Toxicity of doxorubicin entrapped within long-circulating liposomes

We studied the effect of doxorubicin entrapped within long-circulating liposomes (Dox-LCL) on the phagocytic capacity and bacterial blood clearance capacity of rat liver macrophages. Dox-LCL (125 nm in diameter) were composed of egg phosphatidylcholine (PC), cholesterol (CH) and poly(ethyleneglycol)distearoylphosphatidylethanolamine (PEG-PE) (55:45:5 molar ratio; MW PEG 1900), and loaded with doxorubicin by means of a trans-membrane pH gradient. The doxorubicin/lipid ratio was 0.36:1 (mol/mol). At different time-points after one, two or three intravenous injections of Dox-LCL, radiolabeled negatively charged test liposomes (egg PC, CH, and phosphatidylserine in a 4:5:1 molar ratio) were injected. After 2 h, liver macrophages were isolated and the amount of macrophage-associated radioactivity was determined. Twenty-four hours after a single injection of 5 mg/kg Dox-LCL, no significant effect was observed. However, 48 h after injection, phagocytic activity was reduced significantly (49%). Recovery of phagocytic capacity of the liver macrophages took 8 days after two injections of Dox-LCL (2 x 5 mg/kg). Seventy-two hours after the last of two injections of Dox-LCL, bacterial blood clearance was significantly reduced as compared to clearance in control rats and in rats injected twice with doxorubicin combined with placebo liposomes. When comparing these Dox-LCL data with previous data on the effects of Dox-L, Dox-LCL appears less toxic than Dox-L for the liver macrophage population following i.v. administration both with respect to specific phagocytic activity and cell numbers. Due to the delay in onset of toxic effects and the faster recovery from Dox-LCL treatment as compared to Dox-L treatment, it is conceivable that therapeutic protocols can be designed with Dox-LCL that circumvent long-term impairment of the liver macrophage population.