Autitumor interaction of short-course endostatin and ionizing radiation

PURPOSE The purpose of this study was to evaluate whether endostatin, an antiangiog enic cleavage fragment of collagen XVIII, enhances the antitumor effects of ionizing radiation (IR). Endostatin was injected to coincide with fraction ated radiotherapy. METHODS Xenografts of radioresistant SQ-20B tumor cells were established in athymic nude mice. Lewis lung carcinoma cells were injected into C57B1/6 mice, Mic e bearing SQ-20B xenografts were injected intraperitoneally with 2.5 mg/kg/ day of murine recombinant endostatin 5 limes per week for 2 weeks 3 hours b efore IR treatment (50 Gy total dose). Mice bearing Lewis lung carcinoma tu mors were injected intraperitoneally with endostatin (2.5 mg/ kg/day) four times; the first injection was given 24 hours before the first IR dose (15 Gy) and then 3 hours before IR (15 Gy/day) for 3 consecutive days. Microvas cular density was assessed on tumor tissue sections by use of CD31 immunohi stochemistry and light microscopy. Endothelial cell survival analyses were employed to evaluate endostatin effects on human aortic endothelial cells a nd human umbilical vein endothelial cells. Endothelial cell apoptosis was e xamined by use of FAGS analysis and DAPI microscopy. RESULTS In SQ-20B xenografts, combined treatment with endostatin and IR produced tu mor growth inhibition that was most pronounced at the nadir of regression ( day 21), By day 35, tumors receiving combined treatment with endostatin and IR were 47% smaller than tumors treated with endostatin alone. Interactive cytotoxic treatment effects between endostatin and IR were also demonstrat ed in mice bearing Lewis lung carcinoma tumors. Significant tumor growth in hibition was observed in the endostatin/IR group at days 11 and 13 compared with IR alone. Histologic analyses demonstrated a reduction in microvascul ar density after combined treatment with endostatin and IR compared with en dostatin treatment alone. Survival analyses confirmed interactive cytotoxic ity between endostatin and IR in both human aortic endothelial cells and hu man umbilical vein endothelial cells but not in SQ-20B tumor cells. Combine d treatment with endostatin and IR produced an increase in cow pulmonary ar tery endotheljal apoptosis compared with either treatment alone. DISCUSSION The tumor regression observed after combined treatment with endostatin and IR suggests additive antitumor effects in both human and murine tumors. imp ortantly, the concentrations of endostatin employed produced little tumor r egression when endostatin was employed as a single agent. The results from the clonogenic and apoptosis assays support the hypothesis that the endothe lial compartment is the target for the endostatin/IR interaction.