Cannabinoid Receptor Type 2 (CB2) Selective N-Aryl-oxadiazolyl-propionamides: Synthesis, Radiolabeling, Molekular Modelling and Biological Evaluation

Background: The endocannabinoid system is involved in many physiological and pathological processes. Two receptors (cannabinoid receptor type 1, CB1, and type 2, CB2) are known so far. Many unwanted psychotic side effects of inhibitors of this system can be addressed to the interaction with CB1. While CB1 is one of the most abundant neuroreceptors, CB2 is expressed in the brain only at very low levels. Thus, high potent and selective compounds for CB2 are desired. N-Aryl-((hetero)aromatic)-oxadiazolyl-propionamides represent a promising class of such selective ligands for the human CB2. Here, a library of various derivatives is studied for suitable routes for labelling with [18F]. Such [18F]-labelled compounds can then be employed as CB2 selective radiotracers for molecular imaging studies employing positron emission tomography (PET).
Results: By varying the N-arylamide substructure we could explore the binding pocket of the human CB2 receptor and identified the 9-ethyl-9H-carbazole amide as the group with optimal size. Radioligand replacement experiments revealed that the modification of the (hetero)aromatic moiety in 3-position of the 1,2,4-oxadiazoles shows only moderate impact on affinity to CB2 but high impact on selectivity towards the CB2 with respect to CB1. Further, we could show by autoradiography studies, that the most promising compounds bind selectively on CB2 receptors in mouse spleen tissue. Molecular docking studies based in a novel 3D structural model of the human CB2 receptor in its activated form indicate that the compounds bind with the N-arylamide substructure in the binding pocket. [18F]-labelling at (hetero)aromatic moiety at the opposite site of the compounds via radiochemistry were carried out.
Conclusions: The synthesized selective CB2 compounds have high affinity towards CB2 and good selectivity against CB1 receptors. The introduction of labelling groups at the (hetero)aromatic moiety shows only moderate impact on CB2 affinity, indicating the introduction of potential labelling groups at this position as a promising approach to develop selective CB2 ligands suitable for molecular imaging with PET. The high affinity for human CB2 and selectivity against human CB1 of the herein presented compounds renders them as suitable candidates for molecular imaging studies.