INTERACTION OF AMINOGLYCOSIDES AND COLISTIN WITH MODEL MEMBRANES - LIPOSOMES AND MONOLAYERS

The interaction of gentamicin (G), kanamycin (K), spectinomycin (Sp), streptomycin (St) and a polymixin, colistin (C), with lipids was studied by using liposomes and monomolecular layers as membrane models. The lipids used were phosphatidylcholine (PC), phosphatidylserine (PS), phosphatidylinositol (PI), phosphatidylinositol disphosphate (PIP2), gangliosides (Gan) and cholesterol (Chol). The results show that only colistin is able to induce the leakage of entrapped carboxyfluorescein (CF) from the liposomes after incubation. Moreover, this interaction is not clearly related to a single type of phospholipid, is dependent on the PL/colistin ratio and is slightly modified by the pH or the temperature of the incubation media. The interactions of these molecules with the polar heads of phospholipids were studied using 1-anilino-8-napthalenesulphonic acid (ANS) as fluorescent probe. The polarization values indicate that these antibiotics induce rigidification of the membrane. Monolayers having the same lipid composition of liposomes were spread on aqueous subphases and pressure increases, after injection of drug molecules, were recorded. In this set of experiments all molecules exhibit interaction, colistin and kanamycin being those with the maximum activity. Moreover, no specific differences could be assigned to the different lipids used except for kanamycin and gangliosides. The mathematical equations of the penetration kinetics for all the assays carried out were determined. The Lineweaver-Burk equation gave the best fit to the experimental values, the regression coefficient being higher than 0.98 in all cases. Compression isotherms of the same phospholipid mixtures were recorded, the antibiotic molecules being dissolved in the subphase. The compressibility and area/molecule were slightly affected by the presence of aminoglycosides in the aqueous phase. In contrast, the presence of colistin induced expansion of the monolayer especially at low pressures, thus indicating the existence of interactions. Comparing these results with those in the literature, it appears clear that the membrane model used exerts a strong influence on the results obtained.