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 A)

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 line of any CDK4/6 inhibitor-based regimen is required (independent of the setting eg, adjuvant or advanced/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 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