Differences in drug efficacy and prognosis between primary and metastatic sites for de novo stage IV breast cancer: an exploratory analysis of a phase III trial, JCOG1017

Background Breast cancer is a highly heterogeneous disease, with biological factors like estrogen receptor, progesterone receptor, and HER2 receptor differing between primary and metastatic sites, potentially affecting treatment response.
This exploratory analysis aims to differentiate drug efficacy and long-term prognosis between these sites in stage IV breast cancer.
Methods Patients from JCOG1017, a phase III trial evaluating the role of primary tumor resection, who received primary systemic therapy (PST) were evaluated at three months. In this analysis, treatment response was assessed separately in primary and metastatic sites. Patients were categorized into four groups: discordant I (primary non-PD, metastatic PD), concordant I (both PD), discordant II (primary PD, metastatic non-PD), and concordant II (both non-PD).
Results Among 271 patients, overall discordance proportion of treatment response between primary and metastatic sites was 25.1%. Group distribution was 24.7% (discordant I), 9.6% (concordant I), 0.4% (discordant II), and 65.3% (concordant II). Discordance was more frequent in luminal (28.9%), triple-negative (25.0%), and luminal-HER2 (22.0%) subtypes than in HER2-enriched (11.1%). Survival analysis showed prognostic differences: concordant II, with both sites non-PD, demonstrated the most favorable outcome compared with discordant I (HR 0.556; 95% confidence interval, 0.396–0.782).
Conclusions One-fourth of patients exhibited discordant responses between primary and metastatic sites in early treatment phases. These discrepancies were associated with survival differences, emphasizing the importance of evaluating both primary and metastatic lesions when assessing efficacy and determining treatment strategies in de novo stage IV breast cancer.