Neoadjuvant and Adjuvant Ribociclib and ET for Clinically High-risk ER+ and HER2- Breast Cancer

All patients will receive letrozole plus ribociclib as neoadjuvant therapy. Treatment will consist of six 28-days cycles of daily letrozole (2.5mg; continuous) and ribociclib (600 mg/day; 3 weeks ON and 1 week OFF). In pre-menopausal and men patients, monthly LHRH agonists will be added to letrozole and ribociclib, beginning at least two weeks before starting letrozole and ribociclib. After finalization of neoadjuvant treatment, patients will undergo surgery. Surgery samples of the residual tumor tissue (or tumor bed if pathological complete response \[pCR\] is achieved) will be collected regardless of whether they completed full neoadjuvant treatment. This is not a randomized study; therefore, adjuvant treatment will be decided according to centrally assessed ROR and pathological stage after surgery. Patients are considered responders if they achieve a pCR or have ypN0 and ROR ≤ 30 or ypN1mi (cancer the lymph node is \> 0.2 mm but \< 2 mm) and ROR ≤ 20 or ypN1 and ROR ≤ 10. All patients with ypN0 and ROR \> 30, ypN1mi and ROR \> 20, ypN1 and ROR \> 10 or ypN2-3 are considered non-responders. Patients who progress during neoadjuvant treatment with ribociclib will be considered non-responders. If indicated, adjuvant radiotherapy will be performed after surgery in the responder group and after adjuvant chemotherapy in the non-responders group. Patients considered as responders will continue on treatment after optimal recovery of surgery and radiotherapy if indicated. Treatment with ribociclib (400 mg/day; 3 weeks ON and 1 week OFF) in the adjuvant setting will be maintained for 30 months approximately corresponding to 33 cycles. Letrozole treatment duration must be of at least 5 years. Visits during ribociclib treatment will be scheduled every three cycles. At the end of ribociclib treatment, visits will be every 6 months until 5 years from last patient's surgery. Patients considered as non-responders will be treated with standard chemotherapy regimens. Patients will continue treatment with ribociclib and letrozole after optimal recovery of adjuvant chemotherapy and radiotherapy if indicated. Treatment with ribociclib (400 mg/day; 3 weeks ON and 1 week OFF) in the adjuvant setting will be maintained for 30 months approximately corresponding to 33 cycles after adjuvant chemotherapy. Endocrine therapy treatment duration must be of at least 5 years. Visits during ribociclib treatment will be scheduled every three cycles. At the end of ribociclib treatment, visits will be every 6 months until 5 years from last patient's surgery. During adjuvant treatment (both responders and non-responders), letrozole can be switched to another aromatase inhibirtor (AI). Tamoxifen is only permitted after the 30-day post ribociclib visit, according to investigator criteria. Maintaining suppression of ovarian function by luteinizing hormone releasing hormone (LHRH) agonists during adjuvant treatment is mandatory (if AI are taken)/ recommended (if tamoxifen is taken) in premenopausal and men patients unless there is unmanageable toxicity. Adjuvant hormonal treatment of patients who progress during neoadjuvant Ribociclib will be at the investigator's discretion. Blood samples for ctDNA will be collected at screening, C2D1, pre-surgery, post-surgery, and every 6 months during the adjuvant period. Blood samples will be also collected in case of recurrence. The global end of the study is defined as the date when the last patient accomplishes 5 years of follow up after surgery. The total duration of the study is expected to be 32 months for enrollment, 3 years of adjuvant treatment (including 2.5 years of ribociclib treatment), and additional 2.5 years of follow-up.