The Isolation and Structure Elucidation of Spirotetronate Lobophorins A, B, and H8 from Streptomyces sp. CB09030 and Their Biosynthetic Gene Cluster

Lobophorins (LOBs) are a growing family of spirotetronate natural products with significant cytotoxicity, anti-inflammatory, and antibacterial activities. Herein, we report the transwell-based discovery of Streptomyces sp. CB09030 from a panel of 16 in-house Streptomyces strains, which has significant anti-mycobacterial activity and produces LOB A (1), LOB B (2), and LOB H8 (3). Genome sequencing and bioinformatic analyses revealed the potential biosynthetic gene cluster (BGC) for 1-3, which is highly homologous with the reported BGCs for LOBs. However, the glycosyltransferase LobG1 in S. sp. CB09030 has certain point mutations compared to the reported LobG1. Finally, LOB analogue 4 (O-β-D-kijanosyl-(1→17)-kijanolide) was obtained through an acid-catalyzed hydrolysis of 2. Compounds 1-4 showed different antibacterial activities against Mycobacterium smegmatis and Bacillus subtilis, which revealed the varying roles of different sugars in their antibacterial activities.

Antibacterial Spirotetronate Polyketides from an Actinomadura sp. Strain A30804

Large scale cultivation and chemical investigation of an extract obtained from Actimonadura sp. resulted in the identification of six previously undescribed spirotetronates (pyrrolosporin B and decatromicins C-G; 7-12), along with six known congeners, namely decatromicins A-B (1-2), BE-45722B-D (3-5), and pyrrolosporin A (6). The chemical structures of compounds 1-12 were characterized via comparison with previously reported data and analysis of 1D/2D NMR and MS data. The structures of all new compounds were highly related to the spirotetronate type compounds, decatromicin and pyrrolosporin, with variations in the substituents on the pyrrole and aglycone moieties. All compounds were evaluated for antibacterial activity against the Gram-negative bacteria, Acinetobacter baumannii and Gram-positive bacteria, Staphylococcus aureus and were investigated for their cytotoxicity against the human cancer cell line A549. Of these, decatromicin B (2), BE-45722B (3), and pyrrolosporin B (7) exhibited potent antibacterial activities against both Gram-positive (MIC₉₀ between 1-3 μM) and Gram-negative bacteria (MIC₉₀ values ranging from 12-36 μM) with weak or no cytotoxic activity against A549 cells.

Nomimicins B-D, new tetronate-class polyketides from a marine-derived actinomycete of the genus Actinomadura

Three new tetronate-class polyketides, nomimicins B, C, and D, along with nomimicin, hereafter named nomimicin A, were isolated from the culture extract of Actinomadura sp. AKA43 collected from floating particles in the deep-sea water of Sagami Bay, Japan. The structures of nomimicins B, C, and D were elucidated through the interpretation of NMR and MS analytical data, and the absolute configuration was determined by combination of NOESY/ROESY and ECD analyses. Nomimicins B, C, and D showed antimicrobial activity against Gram-positive bacteria, Kocuria rhizophila and Bacillus subtilis, with MIC values in the range of 6.5 to 12.5 μg/mL. Nomimicins B and C also displayed cytotoxicity against P388 murine leukemia cells with IC₅₀ values of 33 and 89 μM, respectively.

Marine Spirotetronates: Biosynthetic Edifices That Inspire Drug Discovery

Spirotetronates are actinomyces-derived polyketides that possess complex structures and exhibit potent and unexplored bioactivities. Due to their anticancer and antimicrobial properties, they have potential as drug hits and deserve further study. In particular, abyssomicin C and tetrocarcin A have shown significant promise against antibiotic-resistant S. aureus and tuberculosis, as well as for the treatment of various lymphomas and solid tumors. Improved synthetic routes to these compounds, particularly the class II spirotetronates, are needed to access sufficient quantities for structure optimization and clinical applications.

In vitro antiviral activity of spirotetronate compounds against dengue virus serotype 2

Spirotetronate compounds are polyketide secondary metabolites with diverse biological functions, such as antibacterial, antitumor and antiviral activities. Three pure spirotetronate compounds (2EPS-A, -B, -C) isolated from Actinomadura strain 2EPS showed inhibitory activity against dengue virus serotype 2 (DENV-2). 2EPS-A, -B and -C demonstrated the LC₅₀ values of 11.6, 27.5 and 12.0 μg/ml, respectively, in a test of cytotoxicity to Vero cells. The least cytotoxic, 2EPS-B, was further analyzed for its impact on viral propagation in a cell-based replication assay. At a concentration of 6.25 μg/ml, it could reduce the DENV-2 infection in Vero cells by about 94% when cells infected with DENV-2 were exposed to 2EPS-B, whereas direct treatment of DENV-2 with 2EPS-B at the same concentration prior to subsequent infection to Vero cell yielded no inhibition. 2EPS-A, -B an -C showed strong DENV-2 NS2B-NS3 protease inhibition in an in vitro assay, with IC₅₀ values of 1.94 ± 0.18, 1.47 ± 0.15 and 2.51 ± 0.21 μg/ml, respectively. Therefore, the spirotetronate compounds appear to prevent viral replication and viral assembly by inhibition of the viral protease.

Review on Abyssomicins: Inhibitors of the Chorismate Pathway and Folate Biosynthesis

Antifolates targeting folate biosynthesis within the shikimate-chorismate-folate metabolic pathway are ideal and selective antimicrobials, since higher eukaryotes lack this pathway and rely on an exogenous source of folate. Resistance to the available antifolates, inhibiting the folate pathway, underlines the need for novel antibiotic scaffolds and molecular targets. While para-aminobenzoic acid synthesis within the chorismate pathway constitutes a novel molecular target for antifolates, abyssomicins are its first known natural inhibitors. This review describes the abyssomicin family, a novel spirotetronate polyketide Class I antimicrobial. It summarizes synthetic and biological studies, structural, biosynthetic, and biological properties of the abyssomicin family members. This paper aims to explain their molecular target, mechanism of action, structure⁻activity relationship, and to explore their biological and pharmacological potential. Thirty-two natural abyssomicins and numerous synthetic analogues have been reported. The biological activity of abyssomicins includes their antimicrobial activity against Gram-positive bacteria and mycobacteria, antitumor properties, latent human immunodeficiency virus (HIV) reactivator, anti-HIV and HIV replication inducer properties. Their antimalarial properties have not been explored yet. Future analoging programs using the structure⁻activity relationship data and synthetic approaches may provide a novel abyssomicin structure that is active and devoid of cytotoxicity. Abyssomicin J and atrop-o-benzyl-desmethylabyssomicin C constitute promising candidates for such programs.