Structural characterization and pharmacological assessment in vitro/in vivo of a new copper(ii)-based derivative of enrofloxacin†

Enrofloxacin (EFX) was selected as the medicinal ligand to afford a new copper(II)-based complex, EFX-Cu, which was structurally characterized by spectroscopic analyses including X-ray single crystal diffraction. It was also stable and could retain the coordination state in aqueous solution. The in vitro antibacterial activity of EFX-Cu against a panel of pathogenic bacteria was about the same as that of EFX, except that it was twice as active against E. coli. The in vivo test on mice gave a LD₅₀ value of 8148 mg kg^?1 for EFX-Cu, which was much lower than those for EFX (LD₅₀, 5312 mg kg^?1) and its clinically used sodium salt, EFX-Na (LD₅₀, 1421 mg kg^?1). In addition, no obvious lesions in the organs of the dead mice were found by histopathological examination. Pharmacokinetic studies on rats suggested similar pharmacokinetics between EFX-Cu and EFX. On the other hand, EFX-Cu showed higher acute toxicity than EFX-Na in zebrafish, which was inconsistent with that in mice. The ROS-related inflammation and anti-inflammatory assay of EFX-Cu, respectively, in normal cells and zebrafish could be ascribed to its ROS-related redox property. Unfortunately, the final in vivo therapeutic assay in the E. coli-infected mouse model indicated that the therapeutic effect of EFX-Cu, mainly in terms of mortality in mice, was found to be lower than that of EFX-Na at the same dosage (800 mg kg^?1, continuous gavage), although the contradictory factors between toxicity and antibacterial activity could not be excluded in this trial.