Pharmacological characterization of the cannabinoid CB1 receptor PET ligand ortholog, [3H]MePPEP

Abstract MePPEP ((3 R ,5 R )-5-(3-methoxy-phenyl)-3-((R)-1-phenyl-ethylamino)-1-(4-trifluoromethyl-phenyl)-pyrrolidin-2-one) is an inverse agonist shown to be an effective PET ligand for labeling cannabinoid CB 1 receptors in vivo. [ 11 C]MePPEP and structurally related analogs have been reported to specifically and reversibly label cannabinoid CB 1 receptors in rat and non-human primate brains, and [ 11 C]MePPEP has been used in human subjects as a PET tracer. We have generated [ 3 H]MePPEP, an ortholog of [ 11 C]MePPEP, to characterize the molecular pharmacology of the cannabinoid CB 1 receptor across preclinical and clinical species. [ 3 H]MePPEP demonstrates saturable, reversible, and single-site high affinity binding to cannabinoid CB 1 receptors. In cerebellar membranes purified from brains of rat, non-human primate and human, and cells ectopically expressing recombinant human cannabinoid CB 1 receptor, [ 3 H]MePPEP binds cannabinoid CB 1 receptors with similar affinity with K d values of 0.09 nM, 0.19 nM, 0.14 nM and 0.16 nM, respectively. Both agonist and antagonist cannabinoid ligands compete [ 3 H]MePPEP with predicted rank order potency. No specific binding is present in autoradiographic sections from cannabinoid CB 1 receptor knockout mouse brains, demonstrating that [ 3 H]MePPEP selectively binds cannabinoid CB 1 receptors in native mouse tissue. Furthermore, [ 3 H]MePPEP binding to anatomical sites in mouse and rat brain is comparable to the anatomical profiles of [ 11 C]MePPEP in non-human primate and human brain in vivo, as well as the binding profiles of other previously described cannabinoid CB 1 receptor agonist and antagonist radioligands. Therefore, [ 3 H]MePPEP is a promising tool for translation of preclinical cannabinoid CB 1 receptor pharmacology to clinical PET ligand and cannabinoid CB 1 receptor inverse agonist therapeutic development.