The purpose of this study is to learn about the safety and effects of the study medicine called vepdegestrant. The safety and effects of vepdegestrant will be see when given with other medicines. Vepdegestrant is studied to see if it can be a possible treatment for advanced metastatic breast cancer. This type of cancer would have spread from where it started (breast) to other parts of the body and would be tough to treat.

The study is seeking for participants who have breast cancer that:

• is hard to treat (advanced) and may have spread to other organs (metastatic). is sensitive to hormonal therapy (it is called estrogen receptor positive).
• is no longer responding to treatments taken before starting this study.

This study is divided into separate sub-studies.

For Sub-Study C:

All the participants will receive vepdegestrant and a medicine called samuraciclib.

Vepdegestrant and samuraciclib will be taken once in a day by mouth. The medicines will be taken at home. The experience of people receiving the study medicines will be studied. This will help see if the study medicine is safe and effective.

Participant will continue to take vepdegestrant and samuraciclib 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.

C4891024 is a prospective, open-label, multicenter, Phase 1b/2 sub-study to evaluate the safety, antitumor activity, and PK of ARV-471 with samuraciclib 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 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.

Inclusion Criteria:

• Histological or cytological diagnosis of breast cancer. At time of enrollment this must not be amendable to surgical resection with curative intent (≥1% ER+ stained cells as per local practice on the most recent tumor biopsy HER2- tumor by IHC or in-situ hybridization per ASCO/CAP).
• prior anticancer therapies: up to 2 lines of prior therapies for advanced/metastatic disease; 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 28 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/inducers of CYP3A, strong CYP2D6 inhibitors 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

Pfizer will provide access to individual de-identified participant data and related study documents (e.g. protocol, Statistical Analysis Plan (SAP), Clinical Study Report (CSR)) upon request from qualified researchers, and subject to certain criteria, conditions, and exceptions. Further details on Pfizer's data sharing criteria and process for requesting access can be found at: https://www.pfizer.com/science/clinical\_trials/trial\_data\_and\_results/data\_requests.