A Direct Comparison of the Anticancer Activities of Digitoxin MeON-Neoglycosides and O-Glycosides: Oligosaccharide Chain Length-Dependent Induction of Caspase-9-Mediated Apoptosis

Cardiac glycoside ouabain efficiently targets leukemic stem cell apoptotic machinery independent of cell differentiation status

BACKGROUND

Acute myeloid leukemia (AML) is an aggressive hematologic malignancy characterized by an accumulation of immature leukemic myeloblasts initiating from leukemic stem cells (LSCs)-the subpopulation that is also considered the root cause of chemotherapy resistance. Repurposing cardiac glycosides to treat cancers has gained increasing attention and supporting evidence, but how cardiac glycosides effectively target LSCs, e.g., whether it involves cell differentiation, remains largely unexplored.

METHODS

Digoxin, a user-designed digitoxigenin-α-L-rhamnoside (D6-MA), and ouabain were tested against various human AML-derived cells with different maturation phenotypes. Herein, we established two study models to specifically determine the effects of cardiac glycosides on LSC death and differentiation-one allowed change in dynamics of LSCs and leukemic progenitor cells (LPCs), while another maintained their undifferentiated status. Regulatory mechanisms underlying cardiac glycoside-induced cytotoxicity were investigated and linked to cell cycle distribution and apoptotic machinery.

RESULTS

Primitive AML cells containing CD34+ LSCs/LPCs were very responsive to nanomolar concentrations of cardiac glycosides, with ouabain showing the greatest efficiency. Ouabain preferentially induces caspase-dependent apoptosis in LSCs, independent of its cell differentiation status, as evidenced by (i) the tremendous induction of apoptosis by ouabain in AML cells that acquired less than 15% differentiation and (ii) the higher rate of apoptosis in enriched LSCs than in LPCs. We sorted LSCs and LPCs according to their cell cycle distribution into G0/G1, S, and G2/M cells and revealed that G0/G1 cells in LSCs, which was its major subpopulation, were the top ouabain responders, indicating that the difference in ouabain sensitivity between LSCs and LPCs involved both distinct cell cycle distribution and intrinsic apoptosis regulatory mechanisms. Further, Mcl-1 and c-Myc, which were differentially expressed in LSCs and LPCs, were found to be the key apoptosis mediators that determined ouabain sensitivity in AML cells. Ouabain induces a more rapid loss of Mcl-1 and c-Myc in LSCs than in LPCs via the mechanisms that in part involve an inhibition of Mcl-1 protein synthesis and an induction of c-Myc degradation.

CONCLUSIONS

Our data provide new insight for repurposing cardiac glycosides for the treatment of relapsed/refractory AML through targeting LSCs via distinct cell cycle and apoptosis machinery. Video Abstract.

A Short Route to the Synthesis of Digoxose Trisaccharide Glycal Donor via Mislow-Evans Rearrangement

The Mislow-Evans rearrangement was used as a key reaction to construct digitoxose-derived glycals. The same rearrangement was iteratively performed on di- and trisaccharides to form the digoxose glycal donor component present in the cardenolides digitoxin, digoxin, and gitoxin. The scalability of the trisaccharide synthesis was shown by performing the reactions on a multigram scale. Glycosylation reactions were also performed between the synthesized digoxin glycal donor and aglycons digoxigenin and gitoxigenin to synthesize novel cardenolide derivatives.

The mechanistic role of cardiac glycosides in DNA damage response and repair signaling

Cardiac glycosides (CGs) are a class of bioactive organic compounds well-known for their application in treating heart disease despite a narrow therapeutic window. Considerable evidence has demonstrated the potential to repurpose CGs for cancer treatment. Chemical modification of these CGs has been utilized in attempts to increase their anti-cancer properties; however, this has met limited success as their mechanism of action is still speculative. Recent studies have identified the DNA damage response (DDR) pathway as a target of CGs. DDR serves to coordinate numerous cellular pathways to initiate cell cycle arrest, promote DNA repair, regulate replication fork firing and protection, or induce apoptosis to avoid the survival of cells with DNA damage or cells carrying mutations. Understanding the modus operandi of cardiac glycosides will provide critical information to better address improvements in potency, reduced toxicity, and the potential to overcome drug resistance. This review summarizes recent scientific findings of the molecular mechanisms of cardiac glycosides affecting the DDR signaling pathway in cancer therapeutics from 2010 to 2022. We focus on the structural and functional differences of CGs toward identifying the critical features for DDR targeting of these agents.

Stereoselective glycosylation reactions with 2-deoxyglucose: a computational study of some catalysts

2-Deoxy glycosides are important components of many oligosaccharides with antibiotic and anti-cancer activity, but their synthesis can be very challenging. Phenanthrolines and substituted pyridines promote stereoselective glycosylation of 1-bromo sugars via a double SN2 mechanism. Pyridine reacting with α-bromo, 2-deoxyglucose was chosen to model this reaction. The first step involves displacement of bromide by pyridine which can be rate limiting because bromide ion is poorly solvated in the non-polar solvents used for these reactions. We examined a series of small molecules to bind bromide and stabilize this transition state. Geometry optimization and vibrational frequencies were calculated using M06-2X/6-31+G(d,p) and SMD implicit solvation for diethyl ether. More accurate energies were obtained with M06-2X/aug-cc-pVTZ and implicit solvation. Urea, thiourea, guanidine and cyanoguanidine bind bromide more strongly than alkylamines, (NH2CH2CH2)nNH3-n. Compared to the uncatalyzed reaction, urea, thiourea and cyanoguanidine lower the free energy of the transition state by 3 kcal/mol while guanidine lowers the barrier by 2 kcal/mol.

Application of Redox-Active Ester Catalysis to the Synthesis of Pyranose Alkyl C-Glycosides

The direct coupling of shelf-stable, tetrachloro-N-hydroxyphthalimide ester (TCNHPI) glycosyl donors with a variety of alkylzinc reagents under redox catalysis is described. Alkyl C-glycosides are formed directly by a decarboxylative, Negishi-type process in 31-73% yields without the need for photocatalytic activation or additional reductants. Extension of this approach to the coupling of TCNHPI donors with stereodefined α-alkoxy furan-containing alkylzinc halides enabled de novo synthesis of methylene-linked exo-C-disaccharides via an Achmatowicz rearrangement.

De novo asymmetric Achmatowicz approach to oligosaccharide natural products

The development and application of the asymmetric synthesis of oligosaccharides from achiral starting materials is reviewed. This de novo asymmetric approach centers around the use of asymmetric catalysis for the synthesis of optically pure furan alcohols in conjunction with Achmatowicz oxidative rearrangement for the synthesis of various pyranones. In addition, the use of a diastereoselective palladium-catalyzed glycosylation and subsequent diastereoselective post-glycosylation transformation was used for the synthesis of oligosaccharides. The application of this approach to oligosaccharide synthesis is discussed.

Synthesis and Biological Evaluation of Cardiac Glycosides for Cancer Therapy by Targeting the DNA Damage Response

Cardiac glycosides (CGs) are bioactive compounds originally used to treat heart diseases, but recent studies have demonstrated their anticancer activity. We previously demonstrated that Antiaris toxicaria 2 (AT2) possesses anticancer activity in KRAS mutated lung cancers via impinging on the DNA damage response (DDR) pathway. Toward developing this class of molecules for cancer therapy, herein we report a multistep synthetic route utilizing k-strophanthidin as the initial building block for determination of structure-activity relationships (SARs). A systematic structural design approach was applied that included modifications of the sugar moiety, the glycoside linker, stereochemistry, and lactone ring substitutions to generate a library of O-glycosides and MeON-neoglycosides derivatives. These molecules were screened for their anticancer activities and their impact on DDR signaling in KRAS mutant lung cancer cells. These results demonstrate the ability to chemically synthesize CG derivatives and define the SARs to optimize AT2 as a cancer therapeutic.

Calotropin and corotoxigenin 3-O-glucopyranoside from the desert milkweed Asclepias subulata inhibit the Na+/K+-ATPase activity

Na⁺/K⁺-ATPase is an essential transmembrane enzyme found in all mammalian cells with critical functions for cell ion homeostasis. The inhibition of this enzyme by several cardiotonic steroids (CTS) has been associated with the cytotoxic effect on cancer cell lines of phytochemicals such as ouabain and digitoxin. This study evaluated the inhibitory capacity of cardenolides calotropin and corotoxigenin 3-O-glucopyranoside (C3OG) from Asclepias subulata over the Na⁺/K⁺-ATPase activity in vitro and silico. The inhibitory assays showed that calotropin and C3OG decreased the Na⁺/K⁺-ATPase activity with IC₅₀ values of 0.27 and 0.87 μM, respectively. Furthermore, the molecules presented an uncompetitive inhibition on Na⁺/K⁺-ATPase activity, with K_i values of 0.2 μM to calotropin and 0.5 μM to C3OG. Furthermore, the molecular modeling indicated that calotropin and C3OG might interact with the Thr⁷⁹⁷ and Gln¹¹¹ residues, considered essential to the interaction with the Na⁺/K⁺-ATPase. Besides, these cardenolides can interact with amino acid residues such as Phe⁷⁸³, Leu¹²⁵, and Ala³²³, to establish hydrophobic interactions on the binding site. Considering the results, these provide novel evidence about the mechanism of action of cardenolides from A. subulata, proposing that C3OG is a novel cardenolide that deserves further consideration for in vitro cellular antiproliferative assays and in vivo studies as an anticancer molecule.

Cytotoxic effect of carbohydrate derivatives of digitoxigenin involves modulation of plasma membrane Ca2+ -ATPase

Cardiac glycosides, such as digoxin and digitoxin, are compounds that interact with Na+ /K+ -ATPase to induce anti-neoplastic effects; however, these cardiac glycosides have narrow therapeutic index. Thus, semi-synthetic analogs of digitoxin with modifications in the sugar moiety has been shown to be an interesting approach to obtain more selective and more effective analogs than the parent natural product. Therefore, the aim of this study was to assess the cytotoxic potential of novel digitoxigenin derivatives, digitoxigenin-α-L-rhamno-pyranoside (1) and digitoxigenin-α-L-amiceto-pyranoside (2), in cervical carcinoma cells (HeLa) and human diploid lung fibroblasts (Wi-26-VA4). In addition, we studied the anticancer mechanisms of action of these compounds by comparing its cytotoxic effects with the potential to modulate the activity of three P-type ATPases; Na+ /K+ -ATPase, sarco/endoplasmic reticulum Ca2+ -ATPase (SERCA), and plasma membrane Ca2+ -ATPase (PMCA). Briefly, the results showed that compounds 1 and 2 were more cytotoxic and selectivity for HeLa tumor cells than the nontumor cells Wi-26-VA4. While the anticancer cytotoxicity in HeLa cells involves the modulation of Na+ /K+ -ATPase, PMCA and SERCA, the modulation of these P-type ATPases was completely absent in Wi-26-VA4 cells, which suggest the importance of their role in the cytotoxic effect of compounds 1 and 2 in HeLa cells. Furthermore, the compound 2 inhibited directly erythrocyte ghosts PMCA and both compounds were more cytotoxic than digitoxin in HeLa cells. These results provide a better understanding of the mode of action of the synthetic cardiac glycosides and highlights 1 and 2 as potential anticancer agents.

Implications of Synthetic Modifications of the Cardiotonic Steroid Lactone Ring on Cytotoxicity

Na,K-ATPase (NKA) and cardiotonic steroids (CTS) have shown potent cytotoxic and anticancer effects. Here, we have synthesized a series of CTS digoxin derivatives (γ-benzylidene) with substitutions in the lactone ring and evaluated the cytotoxicity caused by digoxin derivatives in tumor and non-tumor cells lines, as well as their effects on NKA. The cytotoxicity assay was determined in HeLa, A549, and WI-26 VA4 after they were treated for 48 h with increased concentrations of CTS. The effects of CTS on NKA activity and immunoblotting of α1 and β1 isoforms were evaluated at IC₅₀ concentrations in A549 cell membrane. NKA activity from mouse brain cortex was also measured. The majority of CTS exhibited low cytotoxicity in tumor and non-tumor cells, presenting IC₅₀ values at micromolar concentrations, while digoxin showed cytotoxicity at nanomolar concentrations. BD-15 presented the lowest IC₅₀ value (8 µM) in A549 and reduced its NKA activity in 28%. In contrast, BD-7 was the compound that most inhibited NKA (56% inhibition) and presented high IC₅₀ value for A549. In mouse cortex, only BD-15 modulated the enzyme activity in a concentration-dependent inhibition curve. These results demonstrate that the cytotoxicity of these compounds is not related to NKA inhibition. The substitutions in the lactone ring of digoxin led to an increase in the cytotoxic concentration in tumor cells, which may not be interesting for cancer, but it has the advantage of increasing the therapeutic margin of these molecules when compared to classic CTS, and can be used safely in research for other diseases.

In vitro chemotherapeutic and antiangiogenic properties of cardenolides from Acokanthera oblongifolia (Hochst.) Codd

Acovenoside A (Acov-A) and acobioside A (Acob-A) were isolated from Acokanthera oblongifolia. Their anticancer properties were explored regarding, antiproliferative and antiangiogenic activities. The study included screening phase against six cancer cell lines followed by mechanistic investigation against HepG2 cancer cell line. The sulforhodamine-B (SRB) was used to determine their growth inhibitory power. In the other hand, flow cytometry techniques were recorded the cell death type and cell cycle analysis. The clonogenic (colony formation) and wound healing assays, enzyme-linked immunosorbent assay (ELISA) and molecular docking, were performed to evaluate the antiangiogenesis capability. Both compounds were strongly, inhibited four cancer cell lines at GI₅₀ less than 100 nM. The in vitro mechanistic investigation against HepG2 resulted in cell accumulations at G2M phase and induction of apoptosis upon treating cells separately, with 400 nM Acov-A and 200 nM Acob-A. Interestingly, the same concentrations were able to activate caspase-3 by 7.2 and 4.8-fold, respectively. Suppressing the clonogenic capacity of HepG2 cells (20 and 40 nM) and inhibiting the migration of the colon Caco-2 cancer cells were provoke the results of vascular endothelial growth factor receptor2 (VEGFR2) kinase enzyme inactivation. The docked study was highly supportive, to the antiangiogenic approach of both cardenolides. The isolated cardenolides could orchestrate pivotal events in fighting cancer.

Na+/K+-ATPase Revisited: On Its Mechanism of Action, Role in Cancer, and Activity Modulation

Maintenance of Na⁺ and K⁺ gradients across the cell plasma membrane is an essential process for mammalian cell survival. An enzyme responsible for this process, sodium-potassium ATPase (NKA), has been currently extensively studied as a potential anticancer target, especially in lung cancer and glioblastoma. To date, many NKA inhibitors, mainly of natural origin from the family of cardiac steroids (CSs), have been reported and extensively studied. Interestingly, upon CS binding to NKA at nontoxic doses, the role of NKA as a receptor is activated and intracellular signaling is triggered, upon which cancer cell death occurs, which lies in the expression of different NKA isoforms than in healthy cells. Two major CSs, digoxin and digitoxin, originally used for the treatment of cardiac arrhythmias, are also being tested for another indication—cancer. Such drug repositioning has a big advantage in smoother approval processes. Besides this, novel CS derivatives with improved performance are being developed and evaluated in combination therapy. This article deals with the NKA structure, mechanism of action, activity modulation, and its most important inhibitors, some of which could serve not only as a powerful tool to combat cancer, but also help to decipher the so-far poorly understood NKA regulation.

Steroid Glycosides Hyrcanoside and Deglucohyrcanoside: On Isolation, Structural Identification, and Anticancer Activity

Cardiac glycosides (CGs) represent a group of sundry compounds of natural origin. Most CGs are potent inhibitors of Na⁺/K⁺-ATPase, and some are routinely utilized in the treatment of various cardiac conditions. Biological activities of other lesser known CGs have not been fully explored yet. Interestingly, the anticancer potential of some CGs was revealed and thereby, some of these compounds are now being evaluated for drug repositioning. However, high systemic toxicity and low cancer cell selectivity of the clinically used CGs have severely limited their utilization in cancer treatment so far. Therefore, in this study, we have focused on two poorly described CGs: hyrcanoside and deglucohyrcanoside. We elaborated on their isolation, structural identification, and cytotoxicity evaluation in a panel of cancerous and noncancerous cell lines, and on their potential to induce cell cycle arrest in the G2/M phase. The activity of hyrcanoside and deglucohyrcanoside was compared to three other CGs: ouabain, digitoxin, and cymarin. Furthermore, by in silico modeling, interaction of these CGs with Na⁺/K⁺-ATPase was also studied. Hopefully, these compounds could serve not only as a research tool for Na⁺/K⁺-ATPase inhibition, but also as novel cancer therapeutics.

Glycorandomization: A promising diversification strategy for the drug development

Glycorandomization is a natural product derivatization strategy in which different sugar moieties are linked to the aglycone part of the naturally existing glycosides to create glycorandomized libraries. Sugars attached to the natural products are responsible for affecting their solubility, mechanism of action, target recognition, and toxicity and thus, by changing the sugar part, these properties could be modified. Glycorandomization can be done via two approaches (i) a synthetic approach known as neoglycorandomization, and (ii) chemoenzymatic approach including in-vitro and in-vivo glycorandomization. Glycorandomization can be a promising technology for the drug discovery that has proved its potential to improve pharmacokinetic (solubility) and pharmacodynamic profile (mechanism of action, toxicity, and target recognition) of the parent compounds. The substrate flexibility of glycosyltransferases and other enzymes towards sugars and/or aglycone substrates has made this technique versatile. Further, the enzymes can be altered by genetic engineering to generate glycorandomized libraries of diverse natural product scaffolds. This technique has the potential to produce new compounds that can be helpful to the mankind by treating the threatening disease states. This review covers the different strategies for glycorandomization as a tool in drug discovery and development. The fundamentals of glycorandomization, different types, and further development of differentially glycorandomized libraries of natural products and small molecule based drugs have been discussed.

Prospects and Therapeutic Applications of Cardiac Glycosides in Cancer Remediation

Active metabolites from natural sources are the predominant molecular targets in numerous biological studies owing to their appropriate compatibility with biological systems and desirable selective toxicities. Thus, their potential for therapeutic development could span a broad scope of disease areas, including pathological and neurological dysfunctions. Cardiac glycosides are a unique class of specialized metabolites that have been extensively applied as therapeutic agents for the treatment of numerous heart conditions, and more recently, they have also been explored as probable antitumor agents. They are a class of naturally derived compounds that bind to and inhibit Na⁺/K⁺-ATPase. This study presents cardiac glycosides and their analogues with highlights on their applications, challenges, and prospects as lead compounds for cancer treatment.

Anticancer and Antiviral Properties of Cardiac Glycosides: A Review to Explore the Mechanism of Actions

Cardiac glycosides (CGs) have a long history of treating cardiac diseases. However, recent reports have suggested that CGs also possess anticancer and antiviral activities. The primary mechanism of action of these anticancer agents is by suppressing the Na+/k+-ATPase by decreasing the intracellular K+ and increasing the Na+ and Ca2+. Additionally, CGs were known to act as inhibitors of IL8 production, DNA topoisomerase I and II, anoikis prevention and suppression of several target genes responsible for the inhibition of cancer cell proliferation. Moreover, CGs were reported to be effective against several DNA and RNA viral species such as influenza, human cytomegalovirus, herpes simplex virus, coronavirus, tick-borne encephalitis (TBE) virus and Ebola virus. CGs were reported to suppress the HIV-1 gene expression, viral protein translation and alters viral pre-mRNA splicing to inhibit the viral replication. To date, four CGs (Anvirzel, UNBS1450, PBI05204 and digoxin) were in clinical trials for their anticancer activity. This review encapsulates the current knowledge about CGs as anticancer and antiviral drugs in isolation and in combination with some other drugs to enhance their efficiency. Further studies of this class of biomolecules are necessary to determine their possible inhibitory role in cancer and viral diseases.

Potential antitumor activity of digitoxin and user-designed analog administered to human lung cancer cells

BACKGROUND

Cardiac glycosides (CGs), such as digitoxin, are traditionally used for treatment of congestive heart failure; recently they also gained attention for their anticancer properties. Previous studies showed that digitoxin and a synthetic L-sugar monosaccharide analog treatment decreases cancer cell proliferation, increases apoptosis, and pro-adhesion abilities; however, no reports are available on their potential to alter lung cancer cell cytoskeleton structure and reduce migratory ability. Herein, we investigated the anticancer effects of digitoxin and its analog, digitoxigenin-α-L-rhamnoside (D6MA), to establish whether cytoskeleton reorganization and reduced motility are drug-induced cellular outcomes.

METHODS

We treated non-small cell lung carcinoma cells (NSCLCs) with sub-therapeutic, therapeutic, and toxic concentrations of digitoxin and D6MA respectively, followed by both single point and real-time assays to evaluate changes in cellular gene and protein expression, adhesion, elasticity, and migration.

RESULTS

Digitoxin and D6MA induced a decrease in matrix metalloproteinases expression via altered focal adhesion signaling and a suppression of the phosphoinositide 3-kinases / protein kinase B pathway which lead to enhanced adhesion, altered elasticity, and reduced motility of NSCLCs. Global gene expression analysis identified dose-dependent changes to nuclear factor kappa-light-chain-enhancer, epithelial tumor, and microtubule dynamics signaling.

CONCLUSIONS

Our study demonstrates that digitoxin and D6MA can target antitumor signaling pathways to alter NSCLC cytoskeleton and migratory ability to thus potentially reduce their tumorigenicity.

SIGNIFICANCE

Discovering signaling pathways that control cancer's cell phenotype and how such pathways are affected by CG treatment will potentially allow for active usage of synthetic CG analogs as therapeutic agents in advanced lung conditions.

Copper(ii)-catalyzed stereoselective 1,2-addition vs. Ferrier glycosylation of "armed" and "disarmed" glycal donors

Selective activation of "armed' and ''disarmed" glycal donors enabling the stereo-controlled glycosylations by employing Cu(ii)-catalyst as the promoter has been realized. The distinctive stereochemical outcome in the process is mainly influenced by the presence of diverse protecting groups on the donor and the solvent system employed. The protocol is compatible with a variety of aglycones including carbohydrates, amino acids, and natural products to access deoxy-glycosides and glycoconjugates with high α-anomeric selectivity. Notably, the synthetic practicality of the method is amply verified for the stereoselective assembling of trisaccharides comprising 2-deoxy components. Mechanistic studies involving deuterated experiments validate the syn-diastereoselective 1,2-addition of acceptors on the double bond of armed donors.

Digitoxin inhibits HeLa cell growth through the induction of G2/M cell cycle arrest and apoptosis in vitro and in vivo

Cervical cancer is the fourth most common gynecological malignancy affecting the health of women worldwide and the second most common cause of cancer‑related mortality among women in developing regions. Thus, the development of effective chemotherapeutic drugs for the treatment of cervical cancer has become an important issue in the medical field. The application of natural products for the prevention and treatment of various diseases, particularly cancer, has always attracted widespread attention. In the present study, a library of natural products composed of 78 single compounds was screened and it was found that digitoxin exhibited the highest cytotoxicity against HeLa cervical cancer cells with an IC50 value of 28 nM at 48 h. Furthermore, digitoxin exhibited extensive antitumor activities in a variety of malignant cell lines, including the lung cancer cell line, A549, the hepatoma cell line, MHCC97H, and the colon cancer cell line, HCT116. Mechanistically, digitoxin caused DNA double‑stranded breaks (DSBs), inhibited the cell cycle at the G2/M phase via the ataxia telangiectasia mutated serine/threonine kinase (ATM)/ATM and Rad3‑related serine/threonine kinase (ATR)‑checkpoint kinase (CHK1)/checkpoint kinase 2 (CHK2)‑Cdc25C pathway and ultimately triggered mitochondrial apoptosis, which was characterized by the disruption of Bax/Bcl‑2, the release of cytochrome c and the sequential activation of caspases and poly(ADP‑ribose) polymerase (PARP). In addition, the in vivo anticancer effect of digitoxin was confirmed in HeLa cell xenotransplantation models. On the whole, the findings of the present study demonstrate the efficacy of digitoxin against cervical cancer in vivo and elucidate its molecular mechanisms, including DSBs, cell cycle arrest and mitochondrial apoptosis. These results will contribute to the development of digitoxin as a chemotherapeutic agent in the treatment of cervical cancer.

Sugar-Pirating as an Enabling Platform for the Synthesis of 4,6-Dideoxyhexoses

An efficient divergent synthetic strategy that leverages the natural product spectinomycin to access uniquely functionalized monosaccharides is described. Stereoselective 2'- and 3'-reduction of key spectinomycin-derived intermediates enabled facile access to all eight possible 2,3-stereoisomers of 4,6-dideoxyhexoses as well as representative 3,4,6-trideoxysugars and 3,4,6-trideoxy-3-aminohexoses. In addition, the method was applied to the synthesis of two functionalized sugars commonly associated with macrolide antibiotics-the 3-O-alkyl-4,6-dideoxysugar d-chalcose and the 3-N-alkyl-3,4,6-trideoxysugar d-desosamine.

Reagent-Controlled α-Selective Dehydrative Glycosylation of 2,6-Dideoxy Sugars: Construction of the Arugomycin Tetrasaccharide

The first synthesis of the tetrasaccharide fragment of the anthracycline natural product Arugomycin is described. A reagent controlled dehydrative glycosylation method involving cyclopropenium activation was utilized to synthesize the α-linkages with complete anomeric selectivity. The synthesis was completed in 20 total steps, and in 2.5% overall yield with a longest linear sequence of 15 steps.

Stereoselective Phenylselenoglycosylation of Glycals Bearing a Fused Carbonate Moiety toward the Synthesis of 2-Deoxy-β-galactosides and β-Mannosides

A phenylselenoglycosylation reaction of glycal derivatives mediated by diphenyl diselenide and phenyliodine(III) bis(trifluoroacetate) under mild conditions is described. Stereoselective glycosylation has been achieved by installing fused carbonate on those glycals. 3,4-O-Carbonate galactals and 2,3-O-carbonate 2-hydroxyglucals are converted into corresponding glycosides in good yields with excellent β-selectivity, resulting in 2-phenylseleno-2-deoxy-β-galactosides and 2-phenylseleno-β-mannosides which are good precursors of 2-deoxy-β-galactosides and β-mannosides, respectively.

Strophanthidin Attenuates MAPK, PI3K/AKT/mTOR, and Wnt/β-Catenin Signaling Pathways in Human Cancers

Lung cancer is the most prevalent in cancer-related deaths, while breast carcinoma is the second most dominant cancer in women, accounting for the most number of deaths worldwide. Cancers are heterogeneous diseases that consist of several subtypes based on the presence or absence of hormone receptors and human epidermal growth factor receptor 2. Several drugs have been developed targeting cancer biomarkers; nonetheless, their efficiency are not adequate due to the high reemergence rate of cancers and fundamental or acquired resistance toward such drugs, which leads to partial therapeutic possibilities. Recent studies on cardiac glycosides (CGs) positioned them as potent cytotoxic agents that target multiple pathways to initiate apoptosis and autophagic cell death in many cancers. In the present study, our aim is to identify the anticancer activity of a naturally available CG (strophanthidin) in human breast (MCF-7), lung (A549), and liver cancer (HepG2) cells. Our results demonstrate a dose-dependent cytotoxic effect of strophanthidin in MCF-7, A549, and HepG2 cells, which was further supported by DNA damage on drug treatment. Strophanthidin arrested the cell cycle at the G2/M phase; this effect was further validated by checking the inhibited expressions of checkpoint and cyclin-dependent kinases in strophanthidin-induced cells. Moreover, strophanthidin inhibited the expression of several key proteins such as MEK1, PI3K, AKT, mTOR, Gsk3α, and β-catenin from MAPK, PI3K/AKT/mTOR, and Wnt/β-catenin signaling. The current study adequately exhibits the role of strophanthidin in modulating the expression of various key proteins involved in cell cycle arrest, apoptosis, and autophagic cell death. Our in silico studies revealed that strophanthidin can interact with several key proteins from various pathways. Taken together, this study demonstrates the viability of strophanthidin as a promising anticancer agent, which may serve as a new anticancer drug.

Recent progress in the synthesis of limonoids and limonoid-like natural products

Recent progress in syntheses of limonoids and limonoid-like natural products is reviewed. The current “state-of-art” advance on novel synthetic strategy are summarized and future outlook will be presented.

Rapid de Novo Preparation of 2,6-Dideoxy Sugar Libraries through Gold-Catalyzed Homopropargyl Orthoester Cyclization

A flexible de novo route capable of producing libraries of 2,6-dideoxy sugars is described. We have found that Au(JackiePhos)SbF6MeCN promotes the conversion of homopropargyl orthoesters into functionalized 2,3-dihydro-4H-pyran-4-ones in good to excellent yields (71-90%). These latter compounds can be easily converted into a number of otherwise difficult to access 2,6-dideoxy sugars.

Cloning and characterization of a glycosyltransferase from Catharanthus roseus for glycosylation of cardiotonic steroids and phenolic compounds

OBJECTIVES

To characterize a glycosyltransferase (UGT74AN3) from Catharanthus roseus and investigate its specificity toward cardiotonic steroids and phenolic compounds.

RESULTS

UGT74AN3, a novel permissive GT from C. roseus, displayed average high conversion rate (> 90%) toward eight structurally different cardiotonic steroids. Among them, resibufogenin, digitoxigenin, and uzarigenin gave 100% yield. Based on LC-MS, ¹H-NMR and ¹³C-NMR analysis, structure elucidation of eight glycosides was consistent with 3-O-β-D-glucosides. We further confirmed UGT74AN3 was permissive enough to glycosylate curcumin, resveratrol, and phloretin. The cDNA sequence of UGT74AN3 contained an ORF of 1,425 nucleotides encoding 474 amino acids. UGT74AN3 performed the maximum catalytic activity at 40 °C, pH 8.0, and was divalent cation-independent. K_m values of UGT74AN3 toward resibufogenin, digitoxigenin, and uzarigenin were 7.0 µM, 12.3 µM, and 17.4 µM, respectively.

CONCLUSIONS

UGT74AN3, a glycosyltransferase from a noncardenolide-producing plant, displayed catalytic efficiency toward cardiotonic steroids and phenolic compounds, which would make it feasible for glycosylation of bioactive molecules.

Synthesis and direct comparison of the anticancer activities of phomopsolides D and E and two 7-oxa-/7-aza-analogues

The synthesis of two stable phomopsolide natural products (D and E) and two analogues is presented. The cytotoxicities of these four compounds are surveyed and compared across a panel of NCI-cancer cell lines. This analysis found moderate cytotoxicities (2-50 μM) for the majority of the cell lines with phomopsolide D being more active than phomopsolide E and the 7-oxa analogue being commensurately more active than the 7-aza analogue.

Regioselective single pot C3-glycosylation of strophanthidol using methylboronic acid as a transient protecting group

This manuscript describes a single pot protocol for the selective introduction of unprotected sugars to the C3 position of the cardiotonic steroid strophanthidol. These reactions proceed with high levels of regiocontrol (>20:1 rr) in the presence of three other hydroxyl functionalities including the C19 primary hydroxyl group and could be applied to different sugars to provide the deprotected cardiac glycosides upon work up (5 examples, 77-69% yield per single operation). The selective glycosylation of the less reactive C3 position is accomplished by the use of traceless protection with methylboronic acid that blocks the C5 and C19 hydroxyls by forming a cyclic boronic ester, followed by in situ glycosylation and a work up with ammonia in methanol to remove the boronic ester and the carbohydrate ester protecting groups.

Modular Total Synthesis and Cell-Based Anticancer Activity Evaluation of Ouabagenin and Other Cardiotonic Steroids with Varying Degrees of Oxygenation

A Cu(II)-catalyzed diastereoselective Michael/aldol cascade approach is used to accomplish concise total syntheses of cardiotonic steroids with varying degrees of oxygenation including cardenolides ouabagenin, sarmentologenin, 19-hydroxysarmentogenin, and 5- epi-panogenin. These syntheses enabled the subsequent structure activity relationship (SAR) studies on 37 synthetic and natural steroids to elucidate the effect of oxygenation, stereochemistry, C3-glycosylation, and C17-heterocyclic ring. Based on this parallel evaluation of synthetic and natural steroids and their derivatives, glycosylated steroids cannogenol-l-α-rhamnoside (79a), strophanthidol-l-α-rhamnoside (92), and digitoxigenin-l-α-rhamnoside (97) were identified as the most potent steroids demonstrating broad anticancer activity at 10-100 nM concentrations and selectivity (nontoxic at 3 μM against NIH-3T3, MEF, and developing fish embryos). Further analyses indicate that these molecules show a general mode of anticancer activity involving DNA-damage upregulation that subsequently induces apoptosis.

An Improved Approach to the Direct Construction of 2-Deoxy-β-Linked Sugars: Applications to Oligosaccharide Synthesis

A next-generation reagent-controlled approach for the synthesis of 2,6-dideoxy and 2,3,6-trideoxy sugar donors in good yield and high β-selectivity is reported. The use of p-toluenesulfonyl chloride and potassium hexamethyldisilazide (KHMDS) greatly simplifies deoxy-sugar glycoside construction, and can be used for gram-scale glycosylation reactions. The development of this approach and its application to the construction of β-linked deoxy-sugar oligosaccharides are described.

Stereochemical Structure Activity Relationship Studies (S-SAR) of Tetrahydrolipstatin

Tetrahydrolipstatin (THL), its enantiomer, and an additional six diastereomers were evaluated as inhibitors of the hydrolysis of p-nitrophenyl butyrate by porcine pancreatic lipase. IC₅₀s were found for all eight stereoisomers ranging from a low of 4.0 nM for THL to a high of 930 nM for the diastereomer with the inverted stereocenters at the 2,3,2'-positions. While the enantiomer of THL was also significantly less active (77 nM) the remaining five stereoisomers retained significant inhibitory activities (IC₅₀s = 8.0 to 20 nM). All eight compounds were also evaluated against three human cancer cell lines (human breast cancers MCF-7 and MDA-MB-231, human large-cell lung carcinoma H460). No appreciable cytotoxicity was observed for THL and its seven diastereomers, as their IC₅₀s in a MTT cytotoxicity assay were all greater than 3 orders of magnitude of camptothecin.

Pre-activation Based Stereoselective Glycosylations

Due to the wide presence of carbohydrates in nature and their crucial roles in numerous important biological processes, oligosaccharides have attracted a lot of attention in synthetic organic chemistry community. Many innovative synthetic methods have been developed for oligosaccharide synthesis, among which the pre-activation based glycosylation is particularly noteworthy. Traditionally, glycosylation reactions are carried out when the glycosyl donor and the acceptor are both present when the promoter is added. In comparison, the pre-activation based glycosylation is unique, where the glycosyl donor is activated by the promoter in the absence of the acceptor. Upon complete donor activation, the acceptor is added to the reaction mixture enabling glycosylation. The key step in any oligosaccharide synthesis is the stereoselective formation of the glycosidic bond. As donor activation and acceptor glycosylation are temporally separated, pre-activation based glycosylation can bestow unique stereochemical control. This review systematically discusses factors impacting the stereochemical outcome of a pre-activation based glycosylation reaction including substituents on the glycosyl donor, reaction solvent, and additives. Applications of pre-activation based stereoselective glycosylation in assembly of complex oligosaccharides are also discussed.

Synthesis of tigogenin MeON-Neoglycosides and their antitumor activity

To discover new potent cytotoxic steroidal saponins, a series of tigogenin neoglycosides were synthesized via oxyamine neoglycosylation for the first time. The preliminary bioassays for their in vitro antitumor activities against five human cancer cell lines (A375, A-549, HCT-116, HepG2 and MCF-7) were conducted. The results revealed a sugar-dependent activity profile of their cytotoxicity, the glycoconjugation converted the non-active tigogenin to the most potential product Tg29 ((3R)-N-methoxyamino-tigogenin-β-2-deoxy-d-galactoside) with IC₅₀ value of 2.7μM and 4.6μM against HepG2 and MCF-7 cells respectively. And the 3R-tigogenin neoglycosides exhibited enhanced antitumor activity while the 3S-tigogenin almost showed no activity. Among the five cell lines, HepG2 and MCF-7 cells showed more sensitive cytotoxic responses to the products. Therefore, the neoglycosylation could be a promising strategy for the synthesis of antitumor steroidal saponins and it also proved the essential role of carbohydrate moiety of steroidal saponins in the biological activity.

Plant bioactive molecules bearing glycosides as lead compounds for the treatment of fungal infection: A review

Despite therapeutic advancement in the treatment of fungal infections, morbidity and mortality caused by these infections are still very high. There are approximately 300 fungal species that are infectious and can cause a variety of diseases. At present, several synthetic antifungal drugs are in clinical practice, many of them, however, are vulnerable to multidrug-resistant strains of microbes, and thus compromising the overall treatment outcomes. Glycosides are naturally occurring plant secondary metabolites with important therapeutic potential and clinical utility. The aim of this review was to focus on the antifungal effects of glycosides in preclinical studies with possible mechanism(s) wherein described. Published research show significant susceptibility of different fungi towards phytoglycosides, mediated through multiple mechanisms. Further detailed studies are needed to explain the clinical applications and limitations of these glycosides.

Anti-tumorigenic effects of a novel digitoxin derivative on both estrogen receptor-positive and triple-negative breast cancer cells

While there are targeted treatments for triple positive breast cancers, lack of specific biomarkers for triple-negative breast cancers (TNBC) has hindered the development of therapies for this subset of cancers. In this study, we evaluated the anticancer properties of cardiac glycoside Digitoxin (Dtx) and its synthetic analog MonoD on breast cancer cell lines MCF-7 (estrogen receptor-positive breast cancer) and MDA-MB-468 (triple-negative breast cancer). Both cardiac glycosides, at concentrations within the therapeutic range, increased the fraction of cells in the G0/G1 phase of the cell cycle, decreased viability, and inhibited the migration of MCF-7 and MDA-MB-468 cells. Both cardiac glycosides increased production of superoxide and induced apoptosis in both cell types. Reduced protein levels of nuclear factor kappa B and IkappaB kinase-beta were found in cardiac glycoside-treated cells, indicating that the cellular effects of these compounds are mediated via nuclear factor kappa B pathway. This study demonstrates the cytotoxic potential of digitoxin, and more importantly its synthetic analog MonoD, in the treatment of triple-positive breast cancer and more importantly the aggressive triple-negative breast cancer. Collectively, this study provides a basis for the reevaluation of cardiac glycosides in the treatment of breast cancer and more importantly reveals their potential in the treatment of triple-negative breast cancers.

Autophagy-Induced Apoptosis in Lung Cancer Cells by a Novel Digitoxin Analog

We have synthesized a novel derivative of Digitoxin, termed "MonoD", which demonstrates cytotoxic effects in lung cancer cells with much higher potency as compared to Digitoxin. Our data show that within 1 h of MonoD treatment, H460 cells showed increased oxidative stress, increased formation of autophagic vacuoles, and increased expression of pro-autophagic markers Beclin-1 and LC3-II. Cells pretreated with MnTBAP, a superoxide scavenger not only lowered superoxide production, but also had lower levels of LC3-II and Beclin-1. Prolonged treatment with MonoD-induced apoptosis in lung cancer cells. We investigated MonoD-dependent regulation of Akt and Bcl2, proteins that are known regulators of both autophagy and apoptosis. Molecular and pharmacologic inhibitors of Bcl2 and Akt, when combined with MonoD, led to higher expression of LC3-II and Beclin-1 as compared to MonoD alone, suggesting a repressive effect for these proteins in MonoD-dependent autophagy. Pretreatment of cells with an autophagy inhibitor repressed the apoptotic potential of MonoD, confirming that early autophagic flux is important to drive apoptosis. Therapeutic entities such as MonoD that target multiple pathways such as autophagy and apoptosis may prove advantageous over current therapies that have unimodal basis for action and may drive sustained tumor regression, which is highly desirable. J. Cell. Physiol. 231: 817-828, 2016. © 2015 Wiley Periodicals, Inc.

Stereoselective Synthesis and Evaluation of C6″-Substituted 5a-Carbasugar Analogues of SL0101 as Inhibitors of RSK1/2

A convergent synthesis of 5a-carbasugar analogues of the n-Pr-variant of SL0101 is described. The analogues were synthesized in an effort to find compounds with potent in vivo efficacy in the inhibition of p90 ribosomal s6 kinase (RSK1/2). The synthesis derived the desired C-4 L-rhamnose stereochemistry from quinic acid and used a highly selective cuprate addition, NaBH₄ reduction, Mitsunobu inversion, and alkene dihydroxylation to install the remaining stereochemistry. A Pd-catalyzed cyclitolization stereoselectively installed the aglycon at the anomeric position. The analogues were evaluated as RSK1/2 inhibitors and found to have 3- to 6-fold improved activity.

Nigericin decreases the viability of multidrug-resistant cancer cells and lung tumorspheres and potentiates the effects of cardiac glycosides

Multiple factors including tumor heterogeneity and intrinsic or acquired resistance have been associated with drug resistance in lung cancer. Increased stemness and the plasticity of cancer cells have been identified as important mechanisms of resistance; therefore, treatments targeting cancer cells independent of stemness phenotype would be much more effective in treating lung cancer. In this article, we have characterized the anticancer effects of the antibiotic Nigericin in cells displaying varying degrees of stemness and resistance to anticancer drugs, arising from (1) routine culture conditions, (2) prolonged periods of serum starvation. These cells are highly resistant to conventional anticancer drugs such as Paclitaxel, Hydroxyurea, Colchicine, Obatoclax, Wortmannin, and LY294002, and the multidrug-resistant phenotype of cells growing under prolonged periods of serum starvation is likely the result of extensive rewiring of signaling pathways, and (3) lung tumorspheres that are enriched for cancer stem-like cells. We found that Nigericin potently inhibited the viability of cells growing under routine culture conditions, prolonged periods of serum starvation, and lung tumorspheres. In addition, we found that Nigericin downregulated the expression of key proteins in the Wnt canonical signaling pathway such as LRP6, Wnt5a/b, and β-catenin, but promotes β-catenin translocation into the nucleus. The antitumor effects of Nigericin were potentiated by the Wnt activator HLY78 and by therapeutic levels of the US Food and Drug Administration–approved drug Digitoxin and its novel synthetic analog MonoD. We believe that Nigericin may be used in a co-therapy model in combination with other novel chemotherapeutic agents in order to achieve potent inhibition of cancers that display varying degrees of stemness, potentially leading to sustained anticancer effects.