Enhanced delivery of doxorubicin to tumor by long-circulating thermosensitive liposomes and local hyperthermia

Doxorubicin (DXR) was encapsulated in long-circulating, thermosensitive liposomes (180-200 nm), prepared from dipalmitoylphosphatidylcholine (DPPC)/distearoylphosphatidylcholine (DSPC) (9:1 (m/m)) and 6 mol% of ganglioside G_M1 (G_M1), with 95-98% entrapping efficiency by the pH-gradient method. 45% of the entrapped DXR was released from these G_M1/DPPC/DSPC liposomes by incubation at 42°C for 5 min in 20% serum or saline (this degree of release was lower than that of hydrophilic drugs such as cisplatin, due to the basic and amphiphilic nature of DXR). Inclusion of G_M1 (6 mol%) endowed DPPC/DSPC liposomes with prolonged circulation ability, resulting in increased blood levels of liposomes and decreased reticuloendothelial system uptake over 6 h after injection. Concomitantly, DXR levels in blood remained high for long time. Accumulation of DXR into tumor tissue of tumor-bearing mice (mouse colon carcinoma 26) by local hyperthermia after injection of DXR loaded, long-circulating, thermosensitive (DXR-G_M1/DPPC/DSPC) liposomes was 2.5-times or 6-times higher than that after treatment with DXR-DPPC/DSPC liposomes or free DXR in combination with hyperthermia, respectively. Furthermore, the treatment with DXR-G_M1/DPPC/DSPC liposomes and hyperthermia resulted in effective tumor-growth retardation and increased survival time. Our results indicate that the combination of drug-loaded, 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.