Solid-state compatibility studies of a drug without melting point: The case of omeprazole sodium

In preliminary pre-formulation studies, the evaluation of solid-state interactions between an active pharmaceutical ingredient and different excipients is essential to guarantee the quality of the final product. Studies of drug–excipient compatibility represent an important phase in the development of all dosage forms. In this study, to evaluate the possible solid-state interaction from thermal events other than the usual melting point, physical mixtures (1:1, w/w) of a model drug without melting point (omeprazole sodium) and a number of pharmaceutical excipients were investigated. Differential scanning calorimetry (DSC), differential diffuse reflectance infrared Fourier transform (DRIFT) and high-performance liquid chromatography (HPLC) methods were used. The properties of the pure drug were compared with those of a binary drug–excipient mixture (1:1; w/w). Evidence of solid–solid incompatibility of omeprazole sodium and Acryl-Eze ® , stearic acid and citric acid was observed from the first decomposition step in the DSC curve and subsequently confirmed by DRIFT and HPLC analyses.