Bufadienolides of plant and animal origin

Assessment of neuropharmacological potential of low molecular weight components extracted from Rhinella schneideri toad poison

BACKGROUND

Studies on toad poison are relevant since they are considered a good source of toxins that act on different biological systems. Among the molecules found in the toad poison, it can be highlighted the cardiotonic heterosides, which have a known mechanism that inhibit Na+/K+-ATPase enzyme. However, these poisons have many other molecules that may have important biological actions. Therefore, this work evaluated the action of the low molecular weight components from Rhinella schneideri toad poison on Na+/K+-ATPase and their anticonvulsive and / or neurotoxic effects, in order to detect molecules with actions of biotechnological interest.

METHODS

Rhinella schneideri toad (male and female) poison was collected by pressuring their parotoid glands and immediately dried and stored at -20 °C. The poison was dialysed and the water containing the low molecular mass molecules (< 8 kDa) that permeate the dialysis membrane was collected, frozen and lyophilized, resulting in the sample used in the assays, named low molecular weight fraction (LMWF). Na+/K+ ATPase was isolated from rabbit kidneys and enzyme activity assays performed by the quantification of phosphate released due to enzyme activity in the presence of LMWF (1.0; 10; 50 and 100 µg/mL) from Rhinella schneideri poison. Evaluation of the L-Glutamate (L-Glu) excitatory amino acid uptake in brain-cortical synaptosomes of Wistar rats was performed using [3H]L-glutamate and different concentration of LMWF (10-5 to 10 µg/µL). Anticonvulsant assays were performed using pentylenetetrazole (PTZ) and N-methyl-D-aspartate (NMDA) to induce seizures in Wistar rats (n= 6), which were cannulated in the lateral ventricle and treated with different concentration of LMWF (0.25; 0.5; 1.0; 2.0; 3.0 and 4.0 µg/µL) 15 min prior to the injection of the seizure agent.

RESULTS

LMWF induced a concentration-dependent inhibition of Na+/K+-ATPase (IC50% = 107.5 μg/mL). The poison induces an increased uptake of the amino acid L-glutamate in brain-cortical synaptosomes of Wistar rats. This increase in the L-glutamate uptake was observed mainly at the lowest concentrations tested (10-5 to 10-2 µg/µL). In addition, this fraction showed a very relevant central neuroprotection on seizures induced by PTZ and NMDA.

CONCLUSIONS

LMWF from Rhinella schneideri poison has low molecular weight compounds, which were able to inhibit Na+/K+-ATPase activity, increase the L-glutamate uptake and reduced seizures induced by PTZ and NMDA. These results showed that LMWF is a rich source of components with biological functions of high medical and scientific interest.

Evaluation of in vitro neutralization of epoxyscillirosidine by antibodies raised in sheep

Intoxication by Moraea pallida Bak. (yellow tulp) in livestock is of great importance in South Africa, ranking top among all plant-induced cardiac glycoside toxicosis. The toxic principle, a bufadienolide, is 1α, 2α-epoxyscillirosidine. Treatment of poisoning is challenging and affected livestock often succumbs due to the stress of handling. Manipulating animals to resist poisoning is a potential management strategy. The goal of this study was to explore the potential to develop a vaccine against epoxyscillirosidine by raising antibodies against epoxyscillirosidine in sheep and to assess the neutralization ability of the antibodies in vitro. Epoxyscillirosidine was successfully conjugated to keyhole limpet haemocyanin (KLH) and bovine serum albumin (BSA) rendering them immunogenic. The sheep, vaccinated with epoxyscillirosidine-KLH conjugate (n = 4) and KLH (n = 2) with Montanide, developed antibodies as determined with an indirect enzyme linked immunosorbent assay (ELISA). Total immunoglobulins from sera of vaccinated and control sheep that were purified and concentrated using ammonium sulphate precipitation were 11,940 and 7850 μg, respectively. The in vitro neutralization assay using the methyl blue tetrazolium bromide (MTT) cell viability assay indicated no significant difference (p > 0.05) between anti-epoxyscillirosidine-KLH and KLH antibodies. Rather, the antibodies seemed to enhance the cytotoxicity of epoxyscillirosidine in H9c2 cells. Thus, it is necessary to develop improved vaccination methods to generate antibodies capable of neutralizing the functional group responsible for epoxyscillirosidine toxicity.

Changes in Toxin Quantities Following Experimental Manipulation of Toxin Reserves in Bufo bufo Tadpoles

Possessing toxins can contribute to an efficient defence against various threats in nature. However, we generally know little about the energy- and time-demands of developing toxicity in animals, which determines the efficiency of chemical defence and its trade-off with other risk-induced phenotypic responses. In this study we examined how immersion into norepinephrine solution inducing the release of stored toxins, administration of mild stress mimicking predator attack or simple handling during experimental procedure affected the quantity and number of toxin compounds present in common toad (Bufo bufo) tadpoles as compared to undisturbed control individuals, and investigated how fast toxin reserves were restored. We found that total bufadienolide quantity (TBQ) significantly decreased only in the norepinephrine treatment group immediately after treatment compared to the control, but this difference disappeared after 12 h; there were no consistent differences in TBQ between treatments at later samplings. Interestingly, in the norepinephrine treatment approximately half of the compounds characterized by >700 m/z values showed the same changes in time as TBQ, but several bufadienolides characterized by <600 m/z values showed the opposite pattern: they were present in higher quantities immediately after treatment. The number of bufadienolide compounds was not affected by any treatments, but was positively related to TBQ. Our study represents the first experimental evidence that toxin quantities returned to the original level following induced toxin release within a very short period of time in common toad tadpoles and provide additional insights into the physiological background of chemical defence in this model vertebrate species.

Marinobufagenin extraction from Rhinella marina toad glands: Alternative approaches for a systematized strategy

Marinobufagenin is a bufadienolide compound detected mainly in skin and parotoid gland secretions of Rhinella marina (L.) toad. Bufadienolides regulate the Na⁺ /K⁺ -ATPase pump by inhibiting the cardiotonic steroid dependent-site and act as cardiac inotropes with vasoconstrictive properties. Marinobufagenin and other bufadienolides, such as telocinobufagin and bufalin, are thought to be found endogenously in mammals in salt-sensitive hypertensive states such as essential hypertension, congestive heart-failure, and preeclampsia. The role of marinobufagenin as antimicrobial agent and its cytotoxic potential have also been recognized. The particular interest around marinobufagenin prompts us to consider the Rhinella marina toad venom as a possible source for molecules with pharmacological and/or diagnostic potential. In this article, two different approaches of extraction and purification of marinobufagenin from Rhinella marina (L.) venom are studied: (i) Preparative thin-layer chromatography combined to mass spectrometry and/or ultraviolet detection and (ii) solid-phase extraction coupled with fractionation on high-performance liquid chromatography. Different chromatographic conditions are tested for each approach. The solid-phase extraction combined with high-performance liquid chromatography fractionation approach was preferred as it offered a greater yield, was less time-consuming and allowed us to selectively isolate marinobufagenin. Both protocols aim to provide efficient and convenient methods for toad venom extraction, based on an easily automatable and systematized strategy.

Asymmetric synthesis of the tetracyclic core of bufogargarizin C by an intramolecular [5 + 2] cycloaddition

A concise asymmetric synthesis of the synthetically challenging 7/5/6/5-tetracyclic core of bufogargarizin C by a unique intramolecular [5 + 2] cycloaddition was reported.

(±)-Peniorthoesters A and B, Two Pairs of Novel Spiro-Orthoester en-antiomers With an Unusual 1,4,6-Trioxaspi-ro[4.5]decane-7-One Unit From Penicillium minioluteum

(±)-Peniorthoesters A and B (±1 and ±2), two pairs of unprecedented spiro-orthoester enantiomers with a 1,4,6-trioxaspiro[4. 5]decane-7-one unit, were obtained from Penicillium minioluteum. Their structures were determined by spectroscopic methods, X-ray diffraction analyses, and ECD calculations. (±)-Peniorthoesters A and B are the first examples of spiro-orthoester enantiomers, and they represent the first spiro-orthoesters originating from fungi. All compounds showed potential inhibitory activities comparable to dexamethasone against NO production with IC₅₀ values ranging from 14.2 to 34.5 μM.

Cardenolides: Insights from chemical structure and pharmacological utility

Cardiac glycosides (CGs) are a class of naturally occurring steroid-like compounds, and members of this class have been in clinical use for more than 1500 years. They have been used in folk medicine as arrow poisons, abortifacients, heart tonics, emetics, and diuretics as well as in other applications. The major use of CGs today is based on their ability to inhibit the membrane-bound Na⁺/K⁺-ATPase enzyme, and they are regarded as an effective treatment for congestive heart failure (CHF), cardiac arrhythmia and atrial fibrillation. Furthermore, increasing evidence has indicated the potential cytotoxic effects of CGs against various types of cancer. In this review, we highlight some of the structural features of this class of natural products that are crucial for their efficacy, some methods of isolating these compounds from natural resources, and the structural elucidation tools that have been used. We also describe their physicochemical properties and several modern biotechnological approaches for preparing CGs that do not require plant sources.

Traditional medical uses of Drimia species in terms of phytochemistry, pharmacology and toxicology

Drimia genus includes plants that used from ancient time for various ailments such as dropsy, respiratory ailment, bone and joint complications, skin disorders, epilepsy and cancer. Toxic properties of some Drimia species also were noted by ancient scientists and these plants have been traditionally used for rat control. Bufadienolides have been identified as the main constituents in the genus of Drimia. Phenolics, sterols, protein and some of other phytochemicals have been also isolated from these plants. Pharmacological and clinical studies have strongly approved their effect on cardiovascular system. Extracts and compounds isolated from Drimia species showed biological activities such as antibacterial, antifungal, antiviral, antioxidant, anti-inflammatory and insecticidal effects through several in vivo and in vitro studies. Moreover, cytotoxic and antitumor activities which may be related to bufadienolide content of these plants have been considered by many researchers. Traditional therapeutic values of these plants for treating respiratory and rheumatic ailments as well as skin disorders are needed to be validated through more researches. Toxic effects of these plants and isolated compounds have been investigated through several in vivo studies. Drimia plants and their isolated compounds have narrow therapeutic index, so patients should be prohibited from applying these plants without medical supervision and should be informed about the main intoxication symptoms before starting treatment. Moreover, interaction of Drimia plants with other constituents of traditional herbal mixtures as well as chemical and biological modalities for reducing toxicity of bufadienolide compounds can be subjected for future studies.

Bufadienolides from the Venom of Bufo Bufo gargarizans and Their Enzyme Inhibition Activities and Brine Shrimp Lethality

A new bufadienolide named dyscinobufotalin (1), along with twenty known bufadienolides (2–21) were isolated from the venom of Bufo bufo gargarizans. Their structures were elucidated by spectroscopic analysis. The absolute configuration of the new natural product 2 was determined by X-ray single crystal diffraction and the complete NMR data for 3 was delivered for the first time. Both compounds 4 and 8 showed comparable α-glucosidase inhibitory activity (IC50 values of 0.25 and 0.26 μM, respectively) to the positive control acarbose (IC50 value of 0.42 μM), and 5 displayed potent inhibitory activity on acetylcholinesterase with an IC50 value of 0.12 μM. Moreover, 5, 8 and 13 presented moderate toxicity against brine shrimp.

Toxic activity and protein identification from the parotoid gland secretion of the common toad Bufo bufo

Anuran toxins released from the skin glands are involved in defence against predators and microorganisms. Secretion from parotoid macroglands of bufonid toads is a rich source of bioactive compounds with the cytotoxic, cardiotoxic and hemolytic activity. Bufadienolides are considered the most toxic components of the toad poison, whereas the protein properties are largely unknown. In the present work, we analysed the cardio-, myo-, and neurotropic activity of extract and the selected proteins from Bufo bufo parotoids in in vitro physiological bioassays carried out on two standard model organisms: beetles and frogs. Our results demonstrate a strong cardioactivity of B. bufo gland extract. The toad poison stimulates (by 16%) the contractility of the insect heart and displays the cardioinhibitory effect on the frog heartbeat frequency (a 27% decrease), coupled with an irreversible cardiac arrest. The gland extract also exhibits significant myotropic properties (a 10% decrease in the muscle contraction force), whereas its neuroactivity remains low (a 4% decrease in the nerve conduction velocity). Among identified peptides present in the B. bufo parotoid extract are serine proteases, muscle creatine kinase, phospholipid hydroperoxide glutathione peroxidase, cytotoxic T-lymphocyte protein, etc. Some proteins contribute to the cardioinhibitory effect. Certain compounds display the paralytic (myo- and neurotropic) properties. As the toad gland extract exhibits a strong cardiotoxic activity, we conclude that the poison is a potent agent capable of slaying a predator. Our results also provide the guides for the use of toad poison-peptides in therapeutics and new drug development.

Probing the stereoselectivity of OleD-catalyzed glycosylation of cardiotonic steroids

The glycosyltransferase OleD variant as a catalyst for the glycosylation of four pairs of epimers of cardiotonic steroids (CTS) are assessed. The results of this study demonstrated that the OleD-catalyze glycosylation of CTS is significantly influenced by the configuration at C-3 and the A/B fusion mode. 3β-OH and A/B ring cis fusion are favoured by OleD (ASP). An epoxide ring at C-14 and C-15 further increases the bioconversion rate; while an acetyl group at C-16 and lactone ring type at C-17 did not influence the biotransformation. A high conversion rate corresponded to a low K m value. A molecular docking simulation showed that filling of hydrophobic pocket II and interaction with residue Tyr115 may play an important role in the glycosylation reactions catalyzed by OleD glycosyltransferases. Furthermore, the glycosylation products showed a stronger inhibitory activity for Na+, K+-ATPase than the corresponding aglycones. This study provides the first stereoselective properties for OleD (ASP) catalyzed glycosylation.

Helleborus sp. an ethnopharmacological and toxicological review

Helleborus sp. are healing plants from the family Ranunculaceae. The aim of this paper is to consider ethnopharmacology and the toxicity of the hellebores which is focused on a detailed research of the value of phytochemicals, as pharmacological attributes of phytomedicine herbs. The data is based on active components. Also, this paper presents the chemical composition of the root in the overall effect of the drug, as well as the importance of the hellebores in pharmacy. It shows a wide range of pharmacological effects such as cardiotonic, immunostimulative, antibacterial, antitumor, diuretic, and emetic. Helleborus odorus Waldst. et Kit. is rich with secondary metabolites such as bufadenolides, flavonoids, phenolic heterozids. With this research it was concluded that these plants have great significance and great application in the formulation of pharmaceutical dosage forms. Keywords: Helleborus sp., phytomedicine, folk medicinal uses, cardiotonic, toxicity, hellebore

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.

Snakes exhibit tissue-specific variation in cardiotonic steroid sensitivity of Na+/K+-ATPase

Toads are among several groups of organisms chemically defended with lethal concentrations of cardiotonic steroids. As a result, most predators that prey on amphibians avoid toads. However, several species of snakes have gained resistance-conferring mutations of Na⁺/K⁺-ATPase, the molecular target of cardiotonic steroids, and can feed on toads readily. Despite recent advances in our understanding of this adaptation at the genetic level, we have lacked functional evidence for how mutations of Na⁺/K⁺-ATPase account for cardiotonic steroid resistance in snake tissues. To address this issue, it is necessary to determine how the Na⁺/K⁺-ATPases of snakes react to the toxins. Some tissues might have Na⁺/K⁺-ATPases that are more susceptible than others and can thus provide clues about how the toxins influence organismal function. Here we provide a mechanistic link between observed Na⁺/K⁺-ATPase substitutions and observed resistance using actual snake Na⁺/K⁺-ATPases. We used an in vitro approach to determine the tissue-specific levels of sensitivity to cardiotonic steroids in select resistant and non-resistant snakes. We compared the sensitivities of select tissues within and between species. Our results suggest that resistant snakes contain highly resistant Na⁺/K⁺-ATPases in their heart and kidney, both of which rely heavily on the enzymes to function, whereas tissues that do not rely as heavily on Na⁺/K⁺-ATPases or might be protected from cardiotonic steroids by other means (liver, gut, and brain) contain non-resistant forms of the enzyme. This study reveals functional evidence that tissue-level target-site insensitivity to cardiotonic steroids varies not only among species but also across tissues within resistant taxa.

De Novo Synthesis of Possible Candidates for the Inagami-Tamura Endogenous Digitalis-like Factor

De novo synthesis of possible candidates for the Inagami-Tamura endogenous digitalis-like factor (EDLF) was achieved to validate a previously proposed structure. Our synthetic approach involves a highly regio- and diastereoselective Mizoroki-Heck reaction and a Friedel-Crafts-type cyclodehydration to construct steroidal tetracycle 14 as a versatile common intermediate leading to seven 2,14β-dihydroxyestradiol analogues 1a-c, 2a-c, and 3 as possible candidates. By comparing the potency of inhibitory activity against Na⁺/K⁺-ATPase between the synthesized candidates and the EDLF, it was found that the proposed structure is not likely to be a true structure of the Inagami-Tamura EDLF.

Microbiome-Mediated Biotransformation of Cane Toad Bufagenins

Cane toads are an invasive pest species in which all life stages employ cardiotoxic bufagenins as a chemical defense against predators. Curiously, the bufagenin profiles of eggs and tadpoles are more complex than those of parotoid secretion, the principle mechanism of toxin delivery in adult toads. In an effort to understand this complexity, we determined that selected strains of adult toad parotoid-gland-associated Gram-positive bacteria, Bacillus spp., were capable of biotransforming secreted bufagenins, marinobufagenin (1), telocinobufagenin (2), bufalin (3), and resibufagenin (4), to hydroxylated scaffolds commonly encountered in cane toad eggs and tadpoles. Scaled-up cultivation, preparative chromatography, and detailed spectroscopic analysis identified Bacillus sp. CMB-TD29 biotransformation products of 1, as 11α-hydroxymarinobufagenin (6), 12β-hydroxymarinobufagenin (7), and 17α-hydroxymarinobufagenin (8). Comparative bufagenin profiles across the cane toad life cycle suggest that bacterial biotransformation mediates the oxidative adaptation of adult toad bufagenins to hydroxylated bufagenins encountered in eggs and tadpoles. We speculate that knowledge of a relationship between the cane toad microbiome and bufagenin chemical defenses could inspire the development of a natural, nontoxic, environmentally sustainable bacterial biocontrol for this toxic invasive species.

Chronic exposure to a glyphosate-based herbicide makes toad larvae more toxic

Chemical pollutants can exert various sublethal effects on wildlife, leading to complex fitness consequences. Many animals use defensive chemicals as protection from predators and diseases, yet the effects of chemical contaminants on this important fitness component are poorly known. Understanding such effects is especially relevant for amphibians, the globally most threatened group of vertebrates, because they are particularly vulnerable to chemical pollution. We conducted two experiments to investigate how exposure to glyphosate-based herbicides, the most widespread agrochemicals worldwide, affects the production of bufadienolides, the main compounds of chemical defence in common toads (Bufo bufo). In both experiments, herbicide exposure increased the amount of bufadienolides in toad tadpoles. In the laboratory, individuals exposed to 4 mg a.e./L glyphosate throughout their larval development had higher bufadienolide content at metamorphosis than non-exposed tadpoles, whereas exposure for 9 days to the same concentration or to 2 mg a.e./L throughout larval development or for 9 days had no detectable effect. In outdoor mesocosms, tadpoles from 16 populations exhibited elevated bufadienolide content after three-weeks exposure to both concentrations of the herbicide. These results show that pesticide exposure can have unexpected effects on non-target organisms, with potential consequences for the conservation management of toxin-producing species and their predators.

Identification of Na+/K+-ATPase inhibition-independent proarrhythmic ionic mechanisms of cardiac glycosides

The current study explored the Na⁺/K⁺-ATPase (NKA) inhibition-independent proarrhythmic mechanisms of cardiac glycosides (CGs) which are well-known NKA inhibitors. With the cytosolic Ca²⁺ chelated by EGTA and BAPTA or extracellular Ca²⁺ replaced by Ba²⁺, effects of bufadienolides (bufalin (BF) and cinobufagin (CBG)) and cardenolides (ouabain (Oua) and pecilocerin A (PEA)) on the L-type calcium current (I_Ca,L) were recorded in heterologous expression Cav1.2-CHO cells and human embryonic stem cell-derived cardiomyocytes (hESC-CMs). BF and CBG demonstrated a concentration-dependent (0.1 to 100 µM) I_Ca,L inhibition (maximal ≥50%) without and with the NKA activity blocked by 10 µM Oua. BF significantly shortened the action potential duration at 1.0 µM and shortened the extracellular field potential duration at 0.01~1.0 µM. On the other hand, BF and CBG at 100 µM demonstrated a strong inhibition (≥40%) of the rapidly activating component of the delayed rectifier K⁺ current (I_Kr) in heterologous expression HEK293 cells and prolonged the APD of the heart of day-3 Zebrafish larva with disrupted rhythmic contractions. Moreover, hESC-CMs treated with BF (10 nM) for 24 hours showed moderate yet significant prolongation in APD90. In conclusion, our data indicate that CGs particularly bufadienolides possess cytosolic [Ca²⁺]_i- and NKA inhibition- independent proarrhythmic potential through I_Ca,L and I_Kr inhibitions.

MK2206 enhances the cytocidal effects of bufalin in multiple myeloma by inhibiting the AKT/mTOR pathway

Despite the development of promising cancer therapeutic drugs, multiple myeloma (MM) remains an incurable disease. Bufalin is a bufanolide steroid compound of the traditional Chinese medicine Chan Su that was previously shown to exert growth suppression effects on myeloma cell lines. Previous studies conducted by our group demonstrated that bufalin activated the AKT/mTOR pathway in myeloma cells, which is considered an essential pathway to disease progression and is related to drug resistance in MM. In view of the significant role of AKT in MM, the allosteric AKT inhibitor MK2206 was selected in order to enhance the antitumor effects of bufalin in different MM cell lines (NCI-H929, U266, LP-1 and RPMI8226). The data indicated that MK2206 enhanced the cytotoxicity of bufalin in MM cells, via the suppression of cellular proliferation and the induction of apoptosis, as demonstrated by cleavage of apoptosis-related proteins. This effect was further noted in the presence of exogenous interleukin-6 and/or following the co-culture of MM cells with bone marrow stromal cells (BMSC). This process was associated with the inhibition of the AKT/mTOR pathway. The combination of bufalin with MK2206 reduced the secretion of IL-6 in U266 cells. The combined treatment exhibited similar anti-MM effects in bortezomib-resistant cell lines (NCI-H929R, U266R). In addition to the in vitro cell line models, the synergistic effect was noted in primary MM cells and in MM xenografts of BALB-c and NOD-SCID mice. In conclusion, the data suggested that MK2206 significantly enhanced the cytocidal effects of bufalin in MM cells, regardless of the sensitivity to bortezomib, via the inhibition of the AKT/mTOR pathway. The study provided the basis of a promising treatment approach for MM.

The synthesis of cardenolide and bufadienolide aglycones, and related steroids bearing a heterocyclic subunit

Covering: early studies through to March 2016Cardenolides and bufadienolides constitute an attractive class of biologically active steroid derivatives which have been used for the treatment of heart disease in traditional remedies as well as in modern medicinal therapy. Due to their application as therapeutic agents and their unique molecular structures, bearing unsaturated 5- or 6-membered lactones (or other heterocycles) attached to the steroid core, cardio-active steroids have received great attention, which has intensified during the last decade, in the synthetic organic community. Advances in the field of cross-coupling reactions have provided a powerful tool for the attachment of lactone subunits to the steroid core. This current review covers a methodological analysis of synthetic efforts to cardenolide and bufadienolide aglycones. Special emphasis is given to cross-coupling reactions applied for the attachment of lactone subunits at sterically very hindered positions of the steroid core. The carefully selected partial and total syntheses of representative cardio-active steroids will also be presented to exemplify recent achievements (improvements) in the field.

A Strategy Combining Higher Energy C-Trap Dissociation with Neutral Loss- and Product Ion-Based MSn Acquisition for Global Profiling and Structure Annotation of Fatty Acids Conjugates

Fatty acids conjugates (FACs) are ubiquitous but found in trace amounts in the natural world. They are composed of multiple unknown substructures and side chains. Thus, FACs are difficult to be analyzed by traditional mass spectrometric methods. In this study, an integrated strategy was developed to global profiling and targeted structure annotation of FACs in complex matrix by LTQ Orbitrap. Dicarboxylic acid conjugated bufotoxins (DACBs) in Venenum bufonis (VB) were used as model compounds. The new strategy (abbreviated as HPNA) combined higher-energy C-trap dissociation (HCD) with product ion- (PI), neutral loss- (NL) based MSⁿ (n ≥ 3) acquisition in both positive-ion mode and negative-ion mode. Several advantages are presented. First, various side chains were found under HCD in negative-ion mode, which included both known and unknown side chains. Second, DACBs with multiple side chains were simultaneously detected in one run. Compared with traditional quadrupole-based mass method, it greatly increased analysis throughput. Third, the fragment ions of side chain and steroids substructure could be obtained by PI- and NL-based MSⁿ acquisition, respectively, which greatly increased the accuracy of the structure annotation of DACBs. In all, 78 DACBs have been discovered, of which 68 were new compounds; 25 types of substructure formulas and seven dicarboxylic acid side chains were found, especially five new side chains, including two saturated dicarboxylic acids [(azelaic acid (C₉) and sebacic acid (C₁₀)] and three unsaturated dicarboxylic acids (u-C₈, u-C₉, and u-C₁₀). All these results greatly enriched the structures of DACBs in VB. Graphical Abstract ᅟ.

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.

A Novel Bufalin Derivative Exhibited Stronger Apoptosis-Inducing Effect than Bufalin in A549 Lung Cancer Cells and Lower Acute Toxicity in Mice

BF211 is a synthetic molecule derived from bufalin (BF). The apoptosis-inducing effect of BF211 was stronger than that of BF while the acute toxicity of BF211 was much lower than that of BF. BF211 exhibited promising concentration-dependent anti-cancer effects in nude mice inoculated with A549 cells in vivo. The growth of A549 tumor xenografts was almost totally blocked by treatment with BF211 at 6 mg/kg. Notably, BF and BF211 exhibited differences in their binding affinity and kinetics to recombinant proteins of the α subunits of Na+/K+-ATPase. Furthermore, there was a difference in the effects of BF or BF211 on inhibiting the activity of porcine cortex Na+/K+-ATPase and in their time-dependent effects on intracellular Ca2+ levels in A549 cells. The time-dependent effects of BF or BF211 on the activation of Src, which was mediated by the Na+/K+-ATPase signalosome, in A549 cells were also different. Both BF and BF211 could induce apoptosis-related cascades, such as activation of caspase-3 and the cleavage of PARP (poly ADP-ribose polymerase) in A549 cells, in a concentration-dependent manner; however, the effects of BF211 on apoptosis-related cascades was stronger than that of BF. The results of the present study supported the importance of binding to the Na+/K+-ATPase α subunits in the mechanism of cardiac steroids and also suggested the possibility of developing new cardiac steroids with a stronger anti-cancer activity and lower toxicity as new anti-cancer agents.

Inhibitory efficacy of bufadienolides on Na+,K+-pump activity versus cell proliferation

Bufadienolides are cytotoxic drugs that may form the basis for anticancer agents. Due to structural and functional similarity to cardiotonic glycosides, application is restricted. We, therefore, investigated correlation of their putative anticancer effects with inhibition of Na⁺,K⁺pumps. The natural bufalin and three derivatives were tested. The anticancer effects of the drugs were checked by observing their inhibitory effects on proliferation of rat liver cancer cells using MTT assay. Inhibition of Na⁺,K⁺-pump was determined by measuring pump-mediated current of rat α1/β1 and α2/β1 Na⁺,K⁺pumps expressed in Xenopus oocytes.

All tested bufadienolides inhibited cell proliferation and Na⁺,K⁺pump activity. An activity coefficient A=100xIC₅₀^{Na,K pump}/IC₅₀^{proliferation} was used to describe drug effectivity as anticancer drug. Natural bufalin exhibited lowest effectivity on cell proliferation, and also the A value for rat α1 isoform was the lowest (0.08), the α2 isoform was much less sensitive (A=1.00). The highest A values were obtained for the BF238 derivative with A=0.88 and 2.64 for the α1 and α2 isoforms, respectively. Therefore, we suggest that search for bufalin derivatives with high anticancer effect and low affinity for both Na⁺,K⁺pump isoforms may be a promising strategy for development of anticancer drugs.

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Effects of bufadienolides on Na pump are not correlated with their cytotoxicity.

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Bufadienolides with high anticancer effect but low side effect may exist.

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BF238 may form a basis for further anticancer drug research and development.

Potential Health Risks Posed by Plant-Derived Cumulative Neurotoxic Bufadienolides in South Africa

Bufadienolide-type cardiac glycosides have a worldwide distribution and are mainly synthesized by plants, but there are also animal sources. In South Africa, members of three genera of the Crassulaceae (Cotyledon, Tylecodon and Kalanchoe) cause a unique chronic form of cardiac glycoside poisoning, predominantly in small stock. This paretic/paralytic condition is referred to as “krimpsiekte”, cotyledonosis or “nenta”. “Krimpsiekte” is a plant poisoning only reported from South Africa and is regarded as the most important plant poisoning of small stock in the semi-arid Little Karoo and southern fringes of the Great Karoo. The toxicosis is caused by cumulative bufadienolides which have neurotoxic properties. Four types of cumulative neurotoxic bufadienolides, namely cotyledoside, and the tyledosides, orbicusides and lanceotoxins, have been isolated. Based on the structure activity relationships and certain toxicokinetic parameters possible reasons for their accumulation are presented. Consumption of edible tissues from animals that have ingested these plants poses a potential risk to humans.

TXNIP/TRX/NF-κB and MAPK/NF-κB pathways involved in the cardiotoxicity induced by Venenum Bufonis in rats

Venenum Bufonis (VB) is a widely used traditional medicine with serious cardiotoxic effects. The inflammatory response has been studied to clarify the mechanism of the cardiotoxicity induced by VB for the first time. In the present study, Sprague Dawley (SD) rats, were administered VB (100, 200, and 400 mg/kg) intragastrically, experienced disturbed ECGs (lowered heart rate and elevated ST-segment), increased levels of serum indicators (creatine kinase (CK), creatine kinase isoenzyme-MB (CK-MB), alanine aminotransferase (ALT), aspartate aminotransferase (AST)) and serum interleukin (IL-6, IL-1β, TNF-α) at 2 h, 4 h, 6 h, 8 h, 24 h, and 48 h, which reflected that an inflammatory response, together with cardiotoxicity, were involved in VB-treated rats. In addition, the elevated serum level of MDA and the down-regulated SOD, CAT, GSH, and GPx levels indicated the appearance of oxidative stress in the VB-treated group. Furthermore, based on the enhanced expression levels of TXNIP, p-NF-κBp65, p-IκBα, p-IKKα, p-IKKβ, p-ERK, p-JNK, and p-P38 and the obvious myocardial degeneration, it is proposed that VB-induced cardiotoxicity may promote an inflammatory response through the TXNIP/TRX/NF-κB and MAPK/NF-κB pathways. The observed inflammatory mechanism induced by VB may provide a theoretical reference for the toxic effects and clinical application of VB.

ψ-Bufarenogin, a novel anti-tumor compound, suppresses liver cancer growth by inhibiting receptor tyrosine kinase-mediated signaling

Resistance of hepatocellular carcinoma (HCC) to existing chemotherapeutic agents largely contributes to the poor prognosis of patients, and discovery of novel anti-HCC drug is in an urgent need. Herein we report ψ-Bufarenogin, a novel active compound that we isolated from the extract of toad skin, exhibited potent therapeutic effect in xenografted human hepatoma without notable side effects. In vitro, ψ-Bufarenogin suppressed HCC cells proliferation through impeding cell cycle progression, and it facilitated cell apoptosis by downregulating Mcl-1 expression. Moreover, ψ-Bufarenogin decreased the number of hepatoma stem cells through Sox2 depression and exhibited synergistic effect with conventional chemotherapeutics. Mechanistic study revealed that ψ-Bufarenogin impaired the activation of MEK/ERK pathway, which is essential in the proliferation of hepatoma cells. ψ-Bufarenogin notably suppressed PI3-K/Akt cascade, which was required in ψ-Bufarenogin-mediated reduction of Mcl-1 and Sox2. ψ-Bufarenogin inhibited the auto-phosphorylation and activation of epithelial growth factor receptor (EGFR) and hepatocyte growth factor receptor (c-Met), thereafter suppressed their primary downstream cascades Raf/MEK/ERK and PI3-K/Akt signaling. Taken together, ψ-Bufarenogin suppressed HCC growth via inhibiting, at least partially, receptor tyrosine kinases-regulated signaling, suggesting that ψ-Bufarenogin could be a novel lead compound for anti-HCC drug.

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.

Molecular structure-affinity relationship of bufadienolides and human serum albumin in vitro and molecular docking analysis

The development of bufadienolides as anti-tumor agents is limited due to poor pharmacokinetic properties regarding drug half-lives and toxicity in vivo. These serious factors might be improved by increasing the drug/albumin-binding ratio. This study therefore investigated the relationship between the structural properties of nine bufadienolides and their affinities for human serum albumin (HSA) by a fluorescence spectroscopy-based analysis and molecular docking. Fluorescence quenching data showed that the interaction of each bufadienolide with HSA formed a non-fluorescent complex, while thermodynamic parameters revealed negative ΔS and ΔH values, corresponding to changes in enthalpy and entropy, respectively. The structural differences between the various bufadienolides markedly influenced their binding affinity for HSA. With the exception of a C = O bond at the C12 position that decreased the binding affinity for HSA, other polar groups tended to increase the affinity, especially a hydroxyl (OH) group at assorted bufadienolide sites. The rank order of binding affinities for drugs with tri-hydroxyl groups was as follows: 11-OH > 5-OH > 16-OH; in addition, 16-acetoxy (OAc), 10-aldehyde and 14-epoxy constituents notably enhanced the binding affinity. Among these groups, 11-OH and 16-acetyl were especially important for a seamless interaction between the bufadienolides and HSA. Furthermore, molecular docking analysis revealed that either an 11-OH or a 16-OAc group spatially close to a five-membered lactone ring significantly facilitated the anchoring of these compounds within site I of the HSA pocket via hydrogen bonding (H-bonding) with Tyr150 or Lys199, respectively. In summary, bufadienolide structure strongly affects binding with HSA, and 11-OH or 16-OAc groups improve the drug association with key amino acid residues. This information is valuable for the prospective development of bufadienolides with improved pharmacological profiles as novel anti-tumor drugs.

Study of the cardiotoxicity of Venenum Bufonis in rats using an 1H NMR-based metabolomics approach

Venenum Bufonis, a well-known traditional Chinese medicine, has been widely used in Asia and has gained popularity in Western countries over the last decade. Venenum Bufonis has obvious side effects that have been observed in clinical settings, but few studies have reported on its cardiotoxicity. In this work, the cardiotoxicity of Venenum Bufonis was investigated using a ¹¹H NMR-based metabolomics approach. The ¹H NMR profiles of the serum, myocardial extracts and liver extracts of specific-pathogen-free rats showed that Venenum Bufonis produced significant metabolic perturbations dose-dependently with a distinct time effect, peaking at 2 hr after dosing and attenuating gradually. Clinical chemistry, electrocardiographic recordings, and histopathological evaluation provided additional evidence of Venenum Bufonis-induced cardiac damage that complemented and supported the metabolomics findings. The combined results demonstrated that oxidative stress, mitochondrial dysfunction, and energy metabolism perturbations were associated with the cardiac damage that results from Venenum Bufonis.

Effects of Sumsu (Bufonis venenum) Pharmacopuncture Treatment on Depression in Mice

Objectives:

The main objective of this study was to evaluate the anti-depressant effects of pharmacopuncture using sumsu (Bufonis venenum).

Methods:

Animals were divided into three groups (control, sham, and experimental), with eight mice per group. The sham and the experimental groups were exposed to 2 hours of immobilization stress daily for 14 days. They were also injected with normal saline (sham) or subjected to pharmacopuncture with sumsu at the acupoints HT7, SP6, and GV20 (experimental). The depression or anxiety-like behaviors of the mice in each group were evaluated 1 day after treatment.

Results:

There was no difference in locomotor activity between the groups during the open-field test; i.e., all groups had normal motor function. However, the open-field and the forced-swimming tests revealed that depression and anxiety-like behaviors were decreased significantly in the group treated with sumsu pharmacopuncture.

Conclusion:

Sumsu pharmacopuncture attenuated depressive or anxiety-like behavior in mice stressed with chronic immobilization. These results suggest that sumsu pharmacopuncture has therapeutic potential for treating neuropsychiatric disorders such as anxiety or depression disorder.

Development of a concise synthesis of ouabagenin and hydroxylated corticosteroid analogues

The natural product ouabagenin is a complex cardiotonic steroid with a highly oxygenated skeleton. This full account describes the development of a concise synthesis of ouabagenin, including the evolution of synthetic strategy to access hydroxylation at the C19 position of a steroid skeleton. In addition, approaches to install the requisite butenolide moiety at the C17 position are discussed. Lastly, methodology developed in this synthesis has been applied in the generation of novel analogues of corticosteroid drugs bearing a hydroxyl group at the C19 position.

Biotransformation of resibufogenin by Actinomucor elegans

Resibufogenin (RB), a major bioactive bufadienolide, has the potential anticancer activity. In the present work, biotransformation of RB by Actinomucor elegans AS 3.2778 yielded five products, namely 3-oxo-resibufogenin (1), 3-epi-resibufogenin (2), 3-epi-12-oxo-hydroxylresibufogenin (3), 3α-acetoxy-15α-hydroxylbufalin (4), and 3-epi-12α-hydroxylresibufogenin (5), respectively. Among them, metabolites 3 and 4 are previously unreported. The chemical structures of metabolites 1-5 were fully elucidated on the basis of 2D NMR and HR-MS. The highly stereo- and regio-specific isomerization, hydroxylation, and esterification reactions were observed in the biotransformation process of RB by A. elegans. Their cytotoxicities against A549 and H1299 cells were evaluated.

Spatial variation of epoxyscillirosidine concentrations in Moraea pallida (yellow tulp) in South Africa

Moraea pallida (yellow tulp) poisoning is economically the most important intoxication of livestock in South Africa. Poisoning varies according to locality, climatic conditions and growth stage of the plant. The primary objective of this study was to determine the concentration of the toxic principle, epoxyscillirosidine, in yellow tulp leaves and to ascertain the variability of epoxyscillirosidine concentrations within and between different locations. A secondary objective was to utilise Geographic Information Systems in an attempt to explain the variability in toxicity. Flowering yellow tulp plants were collected at 26 sampling points across 20 districts of South Africa. The leaves of five plants per sampling point were extracted and submitted for liquid chromatography/mass spectrometry analysis. A large variation in mean epoxyscillirosidine concentrations, ranging from 3.32 μg/g - 238.27 μg/g, occurred between different geographical regions. The epoxyscillirosidine concentrations also varied tremendously between individual plants (n = 5) collected at the same sampling point, with up to a 24 times difference between the lowest and highest concentration detected. No generalised correlation between epoxyscillirosidine concentrations and soil elemental concentrations could be established. However, samples obtained from the north-eastern part of the sampling region tended to have higher epoxyscillirosidine concentrations compared to samples obtained from the south-western part of the sampling region. Higher toxin concentrations in the north-east were associated with statistically significant higher soil concentrations of iron, bismuth, bromide, cadmium, chromium, rubidium, tellurium, thallium, titanium and zinc, whilst soil concentrations of strontium and soil pH, were significantly lower. This study corroborated the contention that epoxyscillirosidine concentration in yellow tulp fluctuates and may explain the variability in toxicity.

C23 steroids from the venom of Bufo bufo gargarizans

Five new C23 steroids (1-5) together with five known bufadienolides (6-10) were isolated from the venom of Bufo bufo gargarizans (ChanSu in Chinese). The structures of the new steroids were elucidated by extensive spectroscopic methods in combination with X-ray diffraction analysis. Among these C23 steroids, only compound 3 showed cytotoxicities against HepG2 and A549 cancer cells, with respective IC50 values of 26.8 ± 8.3 and 45.6 ± 2.5 μM. In contrast, the bufadienolides (7-10) displayed potent inhibitory activities against these cancer cells, with respective IC50 values in the ranges 0.5-5.5 and 0.6-6.5 μM, but relatively less cytotoxicity on normal mouse spleen cells. In addition, the Na(+)/K(+)-ATPase inhibitory activities of 2, 5, and 7 revealed that the lactone moiety of a bufadienolide was important for the inhibitory activity.

Quantitative determination of arenobufagin in rat plasma by ultra fast liquid chromatography-tandem mass spectrometry and its application in a pharmacokinetic study

A rapid, sensitive, and selective ultra fast liquid chromatography-tandem mass spectrometry method was developed for quantitative determination of arenobufagin in rat plasma. Sample pretreatment involved a one-step protein precipitation with methanol using 0.1mL rat plasma. The separation was carried out on a Shim-pack XR-ODS II (75mm×2.0mm, i.d. 2.1μm) column with gradient elution at a flow rate of 0.30mLmin(-1). The mobile phase was acetonitrile and 0.1% formic acid in water. A post-column switching valve was applied to reduce the matrix effect. The detection was performed on a triple-quadruple tandem mass spectrometer in the multiple reaction monitoring mode after electrospray ionization. Linear calibration curves for arenobufagin were obtained over the concentration range 1.056-1056ngmL(-1), with a lower limit of quantification of 1.056ngmL(-1). The intra-day and inter-day precision values were lower than 15% and the accuracy ranged from 5.4% to 9.8% at all quality control levels. The method was successfully applied to the determination and pharmacokinetic study of arenobufagin in rat plasma following intraperitoneal administration.

Antiproliferative Homoisoflavonoids and Bufatrienolides from Urginea depressa

Investigation of the South African plant Urginea depressa Baker (Asparagaceae Juss.) for antiproliferative activity against the A2780 ovarian cancer cell line led to the isolation of the six new homoisoflavonoids urgineanins A-F (1-6), the two known bufatrienolides 7 and 9, and the new bufatrienolides urginins B and C (8 and 10). Structures were elucidated based on analysis of their 1D and 2D NMR spectra, electronic circular dichroism, and mass spectrometric data. Five of the six new homoisoflavonoids had good antiproliferative activity against the A2780 ovarian cancer, A2058 melanoma, and H522-T1 human non-small-cell lung cancer cells, and urgineanin A (1) had submicromolar activity against all three cell lines. The four bufatrienolides 7-10 had strong antiproliferative activity against the same cell line, with IC50 values of 24.1, 11.2, 111, and 40.6 nM, respectively.

Anticancer steroids: linking natural and semi-synthetic compounds

Steroids, a widespread class of natural organic compounds occurring in animals, plants and fungi, have shown great therapeutic value for a broad array of pathologies. The present overview is focused on the anticancer activity of steroids, which is very representative of a rich structural molecular diversity and ability to interact with various biological targets and pathways. This review encompasses the most relevant discoveries on steroid anticancer drugs and leads through the last decade and comprises 668 references.