Abstract
Crystal form screening of the antiepileptic drug Retigabine was undertaken to establish the stability order among the known trimorphs and design new cocrystals of the drug. X-ray crystal structures of polymorphs A and B have been determined, and the thermodynamic stability among the trimorphs at ambient conditions was found to be in the order B (least stable) < C < A (most stable) from thermal and phase transformation experiments. Two anhydrous cocrystals, an anhydrous salt and a eutectic of Retigabine were successfully designed and characterized. Interestingly, Retigabine forms a cocrystal with 4-hydroxybenzoic acid only with a water of crystallization (i.e., as a cocrystal hydrate), or else the binary combination manifests as a eutectic. The number of hydroxyl groups and water is implicated in the formation of anhydrous/hydrated cocrystals and eutectic respectively among the combinations explored.
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