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Editors-in-Chief Kelvin Lam – Simplex Pharma Advisors, Inc., Arlington, MA, USA Henk Timmerman – Vrije Universiteit, The Netherlands DRUG DISCOVERY

TODAY

TECHNOLOGIES

Drug resistance

Antimalarial drug resistance: new treatments options for Plasmodium Francisco-Javier Gamo Tres Cantos Medicines Development Campus, Diseases of the Developing World, GlaxoSmithKline, Tres Cantos, Madrid, Spain

Malaria is one of the world’s most deadly infectious

Section editor: Jurgen Moll – Boehringer-Ingelheim, Vienna, Austria.

diseases. Millions of lives are threatened by the continued development of resistance in the malaria parasite which is overcoming the effectiveness of current antimalarial treatments. The scientific community is facing this challenge by developing new and superior therapies to combat, and potentially eradicate, this wide spread plague. New anti-Plasmodium agents derived from phenotypic screening hits (e.g. spiroindolones) or from target based projects (e.g. DSM265) have recently entered into clinical development and hopefully will provide soon a new wave of antimalarial treatments. Introduction Malaria is an infectious disease caused by protozoa of the genus Plasmodium and is transmitted by infected female Anopheles mosquitoes. Five different Plasmodium species can cause human malaria; falciparum, vivax, ovale, malariae, and lastly knowlesi, which has only recently been described as a human pathogen [1]. Malaria affects mainly countries located in tropical and subtropical climates. It is difficult to accurately estimate malaria incidence, though more than 219 million cases and around 660,000 deaths are the alarming data provided by the World Health Organization in the last World Malaria Report (http:// www.who.int/malaria/pu blications/ world_malaria_ report_2012/wmr2012_full_report.pdf). Most deaths occur in sub-Saharan Africa as result of Plasmodium falciparum infections, whereas Plasmodium vivax is the most widespread speE-mail address: ([email protected]) 1740-6749/$ ß 2014 Published by Elsevier Ltd.

cies. Significant progress has been made during recent years to reduce the number of deaths caused by this disease. The fight against malaria encompasses diverse approaches, such as vector control, vaccine development and chemotherapy. Increasingly, vector control is limited by mosquitoes becoming resistant to the most commonly used insecticides. Vaccines are still in development, though not providing full protection from malaria, are potentially life-saving for children, reducing their risk of developing severe malaria and the most dangerous form of the disease, cerebral malaria [2]. Effective drug treatment remains, therefore, the cornerstone of malaria control. However, clinical resistance is emerging continuously and threatens the efficacy of current treatments. New chemotherapeutic options are, therefore, required urgently [3]. The most relevant antimalarial resistance mechanisms and recent and novel options for treatment are the main subjects of this review.

Current status of antimalarial chemotherapy: understanding resistance mechanisms Current antimalarial therapy is built upon only a few different chemotypes. Representative examples of the most commonly marketed antimalarials are shown in Fig. 1. At a first glance, it can be seen that both the number of different structural classes and the diversity in their mechanisms of action are very limited.

Atovaquone Atovaquone, is the only clinical antimalarial operating through inhibition of mitochondrial cytochrome b (Cytb).

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1. Name 2. Chemical family 3. Mode of action 4. Other drugs from the family

Cl

O O OH O

O N S N H

N O

N

O H2N

1. Atovaquone 2. Napthoquinone 3. Mitochondrial cytochrome bc1 inhibitor 4. None

Cl

NH2

N

H2N

1. Pyrimethamine 1. Sulfadoxine 2. Diaminopyrimidine 2. Aminosulphonamide 3. Folate pathway inhibitor (DHPS) 3. Folate pathway inhibitor (DHFR) 4. Cycloguanil, proguanil 4. Dapsone NH OH

HN

H

N O

Cl

N

N HN

1. Chloroquine 2. Aminoisoquinoline 3. Interference with heme polymerization 4. Amodiaquine

NH2

1. Primaquine 2. 8-aminoquinoline 3. Unknown 4. Pamaquine, bulaquine

H CH3 O O O H O

CF3

CF3

N

H3C

1. Mefloquine 2. Amino alcohol 3. Interference with heme polymerization 4. Quinine, lumefantrine, halofantrine

O

CH3

1. Artemisinin 2. Endoperoxide 3. Unknown 4. Artesunate, dihydroartemisinin, arteether, artemether

DHFR, dihydrofolate reductase; DHPS, dihydropteroate synthase Drug Discovery Today: Technologies

Figure 1. Representative compounds of the different antimalarial chemotypes that are approved for clinical use. Detailed descriptions of all the drugs can be found at http://www.drugbank.ca.

As part of the cytochrome bc1 complex (or complex III), Cytb is essential for maintenance of a functional electron transport chain. In the parasite, cytochrome bc1 complex reoxidizes mitochondrial coenzyme Q following its reduction by different mitochondrial dehydrogenases. One of these enzymes is the dihydroorotate dehydrogenase (DHODH), which catalyzes a key step in pyrimidine biosynthesis. Plasmodium spp. are unable to salvage pyrimidines from the host, and consequently inhibition of cytochrome bc1 depletes the parasite intracellular pyrimidine pool resulting in parasite death [4]. Atovaquone is currently used in combination with proguanil and marketed as MalaroneTM. Clinical resistance to MalaroneTM has been described. Point mutations at codon 268 of Cytb (Y268S or Y268C) are the most frequent factor associated with resistance and can be used as a molecular marker of resistant parasites (Fig. 2). However, probably because of the fitness loss in the parasite conferred by Cytb mutations, resistance is uncommon and MalaroneTM is still useful for malaria treatment and prophylaxis as it targets both the blood forms as well as the initial liver stages of the disease [5]. In fact, treatment with atovaquone–proguanil has been recently recommended by the e2

World Health Organization for use in areas with emerging P. falciparum artemisinin resistance.

Sulfadoxine–pyrimethamine combination Sulfadoxine, an inhibitor of the folate biosynthesis that impedes dihydropteroate synthase (DHPS) function and pyrimethamine, another antifolate antimalarial, are typically used in combination therapy. Sulfadoxine–pyrimethamine has been one of the most widely prescribed antimalarials and has proved highly effective as prophylaxis and in curative treatment. Pyrimethamine targets dihydrofolate reductase (DHFR), another key enzyme of the folate biosynthetic pathway. Despite the capacity of P. falciparum to salvage reduced folates from the host, DHFR activity is essential for parasite survival. Clinical resistance to antimalarial antifolates is now widespread, though surprisingly, sulfadoxine–pyrimethamine still appears to improve outcomes of treatments during pregnancy when used as intermittent preventive therapy [6]. Characterization of resistant clinical isolates has led to the identification of highly resistant strains with DHFR harbouring quadruple mutations (N51I/C59R/S108N/I64L). However, the

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Erythrocyte Parasitophorus vacuole

Plasmodium A437G, K540E

DHPS

DHFR S108N (N51I, C59R, I64L)

K76T

CRT Y268S, Y268C

Cytb Vacuole Mitochondria

DHPS, dihydropteroate synthase; DHFR, dihydrofolate reductase; Cytb, cytochrome b; CRT, chloroquine resistance transporter Drug Discovery Today: Technologies

Figure 2. Most frequent mutations associated with clinical resistance to antimalarial treatments. Chloroquine, a heme polymerization inhibitor, selected for mutations in the vacuolar CRT protein. Antifolate sulfadoxine–pyrimethamine combination selected for mutations in the DHPS and DHFR enzymes of the folate biosynthesis pathway. Atovaquone, an electron transport inhibitor, selected for mutations in the mitochondrial Cytb protein.

single S108N mutation is sufficient to confer clinically relevant resistance to pyrimethamine and can be used as a molecular marker [7] (Fig. 2). Parasites resistant to antifolate treatment usually also harbour mutations in DHPS; most commonly A437G and K540E (Fig. 2). At present, the electron transfer chain and folate pathways are the only source of clinically validated antimalarial targets. All other approved antimalarials display a complex mode of action and lack a well defined target.

Chloroquine, mefloquine and primaquine 4-Aminoquinolines and aryl aminoalcohols exert their antimalarial effect by interfering with heme polymerization. Polymerization of heme to form hemozoin pigment is a process unique to Plasmodium and it is a detoxification strategy essential for the survival and growth of the malaria parasite. The 4aminoquinoline chloroquine has been by far the most extensively used antimalarial drug owing to its excellent efficacy and low cost of production. Unfortunately, P. falciparum resistance to chloroquine is now widespread and this drug cannot be considered any more as a therapeutic option for this species. Against P. vivax malaria, however, chloroquine remains quite effective in most regions. In fact, chloroquine followed by a 14day course of primaquine, which is the only drug proven to be effective and approved for the elimination of liver stage P. vivax hypnozoites, is recommended by the World Health Organization for the prevention of P. vivax relapse in regions where the

parasite is chloroquine sensitive. Nevertheless, significant P. vivax chloroquine resistance has emerged and appears to be spreading, though good quality surveillance data are scarce for this species [8]. In P. falciparum, chloroquine resistance is mediated by the chloroquine resistance transporter gene (Pfcrt) and is dependent on the presence of the K76T mutation in the PfCRT protein (Fig. 2). The mutated form of this transporter is able to reduce chloroquine accumulation in the digestive vacuole of the pathogen where the heme polymerization process takes place. The K76T mutation is necessary, although perhaps not sufficient, to confer significant chloroquine resistance and can be used as a molecular marker [9]. Levels of multi-drug resistance 1 gene (Pfmdr1) expression are also particularly relevant to chloroquine resistance. Pfmdr1 encodes P-glycoprotein homologue 1 (Pgh1), an ABC efflux pump homologous to the multi-drug resistant (MDR) protein present in chemotherapy-resistant tumor cells. Amplification and overexpression of Pfmdr1 produces resistance not only to chloroquine, but also to aryl aminoalcohols such as mefloquine, lumefantrine and quinine. It is postulated that Pfmdr1 causes these drugs to be sequestered in vacuolar compartments where they are less harmful to the parasite. Consequently, complex patterns of resistance can result based on the different levels of Pfmdr1 combined with mutations on PfCRT. Furthermore, modulation via other mechanisms, such as multi-drug resistance-associated protein (PfMRP), adds www.drugdiscoverytoday.com

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further layers of complexity. Sanchez et al. provide a detailed review of drug resistance mediated by transporters [10]. Primaquine, an 8-aminoquinoline, is the only drug proven to be effective and licensed to eliminate the hypnozoites of Plasmodium vivax and Plasmodium ovale through an unknown mechanism of action. No evidences of resistance have been reported to date.

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community has followed different approaches. The main strategies have been based on: (a) target-based screening, through the assessment and validation of new antimalarial targets as well as seeking for new chemical diversity to interfere with already validated targets or processes, and (b) whole cell screening using large chemical libraries.

Target-based screening Artemisinin Artemisinin-based combination therapy (ACT) is the current gold standard of antimalarial treatment. Artemisinins are fast-acting endoperoxide compounds that display the highest killing rates seen both in vitro and in vivo across all studied antimalarials [11,12]. Endoperoxides have a complex antimalarial mechanism of action, thought to cause multiple effects in Plasmodium parasites that may hamper resistance selection. However, the first cases of artemisinin resistance are currently being confirmed. They are characterized by a delayed parasite clearance observed in treated patients with no clear correlation to a well-defined in vitro phenotype [11,13]. Thus, there are no defined molecular markers that can be used for artemisinin-resistance surveillance. However, the use of the most modern techniques for full genome studies are starting to provide some insights into the molecular descriptors associated with endoperoxide resistance [14]. Recent studies carried out with parasites collected from participants in clinical trials conducted in northwestern Thailand and western Cambodia, two of the places where resistance is emerging, have allowed identification of regions in chromosomes 10 and 13 which are associated with delayed parasite clearance. Ongoing studies could potentially provide useful molecular markers for the surveillance of artemisinin resistance in southeast Asia [15].

Practical considerations for the discovery of new antimalarial treatments From the previous section it is clear that Plasmodium parasites are close to winning the race between available antimalarial treatments and the development of resistance. Even artemisinins, the current last line of defence against the increasing problem of resistance, are showing signs of a possible impending failure. Obviously, new treatments that overcome the current problems of resistance by targeting new mechanisms of action are the most desired option for future antimalarial drugs. These considerations need to be taken into account during target validation as part of the earliest stage of the drug discovery process [16]. The recent proliferation of public-private partnerships (PPPs) in malaria research supported by key players such as the Bill and Melinda Gates Foundation, Wellcome Trust or the Medicines for Malaria Venture (MMV) has provided an invaluable resource in the fight against the disease. To advance the development of new treatments for malaria, the research e4

This has been the favoured approach for antimalarial drug discovery in the past decades. However, success has been limited, as reflected in the poor pipeline of antimalarial drug candidates. Although the complete sequencing of the P. falciparum genome revealed the existence of about 5300 genes (most of them without a known orthologue in humans), the number of new validated antimalarial targets remains frustratingly low. There are some good reasons for this. For example, genetic target validation of putative drug targets in Plasmodium is not easy as manipulation of the parasite genome is extremely difficult. Also, essentiality of protein expression per se does not necessarily assure tractability as an antimalarial target as druggability might be also an issue. Although numerous attempts have been undertaken, small molecule chemical screens, as of today were disappointing. Despite the diversity present in screening compound collections, robust inhibitors against expected essential targets with key roles in parasite biology have remained elusive [17]. An interesting consideration regarding target-based approaches is that those antimalarials that inhibit defined molecular targets (e.g. DHFR or Cytb inhibitors) have historically shown a greater probability to cause resistance compared to those drugs that have more complex modes of action (e.g. endoperoxides or chloroquine) (Fig. 3). Although target-based drug discovery efforts to identify and exploit new drug targets in Plasmodium have been so far disappointing, drug discovery projects based on well-validated targets have provided some new clinical drug candidates, for example, new DHODH, DHFR or Cytb inhibitors have been discovered, which will be described in the next section in more detail. These drugs provide promising treatment alternatives to current therapies.

Whole cell screening A key constraint for the discovery of new treatments has been the relatively small amount of antimalarial chemotypes that could be used for lead optimization programs. However, in the last few years large whole cell screening campaigns have resulted in thousands of hits that could be used as starting points for drug discovery. Moreover, structures, molecular descriptors and relevant biological data have been deposited in public databases for use by the malaria research community within an ‘open source’ model of drug discovery [18–20]. Furthermore, external researchers have been given easier access to physical samples of the most interesting compounds. The clear advantage of whole cell screening over

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Atovaquone

Mefloquine 1980

1990

2000

2010

Artemisinin

1970

Atovaquone

1960

Mefloquine

1950

Proguail

1940

Pyrimethamine

1930

Chloroquine

1920

Quinine

1910

Artemisinin

Chloroquine Proguanil

Quinine 1632

Pyrimethamine

Year introduced into clinical use

First evidence of clinical resistance Drug Discovery Today: Technologies

Figure 3. Year of introduction for the main antimalarial therapies versus the first evidence of clinical resistance to these agents.

other approaches is the likelihood that hits are active against their target in a physiologically relevant context. On the other hand, hits generated without any knowledge on their mode of action will need to be progressed without the information of a target-based assay and if toxicity issues arise further in development, it is much harder to extricate them from the antimalarial effect without a target, or at least mechanism-based assay. Nevertheless, this approach has been very successful, with a good number of chemotypes identified in phenotypic screening which are now in different stages of the drug discovery pipeline. In fact, some of these agents are even in clinical development and will be described in the next section. Data from these collections of compounds have also helped to improve researchers’ understanding of the fundamental biology of malaria, which should underpin wider efforts to fight the disease.

New treatments options under development The current antimalarial pipeline under development is the result of: (a) compounds synthesized in programs based on clinical validated targets like DHFR or genetically validated enzymes like DHODH, and (b) hits found in phenotypic screening (see [21,22] for detailed reviews). The most promising series under development are discussed below.

sensitive (strain NF54) and the chloroquine-resistant (strain K1) P. falciparum strains. A successful lead optimization process was able to solve the initial issues related to stereochemistry observed with this chemical class and improvements of potency, in vivo clearance and oral bioavailability led to the discovery of NITD609 [23]. The molecule shows an outstanding in vitro and in vivo potency with a pharmacokinetic profile that is potentially compatible with a single-dose treatment. Currently, it is in clinical development (Phase II) and is one of the most promising novel antimalarials. Mode of action studies carried out with this molecule suggest that PfATP4 is the potential parasite molecular target for this chemical series [24]. PfATP4 is a vacuolar proton pump (V-H+/Na+) that regulates sodium homeostasis in Plasmodium, a process that seems to be critical to parasite viability. In fact, sodium homeostasis is emerging as a promising source of new antimalarial targets. Researchers at GlaxoSmithKline have already identified at least four additional new scaffolds totally unrelated to the NITD609 spiroindolone that could be acting through the same PfATP4 pathway. Characterization of these molecules is ongoing and full descriptions of the chemotypes will be accessible to the public in due course.

Imidazolopiperazines (GNF156) Spiroindolones (NITD609) This chemical family was discovered by Novartis as result of whole cell screening of Plasmodium infected cells using a small natural product library which consists of approximately 12,000 molecules. After screening of the library, a hit was identified, which displayed moderate potency against both a

These compounds were also identified by Novartis’ researchers starting from a hit series selected via whole cell screening. A successful lead optimization exercise resulted in molecules with improved potency, metabolic stability and potent in vivo efficacy [25]. The mechanism for the potent antimalarial activity of related imidazopyrazines has been recently elucidated. www.drugdiscoverytoday.com

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This chemical family inhibits Plasmodium PI(4)K activity and displays potent activity against multiple stages of Plasmodium lifecycle [26]. One candidate molecule GNF156 has been progressed to Phase I studies (see MMV website: http:// www.mmv.org).

Aminoindoles (Genz668764) This series was identified at Harvard Medical School in a screen of 70,000 compounds, which were composed of the Broad Institute’s small-molecule library and the ICCB-L compound collection [27]. Lead optimization efforts resulted in the identification of molecules with potent activity against the parasite and with an acceptable human pharmacokinetic prediction that supports progression of the series to clinical trials. Mechanism of action is unknown and experiments on in vitro resistant selection with representative compounds of the series have been unable to identify mutants even after nine months of drug exposure. This result could indicate a restricted capacity of the parasite to generate viable genetic variations to bypass effect of this specific inhibitor, a highly desirable virtue for any future antimalarial drug treatment.

Aminopyrimidines (MMV390048) This chemical family was discovered as result of a whole screening of BioFocus SoftFocus chemical libraries [28]. Successful hit-to-lead and lead optimization processes on these compound classes afforded molecules with good bioavailability, long half-life, and the ability to cure P. berghei infected mice with only a single oral dose. The latter compound property makes this chemical class highly attractive, since it has the potential to deliver compounds with a single-dose cure profile. MMV390048 is the most advanced compound of this series and has been recently selected as a pre-clinical candidate for development and is expected to enter Phase I trials in 2014 [29]. Attempts to identify the intracellular target responsible for these outstanding antimalarial effects are ongoing. Efforts will be greatly facilitated as BioFocus libraries were designed to target kinases.

Endoperoxides (OZ439) This chemical family displays the fastest antimalarial effect through all known antimalarials. Treatment produces a rapid clearance of the parasites that translates into an immediate symptomatic relief in the patient. Despite the excellent antimalarial properties of artemisinins, compound supply, though improved, remains a problem. Plant-derived artemisinin production is unstable, resulting in shortages and price fluctuations, complicating drug supply by ACT manufacturers and causing difficulties in providing a low-cost therapy. On the other hand, artemisinin synthesis has a low overall yield and is not commercially viable. Although use of engineered Saccharomyces cerevisiae strains to produce the starting material is giving promising results [30], the antimalarial community e6

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has focused its efforts on identifying novel endoperoxides that are synthetically tractable to ensure a low cost of goods. OZ439 is a second generation ozonide (1,2,4-trioxolane) of the first synthetic peroxide (OZ277) and has been approved for use as antimalarial (Ranbaxy, India). The OZ439 series has a potent antimalarial activity with a superior pharmacokinetic and efficacy profile compared with the OZ277 series [31]. Potentially, compounds in the OZ439 series could be used as singledose therapy [32].

Dihydrofolate reductase inhibitors (P218) New generation DHFR inhibitors have been identified which avoid cross-resistance issues with the already existing DHFRmutant parasites. Design and synthesis of enzymatic inhibitors was guided using X-ray structures of P. falciparum and human DHFR with substrates and inhibitors, along with efficacy and pharmacokinetic studies, and led to the eventual identification of P218 as a drug candidate. P218 is a potent, selective, highly efficacious and orally bioavailable antimalarial drug that inhibits potently both wild-type and clinically relevant mutant forms of P. falciparum DHFR. P218 has a different enzymatic mechanism of inhibition to that of pyrimethamine. Notably, P218 has a novel binding site in DHFR that overlaps the envelope mapped out by the dihydroorotate substrate. This could retard the selection of DHFR resistance and make of P218 an interesting candidate for clinical development [33].

Cytochrome bc1 inhibitors (ELQ-300) Mitochondrial metabolism appears to be a good source of antimalarial targets and several attempts have been made to overcome resistance issues associated with the cytochrome bc1 inhibitor atovaquone. As result of these efforts 4(1H)pyridone (GSK932121), was identified as preclinical candidate and progressed to Phase I studies [34]. However, safety concerns that arose during clinical development with a more soluble pro-drug of this compound caused development of this molecule to be suspended. ELQ-300 is a cytochrome bc1 inhibitor based on a quinolone central scaffold that resembles the structural characteristics in the side-chain present in the previously described 4(1H)-pyridones. ELQ-300 is a potent antimalarial with a high metabolic stability displaying excellent antimalarial efficacy against blood stages and exoerythrocytic forms of the parasite, including liver and mosquito stages [35]. These results offer the hope of an effective treatment not only to cure and protect patients but also to block transmission, thus contributing to the ultimate goal of malaria eradication.

Dihydroorotate dehydrogenase inhibitors (DSM265) DHODH is one of the few genetic validated antimalarial targets displaying a key role in pyrimidine biosynthesis [4]. A biochemical screen using a recombinant PfDHODH

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enzyme identified the triazolopyrimidine scaffold as a potent inhibitor of both enzymatic activity and parasite growth. Optimization of initial hits, assisted by X-ray modelling, resulted in the identification of DSM265, a second generation small molecule [36]. This molecule is a simple and potent inhibitor of PfDHODH with an in vivo efficacy similar to that of chloroquine. Pharmacokinetic studies demonstrated excellent bioavailability and a long half-life supporting the recent progression of this candidate to Phase I clinical studies.

mosquito stages is an important advance goal in achieving malaria elimination (Fig. 4). Robust biological assays have recently been described that target such components of the Plasmodium lifecycle [39]. However, it may be necessary to combine different chemical entities with a variety of therapeutic profiles as part of a single treatment to assure effectiveness against intra-erythrocytic stages, thus relieving clinical symptoms, to block parasite transmission, and to reduce the potential for the emergence of resistance [40].

8-Aminoquinolines (tafenoquine)

Concluding remarks

Tafenoquine is a lead investigational medicine for the treatment of P. vivax liver hypnozoite stages. Structurally, it is an 8-aminoquinoline primaquine analogue, and has demonstrated activity against the dormant liver stages of P. vivax [37]. The main advantage of tafenoquine over primaquine is that its pharmacokinetic profile permits single-dose therapy for the prevention of relapse in P. vivax malaria, versus the 14day regimen required with primaquine. As is the case for primaquine, tafenoquine causes haemolysis in individuals that are glucose-6-phosphate dehydrogenase (G6PD) deficient. Tafenoquine is currently being investigated in Phase IIb clinical trials in combination with chloroquine. Recently published data suggest that co-administration of these two antimalarials does not appear to produce any pharmacokinetic interaction issues or safety concerns [38].

Malaria parasites are escaping pharmacological control as our ability to develop and deploy novel antimalarials in the clinic fails to keep pace with the pathogen’s ability to develop resistance. P. falciparum has developed clinical resistance to all known antimalarials and even our most powerful weapon, the artemisinin endoperoxides, will eventually succumb to the evolution of parasite resistance. Thus, the discovery and development of new antimalarial drugs is imperative. There has been a positive and urgent response in the scientific community to this threat. The identification and public release of thousands of new chemical chemotypes with activity against the parasite, the increasing availability of molecular biology technologies that enable Plasmodium genetic modification, and the deployment of full genome sequencing technologies that will improve identification of molecular markers of resistance and virulence will be the basis for the discovery of a new generation of antimalarial drugs. New antimalarial treatments should display novel mechanisms of action with efficacy against already existing multi-drug resistant strains. Additionally, the interruption of parasite transmission, with the potential to contribute to malaria eradication, should be exploited by the next generation of antimalarial drugs. Whilst malaria vaccines complete their clinical development and all the integrated approaches

New areas for future antimalarials: tackling transmission of the disease Clinical antimalarials have been designed to target the Plasmodium life-cycle stages that take place in humans, specifically the replicative intra-erythrocytic stages that directly cause malarial symptoms. However, the parasite life-cycle is complex and the effective pharmacological disruption of human–mosquito transmission by interfering with any of the

Mosquito stages

Infected human Zygote

Oocysts

New host Gametocytes

Macrogametocytes (exflagellation)

Ookinete

Sporozoites Drug Discovery Today: Technologies

Figure 4. Targeting P. falciparum transmission (adapted with permission from Medicines for Malaria Venture). During intraerythrocytic multiplication some parasites differentiate into sexual forms called female and male gametocytes. When a mosquito bites an infected human, it ingests the gametocytes. In the mosquito gut gametocytes develop further into mature sex cells called gametes. Male and female gametes form diploid zygotes, which develop into moving ookinetes that burrow into the mosquito midgut wall and form oocysts. Division of each oocyst produces thousands of haploid forms called sporozoites. After 1–2 weeks, the oocyst bursts releasing sporozoites that migrate to the salivary glands. Cycle of human infection re-starts when the infected mosquito takes a blood meal, injecting sporozoites into the human bloodstream. More detailed information about the full Plasmodium lifecycle can be obtained from the Medicines for Malaria Venture website (http://www.mmv.org/malaria-medicines/parasite-lifecycle).

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to malaria eradication start to make an impact, the role of drug discovery scientists is critical to finally defeating this deadly disease.

Conflict of interest The author confirms that this article content has no conflict of interest.

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Please cite this article in press as: Gamo FJ. Antimalarial drug resistance: new treatments options for Plasmodium, Drug Discov Today: Technol (2014), http://dx.doi.org/

Antimalarial drug resistance: new treatments options for Plasmodium.

Malaria is one of the world’s most deadly infectious diseases. Millions of lives are threatened by the continued development of resistance in the mala...
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