Inclusion Criteria:

   - 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

Exclusion Criteria:

   - 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)

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

   - Treatment with strong CYP3A inhibitors or inducers within 14 days or 5 drug
   elimination half-lives (whichever is longer) prior to randomization

   - Advanced, symptomatic, visceral spread that is at risk of life-threatening
   complications in the short term

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

   - Active cardiac disease or history of cardiac dysfunction, as defined in the protocol

   - Pregnant or breastfeeding