Toi et al. BMC Cancer 2010, 10:56 http://www.biomedcentral.com/1471-2407/10/56 R E S E A R C H A R T I C L E Open Access Differential survival following trastuzumab treatment based on quantitative HER2 expression and HER2 homodimers in a clinic-based cohort of patients with metastatic breast cancer Masakazu Toi1*, Jeff Sperinde2, Weidong Huang3, Shigehira Saji4, John Winslow2, Xueguang Jin2, Yuping Tan2, Shinji Ohno5, Seigo Nakamura6, Hiroji Iwata7, Norikazu Masuda8, Kenjiro Aogi9, Satoshi Morita10, Christos Petropoulos2, Michael Bates3 Abstract Background: We have recently described the correlation between quantitative measures of HER2 expression or HER2 homodimers by the HERmark assay and objective response (RR), time-to progression (TTP), and overall survival (OS) in an expanded access cohort of trastuzumab-treated HER2-positive patients with metastatic breast cancer (MBC) who were stringently selected by fluorescence in situ hybridization (FISH). Multivariate analyses suggested a continuum of HER2 expression that correlated with outcome following trastuzumab. Here we investigate the relationship between HER2 expression or HER2 homodimers and OS in a clinic-based population of patients with MBC selected primarily by IHC. Methods: HERmark, a proximity-based assay designed to detect and quantitate protein expression and dimerization in formalin-fixed paraffin-embedded (FFPE) tissues, was used to measure HER2 expression and HER2 homodimers in FFPE samples from patients with MBC. Assay results were correlated with OS using univariate Kaplan-Meier, hazard function plots, and multivariate Cox regression analyses. Results: Initial analyses revealed a parabolic relationship between continuous measures of HER2 expression and risk of death, suggesting that the assumption of linearity for the HER2 expression measurements may be inappropriate in subsequent multivariate analyses. Cox regression analyses using the categorized variable of HER2 expression level demonstrated that higher HER2 levels predicted better survival outcomes following trastuzumab treatment in the high HER2-expressing group. Conclusions: These data suggest that the quantitative amount of HER2 expression measured by Hermark may be a new useful marker to identify a more relevant target population for trastuzumab treatment in patients with MBC. Background Over-expression of HER2 has been linked to both adverse prognosis and improved responsiveness to treat- ment with trastuzumab (Herceptin, Genentech) in a sub-population of metastatic breast cancers [1-5]. Tras- tuzumab offers significant disease-free and overall survi- val advantages in both the metastatic and adjuvant settings in patients with HER2 over-expressing tumors [6-11]. Currently, the selection of patients with breast * Correspondence: toi@kuhp.kyoto-u.ac.jp 1Faculty of Medicine, Kyoto University, Kyoto, Japan cancer for treatment with trastuzumab is based on the measurement of HER2 receptor protein expression by immunohistochemistry (IHC), or by the presence of HER2 gene amplification as detected by FISH. Despite the success of trastuzumab, recent data from NCCTG 9831, NSABP B-31, and CALGB 150002 have called into question the accuracy of the current methods used to identify those patients who are most likely to benefit from treatment with trastuzumab [12-14]. New approaches to the accurate quantitation of HER2 expression and HER2 dimerization in FFPE breast © 2010 Toi et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.