Factors affecting the accelerated blood clearance of polyethylene glycol-liposomes upon repeated injection

Previously, we showed that long-circulating polyethylene glycol (PEG)-lipos omes are cleared rapidly from the circulation when injected repeatedly in t he same animal. In this article, we describe the effects of PEG-coating, th e circulation time, the lipid dose, and the presence of encapsulated doxoru bicin on the pharmacokinetics upon repeated injection in rats. Furthermore, the role of liver and splenic macrophages was investigated. Liposomes with out PEG-coating also showed the so-called "enhanced clearance effect": bloo d levels at 4 h post injection decreased from 62.8 +/- 13.7% of injected do se (%ID) after the first injection to 0.54 +/-0.21%ID after the second inje ction. This decrease was independent of the circulation time of the first d ose. Decreasing the first lipid dose of PEG-liposomes to 0.05 mu mol/kg sti ll led to enhanced clearance of a second dose of 5 mu mol/kg. No changes in pharmacokinetics were observed when the second dose was 50 mu mol/kg. When hepatosplenic macrophages were depleted, no enhanced clearance of repeated liposome injections was observed. A dose of doxorubicin containing PEG-lip osomes (Doxil(1) or Caelyx(2)), injected 1 week after injection of empty PE G-liposomes, was cleared rapidly from the circulation in rats. Our results indicate that hepatosplenic macrophages play an essential role in the enhan ced clearance effect and that the change in pharmacokinetic behavior upon r epeated injection is a general characteristic of liposomes, unrelated to th e presence of PEG. Therefore, these findings may have a considerable impact on the clinical application of liposomal formulations that are administere d repeatedly.