Recently, small molecule inhibitors of transforming growth factorb (TGF-b) type I receptor kinase ⁄ activin receptor-like kinase-5 (ALK5) have been developed to target TGF-b signalling as a therapeutic strategy for combating cancer. In the present study, the authors examined a novel small molecule inhibitor of ALK5,[3][4][5]2,4]triazolo [1,5-a]pyridin-6-yl)-4-(6-methylpyridin-2-yl)thiazol-2-ylamino)methyl)benzonitrile (EW-7203) in breast cancer cells to determine if it has potential for cancer treatment. The inhibitory effects of EW-7203 on TGF-b-induced Smad signalling and epithelial-to-mesenchymal transition (EMT) were investigated in mammary epithelial cells using luciferase reporter assays, immunoblotting, confocal microscopy and wound healing assays. In addition, the suppressive effects of EW-7203 on mammary cancer metastasis to the lung were examined using a Balb ⁄ c xenograft model system. The novel ALK5 inhibitor, EW-7203, inhibited the TGF-b1-stimulated transcriptional activation of p3TP-Lux and pCA-GA 12 -Luc. In addition, EW-7203 decreased phosphorylated Smad2 levels and the nuclear translocation of Smad2 was increased by TGF-b1. In addition, EW-7203 inhibited TGF-b1-induced EMT and wound healing of NMuMG cells. Furthermore, in xenografted Balb ⁄ c mice, EW-7203 inhibited metastasis to the lung from breast tumors. The novel ALK5 inhibitor, EW-7203, efficiently inhibited TGF-b1-induced Smad signalling, EMT and breast tumor metastasis to the lung in vivo, demonstrating that EW-7203 has therapeutic potential for breast cancer metastasis to the lung. (Cancer Sci 2011; 102: 1889-1896 M etastasis is the primary cause of death among cancer patients, and breast cancer patients are no exception.(1,2) During breast cancer progression, transforming growth factor-b (TGF-b) levels are elevated (3,4) and the tumor-suppressing function of TGF-b is abrogated.(5) Transforming growth factor-b has at least two roles during carcinogenesis, for although it was initially found to be a tumor-suppressing cytokine, it also acts as a mediator of metastasis to the lung.(6,7) During the initial stage of metastasis, cancer cells undergo epithelial-to-mesenchymal transition (EMT), which involves loss of epithelial cell polarity, acquisition of the mesenchymal phenotype, greater motility and invasiveness, and disruption of cell-to-cell adhesion. The EMT is generally induced in epithelial cells by signals released from mesenchymal cells that compose the stroma of normal and neoplastic tissues. Furthermore, members of the TGF-b cytokine family are the foremost characterized inducers of EMT during embryonic development, wound healing, fibrotic diseases and cancer pathogenesis, (7,9) which suggests that TGFb might induce EMT via multiple signalling mechanisms. The canonical TGF-b signalling pathway is activated when TGF-b binds to TGF-b type II receptor (TbRII). Initially, this binding facilitates activation of TGF-b type I receptor (TbRI) kinase (activin receptor-like kinase 5 [ALK5]), (10) which contains a kinase domain that phospho...