Abstract 2440: Evaluation of structural changes in SN79-derived sigma-2 receptor modulators: effect on apoptotic efficacy in SK-N-SH neuroblastoma

Sigma receptors are a unique class of membrane bound receptors with key roles in mediating cell survival. The sigma-2 subtype has received attention due to its significant upregulation in rapidly proliferating tumor cells as compared to non-cancerous cells. When activated, the sigma-2 receptor induces apoptosis. Previously, we described the use of -NCS derivatives of SN79, a moderately sigma-2 selective antagonist (Ki = 290 and 30 nM at sigma-1 and sigma-2, respectively), to induce apoptosis in breast, brain, and pancreatic cancer cell lines (Nicholson et al., Proc. Amer. Assoc. Cancer Res. 74: #3237, 2014). CM572, a potent selective partial agonist of the sigma-2 receptor (Ki = >10μM and 15nM at sigma-1 and sigma-2, respectively), was found to irreversibly bind and induce apoptosis in several cancer cell lines. Here we present a series of heterocyclic ring structural variants of CM572 and our evaluation of these ligands for use as potential chemotherapeutics. When the methyl ketone group on the heterocyclic ring of SN79 was substituted with -NO2, the resulting ligand showed low or no selectivity (≤2-fold) for the sigma-2 receptor over the sigma-1 receptor, while -NCS substituted ligands showed higher selectivity (≥10-fold) for the sigma-2 receptor. Substitution of the -O- in the heterocyclic ring of SN79 with -S- resulted in excellent sigma-2 receptor affinity but greatly reduced selectivity, suggesting potential interaction at this position in the binding pocket of the sigma-1 receptor. Substituting this position with -N-Me significantly increased selectivity by reducing sigma-1 receptor affinity, suggesting that steric hindrance rendered this position less amenable to sigma-1 receptor binding. -NCS derivatives of SN79 were cytotoxic to SK-N-SH neuroblastoma, while substitution of -NCS for -NO2 reduced efficacy, potency, and eliminated irreversible binding capability. Substitution of the -NCS group with -NH2 eliminated agonist properties, rendering these ligands unable to induce apoptosis up to 50 μM. Interestingly, while high sigma-2 receptor affinity was retained for -NH2-substituted ligands, they were unable to attenuate agonist activity at the sigma-2 receptor, suggesting poor antagonist activity compared to the parent compound. This data implies that the presence of an electron withdrawing group on the heterocyclic ring imparts agonist activity. We conclude that the greatest potential for a potent, highly efficacious, selective sigma-2 receptor -targeted chemotherapeutic agent will result from an SN79 derivative containing an -NCS at the 4 position of the aromatic moiety and an N-Me group in the heterocyclic ring. Future work will focus on the effects of these compounds in tumor xenograft models across a variety of cancer types, as well as using structural variants to predict characteristics of the binding pockets of sigma-1 and sigma-2 receptors. Citation Format: Hilary Elaine Nicholson, Walid Alsharif, Christopher R. McCurdy, Wayne D. Bowen. Evaluation of structural changes in SN79-derived sigma-2 receptor modulators: effect on apoptotic efficacy in SK-N-SH neuroblastoma. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2440. doi:10.1158/1538-7445.AM2015-2440