Differential drug efflux or accumulation does not explain variation in the chloroquine response of Plasmodium falciparum strains expressing the same isoform of mutant PfCRT

Molecular Mechanisms of Drug Resistance in Plasmodium falciparum Malaria

Understanding and controlling the spread of antimalarial resistance, particularly to artemisinin and its partner drugs, is a top priority. Plasmodium falciparum parasites resistant to chloroquine, amodiaquine, or piperaquine harbor mutations in the P. falciparum chloroquine resistance transporter (PfCRT), a transporter resident on the digestive vacuole membrane that in its variant forms can transport these weak-base 4-aminoquinoline drugs out of this acidic organelle, thus preventing these drugs from binding heme and inhibiting its detoxification. The structure of PfCRT, solved by cryogenic electron microscopy, shows mutations surrounding an electronegative central drug-binding cavity where they presumably interact with drugs and natural substrates to control transport. P. falciparum susceptibility to heme-binding antimalarials is also modulated by overexpression or mutations in the digestive vacuole membrane-bound ABC transporter PfMDR1 (P. falciparum multidrug resistance 1 transporter). Artemisinin resistance is primarily mediated by mutations in P. falciparum Kelch13 protein (K13), a protein involved in multiple intracellular processes including endocytosis of hemoglobin, which is required for parasite growth and artemisinin activation. Combating drug-resistant malaria urgently requires the development of new antimalarial drugs with novel modes of action.

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.

Emerging Southeast Asian PfCRT mutations confer Plasmodium falciparum resistance to the first-line antimalarial piperaquine

The widely used antimalarial combination therapy dihydroartemisinin + piperaquine (DHA + PPQ) has failed in Cambodia. Here, we perform a genomic analysis that reveals a rapid increase in the prevalence of novel mutations in the Plasmodium falciparum chloroquine resistance transporter PfCRT following DHA + PPQ implementation. These mutations occur in parasites harboring the K13 C580Y artemisinin resistance marker. By introducing PfCRT mutations into sensitive Dd2 parasites or removing them from resistant Cambodian isolates, we show that the H97Y, F145I, M343L, or G353V mutations each confer resistance to PPQ, albeit with fitness costs for all but M343L. These mutations sensitize Dd2 parasites to chloroquine, amodiaquine, and quinine. In Dd2 parasites, multicopy plasmepsin 2, a candidate molecular marker, is not necessary for PPQ resistance. Distended digestive vacuoles were observed in pfcrt-edited Dd2 parasites but not in Cambodian isolates. Our findings provide compelling evidence that emerging mutations in PfCRT can serve as a molecular marker and mediator of PPQ resistance.

Increasing resistance of Plasmodium falciparum strains to piperaquine (PPQ) in Southeast Asia is of concern and resistance mechanisms are incompletely understood. Here, Ross et al. show that mutations in the P. falciparum chloroquine resistance transporter are rapidly increasing in prevalence in Cambodia and confer resistance to PPQ.

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

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

Cell Swelling Induced by the Antimalarial KAE609 (Cipargamin) and Other PfATP4-Associated Antimalarials

For an increasing number of antimalarial agents identified in high-throughput phenotypic screens, there is evidence that they target PfATP4, a putative Na⁺ efflux transporter on the plasma membrane of the human malaria parasite Plasmodium falciparum For several such "PfATP4-associated" compounds, it has been noted that their addition to parasitized erythrocytes results in cell swelling. Here we show that six structurally diverse PfATP4-associated compounds, including the clinical candidate KAE609 (cipargamin), induce swelling of both isolated blood-stage parasites and intact parasitized erythrocytes. The swelling of isolated parasites is dependent on the presence of Na⁺ in the external environment and may be attributed to the osmotic consequences of Na⁺ uptake. The swelling of the parasitized erythrocyte results in an increase in its osmotic fragility. Countering cell swelling by increasing the osmolarity of the extracellular medium reduces the antiplasmodial efficacy of PfATP4-associated compounds, consistent with cell swelling playing a role in the antimalarial activity of this class of compounds.

Globally prevalent PfMDR1 mutations modulate Plasmodium falciparum susceptibility to artemisinin-based combination therapies

Antimalarial chemotherapy, globally reliant on artemisinin-based combination therapies (ACTs), is threatened by the spread of drug resistance in Plasmodium falciparum parasites. Here we use zinc-finger nucleases to genetically modify the multidrug resistance-1 transporter PfMDR1 at amino acids 86 and 184, and demonstrate that the widely prevalent N86Y mutation augments resistance to the ACT partner drug amodiaquine and the former first-line agent chloroquine. In contrast, N86Y increases parasite susceptibility to the partner drugs lumefantrine and mefloquine, and the active artemisinin metabolite dihydroartemisinin. The PfMDR1 N86 plus Y184F isoform moderately reduces piperaquine potency in strains expressing an Asian/African variant of the chloroquine resistance transporter PfCRT. Mutations in both digestive vacuole-resident transporters are thought to differentially regulate ACT drug interactions with host haem, a product of parasite-mediated haemoglobin degradation. Global mapping of these mutations illustrates where the different ACTs could be selectively deployed to optimize treatment based on regional differences in PfMDR1 haplotypes.

Antimalarial chemotherapy relies on combination therapies (ACTs) consisting of an artemisinin derivative and a partner drug. Here, the authors study the effects of globally prevalent mutations in a multidrug resistance transporter (PfMDR1) on the parasite's susceptibility to ACT drugs.

Membrane transport in the malaria parasite and its host erythrocyte

As it grows and replicates within the erythrocytes of its host the malaria parasite takes up nutrients from the extracellular medium, exports metabolites and maintains a tight control over its internal ionic composition. These functions are achieved via membrane transport proteins, integral membrane proteins that mediate the passage of solutes across the various membranes that separate the biochemical machinery of the parasite from the extracellular environment. Proteins of this type play a key role in antimalarial drug resistance, as well as being candidate drug targets in their own right. This review provides an overview of recent work on the membrane transport biology of the malaria parasite-infected erythrocyte, encompassing both the parasite-induced changes in the membrane transport properties of the host erythrocyte and the cell physiology of the intracellular parasite itself.

Structural polymorphism in the promoter of pfmrp2 confers Plasmodium falciparum tolerance to quinoline drugs

Drug resistance in Plasmodium falciparum remains a challenge for the malaria eradication programmes around the world. With the emergence of artemisinin resistance, the efficacy of the partner drugs in the artemisinin combination therapies (ACT) that include quinoline-based drugs is becoming critical. So far only few resistance markers have been identified from which only two transmembrane transporters namely PfMDR1 (an ATP-binding cassette transporter) and PfCRT (a drug-metabolite transporter) have been experimentally verified. Another P. falciparum transporter, the ATP-binding cassette containing multidrug resistance-associated protein (PfMRP2) represents an additional possible factor of drug resistance in P. falciparum. In this study, we identified a parasite clone that is derived from the 3D7 P. falciparum strain and shows increased resistance to chloroquine, mefloquine and quinine through the trophozoite and schizont stages. We demonstrate that the resistance phenotype is caused by a 4.1 kb deletion in the 5′ upstream region of the pfmrp2 gene that leads to an alteration in the pfmrp2 transcription and thus increased level of PfMRP2 protein. These results also suggest the importance of putative promoter elements in regulation of gene expression during the P. falciparum intra-erythrocytic developmental cycle and the potential of genetic polymorphisms within these regions to underlie drug resistance.

Degrees of chloroquine resistance in Plasmodium - is the redox system involved?

Chloroquine (CQ) was once a very effective antimalarial drug that, at its peak, was consumed in the hundreds of millions of doses per year. The drug acts against the Plasmodium parasite during the asexual intraerythrocytic phase of its lifecycle. Unfortunately, clinical resistance to this drug is now widespread. Questions remain about precisely how CQ kills malaria parasites, and by what means some CQ-resistant (CQR) parasites can withstand much higher concentrations of the drug than others that also fall in the CQR category. In this review we investigate the evidence for and against the proposal that CQ kills parasites by generating oxidative stress. Further, we examine a long-held idea that the glutathione system of malaria parasites plays a role in CQ resistance. We conclude that there is strong evidence that glutathione levels modulate CQ response in the rodent malaria species P. berghei, but that a role for redox in contributing to the degree of CQ resistance in species infectious to humans has not been firmly established.

PfCRT and its role in antimalarial drug resistance

Plasmodium falciparum resistance to chloroquine, the former gold standard antimalarial drug, is mediated primarily by mutant forms of the chloroquine resistance transporter (PfCRT). These mutations impart upon PfCRT the ability to efflux chloroquine from the intracellular digestive vacuole, the site of drug action. Recent studies reveal that PfCRT variants can also affect parasite fitness, protect immature gametocytes against chloroquine action, and alter P. falciparum susceptibility to current first-line therapies. These results highlight the need to be vigilant in screening for the appearance of novel pfcrt alleles that could contribute to new multi-drug resistance phenotypes.

Molecular markers in plasmodium falciparum linked to resistance to anti-malarial drugs in samples imported from Africa over an eight-year period (2002-2010): impact of the introduction of artemisinin combination therapy

BACKGROUND

Drug resistance is a major problem to control Plasmodium falciparum infection in endemic countries. During last decade, African countries have changed first-line treatment to artemisinin-based combinations therapy (ACT); sulphadoxine-pyrimethamine (SP) is recommended for Intermittent Preventive Therapy (IPT). Molecular markers related to P falciparum resistance were analysed for the period of transition from SP to ACT, in isolates imported from Africa.

METHODS

A first group of samples was taken in the period between June 2002 and June 2006 (n = 113); a second group in the period between November 2008 and August 2010 (n = 46). Several alleles were analysed by nested PCR-RFLP: 51, 59, 108, 164, in the pfdhfr gene; 436, 437, 540, 581, in the pfdhps gene; 86, 1246, in the pfmdr1 gene and 76, in the pfcrt gene. The prevalence of alleles in the groups was compared with the chi-squared or Fisher's exact tests.

RESULTS

The pfdhfr N51I, C59R and S108N were over to 90% in the two groups; all samples had the I164. In the pfdhps, 437 G and 581 G, increased up to 80% and 10.9% (p = 0.024), respectively in the second group. The 540 G decreases (24% to 16.%) and the 436A disappears at the end of the follow-up (p = 0.004) in the second group. The 76I-pfcrt stayed over 95% in the two groups. Prevalence of 86Y-pfmdr1 decreased over eight years.

CONCLUSIONS

Pharmacological pressure affects the resistance strains prevalence. As for SP, the disappearance of 436A and the decrease in 540 G suggest that these mutations are not fixed. On the other hand, studies carried out after ACT introduction show there was a selection of strains carrying the SNPs N86Y, D1246Y in pfmdr1. In this work, the prevalence of pfmdr1- D1246Y is increasing, perhaps as a result of selective pressure by ACT. Continued surveillance is essential to monitor the effectiveness of treatments.

Saquinavir inhibits the malaria parasite's chloroquine resistance transporter

The antiretroviral protease inhibitors (APIs) ritonavir, saquinavir, and lopinavir, used to treat HIV infection, inhibit the growth of Plasmodium falciparum at clinically relevant concentrations. Moreover, it has been reported that these APIs potentiate the activity of chloroquine (CQ) against this parasite in vitro. The mechanism underlying this effect is not understood, but the degree of chemosensitization varies between the different APIs and, with the exception of ritonavir, appears to be dependent on the parasite exhibiting a CQ-resistant phenotype. Here we report a study of the role of the P. falciparum chloroquine resistance transporter (PfCRT) in the interaction between CQ and APIs, using transgenic parasites expressing different PfCRT alleles and using the Xenopus laevis oocyte system for the heterologous expression of PfCRT. Our data demonstrate that saquinavir behaves as a CQ resistance reverser and that this explains, at least in part, its ability to enhance the effects of CQ in CQ-resistant P. falciparum parasites.

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

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