The role of neutral lipid nanospheres in Plasmodium falciparum haem crystallization

Application of a calcium and phosphorus biomineralization strategy in tooth repair: a systematic review

Biomineralization is a natural process in which organisms regulate the growth of inorganic minerals to form biominerals with unique layered structures, such as bones and teeth, primarily composed of calcium and phosphorus. Tooth decay significantly impacts our daily lives, and the key to tooth regeneration lies in restoring teeth through biomimetic approaches, utilizing mineralization strategies or materials that mimic natural processes. This review delves into the types, properties, and transformations of calcium and phosphorus minerals, followed by an exploration of the mechanisms behind physiological and pathological mineralization in living organisms. It summarizes the mechanisms and commonalities of biomineralization and discusses the advancements in dental biomineralization research, guided by insights into calcium and phosphorus mineral biomineralization. This review concludes by addressing the current challenges and future directions in the field of dental biomimetic mineralization.

Antimalarial Imidazopyridines Incorporating an Intramolecular Hydrogen Bonding Motif: Medicinal Chemistry and Mechanistic Studies

We previously identified a novel class of antimalarial benzimidazoles incorporating an intramolecular hydrogen bonding motif. The frontrunner of the series, analogue A, showed nanomolar activity against the chloroquine-sensitive NF54 and multi-drug-resistant K1 strains of Plasmodium falciparum (PfNF54 IC₅₀ = 0.079 μM; PfK1 IC₅₀ = 0.335 μM). Here, we describe a cell-based medicinal chemistry structure-activity relationship study using compound A as a basis. This effort led to the identification of novel antimalarial imidazopyridines with activities of <1 μM, favorable cytotoxicity profiles, and good physicochemical properties. Analogue 14 ( PfNF54 IC₅₀ = 0.08 μM; PfK1 IC₅₀ = 0.10 μM) was identified as the frontrunner of the series. Preliminary mode of action studies employing molecular docking, live-cell confocal microscopy, and a cellular heme fractionation assay revealed that 14 does not directly inhibit the conversion of heme to hemozoin, although it could be involved in other processes in the parasite's digestive vacuole.

Leveraging a Fluorescent Fatty Acid Probe to Discover Cell-Permeable Inhibitors of Plasmodium falciparum Glycerolipid Biosynthesis

A sensitive and quantitative fluorescence-based approach is presented for characterizing fatty acid acquisition and lipid biosynthesis by asexually replicating, intraerythrocytic Plasmodium falciparum. We show that a BODIPY-containing, green-fluorescent fatty acid analog is efficiently and rapidly incorporated into parasite neutral lipids and phospholipids. Prelabeling with a red-fluorescent ceramide analog permits normalization and enables reliable quantitation of glycerolipid labeling. Inhibition of lipid labeling by competition with natural fatty acids and by acyl-coenzyme A synthetase and diacylglycerol acyltransferase inhibitors demonstrates that the fluorescent fatty acid probe is acquired, activated, and transferred to lipids through physiologically-relevant pathways. To assess its utility in discovering small molecules that block parasite lipid biosynthesis, the lipid labeling assay was used to screen a panel of mammalian lipase inhibitors and a selection of compounds from the "Malaria Box" anti-malarial collection. Several compounds were identified that inhibited the incorporation of the fluorescent fatty acid probe into lipids in cultured parasites at low micromolar concentrations. Two contrasting profiles of suppression of neutral lipid and phospholipid synthesis were observed, which implies the inhibition of distinct pathways. IMPORTANCE The human malaria parasite Plasmodium falciparum relies on fatty acid scavenging to supply this essential precursor of lipid synthesis during its asexual replication cycle in human erythrocytes. This dependence on host fatty acids represents a potential vulnerability that can be exploited to develop new anti-malarial therapies. The quantitative experimental approach described here provides a platform for simultaneously interrogating multiple facets of lipid metabolism- fatty acid uptake, fatty acyl-CoA synthesis, and neutral lipid and phospholipid biosynthesis- and of identifying cell-permeable inhibitors that are active in situ.

Diffraction contrast in cryo-scanning transmission electron tomography reveals the boundary of hemozoin crystals in situ

Malaria is a potentially fatal infectious disease caused by the obligate intracellular parasite Plasmodium falciparum. The parasite infects human red blood cells (RBC) and derives nutrition by catabolism of hemoglobin. As amino acids are assimilated from the protein component, the toxic heme is released. Molecular heme is detoxified by rapid sequestration to physiologically insoluble hemozoin crystals within the parasite's digestive vacuole (DV). Common antimalarial drugs interfere with this crystallization process, leaving the parasites vulnerable to the by-product of their own metabolism. A fundamental debate with important implications on drug mechanism regards the chemical environment of crystallization in situ, whether aqueous or lipid. This issue had been addressed previously by cryogenic soft X-ray tomography. We employ cryo-scanning transmission electron tomography (CSTET) to probe parasite cells throughout the life cycle in a fully hydrated, vitrified state at higher resolution. During the acquisition of CSTET data, Bragg diffraction from the hemozoin provides a uniquely clear view of the crystal boundary at nanometer resolution. No intermediate medium, such as a lipid coating or shroud, could be detected surrounding the crystals. The present study describes a unique application of CSTET in the study of malaria. The findings can be extended to evaluate new drug candidates affecting hemozoin crystal growth.

Antiplasmodial Cyclodecapeptides from Tyrothricin Share a Target with Chloroquine

Previous research found that the six major cyclodecapeptides from the tyrothricin complex, produced by Brevibacillus parabrevis, showed potent activity against chloroquine sensitive (CQS) Plasmodium falciparum. The identity of the aromatic residues in the aromatic dipeptide unit in cyclo-(D-Phe¹-Pro²-(Phe³/Trp³)-D-Phe⁴/D-Trp⁴)-Asn⁵-Gln⁶-(Tyr⁷/Phe⁷/Trp⁷)-Val⁸-(Orn⁹/Lys⁹)-Leu¹⁰ was proposed to have an important role in activity. CQS and resistant (CQR) P. falciparum strains were challenged with three representative cyclodecapeptides. Our results confirmed that cyclodecapeptides from tyrothricin had significantly higher antiplasmodial activity than the analogous gramicidin S, rivaling that of CQ. However, the previously hypothesized size and hydrophobicity dependent activity for these peptides did not hold true for P. falciparum strains, other than for the CQS 3D7 strain. The Tyr⁷ in tyrocidine A (TrcA) with Phe³-D-Phe⁴ seem to be related with loss in activity correlating with CQ antagonism and resistance, indicating a shared target and/or resistance mechanism in which the phenolic groups play a role. Phe⁷ in phenycidine A, the second peptide containing Phe³-D-Phe⁴, also showed CQ antagonism. Conversely, Trp⁷ in tryptocidine C (TpcC) with Trp³-D-Trp⁴ showed improved peptide selectivity and activity towards the more resistant strains, without overt antagonism towards CQ. However, TpcC lead to similar parasite stage inhibition and parasite morphology changes than previously observed for TrcA. The disorganization of chromatin packing and neutral lipid structures, combined with amorphous hemozoin crystals, could account for halted growth in late trophozoite/early schizont stage and the nanomolar non-lytic activity of these peptides. These targets related to CQ antagonism, changes in neural lipid distribution, leading to hemozoin malformation, indicate that the tyrothricin cyclodecapeptides and CQ share a target in the malaria parasite. The differing activities of these cyclic peptides towards CQS and CQR P. falciparum strains could be due to variable target interaction in multiple modes of activity. This indicated that the cyclodecapeptide activity and parasite resistance response depended on the aromatic residues in positions 3, 4 and 7. This new insight on these natural cyclic decapeptides could also benefit the design of unique small peptidomimetics in which activity and resistance can be modulated.

Adsorption to the surface of hemozoin crystals: Structure-based design and synthesis of new amino-phenoxazine β-hematin inhibitors

In silico adsorption of eight antimalarials that inhibit β-hematin (synthetic hemozoin) formation identified a primary binding site on the (001) face, which accommodates inhibitors via formation of predominantly π-π interactions. A good correlation (r2 = 0.64, P = 0.017) between adsorption energies and the logarithm of β-hematin inhibitory activity was found for this face. Of 53 monocyclic, bicyclic and tricyclic scaffolds, the latter yielded the most favorable adsorption energies. Five new amino-phenoxazine compounds were pursued as β-hematin inhibitors based on adsorption behaviour. The 2-substituted phenoxazines show good to moderate β-hematin inhibitory activity (< 100 μM) and Plasmodium falciparum blood stage activity against the 3D7 strain. N1,N1-diethyl-N4-(10H-phenoxazin-2-yl)pentane-1,4-diamine (P2a) is the most promising hit with IC50 values of 4.7 ± 0.6 and 0.64 ± 0.05 μM, respectively. Adsorption energies are predictive of β-hematin inhibitory activity, and thus the in silico approach is a beneficial tool for structure-based development of new non-quinoline inhibitors.

Docosahexaenoic acid is potent against the growth of mature stages of Plasmodium falciparum; inhibition of hematin polymerization a possible target

The present study investigates the potential effect of externally added unsaturated fatty acids on P. falciparum growth. Our results indicate that polyunsaturated fatty acids (PUFAs) inhibit the growth of Plasmodium in proportional to their degree of unsaturation. At higher concentration the PUFA Docosahexaenoic acid (DHA) induces pyknotic nuclei in infected erythrocytes. When Plasmodium stages were treated transiently with DHA, the ring stage culture recovered from the drug effect and parasitemia was increased post DHA removal with delayed growth of 12 h, compared to untreated control. Schizont stage treated culture displayed a 36 h delay in growth to infect fresh erythrocytes signifying its recovery is less than the ring stage. However the trophozoite stage failed to recover and showed a decrease in parasitemia, similar to that of continuous treated culture. PUFAs inhibited β- hematin polymerization by binding to free heme derived from hemoglobin degradation. Digestive vacuole neutral lipid bodies, which are pivotal for β- hematin polymerization, decreased and subsequently abrogated with increasing concentration of DHA in trophozoite stage treated culture. Our study concludes that DHA interacts with heme monomers and inhibits the β- hematin polymerization and growth of mature stages i.e., trophozoite and schizont stages of plasmodium.

Solvent Structure and Dynamics near the Surfaces of β-Hematin Crystals

Hematin crystallization, which is an essential component of the physiology of malaria parasites and the most successful target for antimalarial drugs, proceeds in mixed organic-aqueous solvents both in vivo and in vitro. Here we employ molecular dynamics simulations to examine the structuring and dynamics of a water-normal octanol mixture (a solvent that mimics the environment hosting hematin crystallization in vivo) in the vicinity of the typical faces in the habit of a hematin crystal. The simulations reveal that the properties of the solvent in the layer adjacent to the crystal are strongly impacted by the distinct chemical and topological features presented by each crystal face. The solvent organizes into at least three distinct layers. We also show that structuring of the solvent near the different faces of β-hematin strongly impacts the interfacial dynamics. The relaxation time of n-octanol molecules is longest in the contact layers and correlates with the degree of structural ordering at the respective face. We show that the macroscopically homogeneous water-octanol solution holds clusters of water and n-octanol connected by hydrogen bonds that entrap the majority of the water but are mostly smaller than 30 water molecules. Near the crystal surface the clusters anchor on hematin carboxyl groups. These results provide a direct example that solvent structuring is not restricted to aqueous and other hydrogen-bonded solutions. Our findings illuminate two fundamental features of the mechanisms of hematin crystallization: the elongated shapes of natural and synthetic hematin crystals and the stabilization of charged groups of hematin and antimalarials by encasing in water clusters. In addition, these findings suggest that hematin crystallization may be controlled by additives that disrupt or reinforce solvent structuring.

An essential vesicular-trafficking phospholipase mediates neutral lipid synthesis and contributes to hemozoin formation in Plasmodium falciparum

Background

Plasmodium falciparum is the pathogen responsible for the most devastating form of human malaria. As it replicates asexually in the erythrocytes of its human host, the parasite feeds on haemoglobin uptaken from these cells. Heme, a toxic by-product of haemoglobin utilization by the parasite, is neutralized into inert hemozoin in the food vacuole of the parasite. Lipid homeostasis and phospholipid metabolism are crucial for this process, as well as for the parasite’s survival and propagation within the host. P. falciparum harbours a uniquely large family of phospholipases, which are suggested to play key roles in lipid metabolism and utilization.

Results

Here, we show that one of the parasite phospholipase (P. falciparum lysophospholipase, PfLPL1) plays an essential role in lipid homeostasis linked with the haemoglobin degradation and heme conversion pathway. Fluorescence tagging showed that the PfLPL1 in infected blood cells localizes to dynamic vesicular structures that traffic from the host-parasite interface at the parasite periphery, through the cytosol, to get incorporated into a large vesicular lipid rich body next to the food-vacuole. PfLPL1 is shown to harbour enzymatic activity to catabolize phospholipids, and its transient downregulation in the parasite caused a significant reduction of neutral lipids in the food vacuole-associated lipid bodies. This hindered the conversion of heme, originating from host haemoglobin, into the hemozoin, and disrupted the parasite development cycle and parasite growth. Detailed lipidomic analyses of inducible knock-down parasites deciphered the functional role of PfLPL1 in generation of neutral lipid through recycling of phospholipids. Further, exogenous fatty-acids were able to complement downregulation of PfLPL1 to rescue the parasite growth as well as restore hemozoin levels.

Conclusions

We found that the transient downregulation of PfLPL1 in the parasite disrupted lipid homeostasis and caused a reduction in neutral lipids essentially required for heme to hemozoin conversion. Our study suggests a crucial link between phospholipid catabolism and generation of neutral lipids (TAGs) with the host haemoglobin degradation pathway.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01042-z.

Artemisinin inspired synthetic endoperoxide drug candidates: Design, synthesis, and mechanism of action studies

Artemisinin combination therapies (ACTs) have been used as the first-line treatments against Plasmodium falciparum malaria for decades. Recent advances in chemical proteomics have shed light on the complex mechanism of action of semi-synthetic artemisinin (ARTs), particularly their promiscuous alkylation of parasite proteins via previous heme-mediated bioactivation of the endoperoxide bond. Alarmingly, the rise of resistance to ART in South East Asia and the synthetic limitations of the ART scaffold have pushed the course for the necessity of fully synthetic endoperoxide-based antimalarials. Several classes of synthetic endoperoxide antimalarials have been described in literature utilizing various endoperoxide warheads including 1,2-dioxanes, 1,2,4-trioxanes, 1,2,4-trioxolanes, and 1,2,4,5-tetraoxanes. Two of these classes, the 1,2,4-trioxolanes (arterolane and artefenomel) and the 1,2,4,5-tetraoxanes (N205 and E209) based antimalarials, have been explored extensively and are still in active development. In contrast, the most recent publication pertaining to the development of the 1,2-dioxane, Arteflene, and 1,2,4-trioxanes fenozan-50F, DU1301, and PA1103/SAR116242 was published in 2008. This review summarizes the synthesis, biological and clinical evaluation, and mechanistic studies of the most developed synthetic endoperoxide antimalarials, providing an update on those classes still in active development.

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.

Malaria Pigment Crystals: The Achilles' Heel of the Malaria Parasite

The biogenic formation of hemozoin crystals, a crucial process in heme detoxification by the malaria parasite, is reviewed as an antimalarial drug target. We first focus on the in‐vivo formation of hemozoin. A model is presented, based on native‐contrast 3D imaging obtained by X‐ray and electron microscopy, that hemozoin nucleates at the inner membrane leaflet of the parasitic digestive vacuole, and grows in the adjacent aqueous medium. Having observed quantities of hemoglobin and hemozoin in the digestive vacuole, we present a model that heme liberation from hemoglobin and hemozoin formation is an assembly‐line process. The crystallization is preceded by reaction between heme monomers yielding hematin dimers involving fewer types of isomers than in synthetic hemozoin; this is indicative of protein‐induced dimerization. Models of antimalarial drugs binding onto hemozoin surfaces are reviewed. This is followed by a description of bromoquine, a chloroquine drug analogue, capping a significant fraction of hemozoin surfaces within the digestive vacuole and accumulation of the drug, presumably a bromoquine–hematin complex, at the vacuole's membrane.

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.

Intrinsic fluorescence properties of antimalarial pyrido[1,2-a]benzimidazoles facilitate subcellular accumulation and mechanistic studies in the human malaria parasite Plasmodium falciparum

The intrinsic fluorescence properties of two related pyrido[1,2-a]benzimidazole antimalarial compounds suitable for the cellular imaging of the human malaria parasite Plasmodium falciparum without the need to attach extrinsic fluorophores are described. Although these compounds are structurally related, they have been shown by confocal microscopy to not only accumulate selectively within P. falciparum but to also accumulate differently in the organelles investigated. Localization to the digestive vacuole and nearby neutral lipids was observed for compound 2 which was shown to inhibit hemozoin formation using a cellular fractionation assay indicating that this is a contributing mechanism of action. By contrast, compound 1, which differs from compound 2 by the replacement of the imidazole[1,2-a:4,5-b']dipyridine core with the benzimidazole core as well as the presence of Cl substituents, shows very different localisation patterns and shows no evidence of hemozoin inhibition, suggesting a different mechanism of antimalarial action. Docking profiles of both compounds on the hemozoin surface further provided insight into their mechanisms of action.

A lipocalin mediates unidirectional heme biomineralization in malaria parasites

During blood-stage development, malaria parasites are challenged with the detoxification of enormous amounts of heme released during the proteolytic catabolism of erythrocytic hemoglobin. They tackle this problem by sequestering heme into bioinert crystals known as hemozoin. The mechanisms underlying this biomineralization process remain enigmatic. Here, we demonstrate that both rodent and human malaria parasite species secrete and internalize a lipocalin-like protein, PV5, to control heme crystallization. Transcriptional deregulation of PV5 in the rodent parasite Plasmodium berghei results in inordinate elongation of hemozoin crystals, while conditional PV5 inactivation in the human malaria agent Plasmodium falciparum causes excessive multidirectional crystal branching. Although hemoglobin processing remains unaffected, PV5-deficient parasites generate less hemozoin. Electron diffraction analysis indicates that despite the distinct changes in crystal morphology, neither the crystalline order nor unit cell of hemozoin are affected by impaired PV5 function. Deregulation of PV5 expression renders P. berghei hypersensitive to the antimalarial drugs artesunate, chloroquine, and atovaquone, resulting in accelerated parasite clearance following drug treatment in vivo. Together, our findings demonstrate the Plasmodium-tailored role of a lipocalin family member in hemozoin formation and underscore the heme biomineralization pathway as an attractive target for therapeutic exploitation.

System-wide biochemical analysis reveals ozonide antimalarials initially act by disrupting Plasmodium falciparum haemoglobin digestion

Ozonide antimalarials, OZ277 (arterolane) and OZ439 (artefenomel), are synthetic peroxide-based antimalarials with potent activity against the deadliest malaria parasite, Plasmodium falciparum. Here we used a "multi-omics" workflow, in combination with activity-based protein profiling (ABPP), to demonstrate that peroxide antimalarials initially target the haemoglobin (Hb) digestion pathway to kill malaria parasites. Time-dependent metabolomic profiling of ozonide-treated P. falciparum infected red blood cells revealed a rapid depletion of short Hb-derived peptides followed by subsequent alterations in lipid and nucleotide metabolism, while untargeted peptidomics showed accumulation of longer Hb-derived peptides. Quantitative proteomics and ABPP assays demonstrated that Hb-digesting proteases were increased in abundance and activity following treatment, respectively. Ozonide-induced depletion of short Hb-derived peptides was less extensive in a drug-treated K13-mutant artemisinin resistant parasite line (Cam3.IIR539T) than in the drug-treated isogenic sensitive strain (Cam3.IIrev), further confirming the association between ozonide activity and Hb catabolism. To demonstrate that compromised Hb catabolism may be a primary mechanism involved in ozonide antimalarial activity, we showed that parasites forced to rely solely on Hb digestion for amino acids became hypersensitive to short ozonide exposures. Quantitative proteomics analysis also revealed parasite proteins involved in translation and the ubiquitin-proteasome system were enriched following drug treatment, suggestive of the parasite engaging a stress response to mitigate ozonide-induced damage. Taken together, these data point to a mechanism of action involving initial impairment of Hb catabolism, and indicate that the parasite regulates protein turnover to manage ozonide-induced damage.

Unsymmetrical Bisquinolines with High Potency against P. falciparum Malaria

Quinoline-based scaffolds have been the mainstay of antimalarial drugs, including many artemisinin combination therapies (ACTs), over the history of modern drug development. Although much progress has been made in the search for novel antimalarial scaffolds, it may be that quinolines will remain useful, especially if very potent compounds from this class are discovered. We report here the results of a structure-activity relationship (SAR) study assessing potential unsymmetrical bisquinoline antiplasmodial drug candidates using in vitro activity against intact parasites in cell culture. Many unsymmetrical bisquinolines were found to be highly potent against both chloroquine-sensitive and chloroquine-resistant Plasmodium falciparum parasites. Further work to develop such compounds could focus on minimizing toxicities in order to find suitable candidates for clinical evaluation.

Molecular targets in cerebral malaria for developing novel therapeutic strategies

Cerebral malaria (CM) is the severe neurological complication associated with Plasmodium falciparum infection. In clinical settings CM is predominantly characterized by fever, epileptic seizures, and asexual forms of parasite on blood smears, coma and even death. Cognitive impairment in the children and adults even after survival is one of the striking consequences of CM. Poor diagnosis often leads to inappropriate malaria therapy which in turn progress into a severe form of disease. Activation of multiple cell death pathways such as Inflammation, oxidative stress, apoptosis and disruption of blood brain barrier (BBB) plays critical role in the pathogenesis of CM and secondary brain damage. Thus, understanding such mechanisms of neuronal cell death might help to identify potential molecular targets for CM. Mitigation strategies for mortality rate and long-term cognitive deficits caused by existing anti-malarial drugs still remains a valid research question to ask. In this review, we discuss in detail about critical neuronal cell death mechanisms and the overall significance of adjunctive therapy with recent trends, which provides better insight towards establishing newer therapeutic strategies for CM.

Leveraging the effects of chloroquine on resistant malaria parasites for combination therapies

Background

Malaria is a major global health problem, with the Plasmodium falciparum protozoan parasite causing the most severe form of the disease. Prevalence of drug-resistant P. falciparum highlights the need to understand the biology of resistance and to identify novel combination therapies that are effective against resistant parasites. Resistance has compromised the therapeutic use of many antimalarial drugs, including chloroquine, and limited our ability to treat malaria across the world. Fortunately, chloroquine resistance comes at a fitness cost to the parasite; this can be leveraged in developing combination therapies or to reinstate use of chloroquine.

Results

To understand biological changes induced by chloroquine treatment, we compared transcriptomics data from chloroquine-resistant parasites in the presence or absence of the drug. Using both linear models and a genome-scale metabolic network reconstruction of the parasite to interpret the expression data, we identified targetable pathways in resistant parasites. This study identified an increased importance of lipid synthesis, glutathione production/cycling, isoprenoids biosynthesis, and folate metabolism in response to chloroquine.

Conclusions

We identified potential drug targets for chloroquine combination therapies. Significantly, our analysis predicts that the combination of chloroquine and sulfadoxine-pyrimethamine or fosmidomycin may be more effective against chloroquine-resistant parasites than either drug alone; further studies will explore the use of these drugs as chloroquine resistance blockers. Additional metabolic weaknesses were found in glutathione generation and lipid synthesis during chloroquine treatment. These processes could be targeted with novel inhibitors to reduce parasite growth and reduce the burden of malaria infections. Thus, we identified metabolic weaknesses of chloroquine-resistant parasites and propose targeted chloroquine combination therapies.

Electronic supplementary material

The online version of this article (10.1186/s12859-019-2756-y) contains supplementary material, which is available to authorized users.

Quantification of Free Ferriprotoporphyrin IX Heme and Hemozoin for Artemisinin Sensitive versus Delayed Clearance Phenotype Plasmodium falciparum Malarial Parasites

We use Plasmodium falciparum culture synchronization, optimized heme and hemozoin extraction protocols, and mass spectrometry to quantify the abundance of free ferriprotoporphyrin IX (FPIX) heme and crystallized FPIX (hemozoin; Hz) for various growth stages of intraerythrocytic P. falciparum malarial parasites. Because of altered cell cycle kinetics for delayed clearance phenotype (DCP) parasites relative to that of the control, we test whether FPIX and Hz abundances differ for DCP and control parasites.

Chloroquine analogs as antimalarial candidates with potent in vitro and in vivo activity

In spite of recent efforts to eradicate malaria in the world, this parasitic disease is still considered a major public health problem, with a total of 216 million cases of malaria and 445,000 deaths in 2016. Artemisinin-based combination therapies remain effective in most parts of the world, but recent cases of resistance in Southeast Asia have urged for novel approaches to treat malaria caused by Plasmodium falciparum. In this work, we present chloroquine analogs that exhibited high activity against sensitive and chloroquine-resistant P. falciparum blood parasites and were also active against P. berghei infected mice. Among the compounds tested, DAQ, a chloroquine analog with a more linear side chain, was shown to be the most active in vitro and in vivo, with low cytotoxicity, and therefore may serve as the basis for the development of more effective chloroquine analogs to aid malaria eradication.

Structuring of Organic Solvents at Solid Interfaces and Ramifications for Antimalarial Adsorption on β-Hematin Crystals

A critical aspect of material synthesis is solvent structuring at solid-liquid interfaces, which can impact the adsorption of solute and growth modifiers on an underlying substrate. In general, the impact of solvent structuring on molecular sorbate interactions with solid sorbents is poorly understood. This is particularly true for processes that occur in organic media, such as hematin crystallization, which is crucial to the survival of malaria parasites. Here, we use chemical force microscopy and molecular modeling to analyze the interactions between functional moieties of known antimalarials and the interface between β-hematin crystals and a mixed organic (octanol)-aqueous solvent. We show that the β-hematin surface, patterned in parallel hydrophobic and hydrophilic stripes, engenders the assembly of up to five layers of octanol molecules aligned parallel to the crystal surface. In contrast, studies of solvent structuring on a disordered glass surface reveal that octanol molecules align perpendicular to the interface. The distinct octanol arrays direct molecule adsorption at the respective interfaces. At both substrates, we also find stabilized pockets of aqueous nanophase lining the surfaces. A combination of experimental analyses and modeling of solvent structuring provides crucial insights into the association of hematin molecules with growing crystals as well as the adsorption and mobility of antimalarial drugs. Moreover, our findings offer a general perspective on the collective behaviors of complex organic solvents that may apply to a broad range of interactions at solid-liquid interfaces.

Study of the antimalarial activity of 4-aminoquinoline compounds against chloroquine-sensitive and chloroquine-resistant parasite strains

This study is concerned with identifying features of 4-aminoquinoline scaffolds that can help pinpoint characteristics that enhance activity against chloroquine-resistant parasites. Statistically valid predictive models are reported for a series of 4-aminoquinoline analogues that are active against chloroquine-sensitive (NF54) and chloroquine-resistant (K1) strains of Plasmodium falciparum. Quantitative structure activity relationship techniques, based on statistical and machine learning methods such as multiple linear regression and partial least squares, were used with a novel pruning method for the selection of descriptors to develop robust models for both strains. Inspection of the dominant descriptors supports the hypothesis that chemical features that enable accumulation in the food vacuole of the parasite are key determinants of activity against both strains. The hydrophilic properties of the compounds were found to be crucial in predicting activity against the chloroquine-sensitive NF54 parasite strain, but not in the case of the chloroquine-resistant K1 strain, in line with previous studies. Additionally, the models suggest that ‘softer’ compounds tend to have improved activity for both strains than do ‘harder’ ones. The internally and externally validated models reported here should also prove useful in the future screening of potential antimalarial compounds for targeting chloroquine-resistant strains.

Graphical Abstract

Drug targets for resistant malaria: Historic to future perspectives

New antimalarial targets are the prime need for the discovery of potent drug candidates. In order to fulfill this objective, antimalarial drug researches are focusing on promising targets in order to develop new drug candidates. Basic metabolism and biochemical process in the malaria parasite, i.e. Plasmodium falciparum can play an indispensable role in the identification of these targets. But, the emergence of resistance to antimalarial drugs is an escalating comprehensive problem with the progress of antimalarial drug development. The development of resistance has highlighted the need for the search of novel antimalarial molecules. The pharmaceutical industries are committed to new drug development due to the global recognition of this life threatening resistance to the currently available antimalarial therapy. The recent developments in the understanding of parasite biology are exhilarating this resistance issue which is further being ignited by malaria genome project. With this background of information, this review was aimed to highlights and provides useful information on various present and promising treatment approaches for resistant malaria, new progresses, pursued by some innovative targets that have been explored till date. This review also discusses modern and futuristic multiple approaches to antimalarial drug discovery and development with pictorial presentations highlighting the various targets, that could be exploited for generating promising new drugs in the future for drug resistant malaria.

Shining new light on ancient drugs: preparation and subcellular localisation of novel fluorescent analogues of Cinchona alkaloids in intraerythrocytic Plasmodium falciparum

Fluorescent derivatives of the archetypal antimalarial quinine and its diastereomer, quinidine, suitable for cellular imaging have been synthesised by attaching the small extrinsic fluorophore, NBD. Interactions of these derivatives with ferriprotoporphyrin IX were evaluated to verify that insights generated by live-cell imaging were relevant to the parent molecules. These analogues are shown by confocal and super-resolution microscopy to accumulate selectively in Plasmodium falciparum. Localisation to the region corresponding to the digestive vacuole supports the putative primary role of these alkaloids as haemozoin inhibitors. Quantitative analysis revealed minimal accumulation within the nucleus, rejecting the disruption of DNA replication as a possible mode of action. While extensive localisation to phospholipid structures and associated organelles was observed, the analogues did not show evidence of association with neutral lipid bodies.

Metabolome-wide association study of peripheral parasitemia in Plasmodium vivax malaria

BACKGROUND

Plasmodium vivax is one of the leading causes of malaria worldwide. Infections with this parasite cause diverse clinical manifestations, and recent studies revealed that infections with P. vivax can result in severe and fatal disease. Despite these facts, biological traits of the host response and parasite metabolism during P. vivax malaria are still largely underexplored. Parasitemia is clearly related to progression and severity of malaria caused by P. falciparum, however the effects of parasitemia during infections with P. vivax are not well understood.

RESULTS

We conducted an exploratory study using a high-resolution metabolomics platform that uncovered significant associations between parasitemia levels and plasma metabolites from 150 patients with P. vivax malaria. Most plasma metabolites were inversely associated with higher levels of parasitemia. Top predicted metabolites are implicated into pathways of heme and lipid metabolism, which include biliverdin, bilirubin, palmitoylcarnitine, stearoylcarnitine, phosphocholine, glycerophosphocholine, oleic acid and omega-carboxy-trinor-leukotriene B4.

CONCLUSIONS

The abundance of several plasma metabolites varies according to the levels of parasitemia in patients with P. vivax malaria. Moreover, our data suggest that the host response and/or parasite survival might be affected by metabolites involved in the degradation of heme and metabolism of several lipids. Importantly, these data highlight metabolic pathways that may serve as targets for the development of new antimalarial compounds.

Exploring Heme and Hemoglobin Binding Regions of Plasmodium Heme Detoxification Protein for New Antimalarial Discovery

Hemoglobin degradation/hemozoin formation, essential steps in the Plasmodium life cycle, are targets of existing antimalarials. The pathway still offers vast possibilities to be explored for new antimalarial discoveries. Here, we characterize heme detoxification protein, PfHDP, a major protein involved in hemozoin formation, as a novel drug target. Using in silico and biochemical approaches, we identified two heme binding sites and a hemoglobin binding site in PfHDP. Treatment of Plasmodium falciparum 3D7 parasites with peptide corresponding to the hemoglobin binding domain in PfHDP resulted in food vacuole abnormalities similar to that seen with a cysteine protease inhibitor, E-64 (I-1). Screening of compounds that bound the modeled PfHDP structure in the heme/hemoglobin-binding pockets from Maybridge Screening Collection identified a compound, ML-2, that inhibited parasite growth in a dose-dependent manner, thus paving the way for testing its potential as a new drug candidate. These results provide functional insights into the role of PfHDP in Hz formation and further suggest that PfHDP could be an important drug target to combat malaria.

Nanocrystallography measurements of early stage synthetic malaria pigment

The recent availability of extremely intense, femtosecond X-ray free-electron laser (XFEL) sources has spurred the development of serial femtosecond nanocrystallography (SFX). Here, SFX is used to analyze nanoscale crystals of β-hematin, the synthetic form of hemozoin which is a waste by-product of the malaria parasite. This analysis reveals significant differences in β-hematin data collected during SFX and synchrotron crystallography experiments. To interpret these differences two possibilities are considered: structural differences between the nanocrystal and larger crystalline forms of β-hematin, and radiation damage. Simulation studies show that structural inhomogeneity appears at present to provide a better fit to the experimental data. If confirmed, these observations will have implications for designing compounds that inhibit hemozoin formation and suggest that, for some systems at least, additional information may be gained by comparing structures obtained from nanocrystals and macroscopic crystals of the same molecule.

Deconstructing Quinoline-Class Antimalarials to Identify Fundamental Physicochemical Properties of Beta-Hematin Crystal Growth Inhibitors

A versatile approach to control crystallization involves the use of modifiers, which are additives that interact with crystal surfaces and alter their growth rates. Elucidating a modifier's binding specificity to anisotropic crystal surfaces is a ubiquitous challenge that is critical to their design. In this study, we select hematin, a byproduct of malaria parasites, as a model system to examine the complementarity of modifiers (i.e., antimalarial drugs) to β-hematin crystal surfaces. We divide two antimalarials, chloroquine and amodiaquine, into segments consisting of a quinoline base, common to both drugs, and side chains that differentiate their modes of action. Using a combination of scanning probe microscopy, bulk crystallization, and analytical techniques, we show that the base and side chain work synergistically to reduce the rate of hematin crystallization. In contrast to general observations that modifiers retain their function upon segmentation, we show that the constituents do not act as modifiers. A systematic study of quinoline isomers and analogues shows how subtle rearrangement and removal of functional moieties can create effective constituents from previously ineffective modifiers, along with tuning their inhibitory modes of action. These findings highlight the importance of specific functional moieties in drug compounds, leading to an improved understanding of modifier-crystal interactions that could prove to be applicable to the design of new antimalarials.

Unraveling heme detoxification in the malaria parasite by in situ correlative X-ray fluorescence microscopy and soft X-ray tomography

A key drug target for malaria has been the detoxification pathway of the iron-containing molecule heme, which is the toxic byproduct of hemoglobin digestion. The cornerstone of heme detoxification is its sequestration into hemozoin crystals, but how this occurs remains uncertain. We report new results of in vivo rate of heme crystallization in the malaria parasite, based on a new technique to measure element-specific concentrations at defined locations in cell ultrastructure. Specifically, a high resolution correlative combination of cryo soft X-ray tomography has been developed to obtain 3D parasite ultrastructure with cryo X-ray fluorescence microscopy to measure heme concentrations. Our results are consistent with a model for crystallization via the heme detoxification protein. Our measurements also demonstrate the presence of considerable amounts of non-crystalline heme in the digestive vacuole, which we show is most likely contained in hemoglobin. These results suggest a tight coupling between hemoglobin digestion and heme crystallization, highlighting a new link in the crystallization pathway for drug development.

The Effects of Quinoline and Non-Quinoline Inhibitors on the Kinetics of Lipid-Mediated β-Hematin Crystallization

The throughput of a biomimetic lipid-mediated assay used to investigate the effects of inhibitors on the kinetics of β-hematin formation has been optimized through the use of 24-well microplates. The rate constant for β-hematin formation mediated by monopalmitoyl-rac-glycerol was reduced from 0.17 ± 0.04 min^-1 previously measured in Falcon tubes to 0.019 ± 0.002 min^-1 in the optimized assay. While this necessitated longer incubation times, transferring aliquots from multiple 24-well plates to a single 96-well plate for final absorbance measurements actually improved the overall turnaround time per inhibitor. This assay has been applied to investigate the effects of four clinically relevant antimalarial drugs (chloroquine, amodiaquine, quinidine, and quinine) as well as several short-chain 4-aminoquinoline derivatives and non-quinoline (benzamide) compounds on the kinetics of β-hematin formation. The adsorption strength of these inhibitors to crystalline β-hematin (K_ads) was quantified using a theoretical kinetic model that is based on the Avrami equation and the Langmuir isotherm. Statistically significant linear correlations between lipid-mediated β-hematin inhibitory activity and K_ads values for quinoline (r² = 0.76, P-value = 0.0046) and non-quinoline compounds (r² = 0.99, P-stat = 0.0006), as well as between parasite inhibitory activity (D10) and K_ads values for quinoline antimalarial drugs and short-chain chloroquine derivatives (r² = 0.64, P-value = 0.0098), provide a strong indication that drug action involves adsorption to the surface of β-hematin crystals. Independent support in this regard is provided by experiments that spectrophotometrically monitor the direct adsorption of antimalarial drugs to preformed β-hematin.

Antimalarials inhibit hematin crystallization by unique drug-surface site interactions

In malaria pathophysiology, divergent hypotheses on the inhibition of hematin crystallization posit that drugs act either by the sequestration of soluble hematin or their interaction with crystal surfaces. We use physiologically relevant, time-resolved in situ surface observations and show that quinoline antimalarials inhibit β-hematin crystal surfaces by three distinct modes of action: step pinning, kink blocking, and step bunch induction. Detailed experimental evidence of kink blocking validates classical theory and demonstrates that this mechanism is not the most effective inhibition pathway. Quinolines also form various complexes with soluble hematin, but complexation is insufficient to suppress heme detoxification and is a poor indicator of drug specificity. Collectively, our findings reveal the significance of drug-crystal interactions and open avenues for rationally designing antimalarial compounds.

Quantitative characterization of hemozoin in Plasmodium berghei and vivax

The incidence and global distribution of chloroquine resistant (CR) Plasmodium vivax infection has increased since emerging in 1989. The mechanism of resistance in CR P. vivax has not been defined. The resistance likely relates to the formation and disposition of hemozoin as chloroquine's primary mechanism of action involves disruption of hemozoin formation. CR P. berghei strains, like CR P. vivax strains, are confined to reticulocyte host cells and reportedly they do not accumulate appreciable intraerythrocytic hemozoin. Reports comparing hemozoin production between P. vivax strains and CR to chloroquine sensitive (CS) P. berghei are absent. Here we compare in vivo patterns of hemozoin formation and distribution in blood, spleen and liver tissue of male Swiss mice infected with CS or CR P. berghei not treated with chloroquine and CR P. berghei also treated with chloroquine. Light microscopy, laser desorption mass spectrometry and a colorimetric hemozoin assay detect trace hemozoin in the blood of CR P. berghei infected mice but significant hemozoin accumulation in liver and spleen tissue. Field emission in lens scanning electron microscopy reveals CR P. berghei hemozoin crystals are morphologically smaller but similar to those formed by CS parasites. CR P. berghei produces approximately five-fold less total hemozoin than CS strain. Lipid analysis of CS and CR P. berghei sucrose gradient purified bloodstage hemozoin indicates a similar lipid environment around the isolated hemozoin, predominately monopalmitic glycerol and monostearic glycerol. In contrast to CR and CS P. berghei, colorimetric hemozoin analysis of P. vivax strains indicates similar amounts of hemozoin are produced despite differing chloroquine sensitivities. These results suggest CR P. berghei forms significant hemozoin which accumulates in liver and spleen tissues and that the P. vivax chloroquine resistance mechanism differs from P. berghei.

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Chloroquine resistant Plasmodium berghei release measurable hemozoin into tissues with blood hemozoin 100 times less per parasite while total in all tissues is only 5 times less than chloroquine sensitive.

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Chloroquine resistant P. bergheihemozoin crystals are morphologically smaller but similar to those formed by chloroquine sensitive parasites.

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Chloroquine resistance in P. vivax is distinct from P. berghei even though both infect reticulocytes.

Multifactorial resistance to aminopeptidase inhibitor prodrug CHR2863 in myeloid leukemia cells: down-regulation of carboxylesterase 1, drug sequestration in lipid droplets and pro-survival activation ERK/Akt/mTOR

Aminopeptidase inhibitors are receiving attention as combination chemotherapeutic agents for the treatment of refractory acute myeloid leukemia. However, the factors determining therapeutic efficacy remain elusive. Here we identified the molecular basis of acquired resistance to CHR2863, an orally available hydrophobic aminopeptidase inhibitor prodrug with an esterase-sensitive motif, in myeloid leukemia cells. CHR2863 enters cells by diffusion and is retained therein upon esterase activity-mediated conversion to its hydrophilic active metabolite drug CHR6768, thereby exerting amino acid depletion. Carboxylesterases (CES) serve as candidate prodrug activating enzymes given CES1 expression in acute myeloid leukemia specimens. We established two novel myeloid leukemia sublines U937/CHR2863(200) and U937/CHR2863(5uM), with low (14-fold) and high level (270-fold) CHR2863 resistance. The latter drug resistant cells displayed: (i) complete loss of CES1-mediated drug activation associated with down-regulation of CES1 mRNA and protein, (ii) marked retention/sequestration of the prodrug, (iii) a substantial increase in intracellular lipid droplets, and (iv) a dominant activation of the pro-survival Akt/mTOR pathway. Remarkably, the latter feature coincided with a gain of sensitivity to the mTOR inhibitor rapamycin. These finding delineate the molecular basis of CHR2863 resistance and offer a novel modality to overcome this drug resistance in myeloid leukemia cells.

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.

Lipid response patterns in acute phase paediatric Plasmodium falciparum malaria

INTRODUCTION

Several studies have observed serum lipid changes during malaria infection in humans. All of them were focused at analysis of lipoproteins, not specific lipid molecules. The aim of our study was to identify novel patterns of lipid species in malaria infected patients using lipidomics profiling, to enhance diagnosis of malaria and to evaluate biochemical pathways activated during parasite infection.

METHODS

Using a multivariate characterization approach, 60 samples were representatively selected, 20 from each category (mild, severe and controls) of the 690 study participants between age of 0.5-6 years. Lipids from patient's plasma were extracted with chloroform/methanol mixture and subjected to lipid profiling with application of the LCMS-QTOF method.

RESULTS

We observed a structured plasma lipid response among the malaria-infected patients as compared to healthy controls, demonstrated by higher levels of a majority of plasma lipids with the exception of even-chain length lysophosphatidylcholines and triglycerides with lower mass and higher saturation of the fatty acid chains. An inverse lipid profile relationship was observed when plasma lipids were correlated to parasitaemia.

CONCLUSIONS

This study demonstrates how mapping the full physiological lipid response in plasma from malaria-infected individuals can be used to understand biochemical processes during infection. It also gives insights to how the levels of these molecules relate to acute immune responses.

Evolution of Fitness Cost-Neutral Mutant PfCRT Conferring P. falciparum 4-Aminoquinoline Drug Resistance Is Accompanied by Altered Parasite Metabolism and Digestive Vacuole Physiology

Southeast Asia is an epicenter of multidrug-resistant Plasmodium falciparum strains. Selective pressures on the subcontinent have recurrently produced several allelic variants of parasite drug resistance genes, including the P. falciparum chloroquine resistance transporter (pfcrt). Despite significant reductions in the deployment of the 4-aminoquinoline drug chloroquine (CQ), which selected for the mutant pfcrt alleles that halted CQ efficacy decades ago, the parasite pfcrt locus is continuously evolving. This is highlighted by the presence of a highly mutated allele, Cam734 pfcrt, which has acquired the singular ability to confer parasite CQ resistance without an associated fitness cost. Here, we used pfcrt-specific zinc-finger nucleases to genetically dissect this allele in the pathogenic setting of asexual blood-stage infection. Comparative analysis of drug resistance and growth profiles of recombinant parasites that express Cam734 or variants thereof, Dd2 (the most common Southeast Asian variant), or wild-type pfcrt, revealed previously unknown roles for PfCRT mutations in modulating parasite susceptibility to multiple antimalarial agents. These results were generated in the GC03 strain, used in multiple earlier pfcrt studies, and might differ in natural isolates harboring this allele. Results presented herein show that Cam734-mediated CQ resistance is dependent on the rare A144F mutation that has not been observed beyond Southeast Asia, and reveal distinct impacts of this and other Cam734-specific mutations on CQ resistance and parasite growth rates. Biochemical assays revealed a broad impact of mutant PfCRT isoforms on parasite metabolism, including nucleoside triphosphate levels, hemoglobin catabolism and disposition of heme, as well as digestive vacuole volume and pH. Results from our study provide new insights into the complex molecular basis and physiological impact of PfCRT-mediated antimalarial drug resistance, and inform ongoing efforts to characterize novel pfcrt alleles that can undermine the efficacy of first-line antimalarial drug regimens.

Point mutations in the Plasmodium falciparum chloroquine resistance transporter (PfCRT) earlier thwarted the clinical efficacy of chloroquine, the former gold standard, and constitute a major determinant of parasite susceptibility to antimalarial drugs. Recently, we reported that the highly mutated Cambodian PfCRT isoform Cam734 is fitness-neutral in terms of parasite growth, unlike other less fit isoforms such as Dd2 that are outcompeted by wild-type parasites in the absence of CQ pressure. Using pfcrt-specific zinc-finger nucleases to genetically dissect the Cam734 allele, we report that its unique constituent mutations directly contribute to CQ resistance and collectively offset fitness costs associated with intermediate mutational steps. We also report that these mutations can contribute to resistance or increased sensitivity to multiple first-line partner drugs. Using isogenic parasite lines, we provide evidence of changes in parasite metabolism associated with the Cam734 allele compared to Dd2. We also observe a close correlation between CQ inhibition of hemozoin formation and parasite growth, and provide evidence that Cam734 PfCRT can modulate drug potency depending on its membrane electrochemical gradient. Our data highlight the capacity of PfCRT to evolve new states of antimalarial drug resistance and to offset associated fitness costs through its impact on parasite physiology and hemoglobin catabolism.

Changes in serum lipid profile in the acute and convalescent Plasmodium vivax malaria: A cohort study

Although serum lipids are known to be altered in Plasmodium falciparum-induced malaria, little is known about such changes due to Plasmodium vivax infection. This cohort study assessed serum concentrations of triglycerides, total cholesterol, low-density lipoprotein (LDL), and high-density lipoprotein (HDL) in 164 patients in the acute phase of malaria caused by P. vivax and characterized these changes in the convalescent phase after treatment with chloroquine and primaquine. Compared to reference values, serum total cholesterol, LDL, and HDL levels were lower and triglyceride levels were higher in the acute phase. Moreover, the parasite density was negatively correlated with LDL (r=-0,189; p=0.027) and HDL (r=-0,256; p=0.001) serum levels. Eighty patients returned for clinical and laboratory revaluation 7-12days after treatment initiation. All patients showed parasite clearance and the absence of symptoms during the convalescent phase. Analysis of the serum lipids of these 80 patients showed significant increases in the serum levels of total cholesterol (p<0.0001), LDL (p<0.0001), and HDL (p<0.0001) as well as a significant reduction in triglycerides (p=0.004), indicating a trend towards a return to normal levels. This transient change in lipid profile between the acute and convalescent stages may be useful for the clinical monitoring of patients treated for vivax malaria.

Influence of LAR and VAR on Para-Aminopyridine Antimalarials Targetting Haematin in Chloroquine-Resistance

Antimalarial chloroquine (CQ) prevents haematin detoxication when CQ-base concentrates in the acidic digestive vacuole through protonation of its p-aminopyridine (pAP) basic aromatic nitrogen and sidechain diethyl-N. CQ export through the variant vacuolar membrane export channel, PFCRT, causes CQ-resistance in Plasmodium falciparum but 3-methyl CQ (sontochin SC), des-ethyl amodiaquine (DAQ) and bis 4-aminoquinoline piperaquine (PQ) are still active. This is determined by changes in drug accumulation ratios in parasite lipid (LAR) and in vacuolar water (VAR). Higher LAR may facilitate drug binding to and blocking PFCRT and also aid haematin in lipid to bind drug. LAR for CQ is only 8.3; VAR is 143,482. More hydrophobic SC has LAR 143; VAR remains 68,523. Similarly DAQ with a phenol substituent has LAR of 40.8, with VAR 89,366. In PQ, basicity of each pAP is reduced by distal piperazine N, allowing very high LAR of 973,492, retaining VAR of 104,378. In another bis quinoline, dichlorquinazine (DCQ), also active but clinically unsatisfactory, each pAP retains basicity, being insulated by a 2-carbon chain from a proximal nitrogen of the single linking piperazine. While LAR of 15,488 is still high, the lowest estimate of VAR approaches 4.9 million. DCQ may be expected to be very highly lysosomotropic and therefore potentially hepatotoxic. In 11 pAP antimalarials a quadratic relationship between logLAR and logResistance Index (RI) was confirmed, while log (LAR/VAR) vs logRI for 12 was linear. Both might be used to predict the utility of structural modifications.

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.

Detergent-Mediated Formation of β-Hematin: Heme Crystallization Promoted by Detergents Implicates Nanostructure Formation for Use as a Biological Mimic

Hemozoin is a unique biomineral that results from the sequestration of toxic free heme liberated as a consequence of hemoglobin degradation in the malaria parasite. Synthetic neutral lipid droplets (SNLDs) and phospholipids were previously shown to support the rapid formation of β-hematin, abiological hemozoin, under physiologically relevant pH and temperature, though the mechanism by which heme crystallization occurs remains unclear. Detergents are particularly interesting as a template because they are amphiphilic molecules that spontaneously organize into nanostructures and have been previously shown to mediate β-hematin formation. Here, 11 detergents were investigated to elucidate the physicochemical properties that best recapitulate crystal formation in the parasite. A strong correlation between the detergent's molecular structure and the corresponding kinetics of β-hematin formation was observed, where higher molecular weight polar chains promoted faster reactions. The larger hydrophilic chains correlated to the detergent's ability to rapidly sequester heme into the lipophilic core, allowing for crystal nucleation to occur. The data presented here suggest that detergent nanostructures promote β-hematin formation in a similar manner to SNLDs and phospholipids. Through understanding mediator properties that promote optimal crystal formation, we are able to establish an in vitro assay to probe this drug target pathway.

Insights into the initial stages of lipid-mediated haemozoin nucleation

Lipid-mediated haemozoin nucleation, as probed by molecular dynamics, proceeds via aggregation of ferrihaem π–π dimers at a lipid–aqueous interface.

Characterization and optimization of the haemozoin-like crystal (HLC) assay to determine Hz inhibiting effects of anti-malarial compounds

Background

The haem-haemozoin biocrystallization pathway is an attractive target where several efficacious and safe anti-malarial drugs act. Consequently, in vitro haemozoin (Hz) inhibition assays have been developed to identify novel compounds. However, results may differ between assays and often require complex methods or sophisticated infrastructure. The recently reported growth of haemozoin-like crystals (HLC) appears to be a simple alternative although the endproduct is structurally different to Hz. This study set out to characterize this assay in depth, optimize it, and assess its performance.

Methods

The HLC assay was used as previously described but a range of different growth conditions were examined. Obtained HLCs were investigated and compared to synthetic (sHz) and natural haemozoin (nHz) using scanning electron microscopy, powder X-ray diffraction (PXRD), Fourier Transform Infrared spectroscopy (FTIR) and Raman spectroscopy (RS). Interactions of HLC with quinolines was analysed using RS. Inhibitory effects of currently used anti-malarial drugs under four final growth conditions were established.

Results

HLC growth requires Mycoplasma Broth Base, Tween 80, pancreatin, and lysed blood or haemin. HLCs are similar to nHz and sHz in terms of solubility, macroscopic and microscopic appearance although PXRD, FTIR and RS confirm that the haem aggregates of HLCs are structurally different. RS reveals that CQ seems to interact with HLCs in similar ways as with Hz. Inhibition of quinoline drugs ranged from 62.5 µM (chloroquine, amodiaquine, piperaquine) to 500 µM in mefloquine.

Conclusions

The HLC assay provides data on inhibiting properties of compounds. Even if the end-product is not structurally identical to Hz, the inhibitory effects appear consistent with those obtained with sHz assays, as illustrated by the results obtained for quinolines. The assay is simple, inexpensive, robust, reproducible and can be performed under basic laboratory conditions with a simple visual positive/negative read-out.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-015-0913-y) contains supplementary material, which is available to authorized users.

Alkoxide coordination of iron(III) protoporphyrin IX by antimalarial quinoline methanols: a key interaction observed in the solid-state and solution

The quinoline methanol antimalarial drug mefloquine is a structural analogue of the Cinchona alkaloids, quinine and quinidine. We have elucidated the single crystal X-ray diffraction structure of the complexes formed between racemic erythro mefloquine and ferriprotoporphyrin IX (Fe(iii)PPIX) and show that alkoxide coordination is a key interaction in the solid-state. Mass spectrometry confirms the existence of coordination complexes of quinine, quinidine and mefloquine to Fe(iii)PPIX in acetonitrile. The length of the iron(iii)-O bond in the quinine and quinidine complexes as determined by Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy unequivocally confirms that coordination of the quinoline methanol compounds to Fe(iii)PPIX occurs in non-aqueous aprotic solution via their benzylic alkoxide functional group. UV-visible spectrophotometric titrations of the low-spin bis-pyridyl-Fe(iii)PPIX complex with each of the quinoline methanol compounds results in the displacement of a single pyridine molecule and subsequent formation of a six-coordinate pyridine-Fe(iii)PPIX-drug complex. We propose that formation of the drug-Fe(iii)PPIX coordination complexes is favoured in a non-aqueous environment, such as that found in lipid bodies or membranes in the malaria parasite, and that their existence may contribute to the mechanism of haemozoin inhibition or other toxicity effects that lead ultimately to parasite death. In either case, coordination is a key interaction to be considered in the design of novel antimalarial drug candidates.

Optimization of a multi-well colorimetric assay to determine haem species in Plasmodium falciparum in the presence of anti-malarials

Background

The activity of several well-known anti-malarials, including chloroquine (CQ), is attributed to their ability to inhibit the formation of haemozoin (Hz) in the malaria parasite. The formation of inert Hz, or malaria pigment, from toxic haem acquired from the host red blood cell of the parasite during haemoglobin digestion represents a pathway essential for parasite survival. Inhibition of this critical pathway therefore remains a desirable target for novel anti-malarials. A recent publication described the results of a haem fractionation assay used to directly determine haemoglobin, free haem and Hz in Plasmodium falciparum inoculated with CQ. CQ was shown to cause a dose-dependent increase in cellular-free haem that was correlated with decreased parasite survival. The method provided valuable information but was limited due to its low throughput and high demand on parasite starting material. Here, this haem fractionation assay has been successfully adapted to a higher throughput method in 24-well plates, significantly reducing lead times and starting material volumes.

Methods

All major haem species in P. falciparum trophozoites, isolated through a series of cellular fractionation steps were determined spectrophotometrically in aqueous pyridine (5 % v/v, pH 7.5) as a low spin complex with haematin. Cell counts were determined using a haemocytometer and a rapid novel fluorescent flow cytometry method.

Results

A higher throughput haem fractionation assay in 24-well plates, containing at most ten million trophozoites was validated against the original published method using CQ and its robustness was confirmed. It provided a minimum six-fold improvement in productivity and 24-fold reduction in starting material volume. The assay was successfully applied to amodiaquine (AQ), which was shown to inhibit Hz formation, while the antifolate pyrimethamine (PYR) and the mitochondrial electron transporter inhibitor atovaquone (Atov) demonstrated no increase in toxic cellular free haem.

Conclusions

This higher throughput cellular haem fractionation assay can easily be applied to novel anti-malarials with a significantly decreased lead time, providing a valuable tool with which to probe the mechanisms of action of both new and established anti-malarials.