Autoradiographic comparison of in vitro binding characteristics of various tritiated adenosine A2A receptor ligands in rat, mouse and pig brain and first ex vivo results

The adenosine A2A receptor in the basal ganglia is involved in the control of movement and plays a role in movement disorders such as Parkinsonism. Developing ligands to evaluate that receptor by noninvasive methods such as positron emission tomography has a high priority. In vitro radioligand binding guides the selection of ligands for in vivo application. This study measured the binding of the adenosine A2A receptor antagonist [3H]MSX-2 (3-(3-hydroxypropyl)-8-m-methoxystyryl)-7-methyl-1-propargylxanthine) to rat, mouse and pig brain by autoradiography. Other studies measured binding to membranes from PC12 pheochromocytoma cells. Those binding parameters were compared to those of the adenosine A2A receptor antagonist [3H]ZM241385 (4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino)ethyl)phenol), the adenosine A2A receptor agonist [3H]CGS21680 (2-[p-(2-carboxyethyl)-phenethylamino]-5′-N-ethylcarboxamidoadenosine) and the unselective adenosine receptor agonist [3H]NECA (5′N-ethylcarboxamido)adenosine). The potency order (Kd) in the three species was [3H]ZM241385 < [3H]MSX-2 < [3H]NECA < [3H]CGS21680. The density of [3H]MSX-2 binding sites was greater in the striatum than in the cortex. Preliminary ex vivo experiments showed that by 10 min after iv injection, [3H]MSX-2 and [3H]CGS21680 crossed the blood-brain barrier to the extent of almost 1% ID/g brain tissue, but [3H]NECA and [3H]ZM241385 to only 0.2% ID/g. The prior administration of unlabeled ZM241385 significantly lowered brain uptake of [3H]MSX-2. In conclusion, [3H]MSX-2 has a high affinity and sufficient selectivity for the adenosine A2A receptor. It penetrates the blood-brain barrier. Sensitivity to photoisomerization is a limitation. Further investigations assess its suitability as a ligand for imaging the brain adenosine A2A receptor.