Clerodane diterpenes from Polyalthia longifolia (Sonn) Thw. var. pendula: Potential antimalarial agents for drug resistant Plasmodium falciparum infection

Anticancer diterpenes of African natural products: Mechanistic pathways and preclinical developments

BACKGROUND

The African continent is home to five biodiversity hotspots, boasting an immense wealth of medicinal flora, fungi and marine life. Diterpenes extracted from such natural products have compelling cytotoxic activities that warrant further exploration for the drug market, particularly in cancer therapy, where mortality rates remain elevated worldwide.

PURPOSE

To demonstrate the potential of African natural products on the global stage for cancer therapy development and provide an in-depth analysis of the current literature on the activity of cancer cytotoxic diterpenes from African natural sources (to our knowledge, the first of its kind); not only to reveal the most promising candidates for clinical development, but to demonstrate the importance of preserving the threatened ecosystems of Africa.

METHODS

A comprehensive search by means of the PRISMA strategy was conducted using electronic databases, namely Web of Science, PubMed, Google Scholar and ScienceDirect. The search terms employed were 'diterpene & mechanism & cancer' and 'diterpene & clinical & cancer'. The selection process involved assessing titles in English, Portuguese and Spanish, adhering to predefined eligibility criteria. The timeframe for inclusion spanned from 2010 to 2023, resulting in 218 relevant papers. Chemical structures were visualized using ChemDraw 21.0, PubChem was utilized to search for CID numbers.

RESULTS

Despite being one of the richest biodiverse zones in the world, African natural products are proportionally underreported compared to Asian countries or otherwise. The diterpenes andrographolide (Andrographis paniculata), forskolin (Coleus forskohlii), ent-kauranes from Isodon spp., euphosorophane A (Euphorbia sororia), cafestol & kahweol (Coffea spp.), macrocylic jolkinol D derivatives (Euphorbia piscatoria) and cyathane erinacine A (Hericium erinaceus) illustrated the most encouraging data for further cancer therapy exploration and development.

CONCLUSIONS

Diterpenes from African natural products have the potential to be economically significant active pharmaceutical and medicinal ingredients, specifically focussed on anticancer therapeutics.

Design, synthesis and evaluation of novel pyrrole-hydroxybutenolide hybrids as promising antiplasmodial and anti-inflammatory agents

In the pursuance of novel scaffolds with promising antiplasmodial and anti-inflammatory activity, a series of twenty-one compounds embraced with most promising penta-substituted pyrrole and biodynamic hydroxybutenolide in single skeleton was designed and synthesized. These pyrrole-hydroxybutenolide hybrids were evaluated against Plasmodium falciparum parasite. Four hybrids 5b, 5d, 5t and 5u exhibited good activity with IC₅₀ of 0.60, 0.88, 0.97 and 0.96 μM for chloroquine sensitive (Pf3D7) strain and 3.92, 4.31, 4.21 and 1.67 μM for chloroquine resistant (PfK1) strain, respectively. In vivo efficacy of 5b, 5d, 5t and 5u was studied against the P. yoelii nigeriensis N67 (a chloroquine-resistant) parasite in Swiss mice at a dose of 100 mg/kg/day for 4 days via oral route. 5u was found to show maximum 100% parasite inhibition with considerably increased mean survival time. Simultaneously, the series of compounds was screened for anti-inflammatory potential. In preliminary assays, nine compounds showed more than 85% inhibition in hu-TNFα cytokine levels in LPS stimulated THP-1 monocytes and seven compounds showed more than 40% decrease in fold induction in reporter gene activity analyzed via Luciferase assay. 5p and 5t were found to be most promising amongst the series, thus were taken up for further in vivo studies. Wherein, mice pre-treated with them showed a dose dependent inhibition in carrageenan induced paw swelling. Moreover, the results of in vitro and in vivo pharmacokinetic parameters indicated that the synthesized pyrrole-hydroxybutenolide conjugates abide by the required criteria for the development of orally active drug and thus this scaffold can be used as pharmacologically active framework that should be considered for the development of potential antiplasmodial and anti-inflammatory agents.

Novel Aporphine- and Proaporphine-Clerodane Hybrids Identified from the Barks of Taiwanese Polyalthia longifolia (Sonn.) Thwaites var. pendula with Strong Anti-DENV2 Activity

Hybrid natural products produced via mixed biosynthetic pathways are unique and often surprise one with unexpected medicinal properties in addition to their fascinating structural complexity/diversity. In view of chemical structures, hybridization is a way of diversifying natural products usually through dimerization of two similar or dissimilar subcomponents through a C-C or N-C covalent linkage. Here, we report four structurally attractive diterpene-alkaloid conjugates polyalongarins A-D (1-4), clerodane-containing aporphine and proaporphine alkaloids, the first of its kind from the barks of Taiwanese Polyalthia longifolia (Sonn.) Thwaites var. pendula. In addition to conventional spectroscopic analysis, single crystal X-ray crystallography was employed to determine the chemical structures and stereo-configurations of 1. Compounds 1-4 were subsequently subjected to in vitro antiviral examination against DENV2 by evaluating the expression level of the NS2B protein in DENV2-infected Huh-7 cells. These compounds display encouraging anti-DENV2 activity with superb EC₅₀ (2.8-6.4 μM) and CC₅₀ values (50.4-200 μM). The inhibitory mechanism of 1-4 on NS2B was further explored drawing on in-silico molecular docking analysis. Based on calculated binding affinities and predicted interactions between the functional groups of 1-4 and the allosteric-site residues of the DENV2 NS2B-NS3 protease, our analysis concludes that the clerodane-aporphine/proaporphine-type hybrids are novel and effective DENV NS2B-NS3 protease inhibitors.

In Vivo Antimalarial Activity of Polyalthia longifolia (Annonaceae) Leaf Extract and Assessment of Its Formulated Oral Dosage Forms

Plant medicine is commonly employed to treat malaria and other infections in Ghana. However, many of these phytomedicines have not been scientifically investigated to justify their use. This study therefore sought to investigate the antimalarial property of Polyalthia longifolia leaves and to formulate suitable dosage forms for ease of administration. A four-day antiplasmodial suppressive and curative study was conducted on ethanol extract of P. longifolia leaves (PLE) using Plasmodium berghei infected albino mice. Tablet and suspension dosage forms of PLE were formulated and evaluated for quality and stability. Statistically significant (P < 0.05) parasitaemia suppression (61.25%) and cure (58.78%) were achieved at a PLE dose of 100 mg/kg, and increases in hematological indices (P < 0.001) were also observed in the PLE-treated mice as compared to the untreated group. The tablets passed the tests for uniformity of weight, friability (<1%), hardness, disintegration (<15 minutes), and in vitro dissolution (>70% release in 45 minutes). The sedimentation volume, rheology, viscosity, and pH of the formulated suspension were within the official specifications. The dosage forms showed consistency in PLE content (85-105%) and no changes in physicochemical properties over the six months period of stability study. The in vivo antimalarial activity of PLE has been established and oral dosage forms that conformed to Pharmacopoeial standards are formulated for use in the management of malaria.

Phytochemicals from Polyalthia Species: Potential and Implication on Anti-Oxidant, Anti-Inflammatory, Anti-Cancer, and Chemoprevention Activities

Polyalthia belong to the Annonaceae family and are a type of evergreen tree distributed across many tropical and subtropical regions. Polyalthia species have been used long term as indigenous medicine to treat certain diseases, including fever, diabetes, infection, digestive disease, etc. Recent studies have demonstrated that not only crude extracts but also the isolated pure compounds exhibit various pharmacological activities, such as anti-oxidant, anti-microbial, anti-tumor, anti-cancer, etc. It is known that the initiation of cancer usually takes several years and is related to unhealthy lifestyle, as well as dietary and environmental factors, such as stress, toxins and smoking. In fact, natural or synthetic substances have been used as cancer chemoprevention to delay, impede, or even stop cancer growing. This review is an attempt to collect current available phytochemicals from Polyalthia species, which exhibit anti-cancer potentials for chemoprevention purposes, providing directions for further research on the interesting agents and possible clinical applications.

Antiplasmodial, antimalarial activities and toxicity of African medicinal plants: a systematic review of literature

BACKGROUND

Malaria still constitutes a major public health menace, especially in tropical and subtropical countries. Close to half a million people mainly children in Africa, die every year from the disease. With the rising resistance to frontline drugs (artemisinin-based combinations), there is a need to accelerate the discovery and development of newer anti-malarial drugs. A systematic review was conducted to identify the African medicinal plants with significant antiplasmodial and/or anti-malarial activity, toxicity, as wells as assessing the variation in their activity between study designs (in vitro and in vivo).

METHODS

Key health-related databases including Google Scholar, PubMed, PubMed Central, and Science Direct were searched for relevant literature on the antiplasmodial and anti-malarial activities of African medicinal plants.

RESULTS

In total, 200 research articles were identified, a majority of which were studies conducted in Nigeria. The selected research articles constituted 722 independent experiments evaluating 502 plant species. Of the 722 studies, 81.9%, 12.4%, and 5.5% were in vitro, in vivo, and combined in vitro and in vivo, respectively. The most frequently investigated plant species were Azadirachta indica, Zanthoxylum chalybeum, Picrilima nitida, and Nauclea latifolia meanwhile Fabaceae, Euphorbiaceae, Annonaceae, Rubiaceae, Rutaceae, Meliaceae, and Lamiaceae were the most frequently investigated plant families. Overall, 248 (34.3%), 241 (33.4%), and 233 (32.3%) of the studies reported very good, good, and moderate activity, respectively. Alchornea cordifolia, Flueggea virosa, Cryptolepis sanguinolenta, Zanthoxylum chalybeum, and Maytenus senegalensis gave consistently very good activity across the different studies. In all, only 31 (4.3%) of studies involved pure compounds and these had significantly (p = 0.044) higher antiplasmodial activity relative to crude extracts. Out of the 198 plant species tested for toxicity, 52 (26.3%) demonstrated some degree of toxicity, with toxicity most frequently reported with Azadirachta indica and Vernonia amygdalina. These species were equally the most frequently inactive plants reported. The leaves were the most frequently reported toxic part of plants used. Furthermore, toxicity was observed to decrease with increasing antiplasmodial activity.

CONCLUSIONS

Although there are many indigenous plants with considerable antiplasmodial and anti-malarial activity, the progress in the development of new anti-malarial drugs from African medicinal plants is still slothful, with only one clinical trial with Cochlospermum planchonii (Bixaceae) conducted to date. There is, therefore, the need to scale up anti-malarial drug discovery in the African region.

Pharmacologic activities of phytosteroids in inflammatory diseases: Mechanism of action and therapeutic potentials

Natural products and their derivatives are known to be useful for treating numerous diseases since ancient times. Because of their high therapeutic potentials, the use of different medicinal plants is possible to treat varied inflammation-mediated chronic diseases. Among natural products, phytosteroids have emerged as promising compounds mostly because they have diverse pharmacological activities. Currently, available medications exert numerous systemic toxicities, including hypertension, immune suppression, osteoporosis, and metabolic abnormalities. Thus, further research on phytosteroids to subside these complications is of significant importance. In this study, the information on phytosteroids, their types, and actions against inflammation, and allergic complications was collected by a systematic survey of literature on several scientific search engines. The literature review suggested that phytosteroids exhibit antiinflammatory action via different modes through transrepression or selective COX-2 enzymes. Also, in silico ADMET analysis was carried out on available phytosteroids to uncover their pharmacokinetic properties. Our analysis has shown that eight compounds: withaferin A, stigmasterol, β-sitosterol, guggulsterone, diosgenin, sarsasapogenin, physalin A, and dioscin, -isolated from medicinal plants show similar pharmacokinetic properties as compared to dexamethasone, commercially available glucocorticoid. These phytosteroids could be useful for the treatment of inflammatory diseases, such as rheumatoid arthritis, inflammatory bowel diseases, multiple sclerosis, asthma, and cardiovascular diseases. Thus, systematic research is required to explore potent phytosteroids with lesser side effects, which might substitute the current medications.

The potential of anti-malarial compounds derived from African medicinal plants: a review of pharmacological evaluations from 2013 to 2019

Background

African Traditional Medicine (ATM) is used for the healthcare of about 80% of the rural populations of the continent of Africa. The practices of ATM make use of plant-products, which are known to contain plant-based secondary metabolites or natural products (NPs), likely to play key roles in drug discovery, particularly as lead compounds. For various reasons, including resistance of strains of Plasmodium to known anti-malarial drugs, local African populations often resort to plant-based treatments and/or a combination of this and standard anti-malarial regimens. Emphasis has been laid in this review to present the anti-malarial virtue of the most recently published phytochemicals or natural products, which have been tested by in vitro and in vivo assays.

Methods

The data was based on the current version of the African Compound Libraries, which are constantly being updated based on inputs from journal articles and student theses (M.Sc/Ph.D) from African University libraries. Emphasis was laid on data published after 2012. In order to carry out the original data collection, currently being included in the African Compounds Database, individual journal websites were queried using the country names in Africa as search terms. Over 40,000 articles “hits” were originally retrieved, then reduced to about 9000 articles. The retained articles/theses was further queried with the search terms “malaria”, “malarial”, “plasmodium”, “plasmodial” and a combination of them, resulting in over 500 articles. Those including compounds with anti-malarial activities for which the measured activities fell within the established cut off values numbered 55, which were all cited in the review as relevant references.

Results and discussion

Pure compounds derived from African medicinal plants with demonstrated anti-malarial/antiplasmodial properties with activities ranging from “very active” to “weakly active” have been discussed. The majority of the 187 natural products were terpenoids (30%), followed by flavonoids (22%), alkaloids (19%) and quinones (15%), with each of the other compound classes being less than 5% of the entire compound collection. It was also observed that most of the plant species from which the compounds were identified were of the families Rubiaceae, Meliaceae and Asphodelaceae. The review is intended to continue laying the groundwork for an African-based anti-malarial drug discovery project.

Two clerodane diterpenes isolated from Polyalthia longifolia leaves: comparative structural features, anti-histaminic and anti-Helicobacter pylori activities

Phytochemical studies of methylene chloride soluble fraction from the 70% aqueous-alcoholic extract of Polyalthia longifolia (leaves) resulted in the isolation of two known clerodane diterpenes, namely: 16α-hydroxycleroda-3,13(14)-dien-16,15-olide (1) and (4→2)-abeo-16-hydroxy-cleroda-2,13(14)-dien-15,16-olide-3-al (2). The isolated compounds were evaluated for their anti-histaminic and anti-Helicobacter pylori activities. Compound 1 was more potent than 2 with IC₅₀ 29.7 μg/mL and 189.2 μg/mL in the anti-histaminic assay and MIC equals to 31.25 μg/mL and 125 μg/mL in the anti-H. pylori assay, as compared with diclofenac (IC₅₀ = 17.9 μg/mL) and clarithromycin (MIC = 1.95 μg/mL), respectively. The differences in their structures, as well as their activity results, indicated important characteristic features necessary for activity. It was suggested that the decalin moiety in 1 was important for both activities. On the other hand, the (4→2)-abeo migration in 2 reduced both activities. The potent activities of 1 suggest its potential application as an anti-histaminic and antimicrobial agent.

Diastereoselective Total Syntheses of (±)-Caseabalansin A and (±)-18-Epicaseabalansin A via Intramolecular Robinson-type Annulation

Highly diastereoselective total syntheses of (±)-caseabalansin A (1) and (±)-18-epicaseabalansin A (2) are described in this paper. We revealed that the intramolecular Robinson-type annulation of an alkynone was effective in the stereocontrolled construction of the bicyclic skeleton of 1 and 2. Further transformation of the resulting enone, including diastereoselective reduction by LiAlH(OtBu)₃ , hydroxy-group-directed hydrogenation, cyclization to form the cyclic acetal moiety, and introduction of a side chain by a C(sp³ )-C(sp³ ) Stille coupling reaction, resulted in the total syntheses of (±)-1 and (±)-2.

The medicinal uses, toxicities and anti-inflammatory activity of Polyalthia species (Annonaceae)

ETHNOPHARMACOLOGICAL RELEVANCE

Polyalthia is one of the largest and notable genera in Annonaceae family. Polyalthia species have been widely used in folklore medicine for the treatment of rheumatic fever, gastrointestinal ulcer and generalized body pain. Numerous in vitro and in vivo studies on Polyalthia Species have also corroborated the significant anti-inflammatory potential of its extracts and secondary metabolites.

AIM OF THE STUDY

This review is an attempt to assess the anti-inflammatory activity of Polyalthia species by giving critical appraisal and establishing evidences of their traditional uses. Moreover this review will highlight the lead compounds for future drug development that can serve as a potential anti-inflammatory drug with comparative efficacy and minimum side effects.

MATERIALS AND METHODS

An extensive literature review, focusing the anti-inflammatory potential of Polyalthia species was conducted using the following databases:PubMed, ScienceDirect, SpringerLink, Ovid, Scopus and ProQuest, as well as the locally available books, journals and relevant documents. The reference lists of retrieved papers were also searched for additional studies.

RESULTS

The Polyalthia species have shown significant anti-inflammatory activity through various mechanism of action. The most significant anti-inflammatory mechanism includes the inhibition of nuclear factor kappa B (NF-κB), prostaglandins (PGs), pro-inflammatory cytokines, inducible nitric oxide synthase (iNOS) and reactive oxygen species (ROS). The data suggests that hydroxycleroda-3,13-dien-15,16-olide and 16-oxocleroda-3,13-dien-15-oic acid, quercetin, rutin, spinasterol, α-spinasterol, goniothalamin and (-)-5-hydroxygoniothalamin are the most potent anti-inflammatory compounds from Polyalthia species with comparable IC₅₀ with positive controls.

CONCLUSIONS

Numerous pharmacological studies have supported the use of Polyalthia species against pain, rheumatic fever, haemorrhages and inflammation in traditional medicine. Flavonoids, diterpenoids, sterols and styrylpyrones from genus Polyalthia are the most significant class of compounds with potent anti-inflammatory activity. Secondary metabolites from these classes should be brought into further research to fill the gaps of knowledge in pharmacokinetics, pharmacodynamics, bioavailability, and toxicity in order to convert the pre-clinical results into clinical data for further investigation.

Diterpenes as lead molecules against neglected tropical diseases

Nowadays, neglected tropical diseases (NTDs) are reported to be present everywhere. Poor and developing areas in the world have received great attention to NTDs. Drug resistance, safety profile, and various challenges stimulate the search for alternative medications. Plant-based drugs are viewed with great interest, as they are believed to be devoid of side effects. Diterpenes, a family of essential oils, have showed attractive biological effects. A systematic review of the literature was carried out to summarize available evidences of diterpenes against NTDs. For this, databases were searched using specific search terms. Among the 2338 collected reports, a total of 181 articles were included in this review. Of them, 148 dealt with investigations using single organisms, and 33 used multiple organisms. No mechanisms of action were reported in the case of 164 reports. A total of 93.92% were related to nonclinical studies, and 4.42% and 1.66% dealt with preclinical and clinical studies, respectively. The review displays that many diterpenes are effective upon Chagas disease, chikungunya, echinococcosis, dengue, leishmaniasis, leprosy, lymphatic filariasis, malaria, schistosomiasis, and tuberculosis. Indeed, diterpenes are amazing drug candidates against NTDs. Copyright © 2016 John Wiley & Sons, Ltd.

Therapeutic Potential of Essential Oils Focusing on Diterpenes

Among all plant derivates, essential oils (EOs) have gained the attention of many scientists. Diterpenes, a family of components present in some EO, are becoming a milestone in the EOs world. The goal of this review is to describe a scenario of diterpenes taking into health-consumption deportment. Previous studies revealed that diterpenes have antioxidant, antimicrobial, antiviral, antiprotozoal, cytotoxic, anticancer, antigenotoxic, antimutagenic, chemopreventive, antiinflammatory, antinociceptive, immunostimulatory, organoprotective, antidiabetic, lipid-lowering, antiallergic, antiplatelet, antithrombotic, and antitoxin activities. In conclusion, diterpenes may be an immense featuring concern in pharmaceutical consumption from a drug discovery point of view. Copyright © 2016 John Wiley & Sons, Ltd.