Crystallographic Characterization of Different Forms of the Salt of Pazufloxacin Mesylate

Pazufloxacin is a fluoroquinolone antibiotic synthesized by Toyama Chemical Co., Ltd. (Tokyo, Japan) in the 1990s. Up until now, the X-ray crystal structure of its mesylate salt had not been determined. The dissolution and recrystallization of pazufloxacin mesylate from different solvents afforded the salts pazufloxacinium mesylate (1), pazufloxacinium mesylate dihydrate (2), pazufloxacinium mesylate hydrate (3) and pazufloxacinium mesylate bis(peroxosolvate) (4), which were all crystallographically characterized. Molecular and crystal structures of these compounds, as well as their thermal behavior, were studied. For all compounds, single-crystal X-ray diffraction confirmed that a proton migrates from methanesulfonic acid to the amino group of pazufloxacin to form a salt. Dehydration of two hydrates occurs as a two-step single-crystal-to-powder process, leading to the formation of two metastable polymorphs of the anhydrous salt. In the solid state, the peroxosolvate compound is stable under ambient conditions for several months, thus making this drug–drug solid suitable for topical application.