Two Novel Hydrate Salts of Norfloxacin with Phenolic Acids and Their Physicochemical Properties

Norfloxacin (NORF) is a broad-spectrum quinolone that is widely utilized for the treatment of various bacterial infections and is considered one of the most commonly used fluoroquinolone antibiotics. However, NORF’s clinical utility is limited by its poor water solubility and relatively low oral bioavailability. This study presents an optimization and synergistic enhancement approach through salt/co-crystal, aiming to maximize the biopharmaceutical properties of NORF with the use of phenolic acid. Following this strategy, two new hydrate salts of NORF with phenolic acid, namely, NORF—3,5-DBA hydrate (salt 1) and NORF—VA hydrate (salt 2), were prepared and systematically confirmed. Two hydrate salts were produced by means of the slow evaporation crystallization method, and the structures were determined through single-crystal X-ray diffraction (SCXRD). Additionally, powder X-ray diffraction (PXRD), Fourier-transform infrared (FT-IR) spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and high-performance liquid chromatography (HPLC) were applied to analyze the features of the two salts. The experimental results indicated that the formation of the two salts could enhance the solubility and improve the release behavior of NORF. Interestingly, the physicochemical properties of NORF were significantly improved as a result, leading to an enhancement in its antibacterial activity. This was demonstrated by the enhanced inhibition of bacterial strains and the lower minimum inhibitory concentration values.