646 Background: The efficacy of immune checkpoint inhibitor has been limited to small portion of colorectal cancer (CRC) patients whose tumors with mismatch repair (MMR) gene abnormalities. There is an urgent need for patients with MMR proficient (pMMR) tumors. Oncolytic immunotherapy represents a novel therapeutic platform for the treatment of cancer with unique activity compared to conventional chemotherapy. The trial is to evaluate if the combination of Pexa-Vec oncolytic virus (PV) with immune checkpoint inhibition enhance antitumor immunity. Methods: Patients with microsatellite-stable and MSI-H mCRC refractory to PD-1 monotherapy were enrolled. Patients received either Arm A treated with PV + Durvalumab or Arm B with PV + Durvalumab and Tremelimumab. Each arm had two dose levels (DL) of PV, 3 x 108 pfu in DL1 and at 109 pfu in DL2, every 2 weeks for total 4 doses. The first dose of PV was administered on Day -12, followed by three more dose administration on Days 2, 16 and 30 in combination with the immune checkpoint inhibition. The primary endpoint is response rate, safety, tolerability and feasibility of these combination therapy in refractory metastatic CRC. Results: Here we report the safety data of Arm A.A total of 9 patients was enrolled so far. The longest follow-up time is 8 months. Four patients received DL1 PV and subsequent five patients received DL2 PV. No DLT was observed at the time of this abstract. No grade 4-5 adverse event (AE) were observed. All patients experienced lymphopenia. All patients with DL2 developed fever, hypotension and papulopastular rashes and were successfully managed with antipyretics, fluid support and skin protection, respectively. The most frequent treatment-related AEs were lymphopenia (8 [100%], fever (7 [87.5%]), chills (6 [75.0%]), hypotension (5 [62.5%]), papulopastular rashes (5 [62.5%], flu-like symptoms (2 [12.5%]), Nausea/vomiting (2 [12.5%]). Conclusions: Pexa-Vec in combination with Durvalumab showed a favorable safety profile. Clinical trial information: NCT03206073.