No abstract

Cocrystals constitute an important class of pharmaceutical solids for their remarkable ability to modulate solubility and pH dependence of water insoluble drugs. Here we show how cocrystals of indomethacin-saccharin (IND-SAC) and carbamazepine-saccharin (CBZ-SAC) enhance solubility and impart a pH-sensitivity different from that of the drugs. IND-SAC exhibited solubilities 13 to 65 times higher than IND at pH values of 1 to 3, whereas CBZ-SAC exhibited a 2 to 10 times higher solubility than CBZ dihydrate. Cocrystal solubility dependence on pH predicted from mathematical models using cocrystal K(sp), and cocrystal component K(a) values, was in excellent agreement with experimental measurements. The cocrystal solubility increase relative to drug was predicted to reach a limiting value for a cocrystal with two acidic components. This limiting value is determined by the ionization constants of cocrystal components. Eutectic constants are shown to be meaningful indicators of cocrystal solubility and its pH dependence. The two contributions to solubility, cocrystal lattice and solvation, were evaluated by thermal and solubility determinations. The results show that solvation is the main barrier for the aqueous solubility of these drugs and their cocrystals, which are orders of magnitude higher than their lattice barriers. Cocrystal increase in solubility is thus a result of decreasing the solvation barrier compared to that of the drug. This work demonstrates the favorable properties of cocrystals and strategies that facilitate their meaningful characterization.

Cocrystals of a nonionizable, water soluble compound (gababentin lactam (GBPL)) with less soluble coformers, are shown to be 2 to 17 times less soluble than GBPL. Cocrystals of GBPL with gentisic acid, 4-hydroxybenzoic acid, 4-aminobenzoic acid and fumaric acid are characterized by carboxylic acid-amide hydrogen bonds between coformer and GBPL, consistent with a previously reported structure of a benzoic acid cocrystal. The lattice and solvation contributions to cocrystal aqueous solubility were evaluated and solvation was found to be the main contribution to solubilization. Cocrystals exhibited pH-dependent solubility and pH max , both of which are described by coformer pK a and cocrystal K sp values. These findings have important implications for the characterization and selection of cocrystals for desired drug delivery behavior.

Co-crystals play a significant role in solving bioavailability problems because they have the ability to fine tune solubility and delivery of drugs. Understanding the link between the structure/composition of co-crystals and their solution properties is important, so that co-crystal and formulation conditions can be selected on a rational basis. This chapter presents the thermodynamic factors that influence co-crystal solubility, the parameters that determine co-crystal stability, and experimental methods that overcome the difficulties of co-crystal conversion to less soluble phases.