Fenofibrate enhances systemic antitumor efficacy of high- and low-dose radiotherapy plus PD-1 blockade through CD8+ T-cell metabolic reprogramming and LDRT tumor conditioning.

Radiotherapy (RT) combined with PD-1/PD-L1 blockade can induce systemic antitumor responses, but robust control of distant lesions remains uncommon. We identified a quadruple regimen combining immunogenic hypofractionated RT (hRT; 8 Gy × 3) to the primary tumor, low-dose RT (LDRT; 3 Gy × 3) to the secondary tumor, anti-PD-1 (αPD-1), and fenofibrate (FFB), a clinically approved PPARα agonist, that enhanced antitumor efficacy in two bilateral tumor models. Only this regimen substantially improved secondary-tumor control, eradicating LDRT-treated secondary tumors in all mice in the immunologically cold C51 model and inducing prolonged control in the aggressive, αPD-1-poorly responsive B16-CD133 model. This effect required CD8+ T cells. Tumor-specific CD8+ TILs showed enhanced effector function, reduced exhaustion, increased mitochondrial fitness, and a shift toward mitochondrial fatty acid oxidation (FAO), with scRNA-seq supporting PPARα/FAO-associated remodeling, including upregulation of Cpt1a, encoding the FAO enzyme CPT1A, in effector-associated, proliferative, and clonally expanded subsets. Both antitumor efficacy and T-cell metabolic remodeling were reversed by the CPT1 inhibitor etomoxir. By contrast, LDRT impaired tumor-cell mitochondrial fitness, while FFB shifted tumor cells from FAO toward glycolytic reliance and induced only slight apoptosis. Consistently, in vivo data argued against direct LDRT-mediated tumor-cell debulking. The quadruple regimen increased vascular MECA-79 expression, while LDRT enhanced endothelial ICAM-1/VCAM-1 and tumor-cell CXCL9/CXCL10 expression in secondary tumors, consistent with local conditioning favoring T-cell recruitment. These findings highlight FFB as a clinically tractable metabolic modulator and link FFB-enhanced hRT/LDRT/αPD-1-induced systemic antitumor efficacy to metabolic reprogramming of tumor-specific CD8+ TILs and conditioning of the LDRT-treated tumor microenvironment.