Development of the first fluorine-18 labelled radioligand for imaging of the adenosine A2B receptor

Objectives
The G protein-coupled A2B receptor differs from other adenosine receptor subtypes (A1, A2A, A3) by its low affinity towards the endogenous ligand adenosine. It is suggested to be involved in various pathological processes accompanied by increased levels of adenosine, e.g. inflammation, hypoxia, and cancer. To enable the investigation of the function and expression of A2B-receptor in living organisms, we developed a fluorine-18 labelled radioligand with the particular aim of imaging of neurooncological and neuroinflammatory processes by PET.
Methods
Based on the pyrazine compound 1 [1] (Fig. A) several novel fluorine-containing derivatives were synthesized in four steps and their affinities and selectivities toward all four adenosine receptor subtypes were determined. The most promising candidate PA51 was radiolabelled by using the corresponding nitro precursor in DMSO with thermal as well as microwave heating (Fig. B). To study the in vivo metabolism of [18F]PA51 plasma and brain samples obtained from mouse at 30 min p.i. were investigated by using (a) conventional extraction procedures and (b) a micellar HPLC approach.
Results
[18F]PA51 (binding affinities in Fig. B) was successfully synthesized with radiochemical yields of 36.1±4.6% (dec. corr., formulated product), molar activities of 10-30 GBq/µmol, and radiochemical purities of =99% (determined by analytical HPLC by UV absorption at ? = 254 nm). In vivo studies in mice revealed high initial brain uptake (5 min p.i.). Fast metabolism was found with formation of a single major radiometabolite able to cross the blood-brain barrier.
Conclusions
[18F]PA51 is unsuitable for imaging of A2B receptors in brain in vivo due to the presence of a radiometabolite. However, the initially high uptake of activity in the brain encourages further structural modifications to improve the selectivity and the metabolic stability.
References
[1] P. Eastwood, et al. ACS Med. Chem. Lett. 2011, 2, 213-218.