Determinants for in vivo anti-tumor effects of PEG liposomal doxorubicin: importance of vascular permeability within tumors.

To elucidate the determinants of the in vivo anti-tumor efficacy of polyethylene glycol (PEG)-modified liposomal doxorubicin (DOX), we examined its anti-tumor effect against three different tumor cell lines (Lewis lung cancer (LLC), Colon-26 (C26) and B16BL6 melanoma (B16)) in vitro and in vivo. In vitro, LLC was the most sensitive tumor to DOX and liposomal DOX based on the MTT assay. However, the strongest in vivo anti-tumor effect was observed in the C26 tumor-bearing mice. The in vivo accumulation of radiolabelled PEG liposome in the C26 tumor after intravenous injection was significantly larger than in other tumors. The extent of vascularity assessed by immunohistochemical staining of CD31 was not directly related with the tumor accumulation of PEG liposome. On the other hand, Evans blue extravasation and secretion of VEGF in C26 tumors were higher than in LLC tumors, clearly demonstrating that the vasculature permeability was higher within C26 tumors. These results indicated that the vascular permeability within the tumor substantially affects the tumor accumulation of PEG liposome and may be one of the important determinants in the in vivo anti-tumor efficacy of PEG liposomal DOX.