An Improved Antagonist Radiotracer for the κ-Opioid Receptor: Synthesis and Characterization of (11)C-LY2459989.
The κ-opioid receptors (KORs) are implicated in several neuropsychiatric diseases and addictive disorders. PET with radioligands provides a means to image the KOR in vivo and investigate its function in health and disease. The purpose of this study was to develop the selective KOR antagonist (11)C-LY2459989 as a PET radioligand and characterize its imaging performance in nonhuman primates.


METHODS
LY2459989 was synthesized and assayed for in vitro binding to opioid receptors. Ex vivo studies in rodents were conducted to assess its potential as a tracer candidate. (11)C-LY2459989 was synthesized by reaction of its iodophenyl precursor with (11)C-cyanide, followed by partial hydrolysis of the resulting (11)C-cyanophenyl intermediate. Imaging experiments with (11)C-LY2459989 were performed in rhesus monkeys with arterial input function measurement. Imaging data were analyzed with kinetic models to derive in vivo binding parameters.


RESULTS
LY2459989 is a full antagonist with high binding affinity and selectivity for KOR (0.18, 7.68, and 91.3 nM, respectively, for κ, μ, and δ receptors). Ex vivo studies in rats indicated LY2459989 as an appropriate tracer candidate with high specific binding signals and confirmed its KOR binding selectivity in vivo. (11)C-LY2459989 was synthesized in high radiochemical purity and good specific activity. In rhesus monkeys, (11)C-LY2459989 displayed a fast rate of peripheral metabolism. Similarly, (11)C-LY2459989 displayed fast uptake kinetics in the brain 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 in the brain. Further, specific binding of (11)C-LY2459989 was dose-dependently reduced by the selective KOR antagonist LY2456302 and the unlabeled LY2459989. Regional binding potential values derived from the multilinear analysis-1 (MA1) method, as a measure of in vivo specific binding signal, were 2.18, 1.39, 1.08, 1.04, 1.03, 0.59, 0.51, and 0.50, respectively, for the globus pallidus, cingulate cortex, insula, caudate, putamen, frontal cortex, temporal cortex, and thalamus.


CONCLUSIONS
The novel PET radioligand (11)C-LY2459989 displayed favorable pharmacokinetic properties, a specific and KOR-selective binding profile, and high specific binding signals in vivo, thus making it a promising PET imaging agent for KOR.
PET Center, Department of Diagnostic Radiology, Yale University, New Haven, Connecticut; and.
Ming-Qiang
MQ
Zheng
PET Center, Department of Diagnostic Radiology, Yale University, New Haven, Connecticut; and.
Su Jin
SJ
Kim
PET Center, Department of Diagnostic Radiology, Yale University, New Haven, Connecticut; and.
Daniel
D
Holden
PET Center, Department of Diagnostic Radiology, Yale University, New Haven, Connecticut; and.
Shu-fei
SF
Lin
Eli Lilly & Company, Indianapolis, Indiana.
Anne
A
Need
Eli Lilly & Company, Indianapolis, Indiana.
Karen
K
Rash
Eli Lilly & Company, Indianapolis, Indiana.
Vanessa
V
Barth
Eli Lilly & Company, Indianapolis, Indiana.
Charles
C
Mitch
Eli Lilly & Company, Indianapolis, Indiana.
Antonio
A
Navarro
PET Center, Department of Diagnostic Radiology, Yale University, New Haven, Connecticut; and.
Michael
M
Kapinos
PET Center, Department of Diagnostic Radiology, Yale University, New Haven, Connecticut; and.
Kathleen
K
Maloney
PET Center, Department of Diagnostic Radiology, Yale University, New Haven, Connecticut; and.
Jim
J
Ropchan
PET Center, Department of Diagnostic Radiology, Yale University, New Haven, Connecticut; and.
Richard E
RE
Carson
PET Center, Department of Diagnostic Radiology, Yale University, New Haven, Connecticut; and henry.huang@yale.edu.
Yiyun
Y
Huang
D000818:Animals; D001549:Benzamides / Q000138:chemical synthesis* / Q000737:chemistry / Q000494:pharmacology; D060326:Chemistry Techniques, Synthetic; D008253:Macaca mulatta; D008297:Male; D049268:Positron-Emission Tomography / Q000379:methods*; D011725:Pyridines / Q000138:chemical synthesis* / Q000737:chemistry / Q000494:pharmacology; D011849:Radioactive Tracers; D011854:Radiochemistry; D019275:Radiopharmaceuticals / Q000138:chemical synthesis* / Q000737:chemistry / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D017473:Receptors, Opioid, kappa / Q000037:antagonists & inhibitors* / Q000378:metabolism*
D001549:Benzamides; C000599322:LY2459989; D011725:Pyridines; D011849:Radioactive Tracers; D019275:Radiopharmaceuticals; D017473:Receptors, Opioid, kappa
PET; antagonist; kappa opioid receptor; radioligand; synthesis and evaluation