The proof of concept of 2-{3-[N-(1-benzylpiperidin-4-yl)propyl]amino}-6-[N-methyl-N-(prop-2-yn-1-yl)amino]-4-phenylpyridine-3,5-dicarbonitrile for the therapy of neuropathic pain

In the search for new small molecules for the therapy of neuropathic pain, we found that 2-{3-[N-(1-benzylpiperidin-4-yl)propyl]amino}-6-[N-methyl-N-(prop-2-yn-1-yl)amino]-4-phenylpyridine-3,5-dicarbonitrile (12) induced a robust antiallodynic effect in capsaicin-induced mechanical allodynia, a behavioural model of central sensitization, through σ1R antagonism. Furthermore, administration of compound 12 to neuropathic animals, fully reversed mechanical allodynia, increasing its mechanical threshold to levels that were not significantly different from those found in paclitaxel-vehicle treated mice or from basal levels before neuropathy was induced. Ligand 12 is thus a promising hit-compound for the therapy of neuropathic pain.

A conformational rearrangement of the SARS-CoV-2 host protein sigma-1 is required for antiviral activity: insights from a combined in-silico/in-vitro approach

The development of effective drugs to treat coronavirus infections remains a significant challenge for the scientific community. Recent evidence reports on the sigma-1 receptor (S1R) as a key druggable host protein in the SARS-CoV-1 and SARS-CoV-2 interactomes and shows a potent antiviral activity against SARS-CoV-2 for the S1R antagonist PB28. To improve PB28 activity, we designed and tested a series of its analogues and identified a compound that is fourfold more potent against SARS-CoV-2 than PB28 itself. Interestingly, we found no direct correlation between S1R affinity and SARS-CoV-2 antiviral activity. Building on this, we employed comparative induced fit docking and molecular dynamics simulations to gain insights into the possible mechanism that occurs when specific ligand-protein interactions take place and that may be responsible for the observed antiviral activity. Our findings offer a possible explanation for the experimental observations, provide insights into the S1R conformational changes upon ligand binding and lay the foundation for the rational design of new S1R ligands with potent antiviral activity against SARS-CoV-2 and likely other viruses.

Development of Fluorescent 4-[4-(3H-Spiro[isobenzofuran-1,4'-piperidin]-1'-yl)butyl]indolyl Derivatives as High-Affinity Probes to Enable the Study of σ Receptors via Fluorescence-Based Techniques

Sigma (σ) receptor subtypes, σ₁ and σ₂, are targets of wide pharmaceutical interest. The σ₂ receptor holds promise for the development of diagnostics and therapeutics against cancer and Alzheimer's disease. Nevertheless, little is known about the mechanisms activated by the σ₂ receptor. To contribute to the exploitation of its therapeutic potential, we developed novel specific fluorescent ligands. Indole derivatives bearing the N-butyl-3H-spiro[isobenzofuran-1,4'-piperidine] portion were functionalized with fluorescent tags. Nanomolar-affinity fluorescent σ ligands, spanning from green to red to near-infrared emission, were obtained. Compounds 19 (σ pan affinity) and 29 (σ₂ selective), which displayed the best compromise between pharmacodynamic and photophysical properties, were investigated in flow cytometry, confocal, and live cell microscopy, demonstrating their specificity for the σ₂ receptor. To the best of our knowledge, these are the first red-emitting fluorescent σ₂ ligands, validated as powerful tools for the study of σ₂ receptors via fluorescence-based techniques.

2-Aminopyridine - an unsung hero in drug discovery

2-Aminopyridine is a simple, low molecular weight and perfectly functionalised moiety known for the synthesis of diverse biological molecules. Many pharmaceutical companies across the globe aim to synthesise low-molecular weight molecules for use as pharmacophores against various biological targets. 2-Aminopyridine can serve as a perfect locomotive in the synthesis and pulling of such molecules towards respective pharmacological goals. The major advantage of this moiety is its simple design, which can be used to produce single products with minimum side reactions. Moreover, the exact weight of synthesised compounds is low, which enables facile identification of toxicity-causing metabolites in drug discovery programmes. This manuscript is a quick review of such pharmacophores derived from 2-aminopyridine.

QSAR-Based Computational Approaches to Accelerate the Discovery of Sigma-2 Receptor (S2R) Ligands as Therapeutic Drugs

S2R overexpression is associated with various forms of cancer as well as both neuropsychiatric disorders (e.g., schizophrenia) and neurodegenerative diseases (Alzheimer’s disease: AD). In the present study, three ligand-based methods (QSAR modeling, pharmacophore mapping, and shape-based screening) were implemented to select putative S2R ligands from the DrugBank library comprising 2000+ entries. Four separate optimization algorithms (i.e., stepwise regression, Lasso, genetic algorithm (GA), and a customized extension of GA called GreedGene) were adapted to select descriptors for the QSAR models. The subsequent biological evaluation of selected compounds revealed that three FDA-approved drugs for unrelated therapeutic indications exhibited sub-1 uM binding affinity for S2R. In particular, the antidepressant drug nefazodone elicited a S2R binding affinity Ki = 140 nM. A total of 159 unique S2R ligands were retrieved from 16 publications for model building, validation, and testing. To our best knowledge, the present report represents the first case to develop comprehensive QSAR models sourced by pooling and curating a large assemblage of structurally diverse S2R ligands, which should prove useful for identifying new drug leads and predicting their S2R binding affinity prior to the resource-demanding tasks of chemical synthesis and biological evaluation.

PB28, the Sigma-1 and Sigma-2 Receptors Modulator With Potent Anti-SARS-CoV-2 Activity: A Review About Its Pharmacological Properties and Structure Affinity Relationships

These unprecedented times have forced the scientific community to gather to face the COVID-19 pandemic. Efforts in diverse directions have been made. A multi-university team has focused on the identification of the host (human) proteins interacting with SARS-CoV-2 viral proteins, with the aim of hampering these interactions that may cause severe COVID-19 symptoms. Sigma-1 and sigma-2 receptors surprisingly belong to the “druggable” host proteins found, with the pan-sigma receptor modulator PB28 displaying the most potent anti–SARS-CoV-2 activity in in vitro assays. Being 20-fold more active than hydroxychloroquine, without cardiac side effects, PB28 is a promising antiviral candidate worthy of further investigation. Our research group developed PB28 in 1996 and have thoroughly characterized its biological properties since then. Structure–affinity relationship (SAfiR) studies at the sigma receptor subtypes were also undertaken with PB28 as the lead compound. We herein report our knowledge of PB28 to share information that may help to gain insight into the antiviral action of this compound and sigma receptors, while providing structural hints that may speed up the translation into therapeutics of this class of ligands.

Discovery of 2-aminopyridine Derivatives with Antichagasic and Antileishmanial Activity Using Phenotypic Assays

Background:

Compounds previously studied as anticancer were screened against trypomastigotes to access the bioactivity. The epimastigote form of Trypanosoma cruzi Y strain and the promastigote form of Leishmania amazonensis and Leishmania infantum were used in this work.

Methods:

Cell-based assays were performed to access the bioactivity of the compounds using MTT and the flow cytometry methods.

Results:

Neq0438, Neq0474 and Neq0440 had the highest potency, with EC50 of 39 μM (L. amazonensis), 52 μM (T. cruzi) and 81 μM (T. cruzi), respectively. These molecules were inactive for Balb/C fibroblast cell line at concentrations above 250 μM, showing selectivity for the parasites.

Conclusion:

This is the first report that demonstrates antiparasitic activity for the 2-aminopyridine scaffold, with cross-activity against cancer cells.

Multifunctional thiosemicarbazones and deconstructed analogues as a strategy to study the involvement of metal chelation, Sigma-2 (σ2) receptor and P-gp protein in the cytotoxic action: In vitro and in vivo activity in pancreatic tumors

The aggressiveness of pancreatic cancer urgently requires more efficient treatment options. Because the sigma-2 (σ2) receptor was recently proposed as a promising target for pancreatic cancer therapy, we explored our previously developed multifunctional thiosemicarbazones, designed to synergistically impair cell energy levels, by targeting σ2 and P-gp proteins and chelating Iron. A deconstruction approach was herein applied by removing one function at a time from the potent multifunctional thiosemicarbazones 1 and 2, to investigate the contribution to cytotoxicity of each target involved. The results from in vitro (panel of pancreatic tumor cells) and in vivo experiments (C57BL/6 bearing KP02 tumor), suggest that while the multifunctional activity was not required for the antitumor activity of these thiosemicarbazones, σ2-targeting appeared to allow alternative tumor cell death mechanisms, leading to potent and less toxic off-targets toxicities compared to other thiosemicarbazones devoid of σ2-targeting.

Quantum Dot Based Luminescent Nanoprobes for Sigma-2 Receptor Imaging

The increasing importance of sigma-2 receptor as target for the diagnosis and therapy of tumors paves the way for the development of innovative optically traceable fluorescent probes as tumor cell contrast and therapeutic agents. Here, a novel hybrid organic-inorganic nanostructure is developed by combining the superior fluorescent properties of inorganic quantum dots (QDs), coated with a hydrophilic silica shell (QD@SiO₂ NPs), the versatility of the silica shell, and the high selectivity for sigma-2 receptor of the two synthetic ligands, namely, the 6-[(6-aminohexyl)oxy]-2-(3-(6,7-dimethoxy-3,4-dihydroisoquinolin-2(1H)-yl)propyl)-3,4-dihydroisoquinolin-1(2H)-one (MLP66) and 6-[1-[3-(4-cyclohexylpiperazin-1-yl)propyl]-1,2,3,4-tetrahydronaphthalen-5-yloxy]hexylamine (TA6). The proposed nanostructures represent a challenging alternative to all previously studied organic small fluorescent molecules, based on the same sigma-2 receptor affinity moieties. Flow cytometry and confocal fluorescence microscopy experiments, respectively, on fixed and living cancerous MCF7 cells, which overexpress the sigma-2 receptor, prove the ability of functionalized (QD@SiO₂-TA6 and QD@SiO₂-MLP66) NPs to be internalized and demonstrate their affinity to the sigma-2 receptor, ultimately validating the targeting properties conveyed to the NPs by sigma-2 ligand conjugation. The presented QD-based nanoprobes possess a great potential as in vitro selective sigma-2 receptor imaging agent and, consequently, could provide a significant impact to future theranostic applications.

The σ1 receptor agonist (+)-pentazocine increases store-operated Ca2+ entry in MCF7σ1 and SK-N-SH cell lines

BACKGROUND

The intracellular [Ca²⁺] is modulated by σ receptors. An important component of the cellular machinery governing the intracellular [Ca²⁺] is Store-Operated Calcium Entry (SOCE). Here we want to investigate whether ligands of σ receptors affect SOCE.

METHODS

The intracellular [Ca²⁺] was monitored, with the fluorescent Ca²⁺-sensitive probe Fura-2, in four cell lines with a different expression of σ receptors, namely MCF7 (expressing σ₁ receptors with a low density and overexpressing σ₂ receptors), MCF7σ₁ (overexpressing σ₁ receptors), SK-N-SH, and HT-29.

RESULTS

When thapsigargin was used to deplete intracellular Ca²⁺ stores, in a Ca²⁺-free incubation medium, the Ca²⁺ influx (following Ca²⁺ re-addition) was significantly increased by 1μM (+)-pentazocine (σ₁ receptor agonist) in MCF7σ₁ (by 22.5%) and SK-N-SH (by 45.6%), but not in HT-29 and MCF7 cells. We have used, as a second approach, the "Mn²⁺ quenching" protocol. In MCF7σ₁ cells, after thapsigargin treatment, the fluorescence quenching induced by Mn²⁺ influx (evidence of Ca²⁺ influx) was significantly increased (by 25.8%) by 1μM (+)-pentazocine, significantly decreased (by 18.0%) by BD1063 (σ₁ receptor antagonist), and not affected by the presence of both ligands. These effects were not observed in MCF7 cells. Finally, in MCF7 cells, 1μM PB28 (σ₂ receptor agonist), did not affect both the Ca²⁺ response after Ca²⁺ re-addition and the fluorescence quenching induced by Mn²⁺ influx.

CONCLUSIONS

We propose that the σ₁ receptor agonist (+)-pentazocine increases SOCE in MCF7σ₁ and SK-N-SH cell lines. The σ₂ receptor agonist PB28 does not affect SOCE in MCF7 cells.

Development of sigma-1 (σ1) receptor fluorescent ligands as versatile tools to study σ1 receptors

Despite their controversial physiology, sigma-1 (σ1) receptors are intriguing targets for the development of therapeutic agents for central nervous system diseases. With the aim of providing versatile pharmacological tools to study σ1 receptors, we developed three σ1 fluorescent tracers by functionalizing three well characterized σ1 ligands with a fluorescent tag. A good compromise between σ1 binding affinity and fluorescent properties was reached, and the σ1 specific targeting of the novel tracers was demonstrated by confocal microscopy and flow cytometry. These novel ligands were also successfully used in competition binding studies by flow cytometry, showing their utility in nonradioactive binding assays as an alternative strategy to the more classical radioligand binding assays. To the best of our knowledge these are the first σ1 fluorescent ligands to be developed and successfully employed in living cells, representing promising tools to strengthen σ1 receptors related studies.

Novel and Selective Fluorescent σ2 -Receptor Ligand with a 3,4-Dihydroisoquinolin-1-one Scaffold: A Tool to Study σ2 Receptors in Living Cells

Although sigma-2 (σ2 ) receptors are still enigmatic proteins, they are promising targets for tumor treatment and diagnosis. With the aim of clarifying their role in oncology, we developed a σ2 -selective fluorescent tracer (compound 5) as a specific tool to study σ2 receptors. By using flow cytometry with 5, we performed competition binding studies on three different cell lines where we also detected the content of the σ2 receptors, avoiding the inconvenient use of radioligands. Comparison with a previously developed mixed σ1 /σ2 fluorescent tracer (1) also allowed for the detection of σ1 receptors within these cells. Results obtained by flow cytometry with tracers 1 and 5 were confirmed by standard methods (western blot for σ1 , and Scatchard analysis for σ2 receptors). Thus, we have produced powerful new tools for research on the σ whose reliability and adaptability to a number of fluorescence techniques will be useful to elucidate the roles of σ receptors in oncology.

Sulphuric acid immobilized on silica gel (H2SO4–SiO2) as an eco-friendly catalyst for transamidation

A novel method of transamidation of carboxamides with amines using catalytic amounts of H₂SO₄–SiO₂ under solvent-free conditions has been developed. The scope of the methodology has been demonstrated with primary and secondary amines.

Deconstruction of 6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline moiety to separate P-glycoprotein (P-gp) activity from σ2 receptor affinity in mixed P-gp/σ2 receptor agents

6,7-Dimethoxytetrahydroisoquinoline is widely used as basic moiety in σ2 receptor ligands, in order to provide σ2versus σ1 selectivity. This same moiety is also widely exploited in modulators of P-glycoprotein (P-gp) efflux pump, so that mixed σ2/P-gp agents are often obtained. Deconstruction of 6,7-dimethoxytetrahydroisoquinoline moiety present in the potent mixed σ2/P-gp agent 6,7-dimethoxy-2-[4-[1-(4-fluorophenyl)-1H-indol-3-yl]butyl]-1,2,3,4-tetrahydroisoquinoline (1) could lead to the separation of σ2 affinity from P-gp activity. Therefore, phenethylamino-, benzylamino- and indanamine series were obtained. The NH group was also methylated in the N-phenethylamino series, and ethylated in the benzylamino series, to better match 6,7-dimethoxytetrahydroisoquinoline. The σ2 affinity drastically decreased with the increase of conformational freedom, whereas alkylation of the NH-group was beneficial for σ2 receptor interaction. By contrast, deconstruction of 6,7-dimethoxytetrahydroisoquinoline slightly reduced P-gp activity, with dimethoxy-substituted derivatives displaying potent P-gp interaction. Therefore, 'ring-opened' 6,7-dimethoxytetrahydroisoquinoline derivatives represent a promising strategy to obtain P-gp selective agents devoid of σ2 receptor affinity.

Novel derivatives of 1-cyclohexyl-4-[3-(5-methoxy-1,2,3,4-tetrahydronaphthalen-1-yl)propyl]piperazine (PB28) with improved fluorescent and σ receptors binding properties

Despite the promising potentials of σ2 receptors in cancer therapy and diagnosis, there are still ambiguities related to the nature and physiological role of the σ2 protein. With the aim of providing potent and reliable tools to be used in σ2 receptor research, we developed a novel series of fluorescent σ2 ligands on the basis of our previous work, where high-affinity σ2 ligand 1-cyclohexyl-4-[3-(5-methoxy-1,2,3,4-tetrahydronaphthalen-1-yl)-n-propyl]piperazine (1, PB28) was used as the pharmacophore. Compared to the previous compounds, these novel ligands displayed improved fluorescence and σ2 binding properties, were σ2-specifically taken up by breast tumor cells, and were successfully employed in confocal microscopy. Compound 14, which was the best compromise between pharmacological and fluorescent properties, was successfully employed in flow cytometry, demonstrating its potential to be used as a tool in nonradioactive binding assays for studying the affinity of putative σ2 receptor ligands.

Sigma-2 receptor agonists as possible antitumor agents in resistant tumors: hints for collateral sensitivity

With the aim of contributing to the development of novel antitumor agents, high-affinity σ2 receptor agonists were developed, with 6,7-dimethoxy-2-[4-[1-(4-fluorophenyl)-1H-indol-3-yl]butyl]-1,2,3,4-tetrahydroisoquinoline (15) and 9-[4-(6,7-dimethoxy-1,2,3,4-tetrahydroisoquinolin-2-yl)butyl]-9H-carbazole (25) showing exceptional selectivity for the σ2 subtype. Most of the compounds displayed notable antiproliferative activity in human MCF7 breast adenocarcinoma cells, with similar activity in the corresponding doxorubicin-resistant MCF7adr cell line. Surprisingly, a few compounds, including 25, displayed enhanced activity in MCF7adr cells over parent cells, recalling the phenomenon of collateral sensitivity, which is under study for the treatment of drug-resistant tumors. All of the compounds showed interaction with P-glycoprotein (P-gp), and 15 and 25, with the greatest activity, were able to revert P-gp-mediated resistance and reestablish the antitumor effect of doxorubicin in MCF7adr cells. We therefore identified a series of σ2 receptor agonists endowed with intriguing antitumor properties; these compounds deserve further investigation for the development of alternate strategies against multidrug- resistant cancers.

Sigma-2 receptor ligands and their perspectives in cancer diagnosis and therapy

The sigma-2 receptor is highly expressed in various rapidly proliferating cancer cells and regarded as a cancer cell biomarker. Selective sigma-2 ligands have been shown to specifically label the tumor sites, induce cancer cells to undergo apoptosis, and inhibit tumor growth. Sigma-2 ligands are potentially useful as cancer diagnostics, anticancer therapeutics, or adjuvant anticancer treatment agents. However, both the cloning of this receptor and the identification of its endogenous ligand have not been successful, and the lack of structural information has severely hindered the understanding of its physiological roles, its signaling pathways, and the development of more selective sigma-2 ligands. Recent data have implicated that sigma-2 binding sites are within the lipid rafts and that PGRMC1 (progesterone receptor membrane component 1) complex and sigma-2 receptor may be coupled with EGFR (epidermal growth factor receptor), mTOR (mammalian target of rapamycin), caspases, and ion channels. Due to its promising applications in cancer management, there are rapidly increasing research efforts that are being directed into this field. This review article updates the current understanding of sigma-2 receptor and its potential physiological roles, applications, interaction with other effectors, with special focuses on the development of sigma-2 ligands, their chemical structures, pharmacological profiles, applications in imaging and anticancer therapy.