Sulfonylpiperazine compounds prevent Plasmodium falciparum invasion of red blood cells through interference with actin-1/profilin dynamics.

With emerging resistance to frontline treatments, it is vital that new antimalarial drugs are identified to target Plasmodium falciparum. We have recently described a compound, MMV020291, as a specific inhibitor of red blood cell (RBC) invasion, and have generated analogues with improved potency. Here, we generated resistance to MMV020291 and performed whole genome sequencing of 3 MMV020291-resistant populations. This revealed 3 nonsynonymous single nucleotide polymorphisms in 2 genes; 2 in profilin (N154Y, K124N) and a third one in actin-1 (M356L). Using CRISPR-Cas9, we engineered these mutations into wild-type parasites, which rendered them resistant to MMV020291. We demonstrate that MMV020291 reduces actin polymerisation that is required by the merozoite stage parasites to invade RBCs. Additionally, the series inhibits the actin-1-dependent process of apicoplast segregation, leading to a delayed death phenotype. In vitro cosedimentation experiments using recombinant P. falciparum proteins indicate that potent MMV020291 analogues disrupt the formation of filamentous actin in the presence of profilin. Altogether, this study identifies the first compound series interfering with the actin-1/profilin interaction in P. falciparum and paves the way for future antimalarial development against the highly dynamic process of actin polymerisation.

Anti-plasmodial Effects of Zanthoxylum zanthoxyloides.

Zanthoxylum zanthoxyloides, syn. Fagara zanthoxyloides, is a tree growing in West Africa and is used in traditional medicine against a variety of diseases, including malaria. In the work reported here, root bark and stem bark extracts of this tree, as well as compounds isolated from the extracts, have been investigated for activity in vitro against chloroquine-sensitive and chloroquine-resistant strains of Plasmodium falciparum. In addition, toxicity against nauplii of the brine shrimp Artemia salina has been studied. Dichloromethane extracts of the root bark and stem bark, and a methanol extract of the stem bark, showed anti-parasitic activity towards chloroquine-sensitive as well as chloroquine-resistant P. falciparum, with IC50 values between 1 and 10 µg/mL. Among the isolated compounds, bis-dihydrochelerythrinyl ether, buesgenine, chelerythrine, γ-fagarine, skimmianine, and pellitorine were the most active, with IC50 values of less than 5 µg/mL. The dichloromethane extracts were toxic to brine shrimp nauplii, with LC50 values of less than 1 µg/mL. Methanol extracts were much less toxic (LC50 between 50 and 100 µg/mL). Among the isolated substances, bis-dihydrochelethrinyl ether was the most toxic (LC50 ca. 2 µg/mL).

Validation of Putative Apicoplast-Targeting Drugs Using a Chemical Supplementation Assay in Cultured Human Malaria Parasites.

Malaria parasites contain a relict plastid, the apicoplast, which is considered an excellent drug target due to its bacterial-like ancestry. Numerous parasiticidals have been proposed to target the apicoplast, but few have had their actual targets substantiated. Isopentenyl pyrophosphate (IPP) production is the sole required function of the apicoplast in the blood stage of the parasite life cycle, and IPP supplementation rescues parasites from apicoplast-perturbing drugs. Hence, any drug that kills parasites when IPP is supplied in culture must have a nonapicoplast target. Here, we use IPP supplementation to discriminate whether 23 purported apicoplast-targeting drugs are on- or off-target. We demonstrate that a prokaryotic DNA replication inhibitor (ciprofloxacin), several prokaryotic translation inhibitors (chloramphenicol, doxycycline, tetracycline, clindamycin, azithromycin, erythromycin, and clarithromycin), a tRNA synthase inhibitor (mupirocin), and two IPP synthesis pathway inhibitors (fosmidomycin and FR900098) have apicoplast targets. Intriguingly, fosmidomycin and FR900098 leave the apicoplast intact, whereas the others eventually result in apicoplast loss. Actinonin, an inhibitor of bacterial posttranslational modification, does not produce a typical delayed-death response but is rescued with IPP, thereby confirming its apicoplast target. Parasites treated with putative apicoplast fatty acid pathway inhibitors could not be rescued, demonstrating that these drugs have their primary targets outside the apicoplast, which agrees with the dispensability of the apicoplast fatty acid synthesis pathways in the blood stage of malaria parasites. IPP supplementation provides a simple test of whether a compound has a target in the apicoplast and can be used to screen novel compounds for mode of action.

Red cells from ferrochelatase-deficient erythropoietic protoporphyria patients are resistant to growth of malarial parasites.

Many red cell polymorphisms are a result of selective pressure by the malarial parasite. Here, we add another red cell disease to the panoply of erythrocytic changes that give rise to resistance to malaria. Erythrocytes from individuals with erythropoietic protoporphyria (EPP) have low levels of the final enzyme in the heme biosynthetic pathway, ferrochelatase. Cells from these patients are resistant to the growth of Plasmodium falciparum malarial parasites. This phenomenon is due to the absence of ferrochelatase and not an accumulation of substrate, as demonstrated by the normal growth of P falciparum parasites in the EPP phenocopy, X-linked dominant protoporphyria, which has elevated substrate, and normal ferrochelatase levels. This observation was replicated in a mouse strain with a hypomorphic mutation in the murine ferrochelatase gene. The parasite enzyme is not essential for parasite growth as Plasmodium berghei parasites carrying a complete deletion of the ferrochelatase gene grow normally in erythrocytes, which confirms previous studies. That ferrochelatase is essential to parasite growth was confirmed by showing that inhibition of ferrochelatase using the specific competitive inhibitor, N-methylprotoporphyrin, produced a potent growth inhibition effect against cultures of P falciparum. This raises the possibility of targeting human ferrochelatase in a host-directed antimalarial strategy.

Natural products from Zanthoxylum heitzii with potent activity against the malaria parasite.

BACKGROUND: Zanthoxylum heitzii (Rutaceae) (olon) is used in traditional medicine in Central and West Africa to treat malaria. To identify novel compounds with anti-parasitic activity and validate medicinal usage, extracts and compounds isolated from this tree were tested against the erythrocytic stages of the human malaria parasite Plasmodium falciparum and for inhibition of transmission in rodent malaria parasite Plasmodium berghei. RESULTS: Hexane bark extract showed activity against P. falciparum (IC50 0.050 µg/ml), while leaf and seed extracts were inactive. Fractionation of the hexane bark extract led to the identification of three active constituents; dihydronitidine, pellitorine and heitziquinone. Dihydronitidine was the most active compound with an IC50 value of 0.0089 µg/ml (25 nM). This compound was slow acting, requiring 50 % longer exposure time than standard anti-malarials to reach full efficacy. Heitziquinone and pellitorine were less potent, with IC50 values of 3.55 µg/ml and 1.96 µg/ml, but were fast-acting. Plasmodium berghei ookinete conversion was also inhibited by the hexane extract (IC50 1.75 µg/ml), dihydronitidine (0.59 µg/ml) and heitziquinone (6.2 µg/ml). Water extracts of Z. heitzii bark contain only low levels of dihydronitidine and show modest anti-parasitic activity. CONCLUSIONS: Three compounds with anti-parasitic activity were identified in Z. heitzii bark extract. The alkaloid dihydronitidine is the most effective of these, accounting for the bulk of activity in both erythrocytic and transmission-blocking assays. These compounds may present good leads for development of novel anti-malarials and add to the understanding of the chemical basis of the anti-parasitic activity in these classes of natural product.

Aryl amino acetamides prevent Plasmodium falciparum ring development via targeting the lipid-transfer protein PfSTART1.

With resistance to most antimalarials increasing, it is imperative that new drugs are developed. We previously identified an aryl acetamide compound, MMV006833 (M-833), that inhibited the ring-stage development of newly invaded merozoites. Here, we select parasites resistant to M-833 and identify mutations in the START lipid transfer protein (PF3D7_0104200, PfSTART1). Introducing PfSTART1 mutations into wildtype parasites reproduces resistance to M-833 as well as to more potent analogues. PfSTART1 binding to the analogues is validated using organic solvent-based Proteome Integral Solubility Alteration (Solvent PISA) assays. Imaging of invading merozoites shows the inhibitors prevent the development of ring-stage parasites potentially by inhibiting the expansion of the encasing parasitophorous vacuole membrane. The PfSTART1-targeting compounds also block transmission to mosquitoes and with multiple stages of the parasite's lifecycle being affected, PfSTART1 represents a drug target with a new mechanism of action.

The effects of anti-bacterials on the malaria parasite Plasmodium falciparum.

Many anti-bacterial drugs inhibit growth of malaria parasites by targeting their bacterium-derived endosymbiotic organelles, the mitochondrion and plastid. Several of these drugs are either in use or being developed as therapeutics or prophylactics, so it is paramount to understand more about their target of action and modality. To this end, we measured in vitro growth and visualized nuclear division and the development of the mitochondrion and apicoplast in Plasmodium falciparum treated with five drugs targeting bacterial housekeeping pathways. This revealed two distinct classes of drug effect. Ciprofloxacin, rifampicin, and thiostrepton had an immediate effect: slowing parasite growth, retarding organellar development and preventing nuclear division. Classic delayed-death, in which the drug has no apparent effect until division and reinvasion of a new host by the daughter merozoites, was only observed for two drugs: clindamycin and tetracycline. These cells had apparently normal division and segregation of organelles in the first cycle but severe defects in apicoplast growth, subtle changes in the mitochondrion and a failure to complete cytokinesis during the second cycle. In two cases, the drug response in P. falciparum directly conflicted with reported responses for the related parasite Toxoplasma gondii, suggesting significant differences in apicoplast biology between the two parasites.

A multidrug resistance-associated protein involved in anthocyanin transport in Zea mays.

Anthocyanin biosynthesis is one of the most thoroughly studied enzymatic pathways in biology, but little is known about the molecular mechanisms of its final stage: the transport of the anthocyanin pigment into the vacuole. We have identified a multidrug resistance-associated protein (MRP), ZmMrp3, that is required for this transport process in maize (Zea mays). ZmMrp3 expression is controlled by the regulators of anthocyanin biosynthesis and mirrors the expression of other anthocyanin structural genes. Localization of ZmMRP3 in vivo shows its presence in the tonoplast, the site at which anthocyanin transport occurs. Mutants generated using antisense constructs have a distinct pigmentation phenotype in the adult plant that results from a mislocalization of the pigment as well as significant reduction in anthocyanin content, with no alteration in the anthocyanin species produced. Surprisingly, mutant plants did not show a phenotype in the aleurone. This appears to reflect the presence of a second, highly homologous gene, ZmMrp4, that is also coregulated with the anthocyanin pathway but is expressed exclusively in aleurone tissue. This description of a plant MRP with a role in the transport of a known endogenous substrate provides a new model system for examining the biological and biochemical mechanisms involved in the MRP-mediated transport of plant secondary metabolites.