In Vivo Positron Emission Tomography Imaging of Protease Activity by Generation of a Hydrophobic Product from a Noninhibitory Protease Substrate

Purpose: To develop an imaging technology for protease activities in patients that could help in prognosis prediction and in design of personalized, protease-based inhibitors and prodrugs for targeted therapy. Experimental Design: Polyethylene glycol (PEG) was covalently attached to the N-terminus of a hydrophilic peptide substrate (GPLGVR) for matrix metalloproteinase (MMP) to increase hydrophilicity. PEG-peptide was then linked to a hydrophobic tetramethylrhodamine (TMR) domain and labeled with (18)F to form a PEG-peptide-(18)F-TMR probe. Specific cleavage of the probe by MMP2 was tested in vitro by matrix-assisted laser desorption/ionization-time-of-flight (MALDI-TOF). In vivo imaging of MMP2-expressing tumors was evaluated by micro-PET. Results: The hydrophobic TMR fragment (948 Da) was specifically generated by MMP2 enzymes and MMP-expressing HT1080 cells but not control MCF-7 cells. MMP-expressing HT1080 cells and tumors selectively accumulated the hydrolyzed, hydrophobic TMR fragment at sites of protease activity. Importantly, we found that (18)F-labeled probe ((18)F-TMR) preferentially localized in HT1080 tumors but not control MCF-7 tumors as shown by micro-PET. Uptake of the probe in HT1080 tumors was 18.4 +/- 1.9-fold greater than in the MCF-7 tumors 30 minutes after injection. These results suggest that the PEG-peptide- (18)F-TMR probe displays high selectivity for imaging MMP activity. Conclusions: This strategy successfully images MMP expression in vivo and may be extended to other proteases to predict patient prognosis and to design personalized, protease-based inhibitors and prodrug-targeted therapies. Clin Cancer Res; 18(1); 238-47. (C) 2011 AACR.