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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 to a maximum of eight cycles of induction therapy; the minimum cycles are defined as either: Phesgo injections + 4 docetaxel infusions, or Phesgo injections + 12 paclitaxel infusions
* 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), single-agent trastuzumab IV or SC, ado-trastuzumab emtansine, lapatinib, and neratinib in the neoadjuvant or adjuvant setting
* Disease progression within 6 months of receiving adjuvant anti-HER2 therapy (such as trastuzumab, with or without pertuzumab \[IV, SC, or fixed-dose combination\], or ado-trastuzumab emtansine, or neratinib)
* 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 consistent with Child-Pugh Class B or C, active liver disease including active viral or other hepatitis virus, autoimmune hepatic disorders, or sclerosing cholangitis, current alcohol abuse, or cirrhosis
* 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 for ≥4 weeks, have a CD4 count ≥350 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
* Treatment with strong CYP3A4 inhibitors or inducers within 14 days or 5 drug-elimination half-lives, whichever is longer, prior to initiation of giredestrant treatment in Arm B
* A documented history of hemorrhagic diathesis, coagulopathy, or thromboembolism, including deep vein thrombosis, unless the condition is adequately treated and under control

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