Antimicrobial Activity of a Library of Thioxanthones and Their Potential as Efflux Pump Inhibitors

Abstract

The overexpression of efflux pumps is one of the causes of multidrug resistance, which leads to the inefficacy of drugs. This plays a pivotal role in antimicrobial resistance, and the most notable pumps are the AcrAB-TolC system (AcrB belongs to the resistance-nodulation-division family) and the NorA, from the major facilitator superfamily. In bacteria, these structures can also favor virulence and adaptation mechanisms, such as quorum-sensing and the formation of biofilm. In this study, the design and synthesis of a library of thioxanthones as potential efflux pump inhibitors are described. The thioxanthone derivatives were investigated for their antibacterial activity and inhibition of efflux pumps, biofilm formation, and quorum-sensing. The compounds were also studied for their potential to interact with P-glycoprotein (P-gp, ABCB1), an efflux pump present in mammalian cells, and for their cytotoxicity in both mouse fibroblasts and human Caco-2 cells. The results concerning the real-time ethidium bromide accumulation may suggest a potential bacterial efflux pump inhibition, which has not yet been reported for thioxanthones. Moreover, in vitro studies in human cells demonstrated a lack of cytotoxicity for concentrations up to 20 µM in Caco-2 cells, with some derivatives also showing potential for P-gp modulation.

Enantioselectivity of Chiral Derivatives of Xanthones in Virulence Effects of Resistant Bacteria

“…In the interactions of (R,R)-8 in NorA, it can be seen that the substituent in the C3 position was predicted to play an important role in binding to this portion of the efflux system. The amide establishes a hydrogen bond with Glu-222, as did structurally-related thioxanthones previously tested [53], and the hydroxyl moiety can do the same with Phe-288, as can be seen in Figure 3A. Besides, this compound seems to fit perfectly in a pocket present in this site (Figure 3B).…”

“…Compound (R)-17 was predicted to interact with several residues previously described for xanthones and thioxanthones, such as Gln-89, Gln-176, and Gly-619 [32,53]. Additionally, this compound also interacts with Ser-180, Gln-181, and Gln-273 (Figure 4C).…”

“…The bacteria chosen for this assay were the Grampositive S. aureus 272123, a clinical strain resistant to methicillin and ofloxacin, and the Gram-negative Salmonella Typhimurium SE03, with the acrA gene deleted, as this was predicted to be the portion of the efflux system to which the compounds would present the least affinity. These models were chosen to compare the activity of the derivatives herein present with the previously described activity of structurally related xanthones and thioxanthones [32,53].…”

“…Lastly, compound (R,S)-18 is predicted to establish hydrogen bonds mostly with residues previously described in other studies with (thio)xanthones, such as Thr-87. Gln-176 and Arg-620 [32,53]. The hydroxyl present in the compound can establish an additional hydrogen bond with Lys-292 (Figure 4D).…”

Metabolites from Marine-Derived Fungi as Potential Antimicrobial Adjuvants

“…For the Gram-positive, S. aureus 272123, a clinical oxacillin- and methicillin-resistant strain, was used. This strain was used to compare the activity of the isolated compounds with that of derivatives previously tested in the same system [ 35 , 36 ]. However, previous studies with this strain suggested that the expression of the norA gene, a core gene of S. aureus , remained unchanged [ 37 ].…”