Background: Despite treatment, many estrogen receptor positive (ER+) breast cancer (BC) patients, relapse with de novo or acquired endocrine resistant disease. Using a kinome siRNA library screen, we identified MPS1, which is required for recruitment of the spindle assembly checkpoint complex, as strongly associated with resistance to both endocrine therapy and CDK4/6 inhibitor palbociclib, a contemporary first-line combination for advanced ER+ BC. Until now, the target population for MPS1 inhibitors has been triple negative BC. Here we show for the first time, potential efficacy of MPS1 inhibitors in endocrine resistant BC models.

Methods: A panel of ER+ BC cell lines adapted to estrogen independent growth (LTED) or after additional resistance to palbociclib (palboR), was subjected to a siRNA kinome screen targeting 709 genes. Kinases causative of resistance were identified using Z-scores. Cell viability upon MPS1 inhibition (MPS1i) with BOS172722 was assessed 2D and 3D, and the class effect confirmed with other compounds targeting MPS1. Impact of MPS1i on ER-mediated transcription was assessed using luciferase reporter assays. Effect of MPS1i on chromosomal alignment and time spent in mitosis was established by time lapse and confocal microscopy. Cell cycle and DNA content were analysed by FACS. Apoptosis was assessed using PARP cleavage. Efficacy of BOS172722 in a xenograft model of LTED was evaluated.

Results: Kinome knockdown identified MPS1 as the top common hit in LTED and palboR cell lines. Increased MPS1 was evident in MCF7-LTED at the transcript and protein level. Notably, BOS172722 significantly reduced viability of the majority of LTED and palboR cell lines tested (IC50 between 25-100nM), an effect shown to be class specific using a second MPS1i. Upon MPS1i, cells demonstrated shorter time in mitosis, aberration of cell cycle, increased mitotic errors, culminating in apoptosis. Of note, as little as 48hr exposure to BOS172722 reduced colony formation over 3 weeks suggesting early accumulation of errors is sufficient to provide a long term anti-proliferative effect. To evaluate the clinical relevance of MPS1 in ER+ BC treated with endocrine therapy, we interrogated publicly available datasets from patients treated with neoajuvant AI therapy. In the anastrozole cohort, on-treatment MPS1 expression was significantly (p<0.0001) associated with poor response to anastrozole based on a 2-week residual Ki67 score <10%. In the letrozole cohort, increased on-treatment expression of MPS1 (p=0.0118) was associated with poor response based on tumor shrinkage <50%. Finally, in a xenograft model of AI relapse, BOS172722 caused significant tumor shrinkage, compared to vehicle.

Conclusion: For the first time, we show that MPS1 inhibitors are capable of inducing mitotic aberrations and apoptosis in ER+ BC resistant to endocrine therapy and palbociclib providing a new therapeutic strategy.

Citation Format: Joanna Nikitorowicz-Buniak, Sunil Pancholi, Nikiana Simigdala, Ricardo Ribas, Spiros Linardopoulos, Mitch Dowsett, Stephen Johnston, Lesley-Ann Martin. MPS1 as a novel target in endocrine- and palbociclib-resistant estrogen receptor positive breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4950.