One-step 18F-fluorination of smart positron emission tomography tracer for sensing furin activity in tumors

Abstract

IntroductionPeptide analogues have attracted considerable attention in the field of developing novel positron emission tomography (PET) imaging agents due to their unique properties. Nevertheless, the complicated radiolabeling process and fast metabolism usually pose challenges to the clinical applications of peptide-based molecular probes. Herein a novel PET tracer containing a specific peptide sequence Arg-Val-Arg-Arg (RVRR), Acetyl-Arg-Val-Arg-Arg-Cys(StBu)-Gly(AMB[18F]F3)-CBT ([18F]1), was designed and radiosynthesized using a simple and convenient one-step 18F-fluorination procedure. The smart tracer can be activated by the protease furin and then undergoes an intermolecular cyclization reaction in tumor cells, leading to improved PET imaging efficiency of tumor. MethodsThe radiosynthesis of the target tracer [18F]1 and the control tracer [18F]1-ctrl was performed under facile conditions in pyridazine-HCl buffer (pH~2.5) at 80 °C within 30 min. The enzyme-controlled condensation was studied for non-radioactive compound 1 in the human breast cancer cell lysates (MDA-MB-468). The cellular uptake of [18F]1 and [18F]1-ctrl was studied and compared by measuring the activity in MDA-MB-468 cells using a γ-counter after incubation with 37 kBq of [18F]1 or [18F]1-ctrl, respectively. In vivo behavior of [18F]1 was examined through PET imaging of MDA-MB-468 tumor-bearing mice and compared with that of [18F]1-ctrl as well as that of [18F]1 co-injected with non-radioactive compound 1. ResultsThe tracer [18F]1 was obtained with a high radiochemical yield (RCY) of 42.5 ± 1.47% and an excellent radiochemical purity (RCP > 99%). Under the activation of furin and GSH, the tracer suffered a condensation reaction to form dimers and then self-assembled into nanoparticles to produce enduring signal. The cellular uptake of [18F]1 and [18F]1-ctrl was determined to be 10.2 ± 0.37 and 1.19 ± 0.25%ID at 120 min, respectively. For in vivo PET imaging, [18F]1 exhibited the optimum tumor uptake of 2.39 ± 0.31%ID/g and the tumor-to-muscle uptake ratio of 2.93 ± 0.92 at 10 min post injection. Co-injection of [18F]1 and non-radioactive compound 1 produced a high tumor uptake ranging from 2.83 ± 0.23%ID/g to 3.40 ± 0.18%ID/g at 10 min and 60 min post injection, respectively. ConclusionsThe one-step labeling method of tracer [18F]1 showed advantage in simplifying the radiolabeling process with high RCY, which could enable a real kit process for the synthesis of 18F-radiopharmaceuticals and was significant for the large-scale production of tracers for clinical applications. PET imaging results suggested that the tracer [18F]1 had good tumor uptake and the co-injection strategy of [18F]1 with 1 could enhance the imaging signal in tumor.