Regorafenib sensitizes human breast cancer cells to radiation by inhibiting multiple kinases and inducing DNA damage

Purpose: Triple-negative breast cancer (TNBC) is the most challenging and aggressive subtype of breast cancer with limited treatment options because of tumor heterogeneity, lack of druggable targets and therapy resistance. TNBCs are characterized by overexpression of growth factor receptors such as epidermal growth factor receptor (EGFR), vascular endothelial growth factor receptor (VEGFR), and platelet derived growth factor receptor (PDGFR) making them promising therapeutic targets. Regorafenib is an FDA approved oral multi-kinase inhibitor that blocks the activity of multiple protein kinases including those involved in the regulation of tumor angiogenesis [VEGFR1-3, TIE2], tumor microenvironment [PDGFR-β, FGFR] and oncogenesis (KIT, RET, RAF-1, BRAF). In the current study we examined the radiosensitizing effects of Regorafenib on TNBC cell lines and explored the mechanism by which Regorafenib enhances radiosensitivity.

Methods: MDA-MB-231 and SUM159PT (human TNBC cell lines) and MCF 10a (human mammary epithelial cell line) were treated with Regorafenib, ionizing radiation or a combination of both. Following treatment with Regorafenib and radiation we conducted clonogenic assay to determine radiosensitivity, immunoblot analysis to assess the effect on key signaling targets, tube formation to evaluate effect on angiogenesis and comet assay as well as western blot for γH2AX to assess DNA damage response (DDR).

Results: Regorafenib reduced cell proliferation and enhanced radiosensitivity of MDA-MB-231 and SUM159PT cell lines but had no effect on the MCF 10a cells. Clonogenic survival assays showed that the surviving fraction at 2 Gy for both MDA-MB-231 and SUM159PT was reduced from 66.4 ± 8.9 and 88.2 ± 1.7 in controls to 38.1 ± 4.9 and 75.1 ± 1.1 following a 24 hr pretreatment with 10 μM and 5 μM Regorafenib respectively. A marked reduction in the expression of VEGFR, PDGFR, EGFR and the downstream target, ERK, was observed with Regorafenib treatment alone or in combination with radiation. We also observed a significant inhibition of VEGF-A production in the TNBC cell lines following treatment with Regorafenib. Further, the addition of conditioned medium from Regorafenib-treated tumor cells onto human umbilical vein endothelial cells (HUVEC) suppressed tube formation, indicating an inhibition of tumor angiogenesis. Regorafenib also decreased migration of TNBC cells and suppressed radiation-induced DNA damage repair in a time-dependent manner.

Conclusions: Our findings demonstrate that Regorafenib enhanced radiosensitivity of breast cancer cells by inhibiting the expression of multiple receptor tyrosine kinases, VEGF-mediated angiogenesis and DNA damage response in TNBC. Therefore, combining Regorafenib with radiation and antiangiogenic agents will be beneficial and effective in controlling TNBC.