Rapid structural identification of cytotoxic bufadienolide sulfates in toad venom from Bufo melanosticus by LC-DAD-MS(n) and LC-SPE-NMR

NMR-Based Chromatography Readouts: Indispensable Tools to “Translate” Analytical Features into Molecular Structures

Gaining structural information is a must to allow the unequivocal structural characterization of analytes from natural sources. In liquid state, NMR spectroscopy is almost the only possible alternative to HPLC-MS and hyphenating the effluent of an analyte separation device to the probe head of an NMR spectrometer has therefore been pursued for more than three decades. The purpose of this review article was to demonstrate that, while it is possible to use mass spectrometry and similar methods to differentiate, group, and often assign the differentiating variables to entities that can be recognized as single molecules, the structural characterization of these putative biomarkers usually requires the use of NMR spectroscopy.

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.

Toad venom: A comprehensive review of chemical constituents, anticancer activities, and mechanisms

Toad venom, a traditional natural medicine, has been used for hundreds of years in China for treating different diseases. Many studies have been performed to elucidate the cardiotonic and analgesic activities of toad venom. Until the last decade, an increasing number of studies have documented that toad venom is a source of lead compound(s) for the development of potential cancer treatment drugs. Research has shown that toad venom contains 96 types of bufadienolide monomers and 23 types of indole alkaloids, such as bufalin, cinobufagin, arenobufagin, and resibufogenin, which exhibit a wide range of anticancer activities in vitro and, in particular, in vivo for a range of cancers. The main antitumor mechanisms are likely to be apoptosis or/and autophagy induction, cell cycle arrest, cell metastasis suppression, reversal of drug resistance, or growth inhibition of cancer cells. This review summarizes the chemical constituents of toad venom, analyzing their anticancer activities and molecular mechanisms for cancer treatments. We also outline the importance of further studies regarding the material basis and anticancer mechanisms of toad venom.

Eviscerated alive: Novel and macabre feeding strategy in Oligodon fasciolatus (Günther, 1864) eating organs of Duttaphrynus melanostictus (Schneider, 1799) in Thailand

A hitherto unknown feeding mode among snakes is described for the colubrid snake Oligodon fasciolatus in Northeast Thailand. Three cases of O. fasciolatus using enlarged posterior maxillary teeth to cut open the abdomen of a live poisonous toad Duttaphrynus melanostictus and eat its organs are described. The snakes and toads fought vigorously, and the toads secreted toxic white liquid on the dorsum and neck. The snakes inserted their heads into the abdomen of the toads, pulled out some of the organs and swallowed them. The snakes and toads were adults. All three cases were documented by extensive photographic material.

In a fourth case from Central Thailand, an adult O. fasciolatus was observed swallowing an entire semi-adult D. melanostictus.

The majority of all snake species swallow their prey in one piece, but to place our observations in a broader context we review a number of exceptions.

Total synthesis, chemical modification and structure-activity relationship of bufadienolides

Bufadienolides are a type of natural cardiac steroids and originally isolated from the Traditional Chinese Medicine Chan'Su, they have been used for the treatment of heart disease in traditional remedies as well as in modern medicinal therapy with potent anti-tumor activities. Due to their unique molecular structures with unsaturated six-membered lactones attached to the steroid core, bufadienolides have received great attention in the synthetic organic community. This review presents total synthetic efforts to some representative bufadienolides, chemical modification of bufadienolides will also be given to discuss their structure-activity relationship in anti-tumor.

Metabolomics in the Context of Plant Natural Products Research: From Sample Preparation to Metabolite Analysis

Plant-derived natural products have long been considered a valuable source of lead compounds for drug development. Natural extracts are usually composed of hundreds to thousands of metabolites, whereby the bioactivity of natural extracts can be represented by synergism between several metabolites. However, isolating every single compound from a natural extract is not always possible due to the complex chemistry and presence of most secondary metabolites at very low levels. Metabolomics has emerged in recent years as an indispensable tool for the analysis of thousands of metabolites from crude natural extracts, leading to a paradigm shift in natural products drug research. Analytical methods such as mass spectrometry (MS) and nuclear magnetic resonance (NMR) are used to comprehensively annotate the constituents of plant natural products for screening, drug discovery as well as for quality control purposes such as those required for phytomedicine. In this review, the current advancements in plant sample preparation, sample measurements, and data analysis are presented alongside a few case studies of the successful applications of these processes in plant natural product drug discovery.

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.

Analysis of proliferation and apoptotic induction by 20 steroid glycosides in 143B osteosarcoma cells in vitro

OBJECTIVES

Osteosarcoma is the most common type of malignant bone tumour in children and adolescents; it has poor prognosis, is highly metastatic and is resistant to current therapeutic approaches. In this study, different herbal extracts used in phytotherapy have been screened after searching innovative natural anti-cancer components.

MATERIALS AND METHODS

Twenty steroid glycosides were examined for accordance to their potential of inhibiting cell proliferation and inducing apoptosis in the osteosarcoma cell line 143B. Cell proliferation was examined using a CASY counter. Effects of cardiac glycosides on induction of apoptosis were evaluated by Annexin V-APC and flow cytometry, caspase activity assay and measurement of mitochondrial membrane potential.

RESULTS

The study revealed that various steroid glycosides suppress cell proliferation in a concentration-dependent manner. Further investigations indicated apoptotic induction by 17 of the 20 tested cardenolides and bufadienolides. Bufadienolide proscillaridin A, arenobufagin, and cardenolides evomonoside, convallatoxol and ouabain waged strongest apoptotic induction, associated with breakdown of mitochondrial membrane potential and activation of caspases -8 and -9. In contrast, the bufadienolide resibufogenin and cardenolide uzarin had no effect on proliferation inhibition, apoptotic induction or change in mitochondrial membrane potential.

CONCLUSION

These results indicate that bufadienolides proscillaridin A and arenobufagin and cardenolide evomonoside, or related natural compounds might be promising new starting points for development of novel anti-cancer agents for treatment of osteosarcoma.

Bufadienolides and polyhydroxycholestane derivatives from Bufo bufo gargarizans

A new polyhydroxycholestane sulfate ester, 3α,12β,25,26-tetrahydroxy-7-oxo-5β-cholestane 26-O-sulfate (1), was isolated from dried skin of Bufo bufo gargarizans Cantor and its structure was elucidated on the basis of extensive 1D and 2D NMR as well as HR-ESI-MS analysis. A comparison of steroidal metabolite profiles, based on HPLC and LC-MS analyses, indicates that the chemical compositions of the various parts of toads, such as venom, skin and stratum corneum, are significantly different.

Efficient purification of active bufadienolides by a class separation method based on hydrophilic solid-phase extraction and reversed-phase high performance liquid chromatography

Traditional Chinese medicines (TCMs) have played a significant role in the process of discovering natural bioactive compounds, especially in anticancer therapeutics. However, the components of TCMs are complex mixtures with wide variation in polarity and content, which leads to inefficiency in the process of active compound discovery from TCMs. In this paper, the popular strategy of utilizing "pre-fractionated natural product libraries" has been improved by a new class separation approach to accelerate the process. As an example, the skin of Bufo bufo gargarizans Cantor, a well-known TCM, mainly contains two distinct bufadienolide classes: amino acid-conjugated bufadienolides (AACBs) and free form bufadienolides (AAUBs). We utilized hydrophilic interaction liquid chromatography solid-phase extraction (HILIC-SPE) to resolve the two types of bufadienolides, which co-eluted on C18 columns. By this strategy, twelve bufadienolides of the two types were purified via prep-HPLC from one active fraction, and eight of them were identified by (1)H NMR and (13)C NMR. These results indicated that the class separation method not only overcame the limited orthogonality in a 2D-RPLC×RPLC system but also accelerated the process of active compound discovery.

Semi-preparative LC-SPE-cryoflow NMR for impurity identifications: use of mother liquor as a better source of impurities

Unambiguous structural elucidation of active pharmaceutical ingredients (API) impurities is a particularly challenging necessity of pharmaceutical development, particularly if the impurities are low level (0.1% level). In many cases, this requires acquiring high-quality NMR data on a pure sample of each impurity. High-quality, high signal-to-noise (S/N) one- and two-dimensional NMR data can be obtained using liquid chromatography-solid phase extraction-cryoflow NMR (LC-SPE-cryoflow NMR) with a combination of semi-preparative column for separation and mother liquor as a source of concentrated impurities. These NMR data, in conjunction with mass spectrometry data, allowed for quick and unambiguous structural elucidations of four impurities found at low level in the crystallized API but found at appreciable levels in the mother liquor that was used as the source for these impurities. These data show that semi-preparative columns can be used at lower than ideal flow rates to facilitate trapping of HPLC components for LC-SPE-cryoflow NMR analysis without compromising chromatographic resolution. Also, despite the complex chromatography encountered with the use of mother liquor as a source of impurities, acceptably pure analytes were obtained for acquiring NMR data for unambiguous structure elucidations.

Mass spectrometry and NMR spectroscopy: modern high-end detectors for high resolution separation techniques--state of the art in natural product HPLC-MS, HPLC-NMR, and CE-MS hyphenations

Current natural product research is unthinkable without the use of high resolution separation techniques as high performance liquid chromatography or capillary electrophoresis (HPLC or CE respectively) combined with mass spectrometers (MS) or nuclear magnetic resonance (NMR) spectrometers. These hyphenated instrumental analysis platforms (CE-MS, HPLC-MS or HPLC-NMR) are valuable tools for natural product de novo identification, as well as the authentication, distribution, and quantification of constituents in biogenic raw materials, natural medicines and biological materials obtained from model organisms, animals and humans. Moreover, metabolic profiling and metabolic fingerprinting applications can be addressed as well as pharmacodynamic and pharmacokinetic issues. This review provides an overview of latest technological developments, discusses the assets and drawbacks of the available hyphenation techniques, and describes typical analytical workflows.

Structure-activity relationship analysis of bufadienolide-induced in vitro growth inhibitory effects on mouse and human cancer cells

The in vitro growth inhibitory effects of 27 bufadienolides and eight degradation products, with two cardenolides (ouabain and digoxin) chosen as reference compounds, were analyzed by means of an MTT colorimetric assay in six human and two mouse cancer cell lines. A structure-activity analysis was then performed to highlight the most important substituents relating to the in vitro growth inhibitory activity of bufadienolides in cancer cells. Thus, the current study revealed that various bufadienolides, including gamabufotalin rhamnoside (1a), bufotalin (2a), and hellebrin (3a), displayed higher growth inhibitory activities for various human cancer cell lines when compared to ouabain and digoxin. Gamabufotalin rhamnoside (1a) was the only compound that displayed growth inhibitory effects of <1 μM in mouse cancer cells that expressed mutated forms of the Na(+),K(+)-ATPase α-1 subunit. In addition, all genins and degradation products displayed weaker (if any) in vitro growth inhibitory effects on cancer cells when compared to their respective glycosylated homologue, with the exception of hellebrigenin (3b), which was as active as hellebrin (3a).

Synthesis and structure-activity relationships study of cytotoxic bufalin 3-nitrogen-containing-ester derivatives

A series of bufalin 3-nitrogen-containing-ester derivatives (2-6) were designed, synthesized, and evaluated for their proliferation inhibition activities against human cervical epithelial adenocarcinoma (HeLa) and non-small-cell lung cancer (A549) cell lines. The structure-activity relationships (SARs) of this new series were described in this paper. Cytotoxicity data revealed that C3 moiety had important influence on cytotoxic activity. On two cell lines, the bufalin-3-piperidinyl-4-carboxylate compound 2 (IC50 values on HeLa and A549 cell lines were 0.76 nM and 0.34 nM, respectively) displayed a significant cytotoxic potency compared to the parent compound bufalin.

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.

Hellebrin and its aglycone form hellebrigenin display similar in vitro growth inhibitory effects in cancer cells and binding profiles to the alpha subunits of the Na+/K+-ATPase

BACKGROUND

Surface-expressed Na+/K+-ATPase (NaK) has been suggested to function as a non-canonical cardiotonic steroid-binding receptor that activates multiple signaling cascades, especially in cancer cells. By contrast, the current study establishes a clear correlation between the IC50in vitro growth inhibitory concentration in human cancer cells and the Ki for the inhibition of activity of purified human α1β1 NaK.

METHODS

The in vitro growth inhibitory effects of seven cardiac glycosides including five cardenolides (ouabain, digoxin, digitoxin, gitoxin, uzarigenin-rhamnoside, and their respective aglycone forms) and two bufadienolides (gamabufotalin-rhamnoside and hellebrin, and their respective aglycone forms) were determined by means of the MTT colorimetric assay and hellebrigenin-induced cytotoxic effects were visualized by means of quantitative videomicroscopy. The binding affinity of ten of the 14 compounds under study was determined with respect to human α1β1, α2β1 and α3β1 NaK complexes. Lactate releases and oxygen consumption rates were also determined in cancer cells treated with these various cardiac glycosides.

RESULTS

Although cardiotonic steroid aglycones usually display weaker binding affinity and in vitro anticancer activity than the corresponding glycoside, the current study demonstrates that the hellebrin / hellebrigenin pair is at odds with respect to this rule. In addition, while some cardiac steroid glycosides (e.g., digoxin), but not the aglycones, display a higher binding affinity for the α2β1 and α3β1 than for the α1β1 complex, both hellebrin and its aglycone hellebrigenin display ~2-fold higher binding affinity for α1β1 than for the α2β1 and α3β1 complexes. Finally, the current study highlights a common feature for all cardiotonic steroids analyzed here, namely a dramatic reduction in the oxygen consumption rate in cardenolide- and bufadienolide-treated cells, reflecting a direct impact on mitochondrial oxidative phosphorylation.

CONCLUSIONS

Altogether, these data show that the binding affinity of the bufadienolides and cardenolides under study is usually higher for the α2β1 and α3β1 than for the α1β1 NaK complex, excepted for hellebrin and its aglycone form, hellebrigenin, with hellebrigenin being as potent as hellebrin in inhibiting in vitro cancer cell growth.

Characterization of chlorinated valepotriates from Valeriana jatamansi

HPLC-PDA-MS and TLC analysis were used to look for minor cytotoxic chlorinated valepotriates from whole plants of Valeriana jatamansi (syn. Valeriana wallichii DC.). This resulted in isolation of 15 chlorinated valepotriates, designated as chlorovaltrates A-O, together with six known analogues, (1S,3R,5R,7S,8S,9S)-3,8-epoxy-1,5-dihydroxyvalechlorine, volvaltrate B, chlorovaltrate, rupesin B, (1S,3R,5R,7S,8S,9S)-3,8-epoxy-1-O-ethyl-5-hydroxyvalechlorine, and (1R,3R,5R,7S,8S,9S)-3,8-epoxy-1-O-ethyl-5-hydroxyvalechlorine. Their structures were elucidated by spectroscopic methods including homo- and heteronuclear two-dimensional NMR experiments. Chlorovaltrates K-N, chlorovaltrate and rupesin B showed moderate cytotoxicity against lung adenocarcinoma (A 549), metastatic prostate cancer (PC-3M), colon cancer (HCT-8) and hepatoma (Bel 7402) cell lines with IC₅₀ values of 0.89-9.76 μM.

Evaluation of the antiproliferative activity of diterpene isonitriles from the sponge Pseudoaxinella flava in apoptosis-sensitive and apoptosis-resistant cancer cell lines

One new (1) and three known (2-4) isonitrile diterpenes, isolated from the Caribbean sponge Pseudoaxinella flava, were assayed in human cancer cell lines in vitro using an MTT colorimetric assay and quantitative videomicroscopy. Compounds 1-4 displayed activity for human PC3 prostate apoptosis-sensitive cancer cell lines. Compounds 3 and 4 demonstrated similar growth inhibitory effects for three apoptosis-sensitive and three apoptosis-resistant cancer cell lines. Quantitative videomicroscopy analysis revealed that compounds 1 and 2 exerted their activity through cytotoxic effects, and compounds 3 and 4 through cytostatic effects. These results identify marine diterpene isonitriles as potential lead compounds for anticancer drug discovery.

Bufadienolide compounds sensitize human breast cancer cells to TRAIL-induced apoptosis via inhibition of STAT3/Mcl-1 pathway

The death receptor ligand TRAIL is considered a promising candidate for cancer therapy because of its preferential toxicity to malignant cells. However its efficacy has been challenged by a number of resistance mechanisms. Therefore, agents that can overcome the resistance to enhance therapeutic efficacy of TRAIL are needed. In the current study, we found that bufalin, bufotalin and gamabufotalin, key members of bufadienolides isolated from a traditional Chinese medicine ChanSu, significantly potentiated human breast cancer cells with different status of ER-alpha to apoptosis induction of TRAIL, as evidenced by enhanced Annexin V/FITC positive cells (apoptotic cells), cytoplasmic histone-associated-DNA-fragments, membrane permeability transition (MPT), caspases activation and PARP cleavage. Further mechanistic investigation demonstrated that bufalin was able to significantly decrease Mcl-1 expression and modestly decrease Bcl-XL expression level. Down-regulations of these anti-apoptotic proteins were well correlated with inhibition of transcription factor STAT3 activation. The important consequence of down-regulation Mcl-1 in the enhancement action by combining bufalin with TRAIL was confirmed by either knockdown or overexpression of Mcl-1 approach. Our findings for the first time provided strong evidences that bufadienolide compounds have excellent potential to be developed as a novel class of sensitizers of TRAIL.

Rapid screening of lignans from Phyllanthus myrtifolius and stilbenoids from Syagrus romanzoffiana by HPLC-SPE-NMR

INTRODUCTION

Application of on-line solid-phase extraction (SPE) as an interface between HPLC and NMR has gained great improvement in solving sensitivity problems and signal interferences by the eluents.

OBJECTIVE

Rapid analysis and characterisation by HPLC-SPE-NMR and LC/MS of the arylnaphthalene-type lignans present in Phyllanthus myrtifolius and the minor stilbenoids present in the polyphenol-rich fraction from the ethanol extract of the seeds of Syagrus romanzoffiana.

METHODOLOGY

Pretreatment of fractions by liquid-liquid partitioning, followed by Sephadex LH-20 fractionation, was found very useful to facilitate the focusing and analysis of the polyphenolic fraction. HPLC-DAD-SPE-NMR (400  MHz and 600 MHz) analysis was carried out using an Agilent 1100 liquid chromatography, followed by a Prospekt 2 automated solid-phase extraction unit, containing 96 HySphere-Resin GP cartridges (10 × 2 mm, 10-12 µm), which was connected to a 120 or 60 µL LC probe.

RESULTS

Seven arylnaphthalene-type lignans from the chloroform-soluble fraction of P. myrtifolius and nine stilbenoids from a polyphenol-rich butanol-soluble fraction of the seeds of S. romanzoffiana were characterised.

CONCLUSIONS

HPLC-SPE-NMR associated with HR-ESI/MS, which consumed only analytical amounts of partially purified mixtures, was demonstrated to be a good tool for rapid screening of both known and new natural products.

Comparison of toad venoms from different Bufo species by HPLC and LC-DAD-MS/MS

ETHNOPHARMACOLOGICAL RELEVANCE

Toad venom, called Chansu in China, has been widely used for the treatment of heart failure, sores, pains, and various cancers for a long time in clinic.

AIM OF THE STUDY

The aim of the study is to investigate the chemical differences among a variety of toad venoms from different geographic locations and related Bufo species.

MATERIALS AND METHODS

Ten batches of commercial toad venom collected from different regions in China, one batch of fresh toad venom obtained from Bufo bufo gargarizans, and six batches of related Bufo species were analyzed by HPLC and LC-DAD-MS/MS. Individual components were identified by comparison of retention times, UV spectra, and mass spectra with authentic compounds, standard addition, as well as summarized MS fragmentation rules. Based on the profile of identified constituents and the content of cinobufagin and resibufogenin, the chemical differences observed among different samples are discussed.

RESULTS

Overall, 43 compounds were identified in the methanolic extracts of the different samples of toad venom. Besides of suberoyl arginine, several free bufadienolides, bufadienolide sulfates, and suberoyl esters of bufadienolides were found. The total amounts of cinobufagin and resibufogenin, which are the only two control markers according to the current Chinese Pharmacopoeia, varied widely from 0.7% to 10.9% in the commercial Chansu samples collected in the different locations in China. Low levels of resibufogenin, but no cinobufagin was observed in the samples from Bufo melanosticus and Bufo marinus, and even neither of both compounds was found in the sample from Bufo viridis.

CONCLUSIONS

The chemical profiles of the different commercial and collected toad venoms from related Bufo species differed significantly, not only in the absolute and relative contents, but also in the number and type of the constituents. The main reason for this variation are species-specific differences, but additional factors, such as the harvest and post-harvest processing, and adaption to environmental factors in different geographic locations, also seem to contribute.