This is a prospective, single-center, randomize-controlled study. The purpose of this study is to evaluate the efficacy of neoadjuvant CDK4/6 inhibitors in patients with high-risk EPclin multigene risk analysis and non-response to Ki-67 2W, and to explore predictive biomarkers for sensitivity to CDK4/6 inhibitor therapy.

China is a country with a high incidence of breast cancer. For operable hormone receptor-positive (HR+) / human epidermal growth factor receptor 2-negative (HER2-) breast cancer, traditional neoadjuvant chemotherapy often fails to achieve clinical complete response (CCR) and has poor tolerability. Endocrine therapy plays a significant role in the treatment of advanced and early-stage HR+/HER2- breast cancer, but its efficacy in neoadjuvant settings needs further exploration. Studies have shown that the CCR and breast conservation rate of neoadjuvant endocrine therapy for HR+/HER2- breast cancer are similar to those of neoadjuvant chemotherapy, with lower toxicity. Therefore, by precisely stratifying the recurrence risk of HR+/HER2- patients and treating them according to different risk levels, the response rate to neoadjuvant endocrine therapy can be further improved, which would be more beneficial for the patients. With the clinical application of CDK4/6 inhibitors, the effectiveness of neoadjuvant endocrine therapy is expected to be enhanced.

In PALLET study, the combination of palbociclib and letrozole significantly inhibited Ki-67 expression, leading to a higher number of patients achieving a state of cell cycle arrest (CCCA) (defined as Ki-67≤2.7%). NeoMONARCH study suggests that a regimen containing abemaciclib is significantly superior to anastrozole monotherapy, with statistical significance (P < 0.001). The NeoPAL and CORALLEEN studies indicate that neoadjuvant endocrine therapy with palbociclib or ribociclib is comparable in efficacy to neoadjuvant chemotherapy, yet has fewer side effects.

EndoPredict (12-gene assay, EPclin) is a second-generation multigene testing scoring tool that combines the molecular biological status of the tumor with clinical factors (tumor size and lymph node status). For early-stage HR+/HER2- breast cancer patients, the EPclin test result helps to distinguish between low and high recurrence risk populations, allowing for a more precise assessment of patient prognosis. In the validation study for premenopausal patients, a total of 385 patients with stage ≤pT3 and pN0~1, who received only endocrine therapy for ER+/HER2- early invasive breast cancer, were enrolled. The results showed that the 10-year distant recurrence-free survival (DRFS) rate for patients in the EPclin low-risk group reached 97%, while the 10-year DRFS rate for the EPclin high-risk group was 76% (P=0.004). For postmenopausal patients, the prognostic value of EPclin was independently validated through the ABCSG6&8 study. The study included 1,702 patients with ER+/HER2- early invasive breast cancer who underwent surgery and received only five years of endocrine therapy. The results showed that the 10-year distant recurrence rate for patients in the EPclin low-risk group was 4%, while the 10-year distant recurrence rates for patients in the EPclin high-risk group were 28% and 22% (P<0.001). A retrospective analysis of ABCSG-34 demonstrated that for patients undergoing neoadjuvant endocrine therapy, the application of EPclin for prognostic assessment revealed a negative correlation between risk levels and residual cancer burden (RCB). That is, among patients receiving neoadjuvant endocrine therapy (NET), a higher proportion of low-risk patients compared to high-risk patients ultimately had an RCB of 0-I.

This study focuses on early-stage HR+/HER2- breast cancer, conducting recurrence risk analysis through EPclin multigene testing, and integrating the dynamic changes of Ki67 after two weeks of neoadjuvant endocrine therapy. Patients are risk-stratified and treated with or without CDK4/6 inhibitors. The study explores the improvement in PEPI scores after surgery and analyzes the effectiveness of multigene testing combined with Ki67 dynamic changes as a predictor for the efficacy of CDK4/6 inhibitor treatment in HR+/HER2- breast cancer with high recurrence risk and insensitivity to endocrine therapy. The study also seeks to explore methods for predicting sensitivity to CDK4/6 inhibitor treatment.

Inclusion Criteria:

1. Females aged 18 to 70 years;
2. Patients with histologically confirmed HR+/HER2- invasive breast cancer that is sensitive to endocrine therapy. Tumor diameter >2 cm, clinically positive axillary lymph nodes not more than 2 (T2N1M0), Ki67≥+20%. Endocrine therapy sensitivity is defined as ER expression >50% by immunohistochemistry. HER-2 negativity is defined as HER-2 results of 0 or 1+ by immunohistochemistry, or negative by FISH;
3. Within 28 days before the first dose of study medication, patients must have at least one measurable lesion according to RECIST 1.1 standards, as assessed by ultrasound or MRI;
4. Laboratory test values: a. Absolute neutrophil count (ANC) ≥ 1,500/mm3 (1.5 × 10^9/L); b. Platelet count (PLT) ≥ 100,000/mm3 (100 × 10^9/L); c. Hemoglobin (Hb) ≥ 9 g/dL (90 g/L); d. Serum creatinine ≤ 1.5 times the upper limit of normal (ULN) or creatinine clearance ≥ 30 ml/min (based on the Cockroft-Gault formula); e. Total bilirubin (BIL) ≤ 1.5 times the upper limit of normal (ULN), Gilbert's syndrome; f. Aspartate aminotransferase (AST) or alanine aminotransferase (ALT) levels ≤ 2.5 times the upper limit of normal (ULN); g. INR or PT ≤1.5 ULN, APTT ≤1.5 ULN.
5. Subject agrees to collect tumor biopsy specimen during the screening period;
6. Subject signed the informed consent form, expressing willingness and ability to comply with the planned visits, study treatments, laboratory tests, and other trial procedures.

Exclusion Criteria:

1. Coexisted with other malignant tumors within 5 years prior to first medication, except for cured squamous cell carcinoma of the skin, basal cell carcinoma, non-muscle-invasive bladder cancer, carcinoma in situ of the cervix/breast, etc;
2. Had a serious infection within 1 month before screening or required systemic treatment for any active infection within 2 weeks before first medication;
3. History or current presence of autoimmune diseases, including but not limited to Crohn's disease, ulcerative colitis, systemic lupus erythematosus, sarcoidosis, Wegener's syndrome (granulomatosis with polyangiitis, Graves' disease, rheumatoid arthritis, hypophysitis, uveitis), autoimmune hepatitis, systemic sclerosis (scleroderma, etc.), Hashimoto's thyroiditis (exceptions are noted below), autoimmune vasculitis, autoimmune neuropathy (Guillain-Barre syndrome), etc. The following exceptions apply: Type I diabetes, thyroid dysfunction requiring only hormone replacement therapy (including thyroid dysfunction caused by autoimmune thyroid disease), psoriasis or vitiligo not requiring systemic treatment;
4. History of organ transplantation or allogeneic hematopoietic stem cell transplantation;
5. Patients who received systemic anti-cancer treatment within 2 weeks before the first dose, including chemotherapy, immunotherapy, hormone therapy, biological therapy (cytokines or growth factors that control the progression of cancer);
6. Metastatic breast cancer;
7. Positive for human immunodeficiency virus antibodies (HIV-Ab) or syphilis antibodies (Anti-TP); positive for hepatitis C virus antibodies (HCV-Ab), with hepatitis C virus RNA quantification > the upper limit of the normal value of the detection unit; positive for hepatitis B surface antigen (HBsAg) and/or hepatitis B core antibodies (HBcAb), with HBV DNA quantification > the lower limit of detection of the detection unit;
8. History of alcohol abuse or drug abuse.