Change in the molecular profile of tumor tissues during treatment with trastuzumab, as analyzed by next-generation sequencing and immunohistochemistry: A multicenter prospective biomarker study on HER2-positive gastric cancer

Abstract

Background: Trastuzumab (Tmab) is an active molecular-targeted drug for HER2- positive gastric cancer (GC) patients. However, continued use of Tmab beyond progression is not established in HER2-positive GC, unlike that of breast cancer. Therefore, we conducted this study to evaluate the resistance mechanism of anti-HER2 drugs in metastatic GC patients. Methods: Metastatic HER2-positive GC patients treated with Tmab were registered prospectively, and tumor tissues were obtained by biopsy from primary lesions at the following points: (1) pre-treatment, (2) post-treatment, and (3) disease progression during chemotherapy with Tmab. Formalin-fixed paraffin-embedded tissue slides were prepared, and the expression of receptor tyrosine kinases (RTKs) such as EGFR, HER2, HER3, c-MET, FGFR2 and IGF-1R was evaluated by immunohistochemistry (IHC). Hot spot mutations and copy number variations (CNVs) were analyzed by nextgeneration sequencing (NGS) using Ion AmpliSeq Cancer Hotspot Panel χ2. Results: Twenty patients were enrolled and evaluated by IHC, and 15 of 20 patients were evaluated by NGS. One patient was excluded because HER2 status was revealed as negative after registration. HER2 expression (≥ 2+) by IHC have disappeared after treatment in 8 patients (42%). FGFR2 expression (≥2+) by IHC was most frequently observed after treatment. Cases with IGF-1R expression (≥2+) were significantly increased after treatment (p<0.05). Somatic mutations of TP53 (n=9), KRAS (n=2), BRAF (n=2), SMAD4 (n=1), CDH1 (n=1), and CDKN2A (n=1) were observed before treatment. Mutations of TP53 (n=2), KRAS (n=2), PIK3CA (n=2), CDH1 (n=2), PDGFRA (n=1), and PTEN (n=1) were newly observed during the treatment. CNV analyses revealed that the cases with gain of KRAS and loss of STK11 were likely to increase during treatment with Tmab. Conclusions: Our study indicated that molecular changes in RTK expression and genomic alternation frequently occur during treatment with Tmab. These findings will contribute to the development of individualized treatment for HER2-positive GC patients.