Abstract PD2-03: CDK4/6 inhibitor-resistant ER+ breast cancer cell lines are hypersensitive to TTK inhibition

Inhibitors of cyclin-dependent kinases 4 and 6 (CDK4/6i), in combination with hormonal therapies, have become standard of care for the treatment of estrogen receptor-positive (ER+)/HER2-negative metastatic breast cancer. Despite demonstrating significant improvements in progression-free survival, acquired resistance to these inhibitors invariably develops. Recent analyses of clinical samples have identified emergent genomic alterations conferring acquired resistance to CDK4/6i, and begin to define the biology of this new clinical entity. Discovery of vulnerabilities of CDK4/6i-resistant tumours is imperative to improve the survival of this group of patients. We modeled CDK4/6i resistance in ER+ breast cancer cell lines using two complementary approaches: (1) spontaneous development of resistance upon continuous exposure to the palbociclib for 6-9 months, and (2) genetic engineering of RB1 loss of function. In both cases, palbociclib resistance was confirmed by colony formation and cell proliferation assays. To identify potential therapeutic strategies for CDK4/6i-resistant cells, we tested the in vitro activity of novel cell cycle inhibitors using sulforhodamine B (SRB) cytotoxicity assays. CFI-402257, a selective TTK inhibitor now in Phase I testing, induced significantly increased cytotoxicity in different CDK4/6i-resistant models compared to parental cell lines, including but not exclusively those with RB1 loss. CFI-402257 treatment caused defects in cell cycle progression and increased DNA damage and genomic instability in CDK4/6i-resistant cells, while these effects were mild in parental, CDK4/6i-sensitive cell lines. In some cases, these phenotypes were accompanied by an increase in apoptotic signaling. Analysis of the molecular determinants of these effects are being evaluated (additional results will be presented). In xenografts derived from MCF7 cells, CFI-402257 treatment completely abrogated the growth of RB1-KO tumours and had a much less pronounced effect on wild-type tumours. In summary, our results nominate the TTK inhibitor CFI-402257 as a promising therapeutic strategy for breast cancer patients who progress after CDK4/6 inhibition. A clinical trial testing this strategy is being launched. Citation Format: Isabel Soria Bretones, Kelsie L Thu, Jennifer Silvester, Reza Kiarash, Graham C Fletcher, Jennifer Cruickshank, Mark R Bray, Tak W Mak, David W Cescon. CDK4/6 inhibitor-resistant ER+ breast cancer cell lines are hypersensitive to TTK inhibition [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr PD2-03.