KS01.6.A INTRATUMORAL IL-12/ANTI-PD-1 ONCOLYTIC HSV1 (MVR-C5252) IS SAFE AND IMMUNOACTIVE IN RECURRENT HIGH-GRADE GLIOMA: PRELIMINARY PHASE 1 RESULTS FROM A MULTICENTER MULTISTAGE CLINICAL TRIAL

Abstract BACKGROUND Oncolytic viruses represent a promising immunotherapeutic strategy by combining direct tumor lysis with local immune activation. MVR-C5252 is a genetically engineered HSV1 encoding interleukin-12 (IL-12) and an anti-PD-1 antibody fragment designed to promote both innate and adaptive anti-tumor responses. This trial investigates the safety, tolerability, and early biological activity of intratumoral MVR-C5252 in patients with high-grade gliomas, delivered via convection-enhanced delivery (CED) to bypass the blood-brain barrier. MATERIALS AND METHODS In Stage 1, two three-patient cohorts with recurrent high-grade glioma (HGG) were treated with a single intratumoral infusion of MVR-C5252 at escalating doses (5×10⁶ or 1×10⁸ PFU) via externalized CED catheters. Safety and feasibility were assessed over a 28-day dose-limiting toxicity (DLT) observation window. CSF was collected at baseline, 1 hour post-infusion, and day 28 for biomarker analyses. Cytokines were quantified using LegendPlex assays. Single-cell RNA sequencing (scRNA-seq) of cryopreserved CSF cells was performed to characterize dynamic changes in immune cell composition and gene expression signatures. Stage 2 will evaluate MVR-C5252 delivery via an implantable Medtronic pump and catheter, administering two doses (one cycle) on days 1 and 28. Stage 3 will escalate to five cycles (ten doses) to determine the recommended Phase 2 dose. RESULTS Stage 1 enrolment was completed 5/2025. Among the 5 patients who completed full analysis to date, no grade 3-5 adverse events or viral shedding were observed, supporting the procedural safety. Grade 1-2 adverse events included fatigue, flu-like symptoms, and cognitive disturbance. CSF cytokine profiling demonstrated increases in IL-12, IFN-γ, and IL-10 at 1 hour post-infusion, with sustained levels at day 28, reflecting persistent immune activation within the CNS. scRNA-seq revealed remodeling of the CSF immune landscape: activated CD4⁺ T cells expressing IL12RB1/2 and IFN-γ expanded by day 28, consistent with Th1 polarization. CD8⁺ T cells also increased, expressing cytotoxic effector molecules (GZMB, PRF1), while myeloid-derived suppressor populations declined, suggesting a shift toward an effector-dominant phenotype. Patients with higher early IL-12 induction exhibited greater IFN-γ⁺ CD4⁺ T cell expansion at day 28, indicating a link between acute cytokine responses and immune reprogramming. CONCLUSION Preliminary data show that MVR-C5252 is safe, feasible, and active in recurrent HGG. Early CSF-based analyses reveal induction of Th1 and cytotoxic CD8⁺ T cell responses and remodeling of the CNS immune milieu. Stages 2-3 will define the optimal regimen for Phase 2 evaluation. Enrollment in Stage 2 is expected to be complete by time of the presentation, with updated clinical outcomes, imaging, and longitudinal CSF immune profiling to be reported.