Mechanisms of arsenical and diamidine uptake and resistance in Trypanosoma brucei

Symmetrical choline-derived dications display strong anti-kinetoplastid activity

OBJECTIVES

to investigate the anti-kinetoplastid activity of choline-derived analogues with previously reported antimalarial efficacy.

METHODS

from an existing choline analogue library, seven antimalarial compounds, representative of the first-, second- and third-generation analogues previously developed, were assessed for activity against Trypanosoma and Leishmania spp. Using a variety of techniques, the effects of choline analogue exposure on the parasites were documented and a preliminary investigation of their mode of action was performed.

RESULTS

the activities of choline-derived compounds against Trypanosoma brucei and Leishmania mexicana were determined. The compounds displayed promising anti-kinetoplastid activity, particularly against T. brucei, to which 4/7 displayed submicromolar EC(50) values for the wild-type strain. Low micromolar concentrations of most compounds cleared trypanosome cultures within 24-48 h. The compounds inhibit a choline transporter in Leishmania, but their entry may not depend only on this carrier; T. b. brucei lacks a choline carrier and the mode of uptake remains unclear. The compounds had no effect on the overall lipid composition of the cells, cell cycle progression or cyclic adenosine monophosphate production or short-term effects on intracellular calcium levels. However, several of the compounds, displayed pronounced effects on the mitochondrial membrane potential; this action was not associated with production of reactive oxygen species but rather with a slow rise of intracellular calcium levels and DNA fragmentation.

CONCLUSIONS

the choline analogues displayed strong activity against kinetoplastid parasites, particularly against T. b. brucei. In contrast to their antimalarial activity, they did not act on trypanosomes by disrupting choline salvage or phospholipid metabolism, instead disrupting mitochondrial function, leading to chromosomal fragmentation.

Selective delivery of 2-hydroxy APA to Trypanosoma brucei using the melamine motif

Trypanosoma brucei, the parasite that causes human African trypanosomiasis, is auxotrophic for purines and has specialist nucleoside transporters to import these metabolites. In particular, the P2 aminopurine transporter can also selectively accumulate melamine derivatives. In this Letter, we report the coupling of the melamine moiety to 2-hydroxy APA, a potent ornithine decarboxylase inhibitor, with the aim of selectively delivering this compound to the parasite. The best compound described here shows an increased in vitro trypanocidal activity compared with the parent.

Evaluation of nucleoside hydrolase inhibitors for treatment of African trypanosomiasis

In this paper, we present the biochemical and biological evaluation of N-arylmethyl-substituted iminoribitol derivatives as potential chemotherapeutic agents against trypanosomiasis. Previously, a library of 52 compounds was designed and synthesized as potent and selective inhibitors of Trypanosoma vivax inosine-adenosine-guanosine nucleoside hydrolase (IAG-NH). However, when the compounds were tested against bloodstream-form Trypanosoma brucei brucei, only one inhibitor, N-(9-deaza-adenin-9-yl)methyl-1,4-dideoxy-1,4-imino-d-ribitol (UAMC-00363), displayed significant activity (mean 50% inhibitory concentration [IC(50)] +/- standard error, 0.49 +/- 0.31 microM). Validation in an in vivo model of African trypanosomiasis showed promising results for this compound. Several experiments were performed to investigate why only UAMC-00363 showed antiparasitic activity. First, the compound library was screened against T. b. brucei IAG-NH and inosine-guanosine nucleoside hydrolase (IG-NH) to confirm the previously demonstrated inhibitory effects of the compounds on T. vivax IAG-NH. Second, to verify the uptake of these compounds by T. b. brucei, their affinities for the nucleoside P1 and nucleoside/nucleobase P2 transporters of T. b. brucei were tested. Only UAMC-00363 displayed significant affinity for the P2 transporter. It was also shown that UAMC-00363 is concentrated in the cell via at least one additional transporter, since P2 knockout mutants of T. b. brucei displayed no resistance to the compound. Consequently, no cross-resistance to the diamidine or the melaminophenyl arsenical classes of trypanocides is expected. Third, three enzymes of the purine salvage pathway of procyclic T. b. brucei (IAG-NH, IG-NH, and methylthioadenosine phosphorylase [MTAP]) were investigated using RNA interference. The findings from all these studies showed that it is probably not sufficient to target only the nucleoside hydrolase activity to block the purine salvage pathway of T. b. brucei and that, therefore, it is possible that UAMC-00363 acts on an additional target.

Immunobiology of African trypanosomes: need of alternative interventions

Trypanosomiasis is one of the major parasitic diseases for which control is still far from reality. The vaccination approaches by using dominant surface proteins have not been successful, mainly due to antigenic variation of the parasite surface coat. On the other hand, the chemotherapeutic drugs in current use for the treatment of this disease are toxic and problems of resistance are increasing (see Kennedy (2004) and Legros et al. (2002)). Therefore, alternative approaches in both treatment and vaccination against trypanosomiasis are needed at this time. To be able to design and develop such alternatives, the biology of this parasite and the host response against the pathogen need to be studied. These two aspects of this disease with few examples of alternative approaches are discussed here.

Proteomics of trypanosomatids of human medical importance

Leishmania spp., Trypanosoma cruzi, and Trypanosoma brucei are protozoan parasites that cause a spectrum of fatal human diseases around the world. Recent completion of the genomic sequencing of these parasites has enormous relevance to the study of their biology and the pathogenesis of the diseases they cause because it opens the door to high-throughput proteomic technologies. This review encompasses studies using diverse proteomic approaches with these organisms to describe and catalogue global protein profiles, reveal changes in protein expression during development, elucidate the subcellular localisation of gene products, and evaluate host-parasite interactions.

Multiple genetic mechanisms lead to loss of functional TbAT1 expression in drug-resistant trypanosomes

The P2 aminopurine transporter, encoded by TbAT1 in African trypanosomes in the Trypanosoma brucei group, carries melaminophenyl arsenical and diamidine drugs into these parasites. Loss of this transporter contributes to drug resistance. We identified the genomic location of TbAT1 to be in the subtelomeric region of chromosome 5 and determined the status of the TbAT1 gene in two trypanosome lines selected for resistance to the melaminophenyl arsenical, melarsamine hydrochloride (Cymelarsan), and in a Trypanosoma equiperdum clone selected for resistance to the diamidine, diminazene aceturate. In the Trypanosoma brucei gambiense STIB 386 melarsamine hydrochloride-resistant line, TbAT1 is deleted, while in the Trypanosoma brucei brucei STIB 247 melarsamine hydrochloride-resistant and T. equiperdum diminazene-resistant lines, TbAT1 is present, but expression at the RNA level is no longer detectable. Further characterization of TbAT1 in T. equiperdum revealed that a loss of heterozygosity at the TbAT1 locus accompanied loss of expression and that P2-mediated uptake of [(3)H]diminazene is lost in drug-resistant T. equiperdum. Adenine-inhibitable adenosine uptake is still detectable in a DeltaTbat1 T. b. brucei mutant, although at a greatly reduced capacity compared to that of the wild type, indicating that an additional adenine-inhibitable adenosine permease, distinct from P2, is present in these cells.

Curcuminoid analogs with potent activity against Trypanosoma and Leishmania species

The natural curcuminoids curcumin (1), demethoxycurcumin (2) and bisdemethoxycurcumin (3) have been chemically modified to give 46 analogs and 8 pairs of 1:1 mixture of curcuminoid analogs and these parent curcuminoids and their analogs were assessed against protozoa of the Trypanosoma and Leishmania species. The parent curcuminoids exhibited low antitrypanosomal activity (EC(50) for our drug-sensitive Trypanosoma brucei brucei line (WT) of compounds 1, 2 and 3 are 2.5, 4.6 and 7.7 microM, respectively). Among 43 curcuminoid analogs and 8 pairs of 1:1 mixture of curcuminoid analogs tested, 8 pure analogs and 5 isomeric mixtures of analogs exhibited high antitrypanosomal activity in submicromolar order of magnitude. Among these highly active analogs, 1,7-bis(4-hydroxy-3-methoxyphenyl)hept-4-en-3-one (40) was the most active compound, with an EC(50) value of 0.053+/-0.007 microM; it was about 2-fold more active than the standard veterinary drug diminazene aceturate (EC(50) 0.12+/-0.01 microM). Using a previously characterized diminazene-resistant T. b. brucei (TbAT1-KO) and a derived multi-drug resistant line (B48), no cross-resistance of curcuminoids was observed to the diamidine and melaminophenyl arsenical drugs that are the current treatments. Indeed, curcuminoids carrying a conjugated keto (enone) motif, including 40, were significantly more active against T. b. brucei B48. This enone motif was found to contribute to particularly high trypanocidal activity against all Trypanosoma species and strains tested. The parent curcuminoids showed low antileishmanial activity (EC(50) values of compounds 1 and 2 for Leishmania mexicana amastigotes are 16+/-3 and 37+/-6 microM, respectively) while the control drug, pentamidine, displayed an EC(50) of 16+/-2 microM. Among the active curcuminoid analogs, four compounds exhibited EC(50) values of less than 5 microM against Leishmania major promastigotes and four against L. mexicana amastigotes. No significant difference in sensitivity to curcuminoids between L. major promastigotes and L. mexicana amastigotes was observed. The parent curcuminoids and most of their analogs were also tested for their toxicity against human embryonic kidney (HEK) cells. All the curcuminoids exhibited lower toxicity to HEK cells than to T. b. brucei bloodstream forms and only one of the tested compounds showed significantly higher activity against HEK cells than curcumin (1). The selectivity index for T. b. brucei ranged from 3-fold to 1500-fold. The selectivity index for the most active analog, the enone 40, was 453-fold.

A luciferase based viability assay for ATP detection in 384-well format for high throughput whole cell screening of Trypanosoma brucei brucei bloodstream form strain 427

Background

Human African Trypanosomiasis (HAT) is caused by two trypanosome species, Trypanosoma brucei rhodesiense and Trypanosoma brucei gambiense. Current drugs available for the treatment of HAT have significant issues related to toxicity, administration regimes with limited effectiveness across species and disease stages, thus there is a considerable need to find alternative drugs. A well recognised approach to identify new drug candidates is high throughput screening (HTS) of large compound library collections.

Results

We describe here the development of a luciferase based viability assay in 384-well plate format suitable for HTS of T.b.brucei. The parameters that were explored to determine the final HTS assay conditions are described in detail and include DMSO tolerability, Z', diluents and cell inoculum density. Reference compound activities were determined for diminazene, staurosporine and pentamidine and compared to previously published IC₅₀ data obtained. The assay has a comparable sensitivity to reference drugs and is more cost effective than the 96-well format currently reported for T.b.brucei.

Conclusion

Due to the reproducibility and sensitivity of this assay it is recommended for potential HTS application. As it is commercially available this assay can also be utilised in many laboratories for both large and small scale screening.

Trypanocidal drugs: mechanisms, resistance and new targets

The protozoan parasitesTrypanosoma bruceiandTrypanosoma cruziare the causative agents of African trypanosomiasis and Chagas disease, respectively. These are debilitating infections that exert a considerable health burden on some of the poorest people on the planet. Treatment of trypanosome infections is dependent on a small number of drugs that have limited efficacy and can cause severe side effects. Here, we review the properties of these drugs and describe new findings on their modes of action and the mechanisms by which resistance can arise. We further outline how a greater understanding of parasite biology is being exploited in the search for novel chemotherapeutic agents. This effort is being facilitated by new research networks that involve academic and biotechnology/pharmaceutical organisations, supported by public–private partnerships, and are bringing a new dynamism and purpose to the search for trypanocidal agents.

Predictive computational models of substrate binding by a nucleoside transporter

Transporters play a vital role in both the resistance mechanisms of existing drugs and effective targeting of their replacements. Melarsoprol and diamidine compounds similar to pentamidine and furamidine are primarily taken up by trypanosomes of the genus Trypanosoma brucei through the P2 aminopurine transporter. In standardized competition experiments with [(3)H]adenosine, P2 transporter inhibition constants (K(i)) have been determined for a diverse dataset of adenosine analogs, diamidines, Food and Drug Administration-approved compounds and analogs thereof, and custom-designed trypanocidal compounds. Computational biology has been employed to investigate compound structure diversity in relation to P2 transporter interaction. These explorations have led to models for inhibition predictions of known and novel compounds to obtain information about the molecular basis for P2 transporter inhibition. A common pharmacophore for P2 transporter inhibition has been identified along with other key structural characteristics. Our model provides insight into P2 transporter interactions with known compounds and contributes to strategies for the design of novel antiparasitic compounds. This approach offers a quantitative and predictive tool for molecular recognition by specific transporters without the need for structural or even primary sequence information of the transport protein.

Genotypic status of the TbAT1/P2 adenosine transporter of Trypanosoma brucei gambiense isolates from Northwestern Uganda following melarsoprol withdrawal

Background

The development of arsenical and diamidine resistance in Trypanosoma brucei is associated with loss of drug uptake by the P2 purine transporter as a result of alterations in the corresponding T. brucei adenosine transporter 1 gene (TbAT1). Previously, specific TbAT1 mutant type alleles linked to melarsoprol treatment failure were significantly more prevalent in T. b. gambiense from relapse patients at Omugo health centre in Arua district. Relapse rates of up to 30% prompted a shift from melarsoprol to eflornithine (α-difluoromethylornithine, DFMO) as first-line treatment at this centre. The aim of this study was to determine the status of TbAT1 in recent isolates collected from T. b. gambiense sleeping sickness patients from Arua and Moyo districts in Northwestern Uganda after this shift in first-line drug choice.

Methodology and results

Blood and cerebrospinal fluids of consenting patients were collected for DNA preparation and subsequent amplification. All of the 105 isolates from Omugo that we successfully analysed by PCR-RFLP possessed the TbAT1 wild type allele. In addition, PCR/RFLP analysis was performed for 74 samples from Moyo, where melarsoprol is still the first line drug; 61 samples displayed the wild genotype while six were mutant and seven had a mixed pattern of both mutant and wild-type TbAT1. The melarsoprol treatment failure rate at Moyo over the same period was nine out of 101 stage II cases that were followed up at least once. Five of the relapse cases harboured mutant TbAT1, one had the wild type, while no amplification was achieved from the remaining three samples.

Conclusions/significance

The apparent disappearance of mutant alleles at Omugo may correlate with melarsoprol withdrawal as first-line treatment. Our results suggest that melarsoprol could successfully be reintroduced following a time lag subsequent to its replacement. A field-applicable test to predict melarsoprol treatment outcome and identify patients for whom the drug can still be beneficial is clearly required. This will facilitate cost-effective management of HAT in rural resource-poor settings, given that eflornithine has a much higher logistical requirement for its application.

Human African trypanosomiasis (HAT) manifests as a chronic infection caused by Trypanosoma brucei gambiense, or as a more acute form due to T. b. rhodesiense. Both manifestations occur in Uganda and melarsoprol use against the former was jeopardised in the 1990s as reports of reduced efficacy increased to the point where it was dismissed as first-line treatment at some treatment centers. Previous work to elucidate possible mechanisms leading to melarsoprol resistance pointed to a P2 type adenosine transporter known to mediate melarsoprol uptake and previously shown to be mutated in significant numbers of patients not responding to the drug. Our present findings indicate that there is a low prevalence of mutants in foci where melarsoprol relapses are infrequent. In addition we observe that at the Omugo focus where the drug was withdrawn as first line over 6 years ago, the mutant alleles have disappeared, suggesting that drug pressure is responsible for fuelling their spread. Thus constant monitoring for mutants could play a key role in cost-effective HAT management by identifying which foci can still use the less logistically demanding melarsoprol as opposed to the alternative drug eflornithine. What is required now is a simple method for identifying such mutants at the point of care, enabling practitioners to make informed prescriptions at first diagnosis.

Preclinical assessment of the treatment of second-stage African trypanosomiasis with cordycepin and deoxycoformycin

Background

There is an urgent need to substitute the highly toxic compounds still in use for treatment of the encephalitic stage of human African trypanosomiasis (HAT). We here assessed the treatment with the doublet cordycepin and the deaminase inhibitor deoxycoformycin for this stage of infection with Trypanosoma brucei (T.b.).

Methodology/Principal Findings

Cordycepin was selected as the most efficient drug from a direct parasite viability screening of a compound library of nucleoside analogues. The minimal number of doses and concentrations of the drugs effective for treatment of T.b. brucei infections in mice were determined. Oral, intraperitoneal or subcutaneous administrations of the compounds were successful for treatment. The doublet was effective for treatment of late stage experimental infections with human pathogenic T.b. rhodesiense and T.b. gambiense isolates. Late stage infection treatment diminished the levels of inflammatory cytokines in brains of infected mice. Incubation with cordycepin resulted in programmed cell death followed by secondary necrosis of the parasites. T.b. brucei strains developed resistance to cordycepin after culture with increasing concentrations of the compound. However, cordycepin-resistant parasites showed diminished virulence and were not cross-resistant to other drugs used for treatment of HAT, i.e. pentamidine, suramin and melarsoprol. Although resistant parasites were mutated in the gene coding for P2 nucleoside adenosine transporter, P2 knockout trypanosomes showed no altered resistance to cordycepin, indicating that absence of the P2 transporter is not sufficient to render the trypanosomes resistant to the drug.

Conclusions/Significance

Altogether, our data strongly support testing of treatment with a combination of cordycepin and deoxycoformycin as an alternative for treatment of second-stage and/or melarsoprol-resistant HAT.

There is an urgent need to substitute the highly toxic arsenic compounds still in use for treatment of the encephalitic stage of African trypanosomiasis, a disease caused by infection with Trypanosoma brucei. We exploited the inability of trypanosomes to engage in de novo purine synthesis as a therapeutic target. Cordycepin was selected from a trypanocidal screen of a 2200-compound library. When administered together with the adenosine deaminase inhibitor deoxycoformycin, cordycepin cured mice inoculated with the human pathogenic subspecies T. brucei rhodesiense or T. brucei gambiense even after parasites had penetrated into the brain. Successful treatment was achieved by intraperitoneal, oral or subcutaneous administration of the compounds. Treatment with the doublet also diminished infection-induced cerebral inflammation. Cordycepin induced programmed cell death of the parasites. Although parasites grown in vitro with low doses of cordycepin gradually developed resistance, the resistant parasites lost virulence and showed no cross-resistance to trypanocidal drugs in clinical use. Our data strongly support testing cordycepin and deoxycoformycin as an alternative for treatment of second-stage and/or melarsoprol-resistant HAT.

New treatment option for second-stage African sleeping sickness: in vitro and in vivo efficacy of aza analogs of DB289

African sleeping sickness is a fatal parasitic disease, and all drugs currently in use for treatment have strong liabilities. It is essential to find new, effective, and less toxic drugs, ideally with oral application, to control the disease. In this study, the aromatic diamidine DB75 (furamidine) and two aza analogs, DB820 and DB829 (CPD-0801), as well as their methoxyamidine prodrugs and amidoxime metabolites, were evaluated against African trypanosomes. The active parent diamidines showed similar in vitro profiles against different Trypanosoma brucei strains, melarsoprol- and pentamidine-resistant lines, and a P2 transporter knockout strain (AT1KO), with DB75 as the most trypanocidal molecule. In the T. b. rhodesiense strain STIB900 acute mouse model, the aza analogs DB820 and DB829 demonstrated activities superior to that of DB75. The aza prodrugs DB844 and DB868, as well as two metabolites of DB844, were orally more potent in the T. b. brucei strain GVR35 mouse central nervous system (CNS) model than DB289 (pafuramidine maleate). Unexpectedly, the parent diamidine DB829 showed high activity in the mouse CNS model by the intraperitoneal route. In conclusion, DB868 with oral and DB829 with parenteral application are potential candidates for further development of a second-stage African sleeping sickness drug.

Unsaturated Mannich bases active against multidrug-resistant Trypanosoma brucei brucei strains

A series of unsaturated Mannich bases possessing two electrophilic sites was recently identified as irreversible inhibitors of trypanothione reductase from Trypanosoma cruzi. New derivatives were synthesized by modifying the substitution pattern on the aromatic ring and by incorporating the melamine motif of melarsoprol. Their affinity to P2 transporter and their trypanocidal properties have been studied using three strains expressing various purine transporters. While the melamine derivatives showed some affinity to the P2 transporter, unsaturated Mannich bases without the melamine motif showed excellent potencies against pentamidine-resistant strains of T. brucei brucei suggesting alternative drug uptake routes. The Michael acceptor properties of the three most active compounds towards glutathione correlated with the observed trypanocidal activities.

Human equilibrative nucleoside transporter (ENT) family of nucleoside and nucleobase transporter proteins

1. The human (h) SLC29 family of integral membrane proteins is represented by four members, designated equilibrative nucleoside transporters (ENTs) because of the properties of the first-characterized family member, hENT1. They belong to the widely distributed eukaryotic ENT family of equilibrative and concentrative nucleoside/nucleobase transporter proteins. 2. A predicted topology of eleven transmembrane helices has been experimentally confirmed for hENT1. The best-characterized members of the family, hENT1 and hENT2, possess similar broad permeant selectivities for purine and pyrimidine nucleosides, but hENT2 also efficiently transports nucleobases. hENT3 has a similar broad permeant selectivity for nucleosides and nucleobases and appears to function in intracellular membranes, including lysosomes. 3. hENT4 is uniquely selective for adenosine, and also transports a variety of organic cations. hENT3 and hENT4 are pH sensitive, and optimally active under acidic conditions. ENTs, including those in parasitic protozoa, function in nucleoside and nucleobase uptake for salvage pathways of nucleotide synthesis and, in humans, are also responsible for the cellular uptake of nucleoside analogues used in the treatment of cancers and viral diseases. 4. By regulating the concentration of adenosine available to cell surface receptors, mammalian ENTs additionally influence physiological processes ranging from cardiovascular activity to neurotransmission.

Two novel nucleobase/pentamidine transporters from Trypanosoma brucei

African trypanosomes are unable to synthesize purines de novo and must salvage preformed purine nucleosides and nucleobases from their hosts. The Trypanosoma brucei genome project has identified 12 members of the equilibrative nucleoside transporter family, most of which have been characterized previously as nucleoside and/or nucleobase transporters. Here the 11th member of this family, TbNT11.1, has been functionally expressed in null mutants of Leishmania that are deficient in purine nucleoside or nucleobase uptake and identified as a high-affinity purine nucleobase transporter. Expression of TbNT11.1 in Xenopus oocytes revealed that it is also a transporter for the diamidine drug pentamidine that is the principal drug employed to treat early stage human African trypanosomiasis and may thus contribute to the uptake of this therapeutically important compound. In addition, characterization of the 12th member of the family, TbNT12.1, reveals that it is an adenine/pentamidine transporter.

Propidium iodide-based methods for monitoring drug action in the kinetoplastidae: comparison with the Alamar Blue assay

The urgent need for new drug development for African trypanosomiasis is widely recognized. This requires reliable and informative high-throughput assays. Currently, drug action is determined with a fluorimetric/colorimetric assay based on the metabolism of the dye Alamar Blue (resazurin) by live cells. However, this assay does not easily distinguish between cell death and growth arrest, or supply information about the rate at which test compounds affect these parameters. We report here an alternative fluorimetric assay, based on the interaction of propidium iodide with DNA, that allows either real-time monitoring of cell viability or the generation of EC(50) values at a predetermined time-point. The assay is highly sensitive and fluorescence readings easily correlate to numbers of parasites or DNA content. The EC(50) values were highly similar to those obtained with the standard Alamar Blue assay. The procedure lends itself readily to applications in drug development or resistance monitoring.

Drugs and transporters in kinetoplastid protozoa

Kinetoplastid protozoa express hundreds of membrane transport proteins that allow them to take up nutrients, establish ion gradients, efflux metabolites, translocate compounds from one intracellular compartment to another, and take up or export drugs. The combination of molecular cloning, genetic approaches, and the completed genome projects for Trypanosoma brucei, Leishmania major, and Trypanosoma cruzi have allowed detailed functional analysis of various transporters and predictions about the likely functions of others. Thus many opportunities exist to define the biological and pharmacological properties of parasite transporters whose genes were often difficult to identify in the pregenomic era. A subset of these transporters that are essential for parasite viability could serve as targets for novel drug therapies by identifying compounds that interfere with their uptake functions. Other permeases provide routes for uptake of selectively cytotoxic compounds and can thus be useful for delivery of drugs. Drug resistance may develop in strains where such drug uptake transporters are nonfunctional or in parasites that over-express other permeases that export a drug. A summary of recent work on Leishmania transporters for glucose and for purines is provided as an example of permeases that are being studied in molecular detail.

Molecular tools for the rapid detection of drug resistance in animal trypanosomes

There are currently 17 African countries in which animal trypanocidal drug resistance has been reported. Large-scale surveys were carried out in only ten of them. The lack of baseline information is mainly due to the fact that the methods currently available for the detection of drug resistance are laborious, expensive and time consuming. In this review the mechanisms involved in resistance to isometamidium and diminazene will be discussed, together with some new molecular detection tools that have been developed recently enabling faster diagnosis of drug resistance than conventional laboratory or field tests.

Antiparasitic compounds that target DNA

Designed, synthetic heterocyclic diamidines have excellent activity against eukaryotic parasites that cause diseases such as sleeping sickness and leishmania and adversely affect millions of people each year. The most active compounds bind specifically and strongly in the DNA minor groove at AT sequences. The compounds enter parasite cells rapidly and appear first in the kinetoplast that contains the mitochondrial DNA of the parasite. With time the compounds are also generally seen in the cell nucleus but are not significantly observed in the cytoplasm. The kinetoplast decays over time and disappears from the mitochondria of treated cells. At this point the compounds begin to be observed in other regions of the cell, such as the acidocalcisomes. The cells typically die in 24-48h after treatment. Active compounds appear to selectively target extended AT sequences and induce changes in kinetoplast DNA minicircles that cause a synergistic destruction of the catenated kinetoplast DNA network and cell death.

Detection of mutant P2 adenosine transporter (TbAT1) gene in Trypanosoma brucei gambiense isolates from northwest Uganda using allele-specific polymerase chain reaction

OBJECTIVE

To assess the application of allele-specific PCR (AS-PCR) as a fast, cheap and reliable method for detecting mutant TbAT1 associated with melarsoprol relapse in Trypanosoma brucei gambiense isolates from northwest Uganda.

METHODS

A total of 105 trypanosome isolates were analysed using SfaN1 restriction fragment length polymorphism (RFLP) and AS-PCR, the former used as the gold standard. Sensitivity, specificity, positive and negative predictive values of AS-PCR as well as agreement between the tests were determined.

RESULTS

Eleven trypanosome isolates had mutant TbAT1 while 94 exhibited the wild-type TbAT1 genes. There was a highly significant agreement between SfaN1 RFLP and AS-PCR with kappa and intra-class correlation values of 1.0. The sensitivity and specificity of AS-PCR were both 100%, while the positive and negative predictive values were found to be equal to 1.0. Cost and time analyses were performed and AS-PCR was 4.3 times cheaper than SfaN1 RFLP, in addition to the less time required for its execution.

CONCLUSION

AS-PCR should be the test of choice for screening for mutant TbAT1 in the ever-increasing numbers of field trypanosome isolates.

Genotypic and phenotypic characterization of Trypanosoma brucei gambiense isolates from Ibba, South Sudan, an area of high melarsoprol treatment failure rate

Resistance of trypanosomes to melarsoprol is ascribed to reduced uptake of the drug via the P2 nucleoside transporter. The aim of this study was to look for evidence of drug resistance in Trypanosoma brucei gambiense isolates from sleeping sickness patients in Ibba, South Sudan, an area of high melarsoprol failure rate. Eighteen T. b. gambiense stocks were phenotypically and only 10 strains genotypically characterized. In vitro, all isolates were sensitive to melarsoprol, melarsen oxide, and diminazene. Infected mice were cured with a 4 day treatment of 2.5mg/kg bwt melarsoprol, confirming that the isolates were sensitive. The gene that codes for the P2 transporter, TbATI, was amplified by PCR and sequenced. The sequences were almost identical to the TbAT1(sensitive) reference, except for one point mutation, C1384T resulting in the amino acid change proline-462 to serine. None of the described TbAT1(resistant)-type mutations were detected. In a T. b. gambiense sleeping sickness focus where melarsoprol had to be abandoned due to the high incidence of treatment failures, no evidence for drug resistant trypanosomes or for TbAT1(resistant)-type alleles of the P2 transporter could be found. These findings indicate that factors other than drug resistance contribute to melarsoprol treatment failures.

Induction of apoptosis-like cell death by pentamidine and doxorubicin through differential inhibition of topoisomerase II in arsenite-resistant L. donovani

The current study has been undertaken to investigate the sensitivity of the topoisomerase II (topo II) of wild type (Ld-Wt) and arsenite-resistant (Ld-As20) L. donovani to an anti-leishmanial agent pentamidine and an anti-cancer drug doxorubicin. We demonstrate that the cross resistance to pentamidine and doxorubicin in Ld-As20, was in part implicated through differential inhibition of topo II in Ld-Wt and Ld-As20. Further, the treatment of promastigotes at drug concentrations inhibiting 50% of topo II activity inflicted a regulated cell death sharing several apoptotic features like externalization of phosphatidylserine, loss of mitochondrial membrane potential, cytochrome C release into the cytosol, activation of cellular proteases and DNA fragmentation. The cytotoxic potential of pentamidine and doxorubicin in L. donovani has been shown to be mediated through topoisomerase II inhibition and results in inciting programmed cell death process.

Adenosine kinase of Trypanosoma brucei and its role in susceptibility to adenosine antimetabolites

Trypanosoma brucei cannot synthesize purines de novo and relies on purine salvage from its hosts to build nucleic acids. With adenosine being a preferred purine source of bloodstream-form trypanosomes, adenosine kinase (AK; EC 2.7.1.20) is likely to be a key player in purine salvage. Adenosine kinase is also of high pharmacological interest, since for many adenosine antimetabolites, phosphorylation is a prerequisite for activity. Here, we cloned and functionally characterized adenosine kinase from T. brucei (TbAK). TbAK is a tandem gene, expressed in both procyclic- and bloodstream-form trypanosomes, whose product localized to the cytosol of the parasites. The RNA interference-mediated silencing of TbAK suggested that the gene is nonessential under standard growth conditions. Inhibition or downregulation of TbAK rendered the trypanosomes resistant to cordycepin (3'-deoxyadenosine), demonstrating a role for TbAK in the activation of adenosine antimetabolites. The expression of TbAK in Saccharomyces cerevisiae complemented a null mutation in the adenosine kinase gene ado1. The concomitant expression of TbAK with the T. brucei adenosine transporter gene TbAT1 allowed S. cerevisiae ado1 ade2 double mutants to grow on adenosine as the sole purine source and, at the same time, sensitized them to adenosine antimetabolites. The coexpression of TbAK and TbAT1 in S. cerevisiae ado1 ade2 double mutants proved to be a convenient tool for testing nucleoside analogues for uptake and activation by T. brucei adenosine salvage enzymes.

2,N6-disubstituted adenosine analogs with antitrypanosomal and antimalarial activities

A library of 2,N(6)-disubstituted adenosine analogs was synthesized and the analogs were tested for their antiprotozoal activities. It was found that 2-methoxy and 2-histamino and N(6)-m-iodobenzyl substitutions generally produced analogs with low levels of antiprotozoal activity. The best antiplasmodial activity was achieved with large aromatic substitutions, such as N(6)-2,2-diphenylethyl and naphthylmethyl, which could indicate a mechanism of action through aromatic stacking with heme in the digestive vacuole of Plasmodium spp. The activities against Trypanosoma cruzi trypomastigotes and Leishmania donovani amastigotes were generally low; but several analogs, particularly those with cyclopentylamino substitutions, displayed potent activities against Trypanosoma brucei rhodesiense and T. b. brucei bloodstream forms in vitro. The most active were 2-cyclopentylamino-N(6)-cyclopentyladenosine (compound NA42) and 2-cyclopentylamino-N(6)-cyclopentyladenine (compound NA134), with the nucleobase an order of magnitude more potent than the nucleoside, at 26 +/- 4 nM. It was determined that the mode of action of these purines was trypanostatic, with the compounds becoming trypanocidal only at much higher concentrations. Those 2,N(6)-disubstituted purines tested for their effects on purine transport in T. b. brucei displayed at best a moderate affinity for the transporters. It is highly probable that the large hydrophobic substitutions, which bestow high calculated octanol-water coefficient values on the analogs, allow them to diffuse across the membrane. Consistent with this view, the analogs were as effective against a T. b. brucei strain lacking the P2 nucleoside transporter as they were against the parental strain. As the analogs were not toxic to human cell lines, the purine analogs are likely to act on a trypanosome-specific target.

Human African trypanosomiasis: pharmacological re-engagement with a neglected disease

This review discusses the challenges of chemotherapy for human African trypanosomiasis (HAT). The few drugs registered for use against the disease are unsatisfactory for a number of reasons. HAT has two stages. In stage 1 the parasites proliferate in the haemolymphatic system. In stage 2 they invade the central nervous system and brain provoking progressive neurological dysfunction leading to symptoms that include the disrupted sleep wake patterns that give HAT its more common name of sleeping sickness. Targeting drugs to the central nervous system offers many challenges. However, it is the cost of drug development for diseases like HAT, that afflict exclusively people of the world's poorest populations, that has been the principal barrier to new drug development and has led to them becoming neglected. Here we review drugs currently registered for HAT, and also discuss the few compounds progressing through clinical trials. Finally we report on new initiatives that might allow progress to be made in developing new and satisfactory drugs for this terrible disease.

Melarsoprol- and pentamidine-resistant Trypanosoma brucei rhodesiense populations and their cross-resistance

Resistance to melarsoprol and pentamidine was induced in bloodstream-form Trypanosoma brucei rhodesiense STIB 900 in vitro, and drug sensitivity was determined for melarsoprol, pentamidine and furamidine. The resistant populations were also inoculated into immunosuppressed mice to verify infectivity and to monitor whether rodent passage selects for clones with altered drug sensitivity. After proliferation in the mouse, trypanosomes were isolated and their IC(50) values to the three drugs were determined. To assess the stability of drug-induced resistance, drug pressure was ceased for 2 months and the drug sensitivity was determined again. Resistance was stable, with a few exceptions that are discussed. Drug IC(50)s indicated cross-resistance among all drugs, but to varying extents: resistance of the melarsoprol-selected and pentamidine-selected trypanosomes to pentamidine was the same, but the pentamidine-selected trypanosome population showed lower resistance to melarsoprol than the melarsoprol-selected trypanosomes. Interestingly, both resistant populations revealed the same intermediate cross-resistance to furamidine. Resistant trypanosome populations were characterised by molecular means, referring to the status of the TbAT1 gene. The melarsoprol-selected population apparently had lost TbAT1, whereas in the pentamidine-selected trypanosome population it was still present.

Pyrimidine transport activities in trypanosomes

Parasites of the Trypanosomatidae family are unable to synthesize purines. Instead, they rely on their hosts to supply these necessary compounds. The article by Gudin et al. identifies three transport mechanisms of the equilibrative nucleoside transporter family by which nucleosides and nucleobases are transported in this medically important family of organisms. The work by Gudin et al. characterizes the dynamics of these transporters and points to further areas for future genetic and therapeutic experiments.

Loss of the high-affinity pentamidine transporter is responsible for high levels of cross-resistance between arsenical and diamidine drugs in African trypanosomes

Treatment of many infectious diseases is under threat from drug resistance. Understanding the mechanisms of resistance is as high a priority as the development of new drugs. We have investigated the basis for cross-resistance between the diamidine and melaminophenyl arsenical classes of drugs in African trypanosomes. We induced high levels of pentamidine resistance in a line without the tbat1 gene that encodes the P2 transporter previously implicated in drug uptake. We isolated independent clones that displayed very considerable cross-resistance with melarsen oxide but not phenylarsine oxide and reduced uptake of [(3)H]pentamidine. In particular, the high-affinity pentamidine transport (HAPT1) activity was absent in the pentamidine-adapted lines, whereas the low affinity pentamidine transport (LAPT1) activity was unchanged. The parental tbat1(-/-) line was sensitive to lysis by melarsen oxide, and this process was inhibited by low concentrations of pentamidine, indicating the involvement of HAPT1. This pentamidine-inhibitable lysis was absent in the adapted line KO-B48. Likewise, uptake of the fluorescent diamidine 4',6-diamidino-2-phenylindole dihydrochloride was much delayed in live KO-B48 cells and insensitive to competition with up to 10 muM pentamidine. No overexpression of the Trypanosoma brucei brucei ATP-binding cassette transporter TbMRPA could be detected in KO-B48. We also show that a laboratory line of Trypanosoma brucei gambiense, adapted to high levels of resistance for the melaminophenyl arsenical drug melarsamine hydrochloride (Cymelarsan), had similarly lost TbAT1 and HAPT1 activity while retaining LAPT1 activity. It seems therefore that selection for resistance to either pentamidine or arsenical drugs can result in a similar phenotype of reduced drug accumulation, explaining the occurrence of cross-resistance.

Targeting of toxic compounds to the trypanosome's interior

Drugs can be targeted into African trypanosomes by exploiting carrier proteins at the surface of these parasites. This has been clearly demonstrated in the case of the melamine-based arsenical and the diamidine classes of drug that are already in use in the treatment of human African trypanosomiasis. These drugs can enter via an aminopurine transporter, termed P2, encoded by the TbAT1 gene. Other toxic compounds have also been designed to enter via this transporter. Some of these compounds enter almost exclusively through the P2 transporter, and hence loss of the P2 transporter leads to significant resistance to these particular compounds. It now appears, however, that some diamidines and melaminophenylarsenicals may also be taken up by other routes (of yet unknown function). These too may be exploited to target new drugs into trypanosomes. Additional purine nucleoside and nucleobase transporters have also been subverted to deliver toxic agents to trypanosomes. Glucose and amino acid transporters too have been investigated with a view to manipulating them to carry toxins into Trypanosoma brucei, and recent work has demonstrated that aquaglyceroporins may also have considerable potential for drug-targeting. Transporters, including those that carry lipids and vitamins such as folate and other pterins also deserve more attention in this regard. Some drugs, for example suramin, appear to enter via routes other than plasma-membrane-mediated transport. Receptor-mediated endocytosis has been proposed as a possible way in for suramin. Endocytosis also appears to be crucial in targeting natural trypanocides, such as trypanosome lytic factor (TLF) (apolipoprotein L1), into trypanosomes and this offers an alternative means of selectively targeting toxins to the trypanosome's interior. Other compounds may be induced to enter by increasing their capacity to diffuse over cell membranes; in this case depending exclusively on selective activity within the cell rather than selective uptake to impart selective toxicity. This review outlines studies that have aimed to exploit trypanosome nutrient uptake routes to selectively carry toxins into these parasites.

SSCP analysis of the P2 purine transporter TcoAT1 gene of Trypanosoma congolense leads to a simple PCR-RFLP test allowing the rapid identification of diminazene resistant stocks

Analyses were made on a Trypanosoma congolense contig coding a putative P2-like nucleoside transporter (the contig was named in this study TcoAT1). The sequence includes a start and stop codon and presents a high similarity with the gene TbAT1 of T. brucei (Smallest Sum Probability 2.8e-136). To investigate a possible link between point mutations and diminazene aceturate (DA) resistance in mice, the TcoAT1 putative genes of 26 T. congolense strains, characterised for DA sensitivity in the single dose mouse test, were screened by means of the Single Strand Conformation Polymorphism technique (SSCP). Results showed that the SSCP profiles of 23 out of 26 (88.5%) T. congolense strains were confirmed by the sensitivity test in mice with the commonly accepted criterion for sensitivity to diminazene being a CD80 of 20mg/kg in the mouse test. The remaining T. congolense strains showed a resistant SSCP profile and relapsed in mice after treatment at doses lower than 20mg/kg indicating that the SSCP is more sensitive than the single dose mouse test for the detection of resistance to diminazene. However, none of the strains used in this study showed a sensitive SSCP profile while they were resistant in the single dose mouse test. The sequencing of the TcoAT1 gene of two sensitive, two intermediate and two resistant strains allowed the set up of a PCR-RFLP test for the discrimination between sensitive and resistant strains confirming the SSCP results for the 26 strains of this study.

Synthesis, DNA affinity, and antiprotozoal activity of linear dications: Terphenyl diamidines and analogues

Diamidines 10a-g and 18a,b were obtained from dinitriles 9a-g and 15a,b by treatment with lithium trimethylsilylamide or upon hydrogenation of bis-O-acetoxyamidoximes. Dinitriles 9a-g were prepared via Suzuki reactions between arylboronic acids and arylnitriles. Potential prodrugs 12a-f and 17 were prepared via methylation of the diamidoximes 11a-f and 16a. Significant DNA affinities for rigid-rod molecules were observed. Compounds 10a, 10b, 10d, 18a, and 18b show IC50 values of 5 nM or less against Trypanosoma brucei rhodesiense (T. b. r.) and 10a, 10b, 10e, 18a, and 18b gave similar ones against Plasmodium falciparum (P.f.). The dications, 10a, 10d, 10f, and 10g are more active than furamidine in vivo. The prodrugs are only moderately effective on oral administration. Mouse liver microsome bioconversion of the methamidoxime prodrugs is significantly reduced from that of pafuramidine and suggests that the in vivo efficacy of these prodrugs is, in part, due to poor bioconversion.

Roles for theTrypanosoma bruceiP2 Transporter in DB75 Uptake and Resistance

A novel trypanocide, 2,5-bis(4-amidinophenyl)furan (DB75), in its prodrug amidoxime-derivative form, 2,5-bis(4-amidinophenyl)furan-bis-O-methylamidoxime (DB289), is in trials as the first orally administered drug for human African trypanosomiasis. DB75 is a diamidine. Resistance to some diamidines correlates to loss of uptake via the P2 aminopurine transporter. We show here that uptake of DB75 into Trypanosoma brucei also occurs principally via the P2 transporter. Uptake of tritiated DB75 occurred via a high-affinity (K(m app), 3.2 microM) carriermediated route that was inhibited by adenosine, adenine, and pentamidine, all known substrates of the P2 transporter. Trypanosomes lacking the TbAT1 gene that encodes the P2 transporter demonstrated an 11-fold reduction in sensitivity to DB75 when measured under controlled in vitro conditions. These knockout cells were also less sensitive to DB75 than wild-type cells in mice. Initial uptake rates of DB75 into the Deltatbat1 knockout cell line were greatly reduced compared with rates in wild-type cells. A trypanosome cell line selected in vitro for DB75 resistance was shown to have lost P2-mediated DB75 uptake. The TbAT1 gene was mapped to chromosome V of the T. brucei genome and the DB75-resistant parasites were shown to have deleted both alleles of this gene. Fluorescence microscopy of DB75-treated trypanosomes revealed that DB75 fluorescence localizes rapidly within the DNA-containing organelles of wild-type trypanosomes, whereas no fluorescence was observed in Deltatbat1-null parasites or in the parasites selected for resistance to DB75.

Dicationic DNA-targeted antiprotozoal agents: naphthalene replacement of benzimidazole

A series of naphthalene analogues of highly active benzimidazole diamidines were synthesized using sequential Stille and Suzuki coupling reactions for preparation of the bis-nitrile intermediates. All of the diamidines showed strong DNA affinities as judged by high DeltaTm values with poly(dA-dT). The dicationic compounds were quite active in vitro versus Trypanosoma brucei rhodesiense (T. b. r.) exhibiting IC50 values ranging from 4 to 98 nM. These compounds were also active versus Plasmodium falciparum (P. f.) giving IC50 values ranging from 4 to 33 nM. Two of the compounds showed good activity in vivo in the STIB900 model for acute African trypanosomiasis; one gave 3/4 cures and the other gave 4/4 cures on ip dosage of 20 mg/kg for 4 days. The amidoxime prodrugs of the naphthalene analogues were essentially ineffective.

DNA binding affinity of bisguanidine and bis(2-aminoimidazoline) derivatives with in vivo antitrypanosomal activity

A new antitrypanosomal hit compound that cures an acute (STIB 900) mouse model of Trypanosoma brucei rhodesiense trypanosomiasis is described. This bis(2-aminoimidazolinium) dicationic compound proved to be an excellent DNA minor groove binder, suggesting a possible mechanism for its trypanocidal activity. From these studies, the 4,4'-diaminodiphenylamine skeleton emerged as a good scaffold for antitrypanosomal drugs.

Molecular and functional characterization of a spermidine transporter (TcPAT12) from Trypanosoma cruzi

Trypanosoma cruzi, the etiological agent of Chagas' disease, is the only eukaryotic cell which lacks the ability to synthesize polyamines de novo. In this work, we describe for the first time the molecular and biochemical properties of a high-affinity spermidine transporter from T. cruzi. The transporter gene TcPAT12 was functionally expressed in Xenopus laevis oocytes, showing high levels of spermidine uptake. Similar apparent affinity constants for spermidine uptake were obtained when comparing T. cruzi epimastigotes and heterologous expressed TcPAT12 in X. laevis. In addition, TcPAT12 also transports putrescine and the amino acid l-arginine at lower rates than spermidine.

3D QSAR on a library of heterocyclic diamidine derivatives with antiparasitic activity

African trypanosomes, Trypanosoma brucei rhodesiense (TBR) and Trypanosoma brucei gambiense (TBG), affect hundreds of thousands of lives in tropical regions of the world. The toxicity of the diamidine pentamidine, an effective drug against TBG, necessitates the design of better drugs. An orally effective prodrug of the diamidine, furamidine (DB75), presently scheduled for phase III clinical trials, has excellent activity against TBG with toxicity lower than that of pentamidine. As part of an effort to develop additional and improved diamidines against African trypanosomes, CoMFA and CoMSIA 3D QSAR analyses have been conducted with furamidine and a set of 25 other structurally related compounds. Two different alignment strategies, based on a putative kinetoplast DNA minor groove target, were used. Due to conserved electrostatic properties across the compounds, models that used only steric and electronic properties did not perform well in predicting biological results. An extended CoMSIA model with additional descriptors for hydrophobic, donor, and acceptor properties had good predictive ability with a q2=0.699, r2=0.974, SEE, standard error of estimate=0.1, and F=120.04. The results have been used as a guide to design compounds that, potentially, have better activity against African trypanosomes.

A proteomic analysis of arsenical drug resistance inTrypanosoma brucei

We have undertaken 2-DE and MS to identify proteins associated with arsenical drug resistance in Trypanosoma brucei. This parasite causes sleeping sickness in humans, and arsenical drug resistance is a significant potential problem. Comparative analysis of approximately 2000 spots resolved by 2-DE in the soluble proteomes of drug-sensitive and drug-resistant isogenic lines of T. brucei identified a protein spot whose absence associated with resistance to the arsenical drug, Cymelarsan. MS matched this protein to an identical pair of tandem genes Tb09.211.0120 and 0130 that encode a putative nascent polypeptide associated complex subunit. This protein also occurs as an isoform located in both resistant and sensitive lines at a similar molecular weight, but different pI. The difference between isogenic lines was confirmed by Western blot using an antibody against recombinant protein. Both genes were identical in sequence between drug-sensitive and drug-resistant lines and both were transcribed as determined by RT-PCR. We postulate that the missing protein isoform arose due to the lack of a PTM.

Aromatic diamidines as antiparasitic agents

Parasitic infections are widespread in developing countries and frequently associated with immunocompromised patients in developed countries. Consequently, such infections are responsible for a significant amount of human mortality, morbidity and economic hardship. A growing consensus has identified the urgent need for the development of new antiparasitic compounds, mostly due to the large number of drug-resistant parasites and the fact that currently available drugs are expensive, highly toxic, require long treatment regimens and frequently exhibit significantly reduced activity towards certain parasite strains and evolutive stages. In this context, the activity of aromatic diamidines has been explored against a widespread range of micro-organisms, and the authors' present aim is to review the current status of chemotherapy with these compounds against human parasitic infections.

Rapid identification of isometamidium-resistant stocks of Trypanosoma b. brucei by PCR-RFLP

Analyses were made on the adenosine transporter-1 gene in Trypanosoma brucei (TbAT1), encoding a P2-like nucleoside transporter, from T. brucei brucei field stocks to investigate a possible link between the presence of mutations in this gene and isometamidium resistance. We have analysed the gene from 11 isometamidium-sensitive field stocks isolated from cattle in Uganda, two sensitive reference clones and two resistant reference clones. A sequence alignment showed that the isometamidium-sensitive T. b. brucei contained the wild-type sequence patterns. In contrast, the isometamidium-resistant T. b. brucei stocks showed the mutant-type sequence patterns with six point mutations that had previously been reported in a laboratory-derived arsenical-resistant T. brucei strain. To analyse the restriction fragment length polymorphism pattern of a fragment of TbAT1 (nucleotides 430-1108), the 677-bp polymerase chain reaction products from eight of the isometamidium-sensitive and two of the isometamidium-resistant T. b. brucei were subjected to digestion with Sfa NI. The results revealed two different banding patterns: the digest resulted in fragment sizes of 566 and 111 bp in the case of TbAT1 from isometamidium-sensitive stocks, whereas it produced fragment sizes of 435 and 242 bp in the case of TbAT1 from isometamidium-resistant stocks. Thus, the isometamidium-sensitive and isometamidium-resistant T. b. brucei could be successfully distinguished by digestion with the restriction endonuclease Sfa NI.

Interaction of monobenzamidine-linked trypanocides with the Trypanosoma brucei P2 aminopurine transporter

Single benzamidine group-carrying compounds were shown to interact with the Trypanosoma brucei P2 aminopurine transporter. Replacement of the amidine with a guanidine group decreased affinity. Trypanocidal activity was evident, but compounds were equally toxic against trypanosomes lacking the P2 transporter, which indicates additional uptake routes for monobenzamidine-derived compounds.

Acquired resistance to berenil in a cloned isolate of Trypanosoma evansi is associated with upregulation of a novel gene, TeDR40

Drug resistance is now a severe and increasing problem in trypanosomes, but molecular details of mechanisms of resistance are only beginning to unveil. There is urgent need to clearly elucidate the different mechanisms of drug resistance in trypanosomes in order to circumvent existing resistance problems and avoid emergence of resistance to the next generation drugs. In this study, we cloned and characterized a novel gene, TeDR40, whose expression is associated with resistance to berenil in Trypanosoma evansi. Expression analysis showed that the gene was at least 1000-fold upregulated in resistant parasites and the encoded protein appeared to have a ubiquitous cellular localization. To investigate the association of TeDR40 with berenil-resistance, we genetically modified wild-type berenil-sensitive T. evansi for inducible over-expression of the TeDR40 gene. Induction of over-expression of TeDR40 in T. evansi led to decreased (P < 0.01) sensitivity to berenil. Our findings indicate a possible correlation between over-expression of a novel gene, TeDR40, and reduced sensitivity to berenil in an in vitro-cultured clonal line of T. evansi.

Dicationic near-linear biphenyl benzimidazole derivatives as DNA-targeted antiprotozoal agents

A series of near-linear biphenyl benzimidazole diamidines 5a-h were synthesized from their respective diamidoximes (4a-h), through the bis-O-acetoxyamidoxime, followed by hydrogenation in glacial acetic acid/ethanol in the presence of Pd-C. Compounds 4a-h were obtained in three steps, starting with the Suzuki coupling reaction of the appropriate haloarylcarbonitriles 1a-g or 4-bromo-2-fluorobenzaldehyde with 4-formylphenylboronic acid or 4-cyanophenylboronic acid to form the anticipated 4-formylbiphenyl carbonitrile analogues 2a-h. Subsequent condensation of the formyl derivatives 2a-h with 3,4-diaminobenzonitrile in the presence of sodium bisulfite or 1,4-benzoquinone gave the desired dinitriles 3a-h, the precursors for 4a-h. All the diamidines showed strong DNA affinities, as judged by high Delta Tm values with poly(dA.dT)2) The compounds were quite active in vitro versus Trypanosoma brucei rhodesiense, giving IC50 values ranging from 3 to 37 nM. These compounds were even more active versus Plasmodium falciparum, exhibiting IC50 values ranging from 0.5 to 23 nM. The compounds showed moderate to good activity in vivo in the STIB900 model for acute African trypanosomiasis. The most active compounds 5b and e gave 3/4 cures on an IP dosage of 20 mg/kg.