Tellurium-based cysteine protease inhibitors: evaluation of novel organotellurium(IV) compounds as inhibitors of human cathepsin B

Enzymatic kinetic resolution of tert-butyl 2-(1-hydroxyethyl)phenylcarbamate, a key intermediate to chiral organoselenanes and organotelluranes

The enzymatic kinetic resolution of tert-butyl 2-(1-hydroxyethyl)phenylcarbamate via lipase-catalyzed transesterification reaction was studied. We investigated several reaction conditions and the carbamate was resolved by Candida antarctica lipase B (CAL-B), leading to the optically pure (R)- and (S)-enantiomers. The enzymatic process showed excellent enantioselectivity (E > 200). (R)- and (S)-tert-butyl 2-(1-hydroxyethyl)phenylcarbamate were easily transformed into the corresponding (R)- and (S)-1-(2-aminophenyl)ethanols.

Fe(II) and sodium nitroprusside induce oxidative stress: a comparative study of diphenyl diselenide and diphenyl ditelluride with their napthyl analog

Here, we compare the influence of molecular structural modifications of diphenyl diselenide (DPDS) and diphenyl ditelluride (DPDT) with their naphthalene analogs, 1-dinapthyl diselenide (1-NapSe)2, 2-dinapthyl diselenide (2-NapSe)2, 1-dinapthyl distelluride (1-NapTe)2, and 2-dinapthyl ditelluride (2-NapTe)2. Fe(II)-induced hepatic thiobarbituric acid reactive species (TBARS) was in the order [(2-NapTe)2] > [(2-NapSe)2] > [(DPDS)] > [(1-NapSe)2] > [(1-NapTe)2]> [(DPDT)]. For sodium nitroprusside (SNP)-induced hepatic TBARS, the order was [(2-NapTe)2] > [(DPDT)] > [(1-NapSe)2] > [(2-NapSe)2] > [(1-NapTe)2] > [(DPDS)]. For Fe(II) and SNP-induced renal TBARS, the orders were [(2-NapTe)2] > [(1-NapTe)2] = [(DPDT)] > [(1-NapSe)2] > [(2-NapSe)2] > [(DPDS)] and [(2-NapTe)2] > [(1-NapTe)2] > [(1-NapSe)2] > [(2-NapSe)2] > [(DPDS)] > [(DPDS)], respectively. The present investigation shows that DPDS was less potent and the change in the organic moiety from an aryl to napthyl group dramatically changed the potency of diselenides. These results suggest that minor changes in the organic moiety of aromatic diselenides can profoundly modify their antioxidant properties. In view of the fact that the pharmacological properties of organochalcogens are linked, at least in part, to their antioxidant properties, it becomes important to explore the pharmacological properties of dinaphtyl diselenides and ditellurides.

Poliovirus 3C proteinase inhibition by organotelluranes

The 3C proteinase, essential for human poliovirus (PV) replication, has unique characteristics as its three-dimensional structure resembles chymotrypsin, but its catalytic nucleophile is a cysteine SH group rather than the OH group of serine. Here, we describe the use of tellurium compounds as inhibitors of PV3C proteinase. A rapid, stoichiometric and covalent inactivation of PV3C was observed with both a chloro-telluroxetane and a bis-vinylic organotellurane. These compounds also inhibit human cathepsins B, L, S, and K with second order rate constants higher than those obtained for PV3C. Chloro-telluroxetane inhibits replication of PV in human embryonic rhabdomyosarcoma cells in the low micromolar range and below the toxic level for the host cells. Bis-vinylic organotellurane is more effective as antiviral agent but reduces the cell viability by 20% at 10 μm, a concentration almost completely inhibiting virus growth. This is the first description of inhibition of viral 3C proteinase with antiviral property by this class of compounds.

Comparison of the antioxidant properties and the toxicity of p,p'-dichlorodiphenyl ditelluride with the parent compound, diphenyl ditelluride

The hypothesis to be tested in this study is whether the introduction of the chloro group into diphenyl ditelluride molecule (p,p'-dichlorodiphenyl ditelluride, compound 1b) alters the antioxidant and scavenging activity of diphenyl ditelluride (compound 1a) in vitro. The results revealed that 1a and 1b had a potent antioxidant activity in vitro. However, the introduction of a functional group, chloro, into diphenyl ditelluride molecule (1b) did not cause great alterations in the antioxidant action of diphenyl ditelluride against lipid peroxidation, protein carbonyl, and scavenging of 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulfonate) and 2,2'-diphenyl-1-picrylhydrazyl radicals. Based on the in vitro results, different doses (0.25 and 0.75 µmol/kg) of 1a and 1b or vehicle (canola oil, 1 ml/kg) were administered to rats to investigate if the presence of chloro into diphenyl ditelluride molecule reduces its toxicity. The data demonstrate that the chloro group introduced into diphenyl ditelluride molecule did not alter the acute oral toxicity in rats. The administration of compound 1a in rats only altered the urea level, while compound 1b caused alterations in all toxicological parameters analyzed (alanine aminotransferase and aspartate aminotransferase activities, urea and creatinine levels) in plasma of rats. The results of the present investigation support similar antioxidant and scavenging activities of 1a and 1b in rat liver homogenate in vitro. Furthermore, the presence of chloro into diphenyl ditelluride molecule did not alter the mortality index but increased toxicity of diphenyl ditelluride in rats.

Hypervalent organochalcogenanes as inhibitors of protein tyrosine phosphatases

A series of organochalcogenanes was synthesized and evaluated as protein tyrosine phosphatases (PTPs) inhibitors. The results indicate that organochalcogenanes inactivate the PTPs in a time- and concentration-dependent fashion, most likely through covalent modification of the active site sulfur-moiety by the chalcogen atom. Consequently, organochalcogenanes represent a new class of mechanism-based probes to modulate the PTP-mediated cellular processes.

Tellurium: an element with great biological potency and potential

Tellurium has long appeared as a nearly 'forgotten' element in Biology, with most studies focusing on tellurite, tellurate and a handful of organic tellurides. During the last decade, several discoveries have fuelled a renewed interest in this element. Bioincorporation of telluromethionine provides a new approach to add heavy atoms to selected sites in proteins. Cadmium telluride (CdTe) nanoparticles are fluorescent and may be used as quantum dots in imaging and diagnosis. The antibiotic properties of tellurite, long known yet almost forgotten, have attracted renewed interest, especially since the biochemical mechanisms of tellurium cytotoxicity are beginning to emerge. The close chemical relationship between tellurium and sulfur also transcends into in vitro and in vivo situations and provides new impetus for the development of enzyme inhibitors and redox modulators, some of which may be of interest in the field of antibiotics and anticancer drug design.

A glimpse on biological activities of tellurium compounds

Tellurium is a rare element which has been regarded as a toxic, non-essential trace element and its biological role is not clearly established to date. Besides of that, the biological effects of elemental tellurium and some of its inorganic and organic derivatives have been studied, leading to a set of interesting and promising applications. As an example, it can be highlighted the uses of alkali-metal tellurites and tellurates in microbiology, the antioxidant effects of organotellurides and diorganoditellurides and the immunomodulatory effects of the non-toxic inorganic tellurane, named AS-101, and the plethora of its uses. Inasmuch, the nascent applications of organic telluranes (organotelluranes) as protease inhibitors and its applications in disease models are the most recent contribution to the scenario of the biological effects and applications of tellurium and its compounds discussed in this manuscript.

Irreversible inhibition of human cathepsins B, L, S and K by hypervalent tellurium compounds

The inhibition of human cysteine cathepsins B, L, S and K was evaluated by a set of hypervalent tellurium compounds (telluranes) comprising both organic and inorganic derivatives. All telluranes studied showed a time- and concentration-dependent irreversible inhibition of the cathepsins, and their second-order inactivation rate constants were determined. The organic derivatives were potent inhibitors of the cathepsins and clear specificities were detected, which were parallel to their known substrate specificities. In all cases, the activity of the tellurane-inhibited cathepsins was recovered by treatment of the inactivated enzymes with reducing agents. The maximum stoichiometry of the reaction between cysteine residues and telluranes were also determined. The presented data indicate that it is possible to design organic compounds with a tellurium(IV) moiety as a novel warhead that covalently modifies the catalytic cysteine, and which also form strong interactions with subsites of cathepsins B, L, S and K, resulting in more specific inhibition.

A novel organotellurium compound (RT-01) as a new antileishmanial agent

Leishmaniasis is a neglected disease and endemic in developing countries. A lack of adequate and definitive chemotherapeutic agents to fight against this infection has led to the investigation of numerous compounds. The aim of this study was to investigate the effect of RT-01, an organotellurane compound presenting biological activities, in 2 experimental systems against Leishmania amazonensis. The in vitro system consisted of promastigotes and amastigotes forms of the parasite, and the in vivo system consisted of L. amazonensis infected BALB/c mice, an extremely susceptible mouse strain. The compound proved to be toxic against promastigotes and amastigotes. The study also showed that treatment with RT-01 produces an effect similar to that treatment with the reference antimonial drug, Glucantime, in L. amazonensis infected mice. The best results were obtained following RT-01 intralesional administration (720 microg/kg/day); mice showed significant delay in the development of cutaneous lesions and decreased numbers of parasites obtained from the lesions. Significant differences in tissue pathology consisted mainly of no expressive accumulation of inflammatory cells and well-preserved structures in the skin tissue of RT-01-treated mice compared with expressive infiltration of infected cells replacing the skin tissue in lesions of untreated mice. These findings highlight the fact that the apparent potency of organotellurane compounds, together with their relatively simple structure, may represent a new avenue for the development of novel drugs to combat parasitic diseases.

Antioxidant effect of a novel class of telluroacetilene compounds: studies in vitro and in vivo

AIMS

The effect of telluroacetylenes a-d on pharmacological assays was investigated in vitro. A second objective of this study was to investigate the antioxidant action of compound b against the oxidative damage induced by sodium nitroprusside (SNP) in mouse brain.

MAIN METHODS

In in vitro experiments, lipid peroxidation (LP) and protein carbonyl (PC) levels and delta-aminolevulinate dehydratase (delta-ALA-D) activity were carried out in rat brain homogenate. The thiol peroxidase-like activity and DPPH radical scavenging of telluroacetylenes a-d were investigated. In in vivo experiments, mice received SNP (0.335 micromol per site) intra cerebroventricular (i.c.v.) thirty minutes after oral administration of telluroacetylene b (10 mg/kg). After 1 h, animals were euthanized. The levels of LP and delta-ALA-D, catalase (CAT), glutathione peroxidase (GPx), glutathione reductase (GR), glutathione S-transferase (GST) activities were carried out in mouse brain homogenate.

KEY FINDINGS

Telluroacetylenes a-d, at low muM range, reduced LP and PC levels in rat brain homogenate. Telluroacetylenes a-d showed effect of scavenging DPPH radicals. delta-ALA-D activity was inhibited by telloruacetylenes a-d, at high muM range, in rat brain homogenate. Brains of mice treated with SNP showed an increase in LP and the reduction in delta-ALA-D, GR and GST activities. Telluroacetylene b protected against the oxidative stress caused by SNP in brain of rats.

SIGNIFICANCE

The results support an antioxidant effect of telluroacetylenes a-d in vitro. Telluroacetylene b protected against oxidative damage caused by SNP in mouse brain, suggesting an antioxidant effect of this compound.

Bcl-2 expression and apoptosis induction in human HL60 leukaemic cells treated with a novel organotellurium(IV) compound RT-04

Organotellurium(IV) compounds have been reported to have multiple biological activities including cysteine protease-inhibitory activity, mainly cathepsin B. As cathepsin B is a highly predictive indicator for prognosis and diagnosis of cancer, a possible antitumor potential for these new compounds is expected. In this work, it was investigated the effectiveness of organotellurium(IV) RT-04 to produce lethal effects in the human promyelocytic leukaemia cell line HL60. Using the MTT tetrazolium reduction test, and trypan blue exclusion assay, the IC50 for the compound after 24 h incubation was 6.8 and 0.35 microM, respectively. Moreover, the compound was found to trigger apoptosis in HL60 cells, inducing DNA fragmentation and caspase-3, -6, and -9 activations. The apoptsosis-induced by RT-04 is probably related to the diminished Bcl-2 expression, observed by RT-PCR, in HL60-treated cells. In vivo studies demonstrated that the RT-04 treatment (2.76 mg/kg given for three consecutive days) produces no significant toxic effects for bone marrow and spleen CFU-GM. However, higher doses (5.0 and 10 mg/kg) produced a dose-dependent reduction in the number of CFU-GM of RT-04-treated mice. These results suggest that RT-04 is able to induce apoptosis in HL60 cells by Bcl-2 expression down-modulation. Further studies are necessary to better clarify the effects of this compound on bone marrow normal cells.

Fluorescence resonance energy transfer (FRET) peptides and cycloretro-inverso peptides derived from bradykinin as substrates and inhibitors of prolyl oligopeptidase

Prolyl oligopeptidase (POP, EC 3.4.21.26) is a member of a family of serine peptidases with post-proline cleaving activity towards peptides. It is located in the cytosol in active form but without hydrolytic activity on proteins or peptides higher than 30 amino acids. Its function is not well defined, but it is involved in central nervous system disorders. Here, we studied the substrate specificity of wild type POP (POPwt) and its C255T variant lacking the non-catalytic Cys(255). This residue is located in the seven-bladed beta-propeller domain that regulates the activity of POP. Fluorescence resonance energy transfer (FRET) peptides were used with sequences derived from bradykinin-containing region of human kininogen and flanked by Abz (ortho-aminobenzoic acid) and EDDnp [N-ethylenediamine-(2,4-dinitrophenyl)]. The peptide Abz-GFSPFRQ-EDDnp was taken as leader substrate for the synthesis of five series of peptides modified at the P(3), P(2), P'(1), P'(2) and P'(3) residues. The optimal amino acids in each position for POPwt resulted in the sequence RRPYIR that is very similar to the C-terminal sequence of neurotensin. The cyclic peptides c(G((n))FSPFR) (n=1-4) were hydrolyzed by POP; their cycloretro and cycloretro-inverso analogues were inhibitors in the micromolar range. The differences between POPwt and its C255T mutant in the hydrolysis of the series derived from Abz-GFSPFRQ-EDDnp were restricted to the non-prime site of the substrates. The kinetic data of hydrolysis and inhibition by the cyclic peptides are consistent with the structures of POP-substrate/inhibitor complexes and with the substrate specificity data obtained with linear FRET peptides. All together, these results give information about the POP-substrate/inhibitor interactions that further complete knowledge of this important oligopeptidase.

Organotellurane-promoted mitochondrial permeability transition concomitant with membrane lipid protection against oxidation

Organotelluranes exhibit potent antioxidant properties as well as the ability to react with protein thiol groups and, thereby, they are good models to study the mechanism of the mitochondrial permeability transition (MPT). We evaluated the effects of the concentration of organotelluranes, namely RT-03 and RT-04, on rat liver mitochondria. At the concentration range of 0.25-1.0 microM, organotelluranes did not cause any mitochondrial dysfunction. At the concentration range of 5-10 microM, RT-03 and RT-04 caused the Ca2+-dependent opening of the (MPT) pore, regulated by Cyclosporin A. At the concentration range of 15-30 microM the swelling was not inhibited by Cyclosporin A and in the absence of Ca2+, a significant decrease of respiratory control ratio was observed due to concomitant phosphorylation impairment and uncoupling, transmembrane potential disruption, depletion of mitochondrial reduced thiol groups, and alterations in the bilayer fluidity. Above 100 microM, the organotelluranes caused complete inhibition of respiratory chain. Over the whole studied concentration range, RT-03 and RT-04 did not induce mitochondrial oxidative stress assessed by using the reactive oxygen and nitrogen species indicator 2',7'-dichlorodihydrofluorescein diacetate. Further, the organotelluranes also exhibited protective effect against t-butyl hydroperoxide-induced oxidative stress as well as against Fe2+/citrate-induced peroxidation of mitochondrial membranes and PCPECL liposomes. These results point out that MPT pore opening can involve damage exclusively to mitochondrial membrane proteins. The exclusive antioxidant activity observed at nanomolar range is also an interesting new finding described in this work.

Neutral and positively charged thiols synergize the effect of the immunomodulator AS101 as a growth inhibitor of Jurkat cells, by increasing its uptake

The immunomodulator amonium trichloro[1,2-ethanediolato-O,O'] tellurate (AS101), a nontoxic tellurium(IV) compound, exhibited antitumoral activity in several preclinical and clinical studies. In this study, we investigated the synergism between thiols and AS101 in its antitumoral activity on Jurkat cells. AS101 induced a G2/M arrest in the cell cycle after 24h. Addition of the thiols 2-mercaptoethanol or cysteamine led to an induction of apoptosis. Other thiols, including glutathione (GSH) and cysteine, did not potentiate the effect of AS101. We propose that this is due to the alpha-carboxylate group present in the compounds formed between AS101 and these thiols. Programmed cell death was associated with the loss of mitochondrial transmembrane potential and activation of caspase-3 and -9. Elevation of intracellular reactive oxygen species (ROS) production was also demonstrated; the antioxidant catalase significantly reduced the apoptosis, suggesting that ROS play a key role in the apoptosis induced by AS101 and the thiols. Finally, we quantified the intracellular concentration of tellurium, using electron microscopy and energy-dispersive spectroscopy (EDS) analysis. The addition of cysteamine to AS101 significantly increased the concentration of tellurium within the cells. The results indicate that neutral or positively charged thiols but not negatively charged ones, increase the antitumoral effect of AS101 by increasing its uptake into the cells.

A biochemical and toxicological study with diethyl 2-phenyl-2-tellurophenyl vinylphosphonate in a sub-chronic intraperitoneal treatment in mice

Diethyl-2-phenyl-2-tellurophenyl vinylphosphonate (DPTVP) is an organotellurium compound with low toxicity after subcutaneous administration in mice. This study evaluated possible in vivo and ex vivo toxicological effects of daily injections of DPTVP for 12 days in mice, using the intraperitoneal administration. This route potentially increases the pharmacokinetics of absorption, distribution, metabolism and toxicity of DPTVP. Treatment with DPTVP (0, 30, 50, 75, 100, 250, 350 or 500 micromol/kg) were not associated with mortality or body weight loss. Nevertheless, the liver and liver-to-body weight ratio increased in groups treated with 350 and 500 micromol/kg of DPTVP. However, plasmatic aspartate and alanine aminotransferase activities (classical markers of hepatotoxicity) were not increased after diethyl-2-phenyl-2-tellurophenyl vinylphosphonate administration. Hepatic, renal and cerebral thiobarbituric acid reactive substances (TBARS), delta-ALA-D activity and Vitamin C levels were not modified after DPTVP treatment. Renal and hepatic superoxide dismutase (SOD) and catalase (CAT) were unchanged after DPTVP treatment. Conversely, SOD activity significantly increased in brain in groups treated with 50, 75, 100 and 500 micromol/kg of DPTVP treated groups. Our findings corroborates that brain is a potential target for organochalcogen action. The absence of severe overt signs of toxicity after sub-chronic exposure to DPTVP reinforces the necessity for more detailed pharmacological studies concerning this new organotellurium compound.

Electrospray ionization mass spectrometric characterization of key Te(IV) cationic intermediates for the addition of TeCl4 to alkynes

Tellurium tetrachloride adds to alkynes via two pathways: a concerted syn-addition that yields Z-tri- and -tetrasubstituted alkenes or an anti-addition that yields E-alkenes. The mechanistic aspects of these divergent pathways for TeCl4 addition to alkynes have been investigated by on-line electrospray ionization (tandem) mass spectrometry (ESI-MS(/MS)). Via ESI-MS(/MS), we have been able to intercept and characterize the active electrophile TeCl3+ in tetrahydrofuran (THF) solutions of TeCl4, as well as its THF complex and several TeClx(OH)y+ derivatives. For the first time, also, key Te(IV) cationic intermediates of the electrophilic addition of TeCl4 to alkynes were captured for gas-phase MS investigation. The detailed structural data of cyclic tellurane intermediates intercepted herein seems to provide insights into the coordinative behavior of the Te(IV) atom and its mode of action towards biological targets.

Diethyl 2-phenyl-2 tellurophenyl vinylphosphonate: An organotellurium compound with low toxicity

It is well-known that organotellurium compounds can have antioxidant activity in vitro, but in vivo these compounds can be potentially toxic to rodents. Here we investigated the potential in vitro and ex vivo toxicity of a new beta-organochalcogenyl vinylphosphonate, the diethyl 2-phenyl-2 tellurophenyl vinylphosphonate. The in vitro antioxidant activity of this organotellurium compound was also investigated. In vitro, the rate of dithiotreitol (DTT) oxidation was increased and the activity of cerebral, renal and hepatic delta-aminolevulinate dehydratase (delta-ALA-D) was decreased by diethyl 2-phenyl-2-tellurophenyl vinylphosphonate (120-1200 microM), indicating that this compound oxidize-SH groups. The antioxidant activity was also observed in brain, liver and kidney, in very low concentrations (0.4, 1.0, 4.0, 10.0 and 40.0 microM), and this capacity was comparable to the antioxidant standard organotellurium compound, diphenyl ditelluride. In vivo, delta-ALA-D activity in liver, kidney and brain of mice treated for 12 days with dimethylsulfoxide (DMSO) as vehicle, 25, 75 or 250 micromol/kg of diethyl 2-phenyl-2-tellurophenyl vinylphosphonate was not affected. Furthermore, only one animal treated with the highest dose died, whereas all animals treated with diphenyl ditteluride died in the fourth day. These results suggest that this novel organotellurium compound interacts with the sulfhydryl groups, however only at higher doses when compared with diphenyl ditelluride. Since diethyl 2-phenyl-2 tellurophenyl vinylphosphonate had low toxicity to mice after sub-chronic exposure, it becomes important to investigate its possible pharmacological properties.