Background: Definitive chemoradiation (CRT) is a standard-of-care for patients (Pts) with medically inoperable esophageal cancer (EC). The NRG/RTOG 0436 study of cisplatin/paclitaxel and RT (+/- cetuximab) as definitive therapy showed that about half of Pts have locally persistent disease. OBP-301 is a conditionally restricted, replication-competent adenovirus derived from human adenovirus type 5 that adds a human Telomerase Reverse Transcriptase (hTERT) gene promoter, replicating only in tumor cells to cause lysis. It may cause immunogenic cell death, enhance RT, and improve local control. OBP-301 was previously studied with RT in Japanese esophageal cancer Pts and shown to be safe and has promising activity.¹Methods: In NRG-GI007, a phase 1 study (NCT04391049), OBP-301 is added to weekly carboplatin/paclitaxel and RT (50.4 Gy/28 fractions) for Pts with medically inoperable EC. Pts receive 1-2mL of intratumoral OBP-301 1×10¹²virus particles/ml via endoscopy 3 days prior to and then at days 12 and 26 of CRT. The primary endpoint is dose-limiting toxicity (DLT), defined as adverse events (AEs, by CTCAE v5) definitely or probably attributed to OBP-301 that meet either of the following: (1) leading to a >14-day cumulative delay in CRT or (2) any grade ≥ 3 AE EXCEPT: grade 3 nausea/vomiting, grade 3 esophagitis or dehydration, first occurrence of grade 3/4 neutropenia, or grade 3/4 lymphopenia. Initially, 6 evaluable Pts (eligible and started protocol treatment) are enrolled. If protocol defined DLT occurs in ≤1 of 6 Pts, the dose will be deemed safe and an expansion cohort of 9 more will be enrolled to further assess safety and obtain a preliminary assessment of clinical complete response. If ≥2 Pts have a DLT then one de-escalated OBP-301 dose will be assessed. Results: From June 2020 to April 2023, 6 evaluable Pts were enrolled. Median age was 73.5 years, all male. Five Pts had adenocarcinoma and 1 had squamous cell carcinoma; 3 Pts had node-positive disease. All Pts received all planned OBP-301 injections, along with 50.4 Gy RT. Four out of 6 Pts received all 5 weekly doses of chemo for > 85% of planned total dose, and 2 got 4 weekly doses for > 70% of planned total dose. No Pt experienced ³7 day treatment delay. The following treatment-related (attributed as definitely, probably, or possibly related to CRT and/or OBP-301) grade 3 AEs were reported across 5 Pts: decreased neutrophil count (3 Pts), decreased lymphocyte count (2 Pts), decreased WBC (2 Pts), and fever and fatigue (1 Pt each). There were 4 Pts with AEs reported as definitely or probably related to OBP-301, all grade ≤2. No DLT occurred. Conclusions: No DLTs in the 6 evaluable Pts were observed and it is concluded that the initial OBP-301 dose level is safe. NRG-GI007 was reopened on August 7, 2023 to the expansion cohort. Full toxicity and treatment data will be presented. 1. Eur J Cancer 2021;153:98. Clinical trial information: NCT04391049.