Novel cocrystals and eutectics of the antiprotozoal tinidazole: Mechanochemical synthesis, cocrystallization, and characterization

Abstract

Antiprotozoal tinidazole (TNZ) exhibits low aqueous solubility (Sw) and poor photochemical stability. In this work, TNZ was used to form cocrystals and eutectics with the aim of enhancing its solubility and physical stability. The choice of coformers was based on a crystal engineering strategy focused on the success rate of supramolecular heterosynthon formation between azoles and carboxylic acids. By liquid-assisted grinding, three cocrystals [with p-aminobenzoic acid (PABA), citric acid (CA), and salicylic acid (SA)] and two eutectics [with nicotinamide and succinic acid] were obtained and characterized using powder X-ray diffraction, 13C solid-state nuclear magnetic resonance, infrared spectroscopy, differential scanning calorimetry, thermogravimetry, and hot-stage microscopy. The crystal structures of TNZ-PABA and TNZ-SA were resolved by means of single-crystal X-ray diffraction, which revealed that they are a solvated and an unsolvated cocrystal, respectively. The Sw values of the obtained samples were determined at 37 °C, with the outcome that only the TNZ-CA cocrystal had a slightly higher Sw than pure TNZ. A physical stability study was also performed under UV-visible irradiation, and it was found that the three cocrystals showed better photostability when compared to TNZ, the two eutectics, and each respective physical mixture, a finding that could be relevant from the pharmacotechnical point of view.