Discovery of a fluorinated 4‐oxo‐quinoline derivative as a potential positron emission tomography radiotracer for imaging cannabinoid receptor type 2

Abstract

The cannabinoid receptor type 2 (CB2) is part of the endocannabinoid system and has gained growing attention in recent years because of its important role in neuroinflammatory/neurodegenerative diseases. Recently, we reported on a carbon-11 labeled 4-oxo-quinoline derivative, designated RS-016, as a promising radiotracer for imaging CB2 using PET. In this study, three novel fluorinated analogs of RS-016 were designed, synthesized, and pharmacologically evaluated. The results of our efforts led to the identification of N-(1-adamantyl)-1-(2-(2-fluoroethoxy)ethyl)-8-methoxy-4-oxo-1,4-dihydroquinoline-3-carboxamide (RS-126) as the most potent candidate for evaluation as a CB2 PET ligand. [(18) F]RS-126 was obtained in ≥ 99% radiochemical purity with an average specific radioactivity of 98 GBq/μmol at the end of the radiosynthesis. [(18) F]RS-126 showed a logD7.4 value of 1.99 and is stable in vitro in rat and human plasma over 120 min, whereas 55% intact parent compound was found in vivo in rat blood plasma at 10 min post injection. In vitro autoradiographic studies with CB2-positive rat spleen tissue revealed high and blockable binding which was confirmed in in vivo displacement experiments with rats by dynamic PET imaging. Ex vivo biodistribution studies confirmed accumulation of [(18) F]RS-126 in rat spleen with a specificity of 79% under blocking conditions. The moderate elevated CB2 levels in LPS-treated mice brain did not permit the detection of CB2 by [(18) F]RS-126 using PET imaging. In summary, [(18) F]RS-126 demonstrated high specificity toward CB2 receptor in vitro and in vivo and is a promising radioligand for imaging CB2 receptor expression. Cannabinoid receptor type 2 (CB2) is an interesting target for PET imaging. Specific binding of [(18) F]RS-126 in CB2-positive spleen tissue (white arrow head) was confirmed in in vivo displacement experiments with rats. Time activity curve of [(18) F]RS-126 in the spleen after the addition of GW405833 (CB2 specific ligand, green) demonstrates faster radiotracer elimination (blue) compared to the tracer only (red).

Molecular Targets for PET Imaging of Activated Microglia: The Current Situation and Future Expectations

“…In 2015, Slavik et al reported a novel carbon-11 radiolabeled tracer 11 C- RS -016 for CB2R imaging, which showed higher specific binding in postmortem ALS patient spinal cord tissues [79,80]. Since then, several other recently synthetized radiotracers are under preclinical investigations and seem to offer promising prospects for imaging CB2R expression [115,116,117]. …”

Radioligands for positron emission tomography imaging of cannabinoid type 2 receptor

“…In addition to the 2‐oxoquinoline derivatives mentioned above, three 4‐oxoquinoline derivatives with different substitutions at the N‐1 position have been developed for the PET imaging of CB2 receptors (Figure 1C) . Similar to other radiotracers already mentioned, specific binding of these 4‐oxoquinoline radioligands to CB2 receptors was successfully demonstrated in in vitro autoradiography and in vivo PET scans again using CB2‐positive spleen .…”

“…In addition to the 2‐oxoquinoline derivatives mentioned above, three 4‐oxoquinoline derivatives with different substitutions at the N‐1 position have been developed for the PET imaging of CB2 receptors (Figure 1C) . Similar to other radiotracers already mentioned, specific binding of these 4‐oxoquinoline radioligands to CB2 receptors was successfully demonstrated in in vitro autoradiography and in vivo PET scans again using CB2‐positive spleen . Furthermore, the N‐1 pentyl derivative, [ 11 C]KD2, and the N ‐ethoxyethyl derivative, [ 11 C]RS‐016, showed specific binding in in vitro autoradiography performed on spinal cord slices derived from ALS patients, suggesting a potential use of CB2 radioligands in this specific neurodegenerative disease …”

“…Structures of compounds A , [ 11 C]A‐836339 (Yao et al, Horti et al, Savonenko et al, and Pottier et al) and [ 18 F]1 (Moldovan et al); B , [ 11 C]NE40 (Evens et al, Ahmad et al, and Vandeputte et al) and [ 11 C]KP23 (Mu et al); C , [ 11 C]KD2 (Mu et al), [ 11 C]RS‐016 (Slavik et al and Meletta et al), and [ 18 F]RS‐126 (Slavik et al); D , RSR‐056 (Slavik et al); E , [ 18 F]‐ d 2 ‐2 (Hortala et al); F , [ 18 F]3 (Yrjölä et al)…”

Radiosynthesis of a Novel 11C‐Labeled Derivative of 4’‐O‐Methylhonokiol and Its Preliminary Evaluation in an LPS Rat Model of Neuroinflammation

“…The radioligand, 7 was chemically stable for up to 40 min, because no radioactive degradation or biotransformation products in human plasma after 40 min incubation time were detected (Supporting information). The shake‐flask method was applied to measure the distribution of [ 11 C]MPbP in n‐octanol/phosphate buffer pH 7.4 . The obtained logD 7.4 value 2.46±0.12 (n=5) suggests that the radioligand is sufficiently lipophilic for passive diffusion through the blood‐brain barrier (BBB).…”