443P Efficacy and safety of CDK 4/6 inhibitors in patients with bone marrow-involved metastatic breast cancer

No data on the efficacy of cyclin-dependent kinase 4/6 inhibitors (CDKi) were presented in randomized clinical trials in patients with metastatic breast cancer (mBC) with bone marrow involvement. This study aimed to investigate the efficacy and safety of CDKi in patients with mBC with bone marrow involvement. The data of patients with bone marrow-involved hormone receptor-positive (HR+) Her2-negative mBC between 2019- 2022 who received ribociclib or palbociclib in combination with endocrine therapy (ET) were retrospectively analyzed within the thirteen centers. The median age of 23 patients included in the study was 48 (range: 29-75). Eleven (48.8%) of the patients were denovo metastatic. Eight of the patients (34.8%) received CDKi treatment in the first line, 5 (21.7%) in the second line, and 10 (43.5%) in ≥ the 3rd line. In addition to bone marrow metastasis, 39.1% of the patients had bone +/- lymph nodes, and 56.5% had bone + visceral organ metastasis. Of the patients, 15 (65.2%) received ribociclib and 8 (34.8%) received palbociclib. When the CDKi response was evaluated, 2 (8.7%) of the patients had a complete response, 11 (47.8%) had a partial response, 8 (34.8%) had stable disease, and 2 (8.7%) had progressive disease. Median follow-up was 15.1 (95% CI: 5.8-24.5) months. The progression-free survival (PFS) of patients who received CDKi in first-line was 21.3 (95% CI: 0.8-41.9) months, those who received CDKi in second-line 13.1 (95% CI: 6.9-19.3) months, and those who received CDKi in ≥3rd line 5.3 (95% CI: 3.9-6.7) months (p=0.004). CDKi dose reduction was required in 60.9% of patients. CDKi was discontinued in 2 (8.7%) patients due to cytopenias and 56.5% due to disease progression. Eleven (47.8%) patients had grade ≥3 neutropenia, and 5 (21.7%) patients had grade ≥3 thrombocytopenia. No treatment-related death was detected. It has been shown that PFS similar to those in randomized clinical studies can be achieved with CDK 4/6 inhibitors combined with ET in treating hormone receptor-positive Her2-negative bone marrow-involved metastatic breast cancer. The CDKi discontinuation associated with cytopenia was also meager showed that CDKi treatment could be safe in this patient group.