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.

Eligibility Criteria:
Inclusion Criteria:

* histological or cytological diagnosis of ER+ and HER2- advanced/metastatic breast cancer that is not amendable to surgical resection with curative intent (≥1% ER+ stained cells on the most recent tumor biopsy).
* prior anticancer therapies: at least 1 and no more than 2 lines of prior therapies for advanced/metastatic disease; 1, and only 1, line of any CDK4/6 inhibitor-based regimen is required (in any setting eg adjuvant, metastatic)
* at least 1 measurable lesion as defined by RECIST v1.1.
* ECOG PS ≤1.

Exclusion Criteria:

* visceral crisis at risk of life-threatening complications in the short term
* known history of drug-induced pneumonitis or other significant symptomatic deterioration of lung functions.
* newly diagnosed brain metastases, or symptomatic CNS metastases, carcinomatous meningitis, or leptomeningeal disease. Participants with a history of CNS metastases or cord compression are eligible if they have been definitively treated, clinically stable and discontinued anti-seizure medications and corticosteroids for at least 14 days prior to enrollment in the of study.
* history of any other tumor malignancies within the past 3 years, except for the following: (1) adequately treated basal or squamous cell carcinoma of the skin; (2) curatively treated in situ carcinoma of the cervix.
* inflammatory breast cancer
* impaired cardiovascular function or clinically significant cardiovascular diseases
* concurrent administration of medications, food, or herb supplements that are strong inhibitors and strong/moderate inducers of CYP3A and drugs known to predispose to Torsade de Pointes or QT interval prolongation.
* renal impairment, not adequate liver function and/or bone marrow function
* known active infection