The novel hydroxynaphthoquinone 566C80 inhibits the development of liver stages of Plasmodium berghei cultured in vitro

Synthesis and activity of β-carboline antimalarials targeting the Plasmodium falciparum heat shock 90 protein

A collection of β-carbolines based on the natural product harmine, a compound known to target the heat shock 90 protein of Plasmodium falciparum, was synthesized and tested for antimalarial activity and potential toxicity. Several of these novel compounds display promising bioactivity, providing a new potential therapeutic with a mode of action that differs versus any currently available clinical treatment.

Pre-clinical evaluation of a P. berghei-based whole-sporozoite malaria vaccine candidate

Whole-sporozoite vaccination/immunization induces high levels of protective immunity in both rodent models of malaria and in humans. Recently, we generated a transgenic line of the rodent malaria parasite P. berghei (Pb) that expresses the P. falciparum (Pf) circumsporozoite protein (PfCS), and showed that this parasite line (PbVac) was capable of (1) infecting and developing in human hepatocytes but not in human erythrocytes, and (2) inducing neutralizing antibodies against the human Pf parasite. Here, we analyzed PbVac in detail and developed tools necessary for its use in clinical studies. A microbiological contaminant-free Master Cell Bank of PbVac parasites was generated through a process of cyclic propagation and clonal expansion in mice and mosquitoes and was genetically characterized. A highly sensitive qRT-PCR-based method was established that enables PbVac parasite detection and quantification at low parasite densities in vivo. This method was employed in a biodistribution study in a rabbit model, revealing that the parasite is only present at the site of administration and in the liver up to 48 h post infection and is no longer detectable at any site 10 days after administration. An extensive toxicology investigation carried out in rabbits further showed the absence of PbVac-related toxicity. In vivo drug sensitivity assays employing rodent models of infection showed that both the liver and the blood stage forms of PbVac were completely eliminated by Malarone^® treatment. Collectively, our pre-clinical safety assessment demonstrates that PbVac possesses all characteristics necessary to advance into clinical evaluation.

PbVac is a transgenic malaria parasite expressing circumsporozoite antigen from the human parasite Plasmodium falciparum. PbVac elicits neutralizing P. falciparum antibodies and can infect human hepatocytes but not erythrocytes, suggesting that humans would be non-permissive. Miguel Prudêncio and colleagues at the Institute of Molecular Medicine in Lisbon perform a detailed in vivo analysis and toxicology of PbVac. Extensive biodistribution analysis using a highly sensitive qPCR in non-permissive rabbit hosts shows PbVac are present at the initial bite site early on with later appearance in the liver, but by day 10 is undetectable. Importantly no PbVac could be detected in the blood at any time-point. PbVac was well tolerated with no apparent pathological signatures. In permissive mouse hosts PbVac could be effectively eliminated from both the blood and liver and could thereby act as a potential clinical ‘safety net’ in the event of an erythrocytic stage or persistence in the liver.

Non-competitive resource exploitation within mosquito shapes within-host malaria infectivity and virulence

Malaria is a fatal human parasitic disease transmitted by a mosquito vector. Although the evolution of within-host malaria virulence has been the focus of many theoretical and empirical studies, the vector’s contribution to this process is not well understood. Here, we explore how within-vector resource exploitation would impact the evolution of within-host Plasmodium virulence. By combining within-vector dynamics and malaria epidemiology, we develop a mathematical model, which predicts that non-competitive parasitic resource exploitation within-vector restricts within-host parasite virulence. To validate our model, we experimentally manipulate mosquito lipid trafficking and gauge within-vector parasite development and within-host infectivity and virulence. We find that mosquito-derived lipids determine within-host parasite virulence by shaping development (quantity) and metabolic activity (quality) of transmissible sporozoites. Our findings uncover the potential impact of within-vector environment and vector control strategies on the evolution of malaria virulence.

The evolution of within-host malaria virulence has been studied, but the vector’s contribution isn’t well understood. Here, Costa et al. show that non-competitive parasitic resource exploitation within-vector, in particular lipid trafficking, restricts within-host infectivity and virulence of the parasite.

Drug screen targeted at Plasmodium liver stages identifies a potent multistage antimalarial drug

Plasmodium parasites undergo a clinically silent and obligatory developmental phase in the host's liver cells before they are able to infect erythrocytes and cause malaria symptoms. To overcome the scarcity of compounds targeting the liver stage of malaria, we screened a library of 1037 existing drugs for their ability to inhibit Plasmodium hepatic development. Decoquinate emerged as the strongest inhibitor of Plasmodium liver stages, both in vitro and in vivo. Furthermore, decoquinate kills the parasite's replicative blood stages and is active against developing gametocytes, the forms responsible for transmission. The drug acts by selectively and specifically inhibiting the parasite's mitochondrial bc(1) complex, with little cross-resistance with the antimalarial drug atovaquone. Oral administration of a single dose of decoquinate effectively prevents the appearance of disease, warranting its exploitation as a potent antimalarial compound.

A chemical genomic analysis of decoquinate, a Plasmodium falciparum cytochrome b inhibitor

Decoquinate has single-digit nanomolar activity against in vitro blood stage Plasmodium falciparum parasites, the causative agent of human malaria. In vitro evolution of decoquinate-resistant parasites and subsequent comparative genomic analysis to the drug-sensitive parental strain revealed resistance was conferred by two nonsynonymous single nucleotide polymorphisms in the gene encoding cytochrome b. The resultant amino acid mutations, A122T and Y126C, reside within helix C in the ubiquinol-binding pocket of cytochrome b, an essential subunit of the cytochrome bc₁ complex. As with other cytochrome bc₁ inhibitors, such as atovaquone, decoquinate has low nanomolar activity against in vitro liver stage P. yoelii and provides partial prophylaxis protection when administered to infected mice at 50 mg kg^–1. In addition, transgenic parasites expressing yeast dihydroorotate dehydrogenase are >200-fold less sensitive to decoquinate, which provides additional evidence that this drug inhibits the parasite’s mitochondrial electron transport chain. Importantly, decoquinate exhibits limited cross-resistance to a panel of atovaquone-resistant parasites evolved to harbor various mutations in cytochrome b. The basis for this difference was revealed by molecular docking studies, in which both of these inhibitors were shown to have distinctly different modes of binding within the ubiquinol-binding site of cytochrome b.

2-Hexadecynoic acid inhibits plasmodial FAS-II enzymes and arrests erythrocytic and liver stage Plasmodium infections

Acetylenic fatty acids are known to display several biological activities, but their antimalarial activity has remained unexplored. In this study, we synthesized the 2-, 5-, 6-, and 9-hexadecynoic acids (HDAs) and evaluated their in vitro activity against erythrocytic (blood) stages of Plasmodium falciparum and liver stages of Plasmodium yoelii infections. Since the type II fatty acid biosynthesis pathway (PfFAS-II) has recently been shown to be indispensable for liver stage malaria parasites, the inhibitory potential of the HDAs against multiple P. falciparum FAS-II (PfFAS-II) elongation enzymes was also evaluated. The highest antiplasmodial activity against blood stages of P. falciparum was displayed by 5-HDA (IC(50) value 6.6 μg/ml), whereas the 2-HDA was the only acid arresting the growth of liver stage P. yoelii infection, in both flow cytometric assay (IC(50) value 2-HDA 15.3 μg/ml, control drug atovaquone 2.5 ng/ml) and immunofluorescence analysis (IC(50) 2-HDA 4.88 μg/ml, control drug atovaquone 0.37 ng/ml). 2-HDA showed the best inhibitory activity against the PfFAS-II enzymes PfFabI and PfFabZ with IC(50) values of 0.38 and 0.58 μg/ml (IC(50) control drugs 14 and 30 ng/ml), respectively. Enzyme kinetics and molecular modeling studies revealed valuable insights into the binding mechanism of 2-HDA on the target enzymes. All HDAs showed in vitro activity against Trypanosoma brucei rhodesiense (IC(50) values 3.7-31.7 μg/ml), Trypanosoma cruzi (only 2-HDA, IC(50) 20.2 μg/ml), and Leishmania donovani (IC(50) values 4.1-13.4 μg/ml) with generally low or no significant toxicity on mammalian cells. This is the first study to indicate therapeutic potential of HDAs against various parasitic protozoa. It also points out that the malarial liver stage growth inhibitory effect of the 2-HDA may be promoted via PfFAS-II enzymes. The lack of cytotoxicity, lipophilic nature, and calculated pharmacokinetic properties suggests that 2-HDA could be a useful compound to study the interaction of fatty acids with these key P. falciparum enzymes.

Activity of a trisubstituted pyrrole in inhibiting sporozoite invasion and blocking malaria infection

Malaria infection is initiated by Plasmodium sporozoites infecting the liver. Preventing sporozoite infection would block the obligatory first step of the infection and perhaps reduce disease severity. In addition, such an approach would decrease Plasmodium vivax hypnozoite formation and therefore disease relapses. Here we describe the activity of a trisubstituted pyrrole, 4-[2-(4-fluorophenyl)-5-(1-methylpiperidine-4-yl)-1H-pyrrol-3-yl] pyridine, in inhibiting motility, invasion, and consequently infection by P. berghei sporozoites. In tissue culture, the compound was effective within the first 3 h of sporozoite addition to HepG2 cells. In vivo, intraperitoneal administration of the compound significantly inhibited liver-stage parasitemia in P. yoelii sporozoite-infected mice and prevented the appearance of blood-stage parasites. P. berghei sporozoites lacking the parasite cGMP-dependent protein kinase, the primary target of the compound in erythrocyte-stage parasites, remained infectious to HepG2 cells and sensitive to the drug. These results suggest that the drug has an additional target(s) in sporozoites. We propose that drugs that inhibit sporozoite infection offer a feasible approach to malaria prophylaxis.

New approach for high-throughput screening of drug activity on Plasmodium liver stages

Plasmodium liver stages represent potential targets for antimalarial prophylactic drugs. Nevertheless, there is a lack of molecules active on these stages. We have now developed a new approach for the high-throughput screening of drug activity on Plasmodium liver stages in vitro, based on an infrared fluorescence scanning system. This method allowed us to count automatically and rapidly Plasmodium-infected hepatocytes, using different hepatic cells and different Plasmodium species, including Plasmodium falciparum. This new technique is well adapted for high-throughput drug screening and should facilitate the identification of new antimalarial compounds active on Plasmodium liver stages.

Mutations in Plasmodium falciparum cytochrome b that are associated with atovaquone resistance are located at a putative drug-binding site

Atovaquone is the major active component of the new antimalarial drug Malarone. Considerable evidence suggests that malaria parasites become resistant to atovaquone quickly if atovaquone is used as a sole agent. The mechanism by which the parasite develops resistance to atovaquone is not yet fully understood. Atovaquone has been shown to inhibit the cytochrome bc(1) (CYT bc(1)) complex of the electron transport chain of malaria parasites. Here we report point mutations in Plasmodium falciparum CYT b that are associated with atovaquone resistance. Single or double amino acid mutations were detected from parasites that originated from a cloned line and survived various concentrations of atovaquone in vitro. A single amino acid mutation was detected in parasites isolated from a recrudescent patient following atovaquone treatment. These mutations are associated with a 25- to 9,354-fold range reduction in parasite susceptibility to atovaquone. Molecular modeling showed that amino acid mutations associated with atovaquone resistance are clustered around a putative atovaquone-binding site. Mutations in these positions are consistent with a reduced binding affinity of atovaquone for malaria parasite CYT b.

Efficacy and pharmacokinetics of atovaquone and proguanil in children with multidrug-resistant Plasmodium falciparum malaria

A trial was conducted in 32 Thai children with uncomplicated multidrug-resistant falciparum malaria to assess the efficacy, safety and pharmacokinetics of atovaquone and proguanil; plasma concentrations of atovaquone, proguanil and its metabolite, cycloguanil, were measured in a subset of 9 children. The children received atovaquone (17 mg/kg/d for 3 d) plus proguanil (7 mg/kg/d for 3 d). Twenty-six children who had only Plasmodium falciparum infection and remained in hospital for 28 d were assessed for drug efficacy. The combination regimen produced a cure rate of 100%. Parasite and fever clearance times were 47 h (range 8-75) and 50 h (range 7-111), respectively. Atovaquone and proguanil were rapidly absorbed, with median time to peak concentrations of 6 h (range 6-24) and 6 h (range 6-12), respectively. Peak concentrations of cycloguanil were achieved between 6 and 12 h (median 6) after administration of proguanil. Mean peak plasma concentration of atovaquone on day 3 was 5.1 micrograms/mL (SD = 2.1). The day 3 mean peak plasma concentration of proguanil was 306 ng/mL (SD = 108) compared with 44.3 ng/mL (SD = 27.3) for cycloguanil. Mean values for the AUC (area under plasma concentration-time curve) were 161.8 micrograms/mL.h (SD = 126.9) for atovaquone, 4646 ng/mL.h (SD = 1226) for proguanil, and 787 ng/mL.h (SD = 397) for cycloguanil. Terminal elimination half-lives of atovaquone, proguanil and cycloguanil were estimated as 31.8 h (SD = 8.9), 14.9 h (SD = 3.3) and 14.6 h (SD = 2.6), respectively. No major adverse effect was attributable to the study drugs. Atovaquone/proguanil combination is safe and highly effective, and should be especially valuable for treatment of multidrug-resistant falciparum malaria.

Atovaquone, a broad spectrum antiparasitic drug, collapses mitochondrial membrane potential in a malarial parasite

At present, approaches to studying mitochondrial functions in malarial parasites are quite limited because of the technical difficulties in isolating functional mitochondria in sufficient quantity and purity. We have developed a flow cytometric assay as an alternate means to study mitochondrial functions in intact erythrocytes infected with Plasmodium yoelii, a rodent malaria parasite. By using a very low concentration (2 nM) of a lipophilic cationic fluorescent probe, 3,3'dihexyloxacarbocyanine iodide, we were able to measure mitochondrial membrane potential(DeltaPsim) in live intact parasitized erythrocytes through flow cytometry. The accumulation of the probe into parasite mitochondria was dependent on the presence of a membrane potential since inclusion of carbonyl cyanide m-chlorophenylhydrazone, a protonophore, dissipated the membrane potential and abolished the probe accumulation. We tested the effect of standard mitochondrial inhibitors such as myxothiazole, antimycin, cyanide and rotenone. All of them except rotenone collapsed the DeltaPsim and inhibited respiration. The assay was validated by comparing the EC50 of these compounds for inhibiting DeltaPsim and respiration. This assay was used to investigate the effect of various antimalarial drugs such as chloroquine, tetracycline and a broad spectrum antiparasitic drug atovaquone. We observed that only atovaquone collapsed DeltaPsim and inhibited parasite respiration within minutes after drug treatment. Furthermore, atovaquone had no effect on mammalian DeltaPsim. This suggests that atovaquone, shown to inhibit mitochondrial electron transport, also depolarizes malarial mitochondria with consequent cellular damage and death.

The mechanisms of action of antiprotozoal and anthelmintic drugs in man

The mechanisms of action of antiprotozoal and anthelmintic drugs are reviewed according to: (1) drugs interfering with metabolic processes; (2) drugs interfering with reproduction and larval physiology; and (3) drugs interfering with neuromuscular physiology of parasites.

Retarded development of exoerythrocytic stages of the rodent malaria parasite Plasmodium berghei in human hepatoma cells by extracts from Dioncophyllaceae and Ancistrocladaceae species

Retarded development of exoerythrocytic stages of the rodent malaria parasite Plasmodium berghei in human hepatoma cells by extracts from Dioncophyllaceae and Ancistrocladaceae species. International Journal for Parasitology 27: 29-32. Naphthylisoquinoline alkaloid-containing extracts (10 micrograms ml-1) of species belonging to the Dioncophyllaceae and the Ancistrocladaceae, 2 small tropical plant families, display pronounced in vitro activities against exoerythrocytic stages of Plasmodium berghei (Anka), developing in human hepatoma cells (Hep G2). The highest activities were obtained with CH2Cl2 root and bark extracts, and a CH2Cl2/NH3 leaf extract from Triphyophyllum peltatum, a CH2Cl2/NH3 root extract from Ancistrocladus abbreviatus, and a CH2Cl2 leaf extract from A. tectorius. The degrees of growth inhibition ranged within 27.7-70.0%. The commercially available drug primaquine diphosphate (25 micrograms ml-1) caused a comparable effect (62.1%) in the same test system.

Effects of the introduction of white sucker, Catostomus commersoni, on the parasite fauna of brook trout, Salvelinus fontinalis

White sucker, Catostomus commersoni, has been introduced in many brook trout, Salvelinus fontinalis, lakes of the Laurentian Shield, Quebec, Canada. The goal of this study was to assess the impact of these introductions on the parasite fauna of brook trout. Three lakes containing brook trout only and three lakes containing both brook trout and white sucker were studied. The objectives were (i) to determine if white sucker parasites were able to colonise the relatively oligotrophic lakes of the Laurentian Shield, (ii) to establish if parasites were exchanged between sucker and trout, and (iii) to study the effect of trout feeding habits on their parasite fauna, since this fish shifts its diet from zoobenthos to Zooplankton when living with white sucker. Eight of the 12 parasite species found on white sucker probably colonised the lakes with their host. Among the 11 parasite species identified from trout, it is unlikely that any were introduced by white sucker. Trout living with white sucker have more parasites transmitted by Zooplankton (Diphyllobothrium ditremum and Eubothrium salvelini) and fewer parasites transmitted by zoobenthos (Crepidostomum farionis and Sterliadochona ephemeridarum) than trout living in allopatry. Local factors such as lake morphometrics also seemed to play an important role in the composition of the trout parasite fauna.

The effect of atovaquone (566C80) on the maturation and viability of Plasmodium falciparum gametocytes in vitro

Atovaquone (566C80), a hydroxynaphthoquinone, was investigated for activity against Plasmodium falciparum gametocytes (NF54 strain) in vitro. After 96 h of continuous exposure to the drug at 1.4 x 10(-7) M (a concentration achievable in humans 14 d after administration of a therapeutic dose of 10 mg/kg) reductions of 75%, 54% and 20% in the number of gametocyte stages 2, 3 and 4, respectively, were achieved. A small increase (14%) in stage 5 gametocytes was seen. At the same concentration, atovaquone showed greater activity against the asexual stages of P. falciparum, reductions of 93%, 96% and 43% in the number of rings, schizonts and trophozoites, respectively, being achieved. These data are consistent with inhibition of maturation of trophozoites. The observed effect on maturation of gametocytes is similarly consistent with blockade of gametocyte recruitment from merozoites produced by the preceding schizogony, or to stasis of intraerythrocytic sexual development before the formation of stage 2 gametocytes.

Effects of atovaquone and other inhibitors on Pneumocystis carinii dihydroorotate dehydrogenase

Dihydroorotate dehydrogenase (DHOD) is a pyrimidine biosynthetic enzyme which is usually directly linked to the mitochondrial respiratory chain. Antimalarial naphthoquinones such as atovaquone (566c80) inhibit malarial DHOD by inhibiting electron transport. Since atovaquone also has therapeutic activity against Pneumocystis carinii, the P. carinii DHOD may also be an important drug target. Organisms were obtained from immunosuppressed rats, incubated for 24 h in a short-term in vitro culture system, and then lysed. P. carinii lysates catalyzed the generation of orotate from dihydroorotate at a rate of 852 pmol/mg of protein per min. Control preparations made from uninfected mice showed much less total enzymatic activity and enzyme specific activity. As expected, P. carinii DHOD activity was susceptible to respiratory inhibitors such as cyanide, antimycin A, and salicylhydroxamic acid (SHAM). Susceptibility to SHAM suggests the presence of an alternative oxidase. In contrast, neither pentamidine nor 5-hydroxy-6-demethylprimaquine (5H6DP), a quinone metabolite of primaquine, inhibited the enzyme. Atovaquone inhibited DHOD by 76.3% at 100 microM and 36.5% at 10 microM. A similar degree of inhibition was found when the organisms were preincubated with the drug. Atovaquone inhibited P. carinii growth in vitro at a somewhat lower concentration (between 0.3 and 3 microM). In contrast, Plasmodium falciparum growth and enzyme activity are susceptible to nanomolar concentrations of atovaquone. Thus, while it is possible that atovaquone acts by inhibiting the P. carinii electron transport chain, the possibility of another drug target cannot be excluded.

Inhibitory action of the anti-malarial compound atovaquone (566C80) against Plasmodium berghei ANKA in the mosquito, Anopheles stephensi

The activity of atovaquone against Plasmodium berghei ANKA during sporogonic development has been examined. Anopheles stephensi mosquitoes were fed on gametocyte infected mice which had been treated 8 h previously with atovaquone or diluent alone. Mosquito midguts were examined for oocysts, and salivary gland infections were estimated using an ELISA for the circumsporozoite protein (CSP). The number of oocysts per midgut fell by at least 97% when mosquitoes were fed on mice dosed with 0.1-10 mg atovaquone/kg body weight. This was paralleled by a decrease in the prevalence of oocyst-infected mosquitoes from 70-90% in controls to 40% or 10% respectively. No oocysts were observed at a dose of 100 mg/kg. CSP ELISA results indicated that mosquitoes fed on atovaquone failed to produce sporozoites. Mosquitoes which fed on gametocytaemic, atovaquone-treated mice (0.1-100 mg/kg) did not transmit malaria to naive mice. These results demonstrate that atovaquone has a highly potent inhibitory activity against the mosquito stages of P. berghei.

Coenzyme Q homologs in parasitic protozoa as targets for chemotherapeutic attack

The central role of coenzyme Q (ubiquinone) in cellular energy metabolism is well established. Recent work has implicated this molecule in a wide range of other cellular functions, including roles in growth control, plasma membrane oxidase and as a cellular antioxidant. In this review, Jayne Ellis presents an overview of the current knowledge of this important cellular component in species of parasitic protozoa, discusses current therapies using its analogs and proposes its potential roles in these organisms.