Synthesis and characterization of [⁷⁶Br]-labeled high-affinity A₃ adenosine receptor ligands for positron emission tomography

Abstract 

Introduction

Bromine-76-radiolabeled analogues of previously reported high-affinity A₃ adenosine receptor (A₃AR) nucleoside ligands have been prepared as potential radiotracers for positron emission tomography.

Methods

The radiosyntheses were accomplished by oxidative radiobromination on the N⁶-benzyl moiety of trimethyltin precursors. Biodistribution studies of the kinetics of uptake were conducted in awake rats.

Results

We prepared an agonist ligand {[⁷⁶Br](1′S,2′R,3′S,4′R,5′S)-4′-{2-chloro-6-[(3-bromophenylmethyl)amino]purin-9-yl}-1′-(methylaminocarbonyl)bicyclo[3.1.0]hexane-2′,3′-diol (MRS3581)} in 59% radiochemical yield with a specific activity of 19.5 GBq/μmol and an antagonist ligand {[⁷⁶Br](1′R,2′R,3′S,4′R,5′S)-4′-(6-(3-bromobenzylamino)-2-chloro-9H-purin-9-yl)bicyclo[3.1.0]hexane-2′,3′-diol (MRS5147)} in 65% radiochemical yield with a specific activity of 22 GBq/μmol. The resultant products exhibited the expected high affinity (K_i∼0.6 nM) and specific binding at the human A₃AR in vitro. Biodistribution studies in the rat showed uptake in the organs of excretion and metabolism. The antagonist MRS5147 exhibited increasing uptake in testes, an organ that contains significant quantities of A₃AR, over a 2-h time course, which suggests the presence of a specific A₃AR retention mechanism.

Conclusion

We were able to compare uptake of the [⁷⁶Br]-labeled antagonist MRS5147 to [⁷⁶Br]agonist MRS3581. The antagonist MRS5147 shows increasing uptake in the testes, an A₃AR-rich tissue, suggesting that this ligand may have promise as a molecular imaging agent.

Keywords: Adenosine A₃ receptor, G-protein-coupled receptor, Nucleoside, Purines, Receptor binding, Bromine-76