Mechanistic Analysis of Cocrystal Dissolution, Surface pH, and Dissolution Advantage as a Guide for Rational Selection

The dissolution behavior of a dibasic drug ketoconazole under the influence of pH has been evaluated and compared to its three 1:1 cocrystals with diacidic coformers, fumaric acid, succinic acid (SUC), and adipic acid. Mass transport models were developed by applying Fick's law of diffusion to dissolution with simultaneous chemical reactions in the hydrodynamic boundary layer adjacent to the dissolving surface to predict the interfacial pH and flux of the parent drug and cocrystals. All 3 cocrystals have the ability to modulate the interfacial pH to different extents compared to the parent drug due to the acidity of the coformers. Dissolution pH dependence of ketoconazole is significantly reduced by the cocrystallization with acidic coformers. Due to the different dissolution pH dependence, there exists a transition pH where the flux of the cocrystal is the same as the parent drug. Below this transition pH, the drug flux is higher, but above it, the cocrystal flux is higher. The development of these mass transport models provide a mechanistic understanding of the dissolution behavior and help identify cocrystalline solids with optimal dissolution characteristics.