For women who are premenopausal or perimenopausal and for men: treatment with approved LHRH agonist therapy for the duration of study treatment

Locally advanced (recurrent or progressed) or metastatic adenocarcinoma of the breast, not amenable to treatment with curative intent

Documented ER-positive tumor and HER2-negative tumor, assessed locally

Patients who have bilateral breast cancers which are both ER-positive and HER2-negative can be included in the study because the metastases are suitably targeted by the study treatments. If patients have bilateral tumors which are of different biomarker status, then proof of the ER and HER2 status of the metastases is required for study entry;

No history of systemic anti-cancer therapy for locally advanced (recurrent or progressed) or metastatic disease

Disease recurrence from early-stage breast cancer after standard adjuvant endocrine therapy meeting the protocol-defined criteria of having received at least 24 months of treatment without disease progression during treatment and a disease-free interval since the completion of treatment that was greater than 12 months

Measurable disease as defined per RECIST v.1.1 or bone only disease which must have at least one predominantly lytic bone lesion confirmed by CT or MRI which can be followed

Eastern Cooperative Oncology Group Performance Status 0-1;

Adequate organ function

Disease recurrence during or within 12 months of completing prior neoadjuvant or adjuvant treatment with an aromatase inhibitor (AI)

Disease recurrence during or within 12 months of completing prior neoadjuvant or adjuvant treatment with any CDK4/6 inhibitor

Prior treatment with a selective estrogen receptor degrader (SERD);

Prior treatment with tamoxifen is permitted, provided the patient did not experience disease recurrence within the first 24 months of treatment with tamoxifen

Treatment with any investigational therapy within 28 days prior to study treatment

Advanced, symptomatic, visceral spread that is at risk of life-threatening complications

Known active uncontrolled or symptomatic CNS metastases, carcinomatous meningitis, or leptomeningeal disease

Active cardiac disease or history of cardiac dysfunction

Pregnant or breastfeeding

• Antineoplastic Agents

• Aromatase Inhibitors

• BO41843

• Breast Diseases

• Breast Neoplasms

• Enzyme Inhibitors

• Estrogen Antagonists

• Hormone Antagonists

• Hormones, Hormone Substitutes, and Hormone Antagonists

• Letrozole

• Molecular Mechanisms of Pharmacological Action

• Neoplasms

• Neoplasms by Site

• Palbociclib

• Physiological Effects of Drugs

• Protein Kinase Inhibitors

• Skin Diseases

• Steroid Synthesis Inhibitors

• Oncology (Adult)

• Hoffmann-La Roche