Evaluation of Several Dosing Regimens of Cefepime, with Various Simulations of Renal Function, against Clinical Isolates of Pseudomonas aeruginosa in a Pharmacodynamic Infection Model

The objectives of this study were as follows: (i) to examine the killing activity of 2-g doses of cefepime against two clinical isolates (mucoid and nonmucoid) of Pseudomonas aeruginosa in a pharmacodynamic in vitro infection model, (ii) to compare the percentage of time above the MIC (T > MIC) for each of the regimens against P. aeruginosa, and (iii) to evaluate the area under the bactericidal curve for each regimen. Cefepime was administered at intervals of 8, 12, and 24 h with and without tobramycin, and two different levels of renal function were simulated: normal (creatinine clearance [CL_CR] = 90 ml/min) and decreased (CR_CL = 60 ml/min). Also, the killing activity of cefepime with and without tobramycin was compared to the killing activity of ceftazidime (2 g every 8 h) with and without tobramycin. The T > MIC was 100% in the central chamber except for the regimen in which cefepime was administered every 12 h and the CL_CR was 90 ml/min, which provided concentrations above the MIC for 92% of the dosing interval against the C31 (mucoid; MIC of cefepime, 4 μg/ml) isolate and for 75% of the interval against the C34 (nonmucoid; MIC of cefepime, 8 μg/ml) isolate. All cefepime and ceftazidime monotherapy simulations resulted in 99.9% killing of the nonmucoid isolate within 4 to 8 h and within 4 to 6 h, respectively. Against the mucoid isolate, 99.9% killing was achieved only with combination therapy. The results of this study indicate that cefepime dosed at 2 g every 12 h under conditions of normal renal function and every 24 h with decreased creatinine clearance (60 ml/min) is effective both as monotherapy and in combination therapy against a nonmucoid strain of P. aeruginosa. With cefepime MICs of 4 and 8 μg/ml, the single-agent regimens provided T > MIC values in the central chamber for 92 and ≥75% of the dosing interval against the mucoid and nonmucoid isolates, respectively. Cefepime dosed at 2 g every 12 h, with a creatinine clearance of 90 ml/min, and every 24 h, with a creatinine clearance of 60 ml/min, resulted in killing activity equivalent to that of ceftazidime dosed at 2 g every 8 h. None of the monotherapies provided adequate killing of the mucoid strain of P. aeruginosa despite drug concentrations being above the MIC for ≥92% of all dosing intervals. Finally, combination therapy with tobramycin and either cefepime or ceftazidime enhanced the killing of both the mucoid and nonmucoid P. aeruginosa isolates.