Radiosynthesis and in vivo evaluation of novel radioligands for PET imaging of cerebral 5-HT7 receptors.

UNLABELLED: The serotonin (5-hydroxytryptamine [5-ΗΤ]) 7 receptor (5-HT7R) is the most recently discovered 5-HT receptor, and its physiologic and possible pathophysiologic roles are not fully elucidated. So far, no suitable 5-HT7R PET radioligand is available, thus limiting the investigation of this receptor in the living brain. Here, we present the radiosynthesis and in vivo evaluation of Cimbi-712 (3-{4-[4-(4-methylphenyl)piperazine-1-yl]butyl}p-1,3-dihydro-2H-indol-2-one) and Cimbi-717 (3-{4-[4-(3-methoxyphenyl)piperazine-1-yl]butyl}-1,3-dihydro-2H-indol-2-one) as selective 5-HT7R PET radioligands in the pig brain. The 5-HT7R distribution in the postmortem pig brain is also assessed. METHODS: In vitro autoradiography with the 5-HT7R selective radioligand (3)H-labeled (R)-3-(2-(2-(4-methylpiperidin-1-yl)ethyl)pyrrolidine-1-sulfonyl)phenol (SB-269970) was performed on pig brain sections to establish the 5-HT7R binding distribution. Radiolabeling of 5-HT7R selective compounds was performed in an automated synthesis module in which we conducted either palladium-mediated cross coupling ((11)C-Cimbi-712) or conventional O-methylation ((11)C-Cimbi-717) using (11)C-MeI and (11)C-MeOTf, respectively. After intravenous injection of the radioligands in Danish Landrace pigs, the in vivo brain distribution of the ligands was studied. Specific binding of (11)C-Cimbi-712 and (11)C-Cimbi717 to 5-HT7R was investigated by intravenous administration of SB-269970 before a second PET scan. RESULTS: High 5-HT7R density was found in the thalamus and cortical regions of the pig brain by autoradiography. The radiosynthesis of both radioligands succeeded after optimization efforts (radiochemical yield, ∼20%-30% at the end of synthesis). Time-activity curves of (11)C-Cimbi-712 and (11)C-Cimbi-717 showed high brain uptake and distribution according to 5-HT7R distribution, but the tracer kinetics of (11)C-Cimbi-717 were faster than (11)C-Cimbi-712. Both radioligands were specific for 5-HT7R, as binding could be blocked by pretreatment with SB-269970 for (11)C-Cimbi-717 in a dose-dependent fashion. For (11)C-Cimbi-717, nondisplaceable binding potentials of 6.4 ± 1.2 (n = 6) were calculated in the thalamus. CONCLUSION: Both (11)C-Cimbi-712 and (11)C-Cimbi-717 generated a specific binding in accordance with 5-HT7R distribution and are potential PET radioligands for 5-HT7R. (11)C-Cimbi-717 is the better candidate because of the more reversible tracer kinetics, and this radioligand showed a dose-dependent decline in cerebral binding after receptor blockade. Thus, (11)C-Cimbi-717 is currently the most promising radioligand for investigation of 5-HT7R binding in the living human brain.