Impact of meropenem in combination with tobramycin in a murine model of Pseudomonas aeruginosa pneumonia

Pseudomonas aeruginosa pneumonia remains a difficult therapeutic problem. Optimal doses and modes of administration of single agents often do not result in acceptable outcomes. Further, emergence of resistance occurs frequently in this setting with single-agent chemotherapy. The purpose of these experiments was to evaluate combination chemotherapy with meropenem plus tobramycin for P. aeruginosa in a murine pneumonia model. Neutropenia was induced by cyclophosphamide. Pharmacokinetics of meropenem and tobramycin were determined using a population pharmacokinetic approach. Both drugs were given at 4-h intervals. Meropenem was administered as total daily doses of 30 to 600 mg/kg of body weight, while tobramycin doses ranged from 50 to 400 mg/kg. Combination therapy evaluated all combinations of 50, 100, and 150 mg/kg/day of tobramycin doses with 60 or 300 mg/kg/day of meropenem. Total and drug-resistant organisms were enumerated. Meropenem alone had a near-maximal effect at 60 mg/kg/day (3.18 log10 [CFU/g] kill from stasis). The time > MIC in epithelial lining fluid (ELF) at this dose was 35.25% of 24 h. For tobramycin alone, the near-maximal effect was at 150 mg/kg/day and the area under the concentration-time curve over 24 h in the steady state divided by the MIC (AUC/MIC ratio) in ELF was 240.3. Resistance suppression occurred at an ELF AUC/MIC ratio of 110.6. For combination therapy, the near-maximal effect was reached at 60 mg/kg/day and 50 mg/kg/day of meropenem and tobramycin, which produced a 35.25% time > MIC in ELF and an ELF AUC/MIC ratio of 80.1. The interaction was additive. All combination regimens suppressed resistance. Combination therapy produced additive drug interaction and suppressed all resistance amplification. It is likely that optimal therapy for Pseudomonas aeruginosa pneumonia will involve a combination of agents.