The impact of the inflammatory microenvironment in predicting the benefit of a CDK4/6 inhibitor in the treatment of locally advanced or first line metastatic breast cancer has not yet been studied.

We propose a multicenter prospective study to confirm the prognostic value of pre-therapy NLR on progression-free survival of patients initiating treatment with a CDK4/6 inhibitor combined with hormonal therapy for locally advanced or metastatic HR-pos / HER2-neg breast cancer.

We will also evaluate other markers of inflammation and their prognostic and predictive value for a better response to CDK4/6 inhibitor therapy in combination with hormone therapy in these patients.

Specific blood tests (lymphocyte typing) will be performed during treatment with CDK4/6.

The impact of the inflammatory microenvironment in predicting the benefit of a CDK4/6 inhibitor in the treatment of locally advanced or first line metastatic breast cancer has not yet been studied.

We propose a multicenter prospective study to confirm the prognostic value of pre-therapy NLR on progression-free survival of patients initiating treatment with a CDK4/6 inhibitor combined with hormonal therapy for locally advanced or metastatic HR-pos / HER2-neg breast cancer.

We will also evaluate other markers of inflammation and their prognostic and predictive value for a better response to CDK4/6 inhibitor therapy in combination with hormone therapy in these patients.

Specific blood tests (lymphocyte typing) will be performed during treatment with CDK4/6.

Inclusion Criteria:

• Patients with locally advanced or metastatic HR-positive, Her2-negative breast cancer
• Patients who are scheduled to receive first-line metastatic CDK4/6 inhibitor therapy in combination with hormone therapy as per the marketing authorization
• Patients who have not received previous anti-neoplastic therapies for metastatic or advanced disease (chemotherapy, targeted therapy or hormonal therapy). However, it will be possible to have initiated 1st line hormonal therapy within 4 to 6 weeks prior to inclusion.
• Prior radiotherapy allowed even in metastatic disease. In case of radiotherapy treatment, side effects attributable to the treatment must be resolved.
• Postmenopausal patients or patients with suppressed ovarian function
• Patient with measurable or non-measurable disease (according to RECIST v1.1 criteria)
• Adequate organ and marrow function to allow prescription of CDK 4/6 inhibitor therapy
• Age of Patient ≥ 18 years
• Willingness and ability to comply with scheduled visits, treatment plan, biologic tests and other trial procedures including assessments requested for inclusion
• Patient affiliated with a social security plan
• Informed consent signed prior to any specific study-related procedures

Exclusion Criteria:

• Men (no marketing authorization for CDK4/6 inhibitors in men in France)
• Previous systemic treatment for metastatic disease (chemotherapy, hormone therapy, etc.)
• Previous treatment with a CDK4/6 inhibitor (adjuvant or for metastatic disease)
• Locally advanced or relapsed breast cancer for which curative treatment would be considered
• Her2-positive tumor status on either the primary or relapsed tumor as defined by ASCO criteria
• Patient with advanced, symptomatic visceral extension who may be at risk for a potentially fatal short-term complication ("visceral crisis") and who requires treatment with chemotherapy
• Patients who are deprived of their liberty, under guardianship, or subject to a legal protection measure or who are unable to express their consent
• Patients who cannot undergo the trial follow-up for geographical, social or psychopathological reasons