Comparative folate metabolism in humans and malaria parasites (part I): pointers for malaria treatment from cancer chemotherapy

CRISPR-based functional profiling of the Toxoplasma gondii genome during acute murine infection

Examining host-pathogen interactions in animals can capture aspects of infection that are obscured in cell culture. Using CRISPR-based screens, we functionally profile the entire genome of the apicomplexan parasite Toxoplasma gondii during murine infection. Barcoded gRNAs enabled bottleneck detection and mapping of population structures within parasite lineages. Over 300 genes with previously unknown roles in infection were found to modulate parasite fitness in mice. Candidates span multiple axes of host-parasite interaction. Rhoptry Apical Surface Protein 1 was characterized as a mediator of host-cell tropism that facilitates repeated invasion attempts. GTP cyclohydrolase I was also required for fitness in mice and druggable through a repurposed compound, 2,4-diamino-6-hydroxypyrimidine. This compound synergized with pyrimethamine against T. gondii and malaria-causing Plasmodium falciparum parasites. This work represents a complete survey of an apicomplexan genome during infection of an animal host and points to novel interfaces of host-parasite interaction.

Identification of Innovative Folate Inhibitors Leveraging the Amino Dihydrotriazine Motif from Cycloguanil for Their Potential as Anti-Trypanosoma brucei Agents

Folate enzymes, namely, dihydrofolate reductase (DHFR) and pteridine reductase (PTR1) are acknowledged targets for the development of antiparasitic agents against Trypanosomiasis and Leishmaniasis. Based on the amino dihydrotriazine motif of the drug Cycloguanil (Cyc), a known inhibitor of both folate enzymes, we have identified two novel series of inhibitors, the 2-amino triazino benzimidazoles (1) and 2-guanidino benzimidazoles (2), as their open ring analogues. Enzymatic screening was carried out against PTR1, DHFR, and thymidylate synthase (TS). The crystal structures of TbDHFR and TbPTR1 in complex with selected compounds experienced in both cases a substrate-like binding mode and allowed the rationalization of the main chemical features supporting the inhibitor ability to target folate enzymes. Biological evaluation of both series was performed against T. brucei and L. infantum and the toxicity against THP-1 human macrophages. Notably, the 5,6-dimethyl-2-guanidinobenzimidazole 2g resulted to be the most potent (Ki = 9 nM) and highly selective TbDHFR inhibitor, 6000-fold over TbPTR1 and 394-fold over hDHFR. The 5,6-dimethyl tricyclic analogue 1g, despite showing a lower potency and selectivity profile than 2g, shared a comparable antiparasitic activity against T. brucei in the low micromolar domain. The dichloro-substituted 2-guanidino benzimidazoles 2c and 2d revealed their potent and broad-spectrum antitrypanosomatid activity affecting the growth of T. brucei and L. infantum parasites. Therefore, both chemotypes could represent promising templates that could be valorized for further drug development.

Tetrahydrobiopterin from cyanide-degrading bacterium Bacillus pumilus strain SVD06 induces apoptosis in human lung adenocarcinoma cell (A549)

Tetrahydrobiopterin (BH4) is an essential biological cofactor and a derivative of pterin which is considered potent anticancer agents. In continuation of our previous study on the identification of BH4 from cyanide-degrading Bacillus pumilus, the present study focuses on evaluating the anticancer properties of BH4 on A549, a human lung adenocarcinoma. Anticancer activity analysis shows that BH4 inhibited A549 cell growth after 24 h of incubation with 0.02 mg/mL. In acridine orange/ethidium bromide staining, BH4-treated A549 cells showed apoptotic morphology. BH4 treatment caused cell cycle arrest at G0/G1 phase compared to control cells. BH4 augmented p53 expression in alveolar cancer cells by downregulating MDM2 levels. There was downregulation of casp-3 and upregulation of iNOS gene in BH4-treated A549 cells. Further, docking studies indicated that BH4 had significant interactions with the above proteins affirming the apoptosis mechanism. Thus, BH4 could be considered a potential anticancer drug.

Functional profiling of the Toxoplasma genome during acute mouse infection

Within a host, pathogens encounter a diverse and changing landscape of cell types, nutrients, and immune responses. Examining host-pathogen interactions in animal models can therefore reveal aspects of infection absent from cell culture. We use CRISPR-based screens to functionally profile the entire genome of the model apicomplexan parasite Toxoplasma gondii during mouse infection. Barcoded gRNAs were used to track mutant parasite lineages, enabling detection of bottlenecks and mapping of population structures. We uncovered over 300 genes that modulate parasite fitness in mice with previously unknown roles in infection. These candidates span multiple axes of host-parasite interaction, including determinants of tropism, host organelle remodeling, and metabolic rewiring. We mechanistically characterized three novel candidates, including GTP cyclohydrolase I, against which a small-molecule inhibitor could be repurposed as an antiparasitic compound. This compound exhibited antiparasitic activity against T. gondii and Plasmodium falciparum, the most lethal agent of malaria. Taken together, we present the first complete survey of an apicomplexan genome during infection of an animal host, and point to novel interfaces of host-parasite interaction that may offer new avenues for treatment.

Design and development of novel N ‐(4‐aminobenzoyl)‐ l ‐glutamic acid conjugated 1,3,5‐triazine derivatives as Pf ‐DHFR inhibitor: An in‐silico and in‐vitro study

In the present work, a library of 120 compounds was prepared using various aliphatic and aromatic amines. Finally, 10 compounds were selected through in silico screening carrying 4-aminobenzoyl-l-glutamic acid and 1,3,5-triazine moiety. The docking results of compounds 4d16 and 4d38 revealed higher binding interaction with amino acids Asp54 (-537.96 kcal/mol) and Asp54, Phe116 (-618.22 kcal/mol) against wild (1J3I) and quadruple mutant (1J3K) type of Pf-DHFR inhibitors and were comparable to standard WR99210. These compounds were developed by facile and microwave-assisted synthesis via nucleophilic substitution reaction and characterized by different spectroscopic methods. In vitro antimalarial assay results also suggested that these two compounds were having higher antimalarial activity against chloroquine-sensitive (3D7) and chloroquine-resistant (Dd2) strain out of the ten synthesized compounds with IC₅₀ 13.25 μM and 14.72 μM, respectively. These hybrid scaffolds might be useful in the lead discovery of a new class of Pf-DHFR inhibitors.

Molecular docking and antimalarial evaluation of novel N-(4-aminobenzoyl)-l-glutamic acid conjugated 1,3,5-triazine derivatives as Pf-DHFR inhibitors

Malaria has been a source of concern for humans for millennia; therefore in the present study we have utilized in-silico approach to generate diverse anti-malarial hit. Towards this, Molinspiration cheminformatics and Biovia Discovery Studio (DS) 2020 were used to conduct molecular modelling studies on 120 designed compounds. Furthermore, the TOPKAT module was used to evaluate the toxicity of the screened compounds. The CDOCKER docking technology was used to investigate protein-ligand docking against the Pf-DHFR-TS protein (PDB ID: 1J3I and 1J3K). These compounds were synthesized using a conventional and microwave-assisted nucleophilic substitution reaction, and they were characterized using a variety of physicochemical and spectroscopic methods. Among the ten compounds tested, Df3 had the highest antimalarial activity against the chloroquine-resistant (Dd2) strain, with an IC50 value of 9.54 μg mL-1 and further demonstrate, molecular dynamics (MD) simulation studies and estimation of MM-PBSA-based free binding energies of docked complexes with 1J3I and 1J3K were carried out. The discovery of a novel class of Pf-DHFR inhibitors can be accomplished using this hybrid scaffold.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-022-03400-2.

Characterization of the dynamics of Plasmodium falciparum deoxynucleotide-triphosphate pool in a stage-specific manner

Understanding and characterizing the molecular background of the maintenance of genomic integrity might be a major factor in comprehending the exceptional ability of the malaria parasite, Plasmodium falciparum to adapt at a fast pace to antimalarials. A balanced nucleotide pool is an essential factor for high-fidelity replication. The lack of detailed studies on deoxynucleotide-triphosphate (dNTP) pools in various intraerythrocytic stages of Plasmodium falciparum motivated our present study. Here, we focused on the building blocks of DNA and utilized an EvaGreen-based dNTP incorporation assay to successfully measure the temporal dynamics of dNTPs in every intraerythrocytic stage and in drug-treated trophozoites. Our findings show that the ratio of dNTPs in the ring-stage parasites significantly differs from the more mature trophozoite and schizont stages. We were also able to detect dGTP levels that have never been shown before and found it to be the least abundant dNTP in all stages. Treatment with WR99210, a TS-DHFR inhibitor drug, affected not only dTTP, but also dGTP levels, despite its presumed selective action on pyrimidine biosynthesis. Results from our studies might assist in a better understanding of genome integrity mechanisms and may potentially lead to novel drug related aspects involving purine and pyrimidine metabolic targets.

Microwave synthesis and antimalarial screening of novel 4-amino benzoic acid (PABA)-substituted pyrimidine derivatives as Plasmodium falciparum dihydrofolate reductase inhibitors

Antimalarial drug resistance is a major threat due to the emerging resistance to all the available drugs in the market. In an approach to develop alternative drugs, a novel class of Pf-DHFR inhibitors was developed using pyrimidine as the core nucleus and substituting the 4- and 6- positions with amines and 4-amino benzoic acid (PABA) to avoid the problem of drug resistance. The resultant compounds 3(a-j) after primary in silico screening and filtering were synthesized using microwave efficiently in high yield and reduced time period compared to conventional synthesis. The antimalarial assay was performed in vitro, against chloroquine-sensitive (3D7) and chloroquine-resistant (Dd2) strains of Plasmodium falciparum using chloroquine as a reference standard. The IC₅₀ values were in the range of 5.26-106.76 µg/ml for 3D7 and in Dd2 the value ranges from 4.71 to 112.98 µg/ml. Compounds 3d, 3e, 3f and 3h showed significant antimalarial activity against both the strains of P. falciparum with no cytotoxicity against fibroblast cell line and 3f was found to be the most potent among them. The hemolysis assay of all the compounds in fresh erythrocytes showed insignificant hemolysis below 5% at a higher dose level. Hence, the present study suggests the possible utility of PABA-substituted pyrimidine scaffold for further development of new Pf-DHFR inhibitors.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-022-03236-w.

Dihydrofolate reductase, thymidylate synthase, and serine hydroxy methyltransferase: successful targets against some infectious diseases

Parasitic diseases have a serious impact on the world in terms of health and economics and are responsible for worldwide mortality and morbidity. The present review features the hybrid targeting involving three main enzymes for the treatment of different parasitic diseases. The enzymes Dihydrofolate reductase, thymidylate synthase, and Serine hydroxy methyltransferase play an essential role in the folate pathway. The present review focuses on these enzymes, which can be targeted against several diseases. It shed light on the past, present, and future of these targets, and it can be assessed that these targets can play a significant role against several infectious diseases. For combating viral and protozoal infectious diseases, these targets in combination should be addressed.

Metabolomic Analysis of Diverse Mice Reveals Hepatic Arginase-1 as Source of Plasma Arginase in Plasmodium chabaudi Infection

Infections disrupt host metabolism, but the factors that dictate the nature and magnitude of metabolic change are incompletely characterized. To determine how host metabolism changes in relation to disease severity in murine malaria, we performed plasma metabolomics on eight Plasmodium chabaudi-infected mouse strains with diverse disease phenotypes. We identified plasma metabolic biomarkers for both the nature and severity of different malarial pathologies. A subset of metabolic changes, including plasma arginine depletion, match the plasma metabolomes of human malaria patients, suggesting new connections between pathology and metabolism in human malaria. In our malarial mice, liver damage, which releases hepatic arginase-1 (Arg1) into circulation, correlated with plasma arginine depletion. We confirmed that hepatic Arg1 was the primary source of increased plasma arginase activity in our model, which motivates further investigation of liver damage in human malaria patients. More broadly, our approach shows how leveraging phenotypic diversity can identify and validate relationships between metabolism and the pathophysiology of infectious disease. IMPORTANCE Malaria is a severe and sometimes fatal infectious disease endemic to tropical and subtropical regions. Effective vaccines against malaria-causing Plasmodium parasites remain elusive, and malaria treatments often fail to prevent severe disease. Small molecules that target host metabolism have recently emerged as candidates for therapeutics in malaria and other diseases. However, our limited understanding of how metabolites affect pathophysiology limits our ability to develop new metabolite therapies. By providing a rich data set of metabolite-pathology correlations and by validating one of those correlations, our work is an important step toward harnessing metabolism to mitigate disease. Specifically, we showed that liver damage in P. chabaudi-infected mice releases hepatic arginase-1 into circulation, where it may deplete plasma arginine, a candidate malaria therapeutic that mitigates vascular stress. Our data suggest that liver damage may confound efforts to increase levels of arginine in human malaria patients.

Microwave-assisted synthesis of hybrid PABA-1,3,5-triazine derivatives as an antimalarial agent

The present manuscript deals with the development of novel p-aminobenzoic acid (PABA) associated 1,3,5-triazine derivatives as antimalarial agents. The molecules were developed via microwave-assisted synthesis and structures of compounds were ascertained via numerous analytical and spectroscopic techniques. The synthesized compounds were also subjected to ADMET analysis. In a docking analysis, the title compounds showed high and diverse binding affinities towards wild (-162.45 to -369.38 kcal/mol) and quadruple mutant (-165.36 to -209.47 kcal/mol) Pf-DHFR-TS via interacting with Phe58, Arg59, Ser111, Ile112, Phe116. The in vitro antimalarial activity suggested that compounds 4e, 4b, and 4h showed IC₅₀ ranging from 4.18 to 8.66 μg/ml against the chloroquine-sensitive (3D7) strain of Plasmodium falciparum. Moreover, compounds 4g, 4b, 4e, and 4c showed IC₅₀ ranging from 8.12 to 12.09 μg/ml against chloroquine-resistant (Dd2) strain. In conclusion, our study demonstrated the development of hybrid PABA substituted 1,3,5-triazines as a novel class of Pf-DHFR inhibitor for antimalarial drug discovery.

AMBER force field parameters for the Zn (II) ions of the tunneling-fold enzymes GTP cyclohydrolase I and 6-pyruvoyl tetrahydropterin synthase

The folate biosynthesis pathway is an essential pathway for cell growth and survival. Folate derivatives serve as a source of the one-carbon units in several intracellular metabolic reactions. Rapidly dividing cells rely heavily on the availability of folate derivatives for their proliferation. As a result, drugs targeting this pathway have shown to be effective against tumor cells and pathogens, but drug resistance against the available antifolate drugs emerged quickly. Therefore, there is a need to develop new treatment strategies and identify alternative metabolic targets. The two de novo folate biosynthesis pathway enzymes, GTP cyclohydrolase I (GCH1) and 6-pyruvoyl tetrahydropterin synthase (PTPS), can provide an alternative strategy to overcome the drug resistance that emerged in the two primary targeted enzymes dihydrofolate reductase and dihydropteroate synthase. Both GCH1 and PTPS enzymes contain Zn²⁺ ions in their active sites, and to accurately study their dynamic behaviors using all-atom molecular dynamics (MD) simulations, appropriate parameters that can describe their metal sites should be developed and validated. In this study, force field parameters of the GCH1 and PTPS metal centers were generated using quantum mechanics (QM) calculations and then validated through MD simulations to ensure their accuracy in describing and maintaining the Zn²⁺ ion coordination environment. The derived force field parameters will provide accurate and reliable MD simulations involving these two enzymes for future in-silico identification of drug candidates against the GCH1 and PTPS enzymes. Communicated by Ramaswamy H. Sarma.

Vitamin and cofactor acquisition in apicomplexans: Synthesis versus salvage

The Apicomplexa phylum comprises diverse parasitic organisms that have evolved from a free-living ancestor. These obligate intracellular parasites exhibit versatile metabolic capabilities reflecting their capacity to survive and grow in different hosts and varying niches. Determined by nutrient availability, they either use their biosynthesis machineries or largely depend on their host for metabolite acquisition. Because vitamins cannot be synthesized by the mammalian host, the enzymes required for their synthesis in apicomplexan parasites represent a large repertoire of potential therapeutic targets. Here, we review recent advances in metabolic reconstruction and functional studies coupled to metabolomics that unravel the interplay between biosynthesis and salvage of vitamins and cofactors in apicomplexans. A particular emphasis is placed on Toxoplasma gondii, during both its acute and latent stages of infection.

Folic Acid Antagonists: Antimicrobial and Immunomodulating Mechanisms and Applications

: Bacterial, protozoan and other microbial infections share an accelerated metabolic rate. In order to ensure a proper functioning of cell replication and proteins and nucleic acids synthesis processes, folate metabolism rate is also increased in these cases. For this reason, folic acid antagonists have been used since their discovery to treat different kinds of microbial infections, taking advantage of this metabolic difference when compared with human cells. However, resistances to these compounds have emerged since then and only combined therapies are currently used in clinic. In addition, some of these compounds have been found to have an immunomodulatory behavior that allows clinicians using them as anti-inflammatory or immunosuppressive drugs. Therefore, the aim of this review is to provide an updated state-of-the-art on the use of antifolates as antibacterial and immunomodulating agents in the clinical setting, as well as to present their action mechanisms and currently investigated biomedical applications.

Design strategy for serine hydroxymethyltransferase probes based on retro-aldol-type reaction

Serine hydroxymethyltransferase (SHMT) is an enzyme that catalyzes the reaction that converts serine to glycine. It plays an important role in one-carbon metabolism. Recently, SHMT has been shown to be associated with various diseases. Therefore, SHMT has attracted attention as a biomarker and drug target. However, the development of molecular probes responsive to SHMT has not yet been realized. This is because SHMT catalyzes an essential yet simple reaction; thus, the substrates that can be accepted into the active site of SHMT are limited. Here, we focus on the SHMT-catalyzed retro-aldol reaction rather than the canonical serine-glycine conversion and succeed in developing fluorescent and 19F NMR molecular probes. Taking advantage of the facile and direct detection of SHMT, the developed fluorescent probe is used in the high-throughput screening for human SHMT inhibitors, and two hit compounds are obtained.

A Structural View on Medicinal Chemistry Strategies against Drug Resistance

The natural phenomenon of drug resistance is a widespread issue that hampers the performance of drugs in many major clinical indications. Antibacterial and antifungal drugs are affected, as well as compounds for the treatment of cancer, viral infections, or parasitic diseases. Despite the very diverse set of biological targets and organisms involved in the development of drug resistance, the underlying molecular mechanisms have been identified to understand the emergence of resistance and to overcome this detrimental process. Detailed structural information on the root causes for drug resistance is nowadays frequently available, so next-generation drugs can be designed that are anticipated to suffer less from resistance. This knowledge-based approach is essential for fighting the inevitable occurrence of drug resistance.

Structure of 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase-dihydropteroate synthase from Plasmodium vivax sheds light on drug resistance

The genomes of the malaria-causing Plasmodium parasites encode a protein fused of 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase (HPPK) and dihydropteroate synthase (DHPS) domains that catalyze sequential reactions in the folate biosynthetic pathway. Whereas higher organisms derive folate from their diet and lack the enzymes for its synthesis, most eubacteria and a number of lower eukaryotes including malaria parasites synthesize tetrahydrofolate via DHPS. Plasmodium falciparum (Pf) and Plasmodium vivax (Pv) HPPK-DHPSs are currently targets of drugs like sulfadoxine (SDX). The SDX effectiveness as an antimalarial drug is increasingly diminished by the rise and spread of drug-resistant mutations. Here, we present the crystal structure of PvHPPK-DHPS in complex with four substrates/analogs, revealing the bifunctional PvHPPK-DHPS architecture in an unprecedented state of enzymatic activation. SDX's effect on HPPK-DHPS is due to 4-amino benzoic acid (pABA) mimicry, and the PvHPPK-DHPS structure sheds light on the SDX-binding cavity, as well as on mutations that effect SDX potency. We mapped five dominant drug resistance mutations in PvHPPK-DHPS: S382A, A383G, K512E/D, A553G, and V585A, most of which occur individually or in clusters proximal to the pABA-binding site. We found that these resistance mutations subtly alter the intricate enzyme/pABA/SDX interactions such that DHPS affinity for pABA is diminished only moderately, but its affinity for SDX is changed substantially. In conclusion, the PvHPPK-DHPS structure rationalizes and unravels the structural bases for SDX resistance mutations and highlights architectural features in HPPK-DHPSs from malaria parasites that can form the basis for developing next-generation anti-folate agents to combat malaria parasites.

Chemoenzymatic Assembly of Isotopically Labeled Folates

Pterin-containing natural products have diverse functions in life, but an efficient and easy scheme for their in vitro synthesis is not available. Here we report a chemoenzymatic 14-step, one-pot synthesis that can be used to generate ¹³C- and ¹⁵N-labeled dihydrofolates (H₂F) from glucose, guanine, and p-aminobenzoyl-l-glutamic acid. This synthesis stands out from previous approaches to produce H₂F in that the average yield of each step is >91% and it requires only a single purification step. The use of a one-pot reaction allowed us to overcome potential problems with individual steps during the synthesis. The availability of labeled dihydrofolates allowed the measurement of heavy-atom isotope effects for the reaction catalyzed by the drug target dihydrofolate reductase and established that protonation at N5 of H₂F and hydride transfer to C6 occur in a stepwise mechanism. This chemoenzymatic pterin synthesis can be applied to the efficient production of other folates and a range of other natural compounds with applications in nutritional, medical, and cell-biological research.

Safety and benefits of interventions to increase folate status in malaria-endemic areas

For decades, folic acid has routinely been given to prevent or treat anaemia in children, pregnant women and people with sickle cell disease. However, there is no conclusive evidence that folate deficiency anaemia constitutes a public health problem in any of these groups. Industrial flour fortification is recommended and implemented in many countries to combat neural tube defects. Dietary folates or folic acid can antagonise the action of antifolate drugs that play a critical role in the prevention and treatment of malaria. Randomised trials have shown that folic acid supplementation increases the rate of treatment failures with sulfadoxine-pyrimethamine. The efficacy of antifolate drugs against Plasmodium is maximized in the absence of exogenous folic acid, suggesting that there is no safe minimum dose of ingested folic acid. We here review the safety and benefits of interventions to increase folate status in malaria-endemic countries. We conclude that formal cost-benefit analyses are required.

Synthesis of (+)-7,20-Diisocyanoadociane and Liver-Stage Antiplasmodial Activity of the Isocyanoterpene Class

7,20-Diisocyanoadociane, a scarce marine metabolite with potent antimalarial activity, was synthesized as a single enantiomer in 13 steps from simple building blocks (17 linear steps). Chemical synthesis enabled identification of isocyanoterpene antiplasmodial activity against liver-stage parasites, which suggested that inhibition of heme detoxification does not exclusively underlie the mechanism of action of this class.

Malaria in the Era of Food Fortification With Folic Acid

Food fortified with folic acid has been available for consumption in North America for over a decade. This strategy has led to an increase in folate levels in the general population and, more importantly, a significant decrease in the incidence of neural tube defects. However, this increase in folate intake has been associated with a greater risk of cancer disease. Many African countries are now embracing this concept; however, because folate promotes malaria parasite division, as it does in cancer cells, there is a possibility of malaria exacerbation if folate intake is increased. A precedent for such a concern is the now compelling evidence showing that an increase in iron intake can lead to a higher malaria risk; as a result, mass administration of iron in malaria-endemic areas is not recommended. In this article, we review work on the effect of folate on malaria parasites. Although this topic has received little research attention, the available data suggest that the increase in folate concentration could be associated with an increase in malaria infection. Thus, the introduction of food fortification with folic acid in malaria-endemic areas should be attended by precautionary programs to monitor the risk of malaria.

Down-regulation of dihydrofolate reductase inhibits the growth of endothelial EA.hy926 cell through induction of G1 cell cycle arrest via up-regulating p53 and p21(waf1/cip1) expression

Folic acid supplementation may meliorate cardiovascular disease risk by improving vascular endothelial structure and function. However, the underlying mechanisms are still lack of a global understanding. To be used, folic acid must be converted to 7,8-dihydrofolate by dihydrofolate reductase to generate one-carbon derivatives serving as important cellular cofactors in the synthesis of nucleotides and amino acids required for cell growth. Therefore, this study explored the effect of dihydrofolate reductase knockdown on endothelial EA.hy926 cell growth and the mechanism involved. We found that down-regulation of dihydrofolate reductase inhibited EA.hy926 cell proliferation, and induced G1 phase arrest. Meanwhile, the expression of regulators necessary for G1/S phase transition, such as cyclin-dependent kinases CDK2, CDK4 and CDK6, were remarkably down-regulated; by contrast, the cell cycle inhibitors p21^waf/cip1, p27^Kip1 and p53 were significantly up-regulated after dihydrofolate reductase knockdown. Furthermore, supplementation of 5-methyltetrahydrofolate to the dihydrofolate reductase knockdown cells could weaken the inhibitory effect of dihydrofolate reductase knockdown on cell proliferation, simultaneously, inducing the expression of p53 and p21^waf/cip1 falling back moderately. Our findings suggest that attenuating dihydrofolate reductase may cause imbalanced expression of cell cycle regulators, especially up-regulation of p53-p21^waf/cip1 pathway, leading to G1 cell cycle arrest, thereby inhibiting the growth of endothelial EA.hy926 cells.

Design, synthesis and biological evaluation of 6-aryl-1,6-dihydro-1,3,5-triazine-2,4-diamines as antiplasmodial antifolates

One-pot multistep synthesis of 1,6-dihydro-1,3,5-triazines displaying potent antiplasmodial activity in vitro.

High Dietary Folate in Mice Alters Immune Response and Reduces Survival after Malarial Infection

Malaria is a significant global health issue, with nearly 200 million cases in 2013 alone. Parasites obtain folate from the host or synthesize it de novo. Folate consumption has increased in many populations, prompting concerns regarding potential deleterious consequences of higher intake. The impact of high dietary folate on the host’s immune function and response to malaria has not been examined. Our goal was to determine whether high dietary folate would affect response to malarial infection in a murine model of cerebral malaria. Mice were fed control diets (CD, recommended folate level for rodents) or folic acid-supplemented diets (FASD, 10x recommended level) for 5 weeks before infection with Plasmodium berghei ANKA. Survival, parasitemia, numbers of immune cells and other infection parameters were assessed. FASD mice had reduced survival (p<0.01, Cox proportional hazards) and higher parasitemia (p< 0.01, joint model of parasitemia and survival) compared with CD mice. FASD mice had lower numbers of splenocytes, total T cells, and lower numbers of specific T and NK cell sub-populations, compared with CD mice (p<0.05, linear mixed effects). Increased brain TNFα immunoreactive protein (p<0.01, t-test) and increased liver Abca1 mRNA (p<0.01, t-test), a modulator of TNFα, were observed in FASD mice; these variables correlated positively (r_s = 0.63, p = 0.01). Bcl-xl/Bak mRNA was increased in liver of FASD mice (p<0.01, t-test), suggesting reduced apoptotic potential. We conclude that high dietary folate increases parasite replication, disturbs the immune response and reduces resistance to malaria in mice. These findings have relevance for malaria-endemic regions, when considering anti-folate anti-malarials, food fortification or vitamin supplementation programs.

Modeling metabolism and stage-specific growth of Plasmodium falciparum HB3 during the intraerythrocytic developmental cycle

The human malaria parasite Plasmodium falciparum goes through a complex life cycle, including a roughly 48-hour-long intraerythrocytic developmental cycle (IDC) in human red blood cells. A better understanding of the metabolic processes required during the asexual blood-stage reproduction will enhance our basic knowledge of P. falciparum and help identify critical metabolic reactions and pathways associated with blood-stage malaria. We developed a metabolic network model that mechanistically links time-dependent gene expression, metabolism, and stage-specific growth, allowing us to predict the metabolic fluxes, the biomass production rates, and the timing of production of the different biomass components during the IDC. We predicted time- and stage-specific production of precursors and macromolecules for P. falciparum (strain HB3), allowing us to link specific metabolites to specific physiological functions. For example, we hypothesized that coenzyme A might be involved in late-IDC DNA replication and cell division. Moreover, the predicted ATP metabolism indicated that energy was mainly produced from glycolysis and utilized for non-metabolic processes. Finally, we used the model to classify the entire tricarboxylic acid cycle into segments, each with a distinct function, such as superoxide detoxification, glutamate/glutamine processing, and metabolism of fumarate as a byproduct of purine biosynthesis. By capturing the normal metabolic and growth progression in P. falciparum during the IDC, our model provides a starting point for further elucidation of strain-specific metabolic activity, host-parasite interactions, stress-induced metabolic responses, and metabolic responses to antimalarial drugs and drug candidates.

The molecular basis of antifolate resistance in Plasmodium falciparum: looking beyond point mutations

Drugs that target the folate-synthesis pathway have a long history of effectiveness against a variety of pathogens. As antimalarials, the antifolates were safe and well tolerated, but resistance emerged quickly and has persisted even with decreased drug pressure. The primary determinants of resistance in Plasmodium falciparum are well-described point mutations in the enzymes dihydropteroate synthase and dihydrofolate reductase targeted by the combination sulfadoxine-pyrimethamine. Recent work has highlighted the contributions of additional parasite adaptation to antifolate resistance. In fact, the evolution of antifolate-resistant parasites is multifaceted and complex. Gene amplification of the first enzyme in the parasite folate synthesis pathway, GTP-cyclohydrolase, is strongly associated with resistant parasites and potentially contributes to persistence of resistant parasites. Further understanding of how parasites adjust flux through the folate pathway is important to the further development of alternative agents targeting this crucial synthesis pathway.

Investigation of antimalarial drug pyrimethamine and its interaction with dsDNA by electrochemical and spectroscopic techniques

The electrochemical behavior of the antimalarial drug pyrimethamine (PMT) was examined at a screen printed carbon electrode (SPCE) in different aqueous supporting electrolytes using cyclic voltammetry (CV) and differential pulse voltammetry (DPV).

Tetrahydrobiopterin in cardiovascular health and disease

Tetrahydrobiopterin (BH4) functions as a cofactor for several important enzyme systems, and considerable evidence implicates BH4 as a key regulator of endothelial nitric oxide synthase (eNOS) in the setting of cardiovascular health and disease. BH4 bioavailability is determined by a balance of enzymatic de novo synthesis and recycling, versus degradation in the setting of oxidative stress. Augmenting vascular BH4 levels by pharmacological supplementation has been shown in experimental studies to enhance NO bioavailability. However, it has become more apparent that the role of BH4 in other enzymatic pathways, including other NOS isoforms and the aromatic amino acid hydroxylases, may have a bearing on important aspects of vascular homeostasis, inflammation, and cardiac function. This article reviews the role of BH4 in cardiovascular development and homeostasis, as well as in pathophysiological processes such as endothelial and vascular dysfunction, atherosclerosis, inflammation, and cardiac hypertrophy. We discuss the therapeutic potential of BH4 in cardiovascular disease states and attempt to address how this modulator of intracellular NO-redox balance may ultimately provide a powerful new treatment for many cardiovascular diseases.

Effect of pyrimethamine treatment on male rat testicular cell population development

Pyrimethamine (PYR) is a drug used in the treatment of newborn with congenital Toxoplasmosis. Even when PYR is highly specific against parasites, it may provoke neutropenia in the patients apart from other affectations, conditions that usually justify its suspension. Moreover, medication against congenital toxoplasmosis coincides with the proliferation stage of Sertoli and germ cells. Although, there are several reports on the effect of this drug on mature testes, records of its effects on the testes of young individuals yet in the process of growth are still lacking. This work was aimed to study the effects of in vivo administration of PYR in the first 21 days of life of male rat pups by evaluating their testicular alterations and its long-term sequels on fertility. Through the determination of the levels of seminiferous epithelium maturity, apoptotic index and cell proliferation index at 7, 14, 35 and 90 days post-natal using immunocytochemical studies. The fertility of the treated rats was evaluated at 90 days. PYR-treated animals were found to undergo some kind of delays in seminiferous epithelium maturity, decreased cell proliferation index and an increase in apoptosis when compared with the control (p < 0.05). Epididymal sperm counts were also affected (p < 0.05). The application of folic acid (FA) in newborns treated with PYR decreased the severity of the problem (p < 0.05). This study provides strong evidence that the effect of PYR on testicular development is specific. It reinforces the importance of FA application in neonates treated with PYR to prevent the effect of the later on spermatogenesis.

Biochemical and functional characterization of Plasmodium falciparum GTP cyclohydrolase I

Background

Antifolates are currently in clinical use for malaria preventive therapy and treatment. The drugs kill the parasites by targeting the enzymes in the de novo folate pathway. The use of antifolates has now been limited by the spread of drug-resistant mutations. GTP cyclohydrolase I (GCH1) is the first and the rate-limiting enzyme in the folate pathway. The amplification of the gch1 gene found in certain Plasmodium falciparum isolates can cause antifolate resistance and influence the course of antifolate resistance evolution. These findings showed the importance of P. falciparum GCH1 in drug resistance intervention. However, little is known about P. falciparum GCH1 in terms of kinetic parameters and functional assays, precluding the opportunity to obtain the key information on its catalytic reaction and to eventually develop this enzyme as a drug target.

Methods

Plasmodium falciparum GCH1 was cloned and expressed in bacteria. Enzymatic activity was determined by the measurement of fluorescent converted neopterin with assay validation by using mutant and GTP analogue. The genetic complementation study was performed in ∆folE bacteria to functionally identify the residues and domains of P. falciparum GCH1 required for its enzymatic activity. Plasmodial GCH1 sequences were aligned and structurally modeled to reveal conserved catalytic residues.

Results

Kinetic parameters and optimal conditions for enzymatic reactions were determined by the fluorescence-based assay. The inhibitor test against P. falciparum GCH1 is now possible as indicated by the inhibitory effect by 8-oxo-GTP. Genetic complementation was proven to be a convenient method to study the function of P. falciparum GCH1. A series of domain truncations revealed that the conserved core domain of GCH1 is responsible for its enzymatic activity. Homology modelling fits P. falciparum GCH1 into the classic Tunnelling-fold structure with well-conserved catalytic residues at the active site.

Conclusions

Functional assays for P. falciparum GCH1 based on enzymatic activity and genetic complementation were successfully developed. The assays in combination with a homology model characterized the enzymatic activity of P. falciparum GCH1 and the importance of its key amino acid residues. The potential to use the assay for inhibitor screening was validated by 8-oxo-GTP, a known GTP analogue inhibitor.

The role of antioxidants treatment on the pathogenesis of malarial infections: a review

Oxidative damage is one of the most important pathological consequences of malarial infections. It affects vital organs of the body manifesting in changes such as splenomegaly, hepatomegaly, endothelial and cognitive damages. The currently used antimalarials often leave traces of these damages after therapy, as evident in memory impairment after cerebral malaria. Hence, some research investigations have focused attention on the use of antioxidants, alone or in combination with antimalarials, as a viable therapeutic strategy aimed at alleviating plasmodium-induced oxidative stress and its associated complications. However, the practical application of this approach often yields conflicting outcomes because some antimalarials specifically act via induction of oxidative stress. This article critically reviews most of the studies conducted on the potential role of antioxidant therapy in malarial infections. The most frequently investigated antioxidants are vitamins C and E, N-acetylcystein, folate and desferroxamine. Some of the investigations measured the effects of direct administration of the antioxidants on the plasmodium parasites while others performed an adjunctive therapy with standard antimalarials. The therapeutic application of each of the antioxidants in malaria management depends on the targeted aspect of malarial pathology. It is hoped that this article will provide an informed basis for future research activities on the therapeutic role of antioxidants on malarial pathogenesis.

Vitamin B6-dependent enzymes in the human malaria parasite Plasmodium falciparum: a druggable target?

Malaria is a deadly infectious disease which affects millions of people each year in tropical regions. There is no effective vaccine available and the treatment is based on drugs which are currently facing an emergence of drug resistance and in this sense the search for new drug targets is indispensable. It is well established that vitamin biosynthetic pathways, such as the vitamin B6 de novo synthesis present in Plasmodium, are excellent drug targets. The active form of vitamin B6, pyridoxal 5-phosphate, is, besides its antioxidative properties, a cofactor for a variety of essential enzymes present in the malaria parasite which includes the ornithine decarboxylase (ODC, synthesis of polyamines), the aspartate aminotransferase (AspAT, involved in the protein biosynthesis), and the serine hydroxymethyltransferase (SHMT, a key enzyme within the folate metabolism).

Targeting the vitamin biosynthesis pathways for the treatment of malaria

The most severe form of malaria is Malaria tropica, caused by Plasmodium falciparum. There are more than 1 billion people that are exposed to malaria parasites leading to more than 500,000 deaths annually. Vaccines are not available and the increasing drug resistance of the parasite prioritizes the need for novel drug targets and chemotherapeutics, which should be ideally designed to target selectively the parasite. In this sense, parasite-specific pathways, such as the vitamin biosyntheses, represent perfect drug-target characteristics because they are absent in humans. In the past, the vitamin B9 (folate) metabolism has been exploited by antifolates to treat infections caused by malaria parasites. Recently, two further vitamin biosynthesis pathways - for the vitamins B6 (pyridoxine) and B1 (thiamine) - have been identified in Plasmodium and analyzed for their suitability to discover new drugs. In this review, the current status of the druggability of plasmodial vitamin biosynthesis pathways is summarized.

Impact of folate supplementation on the efficacy of sulfadoxine/pyrimethamine in preventing malaria in pregnancy: the potential of 5-methyl-tetrahydrofolate

Malaria remains the leading cause of mortality and morbidity in children under the age of 5 years and pregnant women. To counterbalance the malaria burden in pregnancy, an intermittent preventive treatment strategy has been developed. This is based on the use of the antifolate sulfadoxine/pyrimethamine, taken at specified intervals during pregnancy, and reports show that this approach reduces the malaria burden in pregnancy. Pregnancy is also associated with the risk of neural tube defects (NTDs), especially in women with low folate status, and folic acid supplementation is recommended in pregnancy to lower the risk of NTDs. Thus, in malaria-endemic areas, pregnant women have to take both antifolate medication to prevent malaria and folic acid to lower the risk of NTDs. However, the concomitant use of folate and antifolate is associated with a decrease in antifolate efficacy, exposing pregnant women to malaria. Thus, there is genuine concern that this strategy may not be appropriate. We have reviewed work carried out on malaria folate metabolism and antifolate efficacy in the context of folate supplementation. This review shows that: (i) the folate supplementation effect on antifolate efficacy is dose-dependent, and folic acid doses required to protect pregnant women from NTDs will not decrease antifolate activity; and (ii) 5-methyl-tetrahydrofolate, the predominant form of folate in the blood circulation, could be administered (even at high dose) concomitantly with antifolate without affecting antifolate efficacy. Thus, strategies exist to protect pregnant women from malaria while maintaining adequate folate levels in the body to reduce the occurrence of NTDs.

Ordered accumulation of mutations conferring resistance to sulfadoxine-pyrimethamine in the Plasmodium falciparum parasite

BACKGROUND

Monitoring the prevalence of drug resistant Plasmodium falciparum is essential for effective malaria control. Resistance to pyrimethamine and sulfadoxine increases as mutations accumulate in the parasite genes encoding dihydrofolate reductase (dhfr) and dihydropteroate synthase (dhps), respectively. Although parasites are exposed to these antifolate drugs simultaneously, it remains virtually unknown whether dhfr and dhps mutations accumulate along interrelated paths.

METHODS

We investigated the order of step-wise accumulation in dhfr and dhps by cumulative analyses using binomial tests in 575 P. falciparum isolates obtained from 7 countries in Asia and Melanesia.

RESULTS

An initial step in the accumulation of mutations preferentially occurred in dhfr (2 mutations), followed by 1 mutation in dhps. In a subsequent step, mutations were estimated separately for 5 dhfr/dhps-resistant lineages identified using 12 microsatellites flanking dhfr and dhps. Among these lineages, we found 3 major mutational paths, each of which follows a unique stepwise trajectory to produce the most highly resistant form with 4 mutations in dhfr and 3 in dhps.

CONCLUSIONS

The ordered accumulation of mutations in dhfr and dhps elucidated here will assist in predicting the status and progression of antifolate resistance in malaria-endemic regions where antifolate drugs are used for intermittent preventive treatment.

Direct evidence for the adaptive role of copy number variation on antifolate susceptibility in Plasmodium falciparum

Resistance to antimalarials targeting the folate pathway is widespread. GTP-cyclohydrolase (gch1), the first enzyme in this pathway, exhibits extensive copy number variation (CN) in parasite isolates from areas with a history of longstanding antifolate use. Increased CN of gch1 is associated with a greater number of point mutations in enzymes targeted by the antifolates, pyrimethamine and sulphadoxine. While these observations suggest that increases in gch1 CN are an adaptation to drug pressure, changes in CN have not been experimentally demonstrated to directly alter drug susceptibility. To determine if changes in gch1 expression alone modify pyrimethamine sensitivity, we manipulated gch1 CN in several parasite lines to test the effect on drug sensitivity. We report that increases in gch1 CN alter pyrimethamine resistance in most parasites lines. However we find evidence of a detrimental effect of very high levels of gch1 overexpression in parasite lines with high endogenous levels of gch1 expression, revealing the importance of maintaining balance in the folate pathway and implicating changes in gch1 expression in preserving proper metabolic flux. This work expands our understanding of parasite adaptation to drug pressure and provides a possible mechanism for how specific mutations become fixed within parasite populations.

Synthesis and recycling of tetrahydrobiopterin in endothelial function and vascular disease

Nitric oxide, generated by the nitric oxide synthase (NOS) enzymes, plays pivotal roles in cardiovascular homeostasis and in the pathogenesis of cardiovascular disease. The NOS cofactor, tetrahydrobiopterin (BH4), is an important regulator of NOS function, since BH4 is required to maintain enzymatic coupling of L-arginine oxidation, to produce NO. Loss or oxidation of BH4 to 7,8-dihydrobiopterin (BH2) is associated with NOS uncoupling, resulting in the production of superoxide rather than NO. In addition to key roles in folate metabolism, dihydrofolate reductase (DHFR) can 'recycle' BH2, and thus regenerate BH4. It is therefore likely that net BH4 cellular bioavailability reflects the balance between de novo BH4 synthesis, loss of BH4 by oxidation to BH2, and the regeneration of BH4 by DHFR. Recent studies have implicated BH4 recycling in the direct regulation of eNOS uncoupling, showing that inhibition of BH4 recycling using DHFR-specific siRNA and methotrexate treatment leads to eNOS uncoupling in endothelial cells and the hph-1 mouse model of BH4 deficiency, even in the absence of oxidative stress. These studies indicate that not only BH4 level, but the recycling pathways regulating BH4 bioavailability represent potential therapeutic targets and will be discussed in this review.

Dihydrofolate reductase protects endothelial nitric oxide synthase from uncoupling in tetrahydrobiopterin deficiency

Tetrahydrobiopterin (BH4) is a required cofactor for the synthesis of NO by endothelial nitric oxide synthase (eNOS), and endothelial BH4 bioavailability is a critical factor in regulating the balance between NO and superoxide production (eNOS coupling). Biosynthesis of BH4 is determined by the activity of GTP-cyclohydrolase I (GTPCH). However, BH4 levels may also be influenced by oxidation, forming 7,8-dihydrobiopterin (BH2), which promotes eNOS uncoupling. Conversely, dihydrofolate reductase (DHFR) can regenerate BH4 from BH2, but whether DHFR is functionally important in maintaining eNOS coupling remains unclear. To investigate the mechanism by which DHFR might regulate eNOS coupling in vivo, we treated wild-type, BH4-deficient (hph-1), and GTPCH-overexpressing (GCH-Tg) mice with methotrexate (MTX), to inhibit BH4 recycling by DHFR. MTX treatment resulted in a striking elevation in BH2 and a decreased BH4:BH2 ratio in the aortas of wild-type mice. These effects were magnified in hph-1 but diminished in GCH-Tg mice. Attenuated eNOS activity was observed in MTX-treated hph-1 but not wild-type or GCH-Tg mouse lung, suggesting that inhibition of DHFR in BH4-deficient states leads to eNOS uncoupling. Taken together, these data reveal a key role for DHFR in regulating the BH4 vs BH2 ratio and eNOS coupling under conditions of low total biopterin availability in vivo.

Folate metabolism pathway and Plasmodium falciparum malaria infection in pregnancy

Malaria induced by Plasmodium falciparum is a major cause of mortality. P. falciparum has the ability to use host plasma folate as its primary folate source. Folate is a cofactor needed for both malaria parasite growth and host erythrocyte production. This review examines the possible impairment of the folate-mediated one-carbon metabolism pathway as a result of P. falciparum malaria infection during pregnancy. Folate deficiency during malaria infection is presented, with an emphasis on the controversy regarding the decrease of plasma or erythrocyte folate secondary to malaria. Maternal folate deficiency increases the risk of adverse pregnancy outcomes. Functional folate deficiency and/or increased plasma homocysteine levels during pregnancy of infected women in areas endemic for malaria is a probable scenario accentuating the impairment of placenta function leading to the occurrence of neural tube defects, low birth weights, and intrauterine growth retardations. Potential questions that may be answered in future investigations using an appropriate protocol to study pregnant women with malaria are also addressed.

Co-expression of the Plasmodium falciparum molecular chaperone, PfHsp70, improves the heterologous production of the antimalarial drug target GTP cyclohydrolase I, PfGCHI

Molecular chaperones have been used for the improved expression of target proteins within heterologous systems; however, the chaperone and target protein have seldom been matched in terms of origin. We have developed a heterologous co-expression system that allows independent expression of the plasmodial chaperone, PfHsp70, and a plasmodial target protein. In this study, the target was Plasmodium falciparum GTP cyclohydrolase I (PfGCHI), the first enzyme in the plasmodial folate pathway. The sequential expression of the molecular chaperone followed by the target protein increased the expression of soluble functional PfGCHI. His-tagged PfGCHI was successfully purified using nickel affinity chromatography, and the specific activity was determined by high performance liquid chromatography with spectrofluorometeric detection to be 5.93nmol/h/mg. This is the first report of a heterologous co-expression system in which a plasmodial chaperone is harnessed for the improved production and purification of a plasmodial target protein.

Next-Generation Antimalarial Drugs: Hybrid Molecules as a New Strategy in Drug Design

Malaria is a disease that affects nearly 40% of the global population, and chemotherapy remains the mainstay of its control strategy. The global malaria situation is increasingly being exacerbated by the emergence of drug resistance to most of the available antimalarials, necessitating search for novel drugs. A recent rational approach of antimalarial drug design characterized as "covalent bitherapy" involves linking two molecules with individual intrinsic activity into a single agent, thus packaging dual-activity into a single hybrid molecule. Current research in this field seems to endorse hybrid molecules as the next-generation antimalarial drugs. If the selective toxicity of hybrid prodrugs can be demonstrated in vivo with good bioavailability at the target site in the parasite, it would offer various advantages including dosage compliance, minimized toxicity, ability to design better drug combinations, and cheaper preclinical evaluation while achieving the ultimate object of delaying or circumventing the development of resistance. This review is focused on several hybrid molecules that have been developed, with particular emphasis on those deemed to have high potential for development for clinical use. Drug Dev Res 71: 20-32, 2010. © 2009 Wiley-Liss, Inc.

Critical role for tetrahydrobiopterin recycling by dihydrofolate reductase in regulation of endothelial nitric-oxide synthase coupling: relative importance of the de novo biopterin synthesis versus salvage pathways

Tetrahyrobiopterin (BH4) is a required cofactor for the synthesis of nitric oxide by endothelial nitric-oxide synthase (eNOS), and BH4 bioavailability within the endothelium is a critical factor in regulating the balance between NO and superoxide production by eNOS (eNOS coupling). BH4 levels are determined by the activity of GTP cyclohydrolase I (GTPCH), the rate-limiting enzyme in de novo BH4 biosynthesis. However, BH4 levels may also be influenced by oxidation, forming 7,8-dihydrobiopterin (BH2), which promotes eNOS uncoupling. Conversely, dihydrofolate reductase (DHFR) can regenerate BH4 from BH2, but the functional importance of DHFR in maintaining eNOS coupling remains unclear. We investigated the role of DHFR in regulating BH4 versus BH2 levels in endothelial cells and in cell lines expressing eNOS combined with tet-regulated GTPCH expression in order to compare the effects of low or high levels of de novo BH4 biosynthesis. Pharmacological inhibition of DHFR activity by methotrexate or genetic knockdown of DHFR protein by RNA interference reduced intracellular BH4 and increased BH2 levels resulting in enzymatic uncoupling of eNOS, as indicated by increased eNOS-dependent superoxide but reduced NO production. In contrast to the decreased BH4:BH2 ratio induced by DHFR knockdown, GTPCH knockdown greatly reduced total biopterin levels but with no change in BH4:BH2 ratio. In cells expressing eNOS with low biopterin levels, DHFR inhibition or knockdown further diminished the BH4:BH2 ratio and exacerbated eNOS uncoupling. Taken together, these data reveal a key role for DHFR in eNOS coupling by maintaining the BH4:BH2 ratio, particularly in conditions of low total biopterin availability.

Effects of point mutations in Plasmodium falciparum dihydrofolate reductase and dihydropterate synthase genes on clinical outcomes and in vitro susceptibility to sulfadoxine and pyrimethamine

Background

Sulfadoxine-pyrimethamine was a common first line drug therapy to treat uncomplicated falciparum malaria, but increasing therapeutic failures associated with the development of significant levels of resistance worldwide has prompted change to alternative treatment regimes in many national malaria control programs.

Methodology and Finding

We conducted an in vivo therapeutic efficacy trial of sulfadoxine-pyrimethamine at two locations in the Peruvian Amazon enrolling 99 patients of which, 86 patients completed the protocol specified 28 day follow up. Our objective was to correlate the presence of polymorphisms in P. falciparum dihydrofolate reductase and dihydropteroate synthase to in vitro parasite susceptibility to sulfadoxine and pyrimethamine and to in vivo treatment outcomes. Inhibitory concentration 50 values of isolates increased with numbers of mutations (single [108N], sextuplet [BR/51I/108N/164L and 437G/581G]) and septuplet (BR/51I/108N/164L and 437G/540E/581G) with geometric means of 76 nM (35–166 nM), 582 nM (49-6890- nM) and 4909 (3575–6741 nM) nM for sulfadoxine and 33 nM (22–51 nM), 81 nM (19–345 nM), and 215 nM (176–262 nM) for pyrimethamine. A single mutation present in the isolate obtained at the time of enrollment from either dihydrofolate reductase (164L) or dihydropteroate synthase (540E) predicted treatment failure as well as any other single gene alone or in combination. Patients with the dihydrofolate reductase 164L mutation were 3.6 times as likely to be treatment failures [failures 85.4% (164L) vs 23.7% (I164); relative risk = 3.61; 95% CI: 2.14 – 6.64] while patients with the dihydropteroate synthase 540E were 2.6 times as likely to fail treatment (96.7% (540E) vs 37.5% (K540); relative risk = 2.58; 95% CI: 1.88 – 3.73). Patients with both dihydrofolate reductase 164L and dihydropteroate synthase 540E mutations were 4.1 times as likely to be treatment failures [96.7% vs 23.7%; RR = 4.08; 95% CI: 2.45 – 7.46] compared to patients having both wild forms (I164 and K540).

Conclusions

In this part of the Amazon basin, it may be possible to predict treatment failure with sulfadoxine-pyrimethamine equally well by determination of either of the single mutations dihydrofolate reductase 164L or dihydropteroate synthase 540E.

Trial Registration

ClinicalTrials.gov NCT00951106 NCT00951106

Characterization of Plasmodium falciparum serine hydroxymethyltransferase-A potential antimalarial target

Serine hydroxymethyltransferase (SHMT) is a ubiquitous enzyme required for folate recycling and dTMP synthesis. A cDNA encoding Plasmodium falciparum (Pf) SHMT was expressed as a hexa-histidine tagged protein in Escherichia coli BL21-CodonPlus (DE3)-RIL. The protein was purified and the process yielded 3.6 mg protein/l cell culture. Recombinant His(6)-tagged PfSHMT exhibits a visible spectrum characteristic of pyridoxal-5'-phosphate enzyme and catalyzes the reversible conversion of l-serine and tetrahydrofolate (H(4)folate) to glycine and 5,10-methylenetetrahydrofolate (CH(2)-H(4)folate). Steady-state kinetics study indicates that His(6)-tagged PfSHMT catalyzes the reaction by a ternary-complex mechanism. The sequence of substrate binding to the enzyme was also examined by glycine product inhibition. A striking property that is unique for His(6)-tagged PfSHMT is the ability to use D-serine as a substrate in the folate-dependent serine-glycine conversion. Kinetic data in combination with expression result support the proposal of SHMT reaction being a regulatory step for dTMP cycle. This finding suggests that PfSHMT can be a potential target for antimalarial chemotherapy.