Flavonolignan and flavone inhibitors of a Staphylococcus aureus multidrug resistance (MDR) efflux pump: synthesis and structure activity relationships

Abstract

Multidrug resistance (MDR) represents an increasing problem in the treatment of bacterial infections and cancer. It often appears after prolonged exposure of cells to a single drug and is often characterized by resistance to a series of structurally unrelated compounds. One important resistance mechanism involves drug depletion in cells by transmembrane efflux proteins, for example P-glycoprotein (P-gp) in mammalian cells, Bmr in Bacillus subtilis, and NorA in Staphylococcus aureus. There has been extensive work in the field of mammalian tumor research and a multitude of structure-activity relationship (SAR) studies of P-gp inhibitors have been reported. The identification of bacterial MDR efflux inhibitors is in its infancy and only a limited number have been identified. A potent inhibitor of the NorA efflux pump in the gram-positive bacteria S. aureus was previously isolated from Berberis fremontii and identified as the flavonolignan 5'-methoxyhydnocarpin. This flavonolignan was also reported in the mid 1970's as a minor constituent of Hydnocarpus wightiana, but the authors were unable to report a definitive regiochemical assignment of the 1,4-benzodioxane substituents due to the low resolution and lack of two-dimensional NMR techniques available at that time. A regiocontrolled total synthesis of the flavonolignan was performed and it was shown that the original regiochemical assignments were incorrect. The flavonolignan was renamed 5'-methoxyhydnocarpin-D to denote the correct regiochemistry. Because of the high activity of 5-methoxyhydoncarpin-D, a SAR study of structurally reminiscent synthetic flavonolignans was performed to see if a more potent inhibitor of the NorA efflux pump could be found. Numerous inhibitors were synthesized using a Ag2CO3 mediated biomimetic coupling of various flavonoids and hydroxycinnamyl alcohols. Hydnocarpin-D, 5-deoxyhydnocarpin-D, 5,7-deoxyhydnocarpin-D, and 5-deoxyscutelIaprostin-A proved to be more potent inhibitors of the NorA efflux pump than the natural product. It was also shown that some small lipophilic ether derivatives of 4'-hydroxyflavone were also more potent than 5'-methoxyhydoncarpin-D. Preliminary results from bioassays investigating MDR efflux pump inhibitors of the gram-negative bacteria Escherichia coli and Pseudomonus aeruginosa showed that coumarins may potentiate anthraquinones. Harbouria trachypleura belongs to the Apiaceae, a family of plants known to often contain coumarins. Bioassay results showed that H. trachypleura did not contain the desired inhibitors, but the plant belongs to a monotypic genus and this distinction warranted its further investigation. Two new coumarins, (+)-trachypleuranin-A and (±)-trachypleuranin-B were isolated and identified by spectroscopic and synthetic means.