Triggered Crosslinking of Main-Chain Enediyne Polyurethanes via Bergman Cyclization

Crosslinking chemistries occupy an important position in polymer modification with a particular importance when triggered in response to external stimuli. Enediyne (EDY) moieties are used as functional entities in this work, known to undergo a pericyclic Bergman cyclization (BC) to induce a triggered crosslinking of polyurethanes (PU) via the intermediately formed diradicals. Diamino-EDYs, where the distance between the enyne-moieties is known to be critical to induce a BC, are placed repetitively as main-chain structural elements in isophorone-based PUs to induce reinforcement upon heating, compression, or stretching. A 7-day compression under room temperature results in a ≈69% activation of the BC, together with the observation of an increase in tensile strength by 62% after 25 stretching cycles. The occurrence of BC is further proven by the decreased exothermic values in differential scanning calorimetry, together with characteristic peaks of the formed benzene moieties via IR spectroscopy. Purely heat-induced crosslinking contributes to 191% of the maximum tensile strength in comparison to the virgin PU. The BC herein forms an excellent crosslinking strategy, triggered by heat or force in PU materials.

Jejucarbosides B-E, Chlorinated Cycloaromatized Enediynes, from a Marine Streptomyces sp

Four new chlorinated cycloaromatized enediyne compounds, jejucarbosides B-E (1-4), were discovered together with previously-identified jejucarboside A from a marine actinomycete strain. Compounds 1-4 were identified as new chlorinated cyclopenta[a]indene glycosides based on 1D and 2D nuclear magnetic resonance, high-resolution mass spectrometry, and circular dichroism (CD) spectra. Jejucarbosides B and E bear a carbonate functional group whereas jejucarbosides C and D are variants possessing 1,2-diol by losing the carbonate functionality. It is proposed that the production of 1-4 occurs via Bergman cycloaromatization capturing Cl^- and H⁺ in the alternative positions of a p-benzyne intermediate derived from a 9-membered enediyne core. Jejucarboside E (4) displayed significant cytotoxicity against human cancer cell lines including SNU-638, SK-HEP-1, A549, HCT116, and MDA-MB-231, with IC₅₀ values of 0.31, 0.40, 0.25, 0.29, and 0.48 μM, respectively, while jejucarbosides B-D (1-3) showed moderate or no cytotoxic effects.

A discrete intermediate for the biosynthesis of both the enediyne core and the anthraquinone moiety of enediyne natural products

The enediynes are structurally characterized by a 1,5-diyne-3-ene motif within a 9- or 10-membered enediyne core. The anthraquinone-fused enediynes (AFEs) are a subclass of 10-membered enediynes that contain an anthraquinone moiety fused to the enediyne core as exemplified by dynemicins and tiancimycins. A conserved iterative type I polyketide synthase (PKSE) is known to initiate the biosynthesis of all enediyne cores, and evidence has recently been reported to suggest that the anthraquinone moiety also originates from the PKSE product. However, the identity of the PKSE product that is converted to the enediyne core or anthraquinone moiety has not been established. Here, we report the utilization of recombinant E. coli coexpressing various combinations of genes that encode a PKSE and a thioesterase (TE) from either 9- or 10-membered enediyne biosynthetic gene clusters to chemically complement ΔPKSE mutant strains of the producers of dynemicins and tiancimycins. Additionally, 13C-labeling experiments were performed to track the fate of the PKSE/TE product in the ΔPKSE mutants. These studies reveal that 1,3,5,7,9,11,13-pentadecaheptaene is the nascent, discrete product of the PKSE/TE that is converted to the enediyne core. Furthermore, a second molecule of 1,3,5,7,9,11,13-pentadecaheptaene is demonstrated to serve as the precursor of the anthraquinone moiety. The results establish a unified biosynthetic paradigm for AFEs, solidify an unprecedented biosynthetic logic for aromatic polyketides, and have implications for the biosynthesis of not only AFEs but all enediynes.

Transition Metal-Free Regio- and Stereo-Selective trans Hydroboration of 1,3-Diynes: A Phosphine-Catalyzed Access to (E)-1-Boryl-1,3-Enynes

We report a transition metal-free, regio- and stereo-selective, phosphine-catalyzed method for the trans hydroboration of 1,3-diynes with pinacolborane that affords (E)-1-boryl-1,3-enynes. The reaction proceeds with excellent selectivity for boron addition to the external carbon of the 1,3-diyne framework as unambiguously established by NMR and X-ray crystallographic studies. The reaction displays a broad substrate scope including unsymmetrical diynes to generate products in high yield (up to 95 %). Experimental and theoretical studies suggest that phosphine attack on the alkyne is a key process in the catalytic cycle.

Liposome-Encapsulated Tiancimycin A Is Active against Melanoma and Metastatic Breast Tumors: The Effect of cRGD Modification of the Liposomal Carrier and Tiancimycin A Dose on Drug Activity and Toxicity

Enediyne natural products, including neocarzinostatin and calicheamicin γ¹, are used in the form of a copolymer or antibody-drug conjugate to treat hepatomas and leukemia. Tiancimycin (TNM) A is a novel anthraquinone-fused enediyne that can rapidly and completely kill tumor cells. Herein, we encapsulated TNM A in liposomes (Lip-TNM A) and cyclic arginine-glycine-aspartate (cRGD)-functionalized liposomes (cRGD-Lip-TNM A) and demonstrated its antitumor activity using mouse xenografts. Because TNM A causes rapid DNA damage, cell cycle arrest, and apoptosis, these nanoparticles exhibited potent cytotoxicity against multiple tumor cells for 8 h. In B16-F10 and KPL-4 xenografts, both nanoparticles showed superior potency over doxorubicin and trastuzumab. However, cRGD-Lip-TNM A reduced the tumor weight more significantly than Lip-TNM A in B16-F10 xenografts, in which the α_vβ₃-integrin receptors are significantly overexpressed in this melanoma. Lip-TNM A was slightly more active than cRGD-Lip-TNM A against KPL-4 xenografts, which probably reflected the difference of their in vivo fate in this mouse model. In a highly metastatic 4T1 tumor model, cRGD-Lip-TNM A reduced tumor metastasis induced by losartan, a tumor microenvironment-remodeling agent. These findings suggest that targeted delivery of enediynes with unique modes of action may enable more effective translation of anticancer nanomedicines.

Ring-closing metathesis of prochiral oxaenediynes to racemic 4-alkenyl-2-alkynyl-3,6-dihydro-2H-pyrans

The prochiral 4-(allyloxy)hepta-1,6-diynes, optionally modified in the positions 1 and 7 with an alkyl or ester group, undergo a chemoselective ring-closing enyne metathesis yielding racemic 4-alkenyl-2-alkynyl-3,6-dihydro-2H-pyrans. Among the catalysts tested, Grubbs 1st generation precatalyst in the presence of ethene (Mori conditions) gave superior results compared to the more stable Grubbs or Hoveyda-Grubbs 2nd generation precatalysts. This is probably caused by a suppression of the subsequent side-reactions of the enyne metathesis product with ethene. On the other hand, the 2nd generation precatalysts gave better yields in the absence of ethene. The metathesis products, containing both a triple bond and a conjugated system, can be successfully orthogonally modified. For example, the metathesis product of 5-(allyloxy)nona-2,7-diyne reacted chemo- and stereoselectively in a Diels-Alder reaction with N-phenylmaleimide affording the tricyclic products as a mixture of two separable diastereoisomers, the configuration of which was estimated by DFT computations. The reported enediyne metathesis paves the way to the enantioselective enyne metathesis yielding chiral building blocks for compounds with potential biological activity, e.g., norsalvinorin or cacospongionolide B.

C-N-Coupled Metabolites Yield Insights into Dynemicin A Biosynthesis

The biosynthesis of the three structural subclasses of enediyne antitumor antibiotics remains largely unknown beyond a common C16 -hexaene precursor. For the anthraquinone-fused subtype, however, an unexpected iodoanthracene γ-thiolactone was established to be a mid-pathway intermediate to dynemicin A. Having deleted a putative flavin-dependent oxidoreductase from the dynemicin biosynthetic gene cluster, we can now report four metabolites that incorporate the iodoanthracene and reveal the formation of the C-N bond linking the anthraquinone and enediyne halves emblematic of this structural subclass. The coupling of an aryl iodide and an amine is familiar from organometallic chemistry, but has little or no precedent in natural product biosynthesis. These metabolites suggest further that enediyne formation occurs early in the overall biosynthesis, and that even earlier events might convert the C16 -hexaene to a common C15 intermediate that partitions to enediyne and anthraquinone building blocks for the heterodimerization.

Resistance to Enediyne Antitumor Antibiotics by Sequestration

The enediynes, microbial natural products with extraordinary cytotoxicities, have been translated into clinical drugs. Two self-resistance mechanisms are known in the enediyne producers-apoproteins for the nine-membered enediynes and self-sacrifice proteins for the ten-membered enediyne calicheamicin. Here we show that: (1) tnmS1, tnmS2, and tnmS3 encode tiancimycin (TNM) resistance in its producer Streptomyces sp. CB03234, (2) tnmS1, tnmS2, and tnmS3 homologs are found in all anthraquinone-fused enediyne producers, (3) TnmS1, TnmS2, and TnmS3 share a similar β barrel-like structure, bind TNMs with nanomolar K_D values, and confer resistance by sequestration, and (4) TnmS1, TnmS2, and TnmS3 homologs are widespread in nature, including in the human microbiome. These findings unveil an unprecedented resistance mechanism for the enediynes. Mechanisms of self-resistance in producers serve as models to predict and combat future drug resistance in clinical settings. Enediyne-based chemotherapies should now consider the fact that the human microbiome harbors genes encoding enediyne resistance.

Metal-mediated diradical tuning for DNA replication arrest via template strand scission

A series of M(PyED)·X (X = 2Cl-, SO42-) pyridine-metalloenediyne complexes [M = Cu(II), Fe(II), or Zn(II)] and their independently synthesized, cyclized analogs have been prepared to investigate their potential as radical-generating DNA-damaging agents. All complexes possess a 1:1 metal-to-ligand stoichiometry as determined by electronic absorption spectroscopy and X-ray diffraction. Solution structural analysis reveals a pπ Cl [Formula: see text] Cu(II) LMCT (22,026 cm-1) for Cu(PyED)·2Cl, indicating three nitrogens and a chloride in the psuedo-equatorial plane with the remaining pyridine nitrogen and solvent in axial positions. EPR spectra of the Cu(II) complexes exhibit an axially elongated octahedron. This spectroscopic evidence, together with density functional theory computed geometries, suggest six-coordinate structures for Cu(II) and Fe(II) complexes and a five-coordinate environment for Zn(II) analogs. Bergman cyclization via thermal activation of these constructs yields benzannulated product indicative of diradical generation in all complexes within 3 h at 37 °C. A significant metal dependence on the rate of the reaction is observed [Cu(II) > Fe(II) > Zn(II)], which is mirrored in in vitro DNA-damaging outcomes. Whereas in situ chelation of PyED leads to considerable degradation in the presence of all metals within 1 h under hyperthermia conditions, Cu(II) activation produces >50% compromised DNA within 5 min. Additionally, Cu(II) chelated PyED outcompetes DNA polymerase I to successfully inhibit template strand extension. Exposure of HeLa cells to Cu(PyBD)·SO4 (IC50 = 10 μM) results in a G2/M arrest compared with untreated samples, indicating significant DNA damage. These results demonstrate metal-controlled radical generation for degradation of biopolymers under physiologically relevant temperatures on short timescales.

Alkoxy and Enediyne Derivatives Containing 1,4-Benzoquinone Subunits-Synthesis and Antitumor Activity

The compounds produced by a living organism are most commonly as medicinal agents and starting materials for the preparation of new semi-synthetic derivatives. One of the largest groups of natural compounds consists of products containing a 1,4-benzoquinone subunit. This fragment occurs in three enediyne antibiotics, dynemicin A, deoxydynemicin A, and uncilamicin, which exhibit high biological activity. A series of alkoxy derivatives containing 1,4-naphthoquinone, 5,8-quinolinedione, and 2-methyl-5,8-quinolinedione moieties was synthesized. Moreover, the 1,4-benzoquinone subunit was contacted with an enediyne fragment. All obtained compounds were characterized by spectroscopy and spectrometry methods. The resulting alkane, alkene, alkyne and enediyne derivatives were tested as antitumor agents. They showed high cytotoxic activity depending on the type of 1,4-benzoquinone subunit and the employed tumor cell lines. The synthesized derivatives fulfill the Lipinski Rule of Five and have low permeability through the blood–brain barrier.

Structural Characterization of CalS8, a TDP-α-D-Glucose Dehydrogenase Involved in Calicheamicin Aminodideoxypentose Biosynthesis

Classical UDP-glucose 6-dehydrogenases (UGDHs; EC 1.1.1.22) catalyze the conversion of UDP-α-d-glucose (UDP-Glc) to the key metabolic precursor UDP-α-d-glucuronic acid (UDP-GlcA) and display specificity for UDP-Glc. The fundamental biochemical and structural study of the UGDH homolog CalS8 encoded by the calicheamicin biosynthetic gene is reported and represents one of the first studies of a UGDH homolog involved in secondary metabolism. The corresponding biochemical characterization of CalS8 reveals CalS8 as one of the first characterized base-permissive UGDH homologs with a >15-fold preference for TDP-Glc over UDP-Glc. The corresponding structure elucidations of apo-CalS8 and the CalS8·substrate·cofactor ternary complex (at 2.47 and 1.95 Å resolution, respectively) highlight a notably high degree of conservation between CalS8 and classical UGDHs where structural divergence within the intersubunit loop structure likely contributes to the CalS8 base permissivity. As such, this study begins to provide a putative blueprint for base specificity among sugar nucleotide-dependent dehydrogenases and, in conjunction with prior studies on the base specificity of the calicheamicin aminopentosyltransferase CalG4, provides growing support for the calicheamicin aminopentose pathway as a TDP-sugar-dependent process.

Iterative type I polyketide synthases involved in enediyne natural product biosynthesis

Enediyne natural products are potent antibiotics structurally characterized by an enediyne core containing two acetylenic groups conjugated to a double bond in a 9- or 10-membered carbocycle. The biosynthetic gene clusters for enediynes encode a novel iterative type I polyketide synthase (PKSE), which is generally believed to initiate the biosynthetic process of enediyne cores. This review article will cover research efforts made since its discovery to elucidate the role of the PKSE in enediyne core biosynthesis. Topics covered include the unique domain architecture, identification, and characterization of turnover products, and interaction with partner thioesterase protein.

Organosilver(I) framework assembly with trifluoroacetate and enediyne-functionalized alicycles

Eight new silver(I) trifluoroacetate complexes based on a series of designed ligands, each featuring an alicyclic ring with enediyne functionality, have been synthesized and characterized by single-crystal X-ray diffraction. Each ethynide terminal is inserted into an Agn (n = 4-5) basket, leading to the generation of coordination chain or layer structures, but the well shielded ethenyl group does not take part in silver-olefin bonding. Variation in ring size of the alicycles is shown to influence the construction of the organosilver(I) coordination networks, which are consolidated by weak intermolecular interactions in the crystal structures. The effect of adding ancillary N-donor ligands to the reaction system on the coordination and supramolecular network assembly is also investigated.

Highly stereocontrolled synthesis of trans-enediynes via carbocupration of fluoroalkylated diynes

Treatment of readily prepared (Z)-6-benzyloxy-1,1,1,2-tetrafluoro-6-methyl-2-hepten-4-yne with 1.5 equiv of LHMDS in −78 °C for 1 h gave the corresponding trifluoromethylated diyne in an excellent yield. This diyne was found to be a good substrate for the carbocupration with various higher-ordered cyanocuprates to give the corresponding vinylcuprates in a highly regio- and stereoselective manner. The in situ generated vinylcuprates could react very smoothly with an excess amount of iodine, the vinyl iodides being obtained in high yields. Thus-obtained iodides underwent a very smooth Sonogashira cross-coupling reaction to afford various trans-enediynes in high yields.

Neocarzinostatin as a probe for DNA protection activity--molecular interaction with caffeine

Neocarzinostatin (NCS), a potent mutagen and carcinogen, consists of an enediyne prodrug and a protein carrier. It has a unique double role in that it intercalates into DNA and imposes radical-mediated damage after thiol activation. Here we employed NCS as a probe to examine the DNA-protection capability of caffeine, one of common dietary phytochemicals with potential cancer-chemopreventive activity. NCS at the nanomolar concentration range could induce significant single- and double-strand lesions in DNA, but up to 75 ± 5% of such lesions were found to be efficiently inhibited by caffeine. The percentage of inhibition was caffeine-concentration dependent, but was not sensitive to the DNA-lesion types. The well-characterized activation reactions of NCS allowed us to explore the effect of caffeine on the enediyne-generated radicals. Postactivation analyses by chromatographic and mass spectroscopic methods identified a caffeine-quenched enediyne-radical adduct, but the yield was too small to fully account for the large inhibition effect on DNA lesions. The affinity between NCS chromophore and DNA was characterized by a fluorescence-based kinetic method. The drug-DNA intercalation was hampered by caffeine, and the caffeine-induced increases in DNA-drug dissociation constant was caffeine-concentration dependent, suggesting importance of binding affinity in the protection mechanism. Caffeine has been shown to be both an effective free radical scavenger and an intercalation inhibitor. Our results demonstrated that caffeine ingeniously protected DNA against the enediyne-induced damages mainly by inhibiting DNA intercalation beforehand. The direct scavenging of the DNA-bound NCS free radicals by caffeine played only a minor role.

Molecular predictors of human nervous system cancer responsiveness to enediyne chemotherapy

PURPOSE

To identify and mathematically model molecular predictors of response to the enediyne chemotherapeutic agent, neocarzinostatin, in nervous system cancer cell lines.

METHODS

Human neuroblastoma, breast cancer, glioma, and medulloblastoma cell lines were maintained in culture. Content of caspase-3 and Bcl-2, respectively, was determined relative to actin content for each cell line by Western blotting and optical densitometry. For each cell line, sensitivity to neocarzinostatin was determined. Brain tumor cell lines were stably transfected with human Bcl-2 cDNA cloned into the pcDNA3 plasmid vector.

RESULTS

In human tumor cell lines of different tissue origins, sensitivity to neocarzinostatin is proportional to the product of the relative contents of Bcl-2 and caspase-3 (r (2) = 0.9; P < 0.01). Neuroblastoma and brain tumor cell lines are particularly sensitive to neocarzinostatin; the sensitivity of brain tumor lines to neocarzinostatin is enhanced by transfection with an expression construct for Bcl-2 and is proportional in transfected cells to the product of the relative contents of Bcl-2 and caspase-3 (r (2) = 0.7).

CONCLUSION

These studies underscore the potential of molecular profiling in identifying effective chemotherapeutic paradigms for cancer in general and tumors of the nervous system in particular.

Biochemical and structural insights of the early glycosylation steps in calicheamicin biosynthesis

The enediyne antibiotic calicheamicin (CLM) gamma(1)(I) is a prominent antitumor agent that is targeted to DNA by a novel aryltetrasaccharide comprised of an aromatic unit and four unusual carbohydrates. Herein we report the heterologous expression and the biochemical characterization of the two "internal" glycosyltransferases CalG3 and CalG2 and the structural elucidation of an enediyne glycosyltransferase (CalG3). In conjunction with the previous characterization of the "external" CLM GTs CalG1 and CalG4, this study completes the functional assignment of all four CLM GTs, extends the utility of enediyne GT-catalyzed reaction reversibility, and presents conclusive evidence of a sequential glycosylation pathway in CLM biosynthesis. This work also reveals the common GT-B structural fold can now be extended to include enediyne GTs.

Design and characterization of mechanism-based inhibitors for the tyrosine aminomutase SgTAM

The synthesis and evaluation of two classes of inhibitors for SgTAM, a 4-methylideneimidazole-5-one (MIO) containing tyrosine aminomutase, are described. A mechanism-based strategy was used to design analogs that mimic the substrate or product of the reaction and form covalent interactions with the enzyme through the MIO prosthetic group. The analogs were characterized by measuring inhibition constants and X-ray crystallographic structural analysis of the co-complexes bound to the aminomutase, SgTAM.

The radiomimetic enediyne C-1027 induces unusual DNA damage responses to double-strand breaks

Cells lacking the protein kinase ataxia telangiectasia mutated (ATM) have defective responses to DNA double-strand breaks (DSBs), including an inability to activate damage response proteins such as p53. However, we previously showed that cells lacking ATM robustly activate p53 in response to DNA strand breaks induced by the radiomimetic enediyne C-1027. To gain insight into the nature of C-1027-induced ATM-independent damage responses to DNA DSBs, we further examined the molecular mechanisms underlying the cellular response to this unique radiomimetic agent. Like ionizing radiation (IR) and other radiomimetics, breaks induced by C-1027 efficiently activate ATM by phosphorylation at Ser1981, yet unlike other radiomimetics and IR, DNA breaks induced by C-1027 result in normal phosphorylation of p53 and the cell cycle checkpoint kinases (Chk1 and Chk2) in the absence of ATM. In the presence of ATM, but under ATM and Rad3-related kinase (ATR) deficient conditions, C-1027 treatment resulted in a decrease in the level of Chk1 phosphorylation but not in the level of p53 and Chk2 phosphorylation. Only when cells were deficient in both ATM and ATR was there a reduction in the level of phosphorylation of each of these DNA damage response proteins. This reduction was also accompanied by an increased level of cell death in comparison to that of wild-type cells or cells lacking either ATM or ATR. Our findings demonstrate a unique cellular response to C-1027-induced DNA DSBs in that DNA damage response proteins are unaffected by the absence of ATM, as long as ATR is present.