Using Salt Cocrystals to Improve the Solubility of Niclosamide

Abstract

This Article reports the solvent-free synthesis and characterization of a number of different crystal forms of niclosamide (HNic), which is an API belonging to the Salicilamide class. The synthesized compounds are four new salt cocrystals (KNic·HNic·H2O, KNic·HNic·3H2O, NaNic·HNic·3H2O, NaNic·HNic·2H2O), a classic cocrystal with imidazole (IM) (HNic·IM), and two sodium salts, (NaNic·DMSO·H2O and NaNic·DMSO·2H2O). The peculiarity of these salt cocrystals is the API’s concomitant presence as both a neutral component and as a salt coformer and the fact that they interact via hydrogen bond formation. HNic’s poor aqueous solubility makes the enhancement of its dissolution rate via the modulation of its physical properties extremely important. All samples have been investigated using a combination of solid-state experimental techniques which provide complementary information on powdered samples. These techniques are X-ray powder diffraction, solid-state NMR, IR, and Raman. Single crystals were only obtained for KNic·HNic·H2O and NaNic·DMSO·2H2O. The nature of the adducts (whether salt or cocrystal), their stoichiometry and the presence of independent molecules in the unit cell of the other samples were thus all determined by means of solid-state NMR and the comparative analysis of 13C and 15N CPMAS (Cross-Polarization Magic Angle Spinning) and 1H MAS spectra. Furthermore, differential scanning calorimetry, thermogravimetric analysis and intrinsic dissolution rate measurements completed the characterization and enabled us to evaluate the effects of microscopic changes (molecular packing, weak interactions, conformations, etc.) on the macroscopic properties (thermal stability and bioavailability) of the multicomponent forms. The results obtained indicate that the formation of salt cocrystals provides a reliable method with which to improve the HNic intrinsic dissolution rate.