N-(4-Cyanotetrahydro-2H-pyran-4-yl) and N-(1-Cyanocyclohexyl) Derivatives of 1,5-Diarylpyrazole-3-carboxamides Showing High Affinity for 18 kDa Translocator Protein and/or Cannabinoid Receptors

• Published in: Journal of medicinal chemistry Vol. 54; no. 8; pp. 2961 - 2970
• Main Authors: Donohue, Sean R, Dannals, Robert F, Halldin, Christer, Pike, Victor W
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 28-04-2011
• Subjects: Carrier Proteins - antagonists & inhibitors; Humans; Ligands; Magnetic Resonance Spectroscopy; Mass Spectrometry; Medicin och hälsovetenskap; Models, Chemical; Pyrazoles - chemistry; Pyrazoles - pharmacology; Radioligand Assay; Receptor, Cannabinoid, CB1 - drug effects; Structure-Activity Relationship
• ISSN: 0022-2623; 1520-4804; 1520-4804
• DOI: 10.1021/jm2000536

In order to develop improved radioligands for imaging brain CB1 receptors with positron emission tomography (PET) based on rimonabant (5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-N-(piperidin-1-yl)-1H-pyrazole-3-carboxamide, 1), we synthesized compounds 9a−s in which the N-piperidinyl ring was replaced with a 4-(4-cyanotetrahydro-2H-pyranyl) or 1-cyanocyclohexyl ring. Such changes were expected to be almost isosteric with 1, confer greater metabolic resistance, and in the case of the 4-(4-cyanotetrahydro-2H-pyranyl) compounds, substantially reduce lipophilicity. One derivative, 1-(2-bromophenyl)-N-(1-cyanocyclohexyl)-5-(4-methoxyphenyl)-4-methylpyrazole-3-carboxamide (9n), showed high affinity (K i = 15.7 nM) and selectivity for binding to CB1 receptors. The corresponding 4-(4-cyanotetrahydro-2H-pyranyl) derivative (9m) also showed quite high affinity for CB1 receptors (K i = 62 nM) but was found to have even higher affinity (K i = 29 nM) for the structurally unrelated 18 kDa translocator protein (TSPO). Some other minor structural changes among 9a−s were also found to switch binding selectivity from CB1 receptors to TSPO or vice versa. These unexpected findings and their implications for the development of selective ligands or PET radioligands for CB1 receptors or TSPO are discussed in relation to current pharmacophore models of CB1 receptor and TSPO binding sites.