Divergent Cytotoxic and Metabolically Stimulative Functions of Sigma-2 Receptors: Structure-Activity Relationships of 6-Acetyl-3-(4-(4-(4-fluorophenyl)piperazin-1-yl)butyl)benzo[d]oxazol-2(3H)-one (SN79) Derivatives

Abstract

Sigma-2 receptors, recently identified as TMEM97, have been implicated in cancer and neurodegenerative disease. Structurally distinct sigma-2 receptor ligands induce cell death in tumor cells, linking sigma-2 receptors to apoptotic pathways. Recently, we reported that sigma-2 receptors can also stimulate glycolytic hallmarks, effects consistent with a prosurvival function and upregulation in cancer cells. Both apoptotic and metabolically stimulative effects were observed with compounds related to the canonical sigma-2 antagonist SN79. Here we investigate a series of 6-substituted SN79 analogs to assess the structural determinants governing these divergent effects. Substitutions on the benzoxazolone ring of the core SN79 structure resulted in high-affinity sigma-2 receptor ligands (K i 5 0.56-17.9 nM), with replacement of the heterocyclic oxygen by N-methyl (producing N-methylbenzimidazolones) generally decreasing sigma-1 affinity and a sulfur substitution (producing benzothiazolones) imparting high affinity at both subtypes, lowering subtype selectivity. Substitution at the 6-position with COCH 3 , NO 2 , NH 2 , or F resulted in ligands that were not cytotoxic. Five of these ligands induced an increase in metabolic activity, as measured by increased reduction of MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) in human SK-N-SH neuroblastoma cells, further supporting a role for sigma-2 receptors in metabolism. Substitution with 6-isothiocyanate resulted in ligands that were sigma-2 selective and that irreversibly bound to the sigma-2 receptor, but not to the sigma-1 receptor. These ligands induced cell death upon both acute and continuous treatment (EC 50 5 7.6-32.8 mM), suggesting that irreversible receptor binding plays a role in cytotoxicity. These ligands will be useful for further study of these divergent roles of sigma-2 receptors. Part of this work was presented as follows: Nicholson HE, Alsharif W, McCurdy CR, and Bowen WD (2015) Evaluation of structural changes in SN79derived sigma-2 receptor modulators: effect on apoptotic efficacy in SK-N-SH neuroblastoma.

Proceedings from the Fourth International Symposium on σ-2 Receptors: Role in Health and Disease

“…Dr. Bowen and colleagues have identified analogs of the canonical S2R antagonist SN79. Some of these are able to induce apoptotic cell death, while others display a novel metabolically stimulative effect (Nicholson et al, 2015(Nicholson et al, , 2016(Nicholson et al, , 2018. This metabolic effect is characterized by increased reductive capacity as indicated by stimulation of MTT reduction, increase in cellular ATP level, reduction in basal ROS level, and stabilization of HIF-1α, as determined in human SK-N-SH neuroblastoma cells (Nicholson et al, 2016).…”

Development of New Benzylpiperazine Derivatives as σ1 Receptor Ligands with in Vivo Antinociceptive and Anti-Allodynic Effects

“… 18 Conversely, dysregulation of the physiological activities of σRs has been observed in several medical conditions, including drug addiction, neuropsychiatric disorders, cancer, and chronic pain. 18 − 22 With an improving understanding of σRs, σR ligands have recently attracted increased attention from the scientific community for their potential as new medications to treat unmet medical needs, 23 − 25 including the novel coronavirus disease 2019 (COVID-19) as recently reported. 26 , 27 Notably, novel σR selective ligands are currently under evaluation in clinical trials as diagnostic agents (i.e., PET radiotracers) and therapeutic efficacy for some of the diseases mentioned above.…”

Recent Advances in the Development of Sigma Receptor Ligands as Cytotoxic Agents: A Medicinal Chemistry Perspective

“…Finally, other non-isothiocyanate derivatives, including SN79, possibly acting as putative σ 2 R antagonists, were tagged as “neutral” since they produced neither programmed cell death nor metabolic stimulation. 167 …”