Characterization of salmon calcitonin in spray-dried powder for inhalation : effect of formulation and process variables

• To characterize physicochemical properties of salmon calcitonin in spray-dried powder for inhalation and understand the interplay between stability, formulation and process parametersSalmon calcitonin (sCT) was spray-dried together with mannitol and chitosan that acts as stabiliser and absorption enhancer, respectively. Two process variables, i.e. inlet temperature and atomizing air volumetric flow rate, were investigated. Solid state properties of the spray-dried powders were characterized using SEM, TGA, XRPD and DSC. The physicochemical stability of salmon calcitonin in the dry powder was investigated by FTIR, HPLC and LC-MS techniques.A high yield of up to 80 % spray-dried powder was obtained with an improved cyclone assembled with B-290 Mini Spray Drier. Nevertheless, the yield was markedly reduced when addition of chitosan exceeded a certain proportion in spray drying formulation. XRPD and DSC results indicated that crystallinity of mannitol was inhibited with an increase of chitosan in the formulation. Residual moisture levels in the spray dried powders were 1-2%. As indicated by FTIR analysis, sCT retained its structural integrity under the spray drying conditions studied, i.e. 100-180 ºC inlet temperature and 357-742 L/h atomizing air volumetric flow rate. Addition of mannitol and chitosan in the spray drying formulation did not improve stabilization of sCT, in which around 7 % degraded impurities were found at a condition of 180 ºC inlet temperature. Yet no obvious degraded impurities were found in plain sCT spray-dried powder under the conditions studied. The LC-MS analysis showed that oxidation was the main degradation pathway at high inlet temperature. Other minor impurities originated from deamidation of Asn26, N-O acyl migration on Ser29 and dimerization by cross-linkage of the disulfide bonds. Two fragments, i.e. H-(Cys1-Gly28)-OH and H-(Ser29-Pro32)-NH2, could also be found when the degraded ester bond between Gly28 and Ser29 was further hydrolysed in phosphate buffer.Salmon calcitonin can be spray-dried into dry powders with good physical integrity under certain conditions. Chemical stability of sCT in spray-dried powder could be improved by the optimization of formulation and process variables.

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