Background: Five-weekly S-1 plus cisplatin (SP) therapy is the standard care for advanced gastric or esophagogastric junction cancer (GC/EGJC) in East Asia. However, its efficacy and safety when combined with trastuzumab therapy for human epidermal growth factor receptor 2 (HER2)-positive advanced GC/EGJC remains unclear. Methods: Patients received 5-weekly SP therapy (S-1 at 40–60 mg twice daily for 21 days plus cisplatin at 60 mg/m2 on day 8, every 5 weeks) plus trastuzumab therapy (first dose of 8 mg/kg, then 6 mg/kg every 3 weeks). The primary end point was the response rate, and the secondary end points included progression-free survival, overall survival, safety, and serum biomarker levels. Results: Forty-four patients were enrolled. The response rate, progression-free survival, and overall survival were 61% (95% confidence interval 46–76%), 5.9 months, and 16.5 months respectively. The commonest grade 3 or grade 4 adverse events were neutropenia (30%) and anorexia (25%). A significantly higher response rate (92% vs 43%; P = 0.008) and longer progression-free survival (median 14.5 months vs 4.2 months; P = 0.028) were observed in patients with high (n = 14) compared with low (n = 17) pretreatment serum neuregulin 1 levels. Conclusions: Five-weekly SP therapy combined with trastuzumab therapy showed a good antitumor response and acceptable toxicity in HER2-positive advanced GC/EGJC. Serum neuregulin 1 might be associated with the efficacy of this treatment regimen.
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