Abstract PS1-12-10: Real-world effectiveness of CDK4/6 inhibitors in HR+/HER2- advanced breast cancer with bone marrow involvement: A case series from a single institution

Abstract Background: Bone marrow (BM) metastasis is a rare and challenging manifestation of advanced breast cancer (ABC), typically associated with high hematologic toxicity risk and poor outcomes. These patients are frequently excluded from clinical trials, limiting available evidence to guide treatment decisions. While chemotherapy is commonly recommended to achieve a rapid response, including hematologic recovery, CDK4/6 inhibitors (CDK4/6i) are increasingly used due to their efficacy in HR+/HER2- ABC. We aimed to assess the real-world (RW) efficacy of CDKis in this high-risk subgroup. Methods: We conducted a retrospective study of patients with HR+/HER2- ABC and BM involvement treated with CDK4/6i between 2022 and 2025 in a single institution located in Bogota, Colombia. Eligibility required histologic or smear-confirmed BM metastasis and receipt of at least one cycle of CDKi (palbociclib, ribociclib, or abemaciclib) in combination with endocrine therapy. Descriptive statistics were used to summarize clinicopathologic characteristics and hematologic outcomes. RW progression-free survival (rwPFS) was also described using the Kaplan-Meier method. Results: Fourteen patients were identified through retrospective chart review, two patients were excluded for being HER2-positive BC, two had HR positive BC but did not receive CDK4/6i, and one died before completion of the first treatment month with CDK4/6i. Nine patients were included (median age: 67 years). All were post menopausal with confirmed BM metastasis; De novo disease was present in 44.5% of the cases, and 8 Patients were ECOG <2; 55.5% had both bone and visceral disease. CDKis used included ribociclib (n=5), palbociclib (n=3), and abemaciclib (n=1). Hematologic abnormalities at baseline included anemia (66.6%), neutropenia (44.4%), and thrombocytopenia (22.2%). A hematologic response was observed in 44.4% at 6 months, with hemoglobin recovery being the most frequent and clinically relevant outcome. Median rwPFS was 32.9 months, combining progression and censoring by last follow-up. Dose was infrequent, with only one patient requiring dose adjustment. Conclusions: CDK4/6 inhibitors appear to be a feasible and effective treatment option in patients with HR+/HER2- ABC and BM involvement. Despite cytopenias traditionally being considered a contraindication for CDKi, a notable proportion of patients achieved hematologic recovery and clinical benefit. rwPFS was comparable to pivotal trials and similar reported cohorts. These findings support the cautious use of CDKis in this underserved population, and emphasize the value of RW data in informing care for this poor prognosis group. Citation Format: W. A. Mantilla, F. Canro, M. A. Bravo, P. Lopez, D. Rubio, H. Carranza, S. X. Franco. Real-world effectiveness of CDK4/6 inhibitors in HR+/HER2- advanced breast cancer with bone marrow involvement: A case series from a single institution [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS1-12-10.