AbstractPurpose: c-Src has been shown to play a pivotal role in breast cancer progression, metastasis, and angiogenesis. In the clinic, however, the limited efficacy and high toxicity of existing c-Src inhibitors have tempered the enthusiasm for targeting c-Src. We developed a novel c-Src inhibitor (UM-164) that specifically binds the DFG-out inactive conformation of its target kinases. We hypothesized that binding the inactive kinase conformation would lead to improved pharmacologic outcomes by altering the noncatalytic functions of the targeted kinases.Experimental Design: We have analyzed the anti–triple-negative breast cancer (TNBC) activity of UM-164 in a comprehensive manner that includes in vitro cell proliferation, migration, and invasion assays (including a novel patient-derived xenograft cell line, VARI-068), along with in vivo TNBC xenografts.Results: We demonstrate that UM-164 binds the inactive kinase conformation of c-Src. Kinome-wide profiling of UM-164 identified that Src and p38 kinase families were potently inhibited by UM-164. We further demonstrate that dual c-Src/p38 inhibition is superior to mono-inhibition of c-Src or p38 alone. We demonstrate that UM-164 alters the cell localization of c-Src in TNBC cells. In xenograft models of TNBC, UM-164 resulted in a significant decrease of tumor growth compared with controls, with limited in vivo toxicity.Conclusions: In contrast with c-Src kinase inhibitors used in the clinic (1, 2), we demonstrate in vivo efficacy in xenograft models of TNBC. Our results suggest that the dual activity drug UM-164 is a promising lead compound for developing the first targeted therapeutic strategy against TNBC. Clin Cancer Res; 22(20); 5087–96. ©2016 AACR.
