The antitumor activity of doxorubicin (DXR) which had been encapsulated in poly(ethylene glycol) (PEP)-coated long-circulating liposomes was examined in mice inoculated with colon 26 carcinoma cells. Six mol% of the distearoylphosphatidylethanolamine derivative of PEGs with different molecular weights was incorporated in liposomes (90-110 nm, mean diameter) composed of distearoylphosphatidylcholine/cholesterol (1/1, molar ratio), and the encapsulating efficiency of DXR in liposomes was more than 98% by the pH gradient method. Each concentration of DXR in blood and tumor tissue was significantly greater after administration of the drug encapsulated in PEG-coated liposomes (DXR-PEG-liposome) compared to the non-coated control liposomes or non-encapsulated free drug. DXR-PEG-liposome prepared with PEG1000 (DXR-PEG1000-liposome) more effectively increased the level of DXR in blood and tumor than did the preparations with PEG5000 or PEG 12000. A single treatment with DXR-PEG1000-liposome (10 mg DXR/kg) resulted in increased survival time. Further therapeutic improvement in terms of tumor growth retardation and prolongation of survival time were observed following multiple treatments with DXR-PEG1000-liposome (3 x 5 mg DXR/kg). Long-circulating liposome coating optimized PEGs should be useful for the delivery of chemotherapeutic agents for the treatment of solid tumors.
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Unezaki et al. (1995) studied this question.