RedTail: A next-generation systemic platform for tumor-localized expression of BiTEs and immune activation.

e14562 Background: RedTail is a next-generation gene therapy platform engineered for systemic delivery and highly selective tumor-specific payload expression. The platform uses a tumor-specific, extracellular enveloped vaccinia virus (EEV) expressing a chimeric form of CD55, providing resistance to complement and neutralizing antibodies and enabling systemic administration. The viral genome can be engineered to express immune activating payloads such as an IL-15 superagonist (IL-15 SA) and tumor-targeting bispecific T-cell engagers (BiTEs) for localized production within the tumor microenvironment (TME). Tumor selective viral amplification induces cancer cell lysis, infiltration of T cells, and the delivery of high concentrations of immunomodulatory payloads such as IL-15 SA directly into tumors, altering the composition of the TME and potentially overcoming long-standing challenges with T-cell engagement in solid tumors. Methods: RedTail EEVs were produced using selected host cell lines that provide protection against serum mediated inactivation. RedTail vectors engineered to express the IL-15 SA transgene and BiTEs were evaluated in vitro and in vivo to assess tumor localized expression of the payload and therapeutic efficacy. Results: Systemic administration of RedTail expressing IL15 SA resulted in selective tumor-localized viral replication and robust, IL-15 SA production—at levels comparable to locally delivered clinically validated IL-15 SA–Fc therapies—while remaining- undetectable in normal tissues. This effect was accompanied by marked increases in NK and CD8⁺ T cell infiltration and broad remodeling of immune -cell composition, observed only in tumor--bearing hosts, consistent with tumor-localized IL-15–mediated immune activation. Building on this platform, RedTail vectors were further engineered to coexpress tumor-localized BiTEs alongside IL-15 SA. Treatment of tumor cells in vitro resulted in efficient viral amplification, tumor cell lysis, and simultaneously high tumor-production of both IL-15 SA and BiTE payloads. In vivo , systemic administration of dual payload RedTail led to tumor--localized coexpression and generated robust therapeutic responses, demonstrating the platform’s capacity to deliver potent combinations of immune engaging agents to the TME, while minimizing serum payload exposure. Conclusions: The RedTail platform enables efficient systemic delivery and selective tumor targeting, transforming tumors into in situ bioreactors for localized therapeutic production. It drives tumor restricted amplification, immune-mediated tumor cell killing, and robust antitumor immune activation. Together, these features position RedTail as a next-generation platform capable of enhancing T cell recruitment, promoting IL-15–mediated expansion, and BiTEs-meditated redirection T cells for potent, tumor selective cytotoxicity.