Chemotherapy is a type of cancer treatment that uses drugs to destroy cancer cells. In this clinical trial, chemotherapy is being tested as a second line of treatment for patients with ER-positive, HER2-negative metastatic breast cancer. The goal is to see how effective chemotherapy is in treating patients who have already been treated with CDK4/6 inhibitors and have certain unfavorable features or high levels of specific markers in their blood.

Endocrine Therapy is a treatment that blocks or lowers the amount of hormones in the body to slow down or stop the growth of cancer. This therapy is being evaluated in the trial as an alternative second line treatment for the same group of breast cancer patients. The study aims to determine if endocrine therapy is more effective than chemotherapy for these patients after they have received CDK4/6 inhibitors.

Metastatic Breast Cancer – This is a type of breast cancer that has spread beyond the breast to other parts of the body, such as the bones, liver, lungs, or brain. It is characterized by the presence of cancer cells in distant organs, which can lead to various symptoms depending on the organs affected. In the context of ER+ HER2- metastatic breast cancer, the cancer cells grow in response to estrogen but do not have an excess of the HER2 protein. The disease often progresses after initial treatments, such as CDK4/6 inhibitors and aromatase inhibitors, which aim to slow down or stop the growth of cancer cells. As the disease advances, patients may experience increased symptoms and require additional therapies to manage the progression.