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

Studies have shown κ-opioid receptor (KOR) abnormalities in addictive disorders, other central nervous system diseases, and Alzheimer's disease. We have developed the first set of agonist ¹¹ C-GR103545 and antagonist ¹¹ C-LY2795050 radiotracers for PET imaging of KOR in humans. Nonetheless, ¹¹ C-GR103545 displays protracted uptake kinetics and is not an optimal radiotracer. Here, we report the development and evaluation of ¹¹ C - methyl-( R )-4-(2-(3,4-dichlorophenyl)acetyl)-3-((diethylamino)methyl)piperazine-1-carboxylate ( ¹¹ C-EKAP) and its comparison with ¹¹ C-GR103545. Methods: EKAP was synthesized and assayed for in vitro binding affinities and then radiolabeled. PET studies were performed on rhesus monkeys. Blocking studies were performed with naloxone and the selective KOR antagonists LY2795050 and LY2456302. Arterial input functions were generated for use in kinetic modeling. Brain TACs were analyzed with multilinear analysis 1 to derive binding parameters. Results: EKAP has high KOR affinity (inhibition constant, 0.28 nM) and good selectivity in vitro. ¹¹ C-EKAP was prepared in good radiochemical purity. ¹¹ C-EKAP rapidly metabolized in plasma and displayed fast and reversible kinetics in brain, with peak uptake at less than 20 min after injection. Preblocking with naloxone (1 mg/kg) or LY2795050 (0.2 mg/kg) produced 84%-89% receptor occupancy, whereas LY2456302 (0.05 and 0.3 mg/kg) dose-dependently reduced ¹¹ C-EKAP-specific binding, thus demonstrating its binding specificity and selectivity in vivo. Mean multilinear analysis 1-derived nondisplaceable binding potential values were 1.74, 1.79, 1.46, 0.80, and 0.77 for cingulate cortex, globus pallidus, insula, striatum, and frontal cortex, respectively, consistent with the known KOR distribution in primate brains. Conclusion: We have successfully developed ¹¹ C-EKAP as a KOR agonist tracer with dual attractive imaging properties of fast uptake kinetics and high specific binding in vivo.
(© 2019 by the Society of Nuclear Medicine and Molecular Imaging.)