Mechanisms of drug resistance in malaria

Toxins from Animal Venoms as a Potential Source of Antimalarials: A Comprehensive Review

Malaria is an infectious disease caused by Plasmodium spp. and it is mainly transmitted to humans by female mosquitoes of the genus Anopheles. Malaria is an important global public health problem due to its high rates of morbidity and mortality. At present, drug therapies and vector control with insecticides are respectively the most commonly used methods for the treatment and control of malaria. However, several studies have shown the resistance of Plasmodium to drugs that are recommended for the treatment of malaria. In view of this, it is necessary to carry out studies to discover new antimalarial molecules as lead compounds for the development of new medicines. In this sense, in the last few decades, animal venoms have attracted attention as a potential source for new antimalarial molecules. Therefore, the aim of this review was to summarize animal venom toxins with antimalarial activity found in the literature. From this research, 50 isolated substances, 4 venom fractions and 7 venom extracts from animals such as anurans, spiders, scorpions, snakes, and bees were identified. These toxins act as inhibitors at different key points in the biological cycle of Plasmodium and may be important in the context of the resistance of Plasmodium to currently available antimalarial drugs.

Role of cathepsin B of Naegleria fowleri during primary amebic meningoencephalitis

Naegleria fowleri causes primary amoebic meningoencephalitis in humans and experimental animals. It has been suggested that cysteine proteases of parasites play key roles in metabolism, nutrient uptake, host tissue invasion, and immune evasion. The aim of this work was to evaluate the presence, expression, and role of cathepsin B from N. fowleri in vitro and during PAM. Rabbit-specific polyclonal antibodies against cathepsin B were obtained from rabbit immunization with a synthetic peptide obtained by bioinformatic design. In addition, a probe was designed from mRNA for N. fowleri cathepsin B. Both protein and messenger were detected in fixed trophozoites, trophozoites interacted with polymorphonuclear and histological sections of infected mice. The main cathepsin B distribution was observed in cytoplasm or membrane mainly pseudopods and food-cups while messenger was in nucleus and cytoplasm. Surprisingly, both the messenger and enzyme were observed in extracellular medium. To determine cathepsin B release, we used trophozoites supernatant recovered from nasal passages or brain of infected mice. We observed the highest release in supernatant from recovered brain amoebae, and when we analyzed molecular weight of secreted proteins by immunoblot, we found 30 and 37 kDa bands which were highly immunogenic. Finally, role of cathepsin B during N. fowleri infection was determined; we preincubated trophozoites with E-64, pHMB or antibodies with which we obtained 60%, 100%, and 60% of survival, respectively, in infected mice. These results suggest that cathepsin B plays a role during pathogenesis caused by N. fowleri mainly in adhesion and contributes to nervous tissue damage.

Mechanisms of cellular invasion by intracellular parasites

Numerous disease-causing parasites must invade host cells in order to prosper. Collectively, such pathogens are responsible for a staggering amount of human sickness and death throughout the world. Leishmaniasis, Chagas disease, toxoplasmosis, and malaria are neglected diseases and therefore are linked to socio-economical and geographical factors, affecting well-over half the world's population. Such obligate intracellular parasites have co-evolved with humans to establish a complexity of specific molecular parasite-host cell interactions, forming the basis of the parasite's cellular tropism. They make use of such interactions to invade host cells as a means to migrate through various tissues, to evade the host immune system, and to undergo intracellular replication. These cellular migration and invasion events are absolutely essential for the completion of the lifecycles of these parasites and lead to their for disease pathogenesis. This review is an overview of the molecular mechanisms of protozoan parasite invasion of host cells and discussion of therapeutic strategies, which could be developed by targeting these invasion pathways. Specifically, we focus on four species of protozoan parasites Leishmania, Trypanosoma cruzi, Plasmodium, and Toxoplasma, which are responsible for significant morbidity and mortality.

Novel polymorphisms in Plasmodium falciparum ABC transporter genes are associated with major ACT antimalarial drug resistance

Chemotherapy is a critical component of malaria control. However, the most deadly malaria pathogen, Plasmodium falciparum, has repeatedly mounted resistance against a series of antimalarial drugs used in the last decades. Southeast Asia is an epicenter of emerging antimalarial drug resistance, including recent resistance to the artemisinins, the core component of all recommended antimalarial combination therapies. Alterations in the parasitic membrane proteins Pgh-1, PfCRT and PfMRP1 are believed to be major contributors to resistance through decreasing intracellular drug accumulation. The pfcrt, pfmdr1 and pfmrp1 genes were sequenced from a set of P.falciparum field isolates from the Thai-Myanmar border. In vitro drug susceptibility to artemisinin, dihydroartemisinin, mefloquine and lumefantrine were assessed. Positive correlations were seen between the in vitro susceptibility responses to artemisinin and dihydroartemisinin and the responses to the arylamino-alcohol quinolines lumefantrine and mefloquine. The previously unstudied pfmdr1 F1226Y and pfmrp1 F1390I SNPs were associated significantly with artemisinin, mefloquine and lumefantrine in vitro susceptibility. A variation in pfmdr1 gene copy number was also associated with parasite drug susceptibility of artemisinin, mefloquine and lumefantrine. Our work unveils new candidate markers of P. falciparum multidrug resistance in vitro, while contributing to the understanding of subjacent genetic complexity, essential for future evidence-based drug policy decisions.

Identification of putative Plasmodium falciparum mefloquine resistance genes

Mefloquine is an effective antimalarial drug; however, resistant strains of the human malarial pathogen, Plasmodium falciparum, are beginning to arise. The yeast Saccharomyces cerevisiae is sensitive to mefloquine, enabling a screen for P. falciparum genes involved in resistance. Yeast were transformed with a P. falciparum expression library, followed by selection on mefloquine plates and sequencing of plasmids that conferred resistance. We characterized the four genes that conferred the strongest mefloquine-resistant phenotype in yeast. All four (PFD0090c, PFI0195c, PF10_0372 and PF14_0649) are uncharacterized P. falciparum genes from distinct chromosomes (4, 9, 10 and 14, respectively). The mefloquine-resistant phenotype was dependent on induction of the P. falciparum gene and independent of vector context. PFI0195c, which likely encodes a small GTPase activator (GAP), also conferred resistance to cycloheximide and halofantrine in yeast. Immunolocalization of the encoded protein to the Golgi complex in yeast is consistent with potential GAP function. The other three candidate proteins localized to the cytoplasm and plasma membrane (PF14_0649), nuclear envelope/ER (PF10_0372) and Golgi (PFD0090c) of yeast. Analysis of mefloquine-resistant P. falciparum strains and the mefloquine-sensitive strain, W2, by sequencing and semi-quantitative RT-PCR identified no relevant mutations in the resistant strains but showed that PFI0195c was upregulated in two out of three resistant strains and PF14_0649 was upregulated in all resistant strains tested.

Resistance of Entamoeba histolytica to the cysteine proteinase inhibitor E64 is associated with secretion of pro-enzymes and reduced pathogenicity

Cysteine proteinases (CPs) have been considered suitable targets for the development of antiparasitic drugs. To assess the importance of CPs for the growth and pathogenicity of the protozoan parasite Entamoeba histolytica we have cultured amoebae in the presence of various cysteine proteinase inhibitors (CPIs). It was found that broad range CPIs, which are membrane permeable and rapidly enter the cell, are highly toxic at micromolar concentrations, and all attempts to generate E. histolytica mutants resistant to these CPIs were unsuccessful. In contrast, the broad range CPI E64, which does not permeate membranes as well, was deleterious at much higher concentrations, and amoebae rapidly developed resistance to this inhibitor. Compared with sensitive wild-type cells, E64-resistant E. histolytica were substantially reduced in the expression of various CP genes and were able to secrete unprocessed enzyme into the culture medium. Moreover, E64 resistance was associated with a significant reduction in virulence, because these cells were greatly impaired in the ability to generate liver abscesses in experimentally infected gerbils.

Changing ideas on chloroquine in Plasmodium falciparum

For 40 years scientists have hotly debated the questions of how chloroquine kills malarial parasites and how resistance to this once first-line antimalarial drug has evolved. While an end to these debates is not in sight, as a result of the complexity of the subject, new findings have come forward that give the discussion a new direction. In this paper we will summarize current knowledge on chloroquine's antimalarial mode of action and the genesis of the resistant phenotype in the human malarial parasite Plasmodium falciparum, with special emphasis on the most recent developments in this field.

Pleiotropic resistance to diverse antimalarials in actinomycin D-resistant Plasmodium falciparum

The development and spread of multidrug-resistant Plasmodium falciparum are major health concerns. The molecular mechanisms of multidrug resistance, including resistance to many quinoline-based antimalarials, are largely unknown. In this study, we report on the isolation and partial characterization of actinomycin D (actD)-resistant P. falciparum (3D7(R)/actD2.3) from a chloroquine-susceptible strain, 3D7. The stepwise selection of an actD-resistant clone (3D7(R)/actD2.3) led to the isolation and cloning of P. falciparum that grew in the presence of 2 ng/mL of actD. The parental isolate (3D7) did not grow in the presence of a 10-fold lower drug concentration (0.2 ng/mL). The latter estimate of parasite growth was determined by direct counting of parasites in infected red blood cells. Estimates of drug resistance levels to actD, using a [(3)H]hypoxanthine uptake and incorporation method, showed a 3-fold difference in the IC(50) between 3D7 and 3D7(R)/actD2.3. Interestingly, 3D7(R)/actD2.3 P. falciparum parasites were less sensitive to several antimalarials (chloroquine, mefloquine, quinidine, and artemisinin) and to the mitochondrial specific dye Rhodamine 123. Drug transport studies using [(3)H]actD showed that 3D7(R)/actD2.3 accumulated less drug than 3D7. Moreover, the accumulation of [(3)H]actD was energy dependent. To determine if Pfmdr1 expression, previously implicated in drug resistance to certain antimalarials, mediated the resistance phenotype of 3D7(R)/actD2.3, Pfmdr1 levels in 3D7 and 3D7(R)/actD2.3 were compared by Southern and northern blot analyses. Our results revealed no differences in Pfmdr1 copy number or mRNA levels between 3D7 and 3D7(R)/actD2.3. Furthermore, comparison of Pfmdr1 sequences between 3D7 and 3D7(R)/actD2.3 showed no differences. In addition, verapamil, which reverses P-glycoprotein-mediated drug resistance in mammalian cells, did not reverse the resistance of 3D7(R)/actD2.3 to actD or chloroquine. Taken together, the findings of this study demonstrated that in vitro selection of P. falciparum for resistance to actD leads to decreased sensitivity to diverse drugs and that this pleiotropic drug resistance is associated with reduced drug accumulation not mediated by Pfmdr1.

Chloroquine - some open questions on its antimalarial mode of action and resistance

During the digestion of its host cell hemoglobin, large amounts of toxic ferriprotoporphyrin IX (FPIX) are generated in the intraerythrocytic malaria parasite. FPIX is detoxified either by being polymerized into hemozoin inside the food vacuole, or through its degradation by glutathione in the cytosol. Chloroquine is able to complex with FPIX, thus inhibiting both processes and thereby generating receptors for its own uptake. These leads to the accumulation of FPIX in the membrane fraction of infected cells that results in membrane permeabilization and disruption of cation homeostasis and concluded in parasite death. Several unresolved questions, such as the site of FPIX:chloroquine complex formation, the role of pH gradient in drug accumulation and resistance, the role of Pgh-1 in resistance, the mode of action of reversers and the involvement of proteins and their mutants in resistance, are discussed. Copyright 1999 Harcourt Publishers Ltd.

Bisquinoline antimalarials: their role in malaria chemotherapy

Quinoline compounds, such as chloroquine, are used widely to treat malaria; however, the malarial parasite is rapidly becoming resistant to the drugs currently available. Presently, rational drug design is hindered considerably due to the mode of action of chloroquine being poorly understood. We rely on serendipity, rather than solid structural evidence, to generate new antimalarials. Hence any insight into the possible modes of action of quinoline antimalarials, including the bisquinolines, would greatly aid rational drug design. The quinoline antimalarial drugs, chloroquine, quinine and mefloquine, are thought to act by interfering with the digestion of haemoglobin in the blood stages of the malaria life-cycle. These quinoline antimalarials traverse down the pH gradient to accumulate to millimolar concentrations in the acidic vacuole of the parasite. It has been suggested that this high intravacuolar concentration prevents haem sequestration, causing a build up of the toxic haem moiety and the death of the parasite by its own toxic waste. The actual mechanism by which the parasite sequesters haem and the drug target(s) during this process, however, still remains elusive. As a consequence, haem polymerisation and the efficiency of quinoline antimalarials, including the bisquinolines, as inhibitors of this process has been investigated. In this paper, the potential role of the bisquinolines in the fight against chloroquine-resistant malaria is addressed.

Direct binding of chloroquine to the multidrug resistance protein (MRP): possible role for MRP in chloroquine drug transport and resistance in tumor cells

Multidrug resistance protein (MRP) transports a range of compounds that include glutathione S-conjugates, amphiphilic anionic drugs, and natural-product toxins. However, the mechanism of MRP drug binding and transport is presently unclear. We recently demonstrated the direct binding of a quinoline-based photoactive drug, N-[4-[1-hydroxy-2-(dibutylamino)ethyl]quinolin-8-yl]-4-az idosalicylamide (IAAQ), to MRP at a biologically relevant site [Vezmar et al., Biochem Biophys Res Commun 241: 104-111, 1997]. In the present report, we demonstrated that the lysosomotropic or antimalarial drug chloroquine is a substrate for MRP. Specifically, our results showed that chloroquine, similar to leukotriene C4 (LTC4) and 3-(3-(2-(7-chloro-2-quinolinyl)ethenyl-phenyl)((3-(dimethyl amino-3-oxo propyl)thio)methyl)thio) propanoic acid (MK 571), inhibits the photoaffinity labeling of MRP by IAAQ. Furthermore, cell growth assays showed MRP-expressing multidrug-resistant cells (H69/AR and HL60/AR) to be more resistant to chloroquine than their parental cells (i.e., IC50 of 121 microM versus 28 microM chloroquine for H69/AR and H69, respectively). Moreover, MK 571, an LTD4 receptor antagonist, reversed the resistance of H69/AR cells to chloroquine. Drug transport studies using [14C]chloroquine demonstrated that MRP-expressing cells accumulate less drug than the parental drug-sensitive cells. The reduced accumulation of [14C]chloroquine in resistant cells was ATP dependent and was due to enhanced drug efflux. Taken together, the results of this study show that MRP modulates the transport of chloroquine by direct binding.

A biochemical and genetic model for parasite resistance to antifolates. Toxoplasma gondii provides insights into pyrimethamine and cycloguanil resistance in Plasmodium falciparum

We have exploited the experimental accessibility of the protozoan parasite Toxoplasma gondii and its similarity to Plasmodium falciparum to investigate the influence of specific dihydrofolate reductase polymorphisms known from field isolates of drug-resistant malaria. By engineering appropriate recombinant shuttle vectors, it is feasible to examine mutations by transient or stable transformation of T. gondii parasites, in bacterial and yeast complementation assays, and through biochemical analysis of purified enzyme. A series of mutant alleles that mirror P. falciparum variants reveals that the key mutation Asn-108 (Asn-83 in T. gondii) probably confers resistance to pyrimethamine by affecting critical interactions in the ternary complex. Mutations such as Arg-59 (T. gondii 36) have limited effect in isolation, but in combination with other mutations they enhance the competitive ability of folate by increasing the speed of product turnover. Val-16 (T. gondii 10) confers low level resistance to cycloguanil but hypersensitivity to pyrimethamine. This mutation precludes Asn-108, probably because compression of the folate binding pocket introduced by this combination is incompatible with enzyme function. These studies permit detailed biochemical, kinetic, and structural analysis of drug resistance mutations and reconstruction of the probable phylogeny of antifolate resistance in malaria.

Synthesis and antimalarial activity in vitro and in vivo of a new ferrocene-chloroquine analogue

The antimalarial activities of ferrocenic compounds mimicking chloroquine and active upon chloroquine-resistant strains of Plasmodium falciparum were evaluated. Four 7-chloro-4-[[[2-[(N,N-substituted amino)methyl]ferrocenyl]methyl]amino]quinoline derivatives have been synthesized; one of them, 1a, showed high potent antimalarial activity in vivo on mice infected with Plasmodium berghei N. and Plasmodium yoelii NS. and was 22 times more potent against schizontocides than chloroquine in vitro against a drug-resistant strain of P. falciparum.

Variations in frequencies of drug resistance in Plasmodium falciparum

Continual exposure of malarial parasite populations to different drugs may have selected not only for resistance to individual drugs but also for genetic traits that favor initiation of resistance to novel unrelated antimalarials. To test this hypothesis, different Plasmodium falciparum clones having varying numbers of preexisting resistance mechanisms were treated with two new antimalarial agents: 5-fluoroorotate and atovaquone. All parasite populations were equally susceptible in small numbers. However, when large populations of these clones were challenged with either of the two compounds, significant variations in frequencies of resistance became apparent. On one extreme, clone D6 from West Africa, which was sensitive to all traditional antimalarial agents, failed to develop resistance under simple nonmutagenic conditions in vitro. In sharp contrast, the Indochina clone W2, which was known to be resistant to all traditional antimalarial drugs, independently acquired resistance to both new compounds as much as a 1,000 times more frequently than D6. Additional clones that were resistant to some (but not all) traditional antimalarial agents acquired resistance to atovaquone at high frequency, but not to 5-fluoroorotate. These findings were unexpected and surprising based on current views of the evolution of drug resistance in P. falciparum populations. Such new phenotypes, named accelerated resistance to multiple drugs (ARMD), raise important questions about the genetic and biochemical mechanisms related to the initiation of drug resistance in malarial parasites. Some potential mechanisms underlying ARMD phenotypes have public health implications that are ominous.