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

Piperine Enhances Antimalarial Activity of Methyl Gallate and Palmatine Combination

PURPOSE

Artemisinin combination therapies, the first-line antimalarials in Nigeria, have reportedly suffered multiple failures in malaria treatment, hence the search for novel combination of other compounds. Methyl gallate and palmatine have been reported to exhibit antiplasmodial activities but the antimalarial activity of their combination has not been evaluated. Therefore, the evaluation of the combination of methyl gallate and palmatine for antimalarial activity in vitro and in vivo in the presence of piperine was carried out.

MATERIALS AND METHODS

The inhibitory potential of methyl gallate and palmatine combination on β-hematin (hemozoin) formation was studied in vitro. Also, the antimalarial activity of methyl gallate and palmatine combination with/without a bioenhancer (piperine) was evaluated in Plasmodium berghei NK65-infected mice.

RESULTS

Methyl gallate and palmatine in the ratio 3:2 acted synergistically in vitro and had the highest inhibitory effect (IC50 = 0.73 µg/mL) on β-hematin (hemozoin) formation. The 3:2 combination of methyl gallate and palmatine exhibited no antimalarial activity in vivo in the absence of piperine but caused reduction in parasitemia that exceeded 40% in the presence of piperine at the dose of 25 mg/kg body weight on days 6 and 8 post-inoculation in mice.

CONCLUSION

The 3:2 combination of methyl gallate and palmatine in the presence of piperine exhibited antimalarial activity in vivo, possibly by synergistic inhibition of hemozoin formation which may cause accumulation of haem within the food vacuole of Plasmodium spp. and its death.

Exploring the modulatory influence on the antimalarial activity of amodiaquine using scaffold hybridisation with ferrocene integration

Amodiaquine (AQ) is a potent antimalarial drug used in combination with artesunate as part of artemisinin-based combination therapies (ACTs) for malarial treatment. Due to the rising emergence of resistant malaria parasites, some of which have been reported for ACT, the usefulness of AQ as an efficacious therapeutic drug is threatened. Employing the organometallic hybridisation approach, which has been shown to restore the antimalarial activity of chloroquine in the form of an organometallic hybrid clinical candidate ferroquine (FQ), the present study utilises this strategy to modulate the biological performance of AQ by incorporating ferrocene. Presently, we have conceptualised ferrocenyl AQ derivatives and have developed facile, practical routes for their synthesis. A tailored library of AQ derivatives was assembled and their antimalarial activity evaluated against chemosensitive (NF54) and multidrug-resistant (K1) strains of the malaria parasite, Plasmodium falciparum. The compounds generally showed enhanced or comparable activities to those of the reference clinical drugs chloroquine and AQ, against both strains, with higher selectivity for the sensitive phenotype, mostly in the double-digit nanomolar IC50 range. Moreover, representative compounds from this series show the potential to block malaria transmission by inhibiting the growth of stage II/III and V gametocytes in vitro. Preliminary mechanistic insights also revealed hemozoin inhibition as a potential mode of action.

Blood-stage antimalarial activity, favourable metabolic stability and in vivo toxicity of novel piperazine linked 7-chloroquinoline-triazole conjugates

The persistence of drug resistance poses a significant obstacle to the advancement of efficacious malaria treatments. The remarkable efficacy displayed by 1,2,3-triazole-based compounds against Plasmodium falciparum highlights the potential of triazole conjugates, with diverse pharmacologically active structures, as potential antimalarial agents. We aimed to synthesize 7-dichloroquinoline-triazole conjugates and their structure-activity relationship (SAR) derivatives to investigate their anti-plasmodial activity. Among them, QP11, featuring a m-NO2 substitution, demonstrated efficacy against both chloroquine-sensitive and -resistant parasite strains. QP11 selectively inhibited FP2, a cysteine protease involved in hemoglobin degradation, and showed synergistic effects when combined with chloroquine. Additionally, QP11 hindered hemoglobin degradation and hemozoin formation within the parasite. Metabolic stability studies indicated high stability of QP11, making it a promising antimalarial candidate. In vivo evaluation using a murine malaria model demonstrated QP11's efficacy in eradicating parasite growth without neurotoxicity, presenting it as a promising compound for novel antimalarial development.

Antimalarial and neuroprotective ent-abietane diterpenoids from the aerial parts of Phlogacanthus curviflorus

Six new ent-abietane diterpenoids, abientaphlogatones A-F (1-6), along with two undescribed ent-abietane diterpenoid glucosides, abientaphlogasides A-B (7-8) and four known analogs were isolated from the aerial parts ofPhlogacanthus curviflorus (P. curviflorus). The structures of these compounds were determined using high-resolution electrospray ionization mass spectrometry (HR-ESI-MS), one-dimensional and two-dimensional nuclear magnetic resonance (NMR) spectroscopy, electronic circular dichroism (ECD) spectra, and quantum chemical calculations. Notably, compounds 5 and 6 represented the first reported instances of ent-norabietane diterpenoids from the genus Phlogacanthus. In the β-hematin formation inhibition assay, compounds 2, 4, 7-10, and 12 displayed antimalarial activity, with IC50 values of 12.97-65.01 μmol·L-1. Furthermore, compounds 4, 5, 8, and 10 demonstrated neuroprotective activity in PC12 cell injury models induced by H2O2 and MPP+.

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.

Novel 3-Trifluoromethyl-1,2,4-oxadiazole Analogues of Astemizole with Multi-stage Antiplasmodium Activity and In Vivo Efficacy in a Plasmodium berghei Mouse Malaria Infection Model

Iterative medicinal chemistry optimization of an ester-containing astemizole (AST) analogue 1 with an associated metabolic instability liability led to the identification of a highly potent 3-trifluoromethyl-1,2,4-oxadiazole analogue 23 (PfNF54 IC₅₀ = 0.012 μM; PfK1 IC₅₀ = 0.040 μM) displaying high microsomal metabolic stability (HLM CL_int < 11.6 μL·min^-1·mg^-1) and > 1000-fold higher selectivity over hERG compared to AST. In addition to asexual blood stage activity, the compound also shows activity against liver and gametocyte life cycle stages and demonstrates in vivo efficacy in Plasmodium berghei-infected mice at 4 × 50 mg·kg^-1 oral dose. Preliminary interrogation of the mode of action using live-cell microscopy and cellular heme speciation revealed that 23 could be affecting multiple processes in the parasitic digestive vacuole, with the possibility of a novel target at play in the organelles associated with it.

Evaluation of ferrocenyl-containing γ-hydroxy-γ-lactam-derived tetramates as potential antiplasmodials

A series of ferrocenyl-containing γ-hydroxy-γ-lactam tetramates were prepared in 2-3 steps through ring opening-ring closure (RORC) process of γ-ylidene-tetronate derivatives in the presence of ferrocenyl alkylamines. The compounds were screened in vitro for their antiplasmodial activity against chloroquine-sensitive (3D7) and chloroquine-resistant (W2) clones of P. falciparum, displaying activity in the range of 0.12-100 μM, with generally good resistance index. The most active ferrocene in these series exhibited IC₅₀ equal to 0.09 μM (3D7) and 0.12 μM (W2). The low cytotoxicity of the ferrocenyl-containing γ-hydroxy-γ-lactam tetramates against Human Umbilical Vein Endothelial (HUVEC) cell line demonstrated selective antiparasitic activity. The redox properties of these ferrocene-derived tetramates were studied and physico-biochemical studies evidenced that these derivatives can exert potent antimalarial activities via a mechanism distinct from ferroquine.

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.

Antimalarial Activity of Highly Coordinative Fused Heterocycles Targeting β-Hematin Crystallization

The β-hematin formation is a unique process adopted by Plasmodium sp. to detoxify free heme and represents a validated target to design new effective antimalarials. Most of the β-hematin inhibitors are mainly based on 4-aminoquinolines, but the parasite has developed diverse defense mechanisms against this type of chemical system. Thus, the identification of other molecular chemical entities targeting the β-hematin formation pathway is highly needed to evade resistance mechanisms associated with 4-aminoquinolines. Herein, we showed that the highly coordinative character can be a useful tool for the rational design of antimalarial agents targeting β-hematin crystallization. From a small library consisting of five compound families with recognized antitrypanosomatid activity and coordinative abilities, a group of tetradentate 1,4-disubstituted phthalazin-aryl/heteroarylhydrazinyl derivatives were identified as potential antimalarials. They showed a remarkable curative response against Plasmodium berghei-infected mice with a significant reduction of the parasitemia, which was well correlated with their good inhibitory activities on β-hematin crystallization (IC₅₀ = 5-7 μM). Their in vitro inhibitory and in vivo responses were comparable to those found for a chloroquine reference. The active compounds showed moderate in vitro toxicity against peritoneal macrophages, a low hemolysis response, and a good in silico ADME profile, identifying compound 2f as a promising antimalarial agent for further experiments. Other less coordinative fused heterocycles exhibited moderate inhibitory responses toward β-hematin crystallization and modest efficacy against the in vivo model. The complexation ability of the ligands with iron(III) was experimentally and theoretically determined, finding, in general, a good correlation between the complexation ability of the ligand and the inhibitory activity toward β-hematin crystallization. These findings open new perspectives toward the rational design of antimalarial β-hematin inhibitors based on the coordinative character as an alternative to the conventional β-hematin inhibitors.

Methods Used to Investigate the Plasmodium falciparum Digestive Vacuole

Plasmodium falciparum malaria remains a global health problem as parasites continue to develop resistance to all antimalarials in use. Infection causes clinical symptoms during the intra-erythrocytic stage of the lifecycle where the parasite infects and replicates within red blood cells (RBC). During this stage, P. falciparum digests the main constituent of the RBC, hemoglobin, in a specialized acidic compartment termed the digestive vacuole (DV), a process essential for survival. Many therapeutics in use target one or multiple aspects of the DV, with chloroquine and its derivatives, as well as artemisinin, having mechanisms of action within this organelle. In order to better understand how current therapeutics and those under development target DV processes, techniques used to investigate the DV are paramount. This review outlines the involvement of the DV in therapeutics currently in use and focuses on the range of techniques that are currently utilized to study this organelle including microscopy, biochemical analysis, genetic approaches and metabolomic studies. Importantly, continued development and application of these techniques will aid in our understanding of the DV and in the development of new therapeutics or therapeutic partners for the future.

THC shows activity against cultured Plasmodium falciparum

The FDA approved drug Dronabinol was identified in a previous study applying virtual screening using the haemozoin crystal as a target against malaria parasites. The active ingredient of dronabinol is synthetic tetrahydrocannabinol (THC), which is one of the major cannabinoids from Cannabis sativa. Traditional use of cannabis for malaria fever was reported in the world's oldest pharmacopoeia, dating to around 5000 years ago. In this research we report that THC inhibits β-haematin (synthetic haemozoin) and malaria parasite growth. Due the psychoactivity of THC, CBD, the other major naturally occurring cannabinoid that lacks the off-target psychoactive effects of THC, was also tested and inhibited β-haematin but showed only a mild antimalarial activity. To evaluate whether THC inhibit haemozoin formation, we performed a cellular haem fractionation assay that indicated that is not the likely mechanism of action. For the first time, the cannabinoid chemical structure is raised as a new chemical class to be further studied for malaria treatment, aiming to overcome the undesirable psychoactive effects of THC and optimize the antimalarial effects.

Studies of Potency and Efficacy of an Optimized Artemisinin-Quinoline Hybrid against Multiple Stages of the Plasmodium Life Cycle

A recently developed artemisinin-quinoline hybrid, named 163A, has been shown to display potent activity against the asexual blood stage of Plasmodium, the malaria parasite. In this study, we determined its in vitro cytotoxicity to mammalian cells, its potency to suppress P. berghei hepatic infection and to decrease the viability of P. falciparum gametocytes, in addition to determining whether the drug exhibits efficacy of a P. berghei infection in mice. This hybrid compound has a low level of cytotoxicity to mammalian cells and, conversely, a high level of selectivity. It is potent in the prevention of hepatic stage development as well as in killing gametocytes, denoting a potential blockage of malaria transmission. The hybrid presents a potent inhibitory activity for beta-hematin crystal formation, in which subsequent assays revealed that its endoperoxide component undergoes bioactivation by reductive reaction with ferrous heme towards the formation of heme-drug adducts; in parallel, the 7-chloroquinoline component has binding affinity for ferric hemin. Both structural components of the hybrid co-operate to enhance the inhibition of beta-hematin, and this bitopic ligand property is essential for arresting the growth of asexual blood parasites. We demonstrated the in vivo efficacy of the hybrid as an erythrocytic schizonticide agent in comparison to a chloroquine/artemisinin combination therapy. Collectively, the findings suggest that the bitopic property of the hybrid is highly operative on heme detoxification suppression, and this provides compelling evidence for explaining the action of the hybrid on the asexual blood stage. For sporozoite and gametocyte stages, the hybrid conserves the potency typically observed for endoperoxide drugs, and this is possibly achieved due to the redox chemistry of endoperoxide components with ferrous heme.

Modular synthesis of antimalarial quinoline-based PGM metallarectangles

A new ditopic, quinoline-based ligand L (7-chloro-4-(pyridin-4-yl)quinoline) was synthesized via a Suzuki cross-coupling reaction. The ligand was utilized to synthesize the corresponding half-sandwich iridium(III) and ruthenium(II) binuclear complexes (1c and 1d) and the subsequent metallarectangles (2c, 2d, 3c, and 3d), via [2 + 2] coordination-driven self-assembly. Single-crystal X-ray diffraction confirmed the proposed molecular structure of the binuclear complex [{IrCl₂(Cp*)}₂(μ-L)] (1c) and DFT calculations were used to predict the optimized geometry of the rectangular nature of [{Ir(μ-Cl)(Cp*)}₄(μ-L)₂](CF₃SO₃)₄ (2c). All of the metallarectangles were isolated as their triflate salts and characterized using various spectroscopic (¹H, ¹³C{¹H}, DOSY NMR, and IR spectroscopy) and analytical techniques (ESI-MS). The synthesized compounds were screened against the NF54 chloroquine-sensitive (CQS) and K1 chloroquine-resistant (CQR) strains of Plasmodium falciparum. Incorporation of the ubiquitous quinoline core and metal complexation significantly enhanced the in vitro biological activity, with an increase in the nuclearity correlating with an increase in the resultant antiplasmodial activity. This was observed across both parasitic strains, alluding to the potential of supramolecular metallarectangles to act as antiplasmodial agents. Inhibition of haemozoin formation was considered a potential mechanism of action and selected metallarectangles exhibit β-haematin inhibition activity with near comparable activity to chloroquine.

Inhibition of heme detoxification pathway in malaria parasite by 3-hydroxy-11-keto-β-boswellic acid isolated from Boswellia serrata

Malaria eradication is still a major global health problem in developing countries, which has been of more concern ever since the malaria parasite has developed resistance against frontline antimalarial drugs. Historical evidence proves that the plants possess a major resource for the development of novel anti-malarial drugs. In the present study, the bioactivity guided fractionation of the oleogum-resin of Boswellia serrata Roxb. yielded the optimum activity in the ethyl acetate fraction with an IC₅₀ of 22 ± 3.9 μg/mL and 26.5 ± 4.5 μg/mL against chloroquine sensitive (NF54) and resistant (K1) strains of Plasmodium falciparum respectively. Further, upon fractionation, the ethyl acetate fraction yielded four major compounds, of which 3-Hydroxy-11-keto-β-boswellic acid (KBA) was found to be the most potent with IC₅₀ values 4.5 ± 0.60 µg/mL and 6.25 ± 1.02 μg/mL against sensitive and resistant strains respectively. KBA was found to inhibit heme detoxification pathways, one of the most common therapeutic targets, which probably lead to an increase in reactive oxygen species (ROS) and nitric oxide (NO) detrimental to P. falciparum. Further, the induced intracellular oxidative stress affected the macromolecules in terms of DNA damage, increased lipid peroxidation, protein carbonylation as well as loss of mitochondrial membrane potential. However, it did not exhibit any cytotoxic effect in VERO cells. Under in vivo conditions, KBA exhibited a significant reduction in parasitemia, retarding the development of anaemia, resulting in an enhancement of the mean survival time in Plasmodium yoelii nigeriensis (chloroquine-resistant) infected mice. Further, KBA did not exhibit any abnormality in serum biochemistry of animals that underwent acute oral toxicity studies at 2000 mg/kg body weight.

Molecular Mechanisms of Drug Resistance in Plasmodium falciparum Malaria

Understanding and controlling the spread of antimalarial resistance, particularly to artemisinin and its partner drugs, is a top priority. Plasmodium falciparum parasites resistant to chloroquine, amodiaquine, or piperaquine harbor mutations in the P. falciparum chloroquine resistance transporter (PfCRT), a transporter resident on the digestive vacuole membrane that in its variant forms can transport these weak-base 4-aminoquinoline drugs out of this acidic organelle, thus preventing these drugs from binding heme and inhibiting its detoxification. The structure of PfCRT, solved by cryogenic electron microscopy, shows mutations surrounding an electronegative central drug-binding cavity where they presumably interact with drugs and natural substrates to control transport. P. falciparum susceptibility to heme-binding antimalarials is also modulated by overexpression or mutations in the digestive vacuole membrane-bound ABC transporter PfMDR1 (P. falciparum multidrug resistance 1 transporter). Artemisinin resistance is primarily mediated by mutations in P. falciparum Kelch13 protein (K13), a protein involved in multiple intracellular processes including endocytosis of hemoglobin, which is required for parasite growth and artemisinin activation. Combating drug-resistant malaria urgently requires the development of new antimalarial drugs with novel modes of action.

Quinoline-triazole half-sandwich iridium(III) complexes: synthesis, antiplasmodial activity and preliminary transfer hydrogenation studies

Iridium(iii) half-sandwich complexes containing 7-chloroquinoline-1,2,3-triazole hybrid ligands were synthesised and their inhibitory activities evaluated against the Plasmodium falciparum malaria parasite. Supporting computational analysis revealed that metal coordination to the quinoline nitrogen occurs first, forming a kinetic product that, upon heating over time, forms a more stable cyclometallated thermodynamic product. Single crystal X-ray diffraction confirmed the proposed molecular structures of both isolated kinetic and thermodynamic products. Complexation with iridium significantly enhances the in vitro activity of selected ligands against the chloroquine-sensitive (NF54) Plasmodium falciparum strain, with selected complexes being over one hundred times more active than their respective ligands. No cross-resistance was observed in the chloroquine-resistant (K1) strain. No cytotoxicity was observed for selected complexes tested against the mammalian Chinese Hamster Ovarian (CHO) cell line. In addition, speed-of-action assays and β-haematin inhibition studies were performed. Through preliminary qualitative and quantitative cell-free experiments, it was found that the two most active neutral, cyclometallated complexes can act as transfer hydrogenation catalysts, by reducing β-nicotinamide adenine dinucleotide (NAD+) to NADH in the presence of a hydrogen source, sodium formate.

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

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

Artemisinin-Based Drugs Target the Plasmodium falciparum Heme Detoxification Pathway

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

Coumarin-Annulated Ferrocenyl 1,3-Oxazine Derivatives Possessing In Vitro Antimalarial and Antitrypanosomal Potency

A tailored series of coumarin-based ferrocenyl 1,3-oxazine hybrid compounds was synthesized and investigated for potential antiparasitic activity, drawing inspiration from the established biological efficacy of the constituent chemical motifs. The structural identity of the synthesized compounds was confirmed by common spectroscopic techniques: NMR, HRMS and IR. Biological evaluation studies reveal that the compounds exhibit higher in vitro antiparasitic potency against the chemosensitive malarial strain (3D7 P. falciparum) over the investigated trypanosomiasis causal agent (T. b. brucei 427) with mostly single digit micromolar IC50 values. When read in tandem with the biological performance of previously reported structurally similar non-coumarin, phenyl derivatives (i.e., ferrocenyl 1,3-benzoxazines and α-aminocresols), structure-activity relationship analyses suggest that the presence of the coumarin nucleus is tolerated for biological activity though this may lead to reduced efficacy. Preliminary mechanistic studies with the most promising compound (11b) support hemozoin inhibition and DNA interaction as likely mechanistic modalities by which this class of compounds may act to produce plasmocidal and antitrypanosomal effects.

Novel Antimalarial Tetrazoles and Amides Active against the Hemoglobin Degradation Pathway in Plasmodium falciparum

Malaria control programs continue to be threatened by drug resistance. To identify new antimalarials, we conducted a phenotypic screen and identified a novel tetrazole-based series that shows fast-kill kinetics and a relatively low propensity to develop high-level resistance. Preliminary structure-activity relationships were established including identification of a subseries of related amides with antiplasmodial activity. Assaying parasites with resistance to antimalarials led us to test whether the series had a similar mechanism of action to chloroquine (CQ). Treatment of synchronized Plasmodium falciparum parasites with active analogues revealed a pattern of intracellular inhibition of hemozoin (Hz) formation reminiscent of CQ's action. Drug selections yielded only modest resistance that was associated with amplification of the multidrug resistance gene 1 (pfmdr1). Thus, we have identified a novel chemical series that targets the historically druggable heme polymerization pathway and that can form the basis of future optimization efforts to develop a new malaria treatment.

A Diverse Range of Hemozoin Inhibiting Scaffolds Act on Plasmodium falciparum as Heme Complexes

A diverse series of hemozoin-inhibiting quinolines, benzamides, triarylimidazoles, quinazolines, benzimidazoles, benzoxazoles, and benzothiazoles have been found to lead to exchangeable heme levels in cultured Plasmodium falciparum (NF54) that ranged over an order of magnitude at the IC₅₀. Surprisingly, less active compounds often exhibited higher levels of exchangeable heme than more active ones. Quantities of intracellular inhibitor measured using the inoculum effect exhibited a linear correlation with exchangeable heme, suggesting formation of heme-inhibitor complexes in the parasite. In an effort to confirm this, the presence of a Br atom in one of the benzimidazole derivatives was exploited to image its distribution in the parasite using electron spectroscopic imaging of Br, an element not naturally abundant in cells. This showed that the compound colocalized with iron, consistent with its presence as a heme complex. Direct evidence for this complex was then obtained using confocal Raman microscopy. Exchangeable heme and inhibitor were found to increase with decreased rate of killing, suggesting that slow-acting compounds have more time to build up exchangeable heme complexes. Lastly, some but not all compounds evidently cause pro-oxidant effects because their activity could be attenuated with N-acetylcysteine and potentiated with t-butyl hydroperoxide. Collectively, these findings suggest that hemozoin inhibitors act as complexes with free heme, each with its own unique activity.

Diversification of quinoline-triazole scaffolds with CORMs: Synthesis, in vitro and in silico biological evaluation against Plasmodium falciparum

A discrete series of tricarbonyl manganese and rhenium complexes conjugated to a quinoline-triazole hybrid scaffold were synthesised and their inhibitory activities evaluated against Plasmodium falciparum. In general, the complexes show moderate activity with improved inhibitory activities for the photoactivatable manganese(I) tricarbonyl complexes in the malaria parasite. All complexes are active in the dark against the NF54 CQS (chloroquine-sensitive) and K1 MDR (multidrug-resistant) strains of Plasmodium falciparum, with IC₅₀ values in the low micromolar range. Of significance, the complexes retain their activity in the MDR strain with resistance indices ranging between 1.1 and 2.1. The Mn(I) analogues display photodissociation of all three CO ligands upon irradiation at 365 nm. More importantly, the complexes show increased antimalarial activity in vitro upon photoactivation, something not observed by the clinically used reference drug, chloroquine. As a purported mechanism of action, the compounds were evaluated as β-haematin inhibitors. To further understand the interactions of the complexes, in silico hemozoin docking simulations were performed, attesting to the fact that CO-release could be vital for blocking the hemozoin formation pathway. These results show that this strategy may be a valuable, novel route to design antimalarial agents with higher efficacy.

Recent Advances in the Biological Investigation of Organometallic Platinum-Group Metal (Ir, Ru, Rh, Os, Pd, Pt) Complexes as Antimalarial Agents

In the face of the recent pandemic and emergence of infectious diseases of viral origin, research on parasitic diseases such as malaria continues to remain critical and innovative methods are required to target the rising widespread resistance that renders conventional therapies unusable. The prolific use of auxiliary metallo-fragments has augmented the search for novel drug regimens in an attempt to combat rising resistance. The development of organometallic compounds (those containing metal-carbon bonds) as antimalarial drugs has been exemplified by the clinical development of ferroquine in the nascent field of Bioorganometallic Chemistry. With their inherent physicochemical properties, organometallic complexes can modulate the discipline of chemical biology by proffering different modes of action and targeting various enzymes. With the beneficiation of platinum group metals (PGMs) in mind, this review aims to describe recent studies on the antimalarial activity of PGM-based organometallic complexes. This review does not provide an exhaustive coverage of the literature but focusses on recent advances of bioorganometallic antimalarial drug leads, including a brief mention of recent trends comprising interactions with biomolecules such as heme and intracellular catalysis. This resource can be used in parallel with complementary reviews on metal-based complexes tested against malaria.

Antimicrobial evaluation of neutral and cationic iridium(III) and rhodium(III) aminoquinoline-benzimidazole hybrid complexes

A series of neutral and cationic Ir(III) and Rh(III) aminoquinoline-benzimidazole hybrid complexes were synthesised and their inhibitory activities evaluated against Plasmodium falciparum and Mycobacterium tuberculosis. In general, the hybrid complexes display good activity against the chloroquine-sensitive NF54 strain of P. falciparum. The neutral Ir(III)- and Rh(III)-Cp∗ complexes were the most active (IC₅₀ = 0.488 μM for Ir^III), maintaining activity against the multidrug-resistant K1 strain. Low to no cytotoxicity against the Chinese hamster ovarian cell line was observed for the tested complexes. Selected active hybrid complexes demonstrated significant inhibition of β-haematin formation in a cell-free NP-40 assay, suggesting an effect on the host haemoglobin degradation pathway as a potential contributing mechanism of action. When tested against M. tuberculosis H37Rv, most hybrid complexes displayed moderate to good activity. Again, the neutral complexes outperformed the cationic complexes, with the neutral Ir(III)-Cp∗ complexes proving most active (MIC₉₀ = 0.488-1.490 μM).

A computational approach to validate novel drug targets of gentianine from Swertiya chirayita in Plasmodium falciparum

Gentianine is one of the compounds found in the plant Swertiya chirayita that is known for its antimalarial activity. However, its exact molecular mechanism of action is yet to be understood. In our present study, we applied several computational approaches to filter out and determine possible targets of gentianine in Plasmodium falciparum 3D7. Protein-protein networks formed the basis of one of our strategies along with orthologous protein analysis to establish essentiality. Out of 6 essential proteins from unique pathways, haloacid dehalogenase like-hydrolase (PfHAD1), phosphoenolpyruvate carboxykinase (PfPEPCK) and fumarate hydratase (PfFH) were screened as drug targets through this approach. Through our other strategy we established the predicted IC50 (PIC50) value of gentianine with a set of molecular descriptors from 123 Pathogen Box anti-malarial compounds. Afterwards through 2D structural similarity, L-lactate dehydrogenase (PfLDH) was established as another possible target. In our work, we performed in silico docking and analysed the binding of gentianine to the proteins. All of the proteins were reported with favourable binding results and were considered for complex molecular dynamics simulation approach. Our research clears up the molecular mechanism of antimalarial activity of gentianine to some extent paving way for experimental validation of the same in future.

The in Vitro Antiplasmodial and Antiproliferative Activity of New Ferrocene-Based α-Aminocresols Targeting Hemozoin Inhibition and DNA Interaction

The conjugation of organometallic complexes to known bioactive organic frameworks is a proven strategy revered for devising new drug molecules with novel modes of action. This approach holds great promise for the generation of potent drug leads in the quest for therapeutic chemotypes with the potential to overcome the development of clinical resistance. Herein, we present the in vitro antiplasmodial and antiproliferative investigation of ferrocenyl α-aminocresol conjugates assembled by amalgamation of the organometallic ferrocene unit and an α-aminocresol scaffold possessing antimalarial activity. The compounds pursued in the study exhibited higher toxicity towards the chemosensitive (3D7) and -resistant (Dd2) strains of the Plasmodium falciparum parasite than to the human HCC70 triple-negative breast cancer cell line. Indication of cross-resistance was absent for the compounds evaluated against the multi-resistant Dd2 strain. Structure-activity analysis revealed that the phenolic hydroxy group and rotatable σ bond between the α-carbon and NH group of the α-amino-o-cresol skeleton are crucial for the biological activity of the compounds. Spectrophotometric techniques and in silico docking simulations performed on selected derivatives suggest that the compounds show a dual mode of action involving hemozoin inhibition and DNA interaction via minor-groove binding. Lastly, compound 9 a, identified as a possible lead, exhibited preferential binding for the plasmodial DNA isolated from 3D7 P. falciparum trophozoites over the mammalian calf thymus DNA, thereby substantiating the enhanced antiplasmodial activity of the compounds. The presented research demonstrates the strategy of incorporating organometallic complexes into known biologically active organic scaffolds as a viable avenue to fashion novel multimodal compounds with potential to counter the development drug resistance.

Naphthylisoquinoline alkaloids, validated as hit multistage antiplasmodial natural products

The discovery and development of multistage antimalarial drugs targeting intra-erythrocytic asexual and sexual Plasmodium falciparum parasites is of utmost importance to achieve the ambitious goal of malaria elimination. Here, we report the validation of naphthylisoquinoline (NIQ) alkaloids and their synthetic analogues as multistage active antimalarial drug candidates. A total of 30 compounds were tested, of which 17 exhibited IC₅₀ values <1 μM against drug-sensitive P. falciparum parasites (NF54 strain); 15 of these retained activity against a panel of drug-resistant strains. These compounds showed low in vitro cytotoxicity against HepG2 cells, with selectivity indices of >10. The tested compounds showed activity in vitro against both early- and late-stage P. falciparum gametocytes while blocking male gamete formation (>70% inhibition of exflagellation at 2 μM). Additionally, five selected compounds were found to have good solubility (≥170 μM in PBS at pH 6.5), while metabolic stability towards human, mouse, and rat microsomes ranged from >90% to >7% after 30 min. Dioncophylline C (2a) emerged as a front runner from the study, displaying activity against both asexual parasites and gametocytes, a lack of cross-resistance to chloroquine, good solubility, and microsomal stability. Overall, this is the first report on the multistage activity of NIQs and their synthetic analogues including gametocytocidal and gametocidal effects induced by this class of compounds.

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Naphthylisoquinolines (NIQs) validated as antimalarial hit candidates.

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First report on transmission-blocking properties of NIQs and analogues.

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15 compounds active across 9 P. falciparum strains, with acceptable RI <10 and SI >10.

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5 compounds show good solubility and microsomal stability.

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Dioncophylline C is the frontrunner antimalarial candidate with multistage activity.

Lapatinib, Nilotinib and Lomitapide Inhibit Haemozoin Formation in Malaria Parasites

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

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

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

Pharmacophore Modeling, Synthesis, Scaffold Hopping and Biological β- Hematin Inhibition Interaction Studies for Anti-malaria Compounds

Backgound:

Exploring potent compounds is critical to generating multi-target drug discovery. Hematin crystallization is an important mechanism of malaria.

Methods:

A series of chloroquine analogues were designed using a repositioning approach to develop new anticancer compounds. Protein-ligand interaction fingerprints and ADMET descriptors were used to assess docking performance in virtual screenings to design chloroquine hybrid β-hematin inhibitors. A PLS algorithm was applied to correlate the molecular descriptors to IC50 values. The modeling presented excellent predictive power with correlation coefficients for calibration and cross-validation of r2 = 0.93 and q2 = 0.72. Using the model, a series of 4-aminoquinlin hybrids were synthesized and evaluated for their biological activity as an external test series. These compounds were evaluated for cytotoxic cell lines and β-hematin inhibition.

Results:

The target compounds exhibited high β-hematin inhibition activity and were 3-9 times more active than the positive control. Furthermore, all the compounds exhibited moderate to high cytotoxic activity. The most potent compound in the dataset was docked with hemoglobin and its pharmacophore features were generated. These features were used as input to the Pharmit server for screening of six databases.

Conclusion:

The compound with the best score from ChEMBL was 2016904, previously reported as a VEGFR-2 inhibitor. The 11 compounds selected presented the best Gold scores with drug-like properties and can be used for drug development.

New abietane diterpenoid glucosides from underground parts of Isodon taliensis

Two new abietane diterpenoid glucosides, talienosides A and B (1 and 2), along with four known compounds (3 - 6), were isolated from the ethyl acetate soluble portion of 70% acetone extract of the underground parts of Isodon taliensis. Their structures were elucidated based on the analyses of extensive spectroscopic data and physicochemical properties. The two new compounds were tested for their bioactivities against malaria and diabetes by β-hematin formation inhibition and α-glycosidase inhibition experiments, respectively. However, only 2 showed weak β-hematin formation inhibitory activity. [Formula: see text].

Bioisosteric ferrocenyl aminoquinoline-benzimidazole hybrids: Antimicrobial evaluation and mechanistic insights

Phenyl- and bioisosteric ferrocenyl-derived aminoquinoline-benzimidazole hybrid compounds were synthesised and evaluated for their in vitro antiplasmodial activity against the chloroquine-sensitive NF54 and multi-drug resistant K1 strains of the human malaria parasite, Plasmodium falciparum. All compounds were active against the two strains, generally showing enhanced activity in the K1 strain, with resistance indices less than 1. Cytotoxicity studies using Chinese hamster ovarian cells revealed that the hybrids were relatively non-cytotoxic and demonstrated selective killing of the parasite. Based on favourable in vitro antiplasmodial and cytotoxicity data, the most active phenyl (4c) and ferrocenyl (5b) hybrids were tested in vivo against the rodent Plasmodium berghei mouse model. Both compounds caused a reduction in parasitemia relative to the control, with 5c displaying superior activity (92% reduction in parasitemia at 4 × 50 mg/kg oral doses). The most active phenyl and ferrocenyl derivatives showed inhibition of β-haematin formation in a NP-40 detergent-mediated assay, indicating a possible contributing mechanism of antiplasmodial action. The most active ferrocenyl hybrid did not display appreciable reactive oxygen species (ROS) generation in a ROS-induced DNA cleavage gel electrophoresis study. The compounds were also screened for their in vitro activity against Mycobacterium tuberculosis. The hybrids containing a more hydrophobic substituent had enhanced activity (<32.7 μM) compared to those with a less hydrophobic substituent (>62.5 μM).

Multistage Antiplasmodium Activity of Astemizole Analogues and Inhibition of Hemozoin Formation as a Contributor to Their Mode of Action

A drug repositioning approach was leveraged to derivatize astemizole (AST), an antihistamine drug whose antimalarial activity was previously identified in a high-throughput screen. The multistage activity potential against the Plasmodium parasite's life cycle of the subsequent analogues was examined by evaluating against the parasite asexual blood, liver, and sexual gametocytic stages. In addition, the previously reported contribution of heme detoxification to the compound's mode of action was interrogated. Ten of the 17 derivatives showed half-maximal inhibitory concentrations (IC₅₀s) of <0.1 μM against the chloroquine (CQ)-sensitive Plasmodium falciparum NF54 ( PfNF54) strain while maintaining submicromolar potency against the multidrug-resistant strain, PfK1, with most showing low likelihood of cross-resistance with CQ. Selected analogues ( PfNF54-IC₅₀ < 0.1 μM) were tested for cytotoxicity on Chinese hamster ovarian (CHO) cells and found to be highly selective (selectivity index > 100). Screening of AST and its analogues against gametocytes revealed their moderate activity (IC₅₀: 1-5 μM) against late stage P. falciparum gametocytes, while the evaluation of activity against P. berghei liver stages identified one compound (3) with 3-fold greater activity than the parent AST compound. Mechanistic studies showed a strong correlation between in vitro inhibition of β-hematin formation by the AST derivatives and their antiplasmodium IC₅₀s. Analyses of intracellular inhibition of hemozoin formation within the parasite further yielded signatures attributable to a possible perturbation of the heme detoxification machinery.

Hydrazonophenol, a Food Vacuole-Targeted and Ferriprotoporphyrin IX-Interacting Chemotype Prevents Drug-Resistant Malaria

The rapid emergence of resistance against frontline antimalarial drugs essentially warrants the identification of new-generation antimalarials. Here, we describe the synthesis of ( E)-2-isopropyl-5-methyl-4-((2-(pyridin-4-yl)hydrazono)methyl)phenol (18), which binds ferriprotoporphyrin-IX (Fe^III-PPIX) ( K_d = 33 nM) and offers antimalarial activity against chloroquine-resistant and sensitive strains of Plasmodium falciparum in vitro. Structure-function analysis reveals that compound 18 binds Fe^III-PPIX through the -C═N-NH- moiety and 2-pyridyl substitution at the hydrazine counterpart plays a critical role in antimalarial efficacy. Live cell confocal imaging using a fluorophore-tagged compound confirms its accumulation inside the acidic food vacuole (FV) of P. falciparum. Furthermore, this compound concentration-dependently elevates the pH in FV, implicating a plausible interference with Fe^III-PPIX crystallization (hemozoin formation) by a dual function: increasing the pH and binding free Fe^III-PPIX. Different off-target bioassays reduce the possibility of the promiscuous nature of compound 18. Compound 18 also exhibits potent in vivo antimalarial activity against chloroquine-resistant P. yoelii and P. berghei ANKA (causing cerebral malaria) in mice with negligible toxicity.

Evaluation of ferrocenyl phosphines as potent antimalarials targeting the digestive vacuole function of Plasmodium falciparum

Ferrocenyl phosphines targeting the digestive vacuole function of the malaria parasite, Plasmodium falciparum.

Chemical Proteomics and Super-resolution Imaging Reveal That Chloroquine Interacts with Plasmodium falciparum Multidrug Resistance-Associated Protein and Lipids

It is well established that chloroquine, a quinoline antimalarial, inhibits hemozoin formation in the malaria parasite. Counterintuitively, this archetypal antimalarial is also used in the treatment of diseases in which hemozoin biocrystallization does not play a role. Hence, we decided to investigate whether chloroquine possesses binding targets other than Fe(III) protoporphyrin IX in blood stage Plasmodium falciparum parasites and whether these are related to sites of accumulation within the parasite other than the digestive vacuole. A 7-nitrobenz-2-oxa-1,3-diazole (NBD)-labeled fluorescent derivative of chloroquine, especially sensitive to regions outside the digestive vacuole and retaining the antiplasmodial pharmacophore, was synthesized to investigate subcellular localization in the parasite. Super-resolution microscopy revealed association with membranes including the parasite plasma membrane, the endoplasmic reticulum, and possibly also the mitochondrion. A drug-labeled affinity matrix was then prepared to capture protein binding targets of chloroquine. SDS-PAGE revealed a single prominent band between 200 and 250 kDa from the membrane-associated fraction. Subsequent proteomic analysis revealed that this band corresponded to P. falciparum multidrug resistance-associated protein (PfMRP1). Intrigued by this finding, we demonstrated pull-down of PfMRP1 by matrices labeled with Cinchona alkaloids quinine and quinidine. While PfMRP1 has been implicated in resistance to quinolines and other antimalarials, this is the first time that these drugs have been found to bind directly to this protein. Based on previous reports, PfMRP1, the only prominent protein found to bind to quinolines in this work, is likely to modulate the activity of these antimalarials in P. falciparum rather than act as a drug target.

Hemozoin inhibiting 2-phenylbenzimidazoles active against malaria parasites

The 2-phenylbenzimidazole scaffold has recently been discovered to inhibit β-hematin (synthetic hemozoin) formation by high throughput screening. Here, a library of 325,728 N-4-(1H-benzo[d]imidazol-2-yl)aryl)benzamides was enumerated, and Bayesian statistics used to predict β-hematin and Plasmodium falciparum growth inhibition. Filtering predicted inactives and compounds with negligible aqueous solubility reduced the library to 35,124. Further narrowing to compounds with terminal aryl ring substituents only, reduced the library to 18, 83% of which were found to inhibit β-hematin formation <100 μM and 50% parasite growth <2 μM. Four compounds showed nanomolar parasite growth inhibition activities, no cross-resistance in a chloroquine resistant strain and low cytotoxicity. QSAR analysis showed a strong association of parasite growth inhibition with inhibition of β-hematin formation and the most active compound inhibited hemozoin formation in P. falciparum, with consequent increasing exchangeable heme. Pioneering use of molecular docking for this system demonstrated predictive ability and could rationalize observed structure activity trends.

Design, synthesis and bioactivities of phenithionate analogues or derivatives for anti-schistosomiasis

A novel series of phenithionate analogues or derivatives were designed and synthesized using phenithionate as the lead compound, and their bioactivities were studied. Their structures were confirmed by ¹H NMR, ¹³C NMR, HR-ESI-MS, and elemental analysis, respectively. The results of in vitro inhibitory activity measurement proved that compounds 5a, 5c, 5g, 5i, 5m and 5o had a better inhibitory effect on larva and imago schistosoma. Among them, the inhibitory activity of compound 5i for larva schistosoma was IC₅₀ = 5.21 ± 0.04 μg mL^-1, and for imago schistosoma it was IC₅₀ = 6.35 ± 0.08 μg mL^-1. Moreover, the experimental results of in vivo anti-schistosomiasis activity measurement showed that they had good anti-schistosomiasis activity. Therefore, these compounds had better drugability.

NMR relaxation properties of the synthetic malaria pigment β-hematin

200 million patients suffer from malaria, a parasitic disease caused by protozoans of the genus Plasmodium. Reliable diagnosis is crucial since it allows the early detection of the disease. The development of rapid, sensitive and low-cost diagnosis tools is an important research area. Different studies focused on the detection of hemozoin, a major by-product of hemoglobin detoxification by the parasite. Hemozoin and its synthetic analog, β-hematin, form paramagnetic crystals. A new detection method of malaria takes advantage of the paramagnetism of hemozoin through the effect that such magnetic crystals have on Nuclear Magnetic Resonance (NMR) relaxation of water protons. Indeed, magnetic microparticles cause a shortening of the relaxation times. In this work, the magnetic properties of two types of β-hematin are assessed at different temperatures and magnetic fields. The pure paramagnetism of β-hematin is confirmed. The NMR relaxation of β–hematin suspensions is also studied at different magnetic fields and for different echo-times. Our results help to identify the best conditions for β–hematin detection by NMR: T₂ must be selected, at large magnetic fields and for long echo-times. However, the effect of β-hematin on relaxation does not seem large enough to achieve accurate detection of malaria without any preliminary sample preparation, as microcentrifugation.