Ultrasound-Driven Reductive Deoxygenation of N-Oxides for Spatiotemporally Controlled Prodrug Activation

Spatiotemporally controlled, tumor-selective activation of N-oxide prodrugs within tumors remains a longstanding challenge in cancer therapy. Herein, we report a broadly applicable strategy driven by clinical ultrasound for the reductive deoxygenation of N-oxides, enabling externally controlled prodrug activation via a riboflavin tetrabutyrate (TBR)/NADPH redox-relay system. Under mild, clinically translatable ultrasound (1 MHz, 2.0 W/cm2, 50% duty cycle), this platform promotes efficient N-O bond cleavage through single-electron transfer and hydrogen-atom transfer. Density functional theory calculations support that all ground-state steps are thermodynamically favorable with negative Gibbs free-energy changes (ΔG), supporting the feasibility of sonochemical N-oxide reduction. This method exhibits broad substrate generality toward diverse N-oxide compounds, including the clinically investigated hypoxia-activated prodrug banoxantrone (AQ4N), quinoline N-oxide, 8-hydroxyquinoline N-oxide, clozapine N-oxide, and olanzapine N-oxide. In hypoxic tumor cells, the ultrasound/TBR system enhances intracellular AQ4 formation significantly and reduces the IC50 of AQ4N from 35.0 to 2.8 mg/L. In vivo, the combination of AQ4N, TBR, and ultrasound increased intratumoral AQ4 formation by approximately 24.1-fold and achieved a tumor inhibition rate of 113.7%, outperforming AQ4N monotherapy (82.3%). This sonochemically triggered activation platform bypasses the heterogeneity of tumor-microenvironment stimuli and offers a versatile, externally controlled framework for precision prodrug chemotherapy with deep tissue penetration and clinical translatability.