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)

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