Lysis of Plasmodium falciparum by ferriprotoporphyrin IX and a chloroquine-ferriprotoporphyrin IX complex

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.

Mechanistic basis for multidrug resistance and collateral drug sensitivity conferred to the malaria parasite by polymorphisms in PfMDR1 and PfCRT

Polymorphisms in the Plasmodium falciparum multidrug resistance protein 1 (pfmdr1) gene and the Plasmodium falciparum chloroquine resistance transporter (pfcrt) gene alter the malaria parasite’s susceptibility to most of the current antimalarial drugs. However, the precise mechanisms by which PfMDR1 contributes to multidrug resistance have not yet been fully elucidated, nor is it understood why polymorphisms in pfmdr1 and pfcrt that cause chloroquine resistance simultaneously increase the parasite’s susceptibility to lumefantrine and mefloquine—a phenomenon known as collateral drug sensitivity. Here, we present a robust expression system for PfMDR1 in Xenopus oocytes that enables direct and high-resolution biochemical characterizations of the protein. We show that wild-type PfMDR1 transports diverse pharmacons, including lumefantrine, mefloquine, dihydroartemisinin, piperaquine, amodiaquine, methylene blue, and chloroquine (but not the antiviral drug amantadine). Field-derived mutant isoforms of PfMDR1 differ from the wild-type protein, and each other, in their capacities to transport these drugs, indicating that PfMDR1-induced changes in the distribution of drugs between the parasite’s digestive vacuole (DV) and the cytosol are a key driver of both antimalarial resistance and the variability between multidrug resistance phenotypes. Of note, the PfMDR1 isoforms prevalent in chloroquine-resistant isolates exhibit reduced capacities for chloroquine, lumefantrine, and mefloquine transport. We observe the opposite relationship between chloroquine resistance-conferring mutations in PfCRT and drug transport activity. Using our established assays for characterizing PfCRT in the Xenopus oocyte system and in live parasite assays, we demonstrate that these PfCRT isoforms transport all 3 drugs, whereas wild-type PfCRT does not. We present a mechanistic model for collateral drug sensitivity in which mutant isoforms of PfMDR1 and PfCRT cause chloroquine, lumefantrine, and mefloquine to remain in the cytosol instead of sequestering within the DV. This change in drug distribution increases the access of lumefantrine and mefloquine to their primary targets (thought to be located outside of the DV), while simultaneously decreasing chloroquine’s access to its target within the DV. The mechanistic insights presented here provide a basis for developing approaches that extend the useful life span of antimalarials by exploiting the opposing selection forces they exert upon PfCRT and PfMDR1.

Synthesis of New 1,2,3,4-Tetrahydroquinoline Hybrid of Ibuprofen and Its Biological Evaluation

Herein we report the obtaining of 1-(3,4-dihydroquinolin-1(2H)-yl)-2- (4-isobutylphenyl)propan-1-one and its characterization. The newly obtained hybrid and its derivatives (hybrids of ibuprofen with 1,2,3,4-tetrahydroisoquinoline, and piperidine) were screened for their in vitro antioxidant, antitryptic, and inhibition of albumin denaturation activity. The lipophilicity was established using both reversed-phase thin layer chromatography and in silico calculations.

Chloroquine inhibits pro-inflammatory effects of heme on macrophages and invivo

BACKGROUND

Chloroquine has been used successfully to treat Malaria, including by chloroquine-resistant Plasmodium sp., indicating that it has effects on disease itself. Since heme has inflammatory effects and contributes to the pathogenesis of hemolytic diseases, we hypothesize that the anti-inflammatory effect of chloroquine is partially due to its inhibitory effect on heme-induced macrophage activation and on inflammatory tissue damage.

METHODS

Bone marrow derived macrophages (BMDMs) were incubated with chloroquine before stimulation with heme, in different conditions, to evaluate cytokines secretion, ROS production, mitogen activated protein kinases (MAPK) or spleen tyrosine kinase (Syk) activation, alone or combined with LPS. The effects of chloroquine upon heme inflammation were also evaluated in vivo, through simultaneous i.p. injection of LPS and heme, intratracheal instillation of Poly-IC followed by heme injection, and in a rhabdomyolysis model.

RESULTS

Chloroquine inhibited TNF secretion, mitochondrial ROS production, MAPK, and Syk activation induced by heme. Inhibition of TNF production could be mimicked by zinc ionophore quercetin, but not by primaquine, a chloroquine analog with low affinity for heme. IL-6 and IL-1β secretions induced by heme in the presence of PRRs agonists were inhibited by chloroquine, but not by calcium chelator BAPTA or inhibitor of endosomal acidification concamycin B. Chloroquine also protected mice from heme inflammatory effects in vivo, inhibiting lethal synergism with PRR agonists, lung pathology caused by heme injection after intratracheal instillation of Poly-IC, and delaying death after rhabdomyolisis.

CONCLUSION

Our data indicate that chloroquine might be used as a supportive therapy to control heme-induced deleterious inflammation in different hemolytic diseases.

Chloroquine and hydroxychloroquine in the treatment of malaria and repurposing in treating COVID-19

Chloroquine (CQ) and Hydroxychloroquine (HCQ) have been commonly used for the treatment and prevention of malaria, and the treatment of autoimmune diseases for several decades. As their new mechanisms of actions are identified in recent years, CQ and HCQ have wider therapeutic applications, one of which is to treat viral infectious diseases. Since the pandemic of the coronavirus disease 2019 (COVID-19), CQ and HCQ have been subjected to a number of in vitro and in vivo tests, and their therapeutic prospects for COVID-19 have been proposed. In this article, the applications and mechanisms of action of CQ and HCQ in their conventional fields of anti-malaria and anti-rheumatism, as well as their repurposing prospects in anti-virus are reviewed. The current trials and future potential of CQ and HCQ in combating COVID-19 are discussed.

Relaxometric studies of erythrocyte suspensions infected by Plasmodium falciparum: a tool for staging infection and testing anti-malarial drugs

PURPOSE

Malaria is a global health problem with the most malignant form caused by Plasmodium falciparum (P. falciparum). Parasite maturation in red blood cells (RBCs) is accompanied by changes including the formation of paramagnetic hemozoin (HZ) nanocrystals, and increased metabolism and variation in membrane lipid composition. Herein, MR relaxometry (MRR) was applied to investigate water exchange across RBCs' membrane and HZ formation in parasitized RBCs.

METHODS

Transverse water protons relaxation rate constants (R₂ = 1/T₂ ) were measured for assessing HZ formation in P. falciparum-parasitized human RBCs. Moreover, water exchange lifetimes across the RBC membrane (τ_i ) were assessed by measuring longitudinal relaxation rate constants (R₁ = 1/T₁ ) at 21.5 MHz in the presence of a gadolinium complex dissolved in the suspension medium.

RESULTS

τ_i increased after invasion of parasites (ring stage, mean τ_i / τ i 0 = 1.234 ± 0.022) and decreased during maturation to late trophozoite (mean τ_i / τ i 0 = 0.960 ± 0.075) and schizont stages (mean τ_i / τ i 0 = 1.019 ± 0.065). The HZ accumulation in advanced stages was revealed by T₂ -shortening. The curves reporting R₂ (1/T₂ ) vs. magnetic field showed different slopes for non-parasitized RBCs (npRBCs) and parasitized RBCs (pRBCs), namely 0.003 ± 0.001 for npRBCs, 0.009 ± 0.002, 0.028 ± 0.004 and 0.055 ± 0.002 for pRBCs at ring-, early trophozoite-, and late trophozoite stage, respectively. Antimalarial molecules dihydroartemisinin and chloroquine elicited measurable changes in parasitized RBCs, namely dihydroartemisinin modified τ_i , whereas the interference of chloroquine with HZ formation was detectable by a significant T₂ increase.

CONCLUSIONS

MRR can be considered a useful tool for reporting on P. falciparum blood stages and for screening potential antimalarial molecules.

Oxidative and nitrosative stresses in cerebral malaria: can we target them to avoid a bad prognosis?

There is currently a global effort to reduce malaria morbidity and mortality. However, malaria still results in the deaths of thousands of people every year. Malaria is caused by Plasmodium spp., parasites transmitted through the bite of an infected female Anopheles mosquito. Treatment timing plays a decisive role in reducing mortality and sequelae associated with the severe forms of the disease such as cerebral malaria (CM). The available antimalarial therapy is considered effective but parasite resistance to these drugs has been observed in some countries. Antimalarial drugs act by increasing parasite lysis, especially through targeting oxidative stress pathways. Here we discuss the roles of reactive oxygen species and reactive nitrogen intermediates in CM as a result of host-parasite interactions. We also present evidence of the potential contribution of oxidative and nitrosative stress-based antimalarial drugs to disease treatment and control.

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.

Chloroquine exposure triggers distinct cellular responses in sensitive versus resistant Plasmodium falciparum parasites

Chloroquine (CQ) treatment failure in Plasmodium falciparum parasites has been documented for decades, but the pharmacological explanation of this phenotype is not fully understood. Current concepts attribute CQ resistance to reduced accumulation of the drug at a given external CQ concentration ([CQ]_ex) in resistant compared to sensitive parasites. The implication of this explanation is that the mechanisms of CQ-induced toxicity in resistant and sensitive strains are similar once lethal internal concentrations have been reached. To test this hypothesis, we investigated the mechanism of CQ-induced toxicity in CQ-sensitive (CQS) versus CQ-resistant (CQR) parasites by analyzing the time-course of cellular responses in these strains after exposure to varying [CQ]_ex as determined in 72 h toxicity assays. Parasite killing was delayed in CQR parasites for up to 10 h compared to CQS parasites when exposed to equipotent [CQ]_ex. In striking contrast, brief exposure (1 h) to lethal [CQ]_ex in CQS but not CQR parasites caused the appearance of hitherto undescribed hemozoin (Hz)-containing compartments in the parasite cytosol. Hz-containing compartments were very rarely observed in CQR parasites even after CQ exposures sufficient to cause irreversible cell death. These findings challenge current concepts that CQ killing of malaria parasites is solely concentration-dependent, and instead suggest that CQS and CQR strains fundamentally differ in the consequences of CQ exposure.

Revisiting the Therapeutic Potential of Bothrops jararaca Venom: Screening for Novel Activities Using Connectivity Mapping

Snake venoms are sources of molecules with proven and potential therapeutic applications. However, most activities assayed in venoms (or their components) are of hemorrhagic, hypotensive, edematogenic, neurotoxic or myotoxic natures. Thus, other relevant activities might remain unknown. Using functional genomics coupled to the connectivity map (C-map) approach, we undertook a wide range indirect search for biological activities within the venom of the South American pit viper Bothrops jararaca. For that effect, venom was incubated with human breast adenocarcinoma cell line (MCF7) followed by RNA extraction and gene expression analysis. A list of 90 differentially expressed genes was submitted to biosimilar drug discovery based on pattern recognition. Among the 100 highest-ranked positively correlated drugs, only the antihypertensive, antimicrobial (both antibiotic and antiparasitic), and antitumor classes had been previously reported for B. jararaca venom. The majority of drug classes identified were related to (1) antimicrobial activity; (2) treatment of neuropsychiatric illnesses (Parkinson’s disease, schizophrenia, depression, and epilepsy); (3) treatment of cardiovascular diseases, and (4) anti-inflammatory action. The C-map results also indicated that B. jararaca venom may have components that target G-protein-coupled receptors (muscarinic, serotonergic, histaminergic, dopaminergic, GABA, and adrenergic) and ion channels. Although validation experiments are still necessary, the C-map correlation to drugs with activities previously linked to snake venoms supports the efficacy of this strategy as a broad-spectrum approach for biological activity screening, and rekindles the snake venom-based search for new therapeutic agents.

Malaria treatment using novel nano-based drug delivery systems

We reside in an era of technological innovation and advancement despite which infectious diseases like malaria remain to be one of the greatest threats to the humans. Mortality rate caused by malaria disease is a huge concern in the twenty-first century. Multiple drug resistance and nonspecific drug targeting of the most widely used drugs are the main reasons/drawbacks behind the failure in malarial therapy. Dose-related toxicity because of high doses is also a major concern. Therefore, to overcome these problems nano-based drug delivery systems are being developed to facilitate site-specific or target-based drug delivery and hence minimizing the development of resistance progress and dose-dependent toxicity issues. In this review, we discuss about the shortcomings in treating malaria and how nano-based drug delivery systems can help in curtailing the infectious disease malaria.

Fighting fire with fire: mass antimalarial drug administrations in an era of antimalarial resistance

The emergence and spread of antimalarial resistance has been a major liability for malaria control. The spread of chloroquine-resistant Plasmodium falciparum strains had catastrophic consequences for people in malaria-endemic regions, particularly in sub-Saharan Africa. The recent emergence of artemisinin-resistant P. falciparum strains is of highest concern. Current efforts to contain artemisinin resistance have yet to show success. In the absence of more promising plans, it has been suggested to eliminate falciparum malaria from foci of artemisinin resistance using a multipronged approach, including mass drug administrations. The use of mass drug administrations is controversial as it increases drug pressure. Based on current knowledge it is difficult to conceptualize how targeted malaria elimination could contribute to artemisinin resistance, provided a full treatment course is ensured.

Resonance Raman spectroscopy in malaria research

In recent years, the field of Raman spectroscopy has witnessed a surge in technological development, with the incorporation of ultrasensitive, charge-coupled devices, improved laser sources and precision Rayleigh-filter systems. This has led to the development of sensitive confocal micro-Raman spectrometers and imaging spectrometers that are capable of obtaining high spatial-resolution spectra and images of subcellular components within single living cells. This review reports on the application of resonance micro-Raman spectroscopy to the study of malaria pigment (hemozoin), a by-product of hemoglobin catabolization by the malaria parasite, which is an important target site for antimalarial drugs. The review aims to briefly describe recent studies on the application of this technology, elucidate molecular and electronic properties of the malaria pigment and its synthetic analog beta-hematin, provide insight into the mechanism of hemozoin formation within the food vacuole of the parasite, and comment on developing strategies for using this technology in drug-screening protocols.

Chloroquine: modes of action of an undervalued drug

For more than two decades, chloroquine (CQ) was largely and deliberately used as first choice drug for malaria treatment. However, worldwide increasing cases of resistant strains of Plasmodium have hampered its use. Nevertheless, CQ has recently been tested as adjunct therapy in several inflammatory situations, such as rheumatoid arthritis and transplantation procedures, presenting intriguing and promising results. In this review, we discuss recent findings and CQ mechanisms of action vis-à-vis its use as a broad adjunct therapy.

Malaria biology and disease pathogenesis: insights for new treatments

Plasmodium falciparum malaria, an infectious disease caused by a parasitic protozoan, claims the lives of nearly a million children each year in Africa alone and is a top public health concern. Evidence is accumulating that resistance to artemisinin derivatives, the frontline therapy for the asexual blood stage of the infection, is developing in southeast Asia. Renewed initiatives to eliminate malaria will benefit from an expanded repertoire of antimalarials, including new drugs that kill circulating P. falciparum gametocytes, thereby preventing transmission. Our current understanding of the biology of asexual blood-stage parasites and gametocytes and the ability to culture them in vitro lends optimism that high-throughput screenings of large chemical libraries will produce a new generation of antimalarial drugs. There is also a need for new therapies to reduce the high mortality of severe malaria. An understanding of the pathophysiology of severe disease may identify rational targets for drugs that improve survival.

The single crystal X-ray structure of β-hematin DMSO solvate grown in the presence of chloroquine, a β-hematin growth-rate inhibitor

Single crystals of solvated β-hematin were grown from a DMSO solution containing the antimalarial drug chloroquine, a known inhibitor of β-hematin formation. In addition, a kinetics study employing biomimetic lipid-water emulsion conditions was undertaken to further investigate the effect of chloroquine and quinidine on the formation of β-hematin. Scanning electron microscopy shows that the external morphology of the β-hematin DMSO solvate crystals is almost indistinguishable from that of malaria pigment (hemozoin), and single crystal X-ray diffraction confirms the presence of μ-propionato coordination dimers of iron(III) protoporphyrin IX. The free propionic acid functional groups of adjacent dimers hydrogen bond to included DMSO molecules, rather than forming carboxylic acid dimers. The observed exponential kinetics were modeled using the Avrami equation, with an Avrami constant equal to 1. The decreased rate of β-hematin formation observed at low concentrations of both drugs could be accounted for by assuming a mechanism of drug adsorption to sites on the fastest growing face of β-hematin. This behavior was modeled using the Langmuir isotherm. Higher concentrations of drug resulted in decreased final yields of β-hematin, and an irreversible drug-induced precipitation of iron(III) protoporphyrin IX was postulated to account for this. The model permits determination of the equilibrium adsorption constant (K(ads)). The values for chloroquine (log K(ads) = 5.55 ± 0.03) and quinidine (log K(ads) = 4.92 ± 0.01) suggest that the approach may be useful as a relative probe of the mechanism of action of novel antimalarial compounds.

Azithromycin plus chloroquine: combination therapy for protection against malaria and sexually transmitted infections in pregnancy

Introduction:

The first-line therapy for the intermittent preventive treatment of malaria in pregnancy (IPTp) is sulphadoxine-pyrimethamine (SP). There is an urgent need to identify safe, well-tolerated and efficacious alternatives to SP due to widespread Plasmodium falciparum resistance. Combination therapy using azithromycin and chloroquine is one possibility that has demonstrated adequate parasitological response > 95% in clinical trials of non-pregnant adults in sub-Saharan Africa and where IPTp is a government policy in 33 countries.

Areas covered:

Key safety, tolerability and efficacy data are presented for azithromycin and chloroquine, alone and/or in combination, when used to prevent and/or treat P. falciparum, P. vivax, and several curable sexually transmitted and reproductive tract infections (STI/RTI). Pharmacokinetic evidence from pregnant women is also summarized for both compounds.

Expert opinion:

The azithromycin-chloroquine regimen that has demonstrated consistent efficacy in non-pregnant adults has been a 3-day course containing daily doses of 1 g of azithromycin and 600 mg base of chloroquine. The pharmacokinetic evidence of these compounds individually suggests that dose adjustments may not be necessary when used in combination for treatment efficacy against P. falciparum, P. vivax, as well as several curable STI/ RTI among pregnant women, although clinical confirmation will be necessary. Mass trachoma-treatment campaigns have shown that azithromycin selects for macrolide resistance in the pneumococcus, which reverses following the completion of therapy. Most importantly, no evidence to date suggests that azithromycin induces pneumococcal resistance to penicillin.

Bioorganometallic Chemistry and Malaria

This chapter summarizes recent developments in the design, synthesis, and structure–activity relationship studies of organometallic antimalarials. It begins with a general introduction to malaria and the biology of the parasite Plasmodium falciparum, with a focus on the heme detoxification system. Then, a number of metal complexes from the literature are reported for their antiplasmodial activity. The second half of the chapter deals with the serendipitous discovery of ferroquine, its mechanism(s) of action, and the failure to induce a resistance. Last, but not least, we suggest that the bioorganometallic approach offers the potential for the design of novel therapeutic agents.

Characterization of coproporphyrinogen III oxidase in Plasmodium falciparum cytosol

A unique hybrid pathway has been proposed for de novo heme biosynthesis in Plasmodium falciparum involving three different compartments of the parasite, namely mitochondrion, apicoplast and cytosol. While parasite mitochondrion and apicoplast have been shown to harbor key enzymes of the pathway, there has been no experimental evidence for the involvement of parasite cytosol in heme biosynthesis. In this study, a recombinant P. falciparum coproporphyrinogen III oxidase (rPfCPO) was produced in E. coli and confirmed to be active under aerobic conditions. rPfCPO behaved as a monomer of 61kDa molecular mass in gel filtration analysis. Immunofluorescence studies using antibodies to rPfCPO suggested that the enzyme was present in the parasite cytosol. These results were confirmed by detection of enzyme activity only in the parasite soluble fraction. Western blot analysis with anti-rPfCPO antibodies also revealed a 58kDa protein only in this fraction and not in the membrane fraction. The cytosolic presence of PfCPO provides evidence for a hybrid heme-biosynthetic pathway in the malarial parasite.

Localisation of Plasmodium falciparum uroporphyrinogen III decarboxylase of the heme-biosynthetic pathway in the apicoplast and characterisation of its catalytic properties

Uroporphyrinogen decarboxylase (UROD) is a key enzyme in the heme-biosynthetic pathway and in Plasmodium falciparum it occupies a strategic position in the proposed hybrid pathway for heme biosynthesis involving shuttling of intermediates between different subcellular compartments in the parasite. In the present study, we demonstrate that an N-terminally truncated recombinant P. falciparum UROD (r(Delta)PfUROD) over-expressed and purified from Escherichia coli cells, as well as the native enzyme from the parasite were catalytically less efficient compared with the host enzyme, although they were similar in other enzyme parameters. Molecular modeling of PfUROD based on the known crystal structure of the human enzyme indicated that the protein manifests a distorted triose phosphate isomerase (TIM) barrel fold which is conserved in all the known structures of UROD. The parasite enzyme shares all the conserved or invariant amino acid residues at the active and substrate binding sites, but is rich in lysine residues compared with the host enzyme. Mutation of specific lysine residues corresponding to residues at the dimer interface in human UROD enhanced the catalytic efficiency of the enzyme and dimer stability indicating that the lysine rich nature and weak dimer interface of the wild-type PfUROD could be responsible for its low catalytic efficiency. PfUROD was localised to the apicoplast, indicating the requirement of additional mechanisms for transport of the product coproporphyrinogen to other subcellular sites for its further conversion and ultimate heme formation.

Unique properties of Plasmodium falciparum porphobilinogen deaminase

The hybrid pathway for heme biosynthesis in the malarial parasite proposes the involvement of parasite genome-coded enzymes of the pathway localized in different compartments such as apicoplast, mitochondria, and cytosol. However, knowledge on the functionality and localization of many of these enzymes is not available. In this study, we demonstrate that porphobilinogen deaminase encoded by the Plasmodium falciparum genome (PfPBGD) has several unique biochemical properties. Studies carried out with PfPBGD partially purified from parasite membrane fraction, as well as recombinant PfPBGD lacking N-terminal 64 amino acids expressed and purified from Escherichia coli cells (DeltaPfPBGD), indicate that both the proteins are catalytically active. Surprisingly, PfPBGD catalyzes the conversion of porphobilinogen to uroporphyrinogen III (UROGEN III), indicating that it also possesses uroporphyrinogen III synthase (UROS) activity, catalyzing the next step. This obviates the necessity to have a separate gene for UROS that has not been so far annotated in the parasite genome. Interestingly, DeltaPfP-BGD gives rise to UROGEN III even after heat treatment, although UROS from other sources is known to be heat-sensitive. Based on the analysis of active site residues, a DeltaPfPBGDL116K mutant enzyme was created and the specific activity of this recombinant mutant enzyme is 5-fold higher than DeltaPfPBGD. More interestingly, DeltaPfPBGDL116K catalyzes the formation of uroporphyrinogen I (UROGEN I) in addition to UROGEN III, indicating that with increased PBGD activity the UROS activity of PBGD may perhaps become rate-limiting, thus leading to non-enzymatic cyclization of preuroporphyrinogen to UROGEN I. PfPBGD is localized to the apicoplast and is catalytically very inefficient compared with the host red cell enzyme.

Quantitative pH measurements in Plasmodium falciparum-infected erythrocytes using pHluorin

The digestive vacuole of the malaria parasite Plasmodium falciparum is the site of action of several antimalarial drugs, such as chloroquine, which accumulate in this organelle due to their properties as amphiphilic weak bases that inhibit haem detoxification. It has been suggested that changes in the pH of the digestive vacuole, affecting either drug partitioning or haem solubility and/or biomineralization rates, would correlate with reduced intracellular chloroquine accumulation and, hence, would determine the chloroquine-resistance phenotype. The techniques previously used to quantify digestive vacuolar pH mainly relied on lysed or isolated parasites, with unpredictable consequences on internal pH homeostasis. In this study, we have investigated the baseline steady-state pH of the cytoplasm and digestive vacuole of a chloroquine-sensitive (HB3) and a chloroquine-resistant (Dd2) parasite using a pH-sensitive green fluorescent protein, termed pHluorin. This non-invasive technique allows for in vivo pH measurements in intact P. falciparum-infected erythrocytes under physiological conditions. The data suggest that the pH of the cytoplasm is approximately 7.15 +/- 0.07 and that of the digestive vacuole approximately 5.18 +/- 0.05. No significant differences in baseline pH values were recorded for the chloroquine-sensitive and chloroquine-resistant parasites.

Differences in trans-stimulated chloroquine efflux kinetics are linked to PfCRT in Plasmodium falciparum

The mechanism underpinning chloroquine drug resistance in the human malarial parasite Plasmodium falciparum has remained controversial. Currently discussed models include a carrier or a channel for chloroquine, the former actively expelling the drug, the latter facilitating its passive diffusion, out of the parasite's food vacuole, where chloroquine accumulates and inhibits haem detoxification. Here we have challenged both models using an established trans-stimulation efflux protocol. While carriers may demonstrate trans-stimulation, channels do not. Our data reveal that extracellular chloroquine stimulates chloroquine efflux in the presence and absence of metabolic energy in both chloroquine-sensitive and -resistant parasites, resulting in a hyperbolic increase in the apparent initial efflux rates as the concentration of external chloroquine increases. In the absence of metabolic energy, the apparent initial efflux rates were comparable in both parasites. Significant differences were only observed in the presence of metabolic energy, where consistently higher apparent initial efflux rates were found in chloroquine-resistant parasites. As trans-stimulation is characteristic of a carrier, and not a channel, we interpret our data in favour of a carrier for chloroquine being present in both chloroquine-sensitive and -resistant parasites, however, with different transport modalities.

Plasmodium berghei: In vitro and in vivo activity of dequalinium

Bisquinoline compounds have exhibited remarkable activity in vitro and in vivo against Plasmodium parasites by inhibition of heme detoxification. We have tested the ability of dequalinium 1,1'-(1,10-decanediyl)bis(4-amino-2-methylquinoline), a known antimicrobial agent, to inhibit beta-hematin synthesis using a non-emzymatic colorimetric assay and globin proteolysis by electrophoretic analysis (SDS-PAGE-15%). Dequalinium was able to inhibit both processes in vitro with close correlation to a murine malaria model, reducing parasitemia levels, prolonging the survival time post-infection and curing 40% of infected mice using a combination therapy with a loading dose of chloroquine. These results confirm that dequalinium is a promising lead for antimalarial drug development.

Curcumin-artemisinin combination therapy for malaria

Artemisinin and curcumin show an additive interaction in killing Plasmodium falciparum in culture. In vivo, 3 oral doses of curcumin following a single injection of alpha,beta-arteether to Plasmodium berghei-infected mice are able to prevent recrudescence due to alpha,beta-arteether monotherapy and ensure almost 100% survival of the animals.

DFT study of the reductive decomposition of artemisinin

Artemisinin is a sesquiterpene lactone with an endoperoxide function that is essential for its antimalarial activity. The DFT B3LYP method, together with the 6-31G(d) and 6-31+G(d,p) basis set, is employed to calculate a set of radical anions and neutral species supposed to be formed during the rearrangement of artemisinin from the two radicals (C-centered and O-centered) that are supposed to play a relevant role in the mechanism of action. The B3LYP results show that the primary and the secondary radicals centered on C(4), generated by homolytic break of the C(3)-C(4) bond and by 1,5 hydrogen shift, respectively, are more stable than radicals centered on oxygen. The calculations show that the activation barriers for rearrangements are low, leading to a thermodynamically favorable process. These results reinforce our previous conclusions based on semi-empirical calculations but also give additional information on the reductive decomposition of artemisinin.

Dictyostelium discoideum expresses a malaria chloroquine resistance mechanism upon transfection with mutant, but not wild-type, Plasmodium falciparum transporter PfCRT

Chloroquine resistance in Plasmodium falciparum malaria results from mutations in PfCRT, a member of a unique family of transporters present in apicomplexan parasites and Dictyostelium discoideum. Mechanisms that have been proposed to explain chloroquine resistance are difficult to evaluate within malaria parasites. Here we report on the targeted expression of wild-type and mutant forms of PfCRT to acidic vesicles in D. discoideum. We show that wild-type PfCRT has minimal effect on the accumulation of chloroquine by D. discoideum, whereas forms of PfCRT carrying a key charge-loss mutation of lysine 76 (e.g. K76T) enable D. discoideum to expel chloroquine. As in P. falciparum, the chloroquine resistance phenotype conferred on transformed D. discoideum can be reversed by the channel-blocking agent verapamil. Although intravesicular pH levels in D. discoideum show small acidic changes with the expression of different forms of PfCRT, these changes would tend to promote intravesicular trapping of chloroquine (a weak base) and do not account for reduced drug accumulation in transformed D. discoideum. Our results instead support outward-directed chloroquine efflux for the mechanism of chloroquine resistance by mutant PfCRT. This mechanism shows structural specificity as D. discoideum transformants that expel chloroquine do not expel piperaquine, a bisquinoline analog of chloroquine used frequently against chloroquine-resistant parasites in Southeast Asia. PfCRT, nevertheless, may have some ability to act on quinine and quinidine. Transformed D. discoideum will be useful for further studies of the chloroquine resistance mechanism and may assist in the development and evaluation of new antimalarial drugs.

Localization of ferrochelatase in Plasmodium falciparum

Our previous studies have demonstrated de novo haem biosynthesis in the malarial parasite (Plasmodium falciparum and P. berghei). It has also been shown that the first enzyme of the pathway is the parasite genome-coded ALA (delta-aminolaevulinate) synthase localized in the parasite mitochondrion, whereas the second enzyme, ALAD (ALA dehydratase), is accounted for by two species: one species imported from the host red blood cell into the parasite cytosol and another parasite genome-coded species in the apicoplast. In the present study, specific antibodies have been raised to PfFC (parasite genome-coded ferrochelatase), the terminal enzyme of the haem-biosynthetic pathway, using recombinant truncated protein. With the use of these antibodies as well as those against the hFC (host red cell ferrochelatase) and other marker proteins, immunofluorescence studies were performed. The results reveal that P. falciparum in culture manifests a broad distribution of hFC and a localized distribution of PfFC in the parasite. However, PfFC is not localized to the parasite mitochondrion. Immunoelectron-microscopy studies reveal that PfFC is indeed localized to the apicoplast, whereas hFC is distributed in the parasite cytoplasm. These results on the localization of PfFC are unexpected and are at variance with theoretical predictions based on leader sequence analysis. Biochemical studies using the parasite cytosolic and organellar fractions reveal that the cytosol containing hFC accounts for 80% of FC enzymic activity, whereas the organellar fraction containing PfFC accounts for the remaining 20%. Interestingly, both the isolated cytosolic and organellar fractions are capable of independent haem synthesis in vitro from [4-14C]ALA, with the cytosol being three times more efficient compared with the organellar fraction. With [2-14C]glycine, most of the haem is synthesized in the organellar fraction. Thus haem is synthesized in two independent compartments: in the cytosol, using the imported host enzymes, and in the organellar fractions, using the parasite genome-coded enzymes.

The pathway of hemoglobin degradation in malaria parasites

Hemoglobin degradation in malaria parasites appears to be a vital and massive undertaking during intraerythrocytic development. An ordered pathway exists that efficiently catabolizes hemoglobin to yield amino acids that the parasite uses as a nutrient source, and that co-ordinates the resultant heme into a remarkable crystalline structure called hemozoin. Daniel Goldberg and Andrew Slater explain that the enzymes involved are specific and present prime targets for chemotherapeutic intervention.

1,2,4-Triazino-[5,6b]indole derivatives: effects of the trifluoromethyl group on in vitro antimalarial activity

In an attempt to search for new and alternative antimalarial agents, a series of unsubstituted and 6-trifluoromethyl-1,2,4-triazino[5,6b]indole and 5H-1,2,4-triazolo[1',5',2,3]-1,2,4-triazino[5,6b]indole derivatives were synthesized and their chemical structures confirmed by 1H NMR and 13C NMR, elemental, IR and mass spectrophotometric analyses. The in vitro antimalarial activities of these compounds were evaluated against the chloroquine-sensitive (D10) and the chloroquine-resistant (RSA11) strains of Plasmodium falciparum. The 1,2,4-triazino[5,6b]indole derivatives (4, 6 and 8) with a trifluoromethyl group at position 6 exhibit increased in vitro activity when compared to the unsubstituted analogues, which are all devoid of activity. The presence of the trifluoromethyl group in the 5H-1,2,4-triazolo[1',5',2,3]-1,2,4-triazino[5,6b]indole ring system leads to compounds with diminished antimalarial activity when compared to the corresponding unsubstituted analogues. The compounds associate with ferriprotoporphyrin IX and interact with DNA to more or less the same extent.

Arylsulfonyl acridinyl derivatives acting on Plasmodium falciparum

Several arylacridinyl sulfones have been synthesized and their antimalarial action was tested on Plasmodium falciparum. PABA (para-aminobenzoic acid) has no antagonistic effect with these compounds as opposed to the observed effect with dapsone and sulfonamides previously studied. A possible relationship between the ability of cleavage of the S-9C acridinic bond and activity is suggested.

Evidence for Activation of Endogenous Transporters in Xenopus laevis Oocytes Expressing the Plasmodium falciparum Chloroquine Resistance Transporter, PfCRT

A large body of genetic, reverse genetic, and epidemiological data has linked chloroquine-resistant malaria to polymorphisms within a gene termed pfcrt in the human malarial parasite Plasmodium falciparum. To investigate the biological function of the chloroquine resistance transporter, PfCRT, as well as its role in chloroquine resistance, we functionally expressed this protein in Xenopus laevis oocytes. Our data show that PfCRT-expressing oocytes exhibit a depolarized resting membrane potential and a higher intracellular pH compared with control oocytes. Pharmacological and electrophysiological studies link the higher intracellular pH to an enhanced amiloride-sensitive H(+) extrusion and the low membrane potential to an activated nonselective cation conductance. The finding that both properties are independent of each other, together with the fact that they are endogenously present in X. laevis oocytes, supports a model in which PfCRT activates transport systems. Our data suggest that PfCRT plays a role as a direct or indirect activator or modulator of other transporters.

Ferriprotoporphyrin IX, phospholipids, and the antimalarial actions of quinoline drugs

Two subclasses of quinoline antimalarial drugs are used clinically. Both act on the endolysosomal system of malaria parasites, but in different ways. Treatment with 4-aminoquinoline drugs, such as chloroquine, causes morphologic changes and hemoglobin accumulation in endocytic vesicles. Treatment with quinoline-4-methanol drugs, such as quinine and mefloquine, also causes morphologic changes, but does not cause hemoglobin accumulation. In addition, chloroquine causes undimerized ferriprotoporphyrin IX (ferric heme) to accumulate whereas quinine and mefloquine do not. On the contrary, treatment with quinine or mefloquine prevents and reverses chloroquine-induced accumulation of hemoglobin and undimerized ferriprotoporphyrin IX. This difference is of particular interest since there is convincing evidence that undimerized ferriprotoporphyrin IX in malaria parasites would interact with and serve as a target for chloroquine. According to the ferriprotoporphyrin IX interaction hypothesis, chloroquine would bind to undimerized ferriprotoporphyrin IX, delay its detoxification, cause it to accumulate, and allow it to exert its intrinsic biological toxicities. The ferriprotoporphyrin IX interaction hypothesis appears to explain the antimalarial action of chloroquine, but a drug target in addition to ferriprotoporphyrin IX is suggested by the antimalarial actions of quinine and mefloquine. This article summarizes current knowledge of the role of ferriprotoporphyrin IX in the antimalarial actions of quinoline drugs and evaluates the currently available evidence in support of phospholipids as a second target for quinine, mefloquine and, possibly, the chloroquine-ferriprotoporphyrin IX complex.

Spectrophotometric determination of de novo hemozoin/β-hematin formation in an in vitro assay

Formation of hemozoin in the malaria parasite, due to its unique nature, is an attractive molecular target. Several laboratories have been trying to unravel the molecular mechanism of hemozoin biosynthesis within the parasite digestive vacuoles. Use of different assay protocols for in vitro beta-hematin (synthetic identical to hemozoin) formation by these laboratories has led to inconsistent and often contradictory findings. Much of the difficulty may be attributed to oligomeric heme aggregates, which may be indistinguishable in some detection approaches if adequate separation of beta-hemtin is not achieved. Therefore, there is an urgent need for a widely accepted protocol for in vitro beta-hematin formation. We describe here a spectrophotometric assay for in vitro beta-hematin formation. The assay has been validated with the Plasmodium falciparum lysate, the parasite lipid extracts, and some commercially available fatty acids, which are known to initiate/catalyze beta-hematin formation in vitro. The necessity for multiple wash steps for accurate quantification of de novo hemozoin/beta-hematin formation was verified experimentally. It was necessary to wash the pellet, which contains beta-hematin and heme aggregates, sequentially with Tris/SDS buffer and alkaline bicarbonate solution for complete removal of monomeric heme and heme aggregates and accurate quantification of beta-hematin formed during the assay. The pellets and side products in the supernatant were characterized by infrared spectroscopy. No beta-hematin formation occurred in the absence of a catalytic/initiating factor. Based on these findings, a filtration-based assay that uses 96-well microplates, and which has important application in in vitro screening and identification of novel inhibitors of hemozoin formation as potential blood schizontocidal antimalarials, has been developed.

Delta-aminolevulinic acid dehydratase from Plasmodium falciparum: indigenous versus imported

The heme biosynthetic pathway of the malaria parasite is a drug target and the import of host delta-aminolevulinate dehydratase (ALAD), the second enzyme of the pathway, from the red cell cytoplasm by the intra erythrocytic malaria parasite has been demonstrated earlier in this laboratory. In this study, ALAD encoded by the Plasmodium falciparum genome (PfALAD) has been cloned, the protein overexpressed in Escherichia coli, and then characterized. The mature recombinant enzyme (rPfALAD) is enzymatically active and behaves as an octamer with a subunit Mr of 46,000. The enzyme has an alkaline pH optimum of 8.0 to 9.0. rPfALAD does not require any metal ion for activity, although it is stimulated by 20-30% upon addition of Mg2+. The enzyme is inhibited by Zn2+ and succinylacetone. The presence of PfALAD in P. falciparum can be demonstrated by Western blot analysis and immunoelectron microscopy. The enzyme has been localized to the apicoplast of the malaria parasite. Homology modeling studies reveal that PfALAD is very similar to the enzyme species from Pseudomonas aeruginosa, but manifests features that are unique and different from plant ALADs as well as from those of the bacterium. It is concluded that PfALAD, while resembling plant ALADs in terms of its alkaline pH optimum and apicoplast localization, differs in its Mg2+ independence for catalytic activity or octamer stabilization. Expression levels of PfALAD in P. falciparum, based on Western blot analysis, immunoelectron microscopy, and EDTA-resistant enzyme activity assay reveals that it may account for about 10% of the total ALAD activity in the parasite, the rest being accounted for by the host enzyme imported by the parasite. It is proposed that the role of PfALAD may be confined to heme synthesis in the apicoplast that may not account for the total de novo heme biosynthesis in the parasite.