Tablets are solid pharmaceutical dosage forms with a defined shape, each containing an exact amount of one or more active pharmaceutical ingredients (APIs). They are manufactured by compressing powder or granules, with or without excipients, using a tablet press. Tablets were first introduced in 1894 following the invention of the tablet press. With advancements in tablet compression technology and the development of biopharmaceutics, tablets have become increasingly popular and are now the most widely used dosage form, accounting for nearly two-thirds of all pharmaceutical products on the market. This widespread use is primarily due to their suitability for formulation research, manufacturing, transportation, storage, and patient convenience.
The stability of APIs is a crucial factor determining the efficacy and safety of the final drug product. The instability and degradation of APIs are typically caused by chemical reactions such as oxidation, photodegradation, thermal degradation, and hydrolysis, which can lead to a loss of therapeutic effectiveness. Hydrolysis due to ambient humidity is the primary cause of instability for moisture-sensitive APIs.
To protect moisture-sensitive active pharmaceutical ingredients (APIs) from hydrolysis, several approaches have been studied, including:
An effective moisture barrier film coating should meet the following requirements:
Currently, various moisture barrier film coating technologies have been researched and developed. Therefore, this article summarizes the formulation techniques for moisture barrier film-coated tablets and evaluates the advantages and limitations of these techniques, providing readers with a comprehensive overview of moisture barrier film-coating technology.
The mechanism of moisture absorption from ambient humidity into a tablet occurs in three main steps. First, water molecules migrate to and are adsorbed by the surface of the film coating. Then, these water molecules diffuse through the coating layer and desorb upon reaching the interface between the film coating and the tablet core. Finally, the water molecules may penetrate deeper into the core and interact with the active pharmaceutical ingredient (API). This process continues until the moisture levels inside and outside the tablet reach equilibrium.
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From the aforementioned moisture absorption mechanism, we can see the importance of selecting an appropriate film coating, where moisture permeability is closely related to the polarity of the polymer used for coating.
Several methods are used to evaluate the moisture permeability of the film coating, including:
The polymers used in moisture barrier film coatings are typically synthetic polymers, classified into water-soluble polymers, water-insoluble polymers, and enteric-soluble polymers. The commonly used polymers for moisture barrier film coatings are listed in the table below.
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Besides polymers, the choice of solvent is also crucial in forming an effective moisture barrier film coating. The evaporation rate of the selected organic solvent plays a vital role in the film formation process. If the solvent evaporates too slowly, moisture may penetrate the tablet core more easily, reducing the protective effect of the film coating. If the solvent evaporates too quickly, the film may form unevenly, leading to defects such as an "orange peel" texture or other coating issues, which can compromise both the appearance and effectiveness of the film. Both water and organic solvents can be used in the film-coating process, each with distinct properties, advantages, and limitations.
Organic solvent coating
Organic solvents remain the preferred method for certain specialized dosage forms and highly moisture-sensitive active pharmaceutical ingredients (APIs). Commonly used organic solvents include ethanol and isopropanol, which are known for their rapid and efficient evaporation. The evaporation rate of the solvent plays a crucial role in the film formation process and the moisture barrier effectiveness of the coating.
Aqueous polymeric film coating
Aqueous solvents are used to overcome the limitations of organic solvents, such as toxicity, residual solvents, and the risk of flammability. However, aqueous solvents may not be suitable for certain highly moisture-sensitive APIs. To enhance moisture protection when using aqueous film coatings, a combination of a hydrophilic polymer and a hydrophobic agent is sometimes employed to improve the barrier properties of the film. Additionally, since water evaporates more slowly than organic solvents, optimizing coating parameters such as temperature and pan speed during the coating process is crucial to achieving a high-quality film.
Dry coating
Over the past decades, concerns about costs and environmental impact related to solvent use have been mitigated through the adoption of dry coating processes, such as compression coating, hot-melt coating, dry heat coating, and powder coating. These methods are highly valued for their safety, environmental friendliness, and lower overall costs, as they eliminate the need for organic solvents and water.
Overall, film coating remains the preferred approach for providing moisture protection for active pharmaceutical ingredients (APIs). Moisture barrier film coatings can be applied using either organic or aqueous solvents, depending on the properties of the API and the chosen solvent system. Dry coating processes help overcome the drawbacks associated with the use of organic or aqueous solvents.
