Background & Aims: Gemcitabine plus cisplatin (GC) remains the standard, frontline therapy for advanced biliary tract cancer (ABTC). The JCOG1113 study suggested that gemcitabine plus S-1 (GS) had noninferior median overall survival and comparable incidence of significant neutropenia as compared to GC treatments. This study evaluates the efficacy and safety of a modified GS regimen. Methods: The eligible patients with chemonaive, measurable ABTC received 800 mg/m2 of gemcitabine on day 1 and 80 mg/m2/day of S-1 (80/100/120 mg for patients with body surface <1.25/ ≥1.25 and <1.5/ ≥1.5 m2 respectively). The primary endpoint was the 12-week disease control rate (12-week DCR: objective response and stable disease ≥ 12 weeks). Per the p0 = 40% and p1 = 60% (α/β = 0.05/0.2) assumption, Simon's optimal two-stage design indicated 12-week DCR in ≥ 24 of 46 evaluable patients for significant activity. Tumour responses were assessed every 6 weeks. Results: Fifty-one patients were enrolled and most of them had intrahepatic cholangiocarcinoma (64.7%), metastatic disease (84.3%) and disease-related symptoms (82.4%). On intention-to-treat analysis, 11 (21.6%) patients showed partial response, whereas 21 (41.2%) showed stable disease ≥ 12 weeks. The progression-free and overall survival were 5.4 months (95% confidence interval [CI]: 3.5-7.0), and 12.7 months (95% CI: 6.1-15.6) respectively. The study met its primary endpoint with a 12-week DCR of 69.6% in 46 evaluable patients. Grade 3/4 treatment-related adverse eventsoccurred in < 6% of patients of all individual items. The mean dose intensities of S-1 and gemcitabine were 87.1% and 92.5% respectively. Conclusions: Modified GS showed moderate efficacy with a favourable safety profile in ABTC patients, thus mandating further assessment. ClinicalTrials.gov number: NCT02425137.
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