Antimalarial Drugs and Drug Targets Specific to Fatty Acid Metabolic Pathway of Plasmodium falciparum

In vitro and in silico anti-malarial activity and cytotoxicity of n-hexyl 1-O-rutinoside (a glycoside) isolated from Annickia polycarpa (DC.) Setten and Maas ex I.M. Turner (Annonaceae)

ETHNOPHARMACOLOGICAL RELEVANCE

Annickia polycarpa leaf is an effective anti-malarial agent. However, its chemical constituents have not been isolated and assayed against any pathogen.

AIM OF THE STUDY

To isolate and characterize anti-malarial compound(s) from the leaf of A. polycarpa.

MATERIALS AND METHODS

Bioassay-guided fractionation was employed to isolated the compound (AL1) from the chloroform fraction (ALCF) of the basified ethanol extract of A. polycarpa leaf (ALE). AL1 was characterized by LC-MS, 1D and 2D NMR spectroscopic analysis. Anti-malarial activity was evaluated against drug resistance Dd2 and drug sensitive 3D7 Plasmodium falciparum strains using the SYBR green assay. Cytotoxicity and mechanistic studies were determined using tetrazolium-based colorimetric assay and molecular docking respectively.

RESULTS

AL1 was characterized as n-hexyl 1-O-rutinoside. The IC50 values of ALE and ALCF against 3D7 and Dd2 P. falciparum strains ranges from 3.441 (0.3389) - 4.255 (0.2246) μg/mL. The IC50s obtained for n-hexyl 1-O-rutinoside and Artesunate (standard drug) were 7.71 (0.5473) and 0.001 (0.00008) nM against the 3D7 parasite strain respectively. Also, the efficacy of n-hexyl 1-O-rutinoside increased by 24.40% against the chloroquine resistance Dd2 P. falciparum strain whiles that of Artesunate decreased by 98.96%. Furthermore, ALE, ALCF and n-hexyl 1-O-rutinoside were weakly cytotoxic to human RBCs with high selectivity indices. N-hexyl 1-O-rutinoside inhibits P. falciparum chloroquine resistance transporter (PfCRT) and dihydrofolate reductase-thymidylate synthase (PfDHFR-TS) better than chloroquine and pyrimethamine respectively. But, produced similar inhibition of P. falciparum 2-trans-enoyl -ACP-reductase (PfERN) as triclosan.

CONCLUSION

These results show that A. polycarpa leaf and n-hexyl 1-O-rutinoside possessed profound anti-malarial activity and are not cytotoxic. N-hexyl 1-O-rutinoside could therefore, be developed into a new anti-malarial medicine. This is the first study to report the anti-malarial activity of n-hexyl 1-O-rutinoside and its isolation from the genus Annickia.

In silico evaluation and in vitro growth inhibition of Plasmodium falciparum by natural amides and synthetic analogs

Malaria, caused by protozoa of the genus Plasmodium, is a disease that infects hundreds of millions of people annually, causing an enormous social burden in many developing countries. Since current antimalarial drugs are starting to face resistance by the parasite, the development of new therapeutic options has been prompted. The enzyme Plasmodium falciparum enoyl-ACP reductase (PfENR) has a determinant role in the fatty acid biosynthesis of this parasite and is absent in humans, making it an ideal target for new antimalarial drugs. In this sense, the present study aimed at evaluating the in silico binding affinity of natural and synthetic amides through molecular docking, in addition to their in vitro activity against P. falciparum by means of the SYBR Green Fluorescence Assay. The in vitro results revealed that the natural amide piplartine (1a) presented partial antiplasmodial activity (20.54 μM), whereas its synthetic derivatives (1m-IC₅₀ 104.45 μM), (1b, 1g, 1k, and 14f) and the natural amide piperine (18a) were shown to be inactive (IC₅₀ > 200 μM). The in silico physicochemical analyses demonstrated that compounds 1m and 14f violated the Lipinski's rule of five. The in silico analyses showed that 14f presented the best binding affinity (- 13.047 kcal/mol) to PfENR and was also superior to the reference inhibitor triclosan (- 7.806 kcal/mol). In conclusion, we found that the structural modifications in 1a caused a significant decrease in antiplasmodial activity. Therefore, new modifications are encouraged in order to improve the activity observed.

POTENCY OF LUTEOLIN WITH SOLID LIPID NANOPARTICLE (SLN)-POLYETHYLENE GLYCOL (PEG) MODIFICATION FOR ARTEMISININ-RESISTANT PLASMODIUM FALCIPARUM INFECTION

Falciparum malaria is still considered as one of the important global health problems and its causal agent (Plasmodium falciparum) is reported to be the third most common factor for contributing the number of deaths in the world. As we all know, Artemisinins arethe most rapidly acting of currently available antimalarial drugs. Along with Artesunate, these two combining drugs, the so-called Artemisinin-based combination therapies (ACTs) has become the foundation of modern falciparum malaria treatment globally. Nowadays, however, there have been reports about intricate cases of resistance against Artemisinin in various Southeast Asian countries and it is predicted to spread over several other countries, including Indonesia. Therefore, adjuvant therapy is required along with first-line therapy administration to help eradicate both Artemisinin-sensitive and resistant P. falciparum. Luteolin in vitro has a prospective inhibitory activity (IC50<50 μg) in inhibiting the development of parasite’s life cycle. Nonetheless, its poor bioavailability and pharmacokinetics restrict clinical application. The low bioavailability of luteolin requires encapsulation using solid lipid nanoparticle (SLN) and polyethylene glycol (PEG). SLN is useful for improving the bioavailability of luteolin in the body, whereas PEG is needed in order to prevent the destruction of luteolin-SLN substance by the reticuloendothelial system. Here in this literature review, we’re trying to demonstrate the benefits, potential, way of constructions, pharmacokinetics, and pharmacodynamics of luteolin encapsulated with SLN with PEG modification. Thus, it is hoped that the results of this literature study may encourage further research in assisting the development of adjuvant therapy for cases of Artemisinin-resistant P. falciparum infection.

Pravastatin and simvastatin inhibit the adhesion, replication and proliferation of Toxoplasma gondii (RH strain) in HeLa cells

The conventional treatment for toxoplasmosis with pyrimethamine and sulfadiazine shows toxic effects to the host, and it is therefore necessary to search for new drugs. Some studies suggest the use of statins, which inhibit cholesterol synthesis in humans and also the initial processes of isoprenoid biosynthesis in the parasite. Thus, the objective of this study was to evaluate the activity of the statins pravastatin and simvastatin in HeLa cells infected in vitro with the RH strain of T. gondii. HeLa cells (1×10⁵) were infected with T. gondii tachyzoites (5×10⁵) following two different treatment protocols. In the first protocol, T. gondii tachyzoites were pretreated with pravastatin (50 and 100μg/mL) and simvastatin (1.56 and 3.125μg/mL) for 30min prior to infection. In the second, HeLa cells were first infected (5×10⁵) with tachyzoites and subsequently treated with pravastatin and simvastatin for 24h at the concentrations noted above. Initially, we evaluated the cytotoxicity of drugs by the MTT assay, number of tachyzoites adhered to cells, number of infected cells, and viability of tachyzoites by trypan blue exclusion. The supernatant of the cell cultures was collected post-treatment for determination of the pattern of Th1/Th2/Th17 cytokines by cytometric bead array. There was no cytotoxicity to HeLa cells with 50 and 100μg/mL pravastatin and 1.56 and 3.125μg/mL simvastatin. There was no change in the viability of tachyzoites that received pretreatment. Regarding the pre- and post-treatment of the cells with pravastatin and simvastatin alone, there was a reduction in adhesion, invasion and proliferation of cells to T. gondii. As for the production of cytokines, we found that IL-6 and IL-17 were significantly reduced in cells infected with T. gondii and treated with pravastatin and simvastatin, when compared to control. Based on these results, we can infer that pravastatin and simvastatin alone possess antiproliferative effects on tachyzoites forms of T. gondii, giving these drugs new therapeutic uses.

Conformational stability and thermal pathways of relaxation in triclosan (antibacterial/excipient/contaminant) in solid-state: combined spectroscopic ((1)H NMR) and computational (periodic DFT) study

The mechanism of molecular dynamics in the antibacterial/antifungal agent, triclosan (5-chloro-2-(2',4'-dichlorophenoxy)-phenol), in solid state was studied by (1)H NMR spectroscopy and periodic density functional theory (DFT) calculations. Temperature dependencies of the proton spin-lattice relaxation time (T1) in the ranges 86-293 and 90-250 K (at 15 and 24.667 MHz, respectively) and the second moment (M2) of the (1)H NMR resonant line in the range 103-300 K were measured. Two minima in the temperature dependence of T1 revealed a classical Arrhenius governed activation processes. The low temperature shallow minimum T1(T) of 71 s at 115 K, 15 MHz, which shifts with frequency, was assigned to classical hindered jumps of hydroxyl group around OC axis and with respect to a 5-chloro-2-phenol ring. The activation energy of this motion estimated on the basis of the fit of the theoretical model to the experimental points is 9.68 kJ/mol. The pointed high temperature minimum T1(T) of 59 s at 190 K, 15 MHz, which also shifts with frequency, was assigned to the small angle librations by Θlib= ± 9° between two positions of equilibrium differing in energy by 7.42 kJ/mol. The activation energy of this motion estimated on the basis of the fit of the theoretical model to the experimental points is 31.1 kJ/mol. Both motions result in a negligible reduction in the (1)H NMR line second moment, thus the second moment delivers an irrelevant description of the molecular motions in triclosan.

Celastrol inhibits Plasmodium falciparum enoyl-acyl carrier protein reductase

Enoyl-acyl carrier protein reductase (ENR), a critical enzyme in type II fatty acid biosynthesis, is a promising target for drug discovery against hepatocyte-stage Plasmodium falciparum. In order to identify PfENR-specific inhibitors, we docked 70 FDA-approved, bioactive, and/or natural product small molecules known to inhibit the growth of whole-cell blood-stage P. falciparum into several PfENR crystallographic structures. Subsequent in vitro activity assays identified a noncompetitive low-micromolar PfENR inhibitor, celastrol, from this set of compounds.

In silico screening for Plasmodium falciparum enoyl-ACP reductase inhibitors

The need for novel therapeutics against Plasmodium falciparum is urgent due to recent emergence of multi-drug resistant malaria parasites. Since fatty acids are essential for both the liver and blood stages of the malarial parasite, targeting fatty acid biosynthesis is a promising strategy for combatting P. falciparum. We present a combined computational and experimental study to identify novel inhibitors of enoyl-acyl carrier protein reductase (PfENR) in the fatty acid biosynthesis pathway. A small-molecule database from ChemBridge was docked into three distinct PfENR crystal structures that provide multiple receptor conformations. Two different docking algorithms were used to generate a consensus score in order to rank possible small molecule hits. Our studies led to the identification of five low-micromolar pyrimidine dione inhibitors of PfENR.

Does Plasmodium falciparum have an Achilles' heel?

BACKGROUND

Plasmodium falciparum is the parasite that causes the most severe form of malaria responsible for nearly a million deaths a year. Currently, science has been established about its cellular structures, its metabolic processes, and even the molecular structures of its intrinsic membrane proteins responsible for transporting water, nutrient, and waste molecules across the parasite plasma membrane (PPM).

PRESENTATION OF THE HYPOTHESIS

I hypothesize that Plasmodium falciparum has an Achilles' heel that can be attacked with erythritol, the well-known sweetener that is classified as generally safe. This hypothesis is based on the molecular structure of the parasite's membrane and the quantitative mechanics of how erythritol interacts with the multi-functional channel protein expressed in the PPM. Most organisms have in their cell membrane two types of water-channel proteins: aquaporins to maintain hydro-homeostasis across the membrane and aquaglyceroporins to uptake glycerols etc. In contrast, P. falciparum has only one type of such proteins---the multi-functional aquaglyceroporin (PfAQP) expressed in the PPM---to do both jobs. Moreover, the parasite also uses PfAQP to excrete its metabolic wastes (ammonia included) produced at a very high rate in the blood stage. This extremely high efficiency of the bug using one protein for multiple essential tasks makes the parasite fatally vulnerable. Erythritol in the blood stream can kill the parasite by clogging up its PfAQP channel that needs to be open for maintaining hydro-homeostasis and for excreting toxic wastes across the bug's PPM.

TESTING THE HYPOTHESIS

In vitro tests are to measure the growth/death rate of P. falciparum in blood with various erythritol concentrations. In vivo experiments are to administer groups of infected mice with various doses of erythritol and monitor the parasite growth levels from blood samples drawn from each group. Clinic trials can be performed to observe the added effects of administering to patients erythritol along with the known drugs because erythritol was classified as a safe food ingredient.

IMPLICATIONS OF THE HYPOTHESIS

If proven true, erythritol will cure the most severe form of malaria without significant side effects.

Novel type II fatty acid biosynthesis (FAS II) inhibitors as multistage antimalarial agents

Malaria is a potentially fatal disease caused by Plasmodium parasites and poses a major medical risk in large parts of the world. The development of new, affordable antimalarial drugs is of vital importance as there are increasing reports of resistance to the currently available therapeutics. In addition, most of the current drugs used for chemoprophylaxis merely act on parasites already replicating in the blood. At this point, a patient might already be suffering from the symptoms associated with the disease and could additionally be infectious to an Anopheles mosquito. These insects act as a vector, subsequently spreading the disease to other humans. In order to cure not only malaria but prevent transmission as well, a drug must target both the blood- and pre-erythrocytic liver stages of the parasite. P. falciparum (Pf) enoyl acyl carrier protein (ACP) reductase (ENR) is a key enzyme of plasmodial type II fatty acid biosynthesis (FAS II). It has been shown to be essential for liver-stage development of Plasmodium berghei and is therefore qualified as a target for true causal chemoprophylaxis. Using virtual screening based on two crystal structures of PfENR, we identified a structurally novel class of FAS inhibitors. Subsequent chemical optimization yielded two compounds that are effective against multiple stages of the malaria parasite. These two most promising derivatives were found to inhibit blood-stage parasite growth with IC(50) values of 1.7 and 3.0 μM and lead to a more prominent developmental attenuation of liver-stage parasites than the gold-standard drug, primaquine.

In vitro activity of Pheroid vesicles containing antibiotics against Plasmodium falciparum

The macrolide antibiotics, erythromycin and azithromycin, have been studied for their potential antimalarial activity, but only modest activity has been demonstrated. In this study, we investigated the enhancement of the efficacy of these antibiotics in combination with a patented lipid-based drug delivery system, Pheroid technology. A chloroquine resistant strain of Plasmodium falciparum (RSA11) was incubated with the formulations for a prolonged incubation time (144 h). Drug efficacy assays were conducted by analyzing the histidine-rich protein II levels of the parasites. The effects of azithromycin and erythromycin were compared with other antibiotics and standard antimalarial drugs. The poor water soluble nature of the drugs led to the formation of micro scale Pheroid vesicles with average particle sizes of 72.76±10.73 μm for azithromycin and 100.62±29.27 μm for erythromycin. The IC(50) values of erythromycin and azithromycin alone and entrapped in Pheroid vesicles decreased statistically significant (P0.05). Prolonged exposure was also statistically meaningful (P0.05), although it seems that exposure need not exceed 96 h. Pheroid vesicles also proved successful in decreasing the IC(50) values of doxycycline, tetracycline and triclosan. Pheroid vesicles containing antibiotics could prove successful as a malaria treatment option.