Cell Swelling Induced by the Antimalarial KAE609 (Cipargamin) and Other PfATP4-Associated Antimalarials

Recent advances, challenges and updates on the development of therapeutics for malaria

Malaria has developed as a serious worldwide health issue as a result of the introduction of resistant Plasmodium species strains. Because of the common chemo resistance to most of the existing drugs on the market, it poses a severe health problem and significant obstacles in drug research. Malaria treatment has evolved during the last two decades in response to Plasmodium falciparum drug sensitivity and a return of the disease in tropical areas. Plasmodium falciparum is now highly resistant to the majority of antimalarial drugs. The parasite resistance drew focus to developing novel antimalarials to combat parasite resistance. The requirement for many novel antimalarial drugs in the future year necessitates adopting various drug development methodologies. Different innovative strategies for discovering antimalarial drugs are now being examined here. This review is primarily concerned with the description of newly synthesized antimalarial compounds, i.e. Tafenoquine, Cipargamin, Ferroquine, Artefenomel, DSM265, MMV390048 designed to improve the activity of pure antimalarial enantiomers. In this review, we selected the representative malarial drugs in clinical trials, classified them with detailed targets according to their action, discussed the relationship within the human trials, and generated a summative discussion with prospective expectations.

A pH Fingerprint Assay to Identify Inhibitors of Multiple Validated and Potential Antimalarial Drug Targets

New drugs with novel modes of action are needed to safeguard malaria treatment. In recent years, millions of compounds have been tested for their ability to inhibit the growth of asexual blood-stage Plasmodium falciparum parasites, resulting in the identification of thousands of compounds with antiplasmodial activity. Determining the mechanisms of action of antiplasmodial compounds informs their further development, but remains challenging. A relatively high proportion of compounds identified as killing asexual blood-stage parasites show evidence of targeting the parasite's plasma membrane Na+-extruding, H+-importing pump, PfATP4. Inhibitors of PfATP4 give rise to characteristic changes in the parasite's internal [Na+] and pH. Here, we designed a "pH fingerprint" assay that robustly identifies PfATP4 inhibitors while simultaneously allowing the detection of (and discrimination between) inhibitors of the lactate:H+ transporter PfFNT, which is a validated antimalarial drug target, and the V-type H+ ATPase, which was suggested as a possible target of the clinical candidate ZY19489. In our pH fingerprint assays and subsequent secondary assays, ZY19489 did not show evidence for the inhibition of pH regulation by the V-type H+ ATPase, suggesting that it has a different mode of action in the parasite. The pH fingerprint assay also has the potential to identify protonophores, inhibitors of the acid-loading Cl- transporter(s) (for which the molecular identity(ies) remain elusive), and compounds that act through inhibition of either the glucose transporter PfHT or glycolysis. The pH fingerprint assay therefore provides an efficient starting point to match a proportion of antiplasmodial compounds with their mechanisms of action.

Melatonin: A look at protozoal and helminths

Melatonin is a pleiotropic neurohormone found in different animal, plant, and microorganism species. It is a product resulting from tryptophan metabolism in the pineal gland and is widely known for its ability to synchronize the circadian rhythm to antitumor functions in different types of cancers. The molecular mechanisms responsible for its immunomodulatory, antioxidant and cytoprotective effects involve binding to high-affinity G protein-coupled receptors and interactions with intracellular targets that modulate signal transduction pathways. In vitro and in vivo studies have reported the therapeutic potential of melatonin in different infectious and parasitic diseases. In this review, the protective and pathophysiological roles of melatonin in fighting protozoan and helminth infections and the possible mechanisms involved against these stressors will be discussed.

Recent developments in antimalarial drug discovery

Although malaria remains a big burden to many countries that it threatens their socio-economic stability, particularly in the countries where malaria is endemic, there have been great efforts to eradicate this disease with both successes and failures. For example, there has been a great improvement in malaria prevention and treatment methods with a net reduction in infection and mortality rates. However, the disease remains a global threat in terms of the number of people affected because it is one of the infectious diseases that has the highest prevalence rate, especially in Africa where the deadly Plasmodium falciparum is still widely spread. Methods to fight malaria are being diversified, including the use of mosquito nets, the target candidate profiles (TCPs) and target product profiles (TPPs) of medicine for malarial venture (MMV) strategy, the search for newer and potent drugs that could reverse chloroquine resistance, and the use of adjuvants such as rosiglitazone and sevuparin. Although these adjuvants have no antiplasmodial activity, they can help to alleviate the effects which result from plasmodium invasion such as cytoadherence. The list of new antimalarial drugs under development is long, including the out of ordinary new drugs MMV048, CDRI-97/78 and INE963 from South Africa, India and Novartis, respectively.

Optimization of 2,3-Dihydroquinazolinone-3-carboxamides as Antimalarials Targeting PfATP4

There is an urgent need to populate the antimalarial clinical portfolio with new candidates because of resistance against frontline antimalarials. To discover new antimalarial chemotypes, we performed a high-throughput screen of the Janssen Jumpstarter library against the Plasmodium falciparum asexual blood-stage parasite and identified the 2,3-dihydroquinazolinone-3-carboxamide scaffold. We defined the SAR and found that 8-substitution on the tricyclic ring system and 3-substitution of the exocyclic arene produced analogues with potent activity against asexual parasites equivalent to clinically used antimalarials. Resistance selection and profiling against drug-resistant parasite strains revealed that this antimalarial chemotype targets PfATP4. Dihydroquinazolinone analogues were shown to disrupt parasite Na⁺ homeostasis and affect parasite pH, exhibited a fast-to-moderate rate of asexual kill, and blocked gametogenesis, consistent with the phenotype of clinically used PfATP4 inhibitors. Finally, we observed that optimized frontrunner analogue WJM-921 demonstrates oral efficacy in a mouse model of malaria.

Metabolic responses in blood-stage malaria parasites associated with increased and decreased sensitivity to PfATP4 inhibitors

BACKGROUND

Spiroindolone and pyrazoleamide antimalarial compounds target Plasmodium falciparum P-type ATPase (PfATP4) and induce disruption of intracellular Na⁺ homeostasis. Recently, a PfATP4 mutation was discovered that confers resistance to a pyrazoleamide while increasing sensitivity to a spiroindolone. Transcriptomic and metabolic adaptations that underlie this seemingly contradictory response of P. falciparum to sublethal concentrations of each compound were examined to understand the different cellular accommodation to PfATP4 disruptions.

METHODS

A genetically engineered P. falciparum Dd2 strain (Dd2^A211V) carrying an Ala211Val (A211V) mutation in PfATP4 was used to identify metabolic adaptations associated with the mutation that results in decreased sensitivity to PA21A092 (a pyrazoleamide) and increased sensitivity to KAE609 (a spiroindolone). First, sublethal doses of PA21A092 and KAE609 causing substantial reduction (30-70%) in Dd2^A211V parasite replication were identified. Then, at this sublethal dose of PA21A092 (or KAE609), metabolomic and transcriptomic data were collected during the first intraerythrocytic developmental cycle. Finally, the time-resolved data were integrated with a whole-genome metabolic network model of P. falciparum to characterize antimalarial-induced physiological adaptations.

RESULTS

Sublethal treatment with PA21A092 caused significant (p < 0.001) alterations in the abundances of 91 Plasmodium gene transcripts, whereas only 21 transcripts were significantly altered due to sublethal treatment with KAE609. In the metabolomic data, a substantial alteration (≥ fourfold) in the abundances of carbohydrate metabolites in the presence of either compound was found. The estimated rates of macromolecule syntheses between the two antimalarial-treated conditions were also comparable, except for the rate of lipid synthesis. A closer examination of parasite metabolism in the presence of either compound indicated statistically significant differences in enzymatic activities associated with synthesis of phosphatidylcholine, phosphatidylserine, and phosphatidylinositol.

CONCLUSION

The results of this study suggest that malaria parasites activate protein kinases via phospholipid-dependent signalling in response to the ionic perturbation induced by the Na⁺ homeostasis disruptor PA21A092. Therefore, targeted disruption of phospholipid signalling in PA21A092-resistant parasites could be a means to block the emergence of resistance to PA21A092.

Spirofused Tetrahydroisoquinoline-Oxindole Hybrids (Spiroquindolones) as Potential Multitarget Antimalarial Agents: Preliminary Hit Optimization and Efficacy Evaluation in Mice

Previous studies suggest that 3',5'-dihydro-2'H-spiro[indoline-3,1'-isoquinolin]-2-ones (DSIIQs [spiroquindolones]) are multitarget antiplasmodial agents that combine the actions of spiroindolone and naphthylisoquinoline antimalarial agents. In this study, 12 analogues of compound (±)-5 (moxiquindole), the prototypical spiroquindolone, were synthesized and tested for antiplasmodial activity. Compound (±)-11 (a mixture of compounds 11a and 11b), the most potent analogue, displayed low-nanomolar activity against P. falciparum chloroquine-sensitive 3D7 strain (50% inhibitory concentration [IC₅₀] for 3D7 = 21 ± 02 nM) and was active against all major erythrocytic stages of the parasite life cycle (ring, trophozoite, and schizont); it also inhibited hemoglobin metabolism and caused extensive vacuolation in parasites. In drug-resistant parasites, compound (±)-11 exhibited potent activity (IC₅₀ for Dd2 = 58.34 ± 2.04 nM) against the P. falciparum multidrug-resistant Dd2 strain, and both compounds (±)-5 and (±)-11 displayed significant cross-resistance against the P. falciparum ATP4 mutant parasite Dd2 SJ733 but not against the Dd2 KAE609 strain. In mice, both compounds (±)-5 and (±)-11 displayed dose-dependent reduction of parasitemia with suppressive 50% effective dose (ED₅₀) values of 0.44 and 0.11 mg/kg of body weight, respectively. The compounds were also found to be curative in vivo and are thus worthy of further investigation.

PfATP4 inhibitors in the Medicines for Malaria Venture Malaria Box and Pathogen Box block the schizont-to-ring transition by inhibiting egress rather than invasion

The cation efflux pump Plasmodium falciparum ATPase 4 (PfATP4) maintains Na⁺ homeostasis in malaria parasites and has been implicated in the mechanism of action of many structurally diverse antimalarial agents, including >7% of the antimalarial compounds in the Medicines for Malaria Venture's 'Malaria Box' and 'Pathogen Box'. Recent screens of the 'Malaria Box' and 'Pathogen Box' revealed that many PfATP4 inhibitors prevent parasites from exiting their host red blood cell (egress) or entering new host cells (invasion), suggesting that these compounds may have additional molecular targets involved in egress or invasion. Here, we demonstrate that five PfATP4 inhibitors reduce egress but not invasion. These compounds appear to inhibit egress by blocking the activation of protein kinase G, an enzyme that, once stimulated, rapidly activates parasite egress. We establish a direct link between egress and PfATP4 function by showing that the inhibition of egress is attenuated in a Na⁺-depleted environment and in parasites with a mutation in pfatp4. Finally, we show that PfATP4 inhibitors induce host cell lysis when administered prior to the completion of parasite replication. Since host cell lysis mimics egress but is not followed by invasion, this phenomenon likely explains why several PfATP4 inhibitors were previously classified as invasion inhibitors. Collectively, our results confirm that PfATP4-mediated Na⁺ efflux is critical to the regulation of parasite egress.

A G358S mutation in the Plasmodium falciparum Na+ pump PfATP4 confers clinically-relevant resistance to cipargamin

Diverse compounds target the Plasmodium falciparum Na+ pump PfATP4, with cipargamin and (+)-SJ733 the most clinically-advanced. In a recent clinical trial for cipargamin, recrudescent parasites emerged, with most having a G358S mutation in PfATP4. Here, we show that PfATP4G358S parasites can withstand micromolar concentrations of cipargamin and (+)-SJ733, while remaining susceptible to antimalarials that do not target PfATP4. The G358S mutation in PfATP4, and the equivalent mutation in Toxoplasma gondii ATP4, decrease the sensitivity of ATP4 to inhibition by cipargamin and (+)-SJ733, thereby protecting parasites from disruption of Na+ regulation. The G358S mutation reduces the affinity of PfATP4 for Na+ and is associated with an increase in the parasite's resting cytosolic [Na+]. However, no defect in parasite growth or transmissibility is observed. Our findings suggest that PfATP4 inhibitors in clinical development should be tested against PfATP4G358S parasites, and that their combination with unrelated antimalarials may mitigate against resistance development.

Innate immunity to malaria: The good, the bad and the unknown

Malaria is the cause of 600.000 deaths annually. However, these deaths represent only a tiny fraction of total malaria cases. Repeated natural infections with the causative agent, Plasmodium sp. parasites, induce protection from severe disease but not sterile immunity. Thus, immunity to Plasmodium is incomplete. Conversely, immunization with attenuated sporozoite stage parasites can induce sterile immunity albeit after multiple vaccinations. These different outcomes are likely to be influenced strongly by the innate immune response to different stages of the parasite lifecycle. Even small numbers of sporozoites can induce a robust proinflammatory type I interferon response, which is believed to be driven by the sensing of parasite RNA. Moreover, induction of innate like gamma-delta cells contributes to the development of adaptive immune responses. Conversely, while blood stage parasites can induce a strong proinflammatory response, regulatory mechanisms are also triggered. In agreement with this, intact parasites are relatively weakly sensed by innate immune cells, but isolated parasite molecules, notably DNA and RNA can induce strong responses. Thus, the innate response to Plasmodium parasite likely represents a trade-off between strong pro-inflammatory responses that may potentiate immunity and regulatory processes that protect the host from cytokine storms that can induce life threatening illness.

Metabolic adjustments of blood-stage Plasmodium falciparum in response to sublethal pyrazoleamide exposure

Due to the recurring loss of antimalarial drugs to resistance, there is a need for novel targets, drugs, and combination therapies to ensure the availability of current and future countermeasures. Pyrazoleamides belong to a novel class of antimalarial drugs that disrupt sodium ion homeostasis, although the exact consequences of this disruption in Plasmodium falciparum remain under investigation. In vitro experiments demonstrated that parasites carrying mutations in the metabolic enzyme PfATP4 develop resistance to pyrazoleamide compounds. However, the underlying mechanisms that allow mutant parasites to evade pyrazoleamide treatment are unclear. Here, we first performed experiments to identify the sublethal dose of a pyrazoleamide compound (PA21A092) that caused a significant reduction in growth over one intraerythrocytic developmental cycle (IDC). At this drug concentration, we collected transcriptomic and metabolomic data at multiple time points during the IDC to quantify gene- and metabolite-level alterations in the treated parasites. To probe the effects of pyrazoleamide treatment on parasite metabolism, we coupled the time-resolved omics data with a metabolic network model of P. falciparum. We found that the drug-treated parasites adjusted carbohydrate metabolism to enhance synthesis of myoinositol-a precursor for phosphatidylinositol biosynthesis. This metabolic adaptation caused a decrease in metabolite flux through the pentose phosphate pathway, causing a decreased rate of RNA synthesis and an increase in oxidative stress. Our model analyses suggest that downstream consequences of enhanced myoinositol synthesis may underlie adjustments that could lead to resistance emergence in P. falciparum exposed to a sublethal dose of a pyrazoleamide drug.

Assessment in vitro of the antimalarial and transmission blocking activities of Cipargamin and Ganaplacide in artemisinin resistant Plasmodium falciparum

Artemisinin resistance in Plasmodium falciparum has emerged and spread widely in the Greater Mekong Subregion threatening current first line artemisinin combination treatments. New antimalarial drugs are needed urgently. Cipargamin (KAE609) and ganaplacide (KAF156) are promising novel antimalarial compounds in advanced stages of development. Both compounds have potent asexual blood stage activities, inhibit P. falciparum gametocytogenesis and reduce oocyst development in anopheline mosquitoes. In this study, we compared the asexual and sexual stage activities of cipargamin, ganaplacide and artesunate in artemisinin resistant P. falciparum isolates (N=7, K13 mutation; C580Y, G449A and R539T) from Thailand and Cambodia. Asexual blood stage antimalarial activity was evaluated in a SYBR-green I based 72h in vitro assay, and the effects on male and female mature stage V gametocytes were assessed in the P. falciparum dual gamete formation assay. Ganaplacide had higher activities when compared to cipargamin and artesunate, with a mean (SD) IC50 against asexual stages of 5.5 (1.1) nM, 7.8 (3.9) nM for male gametocytes and 57.9 (59.6) nM for female gametocytes. Cipargamin had a similar potency against male and female gametocytes, with a mean (SD) IC50 of 123.1 (80.2) nM for male gametocytes, 88.5 (52.7) nM for female gametocytes and 2.4 (0.6) nM for asexual stages. Both cipargamin and ganaplacide showed significant transmission-blocking activities against artemisinin resistant P. falciparum in vitro .

Associations between Varied Susceptibilities to PfATP4 Inhibitors and Genotypes in Ugandan Plasmodium falciparum Isolates

Among novel compounds under recent investigation as potential new antimalarial drugs are three independently developed inhibitors of the Plasmodium falciparum P-type ATPase (PfATP4): KAE609 (cipargamin), PA92, and SJ733. We assessed ex vivo susceptibilities to these compounds of 374 fresh P. falciparum isolates collected in Tororo and Busia districts, Uganda, from 2016 to 2019. Median IC50s were 65 nM for SJ733, 9.1 nM for PA92, and 0.5 nM for KAE609. Sequencing of pfatp4 for 218 of these isolates demonstrated many nonsynonymous single nucleotide polymorphisms; the most frequent mutations were G1128R (69% of isolates mixed or mutant), Q1081K/R (68%), G223S (25%), N1045K (16%), and D1116G/N/Y (16%). The G223S mutation was associated with decreased susceptibility to SJ733, PA92, and KAE609. The D1116G/N/Y mutations were associated with decreased susceptibility to SJ733, and the presence of mutations at both codons 223 and 1116 was associated with decreased susceptibility to PA92 and SJ733. In all of these cases, absolute differences in susceptibilities of wild-type (WT) and mutant parasites were modest. Analysis of clones separated from mixed field isolates consistently identified mutant clones as less susceptible than WT. Analysis of isolates from other sites demonstrated the presence of the G223S and D1116G/N/Y mutations across Uganda. Our results indicate that malaria parasites circulating in Uganda have a number of polymorphisms in PfATP4 and that modestly decreased susceptibility to PfATP4 inhibitors is associated with some mutations now present in Ugandan parasites.

Efficacy of cipargamin (KAE609) in a randomized, Phase II dose-escalation study in adults in sub-Saharan Africa with uncomplicated Plasmodium falciparum malaria

Background

Cipargamin (KAE609) is a potent antimalarial in a phase II trial. Here we report efficacy, pharmacokinetics, and resistance marker analysis across a range of cipargamin doses. These were secondary endpoints from a study primarily conducted to assess the hepatic safety of cipargamin (hepatic safety data are reported elsewhere).

Methods

This phase II, multicenter, randomized, open-label, dose-escalation trial was conducted in sub-Saharan Africa in adults with uncomplicated Plasmodium falciparum malaria. Cipargamin monotherapy was given as single doses up to 150 mg or up to 50 mg once daily for 3 days, with artemether-lumefantrine as control. Key efficacy endpoints were parasite clearance time (PCT), and polymerase chain reaction (PCR)–corrected and uncorrected adequate clinical and parasitological response (ACPR) at 14 and 28 days. Pharmacokinetics and molecular markers of drug resistance were also assessed.

Results

All single or multiple cipargamin doses ≥50 mg were associated with rapid parasite clearance, with median PCT of 8 hours versus 24 hours for artemether-lumefantrine. PCR-corrected ACPR at 14 and 28 days was >75% and 65%, respectively, for each cipargamin dose. A treatment-emerging mutation in the Pfatp4 gene, G358S, was detected in 65% of treatment failures. Pharmacokinetic parameters were consistent with previous data, and approximately dose proportional.

Conclusions

Cipargamin, at single doses of 50 to 150 mg, was associated with very rapid parasite clearance, PCR-corrected ACPR at 28 days of >65% in adults with uncomplicated P. falciparum malaria, and recrudescent parasites frequently harbored a treatment-emerging mutation. Cipargamin will be further developed with a suitable combination partner.

Clinical Trials Registration

ClinicalTrials.gov (NCT03334747).

An insight into the recent development of the clinical candidates for the treatment of malaria and their target proteins

Malaria is an endemic disease, prevalent in tropical and subtropical regions which cost half of million deaths annually. The eradication of malaria is one of the global health priority nevertheless, current therapeutic efforts seem to be insufficient due to the emergence of drug resistance towards most of the available drugs, even first-line treatment ACT, unavailability of the vaccine, and lack of drugs with a new mechanism of action. Intensification of antimalarial research in recent years has resulted into the development of single dose multistage therapeutic agents which has advantage of overcoming the antimalarial drug resistance. The present review explored the current progress in the development of new promising antimalarials against prominent target proteins that have the potential to be a clinical candidate. Here, we also reviewed different aspects of drug resistance and highlighted new drug candidates that are currently in a clinical trial or clinical development, along with a few other molecules with excellent antimalarial activity overs ACTs. The summarized scientific value of previous approaches and structural features of antimalarials related to the activity are highlighted that will be helpful for the development of next-generation antimalarials.

The Functioning of Na+-ATPases from Protozoan Parasites: Are These Pumps Targets for Antiparasitic Drugs?

The ENA ATPases (from exitus natru: the exit of sodium) belonging to the P-type ATPases are structurally very similar to the sarco/endoplasmic reticulum Ca2+-ATPase (SERCA); they exchange Na+ for H+ and, therefore, are also known as Na+-ATPases. ENA ATPases are required in alkaline milieu, as in the case for Aspergillus, where other transporters cannot mediate an uphill Na+ efflux. They are also important for salt tolerance, as described for Arabidopsis. During their life cycles, protozoan parasites might encounter a high pH environment, thus allowing consideration of ENA ATPases as possible targets for controlling certain severe parasitic diseases, such as Chagas' Disease. Phylogenetic analysis has now shown that, besides the types IIA, IIB, IIC, and IID P-type ATPases, there exists a 5th subgroup of ATPases classified as ATP4-type ATPases, found in Plasmodium falciparum and Toxoplasma gondii. In malaria, for example, some drugs targeting PfATP4 destroy Na+ homeostasis; these drugs, which include spiroindolones, are now in clinical trials. The ENA P-type (IID P-type ATPase) and ATP4-type ATPases have no structural homologue in mammalian cells, appearing only in fungi, plants, and protozoan parasites, e.g., Trypanosoma cruzi, Leishmania sp., Toxoplasma gondii, and Plasmodium falciparum. This exclusivity makes Na+-ATPase a potential candidate for the biologically-based design of new therapeutic interventions; for this reason, Na+-ATPases deserves more attention.

A machine learning approach to define antimalarial drug action from heterogeneous cell-based screens

Drug resistance threatens the effective prevention and treatment of an ever-increasing range of human infections. This highlights an urgent need for new and improved drugs with novel mechanisms of action to avoid cross-resistance. Current cell-based drug screens are, however, restricted to binary live/dead readouts with no provision for mechanism of action prediction. Machine learning methods are increasingly being used to improve information extraction from imaging data. These methods, however, work poorly with heterogeneous cellular phenotypes and generally require time-consuming human-led training. We have developed a semi-supervised machine learning approach, combining human- and machine-labeled training data from mixed human malaria parasite cultures. Designed for high-throughput and high-resolution screening, our semi-supervised approach is robust to natural parasite morphological heterogeneity and correctly orders parasite developmental stages. Our approach also reproducibly detects and clusters drug-induced morphological outliers by mechanism of action, demonstrating the potential power of machine learning for accelerating cell-based drug discovery.

The early preclinical and clinical development of cipargamin (KAE609), a novel antimalarial compound

BACKGROUND

Cipargamin (KAE609) is a novel spiroindolone class drug for the treatment of malaria, currently undergoing phase 2 clinical development. This review provides an overview and interpretation of the pre-clinical and clinical data of this possible next-generation antimalarial drug published to date.

METHODS

We systematically searched the literature for studies on the preclinical and clinical development of cipargamin. PubMed and Google Scholar databases were searched using the terms 'cipargamin', 'KAE609' or 'NITD609' in the English language; one additional article was identified during revision. Nineteen of these in total 43 papers identified reported original studies; 13 of those articles were on pre-clinical studies and 6 reported clinical trials.

RESULTS

A total of 20 studies addressing its preclinical and clinical development have been published on this compound at the time of writing. Cipargamin acts on the PfATP4, which is a P-type Na + ATPase disrupting the Na + homeostasis in the parasite. Cipargamin is a very fast-acting antimalarial, it is active against all intra-erythrocytic stages of the malaria parasite and exerts gametocytocidal activity, with transmission-blocking potential. It is currently undergoing phase 2 clinical trial to assess safety and efficacy, with a special focus on hepatic safety.

CONCLUSION

In the search for novel antimalarial drugs, cipargamin exhibits promising properties, exerting activity against multiple intra-erythrocytic stages of plasmodia, including gametocytes. It exhibits a favourable pharmacokinetic profile, possibly allowing for single-dose treatment with a suitable combination partner. According to the clinical results of the first studies in Asian malaria patients, a possible safety concern is hepatotoxicity.

Delayed death in the malaria parasite Plasmodium falciparum is caused by disruption of prenylation-dependent intracellular trafficking

Apicomplexan parasites possess a plastid organelle called the apicoplast. Inhibitors that selectively target apicoplast housekeeping functions, including DNA replication and protein translation, are lethal for the parasite, and several (doxycycline, clindamycin, and azithromycin) are in clinical use as antimalarials. A major limitation of such drugs is that treated parasites only arrest one intraerythrocytic development cycle (approximately 48 hours) after treatment commences, a phenotype known as the ‘delayed death’ effect. The molecular basis of delayed death is a long-standing mystery in parasitology, and establishing the mechanism would aid rational clinical implementation of apicoplast-targeted drugs. Parasites undergoing delayed death transmit defective apicoplasts to their daughter cells and cannot produce the sole, blood-stage essential metabolic product of the apicoplast: the isoprenoid precursor isopentenyl-pyrophosphate. How the isoprenoid precursor depletion kills the parasite remains unknown. We investigated the requirements for the range of isoprenoids in the human malaria parasite Plasmodium falciparum and characterised the molecular and morphological phenotype of parasites experiencing delayed death. Metabolomic profiling reveals disruption of digestive vacuole function in the absence of apicoplast derived isoprenoids. Three-dimensional electron microscopy reveals digestive vacuole fragmentation and the accumulation of cytostomal invaginations, characteristics common in digestive vacuole disruption. We show that digestive vacuole disruption results from a defect in the trafficking of vesicles to the digestive vacuole. The loss of prenylation of vesicular trafficking proteins abrogates their membrane attachment and function and prevents the parasite from feeding. Our data show that the proximate cause of delayed death is an interruption of protein prenylation and consequent cellular trafficking defects.

After treatment with drugs that target apicoplast functions, malaria parasites are initially superficially healthy and go on to infect new erythrocytes. This cell biology study shows that the parasites subsequently die in their second cycle due to trafficking defects caused by depletion of prenyl groups.

A 4-cyano-3-methylisoquinoline inhibitor of Plasmodium falciparum growth targets the sodium efflux pump PfATP4

We developed a novel series of antimalarial compounds based on a 4-cyano-3-methylisoquinoline. Our lead compound MB14 achieved modest inhibition of the growth in vitro of the human malaria parasite, Plasmodium falciparum. To identify its biological target we selected for parasites resistant to MB14. Genome sequencing revealed that all resistant parasites bore a single point S374R mutation in the sodium (Na⁺) efflux transporter PfATP4. There are many compounds known to inhibit PfATP4 and some are under preclinical development. MB14 was shown to inhibit Na⁺ dependent ATPase activity in parasite membranes, consistent with the compound targeting PfATP4 directly. PfATP4 inhibitors cause swelling and lysis of infected erythrocytes, attributed to the accumulation of Na⁺ inside the intracellular parasites and the resultant parasite swelling. We show here that inhibitor-induced lysis of infected erythrocytes is dependent upon the parasite protein RhopH2, a component of the new permeability pathways that are induced by the parasite in the erythrocyte membrane. These pathways mediate the influx of Na⁺ into the infected erythrocyte and their suppression via RhopH2 knockdown limits the accumulation of Na⁺ within the parasite hence protecting the infected erythrocyte from lysis. This study reveals a role for the parasite-induced new permeability pathways in the mechanism of action of PfATP4 inhibitors.

Multimodal analysis of Plasmodium knowlesi ‐infected erythrocytes reveals large invaginations, swelling of the host cell, and rheological defects

The simian parasite Plasmodium knowlesi causes severe and fatal malaria infections in humans, but the process of host cell remodelling that underpins the pathology of this zoonotic parasite is only poorly understood. We have used serial block‐face scanning electron microscopy to explore the topography of P. knowlesi‐infected red blood cells (RBCs) at different stages of asexual development. The parasite elaborates large flattened cisternae (Sinton Mulligan's clefts) and tubular vesicles in the host cell cytoplasm, as well as parasitophorous vacuole membrane bulges and blebs, and caveolar structures at the RBC membrane. Large invaginations of host RBC cytoplasm are formed early in development, both from classical cytostomal structures and from larger stabilised pores. Although degradation of haemoglobin is observed in multiple disconnected digestive vacuoles, the persistence of large invaginations during development suggests inefficient consumption of the host cell cytoplasm. The parasite eventually occupies ~40% of the host RBC volume, inducing a 20% increase in volume of the host RBC and an 11% decrease in the surface area to volume ratio, which collectively decreases the ability of the P. knowlesi‐infected RBCs to enter small capillaries of a human erythrocyte microchannel analyser. Ektacytometry reveals a markedly decreased deformability, whereas correlative light microscopy/scanning electron microscopy and python‐based skeleton analysis (Skan) reveal modifications to the surface of infected RBCs that underpin these physical changes. We show that P. knowlesi‐infected RBCs are refractory to treatment with sorbitol lysis but are hypersensitive to hypotonic lysis. The observed physical changes in the host RBCs may underpin the pathology observed in patients infected with P. knowlesi.

Characterization of the ATP4 ion pump in Toxoplasma gondii

The Plasmodium falciparum ATPase PfATP4 is the target of a diverse range of antimalarial compounds, including the clinical drug candidate cipargamin. PfATP4 was originally annotated as a Ca²⁺ transporter, but recent evidence suggests that it is a Na⁺ efflux pump, extruding Na⁺ in exchange for H⁺ Here we demonstrate that ATP4 proteins belong to a clade of P-type ATPases that are restricted to apicomplexans and their closest relatives. We employed a variety of genetic and physiological approaches to investigate the ATP4 protein of the apicomplexan Toxoplasma gondii, TgATP4. We show that TgATP4 is a plasma membrane protein. Knockdown of TgATP4 had no effect on resting pH or Ca²⁺ but rendered parasites unable to regulate their cytosolic Na⁺ concentration ([Na⁺]_cyt). PfATP4 inhibitors caused an increase in [Na⁺]_cyt and a cytosolic alkalinization in WT but not TgATP4 knockdown parasites. Parasites in which TgATP4 was knocked down or disrupted exhibited a growth defect, attributable to reduced viability of extracellular parasites. Parasites in which TgATP4 had been disrupted showed reduced virulence in mice. These results provide evidence for ATP4 proteins playing a key conserved role in Na⁺ regulation in apicomplexan parasites.

Antimalarial Transmission-Blocking Interventions: Past, Present, and Future

Malaria remains a major global health challenge. Appropriate use of current antimalarial tools has reduced the disease burden, but morbidity and mortality remain unacceptably high. It is widely accepted that, to achieve long-term control/eradication, it will be necessary to use interventions that inhibit the transmission of parasites to mosquitoes - these tools are termed transmission-blocking interventions (TBIs). This article aims to outline the rationale for the development of TBIs, with a focus on transmission-blocking drugs and (parasite-derived) transmission-blocking vaccines. We describe and summarise the current status of each of these intervention classes and attempt to identify future requirements in development, with a focus on the challenges of establishing each method within an integrated malarial control programme in the future.

Biochemical characterization and chemical inhibition of PfATP4-associated Na+-ATPase activity in Plasmodium falciparum membranes

The antimalarial activity of chemically diverse compounds, including the clinical candidate cipargamin, has been linked to the ATPase PfATP4 in the malaria-causing parasite Plasmodium falciparum The characterization of PfATP4 has been hampered by the inability thus far to achieve its functional expression in a heterologous system. Here, we optimized a membrane ATPase assay to probe the function of PfATP4 and its chemical sensitivity. We found that cipargamin inhibited the Na⁺-dependent ATPase activity present in P. falciparum membranes from WT parasites and that its potency was reduced in cipargamin-resistant PfATP4-mutant parasites. The cipargamin-sensitive fraction of membrane ATPase activity was inhibited by all 28 of the compounds in the "Malaria Box" shown previously to disrupt ion regulation in P. falciparum in a cipargamin-like manner. This is consistent with PfATP4 being the direct target of these compounds. Characterization of the cipargamin-sensitive ATPase activity yielded data consistent with PfATP4 being a Na⁺ transporter that is sensitive to physiologically relevant perturbations of pH, but not of [K⁺] or [Ca²⁺]. With an apparent K_m for ATP of 0.2 mm and an apparent K_m for Na⁺ of 16-17 mm, the protein is predicted to operate at below its half-maximal rate under normal physiological conditions, allowing the rate of Na⁺ efflux to increase in response to an increase in cytosolic [Na⁺]. In membranes from a cipargamin-resistant PfATP4-mutant line, the apparent K_m for Na⁺ is slightly elevated. Our study provides new insights into the biochemical properties and chemical sensitivity of an important new antimalarial drug target.

Diverse antimalarials from whole-cell phenotypic screens disrupt malaria parasite ion and volume homeostasis

Four hundred structurally diverse drug-like compounds comprising the Medicines for Malaria Venture's 'Pathogen Box' were screened for their effect on a range of physiological parameters in asexual blood-stage malaria (Plasmodium falciparum) parasites. Eleven of these compounds were found to perturb parasite Na+, pH and volume in a manner consistent with inhibition of the putative Na+ efflux P-type ATPase PfATP4. All eleven compounds fell within the subset of 125 compounds included in the Pathogen Box on the basis of their having been identified as potent inhibitors of the growth of asexual blood-stage P. falciparum parasites. All eleven compounds inhibited the Na+-dependent ATPase activity of parasite membranes and showed reduced efficacy against parasites carrying mutations in PfATP4. This study increases the number of chemically diverse structures known to show a 'PfATP4-associated' phenotype, and adds to emerging evidence that a high proportion (7-9%) of the structurally diverse antimalarial compounds identified in whole cell phenotypic screens share the same mechanism of action, exerting their antimalarial effect via an interaction with PfATP4.