Author Manuscript NIH Public Access Rajeev K. Mehlotra ...aquadoc.typepad.com/files/grimberg-antimalarial-rx-2011.pdfMore than 40% of the world’s population, much of it socioeconomically

Expanding the Antimalarial Drug Arsenal—Now, But How?

Brian T. Grimberg and Rajeev K. MehlotraCenter for Global Health and Diseases, School of Medicine, Case Western Reserve University,Cleveland, OH 44106, USA; Tel.: +1-216-368-6328 or +1-216-368-6172, Fax: +1-216-368-4825Brian T. Grimberg: [email protected]; Rajeev K. Mehlotra: [email protected]

AbstractThe number of available and effective antimalarial drugs is quickly dwindling. This is mainlybecause a number of drug resistance-associated mutations in malaria parasite genes, such as crt,mdr1, dhfr/dhps, and others, have led to widespread resistance to all known classes of antimalarialcompounds. Unfortunately, malaria parasites have started to exhibit some level of resistance inSoutheast Asia even to the most recently introduced class of drugs, artemisinins. While there ismuch need, the antimalarial drug development pipeline remains woefully thin, with little chemicaldiversity, and there is currently no alternative to the precious artemisinins. It is difficult to predictwhere the next generation of antimalarial drugs will come from; however, there are six majorapproaches: (i) re-optimizing the use of existing antimalarials by either replacement/rotation orcombination approach; (ii) repurposing drugs that are currently used to treat other infections ordiseases; (iii) chemically modifying existing antimalarial compounds; (iv) exploring naturalsources; (v) large-scale screening of diverse chemical libraries; and (vi) through parasite genome-based (“targeted”) discoveries. When any newly discovered effective antimalarial treatment isused by the populus, we must maintain constant vigilance for both parasite-specific and human-related factors that are likely to hamper its success. This article is neither comprehensive norconclusive. Our purpose is to provide an overview of antimalarial drug resistance, associatedparasite genetic factors (1. Introduction; 2. Emergence of artemisinin resistance in P. falciparum),and the antimalarial drug development pipeline (3. Overview of the global pipeline of antimalarialdrugs), and highlight some examples of the aforementioned approaches to future antimalarialtreatment. These approaches can be categorized into “short term” (4. Feasible options for now)and “long term” (5. Next generation of antimalarial treatment—Approaches and candidates).However, these two categories are interrelated, and the approaches in both should be implementedin parallel with focus on developing a successful, long-lasting antimalarial chemotherapy.

Keywordsmalaria; falciparum; artemisinin resistance; natural products; drug discovery; kinases; HDAC;DHODH

© 2011 by the authors; licensee MDPI, Basel, Switzerland.Correspondence to: Brian T. Grimberg, [email protected]; Rajeev K. Mehlotra, [email protected] article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license(http://creativecommons.org/licenses/by/3.0/).

NIH Public AccessAuthor ManuscriptPharmaceuticals (Basel). Author manuscript; available in PMC 2011 May 26.

Published in final edited form as:Pharmaceuticals (Basel). 2011 May 1; 4(5): 681–712. doi:10.3390/ph4050681.

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http://creativecommons.org/licenses/by/3.0/

1. Introduction1.1. Malaria

More than 40% of the world’s population, much of it socioeconomically and politicallychallenged, live in areas where malaria, alone or together with HIV/AIDS and tuberculosis,is a significant health risk [1,2]. According to the World Health Organization (WHO),approximately 250 million clinical cases of malaria occur every year [3]. Malaria isestimated to kill nearly one million people annually, with most of the deaths occurring inchildren under 5 years of age in sub-Saharan Africa [4]. If children survive multipleinfections, such exposure leads to a natural immunity that limits the severity of the diseaselater in life. However, this immunity wanes in the absence of continued exposure to malariainfections. Additionally, pregnant women and newborns have reduced immunity, andtherefore are vulnerable to severe complications of malaria infection and disease [5,6].

Malaria is primarily caused by four species of the protozoan parasite Plasmodium: P.falciparum, P. vivax, P. malariae, and P. ovale, which are transmitted by over 70 species ofAnopheles mosquitoes [7,8]. These parasite species occur sympatrically both in humanpopulations and within infected individuals, with P. falciparum and P. vivax being thepredominant species [9–11]. Approximately 80% of all malaria cases and 90% of malaria-attributed deaths occur in Africa, and are caused by P. falciparum [3]. Outside of Africa, P.vivax is the most widespread species, occurring largely in Asia, including the Middle Eastand the Western Pacific, and in Central and South America [12]. This parasite speciescauses a relatively less lethal form of the disease compared with P. falciparum [12,13].

1.2. Overview of Antimalarial DrugsToday when many mosquito vectors are resistant to insecticides [14,15], and an effectivevaccine is not yet available [14,16], chemoprophylaxis/chemotherapy remains the principalmeans of combating malaria. Over the past 60 to 70 years, since the introduction of syntheticantimalarials, only a small number of compounds, belonging to three broad classes, havebeen found suitable for clinical usage [17,18]. These classes are described below.

1.2.1. Quinine and related drugs—Quinine, originally extracted from cinchona bark inthe early 1800s, along with its dextroisomer quinidine, is still one of the most importantdrugs for the treatment of uncomplicated malaria, and often the drug of last resort for thetreatment of severe malaria. Chloroquine (CQ), a 4-aminoquinoline derivative of quinine,has been the most successful, inexpensive, and therefore the most widely used antimalarialdrug since the 1940s. However, its usefulness has rapidly declined in those parts of theworld where CQ-resistant strains of P. falciparum and P. vivax have emerged and are nowwidespread. Amodiaquine, an analogue of CQ, is a pro-drug that relies on its activemetabolite monodesethylamodiaquine, and is still effective in areas of Africa, but not inregions of South America. Other quinine-related, commonly used drugs include mefloquine,a 4-quinoline-methanol derivative of quinine, and the 8-aminoquinoline derivative,primaquine; the latter is specifically used for eliminating relapse causing, latent hepaticforms (hypnozoites) of P. vivax. Halofantrine and lumefantrine, both structurally related toquinine, were found to be effective against multidrug-resistant falciparum malaria[17,19,20]. While lumefantrine, in combination with an artemisinin derivative, artemether(Coartem), is recommended by the WHO for treating uncomplicated falciparum malaria,halofantrine generally is not recommended because of serious side effects and extensivecross-resistance with mefloquine [21].

1.2.2. Antifolate combination drugs—Antifolate drugs include various combinationsof dihydrofolate reductase enzyme (DHFR) inhibitors, such as pyrimethamine, proguanil,

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and chlorproguanil, and dihydropteroate synthase enzyme (DHPS) inhibitors, such assulfadoxine and dapsone. With rapidly growing sulfadoxine-pyrimethamine (SP) resistance,a new combination drug, Lapdap (chlorproguanil-dapsone), was tested in Africa in the early2000s, but was withdrawn in 2008 because of hemolytic anemia in patients with glucose-6-phosphate dehydrogenase enzyme (G6PD) deficiency [22].

1.2.3. Artemisinin and its derivatives—Artemisinin drugs, which originated from theChinese herb qing hao (Artemisia annua), belong to a unique class of compounds, thesesquiterpene lactone endoperoxides [18]. The parent compound of this class is artemisinin(quinghaosu), whereas dihydroartemisinin (DHA), artesunate, artemether, and β-arteetherare the most common derivatives of artemisinin; DHA is the main bioactive metabolite of allartemisinin derivatives (artesunate, artemether, β-arteether, etc.), and is also available as adrug itself [17,18,20].

Since 2001, the WHO has recommended the use of artemisinin-based combination therapies(ACTs) for treating falciparum malaria in all countries where resistance to monotherapies ornon-artemisinin combination therapies (e.g., SP) is prevalent [23]. The rationale for the useof ACTs is based on the facts that artemisinin derivatives are highly potent and fast acting,and that the partner drug in ACT has a long half-life, which allows killing the parasites thatmay have escaped the artemisinin inhibition. Thus, it is thought that ACTs will delay theonset of resistance by acting as a “double-edged sword” [18,24,25]. ACTs, such asartemether-lumefantrine, artesunate-amodiaquine, and artesunate-mefloquine are being usedin China, Southeast Asia, many parts of Africa, and some parts of South America[17,18,20]. Introduction of ACTs has initiated noticeable reduction in malaria prevalence inthese endemic regions of the world [18]. Although the mechanism(s) of action is poorlyunderstood (described below), the current high level of interest in artemisinin drugs is due totheir well-recognized pharmacological advantages: These drugs act rapidly upon asexualblood stages of CQ-sensitive as well as CQ-resistant strains of both P. falciparum and P.vivax, and reduce the parasite biomass very quickly, by about 4-logs for each asexual cycle.

In addition, these drugs are gametocytocidal. Thus, through rapid killing of asexual bloodstages and developing gametocytes, artemisinin drugs significantly limit the transmissionpotential of the treated infections. These drugs have large therapeutic windows, and basedon extensive human use, they appear to be safe, even in children and mid/late pregnantwomen. Furthermore, there is no reported “added toxicity” when these drugs are used incombination with other types of antimalarial compounds. In addition to these three mainclasses of compounds, the antibiotic tetracycline and its derivatives, such as doxycycline, areconsistently active against all species of malaria, and in combination with quinine, areparticularly useful for the treatment of severe falciparum malaria [17]. Until recently, acombination of atovaquone, a hydroxynaphthoquinone, and proguanil (Malarone) wasconsidered to be effective against CQ- and multidrug-resistant falciparum malaria;atovaquone resistance has recently been noticed in Africa [26]. Piperaquine, anothermember of the 4-aminoquinoline group, in combination with DHA (Artekin), holds thepromise of being successful in CQ-resistant endemic areas of Southeast Asia [17,18,20].

1.3. Overview of Antimalarial Drug ResistanceThis limited antimalarial armament is now severely compromised because of the parasite’sremarkable ability to develop resistance to these compounds [19,27]. In many differentmalaria-endemic areas, low to high-level resistance in the predominant malaria parasites, P.falciparum and P. vivax, has been observed for CQ, amodiaquine, mefloquine, primaquine,and SP. Plasmodium falciparum has developed resistance to nearly all antimalarial drugs incurrent use, although the geographic distribution of resistance to any one particular drug



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varies greatly. In particular, Southeast Asia has a highly variable distribution of falciparumdrug resistance; some areas have a high prevalence of complete resistance to multiple drugs,while elsewhere there is a spectrum of sensitivity to various drugs [19]. Until 2009, nonoticeable clinical resistance to artemisinin drugs was reported. However, as describedbelow, a number of recent studies have raised concerns about the efficacy of ACTs,particularly in Southeast Asia.

1.4. Overview of Genetic Basis for Antimalarial Drug ResistanceIt is believed that the selection of parasites harboring polymorphisms, particularly pointmutations, associated with reduced drug sensitivity, is the primary basis for drug resistancein malaria parasites [28,29]. Drug-resistant parasites are more likely to be selected if parasitepopulations are exposed to sub-therapeutic drug concentrations through (a) unregulated druguse; (b) the use of inadequate drug regimens; and/or (c) the use of long half-life drugs singlyor in non-artemisinin combination therapies. In recent years, significant progress has beenmade to understand the genetic/molecular mechanisms underlying drug resistance in malariaparasites [30,31]. Chloroquine resistance (CQR) in P. falciparum is now linked to pointmutations in the chloroquine resistance transporter (PfCRT [encoded by pfcrt, located onchromosome 7]). Pfcrt-K76T mutation confers resistance in vitro, and is the most reliablemolecular marker for CQR. Polymorphisms, including copy number variation and pointmutations, in another parasite transporter, multidrug resistance (PfMDR1 or Pgh1 [encodedby pfmdr1, located on chromosome 5]), contribute to the parasite’s susceptibility to a varietyof antimalarial drugs. Point mutations in pfmdr1 play a modulatory role in CQR, whichappears to be a parasite strain-dependent phenomenon [32].

Point mutations in the P. falciparum DHPS enzyme (encoded by pf-dhps, located onchromosome 8) are involved in the mechanism of resistance to the sulfa class ofantimalarials, and accumulation of mutations in the P. falciparum DHFR domain (encodedby pf-dhfr, located on chromosome 4) defines the major mechanism of high-levelpyrimethamine resistance. In field studies, a pf-dhps double mutant (437G with either 540Eor 581G), combined with the pf-dhfr triple mutant (108N_51I_59R), was found to befrequently associated with SP treatment failure [28,29]. Orthologues of pfcrt (pvcrt-o),pfmdr1 (pvmdr1), pf-dhps (pv-dhps), and pf-dhfr (pv-dhfr) in P. vivax have been identified,and found to be polymorphic. However, associations of the mutant alleles of pvcrt-o/pvmdr1and pv-dhps/pv-dhfr with clinical resistance to CQ and SP, respectively, are unclear [33].

2. Emergence of Artemisinin Resistance in P. falciparumA major advance in the search for effective treatment for drug-resistant malaria came withthe discovery of artemisinin and its derivatives. These compounds show very rapid parasiteclearance times and faster fever resolution than any other currently licensed antimalarialdrug [18]. Given the recent significant impact of ACTs on malaria morbidity and mortality,it is worrisome that higher recrudescence rates of P. falciparum malaria after artemisinintreatment are seen in some areas. Recrudescence, the reappearance of an infection after aperiod of quiescence, occurs in up to 30% of patients on artemisinin monotherapy, and in upto 10% of patients on ACTs [18,34]. The underlying mechanism of recrudescence afterartemisinin treatment is unclear. As illustrated by recent in vitro studies, the occurrence ofparasite dormancy, where parasites enter a temporary growth-arrested state, may provide aplausible explanation for this phenomenon [35,36].

Furthermore, there are recent concerns that the efficacy of ACTs has declined near the Thai-Cambodia border, a historical “hot spot” for emergence and evolution of multidrug-resistantmalaria parasites [37]. In this region, artemisinin resistance is characterized by significantlyslower parasite clearance in vivo, with or without corresponding reductions on conventional



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in vitro susceptibility testing [38,39]. A recent heritability study found that the observedartemisinin-resistant phenotype of the parasites has a genetic basis [40]. The study showedthat genetically identical parasite strains, defined by microsatellite typing, strongly clusteredin patients with slow versus faster parasite clearance rates. It was suggested that theartemisinin-resistant phenotype is expected to spread within parasite populations that livewhere artemisinins are deployed unless associated fitness costs of the putative resistancemutation(s) outweigh selective benefits. The genetic basis for artemisinin resistance is notknown at present. The main reason for this lack of knowledge is that the molecularmechanism(s) of action of artemisinin drugs remains uncertain and debatable, although anumber of potential targets have been proposed [41,42]. Investigations into potential proteintargets of artemisinin drugs have included PfATP6, a P. falciparum SERCA-type calcium-dependent ATPase in the endoplasmic reticulum [43]; PfCRT; PfMDR1; P. falciparummultidrug resistance-associated protein 1 (PfMRP1), residing on the parasite plasmamembrane; a putative deubiquitinating protease termed UBP-1; and mitochondrial proteins[41,42]. A recent study, conducted on artemisinin-resistant P. falciparum strains fromwestern Cambodia, did not find any correlation between artemisinin-resistant phenotype andpfmdr1 mutations/amplification, as well as mutations in pfatp6 (or pfserca), pfcrt, ubp-1, ormtDNA [44].

At present there is no concrete evidence of artemisinin resistance occurring elsewhere.Preliminary reports of emerging artemisinin resistance in South America and some Africancountries are as yet unconfirmed. For now, the situation appears to be confined to the Thai-Cambodia border, and is most likely a result of distinctive features of malaria treatment inthis geographic area. These include unregulated artemisinin monotherapy for more than 30years, creating massive drug pressure, and counterfeited or substandard tablets that containless active ingredients than stated [45]. However, if the problem is not urgently andappropriately addressed, it is feared that artemisinin resistance could spread worldwide.Given the pivotal role that artemisinin drugs now play, this would be a major blow formalaria treatment and control, since there are currently no alternative drugs to replace them.Even the development pipeline of new drugs consists predominantly of ACTs.

3. Overview of the Global Pipeline of Antimalarial DrugsOlliaro and Wells [46] reviewed the global pipeline of new antimalarial combinations andchemical entities, in various stages of development till February 2009. Overall, the pipelineconsists of 22 projects, either completed (artemether-lumefantrine and artesunate-amodiaquine), in various clinical stages (phase III/registration stage, n = 4; phase II, n = 8;phase I, n = 5), or in preclinical translational stage (n = 3). Chemical novelty is relativelylow among the products in clinical stages of development; the prevailing families are ofartemisinin-type and quinoline compounds, mostly in combination with each other. Amongthe non-artemisinin and/or non-quinoline combinations and single compounds are:Azithromycin-CQ (phase III), fosmidomycin-clindamycin (phase II), methylene blue-amodiaquine (phase II), SAR 97276 (a choline uptake antagonist, phase II), and tinidazole(an anti-parasitic nitroimidazole compound, phase II).

It is clear from the size of the antimalarial drug development pipeline that the pipeline hasnot reached critical mass, which is of concern particularly when we consider the recentemergence of artemisinin resistance, and the apparent decrease in time to resistance to eachnew drug/drug combination. The challenges for now and for the future are: (i) how to ensurethat we have compounds to combat emerging and future antimalarial drug resistance; and(ii) how to initiate and expedite the development of compounds that are as innovative aspossible with respect to their chemical scaffold and molecular target.



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4. Feasible Options for Now4.1. Re-Optimizing Existing Antimalarials

4.1.1. Replacing/rotating drugs—Examples of CQ and SP—Recently, a drugreplacement/rotation approach has been suggested to stem the tide of resistance by changingthe “playing field” for malaria parasites [47,48]. This approach is based on theunderstanding that the drug-resistant mutant forms are likely to be less fit than their wild-type strains in the absence of drug selection [49]. It has been shown that when CQ waswithdrawn from Malawi, where a high level of CQR prevailed, CQ sensitivity returned [50].The return of CQ-sensitive falciparum malaria in Malawi took almost 10 years, and wasassociated with a re-expansion of pfcrt-76K allele-carrying diverse, sensitive parasites[51,52]. Similar association between cessation of CQ use and decrease in pfcrt-76T allele-carrying resistant parasites has been observed in Kenya [53] as well as in China [54,55].However, it is estimated that in these areas, sensitivity to CQ may take many more years toreturn to a level where the drug could be used effectively again [53,54].

Regarding SP, it has been shown that in the Amazon region of Peru, the frequencies of themutant pf-dhfr and pf-dhps alleles, conferring SP resistance, declined significantly 5 yearsafter the drug was replaced [48]. This, however, was not the case in Venezuela [56] andCambodia [57], where the SP resistance-conferring alleles have remained at a highfrequency 8 years and 2 decades, respectively, after the replacement of SP. It appears thatthe outcome of the drug replacement/rotation approach is not universal but malaria-endemicregion dependent; a number of diverse ecological and epidemiological factors are likely toplay a role in determining the outcome of this approach. In addition, the logistics, includingthe cost component, of this approach may pose challenges for some countries.

4.1.2. Combining drugs—Examples of CQ-CQR reversers and multiple-drugtherapy—Improving the use of existing antimalarial drugs by designing new combinationsis another potential approach to combat resistance. CQ entered widespread usage in the1940s [19]. In spite of widespread resistance, this drug is still widely available because it isinexpensive and relatively non-toxic. In a thought-provoking article, Ginsburg [58]suggested that differential use of CQ in holoendemic areas that precludes children andpregnant women could still confer an efficacious protection for semi-immune adults, andmore efficient treatment protocols could be devised to treat even patients infected with CQ-resistant parasite strains. According to the author [58], since the antimalarial activity of CQis pleiotropic, drug resistance may be due to different mechanisms, each amenable toreversal by drug combination. Since the pivotal study by Martin and colleagues [59], whichshowed that CQR in P. falciparum was partly reversible by verapamil, it has beenenvisioned that the usefulness of CQ can be rescued by attempting to reverse the resistance—By co-administering CQ with another drug that chemosensitizes the parasite to its effect[60–64]. Among more than 40 such compounds, only one, chlorpheniramine, a histamineH1 receptor antagonist, in combination with CQ has been tested in patients with somesuccess [58,60]. Of further interest, primaquine and antiretroviral protease inhibitors (PIs)saquinavir, ritonavir, and indinavir, may act as CQR reversers [60]. Fluoxetine, anantidepressant of the selective serotonin reuptake inhibitor class, has been shown to reverseboth CQ and mefloquine resistance, while ketoconazole, a synthetic antifungal drug of theimidazole class, has been shown to reverse mefloquine resistance in a monkey model [60].In addition, novel compounds that are more potent and less toxic than the archetypal CQRreverser verapamil are under investigations [60]. Thus, the reports of new resistancereversers over the last few years have generated considerable interest, and may lead to aneffective stopgap therapy for drug-resistant malaria.



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Use of combination therapy is currently mandated by the WHO [23]. With widespread high-level CQR, SP, as a 2-drug combination, was a successful first-line treatment in Africa foralmost two decades. Currently, our hope rests mainly upon ACTs and atovaquone-proguanil(Malarone). As mentioned before, it is of concern that some of these combinations havestarted showing signs of failure. A further approach to minimize/delay the development ofdrug resistance in malaria is to apply combinations of three or more drugs (“multiple-drugtherapy”), as is practiced for treating HIV/AIDS and tuberculosis. Attempts at combiningexisting drugs with CQ (e.g., CQ-SP) have proved less successful, whereas amodiaquine-SPhas shown encouraging clinical activity [65]. Recently, a triple combination of artesunate-Lapdap, despite the side effects associated with Lapdap, is under evaluation for thetreatment of uncomplicated falciparum malaria in Africa [66]. A limitation of this approachis the increased possibility of drug-drug interactions or the need for a reformulation of thedrugs, both of which can prove costly and time-consuming to resolve. Furthermore, giventhat only a handful of effective antimalarial drugs are now left, this approach seems lesspromising.

4.1.3. (Re) Considering drugs with limitations—Example of tafenoquine—Withany drug, one of the major limitations is side effects. Mefloquine was first synthesized in1969 primarily for the purpose of chemoprophylaxis in the U.S. military following the thenrecently discovered threat of CQ-resistant falciparum malaria. The drug has been in use asmonotherapy and as combination therapy with either SP or artesunate. However, due toincreasing incidences of resistance, particularly in the areas of multidrug resistance [67,68],newer information about toxicity [69,70], and the potential for severe neuropsychiatricadverse events among US military personnel [71], doxycycline is considered as areplacement for mefloquine chemoprophylaxis [72]. Although doxycycline has scanty sideeffects, a major limitation is that in prophylactic therapy, it must be taken daily leading tohigh non-compliance. Therefore, a new investigational drug, tafenoquine, an analogue ofprimaquine, related to mefloquine, was introduced in 1990s [73,74]. Although likeprimaquine, tafenoquine causes hemolysis in patients with G6PD deficiency, the drug hasthe potential advantage of a shorter treatment course, and therefore increased patientcompliance, and higher therapeutic index than primaquine [73–75]. Thus, the interest intafenoquine has been continued and efforts are directed at ameliorating side effects, so thatthis new efficacious compound can be used for malaria prevention as well as treatment [76–78].

4.2. Repurposing Drugs Used for Other Infections or DiseasesOne of the approaches for rapidly identifying and bringing new drugs to the market for thetreatment of malaria is to repurpose existing drugs that are currently approved for treatmentof other infections or diseases.

4.2.1. Drugs used for other infections—Anti-HIV/AIDS drugs—It is known thatsome antimalarial drugs have moderate anti-HIV activities [79,80]. However, recent studieshave also demonstrated that certain antiretroviral agents can inhibit malaria parasite growth.Skinner-Adams et al. [81] showed that antiretroviral PIs saquinavir, ritonavir, and indinavirwere effective against P. falciparum. Subsequently, other antiretroviral PIs were also shownto exhibit potent antimalarial activity [82,83]. The current working hypothesis for theantimalarial activity of antiretroviral PIs is that these compounds inhibit one or more of theparasite’s aspartic proteases (termed plasmepsins), located on the food/digestive vacuole ornon-digestive vacuole [84–86].

Recently, we found that a non-nucleoside reverse transcriptase inhibitor, TMC-125, whichhad been shown to be highly active against HIV [87], demonstrated potent activity against



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P. falciparum [88]. It is theorized that this compound targets the parasite enzyme PfTERT, acatalytic reverse transcriptase component of telomerase, expressed in asexual blood-stageparasites that have begun DNA synthesis [89]. There appears to be only one gene for TERTin P. falciparum [89], P. vivax [90], and P. knowlesi [89], containing some highly conservedregions across species [89,90]. Since telomerase activity is likely to be necessary duringblood-stage parasite proliferation, it is hoped that screening other reverse transcriptaseinhibitors, or designing specific anti-telomerase compounds may lead to novel antimalarialdrugs with potent activity against multiple-species infections.

The potential for crossover between malaria and HIV treatments is undoubtedly intriguing,as most of the malaria-endemic areas also bear the brunt of the HIV pandemic. However, ata time when access to antiretroviral drugs is increasing, and new combinations ofantimalarial drugs, particularly ACTs, are being evaluated, it is important that potentialinteractions between therapies for these 2 infections are also understood [79,80,91]. In thisregard, it is important to mention that human drug-metabolizing enzymes CYP2B6 (phase Imetabolism) and UGT2B7 (phase II metabolism) play important roles in the metabolism ofantiretroviral drugs efavirenz, nevirapine, and zidovudine, as well as antimalarial artemisinindrugs, including their active metabolite DHA [92–94]. Functional polymorphisms inCYP2B6 and UGT2B7 are highly prevalent in the regions where HIV/AIDS and malaria co-occur [93–95]. Phenotypic consequences of polymorphisms in these enzyme genes on thepharmacokinetics and effectiveness of artemisinin drugs are yet to be determined, andcomprehensive studies evaluating the potential interactions between these antiretroviral andantimalarial drugs in individuals with or without polymorphisms are needed.

4.2.2. Drugs used for other diseases—Anti-cancer drugs—A new area inantimalarial drug discovery is the exploration of anti-cancer drugs that target cellularprograms such as cell proliferation, cell differentiation, and cell death [88,96,97]. Theinteresting revelation that artesunate is effective against certain types of cancer, and maytarget one or more of these cellular programs [98–100], also lends credence to this path ofinvestigation. Our recent study has shown that two anti-cancer compounds, SU-11274 andBay 43–9006, exhibit potent activities (IC50 values <1 μM) against P. falciparum [88]. Inhumans, SU-11274 acts as a specific inhibitor of the MET receptor tyrosine kinase activity[101]. Bay 43–9006, a dual-action inhibitor, targets the RAF/MEK/ERK signaling pathwayin tumor cells, and receptor tyrosine kinases VEGFR/PDGFR in tumor vasculature [102].The specific targets for SU-11274 and Bay 43–9006 activities against P. falciparum are notclear yet, but with the analysis of Plasmodium spp. genomes, plasmodial protein kinases(“Plasmodium kinome”) are emerging as highly promising targets for such compounds[103,104]. In addition, inhibition of protein kinases of the human host is being viewed asanother major parasiticidal strategy [97,103,105]. Recently, Doerig and colleagues [97]showed that the host PAK-MEK signaling pathway was selectively activated in P.falciparum-infected red blood cells. Selective inhibitors of PAK (IPA-3, IC50 value 2 μM)and MEK (U0126 [IC50 value 3 μM], PD184352 [IC50 value 7 μM]) inhibited the parasiteproliferation. Moreover, the MEK inhibitors showed in vitro parasiticidal effects onhepatocyte and erythrocyte stages of the rodent malaria parasite P. berghei, indicatingconservation of this subversive strategy in plasmodia [97].

As also discussed by Doerig and colleagues [97], several kinase-inhibiting drugs are nowused clinically, mostly in a variety of cancers, with many more in different stages of clinicaltrials. Evaluating these inhibitors for antimalarial properties would considerably reduce theoverall discovery/development cost and accelerate the process. A potential problemassociated with most of these inhibitors is toxicity, as they tend to have small therapeuticindices. Most fatalities in malaria occur in children under 5, which emphasizes the need fornot only highly efficacious but also very safe drugs. Although the kinase-inhibiting anti-



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cancer drugs have toxic effects, using them in malaria chemotherapy would require muchshorter treatment periods, hopefully limiting the problem of toxicity. It would also be ofinterest to see whether these drugs can be used in combination with artemisinin drugs, whichshow no “added toxicity”, and whether such a combination helps in reducing the effectiveparasiticidal concentration of these drugs, thus reducing their toxicity.

5. Next Generation of Antimalarial Treatment—Approaches and Candidates5.1. Development of New Drugs by Modifying Existing Compounds

5.1.1. Derivatization—One method to synthesize new antimalarial drugs is to start withthe chemical framework of existing antimalarial drugs; the 4- and 8-aminoquinolines andartemisinin drugs are the two common examples of this approach [106]. Recently, a varietyof 4-aminoquinoline derivatives (thiourea, triazine, and bisquinoline) have been found to behighly active against P. falciparum in vitro, and against P. yoelii or P. berghei in mice [107–109]. Synthesis/screening of 4-position modified quinoline-methanol compounds isanticipated to yield an efficacious and less toxic replacement for mefloquine [110–112]. Inaddition to 4-aminoquinoline analogs, extensive derivatization approaches followed bybetter understanding of structure-activity relationships and biotransformation mechanisms oftoxicity have also provided 8-aminoquinoline analogs with better pharmacologic andreduced toxicologic profiles [113].

Derivatives of artemisinin, particularly the water-soluble and efficacious artesunate, areanother obvious choice for the synthesis of new compounds [20,114]. Although highlypotent and fast acting, the currently available semisynthetic artemisinin derivatives are proneto hydrolysis, resulting in a short biological half-life and the generation of potentially toxicDHA [115,116]. Therefore, extensive efforts have been made to synthesize new derivativesof artemisinin and chemically-related compounds, which are more stable and exhibit potentin vitro [117,118] and in vivo [119–122] antimalarial activities. The in vitro activity of thesenew derivatives did not indicate any apparent cytotoxicity [118].

Thus, it appears that derivatization is one possible approach to prolong the clinicalusefulness of aminoquinoline and artemisinin classes of compounds. However, as a potentiallimitation of this approach, toxicity and cross-resistance with the parent compound may stilloccur in some of the derivatives [123].

5.1.2. Hybridization—Burgess et al. reported a highly innovative hybrid molecule thatcombines the pharmacophore of an active 4-amino-7-chloroquinoline antimalarial with aresistance-reversing group based on imipramine [124]. The compound was found to behighly active against both CQ-sensitive and CQ-resistant strains of P. falciparum in vitro,and against P. chabaudi in mice, with no obvious signs of toxicity. The authors named thisclass “reversed chloroquine (RCQ)”. Further studies showed that linking any of severalreversal-agent-like moieties to a 4-amino-7-chloroquinoline yielded good parasiticidalactivity [125]. Structure-activity relationship investigation indicated that RCQ moleculesinhibit hemozoin formation in the parasite’s digestive vacuole in a manner similar to that ofCQ [126].

Recently, in a deliberate rational design of antimalarials acting specifically on multipletargets, several hybrid molecules have been developed in what has been termed “covalentbitherapy” [127]. One interesting report described the synthesis of a novel artemisinin-quinine hybrid with potent antimalarial activity [128]. Artemisinin was reduced to DHA andcoupled to a carboxylic acid derivative of quinine via an ester linkage. This novel hybridmolecule had potent activity against CQ-sensitive and CQ-resistant strains of P. falciparumin vitro. The activity was superior to that of artemisinin alone, quinine alone, or a 1:1



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mixture of artemisinin and quinine [128]. Another example is a trioxane motif ofsesquiterpene lactone artemisinin covalently linked to a quinoline entity of anaminoquinoline (e.g., CQ), to form new modular molecules referred to as trioxaquines [127].The trioxaquines had more improved antimalarial activity than their individual components,indicating a potential additive/synergistic effect of the hybrids, as well as good oralbioavailability and low toxicity [127]. Such hybrid molecules have the potential to delay orcircumvent the development of resistance and reduce the risk of drug-drug adverseinteractions, the latter often seen with multiple-drug therapies. The antimalarial activities,combined with a good drug profile (preliminary absorption, metabolism, and safetyparameters) seem favorable for the selection of hybrid molecules for development as drugcandidates [129].

5.2. Finding New Antimalarial Compounds by Exploring Natural ProductsHistorically, drug discovery and development has greatly benefited from sourcingcompounds from nature. In fact, between 1981 and 2002, 61% of new chemical entitiesbrought to the market can be traced back to, or were inspired by, natural sources [130].Malaria drug discovery is no exception. The isolation of the antimalarial drug quinine fromcinchona bark was accomplished in 1820. The bark had long been used by indigenouspeoples in the Amazon region for the treatment of fevers. The Chinese herb qing hao(Artemisia annua) was also used as a treatment for fevers in China for more than 2,000years, but it was not until 1972 that the active compound artemisinin was extracted, and lateridentified as a potent antimalarial drug. The 1990s saw a demise in research into naturalproducts for drug discovery, due in part to the emergence of high-throughput screening andcombinatorial chemistry. Today, however, the current demand for novel compounds totackle emerging antimalarial resistance has stimulated new interest in their potential [131].Several screening projects utilizing different natural sources, from the rainforest to the deepsea and from exotic microorganisms to plants, have been carried out, resulting in severalinteresting antimalarial lead compounds with remarkable chemical diversity [132–136].Some such projects at the Medicines for Malaria Venture (MMV) are in the early-stagediscovery pipeline [46].

The mode of action of compounds originating from natural products is mostly unknown,and, in order to understand the basis for their pharmacological effects, research focused ontheir synergistic or antagonistic interactions is needed [137]. It is also clear that the muchdesired success of this approach faces several challenges, such as species selection criteria,screening procedures, pharmacological models and fractionation processes, as well asprediction of clinical safety and efficacy [138,139]. Nevertheless, it is hoped that once theactivity of natural medicinal products in humans is characterized, it can be used to identifynew molecular scaffolds which will form the basis of the next generation of antimalarialtherapies [140].

5.3. Finding New Antimalarial Drugs by Screening Diverse Chemical Libraries5.3.1. High-throughput screening—One of the major thrust areas for antimalarial drugdiscovery is to screen diverse chemical libraries using high-throughput assays. In recentyears, impressive efforts by several drug-screening groups at pharmaceutical companies andacademic institutions have generated a plethora of compounds, which may serve as newstarting points for potential antimalarial drugs with novel mechanisms. Here, we summarizefive such studies (Table 1).

i. From a screen of ~1.7 million compounds, Plouffe et al. [141] identified a diversecollection of ~6,000 small molecules comprised of >530 distinct scaffolds, all ofwhich showed potent antimalarial activity (<1.25 μM). Most known antimalarials



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were identified in this screen, thus validating their approach. In addition, theauthors identified many novel chemical scaffolds, which likely act through bothknown and novel pathways.

ii. Gamo et al. [142] screened nearly two million compounds in GlaxoSmithKline’schemical library for inhibitors of P. falciparum, of which 13,533 were confirmed toinhibit parasite growth by at least 80% at 2 μM concentration. More than 8,000 alsoshowed potent activity against the multidrug resistant strain Dd2. Further analysessuggested several novel mechanisms of their antimalarial action, such as inhibitionof protein kinases and host-pathogen interaction related targets. All of these provenplasmodial inhibitors, of which ~11,000 (82%) were previously proprietary andthus unknown to the general research community, were made public to acceleratethe pace of drug development for malaria.

iii. Guiguemde et al. [143] used a phenotypic forward chemical genetic approach toassay 309,474 chemicals to discover new antimalarial chemotypes. The primaryscreen resulted in ~1,300 hits with >80% activity against P. falciparum at 7 μM. Ofthese, antimalarial potencies of 172 compounds were re-confirmed to withintenfold by three laboratories using distinct methods providing the cross-validatedhit set. A reverse chemical genetic study of this set identified 19 new inhibitors offour validated drug targets, and 15 novel binders among 61 malarial proteins. Oneexemplar compound (SJ000025081) at 100 mg kg 1 b.i.d. × 3 days resulted in a90% suppression of P. yoelii parasitemia in mice.

iv. From a library of ~ 12,000 pure natural products and synthetic compounds withstructural features found in natural products, Rottmann et al. [144] identified 275primary hits with sub-micromolar activity against P. falciparum. After a series ofanalyses, a synthetic compound, related to the spiroazepineindole class, wasselected for a medicinal chemistry lead optimization effort. Synthesis andevaluation of about 200 derivatives yielded the spirotetrahydro-β-carboline (orspiroindolone) compound NITD609. This compound displayed potent activityagainst a panel of P. falciparum strains (average IC50 range 0.5 to 1.4 nM), with noevidence of diminished potency against drug-resistant strains. The IC50 valuesagainst a clone of multidrug resistant strain Dd2, however, increased 7- to 24-fold(attaining a mean of 3 to 11 nM) after 3 to 4 months of constant drug pressure.Subsequent passaging of drug-selected parasites in drug-free media for 4 monthsshowed no evidence of revertants, indicating that the resistance was stable. Theresistance was associated with point mutations in the gene encoding the P-typecation-transporter ATPase4 (pfatp4, PFL0590c). Further safety andpharmacological preclinical evaluation of NITD609 is currently ongoing to supportthe initiation of human clinical trials.

v. Using a high-throughput colorimetric assay for the detection of hemecrystallization inhibitors, Rush et al. [145] identified 17 hit compounds out of16,000 small molecules belonging to diverse structural classes. The IC50 values ofthese 17 hit compounds in the P. falciparum growth inhibition assay ranged from0.2 μM to 19 μM, and the compounds belonged to structurally related pyrimidineand 1,3-benzoxathiol-2-one classes. These hit compounds are being tested againstmultidrug-resistant strains of P. falciparum to further confirm that they do not showcross-resistance with currently deployed heme crystallization inhibitors.

5.4. Virtual ScreeningVirtual screening involves the rapid in silico assessment of large libraries of chemicalstructures in order to identify those which are most likely to bind to a drug target, typically a



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protein receptor or enzyme. This approach either alone or combined with high-throughputscreening is gaining popularity in the antimalarial drug discovery area. Plouffe et al. [141]showed that in some cases the mechanism of action of novel antimalarial compounds,identified by high-throughput cell-based screen, can be determined by in silico compoundactivity profiling. Using clustering by historical activities and the guilt-by-associationprinciple, the authors found that the compounds may segregate based on their mechanism ofaction (e.g., clusters of protein synthesis inhibitors or folic acid antagonists). Using an insilico approach, Woynarowski et al. [146] found that the targets for AT-specific DNA-reactive antitumor drugs adozelesin and bizelesin, which inhibited the growth of P.falciparum at picomolar concentrations, were “super-AT island” regions of the parasitegenome. Furthermore, investigation of the essentiality of a reaction in the metabolic networkof P. falciparum by deleting (knocking-out) such a reaction in silico [147], and comparisonof reconstructed metabolic network models from the parasite and its human host [148] hasrevealed essential enzymes from the parasite alone, representing new potential drug targets.As molecular databases of compounds and target structures are becoming larger, more andmore computational screening approaches are becoming available, providing new and betterinsights into in silico antimalarial drug discovery [149,150]. Using such computationalscreening approaches, novel and potent inhibitors of P. falciparum plasmepsins [151,152],glutathione-S-transferase [153], and DHFR [153] have been identified.

Thus, it is anticipated that with the aid of these new screening technologies, ongoing andnew projects, including those at the MMV, will keep the malaria drug discovery anddevelopment pipeline full with the next generation of potential malaria chemotherapeutics[46].

5.5. Genome-Based (“Targeted”) Drug DiscoveryThe complete genomes of the predominant malaria parasites P. falciparum and P. vivax, andthe human host are now known. The P. falciparum genome sequence comprises of 14chromosomes containing 5,300 identified genes [154]. Undoubtedly, the completion of thesePlasmodium spp. genomes, and their comparison with the human genome, provides theopportunity to discover parasite-specific, novel molecular targets for malaria therapy andprevention [155]. The information regarding gene expression and key regulatorycomponents of metabolism in P. falciparum is being assembled into various databases, togenerate and visualize network models of various cellular pathways and processes[156,157]. Efforts are also underway to characterize the importance of many of thesepathways and to predict their potential for pharmacological interventions [158–160].Furthermore, linking this increase in advancing new genetic information with the expressionof drug resistance will greatly assist in identifying sites of drug resistance as well asstrategies to improve the treatment [161].

A wealth of information on these themes has been presented in various recent articles [162–168]. The MMV currently has 30 projects in the discovery phase [46]. Out of these, 14discovery projects are on molecularly defined targets, with defined mechanisms of action,and 16 are on whole-cell activity, and therefore have unknown mechanisms of action. Inaddition to this, the MMV has a target database that currently contains more than 23 newtargets at different stages of screening and validation. The molecular targets in ongoing 14discovery projects at the MMV include P. falciparum cysteine proteases, termed falcipains[169–172], heat shock protein 90 (Hsp90) [173–175], DHFR with new series of inhibitors, avariety of kinases, histone deacetylases (HDAC), and dihydroorotate dehydrogenase(DHODH). Here, we present a summary of the recent key findings related to parasitekinases, HDAC, and DHODH as drug targets. As described below, since these enzymesseem essential for the parasite, they may be highly conserved among Plasmodium spp., andmutations in them are likely to be lethal. Therefore, the probability of the parasite becoming



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resistant to compounds directed against these enzymes should be minimal to none. Inaddition to these enzyme targets, we summarize key findings related to parasitesequestration receptors/anti-sequestration compounds as another interesting possible drugtarget.

5.5.1. Kinases—The life cycle of malaria parasites is very tightly regulated, since it passesthrough a succession of developmental stages that vary in terms of proliferation anddifferentiation. The successful completion of such a complex life cycle requires a strictcontrol of the cellular machinery that carries out phosphorylation, transcriptional control,post-transcriptional control, and protein degradation. These mechanisms most likely involvefine interactions among various cell-cycle proteins (particularly kinases), which mayrepresent strategic targets for pharmacological interventions [96,104,176].

There are a large number and variety of kinases (“Plasmodium kinome”), many of which areclearly distinct from human protein kinases, and thus are considered as targets of choice forantimalarial drug development [176,177]. A serine/threonine protein kinase, casein kinase2α (PfCK2α), crucial for asexual blood-stage parasites, has been identified as a potentialtarget for antimalarial chemotherapeutic intervention [178]. Among cyclin-dependentprotein kinases, Pfmrk is the most attractive target for antimalarial drug development; avariety of inhibitors selectively inhibit Pfmrk at sub-micromolar concentrations [179–181].Calcium-dependent protein kinase 1 (PfCDPK1), essential for parasite survival, can beinhibited by small-molecule compounds at nanomolar concentrations [182,183]. An orphanprotein kinase PfPK7, significant for asexual stage development in humans and oocystproduction in mosquitoes, has recently been identified as a lead target [184–187]. Inaddition, mitogen-activated protein kinase 2 (PfMAP-2), essential for completion of theasexual cycle 188, NIMA-related protein kinase Pfnek-2, predominantly expressed in, andrequired for transmission of, gametocytes [189], and cAMP-dependent protein kinase(PfPKA), essential for parasite growth and survival [190], hold the promise of beingselective and effective targets for the development of new antimalarial drugs.

5.5.2. Histone deacetylases—Histone acetylation plays key roles in regulating genetranscription in both prokaryotes and eukaryotes, the acetylated form inducing geneexpression while deacetylation silences genes. A number of HDAC inhibitors are currentlyin clinical trials, alone or in combination, for the treatment of a variety of cancers, parasitic/infectious diseases, and hemoglobinopathies [191,192]. Plasmodium falciparum has at leastfive putative HDACs, which play a major role in transcriptional regulation of the parasitelife cycle [193]. Among these, PfHDAC1 in particular is considered as a promising newantimalarial drug target. A variety of PfHDAC1 inhibitors showed potent in vitroantimalarial activity (IC50 values in nM range) against CQ-sensitive and CQ-resistant strainsof the parasite, and several of the inhibitors were significantly more toxic to the parasitesthan to mammalian cells [194,195]. Furthermore, an HDAC inhibitor WR301801 (YC-2–88)exhibited cures in P. berghei-infected mice at a dose of 52 mg/kg/day orally, whencombined with subcurative doses of CQ [196]. Recently, using a modified schizontmaturation assay, certain HDAC inhibitors were found to exert potent activity (at nMconcentrations) against multidrug-resistant clinical isolates of both P. falciparum (n = 24)and P. vivax (n = 25) from Papua, Indonesia, suggesting that these inhibitors may bepromising candidates for antimalarial therapy in geographical locations where both speciesare endemic [197].

5.5.3. Dihydroorotate dehydrogenase—Erythrocytic stages of P. falciparum seem tomaintain an active mitochondrial electron transport chain to serve just one metabolicfunction: regeneration of ubiquinone required as the electron acceptor for DHODH, anessential enzyme for de novo pyrimidine biosynthesis [198]. For this very reason,



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PfDHODH is also receiving increasing attention as a new target for antimalarial drugdiscovery [199,200]. A variety of potent inhibitors of PfDHODH showing strong selectivityfor the malarial enzyme over that from the human host have been designed and synthesized,and identified by high-throughput screening [201–204]. Potent activity against Plasmodiumspp. parasites in vitro at nM concentrations, with good correlation between activity againstthe parasites and the enzyme from the parasites has been observed for a number of theseinhibitors [201,205]. Lead optimization of a triazolopyrimidine-based series of inhibitors hasidentified Genz-667348, which is orally bioavailable, with prolonged plasma exposure, andcures P. berghei infection in mice at a total dose of 100 mg/kg/day [201]. Further refinementof the structure-activity relationship within this series is underway, with 2 recent analogs ofGenz-667348 undergoing further testing to determine if they represent suitable candidatesfor preclinical development [201].

5.5.4. Anti-sequestration compounds—An interesting possibility—In addition,new compounds may participate in the combat against falciparum malaria not by directlykilling but preventing the parasite from causing severe forms of the disease. One way toachieve this is to inhibit the parasite’s ability to sequester. Sequestration occurs becauseparasite-derived adhesins, expressed on the surface of mature infected erythrocytes, bind toreceptors on human cells (often referred to simply as cytoadherence or cytoadhesion).Organ-specific sequestration in brain and placenta play an important role in the pathogenesisof cerebral and placental malaria, respectively [5,206]. Plasmodium falciparum-infectederythrocytes have been shown to have the potential for binding to a diverse array ofendothelial receptors; however, only two of these receptors, CD36 and intracellular adhesionmolecule 1 (ICAM1), have been studied in detail [206]. Chondroitin sulfate A (CSA) hasbeen consistently identified as the dominant placental adhesion receptor [5].

In recent years, some progress has been made in inhibiting cytoadherence that is mediatedby these receptors. CD36-mediated cytoadherence can be inhibited in vitro by antiretroviralPIs ritonavir and saquinavir [207], and polysaccharides derived from seaweed(carrageenans) [208]. A randomized clinical trial of Thai patients with uncomplicatedmalaria showed that levamisole, an antihelminth drug, known to block cytoadherence toCD36, when used as an adjunctive therapy with quinine and doxycycline, resulted inincreased numbers of early-mid trophozoites in the peripheral blood [209]. It was suggestedthat levamisole prevented the sequestration of these parasites as they matured from ringstages following treatment [209]. Using the crystal structure of ICAM1, Dormeyer andcolleagues [210] found that (+)-epigalloylcatechin gallate, a naturally occurring polyphenolcompound in green tea, inhibited cytoadherence to ICAM1 in a dose-dependent manner withtwo variant ICAM 1-binding parasite lines. Using CSA-expressing CHO-K1 cells and exvivo human placental tissue, Andrews et al. [211] showed that two carrageenans and acellulose sulfate (CS10) were able to inhibit cytoadhesion to CHO-K1 cells as well ashuman placental tissue; the inhibitory effect on CHO-K1 cells was dose-dependent.

Inhibition of cell adhesion processes is becoming increasingly interesting in the discovery ofnovel therapeutics [212]. Although our understanding of the molecular mechanisms ofparasite adhesion is still incomplete, and many questions remain unanswered [206], thediscoveries outlined above open up the possibility of developing therapeutic interventionsaimed at blocking or reversing parasite adhesion.

6. A Final NoteIn these various approaches to: (i) efficient use of existing as well as forthcoming newantimalarial drugs and (ii) discovery of novel antimalarial drugs, one important aspect notdiscussed here is the pharmacology of antimalarial agents. Since malaria is a blood



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infection, the response to an antimalarial treatment is likely determined by the concentrationof a drug and/or its active metabolite(s) in the blood. Although antimalarial drugs have beenin use since the 1930s, until recently, no data were available on the absorption-distribution-metabolism-excretion parameters of these drugs in humans. Over the past 10–15 years, therehave been many methodological developments in the area of pharmacology related toantimalarials. Through these advancements, pharmacokinetics and the major metabolicpathways of a number of antimalarial drugs have been elucidated [33,167,213]. However,little emphasis has been placed on understanding the basic mechanisms responsible for thepharmacokinetic and pharmacodynamic behaviors of the major classes of antimalarial drugsin various populations. Particularly, what role does human genetic diversity play in theoutcomes of treatment, such as failure, resistance, and side effects, with antimalarial drugs?Since drug combinations are increasingly being considered to successfully treat malariaworldwide, such information is essential to optimize antimalarial drug regimens, particularlynew regimens, in order to achieve high treatment effectiveness across all malaria-endemicregions.

7. ConclusionsIn the battle against malaria, we are currently at a stalemate at best. If and when theresistance to artemisinin drugs further develops and spreads, we will be caught unarmed,without any effective weapon against this deadly parasite. Looking at the antimalarial drugdevelopment pipeline, new classes of agents, and, in the meantime, a re-optimization ofexisting drugs using new creative strategies are needed. Re-optimization of existing drugsthrough replacement/rotation and combination approaches may prolong their life spans foreffective treatment and prophylaxis, until new treatments are found. The future ofantimalarial drug discovery lies in innovative thinking and novel areas, some currently underexploration and some yet to be explored. As described in this article, there are a number ofdifferent promising approaches to the identification of new antimalarials. Among theseapproaches, repurposing drugs that are used for other infections or diseases, exploringnatural products with medicinal significance (in malaria), and target-based drug discoveryare of particular interest. In addition, evolving technologies, such as high-throughput cell-based assays and virtual screening, will certainly be useful in discovering new effectivetreatments for malaria relatively quickly. We do not have any choice but to explore many orall of these approaches in parallel, until existing drugs with renewed potential, as well asentirely new series of compounds are available for deployment in the field. The discovery ofnew treatments for malaria, along with their proper execution in the field, will contribute toan important achievement of controlling, and eventually eradicating, this global infectiousdisease.

AcknowledgmentsThe authors thank Kerry O’Connor Grimberg, Dave McNamara, Kathy Andrews, and Cara Henry-Halldin, forhelpful discussions and critical evaluation of this manuscript. RKM dedicates this article to the loving memories ofhis mentors late B. N. Singh and O. P. Shukla, from whom he learnt philosophy of Science and realities of Life.RKM is also thankful to his teacher Sai Baba, whose unconditional love helps him think, write, and beyond. BTG issupported by grants from; the National Institute of Health (AI079388), The International Society for Advancementof Cytometry Scholars Program, and the Case Western Reserve University Vision Fund.

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Author Manuscript NIH Public Access Rajeev K. Mehlotra ...aquadoc.typepad.com/files/grimberg-antimalarial-rx-2011.pdfMore than 40% of the world’s population, much of it socioeconomically