This is a multicentre real-world experience aimed at verifying the outcome of palbociclib plus ET in an unselected population of MBC patients. The primary endpoint is the clinical benefit rate (CBR); secondary aims are the median PFS (mPFS), overall survival (OS) and safety.

This is an open-label, longitudinal, prospective, multicentre cohort study. Eligible patients are pre- and postmenopausal women with a histologically proven HR+ MBC, candidate to receive palbociclib plus endocrine therapy (ET) as first or subsequent line of therapy according to their contingent clinical situation. Additional inclusion criteria are HER2- disease (immunohistochemistry (IHC) 0-1+ or IHC 2+, confirmed as fluorescence in situ hybridization [FISH] negative), presence of measurable or evaluable lesions and life expectancy of at least 4 months. They need to have adequate bone marrow, hepatic and renal function, according to clinical practice guidelines for antineoplastic drug administration. Previous chemotherapy or ET for metastatic disease is allowed. Patients receive palbociclib 125 mg daily, 3 weeks on/1 week off in a 28-day cycle, combined with letrozole 2.5 mg administered orally on a continuous daily dosing schedule (cohort A) or fulvestrant at the dose of 500 mg intramuscular on days 1, 14, 28, then every 4 weeks thereafter (cohort B). Premenopausal women receive a GnRH analogue in combination with ET and palbociclib. Treatment is administered until documented disease progression (PD), unacceptable toxicity or patient refusal. The tumour assessment is performed approximately every 16 weeks. Treatment efficacy is evaluated by Response Evaluation Criteria In Solid Tumors (RECIST version 1.1). A complete blood count and organ function test is performed before each cycle, through study completion, an avarange of 1 year. No pre-specified treatment modifications are planned; dose reductions, delay or discontinuations of palbociclib are performed according to observed side effects. AEs are recorded and graded according to the National Cancer Institute Common Terminology Criteria for Adverse Events (NCI-CTCAE) (version 5.0).

The primary aim of the study is to analyse the activity of palbociclib plus ET in terms of clinical benefit rate that is defined as the percentage of patients experiencing complete response (CR), partial response (PR), or stable disease (SD) lasting 6 months or more. Secondary aims include the evaluation of the safety of the treatments, progression-free survival and overall survival.

Inclusion Criteria:

• Pre- and postmenopausal women with a histologically proven HR+MBC, candidate to receive palbociclib plus ET as first or subsequent line of therapy according to their contingent clinical situation.
• HER2- disease (IHC 0-1 or IHC 2, confirmed as FISH negative), presence of measurable or evaluable lesions and life expectancy of at least 4 months.
• Adequate bone marrow, hepatic and renal function, according to clinical practice guidelines for antineoplastic drug administration

Exclusion Criteria:

• ER- PgR- disease
• HER2+ disease (IHC 3 or IHC 2, confirmed as FISH positive)
• Any cardiovascular, renal or hepatic condition that would compromise conditions in the opinion of the investigator

One hundred-ninety-one patients (92 in cohort A, 99 in cohort B) were enrolled and treated and 182 were evaluable for the analysis. Median age was 62 years (range 47-79)