Characterization and interconversion of polymorphs of premafloxacin, a new quinolone antibiotic

The quinolone antibiotic premafloxacin crystallizes in at least five solid modifications, including three anhydrous phases (Forms I-III), a hydrate, and a methanolate. The anhydrous phases were studied by optical microscopy, X-ray powder diffraction, HPLC, hot-stage microscopy, dynamic moisture sorption gravimetry, differential scanning calorimetry, thermal gravimetry, and solution and isothermal calorimetry. Dry samples of Form I converted to Form II and ultimately to Form III through a sequence of melts and recrystallizations. Form III was stable to its melting temperature near 200 degrees C. Humidified samples of Form I converted directly to Form III via a moisture-mediated solid-state phase transformation at temperatures as low as 40 degrees C. The calorimetric and solubility data confirmed that Form III was lower in free energy and enthalpy than Form I at room temperature. Our investigation revealed that Form I was not crystallized directly from solution. Rather, Form I was the product of facile solid-state desolvation of the methanol solvate.