Evidence for a pfcrt-associated chloroquine efflux system in the human malarial parasite Plasmodium falciparum

Resistance to the antimalarial drug chloroquine has been linked with polymorphisms within a gene termed pfcrt in the human malarial parasite Plasmodium falciparum, yet the mechanism by which this gene confers the reduced drug accumulation phenotype associated with resistance is largely unknown. To investigate the role of pfcrt in mediating chloroquine resistance, we challenged P. falciparum clones differing only in their pfcrt allelic form with the "varying-trans" procedure. In this procedure, movement of labeled substrate across a membrane is measured when unlabeled substrate is present on the trans side of the membrane. If a transporter is mediating the substrate flow, a stimulation of cis-to-trans movement may be observed with increasing concentrations of trans substrate. We present evidence for an association of those pfcrt alleles found in chloroquine-resistant P. falciparum strains with the phenomenon of stimulated chloroquine accumulation under varying-trans conditions. Such an association is not seen with polymorphisms within pfmdr1, which encodes a homologue of the human multidrug resistance efflux pump. Our data are interpreted in terms of a model in which pfcrt is directly or indirectly involved in carrier-mediated chloroquine efflux from resistant cells.     

Potentiation of antimalarial drug action by chlorpheniramine against multidrug-resistant Plasmodium falciparum in vitro

Chlorpheniramine, a histamine H1 receptor antagonist, was assayed for in vitro antimalarial activity against multidrug-resistant Plasmodium falciparum K1 strain and chloroquine-resistant P. falciparum T9/94 clone, by measuring the 3H-hypoxanthine incorporation. Chlorphenirame inhibited P. falciparum K1 and T9/94 growth with IC50 values of 136.0+/-40.2 microM and 102.0+/-22.6 microM respectively. A combination of antimalarial drug and chlorpheniramine was tested against resistant P. falciparum in vitro. Isobologram analysis showed that chlorpheniramine exerts marked synergistic action on chloroquine against P. falciparum K1 and T9/94. Chlorpheniramine also potentiated antimalarial action of mefloquine, quinine or pyronaridine against both of the resistant strains of P. falciparum. However, chlorpheniramine antagonism with artesunate was obtained in both P. falciparum K1 and T9/94. The results in this study indicate that antihistaminic drugs may be promising candidates for potentiating antimalarial drug action against drug resistant malarial parasites.     

In vitro schizontocidal activity of standard antimalarial drugs on chloroquine-sensitive and chloroquine-resistant isolates of Plasmodium falciparum

The expanding foci of multiple drug resistant malaria and emergence of different strains requires the reassessment of antimalarial activity with various drugs. In vitro response of a chloroquine sensitive and a chloroquine resistant isolate of P. falciparum to a group of 6 quinine derived and 3 artemisinin derived standard drugs has been screened, to evaluate schizontocidal activity of the drugs. In a conventional test system the IC50s were derived from the log dose response curves and evaluated by a rigorous statistical interpretation. Analysis by Tukey's test was significant for the quinine related drugs (Q < or = 0.01) and excludes the statistical significance of artemisinin related drugs in these isolates. The dose-responses of these two isolates vary with quinine derivatives, with some overlap at lower doses for the sensitive isolate than for the resistant one which manifests at higher doses.     

Investigating the activity of quinine analogues versus chloroquine resistant Plasmodium falciparum

Plasmodium falciparum, the deadliest malarial parasite species, has developed resistance against nearly all man-made antimalarial drugs within the past century. However, quinine (QN), the first antimalarial drug, remains efficacious worldwide. Some chloroquine resistant (CQR) P. falciparum strains or isolates show mild cross resistance to QN, but many do not. Further optimization of QN may provide a well-tolerated therapy with improved activity versus CQR malaria. Thus, using the Heck reaction, we have pursued a structure-activity relationship study, including vinyl group modifications of QN. Certain derivatives show good antiplasmodial activity in QN-resistant and QN-sensitive strains, with lower IC(50) values relative to QN.     

Trans stimulation provides evidence for a drug efflux carrier as the mechanism of chloroquine resistance in Plasmodium falciparum

The mechanism underpinning chloroquine drug resistance in the human malarial parasite Plasmodium falciparum has remained controversial. Currently considered models to explain the resistance phenotype include acquisition of a chloroquine efflux pump, changes in intracellular chloroquine partitioning, diminished binding affinity of chloroquine to its intracellular target, heme, and changes in heme crystallization. To challenge these different models, we have investigated chloroquine accumulation under trans-stimulation conditions and in the presence and absence of glucose. We show that, in chloroquine-sensitive strains, labeled chloroquine accumulation is steadily reduced as the pre-equilibrated chloroquine concentration is raised. In the resistant cells, the extent of accumulation is, strikingly, raised at the lower levels of preloading, in comparison with resistant controls in the absence of chloroquine. The trans-stimulation effect observed in chloroquine-resistant cells is strictly energy-dependent. The data are interpreted in terms of a model in which chloroquine is bound to intracellular binding sites, not different as between sensitive and resistant cells, but where, in resistant cells, there exists an energy-dependent carrier that moves chloroquine out of this intracellular compartment. A mathematical model describing the kinetics of these processes is presented.     

Polymorphisms within PfMDR1 alter the substrate specificity for anti-malarial drugs in Plasmodium falciparum

Resistance to several anti-malarial drugs has been associated with polymorphisms within the P-glycoprotein homologue (Pgh-1, PfMDR1) of the human malaria parasite Plasmodium falciparum. Pgh-1, coded for by the gene pfmdr1, is predominately located at the membrane of the parasite's digestive vacuole. How polymorphisms within this transporter mediate alter anti-malarial drug responsiveness has remained obscure. Here we have functionally expressed pfmdr1 in Xenopus laevis oocytes. Our data demonstrate that Pgh-1 transports vinblastine, an established substrate of mammalian MDR1, and the anti-malarial drugs halofantrine, quinine and chloroquine. Importantly, polymorphisms within Pgh-1 alter the substrate specificity for the anti-malarial drugs. Wild-type Pgh-1 transports quinine and chloroquine, but not halofantrine, whereas polymorphic Pgh-1 variants, associated with altered drug responsivenesses, transport halofantrine but not quinine and chloroquine. Our data further suggest that quinine acts as an inhibitor of Pgh-1. Our data are discussed in terms of the model that Pgh-1-mediates, in a variant-specific manner, import of certain drugs into the P. falciparum digestive vacuole, and that this contributes to accumulation of, and susceptibility to, the drug in question.     

Plasmodium falciparum malaria: rosettes are disrupted by quinine, artemisinin, mefloquine, primaquine, pyrimethamine, chloroquine and proguanil.

An assay was developed measuring the disruption of rosettes between Plasmodium falciparuminfected (trophozoites) and uninfected erythrocytes by the antimalarial drugs quinine, artemisinin mefloquine, primaquine, pyrimethamine, chloroquine and proguanil. At 4 hr incubation rosettes were disrupted by all the drugs in a dose dependent manner. Artemisinin and quinine were the most effective anti-malarials at disrupting rosettes at their therapeutic concentrations with South African RSA 14, 15, 17 and The Gambian FCR-3 P. falciparum strains. The least effective drugs were proguanil and chloroquine. A combination of artemisinin and mefloquine was more effective than each drug alone. The combinations of pyrimethamine or primaquine, with quinine disrupted more rosettes than quinine alone. Quinine may be an effective drug in the treatment of severe malaria because the drug efficiently reduces the number of rosettes.     

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.     

Genetic linkage analyses redefine the roles of PfCRT and PfMDR1 in drug accumulation and susceptibility in Plasmodium falciparum

Resistance to quinoline antimalarial drugs has emerged in different parts of the world and involves sets of discrete mutational changes in pfcrt and pfmdr1 in the human malaria parasite Plasmodium falciparum. To better understand how the different polymorphic haplotypes of pfmdr1 and pfcrt contribute to drug resistance, we have conducted a linkage analysis in the F1 progeny of a genetic cross where we assess both the susceptibility and the amount of accumulation of chloroquine, amodiaquine, quinine and quinidine. Our data show that the different pfcrt and pfmdr1 haplotypes confer drug-specific responses which, depending on the drug, may affect drug accumulation or susceptibility or both. These findings suggest that PfCRT and PfMDR1 are carriers of antimalarial drugs, but that the interaction with a drug interferes with the carriers' natural transport function such that they are now themselves targets of these drugs. How well a mutant PfCRT and PfMDR1 type copes with its competing transport functions is determined by its specific sets of amino acid substitutions.     

The antimalarial drug resistance protein Plasmodium falciparum chloroquine resistance transporter binds chloroquine

Recently, mutations in the novel polytopic integral membrane protein PfCRT were shown to cause chloroquine resistance (CQR) in the malarial parasite Plasmodium falciparum. PfCRT is not a member of the well-known family of ABC proteins that have previously been associated with other drug resistance phenomena. Thus, the mechanism(s) whereby mutant PfCRT molecules confer antimalarial drug resistance is (are) unknown. Previously, we succeeded in overexpressing PfCRT to high levels in Pichia pastoris yeast by synthesizing a codon-optimized version of the pfcrt gene. Using purified membranes and inside-out plasma membrane vesicles (ISOV) isolated from strains harboring either wild-type or CQR-associated mutant PfCRT, we now show that under deenergized conditions the PfCRT protein specifically binds the antimalarial drug chloroquine (CQ) with a K(D) near 400 nM but does not measurably bind the related drug quinine (QN) at physiologically relevant concentrations. Transport studies using ISOV show that QN is passively accumulated as expected on the basis of previous measurement of the ISOV DeltapH for the different strains. However, passive accumulation of CQ is lower than expected for ISOV harboring mutant PfCRT, despite higher DeltapH for these ISOV.     

The current status of drug resistance in malaria

Drug resistant malaria is a major health problem; it poses a threat to the lives of millions of people and renders it less possible for the worldwide eradication programme to attain its goal in the foreseeable future. At present Plasmodium falciparum is resistant to varying degrees to all antimalarial drugs available e.g. chloroquine, sulfadoxine and pyrimethamine, quinine and even to the new compound, mefloquine.Chloroquine-resistant P. falciparum originated in Thailand some 25 years ago has spread in all directions to Southeast Asia, Western Pacific, to central and southeast India, East Africa and West Africa. In South America, it started in Colombia and now affects the whole of Central and South America with the exception of Argentina, Paraguay and Peru which practically have no falciparum malaria.The mechanism of drug resistance in malaria parasites is believed to be due to gene mutation selected under drug pressure. It may be one-step as in pyrimethamine or multi-step as in chloroquine. Resistant mutation occurs both in schizogony and sporogony. The parasites lose their S strains through hybridization or overgrowth, shifting in character progressively towards high grade resistance.Policies that may help to minimise further development of resistance to existing compounds and to safeguard any new drugs that may be developed in the future include (1) limit the distribution of antimalarials; (2) select priority groups for prophylaxis; (3) use the gametocidal drug primaquine to restrict transmission of resistant strains; (4) establish an effective drug monitoring system; (5) only deploy drugs for control as part of an integrated campaign; (6) control use of new antimalarial; (7) encourage the use of tested effective drug regimens for treatment and (8) encourage research on antimalarials.     

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.     

Nutritional requirements of Plasmodium falciparum in culture. II. Effects of antimetabolites in a semi-defined medium

A semi-defined minimal medium, in which pantothenic acid is the only vitamin, was used to culture Plasmodium falciparum for the analysis of antimetabolite drugs. Analogs of riboflavin, nicotinamide, pyridoxine, and thiamin inhibited the growth of this parasite; for each drug, effects were much more pronounced after 96 h of exposure compared to 48 h. The most potent drug examined was 8-methylamino-8-desmethyl riboflavin (IC50 value approximately 1.0 X 10(-10) M at 96 h). Avidin, a protein which complexes and thus inactivates biotin, did not affect parasite viability. Other antimalarial drugs, including chloroquine and quinine derivatives and antibiotics, were equipotent in the minimal medium and in RPMI 1640. Four strains of P. falciparum showed only minor differences in sensitivity to these antimetabolites. The use of prolonged drug exposure times and a vitamin-depleted medium allowed the preliminary characterization of antimalarial antimetabolites in vitro.     

Double-drug development against antioxidant enzymes from Plasmodium falciparum

New drugs against malaria are urgently and continuously needed. Plasmodium parasites are exposed to higher fluxes of reactive oxygen species and need high activities of intracellular antioxidant systems. A most important antioxidative system consists of (di)thiols which are recycled by disulfide reductases (DR), namely both glutathione reductases (GR) of the malarial parasite Plasmodium falciparum and man, and the thioredoxin reductase (TrxR) of P. falciparum. The aim of our interdisciplinary research is to substantiate DR inhibitors as antimalarial agents. Such compounds are active per se but, in addition, they can reverse thiol-based resistance against other drugs in parasites. Reversal of drug resistance by DR inhibitors is currently investigated for the commonly used antimalarial drug chloroquine (CQ). Our recent strategy is based on the synthesis of inhibitors of the glutathione reductases from parasite and host erythrocyte. With the expectation of a synergistic or additive effect, double-headed prodrugs were designed to be directed against two different and essential functions of the malarial parasite P. falciparum, namely glutathione regeneration and heme detoxification. The prodrugs were prepared by linking bioreversibly a GR inhibitor to a 4-aminoquinoline moiety which is known to concentrate in the acidic food vacuole of parasites. Drug-enzyme interaction was correlated with antiparasitic action in vitro on strains resistant towards CQ and in vivo in Plasmodium berghei-infected mice as well as absence of cytotoxicity towards human cells. Because TrxR of P. falciparum was recently shown to be responsible for the residual glutathione disulfide-reducing capacity observed after GR inhibition in P. falciparum, future development of antimalarial drug-candidates that act by perturbing the redox equilibrium of parasites is based on the design of new double-drugs based on TrxR inhibitors as potential antimalarial drug candidates.     

Pyronaridine: A new antimalarial drug

The worldwide spread of strains of Plasmodium falciparum that are resistant to chloroquine has highlighted the urgent need for new antimalarial drugs, particularly in less developed tropical countries. However, in the current economic climate the pharmaceutical giants in the developed world are withdrawing from tropical disease research. Consequently, the following article from Fu Sui and Xiao Shuhuo is of particular interest, not only because it summarizes work on on alternative antimalarial drug that is efficacious against multiply resistant Plasmodium but also because this drug has been developed primarily from Chinese research efforts, the results of which have largely only been published in the Chinese scientific literature. The drug under scrutiny is pyronaridine, and is the product of 30 years of chemistry that began with the mepacrine nucleus. This nucleus was selected as the starting point in the search for a chloroquine alternative because the various derivatives synthesized were active against chloroquine-resistant parasites. However, mepacrine itself also needed replacing as it is too toxic for mass use. After synthesizing and screening a huge series of substitutions, the addition of an amodiaquine side-chain to this nucleus was found to give the greatest activity for fewest adverse effects. Being aware of the rapid selection of pyronaridine-resistant Plasmodium strains that occurs in the laboratory, the Chinese efforts have also investigated the use of drug combinations to circumvent or delay the development of drug resistance. In addition to the triple combination described here, pyronaridine and primaquine combinations are under trial against both P. vivax and P. falciparum. Pyronaridine is a highly active blood schizonticide like chloroquine and amodiaquine. It has already undergone extensive trials in humans against both P. falciparum and P. vivax. However, nothing is known of its mode of action, nor the basis for the development of resistance and although it is active against chloroquine-resistant strains of parasite, paradoxically, pyronaridine-resistant Plasmodium is resistant to chloroquine.     

Studies on the antimalarial mode of action of quinoline-containing drugs: time-dependence and irreversibility of drug action, and interactions with compounds that alter the function of the parasite's food vacuole

The quinoline-containing antimalarial drugs chloroquine, quinine and mefloquine exert an irreversible inhibitory effect on erythrocytic stages of Plasmodium falciparum grown in culture. Inhibition is time- and concentration-dependent and the full effect is observed after 2-6 hours of exposure to the drug. Washing of infected cells after drug exposure in the presence of NH4Cl to accelerate drug efflux, intensifies the inhibitory effect of chloroquine, probably due to the pH-dependent release of highly concentrated drug from the acidic food vacuole of the parasite. When both antimalarials and NH4Cl are present in the culture, drug effect is reduced, as expected from the demonstrable alkalinization of the food vacuole and the consequent reduction in drug accumulation. The protease inhibitor leupeptin inhibits digestion of ingested host cell cytosol, and thus inhibits parasite growth, though reversibly so (Rosenthal et al, J. Clin. Invest. 82 1560-1566 (1988)). Thus, although the antimalarials also inhibit the feeding process, this is not the cause of their irreversible action. Leupeptin is found to be antagonistic to antimalarials' action, suggesting that the drugs form complexes with products of host cell digestion that are responsible for irreversible inhibition of parasite growth.     

Interaction of chloroquine and its analogues with heme: An isothermal titration calorimetric study

Quinoline-containing drugs such as chloroquine and quinine have had a long and successful history in antimalarial chemotherapy. Identification of ferriprotoporphyrin IX ([Fe(III)PPIX], haematin) as the drug receptors for these antimalarials called for investigations of the binding affinity, mode of interaction, and the conditions affecting the interaction. The parameters obtained are significant in recent times with the emergence of chloroquine resistant strains of the malaria parasites. This has underlined the need to unravel the molecular mechanism of their action so as to meet the requirement of an alternative to the existing antimalarial drugs. The isothermal titration calorimetric studies on the interaction of chloroquine with haematin lead us to propose an altered mode of binding. The initial recognition is ionic in nature mediated by the propionyl group of haematin with the quaternary nitrogen on CQ. This ionic interaction induces a conformational change, such as to favour binding of subsequent CQ molecules. On the contrary, conditions emulating the cytosolic environment (pH 7.4 and 150 mM salt) reveal the hydrophobic force to be the sole contributor driving the interaction. Interaction of a carefully selected panel of quinoline antimalarial drugs with monomeric ferriprotoporphyrin IX has also been investigated at pH 5.6 mimicking the acidic environment prevalent in the food vacuoles of parasite, the center of drug activity, which are consistent with their antimalarial activity.     

Novel oxazine skeletons as potential antiplasmodial active ingredients: Synthesis, in vitro and in vivo biology of some oxazine entities produced via cyclization of novel chalcone intermediates

A novel series of 6-(2-chloroquinolin-3-yl)-4-substituted-phenyl-6H-1,3-oxazin-2-amines were synthesized and evaluated for in vitro antimalarial efficacy against chloroquine sensitive (MRC-02) as well as chloroquine resistant (RKL9) strains of Plasmodium falciparum. The activity tested was at nanomolar concentration. β-Hematin formation inhibition activity (BHIA(50)) of oxazines were determined and correlated with antimalarial activity. A reasonably good correlation (r?=?0.49 and 0.51, respectively) was observed between antimalarial activity (IC(50)) and BHIA(50). This suggests that antimalarial mode of action of these compounds seems to be similar to that of chloroquine and involves the inhibition of hemozoin formation. Some of the compounds were showing better antimalarial activity than chloroquine against resistant strain of P. falciparum and were also found to be active in the in vivo experiment.     

pfmdr1 mutations contribute to quinine resistance and enhance mefloquine and artemisinin sensitivity in Plasmodium falciparum

The emergence and spread of multidrug resistant Plasmodium falciparum has severely limited the therapeutic options for the treatment of malaria. With ever-increasing failure rates associated with chloroquine or sulphadoxine-pyrimethamine treatment, attention has turned to the few alternatives, which include quinine and mefloquine. Here, we have investigated the role of pfmdr1 3' coding region point mutations in antimalarial drug susceptibility by allelic exchange in the GC03 and 3BA6 parasite lines. Results with pfmdr1-recombinant clones indicate a significant role for the N1042D mutation in contributing to resistance to quinine and its diastereomer quinidine. The triple mutations S1034C/N1042D/D1246Y, highly prevalent in South America, were also found to enhance parasite susceptibility to mefloquine, halofantrine and artemisinin. pfmdr1 3' mutations showed minimal effect on P. falciparum resistance to chloroquine or its metabolite mono-desethylchloroquine in these parasite lines, in contrast to previously published results obtained with 7G8 parasites. This study supports the hypothesis that pfmdr1 3' point mutations can significantly affect parasite susceptibility to a wide range of antimalarials in a strain-specific manner that depends on the parasite genetic background.     

Genome comparison of progressively drug resistant Plasmodium falciparum lines derived from drug sensitive clone

Chloroquine has been the mainstay of malaria chemotherapy for the past five decades, but resistance is now widespread. Pyrimethamine or proguanil form an important component of some alternate drug combinations being used for treatment of uncomplicated Plasmodium falciparum infections in areas of chloroquine resistance. Both pyrimethamine and proguanil are dihydrofolate reductase (DHFR) inhibitors, the proguanil acting primarily through its major metabolite cycloguanil. Resistance to these drugs arises due to specific point mutations in the dhfr gene. Cross resistance between cycloguanil and pyrimethamine is not absolute. It is, therefore, important to investigate mutation rates in P. falciparum for pyrimethamine and proguanil so that DHFR inhibitor with less mutation rate is favored in drug combinations. Hence, we have compared mutation rates in P. falciparum genome for pyrimethamine and cycloguanil. Using erythrocytic stages of P. falciparum cultures, progressively drug resistant lines were selected in vitro and comparing their RFLP profile with a repeat sequence. Our finding suggests that pyrimethamine has higher mutation rate compared to cycloguanil. It enhances the degree of genomic polymorphism leading to diversity of natural parasite population which in turn is predisposes the parasites for faster selection of resistance to some other antimalarial drugs.