Supplementary Material for: Post-Progression Treatments after Palbociclib plus Endocrine Therapy in HR+/HER2– Metastatic Breast Cancer Patients: What Is the Better Choice?

Background: To date, a consensus has not yet been reached about the therapy sequence after disease progression (PD) on CDK4/6 inhibitors in patients with HR+/HER2- metastatic breast cancer (MBC). Objectives: The present study assesses, in a real-world setting, the activity of different subsequent therapies in patients who experienced a PD on palbociclib (P) + endocrine therapy (ET), to evaluate the best therapy sequence. Methods: This is a multicenter retrospective observational study. Records of consecutive HR+/HER2- MBC patients from January 2017 to May 2019 were reviewed. The primary endpoint was the evaluation of progression-free survival (PFS) according to subsequent treatment lines after progression on P+ET. Toxicity data were also collected. Results: The outcomes were analyzed in 89 MBC patients that had progressed on previous P+ET: 17 patients were on hormone therapy (HT) and 31 patients on chemotherapy (CT) as second-line treatments; seven patients were on HT and 34 on CT as third-line therapies. PFS of patients treated with HT as second-line therapy is significantly improved when compared with patients treated with CT (p=0.01). Considering third-line settings, the difference in PFS was not statistically different between HT and CT. A better outcome in terms of toxicity is observed among HT patients for both second- and third-line therapies. Conclusions: patients who were progressive on P+ET could still benefit from a subsequent ET. In patients who experienced a good efficacy from prior ET, without visceral metastatic sites, HT seems the most suitable option, when compared to CT, also in terms of safety.