Real-World Clinical Outcomes and Toxicity in Metastatic Breast Cancer Patients Treated With First or Second Line CDK 4/6 Inhibitors and Endocrine Therapy

Inclusion Criteria: * Breast cancer patients aged 18 years old or more * Female patients * Histologically proven invasive breast cancer * Histologically proven HR-positive and HER2-negative breast cancer * Metastatic breast cancer (Stage 4 according to AJCC 8th edition 2017) * Eastern Cooperative Oncology Group (ECOG) performance status (PS) of 0 to 3. * Adequate hematological and organs function * Patients receiving CDK 4/6 inhibitor combined with endocrine therapy(Aromatase inhibitors , GnRH agonists, fulvastrent). as a first or second line treatment Exclusion Criteria: * \- Male breast cancer patients * Patients in, visceral crisis (defined as : severe organ dysfunction, as assessed by signs and symptoms, laboratory studies and rapid progression of disease ) that are at risk of life threatening complication in the short term. * Prior treatment with any CDK 4/6 inhibitor. * History of any other cancer , unless in complete remission with no therapy for a minimum of 3 years. * Have active bacterial or fungal infection, or detectable viral infection.