Abstract
BACKGROUND: An open-label, single-arm, multicenter Japanese phase 2 study (J-Ph2) found first-line palbociclib plus letrozole to be effective and tolerable in postmenopausal Japanese women with estrogen receptor-positive/human epidermal growth factor receptor 2-negative (ER+/HER2-) advanced breast cancer (ABC), but overall survival (OS) data were immature. Here, we report the final analysis of a follow-up study of J-Ph2 evaluating OS and subsequent therapy. METHODS: Patients (N = 42) who participated in J-Ph2 were included in this follow-up study. Primary endpoint was OS; other endpoints included chemotherapy-free survival (CFS) and type and duration of subsequent therapy. Median OS, CFS, and duration of subsequent therapy were estimated using the Kaplan-Meier method; outcomes were stratified by baseline demographic, disease characteristics, and type of second-line therapies. RESULTS: At median follow up of 101.0 months, median OS was 85.4 months (95% CI, 64.3-not estimable) and median CFS was 69.1 months (95% CI, 24.2-85.4). Eighty-one percent of patients (34/42) received a second-line therapy; of those, 85.3% (29/34) received endocrine-based therapy and 8.8% (3/34) received chemotherapy. Median duration of second-line therapy was 7.6 months. Sixty-nine percent of patients (29/42) received a third-line therapy; of those, 58.6% (17/29) received endocrine-based therapy and 31.0% (9/29) received chemotherapy; median duration of third-line therapy was 6.0 months. CONCLUSION: This analysis showed a median OS of > 7 years with first-line palbociclib plus letrozole in Japanese patients with ER+/HER2- ABC. Patient demographics, disease characteristics, and subsequent therapy decisions may have contributed to the extended median OS observed in this study. CLINICAL TRIAL REGISTRATION: ClinicalTrials.gov, NCT04735367.