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

Abstract

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 18F-labeled iCREKA and make preclinical evaluation of the 18F-iCREKA with comparison of its contrasted linear peptide (LP). Methods CREKA, LP, and iCREKA were labeled by the Al18F labeling method, respectively. These 18F-labeled peptides were evaluated by the radiochemistry, binding affinity, in vitro stability, in vivo stability, micro-PET imaging, and biodistribution tests. Results 18F-NOTA-iCREKA was stable both in vitro and in vivo. However, 18F-NOTA-CREKA and 18F-NOTA-LP were both unstable. The FITC or 18F-labeled iCREKA could be abundantly discovered only in matrix metalloproteinases- (MMPs-) 2/9 highly expressed U87MG cells, while the FITC or 18F-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 18F-NOTA-iCREKA than 18F-NOTA-LP while the Caov3 xenografts showed very low uptake of both 18F-NOTA-iCREKA and 18F-NOTA-LP. The tumor-to-muscle (T/M) ratio of 18F-NOTA-iCREKA (9.93 ± 0.42) was obviously higher than 18F-NOTA-LP (2.69 ± 0.35) in U87MG xenografts. Conclusions The novel CREKA-based probe 18F-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 18F-NOTA-LP.