Blood test may predict breast cancer drug response

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

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