Significance: Oxygen tension plays a critical role in the response to radiation therapy (RT). Here we show that hypoxic tumors can be sensitized to RT by targeting mitochondrial respiration. We identified the 150-year-old FDA-approved drug papaverine as a mitochondrial complex I inhibitor. A single dose of the drug prior to RT alleviates hypoxia in model tumors and strikingly enhances the response to RT. Well-oxygenated normal tissues are not radiosensitized. Removal of papaverine’s phosphodiesterase 10A inhibitory activity by structural modification has identified potentially safer generation of complex I-inhibiting radiosensitizers.

We report a methodology which enables the preparation of dendrimeric contrast agents sensitive to Ca²⁺ when starting from the monomeric analogue. The Ca-triggered longitudinal relaxivity response of these agents is not compromised by undertaking synthetic transformations, despite structural changes. The in vivo MRI studies in the rat cerebral cortex indicate that diffusion properties of dendrimeric contrast agents have great advantages as compared to their monomeric equivalents.