NCMP-16. CNS METASTATIC PATTERNS IN BREAST CANCER TREATED WITH CDK4/6 INHIBITORS: PRELIMINARY RESULTS OF A RETROSPECTIVE ANALYSIS

Abstract BACKGROUND CDK4/6 inhibitors combined with hormonal therapy or as monotherapy (abemaciclib) play an important role in the management of hormone receptor positive, HER-2 negative breast cancer (BC). To date, these drugs were shown to improve progression-free survival, whereas their comparative effect modulating breast cancer central nervous system metastases (CNSm) has not yet been extensively studied. Here, we present the early findings and analytic plan of our retrospective study to evaluate CNSm patterns and differences among CDK4/6-inhibitors in BC. METHODS In this retrospective chart review study, we queried the Johns Hopkins Electronic Medical Record from 1/1/2014 to 4/25/2024 using the following prescreening criteria: 1) Age ≥18 years; 2) Diagnosis and/or problem list including BC; 3) Palbociclib, abemaciclib, or ribociclib included in the medication list. The identified patients were dichotomized based on the presence/absence of ICD10 codes for CNSm and are currently being chart-reviewed. Patients without longitudinal data or confirmed CNS or with CNSm diagnosed before/within 1 month of CDK-inhibitor initiation were excluded. Population: Prescreening identified 1,419 patients (1,055 palbociclib, 301 abemaciclib, 145 ribociclib) of whom 239, 36, and 19 patients had ICD10 codes for brain metastases, respectively. After exclusion of false positives from this cohort, we confirmed 53 patients with CNSm in the palbociclib, 3 in the ribociclib, and 1 in the abemaciclib group. RESULTS We present the current data from the first-line palbociclib cohort (N=53) where, median time to CNSm on was 24 months (interquartile range 13-46). Fifty-two (98.1%) patients had documented bone, 31 (58.5%) dural, 7 (13.2%) leptomeningeal, and 33 (62.3%) calvarial involvement. Dural and calvarial metastases were strongly associated with each other (pearson-chi2=10.7; p=0.001). CONCLUSIONS This is a work-in-progress, with early preliminary findings showing a high rate of dural and calvarial metastases in this cohort. A more detailed review of the dataset that will also include the CNS-metastasis-free cohort is underway.