Abstract 6639: Preclinical characterization and clinical biomarker studies with RP3, a novel oncolytic immunotherapy expressing a fusogenic protein, a human anti-CTLA-4 antibody, hCD40L and h4-1BBL (2023)

Abstract Introduction: RP3, a tumor-directed oncolytic immunotherapy (TDOI), is a selectively replication-competent HSV-1 that contains the codon-optimized sequence for the fusogenic GALV-GP-R- protein, an anti-CTLA-4 antibody-like molecule, CD40L, and 4-1BBL. The expression of GALV-GP-R- potentiates immunogenic tumor cell death through the induction of cell-to-cell fusion. CD40L enhances dendritic cell and macrophage activation and 4-1BBL promotes CD8+ T-cell activation and co-stimulation. The anti-CTLA-4 antibody-like molecule blocks the interaction of CTLA-4 with its ligands B7.1 and B7.2, further enhancing T cell activation. RP3 is currently being assessed in a Phase 1 clinical trial with or without nivolumab in patients with advanced solid tumors (NCT04735978). Methods: CD1 nude mice with A375 melanoma xenograft tumors (Group 1) were treated with a single intratumoral injection of 1 × 107 PFU/mL of RP3. Tumors and blood samples were collected 3 days later, and immunohistochemistry (IHC) was performed to detect CD40L, 4-1BBL and HSV-1. For anti-CTLA-4 detection, two doses of 107 PFU/mL of RP3 were injected intratumorally into C57BL/6 mice bearing 4434 murine BRAFV600E mutant melanoma tumors (Group 2). Tumors and blood samples were collected at 3, 6, 9, 12 and 15-days post-dosing and anti-CTLA-4 ELISA was performed. For clinical biomarker studies, tumor biopsies were collected pre- and post-RP3 treatment. CD8 and PD-L1 were characterized by IHC, and gene expression analysis (GEA) was performed. TCR analysis was also performed using PBMCs. Results: IHC analysis from Group 1 confirmed abundant and widespread expression of CD40-L and 4-1BBL, which were detected alongside with HSV-1 antigen. In the blood samples collected from Group 2, robust levels of anti-CTLA-4 antibody were detected, which stayed in detectable range up to 15 days after the 2nd RP3 injection. IHC analysis of patient tumor samples demonstrated a marked increase in CD8 T cells and PD-L1 expression. GEA of patient tumor biopsies demonstrated increased levels of genes associated with innate and adaptive immune activation and genes previously reported to be associated with responsiveness to anti-PD1 therapy. TCR sequencing of PBMCs and multiplex IHC analysis are ongoing and updated results will be presented. Conclusion: Preclinical experiments from mice models confirmed the robust expression of CD40-L, 4-1BBL and anti-CTLA-4 in RP3-injected tumors. Biomarker studies from patients demonstrate increases in CD8-T cell infiltration and PD-L1 expression post RP3 +/- nivolumab treatment, including increases in the expression of genes associated with response to anti-PD-1 therapy. These data indicate the potential for broad utility of RP3 in a range of tumor types, including in tumors that are not sensitive to PD-1 blockade. Citation Format: Praveen K. Bommareddy, Kevin Harrington, Victoria Roulstone, Joan Kyula, Aaron Clack, Linta Kuncheria, Sylwia Jones, Robert Coffin. Preclinical characterization and clinical biomarker studies with RP3, a novel oncolytic immunotherapy expressing a fusogenic protein, a human anti-CTLA-4 antibody, hCD40L and h4-1BBL. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6639.