Cytotoxic diterpenoids from Croton argyrophylloides

A review of natural products and small-molecule therapeutics acting on central nervous system malignancies: Approaches for drug development, targeting pathways, clinical trials, and challenges

In 2021, the World Health Organization released the fifth edition of the central nervous system (CNS) tumor classification. This classification uses histopathology and molecular pathogenesis to group tumors into more biologically and molecularly defined entities. The prognosis of brain cancer, particularly malignant tumors, has remained poor worldwide, approximately 308,102 new cases of brain and other CNS tumors were diagnosed in the year 2020, with an estimated 251,329 deaths. The cost and time-consuming nature of studies to find new anticancer agents makes it necessary to have well-designed studies. In the present study, the pathways that can be targeted for drug development are discussed in detail. Some of the important cellular origins, signaling, and pathways involved in the efficacy of bioactive molecules against CNS tumorigenesis or progression, as well as prognosis and common approaches for treatment of different types of brain tumors, are reviewed. Moreover, different study tools, including cell lines, in vitro, in vivo, and clinical trial challenges, are discussed. In addition, in this article, natural products as one of the most important sources for finding new chemotherapeutics were reviewed and over 700 reported molecules with efficacy against CNS cancer cells are gathered and classified according to their structure. Based on the clinical trials that have been registered, very few of these natural or semi-synthetic derivatives have been studied in humans. The review can help researchers understand the involved mechanisms and design new goal-oriented studies for drug development against CNS malignancies.

New halimane and clerodane diterpenoids from Croton cnidophyllus

Three new halimane furanoditerpenoids (1-3) and three new clerodane furanoditerpenoids (4-6), along with seven known terpenoids including four pimarane diterpenoids (7-10) and three norisoprenoids (11-13) were isolated from the 95% EtOH extracts of the plants of Croton cnidophyllus. The 2D structures including absolute configuration of new furanoditerpenoids (1-6) were elucidated by analysis of their HRMS and NMR data as well as comparison of experimental and calculated ECD curves. Bioassay revealed that two compounds (8 and 9) possessed certain inhibitory effects against NO production stimulated by LPS, with IC₅₀ values of 19.00 ± 1.76 and 21.61 ± 1.11 μM, respectively.

Assessment of antioxidant and antiparkinsonian potential of a new diterpene isolated from Croton argyrophylloides

Oxidative stress is related to health problems including neurological and neurodegenerativedisturbs, such as Parkinson's disease. Natural compounds are reported as source of antioxidant molecules. Therefore, this study aimed to analyze the antioxidant and neuroprotective potential of a new diterpene isolated from C. argyrophylloides (MP-1). Male Wistar rats (250-300 g) were used to evaluate MP-1 antiparkinsonian potential through neurodegenerative model induced by the neurotoxin 6-hydroxydopamine (21 μg). On the 14th day, animals were submitted to behavioral tests and on the 15th day, brain areas were dissected to neurochemical analyzes. MP-1 demonstrated a high antioxidant capacity in vitro and decreased the parkinsonian effects, such as behavioral changes, motor alterations, and body weight loss. MP-1 was also able to control the upregulated levels of nitrosative stress and lipid peroxidation. These findings suggest MP-1 as a diterpene with high antioxidant capacity which might be used to development of new approach against Parkinson's disease.

Evaluation of antibacterial and enhancement of antibiotic action by the flavonoid kaempferol 7-O-β-D-(6″-O-cumaroyl)-glucopyranoside isolated from Croton piauhiensis müll

There has been a rapid increase in the incidence and prevalence of opportunistic bacterial infections. Inappropriate use of current antibiotics has continuously contributed to the emergence of resistance to conventional antibiotic therapy. Therefore, the search for natural molecules that are able to combat infections is of great public interest, and many of these compounds with antimicrobial properties can be obtained from phytochemical studies of medicinal plants. In this context, this study reports the isolation and characterization of the flavonoid, kaempferol 7-O-β-D-(6″-O-cumaroyl)-glucopyranoside, from Croton piauhiensis leaves. Additionally, the intrinsic antimicrobial action of the compound and its enhancement against Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus strains were assessed. The minimum inhibitory concentration (MIC) of the compound was determined using broth microdilution assays. To evaluate the modulatory effect of the flavonoid, the MIC of antibiotics amikacin and gentamicin, belonging to the class aminoglycosides was assessed, with and without the compound in sterile microplates. The results of intrinsic antibacterial activity tests revealed that the compound had no antibacterial activity against strains tested at concentrations <1024 μg/mL. The combination of the flavonoid at a concentration of 128 μg/mL with gentamicin presented synergistic effects against S. aureus 10 and E. coli 06, and also reduced the MIC from 16 μg/mL to 4 μg/mL and 8 μg/mL, respectively. Amikacin also showed synergistic effects against S. aureus 10 and E. coli 06. We also observed reduced MIC for both, from 128 μg/mL to 32 μg/mL; however, antagonism for P. aeruginosa increased the MIC from 16 μg/mL to 64 μg/mL. The combination of the flavonoid with the aminoglycosides may be an alternative to potentiate the expected results in treatment against S. aureus and E. coli, since their association leads to a synergistic effect, reducing the MIC of these drugs and decreasing the dose necessary for therapeutic success.

Two new bioactive diterpenes identified from Isodon interruptus

In the course of our ongoing studies to discover bioactive chemical constituents from plants in the genus Isodon, two new diterpenes, kunminolide A (1) and rabdokunmin F (2) were isolated from the leaves of the medicinal plant Isodon interruptus. Kunminolide A (1) is a novel abietane-like diterpene with a novel skeleton, herein designated as 9, 10-seco-neoabietane. Rabdokunmin F (2) is an ent-kaurene diterpene with C-18 oxidized to a carboxylic acid group. The structures were determined by spectroscopic means including analysis of 1D- and 2D-NMR spectral data. Crystals of 1 obtained from methanol were suitable for X-ray analysis, which confirmed the chemical structure. Kunminolide A (1) demonstrated chemopreventive potential by inducing QR1 activity with a CD value of 14.3 µM, and rabdokunmin F (2) was found to have cytotoxic activities with IC₅₀ values in the range of 1.1-3.0 µM.

Chemical Constituents from Croton Species and Their Biological Activities

The genus Croton belongs to the Euphorbiaceae family, which comprises approximately 1300 species. Many Croton species have been used as folk medicines. This review focuses on the chemical constituents from Croton species and their relevant biological activities, covering the period from 2006 to 2018. A total of 399 new compounds, including 339 diterpenoids, were reported. Diterpenoids are characteristic components of the Croton species. These isolated compounds exhibited a broad spectrum of bioactivities, including cytotoxic, anti-inflammatory, antifungal, acetylcholinesterase inhibitory, and neurite outgrowth-promoting properties. The present review provides a significant clue for further research of the chemical constituents from the Croton species as potential medicines.

Norcrassin A, a novel C16 tetranorditerpenoid, and bicrotonol A, an unusual dimeric labdane-type diterpenoid, from the roots of Croton crassifolius

Norcrassin A (1), a novel C₁₆ tetranorditerpenoid, featuring a new carbon skeleton, and bicrotonol A (2), an unusual dimeric labdane-type diterpenoid, were isolated from Croton crassifolius.

Pteisolic acid G, a novel ent-kaurane diterpenoid, inhibits viability and induces apoptosis in human colorectal carcinoma cells

Human colorectal cancer (CRC) is a major cause of cancer morbidity and mortality, and its incidence rates are increasing in economical transitioning areas globally. To develop efficient chemotherapy drugs for CRC, the present study isolated and identified a novel ent-kaurane diterpenoid from Pteris semipinnata, termed pterisolic acid G (PAG). This ent-kaurane diterpenoid was demonstrated to significantly inhibit the growth of human CRC HCT116 cells in a time- and dose-dependent manner, determined using the Cell Counting Kit-8 assay. Additionally, western blot analysis, Hoechst 33342 staining and cytometry analysis revealed that PAG not only inhibited the viability of HCT116 cells by suppressing the dishevelled segment polarity protein 2/glycogen synthase kinase 3 β/β-catenin pathway, but also induced the apoptosis of HCT116 cells by downregulating nuclear factor-κB p65 activity, stimulating p53 expression and promoting the generation of intracellular reactive oxygen species. These results suggest that PAG, a novel inhibitor of the Wnt/β-catenin pathway and inducer of apoptosis, should be investigated in more detail using in vivo experiments and comprehensive mechanistic studies in order to examine the potential use of PAG as a novel therapeutic agent for the treatment of CRC.

Identification and Modulatory Activity Assessment of 2-Hydroxy-3,4,6-trimethoxyacetophenone Isolated fromCroton anisodontusMull. Arg.(Euphorbiaceae)

The n-hexane extract of the stem bark of Croton anisodontus yielded 2-hydroxy-3,4,6-trimethoxyacetophenone, a well-known substance, but isolated from this species for the first time. The antimicrobial and modulatory activities of the compound towards Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, Candida albicans, C krusei and C tropicalis strains were assessed. Antibiotics such as amikacin, gentamicin and neomycin were used in a subinhibitory concentration. Significant activity was observed towards P. aeruginosa and S. aureus 358, with p < 0.001 in association with amikacin. The present results place C anisodontus as an alternative source of 2-hydroxy-3,4,6-trimethoxyacetophenone with antibacterial potential.

Crotofolane Diterpenoids from Croton caracasanus

Four new crotofolane-type diterpenoids, crotocarasin (A-D) (1–4), together with the known crotofolin E, were isolated from a dichloromethane extract of the stems of Croton caracasanus Pittier. The structures of the new compounds were determined by spectroscopic methods, and the structure of 3 was further confirmed by single-crystal X-ray data analyses.

ent-Kaurane diterpenoids from the plant Wedelia trilobata

Four new ent-kaurane diterpenoids, namely, 3α-tigloyloxypterokaurene L₃ (1), ent-17-hydroxy-kaura-9(11),15-dien-19-oic acid (2), and wedelobatins A (3) and B (4), together with 11 known ent-kaurane diterpenoids (5-15), were isolated from the ethanol extract of Wedelia trilobata. All the structures of 1–15 were elucidated on the basis of spectroscopic studies.

Antispasmodic effects of a new kaurene diterpene isolated from Croton argyrophylloides on rat airway smooth muscle

OBJECTIVES

The effects of rel-(1S,4aS,7S,8aS)-7-(1-vinyl)-tetradecahydro-1,4a-dimethylphenanthrene-7,8a-carbolactone-1-carboxylic acid (TCCA), a new ent-kaurene diterpene isolated from Croton argyrophylloides, on rat tracheal preparations were investigated.

METHODS

Tracheae were removed and cut into two-cartilage segments that were mounted in organ baths containing Tyrode's solution.

RESULTS

TCCA reduced the contractions induced by electrical field stimulation, relaxed K(+)-induced contractions, and inhibited both phasic and tonic components of the K(+)- and ACh-induced contractions. TCCA reduced the serotonin-induced contraction, abolished that evoked by K(+) in the presence of epinephrine, and also reduced the ACh-induced contractions under Ca(2+)-free conditions. TCCA blocked contractions that depend on divalent cation inflow through voltage-operated Ca(2+) channels (VOCCs) and receptor-operated Ca(2+) channels (ROCCs), but had greater potency to block VOCC- than ROCC-dependent contractions or contractions induced by ACh in Ca(2+)-free conditions. TCCA relaxed the phorbol 12,13 dibutyrate (1 µm) induced contraction, but with slight potency.

CONCLUSIONS

TCCA induces an antispasmodic effect through several mechanisms including blockade of either VOCCs (with greater potency) or ROCCs, blockade of IP(3)-induced Ca(2+) release from sarcoplasmic reticulum (with intermediate potency) and reduction of the sensitivity of contractile proteins to Ca(2+).

Geopyxins A-E, ent-kaurane diterpenoids from endolichenic fungal strains Geopyxis aff. majalis and Geopyxis sp. AZ0066: structure-activity relationships of geopyxins and their analogues

Four new ent-kaurane diterpenoids, geopyxins A-D (1-4), were isolated from Geopyxis aff. majalis, a fungus occurring in the lichen Pseudevernia intensa, whereas Geopyxis sp. AZ0066 inhabiting the same host afforded two new ent-kaurane diterpenoids, geopyxins E and F (5 and 6), together with 1 and 3. The structures of 1-6 were established on the basis of their spectroscopic data, while the absolute configurations were assigned using modified Mosher's ester method. Methylation of 1-3, 5, and 6 gave their corresponding methyl esters 7-11. On acetylation, 1 and 7 yielded their corresponding monoacetates 12 and 14 and diacetates 13 and 15. All compounds were evaluated for their cytotoxic and heat-shock induction activities. Compounds 2, 7-10, 12, 14, and 15 showed cytotoxic activity in the low micromolar range against all five cancer cell lines tested, but only compounds 7-9, 14, and 15 were found to activate the heat-shock response at similar concentrations. From a preliminary structure-activity perspective, the electrophilic α,β-unsaturated ketone carbonyl motif present in all compounds except 6 and 11 was found to be necessary but not sufficient for both cytotoxicity and heat-shock activation.

New 3,4-Seco-ent-kaurene Dimers from Croton micans

From the stems of Croton micans Sw., five new 3,4- seco-ent-kaurene dimers: micansinoic acid (1), isomicansinoic acid (2), and the dimethyl (3), monomethyl (4) and monoethyl ester (5) of micansinoic acid were isolated. The structures of the new compounds were elucidated by spectroscopic data interpretation, mainly 1D and 2D NMR experiments and MS. These compounds are the first 3,4- seco-ent-kaurene dimers from a Croton species.

Casbane diterpene as a promising natural antimicrobial agent against biofilm-associated infections

Croton nepetaefolius is a native plant from northeastern Brazil that belongs to the Euphorbiaceae family. The biological action of this plant has been extensively explored, being the secondary metabolites responsible for its properties alkaloids, diterpenes, and triterpenes. This study aimed to evaluate the ability of casbane diterpene (CD), isolated from the ethanolic extract of C. nepetaefolius, to inhibit microbial growth and biofilm formation of several clinical relevant species (bacteria and yeasts). It was found that CD possessed biocidal and biostatic activity against the majority of the species screened, with minimal active concentrations ranging between 125 and 500 µg/mL. In addition, it was observed that biofilm formation was inhibited even when the planktonic growth was not significantly affected. In conclusion, CD showed potential to be a natural tool for the treatment of diseases caused by different infectious microorganisms.