Biological Studies and Target Engagement of the 2- C-Methyl-d-Erythritol 4-Phosphate Cytidylyltransferase (IspD)-Targeting Antimalarial Agent (1 R,3 S)-MMV008138 and Analogs

Development of potent and selective tetrahydro-β-carboline-based HDAC6 inhibitors with promising activity against triple-negative breast cancer

Overexpression of histone deacetylase 6 (HDAC6) is implicated in tumorigenesis, invasion, migration, survival, apoptosis, and growth of various malignancies, making it a promising target for cancer treatment. Building on our previous work, we report a novel series of tetrahydro-β-carboline-piperazinedione derivatives as HDAC6 inhibitors. Structural modifications were introduced at the 6-aryl group, with the m-bromophenyl derivative (9c) emerging as the most potent HDAC6 inhibitor, exhibiting an IC50 of 7 nM. Compound 9c demonstrated robust growth inhibitory activity across 60 cancer cell lines from the NCI panel, with a mean GI50 of 2.64 μM and a GI50 below 5 μM for nearly all tested lines, while exhibiting significantly lower cytotoxicity towards non-tumor cell lines. The triple-negative breast cancer cell line MDA-MB-231 was selected for further investigation of 9c's cellular effects. 9c selectively increased the acetylation of non-histone α-tubulin in MDA-MB-231 cells, confirming its HDAC6 selectivity. Furthermore, 9c effectively induced apoptosis, caused apoptotic sub-G1 phase accumulation, upregulated pro-apoptotic caspase-3, and downregulated anti-apoptotic Bcl-2. Notably, 9c reduced the expression of programmed death-ligand 1 (PD-L1), a key immune checkpoint protein that enables tumor cells to evade immune surveillance, highlighting its potential role in enhancing anti-tumor immunity. In addition, 9c inhibited phosphorylated extracellular signal-regulated kinase (ERK)1/2, a central signaling pathway that drives cell proliferation, survival, and migration, further highlighting its significance in suppressing tumor progression and growth. In migration assays, 9c impaired cell motility, achieving 80% gap closure inhibition in a wound-healing assay. Collectively, these findings underline compound 9c as a highly promising candidate for the treatment of triple-negative breast cancer, with the added benefits of PD-L1 and ERK inhibition for potential synergy in enhancing anti-tumor immunity and reducing tumor cell proliferation.

Stereospecific Resistance to N2-Acyl Tetrahydro-β-carboline Antimalarials Is Mediated by a PfMDR1 Mutation That Confers Collateral Drug Sensitivity

Half the world's population is at risk of developing a malaria infection, which is caused by parasites of the genus Plasmodium. Currently, resistance has been identified to all clinically available antimalarials, highlighting an urgent need to develop novel compounds and better understand common mechanisms of resistance. We previously identified a novel tetrahydro-β-carboline compound, PRC1590, which potently kills the malaria parasite. To better understand its mechanism of action, we selected for and characterized resistance to PRC1590 in Plasmodium falciparum. Through in vitro selection of resistance to PRC1590, we have identified that a single-nucleotide polymorphism on the parasite's multidrug resistance protein 1 (PfMDR1 G293V) mediates resistance to PRC1590. This mutation results in stereospecific resistance and sensitizes parasites to other antimalarials, such as mefloquine, quinine, and MMV019017. Intraerythrocytic asexual stage specificity assays have revealed that PRC1590 is most potent during the trophozoite stage when the parasite forms a single digestive vacuole (DV) and actively digests hemoglobin. Moreover, fluorescence microscopy revealed that PRC1590 disrupts the function of the DV, indicating a potential molecular target associated with this organelle. Our findings mark a significant step in understanding the mechanism of resistance and the mode of action of this emerging class of antimalarials. In addition, our results suggest a potential link between resistance mediated by PfMDR1 and PRC1590's molecular target. This research underscores the pressing need for future research aimed at investigating the intricate relationship between a compound's chemical scaffold, molecular target, and resistance mutations associated with PfMDR1.

Synthetic account on indoles and their analogues as potential anti-plasmodial agents

Malaria caused by P. falciparum, has been recognized as one of the major infectious diseases causing the death of several patients as per the reports from the World Health Organization. In search of effective therapeutic agents against malaria, several research groups have started working on the design and development of novel heterocycles as anti-malarial agents. Heterocycles have been recognized as the pharmacophoric features for the different types of medicinally important activities. Among all these heterocycles, nitrogen containing aza-heterocycles should not be underestimated owing to their wide therapeutic window. Amongst the aza-heterocycles, indoles and fused indoles such as marinoquinolines, isocryptolepines and their regioisomers, manzamines, neocryptolenines, and indolones have been recognized as anti-malarial agents active against P. falciparum. The present work unleashes the synthetic attempts of anti-malarial indoles and fused indoles through cyclocondensation, Fischer-indole synthesis, etc. along with the brief discussions on structure-activity relationships, in vitro or in vivo studies for the broader interest of these medicinal chemists, working on their design and development as potential anti-malarial agents.

Targeting IspD for Anti-infective and Herbicide Development: Exploring Its Role, Mechanism, and Structural Insights

Antimicrobial resistance (AMR) and herbicide resistance pose threats to society, necessitating novel anti-infectives and herbicides exploiting untapped modes of action like inhibition of IspD, the third enzyme in the MEP pathway. The MEP pathway is essential for a wide variety of human pathogens, including Pseudomonas aeruginosa, Mycobacterium tuberculosis, and Plasmodium falciparum, as well as plants. Within the current perspective, we focused our attention on the third enzyme in this pathway, IspD, offering a comprehensive summary of the reported modes of inhibition and common trends, with the goal to inspire future research dedicated to this underexplored target. In addition, we included an overview of the history, catalytic mechanism, and structure of the enzyme to facilitate access to this attractive target.

Genomic profiling and assessment of the plant growth-enhancing traits in the novel actinomycete, Plantactinospora soli sp. nov

Strain KLBMP 9567T, an isolate from weathered soil, was identified as a new actinobacterial species through a comprehensive polyphasic approach. Phylogenetic evaluations relying on 16S rRNA gene sequence placed KLBMP 9567T within the genus Plantactinospora, alongside its close relatives Plantactinospora veratri NEAU-FHS4T and Plantactinospora sonchi NEAU-QY2T, both sharing 98.6% sequence similarity. However, KLBMP 9567T demonstrated low digital DNA-DNA hybridization values with P. veratri NEAU-FHS4T and P. sonchi NEAU-QY2T, recorded at 48.5 and 29.1%, respectively. Meanwhile, the average nucleotide identity values were correspondingly recorded at 91.1 and 83.1%. The cell wall of KLBMP 9567T contained meso-diaminopimelic acid, with xylose, mannose, glucose and galactose as diagnostic sugars. The diagnostic dominant phospholipids included diphosphatidylglycerol, phosphatidylethanolamine and phosphatidylinositol. Given phenotypic and genotypic data, KLBMP 9567T is recognized as a new species within the Plantactinospora genus, named Plantactinospora soli sp. nov., with the type strain KLBMP 9567T (= CGMCC 4.7773T = NBRC 115787T). Genome mining revealed genes associated with plant growth promotion, specifically those encoding enzymes for synthesizing plant hormones like indole acetic acid and gibberellic acid. Experimental validation demonstrated that KLBMP 9567T can produce significant quantities of indole acetic acid (20.6 mg l-1) and gibberellic acid (42.6 mg l-1). Moreover, co-culture with Arabidopsis showed marked growth enhancements: a 56.3% increase in lateral root proliferation, a 16.7% elongation in primary root length and a 44.8% boost in biomass accumulation. These findings underscore the strain's potent plant growth-promoting attributes, providing profound insights into the mechanisms by which KLBMP 9567T enhances plant growth.

Targeting Plasmodium falciparum IspD in the Methyl-d-erythritol Phosphate Pathway: Urea-Based Compounds with Nanomolar Potency on Target and Low-Micromolar Whole-Cell Activity

The methyl-d-erythritol phosphate (MEP) pathway has emerged as an interesting target in the fight against antimicrobial resistance. The pathway is essential in many human pathogens, including Plasmodium falciparum (Pf), but is absent in human cells. In the present study, we report on the discovery of a new chemical class targeting IspD, the third enzyme in the pathway. Exploration of the structure-activity relationship yielded inhibitors with potency in the low-nanomolar range. Moreover, we investigated the whole-cell activity, mode of inhibition, metabolic, and plasma stability of this compound class, and conducted in vivo pharmacokinetic profiling on selected compounds. Lastly, we disclosed a new mass spectrometry (MS)-based enzymatic assay for direct IspD activity determination, circumventing the need for auxiliary enzymes. In summary, we have identified a readily synthesizable compound class, demonstrating excellent activity and a promising profile, positioning it as a valuable tool compound for advancing research on IspD.

Apicoplast-Resident Processes: Exploiting the Chink in the Armour of Plasmodium falciparum Parasites

The discovery of a relict plastid, also known as an apicoplast (apicomplexan plastid), that houses housekeeping processes and metabolic pathways critical to Plasmodium parasites' survival has prompted increased research on identifying potent inhibitors that can impinge on apicoplast-localised processes. The apicoplast is absent in humans, yet it is proposed to originate from the eukaryote's secondary endosymbiosis of a primary symbiont. This symbiotic relationship provides a favourable microenvironment for metabolic processes such as haem biosynthesis, Fe-S cluster synthesis, isoprenoid biosynthesis, fatty acid synthesis, and housekeeping processes such as DNA replication, transcription, and translation, distinct from analogous mammalian processes. Recent advancements in comprehending the biology of the apicoplast reveal it as a vulnerable organelle for malaria parasites, offering numerous potential targets for effective antimalarial therapies. We provide an overview of the metabolic processes occurring in the apicoplast and discuss the organelle as a viable antimalarial target in light of current advances in drug discovery. We further highlighted the relevance of these metabolic processes to Plasmodium falciparum during the different stages of the lifecycle.

Two natural compounds as potential inhibitors against the Helicobacter pylori and Acinetobacter baumannii IspD enzymes

In a vast majority of bacteria, protozoa and plants, the methylerythritol phosphate (MEP) pathway is utilized for the synthesis of isopentenyl diphosphate (IDP) and dimethylallyl diphosphate (DMADP), which are precursors for isoprenoids. Isoprenoids, such as cholesterol and coenzyme Q, play a variety of crucial roles in physiological activities, including cell-membrane formation, protein degradation, cell apoptosis, and transcription regulation. In contrast, humans employ the mevalonate (MVA) pathway for the production of IDP and DMADP, rendering proteins in the MEP pathway appealing targets for antimicrobial agents. This pathway consists of seven consecutive enzymatic reactions, of which 4-diphosphocytidyl-2C-methyl-D-erythritol synthase (IspD) and 2C-methyl-D-erythritol 2,4-cyclodiphosphate synthase (IspF) catalyze the third and fifth steps, respectively. In this study, we characterized the enzymatic activities and protein structures of Helicobacter pylori IspDF and Acinetobacter baumannii IspD. Then, using the direct interaction-based thermal shift assay, we conducted a compound screening of an approved drug library and identified 27 hit compounds potentially binding to AbIspD. Among them, two natural products, rosmarinic acid and tanshinone IIA sodium sulfonate, exhibited inhibitory activities against HpIspDF and AbIspD, by competing with one of the substrates, MEP. Moreover, tanshinone IIA sodium sulfonate also demonstrated certain antibacterial effects against H. pylori. In summary, we identified two IspD inhibitors from approved ingredients, broadening the scope for antibiotic discovery targeting the MEP pathway.

Bibliometric analysis of antimalarial drug resistance

Background

Malaria has always been a serious infectious disease prevalent in the world. Antimalarial drugs such as chloroquine and artemisinin have been the main compounds used to treat malaria. However, the massive use of this type of drugs accelerates the evolution and spread of malaria parasites, leading to the development of resistance. A large number of related data have been published by researchers in recent years. CiteSpace software has gained popularity among us researchers in recent years, because of its ability to help us obtain the core information we want in a mass of articles. In order to analyze the hotspots and develop trends in this field through visual analysis, this study used CiteSpace software to summarize the available data in the literature to provide insights.

Method

Relevant literature was collected from the Web of Science Core Collection (WOSCC) from 1 January 2015 to 29 March 2023. CiteSpace software and Microsoft Excel were used to analyze and present the data, respectively.

Results

A total of 2,561 literatures were retrieved and 2,559 literatures were included in the analysis after the removal of duplicates. An irrefutable witness of the ever-growing interest in the topic of antimalarial drug resistance could be expressed by the exponentially increased number of publications and related citations from 2015 to 2022, and its sustained growth trend by 2023. During the past 7 years, USA, Oxford University, and David A Fidock are the country, institution, and author with the most publications in this field of research, respectively. We focused on the references and keywords from literature and found that the research and development of new drugs is the newest hotspot in this field. A growing number of scientists are devoted to finding new antimalarial drugs.

Conclusion

This study is the first visual metrological analysis of antimalarial drug resistance, using bibliometric methods. As a baseline information, it is important to analyze research output published globally on antimalarial drug resistance. In order to better understand the current research situation and future research plan agenda, such baseline data are needed accordingly.

Cloning and Functional Characterization of 2-C-methyl-D-erythritol-4-phosphate cytidylyltransferase (LiMCT) Gene in Oriental Lily (Lilium 'Sorbonne')

2-C-methyl-D-erythritol-phosphate cytidylyltransferase (MCT) is a key enzyme in the MEP pathway of monoterpene synthesis, catalyzing the generation of 4- (5'-pyrophosphate cytidine)-2-C-methyl-D-erythritol from 2-C-methyl-D-erythritol-4-phosphate. We used homologous cloning strategy to clone gene, LiMCT, in the MEP pathway that may be involved in the regulation of floral fragrance synthesis in the Lilium oriental hybrid 'Sorbonne.' The full-length ORF sequence was 837 bp, encoding 278 amino acids. Bioinformatics analysis showed that the relative molecular weight of LiMCT protein is 68.56 kD and the isoelectric point (pI) is 5.12. The expression pattern of LiMCT gene was found to be consistent with the accumulation sites and emission patterns of floral fragrance monoterpenes in transcriptome data (unpublished). Subcellular localization indicated that the LiMCT protein is located in chloroplasts, which is consistent with the location of MEP pathway genes functioning in plastids to produce isoprene precursors. Overexpression of LiMCT in Arabidopsis thaliana affected the expression levels of MEP and MVA pathway genes, suggesting that overexpression of the LiMCT in A. thaliana affected the metabolic flow of C5 precursors of two different terpene synthesis pathways. The expression of the monoterpene synthase AtTPS14 was elevated nearly fourfold in transgenic A. thaliana compared with the control, and the levels of carotenoids and chlorophylls, the end products of the MEP pathway, were significantly increased in the leaves at full bloom, indicating that LiMCT plays an important role in regulating monoterpene synthesis and in the synthesis of other isoprene-like precursors in transgenic A. thaliana flowers. However, the specific mechanism of LiMCT in promoting the accumulation of isoprene products of the MEP pathway and the biosynthesis of floral monoterpene volatile components needs further investigation.

Aptamer-Based Imaging of Polyisoprenoids in the Malaria Parasite

Dolichols are isoprenoid end-products of the mevalonate and 2C-methyl-D-erythritol-4-phosphate pathways. The synthesis of dolichols is initiated with the addition of several molecules of isopentenyl diphosphate to farnesyl diphosphate. This reaction is catalyzed by a cis-prenyltransferase and leads to the formation of polyprenyl diphosphate. Subsequent steps involve the dephosphorylation and reduction of the α-isoprene unit by a polyprenol reductase, resulting in the generation of dolichol. The size of the dolichol varies, depending on the number of isoprene units incorporated. In eukaryotes, dolichols are synthesized as a mixture of four or more different lengths. Their biosynthesis is predicted to occur in the endoplasmic reticulum, where dolichols play an essential role in protein glycosylation. In this study, we have developed a selection of aptamers targeting dolichols and enhanced their specificity by incorporating fatty acids for negative selection. One aptamer showed high enrichment and specificity for linear polyisoprenoids containing at least one oxygen atom, such as an alcohol or aldehyde, in the α-isoprene unit. The selected aptamer proved to be a valuable tool for the subcellular localization of polyisoprenoids in the malaria parasite. To the best of our knowledge, this is the first time that polyisoprenoids have been localized within a cell using aptamer-based imaging techniques.

New insights into apicoplast metabolism in blood-stage malaria parasites

The apicoplast of Plasmodium falciparum is the only source of essential isoprenoid precursors and Coenzyme A (CoA) in the parasite. Isoprenoid precursor synthesis relies on the iron-sulfur cluster (FeS) cofactors produced within the apicoplast, rendering FeS synthesis an essential function of this organelle. Recent reports provide important insights into the roles of FeS cofactors and the use of isoprenoid precursors and CoA both inside and outside the apicoplast. Here, we review the recent insights into the roles of these metabolites in blood-stage malaria parasites and discuss new questions that have been raised in light of these discoveries.

Characterization of Domiphen Bromide as a New Fast-Acting Antiplasmodial Agent Inhibiting the Apicoplastidic Methyl Erythritol Phosphate Pathway

The evolution of resistance by the malaria parasite to artemisinin, the key component of the combination therapy strategies that are at the core of current antimalarial treatments, calls for the urgent identification of new fast-acting antimalarials. The apicoplast organelle is a preferred target of antimalarial drugs because it contains biochemical processes absent from the human host. Fosmidomycin is the only drug in clinical trials targeting the apicoplast, where it inhibits the methyl erythritol phosphate (MEP) pathway. Here, we characterized the antiplasmodial activity of domiphen bromide (DB), another MEP pathway inhibitor with a rapid mode of action that arrests the in vitro growth of Plasmodium falciparum at the early trophozoite stage. Metabolomic analysis of the MEP pathway and Krebs cycle intermediates in 20 µM DB-treated parasites suggested a rapid activation of glycolysis with a concomitant decrease in mitochondrial activity, consistent with a rapid killing of the pathogen. These results present DB as a model compound for the development of new, potentially interesting drugs for future antimalarial combination therapies.

Enantiopure Benzofuran-2-carboxamides of 1-Aryltetrahydro-β-carbolines Are Potent Antimalarials In Vitro

The tetrahydro-β-carboline scaffold has proven fertile ground for the discovery of antimalarial agents (e.g., MMV008138 (1) and cipargamin (2)). Similarity searching of a publicly disclosed collection of antimalarial hits for molecules resembling 1 drew our attention to N2-acyl tetrahydro-β-carboline GNF-Pf-5009 ((±)-3b). Compound purchase, "analog by catalog", and independent synthesis of hits indicated the benzofuran-2-yl amide portion was required for in vitro efficacy against P. falciparum. Preparation of pure enantiomers demonstrated the pharmacological superiority of (R)-3b. Synthesis and evaluation of D- and F-ring substitution variants and benzofuran isosteres indicated a clear structure-activity relationship. Ultimately (R)-3b was tested in Plasmodium berghei-infected mice; unfavorable physicochemical properties may be responsible for the lack of oral efficacy.

Malaria Box-Inspired Discovery of N-Aminoalkyl-β-carboline-3-carboxamides, a Novel Orally Active Class of Antimalarials

Virtual ligand screening of a publicly available database of antimalarial hits using a pharmacophore derived from antimalarial MMV008138 identified TCMDC-140230, a tetrahydro-β-carboline amide, as worthy of exploration. All four stereoisomers of this structure were synthesized, but none potently inhibited growth of the malaria parasite Plasmodium falciparum. Interestingly, 7e, a minor byproduct of these syntheses, proved to be potent in vitro against P. falciparum and was orally efficacious (40 mg/kg) in an in vivo mouse model of malaria.

Antimalarial Natural Products

Natural products have made a crucial and unique contribution to human health, and this is especially true in the case of malaria, where the natural products quinine and artemisinin and their derivatives and analogues, have saved millions of lives. The need for new drugs to treat malaria is still urgent, since the most dangerous malaria parasite, Plasmodium falciparum, has become resistant to quinine and most of its derivatives and is becoming resistant to artemisinin and its derivatives. This volume begins with a short history of malaria and follows this with a summary of its biology. It then traces the fascinating history of the discovery of quinine for malaria treatment and then describes quinine's biosynthesis, its mechanism of action, and its clinical use, concluding with a discussion of synthetic antimalarial agents based on quinine's structure. The volume then covers the discovery of artemisinin and its development as the source of the most effective current antimalarial drug, including summaries of its synthesis and biosynthesis, its mechanism of action, and its clinical use and resistance. A short discussion of other clinically used antimalarial natural products leads to a detailed treatment of other natural products with significant antiplasmodial activity, classified by compound type. Although the search for new antimalarial natural products from Nature's combinatorial library is challenging, it is very likely to yield new antimalarial drugs. The chapter thus ends by identifying over ten natural products with development potential as clinical antimalarial agents.

Synthesis, Molecular Docking and Antiplasmodial Activities of New Tetrahydro-β-Carbolines

Malaria is still one of the most dangerous infectious diseases and the emergence of drug resistant parasites only worsens the situation. A series of new tetrahydro-β-carbolines were designed, synthesized by the Pictet–Spengler reaction, and characterized. Further, the compounds were screened for their in vitro antiplasmodial activity against chloroquine-sensitive (D10) and chloroquine-resistant (W2) strains of Plasmodium falciparum. Moreover, molecular modeling studies were performed to assess the potential action of the designed molecules and toxicity assays were conducted on the human microvascular endothelial (HMEC-1) cell line and human red blood cells. Our studies identified N-(3,3-dimethylbutyl)-1-octyl-2,3,4,9-tetrahydro-1H-pyrido[3,4-b] indole-3-carboxamide (7) (a mixture of diastereomers) as the most promising compound endowed with the highest antiplasmodial activity, highest selectivity, and lack of cytotoxicity. In silico simulations carried out for (1S,3R)-7 provided useful insights into its possible interactions with enzymes essential for parasite metabolism. Further studies are underway to develop the optimal nanosized lipid-based delivery system for this compound and to determine its precise mechanism of action.

Synthesis, Molecular Docking and Antiplasmodial Activities of New Tetrahydro-β-Carbolines

Malaria is still one of the most dangerous infectious diseases and the emergence of drug resistant parasites only worsens the situation. A series of new tetrahydro-β-carbolines were designed, synthesized by the Pictet-Spengler reaction, and characterized. Further, the compounds were screened for their in vitro antiplasmodial activity against chloroquine-sensitive (D10) and chloroquine-resistant (W2) strains of Plasmodium falciparum. Moreover, molecular modeling studies were performed to assess the potential action of the designed molecules and toxicity assays were conducted on the human microvascular endothelial (HMEC-1) cell line and human red blood cells. Our studies identified N-(3,3-dimethylbutyl)-1-octyl-2,3,4,9-tetrahydro-1H-pyrido[3,4-b] indole-3-carboxamide (7) (a mixture of diastereomers) as the most promising compound endowed with the highest antiplasmodial activity, highest selectivity, and lack of cytotoxicity. In silico simulations carried out for (1S,3R)-7 provided useful insights into its possible interactions with enzymes essential for parasite metabolism. Further studies are underway to develop the optimal nanosized lipid-based delivery system for this compound and to determine its precise mechanism of action.

A review of synthetic bioactive tetrahydro-β-carbolines: A medicinal chemistry perspective

1, 2, 3, 4-Tetrahydro-β-carboline (THβC) scaffold is widespread in many natural products (NPs) and synthetic compounds which show a variety of pharmacological activities. In this article, we reviewed the design, structures and biological characteristics of reported synthetic THβC compounds, and structure and activity relationship (SAR) of them were also discussed. This work might provide a reference for subsequent drug development based on THβC.

Antiplasmodial 2-thiophenoxy-3-trichloromethyl quinoxalines target the apicoplast of Plasmodium falciparum

The identification of a plant-like Achille's Heel relict, i.e. the apicoplast, that is essential for Plasmodium spp., the causative agent of malaria lead to an attractive drug target for new antimalarials with original mechanism of action. Although it is not photosynthetic, the apicoplast retains several anabolic pathways that are indispensable for the parasite. Based on previously identified antiplasmodial hit-molecules belonging to the 2-trichloromethylquinazoline and 3-trichloromethylquinoxaline series, we report herein an antiplasmodial Structure-Activity Relationships (SAR) study at position two of the quinoxaline ring of 16 newly synthesized compounds. Evaluation of their activity toward the multi-resistant K1 Plasmodium falciparum strain and cytotoxicity on the human hepatocyte HepG2 cell line revealed a hit compound (3k) with a PfK1 EC₅₀ value of 0.3 μM and a HepG2 CC₅₀ value of 56.0 μM (selectivity index = 175). Moreover, hit-compound 3k was not cytotoxic on VERO or CHO cell lines and was not genotoxic in the in vitro comet assay. Activity cliffs were observed when the trichloromethyl group was replaced by CH₃, CF₃ or H, showing that this group played a key role in the antiplasmodial activity. Biological investigations performed to determine the target and mechanism of action of the compound 3k strongly suggest that the apicoplast is the putative target as showed by severe alteration of apicoplaste biogenesis and delayed death response. Considering that there are very few molecules that affect the Plasmodium apicoplast, our work provides, for the first time, evidence of the biological target of trichloromethylated derivatives.

Recent Update on the Anti-infective Potential of β-carboline Analogs

β-Carboline, a naturally occurring indole alkaloid, holds a momentous spot in the field of medicinal chemistry due to its myriad of pharmacological actions like anticancer, antiviral, antibacterial, antifungal, antileishmanial, antimalarial, neuropharmacological, anti-inflammatory and antithrombotic among others. β-Carbolines exhibit their pharmacological activity via diverse mechanisms. This review provides a recent update (2015-2020) on the anti-infective potential of natural and synthetic β-carboline analogs focusing on its antibacterial, antifungal, antiviral, antimalarial, antileishmanial and antitrypanosomal properties. In cases where enough details are available, a note on its mechanism of action is also added.

Immuno-antibiotics: targeting microbial metabolic pathways sensed by unconventional T cells

Human Vγ9/Vδ2 T cells, mucosal-associated invariant T (MAIT) cells, and other unconventional T cells are specialised in detecting microbial metabolic pathway intermediates that are absent in humans. The recognition by such semi-invariant innate-like T cells of compounds like (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMB-PP), the penultimate metabolite in the MEP isoprenoid biosynthesis pathway, and intermediates of the riboflavin biosynthesis pathway and their metabolites allows the immune system to rapidly sense pathogen-associated molecular patterns that are shared by a wide range of micro-organisms. Given the essential nature of these metabolic pathways for microbial viability, they have emerged as promising targets for the development of novel antibiotics. Here, we review recent findings that link enzymatic inhibition of microbial metabolism with alterations in the levels of unconventional T cell ligands produced by treated micro-organisms that have given rise to the concept of 'immuno-antibiotics': combining direct antimicrobial activity with an immunotherapeutic effect via modulation of unconventional T cell responses.

Probing the B- & C-rings of the antimalarial tetrahydro-β-carboline MMV008138 for steric and conformational constraints

The antimalarial candidate MMV008138 (1a) is of particular interest because its target enzyme (IspD) is absent in human. To achieve higher potency, and to probe for steric demand, a series of analogs of 1a were prepared that featured methyl-substitution of the B- and C-rings, as well as ring-chain transformations. X-ray crystallography, NMR spectroscopy and calculation were used to study the effects of these modifications on the conformation of the C-ring and orientation of the D-ring. Unfortunately, all the B- and C-ring analogs explored lost in vitro antimalarial activity. The possible role of steric effects and conformational changes on target engagement are discussed.

Resistance to Some But Not Other Dimeric Lindenane Sesquiterpenoid Esters Is Mediated by Mutations in a Plasmodium falciparum Esterase

Unique lindenane sesquiterpenoid dimers from Chloranthecae spp. were recently identified with promising in vitro antiplasmodial activity and potentially novel mechanisms of action. To gain mechanistic insights to this new class of natural products, in vitro selection of Plasmodium falciparum resistance to the most active antiplasmodial compound, chlorajaponilide C, was explored. In all selected resistant clones, the half-maximal effective concentration (EC50) of chlorajaponilide C increased >250-fold, and whole genome sequencing revealed mutations in the recently discovered P. falciparum prodrug activation and resistance esterase (PfPARE). Chlorajaponilide C was highly potent (mean EC50 = 1.6 nM, n = 34) against fresh Ugandan P. falciparum isolates. The analysis of the structure-resistance relationships revealed that in vitro potency of a subset of lindenane sesquiterpenoid dimers was not mediated by PfPARE mutations. Thus, chlorajaponilide C, but not some related compounds, required parasite esterase activity for in vitro potency, and those compounds serve as the foundation for development of potent and selective antimalarials.

Metabolomics profiling reveals new aspects of dolichol biosynthesis in Plasmodium falciparum

The cis-polyisoprenoid lipids namely polyprenols, dolichols and their derivatives are linear polymers of several isoprene units. In eukaryotes, polyprenols and dolichols are synthesized as a mixture of four or more homologues of different length with one or two predominant species with sizes varying among organisms. Interestingly, co-occurrence of polyprenols and dolichols, i.e. detection of a dolichol along with significant levels of its precursor polyprenol, are unusual in eukaryotic cells. Our metabolomics studies revealed that cis-polyisoprenoids are more diverse in the malaria parasite Plasmodium falciparum than previously postulated as we uncovered active de novo biosynthesis and substantial levels of accumulation of polyprenols and dolichols of 15 to 19 isoprene units. A distinctive polyprenol and dolichol profile both within the intraerythrocytic asexual cycle and between asexual and gametocyte stages was observed suggesting that cis-polyisoprenoid biosynthesis changes throughout parasite’s development. Moreover, we confirmed the presence of an active cis-prenyltransferase (PfCPT) and that dolichol biosynthesis occurs via reduction of the polyprenol to dolichol by an active polyprenol reductase (PfPPRD) in the malaria parasite.

β-Carboline Derivatives Tackling Malaria: Biological Evaluation and Docking Analysis

Increasing resistance to presently available antimalarial drugs urges the need to look for new promising compounds. The β-carboline moiety, present in several biologically active natural products and drugs, is an important scaffold for antimalarial drug discovery. The present study explores the antimalarial activity of a β-carboline derivative (1R,3S)-methyl 1-(benzo[d][1,3]dioxol-5-yl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylate (9a) alone in vitro against Plasmodium falciparum and in vivo in combination therapy with the standard drug artesunate against Plasmodium berghei. Compound 9a inhibited both 3D7 and RKL-9 strains of P. falciparum with half-maximal inhibitory concentration (IC50) < 1 μg/mL, respectively. The compound was nontoxic (50% cytotoxic concentration (CC50) > 640 μg/mL) to normal dermal fibroblasts. Selectivity index was >10 against both the strains. The compound exhibited considerable in vivo antimalarial activity (median effective dose (ED50) = 27.74 mg/kg) in monotherapy. The combination of 9a (100 mg/kg) and artesunate (50 mg/kg) resulted in 99.69% chemosuppression on day 5 along with a mean survival time of 25.8 ± 4.91 days with complete parasite clearance. Biochemical studies indicated the safety of the HIT compound to hepatic and renal functions of mice. Molecular docking also highlighted the suitability of 9a as a potential antimalarial candidate.

Structure-based discovery of small molecule APC-Asef interaction inhibitors: In silico approaches and molecular dynamics simulations

Colorectal cancer, which is considered one of the leading causes of mortality worldwide, develops through the formation of benign polyps on the inner colon or rectum wall. Truncations in adenomatous polyposis coli (APC) gene lead to the spread of the disease in the entire colon region when combined with the guanine nucleotide exchange factor (GEF) Asef. A series of peptidomimetic agents were previously discovered as protein-protein interaction inhibitors that can target the APC-Asef interface. Structure-based virtual screening (SBVS), using a set of docking methods combined with molecular dynamics simulations, was carried out to identify new small drug-like agents. After the initial screening process, compounds with diverse chemical scaffolds and direct interaction with Arg549 and other active site residues were chosen and subjected to induce fit. The amide functional group found in the ligand hit structures showed strong interactions with Arg549, leading to observable conformational changes that allow suitable positioning within the peptide binding site. Furthermore, the pH-specific MD simulations of the top hit 838 within the APC-Asef binding site depicted significant interactions required for biochemical recognition in changing microenvironment. Predicted inhibitory constant (Ki) values and binding free energies of hits further described the significance of the amide group over the other chemical scaffolds. This combination of in silico approaches provides key insights for colorectal drug discovery programs targeting the APC-Asef interaction. Graphical abstract The common active site residues involved in interaction with ligands.

Inhibitory Effects of Fosmidomycin Against Babesia microti in vitro

Babesia microti, the main pathogen causing human babesiosis, has been reported to exhibit resistance to the traditional treatment of azithromycin + atovaquone and clindamycin + quinine, suggesting the necessity of developing new drugs. The methylerythritol 4-phosphate (MEP) pathway, a unique pathway in apicomplexan parasites, was shown to play a crucial function in the growth of Plasmodium falciparum. In the MEP pathway, 1-deoxy-D-xylulose 5-phosphate reductoisomerase (DXR) is a rate-limiting enzyme and fosmidomycin (FSM) is a reported inhibitor for this enzyme. DXR has been shown as an antimalarial drug target, but no report is available on B. microti DXR (BmDXR). Here BmDXR was cloned, sequenced, analyzed by bioinformatics, and evaluated as a potential drug target for inhibiting the growth of B. micorti in vitro. Drug assay was performed by adding different concentrations of FSM in B. microti in vitro culture. Rescue experiment was done by supplementing 200 μM isopentenyl pyrophosphate (IPP) or 5 μM geranylgeraniol (GG-ol) in the culture medium together with 5 μM FSM or 10 μM diminazene aceturate. The results indicated that FSM can inhibit the growth of B. microti in in vitro culture with an IC50 of 4.63 ± 0.12 μM, and growth can be restored by both IPP and GG-ol. Additionally, FSM is shown to inhibit the growth of parasites by suppressing the DXR activity, which agreed with the reported results of other apicomplexan parasites. Our results suggest the potential of DXR as a drug target for controlling B. microti and that FSM can inhibit the growth of B. microti in vitro.

Harmicines - harmine and cinnamic acid hybrids as novel antiplasmodial hits

Harmicines constitute novel hybrid compounds that combine two agents with reported antiplasmodial properties, namely β-carboline harmine and a cinnamic acid derivative (CAD). Cu(I) catalyzed azide-alkyne cycloaddition was employed for the preparation of three classes of hybrid molecules: N-harmicines 6a-i, O-harmicines 7a-i and N,O-bis-harmicines 8a-g,i. In vitro antiplasmodial activities of harmicines against the erythrocytic stage of Plasmodium falciparum (chloroquine-sensitive Pf3D7 and chloroquine-resistant PfDd2 strains) and hepatic stage of P. berghei, as well as cytotoxicity against human liver hepatocellular carcinoma cell line (HepG2), were evaluated. Remarkably, most of the compounds exerted significant activities against both stages of the Plasmodium life cycle. The conjugation of various CADs to harmine resulted in the increased antiplasmodial activity relative to harmine. In general, O-harmicines 7 exhibited the highest activity against the erythrocytic stage of both P. falciparum strains, whereas N,O-bis harmicines 8 showed the most pronounced activity against P. berghei hepatic stages. For the latter compound, molecular dynamics simulations confirmed binding within the ATP binding site of PfHsp90, while the weaker binders, namely 6b and harmine, were found to be positioned away from this structural element. In addition, decomposition of the computed binding free energies into contributions from individual residues suggested guidelines for further derivatization of harmine towards more efficient compounds. Cytotoxicity screening revealed N-harmicines 6 as the least, and O-harmicines 7 as the most toxic compounds. Harmicines 6g, 8b and 6d exerted the most selective action towards Plasmodium over human cells, respectively. These results establish harmicines as hits for future optimisation and development of novel antiplasmodial agents.

Metabolic dependency of chorismate in Plasmodium falciparum suggests an alternative source for the ubiquinone biosynthesis precursor

The shikimate pathway, a metabolic pathway absent in humans, is responsible for the production of chorismate, a branch point metabolite. In the malaria parasite, chorismate is postulated to be a direct precursor in the synthesis of p-aminobenzoic acid (folate biosynthesis), p-hydroxybenzoic acid (ubiquinone biosynthesis), menaquinone, and aromatic amino acids. While the potential value of the shikimate pathway as a drug target is debatable, the metabolic dependency of chorismate in P. falciparum remains unclear. Current evidence suggests that the main role of chorismate is folate biosynthesis despite ubiquinone biosynthesis being active and essential in the malaria parasite. Our goal in the present work was to expand our knowledge of the ubiquinone head group biosynthesis and its potential metabolic dependency on chorismate in P. falciparum. We systematically assessed the development of both asexual and sexual stages of P. falciparum in a defined medium in the absence of an exogenous supply of chorismate end-products and present biochemical evidence suggesting that the benzoquinone ring of ubiquinones in this parasite may be synthesized through a yet unidentified route.

Targeting the apicoplast in malaria

Malaria continues to be one of the leading causes of human mortality in the world, and the therapies available are insufficient for eradication. Severe malaria is caused by the apicomplexan parasite Plasmodium falciparum Apicomplexan parasites, including the Plasmodium spp., are descendants of photosynthetic algae, and therefore they possess an essential plastid organelle, named the apicoplast. Since humans and animals have no plastids, the apicoplast is an attractive target for drug development. Indeed, after its discovery, the apicoplast was found to host the target pathways of some known antimalarial drugs, which motivated efforts for further research into its biological functions and biogenesis. Initially, many apicoplast inhibitions were found to result in 'delayed death', whereby parasite killing is seen only at the end of one invasion-egress cycle. This slow action is not in line with the current standard for antimalarials, which seeded scepticism about the potential of compounds targeting apicoplast functions as good candidates for drug development. Intriguingly, recent evidence of apicoplast inhibitors causing rapid killing could put this organelle back in the spotlight. We provide an overview of drugs known to inhibit apicoplast pathways, alongside recent findings in apicoplast biology that may provide new avenues for drug development.

An ImmunoPEGliposome for Targeted Antimalarial Combination Therapy at the Nanoscale

Combination therapies, where two drugs acting through different mechanisms are administered simultaneously, are one of the most efficient approaches currently used to treat malaria infections. However, the different pharmacokinetic profiles often exhibited by the combined drugs tend to decrease treatment efficacy as the compounds are usually eliminated from the circulation at different rates. To circumvent this obstacle, we have engineered an immunoliposomal nanovector encapsulating hydrophilic and lipophilic compounds in its lumen and lipid bilayer, respectively. The antimalarial domiphen bromide has been encapsulated in the liposome membrane with good efficiency, although its high IC₅₀ of ca. 1 µM for living parasites complicates its use as immunoliposomal therapy due to erythrocyte agglutination. The conjugation of antibodies against glycophorin A targeted the nanocarriers to Plasmodium-infected red blood cells and to gametocytes, the sole malaria parasite stage responsible for the transmission from the human to the mosquito vector. The antimalarials pyronaridine and atovaquone, which block the development of gametocytes, have been co-encapsulated in glycophorin A-targeted immunoliposomes. The co-immunoliposomized drugs have activities significantly higher than their free forms when tested in in vitro Plasmodium falciparum cultures: Pyronaridine and atovaquone concentrations that, when encapsulated in immunoliposomes, resulted in a 50% inhibition of parasite growth had no effect on the viability of the pathogen when used as free drugs.

Questioning the γ-gauche effect: stereoassignment of 1,3-disubstituted-tetrahydro-β-carbolines using 1H-1H coupling constants

The Pictet-Spengler reaction of tryptophan esters and aldehydes has been widely applied in natural product synthesis and medicinal chemistry. To date, the trans- or cis-configuration of 1,3-disubstituted tetrahydro-β-carbolines (THβCs) formed in this reaction has most often been assigned based on the relative ¹³C chemical shifts of C1 and C3 in the diastereomers. Although the upfield shifts of C1 and C3 in trans-THβCs relative to cis-THβCs has been attributed to steric compression associated with the "γ-gauche" effect, we show that this effect is not borne out experimentally for other carbons that should suffer this same compression. Thus we developed a robust alternative method for stereochemical assignment based on ¹H NMR coupling constants (31 examples) and supported by extensive DFT-based conformational analysis and calculation of ¹H-¹H coupling constants. DFT calculations of ¹³C NMR chemical shifts also cast doubt upon the role of the "γ-gauche" effect on C1 and C3 chemical shifts in trans-THβCs.

Synthesis and Evaluation of Antiplasmodial Efficacy of β-Carboline Derivatives against Murine Malaria

The difficulty of developing an efficient malaria vaccine along with increasing spread of multidrug resistant strain of Plasmodium falciparum to the available antimalarial drugs poses the need to discover safe and efficacious antimalarial drugs to control malaria. An alternative strategy is to synthesize compounds possessing structures similar to the active natural products or marketed drugs. Several biologically active natural products and drugs contain β-carboline moiety. In the present study, few selected β-carboline derivatives have been synthesized and tested for their in vitro and in vivo antiplasmodial activity against the rodent malaria parasite Plasmodium berghei (NK-65). The designed analogs exhibited considerable in vitro antimalarial activity. Two compounds (1R,3S)-methyl 1-(benzo[d][1,3]dioxol-5-yl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylate (9a) and (1R,3S)-methyl 1-(pyridin-3-yl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylate (9b) were further selected for in vivo studies. Both the lead compounds (9a and 9b) were observed to be safe for oral administration. The therapeutic effective dose (ED50) for 9a and 9b were determined and in the animal model, 9a (at 50 mg/kg dose) exhibited better activity in terms of parasite clearance and enhancement of host survival. Biochemical investigations also point toward the safety of the compound to the hepatic and renal functions of the rodent host. Further studies are underway to explore its activity alone as well as in combination therapy with artesunate against the human malaria parasite P. falciparum.

Deciphering the role of IspD (2‑C‑methyl‑D‑erythritol 4‑phosphate cytidyltransferase) enzyme as a potential therapeutic drug target against Plasmodium vivax

Biosynthesis of isoprenoids (MEP Pathway) in apicoplast has an important role during the erythrocytic stages of Plasmodium, as it is the sole pathway to provide the major isoprene units required as metabolic precursor for various housekeeping activities. With the intensifying need to identify a novel therapeutic drug target against Plasmodium, the MEP pathway and its components are considered as potential therapeutic targets, due to the difference in the isoprenoid synthesis route (MVA) functional in the host cells. While few major components have already been studied from this pathway for their potential as a drug target, IspD (2-C-methyl-D-erythritol-4-phosphate cytidyltransferase) enzyme, the enzyme catalyzing the third step of the pathway has only been tested against a synthetic compound from Malaria box called MMV008138, which also has not shown adequate inhibitory activity against P. vivax IspD. In the present study, to validate the potential of PvIspD as a drug target, various antimicrobial agents were screened for their inhibition possibilities, using in-vitro High Throughput Screening (HTS) technique. Shortlisted antimicrobial drug molecules like Cefepime, Tunicamycin and Rifampicin were further validated by in-vitro biochemical enzyme inhibition assays where they showed activity at nanomolar concentrations suggesting them or their derivatives as prospective future antimalarials. This study also confirmed the in-vivo expression of PvIspD protein during asexual stages by sub-cellular localization in apicoplast and explores the importance of the IspD enzyme in the development of new therapeutics.