Background: Adjuvant anti-PD-1 therapy with nivolumab has shown efficacy in reducing recurrence rates in patients with locoregional esophageal cancer including squamous cell carcinoma who achieved R0 resection but harboring pathologic residual disease following neoadjuvant CRT. However, the efficacy of adjuvant anti-PD-1 therapy in improving outcomes for patients at higher risk of recurrence, especially those with involved or close resection margin (≤1 mm), remains unclear. Methods: We conducted a phase II trial of adjuvant cisplatin (30mg/m2 IV every week for 2 cycles)-based CRT (180-200cGy per fraction for 10-13 fractions) followed by pembrolizumab (200mg IV every 3 weeks for 18 cycles) in patients with locally advanced ESCC who had high-risk recurrence features in esophagectomy tumor tissues, including involved or close margin (≤1 mm), extranodal extension (ENE) of involved lymph node, and ypN2-3, following neoadjuvant CRT (ClinicalTrials.gov Identifier: NCT03322267). Primary endpoint was 1-year relapse-free survival (RFS) rate in comparison with a historical control of 32%.¹ Secondary endpoints included RFS, overall survival (OS), and toxicity. Results: Between Nov 2018 and Dec 2022, we enrolled a total of 25 patients with high risk recurrence features, including margin ≤1 mm (18 patients), ENE of involved lymph node (9), and ypN2-3 (9). Majority of patients were male (88%) and current smokers (84%). The median follow-up duration was 16.5 months (95% CI 10.6-27.1). One-year RFS rate was 63.3% (95% CI 44.5-79.8). Median RFS and OS were 15.3 months (95% CI 10.2-20.4) and 38.3 months (95% CI 10.2-66.3), respectively. Any grade treatment emergent adverse events (TEAEs) occurred in 56% and 75% of patients during cisplatin-based CRT and pembrolizumab treatment, respectively. Grade 3 TEAEs were infrequent, including leukopenia (4%), pancreatitis (4%), infection (8%), and colitis (4%). There was no grade 4 TRAE. Conclusions: Adjuvant cisplatin-based CRT followed by 1-year treatment of pembrolizumab was well-tolerated, and was associated with an improved 1-year RFS rate in patients with pathologic residual disease and high risk for recurrence following trimodality therapy for locally advanced ESCC. 1. Guo JC et al: J Thorac Oncol 2015;10:1481-9. Clinical trial information: NCT03322267.