Overview of Enteric-Coated Tablets and Commonly Used Enteric Coating Polymers

1. What are enteric-coated tablets?

Enteric-coated tablets are a dosage form coated with a polymer film capable of resisting the acidic environment of the stomach and releasing the active ingredient when it reaches a higher pH environment in the gastrointestinal (GI) tract. This technology is used to control the site of active ingredient release, protect acid-sensitive active ingredients, or limit the contact of active ingredients with the gastric mucosa.

Compared to some other GI drug delivery systems, enteric film coating is a relatively common technique in tablet manufacturing. However, formulation development still requires strict control over the polymer type, coating thickness, film-forming capability, coating conditions, and dissolution characteristics of the coating system to achieve the desired release profile.

a. Purpose of enteric coating

• Protect active ingredients sensitive to the acidic environment of the stomach, especially certain enzymes or active ingredients with low stability at acidic pH.
• Limit the direct contact of certain active ingredients with the gastric mucosa, which can contribute to improved gastrointestinal tolerance.
• Deliver the active ingredient to the desired release or absorption site in the GI tract, especially when drug release in the stomach must be restricted.
• For drugs acting locally in the intestine or colon, the coating system can be designed to control the release site based on pH changes along the GI tract.
• It may improve some organoleptic properties of the tablet, such as gloss, surface uniformity, and swallowability. However, this is not the primary goal of enteric coating.
• Note that enteric coating does not inherently reduce hepatic first-pass metabolism. For drugs absorbed in the small intestine, the active ingredient can still pass through the portal vein system and undergo first-pass metabolism in the liver. Therefore, the impact on bioavailability must be evaluated individually for each active ingredient and formulation.

b. Characteristics of enteric coating films

An ideal enteric coating film must maintain its integrity in an acidic environment for the required duration, then rapidly dissolve or lose its release-hindering properties when transitioning to a higher pH environment.

Evaluation criteria typically include:

• Acid resistance: The coating must restrict the release of the active ingredient during exposure to the acidic environment.
• Release capability at higher pH: After the acidic phase, the coating must dissolve or alter its properties sufficiently to allow the active ingredient to be released as required.
• Film continuity: The coating must be uniform, without defects causing leakage or premature release.
• Adhesion: The coating must adhere stably to the tablet core during manufacturing, packaging, and storage.
• Stability: The coating system must maintain its acid resistance and release characteristics throughout the product's shelf life.
• In pharmacopoeial testing, the conditions for acid resistance and subsequent release testing depend on the dosage form, monograph, and applied pharmacopoeia. Therefore, a rigid condition like "2 hours at pH 1.2 and disintegration in 60 minutes at pH 6.8" should not be applied to all enteric-coated preparations.
• For enteric-coated tablets, evaluating dissolution is more significant than simply testing disintegration time.
• The weight gain due to the coating layer also does not have a fixed value for all formulations. It depends on the polymer type, polymer concentration, plasticizer, solvent/dispersion system, tablet core characteristics, coating equipment, and the product's acid resistance–release requirements. Thus, a range of 5–15% should only be considered a reference for certain formulations, not a universal standard.

2. Active ingredients suitable for enteric coating systems

Active ingredients easily destroyed by gastric juice

• Digestive enzymes: Pancreatin, Trypsin.
• Acid-sensitive antibiotics: Erythromycin and select others.
• Proton pump inhibitors (PPIs): Omeprazole, Esomeprazole, Lansoprazole, Rabeprazole.
• Certain bioactive ingredients with low stability in acidic environments.

Active ingredients causing irritation or damage to the gastric mucosa

• Non-steroidal anti-inflammatory drugs (NSAIDs): Aspirin, Diclofenac, Naproxen, Ibuprofen.
• Certain iron supplements may be developed with a coating or appropriate release system depending on formulation goals.

Active ingredients requiring local action or release at specific sites in the intestine

• Drugs for intestinal diseases: Sulfasalazine, Mesalamine.
• Stimulant laxatives: Bisacodyl.

Food oils and supplements

• Fish oil / Omega-3: Some products use appropriate coatings or controlled-release systems to minimize reflux and unpleasant fishy burps.

3. Active ingredients requiring local action or release at specific sites in the intestine

Some medications for GI diseases are designed to release the active ingredient in the small intestine, ileum, or colon to increase the drug concentration at the site of action.

Examples:

• Sulfasalazine
• Mesalamine: For Mesalamine, commercial products may utilize various controlled-release mechanisms, including pH-dependent polymer systems, extended-release systems, or colon-targeted delivery technologies.
• Bisacodyl: Typically formulated as an enteric-coated tablet to prevent active ingredient release in the stomach and small intestine, allowing release in the distal segments of the GI tract.

4. Commonly used polymers for enteric-coated tablets

Choosing an enteric coating polymer depends on multiple factors, including the active ingredient's properties, desired release site, polymer dissolution pH, film-forming ability, compatibility with excipients, coating method, and product stability requirements.

a. Ideal properties of an enteric coating polymer

An enteric coating polymer should:

• Resist acidic environments for the necessary duration.
• Dissolve or alter properties at a pH suitable for the desired release site.
• Form a continuous, uniform film with appropriate mechanical strength.
• Be compatible with the active ingredient and other coating components.
• Be stable during manufacturing and storage.
• Have a toxicity profile suitable for the administration route and regulatory requirements.
• Be viable for stable, industrial-scale production at a reasonable cost.

b. Commonly used polymers for enteric-coated tablets

Conclusion

Enteric film coating is a critical technology in oral dosage form development, especially for active ingredients that are acid-sensitive, cause GI irritation, or require targeted release within the GI tract.

In R&D practice, polymer selection should not rely solely on a single dissolution pH value. Formulators must concurrently consider the specific polymer type and grade, active ingredient properties, absorption/action site, coating thickness, plasticizer, coating method, storage conditions, and the desired release profile.

Polymer groups such as CAP, HPMCP, HPMCAS, PVAP, and methacrylic acid polymers remain essential choices in developing enteric coating systems. Selecting the correct polymer and optimizing the coating system are decisive factors in achieving the targeted acid resistance, stability, and active ingredient release.