Background: CF33-hNIS is a novel chimeric DNA vaccinia virus engineered with the human sodium-iodide symporter (hNIS) gene. CF33-hNIS selectively replicates in tumor cells leading to cell lysis and the release of tumor- and virus-associated antigens stimulating antitumor immunity. Here, we report results of patients with GI malignancies enrolled on a first-in-human phase 1 ascending, multiple dose, safety and tolerability study of CF33-hNIS, administered intratumorally (IT) or intravenously (IV) in adult patients with metastatic or advanced solid tumors (MAST). Methods: The MAST study is evaluating the safety of CF33-hNIS administered IT or IV, alone or in combination with pembrolizumab in patients with advanced or metastatic solid tumors with ≥ 2 prior lines of therapy (NCT05346484). CF33-hNIS is administered in 21-day cycles on C1D1 and C1D8, then D1 of each cycle thereafter. Pembrolizumab begins C2D1 for the combination groups and is administered Q3W. The co-primary endpoints are safety and to identify the recommended phase 2 dose. Secondary endpoints include objective response rate according to RECIST v1.1 and iRECIST, and to assess levels of viral replication via Single-Photon Emission Computerized Tomography (SPECT) with technetium-99. Results: As of Aug 2023, we report early results from seven patients with GI malignancies who received CF33-hNIS alone, including: colorectal cancer (3), bile duct (2), pancreatic (1) and hepatocellular carcinoma (1). The disease control rate is 86%. Notably one patient with cholangiocarcinoma, treated IT with 3.0x10⁷ PFU, presented with pseudoprogression with a 49% increase in tumor burden after two cycles of therapy. By the 4^th cycle they achieved a complete response (CR) with no known recurrence. A second patient with bile duct cancer, who previously progressed on checkpoint blockade therapy, achieved stable disease for >5 months upon receiving IV-administered CF33-hNIS (3.0x10⁷ PFU). Treatment-related adverse events for the CR patient included grade 1 fever and injection site reaction. All other patients experienced no treatment-related adverse events. SPECT analysis of the CR patient reveals intense levels of viral replication within the injected lesion and a concomitant increase in Th1 cytokines in cycle three that coordinated with tumor reduction. In-depth immunological changes in peripheral blood and on-treatment tumor biopsies will be presented. Conclusions: CF33-hNIS can replicate within cholangiocarcinoma lesions, as shown by SPECT imaging, and drive immunological changes known to promote antitumor immunity. CF33-hNIS monotherapy may be an effective and safe treatment option for GI malignancies, including cholangiocarcinoma, a rare disease with an unmet medical need. Treatment with CF33-hNIS monotherapy was well tolerated and the study has advanced to examine combination therapy with pembrolizumab. Clinical trial information: NCT05346484.