PAlbociclib Plus Tamoxifen for the Treatment of Hormone Receptor-positive, HER2-negative Advanced Breast Cancer Women - Asian studY

Inclusion Criteria: * Women 18 years of age or older with histologically or cytologically proven locally advanced or metastatic breast cancer, not amenable to resection or radiation therapy with curative intent * Documented diagnosis of HR+/HER2- breast cancer * Any menopausal status * Previously untreated with any endocrine therapy for their HR+/HER2- advanced breast cancer; or progressed while on or within 3 month from prior endocrine therapy other than tamoxifen for advanced breast cancer. If patients have adjuvant endocrine therapy, they must satisfy as follows: progressed 12 months or more since prior adjuvant endocrine therapy with tamoxifen; or progressed during or after adjuvant endocrine therapy with an aromatase inhibitor. * Measurable disease or non-measurable disease as defined by RECIST ver.1.1 * Eastern Cooperative Oncology Group (ECOG) PS 0-1 * Adequate organ and marrow function, resolution of all toxic effects of prior therapy or surgical procedures Exclusion Criteria: * Prior treatment with any CDK inhibitor, tamoxifen, everolimus, or agent that inhibits the PI3K-mTOR pathway * Patients with extensive advanced/metastatic, symptomatic visceral disease, or known uncontrolled or symptomatic CNS metastases * Use of strong or moderate CYP3A4 and/or CYP2D6 inhibitors or inducers * Major surgery or any anti-cancer therapy within 2 weeks of randomization * Prior stem cell or bone marrow transplantation