Spiropiperidyl rifabutins: expanded in vitro testing against ESKAPE pathogens and select bacterial biofilms

The expanded microbiological evaluation of a series of rifastures, novel spiropiperidyl rifamycin derivatives, against clinically relevant ESKAPE bacteria has identified several analogs with promising in vitro bioactivities against antibiotic-resistant strains of Enterococcus faecium and Staphylococcus aureus. Thirteen of the rifastures displayed minimum inhibitory concentrations (MICs) below 1 µg/ml against the methicillin- and vancomycin-resistant forms of S. aureus and E. faecium (MRSA, VRSA, VRE). Aryl-substituted rifastures 1, 11, and 12 offered the greatest bioactivity, with MICs reaching ≤0.063 µg ml^-1 for these human pathogens. Further analysis indicates that diphenyl rifasture 1 had greater antibiofilm activity against S. aureus and lower cytotoxicity in mammalian HEK cells than rifabutin.