Syntheses and Biological Activities of Disaccharide Daunorubicins

Cancer chemotherapy and beyond: Current status, drug candidates, associated risks and progress in targeted therapeutics

Cancer is an abnormal state of cells where they undergo uncontrolled proliferation and produce aggressive malignancies that causes millions of deaths every year. With the new understanding of the molecular mechanism(s) of disease progression, our knowledge about the disease is snowballing, leading to the evolution of many new therapeutic regimes and their successive trials. In the past few decades, various combinations of therapies have been proposed and are presently employed in the treatment of diverse cancers. Targeted drug therapy, immunotherapy, and personalized medicines are now largely being employed, which were not common a few years back. The field of cancer discoveries and therapeutics are evolving fast as cancer type-specific biomarkers are progressively being identified and several types of cancers are nowadays undergoing systematic therapies, extending patients' disease-free survival thereafter. Although growing evidence shows that a systematic and targeted approach could be the future of cancer medicine, chemotherapy remains a largely opted therapeutic option despite its known side effects on the patient's physical and psychological health. Chemotherapeutic agents/pharmaceuticals served a great purpose over the past few decades and have remained the frontline choice for advanced-stage malignancies where surgery and/or radiation therapy cannot be prescribed due to specific reasons. The present report succinctly reviews the existing and contemporary advancements in chemotherapy and assesses the status of the enrolled drugs/pharmaceuticals; it also comprehensively discusses the emerging role of specific/targeted therapeutic strategies that are presently being employed to achieve better clinical success/survival rate in cancer patients.

Selective Reduction Leading to 3,5-cis-3-Aminosugars: Synthesis and Stereoselective Glycosylation

Synthesis of 3,5-cis-3-amino glycals with a cis-fused cyclic sulfamidate group has been achieved by selective reduction of sulfamidate ketimine groups. The efficient access to the structurally unique glycals allowed the subsequent divergent synthesis of various naturally occurring 3-amino-2,3,6-trideoxysugars. In addition, Lewis acid-promoted glycosylation of the glycals provided a simple solution for the stereoselective installation of O- and C-linked aglycons on the amino sugar scaffolds.

Selective Transformations of Carbohydrates Inspired by Radical-Based Enzymatic Mechanisms

Enzymes are a longstanding source of inspiration for synthetic reaction development. However, enzymatic reactivity and selectivity are frequently untenable in a synthetic context, as the principles that govern control in an enzymatic setting often do not translate to small molecule catalysis. Recent synthetic methods have revealed the viability of using small molecule catalysts to promote highly selective radical-mediated transformations of minimally protected sugar substrates. These transformations share conceptual similarities with radical SAM enzymes found in microbial carbohydrate biosynthesis and present opportunities for synthetic chemists to access microbial and unnatural carbohydrate building blocks without the need for protecting groups or lengthy synthetic sequences. Here, we highlight strategies through which radical reaction pathways can enable the site-, regio-, and diastereoselective transformation of minimally protected carbohydrates in both synthetic and enzymatic systems.

Synthesis and Cytotoxic Evaluation of Arimetamycin A and Its Daunorubicin and Doxorubicin Hybrids

The arimetamycin A glycan governs the compound's cytotoxicity (IC50). To study this branched, deoxy-amino disaccharide, we designed and synthesized a modified acyl donor that underwent glycosylation with three anthracycline aglycones: steffimycinone, daunorubicinone, and doxorubicinone. The result of the approach was a synthesis of arimetamycin A and two novel hybrid anthracyclines. Each molecule exhibited enhanced cytotoxicity in comparison to the parent anthracyclines, steffimycin B, daunorubicin, and doxorubicin. An orienting mechanistic evaluation revealed that the daunorubicin hybrid inhibits the ability of human topoisomerase IIα to relax negatively and positively supercoiled DNA.

Inhibition of glucuronomannan hexamer on the proliferation of lung cancer through binding with immunoglobulin G

The anti-lung cancer activity of oligosaccharides derived from glucuronomannan was investigated. The inhibition of A549 cell proliferation by glucuronomannan (Gn) and its oligomers (dimer (G2), tetramer (G4) and hexamer (G6)) were concentration dependent. In vivo activities on the A549-derived tumor xenografts showed the tumor inhibition of G2, G4 and G6 were 17 %, 40 % and 46 %, respectively. Organ coefficients in nude mice showed an increase in the kidney with G4, the brain with G6, and the spleen with G6. An advanced tandem mass tag labeled proteomics approach was performed. A significant differential expression was found in 59 out of the 4371 proteins, which involved the immune system. Surface plasmon resonance (SPR) studies revealed G6 was strongly bound to immunoglobulin G. This suggests that glucuronomannan hexamer inhibits the proliferation of lung cancer through its binding to immunoglobulin.

Hydrogen-Bonding-Assisted Exogenous Nucleophilic Reagent Effect for β-Selective Glycosylation of Rare 3-Amino Sugars

Challenges for stereoselective glycosylation of deoxy sugars are notorious in carbohydrate chemistry. We herein report a novel strategy for the construction of the less investigated β-glycosidic bonds of 3,5- trans-3-amino-2,3,6-trideoxy sugars (3,5- trans-3-ADSs), which constitute the core structure of several biologically important antibiotics. Current protocol leverages a C-3 axial sulfonamide group in 3,5- trans-3-ADSs as a hydrogen-bond (H-bond) donor and repurposes substoichiometric phosphine oxide as an exogenous nucleophilic reagent (exNu) to establish an intramolecular H-bond between the former and the derived α-oxyphosphonium ion. This pivotal interaction stabilizes the α-face-covered intermediate to inhibit the formation of the more reactive β-intermediate, thereby yielding reversed β-selectivity, which is unconventional for an exNu-mediated glycosylation system. A wide range of substrates was accommodated, and good to excellent β-selectivities were ensured by this H-bonding-assisted exNu effect. The robustness of the current strategy was further attested by the architectural modification of natural products and drugs containing 3,5- trans-3-ADSs, as well as the synthesis of a trisaccharide unit in avidinorubicin.

Synthesis of the Non-Reducing Hexasaccharide Fragment of Saccharomicin B

A synthesis of the nonreducing end hexasaccharide of saccharomicin B, α-l-Eva-(1→4)-α-l-Eva-(1→4)-α-l-Dig-(1→4)-α-l-Eva-(1→4)-α-l-Dig-(1→4)-β-d-Fuc, has been developed. Selective glycosylations of l-digitoxose (l-Dig) using AgPF₆/TTBP-mediated thioether activation and l-4-e pi-vancosamine (l-Eva) using Tf₂O/DTBMP-mediated sulfoxide activation produced the hexasaccharide as a single diastereomer in very good yield. This hexasaccharide is properly functionalized to serve as a glycosyl donor for the total synthesis of saccharomicin B.

Reagent-Controlled Synthesis of the Branched Trisaccharide Fragment of the Antibiotic Saccharomicin B

A concise synthesis of a branched trisaccharide, α-l-Dig-(1 → 3)-[α-l-Eva-(1 → 4)]-β-d-Fuc, corresponding to saccharomicin B, has been developed via reagent-controlled α-selective glycosylations. Starting from the d-fucose acceptor, l- epi-vancosamine was selectively installed using 2,3-bis(2,3,4-trimethoxyphenyl)cyclopropene-1-thione/oxalyl bromide mediated dehydrative glycosylation. Following deprotection, l-digitoxose was installed using the AgPF₆/TTBP thioether-activation method to produce the trisaccharide as a single α-anomer. This highly functionalized trisaccharide can potentially serve as both a donor and an acceptor for the total synthesis of the antibiotic saccharomicin B.

Diversified synthesis and α-selective glycosylation of 3-amino-2,3,6-trideoxy sugars

Quick access to various unnatural 3-amino-2,3,6-trideoxy sugars was achieved by sequential functionalization of a glycal intermediate. This strategy and the further glycosylation method allowed the efficient late-stage modification of bioactive natural products and drugs.

Hydroxyl regioisomerization of anthracycline catalyzed by a four-enzyme cascade

Ranking among the most effective anticancer drugs, anthracyclines represent an important family of aromatic polyketides generated by type II polyketide synthases (PKSs). After formation of polyketide cores, the post-PKS tailoring modifications endow the scaffold with various structural diversities and biological activities. Here we demonstrate an unprecedented four-enzyme-participated hydroxyl regioisomerization process involved in the biosynthesis of kosinostatin. First, KstA15 and KstA16 function together to catalyze a cryptic hydroxylation of the 4-hydroxyl-anthraquinone core, yielding a 1,4-dihydroxyl product, which undergoes a chemically challenging asymmetric reduction-dearomatization subsequently acted by KstA11; then, KstA10 catalyzes a region-specific reduction concomitant with dehydration to afford the 1-hydroxyl anthraquinone. Remarkably, the shunt product identifications of both hydroxylation and reduction-dehydration reactions, the crystal structure of KstA11 with bound substrate and cofactor, and isotope incorporation experiments reveal mechanistic insights into the redox dearomatization and rearomatization steps. These findings provide a distinguished tailoring paradigm for type II PKS engineering.

Gold-catalyzed diversified synthesis of 3-aminosugar analogues of digitoxin and digoxin

A small library containing 3-aminosugar analogues of digitoxin and digoxin with potent anticancer activities was constructed by gold-catalyzed glycosylation.

TMSBr-mediated solvent- and work-up-free synthesis of α-2-deoxyglycosides from glycals

The thio-additions of glycals were efficiently promoted by a stoichiometric amount of trimethylsilyl bromide (TMSBr) to produce S-2-deoxyglycosides under solvent-free conditions in good to excellent yields. In addition, with triphenylphosphine oxide as an additive, the TMSBr-mediated direct glycosylations of glycals with a large range of alcohols were highly α-selective.

2-Deoxyglucose conjugated platinum (II) complexes for targeted therapy: design, synthesis, and antitumor activity

Malignant neoplasms exhibit an elevated rate of glycolysis over normal cells. To target the Warburg effect, we designed a new series of 2-deoxyglucose (2-DG) conjugated platinum (II) complexes for glucose transporter 1 (GLUT1)-mediated anticancer drug delivery. The potential GLUT1 transportability of the complexes was investigated through a comparative molecular docking analysis utilizing the latest GLUT1 protein crystal structure. The key binding site for 2-DG as GLUT1's substrate was identified with molecular dynamics simulation, and the docking study demonstrated that the 2-DG conjugated platinum (II) complexes can be recognized by the same binding site as potential GLUT1 substrate. The conjugates were synthesized and evaluated for in vitro cytotoxicity study with seven human cancer cell lines. The results of this study revealed that 2-DG conjugated platinum (II) complexes are GLUT1 transportable substrates and exhibit improved cytotoxicities in cancer cell lines that over express GLUT1 when compared to the clinical drug, Oxaliplatin. The correlation between GLUT1 expression and antitumor effects are also confirmed. The study provides fundamental information supporting the potential of the 2-DG conjugated platinum (II) complexes as lead compounds for further pharmaceutical R&D.

Enzymatic production of three 6-deoxy-aldohexoses from L-rhamnose

6-Deoxy-L-glucose, 6-deoxy-L-altrose, and 6-deoxy-L-allose were produced from L-rhamnose with an immobilized enzyme that was partially purified (IE) and an immobilized Escherichia coli recombinant treated with toluene (TT). 6-Deoxy-L-psicose was produced from L-rhamnose by a combination of L-rhamnose isomerase (TT-PsLRhI) and D-tagatose 3-epimerase (TT-PcDTE). The purified 6-deoxy-L-psicose was isomerized to 6-deoxy-L-altrose and 6-deoxy-L-allose with L-arabinose isomerase (TT-EaLAI) and L-ribose isomerase (TT-AcLRI), respectively, and then was epimerized to L-rhamnulose with immobilized D-tagatose 3-epimerase (IE-PcDTE). Following purification, L-rhamnulose was converted to 6-deoxy-L-glucose with D-arabinose isomerase (TT-BpDAI). The equilibrium ratios of 6-deoxy-L-psicose:6-deoxy-L-altrose, 6-deoxy-L-psicose:6-deoxy-L-allose, and L-rhamnulose:6-deoxy-L-glucose were 60:40, 40:60, and 27:73, respectively. The production yields of 6-deoxy-L-glucose, 6-deoxy-L-altrose, and 6-deoxy-L-allose from L-rhamnose were 5.4, 14.6, and 25.1%, respectively. These results indicate that the aldose isomerases used in this study acted on 6-deoxy aldohexoses.

Anthracyclines as effective anticancer drugs

Anthracyclines have received significant attention due to their effectiveness and extensive use as anticancer agents. At present, the clinical use of these drugs is offset by drug resistance in tumours and cardiotoxicity. Therefore, a relentless search for the 'better anthracycline' has been ongoing since the inception of these drugs > 30 years ago. This review focuses on the most recent pharmacology and medicinal chemistry developments on the design and use of anthracyclines. Based on their crystal structures as well as molecular modelling, a more detailed mechanism of topoisomerase poisoning by these new anthracyclines has emerged. Chemical modifications of anthracyclines have been found to possibly change the target selectivity among various topoisomerases and, thus, vary their anticancer activity. Additionally, recent sugar modifications of anthracyclines have also been found to overcome P-glycoprotein-mediated drug resistance in cancer therapy. The continued improvement of anthracycline clinical applications so far and the clinical trials of the 'third generation' of anthracyclines (such as sabarubicin) are also discussed. To finally find the 'better' anthracycline, further areas of research still need to be explored such as: the elucidation of the topoisomerase and P-glycoprotein crystal structures, molecular modelling based on crystal structure in order to design the next generation of better anthracycline drugs, the continued modifications of the anthracycline sugar moieties, and the further improvement of anthracycline drug delivery methods.

Synthesis and biological activities of a 3'-azido analogue of Doxorubicin against drug-resistant cancer cells

Doxorubicin (DOX), an anthracycline antibiotic, is one of the most active anticancer chemotherapeutic agents. The clinical use of DOX, however, is limited by the dose-dependant P-glycoprotein (P-gp)-mediated resistance. Herein, a 3'-azido analogue of DOX (ADOX) was prepared from daunorubicin (DNR). ADOX exhibited potent antitumor activities in drug-sensitive (MCF-7 and K562) and drug-resistant cell lines (MCF-7/DNR, K562/DOX), respectively. The drug resistance index (DRI) values of ADOX were much lower than that of DOX. The cytotoxicity experiments of ADOX or DOX against K562/DOX, with or without P-gp inhibitor, indicated that ADOX circumvents resistance by abolishing the P-gp recognition. This conclusion was further supported by drug influx/efflux flow cytometry experiments, as well as by molecular docking of ADOX to P-gp. In vivo animal tests, ADOX exhibited higher activity and less toxicity than DOX. The current data warranted ADOX for additional pre-clinical evaluations for new drug development.

Interaction of an anthracycline disaccharide with ctDNA: Investigation by spectroscopic technique and modeling studies

This study was designed to examine the interaction of an anthracycline disaccharide, 4'-O-(beta-L-oleandrosyl) daunorubicin (DNR-D2), with calf thymus deoxyribonucleic acid (ctDNA) by UV-vis in combination with fluorescence spectroscopy and molecular modeling techniques under physiological conditions (Britton-Robinson buffer solutions, pH 7.4). By the analysis of UV-vis and fluorescence spectrum, it was observed that the binding mode between DNR-D2 and ctDNA might be intercalation, and fluorescence quenching mechanism of DNR-D2 by ctDNA was a static quenching type. Upon binding to ctDNA, the anthraquinone chromophore of DNR-D2 could slide into the C-G rich region of ctDNA. Hydrogen bonding forces may play an essential role in the binding of DNR-D2 to ctDNA. Furthermore, the results obtained from computational modeling corroborated the experimental results obtained from spectroscopic investigations. These studies are valuable for a better understanding the datailed mode of DNR-D2-DNA interaction, which should be important in deeper insight into the therapeutic efficiency of DNR-D2.

Synthesis and DNA-binding affinity studies of glycosylated intercalators designed as functional mimics of the anthracycline antibiotics

Anthracycline antibiotics such as daunomycin (Dauno) and doxorubicin (Dox) are well-known clinically used cancer chemotherapeutics, which, among other mechanisms, bind to DNA, thereby triggering a cascade of biological responses leading to cell death. However, anthracyclines are cardiotoxic, and drug resistance develops rapidly, thus limiting their clinical use. We report here the synthesis and DNA-binding affinity of a novel class of functional anthracycline mimetics consisting of an aromatic moiety linked to a carbohydrate (1-12). In the targets, the aromatic core consists of a 2-phenylbenzo[b]furan-3-yl, 2-phenylbenzo[b]thiophen-3-yl, 1-tosyl-2-phenylindol-3-yl, or 2-phenylindol-3-yl group that is bound to one of three aminosugars (daunosamine, acosamine, or 4-amino-2,3,4,6-tetradeoxy-alpha-l-hexopyranoside) via a propargyl linker. The DNA binding affinity of these twelve compounds has been evaluated by using both direct and indirect fluorescence measurements. Compared to Dauno and Dox, the DNA binding affinity of these analogues is weaker. However, both aromatic and aminosugar motifs are critical to DNA binding, with more influence coming from the structural features of the aromatic portion.

Sweet antibiotics - the role of glycosidic residues in antibiotic and antitumor activity and their randomization

A large number of antibiotics are glycosides. In numerous cases the glycosidic residues are crucial to their activity; sometimes, glycosylation only improves their pharmacokinetic parameters. Recent developments in molecular glycobiology have improved our understanding of aglycone vs. glycoside activities and made it possible to develop new, more active or more effective glycodrugs based on these findings - a very illustrative recent example is vancomycin. The majority of attention has been devoted to glycosidic antibiotics including their past, present, and probably future position in antimicrobial therapy. The role of the glycosidic residue in the biological activity of glycosidic antibiotics, and the attendant targeting and antibiotic selectivity mediated by glycone and aglycone in antibiotics some antitumor agents is discussed here in detail. Chemical and enzymatic modifications of aglycones in antibiotics, including their synthesis, are demonstrated on various examples, with particular emphasis on the role of specific and mutant glycosyltransferases and glycorandomization in the preparation of these compounds. The last section of this review describes and explains the interactions of the glycone moiety of the antibiotics with DNA and especially the design and structure-activity relationship of glycosidic antibiotics, including their classification based on their aglycone and glycosidic moiety. The new enzymatic methodology 'glycorandomization' enabled the preparation of glycoside libraries and opened up new ways to prepare optimized or entirely novel glycoside antibiotics.

Syntheses and biological activities of daunorubicin analogs with uncommon sugars

To study the effects of the sugar structure on the activity of anthracycline against cancer cells, six daunorubicin analogs containing different uncommon sugars were synthesized. Their cytotoxicities were tested against colon cancer cells by MTS assay. The results showed that the aglycon without sugar moiety has 70-100-fold lower activity against cancer cells than daunorubicin derivatives with various uncommon sugars. It suggests that the sugar structure in daunorubicin plays a critical role in determining its anticancer activity. In the compounds with various sugars, the 4'-OH of the sugar is an important determinant for their activity, while the axial-3'-substituent in the sugar interferes with the binding of daunorubicins to DNA. Therefore, 2,6-dideoxy sugars are a better choice for generating biologically active daunorubicin analogs than 6-deoxysugars, 2,3,6-trideoxysugars, or 2,3,4,6-tetradeoxysugars.