Exploring the Solubility of the Carbamazepine–Saccharin Cocrystal: A Charge Density Study

Carbamazepine (CBZ) is used in the treatment of multiple neurological conditions. Although efficacious, its potential has been limited by its poor solubility, which means that patients are required to take very large doses to gain the desired effect. Co-crystals have been proposed as a means of improving the physicochemical properties of pharmaceutical compounds while maintaining their efficacy. CBZ cocrystallized with saccharin (SAC) and nicotinamide (NIC) have previously been studied, with the CBZ-SAC crystal being more soluble than the commercially available product Tegretol, which only contains CBZ, while the nicotinamide cocrystal was found to be less soluble. High-resolution X-ray crystallography has been carried out on the CBZ-SAC cocrystal and its individual constituents to determine which features of the electron density distribution contribute to the differing physical properties. The number of hydrogen bonds found for the CBZ, SAC, and CBZ-SAC systems were 8, 5, and 10, respectively. Homosynthons (interactions between a pair of identical functional groups) are the primary bonding motif in CBZ and SAC, while a heterosynthon is also present in the cocrystal. Molecular electrostatic potential (MEP) maps show that cocrystallization results in changes in distribution around the carboxamide group, thus accommodating heterosynthon formation and leading to subsequent charge redistribution across the CBZ molecule. Additional lattice energy calculations were not able to provide a definitive answer as to which system was most stable. Solid state entropy calculations revealed that the CBZ-SAC cocrystal had a higher entropy, providing explanations for the lower melting point and improved dissolution profile previously described. These investigations at an electronic level help to explain the greater solubility of the CBZ-SAC cocrystal compared to CBZ alone.