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 (Ki = 4.6 nM) over σ2 (Ki = 63.1 nM) sites in guinea pig brain homogenates. Further, FE‐SA4503 exhibits the same 14‐fold selectivity for σ1 (Ki = 8.0 nM) over σ2 (Ki = 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 pKi 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. Synapse 59:350–358, 2006. © 2006 Wiley‐Liss, Inc.