Active Natural Product Scaffolds against Trypanosomatid Parasites: A Review

Anti-protozoal activity and metabolomic analyses of Cichorium intybus L. against Trypanosoma cruzi

Chagas disease, caused by the protozoa Trypanosoma cruzi, is a potentially life-threatening parasitic zoonosis infecting 6-7 million people worldwide, mainly in Latin America. Due to the limited numbers of drugs available against this neglected disease and their frequent adverse effects, novel anti-chagasic agents are urgently needed. Cichorium intybus L. (chicory) is a bioactive plant with potent activity against parasitic nematodes, but its effects on protozoans are poorly known and no studies have explored its trypanocidal potential. Here, we investigated the activity of C. intybus against extracellular and intracellular stages of T. cruzi, including the prediction of trypanocidal compounds by metabolomic analyses and bioactivity-based molecular networking. Purified C. intybus extracts were prepared from leaves and roots of five C. intybus cultivars (cv. 'Benulite', 'Goldine', 'Larigot', 'Maestoso' and 'Spadona'). All C. intybus extracts induced concentration-dependent effects against T. cruzi trypomastigotes. C. intybus leaf extracts had higher trypanocidal selectivity and lower cytotoxicity on mammalian cells than root extracts. The leaf extract of C. intybus cv. Goldine also significantly reduced the number of mammalian cells infected with T. cruzi amastigotes. Metabolomic and bioactivity-based molecular networking analyses revealed 11 compounds in C. intybus leaves strongly linked with activity against trypomastigotes, including the sesquiterpene lactone lactucin, and flavonoid- and fatty acid-derivatives. Furthermore, seven distinct C. intybus molecules (including two sesquiterpene lactone-derivatives) were predicted to be involved in reducing the number of mammalian cells infected with amastigotes. This is the first report of the anti-protozoal activity of C. intybus against trypanosomatid parasites and expands our understanding of the anti-parasitic effects of this plant and its bioactive metabolites. Further studies to elucidate the anti-protozoal compound(s) in C. intybus and their mode(s) of action will improve our knowledge of using this bioactive plant as a promising source of novel broad-spectrum anti-parasitic compounds with associated health benefits and biomedical potential.

Steroidal Antimetabolites Protect Mice against Trypanosoma brucei

Trypanosoma brucei, the causative agent for human African trypanosomiasis, is an emerging ergosterol-dependent parasite that produces chokepoint enzymes, sterol methyltransferases (SMT), not synthesized in their animal hosts that can regulate cell viability. Here, we report the lethal effects of two recently described natural product antimetabolites that disrupt Acanthamoeba sterol methylation and growth, cholesta-5,7,22,24-tetraenol (CHT) and ergosta-5,7,22,24(28)-tetraenol (ERGT) that can equally target T. brucei. We found that CHT/ERGT inhibited cell growth in vitro, yielding EC₅₀ values in the low nanomolar range with washout experiments showing cidal activity against the bloodstream form, consistent with their predicted mode of suicide inhibition on SMT activity and ergosterol production. Antimetabolite treatment generated altered T. brucei cell morphology and death rapidly within hours. Notably, in vivo ERGT/CHT protected mice infected with T. brucei, doubling their survival time following daily treatment for 8-10 days at 50 mg/kg or 100 mg/kg. The current study demonstrates a new class of lead antibiotics, in the form of common fungal sterols, for antitrypanosomal drug development.

Stereocontrolled Total Synthesis of Bastimolide B Using Iterative Homologation of Boronic Esters

Bastimolide B is a polyhydroxy macrolide isolated from marine cyanobacteria displaying antimalarial activity. It features a dense array of hydroxylated stereogenic centers with 1,5-relationships along a hydrocarbon chain. These 1,5-polyols represent a particularly challenging motif for synthesis, as the remote position of the stereocenters hampers stereocontrol. Herein, we present a strategy for 1,5-polyol stereocontrolled synthesis based on iterative boronic ester homologation with enantiopure magnesium carbenoids. By merging boronic ester homologation and transition-metal-catalyzed alkene hydroboration and diboration, the acyclic backbone of bastimolide B was rapidly assembled from readily available building blocks with full control over the remote stereocenters, enabling the total synthesis to be completed in 16 steps (LLS).

Plant Terpenoids as Hit Compounds against Trypanosomiasis

Human African trypanosomiasis (sleeping sickness) and American trypanosomiasis (Chagas disease) are vector-borne neglected tropical diseases, caused by the protozoan parasites Trypanosoma brucei and Trypanosoma cruzi, respectively. These diseases were circumscribed to South American and African countries in the past. However, human migration, military interventions, and climate changes have had an important effect on their worldwide propagation, particularly Chagas disease. Currently, the treatment of trypanosomiasis is not ideal, becoming a challenge in poor populations with limited resources. Exploring natural products from higher plants remains a valuable approach to find new hits and enlarge the pipeline of new drugs against protozoal human infections. This review covers the recent studies (2016–2021) on plant terpenoids, and their semi-synthetic derivatives, which have shown promising in vitro and in vivo activities against Trypanosoma parasites.

Convenient Synthesis of Alternatively Bridged Tryptophan Ketopiperazines and their Activities Against Trypanosomatid Parasites

There is an urgent need for the development of new treatments against trypanosomatid parasites; the causative agents of some of the most debilitating diseases in the developing world. This work targets an interesting 6-5-6-6 fused carboline scaffold, accessing a range of substituted derivatives through stereospecific intramolecular Pictet-Spengler condensation. Modification of the cyclisation conditions allowed retention of the carbamate protecting group and gave insight into the reaction mechanism. Compounds' bioactivities were measured against T. brucei, T. cruzi, L. majorand HeLa cells. We have identified promising pan-trypanocidal lead compounds based on the core scaffold, and highlight key SAR trends which will be useful for the future development of these compounds as potent trypanocidal agents.

Marine alkaloids as bioactive agents against protozoal neglected tropical diseases and malaria

Covering: 2000 up to 2021Natural products are an important resource in drug discovery, directly or indirectly delivering numerous small molecules for potential development as human medicines. Among the many classes of natural products, alkaloids have a rich history of therapeutic applications. The extensive chemodiversity of alkaloids found in the marine environment has attracted considerable attention for such uses, while the scarcity of these natural materials has stimulated efforts towards their total synthesis. This review focuses on the biological activity of marine alkaloids (covering 2000 to up to 2021) towards Neglected Tropical Diseases (NTDs) caused by protozoan parasites, and malaria. Chemotherapy represents the only form of treatment for Chagas disease, human African trypanosomiasis, leishmaniasis and malaria, but there is currently a restricted arsenal of drugs, which often elicit severe adverse effects, show variable efficacy or resistance, or are costly. Natural product scaffolds have re-emerged as a focus of academic drug discovery programmes, offering a different resource to discover new chemical entities with new modes of action. In this review, the potential of a range of marine alkaloids is analyzed, accompanied by coverage of synthetic efforts that enable further studies of key antiprotozoal natural product scaffolds.

Natural-Product-Based Solutions for Tropical Infectious Diseases

About half of the world's population and 80% of the world's biodiversity can be found in the tropics. Many diseases are specific to the tropics, with at least 41 diseases caused by endemic bacteria, viruses, parasites, and fungi. Such diseases are of increasing concern, as the geographic range of tropical diseases is expanding due to climate change, urbanization, change in agricultural practices, deforestation, and loss of biodiversity. While traditional medicines have been used for centuries in the treatment of tropical diseases, the active natural compounds within these medicines remain largely unknown. In this review, we describe infectious diseases specific to the tropics, including their causative pathogens, modes of transmission, recent major outbreaks, and geographic locations. We further review current treatments for these tropical diseases, carefully consider the biodiscovery potential of the tropical biome, and discuss a range of technologies being used for drug development from natural resources. We provide a list of natural products with antimicrobial activity, detailing the source organisms and their effectiveness as treatment. We discuss how technological advancements, such as next-generation sequencing, are driving high-throughput natural product screening pipelines to identify compounds with therapeutic properties. This review demonstrates the impact natural products from the vast tropical biome have in the treatment of tropical infectious diseases and how high-throughput technical capacity will accelerate this discovery process.

A comprehensive survey upon diverse and prolific applications of chitosan-based catalytic systems in one-pot multi-component synthesis of heterocyclic rings

Heterocyclic compounds are among the most prestigious and valuable chemical molecules with diverse and magnificent applications in various sciences. Due to the remarkable and numerous properties of the heterocyclic frameworks, the development of efficient and convenient synthetic methods for the preparation of such outstanding compounds is of great importance. Undoubtedly, catalysis has a conspicuous role in modern chemical synthesis and green chemistry. Therefore, when designing a chemical reaction, choosing and or preparing powerful and environmentally benign simple catalysts or complicated catalytic systems for an acceleration of the chemical reaction is a pivotal part of work for synthetic chemists. Chitosan, as a biocompatible and biodegradable pseudo-natural polysaccharide is one of the excellent choices for the preparation of suitable catalytic systems due to its unique properties. In this review paper, every effort has been made to cover all research articles in the field of one-pot synthesis of heterocyclic frameworks in the presence of chitosan-based catalytic systems, which were published roughly by the first quarter of 2020. It is hoped that this review paper can be a little help to synthetic scientists, methodologists, and catalyst designers, both on the laboratory and industrial scales.

Synthesis and evaluation of trypanocidal activity of derivatives of naturally occurring 2,5-diphenyloxazoles

African trypanosomiasis is a zoonotic protozoan disease affecting the nervous system. Various natural products reportedly exhibit trypanocidal activity. Naturally occurring 2,5-diphenyloxazoles present in Oxytropis lanata, and their derivatives, were synthesized. The trypanocidal activities of the synthesized compounds were evaluated against Trypanosoma brucei brucei, T. b. gambiense, T. b. rhodesiense, T. congolense, and T. evansi. Natural product 1 exhibited trypanocidal activity against all the species/subspecies of trypanosomes, exhibiting half-maximal inhibitory concentrations (IC₅₀) of 1.1-13.5 μM. Modification of the oxazole core improved the trypanocidal activity. The 1,3,4-oxadiazole (7) and 2,4-diphenyloxazole (9) analogs exhibited potency superior to that of 1. However, these compounds exhibited cytotoxicity in Madin-Darby bovine kidney cells. The O-methylated analog of 1 (12) was non-cytotoxic and exhibited selective trypanocidal activity against T. congolense (IC₅₀ = 0.78 µM). Structure-activity relationship studies of the 2,5-diphenyloxazole analogs revealed aspects of the molecular structure critical for maintaining selective trypanocidal activity against T. congolense.

Temperate Zone Plant Natural Products-A Novel Resource for Activity against Tropical Parasitic Diseases

The use of plant-derived natural products for the treatment of tropical parasitic diseases often has ethnopharmacological origins. As such, plants grown in temperate regions remain largely untested for novel anti-parasitic activities. We describe here a screen of the PhytoQuest Phytopure library, a novel source comprising over 600 purified compounds from temperate zone plants, against in vitro culture systems for Plasmodium falciparum, Leishmania mexicana, Trypanosoma evansi and T. brucei. Initial screen revealed 6, 65, 15 and 18 compounds, respectively, that decreased each parasite's growth by at least 50% at 1-2 µM concentration. These initial hits were validated in concentration-response assays against the parasite and the human HepG2 cell line, identifying hits with EC50 < 1 μM and a selectivity index of >10. Two sesquiterpene glycosides were identified against P. falciparum, four sterols against L. mexicana, and five compounds of various scaffolds against T. brucei and T. evansi. An L. mexicana resistant line was generated for the sterol 700022, which was found to have cross-resistance to the anti-leishmanial drug miltefosine as well as to the other leishmanicidal sterols. This study highlights the potential of a temperate plant secondary metabolites as a novel source of natural products against tropical parasitic diseases.

Total synthesis of the actinoallolides and a designed photoaffinity probe for target identification

The actinoallolides are a family of polyketide natural products isolated from the bacterium Actinoallomurus fulvus. They show potent biological activity against trypanosomes, the causative agents of the neglected tropical diseases human African trypanosomiasis (sleeping sickness) and Chagas disease, while exhibiting no cytotoxicity against human cell lines. Herein, we give a full account of our strategy evolution towards the synthesis of this structurally unique class of 12-membered macrolides, which culminated in the first total synthesis of (+)-actinoallolide A in 20 steps and 8% overall yield. Subsequent late-stage diversification then provided ready access to the congeneric (+)-actinoallolides B-E. Enabled by this flexible and efficient endgame sequence, we also describe the design and synthesis of a photoaffinity probe based on actinoallolide A to investigate its biological mode of action. This will allow ongoing labelling studies to identify their protein binding target(s).

Isolation and evaluation of trypanocidal activity of sesquiterpenoids, flavonoids, and lignans in Artemisia sieversiana collected in Mongolia

Artemisia sieversiana is an annual herbaceous plant distributed throughout Central and East Eurasia and is regarded as an undesirable forage plant in Mongolia. It affects livestock, so information about its chemical composition is needed. We isolated three new sesquiterpenoids (1-3) and known compounds from A. sieversiana and investigated their activities. The absolute configuration of 1 was established using single-crystal X-ray diffraction crystallography, and its configuration differed from those of reported compounds with similar structures. Two additional new sesquiterpenoids (2 and 3) with similar structures were identified, and their configurations were determined. The trypanocidal activities of the isolated compounds (1-18) against Trypanosoma congolense and the pathogen responsible for fatal trypanosomosis in animals were estimated. Flavonoids and lignans were identified as active compounds with IC₅₀ values ranging from 2.9 to 90.2 µM.

An investigation of the antileishmanial properties of semi-synthetic saponins

Leishmaniasis is a neglected tropical disease caused by insect-vector borne protozoan parasites of the, Leishmania species. Whilst infection threatens and affects millions of the global poor, vaccines are absent and drug therapy limited. Extensive efforts have recently been made to discover new leads from small molecule synthetic compound libraries held by industry; however, the number of new chemical entities identified and entering development as anti-leishmanials has been very low. This has led to increased interest in the possibility of discovering naturally derived compounds with potent antileishmanial activity which may be developed towards clinical applications. Plant-derived triterpenoid and steroidal saponins have long been considered as anti-microbials and here we describe an investigation of a library of 137 natural (9) and semi-synthetic saponins (128) for activity against Leishmania mexicana, a causative agent of cutaneous leishmaniasis. The triterpenoid sapogenin, hederagenin, readily obtained in large quantities from Hedera helix (common ivy), was converted into a range of 128 derivatives. These semi-synthetic compounds, as well as saponins isolated from ivy, were examined with a phenotypic screening approach to identify potent and selective anti-leishmanial hits. This led to the identification of 12 compounds, including the natural saponin gypsogenin, demonstrating high potency (ED50 < 10.5 μM) against axenic L. mexicana amastigotes, the mammalian pathogenic form. One of these, hederagenin disuccinate, was sufficiently non-toxic to the macrophage host cell to facilitate further analyses, selectivity index (SI) > 10. Whilst this was not active in an infected cell model, the anti-leishmanial properties of hederagenin-derivatives have been demonstrated, and the possibility of improving the selectivity of natural hederagenin through chemical modification has been established.

Hyaluronidase inhibitory saponins and a trypanocidal isoflavonoid from the aerial parts of Oxytropis lanata

A chemical examination of an extract from the aerial part of Oxytropis lanata led to the isolation and identification of 36 compounds, including saponins, isoflavonoids, oxazoles, and glycosides. The three among them were previously unreported oleanane-type saponins. In trypanocidal screening, 5,7,4'-trihydroxyisoflavone showed inhibitory activity against Trypanosoma congolense (IC₅₀ = 10.5 μM), the causative agent of African trypanosomosis in animals; this activity was similar to that of active compounds from the roots of this plant. O. lanata is known to be a traditional medicinal plant in Mongolia for the treatment of inflammatory diseases. The anti-hyaluronidase effect of saponins 3, 5, 8, and 9, (IC₅₀ = 0.15-0.22 mM) was stronger than that of sodium cromoglicate, which was used as a reference drug (IC₅₀ = 0.37 mM). The chemical structures of the new saponins were determined based on HRFABMS, ¹H and ¹³C NMR, ¹H-¹H COSY, HMQC, HMBC, and ROESY spectroscopic data along with chemical procedures.

Synthesis and evaluation of novel 2,4-disubstituted arylthiazoles against T. brucei

2-{2-[3-(1-Adamantyl)-4-fluorophenyl]thiazol-4-yl}ethan-1-amine (1a) and 2-{2-[4-(1-adamantyl)phenyl]thiazol-4-yl}ethan-1-amine (2a) exhibit activity against T. brucei in the range of IC₅₀ = 0.42 μM and IC₅₀ = 0.80 μM, respectively.

The design, synthesis and pharmacological evaluation of the 4-substituted-2-[3-(adamant-1-yl)-4-fluorophenyl]thiazoles 1a–j, the 4-substituted-2-[4-(adamant-1-yl)phenyl]thiazoles 2a–h, the 2-substituted-4-[4-(adamant-1-yl)phenyl]thiazoles 3a–e, the N-substituted 2-phenylthiazol-4-ethylamides 4a, b and the N-substituted 4-phenylthiazol-2-ethylamides 4c, d is described. Compounds 1a and 2a exhibit trypanocidal activity in the range of IC₅₀ = 0.42 μM and IC₅₀ = 0.80 μM, respectively. Both of these derivatives bear a lipophilic end, which consists of a 4-(1-adamantyl) phenyl or a 3-(1-adamantyl)phenyl moiety, a 1,3-thiazole ring and a functional end, which comprises of an alkylamine and can be considered as promising candidates for the treatment of Trypanosoma brucei infections.

Trypanocidal Mechanism of Action and in silico Studies of p-Coumaric Acid Derivatives

Trypanosoma species are responsible for chronic and systemic infections in millions of people around the world, compromising life quality, and family and government budgets. This group of diseases is classified as neglected and causes thousands of deaths each year. In the present study, the trypanocidal effect of a set of 12 ester derivatives of the p-coumaric acid was tested. Of the test derivatives, pentyl p-coumarate (7) (5.16 ± 1.28 μM; 61.63 ± 28.59 μM) presented the best respective trypanocidal activities against both epimastigote and trypomastigote forms. Flow cytometry analysis revealed an increase in the percentage of 7-AAD labeled cells, an increase in reactive oxygen species, and a loss of mitochondrial membrane potential; indicating cell death by necrosis. This mechanism was confirmed by scanning electron microscopy, noting the loss of cellular integrity. Molecular docking data indicated that of the chemical compounds tested, compound 7 potentially acts through two mechanisms of action, whether by links with aldo-keto reductases (AKR) or by comprising cruzain (CZ) which is one of the key Trypanosoma cruzi development enzymes. The results indicate that for both enzymes, van der Waals interactions between ligand and receptors favor binding and hydrophobic interactions with the phenolic and aliphatic parts of the ligand. The study demonstrates that p-coumarate derivatives are promising molecules for developing new prototypes with antiprotozoal activity.

Synthesis of quinones with highlighted biological applications: A critical update on the strategies towards bioactive compounds with emphasis on lapachones

Naphthoquinones are of key importance in organic synthesis and medicinal chemistry. In the last few years, various synthetic routes have been developed to prepare bioactive compounds derived or based on lapachones. In this sense, this review is mainly focused on the synthetic aspects and strategies used for the design of these compounds on the basis of their biological activities for the development of drugs against the neglected diseases leishmaniases and Chagas disease and also cancer. Three strategies used to develop bioactive quinones are discussed and categorized: (i) C-ring modification, (ii) redox centre modification and (iii) A-ring modification. Framed within these strategies for the development of naphthoquinoidal compounds against T. cruzi. Leishmania and cancer, reactions including copper-catalyzed azide-alkyne cycloaddition (click chemistry), palladium-catalysed cross couplings, C-H activation reactions, Ullmann couplings and heterocyclisations reported up to July 2019 will be discussed. The aim of derivatisation is the generation of novel molecules that can potentially inhibit cellular organelles/processes, generate reactive oxygen species and increase lipophilicity to enhance penetration through the plasma membrane. Modified lapachones have emerged as promising prototypes for the development of drugs against leishmaniases, Chagas disease and cancer.

Mining for natural product antileishmanials in a fungal extract library

Leishmaniasis is a Neglected Tropical Disease caused by the insect-vector borne protozoan parasite, Leishmania species. Infection affects millions of the World's poorest, however vaccines are absent and drug therapy limited. Recently, public-private partnerships have developed to identify new modes of controlling leishmaniasis. Most of these collaborative efforts have relied upon the small molecule synthetic compound libraries held by industry, but the number of New Chemical Entities (NCE) identified and entering development as antileishmanials has been very low. In light of this, here we describe a public-private effort to identify natural products with activity against Leishmania mexicana, a causative agent of cutaneous leishmanaisis (CL). Utilising Hypha Discovery's fungal extract library which is rich in small molecule (<500 molecular weight) secondary metabolites, we undertook an iterative phenotypic screening and fractionation approach to identify potent and selective antileishmanial hits. This led to the identification of a novel oxidised bisabolane sesquiterpene which demonstrated activity in an infected cell model and was shown to disrupt multiple processes using a metabolomic approach. In addition, and importantly, this study also sets a precedent for new approaches for CL drug discovery.

New Leishmania donovani nucleoside hydrolase inhibitors from Brazilian flora

This study presents new inhibitors of the nucleoside hydrolase from Leishmania donovani (LdNH) with in vitro leishmanicidal activity. Biological screening of 214 Brazilian plant extracts was performed to select plants with enzyme inhibitory activity. Two plants were selected for their results, and for their lack of prior phytochemical description: Leandra amplexicaulis DC. (Melastomataceae) and Urvillea rufescens Cambess (Sapindaceae). Three flavonoids were isolated by bioguided fractionation of the hydroethanolic extracts: kaempferol 3-O-α-l-rhamnopyranoside (1) and kaempferol 3-O-β-d-xylopyranosyl-(1→2)-α-l-rhamnopyranoside (2) from L. amplexicaulis, as well as tricetin-4′-O-methyl flavone (3) from U. rufescens. These flavonoids showed inhibitory activities (IC₅₀) of 197.4 μM (1), 74.7 μM (2) and 1.1 μM (3) on the LdNH. Their binding mode was proposed based on molecular docking with LdNH and by NMR Saturation Transfer Difference studies. Kinetic studies demonstrate that the most potent inhibitor (3) acts by uncompetitive inhibition. This study reports for the first time the inhibition of LdNH by naturally sourced flavonoids.

This study presents flavonoids as new inhibitors of the nucleoside hydrolase from Leishmania donovani (LdNH) and the first uncompetitive inhibitor described for LdNH.