Time–Kill Analysis of Ceftolozane/Tazobactam Efficacy Against Mucoid Pseudomonas aeruginosa Strains from Cystic Fibrosis Patients

INTRODUCTION: Mucoid Pseudomonas aeruginosa (MP) strains in cystic fibrosis (CF) patients are thought to initiate the chronic infection stage of CF and are associated with pulmonary function decline. OBJECTIVES: The purpose of this study was to assess the susceptibility of MP strains to ceftolozane/tazobactam and the efficacy of ceftolozane/tazobactam against MP strains compared with those for standard-of-care antipseudomonal antibiotics. METHODS: Ten clinical isolates of MP from CF patients were tested for susceptibility with Etest and time–kill analysis with ceftolozane/tazobactam compared with ceftazidime, cefepime, ciprofloxacin, meropenem, tobramycin, and polymyxin B. The physiologic free peak concentrations were used in the time–kill experiments. RESULTS: Ceftolozane/tazobactam minimum inhibitory concentrations ranged from 0.032 to 1.5 mg/L. In the time–kill analysis, the mean starting inoculum for the isolates was 6.29 ± 0.22 log(10) colony forming units (CFU) per milliliter. On average, ceftolozane/tazobactam, cefepime, ciprofloxacin, meropenem, tobramycin, and polymyxin B all demonstrated bactericidal activity. With all isolates taken into account, polymyxin B, tobramycin, meropenem, and ceftolozane/tazobactam 3 g were the most potent, with reductions in inoculum of 5.07 ± 0.45, 4.58 ± 2.2, 4.76 ± 0.71, and 4.17 ± 0.94 log(10) CFU/mL, respectively. Ceftolozane/tazobactam 1.5 g, cefepime, and ciprofloxacin reduced the starting inoculum by 3.74 ± 0.99, 3.42 ± 1.4, and 3.23 ± 2.0 log(10) CFU/mL, respectively. Despite 90% susceptibility, ceftazidime was bactericidal against seven of ten strains, with an average reduction in starting inoculum of 2.91 ± 2.2 log(10) CFU/mL. CONCLUSION: Ceftolozane/tazobactam activity against MP strains derived from CF patients was comparable to that of standard-of-care agents at both the 1.5-g dose and the 3-g dose. Further in vitro modeling and clinical trials are warranted.