In this work, three new pharmaceutical salts of fenbendazole (FNB), a benzimidazole-based anthelmintic drug, with sulfonic acids have been obtained and thoroughly investigated by different analytical techniques, including thermal methods, infrared/Raman spectroscopy, and theoretical methods (periodic DFT computations and Bader analyses of the crystalline electronic density). Single-crystal and high-resolution synchrotron powder X-ray diffraction data for the first time made it possible to determine the crystal structures of mesylate and tosylate salts of the drug, which were further validated by dispersion-corrected density functional theory calculations. All the solid forms were stabilized by a robust R-2(2)(8) supramolecular motif formed by relatively strong N-H center dot center dot center dot O hydrogen bonds. In the monohydrate of FNB tosylate, a considerable gain in the stabilization energy was due to the intermolecular interactions generated by the water molecules. A careful examination of the solubility-pH profile of the FNB salts revealed that, despite being thermodynamically unstable within the physiologically relevant pH range, the new solid forms demonstrated superior dissolution performance in terms of both the apparent solubility and the release rate in comparison to the parent drug. Since FNB has also been reported to possess anticancer activity, improving the drug's poor physicochemical properties through salt formation with the selected sulfonic acids is expected to promote further investigations toward repurposing of this potent compound.
