Clinical Outcomes of Patients Treated with Ribociclib in Combination with Aromatase Inhibitors or Fulvestrant for HR-Positive, HER2-Negative Metastatic Breast Cancer, Real-World Data from a Low-Resourced Country.

Background: Cyclin-dependent kinase (CDK) 4/6 inhibitors have revolutionized the treatment landscape of hormone receptor-positive (HR+) and human epidermal growth factor receptor 2-negative (HER2 -) metastatic breast cancer (MBC). Here, we present the real-world clinical outcomes and toxicity data of patients treated at a single cancer center. Methods: A retrospective analysis was conducted on patients with HR+/HER2- MBC treated with ribociclib plus endocrine therapy (ET). Outcomes measured included progression-free survival (PFS), overall survival (OS), and adverse events. Results: A total of 356 patients (median age 52, range 27-91 years) were enrolled, all with metastatic disease; 204 (57.5%) had de novo metastasis, and 183 (51.4%) had visceral metastasis. Ribociclib was combined with aromatase inhibitors in 321 patients (90.2%) and with fulvestrant in 35 patients (9.8%). Dose reduction was needed in 101 patients (28.4%), primarily due to neutropenia (21.3%) and abnormal liver enzymes (5.9%). After a median follow-up of 36.3 months, median PFS was 27.3 months (95% CI: 21.3-31.7). PFS was significantly better in patients receiving ribociclib as first-line therapy (32.1 months, 95% CI: 27.7-42.1, p < 0.0001) and those with non-visceral metastasis (38.6 months, 95% CI: 29.8-NR, p < 0.0001). Similarly, OS was significantly better in first-line treatment (48.6 months, 95% CI: 39.1-NR) and non-visceral metastasis cases (NR, 95% CI: 40.6-NR, p < 0.0001). No significant differences in 3-year PFS and OS were found between patients with and without dose reductions. Conclusion: In real-world settings, and away from the stringency of controlled clinical trials, endocrine therapy in combination with ribociclib in patients with HR-positive/HER2-negative MBC is an effective and well-tolerated therapy with a manageable toxicity profile and a low drug discontinuation rate. Dose reduction due to toxicity did not worsen the outcome.