Enhancing the pharmaceutical behavior of poorly soluble drugs through the formation of cocrystals and mesophases.

Abstract

This dissertation focuses on understanding the role of molecular recognition, kinetic, and thermodynamic events in forming solid-state supramolecular assemblies. Poorly water soluble drugs present one of the greater challenges during development and stand to benefit from changes in crystal structure that enhance oral bioavailability. Single-component systems of ritonavir (crystalline, mesophase, and amorphous) and multiple-component systems of carbamazepine (binary cocrystals) have been studied with the specific objectives of (1) determining the potential for pH-induced precipitation of ritonavir single-component phases, identifying crystallization pathways from aqueous solution and from the amorphous phase, and characterizing the solid-state properties of phases formed; (2) identifying methods for forming carbamazepine cocrystal from solution and evaluating pharmaceutical behavior relative to the single component crystals (solubility, dissolution, hygroscopicity, physical and chemical stability); and (3) determining the solubility product of carbamazepine cocrystals in organic solvents and relating it to the crystallization in solutions of non-stoichiometric composition. Metastable solid phases of ritonavir were prepared by precipitation from aqueous solution and from the amorphous state exposed to moisture. Some of these phases appear to be lyotropic mesophases, since solvent is required for formation and a higher degree of long-range order is evident relative to amorphous ritonavir. Transformation pathways in suspension are consistent with measured dissolution rates and solubilities: amorphous > mesophase > Form I > Form II. Cocrystals of carbamazepine with nicotinamide and saccharin (CBZ:NCT (1:1) and CBZ:SAC (1:1)) crystallized from organic solvents by solvothermal methods exhibit enhanced physical stability following prolonged exposure to relative humidities ≥75% at 22°C and improved chemical stability following photo-irradiation. Aqueous intrinsic dissolution rates and solubilities of CBZ:NCT and CBZ:SAC show a 1.75 and 2.3-fold increase compared with the dihydrate of carbamazepine (CBZ(D)) at 25°C. Although the cocrystals undergo a solvent-mediated transformation in water to CBZ(D), solution concentrations remain elevated by 1.5 and 2 times the solubility of CBZ(D) for at least 48h. The solubility of cocrystals in ethanol, 2-propanol, and ethyl acetate can be described by the solubility product theory. Conditions for the crystallization of a desired solid-state form (cocrystal or single-component crystal) can be defined from the supersaturation and solution composition of cocrystal components.