Anti-HER2 Immunoliposomes: Antitumor Efficacy Attributable to Targeted Delivery of Anthraquinone-Fused Enediyne

Although natural products are essential sources of small-molecule antitumor drugs, some can exert substantial toxicities, limiting their clinical utility. Anthraquinone-fused enediyne natural products are remarkably potent antitumor drug candidates, and uncialamycin and tiancimycin (TNM) A are under development as antibody-drug conjugates. Herein, a novel drug delivery system is introduced for TNM A using anti-human epidermal growth factor receptor 2 (HER2) immunoliposomes (ILs). Trastuzumab-coated TNM A-loaded ILs (HER2-TNM A-ILs) is engineered with an average particle size of 182.8 ± 2.1 nm and a zeta potential of 1.75 ± 0.12 mV. Compared with liposomes lacking trastuzumab, HER2-TNM A-ILs exhibited selective toxicity against HER2-positive KPL-4 and SKBR3 cells. Coumarin-6, a fluorescent TNM A surrogate, is encapsulated within anti-HER2 ILs; the resultant ILs have enhanced cellular uptake in KPL-4 and SKBR3 cells when compared with control liposomes. Furthermore, ILs loaded with more Cy5.5 accumulated in KPL-4 mouse tumors. A single HER2-TNM A-IL dose (0.02 mg kg-1) suppressed the growth of HER2-positive KPL-4 mouse tumors without apparent toxicity. This study not only provides a straightforward method for the effective delivery of TNM A against HER2-positive breast tumors but also underscores the potential of IL-based drug delivery systems when employing highly potent cytotoxins as payloads.

Cofactorless oxygenases guide anthraquinone-fused enediyne biosynthesis

The anthraquinone-fused enediynes (AFEs) combine an anthraquinone moiety and a ten-membered enediyne core capable of generating a cytotoxic diradical species. AFE cyclization is triggered by opening the F-ring epoxide, which is also the site of the most structural diversity. Previous studies of tiancimycin A, a heavily modified AFE, have revealed a cryptic aldehyde blocking installation of the epoxide, and no unassigned oxidases could be predicted within the tnm biosynthetic gene cluster. Here we identify two consecutively acting cofactorless oxygenases derived from methyltransferase and α/β-hydrolase protein folds, TnmJ and TnmK2, respectively, that are responsible for F-ring tailoring in tiancimycin biosynthesis by comparative genomics. Further biochemical and structural characterizations reveal that the electron-rich AFE anthraquinone moiety assists in catalyzing deformylation, epoxidation and oxidative ring cleavage without exogenous cofactors. These enzymes therefore fill important knowledge gaps for the biosynthesis of this class of molecules and the underappreciated family of cofactorless oxygenases.

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.

Targeted Discovery of Cryptic Enediyne Natural Products via FRET-Coupled High-Throughput Elicitor Screening

Enediyne antibiotics are a striking family of DNA-cleaving natural products with high degrees of cytotoxicity and structural complexity. Microbial genome sequences, which have recently accumulated, point to an untapped trove of "cryptic" enediynes. Most of the cognate biosynthetic gene clusters (BGCs) are sparingly expressed under standard growth conditions, making it difficult to characterize their products. Herein, we report a fluorescence-based DNA cleavage assay coupled with high-throughput elicitor screening for the rapid, targeted discovery of cryptic enediyne metabolites. We applied the approach to Streptomyces clavuligerus, which harbors two such BGCs with unknown products, identified steroids as effective elicitors, and characterized 10 cryptic enediyne-derived natural products, termed clavulynes A-J with unusual carbonate and terminal olefin functionalities, with one of these congeners matching the recently reported jejucarboside. Our results contribute to the growing repertoire of enediynes and provide a blueprint for identifying additional ones in the future.

Normal and Aberrant Methyltransferase Activities Give Insights into the Final Steps of Dynemicin A Biosynthesis

The naturally occurring enediynes are notable for their complex structures, potent DNA cleaving ability, and emerging usefulness in cancer chemotherapy. They can be classified into three distinct structural families, but all are thought to originate from a common linear C15-heptaene. Dynemicin A (DYN) is the paradigm member of anthraquinone-fused enediynes, one of the three main classes and exceptional among them for derivation of both its enediyne and anthraquinone portions from this same early biosynthetic building block. Evidence is growing about how two structurally dissimilar, but biosynthetically related, intermediates combine in two heterodimerization reactions to create a nitrogen-containing C30-coupled product. We report here deletions of two genes that encode biosynthetic proteins that are annotated as S-adenosylmethionine (SAM)-dependent methyltransferases. While one, DynO6, is indeed the required O-methyltransferase implicated long ago in the first studies of DYN biosynthesis, the other, DynA5, functions in an unanticipated manner in the post-heterodimerization events that complete the biosynthesis of DYN. Despite its removal from the genome of Micromonospora chersina, the ΔdynA5 strain retains the ability to synthesize DYN, albeit in reduced titers, accompanied by two unusual co-metabolites. We link the appearance of these unexpected structures to a substantial and contradictory body of other recent experimental data to advance a biogenetic rationale for the downstream steps that lead to the final formation of DYN. A sequence of product-forming transformations that is in line with new and existing experimental results is proposed and supported by a model reaction that also encompasses the formation of the crucial epoxide essential for the activation of DYN for DNA cleavage.

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.

Natural Products: Review for Their Effects of Anti-HBV

Hepatitis B is a global infectious disease, seriously endangering human health. Currently, there are mainly interferons and nucleoside analogues treatment of hepatitis B in the clinic, which have certain therapeutic effects on hepatitis B, but their side effects and drug resistance are increasingly prominent. Therefore, it is urgently needed to discover and develop new anti-HBV drugs, especially natural products, which have novel, high efficiency, and low toxicity anti-HBV compounds with novel antiviral mechanisms. In this manuscript, the natural products (polysaccharides and 165 compounds) with the activity of antihepatitis B virus are discussed according to their chemical classes, including 14 phenylpropanoids, 8 flavonoids,12 xanthones, 13 anthroquinones, 47 terpenoids, 6 alkaloids, 15 enediynes, 11 aromatics, 18 phenylalanine dipeptides compounds, and 13 others. In addition, the anti-HBV mechanism and targets of natural product were also discussed. The aim of this review is to report new discoveries about anti-HBV natural products and to provide reference for researchers.

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.

Light-Cross-linked Enediyne Small-Molecule Micelle-Based Drug-Delivery System

Light-cross-linked small-molecule micelles with enediyne units are designed for developing efficient drug-delivery systems. Gemcitabine (GEM) is chosen as a model hydrophilic drug and tethered with a maleimide-based enediyne (EDY) as a hydrophobic tail in the preparation of amphiphilic EDY-GEM. The stable micellar particles are obtained by cross-linking the enediyne moieties via photoinduced Bergman cyclization polymerization in aqueous media. The light-cross-linked spherical micelles with a size of 80 nm are characterized with dynamic light scattering and electron microscopy, showing robust micellar stability, bright fluorescent emission due to their intrinsic conjugated structure, and potential passive tumor-targeting ability through the enhanced permeability and retention effect. The drug-loaded micelles, as an example of light-cross-linked small-molecule micelle-based drug-delivery system, exhibit high drug-loading contents (50%) and greatly improved cytotoxicity toward A549 cells (decreasing the IC₅₀ value of Gemcitabine by 10 times), thanks to the greatly increased cellular uptake of the drug-loaded micelles as confirmed by confocal laser scanning microscopy. The light-cross-linked enediyne-based small-molecule micelles system therefore provides a simple yet efficient drug-delivery platform for cancer chemotherapy.

A Macropinocytosis-Intensifying Albumin Domain-Based scFv Antibody and Its Conjugate Directed against K-Ras Mutant Pancreatic Cancer

Enhanced macropinocytosis has been found in K-Ras mutant pancreatic cancer cells, through which albumin can massively enter into the K-Ras-driven cancer cells, suggesting its role in serving as a macropinocytosis-intensifying drug delivery carrier. In the present study, a novel recombinant protein Fv-LDP-D3 and its reconstituted analogue Fv-LDP-D3-AE were designed and prepared. Fv is the fragment of an anti-EGFR antibody, D3 is the domain III of human serum albumin (HSA), LDP is the apoprotein of the antitumor antibiotic lidamycin (LDM), and AE is an extremely cytotoxic enediyne chromophore derived from LDM. As shown, the recombinant protein Fv-LDP-D3 presented intensive and selective binding capacity to pancreatic cancer cells and inhibited cell proliferation by blocking EGFR signaling. Moreover, Fv-LDP-D3 showed prominent tumor imaging in pancreatic carcinoma xenograft. The reconstituted, enediyne-integrated analogue Fv-LDP-D3-AE displayed highly potent cytotoxicity to pancreatic cancer cells through apoptosis induction and G2/M arrest. Fv-LDP-D3 and Fv-LDP-D3-AE markedly inhibited the tumor growth of the pancreatic carcinoma AsPC-1 xenograft. Study results indicated that the novel recombinant protein displays both EGFR-targeting and macropinocytosis-intensifying attributes, presenting a new format of scFv antibody that integrates with albumin domain III. It might be a feasible strategy to develop targeted drugs for K-Ras mutant pancreatic cancer.

Characterization of an Anthracene Intermediate in Dynemicin Biosynthesis

Despite the identification of a β-hydroxyhexaene produced by the enediyne polyketide synthases (PKSs), the post-PKS biosynthetic steps to the individual members of this antitumor and antibiotic family remain largely unknown. The massive biosynthetic gene clusters (BGCs) that direct the formation of each product caution that many steps could be required. It was recently demonstrated that the enediyne PKS in the dynemicin A BGC from Micromonospora chersina gives rise to both the anthraquinone and enediyne halves of the molecule. We now present the first evidence for a mid-pathway intermediate in dynemicin A biosynthesis, an iodoanthracene bearing a fused thiolactone, which was shown to be incorporated selectively into the final product. This unusual precursor reflects just how little is understood about these biosynthetic pathways, yet constrains the mechanisms that can act to achieve the key heterodimerization to the anthraquinone-containing subclass of enediynes.

Liver Microvascular Injury and Thrombocytopenia of Antibody-Calicheamicin Conjugates in Cynomolgus Monkeys-Mechanism and Monitoring

Purpose: Adverse reactions reported in patients treated with antibody-calicheamicin conjugates such as gemtuzumab ozogamicin (Mylotarg) and inotuzumab ozogamicin include thrombocytopenia and sinusoidal obstruction syndrome (SOS). The objective of this experimental work was to investigate the mechanism for thrombocytopenia, characterize the liver injury, and identify potential safety biomarkers.Experimental Design: Cynomolgus monkeys were dosed intravenously at 6 mg/m²/dose once every 3 weeks with a nonbinding antibody-calicheamicin conjugate (PF-0259) containing the same linker-payload as gemtuzumab ozogamicin and inotuzumab ozogamicin. Monkeys were necropsied 48 hours after the first administration (day 3) or 3 weeks after the third administration (day 63).Results: PF-0259 induced acute thrombocytopenia (up to 86% platelet reduction) with nadirs on days 3 to 4. There was no indication of effects on megakaryocytes in bone marrow or activation of platelets in peripheral blood. Microscopic evaluation of liver from animals necropsied on day 3 demonstrated midzonal degeneration and loss of sinusoidal endothelial cells (SECs) associated with marked platelet accumulation in sinusoids. Liver histopathology on day 63 showed variable endothelial recovery and progression to a combination of sinusoidal capillarization and sinusoidal dilation/hepatocellular atrophy, consistent with early SOS. Among biomarkers evaluated, there were early and sustained increases in serum hyaluronic acid (HA) that correlated well with serum aspartate aminotransferase and liver microscopic changes, suggesting that HA may be a sensitive diagnostic marker of the liver microvascular injury.Conclusions: These data support the conclusion that target-independent damage to liver SECs may be responsible for acute thrombocytopenia (through platelet sequestration in liver sinusoids) and development of SOS. Clin Cancer Res; 23(7); 1760-70. ©2016 AACR.

Genome neighborhood network reveals insights into enediyne biosynthesis and facilitates prediction and prioritization for discovery

The enediynes are one of the most fascinating families of bacterial natural products given their unprecedented molecular architecture and extraordinary cytotoxicity. Enediynes are rare with only 11 structurally characterized members and four additional members isolated in their cycloaromatized form. Recent advances in DNA sequencing have resulted in an explosion of microbial genomes. A virtual survey of the GenBank and JGI genome databases revealed 87 enediyne biosynthetic gene clusters from 78 bacteria strains, implying that enediynes are more common than previously thought. Here we report the construction and analysis of an enediyne genome neighborhood network (GNN) as a high-throughput approach to analyze secondary metabolite gene clusters. Analysis of the enediyne GNN facilitated rapid gene cluster annotation, revealed genetic trends in enediyne biosynthetic gene clusters resulting in a simple prediction scheme to determine 9- versus 10-membered enediyne gene clusters, and supported a genomic-based strain prioritization method for enediyne discovery.

Enediynes: Exploration of microbial genomics to discover new anticancer drug leads

The enediyne natural products have been explored for their phenomenal cytotoxicity. The development of enediynes into anticancer drugs has been successfully achieved through the utilization of polymer- and antibody-drug conjugates (ADCs) as drug delivery systems. An increasing inventory of enediynes would benefit current application of ADCs in many oncology programs. Innovations in expanding the enediyne inventory should take advantage of the current knowledge of enediyne biosynthesis and post-genomics technologies. Bioinformatics analysis of microbial genomes reveals that enediynes are underexplored, in particular from Actinomycetales. This digest highlights the emerging opportunities to explore microbial genomics for the discovery of novel enediyne natural products.

Tuftsin-based, EGFR-targeting fusion protein and its enediyne-energized analog show high antitumor efficacy associated with CD47 down-regulation

Tuftsin (TF) is an immunomodulator tetrapeptide (Thr-Lys-Pro-Arg) that binds to the receptor neuropilin-1 (Nrp1) on the surface of cells. Many reports have described anti-tumor activity of tuftsin to relate with nonspecific activation of the host immune system. Lidamycin (LDM) that displays extremely potent cytotoxicity to cancer cells is composed of an apoprotein (LDP) and an enediyne chromophore (AE). In addition, Ec is an EGFR-targeting oligopeptide. In the present study, LDP was used as protein scaffold and the specific carrier for the highly potent AE. Genetically engineered fusion proteins LDP-TF and Ec-LDP-TF were prepared; then, the enediyne-energized fusion protein Ec-LDM-TF was generated by integration of AE into Ec-LDP-TF. The tuftsin-based fusion proteins LDP-TF and Ec-LDP-TF significantly enhanced the phagocytotic activity of macrophages as compared with LDP (P < 0.05). Ec-LDP-TF effectively bound to tumor cells and macrophages; furthermore, it markedly suppressed the growth of human epidermoid carcinoma A431 xenograft in athymic mice by 84.2 % (P < 0.05) with up-regulated expression of TNF-α and IFN-γ. Ec-LDM-TF further augmented the therapeutic efficacy, inhibiting the growth of A431 xenograft by 90.9 % (P < 0.05); notably, the Ec-LDM-TF caused marked down-regulation of CD47 in A431 cells. Moreover, the best therapeutic effect was recorded in the group of animals treated with the combination of Ec-LDP-TF with Ec-LDM-TF. The results suggest that tuftsin-based, enediyne-energized, and EGFR-targeting fusion proteins exert highly antitumor efficacy with CD47 modulation. Tuftsin-based fusion proteins are potentially useful for treatment of EGFR- and CD47-overexpressing cancers.

Thermal reactivity of neutral and oxidized ferrocenyl-substituted enediynes

The coupling of two equivalents of ethynylferrocene (2) with one equivalent of 1,2-diiodocyclohexene (1) and 1,2-diiodobenzene (4) using Sonogashira cross-coupling conditions led to 1,2-bis(ferrocenylethynyl)cyclohexene (3) and 1,2-bis(ferrocenylethy-nyl)benzene (5), respectively. At high temperatures enediynes 3 and 5 showed exothermic signals in differential scanning calorimetry (DSC) measurements, suggestive of intramolecular diradicaloid ring formation (Bergman (C1-C6) or Schreiner-Pascal (C1-C5) cyclizations). The oxidation of 3 and 5 to the mono-oxidized enediynes 3+ and 5+ decreased the onset temperatures drastically. Equally, 1-ferrocenylethynyl-2-(p-nitro-phenyl)ethynylbenzene (8) displayed a significant decrease in the onset temperature after oxidation to 8+. Because the insoluble nature of the polymeric material formed in the thermolysis of the oxidized enediynes prevented characterization, the origin of this drastic effect was studied by DFT. Contrary to expectations, one-electron oxidation does not lower the barrier for intramolecular cyclization. Rather, the computations suggest that the polymerization is initiated by a bimolecular process.

An EGFR/HER2-Bispecific and enediyne-energized fusion protein shows high efficacy against esophageal cancer

Esophageal cancer is one of the most common cancers, and the 5-year survival rate is less than 10% due to lack of effective therapeutic agents. This study was to evaluate antitumor activity of Ec-LDP-Hr-AE, a recently developed bispecific enediyne-energized fusion protein targeting both epidermal growth factor receptor (EGFR) and epidermal growth factor receptor 2 (HER2), on esophageal cancer. The fusion protein Ec-LDP-Hr-AE consists of two oligopeptide ligands and an enediyne antibiotic lidamycin (LDM) for receptor binding and cell killing, respectively. The current study demonstrated that Ec-LDP-Hr had high affinity to bind to esophageal squamous cell carcinoma (ESCC) cells, and enediyne-energized fusion protein Ec-LDP-Hr-AE showed potent cytotoxicity to ESCC cells with differential expression of EGFR and HER2. Ec-LDP-Hr-AE could cause significant G2-M arrest in EC9706 and KYSE150 cells, and it also induced apoptosis in ESCC cells in a dosage-dependent manner. Western blot assays showed that Ec-LDP-Hr-AE promoted caspase-3 and caspase-7 activities as well as PARP cleavage. Moreover, Ec-LDP-Hr-AE inhibited cell proliferation via decreasing phosphorylation of EGFR and HER2, and further exerted inhibition of the activation of their downstream signaling molecules. In vivo, at a tolerated dose, Ec-LDP-Hr-AE inhibited tumor growth by 88% when it was administered to nude mice bearing human ESCC cell KYSE150 xenografts. These results indicated that Ec-LDP-Hr-AE exhibited potent anti-caner efficacy on ESCC, suggesting it could be a promising candidate for targeted therapy of esophageal cancer.

[Optimization of the preparation process for fusion protein Fv-LDP that composes lidamycin apoprotein and single-chain Fv antibody directed against type IV collagenase]

This study is to optimize the preparation process of fusion protein Fv-LDP which was expressed in the form of inclusion body and consisted of lidamycin apoprotein LDP and single-chain Fv antibody (scFv) directed against type IV collagenase. The preparation and the dissolution of inclusion body, the immobilized metal affinity chromatography of the target protein and the renaturization by stepwise dialysis were optimized by single-factor analysis or orthogonal design. In addition, the refolded fusion protein Fv-LDP was refined by Sephadex G-75 chromatography followed by fluorescence-activated cell sorter (FACS)-based saturation binding assay to measure its antigen-binding activity. After optimization of the process, the purity of fusion protein Fv-LDP existed in the inclusion body was 63.9% and the corresponding solubility was 95.7%; Under denaturing conditions, the purity of fusion protein Fv-LDP was more than 95% after the purification process. The percentage of monomeric fusion protein Fv-LDP was 60% after the refolding process, while it was further refined to 85% which was 5.6-fold higher than that of the initial refolding condition. The refined fusion protein Fv-LDP could bind to human lung adenocarcinoma PAa cells and human hepatoma BEL-7402 cells with the dissociation constants (Kd) of 0.176 micromol x L(-1) and 0.904 micromol x L(-1), respectively. The preparation process of fusion protein Fv-LDP has been successfully optimized, which provides the experimental basis for the production and future development of fusion protein Fv-LDP, and might serve as a relatively practical system for the preparation of other scFv-based proteins expressed in the form of inclusion body.

Generation and characterization of a distonic biradical anion formed from an enediynone prodrug in the gas phase

A negatively charged biradical intermediate was successfully generated in the gas phase via cyclization of the deprotonated bicyclo[8.3.0]trideca-12-ene-2,7-diyn-1-one precursor. The inherent negative charge of this biradical allows its characterization via collision-activated dissociation and reactions with a variety of neutral substrates in an FT-ICR mass spectrometer. Although the biradical is unreactive toward reagents that usually react rapidly with positively charged biradicals, such as dimethyl disulfide, it reacts with the halogen-containing substrates carbon tetrachloride, carbon tetrabromide, and bromotrichloromethane via bromine or chlorine atom abstraction, which supports its biradical structure. The results presented in this study indicate that cyclizations commonly used in solution to form biradical intermediates from enediyne compounds may also occur in the gas phase.

Falcarindiol allosterically modulates GABAergic currents in cultured rat hippocampal neurons

Falcarindiol (1), a C-17 polyacetylenic diol, shows a pleiotropic profile of bioactivity, but the mechanism(s) underlying its actions are largely unknown. Large amounts of 1 co-occur in water hemlock (Oenanthe crocata) along with the convulsant polyacetylenic toxin oenanthotoxin (2), a potent GABA(A) receptor (GABA(A)R) inhibitor. Since these compounds are structurally and biogenetically related, it was considered of interest to evaluate whether 1 could affect GABAergic activity, and for this purpose a model of hippocampal cultured neurons was used. Compound 1 significantly increased the amplitude of miniature inhibitory postsynaptic currents, accelerated their onset, and prolonged the decay kinetics. This compound enhanced also the amplitude of currents elicited by 3 μM GABA and accelerated their fading, reducing, however, currents evoked by a saturating (10 mM) GABA concentration. Moreover, kinetic analysis of responses to 10 mM GABA revealed that 1 upregulated the rate and extent of desensitization and slowed the current onset and deactivation. Taken together, these data show that 1 exerts a potent modulatory action on GABA(A)Rs, possibly by modulating agonist binding and desensitization, overall potentially decreasing the toxicity of co-occurring GABA-inhibiting convulsant toxins.

Optimization of the assembly efficiency for lidamycin chromophore bound to its apoprotein: a case study using orthogonal array

OBJECTIVE

Lidamycin (LDM) can be dissociated to an apoprotein (LDP) and an active enediyne chromophore (AE). The detached AE can reassemble with its LDP-containing fusion protein to endow the latter with potent antitumor activity. However, the reassembly of AE with LDP is affected by several factors. Our aim was to optimize the assembly efficiency of the AE with a LDP-containing fusion protein and investigate the influence of several factors on the assembly efficacy.

METHODS

A method based on RP-HPLC was developed to analyze the assembly rate, and an orthogonal experimental design L(9) (3(4)) was used to investigate the effects of temperature, assembly time, pH and molecular ratio of LDP-containing fusion protein to AE on the assembly rate. Furthermore, the determined optimum conditions for the assembly rate of the LDP-containing fusion protein with AE were applied and evaluated.

RESULTS

A calibration curve based on the LDM micromolar concentration against the peak-area of AE by HPLC was obtained. The order in which individual factors in the orthogonal experiment affected the assembly rate were temperature>time>pH>molar ratio of AE to protein and all were statistically significant (P<0.01). The optimal assembly conditions were temperature at 10°C, time of 12 h, pH 7.0, and the molar ratio of AE: protein of 5:1. The assembly rate of AE with a LDP-containing fusion protein was improved by 23% after condition optimization.

CONCLUSION

The assembly rate of chromophore of lidamycin with its LDP-containing fusion protein was improved after condition optimization by orthogonal design, and the optimal conditions described herein should prove useful for the development of this type of LDP-containing fusion protein.

Improvement of the enediyne antitumor antibiotic C-1027 production by manipulating its biosynthetic pathway regulation in Streptomyces globisporus

The production of C-1027 in Streptomyces globisporus was previously increased 2- to 3-fold by manipulating three pathway-specific activators, SgcR1, SgcR2, and SgcR3. In this study, we have further characterized two putative C-1027 regulatory genes, sgcE1 and sgcR, by in vivo inactivation. The HxlR family DNA-binding protein SgcE1 was not essential for C-1027 biosynthesis, since inactivation of sgcE1 showed no effect on C-1027 production. In contrast, the proposed repressive role of the sgcR gene was confirmed by a 3-fold increase in C-1027 production in the ΔsgcR mutant S. globisporus SB1022 strain relative to the wild-type strain. Considering SgcR shows no significant similarity to any protein of known function, it may be representative of a new family of regulatory proteins. Finally, overexpression of the previously characterized activator sgcR1 in S. globisporus SB1022 increased the C-1027 yield to 37.5 ± 7.7 mg/L, which is about 7-fold higher than the wild-type strain.

Probing the active site of MIO-dependent aminomutases, key catalysts in the biosynthesis of beta-amino acids incorporated in secondary metabolites

The tyrosine aminomutase SgTAM produces (S)-ss-tyrosine from L-tyrosine in the biosynthesis of the enediyne antitumor antibiotic C-1027. This conversion is promoted by the methylideneimidazole-5-one (MIO) prosthetic group. MIO was first identified in the homologous family of ammonia lyases, which deaminate aromatic amino acids to form alpha,ss-unsaturated carboxylates. Studies of substrate specificity have been described for lyases but there have been limited reports in altering the substrate specificity of aminomutases. Furthermore, it remains unclear as to what structural properties are responsible for catalyzing the presumed readdition of the amino group into the alpha,ss-unsaturated intermediates to form ss-amino acids. Attempts to elucidate specificity and mechanistic determinants of SgTAM have also proved to be difficult as it is recalcitrant to perturbations to the active site via mutagenesis. An X-ray cocrystal structure of the SgTAM mutant of the catalytic base with L-tyrosine verified important substrate binding residues as well as the enzymatic base. Further mutagenesis revealed that removal of these crucial interactions renders the enzyme inactive. Proposed structural determinants for mutase activity probed via mutagenesis, time-point assays and X-ray crystallography revealed a complicated role for these residues in maintaining key quaternary structure properties that aid in catalysis.

SAR studies of gymnasterkoreayne derivatives with cancer chemopreventive activities

We synthesized diyne triols based on gymnasterkoreayne and evaluated their cancer chemopreventive activities in terms of the chemopreventive index (CI) to reveal the structure-activity relationship, and discovered more active compounds than natural diynes.

Determination of the geometry of acetoxyendiynes and acetoxyenynes by NMR heteronuclear (13)C-(1)H scalar couplings and (13)C NMR chemical shifts. Structural assignment of the oxylipin natural products peyssonenynes A and B

Solution NMR methods were used for the structural characterization of the acetoxyendiyne E/Z configuration of the marine natural products peyssonenynes A and B and their synthetic analogs derived from palmitic acid. The scarcity of protons in the proximity of the olefin precluded the determination of the double bond geometry using (1)H NMR methods that rely on proton-proton scalar couplings or experiments such as NOESY or ROESY. Long range (1)H-(13)C heteronuclear scalar couplings, (n)J(CH), measured with the 2D excitation sculptured indirect detection experiment (EXSIDE) proved useful and highly reliable for the analysis of the enol acetate geometry. In addition, it was found that the chemical shift of some carbon atoms in the proximity of the olefin was also sensitive to the double bond configuration of these molecules providing an even simpler way to determine their geometry. This protocol showed its robustness by similar analysis of simpler silyl-protected acetoxyenynes derived from fatty acids. These NMR experimental results and stereochemical predictions were rationalized by DFT calculations.

Multi-component analysis: blind extraction of pure components mass spectra using sparse component analysis

The paper presents sparse component analysis (SCA)-based blind decomposition of the mixtures of mass spectra into pure components, wherein the number of mixtures is less than number of pure components. Standard solutions of the related blind source separation (BSS) problem that are published in the open literature require the number of mixtures to be greater than or equal to the unknown number of pure components. Specifically, we have demonstrated experimentally the capability of the SCA to blindly extract five pure components mass spectra from two mixtures only. Two approaches to SCA are tested: the first one based on l(1) norm minimization implemented through linear programming and the second one implemented through multilayer hierarchical alternating least square nonnegative matrix factorization with sparseness constraints imposed on pure components spectra. In contrast to many existing blind decomposition methods no a priori information about the number of pure components is required. It is estimated from the mixtures using robust data clustering algorithm together with pure components concentration matrix. Proposed methodology can be implemented as a part of software packages used for the analysis of mass spectra and identification of chemical compounds.

Structure and catalytic mechanism of the thioesterase CalE7 in enediyne biosynthesis

The biosynthesis of the enediyne moiety of the antitumor natural product calicheamicin involves an iterative polyketide synthase (CalE8) and other ancillary enzymes. In the proposed mechanism for the early stage of 10-membered enediyne biosynthesis, CalE8 produces a carbonyl-conjugated polyene with the assistance of a putative thioesterase (CalE7). We have determined the x-ray crystal structure of CalE7 and found that the subunit adopts a hotdog fold with an elongated and kinked substrate-binding channel embedded between two subunits. The 1.75-A crystal structure revealed that CalE7 does not contain a critical catalytic residue (Glu or Asp) conserved in other hotdog fold thioesterases. Based on biochemical and site-directed mutagenesis studies, we proposed a catalytic mechanism in which the conserved Arg(37) plays a crucial role in the hydrolysis of the thioester bond, and that Tyr(29) and a hydrogen-bonded water network assist the decarboxylation of the beta-ketocarboxylic acid intermediate. Moreover, computational docking suggested that the substrate-binding channel binds a polyene substrate that contains a single cis double bond at the C4/C5 position, raising the possibility that the C4=C5 double bond in the enediyne moiety could be generated by the iterative polyketide synthase. Together, the results revealed a hotdog fold thioesterase distinct from the common type I and type II thioesterases associated with polyketide biosynthesis and provided interesting insight into the enediyne biosynthetic mechanism.

Polyacetylenes from sardinian Oenanthe fistulosa: a molecular clue to risus sardonicus

An investigation of Oenanthe fistulosa from Sardinia afforded oenanthotoxin (1a) and dihydrooenanthotoxin (1b) from the roots and the diacetylenic epoxydiol 2 from the seeds. The absolute configuration of 1a and 1b was established as R by the modified Mosher's method, and the structure of 2 by chemical correlation with (+)-(3R,8S)-falcarindiol. Oenanthotoxin (1a) and dihydrooenanthotoxin (1b) were found to potently block GABAergic responses, providing a molecular rationale for the symptoms of poisoning from water-dropwort (Oenanthe crocata) and related plants. These observations bear relevance for a series of historical and ethnopharmacological observations on the identification of the Sardonic herb and the molecular details of the facial muscular contraction caused by its ingestion (risus sardonicus).

Efficient and accurate characterization of the Bergman cyclization for several enediynes including an expanded substructure of esperamicin A1

Incorporation of enediynes into anticancer drugs remains an intriguing yet elusive strategy for the design of therapeutically active agents. Density functional theory was used to locate reactants, products, and transition states along the Bergman cyclization pathways connecting enediynes to reactive para-biradicals. Sum method correction to low-level calculations confirmed B3LYP/6-31G(d,p) as the method of choice in investigating enediynes. Herein described as MI:Sum, calculated reaction enthalpies differed from experiment by an average of 2.1 kcal x mol(-1) (mean unsigned error). A combination of strain energy released across the reaction coordinate and the critical intramolecular distance between reacting diynes explains reactivity differences. Where experimental and calculated barrier heights are in disagreement, higher level multireference treatment of the enediynes confirms lower level estimates. Previous work concerning the chemically reactive fragment of esperamcin, MTC, is expanded to our model system MTC2.

Synthesis and DNA-cleaving activity of lactenediynes conjugated with DNA-complexing moieties

Lactenediynes are compounds characterized by the fusion of a beta-lactam with a cyclodeca-3-ene-1,5-diyne. In this work the most promising members of this family have been activated by attaching a carbalkoxy or a carbamoyl group to the azetidinone nitrogen, and conjugated to various DNA-complexing moieties, either acting by intercalation or through groove binding. These conjugated artificial enediynes have been demonstrated to possess in vitro ability to produce single and double strand cleavage of plasmid DNA. As potency and capacity to induce double cut, they rank among the best simple enediyne analogues ever prepared. A thorough investigation was carried out in order to develop the best suited linkers for assembling these conjugates.

The Reactivity of Calicheamicin γ1I in the Minor Groove of DNA: The Decisive Role of the Environment

Triggering and Bergman cyclization of calicheamicin gamma(1) (I) outside and inside the minor groove of the duplex 9mer-B-DNA sequence d(CACTCCTGG).d(CCAGGAGTG) were investigated by using density functional theory and molecular mechanics (DFT and MM) descriptions in which the ligand is completely described at the DFT and the receptor at the MM level. The calculated docking energy of calicheamicin gamma(1) (I) (-12.5 kcal mol(-1)) is close to the measured value of -9.7 kcal mol(-1) and the site specificity is in line with experimental observations. Calicheamicin is triggered in the minor groove in such a way that out of a cyclohexenone by Michael addition an E rather than a Z form of a cyclohexanone is formed, which in turn adopts a chair rather than a twistboat form. Decisive for the stereochemistry of the Michael addition is the orientation of the carbamate substituent at the headgroup of calicheamicin. Triggered calicheamicin can undergo the Bergman cyclization at body temperature only if present in its E chair form (activation enthalpy 16.4 kcal mol(-1)). An intermediate biradical is formed (docking energy -13.6 kcal mol(-1)), which has a sufficient lifetime to abstract two hydrogen atoms. Hydrogen abstraction is a two- rather than one-step process and involves the C5(H5') atom first and then the T22(H4') atom in line with experimental observations. The decisive role of using a DFT rather than an MM description for the ligand is documented.

Characterization of the maduropeptin biosynthetic gene cluster from Actinomadura madurae ATCC 39144 supporting a unifying paradigm for enediyne biosynthesis

The biosynthetic gene cluster for the enediyne antitumor antibiotic maduropeptin (MDP) from Actinomadura madurae ATCC 39144 was cloned and sequenced. Cloning of the mdp gene cluster was confirmed by heterologous complementation of enediyne polyketide synthase (PKS) mutants from the C-1027 producer Streptomyces globisporus and the neocarzinostatin producer Streptomyces carzinostaticus using the MDP enediyne PKS and associated genes. Furthermore, MDP was produced, and its apoprotein was isolated and N-terminal sequenced; the encoding gene, mdpA, was found to reside within the cluster. The biosynthesis of MDP is highlighted by two iterative type I PKSs--the enediyne PKS and a 6-methylsalicylic acid PKS; generation of (S)-3-(2-chloro-3-hydroxy-4-methoxyphenyl)-3-hydroxypropionic acid derived from L-alpha-tyrosine; a unique type of enediyne apoprotein; and a convergent biosynthetic approach to the final MDP chromophore. The results demonstrate a platform for engineering new enediynes by combinatorial biosynthesis and establish a unified paradigm for the biosynthesis of enediyne polyketides.

Regiospecific chlorination of (S)-beta-tyrosyl-S-carrier protein catalyzed by SgcC3 in the biosynthesis of the enediyne antitumor antibiotic C-1027

C-1027 is a potent antitumor antibiotic composed of an apo-protein and a reactive enediyne chromophore. The chromophore consists of four different chemical subunits including an (S)-3-chloro-4,5-dihydroxy-beta-phenylalanine moiety, the biosynthesis of which from l-alpha-tyrosine is catalyzed by six proteins, SgcC, SgcC1, SgcC2, SgcC3, SgcC4, and SgcC5. Biochemical characterization of SgcC3 unveiled the following: (i) SgcC3 is a flavin adenine dinucleotide (FAD)-dependent halogenase; (ii) SgcC3 acts only on the SgcC2 peptidyl carrier protein-tethered substrates; (iii) SgcC3-catalyzed halogenation requires O2 and reduced FAD and either the C-1027 pathway-specific flavin reductase SgcE6 or E. coli flavin reductase (Fre) can support the SgcC3 activity; (iv) SgcC3 also efficiently catalyzes bromination but not fluorination or iodination; (v) SgcC3 can utilize both (S)- and (R)-beta-tyrosyl-S-SgcC2 but not 3-hydroxy-beta-tyrosyl-S-SgcC2 as a substrate. These results establish that SgcC3 catalyzes the third enzymatic transformation during the biosynthesis of the (S)-3-chloro-4,5-dihydroxy-beta-phenylalanine moiety of C-1027 from l-alpha-tyrosine. SgcC3 now represents the second biochemically characterized flavin-dependent halogenase that acts on a carrier protein-tethered substrate. These findings will facilitate the engineering of new C-1027 analogs by combinatorial biosynthesis methods.

A QM/MM Study of the Bergman Reaction of Dynemicin A in the Minor Groove of DNA

The Bergman cyclization of the natural enediyne dynemicin A in its triggered form (2) bound to the minor groove of DNA is compared with the corresponding reaction of its open isomer (4) utilizing QM/MM methodology. The two isomers are typical representatives of 10-membered cyclic (2) and acyclic (4) enediynes, which possess significantly different barriers for the Bergman reaction in the gas phase (2, 20.4 kcal/mol; 4, 31.3 kcal/mol). In the case of the cyclic enediyne (2) the explicit consideration of environmental factors such as the receptor DNA, the solvent water, and charge neutralization by counterions has only minor effects on the energy profile of the cyclization reaction and the corresponding optimized structures when compared with the gas phase. The energetics of the reaction is predominantly determined by QM (electronic) effects. This makes it possible to replace the explicit description of the environment by an implicit one, thus avoiding costly QM/MM calculations and using instead a decoupled QM+MM approach. A conformationally driven hinge mechanism is identified for 2 that makes it possible for the ligand to adjust to the dimensions of the minor groove without significant energy loss. In the case of the acyclic enediyne 4 a QM/MM treatment is necessary to describe the Bergman cyclization in the minor groove. QM/MM corrects the cyclization barrier from 31.3 to 23.7 kcal/mol, which makes the reaction feasible under physiological conditions. The reduction of the barrier is a result of transition-state stabilization, which is caused by an increased dipole moment and hence stronger electrostatic interactions with the environment. In both cases the anionic charge of dynemicin A is largely shielded by water solvation and ion pair formation so that it does not significantly affect the energetics of the Bergman cyclization.

Syntheses and antibacterial activity of phendioxy substituted cyclic enediynes

Syntheses and antibacterial activity of 13-membered 1,3-phendioxy substituted cyclic enediynes are reported. The compounds were screened against gram-positive and gram-negative strains and some of the compounds exhibit potent antibacterial activity.

Enediyne compounds - new promises in anticancer therapy

Scientists of all kinds have long been intrigued by the nature, action and potential of natural toxins that possess exceptional antibacterial and anticancer activities. These compounds, named enediynes, are among the most effective chemotherapeutic agents known. Often compared with intelligent weapons, due to the unique structure and sophisticated mechanism by which they destroy double-helical DNA, enediyne antibiotics are nowadays the most promising leaders in the anticancer therapy. Apart from their diversity, enediyne compounds share some structural and functional similarities. One fragment of a structure is responsible for the recognition and transport, another part acts as molecular trigger while the third, reactive enediyne unit, undergoes Bergman cycloaromatization and causes DNA breakage. Members of the enediyne family are already in clinical use to treat various cancers, but more general use is limited by their complex structure, which makes them formidable targets for synthetic chemists. There are three main approaches in the design of new enediyne-related compounds: improvement of enediyne >>warheads<<, increasing the selectivity and control of chemical or photo-induced activation. This paper gives an overview of naturally occurring enediynes, their mode of action and efforts undertaken to design artificial enediyne-related DNA cleaving agents.

Synthesis of L-kedarosamine in protected form and its efficient incorporation into an advanced intermediate to kedarcidin chromophore

An efficient route to the complex L-kedarosamine alpha-glycosidic ether 2, a synthetic precursor to kedarcidin chromophore, is described. Central to the route, which is suitable for the preparation of multigram amounts of material, is a short synthetic sequence from D-threonine to protected L-kedarosamine derivatives and methodology for their alpha-selective coupling with appropriate hydroxyl acceptors.

Kedarcidin chromophore: synthesis of its proposed structure and evidence for a stereochemical revision

A convergent, enantioselective synthesis of the proposed structure of kedarcidin chromophore (1 ) is described. The route is 24 steps in the longest linear sequence (beginning with the commercial reagent 2,3-O -isopropylidene-d -erythronolactone) with an average yield of 75% per step (overall yield: 0.1%). Our 1H NMR data for 1 do not coincide with the data reported for kedarcidin chromophore. We have re-analyzed the original data and here propose a stereochemical revision at position C10, the site of attachment of the l -mycarose carbohydrate residue to the chromophore core (structure 2 ).

Solution structure of a designed spirocyclic helical ligand binding at a two-base bulge site in DNA

The solution structure of the complex formed between an oligodeoxynucleotide containing a two-base bulge (5'-CCATCGTCTACCTTTGGTAGGATGG) and SCA-alpha2, a designed spirocyclic helical molecule, has been elucidated. SCA-alpha2, a close mimic of the metabolite, NCSi-gb, of the DNA bulge-specific enediyne antibiotic neocarzinostatin, differs in possessing a more stable spirocyclic ring system and in lacking certain bulky groupings that compromise bulged DNA binding. This study provides a detailed comparison of the binding modes of the two complexes and provides new insights into the importance of shape and space, as opposed to simple nucleotide sequence, in complex formation at the bulge site. The two rigidly held aromatic rings of SCA-alpha2 form a right-handed helical molecular wedge that specifically penetrates the bulge-binding pocket and immobilizes the two bulge residues (GT), which point toward the minor groove, rather than the major groove as in the NCSi-gb.bulged DNA complex. The ligand aromatic ring systems stack on the DNA bulge-flanking base pairs that define the long sides of the triangular prism binding pocket. Like NCSi-gb, SCA-alpha2 possesses the natural N-methylfuranose moiety, alpha-linked to the benzindanol (BI) moiety. The amino sugar anchors in the major groove of the DNA and points toward the 3'-bulge-flanking base pair. Lacking the bulky cyclocarbonate of NCSi-gb, the SCA-alpha2.bulged DNA complex has a much less twisted and buckled 3'-bulge-flanking base pair (dG20.dC8), and the G20 residue stacks directly above the BI ring platform. Also, the absence of the methyl group and the free rotation of the methoxy group on the dihydronaphthanone (NA) moiety of SCA-alpha2 allow better stacking geometry of the NA ring above the 5'-bulge-flanking dG21.dC5 base pair. These and other considerations help to explain why NCSi-gb binds very poorly to bulged RNA and are consistent with the recent observation of good binding with SCA-alpha2. Thus, although the two complexes resemble each other closely, they differ in important local environmental details. SCA-alpha2 has a better hand-in-glove fit at the bulge site, making it an ideal platform for the placement of moieties that can react covalently with the DNA and for generating congeners specific for bulges in RNA.

Nucleophilic addition to a p-benzyne derived from an enediyne: a new mechanism for halide incorporation into biomolecules

Biosynthesis of haloaromatics ordinarily occurs by electrophilic attack of an activated halogen species on an electron-rich aromatic ring. We now present the discovery of a new reaction whereby a nucleophilic halide anion can be attached even to an aromatic ring without activating substituents. We show that the enediyne cyclodeca-1,5-diyn-3-ene, in the presence of lithium halide and a weak acid, is converted to 1-halotetrahydronaphthalene. The kinetics are consistent with rate-limiting cyclization to a p-benzyne biradical that rapidly adds halide and is then protonated. This reaction has interesting mechanistic features and important implications for incorporation of halide into biomolecules.

Tuning the Bergman Cyclization by Introduction of Metal Fragments at Various Positions of the Enediyne. Metalla-Bergman Cyclizations

We expand the scope of the Bergman cyclization by exploring computationally the rearrangement of two osmaenediynes and one rhodaenediyne. The three hypothetical metallaenediynes are constructed by substituting the 14-electron Os(PH3)3 fragment for the C fragment, or the 15-electron Os(PH3)3H or Rh(PH3)3 fragments for the sp2 CH fragment, of 3-ene-1,5-diyne. This replacement is guided by the isolobal analogy and previous metallabenzene chemistry. The rearrangement of osmaenediyne with an Os(PH3)3 fragment in place of C is exothermic by 3 kcal/mol (the parent Bergman reaction is computed to be endothermic by 5 kcal/mol) and associated with a significant decrease in the barrier to rearrangement to 13 kcal/mol (the Ea of the parent reaction computed at the same level of theory is 33 kcal/mol). The replacement of a CH by the isolobal analogue Os(PH3)3H reduces the energy of activation for the rearrangement to 23 kcal/mol and produces a corresponding metalladiradical that is 8 kcal/mol less stable that the corresponding osmaenediyne. The activation energy corresponding to the rearrangement of the rhodaenediyne is the same as that of the organic parent enediyne. Interesting polytopal rearrangements of metallaenediynes and the diradical nature of the resulting intermediates are also explored.

Single chemical modifications of the C-1027 enediyne core, a radiomimetic antitumor drug, affect both drug potency and the role of ataxia-telangiectasia mutated in cellular responses to DNA double-strand breaks

The radiomimetic enediyne C-1027 induces almost exclusively DNA double-strand breaks (DSB) and is extremely cytotoxic. Unique among radiomimetics, ataxia-telangiectasia mutated (ATM) is dispensable for cellular responses to C-1027-induced DNA damage. This study explores the biological activity of three recently bioengineered C-1027 analogues: 7''-desmethyl-C-1027 (desmethyl), 20'-deschloro-C-1027 (deschloro), and 22'-deshydroxy-C-1027 (deshydroxy). Each compound maintains the characteristic ability of radiomimetics to cleave DNA in cell-free systems, varying in activity from 2-fold (deschloro) to 55-fold (desmethyl) less than C-1027. The induction of cellular DNA breaks based on pulsed field gel electrophoresis, comet analysis, and gammaH2AX activation was in the same rank order as cell-free DNA break induction, although the amount of breaks induced by desmethyl is greatly reduced compared with the other analogues. Despite the disparity in inducing DNA DSBs, all of the analogues produced G2-M cell cycle arrest and activated DNA DSB damage response proteins, such as p53-Ser15 and Chk2-Thr68, at concentrations in concordance with their ability to inhibit cell growth. Interestingly, of the three analogues, only the desmethyl-induced DNA damage response was similar to C-1027, as it did not cause hypersensitive cell growth inhibition in the absence of ATM nor require the kinase to phosphorylate p53 or Chk2. These findings show that simple modifications of the chromophore of C-1027 can result in varied induction of, and cellular response to, DNA DSBs.

Synthesis of the entire carbon framework of the kedarcidin chromophore aglycon

In advanced studies directed toward the total synthesis of the kedarcidin chromophore, we have successfully achieved the late-stage installation of the nine-membered diyne ring in the presence of the highly functionalised ansamacrocyclic bridge.

An Efficient Synthesis of Epoxydiynes and a Key Fragment of Neocarzinostatin Chromophore

[reaction: see text] A key structural feature of the Neocarzinostatin chromophore is a reactive epoxydiyne. We present here a new method for the preparation of epoxydiynes by the addition of an allenyl zinc bromide to a propargylic ketone.

Deciphering indolocarbazole and enediyne aminodideoxypentose biosynthesis through comparative genomics: insights from the AT2433 biosynthetic locus

AT2433, an indolocarbazole antitumor antibiotic, is structurally distinguished by its aminodideoxypentose-containing disaccharide and asymmetrically halogenated N-methylated aglycon. Cloning and sequence analysis of AT2433 gene cluster and comparison of this locus with that encoding for rebeccamycin and the gene cluster encoding calicheamicin present an opportunity to study the aminodideoxypentose biosynthesis via comparative genomics. The locus was confirmed via in vitro biochemical characterization of two methyltransferases--one common to AT2433 and rebeccamycin, the other unique to AT2433--as well as via heterologous expression and in vivo bioconversion experiments using the AT2433 N-glycosyltransferase. Preliminary studies of substrate tolerance for these three enzymes reveal the potential to expand upon the enzymatic diversification of indolocarbazoles. Moreover, this work sets the stage for future studies regarding the origins of the indolocarbazole maleimide nitrogen and indolocarbazole asymmetry.

Enantioselective synthesis of N1999A2

An enantioselective synthetic route to the enediyne antibiotic N1999A2 (1) is described, proceeding in 21 steps (0.4% yield, 77% average yield per step) from (R)-(+)-glycidol. The route involves the convergent assembly of three components: a (1-iodovinyl) stannane (2), a 1,5-hexadiyne-3,4-diol derivative (3), and a substituted naphthoic acid (4). Important transformations in the synthetic sequence include the palladium-catalyzed coupling of 2 and 3, an intramolecular oxidative cyclization of a terminal bisacetylene, and a transannular anionic (bi)cyclization of a cyclic bromoenetriyne. The careful selection and manipulation of protective groups throughout the sequence proved to be critical to the development of the synthetic route, where all late-stage intermediates were unstable and could not be concentrated. In the final step of the sequence, three protective groups were removed in a single operation, providing synthetic N1999A2 (1) in 76% yield. Conditions were found that, for the first time, led to the precipitation of 1 as a solid.

Spin-Trapping of the p-Benzyne Intermediates from Ten-Membered Enediyne Calicheamicin γ1I

In the presence of thiols, the ten-membered-ring enediyne calicheamicin gamma1I generates a p-benzyne biradical that initiates oxidative cleavage of double-stranded DNA. Application of spin-trapping has successfully provided ESR and mass spectroscopic evidence for the formation of the monoadducts with phenyl tert-butyl nitrone (PBN). [reaction: see text].