ABSTRACT
OBJECTIVES: Hypermutable Pseudomonas aeruginosa strains are a major challenge in cystic fibrosis. We investigated bacterial killing and resistance emergence for approved ceftazidime and tobramycin regimens, alone and in combination. METHODS: Pseudomonas aeruginosa PAOΔmutS and six hypermutable clinical isolates were examined using 48-h static concentration time-kill (SCTK) studies (inoculum ~107.5 CFU/mL); four strains were also studied in a dynamic in vitro model (IVM) (inoculum ~108 CFU/mL). The IVM simulated concentration-time profiles in epithelial lining fluid following intravenous administration of ceftazidime (3 g/day and 9 g/day continuous infusion), tobramycin (5 mg/kg and 10 mg/kg via 30-min infusion 24-hourly; half-life 3.5 h), and their combinations. Time courses of total and less-susceptible populations were determined. RESULTS: Ceftazidime plus tobramycin demonstrated synergistic killing in SCTK studies for all strains, although to a lesser extent for ceftazidime-resistant strains. In the IVM, ceftazidime and tobramycin monotherapies provided ≤5.4 and ≤3.4 log10 initial killing, respectively; however, re-growth with resistance occurred by 72 h. Against strains susceptible to one or both antibiotics, high-dose combination regimens provided >6 log10 initial killing, which was generally synergistic from 8-24 h, and marked suppression of re-growth and resistance at 72 h. The time course of bacterial density in the IVM was well described by mechanism-based models, enabling Monte Carlo simulations (MCSs) to predict likely effectiveness of the combination in patients. CONCLUSION: Results of the IVM and MCS suggested antibacterial effect depends both on the strain's susceptibility and hypermutability. Further investigation of the combination against hypermutable P. aeruginosa strains is warranted.
