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

Troels E Jeppesen,Jacob Madsen,Andreas Kjaer,Julie Maja Leth,Michael Ploug,Line B S Knudsen,Josephine Torp,Matthias M Herth,François Crestey,Marina Simón,Jesper T Jørgensen

doi:10.3389/fnume.2021.799533

Troels E Jeppesen, Jacob Madsen + Show 9 more

Open Access

https://doi.org/10.3389/fnume.2021.799533

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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.

Highlights

For positron emission tomography (PET) imaging, peptides, and other smaller biomolecules are often labeled with 68Ga, exemplified by [68Ga]Ga-DOTATOC/TATE (1), due to the ease of radiolabelling and access to 68Ga/68Ge generators
A direct labeling approach with Al18F was previously demonstrated, where aluminium is coordinated in a chelator and bound to 18F (3)
The results showed that RESCA1-AE105 and NOTA-AE105 exhibited comparable binding kinetics to uPAR with equal association and dissociation rate constants, and with equal KD and IC50

Summary

Introduction

For PET imaging, peptides, and other smaller biomolecules are often labeled with 68Ga, exemplified by [68Ga]Ga-DOTATOC/TATE (1), due to the ease of radiolabelling and access to 68Ga/68Ge generators. The generator is, not suited for large scale production and it would be advantageous if 18F-labeling of the targeting probe was feasible. A direct labeling approach with Al18F was previously demonstrated, where aluminium is coordinated in a chelator (typically NOTA or similar TACN-based chelators) and bound to 18F (3). This labeling approach proceeds in aqueous media at pH 4–5 with heating to 90–110◦C (4). The peptide AE105, which is an antagonist for uPA-binding to its receptor uPAR, was used as the targeting moeity (6). Labeling with Al18F of RESCA1 was optimized and compared to NOTA, and Al18F labeling of RESCA1-AE105 and NOTA-AE105 was performed, compared and evaluated in a cell-binding assay

Methods

Results

Conclusion