Re-evaluation of HER2 status in patients with HER2-positive advanced or recurrent gastric cancer refractory to trastuzumab (KSCC1604)

Highlights

• The HER2 status was re-evaluated using re-biopsied samples from the patients with HER2-positive gastric cancer.

• Among 33 eligible patients, the loss of HER2 was identified in 20 patients (60.6%) with refractory disease.

• The use of fixatives, other than 10% neutral buffered formalin, reduced the HER2-positive rate.

Abstract

Background

Anti-HER2 therapy has not demonstrated a survival advantage in the second-line setting of patients with HER2-positive gastric cancer. We conducted this study to assess changes in HER2 status and to identify possible biomarkers for acquired resistance after the use of trastuzumab as the first-line therapy.

Patients and methods

Patients with advanced or recurrent HER2-positive gastric adenocarcinoma who were diagnosed with progressive disease after the first-line trastuzumab-based therapy and developed pathologically confirmed adenocarcinoma within 3 months after completion of trastuzumab-based therapy were enrolled in this study. We collected re-biopsied samples from the HER2-positive patients who had developed resistance to trastuzumab and re-evaluated their HER2 status. Amplification of EGFR and c-met, as well as PIK3CA mutation, were comparatively analysed when samples were available.

Results

Among 33 eligible patients, loss of HER2 was identified in 20 patients (60.6%) with refractory disease. Immunohistochemistry showed that the rate of HER2 overexpression was greatly reduced after therapy (pre-HER2 3+: 24 [72.7%] vs. post-HER2 3+: 13 [39.4%]). We found that the use of fixatives other than 10% neutral buffered formalin significantly reduced the HER2-positive rate. EGFR amplification, c-met amplification and PIK3CA mutation before and after trastuzumab-based therapy were observed in 10.3% and 3.8%, 17.9% and 4.2% and 4.0% and 4.2% of cases, respectively.

Conclusion

Re-evaluation of HER2 status is needed to determine the appropriate use of anti-HER2–targeted therapy after disease progression. Our results also highlight the importance of formalin fixation conditions for HER2 testing.

Introduction

Human epidermal growth factor receptor 2 (HER2) overexpression/amplification is observed in 7%–34% of patients with gastric cancer [1], [2], [3], [4], [5], [6]. In the phase III ToGA trial of trastuzumab with cisplatin and fluoropyrimidine-based doublet chemotherapy, a significant improvement in survival was observed for patients with HER2-positive gastric cancer [7]. Therefore, addition of trastuzumab to chemotherapy is now the standard first-line treatment for patients with HER2-positive advanced or recurrent gastric cancer. Although trastuzumab prolongs survival in this population, responses are rarely complete and resistance develops invariably.

On the other hand, in the second-line setting, anti-HER2 therapy has not demonstrated a survival advantage until now. In the TyTAN trial, the addition of lapatinib to second-line paclitaxel was not superior to the placebo plus paclitaxel [8]. In the GATSBY trial, trastuzumab emtansine (T-DM1) did not demonstrate superiority to taxane monochemotherapy [9]. There are several possible explanations for these unexpected results. In breast cancer, therapies which include trastuzumab may preferentially eradicate HER2-positive cancer cells, allowing HER2-negative clones to become dominant [10], [11], [12]. This phenomenon may be more frequently observed in gastric cancer because of the heterogeneity of HER2 expression [13]. It is also possible that there are coexisting oncogenic drivers that may function as negative predictors of trastuzumab benefit and could potentially be exploited as therapeutic cotargets. Several candidate alterations (EGFR and/or MET amplification, EGFR/MET/HER3/PI3K/PTEN mutations, etc.) may co-occur with HER2 overexpression/amplification and putatively confer trastuzumab resistance [14]. However, the biological mechanisms of acquired resistance to anti-HER2 therapy after use of trastuzumab for gastric cancer are not well understood.

The purpose of this study was to detect changes in HER2 status and to identify possible biomarkers for acquired resistance after the use of trastuzumab as the first-line therapy for advanced or recurrent gastric cancer. We demonstrate that HER2 loss occurred in a considerable number of patients who were initially diagnosed with HER2-positive gastric cancer and developed resistance to trastuzumab.