Expanded Access Study of Palbociclib Plus Letrozole for Postmenopausal Women with HR+/HER2− Advanced Breast Cancer in Latin America for Whom Letrozole Therapy is Deemed Appropriate

Background and Objectives Palbociclib is a cyclin-dependent kinase 4/6 inhibitor that is approved in the United States for the treatment of hormone receptor‒positive (HR+)/human epidermal growth factor receptor‒2 negative (HER2−) advanced breast cancer (ABC). The objectives of this expanded access trial were to provide palbociclib in combination with letrozole to patients with HR+/HER2− ABC in Argentina, Brazil, Colombia, and Mexico who were candidates for letrozole therapy before commercial availability of palbociclib, and to evaluate the safety and tolerability of palbociclib plus letrozole. Patients and Methods Postmenopausal women aged ≥ 18 years with HR+/HER2− ABC were eligible to participate in this study. Patients received palbociclib 125 mg once daily (3/1 schedule) and letrozole 2.5 mg once daily (continuous schedule). Safety, objective response rate (ORR), and duration of treatment were evaluated. Results A total of 130 patients were treated with palbociclib plus letrozole (Argentina, n = 33; Brazil, n = 35; Colombia, n = 28; Mexico, n = 34). The most common treatment-emergent adverse events (TEAEs) of any grade were neutropenia (70.0%), leukopenia (34.6%), anemia (33.8%), decreased neutrophil count (27.7%), and thrombocytopenia (24.6%); 22.3% of patients required a palbociclib dose reduction due to adverse events (AEs). Serious AEs were reported in 32 patients (24.6%). The ORR was 24.8% (95% confidence interval 17.6‒33.2), and the median duration of treatment was 10.6 months (range 0.1‒29.3). Conclusion Palbociclib in combination with letrozole was generally well tolerated with a clinically manageable safety profile; the observed ORR supported treatment benefit in Latin American women with HR+/HER2− ABC. Trial Registry ClinicalTrials.gov, NCT02600923.