Hematin polymerization assay as a high-throughput screen for identification of new antimalarial pharmacophores

Development and Characterization of Novel Chitosan-Coated Curcumin Nanophytosomes for Treating Drug-Resistant Malaria

This study aimed at enhancing the efficacy of curcumin (CR) by formulating and coating it with chitosan. In silico molecular docking studies revealed that CR exhibited almost similar and low binding energies when compared to artemisinin, indicating high stability at the target site. It can be confirmed that CR is effective in treating and reducing Plasmodium falciparum parasites. Fourier transform infrared studies confirmed that there was a shift and disappearance of some drug peaks in the formulation which revealed complexation with phospholipids. The F2EXT3-developed formulation exhibited greater solubility (24.31 ± 3.47 μg/mL) when compared to pure CR (7.99 ± 1.95 μg/mL). Proton nuclear magnetic resonance studies confirmed the formation of Curcumin-phospholipid hydrogen bonding in F2EXT3. The in vitro drug release studies revealed that the developed formulation F2EXT3 exhibited better drug release at 71.98% at 48 h; this might be due to the effective entrapment efficiency of the drug inside the phospholipid, presence of polyethylene glycol 4000 and chitosan further assisted in sustained release of the drug. Scanning electron microscopy studies revealed that optimized F2EXT3 CR nanophytosomes were nearly spherical with narrow size distribution and smooth surface. The zeta potential of the F2EXT3 showed -3.5 mV. Stability studies revealed that the formulation remained stable even after 6 months. It was observed from the hemin assay that CR and F2EXT3 exhibited (50 μg/mL curcumin) exhibited IC50 values of 47 ± 2.45 and 22 ± 1.58 μM, respectively. Further in vivo antimalarial activity on resistant and sensitive strains needs to be performed to evaluate the efficacy of the developed formulation.

Tafenoquine Is a Promising Drug Candidate for the Treatment of Babesiosis

Due to drug resistance, commonly used anti-Babesia drugs have limited efficacy against babesiosis and inflict severe side effects. Tafenoquine (TAF) was approved by the U.S. Food and Drug Administration in 2018 for the radical cure of Plasmodium vivax infection and for malaria prophylaxis. Here, we evaluated the efficacy of TAF for the treatment of Babesia infection and elucidated the suspected mechanisms of TAF activity against Babesia parasites. Parasitemia and survival rates of Babesia rodhaini-infected BALB/c and SCID mice were used to explore the role of the immune response in Babesia infection after TAF treatment. Parasitemia, survival rates, body weight, vital signs, complete blood count, and blood biochemistry of B. gibsoni-infected splenectomized dogs were determined to evaluate the anti-Babesia activity and side effects of TAF. Then, to understand the mechanism of TAF activity, hydrogen peroxide was used as an oxidizer for short-term B. rodhaini incubation in vitro, and the expression levels of antioxidant enzymes were confirmed using B. microti-infected mice by reverse transcription-quantitative PCR (qRT-PCR). Acute B. rodhaini and B. gibsoni infections were rapidly eliminated with TAF administration. Repeated administration of TAF or a combination therapy with other antibabesial agents is still needed to avoid a potentially fatal recurrence for immunocompromised hosts. Caution about hyperkalemia should be taken during TAF treatment for Babesia infection. TAF possesses a babesicidal effect that may be related to drug-induced oxidative stress. Considering the lower frequency of glucose-6-phosphate dehydrogenase deficiency in animals compared to that in humans, TAF use on Babesia-infected farm animals and pets is eagerly anticipated.

Heme Detoxification in the Malaria Parasite: A Target for Antimalarial Drug Development

Over the last century, malaria deaths have decreased by more than 85%. Nonetheless, there were 405 000 deaths in 2018, mostly resulting from Plasmodium falciparum infection. In the 21st century, much of the advance has arisen from the deployment of insecticide-treated bed nets and artemisinin combination therapy. However, over the past few decades parasites with a delayed artemisinin clearance phenotype have appeared in Southeast Asia, threatening further gains. The effort to find new drugs is thus urgent. A prominent process in blood stage malaria parasites, which we contend remains a viable drug target, is hemozoin formation. This crystalline material consisting of heme can be readily seen when parasites are viewed microscopically. The process of its formation in the parasite, however, is still not fully understood.In early work, we recognized hemozoin formation as a biomineralization process. We have subsequently investigated the kinetics of synthetic hemozoin (β-hematin) crystallization catalyzed at lipid-aqueous interfaces under biomimetic conditions. This led us to the use of neutral detergent-based high-throughput screening (HTS) for inhibitors of β-hematin formation. A good hit rate against malaria parasites was obtained. Simultaneously, we developed a pyridine-based assay which proved successful in measuring the concentrations of hematin not converted to β-hematin.The pyridine assay was adapted to determine the effects of chloroquine and other clinical antimalarials on hemozoin formation in the cell. This permitted the determination of the dose-dependent amounts of exchangeable heme and hemozoin in P. falciparum for the first time. These studies have shown that hemozoin inhibitors cause a dose-dependent increase in exchangeable heme, correlated with decreased parasite survival. Electron spectroscopic imaging (ESI) showed a relocation of heme iron into the parasite cytoplasm, while electron microscopy provided evidence of the disruption of hemozoin crystals. This cellular assay was subsequently extended to top-ranked hits from a wide range of scaffolds found by HTS. Intriguingly, the amounts of exchangeable heme at the parasite growth IC₅₀ values of these scaffolds showed substantial variation. The amount of exchangeable heme was found to be correlated with the amount of inhibitor accumulated in the parasitized red blood cell. This suggests that heme-inhibitor complexes, rather than free heme, lead to parasite death. This was supported by ESI using a Br-containing compound which showed the colocalization of Fe and Br as well as by confocal Raman microscopy which confirmed the presence of a complex in the parasite. Current evidence indicates that inhibitors block hemozoin formation by surface adsorption. Indeed, we have successfully introduced molecular docking with hemozoin to find new inhibitors. It follows that the resulting increase in free heme leads to the formation of the parasiticidal heme-inhibitor complex. We have reported crystal structures of heme-drug complexes for several aryl methanol antimalarials in nonaqueous media. These form coordination complexes but most other inhibitors interact noncovalently, and the determination of their structures remains a major challenge.It is our view that key future developments will include improved assays to measure cellular heme levels, better in silico approaches for predicting β-hematin inhibition, and a concerted effort to determine the structure and properties of heme-inhibitor complexes.

Artemisinin-Based Drugs Target the Plasmodium falciparum Heme Detoxification Pathway

Artemisinin (ART)-based antimalarial drugs are believed to exert lethal effects on malarial parasites by alkylating a variety of intracellular molecular targets. Recent work with live parasites has shown that one of the alkylated targets is free heme within the parasite digestive vacuole, which is liberated upon hemoglobin catabolism by the intraerythrocytic parasite, and that reduced levels of heme alkylation occur in artemisinin-resistant parasites. One implication of heme alkylation is that these drugs may inhibit parasite detoxification of free heme via inhibition of heme-to-hemozoin crystallization; however, previous reports that have investigated this hypothesis present conflicting data. By controlling reducing conditions and, hence, the availability of ferrous versus ferric forms of free heme, we modify a previously reported hemozoin inhibition assay to quantify the ability of ART-based drugs to target the heme detoxification pathway under reduced versus oxidizing conditions. Contrary to some previous reports, we find that artemisinins are potent inhibitors of hemozoin crystallization, with effective half-maximal concentrations approximately an order of magnitude lower than those for most quinoline-based antimalarial drugs. We also examine hemozoin and in vitro parasite growth inhibition for drug pairs found in the most commonly used ART-based combination therapies (ACTs). All ACTs examined inhibit hemozoin crystallization in an additive fashion, and all but one inhibit parasite growth in an additive fashion.

Lapatinib, Nilotinib and Lomitapide Inhibit Haemozoin Formation in Malaria Parasites

With the continued loss of antimalarials to resistance, drug repositioning may have a role in maximising efficiency and accelerating the discovery of new antimalarial drugs. Bayesian statistics was previously used as a tool to virtually screen USFDA approved drugs for predicted β-haematin (synthetic haemozoin) inhibition and in vitro antimalarial activity. Here, we report the experimental evaluation of nine of the highest ranked drugs, confirming the accuracy of the model by showing an overall 93% hit rate. Lapatinib, nilotinib, and lomitapide showed the best activity for inhibition of β-haematin formation and parasite growth and were found to inhibit haemozoin formation in the parasite, providing mechanistic insights into their mode of antimalarial action. We then screened the USFDA approved drugs for binding to the β-haematin crystal, applying a docking method in order to evaluate its performance. The docking method correctly identified imatinib, lapatinib, nilotinib, and lomitapide. Experimental evaluation of 22 of the highest ranked purchasable drugs showed a 24% hit rate. Lapatinib and nilotinib were chosen as templates for shape and electrostatic similarity screening for lead hopping using the in-stock ChemDiv compound catalogue. The actives were novel structures worthy of future investigation. This study presents a comparison of different in silico methods to identify new haemozoin-inhibiting chemotherapeutic alternatives for malaria that proved to be useful in different ways when taking into consideration their strengths and limitations.

Virtual screening as a tool to discover new β-haematin inhibitors with activity against malaria parasites

Malaria remains a major public health problem. With the loss of antimalarials to resistance, the malaria burden will likely continue for decades. New antimalarial scaffolds are crucial to avoid cross-resistance. Here, we present the first structure based virtual screening using the β-haematin crystal as a target for new inhibitor scaffolds by applying a docking method. The ZINC15 database was searched for compounds with high binding affinity with the surface of the β-haematin crystal using the PyRx Virtual Screening Tool. Top-ranked compounds predicted to interact with β-haematin were submitted to a second screen applying in silico toxicity and drug-likeness predictions using Osiris DataWarrior. Fifteen compounds were purchased for experimental testing. An NP-40 mediated β-haematin inhibition assay and parasite growth inhibition activity assay were performed. The benzoxazole moiety was found to be a promising scaffold for further development, showing intraparasitic haemozoin inhibition using a cellular haem fractionation assay causing a decrease in haemozoin in a dose dependent manner with a corresponding increase in exchangeable haem. A β-haematin inhibition hit rate of 73% was found, a large enrichment over random screening, demonstrating that virtual screening can be a useful and cost-effective approach in the search for new haemozoin inhibiting antimalarials.

Quantitative imaging of intraerythrocytic hemozoin by transient absorption microscopy

Hemozoin, the heme detoxification end product in malaria parasites during their growth in the red blood cells (RBCs), serves as an important marker for diagnosis and treatment target of malaria disease. However, the current method for hemozoin-targeted drug screening mainly relies on in vitro β-hematin inhibition assays, which may lead to false-positive events due to under-representation of the real hemozoin crystal. Quantitative in situ imaging of hemozoin is highly desired for high-throughput screening of antimalarial drugs and for elucidating the mechanisms of antimalarial drugs. We present transient absorption (TA) imaging as a high-speed single-cell analysis platform with chemical selectivity to hemozoin. We first demonstrated that TA microscopy is able to identify β-hematin, the artificial form of hemozoin, from the RBCs. We further utilized time-resolved TA imaging to in situ discern hemozoin from malaria-infected RBCs with optimized imaging conditions. Finally, we quantitatively analyzed the hemozoin amount in RBCs at different infection stages by single-shot TA imaging. These results highlight the potential of TA imaging for efficient antimalarial drug screening and drug mechanism investigation.

Targeting virulence factors as an antimicrobial approach: Pigment inhibitors

The fascinating and dangerous colored pathogens contain unique chemically pigmented molecules, which give varied and efficient assistance as virulence factors to the crucial reproduction and growth of microbes. Therefore, multiple novel strategies and inhibitors have been developed in recent years that target virulence factor pigments. However, despite the importance and significance of this topic, it has not yet been comprehensively reviewed. Moreover, research groups around the world have made successful progress against antibacterial infections by targeting pigment production, including our serial works on the discovery of CrtN inhibitors against staphyloxanthin production in Staphylococcus aureus. On the basis of the previous achievements and recent progress of our group in this field, this article will be the first comprehensive review of pigment inhibitors against colored pathogens, especially S. aureus infections, and this article includes design strategies, representative case studies, advantages, limitations, and perspectives to guide future research.

Development in Assay Methods for in Vitro Antimalarial Drug Efficacy Testing: A Systematic Review

The emergence and spread of drug resistance are the major challenges in malaria eradication mission. Besides various strategies laid down by World Health Organization, such as vector management, source reduction, early case detection, prompt treatment, and development of new diagnostics and vaccines, nevertheless the need for new and efficacious drugs against malaria has become a critical priority on the global malaria research agenda. At several screening stages, millions of compounds are screened (1,000–2,000,000 compounds per screening campaign), before pre-clinical trials to select optimum lead. Carrying out in vitro screening of antimalarials is very difficult as different assay methods are subject to numerous sources of variability across different laboratories around the globe. Despite this, in vitro screening is an essential part of antimalarial drug development as it enables to resource various confounding factors such as host immune response and drug–drug interaction. Therefore, in this article, we try to illustrate the basic necessity behind in vitro study and how new methods are developed and subsequently adopted for high-throughput antimalarial drug screening and its application in achieving the next level of in vitro screening based on the current approaches (such as stem cells).

Evidence for a role of hemozoin in metabolism and gametocytogenesis

Hemozoin is generally considered a waste deposit that is formed for the sole purpose of detoxification of free heme that results from the digestion of hemoglobin by Plasmodium parasites. However, several observations of parasite multiplication, both in vertebrate and invertebrate hosts are suggestive of a wider, but overlooked, metabolic role for this product. The presence of clinical peripheral blood samples of P. falciparum with high parasitemia containing only hemozoin-deficient (non-pigmented) asexual forms has been repeatedly confirmed. Such samples stand in contrast with other samples that contain mostly pigmented circulating trophozoites and gametocytes, indicating that pigment accumulation is a prominent feature of gametocytogenesis. The restricted size, i.e. below detection by light microscopy, of hemozoin in asexual merozoites and ringforms of P. falciparum implies its continuous turnover, supporting a role in metabolism. The prominent interaction of hemozoin with several antimalarial drugs, the involvement of proteins in hemozoin formation, and the finding of plasmodial genes coding for a heme-oxygenase-like protein argue for a wider and more active role for hemozoin in the parasite's metabolism. The observed association of hemozoin with crystalloids during ookinete development is consistent with a useful function to it during parasite multiplication in the invertebrate host. Finally, alternative mechanisms, other than hemozoin formation, provide substitute or additional routes for heme detoxification.

Identification and Mechanistic Evaluation of Hemozoin-Inhibiting Triarylimidazoles Active against Plasmodium falciparum

In a previous study, target based screening was carried out for inhibitors of β-hematin (synthetic hemozoin) formation, and a series of triarylimidazoles were identified as active against Plasmodium falciparum. Here, we report the subsequent synthesis and testing of derivatives with varying substituents on the three phenyl rings for this series. The results indicated that a 2-hydroxy-1,3-dimethoxy substitution pattern on ring A is required for submicromolar parasite activity. In addition, cell-fractionation studies revealed uncommonly large, dose-dependent increases of P. falciparum intracellular exchangeable (free) heme, correlating with decreased parasite survival for β-hematin inhibiting derivatives.

Gancidin W, a potential low-toxicity antimalarial agent isolated from an endophytic Streptomyces SUK10

Endophytic Streptomyces strains are potential sources for novel bioactive molecules. In this study, the diketopiperazine gancidin W (GW) was isolated from the endophytic actinobacterial genus Streptomyces, SUK10, obtained from the bark of Shorea ovalis tree, and it was tested in vivo against Plasmodium berghei PZZ1/100. GW exhibited an inhibition rate of nearly 80% at 6.25 and 3.125 μg kg-1 body weight on day four using the 4-day suppression test method on male ICR strain mice. Comparing GW at both concentrations with quinine hydrochloride and normal saline as positive and negative controls, respectively, 50% of the mice treated with 3.125 μg kg-1 body weight managed to survive for more than 11 months after infection, which almost reached the life span of normal mice. Biochemical tests of selected enzymes and proteins in blood samples of mice treated with GW were also within normal levels; in addition, no abnormalities or injuries were found on internal vital organs. These findings indicated that this isolated bioactive compound from Streptomyces SUK10 exhibits very low toxicity and is a good candidate for potential use as an antimalarial agent in an animal model.

High-Throughput Screening and Prediction Model Building for Novel Hemozoin Inhibitors Using Physicochemical Properties

It is essential to continue the search for novel antimalarial drugs due to the current spread of resistance against artemisinin by Plasmodium falciparum parasites. In this study, we developed in silico models to predict hemozoin inhibitors as a potential first-step screening for novel antimalarials. An in vitro colorimetric high-throughput screening assay of hemozoin formation was used to identify hemozoin inhibitors from 9,600 structurally diverse compounds. The physicochemical properties of positive hits and randomly selected compounds were extracted from the ChemSpider database; they were used for developing prediction models to predict hemozoin inhibitors using two different approaches, i.e., traditional multivariate logistic regression and Bayesian model averaging. Our results showed that a total of 224 positive-hit compounds exhibited the ability to inhibit hemozoin formation, with 50% inhibitory concentrations (IC₅₀s) ranging from 3.1 μM to 199.5 μM. The best model according to traditional multivariate logistic regression included the three variables octanol-water partition coefficient, number of hydrogen bond donors, and number of atoms of hydrogen, while the best model according to Bayesian model averaging included the three variables octanol-water partition coefficient, number of hydrogen bond donors, and index of refraction. Both models had a good discriminatory power, with area under the curve values of 0.736 and 0.781 for the traditional multivariate model and Bayesian model averaging, respectively. In conclusion, the prediction models can be a new, useful, and cost-effective approach for the first screen of hemozoin inhibition-based antimalarial drug discovery.

Schistosoma hemozoin and its possible roles

More than 95years ago Schistosoma pigment had been deemed as a degradation product of haemoglobin. Until the 1950s, scientists initiated to pay attention to understand the hematophagous habit of schistosomes, and to study the degradation of haemoglobin as well as the formation of hemozoin inside the gut of the worms. For a long time, the formation of hemozoin in both Plasmodium and in Schistosoma was considered to be the major route of heme detoxification, and hemozoin served a role in waste disposal. At the beginning of this century, the chemical structure of Schistosoma pigment was confirmed to be identical to that of malarial pigment (hemozoin) and its synthetic analogue, β-hematin. Since then, studies on Schistosoma hemozoin have been investigated by some workers and the results showed that Schistosoma hemozoin may play important roles in pathogenicity, immune modulation, iron supply for egg formation, and interaction with some anti-schistosomal drugs. In this review, we briefly review and discuss the hematophagous habit of schistosomes, degradation of haemoglobin, formation of hemozoin in the worm gut, and possible roles of hemozoin.

Understanding malarial toxins

Recognized since antiquity, malaria is one of the most infamous and widespread infectious diseases in humans and, although the death rate during the last century has been diminishing, it still accounts for more than a half million deaths annually. It is caused by the Plasmodium parasite and typical symptoms include fever, shivering, headache, diaphoresis and nausea, all resulting from an excessive inflammatory response induced by malarial toxins released into the victim's bloodstream. These toxins are hemozoin and glycosylphosphatidylinositols. The former is the final product of the parasite's detoxification of haeme, a by-product of haemoglobin catabolism, while the latter anchor proteins to the Plasmodium cell surface or occur as free molecules. Currently, only two groups of antimalarial toxin drugs exist on the market, quinolines and artemisinins. As we describe, they both target biosynthesis of hemozoin. Other substances, currently in various phases of clinical trials, are directed towards biosynthesis of glycosylphosphatidylinositol, formation of hemozoin, or attenuation of the inflammatory response of the patient. Among the innovative approaches to alleviating the effects of malarial toxins, is the development of antimalarial toxin vaccines. In this review the most important lessons learned from the use of treatments directed against the action of malarial toxins in antimalarial therapy are emphasized and the most relevant and promising directions for future research in obtaining novel antimalarial agents acting on malarial toxins are discussed.

Identification and SAR Evaluation of Hemozoin-Inhibiting Benzamides Active against Plasmodium falciparum

Quinoline antimalarials target hemozoin formation causing a cytotoxic accumulation of ferriprotoporphyrin IX (Fe(III)PPIX). Well-developed SAR models exist for β-hematin inhibition, parasite activity, and cellular mechanisms for this compound class, but no comparably detailed investigations exist for other hemozoin inhibiting chemotypes. Here, benzamide analogues based on previous HTS hits have been purchased or synthesized. Only derivatives containing an electron deficient aromatic ring and capable of adopting flat conformations, optimal for π-π interactions with Fe(III)PPIX, inhibited β-hematin formation. The two most potent analogues showed nanomolar parasite activity, with little CQ cross-resistance, low cytotoxicity, and high in vitro microsomal stability. Selected analogues inhibited hemozoin formation in Plasmodium falciparum causing high levels of free heme. In contrast to quinolines, introduction of amine side chains did not lead to benzamide accumulation in the parasite. These data reveal complex relationships between heme binding, free heme levels, cellular accumulation, and in vitro activity of potential novel antimalarials.

In vivo antimalarial activity of the endophytic actinobacteria, Streptomyces SUK 10

Endophytic bacteria, such as Streptomyces, have the potential to act as a source for novel bioactive molecules with medicinal properties. The present study was aimed at assessing the antimalarial activity of crude extract isolated from various strains of actinobacteria living endophytically in some Malaysian medicinal plants. Using the four day suppression test method on male ICR strain mice, compounds produced from three strains of Streptomyces (SUK8, SUK10, and SUK27) were tested in vivo against Plasmodium berghei PZZ1/100 in an antimalarial screen using crude extracts at four different concentrations. One of these extracts, isolated from Streptomyces SUK10 obtained from the bark of Shorea ovalis tree, showed inhibition of the test organism and was further tested against P. berghei-infected mice for antimalarial activity at different concentrations. There was a positive relationship between the survival of the infected mouse group treated with 50 µg/kg body weight (bw) of ethyl acetate-SUK10 crude extract and the ability to inhibit the parasites growth. The parasite inhibition percentage for this group showed that 50% of the mice survived for more than 90 days after infection with the parasite. The nucleotide sequence and phylogenetic tree suggested that Streptomyces SUK10 may constitute a new species within the Streptomyces genus. As part of the drug discovery process, these promising finding may contribute to the medicinal and pharmaceutical field for malarial treatment.

A modified precise analytical method for anti-malarial screening: Heme polymerization assay

Malarial parasite detoxifies the heme generated in its food vacuole in many ways one of which involves heme polymerization to hemozoin. The existing heme polymerization assays involve use of activators along with buffers for polymerization of heme leading to its precipitation. Such assays then involve special instruments and laborious work of isolating the precipitated polymer and its detection. Simple and precise spectrophotometric and HTS methods were developed for heme polymerization using tween 20 as the activator without isolation of polymerized heme.

Identification of β-hematin inhibitors in a high-throughput screening effort reveals scaffolds with in vitro antimalarial activity

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Hemozoin formation is a prime drug target pathway to probe for new lead compounds.

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We examined the VICB library of compounds for in vitro β-hematin inhibition.

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β-Hematin inhibitors were tested for in vitro antimalarial activity in two P. falciparum strains.

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Chemical scaffolds with target-specific and in vitro antimalarial activity were identified.

The emergence of drug resistant strains of Plasmodium spp. creates a critical need for the development of novel antimalarials. Formation of hemozoin, a crystalline heme detoxification process vital to parasite survival serves as an important drug target. The quinoline antimalarials including chloroquine and amodiaquine owe their antimalarial activity to inhibition of hemozoin formation. Though in vivo formation of hemozoin occurs within the presence of neutral lipids, the lipophilic detergent NP-40 was previously shown to serve as a surrogate in the β-hematin (synthetic hemozoin) formation process. Consequently, an NP-40 mediated β-hematin formation assay was developed for use in high-throughput screening. Here, the assay was utilized to screen 144,330 compounds for the identification of inhibitors of crystallization, resulting in 530 hits. To establish the effectiveness of these target-based β-hematin inhibitors against Plasmodiumfalciparum, each hit was further tested in cultures of parasitized red blood cells. This effort revealed that 171 of the β-hematin inhibitors are also active against the parasite. Dose–response data identified 73 of these β-hematin inhibitors have IC₅₀ values ⩽5 μM, including 25 compounds with nanomolar activity against P. falciparum. A scaffold-based analysis of this data identified 14 primary scaffolds that represent 46% of the 530 total hits. Representative compounds from each of the classes were further assessed for hemozoin inhibitory activity in P. falciparum infected human erythrocytes. Each of the hit compounds tested were found to be positive inhibitors, while a negative control did not perturb this biological pathway in culture.

Growth of Large Hematin Crystals in Biomimetic Solutions

Hematin crystallization is an essential component of the physiology of malaria parasites. Several antimalarial drugs are believed to inhibit crystallization and expose the parasites to toxic soluble hematin. Hence, understanding the mechanisms of hematin crystal growth and inhibition is crucial for the design of new drugs. A major obstacle to microscopic, spectroscopic, and crystallographic studies of hematin crystallization has been the unavailability of large hematin crystals grown under conditions representative of the parasite anatomy. We have developed a biomimetic method to reproducibly grow large hematin crystals reaching 50 μm in length. We imitate the digestive vacuole of Plasmodium falciparum and employ a two-phase solution of octanol and citric buffer. The nucleation of seeds is enhanced at the interface between the aqueous and organic phases, where an ordered layer of octanol molecules is known to serve as substrate for nucleation. The seeds are transferred to hematin-saturated octanol in contact with citric buffer. We show that the crystals grow in the octanol layer, while the buffer supplies hydrogen ions needed for bonds that link the hematin molecules in the crystal. The availability of large hematin crystals opens new avenues for studies of hematin detoxification of malaria parasites in host erythrocytes.

Hemozoin and antimalarial drug discovery

Recent initiatives to develop more effective and affordable drugs, controlling mosquitoes and development of a preventative vaccine have been launched with the goal of completely eradicating malaria. To this end, Novartis (Surrey, UK) and GlaxoSmithKline (Middlesex, UK) screened their chemical libraries of approximately two million small molecules for antimalarial properties, which resulted in a set of over 20,000 'highly druggable' initial hits. Efforts in academia are centered on specific pathway targets. One such high-throughput screening effort has been focused on hemozoin formation, a unique heme detoxification pathway found in the malaria parasite. This review discusses the current approaches and limitations of high-throughput screening discovery of hemozoin inhibitors. In the future, new methods must be developed to validate the mechanism of action of these hit compounds within the parasite.

Plasmodium falciparum gametocytes: with a view to a kill

Drugs that kill or inhibit the sexual stages of Plasmodium in order to prevent transmission are important components of malaria control programmes. Reducing gametocyte carriage is central to the control of Plasmodium falciparum transmission as infection can result in extended periods of gametocytaemia. Unfortunately the number of drugs with activity against gametocytes is limited. Primaquine is currently the only licensed drug with activity against the sexual stages of malaria parasites and its use is hampered by safety concerns. This shortcoming is likely the result of the technical challenges associated with gametocyte studies together with the focus of previous drug discovery campaigns on asexual parasite stages. However recent emphasis on malaria eradication has resulted in an upsurge of interest in identifying compounds with activity against gametocytes. This review examines the gametocytocidal properties of currently available drugs as well as those in the development pipeline and examines the prospects for discovery of new anti-gametocyte compounds.

Plasmodium cell biology should inform strategies used in the development of antimalarial transmission-blocking drugs

Malaria is a disease with a devastating impact affecting 216 million people each year and causing 655,000 deaths, most of which are children under 5 years old. Recent appreciation that malaria eradication will require novel interventions to target the parasite during transmission from the human host to the mosquito has lead to an exciting surge in activity to develop transmission-blocking drugs and the high-throughput assays to screen for them. This article presents an overview of transmission-stage cell biology and discusses its impact on assay development to provide a context for researchers to evaluate the relative merits/drawbacks of both screening data obtained from current assays and considerations for future assay design. The most recent knowledge of the transmission-blocking properties of current antimalarial classes is also summarized and, underdeveloped targets for transmission-stage drug discovery are highlighted.

Quinoline drug-heme interactions and implications for antimalarial cytostatic versus cytocidal activities

Historically, the most successful molecular target for antimalarial drugs has been heme biomineralization within the malarial parasite digestive vacuole. Heme released from catabolized host red blood cell hemoglobin is toxic, so malarial parasites crystallize heme to nontoxic hemozoin. For years it has been accepted that a number of effective quinoline antimalarial drugs (e.g., chloroquine, quinine, amodiaquine) function by preventing hemozoin crystallization. However, recent studies over the past decade have revealed a surprising molecular diversity in quinoline-heme molecular interactions. This diversity shows that even closely related quinoline drugs may have quite different molecular pharmacology. This paper reviews the molecular diversity and highlights important implications for understanding quinoline antimalarial drug resistance and for future drug design.

Cytostatic versus cytocidal activities of chloroquine analogues and inhibition of hemozoin crystal growth

We report an improved, nonhazardous, high-throughput assay for in vitro quantification of antimalarial drug inhibition of β-hematin (hemozoin) crystallization performed under conditions that are more physiological relative to previous assays. The assay uses the differential detergent solubility of crystalline and noncrystalline forms of heme and is optimized via the use of lipid catalyst. Using this assay, we quantify the effect of pH on the crystal growth-inhibitory activities of current quinoline antimalarials, evaluate the catalytic efficiencies of different lipids, and test for a possible correlation between hemozoin inhibition by drugs versus their antiplasmodial activity. Consistent with several previous reports, we found a good correlation between hemozoin inhibition potency versus cytostatic antiplasmodial potency (50% inhibitory concentration) for a series of chloroquine (CQ) analogues. However, we found no correlation between hemozoin inhibition potency and cytocidal antiplasmodial potency (50% lethal dose) for the same drugs, suggesting that cellular targets for these two layers of 4-aminoquinoline drug activity differ. This important concept is also explored further for QN and its stereoisomers in the accompanying paper (A. P. Gorka, K. S. Sherlach, A. C. de Dios, and P. D. Roepe, Antimicrob. Agents Chemother. 57:365-374, 2013).

Neutral lipids associated with haemozoin mediate efficient and rapid β-haematin formation at physiological pH, temperature and ionic composition

Background

The malaria parasite disposes of host-derived ferrihaem (iron(III)protoporphyrin IX, Fe(III)PPIX) by conversion to crystalline haemozoin in close association with neutral lipids. Lipids mediate synthetic haemozoin (β-haematin) formation very efficiently. However, the effect on reaction rates of concentrations of lipid, Fe(III)PPIX and physiologically relevant ions and biomolecules are unknown.

Methods

Lipid emulsions containing Fe(III)PPIX were prepared in aqueous medium (pH 4.8, 37°C) to mediate β-haematin formation. The reaction was quenched at various times and free Fe(III)PPIX measured colorimetrically as a pyridine complex and the kinetics and yields analysed. Products were also characterized by FTIR, TEM and electron diffraction. Autofluorescence was also used to monitor β-haematin formation by confocal microscopy.

Results

At fixed Fe(III)PPIX concentration, β-haematin yields remained constant with decreasing lipid concentration until a cut-off ratio was reached whereupon efficiency decreased dramatically. For the haemozoin-associated neutral lipid blend (NLB) and monopalmitoylglycerol (MPG), this occurred below a lipid/Fe(III)PPIX (L/H) ratio of 0.54. Rate constants were found to increase with L/H ratio above the cut-off. At 16 μM MPG, Fe(III)PPIX concentration could be raised until the L/H ratio reached the same ratio before a sudden decline in yield was observed. MPG-mediated β-haematin formation was relatively insensitive to biologically relevant cations (Na⁺, K⁺, Mg²⁺, Ca²⁺), or anions (H₂PO₄⁻, HCO₃⁻, ATP, 2,3-diphosphoglycerate, glutathione). Confocal microscopy demonstrated β-haematin formation occurs in association with the lipid particles.

Conclusions

Kinetics of β-haematin formation have shown that haemozoin-associated neutral lipids alone are capable of mediating β-haematin formation at adequate rates under physiologically realistic conditions of ion concentrations to account for haemozoin formation.

A simple and inexpensive haemozoin-based colorimetric method to evaluate anti-malarial drug activity

BACKGROUND

The spread of drug resistance in malaria parasites and the limited number of effective drugs for treatment indicates the need for new anti-malarial compounds. Current assays evaluating drugs against Plasmodium falciparum require expensive materials and equipment, thus limiting the search for new drugs, particularly in developing countries. This study describes an inexpensive procedure that is based on the advantage of a positive correlation between the haemozoin level of infected erythrocytes and parasite load.

METHODS

The relationship between parasitaemia and the haemozoin level of infected erythrocytes was investigated after converting haemozoin into monomeric haem. The 50% inhibitory concentration (IC50) values of chloroquine, quinine, artemisinin, quinidine and clotrimazole against P. falciparum K1 and 9A strains were determined using the novel assay method.

RESULTS

The haemozoin of parasites was extracted and converted into monomeric haem, allowing the use of a colorimeter to efficiently and rapidly measure the growth of the parasites. There was a strong and direct linear relationship between the absorbance of haem converted from haemozoin and the percentage of the parasite (R2 = 0.9929). Furthermore, the IC50 values of drugs were within the range of the values previously reported.

CONCLUSION

The haemozoin-based colorimetric assay can be considered as an alternative, simple, robust, inexpensive and convenient method, making it applicable in developing countries.

Detection, characterization, and screening of heme-binding molecules by mass spectrometry for malaria drug discovery

Drug screening for antimalarials uses heme biocrystallization inhibition methods as an alternative to parasite cultures, but they involve complex processes and cannot detect artemisinin-like molecules. The described method detects heme-binding compounds by mass spectrometry, using dissociation of the drug-heme adducts to evaluate putative antiplasmodial activity. Applied to a chemical library, it showed a good hit-to-lead ratio and is an efficient early stage screening for complex mixtures like natural extracts.

Screening medicinal plants for the detection of novel antimalarial products applying the inhibition of β-hematin formation

The identification of novel scaffolds for the development of effective and safe treatments to fight malaria is urgently needed. One of the main opportunities is the discovery of new molecules from natural origin. A simple, robust and cost-effective colorimetric assay based on the inhibition of β-hematin has been adapted to routinely screen plant extracts with the ultimate goal to identify novel antimalarial ingredients. The development of this assay has included a careful optimization of all critical experimental parameters. The β-hematin assay can be completed in less than one working day, requiring a 96-well UV-vis plate reader and low-cost commercially available reagents using a standard operating protocol. It can be used on its own or in combination with the well-known Plasmodium growth inhibition assay and has the obvious merit to be informative at the early stage of drug discovery regarding the mechanism of action of the actives. A total of 40 diverse natural products and 219 plants extracts were tested. Good correlations in respect with specificity (pure compounds 85%, extracts 93%) and positive predictive value (pure compounds 72%, extracts 50%) were obtained in comparison with Plasmodium growth inhibition assay that was used as the reference assay.

Use of the NP-40 detergent-mediated assay in discovery of inhibitors of beta-hematin crystallization

The protozoan parasite responsible for malaria affects over 500 million people each year. Current antimalarials have experienced decreased efficacy due to the development of drug-resistant strains of Plasmodium spp., resulting in a critical need for the discovery of new antimalarials. Hemozoin, a crystalline by-product of heme detoxification that is necessary for parasite survival, serves as an important drug target. The quinoline antimalarials, including amodiaquine and chloroquine, act by inhibiting the formation of hemozoin. The formation of this crystal does not occur spontaneously, and recent evidence suggests crystallization occurs in the presence of neutral lipid particles located in the acidic digestive vacuole of the parasite. To mimic these conditions, the lipophilic detergent NP-40 has previously been shown to successfully mediate the formation of β-hematin, synthetic hemozoin. Here, an NP-40 detergent-based assay was successfully adapted for use as a high-throughput screen to identify inhibitors of β-hematin formation. The resulting assay exhibited a favorable Z' of 0.82 and maximal drift of less than 4%. The assay was used in a pilot screen of 38,400 diverse compounds at a screening concentration of 19.3 μM, resulting in the identification of 161 previously unreported β-hematin inhibitors. Of these, 48 also exhibited ≥ 90% inhibition of parasitemia in a Plasmodium falciparum whole-cell assay at a screening concentration of 23 μM. Eight of these compounds were identified to have nanomolar 50% inhibitory concentration values near that of chloroquine in this assay.

Recent advances in the discovery of haem-targeting drugs for malaria and schistosomiasis

Haem is believed to be the target of some of the historically most important antimalarial drugs, most notably chloroquine. This target is almost ideal as haem is host-derived and the process targeted, haemozoin formation, is a physico-chemical process with no equivalent in the host. The result is that the target remains viable despite resistance to current drugs, which arises from mutations in parasite membrane transport proteins. Recent advances in high-throughput screening methods, together with a better understanding of the interaction of existing drugs with this target, have created new prospects for discovering novel haem-targeting chemotypes and for target-based structural design of new drugs. Finally, the discovery that Schistosoma mansoni also produces haemozoin suggests that new drugs of this type may be chemotherapeutic not only for malaria, but also for schistosomiasis. These recent developments in the literature are reviewed.

Colorimetric high-throughput screen for detection of heme crystallization inhibitors

Malaria infects 500 million people annually, a number that is likely to rise as drug resistance to currently used antimalarials increases. During its intraerythrocytic stage, the causative parasite, Plasmodium falciparum, metabolizes hemoglobin and releases toxic heme, which is neutralized by a parasite-specific crystallization mechanism to form hemozoin. Evidence suggests that chloroquine, the most successful antimalarial agent in history, acts by disrupting the formation of hemozoin. Here we describe the development of a 384-well microtiter plate screen to detect small molecules that can also disrupt heme crystallization. This assay, which is based on a colorimetric assay developed by Ncokazi and Egan (K. K. Ncokazi and T. J. Egan, Anal. Biochem. 338:306-319, 2005), requires no parasites or parasite-derived reagents and no radioactive materials and is suitable for a high-throughput screening platform. The assay's reproducibility and large dynamic range are reflected by a Z factor of 0.74. A pilot screen of 16,000 small molecules belonging to diverse structural classes was conducted. The results of the target-based assay were compared with a whole-parasite viability assay of the same small molecules to identify small molecules active in both assays.

The crystal structure of halofantrine-ferriprotoporphyrin IX and the mechanism of action of arylmethanol antimalarials

The crystal structure of the complex formed between the antimalarial drug halofantrine and ferriprotoporphyrin IX (Fe(III)PPIX) has been determined by single crystal X-ray diffraction. The structure shows that halofantrine coordinates to the Fe(III) center through its alcohol functionality in addition to pi-stacking of the phenanthrene ring over the porphyrin. The length of the Fe(III)-O bond is consistent with an alkoxide and not an alcohol coordinating group. The iron porphyrin is five coordinate and monomeric. Changes in the electronic spectrum of Fe(III)PPIX upon addition of halofantrine base in acetonitrile solution are almost identical to those observed upon addition of quinidine free base in the same solvent. This suggests homologous binding. Molecular mechanics modeling of Fe(III)PPIX complexes of quinidine, quinine, 9-epiquinine and 9-epiquinidine based on this homology suggests that the antimalarially active quinidine and quinine can readily adopt conformations that permit formation of an intramolecular salt bridge between the protonated quinuclidine tertiary amino group and unprotonated heme propionate group, while the inactive epimers 9-epiquinidine and 9-epiquinine have to adopt high energy conformations in order to accommodate such salt bridge formation. We propose that salt bridge formation may interrupt formation of the hemozoin precursor dimer formed during the heme detoxification pathway and so account for the strong activity of the two active isomers.

Ethnopharmacology and malaria: New hypothetical leads or old efficient antimalarials?

New treatments are urgently needed to curb and eradicate malaria in developing countries. As most people living in malarial endemic areas use traditional medicine to fight this disease, why have new treatments not emerged recently from ethnopharmacology-oriented research? The rationale and limitations of the ethnopharmacological approach are discussed in this paper, focusing on ethnopharmacology methodologies and techniques used for assessing botanical samples for their antimalarial properties. Discrepancies often observed between strong ethnopharmacological reputation and laboratory results are discussed, as well as new research perspectives.

Haemozoin: from melatonin pigment to drug target, diagnostic tool, and immune modulator

Plasmodium spp produce a pigment (haemozoin) to detoxify the free haem that is generated by haemoglobin degradation. Haemozoin was originally thought to be an inert waste byproduct of the parasite. However, recent research has led to the recognition that haemozoin is possibly of great importance in various aspects of malaria. Haemozoin is the target of many antimalarial drugs, and the unravelling of the exact modes of action may allow the design of novel antimalarial compounds. The detection of haemozoin in erythrocytes or leucocytes facilitates the diagnosis of malaria. The number of haemozoin-containing monocytes and granulocytes has been shown to correlate well with disease severity and may hold the potential for becoming a novel, automated laboratory marker in the assessment of patients. Finally, haemozoin has a substantial effect on the immune system. Further research is needed to clarify these aspects, many of which are important in clinical practice.

A non-radiolabeled heme–GSH interaction test for the screening of antimalarial compounds

Intraerythrocytic Plasmodium produces large amounts of toxic heme during the digestion of hemoglobin, a parasite specific pathway. Heme is then partially biocristallized into hemozoin and mostly detoxified by reduced glutathione. We proposed an in vitro micro assay to test the ability of drugs to inhibit heme-glutathione dependent degradation. As glutathione and o-phthalaldehyde form a fluorescent adduct, we followed the extinction of the fluorescent signal when heme was added with or without antimalarial compounds. In this assay, 50 microM of amodiaquine, arthemether, chloroquine, methylene blue, mefloquine and quinine inhibited the interaction between glutathione (50 microM) and heme (50 microM), while atovaquone did not. Consequently, this test could detect drugs that can inhibit heme-GSH degradation in a fast, simple and specific way, making it suitable for high throughput screening of potential antimalarials.

Simple colorimetric inhibition assay of heme crystallization for high-throughput screening of antimalarial compounds

Current assays for screening new antimalarials need initiators of beta-hematin formation that require laborious preparation, special devices, and substrates. In this study, based on reduction of heme absorption in beta-hematin formation, we developed a simple colorimetric assay using Tween 20 as an initiator and a microplate reader for high-throughput screening of inhibitors of beta-hematin formation.

Antimalarial drugs - host targets (re)visited

Every year, forty percent of the world population is at risk of contracting malaria. Hopes for the erradication of this disease during the 20th century were dashed by the ability of Plasmodium falciparum, its most deadly causative agent, to develop resistance to available drugs. Efforts to produce an effective vaccine have so far been unsuccessful, enhancing the need to develop novel antimalarial drugs. In this review, we summarize our knowledge concerning existing antimalarials, mechanisms of drug-resistance development, the use of drug combination strategies and the quest for novel anti-plasmodial compounds. We emphasize the potential role of host genes and molecules as novel targets for newly developed drugs. Recent results from our laboratory have shown Hepatocyte Growth Factor/MET signaling to be essential for the establishment of infection in hepatocytes. We discuss the potential use of this pathway in the prophylaxis of malaria infection.

Bioavailable iron and heme metabolism in Plasmodium falciparum

Iron metabolism is essential for cell function and potentially toxic because iron can catalyze oxygen radical production. Malaria-attributable anemia and iron deficiency anemia coincide as being treatable diseases in the developing world. In absolute amounts, more than 95% of Plasmodium metal biochemistry occurs in the acidic digestive vacuole where heme released from hemoglobin catabolism forms heme crystals. The antimalarial quinolines interfere with crystallization. Despite the completion of the Plasmodium genome, many 'gene gaps' exist in components of the metal pathways described in mammalian or yeast cells. Present evidence suggests that parasite bioavailable iron originates from a labile erythrocyte cytosolic pool rather than from abundant heme iron. Indeed the parasite has to make its own heme within two separate organelles, the mitochondrion and the apicomplast. Paradoxically, despite the abundance of iron within the erythrocyte, iron chelators are cytocidal to the Plasmodium parasite. Hemozoin has become a sensitive biomarker for laser desorption mass spectrometry detection of Plasmodium infection in both mice and humans.

Mapping Antimalarial Pharmacophores as a Useful Tool for the Rapid Discovery of Drugs Effective in Vivo: Design, Construction, Characterization, and Pharmacology of Metaquine

Resistant strains of Plasmodium falciparum and the unavailability of useful antimalarial vaccines reinforce the need to develop new efficacious antimalarials. This study details a pharmacophore model that has been used to identify a potent, soluble, orally bioavailable antimalarial bisquinoline, metaquine (N,N'-bis(7-chloroquinolin-4-yl)benzene-1,3-diamine) (dihydrochloride), which is active against Plasmodium berghei in vivo (oral ID(50) of 25 micromol/kg) and multidrug-resistant Plasmodium falciparum K1 in vitro (0.17 microM). Metaquine shows strong affinity for the putative antimalarial receptor, heme at pH 7.4 in aqueous DMSO. Both crystallographic analyses and quantum mechanical calculations (HF/6-31+G) reveal important regions of protonation and bonding thought to persist at parasitic vacuolar pH concordant with our receptor model. Formation of drug-heme adduct in solution was confirmed using high-resolution positive ion electrospray mass spectrometry. Metaquine showed strong binding with the receptor in a 1:1 ratio (log K = 5.7 +/- 0.1) that was predicted by molecular mechanics calculations. This study illustrates a rational multidisciplinary approach for the development of new 4-aminoquinoline antimalarials, with efficacy superior to chloroquine, based on the use of a pharmacophore model.

A colorimetric high-throughput β-hematin inhibition screening assay for use in the search for antimalarial compounds

Antimalarial drugs such as chloroquine are believed to act by inhibiting hemozoin formation in the food vacuole of the malaria parasite. We have developed a new assay for measuring and detecting inhibition of synthetic hemozoin (beta-hematin) formation. Aqueous pyridine (5% v/v, pH 7.5) forms a low-spin complex with hematin but not with beta-hematin. Its absorbance obeys Beer's law, making it useful for quantitating hematin concentration in hematin/beta-hematin mixtures, allowing compounds to be investigated for inhibition of beta-hematin formation. The assay is rapid (60 min incubation) and requires no centrifugation. The beta-hematin inhibition data show good agreement with alternative assay methods reported by four laboratories. The assay was adapted for high-throughput colorimetric screening, allowing visual identification of beta-hematin inhibitors. In this mode, the assay successfully detected all 18 beta-hematin inhibitors in a set of 47 compounds tested, with no false positive results. The quantitative in vitro antimalarial activities of a set of 13 aminoquinolines and quinoline methanols were found to correlate significantly with beta-hematin inhibition values determined using the assay.