Enteric film-coated tablets are pharmaceutical dosage forms coated with a polymeric film designed to resist the acidic environment of the stomach and subsequently release the active pharmaceutical ingredient (API) when exposed to the higher-pH environment of the gastrointestinal tract. This technology is primarily used to control the site of drug release, protect acid-sensitive APIs, or minimize direct exposure of the API to the gastric mucosa.
Compared with other gastrointestinal drug-delivery systems, enteric film coating is a well-established and widely applied approach in oral solid dosage-form development and manufacturing. Nevertheless, successful formulation development requires careful optimization of the polymer type and grade, coating weight gain, film thickness, film-forming properties, coating process parameters, and dissolution characteristics to achieve the intended drug-release profile.
It should be emphasized that enteric coating does not inherently reduce hepatic first-pass metabolism. For APIs absorbed in the small intestine, the drug may still enter the portal circulation and undergo hepatic first-pass metabolism. Therefore, any effect on bioavailability or systemic exposure must be evaluated on a case-by-case basis according to the API and formulation characteristics.
An effective enteric film should maintain its structural integrity under acidic gastric conditions for the required duration and subsequently dissolve or undergo a sufficient change in its physicochemical properties when exposed to a higher-pH environment, thereby permitting drug release according to the intended formulation design.
The key performance characteristics generally include:
For pharmacopoeial testing, the conditions used to evaluate acid resistance and subsequent drug release depend on the dosage form, applicable monograph, and pharmacopoeia being applied. Therefore, a fixed condition such as “2 hours at pH 1.2 followed by disintegration within 60 minutes at pH 6.8” should not be regarded as a universal requirement for all enteric-coated products.
For enteric-coated tablets, dissolution/drug-release testing is generally more relevant than evaluation of tablet disintegration time alone, because the primary purpose of the coating is to control the release of the API under defined gastrointestinal conditions.
Coating weight gain likewise does not have a single value applicable to all enteric-coated formulations. The required coating level depends on the polymer type and grade, polymer concentration, plasticizer, solvent or dispersion system, tablet-core characteristics, coating equipment, process parameters, and the required balance between acid resistance and drug release. Accordingly, a coating weight gain of approximately 5–15% should be considered only a reference range for certain formulation systems and not a universal standard.
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Certain drugs used in the treatment of gastrointestinal disorders are formulated to release the API in specific regions of the gastrointestinal tract, such as the small intestine, ileum, or colon, in order to achieve the desired local drug concentration at the site of action.
Examples:
For mesalamine, commercial products may employ different drug-release mechanisms, including pH-dependent polymer systems, extended-release technologies, or other formulation approaches designed to deliver the API to specific regions of the gastrointestinal tract.
Bisacodyl:
Bisacodyl is commonly formulated as an enteric-coated dosage form to minimize drug release in the stomach and proximal gastrointestinal tract while allowing the API to be released in more distal regions of the gastrointestinal tract.
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The selection of an enteric-coating polymer should be based on multiple formulation and process considerations, including the physicochemical characteristics of the API, the intended site of drug release, the dissolution characteristics of the polymer, film-forming performance, compatibility with other excipients, coating process, and product stability requirements.
A polymer intended for use in an enteric-coating system should ideally demonstrate the following characteristics:
Polymer Group | Polymer | Characteristics / Applications | Dissolution pH / Notes |
|---|---|---|---|
Cellulose derivatives | Cellulose Acetate Phthalate (CAP) | A classical enteric-coating polymer. The phthalyl groups can ionize as the pH increases, resulting in increased polymer solubility under intestinal conditions. | Approximately pH ≥ 6.0. The actual dissolution behavior depends on polymer characteristics and test conditions. |
Cellulose derivatives | Hydroxypropyl Methylcellulose Phthalate (HPMCP) | An established enteric-coating polymer available in multiple grades with different dissolution and film-forming characteristics. | Dependent on polymer grade and test conditions. |
Cellulose derivatives | Hydroxypropyl Methylcellulose Acetate Succinate (HPMCAS) | Available in multiple grades with different pH-dependent dissolution profiles, allowing selection of a grade according to the intended release region. | Some grades exhibit dissolution thresholds around pH 5.5, while other grades have higher pH thresholds. |
Cellulose derivatives | Cellulose Acetate Trimellitate (CAT) | A cellulose derivative containing trimellitate groups and exhibiting pH-dependent dissolution characteristics; may be used in enteric-coating systems. | Dependent on polymer characteristics and test conditions. |
Polyvinyl derivatives | Polyvinyl Acetate Phthalate (PVAP) | Provides resistance to gastric acid and forms a protective film around the API while allowing drug release under higher-pH intestinal conditions. | Approximately pH ≥ 5.0. |
Methacrylic acid polymers | Eudragit L 100-55 | A copolymer of methacrylic acid and ethyl acrylate commonly used for enteric drug release in the small intestine. | Approximately pH ≥ 5.5. |
Methacrylic acid polymers | Eudragit L 30 D-55 | An aqueous dispersion of a polymer with pH-dependent dissolution characteristics comparable to L 100-55 and suitable for aqueous film-coating processes. | Approximately pH ≥ 5.5. |
Methacrylic acid polymers | Eudragit L 100 | An enteric-coating polymer used when drug release is intended at a higher intestinal pH. | Approximately pH ≥ 6.0. |
Methacrylic acid polymers | Eudragit S 100 | Exhibits a higher dissolution threshold than Eudragit L 100 and may be considered when targeting release in the distal intestine or colon. | Approximately pH ≥ 7.0. |
Naturally derived polymers | Shellac | A naturally derived polymer with pH-dependent solubility that has historically been used in enteric-coating systems. Its performance may vary depending on the raw-material source, degree of esterification, processing conditions, and storage conditions. Aging may affect dissolution and drug-release behavior. | Variable; dependent on material characteristics and processing conditions. |
Naturally derived polymers | Sodium Alginate | A polysaccharide capable of forming gels and interacting with ions. It is more commonly used in matrix systems, microparticles, beads, and controlled-release systems than as a conventional enteric-coating polymer. | May contribute to pH-dependent release systems but is not considered a classical enteric polymer equivalent to CAP, HPMCP, or methacrylic acid polymers. |
Enteric film coating is an established pharmaceutical technology for the development of oral solid dosage forms, particularly for APIs that are sensitive to acidic conditions, may cause gastric irritation, or require release at a defined location within the gastrointestinal tract.
From a pharmaceutical R&D perspective, polymer selection should not be based solely on a nominal dissolution-pH value. The development process should consider the specific polymer and grade, API physicochemical properties, intended site of absorption or action, coating weight gain and film thickness, plasticizer system, coating process, tablet-core characteristics, storage conditions, and target drug-release profile.
Polymer systems such as CAP, HPMCP, HPMCAS, PVAP, and methacrylic acid polymersremain important options for the development of enteric-coated dosage forms. Appropriate polymer selection, combined with optimization of the coating formulation and process parameters, is critical to achieving the required acid resistance, film integrity, stability, and drug-release performance.
The final enteric-coated formulation should therefore be evaluated using an appropriate combination of pharmaceutical development studies, dissolution testing, coating characterization, process optimization, and stability assessment to demonstrate that the dosage form consistently meets its intended performance requirements.
