σ₁ and σ₂ receptor binding affinity and selectivity of SA4503 and fluoroethyl SA4503

SA4503, a potent σ₁ receptor agonist, is reported as having 103-fold higher affinity for σ₁ (IC₅₀ = 17.4 nM) than σ₂ (IC₅₀ = 1784 nM) sites in guinea pig brain membranes. Modest structural changes appear to have major effects on σ₁/σ₂ selectivity. The fluoroethyl analog, FE-SA4503, is described as having high primary affinity for σ₂ sites (IC₅₀ = 2.11 nM) and a weaker interaction with σ₁ sites (IC₅₀ = 6.48 nM). SA4503 and FE-SA4503 have been radiolabeled for PET studies, and both bind selectively to σ₁ receptors in animal and human brain in vivo. We prepared SA4503 and FE-SA4503 as reference compounds for radioligand development efforts. In our hands, SA4503 is 14-fold selective for σ₁ (K _i = 4.6 nM) over σ₂ (K_i = 63.1 nM) sites in guinea pig brain homogenates. Further, FE-SA4503 exhibits the same 14-fold selectivity for σ₁ (K_i = 8.0 nM) over σ₂ (K_i = 113.2 nM) receptors. The main differences from previously reported values stem from σ₂ affinity determinations. This protocol, displacement of [³H]DTG binding to σ₂ sites using (+)-pentazocine (200 nM) to mask σ₁ sites, was validated by the proper rank order of σ₂ inhibitory potencies shown by a panel of additional ligands: ifenprodil > haloperidol > DTG ≫ (+)-pentazocine. Robust Pearson correlation (r = 1.0, P = 0.002; slope = 0.97) was observed for our pK _i values against those from a prior study by others. The findings have bearing on structure-activity relationships for this active series, and on conclusions that might be drawn from experiments relying upon defined σ₁/σ₂ binding selectivity.