6-thio-dG enhances standard-of-care radiation therapy by reprogramming the tumor microenvironment in glioblastoma multiforme

Background/Objectives - Glioblastoma multiforme (GBM) remains highly lethal, with a five-year survival rate of only 6.9%. Standard-of-care treatments such as ionizing radiation (IR) and temozolomide frequently fail to produce durable responses due to poor drug penetration across the blood-brain barrier (BBB) and the immunosuppressive tumor microenvironment (TME). 6-thio-2′-deoxyguanosine (6-thio-dG) is a telomerase-mediated telomere-targeting guanine analog that induces telomeric DNA damage selectively in telomerase-positive tumor cells. This process activates the cGAS-STING pathway, triggers innate and adaptive immune responses and has previously resensitized resistant tumors to immunotherapy in a Phase 2 NSCLC clinical trial. We hypothesized that 6-thio-dG penetrates the BBB and enhances the therapeutic efficacy of IR by reprogramming the GBM TME. Methods - GBM models were treated with 6-thio-dG alone, IR alone, or sequential 6-thio-dG followed by IR. Immune signaling, microglia/macrophage phenotypes, and tumor responses were assessed via molecular, histological, and functional analyses. Results - Sequential 6-thio-dG + IR treatment significantly increased type I interferon (IFN-I) activation and shifted microglia/macrophages toward a pro-inflammatory M1 phenotype. This TME reprogramming resulted in a statistically significant anti-tumor effect relative to monotherapy. Conclusions - 6-thio-dG enhances IR efficacy in GBM by inducing telomere-driven immune activation and promoting an anti-tumor TME. These findings support 6-thio-dG as a promising adjuvant to standard-of-care and justify further investigation of telomere-targeted combination strategies.