Synthesis and Preclinical Evaluation of the Fibrin-Binding Cyclic Peptide (18)F-iCREKA: Comparison with Its Contrasted Linear Peptide

PURPOSE: Cys-Arg-Glu-Lys-Ala (CREKA) is a pentapeptide which can target fibrin-fibronectin complexes. Our previous study has built a probe called iCREKA which was based on CREKA and has proved the feasibility and specificity of iCREKA by the fluorescence experiment. The purpose of this study is to achieve the (18)F-labeled iCREKA and make preclinical evaluation of the (18)F-iCREKA with comparison of its contrasted linear peptide (LP). METHODS: CREKA, LP, and iCREKA were labeled by the Al(18)F labeling method, respectively. These (18)F-labeled peptides were evaluated by the radiochemistry, binding affinity, in vitro stability, in vivo stability, micro-PET imaging, and biodistribution tests. RESULTS: (18)F-NOTA-iCREKA was stable both in vitro and in vivo. However, (18)F-NOTA-CREKA and (18)F-NOTA-LP were both unstable. The FITC or (18)F-labeled iCREKA could be abundantly discovered only in matrix metalloproteinases- (MMPs-) 2/9 highly expressed U87MG cells, while the FITC or (18)F-labeled LP could also be abundantly discovered in MMP-2/9 lowly expressed Caov3 cells. Biodistribution and micropositron emission tomography (PET) imaging revealed that the U87MG xenografts showed a higher uptake of (18)F-NOTA-iCREKA than (18)F-NOTA-LP while the Caov3 xenografts showed very low uptake of both (18)F-NOTA-iCREKA and (18)F-NOTA-LP. The tumor-to-muscle (T/M) ratio of (18)F-NOTA-iCREKA (9.93 ± 0.42) was obviously higher than (18)F-NOTA-LP (2.69 ± 0.35) in U87MG xenografts. CONCLUSIONS: The novel CREKA-based probe (18)F-NOTA-iCREKA could get a high uptake in U87MG cells and high T/M ratio in U87MG mice. It was more stable and specific than the (18)F-NOTA-LP.