• 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