Abemaciclib combined with endocrine therapy has become one of the important adjuvant treatment regimens for patients with HR+/HER2- high-risk early breast cancer. However, adverse events such as diarrhea, fatigue, neutropenia and elevated liver enzymes are concentrated in the early stage of adjuvant therapy, which often lead to dose reduction, temporary drug interruption or even permanent discontinuation. This further affects treatment adherence, relative dose intensity (RDI) and treatment completion rate.

Findings from the TRADE study suggest that a step-up dosing strategy, initiating at a lower dose followed by gradual titration to the standard dose, combined with standardized patient education and symptomatic management, may improve early treatment tolerance, reduce the burden of partial toxicities, and increase the likelihood of patients achieving and maintaining abemaciclib 150 mg twice daily. Based on the above evidence and clinical experience, step-up dosing has been adopted by some clinicians for real-world clinical practice.

Nevertheless, existing evidence is mainly derived from non-Chinese populations. There is still a lack of systematic real-world data on step-up dosing among Chinese breast cancer patients under routine outpatient management, including the early toxicity profile, dose escalation achievement rate at each stage, dose adjustment pathways (prolonged escalation, treatment pause or dose de-escalation), RDI distribution, correlation with quality of life, and baseline factors affecting treatment tolerance and dose target attainment. Therefore, it is necessary to conduct a real-world study focused on Chinese patients to fill the gap in local clinical evidence, and provide a basis for clinical pathway formulation, patient education, and subsequent multicenter validation studies.

Inclusion Criteria:

• Aged ≥18 years;
• Pathologically confirmed breast cancer with ER and/or PR positive, HER2 negative;
• Completed radical mastectomy/radiacal breast surgery;
• Plan to receive abemaciclib combined endocrine therapy for at least 12 weeks, with abemaciclib administered via a step-up dosing regimen;
• ECOG performance status 0-1;
• Adequate organ function meeting medication requirements (routine blood test, liver and renal function, and other indicators consistent with clinical medication safety criteria);
• Signed written informed consent.

Exclusion Criteria:

• Previous exposure to any CDK4/6 inhibitor;
• Active infection or severe infection requiring systemic anti-infective treatment;
• Significant gastrointestinal diseases such as chronic diarrhea, inflammatory bowel disease, short bowel syndrome, which may affect drug absorption or increase the risk of diarrhea;
• Significant baseline hepatic or renal dysfunction, or uncontrolled severe comorbidities judged by the investigator to affect safety or treatment adherence;
• Pregnancy or lactation;
• Any other condition deemed ineligible for enrollment by the investigator.

Considering the protection of patient privacy, relevant clinical data will not be released publicly.