Atovaquone resistance in malaria parasites

Atovaquone is a broad-spectrum antiparasitic agent active against malaria, Pneumocystis carinii pneumonia, toxoplasmosis and babesiosis. When used as a single agent, resistance to atovaquone arose rapidly in falciparum malaria, requiring the development of a new antimalarial drug combination of atovaquone and proguanil. Recent laboratory investigations have provided insights into the mode of atovaquone action, and identified the molecular basis for the resistance development. Mutations within a catalytic domain of the cytochrome bc₁complex present within the parasite mitochondrial inner membrane were shown to be responsible for atovaquone resistance. Here, we review these studies and propose a mechanism by which atovaquone resistance may arise quickly in malaria parasites.     

The pathological bases of immunomodulatory therapy in malaria

ObjectivesThe objectives of this review are an attempt to analyse the potential bases for immunomodulary therapy in malaria. Malaria is a major cause of suffering and death and one of the most important problems of public health in vast areas of the world, mainly due to the severe forms of Plasmodium falciparum infection.EpidemiologyAccording to the World Health Organization, it is currently estimated that a total of 350 to 500 million clinical cases occur annually, which cause something between 1.1 and 1.3 million deaths every year.PathogenesisThe hyperactivation of the immune system, with enhanced production of cytokines, mainly TNF, has an important role in the complex pathogenesis of the disease. Enhanced sequestration of parasitized erythrocytes within the small vessels of major organs is a central feature of P. falciparum infection. The increased production of pro-inflammatory cytokines and nitric oxide, followed by the up-regulation of endothelial cell adhesion molecules, influences the progression of cerebral lesions.ImmunomodulationTherefore, different approaches have been attempted to downmodulate the hyperactive immune system in malaria, including the administration of cytokines or anti-cytokine antibodies, antibodies against molecules of adherence, drugs that reduce the excessive synthesis of reactive oxygen species, and drugs with pleiotropic action on the immune system. The impact of these immunomodulatory therapies on the course of malaria has reached variable success.ApplicationWe submitted this subject to a critical assessment and it was proposed ways to take advantage of immunomodulatory drugs, associated to anti-parasite therapy, to reduce the morbimortality of malaria.     

Wheat germ cell-free technology for accelerating the malaria vaccine research

Background: Malaria causes about 300 million illnesses and 1 million deaths annually. The likeliest scenario is the aggravation of this disease due to the re-emergence of drug-resistant parasites and insecticide-resistant mosquitoes. One of the promising solutions to this disease are vaccines. However, until now, not even a single licensed malaria vaccine has been developed despite intensive efforts. Even the efficacy of RTS,S, the most advanced vaccine candidate in the pipeline of malaria vaccine development, is only around 50%. Objective: Against this backdrop, there is an urgency to rapidly enrich the pipeline of vaccine development with novel vaccine candidates that can be discovered by synthesizing and screening a multitude of malaria proteins. Methods: However, to achieve this objective, we require optimal technologies for high-throughput synthesis of quality malaria proteins. Among the various protein synthesis systems, the wheat germ cell-free protein synthesis system is advantageous and successful to this end. Results/conclusion: The wheat germ cell-free protein synthesis system is optimal for accelerating the decoding of malaria genome and hence characterization of malaria proteins and discovery of malaria vaccine candidates.     

The mechanism of resistance to sulfa drugs in Plasmodium falciparum

The sulfonamide and sulfone (sulfa) group of antimalarials has been used extensively throughout malaria endemic regions of the world to control this important infectious disease of humans. Sulfadoxine is the most extensively used drug of this group of drugs and is usually combined with pyrimethamine (Fansidar), particularly for the control of Plasmodium falciparum, the causative agent of the most lethal form of malaria. Resistance to the sulfadoxine/pyrimethamine combination is widespread. Analysis using molecular, genetic and biochemical approaches has shown that the mechanism of resistance to sulfadoxine involves mutation of dihydropteroate synthase, the enzyme target of this group of drugs. Understanding the mechanism of resistance of P. falciparum to sulfa drugs has allowed detailed analysis of the epidemiology of the spread of drug resistance alleles in the field¹and, in the future, opens the way to the development of novel antimalarials to this target enzyme.     

In vitro interactions between antiretroviral protease inhibitors and artemisinin endoperoxides against Plasmodium falciparum

Antiretroviral protease inhibitors (APIs), which are effective at controlling the effects of human immunodeficiency virus (HIV) in patients, have also proven efficacious in inhibiting Plasmodium falciparum growth in vitro. Use of artemisinin-based combination therapies is being encouraged to reduce malaria mortality in areas of P. falciparum resistance to conventional antimalarial drugs. The aim of this study was to investigate drug interactions between HIV protease inhibitors and artemisinin drugs against malaria. In vitro cultures of P. falciparum provide a screen system for identifying and evaluating drug combinations. The derived isobolograms provide clear evidence of antagonistic interactions between artemisinin endoperoxides and several different APIs.     

Current knowledge and challenges of antimalarial drugs for treatment and prevention in pregnancy

Importance of the field: Malaria infection during pregnancy is a major public health problem worldwide, with 50 million pregnancies exposed to the infection every year. Approximately 25,000 maternal deaths and between 75,000 and 200,000 infant deaths could be prevented each year by effective malaria control in pregnancy. Antimalarial drug treatment and prevention has been hampered by the appearance of drug resistance, which has been a particular problem in pregnancy due to the inherent safety issues.

Areas covered in this review: New antimalarial drugs and combinations are being studied but there is not yet sufficient information on their efficacy or, more importantly, on their safety in pregnancy. This article provides an overview of the relevance of the topic and reviews the current antimalarial drugs recommended for pregnancy, as well as the guidelines for both treatment and prevention in women living in endemic areas and for travellers.

What the reader will gain: Updated information on the drugs currently used for malaria treatment and prevention in pregnancy, including new drugs under development, is provided. The gaps on efficacy and safety information for use during pregnancy are also discussed.

Take home message: Prevention and case management of malaria during pregnancy is based on risk–benefit criteria and poses one of the greatest challenges to current malaria control.     

The role of asymptomatic P. falciparum parasitaemia in the evolution of antimalarial drug resistance in areas of seasonal transmission

In areas with seasonal transmission, proper management of acute malaria cases that arise in the transmission season can markedly reduce the disease burden. However, asymptomatic carriage of Plasmodium falciparum sustains a long-lasting reservoir in the transmission-free dry season that seeds cyclical malaria outbreaks. Clinical trials targeting asymptomatic parasitaemia in the dry season failed to interrupt the malaria epidemics that follow annual rains. These asymptomatic infections tend to carry multiple-clones, capable of producing gametocytes and infecting Anopheles mosquitoes. Different clones within an infection fluctuate consistently, indicative of interaction between clones during the long course of asymptomatic carriage. However, the therapy-free environment that prevails in the dry season dis-advantages the drug resistant lineages and favors the wild-type parasites. This review highlights some biological and epidemiological characteristics of asymptomatic parasitaemia and calls for consideration of polices to diminish parasite exposure to drugs “therapy-free” and allow natural selection to curb drug resistance in the above setting.     

Novel targets for malaria therapy

Malaria has emerged as one of the most debilitating parasitic infection with about 500 million cases reported annually and one million deaths worldwide. Currently, Plasmodium falciparum has developed resistance to almost all classes of antimalarials, thus precluding the use of those agents which once formed the cornerstone of malaria therapy. In lieu of this phenomenon, and taking into consideration the absence of an effective vaccine for malaria, the only way to combat the deadly parasite is to enrich the antimalarial cache with new molecules acting on fresh targets in the parasite. After potential targets have been validated, these targets can be used as basis for screening compounds to identify new leads followed by lead optimization. This review discusses novel targets of the malaria parasite that can be utilized to treat the disease.     

Recent advances in malaria drug discovery

Malaria is responsible for over 300 million clinical cases annually and claims the lives of approximately 1-2 million. With a disease that has plagued humanity throughout history, one would think that better control measures would be in place to decrease the mortality and morbidity associated with malaria. Due to malaria drug resistance, an increase in the number of clinical infections and deaths is soon likely to be observed. Therefore, there is a push to identify and introduce new drug entities for malaria treatment and prophylaxis. In an effort to develop new malaria drugs, several different approaches have been implemented. These include the use of drug combinations of either new or existing antimalarials, exploitation of natural products, identification of resistance reversal or sensitizing agents and the targeting of specific malarial enzymes. Past experience has shown that introduction of the same chemical entities, such as quinolines and antifolates, results in only limited efficacy with resistance developing rapidly within one year of introduction. New approaches to drug discovery should identify novel chemotypes which circumvent the parasite's disposition to drug resistance. This review summarizes current efforts in malaria drug discovery as uncovered in recent patent literature.     

Malaria: an update on current chemotherapy

Introduction: Chemotherapy of malaria has become a rapidly changing field. Less than two decades ago, treatment regimens were increasingly bound to fail due to emerging drug resistance against 4-aminoquinolines and sulfa compounds. By now, artemisinin-based combination therapies (ACTs) constitute the standard of care for uncomplicated falciparum malaria and are increasingly also taken into consideration for the treatment of non-falciparum malaria.

Areas covered: This narrative review provides an overview of the state-of-art antimalarial drug therapy, highlights the global portfolio of current Phase III/IV clinical trials and summarizes current developments.

Expert opinion: Malaria chemotherapy remains a dynamic field, with novel drugs and drug combinations continuing to emerge in order to outpace the development of large-scale drug resistance against the currently most important drug class, the artemisinin derivatives. More randomized controlled studies are urgently needed especially for the treatment of malaria in first trimester pregnant women. ACTs should be used for the treatment of imported malaria more consequently. Gaining sufficient efficacy and safety information on ACT use for non-falciparum species including Plasmodium ovale and malariae should be a research priority. Continuous investment into malaria drug development is a vital factor to combat artemisinin resistance and successfully improve malaria control toward the ultimate goal of elimination.     

The Biological Control of the Malaria Vector

The call for malaria control, over the last century, marked a new epoch in the history of this disease. Many control strategies targeting either the Plasmodium parasite or the Anopheles vector were shown to be effective. Yet, the emergence of drug resistant parasites and insecticide resistant mosquito strains, along with numerous health, environmental, and ecological side effects of many chemical agents, highlighted the need to develop alternative tools that either complement or substitute conventional malaria control approaches. The use of biological means is considered a fundamental part of the recently launched malaria eradication program and has so far shown promising results, although this approach is still in its infancy. This review presents an overview of the most promising biological control tools for malaria eradication, namely fungi, bacteria, larvivorous fish, parasites, viruses and nematodes.     

Functional genomic technologies applied to the control of the human malaria parasite, Plasmodium falciparum

Infection with any of the four species of Plasmodium single cell parasites that infects humans causes the clinical disease, malaria. Of these, it is Plasmodium falciparum that is responsible for the majority of the 1.5-2.3 million deaths due to this disease each year. Worldwide there are between 300-500 million cases of malaria annually. To date there is no licensed vaccine and resistance to most of the available drugs used to prevent and/or treat malaria is spreading. There is therefore an urgent need to develop new and effective drugs and vaccines against this devastating parasite. We have outlined a strategy using a combination of DNA-based vaccines and the data derived from the soon-to-be completed P. falciparum genome and the genomes of other species of Plasmodium to develop new vaccines against malaria. Much of the technology that we are developing for vaccine target identification is directly applicable to the identification of potential targets for drug discovery. The publicly available genome sequence data also provides a means for researchers whose focus may not be primarily malaria to leverage their research on cancer, yeast biology and other research areas to the biological problems of malaria.     

Insights into the preclinical treatment of blood-stage malaria by the antibiotic borrelidin

Background and Purpose

Blood-stage Plasmodium parasites cause morbidity and mortality from malaria. Parasite resistance to drugs makes development of new chemotherapies an urgency. Aminoacyl-tRNA synthetases have been validated as antimalarial drug targets. We explored long-term effects of borrelidin and mupirocin in lethal P. yoelii murine malaria.

Experimental Approach

Long-term (up to 340 days) immunological responses to borrelidin or mupirocin were measured after an initial 4 day suppressive test. Prophylaxis and cure were evaluated and the inhibitory effect on the parasites analysed.

Key Results

Borrelidin protected against lethal malaria at 0.25 mg·kg⁻¹·day⁻¹. Antimalarial activity of borrelidin correlated with accumulation of trophozoites in peripheral blood. All infected mice treated with borrelidin survived and subsequently developed immunity protecting them from re-infection on further challenges, 75 and 340 days after the initial infection. This long-term immunity in borrelidin-treated mice resulted in negligible parasitaemia after re-infections and marked increases in total serum levels of antiparasite IgGs with augmented avidity. Long-term memory IgGs mainly reacted against high and low molecular weight parasite antigens. Immunofluorescence microscopy showed that circulating IgGs bound predominantly to late intracellular stage parasites, mainly schizonts.

Conclusions and Implications

Low borrelidin doses protected mice from lethal malaria infections and induced protective immune responses after treatment. Development of combination therapies with borrelidin and selective modifications of the borrelidin molecule to specifically inhibit plasmodial threonyl tRNA synthetase should improve therapeutic strategies for malaria.     

Intermittent preventive treatment of malaria in pregnant women and infants: making best use of the available evidence

Introduction: Malaria continues to represent a huge global health burden on the most vulnerable populations. The Intermittent Preventive Treatment (IPT) strategy has been shown to be an efficacious intervention in preventing most of the deleterious effects of malaria in pregnant women and infants. Yet, the effectiveness of the IPT strategy may be impaired by the increasing resistance to sulfadoxine-pyrimethamine (SP), and the scarcity of alternative antimalarial drugs.

Areas covered: This review examines all the available information on IPT, in an aim to provide the scientific community with a framework to understand the benefits and limitations of this malaria control strategy. It includes the understanding of the historical background of the IPT strategy, the drug's mechanisms of actions, updated information on current available evidence, the implications of drug resistance and choice of alternative drugs, and a comprehensive discussion on the perspectives of IPT for malaria control in pregnant women and infants.

Expert opinion: IPT in pregnancy and infants is a cost-effective strategy that can contribute significantly to the control of malaria in endemic areas. Monitoring its effectiveness will allow tracking of progress, evaluation of the adequacy of currently used drugs and will highlight the eventual need for new therapies or alternative interventions.     

Mutational ‘hot-spots’ in mammalian,bacterial and protozoal dihydrofolate reductases associated with antifolate resistance: Sequence and structural comparison

Human dihydrofolate reductase (DHFR) is a primary target for antifolate drugs in cancer treatment, while DHFRs from Plasmodium falciparum, Plasmodium vivax and various bacterial species are primary targets in the treatment of malaria and bacterial infections. Mutations in each of these DHFRs can result in resistance towards clinically relevant antifolates. We review the structural and functional impact of active-site mutations with respect to enzyme activity and antifolate resistance of DHFRs from mammals, protozoa and bacteria. The high structural homology between DHFRs results in a number of cross-species, active-site ‘hot-spots’ for broad-based antifolate resistance. In addition, we identify mutations that confer species-specific resistance, or antifolate-specific resistance. This comparative review of antifolate binding in diverse species provides new insights into the relationship between antifolate design and the development of mutational resistance. It also presents avenues for designing antifolate-resistant mammalian DHFRs as chemoprotective agents.     

Prospects of intermittent preventive treatment of adults against malaria in areas of seasonal and unstable malaria transmission,and a possible role for chloroquine

Chloroquine (CQ) is outmoded as an antimalarial drug in most of the malarial world because of the high resistance rate of parasites. The parasite resistance to CQ is attributed to pfcrt/pfmdr1 gene mutations. Recent studies showed that parasites with mutations of pfcrt/pfmdr1 genes are less virulent, and that those with dhfr/dhps mutations are more susceptible to host immune clearance; the former and latter mutations are linked. In the era of artemisinin-based combination therapy, the frequency of pfcrt/pfmdr1 wild variants is expected to rise. In areas of unstable malaria transmission, the unpredictable severe epidemics of malaria and epidemics of severe malaria could result in high mortality rate among the semi-immune population. With this in mind, the use of CQ for intermittent preventive treatment of adults (IPTa) is suggested as a feasible control measure to reduce malaria mortality in adults and older children without reducing uncomplicated malaria morbidity. The above is discussed in a multidisciplinary approach validating the deployment of molecular techniques in malaria control and showing a possible role for CQ as a rescue drug after being abandoned.     

Pharmacodynamics of antimalarial chemotherapy

Malaria chemotherapy is under constant threat from the emergence and spread of multidrug resistance of Plasmodium falciparum. Resistance has been observed to almost all currently used antimalarials. Some drugs are also limited by toxicity. A fundamental component of the strategy for malaria chemotherapy is based on prompt, effective and safe antimalarial drugs. To counter the threat of resistance of P. falciparum to existing monotherapeutic regimens, current malaria treatment is based principally on the artemisinin group of compounds, either as monotherapy or artemisinin-based combination therapies for treatment of both uncomplicated and severe falciparum malaria. Key advantages of artemisinins over the conventional antimalarials include their rapid and potent action, with good tolerability profiles. Their action also covers transmissible gametocytes, resulting in decreased disease transmission. Up to now there has been no prominent report of drug resistance to this group of compounds. Treatment of malaria in pregnant women requires special attention in light of limited treatment options caused by potential teratogenicity coupled with a paucity of safety data for the mother and fetus. Treatment of other malaria species is less problematic and chloroquine is still the drug of choice, although resistance of P. vivax to chloroquine has been reported. Multiple approaches to the identification of new antimalarial targets and promising antimalarial drugs are being pursued in order to cope with drug resistance.     

深圳市1980～2009年疟疾发病率变化趋势的Cox-Stuart检验分析

目的分析深圳自建市以来历年疟疾的流行趋势,为该市疟疾防治提供科学依据。方法采用Cox-Stuart检验的升降趋势检验法对1980～2009年深圳疟疾的流行变化趋势进行分析。结果深圳市疟疾下降幅度明显,其发病率由20世纪80年代最高的1111.33/10万下降至目前的1/10万以下,下降幅度最大达99.98%,30年中疟疾发病率下降趋势经Cox-Stuart检验差异有统计学意义（P〈0.005）。结论深圳市30年的疟防工作卓有成效,Cox-Stuart检验分析在传染病的趋势检验分析中有一定的应用价值。     

A systems biology approach to antimalarial drug discovery

Introduction: In spite of significant efforts to reduce malaria deaths, this disease still kills around 445,000 people every year. Overcoming drug resistance is one of the main goals of current malaria research programs. This is challenging, since the biology of Plasmodium is not fully understood, requiring the development of advanced models for data analysis in the search for new antimalarials.

Areas covered: In this review the authors introduce the importance of computational models to address the challenges of drug discovery, presenting examples of pioneering systems biology approaches in the search for new antimalarial drugs and their role in the future of drug research programs. Other related topics are discussed, e.g. regulation of malaria pathogenesis by epigenetics and the importance of new platforms for malaria network.

Expert opinion: The use of a systems biology approach in antimalarial drug discovery emerges in a scenario where the most efficient antimalarial chemotherapies are showing resistance in Southeast Asia. New models for a better understanding of Plasmodium cell function have already proved to be powerful tools for uncovering complex mechanisms of resistance, and have great potential to inform the design of novel small molecules with both high antimalarial activity and transmission-blocking potential to improve the control of malaria.     

An evaluation of rectal artesunate for the pre-hospital management of severe malaria

ABSTRACT

Introduction

Severe falciparum malaria stills accounts for around half a million childhood deaths per year in sub-Saharan Africa. Prompt treatment of sick children close to home starting with artesunate given rectally by appropriately trained people can be lifesaving.