Potent Plasmodium falciparum Gametocytocidal Activity of Diaminonaphthoquinones,Lead Antimalarial Chemotypes Identified in an Antimalarial Compound Screen

Forty percent of the world''s population is threatened by malaria, which is caused by Plasmodium parasites and results in an estimated 200 million clinical cases and 650,000 deaths each year. Drug resistance has been reported for all commonly used antimalarials and has prompted screens to identify new drug candidates. However, many of these new candidates have not been evaluated against the parasite stage responsible for transmission, gametocytes. If Plasmodium falciparum gametocytes are not eliminated, patients continue to spread malaria for weeks after asexual parasite clearance. Asymptomatic individuals can also harbor gametocyte burdens sufficient for transmission, and a safe, effective gametocytocidal agent could also be used in community-wide malaria control programs. Here, we identify 15 small molecules with nanomolar activity against late-stage gametocytes. Fourteen are diaminonaphthoquinones (DANQs), and one is a 2-imino-benzo[d]imidazole (IBI). One of the DANQs identified, SJ000030570, is a lead antimalarial candidate. In contrast, 94% of the 650 compounds tested are inactive against late-stage gametocytes. Consistent with the ineffectiveness of most approved antimalarials against gametocytes, of the 19 novel compounds with activity against known anti-asexual-stage targets, only 3 had any strong effect on gametocyte viability. These data demonstrate the distinct biology of the transmission stages and emphasize the importance of screening for gametocytocidal activity. The potent gametocytocidal activity of DANQ and IBI coupled with their efficacy against asexual parasites provides leads for the development of antimalarials with the potential to prevent both the symptoms and the spread of malaria.     

Maduramicin Rapidly Eliminates Malaria Parasites and Potentiates the Gametocytocidal Activity of the Pyrazoleamide PA21A050

New strategies targeting Plasmodium falciparum gametocytes, the sexual-stage parasites that are responsible for malaria transmission, are needed to eradicate this disease. Most commonly used antimalarials are ineffective against P. falciparum gametocytes, allowing patients to continue to be infectious for over a week after asexual parasite clearance. A recent screen for gametocytocidal compounds demonstrated that the carboxylic polyether ionophore maduramicin is active at low nanomolar concentrations against P. falciparum sexual stages. In this study, we showed that maduramicin has an EC₅₀ (effective concentration that inhibits the signal by 50%) of 14.8 nM against late-stage gametocytes and significantly blocks in vivo transmission in a mouse model of malaria transmission. In contrast to other reported gametocytocidal agents, maduramicin acts rapidly in vitro, eliminating gametocytes and asexual schizonts in less than 12 h without affecting uninfected red blood cells (RBCs). Ring stage parasites are cleared by 24 h. Within an hour of drug treatment, 40% of the normally crescent-shaped gametocytes round up and become spherical. The number of round gametocytes increases to >60% by 2 h, even before a change in membrane potential as monitored by MitoProbe DiIC1 (5) is detectable. Maduramicin is not preferentially taken up by gametocyte-infected RBCs compared to uninfected RBCs, suggesting that gametocytes are more sensitive to alterations in cation concentration than RBCs. Moreover, the addition of 15.6 nM maduramicin enhanced the gametocytocidal activity of the pyrazoleamide PA21A050, which is a promising new antimalarial candidate associated with an increase in intracellular Na⁺ concentration that is proposed to be due to inhibition of PfATP4, a putative Na⁺ pump. These results underscore the importance of cation homeostasis in sexual as well as asexual intraerythrocytic-stage P. falciparum parasites and the potential of targeting this pathway for drug development.     

Luciferase-Based,High-Throughput Assay for Screening and Profiling Transmission-Blocking Compounds against Plasmodium falciparum Gametocytes

The discovery of new antimalarial drugs able to target both the asexual and gametocyte stages of Plasmodium falciparum is critical to the success of the malaria eradication campaign. We have developed and validated a robust, rapid, and cost-effective high-throughput reporter gene assay to identify compounds active against late-stage (stage IV and V) gametocytes. The assay, which is suitable for testing compound activity at incubation times up to 72 h, demonstrates excellent quality and reproducibility, with average Z′ values of 0.85 ± 0.01. We used the assay to screen more than 10,000 compounds from three chemically diverse libraries. The screening outcomes highlighted the opportunity to use collections of compounds with known activity against the asexual stages of the parasites as a starting point for gametocytocidal activity detection in order to maximize the chances of identifying gametocytocidal compounds. This assay extends the capabilities of our previously reported luciferase assay, which tested compounds against early-stage gametocytes, and opens possibilities to profile the activities of gametocytocidal compounds over the entire course of gametocytogenesis.     

Identification of MMV Malaria Box Inhibitors of Plasmodium falciparum Early-Stage Gametocytes Using a Luciferase-Based High-Throughput Assay

The design of new antimalarial combinations to treat Plasmodium falciparum infections requires drugs that, in addition to resolving disease symptoms caused by asexual blood stage parasites, can also interrupt transmission to the mosquito vector. Gametocytes, which are essential for transmission, develop as sexual blood stage parasites in the human host over 8 to 12 days and are the most accessible developmental stage for transmission-blocking drugs. Considerable effort is currently being devoted to identifying compounds active against mature gametocytes. However, investigations on the drug sensitivity of developing gametocytes, as well as screening methods for identifying inhibitors of early gametocytogenesis, remain scarce. We have developed a luciferase-based high-throughput screening (HTS) assay using tightly synchronous stage I to III gametocytes from a recombinant P. falciparum line expressing green fluorescent protein (GFP)-luciferase. The assay has been used to evaluate the early-stage gametocytocidal activity of the MMV Malaria Box, a collection of 400 compounds with known antimalarial (asexual blood stage) activity. Screening this collection against early-stage (I to III) gametocytes yielded 64 gametocytocidal compounds with 50% inhibitory concentrations (IC₅₀s) below 2.5 μM. This assay is reproducible and suitable for the screening of large compound libraries, with an average percent coefficient of variance (%CV) of ≤5%, an average signal-to-noise ratio (S:N) of >30, and a Z′ of ∼0.8. Our findings highlight the need for screening efforts directed specifically against early gametocytogenesis and indicate the importance of experimental verification of early-stage gametocytocidal activity in the development of new antimalarial candidates for combination therapy.     

Transmission-Blocking Potential of MEFAS,a Hybrid Compound Derived from Artesunate and Mefloquine

Most antimalarial drugs target asexual parasites without reducing gametocyte formation or development. Drugs with dual roles, i.e., those that can target both asexual parasites and gametocytes, would improve the control of malaria. In the current study, MEFAS, a hybrid drug derived from mefloquine and artesunate that has been shown to be an active blood schizonticidal drug, was assessed to determine its ability to block the infectivity of Plasmodium falciparum gametocytes. MEFAS was 280 and 15 times more effective than mefloquine alone and artesunate alone, respectively.     

Effect of Fluorescent Dyes on In Vitro-Differentiated,Late-Stage Plasmodium falciparum Gametocytes

Plasmodium falciparum gametocytes are not associated with clinical symptoms, but they are responsible for transmitting the pathogen to mosquitoes. Therefore, gametocytocidal interventions are important for malaria control and resistance containment. Currently available drugs and vaccines are not well suited for that purpose. Several dyes have potent antimicrobial activity, but their use against gametocytes has not been investigated systematically. The gametocytocidal activity of nine synthetic dyes and four control compounds was tested against stage V gametocytes of the laboratory strain 3D7 and three clinical isolates of P. falciparum with a bioluminescence assay. Five of the fluorescent dyes had submicromolar 50% inhibitory concentration (IC₅₀) values against mature gametocytes. Three mitochondrial dyes, MitoRed, dihexyloxacarbocyanine iodide (DiOC6), and rhodamine B, were highly active (IC₅₀s < 200 nM). MitoRed showed the highest activity against gametocytes, with IC₅₀s of 70 nM against 3D7 and 120 to 210 nM against clinical isolates. All compounds were more active against the laboratory strain 3D7 than against clinical isolates. In particular, the endoperoxides artesunate and dihydroartemisinin showed a 10-fold higher activity against 3D7 than against clinical isolates. In contrast to all clinically used antimalarials, several fluorescent dyes had surprisingly high in vitro activity against late-stage gametocytes. Since they also act against asexual blood stages, they shall be considered starting points for the development of new antimalarial lead compounds.     

In Vitro Activities of Primaquine-Schizonticide Combinations on Asexual Blood Stages and Gametocytes of Plasmodium falciparum

Currently, the World Health Organization recommends addition of a 0.25-mg base/kg single dose of primaquine (PQ) to artemisinin combination therapies (ACTs) for Plasmodium falciparum malaria as a gametocytocidal agent for reducing transmission. Here, we investigated the potential interactions of PQ with the long-lasting components of the ACT drugs for eliminating the asexual blood stages and gametocytes of in vitro-cultured P. falciparum strains. Using the SYBR green I assay for asexual parasites and a flow cytometry-based assay for gametocytes, we determined the interactions of PQ with the schizonticides chloroquine, mefloquine, piperaquine, lumefantrine, and naphthoquine. With the sums of fractional inhibitory concentrations and isobolograms, we were able to determine mostly synergistic interactions for the various PQ and schizonticide combinations on the blood stages of P. falciparum laboratory strains. The synergism in inhibiting asexual stages and gametocytes was highly evident with PQ-naphthoquine, whereas synergism was moderate for the PQ-piperaquine, PQ-chloroquine, and PQ-mefloquine combinations. We have detected potentially antagonistic interactions between PQ and lumefantrine under certain drug combination ratios, suggesting that precautions might be needed when PQ is added as the gametocytocide to the artemether-lumefantrine ACT (Coartem).     

The Proteasome Inhibitor Epoxomicin Has Potent Plasmodium falciparum Gametocytocidal Activity

Malaria continues to be a major global health problem, but only a limited arsenal of effective drugs is available. None of the antimalarial compounds commonly used clinically kill mature gametocytes, which is the form of the parasite that is responsible for malaria transmission. The parasite that causes the most virulent human malaria, Plasmodium falciparum, has a 48-h asexual cycle, while complete sexual differentiation takes 10 to 12 days. Once mature, stage V gametocytes circulate in the peripheral blood and can be transmitted for more than a week. Consequently, if chemotherapy does not eliminate gametocytes, an individual continues to be infectious for several weeks after the clearance of asexual parasites. The work reported here demonstrates that nanomolar concentrations of the proteasome inhibitor epoxomicin effectively kill all stages of intraerythrocytic parasites but do not affect the viability of human or mouse cell lines. Twenty-four hours after treatment with 100 nM epoxomicin, the total parasitemia decreased by 78%, asexual parasites decreased by 86%, and gametocytes decreased by 77%. Seventy-two hours after treatment, no viable parasites remained in the 100 or 10 nM treatment group. Epoxomicin also blocked oocyst production in the mosquito midgut. In contrast, the cysteine protease inhibitors epoxysuccinyl-l-leucylamido-3-methyl-butane ethyl ester and N-acetyl-l-leucyl-l-leucyl-l-methioninal blocked hemoglobin digestion in early gametocytes but had no effect on later stages. Moreover, once the cysteine protease inhibitor was removed, sexual differentiation resumed. These findings provide strong support for the further development of proteasome inhibitors as antimalaria agents that are effective against asexual, sexual, and mosquito midgut stages of P. falciparum.The current recommended treatments for malaria caused by Plasmodium falciparum, including artemisinin combination therapy, eliminate intraerythrocytic asexual parasites that are responsible for the clinical symptoms. However, these treatments do not kill mature intraerythrocytic gametocytes that are required for the transmission of the parasite (24). In contrast to the 2-day asexual cycle of P. falciparum, the production of a mature stage V gametocyte takes 10 to 12 days. Once mature gametocytes are taken up by a mosquito during a blood meal, fertilization is stimulated. The resulting zygotes develop into oocysts where thousands of sporozoites are produced that can be transmitted to humans during a subsequent blood meal. The prolonged period required for P. falciparum gametocyte maturation in the human host suggests that malaria can be transmitted for several weeks after asexual parasites are eliminated (23). Thus, the development of drugs that are effective against both asexual-stage parasites and gametocytes may directly decrease malaria morbidity and mortality and reduce the spread of the disease.Cysteine protease and proteasome inhibitors have been found to affect asexual intraerythrocytic parasites and are being evaluated as possible antimalarial agents (4, 7, 10, 15, 19-21, 25). However, their effect on the 10- to 12-day course of intraerythrocytic gametocyte development has not been reported. Proteasome inhibitors also have not been tested on parasites taken up by a mosquito, while cysteine protease inhibitors have been shown to significantly decrease P. falciparum gamete surface antigen processing, oocyst production, and sporozoite maturation (7, 10). The dual cysteine and serine protease inhibitors l-1-tosylamide-2-phenylethyl-chloromethyl ketone (TPCK) and Nα-p-tosyl-l-lysine chloromethyl ketone (TLCK) also have been reported to reduce P. falciparum exflagellation and the transmission of Plasmodium berghei to mosquitoes (22, 26).Genes predicted to code for cysteine proteases and the proteasome are expressed throughout gametocytogenesis, providing targets for both classes of compounds (12, 28). Falcipain 1 and the P. berghei orthologs of PfSERA8 (PbECP1) and metacaspase 1 (PbMC1) are the only proteases whose function has been studied directly during gametocytogenesis by targeted gene disruption (3, 9, 14). The disruption of falcipain 1 and PfECP1 affected oocyst production in the mosquito midgut but not the asexual or sexual intraerythrocytic stage, while no stage of the life cycle was affected by the PbMC1 knockout. The work described here evaluates the effect of cysteine and proteasome inhibitors during P. falciparum sexual differentiation and development in the mosquito midgut.     

Imaging-Based High-Throughput Screening Assay To Identify New Molecules with Transmission-Blocking Potential against Plasmodium falciparum Female Gamete Formation

In response to a call for the global eradication of malaria, drug discovery has recently been extended to identify compounds that prevent the onward transmission of the parasite, which is mediated by Plasmodium falciparum stage V gametocytes. Lately, metabolic activity has been used in vitro as a surrogate for gametocyte viability; however, as gametocytes remain relatively quiescent at this stage, their ability to undergo onward development (gamete formation) may be a better measure of their functional viability. During gamete formation, female gametocytes undergo profound morphological changes and express translationally repressed mRNA. By assessing female gamete cell surface expression of one such repressed protein, Pfs25, as the readout for female gametocyte functional viability, we developed an imaging-based high-throughput screening (HTS) assay to identify transmission-blocking compounds. This assay, designated the P. falciparum female gametocyte activation assay (FGAA), was scaled up to a high-throughput format (Z′ factor, 0.7 ± 0.1) and subsequently validated using a selection of 50 known antimalarials from diverse chemical families. Only a few of these agents showed submicromolar 50% inhibitory concentrations in the assay: thiostrepton, methylene blue, and some endoperoxides. To determine the best conditions for HTS, a robustness test was performed with a selection of the GlaxoSmithKline Tres Cantos Antimalarial Set (TCAMS) and the final screening conditions for this library were determined to be a 2 μM concentration and 48 h of incubation with gametocytes. The P. falciparum FGAA has been proven to be a robust HTS assay faithful to Plasmodium transmission-stage cell biology, and it is an innovative useful tool for antimalarial drug discovery which aims to identify new molecules with transmission-blocking potential.     

Antimalarial Iron Chelator FBS0701 Blocks Transmission by Plasmodium falciparum Gametocyte Activation Inhibition

Reducing the transmission of the malarial parasite by Anopheles mosquitoes using drugs or vaccines remains a main focus in the efforts to control malaria. Iron chelators have been studied as potential antimalarial drugs due to their activities against different stages of the parasite. The iron chelator FBS0701 affects the development of Plasmodium falciparum early gametocytes and lowers blood-stage parasitemia. Here, we tested the effect of FBS0701 on stage V gametocyte infectivity for mosquitoes. The incubation of stage V gametocytes for up to 3 days with increasing concentrations of FBS0701 resulted in a significant dose-related reduction in mosquito infectivity, as measured by the numbers of oocysts per mosquito. The reduction in mosquito infectivity was due to the inhibition of male and female gametocyte activation. The preincubation of FBS0701 with ferric chloride restored gametocyte infectivity, showing that the inhibitory effect of FBS0701 was quenched by iron. Deferoxamine, another iron chelator, also reduced gametocyte infectivity but to a lesser extent. Finally, the simultaneous administration of drug and gametocytes to mosquitoes without previous incubation did not significantly reduce the numbers of oocysts. These results show the importance of gametocyte iron metabolism as a potential target for new transmission-blocking strategies.     

Histone Methyltransferase Inhibitors Are Orally Bioavailable,Fast-Acting Molecules with Activity against Different Species Causing Malaria in Humans

Current antimalarials are under continuous threat due to the relentless development of drug resistance by malaria parasites. We previously reported promising in vitro parasite-killing activity with the histone methyltransferase inhibitor BIX-01294 and its analogue TM2-115. Here, we further characterize these diaminoquinazolines for in vitro and in vivo efficacy and pharmacokinetic properties to prioritize and direct compound development. BIX-01294 and TM2-115 displayed potent in vitro activity, with 50% inhibitory concentrations (IC₅₀s) of <50 nM against drug-sensitive laboratory strains and multidrug-resistant field isolates, including artemisinin-refractory Plasmodium falciparum isolates. Activities against ex vivo clinical isolates of both P. falciparum and Plasmodium vivax were similar, with potencies of 300 to 400 nM. Sexual-stage gametocyte inhibition occurs at micromolar levels; however, mature gametocyte progression to gamete formation is inhibited at submicromolar concentrations. Parasite reduction ratio analysis confirms a high asexual-stage rate of killing. Both compounds examined displayed oral efficacy in in vivo mouse models of Plasmodium berghei and P. falciparum infection. The discovery of a rapid and broadly acting antimalarial compound class targeting blood stage infection, including transmission stage parasites, and effective against multiple malaria-causing species reveals the diaminoquinazoline scaffold to be a very promising lead for development into greatly needed novel therapies to control malaria.     

Splenic Retention of Plasmodium falciparum Gametocytes To Block the Transmission of Malaria

Plasmodium falciparum is transmitted from humans to Anopheles mosquito vectors via the sexual erythrocytic forms termed gametocytes. Erythrocyte filtration through microsphere layers (microsphiltration) had shown that circulating gametocytes are deformable. Compounds reducing gametocyte deformability would induce their splenic clearance, thus removing them from the blood circulation and blocking malaria transmission. The hand-made, single-sample prototype for microsphiltration was miniaturized to a 96-well microtiter plate format, and gametocyte retention in the microsphere filters was quantified by high-content imaging. The stiffening activity of 40 pharmacological compounds was assessed in microtiter plates, using a small molecule (calyculin) as a positive control. The stiffening activity of calyculin was assessed in spleen-mimetic microfluidic chips and in macrophage-depleted mice. Marked mechanical retention (80% to 90%) of mature gametocytes was obtained in microplates following exposure to calyculin at concentrations with no effect on parasite viability. Of the 40 compounds tested, including 20 antimalarials, only 5 endoperoxides significantly increased gametocyte retention (1.5- to 2.5-fold; 24 h of exposure at 1 μM). Mature gametocytes exposed to calyculin accumulated in microfluidic chips and were cleared from the circulation of macrophage-depleted mice as rapidly as heat-stiffened erythrocytes, thus confirming results obtained using the microsphiltration assay. An automated miniaturized approach to select compounds for their gametocyte-stiffening effect has been established. Stiffening induces gametocyte clearance both in vitro and in vivo. Based on physiologically validated tools, this screening cascade can identify novel compounds and uncover new targets to block malaria transmission. Innovative applications in hematology are also envisioned.     

A Class of Tricyclic Compounds Blocking Malaria Parasite Oocyst Development and Transmission

Malaria is a deadly infectious disease in many tropical and subtropical countries. Previous efforts to eradicate malaria have failed, largely due to the emergence of drug-resistant parasites, insecticide-resistant mosquitoes and, in particular, the lack of drugs or vaccines to block parasite transmission. ATP-binding cassette (ABC) transporters are known to play a role in drug transport, metabolism, and resistance in many organisms, including malaria parasites. To investigate whether a Plasmodium falciparum ABC transporter (Pf14_0244 or PfABCG2) modulates parasite susceptibility to chemical compounds or plays a role in drug resistance, we disrupted the gene encoding PfABCG2, screened the recombinant and the wild-type 3D7 parasites against a library containing 2,816 drugs approved for human or animal use, and identified an antihistamine (ketotifen) that became less active against the PfABCG2-disrupted parasite in culture. In addition to some activity against asexual stages and gametocytes, ketotifen was highly potent in blocking oocyst development of P. falciparum and the rodent parasite Plasmodium yoelii in mosquitoes. Tests of structurally related tricyclic compounds identified additional compounds with similar activities in inhibiting transmission. Additionally, ketotifen appeared to have some activity against relapse of Plasmodium cynomolgi infection in rhesus monkeys. Further clinical evaluation of ketotifen and related compounds, including synthetic new derivatives, in blocking malaria transmission may provide new weapons for the current effort of malaria eradication.     

Multistage inhibitors of the malaria parasite: Emerging hope for chemoprotection and malaria eradication

Over time, several exciting advances have been made in the treatment and prevention of malaria; however, this devastating disease continues to be a major global health problem and affects millions of people every year. Notably, the paucity of new efficient drug molecules and the inevitable drug resistance of the malaria parasite, Plasmodium falciparum, against frontline therapeutics are the foremost struggles facing malaria eradication initiatives. According to the malaria eradication agenda, the discovery of new chemical entities that can destroy the parasite at the liver stage, the asexual blood stage, the gametocyte stage, and the insect ookinete stage of the parasite life cycle (i.e., compounds exhibiting multistage activity) are in high demand, preferably with novel and multiple modes of action. Phenotypic screening of chemical libraries against the malaria parasite is certainly a crucial step toward overcoming these crises. In the last few years, various research groups, including industrial research laboratories, have performed large‐scale phenotypic screenings that have identified a wealth of chemical entities active against multiple life stages of the malaria parasite. Vital scientific and technological developments have led to the discovery of multistage inhibitors of the malaria parasite; these compounds, considered highly valuable starting points for subsequent drug discovery and eradication of malaria, are reviewed.     

Gametocyte Clearance Kinetics Determined by Quantitative Magnetic Fractionation in Melanesian Children with Uncomplicated Malaria Treated with Artemisinin Combination Therapy

Quantitative magnetic fractionation and a published mathematical model were used to characterize between-treatment differences in gametocyte density and prevalence in 70 Papua New Guinean children with uncomplicated Plasmodium falciparum and/or Plasmodium vivax malaria randomized to one of two artemisinin combination therapies (artemether-lumefantrine or artemisinin-naphthoquine) in an intervention trial. There was an initial rise in peripheral P. falciparum gametocyte density with both treatments, but it was more pronounced in the artemisinin-naphthoquine group. Model-derived estimates of the median pretreatment sequestered gametocyte population were 21/μl for artemether-lumefantrine and 61/μl for artemisinin-naphthoquine (P < 0.001). The median time for P. falciparum gametocyte density to fall to <2.5/μl (below which transmission becomes unlikely) was 16 days in the artemether-lumefantrine group and 20 days in artemisinin-naphthoquine group (P < 0.001). Gametocyte prevalence modeling suggested that artemisinin-naphthoquine-treated children became gametocytemic faster (median, 2.2 days) than artemether-lumefantrine-treated children (median, 5.3 days; P < 0.001) and had a longer median P. falciparum gametocyte carriage time per individual (20 versus 13 days; P < 0.001). Clearance of P. vivax gametocytes was rapid (within 3 days) in both groups; however, consistent with the reappearance of asexual forms in the main trial, nearly 40% of children in the artemether-lumefantrine group developed P. vivax gametocytemia between days 28 and 42 compared with 3% of children in the artemisinin-naphthoquine group. These data suggest that artemisinin is less active than artemether against sequestered gametocytes. Greater initial gametocyte release after artemisinin-naphthoquine increases the period of potential P. falciparum transmission by 4 days relative to artemether-lumefantrine, but the longer elimination half-life of naphthoquine than of lumefantrine suppresses P. vivax recurrence and consequent gametocytemia.     

Thiostrepton and derivatives exhibit antimalarial and gametocytocidal activity by dually targeting parasite proteasome and apicoplast

Ribosome-targeting antibiotics exert their antimalarial activity on the apicoplast of the malaria parasite, an organelle of prokaryote origin having essential metabolic functions. These antibiotics typically cause a delayed-death phenotype, which manifests in parasite killing during the second replication cycle following administration. As an exception, treatment with the antibiotic thiostrepton results in an immediate killing. We recently demonstrated that thiostrepton and its derivatives interfere with the eukaryotic proteasome, a multimeric protease complex that is important for the degradation of ubiquitinated proteins. Here, we report that the thiostrepton-based compounds are active against chloroquine-sensitive and -resistant Plasmodium falciparum, where they rapidly eliminate parasites before DNA replication. The minor parasite fraction that escapes the fast killing of the first replication cycle is arrested in the schizont stage of the following cycle, displaying a delayed-death phenotype. Thiostrepton further exhibits gametocytocidal activity by eliminating gametocytes, the sexual precursor cells that are crucial for parasite transmission to the mosquito. Compound treatment results in an accumulation of ubiquitinated proteins in the blood stages, indicating an effect on the parasite proteasome. In accordance with these findings, expression profiling revealed that the proteasome is present in the nucleus and cytoplasm of trophozoites, schizonts, and gametocytes. In conclusion, thiostrepton derivatives represent promising candidates for malaria therapy by dually acting on two independent targets, the parasite proteasome and the apicoplast, with the capacity to eliminate both intraerythrocytic asexual and transmission stages of the parasite.     

Inhibition of Plasmodium falciparum oocyst production by membrane-permeant cysteine protease inhibitor E64d

During asexual intraerythrocytic growth, Plasmodium falciparum utilizes hemoglobin obtained from the host red blood cell (RBC) as a nutrient source. Papain-like cysteine proteases, falcipains 2 and 3, have been reported to be involved in hemoglobin digestion and are targets of current antimalarial drug development efforts. However, their expression during gametocytogenesis, which is required for malaria parasite transmission, has not been studied. Many of the available antimalarials do not inhibit development of sexual stage parasites, and therefore, the persistence of gametocytes after drug treatment allows continued transmission of the disease. In the work reported here, incubation of stage V gametocytes with membrane-permeant cysteine protease inhibitor E64d significantly inhibited oocyst production (80 to 100%). The same conditions inhibited processing of gametocyte-surface antigen Pfs230 during gametogenesis but did not alter the morphology of the food vacuole in gametocytes, inhibit emergence, or block male exflagellation. E64d reduced the level of oocyst production more effectively than that reported previously for falcipain 1-knockout parasites, suggesting that falcipains 2 and 3 may also be involved in malaria parasite transmission. However, in this study only falcipain 3 and not falcipain 2 was found to be expressed in stage V gametocytes. Interestingly, during gametocytogenesis falcipain 3 was transported into the red blood cell and by stage V was localized in vesicles along the RBC surface, consistent with a role during gamete emergence. The ability of a membrane-permeant cysteine protease inhibitor to significantly reduce malaria parasite transmission suggests that future drug design should include evaluation of gametogenesis and sporogonic development.     

Male and Female Plasmodium falciparum Mature Gametocytes Show Different Responses to Antimalarial Drugs

It is the mature gametocytes of Plasmodium that are solely responsible for parasite transmission from the mammalian host to the mosquito. They are therefore a logical target for transmission-blocking antimalarial interventions, which aim to break the cycle of reinfection and reduce the prevalence of malaria cases. Gametocytes, however, are not a homogeneous cell population. They are sexually dimorphic, and both males and females are required for parasite transmission. Using two bioassays, we explored the effects of 20 antimalarials on the functional viability of both male and female mature gametocytes of Plasmodium falciparum. We show that mature male gametocytes (as reported by their ability to produce male gametes, i.e., to exflagellate) are sensitive to antifolates, some endoperoxides, methylene blue, and thiostrepton, with submicromolar 50% inhibitory concentrations (IC₅₀s), whereas female gametocytes (as reported by their ability to activate and form gametes expressing the marker Pfs25) are much less sensitive to antimalarial intervention, with only methylene blue and thiostrepton showing any significant activity. These findings show firstly that the antimalarial responses of male and female gametocytes differ and secondly that the mature male gametocyte should be considered a more vulnerable target than the female gametocyte for transmission-blocking drugs. Given the female-biased sex ratio of Plasmodium falciparum (∼3 to 5 females:1 male), current gametocyte assays without a sex-specific readout are unlikely to identify male-targeted compounds and prioritize them for further development. Both assays reported here are being scaled up to at least medium throughput and will permit identification of key transmission-blocking molecules that have been overlooked by other screening campaigns.     

An antimalarial neem leaf extract has both schizonticidal and gametocytocidal activities

A crude acetone/water (50/50) extract of neem leaves (IRAB) was evaluated for activity against the asexual (trophozoites/schizonts) and the sexual (gametocytes) forms of the malarial parasite, Plasmodium falciparum, in vitro. In separate 72 hour cultures of both asexual parasites and mature gametocytes treated with IRAB (0.5 microg/mL), parasite numbers were less than 50% of the numbers in control cultures, which had 8.0% and 8.5% parasitemia, respectively. In cultures containing 2.5 microg/mL, asexual parasites and mature and immature gametocytes were reduced to 0.1%, 0.2%, and 0% parasitemia, respectively. There were no parasites in the cultures containing 5.0 microg/mL. This extract, if found safe, may provide materials for development of new antimalarial drugs that may be useful both in treatment of malaria as well as the control of its transmission through gametocytes.