Micro-Raman Spectroscopy: Unveiling Coating Differences in Enteric-Coated Pellets

Enteric capsules frequently contain multi-layered pellets engineered for delayed drug release. This intricate structure facilitates controlled interlayer composition and thickness, crucial for targeted drug delivery. Micro-Raman spectroscopy serves as a powerful technique to meticulously identify components and analyze layer thickness within these complex pharmaceutical structures.

Study Objective

This study harnessed Micro-Raman spectroscopy to critically compare coating differences between pellets sourced from a reference-listed drug (RLD) and its generic counterpart.

The primary goal was to assess generic pellets and pinpoint variations in component and layer thickness, particularly those relevant to coating consistency and overall drug performance.

Pellet Design and Function

Pellets are sophisticated multi-unit dosage forms, typically spherical particles ranging from 500-1500 μm in size. They offer versatility, being formulated into capsules, tablets, or even used directly for sustained, controlled, or delayed release applications. Their multi-layered architecture is key, allowing for precise customization to specific microenvironments, such as different segments of the gastrointestinal (GI) tract.

For instance, enteric-coated capsules are specifically engineered to remain intact in the harsh acidic environment of the stomach. Drug release is designed to commence only upon reaching the small intestine and conclude in the colon. The overall performance and efficacy of these formulations are profoundly influenced by factors like the physicochemical properties of coating layers, their precise thickness, and the production technology employed.

Consistent coating composition and robust process parameters are paramount, as they directly impact drug release kinetics and therapeutic outcomes.

Methodology

While traditional methods like Scanning Electron Microscopy (SEM) offer valuable insights, they are often limited to characterizing thickness alone. Micro-Raman imaging stands out by its ability to not only determine interlayer thickness but also to definitively identify the compositional makeup of each individual layer.

• Samples: Reference and generic enteric-coated pellets were carefully obtained from a pharmaceutical company for comparative analysis.
• Method: Raman imaging was utilized to comprehensively map the component distribution across pellet cross-sections. Layer thickness measurements were subsequently derived from these detailed compositional maps.
• Instrumentation: A high-sensitivity and high-spatial-resolution HORIBA XploRA™ PLUS Raman system was employed for all analyses.
• Data Analysis: Multivariate analysis, specifically CLS fitting, was performed using LabSpec 6 software to enable rapid component imaging. The integrated Layers app within the software was then used to precisely quantify coating layer thickness.

Results

Both the reference and generic pellets demonstrated remarkably similar coating compositions. The following distinct layers were successfully identified:

• Pellet core: Composed primarily of sucrose and starch.
• First layer: Contained the Active Pharmaceutical Ingredient (API) along with various excipients (API layer).
• Second layer: Identified as primarily talc, with minor amounts of API and excipients (alkaline layer).
• Third layer: A complex mixture of polymers and talc (barrier layer).
• Outermost layer: Consisted of a TiO2 composite.

Raman imaging conclusively confirmed the presence of five concentric layers, perfectly aligning with the expected stratified structure: the central core, followed sequentially by the API, alkaline, barrier, and outermost TiO2 composite layers.

Analysis leveraging the Layers app revealed notable variations in layer thickness between the generic and reference pellets. The most significant discrepancy was observed in the outermost layer:

• First layer (API): Reference ~105.2 μm, Generic ~109.4 μm
• Second layer (Alkaline): Reference ~25.9 μm, Generic ~26.7 μm
• Third layer (Barrier): Reference ~36.7 μm, Generic ~32.6 μm
• Outermost layer (TiO2 composite): Reference ~17.5 μm, Generic ~9.2 μm

While minor variations were noted within the internal layers, the reference pellet's outermost layer was considerably thicker than that found in the generic pellet.

Conclusion

Micro-Raman imaging technology proves to be an exceptionally effective tool for evaluating discrepancies in coating composition and thickness between reference and generic enteric-coated capsule pellets.

This powerful technique offers invaluable analytical insight for comprehensively assessing coating uniformity and quality within a wide range of pharmaceutical formulations.