This is a Phase 1b open-label, 2-part study in 3 treatment groups. The 3 treatment groups are as follows:
Treatment Group 1: Palazestrant (OP-1250) in combination with ribociclib (KISQALI®, Novartis Pharmaceuticals Corporation).
Treatment Group 2: Palazestrant (OP-1250) in combination with alpelisib (PIQRAY®, Novartis Pharmaceuticals Corporation).
Treatment Group 3: Palazestrant (OP-1250) in combination with everolimus.
Treatment Group 4: Palazestrant (OP-1250) in combination with atirmociclib.
Female or male aged >18 years.
Willing and able to participate and comply with all study requirements.
Histologically- or cytologically-confirmed advanced or metastatic Breast Cancer (mBC).
ER+/HER2- disease, as determined in the most recently obtained archival tumor tissue sample from a metastatic site, using locally accepted criteria by the local pathology report.
Prior or concurrent malignancy whose natural history or treatment may interfere with the safety or efficacy assessment of the investigational regimen.
Clinically significant, uncontrolled heart disease and/or cardiac repolarization abnormality.
History of cerebral vascular disease within 6 months prior to the first administration of study drug dose.
History of a pulmonary embolism, or deep venous thrombosis within the last 6 months, or subject has an increased risk of thrombosis as determined by the investigator.