Abstract 6625: Photodynamic activation of CX-5461 enhances its anti-tumor efficacy.

Abstract Introduction: CX-5461 (Pidnarulex) is the first selective inhibitor of RNA Polymerase I (Pol I) and a potent stabilizer of DNA G-quadruplexes (G4s), with additional activity as a TOP2 poison. Beyond its transcriptional inhibition, CX-5461 induces cell-cycle arrest, apoptosis, and DNA double-strand breaks, exhibiting broad anti-proliferative effects across diverse cancer models. Clinically, CX-5461 shows promising efficacy in advanced hematologic and solid tumors, such as breast, pancreatic, and ovarian cancers, with phototoxicity as the main adverse effect, manageable through sun protection. G4 structures are guanine-rich and inherently prone to oxidative modification, which has recently positioned them as promising targets for light-activated therapeutics. CX-5461 is known to be light-sensitive and capable of inducing phototoxic reactions; however, in this study we leverage this property as a functional advantage rather than a limitation. We show that CX-5461 acts as an efficient photosensitizer: upon UV irradiation, it generates reactive oxygen species (ROS), leading to enhanced oxidative DNA damage and an approximately ten-fold increase in cytotoxicity relative to its dark state, at concentrations that are otherwise minimally toxic. These findings reveal a photo-responsive mechanism of action for CX-5461, in which G4 binding and light-induced ROS generation converge to potentiate its anti-tumor activity. Materials and Methods: CT26 and B16F10 cells were treated with a serial dilution of CX-5461, with or without UV exposure. DNA damage, ROS levels, and cell viability were analyzed. In the CT26 syngeneic model, CX-5461 was administered at 25 mg/kg, iv, weekly, alongside UV exposure (10 J/cm2) for the first three days. B16F10 models received CX-5461 at 25 or 50 mg/kg, iv, weekly, with UV exposure (5 J/cm2) on Day 1. Tumor size and survival were measured. Results: CX-5461 exhibits strong absorbance in the UV region and generates both type I and type II reactive oxygen species (ROS), including superoxide anion, hydroxyl radicals, and singlet oxygen, upon UV irradiation. This exposure increased oxidative damage to 8-oxoG, raised ROS levels, and enhanced cytotoxicity in CT26 and B16F10 cells. In vivo studies confirmed that the combination of CX-5461 and UV light significantly inhibited tumor growth in the B16F10 model, with a tumor growth inhibition (TGI) of approximately 100% on Day 12, and in the CT26 model, with a TGI of around 74% on Day 14. Conclusion: CX-5461 is a photosensitizer that produces both type I and type II ROS, which increases its cytotoxic effects in vitro when exposed to UV light. The efficacy of CX-5461 in combination with UV treatment has been demonstrated in syngeneic models like CT26 and B16F10. These findings suggest that the phototoxic properties of CX-5461 could represent a novel therapeutic strategy for treating of cancers that are amenable to UV light exposure. Citation Format: Jakub Trojnar, Marta Dudek, Kai-Wei Hsueh, Ping-Yen Huang, Marco Deiana. Photodynamic activation of CX-5461 enhances its anti-tumor efficacy abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 6625.