Real-World Study of Post-Resistance Treatment Strategies in Advanced Breast Cancer Following CDK4/6i, PIK3CA Inhibitors, or T-DXd

Breast cancer is a heterogeneous disease, and advances in targeted therapies have significantly improved clinical outcomes, especially in hormone receptor-positive (HR+), HER2-positive, and selected triple-negative breast cancer (TNBC) subtypes. However, disease progression following treatment with targeted agents such as CDK4/6 inhibitors, PIK3CA inhibitors, and antibody-drug conjugates like trastuzumab deruxtecan (T-DXd) remains a major clinical challenge. With a growing number of post-resistance treatment options available, real-world data are urgently needed to inform evidence-based sequencing strategies in routine clinical practice. This real-world observational study aims to investigate the treatment patterns and clinical outcomes of patients with advanced or metastatic breast cancer who experience disease progression after receiving CDK4/6 inhibitors, PIK3CA inhibitors, T-DXd, or other relevant targeted agents. The study will focus on characterizing the effectiveness of subsequent treatment regimens, including chemotherapy, endocrine therapy, additional targeted therapies, or their combinations. In addition to evaluating clinical outcomes such as progression-free survival (PFS), overall survival (OS), objective response rate (ORR), and duration of response (DoR), the study will collect detailed baseline patient characteristics, tumor biological features, and prior treatment histories. These data will be used to identify potential prognostic and predictive factors and to develop outcome prediction models that may support personalized treatment planning in the post-resistance setting. Patients will be identified based on medical record review, with retrospective and/or prospective data collection depending on institutional capabilities. Treatment decisions are made at the discretion of the treating physicians as part of standard clinical care. The study does not involve any experimental intervention, and no additional diagnostic or therapeutic procedures will be imposed on participants. The ultimate goal of this study is to generate clinically relevant, real-world evidence to guide optimal treatment sequencing after resistance to targeted therapy in advanced breast cancer, and to support future clinical research and decision-making frameworks.