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

Indazole-Based Potent and Cell-Active Mps1 Kinase Inhibitors: Rational Design from Pan-Kinase Inhibitor Anthrapyrazolone (SP600125)

Monopolar spindle 1 (Mps1) is essential for centrosome duplication, the spindle assembly check point, and the maintenance of chromosomal instability. Mps1 is highly expressed in cancer cells, and its expression levels correlate with the histological grades of cancers. Thus, selective Mps1 inhibitors offer an attractive opportunity for the development of novel cancer therapies. To design novel Mps1 inhibitors, we utilized the pan-kinase inhibitor anthrapyrazolone (<b>4</b>, SP600125) and its crystal structure bound to JNK1. Our design efforts led to the identification of indazole-based lead <b>6</b> with an Mps1 IC₅₀ value of 498 nM. Optimization of the 3- and 6-positions on the indazole core of <b>6</b> resulted in <b>23c</b> with improved Mps1 activity (IC₅₀ = 3.06 nM). Finally, application of structure-based design using the X-ray structure of <b>23d</b> bound to Mps1 culminated in the discovery of <b>32a</b> and <b>32b</b> with improved potency for cellular Mps1 and A549 lung cancer cells. Moreover, <b>32a</b> and <b>32b</b> exhibited reasonable selectivities over 120 and 166 kinases, respectively

Diaminopyridine-Based Potent and Selective Mps1 Kinase Inhibitors Binding to an Unusual Flipped-Peptide Conformation

Monopolar spindle 1 (Mps1) is an attractive cancer drug target due to the important role that it plays in centrosome duplication, the spindle assembly checkpoint, and the maintenance of chromosomal stability. A design based on JNK inhibitors with an aminopyridine scaffold and subsequent modifications identified diaminopyridine <b>9</b> with an IC₅₀ of 37 nM. The X-ray structure of <b>9</b> revealed that the Cys604 carbonyl group of the hinge region flips to form a hydrogen bond with the aniline NH group in <b>9</b>. Further optimization of <b>9</b> led to <b>12</b> with improved cellular activity, suitable pharmacokinetic profiles, and good in vivo efficacy in the mouse A549 xenograft model. Moreover, <b>12</b> displayed excellent selectivity over 95 kinases, indicating the contribution of its unusual flipped-peptide conformation to its selectivity