Selectivity of the anthracyclines for negatively charged model membranes: role of the amino group.

The equilibrium-binding affinities of six adriamycin analogues and four daunomycin derivatives for negatively charged dimyristoyl phosphatidylcholine/dimyristoyl phosphatidic acid (DMPC/DMPA) small unilamellar vesicles are compared with values for electroneutral DMPC liposomes. Binding of the daunomycin series to negatively charged dimyristoyl phosphatidyl glycerol (DMPG) vesicles was also examined. Under physiological conditions of pH and ionic strength, substitution of the amino group of adriamycin or daunomycin resulted in a reduced affinity for negatively charged bilayers, even if the substituent enhanced the degree of ionization of the amine. Decreasing the ionic strength increases the binding affinity for acidic membranes but decreases the drug affinity for neutral membranes. We propose that the electrostatic bond of the phosphate-amino group that has been shown to exist between anthracyclines and phosphatidic acid is sterically destabilized by substitution of the amino group. The results are consistent with a mode of anthracycline binding to negatively charged membranes which is driven by hydrophobic and electrostatic considerations but is destabilized by steric bulk at the amino group. The data also provide insight into the design of new anthracyclines with high membrane affinities and reduced uptake; such directed interaction with plasma membranes may enhance antineoplastic potential while reducing cardiac toxicity.