Menoctone Resistance in Malaria Parasites Is Conferred by M133I Mutations in Cytochrome b That Are Transmissible through Mosquitoes

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.

A Theileria annulata parasite with a single mutation, methionine 128 to isoleucine (M128I), in cytochrome B is resistant to buparvaquone

Tropical theileriosis is a fatal leukemic-like disease of cattle caused by the tick-transmitted protozoan parasite Theileria annulata. The economics of cattle meat and milk production is severely affected by theileriosis in endemic areas. The hydroxynaphtoquinone buparvaquone (BPQ) is the only available drug currently used to treat clinical theileriosis, whilst BPQ resistance is emerging and spreading in endemic areas. Here, we chronically exposed T. annulata-transformed macrophages in vitro to BPQ and monitored the emergence of drug-resistant parasites. Surviving parasites revealed a significant increase in BPQ IC50 compared to the wild type parasites. Drug resistant parasites from two independent cloned lines had an identical single mutation, M128I, in the gene coding for T. annulata cytochrome B (Tacytb). This in vitro generated mutation has not been reported in resistant field isolates previously, but is reminiscent of the methionine to isoleucine mutation in atovaquone-resistant Plasmodium and Babesia. The M128I mutation did not appear to exert any deleterious effect on parasite fitness (proliferation and differentiation to merozoites). To gain insight into whether drug-resistance could have resulted from altered drug binding to TaCytB we generated in silico a 3D-model of wild type TaCytB and docked BPQ to the predicted 3D-structure. Potential binding sites cluster in four areas of the protein structure including the Q01 site. The bound drug in the Q01 site is expected to pack against an alpha helix, which included M128, suggesting that the change in amino acid in this position may alter drug-binding. The in vitro generated BPQ resistant T. annulata is a useful tool to determine the contribution of the various predicted docking sites to BPQ resistance and will also allow testing novel drugs against theileriosis for their potential to overcome BPQ resistance.

Clinically relevant atovaquone-resistant human malaria parasites fail to transmit by mosquito

Long-acting injectable medications, such as atovaquone, offer the prospect of a "chemical vaccine" for malaria, combining drug efficacy with vaccine durability. However, selection and transmission of drug-resistant parasites is of concern. Laboratory studies have indicated that atovaquone resistance disadvantages parasites in mosquitoes, but lack of data on clinically relevant Plasmodium falciparum has hampered integration of these variable findings into drug development decisions. Here we generate atovaquone-resistant parasites that differ from wild type parent by only a Y268S mutation in cytochrome b, a modification associated with atovaquone treatment failure in humans. Relative to wild type, Y268S parasites evidence multiple defects, most marked in their development in mosquitoes, whether from Southeast Asia (Anopheles stephensi) or Africa (An. gambiae). Growth of asexual Y268S P. falciparum in human red cells is impaired, but parasite loss in the mosquito is progressive, from reduced gametocyte exflagellation, to smaller number and size of oocysts, and finally to absence of sporozoites. The Y268S mutant fails to transmit from mosquitoes to mice engrafted with human liver cells and erythrocytes. The severe-to-lethal fitness cost of clinically relevant atovaquone resistance to P. falciparum in the mosquito substantially lessens the likelihood of its transmission in the field.

Structure-Activity Relationship Studies on Cell-Potent Nicotinamide N-Methyltransferase Bisubstrate Inhibitors

Nicotinamide N-methyltransferase (NNMT) is a metabolic enzyme implicated in multiple diseases, making it a promising therapeutic target. Building upon our recently reported NNMT inhibitor II399, we systematically investigate the structure-activity relationship by designing and synthesizing a series of analogues. Among them, two top inhibitors II559 (Ki = 1.2 nM) and II802 (Ki = 1.6 nM) displayed over 5000-fold selectivity for NNMT over closely related methyltransferases. Moreover, II559 and II802 showed enhanced cellular inhibition, with a cellular IC50 value of approximately 150 nM, making them the most cell-potent bisubstrate inhibitors reported to date. Furthermore, both inhibitors reduced the cell viability with a GI50 value of ∼10 μM and suppressed the migration of aggressive clear cell renal cancer cell carcinoma cell lines. Overall, II559 and II802 would serve as valuable probes to investigate the enzymatic function of NNMT in health and diseases.

Using an antimalarial in mosquitoes overcomes Anopheles and Plasmodium resistance to malaria control strategies

The spread of insecticide resistance in Anopheles mosquitoes and drug resistance in Plasmodium parasites is contributing to a global resurgence of malaria, making the generation of control tools that can overcome these roadblocks an urgent public health priority. We recently showed that the transmission of Plasmodium falciparum parasites can be efficiently blocked when exposing Anopheles gambiae females to antimalarials deposited on a treated surface, with no negative consequences on major components of mosquito fitness. Here, we demonstrate this approach can overcome the hurdles of insecticide resistance in mosquitoes and drug resistant in parasites. We show that the transmission-blocking efficacy of mosquito-targeted antimalarials is maintained when field-derived, insecticide resistant Anopheles are exposed to the potent cytochrome b inhibitor atovaquone, demonstrating that this drug escapes insecticide resistance mechanisms that could potentially interfere with its function. Moreover, this approach prevents transmission of field-derived, artemisinin resistant P. falciparum parasites (Kelch13 C580Y mutant), proving that this strategy could be used to prevent the spread of parasite mutations that induce resistance to front-line antimalarials. Atovaquone is also highly effective at limiting parasite development when ingested by mosquitoes in sugar solutions, including in ongoing infections. These data support the use of mosquito-targeted antimalarials as a promising tool to complement and extend the efficacy of current malaria control interventions.

Effective control of malaria is hampered by resistance to vector-targeted insecticides and parasite-targeted drugs. This situation is exacerbated by a critical lack of chemical diversity in both interventions and, as such, new interventions are urgently needed. Recent laboratory studies have shown that an alternative approach based on treating Anopheles mosquitoes directly with antimalarial compounds can make mosquitoes incapable of transmitting the Plasmodium parasites that cause malaria. While promising, showing that mosquito-targeted antimalarials remain effective against wild parasites and mosquitoes, including drug- and insecticide-resistant populations in malaria-endemic countries, is crucial to the future viability of this approach. In this study, carried out in the US and Burkina Faso, we show that insecticide-resistance mechanisms found in highly resistant, natural Anopheles mosquito populations do not interfere with the transmission blocking activity of tarsal exposure to the antimalarial atovaquone, and that mosquito-targeted antimalarial exposure can block transmission of parasites resistant to the main therapeutic antimalarial drug artemisinin. By combining lab, and field-based studies in this way we have demonstrated that this novel approach can be effective in areas where conventional control measures are no longer as effective.

Cytochrome b Drug Resistance Mutation Decreases Babesia Fitness in the Tick Stages But Not the Mammalian Erythrocytic Cycle

Human babesiosis is an emerging tick-borne malaria-like illness caused by Babesia parasites following their development in erythrocytes. Here, we show that a mutation in the Babesia microti mitochondrial cytochrome b (Cytb) that confers resistance to the antibabesial drug ELQ-502 decreases parasite fitness in the arthropod vector. Interestingly, whereas the mutant allele does not affect B. microti fitness during the mammalian blood phase of the parasite life cycle and is genetically stable as parasite burden increases, ELQ-502-resistant mutant parasites developing in the tick vector are genetically unstable with a high rate of the wild-type allele emerging during the nymphal stage. Furthermore, we show that B. microti parasites with this mutation are transmitted from the tick to the host, raising the possibility that the frequency of Cytb resistance mutations may be decreased by passage through the tick vector, but could persist in the environment if present when ticks feed.

Selection of Cytochrome b Mutants Is Rare among Plasmodium falciparum Patients Failing Treatment with Atovaquone-Proguanil in Cambodia

Atovaquone-proguanil remains effective against multidrug-resistant Plasmodium falciparum in Southeast Asia, but resistance is mediated by a single point mutation in cytochrome b (cytb) that can arise during treatment. Among 14 atovaquone-proguanil treatment failures in a clinical trial in Cambodia, only one recrudescence harbored the cytb mutation Y268C. Deep sequencing did not detect the mutation at baseline or in the first 3 days of treatment, suggesting that it arose de novo Further sequencing across cytb similarly found no low-frequency cytb mutations that were up-selected from baseline to recrudescence. Copy number amplification in dihydroorotate dehydrogenase (DHODH) and cytb as markers of atovaquone tolerance was also absent. Cytb mutation played a minor role in atovaquone-proguanil treatment failures in an active comparator clinical trial.

KASP: a genotyping method to rapid identification of resistance in Plasmodium falciparum

BACKGROUND

The emergence and spread of anti-malarial resistance continues to hinder malaria control. Plasmodium falciparum, the species that causes most human malaria cases and most deaths, has shown resistance to almost all known anti-malarials. This anti-malarial resistance arises from the development and subsequent expansion of Single Nucleotide Polymorphisms (SNPs) in specific parasite genes. A quick and cheap tool for the detection of drug resistance can be crucial and very useful for use in hospitals and in malaria control programmes. It has been demonstrated in different contexts that genotyping by Kompetitive Allele Specific PCR (KASP), is a simple, fast and economical method that allows a high-precision biallelic characterization of SNPs, hence its possible utility in the study of resistance in P. falciparum.

METHODS

Three SNPs involved in most cases of resistance to the most widespread anti-malarial treatments have been analysed by PCR plus sequencing and by KASP (C580Y of the Kelch13 gene, Y86N of the Pfmdr1 gene and M133I of the Pfcytb gene). A total of 113 P. falciparum positive samples and 24 negative samples, previously analysed by PCR and sequencing, were selected for this assay. Likewise, the samples were genotyped for the MSP-1 and MSP-2 genes, and the Multiplicity of Infection (MOI) and parasitaemia were measured to observe their possible influence on the KASP method.

RESULTS

The KASP results showed the same expected mutations and wild type genotypes as the reference method, with few exceptions that correlated with very low parasitaemia samples. In addition, two cases of heterozygotes that had not been detected by sequencing were found. No correlation was found between the MOI or parasitaemia and the KASP values of the sample. The reproducibility of the technique shows no oscillations between repetitions in any of the three SNPs analysed.

CONCLUSIONS

The KASP assays developed in this study were efficient and versatile for the determination of the Plasmodium genotypes related to resistance. The method is simple, fast, reproducible with low cost in personnel, material and equipment and scalable, being able to core KASP arrays, including numerous SNPs, to complete the main pattern of mutations associated to P. falciparum resistance.

Exposing Anopheles mosquitoes to antimalarials blocks Plasmodium parasite transmission

Every year the bites of Anopheles mosquitoes kill hundreds of thousands of people, mostly young African children, by transmitting deadly Plasmodium falciparum malaria parasites. Since the turn of the century, efforts to prevent transmission of these parasites via the mass distribution of insecticide-treated bed nets have been extremely successful, causing an unprecedented reduction in malaria deaths¹. However, resistance to insecticides has become widespread in Anopheles populations^2–4, threatening a global resurgence of the disease and making the generation of effective new malaria control tools an urgent public health priority. Here, we show that development of P. falciparum can be rapidly and completely blocked when Anopheles gambiae females uptake low concentrations of specific antimalarials from treated surfaces, simulating contact with a bed net. Mosquito exposure to atovaquone prior to or shortly after P. falciparum infection causes full parasite arrest in the female midgut, preventing transmission of infection. Similar transmission-blocking effects are achieved with other cytochrome B inhibitors, demonstrating that parasite mitochondrial function is a good target for parasite killing. Incorporating these effects into a model of malaria transmission dynamics predicts that the inclusion of Plasmodium inhibitors on mosquito nets would significantly mitigate the global health impact of insecticide resistance. This study identifies a powerful new strategy for blocking Plasmodium transmission by Anopheles females, with promising implications for malaria eradication efforts.

Selection of Plasmodium falciparum cytochrome B mutants by putative PfNDH2 inhibitors

Malaria control is threatened by a limited pipeline of effective pharmaceuticals against drug-resistant strains of Plasmodium falciparum Components of the mitochondrial electron transport chain (ETC) are attractive targets for drug development, owing to exploitable differences between the parasite and human ETC. Disruption of ETC function interferes with metabolic processes including de novo pyrimidine synthesis, essential for nucleic acid replication. We investigated the effects of ETC inhibitor selection on two distinct P. falciparum clones, Dd2 and 106/1. Compounds CK-2-68 and RYL-552, substituted quinolones reported to block P. falciparum NADH dehydrogenase 2 (PfNDH2; a type II NADH:quinone oxidoreductase), unexpectedly selected mutations at the quinol oxidation (Q_o) pocket of P. falciparum cytochrome B (PfCytB). Selection experiments with atovaquone (ATQ) on 106/1 parasites yielded highly resistant PfCytB Y268S mutants seen in clinical infections that fail ATQ-proguanil treatment. In contrast, ATQ pressure on Dd2 yielded moderately resistant parasites carrying a PfCytB M133I or K272R mutation. Strikingly, all ATQ-selected mutants demonstrated little change or slight increase of sensitivity to CK-2-68 or RYL-552. Molecular docking studies demonstrated binding of all three ETC inhibitors to the Q_o pocket of PfCytB, where Y268 forms strong van der Waals interactions with the hydroxynaphthoquinone ring of ATQ but not the quinolone ring of CK-2-68 or RYL-552. Our results suggest that combinations of suitable ETC inhibitors may be able to subvert or delay the development of P. falciparum drug resistance.