Antimalarial drug discovery: progress and approaches

Differential effects of translation inhibitors on Plasmodium berghei liver stage parasites

Increasing numbers of antimalarial compounds are being identified that converge mechanistically at inhibition of cytoplasmic translation, regardless of the molecular target or mechanism. A deeper understanding of how their effectiveness as liver stage translation inhibitors relates to their chemoprotective potential could prove useful. Here, we probed that relationship using the Plasmodium berghei-HepG2 liver stage infection model. After determining translation inhibition EC50s for five compounds, we tested them at equivalent effective concentrations to compare the parasite response to, and recovery from, a brief period of translation inhibition in early schizogony, followed by parasites to 120 h post-infection to assess antiplasmodial effects of the treatment. We show compound-specific heterogeneity in single parasite and population responses to translation inhibitor treatment, with no single metric strongly correlated to the release of hepatic merozoites for all compounds. We also demonstrate that DDD107498 is capable of exerting antiplasmodial effects on translationally arrested liver stage parasites and uncover unexpected growth dynamics during the liver stage. Our results demonstrate that translation inhibition efficacy does not determine antiplasmodial efficacy for these compounds.

Characterising the blood-stage antimalarial activity of pyronaridine in healthy volunteers experimentally infected with Plasmodium falciparum

With the spread of artemisinin resistance throughout Southeast Asia and now in Africa, the antimalarial drug pyronaridine is likely to become an increasingly important component of new antimalarial drug regimens. However, the antimalarial activity of pyronaridine in humans has not been completely characterised. This volunteer infection study aimed to determine the pharmacokinetic/pharmacodynamic (PK/PD) relationship of pyronaridine in malaria naïve adults. Volunteers were inoculated with Plasmodium falciparum-infected erythrocytes on day 0 and administered different single oral doses of pyronaridine on day 8. Parasitaemia and concentrations of pyronaridine were measured and standard safety assessments performed. Curative artemether-lumefantrine therapy was administered if parasite regrowth occurred, or on day 47 ± 2. Outcomes were parasite clearance kinetics, PK and PK/PD parameters from modelling. Ten participants were inoculated and administered 360 mg (n = 4), 540 mg (n = 4) or 720 mg (n = 1) pyronaridine. One participant was withdrawn without receiving pyronaridine. The time to maximum pyronaridine concentration was 1-2 h, the elimination half-life was 8-9 d, and the parasite clearance half-life was approximately 5 h. Parasite regrowth occurred with 360 mg (4/4 participants) and 540 mg (2/4 participants). Key efficacy parameters including the minimum inhibitory concentration (5.5 ng/mL) and minimum parasiticidal concentration leading to 90% of maximum effect (MPC90: 8 ng/mL) were derived from the PK/PD model. Adverse events considered related to pyronaridine were predominantly mild to moderate gastrointestinal symptoms. There were no serious adverse events. Data obtained in this study will support the use of pyronaridine in new antimalarial combination therapies by informing partner drug selection and dosing considerations.

Design, synthesis and Anti-Plasmodial activity of Mortiamide-Lugdunin conjugates

In this study, two linear and corresponding cyclic heptapeptide versions of mortiamide A-lugdunin hybrids were designed and synthesized by integrating an anti-malarial peptide epitope derived from Mortiamide A, combined with four residues known for their membrane interactions. Using this synthetic strategy, the sequence of mortiamide A was partly re-engineered with an epitope sequence of lugdunin along with an amino acid replacement using all-L and D/L configurations. Importantly, the re-engineered cyclic mortiamides with all-L (3) and D/L (4) configurations exhibited promising anti-malarial activities against the P. falciparum drug-sensitive TM4/8 strain with half-maximal inhibitory concentration (IC50) values of 6.2 ± 0.5 and 4.8 ± 0.1 μM, respectively. Additionally, they exhibited anti-malarial activities against the P. falciparum multidrug-resistant V1/S strain with IC50 values of 5.0 ± 2.6 and 3.7 ± 0.7 μM, respectively. Interestingly, a linear re-engineered mortiamide with D/L configuration (2) exhibited promising anti-malarial activities, surpassing those of the re-engineered cyclic mortiamides (3 and 4), against both the P. falciparum sensitive TM4/8 and multidrug-resistant V1/S strains with IC50 values of 3.6 ± 0.5 and 2.8 ± 0.7 μM (IC50 of Mortiamide A = 7.85 ± 0.97, 5.31 ± 0.24 μM against 3D7 and Dd2 strains) without any cytotoxicity at >100 µM. The presence of D/L forms in a linear structure significantly impacted the anti-malarial activity against both the P. falciparum sensitive TM4/8 strain and the multidrug-resistant V1/S strain.

Disclosure of cinnamic acid/4,9-diaminoacridine conjugates as multi-stage antiplasmodial hits

4,9-diaminoacridines with reported antiplasmodial activity were coupled to different trans-cinnamic acids, delivering a new series of conjugates inspired by the covalent bitherapy concept. The new compounds were more potent than primaquine against hepatic stages of Plasmodium berghei, although this was accompanied by cytotoxic effects on Huh-7 hepatocytes. Relevantly, the conjugates displayed nanomolar activities against blood stage P. falciparum parasites, with no evidence of hemolytic effects below 100 µM. Moreover, the new compounds were at least 25-fold more potent than primaquine against P. falciparum gametocytes. Thus, the new antiplasmodial hits disclosed herein emerge as valuable templates for the development of multi-stage antiplasmodial drug candidates.

Selective Bias Virtual Screening for Discovery of Promising Antimalarial Candidates targeting Plasmodium N-Myristoyltransferase

Malaria remains a significant public health challenge, with Plasmodium vivax being the species responsible for the most prevalent form of the disease. Given the limited therapeutic options available, the search for new antimalarials against P. vivax is urgent. This study aims to identify new inhibitors for P. vivax N-myristoyltransferase (PvNMT), an essential drug target against malaria. Through a validated virtual screening campaign, we prioritized 23 candidates for further testing. In the yeast NMT system, seven compounds exhibit a potential inhibitor phenotype. In vitro antimalarial phenotypic assays confirmed the activity of four candidates while demonstrating an absence of cytotoxicity. Enzymatic assays reveal LabMol-394 as the most promising inhibitor, displaying selectivity against the parasite and a strong correlation within the yeast system. Furthermore, molecular dynamics simulations shed some light into its binding mode. This study constitutes a substantial contribution to the exploration of a selective quinoline scaffold and provides valuable insights into the development of new antimalarial candidates.

Emerging Drug Targets for Antimalarial Drug Discovery: Validation and Insights into Molecular Mechanisms of Function

Approximately 619,000 malaria deaths were reported in 2021, and resistance to recommended drugs, including artemisinin-combination therapies (ACTs), threatens malaria control. Treatment failure with ACTs has been found to be as high as 93% in northeastern Thailand, and parasite mutations responsible for artemisinin resistance have already been reported in some African countries. Therefore, there is an urgent need to identify alternative treatments with novel targets. In this Perspective, we discuss some promising antimalarial drug targets, including enzymes involved in proteolysis, DNA and RNA metabolism, protein synthesis, and isoprenoid metabolism. Other targets discussed are transporters, Plasmodium falciparum acetyl-coenzyme A synthetase, N-myristoyltransferase, and the cyclic guanosine monophosphate-dependent protein kinase G. We have outlined mechanistic details, where these are understood, underpinning the biological roles and hence druggability of such targets. We believe that having a clear understanding of the underlying chemical interactions is valuable to medicinal chemists in their quest to design appropriate inhibitors.

Umbelliferone and Its Synthetic Derivatives as Suitable Molecules for the Development of Agents with Biological Activities: A Review of Their Pharmacological and Therapeutic Potential

Umbelliferone (UMB), known as 7-hydroxycoumarin, hydrangine, or skimmetine, is a naturally occurring coumarin in the plant kingdom, mainly from the Umbelliferae family that possesses a wide variety of pharmacological properties. In addition, the use of nanoparticles containing umbelliferone may improve anti-inflammatory or anticancer therapy. Also, its derivatives are endowed with great potential for therapeutic applications due to their broad spectrum of biological activities such as anti-inflammatory, antioxidant, neuroprotective, antipsychotic, antiepileptic, antidiabetic, antimicrobial, antiviral, and antiproliferative effects. Moreover, 7-hydroxycoumarin ligands have been implemented to develop 7-hydroxycoumarin-based metal complexes with improved pharmacological activity. Besides therapeutic applications, umbelliferone analogues have been designed as fluorescent probes for the detection of biologically important species, such as enzymes, lysosomes, and endosomes, or for monitoring cell processes and protein functions as well various diseases caused by an excess of hydrogen peroxide. Furthermore, 7-hydroxy-based chemosensors may serve as a highly selective tool for Al3+ and Hg2+ detection in biological systems. This review is devoted to a summary of the research on umbelliferone and its synthetic derivatives in terms of biological and pharmaceutical properties, especially those reported in the literature during the period of 2017-2023. Future potential applications of umbelliferone and its synthetic derivatives are presented.

Parallel Synthesis of Piperazine Tethered Thiazole Compounds with Antiplasmodial Activity

Thiazole and piperazine are two important heterocyclic rings that play a prominent role in nature and have a broad range of applications in agricultural and medicinal chemistry. Herein, we report the parallel synthesis of a library of diverse piperazine-tethered thiazole compounds. The reaction of piperazine with newly generated 4-chloromethyl-2-amino thiazoles led to the desired piperazine thiazole compounds with high purities and good overall yields. Using a variety of commercially available carboxylic acids, the parallel synthesis of a variety of disubstituted 4-(piperazin-1-ylmethyl)thiazol-2-amine derivatives is described. the screening of the compounds led to the identification of antiplasmodial compounds that exhibited interesting antimalarial activity, primarily against the Plasmodium falciparum chloroquine-resistant Dd2 strain. The hit compound 2291-61 demonstrated an antiplasmodial EC50 of 102 nM in the chloroquine-resistant Dd2 strain and a selectivity of over 140.

Translation inhibition efficacy does not determine the Plasmodium berghei liver stage antiplasmodial efficacy of protein synthesis inhibitors

Protein synthesis is a core cellular process, necessary throughout the complex lifecycle of Plasmodium parasites, thus specific translation inhibitors would be a valuable class of antimalarial drugs, capable of both treating symptomatic infections in the blood and providing chemoprotection by targeting the initial parasite population in the liver, preventing both human disease and parasite transmission back to the mosquito host. As increasing numbers of antiplasmodial compounds are identified that converge mechanistically at inhibition of cytoplasmic translation, regardless of molecular target or mechanism, it would be useful to gain deeper understanding of how their effectiveness as liver stage translation inhibitors relates to their chemoprotective potential. Here, we probed that relationship using the P. berghei-HepG2 liver stage infection model. Using o-propargyl puromycin-based labeling of the nascent proteome in P. berghei-infected HepG2 monolayers coupled with automated confocal feedback microscopy to generate unbiased, single parasite image sets of P. berghei liver stage translation, we determined translation inhibition EC50s for five compounds, encompassing parasite-specific aminoacyl tRNA synthetase inhibitors, compounds targeting the ribosome in both host and parasite, as well as DDD107498, which targets Plasmodium eEF2, and is a leading antimalarial candidate compound being clinically developed as cabamiquine. Compounds were then tested at equivalent effective concentrations to compare the parasite response to, and recovery from, a brief period of translation inhibition in early schizogony, with parasites followed up to 120 hours post-infection to assess liver stage antiplasmodial effects of the treatment. Our data conclusively show that translation inhibition efficacy per se does not determine a translation inhibitor's antiplasmodial efficacy. DDD107498 was the least effective translation inhibitor, yet exerted the strongest antimalarial effects at both 5x- and 10x EC50 concentrations. We show compound-specific heterogeneity in single parasite and population responses to translation inhibitor treatment, with no single metric strongly correlated to release of hepatic merozoites for all compound, demonstrate that DDD107498 is capable of exerting antiplasmodial effects on translationally arrested liver stage parasites, and uncover unexpected growth dynamics during the liver stage. Our results demonstrate that translation inhibition efficacy cannot function as a proxy for antiplasmodial effectiveness, and highlight the importance of exploring the ultimate, as well as proximate, mechanisms of action of these compounds on liver stage parasites.

Editorial for the Special Issue "Antiprotozoal Activity of Natural Products"

Neglected tropical diseases (NTDs), a diverse group of infectious diseases, represent the leading cause of morbidity and mortality among the world's low-income populations [...].

Design of Novel Series of Antimalarial PMX Inhibitors with Increased Half-Life via Molecular Property Optimization

Plasmepsin X (PMX) has been identified as a multistage antimalarial target. PMX is a malarial aspartyl protease essential for merozoite egress from infected red blood cells and invasion of the host erythrocytes. Previously, we reported the identification of PMX inhibitors by structure-based optimization of a cyclic guanidine core. Preclinical assessment of UCB7362, which displayed both in vitro and in vivo antimalarial activity, revealed a suboptimal dose paradigm (once daily dosing of 50 mg for 7 days for treatment of uncomplicated malaria) relative to current standard of care (three-dose regime). We report here the efforts toward extending the half-life (t1/2) by reducing metabolic clearance and increasing volume of distribution (Vss). Our efforts culminated in the identification of a biaryl series, with an expected longer t1/2 in human than UCB7362 while maintaining a similar in vitro off-target hit rate.

Recent advances on patents of Plasmodium falciparum dihydroorotate dehydrogenase (PfDHODH) inhibitors as antimalarial agents

INTRODUCTION

Pyrimidine nucleotides are essential for the parasite's growth and replication. Parasites have only a de novo pathway for the biosynthesis of pyrimidine nucleotides. Dihydroorotate dehydrogenase (DHODH) enzyme is involved in the rate-limiting step of the pyrimidine biosynthesis pathway. DHODH is a biochemical target for the discovery of new antimalarial agents.

AREA COVERED

This review discussed the development of patented PfDHODH inhibitors published between 2007 and 2023 along with their chemical structures and activities.

EXPERT OPINION

PfDHODH enzyme is involved in the rate-limiting fourth step of the pyrimidine biosynthesis pathway. Thus, inhibition of PfDHODH using species-selective inhibitors has drawn much attention for treating malaria because they inhibit parasite growth without affecting normal human functions. Looking at the current scenario of antimalarial drug resistance with most of the available antimalarial drugs, there is a huge need for targeted newer agents. Newer agents with unique mechanisms of action may be devoid of drug toxicity, adverse effects, and the ability of parasites to quickly gain resistance, and PfDHODH inhibitors can be those newer agents. Many PfDHODH inhibitors were patented in the past, and the dependency of Plasmodium on de novo pyrimidine provided a new approach for the development of novel antimalarial agents.