Patterns of Treatment and Outcome of Palbociclib Plus Endocrine Therapy in Hormone Receptor-positive (HR+)/HER2 Receptor-negative (HER2-) Metastatic Breast Cancer (MBC): a Real World Multicentre Italian Study

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.