Abstract 366: CDK7 inhibition as a synthetic lethal strategy in Rb-deficient ER+/HER2- breast cancer and sarcoma cells resistant to CDK4/6 inhibitors

Abstract Purpose: Resistance to CDK4/6 inhibitors (CDK4/6i) presents a significant barrier in the treatment of ER+/HER2-breast cancers, affecting approximately 50%-60% of patients initially responsive to therapy. Moreover, soft tissue sarcomas (STS), representing over 13,000 new cases annually in the U.S. alone, are frequently characterized by Rb pathway disruption and have poor survival outcomes due to limited therapeutic options. Given the role of CDK7 as an upstream regulator of CDK2, and emerging evidence that Rb deficient cells are sensitive to CDK7 inhibition, we hypothesize that the novel CDK7 inhibitor REC-617may act as a synthetic lethal agent in Rb-deficient, CDK4/6i-resistant cancer models. This study aims to evaluate the efficacy of REC-617 in both ER+/HER2-breast cancer and STS cell lines, especially within Rb-deficient contexts, as a therapeutic strategy for overcoming resistance and broadening targeted treatment options. Methods: CDK4/6i-resistant cell lines were generated by progressively exposing MCF7 breast cancer cells to increasing concentrations of palbociclib (1.2-4.8 μmol/L), ribociclib (1.2-4.8 μmol/L), and abemaciclib (0.5 and 1 μmol/L). CRISPR-Cas9 technology was used to create Rb-deficient models in resistant cells to assess REC-617’s impact. Syngeneic and non-syngeneic sarcoma cell lines with differential Rb status were also treated to evaluate the drug’s broader applicability in Rb-deficient cancers. Cell cycle progression, apoptosis, and toxicity were analyzed, and Western blotting was performed to monitor changes in cell cycle proteins, DNA damage markers, and signaling pathways. Results: REC-617 induced a marked G2 phase arrest and increased apoptosis in CDK4/6i-resistant MCF7 breast cancer cells compared to parental cells. This effect correlated with decreases in CDK1 and CDC25C and an increase in cyclin B1, confirming G2/M arrest. DNA damage response was heightened, as shown by elevated γH2AX levels, while Rb and p53BP1 levels were diminished. Notably, both Rb-deficient CRISPR-engineered breast cancer cells and Rb-deficient sarcoma models exhibited heightened sensitivity to REC-617 compared to their Rb-intact counterparts. This increased susceptibility was consistently observed across palbociclib-, abemaciclib-, and palbociclib-resistant cells, further validating the relevance of Rb status in CDK7-targeted responses. Conclusion: These findings demonstrate that CDK7 inhibition via REC-617 offers a potent synthetic lethal approach in Rb-deficient, CDK4/6i-resistant ER+/HER2-breast cancer and soft tissue sarcomas. Reduced Rb expression may be predictive of the level of response to REC-617, thus providing a targeted treatment option for overcoming resistance in both cancer types and addressing a critical need in resistant ER+/HER2-breast cancers and soft tissue sarcomas. Citation Format: Rezvan Esmaeili, Tuyen N Bui, Tuyen Duong Thanh Nguyen, Jim Joseph, Fred Aswad, Kelly K Hunt, Elena Fonfria, Khandan Keyomarsi. CDK7 inhibition as a synthetic lethal strategy in Rb-deficient ER+/HER2- breast cancer and sarcoma cells resistant to CDK4/6 inhibitors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 366.