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.

This is a multicenter, Phase 1b/2 trial in participants with estrogen receptor positive/human epidermal growth factor receptor 2 negative (ER+/HER2-) advanced/metastatic breast cancer. The phase 1b part of the trial will determine the recommended Phase 2 dose (RP2D) of elacestrant when administered in combination with alpelisib, everolimus, palbociclib, capivasertib, and ribociclib. The Phase 2 part of the trial will evaluate the efficacy and safety of the various combinations.