Novel Intracellular SbV reducing activity correlates with antimony susceptibility in Leishmania donovani

Insights into the trypanothione system in antimony-resistant and sensitive Leishmania tropica clinical isolates

Pentavalent antimonials are the mainstay treatment against different clinical forms of leishmaniasis. The emergence of resistant isolates in endemic areas has led to treatment failure. Unraveling the underlying resistance mechanism would assist in improving the treatment strategies against resistant isolates. This study aimed to investigate the RNA expression level of glutathione synthetase (GS), Spermidine synthetase (SpS), trypanothione synthetase (TryS) genes involved in trypanothione synthesis, and thiol-dependent reductase (TDR) implicated in drug reduction, in antimony-sensitive and -resistant Leishmania tropica isolates. We investigated 11 antimony-resistant and 11 antimony-sensitive L. tropica clinical isolates from ACL patients. Drug sensitivity of amastigotes was determined in mouse macrophage cell line J774A.1. The RNA expression level in the promastigote forms was analyzed by quantitative real-time PCR. The results revealed a significant increase in the average expression of GS, SpS, and TrpS genes by 2.19, 1.56, and 2.33-fold in resistant isolates compared to sensitive ones. The average expression of TDR was 1.24-fold higher in resistant isolates, which was insignificant. The highest correlation coefficient between inhibitory concentration (IC50) values and gene expression belonged to the TryS, GS, SpS, and TDR genes. Moreover, the intracellular thiol content was increased 2.17-fold in resistant isolates compared to sensitive ones and positively correlated with IC50 values. Our findings suggest that overexpression of trypanothione biosynthesis genes and increased thiol content might play a key role in the antimony resistance of L. tropica clinical isolates. In addition, the diversity of gene expression in the trypanothione system and thiol content among L. tropica clinical isolates highlighted the phenotypic heterogeneity of antimony resistance among the parasite population.

Antimony resistance and gene expression in Leishmania: spotlight on molecular and proteomic aspects

Leishmaniasis is a vector-borne parasitic disease caused by Leishmania parasites with a spectrum of clinical manifestations, ranging from skin lesions to severe visceral complications. Treatment of this infection has been extremely challenging with the concurrent emergence of drug resistance. The differential gene expression and the discrepancies in protein functions contribute to the appearance of 2 distinct phenotypes: resistant and sensitive, but the current diagnostic tools fail to differentiate between them. The identification of gene expression patterns and molecular mechanisms coupled with antimony (Sb) resistance can be leveraged to prompt diagnosis and select the most effective treatment methods. The present study attempts to use comparative expression of Sb resistance-associated genes in resistant and sensitive Leishmania, to disclose their relative abundance in clinical or in vitro selected isolates to gain an understanding of the molecular mechanisms of Sb response/resistance. Data suggest that the analysis of resistance gene expression would verify the Sb resistance or susceptibility only to a certain extent; however, none of the individual expression patterns of the studied genes was diagnostic as a biomarker of Sb response of Leishmania. The findings highlighted will be useful in bridging the knowledge gap and discovering innovative diagnostic tools and novel therapeutic targets.

Novel Schiff bases of Vanillin: potent inhibitors of macrophage harbored Leishmania tropica

Due to limited chemotherapeutic options for leishmaniasis, novel synthetic compounds are gaining attention for evaluation against leishmaniasis. This study aimed to synthesize the compound's Schiff bases of Vanillin to investigate and evaluate their anti-leishmanial potentials against intracellular protozoan parasites Leishmania tropica. In the current study, the phenomena of synergism by designing Schiff bases with Vanillin enhances their desired importance. A total of five compounds Schiff bases of Vanillin were synthesized using different aromatic amines and Vanillin. The structural analysis of all the compounds was done through FT-IR (Fourier Transformer-Infrared), thin layer chromatography, and spectroscopic techniques such as 13C-NMR, mass spectrometry, and 1H-NMR. The antimicrobial properties of all the compounds ZI-1, ZI-2, BS-1, KH-1, and FA-2 against promastigotes and amastigotes forms of L. tropica were analyzed at three different concentrations 25, 50, and 100 µg/ml. The in-vitro MTT assay was performed to calculate the percent inhibition, IC50 values, and their cytotoxicity. The highest percent inhibition values against promastigote form of L. tropica were BS-1 53.78% at 25 µg/ml, ZI-2 66.95% at 50 µg/ml, and again ZI-2 76.92% at 100 µg/ml. Similarly, the highest percent inhibition values against intracellular amastigote stage were BS-1 55.77% at 25 µg/ml, ZI-2 67.78% at 50 µg/ml and again ZI-2 84.93% 100 µg/ml. The highest potency was recorded for BS-1 in both stages, with IC50 values of 9.83 and 4.27 µg/ml against promastigotes and intracellular amastigotes, respectively. The percent hemolysis as toxicity; the lowest percent hemolysis was recorded for ZI-1 at three different concentrations of 25, 50, 100 µg/ml of 2.60, 3.50, and 6.31, respectively. These results suggested that all the compounds exhibited anti-leishmanial activity, with BS-1 as the most potent. Further studies are suggested to increase the activity of compounds with structural modifications by the addition of some other synergistic, novel, and analogue compounds.

Antileishmanial evaluation of triazole-butenolide conjugates: design, synthesis, in vitro screening, SAR and in silico ADME predictions

In the quest to discover novel scaffolds with leishmanicidal effects, a series of 23 compounds containing the most promising 1,2,3-triazole and highly potent butenolide in one framework were synthesized. The synthesized conjugates were screened against Leishmania donovani parasite; five of them showed moderate antileishmanial activity against promastigotes (IC50 30.6 to 35.5 μM) and eight of them exhibited significant activity against amastigotes (IC50 ≤12 μM). Compound 10u was found to be the most active (IC50 8.4 ± 0.12 μM) with the highest safety index (20.47). The series was further evaluated against Plasmodium falciparum (3D7 strain) and seven compounds were found to be moderately active. Among them, again 10u emerged as the most active compound (IC50 3.65 μM). In antifilarial assays against adult female Brugia malayi, five compounds showed grade II inhibition (50-74%). Structure-activity relationship (SAR) analysis suggested a substituted phenyl ring, triazole and butenolide as essential structural features for bioactivity. Moreover, the results of in silico ADME parameter and pharmacokinetic studies indicated that the synthesized triazole-butenolide conjugates abide by the required criteria for the development of orally active drugs, and thus this scaffold can be used as a pharmacologically active framework that should be considered for the development of potential antileishmanial hits.

Recent Advances in Chemotherapeutics for Leishmaniasis: Importance of the Cellular Biochemistry of the Parasite and Its Molecular Interaction with the Host

Leishmaniasis, a category 1 neglected protozoan disease caused by a kinetoplastid pathogen called Leishmania, is transmitted through dipteran insect vectors (phlebotomine, sand flies) in three main clinical forms: fatal visceral leishmaniasis, self-healing cutaneous leishmaniasis, and mucocutaneous leishmaniasis. Generic pentavalent antimonials have long been the drug of choice against leishmaniasis; however, their success is plagued with limitations such as drug resistance and severe side effects, which makes them redundant as frontline therapy for endemic visceral leishmaniasis. Alternative therapeutic regimens based on amphotericin B, miltefosine, and paromomycin have also been approved. Due to the unavailability of human vaccines, first-line chemotherapies such as pentavalent antimonials, pentamidine, and amphotericin B are the only options to treat infected individuals. The higher toxicity, adverse effects, and perceived cost of these pharmaceutics, coupled with the emergence of parasite resistance and disease relapse, makes it urgent to identify new, rationalized drug targets for the improvement in disease management and palliative care for patients. This has become an emergent need and more relevant due to the lack of information on validated molecular resistance markers for the monitoring and surveillance of changes in drug sensitivity and resistance. The present study reviewed the recent advances in chemotherapeutic regimens by targeting novel drugs using several strategies including bioinformatics to gain new insight into leishmaniasis. Leishmania has unique enzymes and biochemical pathways that are distinct from those of its mammalian hosts. In light of the limited number of available antileishmanial drugs, the identification of novel drug targets and studying the molecular and cellular aspects of these drugs in the parasite and its host is critical to design specific inhibitors targeting and controlling the parasite. The biochemical characterization of unique Leishmania-specific enzymes can be used as tools to read through possible drug targets. In this review, we discuss relevant metabolic pathways and novel drugs that are unique, essential, and linked to the survival of the parasite based on bioinformatics and cellular and biochemical analyses.

Molecular-level strategic goals and repressors in Leishmaniasis - Integrated data to accelerate target-based heterocyclic scaffolds

Leishmaniasis is a complex of neglected tropical diseases caused by various species of leishmanial parasites that primarily affect the world's poorest people. A limited number of standard medications are available for this disease that has been used for several decades, these drugs have many drawbacks such as resistance, higher cost, and patient compliance, making it difficult to reach the poor. The search for novel chemical entities to treat leishmaniasis has led to target-based scaffold research. Among several identified potential molecular targets, enzymes involved in the purine salvage pathway include polyamine biosynthetic process, such as arginase, ornithine decarboxylase, S-adenosylmethionine decarboxylase, spermidine synthase, trypanothione reductase as well as enzymes in the DNA cell cycle, such as DNA topoisomerases I and II plays vital role in the life cycle survival of leishmanial parasite. This review mainly focuses on various heterocyclic scaffolds, and their specific inhibitory targets against leishmaniasis, particularly those from the polyamine biosynthesis pathway and DNA topoisomerases with estimated activity studies of various heterocyclic analogs in terms of their IC₅₀ or EC₅₀ value, reported molecular docking analysis from available published literatures.

Translational reprogramming as a driver of antimony-drug resistance in Leishmania

Leishmania is a unicellular protozoan that has a limited transcriptional control and mostly uses post-transcriptional regulation of gene expression, although the molecular mechanisms of the process are still poorly understood. Treatments of leishmaniasis, pathologies associated with Leishmania infections, are limited due to drug resistance. Here, we report dramatic differences in mRNA translation in antimony drug-resistant and sensitive strains at the full translatome level. The major differences (2431 differentially translated transcripts) were demonstrated in the absence of the drug pressure supporting that complex preemptive adaptations are needed to efficiently compensate for the loss of biological fitness once they are exposed to the antimony. In contrast, drug-resistant parasites exposed to antimony activated a highly selective translation of only 156 transcripts. This selective mRNA translation is associated with surface protein rearrangement, optimized energy metabolism, amastins upregulation, and improved antioxidant response. We propose a novel model that establishes translational control as a major driver of antimony-resistant phenotypes in Leishmania.

Cardiovascular Effects of Environmental Metal Antimony: Redox Dyshomeostasis as the Key Pathogenic Driver

Significance: Cardiovascular diseases (CVDs) are the leading cause of death worldwide, which may be due to sedentary lifestyles with less physical activity and over nutrition as well as an increase in the aging population; however, the contribution of pollutants, environmental chemicals, and nonessential metals to the increased and persistent CVDs needs more attention and investigation. Among environmental contaminant nonessential metals, antimony has been less addressed. Recent Advances: Among environmental contaminant nonessential metals, several metals such as lead, arsenic, and cadmium have been associated with the increased risk of CVDs. Antimony has been less addressed, but its potential link to CVDs is being gradually recognized. Critical Issues: Several epidemiological studies have revealed the significant deleterious effects of antimony on the cardiovascular system in the absence or presence of other nonessential metals. There has been less focus on whether antimony alone can contribute to the pathogenesis of CVDs and the proposed mechanisms of such possible effects. This review addresses this gap in knowledge by presenting the current available evidence that highlights the potential role of antimony in the pathogenesis of CVDs, most likely via antimony-mediated redox dyshomeostasis. Future Directions: More direct evidence from preclinical and mechanistic studies is urgently needed to evaluate the possible roles of antimony in mitochondrial dysfunction and epigenetic regulation in CVDs. Antioxid. Redox Signal. 38, 803-823.

Further Investigations of Nitroheterocyclic Compounds as Potential Antikinetoplastid Drug Candidates

Due to the lack of specific vaccines, management of the trypanosomatid-caused neglected tropical diseases (sleeping sickness, Chagas disease and leishmaniasis) relies exclusively on pharmacological treatments. Current drugs against them are scarce, old and exhibit disadvantages, such as adverse effects, parenteral administration, chemical instability and high costs which are often unaffordable for endemic low-income countries. Discoveries of new pharmacological entities for the treatment of these diseases are scarce, since most of the big pharmaceutical companies find this market unattractive. In order to fill the pipeline of compounds and replace existing ones, highly translatable drug screening platforms have been developed in the last two decades. Thousands of molecules have been tested, including nitroheterocyclic compounds, such as benznidazole and nifurtimox, which had already provided potent and effective effects against Chagas disease. More recently, fexinidazole has been added as a new drug against African trypanosomiasis. Despite the success of nitroheterocycles, they had been discarded from drug discovery campaigns due to their mutagenic potential, but now they represent a promising source of inspiration for oral drugs that can replace those currently on the market. The examples provided by the trypanocidal activity of fexinidazole and the promising efficacy of the derivative DNDi-0690 against leishmaniasis seem to open a new window of opportunity for these compounds that were discovered in the 1960s. In this review, we show the current uses of nitroheterocycles and the novel derived molecules that are being synthesized against these neglected diseases.

Global distribution of treatment resistance gene markers for leishmaniasis

Background

Pentavalent antimonials (Sb(V)) such as meglumine antimoniate (Glucantime®) and sodium stibogluconate (Pentostam®) are used as first‐line treatments for leishmaniasis, either alone or in combination with second‐line drugs such as amphotericin B (Amp B), miltefosine (MIL), methotrexate (MTX), or cryotherapy. Therapeutic aspects of these drugs are now challenged because of clinical resistance worldwide.

Methods

We reviewedthe recent original studies were assessed by searching in electronic databases such as Scopus, Pubmed, Embase, and Web of Science.

Results

Studies on molecular biomarkers involved in drug resistance are essential for monitoring the disease. We reviewed genes and mechanisms of resistance to leishmaniasis, and the geographical distribution of these biomarkers in each country has also been thoroughly investigated.

Conclusion

Due to the emergence of resistant genes mainly in anthroponotic Leishmania species such as L. donovani and L. tropica, as the causative agents of ACL and AVL, respectively, selection of an appropriate treatment modality is essential. Physicians should be aware of the presence of such resistance for the selection of proper treatment modalities in endemic countries.

Involvement of tryparedoxin peroxidase (TryP) and trypanothione reductase (TryR) in antimony unresponsive of Leishmania tropica clinical isolates of Iran

Clinical resistance to pentavalent antimonial compounds has long been recognized as a major problem in the treatment of human leishmaniasis. Trypanothione metabolism, the main form of thiol, has shown to play a central role in antimony resistance of laboratory-generated resistant Leishmania spp. and field-isolated resistant L. donovani; but the mechanism of antimony resistance in the clinical isolates of L. tropica causing anthroponotic cutaneous leishmaniasis (ACL) is less studied. Patients were selected among confirmed positive ACL cases who referred to Pasteur Institute of Iran, Tehran, from endemic regions of north-east and south of Iran. L. tropica clinical isolates were collected from patients who were either treatment-responsive (MAS=S1 to S5) or unresponsive (MAR=R1 to R4) to Glucantime® (meglumine antimoniate=MA). Isolates were tested for sensitivity to trivalent antimony (SbIII) in promastigotes and to pentavalent antimony (SbV) in intracellular amastigotes stages. Intracellular thiol levels were assayed and trypanothione-dependent components, including trypanothione reductase (TR) and tryparedoxin peroxidase I (TryP) were analysed at protein level and enzymatic activity in isolates. The MAR isolates had an approximate two fold increase in the levels of intracellular thiols (P< 0.05) accompanied by an average 5-10 fold increase in in vitro resistance to antimony. TryP was amplified at the protein level in all MAR strains as compared to the MAS strains (range: 2.8-5.6 fold). All MAR isolates metabolized H₂O₂ at higher rates than MAS isolates (8.55±0.75 nmol/min/mg vs. 3.14±0.36 nmol/min/mg) (P< 0.05). In addition, levels of TryR protein were also markedly elevated in 3 out of 4 MAR isolates (range: 2.2-4.1 fold). This was accompanied by overexpressed TryR activity (mean level of 46.83±2.43 for extracts of MAR vs. 20.98±3.02 for MAS strains) (P< 0.05). Elevated levels of TryP, active enzyme in peroxide detoxification, were observed in MAR parasites resulting in an increased metabolism of H₂O₂. TryR activity was overexpressed on average in extracts of MAR strains, but not in all isolates. Enhanced anti-oxidant defenses through thiol metabolism may play a significant role in clinical resistance of ACL patients to Glucantime.

Discovery and Validation of Lmj_04_BRCT Domain, a Novel Therapeutic Target: Identification of Candidate Drugs for Leishmaniasis

Since many of the currently available antileishmanial treatments exhibit toxicity, low effectiveness, and resistance, search and validation of new therapeutic targets allowing the development of innovative drugs have become a worldwide priority. This work presents a structure-based drug discovery strategy to validate the Lmj_04_BRCT domain as a novel therapeutic target in Leishmania spp. The structure of this domain was explored using homology modeling, virtual screening, and molecular dynamics studies. Candidate compounds were validated in vitro using promastigotes of Leishmania major, L. amazonensis, and L. infantum, as well as primary mouse macrophages infected with L. major. The novel inhibitor CPE2 emerged as the most active of a group of compounds against Leishmania, being able to significantly reduce the viability of promastigotes. CPE2 was also active against the intracellular forms of the parasites and significantly reduced parasite burden in murine macrophages without exhibiting toxicity in host cells. Furthermore, L. major promastigotes treated with CPE2 showed significant lower expression levels of several genes (α-tubulin, Cyclin CYCA, and Yip1) related to proliferation and treatment resistance. Our in silico and in vitro studies suggest that the Lmj_04_BRCT domain and its here disclosed inhibitors are new potential therapeutic options against leishmaniasis.

A Critical Review of Resistance and Oxidation Mechanisms of Sb-Oxidizing Bacteria for the Bioremediation of Sb(III) Pollution

Antimony (Sb) is a priority pollutant in many countries and regions due to its chronic toxicity and potential carcinogenicity. Elevated concentrations of Sb in the environmental originating from mining and other anthropogenic sources are of particular global concern, so the prevention and control of the source of pollution and environment remediation are urgent. It is widely accepted that indigenous microbes play an important role in Sb speciation, mobility, bioavailability, and fate in the natural environment. Especially, antimony-oxidizing bacteria can promote the release of antimony from ore deposits to the wider environment. However, it can also oxidize the more toxic antimonite [Sb(III)] to the less-toxic antimonate [Sb(V)], which is considered as a potentially environmentally friendly and efficient remediation technology for Sb pollution. Therefore, understanding its biological oxidation mechanism has great practical significance to protect environment and human health. This paper reviews studies of the isolation, identification, diversity, Sb(III) resistance mechanisms, Sb(III) oxidation characteristics and mechanism and potential application of Sb-oxidizing bacteria. The aim is to provide a theoretical basis and reference for the diversity and metabolic mechanism of Sb-oxidizing bacteria, the prevention and control of Sb pollution sources, and the application of environment treatment for Sb pollution.

ASSESSMENT OF THE OXIDATIVE AND NITROSATIVE STRESS IN THE SERUM OF SAUDI PATIENTS WITH CUTANEOUS LEISHMANIASIS BEFORE AND AFTER TREATMENT

Macrophages, within which Leishmania species replicate, generate large amounts of reactive oxygen species (ROS) and reactive nitrogen species (RNS) to kill these parasites. The present study assessed the oxidative and nitrosative stress, and specific immune enzymes in the serum of patients with cutaneous leishmaniasis (Cl) before and after treatment and in the control individuals. Serum activities of superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-Px), L-arginase, myeloperoxidase (MPO), and adenosine deaminase (ADA) and the levels of reduced glutathione, malondialdehyde (MDA), and nitric oxide (NO) were studied. The activities of L-arginase, MPO, and ADA and the levels of MDA and NO were significantly elevated (P < 0.001), while the activities of SOD, CAT, and GSH-Px, and the levels of reduced glutathione (GSH) were significantly (P < 0.001) reduced in untreated patients as compared with values of patients after treatment and of control individuals. The treatment, which included intramuscular injection of sodium stibogluconate and meglumine antimoniate, ameliorated these factors in comparison to the untreated group. These results suggest that oxidative and nitrosative stress may play an important role in the pathogenesis of untreated cutaneous leishmaniasis. Furthermore, the reduction in oxidative and nitrosative stress in the treated Cl patients may be due to the drug decreasing energy production by the parasite, which eventually leads to its death.

Immuno-pharmacokinetics of Meglumine Antimoniate in Patients With Cutaneous Leishmaniasis Caused by Leishmania (Viannia)

BACKGROUND

Control of cutaneous leishmaniasis (CL) relies on chemotherapy, yet gaps in our understanding of the determinants of therapeutic outcome impede optimization of antileishmanial drug regimens. Pharmacodynamic (PD) parameters of antimicrobials are based on the relationship between drug concentrations/exposure and microbial kill. However, viable Leishmania persist in a high proportion of individuals despite clinical resolution, indicating that determinants other than parasite clearance are involved in drug efficacy.

METHODS

In this study, the profiles of expression of neutrophils, monocytes, Th1 and Th17 gene signatures were characterized in peripheral blood mononuclear cells (PBMCs) during treatment with meglumine antimoniate (MA) and clinical cure of human CL caused by Leishmania (Viannia). We explored relationships of immune gene expression with plasma and intracellular antimony (Sb) concentrations.

RESULTS

Our findings show a rapid and orchestrated modulation of gene expression networks upon exposure to MA. We report nonlinear pharmacokinetic/pharmacodynamic (PK/PD) relationships of Sb and gene expression dynamics in PBMCs , concurring with a time lag in the detection of intracellular drug concentrations and with PK evidence of intracellular Sb accumulation.

CONCLUSIONS

Our results quantitatively portray the immune dynamics of therapeutic healing, and provide the knowledge base for optimization of antimonial drug treatments, guiding the selection and/or design of targeted drug delivery systems and strategies for targeted immunomodulation.

Determinants of Unresponsiveness to Treatment in Cutaneous Leishmaniasis: A Focus on Anthroponotic Form Due to Leishmania tropica

Cutaneous leishmaniasis (CL) is a curable disease; however, due to various risk factors, unresponsiveness to CL treatments is inevitable. The treatment of CL has been firmly correlated with multiple determinants, such as demographical, clinical, and environmental factors, the host’s immune response, poor treatment adherence, the parasite’s genetic make-up, and Leishmania RNA virus. This study primarily focuses on the risk factors associated with different therapeutic outcomes following meglumine antimoniate (MA; Glucantime^®) treatment and policy approaches to prevent unresponsiveness in CL patients with a focus on anthroponotic form (ACL). Findings suggest that effective preventive and therapeutic measures should be more vigorously implemented, particularly in endemic areas. Accordingly, extensive training is essential to monitor drug unresponsiveness regularly, especially in tropical regions where the disease is prevalent. Since humans are the fundamental reservoir host of ACL due to L. tropica, prompt detection, early diagnosis, and timely and effective treatment could help control this disease. Furthermore, major challenges and gaps remain: efficacious vaccine, new tools, and expert staff are crucial before CL can be definitively controlled.

Epigallocathechin-O-3-Gallate Inhibits Trypanothione Reductase of Leishmania infantum, Causing Alterations in Redox Balance and Leading to Parasite Death

Leishmania infantum is a protozoan parasite that causes a vector borne infectious disease in humans known as visceral leishmaniasis (VL). This pathology, also caused by L. donovani, presently impacts the health of 500,000 people worldwide, and is treated with outdated anti-parasitic drugs that suffer from poor treatment regimens, severe side effects, high cost and/or emergence of resistant parasites. In previous works we have disclosed the anti-Leishmania activity of (-)-Epigallocatechin 3-O-gallate (EGCG), a flavonoid compound present in green tea leaves. To date, the mechanism of action of EGCG against Leishmania remains unknown. This work aims to shed new light into the leishmanicidal mode of action of EGCG. Towards this goal, we first confirmed that EGCG inhibits L. infantum promastigote proliferation in a concentration-dependent manner. Second, we established that the leishmanicidal effect of EGCG was associated with i) mitochondria depolarization and ii) decreased concentration of intracellular ATP, and iii) increased concentration of intracellular H₂O₂. Third, we found that the leishmanicidal effect and the elevated H₂O₂ levels induced by of EGCG can be abolished by PEG-catalase, strongly suggesting that this flavonoid kills L. infantum promastigotes by disturbing their intracellular redox balance. Finally, we gathered in silico and in vitro evidence that EGCG binds to trypanothione reductase (TR), a central enzyme of the redox homeostasis of Leishmania, acting as a competitive inhibitor of its trypanothione substrate.

An Overview on the Therapeutics of Neglected Infectious Diseases-Leishmaniasis and Chagas Diseases

Neglected tropical diseases (NTDs) as termed by WHO include twenty different infectious diseases that are caused by bacteria, viruses, and parasites. Among these NTDs, Chagas disease and leishmaniasis are reported to cause high mortality in humans and are further associated with the limitations of existing drugs like severe toxicity and drug resistance. The above hitches have rendered researchers to focus on developing alternatives and novel therapeutics for the treatment of these diseases. In the past decade, several target-based drugs have emerged, which focus on specific biochemical pathways of the causative parasites. For leishmaniasis, the targets such as nucleoside analogs, inhibitors targeting nucleoside phosphate kinases of the parasite’s purine salvage pathway, 20S proteasome of Leishmania, mitochondria, and the associated proteins are reviewed along with the chemical structures of potential drug candidates. Similarly, in case of therapeutics for Chagas disease, several target-based drug candidates targeting sterol biosynthetic pathway (C14-ademethylase), L-cysteine protease, heme peroxidation, mitochondria, farnesyl pyrophosphate, etc., which are vital and unique to the causative parasite are discussed. Moreover, the use of nano-based formulations towards the therapeutics of the above diseases is also discussed.

Development of new combination anti-leishmanial complexes: Triphenyl Sb(V) mono-hydroxy mono-quinolinolates

In seeking to develop single entity combination anti-Leishmanial complexes six heteropletic organometallic Sb(V) hydroxido quinolinolate complexes of general formula [SbPh₃(C₉H₄NORR')(OH)] have been synthesised and characterised, derived from a series of halide substituted quinolinols (8-hydroxyquinolines). Single crystal X-ray diffraction on all the complexes show a common distorted six-coordinate octahedral environment at the Sb(V) centre, with the aryl groups and nitrogen atom of quinolinolate ligand bonding in the equatorial planes, with the two oxygen atoms (hydroxyl and quinolinolate) occupying the axial plane in an almost linear configuration. Each complex was tested for their anti-promastigote activity and mammalian cytotoxicity and a selectivity indices established. The complexes displayed excellent anti-promastigote activity (IC₅₀: 2.03-3.39 μM) and varied mammalian cytotoxicity (IC₅₀: 12.7-46.9 μM), leading to a selectivity index range of 4.52-16.7. All complexes displayed excellent anti-amastigote activity with a percentage infection range of 2.25%-9.00%. All complexes performed substantially better than the parent quinolinols and comparable carboxylate complexes [SbPh₃(O₂CRR')₂] indicating the synergistic role of the Sb(V) and quinolinol moieties in increasing parasite mortality. Two of the complexes [SbPh₃(C₉H₄NOBr₂)(OH)] 4, [SbPh₃(C₉H₄NOI₂)(OH)] 5, provide an ideal combination of high selective and good activity towards the leishmanial amastigotes and offer the potential as good lead compounds.

Comparative transcriptomic analysis of antimony resistant and susceptible Leishmania infantum lines

Background

One of the major challenges to leishmaniasis treatment is the emergence of parasites resistant to antimony. To study differentially expressed genes associated with drug resistance, we performed a comparative transcriptomic analysis between wild-type and potassium antimonyl tartrate (Sb^III)-resistant Leishmania infantum lines using high-throughput RNA sequencing.

Methods

All the cDNA libraries were constructed from promastigote forms of each line, sequenced and analyzed using STAR for mapping the reads against the reference genome (L. infantum JPCM5) and DESeq2 for differential expression statistical analyses. All the genes were functionally annotated using sequence similarity search.

Results

The analytical pipeline considering an adjusted p-value < 0.05 and fold change > 2.0 identified 933 transcripts differentially expressed (DE) between wild-type and Sb^III-resistant L. infantum lines. Out of 933 DE transcripts, 504 presented functional annotation and 429 were assigned as hypothetical proteins. A total of 837 transcripts were upregulated and 96 were downregulated in the Sb^III-resistant L. infantum line. Using this DE dataset, the proteins were further grouped in functional classes according to the gene ontology database. The functional enrichment analysis for biological processes showed that the upregulated transcripts in the Sb^III-resistant line are associated with protein phosphorylation, microtubule-based movement, ubiquitination, host–parasite interaction, cellular process and other categories. The downregulated transcripts in the Sb^III-resistant line are assigned in the GO categories: ribonucleoprotein complex, ribosome biogenesis, rRNA processing, nucleosome assembly and translation.

Conclusions

The transcriptomic profile of L. infantum showed a robust set of genes from different metabolic pathways associated with the antimony resistance phenotype in this parasite. Our results address the complex and multifactorial antimony resistance mechanisms in Leishmania, identifying several candidate genes that may be further evaluated as molecular targets for chemotherapy of leishmaniasis.

Can We Harness Immune Responses to Improve Drug Treatment in Leishmaniasis?

Leishmaniasis is a vector-borne parasitic disease that has been neglected in priority for control and eradication of malaria, tuberculosis, and HIV/AIDS. Collectively, over one seventh of the world’s population is at risk of being infected with 0.7–1.2 million new infections reported annually. Clinical manifestations range from self-healing cutaneous lesions to fatal visceral disease. The first anti-leishmanial drugs were introduced in the 1950′s and, despite several shortcomings, remain the mainstay for treatment. Regardless of this and the steady increase in infections over the years, particularly among populations of low economic status, research on leishmaniasis remains under funded. This review looks at the drugs currently in clinical use and how they interact with the host immune response. Employing chemoimmunotherapeutic approaches may be one viable alternative to improve the efficacy of novel/existing drugs and extend their lifespan in clinical use.

Screening Marine Natural Products for New Drug Leads against Trypanosomatids and Malaria

Neglected Tropical Diseases (NTD) represent a serious threat to humans, especially for those living in poor or developing countries. Almost one-sixth of the world population is at risk of suffering from these diseases and many thousands die because of NTDs, to which we should add the sanitary, labor and social issues that hinder the economic development of these countries. Protozoan-borne diseases are responsible for more than one million deaths every year. Visceral leishmaniasis, Chagas disease or sleeping sickness are among the most lethal NTDs. Despite not being considered an NTD by the World Health Organization (WHO), malaria must be added to this sinister group. Malaria, caused by the apicomplexan parasite Plasmodium falciparum, is responsible for thousands of deaths each year. The treatment of this disease has been losing effectiveness year after year. Many of the medicines currently in use are obsolete due to their gradual loss of efficacy, their intrinsic toxicity and the emergence of drug resistance or a lack of adherence to treatment. Therefore, there is an urgent and global need for new drugs. Despite this, the scant interest shown by most of the stakeholders involved in the pharmaceutical industry makes our present therapeutic arsenal scarce, and until recently, the search for new drugs has not been seriously addressed. The sources of new drugs for these and other pathologies include natural products, synthetic molecules or repurposing drugs. The most frequent sources of natural products are microorganisms, e.g., bacteria, fungi, yeasts, algae and plants, which are able to synthesize many drugs that are currently in use (e.g. antimicrobials, antitumor, immunosuppressants, etc.). The marine environment is another well-established source of bioactive natural products, with recent applications against parasites, bacteria and other pathogens which affect humans and animals. Drug discovery techniques have rapidly advanced since the beginning of the millennium. The combination of novel techniques that include the genetic modification of pathogens, bioimaging and robotics has given rise to the standardization of High-Performance Screening platforms in the discovery of drugs. These advancements have accelerated the discovery of new chemical entities with antiparasitic effects. This review presents critical updates regarding the use of High-Throughput Screening (HTS) in the discovery of drugs for NTDs transmitted by protozoa, including malaria, and its application in the discovery of new drugs of marine origin.

Current and promising novel drug candidates against visceral leishmaniasis

Leishmaniasis is a group of zoonotic diseases caused by a trypanosomatid parasite mostly in impoverished populations of low-income countries. In their different forms, leishmaniasis is prevalent in more than 98 countries all over the world and approximately 360-million people are at risk. Since no vaccine is currently available to prevent any form of the disease, the control strategy of leishmaniasis mainly relies on early case detection followed by adequate pharmacological treatment that may improve the prognosis and can reduce transmission. A handful of compounds and formulations are available for the treatment of leishmaniasis in humans, but only few of them are currently in use since most of these agents are associated with toxicity problems such as nephrotoxicity and cardiotoxicity in addition to resistance problems. In recent decades, very few novel drugs, new formulations of standard drugs or combinations of them have been approved against leishmaniasis. This review highlights the current drugs and combinations that are used medical practice and recent advances in new treatments against leishmaniasis that were pointed out in the recent 2nd Conference, Global Challenges in Neglected Tropical Diseases, held in San Juan, Puerto Rico in June 2018, emphasizing the plethora of new families of molecules that are bridging the gap between preclinical and first-in-man trials in next future.

Validation of NAD synthase inhibitors for inhibiting the cell viability of Leishmania donovani: In silico and in vitro approach

NAD (nicotinamide adenine dinucleotide) synthase catalyses the biochemical synthesis of NAD, from nicotinic acid adenine dinucleotide (NAAD). NAD may be synthesized through the de novo pathways and/or the salvage pathways in cells. However, in Leishmania parasite, the synthesis of NAD solely depends on the salvage pathways. NAD synthetase is widely explored as a drug target in various microorganisms. In Bacillus anthracis, a group of sulphonamides 5599, 5617 and 5824 and complex amide 5833 were reported to have activity at micromolar range against NAD synthetase. Hence, in the present study, the same group of sulphonamides and complex amide were validated through in silico and in vitro studies for its efficiency towards Leishmania donovani NAD synthase. In silico study revealed the ligands 5824 and 5833 to have better docking score. Molecular dynamics simulation for a duration of 50 ns of all the ligand-protein complexes suggested that the complexes with the ligands 5824 and 5833 were stable and interacting. In vitro and ex vivo studies have shown that 5824 and 5833 inhibit the cell viability of the organism at a lower concentration than 5599 and 5617. Hence, with further in vivo validation, 5824 (or its synthetic analogues) and 5833 could be the choice that may work synergistically with other potential drugs in treating drug-resistant cases of leishmaniasis. Communicated by Ramaswamy H. Sarma.

Gene expression analysis of antimony resistance in Leishmania tropica using quantitative real-time PCR focused on genes involved in trypanothione metabolism and drug transport

Pentavalent antimonials remain the treatment of choice for all the clinical forms of leishmaniasis. The increasing rates of antimony resistance are becoming a serious health problem in treatment of anthroponotic cutaneous leishmaniasis (ACL). Accordingly, unraveling molecular markers is crucial for improving medication strategies and monitoring of drug-resistant parasites. Different studies have suggested the importance of genes involved in trypanothione metabolism and drug transport. In this regard, present study was designed to investigate the RNA expression level of five genes including γ-GCS, ODC, TRYR (involved in trypanothione metabolism), AQP1 (acts in drug uptake) and MRPA (involved in sequestration of drug) in sensitive and resistant Leishmania tropica isolates. Seven antimony-resistant and seven antimony-sensitive L. tropica clinical isolates were collected from ACL patients. Drug sensitivity test was performed on the samples as well as reference strains; afterwards, gene expression analysis was performed on clinical isolates by quantitative real-time PCR. The results revealed that the average expression level of AQP1 gene was decreased (0.47-fold) in resistant isolates compared to sensitive ones whereas MRPA (2.45), γ-GCS (2.1) and TRYR (1.97) was upregulated in resistant isolates. The average expression of ODC (1.24-fold) gene was not different significantly between sensitive and resistant isolates. Our findings suggest that AQP1, MRPA, GSH1 and TRYR can be considered as potential molecular markers for screening of antimony resistance in some L. tropica clinical isolates.

Unresponsiveness to meglumine antimoniate in anthroponotic cutaneous leishmaniasis field isolates: analysis of resistance biomarkers by gene expression profiling

BACKGROUND

Resistance to antimonials is a fundamental determinant of treatment failure in anthroponotic cutaneous leishmaniasis (ACL). Detection of reliable molecular markers to distinguish unresponsive and responsive parasites is critical for consolidating strategies to monitor drug efficacy.

METHODS

We analysed the expression of five major antimony resistance-associated genes that is aquaglyceroporin1 (AQP1), γ-glutamylcysteine synthetase (γ-GCS), multidrug resistance protein A (MRPA), trypanothione reductase (TR) and thiol-dependent reductase 1 (TDR1), in unresponsive and responsive Leishmania tropica field isolates by quantitative real-time PCR in comparison with sensitive and resistant reference strains.

RESULTS

Gene expression analysis showed the down-regulation of AQP1, γ-GCS and TDR1 by a factor of 1.9, 1.7 and 3.55, respectively, in unresponsive isolates vs. responsive ones. The average RNA expression level of MRPA increased by a factor of 1.9 in the unresponsive group. Isolates exhibited a strong positive linear correlation between gene expression of AQP1 and γ-GCS. A negative correlation between the AQP1 and γ-GCS expression level and lesion duration in responsive patients indicated the potential role in diagnosing drug-unresponsive parasites in endemic areas of ACL.

CONCLUSION

In cases of inconclusive outcomes of resistance tests in clinical isolates, expression analysis of a set of influential genes can be beneficial to identify distinctive biomarkers between antimony-unresponsive and responsive parasites.

Slow pace of antileishmanial drug development

The protozoan parasiteLeishmaniais endemic in large parts of the world which causes leishmaniasis. Its visceral form is fatal if not treated and is caused mostly byLeishmania donovani,Leishmania infantumandLeishmania chagasi. Given the difficulties linked to vector (sandfly) control and the lack of an effective vaccine, the control of leishmaniasis relies mostly on chemotherapy. Unfortunately, the prevalence of parasites becoming resistant to the first-line drug pentavalent antimony (SbV) is increasing worldwide. Few alternative drugs are available that includes amphotericin B, pentamidine and miltefosine (oral). Already, decreases in efficacy, resistance and toxicity have been noted against these drugs. Dry antileishmanial pipeline further indicates the slow pace of drug discovery in this field where resistance as a major barrier. Full understanding of the genetic and molecular basis of the parasite is lagging. Since leishmaniasis is a neglected disease and occurs predominantly in the developing world largely, therefore, it is unaddressed. The pharma industry argues that development of the new drug is too costly and risky to invest in low return neglected diseases is very high. Research is also needed to identify new and effective drug targets. The lack of drug research and development for neglected diseases will require some new strategies. We have discussed the various cause of slow pace of antileishmanial drug discovery in this review to pay attention of researchers and also take the public and private initiative to make the process fast for new antileishmanial drug development.

Repurposing as a strategy for the discovery of new anti-leishmanials: the-state-of-the-art

Leishmaniasis is a vector-borne neglected tropical disease caused by protozoan parasites of the genus Leishmania for which there is a paucity of effective viable non-toxic drugs. There are 1·3 million new cases each year causing considerable socio-economic hardship, best measured in 2·4 million disability adjusted life years, with greatest impact on the poorest communities, which means that desperately needed new antileishmanial treatments have to be both affordable and accessible. Established medicines with cheaper and faster development times may hold the cure for this neglected tropical disease. This concept of using old drugs for new diseases may not be novel but, with the ambitious target of controlling or eradicating tropical diseases by 2020, this strategy is still an important one. In this review, we will explore the current state-of-the-art of drug repurposing strategies in the search for new treatments for leishmaniasis.

Removal of antimonate from wastewater by dissimilatory bacterial reduction: Role of the coexisting sulfate

The priority pollutant antimony (Sb) exists primarily as Sb(V) and Sb(III) in natural waters, and Sb(III) is generally with greater mobility and toxicity than Sb(V). The bio-reduction of Sb(V) would not become a meaningful Sb-removal process unless the accumulation of produced dissolved Sb(III) could be controlled. Here, we examined the dissimilatory antimonate bio-reduction with or without the coexistence of sulfate using Sb-acclimated biomass. Results demonstrated that 0.8mM Sb(V) was almost completely bio-reduced within 20h along with 48.6% Sb(III) recovery. Kinetic parameters q_max and K_s calculated were 0.54mg-Sb mg-DW^-1h^-1 and 41.96mgL^-1, respectively. When the concentrations of coexisting sulfate were 0.8mM, 1.6mM, and 4mM, the reduction of 0.8mM Sb(V) was accomplished within 17, 9, and 5h, respectively, along with no final Sb(III) recovery. Also, the bio-reduction of sulfate occurred synchronously. The precipitated Sb₂O₃ and Sb₂S₃ were characterized by scanning electron microscopy coupled with energy dispersive spectrometer, X-ray diffraction, and X-ray photoelectron spectroscopy. Compared with bacterial compositions of the seed sludge obtained from anaerobic digestion tank in a wastewater treatment plant, new genera of Pseudomonas and Geobacter emerged with large proportions in both Sb-fed and Sb-sulfate-fed sludge, and a small portion of sulfate reduction bacteria emerged only in Sb-sulfate-fed culture.

Profiling gene expression of antimony response genes in Leishmania (Viannia) panamensis and infected macrophages and its relationship with drug susceptibility

The mechanisms of Leishmania resistance to antimonials have been primarily determined in experimentally derived Leishmania strains. However, their participation in the susceptibility phenotype in field isolates has not been conclusively established. Being an intracellular parasite, the activity of antileishmanials is dependent on internalization of drugs into host cells and effective delivery to the intracellular compartments inhabited by the parasite. In this study we quantified and comparatively analyzed the gene expression of nine molecules involved in mechanisms of xenobiotic detoxification and Leishmania resistance to antimonial drugs in resistant and susceptible laboratory derived and clinical L.(Viannia) panamensis strains(n=19). In addition, we explored the impact of Leishmania susceptibility to antimonials on the expression of macrophage gene products having putative functions in transport, accumulation and metabolism of antimonials. As previously shown for other Leishmania species, a trend of increased abcc3 and lower aqp-1 expression was observed in the laboratory derived Sb-resistant L.(V.) panamensis line. However, this was not found in clinical strains, in which the expression of abca2 was significantly higher in resistant strains as both, promastigotes and intracellular amastigotes. The effect of drug susceptibility on host cell gene expression was evaluated on primary human macrophages from patients with cutaneous leishmaniasis (n=17) infected ex-vivo with the matched L.(V.) panamensis strains isolated at diagnosis, and in THP-1 cells infected with clinical strains (n=6) and laboratory adapted L.(V.) panamensis lines. Four molecules, abcb1 (p-gp), abcb6, aqp-9 and mt2a were differentially modulated by drug resistant and susceptible parasites, and among these, a consistent and significantly increased expression of the xenobiotic scavenging molecule mt2a was observed in macrophages infected with Sb-susceptible L. (V.) panamensis. Our results substantiate that different mechanisms of drug resistance operate in laboratory adapted and clinical Leishmania strains, and provide evidence that parasite-mediated modulation of host cell gene expression of molecules involved in drug transport and metabolism could contribute to the mechanisms of drug resistance and susceptibility in Leishmania.

Nanostructured delivery systems with improved leishmanicidal activity: a critical review

Leishmaniasis is a vector-borne zoonotic disease caused by protozoan parasites of the genus Leishmania, which are responsible for numerous clinical manifestations, such as cutaneous, visceral, and mucocutaneous leishmaniasis, depending on the site of infection for particular species. These complexities threaten 350 million people in 98 countries worldwide. Amastigotes living within macrophage phagolysosomes are the principal target of antileishmanial treatment, but these are not an easy target as drugs must overcome major structural barriers. Furthermore, limitations on current therapy are related to efficacy, toxicity, and cost, as well as the length of treatment, which can increase parasitic resistance. Nanotechnology has emerged as an attractive alternative as conventional drugs delivered by nanosized carriers have improved bioavailability and reduced toxicity, together with other characteristics that help to relieve the burden of this disease. The significance of using colloidal carriers loaded with active agents derives from the physiological uptake route of intravenous administered nanosystems (the phagocyte system). Nanosystems are thus able to promote a high drug concentration in intracellular mononuclear phagocyte system (MPS)-infected cells. Moreover, the versatility of nanometric drug delivery systems for the deliberate transport of a range of molecules plays a pivotal role in the design of therapeutic strategies against leishmaniasis. This review discusses studies on nanocarriers that have greatly contributed to improving the efficacy of antileishmaniasis drugs, presenting a critical review and some suggestions for improving drug delivery.

Ornithine decarboxylase or gamma-glutamylcysteine synthetase overexpression protects Leishmania (Vianna) guyanensis against antimony

Trypanosomatids present a unique mechanism for detoxification of peroxides that is dependent on trypanothione (bisglutathionylspermidine). Ornithine decarboxylase (ODC) and γ-glutamylcysteine synthetase (GSH1) produce molecules that are direct precursors of trypanothione. In this study, Leishmania guyanensis odc and gsh1 overexpressor cell lines were generated to investigate the contribution of these genes to the trivalent antimony (Sb^III)-resistance phenotype. The ODC- or GSH1-overexpressors parasites presented an increase of two and four-fold in Sb^III-resistance index, respectively, when compared with the wild-type line. Pharmacological inhibition of ODC and GSH1 with the specific inhibitors α-difluoromethylornithine (DFMO) and buthionine sulfoximine (BSO), respectively, increased the antileishmanial effect of Sb^III in all cell lines. However, the ODC- and GSH1-overexpressor were still more resistant to Sb^III than the parental cell line. Together, our data shows that modulation of ODC and GSH1 levels and activity is sufficient to affect L. guyanensis susceptibility to Sb^III, and confirms a role of these genes in the Sb^III-resistance phenotype.

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.

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.

Microbial Antimony Biogeochemistry: Enzymes, Regulation, and Related Metabolic Pathways

Antimony (Sb) is a toxic metalloid that occurs widely at trace concentrations in soil, aquatic systems, and the atmosphere. Nowadays, with the development of its new industrial applications and the corresponding expansion of antimony mining activities, the phenomenon of antimony pollution has become an increasingly serious concern. In recent years, research interest in Sb has been growing and reflects a fundamental scientific concern regarding Sb in the environment. In this review, we summarize the recent research on bacterial antimony transformations, especially those regarding antimony uptake, efflux, antimonite oxidation, and antimonate reduction. We conclude that our current understanding of antimony biochemistry and biogeochemistry is roughly equivalent to where that of arsenic was some 20 years ago. This portends the possibility of future discoveries with regard to the ability of microorganisms to conserve energy for their growth from antimony redox reactions and the isolation of new species of "antimonotrophs."

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).

Current status on prevention and treatment of canine leishmaniasis

Canine leishmaniasis (CanL) is a parasite-borne disease mainly induced by Leishmania infantum in the Old World and Leishmania chagasi (infantum) in the New World. CanL is a zoonosis transmitted by the bite of infected Phlebotominae flies that act as vectors. CanL is a very serious disease that usually produces death when remains untreated and can be a focus of transmission to other dogs or humans. Infected dogs and other domestic and wild animals act as reservoirs and are a real threat to uninfected/healthy dogs and humans in endemic areas where the sand flies are present. Prevention of new infections in dogs can help to stop the current increase of the disease in humans, reinforcing the concept of "One Health" approach. The management of CanL is being performed using prophylactic measures in healthy dogs - insecticides impregnated in collars or immunostimulants applied by spot-on devices - and chemotherapy in animals that suffer from the disease. Antimonials as first-line monotherapy have proven efficacy in reducing most of the clinical signs of CanL, but they need to be administered during several days, and no complete parasite clearance is achieved, favouring the presence of relapses among treated dogs. Therefore, new drugs, such as miltefosine, or combinations of this drug or antimonials with allopurinol are in the pipeline of clinical treatment of CanL. Recently, there has been an emergence of protective - prophylactic - and curative - autogenous vaccines - immunotherapy tools to face CanL, whose results are still under study. This review highlights the current use of preventive and eradicative weapons to fight against this disease, which is a scourge for dogs and a continuous threat to human beings.

In vivo and in vitro effects of pentavalent antimony on mouse liver cytochrome P450s

Pentavalent antimonial (Sb⁵⁺) drugs such as meglumine antimoniate (MA) are the mainstay treatment of leishmaniases in developing countries. The effects of these compounds on drug-metabolizing enzymes have not been characterized and their potential pharmacokinetic interactions with other drugs are therefore unknown. The present study investigated whether treatment with MA (300 mg Sb⁵⁺/kg body weight/day, subcutaneously) for 24 days affected the activities of cytochrome P450 (CYP)1A (ethoxyresorufin- O-deethylase), CYP2A5 (coumarin 7-hydroxylase), CYP2E1 ( p-nitrophenol-hydroxylase), CYP2B9/10 (benzyloxy-resorufin- O-debenzylase), or CYP3A11 (erythromycin- N-demethylase) in the livers of Swiss Webster (SW) and DBA-2 male and female mice. The results showed that CYP2A5-, CYP2E1-, and CYP3A11-catalyzed reactions were unaffected by MA treatment. A decrease in CYP2B9/10 activity was noted in DBA-2 females (but not males) and was not observed in SW males or females. However, repeated MA administration reduced mouse liver CYP1A activity. CYP1A2 messenger RNA (mRNA) levels were not affected by MA and in vitro exposure of mouse liver microsomes to Sb³⁺ and Sb⁵⁺ did not reduce CYP1A activity. These findings suggested that in vivo treatment with Sb⁵⁺ drugs depressed CYP1A activity, without downregulating CYP1A2 mRNA expression. Since in vitro treatment of liver microsomes failed to inhibit CYP1A activity, this effect may require intact cells.

Drug resistance in eukaryotic microorganisms

Eukaryotic microbial pathogens are major contributors to illness and death globally. Although much of their impact can be controlled by drug therapy as with prokaryotic microorganisms, the emergence of drug resistance has threatened these treatment efforts. Here, we discuss the challenges posed by eukaryotic microbial pathogens and how these are similar to, or differ from, the challenges of prokaryotic antibiotic resistance. The therapies used for several major eukaryotic microorganisms are then detailed, and the mechanisms that they have evolved to overcome these therapies are described. The rapid emergence of resistance and the restricted pipeline of new drug therapies pose considerable risks to global health and are particularly acute in the developing world. Nonetheless, we detail how the integration of new technology, biological understanding, epidemiology and evolutionary analysis can help sustain existing therapies, anticipate the emergence of resistance or optimize the deployment of new therapies.

Visceral leishmaniasis: Revisiting current treatments and approaches for future discoveries

The current treatments for visceral leishmaniasis are old and toxic with limited routes of administration. The emergence of drug-resistant Leishmania threatens the efficacy of the existing reservoir of antileishmanials, leading to an urgent need to develop new treatments. It is particularly important to review and understand how the current treatments act against Leishmania in order to identify valid drug targets or essential pathways for next-generation antileishmanials. It is equally important to adapt newly emerging biotechnologies to facilitate the current research on the development of novel antileishmanials in an efficient fashion. This review covers the basic background of the current visceral leishmaniasis treatments with an emphasis on the modes of action. It briefly discusses the role of the immune system in aiding the chemotherapy of leishmaniasis, describes potential new antileishmanial drug targets and pathways, and introduces recent progress on the utilization of high-throughput phenotypic screening assays to identify novel antileishmanial compounds.

Protein kinase A signaling during bidirectional axenic differentiation in Leishmania

Parasitic protozoa of the genus Leishmania are obligatory intracellular parasites that cycle between the phagolysosome of mammalian macrophages, where they proliferate as intracellular amastigotes, and the midgut of female sand flies, where they proliferate as extracellular promastigotes. Shifting between the two environments induces signaling pathway-mediated developmental processes that enable adaptation to both host and vector. Developmentally regulated expression and phosphorylation of protein kinase A subunits in Leishmania and in Trypanosoma brucei point to an involvement of protein kinase A in parasite development. To assess this hypothesis in Leishmania donovani, we determined proteome-wide changes in phosphorylation of the conserved protein kinase A phosphorylation motifs RXXS and RXXT, using a phospho-specific antibody. Rapid dephosphorylation of these motifs was observed upon initiation of promastigote to amastigote differentiation in culture. No phosphorylated sites were detected in axenic amastigotes. To analyse the kinetics of (re)phosphorylation during axenic reverse differentiation from L. donovani amastigotes to promastigotes, we first established a map of this process with morphological and molecular markers. Upon initiation, the parasites rested for 6-12 h before proliferation of an asynchronous population resumed. After early changes in cell shape, the major changes in molecular marker expression and flagella biogenesis occurred between 24 and 33 h after initiation. RXXS/T re-phosphorylation and expression of the regulatory subunit PKAR1 correlated with promastigote maturation, indicating a promastigote-specific function of protein kinase A signaling. This is supported by the localization of PKAR1 to the flagellum, an organelle reduced to a remnant in amastigote forms. We conclude that a significant increase in protein kinase A-mediated phosphorylation is part of the ordered changes that characterise the amastigote to promastigote differentiation.

Antileishmanial Effect of 5,3'-Hydroxy-7,4'-dimethoxyflavanone of Picramnia gracilis Tul. (Picramniaceae) Fruit: In Vitro and In Vivo Studies

Species of Picramnia genus are used in folk medicine to treat or prevent skin disorders, but only few species have been studied for biological activity and chemical composition. P. gracilis Tul. is a native species from Central and South America and although its fruits are edible, phytochemical analysis or medicinal uses of this species are not known. In the search of candidates to antileishmanial drugs, this work aimed to evaluate the antileishmanial activity of P. gracilis Tul. in in vitro and in vivo studies. Only ethanolic extract of fruits showed leishmanicidal activity. The majoritarian metabolite was 5,3′-hydroxy-7,4′-dimethoxyflavanone ether that exhibited high activity against L. (V.) panamensis (EC₅₀ 17.0 + 2.8 mg/mL, 53.7 μM) and low toxicity on mammalian U-937 cells, with an index of selectivity >11.8. In vivo studies showed that the flavanone administered in solution (2 mg/kg/day) or cream (2%) induces clinical improvement and no toxicity in hamsters with CL. In conclusion, this is the first report about isolation of 5,3′-hydroxy-7,4′-dimethoxyflavanone of P. gracilis Tul. The leishmanicidal activity attributed to this flavanone is also reported for the first time. Finally, the in vitro and in vivo leishmanicidal activity reported here for 5,3′-hydroxy-7,4′-dimethoxyflavanone offers a greater prospect towards antileishmanial drug discovery and development.

Species-specific antimonial sensitivity in Leishmania is driven by post-transcriptional regulation of AQP1

Leishmania is a digenetic protozoan parasite causing leishmaniasis in humans. The different clinical forms of leishmaniasis are caused by more than twenty species of Leishmania that are transmitted by nearly thirty species of phlebotomine sand flies. Pentavalent antimonials (such as Pentostam or Glucantime) are the first line drugs for treating leishmaniasis. Recent studies suggest that pentavalent antimony (Sb(V)) acts as a pro-drug, which is converted to the more active trivalent form (Sb(III)). However, sensitivity to trivalent antimony varies among different Leishmania species. In general, Leishmania species causing cutaneous leishmaniasis (CL) are more sensitive to Sb(III) than the species responsible for visceral leishmaniasis (VL). Leishmania aquaglyceroporin (AQP1) facilitates the adventitious passage of antimonite down a concentration gradient. In this study, we show that Leishmania species causing CL accumulate more antimonite, and therefore exhibit higher sensitivity to antimonials, than the species responsible for VL. This species-specific differential sensitivity to antimonite is directly proportional to the expression levels of AQP1 mRNA. We show that the stability of AQP1 mRNA in different Leishmania species is regulated by their respective 3’-untranslated regions. The differential regulation of AQP1 mRNA explains the distinct antimonial sensitivity of each species.

The degree of response to antimonial drugs varies widely between species and even among strains of the same species of the protozoan parasite Leishmania. However, the molecular mechanism(s) is unknown. In this study, we show that Leishmania aquaglyceroporin AQP1 drives this species-specific antimonial resistance. Aquaglyceroporins are channel proteins that facilitate the passage of small uncharged molecules, such as glycerol and water, across the biological membranes. AQP1 helps the parasite cope with the osmotic challenges it faces during its life cycle. Additionally, AQP1 is an adventitious facilitator of antimonite, the active form of pentavalent antimonial drugs. We show that AQP1 expression level is species-specific, and less AQP1 in visceral species compared to the cutaneous species results in increased resistance to antimonials. We also demonstrate that the 3’-untranslated regions (3’-UTR) of the AQP1 mRNA is a major determining factor of species-specific regulation of AQP1. Along with water homeostasis, aquaglyceroporins are also involved in directed cell migration. The variable levels of AQP1 in different Leishmania species may enable them to find their appropriate niches in vertebrate hosts and cope with the species-specific osmotic challenges during their life cycles.

Intrachromosomal amplification, locus deletion and point mutation in the aquaglyceroporin AQP1 gene in antimony resistant Leishmania (Viannia) guyanensis

BACKGROUND

Antimony resistance complicates the treatment of infections caused by the parasite Leishmania.

METHODOLOGY/PRINCIPAL FINDINGS

Using next generation sequencing, we sequenced the genome of four independent Leishmania guyanensis antimony-resistant (SbR) mutants and found different chromosomal alterations including aneuploidy, intrachromosomal gene amplification and gene deletion. A segment covering 30 genes on chromosome 19 was amplified intrachromosomally in three of the four mutants. The gene coding for the multidrug resistance associated protein A involved in antimony resistance was also amplified in the four mutants, most likely through chromosomal translocation. All mutants also displayed a reduced accumulation of antimony mainly due to genomic alterations at the level of the subtelomeric region of chromosome 31 harboring the gene coding for the aquaglyceroporin 1 (LgAQP1). Resistance involved the loss of LgAQP1 through subtelomeric deletions in three mutants. Interestingly, the fourth mutant harbored a single G133D point mutation in LgAQP1 whose role in resistance was functionality confirmed through drug sensitivity and antimony accumulation assays. In contrast to the Leishmania subspecies that resort to extrachromosomal amplification, the Viannia strains studied here used intrachromosomal amplification and locus deletion.

CONCLUSIONS/SIGNIFICANCE

This is the first report of a naturally occurred point mutation in AQP1 in antimony resistant parasites.

Sb(V) reactivity with human blood components: redox effects

We assessed the reactivity of Sb(V) in human blood. Sb(V) reactivity was determined using an HPLC-HG-AFS hyphenated system. Sb(V) was partially reduced to Sb(III) in blood incubation experiments; however, Sb(III) was a highly unstable species. The addition of 0.1 mol L⁻¹ EDTA prevented Sb(III) oxidation, thus enabling the detection of the reduction of Sb(V) to Sb(III). The transformation of Sb(V) to Sb(III) in human whole blood was assessed because the reduction of Sb(V) in human blood may likely generate redox side effects. Our results indicate that glutathione was the reducing agent in this reaction and that Sb(V) significantly decreased the GSH/GSSG ratio from 0.32±0.09 to 0.07±0.03. Moreover, the presence of 200 ng mL⁻¹ of Sb(V) increased the activity of superoxide dismutase from 4.4±0.1 to 7.0±0.4 U mL⁻¹ and decreased the activity of glutathione peroxidase from 62±1 to 34±2 nmol min⁻¹ mL⁻¹.

Nanoparticle phosphate-based composites as vehicles for antimony delivery to macrophages: possible use in leishmaniasis

Nontoxic NPC containing Sb(v) boosts the infected macrophage recovery.

The overexpression of genes of thiol metabolism contribute to drug resistance in clinical isolates of visceral leishmaniasis (kala azar) in India

Background

Visceral leishmaniasis (VL), also called Kala Azar (KA) or black fever in India, claims around 20,000 lives every year. Chemotherapy remains one of the most important tools in the control of VL. Current chemotherapy for Kala Azar in India relies on a rather limited arsenal of drugs including sodium antimony gluconate and amphotericin B in addition to the very expensive drug miltefosine. Pentavalent antimonials have been used for more than half a century in the therapy of leishmaniasis as it is relatively safe and inexpensive, however, the spread of resistance to this drug is forcing clinicians in India to abandon this treatment. Consequently, improvement of antimonial chemotherapy has become a major challenging area of study by leishmaniacs worldwide. The alarming emergence of resistance to the commonly used antleishmanial drug, sodium antimony gluconate, in India, has led us to elucidate the resistance mechanism(s) in clinical isolates. Studies on laboratory mutants have shown that resistance to antimonials is highly dependent on thiol levels. The parasite evades cytotoxic effects of antimonial therapy by enhanced efflux of drug upon conjugation with thiols, through overexpressed membrane proteins belonging to the superfamily of ABC transporters.

Methods

We have carried out functional studies to determine the activity of the efflux pumps in antimonial resistant clinical isolates collected from disease endemic areas in India and also carried out molecular characterization of thiol levels in these parasites.

Results

Overexpression of the gene coding for γ glutamylcysteine synthetase was observed in these resistant clinical isolates thereby establishing that thiols represent the key determinants of antimonial resistance. The SbIII/thiol conjugates can be sequestered by ABC transporter multidrug resistance protein A (MRPA) into intracellular organelles or can be directly pumped out by an uncharacterized transporter.

Conclusions

Our studies investigating antimonial resistance in different L. donovani clinical isolates suggest that over functioning of MRP plays a role in generation of antimony resistance phenotype in some L. donovani clinical isolates.

Quinone-amino acid conjugates targeting Leishmania amino acid transporters

The aim of the present study was to investigate the feasibility of targeting Leishmania transporters via appropriately designed chemical probes. Leishmania donovani, the parasite that causes visceral leishmaniasis, is auxotrophic for arginine and lysine and has specific transporters (LdAAP3 and LdAAP7) to import these nutrients. Probes 1–15 were originated by conjugating cytotoxic quinone fragments (II and III) with amino acids (i.e. arginine and lysine) by means of an amide linkage. The toxicity of the synthesized conjugates against Leishmania extracellular (promastigotes) and intracellular (amastigotes) forms was investigated, as well their inhibition of the relevant amino acid transporters. We observed that some conjugates indeed displayed toxicity against the parasites; in particular, 7 was identified as the most potent derivative (at concentrations of 1 µg/mL and 2.5 µg/mL residual cell viability was reduced to 15% and 48% in promastigotes and amastigotes, respectively). Notably, 6, while retaining the cytotoxic activity of quinone II, displayed no toxicity against mammalian THP1 cells. Transport assays indicated that the novel conjugates inhibited transport activity of lysine, arginine and proline transporters. Furthermore, our analyses suggested that the toxic conjugates might be translocated by the transporters into the cells. The non-toxic probes that inhibited transport competed with the natural substrates for binding to the transporters without being translocated. Thus, it is likely that 6, by exploiting amino acid transporters, can selectively deliver its toxic effects to Leishmania cells. This work provides the first evidence that amino acid transporters of the human pathogen Leishmania might be modulated by small molecules, and warrants their further investigation from drug discovery and chemical biology perspectives.

A novel marker, ARM58, confers antimony resistance to Leishmania spp

Protozoa of the Leishmania genus cause a variety of disease forms that rank at the top of the list of neglected tropical diseases. Anti-leishmanial drugs based on pentavalent antimony have been the mainstay of therapy for over 60 years and resistance against them is increasingly encountered in the field. The biochemical basis for this is poorly understood and likely diverse. No stringent correlation between genetic markers and antimony resistance has so far been shown, prompting us to use a functional cloning approach to identify markers of resistance. Using gene libraries derived from drug-resistant and drug-sensitive Leishmania braziliensis clinical isolates in a functional cloning strategy, we repeatedly selected one gene locus located on chromosome 20 whose amplification confers increased antimony (III) resistance in vitro to an otherwise sensitive L. braziliensis clone. The gene responsible for the effect encodes a previously hypothetical protein that we dubbed LbrARM58. It comprises four repeats of a domain of unknown function, DUF1935, one of them harbouring a potential trans-membrane domain. The gene is so far unique to the Leishmania genus, while a structurally related gene without antimony resistance functionality is also found in Trypanosoma spp. Overexpression of LbrARM58 also confers antimony resistance to promastigotes and intracellular amastigotes of the related species Leishmania infantum, indicating a conserved function in Old World and New World Leishmania species. Our results also show that in spite of their RNAi system, L. braziliensis promastigotes can serve as acceptor cells for episomally propagated cosmid libraries, at least for the initial stages of functional cloning efforts.