(CDK) 4/6 inhibitors: An effective treatment for homologous recombination deficient (HRD) luminal breast cancers (BC)?

e13100 Background: Cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) with endocrine therapy (ET) still represents the best 1st-line choice for hormone receptor (HR) positive(+)/HER2 negative(-) metastatic breast cancer (MBC) patients (pts). Approximately 8% of pts with HR+ BC harbors germline BRCA mutation (g BRCAmt). The interaction between Cyclin/CDK pathway, endocrine receptor (ER) and Breast Cancer gene 1/2 (BRCA1/2) is complex and evidence reported so far, await confirmation in the context of future randomized clinical trials. So far, limited data exist about treatment effectiveness of CDK4/6i in homologous recombination deficiency (HRD) Luminal BC. A current frequent question in clinical practice is which additional adjuvant therapy to prioritize for luminal BRCA tumors (tu), considering the approval of both CDK4/6i andpoly ADP ribose polymerase (PARP) inhibitors in this scenario. Considering potential impacts of genetic instability in tu biology, such as potential loss of Rb protein related to Loss of Heterozygosity (LOH) of BRCA2 as a primary resistance mechanism in CDK4/6i, we sought to analyze the clinical results of that drug in tu with genetic instability. Methods: We retrospectively analyzed real-world outcomes of CDK4/6i performance in HR+/HER2- MBC treated in a single institution, with focus in clinical outcomes of HRD BC. germline HR genes status and somatic phosphatidylinositol 3-kinase (PIK3CA) status were also analyzed. Results: A total of 384 consecutive MBC pts treated with CDK4/6i were included in this study. The most used CDK4/6i was palbociclib (40,6%), followed by ribociclib (29,8%) and abemaciclib (29,2%). Considering the whole cohort PFSm 30months (95%IC 26-34) and OSm were 56 months (95%IC 52-60) No differences in PFS or OS was observed among the 3 CDK4/6i. Median age of the cohort was 60 years old. 38.5% were premenopause Pts and 59.9% were postmenopause. A total of 149 (39%) pts performed germline screening. Between 19 pts with gHR mt, 12 were BRCA1/2 mt (just one BRCA1). The other were: 2 PALB2, 2 CHEK2, 1 ATM, 1 RAD51D, 1 RAD51C mt. So far, no differences in PFS ou OS was observed in the presence of BRCA 1/2 or other gHR mt. Nevertheless, PIK3CA mt was tested in 142 pts, 51 were positives. That mt was a predictor of worse OS (55m; 95%IC 45-65) x 63m (95%IC 56-69), p 0,019. Conclusions: The results of this real-world study suggest that treatment outcomes with CDK4/6i may be worse in pts with HRD tu due to potential differences in tu biology. Future exploration in larger samples of patients who have had biomarker testing is ongoing. Patients with Pik3CA mt treated with CDK4/6i presented worse clinical outcomes.