This clinical trial is focused on studying metastatic breast cancer, a type of breast cancer that has spread to other parts of the body. The study is investigating the effectiveness and safety of a treatment combination that includes palbociclib, an oral medication, along with anti-HER2 therapy and endocrine therapy. The goal is to see if this combination is better at delaying the progression of the disease compared to just using anti-HER2 therapy and endocrine therapy alone. Participants in the study will receive either the combination treatment or the standard treatment after an initial phase of treatment known as induction treatment.

The study will involve taking medications in the form of capsules or tablets, such as IBRANCE (which contains palbociclib) and other hormone therapies like exemestane, anastrozole, and letrozole. Some treatments, like fulvestrant, triptorelin, leuprorelin, and goserelin, will be given as injections. The study will also include anti-HER2 therapies like trastuzumab and pertuzumab, which are used to target specific proteins on cancer cells. The trial is designed to last for a period of time, during which participants will be monitored for any changes in their condition and any side effects they may experience.

The purpose of this study is to determine if the combination of palbociclib with anti-HER2 and endocrine therapies can provide better outcomes for patients with hormone receptor-positive, HER2-positive metastatic breast cancer. The study will assess how long patients live without their cancer getting worse, as well as overall survival rates and the response to treatment. Participants will be closely monitored by healthcare professionals throughout the study to ensure their safety and to gather important information about the effectiveness of the treatment.