Ciprofloxacin metalloantibiotic: an effective antibiotic with an influx route strongly dependent on lipid interaction?

Fluoroquinolones are antibiotics that have a large spectrum of action against bacteria, especially Gram-negative. A strategy to enhance their pharmacological behavior, and try to counteract bacterial resistance, is their coordination to divalent metal ions and 1,10-phenanthroline. These stable complexes modify fluoroquinolones potency and specificity, possibly due to their alternative translocation through the bacterial membranes. In this work, we determined the interaction of ciprofloxacin and its copper(II) ternary complex with unilamellar liposomes of DMPC, POPE/POPG (0.75:0.25), POPE/POPG/cardiolipin (0.67:0.23:0.10), and E. coli total extract, using time-resolved and steady-state fluorescence spectroscopy. The association constants determined show that the interaction of both compounds depends on membrane lipids composition and is always higher for the metalloantibiotic, a trend already observed for other fluoroquinolone metalloantibiotics. Nevertheless, the interaction of ciprofloxacin metalloantibiotic is, normally, higher, which reflects the fluoroquinolone species that are present in solution at physiological pH. In overall, the results obtained suggest that ciprofloxacin and its metalloantibiotic have different translocation pathways, proposing that the diffusion of the metalloantibiotic is a hydrophobic mechanism and suggesting that this new metalloantibiotic may be a good choice to replace the pure ciprofloxacin and bypass, at least, one of the mechanisms of the bacterial resistance to fluoroquinolones.