TY - JOUR T1 - Synthesis and Evaluation of <sup>11</sup>C-LY2795050 as a κ-Opioid Receptor Antagonist Radiotracer for PET Imaging JF - Journal of Nuclear Medicine JO - J Nucl Med SP - 455 LP - 463 DO - 10.2967/jnumed.112.109512 VL - 54 IS - 3 AU - Ming-Qiang Zheng AU - Nabeel Nabulsi AU - Su Jin Kim AU - Giampaolo Tomasi AU - Shu-fei Lin AU - Charles Mitch AU - Steven Quimby AU - Vanessa Barth AU - Karen Rash AU - John Masters AU - Antonio Navarro AU - Eric Seest AU - Evan D. Morris AU - Richard E. Carson AU - Yiyun Huang Y1 - 2013/03/01 UR - http://jnm.snmjournals.org/content/54/3/455.abstract N2 - 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 11C-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 11C-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 11C-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. 11C-LY2795050 was prepared in an average yield of 12% and greater than 99% radiochemical purity. In rhesus monkeys, 11C-LY2795050 displayed a moderate rate of peripheral metabolism, with approximately 40% of parent compound remaining at 30 min after injection. In the brain, 11C-LY2795050 displayed fast uptake kinetics (regional activity peak times of &lt;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 11C-LY2795050 was reduced by the selective KOR antagonist LY2456302 in a dose-dependent manner. Conclusion: 11C-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. ER -