Design, Synthesis, and Activity of a Novel Series of Pyridazine-Based ALK5 Inhibitors.

ALK5 inhibitors represent an attractive therapeutic approach for the treatment of a variety of pathologies, including cancer and fibrosis. Herein, we report the design and in vitro characterization of a novel series of ALK5 modulators featuring a 4,6-disubstituted pyridazine core. A knowledge-based scaffold-hopping exploration was initially conducted on a restricted set of heteroaromatic cores using available ligand- and structure-based information. The most potent structurally novel hit compound 2A was subsequently subjected to a preliminary optimization for the inhaled delivery, applying physicochemical criteria aimed at minimizing systemic exposure to limit the risk of adverse side effects. The resulting inhibitors showed a marked boost in potency against ALK5 and in vitro ADME properties, potentially favoring lung retention. The optimized hits 20 and 23 might thus be considered promising starting points for the development of novel inhaled ALK5 inhibitors.