Rapid chloroquine efflux phenotype in both chloroquine-sensitive and chloroquine-resistant Plasmodium falciparum. A correlation of chloroquine sensitivity with energy-dependent drug accumulation

Synthesis, Biological Evaluation, and Molecular Modeling Studies of Chiral Chloroquine Analogues as Antimalarial Agents

In a focused exploration, we designed, synthesized, and biologically evaluated chiral conjugated new chloroquine (CQ) analogues with substituted piperazines as antimalarial agents. In vitro as well as in vivo studies revealed that compound 7c showed potent activity (in vitro 50% inhibitory concentration, 56.98 nM for strain 3D7 and 97.76 nM for strain K1; selectivity index in vivo [up to at a dose of 12.5 mg/kg of body weight], 3,510) as a new lead antimalarial agent. Other compounds (compounds 6b, 6d, 7d, 7h, 8c, 8d, 9a, and 9c) also showed moderate activity against a CQ-sensitive strain (3D7) and superior activity against a CQ-resistant strain (K1) of Plasmodium falciparum Furthermore, we carried out docking and three-dimensional quantitative structure-activity relationship (3D-QSAR) studies of all in-house data sets (168 molecules) of chiral CQ analogues to explain the structure-activity relationships (SAR). Our new findings specify the significance of the H-bond interaction with the side chain of heme for biological activity. In addition, the 3D-QSAR study against the 3D7 strain indicated the favorable and unfavorable sites of CQ analogues for incorporating steric, hydrophobic, and electropositive groups to improve the antimalarial activity.

Antimalarial activity of novel 4-aminoquinolines active against drug resistant strains

In the present study we have synthesized a new class of 4-aminoquinolines and evaluated against Plasmodium falciparum in vitro (3D7-sensitive strain & K1-resistant strain) and Plasmodium yoelii in vivo (N-67 strain). Among the series, eleven compounds (5, 6, 7, 8, 9, 11, 12, 13, 14, 15 and 21) showed superior antimalarial activity against K1 strain as compared to CQ. In addition, all these analogues showed 100% suppression of parasitemia on day 4 in the in vivo mouse model against N-67 strain when administered orally. Further, biophysical studies suggest that this series of compounds act on heme polymerization target.

Efflux pump-mediated resistance in chemotherapy

Efflux pump mechanisms perform important physiological functions such as prevention of toxin absorption from the gastrointestinal tract, elimination of bile from the hepatocytes, effective functioning of the blood-brain barrier and placental barrier, and renal excretion of drugs. They exist in all living cells, but those in the bacterial and mammalian cells are more important to the clinician and pharmacologist, as they constitute an important cause of antimicrobial drug resistance, which contributes to treatment failure, high medical bills, and increased mortality / morbidity. This review was aimed at highlighting the role of efflux pump mechanisms in microbial resistance to chemotherapeutic agents. It was also aimed to elucidate their structure and mechanisms of action so as to integrate the efflux pump mechanisms in the design and development of novel antimicrobial agents. Findings from previous studies and research on this subject assessed through Google search, Pubmed, Hinari websites, as well as standard textbooks on chemotherapy, provided the needed information in the process of this review. Efflux pump inhibitors are promising strategies for preventing and reverting efflux-mediated resistance to chemotherapeutic agents. They are usually employed as adjuncts in antimicrobial and cancer chemotherapy. Toxicity, more common with the older-generation inhibitors such as verapamil and reserpine, constitutes the greatest impediment to their clinical applications. No efflux pump inhibitor has been approved for routine clinical use, as a result of doubtful clinical efficacy and unacceptably high incidence of adverse effects, particularly inhibition of the P-450 drug metabolizing enzyme. At present, their applications are mainly restricted to epidemiological studies. Nonetheless, the search for efficacious and tolerable efflux pump inhibitors continues because of the potential benefits. There is a need to consider efflux pump substrate selectivity in the design and development of novel chemotherapeutic agents.

Deciphering the Resistance-Counteracting Functions of Ferroquine in Plasmodium falciparum-Infected Erythrocytes

The aminoquinoline chloroquine (CQ) has been widely used for treating malaria since World War II. Resistance to CQ began to spread around 1957 and is now found in all malarious areas of the world. CQ resistance is caused by multiple mutations in the Plasmodium falciparum chloroquine resistance transporter (PfCRT). These mutations result in an increased efflux of CQ from the acidic digestive vacuole (DV) to the cytosol of the parasite. This year, we proposed a strategy to locate and quantify the aminoquinolines in situ within infected red blood cells (iRBCs) using synchrotron based X-ray nanoprobe fluorescence. Direct measurements of unlabeled CQ and ferroquine (FQ) (a ferrocene-CQ conjugate, extremely active against CQ-resistant strains) enabled us to evidence fundamentally different transport mechanisms from the cytosol to the DV between CQ and FQ in the CQ-susceptible strain HB3. These results inspired the present study of the localization of CQ and FQ in the CQ-resistant strain W2. The introduction of the ferrocene core in the lateral side chain of CQ has an important consequence: the transporter is unable to efflux FQ from the DV. We also found that resistant parasites treated by FQ accumulate a sulfur-containing compound, credibly glutathion, in their DV.

PfCRT-mediated drug transport in malarial parasites

A wide range of drug transport studies using intact infected red blood cells, isolated malarial parasites, heterologous expression systems, and purified protein, combined with elegant genetic experiments, have suggested that chloroquine transport by the Plasmodium falciparum chloroquine resistance transporter (PfCRT) is a key aspect of the molecular mechanism of quinoline antimalarial drug resistance. However, many questions remain. This short review summarizes data that have led to drug channel versus drug pump hypotheses for PfCRT and suggests ways in which recent contrasting interpretations might be reconciled.

On the mechanism of chloroquine resistance in Plasmodium falciparum

Resistance to chloroquine of malaria strains is known to be associated with a parasite protein named PfCRT, the mutated form of which is able to reduce chloroquine accumulation in the digestive vacuole of the pathogen. Whether the protein mediates extrusion of the drug acting as a channel or as a carrier and which is the protonation state of its chloroquine substrate is the subject of a scientific debate. We present here an analytical approach that explores which combination of hypotheses on the mechanism of transport and the protonation state of chloroquine are consistent with available equilibrium experimental data. We show that the available experimental data are not, by themselves, sufficient to conclude whether the protein acts as a channel or as a transporter, which explains the origin of their different interpretation by different authors. Interestingly, though, each of the two models is only consistent with a subset of hypotheses on the protonation state of the transported molecule. The combination of these results with a sequence and structure analysis of PfCRT, which strongly suggests that the molecule is a carrier, indicates that the transported species is either or both the mono and di-protonated forms of chloroquine. We believe that our results, besides shedding light on the mechanism of chloroquine resistance in P. falciparum, have implications for the development of novel therapies against resistant malaria strains and demonstrate the usefulness of an approach combining systems biology strategies with structural bioinformatics and experimental data.

Chloroquine transport in Plasmodium falciparum. 1. Influx and efflux kinetics for live trophozoite parasites using a novel fluorescent chloroquine probe

Several models for how amino acid substitutions in the Plasmodium falciparum chloroquine resistance transporter (PfCRT) confer resistance to chloroquine (CQ) and other antimalarial drugs have been proposed. Distinguishing between these models requires detailed analysis of high-resolution CQ transport data that is unfortunately impossible to obtain with traditional radio-tracer methods. Thus, we have designed and synthesized fluorescent CQ analogues for drug transport studies. One probe places a NBD (6-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)hexanoic acid) group at the tertiary aliphatic N of CQ, via a flexible 6 C amide linker. This probe localizes to the malarial parasite digestive vacuole (DV) during initial perfusion under physiologic conditions and exhibits similar pharmacology relative to CQ, vs both CQ-sensitive (CQS) and CQ-resistant (CQR) parasites. Using live, synchronized intraerythrocytic parasites under continuous perfusion, we define NBD-CQ influx and efflux kinetics for CQS vs CQR parasites. Since this fluorescence approach provides data at much higher kinetic resolution relative to fast-filtration methods using (3)H-CQ, rate constants vs linear initial rates for CQ probe flux can be analyzed in detail. Importantly, we find that CQR parasites have a decreased rate constant for CQ influx into the DV and that this is due to mutation of PfCRT. Analysis of zero trans efflux for CQS and CQR parasites suggests that distinguishing between bound vs free pools of intra-DV drug probe is essential for proper kinetic analysis of efflux. The accompanying paper (DOI 10.1021/bi901035j ) further probes efflux kinetics for proteoliposomes containing purified, reconstituted PfCRT.

Antimalarial activity of thiosemicarbazones and purine derived nitriles

Malaria is a devastating illness caused by multiple species of the Plasmodium genus. The parasite's falcipain proteases have been extensively studied as potential drug targets. Here we report the testing of two established cysteine protease inhibitor scaffolds against both chloroquine sensitive and chloroquine resistant parasites. A subset of purine derived nitriles killed the parasite with moderate potency, and these inhibitors do not seem to exert their antiproliferative effects as cysteine protease inhibitors. Compound potency was determined to be similar against both parasite strains, indicating a low probability of cross resistance with chloroquine. These compounds represent a novel antimalarial scaffold, and a potential starting point for the development new inhibitors.

Comparative study of interactions between chloroquine and chlorpheniramine or promethazine in healthy volunteers: a potential combination-therapy phenomenon for resuscitating chloroquine for malaria treatment in Africa

Although, in in-vitro and limited in-vivo studies, chlorpheniramine (CP) and promethazine (PR) have each been shown to reverse chloroquine (CQ) resistance, the pharmacokinetic basis of this reversal has not been fully elucidated. In the present study, 15 healthy volunteers were randomly allotted to receive standard doses of CQ alone or in combination with CP or PR. Blood samples were collected from each volunteer at 21 time-points, from immediately before to 168 h after the initial dose. These samples were used to follow the changes in the plasma and erythrocytic concentrations of CQ. The ratio between the mean maximum CQ concentration in the erythrocytes and that in the plasma was 4.2 for the volunteers given CQ alone, 7.3 in those given CQ-CP, and 3.2 in those given CQ-PR. CP significantly enhanced the erythrocytic accumulation of CQ, increasing the maximum CQ concentration observed in the erythrocytes by 24% (P = 0.02). The bio-availability of CQ was also significantly increased in the presence of CP, with the mean value for the area under the curve, of erythrocytic concentration v. time, increasing from 99,921 to 214,516 ng/ml.h (P=0.001). The mean half-life of CQ in the erythrocytes also increased when CP was used, from 51 to 100 h, but this change was not statistically significant (P=0.83). In contrast to CP, PR had no statistically significant effect on the disposition of CQ. As CP clearly enhances disposition of CQ, a combination of CQ with CP may be useful in the management of CQ-resistant infections. Detailed toxicological studies are required to understand the full clinical implications of CP's elevation of erythrocytic CQ concentrations.

Recent advances in antimalarial drug development

Malaria caused by protozoa of the genus Plasmodium, because of its prevalence, virulence, and drug resistance, is the most serious and widespread parasitic disease encountered by mankind. The inadequate armory of drugs in widespread use for the treatment of malaria, development of strains resistant to commonly used drugs such as chloroquine, and the lack of affordable new drugs are the limiting factors in the fight against malaria. These factors underscore the continuing need of research for new classes of antimalarial agents, and a re-examination of the existing antimalarial drugs that may be effective against resistant strains. This review provides an in-depth look at the most significant progress made during the past 10 years in antimalarial drug development.

PfCRT and the trans-vacuolar proton electrochemical gradient: regulating the access of chloroquine to ferriprotoporphyrin IX

It is accepted that resistance of Plasmodium falciparum to chloroquine (CQ) is caused primarily by mutations in the pfcrt gene. However, a consensus has not yet been reached on the mechanism by which resistance is achieved. CQ-resistant (CQR) parasite lines accumulate less CQ than do CQ-sensitive (CQS) parasites. The CQR phenotype is complex with a component of reduced energy-dependent CQ uptake and an additional component that resembles energy-dependent CQ efflux. Here we show that the required energy input is in the form of the proton electrochemical gradient across the digestive vacuole (DV) membrane. Collapsing the DV proton gradient (or starving the parasites of glucose) results in similar levels of CQ accumulation in CQS and CQR lines. Under these conditions the accumulation of CQ is stimulated in CQR parasite lines but is reduced in CQS lines. Energy deprivation has no effect on the rate of CQ efflux from CQR lines implying that mutant PfCRT does not function as an efflux pump or active carrier. Using pfcrt-modified parasite lines we show that the entire CQ susceptibility phenotype is switched by the single K76T amino acid change in PfCRT. The efflux of CQ in CQR lines is not directly coupled to the energy supply, consistent with a model in which mutant PfCRT functions as a gated channel or pore, allowing charged CQ species to leak out of the DV.

Quinoline-resistance reversing agents for the malaria parasite Plasmodium falciparum

Resistance to quinoline antimalarials, especially to chloroquine and mefloquine has had a major impact on the treatment of malaria worldwide. In the period since 2000, significant progress has been made in understanding the origins of chloroquine resistance and to a lesser extent mefloquine resistance in Plasmodium falciparum. Chloroquine resistance correlates directly with mutations in the pfcrt gene of the parasite, while changes in another gene, pfmdr1, may also be related to chloroquine resistance in some strains. Mutations in pfcrt do not appear to correlate with mefloquine resistance, but some studies have implicated pfmdr1 in mefloquine resistance. Its involvement however, has not been definitively demonstrated. The protein products of these genes, PfCRT and Pgh-1 are both located in the food vacuole membrane of the parasite. Current evidence suggests that PfCRT is probably a transporter protein. Chloroquine appears to exit the food vacuole via this transporter in resistant PfCRT mutants. Pgh-1 on the other hand, resembles mammalian multi-drug resistance proteins and appears to be involved in expelling hydrophobic drugs from the food vacuole. Resistance reversing agents are believed to act by inhibiting these proteins. The currently known chloroquine- and mefloquine-resistance reversing agents are discussed in this review. This includes a discussion of structure-activity relationships in these compounds and hypotheses on their possible mechanisms of action. The status of current clinical applications is also briefly discussed.

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.

Antimalarial activity of 4-(5-trifluoromethyl-1H-pyrazol-1-yl)-chloroquine analogues

The antimalarial activity of chloroquine-pyrazole analogues, synthesized from the reaction of 1,1,1-trifluoro-4-methoxy-3-alken-2-ones with 4-hydrazino-7-chloroquinoline, has been evaluated in vitro against a chloroquine resistant Plasmodium falciparum clone. Parasite growth in the presence of the test drugs was measured by incorporation of [(3)H]hypoxanthine in comparison to controls with no drugs. All but one of the eight (4,5-dihydropyrazol-1-yl) chloroquine 2 derivatives tested showed a significant activity in vitro, thus, are a promising new class of antimalarials. The three most active ones were also tested in vivo against Plasmodium berghei in mice. However, the (pyrazol-1-yl) chloroquine 3 derivatives were mostly inactive, suggesting that the aromatic functionality of the pyrazole ring was critical.

Design and synthesis of new antimalarial agents from 4-aminoquinoline

This study describes the synthesis of new 4-aminoquinoline derivatives and evaluation of their activity against a chloroquine sensitive strain of P. falciparum in vitro and chloroquine resistant N-67 strain of P. yoelii in vivo. All the analogues were found to form strong complex with hematin and inhibit the beta-hematin formation in vitro. These results suggest that these compounds act on heme polymerization target.

Reversal of chloroquine resistance in Plasmodium falciparum by 9H-xanthene derivatives

Four new chemosensitisers against chloroquine-resistant Plasmodium falciparum based on the 9H-xanthene tricyclic scaffold were designed and synthesised in an attempt to identify simplified compounds that are easily accessible from commercially available starting materials. The compounds contain a common hydrophobic tricyclic 9H-xanthene moiety and an alkyl side chain with two amino groups, one of which is a tertiary substituted terminal amine, separated by three carbons and differing only in the chemical nature of the intermediary nitrogen atom. The best chemosensitising compound has a secondary amino group, showed a response modification index of 0.36 and caused a four-fold increase in chloroquine accumulation in a resistant strain of P. falciparum as well as having the highest selective therapeutic index when tested against a mammalian cell line.

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.

ATPase activity of purified plasma membranes and digestive vacuoles from Plasmodium falciparum

The ATPase activity of the human malaria parasite, Plasmodium falciparum was investigated using two experimental systems, (i) digestive vacuoles, and (ii) purified plasma membranes isolated from a chloroquine-sensitive and a chloroquine-resistant strain. No correlation between the level of ATPase activity and chloroquine sensitivity could be detected. In both systems, the ATPase activity of the chloroquine-resistant and -sensitive strain was decreased in the presence of the P-glycoprotein inhibitor vanadate. Susceptibility to inhibition by vanadate together with the lack of effect of ouabain implies a P-type ATPase activity in the plasma membrane. Furthermore, the inhibition of Fac8 ATPase activity by oligomycin both in the digestive vacuoles and the plasma membranes would be consistent with higher levels of Pgh1 in Fac8. Our data are consistent with the presence of a V-type H+-ATPase in the parasite food vacuole. Bafilomycin A1 and N-ethylmaleimide decreased the vacuolar ATPase activity in both chloroquine-resistant and -sensitive strains. Interestingly, a 30% decrease was observed between the ATPase activity of plasma membranes isolated from Fac8 and D10 in the presence of bafilomycin A1, suggesting the presence of a V-type ATPase in D10 plasma membrane that is underexpressed or altered in the plasma membrane of the chloroquine-resistant Fac8. The chemosensitisers tested had no effect on the ATPase activity of chloroquine-resistant P. falciparum in both systems suggesting that their activity is not mediated through an ATP-dependent mechanism. No effect was observed on the vacuolar ATPase activity in the presence of the antimalarials tested indicating that an ATP-dependent transport has not been activated.

Evidence for a central role for PfCRT in conferring Plasmodium falciparum resistance to diverse antimalarial agents

Chloroquine resistance in Plasmodium falciparum is primarily conferred by mutations in pfcrt. Parasites resistant to chloroquine can display hypersensitivity to other antimalarials; however, the patterns of crossresistance are complex, and the genetic basis has remained elusive. We show that stepwise selection for resistance to amantadine or halofantrine produced previously unknown pfcrt mutations (including S163R), which were associated with a loss of verapamil-reversible chloroquine resistance. This was accompanied by restoration of efficient chloroquine binding to hematin in these selected lines. This S163R mutation provides insight into a mechanism by which PfCRT could gate the transport of protonated chloroquine through the digestive vacuole membrane. Evidence for the presence of this mutation in a Southeast Asian isolate supports the argument for a broad role for PfCRT in determining levels of susceptibility to structurally diverse antimalarials.

Polymorphism in the Plasmodium falciparum chloroquine-resistance transporter protein links verapamil enhancement of chloroquine sensitivity with the clinical efficacy of amodiaquine

Background

Chloroquine accumulates in the acidic digestive vacuole of the intraerythrocytic malaria parasite, and prevents the detoxication of haematin released during haemoglobin digestion. Changes in protein PfCRT in the digestive vacuole membrane of growing intra-erythrocytic stages of Plasmodium falciparum are crucial for resistance. Expressed in yeast, PfCRT resembles an anion channel. Depressed anion channel function could increase intralysosomal pH to reduce entry of basic drug, or enhanced function could reduce drug interaction with target haematin. The most important resistance-associated change is from positively-charged lysine-76 to neutral threonine which could facilitate drug efflux through a putative channel. It has been proposed that the resistance-reversing effect of verapamil is due to hydrophobic binding to the mutated PfCRT protein, and replacement of the lost positive charge, which repels the access of 4-aminoquinoline cations, thus partially restoring sensitivity. Desethylamodiaquine, the active metabolite of amodiaquine, which has significant activity in chloroquine-resistance, may also act similarly on its own.

Methods

Changes in physicochemical parameters in different CQ-resistant PfCRT sequences are analysed, and correlations with drug activity on lines transfected with different alleles of the pfcrt gene are examined.

Results and conclusions

The results support the idea that PfCRT is a channel which, in resistant parasites, can allow efflux of chloroquine from the digestive vacuole. Activity of the chloroquine/verapamil combination and of desethylamodiaquine both correlate with the mean hydrophobicity of PfCRT residues 72-76. This may partly explain clinical-resistance to amodiaquine found in the first chloroquine-resistant malaria cases from South America and enables tentative prediction of amodiaquine's clinical activity against novel haplotypes of PfCRT.

The relationship of physico-chemical properties and structure to the differential antiplasmodial activity of the cinchona alkaloids

Background

The 8-amino and 9-hydroxy substituents of antimalarial cinchona alkaloids have the erythro orientation while their inactive 9-epimers are threo. From the X-ray structures a 90° difference in torsion angle between the N1-H1 and C9-O12 bonds in the two series is believed to be important. In order to kill the malaria parasite, alkaloids must cross the erythrocyte and parasite membranes to accumulate in the acid digestive vacuole where they prevent detoxication of haematin produced during haemoglobin breakdown.

Methods

Ionization constants, octanol/water distribution and haematin interaction are examined for eight alkaloids to explain the influence of small structural differences on activity.

Results

Erythro isomers have a high distribution ratio of 55:1 from plasma to the erythrocyte membrane, while for the more basic threo epimers this is only 4.5:1. This gives an increased transfer rate of the erythro drugs into the erythrocyte and thence into the parasite vacuole where their favourable conformation allows interaction with haematin, inhibiting its dimerization strongly (90 ± 7%) and thereby killing the parasite. The threo compounds not only enter more slowly but are then severely restricted from binding to haematin by the gauche alignment of their N1-H1 and C9-O12 bonds. Confirmatory molecular models allowed measurement of angles and bond lengths and computation of the electronic spectrum of a quinine-haematin complex.

Conclusion

Differences in the antiplasmodial activity of the erythro and threo cinchona alkaloids may therefore be attributed to the cumulative effects of lipid/aqueous distribution ratio and drug-haematin interaction. Possible insights into the mechanism of chloroquine-resistance are discussed.

In vitro increase in chloroquine accumulation induced by dihydroethano- and ethenoanthracene derivatives in Plasmodium falciparum-parasitized erythrocytes

The effects of a series of dihydroethano- and ethenoanthracene derivatives on chloroquine (CQ) accumulation in CQ-susceptible strain 3D7 and CQ-resistant clone W2 were assessed. The levels of CQ accumulation increased little or none in CQ-susceptible strain 3D7 and generally increased markedly in CQ-resistant strain W2. At 10 microM, 28 compounds yielded cellular accumulation ratios (CARs) greater than that observed with CQ alone in W2. At 10 microM, in strain W2, 21 of 31 compounds had CQ CARs two or more times higher than that of CQ alone, 15 of 31 compounds had CQ CARs three or more times higher than that of CQ alone, 13 of 31 compounds had CQ CARs four or more times higher than that of CQ alone, and 9 of 31 compounds had CQ CARs five or more times higher than that of CQ alone. At 1 microM, 17 of 31 compounds had CQ CARs two or more times higher than that of CQ alone, 12 of 31 compounds had CQ CARs three or more times higher than that of CQ alone, 6 of 31 compounds had CQ CARs four or more times higher than that of CQ alone, and 3 of 31 compounds had CQ CARs five or more times higher than that of CQ alone. At 1 microM, 17 of 31 compounds were more potent inducers of CQ accumulation than verapamil and 12 of 31 compounds were more potent inducers of CQ accumulation than promethazine. The nature of the basic group seems to be associated with increases in the levels of CQ accumulation. At 1 and 10 microM, 10 of 14 and 13 of 14 compounds with amino group (amines and diamines), respectively, had CARs >or=3, while at 1 and 10 microM, only 1 of the 13 derivatives with amido groups had CARs >or=3. Among 12 of the 31 compounds which were more active inducers of CQ accumulation than promethazine at 1 microM, 10 had amino groups and 1 had an amido group.

Role of the neurotransmitter reuptake-blocking activity of antidepressants in reversing chloroquine resistance in vitro in Plasmodium falciparum

Since the discovery of the chloroquine (CQ) resistance reversal properties of several different, structurally unrelated classes of compounds, including antidepressants, the way is again open to employ the aminoquinoline drugs to combat malaria effectively. In this study, CQ sensitivity was restored to varying extents in vitro in the CQ-resistant Plasmodium falciparum strain RSA11 by using the antidepressants amitriptyline, citalopram, oxaprotiline, and nomifensine. The 50% inhibitory concentrations (IC(50)) of CQ were reduced from 360 to as low as 11 nM when antidepressants were present. These particular antidepressants are highly specific for blocking the ATP-binding cassette transport protein-mediated reuptake of different neurotransmitters at the synaptic level. This study was aimed at determining the extent to which the neurotransmitter reuptake-blocking properties of these antidepressants play a role in the reversal process. None of the compounds or CQ-antidepressant combinations tested had innate antimalarial activity. No chemosensitizer or combination showed an increased CQ accumulation or significant shift in the IC(50) in the CQ-sensitive clone D10. Of the compounds tested, citalopram, a highly specific serotonin reuptake blocker, produced the largest shift observed in the IC(50) for the resistant isolate RSA11. No particular class of antidepressant was found to be better than any other at restoring CQ sensitivity. We conclude that the resistance-reversing properties of these compounds do not correlate with their activities as reuptake blockers.

Antimalarial drug development and new targets

The Molecular Approaches to Malaria (MAM2000) conference, Lorne, Australia, 2-5 February 2000, brought together world-class malaria research scientists. The development of new tools and technologies - transfection, DNA microarrays and proteomic analysis - and the availability of DNA sequences generated by the Malaria Genome Project, along with more classic approaches, have facilitated the identification of novel drug targets, the development of new antimalarials and the generation of a deeper understanding of the molecular mechanism(s) of drug resistance in malaria. It is hoped that combinations of these technologies could lead to strategies that enable the development of effective, efficient and affordable new drugs to overcome drug-resistant malaria, as discussed at MAM2000 and outlined here by Ian Macreadie and colleagues.

Cellular uptake of chloroquine is dependent on binding to ferriprotoporphyrin IX and is independent of NHE activity in Plasmodium falciparum

Here we provide definitive evidence that chloroquine (CQ) uptake in Plasmodium falciparum is determined by binding to ferriprotoporphyrin IX (FPIX). Specific proteinase inhibitors that block the degradation of hemoglobin and stop the generation of FPIX also inhibit CQ uptake. Food vacuole enzymes can generate cell-free binding, using human hemoglobin as a substrate. This binding accounts for CQ uptake into intact cells and is subject to identical inhibitor specificity. Inhibition of CQ uptake by amiloride derivatives occurs because of inhibition of CQ-FPIX binding rather than inhibition of the Na+/H+ exchanger (NHE). Inhibition of parasite NHE using a sodium-free medium does not inhibit CQ uptake nor does it alter the ability of amilorides to inhibit uptake. CQ resistance is characterized by a reduced affinity of CQ-FPIX binding that is reversible by verapamil. Diverse compounds that are known to disrupt lysosomal pH can mimic the verapamil effect. These effects are seen in sodium-free medium and are not due to stimulation of the NHE. We propose that these compounds increase CQ accumulation and overcome CQ resistance by increasing the pH of lysosomes and endosomes, thereby causing an increased affinity of binding of CQ to FPIX.

Altered binding of chloroquine to ferriprotoporphyrin IX is the basis for chloroquine resistance

The antimalarial specificity of chloroquine (CQ) stems from the saturable uptake of the drug into malaria parasites. Strains of Plasmodium falciparum that are resistant to CQ have evolved a mechanism to reduce the saturable uptake of CQ and several biochemical models have been proposed to explain this. These include an efflux process analogous to multi-drug resistance (MDR) in cancer cells, reduced proton trapping due to elevated vacuolar pH, reduced binding to an intracellular receptor and reduced activity of a permease or drug importer. Here, we attempt to reconcile many of the apparently conflicting data used to support these models. Previous data are analysed in the context of our own model in which CQ uptake is determined by access of the drug to ferriprotoporphyrin IX (FPIX), the intracellular receptor. Copyright 1999 Harcourt Publishers Ltd.

An overview of chemotherapeutic targets for antimalarial drug discovery

The need for new antimalarials comes from the widespread resistance to those in current use. New antimalarial targets are required to allow the discovery of chemically diverse, effective drugs. The search for such new targets and new drug chemotypes will likely be helped by the advent of functional genomics and structure-based drug design. After validation of the putative targets as those capable of providing effective and safe drugs, targets can be used as the basis for screening compounds in order to identify new leads, which, in turn, will qualify for lead optimization work. The combined use of combinatorial chemistry--to generate large numbers of structurally diverse compounds--and of high throughput screening systems--to speed up the testing of compounds--hopefully will help to optimize the process. Potential chemotherapeutic targets in the malaria parasite can be broadly classified into three categories: those involved in processes occurring in the digestive vacuole, enzymes involved in macromolecular and metabolite synthesis, and those responsible for membrane processes and signalling. The processes occurring in the digestive vacuole include haemoglobin digestion, redox processes and free radical formation, and reactions accompanying haem release followed by its polymerization into haemozoin. Many enzymes in macromolecular and metabolite synthesis are promising potential targets, some of which have been established in other microorganisms, although not yet validated for Plasmodium, with very few exceptions (such as dihydrofolate reductase). Proteins responsible for membrane processes, including trafficking and drug transport and signalling, are potentially important also to identify compounds to be used in combination with antimalarial drugs to combat resistance.

Role for the plasmodium falciparum digestive vacuole in chloroquine resistance

We have developed a method for the isolation of pure and intact Plasmodium falciparum digestive vacuoles capable of ATP-dependent chloroquine (CQ) accumulation in vitro. The method is rapid and reliable, and it produces a high yield of vacuoles (20%). CQ accumulation in isolated vacuoles was found to be ATP-, Mg2+-, and temperature-dependent. We then investigated the CQ-accumulating capabilities of vacuoles isolated from CQ-resistant (CQR) and CQ-sensitive (CQS) parasites. At external CQ concentrations of 100 and 250 nM, vacuoles isolated from two CQS strains (D10 and RSA3) (Vm: 380-424 fmol/10(6) vacuoles/hr) accumulated significantly more CQ (approximately 3 times) than those isolated from three (FAC8, RSA11, and RSA15) of the four CQ-resistant strains of P. falciparum tested (Vmax: 127-156 fmol/10(6) vacuoles/hr) (P < or = 0.05). We propose that the low level of CQ accumulation observed in vacuoles isolated from most of the CQ-resistant parasites tested contributes to the decreased CQ accumulation seen in these strains and, hence, to CQ resistance. Although it is often suggested that the digestive vacuole of the P. falciparum parasite is involved in the mechanism of CQ resistance, to our knowledge this is the first direct confirmation.

Access to hematin: the basis of chloroquine resistance

The saturable uptake of chloroquine by parasites of Plasmodium falciparum has been attributed to specific carrier-mediated transport of chloroquine. It is suggested that chloroquine is transported in exchange for protons by the parasite membrane Na+/H+ exchanger [J Biol Chem 272:2652-2658 (1997)]. Once inside the parasite, it is proposed that chloroquine inhibits the polymerization of hematin, allowing this toxic hemoglobin metabolite to accumulate and kill the cell [Pharmacol Ther 57:203-235 (1993)]. To date, the contribution of these proposed mechanisms to the uptake and antimalarial activity of chloroquine has not been assessed. Using sodium-free medium, we demonstrate that chloroquine is not directly exchanged for protons by the plasmodial Na+/H+ exchanger. Furthermore, we show that saturable chloroquine uptake at equilibrium is due solely to the binding of chloroquine to hematin rather than active uptake: using Ro 40-4388, a potent and specific inhibitor of hemoglobin digestion and, by implication, hematin release, we demonstrate a concentration-dependent reduction in the number of chloroquine binding sites. An equal number of chloroquine binding sites are found in both resistant and susceptible clones, but the apparent affinity of chloroquine binding is found to correlate with drug activity (r2 = 0.93, p < 0.0001). This completely accounts for both the reduced drug accumulation and activity observed in resistant clones and the "reversal" of resistance produced by verapamil. The data presented here reconcile most of the available biochemical data from studies of the mode of action of chloroquine and the mechanism of chloroquine resistance. We show that the activity of chloroquine and amodiaquine is directly dependent on the saturable binding of the drugs to hematin and that the inhibition of hematin polymerization may be secondary to this binding. The chloroquine-resistance mechanism regulates the access of chloroquine to hematin. Our model is consistent with a resistance mechanism that acts specifically at the food vacuole to alter the binding of chloroquine to hematin rather than changing the active transport of chloroquine across the parasite plasma membrane.

High-level chloroquine resistance of Plasmodium berghei is associated with multiple drug resistance and loss of reversal by calcium antagonists

The chloroquine resistance of Plasmodium falciparum is reversed in vitro by numerous compounds, including calcium antagonists, which could enhance the accumulation of the drug in the parasite food vacuole. However, this mechanism of resistance could be insufficient when the resistance level increases. Using in vitro drug trials on strains of Plasmodium berghei displaying various chloroquine-resistance levels, we confirmed previous results obtained in vivo in the chloroquine-resistant strains of P. berghei are cross-resistant to related drugs (amodiaquine, quinine and mefloquine), the resistance levels to these drugs being related to their analogy to chloroquine. Furthermore, we showed that high-level resistant lines were associated with a loss of drug potentiation by verapamil and nicardipine in vivo, but that the reversal rates obtained in vitro are of low significance. We conclude that the parasite is able to escape the activity of these reversing agents.

In vitro antimalarial activity of a new organometallic analog, ferrocene-chloroquine

The in vitro activities of new organometallic chloroquine analogs, based on 4-amino-quinoleine compounds bound to a molecule of ferrocene, were evaluated against chloroquine-susceptible, chloroquine-intermediate, and chloroquine-resistant, culture-adapted Plasmodium falciparum lineages by a proliferation test. One of the ferrocene analogs totally restored the activity of chloroquine against chloroquine-resistant parasites. This compound, associated with tartaric acid for better solubility, was highly effective. The role of the ferrocene in reversing chloroquine resistance is discussed, as is its potential use for human therapy.

Synergy between two calcium channel blockers, verapamil and fantofarone (SR33557), in reversing chloroquine resistance in Plasmodium falciparum

This study describes the synergistic interaction of two calcium channel blockers, verapamil (VR) and SR33557 or fantofarone (SR), in reversing chloroquine resistance in Plasmodium falciparum, the causative agent of human malaria. The two calcium channel blockers exhibited an intrinsic antimalarial activity at 10 and 1 microM for verapamil and fantofarone, respectively. Isobolograms revealed that chloroquine and verapamil, and chloroquine and fantofarone, acted synergistically against chloroquine-resistant strains of P. falciparum. When used at subinhibitory concentrations, verapamil appeared 2 to 3 times more potent than fantofarone in reversing chloroquine resistance. Indeed, verapamil completely reversed the chloroquine resistance in P. falciparum, while fantofarone did so only partially. In the highly chloroquine-resistant strain FcB1, VR and SR acted synergistically to reverse CQ resistance, and the concentrations of VR used in these combinations could be reduced 10- or 100-fold (e.g. 100 nM and 10 nM) those required when this drug was used alone. In the moderately chloroquine-resistant strain K1, a combination of VR and SR for CQ resistance reversal allowed us to reduce the concentration of these chemosensitizers 1000- and 100-fold, respectively. The maximum tolerable plasma level beyond which side-effects occurred when using verapamil is 2.5 microM. Thus, the approach described, which allowed us to lower the doses of chemosensitizers, could well prevent toxic effects in humans and enlighten the advantages of polychemotherapy.

4-Aminoquinolines--past, present, and future: a chemical perspective

The 4-aminoquinoline chloroquine (1) can be considered to be one of the most important synthetic chemotherapeutic agents in history. Since its discovery, chloroquine has proved to be a highly effective, safe, and well-tolerated drug for the treatment and prophylaxis of malaria. However, the emergence of chloroquine-resistant strains of the malarial parasite has underlined the requirement for a synthetic alternative to chloroquine. This review describes structure-activity relationships for the 4-aminoquinolines, along with views on the mechanism of action and parasite resistance. A description of drug metabolism and toxicity also is included, with a brief description of potential approaches to the design of new synthetic derivatives.

A comparison of the phenomenology and genetics of multidrug resistance in cancer cells and quinoline resistance in Plasmodium falciparum

Plasmodium falciparum is the causative agent of the most deadly form of human malaria. Chemotherapy traditionally has been the main line of defense against this parasite, and chloroquine, the drug of choice, has been one of the most successful drugs ever developed. Unfortunately, the evolution and spread of resistance to chloroquine and other quinoline-containing drugs means that these compounds are now virtually useless in many endemic areas. Future prospects for the use of quinoline compounds improved considerably when it was demonstrated that chloroquine resistance could be circumvented in vitro by a number of structurally and functionally unrelated compounds such as verapamil and desipramine. The phenomenon of resistance reversal by compounds such as verapamil is also a key feature of drug resistance in mammalian cells, and this has raised the possibility that the underlying mechanisms of drug resistance of the two cell types could be similar. This hypothesis has prompted a large number of studies into the genetics and biochemistry of resistance to quinoline-containing drugs in P. falciparum. Both the genetic and the biochemical studies have raised issues of controversy and stimulated much debate. These issues are discussed in this review, in the context of a comparison with the genetics and biochemistry of multidrug resistance in mammalian cells.

Complex polymorphisms in an approximately 330 kDa protein are linked to chloroquine-resistant P. falciparum in Southeast Asia and Africa

Chloroquine resistance in a P. falciparum cross maps as a Mendelian trait to a 36 kb segment of chromosome 7. This segment harbors cg2, a gene encoding a unique approximately 330 kDa protein with complex polymorphisms. A specific set of polymorphisms in 20 chloroquine-resistant parasites from Asia and Africa, in contrast with numerous differences in 21 sensitive parasites, suggests selection of a cg2 allele originating in Indochina over 40 years ago. One chloroquine-sensitive clone exhibited this allele, suggesting another resistance component. South American parasites have cg2 polymorphisms consistent with a separate origin of resistance. CG2 protein is found at the parasite periphery, a site of chloroquine transport, and in association with hemozoin of the digestive vacuole, where chloroquine inhibits heme polymerization.

In vitro selection of halofantrine resistance in Plasmodium falciparum is not associated with increased expression of Pgh1

Recent investigations into quinoline and phenanthrene methanol resistance in Plasmodium falciparum have described a linkage between amplification of the mdr homologue pfmdr1 and decreased susceptibility to mefloquine (MQ) and halofantrine (HF). We have examined the current theories on cross-resistance patterns and pfmdr1 gene expression by comparing the chloroquine (CQ) resistant isolate K1 with K1Hf, developed from the K1 isolate by intermittent exposure to halofantrine. Reduced halofantrine susceptibility in K1Hf was accompanied by reduced sensitivity to mefloquine and increased sensitivity to chloroquine. These sensitivity changes were reflected by changes in parasite drug accumulation. The loss of high level chloroquine resistance in K1Hf was associated with an inability of verapamil to enhance chloroquine sensitivity or accumulation. In contrast verapamil retained the chemosensitising potential against quinine in this isolate. The changes in phenotype were achieved without any amplification or increased expression of pfmdr1 or reversion of the Tyr86 mutation in the gene. Our data indicates that acquisition of halofantrine and mefloquine resistance and the loss of high level chloroquine resistance can be achieved without enhanced expression of the pfmdr1 gene product.

The role of drug accumulation in 4-aminoquinoline antimalarial potency. The influence of structural substitution and physicochemical properties

We have investigated a series of novel 4-aminoquinoline analogues related to amodiaquine, that possess side chain modifications designed to influence both drug pKa and lipophilicity. These compounds have been used to determine the influence of physicochemical properties on antimalarial activity against, and accumulation by, both chloroquine-susceptible and chloroquine-resistant isolates of Plasmodium falciparum. The compounds tested exhibited a 500-fold range of absolute antimalarial potency. Absolute drug potency and drug accumulation were found to be significantly correlated in each of the four isolates of Plasmodium falciparum studied. The level of accumulation was unrelated to lipophilicity and was significantly greater than the predicted levels of accumulation based on drug pKa, compartmental pH, and Henderson-Hasselbach considerations. Further analysis of the relationship between 4-aminoquinoline accumulation and activity implicated the involvement of additional forces in the accumulation process.

The pfmdr1 gene of Plasmodium falciparum confers cellular resistance to antimalarial drugs in yeast cells

The exact role of the pfmdr1 gene in the emergence of drug resistance in the malarial parasite Plasmodium falciparum remains controversial. pfmdr1 is a member of the ATP binding cassette (ABC) superfamily of transporters that includes the mammalian P-glycoprotein family. We have introduced wild-type and mutant variants of the pfmdr1 gene in the yeast Saccharomyces cerevisiae and have analyzed the effect of pfmdr1 expression on cellular resistance to quinoline-containing antimalarial drugs. Yeast transformants expressing either wild-type or a mutant variant of mouse P-glycoprotein were also analyzed. Dose-response studies showed that expression of wild-type pfmdr1 causes cellular resistance to quinine, quinacrine, mefloquine, and halofantrine in yeast cells. Using quinacrine as substrate, we observed that increased resistance to this drug in pfmdr1 transformants was associated with decreased cellular accumulation and a concomitant increase in drug release from preloaded cells. The introduction of amino acid polymorphisms in TM11 of Pgh-1 (pfmdr1 product) associated with drug resistance in certain field isolates of P. falciparum abolished the capacity of this protein to confer drug resistance. Thus, these findings suggest that Pgh-1 may act as a drug transporter in a manner similar to mammalian P-glycoprotein and that sequence variants associated with drug-resistance pfmdr1 alleles behave as loss of function mutations.

Amodiaquine accumulation in Plasmodium falciparum as a possible explanation for its superior antimalarial activity over chloroquine

Amodiaquine is a 4-aminoquinoline antimalarial whose structure is similar to chloroquine. In contrast to the wealth of information available about chloroquine accumulation and its relationship to activity, little is known about the uptake characteristics of amodiaquine, a drug that is inherently more active against malaria parasites. In this study we have investigated the accumulation of amodiaquine in Plasmodium falciparum in vitro, in order to gain an insight into the mechanisms responsible for its superior activity over chloroquine. The driving force for parasite accumulation of the 4-aminoquinolines is proposed to be a transmembrane proton gradient maintained by a vacuolar ATPase. In the present study, amodiaquine accumulation was greatly reduced, at steady state, in the absence of glucose and at 0 degrees C indicating a clear energy dependence of uptake. Amodiaquine accumulation in Plasmodium falciparum was shown to be 2- to 3-fold greater than chloroquine accumulation. This observation probably accounts for amodiaquine's greater inherent activity but is surprising given that amodiaquine is a weaker base than chloroquine. With this in mind we present evidence for an intraparasitic binding component in the accumulation of the 4-aminoquinolines. Differences in binding affinity of this 'receptor' for amodiaquine and chloroquine may partially explain the greater accumulation and in vitro potency of amodiaquine compared to chloroquine.

Mechanisms of drug resistance in malaria

Plasmodium falciparum causes the most severe form of human malaria which directly results in over two million deaths per year. As there is not yet a useful vaccine against this disease the major form of treatment and control is the use of chemotherapeutic agents. Unfortunately the parasite has managed to devise mechanisms that allow it to evade the action of almost all the antimalarials in our arsenal. The antifolate drugs include the dihydrofolate inhibitors pyrimethamine and proguanil as well as the sulfones and sulfonamides. These antimalarials act on enzymes in the folate pathway. The mechanism of resistance to these compounds involve mutations in the target enzyme that decrease the affinity of binding of the drug. A second major group of antimalarials include the quinine-like compounds. Quinine was one of the first compounds used to treat malaria and the related drug chloroquine is the most important antimalarial. Mefloquine and halofantrine were developed in response to major problems with the spread of chloroquine resistance. Chloroquine resistance is due to the ability of the parasite to decrease the accumulation of the drug in the cell. The exact mechanism that allows this is still under investigation although at least one protein has been identified that affects the accumulation of this important antimalarial.

Verapamil reversal of chloroquine resistance in the malaria parasite Plasmodium falciparum is specific for resistant parasites and independent of the weak base effect

Verapamil increases the net uptake and cytotoxicity of structurally diverse hydrophobic molecules in many multidrug-resistant mammalian cell lines. This compound has also been reported to reverse chloroquine resistance in the human malaria parasite Plasmodium falciparum (Martin, S.K., Oduola, A.M.J., and Milhous, W.K. (1987) Science 235, 899-901). Although the mechanism of this reversal is unknown, it apparently involves an increase in the amount of chloroquine present in erythrocytes infected with the resistant parasites. Chloroquine is a diprotic weak base that accumulates in acidic organelles as a function of the pH gradient present between the organelle and the external medium. By changing the external medium pH, this property of chloroquine was used to alter the cytotoxicity phenotype of genetically chloroquine-sensitive and -resistant trophozoites. Verapamil was also found to be toxic for malaria trophozoites, although this toxicity was independent of external pH and consistently about 3-4-fold higher against resistant strains. When verapamil was tested for its effects on chloroquine cytotoxicity under conditions of phenotypic reversal, it was still found to exert only a measurable effect on the genetically resistant trophozoites. In short time incubations, verapamil was found to increase net chloroquine accumulation in erythrocytes infected with both chloroquine-sensitive and -resistant organisms, but only to affect the chloroquine susceptibility of the latter. Analysis of our data demonstrates that verapamil works independently of the overall pH gradient concentrating chloroquine into a trophozoite's lysosome. Instead, we propose that it inhibits the activity of a membrane ion channel indirectly responsible for determining chloroquine transit within the parasite's cytoplasm.

Functional complementation of the ste6 gene of Saccharomyces cerevisiae with the pfmdr1 gene of Plasmodium falciparum

The pfmdr1 gene has been associated with a drug-resistant phenotype in Plasmodium falciparum, and overexpression of pfmdr1 has been associated with mefloquine- and halofantrine-resistant parasites, but little is known about the functional role of pfmdr1 in this process. Here, we demonstrate that the pfmdr1 gene expressed in a heterologous yeast system functions as a transport molecule and complements a mutation in ste6, a gene which encodes a mating pheromone a-factor export molecule. In addition, the pfmdr1 gene containing two mutations which are associated with naturally occurring chloroquine resistance abolishes this mating phenotype, suggesting that these genetic polymorphisms alter this transport function. Our results support the functional role of pfmdr1 as a transport molecule in the mediation of drug resistance and provide an assay system to address the nature of this transport function.

Drug-resistant malaria in children and in travelers

Malaria remains a significant cause of childhood morbidity and mortality worldwide. Drug resistance in Plasmodium falciparum has become widespread in the past 30 years, and in some parts of the world multidrug resistance is common. Chloroquine resistance in Plasmodium vivax has recently been recognized in Indonesia. The mechanisms of drug resistance have been defined for the antifolate antimalarial agents but remain incompletely understood for the quinolines. Judicious use of antimalarial compounds will be essential to prevent the emergence and spread of further drug resistance. The history, geographic distribution, and mechanisms of drug resistance are reviewed, together with current recommendations regarding prophylaxis and therapy.

Current views on the mechanisms of resistance to quinoline-containing drugs in Plasmodium falciparum

The issue of chloroquine resistance in Plasmodium falciparum and cross-resistance patterns with other related chemotherapeutic agents has been the subject of intense interest for many years. Despite this level of investigation, the picture remains very unclear. Although it is accepted that chloroquine resistance is, at least in part, a function of reduced drug accumulation, the question of reduced drug uptake versus enhanced efflux is yet to be resolved at both the molecular and biochemical levels. Further, the absolute cross-resistance patterns of chloroquine-resistant isolates to closely related analogues is a matter for debate, although there appears to be a reciprocal arrangement between resistance to chloroquine and resistance to mefloquine, halofantrine and possibly quinine. Evidence is presented for the coexistence of two or more chloroquine-resistance mechanisms in isolates of P. falciparum, only one of which is verapamil sensitive. In addition, an analysis of cross-resistance patterns, as measured by the inoculum effect, is presented.

Molecular cloning and sequence of two novel P-type adenosinetriphosphatases from Plasmodium falciparum

We have identified two novel P-type ATPase genes from Plasmodium falciparum and report the full-length nucleotide and derived amino acid sequence of the ATPase2 gene from P. falciparum (PfATPase2). PfATPase2 is phylogenetically remote from the different members of prokaryotic and mammalian P-type ATPases but shares features with a putative membrane-spanning Ca2+ ATPase involved in ribosome function in yeast. PfATPase2 is expressed during the intraerythrocytic life cycle of the parasite and appears to be required in the late stages of its asexual development. We also present the partial sequence of another malarial gene displaying sequence similarity with the family of P-type transporting ATPases (PfATPase4). We have analysed the organisation of the genes encoding the P-type ATPases of P. falciparum and show that they are a highly dispersed gene family.

Enhanced lysosomal acidification leads to increased chloroquine accumulation in CHO cells expressing the pfmdr1 gene

Expression of the pfmdr1-encoded Pgh1 protein of Plasmodium falciparum in CHO cells confers a phenotype of increased sensitivity to chloroquine due to an increased Pgh1-mediated accumulation of this antimalarial. Pgh1 carrying amino acid substitutions associated with chloroquine resistance in P. falciparum does not confer this phenotype. Here, we present studies on the underlying mechanism of Pgh1 mediated chloroquine influx into CHO cells. First, we measured intralysosomal pH using FITC-labelled dextran and found the intralysosomal pH in Pgh1 expressing cells to be decreased. A decreased lysosomal pH was not observed in cells expressing Pgh1 carrying the S1034C and N1042D double substitution found in some chloroquine-resistant P. falciparum parasites. Secondly, Pgh1-mediated uptake of chloroquine was abolished in the presence of bafilomycin A1, a specific inhibitor of vacuolar [H+]ATPases and was nearly abrogated in the presence of NH4Cl. Finally, cells expressing wild-type Pgh1 showed increased uptake of both (+)- and (-)[3H]chloroquine enantiomers, indicating that Pgh1-mediated uptake of chloroquine is not enantioselective and in agreement with a pH-driven process. We conclude from these studies that Pgh1 does not transport chloroquine, but instead influences chloroquine accumulation by modulating the pH of acidic organelles. This function is abolished in Pgh1 carrying amino acid substitutions S1034C and N1042D. We speculate that the pfmdr1 gene encodes a vacuolar chloride channel.

Uptake and efflux of chloroquine by chloroquine-resistant Plasmodium falciparum clones recently isolated in Africa

In recently isolated African Plasmodium falciparum clones, the intracellular chloroquine concentration at steady-state, under standard culture conditions, could not differentiate chloroquine-sensitive from resistant parasites. However, under an atmosphere of air the chloroquine-resistant P. falciparum clones released pre-accumulated [3H]chloroquine more rapidly than sensitive clones. The very fast efflux of the pre-accumulated drug from chloroquine-resistant (CQR) parasites resulted in a differential in the drug retained by resistant and sensitive parasites. The chloroquine-sensitive parasites retained 2-3 times more chloroquine than resistant parasites. The steady-state uptake of [3H]chloroquine appeared to be enhanced by verapamil and desipramine in the chloroquine-resistant clones, while the opposite was observed with sensitive clones. This confirmed the suggestion that verapamil inhibits the rapid efflux in CQR parasites resulting in a readily detectable increase in chloroquine accumulation. These observations indicate that the biochemical phenotypes of African chloroquine-resistant P. falciparum are similar to those reported from S.E. Asia and Latin America and are consistent with a common molecular basis for the phenomenon.

Cloning and characterization of the vacuolar ATPase B subunit from Plasmodium falciparum

The transvacuolar pH gradient determines, to a significant extent, the distribution of the antimalarial drug chloroquine in Plasmodium falciparum. A proton pump, similar to the vacuolar ATPase found in many cell types, appears to regulate a pH gradient across the membranes of acidic compartments of the parasite. In order to understand and define the components involved in the maintenance of the vacuolar pH gradient, we have cloned and characterized a gene, designated VAP B, encoding a P. falciparum homologue of the B subunit of the vacuolar ATPase. The VAP B gene encodes a protein of 494 amino acids which has between 69% and 74% amino acid identity with the sequences of vacuolar ATPase B subunits of other organisms. The VAP B gene exists as a single copy gene on chromosome 4 that gives rise to a RNA transcript of 2.4 kb. Antibodies raised to the VAP B protein react specifically with a protein of 56-kDa, consistent with the size predicted from the gene sequence and with the homologous protein from other organisms. The 56-kDa protein is expressed throughout the asexual life cycle and subcellular localization by indirect immunofluorescence shows that the protein has a heterogeneous distribution over most of the parasite. This suggests that the function of the vacuolar proton ATPase is not confined to the regulation of the pH of the digestive vacuole.