Structural Changes To Sn79 Result In Ligands That Stimulate A Metabolic Function Of Sigma-2 Receptors

Sigma-2 receptors have been of interest as targets for imaging, diagnosis, and treatment of cancer nearly since their discovery. These membrane-bound proteins are highly upregulated in cancer cells compared to healthy tissue, with further upregulation in rapidly proliferating cancer cells compared to quiescent cells. Activation of sigma-2 receptors by traditional agonists induces programmed cell death. However, this function of sigma-2 receptors is inconsistent with upregulation in cancer cells, which is typically associated with survival benefit. We have recently shown the ability of sigma-2 receptors to induce an independent, non-toxic, metabolically stimulative function that is consistent with survival benefit to both cancer cells and rapidly proliferating normal cells, indicated initially by an increase in MTT reduction. CM764, a novel analog of the sigma-2 antagonist SN79, increased total NADH/NAD+ and ATP levels, decreased basal ROS, and increased VEGF levels via induction of HIF1a expression in SK-N-SH neuroblastoma cells (Nicholson et al., JPET, 2015). We also previously described single-element structural changes to the core structure of SN79 and the effects of these changes on sigma receptor binding affinity and ability to induce cell death (Nicholson et al., Proc. Amer. Assoc. Cancer Res. 75: #2440, 2015). Here, we describe further characterization of the metabolic stimulatory effect induced by binding of members of this series to sigma-2 receptors in SK-N-SH neuroblastoma cells. Substitution of the oxazolone oxygen or methyl ketone moieties of the heterocyclic ring system of the SN79 core structure resulted in ligands that all retained significant binding affinity for sigma-2 receptors. Drastic changes in cell viability effects were observed in response to changes to the methyl ketone position. As previously reported, -NCS substitutions to the methyl ketone position resulted in cytotoxic ligands. By contrast, substitutions of the SN79 methyl ketone moiety with an -NH2, either alone (CM571) or combined with heterocyclic ring oxygen atom substitution for an -NMe group (WA403), resulted in ligands that stimulated reduction of MTT, consistent with the metabolically stimulative phenotype previously described. This phenotype was also observed with -NMe and -S- substitutions to the heterocyclic ring oxygen atom without combined substitution of the methyl ketone group (NF7 and WA504, respectively), and for the nitro-substituted methyl ketone group alone (CM458). All stimulative ligands demonstrated maximal increase in MTT reduction after 24 h incubation at 30 μM, with WA504 exhibiting the greatest stimulation of 161% of the amount of MTT reduced by cells that were not treated. These data further support a dual role for sigma-2 receptor activation, inducing both pro-survival and apoptotic pathways, and highlight the impact of single-element changes to the binding and action of SN79 analogs at sigma-2 receptors. Citation Format: Hilary Elaine Nicholson, Walid Alsharif, Christopher R. McCurdy, Wayne D. Bowen. Structural changes to SN79 result in ligands that stimulate a metabolic function of sigma-2 receptors. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3804.