Phase IB & II Study of Ribociclib With Trastuzumab Plus Letrozole in Postmenopausal HR+, HER2+ Advanced Breast Cancer Patients

This trial is performed in a multicenter, single-group, and phases IB and II clinical trial. This is a single-group clinical trial of the effects of ribociclib, letrozole and trastuzumab in postmenopausal women with HR+, HER2+ advanced breast cancer. This trial consists of two parts. The first part is a phase IB study of increasing the dose of ribociclib from 200mg to 600mg in combination with letrozole 2.5mg and trastuzumab (8mg/kg followed by 6mg/kg every 3 weeks). In phase IB study, three to six patients will be enrolled for each dose group. When the IB phase ends, patient enrollment will be discontinued for intermediate safety analysis, and after DLT analysis will determine recommended phase II phase II part will be started. The recommended phase II dose of ribocicib determined in phase IB part with ribociclib and trastuzumab (8mg/kg followed by 6mg/kg every 3 weeks) will be administered. Treatment is discontinued in the following cases. * Progression of disease * Uncontrolled toxicity * Patient's death * Withdrawal of consent In phase II part, 61 patients will be enrolled and the first 20 patients will be analyzed for run-in phase to confirm the safety of the therapy. Regardless of treatment cycle, the disease will be evaluated in the same manner as the method used at the time of registration at intervals of 8 weeks for the first 18 months and at intervals of 12 weeks after that. If the progress of the disease is clinically suspected, additional tests can be performed