3H]N-[4-(3,4-dihydro-6,7-dimethoxyisoquinolin-2(1H)-yl)butyl]-2-methoxy-5-methylbenzamide: a novel sigma-2 receptor probe

N-4-(3,4-dihydro-6,7-dimethoxyisoquinolin-2(1H)-yl)butyl]-2-methoxy-5-methyl-benzamide (RHM-1) and N-2-(3,4-dihydro-6,7-dimethoxyisoquinolin-2(1H)-yl)ethyl]-2-methoxy-5-methylbenzamide (RHM-2), two conformationally flexible benzamide analogues, were radiolabeled with tritium (specific activity=80 Ci/mmol) and the binding of 3H]RHM-1 and 3H]RHM-2 to sigma-2 (sigma2) receptors was evaluated in vitro. 3H]RHM-1 was found to have a higher affinity for sigma2 receptors compared to 3H]RHM-2 and 3H]1,3-di-o-tolylguanidine (3H]DTG). 3H]RHM-1 had a dissociation constant (Kd) of 0.66+/-0.12 nM in rat liver membrane homogenates, which was 30-fold higher than that of 3H]RHM-2 (Kd=19.48+/-0.51 nM). The lower affinity of 3H]RHM-2 can be attributed to its faster K(off) rate since both radioligands have similar K(on) rates. Competitive binding assays were also conducted using a panel of compounds with known affinity for sigma2 receptors. The pharmacologic profile of 3H]RHM-1 was in agreement with that of 3H]DTG. The results of this study indicate that 3H]RHM-1 is a useful ligand for studying sigma2 receptors in vitro.