Doxorubicin Encapsulated in Long-Circulating Thermosensitive Liposomes

Doxorubicin (DXR) was encapsulated in long-circulating, thermosensitive liposomes (TSL, 180-200 nm in mean diameter), prepared from dipalmitoyl phosphatidyl choline (DPPC)/distearoyl phosphatidyl choline (DSPC) (9:1, m/m) and either 3 mol% of amphipathic polyethylene glycol (PEG) with 1000 in average molecular weight or 6 mol% of ganglioside GMI (GMI), with 95-98% entrapping efficiency by the pH gradient method. 57% or 45% of the entrapped DXR was released from PEG/DPPC/DSPC or GM1/DPPC/DSPC liposomes, respectively, by incubation with 20% serum at 42°C for 5 min. Inclusion of PEG or GM1 endowed TSL with prolonged circulation ability, resulting in increased blood levels of liposomes and decreased reticuloendothelial system (RES) uptake over 6 hours after injection. Concomitantly, high DXR level in blood was kept for long time.Accumulation of DXR into tumor tissue of tumor-bearing mice (mouse colon carcinoma 26) by local hyperthermia after injection of DXR-long-circulating TSL was 2 times or 7 times higher than that after treatment with DXR-TSL liposomes or free DXR in combination with hyperthermia, respectively. Furthermore, the systemic treatment with DXR-long-circulating TSL and hyperthermia resulted in effective tumor growth retardation and increased survival time. These results indicate that the combination of long-circulating, thermosensitive liposomes with local hyperthermia at the tumor site could be clinically useful for delivering a wide range of chemotherapeutic agents in the treatment of solid tumors.