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Polymorphism of Spray-dried Mannitol as a Function of Particle Size: Effect of Ethanol
1 , 2 , 3 , * 1 , 1 , 1, 3
1  Department of Pharmaceutics and Analytical Chemistry, Faculty of Pharmaceutical Sciences, University of Copenhagen, Universitetsparken, 2100 Copenhagen, Denmark
2  Quality and technology, section spectroscopy and chemometrics, Faculty of Life Sciences, University of Copenhagen, Rolighedsvej 30, 1958 Frederiksberg, Denmark
3  Advanced Drug Delivery Group, Faculty of Pharmacy, University of Sydney, Sydney, NSW, 2006, Australia

Abstract: Solid state properties of pharmaceutical materials can influence their physical and chemical stability and other performance characteristics of the final product. Spray drying generally produces particles with a given size distribution, which can be subjected to solid state changes during spray drying operation. Solid state can be affected by the process conditions (i.e. fast or slow evaporation rate) inside the spray dryer. The aim of this study was to investigate the polymorphic variation of spray dried mannitol as a function of particle size. A spray dried system with a mixed mannitol polymorphs was produced by using 10% ethanol solution. The obtained dry powders were then dispensed into different size fractions using a Next Generation Pharmaceutical Impactor (NGI). The morphology and polymorphic form of mannitol were analyzed using SEM and XRPD and Raman Microscopy, respectively. Chemometrics was also applied to interpret the Raman spectra. A PLS-DA model with 3 principal components was built to describe the variation of Raman spectra and distinguish different solid state species. XRPD studies indicated that α- and β-mannitol were present in the 10% ethanol spray dried system. Further investigations with Raman studies revealed that more α-mannitol was present in smaller particles, suggesting that the presence of ethanol may affect the evaporation rate in smaller droplets, and by this means crystal arrangement of mannitol. Image analysis from the SEM figures showed a variation of size distribution between NGI stages. In conclusion, the polymorphic forms of spray-dried mannitol could change as a function of particle size. This finding provides guidance to improve dry powder formulations, especially for inhalation purposes, as produced by spray drying, since particle size is a critical component for therapeutic delivery.
Keywords: Spray-drying; Polymorphic form; Mannitol; X-ray powder diffraction; Raman microscopy