Distribution of sigma receptors in EMT-6 cells: preliminary biological evaluation of PB167 and potential for in-vivo PET

σ1 and σ2 receptors have been detected in many tissues and are highly expressed in several tumour cell lines from various tissues. The high level of expression observed for σ receptors and their involvement in cell proliferation and apoptosis has led to the development of several σ ligands in order to obtain a molecular probe for in‐vivo diagnostic imaging techniques such as positron emission tomography (PET) and single photon emission computerized tomography (SPECT). The EMT‐6 cells implanted in mice were a good model for evaluating the proliferation of solid tumours by in‐vivo PET. Moreover, we developed the σ ligand PB167, a cyclohexylpiperazine derivative, previously evaluated for σ2 receptor affinity and activity in standard protocols. The related results encouraged us to verify if this compound could be developed as a radiotracer for in‐vivo PET in order to visualize σ2 receptors expressed in EMT‐6 cells when implanted in mice. This perspective was thought to be favourable because PB167 bears a methoxy substituent on the tetraline nucleus, an easy point for 11C labelling. The aims of this preliminary study were both to assess the relative distribution of σ1 and σ2 receptors in EMT‐6 cells and to verify if PB167 could be developed as a σ2 radiotracer for in‐vivo PET. The results showed that both σ1 and σ2 receptors were overexpressed in EMT‐6 cells and that the ligand PB167 can be positively considered for radiosynthesis preparation in order to suitably visualize σ2 receptors by the in‐vivo PET technique and correlate their presence to tumour proliferation.