Enteric coating formulations
Enteric coating is aimed to prevent the formulations from gastric fluid in the stomach and release the drug component in the intestinal region or once it has passed into the duodenum.
Based on this approach, enteric coating is suitably applied for drugs which cause gastric irritation or are deteriorated by the gastric fluid or gastric enzyme.

Some of the most important reasons for the enteric coating are as follows:
  1. To protect the acid-liable drugs from the gastric fluid. Example : enzymes and certain antibiotics
  2. To prevent gastric distress or nausea due to irritation from a drug. Example : sodium salicylate
  3. To deliver drugs intended for local action in the intestines. Example : intestinal antiseptics could be delivered to their site of action in a concentrated form and bypass systemic absorption in the stomach.
  4. To deliver drugs that are optimally absorbed in the small intestine to their primary absorption site in their most concentrated form.
  5. To provide a delayed-release component for repeat action tablets.

Enteric Coating Polymer/ Enteric Coating Materials

An ideal enteric coated material should have the following properties:
  1. Resistance to gastric fluids.
  2. Ready susceptibility to or permeability to intestinal fluids.
  3. Compatibility with most coating solution components and the drug substrates.
  4. Stability alone and in coating solutions. The films should not change on aging.
  5. Formation of a continuous (uninterrupted) film.
  6. Nontoxicity.
  7. Low cost.
  8. Ease of application without specialized equipment.
  9. Ability to be readily printed or to allow film to be applied to debossed tablets.

With an acid-resistant property, enteric coating polymers generally possess free carboxylic acid groups on the polymer backbone. They are insoluble in acidic media but become deprotonated and dissolved in basic media at nearly neutral pH values (pH>5). Enteric coating polymers can be classified into 3 groups based on chemical compositions as listed below:

1. Polymethacrylates
• Methacrylic acid/ethyl acrylate

2. Cellulose esters
• Cellulose acetate phthalate (CAP)
• Cellulose acetate trimellitate (CAT)
• cellulose acetate succinate

• Hydroxypropylmethylcellulose acetate succinate (HPMCAS)/ hypromellose acetate succinate
• Hydroxypropyl methylcellulose phthalate

3. Polyvinyl derivatives
• Polyvinyl acetate phthalate (PVAP)

Solubility of the polymers depends on the number of carboxylic acid groups varied in the composition. Commercial enteric coating polymers are available as powder, aqueous dispersion and organic solution.

Enteric formulations should have less than 10% drug release after 2 hours in the acid stage. The completion of the drug release in the continuation testing in the buffer stage should take place within 45 min.

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