(-)-3-(4-Chlorophenyl)-N'-[(4-[¹¹C]cyanophenyl)sulfonyl]-4-phenyl-4,5-dihydro-1H-pyrazole-1-carboxamidine

[[¹¹C]CB1-(-)-12a]

Background

[PubMed]

There are two subtypes of cannabinoid receptors in mammalian tissues: CB1 and CB2 (1, 2). CB1 receptors are expressed abundantly in neuronal terminals in the central nervous system (CNS) and some peripheral tissues to inhibit neurotransmitter release. CB1 receptors are found predominantly in the striatum, hippocampus, substantia nigra, globus pallidus, and cerebellum. The CB2 receptors are present mainly on immune cells to modulate cytokine release. Both receptor subtypes are coupled through G_i/o proteins to inhibit adenylate cyclase and to modulate potassium and calcium channels. CB1 receptors have been demonstrated to be involved in analgesia, regulation of food intake, and control of movement in normal subjects (3). Alteration of CB1 receptor function has been implicated in a number of human diseases such as depression, schizophrenia, and obesity (4-6).

Δ9-Tetrahydrocannabinol (THC) is a major active cannabinoid that is found in marijuana and activates CB1 receptors (7). THC has a very high lipophilicity (log D_7.4 value of 7), which causes imaging studies using radiolabeled THC to be unsuccessful because of slow entry into the brain and high nonspecific binding in the brain. However, a high lipophilicity is essential for binding to CB1 receptors, and an optimal lipophilicity (log D_7.4 1–4) is required for crossing the blood–brain barrier (BBB). Existing radiolabeled ligands are mainly analogs of the antagonist rimonabant (SR141716A) and the agonist WIN 55,212-2, which also exhibit high nonspecific binding and lipophilicity, limiting their application in imaging (8). Therefore, there is a need to lower the lipophilicity of the CB1 radioligands with little effect on binding affinity and ability to cross the BBB. (-)-3-(4-chlorophenyl)-N'-[(4-[¹¹C]cyanophenyl)sulfonyl]-4-phenyl- 4,5-dihydro-1H-pyrazole-1-carboxamidine ([¹¹C]CB1-(-)-12a) is a potent and selective CB1 antagonist with nanomolar affinity and lower lipophilicity than SR141716A (9). [¹¹C]CB1-(-)-12a is being developed as a positron emission tomography (PET) agent for the non-invasive study of CB1 receptors in the brain.

Synthesis

[PubMed]

Donohue et al. (9) reported synthesis of [¹¹C]CB1-(+/-)-12a by reaction of the bromo precursor (-)-3-(4-chlorophenyl)-N'-[(4-bromophenyl)sulfonyl]-4-phenyl- 4,5-dihydro-1H-pyrazole-1-carboxamidine with [¹¹C]HCN in DMSO in the presence of KOH, Pd(PPh₃)₄ and K2.2.2 at 135°C for 5 min. An average radiochemical yield was 36% with a total synthesis time of 30 min after purification by high-performance liquid chromatography. The specific activity was 56 GBq/μmol (1.51 Ci/μmol; n = 2) at the end of synthesis with a radiochemical purity of >97%. Using NaHCO₃ instead of KOH gave [¹¹C]CB1-(-)-12a to [¹¹C]CB1-(+)-12a in a 9:1 ratio. Using the (+)-iodo precursor and KH₂PO₄ gave [¹¹C]CB1-(+)-12a to [¹¹C]CB1-(-)-12a in a 9.7: 0.3 ratio. [¹¹C]CB1-(+)-12a and [¹¹C]CB1-(-)-12a were obtained in >94% chiral purity.

In Vitro Studies: Testing in Cells and Tissues

[PubMed]

Donohue et al. (9) reported that CB1-(-)-12a had inhibition constant (K_i) values of 0.5 ± 0.1 nM for CB1 and >5,000 nM for CB2. [¹¹C]CB1-(+)-12a showed K_i values of 16.9 ± 2.0 nM for CB1 and >5,000 nM for CB2. Each enantiomer exhibited a Clog D_7.4 (lipophilicity) value of 3.85.

Animal Studies

Human Studies

[PubMed]

No publication is currently available.

NIH Support

Intramural research program