Novel anthracycline oligosaccharides: influence of chemical modifications of the carbohydrate moiety on biological activity

Topoisomerase II-targeting anticancer clinical candidates and drugs: A critical analysis, unravelling molecular medicinal insights and promising research roadmap

In recent years, the USFDA-approved drug molecules are being frequently analyzed to provide perspectives and strategies for novel therapeutic discovery and development. Some of the remarkable analyses include physicochemical properties of drugs relevant to oral bioavailability, frequent presence of drug relevant-structural motifs, natural products as sources of new drugs, and synthetic approaches to new drugs. In this review article, for the first time, we present a structure-function analysis of human topoisomerase II (hTopo II) inhibitors those are currently clinically used or under clinical trials for anticancer treatment. The case studies and a critical molecular medicinal insight for their therapeutic development have been presented. The review illustrates various key aspects: the hTopo II inhibitors' molecular modulations, common pharmacophores, interactions at molecular level crucial for inhibition of enzyme at its various stages of catalytic function, and network polypharmacology of Topo II with different targets. Numerous toxicophore motifs have been identified, which provide important alerts while designing and discovering novel therapeutic agents. A range of innovative approaches including property-focused strategies, ADCs, and Click Activated Protodrugs Against Cancer (CAPAC) that have addressed challenges faced in the hTopo II-based therapeutic development have been discussed. The analysis with perspectives represents a valuable educational and research resource that will encourage hTopo II-inhibition and its network polypharmacology based drug discovery studies.

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.

The Critical Role of 12-Methyl Group of Anthracycline Dutomycin to Its Antiproliferative Activity

Anthracycline dutomycin is a tetracyclic quinone glycoside produced by Streptomyces minoensis NRRL B-5482. SW91 is a C-12 demethylated dutomycin derivative, which was identified in our previous research. In vitro cytotoxicity and apoptosis assays of these two compounds were conducted to demonstrate their antiproliferation activities. The results showed that both dutomycin and SW91 block cells at the S phase, whereas dutomycin shows more significant inhibition of cell growth. Their interactions with calf thymus DNA (CT-DNA) were investigated, with dutomycin exhibiting higher binding affinity. The molecular docking demonstrated that the 12-methyl group makes dutomycin attach to the groove of DNA. These findings suggest that dutomycin has binding higher affinity to DNA and impairs DNA replication resulting in more significant antitumor activity.

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.

Doxorubicin and Aclarubicin: Shuffling Anthracycline Glycans for Improved Anticancer Agents

Anthracycline anticancer drugs doxorubicin and aclarubicin have been used in the clinic for several decades to treat various cancers. Although closely related structures, their molecular mode of action diverges, which is reflected in their biological activity profile. For a better understanding of the structure-function relationship of these drugs, we synthesized ten doxorubicin/aclarubicin hybrids varying in three distinct features: aglycon, glycan, and amine substitution pattern. We continued to evaluate their capacity to induce DNA breaks, histone eviction, and relocated topoisomerase IIα in living cells. Furthermore, we assessed their cytotoxicity in various human tumor cell lines. Our findings underscore that histone eviction alone, rather than DNA breaks, contributes strongly to the overall cytotoxicity of anthracyclines, and structures containing N,N-dimethylamine at the reducing sugar prove that are more cytotoxic than their nonmethylated counterparts. This structural information will support further development of novel anthracycline variants with improved anticancer activity.

Temporary ether protecting groups at the anomeric center in complex carbohydrate synthesis

The synthesis of a carbohydrate building block usually starts with introduction of a temporary protecting group at the anomeric center and ends with its selective cleavage for further transformation. Thus, the choice of the anomeric temporary protecting group must be carefully considered because it should retain intact during the whole synthetic manipulation, and it should be chemoselectively removable without affecting other functional groups at a late stage in the synthesis. Etherate groups are the most widely used temporary protecting groups at the anomeric center, generally including allyl ethers, MP (p-methoxyphenyl) ethers, benzyl ethers, PMB (p-methoxybenzyl) eithers, and silyl ethers. This chapter provides a comprehensive review on their formation, cleavage, and applications in the synthesis of complex carbohydrates.

Methods for 2-Deoxyglycoside Synthesis

Deoxy-sugars often play a critical role in modulating the potency of many bioactive natural products. Accordingly, there has been sustained interest in methods for their synthesis over the past several decades. The focus of much of this work has been on developing new glycosylation reactions that permit the mild and selective construction of deoxyglycosides. This Review covers classical approaches to deoxyglycoside synthesis, as well as more recently developed chemistry that aims to control the selectivity of the reaction through rational design of the promoter. Where relevant, the application of this chemistry to natural product synthesis will also be described.

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.

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.

Chemoenzymatic synthesis of novel C-ribosylated naphthoquinones

The biological activity of many natural products is dependent on the presence of carbohydrate units, which are usually attached via an O-glycosidic linkage by glycosyltransferases. Recently, an exceptional C-ribosylation event was discovered in the biosynthesis of the polyketide antibiotic alnumycin A. The two-step process involves initial attachment of d-ribose-5-phosphate to the polyaromatic aglycone by the C-glycosynthase AlnA and subsequent dephosphorylation by AlnB, an enzyme of the haloacid dehalogenase family. Here, we tested 23 unnatural substrates to probe the C-ribosylation reaction. The chemoenzymatic synthesis of C-ribosylated juglone, 7-methyl juglone, monomethyl naphthazarin, 8-chloro-7-methyl juglone, and 9-hydroxy-1,4-anthraquinone revealed the importance of a 1,4-quinoid system with an adjacent phenolic ring in order for reaction to occur. To further rationalize the molecular basis for reactivity, factors governing substrate recognition were investigated by NMR binding experiments. Additionally, the suitability of substrates for nucleophilic substitution was assessed by molecular modeling using density functional theory (DFT) calculations.

"Function-first" lead discovery: mode of action profiling of natural product libraries using image-based screening

Cytological profiling is a high-content image-based screening technology that provides insight into the mode of action (MOA) for test compounds by directly measuring hundreds of phenotypic cellular features. We have extended this recently reported technology to the mechanistic characterization of unknown natural products libraries for the direct prediction of compound MOAs at the primary screening stage. By analyzing a training set of commercial compounds of known mechanism and comparing these profiles to those obtained from natural product library members, we have successfully annotated extracts based on MOA, dereplicated known compounds based on biological similarity to the training set, and identified and predicted the MOA of a unique family of iron siderophores. Coupled with traditional analytical techniques, cytological profiling provides an avenue for the creation of "function-first" approaches to natural products discovery.

Sabarubicin

Disaccharide derivatives in the daunorubicin and in the 4-demethoxy (idarubicin) series in whichthe first sugar moiety linked to the aglycone is a non-aminated sugar, namely 2-deoxy-L-rhamnose or 2-deoxy-L-fucose andthe second moiety is daunosamine, have been obtained upon synthesis of the appropriate activated sugarintermediate and glycosylation of the corresponding aglycones. The compounds containing 2-deoxy-L-fucose exhibit superior pharmacological properties with respect to thestereoisomers containing 2-deoxy-L-rhamnose. The doxorubicinanalog 7-O-(α-L-daunosaminyl-α(1-4)-2-deoxy-L-fucosyl)-4-demethoxy-adriamycinone (sabarubicin) was prepared startingfrom 14-acetoxyidarubicinone. Solution properties and molecular interactions are compared with thoseof doxorubicin. Sabarubicin exhibits a superior antitumor efficacy, presumably related to theactivation of p53-independent apoptosis. A number of sabarubicin analogues have also been synthesized.

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.

Structural basis for C-ribosylation in the alnumycin A biosynthetic pathway

Alnumycin A is an exceptional aromatic polyketide that contains a carbohydrate-like 4'-hydroxy-5'-hydroxymethyl-2',7'-dioxane moiety attached to the aglycone via a carbon-carbon bond. Recently, we have identified the D-ribose-5-phosphate origin of the dioxane unit and demonstrated that AlnA and AlnB are responsible for the overall C-ribosylation reaction. Here, we provide direct evidence that AlnA is a natural C-glycosynthase, which catalyzes the attachment of D-ribose-5-phosphate to prealnumycin by formation of the C(8)-C(1') bond as demonstrated by the structure of the intermediate alnumycin P. This compound is subsequently dephosphorylated by AlnB, an enzyme of the haloacid dehalogenase superfamily. Structure determination of the native trimeric AlnA to 2.1-Å resolution revealed a highly globular fold encompassing an α/β/α sandwich. The crystal structure of the complex with D-ribose-5-phosphate indicated that the phosphosugar is bound in the open-chain configuration. Identification of residues E29, K86, and K159 near the C-1 carbonyl of the ligand led us to propose that the carbon-carbon bond formation proceeds through a Michael-type addition. Determination of the crystal structure of the monomeric AlnB in the open conformation to 1.25-Å resolution showed that the protein consists of core and cap domains. Modeling of alnumycin P inside the cap domain positioned the phosphate group next to a Mg(2+) ion present at the junction of the domains. Mutagenesis data were consistent with the canonical reaction mechanism for this enzyme family revealing the importance of residues D15 and D17 for catalysis. The characterization of the prealnumycin C-ribosylation illustrates an alternative means for attachment of carbohydrates to natural products.

Biosynthetic pathway toward carbohydrate-like moieties of alnumycins contains unusual steps for C-C bond formation and cleavage

Carbohydrate moieties are important components of natural products, which are often imperative for the solubility and biological activity of the compounds. The aromatic polyketide alnumycin A contains an extraordinary sugar-like 4'-hydroxy-5'-hydroxymethyl-2',7'-dioxane moiety attached via a carbon-carbon bond to the aglycone. Here we have extensively investigated the biosynthesis of the dioxane unit through (13)C labeling studies, gene inactivation experiments and enzymatic synthesis. We show that AlnA and AlnB, members of the pseudouridine glycosidase and haloacid dehalogenase enzyme families, respectively, catalyze C-ribosylation conceivably through Michael-type addition of d-ribose-5-phosphate and dephosphorylation. The ribose moiety may be attached both in furanose (alnumycin C) and pyranose (alnumycin D) forms. The C(1')-C(2') bond of alnumycin C is subsequently cleaved and the ribose unit is rearranged into an unprecedented dioxolane (cis-bicyclo[3.3.0]-2',4',6'-trioxaoctan-3'β-ol) structure present in alnumycin B. The reaction is catalyzed by Aln6, which belongs to a previously uncharacterized enzyme family. The conversion was accompanied with consumption of O(2) and formation of H(2)O(2), which allowed us to propose that the reaction may proceed via hydroxylation of C1' followed by retro-aldol cleavage and acetal formation. Interestingly, no cofactors could be detected and the reaction was also conducted in the presence of metal chelating agents. The last step is the conversion of alnumycin B into the final end-product alnumycin A catalyzed by Aln4, an NADPH-dependent aldo-keto reductase. This characterization of the dioxane biosynthetic pathway sets the basis for the utilization of C-C bound ribose, dioxolane and dioxane moieties in the generation of improved biologically active compounds.

Cosmomycin C inhibits signal transducer and activator of transcription 3 (STAT3) pathways in MDA-MB-468 breast cancer cell

The signal transducer and activator of transcription 3 (STAT3) is constitutively activated in cancer cells. Therefore, blocking the aberrant activity of STAT3 in tumor cells is a validated therapeutic strategy. To discover novel inhibitors of STAT3 activity, we screened against microbial natural products using a dual-luciferase assay. Using the microbial metabolome library, we identified cosmomycin C (CosC), which was isolated from the mycelium extract of Streptomyces sp. KCTC19769, as a STAT3 pathway inhibitor. CosC inhibited STAT3 (Tyr705) phosphorylation and subsequent nuclear translocation in MDA-MB-468 breast cancer cells. CosC-mediated inhibition of STAT3 signaling pathway was confirmed by suppressed expression of STAT3 downstream target proteins including cyclin D1, Bcl-xL, survivin, Mcl-1, and VEGF in CosC-treated MDA-MB-468 cells. Flow cytometry showed that CosC caused accumulation in the G(0)-G(1) phase of the cell cycle and induced apoptosis via PARP cleavage and caspase-3 activation. Based on these findings, CosC may be a potential candidate for modulation of STAT3 pathway.

Natural glycoconjugates with antitumor activity

Cancer is one of the major causes of death worldwide. As a consequence, many different therapeutic approaches, including the use of glycosides as anticancer agents, have been developed. Various glycosylated natural products exhibit high activity against a variety of microbes and human tumors. In this review we classify glycosides according to the nature of their aglycone (non-saccharidic) part. Among them, we describe anthracyclines, aureolic acids, enediyne antibiotics, macrolide and glycopeptides presenting different strengths and mechanisms of action against human cancers. In some cases, the glycosidic residue is crucial for their activity, such as in anthracycline, aureolic acid and enediyne antibiotics; in other cases, Nature has exploited glycosylation to improve solubility or pharmacokinetic properties, as in the glycopeptides. In this review we focus our attention on natural glycoconjugates with anticancer properties. The structure of several of the carbohydrate moieties found in these conjugates and their role are described. The structure–activity relationship of some of these compounds, together with the structural features of their interaction with the biological targets, are also reported. Taken together, all this information is useful for the design of new potential anti-tumor drugs.

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.

Insights in the glycosylation steps during biosynthesis of the antitumor anthracycline cosmomycin: characterization of two glycosyltransferase genes

Glycosylation pattern in cosmomycins is a distinctive feature among anthracyclines. These antitumor compounds possess two trisaccharide chains attached at C-7 and C-10, each of them with structural variability, mainly at the distal deoxysugar moieties. We have characterized a 14-kb chromosomal region from Streptomyces olindensis containing 13 genes involved in cosmomycin biosynthesis. Two of the genes, cosG and cosK, coding for glycosyltransferase were inactivated with the generation of five new derivatives. Structural elucidation of these compounds showed altered glycosylation patterns indicating the capability of both glycosyltransferases of transferring deoxysugars to both sides of the aglycone and the flexibility of CosK with respect to the deoxysugar donor. A model is proposed for the glycosylation steps during cosmomycins biosynthesis.

Syntheses and biological activity of bisdaunorubicins

To study the length and flexibility of the linkers between two monomers of bisdaunorubicins for their activity against cancer cells, seven bisdaunorubicins were rationally designed and synthesized through click chemistry. Their cytotoxicity was tested in leukemia cells with MTS assay. The results showed that the compounds with short linkers exhibited higher activity than the compounds with long linkers, while the flexibility of the linker also contributed to their activity. These results indicated that the length and flexibility of the linkers between two monomers in bisdaunorubicins are very critical to maintain their activity against cancer cells.

Syntheses and Biological Activities of Disaccharide Daunorubicins

Carbohydrate moiety is found in many anticancer nature products. To explore the carbohydrate moiety of daunorubicin in enhancing anticancer efficacy, several daunorubicin derivatives bearing disaccharide (1-8) have been synthesized. Their cytotoxicities were tested in leukemia K562 and colon cancer SW620 cells. Topoisomerase II (topo II) poisoning was performed with the in vivo complex of topoisomerase bioassay. In both cell lines, compounds with various terminal 2,6-dideoxy sugars (compounds 1, 3, 5, and 8) showed 30- to 60-fold higher anticancer activity than compounds with 2-deoxy- or 6-deoxy sugar (compounds 6 and 7). Compounds with an alpha-linkage between two sugar units (compound 3) showed 35-fold higher anticancer activity than compounds with a beta-linkage (compound 4). In addition, the anticancer activities of these compounds correlated with their ability to target topo II mediated genomic DNA damage in vivo. Compounds 1 and 3 with 2,6-dideoxy sugars produced more covalent topo-DNA complex than compounds with 2-deoxy sugar (6) and 6-deoxy sugar (7). Compounds with an alpha-configuration of terminal 2,6-dideoxy sugar (compounds 1 and 3) showed higher topo II poisoning than their counterparts with the beta-configuration (compounds 2 and 4). These results indicate that sugar moieties in daunorubicin play a significant role in its anticancer activity and topo II inhibition. The second sugar of disaccharide daunorubicin should possess 2,6-dideoxy with alpha-linkage to the first sugar to exhibit better anticancer activity.

Activity of topoisomerase inhibitors daunorubicin, idarubicin, and aclarubicin in the Drosophila Somatic Mutation and Recombination Test

Anthracyclines have been widely used as anticancer drugs against different types of human cancers. The present study evaluated the mutagenic and recombinagenic properties of two anthracycline topoisomerase II (topo II) poisons, daunorubicin (DNR) and idarubicin (IDA), as well as the related topo II catalytic inhibitor aclarubicin (ACLA), using the wing Somatic Mutation and Recombination Test (SMART) in Drosophila melanogaster. The three anthracyclines were positive in this bioassay, producing mainly mitotic homologous recombination. The results for spot-size distribution and recombinagenic activity indicate that recombinational DNA damage accounts for approximately 91, 86, and 62% of DNR, IDA, and ACLA genotoxicity, respectively. Besides being a catalytic inhibitor of topo II, ACLA is also a topoisomerase I (topo I) poison. This dual topo I and II inhibitory effect, associated with its DNA-intercalating activity, could contribute to the activity of ACLA in the SMART assay.

Doxorubicin and two of its analogues are preferential inducers of homologous recombination compared with mutational events in somatic cells of Drosophila melanogaster

The genotoxic effects of the anthracycline doxorubicin (DOX) and two of its analogues, epirubicin (EPI) and pirarubicin (THP) were studied using the wing Somatic Mutation and Recombination Test (SMART) in Drosophila melanogaster. These compounds are classified as topoisomerase II (topo II) poisons, acting by stabilizing a topoisomerase II-cleaved DNA complex. Using the standard version of the SMART test it was possible to estimate the quantitative and qualitative genotoxic effects of these compounds, comparing the wing spot frequencies in marker- and balancer-heterozygous flies. The results obtained indicate that all three compounds induce a high frequency of spots related to homologous recombination (HR), which is the major event responsible for their genetic toxicity. Pirarubicin was the most genotoxic anthracycline, inducing approximately 21 times more genetic lesions than doxorubicin, probably due to the presence of a second sugar ring in the amino sugar moiety in its chemical structure. Although the only difference between epirubicin and doxorubicin is the steric position of the amino sugar 4'-OH in the molecule, epirubicin is approximately 1.6 times as genotoxic as doxorubicin.