Tom S. Chen

Synthesis and evaluation of isatin derivatives as effective SARS coronavirus 3CL protease inhibitors.

Abstract N-Substituted isatin derivatives were prepared from the reaction of isatin and various bromides via two steps. Bioactivity assay results (in vitro tests) demonstrated that some of these compounds are potent and selective inhibitors against SARS coronavirus 3CL protease with IC50 values ranging from 0.95 to 17.50μM. Additionally, isatin 4o exhibited more potent inhibition for SARS coronavirus protease than for other proteases including papain, chymotrypsin, and trypsin.

D-501036, a novel selenophene-based triheterocycle derivative, exhibits potent in vitro and in vivo antitumoral activity which involves DNA damage and ataxia telangiectasia–mutated nuclear protein kinase activation

D-501036 [2,5-bis(5-hydroxymethyl-2-selenienyl)-3-hydroxymethyl-N-methylpyrrole] is herein identified as a novel antineoplastic agent with a broad spectrum of antitumoral activity against several human cancer cells and an IC50 value in the nanomolar range. The IC50 values for D-501036 in the renal proximal tubule, normal bronchial epithelial, and fibroblast cells were >10 μmol/L. D-501036 exhibited no cross-resistance with vincristine- and paclitaxel-resistant cell lines, whereas a low level of resistance toward the etoposide-resistant KB variant was observed. Cell cycle analysis established that D-501036 treatment resulted in a dose-dependent accumulation in S phase with concomitant loss of both the G0-G1 and G2-M phase in both Hep 3B and A-498 cells. Pulsed-field gel electrophoresis showed D-501036–induced, concentration-dependent DNA breaks in both Hep 3B and A-498 cells. These breaks did not involve interference with either topoisomerase-I and topoisomerase-II function or DNA binding. Rapid reactive oxygen species production and formation of Se-DNA adducts were evident following exposure of cells to D-501036, indicating that D-501036–mediated DNA breaks were attributable to the induction of reactive oxygen species and DNA adduct formation. Moreover, D-501036–induced DNA damage activated ataxia telangiectasia–mutated nuclear protein kinase, leading to hyperphosphorylation of Chk1, Chk2, and p53, decreased expression of CDC25A, and up-regulation of p21WAF1 in both p53-proficient and p53-deficient cells. Collectively, the results indicate that D-501036–induced cell death was associated with DNA damage–mediated induction of ataxia telangiectasia–mutated activation, and p53-dependent and -independent apoptosis pathways. Notably, D-501036 shows potent activity against the growth of xenograft tumors of human renal carcinoma A-498 cells. Thus, D-501036 is a promising anticancer compound that has strong potential for the management of human cancers. [Mol Cancer Ther 2007;6(1):193–202]

Studies on the biosynthesis of avermectins

To elucidate the pathway of avermectin biosynthesis, the biosynthetic relationships of avermectins A1a, A2a, B1a, B2a, and their respective monosaccharides and aglycones were studied. 14C-labeled avermectin compounds prepared from [1-14C]acetate were fed to Streptomyces avermitilis strain MA5502 and their metabolites were determined. Two furan ring-free aglycones, 6,8a-seco-6,8a-deoxy-5-keto avermectin B1a and B2a, have been isolated from the fermentation broth of a blocked mutant of S. avermitilis. Addition of the compounds and a semisynthetic compound, 5-keto avermectin B2a aglycone, to the fermentation medium of a second blocked mutant established that the two compounds are intermediates in the avermectin biosynthetic pathway immediately preceding avermectin aglycones.

Further studies on the biosynthesis of the avermectins

SummaryThe biosynthesis of avermectins was studied further inStreptomyces avermitilis MA5502 by feeding experiments with labeled precursors.13C-NMR analysis of the compounds biosynthesized from [2-13C]acetate, [1,2-13C2]acetate, [3-13C]propionate and [2,3-13C2]propionate confirmed that the aglycone of avermectins is made from seven intact acetate and five propionate units. Feeding experiments with [1-13C]2-methylbutyrate and [1-13C]isobutyrate have shown that 2-methylbutyrate and isobutyrate are immediate precursors of the starter units of the polyketide chains of avermectin ‘a’ and ‘b’ components, respectively. The3H/14C doublelabeling experiments suggest that the two oleandrose moieties are derived from glucose.

Biotransformation of taxol

Abstract Bioconversion of taxol/cephalomannine by Streptomyces sp. MA 7065 resulted in hydroxylation on the 10-acetyl methyl group in 60% yield and on the benzene ring at the para position of the phenylisoserine side chain in 10% yield. This culture could also hydroxylate the allylic methyl group of the phenylisoserine side chain of cephalomannine quantitatively. All three metabolites were cytotoxic toward human lung, breast and colon tumor cell lines.

Antifungal spectrum, in vivo efficacy, and structure-activity relationship of ilicicolin h.

Ilicicolin H is a polyketide-nonribosomal peptide synthase (NRPS)-natural product isolated from Gliocadium roseum, which exhibits potent and broad spectrum antifungal activity, with sub-μg/mL MICs against Candida spp., Aspergillus fumigatus, and Cryptococcus spp. It showed a novel mode of action, potent inhibition (IC50 = 2-3 ng/mL) of the mitochondrial cytochrome bc1 reductase, and over 1000-fold selectivity relative to rat liver cytochrome bc1 reductase. Ilicicolin H exhibited in vivo efficacy in murine models of Candida albicans and Cryptococcus neoformans infections, but efficacy may have been limited by high plasma protein binding. Systematic structural modification of ilicicolin H was undertaken to understand the structural requirement for the antifungal activity. The details of the biological activity of ilicicolin H and structural modification of some of the key parts of the molecule and resulting activity of the derivatives are discussed. These data suggest that the β-keto group is critical for the antifungal activity.

Microbial hydroxylation of rustmicin (galbonolide A) and galbonolide B, two antifungal products produced by Micromonospora sp.

In order to synthesize derivatives of galbonolide A and B with improved chemical stability and antifungal activity profiles, a panel of microorganisms consisting of various species of actinomycetes and fungi were screened. As a result, an organism, Streptomyces halstedii, was identified, which catalyzed the formation of two polar compounds, one from each of the galbonolides. The synthesis and the relative stability of these compounds were optimized by using washed cells, which had been prepared from the transforming organism, and reaction conditions, which included the usage of MES buffer with pH adjusted to 5.5 and incubation at 27°C. Under conditions thus established, two compounds were isolated and characterized by a combination of UV, mass, and NMR spectroscopic analysis. The data indicate the synthesis of 21-hydroxy derivatives of galbonolides A and B with reduced but still significant anti-fungal activity when compared to the parent galbonolides.

Microbial transformation of immunosuppressive compounds III. Glucosylation of immunomycin (FR 900520) and FK 506 byBacillus subtilis ATCC 55060

SummaryThe regiospecific glucosylation of FK 506 and immunomycin (FR 900520) at the 24-hydroxy position was performed using resting cells ofBacillus subtilis ATCC 55060. 24-Glucopyranosyl FK 506 and 24-glucopyranosyl immunomycin were isolated by methylene chloride extraction and purification using reverse phase HPLC. The metabolite structures were established using spectroscopic techniques including MS and NMR. The glucose conjugate was further confirmed by chemical degradation. Enzymatic glucosylation was demonstrated using cell-free extracts derived fromBacillus subtilis ATCC 55060. The 24-glucosyltransferase, which appears UDP-glucose dependent, was solubilized from cell membranes by treatment with 0.1% Nonidet P-40 detergent. The optimal conditions for assay of the enzyme have been determined.

A cofactor for thienamycin biosynthesis produced by a blocked mutant ofStreptomyces cattleya

Thienamycin (THM; Fig. 1), the first naturally occurring [3-1actam antibiotic discovered to possess the novel carbapenem ring system, was isolated from the culture filtrate of Streptomyces cattleya [1]. The biosynthesis of THM was investigated by Williamson et al. [3]. They established that the molecule is made up of cysteine, glutamate, acetate, and two methyl groups of methionine. The methyl groups were found to give rise to the hydroxyethyl side-chain of THM. Our intent was to further explore the biosynthetic route of THM using blocked mutants. During our studies we have isolated non-THM-producing strains by NTG, NMU, EMS and/or UV treatment of a THM-producing strain of S. cattleya. Applying an agar strip method [2], these nonproducers were tested for cosynthetic ability and classified into five groups. The complementation patterns between each group are shown in Table 1. In this communication, we wish to report the isolation of a bioconvertible substance produced by a Class I mutant. The blocked mutants used in these studies were MA 5952 (Class I) and M A 5953 (Class XVI), both of which lacked the ability to elaborate THM. However, MA 5953 growing in the presence of previously prepared MA 5952 filtrate was able to produce THM. Utilizing this response as an analytical tool, we were able to determine the presence of the material produced by MA 5952 that allowed MA

Microbial transformation of N-heptyl physostigmine, a semisynthetic alkaloid inhibitor of cholinesterase.

The microbiological transformation ofN-heptyl physostigmine (L-693, 487) (1), a semisynthetic physostigmine cholinesterase inhibitor, was investigated usingVerticillium lecanii MF 5713 (ATCC 74148),Acremonium sp MF 5723 (ATCC 74164) andActinoplanes sp MA 6559 (ATCC 53771). Nine microbial metabolites (2–10) of 1 were isolated and purified using reversed-phase HPLC. The structures of the metabolites were established using spectroscopic techniques including MS and NMR. Some of the microbial metabolites were identical to metabolites present in urine of a dog treated with 1.