Synthesis, Biological Evaluation, and Molecular Modeling Studies of Rebeccamycin Analogues Modified in the Carbohydrate Moiety

Unified Strategy toward Stereocontrolled Assembly of Various Glucans Based on Bimodal Glycosyl Donors

Polymers of glucose, the most abundant and one of the biologically important natural products, named glucans are widely present in fungi, bacteria, mammals, and plants with various anomeric configurations and glycosidic linkages. Because of their structural diversity, the unified strategy for the assembly of pure glucans is yet to be developed. Herein, we describe a general strategy that is applicable to construction of all types of glucans by exploiting a bimodal glycosyl donor equipped with C2-o-TsNHbenzyl ether (TAB), which enables stereocontrolled synthesis of both α- and β-glycosides by switching reaction conditions.

Topoisomerase 1B poisons: Over a half-century of drug leads, clinical candidates, and serendipitous discoveries

Topoisomerases are DNA processing enzymes that relieve supercoiling (torsional strain) in DNA, are necessary for normal cellular division, and act by nicking (and then religating) DNA strands. Type 1B topoisomerase (Top1) is overexpressed in certain tumors, and the enzyme has been extensively investigated as a target for cancer chemotherapy. Various chemical agents can act as "poisons" of the enzyme's religation step, leading to Top1-DNA lesions, DNA breakage, and eventual cellular death. In this review, agents that poison Top1 (and have thus been investigated for their anticancer properties) are surveyed, including natural products (such as camptothecins and indolocarbazoles), semisynthetic camptothecin and luotonin derivatives, and synthetic compounds (such as benzonaphthyridines, aromathecins, and indenoisoquinolines), as well as targeted therapies and conjugates. Top1 has also been investigated as a therapeutic target in certain viral and parasitic infections, as well as autoimmune, inflammatory, and neurological disorders, and a summary of literature describing alternative indications is also provided. This review should provide both a reference for the medicinal chemist and potentially offer clues to aid in the development of new Top1 poisons.

Enzymatic synthesis of avermectin B1a glycosides for the effective prevention of the pine wood nematode Bursaphelenchus xylophilus

Avermectin produced by Streptomyces avermitilis is an anti-nematodal agent against the pine wood nematode Bursaphelenchus xylophilus. However, its potential usage is limited by its poor water solubility. For this reason, continuous efforts are underway to produce new derivatives that are more water soluble. Here, the enzymatic glycosylation of avermectin was catalyzed by uridine diphosphate (UDP)-glycosyltransferase from Bacillus licheniformis with various UDP sugars. As a result, the following four avermectin B_1a glycosides were produced: avermectin B_1a 4″-β-D-glucoside, avermectin B_1a 4″-β-D-galactoside, avermectin B_1a 4″-β-L-fucoside, and avermectin B_1a 4″-β-2-deoxy-D-glucoside. The avermectin B_1a glycosides were structurally analyzed based on HR-ESI MS and 1D and 2D nuclear magnetic resonance spectra, and the anti-nematodal effect of avermectin B_1a 4″-β-D-glucoside was found to exhibit the highest activity (IC₅₀ = 0.23 μM), which was approximately 32 times greater than that of avermectin B_1a (IC₅₀ = 7.30 μM), followed by avermectin B_1a 4″-β-2-deoxy-D-glucoside (IC₅₀ = 0.69 μM), avermectin B_1a 4″-β-L-fucoside (IC₅₀ = 0.89 μM), and avermectin B_1a 4″-β-D-galactoside (IC₅₀ = 1.07 μM). These results show that glycosylation of avermectin B_1a effectively enhances its in vitro anti-nematodal activity and that avermectin glycosides can be further applied for treating infestations of the pine wood nematode B. xylophilus.

Improvement of the pharmacological activity of menthol via enzymatic β-anomer-selective glycosylation

Menthol has a considerable cooling effect, but the use range of menthol is limited because of its extremely low solubility in water and inherent flavor. (−)-Menthol β-glucoside was determined to be more soluble in water (>27 times) than (−)-menthol α-glucoside; hence, β-anomer-selective glucosylation of menthol is necessary. The in vitro glycosylation of (−)-menthol by uridine diphosphate glycosyltransferase (BLC) from Bacillus licheniformis generated (−)-menthol β-glucoside and new (−)-menthol β-galactoside and (−)-menthol N-acetylglucosamine. The maximum conversion rate of menthol to (−)-menthol β-d-glucoside by BLC was found to be 58.9%. Importantly, (−)-menthol β-d-glucoside had a higher cooling effect and no flavor compared with menthol. In addition, (−)-menthol β-d-glucoside was determined to be a non-sensitizer in a skin allergy test in the human cell line activation test, whereas menthol was a sensitizer.

Indimicins A-E, Bisindole Alkaloids from the Deep-Sea-Derived Streptomyces sp. SCSIO 03032

Five new bisindole alkaloids, indimicins A-E (1-5), bearing a unique 1',3'-dimethyl-2'-hydroindole moiety, were isolated from the marine-derived Streptomyces sp. SCSIO 03032, along with two new compounds, lynamicins F and G (6 and 7). Their planar structures were elucidated by detailed interpretation of their MS and NMR spectroscopic data, and the absolute configurations were determined by X-ray crystallographic analysis (for 1), comparison of CD spectra (for 2-4), and quantum chemical calculations (for 5). Indimicin B (2) exhibited moderate cytotoxic activity toward the MCF-7 cell line.

Enzymatic synthesis of epothilone A glycosides

Epothilones are extremely cytotoxic chemotherapeutic agents with epoxide, thiazole, and ketone groups that share equipotent kinetic similarity with taxol. The in vitro glycosylation catalyzed by uridine diphosphate glucosyltransferase (YjiC) from Bacillus licheniformis generated six novel epothilone A glycoside analouges including epothilone A 7-O-β-D-glucoside, epothilone A 7-O-β-D-galactoside, epothilone A 3,7-O-β-D-digalactoside, epothilone A 7-O-β-D-2-deoxyglucoside, epothilone A 7-O-β-L-rhamnoside, and epothilone A 7-O-β-L-fucoside. Epothilone A 7-O-β-D-glucoside was structurally elucidated by ultra-high performance liquid chromatography-photo diode array (UPLC-PDA) conjugated with high resolution quantitative time-of-flight-electrospray ionization mass spectroscopy (HR-QTOF ESI-MS/MS) supported by one-and two-dimensional nuclear magnetic resonance studies whereas other epothilone A glycosides were characterized by UPLC-PDA and HR-QTOF ESI-MS/MS analyses. The time dependent conversion study of epothilone A to epothilone A 7-O-β-D-glucoside found to be maximum (~26%) between 3 h to 5 h incubation.

Microbial natural products: molecular blueprints for antitumor drugs

Microbes from two of the three domains of life, the Prokarya, and Eukarya, continue to serve as rich sources of structurally complex chemical scaffolds that have proven to be essential for the development of anticancer therapeutics. This review describes only a handful of exemplary natural products and their derivatives as well as those that have served as elegant blueprints for the development of novel synthetic structures that are either currently in use or in clinical or preclinical trials together with some of their earlier analogs in some cases whose failure to proceed aided in the derivation of later compounds. In every case, a microbe has been either identified as the producer of secondary metabolites or speculated to be involved in the production via symbiotic associations. Finally, rapidly evolving next-generation sequencing technologies have led to the increasing availability of microbial genomes. Relevant examples of genome mining and genetic manipulation are discussed, demonstrating that we have only barely scratched the surface with regards to harnessing the potential of microbes as sources of new pharmaceutical leads/agents or biological probes.

3,6-bis(3-alkylguanidino)acridines as DNA-intercalating antitumor agents

A series of 3,6-bis(3-alkylguanidino) acridines was prepared and the interaction of these novel compounds with calf thymus DNA was investigated with UV-vis, fluorescence and circular dichroism spectroscopy, in addition to DNA melting techniques. The binding constants K were estimated to range from 1.25 to 5.26 × 10(5) M(-1), and the percentage of hypochromism was found to be 17-42% (from spectral titration). UV-vis, fluorescence and circular dichroism measurements indicated that the compounds act as effective DNA-intercalating agents. Electrophoretic separation proved that ligands 6a-e relaxed topoisomerase I at a concentration of 60 μM, although only those with longer alkyl chains were able to penetrate cell membranes and suppress cell proliferation effectively. The biological activity of novel compounds was assessed using different techniques (cell cycle distribution, phosphatidylserine externalization, caspase-3 activation, changes in mitochondrial membrane potential) and demonstrated mostly transient cytostatic action of the ethyl 6c and pentyl 6d derivatives. The hexyl derivative 6e proved to be the most cytotoxic. Different patterns of cell penetration were also observed for individual derivatives. Principles of molecular dynamics were applied to explore DNA-ligand interactions at the molecular level.

Synthesis and biological evaluation of rebeccamycin analogues modified at the imide moiety

Glycosylated indolocarbazoles related to the antibiotic rebeccamycin represent an important class of antitumour drugs. In the course of our structure-activity relationship studies, new rebeccamycin analogues modified at the imide moiety were synthesised. The antiproliferative activity of the compounds was evaluated on three human cancer cell lines, A2780 (ovarian cancer), H460 (lung cancer), and GLC4 (small-cell lung cancer). The in vitro cytotoxicity of compounds 2 and 4, characterised respectively by a 1,3-dioxolan and (1,3-dioxolan-4-yl)methylene groups linked to the imide moiety, was higher than the reference compound, edotecarin. The effect of compound 2 in inducing tumour regression in the A2780 xenograft model was also investigated.

Neomycin-neomycin dimer: an all-carbohydrate scaffold with high affinity for AT-rich DNA duplexes

A dimeric neomycin-neomycin conjugate 3 with a flexible linker, 2,2'-(ethylenedioxy)bis(ethylamine), has been synthesized and characterized. Dimer 3 can selectively bind to AT-rich DNA duplexes with high affinity. Biophysical studies have been performed between 3 and different nucleic acids with varying base composition and conformation by using ITC (isothermal calorimetry), CD (circular dichroism), FID (fluorescent intercalator displacement), and UV (ultraviolet) thermal denaturation experiments. A few conclusions can be drawn from this study: (1) FID assay with 3 and polynucleotides demonstrates the preference of 3 toward AT-rich sequences over GC-rich sequences. (2) FID assay and UV thermal denaturation experiments show that 3 has a higher affinity for the poly(dA)·poly(dT) DNA duplex than for the poly(dA)·2poly(dT) DNA triplex. Contrary to neomycin, 3 destabilizes poly(dA)·2poly(dT) triplex but stabilizes poly(dA)·poly(dT) duplex, suggesting the major groove as the binding site. (3) UV thermal denaturation studies and ITC experiments show that 3 stabilizes continuous AT-tract DNA better than DNA duplexes with alternating AT bases. (4) CD and FID titration studies show a DNA binding site size of 10-12 base pairs/drug, depending upon the structure/sequence of the duplex for AT-rich DNA duplexes. (5) FID and ITC titration between 3 and an intramolecular DNA duplex [d(5'-A(12)-x-T(12)-3'), x = hexaethylene glycol linker] results in a binding stoichiometry of 1:1 with a binding constant ∼10(8) M(-1) at 100 mM KCl. (6) FID assay using 3 and 512 hairpin DNA sequences that vary in their AT base content and placement also show a higher binding selectivity of 3 toward continuous AT-rich than toward DNA duplexes with alternate AT base pairs. (7) Salt-dependent studies indicate the formation of three ion pairs during binding of the DNA duplex d[5'-A(12)-x-T(12)-3'] and 3. (8) ITC-derived binding constants between 3 and DNA duplexes have the following order: AT continuous, d[5'-G(3)A(5)T(5)C(3)-3'] > AT alternate, d[5'-G(3)(AT)(5)C(3)-3'] > GC-rich d[5'-A(3)G(5)C(5)T(3)-3']. (9) 3 binds to the AT-tract-containing DNA duplex (B* DNA, d[5'-G(3)A(5)T(5)C(3)-3']) with 1 order of magnitude higher affinity than to a DNA duplex with alternating AT base pairs (B DNA, d[5'-G(3)(AT)(5)C(3)-3']) and with almost 3 orders of magnitude higher affinity than a GC-rich DNA (A-form, d[5'-A(3)G(5)C(5)T(3)-3']).

Synthesis and antiproliferative activity of oxazinocarbazole and N,N-bis(carbazolylmethyl)amine derivatives

The synthesis, structure elucidation and antitumoral activity of novel heterocyclic compounds containing a carbazole nucleus are reported. Oxazinocarbazoles were synthesized by application of the Mannich reaction to the corresponding hydroxylated derivatives leading to 41 new molecules. Their cytotoxic activity was evaluated against various human tumor cell lines including three leukemic cell lines: CEM and Jurkat (type T), Raji (type B); breast cancer cell line (MCF-7); colorectal cancer cell line (Caco-2). A primary screening at 100 microM allowed the selection of the 10 most active compounds, which showed an antiproliferative activity on all the cell lines. A dose-effect study between 12.5 and 100 microM sorted two compounds with a significant activity: 5t and 7e against leukemic cell lines CEM, Jurkat and Raji with IC50 values around 12 microM.