• Patients must be capable to understand the purpose of the study and have signed written informed consent form (ICF) prior to beginning specific protocol procedures.

• Female or male patients ≥ 18 years of age at the time of signing ICF.

• ECOG performance status of 0-1.

• Minimum life expectancy of ≥ 12 weeks at screening.

• Evidence of HER2-low expression (1+ by immunohistochemistry (IHC) or 2+ and negative by an in situ hybridization \[ISH\] test) or HER2-ultralow (IHC 0 with faint membrane staining and in ≤ 10% of tumor cells) breast cancer according to the most recent American Society of Clinical Oncology (ASCO)/College of American Pathologists (CAP) guidelines determined by a MEDSIR's designated central laboratory, using Ventana 4B5 antibody. This assessment has to be done on the most recently available (archived or newly collected) formalin-fixed, paraffin-embedded (FFPE) tumor tissue blocks (≤ 6 weeks or FFPE of a tumor sample obtained after last prior systemic therapy) from core or excisional biopsy from a locally recurrent (breast or locoregional lymph nodes) or metastatic tumor lesion, excluding bone metastases.

• Non-luminal breast cancer subtype as per central PAM50 analysis determined in the most recently available (archived or newly collected) FFPE tumor tissue blocks (≤ 6 weeks or FFPE of a tumor sample obtained after last prior systemic therapy) from core or excisional biopsy from a locally recurrent (breast or locoregional lymph nodes) or metastatic tumor lesion with the exception of bone metastases.

• Patients must have HR-positive (estrogen receptor \[ER\] and/or progesterone receptor \[PgR\]-positive defined as ≥ 1% positive stained cells) status according to the most recent ASCO/CAP guidelines locally determined prior to study entry.

• Unresectable locally recurrent or metastatic breast cancer documented by computerized tomography (CT) scan or magnetic resonance imaging (MRI) that is not amenable to resection with curative intent.

• Evaluable disease according to RECIST v.1.1. Patients with bone-only disease are not allowed. Patients with bone metastases with soft tissue masses measuring \> 10 mm are eligible.

⁃ Patients must have endocrine resistance criteria:

⁃ • disease progression during adjuvant ET or within the first year of completing adjuvant ET;

⁃ or endocrine sensitivity criteria:

⁃ • de novo metastatic disease or disease progression ≥ 12 months after completing adjuvant ET with at least one of the following requirements:

‣ Estrogen receptor ≤ 50% positive stained cells;

‣ and/or high histological grade or Ki67 \> 50% on primary tumor;

‣ and/or liver metastases;

‣ and/or known non-luminal subtype as per local PAM50 analysis.

⁃ No prior treatment with any systemic therapy for advanced disease.

⁃ Patients treated with a CDK4/6i in the adjuvant setting with a treatment-free interval (TFI) ≥ 12 months following CDK4/6i treatment completion are eligible.

⁃ Patients have adequate bone marrow, liver, and renal function:

∙ Hematological (without platelet, red blood cell transfusion, and/or granulocyte colony-stimulating factor support within 14 days before first study treatment dose): White blood cell (WBC) count \> 3.0 x 109/L, absolute neutrophil count (ANC) ≥ 1.5 x 109/L, platelet count ≥ 100.0 x 109/L, and hemoglobin ≥ 9.0 g/dL (≥ 5.6mmol/L).

‣ Hepatic: Serum albumin ≥ 2.5 g/dL; total bilirubin ≤ 1.5 times upper limit of normal (x ULN) (≤ 3 x ULN in patients with liver metastases or know history of Gilbert's disease); alkaline phosphatase (ALP) ≤ 2.5 x ULN (≤ 5 x ULN in patients with liver/or bone metastases); aspartate transaminase (AST) and alanine transaminase (ALT) ≤ 3 x ULN (≤ 5 x ULN in patients with liver metastases).

‣ Renal: Creatinine clearance ≥ 30 mL/min as determined by Cockcroft Gault (using actual body weight).

‣ Coagulation: International normalized ratio or prothrombin time and either partial thromboplastin or activated partial thromboplastin time ≤ 1.5 × ULN.

⁃ Resolution of all acute toxic effects of prior anti-cancer therapy to Grade ≤ 1 as determined by the US National Cancer Institute (NCI)-Common Terminology Criteria for Adverse Events (CTCAE) version 5.0 (v.5.0) (except for alopecia or other toxicities not considered a safety risk for the patient at investigator's discretion).

⁃ Women of childbearing potential who are sexually active with a non-sterilized male partner must have a negative serum pregnancy test within 14 days before study treatment initiation. In addition, they must agree to use one highly effective method of birth control from the time of screening until 7 months after the last dose of T-DXd, or within the time period specified per local prescribing guidelines after the final dose of physician's choice of CDK4/6i plus ET. Female patients must refrain from egg cell donation and breastfeeding during this same period.

⁃ Male participants who are sexually active with a female partner of childbearing potential must be surgically sterile or using an acceptable method of contraception from the time of screening until 4 months after the last dose of T-DXd, or within the time period specified per local prescribing guidelines after the final dose of physician's choice of CDK4/6i plus ET. Male participants must not donate or bank sperm during this same period.

⁃ Patients must be accessible for treatment and follow-up.