Pharmaceutical Salts of Biologically Active Hydrazone Compound Salinazid: Crystallographic, Solubility, and Thermodynamic Aspects

The crystal structures of salts of the active pharmaceutical ingredient (API) called salinazid with dicarboxylic acids and acesulfame were determined by single-crystal X-ray diffraction method. The crystals contain hydrogen bond motifs of different structure and complexity, the energies of which were estimated by using the quantum theory of atoms in molecules and crystals (QTAIMC) methodology. It was found that the driving force for facile the oxalate and malate salts formation is the bifurcated N⁺–H···O^– and N⁺–H···O hydrogen bond synthon, while salinazid malonate is mainly stabilized via a “classic” pyridinium-carboxylate heterosynthon. The oxalate and acesulfame salts of salinazid were found to be stable during aqueous dissolution experiments, providing a substantial solubility improvement compared to pure API (33 and 18 times, respectively). However, the malonate and malate salts dissolved incongruently and rapidly underwent a solution-mediated transformation to form pure salinazid. Based on the solubility data of the stable salts and of the pure components, the Gibbs free energy of the salts formation were calculated to be −21.2 kJ·mol^–1 for salinazid oxalate and −22.6 kJ·mol^–1 for salinazid acesulfame

Weak Interactions Cause Packing Polymorphism in Pharmaceutical Two-Component Crystals. The Case Study of the Salicylamide Cocrystal

Two polymorphs of the salicylamide cocrystal with oxalic acid have been obtained and described. Form I of the cocrystal was prepared by three alternative methods in various solvents, while formation of form II was achieved only by a special crystallization procedure. Single-crystal X-ray analysis has revealed that polymorphs consist of conformationally identical salicylamide and oxalic acid molecules, which are assembled into supramolecular units connected via a network of very similar hydrogen bonds. The packing arrangements of the cocrystal polymorphs, however, were found to be different, suggesting a rare example of packing polymorphism. The stability relationship between the polymorphs has been rationalized by using a number of experimental methods, including thermochemical analysis, solubility, and solution calorimetry measurements. Similarities and differences in intermolecular contacts across two polymorphs have been visualized using the Hirshfeld surface analysis. The Bader analysis of the theoretical electron density has enabled us to quantify the pattern of noncovalent interactions in the considered cocrystals. Applicability of different theoretical schemes for evaluation of the lattice energy of the two-component organic crystals has been discussed

Evaluation of the Lattice Energy of the Two-Component Molecular Crystals Using Solid-State Density Functional Theory

The lattice energy <i>E</i>_latt of the two-component crystals (three co-crystals, a salt, and a hydrate) is evaluated using two schemes. The first one is based on the total energy of the crystal and its components computed using the solid-state density functional theory method with the plane-wave basis set. The second approach explores intermolecular energies estimated using the bond critical point parameters obtained from the Bader analysis of crystalline electron density or the pairwise potentials. The <i>E</i>_latt values of two-component crystals are found to be lower or equal to the sum of the absolute sublimation enthalpies of the pure components. The computed energies of the supramolecular synthons vary from ∼80 to ∼30 kJ/mol and decrease in the following order: acid–amide > acid–pyridine > hydroxyl–acid > amide–amide > hydroxyl–pyridine. The contributions from different types of noncovalent interactions to the <i>E</i>_latt value are analyzed. We found that at least 50% of the lattice energy comes from the heterosynthon and a few relatively strong H-bonds between the heterodimer and the adjacent molecules

Pharmaceutical Cocrystals of Diflunisal and Diclofenac with Theophylline

Pharmaceutical cocrystals of nonsteroidal anti-inflammatory drugs diflunisal (DIF) and diclofenac (DIC) with theophylline (THP) were obtained, and their crystal structures were determined. In both of the crystal structures, molecules form a hydrogen bonded supramolecular unit consisting of a centrosymmetric dimer of THP and two molecules of active pharmaceutical ingredient (API). Crystal lattice energy calculations showed that the packing energy gain of the [DIC + THP] cocrystal is derived mainly from the dispersion energy, which dominates the structures of the cocrystals. The enthalpies of cocrystal formation were estimated by solution calorimetry, and their thermal stability was studied by differential scanning calorimetry. The cocrystals showed an enhancement of apparent solubility compared to the corresponding pure APIs, while the intrinsic dissolution rates are comparable. Both cocrystals demonstrated physical stability upon storing at different relative humidity