Combination of Vessel-targeting Agents With Fractionated Radiation Therapy: The Role of SDF-1/CXCR4 Pathway

Abstract

To investigate the changes of structures and functions in tumor microvasculature during fractionated radiation therapy (F-RT), and examine the possibility of enhancing RT effects by targeting pericytes and bone marrow derived cells (BMDCs). Tumors in C57BL mice were generated by i.m. inoculation of TRAMP C-1 (Transgenic Adenocarcinoma of Mouse Prostate) cell line. Tumors at size of 4 mm were treated by a F-RT protocol with 4 Gy/per fraction (fx), 60 Gy in 15 fx, and 5 fx/week. Tumors were monitored in size and collected at indicated time points for studying the structural and functional changes of tumor microvasculature. Combination of Gefitinib (an epidermal growth factor receptor inhibitor) or AMD3100 (an antagonist of CXCR4) with F-RT was implanted to study the effects of targeting pericytes or BMDCs, respectively. The transplantation of green fluorescent protein-tagged bone marrow (GFP-BM) into lethally irradiated recipient mice was used to study the roles of BMDCs on vessel formation. In F-RT-treated tumors, the microvascular density (MVD) was reduced, but the remaining vessels were dilated, incorporated with GFP-positive cells, tightly adhered with pericytes, and well perfused with Hoechst 33342 dyes. These changes suggested remaining vessels are in a more mature structure and vasculogenesis is involved in their survival during F-RT. Although the combination of the EGFR inhibitor gefitinib with F-RT affected vascular structure by dissociating pericytes from vascular wall, it had no enhancement effect on tumor growth delay. This combination in fact protected tumor vessels from irradiation damage, in accord with the elevation of MVD and good vascular perfusion leading to less tumor necrosis and hypoxia. In contrast, combination of CXCR4 inhibitor AMD3100 with F-RT had a significant enhancement effect on tumor growth delay. As compared with those treated by F-RT alone, tumors treated by this combination had less infiltration of GFP-positive BM cells, lower MVD, poorer perfusion function in tumor vessels and more hypoxia; these tumor vessels were not covered by pericytes. Our results suggest that vasculogenesis is involved in surviving tumor vessels during fractionated radiation therapy. There are complex interactions between combination of vessel-targeting therapy with RT and enhancement effect do not always exist. However, blockage of the influx of BMDCs by CXCR4 inhibitor intervenes the vascular maturation via vasculogenesis and has an enhancement effect on RT.