Abstract 4622: Anti-tumor activity of CDK2 inhibitor BLU-222 in combination with CDK4/6 inhibitors for overcoming resistance in HR positive and triple negative metastatic breast cancers models

Abstract Background: Cyclin-dependent-kinase-4/6 inhibitors (CDK4/6is) plus endocrine therapy (ET) are standard of care first-line treatment for patients with hormone receptor (HR)-positive, HER2-negative metastatic breast cancer (mBC). However, the emergence of resistance to CDK4/6is plus ET presents a clinical challenge with few treatment alternatives. The effectiveness of CDK4/6is in patients with triple-negative (TN) breast cancer remains uncertain, although the potential synergy with other targeted therapies is currently under investigation. Our study demonstrates the enhanced and synergistic activity of BLU-222, a selective CDK2 inhibitor, when combined with CDK4/6is in preclinical models of both HR+/HER2- and TN breast cancer resistant to CDK4/6is. Methods: Palbociclib resistant (PR) HR+/HER2- (MCF7 and T47D) and TN (MDA-MB-231 and BT-20) breast cancer cell lines were generated by escalating palbociclib concentrations in culture. Using SynergyFinder, we assessed the effect of BLU-222 alone and in combination with palbociclib in the highest single-agent model in vitro. The effectiveness of BLU-222, alone or in combination with palbociclib, was further evaluated in four patient-derived xenograft (PDX) models from HR+/HER2- patients whose tumors progressed after palbociclib plus ET, two TN PDX models, and a TN breast cancer transgenic model driven by tumor-specific forms of cyclin E. Results: The PR HR+/HER2- and TN breast cancer cell lines, unresponsive to palbociclib, exhibited significantly increased sensitivity to BLU-222. The combination of BLU-222 and palbociclib in all four cell lines revealed a robust synergistic effect in PR cells, inducing enhanced apoptosis and cell cycle alterations in G1 or G2/M phases. Treatment with BLU-222 and palbociclib demonstrated substantial antitumor activity in all six PDX models and the cyclin E high TN transgenic models, surpassing the effects of individual treatments in each model. This combination led to lasting tumor regression and extended survival, even after treatment discontinuation. Mechanistically, treatment with BLU-222, alone or combined with palbociclib, induced the expression of p21 and/or p27 in all in vivo models, which we hypothesize sensitizes tumors to palbociclib. In vitro, the CRISPR knockout of p21 or p27 in MCF-7 PR cells abolished the synergistic activity of BLU-222 and palbociclib, confirming the crucial involvement of p21 and p27 in influencing the treatment's effectiveness. Conclusions: We found robust activity of the CDK2 inhibitor, BLU-222 when combined with CDK4/6is in resistant HR+/HER2- and TN breast cancer cell lines and in vivo models. These results support the potential clinical utility of BLU-222 in combination with CDK4/6is for the treatment of both subtypes of breast cancer. Citation Format: Linjie Luo, Yan Wang, Tuyen Bui, Mi Li, Serena Kim, Juliana Navarro-Yepes, Nicole M. Kettner, Debasish Tripathy, Kelly K. Hunt, Kerrie Faia, Khandan Keyomarsi. Anti-tumor activity of CDK2 inhibitor BLU-222 in combination with CDK4/6 inhibitors for overcoming resistance in HR positive and triple negative metastatic breast cancers models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 4622.