Polymorphism of levofloxacin: structure, properties and phase transformation†
Levofloxacin is a fluoroquinolone and bears a broad-spectrum antibacterial effect. The polymorphism of levofloxacin was intensively studied, nevertheless, the landscape of polymorphs is not fully understood. Herein we reported for the first time the fully solved crystal structure of an anhydrous α form and three newly discovered solvates including n-propanol solvate, ethylene glycol solvate and acetic acid solvate. The successfully solved crystal structure of the anhydrous α form by single crystal X-ray diffraction was found to be in good agreement with the reported data by the SDPD (structure determination from powder X-ray diffraction data) method, but is significantly different from the one by the crystal structure prediction (CSP) approach. The comparison results suggest the wrong solved crystal structure of the α form using the CSP method. The newly reported n-propanol solvate, ethylene glycol solvate and acetic acid solvate of levofloxacin were fully characterized by powder X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry, and single crystal X-ray diffraction (except for the acetic acid solvate). It was found that the solvent molecules of the n-propanol solvate fill the channels and interact weakly with the active pharmaceutical ingredient (API) molecules, while they play crucial roles in the stability of the crystal structure. Further, the phase transformations among different solid-state crystal forms of levofloxacin were investigated in detail by variable-temperature powder X-ray diffraction. The results reveal that all of the newly discovered solvates can be directly dehydrated to an anhydrous γ form, which eventually converted into an anhydrous α form during subsequent heating.