Torezolid phosphate (TR-701) is the
phosphate monoester
prodrug of the
oxazolidinone TR-700 which demonstrates potent in vitro activity against Gram-positive bacteria, including
methicillin-susceptible Staphylococcus aureus (MSSA) and methicillin-resistant S. aureus (MRSA). The pharmacodynamics of
TR-701 or
TR-700 (
TR-701/700) against S. aureus is incompletely defined. Single-dose pharmacokinetic studies were conducted in mice for
TR-701/700. Forty-eight-hour dose range and 24-hour dose fractionation studies were conducted in a neutropenic mouse thigh model of S. aureus
infection using MRSA ATCC 33591 to identify the dose and schedule of administration of
TR-701/700 that was linked with optimized antimicrobial effect. Additional dose range studies compared the efficacies of
TR-701/700 and
linezolid for one MSSA strain and one community-associated MRSA strain. In dose range studies,
TR-701/700 was equally bactericidal against MSSA and MRSA. Mean doses of 37.6 and 66.9 mg/kg of
body weight/day of
TR-701/700 resulted in stasis and 1 log CFU/g decreases in bacterial densities, respectively, at 24 h, and mean doses of 35.3, 46.6, and 71.1 mg/kg/day resulted in stasis and 1 and 2 log CFU/g reductions, respectively, at 48 h.
Linezolid administered at doses as high as 150 mg/kg/day did not achieve stasis at either time point. Dose fractionation studies demonstrated that the area under the concentration-time curve over 24 h in the steady state divided by the MIC (AUC/MIC ratio) was the pharmacodynamic index for
TR-701/700 that was linked with efficacy.
TR-701/700 was highly active against MSSA and MRSA, in vivo, and was substantially more efficacious than
linezolid, although
linezolid's top exposure has half the human exposure. Dose fractionation studies showed that AUC/MIC was the pharmacodynamic index linked with efficacy, indicating that once-daily dosing in humans is feasible.