Discovery of Two Highly Selective Structurally Orthogonal Chemical Probes for Activin Receptor-like Kinases 1 and 2

Activin receptor-like kinases 1-7 (ALK1-7) play a key role in regulating a complex network of both SMAD-dependent and SMAD-independent signaling pathways. LDN-193189 is commonly used as an inhibitor in studies of these pathways, particularly for bone morphogenetic protein (BMP) signaling. However, its limited kinome-wide selectivity presents challenges in its application for cellular target validation assays. In this study, we present the identification and detailed characterization of two chemically distinct, highly selective inhibitors of ALK1 and ALK2, M4K2234 and MU1700, along with their respective negative controls. We demonstrate that both MU1700 and M4K2234 effectively inhibit the BMP pathway by selectively targeting ALK1/2 kinases in cells, and show promising in vivo efficacy in mice. Notably, MU1700 exhibits significant brain penetration, with remarkable accumulation in the brain. These high-quality, orthogonal chemical probes provide the necessary selectivity to serve as valuable tools for both in vitro and in vivo investigations of BMP signaling.