First-Line Ribociclib Plus Endocrine Therapy Versus Chemotherapy With or Without Subsequent Endocrine Therapy in Patients With Rapidly Progressive HR-Positive/HER2-Negative Advanced Breast Cancer

This study focuses on patients with rapidly progressive hormone receptor-positive, HER2-negative advanced breast cancer, a population for whom prompt systemic disease control is clinically important. Rapid progression may include symptomatic visceral metastases, rapidly increasing tumor burden, impending organ dysfunction, or highly symptomatic non-visceral disease. The study was initially designed as a prospective, multicenter, randomized phase II trial. Owing to difficulties in prospectively recruiting patients to the chemotherapy control arm, the protocol was amended to include a prospective ribociclib plus endocrine therapy cohort and a retrospective external control cohort. Following statistical consultation, objective response rate was designated as the primary endpoint, with progression-free survival retained as a key secondary endpoint. Under the amended design, eligible patients receiving first-line ribociclib plus endocrine therapy are enrolled prospectively. Their outcomes are compared with an external control cohort selected from patients treated at the same participating hospitals who received first-line combination chemotherapy, with or without subsequent endocrine maintenance therapy. External control patients are identified from electronic medical records and institutional clinical databases and are required to meet eligibility criteria comparable to those applied to the prospective cohort. The historical control period extends from January 2020 to January 2024. Objective response rate was designated as the primary endpoint because it directly evaluates early antitumor activity in this phase II setting and was used as the basis for the revised sample-size calculation. Progression-free survival remains a key secondary endpoint. To reduce confounding associated with the nonrandomized external-control design, the study will use predefined eligibility criteria, standardized outcome definitions, and propensity score-based methods, including inverse probability weighting and propensity score matching, with additional multivariable analyses as appropriate. The major protocol amendment was incorporated into Protocol Version 4.0, dated 28 December 2025, and was approved by the Ethics Committee of the First Affiliated Hospital of Nanjing Medical University on 21 January 2026 under approval number 2023-SR-880.A1.