Effects of regorafenib on the mononuclear/phagocyte system and how these contribute to the inhibition of colorectal tumors in mice

Abstract

AbstractBackground Regorafenib was previously shown to reduce tumor-associated macrophages and potently inhibit colony-stimulating factor 1 receptor (CSF1R), also known as CD115, in biochemical assays. The CSF1R signaling pathway is essential in the biology of the mononuclear/phagocyte system, which itself can promote the development of cancer. Methods A deeper investigation of regorafenib’s effects on CSF1R signaling was performed using preclinicalin vitroandin vivostudies with syngeneic CT26 and MC38 mouse models of colorectal cancer. Peripheral blood and tumor tissue were analyzed mechanistically by flow cytometry using antibodies against CD115/CSF1R and F4/80 and by ELISA for chemokine (C-C motif) ligand 2 (CCL2) levels. These read-outs were correlated with drug levels for the detection of pharmacokinetic/pharmacodynamic relationships. Results Potent inhibition of CSF1R by regorafenib and its metabolites M-2 and M-5 was confirmedin vitroin RAW264.7 macrophages. The dose-dependent growth inhibition of subcutaneous CT26 tumors by regorafenib was associated with a significant reduction in both the number of CD115himonocytes in peripheral blood and the number of selective subpopulations of intratumoral F4/80hitumor-associated macrophages. CCL2 levels in blood were not affected by regorafenib but increased in tumor tissue, which may contribute to drug resistance and prevent complete tumor remission. An inverse relationship between regorafenib concentration and the number of CD115himonocytes and CCL2 levels was observed in peripheral blood, supporting the mechanistic involvement of regorafenib. Conclusions These findings may be clinically useful in optimizing drug dosing by blood-based pharmacodynamic markers, and in identifying resistance mechanisms and ways to overcome them by appropriate drug combinations.