• Histologically or cytologically confirmed diagnosis of metastatic or locally advanced HR+/HER2- breast cancer

• Adult females, pre- and/or post-menopausal, and adult males. Pre-menopausal (and peri-menopausal) women can be enrolled if amenable to treatment with an LHRH agonist. Patients are to have commenced concomitant treatment with LHRH agonist prior to or on Cycle 1, Day 1 and must be willing to continue for the duration of the study.

• Negative pregnancy test for females of childbearing potential. Female subjects who are not surgically sterile must use a medically effective contraceptive method from screening until 2 years after the last dose of study treatment.

• Progression of disease during or within 12 months of completing (neo)adjuvant ET.

• Adequate archival, fresh tumor tissue, or liquid biopsy for the analysis of PIK3CA mutational status.

• Permitted prior therapies:

∙ (neo)adjuvant fulvestrant or any selective ER degrader only if the treatment duration \< 6 months

‣ (neo)adjuvant chemotherapy

‣ (neo)adjuvant CDK4/6 inhibitor, unless PD was on or within 6 months of discontinuation of CDK4/6i

• Subject has radiologically measurable disease according to RECIST v1.1, per local assessment. Patients with evaluable bone-only disease are not eligible. Patients with bone-only disease that has lytic or mixed lytic/blastic lesions and at least one measurable soft tissue component per RECIST v1.1 may be eligible.

• Eastern Cooperative Oncology Group (ECOG) performance status of 0-2.

• Life expectancy of at least \>6 months.

⁃ Adequate bone marrow, hepatic, renal and coagulation function.