Moisture protection in oral solid dosage forms: The role of film coatings

Maintaining the long-term stability of Oral Solid Dosage Form (OSDF) is a multi-dimensional challenge, primarily dictated by a formulation’s stability against environmental stressors. As the global standard for drug delivery, the OSDF must frequently overcome the industrial challenge of moisture sensitivity.

Atmospheric humidity remains the most pervasive threat, acting as a critical catalyst for both chemical and physical degradation and even the alteration of the microbiological properties of the finished product. At a molecular level, water vapor facilitates the hydrolysis of functional chemical groups, while simultaneously triggering physical instabilities like recrystallization, tablet swelling, or a deleterious loss of mechanical hardness.

While moisture-protective packaging provides a macro-barrier, it is often insufficient for the “in-use” period when the dosage form is repeatedly exposed to varying humidity conditions after the primary seal is breached. The introduction of moisture barrier film coatings represents a strategic solution to this challenge; by providing low water vapor permeability without compromising dissolution functionality, these coatings maintain chemical integrity and preserve the formulation’s properties throughout its entire lifecycle, from storage and transport to the point of administration.

The challenge of moisture-sensitive OSDFs

When hygroscopic tablet cores are exposed to environmental humidity, the resulting degradation follows several critical physico-chemical mechanisms:

Chemical degradation

• Hydrolysis: In many moisture-sensitive APIs, water molecules act as nucleophiles that attack susceptible chemical bonds. This process is particularly reactive toward functional groups such as esters, amides, and lactams, leading to the formation of degradation products that can attenuate therapeutic effect or increase toxicity profiles.
• Oxidation: While oxidation is an oxygen-driven process, moisture often acts as a critical facilitator. Absorbed water vapor can mobilize trace metallic impurities or provide the necessary medium for free radicals to interact with the API. This is a significant risk for compounds containing unsaturated bonds, thiols, or phenolic groups, where moisture effectively accelerates the rate of oxidative degradation.

Physical degradation

• Amorphous state stability: Absorbed moisture acts as a plasticizer, significantly lowering the glass transition temperature of amorphous solids. This increased molecular mobility often triggers an unwanted transition from an amorphous state to a crystalline state, which typically results in a marked decrease in the API’s solubility and dissolution rate.
• Structural compromise: Moisture sorption alters the mechanical properties of the dosage form. The intake of water vapor into the tablet matrix leads to localized swelling and a disruption of inter-particle bonding, resulting in a loss of tablet hardness and increased friability. These physical deformations not only affect the aesthetic quality but can also lead to failure during downstream handling or storage.

Optimizing performance with film coating solutions

The primary objective of a moisture barrier is to minimize the penetration of water vapor into the tablet core. Achieving this requires a precise calibration of the film’s properties: the matrix must be sufficiently dense to retard moisture diffusion during storage, yet remain compatible with the required disintegration and dissolution kinetics for an optimized bioavailability.

Physico-chemical principles of moisture barrier coatings

Traditional film coatings typically utilize water-soluble cellulose derivatives, such as Hydroxypropyl Methylcellulose (HPMC). HPMC is the industry standard due to its excellent film-forming capabilities, high mechanical strength, and superior adhesion to the tablet core. However, from a barrier perspective, standard HPMC possesses a relatively high free volume, empty space of mobility, at the molecular level, which provides limited resistance to water vapor diffusion.

While alternative coating agents are sometimes explored for lower permeability, they can introduce industrial challenges such as reduced spray rates or the risk of moisture “trapping.” If a coating is too restrictive during the desorption phase, any residual moisture within the core remains locked inside, potentially accelerating hydrolytic degradation. Consequently, one effective technical approach involves the functional modification of the HPMC matrix to enhance its barrier properties while preserving its inherent processing advantages.

Coating systems for pharmaceuticals: Designing a low permeability system

To achieve enhanced protection, modern coating formulations focus on optimizing molecular density. This is achieved by creating a composite matrix where the HPMC matrix is integrated with hydrophobic agents, such as long-chain fatty acids. At a microscopic level, these hydrophobic molecules occupy the free volume within the polymer structure, forcing water molecules to navigate a significantly extended diffusional path to reach the tablet core. This increased resistance prevents moisture infiltration without necessitating thick coating layers that could compromise dissolution kinetics.

This technology serves as a robust primary defense, maintaining stability during the critical “in-use” period and potentially allowing for the use of higher permeability packaging materials instead. Furthermore, these systems are engineered for rapid dispersion, ensuring a homogeneous suspension and a uniform barrier across the entire batch.

Technical insights

Barrier protection vs. moisture trapping

A common limitation in coating selection is focusing solely on moisture exclusion. High-level performance assessment requires an analysis of Dynamic Vapor Sorption (DVS) kinetics, a gravimetric technique that measures the rate and quantity of solvent absorbed by a sample.

DVS data reveals that some PVA-based coatings, while offering an initial barrier, exhibit a “trapping” effect. If moisture enters the tablet during the coating process or through environmental exposure, the PVA matrix can act as a barrier, preventing that moisture from escaping during dry environmental cycles (desorption). This residual humidity creates a micro-environment of instability within the core.

By utilizing stearic acid within the HPMC matrices, the SEPIFILM™ LP range is able to address the issue of water entrapment, demonstrating superior desorption efficiency in comparative studies. Performance data indicates that SEPIFILM™ LP not only absorbs less water at 90% RH than PVA-based alternatives but also returns to a lower baseline moisture content during desorption. This prevents the API from being exposed to trapped humidity, a decisive factor in preventing hydrolytic degradation.

Performance data: Comparative case study

To provide empirical evidence, the following study highlights the efficacy of the SEPIFILM™ LP range against benchmark PVA polymers:

Ranitidine swelling study (High sensitivity API model)

Ranitidine is a highly moisture-sensitive API used as a benchmark for stability testing. In a study conducted at 25°C and 75% RH, measuring the swelling of the tablet over time.

The study reveals a stark contrast in protection: while uncoated tablets lost structural integrity within days and standard PVA-based coatings showed limited resistance, SEPIFILM™ LP 030 demonstrated superior barrier properties. By reaching a stable swelling plateau, it effectively shields sensitive cores from environmental humidity, ensuring long-term stability.

Continue reading  the original SEPPIC article here

Source: SEPPIC, website Moisture protection in oral solid dosage forms: The role of film coatings

If you have any questions or need a sample by SEPPIC, please feel free to contact us: