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

Effect of liquiritigenin on chloroquine accumulation in digestive vacuole leading to apoptosis-like death of chloroquine-resistant P. falciparum

BACKGROUND

Malaria remains one of the major health concerns, especially in tropical countries. Although drugs such as artemisinin-based combinations are efficient for treating Plasmodium falciparum, the growing threat from multi-drug resistance has become a major challenge. Thus, there is a constant need to identify and validate new combinations to sustain current disease control strategies to overcome the challenge of drug resistance in the malaria parasites. To meet this demand, liquiritigenin (LTG) has been found to positively interact in combination with the existing clinically used drug chloroquine (CQ), which has become unfunctional due to acquired drug resistance.

PURPOSE

To evaluate the best interaction between LTG and CQ against CQ- resistant strain of P. falciparum. Furthermore, the in vivo antimalarial efficacy and possible mechanism of action of the best combination was also assessed.

METHODS

The in vitro anti-plasmodial potential of LTG against CQ- resistant strain K1 of P. falciparum was tested using Giemsa staining method. The behaviour of the combinations was evaluated using the fix ratio method and evaluated the interaction of LTG and CQ by calculating the fractional inhibitory concentration index (FICI). Oral toxicity study was carried out in a mice model. In vivo antimalarial efficacy of LTG alone and in combination with CQ was evaluated using a four-day suppression test in a mouse model. The effect of LTG on CQ accumulation was measured using HPLC and the rate of alkalinization of the digestive vacuole. Cytosolic Ca²⁺ level, mitochondrial membrane potential, caspase-like activity, terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay, and Annexin V Apoptosis assay to assess anti-plasmodial potential. Proteomics analysis was evaluated by LC-MS/MS analysis.

RESULTS

LTG possesses anti-plasmodial activity on its own and it showed to be an adjuvant of CQ. In in vitro studies, LTG showed synergy with CQ only in the ratio (CQ: LTG-1:4) against CQ-resistant strain (K1) of P. falciparum. Interestingly, in vivo studies, LTG in combination with CQ showed higher chemo-suppression and enhanced mean survival time at much lower concentrations compared to individual doses of LTG and CQ against CQ- resistant strain (N67) of Plasmodium yoelli nigeriensis. LTG was found to increase the CQ accumulation into digestive vacuole, reducing the rate of alkalinization, in turn increasing cytosolic Ca²⁺ level, loss of mitochondrial potential, caspase-3 activity, DNA damage and externalization of phosphatidylserine of the membrane (in vitro). These observations indicate the involvement of apoptosis-like death of P. falciparum that might be due to the accumulation of CQ.

CONCLUSION

LTG showed synergy with CQ in the ratio LTG: CQ, 4:1) in vitro and was able to curtail the IC₅₀ of CQ and LTG. Interestingly, in vivo in combination with CQ, LTG showed higher chemo-suppression as well as enhanced mean survival time at a much lower concentrations of both the partners as compared to an individual dose of CQ and LTG. Thus, synergistic drug combination offers the possibility to enhance CQ efficacy in chemotherapy.

Knowledge Mapping of Drug Repositioning's Theme and Development

Background

In recent years, the emergence of new diseases and resistance to known diseases have led to increasing demand for new drugs. By means of bibliometric analysis, this paper studied the relevant articles on drug repositioning in recent years and analyzed the current research foci and trends.

Methodology

The Web of Science database was searched to collect all relevant literature on drug repositioning from 2001 to 2022. These data were imported into CiteSpace and bibliometric online analysis platforms for bibliometric analysis. The processed data and visualized images predict the development trends in the research field.

Results

The quality and quantity of articles published after 2011 have improved significantly, with 45 of them cited more than 100 times. Articles posted by journals from different countries have high citation values. Authors from other institutions have also collaborated to analyze drug rediscovery. Keywords found in the literature include molecular docking (N=223), virtual screening (N=170), drug discovery (N=126), machine learning (N=125), and drug-target interaction (N=68); these words represent the core content of drug repositioning.

Conclusion

The key focus of drug research and development is related to the discovery of new indications for drugs. Researchers are starting to retarget drugs after analyzing online databases and clinical trials. More and more drugs are being targeted at other diseases to treat more patients, based on saving money and time. It is worth noting that researchers need more financial and technical support to complete drug development.

Protecting future antimalarials from the trap of resistance: Lessons from artemisinin-based combination therapy (ACT) failures

Having faced increased clinical treatment failures with dihydroartemisinin-piperaquine (DHA-PPQ), Cambodia swapped the first line artemisinin-based combination therapy (ACT) from DHA-PPQ to artesunate-mefloquine given that parasites resistant to piperaquine are susceptible to mefloquine. However, triple mutants have now emerged, suggesting that drug rotations may not be adequate to keep resistance at bay. There is, therefore, an urgent need for alternative treatment strategies to tackle resistance and prevent its spread. A proper understanding of all contributors to artemisinin resistance may help us identify novel strategies to keep artemisinins effective until new drugs become available for their replacement. This review highlights the role of the key players in artemisinin resistance, the current strategies to deal with it and suggests ways of protecting future antimalarial drugs from bowing to resistance as their predecessors did.

P-Glycoprotein Inhibitors Differently Affect Toxoplasma gondii, Neospora caninum and Besnoitia besnoiti Proliferation in Bovine Primary Endothelial Cells

Apicomplexan parasites are obligatory intracellular protozoa. In the case of Toxoplasma gondii, Neospora caninum or Besnoitia besnoiti, to ensure proper tachyzoite production, they need nutrients and cell building blocks. However, apicomplexans are auxotrophic for cholesterol, which is required for membrane biosynthesis. P-glycoprotein (P-gp) is a transmembrane transporter involved in xenobiotic efflux. However, the physiological role of P-gp in cholesterol metabolism is unclear. Here, we analyzed its impact on parasite proliferation in T. gondii-, N. caninum- and B. besnoiti-infected primary endothelial cells by applying different generations of P-gp inhibitors. Host cell treatment with verapamil and valspodar significantly diminished tachyzoite production in all three parasite species, whereas tariquidar treatment affected proliferation only in B. besnoiti. 3D-holotomographic analyses illustrated impaired meront development driven by valspodar treatment being accompanied by swollen parasitophorous vacuoles in the case of T. gondii. Tachyzoite and host cell pre-treatment with valspodar affected infection rates in all parasites. Flow cytometric analyses revealed verapamil treatment to induce neutral lipid accumulation. The absence of a pronounced anti-parasitic impact of tariquidar, which represents here the most selective P-gp inhibitor, suggests that the observed effects of verapamil and valspodar are associated with mechanisms independent of P-gp. Out of the three species tested here, this compound affected only B. besnoiti proliferation and its effect was much milder as compared to verapamil and valspodar.

Effects of combined drug treatments on Plasmodium falciparum: In vitro assays with doxycycline, ivermectin and efflux pump inhibitors

There is great concern regarding the rapid emergence and spread of drug-resistance in Plasmodium falciparum, the parasite responsible for the most severe form of human malaria. Parasite populations resistant to some or all the currently available antimalarial treatments are present in different world regions. Considering the need for novel and integrated approaches to control malaria, combinations of drugs were tested on P. falciparum. The primary focus was on doxycycline, an antibiotic that specifically targets the apicoplast of the parasite. In combination with doxycycline, three different drugs known to inhibit efflux pumps (verapamil, elacridar and ivermectin) were tested, with the assumption that they could increase the intracellular concentration of the antibiotic and consequently its efficacy against P. falciparum. We emphasize that elacridar is a third-generation ABC transporters inhibitor, never tested before on malaria parasites. In vitro experiments were performed on asexual stages of two strains of P. falciparum, chloroquine-sensitive (D10) and chloroquine-resistant (W2). Incubation times on asynchronous or synchronous cultures were 72h or 96h, respectively. The antiplasmodial effect (i.e. the IC₅₀) was determined by measuring the activity of the parasite lactate dehydrogenase, while the interaction between drugs was determined through combination index (CI) analyses. Elacridar achieved an IC₅₀ concentration comparable to that of ivermectin, approx. 10-fold lower than that of verapamil, the other tested ABC transporter inhibitor. CI results showed synergistic effect of verapamil plus doxycycline, which is coherent with the starting hypothesis, i.e. that ABC transporters represent potential targets, worth of further investigations, towards the development of companion molecules useful to enhance the efficacy of antimalarial drugs. At the same time, the observed antagonistic effect of doxycycline in combination with ivermectin or elacridar highlighted the importance of drug testing, to avoid the de-facto generation of a sub-dosage, a condition that facilitates the development of drug resistance.

New solutions using natural products

Most antibiotics are derived from natural products, like penicillin, as well as recent insecticides, like pyrethroids. Secondary metabolites are produced by plants as ecological chemical mediators, and can therefore possess intrinsic physiological properties against other organisms. These benefits are far from being fully explored. In particular, attention is here focused on the multipurpose neem tree (Azadirachta indica), reporting several experiments of applications in the field of seed oil and neem cake. The latter product seems to be promising because of the low cost, the possible production on a large scale, and the selection of effects in favor of beneficial organisms. Neem cake is able to act on different sites, as required by integrated pest management. Several utilizations of neem products are reported and their potentiality evidenced. Some considerations in this chapter may appear distant from the title of the book, but only by applying the general natural rules can the reason of the single phenomenon be understood. Other studies on resistance mechanisms of Plasmodium are enabling new possible methods of control always based on natural products activity.

Uptake of a fluorescently tagged chloroquine analogue is reduced in CQ-resistant compared to CQ-sensitive Plasmodium falciparum parasites

Background

Chloroquine (CQ) was the drug of choice for decades in the treatment of falciparum malaria until resistance emerged. CQ is suggested to accumulate in the parasite’s digestive vacuole (DV), where it unfolds its anti-malarial properties. Discrepancies of CQ accumulation in CQ-sensitive (CQS) and CQ-resistant (CQR) strains are thought to play a significant role in drug susceptibility. Analysis of CQ transport and intracellular localization using a fluorescently tagged CQ analogue could provide much needed information to distinguish susceptible from resistant parasite strains. The fluorescently tagged CQ analogue LynxTag-CQ™_GREEN (CQ_GREEN) is commercially available and was assessed for its suitability.

Methods

IC₅₀ values were determined for both CQ and CQ_GREEN in two CQS and two CQR Plasmodium falciparum strains. Buffer solutions with varying pH were used to determine pH-dependent localization of CQ_GREEN in infected red blood cells. Before CQS or CQR parasites were exposed to different pH buffers, they were pre-loaded with varying concentrations of CQ_GREEN for up to 7 h. Intracellular accumulation was analysed using live cell confocal microscopy. CQ_GREEN uptake rates were determined for the cytosol and DV in the presence and absence of verapamil.

Results

In CQS strains, twofold higher IC₅₀ values were determined for the CQ_GREEN analogue compared to CQ. No significant differences in IC₅₀ values were observed in CQR strains. Addition of verapamil reversed drug resistance of CQR strains to both CQ and CQ_GREEN. Live cell imaging revealed that CQ_GREEN fluorescence was mainly seen in the cytosol of most parasites, independent of the concentration used. Incubation periods of up to 7 h did not influence intracellular localization of CQ_GREEN. Nevertheless, CQ_GREEN uptake rates in CQR strains were reduced by 50% compared to CQS strains.

Conclusion

Although fluorescence of CQ_GREEN was mainly seen in the cytosol of parasites, IC₅₀ assays showed comparable efficacy of CQ_GREEN and CQ in parasite killing of CQS and CQR strains. Reduced uptake rates of CQ_GREEN in CQR strains compared to CQS strains indicate parasite-specific responses to CQ_GREEN exposure. The data contains valuable information when CQ_GREEN is used as an analogue for CQ.

Plasmodium falciparum and Plasmodium vivax Demonstrate Contrasting Chloroquine Resistance Reversal Phenotypes

High-grade chloroquine (CQ) resistance has emerged in both Plasmodium falciparum and P. vivax The aim of the present study was to investigate the phenotypic differences of CQ resistance in both of these species and the ability of known CQ resistance reversal agents (CQRRAs) to alter CQ susceptibility. Between April 2015 and April 2016, the potential of verapamil (VP), mibefradil (MF), L703,606 (L7), and primaquine (PQ) to reverse CQ resistance was assessed in 46 P. falciparum and 34 P. vivax clinical isolates in Papua, Indonesia, where CQ resistance is present in both species, using a modified schizont maturation assay. In P. falciparum, CQ 50% inhibitory concentrations (IC₅₀s) were reduced when CQ was combined with VP (1.4-fold), MF (1.2-fold), L7 (4.2-fold), or PQ (1.8-fold). The degree of CQ resistance reversal in P. falciparum was highly correlated with CQ susceptibility for all CQRRAs (R² = 0.951, 0.852, 0.962, and 0.901 for VP, MF, L7, and PQ, respectively), in line with observations in P. falciparum laboratory strains. In contrast, no reduction in the CQ IC₅₀s was observed with any of the CQRRAs in P. vivax, even in those isolates with high chloroquine IC₅₀s. The differential effect of CQRRAs in P. falciparum and P. vivax suggests significant differences in CQ kinetics and, potentially, the likely mechanism of CQ resistance between these two species.

Comprehensive review on various strategies for antimalarial drug discovery

The resistance of malaria parasites to existing drugs carries on growing and progressively limiting our ability to manage this severe disease and finally lead to a massive global health burden. Till now, malaria control has relied upon the traditional quinoline, antifolate and artemisinin compounds. Very few new antimalarials were developed in the past 50 years. Among recent approaches, identification of novel chemotherapeutic targets, exploration of natural products with medicinal significance, covalent bitherapy having a dual mode of action into a single hybrid molecule and malaria vaccine development are explored heavily. The proper execution of these approaches and proper investment from international agencies will accelerate the discovery of drugs that provide new hope for the control or eventual eradication of this global infectious disease. This review explores various strategies for assessment and development of new antimalarial drugs. Current status and scientific value of previous approaches are systematically reviewed and new approaches provide a pragmatic forecast for future developments are introduced as well.

Blood Stage Plasmodium falciparum Exhibits Biological Responses to Direct Current Electric Fields

The development of resistance to insecticides by the vector of malaria and the increasingly faster appearance of resistance to antimalarial drugs by the parasite can dangerously hamper efforts to control and eradicate the disease. Alternative ways to treat this disease are urgently needed. Here we evaluate the in vitro effect of direct current (DC) capacitive coupling electrical stimulation on the biology and viability of Plasmodium falciparum. We designed a system that exposes infected erythrocytes to different capacitively coupled electric fields in order to evaluate their effect on P. falciparum. The effect on growth of the parasite, replication of DNA, mitochondrial membrane potential and level of reactive oxygen species after exposure to electric fields demonstrate that the parasite is biologically able to respond to stimuli from DC electric fields involving calcium signaling pathways.

Characterization of the commercially-available fluorescent chloroquine-BODIPY conjugate, LynxTag-CQGREEN, as a marker for chloroquine resistance and uptake in a 96-well plate assay

Chloroquine was a cheap, extremely effective drug against Plasmodium falciparum until resistance arose. One approach to reversing resistance is the inhibition of chloroquine efflux from its site of action, the parasite digestive vacuole. Chloroquine accumulation studies have traditionally relied on radiolabelled chloroquine, which poses several challenges. There is a need for development of a safe and biologically relevant substitute. We report here a commercially-available green fluorescent chloroquine-BODIPY conjugate, LynxTag-CQ_GREEN, as a proxy for chloroquine accumulation. This compound localized to the digestive vacuole of the parasite as observed under confocal microscopy, and inhibited growth of chloroquine-sensitive strain 3D7 more extensively than in the resistant strains 7G8 and K1. Microplate reader measurements indicated suppression of LynxTag-CQ_GREEN efflux after pretreatment of parasites with known reversal agents. Microsomes carrying either sensitive- or resistant-type PfCRT were assayed for uptake; resistant-type PfCRT exhibited increased accumulation of LynxTag-CQ_GREEN, which was suppressed by pretreatment with known chemosensitizers. Eight laboratory strains and twelve clinical isolates were sequenced for PfCRT and Pgh1 haplotypes previously reported to contribute to drug resistance, and pfmdr1 copy number and chloroquine IC₅₀s were determined. These data were compared with LynxTag-CQ_GREEN uptake/fluorescence by multiple linear regression to identify genetic correlates of uptake. Uptake of the compound correlated with the logIC₅₀ of chloroquine and, more weakly, a mutation in Pgh1, F1226Y.

Exploiting large-scale drug-protein interaction information for computational drug repurposing

BACKGROUND

Despite increased investment in pharmaceutical research and development, fewer and fewer new drugs are entering the marketplace. This has prompted studies in repurposing existing drugs for use against diseases with unmet medical needs. A popular approach is to develop a classification model based on drugs with and without a desired therapeutic effect. For this approach to be statistically sound, it requires a large number of drugs in both classes. However, given few or no approved drugs for the diseases of highest medical urgency and interest, different strategies need to be investigated.

RESULTS

We developed a computational method termed "drug-protein interaction-based repurposing" (DPIR) that is potentially applicable to diseases with very few approved drugs. The method, based on genome-wide drug-protein interaction information and Bayesian statistics, first identifies drug-protein interactions associated with a desired therapeutic effect. Then, it uses key drug-protein interactions to score other drugs for their potential to have the same therapeutic effect.

CONCLUSIONS

Detailed cross-validation studies using United States Food and Drug Administration-approved drugs for hypertension, human immunodeficiency virus, and malaria indicated that DPIR provides robust predictions. It achieves high levels of enrichment of drugs approved for a disease even with models developed based on a single drug known to treat the disease. Analysis of our model predictions also indicated that the method is potentially useful for understanding molecular mechanisms of drug action and for identifying protein targets that may potentiate the desired therapeutic effects of other drugs (combination therapies).

A 2-amino quinoline, 5-(3-(2-(7-chloroquinolin-2-yl)ethenyl)phenyl)-8-dimethylcarbamyl-4,6-dithiaoctanoic acid, interacts with PfMDR1 and inhibits its drug transport in Plasmodium falciparum

Malaria is a major disease in the tropics where chemotherapy remains the main mode of treatment and as such the rise and spread of drug-resistant malaria can lead to human tragedy. Two membrane transport proteins, PfMDR1 (Plasmodium falciparum multidrug resistance protein 1) and PfCRT (P. falciparum chloroquine resistance transporter), have been shown to cause resistance to several antimalarials. Both PfMDR1 and PfCRT are localized to the digestive vacuolar membrane and appear to regulate the transport of drugs and physiological metabolites. In this study we have used MK571, a 2-amino quinoline, to explore its interaction with PfMDR1 and PfCRT in chloroquine-sensitive and -resistant strains of P. falciparum. Our results show that chloroquine-resistant strains (e.g., K1, Dd2, and 7G8) are consistently more sensitive to MK571 than chloroquine-sensitive strains (e.g., 3D7, 106/1 and D10). This association, however, was not maintained with the chloroquine-resistant strain FCB which IC50 value was similar to chloroquine-sensitive strains. Moreover, the susceptibility of chloroquine-sensitive and -resistant strains to MK571 does not correlate with mutated PfCRT, nor is it reversible with verapamil; but correlates with mutations in PfMDR1. Furthermore, MK571 appears to target the parasite's digestive vacuole (DV), as demonstrated by the ability of MK571 to: (1) block the accumulation of the fluorescent dye Fluo-4 AM, a PfMDR1 substrate, into the digestive vacuole; (2) reduce the transvacuolar pH gradient; and (3) inhibit the formation of β-hematin in vitro. Moreover, the presence of non-toxic concentrations of MK571 sensitized both chloroquine-sensitive and -resistant parasites to mefloquine and halofantrine, likely by competing against PfMDR1-mediated sequestering of the drugs into the DV compartment and away from the drugs' cytosolic targets. Our data, nevertheless, found only a minimal decrease in MK571 IC50 value in FCB parasite which second pfmdr1 copy was inactivated via gene disruption. Taken together, the findings of this study suggest that MK571 interacts with native and mutant PfMDR1 and modulates the import of drugs or solutes into the parasite's DV and, as such, MK571 may be a useful tool in the characterization of PfMDR1 drug interactions and substrate specificity.

Harnessing evolutionary fitness in Plasmodium falciparum for drug discovery and suppressing resistance

Drug resistance emerges in an ecological context where fitness costs restrict the diversity of escape pathways. These pathways are targets for drug discovery, and here we demonstrate that we can identify small-molecule inhibitors that differentially target resistant parasites. Combining wild-type and mutant-type inhibitors may prevent the emergence of competitively viable resistance. We tested this hypothesis with a clinically derived chloroquine-resistant (CQ(r)) malaria parasite and with parasites derived by in vitro selection with Plasmodium falciparum dihydroorotate dehydrogenase (PfDHODH) inhibitors. We screened a chemical library against CQ(s) and CQ(r) lines and discovered a drug-like compound (IDI-3783) that was potent only in the CQ(r) line. Surprisingly, in vitro selection of Plasmodium falciparum resistant to IDI-3783 restored CQ sensitivity, thereby indicating that CQ might once again be useful as a malaria therapy. In parallel experiments, we selected P. falciparum lines resistant to structurally unrelated PfDHODH inhibitors (Genz-666136 and DSM74). Both selections yielded resistant lines with the same point mutation in PfDHODH:E182D. We discovered a compound (IDI-6273) more potent against E182D than wild-type parasites. Selection of the E182D mutant with IDI-6273 yielded a reversion to the wild-type protein sequence and phenotype although the nucleotide sequence was different. Importantly, selection with a combination of Genz-669178, a wild-type PfDHODH inhibitor, and IDI-6273, a mutant-selective PfDHODH inhibitor, did not yield resistant parasites. These two examples demonstrate that the compromise between resistance and evolutionary fitness can be exploited to design therapies that prevent the emergence and spread of resistant organisms.

Chloroquine susceptibility and reversibility in a Plasmodium falciparum genetic cross

Mutations in the Plasmodium falciparum chloroquine (CQ) resistance transporter (PfCRT) are major determinants of verapamil (VP)-reversible CQ resistance (CQR). In the presence of mutant PfCRT, additional genes contribute to the wide range of CQ susceptibilities observed. It is not known if these genes influence mechanisms of chemosensitization by CQR reversal agents. Using quantitative trait locus (QTL) mapping of progeny clones from the HB3 × Dd2 cross, we show that the P. falciparum multidrug resistance gene 1 (pfmdr1) interacts with the South-East Asia-derived mutant pfcrt haplotype to modulate CQR levels. A novel chromosome 7 locus is predicted to contribute with the pfcrt and pfmdr1 loci to influence CQR levels. Chemoreversal via a wide range of chemical structures operates through a direct pfcrt-based mechanism. Direct inhibition of parasite growth by these reversal agents is influenced by pfcrt mutations and additional loci. Direct labelling of purified recombinant PfMDR1 protein with a highly specific photoaffinity CQ analogue, and lack of competition for photolabelling by VP, supports our QTL predictions. We find no evidence that pfmdr1 copy number affects CQ response in the progeny; however, inheritance patterns indicate that an allele-specific interaction between pfmdr1 and pfcrt is part of the complex genetic background of CQR.

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.

Molecular and physiologic basis of quinoline drug resistance in Plasmodium falciparum malaria

30 years before the discovery of the pfcrt gene, altered cellular drug accumulation in drug-resistant malarial parasites had been well documented. Heme released from catabolized hemoglobin was thought to be a key target for quinoline drugs, and additional modifications to quinoline drug structure in order to improve activity against chloroquine-resistant malaria were performed in a few laboratories. However, parasite cell culture methods were still in their infancy, assays for drug susceptibility were not well standardized, and the power of malarial genetics was decades away. The last 10 years have witnessed explosive progress in elucidation of the biochemistry of chloroquine resistance. This review briefly summarizes that progress, and discusses where additional work is needed.

In vitro chemosensitization of Plasmodium falciparum to antimalarials by verapamil and probenecid

We tested the effect of probenecid and verapamil in chemosensitizing Plasmodium falciparum to 14 antimalarials using the multidrug-resistant strain V1S and the drug-sensitive 3D7. Verapamil chemosensitizes V1S to quinine and chloroquine. Interestingly, probenecid profoundly chemosensitizes V1S to piperaquine. Thus, probenecid could be used to increase piperaquine efficacy in vivo.

Design of aminated poly(1-vinylimidazole) for a new pH-sensitive polycation to enhance cell-specific gene delivery

Poly(1-vinylimidazole) (PVIm) with aminoethyl groups has been synthesized as a new pH-sensitive polycation to enhance cell-specific gene delivery. The resulting aminated PVIm (PVIm-NH2) was water-soluble despite deprotonation of the imidazole groups at physiological pH, as determined by acid-base titration and solution turbidity measurement. Hemolysis assay showed that PVIm-NH2 enhanced membrane disruptive ability at endosomal pH, owing to the protonation of the imidazole groups with a pKa value around 6.0. Agarose gel retardation assay proved that the introduced aminoethyl groups worked as anchor groups to retain DNA. Furthermore, the ternary complex of DNA, PVIm-NH2, and a poly(L-lysine) conjugated with lactose molecules, PLL-Lac, at pH 7.4 dissociated the PLL-Lac polycation by protonation of the imidazole groups of PVIm-NH2 at pH 6.0. The resulting PVIm-NH2/DNA binary complexes easily released DNA, as compared with the PLL-Lac/PVIm-NH2/DNA ternary complex, which was examined by competitive exchange with dextran sulfate. By using PVIm-NH2 as a pH-sensitive DNA carrier, as well as PLL-Lac as a cell-targeting DNA carrier, the resulting ternary complex specifically mediated the gene expression, which depended on the protonation of the imidazole groups, on human hepatoma HepG2 cells with asialoglycoprotein receptors. These results suggest that the cell-specific gene delivery mediated by PLL-Lac was enhanced by PVIm-NH2 as a new pH-sensitive polycation.

Spinning Disk Confocal Microscopy of Live, Intraerythrocytic Malarial Parasites. 1. Quantification of Hemozoin Development for Drug Sensitive versus Resistant Malaria

We have customized a Nipkow spinning disk confocal microscope (SDCM) to acquire three-dimensional (3D) versus time data for live, intraerythrocytic malarial parasites. Since live parasites wiggle within red blood cells, conventional laser scanning confocal microscopy produces blurred 3D images after reconstruction of z stack data. In contrast, since SDCM data sets at high x, y, and z resolution can be acquired in hundreds of milliseconds, key aspects of live parasite cellular biochemistry can be much better resolved on physiologically meaningful times scales. In this paper, we present the first 3D DIC transmittance "z stack" images of live malarial parasites and use those to quantify hemozoin (Hz) produced within the living parasite digestive vacuole, under physiologic conditions. Using live synchronized cultures and voxel analysis of sharpened DIC z stacks, we present the first quantitative in vivo analysis of the rate of Hz growth for chloroquine sensitive (CQS) versus resistant (CQR) malarial parasites. We present data for laboratory strains, as well as pfcrt transfectants expressing a CQR conferring mutant pfcrt gene. We also analyze the rate of Hz growth in the presence and absence of physiologically relevant doses of chloroquine (CQ) and verapamil (VPL) and thereby present the first in vivo quantification of key predictions from the well-known Fitch hypothesis for CQ pharmacology. In the following paper [Gligorijevic, B., et al. (2006) Biochemistry 45, pp 12411-12423], we acquire fluorescent images of live parasite DV via SDCM and use those to quantify DV volume for CQS versus CQR parasites.

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.

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.

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

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

Evidence for a Substrate Specific and Inhibitable Drug Efflux System in Chloroquine Resistant Plasmodium falciparum Strains

The mechanism underpinning chloroquine drug resistance in the human malarial parasite Plasmodium falciparum remains controversial. By investigating the kinetics of chloroquine accumulation under varying-trans conditions, we recently presented evidence for a saturable and energy-dependent chloroquine efflux system present in chloroquine resistant P. falciparum strains. Here, we further characterize the putative chloroquine efflux system by investigating its substrate specificity using a broad range of different antimalarial drugs. Our data show that preloading cells with amodiaquine, primaquine, quinacrine, quinine, and quinidine stimulates labeled chloroquine accumulation under varying-trans conditions, while mefloquine, halofantrine, artemisinin, and pyrimethamine do not induce this effect. In the reverse of the varying-trans procedure, we show that preloaded cold chloroquine can stimulate quinine accumulation. On the basis of these findings, we propose that the putative chloroquine efflux system is capable of transporting, in addition to chloroquine, structurally related quinoline and methoxyacridine antimalarial drugs. Verapamil and the calcium/calmodulin antagonist W7 abrogate stimulated chloroquine accumulation and energy-dependent chloroquine extrusion. Our data are consistent with a substrate specific and inhibitible drug efflux system being present in chloroquine resistant P. falciparum strains.

Drug targets in malaria parasites

Malaria ranks with tuberculosis and AIDS in terms of its ability to destroy human health. In India there are at least two million cases seen annually. Although mortality may not be as high as it is in Africa, the trauma due to morbidity and debility and loss of productive man hours are colossal. Since resistance to chloroquine and antifolates is spreading rapidly, there is need to develop new pharmacophores, for which identification of new drug targets is essential. This review focuses on targets arising from classical and unique metabolic pathways in the malaria parasite, highlighting the research being carried out in India in the context of the global scenario. A significant amount of research in India and elsewhere has provided new knowledge on parasite biology, that could pave the way for the development of new pharmacophores. However, it is a matter of regret to record that malaria being a poor man's disease does not enthuse pharmaceutical companies in general to invest and bring out new molecules. Developing countries like India should take a lead in developing new but affordable antimalarials.

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.

Cellular uptake of a catechol iron chelator and chloroquine into Plasmodium falciparum infected erythrocytes

Our study demonstrates the capacity of FR160, a catechol iron chelator, to reach and accumulate into infected Plasmodium falciparum erythrocytes and parasites (cellular accumulation ratio between 12 and 43). Steady-state FR160 accumulation is obtained after 2 hr of exposure. After 2 hr exposure, it reaches intracellular levels that are 4- to 10-fold higher in infected red blood cells than those attained in normal erythrocytes. There is quite a good correlation between the accumulation of chloroquine and FR160 in the different strains (r=0.939) and in the IC(50) values (r=0.719). In contrast, the accumulation of FR160 and its activity is poorly correlated (r=0.500), suggesting that activity of FR160 may be independent of its penetration into infected erythrocytes. The mechanism of accumulation is yet unknown but based on inhibitor studies, the uptake of FR160 seems to be not associated with the calcium pump or channel, the potassium channel or the Na(+)/H(+) exchanger. Combinations of FR160 with verapamil, diltiazem, clotrimazole, amiloride, diazoxide, 4-aminopyridine, and picrotoxin should be avoided (antagonistic effects). The potent in vitro activity of FR160 on chloroquine-resistant strains or isolates, its lower toxicity against Vero cells, its mechanisms of action, its capacity to reach rapidly and accumulate into infected erythrocytes suggest that FR160 holds much promise as a new structural lead and effective antimalarial agent or at least a promising adjuvant in treatment of malaria.

Analysis of the antimalarial drug resistance protein Pfcrt expressed in yeast

Mutations in the novel membrane protein Pfcrt were recently found to be essential for chloroquine resistance (CQR) in Plasmodium falciparum, the parasite responsible for most lethal human malaria (Fidock, D. A., Nomura, T., Talley, A. K., Cooper, R. A., Dzekunov, S. M., Ferdig, M. T., Ursos, L. M., Sidhu, A. B., Naude, B., Deitsch, K. W., Su, X. Z., Wootton, J. C., Roepe, P. D., and Wellems, T. E. (2000) Mol. Cell 6, 861-871). Pfcrt is localized to the digestive vacuolar membrane of the intraerythrocytic parasite and may function as a transporter. Study of this putative transport function would be greatly assisted by overexpression in yeast followed by characterization of membrane vesicles. Unfortunately, the very high AT content of malarial genes precludes efficient heterologous expression. Thus, we back-translated Pfcrt to design idealized genes with preferred yeast codons, no long poly(A) sequences, and minimal stem-loop structure. We synthesized a designed gene with a two-step PCR method, fused this to N- and C-terminal sequences to aid membrane insertion and purification, and now report efficient expression of wild type and mutant Pfcrt proteins in the plasma membrane of Saccharomyces cerevisiae and Pichia pastoris yeast. To our knowledge, this is the first successful expression of a full-length malarial parasite integral membrane protein in yeast. Purified membranes and inside-out plasma membrane vesicle preparations were used to analyze wild type versus CQR-conferring mutant Pfcrt function, which may include effects on H(+) transport (Dzekunov, S., Ursos, L. M. B., and Roepe, P. D. (2000) Mol. Biochem. Parasitol. 110, 107-124), and to perfect a rapid purification of biotinylated Pfcrt. These data expand on the role of Pfcrt in conferring CQR and define a productive route for analysis of important P. falciparum transport proteins and membrane associated vaccine candidates.

Chloroquine resistance in the malarial parasite, Plasmodium falciparum

Malarial parasites remain a health problem of staggering proportions. Worldwide, they infect about 500 million, incapacitate tens of millions, and kill approximately 2.5 million (mostly children) annually. Four species infect humans, but most deaths are caused by one particular species, Plasmodium falciparum. The rising number of malarial deaths is due in part to increased drug resistance in P. falciparum. There are many varieties of antimalarial drug resistance, and there may very well be several molecular level contributions to each variety. This is because there are a number of different drugs with different mechanisms of action in use, and more than one molecular event may sometimes be relevant for resistance to any one class of drugs. Thus, "multidrug" resistance in a clinical setting likely entails complex combinations of overlapping resistance pathways, each specific for one class of drug, that then add together to confer the particular multidrug resistance phenotype. Nonetheless, rapid progress has been made in recent years in elucidating mechanisms of resistance to specific classes of antimalarial drugs. As one example, resistance to the antimalarial drug chloroquine, which has been the mainstay therapy for decades, is becoming well understood. This article focuses on recent advances in determining the molecular mechanism of chloroquine resistance, with particular attention to the biochemistry and biophysics of the P. falciparum digestive vacuole, wherein changes in pH have recently been found to be associated with chloroquine resistance.

Cardiac glycosides induce resistance to tubulin-dependent anticancer drugs in androgen-independent human prostate cancer

Due to high prevalence and mortality and the lack of effective therapies, prostate cancer is one of the most crucial health problems in men. Drug resistance aggravates the situation, not only in human prostate cancer but also in other cancers. In this study, we report for the first time that cardiac glycosides (e.g. ouabain and digitoxin) induced resistance of human prostate cancer cells (PC-3) in vitro to tubulin-binding anticancer drugs, such as paclitaxel, colchicine, vincristine and vinblastine. Cardiac glycosides exhibited amazing ability to reverse the G2/M arrest of the cell cycle and cell apoptosis induced by tubulin-binding agents. However, neither ionomycin (a Ca(2+) ionophore) nor veratridine (a Na(+) ionophore) mimicked the preventive action of cardiac glycosides, indicating that elevation of the intracellular Ca(2+) concentration and Na(+) accumulation were not involved in the cardiac glycoside action. Furthermore, cardiac glycosides showed little influence on the effects induced by actinomycin D, anisomycin and doxorubicin, suggesting selectivity for microtubule-targeted anticancer drugs. Using in situ immunofluorescent detection of mitotic spindles, our data showed that cardiac glycosides diminished paclitaxel-induced accumulation of microtubule spindles; however, in a non-cell assay system, cardiac glycosides had little influence on colchicine- and paclitaxel-induced microtubule dynamics. Using an isotope-labeled assay method, we found that ouabain modestly but significantly inhibited the transport of [(14)C]paclitaxel from the cytosol into the nucleus. It is suggested that cardiac glycosides inhibit the G2/M arrest induced by tubulin-binding anticancer drugs via an indirect blockade on microtubule function. The decline in transport of these drugs into the nucleus may partly explain the action of cardiac glycosides.

Does chloroquine really act through oxidative stress?

To assess whether molecular oxygen and oxidative stress contribute to chloroquine activity, we cultivated strains of Plasmodium falciparum in erythrocytes with carboxyhemoglobin and an atmosphere containing 2% CO, 5% CO(2) and 93% N(2). Results indicate that, contrary to common belief, oxygen is not involved in the activity of chloroquine. Reactive radicals formation is suggested.

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.

Identification of two distinct intracellular sites that contribute to the modulation of multidrug resistance in P388/ADR cells expressing P-glycoprotein

Although the ability of chemosensitizers to modulate P-glycoprotein (PGP)-based multidrug resistance (MDR) has been extensively studied, relatively little is known about the cellular pharmacology of the PGP inhibitors themselves in MDR cells. The studies described here have correlated the in vitro accumulation and retention properties of verapamil (VRP) in murine P388 (sensitive) and P388/ADR (MDR) cells with doxorubicin (DOX) uptake and cytotoxicity modulation characteristics in order to better understand VRP-tumor cell interactions that give rise to MDR modulation. VRP is rapidly taken up by DOX-sensitive and -resistant P388 cells where greater than 50% maximal VRP uptake occurs within 10 min of initial exposure at 37 degrees C. Whereas chemosensitization and DOX uptake in P388/ADR cells increase with increasing VRP concentration until a plateau is achieved at approximately 5 microM VRP, cellular modulator levels increase proportionally with increasing VPR concentrations beyond 20 microM. Subsequent to removal of noncell-associated modulator, VRP levels in both sensitive and resistant cells rapidly fall below 10% of those obtained at uptake equilibrium. However, a residual amount of VRP remains associated with the cells for extended time periods after the cells are washed. Pulse exposures of P388/ADR cells to high concentrations of VRP (50-100 microM) are capable of providing extended cell-associated VRP levels comparable to those obtained with continuous exposure at biologically active VRP concentrations (1-3 microM) and this leads to chemosensitization. These results are consistent with the existence of high- and low-affinity intracellular VRP pools in P388 MDR cells, both of which can contribute to the reversal of drug resistance. It is suggested that these properties should be taken into consideration during the design and evaluation of preclinical in vivo MDR models where pulsed exposure to high concentrations of resistance modulators often occurs. Special attention must be given to whether such high concentration pulses are desirable and/or achievable in relevant clinical settings.

Novel phenothiazine antimalarials: synthesis, antimalarial activity, and inhibition of the formation of beta-haematin

We report the synthesis of a series of novel phenothiazine compounds that inhibit the growth of both chloroquine-sensitive and chloroquine-resistant strains of Plasmodium falciparum. We found that the antimalarial activity of these phenothiazines increased with an increase in the number of basic groups in the alkylamino side chain, which may reflect increased uptake into the parasite food vacuole or differences in the toxicities of individual FP-drug complexes. We have examined the ability of the parent phenothiazine, chlorpromazine, and some novel phenothiazines to inhibit the formation of beta-haematin. The degree of antimalarial potency was loosely correlated with the efficacy of inhibition of beta-haematin formation, suggesting that these phenothiazines exert their antimalarial activities in a manner similar to that of chloroquine, i.e. by antagonizing the sequestration of toxic haem (ferriprotoporphyrin IX) moieties within the malaria parasite. Chlorpromazine is an effective modulator of chloroquine resistance; however, the more potent phenothiazine derivatives were more active against chloroquine-sensitive parasites than against chloroquine-resistant parasites and showed little synergy of action when used in combination with chloroquine. These studies point to structural features that may determine the antimalarial activity and resistance modulating potential of weakly basic amphipaths.

Emerging targets for antimalarial drugs

The absence of an effective vaccine against malaria and the ability of the parasite to develop resistance to known antimalarial drugs makes it mandatory to unravel newer drug targets with a view to developing newer pharmacophores. While conventional targets such as the purine, pyrimidine and folate pathways are still being investigated in the light of newer knowledge, a new opportunity has emerged from an understanding of certain unique features of the parasite biology. These include the food vacuole, haemoglobin catabolism, haeme biosynthesis, apicoplasts and their metabolism as well as macromolecular transactions, import of host proteins, parasite induced alterations in the red cell surface and transport phenomena. This review seeks to emphasise the new and emerging targets, while giving a brief account of the targets that have already been exploited.

Membrane transport in the malaria-infected erythrocyte

The malaria parasite is a unicellular eukaryotic organism which, during the course of its complex life cycle, invades the red blood cells of its vertebrate host. As it grows and multiplies within its host blood cell, the parasite modifies the membrane permeability and cytosolic composition of the host cell. The intracellular parasite is enclosed within a so-called parasitophorous vacuolar membrane, tubular extensions of which radiate out into the host cell compartment. Like all eukaryote cells, the parasite has at its surface a plasma membrane, as well as having a variety of internal membrane-bound organelles that perform a range of functions. This review focuses on the transport properties of the different membranes of the malaria-infected erythrocyte, as well as on the role played by the various membrane transport systems in the uptake of solutes from the extracellular medium, the disposal of metabolic wastes, and the origin and maintenance of electrochemical ion gradients. Such systems are of considerable interest from the point of view of antimalarial chemotherapy, both as drug targets in their own right and as routes for targeting cytotoxic agents into the intracellular parasite.

Mutations in the P. falciparum digestive vacuole transmembrane protein PfCRT and evidence for their role in chloroquine resistance

The determinant of verapamil-reversible chloroquine resistance (CQR) in a Plasmodium falciparum genetic cross maps to a 36 kb segment of chromosome 7. This segment harbors a 13-exon gene, pfcrt, having point mutations that associate completely with CQR in parasite lines from Asia, Africa, and South America. These data, transfection results, and selection of a CQR line harboring a novel K761 mutation point to a central role for the PfCRT protein in CQR. This transmembrane protein localizes to the parasite digestive vacuole (DV), the site of CQ action, where increased compartment acidification associates with PfCRT point mutations. Mutations in PfCRT may result in altered chloroquine flux or reduced drug binding to hematin through an effect on DV pH.

Digestive vacuolar pH of intact intraerythrocytic P. falciparum either sensitive or resistant to chloroquine

We present the first single cell-level analysis of digestive vacuolar pH for representative chloroquine resistant (strain Dd2) versus sensitive (strain HB3) malarial parasites. Human red blood cells harboring intact intraerythrocytic parasites were attached to glass substrate, continuously perfused with appropriate buffer, and pH was analyzed via single cell imaging and photometry techniques. We find that digestive vacuolar pH (pH(vac)) is near 5.6 for HB3 parasites. Surprisingly, we also find that pH(vac) of Dd2 is more acidic relative to HB3. Notably, in vitro pH titration of hematin confirms a very steep transition between soluble heme (capable of binding chloroquine) and insoluble heme (not capable of binding chloroquine, but still capable of polymerization to hemozoin) with a distinct midpoint at pH 5.6. We suggest the similarity between the hematin pH titration midpoint and the measured value of HB3 pH(vac) is not coincidental, and that decreased pH(vac) for Dd2 titrates limited initial drug target (i.e. soluble heme) to lower concentration. That is, changes in pH(vac) for drug resistant Dd2 relative to drug sensitive HB3 are consistent with lowering drug target levels, but not directly lowering vacuolar concentrations of drug via the predictions of weak base partitioning theory. Regardless, lowering either would of course decrease the efficiency of drug/target interaction and hence the net cellular accumulation of drug over time, as is typically observed for resistant parasites. These observations contrast sharply with the common expectation that decreased chloroquine accumulation in drug resistant malarial parasites is likely linked to elevated pH(vac,) but nonetheless illustrate important differences in vacuolar ion transport for drug resistant malarial parasites. In the accompanying paper (Ursos, L. et al., following paper this issue) we describe how pH(vac) is affected by exposure to chloroquine and verapamil for HB3 versus Dd2.

The effects of chloroquine and verapamil on digestive vacuolar pH of P. falciparum either sensitive or resistant to chloroquine

In the preceding paper, we present a novel method for measuring the digestive vacuolar pH (pH(vac)) of the malarial parasite Plasmodium falciparum, and show that, surprisingly, pH(vac) is lower for chloroquine resistant (CQR) Dd2 parasites relative to chloroquine sensitive (CQS) HB3. These data may have important consequences for elucidating mechanisms of antimalarial drug resistance and for developing new antimalarial therapy. Additional issues central to a better understanding of antimalarial pharmacology and antimalarial drug resistance require detailed comparative data on the effects of key drugs and other compounds on parasite biophysical parameters such as pH(vac), measured under close-to-physiologic conditions. Since the methods we develop in the previous paper allow us to record fluorescence signals from spatially well-defined regions of the living parasite while they are under continuous perfusion, it is relatively straightforward for us to test how antimalarial drugs (e. g. chloroquine, CQ) and other compounds (e.g. the chemoreversal agent verapamil [VPL]) affect pH(vac). In this paper, we measure both short term (i.e. initial perfusion conditions) and longer-term effects of CQ and VPL for living, intraerythrocytic CQS (HB3) and CQR (Dd2) malarial parasites under constant perfusion with physiologically relevant buffers. We find that VPL normalizes pH(vac) for Dd2 to a value near that measured for HB3, but has no effect on pH(vac) for HB3. Longer term CQ exposure is found to alter pH(vac) for HB3 but not Dd2, and short-term exposure to the drug has no significant effect in either strain. The results may help resolve longstanding debate regarding the effects of CQ and VPL on parasite physiology, and further support our evolving hypothesis for the mechanism of CQ resistance.

Reversal activity of the naturally occurring chemosensitizer malagashanine in Plasmodium malaria

Malagashanine (MG) is the parent compound of a new type of indole alkaloids, the N(b)C(21)-secocuran, isolated so far from the Malagasy Strychnos species traditionally used as chloroquine adjuvants in the treatment of chronic malaria. Previously, it was shown to have weak in vitro intrinsic antiplasmodial activity (IC(50) = 146.5 +/- 0.2 microM), but did display marked in vitro chloroquine-potentiating action against the FcM29 chloroquine-resistant strain of Plasmodium falciparum. The purpose of the present study was to further investigate its reversal activity. Thus, the previous in vitro results were tested in vivo. The interaction of MG with several antimalarials against various strains of P. falciparum was also assessed. As expected, MG enhanced the effect of chloroquine against the resistant strain W2, but had no action on the susceptible strain 3D7 and two sensitive isolates. Interestingly, MG was found to exhibit significant chloroquine-potentiating action against the FcB1 strain formerly described as a resistant strain but one which has since lost its resistance for unknown reasons. One other relevant result that arose from our study was the observation of the selective enhancing action of MG on quinolines (chloroquine, quinine, and mefloquine), aminoacridines (quinacrine and pyronaridine), and a structurally unrelated drug (halofantrine), all of which are believed to exert their antimalarial effect by binding with haematin. MG was finally found to specifically act with chloroquine on the old trophozoite stage of the P. falciparum cycle. Similarities and differences between verapamil and MG reversal activity are briefly presented.

Chloroquine uptake, altered partitioning and the basis of drug resistance: evidence for chloride-dependent ionic regulation

The biochemical mechanism of chloroquine resistance in Plasmodium falciparum remains unknown. We postulated that chloroquine-resistant strains could alter ion fluxes that then indirectly control drug accumulation within the parasite by affecting pH and/or membrane potential ('altered partitioning mechanism'). Two principal intracellular pH-regulating systems in many cell types are the amiloride-sensitive Na+/H+ exchanger (NHE), and the sodium-independent, stilbene-sensitive Cl-/HCO3- antiporter (AE). We report that under physiological conditions (balanced CO2 and HCO3-) chloroquine uptake and susceptibility are not altered by amiloride analogues. We also do not detect a significant difference in NHE activity between chloroquine-sensitive and chloroquine-resistant strains via single cell photometry methods. AE activity is dependent on the intracellular and extracellular concentrations of Cl- and HCO3- ions. Chloroquine-resistant strains differentially respond to experimental modifications in chloride-dependent homeostasis, including growth, cytoplasmic pH and pH regulation. Chloroquine susceptibility is altered by stilbene DIDS only on chloroquine-resistant strains. Our results suggest that a Cl(-)-dependent system (perhaps AE) has a significant effect on the uptake of chloroquine by the infected erythrocyte, and that alterations of this biophysical parameter may be part of the mechanism of chloroquine resistance in P. falciparum.

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.

In vitro biotransformation of a novel antimalarial cysteine protease inhibitor in human liver microsomes

4-Dimethylamino-4'-(imidazol-1-yl)chalcone (RL3142) is a newly developed antimalarial cysteine protease inhibitor. Four metabolites (M1-M4) were found in human liver microsomes and their structures were identified by LC/MS/MS. Two primary metabolites, M2 (minor) and M4 (major), were determined to be the N-demethylated product (M2) and the product (M4) resulting from 1,2-hydrogenation of the alpha, beta-unsaturated ketone moiety of the parent compound. A combined approach utilizing selective P450 inhibitors, immunoinhibition with CYP3A and NADPH P450 reductase antibodies, and cDNA expressed human CYP3A4 and NADPH P450 reductase, was used for identification of enzymes responsible for the biotransformation. For formation of M2, both a rabbit CYP3A polyclonal antibody (110 microliter/mg microsomal protein) and ketoconazole (2 micromol/l), a CYP3A inhibitor, showed about 50% inhibitory effects; other specific inhibitors of CYP1A2, CYP2B6, CYP2C9, CYP2C19, CYP2D6 and CYP2E1 showed no significant effects. For formation of M4, neither CYP3A antibody nor the above mentioned CYP inhibitors exhibited inhibitory effects. Anti-rat NADPH P450 reductase serum (50 microliter/100 microgram microsomal protein) exhibited 70 and 58% inhibitory effects on M2 and M4 formation, respectively. Incubation of RL3142 with cDNA expressed human NADPH P450 reductase yielded formation of M4, but not M2. Carbon monoxide inhibited formation of M2 and M1 (the reduced product of M2), but had no effect on M4 and M3 (the reduced product of M4) formation. Collectively, NADPH P450 reductase solely catalyzed reduction of RL3142 to M4, whereas CYP3A contributed in part to formation of M2.