Malarial parasites vs. antimalarials: never-ending rumble in the jungle

Development of resistance is an increasing problem for antimalarial chemotherapy because resistance against most available drugs has developed in the majority of world-wide parasite populations. Therefore, several strategies to counteract resistance-development are in place. From the pharmaceutical side, identification of new targets and compounds, development of structural relatives of known antimalarials, and fixed combination therapy are pursued. On the other hand, clinical studies focus on novel regimens, distribution schemes and drug combinations. A third possibility to diminish progression of resistance is the application of evolutionary concepts to design new strategies for validation, monitoring and interference with the selection-process that leads to the spread of multidrug-resistance. Since the pharmacologic and clinical side of antimalarial chemotherapy is covered by recent reviews we refer to the newest developments only and lay our focus on determinants of selection for drug resistance in human malaria.     

Strategies for the prevention of antimalarial drug resistance: rationale for combination chemotherapy for malaria

Among the several 'tropical' diseases that affect humans, malarin poses special control problems due to the increasing population at risk from the disease, the difficulties in eradicating the mosquito vector in the tropics and the emergence and spread of parasite resistance to commonly used antimalarial drugs. There is both clinical experience and experimental evidence that, however effective when first introduced, the lifespan of drugs is inevitably curtailed by the emergence of resistant parasites. Resistance is the most important factor in determining the useful lifespan of antimalarial drugs. In this review, Nick White and Piero Olliaro discuss the rationale for combination chemotherapy.     

Reversal of chloroquine resistance in falciparum malaria

Control of falciparum malaria infections has been increasingly hampered by the emergence of parasites resistant to chloroquine, pyrimethomine and other standard anti-malarials. Chloroquine-resistant strains of Plasmodium falciparum, for example, which originally appeared in South-East Asia and South America are now found in East Asia and sub-Saharan Africa(1). Attempts to combat this alarming development have to date taken two main forms: (1) the judicious use of existing ontimalarials, preferably in combinations, in an attempt to delay the emergence of resistance; and (2) on aggressive research effort aimed at identifying a new generation of antimalarial drugs. But what i f it became possible to administer an antimalarial drug together with a second drug capable of overcoming resistance to the first? A recent report from Samuel Martin and co-workers at The Walter Reed Army Institute of Research in Washington DC raises just such an intriguing possibility.     

A network to monitor antimalarial drug resistance: a plan for moving forward

The spread of resistance to antimalarial drugs has required changes in the recommended first-line treatment for falciparum malaria in almost all regions. Most drugs recommended currently are combinations of a long-acting antimalarial and an artemisinin derivative. This article presents the rationale for establishing a web-based, open-access database of antimalarial drug resistance and efficacy: the World Antimalarial Resistance Network (WARN). The goal of this network is to assemble the tools and information that will enable the malaria community to collate, analyze and share contemporary information on antimalarial-drug efficacy in all endemic regions so that decisions on antimalarial-drug use are based on solid evidence.     

Management of severe malaria

The case fatality of WHO-defined 'severe falciparum malaria' remains unacceptably high, at 10-20%. However, a gradual decline in case fatality in adults and children treated in hospitals may reflect use of improved regimens of antimalarial chemotherapy and increased awareness of important complications of the disease. The development of severe, perhaps inevitably-fatal, malaria might be prevented by early appropriate chemotherapy of uncomplicated disease. At the most peripheral levels of the health service, suppository formulations of artemisinin derivatives can be administered even to patients who are vomiting or prostrated. At dispensaries, clinics or hospitals, where intramuscular or intravenous administration of antimalarial drugs is possible, quinine and artemisinin derivatives are the treatments of choice. There is growing evidence of the safety and efficacy of the quinine loading dose and of the use of artemether and artesunate, based on large, randomised, controlled clinical studies. No safe and effective form of prophylactic ancillary treatment has yet emerged. Results of studies of antipyretics, anticonvulsants (phenobarbitone), anticytokine/anti-inflammatory agents (anti-TNF antibodies, pentoxifylline, dexamethasone), iron chelators and hyperimmune sera have been disappointing. Only blood transfusion and treatment of respiratory, circulatory and renal failure are of obvious benefit. New ideas are needed, based on what is known of the pathophysiology of severe disease.     

Systematic Review: Insight into Antimalarial Peptide

Antimalarial peptides varying in size, sequence, charge, conformation and structure, hydrophobicity and amphipathicity reflect their heterogeneity in antimalarial activity. Due to global concern of antimalarial drug resistance, these peptides are seldom in attention for therapeutic values as this microbial and synthetic peptide are likely known for delaying the drug resistance phenomenon. Despite of this, among most of the peptides that have shown activity in cultured parasitized erythrocytes were failing to show its efficacy on in vivo models and few of them that are efficacious are not clinically significant on the host. A systematic literature search was carried out to obtain all related studies in PubMed, EMBASE and GOOGLE SCHOLAR from year 1989 to till date 2015 and we found only 63 studies that focus on antimalarial activity of different peptides originated from different sources under in vitro and in vivo conditions. Antimalarial peptides that are mostly included in this review is the naturally occurring along with their derivatives obtained from different sources ranging from lower prokaryotes to higher eukaryotes. Most of the antimalarial peptides had undergone only in vitro testing on Plasmodium falciparum strains having very less potency, but higher selectivity in comparison to standard drugs. The study included in this article will give future direction for development of more antimalarial peptide with desired efficacy and safety.     

The P-glycoprotein homologues of Plasmodium falciparum: Are they involved in chloroquine resistance?

Chloroquine has been the mainstay of antimalarial chemotherapy but the rapid spread of resistance to this important drug has now compromised its efficacy. The mechanism of chloroquine resistance has not been known but recent evidence from Plasmodium falciparum, the causative agent of the most severe form of human malaria, suggested similarities to the multidrug resistance phenotype (MDR) of mammalian tumour cells which is mediated by a protein molecule termed P-glycoprotein. Two mdr genes (pfmdr1 and pfmdr2) encoding P-glycoprotein homologues have been identified in P. falciparum and one of these (pfmdr1) has several alleles that have been linked to the chloroquine resistance phenotype. In contrast analysis of a genetic cross between chloroquine-resistant and -sensitive P. falciparum has suggested that the genes encoding the known P-glycoprotein homologues are not linked. This review outlines the similarities of the chloroquine resistance phenotype with the MDR phenotype of mammalian tumour cells and explores the possible role of the pfmdr genes.     

Drug-resistant malaria: molecular mechanisms and implications for public health

Resistance to antimalarial drugs has often threatened malaria elimination efforts and historically has led to the short-term resurgence of malaria incidences and deaths. With concentrated malaria eradication efforts currently underway, monitoring drug resistance in clinical settings complemented by in vitro drug susceptibility assays and analysis of resistance markers, becomes critical to the implementation of an effective antimalarial drug policy. Understanding of the factors, which lead to the development and spread of drug resistance, is necessary to design optimal prevention and treatment strategies. This review attempts to summarize the unique factors presented by malarial parasites that lead to the emergence and spread of drug resistance, and gives an overview of known resistance mechanisms to currently used antimalarial drugs.     

Antimalarial chemotherapy: young guns or back to the future?

The burgeoning global problem of malaria is largely due to the emergence of parasite resistance to our limited armamentarium of antimalarial drugs. The recognition of this impending disaster at the international level and the engagement of the pharmaceutical industry promise a more optimistic future for antimalarial drug development. This is particularly exciting when considering the advances in our understanding of parasite biology, which are currently being fuelled by the malaria genome project. This article discusses recent developments in the area of antimalarial drug discovery and evaluation. New advances, based on traditional antimalarial drug classes including the quinolines, peroxides and antifolates (‘back to the future’), are discussed, followed by a presentation of some novel targets (‘young guns’) that have been shown to be good candidates for chemotherapeutic attack.     

Investigation of some medicinal plants traditionally used for treatment of malaria in Kenya as potential sources of antimalarial drugs

Malaria is a major public health problem in many tropical and subtropical countries and 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 discovery of new antimalarial agents. Herbal medicines for the treatment of various diseases including malaria are an important part of the cultural diversity and traditions of which Kenya′s biodiversity has been an integral part. Two major antimalarial drugs widely used today came originally from indigenous medical systems, that is quinine and artemisinin, from Peruvian and Chinese ancestral treatments, respectively. Thus ethnopharmacology is a very important resource in which new therapies may be discovered. The present review is an analysis of ethnopharmacological publications on antimalarial therapies from some Kenyan medicinal plants.     

Drug discovery for malaria: a very challenging and timely endeavor

The prevalence of resistance to known antimalarial drugs has resulted in the expansion of antimalarial drug discovery efforts. Academic and nonprofit institutions are partnering with the pharmaceutical industry to develop new antimalarial drugs. Several new antimalarial agents are undergoing clinical trials, mainly those resurrected from previous antimalarial drug discovery programs. Novel antimalarials are being advanced through the drug development process, of course, with the anticipated high failure rate typical of drug discovery. Many of these are summarized in this review. Mechanisms for funding antimalarial drug discovery and genomic information to aid drug target selection have never been better. It remains to be seen whether ongoing efforts will be sufficient for reducing malaria burden in the developing world.     

An Epigenetic Antimalarial Resistance Mechanism Involving Parasite Genes Linked to Nutrient Uptake

Acquired antimalarial drug resistance produces treatment failures and has led to periods of global disease resurgence. In Plasmodium falciparum, resistance is known to arise through genome-level changes such as mutations and gene duplications. We now report an epigenetic resistance mechanism involving genes responsible for the plasmodial surface anion channel, a nutrient channel that also transports ions and antimalarial compounds at the host erythrocyte membrane. Two blasticidin S-resistant lines exhibited markedly reduced expression of clag genes linked to channel activity, but had no genome-level changes. Silencing aborted production of the channel protein and was directly responsible for reduced uptake. Silencing affected clag paralogs on two chromosomes and was mediated by specific histone modifications, allowing a rapidly reversible drug resistance phenotype advantageous to the parasite. These findings implicate a novel epigenetic resistance mechanism that involves reduced host cell uptake and is a worrisome liability for water-soluble antimalarial drugs.     

Comparative protein modeling of spermidine synthase from Plasmodium falciparum: A potential target for anti-malarial drug therapy

Malaria, caused by protozoan parasites of the genus Plasmodium, affects up to 500 million individuals and kills over 1 million people every year. The increasing resistance of the malaria parasites has enforced strategies for finding new drug targets. In recent years, enzymes associated with the polyamine metabolism have attracted attention as drug targets. Cytosolic Plasmodium falciparum spermidine synthase (PfPAPT) is a potential target for antimalarial chemotherapy. Contrasting with the other enzymes involved in the parasite polyamine amine biosynthesis, little information is available about this enzyme, and its crystallographic structure is unknown yet. In this paper I propose a theoretical low-resolution 3D model for PfPAPT based on crystal structure of the Arabidopsis thaliana, by multiple alignment followed by intensive optimization; validation and dynamic simulations in water. Comparison between the active sites of PfPAPT and human PAPT revealed key differences that could be useful for the design of new selective inhibitors of Plasmodium PAPT.     

The de novo selection of drug-resistant malaria parasites

Antimalarial drug resistance emerges de novo predominantly in areas of low malaria transmission. Because of the logarithmic distribution of parasite numbers in human malaria infections, inadequately treated high biomass infections are a major source of de novo antimalarial resistance, whereas use of antimalarial prophylaxis provides a low resistance selection risk. Slowly eliminated antimalarials encourage resistance largely by providing a selective filter for resistant parasites acquired from others, and not by selecting resistance de novo. The de novo emergence of resistance can be prevented by use of antimalarial combinations. Artemisinin derivative combinations are particularly effective. Ensuring adequate treatment of the relatively few heavily infected patients would slow the emergence of resistance.     

Structure of Plasmodium vivax dihydrofolate reductase determined by homology modeling and molecular dynamics refinement

The structure of Plasmodium vivax dihydrofolate reductase (PvDHFR), a potentially important target for antimalarial chemotherapy, was determined by means of homology modeling and molecular dynamics refinement. The structure proved to be consistent with DHFRs of known crystal structure. The comparison of the complexes of the antifolate inhibitor pyrimethamine bound at the active sites of PvDHFR and PfDHFR, the related enzyme from Plasmodium falciparum, prospected the possibility of using structure-based drug design to develop inhibitors that are effective against both malarial enzymes. This study constitutes a first step toward understanding of the antifolate-PvDHFR molecular interactions and possible rationalization of resistance in vivax malaria.     

Heterologous expression and ATPase activity of mutant versus wild type PfMDR1 protein

Mutation of the P. falciparum chloroquine resistance transporter (PfCRT) causes resistance to chloroquine (CQ) and other antimalarial drugs. Mutation and/or overexpression of one of the multidrug resistance protein homologues found in this malarial parasite (PfMDR1) may further modify or tailor the degree of multidrug resistance. However, considerable controversy surrounds the precise contribution of PfMDR1, in part because no direct biochemical studies of PfMDR1 have yet been possible. Using codon optimization and other principles, we have designed and constructed a yeast optimized version of the wild type pfmdr1 gene and have successfully overexpressed PfMDR1 protein in P. pastoris yeast. The protein is well expressed in either full length form or as two separate half transporters, is well localized to the yeast plasma membrane and is fully functional as evidenced by ATPase activity measurements. We have also expressed mutants that have previously been hypothesized to influence drug resistance in parasites. Using purified plasma membrane fractions, we have analyzed antimalarial drug effects on ATPase activity for wild type versus mutant proteins. Relative to other ABCB transporters involved in drug resistance, PfMDR1 is unusual. It has similar pH, [ATP], and Mg++ dependencies for ATP hydrolysis, yet relatively high Km and Vmax values for ATP hydrolysis, and ATPase activity is only mildly stimulated by antimalarial drugs. The largest measured drug effect is for CQ (to which PfMDR1 is not believed to confer resistance), and it is strongly inhibitory for WT PfMDR1. Drug resistance associated PfMDR1 mutants show either elevated (Dd2 allele encoded) or reduced (7G8 allele) basal ATPase activity and different patterns of drug stimulation or inhibition, relative to WT PfMDR1. The Dd2 PfMDR1 isoform also shows a slightly more alkaline pH optimum. Surprisingly, verapamil alone (1-300 microM) does not significantly affect WT ATPase activity but inhibits the Dd2 isoform at 1 microM. These data should assist ongoing analysis of the contribution of PfMDR1 to antimalarial drug resistance.     

Antimalarial dosing regimens and drug resistance

The contribution of underdosing to antimalarial treatment failure has been underappreciated. Most recommended dosage regimens are based on studies in non-pregnant adult patients. Young children and pregnant women, who bear the heaviest malaria burden, have the highest treatment failure rates. This has been attributed previously to lower immunity, although blood concentrations of many antimalarial drugs are significantly lower in pregnant women and young children than in non-pregnant adults. Nevertheless, there have been no studies of higher dosages. Sub-therapeutic concentrations will certainly contribute to poorer responses to treatment and will fuel the emergence and spread of antimalarial drug resistance. There is an urgent need for studies to optimise antimalarial dosage regimens in infants, young children and pregnant women, both to improve cure rates and to prolong the useful therapeutic lives of antimalarial drugs.     

A chemical approach towards understanding the mechanism and reversal of drug resistance in Plasmodium falciparum: is it viable?

Genetic and biochemical approaches to studies of drug resistance mechanisms in Plasmodium falciparum have raised controversies and contradictions over the past several years. A different and novel chemical approach to this important problem is desirable at this point in time. Recently, the molecular basis of drug resistance in P. falciparum has been associated with mutations in the resistance genes, Chloroquine Resistance Transporter (PfCRT) and the P-glycoprotein homologue (Pgh1). Although not the determinant of chloroquine resistance in P. falciparum, mutations in Pgh1 have important implications for resistance to other antimalarial drugs. Because it is mutations in the aforementioned resistance genes rather than overexpression that has been associated with drug resistance in malaria, studies on mechanisms of drug resistance and its reversal by chemosensitisers should benefit from a chemical approach. Target-oriented organic synthesis of chemosensitisers against proteins implicated in drug resistance in malaria should shed light on mechanism of drug resistance and its reversal in this area. The effect of structurally diverse chemosensitisers should be examined on several putative resistance genes in P. falciparum to deal with antimalarial drug resistance in the broadest sense. Therefore, generating random mutations of these resistance proteins and subsequent screening in search of a specific phenotype followed by a search for mutations and/or chemosensitisers that affect a specific drug resistance pathway might be a viable strategy. This diversity-oriented organic synthesis approach should offer the means to simultaneously identify resistance proteins that can serve as targets for therapeutic intervention (therapeutic target validation) and chemosensitisers that modulate the functions of these proteins (chemical target validation).     

Plasmodium falciparum transmission rate and selection for drug resistance: a vexed association or a key to successful control?

While malaria eradication campaigns once adopted a combination of vector control and chemotherapy to overcome the disease, today's opinion on the matter is equivocal. So what has changed? This paper reviews some of the confusing hypotheses on the relationship between Plasmodium falciparum transmission and levels of drug resistance. New field evidence showing variations of in vivo chloroquine resistance in relation to indoor residual spraying and natural endemicity patterns, is considered with a view to how these phenomena implicate on control.     

Efficacy of amodiaquine and sulfadoxine/pyrimethamine in the treatment of malaria not complicated by Plasmodium falciparum in Nariño,Colombia, 1999-2002

The resurgence and spread of antimalarial drug resistance is one of the causes of the worldwide increase of malaria. In Colombia, uncomplicated Plasmodium falciparum malaria has been treated with a combination of amodiaquine (AQ) and sulfadoxine/pyrimethamine (SP) since 2000. The efficacy of these two antimalarials was evaluated after the implementation of the new malaria treatment scheme. In the municipalities of El Charco and Tumaco (Nari?o) on the Pacific Coast region, the standard PAHO protocol was used to evaluate antimalarial efficacy in areas of low to moderate malaria transmission. Patients were randomly allocated to treatment regime in two cities of Nari?o, El Charco (n = 48) and Tumaco (n = 50). After 14 days none of El Charco patients presented therapeutic failure to either antimalarial. However, in Tumaco after 28 days, 12 of 24 (95% CI: 30.6-69.4) patients presented AQ treatment failure while 4 of 26 (95% CI: 5.1-33.1) patients had SP treatment failure. The high level of AQ treatment failure in Tumaco was unexpected because it had been introduced only recently as an antimalarial treatment in Colombia. The results suggest that the use of the current dose of AQ in combination with SP will be therapeutically useful for less time than expected. Use of combined therapies is a key strategy to delay antimalarial resistance. Unfortunately, its success depends on the efficacy of antimalarial drugs individually.