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 ormetastatic disease, which is not amenable to surgical resection or radiation therapywith 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 advancedor 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 theshort term.

• Current or past history of central nervous system metastases.

• Have received prior (neo)adjuvant endocrine therapy (ET) and had recurrence duringor within 12 months after the last dose of ET.

• Have received prior (neo)adjuvant CDK4/6i and had recurrence during or within 12months after the last dose of CDK4/6i.

• Inadequate renal function, hepatic dysfunction, or hematologic abnormalities.