Discovery and Labeling of High-Affinity 3,4-Diarylpyrazolines as Candidate Radioligands for In Vivo Imaging of Cannabinoid Subtype-1 (CB₁) Receptors

Abstract

Imaging of cannabinoid subtype-1 (CB1) receptors in vivo with positron emission tomography (PET) is likely to be important for understanding their role in neuropsychiatric disorders and for drug development. Radioligands for imaging with PET are required for this purpose. We synthesized new ligands from a 3,4-diarylpyrazoline platform of which (-)-12a ((-)-3-(4-chlorophenyl)-N’-[(4-cyanophenyl)sulfonyl]-4-phenyl-4,5-dihydro-1H-pyrazole-1-carboxamidine) was found to have high-affinity and selectivity for binding to CB1 receptors. (-)-12a and its lower affinity enantiomer ((+)-12a) were labeled with carbon-11 (t1/2 = 20.4 min) using [11C]cyanide ion as labeling agent and evaluated as PET radioligands in cynomolgus monkey. After injection of [11C](-)-12a there was high uptake and retention of radioactivity across brain according to the rank order of CB1 receptor densities. The distomer, [11C](+)-12a, failed to give a sustained CB1 receptor-specific distribution. Polar radiometabolites of [11C](-)-12a appeared moderately slowly in plasma. Radioligand [11C](-)-12a is promising for the study of brain CB1 receptors and merits further investigation in human subjects.

In vitro and in vivo evaluation of 11C-SD5024, a novel PET radioligand for human brain imaging of cannabinoid CB1 receptors

“…11 C-SD5024 was prepared from 11 C-cyanide ion as labeling agent (Donohue et al, 2008) as detailed in the Investigational New Drug Application (112094) submitted to the U.S. Food and Drug Administration and which is now available at: http://pdsp.med.unc.edu/snidd/. The radioligand was obtained with high radiochemical purity (99.7%) and a specific activity of 41±31 GBq/μmol at times of injection ( n =4 batches).…”

“…We recently developed a novel CB 1 receptor ligand labeled with 11 C from a 3,4-diarylpyrazoline structural class, namely 11 C-SD5024, [ cyano - 11 C](−)-3-(4-chlorophenyl)- N -[(4-cyanophenyl)sulfonyl]-4-phenyl-4,5-dihydro-1 H -pyrazole-1-carboxamidine (Donohue et al, 2008). The purposes of this study were two folds, first, to evaluate both in vitro and in vivo the ability of 11 C-SD5024 to quantify CB 1 receptors, and second, to evaluate the utility of 11 C-SD5024 relative to other published ligands, particularly the 11 C-labeled ones.…”

Pyrazoles with a “click” 4-[N-(4-fluorobutyl)-1,2,3-triazole] substituent in position 3 are nanomolar CB1 receptor ligands

“…The crude product was purified by flash chromatography (Hexane/EtOAc 8:2) to give compound 3 4, 14.0, 52.6, 63.1, 128.6, 133.5, 135.3, 142.4, 160.4, 187.5, 190 (4) The a,g-diketoester 3 (4.63 g, 20.51 mmol) was dissolved in absolute EtOH (36 mL) and 2,4-dichlorophenylhydrazine hydrochloride (4.38 g, 20.51 mmol) was added in one portion, then the mixture was refluxed overnight. The solvent was removed under reduced pressured and the crude product was purified by flash chromatography (Hexane/EtOAc 8:2 (d,1H,J = 8.5 Hz),7.46 (d,1H, J = 2.2 Hz); 13 C NMR (100 MHz, CDCl 3 ) d: 9.9, 14.5, 61.0, 120.0, 127.2, 127.7, 127.8, 128.6, 128.9, 130.0, 131.0, 133.9, 136.0, 136.3, 137.8, 142.9, 162.7 (5) Compound 4 (2.21 g, 1.31 mmol) was dissolved in acetonitrile (4.5 mL) and the solution was cooled to 0 8C. NBS (0.39 g, 2.23 mmol) was added in small portions, than the mixture was stirred overnight at r.t. A saturated solution of Na 2 S 2 O 3 (5 mL) was added and the solvent was removed under reduced pressure.…”

“…The crude product was purified by flash chromatography (Hexane/EtOAc 6:4) to give triazole 8 (0.12 g, 55%) as a white solid. Rf 0.24 (Hexane/ (d,114.5,121.3,128.1,128.9,130.3,130.4,131.2,134.1,136.2,136.3,140.2,142.4,144.1,147.0,150.5;MS (ESI) …”

“…The iodinated derivative 9 was obtained (570 mg, 74%) as a yellow oil and was used without further purification. Rf 0.84 (Hexane/EtOAc 7:3); 1 H NMR (400 MHz, CDCl 3 ) d: 2.16 (s, 3H), 6.51 (d, 1H, J = 3.9 Hz), 6.83 (d,1H,J = 3.9 Hz),7.25 (m,2H),7.38 (m,1H); MS (ESI), calculated m/z C 16 H 8 79 )-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazol-3-yl]-1-(4-fluorobutyl)-5-iodo-1H-1,2,3-triazole (10) 1-Azido-4-fluorobutane (400 mg, 0.37 mmol,) CuI (40 mg, 0.19 mmol) and TEA (10 mL, 0.74 mmol) were added to a solution of propargyl iodide 9 (200 mg, 0.37 mmol) in THF (5 mL). The reaction was stirred at r.t. for 72 h, then quenched with a 10% aqueous solution of NH 4 OH (10 mL) and the solvent was removed under reduced pressure.…”