Comparative in vitro activity of carbapenems against clinical isolates of Acinetobacter baumannii.

Aims: The aim of this multi-hospital study was to assess the in vitro activity of doripenem and its comparators, imipenem and meropenem, using the new CLSI breakpoints against a large population of a frequently isolated nosocomial pathogen, Acinetobacter baumannii.

Methods And Results: During a 2-year period, four referral or tertiary hospitals submitted 400 isolates of Ac. baumannii for susceptibility testing using imipenem, meropenem and doripenem via disc diffusion and E-test methods. A subset of 390 isolates was resistant to all three tested carbapenems. Doripenem and meropenem (MIC50 , 32 μg ml(-1) ) had comparable activity, albeit doripenem's activity was greater than imipenem (MIC50 , >32 μg ml(-1) ). A significantly higher proportion of the isolates were inhibited by doripenem than by imipenem at MIC values of 12, 16, 24 and 32 μg ml(-1) (P < 0·05). The cumulative percentage of imipenem MICs was lower compared to its comparators. The comparison of resistance rate to imipenem and meropenem based on old and new breakpoints showed <1% difference. The overall agreement between the two susceptibility testing methods was ≥95%.

Conclusion: Doripenem has a slightly greater in vitro activity than imipenem in terms of zone breakpoints and MIC values, but its activity is comparable to meropenem.

Significance And Impact Of The Study: Doripenem should be considered as a therapeutic option for monotherapy or combination therapy, particularly when the therapeutic options are limited.