The protein kinase TTK is a critical component of the spindle assembly checkpoint (SAC), which regulates the proper attachment of sister chromatids during mitosis. TTK is essential for normal progression of the cell cycle and is upregulated in various aggressive cancers such as triple negative breast cancer (TNBC). Inhibition of TTK leads to an overriding of the SAC, premature progression of the cell cycle and mitotic catastrophe. TTK inhibition therefore contrasts with e.g. Aurora and PLK1 inhibition, which block the cell cycle [1]. Various studies have shown the clinical utility of TTK inhibition, particularly in combination therapy with paclitaxel [1,2]. In vivo, TTK inhibitor monotherapy has shown partial tumor growth inhibition without weight loss [1,2].

We have developed a novel class of TTK inhibitors based on a pyrimido-indolizine scaffold. These bind selectively and with high affinity to the ATP pocket of TTK and potently inhibit the proliferation of a variety of cell lines [1]. Previously we published a representative of this class, NTRC 0066-0, which has an IC50 of 0.6 nM in a biochemical assay for TTK activity and which is more than 200 times selective over a panel of 276 kinases [1]. NTRC 0066-0 has an average IC50 of 98 nM in proliferation assays in 66 different cell lines (Oncolines™ panel) [3].

To further improve the potency of the pyrimido-indolizine series, we determined the X-ray structures of a dozen of class representatives in complex with TTK. The results were compared to 3D complexes of other chemical scaffolds such as BAY-1161909, Mps-Bay2b, MPI-0479605, NMS-P715 and Mps1-IN1. The pyrimido-indolizine series uses an aromatic moiety to trap the catalytic lysine in the active site, enforcing a catalytically incompetent conformation of TTK. Thermal melting and surface plasmon resonance experiments demonstrate that this leads to a strong stabilization of the kinase domain and a slow dissociation rate for the compounds, which is one of the key determinants for potent cellular activity.

We took advantage of these structure-activity relationships to develop analogs of NTRC 0066-0 with increased residence time and cellular potency. One such analog, NTRC 1501-0, inhibits the proliferation of 66 cancer cell lines with an average IC50 of 18 nM, while retaining selectivity over the kinome. It has good pharmacokinetic properties and shows no cross-reactivity with drug safety targets in vitro. In a xenograft model of the human TNBC cell line MDA-MB-231, it completely inhibited tumor growth at a low oral dose, without effect on body weight, indicating good tolerability. These data show, for the first time, that TTK inhibition as monotherapy can achieve complete inhibition of tumor growth.

[1] Maia et al. (2015) Annals of Oncology 26, 2180-2192; [2] Wengner et al (2016) Mol. Canc. Therap. 15, 583-592; [3] Uitdehaag et al. (2016) Mol. Canc. Therap., in the press.

Citation Format: Joost C. Uitdehaag, Jos de Man, Marion Libouban, Nicole Willemsen-Seegers, Jan Gerard Sterrenburg, Joeri J. de Wit, Jeroen A.D de Roos, Martine B. Prinsen, Rogier C. Buijsman, Guido J. Zaman. NTRC 1501-0, a TTK kinase inhibitor selected for its long target residence time, completely inhibits tumor growth in the MDA-MB-231 xenograft model for triple-negative breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4185. doi:10.1158/1538-7445.AM2017-4185