Recent highlights in antimalarial drug resistance and chemotherapy research

Inhibitory effect of ursolic acid derivatives on hydrogen peroxide- and glutathione-mediated degradation of hemin: a possible additional mechanism of action for antimalarial activity

Compounds obtained by the condensation of ursolic acid (UA) with 1,4-bis(3-aminopropyl)piperazines have previously been shown as cytocidal to Plasmodium falciparum strains. Preliminary results indicated that the inhibition of beta-hematin formation (one of the possible mechanisms of action of antimalarial drugs) was achieved by a few of these molecules with varying efficiencies. To gain further insight in the antimalarial action of UA derivatives, we report here the results of additional pathways that may explain their in vitro cytocidal activity such as inhibition of hemin degradation by H(2)O(2) or glutathione (GSH). H(2)O(2)-mediated hemin degradation was drastically reduced by hydroxybenzyl-substituted UA derivatives while UA and intermediate compounds displayed weaker inhibitory actions. The results of GSH-mediated hemin degradation inhibition did not parallel those of H(2)O(2) degradation as hydroxybenzyl-substituted UA only proved to be a weak inhibitor. As H(2)O(2) interaction with the iron moiety of hemin is the first step towards its degradation, we assume that the interaction of our products with the ferric ion in the hemin structure is of upmost importance in inhibiting its peroxidative degradation. A two-step mechanism of action implying (1) stacking of the acetylursolic acid structure to hemin and (2) additive protection of hemin ferric iron from H(2)O(2) by hydroxyphenyl groups through steric hindrance and/or trapping of oxygen reactive species in the direct neighborhood of ferric iron can be put forward. For GSH degradation pathway, grafting of UA structure with a piperazine structure gave the best inhibition, pleading for the implication of this latter moiety in the inhibitory process.

A repurposing strategy identifies novel synergistic inhibitors of Plasmodium falciparum heat shock protein 90

Malaria is responsible for 3 million deaths annually. Antimalarial drug resistance is widespread, and few novel, well-defined targets exist. A robotic high throughput screen (HTS) was performed using 4000 small molecules from a natural compound (Spectrum), pharmacologically active (Lopac), and Food and Drug Administration (FDA) approved drug library (Prestwick) for competitive inhibition of the ATP-binding (GHKL) domain of Plasmodium falciparum (Pf) Hsp90, a highly conserved chaperone. Hits were further screened for specificity based on differential inhibition of PfHsp90 in comparison to human (Hs) Hsp90. PfHsp90-specific inhibitors showed 50% inhibitory concentrations (IC(50)) in the nanomolar range when tested using a cell-based antimalarial validation assay. Three hits, identified as selective PfHsp90 inhibitors in the HTS, also demonstrated synergistic activity in the presence of the known antimalarial drug chloroquine. These data support PfHsp90 as a specific antimalarial target with potential for synergy with known antimalarials.

Purified E255L mutant SERCA1a and purified PfATP6 are sensitive to SERCA-type inhibitors but insensitive to artemisinins

The antimalarial drugs artemisinins have been described as inhibiting Ca(2+)-ATPase activity of PfATP6 (Plasmodium falciparum ATP6) after expression in Xenopus oocytes. Mutation of an amino acid residue in mammalian SERCA1 (Glu(255)) to the equivalent one predicted in PfATP6 (Leu) was reported to induce sensitivity to artemisinin in the oocyte system. However, in the present experiments, we found that artemisinin did not inhibit mammalian SERCA1a E255L either when expressed in COS cells or after purification of the mutant expressed in Saccharomyces cerevisiae. Moreover, we found that PfATP6 after expression and purification from S. cerevisiae was insensitive to artemisinin and significantly less sensitive to thapsigargin and 2,5-di(tert-butyl)-1,4-benzohydroquinone than rabbit SERCA1 but retained higher sensitivity to cyclopiazonic acid, another type of SERCA1 inhibitor. Although mammalian SERCA and purified PfATP6 appear to have different pharmacological profiles, their insensitivity to artemisinins suggests that the mechanism of action of this class of drugs on the calcium metabolism in the intact cell is complex and cannot be ascribed to direct inhibition of PfATP6. Furthermore, the successful purification of PfATP6 affords the opportunity to develop new antimalarials by screening for inhibitors against PfATP6.

Staurosporine-induced programmed cell death in Blastocystis occurs independently of caspases and cathepsins and is augmented by calpain inhibition

Previous studies have shown that the protozoan parasite Blastocystis exhibits apoptotic features with caspase-like activity upon exposure to a cytotoxic monoclonal antibody or the anti-parasitic drug metronidazole. The present study reports that staurosporine (STS), a common apoptosis inducer in mammalian cells, also induces cytoplasmic and nuclear features of apoptosis in Blastocystis, including cell shrinkage, phosphatidylserine (PS) externalization, maintenance of plasma membrane integrity, extensive cytoplasmic vacuolation, nuclear condensation and DNA fragmentation. STS-induced PS exposure and DNA fragmentation were abolished by the mitochondrial transition pore blocker cyclosporine A and significantly inhibited by the broad-range cysteine protease inhibitor iodoacetamide. Interestingly, the apoptosis phenotype was insensitive to inhibitors of caspases and cathepsins B and L, while calpain-specific inhibitors augmented the STS-induced apoptosis response. While the identities of the proteases responsible for STS-induced apoptosis warrant further investigation, these findings demonstrate that programmed cell death in Blastocystis is complex and regulated by multiple mediators.

Mitochondrial translation in absence of local tRNA aminoacylation and methionyl tRNA Met formylation in Apicomplexa

Apicomplexans possess three translationally active compartments: the cytosol, a single tubular mitochondrion, and a vestigial plastid organelle called apicoplast. Mitochondrion and apicoplast are of bacterial evolutionary origin and therefore depend on a bacterial-like translation machinery. The minimal mitochondrial genome contains only three ORFs, and in Toxoplasma gondii the absence of mitochondrial tRNA genes is compensated for by the import of cytosolic eukaryotic tRNAs. Although all compartments require a complete set of charged tRNAs, the apicomplexan nuclear genomes do not hold sufficient aminoacyl-tRNA synthetase (aaRSs) genes to be targeted individually to each compartment. This study reveals that aaRSs are either cytosolic, apicoplastic or shared between the two compartments by dual targeting but are absent from the mitochondrion. Consequently, tRNAs are very likely imported in their aminoacylated form. Furthermore, the unexpected absence of tRNA(Met) formyltransferase and peptide deformylase implies that the requirement for a specialized formylmethionyl-tRNA(Met) for translation initiation is bypassed in the mitochondrion of Apicomplexa.

Nanotechnology applied to the treatment of malaria

Despite the fact that we live in an era of advanced technology and innovation, infectious diseases, like malaria, continue to be one of the greatest health challenges worldwide. The main drawbacks of conventional malaria chemotherapy are the development of multiple drug resistance and the non-specific targeting to intracellular parasites, resulting in high dose requirements and subsequent intolerable toxicity. Nanosized carriers have been receiving special attention with the aim of minimizing the side effects of drug therapy, such as poor bioavailability and the selectivity of drugs. Several nanosized delivery systems have already proved their effectiveness in animal models for the treatment and prophylaxis of malaria. A number of strategies to deliver antimalarials using nanocarriers and the mechanisms that facilitate their targeting to Plasmodium spp.-infected cells are discussed in this review. Taking into account the peculiarities of malaria parasites, the focus is placed particularly on lipid-based (e.g., liposomes, solid lipid nanoparticles and nano and microemulsions) and polymer-based nanocarriers (nanocapsules and nanospheres). This review emphasizes the main requirements for developing new nanotechnology-based carriers as a promising choice in malaria treatment, especially in the case of severe cerebral malaria.

Plasmodium falciparum: new molecular targets with potential for antimalarial drug development

Malaria remains one of the world's most devastating infectious diseases. Drug resistance to all classes of antimalarial agents has now been observed, highlighting the need for new agents that act against novel parasite targets. The complete sequencing of the Plasmodium falciparum genome has allowed the identification of new molecular targets within the parasite that may be amenable to chemotherapeutic intervention. In this review, we investigate four possible targets for the future development of new classes of antimalarial agents. These targets include histone deacetylase, the aspartic proteases or plasmepsins, aminopeptidases and the purine salvage enzyme hypoxanthine-xanthine-guanine phosphoribosyltransferase.

Artemisinin-based combination therapies: a vital tool in efforts to eliminate malaria

Plasmodium falciparum resistance to chloroquine and sulphadoxine-pyrimethamine has led to the recent adoption of artemisinin-based combination therapies (ACTs) as the first line of treatment against malaria. ACTs comprise semisynthetic artemisinin derivatives paired with distinct chemical classes of longer acting drugs. These artemisinins are exceptionally potent against the pathogenic asexual blood stages of Plasmodium parasites and also act on the transmissible sexual stages. These combinations increase the rates of clinical and parasitological cures and decrease the selection pressure for the emergence of antimalarial resistance. This Review article discusses our current knowledge about the mode of action of ACTs, their pharmacological properties and the proposed mechanisms of drug resistance.

In vitro and in vivo activities of 1-hydroxy-2-alkyl-4(1H)quinolone derivatives against Toxoplasma gondii

1-Hydroxy-2-dodecyl-4(1H)quinolone (HDQ) was recently identified as a Toxoplasma gondii inhibitor. We describe here two novel 1-hydroxyquinolones, which displayed 50% inhibitory concentrations 10- and 5-fold lower than that of HDQ. In a mouse model of acute toxoplasmosis, these two compounds and HDQ reduced the percentages of infected peritoneal cells and decreased the parasite loads in lungs and livers. Compound B showed a tendency toward lowering parasite loads in brains in a mouse model of toxoplasmic encephalitis.

Discovery, mechanisms of action and combination therapy of artemisinin

Despite great international efforts, malaria still inflicts an enormous toll on human lives, especially in Africa. Throughout history, antimalarial medicines have been one of the most powerful tools in malaria control. However, the acquisition and spread of parasite strains that are resistant to multiple antimalarial drugs have become one of the greatest challenges to malaria treatment, and are associated with the increase in morbidity and mortality in many malaria-endemic countries. To deal with this grave situation, artemisinin-based combinatory therapies (ACTs) have been introduced and widely deployed in malarious regions. Artemisinin is a new class of antimalarial compounds discovered by Chinese scientists from the sweet wormwood Artemisia annua. The potential development of resistance to artemisinins by Plasmodium falciparum threatens the usable lifespan of ACTs, and therefore is a subject of close surveillance and extensive research. Studies at the Thai-Cambodian border, a historical epicenter of multidrug resistance, have detected reduced susceptibility to artemisinins as manifested by prolonged parasite-clearance times, raising considerable concerns on resistance development. Despite this significance, there is still controversy on the mode of action of artemisinins. Although a number of potential cellular targets of artemisinins have been proposed, they remain to be verified experimentally. Here, we review the history of artemisinin discovery, discuss the mode of action and potential drug targets, and present strategies to elucidate resistance mechanisms.

Design, synthesis and structure-activity relationships of (1H-pyridin-4-ylidene)amines as potential antimalarials

(1H-Pyridin-4-ylidene)amines containing lipophilic side chains at the imine nitrogen atom were prepared as potential clopidol isosteres in the development of antimalarials. Their antiplasmodial activity was evaluated in vitro against the Plasmodium falciparum W2 (chloroquine-resistant) and FCR3 (atovaquone-resistant) strains. The most active of these derivatives, 4m, had an IC(50) of 1microM against W2 and 3microM against FCR3. Molecular modeling studies suggest that (1H-pyridin-4-ylidene)amines may bind to the ubiquinol oxidation Q(o) site of cytochrome bc(1).

Spread and evolution of Plasmodium falciparum drug resistance

Worldwide spread of Plasmodium falciparum drug resistance to conventional antimalarials, chloroquine and sulfadoxine/pyrimethamine, has been imposing a serious public health problem in many endemic regions. Recent discovery of drug resistance-associated genes, pfcrt, pfmdr1, dhfr, and dhps, and applications of microsatellite markers flanking the genes have revealed the evolution of parasite resistance to these antimalarials and the geographical spread of drug resistance. Here, we review our recent knowledge of the evolution and spread of parasite resistance to chloroquine and sulfadoxine/pyrimethamine. In both antimalarials, resistance appears to be largely explained by the invasion of limited resistant lineages to many endemic regions. However, multiple, indigenous evolutionary origins of resistant lineages have also been demonstrated. Further molecular evolutionary and population genetic approaches will greatly facilitate our understanding of the evolution and spread of parasite drug resistance, and will contribute to developing strategies for better control of malaria.

Polymorphism in PfMRP1 (Plasmodium falciparum multidrug resistance protein 1) amino acid 1466 associated with resistance to sulfadoxine-pyrimethamine treatment

Sulfadoxine-pyrimethamine (SP) remains widely recommended for intermittent preventive treatment against Plasmodium falciparum malaria for pregnant women and infants in Africa. Resistance to SP is increasing and associated primarily with mutations in the P. falciparum dhfr (Pfdhfr) and Pfdhps genes. This study aimed to explore the hypothetical association of genetic alterations in the P. falciparum multidrug resistance protein gene (Pfmrp1) with the in vivo response to SP by detecting the selection of single nucleotide polymorphisms (SNPs) following standard single-dose treatment administered to children with acute uncomplicated P. falciparum malaria in Tanzania. We detected significant selection of parasites carrying the Pfmrp1 1466K allele in samples from children with recrudescent infections, with 12 (100%) of 12 such samples being positive for this allele, compared to 52 (67.5%) of 77 baseline samples (P = 0.017), in parallel with the selection of the Pfdhfr Pfdhps quintuple mutant haplotype in cases of recrudescence (P = 0.001). There was no association between the 1466K SNP and the Pfdhfr Pfdhps quintuple mutation, indicating independent selections. Our data point for the first time to a role for a P. falciparum multidrug resistance protein homologue in the antimalarial activity of SP. Moreover, they add to the growing evidence of the potential importance of Pfmrp1 in antimalarial drug resistance.

Artemisinin Antimalarials: Preserving the "Magic Bullet"

The artemisinins are the most effective antimalarial drugs known. They possess a remarkably wide therapeutic index. These agents have been used in traditional Chinese herbal medicine for more than 2,000 years but were not subjected to scientific scrutiny until the 1970s. The first formal clinical trials of the artemisinins, and the development of methods for their industrial scale production, followed rapidly. A decade later, Chinese scientists shared their findings with the rest of the world; since then, a significant body of international trial evidence has confirmed these drugs to be far superior to any available alternatives. In particular, they have the ability to rapidly kill a broad range of asexual parasite stages at safe concentrations that are consistently achievable via standard dosing regimens. As their half-life is very short, there was also thought to be a low risk of resistance. These discoveries coincided with the appearance and spread of resistance to all the other major classes of antimalarials. As a result, the artemisinins now form an essential element of recommended first-line antimalarial treatment regimens worldwide. To minimize the risk of artemisinin resistance, they are recommended to be used to treat uncomplicated malaria in combination with other antimalarials as artemisinin combination therapies (ACTs). Their rollout has resulted in documented reductions in malaria prevalence in a number of African and Asian countries. Unfortunately, there are already worrisome early signs of artemisinin resistance appearing in western Cambodia. If this resistance were to spread, it would be disastrous for malaria control efforts worldwide. The enormous challenge for the international community is how to avert this catastrophe and preserve the effectiveness of this antimalarial “magic bullet”. Drug Dev Res 71: 12–19, 2010. © 2009 Wiley-Liss, Inc.