Bufadienolides with cytotoxic activity from the skins of Bufo bufo gargarizans

3α-Hydroxybufadienolides in Bufo gallbladders: structural insights and biotransformation

Bufadienolides, naturally occurring steroids primarily found in toads, have garnered attention for their pharmacological properties and ecological significance. In this study, we isolated and identified 21 bufadienolides from the gallbladders of Bufo gargarizans, comprising four new compounds and 17 known ones. Notably, the predominance of 15 bufadienolides with a 3α-OH configuration in toad bile differs significantly from the 3β-OH bufadienolides found in venom secreted by toad glands. Moreover, our investigation into the biotransformation of 3β-OH and 3α-OH bufadienolides in the liver and kidney tissues of toads revealed an irreversible conversion from 3β-OH to 3α-OH bufadienolides, suggesting a crucial role in toad self-detoxification. These findings provide valuable insights into the structural diversity of bufadienolides and advance our understanding of their medical and ecological significance.

Bufogarlides A-C, three new bufadienolides with Δ14,15 double bond from the skins of Bufo bufo gargarizans

Three new bufadienolides with a Δ14,15 double bond, named bufogarlides A-C (1-3), together with three known analogs (4-6), were isolated from the skins of Bufo bufo gargarizans. Their structures were identified by analyses of spectroscopic data (1 D and 2 D NMR, HR-ESIMS), and comparison with the literature data. All the isolates were evaluated for their cytotoxic activities against ovarian carcinoma cell lines A2780 and SKOV3. Among them, compound 5 showed the highest potential for the growth inhibition of cancerous cells A2780 and SKOV3 with the IC50 values of 21.09 and 67.08 nM, respectively.

Bufadienolides originated from toad source and their anti-inflammatory activity

Bufadienolide, an essential member of the C-24 steroid family, is characterized by an α-pyrone positioned at C-17. As the predominantly active constituent in traditional Chinese medicine of Chansu, bufadienolide has been prescribed in the treatment of numerous ailments. It is a specifically potent inhibitor of Na+/K+ ATPase with excellent anti-inflammatory activity. However, the severe side effects triggered by unbiased inhibition of the whole-body cells distributed α1-subtype of Na+/K+ ATPase, restrict its future applicability. Thus, researchers have paved the road for the structural alteration of desirable bufadienolide derivatives with minimal adverse effects via biotransformation. In this review, we give priority to the present evidence for structural diversity, MS fragmentation principles, anti-inflammatory efficacy, and structure modification of bufadienolides derived from toads to offer a scientific foundation for future in-depth investigations and views.

Bufadienolides from the Eggs of the Toad Bufo bufo gargarizans and Their Antimelanoma Activities

Toads produce potent toxins, named bufadienolides, to defend against their predators. Pharmacological research has revealed that bufadienolides are potential anticancer drugs. In this research, we reported nine bufadienolides from the eggs of the toad Bufo bufo gargarizans, including two new compounds (1 and 3). The chemical structures of 1 and 3, as well as of one previously reported semisynthesized compound (2), were elucidated on the basis of extensive spectroscopic data interpretation, chemical methods, and X-ray diffraction analysis. Compound 1 is an unusual 19-norbufadienolide with rearranged A/B rings. A biological test revealed that compounds 2 and 4-8 showed potent cytotoxic activities toward human melanoma cell line SK-MEL-1 with IC₅₀ values less than 1.0 μM. A preliminary mechanism investigation revealed that the most potent compound, 8, could induce apoptosis via PARP cleavage, while 5 and 6 significantly suppressed angiogenesis in zebrafish. Furthermore, an in vivo biological study showed that 5, 6, and 8 inhibit SK-MEL-1 cell growth significantly.

Chemical and Pharmacological Screening of Rhinella icterica (Spix 1824) Toad Parotoid Secretion in Avian Preparations

The biological activity of Rhinella icterica parotoid secretion (RIPS) and some of its chromatographic fractions (RI18, RI19, RI23, and RI24) was evaluated in the current study. Mass spectrometry of these fractions indicated the presence of sarmentogenin, argentinogenin, (5β,12β)-12,14-dihydroxy-11-oxobufa-3,20,22-trienolide, marinobufagin, bufogenin B, 11α,19-dihydroxy-telocinobufagin, bufotalin, monohydroxylbufotalin, 19-oxo-cinobufagin, 3α,12β,25,26-tetrahydroxy-7-oxo-5β-cholestane-26-O-sulfate, and cinobufagin-3-hemisuberate that were identified as alkaloid and steroid compounds, in addition to marinoic acid and N-methyl-5-hydroxy-tryptamine. In chick brain slices, all fractions caused a slight decrease in cell viability, as also seen with the highest concentration of RIPS tested. In chick biventer cervicis neuromuscular preparations, RIPS and all four fractions significantly inhibited junctional acetylcholinesterase (AChE) activity. In this preparation, only fraction RI23 completely mimicked the pharmacological profile of RIPS, which included a transient facilitation in the amplitude of muscle twitches followed by progressive and complete neuromuscular blockade. Mass spectrometric analysis showed that RI23 consisted predominantly of bufogenins, a class of steroidal compounds known for their cardiotonic activity mediated by a digoxin- or ouabain-like action and the blockade of voltage-dependent L-type calcium channels. These findings indicate that the pharmacological activities of RI23 (and RIPS) are probably mediated by: (1) inhibition of AChE activity that increases the junctional content of Ach; (2) inhibition of neuronal Na⁺/K⁺-ATPase, leading to facilitation followed by neuromuscular blockade; and (3) blockade of voltage-dependent Ca²⁺ channels, leading to stabilization of the motor endplate membrane.

Two new 19-hydroxy bufadienolides with cytotoxic activity from the skins of Bufo melanosticus

Two new 19-hydroxy bufadienolides, named bufohydrolide A (1) and bufohydrolide B (2), and four known analogs (3-6) were isolated from the aqueous extracts of the skins of Bufo melanosticus. Their structures were established by spectral data analyses, such as UV, IR, 1 D/2D NMR and mass spectra. Compounds 1-6 were evaluated for their cytotoxic activity against SMMC-7721, HT-29 and A549 cells. Noteworthily, all six isolates exhibited various levels of anti-proliferative activities with IC₅₀ values ranging from 0.02 ± 0.0002 to 25 ± 0.5 μM.

Comparative study of the chemical profile of the parotoid gland secretions from Rhaebo guttatus from different regions of the Brazilian Amazon

Amphibian cutaneous secretion has great potential for bioprospection and is a great tool in the development of bioproducts. Thus, the objective of the present work was to evaluate the comparative study of the chemical profile parotoid gland secretions from Rhaebo guttatus collected in two distinct regions of the Brazilian Amazon. For this, the chemical composition of six methanolic extracts of this species were analyzed by Liquid Chromatography in UV and MS Detection Ultra-Chromatography Systems (UFLC-DAD-micrOTOF). All obtained chromatograms presented two distinct regions; one referring to the more hydrophilic molecules (alkaloids), while the other refers to the more hydrophobic compounds (steroids). The steroid region resembles all samples, regardless of where they were collected. In the alkaloid region, there was a standardized variation for the samples from the southern Brazilian Amazon, but the same was not true for the samples collected in the Amazon-Cerrado transition region. Thus, the data suggest that the environment and diet of R. guttatus may be important in alkaloid production, but do not influence steroid content. These results add new information about the poison of the toad R. guttatus and raises new questions to be further investigated, thus contributing to the knowledge of the anuran fauna of the Brazilian Amazon.

Venenum bufonis: An overview of its traditional use, natural product chemistry, pharmacology, pharmacokinetics and toxicology

ETHNOPHARMACOLOGICAL RELEVANCE

The animal medicine of Venenum Bufonis (VB), a product of the secretions of Bufo gargarizans Cantor or B. melanostictus Schneider, has long been used as a traditional Chinese medicine (TCM) for the treatment of sunstroke and faint, acute filthy disease - abdominal pain or vomiting and diarrhea, etc. AIM OF THE REVIEW: This review is aimed at providing the comprehensive and up-to-date information of VB as regards its ethnopharmacological uses, constituents and their metabolism, pharmacokinetics, pharmacology and toxicology, all of which could be used as fundamental data for future research as well as development of new drugs.

MATERIALS AND METHODS

The information and data about the studies of VB were collected from scientific journals, material medica, historical documents, library, and electronic databases (PubMed, Google Scholar, Science Direct, Researchgate, Web of Science and CNKI).

RESULTS

To date, about 142 bufadienolides and 16 indole alkaloids have been isolated from VB in total. The extract and isolated compounds showed a wide range of in vitro and in vivo pharmacologic effects, such as cardiotonic, anti-tumor, antinociceptive, anti-inflammatory, anesthetic and antimicrobial activities. Especially, bufadienolides have been extensively studied due to its powerful anti-tumor activities against various cancer cells. Furthermore, their metabolites and metabolic pathways were concluded in detail, and the main metabolic pathways of bufadienolides were hydroxylation, 3-isomerization, 3-keto, 16-hydrolyzation, 3-O-sulfate and 3-O-glucuronide.

CONCLUSIONS

Although VB possesses significant anti-tumor effect against various cancer cell lines, the development of new drugs still remains to be a challenge due to its pharmacodynamic effects in vivo, druggability and toxicology. The main problem lies in its side effects in vivo, poor bioavailability, fast metabolism, cardiotoxicity and neurovirulence. Besides, studies on its metabolism and toxicology in vitro and in vivo, as well as clinical trials should be further conducted for the new drug development and the establishment of optimal dosage of consumption of its administration.

Induction of phytoalexins and proteins related to pathogenesis in plants treated with extracts of cutaneous secretions of southern Amazonian Bufonidae amphibians

Cutaneous secretions produced by amphibians of the family Bufonidae are rich sources of bioactive compounds that can be useful as new chemical templates for agrochemicals. In crop protection, the use of elicitors to induce responses offers the prospect of durable, broad-spectrum disease control using the plant's own resistance. Therefore, we evaluated the potential of methanolic extracts of cutaneous secretions of two species of amphibians of the family Bufonidae found in the Amazon biome-Rhaebo guttatus (species 1) and Rhinella marina (species 2)-in the synthesis of phytoalexins in soybean cotyledons, bean hypocotyls, and sorghum mesocotyls. Additionally, changes in the enzyme activity of β-1,3-glucanase, peroxidase (POX), and polyphenol oxidase (PPO) and in the total protein content of soybean cotyledons were determined. In the soybean cultivar 'TMG 132 RR', our results indicated that the methanolic extract of R. guttatus cutaneous secretions suppressed glyceollin synthesis and β-1,3-glucanase activity and increased POX and PPO activities at higher concentrations and total protein content at a concentration of 0.2 mg/mL. On the other hand, the methanolic extract of R. marina cutaneous secretions induced glyceollin synthesis in the soybean cultivars 'TMG 132 RR' and 'Monsoy 8372 IPRO' at 0.1-0.2 mg/mL and 0.2 mg/mL, respectively. The methanolic extract of R. marina cutaneous secretions also increased the specific activity of POX and PPO in 'Monsoy 8372 IPRO' and 'TMG 132 RR', respectively, and decreased the activity of β-1,3-glucanases in 'Monsoy 8372 IPRO'. At 0.3 mg/mL, it stimulated phaseolin synthesis. The extracts did not express bioactivity in the synthesis of deoxyanthocyanidins in sorghum mesocotyls. The study in soybean suggests that the bioactivity in defense responses is influenced by cultivar genotypes. Therefore, these results provide evidence that extracts of cutaneous secretions of these amphibians species may contribute to the bioactivity of defense metabolites in plants.

An Efficient Strategy Based on Liquid-Liquid Extraction with Three-Phase Solvent System and High Speed Counter-Current Chromatography for Rapid Enrichment and Separation of Epimers of Minor Bufadienolide from Toad Meat

This study presents an efficient strategy based on liquid-liquid extraction with three-phase solvent system and high speed counter-current chromatography for rapid enrichment and separation of epimers of minor bufadienolide from toad meat. The reflux extraction conditions were optimized by response surface methodology first, and a novel three-phase solvent system composed of n-hexane/methyl acetate/acetonitrile/water (3:6:5:5, v/v) was developed for liquid-liquid extraction of the crude extract. This integrative extraction process could enrich minor bufadienolide from complex matrix efficiently and minimize the loss of minor targets induced by repeated extraction with different kinds of organic solvents occurring in the classical liquid two-phase extraction. As a result, four epimers of minor bufadienolide were greatly enriched in the middle phase and total content of these epimers of minor bufadienolide was increased from 3.25% to 46.23%. Then, the enriched four epimers were separated by HSCCC with a two-phase solvent system composed of chloroform/methanol/water (4:2:2, v/v) successfully. Furthermore, we tested Na⁺,K⁺-ATPase (NKA) inhibitory effect of the four epimers. 3β-Isomers of bufadienolide showed stronger (>8-fold) inhibitory activity than 3α-isomers. The characterization of minor bufadienolide in toad meat and their significant difference of inhibitory effect on NKA would promote the further quantitative analysis and safety evaluation of toad meat as a food source.

Marinobufagin, a molecule from poisonous frogs, causes biochemical, morphological and cell cycle changes in human neoplasms and vegetal cells

Skin toad secretion present physiologically active molecules to protect them against microorganisms, predators and infections. This work detailed the antiproliferative action of marinobufagin on tumor and normal lines, investigate its mechanism on HL-60 leukemia cells and its toxic effects on Allium cepa meristematic cells. Initially, cytotoxic action was assessed by colorimetric assays. Next, HL-60 cells were analyzed by morphological and flow cytometry techniques and growing A. cepa roots were examined after 72 h exposure. Marinobufagin presented high antiproliferative action against all human tumor lines [IC₅₀ values ranging from 0.15 (leukemia) to 7.35 (larynx) μM] and it failed against human erythrocytes and murine lines. Human normal peripheral blood mononuclear cells (PBMC) were up to 72.5-fold less sensitive [IC_50: 10.88 μM] to marinobufagin than HL-60 line, but DNA strand breaks were no detected. Leukemia treaded cells exhibited cell viability reduction, DNA fragmentation, phosphatidylserine externalization, binucleation, nuclear condensation and cytoplasmic vacuoles. Marinobufagin also reduced the growth of A. cepa roots (EC₅₀: 7.5 μM) and mitotic index, caused cell cycle arrest and chromosomal alterations (micronuclei, delays and C-metaphases) in meristematic cells. So, to find out partially targeted natural molecules on human leukemia cells, like marinobufagin, is an amazing and stimulating way to continue the battle against cancer.

Arenobufagin activates p53 to trigger esophageal squamous cell carcinoma cell apoptosis in vitro and in vivo

Esophageal squamous cell carcinoma (ESCC) is often diagnosed at late incurable stage and lacks effective treatment strategy. Bufadienolides are cardiotonic steroids isolated from the skin and parotid venom glands of the toad Bufo bufo gargarizans Cantor with novel anticancer activity. However, there is little information about the effects and action mechanisms of bufadienolides on ESCC cells. In this study, the in vitro and in vivo anti-ESCC activities of bufadienolides, including bufalin (Bu) and arenobufagin (ArBu), were examined and the underlying molecular mechanisms were elucidated. The results showed that ArBu exhibited higher anticancer efficacy than Bu against a panel of five ESCC cells, with IC₅₀ values ranging from 0.8 μM to 3.6 μM. However, ArBu showed lower toxicity toward Het-1A human normal esophageal squamous cells, indicating its great selectivity between cancer and normal cells. Moreover, ArBu effectively induced ESCC cell apoptosis mainly by triggering caspase activation through intrinsic and extrinsic pathways. Treatment of ESCC cells also significantly activated p53 signaling by enhancing its phosphorylation. Interestingly, transfection of cells with p53 small interfering RNA significantly inhibited the ArBu-induced p53 phosphorylation and the overall apoptotic cell death. Furthermore, ArBu also demonstrated novel in vivo anticancer efficacy by inhibiting the tumor growth through activation of p53 pathway. Taken together, these results demonstrate the p53-targeting therapeutic potential of bufadienolides against ESCC.

Bufadienolides from amphibians: A promising source of anticancer prototypes for radical innovation, apoptosis triggering and Na+/K+-ATPase inhibition

Amphibians present pharmacologically active aliphatic, aromatic and heterocyclic molecules in their skin as defense against microorganisms, predators and infections, such as steroids, alkaloids, biogenic amines, guanidine derivatives, proteins and peptides. Based on the discovered bioactive potential of bufadienolides, this work reviewed the contribution of amphibians, especially from members of Bufonidae family, as source of new cytotoxic and antitumor molecules, highlighting the mechanisms responsible for such amazing biological potentialities. Bufonidae species produce bufadienolides related to cholesterol through the mevalonate-independent and acidic bile acid pathways as polyhydroxy steroids with 24 carbons. In vitro antitumor studies performed with skin secretions and its isolated components (specially marinobufagin, telocinobufagin, bufalin and cinobufagin) from Rhinella, Bufo and Rhaebo species have shown remarkable biological action on hematological, solid, sensitive and/or resistant human tumor cell lines. Some compounds revealed higher selectivity against neoplastic lines when compared to dividing normal cells and some molecules may biochemically associate with Na⁺/K⁺-ATPase and there is structural similarity to the digoxin- and ouabain-Na⁺/K⁺-ATPase complexs, implying a similar mechanism of the Na⁺/K⁺-ATPase inhibition by cardenolides and bufadienolides. Some bufadienolides also reduce levels of antiapoptotic proteins and DNA synthesis, cause morphological changes (chromatin condensation, nuclear fragmentation, cytoplasm shrinkage, cytoplasmic vacuoles, stickiness reduction and apoptotic bodies), cell cycle arrest in G₂/M or S phases, mitochondrial depolarization, PARP [poly (ADPribose) polymerase] and Bid cleavages, cytochrome c release, activation of Bax and caspases (-3, -9, -8 and -10), increased expression of the Fas-Associated protein with Death Domain (FADD), induce topoisomerase II inhibition, DNA fragmentation, cell differentiation, angiogenesis inhibition, multidrug resistance reversion, and also regulate immune responses. Then, bufadienolides isolated from amphibians, some of them at risk of extinction, emerge as a natural class of incredible chemical biodiversity, has moderate selectivity against human tumor cells and weak activity on murine cells, probably due to structural differences between subunits of human and mice Na⁺/K⁺-ATPases.

Isolation and identification ofl/d-lactate-conjugated bufadienolides from toad eggs revealing lactate racemization in amphibians

Three pairs of bufadienolidel/d-lactate epimers (1–6) were isolated from the eggs of the toadBufo bufo gargarizans.

Cinobufagin, a bufadienolide, activates ROS-mediated pathways to trigger human lung cancer cell apoptosis in vivo

Lung cancer, as the most common malignancy worldwide, is one of the most threatening diseases for human beings.

Bufadienolides from parotoid gland secretions of Cuban toad Peltophryne fustiger (Bufonidae): Inhibition of human kidney Na(+)/K(+)-ATPase activity

Parotoid gland secretions of toad species are a vast reservoir of bioactive molecules with a wide range of biological properties. Herein, for the first time, it is described the isolation by preparative reversed-phase HPLC and the structure elucidation by NMR spectroscopy and/or mass spectrometry of nine major bufadienolides from parotoid gland secretions of the Cuban endemic toad Peltophryne fustiger: ψ-bufarenogin, gamabufotalin, bufarenogin, arenobufagin, 3-(N-suberoylargininyl) marinobufagin, bufotalinin, telocinobufagin, marinobufagin and bufalin. In addition, the secretion was analyzed by UPLC-MS/MS which also allowed the identification of azelayl arginine. The effect of arenobufagin, bufalin and ψ-bufarenogin on Na(+)/K(+)-ATPase activity in a human kidney preparation was evaluated. These bufadienolides fully inhibited the Na(+)/K(+)-ATPase in a concentration-dependent manner, although arenobufagin (IC50 = 28.3 nM) and bufalin (IC50 = 28.7 nM) were 100 times more potent than ψ-bufarenogin (IC50 = 3020 nM). These results provided evidence about the importance of the hydroxylation at position C-14 in the bufadienolide skeleton for the inhibitory activity on the Na(+)/K(+)-ATPase.