Enteric coated tablets, also called gastro-resistant tablets, have delayed-release features. They are designed to pass unchanged through the stomach to the intestines, where the tablets are insoluble at the low pH of the stomach but dissolve at the higher pH values of the intestine.

Some of the most important reasons for enteric coating are:

1. To protect acid-labile drugs from gastric fluids, such as enzymes and certain antibiotics.
2. To prevent gastric distress or nausea caused by drug irritation, for example, sodium salicylate.
3. To deliver drugs intended for local action in the intestine, such as intestinal antiseptics, in a concentrated form.
4. To deliver drugs to their primary absorption site in their most concentrated form, such as drugs optimally absorbed in the small intestine or colon.
5. To provide a delayed release component for repeat-action tablets.

Materials used in enteric coated tablets

Cellulose acetate phthalate (CAP)

This material is widely used in the industry and only dissolves above pH 6, which delays drug absorption. Compared to other enteric polymers, CAP is hygroscopic and relatively permeable to gastric fluids. Additionally, it is susceptible to hydrolytic removal of phthalic and acetic acids, leading to changes in film properties. Due to its brittleness, CAP is typically formulated with hydrophobic film-forming materials or adjuvants to improve its enteric film. It is available as an aqueous colloidal dispersion of latex particles.

Methacrylic acid polymers (Eudragits)

Polymethacrylates are synthetic polymers composed of dimethylaminoethyl methacrylates, methacrylic acid, and methacrylic acid esters in various ratios, resulting in both cationic and anionic forms. They are available commercially in various forms, such as dry powder, aqueous dispersion, or organic solution. Specifically, Eudragit E 12.5 and E 100 dissolve in gastric fluid from pH 5 and are typically used in film coatings. For enteric coatings, a variety of polymers can be chosen based on the desired release pH range. For instance, Eudragit L 12.5 P and L 12.5 are both soluble in intestinal fluid at pH 6, while Eudragit L 100 also dissolves at pH 6. In contrast, Eudragit L 100-55 and L 30 D-55 are soluble at a slightly lower pH of 5.5. Additionally, Eudragit S polymers, including S 12.5 P, S 12.5, and S 100, dissolve at pH 7.

HPMC phthalate

This polymer type has molecular weight ranges from 20,000 to 200,000 Da. Three grades are available: HP-50, -55, and -55S. These dissolve in aqueous buffer solutions at pH 5, 5.5, and 5.5, respectively. The “S” designation indicates a higher-molecular-weight grade that produces films with greater resistance to cracking.

Polyvinyl actetate phthalate (PVAP)

This is commonly used at concentrations of 9 to 10 percent for tablet enteric film coating. It is insoluble in buffer solutions below pH 5 and soluble at pH values above 5, displaying a sharp solubility response within the pH range of 4.5 to 5. Its solubility can also be influenced by the ionic strength in addition to environmental pH.

Shellac

This is a naturally occurring polymer obtained from a gummy exudation produced by female insects. The pH at which the drug is released is around 7, which might be too high for most enteric-coated products. Therefore, it is not recommended for developing a new product.

Reference:

• Li. X. & Jasti. B. (2006). Design of Controlled Release Drug Delivery Systems. New York: McGraw-Hill.