Molecular Interactions of Fluoroquinolone Antibiotics with Lipid Membranes

Levofloxacin is a broad-spectrum fluoroquinolone antibiotic in clinical use that targets DNA gyrase in the cytosol. It is used in systemic applications via oral or intravenous route, and its pharmacokinetics and access to its molecular targets are strongly influenced by interactions with cellular membranes. We used NMR and MD simulations to investigate the physical state of levofloxacin in solution and its interactions with lipid membranes, assessing the role of membrane charge and antibiotic concentration. Using zwitterionic DOPC and negatively charged DOPC/DOPG lipid membranes, we observe concentration-dependent self-association of levofloxacin in solution below its solubility limits and association with lipid membranes with a preference for negatively charged bilayers. Below the solubility limit, levofloxacin solutions that appear clear contain self-associated molecular assemblies in fast exchange equilibrium with a monomeric soluble population. In the presence of lipid membranes, this equilibrium is shifted in favor of a membrane-associated population with preference for negative lipids. MD simulations show levofloxacin condensation in solution and fractional membrane insertion, which suggest the presence of a molecular population embedded in the lipid bilayer, coexisting with self-associated levofloxacin molecular "droplets" in exchange with a solvated population.