The accumulation and compartmentalization of isometamidium chloride in Trypanosoma congolense, monitored by its intrinsic fluorescence

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.

Vector competence of sterile male Glossina fuscipes fuscipes for Trypanosoma brucei brucei: implications for the implementation of the sterile insect technique in a sleeping sickness focus in Chad

BACKGROUND

Human African trypanosomiasis (HAT) is a neglected tropical disease caused by Trypanosoma brucei gambiense transmitted by tsetse flies in sub-Saharan West Africa. In southern Chad the most active and persistent focus is the Mandoul focus, with 98% of the reported human cases, and where African animal trypanosomosis (AAT) is also present. Recently, a control project to eliminate tsetse flies (Glossina fuscipes fuscipes) in this focus using the sterile insect technique (SIT) was initiated. However, the release of large numbers of sterile males of G. f. fuscipes might result in a potential temporary increase in transmission of trypanosomes since male tsetse flies are also able to transmit the parasite. The objective of this work was therefore to experimentally assess the vector competence of sterile males treated with isometamidium for Trypanosoma brucei brucei.

METHODS

An experimental infection was set up in the laboratory, mimicking field conditions: the same tsetse species that is present in Mandoul was used. A T. b. brucei strain close to T. b. gambiense was used, and the ability of the sterile male tsetse flies fed on blood with and without a trypanocide to acquire and transmit trypanosomes was measured.

RESULTS

Only 2% of the experimentally infected flies developed an immature infection (midgut) while none of the flies developed a metacyclic infection of T. b. brucei in the salivary glands. We did not observe any effect of the trypanocide used (isometamidium chloride at 100 mg/l) on the development of infection in the flies.

CONCLUSIONS

Our results indicate that sterile males of the tested strain of G. f. fuscipes were unable to cyclically transmit T. b. brucei and might even be refractory to the infection. The data of the research indicate that the risk of cyclical transmission of T. brucei by sterile male G. f. fuscipes of the strain colonized at IAEA for almost 40 years appears to be small.

Genomic analysis of Isometamidium Chloride resistance in Trypanosoma congolense

Isometamidium Chloride (ISM) is one of the principal drugs used to counteract Trypanosoma congolense infection in livestock, both as a prophylactic as well as a curative treatment. However, numerous cases of ISM resistance have been reported in different African regions, representing a significant constraint in the battle against Animal African Trypanosomiasis. In order to identify genetic signatures associated with ISM resistance in T. congolense, the sensitive strain MSOROM7 was selected for induction of ISM resistance in a murine host. Administered ISM concentrations in immune-suppressed mice were gradually increased from 0.001 mg/kg to 1 mg/kg, the maximal dose used in livestock. As a result, three independent MSOROM7 lines acquired full resistance to this concentration after five months of induction, and retained this full resistant phenotype following a six months period without drug pressure. In contrast, parasites did not acquire ISM resistance in immune-competent animals, even after more than two years under ISM pressure, suggesting that the development of full ISM resistance is strongly enhanced when the host immune response is compromised. Genomic analyses comparing the ISM resistant lines with the parental sensitive line identified shifts in read depth at heterozygous loci in genes coding for different transporters and transmembrane products, and several of these shifts were also found within natural ISM resistant isolates. These findings suggested that the transport and accumulation of ISM inside the resistant parasites may be modified, which was confirmed by flow cytometry and ex vivo ISM uptake assays that showed a decrease in the accumulation of ISM in the resistant parasites.

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.

Reduced Mitochondrial Membrane Potential Is a Late Adaptation of Trypanosoma brucei brucei to Isometamidium Preceded by Mutations in the γ Subunit of the F1Fo-ATPase

Background

Isometamidium is the main prophylactic drug used to prevent the infection of livestock with trypanosomes that cause Animal African Trypanosomiasis. As well as the animal infective trypanosome species, livestock can also harbor the closely related human infective subspecies T. b. gambiense and T. b. rhodesiense. Resistance to isometamidium is a growing concern, as is cross-resistance to the diamidine drugs diminazene and pentamidine.

Methodology/Principal Findings

Two isometamidium resistant Trypanosoma brucei clones were generated (ISMR1 and ISMR15), being 7270- and 16,000-fold resistant to isometamidium, respectively, which retained their ability to grow in vitro and establish an infection in mice. Considerable cross-resistance was shown to ethidium bromide and diminazene, with minor cross-resistance to pentamidine. The mitochondrial membrane potentials of both resistant cell lines were significantly reduced compared to the wild type. The net uptake rate of isometamidium was reduced 2-3-fold but isometamidium efflux was similar in wild-type and resistant lines. Fluorescence microscopy and PCR analysis revealed that ISMR1 and ISMR15 had completely lost their kinetoplast DNA (kDNA) and both lines carried a mutation in the nuclearly encoded γ subunit gene of F₁ ATPase, truncating the protein by 22 amino acids. The mutation compensated for the loss of the kinetoplast in bloodstream forms, allowing near-normal growth, and conferred considerable resistance to isometamidium and ethidium as well as significant resistance to diminazene and pentamidine, when expressed in wild type trypanosomes. Subsequent exposure to either isometamidium or ethidium led to rapid loss of kDNA and a further increase in isometamidium resistance.

Conclusions/Significance

Sub-lethal exposure to isometamidium gives rise to viable but highly resistant trypanosomes that, depending on sub-species, are infective to humans and cross-resistant to at least some diamidine drugs. The crucial mutation is in the F₁ ATPase γ subunit, which allows loss of kDNA and results in a reduction of the mitochondrial membrane potential.

Isometamidium is the only prophylactic treatment of Animal African Trypanosomiasis, a wasting disease of livestock and domestic animals in sub-Saharan Africa. Unfortunately resistance threatens the continued utility of this drug after decades of use. Not only does this disease have severe impacts on agriculture, but some subspecies of Trypanosoma brucei are human-infective as well (causing sleeping sickness) and there is concern that cross-resistance with trypanocides of the diamidine class could further undermine treatment of both veterinary and human infections. It is therefore essential to understand the mechanism of isometamidium resistance and the likelihood for cross-resistance with other first-line trypanocides. Here, we report that isometamidium resistance can be caused by a mutation in an important mitochondrial protein, the γ subunit of the F₁ ATPase, and that this mutation alone is sufficient for high levels of resistance, cross-resistance to various drugs, and a strongly reduced mitochondrial membrane potential. This report will for the first time enable a structural assessment of isometamidium resistance genes in T. brucei spp.

Metabolism and distribution of phenanthridine trypanocides in Trypanosoma brucei

Phenanthridine trypanocides (isometamidium chloride hydrochloride, ISM, and Ethidium bromide, EBr) have been widely used to treat African trypanosomiasis in livestock for more than 40 years. Their main action is to inhibit nucleic acid synthesis in trypanosome parasites, by intercalation between the DNA base pairs. They can also linearise selectively kinetoplast DNA minicircles; a form of mitochondrial DNA unique to this group of parasites. However, the metabolism of these compounds by trypanosomes has not been reported. Indeed, it is not known whether or not their metabolism by the parasite contributes to their activity, selective toxicity for these parasites or to the development of chemoresistance. Therefore, we studied the metabolism of EBr and ISM, and their distribution in Trypanosoma brucei (TREU 927) using high performance liquid chromatography (HPLC), liquid chromatography combined with mass spectrometry (LC-MS) and confocal laser scanning microscopy (CLSM). Incubation of EBr with trypanosomes led to the formation of a small amount (0.606+/-0.191%) of one metabolite (MI). Ion chromatograms extracted from an LC-MS analysis using electrospray ionisation (ESI), showed that the difference in mass between the parent compound and its metabolite was 30. This may correspond to the addition of a hydroxyl and a methyl group. No metabolites could be detected for ISM. The distribution of the two drugs in trypanosomes was investigated by CLSM, using their intrinsic fluorescence. ISM and EBr showed differences in their distribution in trypanosomes. ISM had a greater affinity for the kinetoplast than EBr and it stained other organelles like the flagellum; in contrast the distribution of EBr was more diffuse.

HPTLC and HPLC determination of isometamidium in the presence of its manufacturing and degradation impurities

The determination of the phenanthridine trypanocide, isometamidium chloride hydrochloride (ISM), in the presence of four process-related and degradation impurities, by RP-HPLC using a Licrospher-60 RP-select B column with a mobile phase composition of acetonitrile/KH2PO4 (PH 3.0, 20 mM) (25:75 v/v) with UV detection at 320 nm, is described. The method is selective, reproducible and precise with a limit of detection of 45 ng ml-1 for ISM. A HPTLC system (Kieselgel 60 F254, pyridine/acetonitrile/butanol/formic acid, 6:6:4:1, v/v), with UV densitometric evaluation at 320 nm, suitable for the separation of ISM and the related substances is reported.

Modulation of mitochondrial electrical potential: a candidate mechanism for drug resistance in African trypanosomes

Bloodstream forms of four populations of the livestock pathogen Trypanosoma congolense, isolated from different natural infections, have been shown to exhibit a wide range of sensitivities to the trypanocide isometamidium chloride (Samorin(R)). In mice the 50% curative doses (CD50) for Samorin range from 0.007 to 20 mg/kg body weight. Uptake of isometamidium chloride demonstrated Michaelis-Menten-type kinetics in all the populations, with Km values in the range 0.35-0.87 microM, and Vmax varied from 17 to 216 pmol/min per 10(8) cells. The magnitude of Vmax was correlated with sensitivity to the drug. In contrast, no correlation was observed between Km values and drug sensitivity. Pulse-chase experiments indicated two compartments for accumulation of drug. The first consists of freely diffusible drug that is invariant between populations; the other consists of retained isometamidium, which is of variable magnitude between the populations and is correlated with drug sensitivity. Autoradiography and fluorescence microscopy demonstrated initial, rapid accumulation of the drug within the mitochondrion, specifically the kinetoplast. In a drug-sensitive population of T. congolense, agents affecting mitochondrial function were shown to produce dose-dependent inhibition of mitochondrial membrane potential (DeltaPsimito), as measured by the accumulation of the lipophilic cations [3H]methyltriphenylphosphonium iodide or rhodamine 123. The agents also produced parallel inhibition of isometamidium uptake, suggesting an involvement of DeltaPsimito in the accumulation of the drug. When characterized in each of the four populations, the spontaneous DeltaPsimito was shown to be characteristic of each population and was correlated with Vmax for drug uptake and sensitivity to the drug in vitro and in vivo. We therefore conclude that in T. congolense DeltaPsimito is an important determinant of the rate and accumulation of the trypanocide isometamidium chloride. Populations of this trypanosome species vary with respect to DeltaPsimito, which is correlated with sensitivity to isometamidium. We suggest that when exposed to drug, the selection of such populations represents a novel mechanism of drug resistance in protozoan parasites.

Cross-resistance associated with development of resistance to isometamidium in a clone of Trypanosoma congolense

Resistance to isometamidium was increased 94-fold in a clone of Trypanosoma congolense (clone IL 1180) by repeated subcurative treatment of infected mice for 11 months. This was associated with 3.4-, 33-, and 4.2-fold increases in resistance to diminazene, homidium, and quinapyramine, respectively. Both T. congolense IL 1180 and the resistant derivative were able to undergo cyclical development in Glossina morsitans centralis tsetse flies, producing hypopharyngeal infection rates of 40.0 and 39.8%, respectively.

Concentrations of isometamidium chloride (Samorin) in sera of Zebu cattle which showed evidence of hepatotoxicity following frequent trypanocidal treatments

The concentrations of isometamidium circulating in poorly nourished Zebu cattle which showed morbidity, mortality, and biochemical and histopathological evidence of hepatotoxicity, following frequent treatments with isometamidium chloride and diminazene aceturate were investigated using the isometamidium-ELISA. As few as two isometamidium treatments one month apart were associated with significant weight loss, and cattle treated with diminazene aceturate after three or four isometamidium treatments suffered a 50% mortality. Although there were no obvious, marked elevations in isometamidium concentration which might have allowed the use of the ELISA as a predictor of a potential toxicity problem, concentrations did increase significantly with the number of monthly treatments administered, suggesting drug accumulation, and the increases were significantly higher in cattle to which diminazene had also been administered. In cattle treated with both trypanocides, weight loss and serum glutamate dehydrogenase levels were correlated with isometamidium concentrations. These observations, together with the histopathological findings, support the hypothesis that the morbidity and mortality observed were related to the repeated treatment with isometamidium in conjunction with diminazene aceturate, and that the pathogenesis involved a component of hepatic damage. It is therefore recommended that cattle, particularly those under nutritional stress, are not subjected to repeated treatments with isometamidium at intervals as short as one month, and particularly not with concurrent administration of diminazene.