KAF156 is an antimalarial clinical candidate with potential for use in prophylaxis, treatment, and prevention of disease transmission

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.

Discovery of novel anti-infective agents

Academic and other non-profit institutions have a long-term vision to improve human health where commercial interests can be limited for profit organizations. Medicinal chemistry to these diseases with no commercial benefit needs is well suited in the academic environment and this chapter outlines some work conducted at Calibr-Skaggs around antibiotic drug development that has led to initiation of multiple clinical trials over the last decade.

Defining the next generation of severe malaria treatment: a target product profile

BACKGROUND

Severe malaria is a life-threatening infection, particularly affecting children under the age of 5 years in Africa. Current treatment with parenteral artemisinin derivatives is highly efficacious. However, artemisinin partial resistance is widespread in Southeast Asia, resulting in delayed parasite clearance after therapy, and has emerged independently in South America, Oceania, and Africa. Hence, new treatments for severe malaria are needed, and it is prudent to define their characteristics now. This manuscript focuses on the target product profile (TPP) for new treatments for severe malaria. It also highlights preparedness when considering ways of protecting the utility of artemisinin-based therapies.

TARGET PRODUCT PROFILE

Severe malaria treatments must be highly potent, with rapid onset of antiparasitic activity to clear the infection as quickly as possible to prevent complications. They should also have a low potential for drug resistance selection, given the high parasite burden in patients with severe malaria. Combination therapies are needed to deter resistance selection and dissemination. Partner drugs which are approved for uncomplicated malaria treatment would provide the most rapid development pathway for combinations, though new candidate molecules should be considered. Artemisinin combination approaches to severe malaria would extend the lifespan of current therapy, but ideally, completely novel, non-artemisinin-based combination therapies for severe malaria should be developed. These should be advanced to at least phase 2 clinical trials, enabling rapid progression to patient use should current treatment fail clinically. New drug combinations for severe malaria should be available as injectable formulations for rapid and effective treatment, or as rectal formulations for pre-referral intervention in resource-limited settings.

CONCLUSION

Defining the TPP is a key step to align responses across the community to proactively address the potential for clinical failure of artesunate in severe malaria. In the shorter term, artemisinin-based combination therapies should be developed using approved or novel drugs. In the longer term, novel combination treatments should be pursued. Thus, this TPP aims to direct efforts to preserve the efficacy of existing treatments while improving care and outcomes for individuals affected by this life-threatening disease.

Identification of an inhibitory pocket in falcilysin provides a new avenue for malaria drug development

Identification of new druggable protein targets remains the key challenge in the current antimalarial development efforts. Here we used mass-spectrometry-based cellular thermal shift assay (MS-CETSA) to identify potential targets of several antimalarials and drug candidates. We found that falcilysin (FLN) is a common binding partner for several drug candidates such as MK-4815, MMV000848, and MMV665806 but also interacts with quinoline drugs such as chloroquine and mefloquine. Enzymatic assays showed that these compounds can inhibit FLN proteolytic activity. Their interaction with FLN was explored systematically by isothermal titration calorimetry and X-ray crystallography, revealing a shared hydrophobic pocket in the catalytic chamber of the enzyme. Characterization of transgenic cell lines with lowered FLN expression demonstrated statistically significant increases in susceptibility toward MK-4815, MMV000848, and several quinolines. Importantly, the hydrophobic pocket of FLN appears amenable to inhibition and the structures reported here can guide the development of novel drugs against malaria.

The problem of antimalarial resistance and its implications for drug discovery

INTRODUCTION

Malaria remains a devastating infectious disease with hundreds of thousands of casualties each year. Antimalarial drug resistance has been a threat to malaria control and elimination for many decades and is still of concern today. Despite the continued effectiveness of current first-line treatments, namely artemisinin-based combination therapies, the emergence of drug-resistant parasites in Southeast Asia and even more alarmingly the occurrence of resistance mutations in Africa is of great concern and requires immediate attention.

AREAS COVERED

A comprehensive overview of the mechanisms underlying the acquisition of drug resistance in Plasmodium falciparum is given. Understanding these processes provides valuable insights that can be harnessed for the development and selection of novel antimalarials with reduced resistance potential. Additionally, strategies to mitigate resistance to antimalarial compounds on the short term by using approved drugs are discussed.

EXPERT OPINION

While employing strategies that utilize already approved drugs may offer a prompt and cost-effective approach to counter antimalarial drug resistance, it is crucial to recognize that only continuous efforts into the development of novel antimalarial drugs can ensure the successful treatment of malaria in the future. Incorporating resistance propensity assessment during this developmental process will increase the likelihood of effective and enduring malaria treatments.

A conserved metabolic signature associated with response to fast-acting anti-malarial agents

IMPORTANCE

In malaria drug discovery, understanding the mode of action of lead compounds is important as it helps in predicting the potential emergence of drug resistance in the field when these drugs are eventually deployed. In this study, we have employed metabolomics technologies to characterize the potential targets of anti-malarial drug candidates in the developmental pipeline at NITD. We show that NITD fast-acting leads belonging to spiroindolone and imidazothiadiazole class induce a common biochemical theme in drug-exposed malaria parasites which is similar to another fast-acting, clinically available drug, DHA. These biochemical features which are absent in a slower acting NITD lead (GNF17) point to hemoglobin digestion and inhibition of the pyrimidine pathway as potential action points for these drugs. These biochemical themes can be used to identify and inform on the mode of action of fast drug candidates of similar profiles in future drug discovery programs.

Imidazolopiperazine (IPZ)-Induced Differential Transcriptomic Responses on Plasmodium falciparum Wild-Type and IPZ-Resistant Mutant Parasites

Imidazolopiperazine (IPZ), KAF156, a close analogue of GNF179, is a promising antimalarial candidate. IPZ is effective against Plasmodium falciparum and Plasmodium vivax clinical malaria in human with transmission blocking property in animal models and effective against liver stage parasites. Despite these excellent drug efficacy properties, in vitro parasites have shown resistance to IPZ. However, the mechanism of action and resistance of IPZ remained not fully understood. Here, we used transcriptomic analysis to elucidate mode of action of IPZs. We report, in wild-type parasites GNF179 treatment down regulated lipase enzymes, two metabolic pathways: the hydrolysis of Phosphoinositol 4,5-bipohosphate (PIP2) that produce diacyglycerol (DAG) and the cytosolic calcium Ca2+ homeostasis which are known to be essential for P. falciparum survival and proliferation, as well for membrane permeability and protein trafficking. Furthermore, in wild-type parasites, GNF179 repressed expression of Acyl CoA Synthetase, export lipase 1 and esterase enzymes. Thus, in wild-type parasites only, GNF179 treatment affected enzymes leading lipid metabolism, transport, and synthesis. Lastly, our data revealed that IPZs did not perturb known IPZ resistance genes markers pfcarl, pfact, and pfugt regulations, which are all instead possibly involved in the drug resistance that disturb membrane transport targeted by IPZ.

Antimalarial drug discovery: progress and approaches

Recent antimalarial drug discovery has been a race to produce new medicines that overcome emerging drug resistance, whilst considering safety and improving dosing convenience. Discovery efforts have yielded a variety of new molecules, many with novel modes of action, and the most advanced are in late-stage clinical development. These discoveries have led to a deeper understanding of how antimalarial drugs act, the identification of a new generation of drug targets, and multiple structure-based chemistry initiatives. The limited pool of funding means it is vital to prioritize new drug candidates. They should exhibit high potency, a low propensity for resistance, a pharmacokinetic profile that favours infrequent dosing, low cost, preclinical results that demonstrate safety and tolerability in women and infants, and preferably the ability to block Plasmodium transmission to Anopheles mosquito vectors. In this Review, we describe the approaches that have been successful, progress in preclinical and clinical development, and existing challenges. We illustrate how antimalarial drug discovery can serve as a model for drug discovery in diseases of poverty.

Ganaplacide (KAF156) plus lumefantrine solid dispersion formulation combination for uncomplicated Plasmodium falciparum malaria: an open-label, multicentre, parallel-group, randomised, controlled, phase 2 trial

BACKGROUND

Emergence of drug resistance demands novel antimalarial drugs with new mechanisms of action. We aimed to identify effective and well tolerated doses of ganaplacide plus lumefantrine solid dispersion formulation (SDF) in patients with uncomplicated Plasmodium falciparum malaria.

METHODS

This open-label, multicentre, parallel-group, randomised, controlled, phase 2 trial was conducted at 13 research clinics and general hospitals in ten African and Asian countries. Patients had microscopically-confirmed uncomplicated P falciparum malaria (>1000 and <150 000 parasites per μL). Part A identified the optimal dose regimens in adults and adolescents (aged ≥12 years) and in part B, the selected doses were assessed in children (≥2 years and <12 years). In part A, patients were randomly assigned to one of seven groups (once a day ganaplacide 400 mg plus lumefantrine-SDF 960 mg for 1, 2, or 3 days; ganaplacide 800 mg plus lumefantrine-SDF 960 mg as a single dose; once a day ganaplacide 200 mg plus lumefantrine-SDF 480 mg for 3 days; once a day ganaplacide 400 mg plus lumefantrine-SDF 480 mg for 3 days; or twice a day artemether plus lumefantrine for 3 days [control]), with stratification by country (2:2:2:2:2:2:1) using randomisation blocks of 13. In part B, patients were randomly assigned to one of four groups (once a day ganaplacide 400 mg plus lumefantrine-SDF 960 mg for 1, 2, or 3 days, or twice a day artemether plus lumefantrine for 3 days) with stratification by country and age (2 to <6 years and 6 to <12 years; 2:2:2:1) using randomisation blocks of seven. The primary efficacy endpoint was PCR-corrected adequate clinical and parasitological response at day 29, analysed in the per protocol set. The null hypothesis was that the response was 80% or lower, rejected when the lower limit of two-sided 95% CI was higher than 80%. This study is registered with EudraCT (2020-003284-25) and ClinicalTrials.gov (NCT03167242).

FINDINGS

Between Aug 2, 2017, and May 17, 2021, 1220 patients were screened and of those, 12 were included in the run-in cohort, 337 in part A, and 175 in part B. In part A, 337 adult or adolescent patients were randomly assigned, 326 completed the study, and 305 were included in the per protocol set. The lower limit of the 95% CI for PCR-corrected adequate clinical and parasitological response on day 29 was more than 80% for all treatment regimens in part A (46 of 50 patients [92%, 95% CI 81-98] with 1 day, 47 of 48 [98%, 89-100] with 2 days, and 42 of 43 [98%, 88-100] with 3 days of ganaplacide 400 mg plus lumefantrine-SDF 960 mg; 45 of 48 [94%, 83-99] with ganaplacide 800 mg plus lumefantrine-SDF 960 mg for 1 day; 47 of 47 [100%, 93-100] with ganaplacide 200 mg plus lumefantrine-SDF 480 mg for 3 days; 44 of 44 [100%, 92-100] with ganaplacide 400 mg plus lumefantrine-SDF 480 mg for 3 days; and 25 of 25 [100%, 86-100] with artemether plus lumefantrine). In part B, 351 children were screened, 175 randomly assigned (ganaplacide 400 mg plus lumefantrine-SDF 960 mg once a day for 1, 2, or 3 days), and 171 completed the study. Only the 3-day regimen met the prespecified primary endpoint in paediatric patients (38 of 40 patients [95%, 95% CI 83-99] vs 21 of 22 [96%, 77-100] with artemether plus lumefantrine). The most common adverse events were headache (in seven [14%] of 51 to 15 [28%] of 54 in the ganaplacide plus lumefantrine-SDF groups and five [19%] of 27 in the artemether plus lumefantrine group) in part A, and malaria (in 12 [27%] of 45 to 23 [44%] of 52 in the ganaplacide plus lumefantrine-SDF groups and 12 [50%] of 24 in the artemether plus lumefantrine group) in part B. No patients died during the study.

INTERPRETATION

Ganaplacide plus lumefantrine-SDF was effective and well tolerated in patients, especially adults and adolescents, with uncomplicated P falciparum malaria. Ganaplacide 400 mg plus lumefantrine-SDF 960 mg once daily for 3 days was identified as the optimal treatment regimen for adults, adolescents, and children. This combination is being evaluated further in a phase 2 trial (NCT04546633).

FUNDING

Novartis and Medicines for Malaria Venture.

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.

CRISPR/Cas advancements for genome editing, diagnosis, therapeutics, and vaccine development for Plasmodium parasites, and genetic engineering of Anopheles mosquito vector

Malaria as vector-borne disease remains important health concern with over 200 million cases globally. Novel antimalarial medicines and more effective vaccines must be developed to eliminate and eradicate malaria. Appraisal of preceding genome editing approaches confirmed the CRISPR/Cas nuclease system as a novel proficient genome editing system and a tool for species-specific diagnosis, and drug resistance researches for Plasmodium species, and gene drive to control Anopheles population. CRISPR/Cas technology, as a handy tool for genome editing can be justified for the production of transgenic malaria parasites like Plasmodium transgenic lines expressing Cas9, chimeric Plasmodium transgenic lines, knockdown and knockout transgenic parasites, and transgenic parasites expressing alternative alleles, and also mutant strains of Anopheles such as only male mosquito populations, generation of wingless mosquitoes, and creation of knock-out/ knock-in mutants. Though, the incorporation of traditional methods and novel molecular techniques could noticeably enhance the quality of results. The striking development of a CRISPR/Cas-based diagnostic kit that can specifically diagnose the Plasmodium species or drug resistance markers is highly required in malaria settings with affordable cost and high-speed detection. Furthermore, the advancement of genome modifications by CRISPR/Cas technologies resolves contemporary restrictions to culturing, maintaining, and analyzing these parasites, and the aptitude to investigate parasite genome functions opens up new vistas in the better understanding of pathogenesis.

Drug discovery for parasitic diseases: powered by technology, enabled by pharmacology, informed by clinical science

While prevention is a bedrock of public health, innovative therapeutics are needed to complement the armamentarium of interventions required to achieve disease control and elimination targets for neglected diseases. Extraordinary advances in drug discovery technologies have occurred over the past decades, along with accumulation of scientific knowledge and experience in pharmacological and clinical sciences that are transforming many aspects of drug R&D across disciplines. We reflect on how these advances have propelled drug discovery for parasitic infections, focusing on malaria, kinetoplastid diseases, and cryptosporidiosis. We also discuss challenges and research priorities to accelerate discovery and development of urgently needed novel antiparasitic drugs.

Fast-Killing Tyrosine Amide ((S)-SW228703) with Blood- and Liver-Stage Antimalarial Activity Associated with the Cyclic Amine Resistance Locus (PfCARL)

Current malaria treatments are threatened by drug resistance, and new drugs are urgently needed. In a phenotypic screen for new antimalarials, we identified (S)-SW228703 ((S)-SW703), a tyrosine amide with asexual blood and liver stage activity and a fast-killing profile. Resistance to (S)-SW703 is associated with mutations in the Plasmodium falciparum cyclic amine resistance locus (PfCARL) and P. falciparum acetyl CoA transporter (PfACT), similarly to several other compounds that share features such as fast activity and liver-stage activity. Compounds with these resistance mechanisms are thought to act in the ER, though their targets are unknown. The tyramine of (S)-SW703 is shared with some reported PfCARL-associated compounds; however, we observed that strict S-stereochemistry was required for the activity of (S)-SW703, suggesting differences in the mechanism of action or binding mode. (S)-SW703 provides a new chemical series with broad activity for multiple life-cycle stages and a fast-killing mechanism of action, available for lead optimization to generate new treatments for malaria.

A progeroid syndrome caused by a deep intronic variant in TAPT1 is revealed by RNA/SI-NET sequencing

Exome sequencing has introduced a paradigm shift for the identification of germline variations responsible for Mendelian diseases. However, non-coding regions, which make up 98% of the genome, cannot be captured. The lack of functional annotation for intronic and intergenic variants makes RNA-seq a powerful companion diagnostic. Here, we illustrate this point by identifying six patients with a recessive Osteogenesis Imperfecta (OI) and neonatal progeria syndrome. By integrating homozygosity mapping and RNA-seq, we delineated a deep intronic TAPT1 mutation (c.1237-52 G>A) that segregated with the disease. Using SI-NET-seq, we document that TAPT1's nascent transcription was not affected in patients' fibroblasts, indicating instead that this variant leads to an alteration of pre-mRNA processing. Predicted to serve as an alternative splicing branchpoint, this mutation enhances TAPT1 exon 12 skipping, creating a protein-null allele. Additionally, our study reveals dysregulation of pathways involved in collagen and extracellular matrix biology in disease-relevant cells. Overall, our work highlights the power of transcriptomic approaches in deciphering the repercussions of non-coding variants, as well as in illuminating the molecular mechanisms of human diseases.

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.

Current and emerging target identification methods for novel antimalarials

New antimalarial compounds with novel mechanisms of action are urgently needed to combat the recent rise in antimalarial drug resistance. Phenotypic high-throughput screens have proven to be a successful method for identifying new compounds, however, do not provide mechanistic information about the molecular target(s) responsible for antimalarial action. Current and emerging target identification methods such as in vitro resistance generation, metabolomics screening, chemoproteomic approaches and biophysical assays measuring protein stability across the whole proteome have successfully identified novel drug targets. This review provides an overview of these techniques, comparing their strengths and weaknesses and how they can be utilised for antimalarial target identification.

Adapt or Die: Targeting Unique Transmission-Stage Biology for Malaria Elimination

Plasmodium parasites have a complex life cycle that includes development in the human host as well as the Anopheles vector. Successful transmission of the parasite between its host and vector therefore requires the parasite to balance its investments in asexual replication and sexual reproduction, varying the frequency of sexual commitment to persist within the human host and generate future opportunities for transmission. The transmission window is extended further by the ability of stage V gametocytes to circulate in peripheral blood for weeks, whereas immature stage I to IV gametocytes sequester in the bone marrow and spleen until final maturation. Due to the low gametocyte numbers in blood circulation and with the ease of targeting such life cycle bottlenecks, transmission represents an efficient target for therapeutic intervention. The biological process of Plasmodium transmission is a multistage, multifaceted process and the past decade has seen a much deeper understanding of the molecular mechanisms and regulators involved. Clearly, specific and divergent processes are used during transmission compared to asexual proliferation, which both poses challenges but also opportunities for discovery of transmission-blocking antimalarials. This review therefore presents an update of our molecular understanding of gametocyte and gamete biology as well as the status of transmission-blocking activities of current antimalarials and lead development compounds. By defining the biological components associated with transmission, considerations for the development of new transmission-blocking drugs to target such untapped but unique biology is suggested as an important, main driver for transmission-blocking drug discovery.

Defining ER-mitochondria contact dynamics in Plasmodium falciparum by targeting component of phospholipid synthesis pathway, Phosphatidylserine synthase (PfPSS)

The malaria parasite completes the asexual cycle inside the host erythrocyte, which requires extensive membrane biogenesis for its development and multiplication. Metabolic pathways for the synthesis of membrane phospholipids (PL), including phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphatidylserine (PS), are crucial for parasite survival. Here, we have studied the P. falciparum enzyme responsible for PS synthesis, Phosphatidylserine synthase (PfPSS), GFP targeting approach confirmed it to be localized in the parasite ER as well as in ER-protrusions. Detailed high resolution microscopy, using these transgenic parasites expressing PfPSS-GFP, redefined the dynamics of ER during the intraerythrocytic life cycle and its association with the mitochondria. We report for the first time presence of ER-mitochondria contact (ERMC) in Plasmodium; ERMC is formed by PfPSS containing ER-protrusions, which associate with the mitochondria surface throughout the parasite growth cycle. Further, ERMC is found to be stable and refractory to ER and mitochondrial stresses, suggesting that it is formed through strong tethering complexes. PfPSS was found to interact with other major key enzyme involved in PL synthesis, choline/Etn-phosphotransferase (CEPT), which suggest that ER is the major site for PL biosynthesis. Overall, this study defines the morphological organisation of ERMC which mediates PL synthesis/transport in the Plasmodium.

Current status of experimental models for the study of malaria

Infection by malaria parasites (Plasmodium spp.) remains one of the leading causes of morbidity and mortality, especially in tropical regions of the world. Despite the availability of malaria control tools such as integrated vector management and effective therapeutics, these measures have been continuously undermined by the emergence of vector resistance to insecticides or parasite resistance to frontline antimalarial drugs. Whilst the recent pilot implementation of the RTS,S malaria vaccine is indeed a remarkable feat, highly effective vaccines against malaria remain elusive. The barriers to effective vaccines result from the complexity of both the malaria parasite lifecycle and the parasite as an organism itself with consequent major gaps in our understanding of their biology. Historically and due to the practical and ethical difficulties of working with human malaria infections, research into malaria parasite biology has been extensively facilitated by animal models. Animals have been used to study disease pathogenesis, host immune responses and their (dys)regulation and further disease processes such as transmission. Moreover, animal models remain at the forefront of pre-clinical evaluations of antimalarial drugs (drug efficacy, mode of action, mode of resistance) and vaccines. In this review, we discuss commonly used animal models of malaria, the parasite species used and their advantages and limitations which hinder their extrapolation to actual human disease. We also place into this context the most recent developments such as organoid technologies and humanized mice.

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 .

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.

Review of the Current Landscape of the Potential of Nanotechnology for Future Malaria Diagnosis, Treatment, and Vaccination Strategies

Malaria eradication has for decades been on the global health agenda, but the causative agents of the disease, several species of the protist parasite Plasmodium, have evolved mechanisms to evade vaccine-induced immunity and to rapidly acquire resistance against all drugs entering clinical use. Because classical antimalarial approaches have consistently failed, new strategies must be explored. One of these is nanomedicine, the application of manipulation and fabrication technology in the range of molecular dimensions between 1 and 100 nm, to the development of new medical solutions. Here we review the current state of the art in malaria diagnosis, prevention, and therapy and how nanotechnology is already having an incipient impact in improving them. In the second half of this review, the next generation of antimalarial drugs currently in the clinical pipeline is presented, with a definition of these drugs' target product profiles and an assessment of the potential role of nanotechnology in their development. Opinions extracted from interviews with experts in the fields of nanomedicine, clinical malaria, and the economic landscape of the disease are included to offer a wider scope of the current requirements to win the fight against malaria and of how nanoscience can contribute to achieve them.

Longitudinal ex vivo and molecular trends of chloroquine and piperaquine activity against Plasmodium falciparum and P. vivax before and after introduction of artemisinin-based combination therapy in Papua, Indonesia

Drug resistant Plasmodium parasites are a major threat to malaria control and elimination. After reports of high levels of multidrug resistant P. falciparum and P. vivax in Indonesia, in 2005, the national first-line treatment policy for uncomplicated malaria was changed in March 2006, to dihydroartemisinin-piperaquine against all species. This study assessed the temporal trends in ex vivo drug susceptibility to chloroquine (CQ) and piperaquine (PIP) for both P. falciparum and P. vivax clinical isolates collected between 2004 and 2018, by using schizont maturation assays, and genotyped a subset of isolates for known and putative molecular markers of CQ and PIP resistance by using Sanger and next generation whole genome sequencing. The median CQ IC50 values varied significantly between years in both Plasmodium species, but there was no significant trend over time. In contrast, there was a significant trend for increasing PIP IC50s in both Plasmodium species from 2010 onwards. Whereas the South American CQ resistant 7G8 pfcrt SVMNT isoform has been fixed since 2005 in the study area, the pfmdr1 86Y allele frequencies decreased and became fixed at the wild-type allele in 2015. In P. vivax isolates, putative markers of CQ resistance (no pvcrt-o AAG (K10) insertion and pvmdr1 Y967F and F1076L) were fixed at the mutant alleles since 2005. None of the putative PIP resistance markers were detected in P. falciparum. The ex vivo drug susceptibility and molecular analysis of CQ and PIP efficacy for P. falciparum and P. vivax after 12 years of intense drug pressure with DHP suggests that whilst the degree of CQ resistance appears to have been sustained, there has been a slight decline in PIP susceptibility, although this does not appear to have reached clinically significant levels. The observed decreasing trend in ex vivo PIP susceptibility highlights the importance of ongoing surveillance.

Ex vivo susceptibility to new antimalarial agents differs among human-infecting Plasmodium species

Several promising antimalarial drugs are currently being tested in human trials, such as artefenomel, cipargamin, ferroquine and ganaplacide. Many of these compounds were identified using high throughput screens against a single species of human malaria, Plasmodium falciparum, under the assumption that effectiveness against all malaria species will be similar, as has been observed for other antimalarial drugs. However, using our in vitro adapted line, we demonstrated recently that P. knowlesi is significantly less susceptible than P. falciparum to some new antimalarial drugs (e.g., cipargamin and DSM265), and more susceptible to others (e.g., ganaplacide). There is, therefore, an urgent need to determine the susceptibility profile of all human malaria species to the current generation of antimalarial compounds. We obtained ex vivo malaria samples from travellers returning to the United Kingdom and, using the [³H]hypoxanthine incorporation method, compared susceptibility to select established and experimental antimalarial agents among all major human infective Plasmodium species. We demonstrate that P. malariae and P. ovale spp. are significantly less susceptible than P. falciparum to cipargamin, DSM265 and AN13762, but are more susceptible to ganaplacide. Preliminary ex vivo data from single isolates of P. knowlesi and P. vivax demonstrate a similar profile. Our findings highlight the need to ensure cross species susceptibility profiles are determined early in the drug development pipeline. Our data can also be used to inform further drug development, and illustrate the utility of the P. knowlesi in vitro model as a scalable approach for predicting the drug susceptibility of non-falciparum malaria species in general.

A New Thienopyrimidinone Chemotype Shows Multistage Activity against Plasmodium falciparum, Including Artemisinin-Resistant Parasites

Human malaria infection begins with a one-time asymptomatic liver stage followed by a cyclic symptomatic blood stage. For decades, the research for novel antimalarials focused on the high-throughput screening of molecules that only targeted the asexual blood stages. In a search for new effective compounds presenting a triple action against erythrocytic and liver stages in addition to the ability to block the transmission of the disease via the mosquito vector, 2-amino-thienopyrimidinone derivatives were synthesized and tested for their antimalarial activity. One molecule, named gamhepathiopine (denoted as “M1” herein), was active at submicromolar concentrations against both erythrocytic (50% effective concentration [EC₅₀] = 0.045 μM) and liver (EC₅₀ = 0.45 μM) forms of Plasmodium falciparum. Furthermore, gamhepathiopine efficiently blocked the development of the sporogonic cycle in the mosquito vector by inhibiting the exflagellation step. Moreover, M1 was active against artemisinin-resistant forms (EC₅₀ = 0.227 μM), especially at the quiescent stage. Nevertheless, in mice, M1 showed modest activity due to its rapid metabolization by P450 cytochromes into inactive derivatives, calling for the development of new parent compounds with improved metabolic stability and longer half-lives. These results highlight the thienopyrimidinone scaffold as a novel antiplasmodial chemotype of great interest to search for new drug candidates displaying multistage activity and an original mechanism of action with the potential to be used in combination therapies for malaria elimination in the context of artemisinin resistance.

IMPORTANCE This work reports a new chemical structure that (i) displays activity against the human malaria parasite Plasmodium falciparum at 3 stages of the parasitic cycle (blood stage, hepatic stage, and sexual stages), (ii) remains active against parasites that are resistant to the first-line treatment recommended by the World Health Organization (WHO) for the treatment of severe malaria (artemisinins), and (iii) reduces transmission of the parasite to the mosquito vector in a mouse model. This new molecule family could open the way to the conception of novel antimalarial drugs with an original multistage mechanism of action to fight against Plasmodium drug resistance and block interhuman transmission of malaria.

Probing the distinct chemosensitivity of Plasmodium vivax liver stage parasites and demonstration of 8-aminoquinoline radical cure activity in vitro

Improved control of Plasmodium vivax malaria can be achieved with the discovery of new antimalarials with radical cure efficacy, including prevention of relapse caused by hypnozoites residing in the liver of patients. We screened several compound libraries against P. vivax liver stages, including 1565 compounds against mature hypnozoites, resulting in one drug-like and several probe-like hits useful for investigating hypnozoite biology. Primaquine and tafenoquine, administered in combination with chloroquine, are currently the only FDA-approved antimalarials for radical cure, yet their activity against mature P. vivax hypnozoites has not yet been demonstrated in vitro. By developing an extended assay, we show both drugs are individually hypnozonticidal and made more potent when partnered with chloroquine, similar to clinically relevant combinations. Post-hoc analyses of screening data revealed excellent performance of ionophore controls and the high quality of single point assays, demonstrating a platform able to support screening of greater compound numbers. A comparison of P. vivax liver stage activity data with that of the P. cynomolgi blood, P. falciparum blood, and P. berghei liver stages reveals overlap in schizonticidal but not hypnozonticidal activity, indicating that the delivery of new radical curative agents killing P. vivax hypnozoites requires an independent and focused drug development test cascade.

Protecting future antimalarials from the trap of resistance: Lessons from artemisinin-based combination therapy (ACT) failures

Having faced increased clinical treatment failures with dihydroartemisinin-piperaquine (DHA-PPQ), Cambodia swapped the first line artemisinin-based combination therapy (ACT) from DHA-PPQ to artesunate-mefloquine given that parasites resistant to piperaquine are susceptible to mefloquine. However, triple mutants have now emerged, suggesting that drug rotations may not be adequate to keep resistance at bay. There is, therefore, an urgent need for alternative treatment strategies to tackle resistance and prevent its spread. A proper understanding of all contributors to artemisinin resistance may help us identify novel strategies to keep artemisinins effective until new drugs become available for their replacement. This review highlights the role of the key players in artemisinin resistance, the current strategies to deal with it and suggests ways of protecting future antimalarial drugs from bowing to resistance as their predecessors did.

MEFAS, a hybrid of artesunate-mefloquine active against asexual stages of Plasmodium vivax in field isolates, inhibits malaria transmission

Human malaria continues to be a public health problem and an important cause of morbidity and mortality in the world. Malaria control is achieved through both individual protection against mosquito bites and drug treatment, which is hampered by the spread of Plasmodium falciparum resistance to most antimalarials, including artemisinin derivatives. One of the key pharmacological strategies for controlling malaria is to block transmission of the parasites to their mosquito vectors. Following this rational, MEFAS, a synthetic hybrid salt derived from artesunate (AS) and mefloquine has been previously reported for its activity against asexual P. falciparum parasites in vitro, in addition to a pronounced reduction in the viability of mature gametocytes. Herein, MEFAS was tested against asexual forms of Plasmodium vivax and for its ability to block malaria transmission in Anopheles darlingi mosquitoes in a membrane feeding assay using P. vivax field isolates. MEFAS demonstrated high potency, with a IC50 of 6.5 nM against asexual forms of P. vivax. At 50 μM, MEFAS completely blocked oocyst formation in mosquitoes, regardless of the oocyst number in the control group. At lower doses, MEFAS reduced oocyst prevalence by greater than 20%. At equivalent doses, AS irregularly reduced oocyst formation and caused only slight inhibition of mosquito infections. These results highlight the potential of MEFAS as a novel transmission-blocking molecule, as well as its high blood schizonticidal activity against P. vivax and P. falciparum field isolates, representing a starting point for further development of a new drug with dual antimalarial activity.

Efforts Made to Eliminate Drug-Resistant Malaria and Its Challenges

Since 2000, a good deal of progress has been made in malaria control. However, there is still an unacceptably high burden of the disease and numerous challenges limiting advancement towards its elimination and ultimate eradication. Among the challenges is the antimalarial drug resistance, which has been documented for almost all antimalarial drugs in current use. As a result, the malaria research community is working on the modification of existing treatments as well as the discovery and development of new drugs to counter the resistance challenges. To this effect, many products are in the pipeline and expected to be marketed soon. In addition to drug and vaccine development, mass drug administration (MDA) is under scientific scrutiny as an important strategy for effective utilization of the developed products. This review discusses the challenges related to malaria elimination, ongoing approaches to tackle the impact of drug-resistant malaria, and upcoming antimalarial drugs.

Antimalarial drug discovery: from quinine to the most recent promising clinical drug candidates

Malaria is a tropical threatening disease caused by Plasmodium parasites, resulting in 409,000 deaths in 2019. The delay of mortality and morbidity has been compounded by the widespread of drug resistant parasites from Southeast Asia since two decades. The emergence of artemisinin-resistant Plasmodium in Africa, where most cases are accounted, highlights the urgent need for new medicines. In this effort, the World Health Organization and Medicines for Malaria Venture joined to define clear goals for novel therapies and characterized the target candidate profile. This ongoing search for new treatments is based on imperative labor in medicinal chemistry which is summarized here with particular attention to hit-to-lead optimizations, key properties, and modes of action of these novel antimalarial drugs. This review, after presenting the current antimalarial chemotherapy, from quinine to the latest marketed drugs, focuses in particular on recent advances of the most promising antimalarial candidates in clinical and preclinical phases.

MalDA, Accelerating Malaria Drug Discovery

The Malaria Drug Accelerator (MalDA) is a consortium of 15 leading scientific laboratories. The aim of MalDA is to improve and accelerate the early antimalarial drug discovery process by identifying new, essential, druggable targets. In addition, it seeks to produce early lead inhibitors that may be advanced into drug candidates suitable for preclinical development and subsequent clinical testing in humans. By sharing resources, including expertise, knowledge, materials, and reagents, the consortium strives to eliminate the structural barriers often encountered in the drug discovery process. Here we discuss the mission of the consortium and its scientific achievements, including the identification of new chemically and biologically validated targets, as well as future scientific directions.

Assessing risks of Plasmodium falciparum resistance to select next-generation antimalarials

Strategies to counteract or prevent emerging drug resistance are crucial for the design of next-generation antimalarials. In the past, resistant parasites were generally identified following treatment failures in patients, and compounds would have to be abandoned late in development. An early understanding of how candidate therapeutics lose efficacy as parasites evolve resistance is important to facilitate drug design and improve resistance detection and monitoring up to the postregistration phase. We describe a new strategy to assess resistance to antimalarial compounds as early as possible in preclinical development by leveraging tools to define the Plasmodium falciparum resistome, predict potential resistance risks of clinical failure for candidate therapeutics, and inform decisions to guide antimalarial drug development.

The molecular basis of antimalarial drug resistance in Plasmodium vivax

Plasmodium vivax is the most geographically widespread cause of human malaria and is responsible for the majority of cases outside of the African continent. While great progress has been made towards eliminating human malaria, drug resistant parasite strains pose a threat towards continued progress. Resistance has arisen to multiple antimalarials in P. vivax, including to chloroquine, which is currently the first line therapy for P. vivax in most regions. Despite its importance, an understanding of the molecular mechanisms of drug resistance in this species remains elusive, in large part due to the complex biology of P. vivax and the lack of in vitro culture. In this review, we will cover the extent and challenges of measuring clinical and in vitro drug resistance in P. vivax. We will consider the roles of candidate drug resistance genes. We will highlight the development of molecular approaches for studying P. vivax biology that provide the opportunity to validate the role of putative drug resistance mutations as well as identify novel mechanisms of drug resistance in this understudied parasite. Validated molecular determinants and markers of drug resistance are essential for the rapid and cost-effective monitoring of drug resistance in P. vivax, and will be useful for optimizing drug regimens and for informing drug policy in control and elimination settings.

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Drug resistance is emerging in Plasmodium vivax, an important cause of malaria.

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The complex biology of P. vivax and the limited range of research tools make it difficult to identify drug resistance.

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The molecular mechanisms of drug resistance in P. vivax remain elusive.

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This review highlights the extent of drug resistance, the putative mechanisms of resistance and new technologies for the study of P. vivax drug resistance.

Potent Antimalarials with Development Potential Identified by Structure-Guided Computational Optimization of a Pyrrole-Based Dihydroorotate Dehydrogenase Inhibitor Series

Dihydroorotate dehydrogenase (DHODH) has been clinically validated as a target for the development of new antimalarials. Experience with clinical candidate triazolopyrimidine DSM265 (1) suggested that DHODH inhibitors have great potential for use in prophylaxis, which represents an unmet need in the malaria drug discovery portfolio for endemic countries, particularly in areas of high transmission in Africa. We describe a structure-based computationally driven lead optimization program of a pyrrole-based series of DHODH inhibitors, leading to the discovery of two candidates for potential advancement to preclinical development. These compounds have improved physicochemical properties over prior series frontrunners and they show no time-dependent CYP inhibition, characteristic of earlier compounds. Frontrunners have potent antimalarial activity in vitro against blood and liver schizont stages and show good efficacy in Plasmodium falciparum SCID mouse models. They are equally active against P. falciparum and Plasmodium vivax field isolates and are selective for Plasmodium DHODHs versus mammalian enzymes.

Absence of Association between Methylene Blue Reduced Susceptibility and Polymorphisms in 12 Genes Involved in Antimalarial Drug Resistance in African Plasmodium falciparum

Half the human population is exposed to malaria. Plasmodium falciparum antimalarial drug resistance monitoring and development of new drugs are major issues related to the control of malaria. Methylene blue (MB), the oldest synthetic antimalarial, is again a promising drug after the break of its use as an antimalarial drug for more than 80 years and a potential partner for triple combination. Very few data are available on the involvement of polymorphisms on genes known to be associated with standard antimalarial drugs and parasite in vitro susceptibility to MB (cross-resistance). In this context, MB susceptibility was evaluated against 482 isolates of imported malaria from Africa by HRP2-based ELISA chemosusceptibility assay. A total of 12 genes involved in antimalarial drug resistance (Pfcrt, Pfdhfr, Pfmdr1, Pfmdr5, Pfmdr6, PfK13, Pfubq, Pfcarl, Pfugt, Pfact, Pfcoronin, and copy number of Pfpm2) were sequenced by Sanger method and quantitative PCR. On the Pfmdr1 gene, the mutation 86Y combined with 184F led to more susceptible isolates to MB (8.0 nM vs. 11.6 nM, p = 0.03). Concerning Pfmdr6, the isolates bearing 12 Asn repetitions were more susceptible to MB (4.6 nM vs. 11.6 nM, p = 0.005). None of the polymorphisms previously described as involved in antimalarial drug resistance was shown to be associated with reduced susceptibility to MB. Some genes (particularly PfK13, Pfugt, Pfact, Pfpm2) did not present enough genetic variability to draw conclusions about their involvement in reduced susceptibility to MB. None of the polymorphisms analyzed by multiple correspondence analysis (MCA) had an impact on the MB susceptibility of the samples successfully included in the analysis. It seems that there is no in vitro cross-resistance between MB and commonly used antimalarial drugs.

Novel Antimalarial Tetrazoles and Amides Active against the Hemoglobin Degradation Pathway in Plasmodium falciparum

Malaria control programs continue to be threatened by drug resistance. To identify new antimalarials, we conducted a phenotypic screen and identified a novel tetrazole-based series that shows fast-kill kinetics and a relatively low propensity to develop high-level resistance. Preliminary structure-activity relationships were established including identification of a subseries of related amides with antiplasmodial activity. Assaying parasites with resistance to antimalarials led us to test whether the series had a similar mechanism of action to chloroquine (CQ). Treatment of synchronized Plasmodium falciparum parasites with active analogues revealed a pattern of intracellular inhibition of hemozoin (Hz) formation reminiscent of CQ's action. Drug selections yielded only modest resistance that was associated with amplification of the multidrug resistance gene 1 (pfmdr1). Thus, we have identified a novel chemical series that targets the historically druggable heme polymerization pathway and that can form the basis of future optimization efforts to develop a new malaria treatment.

Identification and Profiling of a Novel Diazaspiro[3.4]octane Chemical Series Active against Multiple Stages of the Human Malaria Parasite Plasmodium falciparum and Optimization Efforts

A novel diazaspiro[3.4]octane series was identified from a Plasmodium falciparum whole-cell high-throughput screening campaign. Hits displayed activity against multiple stages of the parasite lifecycle, which together with a novel sp³-rich scaffold provided an attractive starting point for a hit-to-lead medicinal chemistry optimization and biological profiling program. Structure-activity-relationship studies led to the identification of compounds that showed low nanomolar asexual blood-stage activity (<50 nM) together with strong gametocyte sterilizing properties that translated to transmission-blocking activity in the standard membrane feeding assay. Mechanistic studies through resistance selection with one of the analogues followed by whole-genome sequencing implicated the P. falciparum cyclic amine resistance locus in the mode of resistance.

Antimalarial Drug Resistance and Novel Targets for Antimalarial Drug Discovery

Malaria is among the most devastating and widespread tropical parasitic diseases in which most prevalent in developing countries. Antimalarial drug resistance is the ability of a parasite strain to survive and/or to multiply despite the administration and absorption of medicine given in doses equal to or higher than those usually recommended. Among the factors which facilitate the emergence of resistance to existing antimalarial drugs: the parasite mutation rate, the overall parasite load, the strength of drug selected, the treatment compliance, poor adherence to malaria treatment guideline, improper dosing, poor pharmacokinetic properties, fake drugs lead to inadequate drug exposure on parasites, and poor-quality antimalarial may aid and abet resistance. Malaria vaccines can be categorized into three categories: pre-erythrocytic, blood-stage, and transmission-blocking vaccines. Molecular markers of antimalarial drug resistance are used to screen for the emergence of resistance and assess its spread. It provides information about the parasite genetics associated with resistance, either single nucleotide polymorphisms or gene copy number variations which are associated with decreased susceptibility of parasites to antimalarial drugs. Glucose transporter PfHT1, kinases (Plasmodium kinome), food vacuole, apicoplast, cysteine proteases, and aminopeptidases are the novel targets for the development of new antimalarial drugs. Therefore, this review summarizes the antimalarial drug resistance and novel targets of antimalarial drugs.

Safety, pharmacokinetics and causal prophylactic efficacy of KAF156 in a Plasmodium falciparum human infection study

BACKGROUND

KAF156 is a novel antimalarial drug that is active against both liver- and blood- stage Plasmodium parasites, including drug-resistant strains. Here, we investigated the causal prophylactic efficacy of KAF156 in a controlled human malaria infection (CHMI) model.

METHODS

In Part 1, healthy, malaria-naïve participants received 800 mg KAF156 or placebo three hr before CHMI with Pf-infected mosquitoes. In Part 2, KAF156 was administered as single doses of 800, 300, 100, 50, or 20 mg 21 hr post-CHMI. All participants received atovaquone/proguanil treatment if blood-stage infection was detected or on day 29. For each cohort, 7-14 subjects were enrolled to KAF156 treatment and up to four subjects to placebo.

RESULTS

KAF156 at all dose levels was safe and well tolerated. Two serious adverse events were reported - both resolved without sequelae and neither was considered related to KAF156. In Part 1, all participants treated with KAF156 and none of those randomized to placebo were protected against malaria infection. In Part 2, all participants treated with placebo or 20 mg KAF156 developed malaria infection. In contrast, 50 mg KAF156 protected 3/14 participants from infection, and doses of 800, 300, and 100 mg KAF156 protected all subjects against infection. An exposure-response analysis suggested that a 24-hr post-dose concentration of KAF156 of 21·5 ng/mL (90% CI 17.66 to 25.32 ng/mL) would ensure a 95% chance of protection from malaria parasite infection.

CONCLUSIONS

KAF156 was safe and well tolerated and demonstrated high levels of pre- and post-CHMI protective efficacy.

Cellular thermal shift assay for the identification of drug-target interactions in the Plasmodium falciparum proteome

Despite decades of research, little is known about the cellular targets and the mode of action of the vast majority of antimalarial drugs. We recently demonstrated that the cellular thermal shift assay (CETSA) protocol in its two variants: the melt curve and the isothermal dose-response, represents a comprehensive strategy for the identification of antimalarial drug targets. CETSA enables proteome-wide target screening for unmodified antimalarial compounds with undetermined mechanisms of action, providing quantitative evidence about direct drug-protein interactions. The experimental workflow involves treatment of P. falciparum-infected erythrocytes with a compound of interest, heat exposure to denature proteins, soluble protein isolation, enzymatic digestion, peptide labeling with tandem mass tags, offline fractionation, and liquid chromatography-tandem mass spectrometry analysis. Methodological optimizations necessary for the analysis of this intracellular parasite are discussed, including enrichment of parasitized cells and hemoglobin depletion strategies to overcome high hemoglobin abundance in the host red blood cells. We outline an effective data processing workflow using the mineCETSA R package, which enables prioritization of drug-target candidates for follow-up studies. The entire protocol can be completed within 2 weeks.

Lead Optimization of a Pyrrole-Based Dihydroorotate Dehydrogenase Inhibitor Series for the Treatment of Malaria

Malaria puts at risk nearly half the world's population and causes high mortality in sub-Saharan Africa, while drug resistance threatens current therapies. The pyrimidine biosynthetic enzyme dihydroorotate dehydrogenase (DHODH) is a validated target for malaria treatment based on our finding that triazolopyrimidine DSM265 (1) showed efficacy in clinical studies. Herein, we describe optimization of a pyrrole-based series identified using a target-based DHODH screen. Compounds with nanomolar potency versus Plasmodium DHODH and Plasmodium parasites were identified with good pharmacological properties. X-ray studies showed that the pyrroles bind an alternative enzyme conformation from 1 leading to improved species selectivity versus mammalian enzymes and equivalent activity on Plasmodium falciparum and Plasmodium vivax DHODH. The best lead DSM502 (37) showed in vivo efficacy at similar levels of blood exposure to 1, although metabolic stability was reduced. Overall, the pyrrole-based DHODH inhibitors provide an attractive alternative scaffold for the development of new antimalarial compounds.

Pan-active imidazolopiperazine antimalarials target the Plasmodium falciparum intracellular secretory pathway

A promising new compound class for treating human malaria is the imidazolopiperazines (IZP) class. IZP compounds KAF156 (Ganaplacide) and GNF179 are effective against Plasmodium symptomatic asexual blood-stage infections, and are able to prevent transmission and block infection in animal models. But despite the identification of resistance mechanisms in P. falciparum, the mode of action of IZPs remains unknown. To investigate, we here combine in vitro evolution and genome analysis in Saccharomyces cerevisiae with molecular, metabolomic, and chemogenomic methods in P. falciparum. Our findings reveal that IZP-resistant S. cerevisiae clones carry mutations in genes involved in Endoplasmic Reticulum (ER)-based lipid homeostasis and autophagy. In Plasmodium, IZPs inhibit protein trafficking, block the establishment of new permeation pathways, and cause ER expansion. Our data highlight a mechanism for blocking parasite development that is distinct from those of standard compounds used to treat malaria, and demonstrate the potential of IZPs for studying ER-dependent protein processing.

Probing the Open Global Health Chemical Diversity Library for Multistage-Active Starting Points for Next-Generation Antimalarials

Most phenotypic screens aiming to discover new antimalarial chemotypes begin with low cost, high-throughput tests against the asexual blood stage (ABS) of the malaria parasite life cycle. Compounds active against the ABS are then sequentially tested in more difficult assays that predict whether a compound has other beneficial attributes. Although applying this strategy to new chemical libraries may yield new leads, repeated iterations may lead to diminishing returns and the rediscovery of chemotypes hitting well-known targets. Here, we adopted a different strategy to find starting points, testing ∼70,000 open source small molecules from the Global Health Chemical Diversity Library for activity against the liver stage, mature sexual stage, and asexual blood stage malaria parasites in parallel. In addition, instead of using an asexual assay that measures accumulated parasite DNA in the presence of compound (SYBR green), a real time luciferase-dependent parasite viability assay was used that distinguishes slow-acting (delayed death) from fast-acting compounds. Among 382 scaffolds with the activity confirmed by dose response (<10 μM), we discovered 68 novel delayed-death, 84 liver stage, and 68 stage V gametocyte inhibitors as well. Although 89% of the evaluated compounds had activity in only a single life cycle stage, we discovered six potent (half-maximal inhibitory concentration of <1 μM) multistage scaffolds, including a novel cytochrome bc1 chemotype. Our data further show the luciferase-based assays have higher sensitivity. Chemoinformatic analysis of positive and negative compounds identified scaffold families with a strong enrichment for activity against specific or multiple stages.

Origin and Spread of Evolving Artemisinin-Resistant Plasmodium falciparum Malarial Parasites in Southeast Asia

In this review, we provide an epidemiological history of the emergence and ongoing spread of evolving Plasmodium falciparum artemisinin resistance (ARTR). Southeast Asia has been the focal point for emergence and spread of multiple antimalarial drug resistance phenomena, and is once again for evolving ARTR, also known as the "delayed clearance phenotype" (DCP). The five countries most impacted, Cambodia, Thailand, Myanmar, Laos, and Vietnam, each have complex histories of antimalarial drug use over many decades, which have in part molded the use of various artemisinin combination therapies (ACTs) within each country. We catalog the use of ACTs, evolving loss of ACT efficacy, and the frequency of pfk13 mutations (mutations associated with ARTR) in the Greater Mekong Subregion and map the historical spread of ARTR/DCP parasites. These data should assist improved surveillance and deployment of next-generation ACTs.

An adaptable soft-mold embossing process for fabricating optically-accessible, microfeature-based culture systems and application toward liver stage antimalarial compound testing

Advanced cell culture methods for modeling organ-level structure have been demonstrated to replicate in vivo conditions more accurately than traditional in vitro cell culture. Given that the liver is particularly important to human health, several advanced culture methods have been developed to experiment with liver disease states, including infection with Plasmodium parasites, the causative agent of malaria. These models have demonstrated that intrahepatic parasites require functionally stable hepatocytes to thrive and robust characterization of the parasite populations' response to investigational therapies is dependent on high-content and high-resolution imaging (HC/RI). We previously reported abiotic confinement extends the functional longevity of primary hepatocytes in a microfluidic platform and set out to instill confinement in a microtiter plate platform while maintaining optical accessibility for HC/RI; with an end-goal of producing an improved P. vivax liver stage culture model. We developed a novel fabrication process in which a PDMS soft mold embosses hepatocyte-confining microfeatures into polystyrene, resulting in microfeature-based hepatocyte confinement (μHEP) slides and plates. Our process was optimized to form both microfeatures and culture wells in a single embossing step, resulting in a 100 μm-thick bottom ideal for HC/RI, and was found inexpensively amendable to microfeature design changes. Microfeatures improved intrahepatic parasite infection rates and μHEP systems were used to reconfirm the activity of reference antimalarials in phenotypic dose-response assays. RNAseq of hepatocytes in μHEP systems demonstrated microfeatures sustain hepatic differentiation and function, suggesting broader utility for preclinical hepatic assays; while our tailorable embossing process could be repurposed for developing additional organ models.

Recent advances in transmission-blocking drugs for malaria elimination

The scientific community worldwide has realized that malaria elimination will not be possible without development of safe and effective transmission-blocking interventions. Primaquine, the only WHO recommended transmission-blocking drug, is not extensively utilized because of the toxicity issues in G6PD deficient individuals. Therefore, there is an urgent need to develop novel therapeutic interventions that can target malaria parasites and effectively block transmission. But at first, it is imperative to unravel the existing portfolio of transmission-blocking drugs. This review highlights transmission-blocking potential of current antimalarial drugs and drugs that are in various stages of clinical development. The collective analysis of the relationships between the structure and the activity of transmission-blocking drugs is expected to help in the design of new transmission-blocking antimalarials.

Low polymorphisms in pfact, pfugt and pfcarl genes in African Plasmodium falciparum isolates and absence of association with susceptibility to common anti-malarial drugs

Background

Resistance to all available anti-malarial drugs has emerged and spread including artemisinin derivatives and their partner drugs. Several genes involved in artemisinin and partner drugs resistance, such as pfcrt, pfmdr1, pfK13 or pfpm2, have been identified. However, these genes do not properly explain anti-malarial drug resistance, and more particularly clinical failures observed in Africa. Mutations in genes encoding for Plasmodium falciparum proteins, such as P. falciparum Acetyl-CoA transporter (PfACT), P. falciparum UDP-galactose transporter (PfUGT) and P. falciparum cyclic amine resistance locus (PfCARL) have recently been associated to resistance to imidazolopiperazines and other unrelated drugs.

Methods

Mutations on pfugt, pfact and pfcarl were characterized on 86 isolates collected in Dakar, Senegal and 173 samples collected from patients hospitalized in France after a travel in African countries from 2015 and 2016 to assess their potential association with ex vivo susceptibility to chloroquine, quinine, lumefantrine, monodesethylamodiaquine, mefloquine, dihydroartemisinin, artesunate, doxycycline, pyronaridine and piperaquine.

Results

No mutations were found on the genes pfugt and pfact. None of the pfcarl described mutations were identified in these samples from Africa. The K784N mutation was found in one sample and the K734M mutation was identified on 7.9% of all samples for pfcarl. The only significant differences in ex vivo susceptibility according to the K734M mutation were observed for pyronaridine for African isolates from imported malaria and for doxycycline for Senegalese parasites.

Conclusion

No evidence was found of involvement of these genes in reduced susceptibility to standard anti-malarial drugs in African P. falciparum isolates.

BIM-46174 fragments as potential ligands of G proteins

The 5,6,7,8-tetrahydroimidazo[1,2-a]pyrazine derivative BIM-46174 has received attention as Gα_q inhibitor. We conducted structural reductions to monocyclic and bicyclic substructures to explore the chemical space of BIM fragments and to gain insights into the pharmacophore of BIM-type Gα_q inhibitors. Two piperazin-2-one-containing fragments and a small library of bicyclic lactams featuring fused pyrazine and diazepine rings were synthesized and evaluated. The results of a second messenger-based cellular assay indicate that the entire BIM structure is required for efficient Gα_q inhibition.