p-Aminobenzoic acid transport by normal and Plasmodium falciparum-infected erythrocytes

Exploring Ubiquinone Biosynthesis Inhibition as a Strategy for Improving Atovaquone Efficacy in Malaria

Atovaquone (AV) acts on the malaria parasite by competing with ubiquinol (UQH₂) for its union to the mitochondrial bc₁ complex, preventing the ubiquinone-8 and ubiquinone-9 (UQ-8 and UQ-9) redox recycling, which is a necessary step in pyrimidine biosynthesis. This study focused on UQ biosynthesis in Plasmodium falciparum and adopted proof-of-concept research to better elucidate the mechanism of action of AV and improve its efficacy. Initially, UQ biosynthesis was evaluated using several radioactive precursors and chromatographic techniques. This methodology was suitable for studying the biosynthesis of both UQ homologs and its redox state. Additionally, the composition of UQ was investigated in parasites cultivated at different oxygen saturations or in the presence of AV. AV affected the redox states of both UQ-8 and UQ-9 homologs by increasing the levels of the respective reduced forms. Conversely, low-oxygen environments specifically inhibited UQ-9 biosynthesis and increased the antimalarial efficacy of AV. These findings encouraged us to investigate the biological importance and the potential of UQ biosynthesis as a drug target based on its inhibition by 4-nitrobenzoate (4-NB), a 4-hydroxybenzoate (4-HB) analog. 4-NB effectively inhibits UQ biosynthesis and enhances the effects of AV on parasitic growth and respiration rate. Although 4-NB itself exhibits poor antimalarial activity, its 50% inhibitory concentration (IC₅₀) value increased significantly in the presence of a soluble UQ analog, p-aminobenzoic acid (pABA), or 4-HB. These results indicate the potential of AV combined with 4-NB as a novel therapy for malaria and other diseases caused by AV-sensitive pathogens.

The emergence of pathogenic TNF/iNOS producing dendritic cells (Tip-DCs) in a malaria model of acute respiratory distress syndrome (ARDS) is dependent on CCR4

Malaria-associated acute respiratory distress syndrome (MA-ARDS) and acute lung injury (ALI) are complications that cause lung damage and often leads to death. The MA-ARDS/ALI is associated with a Type 1 inflammatory response mediated by T lymphocytes and IFN-γ. Here, we used the Plasmodium berghei NK65 (PbN)-induced MA-ALI/ARDS model that resembles human disease and confirmed that lung CD4⁺ and CD8⁺ T cells predominantly expressed Tbet and IFN-γ. Surprisingly, we found that development of MA-ALI/ARDS was dependent on functional CCR4, known to mediate the recruitment of Th2 lymphocytes and regulatory T cells. However, in this Type 1 inflammation-ARDS model, CCR4 was not involved in the recruitment of T lymphocytes, but was required for the emergence of TNF-α/iNOS producing dendritic cells (Tip-DCs) in the lungs. In contrast, recruitment of Tip-DCs and development of MA-ALI/ARDS were not altered in CCR2^-/- mice. Importantly, we showed that NOS2^-/- mice are resistant to PbN-induced lung damage, indicating that reactive nitrogen species produced by Tip-DCs play an essential role in inducing MA-ARDS/ALI. Lastly, our experiments suggest that production of IFN-γ primarily by CD8⁺ T cells is required for inducing Tip-DCs differentiation in the lungs and the development of MA-ALI/ARDS model.

Membrane transport in the malaria parasite and its host erythrocyte

As it grows and replicates within the erythrocytes of its host the malaria parasite takes up nutrients from the extracellular medium, exports metabolites and maintains a tight control over its internal ionic composition. These functions are achieved via membrane transport proteins, integral membrane proteins that mediate the passage of solutes across the various membranes that separate the biochemical machinery of the parasite from the extracellular environment. Proteins of this type play a key role in antimalarial drug resistance, as well as being candidate drug targets in their own right. This review provides an overview of recent work on the membrane transport biology of the malaria parasite-infected erythrocyte, encompassing both the parasite-induced changes in the membrane transport properties of the host erythrocyte and the cell physiology of the intracellular parasite itself.

Antimalarial drug targets in Plasmodium falciparum predicted by stage-specific metabolic network analysis

BACKGROUND

Despite enormous efforts to combat malaria the disease still afflicts up to half a billion people each year of which more than one million die. Currently no approved vaccine is available and resistances to antimalarials are widely spread. Hence, new antimalarial drugs are urgently needed.

RESULTS

Here, we present a computational analysis of the metabolism of Plasmodium falciparum, the deadliest malaria pathogen. We assembled a compartmentalized metabolic model and predicted life cycle stage specific metabolism with the help of a flux balance approach that integrates gene expression data. Predicted metabolite exchanges between parasite and host were found to be in good accordance with experimental findings when the parasite's metabolic network was embedded into that of its host (erythrocyte). Knock-out simulations identified 307 indispensable metabolic reactions within the parasite. 35 out of 57 experimentally demonstrated essential enzymes were recovered and another 16 enzymes, if additionally the assumption was made that nutrient uptake from the host cell is limited and all reactions catalyzed by the inhibited enzyme are blocked. This predicted set of putative drug targets, shown to be enriched with true targets by a factor of at least 2.75, was further analyzed with respect to homology to human enzymes, functional similarity to therapeutic targets in other organisms and their predicted potency for prophylaxis and disease treatment.

CONCLUSIONS

The results suggest that the set of essential enzymes predicted by our flux balance approach represents a promising starting point for further drug development.

Getting infectious: formation and maturation of Plasmodium sporozoites in the Anopheles vector

Research on Plasmodium sporozoite biology aims at understanding the developmental program steering the formation of mature infectious sporozoites - the transmission stage of the malaria parasite. The recent identification of genes that are vital for sporozoite egress from oocysts and subsequent targeting and transmigration of the mosquito salivary glands allows the identification of mosquito factors required for life cycle completion. Mature sporozoites appear to be equipped with the entire molecular repertoire for successful transmission and subsequent initiation of liver stage development. Innovative malaria intervention strategies that target the early, non-pathogenic phases of the life cycle will crucially depend on our insights into sporozoite biology and the underlying molecular mechanisms that lead the parasite from the mosquito midgut to the liver.

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.

Chemosensitization of Plasmodium falciparum by probenecid in vitro

Resistance to drugs can result from changes in drug transport, and this resistance can sometimes be overcome by a second drug that modifies the transport mechanisms of the cell. This strategy has been exploited to partly reverse resistance to chloroquine in Plasmodium falciparum. Studies with human tumor cells have shown that probenecid can reverse resistance to the antifolate methotrexate, but the potential for reversal of antifolate resistance has not been studied in P. falciparum. In the present study we tested the ability of probenecid to reverse antifolate resistance in P. falciparum in vitro. Probenecid, at concentrations that had no effect on parasite viability alone (50 microM), was shown to increase the sensitivity of a highly resistant parasite isolate to the antifolates pyrimethamine, sulfadoxine, chlorcycloguanil, and dapsone by seven-, five-, three-, and threefold, respectively. The equivalent effects against an antifolate-sensitive isolate were activity enhancements of approximately 3-, 6-, 1.2-, and 19-fold, respectively. Probenecid decreased the level of uptake of radiolabeled folic acid, suggesting a transport-based mechanism linked to folate salvage. When probenecid was tested with chloroquine, it chemosensitized the resistant isolate to chloroquine (i.e., enhanced the activity of chloroquine). This enhancement of activity was associated with increased levels of chloroquine accumulation. In conclusion, we have shown that probenecid can chemosensitize malaria parasites to antifolate compounds via a mechanism linked to reduced folate uptake. Notably, this effect is observed in both folate-sensitive and -resistant parasites. In contrast to the activities of antifolate compounds, the effect of probenecid on chloroquine sensitivity was selective for chloroquine-resistant parasites (patent P407595GB [W. P. Thompson & Co., Liverpool, United Kingdom] has been filed to protect this intellectual property).

Membrane transport in the malaria-infected erythrocyte

The malaria parasite is a unicellular eukaryotic organism which, during the course of its complex life cycle, invades the red blood cells of its vertebrate host. As it grows and multiplies within its host blood cell, the parasite modifies the membrane permeability and cytosolic composition of the host cell. The intracellular parasite is enclosed within a so-called parasitophorous vacuolar membrane, tubular extensions of which radiate out into the host cell compartment. Like all eukaryote cells, the parasite has at its surface a plasma membrane, as well as having a variety of internal membrane-bound organelles that perform a range of functions. This review focuses on the transport properties of the different membranes of the malaria-infected erythrocyte, as well as on the role played by the various membrane transport systems in the uptake of solutes from the extracellular medium, the disposal of metabolic wastes, and the origin and maintenance of electrochemical ion gradients. Such systems are of considerable interest from the point of view of antimalarial chemotherapy, both as drug targets in their own right and as routes for targeting cytotoxic agents into the intracellular parasite.

Structure-activity relationships of the antimalarial agent artemisinin. 5. Analogs of 10-deoxoartemisinin substituted at C-3 and C-9

Novel 3- and 9-substituted analogs (4-19) of 10-deoxoartemisinin, 3, were prepared from the corresponding known lactones by one-pot reduction with sodium borohydride and boron trifluoride etherate. Reproducibility problems associated with this heterogeneous reaction were encountered on small reaction scales, and thus alternative methodology was sought for this reduction. Conversion of the lactones to tetrahydropyrans via the corresponding intermediate lactols was made more reproducible using a two-step sequence involving low-temperature reduction with diisobutylaluminum hydride followed by deoxygenation with boron trifluoride etherate in the presence of triethylsilane. In this manner, 10-deoxoartemisinin (3) could be obtained from artemisinin (1) in greater than 95% overall yield. All analogs were tested in vitro against W-2 and D-6 strains of Plasmodium falciparum. Several of the analogs were much more active than the natural product (+)-artemisinin (1) or 10-deoxoartemisinin (3). Conventional structure-activity relationships are discussed in relation to the bioassay data.

Structure-activity relationships of the antimalarial agent artemisinin. 4. Effect of substitution at C-3

Novel antimalarial artemisinin analogs, 3-alkylartemisinins as well as 3-(arylalkyl)- and 3-(carboxyalkyl)artemisinins, were prepared via the synthetic intermediate 2. Formation of the N,N-dimethylhydrazones 5 and 24 and then regio- and chemoselective deprotonation followed by alkylation provided initially alkylated hydrazones that upon chromatography gave ketones 6-13 and 25-30. Direct ozonolysis of the ketones followed by in situ acidification lead directly to the formation of title compounds 14-21 and 31-36. The analogs were tested in vitro against W-2 and D-6 strains of Plasmodium falciparum and found to be in some cases much more active than the natural product (+)-artemisinin. The results were included in structure-activity relationship (CoMFA) studies for further analog design.

Pteroylpolyglutamate synthesis by lung- and culture-derived Pneumocystis carinii

Pneumocystis carinii synthesizes folates de novo from exogenous p-aminobenzoic acid (pABA). Lung-derived organisms take up [3H]pABA in vitro except in the presence of sulfamethoxazole. Supernatants from spinner-flask cultures take up [3H]pABA if they were inoculated with lungs from infected rats, but not if they were inoculated with lungs from uninfected rats. P. carinii folates consist primarily of pteroylpentaglutamates. Plasmodium falciparum, in contrast, contains primarily pteroyltetraglutamates. Culture-derived organisms synthesize folates at a four-fold higher specific activity than lung-derived organisms, possibly because they contain less contaminating lung debris. These data suggest that P. carinii remains metabolically active in culture for at least 4 days.

Transport pathways in the malaria-infected erythrocyte. Their characterization and their use as potential targets for chemotherapy

The intraerythrocytic malarial parasite is involved in an extremely intensive anabolic activity while it resides in its metabolically quiescent host cell. The necessary fast uptake of nutrients and the discharge of waste products are guaranteed by parasite-induced alterations of the constitutive transporters of the host cell and the production of new parallel pathways. The membrane of the host cell thus becomes permeable to phospholipids, purine bases and nucleosides, small non-electrolytes, anions and cations. While the new pathways are quantitatively unimportant for the translocation of a particular solute, classical inhibitors of native transporters can be used to inhibit parasite growth. Several compounds were found to inhibit effectively the new pathways and, consequently, parasite growth. The pathways have also been used to introduce cytotoxic agents. The parasitophorous membrane consists of channels that are highly permeable to small solutes and display no ion selectivity. Transport of some cations and anions across the parasite membrane is rapid and insensitive to classical inhibitors, and in some cases it is mediated by specific antiporters that respond to their respective inhibitors. Macromolecules have been shown to reach the parasitophorous space through a duct contiguous with the host cell membrane, and subsequently to be endocytosed at the parasite membrane. The simultaneous presence of the parasitophorous membrane channels and the duct, however, is incompatible with experimental evidence. No specific inhibitors have been found as yet that would efficiently inhibit transport through the channels or the duct.

Auxotrophs of Plasmodium falciparum dependent on p-aminobenzoic acid for growth

The isolation of auxotrophic strains of a parasite offers new opportunities for studying parasitology. We have isolated cloned lines of Plasmodium falciparum that, unlike the parent line from which they were derived, rely on exogenous p-aminobenzoic acid (PABA) for growth. Isolation involved random mutagenesis of a cloned line of P. falciparum and subsequent selection of PABA-dependent parasites. Both parent and PABA-dependent clones were analyzed for PABA uptake and synthesis. Each clone takes up comparable amounts of PABA from the medium. The parent line, clone 3D7, can synthesize PABA de novo, whereas the PABA-dependent clones cannot. The requirement of exogenous PABA for growth by the auxotrophic strains coupled with their inability to synthesize PABA indicates that normal parasite growth can be completely supported by either synthesis or salvage. This work further clarifies the relationship between the availability of PABA and success of the parasite, an issue of debate from classic studies showing reduced parasite load in individuals on milk-fed diets.