Patients presenting with a de novo diagnosis of luminal-like advanced breast cancer (ABC) or with disease recurrence after >12 months from the end of adjuvant ET, are generally candidate to a first line therapy with an aromatase inhibitor in association with a CDK4/6i. Disease recurrence in <12 months from the end of adjuvant ET defines the disease as "endocrine resistant" and identifies patients that should receive a first line therapy with the selective estrogen receptor degrader (SERD) Fulvestrant in association with the CDK4/6i Ribociclib, according to the results of the MONALEESA-3 trial.

A significant percentage of ABC patients develops a primary resistance with disease progression within the first 6 months from the beginning of the treatment. Furthermore, another relevant percentage of patients initially responding to the therapy, will later develop a secondary resistance, thus progressing after a median of 2 years from the beginning of the treatment. Thereby, it is crucial to identify biomarkers that could be predictive of a response or a resistance to ET and/or CDK4/6i, to provide the best therapeutic strategy, tailored upon both clinico-pathological and molecular characteristics.

Numerous pathways associated with resistance to CDK4/6i have been investigated by means of liquid biopsy analysis. The aim of this study is to identify potential biomarkers predictive of a clinical benefit in patients receiving a first line therapy with AI/fulvestrant (+/- LH-RH analogue) in association with a CDK4/6i for luminal-like advanced breast cancer.

Patients presenting with a de novo diagnosis of luminal-like advanced breast cancer (ABC) or with disease recurrence after >12 months from the end of adjuvant ET, are generally candidate to a first line therapy with an aromatase inhibitor (+/- LH-RH analogue depending from the menopausal status) in association with a CDK4/6i. Disease recurrence in <12 months from the end of adjuvant ET defines the disease as "endocrine resistant" and identifies patients that should receive a first line therapy with the selective estrogen receptor degrader (SERD) Fulvestrant in association with the CDK4/6i Ribociclib, according to the results of the MONALEESA-3 trial.

The choice of the endocrine backbone and of the CDK4/6i is mostly influenced by the patient's clinical characteristics and by disease factors.

However, a significant percentage of ABC patients develops a primary resistance with disease progression within the first 6 months from the beginning of the treatment. Furthermore, another relevant percentage of patients initially responding to the therapy, will later develop a secondary resistance, thus progressing after a median of 2 years from the beginning of the treatment. Thereby, it is crucial to identify biomarkers that could be predictive of a response or a resistance to ET and/or CDK4/6i, to provide the best therapeutic strategy, tailored upon both clinico-pathological and molecular characteristics.

Numerous pathways associated with resistance to CDK4/6i have been investigated by means of liquid biopsy analysis. The aim of this study is to identify potential biomarkers predictive of a clinical benefit in patients receiving a first line therapy with AI/fulvestrant (+/- LH-RH analogue) in association with a CDK4/6i for luminal-like advanced breast cancer.

Inclusion Criteria:

• Histologically proven diagnosis of adenocarcinoma of the breast with evidence of metastatic disease.
• ER positive tumor ≥ 1%
• HER2 negative breast cancer by FISH or IHC (IHC 0,1+, 2+ and/or FISH HER2: CEP17 ratio < 2.0)
• Females, 18 years of age or older
• Candidate to first-line endocrine therapy (LH-RH analogue for pre-menopausal women is allowed)
• Signed and dated informed consent document indicating that the subject (or legally acceptable representative) has been informed of all the pertinent aspects of the trial prior to enrollment.
• Willingness and ability to comply with scheduled visits, treatment plan, laboratory tests, and other trial procedures.

Exclusion Criteria:

• Diagnosis of any secondary malignancy within the last 3 years, except for adequately treated basal cell or squamous cell skin cancer, or carcinoma in situ of the cervix.
• Prior endocrine therapy for metastatic disease
• Prior chemotherapy for metastatic disease
• Patients unwilling to or unable to comply with the protocol.
• Known CNS metastases

Women with hormone receptor-positive ABC, considered eligible for endocrine therapy as first line endocrine treatment