Using structural mimics for accessing and exploring structural landscapes of poorly soluble molecular solids

The importance of using structural mimics for mapping out the structural landscape of a poorly soluble active pharmaceutical ingredient was investigated using erlotinib as an example. A mimic was synthesized by preserving the main molecular functionalities responsible for creating the most probable structural arrangements in the solid state. Calculated molecular electrostatic potentials on both erlotinib and the mimic showed very comparable values indicating that the latter would be able to form hydrogen bonds of similar probability and strength as those of erlotinib. In order to establish the binding preference in co-crystallization experiments, the mimic molecule was co-crystallized with US Food and Drug Administration approved dicarboxylic acids. The crystal structures of the mimic and of four co-crystals thereof were obtained. The mimic forms hydrogen bonds in a way that closely resembles those found in the crystal structure of erlotinib. The four co-crystals display self-consistent hydrogen-bond interactions. Thermal and solubility data for the co-crystals demonstrate that by making systematic and controllable changes to the solid forms of the mimic, it is also possible to alter the behaviour and properties of the new solid forms. The use of a suitable structural mimic can allow for a systematic structural examination of a compound that is otherwise not amenable to such investigations by facilitating the elucidation and mapping out of a closely related structural landscape.