C4891006 is a sub-study from the Umbrella platform, TACTIVE-U, comprising multiple sub-studies that independently evaluate ARV-471 in participants with Estrogen Receptor Positive (ER+) Advanced or Metastatic Breast Cancer (A/MBC). ARV-471 will act as the backbone therapy given in combination with other anticancer agents thought to have clinical relevance in ER+ breast cancer.
The purpose of this clinical trial is to learn about the safety and effects of the study medicine (called ARV-471) when given together with other medicines for the potential treatment of advanced or metastatic breast cancer.~This study is seeking participants who have breast cancer that:~is advanced, may have spread to other organs (metastatic) and cannot be fully treated by surgery or radiation therapy~is sensitive to hormonal therapy (it is called estrogen receptor positive); and~is no longer responding to previous treatments This study is divided into separate sub-studies.~For Sub-Study A:~All participants will receive ARV-471 and a medicine called abemaciclib. ARV-471 will be given by mouth, at home, 1 time a day. Abemaciclib will be given by mouth, at home, 2 times a day. We will examine the experiences of people receiving the study medicines. This will help us determine if the study medicines are safe and effective.~Participants will continue to take ARV-471 and abemaciclib until their cancer is no longer responding, or side effects become too severe. They will have visits at the study clinic about every 4 weeks.