Inclusion Criteria:

• * Signed and dated written informed consent.
• * Subjects ≥ 18 years of age.
• * Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1.
• * Clinical stage IV invasive mammary carcinoma or unresectable locoregional recurrence of invasive mammary carcinoma that is:
• * ER (\>/=1%) and/or PR (\>/= 1%) by IHC and HER2 negative (by IHC or FISH)
• * Previously exposed to an aromatase inhibitor (AI) or a selective estrogenreceptor modulator/ downregulator (SERM; SERD) + a CDK4/6 inhibitor.
• * Prior radiation permitted (if completed at least 2 weeks prior to study entry. Patients who have received prior radiotherapy must have recovered from toxicity (≤ grade 1) induced by this treatment (except for alopecia)
• * Patients with brain metastasis secondary to breast cancer and clinically stable for more than 4 weeks from completion of radiation treatment and off steroids
• * Evaluable disease (measurable or non-measurable)
• * Measurable disease, ie, at least 1 measurable lesion as per RECIST 1.1 (a lesion at a previously irradiated site may only be counted as a target lesion if there is clear sign of progression since the irradiation)
• * Patients with bone only disease allowed if possible to evaluate on radiological exams (eg.bone scan, PET/CT, CT, MRI) even if lesions are non-measurable according to RECIST1.1.
• * Adequate organ function including:
• * Absolute neutrophil count (ANC) ≥ 1.5 × 10\^9/L
• * Platelets ≥ 100 × 10\^9/L
• * Hemoglobin ≥ 8/g/dL (may have been transfused)
• * Total serum bilirubin ≤ 1.5 times upper limit of normal (ULN)
• * Aspartate aminotransferase (AST/SGOT) and alanine aminotransferase (ALT/SGPT) ≤ 2.5 × ULN (or ≤ 5 × ULN if liver metastases are present)
• * Serum creatinine ≤ 1.5 x ULN or estimated creatinine clearance ≥ 50mL/min as calculated using the Cockcroft-Gault (CG) equation
• * For randomized patients only: tumors must be diagnosed as non-Luminal A using the Blueprint® and Mammaprint® tests

Exclusion Criteria:

• * Prior chemotherapy in the metastatic setting
• * Previous malignant disease other than breast cancer within the last 2 years with associated competing risk, with the exception of basal or squamous cell carcinoma of the skin, cervical carcinoma in situ, or low-risk cancers considered curatively treated (i.e. complete remission achieved at least 2 years prior to first dose of study drugs AND additional therapy not required while receiving study treatment).
• * Persisting symptoms related to prior therapy that has not reduced to Grade 1 \[National Cancer Institute Common Toxicity Criteria for Adverse Events (NCI CTCAE) version 5.0\]; however, menopausal symptoms, alopecia, and sensory neuropathy Grade ≤ 2 is acceptable
• * Pregnant or breastfeeding females.