TACTIVE-U: A Study to Learn About the Study Medicine (Vepdegestrant) When Given With Other Medicines in People With Advanced or Metastatic Breast Cancer (Sub-Study B)

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 B:

All participants will receive ARV-471 and a medicine called ribociclib. ARV-471 and ribociclib will be given at the same time by mouth, at home, 1 time a day.

The experiences of people receiving the study medicine will be examined. This will help determine if the study medicine is safe and effective.

Participants will continue to take ARV-471 and ribociclib 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.

C4891023 is a prospective, open-label, multicenter, Phase 1b/2 sub-study to evaluate the safety, antitumor activity, and PK of ARV-471 with ribociclib in the treatment of participants with A/MBC. The sub-study is part of Umbrella platform, TACTIVE-U, comprising multiple sub-studies that independently evaluate ARV-471 in participants with with Estrogen Receptor Positive/Human Epidermal Growth Factor Receptor 2 Negative (ER+/HER2-) Advanced or Metastatic Breast Cancer. ARV-471 will act as the backbone therapy given in combination with other anticancer agents thought to have clinical relevance in ER+ breast cancer.