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

MePPEP ((3R,5R)-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 CB1 receptors in vivo. [11C]MePPEP and structurally related analogs have been reported to specifically and reversibly label cannabinoid CB1 receptors in rat and non-human primate brains, and [11C]MePPEP has been used in human subjects as a PET tracer. We have generated [3H]MePPEP, an ortholog of [11C]MePPEP, to characterize the molecular pharmacology of the cannabinoid CB1 receptor across preclinical and clinical species. [3H]MePPEP demonstrates saturable, reversible, and single-site high affinity binding to cannabinoid CB1 receptors. In cerebellar membranes purified from brains of rat, non-human primate and human, and cells ectopically expressing recombinant human cannabinoid CB1 receptor, [3H]MePPEP binds cannabinoid CB1 receptors with similar affinity with Kd values of 0.09 nM, 0.19 nM, 0.14 nM and 0.16 nM, respectively. Both agonist and antagonist cannabinoid ligands compete [3H]MePPEP with predicted rank order potency. No specific binding is present in autoradiographic sections from cannabinoid CB1 receptor knockout mouse brains, demonstrating that [3H]MePPEP selectively binds cannabinoid CB1 receptors in native mouse tissue. Furthermore, [3H]MePPEP binding to anatomical sites in mouse and rat brain is comparable to the anatomical profiles of [11C]MePPEP in non-human primate and human brain in vivo, as well as the binding profiles of other previously described cannabinoid CB1 receptor agonist and antagonist radioligands. Therefore, [3H]MePPEP is a promising tool for translation of preclinical cannabinoid CB1 receptor pharmacology to clinical PET ligand and cannabinoid CB1 receptor inverse agonist therapeutic development.