Central role of hemoglobin degradation in mechanisms of action of 4-aminoquinolines, quinoline methanols, and phenanthrene methanols

Stereospecific Resistance to N2-Acyl Tetrahydro-β-carboline Antimalarials Is Mediated by a PfMDR1 Mutation That Confers Collateral Drug Sensitivity

Half the world's population is at risk of developing a malaria infection, which is caused by parasites of the genus Plasmodium. Currently, resistance has been identified to all clinically available antimalarials, highlighting an urgent need to develop novel compounds and better understand common mechanisms of resistance. We previously identified a novel tetrahydro-β-carboline compound, PRC1590, which potently kills the malaria parasite. To better understand its mechanism of action, we selected for and characterized resistance to PRC1590 in Plasmodium falciparum. Through in vitro selection of resistance to PRC1590, we have identified that a single-nucleotide polymorphism on the parasite's multidrug resistance protein 1 (PfMDR1 G293V) mediates resistance to PRC1590. This mutation results in stereospecific resistance and sensitizes parasites to other antimalarials, such as mefloquine, quinine, and MMV019017. Intraerythrocytic asexual stage specificity assays have revealed that PRC1590 is most potent during the trophozoite stage when the parasite forms a single digestive vacuole (DV) and actively digests hemoglobin. Moreover, fluorescence microscopy revealed that PRC1590 disrupts the function of the DV, indicating a potential molecular target associated with this organelle. Our findings mark a significant step in understanding the mechanism of resistance and the mode of action of this emerging class of antimalarials. In addition, our results suggest a potential link between resistance mediated by PfMDR1 and PRC1590's molecular target. This research underscores the pressing need for future research aimed at investigating the intricate relationship between a compound's chemical scaffold, molecular target, and resistance mutations associated with PfMDR1.

Optical diffraction tomography and image reconstruction to measure host cell alterations caused by divergent Plasmodium species

Malaria is a life-threatening infectious disease caused by parasites of the genus Plasmodium. Understanding the biological features of various parasite forms is important for the optical diagnosis and defining pathological states, which are often constrained by the lack of ambient visualization approaches. Here, we employ a label-free tomographic technique to visualize the host red blood cell (RBC) remodeling process and quantify changes in biochemical properties arising from parasitization. Through this, we provide a quantitative body of information pertaining to the influence of host cell environment on growth, survival, and replication of P. falciparum and P. vivax in their respective host cells: mature erythrocytes and young reticulocytes. These exquisite three-dimensional measurements of infected red cells demonstrats the potential of evolving 3D imaging to advance our understanding of Plasmodium biology and host-parasite interactions.

Complementary crosstalk between palmitoylation and phosphorylation events in MTIP regulates its role during Plasmodium falciparum invasion

Post-translational modifications (PTMs) including phosphorylation and palmitoylation have emerged as crucial biomolecular events that govern many cellular processes including functioning of motility- and invasion-associated proteins during Plasmodium falciparum invasion. However, no study has ever focused on understanding the possibility of a crosstalk between these two molecular events and its direct impact on preinvasion- and invasion-associated protein–protein interaction (PPI) network-based molecular machinery. Here, we used an integrated in silico analysis to enrich two different catalogues of proteins: (i) the first group defines the cumulative pool of phosphorylated and palmitoylated proteins, and (ii) the second group represents a common set of proteins predicted to have both phosphorylation and palmitoylation. Subsequent PPI analysis identified an important protein cluster comprising myosin A tail interacting protein (MTIP) as one of the hub proteins of the glideosome motor complex in P. falciparum, predicted to have dual modification with the possibility of a crosstalk between the same. Our findings suggested that blocking palmitoylation led to reduced phosphorylation and blocking phosphorylation led to abrogated palmitoylation of MTIP. As a result of the crosstalk between these biomolecular events, MTIP’s interaction with myosin A was found to be abrogated. Next, the crosstalk between phosphorylation and palmitoylation was confirmed at a global proteome level by click chemistry and the phenotypic effect of this crosstalk was observed via synergistic inhibition in P. falciparum invasion using checkerboard assay and isobologram method. Overall, our findings revealed, for the first time, an interdependence between two PTM types, their possible crosstalk, and its direct impact on MTIP-mediated invasion via glideosome assembly protein myosin A in P. falciparum. These insights can be exploited for futuristic drug discovery platforms targeting parasite molecular machinery for developing novel antimalarial therapeutics.

Different Drugs, Same End: Ultrastructural Hallmarks of Autophagy in Pathogenic Protozoa

Protozoan parasites interact with a wide variety of organisms ranging from bacteria to humans, representing one of the most common causes of parasitic diseases and an important public health problem affecting hundreds of millions of people worldwide. The current treatment for these parasitic diseases remains unsatisfactory and, in some cases, very limited. Treatment limitations together with the increased resistance of the pathogens represent a challenge for the improvement of the patient’s quality of life. The continuous search for alternative preclinical drugs is mandatory, but the mechanisms of action of several of these compounds have not been described. Electron microscopy is a powerful tool for the identification of drug targets in almost all cellular models. Interestingly, ultrastructural analysis showed that several classes of antiparasitic compounds induced similar autophagic phenotypes in trypanosomatids, trichomonadids, and apicomplexan parasites as well as in Giardia intestinalis and Entamoeba spp. with the presence of an increased number of autophagosomes as well as remarkable endoplasmic reticulum profiles surrounding different organelles. Autophagy is a physiological process of eukaryotes that maintains homeostasis by the self-digestion of nonfunctional organelles and/or macromolecules, limiting redundant and damaged cellular components. Here, we focus on protozoan autophagy to subvert drug effects, discussing its importance for successful chemotherapy.

Recent contributions of quinolines to antimalarial and anticancer drug discovery research

Quinoline, a privileged scaffold in medicinal chemistry, has always been associated with a multitude of biological activities. Especially in antimalarial and anticancer research, quinoline played (and still plays) a central role, giving rise to the development of an array of quinoline-containing pharmaceuticals in these therapeutic areas. However, both diseases still affect millions of people every year, pointing to the necessity of new therapies. Quinolines have a long-standing history as antimalarial agents, but established quinoline-containing antimalarial drugs are now facing widespread resistance of the Plasmodium parasite. Nevertheless, as evidenced by a massive number of recent literature contributions, they are still of great value for future developments in this field. On the other hand, the number of currently approved anticancer drugs containing a quinoline scaffold are limited, but a strong increase and interest in quinoline compounds as potential anticancer agents can be seen in the last few years. In this review, a literature overview of recent contributions made by quinoline-containing compounds as potent antimalarial or anticancer agents is provided, covering publications between 2018 and 2020.

Xylopic acid-amodiaquine and xylopic acid-artesunate combinations are effective in managing malaria in Plasmodium berghei-infected mice

Background

Evidence of Plasmodium resistance to some of the current anti-malarial agents makes it imperative to search for newer and effective drugs to combat malaria. Therefore, this study evaluated whether the co-administrations of xylopic acid-amodiaquine and xylopic acid-artesunate combinations will produce a synergistic anti-malarial effect.

Methods

Antiplasmodial effect of xylopic acid (XA: 3, 10, 30, 100, 150 mg kg⁻¹), artesunate (ART: 1, 2, 4, 8, 16 mg kg⁻¹), and amodiaquine (AQ: 1.25, 2.5, 5, 10, 20 mg kg⁻¹) were evaluated in Plasmodium berghei (strain ANKA)-infected mice to determine respective ED₅₀s. Artemether/lumefantrine was used as the positive control. XA/ART and XA/AQ were subsequently administered in a fixed-dose combination of their ED₅₀s (1:1) and the combination fractions of their ED₅₀s (1/2, 1/4, 1/8, 1/16, and 1/32) to determine the experimental ED₅₀s (Z_exp). An isobologram was constructed to determine the nature of the interaction between XA/ART, and XA/AQ combinations by comparing Z_exp with the theoretical ED₅₀ (Z_add). Bodyweight and 30-day survival post-treatment were additionally recorded.

Results

ED₅₀s for XA, ART, and AQ were 9.0 ± 3.2, 1.61 ± 0.6, and 3.1 ± 0.8 mg kg⁻¹, respectively. The Z_add, Z_exp, and interaction index for XA/ART co-administration was 5.3 ± 2.61, 1.98 ± 0.25, and 0.37, respectively while that of XA/AQ were 6.05 ± 2.0, 1.69 ± 0.42, and 0.28, respectively. The Z_exp for both combination therapies lay significantly (p < 0.001) below the additive isoboles showing XA acts synergistically with both ART and AQ in clearing the parasites. High doses of XA/ART combination significantly (p < 0.05) increased the survival days of infected mice with a mean hazard ratio of 0.40 while all the XA/AQ combination doses showed a significant (p < 0.05) increase in the survival days of infected mice with a mean hazard ratio of 0.27 similar to AL. Both XA/ART and XA/AQ combined treatments significantly (p < 0.05) reduced weight loss.

Conclusion

Xylopic acid co-administration with either artesunate or amodiaquine produces a synergistic anti-plasmodial effect in mice infected with P. berghei.

An insight into the recent development of the clinical candidates for the treatment of malaria and their target proteins

Malaria is an endemic disease, prevalent in tropical and subtropical regions which cost half of million deaths annually. The eradication of malaria is one of the global health priority nevertheless, current therapeutic efforts seem to be insufficient due to the emergence of drug resistance towards most of the available drugs, even first-line treatment ACT, unavailability of the vaccine, and lack of drugs with a new mechanism of action. Intensification of antimalarial research in recent years has resulted into the development of single dose multistage therapeutic agents which has advantage of overcoming the antimalarial drug resistance. The present review explored the current progress in the development of new promising antimalarials against prominent target proteins that have the potential to be a clinical candidate. Here, we also reviewed different aspects of drug resistance and highlighted new drug candidates that are currently in a clinical trial or clinical development, along with a few other molecules with excellent antimalarial activity overs ACTs. The summarized scientific value of previous approaches and structural features of antimalarials related to the activity are highlighted that will be helpful for the development of next-generation antimalarials.

Spirofused tetrahydroisoquinoline-oxindole hybrids as a novel class of fast acting antimalarial agents with multiple modes of action

Molecular hybridization of privileged scaffolds may generate novel antiplasmodial chemotypes that display superior biological activity and delay drug resistance. In the present study, we describe the in vitro activities and mode of action of 3′,4′-dihydro-2′H-spiro[indoline-3,1′-isoquinolin]-2-ones, a novel class of spirofused tetrahydroisoquinoline–oxindole hybrids, as novel antimalarial agents. Whole cell phenotypic screening of these compounds identified (14b), subsequently named (±)-moxiquindole, as the most potent compound in the current series with equipotent antiplasmodial activity against both chloroquine sensitive and multidrug resistant parasite strains with good selectivity. The compound was active against all asexual stages of the parasite including inhibition of merozoite egress. Additionally, (±)-moxiquindole exhibited significant inhibitory effects on hemoglobin degradation, and disrupted vacuolar lipid dynamics. Taken together, our data confirm the antiplasmodial activity of (±)-moxiquindole, and identify 3′4′-dihydro-2′H-spiro[indoline-3,1′-isoquinolin]-2-ones as a novel class of antimalarial agents with multiple modes of action.

Synthesis and biological evaluation of novel quinoline-piperidine scaffolds as antiplasmodium agents

The parasitic disease malaria places almost half of the world's population at risk of infection and is responsible for more than 400,000 deaths each year. The first-line treatment, artemisinin combination therapies (ACT) regimen, is under threat due to emerging resistance of Plasmodium falciparum strains in e.g. the Mekong delta. Therefore, the development of new antimalarial agents is crucial in order to circumvent the growing resistance. Chloroquine, the long-established antimalarial drug, still serves as model compound for the design of new quinoline analogues, resulting in numerous new active derivatives against chloroquine-resistant P. falciparum strains over the past twenty years. In this work, a set of functionalized quinoline analogues, decorated with a modified piperidine-containing side chain, was synthesized. Both amino- and (aminomethyl)quinolines were prepared, resulting in a total of 18 novel quinoline-piperidine conjugates representing four different chemical series. Evaluation of their in vitro antiplasmodium activity against a CQ-sensitive (NF54) and a CQ-resistant (K1) strain of P. falciparum unveiled highly potent activities in the nanomolar range against both strains for five 4-aminoquinoline derivatives. Moreover, no cytotoxicity was observed for all active compounds at the maximum concentration tested. These five new aminoquinoline hit structures are therefore of considerable value for antimalarial research and have the potency to be transformed into novel antimalarial agents upon further hit-to-lead optimization studies.

Drug targets for resistant malaria: Historic to future perspectives

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

Molecular Mechanisms for Drug Hypersensitivity Induced by the Malaria Parasite's Chloroquine Resistance Transporter

Mutations in the Plasmodium falciparum ‘chloroquine resistance transporter’ (PfCRT) confer resistance to chloroquine (CQ) and related antimalarials by enabling the protein to transport these drugs away from their targets within the parasite’s digestive vacuole (DV). However, CQ resistance-conferring isoforms of PfCRT (PfCRT^CQR) also render the parasite hypersensitive to a subset of structurally-diverse pharmacons. Moreover, mutations in PfCRT^CQR that suppress the parasite’s hypersensitivity to these molecules simultaneously reinstate its sensitivity to CQ and related drugs. We sought to understand these phenomena by characterizing the functions of PfCRT^CQR isoforms that cause the parasite to become hypersensitive to the antimalarial quinine or the antiviral amantadine. We achieved this by measuring the abilities of these proteins to transport CQ, quinine, and amantadine when expressed in Xenopus oocytes and complemented this work with assays that detect the drug transport activity of PfCRT in its native environment within the parasite. Here we describe two mechanistic explanations for PfCRT-induced drug hypersensitivity. First, we show that quinine, which normally accumulates inside the DV and therewithin exerts its antimalarial effect, binds extremely tightly to the substrate-binding site of certain isoforms of PfCRT^CQR. By doing so it likely blocks the normal physiological function of the protein, which is essential for the parasite’s survival, and the drug thereby gains an additional killing effect. In the second scenario, we show that although amantadine also sequesters within the DV, the parasite’s hypersensitivity to this drug arises from the PfCRT^CQR-mediated transport of amantadine from the DV into the cytosol, where it can better access its antimalarial target. In both cases, the mutations that suppress hypersensitivity also abrogate the ability of PfCRT^CQR to transport CQ, thus explaining why rescue from hypersensitivity restores the parasite’s sensitivity to this antimalarial. These insights provide a foundation for understanding clinically-relevant observations of inverse drug susceptibilities in the malaria parasite.

In acquiring resistance to one drug, many pathogens and cancer cells become hypersensitive to other drugs. This phenomenon could be exploited to combat existing drug resistance and to delay the emergence of resistance to new drugs. However, much remains to be understood about the mechanisms that underlie drug hypersensitivity in otherwise drug-resistant microbes. Here, we describe two mechanisms by which the Plasmodium falciparum ‘chloroquine resistance transporter’ (PfCRT) causes the malaria parasite to become hypersensitive to structurally-diverse drugs. First, we show that an antimalarial drug that normally exerts its killing effect within the parasite’s digestive vacuole is also able to bind extremely tightly to certain forms of PfCRT. This activity will block the natural, essential function of the protein and thereby provide the drug with an additional killing effect. The second mechanism arises when a cytosolic-acting drug that normally sequesters within the digestive vacuole is leaked back into the cytosol via PfCRT. In both cases, mutations that suppress hypersensitivity also abrogate the ability of PfCRT to transport chloroquine, thus explaining why rescue from hypersensitivity restores the parasite’s sensitivity to this antimalarial. These insights provide a foundation for understanding and exploiting the hypersensitivity of chloroquine-resistant parasites to several of the current antimalarials.

Lopinavir/ritonavir enhanced the antimalarial activity of amodiaquine and artesunate in a mouse model of Plasmodium berghei

Treatment of malaria and HIV in co-infected patients remains a challenge due to the limited information on interaction between drugs used for the treatment of the two infections. Thus, this study evaluated the interaction between lopinavir/ritonavir (LR) and artesunate (AS), amodiaquine (AQ) or a fixed dose of AS/AQ in a mouse model of chloroquine-resistant Plasmodium berghei. Combination of LR with graded doses of AS or AQ resulted in a significant reduced ED₅₀. In addition, parasites cleared completely from day 3 till day 21 post-infection in animals infected, treated with AS/AQ alone or AS/AQ with LR and all the animals survived till day 21 post-infection. In contrast, survival on day 21 in animals treated with AQ alone or AQ with LR was 20 and 60%, respectively. It appears that the protease inhibitor LR enhanced the antimalarial drugs AS and AQ.

An integrative analysis of small molecule transcriptional responses in the human malaria parasite Plasmodium falciparum

Background

Transcriptional responses to small molecules can provide insights into drug mode of action (MOA). The capacity of the human malaria parasite, Plasmodium falciparum, to respond specifically to transcriptional perturbations has been unclear based on past approaches. Here, we present the most extensive profiling to date of the parasite’s transcriptional responsiveness to thirty-one chemically and functionally diverse small molecules.

Methods

We exposed two laboratory strains of the human malaria parasite P. falciparum to brief treatments of thirty-one chemically and functionally diverse small molecules associated with biological effects across multiple pathways based on various levels of evidence. We investigated the impact of chemical composition and MOA on gene expression similarities that arise between perturbations by various compounds. To determine the target biological pathways for each small molecule, we developed a novel framework for encoding small molecule effects on a spectra of biological processes or GO functions that are enriched in the differentially expressed genes of a given small molecule perturbation.

Results

We find that small molecules associated with similar transcriptional responses contain similar chemical features, and/ or have a shared MOA. The approach also revealed complex relationships between drugs and biological pathways that are missed by most exisiting approaches. For example, the approach was able to partition small molecule responses into drug-specific effects versus non-specific effects.

Conclusions

Our work provides a new framework for linking transcriptional responses to drug MOA in P. falciparum and can be generalized for the same purpose in other organisms.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2165-1) contains supplementary material, which is available to authorized users.

Asparagine requirement in Plasmodium berghei as a target to prevent malaria transmission and liver infections

The proteins of Plasmodium, the malaria parasite, are strikingly rich in asparagine. Plasmodium depends primarily on host haemoglobin degradation for amino acids and has a rudimentary pathway for amino acid biosynthesis, but retains a gene encoding asparagine synthetase (AS). Here we show that deletion of AS in Plasmodium berghei (Pb) delays the asexual- and liver-stage development with substantial reduction in the formation of ookinetes, oocysts and sporozoites in mosquitoes. In the absence of asparagine synthesis, extracellular asparagine supports suboptimal survival of PbAS knockout (KO) parasites. Depletion of blood asparagine levels by treating PbASKO-infected mice with asparaginase completely prevents the development of liver stages, exflagellation of male gametocytes and the subsequent formation of sexual stages. In vivo supplementation of asparagine in mice restores the exflagellation of PbASKO parasites. Thus, the parasite life cycle has an absolute requirement for asparagine, which we propose could be targeted to prevent malaria transmission and liver infections.

Malaria parasites obtain amino acids primarily from the host, but possess a gene encoding a putative asparagine synthetase. Here, the authors show that this enzyme is functional and that asparagine is crucial for the development of the parasite's sexual stages in mosquitoes and liver stages in mice.

Verapamil-Sensitive Transport of Quinacrine and Methylene Blue via the Plasmodium falciparum Chloroquine Resistance Transporter Reduces the Parasite's Susceptibility to these Tricyclic Drugs

BACKGROUND

It is becoming increasingly apparent that certain mutations in the Plasmodium falciparum chloroquine resistance transporter (PfCRT) alter the parasite's susceptibility to diverse compounds. Here we investigated the interaction of PfCRT with 3 tricyclic compounds that have been used to treat malaria (quinacrine [QC] and methylene blue [MB]) or to study P. falciparum (acridine orange [AO]).

METHODS

We measured the antiplasmodial activities of QC, MB, and AO against chloroquine-resistant and chloroquine-sensitive P. falciparum and determined whether QC and AO affect the accumulation and activity of chloroquine in these parasites. We also assessed the ability of mutant (PfCRT(Dd2)) and wild-type (PfCRT(D10)) variants of the protein to transport QC, MB, and AO when expressed at the surface of Xenopus laevis oocytes.

RESULTS

Chloroquine resistance-conferring isoforms of PfCRT reduced the susceptibility of the parasite to QC, MB, and AO. In chloroquine-resistant (but not chloroquine-sensitive) parasites, AO and QC increased the parasite's accumulation of, and susceptibility to, chloroquine. All 3 compounds were shown to bind to PfCRT(Dd2), and the transport of QC and MB via this protein was saturable and inhibited by the chloroquine resistance-reverser verapamil.

CONCLUSIONS

Our findings reveal that the PfCRT(Dd2)-mediated transport of tricyclic antimalarials reduces the parasite's susceptibility to these drugs.

PfMDR2 and PfMDR5 are dispensable for Plasmodium falciparum asexual parasite multiplication but change in vitro susceptibility to anti-malarial drugs

Background

Membrane-associated ATP binding cassette (ABC) transport proteins hydrolyze ATP in order to translocate a broad spectrum of substrates, from single ions to macromolecules across membranes. In humans, members from this transport family have been linked to drug resistance phenotypes, e.g., tumour resistance by enhanced export of chemotherapeutic agents from cancer cells due to gene amplifications or polymorphisms in multidrug resistance (MDR) protein 1. Similar mechanisms have linked the Plasmodium falciparum PfMDR1 transporter to anti-malarial drug resistance acquisition. In this study, the possible involvement of two related MDR proteins, PfMDR2 and PfMDR5, to emerging drug resistance is investigated by a reverse genetics approach.

Methods

A homologous double crossover strategy was used to generate P. falciparum parasites lacking the Pfmdr2 (PfΔmdr2) or Pfmdr5 (PfΔmdr5) gene. Plasmodium lactate dehydrogenase activity was used as read-out for sensitivity to artemisinin (ART), atovaquone (ATO), dihydroartemisinin (DHA), chloroquine (CQ), lumefantrine (LUM), mefloquine (MQ), and quinine (QN). Differences in half maximal inhibitory concentration (IC₅₀) values between wild type and each mutant line were determined using a paired t-test.

Results

Both PfΔmdr2 and PfΔmdr5 clones were capable of asexual multiplication. Upon drug exposure, PfΔmdr2 showed a marginally decreased sensitivity to ATO (IC₅₀ of 1.2 nM to 1.8 nM), MQ (124 nM to 185 nM) and QN (40 nM to 70 nM), as compared to wild type (NF54) parasites. On the other hand, PfΔmdr5 showed slightly increased sensitivity to ART (IC₅₀ of 26 nM to 19 nM).

Conclusion

Both Pfmdr2 and Pfmdr5 are dispensable for blood stage development while the deletion lines show altered sensitivity profiles to commonly used anti-malarial drugs. The findings show for the first time that next to PfMDR2, the PfMDR5 transport protein could play a role in emerging drug resistance.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-015-0581-y) contains supplementary material, which is available to authorized users.

4-Aminoquinoline-hybridization en route towards the development of rationally designed antimalarial agents

Recent developments in 4-aminoquinoline-hybridization, as an attractive strategy for averting and delaying the drug resistance along with improvement in efficacy of new antimalarials, are described.

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

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

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

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

Mutation in the Plasmodium falciparum CRT protein determines the stereospecific activity of antimalarial cinchona alkaloids

The Cinchona alkaloids are quinoline aminoalcohols that occur as diastereomer pairs, typified by (-)-quinine and (+)-quinidine. The potency of (+)-isomers is greater than the (-)-isomers in vitro and in vivo against Plasmodium falciparum malaria parasites. They may act by the inhibition of heme crystallization within the parasite digestive vacuole in a manner similar to chloroquine. Earlier studies showed that a K76I mutation in the digestive vacuole-associated protein, PfCRT (P. falciparum chloroquine resistance transporter), reversed the normal potency order of quinine and quinidine toward P. falciparum. To further explore PfCRT-alkaloid interactions in the malaria parasite, we measured the in vitro susceptibility of eight clonal lines of P. falciparum derived from the 106/1 strain, each containing a unique pfcrt allele, to four Cinchona stereoisomer pairs: quinine and quinidine; cinchonidine and cinchonine; hydroquinine and hydroquinidine; 9-epiquinine and 9-epiquinidine. Stereospecific potency of the Cinchona alkaloids was associated with changes in charge and hydrophobicity of mutable PfCRT amino acids. In isogenic chloroquine-resistant lines, the IC(50) ratio of (-)/(+) CA pairs correlated with side chain hydrophobicity of the position 76 residue. Second-site PfCRT mutations negated the K76I stereospecific effects: charge-change mutations C72R or Q352K/R restored potency patterns similar to the parent K76 line, while V369F increased susceptibility to the alkaloids and nullified stereospecific differences between alkaloid pairs. Interactions between key residues of the PfCRT channel/transporter with (-) and (+) alkaloids are stereospecifically determined, suggesting that PfCRT binding plays an important role in the antimalarial activity of quinine and other Cinchona alkaloids.

Inhibitory effects of pepstatin A and mefloquine on the growth of Babesia parasites

We evaluated the inhibitory effects of pepstatin A and mefloquine on the in vitro and in vivo growths of Babesia parasites. The in vitro growth of Babesia bovis, B. bigemina, B. caballi, and B. equi was significantly inhibited (P < 0.05) by micromolar concentrations of pepstatin A (50% inhibitory concentrations = 38.5, 36.5, 17.6, and 18.1 μM, respectively) and mefloquine (50% inhibitory concentrations = 59.7, 56.7, 20.7, and 4 μM, respectively). Furthermore, both reagents either alone at a concentration of 5 mg/kg or in combinations (2.5/2.5 and 5/5 mg/kg) for 10 days significantly inhibited the in vivo growth of B. microti in mice. Mefloquine treatment was highly effective and the combination treatments were less effective than other treatments. Therefore, mefloquine may antagonize the actions of pepstatin A against babesiosis and aspartic proteases may play an important role in the asexual growth cycle of Babesia parasites.

Know your enemy: understanding the role of PfCRT in drug resistance could lead to new antimalarial tactics

The prevention and treatment of malaria is heavily dependent on antimalarial drugs. However, beginning with the emergence of chloroquine (CQ)-resistant Plasmodium falciparum parasites 50 years ago, efforts to control the disease have been thwarted by failed or failing drugs. Mutations in the parasite's 'chloroquine resistance transporter' (PfCRT) are the primary cause of CQ resistance. Furthermore, changes in PfCRT (and in several other transport proteins) are associated with decreases or increases in the parasite's susceptibility to a number of other antimalarial drugs. Here, we review recent advances in our understanding of CQ resistance and discuss these in the broader context of the parasite's susceptibilities to other quinolines and related drugs. We suggest that PfCRT can be viewed both as a 'multidrug-resistance carrier' and as a drug target, and that the quinoline-resistance mechanism is a potential 'Achilles' heel' of the parasite. We examine a number of the antimalarial strategies currently undergoing development that are designed to exploit the resistance mechanism, including relatively simple measures, such as alternative CQ dosages, as well as new drugs that either circumvent the resistance mechanism or target it directly.

Antischistosomal activities of mefloquine-related arylmethanols

Interesting antischistosomal properties have been documented for the antimalarial mefloquine, a 4-quinolinemethanol. We evaluated the antischistosomal activities of nine mefloquine-related compounds belonging to the 4-pyridinemethanols, 9-phenanthrenmethanols, and 4-quinolinemethanols. Eight compounds revealed high activities against Schistosoma mansoni in vitro, with two drugs (the 4-quinolinemethanols WR7573 and WR7930) characterized by significantly lower half-maximal inhibitory concentrations (IC(50)s) (2.7 and 3.5 μM, respectively) compared to mefloquine (11.4 μM). Mefloquine and WR7930 showed significantly decreased IC(50)s when incubated in the presence of hemoglobin. High worm burden reductions (WBR) were obtained with enpiroline (WBR, 82.7%; dosage, 200 mg/kg of body weight) and its threo isomers (+)-threo (WBR, 100%) and (-)-threo (WBR, 89%) and with WR7930 (WBR, 87%; dosage, 100 mg/kg) against adult S. mansoni in mice. Furthermore, excellent in vitro and in vivo antischistosomal activity was observed for two WR7930-related structures (WR29252 and WR7524). In addition, mefloquine (WBR, 81%), enpiroline (WBR, 77%), and WR7930 (WBR, 100%) showed high activities against S. haematobium harbored in mice following single oral doses of 200 mg/kg. These results provide a deeper insight into the structural features of the arylmethanols that rule antischistosomal activity. Further studies should be launched with enpiroline and WR7930.

Activity of clinically relevant antimalarial drugs on Plasmodium falciparum mature gametocytes in an ATP bioluminescence "transmission blocking" assay

Background

Current anti-malarial drugs have been selected on the basis of their activity against the symptom-causing asexual blood stage of the parasite. Which of these drugs also target gametocytes, in the sexual stage responsible for disease transmission, remains unknown. Blocking transmission is one of the main strategies in the eradication agenda and requires the identification of new molecules that are active against gametocytes. However, to date, the main limitation for measuring the effect of molecules against mature gametocytes on a large scale is the lack of a standardized and reliable method. Here we provide an efficient method to produce and purify mature gametocytes in vitro. Based on this new procedure, we developed a robust, affordable, and sensitive ATP bioluminescence-based assay. We then assessed the activity of 17 gold-standard anti-malarial drugs on Plasmodium late stage gametocytes.

Methods and Findings

Difficulties in producing large amounts of gametocytes have limited progress in the development of malaria transmission blocking assays. We improved the method established by Ifediba and Vanderberg to obtain viable, mature gametocytes en masse, whatever the strain used. We designed an assay to determine the activity of antimalarial drugs based on the intracellular ATP content of purified stage IV–V gametocytes after 48 h of drug exposure in 96/384-well microplates. Measurements of drug activity on asexual stages and cytotoxicity on HepG2 cells were also obtained to estimate the specificity of the active drugs.

Conclusions

The work described here represents another significant step towards determination of the activity of new molecules on mature gametocytes of any strain with an automated assay suitable for medium/high-throughput screening. Considering that the biology of the forms involved in the sexual and asexual stages is very different, a screen of our 2 million-compound library may allow us to discover novel anti-malarial drugs to target gametocyte-specific metabolic pathways.

Polymorphisms of the pfmdr1 but not the pfnhe-1 gene is associated with in vitro quinine sensitivity in Thai isolates of Plasmodium falciparum

BACKGROUND

The emergence of Plasmodium falciparum resistance to most currently used anti-malarial drugs is a major problem in malaria control along the Thai-Myanmar and Thai-Cambodia borders. Quinine (QN) with tetracycline/doxycycline has been used as the second-line treatment for uncomplicated falciparum malaria. In addition, QN monotherapy has been the first-line treatment for falciparum malaria in pregnant women. However, reduced in vitro and in vivo responses to QN have been reported. To date, a few genetic markers for QN resistance have been proposed including Plasmodium falciparum chloroquine resistance transporter (pfcrt), P. falciparum multidrug resistance 1 (pfmdr1), and P. falciparum Na+/H+ exchanger (pfnhe-1). This study was to investigate the role of the pfmdr1 and pfnhe-1 gene on in vitro QN sensitivity in Thai isolates of P. falciparum.

METHODS

Eighty-five Thai isolates of P. falciparum from the Thai-Myanmar and Thai-Cambodia borders from 2003-2008 were determined for in vitro QN sensitivity using radioisotopic assay. Polymorphisms of the pfmdr1 and pfnhe-1 gene were determined by PCR-RFLP and sequence analysis. Associations between the in vitro QN sensitivity and the polymorphisms of the pfmdr1 and pfnhe-1 gene were evaluated.

RESULTS

The mean QN IC50 was 202.8 nM (range 25.7-654.4 nM). Only four isolates were QN resistant when the IC50 of >500 nM was used as the cut-off point. Significant associations were found between the pfmdr1 mutations at codons N86Y and N1042D and in vitro QN sensitivity. However, no associations with the number of DNNND, DDNNNDNHNDD, and NHNDNHNNDDD repeats in the microsatellite ms4760 of the pfnhe-1 gene were identified.

CONCLUSION

Data from the present study put doubt regarding the pfnhe-1 gene as to whether it could be used as the suitable marker for QN resistance in Thailand. In contrast, it confirms the influence of the pfmdr1 gene on in vitro QN sensitivity.

Chemoprophylaxis in malaria: drugs, evidence of efficacy and costs

This review concentrates on different aspects of malaria chemoprophylaxis, namely drug combinations, resistance, impact of malaria prevention in pregnancy and cost effectiveness. A MEDLINE search was performed for all articles with the key word 'Malaria' in the title field and 'Prophylaxis' in any field. The search was restricted to articles published in English within the last decade (1999-2009). Data sources included review articles published in core clinical journals, cohort studies, interventional studies, case control studies and cross sectional analyses. The mechanism of action, trial evidence of efficacy, side effects and geographical distribution of resistance is discussed for each prophylactic drug regimen. Impact of prophylaxis in pregnancy and the cost considerations are discussed under two separate sub topics.

Additive in vitro antiplasmodial effect of N-alkyl and N-benzyl-1,10-phenanthroline derivatives and cysteine protease inhibitor e64

Potential new targets for antimalarial chemotherapy include parasite proteases, which are required for several cellular functions during the Plasmodium falciparum life cycle. Four new derivatives of N-alkyl and N-benzyl-1,10-phenanthroline have been synthesized. Those are (1)-N-methyl-1,10-phenanthrolinium sulfate, (1)-N-ethyl-1,10-phenanthrolinium sulfate, (1)-N-benzyl-1,10-phenanthrolinium chloride, and (1)-N-benzyl-1,10-phenanthrolinium iodide. Those compounds had potential antiplasmodial activity with IC₅₀ values from 260.42 to 465.38 nM. Cysteine proteinase inhibitor E64 was used to investigate the mechanism of action of N-alkyl and N-benzyl-1,10-phenanthroline derivatives. A modified fixed-ratio isobologram method was used to study the in vitro interactions between the new compounds with either E64 or chloroquine. The interaction between N-alkyl and N-benzyl-1,10-phenanthroline derivatives and E64 was additive as well as their interactions with chloroquine were also additive. Antimalarial mechanism of chloroquine is mainly on the inhibition of hemozoin formation. As the interaction of chloroquine and E64 was additive, the results indicated that these new compounds had a mechanism of action by inhibiting Plasmodium proteases.

Role of Plasmodium falciparum digestive vacuole plasmepsins in the specificity and antimalarial mode of action of cysteine and aspartic protease inhibitors

Hemoglobin (Hb) degradation is essential for the growth of the intraerythrocytic stages of malarial parasites. This process, which occurs inside an acidic digestive vacuole (DV), is thought to involve the action of four aspartic proteases, termed plasmepsins (PMs). These enzymes have received considerable attention as potential antimalarial drug targets. Leveraging the availability of a set of PM-knockout lines generated in Plasmodium falciparum, we report here that a wide range of previously characterized or novel aspartic protease inhibitors exert their antimalarial activities independently of their effect on the DV PMs. We also assayed compounds previously shown to inhibit cysteine proteases residing in the DV. The most striking observation was a ninefold increase in the potency of the calpain inhibitor N-acetyl-leucinyl-leucinyl-norleucinal (ALLN) against parasites lacking all four DV PMs. Genetic ablation of PM III or PM IV also decreased the level of parasite resistance to the beta-hematin binding antimalarial chloroquine. On the basis of the findings of drug susceptibility and isobologram assays, as well as the findings of studies of the inhibition of Hb degradation, morphological analyses, and stage specificity, we conclude that the DV PMs and falcipain cysteine proteases act cooperatively in Hb hydrolysis. We also identify several aspartic protease inhibitors, designed to target DV PMs, which appear to act on alternative targets early in the intraerythrocytic life cycle. These include the potent diphenylurea compound GB-III-32, which was found to be fourfold less potent against a P. falciparum line overexpressing plasmepsin X than against the parental nontransformed parasite line. The identification of the mode of action of these inhibitors will be important for future antimalarial drug discovery efforts focusing on aspartic proteases.

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

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

Differential effects of quinoline antimalarials on endocytosis in Plasmodium falciparum

The effects of quinoline antimalarials on endocytosis by Plasmodium falciparum was investigated by measuring parasite hemoglobin levels, peroxidase uptake, and transport vesicle content. Mefloquine, quinine, and halofantrine inhibited endocytosis, and chloroquine inhibited vesicle trafficking, while amodiaquine shared both effects. Protease inhibitors moderated hemoglobin perturbations, suggesting a common role for heme binding.

Dissecting the components of quinine accumulation in Plasmodium falciparum

Although quinine, the active ingredient of chinchona bark, has been used in the treatment of malaria for several centuries, there is little information regarding the interactions of this drug with the human malaria parasite Plasmodium falciparum. To better understand quinine's mode of action and the mechanism underpinning reduced responsiveness, we have investigated the factors that contribute to quinine accumulation by parasites that differ in their susceptibility to quinine. Interestingly, passive distribution, in accordance with the intracellular pH gradients, and intracellular binding could account for only a small fraction of the high amount of quinine accumulated by the parasites investigated. The results of trans-stimulation kinetics suggest that high accumulation of quinine is brought about by a carrier-mediated import system. This import system seems to be weakened in parasites with reduced quinine susceptibility. Other data show that polymorphisms within PfCRT are causatively linked with an increased verapamil-sensitive quinine efflux that, depending on the genetic background, resulted in reduced quinine accumulation. The polymorphisms within PfMDR1 investigated did not affect quinine accumulation. Our data are consistent with the model that several factors, including acidotropic trapping, binding to intracellular sites and carrier-mediated import and export transport systems, contribute to steady-state intracellular quinine accumulation.

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.

Synergistic interactions of the antiretroviral protease inhibitors saquinavir and ritonavir with chloroquine and mefloquine against Plasmodium falciparum in vitro

The antimalarial activity of several antiretroviral protease inhibitor combinations was investigated. Data demonstrate that ritonavir and saquinavir behave synergistically with chloroquine and mefloquine. These data, and interactions with pepstatin-A, E-64, and bestatin, suggest that human immunodeficiency virus protease inhibitors do not target digestive-vacuole plasmepsins.

Incorporation of an intramolecular hydrogen-bonding motif in the side chain of 4-aminoquinolines enhances activity against drug-resistant P. falciparum

Previous data showing that several chloroquine analogues containing an intramolecular hydrogen-bonding motif were potent against multidrug-resistant P. falciparum led to the exploration of the importance of this motif. A series of 116 compounds containing four different alkyl linkers and various aromatic substitutions with hydrogen bond accepting capability was synthesized. The series showed broad potency against the drug-resistant W2 strain of P. falciparum. In particular, a novel series containing variations of the alpha-aminocresol motif gave eight compounds with IC50 values more potent than 5 nM against the W2 strain. Such simple modifications, significantly altering the pKa and sterics of the basic side chain in chloroquine analogues, may prove to be part of a strategy for overcoming the problem of worldwide resistance to affordable antimalarial drugs.

Genetic linkage of pfmdr1 with food vacuolar solute import in Plasmodium falciparum

The P-glycoprotein homolog of the human malaria parasite Plasmodium falciparum (Pgh-1) has been implicated in decreased susceptibility to several antimalarial drugs, including quinine, mefloquine and artemisinin. Pgh-1 mainly resides within the parasite's food vacuolar membrane. Here, we describe a surrogate assay for Pgh-1 function based on the subcellular distribution of Fluo-4 acetoxymethylester and its free fluorochrome. We identified two distinct Fluo-4 staining phenotypes: preferential staining of the food vacuole versus a more diffuse staining of the entire parasite. Genetic, positional cloning and pharmacological data causatively link the food vacuolar Fluo-4 phenotype to those Pgh-1 variants that are associated with altered drug responses. On the basis of our data, we propose that Pgh-1 imports solutes, including certain antimalarial drugs, into the parasite's food vacuole. The implications of our findings for drug resistance mechanisms and testing are discussed.

Hemoglobin degradation

Hemoglobin degradation by Plasmodium is a massive catabolic process within the parasite food vacuole that is important for the organism's survival in its host erythrocyte. A proteolytic pathway is responsible for generating amino acids from hemoglobin. Each of the enzymes involved has its own peculiarities to be exploited for development of antimalarial agents that will starve the parasite or result in build-up of toxic intermediates. There are a number of unanswered questions concerning the cell biology, biochemistry and metabolic roles of this crucial pathway.

The pH of the digestive vacuole of Plasmodium falciparum is not associated with chloroquine resistance

Chloroquine resistance in the human malaria parasite, Plasmodium falciparum, arises from decreased accumulation of the drug in the ;digestive vacuole' of the parasite, an acidic compartment in which chloroquine exerts its primary toxic effect. It has been proposed that changes in the pH of the digestive vacuole might underlie the decreased accumulation of chloroquine by chloroquine-resistant parasites. In this study we have investigated the digestive vacuole pH of a chloroquine-sensitive and a chloroquine-resistant strain of P. falciparum, using a range of dextran-linked pH-sensitive fluorescent dyes. The estimated digestive vacuole pH varied with the concentration and pK(a) of the dye, ranging from approximately 3.7-6.5. However, at low dye concentrations the estimated digestive vacuole pH of both the chloroquine-resistant and chloroquine-sensitive strains converged in the range 4.5-4.9. The results suggest that there is no significant difference in digestive vacuole pH of chloroquine-sensitive and chloroquine-resistant parasites, and that digestive vacuole pH does not play a primary role in chloroquine resistance.

Quinolines and artemisinin: chemistry, biology and history

Plasmodium falciparum is the most important parasitic pathogen in humans, causing hundreds of millions of malaria infections and millions of deaths each year. At present there is no effective malaria vaccine and malaria therapy is totally reliant on the use of drugs. New drugs are urgently needed because of the rapid evolution and spread of parasite resistance to the current therapies. Drug resistance is one of the major factors contributing to the resurgence of malaria, especially resistance to the most affordable drugs such as chloroquine. We need to fully understand the antimalarial mode of action of the existing drugs and the way that the parasite becomes resistant to them in order to design and develop the new therapies that are so urgently needed. In respect of the quinolines and artemisinins, great progress has been made recently in studying the mechanisms of drug action and drug resistance in malaria parasites. Here we summarize from a historical, biological and chemical, perspective the exciting new advances that have been made in the study of these important antimalarial drugs.

PfCG2, a Plasmodium falciparum protein peripherally associated with the parasitophorous vacuolar membrane, is expressed in the period of maximum hemoglobin uptake and digestion by trophozoites

A Plasmodium falciparum gene closely linked to the chloroquine resistance locus encodes PfCG2, a predicted 320-330kDa protein. In the parasitized erythrocyte, PfCG2 expression rises sharply in the trophozoite stage and is detected in electron-dense patches along the parasitophorous vacuolar membrane (PVM), in the cytoplasm and in the digestive vacuole (DV). Results of extraction and partitioning experiments show that PfCG2 is a peripheral membrane protein. Exposure of trophozoite-infected erythrocytes to trypsin-containing buffer after streptolysin O permeabilization indicates that PfCG2 is exposed to the erythrocyte cytosol at the outer face of the PVM. PfCG2 is highly susceptible to hydrolysis by aspartic and cysteine proteases and shows dose-dependent accumulation in the presence of protease inhibitors. These results suggest that PfCG2 is delivered from the outside face of the PVM to the DV, where it is broken down by parasite proteases. PfCG2 interacts with erythrocyte cytoplasm and may be associated with processes of hemoglobin uptake and digestion by erythrocytic-stage parasites.

Mutations in pfmdr1 modulate the sensitivity of Plasmodium falciparum to the intrinsic antiplasmodial activity of verapamil

As well as having the ability to reverse chloroquine resistance in the human malaria parasite Plasmodium falciparum, verapamil has itself an innate antiplasmodial activity. We show here that mutations in Pgh1, the product of the malaria parasite's pfmdr1 gene, influence the parasite's susceptibility to the toxic effects of verapamil.

Antimalarial quinolines and artemisinin inhibit endocytosis in Plasmodium falciparum

Endocytosis is a fundamental process of eukaryotic cells and fulfills numerous functions, most notably, that of macromolecular nutrient uptake. Malaria parasites invade red blood cells and during their intracellular development endocytose large amounts of host cytoplasm for digestion in a specialized lysosomal compartment, the food vacuole. In the present study we have examined the effects of artemisinin and the quinoline drugs chloroquine and mefloquine on endocytosis in Plasmodium falciparum. By using novel assays we found that mefloquine and artemisinin inhibit endocytosis of macromolecular tracers by up to 85%, while the latter drug also leads to an accumulation of undigested hemoglobin in the parasite. During 5-h incubations, chloroquine inhibited hemoglobin digestion but had no other significant effect on the endocytic pathway of the parasite, as assessed by electron microscopy, the immunofluorescence localization of hemoglobin, and the distribution of fluorescent and biotinylated dextran tracers. By contrast, when chloroquine was added to late ring stage parasites, followed by a 12-h incubation, macromolecule endocytosis was inhibited by more than 40%. Moreover, there is an accumulation of transport vesicles in the parasite cytosol, possibly due to a disruption in vacuole-vesicle fusion. This fusion block is not observed with mefloquine, artemisinin, quinine, or primaquine but is mimicked by the vacuole alkalinizing agents ammonium chloride and monensin. These results are discussed in the light of present theories regarding the mechanisms of action of the antimalarials and highlight the potential use of drugs in manipulating and studying the endocytic pathway of malaria parasites.