In Vitro and In Vivo Activities of Amikacin, Cefepime, Amikacin plus Cefepime, and Imipenem against an SHV-5 Extended-Spectrum β-Lactamase-Producing Klebsiella pneumoniae Strain

Abstract

The in vitro and in vivo effectiveness of amikacin, cefepime, and imipenem was studied using a high inoculum of an extended-spectrum ␤-lactamase-producing Klebsiella pneumoniae strain. An in vitro susceptibility test at the standard inoculum predicted the in vivo outcome of amikacin or imipenem while it did not do so for cefepime due to the inoculum effect.Extended-spectrum ␤-lactamase (ESBL) production is one of the main mechanisms of resistance to ␤-lactam antibiotics among the strains of the family Enterobacteriaceae (14). The therapeutic choices in infections caused by such strains remain limited because of cross-resistance (4). Attempts have been made to compare the activities of ␤-lactams with ␤-lactamase inhibitors, showing inconsistent results (5,20,24,27). Conflicting reports have been published concerning the activities of the broad-spectrum and "fourth generation" cephalosporins with an explanation of the inoculum effect (5,15,27).Our aim was to compare the activities of amikacin, cefepime, amikacin plus cefepime, and imipenem in septic mice infected by an SHV-5 ESBL-producing Klebsiella pneumoniae strain using a high initial inoculum.The SHV-5 ESBL-producing K. pneumoniae strain originated in a premature intensive-care unit (26). Amikacin and cefepime (Bristol-Myers Squibb), imipenem (Merck, Sharp & Dohme), and cisplatin (Ebewe) were used according to the manufacturers' instructions. The MICs and the minimal bactericidal concentrations (MBCs) were determined by the microdilution method with inocula of 10 5 and 10 7 CFU/ml, respectively (20, 21). For the killing curve, the initial bacterial concentration was 8 log 10 CFU/ml. The concentrations of antibiotics chosen were close to the in vivo mean levels in serum: amikacin, 4 g/ml; cefepime, 40 g/ml; amikacin plus cefepime in the same concentrations listed above; and imipenem, 16 g/ml. Synergy was defined as a Ն2-log 10 decrease in the number of CFU per milliliter between the combination and its most active constituent after 24 h (17).Randomly selected male CD-1 mice (30 to 35 g) were used for the pharmacokinetic study, for the determination of blood bacterial counts, and for survival analysis. Each group contained 15 mice. Cisplatin (18 mg/kg of body weight as determined in a pilot study) had been administered by intraperitoneal (i.p.) injection 3 days before infection in order to cause renal impairment (18). The mice were infected i.p. with 10 7 CFU/g; the uninfected group received only cisplatin. Three groups of uninfected mice were used for pharmacokinetic analysis, and they were followed up for 48 h (3,6,9,10,16,19,28). Single doses of amikacin (7.5 mg/kg), cefepime (80 mg/kg), and imipenem (40 mg/kg) were administered i.p. Blood samples were taken 15 and 30 min and 1, 2, and 3 h after drug administration. Antibiotic levels in sera were determined by a paper disk method for imipenem and cefepime with Bacillus subtilis ATCC 6633 and Escherichia coli ATCC 25922, respectively, as indicator organisms on antibiotic medium 1 (Becton Dickinson) (1). The a...

Effects of Inoculum and β-Lactamase Activity in AmpC- and Extended-Spectrum β-Lactamase (ESBL)-Producing Escherichia coli and Klebsiella pneumoniae Clinical Isolates Tested by Using NCCLS ESBL Methodology

“…However, inoculum effects in ESBL-producing Klebsiella spp. have been reported for cefepime both in vitro and in vivo (12,29), and they could be due, at least in part, to the hydrolysis of this drug by ESBLs.…”

“…It has been observed that an increase in inoculum size can be correlated with an increase in ␤-lactam MICs and a decrease in efficacy in vivo (11,12,29,30). Factors that could contribute to these effects are the number, type, and amount of ␤-lactamases; outer membrane permeability; efflux; number and susceptibility of penicillin-binding protein targets; and phase of growth.…”

The Use of Noncarbapenem β-Lactams for the Treatment of Extended-Spectrum β-Lactamase Infections

“…This so called "inoculum effect, " in which drug MICs increase dramatically in the presence of an increased bacterial load despite apparent initial susceptibility, has been observed in both in vitro and animal studies [25][26][27][28][29][30][31]. Highlighting one experience, Burgess and colleagues evaluated cefepime and meropenem against standard-and high-inoculum ESBL Klebsiella pneumoniae infections [32].…”

The effect of different doses of cisplatin on the pharmacokinetic parameters of cefepime in mice

“…In our previous study, human serum levels of other renally excreted drugs, amikacin and imipenem, could be approximated using 18 mg/kg of cisplatin pretreatment and by a slightly more frequent administration than in humans (Szabó et al 2001). We suggest that human serum levels can be achieved in animal experiments with the use of cisplatin pretreatment without labour-intensive procedures.…”