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Metabolic chemical probes are small-molecule reagents that utilize naturally occurring biosynthetic enzymes for in situincorporation into biomolecules of interest. These reagents can be used to label, detect, and track important biological processes within living cells including protein synthesis, protein glycosylation, and nucleic acid proliferation. A limitation of current chemical probes, which have largely focused on mammalian cells, is that they often cannot be applied to other organisms due to metabolic differences. For example, the thymidine derivative 5-ethynyl-2′-deoxyuridine (EdU) is a gold standard metabolic chemical probe for assessing DNA proliferation in mammalian cells; however, it is unsuitable for the study of malaria parasites due to Plasmodiumspecies lacking the thymidine kinase enzyme that is essential for metabolism of EdU. Herein, we report the design and synthesis of new thymidine-based probes that sidestep the requirement for a thymidine kinase enzyme in Plasmodium. Two of these DNADetect probes exhibit robust labeling of replicating asexual intraerythrocytic Plasmodium falciparumparasites, as determined by flow cytometry and fluorescence microscopy using copper-catalyzed azide–alkyne cycloaddition to a fluorescent azide. The DNADetect chemical probes are synthetically accessible and thus can be made widely available to researchers as tools to further understand the biology of different Plasmodiumspecies, including laboratory lines and clinical isolates.

Chemical investigation of the antimalarial medicinal plant Clerodendrum polycephalumled to the isolation of five new diterpenoids, including ajugarins VII–X (1–4) and teuvincenone K (5), along with four known compounds, namely, 12,16-epoxy-6,11,14,17-tetrahydroxy-17(15 → 16)-abeo-5,8,11,13,15-abietapentaen-7-one (6), methyl pheophorbide A (7), loliolide (8), and acacetin (9). The chemical structures of the new compounds were elucidated using NMR spectroscopy, mass spectrometry, circular dichroism, as well as density functional theory calculations. All compounds were evaluated for in vitroactivity against Plasmodium falciparum3D7 malaria parasites with methyl pheophorbide A (7) showing the strongest activity (IC504.49 μM). Subsequent in vivotesting in a Plasmodium bergheichemosuppression model showed that compound 7significantly attenuated peripheral blood parasitemia, leading to 79% and 87% chemosuppression following oral doses at 10 and 20 mg/kg, respectively.

Five new isocyano/isothiocyanato sesquiterpenes (1–5) with tri- or bicyclic carbon skeletons have been characterized from Australian specimens of the nudibranch Phyllidia ocellata.Spectroscopic analyses at 900 MHz were informed by DFT calculations. The 1S, 5S, 8Rconfiguration of 2-isocyanoclovene (1) was determined by X-ray crystallographic analysis of formamide 6. A biosynthetic pathway to clovanes 1and 2from epicaryolane precursors is proposed. Isocyanides 1, 2, and 4showed activity against Plasmodium falciparum(IC500.26–0.30 μM), while isothiocyanate 3and formamide 6had IC50values of >10 μM.

ABSTRACTTherapies to prevent transmission of malaria parasites to the mosquito vector are a vital part of the global malaria elimination agenda. Primaquine is currently the only drug with such activity; however, its use is limited by side effects. The development of transmission-blocking strategies requires an understanding of sexual stage malaria parasite (gametocyte) biology and the identification of new drug leads. Lysine acetylation is an important posttranslational modification involved in regulating eukaryotic gene expression and other essential processes. Interfering with this process with histone deacetylase (HDAC) inhibitors is a validated strategy for cancer and other diseases, including asexual stage malaria parasites. Here we confirm the expression of at least one HDAC protein in Plasmodium falciparumgametocytes and show that histone and nonhistone protein acetylation occurs in this life cycle stage. The activity of the canonical HDAC inhibitors trichostatin A (TSA) and suberoylanilide hydroxamic acid (SAHA; Vorinostat) and a panel of novel HDAC inhibitors on early/late-stage gametocytes and on gamete formation was examined. Several compounds displayed early/late-stage gametocytocidal activity, with TSA being the most potent (50% inhibitory concentration, 70 to 90 nM). In contrast, no inhibitory activity was observed in P. falciparumgametocyte exflagellation experiments. Gametocytocidal HDAC inhibitors caused hyperacetylation of gametocyte histones, consistent with a mode of action targeting HDAC activity. Our data identify HDAC inhibitors as being among a limited number of compounds that target both asexual and sexual stage malaria parasites, making them a potential new starting point for gametocytocidal drug leads and valuable tools for dissecting gametocyte biology.

Fragment-based screening is commonly used to identify compounds with relatively weak but efficient localized binding to protein surfaces. We used mass spectrometry to study fragment-sized three-dimensional natural products. We identified seven securinine-related compounds binding to Plasmodium falciparum2′-deoxyuridine 5′-triphosphate nucleotidohydrolase (PfdUTPase). Securinine bound allosterically to PfdUTPase, enhancing enzyme activity and inhibiting viability of both P. falciparumgametocyte (sexual) and blood (asexual) stage parasites. Our results provide a new insight into mechanisms that may be applicable to transmission-blocking agents.

Twenty-one new psammaplysin derivatives (4–24) exhibiting a variety of side chains, as well as six previously known psammaplysins, were identified from the Indonesian marine sponge Aplysinella strongylata. The double bond on the side chain of the fatty acid-containing psammaplysins was located by GC-MS analysis of the fatty acid methyl esters and their pyrrolidide derivatives. HPLC and Mosher ester studies confirmed that the isolated metabolites possessing a 19-OH substituent were mixtures of diastereomers. Selected compounds (4, 5, 7, 8, 12, 18, and 22) were screened for in vitro activity against chloroquine-sensitive (3D7) P. falciparummalaria parasites. Of the new psammaplysins, 19-hydroxypsammaplysin E (4) showed the best antimalarial activity, with an IC50value of 6.4 μM.

ABSTRACTHistone deacetylase (HDAC) enzymes posttranslationally modify lysines on histone and nonhistone proteins and play crucial roles in epigenetic regulation and other important cellular processes. HDAC inhibitors (e.g., suberoylanilide hydroxamic acid [SAHA; also known as vorinostat]) are used clinically to treat some cancers and are under investigation for use against many other diseases. Development of new HDAC inhibitors for noncancer indications has the potential to be accelerated by piggybacking onto cancer studies, as several HDAC inhibitors have undergone or are undergoing clinical trials. One such compound, SB939, is a new orally active hydroxamate-based HDAC inhibitor with an improved pharmacokinetic profile compared to that of SAHA. In this study, the in vitroand in vivoantiplasmodial activities of SB939 were investigated. SB939 was found to be a potent inhibitor of the growth of Plasmodium falciparumasexual-stage parasites in vitro(50% inhibitory concentration [IC50], 100 to 200 nM), causing hyperacetylation of parasite histone and nonhistone proteins. In combination with the aspartic protease inhibitor lopinavir, SB939 displayed additive activity. SB939 also potently inhibited the in vitrogrowth of exoerythrocytic-stage Plasmodiumparasites in liver cells (IC50, ∼150 nM), suggesting that inhibitor targeting to multiple malaria parasite life cycle stages may be possible. In an experimental in vivomurine model of cerebral malaria, orally administered SB939 significantly inhibited P. bergheiANKA parasite growth, preventing development of cerebral malaria-like symptoms. These results identify SB939 as a potent new antimalarial HDAC inhibitor and underscore the potential of investigating next-generation anticancer HDAC inhibitors as prospective new drug leads for treatment of malaria.

ABSTRACTThe stage-specific antimalarial activities of a panel of antiretroviral protease inhibitors (PIs), including two nonpeptidic PIs (tipranavir and darunavir), were tested in vitroagainst Plasmodium falciparum. While darunavir demonstrated limited antimalarial activity (effective concentration [EC50], >50 μM), tipranavir was active at clinically relevant concentrations (EC50, 12 to 21 μM). Saquinavir, lopinavir, and tipranavir preferentially inhibited the growth of mature asexual-stage parasites (24 h postinvasion). While all of the PIs tested inhibited gametocytogenesis, tipranavir was the only one to exhibit gametocytocidal activity.

ABSTRACTThe zoonotic malaria parasite Plasmodium knowlesihas recently been established in continuous in vitroculture. Here, the Plasmodium falciparum[3H]hypoxanthine uptake assay was adapted for P. knowlesiand used to determine the sensitivity of this parasite to chloroquine, cycloguanil, and clindamycin. The data demonstrate that P. knowlesiis sensitive to all drugs, with 50% inhibitory concentrations (IC50s) consistent with those obtained with P. falciparum. This assay provides a platform to use P. knowlesi in vitrofor drug discovery.

ABSTRACTThe antimalarial activity and pharmacology of a series of phenylthiazolyl-bearing hydroxamate-based histone deacetylase inhibitors (HDACIs) was evaluated. In in vitro growth inhibition assays approximately 50 analogs were evaluated against four drug resistant strains of Plasmodium falciparum. The range of 50% inhibitory concentrations (IC50s) was 0.0005 to >1 μM. Five analogs exhibited IC50s of <3 nM, and three of these exhibited selectivity indices of >600. The most potent compound, WR301801 (YC-2-88) was shown to cause hyperacetylation of P. falciparumhistones, which is a marker for HDAC inhibition in eukaryotic cells. The compound also inhibited malarial and mammalian HDAC activity in functional assays at low nanomolar concentrations. WR301801 did not exhibit cures in P. berghei-infected mice at oral doses as high as 640 mg/kg/day for 3 days or in P. falciparum-infected Aotus lemurinus lemurinusmonkeys at oral doses of 32 mg/kg/day for 3 days, despite high relative bioavailability. The failure of monotherapy in mice may be due to a short half-life, since the compound was rapidly hydrolyzed to an inactive acid metabolite by loss of its hydroxamate group in vitro (half-life of 11 min in mouse microsomes) and in vivo (half-life in mice of 3.5 h after a single oral dose of 50 mg/kg). However, WR301801 exhibited cures in P. berghei-infected mice when combined at doses of 52 mg/kg/day orally with subcurative doses of chloroquine. Next-generation HDACIs with greater metabolic stability than WR301801 may be useful as antimalarials if combined appropriately with conventional antimalarial drugs.

ABSTRACTParasite resistance to antimalarial drugs is a serious threat to human health, and novel agents that act on enzymes essential for parasite metabolism, such as proteases, are attractive targets for drug development. Recent studies have shown that clinically utilized human immunodeficiency virus (HIV) protease inhibitors can inhibit the in vitro growth of Plasmodium falciparumat or below concentrations found in human plasma after oral drug administration. The most potent in vitro antimalarial effects have been obtained for parasites treated with saquinavir, ritonavir, or lopinavir, findings confirmed in this study for a genetically distinct P. falciparumline (3D7). To investigate the potential in vivo activity of antiretroviral protease inhibitors (ARPIs) against malaria, we examined the effect of ARPI combinations in a murine model of malaria. In mice infected with Plasmodium chabaudiAS and treated orally with ritonavir-saquinavir or ritonavir-lopinavir, a delay in patency and a significant attenuation of parasitemia were observed. Using modeling and ligand docking studies we examined putative ligand binding sites of ARPIs in aspartyl proteases of P. falciparum(plasmepsins II and IV) and P. chabaudi(plasmepsin) and found that these in silico analyses support the antimalarial activity hypothesized to be mediated through inhibition of these enzymes. In addition, in vitro enzyme assays demonstrated that P. falciparumplasmepsins II and IV are both inhibited by the ARPIs saquinavir, ritonavir, and lopinavir. The combined results suggest that ARPIs have useful antimalarial activity that may be especially relevant in geographical regions where HIV and P. falciparuminfections are both endemic.

Adhesion of Plasmodium falciparum-infected erythrocytes to placental chondroitin 4-sulfate (CSA) has been linked to the severe disease outcome of pregnancy-associated malaria. Soluble polysaccharides that release mature-stage parasitized erythrocytes into the peripheral circulation may help elucidate these interactions and have the potential to aid in developing therapeutic strategies. We have screened a panel of 11 sulfated polysaccharides for their capacities to inhibit adhesion of infected erythrocytes to CSA expressed on CHO-K1 cells and ex vivo human placental tissue. Two carrageenans and a cellulose sulfate (CS10) were able to inhibit adhesion to CSA and to cause already bound infected erythrocytes to de-adhere in a dose-dependent manner. CS10, like CSA and in contrast to all other compounds tested, remained bound to infected erythrocytes after washing and continued to inhibit binding. Both carrageenans and CS10 inhibited adhesion to placental tissue. Although highly sulfated dextran sulfate can inhibit CSA-mediated adhesion to CHO cells, this polysaccharide amplified adhesion to placental tissue severalfold, demonstrating the importance of evaluating inhibitory compounds in systems as close to in vivo as possible. Interestingly, and in contrast to all other compounds tested, which had a random distribution of sulfate groups, CS10 exhibited a clustered sulfate pattern along the polymer chain, similar to that of the undersulfated placental CSA preferred by placental-tissue-binding infected erythrocytes. Therefore, the specific antiadhesive capacity observed here seems to depend not only on the degree of charge and sulfation but also on a particular pattern of sulfation.

Abstract Hepatic cell populations play an important role during the malaria life cycle. L-SIGN, a homologue of DC-SIGN, mediating leukocyte and pathogen binding, is selectively expressed on liver endothelial cells. Here, we present evidence that L-SIGN acts as an endocytic cell surface receptor. However, P. falciparum-infected erythrocytes did not cytoadhere to L-SIGN. Thus, L-SIGN contributes to elimination of mannosylated ligands but does not participate in hepatic clearance of P. falciparum-infected erythrocytes.

Cytoadherence of Plasmodium falciparum-infected erythrocytes (PRBC) to endothelial cells causes severe clinical disease, presumably as a of result perfusion failure and tissue hypoxia. Cytoadherence to endothelial cells is increased by endothelial cell activation, which is believed to occur in a paracrine fashion by mediators such as tumor necrosis factor alpha (TNF-α) released from macrophages that initially recognize PRBC. Here we provide evidence that PRBC directly stimulate human endothelial cells in the absence of macrophages, leading to increased expression of adhesion-promoting molecules, such as intercellular adhesion molecule 1. Endothelial cell stimulation by PRBC required direct physical contact for a short time (30 to 60 min) and was correlated with parasitemia. Gene expression profiling of endothelial cells stimulated by PRBC revealed increased expression levels of chemokine and adhesion molecule genes. PRBC-stimulated endothelial cells especially showed increased expression of molecules involved in parasite adhesion but failed to express molecules promoting leukocyte adhesion, such as E-selectin and vascular cell adhesion molecule 1, even after challenge with TNF-α. Collectively, our data suggest that stimulation of endothelial cells by PRBC may have two effects: prevention of parasite clearance through increased cytoadherence and attenuation of leukocyte binding to endothelial cells, thereby preventing deleterious immune reactivity.

Abstract. The human malarial parasite Plasmodium falciparum is responsible for an estimated 300-500 million clinical cases and 1-3 million deaths annually. At particular risk of developing severe, life-threatening malaria-associated complications are women during their first pregnancy. The observed pathologies, such as premature delivery, intrauterine growth retardation, abortion, and death of the mother and the newborn, are in large parts due to the parasite's ability to render infected erythrocytes adhesive and sequester in the intervillous space of infected placentas. In subsequent pregnancies, women are protected from maternal malaria through antibodies that prevent cytoadhesion of P. falciparum-infected erythrocytes in the placenta. Here, we summarize our current knowledge of the pathophysiological processes underpinning maternal malaria and discuss emerging concepts for intervention.

Malaria, caused by Plasmodiumparasites, continues to be a devastating global health issue, causing 405,000 deaths and 228 million cases in 2018. Understanding key metabolic processes in malaria parasites is critical to the development of new drugs to combat this major infectious disease. The Plasmodiumglycolytic pathway is essential to the malaria parasite, providing energy for growth and replication and supplying important biomolecules for other essential Plasmodiumanabolic pathways. Despite this overreliance on glycolysis, no current drugs target glycolysis, and there is a paucity of information on critical glycolysis targets. Our work addresses this unmet need, providing new mechanistic insights into this key pathway.

Given that aminoglycosides, such as amikacin, may be used for multidrug-resistant Pseudomonas aeruginosainfections, optimization of therapy is paramount for improved treatment outcomes. This study aims to investigate the pharmacodynamics of different simulated intravenous amikacin doses on susceptible P. aeruginosato inform ventilator-associated pneumonia (VAP) and sepsis treatment choices. A hollow-fiber infection model with two P. aeruginosaisolates (MICs of 2 and 8 mg/liter) with an initial inoculum of ∼108CFU/ml was used to test different amikacin dosing regimens.

ABSTRACTA panel of sulfated oligosaccharides was tested for antimalarial activity and inhibition of adhesion to the placental malaria receptor chondroitin-4-sulfate (CSA). The heparan sulfate mimetic PI-88, currently undergoing phase II anticancer trials, displayed the greatest in vitro antimalarial activity against Plasmodium falciparum(50% inhibitory concentration of 7.4 μM) and demonstrated modest adhesion inhibition to cell surface CSA.

Histones are abundant nuclear core proteins that are present in all eukararyotes and are responsible for linking chromosomes and packaging them into tight chromatin aggregates. The histone H2A, H2B, and H3 genes and a partial sequence of the histone H4 gene from Plasmodium falciparum have been previously identified and share a high level of nucleotide sequence identity. In this study, we compare the histone H4 sequence of the human malaria P. falciparum with the sequences of two mouse malarias, Plasmodium berghei and Plasmodium yoelii, revealing at least 91% identity at the nucleotide level and 100% conservation at the amino acid level. Furthermore, we show the P. falciparum histone H4 is developmentally transcribed in late stage asexual parasites, completing the transcription profile for the genes comprising the histone octamer of P. falciparum and adding support to suggestions that a novel histone mRNA control mechanism exists in this parasite.

The title compounds (I) and (II) are the first C‐7 alkylated caprolactam natural products reported.