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This study focuses on the characterization of nasal suspensions for regulatory and scientific purposes. Nasal suspension drug products consist of Active Pharmaceutical Ingredient (API) particles suspended in an aqueous system in the presence of different excipients. The particle size of the API is a critical material attribute, affecting the emitted API particle size and regional deposition of API in the nose. This, in turn, influences the rate of dissolution and absorption at the site of deposition into the nasal epithelium and systemic circulation. The study uses a combination of techniques to investigate API particle size in nasal suspensions, formulation rheological measurements, and force-displacement measurements during product actuation to characterize test and reference nasal suspensions. These approaches are designed to support in vitro bioequivalence investigations of nasal suspension drug products. The study acknowledges the challenges in determining the PSD of the drug substance in the finished nasal aqueous suspension, especially in the presence of undissolved excipients. The study also highlights the potential of ingredient-specific particle sizing using Raman microscopy for characterizing particle size of API in aqueous nasal spray suspension formulations.
