Abemaciclib for Molecular Residual Disease Detected by Circulating Tumor DNA in HR+/HER2- Early Breast Cancer

The purpose of this clinical study is to investigate whether identifying molecular relapse through ctDNA monitoring and initiating abemaciclib treatment at the time of ctDNA positivity can reduce the risk of recurrence in patients with hormone receptor-positive, HER2-negative early breast cancer who have completed curative surgery and standard adjuvant therapy. The study also aims to evaluate the safety profile of abemaciclib in this early-stage setting and to clarify how patients tolerate the treatment over time. Participants will undergo routine postoperative surveillance, including scheduled imaging and laboratory testing, as well as regular blood sampling for ctDNA analysis. When ctDNA becomes detectable, indicating minimal residual disease, participants will begin a predefined course of abemaciclib and attend clinic visits at regular intervals for physical examinations, toxicity assessments, and laboratory monitoring. In addition to determining whether early intervention with abemaciclib can delay or prevent clinical recurrence, researchers will examine several key outcomes. These include invasive disease-free survival, distant recurrence-free survival, and the incidence of adverse events throughout the treatment period. The study will also measure the "lead time" between ctDNA positivity and radiologic or clinical recurrence, which may provide insight into the biological dynamics of disease relapse. Furthermore, researchers will evaluate ctDNA clearance at the completion of abemaciclib therapy, as ctDNA clearance may serve as an early indicator of treatment efficacy. Collectively, these results are expected to clarify the clinical utility of ctDNA-guided therapy escalation and to inform future strategies for preventing recurrence in early breast cancer.