Recent advances in transmission-blocking drugs for malaria elimination

Transmission-Blocking Strategies for Malaria Eradication: Recent Advances in Small-Molecule Drug Development

Malaria drug research and development efforts have resurged in the last decade following the decelerating rate of mortality and malaria cases in endemic regions. The inefficiency of malaria interventions is largely driven by the spreading resistance of the Plasmodium falciparum parasite to current drug regimens and that of the malaria vector, the Anopheles mosquito, to insecticides. In response to the new eradication agenda, drugs that act by breaking the malaria transmission cycle (transmission-blocking drugs), which has been recognized as an important and additional target for intervention, are being developed. These drugs take advantage of the susceptibility of Plasmodium during population bottlenecks before transmission (gametocytes) and in the mosquito vector (gametes, zygotes, ookinetes, oocysts, sporozoites). To date, compounds targeting stage V gametocytes predominate in the chemical library of transmission-blocking drugs, and some of them have entered clinical trials. The targeting of Plasmodium mosquito stages has recently renewed interest in the development of innovative malaria control tools, which hold promise for the application of compounds effective at these stages. In this review, we highlight the major achievements and provide an update on the research of transmission-blocking drugs, with a particular focus on their chemical scaffolds, antiplasmodial activity, and transmission-blocking potential.

Approaches to Address the Anemia Challenge

Anemia is a multifactorial condition; approaches to address it must recognize that the causal factors represent an ecology consisting of internal (biology, genetics, and health) and external (social/behavioral/demographic and physical) environments. In this paper, we present an approach for selecting interventions, followed by a description of key issues related to the multiple available interventions for prevention and reduction of anemia. We address interventions for anemia using the following 2 main categories: 1) those that address nutrients alone, and, 2) those that address nonnutritional causes of anemia. The emphasis will be on interventions of public health relevance, but we also consider the clinical context. We also focus on interventions at different stages of the life course, with a particular focus on women of reproductive age and preschool-age children, and present evidence on various factors to consider when selecting an intervention-inflammation, genetic mutations, nutrient delivery, bioavailability, and safety. Each section on an intervention domain concludes with a brief discussion of key research areas.

Future antimalarials from Artemisia? A rationale for natural product mining against drug-refractory Plasmodium stages

Covering: up to 2023Infusions of the plants Artemisia annua and A. afra are gaining broad popularity to prevent or treat malaria. There is an urgent need to address this controversial public health question by providing solid scientific evidence in relation to these uses. Infusions of either species were shown to inhibit the asexual blood stages, the liver stages including the hypnozoites, but also the sexual stages, the gametocytes, of Plasmodium parasites. Elimination of hypnozoites and sterilization of mature gametocytes remain pivotal elements of the radical cure of P. vivax, and the blockage of P. vivax and P. falciparum transmission, respectively. Drugs active against these stages are restricted to the 8-aminoquinolines primaquine and tafenoquine, a paucity worsened by their double dependence on the host genetic to elicit clinical activity without severe toxicity. Besides artemisinin, these Artemisia spp. contain many natural products effective against Plasmodium asexual blood stages, but their activity against hypnozoites and gametocytes was never investigated. In the context of important therapeutic issues, we provide a review addressing (i) the role of artemisinin in the bioactivity of these Artemisia infusions against specific parasite stages, i.e., alone or in association with other phytochemicals; (ii) the mechanisms of action and biological targets in Plasmodium of ca. 60 infusion-specific Artemisia phytochemicals, with an emphasis on drug-refractory parasite stages (i.e., hypnozoites and gametocytes). Our objective is to guide the strategic prospecting of antiplasmodial natural products from these Artemisia spp., paving the way toward novel antimalarial "hit" compounds either naturally occurring or Artemisia-inspired.

Arsinothricin Inhibits Plasmodium falciparum Proliferation in Blood and Blocks Parasite Transmission to Mosquitoes

Malaria, caused by Plasmodium protozoal parasites, remains a leading cause of morbidity and mortality. The Plasmodium parasite has a complex life cycle, with asexual and sexual forms in humans and Anopheles mosquitoes. Most antimalarials target only the symptomatic asexual blood stage. However, to ensure malaria eradication, new drugs with efficacy at multiple stages of the life cycle are necessary. We previously demonstrated that arsinothricin (AST), a newly discovered organoarsenical natural product, is a potent broad-spectrum antibiotic that inhibits the growth of various prokaryotic pathogens. Here, we report that AST is an effective multi-stage antimalarial. AST is a nonproteinogenic amino acid analog of glutamate that inhibits prokaryotic glutamine synthetase (GS). Phylogenetic analysis shows that Plasmodium GS, which is expressed throughout all stages of the parasite life cycle, is more closely related to prokaryotic GS than eukaryotic GS. AST potently inhibits Plasmodium GS, while it is less effective on human GS. Notably, AST effectively inhibits both Plasmodium erythrocytic proliferation and parasite transmission to mosquitoes. In contrast, AST is relatively nontoxic to a number of human cell lines, suggesting that AST is selective against malaria pathogens, with little negative effect on the human host. We propose that AST is a promising lead compound for developing a new class of multi-stage antimalarials.

A Novel Ex Vivo Drug Assay for Assessing the Transmission-Blocking Activity of Compounds on Field-Isolated Plasmodium falciparum Gametocytes

The discovery and development of transmission-blocking therapies challenge malaria elimination and necessitate standard and reproducible bioassays to measure the blocking properties of antimalarial drugs and candidate compounds. Most of the current bioassays evaluating the transmission-blocking activity of compounds rely on laboratory-adapted Plasmodium strains. Transmission-blocking data from clinical gametocyte isolates could help select novel transmission-blocking candidates for further development. Using freshly collected Plasmodium falciparum gametocytes from asymptomatic individuals, we first optimized ex vivo culture conditions to improve gametocyte viability and infectiousness by testing several culture parameters. We next pre-exposed ex vivo field-isolated gametocytes to chloroquine, dihydroartemisinin, primaquine, KDU691, GNF179, and oryzalin for 48 h prior to direct membrane feeding. We measured the activity of the drug on the ability of gametocytes to resume the sexual life cycle in Anopheles after drug exposure. Using 57 blood samples collected from Malian volunteers aged 6 to 15 years, we demonstrate that the infectivity of freshly collected field gametocytes can be preserved and improved ex vivo in a culture medium supplemented with 10% horse serum at 4% hematocrit for 48 h. Moreover, our optimized drug assay displays the weak transmission-blocking activity of chloroquine and dihydroartemisinin, while primaquine and oryzalin exhibited a transmission-blocking activity of ~50% at 1 μM. KDU691 and GNF179 both interrupted Plasmodium transmission at 1 μM and 5 nM, respectively. This new approach, if implemented, has the potential to accelerate the screening of compounds with transmission-blocking activity.

Blood-stage antiplasmodial activity and oocyst formation-blockage of metallo copper-cinchonine complex

In the fight against malaria, the key is early treatment with antimalarial chemotherapy, such as artemisinin-based combination treatments (ACTs). However, Plasmodium has acquired multidrug resistance, including the emergence of P. falciparum strains with resistance to ACT. The development of novel antimalarial molecules, that are capable of interfering in the asexual and sexual blood stages, is important to slow down the transmission in endemic areas. In this work, we studied the ability of the mettalo copper-cinchonine complex to interfere in the sexual and asexual stages of Plasmodium. The tested compound in the in vitro assay was a cinchonine derivative, named CinCu (Bis[Cinchoninium Tetrachlorocuprate(II)]trihydrate). Its biological functions were assessed by antiplasmodial activity in vitro against chloroquine-resistant P. falciparum W2 strain. The mice model of P. berghei ANKA infection was used to analyze the antimalarial activity of CinCu and chloroquine and their acute toxicity. The oocyst formation-blocking assay was performed by experimental infection of Anopheles aquasalis with P. vivax infected blood, which was treated with different concentrations of CinCu, cinchonine, and primaquine. We found that CinCu was able to suppress as high as 81.58% of parasitemia in vitro, being considered a molecule with high antiplasmodial activity and low toxicity. The in vivo analysis showed that CinCu suppressed parasitemia at 34% up to 87.19%, being a partially active molecule against the blood-stage forms of P. berghei ANKA, without inducing severe clinical signs in the treated groups. The transmission-blocking assay revealed that both cinchonine and primaquine were able to reduce the infection intensity of P. vivax in A. aquasalis, leading to a decrease in the number of oocysts recovered from the mosquitoes' midgut. Regarding the effect of CinCu, the copper-complex was not able to induce inhibition of P. vivax infection; however, it was able to induce an important reduction in the intensity of oocyst formation by about 2.4 times. It is plausible that the metallo-compound also be able to interfere with the differentiation of parasite stages and/or ookinete-secreted chitinase into the peritrophic matrix of mosquitoes, promoting a reduction in the number of oocysts formed. Taken together, the results suggest that this compound is promising as a prototype for the development of new antimalarial drugs. Furthermore, our study can draw a new pathway for repositioning already-known antimalarial drugs by editing their chemical structure to improve the antimalarial activity against the asexual and sexual stages of the parasite.

Synthesis and antimalarial activity of 7-chloroquinoline-tethered sulfonamides and their [1,2,3]-triazole hybrids

Aim & background: Drugs with multiple bioactive moieties have the advantages of multiple modes of action and fewer chances of drug resistance. In continuation of our previous work of developing hybrid antimalarials, we present herein the synthesis and antimalarial activity of two different series of 7-chloroquinoline-sulfonamide hybrids. Materials & methods: The first series of compounds were synthesized by using p-dodecylbenzenesulfonic acid as a Bronsted acid catalyst in ethanol. The second series' compounds were synthesized by 1,3-dipolar cycloaddition of azides and alkynes under click reaction conditions. Results & conclusion: The majority of these compounds demonstrated noncytotoxicity and significant antimalarial activity against Plasmodium falciparum (3D7) with IC₅₀ values in the range of 1.49-13.49 μM. The most promising hybrids (12d, 13a and 13c) may be good starting points for next-generation antimalarials.

An Optimal Control Model to Understand the Potential Impact of the New Vaccine and Transmission-Blocking Drugs for Malaria: A Case Study in Papua and West Papua, Indonesia

Malaria is one of the major causes of a high death rate due to infectious diseases every year. Despite attempts to eradicate the disease, results have not been very successful. New vaccines and other treatments are being constantly developed to seek optimal ways to prevent malaria outbreaks. In this article, we formulate and analyze an optimal control model of malaria incorporating the new pre-erythrocytic vaccine and transmission-blocking treatment. Sufficient conditions to guarantee local stability of the malaria-free equilibrium were derived based on the controlled reproduction number condition. Using the non-linear least square fitting method, we fitted the incidence data from the province of Papua and West Papua in Indonesia to estimate the model parameter values. The optimal control characterization and optimality conditions were derived by applying the Pontryagin Maximum Principle, and numerical simulations were also presented. Simulation results show that both the pre-erythrocytic vaccine and transmission-blocking treatment significantly reduce the spread of malaria. Accordingly, a high doses of pre-erythrocytic vaccine is needed if the number of infected individuals is relatively small, while transmission blocking is required if the number of infected individuals is relatively large. These results suggest that a large-scale implementation of both strategies is vital as the world continues with the effort to eradicate malaria, especially in endemic regions across the globe.

Malaria transmission blocking compounds: a patent review

INTRODUCTION

Despite substantial progress in the field, malaria remains a global health issue and currently available control strategies are not sufficient to achieve eradication. Agents able to prevent transmission are likely to have a strong impact on malaria control and have been prioritized as a primary objective to reduce the number of secondary infections. Therefore, there is an increased interest in finding novel drugs targeting sexual stages of Plasmodium and innovative methods to target malaria transmission from host to vector, and vice versa.

AREAS COVERED

This review covers innovative transmission-blocking inventions patented between 2015 and October 2021. The focus is on chemical interventions which could be used as "chemical vaccines" to prevent transmission (small molecules, carbohydrates, and polypeptides).

EXPERT OPINION

Even though the development of novel strategies to block transmission still requires fundamental additional research and a deeper understanding of parasite sexual stages biology, the research in this field has significantly accelerated. Among innovative inventions patented over the last six years, the surface-delivery of antimalarial drugs to kill transmission-stages parasites in mosquitoes holds the highest promise for success in malaria control strategies, opening completely new scenarios in malaria transmission-blocking drug discovery.

Transmission-Blocking Strategies Against Malaria Parasites During Their Mosquito Stages

Malaria is still the most widespread parasitic disease and causes the most infections globally. Owing to improvements in sanitary conditions and various intervention measures, including the use of antimalarial drugs, the malaria epidemic in many regions of the world has improved significantly in the past 10 years. However, people living in certain underdeveloped areas are still under threat. Even in some well-controlled areas, the decline in malaria infection rates has stagnated or the rates have rebounded because of the emergence and spread of drug-resistant malaria parasites. Thus, new malaria control methods must be developed. As the spread of the Plasmodium parasite is dependent on the part of its life cycle that occurs in mosquitoes, to eliminate the possibility of malaria infections, transmission-blocking strategies against the mosquito stage should be the first choice. In fact, after the gametocyte enters the mosquito body, it undergoes a series of transformation processes over a short period, thus providing numerous potential blocking targets. Many research groups have carried out studies based on targeting the blocking of transmission during the mosquito phase and have achieved excellent results. Meanwhile, the direct killing of mosquitoes could also significantly reduce the probability of malaria infections. Microorganisms that display complex interactions with Plasmodium, such as Wolbachia and gut flora, have shown observable transmission-blocking potential. These could be used as a biological control strategy and play an important part in blocking the transmission of malaria.

The Nitrobenzoxadiazole Derivative NBDHEX Behaves as Plasmodium falciparum Gametocyte Selective Inhibitor with Malaria Parasite Transmission Blocking Activity

This work describes the activity of 6-((7-nitrobenzo[c][1,2,5]oxadiazol-4-yl)thio)hexan-1-ol (NBDHEX) and of its newly identified carboxylic acid metabolite on the human malaria parasite Plasmodium falciparum. NBDHEX has been previously identified as a potent cytotoxic agent against murine and human cancer cells as well as towards the protozoan parasite Giardia duodenalis. We show here that NBDHEX is active in vitro against all blood stages of P. falciparum, with the rare feature of killing the parasite stages transmissible to mosquitoes, the gametocytes, with a 4-fold higher potency than that on the pathogenic asexual stages. This activity importantly translates into blocking parasite transmission through the Anopheles vector in mosquito experimental infections. A mass spectrometry analysis identified covalent NBDHEX modifications in specific cysteine residues of five gametocyte proteins, possibly associated with its antiparasitic effect. The carboxylic acid metabolite of NBDHEX retains the gametocyte preferential inhibitory activity of the parent compound, making this novel P. falciparum transmission-blocking chemotype at least as a new tool to uncover biological processes targetable by gametocyte selective drugs. Both NBDHEX and its carboxylic acid metabolite show very limited in vitro cytotoxicity on VERO cells. This result and previous evidence that NBDHEX shows an excellent in vivo safety profile in mice and is orally active against human cancer xenografts make these molecules potential starting points to develop new P. falciparum transmission-blocking agents, enriching the repertoire of drugs needed to eliminate malaria.

Boromycin has Rapid-Onset Antibiotic Activity Against Asexual and Sexual Blood Stages of Plasmodium falciparum

Boromycin is a boron-containing macrolide antibiotic produced by Streptomyces antibioticus with potent activity against certain viruses, Gram-positive bacteria and protozoan parasites. Most antimalarial antibiotics affect plasmodial organelles of prokaryotic origin and have a relatively slow onset of action. They are used for malaria prophylaxis and for the treatment of malaria when combined to a fast-acting drug. Despite the success of artemisinin combination therapies, the current gold standard treatment, new alternatives are constantly needed due to the ability of malaria parasites to become resistant to almost all drugs that are in heavy clinical use. In vitro antiplasmodial activity screens of tetracyclines (omadacycline, sarecycline, methacycline, demeclocycline, lymecycline, meclocycline), macrolides (oleandomycin, boromycin, josamycin, troleandomycin), and control drugs (chloroquine, clindamycin, doxycycline, minocycline, eravacycline) revealed boromycin as highly potent against Plasmodium falciparum and the zoonotic Plasmodium knowlesi. In contrast to tetracyclines, boromycin rapidly killed asexual stages of both Plasmodium species already at low concentrations (~ 1 nM) including multidrug resistant P. falciparum strains (Dd2, K1, 7G8). In addition, boromycin was active against P. falciparum stage V gametocytes at a low nanomolar range (IC₅₀: 8.5 ± 3.6 nM). Assessment of the mode of action excluded the apicoplast as the main target. Although there was an ionophoric activity on potassium channels, the effect was too low to explain the drug´s antiplasmodial activity. Boromycin is a promising antimalarial candidate with activity against multiple life cycle stages of the parasite.

An update on pharmacogenetic factors influencing the metabolism and toxicity of artemisinin-based combination therapy in the treatment of malaria

INTRODUCTION

Artemisinin-based combination therapies (ACTs) are recommended first-line antimalarials for uncomplicated Plasmodium falciparum malaria. Pharmacokinetic/pharmacodynamic variation associated with ACT drugs and their effect is documented. It is accepted to an extent that inter-individual variation is genetically driven, and should be explored for optimized antimalarial use.

AREAS COVERED

We provide an update on the pharmacogenetics of ACT antimalarial disposition. Beyond presently used antimalarials, we also refer to information available for the most notable next-generation drugs under development. The bibliographic approach was based on multiple Boolean searches on PubMed covering all recent publications since our previous review.

EXPERT OPINION

The last 10 years have witnessed an increase in our knowledge of ACT pharmacogenetics, including the first clear examples of its contribution as an exacerbating factor for drug-drug interactions. This knowledge gap is still large and is likely to widen as a new wave of antimalarial drug is looming, with few studies addressing their pharmacogenetics. Clinically useful pharmacogenetic markers are still not available, in particular, from an individual precision medicine perspective. A better understanding of the genetic makeup of target populations can be valuable for aiding decisions on mass drug administration implementation concerning region-specific antimalarial drug and dosage options.

Chemical Biology Tools To Investigate Malaria Parasites

Parasitic diseases like malaria tropica have been shaping human evolution and history since the beginning of mankind. After infection, the response of the human host ranges from asymptomatic to severe and may culminate in death. Therefore, proper examination of the parasite's biology is pivotal to deciphering unique molecular, biochemical and cell biological processes, which in turn ensure the identification of treatment strategies, such as potent drug targets and vaccine candidates. However, implementing molecular biology methods for genetic manipulation proves to be difficult for many parasite model organisms. The development of fast and straightforward applicable alternatives, for instance small-molecule probes from the field of chemical biology, is essential. In this review, we will recapitulate the highlights of previous molecular and chemical biology approaches that have already created insight and understanding of the malaria parasite Plasmodium falciparum. We discuss current developments from the field of chemical biology and explore how their application could advance research into this parasite in the future. We anticipate that the described approaches will help to close knowledge gaps in the biology of P. falciparum and we hope that researchers will be inspired to use these methods to gain knowledge - with the aim of ending this devastating disease.

Tadalafil impacts the mechanical properties of Plasmodium falciparum gametocyte-infected erythrocytes

Plasmodium falciparum gametocytes modify the mechanical properties of their erythrocyte host to persist for several weeks in the blood circulation and to be available for mosquitoes. These changes are tightly regulated by the plasmodial phosphodiesterase delta that decreases both the stiffness and the permeability of the infected host cell. Here, we address the effect of the phosphodiesterase inhibitor tadalafil on deformability and permeability of gametocyte-infected erythrocytes. We show that this inhibitor drastically increases isosmotic lysis of gametocyte-infected erythrocytes and impairs their ability to circulate in an in vitro model for splenic retention. These findings indicate that tadalafil represents a novel drug lead potentially capable of blocking malaria parasite transmission by impacting gametocyte circulation.

Discovery of Potent and Fast-Acting Antimalarial Bis-1,2,4-triazines

Novel 3,3'-disubstituted-5,5'-bi(1,2,4-triazine) compounds with potent in vitro activity against Plasmodium falciparum parasites were recently discovered. To improve the pharmacokinetic properties of the triazine derivatives, a new structure-activity relationship (SAR) investigation was initiated with a focus on enhancing the metabolic stability of lead compounds. These efforts led to the identification of second-generation highly potent antimalarial bis-triazines, exemplified by triazine 23, which exhibited significantly improved in vitro metabolic stability (8 and 42 μL/min/mg protein in human and mouse liver microsomes). The disubstituted triazine dimer 23 was also observed to suppress parasitemia in the Peters 4-day test with a mean ED50 value of 1.85 mg/kg/day and exhibited a fast-killing profile, revealing a new class of orally available antimalarial compounds of considerable interest.

Investigation of factors affecting the production of P. falciparum gametocytes in an Indian isolate

The fundamental requirement of every gametocytocidal drug screening assay is the sufficient numbers of healthy and viable gametocytes. The number of in vitro gametocytes grossly depends on the genetic capacity of parasites to produce gametocytes and on various environmental factors that are not precisely elucidated. In the present study, we tested multiple environmental factors that are reported, hypothesized, or predicted to influence gametocyte numbers. We observed that hypoxanthine and the use of freshly drawn human blood significantly enhance gametocytemia (p < 0.05) in vitro. However, other tested factors did not significantly affect gametocytemia. The addition of N-acetyl glucosamine to the culture enriched the gametocytes but d-sorbitol (5% v/v) in amounts and duration of incubation tested was unable to do so without negatively affecting the maturity and health of the gametocytes. Although the in vitro gametocyte production depends on the genetic capability of the parasite strain tested, various environmental factors also control the ability of the strain to produce gametocytes up to a certain extent. This is the first study testing the role of various environmental factors that might affect the gametocyte development in a gametocyte producing strain. The results presented herein will help in the optimization of gametocyte production procedures for various gametocytocidal drug screening assays.

The transmission-blocking effects of antimalarial drugs revisited: fitness costs and sporontocidal effects of artesunate and sulfadoxine-pyrimethamine

Assays used to evaluate the transmission-blocking activity of antimalarial drugs are largely focused on their potential to inhibit or reduce the infectivity of gametocytes, the blood stages of the parasite that are responsible for the onward transmission to the mosquito vector. For this purpose, the drug is administered concomitantly with gametocyte-infected blood, and the results are evaluated as the percentage of reduction in the number of oocysts in the mosquito midgut. We report the results of a series of experiments that explore the transmission-blocking potential of two key antimalarial drugs, artesunate and sulfadoxine-pyrimethamine, when administered to mosquitoes already infected from a previous blood meal. For this purpose, uninfected mosquitoes and mosquitoes carrying a 6 day old Plasmodium relictum infection (early oocyst stages) were allowed to feed either on a drug-treated or an untreated host in a fully factorial experiment. This protocol allowed us to bypass the gametocyte stages and establish whether the drugs have a sporontocidal effect, i.e. whether they are able to arrest the ongoing development of oocysts and sporozoites, as would be the case when a mosquito takes a post-infection treated blood meal. In a separate experiment, we also explored whether a drug-treated blood meal impacted key life history traits of the mosquito relevant for transmission, and if this depended on their infection status. Our results showed that feeding on an artesunate- or sulfadoxine-pyrimethamine-treated hosts has no epidemiologically relevant effects on the fitness of infected or uninfected mosquitoes. In contrast, when infected mosquitoes fed on an sulfadoxine-pyrimethamine-treated host, we observed both a significant increase in the number of oocysts in the midgut, and a drastic decrease in both sporozoite prevalence (-30%) and burden (-80%) compared with the untreated controls. We discuss the potential mechanisms underlying these seemingly contradictory results and contend that, provided the results are translatable to human malaria, the potential epidemiological and evolutionary consequences of the current preventive use of sulfadoxine-pyrimethamine in malaria-endemic countries could be substantial.

Antiplasmodial activity of sulfonylhydrazones: in vitro and in silico approaches

Malaria is still a life-threatening public health issue, and the upsurge of resistant strains requires continuous generation of active molecules. In this work, 35 sulfonylhydrazone derivatives were synthesized and evaluated against Plasmodium falciparum chloroquine-sensitive (3D7) and resistant (W2) strains. The most promising compound, 5b, had an IC₅₀ of 0.22 μM against W2 and was less cytotoxic and 26-fold more selective than chloroquine. The structure-activity relationship model, statistical analysis and molecular modeling studies suggested that antiplasmodial activity was related to hydrogen bond acceptor count, molecular weight and partition coefficient of octanol/water and displacement of frontier orbitals to the heteroaromatic ring beside the imine bond. This study demonstrates that the synthesized molecules with a simple scaffold allow the hit-to-lead process for new antimalarials to commence.

Organoarsenic Compounds with In Vitro Activity against the Malaria Parasite Plasmodium falciparum

The rapid development of parasite drug resistance as well as the lack of medications targeting both the asexual and the sexual blood stages of the malaria parasite necessitate the search for novel antimalarial compounds. Eleven organoarsenic compounds were synthesized and tested for their effect on the asexual blood stages and sexual transmission stages of the malaria parasite Plasmodium falciparum using in vitro assays. The inhibitory potential of the compounds on blood stage viability was tested on the chloroquine (CQ)-sensitive 3D7 and the CQ-resistant Dd2 strain using the Malstat assay. The most effective compounds were subsequently investigated for their effect on impairing gametocyte development and gametogenesis, using the gametocyte-producing NF54 strain in respective cell-based assays. Their potential toxicity was investigated on leukemia cell line Nalm-6 and non-infected erythrocytes. Five out of the 11 compounds showed antiplasmodial activities against 3D7, with half-maximal inhibitory concentration (IC₅₀) values ranging between 1.52 and 8.64 µM. Three of the compounds also acted against Dd2, with the most active compound As-8 exhibiting an IC₅₀ of 0.35 µM. The five compounds also showed significant inhibitory effects on the parasite sexual stages at both IC₅₀ and IC₉₀ concentrations with As-8 displaying the best gametocytocidal activity. No hemolytic and cytotoxic effect was observed for any of the compounds. The organoarsenic compound As-8 may represent a good lead for the design of novel organoarsenic drugs with combined antimalarial and transmission blocking activities.

Malaria transmission-blocking drugs: implications and future perspectives

As the world gets closer to eliminating malaria, the scientific community worldwide has begun to realize the importance of malaria transmission-blocking interventions. The onus of breaking the life cycle of the human malaria parasite Plasmodium falciparum predominantly rests upon transmission-blocking drugs because of emerging resistance to commonly used schizonticides and insecticides. This third part of our review series on malaria transmission-blocking entails transmission-blocking potential of preclinical transmission-blocking antimalarials and other non-malaria drugs/experimental compounds that are not in clinical or preclinical development for malaria but possess transmission-blocking potential. Collective analysis of the structure and the activity of these experimental compounds might pave the way toward generation of novel prototypes of next-generation transmission-blocking drugs.