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Differential inhibition of high and low Mr thioredoxin reductases of parasites by organotelluriums supports the concept that low Mr thioredoxin reductases are good drug targets

Published online by Cambridge University Press:  04 November 2008

P. J. McMILLAN ,
E. M. PATZEWITZ ,
S. E. YOUNG ,
G. D. WESTROP ,
G. H. COOMBS ,
L. ENGMAN  and
S. MÜLLER
P. J. McMILLAN
Affiliation:
Division of Infection and Immunity and Wellcome Centre for Molecular Parasitology, Glasgow Biomedical Research Centre, 120 University Place, University of Glasgow, Glasgow G12 8TA, UK
E. M. PATZEWITZ
Affiliation:
Division of Infection and Immunity and Wellcome Centre for Molecular Parasitology, Glasgow Biomedical Research Centre, 120 University Place, University of Glasgow, Glasgow G12 8TA, UK
S. E. YOUNG
Affiliation:
Division of Infection and Immunity and Wellcome Centre for Molecular Parasitology, Glasgow Biomedical Research Centre, 120 University Place, University of Glasgow, Glasgow G12 8TA, UK
G. D. WESTROP
Affiliation:
Strathclyde Institute of Pharmacy and Biomedical Sciences, University of Strathclyde, The John Arbuthnott Building, 27 Taylor Street, Glasgow G4 0NR, UK
G. H. COOMBS
Affiliation:
Strathclyde Institute of Pharmacy and Biomedical Sciences, University of Strathclyde, The John Arbuthnott Building, 27 Taylor Street, Glasgow G4 0NR, UK
L. ENGMAN
Affiliation:
Department of Biochemistry and Organic Chemistry, University of Uppsala, Box 576, SE-751 23 Uppsala, Sweden
S. MÜLLER*
Affiliation:
Division of Infection and Immunity and Wellcome Centre for Molecular Parasitology, Glasgow Biomedical Research Centre, 120 University Place, University of Glasgow, Glasgow G12 8TA, UK
*
*Corresponding author: Division of Infection and Immunity and Wellcome Centre for Molecular Parasitology, Glasgow Biomedical Research Centre, 120 University Place, University of Glasgow, Glasgow G12 8TA, UK. Tel: (+)44 141 330 2383. Fax: (+) 44 141 330 4600. E-mail: s.muller@bio.gla.ac.uk
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Summary

Thioredoxin reductase (TrxR), a NADPH-dependent disulfide oxidoreductase, is vital in numerous cellular processes including defence against reactive oxygen species, cell proliferation and signal transduction. TrxRs occur in 2 forms, a high Mr enzyme characterized by those of mammals, the malaria parasite Plasmodium falciparum and some worms, and a low Mr form is present in bacteria, fungi, plants and some protozoan parasites. Our hypothesis is that the differences between the forms can be exploited in the development of selective inhibitors. In this study, cyclodextrin- and sulfonic acid-derived organotelluriums known to inhibit mammalian TrxR were investigated for their relative efficacy against P. falciparum TrxR (PfTrxR), a high Mr enzyme, and Trichomonas vaginalis TrxR (TvTrxR), a low Mr form of TrxR. The results suggest that selective inhibition of low Mr TrxRs is a feasible goal.

Keywords

inhibitionthioredoxin systemmalariaorganotellurium compoundsTrichomonasPlasmodium
Type
Research Article
Information
Parasitology , Volume 136 , Issue 1 , January 2009 , pp. 27 - 33
Copyright
Copyright © 2008 Cambridge University Press

INTRODUCTION

The NADPH-dependent disulfide oxidoreductase thioredoxin reductase (TrxR) functions in numerous redox reactions, primarily by reducing the small protein thioredoxin (Trx) (Becker et al. Reference Becker, Gromer, Schirmer and Müller2000). Trx interacts with a wide variety of acceptor proteins including ribonucleotide reductase, methionine sulfoxide reductase, peroxiredoxins and several transcription factors (Powis and Montfort, Reference Powis and Montfort2001; Nakamoto and Bardwell, Reference Nakamoto and Bardwell2004; Baier and Dietz, Reference Baier and Dietz2005; Kabe et al. Reference Kabe, Ando, Hirao, Yoshida and Handa2005; Arner and Holmgren, Reference Arner and Holmgren2006). Thus, the adequate functioning of the TrxR/Trx system is crucial for intracellular redox homeostasis, cell proliferation, protection against reactive oxygen species, and signal transduction. TrxRs generally are of 2 types – high Mr enzymes characterized by the selenocysteine-containing enzyme of mammals and a high Mr variant, in which the active site selenocysteine is replaced by a cysteine, which occurs in apicomplexan parasites, and low Mr forms present in bacteria, fungi, plants and some protozoan parasites including Trichomonas vaginalis (Williams et al. Reference Williams, Arscott, Müller, Lennon, Ludwig, Wang, Veine, Becker and Schirmer2000; Hirt et al. Reference Hirt, Müller, Embley and Coombs2002; Coombs et al. Reference Coombs, Westrop, Suchan, Puzova, Hirt, Embley, Mottram and Müller2004). The fundamental differences between the forms, together with their importance, suggest that low Mr TrxRs could be good drug targets.

Thioredoxin-dependent reactions are known to be of importance for the survival of a variety of parasitic protozoa. The most notable example is Plasmodium falciparum, the causative agent of severe malaria responsible for more than 2 million deaths each year, for which disruption of the TrxR gene or protein is lethal (Krnajski et al. Reference Krnajski, Gilberger, Walter, Cowman and Müller2002; Andricopulo et al. Reference Andricopulo, Akoachere, Krogh, Nickel, McLeish, Kenyon, Arscott, Williams, Davioud-Charvet and Becker2006). The Trx-dependent reactions of T. vaginalis, a sexually transmitted protozoon that infects 170 million people annually (WHO Report, 2001), have also been characterized. This parasite is an aerotolerant anaerobe and can only endure oxygen at low concentrations, which reiterates that the parasites should be vulnerable to interference with systems such as TrxR/Trx involved in protecting them against oxidative insults (Coombs et al. Reference Coombs, Westrop, Suchan, Puzova, Hirt, Embley, Mottram and Müller2004; Cartlon et al. Reference Carlton, Hirt and Silva2007). Another important discovery was that TrxR has been implicated in susceptibility of another aerotolerant anaerobic parasitic protozoon, Entamoeba histolytica, to the main drug metronidazole, which is also used against Trichomonas and other anaerobes (Leitsch et al. Reference Leitsch, Kolarich, Wilson, Altmann and Duchêne2007).

Inhibition of TrxR represents an attractive approach for anti-cancer chemotherapy, as reviewed by Becker and colleagues (Becker et al. Reference Becker, Gromer, Schirmer and Müller2000; Gromer et al. Reference Gromer, Urig and Becker2004; Urig and Becker, Reference Urig and Becker2006), which has resulted in a series of inhibitors of TrxR that can be used as experimental tools in studies such as this current one. A number of agents, such as chloro-2,4-dinitrobenzene and 1,3-bis-(2-chloroethyl)1-nitrosourea, are irreversible inhibitors of the selenocysteine-containing mammalian TrxR (see Becker et al. Reference Becker, Gromer, Schirmer and Müller2000; Gromer et al. Reference Gromer, Urig and Becker2004; Urig and Becker, Reference Urig and Becker2006); alkylation of the reactive selenocysteine residue is likely to be the mechanism of inactivation. However, these inhibitors of mammalian TrxR are not suitable for future drug development because they are not very specific – they also were shown to alkylate other cysteine-containing proteins, as well as DNA (Urig and Becker, Reference Urig and Becker2006). More promising compounds interacting with the selenocysteine residue of mammalian TrxR are auranofin, aurothioglucose and other organogold compounds; they inhibit TrxR activity very efficiently (Gromer et al. Reference Gromer, Arscott, Williams, Schirmer and Becker1998; Smith et al. Reference Smith, Guidry, Morris and Levander1999; Engman et al. Reference Engman, McNaughton, Gajewska, Kumar, Birmingham and Powis2006) and are being pursued as lead anti-cancer agents.

The lack of the selenocysteine residue in the active site of parasite TrxRs makes the use of these inhibitors with promise against the mammalian enzyme unattractive against the parasites. Thus in this study we have used organotelluriums. These have been previously proven to efficiently inhibit mammalian TrxR, but the mode of action of these compounds, although yet to be established fully, is thought not to depend upon selenocysteine, and thus activity against the parasite enzymes seemed likely. A number of potential mechanisms have been proposed for organotelluriums; it is accepted that they can readily undergo redox cycling and that they are able to act as catalysts for reduction of hydrogen peroxide and organic hydroperoxides in the presence of thiols (Engman et al. Reference Engman, Kandra, Gallegos, Williams and Powis2000, Reference Engman, Al-Maharik, McNaughton, Birmingham and Powis2003a, Reference Engman, Al-Maharik, McNaughton, Birmingham and Powisb; McNaughton et al. Reference McNaughton, Engman, Birmingham, Powis and Cotgreave2004).

The high Mr and low Mr TrxRs have some similar functions and yet mechanistically they are very different (Williams et al. Reference Williams, Arscott, Müller, Lennon, Ludwig, Wang, Veine, Becker and Schirmer2000). The high Mr proteins have 3 redox active centres that are required for electron transfer from NADPH to thioredoxin, whereas the low Mr proteins have 2 redox centres and the transfer of reducing equivalents requires a conformational change (Williams et al. Reference Williams, Arscott, Müller, Lennon, Ludwig, Wang, Veine, Becker and Schirmer2000). It is likely that a discriminating interference of these very different processes is possible by selectively inhibiting the way in which electrons are transferred through the proteins' active sites. This study investigated whether the 2 classes of TrxR are inhibited by 2 series of organotelluriums and whether selective inhibition of either protein might be possible by these molecules.

MATERIALS AND METHODS

Parasites

P. falciparum 3D7 (The Netherlands) were cultivated according to the method described by Trager and Jensen (Reference Trager and Jensen1976) in human erythrocytes using RPMI 1640 containing 11 mm glucose and with the addition of 0·5% Albumax II (Invitrogen). IC50 values of inhibitors were determined using the hypoxanthine incorporation assay, as outlined previously (Fidock et al. Reference Fidock, Nomura and Wellems1998). Briefly, parasite cultures of 0·25% parasitaemia, 1% haematocrit were prepared in 96-well plates with inhibitor concentrations between 0·05 and 500 μm. After 24 h, 0·5 μCi of [3H]-hypoxanthine (10–30 Ci mmol−1; GE Healthcare) was added to each well and the cultures were incubated for an additional 24 h before harvesting the parasites onto filter mats. The compounds under investigation were solubilized at 50 mm in dimethylsulphoxide (DMSO) and during testing the DMSO level never exceeded 1% of the culture volume. DMSO controls were included and it was confirmed that the solvent did not have a negative effect on parasite viability. IC50 values were determined using GraFit 5 (Erithacus Software) by plotting the mean hypoxanthine incorporation of 3 replicates per independent experiment against the log of the inhibitor concentration.

T. vaginalis were cultured as described previously (Coombs et al. Reference Coombs, Westrop, Suchan, Puzova, Hirt, Embley, Mottram and Müller2004). IC50 values were determined using the CellTiter-Glo® Assay (Promega), which measures the ATP concentration in cells and can be used to assess cell viability and cytotoxicity of inhibitors. The compounds under investigation were dissolved in DMSO to a final stock solution concentration of 25 mm; it was confirmed that the solvent concentration used did not have any effect on parasite viability. Inhibition assays were performed in 96-well plates with varying concentrations of inhibitors (0·24 μm–500 μm) in 100 μl of Modified Diamond's Medium with 10% (v/v) heat inactivated horse serum. Then 100 μl of T. vaginalis cell suspension was added to give a final concentration of 1×105 cells ml−1. The plates were sealed with NescoFilm and incubated overnight (18–22 h) at 37°C in a humidified box. After incubation, 100 μl aliquots were transferred to a black, 96-well luminometer plate and luminescence was measured with the CellTiter-Glo® Assay (Promega) according to the manufacturer's instructions. IC50 values were determined using GraFit 5 (Erithacus Software) by plotting the mean luminescent signal (of 3 replicates per independent experiment) against the log of the inhibitor concentration.

Inhibitors

The organotellurium compounds used in this study were of 2 kinds. The cyclodextrin-derived diorganyl tellurides 1 as well as the sulfonic acid-derived compounds 2 (Fig. 1) were originally designed and synthesized in order to increase water solubility and bioavailability compared with organotellurium inhibitors previously prepared (Kanda et al. Reference Kanda, Engman, Cotgreave and Powis1999; Engman et al. Reference Engman, Kandra, Gallegos, Williams and Powis2000, Reference Engman, Al-Maharik, McNaughton, Birmingham and Powis2003a, Reference Engman, Al-Maharik, McNaughton, Birmingham and Powisb; McNaughton et al. Reference McNaughton, Engman, Birmingham, Powis and Cotgreave2004).

Fig. 1. Structures of organotellurium inhibitors used in this study.

Proteins

Recombinant P. falciparum TrxR (PfTrxR) and P. falciparum Trx (PfTrx)were generated as described previously (McMillan et al. Reference McMillan, Arscott, Ballou, Becker, Williams and Müller2006). Recombinant T. vaginalis TrxR (TvTrxR) and T. vaginalis thioredoxin (TvTrx) were produced as previously described (Coombs et al. Reference Coombs, Westrop, Suchan, Puzova, Hirt, Embley, Mottram and Müller2004). Plasmodium glutathione reductase (GR) was generated as described by Gilberger et al. (Reference Gilberger, Schirmer, Walter and Müller2000).

Enzyme assays

TrxR activities were determined using 2 assay systems.

(i) DTNB assay

The DTNB assay measures the ability of the TrxR to be reduced by NADPH and in turn to reduce DTNB. DTNB-reduction was followed spectrophotometrically (Shimadzu UVPC 2501) in a 1 ml assay at 25°C by monitoring the increase of absorbance at 412 nm. The specific activities were determined using the molar extinction coefficient of TNB− released during the reaction (13 600 M−1 cm−1) (Holmgren, Reference Holmgren1977). IC50 values were determined for each inhibitor by varying the concentration between 0·3 μm and 500 μm. TvTrxR activity was measured in a reaction mix containing 50 mm potassium phosphate buffer, pH 7·0, 1 mm EDTA, 5 mm DTNB, 200 μm NADPH, 330 nmTvTrxR and varying concentrations of inhibitor. PfTrxR activity was measured in a reaction mix containing 50 mm potassium phosphate buffer, pH 7·0, 1 mm EDTA, 3 mm DTNB, 200 μm NADPH, 33 nm PfTrxR and varying concentrations of inhibitor.

(ii) Thioredoxin assay

The thioredoxin assay measures the catalytic reduction of Trx by NADPH that is catalysed by TrxR. Insulin was included in the assay to re-oxidize reduced Trx and so prevent product inhibition. The thioredoxin assay was performed in a 1 ml reaction mix at 25°C according to the method of Luthman and Holmgren (Reference Luthman and Holmgren1982). The change in absorbance due to the oxidation of NADPH was followed spectrophotometrically at 340 nm (Shimadzu UVPC 2501) and activities were calculated using the molar extinction coefficient of 6220 M−1 cm−1. IC50 values were determined for each inhibitor by varying the concentration between 0·3 μm and 500 μm. TvTrxR activity was measured in a reaction mix containing 50 mm potassium phosphate, pH 7·0, 1 mm EDTA, 0·2 mg ml−1 insulin, 66 nmTvTrxR and 40 μmTvTrx at 200 μm NADPH with varying concentrations of inhibitor. PfTrxR activity was measured in a reaction mix containing 50 mm potassium phosphate, pH 7·6, 1 mm EDTA, 0·2 mg ml−1 insulin, 16 nmPfTrxR and 40 μmPfTrx at 200 μm NADPH with varying concentrations of inhibitor. IC50s were determined using Grafit 5.0 (Erithacus Software) by plotting the mean enzyme activity (of 3 replicates per independent experiment) against the log of the inhibitor concentration.

The effect of the compounds on P. falciparum GR was tested in a spectrophotometric assay system as described (Gilberger et al. Reference Gilberger, Schirmer, Walter and Müller2000). The inhibitors were used at 10 μm, 50 μm and 100 μm.

RESULTS

Organotelluriums are efficient inhibitors of high Mr mammalian TrxR in vitro, and they also inhibit human cancer cell growth in culture (Engman et al. Reference Engman, Al-Maharik, McNaughton, Birmingham and Powis2003a, Reference Engman, Al-Maharik, McNaughton, Birmingham and Powisb). However, poor solubility has restricted their use in animal models. Therefore, the synthesis of more water-soluble, cyclodextrin- and sulfonic acid-derived organotellurium inhibitors was undertaken (Kanda et al. Reference Kanda, Engman, Cotgreave and Powis1999; McNaughton et al. Reference McNaughton, Engman, Birmingham, Powis and Cotgreave2004). In this study the effects of these 2 series of water-soluble organotelluriums on the 2 distinct types of TrxR, represented by the enzymes from P. falciparum and T. vaginalis, were analysed.

Effect of cyclodextrin-derived organotelluriums 1 on TrxR activities and parasite growth

The inhibitory effects of cyclodextrin-derived organotelluriums 1a–1d (see Fig. 1) were tested against P. falciparum and T. vaginalis TrxRs and their effects on the viability of the 2 parasites in culture were analysed. The effect of the tellurides against the parasite enzymes was analysed using 2 enzyme assays. The specific activity of the control (non-inhibited) Plasmodium enzyme in the DTNB assay was 9·8±0·1 units mg−1 protein and in the thioredoxin assay it was 18·6±2·5 unit mg−1 protein. The control (non-inhibited) Trichomonas enzyme had a specific activity of 1·3±0·05 units mg−1 protein in the DTNB assay and 25·9±2·4 units mg−1 protein in the thioredoxin assay.

The IC50 values determined using the DTNB and thioredoxin assays are generally similar (Table 1). The greatest differences between the two assay systems were found with compound 1a, which showed a higher IC50 for both parasite enzymes in the thioredoxin assay. Overall the IC50 values determined in this study with both assays were between 1 and 21 μm (see Table 1). Compound 1b showed some selectivity between the TrxRs of Plasmodium and Trichomonas. The IC50 determined for the inhibition of Trichomonas TrxR by 1b was about 14·0 μm and thus about 4 times higher than that determined for the Plasmodium enzyme, which was found to be between 3·3 and 3·9 μm. The effect of this compound against parasite viability was found to be in the same range as the inhibitory effects against the two enzymes. This suggests that some selectivity between inhibition of high and low Mr TrxR is achievable with only slight modifications of the inhibitor structure (see Fig. 1). Interestingly, the human high Mr TrxR is also inhibited by this compound in the low micromolar range and the efficacy on cancer cell viability (McNaughton et al. Reference McNaughton, Engman, Birmingham, Powis and Cotgreave2004) is similar to that found here for the inhibition of Plasmodium growth.

Table 1. Inhibitory effects of cyclodextrin-derived organotelleriums

(The assays were performed in triplicate and means±standard deviations of representative experiments are shown. PfTrxR: P. falciparum thioredoxin reductase; TvTrxR: T. vaginalis thioredoxin reductase; Pf in vitro: IC50 values of compounds tested on in vitro growth of P. falciparum using the hypoxanthine incorporation assay; Tv in vitro: IC50 values of compounds tested on in vitro growth of T. vaginalis by the CellTiter-Glo® assay.)

Effect of sulfonic acid-derived organotelluriums 2 on TrxR activities and parasite growth

Organotelluriums 2 proved to be generally more selective in their inhibition of low and high Mr TrxRs (Table 2). All 4 compounds tested showed relatively good inhibition of the Trichomonas enzyme, although their efficacy against parasite growth was low (Table 2). Compounds 2a and 2c were also moderately good inhibitors of the Plasmodium enzyme. However, 2b was less effective and 2d was relatively poor against the Plasmodium enzyme: IC50 values between 16 and 95 μm in comparison to 2 to 7 μm for the Trichomonas enzyme in the DTNB and thioredoxin assays, respectively. This result clearly shows that differential inhibition of the low MrTrichomonas TrxR and the high MrPlasmodium TrxR is possible. Another interesting finding was that 2a, which has an IC50 of 3 μm against Plasmodium TrxR in the DTNB assay, showed a similar IC50 for inhibition of Plasmodium growth in vitro. As the compound's effect on Trichomonas growth was poor, despite similar inhibition of the low Mr enzyme, it may be that compound 2a is taken up selectively by the Plasmodium-infected erythrocyte and thereby accumulates within the infected cell and thus is able to exert an effect. This suggestion is corroborated by the finding that this compound also has a poor effect on the viability of a cancer cell line (with an IC50 of >30 μm) despite the fact that the mammalian enzyme is inhibited equally well (IC50 0·6 μm, McNaughton et al. Reference McNaughton, Engman, Birmingham, Powis and Cotgreave2004) as the Plasmodium protein.

Compound 2c was only tested in the DTNB assay as its absorbance at 340 nm interfered with the insulin assay system. Although this compound inhibited both parasite enzymes in the low micromolar range, it only had a weak effect on Plasmodium and Trichomonas growth, suggesting poor uptake into the parasites.

Table 2. Inhibitory effects of sulfonic acid-derived organotellurium derivatives

(The assays were performed in triplicate and means±standard deviations of representative experiments are shown. n.d. – not determined. * The activity of TrxRs in the thioredoxin assay was not possible with compound 2c because of its strong absorption at 340 nm which interfered with the detection of NADPH oxidation. PfTrxR: P. falciparum thioredoxin reductase; TvTrxR: T. vaginalis thioredoxin reductase; Pf in vitro: IC50 values of compounds tested on in vitro growth of P. falciparum using the hypoxanthine incorporation assay; Tv in vitro: IC50 values of compounds tested on in vitro growth of T. vaginalis by the CellTiter-Glo® assay.)

In order to analyse the specificity of the inhibitors towards TrxRs, we also tested their effect against glutathione reductase from Plasmodium and found that they all have no inhibitory effect against this related enzyme even at 100 μm. This shows that these compounds have some specificity towards TrxR over other important disulphide oxidoreductases.

DISCUSSION

Thioredoxin reductase has long been proposed as an interesting target for anti-protozoan drug development because parasitic protozoa depend on a highly efficient antioxidant system to survive in their hosts (Krauth-Siegel and Coombs, Reference Krauth-Siegel and Coombs1999; Müller, Reference Müller2004). The Plasmodium enzyme was shown to be essential for parasite growth and the enzyme from the microaerophile T. vaginalis has been suggested to have key functions in protecting the parasite against oxidative damage (Krnajski et al. Reference Krnajski, Gilberger, Walter, Cowman and Müller2002; Coombs et al. Reference Coombs, Westrop, Suchan, Puzova, Hirt, Embley, Mottram and Müller2004). The fact that the 2 parasites have distinct types of TrxR, which differ fundamentally from each other, prompted this study, which primarily wanted to test the concept that selective inhibition of different types of TrxR can be achieved. Recent work by Engman and Powis showed good activities of organotelluriums against both human TrxR in vitro and cancer cell growth (Engman et al. Reference Engman, Al-Maharik, McNaughton, Birmingham and Powis2003a, Reference Engman, Al-Maharik, McNaughton, Birmingham and Powisb). These results were very encouraging and it was decided to take advantage of the existing knowledge on the compounds and analyse their effects against the 2 parasite TrxRs.

To test the selectivity of these compounds for TrxRs, their ability to inhibit the related disulphide flavo-oxidoreductase glutathione reductase was analysed and we found that the compounds showed very little effect against this related enzyme even at concentrations of 100 μm. Both series of organotelluriums were, however, effective inhibitors of the parasite TrxRs, with the results clearly showing some selectivity between the two enzyme types, supporting the hypothesis that selective inhibition of different types of TrxR is possible. A notable additional finding was that compounds 2a and 2b were much more effective against P. falciparum than against MCF cancer cells (McNaughton et al. Reference McNaughton, Engman, Birmingham, Powis and Cotgreave2004), thus suggesting that selective uptake of such compounds into parasitized erythrocytes is achievable and that this may be a worthwhile approach to adopt in seeking specificity towards malaria parasites. Further, we have shown, using, for instance, compound 2d, differential inhibition of the low Mr TrxR (IC50T. vaginalis TrxR, 7·4 μm; IC50P. falciparum TrxR, 95·5 μm), which suggests that this molecule could be an interesting lead in the search of TrxR inhibitors that act specifically against low Mr TrxRs. This is important when considering that the low Mr TrxR is the principal type of bacterial, fungal and plant TrxR, and thus inhibitors of this enzyme class might be useful for the development of bactericides, fungicides or herbicides. Thus selective inhibition of low Mr TrxR seems to offer opportunities in a number of areas.

Thus, potentially the organotelluriums are interesting in terms of future inhibitor design, albeit their general toxicity is largely unexplored. There are concerns about the teratogenicity of tellurides, which clearly is not acceptable in a drug and would need to be addressed. Further, the scarcity of the element tellurium is a disadvantage because future drugs against diseases prevalent in the poorest areas of the world should be cost effective (Nogueira et al. Reference Nogueira, Zeni and Rocha2004; Stangherlin et al. Reference Stangherlin, Favero, Zeni, Rocha and Nogueira2005). On the other hand, it has been reported that tellurides have immunomodulatory effects that could be beneficial in, for instance, the response to an infection with the malaria parasite (Brodsky et al. Reference Brodsky, Yosef, Galit, Albeck, Longo, Albeck and Sredni2007). Therefore, the tellurides are worth considering for future drug development as long as it will be possible to address their toxicity and the issue of cost efficiency.

This research was funded by the Wellcome Trust (WT061173MA, S. M.) and the MRC (G. H. C.) and the Swedish Research Council (L. E.).

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Figure 0

Fig. 1. Structures of organotellurium inhibitors used in this study.

Figure 1

Table 1. Inhibitory effects of cyclodextrin-derived organotelleriums(The assays were performed in triplicate and means±standard deviations of representative experiments are shown. PfTrxR: P. falciparum thioredoxin reductase; TvTrxR: T. vaginalis thioredoxin reductase; Pf in vitro: IC50 values of compounds tested on in vitro growth of P. falciparum using the hypoxanthine incorporation assay; Tv in vitro: IC50 values of compounds tested on in vitro growth of T. vaginalis by the CellTiter-Glo® assay.)

Figure 2

Table 2. Inhibitory effects of sulfonic acid-derived organotellurium derivatives(The assays were performed in triplicate and means±standard deviations of representative experiments are shown. n.d. – not determined. * The activity of TrxRs in the thioredoxin assay was not possible with compound 2c because of its strong absorption at 340 nm which interfered with the detection of NADPH oxidation. PfTrxR: P. falciparum thioredoxin reductase; TvTrxR: T. vaginalis thioredoxin reductase; Pf in vitro: IC50 values of compounds tested on in vitro growth of P. falciparum using the hypoxanthine incorporation assay; Tv in vitro: IC50 values of compounds tested on in vitro growth of T. vaginalis by the CellTiter-Glo® assay.)