Abstract 2307: Sequential treatment with abemaciclib plus endocrine therapy inhibits cell proliferation and triggers apoptosis in cell lines resistant to CDK4/6i

Abstract CDK4/6 inhibitors combined with endocrine therapy (ET) have shown clinical benefit in HR+, HER2- breast cancer. However, development of resistance highlights the need for treatment strategies to improve outcomes. To study resistance mechanisms, breast cancer cell lines were treated with CDK4/6i (abemaciclib or palbociclib) in combination with 4-OH-tamoxifen (tamoxifen) for 120-144h and sorted for resistant cells defined as geminin positive (GEM+), a marker of S/G2/M cell cycle accumulation. To confirm the resistant phenotype, cell lines were treated with tamoxifen plus the CDK4/6 inhibitor used to drive resistance. To understand if sequential CDK4/6i treatment is effective in controlling cell proliferation, resistant cells generated through treatment with palbociclib + tamoxifen or abemaciclib + tamoxifen were treated with abemaciclib + ET (fulvestrant or tamoxifen) or palbociclib + ET, respectively. Geminin/Ki67, annexin V and colony formation assays were performed to evaluate cell proliferation and viability. Molecular characterization by western blot and RNAseq analysis were utilized to provide insights into mechanisms of resistance and the effects of sequential treatment with CDK4/6i + ET. Cell lines resistant to palbociclib + tamoxifen subsequently treated with abemaciclib + ET showed decreased %GEM+ and colony formation ability, decreased Ki67 levels, and increased apoptosis. These effects were not observed in cell lines resistant to abemaciclib + tamoxifen following subsequent treatment with palbociclib + ET. Western blot analysis showed that palbociclib and abemaciclib-resistant cells had increased CDK6 and pERK levels, compared to control. Importantly, treatment of palbociclib-resistant cells with abemaciclib + ET decreased FOXM1, a key regulator of senescence and apoptosis. Cyclin A, a marker of mitosis, was also decreased, consistent with decreased %GEM+ subpopulation in palbociclib + ET resistant cells. These effects were not observed in abemaciclib-resistant cells treated with palbociclib + ET.In summary, our in vitro data provides mechanistic insights into resistance to combination CDK4/6i + tamoxifen and suggests the potential benefit of sequential treatment with abemaciclib + ET to overcome resistance in breast cancer. Citation Format: Elisabet Zapatero-Solana, Maria P. Ganado, Maria J. Ortiz-Ruiz, Cecilia Mur, Lacey Litchfield, Farhana Merzoug, Oscar Puig, Maria Jsoe Lallena. Sequential treatment with abemaciclib plus endocrine therapy inhibits cell proliferation and triggers apoptosis in cell lines resistant to CDK4/6i [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2307.