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

zeta-Potential measurements were obtained in order to determine whether ele ctrostatic interactions occur between the tuberculostatics ofloxacin and ri fampicin and the molecular species of different lipid bilayers. The results obtained showed that both ofloxacin and rifampicin modify the surface char ge of lipid vesicles. The extent of these modifications depends on the natu re of both the bilayer constituents and the tuberculostatics: a relationshi p was found between the initial charge at the bilayer surface and the zeta- potential measured changes and so, no significant changes were observed whe n the presence of anionic lipids in the bilayer was lower than 10%. The hig hest percentage of variation was for N-palmitoylphosphatidylethanolamine (N PPE)-containing liposomes, which have the highest negative charge. P-31-NMR experiments showed the ability to adopt the bilayer structure of all the m ixtures assayed, although changes in the shape of the resulting spectra wer e 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 bil ayers, when the temperature was below that of their transition, whereas the incorporation of NPPE allowed fluid structures above and below the transit ion temperature of the NPPE-containing mixtures to be obtained. In the pres ence of the tuberculostatic drugs ofloxacin and rifampicin no fundamental s tructural changes, e.g. the formation of hexagonal arrangements were observ ed in the structure of the membrane. However, 31P-NMR spectra of NPPE-conta ining 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-dr ug interactions observed by zeta-potential measurements when liposomes cont ained NPPE. The DSC study of the effects of ofloxacin and rifampicin on the thermotropic behaviour of the different lipid bilayers showed no significa nt changes in the main transition temperature of the mixture after the inco rporation of these tuberculostatics. The Delta H-cal and the Delta S-cal va lues 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 cooper ativity only showed appreciable changes when ofloxacin was incorporated int o NPPE- and GM1-containing liposomes. zeta-Potential measurements, P-31-NMR experiments and DSC thermotropic studies prove that it is possible to obta in stable preparations of liposomes containing ofloxacin and rifampicin for use in tuberculosis therapy. (C) 1999 Elsevier Science B.V. All rights res erved.