Crystal Engineering of Molecular and Ionic Cocrystals

Solubility enhancement of poorly-soluble active pharmaceutical ingredients (APIs) remains a scientific challenge and poses a practical issue in the pharmaceutical industry. The emergence of pharmaceutical cocrystals has contributed another dimension to the diversity of crystal forms available at the disposal of the pharmaceutical scientist. That pharmaceutical cocrystals are amenable to the design principles of crystal engineering means that the number of crystal forms offered by pharmaceutical cocrystals is potentially greater than the combined numbers of polymorphs, salts, solvates and hydrates for an API. The current spotlight and early-onset dissolution profile ( spring-and-parachute effect) exhibited by certain pharmaceutical cocrystals draw attention to an immediate question: How big is the impact of cocrystals on aqueous solubility? The scientific literature and in-house data on pharmaceutical cocrystals that are thermodynamically stable in water are reviewed and analyzed for trends in aqueous solubility and melting point between the cocrystal and the cocrystal formers. There is poor correlation between the aqueous solubility of cocrystal and cocrystal former with respect to the API. The log of the aqueous solubility ratio between cocrystal and API has a poor correlation with the melting point difference between cocrystal and API. Structure-property relationships between the cocrystal and the cocrystal formers remain elusive and the actual experiments are still necessary to investigate the desired physicochemical properties. Crystal form (cocrystals, polymorphs, salts, hydrates and solvates) diversity is and will continue to be a contentious issue for the pharmaceutical industry. That the crystal form of an API dramatically impacts its aqueous solubility (a fixed thermodynamic property) is illustrated by the histamine H2-receptor antagonist ranitidine hydrochloride and HIV protease inhibitor ritonavir. For more than a century, the dissolution rate of a solid has been shown to be directly dependent on its solubility, cçterîs paribus. A century later, it remains impossible to predict the properties of a solid, given its molecular structure. If delivery or absorption of an API are limited by its aqueous solubility, aqueous solubility then becomes a critical parameter linking bioavailability and pharmacokinetics of an API. Since the majority of APIs are Biopharmaceutical Classification System (BCS) Class II (low solubility and high permeability) compounds, crystal form screening, optimization and selection have thus received more efforts, attention and investment. Given that the dissolution rate, aqueous solubility and crystal form of an API are intricately linked, it remains a scientific challenge to understand the nature of crystal packing forces and their impact upon physicochemical properties of different crystal forms. Indeed, the selection of an optimal crystal form of an API is an indispensable part of the drug development program. The impact of cocrystals on crystal form diversity is addressed with molecular and ionic targets in ellagic acid and lithium salts. A supramolecular heterosynthon approach was adopted for crystal form screening. Crystal form screening of ellagic acid yields molecular cocrystals, cocrystal solvates/hydrates and solvates. Crystal form screening of lithium salts (chloride, bromide and nitrate salts) afforded ionic cocrystals and cocrystal hydrates