Oral route

Oral route

Oral route :- The digestive tract goes from the mouth to the rectum. The membranes which the drug must cross are the digestive epithelium and the vascular endothelium.

The oral route can be used for a local or general treatment:

• Local treatment: gastrointestinal protectants of the digestive tract itself, treatment of an intestinal infection or a parasitosis. In this case, one wishes, in general, that the drug will not be absorbed or only poorly absorbed.
• General treatment: it is the usual route of administration of drugs and digestive absorption is followed of their diffusion in the body.

The oral route is not usable if the drug intended for a general treatment is degraded in the digestive tract (pH, microbial flora, enzymes of the digestive tract) or is not absorbed or if the patient refuses to take it or vomits.

Digestive absorption can take place at any level of the digestive tract.

1.      Mouth: the absorption of the drug by the oral mucous membrane allows a fast absorption and avoids the hepatic transfer. It is generally called sublingual route. Nitroglycerin, in the form of sugar-coated tablets to be crunched, intended for the treatment of attacks of angina pectoris, is absorbed at this level.

2.      Stomach: the area of the stomach is approximately 1m2. The pH of the gastric fluid is acid. The flow rate of blood drainage of the stomach is low, approximately 0.2 L/min.

o    Neutral molecules and acids not ionized in an acid pH are absorbed from the stomach.

o    Many molecules are secreted into the gastric fluid from the blood , in particular bases which are ionized by protonation  in the acid gastric fluid.

3.      Intestine: the area of the intestine is large: from 200 to 300m2.  The pH is alkaline: 6 to 8. The blood irrigation is important, 1 L/minute. The majority of drugs are absorbed at this level.

4.      Rectum: although absorption can occur at this level, the use of suppositories to obtain a general effect is disadvised because the bioavailability is variable.

At the level of the digestive tract are absorbed:

1.      by passive diffusion through the lipid bilayer , neutral, liposoluble molecules but not those completely insoluble in water.

2.      by secondary active transport, amino acids and sugars, certain peptides.

3.      by complex mechanisms, elements in the form of ions, cations and anions, such as sodium, potassium, calcium, chloride.

The ionized molecules containing, for example, a quaternary ammonium are not or poorly absorbed, in a not very reproducible and variable way according to patients.

A characteristic of digestive absorption is first-pass metabolism: the drug, absorbed from the digestive tract, passes through the liver, reaches the right heart and after pulmonary transfer is distributed by the left heart to the whole body. Both in the intestinal mucous membrane and in the liver, the drug meets enzymes able to transform it into one or more metabolites, sometimes active but generally inactive. This first-pass metabolism explains the low efficacy of certain drugs, especially when they are given in low doses, because they are mainly metabolized before arriving into the blood.