A Study to Learn About Vepdegestrant When Given With PF-07220060 to People With Advanced or Metastatic Breast Cancer.

Inclusion Criteria:

* Histological or cytological diagnosis of breast cancer. At time of enrollment this must not be amenable 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: Phase 1b: at least 1 line of SOC for A/MBC; Prior fulvestrant allowed; ≤1 prior chemotherapy line (no antibody-drug conjugates permitted) for A/MBC setting allowed. Phase 2: At least one and maximum 2 lines of ET in A/MBC setting and most recent ET-based regimen for \>6 months.

  1, and only 1, prior CDK4/6 inhibitor-based regimen required. Up to 1 prior regimen of cytotoxic chemotherapy (no antibody-drug conjugates permitted) in the A/MBC setting; Prior fulvestrant allowed.
* Participant with only non-measurable lesion (Phase1b) or at least 1 measurable lesion as defined by RECIST v1.1. (Phase2) are eligible.
* ECOG PS = 0 or 1 (Phase1b) ; ≤2 (Phase2)

Exclusion Criteria:

* visceral crisis at risk of life-threatening complications in the short term.
* Any condition precluding an adeguate absorption of study interventions.
* newly diagnosed brain metastases, or symptomatic central nervous system (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 are excluded
* impaired cardiovascular function or clinically significant cardiovascular diseases.
* concurrent administration of medications, food, or herb supplements that are strong inhibitors/inducers of CYP3A or UGT2B7, moderate inducers of CYP34 (Phase1b only) 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