Solubility Advantage of Pharmaceutical Cocrystals

Pharmaceutical cocrystals can improve solubility, dissolution, and bioavailability of poorly water soluble drugs. However, true cocrystal solubility is not readily measured for highly soluble cocrystals because they can transform to the most stable drug form in solution. The objectives of this study are to develop a method to estimate the cocrystal solubility in pure solvent and establish the influence of constituent drug and ligand (i.e., coformer) properties. Cocrystal solubility and solubility product were derived from transition concentration measurements where a solution is in equilibrium with solid drug and cocrystal. Transition concentrations and solubilities are reported for carbamazepine cocrystals in water, ethanol, isopropanol, and ethyl acetate. The aqueous solubility for seven carbamazepine cocrystals was estimated to be 2-152 times greater than the solubility of the stable carbamazepine dihydrate form. Cocrystal solubility is shown to be directly proportional to the solubility of constituent reactants for carbamazepine, caffeine, and theophylline cocrystals. The ligand transition concentration is also correlated with ligand solubility. Transition concentration measurements reveal drug solubilization by ligand for several of the cocrystals studied. The correlation established between constituent and cocrystal solubility was not effectively predicted by fusion properties of the various crystal forms considered.