[¹⁸F] PTTCO-Cys⁴⁰-Exendin-4, a newly synthesized GLP-1R probe, effectively targets insulinomas in NOD/SCID mice

Objectives: Glucagon-like peptide 1 receptor (GLP-1R) has become a reliable PET biomarker for insulinoma and pancreatic islets. Exendin-4 is a peptide mimetic of GLP-1 with enhanced target affinity and metabolic stability compared to GLP-1. Currently, a major limitation in the development of radiolabeled Exendin-4 analogues is an inability to separate final products from precursors. This is a critical issue as lack of purity in the final product decreases probe efficiency. The purpose of this study was to develop a method that would yield high-purity [¹⁸F] PTTCO-Cys⁴⁰-Exendin-4, and evaluate the in vivo effectiveness of the product obtained to visualize GLP-1R over-expressing insulinomas by microPET as well as to determine the exact biodistribution of the product. Methods: The synthesis of [¹⁸F] PTTCO-Cys⁴⁰-Exendin-4 was accomplished employing a two-step reaction. First, nucleophilic substitution of the tosylate precursor was performed in a TRACERlab FN Chemistry Synthesizer to produce [¹⁸F] PEG4-TCO. Second, [4+2] inverse electron demand Diels-Alder cycloaddition using [¹⁸F] PEG4-TCO and tetrazines was performed to obtain crude [¹⁸F] PTTCO-Cys⁴⁰-Exendin-4. To increase product purity and activity, magnetic TCO-beads were incubated with the crude product to remove unlabeled Exendin-4. The in vivo binding of the final product was evaluated using micro-PET scans in insulinoma-bearing mice. Also, organs of interest were collected and evaluated to assess in more detail agent biodistribution. Results: [¹⁸F] PTTCO-Cys⁴⁰-Exendin-4 was radiosynthesized with a good radiochemical yield and high radiochemical purity (>98%). MicroPET clearly visualized insulinomas in tumor-bearing animals pre-treated with [¹⁸F] PTTCO-Cys⁴⁰-Exendin-4 (~1.85 MBq). Moreover, biomarker uptake was successfully blocked by administering non-labelled “cold” Exendin-4. Biodistribution data revealed that [¹⁸F] PTTCO-Cys⁴⁰-Exendin-4 specifically accumulated in GLP-1R-enriched insulinomas in mice, confirming results obtained using miroPET. Conclusions: A method is described to synthesize high yield [¹⁸F] PTTCO-Cys⁴⁰-Exendin-4. Systemic administration of [¹⁸F] PTTCO-Cys⁴⁰-Exendin-4 showed persistent and specific uptake of the biomarker in insulinoma-bearing mice. Investigation of [¹⁸F] PTTCO-Cys⁴⁰-Exendin-4 as a biomarker of portal vein-transplanted pancreatic islets is currently underway. Research Support: This work was supported by the grant from Larry L. Hillblom Foundation (2015-D-007-SUP)