Induction of resistance to melarsenoxide cysteamine (Mel Cy) in Trypanosoma brucei brucei

An Update on African Trypanocide Pharmaceutics and Resistance

African trypanosomiasis is associated with Trypanosoma evansi, T. vivax, T. congolense, and T. brucei pathogens in African animal trypanosomiasis (AAT) while T. b gambiense and T. b rhodesiense are responsible for chronic and acute human African trypanosomiasis (HAT), respectively. Suramin sodium suppresses ATP generation during the glycolytic pathway and is ineffective against T. vivax and T. congolense infections. Resistance to suramin is associated with pathogen altered transport proteins. Melarsoprol binds irreversibly with pyruvate kinase protein sulfhydryl groups and neutralizes enzymes which interrupts the trypanosome ATP generation. Melarsoprol resistance is associated with the adenine-adenosine transporter, P2, due to point mutations within this transporter. Eflornithine is used in combination with nifurtimox. Resistance to eflornithine is caused by the deletion or mutation of TbAAT6 gene which encodes the transmembrane amino acid transporter that delivers eflornithine into the cell, thus loss of transporter protein results in eflornithine resistance. Nifurtimox alone is regarded as a poor trypanocide, however, it is effective in melarsoprol-resistant gHAT patients. Resistance is associated with loss of a single copy of the genes encoding for nitroreductase enzymes. Fexinidazole is recommended for first-stage and non-severe second-stage illnesses in gHAT and resistance is associated with trypanosome bacterial nitroreductases which reduce fexinidazole. In AAT, quinapyramine sulfate interferes with DNA synthesis and suppression of cytoplasmic ribosomal activity in the mitochondria. Quinapyramine sulfate resistance is due to variations in the potential of the parasite's mitochondrial membrane. Pentamidines create cross-links between two adenines at 4–5 pairs apart in adenine-thymine-rich portions of Trypanosoma DNA. It also suppresses type II topoisomerase in the mitochondria of Trypanosoma parasites. Pentamidine resistance is due to loss of mitochondria transport proteins P2 and HAPT1. Diamidines are most effective against Trypanosome brucei group and act via the P2/TbAT1 transporters. Diminazene aceturate resistance is due to mutations that alter the activity of P2, TeDR40 (T. b. evansi). Isometamidium chloride is primarily employed in the early stages of trypanosomiasis and resistance is associated with diminazene resistance. Phenanthridine (homidium bromide, also known as ethidium bromide) acts by a breakdown of the kinetoplast network and homidium resistance is comparable to isometamidium. In humans, the development of resistance and adverse side effects against monotherapies has led to the adoption of nifurtimox-eflornithine combination therapy. Current efforts to develop new prodrug combinations of nifurtimox and eflornithine and nitroimidazole fexinidazole as well as benzoxaborole SCYX-7158 (AN5568) for HAT are in progress while little comparable progress has been done for the development of novel therapies to address trypanocide resistance in AAT.

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.

The animal trypanosomiases and their chemotherapy: a review

Pathogenic animal trypanosomes affecting livestock have represented a major constraint to agricultural development in Africa for centuries, and their negative economic impact is increasing in South America and Asia. Chemotherapy and chemoprophylaxis represent the main means of control. However, research into new trypanocides has remained inadequate for decades, leading to a situation where the few compounds available are losing efficacy due to the emergence of drug-resistant parasites. In this review, we provide a comprehensive overview of the current options available for the treatment and prophylaxis of the animal trypanosomiases, with a special focus on the problem of resistance. The key issues surrounding the main economically important animal trypanosome species and the diseases they cause are also presented. As new investment becomes available to develop improved tools to control the animal trypanosomiases, we stress that efforts should be directed towards a better understanding of the biology of the relevant parasite species and strains, to identify new drug targets and interrogate resistance mechanisms.

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.

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.

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.

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.

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.

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.

Genetic variants of the TbAT1 adenosine transporter from African trypanosomes in relapse infections following melarsoprol therapy

We have analyzed the TbAT1 gene, which codes for the P2 adenosine transporter, from Trypanosoma brucei field isolates to investigate a possible link between the presence of mutations in this gene and melarsoprol treatment failure. Of 65 T. b. gambiense isolates analyzed from a focus in north-western Uganda with high treatment failure rates following melarsoprol therapy, 38 had a mutated TbAT1. Unexpectedly, all individual isolates contained the same set of nine mutations in their TbAT1 genes. Of these, five point mutations resulted in amino acid substitutions, one resulted in the deletion of an entire codon, and three were silent point mutations. Eight of these mutations had previously been reported in a laboratory-derived Cymelarsan-resistant T. b. brucei clone. Identical sets of mutations were also found in a drug-resistant T.b.rhodesiense isolate from south-eastern Uganda and in a T.b.gambiense isolate from a relapsing patient from northern Angola. A deletion of the TbAT1 gene was found in a single T. b. gambiense isolate from a relapsing patient from northern Angola. The data presented demonstrate the surprising finding that trypanosomes from individual relapse patients of one area, as well as from geographically distant localities, contain an identical set of point mutations in the transporter gene TbAT1. They further demonstrate that many isolates from relapse patients contained the wild-type TbAT1 genes, suggesting that melarsoprol refractoriness is not solely due to a mutational inactivation of TbAT1.

Melarsoprol refractory T. b. gambiense from Omugo, north-western Uganda

Culture adapted T. b. gambiense isolated from Northwest Uganda were exposed to 0.001-0.14 microg/ml melarsoprol or 1.56-100 microg/ml DL-alpha-difluoromethylornithine (DFMO). Minimum inhibitory concentrations (MICs) of each drug were scored for each isolate after a period of 10 days drug exposure. The results indicate that T. b. gambiense isolates from Northwest Uganda had elevated MIC values for melarsoprol ranging from 0.009 to 0.072 microg/ml as compared with T. b. gambiense isolates from Cote d'Ivoire with MIC values ranging from 0.001 to 0.018 microg/ml or with T. b. rhodesiense from Southeast Uganda with MIC values from 0.001 to 0.009 microg/ml. All MIC values obtained fell below expected peak melarsoprol concentrations in serum of treated patients. However, it may not be possible to maintain constant drug concentrations in serum of patients as was the case in our in vitro experiments. Importantly, the MIC of 0.072 microg/ml exhibited by one of the isolates from Northwest Uganda was above levels attainable in CSF indicating that this isolate would probably not be eliminated from CSF of treated patients. PCR amplification of the gene encoding the P2-like adenosine transporter followed by restriction digestion with Sfa NI enzyme revealed presence of fragments previously observed in a trypanosome clone with laboratory-induced arsenic resistance. From our findings it appears that reduced drug susceptibility may be one factor for the frequent relapses of sleeping sickness after melarsoprol treatment occurring in Northwest Uganda.

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.

Drug resistance in pathogenic African trypanosomes: what hopes for the future?

Trypanosomosis is a serious threat to both man and animals mostly in Africa. Although the first pathogenic trypanosome was discovered over a hundred years ago, there is still no prospect for effective control or eradication of the disease through the development and use of vaccines because of the phenomenon of antigenic variation. Control continues to rely heavily on chemotherapy and vector control strategies. This therapy and prophylaxis depends on the use of drugs which, apart from having been developed over 5 decades ago, suffer from such limitations as toxicity and with their continued use, drug resistance. Resistance to currently used drugs is a serious problem in most fields of anti-microbial chemotherapy, particularly in the case of trypanosomosis where resistance and cross-resistance in animals and man have been developing rapidly. The frequently and widely reported decreasing efficiency of available trypanocides, difficulties of sustaining tsetse control and little hope that a conventional, anti-trypanosome vaccine will be produced in the near future, increase the imperative need for new drugs and alternative effective ways for the control of trypanosomosis. This review examines aspects of drug resistance in pathogenic trypanosomes, measures to minimise it, areas of future research in new drug targets and alternative control strategies. Based on these, it is our opinion that for now the management and control of trypanosomosis will continue to depend on proper usage of the few available trypanocides, especially strategic deployment of the sanative drugs in order to reduce the development of drug resistance, in addition to the continued use of environmentally friendly vector control programmes such tsetse trapping.

A nucleoside transporter from Trypanosoma brucei involved in drug resistance

Drug resistance of pathogens is an increasing problem whose underlying mechanisms are not fully understood. Cellular uptake of the major drugs against Trypanosoma brucei spp., the causative agents of sleeping sickness, is thought to occur through an unusual, so far unidentified adenosine transporter. Saccharomyces cerevisiae was used in a functional screen to clone a gene (TbAT1) from Trypanosoma brucei brucei that encodes a nucleoside transporter. When expressed in yeast, TbAT1 enabled adenosine uptake and conferred susceptibility to melaminophenyl arsenicals. Drug-resistant trypanosomes harbor a defective TbAT1 variant. The molecular identification of the entry route of trypanocides opens the way to approaches for diagnosis and treatment of drug-resistant sleeping sickness.

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.

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

We have examined cross-resistance between trypanocidal drugs using a well-characterised drug-sensitive line, 247, and its cymelarsan-resistant derivative, 247melCyR. The cymelarsan-resistant line was cross-resistant to trimelarsen and melarsen oxide, and partially cross-resistant to two diamidines, pentamidine and berenil (diminazene aceturate). It was cross-resistant to lipid-soluble melarsoprol in vivo but to only a trivial degree in two in vitro assays. The potential role of adenosine transport in arsenical-induced killing of parasites was investigated. Adenosine, adenine, and the diamidines, but not inosine, were able to inhibit killing of drug-sensitive STIB 247 trypanosomes by cymelarsan and melarsen oxide in a concentration-dependent manner. These results are consistent with the view that these arsenical compounds enter trypanosomes via an adenosine-specific transporter. Melarsoprol-induced killing of trypanosomes was unaffected, however, by either purine and to only a slight degree by the diamidines. These data suggest that melarsoprol can enter trypanosomes by a route other than through an adenosine transporter and that there may be two mechanisms contributing to arsenical resistance in this drug-resistant line of trypanosomes.

Characterisation of cloned lines of Trypanosoma brucei expressing stable resistance to MelCy and suramin

Two cloned drug-sensitive stocks of Trypanosoma brucei (STIB 247 and STIB 386) were each used to generate cloned lines expressing resistance to the melaminophenyl arsenical drug cymelarsan (247MelCyR and 386MelCyR) and to suramin (247SurR and 386SurR). The drug-resistance phenotypes were stable after passaging in mice in the absence of drug pressure and three of the lines were transmitted through tsetse flies with no alteration of drug-resistance. There was no evidence of cross-resistance between melCy and suramin in vivo. Twenty-four hour growth inhibition assays were conducted on bloodstream and procyclic forms in axenic in vitro cultures. Suramin-resistance was expressed in bloodstream forms but not in the procyclic stage, while the melCy-resistant lines expressed melCy-resistance in both stages. No cross-resistance between melCy and suramin was observed. Cross-resistance between melCy and another arsenical drug, melB (melarsoprol), was observed in vivo, but to only a very limited extent in vitro. We propose that this difference between the in vivo and in vitro results for melB may indicate that an alteration in a surface adenosine transporter responsible for reduced melCy uptake was bypassed by melB over 24 hours in vitro.

Uptake of diamidine drugs by the P2 nucleoside transporter in melarsen-sensitive and -resistant Trypanosoma brucei brucei

The African trypanosome, Trypanosoma brucei brucei, possesses at least two nucleoside transporter systems designated P1 and P2, the latter being implicated in the selective uptake of melaminophenyl arsenical drugs. Since arsenical-resistant trypanosomes show cross-resistance in vivo to aromatic diamidines, we have investigated whether these drugs are also substrates for the P2 nucleoside transporter. In melarsen-sensitive T. b. brucei, the diamidines, including the commonly used trypanocides, pentamidine and berenil, were found to abrogate lysis induced by the P2 transport of melarsen oxide in vitro. Measurement of [ring-3H]pentamidine transport in melarsen-sensitive T. b. brucei, demonstrated that uptake is carrier-mediated, with a Km of 0.84 microM and a Vmax of 9.35 pmol s-1 (10(8) cells)-1. Pentamidine transport appears to be P2-mediated in these cells, as pentamidine strongly inhibited uptake of [2',5',8-3H]adenosine by the P2 transporter, with a Ki of 0.56 microM. Furthermore, [ring-3H]pentamidine transport was blocked by a number of P2 transporter substrates and inhibitors, as well as by other diamidine drugs. Analysis of the uptake of pentamidine and other diamidines in melarsen-resistant trypanosomes in vitro and in vivo, which also show differential levels of resistance to these compounds in vivo, indicated that P2 transport was altered in these cells and that accumulation of these drugs was markedly reduced.