A drug incubation infectivity test (DIIT) for assessing resistance in trypanosomes

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.

Antitrypanosomal properties of Anogeissus leiocarpa extracts and their inhibitory effect on trypanosome alternative oxidase

Background

African trypanosomiasis is a protozoan disease with huge socio-economic burden to sub-Saharan African exceeding US$4.6 annual loss. To mitigate the incidence of trypanosomal drug resistance, efforts are geared towards discovery of molecules, especially from natural products, with potential to inhibit important molecular target (trypanosome alternative oxidase, TAO) in trypanosomes that are critical to their survival.

Method

Crude methanol extract of Anogeissus leiocarpa was subjected to in vitro bioassay-guided antitrypanosomal assay to identify the most active extract with trypanocidal activity. The most active extract was run on a column chromatography yielding five fractions, F1-F5. The fractions were assayed for inhibitory effect on TAO. The most promising TAO inhibitor was subjected to antitrypanosomal evaluation by trypanosome count, drug incubation infectivity test (DIIT) and in vivo studies. Gas chromatography-mass spectrometry (GC-MS) was used to identify and quantify phytochemical constituents of the potential TAO-inhibiting fraction.

Results

Ethyl acetate extract (EtOAc) significantly (p<0.05) produced trypanocidal effect and was the most active extract. Of the five fractions, only F4 significantly (p<0.05) inhibited TAO compared to the control. F4 completely immobilised the trypanosomes up to 0.5 μg/μl, yielding an EC₅₀ of 0.024 μg/μl compared to the 0.502 μg/μl of diminazene aceturate positive control group. The DIIT showed that F4 was significantly (p<0.05) potent up to 0.1 μg/μl. F4 significantly (p<0.05) suppressed parasite multiplication in systemic circulation of the treated rats and significantly (p<0.05) maintained high PCV when compared to the 5% DMSO group. Furthermore, F4 significantly (p<0.05) lowered serum concentrations of malondialdehyde. Phytoconstituents identified by the GC-MS include tetradecene; cetene; 3-(benzylthio) acrylic acid, methyl ester; 1-octadecene; 9-heptadecanone; hexadecanoic acid, methyl ester; dibutyl phthalate; eicosene; octadecenoic acid, methyl ester; oleic acid; 2-methyl-Z,Z-3,13-octadecadienol; 1-docosene; 3-phenylthiane, s-oxide; phenol, 3-methyl; phthalic acid, di(2-propylpentyl) ester and 1,4-benzenedicarboxylic acid, bis (2-ethylhexyl) ester.

Conclusion

F4 from EtOAc contains six carbohydrates (9.58%), two free fatty acids (6.48%), five fatty acid esters (27.73%), two aromatic compounds (50.63%) and one organosulphide (5.61%). It inhibited TAO and demonstrated antitrypanosomal effects.

Partially Purified Leaf Fractions of Azadirachta indica Inhibit Trypanosome Alternative Oxidase and Exert Antitrypanosomal Effects on Trypanosoma congolense

Introduction

Trypanosomiasis is a neglected disease of humans and livestock caused by single-celled flagellated haemo-protozoan parasites belonging to the genus Trypanosoma.

Purpose

Widespread resistance to trypanocidal drugs creates urgent need for new, more effective drugs with potential to inhibit important trypanosome molecular targets.

Methods

Nine column chromatographic, partially purified leaf fractions of Azadirachta indica (AIF) were subjected to trypanosome alternative oxidase (TAO) inhibition assay using ubiquinol oxidase assay. The potent TAO inhibitors were evaluated for trypanocidal activities against T. congolense in rat model using in vitro, ex vivo, and in vivo assays. Complete cessation or reduction in parasite motility was scored from 0 (no parasite) to 6 (greater than or equal to 6 × 10⁷ trypanosomes/milliliter of blood), and was used to evaluate the efficacy of in vitro treatments.

Results

Only AIF1, AIF2, and AIF5 significantly inhibited TAO. AIF1 and AIF5 produced significant, dose-dependent suppression of parasite motility reaching score zero within 1 h with EC₅₀ of 0.005 and 0.004 µg/µL, respectively, while trypanosome-laden blood was still at score six with an EC₅₀ of 44,086 µg/µL. Mice inoculated with the concentrations at scores 0 and 1 (1–2 moribund parasites) at the end of the experiment did not develop parasitaemia. The two fractions significantly (p < 0.05) lowered parasite burden, with the AIF5 exhibiting highest in vivo trypanocidal effects. Packed cell volume was significantly higher in AIF1 (p < 0.05) and AIF5 (p < 0.001) groups compared to DMSO-treated group. Only AIF5 significantly (p < 0.05) lowered malondialdehyde.

Conclusion

AIF1 and AIF5 offer prospects for the discovery of TAO inhibitor(s).

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.

Whole-organism high-throughput screening against Trypanosoma brucei brucei

INTRODUCTION

Human African trypanosomiasis (HAT) occurs as a result of infection with the protozoan parasites Trypanosoma brucei gambiense and T.b. rhodesiense and is nearly always fatal without treatment. However, current therapeutic options are severely limited and there is a desperate need for new compounds to treat the disease. Whole-cell high-throughput screening (HTS) is a technique frequently used to identify compounds with trypanocidal activity.

AREAS COVERED

The authors examine the development of whole-organism HTS assays for T.b. brucei. The authors describe the successes achieved through HTS and discuss the advantages and disadvantages of whole-organism HTS.

EXPERT OPINION

Despite hundreds of trypanocidal molecules being identified by whole-organism HTS, very few have progressed into preclinical development. The failure of molecules identified by HTS to progress along the drug development pathway is due to a multitude of factors including undrug-like molecules and molecules having poor pharmacodynamics/kinetic properties. Future studies should focus on screening libraries that contain drug-like molecules that possess some of the properties required in the final compound.

Isometamidium sensitivity of Trypanosoma congolense stocks from cattle in West Africa tested in mice and the drug incubation infectivity test

Four Trypanosoma (T.) congolense reference clones with known isometamidium sensitivity and 16 T. congolense stocks from cattle in Kénédougou in south-western Burkina Faso, an area with known history of drug resistance, were characterised with the standard mouse test (SMT) and the drug incubation infectivity test (DIIT). All field stocks from Kénédougou were resistant to 1.0 mg/kg bw isometamidium in the SMT. Fourteen stocks (87.5%) also proved to be refractory to 10 mg/kg bw. Testing with the DIIT confirmed the results of the SMT. By comparison to reference clones, all the Kénédougou populations expressed high levels of resistance to isometamidium.

Field studies of drug-resistant cattle trypanosomes in Kénédougou Province, Burkina Faso

Field studies were conducted to assess the occurrence of resistance to isometamidium chloride and diminazene aceturate in trypanosomes infecting cattle in Kénédougou Province of Burkina Faso. Forty-five of the 166 villages in Kénédougou were randomly sampled and visited to assess livestock numbers, trypanosomosis risk, and tsetse challenge. The proportion of cattle infections associated with drug-resistant trypanosomes was assessed in the nine villages with the highest trypanosome infection prevalence and one village with a confirmed history of drug-resistant infections. These studies showed that resistance to both isometamidium and diminazene was widespread. However, there was considerable variation between villages in drug-resistance parameters, with the proportion of treated cattle with trypanosome infections 3 months after isometamidium prophylaxis varying from 6.9 to 63.8% and the proportion of cattle having infections 2 weeks after treatment with diminazene varying from 0 to 36.8%. The demonstration of widespread resistance to both isometamidium and diminazene has important implications, as administration of trypanocides is the most commonly employed method to control trypanosomosis in this area.

Trypanosoma brucei: anti-tubulin antibodies specifically inhibit trypanosome growth in culture

We previously observed that trypanosome tubulin immunizes mice against infection by this parasite. Here we describe the direct effect of anti-tubulin antibodies on trypanosomes, using rabbit antibodies to renatured (nTbTub) or SDS-PAGE denatured (dTbTub) Trypanosoma brucei tubulin. We also evaluate antibodies to synthetic tubulin peptides (STP) and rat brain tubulin (RbTub). The anti-nTbTub serum strongly inhibited trypanosome proliferation in culture, and immunoagglutinated trypanosomes even after heat inactivation of complement. The anti-dTbTub and the anti-STP sera also inhibited trypanosome growth and immunoagglutinated trypanosomes, but to a lesser extent than the anti-nTbTub, whereas the anti-RbTub serum had no effect. In Western blots these antibodies were species specific. Immunofluorescence showed that the surface of intact trypanosomes was not uniformly stained by any of these antibodies, but cells that had been permeabilised were labeled throughout the cytoplasm. This suggests that the variant surface glycoproteins (VSG) played no part in the generation of these inhibitory antibodies.

The in vitro effects of isometamidium chloride (Samorin) on the piscine hemoflagellate Cryptobia salmositica (Kinetoplastida, Bodonina)

Isometamidium chloride (Samorin) is therapeutic in rainbow trout (Oncorhynchus mykiss) during preclinical and chronic cryptobiosis. However, the toxic mechanism of isometamidium on Cryptobia salmositica has not been elucidated. The objective of the present study was to examine the in vitro effects of isometamidium on C. salmositica. Under in vitro conditions, isometamidium chloride reduced the infectivity of C. salmositica suspended in whole fish blood. It accumulated rapidly in the kinetoplast (within 1 min) and caused disruption and decantenation of kinetoplast DNA. The in vitro cryptobiacidal activity of isometamidium was reduced when parasites were incubated in medium containing serum supplement, suggesting that isometamidium also binds to plasma proteins. Isometamidium altered glycoprotein receptors (epitopes) for antibodies on the surface of C. salmositica and thus protected some of the parasites from lysis by complement-fixing antibodies. In vitro oxygen consumption and carbon dioxide production decreased in drug-exposed C. salmositica, with increased products of glycolysis, i.e., lactate and pyruvate, after exposure to isometamidium. This suggests that some C. salmositica switched from aerobic respiration to glycolysis when the mitochondrion was damaged by isometamidium.

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.

A drug incubation Glossina infectivity test (DIGIT) to assess the susceptibility of Trypanosoma congolense bloodstream forms to trypanocidal drugs (Xenodiagnosis)

Blood was collected from two Sahelian goats, experimentally infected with either a drug-sensitive cloned population of Trypanosoma congolense (IL 1180) or a multiple drug-resistant T. congolense stock (Samorogouan/89/CRTA/267) and incubated at 37 degrees C for 30 min and 12 h, respectively, in the presence of different drug concentrations (0.5, 1.0, 10.0 and 100.0 microg/ml blood) of diminazene aceturate or isometamidium chloride. After that, the trypanosome/blood/drug suspensions were offered to tsetse flies (2100 teneral Glossina morsitans submorsitans) through an in vitro feeding system, using a silicone membrane. All tsetse flies were dissected and examined for the presence of trypanosomes in labrum, hypopharynx and midgut 20 days after their infective blood-meals. Infectivity of the drug-sensitive cloned population was already completely abolished after incubation with 0.5 microg/ml of both drugs; however, 13.6-42.2% of tsetse having been fed on untreated blood had developed an infection. In contrast, no significant differences were observed in the infection rates between the experimental groups and their control groups when fed on blood infected with the multiple drug-resistant stock after incubation for 30 min with up to 10 microg/ml of diminazene or isometamidium. In consequence, tsetse appear to be a useful tool in the assessment of drug susceptibility of typanosome populations.

The effect of spiroarsoranes on Trypanosoma brucei brucei and T. b. rhodesiense

Topical application and intraperitoneal administration of spiroarsoranes were carried out to cure central nervous system (CNS) trypanosomiasis in the chronic Trypanosoma brucei GVR 35 mouse model. Topical application appeared more efficient than intraperitoneal injection. The periods of aparasitaemia after treatment were longer but none of the mice was permanently cured. Combination treatment with eflornithine (DFMO) and the spiroarsoranes failed to show any synergistic effect. In addition, spiroarsorane I was evaluated against the T. b. rhodesiense KETRI 2634 strain, whereby 60-mg/kg treatment produced a noticeable prolongation of the life span of trypanosome-positive animals. These in vivo results suggests that the spiroarsoranes have difficulty in crossing the blood-brain barrier (BBB) and clearing the parasites from the CNS or, alternatively, that these strains are less sensitive to pentavalent arsenicals than the T. b. brucei CMP fast strain, which in the present study was more sensitive to spiroarsoranes whose lipophilicity corresponded to a log-P value ranging from 2.5 to 3.7.

Resistance to diminazine aceturate by Trypanosoma congolense from cattle in the Zambezi Valley of Zimbabwe

The susceptibility of 14 stocks of Trypanosoma congolense, recently isolated from cattle, to therapeutic doses of diminazene aceturate and to isometamidium chloride was assessed in laboratory mice. Eight isolates were readily susceptible to the normal therapeutic dose of diminazene, two were resistant to the drug at 14 mg kg-1, and four were totally resistant at 28 mg kg-1. All the isolates were susceptible to isometamidium chloride at 0.5 mg kg-1. These observations highlight the need for regular evaluation of drugs used in the control of trypanosomosis.

In vitro assays to determine drug sensitivities of African trypanosomes: a review

In vitro test methods have been developed both for determining the sensitivity of isolates of pathogenic African trypanosomes and for evaluating new compounds for antitrypanosomal activity. The principles of the assays and their main advantages or drawbacks are presented and discussed. In vitro assays which do not require trypanosomes pre-adapted to culture conditions are the Drug Incubation Infectivity Test (DIIT) and the [3H]Hypoxanthine Incorporation Assay. Chemosensitivity tests which do require continuous growth of trypanosomes in vitro include photometric, fluorescence, growth inhibition, long-term viability and metacyclic incubation assays. Evidence is presented to use metacyclic or bloodstream forms in such assays but to avoid procyclic trypanosomes. The drug sensitivity of a homogeneous trypanosome population can be quantified by using photometric, fluorescence or growth inhibition assays lasting 1-3 days. Small numbers of resistant organisms hidden in a sensitive population can be detected employing long-term viability assays (7-10 days). Final selection of the assay to be employed will depend on the parameter to be investigated, and the equipment available.

Alterations in drug transport in resistant Trypanosoma congolense

The transport of isometamidium chloride (Samorin) in Trypanosoma congolense which were either sensitive or resistant to this widely used trypanocide was studied in vitro. Significantly lower amounts of drug were accumulated over time by resistant than by sensitive trypanosomes. While no direct evidence could be obtained, indirect observations implied the involvement of an increased efflux of drug from the resistant trypanosomes. In both the resistant and sensitive parasites, drug transport was found to be mediated by an energy-dependent, specific process, presumably receptor-mediated. However, the specificity of the putative receptors was altered in the drug-resistant parasites. It is proposed that an alteration or replacement of a specific receptor in isometamidium chloride-resistant T. congolense results in an increased efflux of the drug and that this increased efflux at least partially mediates the reduction in sensitivity to the compound.

In vivo assessment of drug sensitivity of African trypanosomes using the akinetoplastic induction test

Following treatment of mice infected with Trypanosoma congolense or T brucei brucei with various doses of isometamidium chloride or diminazene aceturate, the induction of akinetoplastic (AK) forms was observed in the trypomastigotes of both species within 10 hours of drug administration. The levels of AK-induction were closely correlated with the levels of resistance to each compound found using a standard in vivo drug assay in mice. In general, ineffective doses of either compound conferred AK-induction rates of less than 30 per cent; relapsing cases had between 30 and 50 per cent while curative doses had AK-induction rates of 50 per cent or more. In vivo determination of AK-induction rates using ordinary light microscopy is thus a potentially feasible alternative indicator to the conventional use of mice infection and treatment methods for assessing drug sensitivity in African trypanosomes.

Trypanosoma congolense: the in vitro akinetoplastic induction sensitivity assay

Incubation of Trypanosoma congolense in diminazene aceturate (Berenil) or isometamidium chloride (Samorin) induced akinetoplastic (AK) forms in vitro. The AK values (expressed in percent) obtained were found to be useful for rapid assessment of relative drug sensitivities. In susceptible clones, AK forms were induced at all drug concentrations tested, whereas in resistant clones they were induced only at higher concentrations. The Berenil-resistant clone exhibited AK values of 0.9% +/- 0.6%--8.9 +/- 2% at concentrations of 1-100 micrograms/ml at 4-10 h post-inoculation (p.i.), whereas the Berenil-susceptible clone displayed values of 9.3% +/- 13%--19.2% +/- 5% at 0.1-50 micrograms/ml. Motile trypanosomes were not seen at 100 micrograms/ml at 4 h p.i. or at 10 or 50 micrograms/ml at 10 h p.i. The Samorin-resistant clone showed AK values of 0.5% +/- 0.1%--43% +/- 3% at concentrations of 0.1-100 micrograms/ml at 4 and 10 h p.i., whereas the Samorin-susceptible clone exhibited values of 5.3% +/- 2%--45% +/- 4% at 0.0005-100 micrograms/ml. These results were supported by the findings obtained using a mouse infectivity test.

Therapeutic effect of Berenil and Samorin in mice infected with four trypanosome populations isolated from Zambian cattle

Four populations of Trypanosoma congolense and Trypanosoma brucei brucei were isolated from cattle under different management practices and environments in Zambia. All four isolates had varied responses to both diminazene aceturate (Berenil) and isometamidium chloride (Samorin) as curative drugs in infected mice. Trypanosomes from a traditionally managed herd in a high-tsetse-challenge area had the strains most resistant to Berenil, with maximum curative dose of 45 mg kg-1 body weight. Another isolate from a high-tsetse-challenge area was evidently resistant both to Berenil at 40 mg kg-1 and to Samorin at 4 mg kg-1. The strains most susceptible to both Berenil and Samorin were from a commercially managed herd of cattle under medium tsetse challenge. They responded to recommended cattle standard doses of 3.5 mg kg-1 or 7 mg kg-1 Berenil and to as little as 0.25 mg kg-1 Samorin. It is evident that trypanosome strains resistant to Berenil and/or partially resistant to Samorin exist, and that both T. congolense and T. b. brucei are implicated.

Effect of isometamidium on Trypanosoma congolense infectivity

Isometamidium chloride (Samorin, RMB, England) is a widely used and highly effective trypanocide for the treatment of bovine trypanosomiases. However, the appearance of isometamidium-resistant populations of T. congolense in Africa makes it necessary to develop methods for the rapid and reliable detection of drug resistance in the laboratory. Currently available tests are time-consuming and/or expensive. In the present study, the short-term in vitro incubation of trypanosomes in a range of isometamidium concentrations and the infectivity of the parasites in mice has been assessed. A series of T. congolense isolates were used which were known to differ in their in vivo sensitivity to the drug. The results showed a close correlation between the known level of resistance and the capability of trypanosomes to remain infective after incubation in isometamidium. Thus isolates displaying a high level of resistance in vivo remained infective following incubation in higher concentrations of drug. This assay may provide a simple and reliable method for detecting drug resistance in T. congolense.

The effect of isometamidium chloride on insect forms of drug-sensitive and drug-resistant stocks of Trypanosoma vivax: studies in vitro and in tsetse flies

Isometamidium chloride-resistant and -sensitive Trypanosoma vivax insect forms were continuously propagated in vitro without feeder-layer cells in a semi-defined liquid medium at 27 degrees C. The effect of isometamidium chloride (Samorin) on T. vivax was assessed by monitoring the viability of epimastigotes and the production of metacyclic forms. Populations of insect forms of T. vivax stock IL 1392 and clone IL 3185 showed reduced growth and died after 10 days when cultivated in the presence of 1 ng/ml isometamidium chloride and after 6 days in the presence of 10 ng/ml. In contrast, populations of the isometamidium-resistant T. vivax stocks CP 2171 and CP 2331 continued to grow for 17 days in the presence of 1 ng/ml isometamidium chloride. The production of metacyclics was inhibited in cultures of T. vivax IL 1392 after incubation in medium containing 1 or 10 ng/ml isometamidium chloride. Epimastigotes of T. vivax CP 2171 and CP 2331 produced metacyclic forms in the presence of 1 ng/ml isometamidium chloride but not 10 ng/ml. When tsetse infected with drug-sensitive T. vivax IL 1392 were fed on a Boran steer that had previously been treated with 1 mg/kg isometamidium chloride, trypanosome infection rates were greatly reduced. In contrast, infection rates in Glossina morsitans centralis infected with drug-resistant T. vivax CP 2171 were not affected when these flies were fed on the same drug-treated animal.