Synthesis and evaluation of 11C-LY2795050 as a κ-opioid receptor antagonist radiotracer for PET imaging.
Kappa-opioid receptors (KOR) are believed to be involved in the pathophysiology of depression, anxiety disorders, drug abuse, and alcoholism. To date, only 1 tracer, the KOR agonist (11)C-GR103545, has been reported to be able to image KOR in primates. The goal of the present study was to synthesize the selective KOR antagonist (11)C-LY2795050 and evaluate its potential as a PET tracer to image KOR in vivo.


METHODS
The in vitro binding affinity of LY2795050 was measured in radioligand competition binding assays. Ex vivo experiments were conducted using microdosing of the unlabeled ligand in Sprague-Dawley rats and in wild-type and KOR knockout mice, to assess the ligand's potential as a tracer candidate. Imaging experiments with (11)C-LY2795050 in monkeys were performed on the Focus-220 scanner with arterial blood input function measurement. Binding parameters were determined with kinetic modeling analysis.


RESULTS
LY2795050 displays full antagonist activity and high binding affinity and selectivity for KOR. Microdosing studies in rodents and ex vivo analysis of tissue concentrations with liquid chromatography-tandem mass spectrometry identified LY2795050 as an appropriate tracer candidate able to provide specific binding signals in vivo. (11)C-LY2795050 was prepared in an average yield of 12% and greater than 99% radiochemical purity. In rhesus monkeys, (11)C-LY2795050 displayed a moderate rate of peripheral metabolism, with approximately 40% of parent compound remaining at 30 min after injection. In the brain, (11)C-LY2795050 displayed fast uptake kinetics (regional activity peak times of <20 min) and an uptake pattern consistent with the distribution of KOR in primates. Pretreatment with naloxone (1 mg/kg, intravenously) resulted in a uniform distribution of radioactivity. Further, specific binding of (11)C-LY2795050 was reduced by the selective KOR antagonist LY2456302 in a dose-dependent manner.


CONCLUSIONS
(11)C-LY2795050 displayed favorable pharmacokinetic properties and binding profiles in vivo and therefore is a suitable ligand for imaging the KOR in primates. This newly developed KOR antagonist tracer has since been advanced to PET imaging of KOR in humans and constitutes the first successful KOR antagonist radiotracer.
PET Center, Department of Diagnostic Radiology, Yale University, New Haven, Connecticut; and.
Ming-Qiang
MQ
Zheng
Nabeel
N
Nabulsi
Su Jin
SJ
Kim
Giampaolo
G
Tomasi
Shu-Fei
SF
Lin
Charles
C
Mitch
Steven
S
Quimby
Vanessa
V
Barth
Karen
K
Rash
John
J
Masters
Antonio
A
Navarro
Eric
E
Seest
Evan D
ED
Morris
Richard E
RE
Carson
Yiyun
Y
Huang
D000818:Animals; D001921:Brain / Q000000981:diagnostic imaging / Q000378:metabolism; D002250:Carbon Radioisotopes; D008024:Ligands; D008253:Macaca mulatta; D008297:Male; D051379:Mice; D057507:Mice, 129 Strain; D018345:Mice, Knockout; D049268:Positron-Emission Tomography / Q000379:methods*; D011869:Radioligand Assay; D019275:Radiopharmaceuticals / Q000138:chemical synthesis* / Q000737:chemistry; D051381:Rats; D017207:Rats, Sprague-Dawley; D017473:Receptors, Opioid, kappa / Q000037:antagonists & inhibitors*
D002250:Carbon Radioisotopes; D008024:Ligands; D019275:Radiopharmaceuticals; D017473:Receptors, Opioid, kappa