Evaluation of Organo [18F]Fluorosilicon Tetrazine as a Prosthetic Group for the Synthesis of PET Radiotracers.

Sofia Otaru,Kerttuli Helariutta,Mirkka Sarparanta,Kristiina Wähälä,Anu Airaksinen,Surachet Imlimthan

doi:10.3390/molecules25051208

Sofia Otaru, Kerttuli Helariutta + Show 4 more

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https://doi.org/10.3390/molecules25051208

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Abstract

Fluorine-18 is the most widely used positron emission tomography (PET) radionuclide currently in clinical application, due to its optimal nuclear properties. The synthesis of 18F-labeled radiotracers often requires harsh reaction conditions, limiting the use of sensitive bio- and macromolecules as precursors for direct radiolabeling with fluorine-18. We aimed to develop a milder and efficient in vitro and in vivo labeling method for trans-cyclooctene (TCO) functionalized proteins, through the bioorthogonal inverse-electron demand Diels-Alder (IEDDA) reaction with fluorine-18 radiolabeled tetrazine ([18F]SiFA-Tz). Here, we used TCO-modified bovine serum albumin (BSA) as the model protein, and isotopic exchange (IE) (19F/18F) chemistry as the labeling strategy. The radiolabeling of albumin-TCO with [18F]SiFA-Tz ([18F]6), providing [18F]fluoroalbumin ([18F]10) in high radiochemical yield (99.1 ± 0.2%, n = 3) and a molar activity (MA) of 1.1 GBq/µmol, confirmed the applicability of [18F]6 as a quick in vitro fluorination reagent for the TCO functionalized proteins. While the biological evaluation of [18F]6 demonstrated defluorination in vivo, limiting the utility for pretargeted applications, the in vivo stability of the radiotracer was dramatically improved when [18F]6 was used for the radiolabeling of albumin-TCO ([18F]10) in vitro, prior to administration. Due to the detected defluorination in vivo, structural optimization of the prosthetic group for improved stability is needed before further biological studies and application of pretargeted PET imaging.

Highlights

Fluorine-18 is an ideal radionuclide for labeling of radiopharmaceuticals for positron emission tomography (PET), due to its nuclear and physical characteristics, including the relatively long half-life (109.7 min), the low energy levels of emitted positrons (Emax = 0.635 MeV), and high positron decay probability (97%) [1]
In 1959, Lindsey et al reported the ability of tetrazines to react chemoselectively with unsaturated compounds through a 1,4-cycloaddition reaction [9]. These findings introduced a new and highly reactive bioorthogonal inverse-electron demand Diels-Alder (IEDDA) click-reaction as a pivotal tool for synthetic modification of biomolecules
The reaction step was the deprotection of 3 acid and an amine, forming the t-Boc protected aminooxy-tetrazine 3, in 65% yield after silica gel with hydrochloric acid in methanol, givenext

Summary

Introduction

Milder, and more selective radiolabeling methodologies are desired, especially for the radiolabeling of compounds sensitive to temperature and higher pH. Fast and efficient catalyst-free click-reactions, such as the inverse electron-demand Diels-Alder (IEDDA) reaction, have been applied as effective tools for the selective incorporation of radiolabels, such as 18 F, into bio- and macromolecules, via small radiolabeled prosthetic groups. Other widely known click-reactions, such as the copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) and the stainpromoted azide-alkyne cycloaddition (SPAAC) have been the starting point for the modification of chemoselective biomolecules [5]. The CuAAC reaction was first reported in 2002 by Sharpless et al [6] and Meldal et al [7]. The aim of these studies was to utilize the

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