Activity of oxazolidinone TR-700 against linezolid-susceptible and -resistant staphylococci and enterococci

Background: TR-700 is the active moiety of TR-701 (DA-7157), a new oral/intravenous oxazolidinone prodrug. We examined its activity against linezolid-susceptible and -resistant staphylococci and enterococci. Methods: MICs were determined by the CLSI agar dilution method. Results: MICs of TR-700 were tightly clustered around 0.5 mg/L for linezolid-susceptible staphylococci and enterococci compared with 2 mg/L for linezolid. MICs for 52 linezolid-resistant isolates were raised, but only exceeded 4 mg/L for two Enterococcus faecium isolates homozygous for the G2576T 23S rRNA mutation and for one with an unknown mechanism of linezolid resistance. Conclusions: TR-700 is 4- to 16-fold more active than linezolid and overcame most linezolid resistance in vitro.