In Vitro Activity of Ceftolozane Alone and in Combination with Tazobactam against Extended-Spectrum-β-Lactamase-Harboring Enterobacteriaceae.

Ceftolozane, formally CXA-101, is a new antipseudomonal cephalosporin that is also active in vitro against Enterobacteriaceae but is vulnerable to extended-spectrum β-lactamases (ESBLs). The addition of tazobactam is intended to broaden coverage to most ESBL-producing Escherichia coli and Klebsiella pneumonia as well as other Enterobacteriaceae. The in vitro activities of ceftolozane-tazobactam combinations against 67 clinically and molecularly characterized ESBL-producing isolates were examined by checkerboard MIC testing to evaluate their potential clinical feasibility and to assess the optimal tazobactam concentrations to be used in MIC determinations of ceftolozane. Isolates included those from E. coli (n = 32), K. pneumoniae (n = 19), Enterobacter cloacae (n = 15), and Citrobacter freundii (n = 1). Checkerboard experiments were performed to study interactions over the range of 0.008 to 64 mg/liter ceftolozane and 0.063 to 32 mg/liter tazobactam using 2-fold-dilution series. The MIC50 and MIC90 of ceftolozane alone for all isolates were 16 and ≥64 mg/liter, respectively. Increasing concentrations of tazobactam resulted in decreasing MICs of ceftolozane. The 50th and 90th percentile concentrations of tazobactam required to reduce the MIC of ceftolozane to 8 mg/liter for all organisms in this ESBL collection were 0.5 and 4 mg/liter, respectively. For E. coli, K. pneumoniae, and E. cloacae, these values were 0.5 and 2, 1 and 16, and 0.5 and 4 mg/liter, respectively. When combined with a fixed amount of 4 mg/liter tazobactam (current CLSI concentration used for susceptibility testing), 90% of the isolates would have an MIC of ≤4 mg/liter. The combination ceftolozane-tazobactam is a promising alternative option for treating infections due to ESBL-harboring isolates.