Structure-Affinity relationships of novel σ₂R/TMEM97 ligands

The sigma 2 receptor (σ₂R), which was recently identified as the transmembrane protein 97 (TMEM97), is increasingly attracting interest as a possible therapeutic target for indications in neuroscience. Toward identifying novel modulators of σ₂R/TMEM97, we prepared a collection of benzoxazocine, benzomorphan, and methanobenzazepine ligands related to the known bioactive norbenzomorphans DKR-1677, FEM-1689, and EES-1686 and determined their K_i values for σ₂R/TMEM97 and the sigma 1 receptor (σ₁R). The σ₂R/TMEM97 binding affinities and selectivities relative to σ₁R of these new benzoxazocine, benzomorphan, and methanobenzazepine analogs are lower, often significantly lower, than their respective norbenzomorphan counterparts, suggesting the spatial orientation of pharmacophoric substituents is critical for binding to the two proteins. The benzoxazocine, benzomorphan, and methanobenzazepine congeners of DKR-1677 and FEM-1689 tend to be weakly selective for σ₂R/TMEM97 versus σ₁R, whereas EES-1686 derivatives exhibit the greatest selectivity, suggesting the size and/or nature of the substituent on the nitrogen atom of the scaffold may be important for selectivity. Computational docking studies were performed for the 1S,5R-and 1R,5S-enantiomers of DKR-1677, FEM-1689, and EES-1686 and their benzoxazocine, benzomorphan, and methanobenzazepine counterparts. These computations predict that the protonated amino group of each ligand forms a highly conserved salt bridge and a H-bonding interaction with Asp29 as well as a cation-π interaction with Tyr150 of σ₂R/TMEM97. These electrostatic interactions are major driving forces for binding to σ₂R/TMEM97 and are similar, though not identical, for each ligand. Other interactions within the well-defined binding pocket also tend to be comparable, but there are some major differences in how the hydrophobic aryl groups of various ligands interact with the protein surface external to the binding pocket. Overall, these studies show that the orientations of aryl and N-substituents on the norbenzomorphan and related scaffolds are important determinants of binding affinity of σ₂R/TMEM97 ligands, and small changes can have significant effects upon binding profiles.