G-quadruplex ligand RHPS4 compromises cellular radioresistance by inhibiting the mitochondrial adaptive response induced by ionizing irradiation.

A major challenge in radiotherapy is to enhance tumour cell sensitivity to radiation while minimizing damage to healthy tissues. Enhancing the effectiveness of radiotherapy can be achieved by combining irradiation with small radiosensitizing molecules, which promote cancer cell death and allow for reduced radiation doses, thereby limiting harm to surrounding healthy tissues. Since mitochondria play a key role in tumour cell proliferation, they represent a promising therapeutic target for cancer treatment. In this study, we characterized the impact of irradiation on mitochondrial function in radioresistant cancer cells. Our findings revealed several adaptive responses that may contribute to radioresistance, including increased mitochondrial DNA (mtDNA) content, mitochondrial mass, enhanced activity, and hyperfusion of the mitochondrial network. Notably, the use of mitochondrial-targeted G-quadruplex (G4) ligands, which block mtDNA replication and transcription, disrupted these responses, reducing cancer cell survival in a mtDNA-dependent manner. These results demonstrate that mitochondrial adaptations contribute to radioresistance and highlight mitochondria as a novel target for the radiosensitizing effects of G4 ligands.