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Repurposing drugs provides a new approach to the fight against multidrug-resistant (MDR) bacteria. We have reported that three major tamoxifen metabolites,  N -desmethyltamoxifen (DTAM), 4-hydroxytamoxifen (HTAM), and endoxifen (ENDX), presented bactericidal activity against  Acinetobacter baumannii  and  Escherichia coli . Here, we aimed to analyze the activity of a mixture of the three tamoxifen metabolites against methicillin-resistant  Staphylococcus epidermidis  (MRSE) and  Enterococcus  species. MRSE ( n  = 17) and  Enterococcus  species ( Enterococcus faecalis   n  = 8 and  Enterococcus faecium   n  = 10) strains were used. MIC of the mixture of DTAM, HTAM, and ENDX and that of vancomycin were determined by microdilution assay. The bactericidal activity of the three metabolites together and of vancomycin against MRSE (SE385 and SE742) and vancomycin-resistant  E. faecalis  (EVR1 and EVR2) strains was determined by time-kill curve assays. Finally, changes in membrane permeability of SE742 and EVR1 strains were analyzed using fluorescence assays. MIC 90  of tamoxifen metabolites was 1 mg/liter for MRSE strains and 2 mg/liter for  E. faecalis  and  E. faecium  strains. In the time-killing assays, tamoxifen metabolites mixture showed bactericidal activity at 4× MIC for MRSE (SE385 and SE742) and at 2× MIC and 4× MIC for  E. faecalis  (EVR1 and EVR2) strains, respectively. SE385 and EVR2 strains treated with the tamoxifen metabolites mixture presented higher membrane permeabilization. Altogether, these results showed that tamoxifen metabolites presented antibacterial activity against MRSE and vancomycin-resistant  E. faecalis , suggesting that tamoxifen metabolites might increase the arsenal of drug treatments against these bacterial pathogens.   IMPORTANCE  The development of new antimicrobial therapeutic strategies requires immediate attention to avoid the tens of millions of deaths predicted to occur by 2050 as a result of MDR bacterial infections. In this study, we assessed the antibacterial activity of three major tamoxifen metabolites,  N -desmethyltamoxifen (DTAM), 4-hydroxytamoxifen (HTAM), and endoxifen (ENDX), against methicillin-resistant  Staphylococcus epidermidis  (MRSE) and  Enterococcus  spp. ( E. faecalis  and  E. faecium ). We found that the tamoxifen metabolites have antibacterial activity against MRSE,  E. faecalis , and  E. faecium  strains by presenting MIC 90  between 1 and 2 mg/liter and bactericidal activity over 24 h. In addition, this antibacterial activity is paralleled by an increased membrane permeability of these strains. Our results showed that tamoxifen metabolites might be potentially used as a therapeutic alternative when treating MRSE and  E. faecalis  strains in an animal model of infection.

microbiology spectrum

Repurposing of the Tamoxifen Metabolites to Treat Methicillin-Resistant Staphylococcus epidermidis and Vancomycin-Resistant Enterococcus faecalis Infections