[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 (K_d) of 0.66 ± 0.12 nM in rat liver membrane homogenates, which was 30-fold higher than that of [ ³H]RHM-2 (K_d = 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 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.