The structure of malaria pigment beta-haematin

The Hematin-dihydroartemisinin Adduct Mobilizes a Potent Mechanism to Suppress β-hematin Crystallization

Malaria remains a significant public health challenge in equatorial regions of the world largely owing to the parasite's emerging resistance to the recently introduced drugs of the artemisinin (ART) family. In the human body most ART-derivative drugs are metabolized to dihydroartemisinin (DHA), which, in the parasite, after activation by heme, can form a hematin-dihydroartemisinin adduct (H-DHA). Here we test whether and how H-DHA inhibits hematin crystallization, the main constituent of the heme detoxification pathway of malaria parasites. We find that H-DHA is a poor inhibitor of classical crystal growth- it weakly blocks the growth sites on crystal surfaces-and, counterproductively, a promotor of β-hematin nucleation, driven by a boost in the formation of precursors. We establish that at elevated hematin concentrations H-DHA activates two non-classical pathways that transform it into a potent β-hematin growth inhibitor. First, β-hematin crystallites, whose nucleation is promoted by H-DHA, incorporate into large β-hematin crystals and suppress their growth, likely by straining the crystal lattice. A second consequence of H-DHA is the generation of macrosteps on β-hematin crystal surfaces that hinder growth. Importantly, the induced growth suppression is irreversible and persists even in the absence of H-DHA. Our findings suggest that a partial resistance mechanism to artemisinin-class drugs in trophozoite-stage parasites may be due to the reduced concentrations of hematin and H-DHA, which deactivate the dual non-classical mode of action of the adduct in the delayed-clearance parasite strains.

Redox Homeostasis within the Drug-Resistant Malarial Parasite Digestive Vacuole

We have developed a cost-effective strategy for the complete synthesis of azetidinyl coumarin fluorophore derivatives that report changes in physiologic levels of glutathione (GSH), which includes a more cost- effective synthesis of the probe precursor hydroxyl derivative and its subsequent derivatization to promote subcellular localization. We functionalize coumarin derivatives with a cyano side chain similar to a previous strategy (Jiang X. et al., Nature Communications 2017, 8; 16087) and validate the 7-azetidinyl conformation as an explanation for enhanced GSH-dependent coumarin fluorescence. We couple the azetidinyl probe to different mass dextrans using either no linker or a 6C linker and also synthesize a morpholino derivative. We titrate the fluorescence of the different functionalized probes vs [GSH] in vitro. We load one dextran-conjugated probe within the digestive vacuole (DV) of live intraerythrocytic P. falciparum malarial parasites and also measure cytosolic localization of the morpholino probe. Using significantly improved single-cell photometry (SCP) methods, we show that the morpholino probe faithfully reports [GSH] from the live parasite cytosol, while the 70 kDa dextran-conjugated probe reports DV redox homeostasis for control chloroquine-sensitive (CQS) and artemisinin-sensitive (ARTS) transfectant parasites vs their genetically matched chloroquine-resistant (CQR)/artemisinin-sensitive (CQR/ARTS) and CQR artemisinin-resistant (CQR/ARTR) strains, respectively. We quantify rapid changes in DV redox homeostasis for these parasites ± drug pulses under live-cell perfusion conditions. The results are important for understanding the pharmacology of antimalarial drugs and the molecular mechanisms underlying CQR and ARTR phenomena.

Chemical propulsion of hemozoin crystal motion in malaria parasites

Malaria parasites infect red blood cells where they digest host hemoglobin and release free heme inside a lysosome-like organelle called the food vacuole. To detoxify excess heme, parasites form hemozoin crystals that rapidly tumble inside this compartment. Hemozoin formation is critical for parasite survival and antimalarial drug activity, but crystal motion and its underlying mechanism are unexplored. We used quantitative image analysis to determine the timescale of motion, which requires the intact vacuole but does not require the parasite itself. Using single-particle tracking and Brownian dynamics simulations with experimentally derived interaction potentials, we found that hemozoin motion exhibits unexpectedly tight confinement but is much faster than thermal diffusion. Hydrogen peroxide, which is generated at high concentrations in the food vacuole, has been shown to stimulate metallic nanoparticle motion via surface-catalyzed peroxide decomposition that generates propulsive kinetic energy. We observed that peroxide stimulated the motion of isolated crystals in solution and that conditions that suppress peroxide formation slowed hemozoin motion inside parasites. These data suggest that surface-exposed metals on hemozoin catalyze peroxide decomposition to drive crystal motion and strengthen oxidative stress protection during blood-stage infection. This work reveals hemozoin motion in malaria parasites as a biological example of a self-propelled nanoparticle.

Hybrid Molecules as Efficient Drugs against Multidrug-Resistant Malaria Parasites

This review is focused on hybrid molecules defined as chemical entities with two or more structural domains, as antimalarial drug-candidates, over the past 25 years. Due to their different pharmacophores, such hybrids can interact with a single biological target by different and complementary mechanisms; they can also act simultaneously on several targets having complementary biological functions (dual mode of action), and can theoretically reduce the selection of parasite drug-resistance. This review is not an exhaustive report of all hybrid drugs tested on malaria parasites but a selection of hybrids with pharmacologically relevant antiplasmodial properties and original chemical structures. The choice of pharmacophore synthons and junction arms is obviously decisive. Among the large varieties of hybrid drugs published, emoquine-1 appears at the moment as a promising antimalarial drug candidate, considering 1) its high activities on several multidrug-resistant Plasmodium lab strains and field isolates, 2) its capacity to eliminate the quiescent forms of the artemisinin-resistant parasites, and 3) its curative properties in a malaria mouse model. Such molecules confirm the synergistic effect of hybrid compounds compared to the combination of the pharmacophores leading to novel chemical structures that meet the critical parameters for new antimalarial drugs.

Distinct Physical Properties of β-Hematin in Two Synthetic Media: Compelling Evidence

It is now widely accepted that detailed knowledge of the physicochemical characteristics of the β-hematin crystals, i.e., the synthetic versions of the natural hemozoin crystals, is important for understanding their formation, the design of antimalarial medicines, and malarial diagnosis. We report that the overall physical properties exhibited by β-hematins greatly depend on the synthetic media. Here, we synthesize β-hematin from hemin in aqueous-acetate and in aqueous-oily media and characterize their properties by several techniques. Infrared spectra clearly demonstrate the formation of β-hematin in both media. The β-hematin crystals prepared in aqueous-acetate are composed by needle-like particles with average lengths around 760-770 nm; their lattice parameters and unit cell volumes are larger than those reported in the literature. They are paramagnetic at 300 K and antiferromagnetic at very low temperatures. Their Mössbauer spectra at 298 K, 77 K, and 10 K are consistent with the presence of high-spin Fe (III) and are less asymmetric as a result of the occurrence of fast spin-spin relaxation time, and their surface composition is complex, showing the presence of a multiplicity of iron oxidation and spin states (although with a majority of high-spin Fe3+ ions). In comparison, the β-hematin crystals prepared in aqueous-oily medium have significantly smaller lengths (ca. 560 nm) and slightly larger unit cell volume in comparison to the previous sample. The magnetic measurements show that they are affected by superparamagnetism and paramagnetism at 298 K and the coexistence of weakly ferromagnetic or possibly ferrimagnetic and paramagnetic phases at 80 K. Their Mössbauer spectra at 298 K, 77 K, and 10 K, also consistent with the presence of high-spin Fe (III), show longer spin-spin relaxation times, and their surface composition is also complex containing less surface OH- groups and higher amounts of Fe (II) ions in low- and high-spin states. The observed differences are discussed in relation to the specific formation conditions present in the synthesis medium. The results reported here are of outmost importance for understanding how the physicochemical properties of β-hematins depend on the synthesis conditions.

Design, Synthesis and in vitro Evaluation of Primaquine and Diaminoquinazoline Hybrid Molecules Against the Malaria Parasite

In this study, we built on the known inhibitory potential of diaminoquinazolines (DAQs) against different stages of Plasmodium development and designed a convenient two-step synthesis to combine DAQ with primaquine (PQ) pharmacophore. The PQ-DAQ hybrids displayed potent in vitro activities in the low nanomolar range (IC50 135.20-398.80 nM) against all intra-erythrocytic stages of the drug-sensitive 3D7 strain, with significant potency enhancement compared to PQ alone (IC50 9370 nM). These hybrids were also potent at killing drug-resistant strains (Dd2, Dd2 R539T, IPC4912, CamWT C580Y, and 7G8) in the nanomolar range, with 11 f being the most effective compound (IC50 172.20-396.60 nM). Notably, for the first time, we present evidence that the DAQ-based compound 8 and its hybrids can inhibit β-hematin formation in vitro with potency (IC50 0.90-27.80 μM), suggesting hemozoin formation to be one of the potential targets of this series. Lastly, two hybrids with potent antiplasmodial activity were also found to be safe up to 10 μM against human HepG2 cells, suggesting the possibility of achieving host vs parasite selectivity with this series.

A Metabolite-Based Resistance Mechanism Against Malaria

Whether jaundice, a common presentation of Plasmodium ( P .) falciparum malaria (1-3) arising from the accumulation of circulating bilirubin, represents an adaptive or maladaptive response to Plasmodium spp. infection is not understood (1-3). We found that asymptomatic P. falciparum infection was associated with a >10-fold higher ratio of unconjugated bilirubin over parasite burden, compared to symptomatic malaria. Genetic suppression of bilirubin synthesis by biliverdin reductase A (BVRA) (4) increased parasite virulence and malaria mortality in mice. Accumulation of unconjugated bilirubin in plasma, via genetic inhibition of hepatic conjugation by UDP glucuronosyltransferase family 1 member A1 (UGT1A1) ( 5 ), was protective against malaria in mice. Unconjugated bilirubin inhibited P. falciparum proliferation in red blood cells (RBC) via a mechanism that suppressed mitochondrial pyrimidine synthesis. Moreover, unconjugated bilirubin inhibited hemozoin (Hz) crystallization and compromised the parasite's food vacuole. In conclusion, jaundice represents a metabolic response to Plasmodium spp . infection that limits malaria severity.

Emoquine-1: A Hybrid Molecule Efficient against Multidrug-Resistant Plasmodium Parasites, Including the Artemisinin-Resistant Quiescent Stage, and Also Active In Vivo

To challenge the multidrug resistance of Plasmodium falciparum malaria parasites, new hybrid compounds were synthesized and evaluated against laboratory strains and multidrug-resistant clinical isolates. Among these hybrids, emoquine-1 was the most active on proliferative P. falciparum, with IC50 values in the range of 20-55 nM and a high selectivity index with respect to mammalian cells. This drug retained its activity on several multiresistant field isolates from Cambodia and Guiana, exhibited no cross-resistance to artemisinin, and is also very active against the quiescent stage of the artemisinin-resistant parasites, three features that constitute the gold standard for new antimalarial drugs. In vivo, emoquine-1 is active against Plasmodium vinckei petteri at 25 mg/kg/d per os and by the intraperitoneal route at 1-5 mg/kg/d, with total cure at 10 mg/kg/d, making emoquine-1 an ideal candidate to fight Plasmodium parasites resistant to artemisinin-based combination therapies (ACTs) with a capacity to eliminate persistent parasites.

Malaria parasite cysteine and aspartic proteases as key drug targets for antimalarial therapy

CONTEXT

Cysteine and aspartic proteases are enzyme families that play crucial roles in the life cycle of Plasmodium, the parasite responsible for malaria. These proteases are involved in vital biological processes, such as hemoglobin degradation within the host's red blood cells, protein turnover, and regulation of parasite development. Inhibiting these proteases with small molecule drugs can block the parasite's growth and survival. Chemically, these enzymes have specific active sites where inhibitors can bind, preventing the breakdown of key proteins, making them attractive targets for the design of novel antimalarial compounds. Understanding the structure and catalytic mechanisms of these proteases is critical for developing selective and potent inhibitors. The degradation of hemoglobin occurs in the parasite's digestive vacuole, and disruption of this process by targeting these proteases can inhibit parasite development, leading to the death of the parasite. Hence, these proteases are critical for maintaining the parasite's metabolic functions, and inhibiting them can disrupt the parasite's life cycle. Malaria remains a major global health problem, particularly in tropical and subtropical regions, where resistance to existing antimalarial drugs, such as chloroquine and artemisinin-based therapies, is an escalating issue. The emergence of drug-resistant Plasmodium strains highlights the urgent need for new therapeutic strategies. Targeting cysteine and aspartic proteases offers a novel approach to antimalarial drug development, as these enzymes are crucial for parasite survival and have not been widely exploited in current therapies. By inhibiting these proteases, researchers aim to develop new antimalarial treatments that could overcome resistance mechanisms and provide more effective options for malaria control and eradication.

METHODS

The application of computational methods such as molecular docking, dynamics simulations, and quantum mechanical calculations, combined with powerful molecular modeling tools, provides a comprehensive framework for discovering and optimizing inhibitors targeting Plasmodium cysteine and aspartic proteases. These methods facilitate the rational design of novel antimalarial drugs, offering a pathway to overcome drug resistance and improve therapeutic outcomes.

Artemisinin-resistant Plasmodium falciparum Kelch13 mutant proteins display reduced heme-binding affinity and decreased artemisinin activation

The potency of frontline antimalarial drug artemisinin (ART) derivatives is triggered by heme-induced cleavage of the endoperoxide bond to form reactive heme-ART alkoxy radicals and covalent heme-ART adducts, which are highly toxic to the parasite. ART-resistant (ART-R) parasites with mutations in the Plasmodium falciparum Kelch-containing protein Kelch13 (PfKekch13) exhibit impaired hemoglobin uptake, reduced yield of hemoglobin-derived heme, and thus decreased ART activation. However, any direct involvement of PfKelch13 in heme-mediated ART activation has not been reported. Here, we show that the purified recombinant PfKelch13 wild-type (WT) protein displays measurable binding affinity for iron and heme, the main effectors for ART activation. The heme-binding property is also exhibited by the native PfKelch13 protein from parasite culture. The two ART-R recombinant PfKelch13 mutants (C580Y and R539T) display weaker heme binding affinities compared to the ART-sensitive WT and A578S mutant proteins, which further translates into reduced yield of heme-ART derivatives when ART is incubated with the heme molecules bound to the mutant PfKelch13 proteins. In conclusion, this study provides the first evidence for ART activation via the heme-binding propensity of PfKelch13. This mechanism may contribute to the modulation of ART-R levels in malaria parasites through a novel function of PfKelch13.

Operando spectroscopy investigations of the redox reactions in heme and heme-proteins

Operando spectroscopic investigations during molecular redox processes provide unique insights into complex molecular structures and their transformations. Herein, a combination of a potentiodynamic method with spectroscopy has been employed to holistically investigate the structural transformations during Fe-redox (Fe3+ ↔ Fe2+) of hemin vis á vis heme-proteins, e.g. myoglobin (Mb), hemoglobin (Hb) and cytochrome-C (Cyt-C). The UV-vis findings reveal the formation of hemozoin (≈heme-dimer), which can be selectively prevented via a high concentration of strongly interacting ligands, e.g. histidine (the fifth coordinating ligand in the heme-based protein). On the other hand, methionine does not prevent the formation of hemozoin. In Mb, Hb, and Cyt-C, as the fifth coordination site is occupied by histidine, hemozoin formation is inhibited. During Fe3+→ Fe2+, operando circular dichroism exhibits a decrease in the initial helical component in Hb from nearly 40% to 28%, which is close to the initial helix component of Mb (≈25%), strongly indicating denaturation of the protein in the redox pathway. The rate of change of the helices versus potential is almost identical for Mb and Hb, but comparatively faster than Cyt-C. In addition, from the Raman bands of M-N dynamics and protein agglomeration, it is concluded that Cyt-C prefers to agglomerate in the 2+ state, whereas Mb/Hb in the 3+ state. In this report, the power of operando spectroscopy is utilized to unearth the dynamics of hemin and heme-based proteins for comprehending the underlying complexities associated with the molecular redox, which have deep implications in electrocatalysis, energy storage, and sensing.

Cryo-tomography and 3D Electron Diffraction Reveal the Polar Habit and Chiral Structure of the Malaria Pigment Crystal Hemozoin

Detoxification of heme in Plasmodium depends on its crystallization into hemozoin. This pathway is a major target of antimalarial drugs. The crystalline structure of hemozoin was established by X-ray powder diffraction using a synthetic analog, β-hematin. Here, we apply emerging methods of in situ cryo-electron tomography and 3D electron diffraction to obtain a definitive structure of hemozoin directly from ruptured parasite cells. Biogenic hemozoin crystals take a striking polar morphology. Like β-hematin, the unit cell contains a heme dimer, which may form four distinct stereoisomers: two centrosymmetric and two chiral enantiomers. Diffraction analysis, supported by density functional theory analysis, reveals a selective mixture in the hemozoin lattice of one centrosymmetric and one chiral dimer. Absolute configuration has been determined by morphological analysis and confirmed by a novel method of exit-wave reconstruction from a focal series. Atomic disorder appears on specific facets asymmetrically, and the polar morphology can be understood in light of water binding. Structural modeling of the heme detoxification protein suggests a function as a chiral agent to bias the dimer formation in favor of rapid growth of a single crystalline phase. The refined structure of hemozoin should serve as a guide to new drug development.

Characterization of antimalarial activity of artemisinin-based hybrid drugs

In response to the spread of artemisinin (ART) resistance, ART-based hybrid drugs were developed, and their activity profile was characterized against drug-sensitive and drug-resistant Plasmodium falciparum parasites. Two hybrids were found to display parasite growth reduction, stage-specificity, speed of activity, additivity of activity in drug combinations, and stability in hepatic microsomes of similar levels to those displayed by dihydroartemisinin (DHA). Conversely, the rate of chemical homolysis of the peroxide bonds is slower in hybrids than in DHA. From a mechanistic perspective, heme plays a central role in the chemical homolysis of peroxide, inhibiting heme detoxification and disrupting parasite heme redox homeostasis. The hybrid exhibiting slow homolysis of peroxide bonds was more potent in reducing the viability of ART-resistant parasites in a ring-stage survival assay than the hybrid exhibiting fast homolysis. However, both hybrids showed limited activity against ART-induced quiescent parasites in the quiescent-stage survival assay. Our findings are consistent with previous results showing that slow homolysis of peroxide-containing drugs may retain activity against proliferating ART-resistant parasites. However, our data suggest that this property does not overcome the limited activity of peroxides in killing non-proliferating parasites in a quiescent state.

Magneto-optical assessment of Plasmodium parasite growth via hemozoin crystal size

Hemozoin is a natural biomarker formed during the hemoglobin metabolism of Plasmodium parasites, the causative agents of malaria. The rotating-crystal magneto-optical detection (RMOD) has been developed for its rapid and sensitive detection both in cell cultures and patient samples. In the current article we demonstrate that, besides quantifying the overall concentration of hemozoin produced by the parasites, RMOD can also track the size distribution of the hemozoin crystals. We establish the relations between the magneto-optical signal, the mean parasite age and the median crystal size throughout one erythrocytic cycle of Plasmodium falciparum parasites, where the latter two are determined by optical and scanning electron microscopy, respectively. The significant correlation between the magneto-optical signal and the stage distribution of the parasites indicates that the RMOD method can be utilized for species-specific malaria diagnosis and for the quick assessment of drug efficacy.

Hematin anhydride (β-hematin): An analogue to malaria pigment hemozoin possesses nonlinearity

Hematin anhydride (β-hematin), the synthetic analogue of the malaria pigment, "hemozoin", is a heme dimer produced by reciprocal covalent bonds among carboxylic acid groups on the protoporphyrin-IX ring and the iron atom present in the two adjacent heme molecules. Hemozoin is a disposal product formed from the digestion of hemoglobin present in the red blood cells infected with hematophagous malaria parasites. Besides, as the parasites invade red blood cells, hemozoin crystals are eventually released into the bloodstream, where they accumulate over time in tissues. Severe malaria infection leads to significant dysfunction in vital organs such as the liver, spleen, and brain in part due to the autoimmune response to the excessive accumulation of hemozoin in these tissues. Also, the amount of these crystals in the vasculature correlates with disease progression. Thus, hemozoin is a unique indicator of infection used as a malaria biomarker and hence, used as a target for the development of antimalarial drugs. Hence, exploring various properties of hemozoin is extremely useful in the direction of diagnosis and cure. The present study focuses on finding one of the unknown properties of β-hematin in physiological conditions by using the Z-scan technique, which is simple, sensitive, and economical. It is observed that hemozoin possesses one of the unique material properties, i.e., nonlinearity with a detection limit of ∼ 15 µM. The self-defocusing action causes β-hematin to exhibit negative refractive nonlinearity. The observed data is analyzed with a thermal lensing model. We strongly believe that our simple and reliable approach to probing the nonlinearity of β-hematin will provide fresh opportunities for malaria diagnostics & cure in the near future.

Hybrid Peptide-Alkoxyamine Drugs: A Strategy for the Development of a New Family of Antiplasmodial Drugs

The emergence and spread of drug-resistant Plasmodium falciparum parasites shed a serious concern on the worldwide control of malaria, the most important tropical disease in terms of mortality and morbidity. This situation has led us to consider the use of peptide-alkoxyamine derivatives as new antiplasmodial prodrugs that could potentially be efficient in the fight against resistant malaria parasites. Indeed, the peptide tag of the prodrug has been designed to be hydrolysed by parasite digestive proteases to afford highly labile alkoxyamines drugs, which spontaneously and instantaneously homolyse into two free radicals, one of which is expected to be active against P. falciparum. Since the parasite enzymes should trigger the production of the active drug in the parasite's food vacuoles, our approach is summarized as "to dig its grave with its fork". However, despite promising sub-micromolar IC50 values in the classical chemosensitivity assay, more in-depth tests evidenced that the anti-parasite activity of these compounds could be due to their cytostatic activity rather than a truly anti-parasitic profile, demonstrating that the antiplasmodial activity cannot be based only on measuring antiproliferative activity. It is therefore imperative to distinguish, with appropriate tests, a genuinely parasiticidal activity from a cytostatic activity.

Soluble meso and deuteroporphyrin analogs of the malaria pigment hematin anhydride

Two soluble heme analogs of the insoluble malaria pigment hematin anhydride (HA, or β-hematin), [Fe(III)(protoporphyrin)]2, with either mesoporphyrin (MHA) or deuteroporphyrin (DHA) are characterized by elemental analysis, SEM, IR spectroscopy, electronic spectroscopy, paramagnetic 1H NMR spectroscopy and solution magnetic susceptibility. While prior single crystal and X-ray powder diffraction results indicate all three have a common propionate linked dimer motif, there is considerable solid state variation in the conformation. This is associated with enhanced solubility of MHA and DHA. As with HA, DHA undergoes thermally promoted reversible hydration/dehydration in the solid state. Solution 1H NMR studies of DHA suggest a high spin dimeric structure with the porphyrin methyls distributed between two isomers which are also present in the solid state. These soluble iron(III)porphyrin dimers allow for the first direct solution studies by NMR and UV-Vis spectroscopies of these key species. Taken together the results illustrate the importance and utility of varying the substituents on the periphery of the porphyrin for studying heme aggregation and malaria pigment formation.

The Digestive Vacuole of the Malaria Parasite: A Specialized Lysosome

The malaria parasite resides within erythrocytes during one stage of its life cycle. During this intraerythrocytic period, the parasite ingests the erythrocyte cytoplasm and digests approximately two-thirds of the host cell hemoglobin. This digestion occurs within a lysosome-like organelle called the digestive vacuole. Several proteases are localized to the digestive vacuole and these proteases sequentially breakdown hemoglobin into small peptides, dipeptides, and amino acids. The peptides are exported into the host cytoplasm via the chloroquine-resistance transporter and an amino acid transporter has also been identified on the digestive vacuole membrane. The environment of the digestive vacuole also provides appropriate conditions for the biocrystallization of toxic heme into non-toxic hemozoin by a poorly understood process. Hemozoin formation is an attribute of Plasmodium and Haemoproteus and is not exhibited by other intraerythrocytic protozoan parasites. The efficient degradation of hemoglobin and detoxification of heme likely plays a major role in the high level of replication exhibited by malaria parasites within erythrocytes. Unique features of the digestive vacuole and the critical importance of nutrient acquisition provide therapeutic targets for the treatment of malaria.

The elementary reactions for incorporation into crystals

The growth rates of crystals are largely dictated by the chemical reaction between solute and kinks, in which a solute molecule severs its bonds with the solvent and establishes new bonds with the kink. Details on this sequence of bond breaking and rebuilding remain poorly understood. To elucidate the reaction at the kinks we employ four solvents with distinct functionalities as reporters on the microscopic structures and their dynamics along the pathway into a kink. We combine time-resolved in situ atomic force microscopy and x-ray and optical methods with molecular dynamics simulations. We demonstrate that in all four solvents the solute, etioporphyrin I, molecules reach the steps directly from the solution; this finding identifies the measured rate constant for step growth as the rate constant of the reaction between a solute molecule and a kink. We show that the binding of a solute molecule to a kink divides into two elementary reactions. First, the incoming solute molecule sheds a fraction of its solvent shell and attaches to molecules from the kink by bonds distinct from those in its fully incorporated state. In the second step, the solute breaks these initial bonds and relocates to the kink. The strength of the preliminary bonds with the kink determines the free energy barrier for incorporation into a kink. The presence of an intermediate state, whose stability is controlled by solvents and additives, may illuminate how minor solution components guide the construction of elaborate crystal architectures in nature and the search for solution compositions that suppress undesirable or accelerate favored crystallization in industry.

Epigenetic and transcriptional regulation of cytokine production by Plasmodium falciparum-exposed monocytes

Plasmodium falciparum infection causes the most severe form of malaria, where excessive production of proinflammatory cytokines can drive the pathogenesis of the disease. Monocytes play key roles in host defense against malaria through cytokine production and phagocytosis; however, they are also implicated in pathogenesis through excessive proinflammatory cytokine production. Understanding the underlying molecular mechanisms that contribute to inflammatory cytokine production in P. falciparum-exposed monocytes is key towards developing better treatments. Here, we provide molecular evidence that histone 3 lysine 4 (H3K4) methylation is key for inflammatory cytokine production in P. falciparum-exposed monocytes. In an established in vitro system that mimics blood stage infection, elevated proinflammatory TNF and IL-6 cytokine production is correlated with increased mono- and tri-methylated H3K4 levels. Significantly, we demonstrate through utilizing a pharmacological inhibitor of H3K4 methylation that TNF and IL-6 expression can be suppressed in P. falciparum-exposed monocytes. This elucidated epigenetic regulatory mechanism, controlling inflammatory cytokine production, potentially provides new therapeutic options for future malaria treatment.

Nanoparticle tracking analysis of natural hemozoin from Plasmodium parasites

BACKGROUND

Hemozoin is a byproduct of hemoglobin digestion crucial for parasite survival. It forms crystals that can be of interest as drug targets or biomarkers of malaria infection. However, hemozoin has long been considered as an amorphous crystal of simple morphology. Studying the consequences of biomineralization of this crystal during the parasite growth may provide more comprehensive evidence of its role during malaria.

OBJECTIVES

This study aimed to investigate the interest of nanoparticles tracker analysis for measuring the concentration and size of hemozoin particles produced from different parasite sources and conditions.

METHODS

Hemozoin was extracted from several clones of Plasmodium falciparum both asexual and sexual parasites. Hemozoin was also extracted from blood samples of malaria patients and from saliva of asymptomatic malaria carriers. Nanoparticles tracking analysis (NTA) was performed to assess the size and concentration of hemozoin.

RESULTS

NTA data showed variation in hemozoin concentration, size, and crystal clusters between parasite clones, species, and stages. Among parasite clones, hemozoin concentration ranged from 131 to 2663 particles/infected red blood cell (iRBC) and size ranged from 149.6 ± 6.3 nm to 234.8 ± 40.1 nm. The mean size was lower for Plasmodium vivax (176 ± 79.2 nm) than for Plasmodium falciparum (254.8 ± 74.0 nm). Sexual NF54 parasites showed a 7.5-fold higher concentration of hemozoin particles (28.7 particles/iRBC) compared to asexual parasites (3.8 particles/iRBC). In addition, the mean hemozoin size also increased by approximately 60 % for sexual parasites. Compared to in vitro cultures of parasites, blood samples showed low hemozoin concentrations.

CONCLUSIONS

This study highlights the potential of NTA as a useful method for analyzing hemozoin, demonstrating its ability to provide detailed information on hemozoin characterization. However, further research is needed to adapt the NTA for hemozoin analysis.

Recent advances in the synthesis and antimalarial activity of 1,2,4-trioxanes

The increasing resistance of various malarial parasite strains to drugs has made the production of a new, rapid-acting, and efficient antimalarial drug more necessary, as the demand for such drugs is growing rapidly. As a major global health concern, various methods have been implemented to address the problem of drug resistance, including the hybrid drug concept, combination therapy, the development of analogues of existing medicines, and the use of drug resistance reversal agents. Artemisinin and its derivatives are currently used against multidrug- resistant P. falciparum species. However, due to its natural origin, its use has been limited by its scarcity in natural resources. As a result, finding a substitute becomes more crucial, and the peroxide group in artemisinin, responsible for the drugs biological action in the form of 1,2,4-trioxane, may hold the key to resolving this issue. The literature suggests that 1,2,4-trioxanes have the potential to become an alternative to current malaria drugs, as highlighted in this review. This is why 1,2,4-trioxanes and their derivatives have been synthesized on a large scale worldwide, as they have shown promising antimalarial activity in vivo and in vitro against Plasmodium species. Consequently, the search for a more convenient, environment friendly, sustainable, efficient, and effective synthetic pathway for the synthesis of 1,2,4-trioxanes continues. The aim of this work is to provide a comprehensive analysis of the synthesis and mechanism of action of 1,2,4-trioxanes. This systematic review highlights the most recent summaries of derivatives of 1,2,4-trioxane compounds and dimers with potential antimalarial activity from January 1988 to 2023.

Annotation of GC-MS Data of Antimicrobial Constituents in the Antarctic Seaweed Phaeurus antarcticus by Molecular Networking

Phaeurus antarcticus is a member of the Desmarestiaceae family endemic to the Antarctic Peninsula. Reports addressing its chemical composition and biological activities are scarce. Herein, bioactive non-polar compounds of P. antarcticus against pathogenic bacteria, Leishmania amazonensis and Neospora caninum parasites were targeted through GC-MS Molecular Networking and multivariate analysis (OPLS-DA). The effects on horseradish peroxidase (HRP) were also evaluated. P. antarcticus exhibited selective bacteriostatic and bactericidal activities against Staphylococcus aureus with MIC and MBC values from 6.25-100 μg mL-1 . Fractions HX-FC and HX-FD were the most active against L. amazonensis with EC50 ranging from 18.5-62.3 μg mL-1 . Additionally, fractions HX-FC and HX-FD showed potent inhibition of N. caninum at EC50 values of 2.8 and 6.3 μg mL-1 , respectively. All fractions inhibited HRP activity, indicating possible interactions with Heme proteins. It was possible to annotate compounds from tree mains clusters, containing terpenoids, steroids, fatty acids, and alcohols by correlating the spectral data of the GC-MS analysis with Molecular Networking and the OPLS-DA results.

Optimization and Enhancement of the Peroxidase-like Activity of Hemin in Aqueous Solutions of Sodium Dodecylsulfate

Iron porphyrins play several important roles in present-day living systems and probably already existed in very early life forms. Hemin (= ferric protoporphyrin IX = ferric heme b), for example, is the prosthetic group at the active site of heme peroxidases, catalyzing the oxidation of a number of different types of reducing substrates after hemin is first oxidized by hydrogen peroxide as the oxidizing substrate of the enzyme. The active site of heme peroxidases consists of a hydrophobic pocket in which hemin is embedded noncovalently and kept in place through coordination of the iron atom to a proximal histidine side chain of the protein. It is this partially hydrophobic local environment of the enzyme which determines the efficiency with which the sequential reactions of the oxidizing and reducing substrates proceed at the active site. Free hemin, which has been separated from the protein moiety of heme peroxidases, is known to aggregate in an aqueous solution and exhibits low catalytic activity. Based on previous reports on the use of surfactant micelles to solubilize free hemin in a nonaggregated state, the peroxidase-like activity of hemin in the presence of sodium dodecyl sulfate (SDS) at concentrations below and above the critical concentration for SDS micelle formation (critical micellization concentration (cmc)) was systematically investigated. In most experiments, 3,3',5,5'-tetramethylbenzidine (TMB) was applied as a reducing substrate at pH = 7.2. The presence of SDS clearly had a positive effect on the reaction in terms of initial reaction rate and reaction yield, even at concentrations below the cmc. The highest activity correlated with the cmc value, as demonstrated for reactions at three different HEPES concentrations. The 4-(2-hydroxyethyl)-1-piperazineethanesulfonate salt (HEPES) served as a pH buffer substance and also had an accelerating effect on the reaction. At the cmc, the addition of l-histidine (l-His) resulted in a further concentration-dependent increase in the peroxidase-like activity of hemin until a maximal effect was reached at an optimal l-His concentration, probably corresponding to an ideal mono-l-His ligation to hemin. Some of the results obtained can be understood on the basis of molecular dynamics simulations, which indicated the existence of intermolecular interactions between hemin and HEPES and between hemin and SDS. Preliminary experiments with SDS/dodecanol vesicles at pH = 7.2 showed that in the presence of the vesicles, hemin exhibited similar peroxidase-like activity as in the case of SDS micelles. This supports the hypothesis that micelle- or vesicle-associated ferric or ferrous iron porphyrins may have played a role as primitive catalysts in membranous prebiotic compartment systems before cellular life emerged.

Molecular Interactions Identified by Two-Dimensional Analysis-Detailed Insight into the Molecular Interactions of the Antimalarial Artesunate with the Target Structure β-Hematin by Means of 2D Raman Correlation Spectroscopy

A thorough understanding of the interaction of endoperoxide antimalarial agents with their biological target structures is of utmost importance for the tailored design of future efficient antimalarials. Detailed insights into molecular interactions between artesunate and β-hematin were derived with a combination of resonance Raman spectroscopy, two-dimensional correlation analysis, and density functional theory calculations. Resonance Raman spectroscopy with three distinct laser wavelengths enabled the specific excitation of different chromophore parts of β-hematin. The resonance Raman spectra of the artesunate-β-hematin complexes were thoroughly analyzed with the help of high-resolution and highly sensitive two-dimensional correlation spectroscopy. Spectral changes in the peak properties were found with increasing artesunate concentration. Changes in the low-frequency, morphology-sensitive Raman bands indicated a loss in crystallinity of the drug-target complexes. Differences in the high-wavenumber region were assigned to increased distortions of the planarity of the structure of the target molecule due to the appearance of various coexisting alkylation species. Evidence for the appearance of high-valent ferryl-oxo species could be observed with the help of differences in the peak properties of oxidation-state sensitive Raman modes. To support those findings, the relaxed ground-state structures of ten possible covalent mono- and di-meso(Cm)-alkylated hematin-dihydroartemisinyl complexes were calculated using density functional theory. A very good agreement with the experimental peak properties was achieved, and the out-of-plane displacements along the lowest-frequency normal coordinates were investigated by normal coordinate structural decomposition analysis. The strongest changes in all data were observed in vibrations with a high participation of Cm-parts of β-hematin.

Nonclassical mechanisms to irreversibly suppress β-hematin crystal growth

Hematin crystallization is an essential element of heme detoxification of malaria parasites and its inhibition by antimalarial drugs is a common treatment avenue. We demonstrate at biomimetic conditions in vitro irreversible inhibition of hematin crystal growth due to distinct cooperative mechanisms that activate at high crystallization driving forces. The evolution of crystal shape after limited-time exposure to both artemisinin metabolites and quinoline-class antimalarials indicates that crystal growth remains suppressed after the artemisinin metabolites and the drugs are purged from the solution. Treating malaria parasites with the same agents reveals that three- and six-hour inhibitor pulses inhibit parasite growth with efficacy comparable to that of inhibitor exposure during the entire parasite lifetime. Time-resolved in situ atomic force microscopy (AFM), complemented by light scattering, reveals two molecular-level mechanisms of inhibitor action that prevent β-hematin growth recovery. Hematin adducts of artemisinins incite copious nucleation of nonextendable nanocrystals, which incorporate into larger growing crystals, whereas pyronaridine, a quinoline-class drug, promotes step bunches, which evolve to engender abundant dislocations. Both incorporated crystals and dislocations are known to induce lattice strain, which persists and permanently impedes crystal growth. Nucleation, step bunching, and other cooperative behaviors can be amplified or curtailed as means to control crystal sizes, size distributions, aspect ratios, and other properties essential for numerous fields that rely on crystalline materials.

Hemoglobin uptake and utilization by human protozoan parasites: a review

The protozoan disease is a major global health concern. Amoebiasis, leishmaniasis, Chagas disease, and African sleeping sickness affect several million people worldwide, leading to millions of deaths annually and immense social and economic problems. Iron is an essential nutrient for nearly all microbes, including invading pathogens. The majority of iron in mammalian hosts is stored intracellularly in proteins, such as ferritin and hemoglobin (Hb). Hb, present in blood erythrocytes, is a very important source of iron and amino acids for pathogenic microorganisms ranging from bacteria to eukaryotic pathogens, such as worms, protozoa, yeast, and fungi. These organisms have developed adequate mechanisms to obtain Hb or its byproducts (heme and globin) from the host. One of the major virulence factors identified in parasites is parasite-derived proteases, essential for host tissue degradation, immune evasion, and nutrient acquisition. The production of Hb-degrading proteases is a Hb uptake mechanism that degrades globin in amino acids and facilitates heme release. This review aims to provide an overview of the Hb and heme-uptake mechanisms utilized by human pathogenic protozoa to survive inside the host.

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.

Mechanistic insights into immunopathogenesis of murine cerebral malaria: Cues from "young" C57BL/6J and BALB/c mice

Cerebral malaria (CM), a major cause of mortality in children <5 years, presents disparity in pathophysiological features and poor prognosis compared to adults. Adult C57BL/6J mice infected with Plasmodium berghei ANKA (PbA) are widely used to understand CM pathogenesis compared to relatively less prone BALB/c mice; however, age and immune status of the host also influence disease sequelae and cerebral manifestations. Murine models of CM known so far do not project complete disease spectrum of pediatric CM. The present study was designed to dissect and differentiate CM immunopathogenesis in "young" BALB/c and C57BL/6J mice infected with PbA, in search of a competent mouse model mimicking pediatric CM. Multipronged approach including the analysis of blood-brain barrier (BBB) permeability and parasite infiltration, histopathology, nitric oxide levels, and pro/anti-inflammatory (TNF-α, IFN-γ, IL-4, and IL-10) cytokine expression were compared in the cortices of both young BALB/c and C57BL/6J mice. The results illustrate severe course of infection and typical CM like histopathological alterations including monocytic plugging in PbA-infected "young" BALB/c compared to C57BL/6J mice. The decreased expression of tight junction proteins (ZO-1 and Claudin-3) and Evan's blue extravasation was also more evident in BALB/c mice indicating a more permeable BBB. The increased cortical expression of TNF-α, IFN-γ, IL-4, IL-10, iNOS, eNOS, nNOS, and associated activation of brain resident cells in cortices of BALB/c with progressive parasitaemia depicts the cumulative involvement of host immune responses and parasite accumulation in progression of CM. Thus, the incongruity of cytokine balance resulted in worsening of disease manifestation in "young" BALB/c similar to pediatric CM.

Identification and characterisation of the haemozoin of Haemonchus contortus

BACKGROUND

Most haematophagous organisms constantly suck the host's haemoglobin, which produces toxic free haem. This toxic haem aggregation into the nontoxic crystallisation complex known as haemozoin represents one of the most important detoxification pathways in living organisms, but very little is known about the features of haemozoin in parasitic nematodes. Here, we identified and characterised the haemozoin of an economically significant blood-sucking nematode, Haemonchus contortus.

METHODS

Using electron microscopy, spectrophotometry analyses and biochemical approaches, haemozoin crystallisation was identified and characterised in parasitic fourth-stage larvae (L4s) and/or adult worms as well as L4s of in vitro culture.

RESULTS

The haemozoin was formed in intestinal lipid droplets of the parasitic L4s and adult worms. The characterisation of the haemozoin showed regularly spherical structures and had a 400-nm absorption peak. Furthermore, the haemozoin in in vitro cultured L4s was associated with the culture time and concentration of red blood cells added into the medium, and its formation could be inhibited by chloroquine-derived drugs.

CONCLUSIONS

This work provides detailed insight into the haemozoin formation of H. contortus and should have important implications for developing novel therapeutic targets against this parasite or related haematophagous organisms.

Lipid peroxidation and its repair in malaria parasites

During its life cycle, the human malaria parasite Plasmodium falciparum is subjected to elevated levels of oxidative stress that cause damage to membrane lipids, a process referred to as lipid peroxidation. Control and repair of lipid peroxidation is critical for survival of P. falciparum. Here, we present an introduction into lipid peroxidation and review the current knowledge about the control and repair of the damage caused by lipid peroxidation in P. falciparum blood stages. We also review the recent identification of host peroxiredoxin 6 (PRDX6), as a key lipid-peroxidation-repair enzyme in P. falciparum blood stages. Such critical host factors provide novel targets for development of drugs against malaria.

Elucidating the phase transitions of decanoic-acid crystal by XRD, Raman, group theory and Gibbs energy analyses combined with DFT calculations

This research reports a series of phase transitions in the decanoic-acid (DA) crystal under low-temperature conditions, which were elucidated by XRD, Raman scattering and DFT calculations in a dimer of DA in the C form (monoclinic structure). The first phase change was noticed within the 210-190 K interval duly characterized as a transition of second-order type, as indicated by Gibbs energy behavior, suggesting that the monoclinic structure (P2₁/c) of the crystal is not changed. The second change was observed nearly 110-90 K, whose transition is first-order type occurring from the C form to an A form (triclinic), possibly belonging to the P1 space group. This new polymorphic phase was duly predicted through DFT calculations. According to Gibbs energy behavior, the third phase change (∼30-10 K) is proposed to be a transition from the A form to a new polymorphic phase that probably is a first-order transition, likely associated with a change from the P1 space group to P-1. Furthermore, group theory and wavenumber vs temperature plots' analyses corroborated the phase transitions undergone by DA crystal. In addition, anharmonicity effects in several Raman bands' behavior were noticed during the cooling. A correct assignment for the Raman and IR modes via DFT calculations at room-temperature conditions is also provided herein.

Identifying inhibitors of β-haematin formation with activity against chloroquine-resistant Plasmodium falciparum malaria parasites via virtual screening approaches

The biomineral haemozoin, or its synthetic analogue β-haematin (βH), has been the focus of several target-based screens for activity against Plasmodium falciparum parasites. Together with the known βH crystal structure, the availability of this screening data makes the target amenable to both structure-based and ligand-based virtual screening. In this study, molecular docking and machine learning techniques, including Bayesian and support vector machine classifiers, were used in sequence to screen the in silico ChemDiv 300k Representative Compounds library for inhibitors of βH with retained activity against P. falciparum. We commercially obtained and tested a prioritised set of inhibitors and identified the coumarin and iminodipyridinopyrimidine chemotypes as potent in vitro inhibitors of βH and whole cell parasite growth.

Ferric heme b in aqueous micellar and vesicular systems: state-of-the-art and challenges

Ferric heme b (= ferric protoporphyrin IX = hemin) is an important prosthetic group of different types of enzymes, including the intensively investigated and widely applied horseradish peroxidase (HRP). In HRP, hemin is present in monomeric form in a hydrophobic pocket containing among other amino acid side chains the two imidazoyl groups of His170 and His42. Both amino acids are important for the peroxidase activity of HRP as an axial ligand of hemin (proximal His170) and as an acid/base catalyst (distal His42). A key feature of the peroxidase mechanism of HRP is the initial formation of compound I under heterolytic cleavage of added hydrogen peroxide as a terminal oxidant. Investigations of free hemin dispersed in aqueous solution showed that different types of hemin dimers can form, depending on the experimental conditions, possibly resulting in hemin crystallization. Although it has been recognized already in the 1970s that hemin aggregation can be prevented in aqueous solution by using micelle-forming amphiphiles, it remains a challenge to prepare hemin-containing micellar and vesicular systems with peroxidase-like activities. Such systems are of interest as cheap HRP-mimicking catalysts for analytical and synthetic applications. Some of the key concepts on which research in this fascinating and interdisciplinary field is based are summarized, along with major accomplishments and possible directions for further improvement. A systematic analysis of the physico-chemical properties of hemin in aqueous micellar solutions and vesicular dispersions must be combined with a reliable evaluation of its catalytic activity. Future studies should show how well the molecular complexity around hemin in HRP can be mimicked by using micelles or vesicles. Because of the importance of heme b in virtually all biological systems and the fact that porphyrins and hemes can be obtained under potentially prebiotic conditions, ideas exist about the possible role of heme-containing micellar and vesicular systems in prebiotic times.

Asian Ancistrocladus Lianas as Creative Producers of Naphthylisoquinoline Alkaloids

This book describes a unique class of secondary metabolites, the mono- and dimeric naphthylisoquinoline alkaloids. They occur in lianas of the paleotropical Ancistrocladaceae and Dioncophyllaceae families, exclusively. Their unprecedented structures include stereogenic centers and rotationally hindered, and thus likewise stereogenic, axes. Extended recent investigations on six Ancistrocladus species from Asia, as reported in this review, shed light on their fascinating phytochemical productivity, with over 100 such intriguing natural products. This high chemodiversity arises from a likewise unique biosynthesis from acetate-malonate units, following a novel polyketidic pathway to plant-derived isoquinoline alkaloids. Some of the compounds show most promising antiparasitic activities. Likewise presented are strategies for the regio- and stereoselective total synthesis of the alkaloids, including the directed construction of the chiral axis.

Assembly of Core/Shell Nanospheres of Amorphous Hemin/Acetone-Derived Carbonized Polymer with Graphene Nanosheets for Room-Temperature NO Sensing

Implementing parts per billion-level nitric oxide (NO) sensing at room temperature (RT) is still in extreme demand for monitoring inflammatory respiratory diseases. Herein, we have prepared a kind of core-shell structural Hemin-based nanospheres (Abbr.: Hemin-nanospheres, defined as HNSs) with the core of amorphous Hemin and the shell of acetone-derived carbonized polymer, whose core-shell structure was verified by XPS with argon-ion etching. Then, the HNS-assembled reduced graphene oxide composite (defined as HNS-rGO) was prepared for RT NO sensing. The acetone-derived carbonized polymer shell not only assists the formation of amorphous Hemin core by disrupting their crystallization to release more Fe-N₄ active sites, but provides protection to the core. Owing to the unique core-shell structure, the obtained HNS-rGO based sensor exhibited superior RT gas sensing properties toward NO, including a relatively higher response (R_a/R_g = 5.8, 20 ppm), a lower practical limit of detection (100 ppb), relatively reliable repeatability (over 6 cycles), excellent selectivity, and much higher long-term stability (less than a 5% decrease over 120 days). The sensing mechanism has also been proposed based on charge transfer theory. The superior gas sensing properties of HNS-rGO are ascribed to the more Fe-N₄ active sites available under the amorphous state of the Hemin core and to the physical protection by the shell of acetone-derived carbonized polymer. This work presents a facile strategy of constructing a high-performance carbon-based core-shell nanostructure for gas sensing.

Anti-malarial activity of HCl salt of SKM13 (SKM13-2HCl)

Malaria is among the most devastating and widespread tropical parasitic diseases in developing countries. To prevent a potential public health emergency, there is an urgent need for new antimalarial drugs, with single-dose cures, broad therapeutic potential, and novel mechanism of action. We synthesized HCl salt of SKM13 (SKM13-2HCl) based on the modification of SKM13 to improve solubility in water. The anti-malarial activity of the synthesized drug was evaluated in both in vitro and in vivo models. The selective index indicated that SKM13-2HCl showed the same effectiveness with SKM13 in Plasmodium falciparum in in-vitro. Even though, in vivo mouse study demonstrated that SKM13 (20 mg/kg) at single dose could not completely inhibit P. berghei growth in blood. The survival rate increased from 33 to 90% at 15 days after infection. However, SKM13-2HCl (20 mg/kg) at a single dose increased the survival rate up to 100% at the same duration. Ultra-High-Performance Liquid Chromatography (UHPLC) showed that solubility in water of SKM13 and SKM13-2HCL was 0.389 mg/mL and 417 mg/mL, respectively. Pharmacokinetics (PK) analysis corresponded to the increased solubility of SKM13-2HCl over SKM13. Haematological parameters [red blood cell (RBC) count, haemoglobin level, and haematocrit level] supported the comparable efficacy of SKM13 and SKM13-2HCl in a 4-day suppression test. One mode of these drugs was found to be activating phosphorylation of eIF2α, hallmark of ER-stress, to kill parasite. Novel salt derivative of SKM13 (SKM13-2HCl) have enhanced anti-malarial activity against P. falciparum with endoplasmic reticulum (ER)-stress and salt form of SKM13 is an excellent direction to develop anti-malarial drug candidate in mice model.

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.

In vitro dual activity of Aloe marlothii roots and its chemical constituents against Plasmodium falciparum asexual and sexual stage parasites

ETHNOPHARMACOLOGICAL RELEVANCE

Aloe marlothii A.Berger (Xanthorrhoeaceae) is indigenous to southern African countries where its aqueous preparations are used in traditional medicine to treat several ailments including hypertension, respiratory infections, venereal diseases, chest pain, sore throat and malaria.

AIM OF THE STUDY

The aims of this study were as follows: (i) isolate and identify the antiplasmodial active compounds in A. marlothii roots. As the water extract was previously inactive, the dichloromethane:methanol (DCM:MeOH) (1:1) was used, (ii) examine the activity of the isolated compounds against Plasmodium falciparum asexual blood stage (ABS) parasites as well as for transmission-blocking activity against gametocytes and gametes, and (iii) to use in silico tools to predict the target(s) of the active molecules.

MATERIALS AND METHODS

The crude DCM:MeOH (1:1) extract of A. marlothii roots was fractionated on a reverse phase C8 column, using a positive pressure solid-phase extraction (ppSPE) workstation to produce seven fractions. The resulting fractions and the crude DCM:MeOH extract were tested in vitro against P. falciparum (NF54) ABS parasites using the malaria SYBR Green I based-fluorescence assay. Flash silica chromatography and mass-directed preparative high-performance liquid chromatography were utilised to isolate the active compounds. The isolated compounds were evaluated in vitro against P. falciparum asexual (NF54 and K1 strains) and sexual (gametocytes and gametes) stage parasites. Molecular docking was then used for the in silico prediction of targets for the isolated active compounds in P. falciparum.

RESULTS

The crude extract and two SPE fractions displayed good antiplasmodial activity with >97% and 100% inhibition of ABS parasites proliferation at 10 and 20 μg/mL, respectively. Following UPLC-MS analysis of these active fractions, a targeted purification resulted in the isolation of six compounds identified as aloesaponol I (1), aloesaponarin I (2), aloesaponol IV (3), β-sorigenin-1-O-methylether (4), emodin (5), and chrysophanol (6). Aloesaponarin I (2) was the most bioactive, compared to other isolated constituents, against P. falciparum ABS parasites exhibiting equipotency against the drug-sensitive (NF54) (IC₅₀ = 1.54 μg/mL (5 μM)) and multidrug-resistant (K1) (IC₅₀ = 1.58 μg/mL (5 μM)) strains. Aloesaponol IV (3) showed pronounced activity against late-stage (>90% stage IV/V) gametocytes (IC₅₀ = 6.53 μg/mL (22.6 μM)) demonstrating a 3-fold selective potency towards these sexual stages compared to asexual forms of the parasite (IC₅₀ = 19.77 ± 6.835 μg/mL (68 μM)). Transmission-blocking potential of aloesaponol IV (3) was validated by in vitro inhibition of exflagellation of male gametes (94% inhibition at 20 μg/mL). In silico studies identified β-hematin and DNA topoisomerase II as potential biological targets of compounds 2 and 3, respectively.

CONCLUSION

The findings from our study substantiate the traditional use of A. marlothii to treat malaria. To our knowledge, this study has provided the first report on the isolation and identification of antiplasmodial compounds from A. marlothii roots. Furthermore, our study has provided the first report on the transmission-blocking potential of one of the compounds from the genus Aloe, motivating for the investigation of other species within this genus for their potential P. falciparum transmission-blocking activity.

Erythropoiesis and Malaria, a Multifaceted Interplay

One of the major pathophysiologies of malaria is the development of anemia. Although hemolysis and splenic clearance are well described as causes of malarial anemia, abnormal erythropoiesis has been observed in malaria patients and may contribute significantly to anemia. The interaction between inadequate erythropoiesis and Plasmodium parasite infection, which partly occurs in the bone marrow, has been poorly investigated to date. However, recent findings may provide new insights. This review outlines clinical and experimental studies describing different aspects of ineffective erythropoiesis and dyserythropoiesis observed in malaria patients and in animal or in vitro models. We also highlight the various human and parasite factors leading to erythropoiesis disorders and discuss the impact that Plasmodium parasites may have on the suppression of erythropoiesis.

An assay procedure to investigate the transformation of toxic heme into inert hemozoin via plasmodial heme detoxification protein

Most antimalarial therapeutics, including chloroquine and artemisinin, induce free heme-mediated toxicity in Plasmodium. This cytotoxic heme is produced as a by-product during the large-scale digestion of host hemoglobin. Conversion of this host-derived heme into inert crystalline hemozoin is the only defense mechanism in Plasmodium against heme-induced cytotoxicity. Heme detoxification protein (HDP), a highly conserved plasmodial protein, is reported to be the most efficient biological mediator for heme to hemozoin transformation. Despite its significance, HDP has never been extensively studied for heme transformation into hemozoin. Therefore, we wish to develop a method to study the HDP-mediated transformation of heme into hemozoin. We have adopted, modified, and optimized the pyridine hemochrome assay to study HDP catalysis and use substrate and time kinetics to study the HDP-mediated transformation of heme into hemozoin. In contrast to the previously reported assay for HDP, we found that the new assay is more precise, accurate, and handy, making it more suitable for kinetic studies. HDP-mediated transformation of heme into hemozoin is not a single-step process, and involves a transient intermediate, most likely a cyclic heme dimer. The kinetics and the manner of HDP-mediated hemozoin production are dependent on the substrate concentration, and a small fraction of substrate remains untransformed to hemozoin irrespective of reaction time. Combining HDP as a catalyst and the pyridine hemochrome assay will facilitate the efficient screening of future antimalarials.

Rhenium(I) derivatives of aminoquinoline and imidazolopiperidine-based ligands: Synthesis, in vitro and in silico biological evaluation against Plasmodium falciparum

A small library of aminoquinoline and imidazolopiperidine (IMP)-based ligands, containing the 1,2,3-triazole moiety, and their corresponding tricarbonyl rhenium complexes were synthesised and their inhibitory activities evaluated against the chloroquine-sensitive (CQS) and multidrug-resistant (MDR) strains (NF54 and K1, respectively) of P. falciparum. The quinoline-based compounds (L1, L2, ReL1, and ReL2) were at least six-fold more potent than their IMP-based counterparts (L3, L4, ReL3, and ReL4) against both strains of P. falciparum, with the most promising compound (L1) displaying activity comparable to chloroquine diphosphate (CQDP) in the MDR strain. Additionally, all of the synthesised compounds have resistance indices less than CQDP. To gain insight into a possible mechanism of action, in silico hemozoin docking simulations were performed. These studies proposed that the tested compounds may act via hemozoin inhibition, as the new aminoquinoline-derivatives, with the exception of complex ReL2 (binding affinity: -12.62 kcal/mol), showed higher binding affinities than the reference drug chloroquine (CQ, -13.56 kcal/mol). Furthermore, the ligands exhibited superior binding affinity relative to their corresponding Re(I) complexes, which is reflected in their antiplasmodial activity.

Towards rainbow portable Cytophone with laser diodes for global disease diagnostics

In vivo, Cytophone has demonstrated the capability for the early diagnosis of cancer, infection, and cardiovascular disorders through photoacoustic detection of circulating disease markers directly in the bloodstream with an unprecedented 1,000-fold improvement in sensitivity. Nevertheless, a Cytophone with higher specificity and portability is urgently needed. Here, we introduce a novel Cytophone platform that integrates a miniature multispectral laser diode array, time-color coding, and high-speed time-resolved signal processing. Using two-color (808 nm/915 nm) laser diodes, we demonstrated spectral identification of white and red clots, melanoma cells, and hemozoin in malaria-infected erythrocytes against a blood background and artifacts. Data from a Plasmodium yoelii murine model and cultured human P. falciparum were verified in vitro with confocal photothermal and fluorescent microscopy. With these techniques, we detected infected cells within 4 h after invasion, which makes hemozoin promising as a spectrally selective marker at the earliest stages of malaria progression. Along with the findings from our previous application of Cytophone with conventional lasers for the diagnosis of melanoma, bacteremia, sickle anemia, thrombosis, stroke, and abnormal hemoglobin forms, this current finding suggests the potential for the development of a portable rainbow Cytophone with multispectral laser diodes for the identification of these and other diseases.

Adsorption to the Surface of Hemozoin Crystals: Structure-Based Design and Synthesis of 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.

The monoterpene 1,8-cineole prevents cerebral edema in a murine model of severe malaria

1,8-Cineole is a naturally occurring compound found in essential oils of different plants and has well-known anti-inflammatory and antimicrobial activities. In the present work, we aimed to investigate its potential antimalarial effect, using the following experimental models: (1) the erythrocytic cycle of Plasmodium falciparum; (2) an adhesion assay using brain microvascular endothelial cells; and (3) an experimental cerebral malaria animal model induced by Plasmodium berghei ANKA infection in susceptible mice. Using the erythrocytic cycle of Plasmodium falciparum, we characterized the schizonticidal effect of 1,8-cineole. This compound decreased parasitemia in a dose-dependent manner with a half maximal inhibitory concentration of 1045.53 ± 63.30 μM. The inhibitory effect of 972 μM 1,8-cineole was irreversible and independent of parasitemia. Moreover, 1,8-cineole reduced the progression of intracellular development of the parasite over 2 cycles, inducing important morphological changes. Ultrastructure analysis revealed a massive loss of integrity of endomembranes and hemozoin crystals in infected erythrocytes treated with 1,8-cineole. The monoterpene reduced the adhesion index of infected erythrocytes to brain microvascular endothelial cells by 60%. Using the experimental cerebral malaria model, treatment of infected mice for 6 consecutive days with 100 mg/kg/day 1,8-cineole reduced cerebral edema with a 50% reduction in parasitemia. Our data suggest a potential antimalarial effect of 1,8-cineole with an impact on the parasite erythrocytic cycle and severe disease.

Structural chemistry at McGill

A brief description of some of the key individuals at McGill University and their contributions to the development of crystallography is given. This is followed by examples of recent structure determinations, which include organic heterocycles, natural products, inorganic salts, and organometallic complexes. Specifically, the bispyridinium adduct of protoporphyrin, the piperidinyl adduct of 3,7-dichloroquinine, trisbenzylmethylthiol, sodium methylarsonium (V), potassium hydroxylamine sulfonate, Vaska’s complex adduct from the oxidative addition of p-tolylmercaptan, and a bis isocyanoiminotriphenylphosphorane adduct of rutheniumdichloridetristriphenylphosphine are described.

CarbonIron Electron Transport Channels in Porphyrin-Graphene Complex for ppb-Level Room Temperature NO Gas Sensing

It is a great challenge to develop efficient room-temperature sensing materials and sensors for nitric oxide (NO) gas, which is a biomarker molecule used in the monitoring of inflammatory respiratory diseases. Herein, Hemin (Fe (III)-protoporphyrin IX) is introduced into the nitrogen-doped reduced graphene oxide (N-rGO) to obtain a novel sensing material HNG-ethanol. Detailed XPS spectra and DFT calculations confirm the formation of carbon-iron bonds in HNG-ethanol during synthesis process, which act as electron transport channels from graphene to Hemin. Owing to this unique chemical structure, HNG-ethanol exhibits superior gas sensing properties toward NO gas (R_a /R_g  = 3.05, 20 ppm) with a practical limit of detection (LOD) of 500 ppb and reliable repeatability (over 5 cycles). The HNG-ethanol sensor also possesses high selectivity against other exhaled gases, high humidity resistance, and stability (less than 3% decrease over 30 days). In addition, a deep understanding of the gas sensing mechanisms is proposed for the first time in this work, which is instructive to the community for fabricating sensing materials based on graphene-iron derivatives in the future.