drug into that organ. Exceptions to this rule are rare – for example, following oral administration, very lipid soluble drugs may be absorbed into the lymphatic system, which then acts as the transport medium.

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Elimination

Elimination is a general term referring to drug removal from the body by any mechanism. A sharp distinction is drawn between two different forms of elimination:

  • Excretion: This is the simple removal of the intact drug molecule from the body. If a patient is treated with the aminoglycoside antibiotic gentamicin, most of the dose will reappear in the urine in an unchanged form and this is a true example of excretion.
  • Metabolism: Here the drug is destroyed by chemically alteration. The metabolites will then probably be disposed of in the urine or bile, but this overall process is referred to as ‘metabolism not ‘excretion’. The latter term is restricted to those cases where the drug is eliminated unchanged. Following a dose of a drug such as theophylline, very little unchanged drug will be present in the urine – the vast majority is metabolized.

It is because of this sharp distinction between two different forms of elimination, that we normally refer to the ADME’ of a drug – its Absorption, Distribution, Metabolism and Excretion.