RT Journal Article SR Electronic T1 In Vivo Imaging of Proteolytic Enzyme Activity Using a Novel Molecular Reporter JF Cancer Research JO Cancer Res FD American Association for Cancer Research SP 4953 OP 4958 VO 60 IS 17 A1 Tung, Ching-Hsuan A1 Mahmood, Umar A1 Bredow, Sebastian A1 Weissleder, Ralph YR 2000 UL http://cancerres.aacrjournals.org/content/60/17/4953.abstract AB The single biggest challenge facing in vivo imaging techniques is to develop biocompatible molecular beacons that are capable of specifically and accurately measuring in vivo targets at the protein, RNA, or DNA level. Our efforts have focused on developing activatable imaging probes to measure specific enzyme activities in vivo. Using cathepsin D as a model target protease, we synthesized a long-circulating, synthetic graft copolymer bearing near-infrared (NIR) fluorochromes positioned on cleavable substrate sequences. In its native state, the reporter probe was essentially nonfluorescent at 700 nm due to energy resonance transfer among the bound fluorochromes (quenching) but became brightly fluorescent when the latter were released by cathepsin D. NIR fluorescence signal activation was linear over at least 4 orders of magnitude and specific when compared with scrambled nonsense substrates. Using matched rodent tumor models implanted into nude mice expressing or lacking the targeted protease, it could be shown that the former generated sufficient NIR signal to be directly detectable and that the signal was significantly different compared with negative control tumors. The developed probes should find widespread applications for real-time in vivo imaging of a variety of clinically relevant proteases, for example, to detect endogenous protease activity in disease, to monitor the efficacy of protease inhibitors, or to image transgene expression.