In vitro and in vivo evaluation of [ 125/123 I]-2-[4-(2-iodophenyl)piperidino]cyclopentanol([ 125/123 I]-OI5V) as a potential sigma-1 receptor ligand for SPECT

Introduction We investigated the characteristics of radio-iodinated 2-[4-(2-iodophenyl)piperidino]cyclopentanol (OI5V) as a single photon emission computed tomography (SPECT) ligand for mapping sigma-1 receptor (σ-1R), which plays an important role in stress remission in many organs. Methods OI5V was synthesized from o-bromobenzaldehyde in three steps. OI5V was evaluated for its affinity to VAChT, σ-1 and σ-2 receptor by in vitro competitive binding assays using rat tissues and radioligands, [ 3 H]vesamicol, ( +)-[ 3 H]pentazocine and [ 3 H]DTG, respectively. [ 125/123 I]OI5V was prepared from o -trimethylstannyl-cyclopentanevesamicol (OT5V) by the iododestannylation reaction under no-carrier-added conditions. In vivo biodistribution study of [ 125 I]OI5V in blood, brain regions and major organs of rats was performed at 2, 10, 30 and 60 min post-injection. In vivo blocking study and ex vivo autoradiography were performed to assess the binding selectivity of [ 125 I]OI5V for σ-1 receptor. SPECT-CT imaging study was performed using [ 123 I]OI5V. Results OI5V demonstrated high selective binding affinity for σ-1R in vitro. In the biodistribution study, the blood–brain barrier (BBB) permeability of [ 125 I]OI5V was high and the accumulation of [ 125 I]OI5V in the rat cortex at 2 min post-injection exceeded 2.00%ID/g. In the in vivo blocking study, the accumulation of [ 125 I]OI5V in the brain was significantly blocked by co-administration of 0.5 μmol of SA4503 and 1.0 μmol of pentazocine. Ex vivo autoradiography revealed that the regional brain accumulation of [ 125 I]OI5V was similar to σ-1R-rich regions of the rat brain. SPECT images of [ 123 I]OI5V in the rat brain reflected the distribution of sigma receptors in the brain. Conclusions This study confirmed that [ 125/123 I]OI5V selectively binds σ-1R in the rat brain in vivo. [ 123 I]OI5V was suggested to be useful as a σ-1R ligand for SPECT.