In vivo anti-tumor effect of PEG liposomal doxorubicin (DOX) in DOX-resistant tumor-bearing mice: Involvement of cytotoxic effect on vascular endothelial cells

We evaluated the in vivo anti-tumor effect of polyethylene glycol-modified liposomal doxorubicin (PEG liposomal DOX) in the DOX-resistant Colon-26 cancer cells (C26/DOX)-bearing mice model. IC50 value of DOX to C26/DOX in vitro (40.0 µM) was about 250 times higher than that to control C26 (C26/control) (0.15 µM). However, in vivo anti-tumor effect of PEG liposomal DOX was similar in both C26/control- and C26/DOX-bearing mice, suggesting that the in vivo anti-tumor effect of PEG liposomal DOX was not directly reflecting the sensitivity of these tumor cells to DOX. IC50 value (0.10 µM) of DOX to HUVEC, a model vascular endothelial cell, was similar to that of C26/control. Double immunohistochemical staining of vascular endothelial cells and apoptotic cells within the tumor tissue after intravenous administration of PEG liposomal DOX showed that the extent of co-localization of apoptotic cells with endothelial cells was significantly higher for C26/DOX tumors (60%) than C26/control ones (20%), suggesting that the apoptosis is caused preferentially for vascular endothelial cells in C26/DOX tumor. From these results, it was suggested that the cytotoxic effect of DOX on vascular endothelial cells in the tumor would be involved in the in vivo anti-tumor effect of PEG liposomal DOX in C26/DOX-bearing mice.