G-quadruplex DNA reprograms the photochemistry of CX-5461 toward radical-driven anticancer activity.

CX-5461 (pidnarulex) is the most clinically advanced small-molecule ligand associated with G-quadruplex (G4) targeting and shows activity in DNA repair-deficient tumors, yet its clinical use is associated with dose-limiting phototoxicity. Here, we repurpose this intrinsic photoreactivity to investigate CX-5461 as a G4-associated, light-activated antitumor scaffold. In solution, CX-5461 produces both Type I and Type II reactive oxygen species (ROS), whereas complexation with G4 DNA markedly decreases detectable singlet oxygen (1O2) formation and favors radical-associated Type I oxidation. This G4-modulated photochemical shift drives oxidative remodeling and destabilization of G4-containing DNA. In cancer cells, photoactivation enhances cytotoxicity by up to two orders of magnitude relative to the dark state and is accompanied by elevated intracellular ROS, increased 8-oxoG formation, γ-H2AX accumulation, and changes in BG4-detected nuclear G4 structures. In vivo, light-activated CX-5461 suppresses tumor growth and extends survival in immunocompetent syngeneic models, eliciting hallmarks consistent with immunogenic genome stress. Collectively, these findings reposition CX-5461 as a clinically relevant scaffold for G4-associated photogenomic cancer therapy, while supporting a model in which broader oxidative DNA damage may also contribute to the biological response.