Background: Ad-RTS-hIL-12 (Ad) is a novel gene therapy candidate conditionally expressing IL-12 under the control of veledimex (V) acting via the proprietary RheoSwitch Therapeutic System (RTS) gene switch with a therapeutic window. Intratumoral Ad + oral V monotherapy (Phase 1 study, NCT02026271) resulted in a new sustained intra-tumor influx of activated cytotoxic T cells, consistent with an immune-mediated anti-tumor effect improving median overall survival (mOS) of subjects with recurrent glioblastoma (rGBM). This correlated with an increased circulating CD8⁺/FoxP3⁺ T cell ratio (“cytoindex”), an emerging biomarker for mOS. PD-1 expression on infiltrating T cells at biopsy after Ad+V, supports combining controlled IL-12 with a PD-1 inhibitor to further augment T-cell-mediated anti-tumor effects. The rationale is also supported by increased OS (100% combo vs 63% for Ad+V vs 40% for anti-PD-1) in mice bearing GL-261 glioma. Methods: An ongoing open label, dose-escalation Phase 1 trial (NCT03636477) is evaluating safety and tolerability of local, controlled IL-12 with nivolumab (nivo) in adult subjects with rGBM. Ad was administered by single intratumoral injection (2 x 10¹¹ viral particles, Day 0) plus V (10-20 mg) PO QD x 15 with nivo (1-3mg/kg) IV on Days -7, 15, then Q2W. Results: Safety data revealed a similar profile as Ad +V monotherapy. Adverse reactions (ARs) during follow-on nivo dosing were consistent with anti-PD-1 reports. ARs were manageable and reversible with no synergistic toxicities. Nivo alone did not alter peripheral IL-12 levels (median baseline (before anti-PD-1) 0.9 pg/mL; Day 0 1 pg/mL) increasing to 5.5 pg/mL on Day 3. Nivo alone increased peripheral T cells (CD3⁺CD8⁺ median baseline 23%; Day 0 26%) and Ad+V elevated peripheral CD3⁺CD8⁺ to 31% at Day 14. Nivo alone decreased regulatory T cells (FoxP3 baseline 1.5% vs Day 0 0.8%). Ad+V decreased these to 0.3% (Day 14). Combination therapy improved the cytoindex (baseline 15; Day 0 29; Day 14 80). Conclusions: Controlled IL-12 production using Ad + V with nivo is a rational combination with initial data consistent with immune-mediated anti-tumor effects with a favorable safety profile, warranting continued investigation in rGBM. Clinical trial information: NCT03636477