Structure of herbimycin A new ansamycin antibiotic

Natural Products and Their Mimics as Targets of Opportunity for Discovery

Diverse structural types of natural products and their mimics have served as targets of opportunity in our laboratory to inspire the discovery and development of new methods and strategies to assemble polyfunctional and polycyclic molecular architectures. Furthermore, our efforts toward identifying novel compounds having useful biological properties led to the creation of new targets, many of which posed synthetic challenges that required the invention of new methodology. In this Perspective, selected examples of how we have exploited a diverse range of natural products and their mimics to create, explore, and solve a variety of problems in chemistry and biology will be discussed. The journey was not without its twists and turns, but the unexpected often led to new revelations and insights. Indeed, in our recent excursion into applications of synthetic organic chemistry to neuroscience, avoiding the more-traveled paths was richly rewarding.

Antioxidant properties of dihydroherbimycin A from a newly isolated Streptomyces sp

During antioxidant screening using 1,1-diphenyl-picrylhydrazyl (DPPH) and a lipid peroxidation assay, a streptomycete strain was found to produce herbimycin A and dihydroherbimycin A as antioxidants in the culture filtrate. These molecules were identified by using spectral analyses, including infrared, ultraviolet, mass spectrum, and nuclear magnetic resonance assays. In the DPPH radical-scavenging assay, dihydroherbimycin A exhibited more potent antioxidant activity (IC(50), 1.3 microM) than alpha-tocopherol (IC(50), 2.7 microM) that was used as a reference compound. In the lipid peroxidation assay, both herbimycin A and dihydroherbimycin A demonstrated antioxidant activities of 61% and 72%, respectively, at 100 microg/ml, while alpha-tocopherol exhibited an activity of 93% at the same concentration. Therefore, dihydroherbimycin A might have the potential to be developed into a new therapeutic agent.

Geldanamycin derivatives and neuroprotective effect on cultured P19-derived neurons

Geldanamycin (1), an antifungal and anticancer ansamycin, was reported as a neurotrophic and neuroprotective substance against antineoplastic drugs, paclitaxel, vincristine, and cisplatin, on cultured dorsal root ganglion neurons from chick embryos. In this study, 1 in a large quantity, together with a known 17-O-demethylgeldanamycin (2), and a new 17-O-demethylgeldanamycin hydroquinone (3) were obtained from a mangrove Streptomyces sp. A series of O-alkyl and N-alkyl derivatives of 1 were prepared by modification of C-17 and/or C-19 on the quinone ring and were evaluated for in vitro activity against P19-derived neurons. Compound 1 and 19-O-methylgeldanamycin (7) at a very low dose (1nM) enhanced survival and neurite outgrowth of P19-derived neurons and prevented neurotoxicity of paclitaxel and vinblastine. Compound 7, possessing the lowest cytotoxicity and neurotoxicity, is serving as the most promising candidate in neurodegenerative therapy against neurotoxic anticancer drugs.

Total Synthesis of Herbimycin A

[structure: see text] Herbimycin A (HA) belongs to a class of antibiotics known as the benzoquinoid ansamycins. Members of this class have shown promising biological activity as Hsp90 inhibitors. An enantioselective synthesis of HA is described, employing asymmetric syn-crotylation methodology to introduce the C10, C11, C14, and C15 stereocenters. The C6-C7 stereocenters were introduced using Brown's alpha-pinene-derived gamma-methoxy allylborane reagent. The C12 stereocenter was established by diastereoselective hydroboration.

Irreversible inhibition of v-src tyrosine kinase activity by herbimycin A and its abrogation by sulfhydryl compounds

Herbimycin A, an antibiotic which reverses Rous sarcoma virus transformation, inhibited irreversibly the auto- and trans-phosphorylation activities of p60v-src in in vitro immune complex kinase assays. The addition of a sulfhydryl compound such as dithiothreitol, 2-mercaptoethanol, glutathione (reduced form) or cysteine abolished the ability of herbimycin A to inactivate p60v-src kinase as well as the ability to reverse transformed cell morphology, whereas the addition of oxidized glutathione, cystine or methionine showed no effect. The sulfhydryl alkylating reagent N-ethylmaleimide also, although less effectively, inactivated p60v-src kinase activity in vitro. These results suggest the likelihood that sulfhydryl groups of p60v-src are involved in the inactivation of v-src tyrosine kinase activity by herbimycin A.

Enhancement of fibronectin expression by herbimycin A

Herbimycin A specifically increased the level of fibronectin mRNA in Rous sarcoma virus-infected rat kidney cells, and the time course of fibronectin expression was found to be closely related to that of morphological change induced by herbimycin A.