Medicinal Hypervalent Tellurium Prodrugs Bearing Different Ligands: A Comparative Study of the Chemical Profiles of AS101 and Its Halido Replaced Analogues

Exploring nickel-catalyzed organochalcogen synthesis via cross-coupling of benzonitrile and alkyl chalcogenols with computational tools

The preparation of organochalcogens has increased in recent times due to their promising biological activity properties. This work studies the reaction mechanism of a nickel(0)-catalyzed cross-coupling between benzonitrile and propanethiol to produce new C-S bonds by computational means. The proposed mechanism follows the classical oxidative addition/transmetalation/reductive elimination cross-coupling sequence, involving an unusual oxidative addition of a Ph-CN bond onto the active species. The computed catalytic cycle for thioether synthesis has been examined to determine whether the same protocol could be employed to build the analogous C-Se and C-Te bonds. The proposed mechanism for C-S coupling is validated by microkinetic modeling and shows a very good agreement with available experimental data. The extension of the proposed mechanism to C-Se and C-Te couplings indicates that these new reactions should be operative, although their reaction rates appear to be significantly slower.

Ovicidal activity of diaryl dichalcogenides and ivermectin on Fasciola hepatica: A novel candidate for a blending-based therapeutic strategy

Fasciolosis is a food and waterborne disease caused by Fasciola spp., representing a global health burden to various hosts, including humans and other animals. This study investigates the in vitro activity of tellurium- and selenium-containing diaryl dichalcogenides: diacetal ditelluride (LQ07), diacetal diselenide (LQ62), and diacetyl diselenide (LQ68) alone and in combination with ivermectin (IVM) against eggs of Fasciola hepatica. The eggs were exposed for 12 h with each organochalcogen (OC) (0.1 - 2 mmol l-1) and IVM (0.01 - 2 mmol l-1) following an incubation of 15 days, allowing embryonation. The inhibitory concentration of 50 % (IC50) of each OC or IVM was tested with the IC10, IC30, and IC50 of IVM or each OC, respectively. LQ07, LQ62, and LQ68, as well as IVM, demonstrated a concentration-dependent ovicidal activity. The peak ovicidal activity of 99.74 % was achieved when IVM was tested at 2.0 mmol l-1. LQ62 and LQ68 demonstrated greater ovicidal activity, having an IC50 < 0.32 mmol l-1 being 6.25-fold more toxic than IVM alone. The percentage of dead eggs was significantly higher in the IVM group (early mortality), as Se-containing OCs led to the (miracidia) embryonation of the eggs with no hatching (late mortality). Blending Se-containing OCs and IVM showed an additive effect of up to 27 % against F. hepatica eggs. The present data contribute to the potential use of blending-based therapeutic strategies to combat F. hepatica infections in eradication programs worldwide. The combinations may also act against multidrug-resistant strains, reinstating drug-based parasite control.

Immunomodulator AS101 restores colistin susceptibility of clinical colistin-resistant Escherichia coli and Klebsiella pneumoniae in vitro and in vivo

OBJECTIVES

Colistin (COL) was once considered to be the last line of defence against multidrug-resistant bacteria belonging to the family Enterobacteriaceae. Due to the misuse of COL, COL-resistant (COL-R) Enterobacteriaceae have emerged. To address this clinical issue and combat COL resistance, novel approaches are urgently needed.

METHODS

In this study, the in vitro and in vivo antimicrobial and antibiofilm effects of the immunomodulator AS101 were investigated in combination with COL against COL-R Escherichia coli (E. coli) and Klebsiella pneumoniae (K. pneumoniae).

RESULTS

Checkerboard assay, time-kill assay, and scanning electron microscopy confirmed the in vitro antimicrobial phenotype, whereas, crystal violet staining and multidimensional confocal laser scanning microscopy with live/dead staining confirmed the antibiofilm capability of the combination therapy. Moreover, the Galleria mellonella infection model and the mouse infection model indicated the high in vivo efficacy of the combination therapy. Additionally, cytotoxicity experiments performed using human kidney-derived HK-2 cells and haemolysis assays performed using human erythrocytes collectively demonstrated safety at effective combination concentrations. Furthermore, quantification of the expression of inflammatory cytokines via enzyme-linked immunosorbent assay confirmed the anti-inflammatory advantage of combination therapy. At the mechanistic level, changes in outer and inner membrane permeability and accumulation of ROS levels, which might be potential mechanisms for synergistic antimicrobial effects.

CONCLUSIONS

This study found that AS101 can restore COL susceptibility in clinical COL-R E. coli and K. pneumoniae and also has synergistic antibiofilm and anti-inflammatory capabilities. This study provided a novel strategy to combat clinical infections caused by COL-R E. coli and K. pneumoniae.

A tellurium-based small compound ameliorates tumor metastasis by downregulating heparanase expression

Tellurium is a rare element, and ammonium trichloro (dioxoethylene-o,o') tellurate (AS101) is the most bioactive molecule among several synthetic tellurium compounds. AS101 was found to be immunomodulatory and can modulate types of cytokines. However, the effect of AS101 on tumor metastasis remains unclear. Heparanase, an endo-glucuronidase, cleaves heparin sulfate side chains of proteoglycans on the cell surface, further leading to the degradation of the extracellular matrix. Heparanase also releases angiogenic factors in the extracellular matrix, is overexpressed in tumor cells, and promotes tumor metastasis and angiogenesis. In this study, we investigated the effect of AS101 in 4T1 and CT26 cells, especially heparanase. Heparanase expression was downregulated in 4T1 and CT26 cells after treatment with AS101 in vitro. The protein level involved in the protein kinase-B/mammalian target of rapamycin (AKT/mTOR) signaling pathway also declined. Cell migration assays revealed the inhibitory effect of AS101 on migration. The results of this study indicate that AS101 inhibits tumor migration by downregulating heparanase through the AKT/mTOR signaling pathway and has positive effects in vivo.

Unveiling the mechanism of activation of the Te(IV) prodrug AS101. New chemical insights towards a better understanding of its medicinal properties

AS101 (Ammonium trichloro (dioxoethylene-O,O') tellurate) is an important hypervalent Te-based prodrug. Recently, we started a systematic investigation on AS101 with the aim to correlate its promising biological effects as a potent immunomodulator drug with multiple medicinal applications and its specific chemical properties. To date, a substantial agreement on the rapid conversion of the initial AS101 species into the corresponding TeOCl3- anion does exist, and this latter species is reputed as the pharmacologically active one. However, we realized that TeOCl3- could quickly undergo further steps of conversion in an aqueous medium, eventually producing the TeO2 species. Using a mixed experimental and theoretical investigation approach, we characterized the conversion process leading to TeO2 occurring both in pure water and in reference buffers at physiological-like pH. Our findings may offer a valuable "chemical tool" for a better description, interpretation -and optimization- of the mechanism of action of AS101 and Te-based compounds. This might be a starting point for improved AS101-based medicinal application.

Screening of AS101 analog, organotellurolate (IV) compound 2 for its in vitro biocompatibility, anticancer, and antibacterial activities

Organotellurium compounds are being well researched as potential candidates for their functional roles in therapeutic and clinical biology. Here, we report the in vitro anticancer and antibacterial activities of an AS101 analog, cyclic zwitterionic organotellurolate (IV) compound 2 [Te^-{CH₂CH(NH₃⁺)COO}(Cl)₃]. Different concentrations of compound 2 were exposed to fibroblast L929 and breast cancer MCF-7 cell lines to study its effect on cell viability. The fibroblast cells with good viability confirmed the biocompatibility, and compound 2 also was less hemolytic on RBCs. A cytotoxic effect on MCF-7 breast cancer cell line investigated compound 2 to be anti-cancerous with IC50 value of 2.86 ± 0.02 µg/mL. The apoptosis was confirmed through the cell cycle phase arrest of the organotellurolate (IV) compound 2. Examination of the antibacterial potency compound 2 was done based on the agar disk diffusion, minimum inhibitory concentration, and time-dependent assay for the Gram-positive Bacillus subtilis and Gram-negative Pseudomonas putida. For both bacterial strains, tests were performed with the concentration range of 3.9-500 μg/mL, and the minimum inhibition concentration value was found to be 125 μg/mL. The time-dependent assay suggested the bactericidal activity of organotellurolate (IV) compound, 2 against the bacterial strains.

Speciation of Tellurium(VI) in Aqueous Solutions: Identification of Trinuclear Tellurates by 17O, 123Te, and 125Te NMR Spectroscopy

Tellurates have attracted the attention of researchers over the past decade due to their properties and as less toxic forms of tellurium derivatives. However, the speciation of Te(VI) in aqueous solutions has not been comprehensively studied. We present a study of the equilibrium speciation of tellurates in aqueous solutions at a wide pH range, 2.5-15 by ¹⁷O, ¹²³Te, and ¹²⁵Te NMR spectroscopy. The coexistence of monomeric, dimeric, and trimeric oxidotellurate species in chemical equilibrium at a wide pH range has been shown. NMR spectroscopy, DFT computations, and single-crystal X-ray diffraction studies confirmed the formation and coexistence of trimeric tellurate anions with linear and triangular structures. Two cesium tellurates, Cs₂[Te₄O₈(OH)₁₀] and Cs₂[Te₂O₄(OH)₆], were isolated from the solution at pH 5.5 and 9.2, respectively, and studied by single-crystal X-ray diffractometry, revealing dimeric and tetrameric tellurate anions in corresponding crystal structures.