Unraveling the mode of action of the antimalarial choline analog G25 in Plasmodium falciparum and Saccharomyces cerevisiae

Biomolecular interactions between Plasmodium and human host: A basis of targeted antimalarial therapy

Malaria is one of the serious health concerns worldwide as it remains a clinical challenge due to the complex life cycle of the malaria parasite and the morphological changes it undergoes during infection. The malaria parasite multiplies rapidly and spreads in the population by changing its alternative hosts. These various morphological stages of the parasite in the human host cause clinical symptoms (anemia, fever, and coma). These symptoms arise due to the preprogrammed biology of the parasite in response to the human pathophysiological response. Thus, complete elimination becomes one of the major health challenges. Although malaria vaccine(s) are available in the market, they still contain to cause high morbidity and mortality. Therefore, an approach for eradication is needed through the exploration of novel molecular targets by tracking the epidemiological changes the parasite adopts. This review focuses on the various novel molecular targets.

Evaluation of the inhibitory efficacy of quaternary ammonium compounds on in vitro growth of Theileria equi parasite in MASP culture

Equine piroplasmosis has become a global problem of the equine husbandry sector. Haemoprotozoans evolved very quickly and developed resistance against most of the current available drugs. Phospholipid membrane synthesis by choline kinase enzyme is vital for propagation of intra-erythrocytic protozoa parasites. This pathway was targeted in the present study. Quaternary ammonium salts (QAS) and their analogues act against choline and hamper the biosynthesis process for phosphatidylcholine. We analysed anti-T. equi activity of three QAS - decamethonium bromide (DMB), decyl trimethyl ammonium bromide (DTAB) and dodecyl trimethyl ammonium bromide (DDTAB). Theileria equi parasites in vitro treated with different concentrations of DMB, DDTAB and DTAB. Drug treated T. equi failed to multiply further in the viability test. The IC₅₀ value of DMB, DDTAB and DTAB for growth inhibition of T. equi was 14.0 μM, 469.51 nM and 558.40 nM, respectively. DMB, DDTAB and DTAB treated T. equi parasites were observed to be devoid of internal structures, showing pyknotic and degenerative appearances. Various concentration of DMB, DDTAB and DTAB were analysed for their cytotoxicity and haemolytic activity on horse's PBMCs and RBCs. DMB was less than 10% cytotoxic to PBMCs, while DDTAB and DTAB were 40%-50% cytotoxic at 1000 μM concentrations. The respective CC₅₀ values were 7202.96 μM, 1026.26 μM and 1263.95 μM. DMB and DTAB showed least haemolytic activity (<3%); whereas DDTAB was more haemolytic to RBCs at highest concentration of 2000 μM. The respective CC₅₀ values of these drugs were 224495.3 μM, and 39101.35 μM; 713.54 μM. Specific selective index for DMB, DDTAB and DTAB values with respect to host's PBMC and RBC cells, were 514.50, 2185.81, 2263.52 and 16035.38, 1519.75, 70023.91, respectively. These data indicated its non-toxicity to host's cells and selective potential of anti-T. equi in vitro activity.

Structural insights into phosphatidylethanolamine formation in bacterial membrane biogenesis

Phosphatidylethanolamine (PE), a major component of the cellular membrane across all domains of life, is synthesized exclusively by membrane-anchored phosphatidylserine decarboxylase (PSD) in most bacteria. The enzyme undergoes auto-cleavage for activation and utilizes the pyruvoyl moiety to form a Schiff base intermediate with PS to facilitate decarboxylation. However, the structural basis for self-maturation, PS binding, and decarboxylation processes directed by PSD remain unclear. Here, we present X-ray crystal structures of PSD from Escherichia coli, representing an apo form and a PE-bound complex, in which the phospholipid is chemically conjugated to the essential pyruvoyl residue, mimicking the Schiff base intermediate. The high-resolution structures of PE-complexed PSD clearly illustrate extensive hydrophobic interactions with the fatty acyl chains of the phospholipid, providing insights into the broad specificity of the enzyme over a wide range of cellular PS. Furthermore, these structures strongly advocate the unique topology of the enzyme in a lipid bilayer environment, where the enzyme associates with cell membranes in a monotopic fashion via the N-terminal domain composed of three amphipathic helices. Lastly, mutagenesis analyses reveal that E. coli PSD primarily employs D90/D142-H144-S254 to achieve auto-cleavage for the proenzyme maturation, where D90 and D142 act in complementary to each other.

Neurological disorder and psychosocial aspects of cerebral malaria: what is new on its pathogenesis and complications? A minireview

Recently, malaria is remain considered as the most prevalent infectious disease, affecting the human health globally. High morbidity and mortality worldwide is often allied with cerebral malaria (CM) based disorders of the central nervous system, especially across many tropical and sub-tropical regions. These disorders are characterised by the infection of Plasmodium species, which leads to acute or chronic neurological disorders, even after having active/effective antimalarial drugs. Furthermore, even during the treatment, individual remain sensitive for neurological impairments in the form of decrease blood flow and vascular obstruction in brain including many more other changes. This review briefly explains and update on the epidemiology, burden of disease, pathogenesis and role of CM in neurological disorders with behaviour and function in mouse and human models. Moreover, the social stigma, which plays an important role in neurological disorders and a factor for assessing CM, is also discussed in this review.

1,2-Diphenoxiethane salts as potent antiplasmodial agents

In this article we present a series of non-cytotoxic potent human choline kinase (CK) inhibitors that exhibit nanomolar antiplasmodial activity in vitro. The most active antiplasmodial compounds, 10a-b, bearing a pyridinium cationic head were inactive against CK, while compounds 10g and 10j with a quinolinium moiety exhibit moderate inhibition of both the parasite and the enzyme. The results point towards an additional mechanism of action unrelated to CK inhibition that remains to be established.

Success stories of natural product-based hybrid molecules for multi-factorial diseases

Complex diseases comprises of highly complicated etiology resulting in limited applicability of conventional targeted therapies. Consequently, conventional medicinal compounds suffer major failure when used for such disease conditions. Additionally, development of multidrug resistance (MDR), adverse drug reactions and clinical specificity of single targeted drug therapy has increased thrust for novel drug therapy. In this rapidly evolving era, natural product-based discovery of hybrid molecules or multi-targeted drug therapies have shown promising results and are trending now a days. Historically, nature has blessed human with different sources viz. plant, animal, microbial, marine and ethnopharmaceutical sources which has given a wide variety of medicinally active compounds. These compounds from natural origin are always choice of interest of medicinal chemists because of their minimum side effects. Hybrid molecules synthesized by fusing or conjugating different active molecules obtained from these sources are reported to synergistically block different pathways which contribute in the pathogenesis of complex diseases. This review strives to encompass all natural product-derived hybrid molecules which act as multi-targeting agents striking various targets involved in different pathways of complex diseased conditions reported in literature.

Bioenergetics-based modeling of Plasmodium falciparum metabolism reveals its essential genes, nutritional requirements, and thermodynamic bottlenecks

Novel antimalarial therapies are urgently needed for the fight against drug-resistant parasites. The metabolism of malaria parasites in infected cells is an attractive source of drug targets but is rather complex. Computational methods can handle this complexity and allow integrative analyses of cell metabolism. In this study, we present a genome-scale metabolic model (iPfa) of the deadliest malaria parasite, Plasmodium falciparum, and its thermodynamics-based flux analysis (TFA). Using previous absolute concentration data of the intraerythrocytic parasite, we applied TFA to iPfa and predicted up to 63 essential genes and 26 essential pairs of genes. Of the 63 genes, 35 have been experimentally validated and reported in the literature, and 28 have not been experimentally tested and include previously hypothesized or novel predictions of essential metabolic capabilities. Without metabolomics data, four of the genes would have been incorrectly predicted to be non-essential. TFA also indicated that substrate channeling should exist in two metabolic pathways to ensure the thermodynamic feasibility of the flux. Finally, analysis of the metabolic capabilities of P. falciparum led to the identification of both the minimal nutritional requirements and the genes that can become indispensable upon substrate inaccessibility. This model provides novel insight into the metabolic needs and capabilities of the malaria parasite and highlights metabolites and pathways that should be measured and characterized to identify potential thermodynamic bottlenecks and substrate channeling. The hypotheses presented seek to guide experimental studies to facilitate a better understanding of the parasite metabolism and the identification of targets for more efficient intervention.

Almost half of the world population is at risk of infection by malaria parasites. The rise in drug-resistant parasites requires better understanding and targeting of their metabolism. In this study, we present a genome-scale metabolic reconstruction (iPfa) of the deadliest malaria parasite, Plasmodium falciparum, and its thermodynamics-based flux analysis (TFA). Our results support and extend the available experimental evidence on the essential genes and nutritional requirements of this organism. Besides, we identify metabolites that give rise to thermodynamic bottlenecks and suggest substrate channeling. Overall, these results provide novel insight into the metabolism of P. falciparum and may guide experimental studies to develop a better characterization of the parasite metabolism and the identification of antimalarial drug targets.

Characterization of Plasmodium phosphatidylserine decarboxylase expressed in yeast and application for inhibitor screening

Phospholipid biosynthesis is critical for the development, differentiation and pathogenesis of several eukaryotic pathogens. Genetic studies have validated the pathway for phosphatidylethanolamine synthesis from phosphatidylserine catalyzed by phosphatidylserine decarboxylase enzymes (PSD) as a suitable target for development of antimicrobials; however no inhibitors of this class of enzymes have been discovered. We show that the Plasmodium falciparum PSD can restore the essential function of the yeast gene in strains requiring PSD for growth. Genetic, biochemical and metabolic analyses demonstrate that amino acids between positions 40 and 70 of the parasite enzyme are critical for proenzyme processing and decarboxylase activity. We used the essential role of Plasmodium PSD in yeast as a tool for screening a library of anti-malarials. One of these compounds is 7-chloro-N-(4-ethoxyphenyl)-4-quinolinamine, an inhibitor with potent activity against P. falciparum, and low toxicity toward mammalian cells. We synthesized an analog of this compound and showed that it inhibits PfPSD activity and eliminates Plasmodium yoelii infection in mice. These results highlight the importance of 4-quinolinamines as a novel class of drugs targeting membrane biogenesis via inhibition of PSD activity.

An Atypical Mitochondrial Carrier That Mediates Drug Action in Trypanosoma brucei

Elucidating the mechanism of action of trypanocidal compounds is an important step in the development of more efficient drugs against Trypanosoma brucei. In a screening approach using an RNAi library in T. brucei bloodstream forms, we identified a member of the mitochondrial carrier family, TbMCP14, as a prime candidate mediating the action of a group of anti-parasitic choline analogs. Depletion of TbMCP14 by inducible RNAi in both bloodstream and procyclic forms increased resistance of parasites towards the compounds by 7-fold and 3-fold, respectively, compared to uninduced cells. In addition, down-regulation of TbMCP14 protected bloodstream form mitochondria from a drug-induced decrease in mitochondrial membrane potential. Conversely, over-expression of the carrier in procyclic forms increased parasite susceptibility more than 13-fold. Metabolomic analyses of parasites over-expressing TbMCP14 showed increased levels of the proline metabolite, pyrroline-5-carboxylate, suggesting a possible involvement of TbMCP14 in energy production. The generation of TbMCP14 knock-out parasites showed that the carrier is not essential for survival of T. brucei bloodstream forms, but reduced parasite proliferation under standard culture conditions. In contrast, depletion of TbMCP14 in procyclic forms resulted in growth arrest, followed by parasite death. The time point at which parasite proliferation stopped was dependent on the major energy source, i.e. glucose versus proline, in the culture medium. Together with our findings that proline-dependent ATP production in crude mitochondria from TbMCP14-depleted trypanosomes was reduced compared to control mitochondria, the study demonstrates that TbMCP14 is involved in energy production in T. brucei. Since TbMCP14 belongs to a trypanosomatid-specific clade of mitochondrial carrier family proteins showing very poor similarity to mitochondrial carriers of mammals, it may represent an interesting target for drug action or targeting.

Human and animal trypanosomiases caused by Trypanosoma brucei parasites represent major burdens to human welfare and agricultural development in rural sub-Saharan Africa. Although the numbers of infected humans have decreased continuously during the last decades, emerging resistance and adverse side effects against commonly used drugs require an urgent need for the identification of novel drug targets and the development of new drugs. Using an unbiased genome-wide screen to search for genes involved in the mode of action of trypanocidal compounds, we identified a member of the mitochondrial carrier family, TbMCP14, as prime candidate to mediate the action of a group of anti-parasitic choline analogs against T. brucei. Ablation of TbMCP14 expression by RNA interference or gene deletion decreases the susceptibility of parasites towards the compounds while over-expression of the carrier shows the opposite effect. In addition, down-regulation of TbMCP14 protects mitochondria from drug-induced decrease in mitochondrial membrane potential and reduces proline-dependent ATP production. Together, the results demonstrate that TbMCP14 is involved in energy production in T. brucei, possibly by acting as a mitochondrial proline carrier, and reveal TbMCP14 as candidate protein for drug action or targeting.

Cucurbit[7]uril host-guest complexes and [2]pseudorotaxanes with N-methylpiperidinium, N-methylpyrrolidinium, and N-methylmorpholinium cations in aqueous solution

The formations of host-guest complexes between cucurbit[7]uril and a series of N-substituted N-methylpiperidinium, N-methylpyrrolidinium, and N-methylmorpholinium cations in aqueous solution have been investigated using (1)H NMR spectroscopy and electrospray ionization mass spectrometry. Dications comprising the N-methylheterocyclic head groups, bridged by a decamethylene chain, form sequential 1 : 1 ([2]pseudorotaxanes) and 2 : 1 host-guest complexes with cucurbit[7]uril. The cucurbituril initially resides over the decamethylene chain, however with further additions of the host molecule a translocation of the hosts to the cationic N-heterocyclic head groups occurs. The order of the magnitude of the cucurbituril host-guest stability constants, determined by competitive (1)H NMR binding experiments, follows the trend in the hydrophobicity of the quaternary ammonium cations.

Lipid synthesis in protozoan parasites: a comparison between kinetoplastids and apicomplexans

Lipid metabolism is of crucial importance for pathogens. Lipids serve as cellular building blocks, signalling molecules, energy stores, posttranslational modifiers, and pathogenesis factors. Parasites rely on a complex system of uptake and synthesis mechanisms to satisfy their lipid needs. The parameters of this system change dramatically as the parasite transits through the various stages of its life cycle. Here we discuss the tremendous recent advances that have been made in the understanding of the synthesis and uptake pathways for fatty acids and phospholipids in apicomplexan and kinetoplastid parasites, including Plasmodium, Toxoplasma, Cryptosporidium, Trypanosoma and Leishmania. Lipid synthesis differs in significant ways between parasites from both phyla and the human host. Parasites have acquired novel pathways through endosymbiosis, as in the case of the apicoplast, have dramatically reshaped substrate and product profiles, and have evolved specialized lipids to interact with or manipulate the host. These differences potentially provide opportunities for drug development. We outline the lipid pathways for key species in detail as they progress through the developmental cycle and highlight those that are of particular importance to the biology of the pathogens and/or are the most promising targets for parasite-specific treatment.

Characterization of choline uptake in Trypanosoma brucei procyclic and bloodstream forms

Choline is an essential nutrient for eukaryotic cells, where it is used as precursor for the synthesis of choline-containing phospholipids, such as phosphatidylcholine (PC). According to published data, Trypanosoma brucei parasites are unable to take up choline from the environment but instead use lyso-phosphatidylcholine as precursor for choline lipid synthesis. We now show that T. brucei procyclic forms in culture readily incorporate [(3)H]-labeled choline into PC, indicating that trypanosomes express a transporter for choline at the plasma membrane. Characterization of the transport system in T. brucei procyclic and bloodstream forms shows that uptake of choline is independent of sodium and potassium ions and occurs with a Km in the low micromolar range. In addition, we demonstrate that choline uptake can be blocked by the known choline transport inhibitor, hemicholinium-3, and by synthetic choline analogs that have been established as anti-malarials. Together, our results show that T. brucei parasites express an uptake system for choline and that exogenous choline is used for PC synthesis.

Novel antimalarial drug targets: hope for new antimalarial drugs

Malaria is a major global threat, that results in more than 2 million deaths each year. The treatment of malaria is becoming extremely difficult due to the emergence of drug-resistant parasites, the absence of an effective vaccine, and the spread of insecticide-resistant vectors. Thus, malarial therapy needs new chemotherapeutic approaches leading to the search for new drug targets. Here, we discuss different approaches to identifying novel antimalarial drug targets. We have also given due attention to the existing validated targets with a view to develop novel, rationally designed lead molecules. Some of the important parasite proteins are claimed to be the targets; however, further in vitro or in vivo structure-function studies of such proteins are crucial to validate these proteins as suitable targets. The interactome analysis among apicoplast, mitochondrion and genomic DNA will also be useful in identifying vital pathways or proteins regulating critical pathways for parasite growth and survival, and could be attractive targets. Molecules responsible for parasite invasion to host erythrocytes and ion channels of infected erythrocytes, essential for intra-erythrocyte survival and stage progression of parasites are also becoming attractive targets. This review will discuss and highlight the current understanding regarding the potential antimalarial drug targets, which could be utilized to develop novel antimalarials.

Choline analogues in malaria chemotherapy

Emerging resistance against well-established anti-malaria drugs warrants the introduction of new therapeutic agents with original mechanisms of action. Inhibition of membrane-based phospholipid biosynthesis, which is crucial for the parasite, has thus been proposed as a novel and promising therapeutic strategy. This review compiles literature concerning the design and study of choline analogues and related cation derivatives as potential anti-malarials. It covers advances achieved over the last two decades and describes: the concept validation, the design and selection of a clinical candidate (Albitiazolium), back-up derivatives while also providing insight into the development of prodrug approaches.

Symmetrical choline-derived dications display strong anti-kinetoplastid activity

OBJECTIVES

to investigate the anti-kinetoplastid activity of choline-derived analogues with previously reported antimalarial efficacy.

METHODS

from an existing choline analogue library, seven antimalarial compounds, representative of the first-, second- and third-generation analogues previously developed, were assessed for activity against Trypanosoma and Leishmania spp. Using a variety of techniques, the effects of choline analogue exposure on the parasites were documented and a preliminary investigation of their mode of action was performed.

RESULTS

the activities of choline-derived compounds against Trypanosoma brucei and Leishmania mexicana were determined. The compounds displayed promising anti-kinetoplastid activity, particularly against T. brucei, to which 4/7 displayed submicromolar EC(50) values for the wild-type strain. Low micromolar concentrations of most compounds cleared trypanosome cultures within 24-48 h. The compounds inhibit a choline transporter in Leishmania, but their entry may not depend only on this carrier; T. b. brucei lacks a choline carrier and the mode of uptake remains unclear. The compounds had no effect on the overall lipid composition of the cells, cell cycle progression or cyclic adenosine monophosphate production or short-term effects on intracellular calcium levels. However, several of the compounds, displayed pronounced effects on the mitochondrial membrane potential; this action was not associated with production of reactive oxygen species but rather with a slow rise of intracellular calcium levels and DNA fragmentation.

CONCLUSIONS

the choline analogues displayed strong activity against kinetoplastid parasites, particularly against T. b. brucei. In contrast to their antimalarial activity, they did not act on trypanosomes by disrupting choline salvage or phospholipid metabolism, instead disrupting mitochondrial function, leading to chromosomal fragmentation.

Vitamin B metabolism in Plasmodium falciparum as a source of drug targets

The malaria parasite Plasmodium falciparum depends primarily on nutrient sources from its human host. Most compounds, such as glucose, purines, amino acids, as well as cofactors and vitamins, are abundantly available in the host cell, and can be readily salvaged by the parasite. However, in some cases the parasite can also synthesize cofactors de novo in reactions that appear to be essential. Importantly, the three biosynthetic pathways that produce vitamins B(1), B(6) and B(9) are absent from the host, but are well established in P. falciparum. This review summarizes and updates the current knowledge of vitamin B de novo synthesis and salvage in P. falciparum and focuses on their potential as targets for drug intervention.

Novel therapeutic targets in Plasmodium falciparum: aquaglyceroporins

BACKGROUND

Malaria is caused by the intracellular parasite Plasmodium falciparum. The constant need for novel malaria therapies is due to the development of resistance against existing drugs.

OBJECTIVE

To summarise attempts to investigate parasitic aquaporins as drug targets in malaria.

METHODS

Starting with a summary of the history of malaria we present aquaporin structure and function relationships. Potential interactions of inhibitors with plasmodial AQP (PfAQP) are discussed. PfAQP blockage is examined in the light of recent work on knock-out parasites. Since PfAQP is able to transport other small solutes the parasites are sensitive to other compounds which are harmless to the human host.

RESULTS/CONCLUSIONS

Total blockage of PfAQP may not lead to the death of the parasite but application of PfAQP as a vehicle for toxic substances may be a further pathway for research.

A systematic approach to understand the mechanism of action of the bisthiazolium compound T4 on the human malaria parasite, Plasmodium falciparum

Background

In recent years, a major increase in the occurrence of drug resistant falciparum malaria has been reported. Choline analogs, such as the bisthiazolium T4, represent a novel class of compounds with strong potency against drug sensitive and resistant P. falciparum clones. Although T4 and its analogs are presumed to target the parasite's lipid metabolism, their exact mechanism of action remains unknown. Here we have employed transcriptome and proteome profiling analyses to characterize the global response of P. falciparum to T4 during the intraerythrocytic cycle of this parasite.

Results

No significant transcriptional changes were detected immediately after addition of T4 despite the drug's effect on the parasite metabolism. Using the Ontology-based Pattern Identification (OPI) algorithm with an increased T4 incubation time, we demonstrated cell cycle arrest and a general induction of genes involved in gametocytogenesis. Proteomic analysis revealed a significant decrease in the level of the choline/ethanolamine-phosphotransferase (PfCEPT), a key enzyme involved in the final step of synthesis of phosphatidylcholine (PC). This effect was further supported by metabolic studies, which showed a major alteration in the synthesis of PC from choline and ethanolamine by the compound.

Conclusion

Our studies demonstrate that the bisthiazolium compound T4 inhibits the pathways of synthesis of phosphatidylcholine from choline and ethanolamine in P. falciparum, and provide evidence for post-transcriptional regulations of parasite metabolism in response to external stimuli.

Antimalarial drugs - what is in use and what is in the pipeline

Malaria continues to be a potentially fatal threat to almost half of the world's population. In light of this threat, the armory to fight this disease is rather limited. Resistance against the most common and affordable antimalarials is widespread. Only few new drugs are in clinical development, most of them belong to long used classes of antimalarial drugs. This review will concisely cover the drugs which are currently in use, and describe the drug candidates which are in clinical evaluation.

Pyridinium cationic-dimer antimalarials, unlike chloroquine, act selectively between the schizont stage and the ring stage of Plasmodium falciparum

Malaria is a leading cause of death in developing countries, and the emergence of strains resistant to the main therapeutic agent, chloroquine, has become a serious problem. We have developed cationic-dimer type antimalarials, MAP-610 and PMAP-H10, which are structurally different from chloroquine. In this study, we introduced several substituents on the terminal phenyl rings of PMAP-H10. The electronic and hydrophobic properties of the substituents were correlated with the antimalarial activity and cytotoxicity of the compounds, respectively. Studies with synchronized cultures of malarial plasmodia showed that our cationic-dimers act selectively between the schizont stage and the ring stage of the parasitic cycle, unlike chloroquine, which has a stage-independent action. Thus, the mechanism of action of our antimalarials appears to be different from that of chloroquine, and our compounds may be effective against chloroquine-resistant strains.

Antimalarial Cation-dimers Synthesized in Two Steps from an Inexpensive Starting Material, Isonicotinic Acid

Malaria is one of the three major serious infectious diseases in the world. As the area affected by malaria includes a large proportion of developing countries, there is a need for new antimalarials that can be synthesized and supplied inexpensively. To generate low-cost antimalarials, the MAP series 6-10, bis-cation dimers, synthesized by amidating the carboxyl group of isonicotinic acid (11) with various amines and by cationizing the nitrogen atoms of the pyridine ring with the corresponding alkyl bromides, were designed. This design enabled expansion of the structural variations of bis-cation-type antimalarial compounds. The compounds bearing alkyl or phenyl groups in the amide moieties showed remarkable antimalarial activities in vitro. Moreover, 1,1'-(1,12-dodecanediyl)bis[4-[(buthylamino)carbonyl]pyridinium bromide], MAP-412 (6 d), exhibited a potent antimalarial activity (ED(50)=8.2 mg kg(-1)). Being prepared at low cost, our bis-cation-type antimalarial compounds may be useful as lead compounds for inexpensive antimalarials.

Malaria Chemotherapeutics Part I: History of Antimalarial Drug Development, Currently Used Therapeutics, and Drugs in Clinical Development

Since ancient times, humankind has had to struggle against the persistent onslaught of pathogenic microorganisms. Nowadays, malaria is still the most important infectious disease worldwide. Considerable success in gaining control over malaria was achieved in the 1950s and 60s through landscaping measures, vector control with the insecticide DDT, and the widespread administration of chloroquine, the most important antimalarial agent ever. In the late 1960s, the final victory over malaria was believed to be within reach. However, the parasites could not be eradicated because they developed resistance against the most widely used and affordable drugs of that time. Today, cases of malaria infections are on the rise and have reached record numbers. This review gives a short description of the malaria disease, briefly addresses the history of antimalarial drug development, and focuses on drugs currently available for malaria therapy. The present knowledge regarding their mode of action and the mechanisms of resistance are explained, as are the attempts made by numerous research groups to overcome the resistance problem within classes of existing drugs and in some novel classes. Finally, this review covers all classes of antimalarials for which at least one drug candidate is in clinical development. Antimalarial agents that are solely in early development stages will be addressed in a separate review.

Novel molecular targets for antimalarial chemotherapy

The emergence and spread of drug-resistant malaria parasites is a serious public health problem in the tropical world. Malaria control has relied upon the traditional quinoline, antifolate and artemisinin compounds. Very few new antimalarials were developed in the last quarter of the 20th century. An alarming increase in drug-resistant strains of the malaria parasite poses a significant problem for effective control. Recent advances in our knowledge of parasite biology as well as the availability of the genome sequence provide a wide range of novel targets for drug design. Gene products involved in controlling vital aspects of parasite metabolism and organelle function could be attractive targets. It is expected that the application of functional genomic tools in combination with modern approaches such as structure-based drug design and combinatorial chemistry will lead to the development of effective new drugs against drug-resistant malaria strains. This review discusses novel molecular targets of the malaria parasite available to the drug discovery scientist.

Inhibition of Plasmodium falciparum choline kinase by hexadecyltrimethylammonium bromide: a possible antimalarial mechanism

Choline kinase is the first enzyme in the Kennedy pathway (CDP-choline pathway) for the biosynthesis of the most essential phospholipid, phosphatidylcholine, in Plasmodium falciparum. In addition, choline kinase also plays a pivotal role in trapping essential polar head group choline inside the malaria parasite. Recently, Plasmodium falciparum choline kinase (PfCK) has been cloned, overexpressed, and purified. However, the function of this enzyme in parasite growth and survival has not been evaluated owing to the lack of a suitable inhibitor. Purified recombinant PfCK enabled us to identify an inhibitor of PfCK, hexadecyltrimethylammonium bromide (HDTAB), which has a very close structural resemblance to hexadecylphosphocholine (miltefosin), the well-known antiproliferative and antileishmanial drug. HDTAB inhibited PfCK in a dose-dependent manner and offered very potent antimalarial activity in vitro against Plasmodium falciparum. Moreover, HDTAB exhibited profound antimalarial activity in vivo against the rodent malaria parasite Plasmodium yoelii (N-67 strain). Interestingly, parasites at the trophozoite and schizont stages were found to be particularly sensitive to HDTAB. The stage-specific antimalarial effect of HDTAB correlated well with the expression pattern of PfCK in P. falciparum, which was observed by reverse transcription-PCR and immunofluorescence microscopy. Furthermore, the antimalarial activity of HDTAB paralleled the decrease in phosphatidylcholine content, which was found to correlate with the decreased phosphocholine generation. These results suggest that inhibition of choline kinase by HDTAB leads to decreased phosphocholine, which in turn causes a decrease in phosphatidylcholine biosynthesis, resulting in death of the parasite.

Molecular characterization and localization of Plasmodium falciparum choline kinase

Generation of phosphocholine by choline kinase is important for phosphatidylcholine biosynthesis via Kennedy pathway and phosphatidylcholine biosynthesis is essential for intraerythrocytic growth of malaria parasite. A putative gene (Gene ID PF14_0020) in chromosome 14, having highest sequence homology with choline kinase, has been identified by BLAST searches from P. falciparum genome sequence database. This gene has been PCR amplified, cloned, over-expressed and characterized. Choline kinase activity of the recombinant protein (PfCK) was validated as it catalyzed the formation of phosphocholine from choline in presence of ATP. The K(m) values for choline and ATP are found to be 145+/-20 microM and 2.5+/-0.3 mM, respectively. PfCK can phosphorylate choline efficiently but not ethanolamine. Southern blotting indicates that PfCK is a single copy gene and it is a cytosolic protein as evidenced by Western immunoblotting and confocal microscopy. A model structure of PfCK was constructed based on the crystal structure of choline kinase of C. elegans to search the structural homology. Consistent with the homology modeling predictions, CD analysis indicates that the alpha and beta content of PfCK are 33% and 14%, respectively. Since choline kinase plays a vital role for growth and multiplication of P. falciparum during intraerythrocytic stages, we can suggest that this well characterized PfCK may be exploited in the screening of new choline kinase inhibitors to evaluate their antimalarial activity.

Localization of the phosphoethanolamine methyltransferase of the human malaria parasite Plasmodium falciparum to the Golgi apparatus

Phosphatidylcholine is the most abundant phospholipid in the membranes of Plasmodium falciparum, the agent of severe human malaria. The synthesis of this phospholipid occurs via two routes, the CDP-choline pathway, which uses host choline as a precursor, and the plant-like serine decarboxylase-phosphoethanolamine methyltransferase (SDPM) pathway, which uses host serine as a precursor. Although various components of these pathways have been identified, their cellular locations remain unknown. We have previously reported the identification and characterization of the phosphoethanolamine methyltransferase, Pfpmt, of P. falciparum and shown that it plays a critical role in the synthesis of phosphatidylcholine via the SDPM pathway. Here we provide the first evidence that the transmethylation step of the SDPM pathway occurs in the parasite Golgi apparatus. We show that the level of Pfpmt protein in the infected erythrocyte is regulated in a stage-specific fashion, with high levels detected during the trophozoite stage at the peak of parasite membrane biogenesis. Confocal microscopy revealed that Pfpmt is not cytoplasmic. Immunoelectron microscopy revealed that Pfpmt localizes to membrane structures that extend from the nuclear membrane but that it only partially co-localizes with the endoplasmic reticulum marker BiP. Using transgenic parasites expressing green fluorescent protein targeted to different cellular compartments, a complete co-localization was detected with Rab6, a marker of the Golgi apparatus. Together these studies provide the first evidence that the transmethylation step of the SDPM pathway of P. falciparum occurs in the Golgi apparatus and indicate an important role for this organelle in parasite membrane biogenesis.

In Vivo Evidence for the Specificity of Plasmodium falciparum Phosphoethanolamine Methyltransferase and Its Coupling to the Kennedy Pathway

Unlike humans and yeast, Plasmodium falciparum, the agent of the most severe form of human malaria, utilizes host serine as a precursor for the synthesis of phosphatidylcholine via a plant-like pathway involving phosphoethanolamine methylation. The monopartite phosphoethanolamine methyltransferase, Pfpmt, plays an important role in the biosynthetic pathway of this major phospholipid by providing the precursor phosphocholine via a three-step S-adenosyl-L-methionine-dependent methylation of phosphoethanolamine. In vitro studies showed that Pfpmt has strong specificity for phosphoethanolamine. However, the in vivo substrate (phosphoethanolamine or phosphatidylethanolamine) is not yet known. We used yeast as a surrogate system to express Pfpmt and provide genetic and biochemical evidence demonstrating the specificity of Pfpmt for phosphoethanolamine in vivo. Wild-type yeast cells, which inherently lack phosphoethanolamine methylation, acquire this activity as a result of expression of Pfpmt. The Pfpmt restores the ability of a yeast mutant pem1Deltapem2Delta lacking the phosphatidylethanolamine methyltransferase genes to grow in the absence of choline. Lipid analysis of the Pfpmt-complemented pem1Deltapem2Delta strain demonstrates the synthesis of phosphatidylcholine but not the intermediates of phosphatidylethanolamine transmethylation. Complementation of the pem1Deltapem2Delta mutant relies on specific methylation of phosphoethanolamine but not phosphatidylethanolamine. Interestingly, a mutation in the yeast choline-phosphate cytidylyltransferase gene abrogates the complementation by Pfpmt thus demonstrating that Pfpmt activity is directly coupled to the Kennedy pathway for the de novo synthesis of phosphatidylcholine.

Current awareness on yeast

In order to keep subscribers up‐to‐date with the latest developments in their field, this current awareness service is provided by John Wiley & Sons and contains newly‐published material on yeasts. Each bibliography is divided into 10 sections. 1 Books, Reviews & Symposia; 2 General; 3 Biochemistry; 4 Biotechnology; 5 Cell Biology; 6 Gene Expression; 7 Genetics; 8 Physiology; 9 Medical Mycology; 10 Recombinant DNA Technology. Within each section, articles are listed in alphabetical order with respect to author. If, in the preceding period, no publications are located relevant to any one of these headings, that section will be omitted. (4 weeks journals ‐ search completed 10th. Nov. 2004)

Characterization of the choline carrier of Plasmodium falciparum: a route for the selective delivery of novel antimalarial drugs

New drugs are urgently needed to combat the growing problem of drug resistance in Plasmodium falciparum malaria. The infected erythrocyte is a multicompartmental system, and its transporters are of interest as drug targets in their own right and as potential routes for the delivery of antimalarial drugs. Choline is an important nutrient that penetrates infected erythrocyte membranes through the endogenous carrier and through parasite-induced permeability pathways, but nothing is known about its transport into the intracellular parasite. Here we present the first characterization of choline transport across the parasite membrane. Transport exhibits Michaelis-Menten kinetics with an apparent Km of 25.0 ± 3.5 μM for choline. The carrier is inhibitor-sensitive, temperature-dependent, and Na+-independent, and it is driven by the proton-motive force. Highly active bis-amidine and bis-quaternary ammonium compounds are also known to penetrate the host erythrocyte membrane through parasite-induced permeability pathways. Here, we demonstrate that the parasite choline transporter mediates the delivery of these compounds to the intracellular parasite. Thus, the induced permeability pathways in the host erythrocyte membrane and the parasite choline transporter described here form a cooperative transport system that shows great promise for the selective targeting of new agents for the chemotherapy of malaria. (Blood. 2004;104: 3372-3377)

Prodrugs of bisthiazolium salts are orally potent antimalarials

We created neutral antimalarial prodrugs that deliver bisthiazolium compounds with antimalarial activity in the nanomolar range. These drugs primarily affect early intraerythrocytic stages through rapid, nonreversible cytotoxicity. The compounds are suitable for both parenteral and oral use and plasma promotes rapid conversion of the prodrug into the drug. We demonstrate that very low doses offer protection in a murine model of malaria. The drugs show great potential for curing high parasitemia with short-course treatments. Oral administration of the TE3 prodrug completely cures Plasmodium cynomolgi infection in rhesus monkeys. The drugs specifically accumulate inside infected erythrocytes, block phosphatidylcholine biosynthesis, and interact with hemozoin. To our knowledge, this class of compounds represents one of the most potent antimalarials tested to date. These unique properties signal a promising future for this class of antimalarial.