THE OVERALL PARTITIONING OF ANTHRACYCLINES INTO PHOSPHATIDYL-CONTAINING MODEL MEMBRANES DEPENDS NEITHER ON THE DRUG CHARGE NOR THE PRESENCEOF ANIONIC PHOSPHOLIPIDS

Anthracyclines are potent anticancer agents. Their use is limited by t he problem of multidrug resistance (MDR) associated with a decreased i ntracellular accumulation of drug correlated with the presence, in the membrane of resistant cells, of the P-glycoprotein responsible for an active efflux of the drug. The activity of a drug depends upon its in tracellular concentration which itself depends on the kinetics (a) of passive influx (b) of passive efflux and (c) of the P-glycoplotein-med iated efflux of drug across the cell membrane. The ability of an anthr acycline to overcome MDR depends largely on the first point. The passi ve drug uptake is governed by their incorporation into the lipid matri x and both electrostatic and hydrophobic forces seem necessary for the stabilization of anthracyclines into lipid bilayers. The aim of the p resent study was to determine the relative importance of these two int eractions. Using microspectrofluorometry and the observation that the fluorescence of anthracycline is enhanced when the dihydroanthraquinon e part is embedded within the lipid bilayer, we have determined the pa rtition coefficient (alternatively, the binding constant) of 12 anthra cycline derivatives in large unilamellar vesicles. The anthracyclines were (a) doxorubicin, daunorubicin and idarubicin which, at pH 7.2, be ar a single positive charge at the level of the amino group on the sug ar, (b) their corresponding neutral 3'-hydroxy derivatives where the a mino group in the sugar has been replaced by a hydroxyl, (c) the three 13-hydroxy derivatives, doxorubicinol, daunorubicinol and idarubicino l, (d) pirarubicin and (e) two permanently positively charged derivati ves. The large unilamellar vesicles contained phosphatidylcholine with various amounts of phosphatidic acid which is negatively charged and of cholesterol. We came to the conclusion that the efficiency of drug incorporation in the bilayers depends neither on the presence of a pos itive charge on the drug nor on the presence of anionic phospholipid b ut on the hydrophobicity of the molecule: the neutral and the positive ly charged form have the same ability to partition into the bilayer. H owever, the percentage of each form present should depend on the elect rostatic parameters.