Serum HER2 extracellular domain (ECD) levels and efficacy of weekly (W) or every 3-weekly (q3W) paclitaxel (P) with or without trastuzumab (T) in patients (pts) with metastatic breast cancer (MBC): CALGB 150002/9840

558 Background: The utility of baseline and serial changes in HER2 ECD levels as predictors of benefit of T-based therapy for MBC is unclear (Kostler et al, Clin Cancer Res 10:1618–24, 2004). We examined the association of ECD with response to WP or q3W P ± T in CALGB 9840. Methods: Plasma samples were collected within 7 days of study entry for 245 pts (WP: 147; q3W P: 98/T: 177, no T: 68), and serially at times of re-staging. Wilcoxon rank sum test was used to correct ECD with the presence of visceral metastases and number (#) of organ systems involved. Cox proportional hazard models were used to evaluate interaction of P schedule and ECD level on DFS and OS. ANOVA approach examined for interaction between P schedule, response, and ECD status. SAS mixed modeling examined serial ECD changes with response. Centralized testing with HercepTest was performed on retrievable blocks. Results: High baseline ECD correlated with presence of visceral metastases and # (>2) of organ systems involved (p = 0.0003, for b...