Characterization of synergy between ofloxacin, ceftazidime, and tobramycin against Pseudomonas aeruginosa

The purposes of this study were to investigate the potential for synergy between ceftazidime, tobramycin, and ofloxacin against two clinical isolates (PSA 9258, and PSA 9263) of Pseudomonas aeruginosa utilizing time-concentration-kill curves. A pharmacodynamic model was used to simulate one-compartment pharmacokinetics for single-, double-, or triple-drug combinations utilizing two different elimination half-lives (T-1/2) Each duplicate experiment was conducted for 24 h in cation-supplemented Mueller-Hinton broth. Synergy, indifference, and antagonism were defined as reductions of greater than or equal to 2.0, greater than or equal to 0 to less than or equal to 2, or less than or equal to 0 in mean log(10) CFU/ml (CFU = colony-forming units) in bacterial counts at any time point during the 24-hour experiment, respectively. Time-concentration-kill curve studies simulating a peak concentration (CP) = 6 mu g/ml and T-1/2 = 5.5 h for ofloxacin combined with ceftazidime (CP = 80 mu g/ml; T-1/2 = 2 h) resulted in 2.18 and 1.81 log CFU/ml mean decreases in bacterial counts for PSA 9258 and 9263, respectively. Simulated ofloxacin pharmacokinetic parameters (CP = 6 mu g/ml; T-1/2 = 5.5 h) combined with tobramycin (CP = 8 mu g/ml; T-1/2 = 2 h) produced 2.26 and 0.6 log CFU/ml mean reductions in bacterial counts for PSA 9258 and 9263! respectively. Time-concentration-kill curve results were inconsistent with checkerboard synergy experiments which indicated antagonism for ofloxacin/tobramycin combinations (fractional inhibitory concentrations = 2.0/2.5) and indifference for ofloxacin/ceftazidime combinations (fractional inhibitory concentrations = 1.0/1.0). In secondary experiments, tobramycin (T-1/2 = 2 h) and ceftazidime (T-1/2 = 2 h) at concentrations of one fourth and equal to the minimum inhibitory concentration were combined with ofloxacin (CP = 6 mu g/ml; T-1/2 = 5.5 h). When ofloxacin was combined with tobramycin equivalent to the minimum inhibitory concentration, mean reductions in bacterial counts were 3.74 and 5.59 CFU/ml. These results suggest that an enhanced antipseudomonal activity may result by the combination of clinically achievable concentrations of ofloxacin with minimum inhibitory concentration equivalent concentrations of tobramycin.