Equilibrium, structural and antibacterial characterization of moxifloxacin-β-cyclodextrin complex

Abstract

Moxifloxacin (MOX), a novel fourth-generation fluoroquinolone antibacterial was characterized in terms of β-cyclodextrin (β-CD) complexation in order to improve its antibacterial activity. The inclusion complexation has been examined with a wide variety of state-of-the-art analytical techniques, such as nuclear magnetic resonance spectroscopy (NMR), affinity capillary electrophoresis (ACE), mass spectrometry (MS), infrared spectroscopy (IR) and differential scanning calorimetry (DSC). The stoichiometry of the complex was investigated by two different techniques. NMR Job plot method indicated 1:1 stoichiometry in liquid state, however MS study revealed that a complex with 2:1 (MOX:β-CD) stoichiometry can also be formed in gas phase. The stability constants were determined by 1H NMR titration and ACE at different pH values, where MOX exists predominantly in monocationic, neutral and monoanionic form, respectively, indicating that the neutral macrospecies forms the most stable complex with the host (logK = 2.51 ± 0.03). Geometric aspects of the inclusion complex were assessed by 2D ROESY NMR and proved that the tricyclic moiety of guest can enter the host cavity. To understand the interaction of different protonation forms of MOX with β-CD at atomic level molecular modeling studies were also performed. Solid state complexation in 1:1 M ratio was carried out by lyophilization and investigated by DSC and IR, which also confirmed the inclusion complex formation in solid state. The antibacterial activity of the complex was tested against Gram-negative and Gram-positive bacteria by determination of minimum inhibitory concentrations, which revealed that supramolecular interactions do not affect significantly the antibacterial activity of the drug.