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.

Abstract The adenosine A 2A 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 A 2A receptor antagonist [ 3 H]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 A 2A receptor antagonist [ 3 H]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 A 2A receptor agonist [ 3 H]CGS21680 (2-[ p -(2-carboxyethyl)-phenethylamino]-5′- N -ethylcarboxamidoadenosine) and the unselective adenosine receptor agonist [ 3 H]NECA (5′ N -ethylcarboxamido)adenosine). The potency order ( K d ) in the three species was [ 3 H]ZM241385  3 H]MSX-2  3 H]NECA  3 H]CGS21680. The density of [ 3 H]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, [ 3 H]MSX-2 and [ 3 H]CGS21680 crossed the blood–brain barrier to the extent of almost 1% ID/g brain tissue, but [ 3 H]NECA and [ 3 H]ZM241385 to only 0.2% ID/g. The prior administration of unlabeled ZM241385 significantly lowered brain uptake of [ 3 H]MSX-2. In conclusion, [ 3 H]MSX-2 has a high affinity and sufficient selectivity for the adenosine A 2A 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 A 2A receptor.