Discovery of Imidazo[1,2‑<i>b</i>]pyridazine Derivatives: Selective and Orally Available Mps1 (TTK) Kinase Inhibitors Exhibiting Remarkable Antiproliferative Activity

Monopolar spindle 1 (Mps1) is an attractive oncology target due to its high expression level in cancer cells as well as the correlation of its expression levels with histological grades of cancers. An imidazo­[1,2-<i>a</i>]­pyrazine <b>10a</b> was identified during an HTS campaign. Although <b>10a</b> exhibited good biochemical activity, its moderate cellular as well as antiproliferative activities needed to be improved. The cocrystal structure of an analogue of <b>10a</b> guided our lead optimization to introduce substituents at the 6-position of the scaffold, giving the 6-aryl substituted <b>21b</b> which had improved cellular activity but no oral bioavailability in rat. Property-based optimization at the 6-position and a scaffold change led to the discovery of the imidazo­[1,2-<i>b</i>]­pyridazine-based <b>27f</b>, an extremely potent (cellular Mps1 IC₅₀ = 0.70 nM, A549 IC₅₀ = 6.0 nM), selective Mps1 inhibitor over 192 kinases, which could be orally administered and was active in vivo. This <b>27f</b> demonstrated remarkable antiproliferative activity in the nanomolar range against various tissue cancer cell lines