Human African trypanosomiasis: potential therapeutic benefits of an alternative suramin and melarsoprol regimen

Anti-trypanosomatid drug discovery: progress and challenges

Leishmaniasis (visceral and cutaneous), Chagas disease and human African trypanosomiasis cause substantial death and morbidity, particularly in low- and middle-income countries. Although the situation has improved for human African trypanosomiasis, there remains an urgent need for new medicines to treat leishmaniasis and Chagas disease; the clinical development pipeline is particularly sparse for Chagas disease. In this Review, we describe recent advances in our understanding of the biology of the causative pathogens, particularly from the drug discovery perspective, and we explore the progress that has been made in the development of new drug candidates and the identification of promising molecular targets. We also explore the challenges in developing new clinical candidates and discuss potential solutions to overcome such hurdles.

Bioluminescence Imaging to Detect Late Stage Infection of African Trypanosomiasis

Human African trypanosomiasis (HAT) is a multi-stage disease that manifests in two stages; an early blood stage and a late stage when the parasite invades the central nervous system (CNS). In vivo study of the late stage has been limited as traditional methodologies require the removal of the brain to determine the presence of the parasites. Bioluminescence imaging is a non-invasive, highly sensitive form of optical imaging that enables the visualization of a luciferase-transfected pathogen in real-time. By using a transfected trypanosome strain that has the ability to produce late stage disease in mice we are able to study the kinetics of a CNS infection in a single animal throughout the course of infection, as well as observe the movement and dissemination of a systemic infection. Here we describe a robust protocol to study CNS infections using a bioluminescence model of African trypanosomiasis, providing real time non-invasive observations which can be further analyzed with optional downstream approaches.

A sensitive and reproducible in vivo imaging mouse model for evaluation of drugs against late-stage human African trypanosomiasis

OBJECTIVES

To optimize the Trypanosoma brucei brucei GVR35 VSL-2 bioluminescent strain as an innovative drug evaluation model for late-stage human African trypanosomiasis.

METHODS

An IVIS® Lumina II imaging system was used to detect bioluminescent T. b. brucei GVR35 parasites in mice to evaluate parasite localization and disease progression. Drug treatment was assessed using qualitative bioluminescence imaging and real-time quantitative PCR (qPCR).

RESULTS

We have shown that drug dose-response can be evaluated using bioluminescence imaging and confirmed quantification of tissue parasite load using qPCR. The model was also able to detect drug relapse earlier than the traditional blood film detection and even in the absence of any detectable peripheral parasites.

CONCLUSIONS

We have developed and optimized a new, efficient method to evaluate novel anti-trypanosomal drugs in vivo and reduce the current 180 day drug relapse experiment to a 90 day model. The non-invasive in vivo imaging model reduces the time required to assess preclinical efficacy of new anti-trypanosomal drugs.

Optimizing bioluminescence imaging to study protozoan parasite infections

Bioluminescence imaging is a non-invasive technique which can be used to monitor infections in real-time. However, its utility is restricted by difficulties in detecting pathogens in deep tissue. 'Red-shifted' luciferases, which emit light of longer wavelength than standard bioluminescence-generating proteins, greatly enhance sensitivity, and have wide applicability for studying parasite infections.

Highly sensitive in vivo imaging of Trypanosoma brucei expressing "red-shifted" luciferase

Background

Human African trypanosomiasis is caused by infection with parasites of the Trypanosoma brucei species complex, and threatens over 70 million people in sub-Saharan Africa. Development of new drugs is hampered by the limitations of current rodent models, particularly for stage II infections, which occur once parasites have accessed the CNS. Bioluminescence imaging of pathogens expressing firefly luciferase (emission maximum 562 nm) has been adopted in a number of in vivo models of disease to monitor dissemination, drug-treatment and the role of immune responses. However, lack of sensitivity in detecting deep tissue bioluminescence at wavelengths below 600 nm has restricted the wide-spread use of in vivo imaging to investigate infections with T. brucei and other trypanosomatids.

Methodology/Principal findings

Here, we report a system that allows the detection of fewer than 100 bioluminescent T. brucei parasites in a murine model. As a reporter, we used a codon-optimised red-shifted Photinus pyralis luciferase (PpyRE9H) with a peak emission of 617 nm. Maximal expression was obtained following targeted integration of the gene, flanked by an upstream 5′-variant surface glycoprotein untranslated region (UTR) and a downstream 3′-tubulin UTR, into a T. brucei ribosomal DNA locus. Expression was stable in the absence of selective drug for at least 3 months and was not associated with detectable phenotypic changes. Parasite dissemination and drug efficacy could be monitored in real time, and brain infections were readily detectable. The level of sensitivity in vivo was significantly greater than achievable with a yellow firefly luciferase reporter.

Conclusions/Significance

The optimised bioluminescent reporter line described here will significantly enhance the application of in vivo imaging to study stage II African trypanosomiasis in murine models. The greatly increased sensitivity provides a new framework for investigating host-parasite relationships, particularly in the context of CNS infections. It should be ideally suited to drug evaluation programmes.

Parasites of the Trypanosoma brucei species complex are the causative agents of human African trypanosomiasis. There is an urgent need for new drugs to treat this debilitating and potentially fatal infection, especially in its late stage, when parasites have entered the central nervous system. Factors which hamper drug development include the limitations of the current murine models for stage II disease. In vivo bioluminescence imaging is a non-invasive technique that can be used to monitor infections in real time and is a powerful new approach for studying drug effectiveness. However, application of this imaging technology to trypanosome infections has been restricted because of lack of sensitivity. In this paper, we have taken a major step to resolve this problem. The enhanced sensitivity in infected mice is based on the high level expression in trypanosomes of a “red-shifted” luciferase variant that greatly improves bioluminescence detection in deep tissue. The system which we have developed should be a widely applicable tool for providing new insights into the infection biology of T. brucei.

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.

The distribution of nifurtimox across the healthy and trypanosome-infected murine blood-brain and blood-cerebrospinal fluid barriers

Nifurtimox, an antiparasitic drug, is used to treat American trypanosomiasis (Chagas disease) and has shown promise in treating central nervous system (CNS)-stage human African trypanosomiasis (HAT; sleeping sickness). In combination with other antiparasitic drugs, the efficacy of nifurtimox against HAT improves, although why this happens is unclear. Studying how nifurtimox crosses the blood-brain barrier (BBB) and reaches the CNS may clarify this issue and is the focus of this study. To study the interaction of nifurtimox with the blood-CNS interfaces, we used the in situ brain/choroid plexus perfusion technique in healthy and trypanosome-infected mice and the isolated incubated choroid plexus. Results revealed that nifurtimox could cross the healthy and infected blood-brain and blood-cerebrospinal fluid (CSF) barriers (K(in) brain parenchyma was 50.8 ± 9.0 μl · min(-1) · g(-1)). In fact, the loss of barrier integrity associated with trypanosome infection failed to change the distribution of [(3)H]nifurtimox to any significant extent, suggesting there is not an effective paracellular barrier for [(3)H]nifurtimox entry into the CNS. Our studies also indicate that [(3)H]nifurtimox is not a substrate for P-glycoprotein, an efflux transporter expressed on the luminal membrane of the BBB. However, there was evidence of [(3)H]nifurtimox interaction with transporters at both the blood-brain and blood-CSF barriers as demonstrated by cross-competition studies with the other antitrypanosomal agents, eflornithine, suramin, melarsoprol, and pentamidine. Consequently, CNS efficacy may be improved with nifurtimox-pentamidine combinations, but over time may be reduced when nifurtimox is combined with eflornithine, suramin, or melarsoprol.

Discovery of novel orally bioavailable oxaborole 6-carboxamides that demonstrate cure in a murine model of late-stage central nervous system african trypanosomiasis

We report the discovery of novel boron-containing molecules, exemplified by N-(1-hydroxy-1,3-dihydrobenzo[c][1,2]oxaborol-6-yl)-2-trifluoromethylbenzamide (AN3520) and 4-fluoro-N-(1-hydroxy-1,3-dihydrobenzo[c][1,2]oxaborol-6-yl)-2-trifluoromethylbenzamide (SCYX-6759), as potent compounds against Trypanosoma brucei in vitro, including the two subspecies responsible for human disease T. b. rhodesiense and T. b. gambiense. These oxaborole carboxamides cured stage 1 (hemolymphatic) trypanosomiasis infection in mice when administered orally at 2.5 to 10 mg/kg of body weight for 4 consecutive days. In stage 2 disease (central nervous system [CNS] involvement), mice infected with T. b. brucei were cured when AN3520 or SCYX-6759 were administered intraperitoneally or orally (50 mg/kg) twice daily for 7 days. Oxaborole-treated animals did not exhibit gross signs of compound-related acute or subchronic toxicity. Metabolism and pharmacokinetic studies in several species, including nonhuman primates, demonstrate that both SCYX-6759 and AN3520 are low-clearance compounds. Both compounds were well absorbed following oral dosing in multiple species and also demonstrated the ability to cross the blood-brain barrier with no evidence of interaction with the P-glycoprotein transporter. Overall, SCYX-6759 demonstrated superior pharmacokinetics, and this was reflected in better efficacy against stage 2 disease in the mouse model. On the whole, oxaboroles demonstrate potent activity against all T. brucei subspecies, excellent physicochemical profiles, in vitro metabolic stability, a low potential for CYP450 inhibition, a lack of active efflux by the P-glycoprotein transporter, and high permeability. These properties strongly suggest that these novel chemical entities are suitable leads for the development of new and effective orally administered treatments for human African trypanosomiasis.

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.

The blood-brain barrier significantly limits eflornithine entry into Trypanosoma brucei brucei infected mouse brain

Drugs to treat African trypanosomiasis are toxic, expensive and subject to parasite resistance. New drugs are urgently being sought. Although the existing drug, eflornithine, is assumed to reach the brain in high concentrations, little is known about how it crosses the healthy and infected blood–brain barrier. This information is essential for the design of drug combinations and new drugs. This study used novel combinations of animal models to address these omissions. Eflornithine crossed the healthy blood–CNS interfaces poorly, but this could be improved by co-administering suramin, but not nifurtimox, pentamidine or melarsoprol. Work using a murine model of sleeping sickness demonstrated that Trypanosoma brucei brucei crossed the blood–CNS interfaces, which remained functional, early in the course of infection. Concentrations of brain parasites increased during the infection and this resulted in detectable blood–brain barrier, but not choroid plexus, dysfunction at day 28 post-infection with resultant increases in eflornithine brain delivery. Barrier integrity was never restored and the animals died at day 37.9 ± 1.2. This study indicates why an intensive treatment regimen of eflornithine is required (poor blood–brain barrier penetration) and suggests a possible remedy (combining eflornithine with suramin). The blood–brain barrier retains functionality until a late, possibly terminal stage, of trypanosoma infection.

The continuing problem of human African trypanosomiasis (sleeping sickness)

Human African trypanosomiasis, also known as sleeping sickness, is a neglected disease, and it continues to pose a major threat to 60 million people in 36 countries in sub-Saharan Africa. Transmitted by the bite of the tsetse fly, the disease is caused by protozoan parasites of the genus Trypanosoma and comes in two types: East African human African trypanosomiasis caused by Trypanosoma brucei rhodesiense and the West African form caused by Trypanosoma brucei gambiense. There is an early or hemolymphatic stage and a late or encephalitic stage, when the parasites cross the blood-brain barrier to invade the central nervous system. Two critical current issues are disease staging and drug therapy, especially for late-stage disease. Lumbar puncture to analyze cerebrospinal fluid will remain the only method of disease staging until reliable noninvasive methods are developed, but there is no widespread consensus as to what exactly defines biologically central nervous system disease or what specific cerebrospinal fluid findings should justify drug therapy for late-stage involvement. All four main drugs used for human African trypanosomiasis are toxic, and melarsoprol, the only drug that is effective for both types of central nervous system disease, is so toxic that it kills 5% of patients who receive it. Eflornithine, alone or combined with nifurtimox, is being used increasingly as first-line therapy for gambiense disease. There is a pressing need for an effective, safe oral drug for both stages of the disease, but this will require a significant increase in investment for new drug discovery from Western governments and the pharmaceutical industry.

Synthesis of bicyclic amines and their activities against Trypanosoma brucei rhodesiense and Plasmodium falciparum K1

New alkenes, aziridines, and diamines were prepared from antiprotozoal 4-dialkylaminobicyclo[2.2.2]octan-2-imines to investigate the influence of several functional groups in position 2 of the ring skeleton on the antitrypanosomal and antiplasmodial activities. They were synthesized from 4-dialkylaminobicyclo[2.2.2]octan-2-imines and tested for their activities against Trypanosoma b. rhodesiense and Plasmodium falciparum K1 (resistant to chloroquine and pyrimethamine) using in vitro microplate assays. 4-Aminobicyclo[2.2.2]oct-2-enes and 3-azatricyclo[3.2.2.0(2,4)]nonylamines exhibit similar antiprotozoal activities as 4-aminobicyclo[2.2.2] octanes. 4-Aminobicyclo[2.2.2]oct-2-ylamines and their N-benzyl derivatives showed decreased antiplasmodial but enhanced antitrypanosomal (IC50 = 0.22-0.41 microM) activities compared to their parent oximes and to formerly synthesized 4-amino-2-azabicyclo[3.2.2]nonanes. Some of the 4-aminobicyclo[2.2.2]oct-2-ylamines exhibit moderate in vivo activity in mice against Trypanosoma brucei brucei.

Novel azabicyclo[3.2.2]nonane derivatives and their activities against Plasmodium falciparum K1 and Trypanosoma brucei rhodesiense

New diaryl substituted 2-azabicyclo[3.2.2]nonane derivatives have been synthesized in order to investigate the influence of the aromatic substitution and of N substitution on the antiprotozoal activities of those compounds. Following a manual method for the Hansch approach, different 4-substituted aryl rings were systematically inserted, and moieties with varying basicity and polarity were attached to the ring nitrogen. All compounds were investigated for their activities against Trypanosoma brucei rhodesiense (STIB 900) and the K(1) strain of Plasmodium falciparum (resistant to chloroquine and pyrimethamine) and for their cytotoxicity using microplate assays. Some of the new compounds are amongst the most active antitrypanosomal agents in this series, and the selectivity index of a single derivative is superior in the 2-azabicyclo-nonane series.

Cosignaling of adenosine and adenosine triphosphate in hypobaric hypoxia-induced hypothermia

Purines, that is, adenosine and ATP, are not only products of metabolism but are also neurotransmitters. Indeed, purinergic neurotransmission is involved in thermoregulatory processes that occur during normoxia. Exposure to severe hypoxia elicits a sharp decrease in body core temperature (T(CO)), and adenosinergic mechanisms have been suspected to be responsible for this hypothermia. Because ATP per se and its metabolite adenosine could have complex interactions in some neural networks, we hypothesize that both adenosine and ATP are involved in the central mechanism of hypoxia-induced hypothermia. Their role in the thermoregulatory process was therefore investigated in a 24-h hypobaric hypoxia (Fi(O2) = 10%), using CGS-15943, a nonselective antagonist of adenosine receptors, and suramin, an ATP receptor antagonist. T(CO) and spontaneous activity (A(S)) were monitored by telemetry in conscious rats, receiving CGS-15943 (10 mg/kg ip), suramin (7 nmol icv), or both. The same treatments were done in normoxia to evaluate the specificity of their thermoregulatory action observed in hypoxia. Suramin/CGS-15943 treatment blunted the profound hypothermia observed in control rats throughout the hypoxia exposure, whereas CGS-15943 treatment blunted hypothermia during only 3 h, and suramin treatment had no effect. These results suggest that suramin potentiates the CGS-15943 effects and consequently that adenosine and ATP signaling act in synergy. In normoxia, suramin/CGS-15943 induced an increase in T(CO) but to a far lesser extent than observed in hypoxia. Thus it might be suggested that the suramin/CGS-15943 blunting of hypoxia-induced hypothermia would be specific to hypoxia-induced mechanisms.

Characterization of [3H]-forskolin binding sites in young and adult rat brain cortex: identification of suramin as a competitive inhibitor of [3H]-forskolin binding

Little is know about forskolin binding in the rat brain during ontogenetic development. For this paper, we have characterized specific binding sites for [3H]-forskolin in cerebrocortical membranes from young (12-day-old) and adult (90-day-old) rats. High-affinity, as well as super-high-affinity, [3H]-forskolin binding sites were detected in samples from both age groups tested, and the binding parameters of these sites differed significantly. Whereas the number of high-affinity [3H]-forskolin binding sites was higher by about 50% in adult than in young rats, their affinity was markedly (about 4 times) lower. In the presence of AlF4-, the number high-affinity [3H]-forskolin binding sites in samples from young rats rose to the level determined in samples from adult animals, and the number of super-high-affinity sites considerably increased in both age groups. The different characteristics of [3H]-forskolin binding found in cerebrocortical membranes from young and adult rats may be closely related to markedly diminished adenyl cyclase activity in preparations from adult animals. Results of our experiments with suramin indicated that this drug may act as a competitive inhibitor of [3H]-forskolin binding.

Inhibition of Myotoxic Activity of Bothrops asper Myotoxin II by the Anti-trypanosomal Drug Suramin

Suramin, a synthetic polysulfonated compound, developed initially for the treatment of African trypanosomiasis and onchocerciasis, is currently used for the treatment of several medically relevant disorders. Suramin, heparin, and other polyanions inhibit the myotoxic activity of Lys49 phospholipase A2 analogues both in vitro and in vivo, and are thus of potential importance as therapeutic agents in the treatment of viperid snake bites. Due to its conformational flexibility around the single bonds that link the central phenyl rings to the secondary amide backbone, the symmetrical suramin molecule binds by an induced-fit mechanism complementing the hydrophobic surfaces of the dimer and adopts a novel conformation that lacks C2 symmetry in the dimeric crystal structure of the suramin-Bothrops asper myotoxin II complex. The simultaneous binding of suramin at the surfaces of the two monomers partially restricts access to the nominal active sites and significantly changes the overall charge of the interfacial recognition face of the protein, resulting in the inhibition of myotoxicity.

Antiprotozoal activities of new bicyclo[2.2.2]octan-2-imines and esters of bicyclo[2.2.2]octan-2-ols

Several bicyclo[2.2.2]octan-2-imines and esters of bicyclo[2.2.2]octan-2-ols were prepared. Their antitrypanosomal and antiplasmodial activities against Trypanosoma brucei rhodesiense (STIB 900) and the K1 strain of Plasmodium falciparum (resistant to chloroquine and pyrimethamine) were determined using microplate assays. Two of the synthesized bicyclo[2.2.2]octan-2-one 4'-phenylthiosemicarbazones showed the highest antitrypanosomal activity (IC(50)<0.3 microM) of the so far prepared 4-amino-6,7-diarylbicyclo[2.2.2]octane derivatives, but they are distinctly less active than suramine (IC(50)=0.0075 microM). Most of the 4'-phenylthiosemicarbazones and a single bicyclo[2.2.2]octan-2-yl benzoate exhibit attractive antimalarial activity (IC(50)=0.23-0.72 microM). Two bicyclooctanone oximes are even as active as chloroquine (IC(50)=0.08-0.15 microM, chloroquine: IC(50)=0.12 microM against sensitive strains).

Megazol combined with suramin improves a new diagnosis index of the early meningo-encephalitic phase of experimental African trypanosomiasis

In human African trypanosomiasis (HAT), the parasites invade the central nervous system (CNS), leading to the development of meningo-encephalitis and an irreversible demyelinating process, which kills the patient unless specific treatment is undertaken. Among the experimental trypanocides, the nitroimidazole derivative megazol alone at optimal doses does not cure late-stage disease tested in mouse models, however the combination of suramin and megazol is able to cure infected mice without CNS involvement. We recently developed an experimental model of HAT with a sharp decrease in both the food intake and the body weight which may constitute an effective index of the early meningo-encephalitic phase. Using this model, we tested this hypothesis by the exclusive effectiveness of a megazol and suramin combination treatment to eliminate CNS trypanosomes. Sprague-Dawley rats were infected with Trypanosoma brucei brucei AnTat 1.1E. Food intake and body weight were measured daily from the day of infection to death. Haematocrit was measured twice a week. Treatment consisted of 20 mg suramin per kg body weight administered intraperitoneally (i.p.) alone, or three daily doses (80 mg/kg) of megazol given per os, or suramin (20 mg/kg, i.p.) followed 24 h later by three daily doses (80 mg/kg) of megazol given per os. Treatment was followed by an increase in daily body weight and food intake similar to those of the control animals, 2 weeks after treatment. The anaemia developed after infection is also cleared as shown by the haematocrit measurements. The rats treated with megazol alone died about 29 days after treatment and those treated with suramin, after about 26 days. Seven months later, no signs of relapse were seen in 10 of 12 rats treated with the therapeutic combination, indicating that this chemotherapy regimen was curative. The results support our previous finding, i.e. the decrease in body weight may constitute a diagnosis index of the early meningo-encephalitic phase.

The trypanosomiases

The trypanosomiases consist of a group of important animal and human diseases caused by parasitic protozoa of the genus Trypanosoma. In sub-Saharan Africa, the final decade of the 20th century witnessed an alarming resurgence in sleeping sickness (human African trypanosomiasis). In South and Central America, Chagas' disease (American trypanosomiasis) remains one of the most prevalent infectious diseases. Arthropod vectors transmit African and American trypanosomiases, and disease containment through insect control programmes is an achievable goal. Chemotherapy is available for both diseases, but existing drugs are far from ideal. The trypanosomes are some of the earliest diverging members of the Eukaryotae and share several biochemical peculiarities that have stimulated research into new drug targets. However, differences in the ways in which trypanosome species interact with their hosts have frustrated efforts to design drugs effective against both species. Growth in recognition of these neglected diseases might result in progress towards control through increased funding for drug development and vector elimination.