Background: Tumor-Specific Immuno Gene Therapy (T-SIGn) vectors are next-generation transgene-armed variants of the adenoviral vector enadenotucirev that selectively replicate in epithelial tumor cells. T-SIGn vectors are blood-stable, allowing IV delivery to be coupled with local transgene expression in the tumor microenvironment (TME), thereby targeting all lesions while limiting systemic exposure. The T-SIGn vector NG-641 encodes four immunostimulatory transgenes: fibroblast activation protein-directed bi-specific T-cell activator antibody to target cancer-associated fibroblasts (CAFs), IFNα2 to promote innate and adaptive immune responses, and CXCL9/10 to induce T-cell infiltration. Through this novel multimodal combination of immunostimulatory effects NG-641 is designed to re-program the TME to allow functional anti-cancer immune responses. In the ongoing STAR study (NCT04053283), NG-641 has been successfully dose-escalated to 1 × 10¹² viral particles (vp) on Day 1 and 3 × 10¹² vp on Days 3 and 5, with promising preliminary safety/tolerability and pharmacodynamic results. Based on these encouraging preliminary data with NG-641 monotherapy, we designed a new study to assess NG-641 + nivolumab. Methods: NEBULA (NCT05043714) is an open-label, dose-escalating, phase 1a/b study of NG-641 + nivolumab in patients (pts) with advanced/metastatic epithelial tumors that have progressed after ≥1 line of systemic therapy and are incurable by local therapy. Pts are eligible for phase 1a if they have received prior PD-1/PD-L1 inhibition as part of any line of therapy. During phase 1a, up to 30 pts will receive escalating doses of IV NG-641 to a maximum dose of 1 × 10¹² viral particles (vp) on Day 1 and 1 × 10¹³ vp on Days 3 and 5 (1 cycle; Bayesian Optimal Interval design). Pts will receive a fixed-dose of nivolumab (480 mg IV) on Day 15 and then every 4 weeks thereafter for up to 8 cycles. In phase 1b, the recommended dose regimen will be further studied in patients with primary resistance to PD-1/PD-L1 inhibition; patients will be enrolled in up to 3 tumor-specific cohorts (Cohorts A-C; Simon 2-stage design). Co-primary objectives are to characterize the safety and tolerability of NG-641 + nivolumab and to identify a recommended dose. Preliminary efficacy and immunogenicity are secondary endpoints. Pharmacodynamic outcomes will also be assessed. Viral replication, transgene expression, immune/inflammatory responses and effects on CAFs by IHC and gene expression analysis will be analyzed using tumor tissue from serial biopsies (taken at baseline and Day 15 of cycles 1-3 [cycles 1-2 only in Phase 1b]). Serial blood samples will be analyzed to study cytokine production and changes in peripheral immune cell subsets. Enrollment to the first dose-escalation cohort is ongoing. Clinical trial information: NCT05043714.