Combination of suboptimal doses of inhibitors targeting different domains of LtrMDR1 efficiently overcomes resistance of Leishmania spp. to Miltefosine by inhibiting drug efflux

3'Nucleotidase/nuclease is required for Leishmania infantum clinical isolate susceptibility to miltefosine

BACKGROUND

Miltefosine treatment failure in visceral leishmaniasis in Brazil has been associated with deletion of the miltefosine susceptibility locus (MSL) in Leishmania infantum. The MSL comprises four genes, 3'-nucleotidase/nucleases (NUC1 and NUC2); helicase-like protein (HLP); and 3,2-trans-enoyl-CoA isomerase (TEI).

METHODS

In this study CRISPR-Cas9 was used to either epitope tag or delete NUC1, NUC2, HLP and TEI, to investigate their role in miltefosine resistance mechanisms. Additionally, miltefosine transporter genes and miltefosine-mediated reactive oxygen species homeostasis were assessed in 26 L. infantum clinical isolates. A comparative lipidomic analysis was also performed to investigate the molecular basis of miltefosine resistance.

FINDINGS

Deletion of both NUC1, NUC2 from the MSL was associated with a significant decrease in miltefosine susceptibility, which was restored after re-expression. Metabolomic analysis of parasites lacking the MSL or NUC1 and NUC2 identified an increase in the parasite lipid content, including ergosterol; these lipids may contribute to miltefosine resistance by binding the drug in the membrane. Parasites lacking the MSL are more resistant to lipid metabolism perturbation caused by miltefosine and NUC1 and NUC2 are involved in this pathway. Additionally, L. infantum parasites lacking the MSL isolated from patients who relapsed after miltefosine treatment were found to modulate nitric oxide accumulation in host macrophages.

INTERPRETATION

Altogether, these data indicate that multifactorial mechanisms are involved in natural resistance to miltefosine in L. infantum and that the absence of the 3'nucleotidase/nuclease genes NUC1 and NUC2 contributes to the phenotype.

FUNDING

MRC GCRF and FAPES.

In-Depth Quantitative Proteomics Characterization of In Vitro Selected Miltefosine Resistance in Leishmania infantum

Visceral leishmaniasis (VL) is a neglected disease caused by Leishmania parasites. Although significant morbidity and mortality in tropical and subtropical regions of the world are associated with VL, the low investment for developing new treatment measures is chronic. Moreover, resistance and treatment failure are increasing for the main medications, but the emergence of resistance phenotypes is poorly understood at the protein level. Here, we analyzed the development of resistance to miltefosine upon experimental selection in a L. infantum strain. Time to miltefosine resistance emergence was ~six months and label-free quantitative mass-spectrometry-based proteomics analyses revealed that this process involves a remodeling of components of the membrane and mitochondrion, with significant increase in oxidative phosphorylation complexes, particularly on complex IV and ATP synthase, accompanied by increased energy metabolism mainly dependent on β-oxidation of fatty acids. Proteins canonically involved in ROS detoxification did not contribute to the resistant process whereas sterol biosynthesis enzymes could have a role in this development. Furthermore, changes in the abundance of proteins known to be involved in miltefosine resistance such as ABC transporters and phospholipid transport ATPase were detected. Together, our data show a more complete picture of the elements that make up the miltefosine resistance phenotype in L. infantum.

Antimicrobial Secretions of Toads (Anura, Bufonidae): Bioactive Extracts and Isolated Compounds against Human Pathogens

Species of the family Bufonidae, better known as true toads, are widespread and produce bioactive substances in the secretions obtained from specialized skin macroglands. Some true toads have been employed as a folk remedy to treat infectious diseases caused by microbial pathogens. Recent publications based on in silico analysis highlighted the Bufonidae as promising sources of antimicrobial peptides. A review of the literature reveals that Bufonidae skin secretion extracts show inhibitory activity in vitro against clinical isolates of bacteria, resistant and standard strains of bacterial, and fungal and parasitic human pathogens. Secondary metabolites belonging to the classes of alkaloids, bufadienolides, and peptides with antimicrobial activity have been isolated from species of the genera Bufo, Bufotes, Duttaphrynus, and Rhinella. Additionally, some antimicrobial extracts and purified compounds display low cytotoxicity against mammal cells.

Assessing the composition of the plasma membrane of Leishmania (Leishmania) infantum and L. (L.) amazonensis using label-free proteomics

Protozoan parasites of the genus Leishmania are causative agents of leishmaniasis, a wide range of diseases affecting 12 million people worldwide. The species L. infantum and L. amazonensis are etiologic agents of visceral and cutaneous leishmaniasis, respectively. Most proteome analyses of Leishmania have been carried out on whole-cell extracts, but such an approach tends to underrepresent membrane-associated proteins due to their high hydrophobicity and low solubility. Considering the relevance of this category of proteins in virulence, invasiveness and the host-parasite interface, this study applied label-free proteomics to assess the plasma membrane sub-proteome of L. infantum and L. amazonensis. The number of proteins identified in L. infantum and L. amazonensis promastigotes was 1168 and 1455, respectively. After rigorous data processing and mining, 157 proteins were classified as putative plasma membrane-associated proteins, of which 56 proteins were detected in both species, six proteins were detected only in L. infantum and 39 proteins were exclusive to L. amazonensis. The quantitative analysis revealed that two proteins were more abundant in L. infantum, including the glucose transporter 2, and five proteins were more abundant in L. amazonensis. The identified proteins associated with distinct processes and functions. In this regard, proteins of L. infantum were linked to metabolic processes whereas L. amazonensis proteins were involved in signal transduction. Moreover, transmembrane transport was a significant process among the group of proteins detected in both species and members of the superfamily of ABC transporters were highly represented. Interestingly, some proteins of this family were solely detected in L. amazonensis, such as ABCA9. GP63, a well-known virulence factor, was the only GPI-anchored protein identified in the membrane preparations of both species. Finally, we found several proteins with uncharacterized functions, including differentially abundant ones, highlighting a gap in the study of Leishmania proteins. Proteins characterization could provide a better biological understanding of these parasites and deliver new possibilities regarding the discovery of therapeutic targets, drug resistance and vaccine candidates.

Antileishmanial Aminopyrazoles: Studies into Mechanisms and Stability of Experimental Drug Resistance

Current antileishmanial treatment is hampered by limitations, such as drug toxicity and the risk of treatment failure, which may be related to parasitic drug resistance. Given the urgent need for novel drugs, the Drugs for Neglected Diseases initiative (DNDi) has undertaken a drug discovery program, which has resulted in the identification of aminopyrazoles, a highly promising antileishmanial chemical series. Multiple experiments have been performed to anticipate the propensity for resistance development.

Current antileishmanial treatment is hampered by limitations, such as drug toxicity and the risk of treatment failure, which may be related to parasitic drug resistance. Given the urgent need for novel drugs, the Drugs for Neglected Diseases initiative (DNDi) has undertaken a drug discovery program, which has resulted in the identification of aminopyrazoles, a highly promising antileishmanial chemical series. Multiple experiments have been performed to anticipate the propensity for resistance development. Resistance selection was performed by successive exposure of Leishmania infantum promastigotes (in vitro) and intracellular amastigotes (both in vitro and in golden Syrian hamsters). The stability of the resistant phenotypes was assessed after passage in mice and Lutzomyia longipalpis sandflies. Whole-genome sequencing (WGS) was performed to identify mutated genes, copy number variations (CNVs), and somy changes. The potential role of efflux pumps (the MDR and MRP efflux pumps) in the development of resistance was assessed by coincubation of aminopyrazoles with specific efflux pump inhibitors (verapamil, cyclosporine, and probenecid). Repeated drug exposure of amastigotes did not result in the emergence of drug resistance either in vitro or in vivo. Selection at the promastigote stage, however, was able to select for parasites with reduced susceptibility (resistance index, 5.8 to 24.5). This phenotype proved to be unstable after in vivo passage in mice and sandflies, suggesting that nonfixed alterations are responsible for the elevated resistance. In line with this, single nucleotide polymorphisms and indels identified by whole-genome sequencing could not be directly linked to the decreased drug susceptibility. Copy number variations were absent, whereas somy changes were detected, which may have accounted for the transient acquisition of resistance. Finally, aminopyrazole activity was not influenced by the MDR and MRP efflux pump inhibitors tested. The selection performed does not suggest the rapid development of resistance against aminopyrazoles in the field. Karyotype changes may confer elevated levels of resistance, but these do not seem to be stable in the vertebrate and invertebrate hosts. MDR/MRP efflux pumps are not likely to significantly impact the activity of the aminopyrazole leads.

Phenotypic adaptations of Leishmania donovani to recurrent miltefosine exposure and impact on sand fly infection

Background

Since the introduction of miltefosine (MIL) as first-line therapy in the kala-azar elimination programme in the Indian subcontinent, treatment failure rates have been increasing. Since parasite infectivity and virulence may become altered upon treatment relapse, this laboratory study assessed the phenotypic effects of repeated in vitro and in vivo MIL exposure.

Methods

Syngeneic Leishmania donovani lines either or not exposed to MIL were compared for drug susceptibility, rate of promastigote multiplication and metacyclogenesis, macrophage infectivity and behaviour in the sand fly vector, Lutzomyia longipalpis.

Results

Promastigotes of both in vitro and in vivo MIL-selected strains displayed a slightly reduced drug susceptibility that was associated with a reduced MIL-accumulation linked to a lower copy number (disomic state) of chromosome 13 harboring the miltefosine transporter (LdMT) gene. In vitro selected promastigotes showed a lower rate of metacyclogenesis whereas the in vivo derived promastigotes displayed a moderately increased growth rate. Repeated MIL exposure did neither influence the parasite load nor metacyclogenesis in the sand fly vector.

Conclusions

Recurrent in vitro and in vivo MIL exposure evokes a number of very subtle phenotypic and genotypic changes which could make promastigotes less susceptible to MIL without attaining full resistance. These changes did not significantly impact on infection in the sand fly vector.

Investigation of the pathways related to intrinsic miltefosine tolerance in Leishmania (Viannia) braziliensis clinical isolates reveals differences in drug uptake

In Brazil, cutaneous leishmaniasis is caused predominantly by L. (V.) braziliensis. The few therapeutic drugs available exhibit several limitations, mainly related to drug toxicity and reduced efficacy in some regions. Miltefosine (MF), the only oral drug available for leishmaniasis treatment, is not widely available and has not yet been approved for human use in Brazil. Our group previously reported the existence of differential susceptibility among L. (V.) braziliensis clinical isolates. In this work, we further characterized three of these isolates of L. (V.) braziliensis chosen because they exhibited the lowest and the highest MF half maximal inhibitory concentrations and were therefore considered less tolerant or more tolerant, respectively. Uptake of MF, and also of phosphocholine, were found to be significantly different in more tolerant parasites compared to the less sensitive isolate, which raised the hypothesis of differences in the MF transport complex Miltefosine Transporter (MT)-Ros3. Although some polymorphisms in those genes were found, they did not correlate with the drug susceptibility phenotype. Drug efflux and compartmentalization were similar in the isolates tested, and amphotericin B susceptibility was retained in MF tolerant parasites, suggesting that increased fitness was also not the basis of observed differences. Transcriptomic analysis revealed that Ros3 mRNA levels were upregulated in the sensitive strain compared to the tolerant ones. Increased mRNA abundance in more tolerant isolates was validated by quantitative PCR. Our results suggest that differential gene expression of the MT transporter complex is the basis of the differential susceptibility in these unselected, naturally occurring parasites.

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Brazilian L. (V.) braziliensis isolates vary in mitefosine susceptibility.

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Diminished drug internalization was observed in more tolerant isolates.

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Drug susceptibility did not correlate with SNPs in MT-Ros3 genes.

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Drug efflux and compartmentalization were similar in the isolates tested.

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Increased drug sensitivity is accompanied by Ros3 mRNA upregulation.

Identification and Characterization of the Entamoeba Histolytica Rab8a Binding Protein: A Cdc50 Homolog

Membrane traffic plays a pivotal role in virulence in the enteric protozoan parasite Entamoeba histolytica. EhRab8A small GTPase is a key regulator of membrane traffic at the endoplasmic reticulum (ER) of this protist and is involved in the transport of plasma membrane proteins. Here we identified the binding proteins of EhRab8A. The Cdc50 homolog, a non-catalytic subunit of lipid flippase, was identified as an EhRab8A binding protein candidate by affinity coimmunoprecipitation. Binding of EhRab8A to EhCdc50 was also confirmed by reciprocal immunoprecipitation and blue-native polyacrylamide gel electrophoresis, the latter of which revealed an 87 kDa complex. Indirect immunofluorescence imaging with and without Triton X100 showed that endogenous EhCdc50 localized on the surface in the absence of permeabilizing agent but was observed on the intracellular structures and overlapped with the ER marker Bip when Triton X100 was used. Overexpression of N-terminal HA-tagged EhCdc50 impaired its translocation to the plasma membrane and caused its accumulation in the ER. As reported previously in other organisms, overexpression and accumulation of Cdc50 in the ER likely inhibited surface transport and function of the plasma membrane lipid flippase P4-ATPase. Interestingly, HA-EhCdc50-expressing trophozoites gained resistance to miltefosine, which is consistent with the prediction that HA-EhCdc50 overexpression caused its accumulation in the ER and mislocalization of the unidentified lipid flippase. Similarly, EhRab8A gene silenced trophozoites showed increased resistance to miltefosine, supporting EhRab8A-dependent transport of EhCdc50. This study demonstrated for the first time that EhRab8A mediates the transport of EhCdc50 and lipid flippase P4-ATPase from the ER to the plasma membrane.

Autophagic-related cell death of Trypanosoma brucei induced by bacteriocin AS-48

The parasitic protozoan Trypanosoma brucei is the causative agent of human African trypanosomiasis (sleeping sickness) and nagana. Current drug therapies have limited efficacy, high toxicity and/or are continually hampered by the appearance of resistance. Antimicrobial peptides have recently attracted attention as potential parasiticidal compounds. Here, we explore circular bacteriocin AS-48's ability to kill clinically relevant bloodstream forms of T. brucei gambiense, T. brucei rhodesiense and T. brucei brucei. AS-48 exhibited excellent anti-trypanosomal activity in vitro (EC₅₀ = 1–3 nM) against the three T. brucei subspecies, but it was innocuous to human cells at 10⁴-fold higher concentrations. In contrast to its antibacterial action, AS-48 does not kill the parasite through plasma membrane permeabilization but by targeting intracellular compartments. This was evidenced by the fact that vital dye internalization-prohibiting concentrations of AS-48 could kill the parasite at 37 °C but not at 4 °C. Furthermore, AS-48 interacted with the surface of the parasite, at least in part via VSG, its uptake was temperature-dependent and clathrin-depleted cells were less permissive to the action of AS-48. The bacteriocin also caused the appearance of myelin-like structures and double-membrane autophagic vacuoles. These changes in the parasite's ultrastructure were confirmed by fluorescence microscopy as AS-48 induced the production of EGFP-ATG8.2-labeled autophagosomes. Collectively, these results indicate AS-48 kills the parasite through a mechanism involving clathrin-mediated endocytosis of VSG-bound AS-48 and the induction of autophagic-like cell death. As AS-48 has greater in vitro activity than the drugs currently used to treat T. brucei infection and does not present any signs of toxicity in mammalian cells, it could be an attractive lead compound for the treatment of sleeping sickness and nagana.

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AS-48 kills Trypanosoma brucei efficiently and is innocuous in mammalian cells.

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It has greater in vitro activity than drugs currently in use.

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AS-48 must be internalized by the parasite in order to exert its trypanocidal effect.

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AS-48 uptake involves VSG binding and clathrin-mediated endocytosis.

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AS-48 induces an autophagic-related cell death.

Different Mutations in a P-type ATPase Transporter in Leishmania Parasites are Associated with Cross-resistance to Two Leading Drugs by Distinct Mechanisms

Leishmania infantum is an etiological agent of the life-threatening visceral form of leishmaniasis. Liposomal amphotericin B (AmB) followed by a short administration of miltefosine (MF) is a drug combination effective for treating visceral leishmaniasis in endemic regions of India. Resistance to MF can be due to point mutations in the miltefosine transporter (MT). Here we show that mutations in MT are also observed in Leishmania AmB-resistant mutants. The MF-induced MT mutations, but not the AmB induced mutations in MT, alter the translocation/uptake of MF. Moreover, mutations in the MT selected by AmB or MF have a major impact on lipid species that is linked to cross-resistance between both drugs. These alterations include changes of specific phospholipids, some of which are enriched with cyclopropanated fatty acids, as well as an increase in inositolphosphoceramide species. Collectively these results provide evidence of the risk of cross-resistance emergence derived from current AmB-MF sequential or co-treatments for visceral leishmaniasis.

New Approaches to Overcome Transport Related Drug Resistance in Trypanosomatid Parasites

Leishmania and Trypanosoma are members of the Trypanosomatidae family that cause severe human infections such as leishmaniasis, Chagas disease, and sleeping sickness affecting millions of people worldwide. Despite efforts to eradicate them, migrations are expanding these infections to developing countries. There are no vaccines available and current treatments depend only on chemotherapy. Drug resistance is a major obstacle for the treatment of these diseases given that existing drugs are old and limited, with some having severe side effects. Most resistance mechanisms developed by these parasites are related with a decreased uptake or increased efflux of the drug due to mutations or altered expression of membrane transporters. Different new approaches have been elaborated that can overcome these mechanisms of resistance including the use of inhibitors of efflux pumps and drug carriers for both active and passive targeting. Here we review new formulations that have been successfully applied to circumvent resistance related to drug transporters, opening alternative ways to solve drug resistance in protozoan parasitic diseases.

The antileishmanial drug miltefosine (Impavido(®)) causes oxidation of DNA bases, apoptosis, and necrosis in mammalian cells

Miltefosine was developed to treat skin cancer; further studies showed that the drug also has activity against Leishmania. Miltefosine is the first oral agent for treating leishmaniasis. However, its mechanism of action is not completely understood. We have evaluated the induction of DNA damage by miltefosine. Cytotoxicity and genotoxicity (comet assay) tests were performed on human leukocytes exposed to the drug in vitro. Apoptosis and necrosis were also evaluated. In vivo tests were conducted in Swiss male mice (Mus musculus) treated orally with miltefosine. Oxidation of DNA bases in peripheral blood cells was measured using the comet assay followed by digestion with formamidopyrimidine glycosylase (FPG), which removes oxidized guanine bases. The micronucleus test was performed on bone marrow erythrocytes. Miltefosine caused DNA damage, apoptosis, and necrosis in vitro. Mice treated with miltefosine showed an increase in the DNA damage score, which was further increased following FPG digestion. The micronucleus test was also positive.

Joining forces: first application of a rapamycin-induced dimerizable Cre system for conditional null mutant analysis in Leishmania

Reverse genetics in Leishmania spp has gained importance beyond basic research as efforts increase to discover and validate new drug targets. Often, the most promising targets are essential for viability of the parasites, defying a genetic analysis by current gene replacement strategies. Duncan et al. demonstrate the applicability of DiCre recombination in Leishmania for induced replacement of the kinase CRK3 gene in promastigotes. DiCre gene replacement leads to the rapid loss of the gene and allows monitoring the phenotypic effects of the loss of function, eliminating the need for prolonged cultivation and selection. Implementation of the DiCre approach will allow functional genetics of the most important of Leishmania genes and is likely to boost genetic research and drug target discovery.

The Oral Antimalarial Drug Tafenoquine Shows Activity against Trypanosoma brucei

The protozoan parasite Trypanosoma brucei causes human African trypanosomiasis, or sleeping sickness, a neglected tropical disease that requires new, safer, and more effective treatments. Repurposing oral drugs could reduce both the time and cost involved in sleeping sickness drug discovery. Tafenoquine (TFQ) is an oral antimalarial drug belonging to the 8-aminoquinoline family which is currently in clinical phase III. We show here that TFQ efficiently kills different T. brucei spp. in the submicromolar concentration range. Our results suggest that TFQ accumulates into acidic compartments and induces a necrotic process involving cell membrane disintegration and loss of cytoplasmic content, leading to parasite death. Cell lysis is preceded by a wide and multitarget drug action, affecting the lysosome, mitochondria, and acidocalcisomes and inducing a depolarization of the mitochondrial membrane potential, elevation of intracellular Ca(2+), and production of reactive oxygen species. This is the first report of an 8-aminoquinoline demonstrating significant in vitro activity against T. brucei.

Untargeted metabolomic analysis of miltefosine action in Leishmania infantum reveals changes to the internal lipid metabolism

There are many theories as to the mode of action of miltefosine against Leishmania including alterations to the membrane lipid content, induction of apoptosis and modulation of macrophage responses. Here we perform untargeted metabolomics to elucidate the metabolic changes involved in miltefosine action. Over 800 metabolites were detected, 10% of which were significantly altered after 3.75 h. Many of the changes related to an increase in alkane fragment and sugar release. Fragment release is synchronised with reactive oxygen species production, but native membrane phospholipids remain intact. Signs of DNA damage were also detected as were changes to the levels of some thiols and polyamines. After 5 h of miltefosine treatment the cells showed depleted levels of most metabolites, indicating that the cells' outer membrane integrity had become compromised and internal metabolites were escaping upon cell death. In miltefosine resistant cells, the drug was not internalised and the changes to the internal metabolite levels were not seen. In contrast, cells resistant to antimony (SbIII) had similar corresponding alterations to the levels of internal metabolites as wild-type cells. A detailed knowledge of the mode of action of miltefosine will be important to inform the design of combination therapies to combat leishmaniasis, something that the research community should be prioritising in the coming years.

Identification of specific reversal agents for Leishmania ABCI4-mediated antimony resistance by flavonoid and trolox derivative screening

OBJECTIVES

To identify reversal agents for the Leishmania ABCI4 transporter that confers resistance to antimony.

METHODS

Selective ABCI4 inhibitors among a series of 15 flavonoid and trolox derivatives or analogues were investigated by evaluating their ability to reverse antimony resistance in Leishmania parasites overexpressing ABCI4. Among the compounds screened, N-ethyltrolox carboxamide (compound D2) produced the highest reversal activity. In order to optimize the activity of D2, we synthesized a series of 10 derivatives by condensation of various amines with trolox.

RESULTS

Analysis of antimony resistance reversal activity showed that N-propyltrolox carboxamide (compound D4) was the most potent ABCI4 inhibitor, with reversal activity being maintained in the intracellular amastigote stage. In addition, trolox derivatives significantly reverted the resistance to zinc protoporphyrin. The mechanism of action of these active derivatives was found to be related to significant reversion of Sb(III) and zinc protoporphyrin accumulation and to a decrease in drug efflux.

CONCLUSIONS

Our findings suggest that trolox derivatives D2 and D4 could be considered to be specific reversal agents targeting the Leishmania ABCI4 transporter. The structure-activity relationship obtained in the present study highlights the importance of the size and length of the alkyl substituent linked to trolox. Furthermore, the structural data obtained provide valuable information for the further development of new, even more specific and potent Leishmania ABCI4 reversal agents.

Miltefosine: a review of its pharmacology and therapeutic efficacy in the treatment of leishmaniasis

Miltefosine is an alkylphosphocholine drug with demonstrated activity against various parasite species and cancer cells as well as some pathogenic bacteria and fungi. For 10 years it has been licensed in India for the treatment of visceral leishmaniasis (VL), a fatal neglected parasitic disease. It is the first and still the only oral drug that can be used to treat VL and cutaneous leishmaniasis (CL). The standard 28 day miltefosine monotherapy regimen is well tolerated, except for mild gastrointestinal side effects, although its teratogenic potential severely hampers its general use in the clinic and roll-out in national elimination programmes. The pharmacokinetics of miltefosine are mainly characterized by its long residence time in the body, resulting in extensive drug accumulation during treatment and long elimination half-lives. At the moment, different combination therapy strategies encompassing miltefosine are being tested in multiple controlled clinical trials in various geographical areas of endemicity, both in South Asia and East Africa. We here review the most salient pre-clinical and clinical pharmacological aspects of miltefosine, its mechanism of action against Leishmania parasites and other pathogens, and provide a systematic overview of the efficacy and safety data from all clinical trials of miltefosine, either alone or in combination, in the treatment of VL and CL.

Defeating Leishmania resistance to miltefosine (hexadecylphosphocholine) by peptide-mediated drug smuggling: a proof of mechanism for trypanosomatid chemotherapy

Miltefosine (hexadecylphosphocholine, HePC), the first orally active drug successful against leishmaniasis, is especially active on the visceral form of the disease. Resistance mechanisms are almost exclusively associated to dysfunction in HePC uptake systems. In order to evade the requirements of its cognate receptor/translocator, HePC-resistant Leishmania donovani parasites (R40 strain) were challenged with constructs consisting of an ω-thiol-functionalized HePC analogue conjugated to the cell-penetrating peptide (CPP) Tat(48-60), either through a disulfide or a thioether bond. The conjugates enter and kill both promastigote and intracellular amastigote forms of the R40 strain. Intracellular release of HePC by reduction of the disulfide-based conjugate was confirmed by means of double tagging at both the CPP (Quasar 670) and HePC (BODIPY) moieties. Scission of the conjugate, however, is not mandatory, as the metabolically more stable thioether conjugate retained substantial activity. The disulfide conjugate is highly active on the bloodstream form of Trypanosoma b. brucei, naturally resistant to HePC. Our results provide proof-of-mechanism for the use of CPP conjugates to avert drug resistance by faulty drug accumulation in parasites, as well as the possibility to extend chemotherapy into other parasites intrinsically devoid of membrane translocation systems.

Use of antimony in the treatment of leishmaniasis: current status and future directions

In the recent past the standard treatment of kala-azar involved the use of pentavalent antimonials Sb(V). Because of progressive rise in treatment failure to Sb(V) was limited its use in the treatment program in the Indian subcontinent. Until now the mechanism of action of Sb(V) is not very clear. Recent studies indicated that both parasite and hosts contribute to the antimony efflux mechanism. Interestingly, antimonials show strong immunostimulatory abilities as evident from the upregulation of transplantation antigens and enhanced T cell stimulating ability of normal antigen presenting cells when treated with Sb(V) in vitro. Recently, it has been shown that some of the peroxovanadium compounds have Sb(V)-resistance modifying ability in experimental infection with Sb(V) resistant Leishmania donovani isolates in murine model. Thus, vanadium compounds may be used in combination with Sb(V) in the treatment of Sb(V) resistance cases of kala-azar.

Sitamaquine overcomes ABC-mediated resistance to miltefosine and antimony in Leishmania

Although oral miltefosine represented an important therapeutic advance in the treatment of leishmaniasis, the appearance of resistance remains a serious threat. LMDR1/LABCB4, a P-glycoprotein-like transporter included in the Leishmania ABC (ATP-binding cassette) family, was the first molecule shown to be involved in experimental miltefosine resistance. LMDR1 pumps drugs out of the parasite, thereby decreasing their intracellular accumulation. Sitamaquine, another promising oral drug for leishmaniasis, is currently in phase 2b clinical trials. The physicochemical features of this drug suggested to us that it could be considered for use as an LMDR1 inhibitor. Indeed, we report herein that nonleishmanicidal concentrations of sitamaquine reverse miltefosine resistance in a multidrug resistance Leishmania tropica line that overexpresses LMDR1. This reversal effect is due to modulation of the LMDR1-mediated efflux of miltefosine. In addition, sitamaquine is not a substrate of LMDR1, as this transporter does not affect sitamaquine accumulation or sensitivity in the parasite. Likewise, we show that ketoconazole, another oral leishmanicidal drug known to interact with ABC transporters, is also able to reverse LMDR1-mediated miltefosine resistance, although with a lower efficiency than sitamaquine. Molecular docking on a three-dimensional homology model of LMDR1 showed different preferential binding sites for each substrate-inhibitor pair, thus explaining this different behavior. Finally, we show that sitamaquine is also able to modulate the antimony resistance mediated by MRPA/LABCC3, another ABC transporter involved in experimental and clinical antimony resistance in this parasite. Taken together, these data suggest that the combination of sitamaquine with miltefosine or antimony could avoid the appearance of resistance mediated by these membrane transporters in Leishmania.

Therapeutic switching in leishmania chemotherapy: a distinct approach towards unsatisfied treatment needs

Current drugs for the treatment of visceral leishmaniasis are inadequate. No novel compound is in the pipeline. Since economic returns on developing a new drug for neglected disease, leishmaniasis is so low that therapeutic switching represents the only realistic strategy. It refers to "alternative drug use" discoveries which differ from the original intent of the drug. Amphotericin B, paromomycin, miltefosine and many other drugs are very successful examples of "new drugs from old". This article reviews the discovery, growth and current status of these drugs and concluded that the potential of this approach (therapeutic switching) may use in the development of new antileishmanials in future also.

A new ATP-binding cassette protein is involved in intracellular haem trafficking in Leishmania

The characterization of LABCG5, a new intracellular ATP-binding cassette protein in Leishmania donovani, is described. Unlike other ABCG half-transporters, LABCG5 is not involved in either drug resistance or phospholipid efflux. However, we provide evidence suggesting that this protein is involved in intracellular haem trafficking. Thus, downregulation of LABCG5 function produced upon overexpression of an inactive version of the protein caused a dramatic growth arrest unless a haemin supplement was added or the mutated gene was eliminated. Supplementation with haemoglobin, an upstream metabolite normally sufficient to meet parasite haem requirements, was unable to rescue the growth defect phenotype. Haemoglobin endocytosis was not hampered in dominant-negative parasites and neither was haem uptake, a process that we show here to be dependent on a specific transporter. In contrast, LABCG5 function was required for the correct intracellular trafficking of haemoglobin-bound porphyrins to the mitochondria, not affecting the routing of free haem. Finally, LABCG5 binds haem through hydrophobic and electrostatic interactions. Altogether, these data suggest that LABCG5 is involved in the salvage of the haem released after the breakdown of internalized haemoglobin. As Leishmania is auxotrophic for haem, the pharmacological targeting of this route could represent a novel approach to control fatal visceral leishmaniasis.

Drug resistance in visceral leishmaniasis

Visceral leishmaniasis remains a public health problem worldwide. This illness was included by the World Health Organization in the list of neglected tropical diseases targeted for elimination by 2015. The widespread emergence of resistance to pentavalent antimonials in India where half cases occur globally and the unavailability of a vaccine in clinical use constitute major obstacles in achieving of this goal. The last decade new antileishmanials became available, including the oral agent miltefosine. However, in poor endemic countries their wide use was curtailed because of the high costs, and also due to concerns of toxicity and emergence of resistance. Various mechanisms of antileishmanial resistance were identified recently in field isolates. Their elucidation will boost the design of new drugs and the molecular surveillance of resistance. Combination regimens should be evaluated in large trials. Overall, the development of antileishmanials has been generally slow; new drugs are needed. In order to control visceral leishmaniasis worldwide, treatment advances should become affordable in the poorest countries, where they are needed most.

Functional characterization of the nucleotide binding domain of the Cryptosporidium parvum CpABC4 transporter: an iron-sulfur cluster transporter homolog

In a previous study, we showed that the Cryptosporidium parvum ATP half-transporter CpABC4 (cgd1_1350) transcript was up-regulated in response to drug treatment with paromomycin and cyclosporine A in an in vitro infection model. CpABC4 may be directly or indirectly involved in the metabolic interactions between host and parasite in response to drug treatment and/or be involved in the intrinsic resistance to chemotherapy. In order to characterize the catalytic site of this transporter, an extended region of the nucleotide-binding domain of CpABC4 (H6-1350NBD) was expressed and purified as an N-terminal hexahistidine-tagged protein in E. coli. The presence of a single tryptophan residue enabled the intrinsic fluorescence to be monitored in response to binding of different compounds. A dose-dependent quenching of the domain's intrinsic fluorescence was observed with its natural substrate, ATP and the fluorescent analogue TNP-ATP. A similar effect was observed with progesterone as well as the flavonoids quercetin and silibinin, previously shown to inhibit parasite development in a cell-based assay. The purified domain also exhibited ATPase activity in the nanomolar range, which further confirmed correct folding and activity of the recombinant domain. The H6-1350NBD serves as a tool to test and design stereospecific inhibitors of the catalytic site, as well as other compounds that bind elsewhere in the domain that may indirectly interact with the catalytic site of the NBD of the CpABC4 transporter.

Characterization of an ABCG-like transporter from the protozoan parasite Leishmania with a role in drug resistance and transbilayer lipid movement

Leishmaniasis treatment is hampered by the increased appearance of treatment failure. ATP-binding cassette (ABC) transporters are usually involved in drug resistance both in tumor cells and in microorganisms. Here we report the characterization of an ABCG-like transporter, LiABCG6, localized mainly at the plasma membrane in Leishmania protozoan parasites. When overexpressed, this half-transporter confers significant resistance to the leishmanicidal agents miltefosine and sitamaquine. This resistance phenotype is mediated by a reduction in intracellular drug accumulation. LiABCG6 also reduces the accumulation of short-chain fluorescent phospholipid analogues of phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine. As a whole, these results suggest that LiABCG6 could be implicated in phospholipid trafficking and drug resistance.

A novel ATP-binding cassette transporter from Leishmania is involved in transport of phosphatidylcholine analogues and resistance to alkyl-phospholipids

ATP-binding cassette (ABC) transporters represent an important family of membrane proteins involved in drug resistance and other biological activities. The present work reports the characterization of the first ABC subfamily G (ABCG)-like transporter, LiABCG4, in the protozoan parasite Leishmania. LiABCG4 localized mainly to the parasite plasma membrane. Overexpression of this half-transporter reduced the accumulation of phosphatidylcholine analogues and conferred resistance to alkyl-phospholipids. Likewise, when expressed in Saccharomyces cerevisiae, the protein localized to the yeast plasma membrane and conferred resistance to alkyl-phospholipids. Post-Golgi secretory vesicles isolated from a LiABCG4-overexpressing yeast mutant contained the leishmanial ABC transporter and exhibited ATP-dependent, vanadate-sensitive transport of phosphatidylcholine analogues from the cytosolic to the lumenal leaflet of the vesicle membrane. Cross-linking showed dimerization of LiABCG4. These results suggest that LiABCG4 is involved in the active transport of phosphatidylcholine and resistance to alkyl-phospholipids in Leishmania.