Efficacy and safety of triple combination therapy with artesunate-amodiaquine-methylene blue for falciparum malaria in children: a randomized controlled trial in Burkina Faso.

BACKGROUND: Methylene blue (MB) has been shown to be safe and effective against falciparum malaria in Africa and to have pronounced gametocytocidal properties. METHODS: Three days of treatment with artesunate (AS)-amodiaquine (AQ) combined with MB was compared with AS-AQ treatment in a randomized controlled phase IIb study; the study included 221 children aged 6-59 months with uncomplicated falciparum malaria in Burkina Faso. The primary end point was gametocyte prevalence during follow-up, as determined by microscopy and real-time quantitative nucleic acid sequence-based amplification (QT-NASBA). RESULTS: The gametocyte prevalence of Plasmodium falciparum at baseline was 3.6% (microscopy) and 97% (QT-NASBA). It was significantly lower in the AS-AQ-MB than in the AS-AQ group on day 7 of follow-up (microscopy, 1.2% vs 8.9% [P < .05]; QT-NASBA, 36.7% vs 63.3% [P < .001]). Hemoglobin values were significantly lower in the AS-AQ-MB group than in the AS-AQ group at days 2 and 7 of follow-up. Vomiting of the study medication occurred significantly more frequently in the AS-AQ-MB group. CONCLUSIONS: The combination of MB with an artemisinin-based combination therapy has been confirmed to be effective against the gametocytes of P. falciparum. MB-based combinations need to be compared with primaquine-based combinations, preferably using MB in an improved pediatric formulation. Clinical Trials Registration: NCT01407887.

Haemolysis risk in methylene blue treatment of G6PD-sufficient and G6PD-deficient West-African children with uncomplicated falciparum malaria: a synopsis of four RCTs.

PURPOSE: Methylene blue (MB), which was recently tested in a number of clinical malaria studies in Burkina Faso, is currently investigated for its benefit when added to artemisinin-based combination therapy. Together with a number of other antimalarials, MB is on the list of drugs which potentially induce haemolysis in patients with G6PD deficiency. Ruling out safety concerns is of major importance during drug development. METHODS: A pooled analysis was performed with patient data from four clinical studies conducted in West African children with falciparum malaria between 2003 and 2007. The primary endpoints were haemoglobin levels over time as well as haemolysis in G6PD-deficient (n = 199) and G6PD-sufficient (n = 806) children treated with MB-containing (n = 844) compared to children without MB-containing (n = 161) drug regimens. RESULTS: In the chosen model, the haemoglobin time course was significantly influenced by the G6PD genotype and the MB dose. In children with hemi- or homozygous G6PD (A-) deficiency, MB treatment with 15 mg/kg per day was associated with a significant reduction in Hb values which reached a minimum of 8.5 g/dl. Two episodes of haemolysis occurred (out of 1005 children); one in a girl heterozygous for G6PD deficiency and one in a hemizygous boy, both had received MB. CONCLUSIONS: MB treatment of malaria in Africa is associated with slightly reduced haemoglobin values in children with a full G6PD defect compared to non-G6PD deficient children. This effect appears to be of limited clinical relevance but needs to be monitored.

Molecular genetics evidence for the in vivo roles of the two major NADPH-dependent disulfide reductases in the malaria parasite.

Malaria-associated pathology is caused by the continuous expansion of Plasmodium parasites inside host erythrocytes. To maintain a reducing intracellular milieu in an oxygen-rich environment, malaria parasites have evolved a complex antioxidative network based on two central electron donors, glutathione and thioredoxin. Here, we dissected the in vivo roles of both redox pathways by gene targeting of the respective NADPH-dependent disulfide reductases. We show that Plasmodium berghei glutathione reductase and thioredoxin reductase are dispensable for proliferation of the pathogenic blood stages. Intriguingly, glutathione reductase is vital for extracellular parasite development inside the insect vector, whereas thioredoxin reductase is dispensable during the entire parasite life cycle. Our findings suggest that glutathione reductase is the central player of the parasite redox network, whereas thioredoxin reductase fulfils a specialized and dispensable role for P. berghei. These results also indicate redundant roles of the Plasmodium redox pathways during the pathogenic blood phase and query their suitability as promising drug targets for antimalarial intervention strategies.

Glutathione reductase-catalyzed cascade of redox reactions to bioactivate potent antimalarial 1,4-naphthoquinones--a new strategy to combat malarial parasites.

Our work on targeting redox equilibria of malarial parasites propagating in red blood cells has led to the selection of six 1,4-naphthoquinones, which are active at nanomolar concentrations against the human pathogen Plasmodium falciparum in culture and against Plasmodium berghei in infected mice. With respect to safety, the compounds do not trigger hemolysis or other signs of toxicity in mice. Concerning the antimalarial mode of action, we propose that the lead benzyl naphthoquinones are initially oxidized at the benzylic chain to benzoyl naphthoquinones in a heme-catalyzed reaction within the digestive acidic vesicles of the parasite. The major putative benzoyl metabolites were then found to function as redox cyclers: (i) in their oxidized form, the benzoyl metabolites are reduced by NADPH in glutathione reductase-catalyzed reactions within the cytosols of infected red blood cells; (ii) in their reduced forms, these benzoyl metabolites can convert methemoglobin, the major nutrient of the parasite, to indigestible hemoglobin. Studies on a fluorinated suicide-substrate indicate as well that the glutathione reductase-catalyzed bioactivation of naphthoquinones is essential for the observed antimalarial activity. In conclusion, the antimalarial naphthoquinones are suggested to perturb the major redox equilibria of the targeted infected red blood cells, which might be removed by macrophages. This results in development arrest and death of the malaria parasite at the trophozoite stage.

Efficacy of methylene blue monotherapy in semi-immune adults with uncomplicated falciparum malaria: a controlled trial in Burkina Faso.

OBJECTIVE: To assess the efficacy of methylene blue (MB) monotherapy in semi-immune adults with uncomplicated malaria in Burkina Faso. METHODS: In an open-label controlled phase II study with 60 semi-immune adults with uncomplicated falciparum malaria in Nouna, north-western Burkina Faso, MB monotherapy (390 mg twice daily) was given sequentially to groups of 20 adults for 7 days (MB7), 5 days (MB5) and 3 days (MB3), respectively. The primary outcome was the rate of adequate clinical and parasitological response (ACPR) on day 28 of follow-up. RESULTS: Of the study population, 27/58 (47%) and 5/51 (10%) patients still had parasites on days 2 and 3, respectively, of follow-up resulting in 9/58 (16%) early treatment failures. By day 14, no recrudescence was observed but in 4/19 (MB5) and 2/20 (MB3) individuals by day 28. The PCR-corrected rate of ACPR was 72%, 58% and 85% in groups 7, 5 and 3, respectively, by per protocol analysis. Self-limiting dysuria was the most frequent adverse event. CONCLUSIONS: MB acts slowly against the blood stages of P. falciparum. MB alone needs to be given for at least 7 days to be efficacious in the treatment of falciparum malaria but should be used in combination with a fast acting antimalarial.

Myristoylated adenylate kinase-2 of Plasmodium falciparum forms a heterodimer with myristoyltransferase.

Adenylate kinases (AK; ATP+AMP<-->2 ADP; E.C. 2.7.4.3.) are enzymes essentially involved in energy metabolism and macromolecular biosynthesis. As we reported previously, the malarial parasite Plasmodium falciparum possesses one genuine AK and one GTP-AMP phosphotransferase. Analysis of the P. falciparum genome suggested the presence of one additional adenylate kinase, which we designated AK2. Recombinantly produced AK2 was found to be a monomeric protein of 33 kDa showing a specific activity of 10 U/mg with ATP and AMP as a substrate pair and to interact with the AK-specific inhibitor P(1),P(5)-(diadenosine-5')-pentaphosphate (IC(50)=200 nM). At its N-terminus AK2 carries a predicted myristoylation sequence. This sequence is only present in AK2 of P. falciparum causing the severe tropical malaria and not in other malarial parasites. We heterologously coexpressed AK2 and P. falciparum N-myristoyltransferase (NMT) in the presence of myristate in Escherichia coli. As demonstrated by protein purification and mass spectrometry, AK2 is indeed myristoylated under catalysis of the parasites' transferase. The modification significantly enhances the stability of the kinase. Furthermore, AK2 and NMT were shown to interact strongly with each other forming a heterodimeric protein in vitro. To our knowledge this is the first direct evidence that P. falciparum NMT myristoylates an intact malarial protein.

CA/C1 peptidases of the malaria parasites Plasmodium falciparum and P. berghei and their mammalian hosts--a bioinformatical analysis.

In genome-wide screens we studied CA/C1 peptidases of malaria-causing plasmodia and their hosts (man and mouse). For Plasmodium falciparum and P. berghei, several new CA/C1 peptidase genes encoding proteases of the L- and B-family with specific promoter modules were identified. In addition, two new human CA/C1 peptidase loci and one new mouse gene locus were found; otherwise, the sets of CA/C1 peptidase genes in man and mouse seem to be complete now. In each species studied there is a multitude of CA/C1 peptidases with lysosomal localization signals and partial functional overlap according to similar but subfamily-specific structures. Individual target structures in plasmodia include residues specifically different in CA/C1 peptidase subsite 2. This is of medical interest considering CA/C1 peptidase inhibition for chemotherapy in malaria, malignancies and other diseases. Promoter structures and mRNA regulation differ widely among CA/C1 peptidase subfamilies and between mammals and plasmodia. We characterized promoter modules conserved in mouse and man for the CA/C1 peptidase families B and L (with the L-like subfamily, F-like subfamily and mouse-specific J-like subfamily). RNA motif searches revealed conserved regulatory elements such as GAIT elements; plasmodial CA/C1 peptidase mRNA elements include ARE elements and mammalian mRNAs contain 15-lox DICE elements.

Cytotoxic interactions of methylene blue with trypanosomatid-specific disulfide reductases and their dithiol products.

Methylene blue (MB) is known to have trypanocidal activity. We tested the interactions of MB with a number of trypanosomatid-specific molecules of the antioxidant metabolism. At pH 7, trypanothione and other (di)thiols were oxidized to disulfides by the phenothiazine drug. MB inhibited Trypanosoma cruzi trypanothione reductase (TR) (K(i)=1.9 microM), and served as a significant subversive substrate of this enzyme (K(M)=30 microM, k(cat)=4.9s(-1)). With lipoamide dehydrogenase, the second thiol-generating flavoenzyme of T. cruzi, the catalytic efficiency for MB reduction was found to be almost 10(6)M(-1)s(-1). When the system MB-enzyme-molecular oxygen acts as a NAD(P)H-driven redox cycler, a reactive oxygen species, H(2)O(2) or superoxide, is produced in each cycle. Since MB is an affordable, available, and accessible drug it might be tested--alone or in drug combinations--against trypanosomatid-caused diseases of animal and man.

Comparative structural analysis of oxidized and reduced thioredoxin from Drosophila melanogaster.

Thioredoxins (Trx) participate in essential antioxidant and redox-regulatory processes via a pair of conserved cysteine residues. In dipteran insects like Drosophila and Anopheles, which lack a genuine glutathione reductase (GR), thioredoxins fuel the glutathione system with reducing equivalents. Thus, characterizing Trxs from these organisms contributes to our understanding of redox control in GR-free systems and provides information on novel targets for insect control. Cytosolic Trx of Drosophila melanogaster (DmTrx) is the first thioredoxin that was crystallized for X-ray diffraction analysis in the reduced and in the oxidized form. Comparison of the resulting structures shows rearrangements in the active-site regions. Formation of the C32-C35 disulfide bridge leads to a rotation of the side-chain of C32 away from C35 in the reduced form. This is similar to the situation in human Trx and Trx m from spinach chloroplasts but differs from Escherichia coli Trx, where it is C35 that moves upon change of the redox state. In all four crystal forms that were analysed, DmTrx molecules are engaged in a non-covalent dimer interaction. However, as demonstrated by gel-filtration analyses, DmTrx does not dimerize under quasi in vivo conditions and there is no redox control of a putative monomer/dimer equilibrium. The dimer dissociation constants K(d) were found to be 2.2mM for reduced DmTrx and above 10mM for oxidized DmTrx as well as for the protein in the presence of reduced glutathione. In human Trx, oxidative dimerization has been demonstrated in vitro. Therefore, this finding may indicate a difference in redox control of GR-free and GR-containing organisms.

Methylene blue for malaria in Africa: results from a dose-finding study in combination with chloroquine.

The development of safe, effective and affordable drug combinations against malaria in Africa is a public health priority. Methylene blue (MB) has a similar mode of action as chloroquine (CQ) and has moreover been shown to selectively inhibit the Plasmodium falciparum glutathione reductase. In 2004, an uncontrolled dose-finding study on the combination MB-CQ was performed in 435 young children with uncomplicated falciparum malaria in Burkina Faso (CQ monotherapy had a > 50% clinical failure rate in this area in 2003). Three serious adverse events (SAE) occurred of which one was probably attributable to the study medication. In the per protocol safety analysis, there were no dose specific effects. The overall clinical and parasitological failure rates by day 14 were 10% [95% CI (7.5%, 14.0%)] and 24% [95% CI (19.4%, 28.3%)], respectively. MB appears to have efficacy against malaria, but the combination of CQ-MB is clearly not effective in the treatment of malaria in Africa.

The conserved histidine 106 of large thioredoxin reductases is likely to have a structural role but not a base catalyst function.

The catalytic activity of selenocysteine-containing thioredoxin reductases can be mimicked by cysteine-variants if the local environment at the C-terminal redox center supports thiol activation. This concept of a linear catalytic site was challenged by structural data suggesting that the invariant residue His106 functions as a base catalyst for the dithiol-disulphide exchange reaction between enzyme and substrate. As reported here, we changed His106 to asparagine, glutamine, and phenylalanine in various C-terminal mutants of Drosophila melanogaster thioredoxin reductase. The catalytic activity dropped considerably, yet pH-profiles did not reveal differences, rendering a function for His106 as a base catalyst unlikely. Interestingly, the phenylalanine-mutants, designed as negative controls were the most active mutants which suggests rather a structural role of His106.

Dithiol proteins as guardians of the intracellular redox milieu in parasites: old and new drug targets in trypanosomes and malaria-causing plasmodia.

Parasitic diseases such as sleeping sickness, Chagas' heart disease, and malaria are major health problems in poverty-stricken areas. Antiparasitic drugs that are not only active but also affordable and readily available are urgently required. One approach to finding new drugs and rediscovering old ones is based on enzyme inhibitors that paralyze antioxidant systems in the pathogens. These antioxidant ensembles are essential to the parasites as they are attacked in the human host by strong oxidants such as peroxynitrite, hypochlorite, and H2O2. The pathogen-protecting system consists of some 20 thiol and dithiol proteins, which buffer the intraparasitic redox milieu at a potential of -250 mV. In trypanosomes and leishmania the network is centered around the unique dithiol trypanothione (N1,N8-bis(glutathionyl)spermidine). In contrast, malaria parasites have a more conservative dual antioxidative system based on glutathione and thioredoxin. Inhibitors of antioxidant enzymes such as trypanothione reductase are, indeed, parasiticidal but they can also delay or prevent resistance against a number of other antiparasitic drugs.

Adenylate kinase and GTP:AMP phosphotransferase of the malarial parasite Plasmodium falciparum. Central players in cellular energy metabolism.

For coping with energetic and synthetic challenges, parasites require high activities of adenylate kinase (AK; ATP + AMP <==> 2 ADP) and GTP:AMP phosphotransferase (GAK; GTP + AMP <==> GDP + ADP). These enzymes were identified in erythrocytic stages of Plasmodium falciparum. The genes encoding PfAK and PfGAK are located on chromosomes 10 and 4, respectively. Molecular cloning and heterologous expression in E. coli yielded enzymatically active proteins of 28.9 (PfAK) and 28.0 kDa (PfGAK). Recombinant PfAK resembles authentic PfAK in its biochemical characteristics including the possible association with a stabilizing protein and the high specificity for AMP as the mononucleotide substrate. Specificity is less stringent for the triphosphate, with ATP as the best substrate (75 U/mg; kcat = 2160 min(-1) at 25 degrees C). PfAK contains the sequence of the amphiphatic helix that is known to mediate translocation of the cytosolic protein into the mitochondrial intermembrane space. PfGAK exhibits substrate preference for GTP and AMP (100 U/mg; kcat = 2800 min(-1) at 25 degrees C); notably, there is no detectable activity with ATP. In contrast to its human orthologue (AK3), PfGAK contains a zinc finger motif and binds ionic iron. The dinucleoside pentaphosphate compounds AP5A and GP5A inhibited PfAK and PfGAK, respectively, with Ki values of approximately 0.2 microM which is more than 250-fold lower than the KM values determined for the nucleotide substrates. The disubstrate inhibitors are useful for studying the enzymatic mechanism of PfAK and PfGAK as well as their function in adenine nucleotide homeostasis; in addition, the chimeric inhibitors represent interesting lead compounds for developing nucleosides to be used as antiparasitic agents.

5-substituted tetrazoles as bioisosteres of carboxylic acids. Bioisosterism and mechanistic studies on glutathione reductase inhibitors as antimalarials.

Plasmodium parasites are exposed to elevated fluxes of reactive oxygen species during intraerythrocytic life. The most important antioxidative systems are based on the glutathione reductases of the malarial parasite Plasmodium falciparum and the host erythrocyte. The development of menadione chemistry has led to the selection of the carboxylic acid 6-[2'-(3'-methyl)-1',4'-naphthoquinolyl] hexanoic acid M(5) as an inhibitor of the parasitic enzyme. As reported here, revisiting the mechanism of M(5) action revealed an uncompetitive inhibition type with respect to both NADPH and glutathione disulfide. Masking the polarity of the acidic function of M(5) by ester or amide bonds improved antiplasmodial activity. Bioisosteric replacement of the carboxylic function by tetrazole to increase bioavailability and to maintain comparable acidity led to improved antimalarial properties as well, but only with the cyanoethyl-protected tetrazoles. Using computed ab initio quantum methods, detailed analyses of the electronic profiles and the molecular properties evidenced the similarity of M(5) and the bioisoteric tetrazole T(4). The potential binding site of these molecules is discussed in light of the recently solved crystallographic structure of P. falciparum enzyme.

Plasmodium falciparum thioredoxins and glutaredoxins as central players in redox metabolism.

Over the last few years, an increasing number of different functions have been ascribed to small redox-active proteins like thioredoxins (Trx) and glutaredoxins (Grx). These functions include redox regulation of transcription and translation, antioxidant defence, involvement in protein folding and cellular signalling, and reduction of ribonucleotide reductase. In the malarial parasite Plasmodium falciparum, a classical Trx and a typical Grx have been described as well as a number of Trx- and Grx-like proteins including monothiol glutaredoxins. Furthermore, plasmoredoxin, a redox-active protein related to Trx, has been characterized; plasmoredoxin is unique for malarial parasites, therefore having great potential as diagnostic tool. In this minireview, we summarize the current knowledge on members of the thioredoxin superfamily and their function in the malarial parasite P. falciparum.

Methylene blue as an antimalarial agent.

Methylene blue has intrinsic antimalarial activity and it can act as a chloroquine sensitizer. In addition, methylene blue must be considered for preventing methemoglobinemia, a serious complication of malarial anemia. As an antiparasitic agent, methylene blue is pleiotropic: it interferes with hemoglobin and heme metabolism in digestive organelles, and it is a selective inhibitor of Plasmodium falciparum glutathione reductase. The latter effect results in glutathione depletion which sensitizes the parasite for chloroquine action. At the Centre de Recherche en Santé de Nouna in Burkina Faso, we study the combination of chloroquine with methylene blue (BlueCQ) as a possible medication for malaria in endemic regions. A pilot study with glucose-6-phosphate dehydrogenase-sufficient adult patients has been conducted recently.

Subcellular localization of adenylate kinases in Plasmodium falciparum.

Adenylate kinases (AK) play a key role in nucleotide signaling processes and energy metabolism by catalyzing the reversible conversion of ATP and AMP to 2 ADP. In the malaria parasite Plasmodium falciparum this reaction is mediated by AK1, AK2, and a GTP:AMP phosphotransferase (GAK). Here, we describe two additional adenylate kinase-like proteins: PfAKLP1, which is homologous to human AK6, and PfAKLP2. Using GFP-fusion proteins and life cell imaging, we demonstrate a cytosolic localization for PfAK1, PfAKLP1, and PfAKLP2, whereas PfGAK is located in the mitochondrion. PfAK2 is located at the parasitophorous vacuole membrane, and this localization is driven by N-myristoylation.

"Lest we forget you--methylene blue...".

Methylene blue (MB), the first synthetic drug, has a 120-year-long history of diverse applications, both in medical treatments and as a staining reagent. In recent years there was a surge of interest in MB as an antimalarial agent and as a potential treatment of neurodegenerative disorders such as Alzheimer's disease (AD), possibly through its inhibition of the aggregation of tau protein. Here we review the history and medical applications of MB, with emphasis on recent developments.