Understanding Polymorphic Control of Pharmaceuticals Using Imidazolium-Based Ionic Liquid Mixtures as Crystallization Directing Agents

Imidazolium-based room temperature ionic liquids (RTILs) were tested to assess their ability to control molecular polymorphic behavior. Mixtures of RTILs with distinct cation/anion combinations revealed promising capabilities in directing the crystallization process toward less stable polymorphs. In our tests, gabapentin (GBP) neuroleptic drug was used as a case study, as it is a well know polymorphic active pharmaceutical ingredient. For the first time, pure “bulk” GBP Form IV, a highly unstable polymorph, was isolated through RTILs. Forms were maintained over time, once they were kept soaked, opening new perspectives for the method presented here. Molecular dynamics (MD) simulations clearly supported the results. In this work the polymorphic behavior of GBP is controlled recurring to the use of different pure imidazolium-based RTILs or mixtures, as crystallization solvents. Molecular dynamics simulations clearly supported the results showing that specific H(acidic(C4/C6mim))···O(carboxylate) interaction between GBP and RTILs drives the formation of Form IV. For the first time, pure “bulk” GBP Form IV, a highly unstable polymorph, was isolated. These results showed the importance of these “directing agents” in the polymorphic process as well as the importance of using MD simulations in predicting the “designed” crystallization environment for the “desired” polymorph.