Comparative study of two related chiral co-crystals: crystallization behaviors, crystal structures and preferential crystallization
Abstract
In a pharmaceutical context, the obtention of a co-crystal of an active pharmaceutical ingredient (API) offers an opportunity for patent term extension but also to improve the attributes of the drug in terms of biological efficacy, chemical stability or manufacturing.[1] A co-crystal is a defined compound made of a stoichiometric association between the target compound (i.e., the API) and one or several co-formers, interacting via non-covalent bonds. The co-crystal has distinct physical properties with reference to the pure components, such as new melting point, different solubilities. If the target molecule is chiral, co-crystallization can also impact the way enantiomers are stacked together in the crystal lattice. For instance, a mixture of enantiomers crystallizing as a racemic compound (i.e., a 1:1 stoichiometric compound) can actually give rise to a conglomerate (i.e., a physical mixture of homochiral particles) when co-crystallized with the suitable coformer (Figure 1).[2] This feature is an asset in the context of enantiomeric separation by crystallization since most techniques require the existence of a stable conglomerate between the enantiomers whereas racemic compounds form in 95% of the cases. Yet, co-crystallization has been almost not considered for the design of Preferential Crystallization (PC).[3]