Recent advances in novel heterocyclic scaffolds for the treatment of drug-resistant malaria

Malaria is a major public health problem all over the world, particularly in tropical and subtropical countries due to the development of resistance and most deadly infection is caused by Plasmodium falciparum. There is a direct need for the discovery of new drugs with unique structures and mechanism of action to treat sensitive and drug-resistant strains of various plasmodia for radical cure of this disease. Traditional compounds such as quinine and related derivatives represent a major source for the development of new drugs. This review presents recent modifications of 4-aminoquinoline and 8-aminoquinolone rings as leads to novel active molecules which are under clinical trials. The review also encompasses the other heterocyclic compounds emerged as potential antimalarial agents with promising results such as acridinediones and acridinone analogues, pyridines and quinolones as antimalarials. Miscellaneous heterocyclics such as tetroxane derivatives, indole derivatives, imidazolopiperazine derivatives, biscationic choline-based compounds and polymer-linked combined antimalarial drugs are also discussed. At last brief introduction to heterocyclics in natural products is also reviewed. Most of them have been under clinical trials and found to be promising in the treatment of drug-resistant strains of Plasmodium and others can be explored for the same purpose.     

Review: Natural Antimalarial Agents (1995-2001)

Malaria is one of the diseases for which even today not many suitable drugs are available. The rapid spread of resistance toward current drugs encourages the study for new active molecules. Ethnobotanical research can be of help to find new leads. Traditional remedies have always been a source of important antimalarial drugs and continue to provide novel and effective treatments, both where pharmaceuticals are not available and also where the disease is highly resistant to commonly prescribed drugs. This article provides a comparative compilation of various studies reported between 1995 and 2001 on natural compounds with antiplasmodial activity, with the goal of providing a basis for further in vitro and in vivo studies, as well as for clinical trials for the development of new antimalarial medicines. Referee: Dr. Robert Verpoorte, Leiden/Amsterdam Center for Drug Research, Gorlaeus Laboratories, Leiden University, Einsteinweg 55, PO Box 9502, 2300 RA Leiden, The Netherlands     

Plant Derived Antimalarial Agents: New Leads and Challenges

New treatments for malaria are urgently needed due to the increasing problem of drug-resistance in malaria parasites. The long-established use of quinine and the more recent introduction of artemisinin and its derivatives as highly effective antimalarials demonstrates that plant species are an important resource for the discovery of new antimalarial agents. Furthermore, many plant species continue to be used in traditional medicines for the treatment of malaria and many people depend on such remedies as they cannot afford and/or do not have access to effective antimalarial drugs. In this paper the potential of plant species to yield new leads to antimalarial drugs will be illustrated with reference to cryptolepine, the main alkaloid present in the species, Cryptolepis sanguinolenta. In addition to this approach, there is currently increasing interest in the use and development of traditional herbal remedies for the treatment of malaria as these may have the potential to provide affordable antimalarial treatment for many who cannot afford the drugs needed to treat chloroquine-resistant Plasmodium falciparum infections. However, little is known with respect to the efficacy and safety of traditional antimalarials and clinical studies are urgently needed to establish their value. Some of the issues pertinent to this area will be briefly reviewed and it is hoped that this will stimulate further discussion and research on this important topic.     

Study of the antimalarial properties of hydroxyethylamine derivatives using green fluorescent protein transformed Plasmodium berghei

A rapid decrease in parasitaemia remains the major goal for new antimalarial drugs and thus, in vivo models must provide precise results concerning parasitaemia modulation. Hydroxyethylamine comprise an important group of alkanolamine compounds that exhibit pharmacological properties as proteases inhibitors that has already been proposed as a new class of antimalarial drugs. Herein, it was tested the antimalarial property of new nine different hydroxyethylamine derivatives using the green fluorescent protein (GFP)-expressing Plasmodium berghei strain. By comparing flow cytometry and microscopic analysis to evaluate parasitaemia recrudescence, it was observed that flow cytometry was a more sensitive methodology. The nine hydroxyethylamine derivatives were obtained by inserting one of the following radical in the para position: H, 4Cl, 4-Br, 4-F, 4-CH3, 4-OCH3, 4-NO2, 4-NH2 and 3-Br. The antimalarial test showed that the compound that received the methyl group (4-CH3) inhibited 70% of parasite growth. Our results suggest that GFP-transfected P. berghei is a useful tool to study the recrudescence of novel antimalarial drugs through parasitaemia examination by flow cytometry. Furthermore, it was demonstrated that the insertion of a methyl group at the para position of the sulfonamide ring appears to be critical for the antimalarial activity of this class of compounds.     

Can estimates of antimalarial efficacy from field studies be improved?

Monitoring therapeutic efficacy of antimalarial drugs is important because treatment failure rates are the primary basis for changing antimalarial treatment policy. An important aspect of efficacy studies is the use of PCR genotyping to distinguish recrudescent from new infections. The conclusions reached using this technique might be misleading if there is insufficient parasite diversity or a non-uniform haplotype frequency distribution in the study area. Statistical techniques can be used to overcome this problem, but only when data describing the haplotype frequency distribution are available. Therefore, assessing haplotype frequency and distribution should form an integral part of all studies investigating the therapeutic efficacy of antimalarial treatment regimes.     

High-content live cell imaging with RNA probes: advancements in high-throughput antimalarial drug discovery

Background  

Malaria, a major public health issue in developing nations, is responsible for more than one million deaths a year. The most lethal species, Plasmodium falciparum, causes up to 90% of fatalities. Drug resistant strains to common therapies have emerged worldwide and recent artemisinin-based combination therapy failures hasten the need for new antimalarial drugs. Discovering novel compounds to be used as antimalarials is expedited by the use of a high-throughput screen (HTS) to detect parasite growth and proliferation. Fluorescent dyes that bind to DNA have replaced expensive traditional radioisotope incorporation for HTS growth assays, but do not give additional information regarding the parasite stage affected by the drug and a better indication of the drug's mode of action. Live cell imaging with RNA dyes, which correlates with cell growth and proliferation, has been limited by the availability of successful commercial dyes.     

Metabolic engineering of artemisinin biosynthesis in Artemisia annua L.

Artemisinin, a sesquiterpene lactone isolated from the Chinese medicinal plant Artemisia annua L., is an effective antimalarial agent, especially for multi-drug resistant and cerebral malaria. To date, A. annua is still the only commercial source of artemisinin. The low concentration of artemisinin in A. annua, ranging from 0.01 to 0.8% of the plant dry weight, makes artemisinin relatively expensive and difficult to meet the demand of over 100 million courses of artemisinin-based combinational therapies per year. Since the chemical synthesis of artemisinin is not commercially feasible at present, another promising approach to reduce the price of artemisinin-based antimalarial drugs is metabolic engineering of the plant to obtain a higher content of artemisinin in transgenic plants. In the past decade, we have established an Agrobacterium-mediated transformation system of A. annua, and have successfully transferred a number of genes related to artemisinin biosynthesis into the plant. The various aspects of these efforts are discussed in this review.     

Combined antimalarial therapy using artemisinin

The existing armamentarium of drugs for the treatment and prevention of malaria is limited primarily by resistance (and cross-resistance between closely related drugs). However, most of these drugs still have a place and their life-span could be prolonged if better deployed and used, and also by rationally combining them based on pharmacodynamic and pharmacokinetic properties. Newer compounds are also being developed. The nature of malaria disease and its prevalence in the developing world call for innovative approaches to develop new affordable drugs and to safeguard the available ones. According to WHO, the concept of combination therapy is based on the synergistic or additive potential of two or more drugs, to improve therapeutic efficacy and also delay the development of resistance to the individual components of the combination. Combination therapy (CT) with antimalarial drugs is the simultaneous use of two or more blood schizontocidal drugs with independent modes of action and different biochemical targets in the parasite. In the context of this definition, multiple-drug therapies that include a nonantimalarial drug to enhance the antimalarial effect of a blood schizontocidal drug are not considered combination therapy. Similarly, certain antimalarial drugs that fit the criteria of synergistic fixed-dose combinations are operationally considered as single products in that neither of the individual components would be given alone for anti-malarial therapy. An example is sulfadoxine-pyrimethamine. Artemisinin-based combination therapies have been shown to improve treatment efficacy and also contain drug resistance in South-East Asia. However, major challenges exist in the deployment and use of antimalarial drug combination therapies, particularly in Africa. These include: 1) the choice of drug combinations best suited for the different epidemiological situations; 2) the cost of combination therapy; 3) the timing of the introduction of combination therapy; 4) the operational obstacles to implementation, especially compliance. As a response to increasing levels of antimalarial resistance, the World Health Organization (WHO) recommends that all countries experiencing resistance to conventional monotherapies, such as chloroquine, amodiaquine or sulfadoxine/pyrimethamine, should use combination therapies, preferably those containing artemisinin derivatives (ACTs--artemisinin-based combination therapies) for malaria caused by Plasmodium falciparum. There is a promising role of such compounds in replacing or complementing current options. Since 1979, several different formulations of artemisinin and its derivatives have been produced and studied in China in several thousand patients for either P. falciparum or P. vivax malaria. To date, there is no evidence of drug resistance to these compounds. The use of artemisinin, artemether, arteether and artesunate for either uncomplicated or severe malaria is now spreading through almost all malarious areas of the world, although some of they have no patent protection, their development (with few exceptions) has not followed yet full international standards. Both artesunate, artemether and arteether are rapidly and extensively converted to their common bioactive metabolite, dihydroarte-misinin. WHO currently recommends the following therapeutic options: 1) artemether/lumefantrine; 2) artesunate plus amodiaquine; 3) artesunate plus sulfadoxine/pyrimethamine (in areas where SP efficacy remains high); 4) artesunate plus mefloquine (in areas with low to moderate transmission); and 5) amodiaquine plus sulfadoxine/pyrimethamine, in areas where efficacy of both amodiaquine and sulfadoxine/pyrimethamine remains high (mainly limited to countries in West Africa). This non artemisinin-based combination therapy is reserved as an interim option for countries, which, for whatever reason, are unable immediately to move to ACTs.     

Structural studies of vacuolar plasmepsins

Plasmepsins (PMs) are pepsin-like aspartic proteases present in different species of parasite Plasmodium. Four Plasmodium spp. (P. vivax, P. ovale, P. malariae, and the most lethal P. falciparum) are mainly responsible for causing human malaria that affects millions worldwide. Due to the complexity and rate of parasite mutation coupled with regional variations, and the emergence of P. falciparum strains which are resistant to antimalarial agents such as chloroquine and sulfadoxine/pyrimethamine, there is constant pressure to find new and lasting chemotherapeutic drug therapies. Since many proteases represent therapeutic targets and PMs have been shown to play an important role in the survival of parasite, these enzymes have recently been identified as promising targets for the development of novel antimalarial drugs. The genome of P. falciparum encodes 10 PMs (PMI, PMII, PMIV-X and histo-aspartic protease (HAP)), 4 of which (PMI, PMII, PMIV and HAP) reside within the food vacuole, are directly involved in degradation of human hemoglobin, and share 50-79% amino acid sequence identity. This review focuses on structural studies of only these four enzymes, including their orthologs in other Plasmodium spp.. Almost all original crystallographic studies were performed with PMII, but more recent work on PMIV, PMI, and HAP resulted in a more complete picture of the structure-function relationship of vacuolar PMs. Many structures of inhibitor complexes of vacuolar plasmepsins, as well as their zymogens, have been reported in the last 15 years. Information gained by such studies will be helpful for the development of better inhibitors that could become a new class of potent antimalarial drugs. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.     

座壳孢及其有性型（子囊菌门）代谢产物研究进展

座壳孢及其有性型是一类重要的虫生真菌,隶属子囊菌门、粪菌纲、肉座菌目、麦角菌科,能寄生粉虱和蚧类昆虫,可开发成一种环境友好型的生物农药。座壳孢及其有性型属成员广泛分布于热带和亚热带地区,该类群物种、栖息环境和生态适应的多样性使它们的代谢产物化学结构及其生物活性彰显出多样性特点,结构和功能多样的真菌代谢产物已成为发现新药先导化合物的重要资源。虫生真菌因其独特的生活方式和多种生存环境形成了与众不同的适应寄主特性及代谢通路,明显提高获得新颖结构、显著活性的代谢产物的几率,目前已是药物重点研发的领域。在座壳孢及其有性型代谢产物研究中已报道了萜类、黄酮类、醌类、环肽和甾醇等多种化合物,并具有抗肿瘤、抗疟疾、抗菌和杀虫等多种生物活性,可在农业、工业和医药保健等方面应用。本文对近年座壳孢及其有性型代谢产物的化学成分和生物活性等方面的研究进展进行概述,为促进座壳孢及其有性型代谢产物的深入研究、开发利用和新药创制提供参考。     

Current opinion on an emergence of drug resistant strains of Plasmodium falciparum through genetic alterations

The human malarial parasite Plasmodium falciparum is one of the world''s most devastating pathogen. Its capability to regulate its genes under various stages of its life cycle as well as under unfavourable environmental conditions has led to the development of vaccine resistant strains. Similarly, under drug pressure it develops mutations in the target genes. These mutations confer mid and high-level resistance to the antimalarial drugs. Increasing a resistance of malaria parasites to conventional antimalarial drugs is an important factor contributing to the persistence of the disease as a major health threat. This article reviews current knowledge of stage specific malarial targets, antimalarial drugs and the mutations that have led to the emergence of resistant strains.     

Antimalarial activity of endoperoxide compound 6-(1,2,6,7-tetraoxaspiro[7.11]nonadec-4-yl)hexan-1-ol

Plasmodium falciparum, the major causative parasite for the disease, has acquired resistance to most of the antimalarial drugs used today, presenting an immediate need for new antimalarial drugs. Here, we report the in vitro and in vivo antimalarial activities of 6-(1,2,6,7-tetraoxaspiro[7.11]nonadec-4-yl)hexan-1-ol (N-251) against P. falciparum and Plasmodium berghei parasites. The N-251 showed high antimalarial potencies both in the in vitro and the in vivo tests (EC₅₀ 2.3 × 10⁻⁸ M; ED₅₀ 15 mg/kg (per oral)). The potencies were similar to that of artemisinin in vitro and greater than artemisinin's activity in vivo (p.o.). In addition, N-251 has little toxicity: a single oral administration at 2000 mg/kg to a rat gave no health problems to it. Administration of N-251 to mice bearing 1% of parasitemia (per oral 68 mg/kg, 3 times a day for 3 consecutive days) resulted in a dramatic decrease in the parasitemia: all the 5 mice given N-251 were cured without any recurrence, with no diarrhea or weight loss occurring in the 60 days of experiment. N-251 deserves more extensive clinical evaluation, desirably including future trials in the human.     

Multiple e-pharmacophore modelling pooled with high-throughput virtual screening,docking and molecular dynamics simulations to discover potential inhibitors of Plasmodium falciparum lactate dehydrogenase (PfLDH)

Development of new antimalarial drugs continues to be of huge importance because of the resistance of malarial parasite towards currently used drugs. Due to the reliance of parasite on glycolysis for energy generation, glycolytic enzymes have played important role as potential targets for the development of new drugs. Plasmodium falciparum lactate dehydrogenase (PfLDH) is a key enzyme for energy generation of malarial parasites and is considered to be a potential antimalarial target. Presently, there are nearly 15 crystal structures bound with inhibitors and substrate that are available in the protein data bank (PDB). In the present work, we attempted to consider multiple crystal structures with bound inhibitors showing affinity in the range of 1.4 × 10²–1.3 × 10⁶ nM efficacy and optimized the pharmacophore based on the energy involved in binding termed as e-pharmacophore mapping. A high throughput virtual screening (HTVS) combined with molecular docking, ADME predictions and molecular dynamics simulation led to the identification of 20 potential compounds which could be further developed as novel inhibitors for PfLDH.     

Double-drug development against antioxidant enzymes from Plasmodium falciparum

Abstract

New drugs against malaria are urgently and continuously needed. Plasmodium parasites are exposed to higher fluxes of reactive oxygen species and need high activities of intracellular antioxidant systems. A most important antioxidative system consists of (di)thiols which are recycled by disulfide reductases (DR), namely both glutathione reductases (GR) of the malarial parasite Plasmodium falciparum and man, and the thioredoxin reductase (TrxR) of P. falciparum. The aim of our interdisciplinary research is to substantiate DR inhibitors as antimalarial agents. Such compounds are active per se but, in addition, they can reverse thiol-based resistance against other drugs in parasites. Reversal of drug resistance by DR inhibitors is currently investigated for the commonly used antimalarial drug chloroquine (CQ). Our recent strategy is based on the synthesis of inhibitors of the glutathione reductases from parasite and host erythrocyte. With the expectation of a synergistic or additive effect, double-headed prodrugs were designed to be directed against two different and essential functions of the malarial parasite P. falciparum, namely glutathione regeneration and heme detoxification. The prodrugs were prepared by linking bioreversibly a GR inhibitor to a 4-aminoquinoline moiety which is known to concentrate in the acidic food vacuole of parasites. Drug-enzyme interaction was correlated with antiparasitic action in vitro on strains resistant towards CQ and in vivo in Plasmodium berghei-infected mice as well as absence of cytotoxicity towards human cells. Because TrxR of P. falciparum was recently shown to be responsible for the residual glutathione disulfide-reducing capacity observed after GR inhibition in P. falciparum, future development of antimalarial drug-candidates that act by perturbing the redox equilibrium of parasites is based on the design of new double-drugs based on TrxR inhibitors as potential antimalarial drug candidates.     

Artemisinin-Ginkgo biloba extract combination therapy for Plasmodium yoelii

Malaria remains a widespread life-threatening infectious disease, leading to an estimated 219 million cases and around 435,000 deaths. After an unprecedented success, the antimalarial progress is at a standstill. Therefore, new methods are urgently needed to decrease drug resistant and enhance antimalarial efficacy. According to the alteration of erythrocyte biomechanical properties and the immune evasion mechanism of parasites, drugs, which can improve blood circulation, can be chosen to combine with antimalarial drugs for malaria treatment. Ginkgo biloba extract (GBE), one of drug for vascular disease, was used to combine with artemisinin for Plasmodium yoelii therapy. Artemisinin-GBE combination therapy (AGCT) demonstrated remarkable antimalarial efficacy by decreasing infection rate, improving blood microcirculation and modulating immune system. Besides, the expression of invasion related genes, such as AMA1, MSP1 and Py01365, can be suppressed by AGCT, hindering invasion process of merozoites. This new antimalarial strategy, combining antimalarial drugs with drugs that improve blood circulation, may enhance the antimalarial efficacy and ameliorate restoration ability, proving a potential method for finding ideal compatible drugs to improve malaria therapy     

Antimalarials from nature

Malaria is a major public health problem mainly due to the development of resistance by the most lethal causative parasitic species, Plasmodium falciparum to the mainstay drugs like chloroquine. New drugs with unique structures and mechanism of action are urgently required to treat sensitive and drug-resistant strains of malaria. Historically, compounds containing novel structure from natural origin represent a major source for the discovery and development of new drugs for several diseases. This review presents recent advances in antimalarial drug discovery from natural sources, including plant extracts, and compounds isolated from plants, bacteria, fungi and marine organisms. These compounds offer new and novel scaffolds for development as antimalarials. The literature from 1998 to October 2008 is reviewed. The review present literature compilation from plant and marine extracts, alkaloids (naphthylisoquinolines, bisbenzylisoquinolines, protoberberines and aporphines, indoles, manzamines, and miscellaneous alkaloids) terpenes (sesquiterpenes, triterpenes, diterpenes, and miscellaneous terpenes) quassinoids, flavonoids, limonoids, chalcones, peptides, xanthones, quinones and coumarines, and miscellaneous antimalarials from nature. The review also provides an outlook to recent semisynthetic approaches to antimalarial drugs discovered from natural sources.     

Cytokine polymorphisms influence treatment outcomes in SLE patients treated with antimalarial drugs

Antimalarial agents have been widely used as disease-modifying antirheumatic drugs in the treatment of systemic lupus erythematosus (SLE) and other rheumatological diseases, although their mechanism of action has not yet been fully defined. It is known, however, that effective response to treatment is variable among patients. Thus, the identification of genetic predictors of treatment response would provide valuable information for therapeutic intervention. The aim of the present study was to analyze the effect of antimalarial treatment on tumor necrosis factor (TNF)α serum levels and evaluate the possible influence of TNFα and IL-10 functional genetic polymorphisms on the response to antimalarial drugs. To this end, TNFα serum levels were quantified in 171 SLE patients and 215 healthy controls by ELISA techniques and polymorphisms at positions -1,082 and -308 of the IL-10 and TNFα gene promoterswere determined by PCR amplification followed by hybridization with fluorescent-labeled allele-specific probes in 192 SLE patients and 343 matched controls. Data were related to clinical features and treatment at the time of sampling and during the course of the disease. Results showed a significantly higher amount of serum TNFα in the entire SLE population compared with controls. However, TNFα serum levels correlated negatively with the use of antimalarial treatment during at least three months before sampling. Patients under single or combined treatment with these drugs had TNFα serum levels similar to healthy controls, whereas untreated patients and those under corticosteroid or immunosuppressive therapies had increased amounts of this cytokine. This suggests, however, that antimalarial-mediated inhibition of TNFα was only significant in patients who were genetically high TNFα or low IL-10 producers. In addition, evaluation of SLE patients administered antimalarial drugs for three or more years who did not require any other specific SLE treatment indicates that patients with the combined genotype low IL-10/high TNFα are the best responders to antimalarial therapy, developing mild disease with a good course under this treatment. In conclusion, we proposed that an antimalarial-mediated downregulation of TNFα levels in SLE patients is influenced by polymorphisms at IL-10 and TNFα promoters. Our results may thus find important clinical application through the identification of patients who are the most likely to benefit from antimalarial therapy.     

Plasmodium yoelii: Correlation of TEP1 with mosquito melanization induced by nitroquine

The antimalarial drug nitroquine is not only an effective antimalarial drug, it is also able to induce the melanization of Plasmodium species. However, the molecular mechanisms of the recognition reaction induced by this drug remain unclear. Silencing of thioester-containing protein-1 (TEP1) significantly compromised the ability of Anopheles gambiae to melanize the Plasmodium, leading to investigation of the involvement of A. stephensi TEP1 in melanization induced by nitroquine. This study shows that (1) binding of AsTEP1 to oocysts, especially melanized oocysts, (2) after ingestion of anti-AsTEP1 antibody, the melanization rate in antibody-treated mosquitoes are significantly lower than in control mosquito (p < 0.05). The results suggest that nitroquine is able to induce Plasmodium recognition by TEP1, possibly triggering the resulting melanotic encapsulation. Further elucidation of the molecular mechanisms of mosquito immunity induced by antimalarial drugs will provide theoretical evidence for the use of antimalarial drugs, and a meaningful pathway for the design of novel antimalarial drugs.     

Microbial transformation of antimalarial terpenoids

The fungal and bacterial transformation of terpenoids derived from plant essential oils, especially the sesquiterpenoid artemisinin from Artemisia annua, has produced several new candidate drugs for the treatment of malaria. Obtaining new derivatives of terpenoids, including artemisinin derivatives with increased antimalarial activity, is an important goal of research in microbial biotechnology and medicinal chemistry.