C20-nor-Abietane and Three Abietane Diterpenoids from Plectranthus mutabilis Leaves as P-Glycoprotein Modulators

Abietane-Type Diterpenoids: Insights into Structural Diversity and Therapeutic Potential

The abietane-type diterpenoids are among the most significant diterpene subsets found in hundreds of plant species belonging to various families. Among which, the members of the genus Salvia and Euphorbia are rich in abietane diterpenoids. Because of the chemical diversity and notable bioactivities, such as anticancer, antiinflammatory, antimicrobial, and antioxidant activities, they are attractive. Herein, recent advances in the isolation and characterization of abietanes from natural sources, as well as their biological activities, from 2015 up to 2024 are reviewed. During this time, over 300 abietanes with diverse structures have been discovered.

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.

Diterpenes Isolated from Three Different Plectranthus Sensu Lato Species and Their Antiproliferative Activities against Gynecological and Glioblastoma Cancer Cells

Fourteen diterpenes were isolated from methanol extracts of the aerial parts ofColeus comosus,Coleus forsteri "Marginatus", and Plectranthus ciliatus. The compounds belong to the abietane (1-4, 9-11, and 13), ent-clerodane (5-8), and ent-kaurane (14, 15) classes. Three new compounds were isolated from C. comosus, including 3-O-acetylornatin G (2), 3,12-di-O-acetylornatin G (3), ornatin B methyl ester (5), and ornatin F (4), for which we proposed a revised structure. The structures of the compounds were determined by comprehensive spectroscopic data analysis. The isolated diterpenes were examined in silico for their physicochemical and early ADME properties. Their antiproliferative effects were determined in vitro using human breast (MDA-MB-231 and MCF-7), cervical (HeLa), and glioblastoma (U-87 MG) cancer cell lines. The royleanone- and hydroquinone-type abietane diterpenes (9-13)exhibited the most potent antiproliferative activity against all cancer cell lines tested, particularly against glioblastoma cells, with IC50 values ranging from 1.1 to 15.6 μM.

Hydrogenation of Quinones to Hydroquinones under Atmospheric Pressure Catalyzed by a Metal-Ligand Bifunctional Iridium Catalyst

A general method for the hydrogenation of quinones to hydroquinones under atmospheric pressure has been developed. In the presence of [Cp*Ir(2,2'-bpyO)(H2O)] (0.5-1 mol %), a range of products were obtained in high yields. Furthemore, the expansion of this catalytic system to the hydrogenation of 1,4-benzoquinone diimines was also presented. Functional groups in the bpy ligand were found to be crucial for the catalytic activity of iridium complexes.

Total Synthesis of Diterpenoid Quinone Methide Tumor Inhibitor, (+)-Taxodione

An asymmetric polyene cyclization (92% ee) strategy has been successfully applied for the first asymmetric total synthesis of oxidized abietane, anticancer agent, taxodione (1) sharing a trans-decalin system. Additionally, the total syntheses of pomiferin B (2) and gaultheric acid (3) (a nor-abietane) were achieved utilizing this unified approach.

Coleon U, Isolated from Plectranthus mutabilis Codd., Decreases P-Glycoprotein Activity Due to Mitochondrial Inhibition

Multidrug resistance in cancer is often mediated by P-glycoprotein. Natural compounds have been suggested as a fourth generation of P-glycoprotein inhibitors. Coleon U, isolated from Plectranthus mutabilis Codd., was reported to modulate P-glycoprotein activity but the underlying mechanism has not yet been revealed. Therefore, the effects of Coleon U on cell viability, proliferation, and cell death induction were studied in a non-small-cell lung carcinoma model comprising sensitive and multidrug-resistant cells with P-glycoprotein overexpression. P-glycoprotein activity and mitochondrial membrane potential were assessed by flow cytometry upon Coleon U, sodium-orthovanadate (an ATPase inhibitor), and verapamil (an ATPase stimulator) treatments. SwissADME was used to identify the pharmacokinetic properties of Coleon U, while P-glycoprotein expression was studied by immunofluorescence. Our results showed that Coleon U is not a P-glycoprotein substrate and is equally efficient in sensitive and multidrug-resistant cancer cells. A decrease in P-glycoprotein activity observed with Coleon U and verapamil after 72 h is antagonized in combination with sodium-orthovanadate. Coleon U induced a pronounced effect on mitochondrial membrane depolarization and showed a tendency to decrease P-glycoprotein expression. In conclusion, Coleon U-delayed effect on the decrease in P-glycoprotein activity is due to P-glycoprotein's functioning dependence on ATP production in mitochondria.

Anti-inflammatory activities of Coleus forsteri (formerly Plectranthus forsteri) extracts on human macrophages and chemical characterization

Introduction: Formerly named Plectranthus forsteri, Coleus forsteri (Benth.) A.J.Paton, 2019 is a Lamiaceae traditionally used to treat flu-like symptoms and shock-related ecchymosis, especially in the Pacific region. Few studies investigated chemical composition and anti-inflammatory potential of this plant. Method: Herein, we investigated anti-inflammatory potential of C. forsteri ethanolic (ePE) and cyclohexane (cPE) plant extract on LPS-induced human macrophages models and quantified cytokines and quinolinic acid (QUIN) as inflammatory markers. Results: Our results show that extract of ePE and cPE significantly inhibit inflammatory cytokine IL-6 and TNF-α induced by LPS on PMA-derived THP-1 macrophages. QUIN production is also diminished under ePE and cPE treatment in activated human monocyte-derived macrophages (MDMs). Seven abietane diterpenes were characterized from C. forsteri cPE including coleon U (1), coleon U-quinone (2), 8α,9α-epoxycoleon U-quinone (3), horminone or 7α-hydroxyroyleanone (4), 6β,7α-dihydroxyroyleanone (5), 7α-acetoxy-6β-hydroxyroyleanone (6) and 7α-formyloxy-6β-hydroxyroyleanone (7). Discussion: We discussed potential contributions of these molecules from C. forsteri extracts for their anti-inflammatory activities.