Aim: The aims of the study are to evaluate the activity of sulbactam, meropenem, and polymyxin B alone and in combination against six isolates of extremely drug resistant Acinetobacter baumannii and to determine dosing regimens that achieve a sufficient joint probability of target attainment (PTA) based on combination antimicrobial pharmacodynamics. Materials and Methods: The combinations were evaluated by the checkerboard method and were considered synergistic when the fractional inhibitory concentration index (FICI) ≤0.5. Pharmacodynamic analyses were carried out by evaluating dosing regimens that achieve ≥90% joint PTA at the percentage of time over a 24-h period wherein the free drug concentration is above the minimum inhibitory concentration (%fT> MIC) of 40% and 60% for meropenem and sulbactam, respectively, and 20 for the ratio of the area under the free drug concentration-time curve over MIC (fAUC/MIC) for polymyxin B. Results: For both polymyxin B-resistant and susceptible isolates, the addition of sulbactam in combination with meropenem and subinhibitory concentration of polymyxin B showed important synergistic activity (five isolates; FICI ≤0.281); the recommended dosing regimens were 2/4 g meropenem/sulbactam q8 hours and 0.5 mg/kg polymyxin B q12 hours. Conclusion: This in vitro study showed that sulbactam can significantly improve the action of meropenem and polymyxin B in OXA-producing A. baumannii isolates, especially when there are no new treatment options available for infections caused by these microorganisms.
Keywords: Acinetobacter baumannii; antimicrobial combination; pharmacodynamic; polymyxin B; probability.