Structural basis of transport and inhibition of the Plasmodium falciparum transporter PfFNT

In “Structural basis of transport and inhibition of the Plasmodium falciparum transporter PfFNT” by Lyu et al (2021), the authors depict the inhibitor MMV007839 in its hemiketal form in Fig 3A and F, Fig 4C, and Appendix Figs S10A, B and S13. We note that Golldack et al (2017) reported that the linear vinylogous acid tautomer of MMV007839 constitutes the binding and inhibitory entity of PfFNT. The authors are currently obtaining higher resolution cryo‐EM structural data of MMV007839‐bound PfFNT to ascertain which of the interconvertible isoforms is bound and the paper will be updated accordingly.     

Structural basis of transport and inhibition of the Plasmodium falciparum transporter PfFNT

The intra‐erythrocyte stage of P. falciparum relies primarily on glycolysis to generate adenosine triphosphate (ATP) and the energy required to support growth and reproduction. Lactic acid, a metabolic byproduct of glycolysis, is potentially toxic as it lowers the pH inside the parasite. Plasmodium falciparum formate–nitrite transporter (PfFNT), a 34‐kDa transmembrane protein, has been identified as a novel drug target as it exports lactate from inside the parasite to the surrounding parasitophorous vacuole within the erythrocyte cytosol. The structure and detailed molecular mechanism of this membrane protein are not yet available. Here we present structures of PfFNT in the absence and presence of the functional inhibitor MMV007839 at resolutions of 2.56 Å and 2.78 Å using single‐particle cryo‐electron microscopy. Genetic analysis and transport assay indicate that PfFNT is able to transfer lactate across the membrane. Combined, our data suggest a stepwise displacement mechanism for substrate transport. The PfFNT membrane protein is capable of picking up lactate ions from the parasite’s cytosol, converting them to lactic acids and then exporting these acids into the extracellular space.     

Structural basis of transport and inhibition of the Plasmodium falciparum transporter PfFNT

Subject Categories: Membranes & Trafficking, Microbiology, Virology & Host Pathogen Interaction, Structural Biology

We recently reported the first structures of the Plasmodium falciparum transporter PfFNT, both in the absence and presence of the inhibitor MMV007839 (Lyu et al, 2021). These structures indicated that PfFNT assembles as a pentamer. The bound MMV007839 was found in the middle of the elongated channel formed by each PfFNT protomer, adjacent to residue G107. MMV007839 exists in two tautomeric forms and can adopt either a cyclic hemiketal‐like structure or a linear vinylogous acid conformation (Fig ​(Fig3A).3A). Unfortunately, these two tautomeric forms could not be clearly distinguished based on the existing cryo‐EM data at 2.78 Å resolution. The bound MMV007839 inhibitor was reported as the cyclic hemiketal‐like form in the structure in Figs ​Figs3A3A and ​andF,F, and ​and4C,4C, Appendix Figs S10A and B, and S13 and in the online synopsis image.Open in a separate windowFigure 3Cryo‐EM structure of the PfFNT‐MMV007839 complex

1. Chemical structure of MMV007839. The compound can either be in cyclic hemiketal‐like or linear vinylogous acid tautomeric forms.
2. Cryo‐EM density map of pentameric PfFNT viewed from the parasite’s cytoplasm. Densities of the five bound MMV007839 within the pentamer are colored red. The five protomers of pentameric PfFNT are colored yellow, slate, orange, purple, and gray.
3. Ribbon diagram of the 2.18‐Å resolution structure of pentameric PfFNT‐MMV007839 viewed from the parasite’s cytoplasm. The five protomers of pentameric PfFNT are colored yellow, slate, orange, purple, and gray.
4. Ribbon diagram of pentameric PfFNT‐MMV007839 viewed from the extracellular side of the parasite. The five protomers of pentameric PfFNT are colored yellow, slate, orange, purple, and gray.
5. Ribbon diagram of pentameric PfFNT‐MMV007839 viewed from the parasite’s membrane plane. The five protomers of pentameric PfFNT are colored yellow, slate, orange, purple, and gray. Densities of the five bound MMV007839 are depicted as red meshes.
6. The MMV007839‐binding site of PfFNT. The bound MMV007839 is colored green. Density of the bound MMV007839 is depicted as black mesh. Residues involved in forming the inhibitor binding site are colored yellow. The hydrogen bonds are highlighted with black dotted lines.

Open in a separate windowFigure 4Structure of the central channel in the PfFNT‐MMV007839 protomer

• CA cartoon of the central channel formed within a PfFNT protomer. The channel contains one constriction site in this conformational state. Residues forming the constriction and the K35‐D103‐N108 and K177‐E229‐N234 triads are illustrated as sticks. Residues F94, I97, and L104, which form the first constriction site in the apo‐PfFNT structure, are also included in the figure.

Eric Beitz alerted us to the findings reported by his group that the linear vinylogous acid tautomer of MMV007839 constitutes the binding and inhibitory entity of PfFNT (Golldack et al, 2017).     

Identification and Functional Analysis of the Primary Pantothenate Transporter,PfPAT, of the Human Malaria Parasite Plasmodium falciparum

The human malaria parasite Plasmodium falciparum is absolutely dependent on the acquisition of host pantothenate for its development within human erythrocytes. Although the biochemical properties of this transport have been characterized, the molecular identity of the parasite-encoded pantothenate transporter remains unknown. Here we report the identification and functional characterization of the first protozoan pantothenate transporter, PfPAT, from P. falciparum. We show using cell biological, biochemical, and genetic analyses that this transporter is localized to the parasite plasma membrane and plays an essential role in parasite intraerythrocytic development. We have targeted PfPAT to the yeast plasma membrane and showed that the transporter complements the growth defect of the yeast fen2Δ pantothenate transporter-deficient mutant and mediates the entry of the fungicide drug, fenpropimorph. Our studies in P. falciparum revealed that fenpropimorph inhibits the intraerythrocytic development of both chloroquine- and pyrimethamine-resistant P. falciparum strains with potency equal or better than that of currently available pantothenate analogs. The essential function of PfPAT and its ability to deliver both pantothenate and fenpropimorph makes it an attractive target for the development and delivery of new classes of antimalarial drugs.     

Structural analyses of the malaria parasite aminoacyl‐tRNA synthetases provide new avenues for antimalarial drug discovery

Malaria is a parasitic illness caused by the genus Plasmodium from the apicomplexan phylum. Five plasmodial species of P. falciparum (Pf), P. knowlesi, P. malariae, P. ovale, and P. vivax (Pv) are responsible for causing malaria in humans. According to the World Malaria Report 2020, there were 229 million cases and ~ 0.04 million deaths of which 67% were in children below 5 years of age. While more than 3 billion people are at risk of malaria infection globally, antimalarial drugs are their only option for treatment. Antimalarial drug resistance keeps arising periodically and thus threatens the main line of malaria treatment, emphasizing the need to find new alternatives. The availability of whole genomes of P. falciparum and P. vivax has allowed targeting their unexplored plasmodial enzymes for inhibitor development with a focus on multistage targets that are crucial for parasite viability in both the blood and liver stages. Over the past decades, aminoacyl‐tRNA synthetases (aaRSs) have been explored as anti‐bacterial and anti‐fungal drug targets, and more recently (since 2009) aaRSs are also the focus of antimalarial drug targeting. Here, we dissect the structure‐based knowledge of the most advanced three aaRSs—lysyl‐ (KRS), prolyl‐ (PRS), and phenylalanyl‐ (FRS) synthetases in terms of development of antimalarial drugs. These examples showcase the promising potential of this family of enzymes to provide druggable targets that stall protein synthesis upon inhibition and thereby kill malaria parasites selectively.     

Mechanistic basis for multidrug resistance and collateral drug sensitivity conferred to the malaria parasite by polymorphisms in PfMDR1 and PfCRT

Polymorphisms in the Plasmodium falciparum multidrug resistance protein 1 (pfmdr1) gene and the Plasmodium falciparum chloroquine resistance transporter (pfcrt) gene alter the malaria parasite’s susceptibility to most of the current antimalarial drugs. However, the precise mechanisms by which PfMDR1 contributes to multidrug resistance have not yet been fully elucidated, nor is it understood why polymorphisms in pfmdr1 and pfcrt that cause chloroquine resistance simultaneously increase the parasite’s susceptibility to lumefantrine and mefloquine—a phenomenon known as collateral drug sensitivity. Here, we present a robust expression system for PfMDR1 in Xenopus oocytes that enables direct and high-resolution biochemical characterizations of the protein. We show that wild-type PfMDR1 transports diverse pharmacons, including lumefantrine, mefloquine, dihydroartemisinin, piperaquine, amodiaquine, methylene blue, and chloroquine (but not the antiviral drug amantadine). Field-derived mutant isoforms of PfMDR1 differ from the wild-type protein, and each other, in their capacities to transport these drugs, indicating that PfMDR1-induced changes in the distribution of drugs between the parasite’s digestive vacuole (DV) and the cytosol are a key driver of both antimalarial resistance and the variability between multidrug resistance phenotypes. Of note, the PfMDR1 isoforms prevalent in chloroquine-resistant isolates exhibit reduced capacities for chloroquine, lumefantrine, and mefloquine transport. We observe the opposite relationship between chloroquine resistance-conferring mutations in PfCRT and drug transport activity. Using our established assays for characterizing PfCRT in the Xenopus oocyte system and in live parasite assays, we demonstrate that these PfCRT isoforms transport all 3 drugs, whereas wild-type PfCRT does not. We present a mechanistic model for collateral drug sensitivity in which mutant isoforms of PfMDR1 and PfCRT cause chloroquine, lumefantrine, and mefloquine to remain in the cytosol instead of sequestering within the DV. This change in drug distribution increases the access of lumefantrine and mefloquine to their primary targets (thought to be located outside of the DV), while simultaneously decreasing chloroquine’s access to its target within the DV. The mechanistic insights presented here provide a basis for developing approaches that extend the useful life span of antimalarials by exploiting the opposing selection forces they exert upon PfCRT and PfMDR1.     

Inhibition of Plasmepsin V Activity Demonstrates Its Essential Role in Protein Export,PfEMP1 Display,and Survival of Malaria Parasites

The malaria parasite Plasmodium falciparum exports several hundred proteins into the infected erythrocyte that are involved in cellular remodeling and severe virulence. The export mechanism involves the Plasmodium export element (PEXEL), which is a cleavage site for the parasite protease, Plasmepsin V (PMV). The PMV gene is refractory to deletion, suggesting it is essential, but definitive proof is lacking. Here, we generated a PEXEL-mimetic inhibitor that potently blocks the activity of PMV isolated from P. falciparum and Plasmodium vivax. Assessment of PMV activity in P. falciparum revealed PEXEL cleavage occurs cotranslationaly, similar to signal peptidase. Treatment of P. falciparum–infected erythrocytes with the inhibitor caused dose-dependent inhibition of PEXEL processing as well as protein export, including impaired display of the major virulence adhesin, PfEMP1, on the erythrocyte surface, and cytoadherence. The inhibitor killed parasites at the trophozoite stage and knockdown of PMV enhanced sensitivity to the inhibitor, while overexpression of PMV increased resistance. This provides the first direct evidence that PMV activity is essential for protein export in Plasmodium spp. and for parasite survival in human erythrocytes and validates PMV as an antimalarial drug target.     

Transporters as mediators of drug resistance in Plasmodium falciparum

Drug resistance represents a major obstacle in the radical control of malaria. Drug resistance can arise in many different ways, but recent developments highlight the importance of mutations in transporter molecules as being major contributors to drug resistance in the human malaria parasite Plasmodium falciparum. While approximately 2.5% of the P. falciparum genome encodes membrane transporters, this review concentrates on three transporters, namely the chloroquine resistance transporter PfCRT, the multi-drug resistance transporter 1 PfMDR1, and the multi-drug resistance-associated protein PfMRP, which have been strongly associated with resistance to the major antimalarial drugs. The studies that identified these entities as contributors to resistance, and the possible molecular mechanisms that can bring about this phenotype, are discussed. A deep understanding of the underpinning mechanisms, and of the structural specificities of the players themselves, is a necessary basis for the development of the new drugs that will be needed for the future armamentarium against malaria.     

Multiple Drugs Compete for Transport via the Plasmodium falciparum Chloroquine Resistance Transporter at Distinct but Interdependent Sites

Mutations in the “chloroquine resistance transporter” (PfCRT) are a major determinant of drug resistance in the malaria parasite Plasmodium falciparum. We have previously shown that mutant PfCRT transports the antimalarial drug chloroquine away from its target, whereas the wild-type form of PfCRT does not. However, little is understood about the transport of other drugs via PfCRT or the mechanism by which PfCRT recognizes different substrates. Here we show that mutant PfCRT also transports quinine, quinidine, and verapamil, indicating that the protein behaves as a multidrug resistance carrier. Detailed kinetic analyses revealed that chloroquine and quinine compete for transport via PfCRT in a manner that is consistent with mixed-type inhibition. Moreover, our analyses suggest that PfCRT accepts chloroquine and quinine at distinct but antagonistically interacting sites. We also found verapamil to be a partial mixed-type inhibitor of chloroquine transport via PfCRT, further supporting the idea that PfCRT possesses multiple substrate-binding sites. Our findings provide new mechanistic insights into the workings of PfCRT, which could be exploited to design potent inhibitors of this key mediator of drug resistance.     

Small Molecule Screen for Candidate Antimalarials Targeting Plasmodium Kinesin-5

Plasmodium falciparum and vivax are responsible for the majority of malaria infections worldwide, resulting in over a million deaths annually. Malaria parasites now show measured resistance to all currently utilized drugs. Novel antimalarial drugs are urgently needed. The Plasmodium Kinesin-5 mechanoenzyme is a suitable “next generation” target. Discovered via small molecule screen experiments, the human Kinesin-5 has multiple allosteric sites that are “druggable.” One site in particular, unique in its sequence divergence across all homologs in the superfamily and even within the same family, exhibits exquisite drug specificity. We propose that Plasmodium Kinesin-5 shares this allosteric site and likewise can be targeted to uncover inhibitors with high specificity. To test this idea, we performed a screen for inhibitors selective for Plasmodium Kinesin-5 ATPase activity in parallel with human Kinesin-5. Our screen of nearly 2000 compounds successfully identified compounds that selectively inhibit both P. vivax and falciparum Kinesin-5 motor domains but, as anticipated, do not impact human Kinesin-5 activity. Of note is a candidate drug that did not biochemically compete with the ATP substrate for the conserved active site or disrupt the microtubule-binding site. Together, our experiments identified MMV666693 as a selective allosteric inhibitor of Plasmodium Kinesin-5; this is the first identified protein target for the Medicines of Malaria Venture validated collection of parasite proliferation inhibitors. This work demonstrates that chemical screens against human kinesins are adaptable to homologs in disease organisms and, as such, extendable to strategies to combat infectious disease.     

Sensitivity of Plasmodium falciparum to Antimalarial Drugs in Hainan Island,China

Pyronaridine and artesunate have been shown to be effective in falciparum malaria treatment. However, pyronaridine is rarely used in Hainan Island clinically, and artesunate is not widely used as a therapeutic agent. Instead, conventional antimalarial drugs, chloroquine and piperaquine, are used, explaining the emergence of chloroquine-resistant Plasmodium falciparum. In this article, we investigated the sensitivity of P. falciparum to antimalarial drugs used in Hainan Island for rational drug therapy. We performed in vivo (28 days) and in vitro tests to determine the sensitivity of P. falciparum to antimalarial drugs. Total 46 patients with falciparum malaria were treated with dihydroartemisinin/piperaquine phosphate (DUO-COTECXIN) and followed up for 28 day. The cure rate was 97.8%. The mean fever clearance time (22.5±10.6 hr) and the mean parasite clearance time (27.3±12.2 hr) showed no statistical significance with different genders, ages, temperatures, or parasite density (P>0.05). The resistance rates of chloroquine, piperaquine, pyronarididine, and artesunate detected in vitro were 71.9%, 40.6%, 12.5%, and 0%, respectively (P<0.0001). The resistance intensities decreased as follows: chloroquine>piperaquine>pyronarididine>artesunate. The inhibitory dose 50 (IC₅₀) was 3.77×10^-6 mol/L, 2.09×10^-6 mol/L, 0.09×10^-6 mol/L, and 0.05×10^-6 mol/L, and the mean concentrations for complete inhibition (CIMC) of schizont formation were 5.60×10^-6 mol/L, 9.26×10^-6 mol/L, 0.55×10^-6 mol/L, and 0.07×10^-6 mol/L, respectively. Dihydroartemisinin showed a strong therapeutic effect against falciparum malaria with a low toxicity.     

Crystal Structure of the Plasmodium falciparum Thioredoxin Reductase–Thioredoxin Complex

Over the last decades, malaria parasites have been rapidly developing resistance against antimalarial drugs, which underlines the need for novel drug targets. Thioredoxin reductase (TrxR) is crucially involved in redox homeostasis and essential for Plasmodium falciparum. Here, we report the first crystal structure of P. falciparum TrxR bound to its substrate thioredoxin 1. Upon complex formation, the flexible C-terminal arm and an insertion loop of PfTrxR are rearranged, suggesting that the C-terminal arm changes its conformation during catalysis similar to human TrxR. Striking differences between P. falciparum and human TrxR are a Plasmodium-specific insertion and the conformation of the C-terminal arm, which lead to considerable differences in thioredoxin binding and disulfide reduction. Moreover, we functionally analyzed amino acid residues involved in substrate binding and in the architecture of the intersubunit cavity, which is a known binding site for disulfide reductase inhibitors. Cell biological experiments indicate that P. falciparum TrxR is indeed targeted in the parasite by specific inhibitors with antimalarial activity. Differences between P. falciparum and human TrxR and details on substrate reduction and inhibitor binding provide the first solid basis for structure-based drug development and lead optimization.     

Dihydroartemisinin-Piperaquine vs. Artemether-Lumefantrine for First-Line Treatment of Uncomplicated Malaria in African Children: A Cost-Effectiveness Analysis

Background

Recent multi-centre trials showed that dihydroartemisinin-piperaquine (DP) was as efficacious and safe as artemether-lumefantrine (AL) for treatment of young children with uncomplicated P. falciparum malaria across diverse transmission settings in Africa. Longitudinal follow-up of patients in these trials supported previous findings that DP had a longer post-treatment prophylactic effect than AL, reducing the risk of reinfection and conferring additional health benefits to patients, particularly in areas with moderate to high malaria transmission.

Methods

We developed a Markov model to assess the cost-effectiveness of DP versus AL for first-line treatment of uncomplicated malaria in young children from the provider perspective, taking into consideration the post-treatment prophylactic effects of the drugs as reported by a recent multi-centre trial in Africa and using the maximum manufacturer drug prices for artemisinin-based combination therapies set by the Global Fund in 2013. We estimated the price per course of treatment threshold above which DP would cease to be a cost-saving alternative to AL as a first-line antimalarial drug.

Results

First-line treatment with DP compared to AL averted 0.03 DALYs (95% CI: 0.006–0.07) and 0.001 deaths (95% CI: 0.00–0.002) and saved $0.96 (95% CI: 0.33–2.46) per child over one year. The results of the threshold analysis showed that DP remained cost-saving over AL for any DP cost below $1.23 per course of treatment.

Conclusions

DP is superior to AL from both the clinical and economic perspectives for treatment of uncomplicated P. falciparum malaria in young children. A paediatric dispersible formulation of DP is under development and should facilitate a targeted deployment of this antimalarial drug. The use of DP as first-line antimalarial drug in paediatric malaria patients in moderate to high transmission areas of Africa merits serious consideration by health policymakers.     

Evidence for the early emergence of piperaquine-resistant Plasmodium falciparum malaria and modeling strategies to mitigate resistance

Multidrug-resistant Plasmodium falciparum parasites have emerged in Cambodia and neighboring countries in Southeast Asia, compromising the efficacy of first-line antimalarial combinations. Dihydroartemisinin + piperaquine (PPQ) treatment failure rates have risen to as high as 50% in some areas in this region. For PPQ, resistance is driven primarily by a series of mutant alleles of the P. falciparum chloroquine resistance transporter (PfCRT). PPQ resistance was reported in China three decades earlier, but the molecular driver remained unknown. Herein, we identify a PPQ-resistant pfcrt allele (China C) from Yunnan Province, China, whose genotypic lineage is distinct from the PPQ-resistant pfcrt alleles currently observed in Cambodia. Combining gene editing and competitive growth assays, we report that PfCRT China C confers moderate PPQ resistance while re-sensitizing parasites to chloroquine (CQ) and incurring a fitness cost that manifests as a reduced rate of parasite growth. PPQ transport assays using purified PfCRT isoforms, combined with molecular dynamics simulations, highlight differences in drug transport kinetics and in this transporter’s central cavity conformation between China C and the current Southeast Asian PPQ-resistant isoforms. We also report a novel computational model that incorporates empirically determined fitness landscapes at varying drug concentrations, combined with antimalarial susceptibility profiles, mutation rates, and drug pharmacokinetics. Our simulations with PPQ-resistant or -sensitive parasite lines predict that a three-day regimen of PPQ combined with CQ can effectively clear infections and prevent the evolution of PfCRT variants. This work suggests that including CQ in combination therapies could be effective in suppressing the evolution of PfCRT-mediated multidrug resistance in regions where PPQ has lost efficacy.     

The Plasmodium knowlesi MAHRP2 ortholog localizes to structures connecting Sinton Mulligan's clefts in the infected erythrocyte

During development within the host erythrocyte malaria parasites generate nascent membranous structures which serve as a pathway for parasite protein transport to modify the host cell. The molecular basis of such membranous structures is not well understood, particularly for malaria parasites other than Plasmodium falciparum. To characterize the structural basis of protein trafficking in the Plasmodium knowlesi-infected erythrocyte, we identified a P. knowlesi ortholog of MAHRP2, a marker of the tether structure that connects membranous structures in the P. falciparum-infected erythrocyte. We show that PkMAHRP2 localizes on amorphous structures that connect Sinton Mulligan's clefts (SMC) to each other and to the erythrocyte membrane. Three dimensional reconstruction of the P. knowlesi-infected erythrocyte revealed that the SMC is a plate-like structure with swollen ends, reminiscent of the morphology of the Golgi apparatus. The PkMAHRP2-localized amorphous structures are possibly functionally equivalent to P. falciparum tether structure. These findings suggest a conservation in the ultrastructure of protein trafficking between P. falciparum and P. knowlesi.     

Associations between Antibodies to a Panel of Plasmodium falciparum Specific Antigens and Response to Sub-Optimal Antimalarial Therapy in Kampala,Uganda

Background

Antibodies are important in the control of blood stage Plasmodium falciparum infection. It is unclear which antibody responses are responsible for, or even associated with protection, partly due to confounding by heterogeneous exposure. Assessment of response to partially effective antimalarial therapy, which requires the host to assist in clearing parasites, offers an opportunity to measure protection independent of exposure.

Methods

A cohort of children aged 1–10 years in Kampala, Uganda were treated with amodiaquine+sulfadoxine-pyrimethamine for uncomplicated malaria. Serum samples from the time of malaria diagnosis and 14 days later were analyzed for total IgG to 8 P. falciparum antigens using a quantitative indirect ELISA. Associations between antibody levels and risk of treatment failure were estimated using Cox proportional hazard regression.

Results

Higher levels of antibodies to apical membrane antigen 1 (AMA-1), but to none of the other 7 antigens were significantly associated with protection against treatment failure (HR 0.57 per 10-fold increase in antibody level, CI 0.41–0.79, p = 0.001). Protection increased consistently across the entire range of antibody levels.

Conclusions

Measurement of antibody levels to AMA-1 at the time of malaria may offer a quantitative biomarker of blood stage immunity to P. falciparum, a tool which is currently lacking.     

Imported Malaria in Korea: a 13-Year Experience in a Single Center

The incidence of imported malaria has been increasing in Korea. We reviewed data retrospectively to evaluate the epidemiology, clinical features, and outcomes of imported malaria from 1995 to 2007 in a university hospital. All patients diagnosed with imported malaria were included. Imported malaria was defined as a positive smear for malaria that was acquired in a foreign country. A total of 49 patients (mean age, 35.7 year; M : F = 38 : 11) were enrolled. The predominant malarial species was Plasmodium falciparum (73.5%), and the most frequent area of acquisition was Africa (55.1%), followed by Southeast Asia (22.4%) and South Asia (18.4%). Fourteen-patients (30.6%) suffered from severe malaria caused by P. falciparum and 1 patient (2.0%) died of multiorgan failure. Most of the patients were treated with mefloquine (79.2%) or quinine (10.2%); other antimalarial agents had to be given in 13.2% treated with mefloquine and 44.4% with quinine due to adverse drug events (ADEs). P. falciparum was the most common cause of imported malaria, with the majority of cases acquired from Africa, and a significant number of patients had severe malaria. Alternative antimalarial agents with lower rates of ADEs might be considered for effective treatment instead of mefloquine and quinine.     

Evaluation of amidoxime derivatives as prodrug candidates of potent bis-cationic antimalarials

Plasmodium falciparum is responsible for most of the cases of malaria and its resistance to established antimalarial drugs is a major issue. Thus, new chemotherapies are needed to fight the emerging multi-drug resistance of P. falciparum malaria, like choline analogues targeting plasmodial phospholipidic metabolism. Here we describe the synthesis of amidoxime derivatives as prodrug candidates of reverse-benzamidines and hybrid compounds able to mimic choline, as well as the design of a new series of asymmetrical bis-cationic compounds. Bioconversion studies were conducted on amidoximes in asymmetrical series and showed that amidoxime prodrug strategy could be applied on C-alkylamidine moieties, like benzamidines and that N-substituents did not alter the bioconversion of amidoximes. The antimalarial activity of the three series of compounds was evaluated in vitro against P. falciparum and in vivo against P. vinckei petteri in mice.