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An enteric coating is a polymer barrier applied to an oral drug that prevents its dissolution or disintegration in the gastric environment. It either protects the drug from stomach acidity, protects the stomach from the harmful effects of the drug, or helps release the drug after the stomach (usually in the upper intestinal tract). Some drugs are unstable at gastric acid pH and need to be protected from degradation. Enteric coating is an effective method to achieve drug targeting (eg gastro-resistant drugs). Other drugs such as some anthelmintics may need to reach a higher concentration in a specific part of the intestine. The intestinal lining can also be used during studies as a research tool to determine drug absorption. Enteric-coated drugs belong to the “delayed-acting” dosage form category. Tablets, mini-tablets, pellets, and granules (usually filled with capsule shells) are the most common enteric-coated dosage forms.
Most of the intestinal lining functions by presenting a surface that is stable at the strongly acidic pH found in the stomach, but breaks down rapidly at higher pH (alkaline pH). For example, they will not dissolve in the gastric acid of the stomach (pH ~3), but they will dissolve in the alkaline (pH 7-9) environment present in the small intestine. The time required for an enteric-coated dosage form to reach the intestine depends largely on the presence and type of food in the stomach. It varies from 30 minutes to 7 hours, with an average time of 6 hours. Although some studies have indicated that larger dosage forms may require additional time for gastric emptying, others have suggested that tablet size, shape, or volume may have no significant effect. The emptying rate of enteric coated granules, however, is less affected by the presence of food and presents a more uniform expression and reproducible transit time for dispersing multiparticulates.
By preventing the drug from dissolving in the stomach, the intestinal lining can protect the gastric mucosa from the irritant effects of the drug. When the drug reaches the neutral or alkaline environment of the gut, its active ingredients can dissolve and become available for absorption into the bloodstream. Drugs that have an irritating effect on the stomach, such as aspirin or potassium chloride, can be coated with a substance that will dissolve only in the small intestine. However, it has been shown that enteric-coated aspirin can cause incomplete inhibition of platelets, which negates the potential effect for those treated for vascular disease. Similarly, some groups of proton pump inhibitors (esomeprazole, omeprazole, pantoprazole and all groups of azoles) are acid-reactive. For such drugs, the enteric coating added to the formulation avoids activation in the mouth and esophagus.
Materials used for gut lining include fatty acids, waxes, shellac, plastics, and plant fibers. The conventional materials used are film resin solutions. However, since such solvents are organic solvents, there is concern about the toxic potential of traces of solvents remaining on the tablet coating.
The first form of gastro-resistant coating was introduced by Unna in 1884 in the form of keratin-coated pills, although it was later discovered that they were unable to resist gastric digestion. Salol was also used by Seppi as one of the first forms of intestinal coating. However, shellac was the first material widely used as an enteric coating agent since its introduction in the 1930s. Properly treated or hydrolyzed shellac shows different intestinal release properties.
Recently, some companies have begun applying enteric coating to fish oil (omega-3 fatty acid) supplements. The coating prevents the fish oil capsules from being digested in the stomach, which is known to cause gastroesophageal reflux.
