Plasmodium falciparum: new molecular targets with potential for antimalarial drug development

Plant-based nanoparticles targeting malaria management

Malaria is one of the most devastating diseases across the globe, particularly in low-income countries in Sub-Saharan Africa. The increasing incidence of malaria morbidity is mainly due to the shortcomings of preventative measures such as the lack of vaccines and inappropriate control over the parasite vector. Additionally, high mortality rates arise from therapeutic failures due to poor patient adherence and drug resistance development. Although the causative pathogen (Plasmodium spp.) is an intracellular parasite, the recommended antimalarial drugs show large volumes of distribution and low-to no-specificity towards the host cell. This leads to severe side effects that hamper patient compliance and promote the emergence of drug-resistant strains. Recent research efforts are promising to enable the discovery of new antimalarial agents; however, the lack of efficient means to achieve targeted delivery remains a concern, given the risk of further resistance development. New strategies based on green nanotechnologies are a promising avenue for malaria management due to their potential to eliminate malaria vectors (Anopheles sp.) and to encapsulate existing and emerging antimalarial agents and deliver them to different target sites. In this review we summarized studies on the use of plant-derived nanoparticles as cost-effective preventative measures against malaria parasites, starting from the vector stage. We also reviewed plant-based nanoengineering strategies to target malaria parasites, and further discussed the site-specific delivery of natural products using ligand-decorated nanoparticles that act through receptors on the host cells or malaria parasites. The exploration of traditionally established plant medicines, surface-engineered nanoparticles and the molecular targets of parasite/host cells may provide valuable insights for future discovery of antimalarial drugs and open new avenues for advancing science toward the goal of malaria eradication.

Novel quinolinepiperazinyl-aryltetrazoles targeting the blood stage of Plasmodium falciparum

The emergence of drug resistance against the frontline antimalarials is a major challenge in the treatment of malaria. In view of emerging reports on drug-resistant strains of Plasmodium against artemisinin combination therapy, a dire need is felt for the discovery of novel compounds acting against novel targets in the parasite. In this study, we identified a novel series of quinolinepiperazinyl-aryltetrazoles (QPTs) targeting the blood stage of Plasmodium. In vitro anti-plasmodial activity screening revealed that most of the compounds showed IC50 < 10 μM against chloroquine-resistant PfINDO strain, with the most promising lead compounds 66 and 75 showing IC50 values of 2.25 and 1.79 μM, respectively. Further, compounds 64-66, 68, 75-77 and 84 were found to be selective (selectivity index >50) in their action against Pf over a mammalian cell line, with compounds 66 and 75 offering the highest selectivity indexes of 178 and 223, respectively. Explorations into the action of lead compounds 66 and 75 revealed their selective cidal activity towards trophozoites and schizonts. In a ring-stage survival assay, 75 showed cidal activity against the early rings of artemisinin-resistant PfCam3.1R539T. Further, 66 and 75 in combination with artemisinin and pyrimethamine showed additive to weak synergistic interactions. Of these two in vitro lead molecules, only 66 restricted rise in the percentage of parasitemia to about 10% in P. berghei-infected mice with a median survival time of 28 days as compared to the untreated control, which showed the percentage of parasitemia >30%, and a median survival of 20 days. Promising antimalarial activity, high selectivity, and additive interaction with artemisinin and pyrimethamine indicate the potential of these compounds to be further optimized chemically as future drug candidates against malaria.

Emerging Drug Targets for Antimalarial Drug Discovery: Validation and Insights into Molecular Mechanisms of Function

Approximately 619,000 malaria deaths were reported in 2021, and resistance to recommended drugs, including artemisinin-combination therapies (ACTs), threatens malaria control. Treatment failure with ACTs has been found to be as high as 93% in northeastern Thailand, and parasite mutations responsible for artemisinin resistance have already been reported in some African countries. Therefore, there is an urgent need to identify alternative treatments with novel targets. In this Perspective, we discuss some promising antimalarial drug targets, including enzymes involved in proteolysis, DNA and RNA metabolism, protein synthesis, and isoprenoid metabolism. Other targets discussed are transporters, Plasmodium falciparum acetyl-coenzyme A synthetase, N-myristoyltransferase, and the cyclic guanosine monophosphate-dependent protein kinase G. We have outlined mechanistic details, where these are understood, underpinning the biological roles and hence druggability of such targets. We believe that having a clear understanding of the underlying chemical interactions is valuable to medicinal chemists in their quest to design appropriate inhibitors.

Gas-Phase Studies of Hypoxanthine-Guanine-(Xanthine) Phosphoribosyltransferase (HG(X)PRT) Substrates

The gas-phase acidity and proton affinity of nucleobases that are substrates for the enzyme Plasmodium falciparum hypoxanthine-guanine-(xanthine) phosphoribosyltransferase (Pf HG(X)PRT) have been examined using both computational and experimental methods. These thermochemical values have not heretofore been measured and provide experimental data to benchmark the theoretical results. Pf HG(X)PRT is a target of interest in the development of antimalarials. We use our gas-phase results to lend insight into the Pf HG(X)PRT mechanism, and also propose kinetic isotope studies that could potentially differentiate between possible mechanisms.

Blood-stage antiplasmodial activity and oocyst formation-blockage of metallo copper-cinchonine complex

In the fight against malaria, the key is early treatment with antimalarial chemotherapy, such as artemisinin-based combination treatments (ACTs). However, Plasmodium has acquired multidrug resistance, including the emergence of P. falciparum strains with resistance to ACT. The development of novel antimalarial molecules, that are capable of interfering in the asexual and sexual blood stages, is important to slow down the transmission in endemic areas. In this work, we studied the ability of the mettalo copper-cinchonine complex to interfere in the sexual and asexual stages of Plasmodium. The tested compound in the in vitro assay was a cinchonine derivative, named CinCu (Bis[Cinchoninium Tetrachlorocuprate(II)]trihydrate). Its biological functions were assessed by antiplasmodial activity in vitro against chloroquine-resistant P. falciparum W2 strain. The mice model of P. berghei ANKA infection was used to analyze the antimalarial activity of CinCu and chloroquine and their acute toxicity. The oocyst formation-blocking assay was performed by experimental infection of Anopheles aquasalis with P. vivax infected blood, which was treated with different concentrations of CinCu, cinchonine, and primaquine. We found that CinCu was able to suppress as high as 81.58% of parasitemia in vitro, being considered a molecule with high antiplasmodial activity and low toxicity. The in vivo analysis showed that CinCu suppressed parasitemia at 34% up to 87.19%, being a partially active molecule against the blood-stage forms of P. berghei ANKA, without inducing severe clinical signs in the treated groups. The transmission-blocking assay revealed that both cinchonine and primaquine were able to reduce the infection intensity of P. vivax in A. aquasalis, leading to a decrease in the number of oocysts recovered from the mosquitoes' midgut. Regarding the effect of CinCu, the copper-complex was not able to induce inhibition of P. vivax infection; however, it was able to induce an important reduction in the intensity of oocyst formation by about 2.4 times. It is plausible that the metallo-compound also be able to interfere with the differentiation of parasite stages and/or ookinete-secreted chitinase into the peritrophic matrix of mosquitoes, promoting a reduction in the number of oocysts formed. Taken together, the results suggest that this compound is promising as a prototype for the development of new antimalarial drugs. Furthermore, our study can draw a new pathway for repositioning already-known antimalarial drugs by editing their chemical structure to improve the antimalarial activity against the asexual and sexual stages of the parasite.

An update mini-review on the progress of azanucleoside analogues

The development of structurally novel nucleoside analogues is an active area in medicinal chemistry, since these drugs have proven clinical efficacy for decades. Azanucleosides are nucleoside analogues in which the sugar moieties are composed of nitrogen-containing rings or chains. In recent years, many azanucleosides have demonstrated therapeutic potential. In this short review, we describe recent advancements in azanucleosides, which may translate in a better understanding of the molecular design, biological activity, structure-activity relationship, and their related mechanism of action. The information summarized in this paper should encourage medicinal chemists in their future efforts to create more potent and effective chemotherapeutic agents.

Drug Repurposing of Quisinostat to Discover Novel Plasmodium falciparum HDAC1 Inhibitors with Enhanced Triple-Stage Antimalarial Activity and Improved Safety

Our previous work found that the clinical histone deacetylase (HDAC) inhibitor quisinostat exhibited a significant antimalarial effect but with severe toxicity. In this work, 35 novel derivatives were designed and synthesized based on quisinostat as the lead compound, and their in vitro antimalarial activities and cytotoxicities were systematically evaluated. Among them, JX35 showed potent inhibition against both wild-type and multidrug-resistant parasite strains and displayed a significant in vivo killing effect against all life cycles of parasites, including the blood stage, liver stage, and gametocyte stage, indicating its potential for the simultaneous treatment, chemoprevention, and blockage of malaria transmission. Compared with quisinostat, JX35 exhibited stronger antimalarial efficacy, more adequate safety, and good pharmacokinetic properties. Additionally, mechanistic studies via molecular docking studies, induced PfHDAC1/2 knockdown assays, and PfHDAC1 enzyme inhibition assays jointly indicated that the antimalarial target of JX35 was PfHDAC1. In summary, we discovered the promising candidate PfHDAC1 inhibitor JX35, which showed stronger triple-stage antimalarial effects and lower toxicity than quisinostat.

Synthesis, and antimalarial and antibacterial activities of marine alkaloids

The activities of marine alkaloids are manifested in antifungus and antimalaria. The optimization process, chemical synthesis, antimalarial activity, and antibacterial activity of various compounds were discussed.

Development and Validation of a Sensitive, Specific and Reproducible UPLC-MS/MS Method for the Quantification of OJT007, A Novel Anti-Leishmanial Agent: Application to a Pharmacokinetic Study

OJT007 is a methionine aminopeptidase 1 (MetAP1) inhibitor with potent anti-proliferative effects against Leishmania Major. In order to study its pharmacokinetics as a part of the drug development process, a sensitive, specific, and reproducible ultra-high performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) method was developed and validated. Voriconazole was used as the internal standard to generate standard curves ranging from 5 to 1000 ng/mL. The separation was achieved using a UPLC system equipped with an Acquity UPLC BEH C₁₈ column (2.1 × 50 mm, 1.7 μm) with 0.1% formic acid in acetonitrile and 0.1% formic acid in water as the mobile phase under gradient elution at a flow rate of 0.4 mL/min. The mass analysis was performed with a 4000 QTRAP^® mass spectrometer using multiple-ion reaction monitoring (MRM) in the positive mode, with the transition of m/z 325 → m/z 205 for OJT007 and m/z 350 → m/z 101 for voriconazole. The intra- and inter-day precision and accuracy were within ±15%. The mean extraction recovery and the matrix effect were 95.1% and 7.96%, respectively, suggesting no significant matrix interfering with the quantification of the drug in rat plasma. This study was successfully used for the pharmacokinetic evaluation of OJT007 using the rat as an animal model.

Retracted Article: The synthesis and biological activity of marine alkaloid derivatives and analogues

The ocean is the origin of life, with a unique ecological environment, which has given birth to a wealth of marine organisms. The ocean is an important source of biological resources and tens of thousands of monomeric compounds have been separated from marine organisms using modern separation technology. Most of these monomeric compounds have some kind of biological activity that has attracted extensive attention from researchers. Marine alkaloids are a kind of compound that can be separated from marine organisms. They have complex and special chemical structures, but at the same time, they can show diversity in biological activities. The biological activities of marine alkaloids mainly manifest in the form of anti-tumor, anti-fungus, anti-viral, anti-malaria, and anti-osteoporosis properties. Many marine alkaloids have good medicinal prospects and can possibly be used as anti-tumor, anti-viral, and anti-fungal clinical drugs or as lead compounds. The limited amounts of marine alkaloids that can be obtained by separation, coupled with the high cytotoxicity and low selectivity of these lead compounds, has restricted the clinical research and industrial development of marine alkaloids. Marine alkaloid derivatives and analogues have been obtained via rational drug design and chemical synthesis, to make up for the shortcomings of marine alkaloids; this has become an urgent subject for research and development. This work systematically reviews the recent developments relating to marine alkaloid derivatives and analogues in the field of medical chemistry over the last 10 years (2010-2019). We divide marine alkaloid derivatives and analogues into five types from the point-of-view of biological activity and elaborated on these activities. We also briefly discuss the optimization process, chemical synthesis, biological activity evaluation, and structure-activity relationship (SAR) of each of these compounds. The abundant SAR data provides reasonable approaches for the design and development of new biologically active marine alkaloid derivatives and analogues.

Selective inhibition of PfA-M1, over PfA-M17, by an amino-benzosuberone derivative blocks malaria parasites development in vitro and in vivo

Background

Plasmodium falciparum M1 family aminopeptidase is currently considered as a promising target for anti-malarial chemotherapy. Several series of inhibitors developed by various research groups display IC50/Ki values down to nM range on native PfA-M1 or recombinant forms and block the parasite development in culture at µM to sub-µM concentrations. A handful of these inhibitors has been tested on murine models of malaria and has shown anti plasmodial in vivo activity. However, most of these inhibitors do also target the other neutral malarial aminopeptidase, PfA-M17, often with lower Ki values, which questions the relative involvement and importance of each enzyme in the parasite biology.

Results

An amino-benzosuberone derivative from a previously published collection of chemicals targeting specifically the M1-aminopeptidases has been identified; it is highly potent on PfA-M1 (Ki = 50 nM) and devoid of inhibitory activity on PfA-M17 (no inhibition up to 100 µM). This amino-benzosuberone derivative (T5) inhibits, in the µM range, the in vitro growth of two P. falciparum strains, 3D7 and FcB1, respectively chloroquino-sensitive and resistant. Evaluated in vivo, on the murine non-lethal model of malaria Plasmodium chabaudi chabaudi, this amino-benzosuberone derivative was able to reduce the parasite burden by 44 and 40% in a typical 4-day Peters assay at a daily dose of 12 and 24 mg/kg by intraperitoneal route of administration.

Conclusions

The evaluation of a highly selective inhibitor of PfA-M1, over PfA-M17, active on Plasmodium parasites in vitro and in vivo, highlights the relevance of PfA-M1 in the biological development of the parasite as well as in the list of promising anti-malarial targets to be considered in combination with current or future anti-malarial drugs.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-017-2032-4) contains supplementary material, which is available to authorized users.

Targeting the active sites of malarial proteases for antimalarial drug discovery: approaches, progress and challenges

Malaria is an infectious disease causing vast mortality and morbidity worldwide. Although antimalarial drugs are effective in several parts of the world, there is a serious threat to malaria control as malaria parasites are continuously developing widespread resistance against currently available antimalarial drugs, including artemisinin. Such widespread antimalarial drug resistance confirms the need to improve the efficacy of existing or new drugs as well as to develop alternative treatments through the identification of novel drug targets and the development of candidate drugs. Similar to proteases in other parasitic diseases such as leishmaniasis, schistosomiasis, Chagas disease and African sleeping sickness, malarial proteases constitute the major virulence factors in malaria. Malarial proteases belong to several classes and many of them have been targeted for the design and discovery of antimalarial agents. This review summarises the approaches, progress and challenges in the design of small-molecule inhibitors as antimalarial drugs targeting the inhibition of various malarial proteases.

Epigenetic landscapes underlining global patterns of gene expression in the human malaria parasite, Plasmodium falciparum

The dynamic chromatin landscape displaying combinatorial complexity of the epigenome impacts gene expression that underlies many events of differentiation and cell cycle progression. In the past few years, epigenetic mechanisms have emerged as important processes involved in the tight gene regulation in malaria parasites, Plasmodium spp. Focusing predominantly on Plasmodium falciparum, the species associated with the most severe form of the disease, many advances have been made in our understanding of the interaction between transcriptional regulation and epigenetic mechanisms as the pivotal processes in regulating life cycle progression, host parasite interactions and parasite adaptation to the host environment. This review focuses on the epigenome and its effect on transcriptional regulation in P. falciparum, highlighting its unique, evolutionary diverse features.

Structure Based Docking and Molecular Dynamic Studies of Plasmodial Cysteine Proteases against a South African Natural Compound and its Analogs

Identification of potential drug targets as well as development of novel antimalarial chemotherapies with unique mode of actions due to drug resistance by Plasmodium parasites are inevitable. Falcipains (falcipain-2 and falcipain-3) of Plasmodium falciparum, which catalyse the haemoglobin degradation process, are validated drug targets. Previous attempts to develop peptide based drugs against these enzymes have been futile due to the poor pharmacological profiles and susceptibility to degradation by host enzymes. This study aimed to identify potential non-peptide inhibitors against falcipains and their homologs from other Plasmodium species. Structure based virtual docking approach was used to screen a small non-peptidic library of natural compounds from South Africa against 11 proteins. A potential hit, 5α-Pregna-1,20-dien-3-one (5PGA), with inhibitory activity against plasmodial proteases and selectivity on human cathepsins was identified. A 3D similarity search on the ZINC database using 5PGA identified five potential hits based on their docking energies. The key interacting residues of proteins with compounds were identified via molecular dynamics and free binding energy calculations. Overall, this study provides a basis for further chemical design for more effective derivatives of these compounds. Interestingly, as these compounds have cholesterol-like nuclei, they and their derivatives might be well tolerated in humans.

New insight-guided approaches to detect, cure, prevent and eliminate malaria

New challenges posed by the development of resistance against artemisinin-based combination therapies (ACTs) as well as previous first-line therapies, and the continuing absence of vaccine, have given impetus to research in all areas of malaria control. This review portrays the ongoing progress in several directions of malaria research. The variants of RTS,S and apical membrane antigen 1 (AMA1) are being developed and test adapted as multicomponent and multistage malaria control vaccines, while many other vaccine candidates and methodologies to produce antigens are under experimentation. To track and prevent the spread of artemisinin resistance from Southeast Asia to other parts of the world, rolling circle-enhanced enzyme activity detection (REEAD), a time- and cost-effective malaria diagnosis in field conditions, and a DNA marker associated with artemisinin resistance have become available. Novel mosquito repellents and mosquito trapping and killing techniques much more effective than the prevalent ones are undergoing field testing. Mosquito lines stably infected with their symbiotic wild-type or genetically engineered bacteria that kill sympatric malaria parasites are being constructed and field tested for stopping malaria transmission. A complementary approach being pursued is the addition of ivermectin-like drug molecules to ACTs to cure malaria and kill mosquitoes. Experiments are in progress to eradicate malaria mosquito by making it genetically male sterile. High-throughput screening procedures are being developed and used to discover molecules that possess long in vivo half life and are active against liver and blood stages for the fast cure of malaria symptoms caused by simple or relapsing and drug-sensitive and drug-resistant types of varied malaria parasites, can stop gametocytogenesis and sporogony and could be given in one dose. Target-based antimalarial drug designing has begun. Some of the putative next-generation antimalarials that possess in their scaffold structure several of the desired properties of malaria cure and control are exemplified by OZ439, NITD609, ELQ300 and tafenoquine that are already undergoing clinical trials, and decoquinate, usnic acid, torin-2, ferroquine, WEHI-916, MMV396749 and benzothiophene-type N-myristoyltransferase (NMT) inhibitors, which are candidates for future clinical usage. Among these, NITD609, ELQ300, decoquinate, usnic acid, torin-2 and NMT inhibitors not only cure simple malaria and are prophylactic against simple malaria, but they also cure relapsing malaria.

Methyl-methoxylpyrrolinone and flavinium nucleus binding signatures on falcipain-2 active site

Following the increasing reports of human toxicity and plasmodium resistance to artemisinin and its derivatives, falcipain-2 (FP-2) is now emerging as the choice antimalarial drug target. Coincidentally, FP-2 is the in vivo target of naturally occurring, therapeutically safe flavonoids (stenopalustroside, myricetin, and fisetin) and symplostatin (symplostatin 4) compounds known to exhibit potent in vitro and in vivo antiplasmodial actions. Here, the structural bases for their inhibitory actions have been studied using molecular dynamics simulation. Myricetin and fisetin act as proton transfer tunnel breakers by inserting between His174 and Cys42, which are key active site residues of FP-2, stenopalustroside inhibits the polarization of His174 by Asn173; a major preparatory step for Cys42/His174 proton transfer process. The roles of flavonoids are favored by T-shaped pi-pi interactions with His174. Symplostatin 4 inserts its methyl-methoxylpyrrolinone moiety into the active site where its proton acceptor function prepares Cys42 for nucleophilic attack on the Michael α,β-unsaturated bonds on its 4(S)-amino-2(E)-pentenoate moiety. Further analyses of the structures identified a unique bridge formed on FP-2 active site groove by stenopalustroside and symplostatin 4 during interaction with the sub-site I of FP-2, whereas fisetin preferentially interacts with sub-site II and myricetin interacts with sub-site III residues. Ultimately, symplostatin-4, myricetin, and fisetin were better than stenopalustroside at trapping FP-2 in its inactive state as revealed by comparative RSMD plots with X-ray structures of FP-2 co-crystallized with inhibitors. Comparative estimates of free energy of binding using the Molecular Mechanics-Poisson Boltzmann Surface Area (MMPBSA) method suggested that His174 protonation may further enhance stenopalustroside-FP-2 interaction. The unique binding signatures of the ligands within the FP-2 active site groove and its sub-sites may explain the subtle differences in their IC50 values and their mechanism of inhibition.

Dynamic epigenetic regulation of gene expression during the life cycle of malaria parasite Plasmodium falciparum

Epigenetic mechanisms are emerging as one of the major factors of the dynamics of gene expression in the human malaria parasite, Plasmodium falciparum. To elucidate the role of chromatin remodeling in transcriptional regulation associated with the progression of the P. falciparum intraerythrocytic development cycle (IDC), we mapped the temporal pattern of chromosomal association with histone H3 and H4 modifications using ChIP-on-chip. Here, we have generated a broad integrative epigenomic map of twelve histone modifications during the P. falciparum IDC including H4K5ac, H4K8ac, H4K12ac, H4K16ac, H3K9ac, H3K14ac, H3K56ac, H4K20me1, H4K20me3, H3K4me3, H3K79me3 and H4R3me2. While some modifications were found to be associated with the vast majority of the genome and their occupancy was constant, others showed more specific and highly dynamic distribution. Importantly, eight modifications displaying tight correlations with transcript levels showed differential affinity to distinct genomic regions with H4K8ac occupying predominantly promoter regions while others occurred at the 5′ ends of coding sequences. The promoter occupancy of H4K8ac remained unchanged when ectopically inserted at a different locus, indicating the presence of specific DNA elements that recruit histone modifying enzymes regardless of their broad chromatin environment. In addition, we showed the presence of multivalent domains on the genome carrying more than one histone mark, highlighting the importance of combinatorial effects on transcription. Overall, our work portrays a substantial association between chromosomal locations of various epigenetic markers, transcriptional activity and global stage-specific transitions in the epigenome.

Malaria is a devastating parasitic disease caused by the protozoan protist Plasmodium falciparum. The complex life cycle of P. falciparum comprises various morphological and functionally distinct forms and is completed in two different hosts. Various regulatory mechanisms are employed by these parasites to complete their life cycle and survive in human hosts. Epigenetic mechanisms, though not fully explored, have been implicated as one of the key players in gene regulation, morphological differentiation and antigenic variation. Here, we present a comprehensive epigenetic map of 12 histone post-translational modifications during the intraerythrocytic life cycle of P. falciparum. We have been able to identify at least eight histone modifications whose dynamic patterns correlate with the transcriptional regulation across the life cycle. In particular, we have shown that a set of euchromatic histone marks work in synergy, creating a dynamic unique histone code that is linked with gene expression during the progression of the Plasmodium intraerythrocytic developmental cycle. These findings enhance our knowledge of complex gene regulation and will help to identify novel targets for fighting malaria.

Structure-activity relationships and blood distribution of antiplasmodial aminopeptidase-1 inhibitors

Malaria is a severe infectious disease that causes between 655,000 and 1.2 million deaths annually. To overcome the resistance to current drugs, new biological targets are needed for drug development. Aminopeptidase M1 (PfAM1), a zinc metalloprotease, has been proposed as a new drug target to fight malaria. Herein, we disclosed the structure-activity relationships of a selective family of hydroxamate PfAM1 inhibitors based on the malonic template. In particular, we performed a "fluoro-scanning" around hit 1 that enlightened the key positions of the halogen for activity. The docking of the best inhibitor 2 is consistent with in vitro results. The stability of 2 was evaluated in microsomes, in plasma, and toward glutathione. The in vivo distribution study performed with the nanomolar hydroxamate inhibitor 2 (BDM14471) revealed that it reaches its site of action. However, it fails to kill the parasite at concentrations relevant to the enzymatic inhibitory potency, suggesting that killing the parasite remains a challenge for potent and druglike catalytic-site binding PfAM1 inhibitors. In all, this study provides important insights for the design of inhibitors of PfAM1 and the validity of this target.

Structural insights into the activation and inhibition of histo-aspartic protease from Plasmodium falciparum

Histo-aspartic protease (HAP) from Plasmodium falciparum is a promising target for the development of novel antimalarial drugs. The sequence of HAP is highly similar to those of pepsin-like aspartic proteases, but one of the two catalytic aspartates, Asp32, is replaced with histidine. Crystal structures of the truncated zymogen of HAP and of the complex of the mature enzyme with inhibitor KNI-10395 have been determined at 2.1 and 2.5 Å resolution, respectively. As in other proplasmepsins, the propeptide of the zymogen interacts with the C-terminal domain of the enzyme, forcing the N- and C-terminal domains apart, thereby separating His32 and Asp215 and preventing formation of the mature active site. In the inhibitor complex, the enzyme forms a tight domain-swapped dimer, not previously seen in any aspartic proteases. The inhibitor is found in an unprecedented conformation resembling the letter U, stabilized by two intramolecular hydrogen bonds. Surprisingly, the location and conformation of the inhibitor are similar to those of the fragment of helix 2 comprising residues 34p-38p in the prosegments of the zymogens of gastric aspartic proteases; a corresponding helix assumes a vastly different orientation in proplasmepsins. Each inhibitor molecule is in contact with two molecules of HAP, interacting with the carboxylate group of the catalytic Asp215 of one HAP protomer through a water molecule, while also making a direct hydrogen bond to Glu278A' of the other protomer. A comparison of the shifts in the positions of the catalytic residues in the inhibitor complex presented here with those published previously gives further hints regarding the enzymatic mechanism of HAP.

Expanding the Antimalarial Drug Arsenal-Now, But How?

The number of available and effective antimalarial drugs is quickly dwindling. This is mainly because a number of drug resistance-associated mutations in malaria parasite genes, such as crt, mdr1, dhfr/dhps, and others, have led to widespread resistance to all known classes of antimalarial compounds. Unfortunately, malaria parasites have started to exhibit some level of resistance in Southeast Asia even to the most recently introduced class of drugs, artemisinins. While there is much need, the antimalarial drug development pipeline remains woefully thin, with little chemical diversity, and there is currently no alternative to the precious artemisinins. It is difficult to predict where the next generation of antimalarial drugs will come from; however, there are six major approaches: (i) re-optimizing the use of existing antimalarials by either replacement/rotation or combination approach; (ii) repurposing drugs that are currently used to treat other infections or diseases; (iii) chemically modifying existing antimalarial compounds; (iv) exploring natural sources; (v) large-scale screening of diverse chemical libraries; and (vi) through parasite genome-based ("targeted") discoveries. When any newly discovered effective antimalarial treatment is used by the populus, we must maintain constant vigilance for both parasite-specific and human-related factors that are likely to hamper its success. This article is neither comprehensive nor conclusive. Our purpose is to provide an overview of antimalarial drug resistance, associated parasite genetic factors (1. Introduction; 2. Emergence of artemisinin resistance in P. falciparum), and the antimalarial drug development pipeline (3. Overview of the global pipeline of antimalarial drugs), and highlight some examples of the aforementioned approaches to future antimalarial treatment. These approaches can be categorized into "short term" (4. Feasible options for now) and "long term" (5. Next generation of antimalarial treatment-Approaches and candidates). However, these two categories are interrelated, and the approaches in both should be implemented in parallel with focus on developing a successful, long-lasting antimalarial chemotherapy.

A research agenda for malaria eradication: basic science and enabling technologies

Today's malaria control efforts are limited by our incomplete understanding of the biology of Plasmodium and of the complex relationships between human populations and the multiple species of mosquito and parasite. Research priorities include the development of in vitro culture systems for the complete life cycle of P. falciparum and P. vivax and the development of an appropriate liver culture system to study hepatic stages. In addition, genetic technologies for the manipulation of Plasmodium need to be improved, the entire parasite metabolome needs to be characterized to identify new druggable targets, and improved information systems for monitoring the changes in epidemiology, pathology, and host-parasite-vector interactions as a result of intensified control need to be established to bridge the gap between bench, preclinical, clinical, and population-based sciences.

CPP-ZFN: a potential DNA-targeting anti-malarial drug

BACKGROUND

Multidrug-resistant Plasmodium is of major concern today. Effective vaccines or successful applications of RNAi-based strategies for the treatment of malaria are currently unavailable. An unexplored area in the field of malaria research is the development of DNA-targeting drugs that can specifically interact with parasitic DNA and introduce deleterious changes, leading to loss of vital genome function and parasite death.

PRESENTATION OF THE HYPOTHESIS

Advances in the development of zinc finger nuclease (ZFN) with engineered DNA recognition domains allow us to design and develop nuclease of high target sequence specificity with a mega recognition site that typically occurs only once in the genome. Moreover, cell-penetrating peptides (CPP) can cross the cell plasma membrane and deliver conjugated protein, nucleic acid, or any other cargo to the cytoplasm, nucleus, or mitochondria. This article proposes that a drug from the combination of the CPP and ZFN systems can effectively enter the intracellular parasite, introduce deleterious changes in its genome, and eliminate the parasite from the infected cells.

TESTING THE HYPOTHESIS

Availability of a DNA-binding motif for more than 45 triplets and its modular nature, with freedom to change number of fingers in a ZFN, makes development of customized ZFN against diverse target DNA sequence of any gene feasible. Since the Plasmodium genome is highly AT rich, there is considerable sequence site diversity even for the structurally and functionally conserved enzymes between Plasmodium and humans. CPP can be used to deliver ZFN to the intracellular nucleus of the parasite. Signal-peptide-based heterologous protein translocation to Plasmodium-infected RBCs (iRBCs) and different Plasmodium organelles have been achieved. With successful fusion of CPP with mitochondrial- and nuclear-targeting peptides, fusion of CPP with 1 more Plasmodium cell membrane translocation peptide seems achievable.

IMPLICATIONS OF THE HYPOTHESIS

Targeting of the Plasmodium genome using ZFN has great potential for the development of anti-malarial drugs. It allows the development of a single drug against all malarial infections, including multidrug-resistant strains. Availability of multiple ZFN target sites in a single gene will provide alternative drug target sites to combat the development of resistance in the future.