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Pharmacodynamics of RWJ-54428 against Staphylococcus aureus, Streptococcus pneumoniae, and Enterococcus faecalis in a Neutropenic Mouse Thigh Infection Model

David C. Griffith,^* David Rodriguez, Erik Corcoran, and Michael N. Dudley

Microcide Pharmaceuticals, Inc. Mountain View, California 94043

Received 15 June 2007/ Returned for modification 29 August 2007/ Accepted 17 October 2007

RWJ-54428 (also known as MC-02,479) is a new cephalosporin with promising activity against gram-positive bacteria. The pharmacodynamics (PDs) of RWJ-54428 against Staphylococcus aureus, Streptococcus pneumoniae, and Enterococcus faecalis were studied in a neutropenic mouse thigh infection model. The RWJ-54428 MICs ranged from 0.25 to 1 mg/liter. Mice with ca. 10⁶ CFU/thigh at the initiation of therapy were treated intraperitoneally with RWJ-54428 at doses that ranged from 3 to 1,200 mg/kg of body weight/day (in 2, 3, 4, 6, or 12 divided doses) for 24 h. The maximal reductions in bacterial counts in thigh tissues at 24 h for the methicillin-resistant S. aureus, penicillin-resistant S. pneumoniae, and E. faecalis strains were –2.8, –3.8, and –1.7 log₁₀ CFU/thigh, respectively. The percentage of a 24-h dosing interval that the unbound serum RWJ-54428 concentrations exceeded the MIC (fT_>MIC) was the pharmacokinetic (PK)-PD parameter that best described the efficacy of RWJ-54428. The fT_>MICs for a bacteriostatic effect (no net change in the numbers of CFU/thigh over 24 h) ranged from 14 to 20% for staphylococci and streptococci; for maximal reductions in the numbers of CFU/thigh, the fT_>MICs ranged from 22 to 36% for these strains. For E. faecalis, the ranges of fT_>MICs for static and maximal effects were 30 to 46% and 55 to 60%, respectively. These data show that treatment with RWJ-54428 results in marked antibacterial effects in vivo, with the PK-PD parameters for efficacy being comparable to those for the efficacy of penicillins and carbapenems active against staphylococci and pneumococci.

Published ahead of print on 22 October 2007.

Present address: Medarex, Inc., South San Francisco, CA.

Present address: Imclone Systems, 180 Varick St., New York, NY 10014.