CDK4/6 Inhibitors and Hormone Therapy in Hormone Receptor-Positive Advanced Breast Cancer: Real-world Data and Intrinsic Subtypes Defined by PAM50.

e13035 Background: CDK 4/6 inhibitors plus hormone therapy(HT) has been approved by FDA and EMA for the treatment of hormone receptor positive HER2 negative advanced breast cancer (HR+/HER2-BC) with improvement in PFS consistently demonstrated in several clinical trials. Benefit in OS has also been demonstrated in Monaleesa3 and Monarch2 clinical trials. To date we don`t have real-world data and no single biomarker has been validated to identify subgroups that would benefit most from this new drugs. Methods: This is a multicenter, real life, observational study. From January 2015 to December 2019 we recruited 98 patients with immunohistochemical(IHC) HR+/HER2-BC treated with CDK4-6 inhibitor plus HT. All patients were classified into intrinsic molecular subtypes based on PAM50 signature done centrally in diagnostic/metastatic biopsies. Results: The clinical and treatment characteristic are in table. In IHC studies all population were classified as luminalA (40%) and LuminalB (60%) but we found HER2 enriched patients (6%) defined by PAM50. The median PFS for all population was 14 months and median PFS for Luminal A subtype was 12 months and 15 months for Luminal B with no statistically significant differences between them. Conclusions: Based on the results obtained, the intrinsic molecular subtype defined by PAM50 does not appear to be associated with differences in PFS in our study group. However, a study with a larger number of patients would be necessary. Inhibitors of CDK4/6, have established a central role in the management of HR+/HER2-BC. There is a clear benefit in PFS and OS but we still don’t know which patients will benefit from this therapy and who will not while the side effects such chronic neutropenia, QTC prolongation and diarrhea could have a negative impact on their quality of life. Its mandatory to explore a good biomarker to direct therapy. [Table: see text]