Development of LC-MS/MS-based receptor occupancy tracers and positron emission tomography radioligands for the nociceptin/orphanin FQ (NOP) receptor

J Med Chem. 2012 Jun 14;55(11):4955-67. doi: 10.1021/jm201629q. Epub 2012 May 16.


Currently, a lack of sufficient tools has limited the understanding of the relationship between neuropsychiatric disorders and the nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor. Herein, we describe the discovery and development of an antagonist NOP receptor occupancy (RO) tracer and a novel positron emission tomography (PET) radioligand suitable to probe the NOP receptor in human clinical studies. A thorough structure-activity relationship (SAR) around the high-affinity 3-(2'-fluoro-4',5'-dihydrospiro[piperidine-4,7'-thieno[2,3-c]pyran]-1-yl)-2-(2-halobenzyl)-N-alkylpropanamide scaffold identified a series of subnanomolar, highly selective NOP antagonists. Subsequently, these unlabeled NOP ligands were evaluated in vivo by liquid chromatography-tandem mass spectrometry (LC-MS/MS) in rat to determine brain uptake, kinetics and specific binding. (S)-27 was identified as a suitable unlabeled preclinical RO tracer to accurately quantify NOP receptor engagement in rat brain. Three compounds were selected for evaluation in nonhuman primates as PET tracers: (-)-26, (-)-30, and (-)-33. Carbon-11 labeling of (+)-31 yielded [(11)C]-(S)-30, which exhibited minimal generation of central nervous system (CNS) penetrant radiometabolites, improved brain uptake, and was an excellent PET radioligand in both rat and monkey. Currently [(11)C]-(S)-30 is being evaluated as a PET radiotracer for the NOP receptor in human subjects.

MeSH terms

  • Animals
  • Brain / diagnostic imaging
  • Brain / metabolism
  • CHO Cells
  • Carbon Radioisotopes
  • Chromatography, Liquid
  • Cricetinae
  • Cricetulus
  • HEK293 Cells
  • Humans
  • Macaca
  • Male
  • Narcotic Antagonists
  • Nociceptin Receptor
  • Positron-Emission Tomography
  • Radioligand Assay
  • Radiopharmaceuticals / chemical synthesis*
  • Radiopharmaceuticals / chemistry
  • Radiopharmaceuticals / pharmacokinetics
  • Rats
  • Rats, Sprague-Dawley
  • Receptors, Opioid / agonists
  • Receptors, Opioid / metabolism*
  • Spiro Compounds / chemical synthesis*
  • Spiro Compounds / chemistry
  • Spiro Compounds / pharmacokinetics
  • Stereoisomerism
  • Structure-Activity Relationship
  • Tandem Mass Spectrometry
  • Thiophenes / chemical synthesis*
  • Thiophenes / chemistry
  • Thiophenes / pharmacokinetics


  • 2-((2-fluorophenyl)methyl)-3-(2-fluorospiro(4,5-dihydrothieno(2,3-c)pyran-7,4'-piperidine)-1'-yl)-N-methylpropanamide
  • Carbon Radioisotopes
  • Narcotic Antagonists
  • Radiopharmaceuticals
  • Receptors, Opioid
  • Spiro Compounds
  • Thiophenes
  • Nociceptin Receptor