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Delayed-release (enteric) coatings
Aug 07, 2018

Enteric coatings are primarily used for the purpose of:

  • Maintaining the stability of APIs that are unstable when exposed to the acidic conditions of the gastric milieu. Such API’s include erythromycin, pancreatin, and the class of proton pump inhibitors, such as omeprazole.

  • Minimizing the side effects (eg, nausea, and gastric irritation and bleeding) that can occur with APIs such aspirin and certain nonsteroidal inflammatory compounds.

  • Creating opportunities for “night-time dosing” strategies, where the intent is to allow the dosage form to be consumed at bed-time, and permit effective blood levels of the API to be attained just prior to waking.

  • Facilitating colonic drug delivery.

The functionality of enteric coatings is, for the most part, mediated by a change in pH of the environment to which the enteric-coated product is exposed. Enteric polymers remain unionized (and thus, insoluble) at low pH values, and begin to dissolve at a pH value of approximately 5.0–5.5. In addition, the functionality of enteric coatings can be greatly affected by many other factors, such as:

  • The nature of the API contained in the dosage form; this is especially true when that API is ionic in nature.

  • The quantity of coating applied; insufficient coating can result in ineffective gastric resistance, while too much applied coating can seriously delay drug release when the dosage form passes into the small intestine.

  • The presence of imperfections in the coating (eg, cracks, “pick marks,” etc.) that can also lead to reduced gastric resistance.

  • The chemistry of the polymer used (especially dissolution pH and dissolution rate at a given pH).

  • The influence of the in-vitro test conditions used (such as pH and ionic strength of the test medium; as well as the agitation rate used in the test).

Enteric film-coating polymers are essentially polyacids (see Fig. 34.16), and typically only dissolve in water above pH=5.0–6.0; these polymers are selected for their ability not only to form robust coatings that adhere strongly to tablet surfaces, but also to permit rapid drug release from dosage form once it passes from the stomach into the small intestine (see Fig. 34.17).

A list of commonly used enteric-coating polymers is given in Table 34.13, and these form the basis of enteric coating formulations used in either organic-solvent-based or aqueous-coating formulations. A breakdown of coating systems specially designed for aqueous-coating applications is shown in Table 34.14.

While enteric-coated products have conventionally taken the form of tablets, recently a preference has been shown for coating multiparticulates because of their more consistent gastrointestinal transit characteristics. Enteric-coated capsules (especially one-piece softgels, containing garlic or fish oils used in nutraceutical applications) have also become quite commonplace.

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