Study of PF-07220060 With Letrozole in Adults With HR-positive HER2-negative Breast Cancer Who Have Not Received Anticancer Treatment for Advanced/Metastatic Disease

Inclusion Criteria: * Histological confirmation of breast cancer with evidence of locally advanced or metastatic disease, which is not amenable to surgical resection or radiation therapy with curative intent. * Documented estrogen receptor (ER) and/or progesterone receptor (PR)-positive tumor * Documented HER2-negative tumor * Previously untreated with any systemic anticancer therapy for their locally advanced or metastatic disease. * Measurable disease or non-measurable bone only disease as defined by RECIST version 1.1 Exclusion Criteria: * In visceral crisis at risk of immediately life-threatening complications in the short term. * Current or past history of central nervous system metastases. * Have received prior (neo)adjuvant endocrine therapy (ET) and had recurrence during or within 12 months after the last dose of ET. * Have received prior (neo)adjuvant CDK4/6i and had recurrence during or within 12 months after the last dose of CDK4/6i. * Inadequate renal function, hepatic dysfunction, or hematologic abnormalities.