Evaluation and lead optimization of anti-malarial acridones

Plasmodium falciparum Mitochondrial Complex III, the Target of Atovaquone, Is Essential for Progression to the Transmissible Sexual Stages

The Plasmodium falciparum mitochondrial electron transport chain (mETC) is responsible for essential metabolic pathways such as de novo pyrimidine synthesis and ATP synthesis. The mETC complex III (cytochrome bc1 complex) is responsible for transferring electrons from ubiquinol to cytochrome c and generating a proton gradient across the inner mitochondrial membrane, which is necessary for the function of ATP synthase. Recent studies have revealed that the composition of Plasmodium falciparum complex III (PfCIII) is divergent from humans, highlighting its suitability as a target for specific inhibition. Indeed, PfCIII is the target of the clinically used anti-malarial atovaquone and of several inhibitors undergoing pre-clinical trials, yet its role in parasite biology has not been thoroughly studied. We provide evidence that the universally conserved subunit, PfRieske, and the new parasite subunit, PfC3AP2, are part of PfCIII, with the latter providing support for the prediction of its divergent composition. Using inducible depletion, we show that PfRieske, and therefore, PfCIII as a whole, is essential for asexual blood stage parasite survival, in line with previous observations. We further found that depletion of PfRieske results in gametocyte maturation defects. These phenotypes are linked to defects in mitochondrial functions upon PfRieske depletion, including increased sensitivity to mETC inhibitors in asexual stages and decreased cristae abundance alongside abnormal mitochondrial morphology in gametocytes. This is the first study that explores the direct role of the PfCIII in gametogenesis via genetic disruption, paving the way for a better understanding of the role of mETC in the complex life cycle of these important parasites and providing further support for the focus of antimalarial drug development on this pathway.

Structure-activity relationships of Toxoplasma gondii cytochrome bc1 inhibitors

INTRODUCTION

Toxoplasma gondii is a prolific apicomplexan parasite that infects human and nonhuman animals worldwide and can cause severe brain and eye disease. Safer, more effective therapies for toxoplasmosis are needed. Cytochrome bc1 inhibitors are remarkably effective against toxoplasmosis and other apicomplexan-caused diseases.

AREAS COVERED

This work reviews T. gondii cytochrome bc1 inhibitors. Emphasis is placed on the structure-activity relationships of these inhibitors with regard to efficacy, pharmacokinetics, selectivity of T. gondii cytochrome bc1 over host, safety, and potential therapeutic strategies.

EXPERT OPINION

Cytochrome bc1 inhibitors are highly promising compounds for toxoplasmosis that have been effective in clinical and preclinical studies. Clinical experience with atovaquone previously validated cytochrome bc1 as a tractable drug target and, over the past decade, optimization of cytochrome bc1 inhibitors has resulted in improved bioavailability, metabolic stability, potency, blood-brain barrier penetration, and selectivity for the T. gondii cytochrome bc1 over the mammalian bc1. Recent studies have demonstrated preclinical safety, identified novel therapeutic strategies for toxoplasmosis using synergistic combinations or long-acting administration and provided insight into their role in chronic infection. This research has identified drug candidates that are more effective than clinically used drugs in preclinical measures of efficacy.

An overview on the synthesis of carbohydrate-based molecules with biological activity related to neurodegenerative diseases

In the context of the search for multitarget drugs with improved efficacy against neurodegenerative disorders, carbohydrate derivatives have emerged as promising candidates for Alzheimer's therapy. Herein we describe the synthesis and biological evaluation of several classes of sugar-based compounds, where most of them contain heterocyclic aromatic moieties that bear known biological properties and high affinity for the cholinesterase active site. This general idea led to the synthesis of compounds with high inhibitory potency against acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE), enzymatic selectivity and combined properties such as antioxidant and neuroprotection, in addition to the absence of toxicity.

Antimalarial drug discovery: from quinine to the most recent promising clinical drug candidates

Malaria is a tropical threatening disease caused by Plasmodium parasites, resulting in 409,000 deaths in 2019. The delay of mortality and morbidity has been compounded by the widespread of drug resistant parasites from Southeast Asia since two decades. The emergence of artemisinin-resistant Plasmodium in Africa, where most cases are accounted, highlights the urgent need for new medicines. In this effort, the World Health Organization and Medicines for Malaria Venture joined to define clear goals for novel therapies and characterized the target candidate profile. This ongoing search for new treatments is based on imperative labor in medicinal chemistry which is summarized here with particular attention to hit-to-lead optimizations, key properties, and modes of action of these novel antimalarial drugs. This review, after presenting the current antimalarial chemotherapy, from quinine to the latest marketed drugs, focuses in particular on recent advances of the most promising antimalarial candidates in clinical and preclinical phases.

Atypical Molecular Basis for Drug Resistance to Mitochondrial Function Inhibitors in Plasmodium falciparum

The continued emergence of drug-resistant Plasmodium falciparum parasites hinders global attempts to eradicate malaria, emphasizing the need to identify new antimalarial drugs. Attractive targets for chemotherapeutic intervention are the cytochrome (cyt) bc ₁ complex, which is an essential component of the mitochondrial electron transport chain (mtETC) required for ubiquinone recycling and mitochondrially localized dihydroorotate dehydrogenase (DHODH) critical for de novo pyrimidine synthesis. Despite the essentiality of this complex, resistance to a novel acridone class of compounds targeting cyt bc ₁ was readily attained, resulting in a parasite strain (SB1-A6) that was panresistant to both mtETC and DHODH inhibitors. Here, we describe the molecular mechanism behind the resistance of the SB1-A6 parasite line, which lacks the common cyt bc ₁ point mutations characteristic of resistance to mtETC inhibitors. Using Illumina whole-genome sequencing, we have identified both a copy number variation (∼2×) and a single-nucleotide polymorphism (C276F) associated with pfdhodh in SB1-A6. We have characterized the role of both genetic lesions by mimicking the copy number variation via episomal expression of pfdhodh and introducing the identified single nucleotide polymorphism (SNP) using CRISPR-Cas9 and assessed their contributions to drug resistance. Although both of these genetic polymorphisms have been previously identified as contributing to both DSM-1 and atovaquone resistance, SB1-A6 represents a unique genotype in which both alterations are present in a single line, suggesting that the combination contributes to the panresistant phenotype. This novel mechanism of resistance to mtETC inhibition has critical implications for the development of future drugs targeting the bc ₁ complex or de novo pyrimidine synthesis that could help guide future antimalarial combination therapies and reduce the rapid development of drug resistance in the field.

The cytochrome bc1 complex as an antipathogenic target

The cytochrome bc₁ complex is a key component of the mitochondrial respiratory chains of many eukaryotic microorganisms that are pathogenic for plants or humans, such as fungi responsible for crop diseases and Plasmodium falciparum, which causes human malaria. Cytochrome bc₁ is an enzyme that contains two (ubi)quinone/quinol-binding sites, which can be exploited for the development of fungicidal and chemotherapeutic agents. Here, we review recent progress in determination of the structure and mechanism of action of cytochrome bc₁ , and the associated development of antimicrobial agents (and associated resistance mechanisms) targeting its activity.

4-Aryl Pyrrolidines as a Novel Class of Orally Efficacious Antimalarial Agents. Part 1: Evaluation of 4-Aryl- N-benzylpyrrolidine-3-carboxamides

Identification of novel chemotypes with antimalarial efficacy is imperative to combat the rise of Plasmodium species resistant to current antimalarial drugs. We have used a hybrid target-phenotype approach to identify and evaluate novel chemotypes for malaria. In our search for drug-like aspartic protease inhibitors in publicly available phenotypic antimalarial databases, we identified GNF-Pf-4691, a 4-aryl- N-benzylpyrrolidine-3-carboxamide, as having a structure reminiscent of known inhibitors of aspartic proteases. Extensive profiling of the two terminal aryl rings revealed a structure-activity relationship in which relatively few substituents are tolerated at the benzylic position, but the 3-aryl position tolerates a range of hydrophobic groups and some heterocycles. Out of this effort, we identified (+)-54b (CWHM-1008) as a lead compound. 54b has EC50 values of 46 and 21 nM against drug-sensitive Plasmodium falciparum 3D7 and drug-resistant Dd2 strains, respectively. Furthermore, 54b has a long half-life in mice (4.4 h) and is orally efficacious in a mouse model of malaria (qd; ED99 ∼ 30 mg/kg/day). Thus, the 4-aryl- N-benzylpyrrolidine-3-carboxamide chemotype is a promising novel chemotype for malaria drug discovery.

Discovery and Structural Optimization of Acridones as Broad-Spectrum Antimalarials

Malaria remains one of the deadliest diseases in the world today. Novel chemoprophylactic and chemotherapeutic antimalarials are needed to support the renewed eradication agenda. We have discovered a novel antimalarial acridone chemotype with dual-stage activity against both liver-stage and blood-stage malaria. Several lead compounds generated from structural optimization of a large library of novel acridones exhibit efficacy in the following systems: (1) picomolar inhibition of in vitro Plasmodium falciparum blood-stage growth against multidrug-resistant parasites; (2) curative efficacy after oral administration in an erythrocytic Plasmodium yoelii murine malaria model; (3) prevention of in vitro Plasmodium berghei sporozoite-induced development in human hepatocytes; and (4) protection of in vivo P. berghei sporozoite-induced infection in mice. This study offers the first account of liver-stage antimalarial activity in an acridone chemotype. Details of the design, chemistry, structure-activity relationships, safety, metabolic/pharmacokinetic studies, and mechanistic investigation are presented herein.

Lack of mitochondrial MutS homolog 1 in Toxoplasma gondii disrupts maintenance and fidelity of mitochondrial DNA and reveals metabolic plasticity

The importance of maintaining the fidelity of the mitochondrial genome is underscored by the presence of various repair pathways within this organelle. Presumably, the repair of mitochondrial DNA would be of particular importance in organisms that possess only a single mitochondrion, like the human pathogens Plasmodium falciparum and Toxoplasma gondii. Understanding the machinery that maintains mitochondrial DNA in these parasites is of particular relevance, as mitochondrial function is a validated and effective target for anti-parasitic drugs. We previously determined that the Toxoplasma MutS homolog TgMSH1 localizes to the mitochondrion. MutS homologs are key components of the nuclear mismatch repair system in mammalian cells, and both yeast and plants possess MutS homologs that localize to the mitochondria where they regulate DNA stability. Here we show that the lack of TgMSH1 results in accumulation of single nucleotide variations in mitochondrial DNA and a reduction in mitochondrial DNA content. Additionally, parasites lacking TgMSH1 function can survive treatment with the cytochrome b inhibitor atovaquone. While the Tgmsh1 knockout strain has several missense mutations in cytochrome b, none affect amino acids known to be determinants of atovaquone sensitivity and atovaquone is still able to inhibit electron transport in the Tgmsh1 mutants. Furthermore, culture of Tgmsh1 mutant in the presence atovaquone leads to parasites with enhanced atovaquone resistance and complete shutdown of respiration. Thus, parasites lacking TgMSH1 overcome the disruption of mitochondrial DNA by adapting their physiology allowing them to forgo the need for oxidative phosphorylation. Consistent with this idea, the Tgmsh1 mutant is resistant to mitochondrial inhibitors with diverse targets and exhibits reduced ability to grow in the absence of glucose. This work shows TgMSH1 as critical for the maintenance and fidelity of the mitochondrial DNA in Toxoplasma, reveals a novel mechanism for atovaquone resistance, and exposes the physiological plasticity of this important human pathogen.

A tetraoxane-based antimalarial drug candidate that overcomes PfK13-C580Y dependent artemisinin resistance

K13 gene mutations are a primary marker of artemisinin resistance in Plasmodium falciparum malaria that threatens the long-term clinical utility of artemisinin-based combination therapies, the cornerstone of modern day malaria treatment. Here we describe a multinational drug discovery programme that has delivered a synthetic tetraoxane-based molecule, E209, which meets key requirements of the Medicines for Malaria Venture drug candidate profiles. E209 has potent nanomolar inhibitory activity against multiple strains of P. falciparum and P. vivax in vitro, is efficacious against P. falciparum in in vivo rodent models, produces parasite reduction ratios equivalent to dihydroartemisinin and has pharmacokinetic and pharmacodynamic characteristics compatible with a single-dose cure. In vitro studies with transgenic parasites expressing variant forms of K13 show no cross-resistance with the C580Y mutation, the primary variant observed in Southeast Asia. E209 is a superior next generation endoperoxide with combined pharmacokinetic and pharmacodynamic features that overcome the liabilities of artemisinin derivatives.

Antibiotics in malaria therapy: which antibiotics except tetracyclines and macrolides may be used against malaria?

Malaria, a parasite vector-borne disease, is one of the most significant health threats in tropical regions, despite the availability of individual chemoprophylaxis. Malaria chemoprophylaxis and chemotherapy remain a major area of research, and new drug molecules are constantly being developed before drug-resistant parasites strains emerge. The use of anti-malarial drugs is challenged by contra-indications, the level of resistance of Plasmodium falciparum in endemic areas, clinical tolerance and financial cost. New therapeutic approaches are currently needed to fight against this disease. Some antibiotics that have shown potential effects on malaria parasite have been recently studied in vitro or in vivo intensively. Two families, tetracyclines and macrolides and their derivatives have been particularly studied in recent years. However, other less well-known have been tested or are being used for malaria treatment. Some of these belong to older families, such as quinolones, co-trimoxazole or fusidic acid, while others are new drug molecules such as tigecycline. These emerging antibiotics could be used to prevent malaria in the future. In this review, the authors overview the use of antibiotics for malaria treatment.

New paradigms for understanding and step changes in treating active and chronic, persistent apicomplexan infections

Toxoplasma gondii, the most common parasitic infection of human brain and eye, persists across lifetimes, can progressively damage sight, and is currently incurable. New, curative medicines are needed urgently. Herein, we develop novel models to facilitate drug development: EGS strain T. gondii forms cysts in vitro that induce oocysts in cats, the gold standard criterion for cysts. These cysts highly express cytochrome b. Using these models, we envisioned, and then created, novel 4-(1H)-quinolone scaffolds that target the cytochrome bc1 complex Qi site, of which, a substituted 5,6,7,8-tetrahydroquinolin-4-one inhibits active infection (IC50, 30 nM) and cysts (IC50, 4 μM) in vitro, and in vivo (25 mg/kg), and drug resistant Plasmodium falciparum (IC50, <30 nM), with clinically relevant synergy. Mutant yeast and co-crystallographic studies demonstrate binding to the bc1 complex Qi site. Our results have direct impact on improving outcomes for those with toxoplasmosis, malaria, and ~2 billion persons chronically infected with encysted bradyzoites.

Subtle changes in endochin-like quinolone structure alter the site of inhibition within the cytochrome bc1 complex of Plasmodium falciparum

The cytochrome bc1 complex (cyt bc1) is the third component of the mitochondrial electron transport chain and is the target of several potent antimalarial compounds, including the naphthoquinone atovaquone (ATV) and the 4(1H)-quinolone ELQ-300. Mechanistically, cyt bc1 facilitates the transfer of electrons from ubiquinol to cytochrome c and contains both oxidative (Qo) and reductive (Qi) catalytic sites that are amenable to small-molecule inhibition. Although many antimalarial compounds, including ATV, effectively target the Qo site, it has been challenging to design selective Qi site inhibitors with the ability to circumvent clinical ATV resistance, and little is known about how chemical structure contributes to site selectivity within cyt bc1. Here, we used the proposed Qi site inhibitor ELQ-300 to generate a drug-resistant Plasmodium falciparum clone containing an I22L mutation at the Qi region of cyt b. Using this D1 clone and the Y268S Qo mutant strain, P. falciparum Tm90-C2B, we created a structure-activity map of Qi versus Qo site selectivity for a series of endochin-like 4(1H)-quinolones (ELQs). We found that Qi site inhibition was associated with compounds containing 6-position halogens or aryl 3-position side chains, while Qo site inhibition was favored by 5,7-dihalogen groups or 7-position substituents. In addition to identifying ELQ-300 as a preferential Qi site inhibitor, our data suggest that the 4(1H)-quinolone scaffold is compatible with binding to either site of cyt bc1 and that minor chemical changes can influence Qo or Qi site inhibition by the ELQs.

Atovaquone tolerance in Plasmodium falciparum parasites selected for high-level resistance to a dihydroorotate dehydrogenase inhibitor

Atovaquone is a component of Malarone, a widely prescribed antimalarial combination, that targets malaria respiration. Here we show that parasites with high-level resistance to an inhibitor of dihydroorotate dehydrogenase demonstrate unexpected atovaquone tolerance. Fortunately, the tolerance is diminished with proguanil, the second partner in Malarone. It is important to understand such "genetic cross talk" between respiration and pyrimidine biosynthesis since many antimalarial drug development programs target these two seemingly independent pathways.

Recent progress in the development of anti-malarial quinolones

Available anti-malarial tools have over the ten-year period prior to 2012 dramatically reduced the number of fatalities due to malaria from one million to less than six-hundred and thirty thousand. Although fewer people now die from malaria, emerging resistance to the first-line anti-malarial drugs, namely artemisinins in combination with quinolines and arylmethanols, necessitates the urgent development of new anti-malarial drugs to curb the disease. The quinolones are a promising class of compounds, with some demonstrating potent in vitro activity against the malaria parasite. This review summarizes the progress made in the development of potential anti-malarial quinolones since 2008. The efficacy of these compounds against both asexual blood stages and other stages of the malaria parasite, the nature of putative targets, and a comparison of these properties with anti-malarial drugs currently in clinical use, are discussed.

Novel inhibitors of the Plasmodium falciparum electron transport chain

Due to an increased need for new antimalarial chemotherapies that show potency against Plasmodium falciparum, researchers are targeting new processes within the parasite in an effort to circumvent or delay the onset of drug resistance. One such promising area for antimalarial drug development has been the parasite mitochondrial electron transport chain (ETC). Efforts have been focused on targeting key processes along the parasite ETC specifically the dihydroorotate dehydrogenase (DHOD) enzyme, the cytochrome bc 1 enzyme and the NADH type II oxidoreductase (PfNDH2) pathway. This review summarizes the most recent efforts in antimalarial drug development reported in the literature and describes the evolution of these compounds.

Antimalarial chemotherapy: artemisinin-derived dimer carbonates and thiocarbonates

Several 2-carbon-linked trioxane dimer secondary alcohol carbonates 14 and thiocarbonates 15, combined with mefloquine and administered in a low single oral dose, prolonged the survival times of malaria-infected mice much more effectively than the popular monomeric antimalarial drug artemether plus mefloquine. Three dimer carbonates 14 and one dimer thiocarbonate 15 partially cured malaria-infected mice.

Discovery, synthesis, and optimization of antimalarial 4(1H)-quinolone-3-diarylethers

The historical antimalarial compound endochin served as a structural lead for optimization. Endochin-like quinolones (ELQ) were prepared by a novel chemical route and assessed for in vitro activity against multidrug resistant strains of Plasmodium falciparum and against malaria infections in mice. Here we describe the pathway to discovery of a potent class of orally active antimalarial 4(1H)-quinolone-3-diarylethers. The initial prototype, ELQ-233, exhibited low nanomolar IC50 values against all tested strains including clinical isolates harboring resistance to atovaquone. ELQ-271 represented the next critical step in the iterative optimization process, as it was stable to metabolism and highly effective in vivo. Continued analoging revealed that the substitution pattern on the benzenoid ring of the quinolone core significantly influenced reactivity with the host enzyme. This finding led to the rational design of highly selective ELQs with outstanding oral efficacy against murine malaria that is superior to established antimalarials chloroquine and atovaquone.

Synthesis and antimalarial bioassay of quinine - peptide conjugates

Amino acid and peptide conjugates of quinine were synthesized using microwave irradiation in 52-95% yields using benzotriazole methodology. The majority of these conjugates retain in vitro antimalarial activity with IC50 values below 100 nm, similar to quinine.

Flavones as isosteres of 4(1H)-quinolones: discovery of ligand efficient and dual stage antimalarial lead compounds

Malaria is responsible for nearly one million deaths annually, and the increasing prevalence of multi-resistant strains of Plasmodium falciparum poses a great challenge to controlling the disease. A diverse set of flavones, isosteric to 4(1H)-quinolones, were prepared and profiled for their antiplasmodial activity against the blood stage of P. falciparum W2 strain, and the liver stage of the rodent parasite Plasmodium berghei. Ligand efficient leads were identified as dual stage antimalarials, suggesting that scaffold optimization may afford potent antiplasmodial compounds.

Cell-based medicinal chemistry optimization of high-throughput screening (HTS) hits for orally active antimalarials. Part 1: challenges in potency and absorption, distribution, metabolism, excretion/pharmacokinetics (ADME/PK)

Malaria represents a significant health issue, and novel and effective drugs are needed to address parasite resistance that has emerged to the current drug arsenal. Antimalarial drug discovery has historically benefited from a whole-cell (phenotypic) screening approach to identify lead molecules. This approach has been utilized by several groups to optimize weakly active antimalarial pharmacophores, such as the quinolone scaffold, to yield potent and highly efficacious compounds that are now poised to enter clinical trials. More recently, GNF/Novartis, GSK, and others have employed the same approach in high-throughput screening (HTS) of large compound libraries to find novel scaffolds that have also been optimized to clinical candidates by GNF/Novartis. This perspective outlines some of the inherent challenges in cell-based medicinal chemistry optimization, including optimization of oral exposure and hERG activity.

Sontochin as a guide to the development of drugs against chloroquine-resistant malaria

Sontochin was the original chloroquine replacement drug, arising from research by Hans Andersag 2 years after chloroquine (known as "resochin" at the time) had been shelved due to the mistaken perception that it was too toxic for human use. We were surprised to find that sontochin, i.e., 3-methyl-chloroquine, retains significant activity against chloroquine-resistant strains of Plasmodium falciparum in vitro. We prepared derivatives of sontochin, "pharmachins," with alkyl or aryl substituents at the 3 position and with alterations to the 4-position side chain to enhance activity against drug-resistant strains. Modified with an aryl substituent in the 3 position of the 7-chloro-quinoline ring, Pharmachin 203 (PH-203) exhibits low-nanomolar 50% inhibitory concentrations (IC(50)s) against drug-sensitive and multidrug-resistant strains and in vivo efficacy against patent infections of Plasmodium yoelii in mice that is superior to chloroquine. Our findings suggest that novel 3-position aryl pharmachin derivatives have the potential for use in treating drug resistant malaria.

Evaluation of synthetic acridones and 4-quinolinones as potent inhibitors of cathepsins L and V

Cathepsins, also known as lysosomal cysteine peptidases, are members of the papain-like peptidase family, involved in different physiological processes. In addition, cathepsins are implicated in many pathological conditions. This report describes the synthesis and evaluation of a series of N-arylanthranilic acids, acridones, and 4-quinolinones as inhibitors of cathepsins V and L. The kinetics revealed that compounds of the classes of acridones are reversible competitive inhibitors of the target enzyme with affinities in the low micromolar range. They represent promising lead candidates for the discovery of novel competitive cathepsin inhibitors with enhanced selectivity and potency. On the other hand, 4-quinolinones were noncompetitive inhibitors and N-arylanthranilic acids were uncompetitive inhibitors.

Lead optimization of 3-carboxyl-4(1H)-quinolones to deliver orally bioavailable antimalarials

Malaria is a protozoal parasitic disease that is widespread in tropical and subtropical regions of Africa, Asia, and the Americas and causes more than 800,000 deaths per year. The continuing emergence of multidrug-resistant Plasmodium falciparum drives the ongoing need for the development of new and effective antimalarial drugs. Our previous work has explored the preliminary structural optimization of 4(1H)-quinolone ester derivatives, a new series of antimalarials related to the endochins. Herein, we report the lead optimization of 4(1H)-quinolones with a focus on improving both antimalarial potency and bioavailability. These studies led to the development of orally efficacious antimalarials including quinolone analogue 20g, a promising candidate for further optimization.

Optimization of 1,2,3,4-tetrahydroacridin-9(10H)-ones as antimalarials utilizing structure-activity and structure-property relationships

Antimalarial activity of 1,2,3,4-tetrahydroacridin-9(10H)-ones (THAs) has been known since the 1940s and has garnered more attention with the development of the acridinedione floxacrine (1) in the 1970s and analogues thereof such as WR 243251 (2a) in the 1990s. These compounds failed just prior to clinical development because of suboptimal activity, poor solubility, and rapid induction of parasite resistance. Moreover, detailed structure-activity relationship (SAR) studies of the THA core scaffold were lacking and SPR studies were nonexistent. To improve upon initial findings, several series of 1,2,3,4-tetrahydroacridin-9(10H)-ones were synthesized and tested in a systematic fashion, examining each compound for antimalarial activity, solubility, and permeability. Furthermore, a select set of compounds was chosen for microsomal stability testing to identify physicochemical liabilities of the THA scaffold. Several potent compounds (EC(50) < 100 nM) were identified to be active against the clinically relevant isolates W2 and TM90-C2B while possessing good physicochemical properties and little to no cross-resistance.

Antimalarial activity of new acridinone derivatives

Malaria is one of the major threats concerning world public health. Resistance to the current antimalarial drugs has led to searches for new antimalarial compounds. Acridinone derivatives have recently demonstrated to be active against malaria parasite. We focused our attention on synthesized new acridinone derivatives, some of them resulting with high antiviral and trypanocidal activity. In this study new derivatives of 10-alyl-, 10-(3-methyl-2-butenyl)- and 10-(1,2-propadienyl)-9(10H)-acridinone were evaluated for their antimalarial activity against Plasmodium falciparum. To assess the selectivity, cytotoxicity was assessed in parallel against human MRC-5 cells. Inhibition of β-hematin formation was determined using a spectrophotometric assay. Mitochondrial bc(1) complexes were isolated from yeast and bovine heart cells to test acridinone inhibitory activity. This study resulted in the identification of three compounds with submicromolar efficacy against P. falciparum and without cytotoxic effects on human cellular line. One compound, IIa (1-fluoro-10-(3-methyl-2-butenyl)-9(10H)-acridinone), can be classified as hit for antimalarial drug development exhibiting IC(50) less than 0.2 μg/mL with SI greater than 100. In molecular tests, no relevant inhibitory activity was obtained for our compounds. The mechanism of acridinones antimalarial action remains unclear.

Synthesis of novel heme-interacting acridone derivatives to prevent free heme-mediated protein oxidation and degradation

Heme is an important prosthetic molecule for various hemoproteins and serves important function in living aerobic organisms. But degradation of hemoprotein, for example, hemoglobin during different pathological conditions leads to the release of heme, which is very toxic as it induces oxidative stress and inflammation due to its pro-oxidant nature. Thus, synthesis of compound that will detoxify free heme by interacting with it would be fruitful for the management of heme-induced pathogenesis. Here, we report the synthesis of a novel natural product arborinine and some other acridone derivatives, which interact with free heme. These acridones in vitro block heme-mediated protein oxidation and degradation, markers for heme-induced oxidative stress.

Acridine and acridinones: old and new structures with antimalarial activity

Since emergence of chloroquine-resistant Plasmodium falciparum and reports of parasite resistance to alternative drugs, there has been renewed interest in the antimalarial activity of acridines and their congeners, the acridinones. This article presents literature compilation of natural acridinone alkaloids and synthetic 9-substituted acridines, acridinediones, haloalcoxyacridinones and 10-N-substituted acridinones with antimalarial activity. The review also provides an outlook to antimalarial modes of action of some described compounds.

Optimization of endochin-like quinolones for antimalarial activity

Our prior work on tricyclic acridones combined with a desire to minimize the tricyclic system led to an interest in antimalarial quinolones and a reexamination of endochin, an experimental antimalarial from the 1940's. In the present article, we show that endochin is unstable in the presence of murine, rat, and human microsomes which may explain its relatively poor antimalarial activity in mammalian systems. We also profile the structure-activity relationships of ≈ 30 endochin-like quinolone (ELQ) analogs and highlight features that are associated with enhanced metabolic stability, potent antiplasmodial activity against multidrug resistant strains of Plasmodium falciparum, and equal activity against an atovaquone-resistant clinical isolate. Our work also features an ELQ construct containing a polyethylene glycol carbonate pro-moiety that is highly efficacious by oral administration in a murine malaria model. These findings provide compelling evidence that development of ELQ therapeutics is feasible.

Mitochondrial electron transport inhibition and viability of intraerythrocytic Plasmodium falciparum

Although mitochondrial electron transport is a validated target of the antimalarial drug atovaquone, the molecular details underlying parasite demise are unclear. We have shown that a critical function of mitochondrial electron transport in blood-stage Plasmodium falciparum is to support pyrimidine biosynthesis. Here, we explore the effects of atovaquone, alone and in combination with proguanil, on P. falciparum viability. Our results suggest that the effects of inhibition depend upon the erythrocytic stage of the parasites and the duration of exposure. Ring- and schizont-stage parasites are most resilient to drug treatment and can survive for 48 h, with a fraction remaining viable even after 96 h. Survival of parasites does not appear to require nutrient uptake. Thus, intraerythrocytic parasites with inhibited mitochondrial electron transport and collapsed mitochondrial membrane potential do not undergo apoptosis but enter an apparent static state. These results have significant implications for desirable properties of antimalarials under development that target mitochondrial functions.

Plant-derived antimalarial agents: new leads and efficient phythomedicines. Part I. Alkaloids

Malaria remains one of the most serious world health problem and the major cause of mortality and morbidity in the endemic regions. Brazil is among the 30 high-burden countries and most of the cases occur in the Legal Amazonian Region. New chemotherapeutical agents are needed for the treatment of malaria. Many plant species are used in traditional medicines of malarious countries and a relatively few number of these have been investigated for evaluation of their antimalarial effect. Still lower is the number of those that have had the active natural compounds isolated and the toxicity determined. This area is, then, of great research interest. discovery project of antimalarial natural products from plants traditionally used to treat malaria must include in vitro and in vivo assays as well as bioguided isolation of active compounds. The final products would be antimalarial chemical entities, potential new drugs or templates for new drugs development, and/or standardized antimalarial extracts which are required for pre-clinical and clinical studies when the aim is the development of effective and safe phythomedicines. This review discusses these two approaches, presents briefly the screening methodologies for evaluation of antimalarial activity and focuses the activity of alkaloids belonging to different structural classes as well as its importance as new antimalarial drugs or leads and chemical markers for phytomedicines.

Plant-derived antimalarial agents: new leads and efficient phytomedicines. Part II. Non-alkaloidal natural products

Malaria is still the most destructive and dangerous parasitic infection in many tropical and subtropical countries. The burden of this disease is getting worse, mainly due to the increasing resistance of Plasmodium falciparum against the widely available antimalarial drugs. There is an urgent need for new, more affordable and accessible antimalarial agents possessing original modes of action. Natural products have played a dominant role in the discovery of leads for the development of drugs to treat human diseases, and this fact anticipates that new antimalarial leads may certainly emerge from tropical plant sources. This present review covers most of the recently-published non-alkaloidal natural compounds from plants with antiplasmodial and antimalarial properties, belonging to the classes of terpenes, limonoids, flavonoids, chromones, xanthones, anthraquinones, miscellaneous and related compounds, besides the majority of papers describing antiplasmodial crude extracts published in the last five years not reviewed before. In addition, some perspectives and remarks on the development of new drugs and phytomedicines for malaria are succinctly discussed.

Probing binding determinants in center P of the cytochrome bc(1) complex using novel hydroxy-naphthoquinones

Atovaquone is a substituted 2-hydroxy-naphthoquinone used therapeutically against Plasmodium falciparum (malaria) and Pneumocystis pathogens. It acts by inhibiting the cytochrome bc(1) complex via interactions with the Rieske iron-sulfur protein and cytochrome b in the ubiquinol oxidation pocket. As the targeted pathogens have developed resistance to this drug there is an urgent need for new alternatives. To better understand the determinants of inhibitor binding in the ubiquinol oxidation pocket of the bc(1) complex we synthesized a series of hydroxy-naphthoquinones bearing a methyl group on the benzene ring that is predicted to interact with the nuclear encoded Rieske iron-sulfur protein. We have also attempted to overcome the metabolic instability of a potent cytochrome bc(1) complex inhibitor, a 2-hydroxy-naphthoquinone with a branched side chain, by fluorinating the terminal methyl group. We have tested these new 2-hydroxy-naphthoquinones against yeast and bovine cytochrome bc(1) complexes to model the interaction with pathogen and human enzymes and determine parameters that affect efficacy of binding of these inhibitors. We identified a hydroxy-naphthoquinone with a trifluoromethyl function that has potential for development as an anti-fungal and anti-parasitic therapeutic.

Recent advances in the discovery of haem-targeting drugs for malaria and schistosomiasis

Haem is believed to be the target of some of the historically most important antimalarial drugs, most notably chloroquine. This target is almost ideal as haem is host-derived and the process targeted, haemozoin formation, is a physico-chemical process with no equivalent in the host. The result is that the target remains viable despite resistance to current drugs, which arises from mutations in parasite membrane transport proteins. Recent advances in high-throughput screening methods, together with a better understanding of the interaction of existing drugs with this target, have created new prospects for discovering novel haem-targeting chemotypes and for target-based structural design of new drugs. Finally, the discovery that Schistosoma mansoni also produces haemozoin suggests that new drugs of this type may be chemotherapeutic not only for malaria, but also for schistosomiasis. These recent developments in the literature are reviewed.

Search for MDR modulators: design, syntheses and evaluations of N-substituted acridones for interactions with p-glycoprotein and Mg2+

By combining the structural features of acridone based anti-cancer drugs (like amsacrine) and MDR modulator propafenone, acridones with hydroxyl amine chain at N-10 have been designed and synthesized. These molecules exhibit appreciable interactions with p-gp and Mg(2+) indicating their suitability to modulate p-gp mediated multi drug resistance.