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Drugs may be given in a number of ways. Moreover oral administration is the most common and the easiest way to give a drug. The drug is absorbed from the gastro-intestinal tract and passes via the portal vein into the liver. The liver metabolises many drugs, sometimes to such an extent that only a small amount of active drug emerges from the liver to the rest of the circulatory system. The first pass metabolism greatly reduces the bioavailability of the drug. First pass metabolism can occur in the gut and the liver.
One way to overcome the first pass metabolism is to administer long-chain lipophilic/self emulsifying prodrug systems via lymphatic absorption. Lymph vessels are present in the lining of the gastro-intestinal tract, predominantly in the small intestine. While most other nutrients absorbed by the small intestine are passed on to the portal venous system to drain, via the portal vein, into the liver for processing, fats are passed on the lymphatic system, to be transported to the blood circulation via the thoracic duct avoiding first pass metabolism in the liver and gut. |
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