Impact of Different Carbapenems and Regimens of Administration on Resistance Emergence for Three Isogenic Pseudomonas aeruginosa Strains with Differing Mechanisms of Resistance

We compared drugs (imipenem and doripenem), doses (500 mg and 1 g), and infusion times (0.5 and 1.0 [imipenem], 1.0 and 4.0 h [doripenem]) in our hollow-fiber model, examining cell kill and resistance suppression for three isogenic strains of Pseudomonas aeruginosa PAO1. The experiments ran for 10 days. Serial samples were taken for total organism and resistant subpopulation counts. Drug concentrations were determined by high-pressure liquid chromatography-tandem mass spectrometry (LC/MS/MS). Free time above the MIC (time > MIC) was calculated using ADAPT II. Time to resistance emergence was examined with Cox modeling. Cell kill and resistance emergence differences were explained, in the main, by differences in potency (MIC) between doripenem and imipenem. Prolonged infusion increased free drug time > MIC and improved cell kill. For resistance suppression, the 1-g, 4-h infusion was able to completely suppress resistance for the full period of observation for the wild-type isolate. For the mutants, control was ultimately lost, but in all cases, this was the best regimen. Doripenem gave longer free time > MIC than imipenem and, therefore, better cell kill and resistance suppression. For the wild-type organism, the 1-g, 4-h infusion regimen is preferred. For organisms with resistance mutations, larger doses or addition of a second drug should be studied.