Mechanisms of resistance of Plasmodium falciparum to antimalarial drugs

Design, synthesis, and evaluation of 10-N-substituted acridones as novel chemosensitizers in Plasmodium falciparum

A series of novel 10-N-substituted acridones, bearing alkyl side chains with tertiary amine groups at the terminal position, were designed, synthesized, and evaluated for the ability to enhance the potency of quinoline drugs against multidrug-resistant (MDR) Plasmodium falciparum malaria parasites. A number of acridone derivatives, with side chains bridged three or more carbon atoms apart between the ring nitrogen and terminal nitrogen, demonstrated chloroquine (CQ)-chemosensitizing activity against the MDR strain of P. falciparum (Dd2). Isobologram analysis revealed that selected candidates demonstrated significant synergy with CQ in the CQ-resistant (CQR) parasite Dd2 but only additive (or indifferent) interaction in the CQ-sensitive (CQS) D6. These acridone derivatives also enhanced the sensitivity of other quinoline antimalarials, such as desethylchloroquine (DCQ) and quinine (QN), in Dd2. The patterns of chemosensitizing effects of selected acridones on CQ and QN were similar to those of verapamil against various parasite lines with mutations encoding amino acid 76 of the P. falciparum CQ resistance transporter (PfCRT). Unlike other known chemosensitizers with recognized psychotropic effects (e.g., desipramine, imipramine, and chlorpheniramine), these novel acridone derivatives exhibited no demonstrable effect on the uptake or binding of important biogenic amine neurotransmitters. The combined results indicate that 10-N-substituted acridones present novel pharmacophores for the development of chemosensitizers against P. falciparum.

Drug-resistant malaria - an insight

Despite intensive research extending back to the 1930s, when the first synthetic antimalarial drugs made their appearance, the repertoire of clinically licensed formulations remains very limited. Moreover, widespread and increasing resistance to these drugs contributes enormously to the difficulties in controlling malaria, posing considerable intellectual, technical and humanitarian challenges. A detailed understanding of the molecular mechanisms underlying resistance to these agents is emerging that should permit new drugs to be rationally developed and older ones to be engineered to regain their efficacy. This review summarizes recent progress in analysing the causes of resistance to the major antimalarial drugs and its spread.

Sulfadoxine-pyrimethamine susceptibilities and analysis of the dihydrofolate reductase and dihydropteroate synthase of Plasmodium falciparum isolates from Côte d'Ivoire

Over a 2-year study period, three methods [a test of therapeutic efficacy, an in-vitro assay, and sequencing of the parasites' dihydrofolate-reductase (dhfr) and dihydropteroate-synthase (dhps) genes] were used to monitor sulfadoxine-pyrimethamine (SP) resistance in the Plasmodium falciparum strains infecting young children near Abidjan, the largest city in Côte d'Ivoire. Overall, 118 children aged<5 years and infected with P. falciparum were treated with SP. Twenty-one (23.5%) of the 89 children who completed the 14 days of follow-up were categorized as therapeutic failures. When P. falciparum isolates from the 61 children with adequate parasitaemias were investigated in the in-vitro assay, 24 (39.5%) were found to be highly resistant to pyrimethamine, each having a median inhibitory concentration (IC50) of at least 2000 nM. Polymorphism analysis of gene fragments of 118 P. falciparum isolates (one from each child enrolled in the study) revealed that 46 (39%) of the isolates had mutant dhfr and 111 (94%) had mutant dhps. The mutations mainly affected DHFR codon 108 (39% of the isolates) and DHPS codons 436 (65%), 437 (52%) and 613 (27%). Of the 37 DHFR mutant isolates from children who completed follow-up, 21 were from children with therapeutic failure, indicating that mutant DHFR was associated with resistance to pyrimethamine in vivo (kappa=0.61). A mutant dhfr genotype was also found to be strongly associated with resistance to pyrimethamine in vitro (kappa=0.79). There was, however, little evidence of an association between SP efficacy and dhps genotype (kappa=0.04). Resistance to SP appears to be an increasing problem in southern Côte d'Ivoire and one which may now justify a change away from this drug combination as the first- or second-line treatment for P. falciparum malaria in this area.

Antifolates can have a role in the treatment of Plasmodium vivax

Plasmodium vivax is a serious health concern in many regions and is sometimes inadvertently treated with sulfadoxine-pyrimethamine (SP). Mutations in the genes that encode dihydrofolate reductase (DHFR) and dihydropteroate synthase (DHPS) confer resistance to pyrimethamine and sulfadoxine, respectively. Numerous studies have examined the prevalence and diversity of mutations in P. vivax dhfr and some have assessed the relationship between dhfr genotype and clinical or in vitro response to pyrimethamine. Other studies have examined the impact of dhps genotype on response to sulfadoxine. These data indicate that, under certain circumstances, SP could be a valuable tool in the fight against P. vivax.

Characterisation of exogenous folate transport in Plasmodium falciparum

Folate salvage by Plasmodium falciparum is an important source of key cofactors, but little is known about the underlying mechanism. Using synchronised parasite cultures, we observed that uptake of this dianionic species against the negative-inward electrochemical gradient is highly dependent upon cell-cycle stage, temperature and pH, but not on mono- or divalent metal ions. Energy dependence was tested with different sugars; glucose was necessary for folate import, although fructose was also able to function in this role, unlike sugars that cannot be processed through the glycolytic pathway. Import into both infected erythrocytes and free parasites was strongly inhibited by the anion-channel blockers probenecid and furosemide, which are likely to be acting predominantly on specific folate transporters in both cases. Import was not affected by high concentrations of the antifolate drugs pyrimethamine and sulfadoxine, but was inhibited by the close folate analogue methotrexate. The pH optimum for folate uptake into infected erythrocytes was 6.5–7.0. Dinitrophenol and nigericin, which strongly facilitate the equilibration of H⁺ ions across biological membranes and thus abolish or substantially reduce the proton gradient, inhibited folate uptake profoundly. The ATPase inhibitor concanamycin A also greatly reduced folate uptake, further demonstrating a link to ATP-powered proton transport. These data strongly suggest that the principal folate uptake pathway in P. falciparum is specific, highly regulated, dependent upon the proton gradient across the parasite plasma membrane, and is likely to be mediated by one or more proton symporters.

Pyrimethamine-resistant dihydrofolate reductase enzymes of Plasmodium falciparum are not enzymatically compromised in vitro

Plasmodium falciparum, the protozoan that causes the most lethal form of human malaria, has been controlled principally by two safe, affordable drugs, chloroquine and sulfadoxine-pyrimethamine (SP). Studies in the laboratory and in the field have demonstrated that resistance to SP depends on non-synonymous point mutations in the dihydrofolate reductase (DHFR), and dihydropteroate synthase (DHPS) coding regions. Parasites that carry dhfr genes with 3 or 4 point mutations (51I/59R/108N triple mutation or 51I/59R/108N/164L quadruple mutation) are resistant to pyrimethamine in vitro and patients infected with these parasites respond poorly to SP treatment. The wide spread of these pyrimethamine-resistant alleles demonstrates the increased fitness over drug-sensitive alleles in the presence of the drug. However, it is not clear whether these alleles might reduce the fitness of parasites in the absence of drug pressure. As a first step, we compared the kinetic properties of the wild type, and three mutant alleles to determine whether the native DHFR-thymidylate synthase form of the mutant proteins showed compromised activity in vitro. The mutant enzymes had K(m) values for their substrate, dihydrofolate that were significantly lower than the wild type, k(cat) values in the same range as the wild type enzyme, and k(cat)/K(m) values higher than wild type. In contrast, the K(m) values for the NADPH cofactor were higher than wild type for the mutant enzymes. These observations suggest that the fitness of these parasites may not be compromised relative to those that carry the wild type allele, even without sustained SP drug pressure.

Assessment and continued validation of the malaria SYBR green I-based fluorescence assay for use in malaria drug screening

Several new fluorescence malaria in vitro drug susceptibility microtiter plate assays that detect the presence of malarial DNA in infected erythrocytes have recently been reported, in contrast to traditional isotopic screens that involve radioactive substrate incorporation to measure in vitro malaria growth inhibition. We have assessed and further characterized the malaria SYBR Green I-based fluorescence (MSF) assay for its ability to monitor drug resistance. In order to use the MSF assay as a drug screen, all assay conditions must be thoroughly examined. In this study we expanded upon the capabilities of this assay by including antibiotics and antifolates in the drug panel and testing folic acid-free growth conditions. To do this, we evaluated a more expansive panel of antimalarials in combination with various drug assay culture conditions commonly used in drug sensitivity screening for their activity against Plasmodium falciparum strains D6 and W2. The detection and quantitation limits of the MSF assay were 0.04 to 0.08% and approximately 0.5% parasitemia, respectively. The MSF assay quality was significantly robust, displaying a Z' range of 0.73 to 0.95. The 50% inhibitory concentrations for each drug and culture condition combination were determined by using the MSF assay. Compared to the standard [(3)H]hypoxanthine assay, the MSF assay displayed the expected parasite drug resistance patterns with a high degree of global and phenotypic correlation (r(2) >/= 0.9238), regardless of which culture condition combination was used. In conclusion, the MSF assay allows for reliable one-plate high-throughput, automated malaria in vitro susceptibility testing without the expense, time consumption, and hazard of other screening assays.

The role of tryptophan-48 in catalysis and binding of inhibitors of Plasmodium falciparum dihydrofolate reductase

Dihydrofolate reductases (DHFRs) from Plasmodium falciparum (Pf) and various species of both prokaryotic and eukaryotic organisms have a conserved tryptophan (Trp) at position 48 in the active site. The role in catalysis and binding of inhibitors of the conserved Trp48 of PfDHFR has been analysed by site-specific mutagenesis, enzyme kinetics and use of a bacterial surrogate system. All 19 mutant enzymes showed undetectable or very low specific activities, with the highest value of k(cat)/K(m) from the Tyr48 (W48Y) mutant (0.12 versus 11.94M(-1)s(-1)), of about 1% of the wild-type enzyme. The inhibition constants for pyrimethamine, cycloguanil and WR99210 of the W48Y mutants are 2.5-5.3 times those of the wild-type enzyme. All mutants, except W48Y, failed to support the growth of Escherichia coli transformed with the parasite gene in the presence of trimethoprim, indicating the loss of functional activity of the parasite enzyme. Hence, Trp48 plays a crucial role in catalysis and inhibitor binding of PfDHFR. Interestingly, W48Y with an additional mutation at Asn188Tyr (N188Y) was found to promote bacterial growth and yielded a higher amount of purified enzyme. However, the kinetic parameters of the purified W48Y+N188Y enzyme were comparable with W48Y and the binding affinities for DHFR inhibitors were also similar to the wild-type enzyme. Due to its conserved nature, Trp48 of PfDHFR is a potential site for interaction with antimalarial inhibitors which would not be compromised by its mutations.

Targeting of toxic compounds to the trypanosome's interior

Drugs can be targeted into African trypanosomes by exploiting carrier proteins at the surface of these parasites. This has been clearly demonstrated in the case of the melamine-based arsenical and the diamidine classes of drug that are already in use in the treatment of human African trypanosomiasis. These drugs can enter via an aminopurine transporter, termed P2, encoded by the TbAT1 gene. Other toxic compounds have also been designed to enter via this transporter. Some of these compounds enter almost exclusively through the P2 transporter, and hence loss of the P2 transporter leads to significant resistance to these particular compounds. It now appears, however, that some diamidines and melaminophenylarsenicals may also be taken up by other routes (of yet unknown function). These too may be exploited to target new drugs into trypanosomes. Additional purine nucleoside and nucleobase transporters have also been subverted to deliver toxic agents to trypanosomes. Glucose and amino acid transporters too have been investigated with a view to manipulating them to carry toxins into Trypanosoma brucei, and recent work has demonstrated that aquaglyceroporins may also have considerable potential for drug-targeting. Transporters, including those that carry lipids and vitamins such as folate and other pterins also deserve more attention in this regard. Some drugs, for example suramin, appear to enter via routes other than plasma-membrane-mediated transport. Receptor-mediated endocytosis has been proposed as a possible way in for suramin. Endocytosis also appears to be crucial in targeting natural trypanocides, such as trypanosome lytic factor (TLF) (apolipoprotein L1), into trypanosomes and this offers an alternative means of selectively targeting toxins to the trypanosome's interior. Other compounds may be induced to enter by increasing their capacity to diffuse over cell membranes; in this case depending exclusively on selective activity within the cell rather than selective uptake to impart selective toxicity. This review outlines studies that have aimed to exploit trypanosome nutrient uptake routes to selectively carry toxins into these parasites.

Evaluation of the activities of pyrimethamine analogs against Plasmodium vivax and Plasmodium falciparum dihydrofolate reductase-thymidylate synthase using in vitro enzyme inhibition and bacterial complementation assays

Pyrimethamine analogs were examined as potential agents against vivax malaria using a bacterial surrogate system carrying Plasmodium vivax dihydrofolate reductase-thymidylate synthase (PvDHFR-TS), in which the PvDHFR complemented chemically knocked out host dihydrofolate reductase. The system was initially tested with P. falciparum dihydrofolate reductase-thymidylate synthase and was found to have good correlation with the parasite-based system. The 50% inhibitory concentrations derived from PvDHFR-TS-dependent bacteria were correlated with their corresponding inhibition constants (Ki) from an enzyme inhibition assay, pointing to the likelihood that the potent enzyme inhibitors will also have potent antimalarial activities. Active compounds against both wild-type and S58R S117N (SP21) double-mutant P. vivax include analogs with structures which can avert a steric clash with the asparagine (S117N) side chain of the mutant, similar to those found for homologous Plasmodium falciparum mutants, raising the possibility that the same compounds can be developed against both types of antifolate-resistant malaria. This rapid and convenient drug screening system should be useful for development of new antifolates against P. vivax, for which a continuous culture system is not yet available.

Susceptibility of Colombian Plasmodium falciparum isolates to 4-aminoquinolines and the definition of amodiaquine resistance in vitro

There are wide variations in the threshold used to define in vitro resistance of Plasmodium falciparum to amodiaquine (AQ), probably due to differences in methodology and interpretation. In vitro susceptibility data of Colombian P. falciparum strains to AQ and N-desethylamodiaquine is used to illustrate the need to standardized methodologies and compare inhibitory concentrations, instead of resistant/susceptible phenotypes, when studying the mechanisms of resistance to AQ and monitoring drug susceptibility trends in the field.

Antiplasmodial and antitrypanosomal activity of new esters and ethers of 4-dialkylaminobicyclo[2.2.2]octan-2-ols

Only three drugs are available for the treatment of East African trypanosomiasis. Patients suffer from painful application, severe side effects and increasing resistance against these drugs. Malaria tropica kills more than 2 million people every year mainly due to growing drug resistance. 4-Dialkylaminobicyclo[2.2.2]octan-2-ols and some of their esters have shown activity against both the causative organisms, Trypanosoma brucei rhodesiense and Plasmodium falciparum. Ethers and new esters with markedly higher lipophilicity were prepared in three-step procedures from acyclic synthons. The new compounds were screened for their antiprotozoal activities against T. b. rhodesiense (STIB 900) and P. falciparum K1 (resistant to chloroquine and pyrimethamine), and for their cytotoxicity with L-6 cells by means of in vitro microplate assays. The results were compared to those of the parent compounds indicating that higher lipophilicity increases the antiprotozoal activities. The pivalate 10a showed the highest antitrypanosomal activity. The 4-chlorobenzoate 9b exhibited good antiplasmodial activity and low cytotoxicity. The most active antiplasmodial agent was the benzhydryl ether 13c which was nearly as active as chloroquine against sensitive strains.

MOLECULAR MECHANISMS OF RESISTANCE IN ANTIMALARIAL CHEMOTHERAPY: The Unmet Challenge

▪ Abstract 

The enormous public health problem posed by malaria has been substantially worsened in recent years by the emergence and worldwide spread of drug-resistant parasites. The utility of two major therapies, chloroquine and the synergistic combination of pyrimethamine/sulfadoxine, is now seriously compromised. Although several genetic mechanisms have been described, the major source of drug resistance appears to be point mutations in protein target genes. Clinically significant resistance to these agents requires the accumulation of multiple mutations, which genetic studies of parasite populations suggest arise focally and sweep through the population. Efforts to circumvent resistance range from the use of combination therapy with existing agents to laboratory studies directed toward discovering novel targets and therapies.

The prevention and management of drug resistance are among the most important practical problems of tropical medicine and public health.

Leonard J. Bruce-Chwatt, 1972

Drug-resistant malaria

In the past 21 years, a modest increase in the range of antimalarial drugs approved for clinical use has been complemented by a more impressive expansion in the analysis and understanding of the molecular mechanisms underlying resistance to these agents. Such resistance is a major factor in the increasing difficulty in controlling malaria, and important developments during this period are recounted here.

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

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

Biochemical characterization of the two nucleosome assembly proteins from Plasmodium falciparum

The human malaria parasite Plasmodium falciparum contains two nucleosome assembly proteins, which we have termed PfNAPS and PfNAPL. We have over-expressed, purified and characterized these proteins using biochemical and biophysical techniques. PfNAPS and PfNAPL exist as dimers in solution and circular dichroism studies suggest that they may have different three-dimensional protein structures. ELISA-based binding data also suggest that PfNAPS and PfNAPL preferentially interact with the H3-H4 tetramer histones over H2A and H2B histones. We show that the parasite lysate phosphorylates only PfNAPL and this phosphorylation can be inhibited by heparin suggesting a potential role of casein kinase II in this process. Immuno-fluorescence experiments revealed that both PfNAPS and PfNAPL were expressed in all erythrocytic stages of the parasite. PfNAPL was predominantly localised in the cytoplasm in asexual and sexual stages of the parasite. PfNAPS did not co-localise with PfNAPL and was more intimately associated with the parasite nucleus, most strikingly in P. falciparum gametocytes. Taken together, our data show that although PfNAPS and PfNAPL share histone chaperone acitivities, they are regulated differently by phosphorylation and are spatially segregated within the parasite. These proteins are therefore likely to play non-redundant roles as nucleosome assembly motors in the parasite.

The phenothiazinium chromophore and the evolution of antimalarial drugs

The phenothiazinium salt methylene blue [3,7-bis(dimethylamino)phenothiazinium chloride] is the oldest known synthetic antimalarial drug, its clinical efficacy having been reported in 1891. The role of methylene blue in the evolution of the modern antimalarial armoury is often unappreciated, yet it can be linked directly to standard drugs such as chloroquine and its congeners. Also, in the face of increasing plasmodial resistance to modern antimalarials, phenothiazinium derivatives have again featured as lead compounds in drug research. The precise mode of action of methylene blue and its commercial analogues against Plasmodium spp. remains a cause for conjecture, having been variously described as nucleic acid intercalation, food vacuole basification, parasite redox cycle interference and haem polymerization inhibition. That the activity of the series may be due to more than one route - i.e. a multifactorial activity - underlines the utility of these compounds in antimalarial research either as single drugs or as adjuvants (partners in a drug combination), particularly in the face of resistant parasitic strains.

Exploring the folate pathway in Plasmodium falciparum

As in centuries past, the main weapon against human malaria infections continues to be intervention with drugs, despite the widespread and increasing frequency of parasite populations that are resistant to one or more of the available compounds. This is a particular problem with the lethal species of parasite, Plasmodium falciparum, which claims some two million lives per year as well as causing enormous social and economic problems. Amongst the antimalarial drugs currently in clinical use, the antifolates have the best defined molecular targets, namely the enzymes dihydrofolate reductase (DHFR) and dihydropteroate synthase (DHPS), which function in the folate metabolic pathway. The products of this pathway, reduced folate cofactors, are essential for DNA synthesis and the metabolism of certain amino acids. Moreover, their formation and interconversions involve a number of other enzymes that have not as yet been exploited as drug targets. Antifolates are of major importance as they currently represent the only inexpensive regime for combating chloroquine-resistant malaria, and are now first-line drugs in a number of African countries. Aspects of our understanding of this pathway and antifolate drug resistance are reviewed here, with a particular emphasis on approaches to analysing the details of, and balance between, folate biosynthesis by the parasite and salvage of pre-formed folate from exogenous sources.

Multidrug resistance in parasites: ABC transporters, P-glycoproteins and molecular modelling

Parasitic diseases, caused by protozoa, helminths and arthropods, rank among the most important problems in human and veterinary medicine, and in agriculture, leading to debilitating sicknesses and loss of life. In the absence of vaccines and with the general failure of vector eradication programs, drugs are the main line of defence, but the newest drugs are being tracked by the emergence of resistance in parasites, sharing ominous parallels with multidrug resistance in bacterial pathogens. Any of a number of mechanisms will elicit a drug resistance phenotype in parasites, including: active efflux, reduced uptake, target modification, drug modification, drug sequestration, by-pass shunting, or substrate competition. The role of ABC transporters in parasitic multidrug resistance mechanisms is being subjected to more scrutiny, due in part to the established roles of certain ABC transporters in human diseases, and also to an increasing portfolio of ABC transporters from parasite genome sequencing projects. For example, over 100 ABC transporters have been identified in the Escherichia coli genome, but to date only about 65 in all parasitic genomes. Long established laboratory investigations are now being assisted by molecular biology, bioinformatics, and computational modelling, and it is in these areas that the role of ABC transporters in parasitic multidrug resistance mechanisms may be defined and put in perspective with that of other proteins. We discuss ABC transporters in parasites, and conclude with an example of molecular modelling that identifies a new interaction between the structural domains of a parasite P-glycoprotein.

Novel Plasmodium vivax dhfr alleles from the Indonesian Archipelago and Papua New Guinea: association with pyrimethamine resistance determined by a Saccharomyces cerevisiae expression system

In plasmodia, the dihydrofolate reductase (DHFR) enzyme is the target of the pyrimethamine component of sulfadoxine-pyrimethamine (S/P). Plasmodium vivax infections are not treated intentionally with antifolates. However, outside Africa, coinfections with Plasmodium falciparum and P. vivax are common, and P. vivax infections are often exposed to S/P. Cloning of the P. vivax dhfr gene has allowed molecular comparisons of dhfr alleles from different regions. Examination of the dhfr locus from a few locations has identified a very diverse set of alleles and showed that mutant alleles of the vivax dhfr gene are prevalent in Southeast Asia where S/P has been used extensively. We have surveyed patient isolates from six locations in Indonesia and two locations in Papua New Guinea. We sequenced P. vivax dhfr alleles from 114 patient samples and identified 24 different alleles that differed from the wild type by synonymous and nonsynonymous point mutations, insertions, or deletions. Most importantly, five alleles that carried four or more nonsynonymous mutations were identified. Only one of these highly mutant alleles had been previously observed, and all carried the 57L and 117T mutations. P. vivax cannot be cultured continuously, so we used a yeast assay system to determine in vitro sensitivity to pyrimethamine for a subset of the alleles. Alleles with four nonsynonymous mutations conferred very high levels of resistance to pyrimethamine. This study expands significantly the total number of novel dhfr alleles now identified from P. vivax and provides a foundation for understanding how antifolate resistance arises and spreads in natural P. vivax populations.

Arylsulfonyl acridinyl derivatives acting on Plasmodium falciparum

Several arylacridinyl sulfones have been synthesized and their antimalarial action was tested on Plasmodium falciparum. PABA (para-aminobenzoic acid) has no antagonistic effect with these compounds as opposed to the observed effect with dapsone and sulfonamides previously studied. A possible relationship between the ability of cleavage of the S-9C acridinic bond and activity is suggested.

Crystal structure of a novel Plasmodium falciparum 1-Cys peroxiredoxin

Plasmodium falciparum, the causative agent of malaria, is sensitive to oxidative stress and therefore the family of antioxidant enzymes, peroxiredoxins (Prxs) represent a target for antimalarial drug design. We present here the 1.8 A resolution crystal structure of P.falciparum antioxidant protein, PfAOP, a Prx that in terms of sequence groups with mammalian PrxV. The structure is compared to all 11 known Prx structures to gain maximal insight into its properties. We describe the common Prx fold and show that the dimeric PfAOP can be mechanistically categorized as a 1-Cys Prx. In the active site the peroxidatic Cys is over-oxidized to cysteine sulfonic acid, making this the first Prx structure seen in that state. Now with structures of Prxs in Cys-sulfenic, -sulfinic and -sulfonic acid oxidation states known, the structural steps involved in peroxide binding and over-oxidation are suggested. We also describe that PfAOP has an alpha-aneurism (a one residue insertion), a feature that appears characteristic of the PrxV-like group. In terms of crystallographic methodology, we enhance the information content of the model by identifying bound water sites based on peak electron densities, and we use that information to infer that the oxidized active site has suboptimal interactions that may influence catalysis. The dimerization interface of PfAOP is representative of an interface that is widespread among Prxs, and has sequence-dependent variation in geometry. The interface differences and the structural features (like the alpha-aneurism) may be used as markers to better classify Prxs and study their evolution.

Metalloantimalarials: synthesis and characterization of a novel agent possessing activity against Plasmodium falciparum

The synthesis, characterization, and antimalarial potency of an amine-phenol complex of gallium(III), [{1,12-bis(2-hydroxy-3-methoxy-5-(quinolin-3-yl)-benzyl)-1,5,8,12-tetraazadodecane}-gallium(III)]+, [Ga-3-M-5-Quadd]+ (7) is described; a novel agent that targets Plasmodium falciparum strains.

Novel alleles of Plasmodium falciparum dhfr that confer resistance to chlorcycloguanil

In Plasmodium falciparum, resistance to folate inhibitors like pyrimethamine is mediated by point mutations in the target gene dihydrofolate reductase (dhfr). The resistance to pyrimethamine increases with the accumulation of particular point mutations. These mutations also confer increased resistance to chlorcycloguanil, the active metabolite of chlorproguanil and one component of a newly introduced DHFR inhibitor, LapDap. One genotype (16V/108T) has been previously identified that confers resistance to cycloguanil but not to pyrimethamine. This study was designed to identify novel alleles that might confer resistance to chlorcycloguanil, but escape the surveillance methods currently in place for common pyrimethamine-resistant alleles. Directed mutagenesis was performed using the wild type and the common pyrimethamine-resistant allele, 51I/59R/108N, to determine the effect of the 16V and 108T mutations on enzyme activity and drug resistance. In addition, we randomly mutagenized the 51I/59R/108N allele and identified nine novel alleles that could confer resistance to chlorcycloguanil. These yeast strains were also resistant to pyrimethamine, but retained sensitivity to the experimental DHFR inhibitor, WR99210. None of the alleles generated in this study was as resistant to chlorcycloguanil as the common quadruple mutant, 51I/59R/108N/164L. In addition, selection of high levels of chlorcycloguanil resistance in parasites that carry the 51I/59R/108N allele will require two directed steps, a change from 108N to 108T followed by a mutation from A16 to 16V. The resulting allele, 16V/51I/59R/108T is highly resistant to chlorcycloguanil, but 200-fold more sensitive to pyrimethamine than the 51I/59R/108N allele.

Proteases in host cell invasion by the malaria parasite

The life cycle of the malaria parasite contains three distinct invasive forms, or zoites. For at least two of these--the sporozoite and the blood-stage merozoite--invasion into their respective host cell requires the activity of parasite proteases. This review summarizes the evidence for this, discusses selected well-described proteolytic modifications linked to invasion, and describes recent progress towards identifying the proteases involved.

Automated solid-phase extraction method for the determination of piperaquine in capillary blood applied onto sampling paper by liquid chromatography

A bioanalytical method for the determination of piperaquine in 100 microL blood applied onto sampling paper, by solid-phase extraction and liquid chromatography, has been developed and validated. Blood spots were cut into small pieces prior to addition of 0.3M perchloric acid, acetonitrile and phosphate buffer containing an internal standard. The liquid phase was loaded onto a mixed phase cation-exchange (MPC) solid-phase extraction column. Piperaquine and the internal standard were analysed by liquid chromatography and separated on a Chromolith Performance (100 mm x 4.6 mm) column with acetonitrile:phosphate buffer pH 2.5, I = 0.1 (8:92, v/v) at the flow of 3.5 mL/min. The UV detection was performed at 345 nm. The intra-assay precision was 12.0% at 0.150 microM, 7.3% at 1.25 microM and 7.3% at 2.25 microM. The inter-assay precision was 1.8% at 0.150 microM, 5.2% at 1.25 microM and 2.8% at 2.25 microM. The lower limit of quantification (LLOQ) was determined to 0.050 microM where the precision was 14.7%.

PLASMODIUM YOELII: IDENTIFICATION AND PARTIAL CHARACTERIZATION OF ANMDR1GENE IN AN ARTEMISININ-RESISTANT LINE

The molecular mechanisms by which the malarial parasite has managed to develop resistance to many antimalarial drugs remain to be completely elucidated. Mutations in the pfmdr1 gene of Plasmodium falciparum, as well as an increase in pfmdr1 copy number, have been associated with resistance to the quinoline-containing antimalarial drugs. We investigated the mechanisms of drug resistance in Plasmodium using a collection of P. yoelii lines with different drug resistance profiles. The mdr1 gene of P. yoelii (pymdr1) was identified and characterized. A 2- to 3-fold increase in the pymdr1 gene copy number was observed in the P. yoelii ART line (artemisinin resistant) when compared with the NS parental line. The pymdr1 gene was mapped to a chromosome of 2.1 Mb in all lines analyzed. Reverse transcriptase-polymerase chain reaction and Western blot experiments confirmed the expression of the gene at the RNA and protein levels.

New 4-aminobicyclo[2.2.2]octane derivatives and their activities against Plasmodium falciparum and Trypanosoma b. rhodesiense

A series of new 2-substituted 4-dialkylaminobicyclo[2.2.2]octane derivatives was prepared and the compounds were investigated for their activity against causative organisms of tropical diseases. The tests were performed as microplate assays using the K1 strain of Plasmodium falciparum (resistant to chloroquine and pyrimethamine) and Trypanosoma brucei rhodesiense (STIB 900). The results were compared to the activities of former tested compounds of the bicyclo[2.2.2]octane series and to known drugs. Most of the 4-amino-6,7-diphenylbicyclo[2.2.2]octan-2-one thiosemicarbazones were compounds with attractive antimalarial potency (IC(50)=0.84-0.99microM, chloroquine: IC(50)=0.12microM). One of the bicyclo[2.2.2]octan-2-yl 4-tert-butylbenzenesulfonates showed the highest antitrypanosomal activity (IC(50)=0.68microM) of the so far prepared 4-amino-6,7-diarylbicyclo[2.2.2]octane derivatives, but is distinctly less active than suramin (IC(50)=0.0075microM).

Anti-plasmodial activities and X-ray crystal structures of rotenoids from Millettia usaramensis subspecies usaramensis

The dichloromethane extract of the stem bark of Millettia usaramensis subspecies usaramensis showed anti-plasmodial activity against the chloroquine sensitive (D6) and chloroquine resistant (W2) strains of Plasmodium falciparum. Chromatographic separation of the extract led to the identification of a new rotenoid, (6aR,12aS)-2,3-methylenedioxy-9-methoxy-8-(3,3-dimethylallyl)-12a-hydroxyrotenoid (trivial name, usararotenoid C) along with known flavonoids (usararotenoid A, 12a-epimillettosin, 6a,12a-dehydromillettone, barbigerone and 4'-O-geranylisoliquiritigenin) as the anti-plasmodial principles. The structures were determined by spectroscopic analyses. CD and X-ray analyses established absolute configurations.

The heat shock protein 90 of Plasmodium falciparum and antimalarial activity of its inhibitor, geldanamycin

BACKGROUND

The naturally occurring benzoquinone ansamycin compound, geldanamycin (GA), is a specific inhibitor of heat shock protein 90 (Hsp90) and is a potential anticancer agent. Since Plasmodium falciparum has been reported to have an Hsp90 ortholog, we tested the possibility that GA might inhibit it and thereby display antiparasitic activity.

RESULTS

We provide direct recombinant DNA evidence for the Hsp90 protein of Plasmodium falciparum, the causative agent of fatal malaria. While the mRNA of Hsp90 was mainly expressed in ring and trophozoite stages, the protein was found in all stages, although schizonts contained relatively lower amounts. In vitro the parasitic Hsp90 exhibited an ATP-binding activity that could be specifically inhibited by GA. Plasmodium growth in human erythrocyte culture was strongly inhibited by GA with an IC50 of 20 nM, compared to the IC50 of 15 nM for chloroquine (CQ) under identical conditions. When used in combination, the two drugs acted synergistically. GA was equally effective against CQ-sensitive and CQ-resistant strains (3D7 and W2, respectively) and on all erythrocytic stages of the parasite.

CONCLUSIONS

Together, these results suggest that an active and essential Hsp90 chaperone cycle exists in Plasmodium and that the ansamycin antibiotics will be an important tool to dissect its role in the parasite. Additionally, the favorable pharmacology of GA, reported in human trials, makes it a promising antimalarial drug.

Clotrimazole, ketoconazole, and clodinafop-propargyl as potent growth inhibitors of equine Babesia parasites during in vitro culture

The antifungal agents clotrimazole (CLT) and ketoconazole (KC) and the herbicide clodinafop-propargyl (CP) inhibit growth of Plasmodium sp., Toxoplasma sp., and Trypanosoma sp. In the present study, we evaluated these drugs against the in vitro growth of the equine protozoan parasites Babesia equi and B. caballi. Clotrimazole (IC50: 2 and 17 microM), KC (IC50: 6 and 22 microM), and CP (IC50: 450 and 354 microM) were effective growth inhibitors. Interestingly, intraerythrocytic KC-treated Babesia sp. were observed to be in immediate contact with the plasma fraction of the blood in electron microscopy. These results demonstrate the babesiacidial activities of these compounds and suggest their chemotherapeutic potential for the treatment of equine babesioses.

Structure-based approach to falcipain-2 inhibitors: synthesis and biological evaluation of 1,6,7-trisubstituted dihydroisoquinolines and isoquinolines

1,4,7-Trisubstituted isoquinolines were designed, synthesized and evaluated for their inhibition against Plasmodium falciparum cysteine protease falcipain-2. The 1-benzyloxyphenyl-dihydroisoquinoline and -isoquinoline derivatives were found to exhibit better activity against falcipain-2 than their corresponding 1-hydroxyphenyl or 1-methoxyphenyl analogues. The docking scores correlate with the IC(50) values of compounds and give a high coefficient correlation of 0.94.

Glutathione reductase of the malarial parasite Plasmodium falciparum: crystal structure and inhibitor development

The malarial parasite Plasmodium falciparum is known to be sensitive to oxidative stress, and thus the antioxidant enzyme glutathione reductase (GR; NADPH+GSSG+H(+) <==> NADP(+)+2 GSH) has become an attractive drug target for antimalarial drug development. Here, we report the 2.6A resolution crystal structure of P.falciparum GR. The homodimeric flavoenzyme is compared to the related human GR with focus on structural aspects relevant for drug design. The most pronounced differences between the two enzymes concern the shape and electrostatics of a large (450A(3)) cavity at the dimer interface. This cavity binds numerous non-competitive inhibitors and is a target for selective drug design. A 34-residue insertion specific for the GRs of malarial parasites shows no density, implying that it is disordered. The precise location of this insertion along the sequence allows us to explain the deleterious effects of a mutant in this region and suggests new functional studies. To complement the structural comparisons, we report the relative susceptibility of human and plasmodial GRs to a series of tricyclic inhibitors as well as to peptides designed to interfere with protein folding and dimerization. Enzyme-kinetic studies on GRs from chloroquine-resistant and chloroquine-sensitive parasite strains were performed and indicate that the structure reported here represents GR of P.falciparum strains in general and thus is a highly relevant target for drug development.

Lansium domesticum: skin and leaf extracts of this fruit tree interrupt the lifecycle of Plasmodium falciparum, and are active towards a chloroquine-resistant strain of the parasite (T9) in vitro

Malaria remains a global problem in the light of chloroquine-resistant strains of Plasmodium falciparum. New compounds are needed for the development of novel antimalarial drugs. Seed, leaf, and fruit skin extracts of Lansium domesticum, a common fruit tree in South-East Asia, are used by indigenous tribes in Sabah, Malaysia for treating malaria. The skin and aqueous leaf extracts of the tree were found to reduce parasite populations of the drug sensitive strain (3D7) and the chloroquine-resistant strain (T9) of P. falciparum equally well. The skin extracts were also found to interrupt the lifecycle of the parasite. The data reported here indicate that extracts of L. domesticum are a potential source for compounds with activity towards chloroquine-resistant strains of P. falciparum.

Cyclin-dependent protein kinases as therapeutic drug targets for antimalarial drug development

Cyclin-dependent protein kinases (CDKs) have been attractive drug targets for the development of anticancer therapies due to their direct and crucial role in the regulation of cellular proliferation. Following this trend, CDKs have been pursued as potential drug targets for several other diseases. Structure-based drug design programmes have focused on the plasmodial CDKs to develop new candidate antimalarial compounds. This review discusses the most recent advances relating to three Plasmodium falciparum CDKs (PfPK5, PfPK6 and Pfmrk) as they are developed as antimalarial drug targets. CDKs are highly conserved, and focus must be placed upon the amino acid differences between human and plasmodial CDKs in order to develop specific inhibitors. Comparisons of the active sites of human and parasite CDKs reveal sequence and potential structural variations. Using sequence analysis, molecular modelling and in vitro drug screening, it is possible to identify and develop inhibitors that specifically target the plasmodial CDKs. These efforts are aimed at identifying new classes of CDK inhibitors that may be exploited for antimalarial drug development.

Characterization of a unique aspartate-rich protein of the SET/TAF-family in the human malaria parasite, Plasmodium falciparum, which inhibits protein phosphatase 2A

A search for physiological inhibitors of protein phosphatases led to the identification of a Plasmodium falciparum (Pf) cDNA that had the potential to code for an aspartate-rich protein and hence named ARP. The PfARP was virtually identical to its Plasmodium berghei counterpart in gene structure and protein sequence. The PfARP coding sequence contained two introns, and the predicted protein contained 269 amino acid residues. Its primary structure showed significant similarity to eukaryotic proteins of the SET and TAF-family that included two inhibitors of mammalian serine/threonine protein phosphatase 2A (PP2A), namely I1(PP2A) and I2(PP2A). Like the SET and TAF proteins, it had an extremely acidic tail. The cDNA was confirmed by recombinant expression in bacteria. Native parasitic ARP was purified and was found to be highly thermostable. PfARP specifically inhibited the parasitic PP2A at nanomolar concentrations, with no effect on PP1, PP2B, PP5, or PPJ. Expression of PfARP in HeLa cells led to elevated phosphorylation of c-Jun, and activation of transcription factors AP1 and NF-kappa B. These functional properties are also characteristic of the SET/TAF-family proteins. The ARP mRNA and protein were detectable in all the erythrocytic asexual stages of the parasite, and the protein was located mainly in the parasitic cytoplasm. Thus, PfARP is a unique cytoplasmic member of the SET/TAF-family and a candidate physiological regulator of the Plasmodium PP2A.

Microbial and viral drug resistance mechanisms

Microorganisms and viruses have developed numerous resistance mechanisms that enable them to evade the effect of antimicrobials and antivirals. As a result, many have become resistant to almost every available means of treatment. This problem, although not new, is becoming increasingly acute and it is now clear that a fundamental understanding of the mechanisms that microbes and viruses deploy in the development of resistance is essential if we are to gain new insights into ways to combat this problem.

Pyrimethamine and WR99210 exert opposing selection on dihydrofolate reductase from Plasmodium vivax

Plasmodium vivax is a major public health problem in Asia and South and Central America where it is most prevalent. Until very recently, the parasite has been effectively treated with chloroquine, but resistance to this drug has now been reported in several areas. Affordable alternative treatments for vivax malaria are urgently needed. Pyrimethamine-sulfadoxine is an inhibitor of dihydrofolate reductase (DHFR) that has been widely used to treat chloroquine-resistant Plasmodium falciparum malaria. DHFR inhibitors have not been considered for treatment of vivax malaria, because initial trials showed poor efficacy against P. vivax. P. vivax cannot be grown in culture; the reason for its resistance to DHFR inhibitors is unknown. We show that, like P. falciparum, point mutations in the dhfr gene can cause resistance to pyrimethamine in P. vivax. WR99210 is a novel inhibitor of DHFR, effective even against the most pyrimethamine-resistant P. falciparum strains. We have found that it is also an extremely effective inhibitor of the P. vivax DHFR, and mutations that confer high-level resistance to pyrimethamine render the P. vivax enzyme exquisitely sensitive to WR99210. These data suggest that pyrimethamine and WR99210 would exert opposing selective forces on the P. vivax population. If used in combination, these two drugs could greatly slow the selection of parasites resistant to both drugs. If that is the case, this novel class of DHFR inhibitors could provide effective and affordable treatment for chloroquine- and pyrimethamine-resistant vivax and falciparum malaria for many years to come.

Medical need, scientific opportunity and the drive for antimalarial drugs

Continued and sustainable improvements in antimalarial medicines through focused research and development are essential for the world's future ability to treat and control malaria. Unfortunately, malaria is a disease of poverty, and despite a wealth of scientific knowledge there is insufficient market incentive to generate the competitive, business-driven industrial antimalarial drug research and development that is normally needed to deliver new products. Mechanisms of partnering with industry have been established to overcome this obstacle and to open up and build on scientific opportunities for improved chemotherapy in the future.