Polymorphism and desolvation of flupirtine maleate.

ABSTRACT

Crystallizates of the analgesic agent flupirtine maleate (Katadolon; ASTA Medica, Dresden, Germany) obtained from isopropanol are examined by X-ray diffractometry, polarization microscopy and thermoanalysis. Depending on the crystallizing conditions, the modifications A and B as well as an isopropanol solvate are observed. The inversion temperature A-->B of the enantiotropic modifications is 164 degrees C (differential scanning calorimetry (DSC) onset). During thermal desolvation, modification B is formed well below the inversion temperature. In concentrated isopropanol suspensions, the solvate and modification B are rapidly transformed into modification A. It is shown how phase-pure products consisting of modification A, which is better wettable with water and stable at room temperature, can be obtained.