Conformational polymorphism in aripiprazole: Preparation, stability and structure of five modifications

Abstract

Five phase-pure modifications of the antipsychotic drug aripiprazole were prepared and characterized by thermal analysis, vibrational spectroscopy and X-ray diffractometry. All modifications can be produced from solvents, form I additionally by heating of form X degrees to approximately 120 degrees C (solid-solid transformation) and form III by crystallization from the melt. Thermodynamic relationships between the polymorphs were evaluated on the basis of thermochemical data and visualized in a semi-schematic energy/temperature diagram. At least six of the ten polymorphic pairs are enantiotropically and two monotropically related. Form X degrees is the thermodynamically stable modification at 20 degrees C, form II is stable in a window from about 62-77 degrees C, and form I above 80 degrees C (high-temperature form). Forms III and IV are triclinic ($P\overline 1$), I and X degrees are monoclinic (P2(1)) and form II orthorhombic (Pna2(1)). Each polymorph exhibits a distinct molecular conformation, and there are two fundamental N-H$\cdots$O hydrogen bond synthons (catemers and dimers). Hirshfeld surface analysis was employed to display differences in intermolecular short contacts. A high kinetic stability was observed for three metastable polymorphs which can be categorized as suitable candidates for the development of solid dosage forms.