Characterization of polymorphism of gepirone hydrochloride

Gepirone hydrochloride, an investigational anxiolytic drug, was found to have at least three polymorphic forms which melted at 180 degrees C (I), 212 degrees C (II), and 200 degrees C (III). Thermal analytical studies showed that forms I and II were an enantiotropic pair, as were forms I and III. Form III was monotropic with form II and there was no temperature at which III was the most stable polymorph. Solubility data from powder dissolution studies were used to estimate a transition temperature of 74 degrees C for the enantiotropic pair of I and II. The difference in enthalpy was 4.5 kcal/mol at 74 degrees C and 2.54 kcal/mol at 25 degrees C. Form I was the most physically stable below 74 degrees C, whereas form II was the most stable above 74 degrees C. Essentially pure samples of I and II could be obtained easily, but pure III could be developed only transiently on the differential scanning calorimeter heating block. Video taping of hot-stage microscopic observations for review was helpful for detecting seed crystals of II or III in samples of I. The information developed is presented in a hypothetical free energy-temperature diagram.