Ligand self-association at the surface of liposomes: a complication during equilibrium-binding studies

Daunomycin and carminomycin, two anthracycline antibiotics known to bind phospholipid bilayers, appear to self-associate at the surface of liposomes at high bound drug/lipid ratios (r). Fluorescence intensity, lifetime, and anisotropy measurements have been used to monitor the equilibrium binding of these drugs to small unilamellar solid-phase dipalmitoylphosphatidylcholine vesicles. Association of an anthracycline with excess liposome (low r) resulted in an increase in both the observed intensity and the fluorescence lifetime. At low vesicle concentrations (high r), a decrease in the total emission intensity was observed which was not paralleled by the excited-state lifetime. The data from these experiments are consistent with the formation of nonfluorescent anthracycline complexes at the surface of liposomes. Such ligand self-association is a potential complication in any studies on the interaction of amphipathic molecules with liposomes conducted at high r values. Because ligand self-association limits the collection of binding data over certain concentration ranges, this consequently results in greater uncertainty in the determination of the maximum value of r (n) in equilibrium binding studies.