[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 (σ2) receptors was evaluated in vitro. [3H]RHM-1 was found to have a higher affinity for σ2 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 Koff rate since both radioligands have similar Kon rates. Competitive binding assays were also conducted using a panel of compounds with known affinity for σ2 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 σ2 receptors in vitro.