Optimization and Evaluation of Al18F Labeling Using a NOTA—or RESCA1-Conjugated AE105 Peptide Antagonist of uPAR

Abstract

Fluorine-18 displays almost ideal decay properties for positron emission tomography (PET) and allows for large scale production. As such, simplified methods to radiolabel peptides with fluorine-18 are highly warranted. Chelation of aluminium fluoride-18 toward specific peptides represents one method to achieve this. With the current methods, chelation of aluminium fluoride-18 can be achieved using NOTA-conjugated peptides. However, the heating to 90–100◦C that is required for this chelation approach may be deleterious to the targeting moiety of the probe. Recently, a new chelator, RESCA1, was developed allowing Al18F chelation at room temperature. Here, we optimize the labeling procedure enabling high chelation efficacy of fluoride-18 at 22◦C, even at full batch labeling. The optimized procedure was tested by Al18F-labeling of RESCA1-AE105—a uPAR targeting peptide. NOTA-AE105 was also labeled with Al18F, and the two peptides were compared head-to-head. [18F]AlF-NOTA-AE105 and [18F]AlF-RESCA1-AE105 could be produced in equal radiochemical yields (RCY), radiochemical purities (RCP) and molar activities. Additionally, the two peptides showed comparable binding affinity to uPAR and uptake in cells expressing the uPAR, when evaluated in vitro. Overall, we found that the performances of [18F]AlF-NOTA-AE105 and [18F]AlF-RESCA1-AE105 were grossly comparable, but importantly RESCA1 can be labeled with aluminium fluoride-18 at 22◦C. Consequently, this study showed that RESCA1 is superior to NOTA with respect to Al18F chelation of temperature sensitive molecules, such as thermolabile peptides and proteins as well as that full batch chelation of RESCA1 with fluoride-18 is possible.

Targeted imaging of uPAR expression in vivo with cyclic AE105 variants

“…The IC 50 -values of antagonistic peptides on the uPAR•uPA interaction were determined with SPR using a Biacore T200™ or Biacore 3000™ instruments (Cytiva) essentially as described 4 , 56 , 77 . In brief, we immobilized high levels of pro-uPA S356A (0.1 pmol pro-uPA/mm 2 ) on a CM5 sensor chip via amine coupling.…”

“…The real-time binding kinetics of uPAR•peptide interactions were measured with SPR using a Biacore T200™(Cytiva); essentially as described in 56 , 77 . In brief, uPAR was covalently immobilised on a CM5 sensor chip via amine coupling resulting in a surface density of 1157–1581 RU (~ 33–44 fmol/mm 2 ).…”

Recent Advances in the Development of Tetrazine Ligation Tools for Pretargeted Nuclear Imaging

“…In general, aluminum [ 18 F]fluoride labeling is in its infancy and is typically difficult to scale. However, many efforts have been reported and are made to improve labeling yields [131][132][133][134]. As such, aluminum [ 18 F]fluoride labeling is a very interesting approach and was also applied to labeling Tzs.…”