First pass metabolism can be defined as metabolism of a drug molecule which occurs during passing through the gut wall and then in liver before a drug molecule goes in to systemic circulation.
That is a metabolism of a drug occurring while a drug is being absorbed from stomach and intestine and then the metabolism of drug occurring in liver after it is absorbed and going before the systemic circulation.
When a drug is administered through oral route it is absorbed from digestive tract and while it is being absorbed , it comes in contact with many enzymes like esterase , and oxidase, and deaminase and so on , which are capable of metabolizing a drug molecule in to an inactive form .
Thus many times the active drug molecules reaching in systemic circulation is very little.
Liver is the main organ in body which is responsible for protecting our body from any of incoming foreign material in to body, be it a drug molecule, or a microorganism or a particulate matter or foreign protein molecule.
Microphages present in liver are capable of imbibing a bacterium within fraction of a minute and then dissolves and digests bacteria or microorganism. Cytochrom p450 enzyme system which is a part of livers metabolic system is mainly responsible for metabolism of organic substances entering in the liver. After a drug is metabolized in to an inactive form it is excretion in to bile , bile is excreted in to intestine , some drugs get reabsorbed from this point from intestine owing to their solubility, a highly fat soluble molecule is more likely to get reabsorbed in to intestine.
A drug molecule in liver is subjected to various metabolic enzymes; some times a drug may be transformed in to an active form in liver, the phenomenon is known as biotransformation, then from liver the drug enters in to systemic circulation.
First pass metabolism can be bypassed or avoided if a drug is administered through Intravenous, intramuscular, subcutaneous, sublingual, rectal, routes of administration, as the drug molecule directly enters in to systemic circulation, from the site of application.
What are the factors affecting absorption of drug:
1.Route of admistration of a drug
2.Drug molecule :
This is one of the main factor which affect the absorption of drug from site of administration, in all routes of administration of drug other than paranteral route
* Polarity of molecule.
*.Pka of drug molecule
*.Persons disease conditions
*.Food taken along with drug or food present in gut.
*.Drug interactions of concomitantly administered other drugs
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