Trypanosoma brucei: lack of cross-resistance to melarsoprol in vitro by cymelarsan-resistant parasites

Drug resistance in animal trypanosomiases: Epidemiology, mechanisms and control strategies

Animal trypanosomiasis (AT) is a complex of veterinary diseases known under various names such as nagana, surra, dourine and mal de caderas, depending on the country, the infecting trypanosome species and the host. AT is caused by parasites of the genus Trypanosoma, and the main species infecting domesticated animals are T. brucei brucei, T. b. rhodesiense, T. congolense, T. simiae, T. vivax, T. evansi and T. equiperdum. AT transmission, again depending on species, is through tsetse flies or common Stomoxys and tabanid flies or through copulation. Therefore, the geographical spread of all forms of AT together is not restricted to the habitat of a single vector like the tsetse fly and currently includes almost all of Africa, and most of South America and Asia. The disease is a threat to millions of companion and farm animals in these regions, creating a financial burden in the billions of dollars to developing economies as well as serious impacts on livestock rearing and food production. Despite the scale of these impacts, control of AT is neglected and under-resourced, with diagnosis and treatments being woefully inadequate and not improving for decades. As a result, neither the incidence of the disease, nor the effectiveness of treatment is documented in most endemic countries, although it is clear that there are serious issues of resistance to the few old drugs that are available. In this review we particularly look at the drugs, their application to the various forms of AT, and their mechanisms of action and resistance. We also discuss the spread of veterinary trypanocide resistance and its drivers, and highlight current and future strategies to combat it.

Systematic Review and Meta-Analysis on Human African Trypanocide Resistance

Background Human African trypanocide resistance (HATr) is a challenge for the eradication of Human African Trypansomiaisis (HAT) following the widespread emergence of increased monotherapy drug treatment failures against Trypanosoma brucei gambiense and T. b. rhodesiense that are associated with changes in pathogen receptors. Methods: Electronic searches of 12 databases and 3 Google search websites for human African trypanocide resistance were performed using a keyword search criterion applied to both laboratory and clinical studies. Fifty-one publications were identified and included in this study using the PRISMA checklist. Data were analyzed using RevMan and random effect sizes were computed for the statistics at the 95% confidence interval. Results: Pentamidine/melarsoprol/nifurtimox cross-resistance is associated with loss of the T. brucei adenosine transporter 1/purine 2 gene (TbAT1/P2), aquaglyceroporins (TbAQP) 2 and 3, followed by the high affinity pentamidine melarsoprol transporter (HAPT) 1. In addition, the loss of the amino acid transporter (AAT) 6 is associated with eflornithine resistance. Nifurtimox/eflornithine combination therapy resistance is associated with AAT6 and nitroreductase loss, and high resistance and parasite regrowth is responsible for treatment relapse. In clinical studies, the TbAT1 proportion of total random effects was 68% (95% CI: 38.0−91.6); I2 = 96.99% (95% CI: 94.6−98.3). Treatment failure rates were highest with melarsoprol followed by eflornithine at 41.49% (95% CI: 24.94−59.09) and 6.56% (3.06−11.25) respectively. HATr-resistant phenotypes used in most laboratory experiments demonstrated significantly higher pentamidine resistance than other trypanocides. Conclusion: The emergence of drug resistance across the spectrum of trypanocidal agents that are used to treat HAT is a major threat to the global WHO target to eliminate HAT by 2030. T. brucei strains were largely resistant to diamidines and the use of high trypanocide concentrations in clinical studies have proved fatal in humans. Studies to develop novel chemotherapeutical agents and identify alternative protein targets could help to reduce the emergence and spread of HATr.

Laboratory Selection of Trypanosomatid Pathogens for Drug Resistance

The selection of parasites for drug resistance in the laboratory is an approach frequently used to investigate the mode of drug action, estimate the risk of emergence of drug resistance, or develop molecular markers for drug resistance. Here, we focused on the How rather than the Why of laboratory selection, discussing different experimental set-ups based on research examples with Trypanosoma brucei, Trypanosoma cruzi, and Leishmania spp. The trypanosomatids are particularly well-suited to illustrate different strategies of selecting for drug resistance, since it was with African trypanosomes that Paul Ehrlich performed such an experiment for the first time, more than a century ago. While breakthroughs in reverse genetics and genome editing have greatly facilitated the identification and validation of candidate resistance mutations in the trypanosomatids, the forward selection of drug-resistant mutants still relies on standard in vivo models and in vitro culture systems. Critical questions are: is selection for drug resistance performed in vivo or in vitro? With the mammalian or with the insect stages of the parasites? Under steady pressure or by sudden shock? Is a mutagen used? While there is no bona fide best approach, we think that a methodical consideration of these questions provides a helpful framework for selection of parasites for drug resistance in the laboratory.

Trypanosoma brucei aquaglyceroporin 2 is a high-affinity transporter for pentamidine and melaminophenyl arsenic drugs and the main genetic determinant of resistance to these drugs

Objectives

Trypanosoma brucei drug transporters include the TbAT1/P2 aminopurine transporter and the high-affinity pentamidine transporter (HAPT1), but the genetic identity of HAPT1 is unknown. We recently reported that loss of T. brucei aquaglyceroporin 2 (TbAQP2) caused melarsoprol/pentamidine cross-resistance (MPXR) in these parasites and the current study aims to delineate the mechanism by which this occurs.

Methods

The TbAQP2 loci of isogenic pairs of drug-susceptible and MPXR strains of T. brucei subspecies were sequenced. Drug susceptibility profiles of trypanosome strains were correlated with expression of mutated TbAQP2 alleles. Pentamidine transport was studied in T. brucei subspecies expressing TbAQP2 variants.

Results

All MPXR strains examined contained TbAQP2 deletions or rearrangements, regardless of whether the strains were originally adapted in vitro or in vivo to arsenicals or to pentamidine. The MPXR strains and AQP2 knockout strains had lost HAPT1 activity. Reintroduction of TbAQP2 in MPXR trypanosomes restored susceptibility to the drugs and reinstated HAPT1 activity, but did not change the activity of TbAT1/P2. Expression of TbAQP2 sensitized Leishmania mexicana promastigotes 40-fold to pentamidine and >1000-fold to melaminophenyl arsenicals and induced a high-affinity pentamidine transport activity indistinguishable from HAPT1 by K_m and inhibitor profile. Grafting the TbAQP2 selectivity filter amino acid residues onto a chimeric allele of AQP2 and AQP3 partly restored susceptibility to pentamidine and an arsenical.

Conclusions

TbAQP2 mediates high-affinity uptake of pentamidine and melaminophenyl arsenicals in trypanosomes and TbAQP2 encodes the previously reported HAPT1 activity. This finding establishes TbAQP2 as an important drug transporter.

In vivo imaging of trypanosome-brain interactions and development of a rapid screening test for drugs against CNS stage trypanosomiasis

HUMAN AFRICAN TRYPANOSOMIASIS (HAT) MANIFESTS IN TWO STAGES OF DISEASE: firstly, haemolymphatic, and secondly, an encephalitic phase involving the central nervous system (CNS). New drugs to treat the second-stage disease are urgently needed, yet testing of novel drug candidates is a slow process because the established animal model relies on detecting parasitemia in the blood as late as 180 days after treatment. To expedite compound screening, we have modified the GVR35 strain of Trypanosoma brucei brucei to express luciferase, and have monitored parasite distribution in infected mice following treatment with trypanocidal compounds using serial, non-invasive, bioluminescence imaging. Parasites were detected in the brains of infected mice following treatment with diminazene, a drug which cures stage 1 but not stage 2 disease. Intravital multi-photon microscopy revealed that trypanosomes enter the brain meninges as early as day 5 post-infection but can be killed by diminazene, whereas those that cross the blood-brain barrier and enter the parenchyma by day 21 survived treatment and later caused bloodstream recrudescence. In contrast, all bioluminescent parasites were permanently eliminated by treatment with melarsoprol and DB829, compounds known to cure stage 2 disease. We show that this use of imaging reduces by two thirds the time taken to assess drug efficacy and provides a dual-modal imaging platform for monitoring trypanosome infection in different areas of the brain.

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.

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.

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.

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.

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.

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.

Effective measures for controlling trypanosomiasis

African trypanosomiasis, otherwise known as sleeping sickness in humans and 'Nagana' in cattle, is a disease that is resurgent in Africa. Research on the disease suggests that the development of a vaccine is still far away; even existing drugs are becoming ineffective on account of the emergence of drug-resistant trypanosomes. All this contributes to heavy economic losses and a sociopolitical crisis in the continent, thus underscoring the pressure to intensify research for inexpensive, less toxic and affordable trypanocides. This review discusses the current treatment of trypanosomiasis and the progress made towards the effective control of trypanosomiasis.

Mechanisms of arsenical and diamidine uptake and resistance in Trypanosoma brucei

Sleeping sickness, caused by Trypanosoma brucei spp., has become resurgent in sub-Saharan Africa. Moreover, there is an alarming increase in treatment failures with melarsoprol, the principal agent used against late-stage sleeping sickness. In T. brucei, the uptake of melarsoprol as well as diamidines is thought to be mediated by the P2 aminopurine transporter, and loss of P2 function has been implicated in resistance to these agents. The trypanosomal gene TbAT1 has been found to encode a P2-type transporter when expressed in yeast. Here we investigate the role of TbAT1 in drug uptake and drug resistance in T. brucei by genetic knockout of TbAT1. Tbat1-null trypanosomes were deficient in P2-type adenosine transport and lacked adenosine-sensitive transport of pentamidine and melaminophenyl arsenicals. However, the null mutants were only slightly resistant to melaminophenyl arsenicals and pentamidine, while resistance to other diamidines such as diminazene was more pronounced. Nevertheless, the reduction in drug sensitivity might be of clinical significance, since mice infected with tbat1-null trypanosomes could not be cured with 2 mg of melarsoprol/kg of body weight for four consecutive days, whereas mice infected with the parental line were all cured by using this protocol. Two additional pentamidine transporters, HAPT1 and LAPT1, were still present in the null mutant, and evidence is presented that HAPT1 may be responsible for the residual uptake of melaminophenyl arsenicals. High-level arsenical resistance therefore appears to involve the loss of more than one transporter.

The trypanocide diminazene aceturate is accumulated predominantly through the TbAT1 purine transporter: additional insights on diamidine resistance in african trypanosomes

Resistance to diminazene aceturate (Berenil) is a severe problem in the control of African trypanosomiasis in domestic animals. It has been speculated that resistance may be the result of reduced diminazene uptake by the parasite. We describe here the mechanisms by which [(3)H]diminazene is transported by Trypanosoma brucei brucei bloodstream forms. Diminazene was rapidly accumulated through a single transporter, with a K(m) of 0.45 +/- 0.11 micro M, which was dose dependently inhibited by pentamidine and adenosine. The K(i) values for these inhibitors were consistent with this transporter being the P2/TbAT1 adenosine transporter. Yeast expressing TbAT1 acquired the ability to take up [(3)H]diminazene and [(3)H]pentamidine. TbAT1-null mutants had lost almost all capacity for [(3)H]diminazene transport. However, this cell line still displayed a small but detectable rate of [(3)H]diminazene accumulation, in a nonsaturable manner. We conclude that TbAT1 mediates [(3)H]diminazene transport almost exclusively and that this explains the observed diminazene resistance phenotypes of TbAT1-null mutants and field isolates.

Drug transport and drug resistance in African trypanosomes

Drug resistance in African trypanosomes has been studied for almost a hundred years. Beginning with Paul Ehrlich's work that led to the chemoreceptor hypothesis, reduction of net drug uptake has emerged as the most frequent cause of resistance. This review, therefore, focuses on trypanosomal drug transporter genes. TbAT1 encodes purine permease P2, which mediates influx of melarsoprol and diamidines. Disruption of TbAT1 in Trypanosoma brucei reduced sensitivity to these trypanocides. TbMRPA encodes a putative trypanothione-conjugate efflux pump, and overexpression of TbMRPA in T. brucei causes melarsoprol resistance. It will be important to determine the role of TbAT1 and TbMRPA in sleeping sickness treatment failures.

Arsenicals (melarsoprol), pentamidine and suramin in the treatment of human African trypanosomiasis

Human African trypanosomiasis (HAT), otherwise known as sleeping sickness, has remained a disease with no effective treatment. Recent progress in HAT research suggests that a vaccine against the disease is far from being successful. Also the emergence of drug-resistant trypanosomes makes further work in this area imperative. So far the treatment for the early stage of HAT involves the drugs pentamidine and suramin which have been very successful. In the second stage of the disease, during which the trypanosomes reside in the cerebrospinal fluid (CSF), treatment is dependent exclusively on the arsenical compound melarsoprol. This is largely due to the inability to find compounds that can cross the blood brain barrier and kill the CSF-residing trypanosomes. This review summarises our current understanding on the treatment of HAT.

Changes in properties of adenosine transporters in Trypanosoma evansi and modes of selection of resistance to the melaminophenyl arsenical drug, Mel Cy

Resistance to arsenical drugs in trypanosomes has been linked to changes in adenosine uptake. The transport of melaminophenyl arsenicals into Trypanosoma brucei was shown to be mediated by an unusual adenosine nucleoside transporter, P2 (Carter and Fairlamb, 1993), and the loss of this transporter is associated with resistance to melaminophenyl arsenicals in these parasites. To further understand the mechanisms of arsenical resistance, we generated several lines of Mel Cy-resistant T. evansi from a drug-sensitive isolate using both in vivo and in vitro selection methods. Uptake of the melaminophenyl arsenical, Mel Cy on the P2 transporter was studied in the drug-sensitive as well as Mel Cy-resistant parasites, by means of inhibition of Mel Cy-induced lysis of trypanosomes, in an in vitro lysis assay. Adenosine uptake was also investigated using competition inhibition assays. Our study shows that T. evansi, TREU 1840, possesses the P1/P2 adenosine transport system as reported in T. brucei and T. equiperdum. However, in T. evansi, the P2 transporter is the larger transport process instead of the P1. The P2 transporter in T. evansi mediated the uptake of Mel Cy in the drug-sensitive parasites. The P2 was retained in all the arsenical-resistant T. evansi lines studied. However, the activity of the transporter was reduced to different extents in the different-resistant lines. The residual P2 activity related well to the levels of drug resistance in each line, suggesting that P2 activity could be an important marker for arsenical resistance. Furthermore, important differences were observed between the in vivo- and the in vitro-selected arsenical-resistant parasites suggesting that there may be differences in resistance phenotypes selected on the field.

Novel alkylpolyamine analogues that possess both antitrypanosomal and antimicrosporidial activity

A novel series of alkyl- or aralkyl-substituted polyamine analogues was synthesized containing a 3-7-3 polyamine backbone. These analogues were evaluated in vitro, and in one case in vivo, for activity as antitrypanosomal agents, and for activity against opportunistic infection caused by Microsporidia. Compound 21 inhibits trypanosomal growth with an IC(50) as low as 31nM, while compound 24 shows promising activity in vitro against trypanosomes, and against Microsporidia in vitro and in vivo.

Transporters in African trypanosomes: role in drug action and resistance

Sleeping sickness is an increasing problem in many parts of sub-Saharan Africa. The problems are compounded by the lack of new medication, and the increasing resistance against traditional drugs such as melarsoprol, berenil and isometamidium. Over the last few years, much progress has been made in understanding how drug action, and the development of resistance, is related to the mechanisms by which the parasite ingests the drugs. In some cases novel transporters have been identified. In other cases, transporters do not appear to be involved in drug uptake, and selectivity must lie with other parasite features, such as a specific target or activation of the drug. Lessons learned from studying the uptake of drugs currently in use may assist the design of a much needed new generation of trypanocides.

Contribution of dithiol ligands to in vitro and in vivo trypanocidal activities of dithiaarsanes and investigation of ligand exchange in an aqueous solution

Twelve new dithiaarsanes were evaluated for their in vitro and in vivo trypanocidal properties in regard to their three parent molecules, 4-amino-phenylarsenoxide, melarsenoxide, and 4-dansylamino-phenylarsenoxide. The most potent dithiaarsane, compound 2b, had a minimum effective concentration of 1.5 nM after 48 h of incubation and at a dose of 0.39 micromol/kg of body weight (0.2 mg/kg) administered subcutaneously cured 100% of mice acutely infected with Trypanosoma brucei brucei CMP. With this model, the chemotherapeutic index of compound 2b was 512, compared to 256 for melarsamine dihydrochloride (Cymelarsan) under the same conditions. With a chronic infection produced by T. brucei brucei GVR, compound 2b cured 100% of mice after treatment at a dose of 25 micromol/kg (12.5 mg/kg) for 4 consecutive days, whereas melarsamine dihydrochloride and potassium melarsonyl (Trimelarsan) cured less than 50% mice at this dose. For both acute and late-stage infections, dithiaarsanes having a melaminophenyl ring exhibited the most-potent trypanocidal activity. Compound 2b is thus one of the most active organoarsenicals described in a mouse trypanosomiasis model. Considering that the main intracellular targets of organoarsenicals are thiol groups, we studied the possibility of ligand exchange between Cymelarsan and several dithiols. In aqueous solution, we observed a rapid exchange of cysteamine from melarsamine with free cysteamine and also with various dithiols always in favor of more stable cyclic derivatives. These ligand exchanges suggest the ability of trivalent organoarsenicals to react with targets such as trypanothione and dihydrolipoic acid. Among several ligands, a 1,3-dimercaptopropane moiety appeared the most suitable for trypanocidal activity.

Differential regulation of nucleoside and nucleobase transporters in Crithidia fasciculata and Trypanosoma brucei brucei

The regulation of the activity of purine transporters in two protozoan species, Crithidia fasciculata and Trypanosoma brucei brucei, was investigated in relation to purine availability and growth cycle. In C. fasciculata, two high-affinity purine nucleoside transporters were identified. The first, designated CfNT1, displayed a K(m) of 9.4 +/- 2.8 microM for adenosine and was inhibited by pyrimidine nucleosides as well as adenosine analogues; a second C. fasciculata nucleoside transporter (CfNT2) recognized inosine (K(m) = 0.38 +/- 0.06 microM) and guanosine but not adenosine. The activity of both transporters increased in cells at mid-logarithmic growth, as compared to cells in the stationary phase, and was also stimulated 5-15-fold following growth in purine-depleted medium. These increased rates were due to increased Vmax values (K(m) remained unchanged) and inhibited by cycloheximide (10 microM). In the procyclic forms of T. b. brucei, adenosine transport by the P1 transporter was upregulated by purine starvation but only after 48 h, whereas hypoxanthine transport was maximally increased after 24 h. The latter effect was due to the expression of an additional hypoxanthine transporter, H2, that is normally absent from procyclic forms of T. b. brucei and was characterised by its high affinity for hypoxanthine (K(m) approximately 0.2 microM) and its sensitivity to inhibition by guanosine. The activity of the H1 hypoxanthine transporter (K(m) approximately 10 microM) was unchanged. These results show that regulation of the capacity of the purine transporters is common in different protozoa, and that, in T. b. brucei, various purine transporters are under differential control.

The biochemical basis of arsenical-diamidine crossresistance in African trypanosomes

Resistance to currently used drugs is a serious problem in most fields of antimicrobial chemotherapy. Crossresistance between two of the major classes of drug used in the treatment of African trypanosomiasis, the melaminophenyl arsenicals and diamidines is easily selected in the laboratory. Here, Mike Barrett and Alan Fairlamb outline the mechanism underlying this crossresistance, which appears to arise as a result of alterations in an unusual adenosine transporter involved in the uptake of these drugs.

Surface receptors and transporters of Trypanosoma brucei

African trypanosomes combine antigenic variation of their surface coat with the ability to take up nutrients from their mammalian hosts. Uptake of small molecules such as glucose or nucleosides is mediated by translocators hidden from host antibodies by the surface coat. The multiple glucose transporters and transporters for nucleobases and nucleosides have been characterized. Receptors for host macromolecules such as transferrin and lipoproteins are visible to antibodies but hidden from the cellular arm of the host immune system in an invagination of the trypanosome surface, the flagellar pocket. The trypanosomal transferrin receptor is a heterodimer that resembles the major component of the surface coat of Trypanosoma brucei. The ability to make several versions of this receptor allows T. brucei to bind transferrins from a range of mammals with high affinity. The proteins required for uptake of nutrients by trypanosomes provide a target for chemotherapy that remains to be fully exploited.