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Preparation and Characterization of Salt Forms of Enalapril

Selection of an optimal salt form of a drug candidate is a vital component of preformulation stage of drug development. In this study, six salts of enalapril – citrate, mesylate, tartrate, malate, besylate and tosylate – were prepared and characterized by Mass Spectroscopy, Differential Scanning Calorimetry, Thermogravimetric Analysis, Microscopy, Powder X-ray Diffraction, Karl Fischer Titration, High Performance Liquid Chromatography, Fourier-Transform Infra-red Spectroscopy and Head Space Gas Chromatography. All the six salts were subjected to a tiered screening involving five stages in the following order: crystallinity, hygroscopicity, solubility, stability and flow/compactability. Enalapril malate showed encouraging profile because of its lower hygroscopicity, higher solubility, good solid state stability, and better flow and compactability, in comparison to the marketed maleate salt.