Significance of accurate HER2 testing as a new biomarker in advanced gastric cancer.

Abstract

106 Background: The Trastuzumab for Gastric Cancer (ToGA) study is the first international trial to include Japanese patients with human epidermal growth factor 2 (HER2)-positive advanced or recurrent gastric or gastroesophageal junction cancer, which demonstrated that trastuzumab plus chemotherapy improved overall survival in the overall population (hazard ratio 0.74). HER2 testing in gastric cancer differs from testing in breast cancer due to inherent differences in tumor biology; gastric cancer more frequently shows HER2 heterogeneity and incomplete membrane staining. The aim of the present study was to evaluate the frequency of HER2-positive cases by application of the standard criteria in Japanese patients with advanced gastric cancer (AGC) and to investigate the relationships between HER2 expression and therapeutic responses. Methods: A total of 199 tumor samples were assessed for HER2 expression both by immunohistochemistry (IHC) and HER2 amplification by fluorescence in situ hybridization (FISH). HER2-positive status was defined as IHC2+ and FISH-positive or IHC3+. Objective responses were evaluated in the patients with AGC who were treated with chemotherapy plus trastuzumab or chemotherapy alone based on the HER2 expression status. Results: HER2-positive tumors were identified in 12 patients (5.5%), less than 28.1% in the Japanese subgroup analyses of ToGA study. The positive rates varied with histological type; 14%, 5.3% and 0.95% in the well, moderately and poorly differentiated adenocarcinoma, respectively. Although high concordance between the results of IHC and FISH in all samples was found, IHC2+ samples retested here showed FISH-negative. Of all 10 patients with AGC, 3 patients with HER2-positive tumor were treated with capecitabine/cisplatin plus trastuzumab, and partial response was found in 2 cases; response rates were 67%. Conclusions: Specific consideration and scoring modification are required before embarking on HER2 testing in gastric cancer. Accurate and reliable HER2 testing and scoring will allow appropriate selection of patients eligible for treatment with trastuzumab.