Screening for pharmaceutical cocrystal hydrates via neat and liquid-assisted grinding

The formation of cocrystal hydrates represents a potential route to achieve molecular materials with improved properties, particularly stability under conditions of high relative humidity. We describe the use of neat and liquid-assisted grinding for screening for hydrated forms of pharmaceutical cocrystals. In the case of liquid-assisted grinding, water is present in the reaction mixture as a liquid, whereas in the case of neat grinding, it is introduced by employing crystalline hydrates as reactants. The ability to form a cocrystal hydrate by either of the two methods appears to be variable, depending on the choice of cocrystal components. Theophylline readily forms a cocrystal hydrate with citric acid. This contrasts with the behavior of caffeine, which provides only an anhydrous cocrystal ("caffeine citrate") even when both reactants are crystalline hydrates. The preference of theophylline to form a cocrystal hydrate is qualitatively explained by similarity between crystal structures of the products and reactant hydrates. Overall, liquid-assisted grinding is less sensitive to the form of the reactant (i.e., hydrate or anhydrate) than neat grinding. For that reason liquid-assisted grinding appears to be a more efficient method of screening for cocrystal hydrates, and it is also applicable to screening for hydrates of APIs.