Synthesis and in vitro antikinetoplastid activity of polyamine–hydroxybenzotriazole conjugates

Thirteen new polyamine derivatives coupled to hydroxybenzotriazole have been synthesized and evaluated for their in vitro antikinetoplastid activity. Trypanosoma Trypanothione reductase (TryR) was envisioned as a potential target. Among all tested molecules, only one compound, a N³-spermidine–benzotriazole derivative, displayed relevant inhibitory activity on this enzyme but was not active on parasites. The corresponding Boc-protected spermidine–benzotriazole was however trypanocidal against Trypanosoma brucei gambiense with an IC₅₀ value of 1 μM and was completely devoid of cytotoxicity. On the intramacrophage amastigotes of Leishmania donovani, a N²-spermidine conjugate of this series, exhibited an interesting IC₅₀ value of 3 μM associated with both low cytotoxicity against axenic Leishmania donovani. These new compounds are promising leads for the development of antikinetoplastid agents and their targets have to be deciphered.     

Significantly Lower Anti-Leishmania IgG Responses in Sudanese versus Indian Visceral Leishmaniasis

Background

Visceral leishmaniasis (VL), a widely distributed systemic disease caused by infection with the Leishmania donovani complex (L. donovani and L. infantum), is almost always fatal if symptomatic and untreated. A rapid point-of-care diagnostic test for anti-Leishmania antibodies, the rK39-immunochromatographic test (rK39-ICT), has high sensitivity and specificity in South Asia but is less sensitive in East Africa. One of the underlying reasons may be continent-specific molecular diversity in the rK39 antigen within the L. donovani complex. However, a second reason may be differences in specific IgG anti-Leishmania levels in patients from different geographical regions, either due to variable antigenicity or immunological response.

Methodology/Principal Findings

We determined IgG titres of Indian and Sudanese VL patients against whole cell lysates of Indian and Sudanese L. donovani strains. Indian VL patients had significantly higher IgG titres against both L. donovani strains compared to Sudanese VL patients (p<0.0001). Mean reciprocal log₁₀ 50% end-point titres (1/log₁₀t₅₀) were i) 3.80 and 3.88 for Indian plasma and ii) 2.13 and 2.09 for Sudanese plasma against Indian and Sudanese antigen respectively (p<0.0001). Overall, the Indian VL patients therefore showed a 46.8–61.7 -fold higher mean ELISA titre than the Sudanese VL patients. The higher IgG titres occurred in children (<16 years old) and adults of either sex from India (mean 1/log₁₀t₅₀: 3.60–4.15) versus Sudan (mean 1/log₁₀t₅₀: 1.88–2.54). The greatest difference in IgG responses was between male Indian and Sudanese VL patients of ≥ 16 years old (mean 1/log₁₀t₅₀: 4.15 versus 1.99 = 144-fold (p<0.0001).

Conclusions/Significance

Anti-Leishmania IgG responses among VL patients in Sudan were significantly lower than in India; this may be due to chronic malnutrition with Zn²⁺ deficiency, or variable antigenicity and capacity to generate IgG responses to Leishmania antigens. Such differential anti-Leishmania IgG levels may contribute to lower sensitivity of the rK39-ICT in East Africa.     

Phlebotomus orientalis Sand Flies from Two Geographically Distant Ethiopian Localities: Biology,Genetic Analyses and Susceptibility to Leishmania donovani

Background

Phlebotomus orientalis Parrot (Diptera: Psychodidae) is the main vector of visceral leishmaniasis (VL) caused by Leishmania donovani in East Africa. Here we report on life cycle parameters and susceptibility to L. donovani of two P. orientalis colonies originating from different sites in Ethiopia: a non-endemic site in the lowlands - Melka Werer (MW), and an endemic focus of human VL in the highlands - Addis Zemen (AZ).

Methodology/Principal Findings

Marked differences in life-cycle parameters between the two colonies included distinct requirements for larval food and humidity during pupation. However, analyses using Random Amplified Polymorphic DNA (RAPD) PCR and DNA sequencing of cytB and COI mitochondrial genes did not reveal any genetic differences. F1 hybrids developed successfully with higher fecundity than the parental colonies. Susceptibility of P. orientalis to L. donovani was studied by experimental infections. Even the lowest infective dose tested (2×10³ per ml) was sufficient for successful establishment of L. donovani infections in about 50% of the P. orientalis females. Using higher infective doses, the infection rates were around 90% for both colonies. Leishmania development in P. orientalis was fast, the presence of metacyclic promastigotes in the thoracic midgut and the colonization of the stomodeal valve by haptomonads were recorded in most P. orientalis females by day five post-blood feeding.

Conclusions

Both MW and AZ colonies of P. orientalis were highly susceptible to Ethiopian L. donovani strains. As the average volume of blood-meals taken by P. orientalis females are about 0.7 µl, the infective dose at the lowest concentration was one or two L. donovani promastigotes per sand fly blood-meal. The development of L. donovani was similar in both P. orientalis colonies; hence, the absence of visceral leishmaniasis in non-endemic area Melka Werer cannot be attributed to different susceptibility of local P. orientalis populations to L. donovani.     

An in vitro study of apoptotic like death in Leishmania donovani promastigotes by withanolides

The aim of this study was to isolate and evaluate the withanolides in inducing apoptotic like death in Leishmania donovani in vitro. Withanolides were fractionated and isolated from the leaves of Withania somnifera and LC-MS/MS analysis of two fractions namely, F5 and F6 of ethanolic extracts, obtained through column chromatography with silica gel, was performed. The antileishmanial effect of withanolides on L. donovani promastigotes was assessed in vitro using PI dye exclusion test. The effect of withanolides on promastigote morphology was determined by scanning electron microscopy. To understand their mode of action against L. donovani, DNA fragmentation, quantification of parasites at sub G₀/G₁ phase, determination of phosphatidylserine externalization, measurement of reactive oxygen species (ROS) and mitochondrial membrane potential (Ψ_m) were done. Results showed that LC–MS/MS analysis confirmed the presence of withanolides in isolated fractions. Treatment with withanolides resulted in morphological alterations from spindle to round shape and loss of flagella/cell integrity in promastigotes. Moreover, it induced DNA nicks, cell cycle arrest at sub G₀/G₁ phase and externalization of phosphatidylserine in dose and time dependent manner via increase in ROS and decrease in Ψ_m. Results of this study indicate that withanolides induce apoptotic like death through the production of ROS from mitochondria and disruption of Ψ_m in promastigotes of L donovani.     

Leishmania donovani lacking the Golgi GDP-Man transporter LPG2 exhibit attenuated virulence in mammalian hosts

Surface phosophoglycans such as lipophosphoglycan (LPG) or proteophosphoglycan (PPG) and glycosylinositol phospholipids (GIPLs) modulate essential interactions between Leishmania and mammalian macrophages. Phosphoglycan synthesis depends on the Golgi GDP-mannose transporter encoded by LPG2. LPG2-null (lpg2⁻) Leishmania major cannot establish macrophage infections or induce acute pathology, whereas lpg2⁻Leishmania mexicana retain virulence. lpg2⁻Leishmaniadonovani has been reported to survive poorly in cultured macrophages but in vivo survival has not been explored. Herein we discovered that, similar to lpg2⁻L. major, lpg2⁻L. donovani promastigotes exhibited diminished virulence in mice, but persisted at consistently low levels. lpg2⁻L. donovani promastigotes could not establish infection in macrophages and could not transiently inhibit phagolysosomal fusion. Furthermore, lpg2⁻ promastigotes of L. major, L. donovani and L. mexicana were highly susceptible to complement-mediated lysis. We conclude that phosphoglycan assembly and expression mediated by L. donovani LPG2 are important for promastigote and amastigote virulence, unlike L. mexicana but similar to L. major.     

Synthesis and biological evaluation of a novel series of aryl S,N-ketene acetals as antileishmanial agents

A series of aryl S,N-ketene acetals 7(a–f) was synthesized and evaluated for their in vitro and in vivo antileishmanial activity against Leishmania donovani. All the 6 compounds exhibited significant in vitro activity against intracellular amastigotes of L. donovani with IC₅₀ values ranging from 1.2 to 3.5 μM and were found promising as compared with reference drugs, sodium stibogluconate (SSG) and paromomycin. On the basis of good selectivity indices (SI), they were further tested for their in vivo potential against L. donovani/hamster model. Two compounds 7a and 7b showed significant inhibition of parasite multiplication, 72% and 83%, respectively. These compounds were comparable with SSG and superior to paromomycin. Preliminary in vitro metabolic investigations were also performed to assess the metabolic stability and in vitro hepatic intrinsic clearance (Cl_int) of compound 7b in hamster liver microsomes.     

Chemotherapy of leishmaniasis. Part IX: Synthesis and bioevaluation of aryl substituted ketene dithioacetals as antileishmanial agents

A new series of aryl substituted ketene dithioacetals 6a–h was synthesized and evaluated for their in vitro and in vivo antileishmanial activity against Leishmania donovani. Two compounds exhibited significant in vitro activity against intracellular amastigotes of L. donovani with IC₅₀ values 3.56 and 5.12 μM and were found promising as compared with reference drug, miltefosine. On the basis of good Selectivity Indices (S.I.), they were further tested for their in vivo response against L. donovani/hamster model and showed significant inhibition of parasite multiplication 78% and 83%, respectively. These compounds were better than the existing antileishmanials in respect to IC₅₀ and SI values, but were less active than miltefosine in vivo.     

Leishmania donovani Attachment Stimulates PKC-Mediated Oxidative Events in Bone Marrow-Derived Macrophages

We investigated activation signaling events in bone marrow-derived macrophages after infection with Leishmania donovani, an intracellular parasite of macrophages. Leishmania donovani infection caused a general suppression of activation parameters like O₂^- and NO production. However, conditions which allow parasite attachment and prevent entry resulted in triggering of O₂^- and NO production and stimulation of O₂ consumption. Optimal NO and O₂^- production occurred when bone marrow-derived macrophages and Leishmania ratio was 1:100. The activation signal for O₂^- production was initiated 15 min after parasite attachment, whereas augmentation of NO production started 6 h after attachment. Activation of O₂^- and NO generation by L. donovani attachment was inhibited by staurosporine as well as by prolonged treatment of phorbol myristate acetate suggesting a protein kinase C-dependent mechanism. Translocation studies showed that protein kinase C activity in cell membrane fraction rapidly and transiently increased following parasite attachment. No such protein kinase C translocation event occurred in L. donovani infected bone marrow-derived macrophages. Phorbol myristate acetate was found to stimulate membrane translocation of protein kinase C in parasite attached cells whereas it was impaired in infected cells. However, both attachment and infection induced a similar shift of phorbol receptors from cytosolic to membrane fraction indicating that in infected cells the translocation of protein kinase C protein was not impaired but the activity of the membrane associated enzyme was somehow inhibited. These results suggest that although internalization of intracellular parasites like L. donovani caused inhibition of nitrite and superoxide release, mere attachment on macrophage surface resulted in an activation of protein kinase C-mediated downstream oxidative events.     

Immunomodulation of dual specificity phosphatase 4 during visceral leishmaniasis

DUSP4, an inducible protein has a substrate specificity toward ERK1/2, a component of MAP kinase which is enhanced during Leishmania infection. The DUSP4^?/? mice show increased susceptibility towards the infection caused by Toxoplasma gondii and Leishmania mexicana. These observations emphatically established the fact that unlike DUSP1, DUSP4 has host protective role. In our study, it has been Leishmania donovani, the causative agent of visceral leishmaniasis (VL) significantly reduced the expression of DUSP4 during infection. In order to find out the host protective role of DUSP4 in macrophages during VL, we silenced DUSP4 prior to infection and the parasite number within macrophage was counted. Under DUSP4 knock-down condition, phosphorylation of p38 MAPK and generation of pro-inflammatory response like IL-12, TNF-α, and iNOS was decreased significantly. Silencing DUSP4 promoted the phosphorylation of ERK1/2 and the generation of anti-inflammatory response like- IL-10, TGF-β with increased Arginase-1 and Cox-2 activity. Glycyrrhizic Acid (GA), an immunomodulator, already known to suppress L. donovani infection, found to up-regulate DUSP4 expression during L. donovani infection. On the other hand, GA failed to increase Th1 cytokine production and decrease Th2 response during DUSP4 knock-down condition suggesting the key role of DUSP4 while providing protection during L. donovani infection.     

Synthesis and anti-leishmanial activity of heterocyclic betulin derivatives

Betulin, a naturally occurring abundant triterpene is converted in four steps to 3,28-di-O-acetyllupa-12,18-diene. When various 4-substituted urazoles were oxidized to the corresponding urazines with iodobenzene diacetate in the presence of 3,28-di-O-acetyllupa-12,18-diene, new heterocyclic betulin derivatives were produced. These betulin derivatives were examined in a microplate assay at 50 μM for their ability to inhibit the growth of Leishmania donovani axenic amastigotes, a species that causes the fatal visceral leishmaniasis. GI₅₀ (concentration for 50% growth inhibition) values of the most effective compounds were determined and their cytotoxicity on the human macrophage cell line THP-1 evaluated. The anti-leishmanial activity on L. donovani amastigotes growing in macrophages was also examined. The heterocycloadduct between 3,28-di-O-acetyllupa-12,18-diene and 4-methylurazine was the most effective derivative with an GI₅₀ = 8.9 μM against L. donovani amastigotes.     

Biological evaluation and structure activity relationship of 9-methyl-1-phenyl-9H-pyrido[3,4-b]indole derivatives as anti-leishmanial agents

A series of piperazinyl-β-carboline-3-carboxamide derivatives were designed through a molecular hybridization approach. Designed analogues were synthesized, characterized and evaluated for anti-leishmanial activity against Leishmania infantum and Leishmania donovani. In L. infantum inhibition assay, compounds 7d, 7g and 7c displayed potent inhibition of promastigotes (EC₅₀ 1.59, 1.47 and 3.73 µM respectively) and amastigotes (EC₅₀ 1.4, 1.9 and 2.6 µM respectively). SAR studies revealed that, para substitution of methoxy, chloro groups and methyl group on ortho position favored anti-leishmanial activity against L. infantum. Among these analogues 7d, 7h, 7n and 7g exhibited potent inhibition against L. donovani promastigotes (EC₅₀ 0.91, 4.0, 4.57 and 5.02 µM respectively), axenic amastigotes (EC₅₀ 0.9, 3.5, 2.2 and 3.8 µM respectively) and intracellular amastigotes (EC₅₀ 1.3, 7.8, 5.6 and 6.3 µM respectively). SAR studies suggested that, para substitution of methoxy group, para and meta substitution of chloro groups and benzyl replacement recommended for significant anti-leishmanial against L. donovani.     

Up regulation of A2B adenosine receptor on monocytes are crucially required for immune pathogenicity in Indian patients exposed to Leishmania donovani

Adenosine, an endogenous purine nucleoside is one such extracellular signalling molecule whose role in regulation of anti-inflammatory cytokines and immune pathogenicity in visceral leishmaniasis is not fully understood. Here, we investigated the relationship between Leishmania donovani infection and expression of A_2B receptor on monocytes in VL patients in their pre and post treatment stage. We also investigated the molecular mechanisms influencing the interaction between immunopathogenicity and infection by exposing Leishmania donovani pulsed macrophages to Adenosine. A direct correlation of up-regulated A_2B expression on monocytes with increased parasite load was also observed. Our results also suggested that A_2B receptor activation is critically required for the stimulatory effect of adenosine on IL-10 production and suppression of nitric oxide release. The stimulatory effect of adenosine on Leishmania donovani induced IL-10 production required ERK1/2 activation and is p-38 MAPK independent.     

Chemotherapy of leishmaniasis. Part XII: Design,synthesis and bioevaluation of novel triazole integrated phenyl heteroterpenoids as antileishmanial agents

A novel series of triazole integrated phenyl heteroterpenoids have been synthesized and screened for their in vitro activity against intracellular amastigote form of Leishmania donovani. Among all tested compounds, compound 3a was found to be the most active with IC₅₀ 6.4 μM and better selectivity index (SI) 18 as compared to reference drugs, miltefosine and miconazole. When evaluated in vivo in L. donovani/hamster model, 3a has exhibited 79 ± 11% inhibition of parasite multiplication at 50 mg kg^?1 × 5 days on day 7 post treatment.     

Anaerobic formation of succinate from glucose and bicarbonate in resting cells of Leishmania donovani

The cell suspension of Leishmania donovani incorporates ¹⁴CO₂ resulting in the formation of [¹⁴C]-succinic acid under anaerobic conditions. The results showed that the [¹⁴C]-succinate formation from [1-¹⁴C]-glucose is much greater than that from [6-¹⁴C]-glucose. [¹⁴C-pyruvate] takes part in the production of succinic acid under anaerobic conditions without decarboxylation. The anaerobic formation of succinate appears to involve the production of malate, which is then converted to succinate via the reduction of fumarate by the reversal of the tricarboxylic acid cycle. Evidence indicated that the active species in this carboxylation reaction was CO₂ although HCO₃ was active to some extent.     

A novel multilocus sequence typing scheme identifying genetic diversity amongst Leishmania donovani isolates from a genetically homogeneous population in the Indian subcontinent

In the Indian subcontinent, infection with Leishmania donovani can cause fatal visceral leishmaniasis. Genetic variation in L. donovani is believed to occur rapidly from environmental changes and through selective drug pressures, thereby allowing continued disease occurrence in this region. All previous molecular markers that are commonly in use multilocus microsatellite typing and multilocus sequence typing, were monomorphic in L. donovani originating from the Indian subcontinent (with only a few exceptions) and hence are not suitable for this region. An multilocus sequence typing scheme consisting of a new set of seven housekeeping genes was developed in this study, based on recent findings from whole genome sequencing data. This new scheme was used to assess the genetic diversity amongst 22 autochthonous L. donovani isolates from Bangladesh. Nineteen additional isolates of the L. donovani complex (including sequences of L. donovani reference strain BPK282A1) from other countries were included for comparison. By using restriction fragment length polymorphism of the internal transcribed spacer 1 region (ITS1-RFLP) and ITS1 sequencing, all Bangladeshi isolates were confirmed to be L. donovani. Population genetic analyses of 41 isolates using the seven new MLST loci clearly separated L. donovani from Leishmania infantum. With this multilocus sequence typing scheme, seven genotypes were identified amongst Bangladeshi L. donovani isolates, and these isolates were found to be phylogenetically different compared with those from India, Nepal, Iraq and Africa. This novel multilocus sequence typing approach can detect intra- and inter-species variations within the L. donovani complex, but most importantly these molecular markers can be applied to resolve the phylogenetically very homogeneous L. donovani strains from the Indian subcontinent. Four of these markers were found suitable to differentiate strains originating from Bangladesh, with marker A2P being the most discriminative one.     

High resolution melting based method for rapid discriminatory diagnosis of co-infecting Leptomonas seymouri in Leishmania donovani-induced leishmaniasis

Leishmania donovani, a protozoan parasite of family Trypanosomatidae, causes fatal visceral leishmaniasis (VL) in the Indian subcontinent and Africa and cutaneous leishmaniasis (CL) in Sri Lanka. Another member of Trypanosomatidae, Leptomonas seymouri, resembling Leishmania was discovered recently to co-exist with L. donovani in the clinical samples from India and Sri Lanka and therefore, interfere with its investigations. We earlier described a method for selective elimination of such co-existing L. seymouri from clinical samples of VL exploiting the differential growth of the parasites at 37 °C in vitro. Here, we explored ways for a rapid discriminatory diagnosis using high resolution melting (HRM) curves to detect co-occurring L. seymouri with L. donovani in clinical samples. Initial attempt with kDNA-minicircle (mitochondrial DNA) based HRM did not display different T_m values between L. donovani and L. seymouri. Surprisingly, all of their minicircle sequences co-existed in similar clades in the dendrogram analysis, although the kDNA sequences are known for its species and strain specific variations among the Trypanosomatids. However, an HRM analysis that targets the HSP70 gene successfully recognized the presence of L. seymouri in the clinical isolates. This discovery will facilitate rapid diagnosis of L. seymouri and further investigations in to this elusive organism, including the clinico-pathological implications of its co-existence with L. donovani in patients.     

Using a Non-Image-Based Medium-Throughput Assay for Screening Compounds Targeting N-myristoylation in Intracellular Leishmania Amastigotes

We have refined a medium-throughput assay to screen hit compounds for activity against N-myristoylation in intracellular amastigotes of Leishmania donovani. Using clinically-relevant stages of wild type parasites and an Alamar blue-based detection method, parasite survival following drug treatment of infected macrophages is monitored after macrophage lysis and transformation of freed amastigotes into replicative extracellular promastigotes. The latter transformation step is essential to amplify the signal for determination of parasite burden, a factor dependent on equivalent proliferation rate between samples. Validation of the assay has been achieved using the anti-leishmanial gold standard drugs, amphotericin B and miltefosine, with EC₅₀ values correlating well with published values. This assay has been used, in parallel with enzyme activity data and direct assay on isolated extracellular amastigotes, to test lead-like and hit-like inhibitors of Leishmania N-myristoyl transferase (NMT). These were derived both from validated in vivo inhibitors of Trypanosoma brucei NMT and a recent high-throughput screen against L. donovani NMT. Despite being a potent inhibitor of L. donovani NMT, the activity of the lead T. brucei NMT inhibitor (DDD85646) against L. donovani amastigotes is relatively poor. Encouragingly, analogues of DDD85646 show improved translation of enzyme to cellular activity. In testing the high-throughput L. donovani hits, we observed macrophage cytotoxicity with compounds from two of the four NMT-selective series identified, while all four series displayed low enzyme to cellular translation, also seen here with the T. brucei NMT inhibitors. Improvements in potency and physicochemical properties will be required to deliver attractive lead-like Leishmania NMT inhibitors.     

Adenylosuccinate Synthetase and Adenylosuccinate Lyase Deficiencies Trigger Growth and Infectivity Deficits in Leishmania donovani

Leishmania are auxotrophic for purines, and consequently purine acquisition from the host is a requisite nutritional function for the parasite. Both adenylosuccinate synthetase (ADSS) and adenylosuccinate lyase (ASL) have been identified as vital components of purine salvage in Leishmania donovani, and therefore Δadss and Δasl null mutants were constructed to test this hypothesis. Unlike wild type L. donovani, Δadss and Δasl parasites in culture exhibited a profoundly restricted growth phenotype in which the only permissive growth conditions were a 6-aminopurine source in the presence of 2′-deoxycoformycin, an inhibitor of adenine aminohydrolase activity. Although both knock-outs showed a diminished capacity to infect murine peritoneal macrophages, only the Δasl null mutant was profoundly incapacitated in its ability to infect mice. The enormous discrepancy in parasite loads observed in livers and spleens from mice infected with either Δadss or Δasl parasites can be explained by selective accumulation of adenylosuccinate in the Δasl knock-out and consequent starvation for guanylate nucleotides. Genetic complementation of a Δasl lesion in Escherichia coli implied that the L. donovani ASL could also recognize 5-aminoimidazole-(N-succinylocarboxamide) ribotide as a substrate, and purified recombinant ASL displayed an apparent K_m of ∼24 μm for adenylosuccinate. Unlike many components of the purine salvage pathway of L. donovani, both ASL and ADSS are cytosolic enzymes. Overall, these data underscore the paramount importance of ASL to purine salvage by both life cycle stages of L. donovani and authenticate ASL as a potential drug target in Leishmania.     

Chemotherapy of leishmaniasis part X: Synthesis and bioevaluation of novel terpenyl heterocycles

Some novel α and β ionone based chalcones and their dihydropyrazolidines/pyrazolidines have been synthesized and evaluated for their in vitro and in vivo antileishmanial activities against Leishmania donovani. Amongest all, one compound (4d) exhibited significant in vitro activity against intracellular amastigotes of Leishmania donovani with IC₅₀ values of 7.49 μM and was found promising as compared to reference drug, miltefosine. On the basis of good Selectivity Index (S.I.), the compound was further tested for its in vivo response against Leishmania donovani/hamster model and has shown significant inhibition of parasite multiplication (81%). The present study has helped us in identifying a new lead that could be exploited as a potential antileishmanial agent.