title: Positron emission tomographic evaluation of the putative dopamine-D3 receptor ligand, [11C]RGH-1756 in the monkey brain.
abstract: The dopamine-D3 receptor is of special interest due to its postulated role in the pathophysiology and treatment of schizophrenia and Parkinson's Disease. Increasing evidences support the assumption that the D3 receptors are occupied to a high degree by dopamine at physiological conditions. Research on the functional role of the D3 receptors in brain has however been hampered by the lack of D3 selective ligands. In the present Positron Emission Tomography (PET) study the binding of the novel, putative dopamine-D3 receptor ligand, [11C]RGH-1756 was characterized in the cynomolgus monkey brain. [11C]RGH-1756 was rather homogenously distributed in brain and the regional binding potential (BP) values ranged between 0.17 and 0.48. Pretreatment with unlabelled RGH-1756 decreased radioligand binding to the level of the cerebellum in most brain areas. The regional BP values were lower after intravenous injection of a higher mass of RGH-1756, indicating saturable binding of [11C]RGH-1756. The D2/D3 antagonist raclopride partly inhibited the binding of [11C]RGH-1756 in several brain areas, including the striatum, mesencephalon and neocortex, whereas the 5HT(1A) antagonist WAY-100635 had no evident effect on [11C]RGH-1756 binding. Despite the promising binding characteristics of RGH-1756 in vitro the present PET-study indicates that [11C]RGH-1756 provides a low signal for specific binding to the D3 receptor in vivo. One explanation is that the favorable binding characteristics of RGH-1756 in vitro are not manifested in vivo. Alternatively, the results may support the hypothesis that the dopamine-D3 receptors are indeed occupied to a high extent by dopamine in vivo and thus not available for radioligand binding.
authors.affiliation: Department of Clinical Neuroscience, Karolinska Institutet, Psychiatry Section, S-171 76 Stockholm, Sweden.
authors.forename: Judit
authors.initials: J
authors.lastname: Sóvágó
authors.forename: Lars
authors.initials: L
authors.lastname: Farde
authors.forename: Christer
authors.initials: C
authors.lastname: Halldin
authors.forename: Oliver
authors.initials: O
authors.lastname: Langer
authors.forename: István
authors.initials: I
authors.lastname: Laszlovszky
authors.forename: Béla
authors.initials: B
authors.lastname: Kiss
authors.forename: Balázs
authors.initials: B
authors.lastname: Gulyás
mesh_terms: D000818:Animals; D001711:Biotransformation; D001921:Brain / Q000000981:diagnostic imaging*; D018492:Dopamine Antagonists / Q000494:pharmacology; D007091:Image Processing, Computer-Assisted; D008252:Macaca fascicularis; D008297:Male; D010879:Piperazines / Q000493:pharmacokinetics* / Q000494:pharmacology; D011725:Pyridines / Q000494:pharmacology; D020891:Raclopride / Q000494:pharmacology; D019275:Radiopharmaceuticals / Q000493:pharmacokinetics*; D017448:Receptors, Dopamine D2 / Q000378:metabolism*; D050637:Receptors, Dopamine D3; D012702:Serotonin Antagonists / Q000494:pharmacology; D013844:Thiazoles / Q000493:pharmacokinetics*; D014055:Tomography, Emission-Computed
chemical_list: C427703:1-(2-methoxy-phenyl)-4-(4-(4-(6-imidazol(2,1-b) thiazolyl)-phenoxy)-butyl-4-(14)C)-piperazine dimethane; D018492:Dopamine Antagonists; D010879:Piperazines; D011725:Pyridines; D019275:Radiopharmaceuticals; D017448:Receptors, Dopamine D2; D050637:Receptors, Dopamine D3; D012702:Serotonin Antagonists; D013844:Thiazoles; D020891:Raclopride; C090413:N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridinyl)cyclohexanecarboxamide