Pharmaceutical cocrystals of nitrofurantoin: screening, characterization and crystal structure analysis

Abstract

The objective of this study was to screen and prepare cocrystals of the poorly soluble drug nitrofurantoin (NTF) with the aim of increasing its solubility. Screening for cocrystals of NTF using 47 coformers was performed by high-throughput (HT) screening using liquid assisted grinding (LAG) methods. Raman spectroscopy and powder X-ray diffraction (PXRD) were used as the primary analytical tools to identify the new crystalline solid forms. Manual LAG and reaction crystallization (RC) experiments were carried out to confirm and scale-up the hits. Seven hits were confirmed to be cocrystals. The cocrystals were characterized by PXRD, Raman and IR spectroscopy, thermal analysis (DSC and TGA) and liquid-state NMR or elemental analysis. The solution stability of the scaled-up cocrystals in water was tested by slurrying the cocrystals at 25 °C for one week. NTF forms cocrystals with a 1 : 1 stoichiometric ratio with urea (1), 4-hydroxybenzoic acid (2), nicotinamide (3), citric acid (4), L-proline (5) and vanillic acid (6). In addition, NTF forms a 1 : 2 cocrystal with vanillin (7). All but one of the NTF cocrystals transformed (dissociated) in water, resulting in NTF hydrate crystalline material or NTF hydrate plus the coformer, which indicates that the transforming cocrystals have a higher solubility than the NTF hydrate under these conditions. The crystal structures of 1 : 1 NTF-citric acid (4) and 1 : 2 NTF-vanillin (7) were solved by single-crystal X-ray diffraction. The crystal structures of these two cocrystals were analyzed in terms of their supramolecular synthons.