Ferric Ion-Release Nanoprobes for Hypoxia Magnetic Resonance Imaging and Radiotherapy Stratification in Solid Tumors

Hypoxia, a hallmark of the tumor microenvironment, promotes cancer aggressiveness and treatment resistance by activating HIF-mediated signaling pathways while compromising the efficacy of radiotherapy, chemotherapy, and targeted therapy. Current hypoxia imaging techniques suffer from invasiveness, limited specificity, or insufficient spatial resolution. To address these challenges, we developed hypoxia-responsive magnetic resonance imaging (MRI) nanoprobes (FAP NPs) composed of 4,4'-azobisbenzoic acid (AZB) and Fe³⁺. These nanoprobes exploit the overexpression of azoreductase in hypoxic tumors, triggering the selective reduction of the azo bond and the subsequent release of paramagnetic Fe³⁺ ions. This structural transformation amplifies proton relaxivity by 12-fold (r₁: 0.1-1.2 mM^-1 s^-1), significantly enhancing T₁-weighted MRI contrast for precise hypoxia mapping. <i>In vivo</i> studies showed that FAP NPs detect hypoxia with high sensitivity in pancreatic, colorectal, and breast tumors of varying sizes (5-14 mm) and allow quantitative monitoring of changes in hypoxia levels following pancreatic tumor chemotherapy. Furthermore, FAP NPs stratified tumors into normoxic and hypoxic subgroups, accurately predicting radiotherapy efficacy. By integrating enzyme-responsive activation with Fe³⁺-mediated signal amplification, FAP NPs provide a noninvasive platform for real-time hypoxia assessment, guiding personalized therapeutic strategies and optimizing outcomes in solid tumors.