Molecular and physicochemical aspects of the interactions of the tuberculostatics ofloxacin and rifampicin with liposomal bilayers: a 31P-NMR and DSC study

Abstract

ζ-Potential measurements were obtained in order to determine whether electrostatic interactions occur between the tuberculostatics ofloxacin and rifampicin and the molecular species of different lipid bilayers. The results obtained showed that both ofloxacin and rifampicin modify the surface charge of lipid vesicles. The extent of these modifications depends on the nature of both the bilayer constituents and the tuberculostatics: a relationship was found between the initial charge at the bilayer surface and the ζ-potential measured changes and so, no significant changes were observed when the presence of anionic lipids in the bilayer was lower than 10%. The highest percentage of variation was for N-palmitoylphosphatidylethanolamine ( NPPE)-containing liposomes, which have the highest negative charge. 31P-NMR experiments showed the ability to adopt the bilayer structure of all the mixtures assayed, although changes in the shape of the resulting spectra were observed in function of both the nature of the molecular constituents of the bilayer and the temperature. The nature of DPPC and DSPC gave rigid bilayers, when the temperature was below that of their transition, whereas the incorporation of NPPE allowed fluid structures above and below the transition temperature of the NPPE-containing mixtures to be obtained. In the presence of the tuberculostatic drugs ofloxacin and rifampicin no fundamental structural changes, e.g. the formation of hexagonal arrangements were observed in the structure of the membrane. However, 31P-NMR spectra of NPPE-containing liposomes underwent a decrease in their intrinsic line width, though no significant changes were observed in the chemical shift anisotropy, when ofloxacin and rifampicin were added to the lipid mixtures at a 1:10 weight ratio. This result was supported by the considerable degree of liposome–drug interactions observed by ζ-potential measurements when liposomes contained NPPE. The DSC study of the effects of ofloxacin and rifampicin on the thermotropic behaviour of the different lipid bilayers showed no significant changes in the main transition temperature of the mixture after the incorporation of these tuberculostatics. The Δ H cal and the Δ S cal values obtained from the calorimetric thermograms were dependent on the lipid composition, were slightly modified in the presence of ofloxacin and fell after the incorporation of rifampicin to the bilayers. The degree of cooperativity only showed appreciable changes when ofloxacin was incorporated into NPPE- and GM1-containing liposomes. ζ-Potential measurements, 31P-NMR experiments and DSC thermotropic studies prove that it is possible to obtain stable preparations of liposomes containing ofloxacin and rifampicin for use in tuberculosis therapy.