The ALK‐5 inhibitor A‐83‐01 inhibits Smad signaling and epithelial‐to‐mesenchymal transition by transforming growth factor‐β

Transforming growth factor (TGF)‐β signaling facilitates tumor growth and metastasis in advanced cancer. Use of inhibitors of TGF‐β signaling may thus be a novel strategy for the treatment of patients with such cancer. In this study, we synthesized and characterized a small molecule inhibitor, A‐83‐01, which is structurally similar to previously reported ALK‐5 inhibitors developed by Sawyer et al. (2003) and blocks signaling of type I serine/threonine kinase receptors for cytokines of the TGF‐β superfamily (known as activin receptor‐like kinases; ALKs). Using a TGF‐β‐responsive reporter construct in mammalian cells, we found that A‐83‐01 inhibited the transcriptional activity induced by TGF‐β type I receptor ALK‐5 and that by activin type IB receptor ALK‐4 and nodal type I receptor ALK‐7, the kinase domains of which are structurally highly related to those of ALK‐5. A‐83‐01 was found to be more potent in the inhibition of ALK5 than a previously described ALK‐5 inhibitor, SB‐431542, and also to prevent phosphorylation of Smad2/3 and the growth inhibition induced by TGF‐β. In contrast, A‐83‐01 had little or no effect on bone morphogenetic protein type I receptors, p38 mitogen‐activated protein kinase, or extracellular regulated kinase. Consistent with these findings, A‐83‐01 inhibited the epithelial‐to‐mesenchymal transition induced by TGF‐β, suggesting that A‐83–01 and related molecules may be useful for preventing the progression of advanced cancers. (Cancer Sci 2005; 96: 791–800)