Two decades of advances in diterpenoid alkaloids with cytotoxicity activities

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.

Micromorphological and phytochemical profiling of Delphinium suave Huth. from Hindukush range, Lower Dir Khyber Pakhtunkhwa Pakistan

This is a very first attempt to study various parameters of a medicinal plant, Delphinium suave Huth. The plant is erect, geophytic, herbaceous, with tuberous root, trifid in a palmatipartite, strigose cuneate leaf and white spurred zygomorphic flower. The root was isodiametric phellem with single non-glandular trichomes. The stem revealed single-layered cuticle, multiseriate epidermis, cortex, pith ray and uniserate bowed non-glandular trichomes. The leaf was amphistomatic, showed tapering trichomes, prismatic crystals and ranunculaceous stomata with circumference 144.66-182.67 μm. Pollen grains in Light Microscopy (LM), were prolate, spheroidal trizonocolpate, isopolar, radiosymmetric, scabrate, elliptic and monads. Scanning Electron Microscope (SEM) pollen surface was scabrate, monad, size varied from 18.06 to 16.67 μm, colpus to inaperturate, tricolpate, ornamented, echinus, isopolar, isodiametric and circular. SEM roots showed sclerenchymatic tissues, stellate, glandular, non-glandular trichomes and crystals. The stem showed scalariform, pitted vessels, warty protuberances, unicellular, silicified, non-glandular trichomes. Leaves powder revealed, simple, unicellular, tapered headed, uniseriate, sessile, capitate, unbranched glandular, non-glandular, trichomes with crystals. Capitate, stellate, circular, unicellular, branchy trichomes were observed for the first time through SEM. Powder drug study of root, stem leaves through LM revealed different tissues. Preliminary phytochemical revealed alkaloids, anthocyanins, anthraquinones, coumarins, flavones, mucilages, saponins, steroids, terpenoids, volatile oils and proteins. GC/MS showed 36 compounds in roots, 33 in stem while 40 in leaves. Fluorescence analysis of roots, stem and leaves showed variations in color when treated with chemicals. This study will assist pharmacognostic exploration, authentication from adulterants/allied species for consistent quality, resulting in safe use, preservation and efficacy. RESEARCH HIGHLIGHTS: This was first attempt on pharmacognostic study on D. suave Huth. which could be used as a foundation for identifying and authenticating the specie from other allied species by these morphological, anatomical, GC/MS profiling, phytochemical analysis and fluorescence analysis.

Alkaloids as Natural NRF2 Inhibitors: Chemoprevention and Cytotoxic Action in Cancer

Being a controller of cytoprotective actions, inflammation, and mitochondrial function through participating in the regulation of multiple genes in response to stress-inducing endogenous or exogenous stressors, the transcription factor Nuclear Factor Erythroid 2-Related Factor 2 (NRF2) is considered the main cellular defense mechanism to maintain redox balance at cellular and tissue level. While a transient activation of NRF2 protects normal cells under oxidative stress, the hyperactivation of NRF2 in cancer cells may help them to survive and to adapt under oxidative stress. This can be detrimental and related to cancer progression and chemotherapy resistance. Therefore, inhibition of NRF2 activity may be an effective approach for sensitizing cancer cells to anticancer therapy. In this review, we examine alkaloids as NRF2 inhibitors from natural origin, their effects on cancer therapy, and/or as sensitizers of cancer cells to anticancer chemotherapeutics, and their potential clinical applications. Alkaloids, as inhibitor of the NRF2/KEAP1 signaling pathway, can have direct (berberine, evodiamine, and diterpenic aconitine types of alkaloids) or indirect (trigonelline) therapeutic/preventive effects. The network linking alkaloid action with oxidative stress and NRF2 modulation may result in an increased NRF2 synthesis, nuclear translocation, as well in a downstream impact on the synthesis of endogenous antioxidants, effects strongly presumed to be the mechanism of action of alkaloids in inducing cancer cell death or promoting sensitivity of cancer cells to chemotherapeutic agents. In this regard, the identification of additional alkaloids targeting the NRF2 pathway is desirable and the information arising from clinical trials will reveal the potential of these compounds as a promising target for anticancer therapy.

Neuropharmacological Potential of Diterpenoid Alkaloids

This study provides a narrative review of diterpenoid alkaloids (DAs), a family of extremely important natural products found predominantly in some species of Aconitum and Delphinium (Ranunculaceae). DAs have long been a focus of research attention due to their numerous intricate structures and diverse biological activities, especially in the central nervous system (CNS). These alkaloids originate through the amination reaction of tetra or pentacyclic diterpenoids, which are classified into three categories and 46 types based on the number of carbon atoms in the backbone structure and structural differences. The main chemical characteristics of DAs are their heterocyclic systems containing β-aminoethanol, methylamine, or ethylamine functionality. Although the role of tertiary nitrogen in ring A and the polycyclic complex structure are of great importance in drug-receptor affinity, in silico studies have emphasized the role of certain sidechains in C13, C14, and C8. DAs showed antiepileptic effects in preclinical studies mostly through Na⁺ channels. Aconitine (1) and 3-acetyl aconitine (2) can desensitize Na⁺ channels after persistent activation. Lappaconitine (3), N-deacetyllapaconitine (4), 6-benzoylheteratisine (5), and 1-benzoylnapelline (6) deactivate these channels. Methyllycaconitine (16), mainly found in Delphinium species, possesses an extreme affinity for the binding sites of α7 nicotinic acetylcholine receptors (nAChR) and contributes to a wide range of neurologic functions and the release of neurotransmitters. Several DAs such as bulleyaconitine A (17), (3), and mesaconitine (8) from Aconitum species have a drastic analgesic effect. Among them, compound 17 has been used in China for decades. Their effect is explained by increasing the release of dynorphin A, activating the inhibitory noradrenergic neurons in the β-adrenergic system, and preventing the transmission of pain messages by inactivating the Na⁺ channels that have been stressed. Acetylcholinesterase inhibitory, neuroprotective, antidepressant, and anxiolytic activities are other CNS effects that have been investigated for certain DAs. However, despite various CNS effects, recent advances in developing new drugs from DAs were insignificant due to their neurotoxicity.

In Vitro Cytotoxic Potential of Selected Jordanian Flora and Their Associated Phytochemical Analysis

Traditional medicines are a significant source of phytochemicals with potential anticancer effects. Ten Jordanian plants were chosen to be tested for cytotoxicity on human colorectal (HT-29) and breast adenocarcinoma (MCF-7) cell lines. The ethanol extracts were screened for their potential cytotoxic effects using a Sulforhodamine B (SRB) colorimetric assay, using doxorubicin as positive control. Plants extracts exhibiting marked cytotoxic activity were further investigated by qualitative and quantitative phytochemical methods. Total phenolics were quantified using the Folin-Ciocalteu reagent, while flavonoids were quantified using aluminum chloride. The total saponins of the n-butanol fraction were estimated using diosgenin as a standard. The total alkaloids and total terpenoids were also evaluated using the gravimetric method. As results, Senecio leucanthemifolius (IC₅₀: 13.84 μg/mL) and Clematis cirrhosa (IC₅₀: 13.28 μg/mL) exhibited marked cytotoxic effects on human colorectal adenocarcinoma (HT-29) cell lines. Total phenolics, flavonoids, saponins, alkaloids, and terpenoids found in Senecio leucanthemifolius were (91.82, 14.90, 14.27, 101, and 135.4 mg/g of dry extract), respectively. They were revealed to be (68.18, 7.16, 31.25, 73.6, and 180 mg/g of dry extract) in Clematis cirrhosa, respectively. Senecio leucanthemifolius and Clematis cirrhosa have been found to possess cytotoxicity against colorectal (HT-29). In conclusion, the findings of this study offer a new perspective on Jordanian plant extracts anticancer activity research.

Rational design and structure-activity relationship studies reveal new esterified C20-diterpenoid alkaloid analogues active against MCF-7 human breast cancer cells

Although various diterpenoid alkaloids have been evaluated recently for antiproliferative activity against human tumor cell lines, little information is available regarding the antiproliferative effects of C20-diterpenoid alkaloids against MCF-7 cells. Six new diterpenoid alkaloid derivatives (13, 14, 22, 23, 25, 26) were prepared by C-11 and 15 esterification of kobusine (6). The natural parent alkaloid 6 and all synthesized derivatives (7 - 27, 12a, 15a, 15b, 18a, 18b) were evaluated for antiproliferative activity against MCF-7 cells. The structure-based design strategy resulted in an initial lead derivative, 11,15-dibenzoylkobusine (7; IC50 8.6 µM). Subsequent synthesized 11,15-diacylkobusine derivatives (9, 16, 20, 21, 23, 25, and 26) showed substantially increased suppressive effects against the MCF-7 cell line (IC50 2.3-4.4 µM). In contrast, parent alkaloid 6, two 11-acylkobusine derivatives (15a, 18a), and two 15-acylkobusine derivatives (15b, 18b) showed no effect. 11,15-Diacylation appears to be critical for producing antiproliferative activity in this alkaloid class and could introduce a new avenue in overcoming breast cancer cell proliferation using natural product derivatives. In a preliminary mechanism of action study, representative derivatives (5, 8, 9, and 17) decreased cyclin D1 mRNA expression.

Design, synthesis and insecticidal activity and mechanism research of Chasmanthinine derivatives

Unrestricted reproduction and spread of pest had caused great damage to the quality and yield of crops in recent years. Besides the use of traditional chemical pesticides, natural products also make a huge contribution against pests. Chasmanthinine, a diterpenoid alkaloid isolated from Aconitum franchetii var. villosulum, shown extremely antifeedant activity against Spodoptera exigua. Therefore, a series of novel Chasmanthinine derivatives were synthesized and their biological activity was studied in this work. Compound 33 showed the strongest antifeedant activity (EC₅₀ = 0.10 mg/cm²) among all the test compounds. The mechanism research of 33 revealed that its antifeedant effect was related to the inhibition of carboxylesterase (CES), and proved the thiophene acyl group could form a strong binding effect with CES by molecular docking. Moreover, compound 10 exhibited the strongest cytotoxicity (IC₅₀ = 12.87 μM) against Sf9 cell line and moderate contact toxicity. The mechanism research indicated that compound 10 could induce Sf9 cells apoptosis. In summary, the results lay a foundation for the application of diterpene alkaloids in plant protection.

Bioactivity inspired C19-diterpenoid alkaloids for overcoming multidrug-resistant cancer

The pharmacological activities of C₁₉-diterpenoid alkaloids are related to their basic skeletons (e.g., aconitine-type or lycoctonine-type). Also, few studies have been reported on the chemosensitizing effects of diterpenoid alkaloids. Consequently, this study was aimed at determining the chemosensitizing effects of synthetic derivatives of lycoctonine-type C₁₉-diterpenoid alkaloids on a P-glycoprotein (P-gp)-overexpressing multidrug-resistant (MDR) cancer cell line KB-VIN. The acyl-derivatives of delpheline and delcosine showed moderate cytotoxicity against chemosensitive cancer cell lines. Among non-cytotoxic synthetic analogs (1-14), several derivatives effectively and significantly sensitized MDR cells by interfering with the drug transport function of P-gp to three anticancer drugs, vincristine, paclitaxel, and doxorubicin. The chemosensitizing effect of derivatives 2, 4, and 6 on KB-VIN cells against vincristine were more potent than 5 μM verapamil, and derivatives 4 and 13 were more effective than 5 μM verapamil for paclitaxel. Among them, 2 in particular increased the sensitivity of KB-VIN cells to vincristine by 253-fold.

The diterpenoid alkaloids

The diterpenoid alkaloids are a family of extremely important natural products that have long been a research hotspot due to their myriad of intricate structures and diverse biological properties. This chapter systematically summarizes the past 11 years (2009-2019) of studies on the diterpenoid alkaloids, including the "so-called" atypical ones, covering the classification and biogenetic relationships, phytochemistry together with 444 new alkaloids covering 32 novel skeletons and the corrected structures, chemical reactions including conversion toward toxoids, synthetic studies, as well as biological activities. It should be noted that the synthetic studies, especially the total syntheses of various diterpenoid alkaloids, are for the first time reviewed in this treatise. This chapter, in combination with our four previous reviews in volumes 42, 59, 67, and 69, will present to the readers a more completed and updated profile of the diterpenoid alkaloids.

Aconitine linoleate, a natural lipo-diterpenoid alkaloid, stimulates anti-proliferative activity reversing doxorubicin resistance in MCF-7/ADR breast cancer cells as a selective topoisomerase IIα inhibitor

Aconitine linoleate (1) is a lipo-diterpenoid alkaloid, isolated from Aconitum sinchiangense W. T. Wang. The study aimed at investigating the anti-proliferative efficacy and the underlying mechanisms of 1 against MCF-7 and MCF-7/ADR cells, as well as obvious the safety evaluation in vivo. The cytotoxic activities of 1 were measured in vitro. Also, we investigated the latent mechanism of 1 by cell cycle analysis in MCF-7/ADR cells and topo I and topo IIα inhibition assay. Molecular docking is done by Discovery Studio 3.5 and Autodock vina 1.1.2. Finally, the acute toxicity of 1 was detected on mice. 1 exhibited significant antitumor activity against both MCF-7 and MCF-7/ADR cells, with IC50 values of 7.58 and 7.02 μM, which is 2.38 times and 5.05 times more active, respectively than etoposide in both cell lines, and being 9.63 times more active than Adriamycin in MCF-7/ADR cell lines. The molecular docking and the topo inhibition test found that it is a selective inhibitor of topoisomerase IIα. Moreover, activation of the damage response pathway of the DNA leads to cell cycle arrest at the G0G1 phase. Furthermore, the in vivo acute toxicity of 1 in mice displayed lower toxicity than aconitine, with LD50 of 2.2 × 105 nmol/kg and only slight pathological changes in liver and lung tissue, 489 times safer than aconitine. In conclusion, compared with aconitine, 1 has more significant anti-proliferative activity against MCF-7 and MCF-7/ADR cells and greatly reduces in vivo toxicity, which suggests this kind of lipo-alkaloids is powerful and promising antitumor compounds for breast cancer.

Chemistry and biological activities of hetisine-type diterpenoid alkaloids

Hetisine-type C₂₀-diterpenoid alkaloids (DAs) are one of the most important DA subtypes. During the past decades, a total of 157 hetisine-type DAs were obtained from plants from seven genera in three families, most of which were isolated from the genera Aconitum and Delphinium in the Ranunculaceae family. Structurally, hetisine-type DAs are characterized by a heptacyclic hetisane skeleton formed by the linkage of C(14)–C(20) and N–C(6) bonds in an atisine-type DA, and their structural diversity is created by the states of the N atom and various substituents. Pharmacological studies have revealed a wide range of pharmacological actions for hetisine-type DAs, including antiarrhythmic, antitumor, antimicrobial and insecticidal activities, as well as effects on peripheral vasculature, which are closely related to their chemical structures. In particular, the prominent antiarrhythmic effects and low toxicity of hetisine-type DAs highlight their potential in antiarrhythmic drug discovery. Hetisine-type DAs with diverse bioactivities are promising lead structures for further development as commercial agents in medicine.

Natural hetisine-type C₂₀-diterpenoid alkaloids with diverse structures showed multiple pharmacological activities, indicating great potential in drug discovery.

Structural diversity, bioactivities, and biosynthesis of natural diterpenoid alkaloids

Covering: 2009 to 2018. Diterpenoid alkaloids, originating from the amination of natural tetracyclic diterpenes, are a diverse class of compounds having complex structural features with many stereocenters. The important pharmacological activities and structural complexity of the diterpenoid alkaloids have long interested scientists due to their medicinal uses, infamous toxicity, and unique biosynthesis. Since 2009, 373 diterpenoid alkaloids, assigned to 46 skeletons, have been isolated and identified from plants mostly in the Ranunculaceae family. The names, classes, molecular weight, molecular formula, NMR data, and plant sources of these diterpene alkaloids are collated here. This review will be a detailed update of the naturally occurring diterpene alkaloids reported from the plant kingdom from 2009-2018, providing an in-depth discussion of their diversity, biological activities, pharmacokinetics, toxicity, application, evolution, and biosynthesis.

Classification, Toxicity and Bioactivity of Natural Diterpenoid Alkaloids

Diterpenoid alkaloids are natural compounds having complex structural features with many stereo-centres originating from the amination of natural tetracyclic diterpenes and produced primarily from plants in the Aconitum, Delphinium, Consolida genera. Corals, Xenia, Okinawan/Clavularia, Alcyonacea (soft corals) and marine sponges are rich sources of diterpenoids, despite the difficulty to access them and the lack of availability. Researchers have long been concerned with the potential beneficial or harmful effects of diterpenoid alkaloids due to their structural complexity, which accounts for their use as pharmaceuticals as well as their lousy reputation as toxic substances. Compounds belonging to this unique and fascinating family of natural products exhibit a broad spectrum of biological activities. Some of these compounds are on the list of clinical drugs, while others act as incredibly potent neurotoxins. Despite numerous attempts to prepare synthetic products, this review only introduces the natural diterpenoid alkaloids, describing 'compounds' structures and classifications and their toxicity and bioactivity. The purpose of the review is to highlight some existing relationships between the presence of substituents in the structure of such molecules and their recognised bioactivity.

Antifungal Activity of New Diterpenoid Alkaloids Isolated by Different Chromatographic Methods from Delphinium peregrinum L. var. eriocarpum Boiss

This paper aimed to investigate the potential antifungal influences of new alkaloids from Delphinium peregrinum L. var. eriocarpum Boiss. New Diterpenoid alkaloids Delcarpum (1), Hydrodavisine (4) and known alkaloids Peregrine (2), Delphitisine (3) were isolated by different chromatographic methods from the aerial parts of D. Peregrinum eriocarpum Boiss, which grows in Syria. The structures of alkaloids were proposed based on 1D NMR spectroscopy 1H-NMR, 13C-NMR, DEPT-135, DEPT-90, 2D NMR spectroscopy DQF-COSY, HMQC, EI-Ms mass spectrum, and IR spectroscopic measurements. The antifungal activity of the isolated alkaloids was evaluated against different dermatophyte fungal isolates compared with fluconazole. In the case of Peregrine (2) the minimum inhibitory concentrations(MICs) recorded 128-256, 32-64, and 32 for Epidermophyton floccosum, Microsporum canis, and Trichophyton rubrum, respectively, compared to 32-64, 16, and 32 μg/mL in the case of fluconazole, respectively. The MICs recorded on application of the four alkaloids mixture were 64, 32, and 16 in the case of E. floccosum, M. canis, and T. rubrum, respectively, which were significantly lower than that measured for each of the individual alkaloid and were compatible for fluconazole. In conclusion, MICs of the tested alkaloids showed a variable potential effect on the investigated fungal isolates. Peregrine (2) was the most effective alkaloid, however, the application of the mixture of alkaloids induced significant synergistic activity that was more pronounced than the application of individual ones.

Synthesis and structure-activity relationship of lipo-diterpenoid alkaloids with potential target of topoisomerase IIα for breast cancer treatment

Aconitine linoleate (11) isolated from the Aconitum sinchiangense W. T. Wang exhibited significant anti-tumor activity. Based on this, a series of novel lipo-diterpenoid alkaloids were synthesized and evaluated for their anticancer activities against MCF-7 and MCF-7/ADR cell lines. Seventeen compounds, including 18-20, 22, 24-32, 36, 39, 41-42 possessed higher anti-proliferative activities (IC₅₀ < 20 μM) against MCF-7 cell lines, which were better than the reference drug etoposide (IC₅₀ = 18.01 ± 1.64 μM), among which compound 24 (IC₅₀ = 4.00 ± 0.30 μM) was found to be the most potent derivative, being 4.5-fold more active than etoposide. Meanwhile, eighteen compounds, including 18-22, 24, 26-32, 36, 38-39, 41-42 presented excellent activities (IC₅₀ < 20 μM) against MCF-7/ADR cell lines, better than etoposide (IC₅₀ = 35.48 ± 0.29 μM) and doxorubicin (IC₅₀ = 67.61 ± 6.5 μM). The most potent compound (19) was 13.5- and 25.7-fold more active than etoposide and doxorubicin against MCF-7/ADR cell lines, respectively. The structure-activity relationship (SAR) studies indicated that the 3-OH, 8-lipo, 14-benzene ring, and nitrogen atom with proper alkaline are crucial elements for anti-proliferative activity of target lipo-diterpenoid compounds. The proper length, the double bonds or di-fluoro-substituted at C-8 fatty acid chain, the para-donating electron group on 14-benzene group, and 13-OH are all favorable for the enhancement of anti-proliferative activities. In conclusion, the introduction of the 8-lipo group into aconitine leads to significant increase of anti-proliferative activity against MCF-7 and MCF-7/ADR cells, which suggests these kinds of lipo-alkaloids are powerful and promising antitumor compounds for breast cancer, especially for drug-resistant breast cancer.

ent-Atisane diterpenoids: isolation, structure and bioactivity

Covering: up to 2020 ent-Atisane diterpenoids are a class of over 150 members with diverse structures and valuable bioactivities. These compounds share a curious history in which the synthesis of the archetypal member preceded its isolation from natural sources. In this review, we provide a comprehensive summary of the isolation, structure, and bioactivity of ent-atisane diterpenoids from their discovery in 1965 to the present day.

A systematic review on the chemical constituents of the genus Consolida (Ranunculaceae) and their biological activities

For centuries, species of the genus Consolida (Ranunculaceae) have been extensively utilized for their extremely high ornamental and medicinal values. Phytochemical investigations of Consolida species have revealed the presence of multiple active ingredients, including diterpenoid alkaloids, flavonoids, phenolic acids, phytosterols, fatty acids, and volatile constituents. These chemical constituents are of great research significance due to their novel structures and broad biological activities. This review addresses, for the first time, the chemical constituents of Consolida plants and the biological activities of these compounds to facilitate future research.

An overview of the chemical constituents from the genus Delphinium reported in the last four decades

Species of the genus Delphinium have been extensively used for different purposes by various civilizations worldwide since antiquity. Phytochemical investigations on Delphinium plants in the last four decades (1980–2019) have afforded a total of 453 new compounds, most of which are diterpenoid alkaloids. These constituents are of great research significance due to their novel structures and broad bioactivities. This review addresses, for the first time, the chemical constituents of Delphinium plants and the biological properties of these compounds to facilitate future research.

Multiple chemical constituents, mainly diterpenoid alkaloids, from Delphinium showed great research significance for their novel structures and broad bioactivities.

Non-alkaloidal constituents from the genus Aconitum: a review

Multiple non-alkaloidal constituents from Aconitum showed great research significance for their novel structures, broad bioactivities and chemotaxonomical significance.