223 Background: Definitive or palliative chemoradiation therapy has been employed in the management of esophageal squamous cell carcinoma (ESCC). Immune checkpoint inhibitor has improved outcomes in metastatic stage IV pts. Here we report the addition of KN046, a PD-L1/CTLA-4 bispecific antibody, to concurrent chemoradiation (CRT) therapy to determine the safety and efficacy of this approach (ChiCTR2000031544). Methods: Pts with recurrent or metastatic ESCC, not been treated by CRT or other systemic treatment within 6 months, were recruited and received palliative CRT consisting of cisplatin (75 mg/m2 IV Q3W for 4~6 cycles), paclitaxel (135~175 mg/m2 IV Q3W for 4-6 cycles) and radiation (SBRT or conventional and dose are determined at the investigator’s discretion according to institutional standard). KN046 at ascending doses of 1, 3 and 5 mg/kg Q3W was added within 7-14 days after the completion of radiation therapy (RT) and concurrently with chemotherapy, followed by KN046 Q2W maintenance. Dose limiting toxicities (DLTs) were assessed for the first treatment cycle of KN046. Anti-tumor activity was assessed according to RECIST 1.1 every 6 weeks within the first year, and every 12 weeks thereafter. Results: As of June 30, 2020, 18 subjects were enrolled and received KN046 treatment (1mg/kg, n = 3; 3mg/kg, n = 11; 5mg/kg, n = 4). The median KN046 exposure was 11.5 weeks. No DLT was reported. 3 (16.7%) subjects experienced Grade 3, KN046 related adverse events (1 Grade 3 pneumonitis and 2 Grade 3 colitis recovered after steroid and antibiotic Tx). For 18 evaluable subjects, the disease control rate and objective response rate were 94.4% and 44.4%, respectively. DOR and PFS were not matured yet as of cutoff date. At 3 mg/kg, objective response was observed in 5 out of 9 subjects (55.6%) with measurable disease and disease control rate was 100%; 7/9 (77.8%) subjects experienced further tumor reduction after initiation of KN046 treatment. It is worth to note that, 2 subjects at 3mg/kg achieved complete response after receiving KN046 treatment. Conclusions: The addition of KN046 to CRT was well tolerated and showed promising efficacy signal in recurrent or metastatic ESCC. This pilot study enables further investigation of a new treatment modality of KN046 with CRT in this detrimental disease with poor prognosis. Clinical trial information: 2000031544.