# Overall survival and subsequent therapy patterns in Japanese patients with ER+/HER2− advanced breast cancer treated with palbociclib plus letrozole in the first-line setting: a final analysis

## TL;DR

A study in Japan found that first-line treatment with palbociclib plus letrozole extended survival for over seven years in patients with a specific type of advanced breast cancer.

## Contribution

The study provides final overall survival data for Japanese patients with ER+/HER2− advanced breast cancer treated with palbociclib plus letrozole.

## Key findings

- Median overall survival was 85.4 months with first-line palbociclib plus letrozole.
- Most patients received endocrine-based therapy in second-line treatment.
- Patient demographics and subsequent therapy decisions may have contributed to extended survival.

## Abstract

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.

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.

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.

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.

ClinicalTrials.gov, NCT04735367.

The online version contains supplementary material available at 10.1007/s12282-025-01760-0.