Effects of methotrexate and other antifolates on the growth and dihydrofolate reductase activity of Leishmania promastigotes

Exploring the ability of dihydropyrimidine-5-carboxamide and 5-benzyl-2,4-diaminopyrimidine-based analogues for the selective inhibition of L. major dihydrofolate reductase

To tackle leishmaniasis, search for efficient therapeutic drug targets should be pursued. Dihydrofolate reductase (DHFR) is considered as a key target for the treatment of leishmaniasis. In current study, we are interested in the design and synthesis of selective antifolates targeting DHFR from L. major. We focused on the development of new antifolates based on 3,4-dihydropyrimidine-2-one and 5-(3,5-dimethoxybenzyl)pyrimidine-2,4-diamine motif. Structure activity relationship (SAR) studies were performed on 4-phenyl ring of dihydropyrimidine (26-30) template. While for 5-(3,5-dimethoxybenzyl)pyrimidine-2,4-diamine, the impact of different amino acids (valine, tryptophan, phenylalanine, and glutamic acid) and two carbon linkers were explored (52-59). The synthesized compounds were assayed against LmDHFR. Compound 59 with the IC₅₀ value of 0.10 μM appeared as potent inhibitors of L. major. Selectivity for parasite DHFR over human DHFR was also determined. Derivatives 55-59 demonstrated excellent selectivity for LmDHFR. Highest selectivity for LmDHFR was shown by compounds 56 (SI = 84.5) and 58 (SI = 87.5). Compounds Antileishmanial activity against L. major and L. donovani promastigotes was also performed. To explore the interaction pattern of the synthesized compounds with biological macromolecules, the docking studies were carried out against homology modelled LmDHFR and hDHFR targets.

Purified Splenic amastigotes of Leishmania donovani-Immunoproteomic approach for exploring Th1 stimulatory polyproteins

Visceral leishmaniasis (VL) represents one of the most challenging infectious diseases worldwide. The reason that once infected, patient develops immunity against Leishmania parasite has paved way to develop prophylactic vaccines against disease, but only some of these have moved ahead for clinical trials. Herein, the study to explore novel and potential vaccine candidates was extended to pathogenic form of parasite, that is, amastigote form which is less explored due to complexity of its purification process. Methods and results. Classical protocol of purification of splenic amastigotes was modified to obtain highly pure amastigotes which was confirmed by Western blotting in support with proteomics studies. Fractionation and sub-fractionation of purified splenic amastigotes revealed four sub-fractions, belonging to 97 to 68 kDa and 68 to 43 kDa ranges, which showed long-lasting protection with remarkable Th1-type cellular responses in hamsters vaccinated with these sub-fractions (LTT, NO, QRT-PCR). Further proteomics analysis, to identify and understand the precise nature and function of these protective protein sub-fractions, identified a total of 47 proteins including twenty-five hypothetical proteins/unknowns. Amastigote stage has potential Th1-stimulatory vaccine candidates, notably, among identified proteins, major were uncharacterized proteins/hypothetical proteins, which once characterized may serve as novel and potential vaccine candidates/drug targets.

Targeting Kinetoplastid and Apicomplexan Thymidylate Biosynthesis as an Antiprotozoal Strategy

Kinetoplastid and apicomplexan parasites comprise a group of protozoans responsible for human diseases, with a serious impact on human health and the socioeconomic growth of developing countries. Chemotherapy is the main option to control these pathogenic organisms and nucleotide metabolism is considered a promising area for the provision of antimicrobial therapeutic targets. Impairment of thymidylate (dTMP) biosynthesis severely diminishes the viability of parasitic protozoa and the absence of enzymatic activities specifically involved in the formation of dTMP (e.g. dUTPase, thymidylate synthase, dihydrofolate reductase or thymidine kinase) results in decreased deoxythymidine triphosphate (dTTP) levels and the so-called thymineless death. In this process, the ratio of deoxyuridine triphosphate (dUTP) versus dTTP in the cellular nucleotide pool has a crucial role. A high dUTP/dTTP ratio leads to uracil misincorporation into DNA, the activation of DNA repair pathways, DNA fragmentation and eventually cell death. The essential character of dTMP synthesis has stimulated interest in the identification and development of drugs that specifically block the biochemical steps involved in thymine nucleotide formation. Here, we review the available literature in relation to drug discovery studies targeting thymidylate biosynthesis in kinetoplastid (genera Trypanosoma and Leishmania) and apicomplexan (Plasmodium spp and Toxoplasma gondii) protozoans. The most relevant findings concerning novel inhibitory molecules with antiparasitic activity against these human pathogens are presented herein.

Pharmacoinformatic Study on the Selective Inhibition of the Protozoan Dihydrofolate Reductase Enzymes

Dihydrofolate reductase (DHFR) is an essential enzyme of the folate metabolic pathway in protozoa and it is a validated, potential drug target in many infectious diseases. Information about unique conserved residues of the DHFR enzyme is required to understand residual selectivity of the protozoan DHFR enzyme. The three dimensional crystal structures are not available for all the protozoan DHFR enzymes. Enzyme-substrate/inhibitor interaction information is required for the binding mode characterization in protozoan DHFR for selective inhibitor design. In this work, multiple sequence analysis was carried out in all the studied species. Homology models were built for protozoan DHFR enzymes, for which 3D structures are not available in PDB. The molecular docking and Prime-MMGBSA calculations of the natural substrate (dihydrofolate, DHF) and classical DHFR inhibitor (methotrexate, MTX) were performed in protozoan DHFR enzymes. Comparative sequence analysis showed that an overall sequence identity between the studied species ranging from 22.94 % (CfDHFR-BgDHFR) to 94.61 % (LdDHFR-LmDHFR). Interestingly, it was observed that most of the active site residues were conserved in all the cases and all the enzymes exhibit similar key binding interactions with DHF and MTX in molecular docking analysis, but there are a few key binding residues which differ in protozoan species that makes it suitable for target selectivity. This information can be used to design selective and potent protozoan DHFR enzyme inhibitors.

Characterization of glycolytic enzymes--rAldolase and rEnolase of Leishmania donovani, identified as Th1 stimulatory proteins, for their immunogenicity and immunoprophylactic efficacies against experimental visceral leishmaniasis

Th1 immune responses play an important role in controlling Visceral Leishmaniasis (VL) hence, Leishmania proteins stimulating T-cell responses in host, are thought to be good vaccine targets. Search of such antigens eliciting cellular responses in Peripheral blood mononuclear cells (PBMCs) from cured/exposed/Leishmania patients and hamsters led to the identification of two enzymes of glycolytic pathway in the soluble lysate of a clinical isolate of Leishmania donovani--Enolase (LdEno) and aldolase (LdAld) as potential Th1 stimulatory proteins. The present study deals with the molecular and immunological characterizations of LdEno and LdAld. The successfully cloned and purified recombinant proteins displayed strong ability to proliferate lymphocytes of cured hamsters' along with significant nitric-oxide production and generation of Th1-type cytokines (IFN-γ and IL-12) from stimulated PBMCs of cured/endemic VL patients. Assessment of their prophylactic potentials revealed ∼ 90% decrease in parasitic burden in rLdEno vaccinated hamsters against Leishmania challenge, strongly supported by an increase in mRNA expression levels of iNOS, IFN-γ, TNF-α and IL-12 transcripts along with extreme down-regulation of TGF-β, IL-4 and IL-10. However, animals vaccinated with rLdAld showed comparatively lesser prophylactic efficacy (∼ 65%) with inferior immunological response. Further, with a possible implication in vaccine design against VL, identification of potential T-cell epitopes of both the proteins was done using computational approach. Additionally, in-silico 3-D modelling of the proteins was done in order to explore the possibility of exploiting them as potential drug targets. The comparative molecular and immunological characterizations strongly suggest rLdEno as potential vaccine candidate against VL and supports the notion of its being effective T-cell stimulatory protein.

Leishmania donovani: immunostimulatory cellular responses of membrane and soluble protein fractions of splenic amastigotes in cured patient and hamsters

Visceral leishmaniasis (VL), caused by the intracellular parasite Leishmania donovani, L. chagasi and L. infantum is characterized by defective cell-mediated immunity (CMI) and is usually fatal if not treated properly. An estimated 350 million people worldwide are at risk of acquiring infection with Leishmania parasites with approximately 500,000 cases of VL being reported each year. In the absence of an efficient and cost-effective antileishmanial drug, development of an appropriate long-lasting vaccine against VL is the need of the day. In VL, the development of a CMI, capable of mounting Th1-type of immune responses, play an important role as it correlate with recovery from and resistance to disease. Resolution of infection results in lifelong immunity against the disease which indicates towards the feasibility of a vaccine against the disease. Most of the vaccination studies in Leishmaniasis have been focused on promastigote- an infective stage of parasite with less exploration of pathogenic amastigote form, due to the cumbersome process of its purified isolation. In the present study, we have isolated and purified splenic amastigotes of L. donovani, following the traditional protocol with slight modification. These were fractionated into five membranous and soluble subfractions each i.e MAF1-5 and SAF1-5 and were subjected for evaluation of their ability to induce cellular responses. Out of five sub-fractions from each of membrane and soluble, only four viz. MAF2, MAF3, SAF2 and SAF3 were observed to stimulate remarkable lymphoproliferative, IFN-γ, IL-12 responses and Nitric Oxide production, in Leishmania-infected cured/exposed patients and hamsters. Results suggest the presence of Th-1 type immunostimulatory molecules in these sub-fractions which may further be exploited for developing a successful subunit vaccine from the less explored pathogenic stage against VL.

Environmental risk and toxicology of human and veterinary waste pharmaceutical exposure to wild aquatic host-parasite relationships

Pollution of the aquatic environment by human and veterinary waste pharmaceuticals is an increasing area of concern but little is known about their ecotoxicological effects on wildlife. In particular the interactions between pharmaceuticals and natural stressors of aquatic communities remains to be elucidated. A common natural stressor of freshwater and marine organisms are protozoan and metazoan parasites, which can have significant effects on host physiology and population structure, especially under the influence of many traditional kinds of toxic pollutants. However, little is known about the effects of waste pharmaceuticals to host-parasite dynamics. In order to assess the risk waste pharmaceuticals pose to aquatic wildlife it has been suggested the use of toxicological data derived from mammals during the product development of pharmaceuticals may be useful for predicting toxic effects. An additional similar source of information is the extensive clinical studies undertaken with numerous classes of drugs against parasites of human and veterinary importance. These studies may form the basis of preliminary risk assessments to aquatic populations and their interactions with parasitic diseases in pharmaceutical-exposed habitats. The present article reviews the effects of the most common classes of pharmaceutical medicines to host-parasite relationships and assesses the risk they may pose to wild aquatic organisms. In addition the effects of pharmaceutical mixtures, the importance of sewage treatment, and the risk of developing resistant strains of parasites are also assessed.

Functional analysis of Leishmania major cyclophilin

A potent immunosuppressive drug cyclosporin A (CsA) is known to inhibit human cell infection by the pathogenic protozoan parasite Leishmania major both in vitro and in vivo. The proposed mechanism of action involves CsA binding to Leishmania major-expressed cyclophilin and subsequent down-regulation of signaling events necessary for establishing productive infection. Recently, we identified a ubiquitously expressed membrane protein, CD147, as a signaling receptor for extracellular cyclophilins in mammalian cells. Here we demonstrate that, while being enzymatically active, the Leishmania cyclophilin, unlike its human homologue, does not interact with CD147 on the cell surface of target cells. CD147 facilitates neither Leishmania binding nor infection. Primary structure and biochemical analyses revealed that the parasite's cyclophilin is defective in heparan binding, an event required for signaling interaction between CD147 and human cyclophilin. When the heparan-binding motif was reconstituted in Leishmania cyclophilin, it regained the CD147-dependent signaling activity. These results underscore a critical role of cyclophilin-heparan interactions in CD147-mediated signaling events and argue against the role of Leishmania cyclophilin in parasite binding to target cells.

Chemotherapy of leishmaniasis. Part VII: synthesis and bioevaluation of substituted terpenyl pyrimidines

Some novel 4-N-substituted terpenyl pyrimidines 5(a-d) and 7(a-g) have been synthesized using novel synthetic methods. The compounds were screened for in vivo antileishmanial screening. When compared to 4-thiomethoxy substituted pyrimidine 2 4-N-substituted terpenyl pyrimidines 5(a-d) and 7(a-g) were found inactive.

Chemotherapy of leishmaniasis. Part V: Synthesis and in vitro bioevaluation of novel pyridinone derivatives

Some novel 2-substituted pyridinone derivatives exemplified by 3 and 4 have been synthesized from 2-methyl-gamma-pyrone and screened towards in vitro antileishmanial activity profile. Some of the compounds such as 3a, 3b, 4i and 4j displayed good antileishmanial profile.

Synthesis and in vitro evaluation of potential anti-leishmanial targeted drugs of pyrimethamine

Pyrimethamine, an antimalarial drug, was found to be able to inhibit both enzymes (DHFR-TS and PTR1) of the leishmanial folate pathway, although this effect in vivo appears only in relatively high concentrations. To reach the parasites inside macrophage cells, where they are sheltered, targeted drugs of pyrimethamine, carboxymethyldextran-thiomannopyranoside-pyrimethamine (CMD-P), and succinyldextran-thiomannopyranoside-pyrimethamine (SD-P), were synthesized and assayed against L.(L.) amazonensis amastigotes. CMD-P has 2.43% and SD-P has 2.58% of pyrimethamine attached. At a CMD-P dose of 200 microg/mL (4.86 microg/mL pyrimethamine), the results were very promising, with a destruction of approximately 50% of the intracellular amastigotes, with no detectable toxicity to macrophage cells. SD-P in similar doses did not show good results, probably due to different patterns of drug release. These results open the possibility of treating leishmaniasis with a safe targeted drug of pyrimethamine released directly inside the macrophage cells, reducing the host systemic toxicity.

Inhibitors of dihydrofolate reductase in Leishmania and trypanosomes

The protozoan diseases leishmaniasis, Chagas' disease and African trypanosomiasis are major health problems in many countries, particularly developing countries, and there are few drugs available to treat these diseases. Dihydrofolate reductase (DHFR) inhibitors have been used successfully in the treatment of a number of other diseases such as cancer, malaria and bacterial infections; however they have not been used for the treatment of these diseases. This article summarises studies on leishmanial and trypanosomal DHFR inhibitor development and evaluation. Possible mechanisms of resistance to DHFR inhibitors are also discussed.

Pteridine salvage throughout the Leishmania infectious cycle: implications for antifolate chemotherapy

Protozoan parasites of the trypanosomatid genus Leishmania are pteridine auxotrophs, and have evolved an elaborate and versatile pteridine salvage network capable of accumulating and reducing pteridines. This includes biopterin and folate transporters (BT1 and FT1), pteridine reductase (PTR1), and dihydrofolate reductase-thymidylate synthase (DHFR-TS). Notably, PTR1 is a novel alternative pteridine reductase whose activity is resistant to inhibition by standard antifolates. In cultured promastigote parasites, PTR1 can function as a metabolic by-pass under conditions of DHFR inhibition and thus reduce the efficacy of chemotherapy. To test whether pteridine salvage occurred in the infectious stage of the parasite, we examined several pathogenic species of Leishmania and the disease-causing amastigote stage that resides within human macrophages. To accomplish this we developed a new sensitive HPLC-based assay for PTR1 activity. These studies established the existence of the pteridine salvage pathway throughout the infectious cycle of Leishmania, including amastigotes. In general, activities were not well correlated with RNA transcript levels, suggesting the occurrence of at least two different modes of post-transcriptional regulation. Thus, pteridine salvage by amastigotes may account for the clinical inefficacy of antifolates against leishmaniasis, and ultimately provide insights into how this may be overcome in the future.

Increased transport of pteridines compensates for mutations in the high affinity folate transporter and contributes to methotrexate resistance in the protozoan parasite Leishmania tarentolae

Functional cloning led to the isolation of a novel methotrexate (MTX) resistance gene in the protozoan parasite Leishmania. The gene corresponds to orfG, an open reading frame (ORF) of the LD1/CD1 genomic locus that is frequently amplified in several Leishmania stocks. A functional ORF G-green fluorescence protein fusion was localized to the plasma membrane. Transport studies indicated that ORF G is a high affinity biopterin transporter. ORF G also transports folic acid, with a lower affinity, but does not transport the drug analog MTX. Disruption of both alleles of orfG led to a mutant strain that became hypersensitive to MTX and had no measurable biopterin transport. Leishmania tarentolae MTX-resistant cells without their high affinity folate transporters have a rearranged orfG gene and increased orfG RNA levels. Overexpression of orfG leads to increased biopterin uptake and, in folate-rich medium, to increased folate uptake. MTX-resistant cells compensate for mutations in their high affinity folate/MTX transporter by overexpressing ORF G, which increases the uptake of pterins and selectively increases the uptake of folic acid, but not MTX.

Expression of the major surface glycoprotein of Leishmania, gp63, in wild-type and sinefungin-resistant promastigotes

In this study, we have surveyed gp63 expression in sinefungin-(SF)-resistant and wild-type Leishmania promastigotes. Documentation of gp63 expression in Leishmania promastigotes was carried out by Western blotting, purification of the protein and assessment of gp63 protease activity. We demonstrated a 3-4-fold and 1.5-2-fold increase of gp63 protein in SF-resistant Leishmania donovani and Leishmania tropica promastigotes compared to wild-type, respectively. Northern blot analysis showed that the increase in the amount of gp63 protein in SF-resistant compared to wild-type parasites was concomitant with an increase in gp63 mRNA. No extrachromosomal DNA was identified by alkaline lysis of isolated DNA samples and Southern blot analysis. Treatment of SF-resistant and wild-type L. donovani promastigotes with cycloheximide resulted in an increase of the steady state levels of gp63 mRNA in the SF-resistant parasites to approximately fivefold that of the wild type. After treating parasites with actinomycin D, estimated gp63 mRNA t1/2 in the wild type was 40 min and increased to 83 min in SF-resistant promastigotes. Therefore, the overexpression of gp63 may be mediated, at least in part, by post-transcriptional stabilization of a gp63 transcript by a protein factor. Down regulation of the latter factor may account for the observed increase in gp63 expression in SF-resistant promastigotes. Attempts to correlate gp63 expression with promastigote virulence suggested that the observed increase in gp63 expression did not result in a significant change in the virulence of SF-resistant compared to wild-type L. donovani promastigotes.

Neoglycoproteins as carriers for receptor-mediated drug targeting in the treatment of experimental visceral leishmaniasis

Methotrexate (MTX) coupled to mannosyl bovine serum albumin (BSA) was taken up efficiently through the mannosyl receptors present on macrophages. Binding experiments indicate that conjugation does not decrease the affinity of the neoglycoprotein for its cell surface receptor. The drug conjugate eliminated intracellular amastigotes of Leishmania donovani in mouse peritoneal macrophages about 100 times more efficiently than free drug on the basis of 50% inhibitory dose. Inhibitory effect of the conjugate was directly proportional to the density of sugar on the neoglycoprotein carrier. Colchicine and monensin, inhibitors of receptor-mediated endocytosis, can prevent the leishmanicidal effect of the conjugate. Antileishmanial effect of the conjugate can be competitively inhibited by mannose-BSA and mannan. In a murine model of experimental visceral leishmaniasis the drug conjugate reduced the spleen parasite burden by more than 85% in a 30-day model whereas the same concentration of free drug caused little effect. These results indicate that MTX-neoglycoprotein conjugate binds specifically to macrophages, and is internalized and degraded in lysosomes releasing the active drug to act on Leishmania parasites. These results also represent the potential for a general approach to intracellular targeting of clinical agents for macrophage-associated disorders.

Antileishmanial activities of 2,4-diaminoquinazoline putative dihydrofolate reductase inhibitors

2,4-Diaminoquinazoline analogs of folate were assessed as antileishmanial agents and as dihydrofolate reductase inhibitors. Against Leishmania major in human macrophages in vitro, two compounds with tertiary amines attached directly to the quinazoline ring were remarkably active. The 50% effective doses were in the picogram per milliliter range (12 to 91 pg/ml), and the in vitro therapeutic indices were approximately 10(5). These compounds were 1,000 times more active on an absolute basis and had a 100 times more favorable therapeutic index than any compound previously tested in this model. Antileishmanial activity was not correlated with activity against Leishmania mexicana promastigote reductase, which suggests that folate utilization in general, rather than reductase activity specifically, was being inhibited.

Selective inhibition of Leishmania dihydrofolate reductase and Leishmania growth by 5-benzyl-2,4-diaminopyrimidines

The classical anti-microbial antifolates trimethoprim, pyrimethamine, and cycloguanil are poor inhibitors of purified dihydrofolate reductase (DHFR) from Leishmania major. They show no selectivity for Leishmania DHFR relative to the human enzyme, and it is not surprising that they are ineffectual as anti-leishmanial agents. Several 5-(substituted-benzyl)-2,4-diaminopyrimidines have been screened as inhibitors for purified L. major and human DHFRs. These compounds inhibit Leishmania DHFR with I50 values ranging from 0.2 to 11 microM, and show about 5 to greater than 100-fold greater selectivity for the parasite DHFR than the human enzyme. These pyrimidine analogs are more potent inhibitors of Leishmania promastigote and amastigote growth than the classical anti-microbial antifolates, and serve as lead compounds for the development of new selective antileishmanial agents.