An Improved Antagonist Radiotracer for the κ-Opioid Receptor: Synthesis and Characterization of ¹¹C-LY2459989

Abstract

The kappa opioid receptors (KOR) are implicated in a number of neuropsychiatric diseases and addictive disorders. Positron Emission Tomography (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 11C-LY2459989 as a PET radioligand and characterize its imaging performance in non-human 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. 11C-LY2459989 was synthesized by reaction of its iodophenyl precursor with 11C-cyanide followed by partial hydrolysis of the resulting 11C-cyanophenyl intermediate. Imaging experiments with 11C-LY2459989 were carried out 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 (Ki = 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. 11C-LY2459989 was synthesized in high radiochemical purity and good specific activity. In rhesus monkeys, 11C-LY2459989 displayed a fast rate of peripheral metabolism. Similarly, 11C-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, i.v.) resulted in a uniform distribution of radioactivity in the brain. Further, specific binding of 11C-LY2459989 was dose-dependently reduced by the selective KOR antagonist LY2456302 and the unlabeled LY2459989. Regional binding potential (BPND) values derived from the multilinear analysis method (MA1), as a measure of in vivo specific binding signal, were 2.18, 1.39, 1.08, 1.04, 1.03, 0.59, 0.51, 0.50, respectively, for the globus pallidus, cingulate cortex, insula, caudate, putamen, frontal cortex, temporal cortex, and thalamus. Conclusion The novel PET radioligand 11C-LY2459989 displayed favorable pharmacokinetic properties, specific and KOR-selective binding profile, and high specific binding signals in vivo, thus making it a promising PET imaging agent for KOR.

11CO bonds made easily for positron emission tomography radiopharmaceuticals

“…209 Furthermore, this strategy and method for radiolabeling has proven useful in development of a second-generation KOR antagonist PET radiotracer with improved KOR selectivity and binding, [ 11 C]LY2459989. 210 …”

Development and In Vivo Evaluation of a κ-Opioid Receptor Agonist as a PET Radiotracer with Superior Imaging Characteristics

“…Previous studies with KOR radiotracers indicated that cerebellum can be used as a reference region in nonhuman primates to calculate BP ND (44). This appeared to be true as well for 11 C-EKAP, as cerebellum V T remained similar when various blocking drugs were administered and regional BP ND values estimated by SRTM using cerebellum as the reference region exhibited an…”

Immediate and Persistent Effects of Salvinorin A on the Kappa Opioid Receptor in Rodents, Monitored In Vivo with PET

“…pharmacologically (Lowe et al, 2014) and some as KOR radiotracers for PET (Zheng et al, 2013(Zheng et al, , 2014. LY2795050 has been evaluated as an antagonist PET tracer in both NHPs (Zheng et al, 2013;Kim et al, 2013) and humans (Naganawa et al, 2014b(Naganawa et al, , 2015.…”