Doxorubicin induces aggregation of small negatively charged liposomes

Abstract

The anticancer drug doxorubicin (DOX) can induce aggregation of liposomes containing a transmembrane ammonium sulfate gradient. This process proved to be dependent on several parameters. (1) Initial vesicle size. Aggregation of liposomes composed of dipalmitoylphosphatidylcholine (DPPC)/dipalmitoylphosphatidyl glycerol (DPPG)/cholesterol (CHOL) (10:1:4), molar ratio with an average diameter of 0.07 μm, was more pronounced than for those of 0.1 and 0.2 μm. (2) Doxorubicin (DOX)/phospholipid (PL) ratio. A certain threshold DOX/PL ratio was required to induce aggregation. (3) Composition of the bilayer. The shorter the fatty acid chain of the PC component, the higher the tendency to aggregate (dimiristoylphosphatidylcholine (DMPC)>DPPC>distearoylphosphatidylcholine (DSPC). Besides, the addition of cholesterol increased the critical DOX/PL ratio required to induce aggregation. Interestingly, no aggregation was observed for egg phosphatidylcholine (EPC)/egg phosphatidylglycerol (EPG)/CHOL (10:1:4) liposomes. The presence and nature of the negative charge inducing lipid in the bilayer also played a role. Liposomes containing DPPG were more susceptible to aggregation than those containing dipalmitoylphosphatidylserine (DPPS), whereas neutral liposomes (DPPC/CHOL (10:4)) did not aggregate. If aggregation occurred with cholesterol free liposomes, it always happened after liposome loading with DOX at elevated temperatures, during the cooling down process, when liposomes reached their phase transition temperature. Then, we suggest that non-entrapped doxorubicin binds to the lipid bilayer (mainly by electrostatic interactions), as well as to other doxorubicin molecules (stacking), which in turn interact with membranes of other liposomes, thus inducing aggregation.