Reevaluation of fenpropimorph as a σ receptor ligand: Structure-affinity relationship studies at human σ₁ receptors

Fenpropimorph (1) is considered a "super high-affinity" σ₁ receptor ligand (K_i=0.005nM for guinea pig σ₁ receptors). Here, we examine the binding of 1 and several of its deconstructed analogs at human σ₁ (hσ₁) receptors. We monitored their subtype selectivity by determining the binding affinity at σ₂ receptors. In addition, we validated an existing pharmacophore model at the molecular level by conducting 3D molecular modeling studies, using the crystal structure of hσ₁ receptors, and Hydrophatic INTeractions (HINT) analysis. Our structure affinity relationship studies showed that 1 binds with lower affinity at hσ₁ receptors (K_i=17.3nM) compared to guinea pig; moreover, we found that none of the fenpropimorph methyl groups is important for its binding at hσ₁ receptors, nor is stereochemistry. For example, removal of all methyl groups as seen in 4 resulted in an almost 5-fold higher affinity at hσ₁ receptors compared to 1 and 350-fold selectivity versus σ₂ receptors. In addition, although the O atom of the morpholine ring does not contribute to affinity at hσ₁ receptors (and might even detract from it), it plays role in subtype (σ₁ versus σ₂ receptor) selectivity.