Repositioning: the fast track to new anti-malarial medicines?
- Julie Lotharius1,
- Francisco Javier Gamo-Benito2,
- Iñigo Angulo-Barturen2,
- Julie Clark3,
- Michele Connelly3,
- Santiago Ferrer-Bazaga2,
- Tanya Parkinson4,
- Pavithra Viswanath5,
- Balachandra Bandodkar5,
- Nikhil Rautela5,
- Sowmya Bharath5,
- Sandra Duffy6,
- Vicky M Avery6,
- Jörg J Möhrle1Email author,
- R Kiplin Guy3 and
- Timothy Wells1
© Lotharius et al.; licensee BioMed Central Ltd. 2014
Received: 5 February 2014
Accepted: 23 March 2014
Published: 14 April 2014
Repositioning of existing drugs has been suggested as a fast track for developing new anti-malarial agents. The compound libraries of GlaxoSmithKline (GSK), Pfizer and AstraZeneca (AZ) comprising drugs that have undergone clinical studies in other therapeutic areas, but not achieved approval, and a set of US Food and Drug Administration (FDA)-approved drugs and other bio-actives were tested against Plasmodium falciparum blood stages.
Molecules were tested initially against erythrocytic co-cultures of P. falciparum to measure proliferation inhibition using one of the following methods: SYBR®I dye DNA staining assay (3D7, K1 or NF54 strains); [3H] hypoxanthine radioisotope incorporation assay (3D7 and 3D7A strain); or 4’,6-diamidino-2-phenylindole (DAPI) DNA imaging assay (3D7 and Dd2 strains). After review of the available clinical pharmacokinetic and safety data, selected compounds with low μM activity and a suitable clinical profile were tested in vivo either in a Plasmodium berghei four-day test or in the P. falciparum Pf3D70087/N9 huSCID ‘humanized’ mouse model.
Of the compounds included in the GSK and Pfizer sets, 3.8% (9/238) had relevant in vitro anti-malarial activity while 6/100 compounds from the AZ candidate drug library were active. In comparison, around 0.6% (24/3,800) of the FDA-approved drugs and other bio-actives were active. After evaluation of available clinical data, four investigational drugs, active in vitro were tested in the P. falciparum humanized mouse model: UK-112,214 (PAF-H1 inhibitor), CEP-701 (protein kinase inhibitor), CEP-1347 (protein kinase inhibitor), and PSC-833 (p-glycoprotein inhibitor). Only UK-112,214 showed significant efficacy against P. falciparum in vivo, although at high doses (ED90 131.3 mg/kg [95% CI 112.3, 156.7]), and parasitaemia was still present 96 hours after treatment commencement. Of the six actives from the AZ library, two compounds (AZ-1 and AZ-3) were marginally efficacious in vivo in a P. berghei model.
Repositioning of existing therapeutics in malaria is an attractive proposal. Compounds active in vitro at μM concentrations were identified. However, therapeutic concentrations may not be effectively achieved in mice or humans because of poor bio-availability and/or safety concerns. Stringent safety requirements for anti-malarial drugs, given their widespread use in children, make this a challenging area in which to reposition therapy.
Effective anti-malarial treatment with artemisinin-based combination therapy (ACT) has been critical for supporting and consolidating recent gains in malaria control, with reductions in the number of cases and in mortality . Malaria elimination is becoming a reality for some countries , and strategies for global malaria eradication are now being considered [3, 4]. This will require new drug regimens with improvements in cost, simplicity and efficacy against resistant strains . In particular, the emergence of Plasmodium falciparum strains that are tolerant to artemisinin in the Thai-Cambodia border area is of great concern . This not only has direct implications for artemisinin therapy, but promotes the selection of strains resistant to partner drugs.
New anti-malarial drugs are needed urgently . Recent improvements in cell-based screening technology have led to over 20,000 new starting points in medicinal chemistry [8–10], and the great majority of these data are open access . This has led to a whole series of new molecules in preclinical development . For example, one series, the spiroindolones, has entered early clinical studies only five years after the initiation of screening .
In general, however, malaria projects take much longer than five years to go from discovery to having a clinical candidate. Sometimes this is because of technical challenges, but more often because of lack of funding or other resources and the attrition rates are high. It is clearly important to search for new approaches to make this process more efficient. An alternative approach is that of drug repositioning or repurposing. Most simply, this is taking a molecule that has been developed for one indication and showing its utility in another. Although the concept is widely discussed as an attractive drug development strategy, meaningful published data on its success rate and the factors determining that success are limited.
Starting with a molecule that has already undergone clinical trials in another indication provides several potential advantages. The clinical safety profile will be understood, and safe therapeutic doses will have been established. Importantly, human pharmacokinetic data will exist and provide some indication of whether therapeutic concentrations in the new indication can be achieved safely and maintained in patients. In addition, there are regulatory fast track processes, such as the US Food and Drug Administration (FDA) 505 (b) (2) process, where the applicant can rely on data from the studies done by others (with or without the right to reference them) to progress the compound for the new indication. This has acted as a spur to finding new activities of old molecules .
Programmes to identify new clinical activities of existing medicines have been conducted in many therapeutic areas, such as oncology  and for orphan diseases , where there is often an extremely high and specific unmet medical need. Approaches have also been successful in infectious disease, such as tuberculosis , schistosomiasis  and onchocerciasis . In human African trypanosomiasis, fexinidazole was not so much repositioned as rediscovered following compound mining efforts of more than 700 new and existing nitroheterocycles; efficacy in animal models was initially reported in the 1980s [20, 21].
In malaria, there have also been initiatives in drug repositioning. Screening a library of 2,687 compounds containing 1,937 FDA-registered medicines and 750 other molecules in clinical development identified astemizole (a histamine H1 antagonist) as the most promising compound, with good activity against P. falciparum blood stages . Unfortunately, this drug was withdrawn because of side effects linked to QTc prolongation, so could not be repositioned as an anti-malarial. A smaller collection of 1,037 existing drugs was tested in an assay for activity against Plasmodium liver stages and decoquinate was identified as a potent inhibitor both in vitro and in vivo[23, 24]. As this drug has a veterinary indication, no human safety information is available, but it remains an interesting possibility.
A further potential source of drugs for repositioning is those molecules where clinical development has been discontinued before approval. Of particular interest are drugs that did not achieve efficacy in their proposed indication even though a safe plasma exposure could be obtained in humans. However, it may be difficult to obtain information on such drugs, or gain access to physical samples of them.
In the course of screening large compound collections from pharmaceutical and biotechnology companies against the blood stages of P. falciparum[8–10], it was apparent that compounds that had progressed to clinical development were often excluded from the test set. The studies outlined in this paper aimed to specifically identify and test molecules that were not clinically available, but for which some clinical development activity had been conducted. Existing libraries of FDA-approved drugs and some selected bio-actives were also tested, with particular emphasis on antineoplastic and antiretroviral agents. Any compounds showing low micromolar activity and with a suitable pharmacokinetic and safety profile were further evaluated in vivo.
Summary of compounds tested, in vitro screening methods, and results
Number of compounds
In vitro testing by:
In vitro testing method
No. hits (%)
No. tested in vivo*
• 800 FDA-approved drugs (2008)
• SYBR® I dye DNA staining assay
• 2,700 bio-actives (Prestwick, Sigma-Lopac, and MSD)
• P. falciparum 3D7 and K1
• 296 FDA-approved drugs (2009)
• 47 ‘anti-proliferative’ compounds
GSK discontinued clinical candidates
• Whole-cell [3H] hypoxanthine radioisotope incorporation
• P. falciparum 3D7A
Pfizer STLAR library of discontinued clinical candidates
• HTS screen using DAPI DNA imaging assay with P. falciparum 3D7 and Dd2
• EC50 determined using SYBR® I dye DNA staining assay with P. falciparum 3D7 and K1
AstraZeneca discontinued clinical candidates
• SYBR® I dye DNA staining assay
• P. falciparum NF54
An initial set of around 3,500 compounds was assembled and tested by St Jude’s Children’s Research Hospital (SJCRH). This comprised a library of approximately 800 FDA-approved drugs registered up to the year 2008, plus about 2,700 bio-active compounds sourced from the complete Prestwick, Sigma-Lopac and Merck Sharp & Dohme (MSD) libraries. Subsequently, a smaller set of 296 FDA-approved drugs updated for 2009 was tested as well as a small library of 47 ‘antiproliferative’ compounds to further assess targets related to protein kinase inhibitors, antineoplastic and antiretroviral agents. Compounds were not deselected based on known toxicities in order to provide information that could inform the identification and selection of related compounds in development, which could be sourced subsequently. In total, the consolidated test set included approximately 3,800 unique compounds, excluding known anti-malarial drugs. Compounds for the SJCRH screens were sourced firstly from the SJCRH drug repository or, if not available, were obtained from commercial vendors or resynthesized. All supplied compounds were assured by the vendor as >90% pure with quality control data provided and were verified internally at SJCRH after plating.
An initial search of the GlaxoSmithKline (GSK) clinical development pipeline on a commercially available database (Thomson Pharma) revealed around 100 compounds that had been taken into clinical development and subsequently been discontinued. Excluding those molecules that had already been screened against P. falciparum in the GSK discovery library , samples were obtained from GSK for 63 new compounds. GSK verified samples for purity and activity, and conducted the in vitro testing for this compound set.
Pfizer Inc were also approached, and offered to screen their STLAR library of 176 drugs, comprised mainly of pre-Phase III discontinued clinical candidates, though Phase III data were available for a few compounds. There were no approved drugs or active clinical candidates in the set. Pfizer provided samples verified for purity and activity. First, the compound set was tested in vitro using high-throughput screening (HTS) by Discovery Biology, Griffith University, Nathan, Australia with subsequent EC50 determination by Pfizer in-house.
AstraZeneca (AZ) identified a set of 100 candidate drugs from other therapeutic areas for testing against P. falciparum. All 100 candidates had been discontinued for the original indication, and Phase I/II data were available for several compounds. AZ verified the samples for purity and conducted in vitro and in vivo testing for the compounds.
None of the test sets described above was prescreened for pharmacokinetics/safety but included in their entirety. This was because identification of any active compound could also have led to testing of related follow-up compounds that did not reach clinical testing (and so would not have been included in the initial test set).
In vitro screening assays
More detailed information on the in vitro methods is provided in Additional file 1.
SJCRH used the SYBR® I dye DNA staining assay, which measures proliferation of P. falciparum in human erythrocytes . Plasmodium falciparum strains 3D7 (chloroquine-sensitive) and K1 (chloroquine-resistant) (American Type Culture Collection [ATCC], Manassas, VA, USA) were maintained using established methods . The assay method is as previously described . Tests were run in triplicate in two independent runs to generate ten-point, dose–response curves to determine the half maximal effective concentration (EC50) against the 3D7 and K1 P. falciparum strains for each drug. EC50 values were calculated with the robust investigation of screening experiments (RISE) algorithm with a four-parameter logistic equation. EC50 values of <1 μM were considered significant.
GSK Tres Cantos used a whole-cell [3H] hypoxanthine radioisotope incorporation assay to determine per cent parasite inhibition at 48 hours and 96 hours [30, 31]. Plasmodium falciparum 3D7A strain (Malaria Research and Reference Reagent Resource Center MR4; ) was maintained as described previously . Parasite growth inhibition assays and EC50 determination were carried out following standard methods . Three independent experiments were conducted for each time duration and test compound. Inactive and active controls were also included. Parasite inhibition of ≥50% at 48 hours relative to non-treated parasitized controls was considered significant.
For the Pfizer STLAR set, initial HTS was performed by Discovery Biology, Griffith University, Australia using a 4′,6-diamidino-2-phenylindole (DAPI) DNA imaging assay . Plasmodium falciparum 3D7 and the Dd2 clone, which has a high propensity to acquire drug resistance were maintained using standard methods with some adaptations [28, 33]. Inhibition values of treated wells were calculated relative to the minimum and maximum inhibition controls . Inhibition of ≥50% at a concentration of 0.784 μM was considered significant. Following the HTS findings, EC50 values were determined for a subset of active compounds by Pfizer using a SYBR® I dye DNA staining assay, similar to that described above for SJCRH, using P. falciparum 3D7 and K1 (both from David Baker, LSHTM). Per cent anti-malarial activity was calculated relative to the minimum and maximum controls for each of the 11 drug concentrations and EC50 values determined from the resulting data plot.
AZ also used a SYBR® I EC50 determination assay, but with P. falciparum NF54 (MRA-1000, MR4, ATCC, Manassas, VA, USA). The per cent inhibition with respect to the control was plotted against the logarithm of the drug concentration. The curve was fitted by non-linear regression using the sigmoidal dose–response (variable slope) formula to yield the concentration–response curves. The concentration at which 50% inhibition was observed was taken as the EC50 value of the compound. A cytotoxicity assay was also performed by AZ, using the human hepatoma Hep G2 cell line and the per cent inhibition and EC50 values were calculated as described for P. falciparum.
For those compounds showing in vitro activity in any of the above tests, the available published and unpublished toxicity, clinical safety and human pharmacokinetic data were reviewed (due diligence).
In vivo assays
Compounds that showed promising activity in vitro and that had an acceptable toxicity/safety/pharmacokinetic profile were progressed to in vivo testing. For the AZ compound set, a Plasmodium berghei four-day suppression test was used. For all other compound sets, activity against P. falciparum in the huSCID mouse was determined (as described below). Animal experiments complied with all national and European Union laws, guidelines and codes of conduct for animal care and research use.
Plasmodium berghei four-day suppression test
AZ compounds were tested by the company for in vivo efficacy in a standard four-day suppression test using the rodent malaria parasite P. berghei. All animal experimentation protocols were approved by the Institutional Animal Ethics Committee registered with the Government of India (Registration No: 5/1999/CPCSEA). Adult male BALB/c mice (purchased from RCC Laboratories, Hyderabad, India) were used for efficacy studies. Animals were randomly distributed to cages quarantined for one week with veterinary examination and then taken into experimentation. Feed and water were given ad libitum. Briefly, male BALB/c mice were infected intraperitoneally with 2×107 infected erythrocytes on day 0. Test compounds were administered orally at a volume of 10 mL/kg as once (UID) or twice daily (BID) doses every 24 hours for four days. On day 3, per cent parasitaemia was estimated microscopically from a Giemsa-stained blood smear. The effect of the test compound on parasite growth was calculated as the difference between the mean value of the control group (taken as 100%) and those of the experimental group and expressed as per cent reduction. Reference anti-malarial compounds (chloroquine and artemisinin) were used as positive controls and the results obtained matched those published in the literature. Pharmacokinetics were analysed in healthy as well as infected mice. Data from healthy mice were used for designing the dosing regimen for the efficacy studies. In infected mice, pharmacokinetics was carried out on day 2 of compound administration. One mouse per time point was sampled according to the fast mouse pharmacokinetic protocol .
Plasmodium falciparum huSCID mouse model
In vivo testing using this model was performed by GSK at Tres Cantos, against P. falciparum 3D7 (in vivo strain Pf3D70087/N9generated by GSK using Pf3D7 obtained from Eduardo Dei-Cas, Institute Pasteur, Lille, France ) growing in peripheral blood of female NOD-scid IL-2Rγnull mice engrafted with human erythrocytes, i e, a ‘humanized’ mouse model, following published protocols [36, 37]. Briefly, animals were infected intravenously with 20×106 infected erythrocytes on day 0. Test compounds were administered orally at a volume of 20 mL/kg or subcutaneously (10 mL/kg) in an appropriate inactive vehicle. Dosing was initiated at the maximum tolerated dose in mice on day 3 after infection and continued once daily for four days. Each experimental group was n = 3 mice unless otherwise stated. Control animals received vehicle only and a quality control assay used chloroquine at target doses of 3 mg/kg and 7 mg/kg. Venous blood samples for parasitology (2 μL) were taken at days 3, 5, and 7 after infection. Anti-malarial efficacy was assessed using a standard four-day test (i e, at day 7) and blood parasitaemia was measured by fluorescence-activated cell sorting (FACS) analysis . The limit of detection (per cent of P. falciparum) was 0.01%. The number of parasites ×106 cells was recorded and data were analysed by non-linear fitting to a logistic equation of log10 (per cent parasitaemia at day 7 after infection) versus the dose level administered.
Per cent parasitaemia at day 7 after infection in treated versus control animals was analysed using a one factor ANOVA with Tukey’s post-test analysis. If there was a significant difference (P < .05) then the ED50 was calculated as the dose in mg/kg that reduced parasitaemia at day 7 after infection by 50% with respect to vehicle-treated mice. ED90 was calculated similarly. Analysis was performed using WinNonlin 5.2 and GraphPad Prism 5.0.
The pharmacokinetics of compounds after oral administration was determined concurrently in the same mice used for the therapeutic efficacy assay. Samples were taken at 0.25, 0.5, 1, 3, 6, 8, and 24 hours after the first dose. Compound levels were measured in 25 μL blood samples that were mixed with 25 μL of saponin (0.1% in water) and processed under standard liquid–liquid extraction conditions . Pharmacokinetic parameters were calculated using WinNonlin 5.2 non-compartmental analysis. The data for the exposure of the drug in blood (area under the curve, AUC) after the first oral administration and parasitaemia at day 7 were fitted to a logistic function to predict the exposure necessary to inhibit parasitaemia at day 7 after infection in compound-treated mice by 90% with respect to vehicle-treated mice (AUCED90).
Most active compounds tested by St Jude’s Children’s Research Hospital
Class (therapeutic area)
Nitric oxide/guanylate cyclase inhibitor (various)
Nucleoside reverse transcriptase inhibitor (oncology)
0.0009 (0, 0.13)
0.001 (0.0003, 0.006)
Dihydrofolate synthetase inhibitor (anti-infective)
0.01 (0.01, 0.01)
0.01 (0.01, 0.01)
Dihydrofolate reductase inhibitor (oncology)
0.01 (0.009, 0.01)
0.02 (0.01, 0.02)
Proteasome inhibitor (oncology)
0.02 (0.01, 0.04)
0.08 (0.07, 0.09)
Post-synaptic receptor agonista (anti-psychotic)
0.04 (0, 233.71)
0.02 (0.01, 0.05)
0.06 (0.002, 1.53)
0.06 (0.03, 0.12)
Topoisomerase II inhibitor, DNA intercalating agent (oncology)
0.21 (0.16, 0.27)
0.20 (0.14, 0.30)
Inhibition of DNA, RNA, phospholipid and protein synthesisb (anti-infective)
0.22 (0.18, 0.27)
0.05 (0.04, 0.06)
Tyrosine kinase inhibitor (oncology)
0.22 (0.016, 3.11)
0.65 (0.36, 1.19)
Dihydrofolate reductase inhibitor (oncology)
0.32 (0.30, 0.33)
1.25 (1.11, 1.41)
Multi-kinase inhibitor (oncology)
0.35 (0.17, 0.71)
0.15 (0.13, 0.17)
FMS-like tyrosine kinase 3 inhibitor (oncology)
0.49 (0.28, 0.84)
0.34 (0.29, 0.41)
Cholinesterase inhibitor (ophthalmology)
0.51 (0.45, 0.57)
0.30 (0.26, 0.36)
Steroidal anti-androgen (oncology)
0.56 (0.54, 0.58)
0.89 (0, 1501.50)
Tyrosine kinase inhibitor (oncology)
0.56 (0.39, 0.80)
Dopamine receptor blocker (anti-psychotic)
0.70 (0.44, 1.11)
HMG-CoA reductase inhibitor (anti-cholesterol)
0.75 (0.51, 1.09)
0.12 (0.10, 0.15)
0.84 (0.71, 0.98)
0.50 (0.21, 1.16)
Inhibition of DNA synthesis (oncology)
0.97 (0.81, 1.17)
0.51 (0.45, 0.57)
Sodium channel modulator (cardiology)
1.22 (0.58, 2.55)
0.33 (0.31, 0.34)
Immune suppressant (oncology)
1.23 (1.06, 1.44)
0.87 (0.62, 1.23)
Histone deacetylase inhibitor (oncology)
1.47 (1.17, 1.84)
0.84 (0.76, 0.93)
Multi-kinase inhibitor (oncology)
2.71 (2.4, 3.1)
0.88 (0.7, 1.1)
Most active compounds in vitro from the GlaxoSmithKline discontinued drugs compound set
Class (therapeutic area)
Dihydrofolate reductase inhibitor (oncology)
0.011 ± 0.001
Phospholipase A2-inhibitor (anti-infective/anti-inflammatory)
1.126 ± 0.146
Topoisomerase I inhibitor (oncology)
0.191 ± 0.062
Muscarinic receptor agonist (asthma)
1.582 ± 0.206
Most active compounds in vitro from the Pfizer STLAR library
Class (therapeutic area)
Dual platelet activating factor/histamine H1 receptor antagonist (allergic rhinitis)
0.55 (0.45, 0.65)
Neuropeptide Y5 receptor antagonist (obesity)
0.40 (0.2, 0.6)
TIE2 tyrosine kinase inhibitor (oncology)
Bradykinin B2 receptor antagonist (pain)
0.65 (0.36, 0.94)
CDK1/2/4/5 inhibitor (oncology)
0.7 (0.11, 1.29)
Most active compounds in vitro from the AstraZeneca discontinued drugs compound set
Status in original indication
Stopped after GLP toxicity
Stopped after GLP toxicity
Stopped after Phase II
Stopped after GLP toxicity
Aurora kinase 1
Stopped after GLP toxicity
Stopped after GLP toxicity
Plasmodium falciparum huSCID mouse model
Therapeutic efficacy of test compounds against Plasmodium falciparum Pf3D7 0087/N9 in a humanized mouse model
Target dose (mg/kg)/route
AUCED90(μg · h · mL−1 · day−1)
100, 300 po
80.1 (99.8, 55.1)
131.3 (112.3, 156.7)
111.5 (106.6, 121.1)
10, 30 sc
10, 30 sc
50, 100, 200 po
Pharmacokinetics of test compounds in Plasmodium falciparum -infected humanized mice
Target dose (mg/kg)
AUC(0–t)(μg · h · mL)
DNAUC(0–t)(μg · h · mL−1 · day−1)
An additional compound, PSC-833 (valspodar), was tested. This is a non-immunosuppressive cyclosporin derivative developed primarily as a p-glycoprotein inhibitor. As cyclosporin had been active during in vitro screening against P. falciparum but cannot be considered because of its immunosuppressive properties, valspodar was considered a potential substitute for addressing the cyclosporin target. This compound was sourced from Novartis AG, and although it had completed Phase III studies as an oncology drug, it had been discontinued for lack of efficacy. Valspodar did not significantly inhibit P. falciparum parasitaemia in vivo (ED90 > 200 mg/kg) (Table 6). The oral pharmacokinetics in the dose range studied was non-linear, with similar values of AUC(0–t) for both dose levels (100 and 200 mg/kg) (Table 7).
In programmes that are currently being conducted in collaboration with or supported by MMV, a significant in vivo potency in the humanized mouse model is considered to be lower than 20 mg/kg. Therefore, none of the drugs tested met the criteria for further development.
Although a large number of approved, investigational and discontinued drugs were evaluated in this project, none of the compounds identified with antiplasmodial activity met the candidate selection criteria warranting further development. From the approximately 3,800 compounds that were tested by SJCRH, there were 24 with EC50 values <1 μM against P. falciparum – a hit rate of about 0.6%, which is similar to that obtained when testing sets of random pharmaceutical diversity. Within the unregistered compound sets of GSK, Pfizer and AZ, 15 of the 338 compounds tested showed significant in vitro activity – a hit rate of 4.4%. This higher hit rate in the unregistered compound sets probably reflects the greater diversity of bio-activity the SJCRH compound set. The unregistered compounds reflect the focus of recent pharmaceutical development in the companies concerned in anti-proliferative, anti-infective and anti-inflammatory disease, areas likely to have biological overlap with processes in the malaria parasite.
Encouragingly, it is clear that a number of different targets in the malaria parasite can be addressed by existing drugs. For example, several protein kinase inhibitors showed in vitro activity against P. falciparum in this study (bosutinib, midostaurin, lestaurtinib, lapatinib, sorafenib, and CE-245677). These compounds were of particular interest as they are essential throughout all stages of the Plasmodium spp. lifecycle [40, 41]. Many protein kinase inhibitors have been registered or investigated, primarily for the treatment of cancer, although these drugs have known toxicities that have discouraged their use in malaria. Antiretroviral protease inhibitors were also of interest and tested in this study, though they had relatively poor in vitro activity. Previous data showed moderate in vitro activity of saquinavir, nevirapine, ritonavir, nelfinavir, amprenavir, and indinavir at clinically relevant concentrations . However, a recent clinical study in HIV-infected women from malaria-endemic regions of sub-Saharan Africa showed no effect of antiretroviral treatment on the incidence of malaria .
Among the licensed products that were active in vitro, none of the compounds were progressed to the in vivo model, mainly because of their unfavourable pharmacokinetic and/or safety profile for use as an oral anti-malarial. However, the scope of this study did not include speculation about the clinical safety and pharmacokinetics that might be discovered should clinical studies in malaria be conducted. In fact, a number of these compounds have been investigated further in malaria. Methotrexate has good activity against P. falciparum and Plasmodium vivax in vitro, although poor activity in vivo against murine malaria species [48–50]. The assumed toxicity of methotrexate and other anticancer drugs when used in short-course, low-dose therapy has been questioned . However, a recent clinical study of methotrexate in healthy volunteers failed to achieve sufficient drug exposures for effective malaria therapy . Methylene blue has also been investigated clinically for malaria, although it is slow acting and there are potential haemolytic effects of this compound in glucose-6-phosphate dehydrogenase-deficient individuals [53–56]. Bortezomib has confirmed in vitro activity against P. falciparum, although clinically its effect as an immunosuppressant probably precludes its use in malaria. Similarly, although cyclosporin A has shown good efficacy in a murine mouse model , its immunosuppressive effect prevents its repositioning as an anti-malarial.
Safety is the greatest impediment to the repositioning of existing drugs to treat malaria. Anti-malarial drugs are taken in possibly many millions of doses every year. Most importantly, an anti-malarial must be safe in children and pregnant women as these groups are most severely affected by the disease. Supply to the patient is often unregulated, self-medication is common and medical resources may be limited. Thus, patients may not be monitored for adverse events or be able to access medical care should these occur.
To achieve the required therapeutic window for an anti-malarial drug, it should have good oral bio-availability, potent activity against the parasite and a high specificity for perturbing parasite metabolic and biochemical processes versus those of the host, ie, few and mild adverse events. These requirements are challenging, particularly for drugs that have been developed to affect human disease processes. In general, unless a drug demonstrates efficacy in malaria at a lower dose than in the ‘parent’ indication, the required therapeutic window cannot be achieved. Thus, repositioning of clinical compounds would seem most appropriate when the new use has a higher tolerance of potential safety signals, such as from malaria to cancer chemotherapy rather than vice versa.
In fact, anti-malarial drugs have been successfully repositioned into other therapeutic areas. Classically, hydroxyl chloroquine has been used to treat inflammatory conditions such as systemic lupus erythematosus, lupus nephritis and rheumatoid arthritis , and may also have utility in other auto-immune diseases . More recently, investigations have been initiated into the use of anti-malarial drugs in cancer, for example, for the sensitization of tumours to enhance the response to conventional treatments [63, 64]. Schistosomiasis is another indication that is being examined . In particular, artemisinins appear to have many potential uses in diverse indications .
In recent years, repositioning of existing drug therapy has been suggested as a fast track to developing new anti-malarial medicines [51, 67, 68]. Studies such as this are necessary in the continuing efforts to explore all potential routes in the search for new effective medicines against this devastating disease. However, the drugs tested in this study did not approach the efficacy requirements for progression or had known safety issues preventing their use in malaria. Thus, it is becoming evident that the development of new drugs for the treatment of uncomplicated P. falciparum infection will probably require the design of molecules specifically targeted at the parasite and pharmacokinetically optimized to provide a sufficient margin of safety.
Thanks to Steve Trusko at Cephalon Inc, West Chester, PA, USA for providing compounds for testing. Thanks to Thierry Diagana at Novartis for providing PSC-833 compound for testing at GSK Tres Cantos. We thank the staff at MMV for the critical discussion and contributions throughout the project and the funding organizations for their support of MMV. RKG, WAG and JAC acknowledge the financial support of the American Lebanese Syrian Associated Charities (ALSAC) and NIH (AI075517 and AI090662). The authors are indebted to María Belén Jiménez-Díaz, Sara Viera, Helen Garuti, Noemí Magán Marchal, Vanesa Gómez, Teresa Mulet, Javier Ibáñez, María Santos Martínez, Leticia Huertas, Maria Jose Lafuente, Sara Prats, and Jaume Vidal at GSK for performing the in vivo experiments in P. falciparum-humanized mouse model. We thank Dr Leonard D Shultz and The Jackson Laboratory for providing access to NOD-scidIL-2Rγnull mice through their collaboration with GlaxoSmithKline-Tres Cantos Medicines Development Campus. Thanks to Achyut Sinha, Bala Subramanian, Suresh Solapure, the DMPK and compound management teams at AstraZeneca for their various invaluable contributions to this project. Naomi Richardson of Magenta Communications Ltd provided writing and editorial assistance on this paper and was funded by Medicines for Malaria Venture.
This work was supported by the Medicines Malaria Venture, St Jude Children's Research Hospital, GlaxoSmithKline Plc and Pfizer Inc.
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