Design of a Bioreductively-Activated Fluorescent pH Probe for Tumor Hypoxia Imaging

We have designed and evaluated UTX-12 as a novel fluorescent pH probe for tumor hypoxia
imaging. UTX-12 consists of a p-nitro benzyl moiety, which is a latent hypoxia-selective leaving
group activated by nitro reduction, directly linked to SNARF. Although UTX-12 itself is colorless and
non-fluorescent in aqueous solution, nitro reduction triggers the release of SNARF which has
well-characterized long wavelength absorption and fluorescence that is sensitive to pH. The
resultant SNARF, released intracellularly by enzymatic reduction of UTX-12, allows measurement of
pH by pH-dependent dual emission shifts. UTX-12 showed clear differences in fluorescence
behavior between hypoxic and aerobic conditions in liver microsomes and inside V79 cells. These
data are confirmation that UTX-12 is biologically reduced inside tumor cells and the released SNARF
should monitor intracellular pH of tumor cells selectively with reduced background signal.