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 monkeys. 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.

Figures

Figure 1

Structures of CB1 receptor ligands (1-3).

Figure 2

Structures of [11C]4-6, [18F]6, [18F]7 and [11C]8.

Figure 3

Regional time-radioactivity curves after i.v. injection of [11C](±)-12a in cynomolgus monkey under baseline condition (Panel A), with 6 (1 mg/kg, i.v.) administered as a displacing agent at 25 min (Panel B), or pretreatment condition with 6 (1 mg/kg, i.v.) (Panel C). Key: ×, striatum; △, cerebellum; ◇, frontal cortex; □, lateral temporal cortex; +, thalamus; ○, medial temporal cortex; ▲, pons.

Figure 4

Regional time-radioactivity curves after i.v. injection of [11C](-)-12a (100 MBq) (Panel A) or [11C](+)-12a (98 MBq) (Panel B) in cynomolgus monkey. Key: ×, striatum; △, cerebellum; ◇, frontal cortex; □, lateral temporal cortex; +, thalamus; ○, medial temporal cortex; ▲, pons.

Figure 5

Horizontal PET images, obtained at the level of the striatum from data acquired between 9 and 93 min after injection of [11C](-)-12a (100 MBq, Panel A) or [11C](+)-12a (98 MBq, Panel B).

Figure 6

Radio-HPLC of plasma of [11C](-)-12a in cynomolgus monkey (15 min after injection, Panel A), and time course of radioactivity in plasma represented by parent radioligand and radiometabolite fractions (Panel B). Key: □, [11C](-)-12a; △, metabolite a; ▽, metabolite b; ○, metabolite c.

Scheme 1

Synthesis of 3,4-diarylpyrazoline derivatives. Conditions: a) methyl chloroformate, TEA, MeCN; b) 3-(4-chlorophenyl)-4,5-dihydro-4-phenyl-1H-pyrazole, toluene, reflux; c) chlorobenzene, PCl5; d) methanolic NH3; e) ChiralPak AD, MeCN, 8 mL/min for 12a and 12b and 6 mL/min for 12c.

Scheme 2

Radiosynthesis of [11C](±)-12a. Conditions, reagents and decay-corrected yield: a) Pt, NH3 (20-30 mL/min), 990 °C. b) 50% H2SO4, 90 °C; c) Pd(PPh3)4, KOH, K2.2.2, DMSO, 110 °C, 36% (n = 2).

Scheme 3

Proposed mechanism for the epimerization of [11C](-)-12a under labeling conditions.