The purpose of this study is to learn about the safety and effects of the study medicine ARV-471 (PF-07850327) compared to fulvestrant (FUL) in participants with advanced breast cancer. FUL is a medicine already used for treatment of breast cancer while ARV-471 is a new medicine.~This study is seeking participants with breast cancer who:~Have a disease that comes back in the place where it started or in the nearby region (locoregional recurrent disease) or have a disease that has spread to other parts of the body (metastatic) and cannot be fully cured by surgery or radiation therapy~Are responsive to hormonal therapy such as tamoxifen (it is called estrogen receptor positive disease)~Have received one line of CDK4/6 inhibitor therapy (for example palbociclib) in combination with endocrine therapy (for example letrozole) for advanced disease.~Allowed up to one additional endocrine therapy (for example exemestane) for advanced disease.~Half of the participants will be given ARV-471 while half of the participants will be given FUL.~Participants will take ARV-471 by mouth with food, one time a day. During the first treatment cycle participants will be given FUL by shots into the muscles on Day 1 and again 2 weeks later. Afterwards, FUL shots will be given on the first day of each new treatment cycle. One treatment cycle is 28 days~Participants will receive the study medicine until their breast cancer worsens or side effects become too severe. Participants will have visits at the study clinic about every 4 weeks.
A study to learn about a new medicine called ARV-471 (PF-07850327) in people who have advanced metastatic breast cancer.