Mechanism of amphotericin B resistance in clinical isolates of Leishmania donovani

Combined treatment of miltefosine and paromomycin delays the onset of experimental drug resistance in Leishmania infantum

Background

Since miltefosine monotherapy against visceral leishmaniasis (VL) caused by Leishmania donovani has been discontinued in the Indian subcontinent due to an increase in the number of treatment failures, single dose liposomal amphotericin B is now advocated as a treatment option of choice. Paromomycin-miltefosine combination therapy can be used as substitute first-line treatment in regions without cold-chain potential. Previous laboratory studies in the closely related species Leishmania infantum have demonstrated that paromomycin monotherapy fairly rapidly selects for resistance producing a phenotype with increased fitness. Given the possible clinical implications of these findings for the current field situation, the present study aimed to identify the potential hazards of paromomycin-miltefosine combination therapy.

Principal findings

Drug interaction studies using the fixed-ratio isobologram method revealed an indifferent interaction between paromomycin and miltefosine. In hamsters infected with L. infantum, the combination resulted in cumulative efficacy in reducing parasite burdens in the liver, spleen and bone marrow. Selected resistant lines against the single drugs did not display cross-resistance. When the intracellular amastigote stage was repeatedly exposed to the paromomycin-miltefosine combination, either in vitro or in vivo, no significant susceptibility decrease towards either drug was noted.

Conclusions

These results suggest that implementation of paromomycin-miltefosine combination therapy indeed could represent a safe and affordable treatment option for L. donovani VL as miltefosine appears to overrule the anticipated rapid development of PMM resistance.

Liposomal amphotericin B is presently being used as first-line treatment option against visceral leishmaniasis in the Indian subcontinent. However, the need for temperature-controlled transport and storage limits its widespread use in rural areas. Previous studies already suggested that paromomycin-miltefosine combination therapy could be a valuable alternative, side passing some of the disadvantages associated with monotherapy, such as development of drug resistance. As the first reports of miltefosine resistant clinical isolates have already surfaced and paromomycin resistance could be easily induced under laboratory conditions, it remains essential to assess the risk of developing resistance against both drugs upon combination therapy. This study evaluated the efficacy of combined therapy against a Leishmania species closely related to the agent found in the Indian subcontinent, using both in vitro and in vivo models with the aim to select multidrug-resistant species by simultaneous exposure to paromomycin and miltefosine. The combination of both drugs in the hamster model resulted in a cumulative efficacy but did not lead to a significant susceptibility decrease, indicating that paromomycin-miltefosine combination therapy may represent a safe and affordable treatment option for visceral leishmaniasis.

Glucose-6-phosphate dehydrogenase and Trypanothione reductase interaction protects Leishmania donovani from metalloid mediated oxidative stress

Exploration of metabolons as viable drug target is rare in kinetoplastid biology. Here we present a novel protein-protein interaction among Glucose-6-phosphate dehydrogenase (LdG6PDH) and Trypanothione reductase (LdTryR) of Leishmania donovani displaying interconnection between central glucose metabolism and thiol metabolism of this parasite. Digitonin fractionation patterns observed through immunoblotting indicated localisation of both LdG6PDH and LdTryR in cytosol. In-silico and in-vitro interaction observed by size exclusion chromatography, co-purification, pull-down assay and spectrofluorimetric analysis revealed LdG6PDH and LdTryR physically interact with each other in a NADPH dependent manner. Coupled enzymatic assay displayed that NADPH generation was severely impaired by addition of Sb^III, As^III and Te^IV extraneously, which hint towards metalloid driven structural changes of the interacting proteins. Co-purification patterns and pull-down assays also depicted that metalloids (Sb^III, As^III and Te^IV) hinder the in-vitro interaction of these two enzymes. Surprisingly, metalloids at sub-lethal concentrations induced the in-vivo interaction of LdG6PDH and LdTryR, as analyzed by pull-down assays and fluorescence microscopy signifying protection against metalloid mediated ROS. Inhibition of LdTryR by thioridazine in LdG6PDH^-/- parasites resulted in metalloid induced apoptotic death of the parasites due to abrupt fall in reduced thiol content, disrupted NADPH/NADP⁺ homeostasis and lethal oxidative stress. Interestingly, clinical isolates of L.donovani resistant to SAG exhibited enhanced interaction between LdG6PDH and LdTryR and showed cross resistivity towards As^III and Te^IV. Thus, our findings propose the metabolon of LdG6PDH and LdTryR as an alternate therapeutic target and provide mechanistic insight about metalloid resistance in Visceral Leishmaniasis.

Therapeutic effect of ursolic acid in experimental visceral leishmaniasis

Leishmaniasis is an important neglected tropical disease, affecting more than 12 million people worldwide. The available treatments are not well tolerated and present diverse side effects in patients, justifying the search for new therapeutic compounds. In the present study, the therapeutic potential and toxicity of ursolic acid (UA), isolated from the leaves of Baccharis uncinella C. DC. (Asteraceae), were evaluated in experimental visceral leishmaniasis. To evaluate the therapeutic potential of UA, hamsters infected with L. (L.) infantum were treated daily during 15 days with 1.0 or 2.0 mg UA/kg body weight, or with 5.0 mg amphotericin B/kg body weight by intraperitoneal route. Fifteen days after the last dose, the parasitism of the spleen and liver was stimated and the main histopathological alterations were recorded. The proliferation of splenic mononuclear cells was evaluated and IFN-γ, IL-4, and IL-10 gene expressions were analyzed in spleen fragments. The toxicity of UA and amphotericin B were evaluated in healthy golden hamsters by histological analysis and biochemical parameters. Animals treated with UA had less parasites in the spleen and liver when compared with the infected control group, and they also showed preservation of white and red pulps, which correlate with a high rate of proliferation of splenic mononuclear cells, IFN-γ mRNA and iNOS production. Moreover, animals treated with UA did not present alterations in the levels of AST, ALT, creatinine and urea. Taken together, these findings indicate that UA is an interesting natural compound that should be considered for the development of prototype drugs against visceral leishmaniasis.

Deciphering the interplay between cysteine synthase and thiol cascade proteins in modulating Amphotericin B resistance and survival of Leishmania donovani under oxidative stress

Leishmania donovani is the causative organism of the neglected human disease known as visceral leishmaniasis which is often fatal, if left untreated. The cysteine biosynthesis pathway of Leishmania may serve as a potential drug target because it is different from human host and regulates downstream components of redox metabolism of the parasites; essential for their survival, pathogenicity and drug resistance. However, despite the apparent dependency of redox metabolism of cysteine biosynthesis pathway, the role of L. donovani cysteine synthase (LdCS) in drug resistance and redox homeostasis has been unexplored. Herein, we report that over-expression of LdCS in Amphotericin B (Amp B) sensitive strain (S1-OE) modulates resistance towards oxidative stress and drug pressure. We observed that antioxidant enzyme activities were up-regulated in S1-OE parasites and these parasites alleviate intracellular reactive oxygen species (ROS) efficiently by maintaining the reduced thiol pool. In contrast to S1-OE parasites, Amp B sensitive strain (S1) showed higher levels of ROS which was positively correlated with the protein carbonylation levels and negatively correlated with cell viability. Moreover, further investigations showed that LdCS over-expression also augments the ROS-primed induction of LdCS-GFP as well as endogenous LdCS and thiol pathway proteins (LdTryS, LdTryR and LdcTXN) in L. donovani parasites; which probably aids in stress tolerance and drug resistance. In addition, the expression of LdCS was found to be up-regulated in Amp B resistant isolates and during infective stationary stages of growth and consistent with these observations, our ex vivo infectivity studies confirmed that LdCS over-expression enhances the infectivity of L. donovani parasites. Our results reveal a novel crosstalk between LdCS and thiol metabolic pathway proteins and demonstrate the crucial role of LdCS in drug resistance and redox homeostasis of Leishmania.

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Over-expression of CS in L. donovani modulates oxidative stress & Amp B resistance.

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Over-expressing parasite possess higher thiol to counteract the oxidative stress.

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Over-expressing parasites showed increased activity of TXNPx, GST, SOD, and APx.

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Expression/activity of LdCS is up-regulated in Amp B resistant clinical isolates.

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Ex vivo results confirm that LdCS over-expression enhance the parasites infectivity.

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Over-expressing parasites survived long time under oxidative stress conditions.

Identification and Characterization of miRNAs in Response to Leishmania donovani Infection: Delineation of Their Roles in Macrophage Dysfunction

The outcome of Leishmania infection depends on parasite abilities to evade host immune response and its survival in hostile environment of host macrophages. Despite a wealth of gained crucial information, parasite strategies by which it dampens host macrophage functions remain poorly understood. Micro RNAs (miRNAs) are evolutionarily conserved class of endogenous 22-nucleotide small non-coding RNA gene products, described to participate in the regulation of almost every cellular process investigated so far. In this study, we identified 940 miRNAs in Leishmania donovani infected macrophages by de novo sequencing out of which levels of 85 miRNAs were found to be consistently modified by parasite infection. Herein, we report the functional characteristics of 10 miRNAs i.e., mir-3620, mir-6385, mir-6973a, mir-6996, mir-328, mir-8113, mir-3473f, mir-763, mir-6540, and mir-1264 that were differentially but constantly regulated in infected macrophages for their role in regulation of macrophage effector functions. The target gene prediction and biological interaction analysis revealed involvement of these miRNAs in various biological processes such as apoptosis inhibition, phagocytosis, drug response, and T cell phenotypic transitions. These findings could contribute for the better understanding of macrophages dysfunction and leishmanial pathogenesis. Further, the identified miRNAs could also be used as biomarker/s in diagnosis, prognosis, and therapeutics of Leishmania infection.

Chemotherapy of leishmaniasis: present challenges

Cutaneous and visceral leishmaniasis are amongst the most devastating infectious diseases of our time, affecting millions of people worldwide. The treatment of these serious diseases rely on a few chemotherapeutic agents, most of which are of parenteral use and induce severe side-effects. Furthermore, rates of treatment failure are high and have been linked to drug resistance in some areas. Here, we reviewed data on current chemotherapy practice in leishmaniasis. Drug resistance and mechanisms of resistance are described as well as the prospects for applying drug combinations for leishmaniasis chemotherapy. It is clear that efforts for discovering new drugs applicable to leishmaniasis chemotherapy are essential. The main aspects on the various steps of drug discovery in the field are discussed.

Development of mannose-anchored thiolated amphotericin B nanocarriers for treatment of visceral leishmaniasis

AIM

Our goal was to improve treatment outcomes for visceral leishmaniasis by designing nanocarriers that improve drug biodistribution and half-life. Thus, long-acting mannose-anchored thiolated chitosan amphotericin B nanocarrier complexes (MTC AmB) were developed and characterized.

MATERIALS & METHODS

A mannose-anchored thiolated chitosan nanocarrier was manufactured and characterized. MTC AmB was examined for cytotoxicity, biocompatibility, uptake and antimicrobial activities.

RESULTS

MTC AmB was rod shaped with a size of 362 nm. MTC AmB elicited 90% macrophage viability and 71-fold enhancement in drug uptake compared with native drug. The antileishmanial IC50 for MTC AmB was 0.02 μg/ml compared with 0.26 μg/ml for native drug.

CONCLUSION

These studies show that MTC can serve as a platform for clearance of Leishmania in macrophages.

Molecular Mechanism of Drug Resistance

The treatment of microbial infections has suffered greatly in this present century of pathogen dominance. Inspite of extensive research efforts and scientific advancements, the worldwide emergence of microbial tolerance continues to plague survivability. The innate property of microbe to resist any antibiotic due to evolution is the virtue of intrinsic resistance. However, the classical genetic mutations and extrachromosomal segments causing gene exchange attribute to acquired tolerance development. Rampant use of antimicrobials causes certain selection pressure which increases the resistance frequency. Genomic duplication, enzymatic site modification, target alteration, modulation in membrane permeability, and the efflux pump mechanism are the major contributors of multidrug resistance (MDR), specifically antibiotic tolerance development. MDRs will lead to clinical failures for treatment and pose health crisis. The molecular mechanisms of antimicrobial resistance are diverse as well as complex and still are exploited for new discoveries in order to prevent the surfacing of “superbugs.” Antimicrobial chemotherapy has diminished the threat of infectious diseases to some extent. To avoid the indiscriminate use of antibiotics, the new ones licensed for use have decreased with time. Additionally, in vitro assays and genomics for anti-infectives are novel approaches used in resolving the issues of microbial resistance. Proper use of drugs can keep it under check and minimize the risk of MDR spread.

Phosphorylation of Translation Initiation Factor 2-Alpha in Leishmania donovani under Stress Is Necessary for Parasite Survival

The transformation of Leishmania donovani from a promastigote to an amastigote during mammalian host infection displays the immense adaptability of the parasite to survival under stress. Induction of translation initiation factor 2-alpha (eIF2α) phosphorylation by stress-specific eIF2α kinases is the basic stress-perceiving signal in eukaryotes to counter stress. Here, we demonstrate that elevated temperature and acidic pH induce the phosphorylation of Leishmania donovani eIF2α (LdeIF2α). In vitro inhibition experiments suggest that interference of LdeIF2α phosphorylation under conditions of elevated temperature and acidic pH debilitates parasite differentiation and reduces parasite viability (P < 0.05). Furthermore, inhibition of LdeIF2α phosphorylation significantly reduced the infection rate (P < 0.05), emphasizing its deciding role in successful invasion and infection establishment. Notably, our findings suggested the phosphorylation of LdeIF2α under H₂O₂-induced oxidative stress. Inhibition of H₂O₂-induced LdeIF2α phosphorylation hampered antioxidant balance by impaired redox homeostasis gene expression, resulting in increased reactive oxygen species accumulation (P < 0.05) and finally leading to decreased parasite viability (P < 0.05). Interestingly, exposure to sodium antimony glucamate and amphotericin B induces LdeIF2α phosphorylation, indicating its possible contribution to protection against antileishmanial drugs in common use. Overall, the results strongly suggest that stress-induced LdeIF2α phosphorylation is a necessary event for the parasite life cycle under stressed conditions for survival.

Leishmaniasis and various immunotherapeutic approaches

Leishmaniasis is a vector-borne infectious disease caused by multiple Leishmania (L.) species with diverse clinical manifestations. There is currently no vaccine against any form of the disease approved in humans, and chemotherapy is the sole approach for treatment. Unfortunately, treatment options are limited to a small number of drugs, partly due to high cost and significant adverse effects. The other obstacle in leishmaniasis treatment is the potential for drug resistance, which has been observed in multiple endemic countries. Immunotherapy maybe another important avenue for controlling leishmaniasis and could help patients control the disease. There are different approaches for immunotherapy in different infectious diseases, generally with low-cost, limited side-effects and no possibility to developing resistance. In this paper, different immunotherapy approaches as alternatives to routine drug treatment will be reviewed against leishmaniasis.

Genetic markers for antimony resistant clinical isolates differentiation from Indian Kala-azar

Visceral Leishmaniasis or Kala-azar is caused by the protozoan parasites belonging to the Genus Leishmania. Once thought eradicated from the Indian subcontinent, the disease came back with drug resistance to almost all prevalent drugs. Molecular epidemiological studies revealed the polymorphic nature of the population of the main player of the disease, Leishmania donovani and involvement of other species (L. tropica) and other genus (Leptomonas) with the disease. This makes control measures almost futile. It also strongly demands the characterization of each and every isolate mandatory which is not done. In this background, the present study has been carried out to assess the genetic attributes of each clinical isolates (n=26) of KA and PKDL patients from India and Bangladesh. All the isolates were characterized through Restriction Fragment Length Polymorphism (RFLP) analysis to ascertain their species identity. 46.2% of the isolates were found to be Sodium Stibogluconate (SSG) resistant by amastigote-macrophage model. When the clinical isolates were subjected to Single Stranded Conformation Polymorphism (SSCP) of Internal Transcribed Spacer 1 (ITS1), Internal Transcribed Spacer 2 (ITS2) and some anonymous markers, the drug resistant Leishmania isolates of SSG can be distinguished from the sensitive isolates distinctly. This study showed for the first time, the genetic markers for SSG drug resistance of Indian Kala-azar clinical isolates.

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.

Evaluating drug resistance in visceral leishmaniasis: the challenges

For decades antimonials were the drugs of choice for the treatment of visceral leishmaniasis (VL), but the recent emergence of resistance has made them redundant as first-line therapy in the endemic VL region in the Indian subcontinent. The application of other drugs has been limited due to adverse effects, perceived high cost, need for parenteral administration and increasing rate of treatment failures. Liposomal amphotericin B (AmB) and miltefosine (MIL) have been positioned as the effective first-line treatments; however, the number of monotherapy MIL-failures has increased after a decade of use. Since no validated molecular resistance markers are yet available, monitoring and surveillance of changes in drug sensitivity and resistance still depends on standard phenotypic in vitro promastigote or amastigote susceptibility assays. Clinical isolates displaying defined MIL- or AmB-resistance are still fairly scarce and fundamental and applied research on resistance mechanisms and dynamics remains largely dependent on laboratory-generated drug resistant strains. This review addresses the various challenges associated with drug susceptibility and -resistance monitoring in VL, with particular emphasis on the choice of strains, susceptibility model selection and standardization of procedures with specific read-out parameters and well-defined threshold criteria. The latter are essential to support surveillance systems and safeguard the limited number of currently available antileishmanial drugs.

Solute carrier protein family 11 member 1 (Slc11a1) activation efficiently inhibits Leishmania donovani survival in host macrophages

Visceral leishmaniasis (kala-azar), a life threatening disease caused by L. donovani, is a latent threat to more than 147 million people living in disease endemic South East Asia region of the Indian subcontinent. The therapeutic option to control leishmanial infections are very limited, and at present comprise only two drugs, an antifungal amphotericin B and an antitumor miltefosine, which are also highly vulnerable for parasitic resistance. Therefore, identification and development of alternate control measures is an exigent requirement to control leishmanial infections. In this study, we report that functionally induced expression of solute carrier protein family 11 member 1 (Slc11a1), a transmembrane divalent cationic transporter recruited on the surface of phagolysosomes after phagocytosis of parasites, effectively inhibits Leishmania donovani growth in host macrophages. Further, the increased Slc11a1 functionality also resulted in increased production of NOx, TNF-α and IL-12 by activated macrophages. The findings of this study signify the importance of interplay between Slc11a1 expression and macrophages activation that can be effectively used to control of Leishmania growth and survival.

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.

Interaction between cysteine synthase and serine O-acetyltransferase proteins and their stage specific expression in Leishmania donovani

Leishmania possess a unique trypanothione redox metabolism with undebated roles in protection from oxidative damage and drug resistance. The biosynthesis of trypanothione depends on l-cysteine bioavailability which is regulated by cysteine biosynthesis pathway. The de novo cysteine biosynthesis pathway is comprised of serine O-acetyltransferase (SAT) and cysteine synthase (CS) enzymes which sequentially mediate two consecutive steps of cysteine biosynthesis, and is absent in mammalian host. However, despite the apparent dependency of redox metabolism on cysteine biosynthesis pathway, the role of SAT and CS in redox homeostasis has been unexplored in Leishmania parasites. Herein, we have characterized CS and SAT to investigate their interaction and relative abundance of these proteins in promastigote vs. amastigote growth stages of L. donovani. CS and SAT genes of L. donovani (LdCS and LdSAT) were cloned, expressed, and fusion proteins purified to homogeneity with affinity column chromatography. Purified LdCS contains PLP as cofactor and showed optimum enzymatic activity at pH 7.5. Enzyme kinetics showed that LdCS catalyses the synthesis of cysteine using O-acetylserine and sulfide with a K_m of 15.86 mM and 0.17 mM, respectively. Digitonin fractionation and indirect immunofluorescence microscopy showed that LdCS and LdSAT are localized in the cytoplasm of promastigotes. Size exclusion chromatography, co-purification, pull down and immuno-precipitation assays demonstrated a stable complex formation between LdCS and LdSAT proteins. Furthermore, LdCS and LdSAT proteins expression/activity was upregulated in amastigote growth stage of the parasite. Thus, the stage specific differential expression of LdCS and LdSAT suggests that it may have a role in the redox homeostasis of Leishmania.

Cross-resistance of Leishmania infantum isolates to nitric oxide from patients refractory to antimony treatment, and greater tolerance to antileishmanial responses by macrophages

Visceral leishmaniasis is a life-threatening disease characterized by intense parasitism of the spleen, liver, and bone marrow. Antimonials have served as front-line antileishmanial therapeutics for decades, but the increasing failure rates under antimonial treatment have challenged the continued use of these drugs. Pentavalent antimonials are known to reinforce the killing mechanisms of macrophages, although the associated mechanism remains unclear. Here, for the first time, we determined whether Leishmania infantum strains isolated from patients refractory to antimony treatment (relapse cases) were cross-resistant to antimonials, liposomal amphotericin B, and/or nitric oxide, and also whether these strains modulate macrophage infection. We selected four clinical isolates from relapse cases and two clinical isolates from antimony-responsive patients (control group) for the present study. The L. infantum promastigotes from all four relapse cases were resistant to trivalent antimonial treatment and nitric oxide, while only one isolate was resistant to liposomal amphotericin B. We evaluated whether the resistant strains from relapse cases showed enhanced infectivity and amastigote survival in macrophages, or macrophage-killing mechanisms in macrophages activated by lipopolysaccharide plus interferon gamma. Infection indexes calculated using macrophages infected with isolates from relapse were higher than those observed with control strains that were stimulated independently. Macrophage infection was higher with L. infantum isolates from relapse cases and correlated with enhanced interleukin 1-β production but showed similar nitrite production. Our results demonstrate that L. infantum field isolates from relapse cases were resistant to antimonials and nitric oxide and that these parasites stimulated inflammatory cytokines and were resistant to macrophage-killing mechanisms, factors that may contribute to disease severity.

The Role of Signaling via Aqueous Pore Formation in Resistance Responses to Amphotericin B

Drug resistance studies have played an important role in the validation of antibiotic targets. In the case of the polyene antibiotic amphotericin B (AmB), such studies have demonstrated the essential role that depletion of ergosterol plays in the development of AmB-resistant (AmB-R) organisms. However, AmB-R strains also occur in fungi and parasitic protozoa that maintain a normal level of ergosterol at the plasma membrane. Here, I review evidence that shows not only that there is increased protection against the deleterious consequences of AmB-induced ion leakage across the membrane in these resistant pathogens but also that a set of events are activated that block the cell signaling responses that trigger the oxidative damage produced by the antibiotic. Such signaling events appear to be the consequence of a membrane-thinning effect that is exerted upon lipid-anchored Ras proteins by the aqueous pores formed by AmB. A similar membrane disturbance effect may also explain the activity of AmB on mammalian cells containing Toll-like receptors. These resistance mechanisms expand our current understanding of the role that the formation of AmB aqueous pores plays in triggering signal transduction responses in both pathogens and host immune cells.

In vitro anti-Leishmania activity of tetracyclic iridoids from Morinda lucida, benth

Leishmaniasis is an infectious disease transmitted by the sand fly. It is caused by over 20 different species of Leishmania and has affected over 14 million people worldwide. One of the main forms of control of leishmaniasis is chemotherapy, but this is limited by the high cost and/or toxicity of available drugs. We previously found three novel compounds with an iridoid tetracyclic skeleton to have activity against trypanosome parasites. In this study, we determined the activity of the three anti-trypanosome compounds against Leishmania using field strain, 010, and the lab strain Leishmania hertigi. The minimum inhibitory concentration (MIC) of the compounds against 010 was determined by microscopy while the IC50 of compounds against L. hertigi was determined by fluorescence-activated cell sorting with Guava viacount analysis. We found two of the three compounds, molucidin and ML-F52, to have anti-Leishmania activity against both strains. The fluor-microscope observation with DAPI stain revealed that both Molucidin and ML-F52 induced abnormal parasites with two sets of nucleus and kinetoplast in a cell, suggesting that compounds might inhibit cytokinesis in Leishmania parasites. Molucidin and ML-F52 might be good lead compounds for the development of new anti-Leishmania chemotherapy.

Silver and Nitrate Oppositely Modulate Antimony Susceptibility through Aquaglyceroporin 1 in Leishmania (Viannia) Species

Antimony (Sb) resistance in leishmaniasis chemotherapy has become one of the major challenges to the control of this spreading worldwide public health problem. Since the plasma membrane pore-forming protein aquaglyceroporin 1 (AQP1) is the major route of Sb uptake in Leishmania, functional studies are relevant to characterize drug transport pathways in the parasite. We generated AQP1-overexpressing Leishmania guyanensis and L. braziliensis mutants and investigated their susceptibility to the trivalent form of Sb (Sb(III)) in the presence of silver and nitrate salts. Both AQP1-overexpressing lines presented 3- to 4-fold increased AQP1 expression levels compared with those of their untransfected counterparts, leading to an increased Sb(III) susceptibility of about 2-fold. Competition assays using silver nitrate, silver sulfadiazine, or silver acetate prior to Sb(III) exposure increased parasite growth, especially in AQP1-overexpressing mutants. Surprisingly, Sb(III)-sodium nitrate or Sb(III)-potassium nitrate combinations showed significantly enhanced antileishmanial activities compared to those of Sb(III) alone, especially against AQP1-overexpressing mutants, suggesting a putative nitrate-dependent modulation of AQP1 activity. The intracellular level of antimony quantified by graphite furnace atomic absorption spectrometry showed that the concomitant exposure to Sb(III) and nitrate favors antimony accumulation in the parasite, increasing the toxicity of the drug and culminating with parasite death. This is the first report showing evidence of AQP1-mediated Sb(III) susceptibility modulation by silver in Leishmania and suggests the potential antileishmanial activity of the combination of nitrate salts and Sb(III).

Molecular detection of infection homogeneity and impact of miltefosine treatment in a Syrian golden hamster model of Leishmania donovani and L. infantum visceral leishmaniasis

Control of visceral leishmaniasis caused by Leishmania infantum and Leishmania donovani primarily relies on chemotherapy using an increasingly compromised repertoire of antileishmanial compounds. For evaluation of novel drugs, the Syrian golden hamster is considered as a clinically relevant laboratory model. In this study, two molecular parasite detection assays were developed targeting cathepsin-like cysteine protease B (CPB) DNA and 18S rRNA to achieve absolute amastigote quantification in the major target organs liver and spleen. Both quantitative PCR (qPCR) techniques showed excellent agreement with a strong correlation with the conventional microscopic reading of Giemsa-stained tissue smears. Using multiple single tissue pieces and all three detection methods, we confirmed homogeneity of infection in liver and spleen and the robustness of extrapolating whole organ burdens from a small single tissue piece. Comparison of pre- and post-treatment burdens in infected hamsters using the three detection methods consistently revealed a stronger parasite reduction in the spleen compared to the liver, indicating an organ-dependent clearance efficacy for miltefosine. In conclusion, this study in the hamster demonstrated high homogeneity of infection in liver and spleen and advocates the use of molecular detection methods for assessment of low (post-treatment) tissue burdens.

Genomic Appraisal of the Multifactorial Basis for In Vitro Acquisition of Miltefosine Resistance in Leishmania donovani

Protozoan parasites of the Leishmania donovani complex are the causative agents of visceral leishmaniasis (VL), the most severe form of leishmaniasis, with high rates of mortality if left untreated. Leishmania parasites are transmitted to humans through the bite of infected female sandflies (Diptera: Phlebotominae), and approximately 500,000 new cases of VL are reported each year. In the absence of a safe human vaccine, chemotherapy, along with vector control, is the sole tool with which to fight the disease. Miltefosine (hexadecylphosphatidylcholine [HePC]), an antitumoral drug, is the only successful oral treatment for VL. In the current study, we describe the phenotypic traits of L. donovani clonal lines that have acquired resistance to HePC. We performed whole-genome and RNA sequencing of these resistant lines to provide an inclusive overview of the multifactorial acquisition of experimental HePC resistance, circumventing the challenge of identifying changes in membrane-bound proteins faced by proteomics. This analysis was complemented by assessment of the in vitro infectivity of HePC-resistant parasites. Our work underscores the importance of complementary "omics" to acquire the most comprehensive insight for multifaceted processes, such as HePC resistance.

Activity of a novel sulfonamide compound 2-nitro-N-(pyridin-2-ylmethyl)benzenesulfonamide against Leishmania donovani

New treatments for visceral leishmaniasis, caused by Leishmania donovani, are needed to overcome sustained toxicity, cost, and drug resistance. The aim of this study was to evaluate the therapeutic effects of 2-nitro-N-(pyridin-2-ylmethyl)benzenesulfonamide (2NB) against promastigote and amastigote forms of L. donovani and examine its effect in combination with amphotericin B (AmB) against AmB-resistant clinical isolates. Effects were assessed against extracellular promastigotes in vitro and intracellular amastigotes in L. donovani-infected macrophages. Levels of inducible nitric oxide and Th1 and Th2 cytokines were measured in infected 2NB-treated macrophages, and levels of reactive oxygen species and NO were measured in 2NB-treated macrophages. 2NB was active against promastigotes and intracellular amastigotes with 50% inhibitory concentration values of 38.5±1.5 µg/mL and 86.4±2.4 µg/mL, respectively. 2NB was not toxic to macrophages. Parasite titer was reduced by >85% in infected versus uninfected macrophages at a 2NB concentration of 120 µg/mL. The parasiticidal activity was associated with increased levels of Th1 cytokines, NO, and reactive oxygen species. Finally, 2NB increased the efficacy of AmB against AmB-resistant L. donovani. These results demonstrate 2NB to be an antileishmanial agent, opening up a new avenue for the development of alternative chemotherapies against visceral leishmaniasis.

Cos-Seq for high-throughput identification of drug target and resistance mechanisms in the protozoan parasite Leishmania

Innovative strategies are needed to accelerate the identification of antimicrobial drug targets and resistance mechanisms. Here we develop a sensitive method, which we term Cosmid Sequencing (or "Cos-Seq"), based on functional cloning coupled to next-generation sequencing. Cos-Seq identified >60 loci in the Leishmania genome that were enriched via drug selection with methotrexate and five major antileishmanials (antimony, miltefosine, paromomycin, amphotericin B, and pentamidine). Functional validation highlighted both known and previously unidentified drug targets and resistance genes, including novel roles for phosphatases in resistance to methotrexate and antimony, for ergosterol and phospholipid metabolism genes in resistance to miltefosine, and for hypothetical proteins in resistance to paromomycin, amphothericin B, and pentamidine. Several genes/loci were also found to confer resistance to two or more antileishmanials. This screening method will expedite the discovery of drug targets and resistance mechanisms and is easily adaptable to other microorganisms.

Current challenges in treatment options for visceral leishmaniasis in India: a public health perspective

Visceral leishmaniasis (VL) is a serious parasitic disease causing considerable mortality and major disability in the Indian subcontinent. It is most neglected tropical disease, particularly in terms of new drug development for the lack of financial returns. An elimination campaign has been running in India since 2005 that aim to reduce the incidence of VL to below 1 per 10,000 people at sub-district level. One of the major components in this endeavor is reducing transmission through early case detection followed by complete treatment. Substantial progress has been made during the recent years in the area of VL treatment, and the VL elimination initiatives have already saved many lives by deploying them effectively in the endemic areas. However, many challenges remain to be overcome including availability of drugs, cost of treatment (drugs and hospitalization), efficacy, adverse effects, and growing parasite resistance. Therefore, better emphasis on implementation research is urgently needed to determine how best to deliver existing interventions with available anti-leishmanial drugs. It is essential that the new treatment options become truly accessible, not simply available in endemic areas so that they may promote healing and save lives. In this review, we highlight the recent advancement and challenges in current treatment options for VL in disease endemic area, and discuss the possible strategies to improve the therapeutic outcome.

In vitro antileishmanial activity of trans-stilbene and terphenyl compounds

Leishmaniasis are globally widespread parasitic diseases which often leads to death if left untreated. Currently available drugs present different drawbacks, so there is an urgent need to develop new, safe and cost-effective drugs against leishmaniasis. In this study we tested a small library of trans-stilbene and terphenyl derivatives against promastigote, amastigotes and intramacrophage amastigote forms of Leishmania infantum. Two compounds of the series, the trans-stilbene 3 and the terphenyl 11, presented the best activity and safety profiles. Terphenyl 11 showed a leshmanicidal activity higher than pentostam and the ability to induce apoptosis selectively in Leishmania infantum while saving macrophages and primary epithelial cells. Our data indicate that terphenyl compounds, as well as stilbenes, are endowed with leishmanicidal activity, showing potential for further studies in the context of leishmanial therapy.

Universal minicircle sequence binding protein of Leishmania donovani regulates pathogenicity by controlling expression of cytochrome-b

BACKGROUND

Leishmania contains a concatenated mitochondrial DNA, kDNA. Universal minicircle sequence binding protein (UMSBP), a mitochondrial protein, initiates kDNA replication by binding with a conserved universal minicircle sequence (UMS) of kDNA. Here, we describe first time in L. donovani the regulation of DNA binding activity of UMSBP and the role of UMSBP in virulence.

METHODS

Insilco and EMSA study were performed to show UMS-binding activity of UMSBP. Tryparedoxin(TXN)-tryparedoxin peroxidase(TXNPx) assay as well as co-overexpression of cytochrome-b5 reductase-like protein (CBRL) and tryparedoxin in L. donovani were done to know the regulation of DNA binding activity of UMSBP. Knockout and episomal-expression constructs of UMSBP were transfected in L. donovani. The cell viability assay and immunofluorescence study to know the status of kDNA were performed. Macrophages were infected with transfected parasites. mRNA level of cytochrome b, activity of complex-III, intracellular ATP level of both transfected promastigotes and amastigotes as well as ROS concentration and the level of apoptosis of transfected promastigotes were measured. Level of oxidative phosphorylation of both transfected and un-transfected amastigotes were compared. Burden of transfected amastigotes in both macrophages and BALB/c mice were measured.

RESULTS

L. donovani UMSBP is capable of binding with UMS, regulated by redox through mitochondrial enzymes, TXN, TXNPx and CBRL. Depletion of UMSBP (LdU(-/-)) caused kDNA loss, which decreased cytochrome-b expression [component of complex-III of electron transport chain (ETC)] and leads to the disruption of complex-III activity, decreased ATP generation, increased ROS level and promastigotes exhibited apoptosis like death. Interestingly, single knockout of UMSBP (LdU(-/+)) has no effect on promastigotes survival. However, single knockout in intracellular amastigotes demonstrate loss of mRNA level of cytochrome-b, disruption in the activity of complex-III and reduced production of ATP in amastigotes than wild type. This process interfere with the oxidative-phosphorylation and thereby completely inhibit the intracellular proliferation of LdU(-/+) amastigotes in human macrophages and in BALB/c mice. Amastigotes proliferation was restored as wild type after episomal expression of LdUMSBP in LdU(-/+) parasites (LdU(-/+)AB).

CONCLUSION

The LdUMSBP regulates leishmanial mitochondrial respiration and pathogenesis. So, LdUMSBP may be an attractive target for rational drug designing and LdU(-/+) parasites could be considered as a live attenuated vaccine candidate against visceral leishmaniasis.

Allopurinol Resistance in Leishmania infantum from Dogs with Disease Relapse

BACKGROUND

Visceral leishmaniasis caused by the protozoan Leishmania infantum is a zoonotic, life threatening parasitic disease. Domestic dogs are the main peridomestic reservoir, and allopurinol is the most frequently used drug for the control of infection, alone or in combination with other drugs. Resistance of Leishmania strains from dogs to allopurinol has not been described before in clinical studies.

METHODOLOGY/PRINCIPAL FINDINGS

Following our observation of clinical disease relapse in dogs under allopurinol treatment, we tested susceptibility to allopurinol of L. infantum isolated from groups of dogs pre-treatment, treated in remission, and with disease relapse during treatment. Promastigote isolates obtained from four treated relapsed dogs (TR group) showed an average half maximal inhibitory concentration (IC50) of 996 μg/mL. A significantly lower IC50 (P = 0.01) was found for isolates from ten dogs before treatment (NT group, 200 μg/mL), as well as for five isolates obtained from treated dogs in remission (TA group, 268 μg/mL). Axenic amastigotes produced from isolates of the TR group also showed significantly higher (P = 0.002) IC50 compared to the NT group (1678 and 671 μg/mL, respectively). The lower sensitivity of intracellular amastigotes from the TR group relative to those from the NT group (P = 0.002) was confirmed using an infected macrophage model (6.3% and 20% growth inhibition, respectively at 300 μg/mL allopurinol).

CONCLUSIONS

This is the first study to demonstrate allopurinol resistance in L. infantum and to associate it with disease relapse in the canine host. These findings are of concern as allopurinol is the main drug used for long term control of the disease in dogs, and resistant L. infantum strains may enhance uncontrolled transmission to humans and to other dogs.

Host-mediated Leishmania donovani treatment using AR-12 encapsulated in acetalated dextran microparticles

Leishmaniasis is a disease caused by parasites of Leishmania sp., which effects nearly 12 million people worldwide and is associated with treatment complications due to widespread parasite resistance toward pathogen-directed therapeutics. The current treatments for visceral leishmaniasis (VL), the systemic form of the disease, involve pathogen-mediated drugs and have long treatment regimens, increasing the risk of forming resistant strains. One way to limit emergence of resistant pathogens is through the use of host-mediated therapeutics. The host-mediated therapeutic AR-12, which is FDA IND-approved for cancer treatment, has shown activity against a broad spectrum of intracellular pathogens; however, due to hydrophobicity and toxicity, it is difficult to reach therapeutic doses. We have formulated AR-12 into microparticles (AR-12/MPs) using the novel biodegradable polymer acetalated dextran (Ace-DEX) and used this formulation for the systemic treatment of VL. Treatment with AR-12/MPs significantly reduced liver, spleen, and bone marrow parasite loads in infected mice, while combinatorial therapies with amphotericin B had an even more significant effect. Overall, AR-12/MPs offer a unique, host-mediated therapy that could significantly reduce the emergence of drug resistance in the treatment of VL.

Deprivation of L-Arginine Induces Oxidative Stress Mediated Apoptosis in Leishmania donovani Promastigotes: Contribution of the Polyamine Pathway

The growth and survival of intracellular parasites depends on the availability of extracellular nutrients. Deprivation of nutrients viz glucose or amino acid alters redox balance in mammalian cells as well as some lower organisms. To further understand the relationship, the mechanistic role of L-arginine in regulation of redox mediated survival of Leishmania donovani promastigotes was investigated. L-arginine deprivation from the culture medium was found to inhibit cell growth, reduce proliferation and increase L-arginine uptake. Relative expression of enzymes, involved in L-arginine metabolism, which leads to polyamine and trypanothione biosynthesis, were downregulated causing decreased production of polyamines in L-arginine deprived parasites and cell death. The resultant increase in reactive oxygen species (ROS), due to L-arginine deprivation, correlated with increased NADP+/NADPH ratio, decreased superoxide dismutase (SOD) level, increased lipid peroxidation and reduced thiol content. A deficiency of L-arginine triggered phosphatidyl serine externalization, a change in mitochondrial membrane potential, release of intracellular calcium and cytochrome-c. This finally led to DNA damage in Leishmania promastigotes. In summary, the growth and survival of Leishmania depends on the availability of extracellular L-arginine. In its absence the parasite undergoes ROS mediated, caspase-independent apoptosis-like cell death. Therefore, L-arginine metabolism pathway could be a probable target for controlling the growth of Leishmania parasites and disease pathogenesis.

A Novel Sterol Isolated from a Plant Used by Mayan Traditional Healers Is Effective in Treatment of Visceral Leishmaniasis Caused by Leishmania donovani

Visceral leishmaniasis (VL), caused by the protozoan parasite Leishmania donovani, is a global health problem affecting millions of people worldwide. Treatment of VL largely depends on therapeutic drugs such as pentavalent antimonials, amphotericin B, and others, which have major drawbacks due to drug resistance, toxicity, and high cost. In this study, for the first time, we have successfully demonstrated the synthesis and antileishmanial activity of the novel sterol pentalinonsterol (PEN), which occurs naturally in the root of a Mexican medicinal plant, Pentalinon andrieuxii. In the experimental BALB/c mouse model of VL induced by infection with L. donovani, intravenous treatment with liposome-encapsulated PEN (2.5 mg/kg) led to a significant reduction in parasite burden in the liver and spleen. Furthermore, infected mice treated with liposomal PEN showed a strong host-protective TH1 immune response characterized by IFN-γ production and formation of matured hepatic granulomas. These results indicate that PEN could be developed as a novel drug against VL.

Novel Agents against Miltefosine-Unresponsive Leishmania donovani

Visceral leishmaniasis is a deadly endemic disease. Unresponsiveness to the only available oral drug miltefosine poses a big challenge for the chemotherapy of the disease. We report a novel molecule, PS-203 {4-(4,4,8-trimethyl-7-oxo-3-oxabicyclo[3.3.1]non-2-yl)-benzoic acid methyl ester}, as effective against a miltefosine-unresponsive strain of the parasite. Further, combinations of PS-203 with miltefosine were also evaluated and showed promising results against a miltefosine-unresponsive strain.

Combination Treatment for Visceral Leishmaniasis Patients Coinfected with Human Immunodeficiency Virus in India

We describe the safety and efficacy of treating visceral leishmaniasis (VL) in patients with human immunodeficiency virus-VL coinfection with concurrent intravenous liposomal amphotericin B (AmBisome) and oral miltefosine (Impavido) in India. The regimen was safe, well tolerated, with lower relapse rates than monotherapy.

Background. There are considerable numbers of patients coinfected with human immunodeficiency virus (HIV) and visceral leishmaniasis (VL) in the VL-endemic areas of Bihar, India. These patients are at higher risk of relapse and death, but there are still no evidence-based guidelines on how to treat them. In this study, we report on treatment outcomes of coinfected patients up to 18 months following treatment with a combination regimen.

Methods. This retrospective analysis included all patients with confirmed HIV-VL coinfection receiving combination treatment for VL at a Médecins Sans Frontières treatment center between July 2012 and September 2014. Patients were treated with 30 mg/kg body weight intravenous liposomal amphotericin B (AmBisome) divided as 6 equal dose infusions combined with 14 days of 100 mg/day oral miltefosine (Impavido). All patients were encouraged to start or continue on antiretroviral therapy (ART).

Results. 102 patients (76% males, 57% with known HIV infection, 54% with a prior episode of VL) were followed-up for a median of 11 months (interquartile range: 4–18). Cumulative incidence of all-cause mortality and VL relapse at 6, 12, and 18 months was 11.7%, 14.5%, 16.6% and 2.5%, 6.0%,13.9%, respectively. Cumulative incidence of poor outcome at 6, 12, and 18 months was 13.9%, 18.4%, and 27.2%, respectively. Not initiating ART and concurrent tuberculosis were independent risk factors for mortality, whereas no factors were associated with relapse.

Conclusions. In this Bihar-based study, combination therapy appeared to be well tolerated, safe, and effective and may be considered as an option for treatment of VL in HIV coinfected patients.

Up-regulation of silent information regulator 2 (Sir2) is associated with amphotericin B resistance in clinical isolates of Leishmania donovani

OBJECTIVE

Silent information regulator 2 (Sir2) is involved in parasite survival and apoptosis. Here, we aimed to explore the involvement of Sir2 in amphotericin B (AmB) resistance mechanism in Leishmania donovani.

METHODS

The expression levels of Sir2, MDR1 and NAD(+) biosynthetic pathway enzymes in AmB-resistant and -susceptible parasites were measured and total intracellular NAD(+)/NADH ratios were compared. Overexpression and knockout constructs of Sir2 were transfected in AmB-resistant and -susceptible parasites. Both resistant and susceptible parasites were inhibited with sirtinol for 4 h. The deacetylase activity of Sir2, the expression level of MDR1, the rate of AmB efflux, concentrations of reactive oxygen species (ROS) and levels of apoptosis were examined in WT, inhibited and transfected parasites, and the AmB susceptibility of the respective parasites was measured by determining the LD50 of AmB.

RESULTS

Levels of mRNA, protein and NAD(+)-dependent deacetylase activity of Sir2 were elevated in resistant versus susceptible parasites. Inhibition and/or deletion of Sir2 allele showed a decreased mRNA level of MDR1, lower drug efflux, increased ROS concentration, apoptosis-like phenomenon and decreased LD50 of AmB in resistant parasites. In contrast, Sir2 overexpression in susceptible parasites reversed drug susceptibility producing a resistant phenotype. This was associated with increased LD50 of AmB along with increased expression levels of MDR1, drug efflux and reduced concentrations of ROS, corresponding to decreased apoptosis of resistant to WT sensitive.

CONCLUSIONS

Sir2 plays a critical role in AmB resistance by regulating MDR1, ROS concentration and apoptosis-like phenomena and may be a new resistance marker for visceral leishmaniasis.

In-vitro sensitivity of Pakistani Leishmania tropica field isolate against buparvaquone in comparison to standard anti-leishmanial drugs

In this study, in vitro anti-leishmanial activity of buparvaquone was evaluated against promastigotes and intracellular amastigotes of Pakistani Leishmania tropica isolate KWH23 in relation to the current standard chemotherapy for leishmaniasis (sodium stibogluconate, sodium stibogluconate, amphotericin B and miltefosine). For buparvaquone, mean % inhibition in intracellular amastigotes at four different concentrations (1.35 µM, 0.51 µM, 0.17 µM and 0.057 µM) was 78%, 44%, 20% and 14% respectively, whereas, against promastigotes it was 89%, 77%, 45% and 35% respectively. IC50 values calculated to estimate the anti-leishmanial activity of buparvaquone against intra-cellular amastigotes and promastigotes was 0.53 µM (95% C.I. = 0.32-0.89) and 0.15 µM (95% C.I. = 0.01-1.84) respectively. Amphotericin B was the most potent in-vitro drug tested, with an IC50 of 0.075 µM (95% C.I. = 0.006-0.907) against promastigotes, and 0.065 µM (95% C.I. = 0.048-0.089) against intra-cellular amastigotes. Amphotericin B was more cytotoxic against THP1 cells, with an IC50 of 0.15 µM (95% C.I. = 0.01-0.95) and an apparent in-vitro therapeutic index of 2.0, than was buparvaquone, with an IC50 of 12.03 µM (95% C.I. = 5.36-26.96) against THP1 cells and a therapeutic index of 80.2. The study proposes that buparvaquone may be further investigated as a candidate drug for treatment of cutaneous leishmaniasis.

Metabolic reconfiguration of the central glucose metabolism: a crucial strategy of Leishmania donovani for its survival during oxidative stress

Understanding the mechanism that allows the intracellular protozoan parasite Leishmania donovani (Ld) to respond to reactive oxygen species (ROS) is of increasing therapeutic importance because of the continuing resistance toward antileishmanial drugs and for determining the illusive survival strategy of these parasites. A shift in primary carbon metabolism is the fastest response to oxidative stress. A (14)CO2 evolution study, expression of glucose transporters together with consumption assays, indicated a shift in metabolic flux of the parasites from glycolysis toward pentose phosphate pathway (PPP) when exposed to different oxidants in vitro/ex vivo. Changes in gene expression, protein levels, and enzyme activities all pointed to a metabolic reconfiguration of the central glucose metabolism in response to oxidants. Generation of glucose-6-phosphate dehydrogenase (G6PDH) (∼5-fold) and transaldolase (TAL) (∼4.2-fold) overexpressing Ld cells reaffirmed that lethal doses of ROS were counterbalanced by effective manipulation of NADPH:NADP(+) ratio and stringent maintenance of reduced thiol content. The extent of protein carbonylation and accumulation of lipid peroxidized products were also found to be less in overexpressed cell lines. Interestingly, the LD50 of sodium antimony gluconate (SAG), amphotericin-B (AmB), and miltefosine were significantly high toward overexpressing parasites. Consequently, this study illustrates that Ld strategizes a metabolic reconfiguration for replenishment of NADPH pool to encounter oxidative challenges.

Experimental resistance to drug combinations in Leishmania donovani: metabolic and phenotypic adaptations

Together with vector control, chemotherapy is an essential tool for the control of visceral leishmaniasis (VL), but its efficacy is jeopardized by growing resistance and treatment failure against first-line drugs. To delay the emergence of resistance, the use of drug combinations of existing antileishmanial agents has been tested systematically in clinical trials for the treatment of visceral leishmaniasis (VL). In vitro, Leishmania donovani promastigotes are able to develop experimental resistance to several combinations of different antileishmanial drugs after 10 weeks of drug pressure. Using an untargeted liquid chromatography-mass spectrometry (LC-MS) metabolomics approach, we identified metabolic changes in lines that were experimentally resistant to drug combinations and their respective single-resistant lines. This highlighted both collective metabolic changes (found in all combination therapy-resistant [CTR] lines) and specific ones (found in certain CTR lines). We demonstrated that single-resistant and CTR parasite cell lines show distinct metabolic adaptations, which all converge on the same defensive mechanisms that were experimentally validated: protection against drug-induced and external oxidative stress and changes in membrane fluidity. The membrane fluidity changes were accompanied by changes in drug uptake only in the lines that were resistant against drug combinations with antimonials, and surprisingly, drug accumulation was higher in these lines. Together, these results highlight the importance and the central role of protection against oxidative stress in the different resistant lines. Ultimately, these phenotypic changes might interfere with the mode of action of all drugs that are currently used for the treatment of VL and should be taken into account in drug development.

Reactive oxygen species regulates expression of iron-sulfur cluster assembly protein IscS of Leishmania donovani

The cysteine desulfurase, IscS, is a highly conserved and essential component of the mitochondrial iron-sulfur cluster (ISC) system that serves as a sulfur donor for Fe-S clusters biogenesis. Fe-S clusters are versatile and labile cofactors of proteins that orchestrate a wide array of essential metabolic processes, such as energy generation and ribosome biogenesis. However, no information regarding the role of IscS or its regulation is available in Leishmania, an evolving pathogen model with rapidly developing drug resistance. In this study, we characterized LdIscS to investigate the ISC system in AmpB-sensitive vs resistant isolates of L. donovani and to understand its regulation. We observed an upregulated Fe-S protein activity in AmpB-resistant isolates but, in contrast to our expectations, LdIscS expression was upregulated in the sensitive strain. However, further investigations showed that LdIscS expression is positively correlated with ROS level and negatively correlated with Fe-S protein activity, independent of strain sensitivity. Thus, our results suggested that LdIscS expression is regulated by ROS level with Fe-S clusters/proteins acting as ROS sensors. Moreover, the direct evidence of a mechanism, in support of our results, is provided by dose-dependent induction of LdIscS-GFP as well as endogenous LdIscS in L. donovani promastigotes by three different ROS inducers: H2O2, menadione, and Amphotericin B. We postulate that LdIscS is upregulated for de novo synthesis or repair of ROS damaged Fe-S clusters. Our results reveal a novel mechanism for regulation of IscS expression that may help parasite survival under oxidative stress conditions encountered during infection of macrophages and suggest a cross talk between two seemingly unrelated metabolic pathways, the ISC system and redox metabolism in L. donovani.

Ascorbate peroxidase, a key molecule regulating amphotericin B resistance in clinical isolates of Leishmania donovani

Amphotericin B (AmB), a polyene macrolide, is now a first-line treatment of visceral leishmaniasis cases refractory to antimonials in India. AmB relapse cases and the emergence of secondary resistance have now been reported. To understand the mechanism of AmB, differentially expressed genes in AmB resistance strains were identified by a DNA microarray and real-time reverse transcriptase PCR (RT-PCR) approach. Of the many genes functionally overexpressed in the presence of AmB, the ascorbate peroxidase gene from a resistant Leishmania donovani strain (LdAPx gene) was selected because the gene is present only in Leishmania, not in humans. Apoptosis-like cell death after exposure to AmB was investigated in a wild-type (WT) strain in which the LdAPx gene was overexpressed and in AmB-sensitive and -resistant strains. A higher percentage of apoptosis-like cell death after AmB treatment was noticed in the sensitive strain than in both the resistant isolate and the strain sensitive to LdAPx overexpression. This event is preceded by AmB-induced formation of reactive oxygen species and elevation of the cytosolic calcium level. Enhanced cytosolic calcium was found to be responsible for depolarization of the mitochondrial membrane potential and the release of cytochrome c (Cyt c) into the cytosol. The redox behavior of Cyt c showed that it has a role in the regulation of apoptosis-like cell death by activating metacaspase- and caspase-like proteins and causing concomitant nuclear alterations, as determined by terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling (TUNEL) and DNA fragmentation in the resistant strain. The present study suggests that constitutive overexpression of LdAPx in the L. donovani AmB-resistant strain prevents cells from the deleterious effect of oxidative stress, i.e., mitochondrial dysfunction and cellular death induced by AmB.

Visceral leishmaniasis and HIV co-infection in Bihar, India: long-term effectiveness and treatment outcomes with liposomal amphotericin B (AmBisome)

Background

Visceral Leishmaniasis (VL; also known as kala-azar) is an ultimately fatal disease endemic in the Indian state of Bihar, while HIV/AIDS is an emerging disease in this region. A 2011 observational cohort study conducted in Bihar involving 55 VL/HIV co-infected patients treated with 20–25 mg/kg intravenous liposomal amphotericin B (AmBisome) estimated an 85.5% probability of survival and a 26.5% probability of VL relapse within 2 years. Here we report the long-term field outcomes of a larger cohort of co-infected patients treated with this regimen between 2007 and 2012.

Methods and Principal Findings

Intravenous AmBisome (20–25 mg/kg) was administered to 159 VL/HIV co-infected patients (both primary infections and relapses) in four or five doses of 5 mg/kg over 4–10 days. Initial cure of VL at discharge was defined as improved symptoms, cessation of fever, improvement of appetite and recession of spleen enlargement. Test of cure was not routinely performed. Antiretroviral treatment (ART) was initiated in 23 (14.5%), 39 (24.5%) and 61 (38.4%) before, during and after admission respectively. Initial cure was achieved in all discharged patients. A total of 36 patients died during follow-up, including six who died shortly after admission. Death occurred at a median of 11 weeks (IQR 4–51) after starting VL treatment. Estimated mortality risk was 14.3% at six months, 22.4% at two years and 29.7% at four years after treatment. Among the 153 patients discharged from the hospital, 26 cases of VL relapse were diagnosed during follow-up, occurring at a median of 10 months (IQR 7–14) after discharge. After accounting for competing risks, the estimated risk of relapse was 16.1% at one year, 20.4% at two years and 25.9% at four years. Low hemoglobin level and concurrent infection with tuberculosis were independent risk factors for mortality, while ART initiated shortly after admission for VL treatment was associated with a 64–66% reduced risk of mortality and 75% reduced risk of relapse.

Significance

This is the largest cohort of HIV-VL co-infected patients reported from the Indian subcontinent. Even after initial cure following treatment with AmBisome, these patients appear to have much higher rates of VL relapse and mortality than patients not known to be HIV-positive, although relapse rates appear to stabilize after 2 years. These results extend the earlier findings that co-infected patients are at increased risk of death and require a multidisciplinary approach for long-term management.

Fifty percent of all visceral leishmaniasis (VL) cases globally occur in India, where up to 90% of cases occur in the state of Bihar. There are also an estimated 300,000 people in Bihar living with HIV/AIDS. Patients with HIV who are treated for VL typically have much worse outcomes than VL patients who are HIV-negative, yet there exists very little evidence suggesting more effective treatments for this group. Between 2007–2012, with support of the Rajendra Memorial Research Institute (RMRI), Médecins Sans Frontières (MSF) treated 8,749 VL patients in Bihar using liposomal amphotericin-B (AmBisome). Here we describe the characteristics and long-term outcomes of a subgroup of 159 HIV-VL co-infected patients treated within this program over the 5-year period. Their estimated mortality risk was 14.3% at six months after treatment, 22.4% at two years and 29.7% at four years. Estimated risk of relapse was 16.1% at one year, 20.4% at two years and 25.9% at four years. We conclude that treatment of HIV-VL co-infected patients with 20–25 mg/kg of liposomal amphotericin-B is well tolerated and relatively effective. However, HIV-VL co-infection is a complex chronic disease with high early mortality and much worse outcomes than VL alone, and requires a multidisciplinary long-term management strategy.

Efficacy of low doses of amphotericin B plus allicin against experimental visceral leishmaniasis

OBJECTIVES

To evaluate the efficacy of the combination of allicin and amphotericin deoxycholate (AmB) in the chemotherapy of Leishmania infantum infection with the final aim of reducing the dose of AmB in the chemotherapy of visceral leishmaniasis.

METHODS

Hamsters were intraperitoneally (ip) infected with L. infantum (10(7) stationary phase promastigotes). On day 45 post-infection animals were treated ip with AmB (1 or 5 mg/kg/day), allicin (5 mg/kg/day) or a combination of AmB (1 mg/kg/day) + allicin (5 mg/kg/day) for 5 days. Animals were clinically and biopathologically monitored and the antibody response (IgG, IgG1, IgG2) was determined. Parasite burdens were estimated by limiting dilution and AmB biodistribution was determined by HPLC in plasma, kidney, spleen and liver.

RESULTS

No clinical signs or liver and kidney alterations were observed. AmB (1 mg/kg/day) did not clear the Leishmania infection and no parasites were detected in two animals treated with 5 mg/kg/day allicin. Combination therapy (5 mg/kg allicin + 1 mg/kg AmB) reduced the L. infantum burden by >95%. Antileishmanial activity of the combination was comparable (P < 0.05) to the standard AmB treatment (5 mg/kg).

CONCLUSIONS

Allicin alone (5 mg/kg/day for 5 days) significantly reduced the Leishmania burden in spleen and liver of infected hamsters. Co-administration of allicin (5 mg/kg/day for 5 days) and AmB (1 mg/kg/day for 5 days) showed a partial additive effect on the reduction of leishmanial burden in both target organs.

Does P-glycoprotein contribute to amphotericin B epithelial transport in Caco-2 cells?

INTRODUCTION

Amphotericin B (AmB) is a highly efficacious therapeutic for invasive fungal infections and protozoal diseases. Increasing prevalence of these conditions warrants the development of an oral AmB formulation. Efflux transporters, such as the ABCB1 gene product P-glycoprotein, affect the oral bioavailability and disposition of a range of clinically relevant compounds. At present, it remains to be determined whether AmB is a substrate of P-glycoprotein mediated efflux. The objective of this study was to determine whether P-glycoprotein contributes to the epithelial transport of AmB in a Caco-2 cell model.

METHODS

Stimulation of P-glycoprotein ATPase activity was assessed using membranes containing human recombinant P-glycoprotein. An ABCB1 knockdown Caco-2 cell model was employed to determine non-toxic concentrations of AmB. AmB cellular association, following a 180 min incubation, was determined using an high performance liquid chromatography-ultraviolet (HPLC-UV) assay.

RESULTS

At the concentrations investigated, AmB did not stimulate P-glycoprotein ATPase activity. Non-toxic concentrations of AmB were 1 μg/mL-5 μg/mL; these were used in subsequent experiments. No significant difference in AmB cellular association was observed for ABCB1 small interfering ribonucleic acid transfected and non-transfected Caco-2 cells, following a 180 min incubation with 1 μg/mL and 2.5 μg/mL AmB. However, significantly greater AmB was associated with transfected cells as compared to non-transfected cells, when cells were incubated with 5 μg/mL AmB.

CONCLUSIONS

These results suggest that AmB is not a substrate of P-glycoprotein mediated efflux in this Caco-2 cell model. P-glycoprotein is not expected to be a major barrier to the oral absorption and disposition of AmB.

Drug resistance in leishmaniasis: Newer developments

Leishmaniasis is a vector borne protozoan disease and it remains a major public health problem world-wide. Lack of an effective vaccine and vector control program makes the chemotherapy as the primary tool for leishmaniasis. Antimonials were used as the first line of treatment for many years. Emergence of resistance against this drug has become a major concern. Literatures and studies published on anti-leishmanial drug resistance, newer drug discovery for leishmanial resistance etc., in PubMed, Medline and Google search and reviewed thoroughly. Various newer drugs have been identified but, are in limited use because of high cost, toxicity, resistance etc., Recently, many newer mechanisms of drug resistance have been identified which may boost in future designing and development of drugs.

Stage-dependent expression and up-regulation of trypanothione synthetase in amphotericin B resistant Leishmania donovani

Kinetoplastids differ from other organisms in their ability to conjugate glutathione and spermidine to form trypanothione which is involved in maintaining redox homeostasis and removal of toxic metabolites. It is also involved in drug resistance, antioxidant mechanism, and defense against cellular oxidants. Trypanothione synthetase (TryS) of thiol metabolic pathway is the sole enzyme responsible for the biosynthesis of trypanothione in Leishmania donovani. In this study, TryS gene of L. donovani (LdTryS) was cloned, expressed, and fusion protein purified with affinity column chromatography. The purified protein showed optimum enzymatic activity at pH 8.0–8.5. The TryS amino acids sequences alignment showed that all amino acids involved in catalytic and ligands binding of L. major are conserved in L. donovani. Subcellular localization using digitonin fractionation and immunoblot analysis showed that LdTryS is localized in the cytoplasm. Furthermore, RT-PCR coupled with immunoblot analysis showed that LdTryS is overexpressed in Amp B resistant and stationary phase promastigotes (∼2.0-folds) than in sensitive strain and logarithmic phase, respectively, which suggests its involvement in Amp B resistance. Also, H₂O₂ treatment upto 150 µM for 8 hrs leads to 2-fold increased expression of LdTryS probably to cope up with oxidative stress generated by H₂O₂. Therefore, this study demonstrates stage- and Amp B sensitivity-dependent expression of LdTryS in L. donovani and involvement of TryS during oxidative stress to help the parasites survival.