TR-700 in vitro activity against and resistance mutation frequencies among Gram-positive pathogens

TR-700, the active component of the oxazolidinone prodrug TR-701, has demonstrated potent activity against numerous Gram-positive species. In this study, single-step mutation frequencies, passaging and the activity of TR-700 were tested against a worldwide collection of linezolid-non-susceptible organisms and matched controls. One hundred and twenty linezolid-non-susceptible and 120 controls matched by genus/species, geographic origin, site of infection and time were susceptibility tested by reference broth microdilution methods. Species of isolates were: Enterococcus faecalis (16 linezolid non-susceptible/16 wild-type strains); Enterococcus faecium (55/55), Staphylococcus aureus (8/8); coagulase-negative staphylococci (at least 7 spp., 40/40) and viridans group streptococci (2 spp., 1/1). 23S rRNA target mutations or cfr genes were detected by PCR and sequencing. Among linezolid-non-susceptible strains, the resistance mechanisms were G2576T (109), cfr (4) and unknown (7), with strains originating from Europe, Far East and North and South America. Most strains were multidrug-resistant and cfr isolates exhibited co-resistance to phenicols, clindamycin, linezolid, pleuromutilins and streptogramin B. TR-700 MIC values, regardless of species, were 4-32-fold lower than those of linezolid. TR-700 MIC results were <= 4, <= 8 or <= 16 mg/L for 88%, 96% and > 99% of linezolid-non-susceptible strains, respectively. Spontaneous single-step mutations were undetected (< 1.1x10(-9)) and 14 day passaging studies produced modest TR-700 MIC elevations compared with linezolid controls. TR-700 exhibited enhanced activity against linezolid-non-susceptible and wild-type control strains of Gram-positive cocci. A significant number (nearly 90%) of linezolid-non-susceptible strains were inhibited by potentially achievable levels (<= 4 mg/L) of TR-700. All strains with the emerging cfr-mediated resistance determinant had TR-700 MIC results at <= 8 mg/L.