Novel Orthosteric/Allosteric Ligands of Cannabinoid Receptors: An Unexpected Pharmacological Profile

The design of dualsteric/bitopic receptor ligands as compounds capable of simultaneously interacting with both the orthosteric and an allosteric binding site has gained importance to achieve enhanced receptor specificity and minimize off-target effects. In this work, we reported the synthesis and biological evaluation of a new series of compounds, namely, the RF series, obtained by chemically combining the CB1R ago-positive allosteric modulators (PAM) GAT211 with the cannabinoid receptors (CBRs) orthosteric agonist FM6b. Therefore, RF compounds were designed as dualsteric/bitopic ligands for hCB1R with the aim of obtaining stronger hCB1R agonists or ago-PAMs, with improved receptor subtype selectivity and reduction of central side effects. Unexpectedly, in vitro assays on hCB1R indicated RF compounds were inverse agonists/antagonists, exhibiting different profiles compared to those of parent compounds FM6b and GAT211 and, furthermore, two compounds behaved as hCB2R PAMs. The unpredictable change in the function of these new ligands suggests that the function of cannabinoids is not simply predicted.