A Study to Evaluate the Efficacy and Safety of Giredestrant in Combination With Phesgo (Pertuzumab, Trastuzumab, and Hyaluronidase-zzxf) Versus Phesgo in Participants With Locally Advanced or Metastatic Breast Cancer (heredERA Breast Cancer)

Inclusion Criteria:

Histologically or cytologically confirmed and documented human epidermal growth factor receptor 2 (HER2)-positive/estrogen receptor (ER)-positive adenocarcinoma of the breast with metastatic or locally-advanced disease not amenable to curative resection
At least one measurable lesion and/or non-measurable disease evaluable according to Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1
Disease-free interval from completion of adjuvant or neoadjuvant systemic non-hormonal treatment to recurrence of ≥6 months
Eastern Cooperative Oncology Group (ECOG) Performance Status 0 or 1
Left ventricular ejection fraction (LVEF) of at least (≥)50% measured by echocardiogram (ECHO) or multiple-gated acquisition scan (MUGA)
Adequate hematologic and end-organ function
For women of childbearing potential: Participants who agree to remain abstinent (refrain from heterosexual intercourse) or use contraception, and agree to refrain from donating eggs, during the treatment period and for 7 months after the final dose of Phesgo
For men: participants who agree to remain abstinent (refrain from heterosexual intercourse) or use a condom, and agree to refrain from donating sperm, during the treatment period and for 7 months after the final dose of Phesgo to avoid exposing the embryo

Maintenance Phase Inclusion Criteria

Complete a minimum of four cycles of induction therapy
Achieve a minimum of stable disease (SD) (or Non-complete response [CR]/Non-progressive disease [PD] for participants with non-measurable disease) (i.e., did not experience PD) according to RECIST v1.1 at the last tumor assessment during the induction therapy phase
LVEF of ≥50% at the last assessment during the induction therapy phase

Exclusion Criteria:

Previous systemic non-hormonal anti-cancer therapy in the metastatic breast cancer (MBC) or advanced breast cancer (ABC) setting. Note: Up to one line of single-agent endocrine therapy given in the metastatic or locally advanced setting will be allowed.
Prior treatment with a selective estrogen receptor degrader (SERD)
Previous treatment with approved or investigative anti-HER2 agents in any breast cancer treatment setting, except Phesgo (or trastuzumab SC with pertuzumab IV, or pertuzumab and trastuzumab IV), ado-trastuzumab emtansine, lapatinib, and neratinib in the neoadjuvant or adjuvant setting
Disease progression within 6 months of receiving trastuzumab, with or without pertuzumab, or ado-trastuzumab emtansine in the adjuvant setting
Non-resolution of all acute toxic effects of prior anti-cancer therapy or surgical procedures to National Cancer Institute Common Terminology Criteria for Adverse Events version 5.0 (NCI CTCAE v5.0) Grade 1 or better
History of persistent Grade ≥2 (NCI-CTC, Version 5.0) hematological toxicity resulting from previous adjuvant or neo-adjuvant therapy
History of exposure to the following cumulative doses of anthracyclines; Doxorubicin >360 mg/m2; Liposomal doxorubicin >500 mg/m2; Epirubucin >720 mg/m2; Mitoxantrone >120 mg/m2; Idarubicin >90 mg/m2.
Known active uncontrolled or symptomatic central nervous system (CNS) metastases, carcinomatous meningitis, or leptomeningeal disease
Dyspnea at rest due to complications of advanced malignancy, or other disease requiring continuous oxygen therapy
Pregnant or breastfeeding, or intending to become pregnant during the study or within 7 months after the final dose of Phesgo
Women of childbearing potential must have a negative serum pregnancy test result within 14 days prior to initiation of induction therapy
Treated with investigational therapy within 28 days prior to initiation of induction therapy
Treated with localized palliative radiotherapy within 14 days prior to initiation of induction therapy
Concurrent participation in any other therapeutic clinical trial
Known hypersensitivity to any of the study medications or to excipients of recombinant human or humanized antibodies
Current chronic daily treatment (continuous for >3 months) with corticosteroids (dose of 10 mg/day methylprednisolone or equivalent)
Poorly controlled hypertension
Known clinically significant history of liver disease
Active cardiac disease or history of cardiac dysfunction
Major surgical procedure or significant traumatic injury within 14 days prior to enrollment or anticipation of need for major surgery during induction therapy
Active inflammatory bowel disease, chronic diarrhea, short bowel syndrome, or major upper gastrointestinal surgery
Concurrent, serious, uncontrolled infections, or known infection with HIV with the following exception: Individuals who are HIV positive are eligible provided they are stable on anti-retroviral therapy, have a CD4 count ≥200 cells/uL, and have an undetectable viral load and no history of AIDS-defining opportunistic infections within 12 months prior to enrollment.
Serious COVID-19 infection within 14 days prior to enrollment; however, no screening testing for SARS-CoV-2 is required
Serious infection requiring oral or IV antibiotics within 7 days prior to screening
Any serious medical condition or abnormality in clinical laboratory tests that precludes an individual's safe participation in the study
History of malignancy within 5 years prior to screening with the exception of the cancer under investigation in this study and malignancies with a negligible risk of metastasis or death
For pre- and perimenopausal women, and men: Known hypersensitivity to luteinizing hormone-releasing hormone agonist (LHRHa); Not willing to undergo and maintain treatment with approved LHRHa therapy for the duration of endocrine therapy that requires gonadal function suppression