Abstract 4639: Predictive clinical-pathological features for CDK 4/6 inhibitors: Outcomes in hormone receptor-positive, HER2-negative metastatic breast cancer patients

Abstract Background: Breast cancer is the most common cancer among Saudi women and around the world, accounting for 30.9% of all Saudi female cancers in 2020. Metastatic breast cancer (MBC) represents a significant health burden. There has been a recent increase in hormone receptor-positive (HR+) breast cancer in young women. When used with endocrine therapy, CDK4/6 inhibitors have become a big step forward in the treatment of hormone receptor-positive breast cancer. There are sparse studies on their response prediction. We aim to determine predictive clinical-pathological features for CDK4/6 inhibitors response in HR+, Her2 negative metastatic breast cancer by getting their impact on the response rate and progression-free survival. The study looked at the medical records of women with HR+, Her2 negative breast cancer who went to the Princes Noora Oncology Centre (PNOC) in King Abdulaziz Medical City-WR between January 2016 and December 2023. The study was approved by the Institutional Review Board (IRB). We collected data for patients' demographics, clinical features, peripheral blood inflammatory cells, pathology, treatment, and follow-up details from patients' files in Electronic Medical Records. We included 18 years or older, histopathology-confirmed, advanced unresectable, and systemic therapy-naïve MBC. We excluded intracranial metastases and nonassessable responses. We cleaned and explored the data using tables, histograms, box plots, and bar charts. We applied the chi-square test, Fisher exact test, and Kaplan-Meier survival curves for survival. Results: We found 3084 breast cancer patients and 1250 hormone receptor-positive patients. We analyzed 162 cases after applying inclusion-exclusion criteria. The median age was 51 years; PFS was 72.5 months. Menopause status, estrogen receptor positivity percentage, body mass index (BMI), tumor grade, and adenocarcinoma histology were clinical factors that could not predict the outcome. A high KI67 index of more than20%, high monocyte lymphocyte ratio (MLR), and a low lymphocyte count before the second cycle predicted the response of the CDK4/6 inhibitors, as shown by a higher response rate (RR) and better progression-free survival (PFS). Conclusion: While clinical features were not predictive, factors such as elevated KI67 by more than 20%, elevated peripheral blood monocytes (PBMC), monocyte lymphocyte ratio (MLR), and a low lymphocyte count before the second cycle were predictive of the response to CDK4/6 inhibitors. Citation Format: Amer N. Radwi, Anwar Khan, Mohammed Essam Mahroos, Fares Alhejaili, Saleh Alsaiari, Ali Nasfan, Ahmad Almuslim, Mohammed Alasri, Yousef Alraddadi. Predictive clinical-pathological features for CDK 4/6 inhibitors: Outcomes in hormone receptor-positive, HER2-negative metastatic breast cancer patients [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 4639.