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Wednesday, August 25, 2010

Drug protein binding drug absorption distribution

Why a drug is bound to protein, What is protein binding?  What is drug absorption , distribution ?

We are trying to answer these questions asked by our readers in a very short manner as far as possible.
Summery:
When a drug is administered it get absorbed from GIT and it is goes to liver and then it is distributed in to all part of body , while it is being absorbed , some times some enzymes act up on drugs and bring about degradation and further a drug is also acted up on by liver enzymes which causes a drug to get degraded , and loose its activity , this phenomenon is also known as first pass effect , which we have already discussed in our earlier post.

This phenomenon of first pass metabolism is avoided when a drug is administered through parental route and topical route, as the drug directly enters in to systemic circulation.

Drug distribution:
Drug distribution is a important and unique phenomenon for any given drug, Distribution of the drug decides the minimum required therapeutic concentration or dose of a drug to be administered.
After drug is absorbed it is distributed through out the all body compartments and a state of equilibrium is established , between , body tissues and blood concentration.
Tissue bound drug is released in to blood when blood concentration of drug is reduced , as a effect of excretion and elimination.

These phenomenon depends up on physical and chemical properties of a drug molecule, if a drug molecule is more lipid soluble , it is likely that it is absorbed more in organs and tissues tissues made up of fatty tissues and elimination too is delayed .

If a drug molecule is predominately hydrophilic then it is likely that, it leaves tissues more rapidly and comes in to blood , and make it self available for desired action.

Also a hydrophilic molecules are more likely to get eliminated faster through as a of lower rate of reabsorption from renal tubules .

Chemical properties are more important, as these impart ability to a drug molecule to get bound to a particular tissue or proteins.

Protein Binding in drug molecule, Why a drug gets bound to protein:
A drug molecule is chemically active entity, It contains many functional groups on it , which are capable of forming , covalent , ionic bonds and some times complexes with proteins in our body.
Drug molecule under goes many reactions like chelation, conjugation , acylation, esterification and dehydration, mostly electron transfer reactions, so that it gets bound to protein (serum albumin, lipoprotein, glycoprotein, α, β‚ and γ globulins. )

Hence a drug molecule when it is in bound form (bound to proteins) mostly do not exert a pharmacological action on body.

The drug bound with protein is always in state of equilibrium with free drug , hence when a free drug concentration goes down bound drug is released in to blood as free drug molecule and vice versa, up to a particular concentration of administered drug , beyond which up on saturation of proteins , the free drug concentration is predominant. Protein binding also affect the biological half life of a drug.

Protein binding is also dependant on number of physiological factors and disease conditions, which induces a condition where, plasma protein concentration is lower , in such case a risk of higher dose of a drug may get available for action , resulting in higher unbound drug available for action on body, it may lead to toxic effect.
Certain infections causes raised plasma protein level in such conditions a higher dose of a drug is required to be administered to attain a desired effect.

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