Pharmaceutical Co-Crystals of Pyrazinecarboxamide (PZA) with Various Carboxylic Acids: Crystallography, Hirshfeld Surfaces, and Dissolution Study

Three new pharmaceutical co-crystals: 1 PZA-MA (malonic acid), 2 PZA-SA (succinic acid, a new polymorph of a reported one), and 3 PZA-GA (glutaric acid) have been prepared and characterized by differential scanning calorimetry (DSC), thermogravimetric analyses (TGA), and single-crystal X-ray diffraction. Wherein, PZA formed 1:1 co-crystals with MA and GA by acid–amide and acid–py heterosynthon, while it formed 2:1 co-crystal with SA by amide–amide homosynthon in addition to acid–amide and acid–py heterosynthon. Their melting points follow the order, PZA-GA < PZA-MA < PZA-SA, which are lower than the melting points of the individual components. Hirshfeld surface analysis revealed that N–H···O hydrogen bonding and π···π interactions for PZA in them follow the order: PZA-MA > PZA-SA > PZA-GA, while H–H and O–O interactions follow the order: PZA-MA < PZA-SA < PZA-GA. We also compared the Hirshfeld surfaces of the present co-crystals with the nine reported PZA co-crystals, which obtained important results. The studies of the solubility and dissolution showed a semiempirical inverse relationship with the melting point: the solubility follows the order, PZA-SA < PZA-GA < PZA-MA and dissolution rate follows the order, PZA-SA < PZA-MA < PZA-GA.