Bischloroanthrabenzoxocinone

Type II fatty acid synthesis (FASII) is essential to bacterial cell viability and is a promising target for the development of novel antibiotics. In the past decade, a few inhibitors have been identified for this pathway, but none of them lend themselves to drug development. To find better inhibitors that are potential drug candidates, we developed a high throughput assay that identifies inhibitors simultaneously against multiple targets within the FASII pathway of most bacterial pathogens. We demonstrated that the inverse t(1/2) value of the FASII enzyme-catalyzed reaction gives a measure of FASII activity. The Km values of octanoyl-CoA and lauroyl-CoA were determined to be 1.1 +/- 0.3 and 10 +/- 2.7 microM in Staphylococcus aureus and Bacillus subtilis, respectively. The effects of free metals and reducing agents on enzyme activity showed an inhibition hierarchy of Zn2+ > Ca2+ > Mn2+ > Mg2+; no inhibition was found with beta-mercaptoethanol or dithiothreitol. We used this assay to screen the natural product libraries and isolated an inhibitor, bischloroanthrabenzoxocinone (BABX) with a new structure. BABX showed IC50 values of 11.4 and 35.3 microg/ml in the S. aureus and Escherichia coli FASII assays, respectively, and good antibacterial activities against S. aureus and permeable E. coli strains with minimum inhibitory concentrations ranging from 0.2 to 0.4 microg/ml. Furthermore, the effectiveness, selectivity, and the in vitro and in vivo correlations of BABX as well as other fatty acid inhibitors were elucidated, which will aid in future drug discovery.

Liver X receptors (LXR) are nuclear hormone receptors that play a critical role in cholesterol homeostasis. They regulate the expression of the ABCA1 gene, which mediates the efflux of cholesterol out of cells. LXR agonists are expected to increase cholesterol efflux, lower LDL, and raise HDL levels. Screening of a natural product library of microbial extracts using a LXR-SPA binding assay and bioassay-guided fractionation of an active extract of a Streptomyces sp. (MA6657) led to the discovery of two new hexacyclic aromatic ketones, (-)-anthrabenzoxocinone [(-)-ABX (1)], an enantiomer of BE-24566B, and (-)-bischloroanthrabenzoxocinone [(-)-BABX (2)]. The IC50 values of LXRalpha-SPA binding are 2 microM for (-)-ABX and 10 microM for (-)-BABX. This extract was also found to inhibit type II fatty acid synthesis, and its active component, (-)-BABX, was responsible for the majority of the inhibition. All three compounds showed good Gram-positive antibacterial activity (MIC 0.5-2 microg/mL). Details of the isolation, structure elucidation, LXR ligand binding, antibacterial activity, and selectivity of inhibition of 1 and 2 are described.