¹⁸F-BODIPY labeled pegylated peptide probe for PET imaging of MT1-MMP activity in cancers

Objectives Membrane type-1 matrix metalloproteinase (MT1-MMP) plays pivotal roles in tumor metastasis. Thus, we aimed to develop a novel ¹⁸F labeled peptide probe for in vivo imaging of MT1-MMP activity. This probe, ¹⁸F-MBP, consists of ¹⁸F-BODIPY, a MT1-MMP substrate sequence, and PEG. Following MT1-MMP-dependent cleavage of the probe to remove the PEG moiety, the radioactivity would be expected to accumulate in tumor cells due to the high lipophilicity of BODIPY. Here, we evaluated five substrate peptides and four PEGs by in vitro FRET assays and biodistribution experiments to obtain ¹⁸F-MBP. Finally, we performed a pre-clinical evaluation of the usefulness of ¹⁸F-MBP for in vivo PET imaging of MT1-MMP.

Methods FRET probes consisting of BODIPY 650/665-SE, five substrate peptides, and a fluorescence quencher were synthesized. The resulting probes were incubated with hMT1-MMP protein followed by fluorescence measurement. Cell microscopy was performed using MT1-MMP-positive HT1080 cells with the MMP inhibitor GM6001. ¹⁹F-MBPs with four different PEG lengths were evaluated by in vivo fluorescence imaging in tumor bearing mice. Synthesized ¹⁸F-BODIPY was reacted with a pegylated peptide to obtain ¹⁸F-MBP and PET/CT scans were performed on tumor bearing mice.

Results Fluorescence was increased by hMT1-MMP treatment in four out of five FRET probes tested. Fluorescence of the four probes in HT1080 cells was inhibited by GM6001, indicating MMP-dependent accumulation. The substrate sequence adopted for ^18/19F-MBPs was determined from the one FRET probe that showed the highest imaging contrast in vivo. Meanwhile, among the ¹⁹F-MBPs tested, ¹⁹F-MBP with 2 kDa PEG showed the highest fluorescence in HT1080 tumors. Compared with MT1-MMP-negative A549 tumors, ¹⁸F-MBP synthesized with a 78% radiochemical yield from ¹⁸F-BODIPY and more than 99% radiochemical purity could clearly depict HT1080 tumors by PET 2 h after injection.

Conclusions We successfully developed ¹⁸F-MBP, which is predicted to have useful applications for in vivo evaluation of MT1-MMP activity in tumors.