A Study of the Gut Microbiome in Hormone Receptor-positive HER2-negative Breast Cancer Treated With CDK4/6 Inhibitors

• Cohort of metastatic HR-positive HER2-negative breast cancer :

Inclusion Criteria:

Patients that respond to each of these criteria can be included :

• Diagnosis of previously untreated HR+ HER2- advanced breast cancer (defined aslocally advanced and unresectable, or metastatic). HR+ defined as positive estrogenreceptors as per local laboratory testing. HER2- defined as negative ISH test or anIHC status of 0 or 1+ as per local laboratory testing.

• Planned first-line treatment with an endocrine therapy (aromatase inhibitor orfulvestrant) and a CDK4/6i.

• Male or female ≥ 18 years of age at the time the informed consent is signed.

• Being able to provide written informed consent.

• Patients with a history of early breast cancer are allowed providing systemictherapy (including adjuvant endocrine therapy) was discontinued more than 6 monthsago.

• Patients are willing and able to comply with the protocol for the duration of thestudy including sample collection.

• Paraffin-embedded tumor tissue available at diagnosis of metastatic disease (inclusion to be discussed if not available).

Exclusion Criteria:

Patients who respond to any of these criteria are excluded :

• Administration of the CDK4/6i already started.

• Concurrent or previous non breast-related malignancy in the last 3 years prior tothe start of the study treatment (with the exception of a history of adequatelytreated cervical carcinoma in situ or non-melanoma skin cancer).

• Treatment or chronic prevention of an infection through oral or intravenousantibiotic administered less than 1 month ago. History of unique antibioticadministration as prophylaxis for an invasive procedure is allowed.

• Active disease requiring treatment with an immunomodulatory agent. Low dose oralcorticosteroids (equivalent to 8 mg or less of prednisone) or topicalcorticosteroids are allowed.

• Serological positivity for human immunodeficiency virus (HIV) or hepatitis C (HCV).

• Known active hepatitis.

• Active inflammatory bowel disease or documented malabsorption.

• Alcohol consumption (>3 glasses/day).

• Cohort of early HR-positive HER2-negative breast cancer at high risk of relapse :

Inclusion criteria :

Patients that respond to each of these criteria can be included :

• Early HR+ HER2- node-positive breast cancer considered at high risk of relapse (≥ 4positive lymph nodes, or 1-3 positive lymph nodes and either or both grade 3 ortumor size > 5 cm). HR+ defined as positive estrogen receptors as per locallaboratory testing. HER2- defined as negative ISH test or an IHC status of 0 or 1+as per local laboratory testing.

• Planned adjuvant treatment with a CDK4/6i, in combination with an endocrine therapy (aromatase inhibitor or tamoxifen, with or without LHRH agonists).

• Male or female ≥ 18 years of age at the time the informed consent is signed.

• Being able to provide written informed consent.

• Patients are willing and able to comply with the protocol for the duration of thestudy including sample collection.

• Paraffin-embedded tumor tissue available at diagnosis of the disease or at surgicalresection (inclusion to be discussed if not available).

Exclusion criteria :

Patients who respond to any of these criteria are excluded :

• Administration of the CDK4/6i already started. Ongoing administration of theendocrine therapy before study inclusion is allowed.

• Concurrent or previous non breast-related malignancy in the last 3 years prior tothe start of the study treatment (with the exception of a history of adequatelytreated cervical carcinoma in situ or non-melanoma skin cancer).

• Treatment or chronic prevention of an infection through oral or intravenousantibiotic administered less than 1 month ago. History of unique antibioticadministration as prophylaxis for an invasive procedure is allowed.

• Active disease requiring treatment with an immunomodulatory agent. Low dose oralcorticosteroids (equivalent to 8 mg or less of prednisone) or topicalcorticosteroids are allowed.

• Serological positivity for human immunodeficiency virus (HIV) or hepatitis C (HCV).

• Known active hepatitis.

• Active inflammatory bowel disease or documented malabsorption.

• Alcohol consumption (>3 glasses/day).