Synthesis and in vivo evaluation of [123I]-4-iodo-N-(4-(4-(2-methoxyphenyl)-piperazin-1-yl)butyl)-benzamide, a potential sigma receptor ligand for SPECT studies

Aim: Sigma receptors which are expressed in the brain as well as in endocrine and immune systems have been the focus of research in the past few years due to their implicated role in psychosis. Many widely used antipsychotics interact with sigma receptors, some exhibit sigma receptor antagonism as their predominant mode of action. There is evidence that sigma receptors modulate several neuroreceptors, including dopaminergic and other catecholaminergic systems. Furthermore there are indications that a decrease of cortical sigma receptors occurs in schizophrenia. They are also present in high densities in various human and rodent cancer cell lines. In light of these findings we report the synthesis and in vivo evaluation of a 123I-labelled selective sigma ligand. Materials and Methods: The 123I-labelled compound was synthesized by electrophilic iododestannylation of the tributyltin derivative. For biodistribution studies 37 kBq of the 123I-labelled compound dissolved in a mixture water/ethanol (99/1) was injected i.v. into the tail vein of NMRI mice. At various time points p.i. the mice were sacrificed and dissected. Biodistribution studies were performed until 48 hours p.i.. For blocking studies the mice were injected with cold product (1mg/kg) 10 minutes before tracer injection. Regional brain distributions were carried out in New Zealand rabbits, for this study 9,25 MBq of the 123I-labelled compound dissolved in a mixture water/ethanol (90/10) was injected i.v. into the ear vein. At various time points up to 1 hour post injection the rabbits were sacrificed and their brain was dissected. Subsequently a regional blocking study was preformed in rabbits using the sigma ligand 1-(3-fluoropropyl)-4-(4-cyanophenoxymethyl)-piperidine (FPS)(0,5 mg/kg) which was injected 5 minutes before tracer injection. Results: Radiochemical yield was 70% ± 5%. Radiochemical purity was >95%. Biodistribution studies showed penetration through the blood brain barrier and accumulation in the brain (ratio brain to blood after 10 min: 10). Pretreatment with cold product resulted in a decrease of accumulation of the tracer in the brain (ratio brain to blood after 10 min: 1.6). As expected the regional brain distribution showed a homogeneous distribution throughout the brain, pretreatment with FPS resulted in a decrease of the uptake in different brain regions (cortex: 91%, striatum: 88% and hypothalamus: 89% decrease). Conclusion: Both biodistribution and blocking studies in mice and rabbits show that 123I-4-iodo-N-(4-(4-(2-methoxyphenyl)-1-piperazinyl)butyl)-benzamide is a potential tracer for in vivo visualization of the sigma receptor