Systematic evaluation of a panel of 30 synthetic cannabinoid receptor agonists structurally related to MMB-4en-PICA, MDMB-4en-PINACA, ADB-4en-PINACA, and MMB-4CN-BUTINACA using a combination of binding and different CB1 receptor activation assays. Part III: The G protein pathway and critical comparison of different assays.

The present work is the last of a three-part study investigating a panel of 30 systematically designed synthetic cannabinoid receptor agonists (SCRAs) including features such as the 4-pentenyl tail and varying head groups including amides and esters of L-valine (MMB, AB), L-tert-leucine (ADB), and L-phenylalanine (APP), as well as adamantyl (A) and cumyl moieties (CUMYL). Here, we evaluated these SCRAs for their capacity to activate the human cannabinoid receptor 1 (CB1 ) via indirect measurement of G protein recruitment. Furthermore, we comparatively evaluated the results obtained from three in vitro assays, based on the recruitment of β-arrestin 2 (βarr2 assay) or Gαi protein (mini-Gαi assay), or binding of [35 S]-GTPγS. The observed efficacies (Emax ) varied depending on the conducted assay. Statistical analysis suggests that the population means of the relative intrinsic activity (RAi ) significantly differ for the [35 S]-GTPγS assay and the other two assays, but the population means of the βarr2 and mini-Gαi assays were not statistically different. Our data suggests that differences observed between the βarr2 and mini-Gαi assays are the best predictor for 'biased agonism' towards βarr or G protein recruitment in our study. SCRAs carrying an ADB or MPP moiety as a head group tended to produce elevated Emax values in the βarr2 assay, which might result in a tendency of these compounds to cause pronounced tolerance in users - a hypothesis that should be evaluated further by future studies. In general, a comparison of efficacies derived from different assays is difficult and should only be conducted very cautiously.