Concise asymmetric syntheses of radicicol and monocillin I.

Radicicol (1) exhibits potent anticancer properties in vitro, which are likely to be mediated through its high affinity (20 nM) for the molecular chaperone Hsp90. Recently, we reported the results of a synthetic program targeting radicicol (1) and monocillin I (2), highlighted by the application of ring-closing metathesis to macrolide formation. These efforts resulted in a highly convergent synthesis of radicicol dimethyl ether but failed in the removal of the two aryl methyl ethers. Simple exchange of these methyl ethers with more labile functionalities disabled a key esterification in the initial route. Through extended experimentation, a successful route to both natural products was secured, along with some intriguing results that emphasize the implications of this design on a broad range of fused benzoaliphatic targets, including analogues of these natural products.

The synthesis, discovery, and development of a highly promising class of microtubule stabilization agents: curative effects of desoxyepothilones B and F against human tumor xenografts in nude mice.

We have evaluated two synthetic epothilone analogues lacking the 12,13-epoxide functionality, 12,13-desoxyepothilone B (dEpoB), and 12,13-desoxyepothilone F (dEpoF). The concentrations required for 50% growth inhibition (IC(50)) for a variety of anticancer agents were measured in CCRF-CEM/VBL1000 cells (2,048-fold resistance to vinblastine). By using dEpoB, dEpoF, aza-EpoB, and paclitaxel, the IC(50) values were 0.029, 0.092, 2.99, and 5.17 microM, respectively. These values represent 4-, 33.5-, 1,423- and 3,133-fold resistance, respectively, when compared with the corresponding IC(50) in the parent [nonmultiple drug-resistant (MDR)] CCRF-CEM cells. We then produced MDR human lung carcinoma A549 cells by continuous exposure of the tumor cells to sublethal concentrations of dEpoB (1.8 yr), vinblastine (1.2 yr), and paclitaxel (1.8 yr). This continued exposure led to the development of 2.1-, 4,848-, and 2,553-fold resistance to each drug, respectively. The therapeutic effect of dEpoB and paclitaxel was also compared in vivo in a mouse model by using various tumor xenografts. dEpoB is much more effective in reducing tumor sizes in all MDR tumors tested. Analysis of dEpoF, an analog possessing greater aqueous solubility than dEpoB, showed curative effects similar to dEpoB against K562, CCRF-CEM, and MX-1 xenografts. These results indicate that dEpoB and dEpoF are efficacious antitumor agents with both a broad chemotherapeutic spectrum and wide safety margins.

Insights into long-range structural effects on the stereochemistry of aldol condensations: a practical total synthesis of desoxyepothilone F.

A processable total synthesis of a potent antitumor agent, desoxyepothilone F (dEpoF, 21-hydroxy-12,13-desoxyepothilone B, 21-hydroxyepothilone D), has been accomplished. The route is highly convergent. The new technology has also been applied to a total synthesis of 12,13-desoxyepothilone (dEpoB). The crucial point of departure from previous syntheses of dEpoB and dEpoF involves presentation of the C1-C11 sector for Suzuki coupling with C3 in reduced form. Hitherto, the required S stereochemistry at C3 had been implemented via reduction of a keto function after Suzuki coupling. Whereas that chemistry worked quite well in a synthesis of dEpoB, it was not transferable to a high-yielding synthesis of dEpoF. The reduction of the keto group at C3 via a Noyori protocol after Suzuki coupling had proved to be very difficult. In our current approach, two consecutive aldol reactions are used to fashion the acyl sector. In the first aldol condensation, C6 becomes attached to C7. Following protection at C7, a two-carbon acetate equivalent is used to join C2 and C3 with very high asymmetric induction at C3. Only after this center has been implemented is the Suzuki reaction conducted. This major advance allowed us to synthesize dEpoF in a straightforward fashion. These findings found ready application in the total synthesis of dEpoB. Another part of the study involved analysis of the factors associated with aldol condensations joining C6 to C7. In the work described herein, the consequences of the status of C3 in promoting the C6-C7 aldol coupling are probed in detail. Dramatic stereochemical long-range effects uncovered during the study are described, and a working model to explain these effects has emerged.

The epothilones, eleutherobins, and related types of molecules.

Taxol is currently one of the most effective anticancer agents available. However, limitations due to multidrug-resistance (MDR) susceptibility and lack of aqueous solubility render it less than an ideal drug. These limitations, coupled with taxol's unique mechanism of tumor inhibition, involving the stabilization of microtubule assembly, have spurred the search for more effective chemotherapeutic agents. This review will discuss the chemistry and biology of some of the most promising new molecules with "taxol-like" activity. The extended family of microtubule-stabilizing agents now includes the epothilones, eleutherobins, discodermolide, laulimalide and WS9885B. The epothilones have emerged as one of the most exciting new candidates for detailed structure-activity-related studies. A review of our efforts in the synthetic and biological aspects of this research is presented, as are the latest developments reported from other laboratories in academia and the pharmaceutical industry. The synthesis and structure-activity studies of eleutherobins, as well as recent progress with discodermolide, laulimalide and WS9885B are also reviewed. An abundance of exciting advances in chemistry and biology have emerged from these studies, and it is hoped that it will ultimately result in the development of new and more effective chemotherapeutic agents in the fight against cancer.

On the interactivity of complex synthesis and tumor pharmacology in the drug discovery process: total synthesis and comparative in vivo evaluations of the 15-aza epothilones.

The total syntheses of 12,13,15-desoxy-15(S)-aza-epothilone B (aza-dEpoB; dEpoB-lactam) and 12,13,15-desoxy-15(R)-aza-epothilone B (15-epi-aza-dEpoB; 15-epi-dEpoB-lactam) have been accomplished via a highly convergent strategy. We have also successfully oxidized 12,13,15-desoxy-15(S)-aza-epothilone B to aza-epothilone B (aza-EpoB; EpoB-lactam). Aza-epothilone B has been advanced to phase I clinical trials by the Bristol-Myers Squibb group. Our synthesis is efficient and was amenable to the production of significant quantities of these lactams. Using our fully synthetically derived lactams, in vitro and in vivo studies were conducted in comparison with advanced clinical candidates, 12,13-desoxyepothilone B and 12,13-desoxyepothilone F, also derived by total synthesis.

Total synthesis and antitumor activity of 12,13-desoxyepothilone F: an unexpected solvolysis problem at C15, mediated by remote substitution at C21.

A new epothilone analogue, 12,13-desoxyepothilone F (dEpoF, 21-hydroxy-12,13-desoxyepothilone B, 21-hydroxyepothilone D), was synthesized and evaluated for antitumor potential. A convergent strategy employed for the semipractical synthesis of 12,13-desoxyepothilone B (dEpoB) has been utilized to yield an amount of dEpoF sufficient for relevant biological studies. The results from an in vitro assay reveal that this new analogue is highly active against various tumor cell lines with a potency comparable to that of dEpoB. In particular, the growth of resistant tumor cells is inhibited by dEpoF at concentrations where paclitaxel (Taxol) is basically ineffective. A preliminary assessment of its in vivo activity is also promising. The new analogue, containing an additional hydroxyl group at C21, exhibits advantages over other epothilones in terms of water solubility, and can serve as a readily functionalizable handle to produce other useful compounds for pertinent biological studies.

Xotx5b, a new member of the Otx gene family, may be involved in anterior and eye development in Xenopus laevis.

We describe the cloning, expression pattern and functional overexpression analysis of Xotx5b, a new member of the Otx gene family in Xenopus laevis. Early expression of Xotx5b resembles that of Xotx2, being detected in the organizer region at early gastrula stage, and, shortly after, also in anterior neuroectoderm. During neurula stages Xotx5b exhibits a changing and dynamic pattern of expression. After neural tube closure, Xotx5b is expressed in the eye and pineal gland, both involved in photoreception. Overexpression of Xotx5b has a similar effect to that of Xotx2, producing posterior truncations and inducing ectopic cement gland and neural tissue in whole embryos. In animal cap assays, Xotx5b and Xotx2 are both able to activate XAG, to strongly suppress the expression of the epidermal marker XK81, and to reciprocally activate each other. Finally, in einsteck transplantation assays, Xotx5b is able to respecify a tail/trunk organizer to a head organizer.

En route to a plant scale synthesis of the promising antitumor agent 12,13-desoxyepothilone B.

[reaction--see text] Efficient and processable syntheses of key building blocks of the antitumor agent 12,13-desoxyepothilone B (dEpoB) by catalytic asymmetric induction are herein described.

On the total synthesis and preliminary biological evaluations of 15(R) and 15(S) aza-dEpoB: a Mitsunobu inversion at C15 in pre-epothilone fragments.

[reaction-see text] The syntheses of two epothilone analogues, 15(S)-aza-12,13-desoxyepothilone B and the epimeric 15(R)-aza-12,13-desoxyepothilone B, are described. A Mitsunobu inversion was utilized for elaboration of pre-epothilone fragments to the corresponding macrolactam. Tubulin binding and cytotoxicity profiles of these analogues are presented.

The fluorescence of scorpions and cataractogenesis.

BACKGROUND: Protein cross-linking and fluorescence are widely recognized markers of oxidative aging in human proteins. Oxidative protein aging is a combinatorial process in which diversity arises from the heterogeneity of the targets and is amplified by the nonselective nature of the reactants. The cross-links themselves defy analysis because they are generally embedded in a covalent matrix. Arthropods rely upon oxidative cross-linking in the hardening of the cuticle - a process known as sclerotization. Among arthropods, scorpions are noteworthy in that the process of sclerotization is accompanied by the buildup of strong visible fluorescence. To date, the nature of the fluorescent species has remained a mystery. RESULTS: We have identified one of the soluble fluorescent components of the scorpions Centuroides vittatus and Pandinus imperator as beta-carboline - a tryptophan derivative that has previously been identified by hydrolysis and oxidation of lens protein. We have also shown that beta-carboline-3-carboxylic acid is released from both scorpion exuvia (the shed cuticle) and human cataracts upon hydrolysis, suggesting that the protein-bound beta-carboline and free beta-carboline have common chemical origins. CONCLUSIONS: Cataractogenesis and cuticular sclerotization are disparate oxidative processes - the former is collateral and the latter is constitutive. The common formation of beta-carbolines shows that similar patterns of reactivity are operative. These fundamental mechanisms provide predictive insight into the consequences of human protein aging.

Manipulation of the C(22)-C(27) region of rapamycin: stability issues and biological implications.

A novel series of rapamycin derivatives with modifications in the C(22)-C(27) region has been prepared. These compounds are evaluated for their ability to prevent ring fragmentation while still retaining immunosuppressive capabilities.

Activin-induced factors maintain goosecoid transcription through a paired homeodomain binding site.

Previous studies in both Xenopus and zebrafish have shown that goosecoid is one of the first genes to be transcribed at the onset of gastrulation. Goosecoid transcription still initiates when embryos are treated with protein synthesis inhibitors, indicating that it is mediated by preexisting factors and suggesting that goosecoid transcription is immediately downstream of the maternal mesoderm-inducing signal. However, goosecoid transcription continues long after this maternal signal has ceased to be active, indicating that there are mechanisms to maintain activin-induced transcription. Our study has focused on understanding the factors required to maintain this transcription. We have defined an element within the zebrafish goosecoid promoter that is sufficient for activin inducibility in both Xenopus and zebrafish embryos. This element, the goosecoid activin element, interacts with two developmentally regulated proteins from Xenopus embryos. A maternal protein interacts through cleavage stages until the midblastula transition, and a second protein binds from the onset of gastrulation. The second protein is zygotically expressed, and its binding is required for activin inducibility in our assay system. We suggest that the zygotic protein we have identified is a good candidate to be involved in the maintenance of goosecoid transcription. Furthermore, this zygotic protein is likely to contain a paired class homeodomain since a consensus binding site for such proteins is present within the goosecoid activin element and is essential for its function.

Stabilization of a C7 equatorial gamma turn in DMSO-d6 by a ditryptophan crosslink.

Covalent crosslinks can control local peptide conformation. In tripeptide sequences of the general formula Cys-Xxx-Cys, cysteine disulfides have been previously shown to enforce a C7 equatorial gamma-turn conformation (also referred to as an inverse gamma-turn). Much less is known about the effects of dityrosine and ditryptophan crosslinks on local peptide structure. In a series of tripeptides, ditryptophan crosslinks were formed using the two-step process of acid-promoted Mannich dimerization followed by oxidative aromatization. In these peptides, with the general formula Trp-Xxx-Trp (Xxx not equal to Gly), ditryptophan crosslinks were found to stabilize a C7 equatorial gamma-turn conformation in DMSO-d6. Rigorous support for a C7 equatorial conformation in the crosslinked sequence Trp-Pro-Trp came from a variety of 1H NMR experiments and molecular modelling. Interproton distances were derived from NOE buildups that were determined through a series of double pulsed field gradient spin echo (DPFGSE) experiments. In addition, the small temperature dependence of the i+2 NH chemical shifts (delta delta/delta T < 2 ppm/degree C) provided further support for the intramolecular hydrogen bond which defines a gamma-turn.

Follistatin and noggin are excluded from the zebrafish organizer.

The patterning activity of the Spemann organizer in early amphibian embryos has been characterized by a number of organizer-specific secreted proteins including Chordin, Noggin, and Follistatin, which all share the same inductive properties. They can neuralize ectoderm and dorsalize ventral mesoderm by blocking the ventralizing signals Bmp2 and Bmp4. In the zebrafish, null mutations in the chordin gene, named chordino, lead to a severe reduction of organizer activity, indicating that Chordino is an essential, but not the only, inductive signal generated by the zebrafish organizer. A second gene required for zebrafish organizer function is mercedes, but the molecular nature of its product is not known as yet. To investigate whether and how Follistatin and Noggin are involved in dorsoventral (D-V) patterning of the zebrafish embryo, we have now isolated and characterized their zebrafish homologues. Overexpression studies demonstrate that both proteins have the same dorsalizing properties as their Xenopus homologues. However, unlike the Xenopus genes, zebrafish follistatin and noggin are not expressed in the organizer region, nor are they linked to the mercedes mutation. Expression of both genes starts at midgastrula stages. While no patterned noggin expression was detectable by in situ hybridization during gastrulation stages, later expression is confined to presumptive cartilage cells in the branchial arches and the neurocranium and to proximal regions of the pectoral fin buds. follistatin transcripts in gastrulating embryos are confined to anterior paraxial regions, which give rise to head mesoderm and the first five somites. The dorsolateral extent of this expression domain is regulated by Bmp2b, Chordino, and Follistatin itself. In addition, transient expression was observed in a subset of cells in the posterior notochord anlage. Later, follistatin is expressed in brain, eyes, and somites. Comparison of the spatiotemporal expression pattern of follistatin and noggin with those of bmp2b and bmp4 and overexpression studies suggest that Noggin and Follistatin may function as Bmp antagonists in later processes of zebrafish development, including late phases of D-V patterning, to refine the early pattern set up by the interaction of Chordino and Bmp2/4. It thus appears that many, but not all, aspects of early dorsoventral patterning are shared among different vertebrate species.

Neural induction by the secreted polypeptide noggin.

The Spermann organizer induces neural tissue from dorsal ectoderm and dorsalizes lateral and ventral mesoderm in Xenopus. The secreted factor noggin, which is expressed in the organizer, can mimic the dorsalizing signal of the organizer. Data are presented showing that noggin directly induces neural tissue, that it induces neural tissue in the absence of dorsal mesoderm, and that it acts at the appropriate stage to be an endogenous neural inducing signal. Noggin induces cement glands and anterior brain markers, but not hindbrain or spinal cord markers. Thus, noggin has the expression pattern and activity expected of an endogenous neural inducer.

Lithium perturbation and goosecoid expression identify a dorsal specification pathway in the pregastrula zebrafish.

The zebrafish dorsoventral axis can first be distinguished at gastrulation, upon formation of the embryonic shield, the site of the organizer. We have asked whether the shield is specified before gastrulation. First, we show that brief exposure of premidblastula embryos to lithium, which is known to shut down the phospho-inositol signaling pathway, produces excessive shield formation and extreme hyper-dorsal development. Second, we show that the zebrafish goosecoid homeobox gene is activated at or just after the midblastula in a localized domain of cells that subsequently populate the most anterior region of the incipient shield and axial hypoblast, goosecoid expression is elevated and radialized by early lithium treatment, suggesting that goosecoid plays a role in establishing the organizer and shield. Our results demonstrate that the zebrafish dorsal axis is signaled by a pathway initiated in the cleavage-stage embryo. Furthermore, they provide novel insights into anterior morphogenesis.

A nontransformable Triticum monococcum monocotyledonous culture produces the potent Agrobacterium vir-inducing compound ethyl ferulate.

Exudates of dicotyledonous plants contain specific phenolic signal molecules, such as acetosyringone, which serve as potent inducers for the expression of the virulence (vir) regulon of the phytopathogen Agrobacterium tumefaciens. This induction activates the Agrobacterium T-DNA transfer process to initiate the genetic transformation of target plant cells. Wounded and metabolically active plant cells are particularly susceptible to Agrobacterium infection, and these cells specifically produce vir-inducing molecules. Most monocotyledonous, as opposed to dicotyledonous, species are resistant to Agrobacterium transformation. One hypothesis for this resistance is that nonsusceptible monocotyledonous cells fail to produce vir signal molecules and, thus, are not recognized by Agrobacterium as transformation targets. Here we demonstrate that monocotyledonous cells make such molecules, and, furthermore, we purify the inducer produced by a Triticum monococcum suspension culture that is resistant to Agrobacterium infection. This molecule is shown to correspond to ethyl ferulate [C12H14O4; 3-(4-hydroxy-3-methoxyphenyl)-2-propenoic acid ethyl ester], to be more active for vir induction at low concentrations than acetosyringone, and to be produced in quantities giving significant levels of induction. Thus, at least for the wheat cell line used in this study, monocotyledonous resistance to Agrobacterium transformation must result from a block to a step of the T-DNA transfer process subsequent to vir induction.

Cloning of a lymphocyte homing receptor reveals a lectin domain.

Lymphocytes express cell surface molecules, termed homing receptors, that mediate their selective attachment to specialized high endothelial venules found within secondary lymphoid organs. Previous work has demonstrated that the adhesive interaction between lymphocytes and the endothelium of peripheral lymph nodes appears to involve a lectin-like activity. Moreover, MEL-14, a monoclonal antibody that blocks lymphocyte-peripheral lymph node binding and presumably recognizes the homing receptor mediating this adhesive interaction, appeared to detect the lectin-like receptor. In this paper we describe the cloning of a murine cDNA that encodes the antigen recognized by the MEL-14 antibody. Characterization of the cDNA encoding the putative mouse peripheral lymph node-specific homing receptor shows that it contains a lectin domain that appears to be involved in the binding of lymphocytes to peripheral lymph node endothelium, thus defining a new type of cellular adhesion molecule. This result supports a novel mechanism for the distribution of lymphocyte populations to various lymphoid organs.

Characterization of Agrobacterium tumefaciens virulence proteins induced by the plant factor acetosyringone.

The Ti plasmid virulence (vir) loci encode functions essential for the transfer of the T-DNA element from Agrobacterium tumefaciens to plant cells. The expression of these loci is specifically signaled by plant phenolics such as acetosyringone. Here, we characterize the protein products that are induced in Agrobacterium grown in the presence of acetosyringone. More than 10 to 15 proteins are induced in strains harboring different Ti plasmids. Two general classes of acetosyringone-induced proteins are observed, encoded either within or outside the vir region. Synthesis of both classes of proteins requires acetosyringone and the products of the vir regulatory genes A and G. Those proteins encoded outside the vir region define a novel category of proteins, the virulence-related proteins, which are both chromosomally and Ti plasmid-encoded. The molecular weight and subcellular localization of several pTiA6 vir-induced proteins are identified. The most abundant induced protein has a molecular weight of 65,000, and is the single product of the virE locus; this protein distributes into both cell envelope and soluble fractions. Three proteins with molecular weights of approximately 33,000, 80,000 and 25,000 fractionate with the cell envelope and are encoded by genes within the 5' half of the virB locus. The envelope localization of the virB proteins suggests that they play a role in directing T-DNA transfer events that occur at the bacterial surface.