Autoradiographic comparison of in vitro binding characteristics of various tritiated adenosine A sub(2A) receptor ligands in rat, mouse and pig brain and first ex vivo results

• Published in: European journal of pharmacology 2009-08, Vol.616 (1-3), p.107-114
• Main Authors: Sihver, Wiebke, Schulze, Annette, Wutz, Walter, Stuesgen, Stefan, Olsson, Ray A, Bier, Dirk, Holschbach, Marcus H
• Format: Article
• Language: English
• ISSN: 0014-2999; 1879-0712
• DOI: 10.1016/j.ejphar.2009.06.025

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