Mutant Prevention Concentrations of Doripenem and Meropenem Alone and in Combination with Colistin (Polymyxin E), Levofloxacin and Tobramycin in Pseudomonas aeruginosa

BACKGROUND: With a limited number of available antimicrobialagents to treat Pseudomonas aeruginosa infections, the prevention ofemergence of antimicrobial resistance and its subsequent spread iscritical. In the present study, the mutant prevention concentration(MPC) of doripenem was examined and compared with meropenemfor its ability to prevent resistant mutant selection for P aeruginosawhen used alone and in combination with the other antipseudomonalagents colistin (polymyxin E), levofloxacin and tobramycin.OBJECTIVE: To determine if two antimicrobial agents that possesseddifferent mechanisms of action and separate demonstrated activitiesagainst P aeruginosa would produce a reduced MPC in combinationcompared with the MPC of each agent alone.METHODS: Twelve clinical isolates of P aeruginosa were plated onMueller-Hinton agar containing 1×, 2×, 4×, 8×, 16× and 32× thedoripenem, imipenem or meropenem minimum inhibitory concentration(MIC), and also on agar containing doripenem or meropenem incombination with either tobramycin (6 μg/mL), colistin (2 μg/mL or8 μg/mL), levofloxacin (8 μg/mL) or azithromycin (0.4 μg/mL). TheMPC for each antimicrobial agent-isolate combination was defined asthe lowest antibiotic concentration that prevented the visible growthof mutant colonies at 48 h of incubation. The MPC/MIC (μg/mL)ratio was defined as the ratio of the MPC obtained to the originalMIC.RESULTS: The MPC/MIC ratios of doripenem alone ranged from 8 to32 for the twelve isolates tested compared with 32 for two isolates andgreater than 32 for 10 isolates with imipenem, and 32 for three isolatesand greater than 32 for nine isolates with meropenem. All antimicrobialstested exhibited markedly elevated MPCs compared with theiroriginal MICs with MPC/MIC ratios of 8 to 32 for doripenem, 32 togreater than 32 for imipenem, 32 to greater than 32 for meropenem,32 to greater than 32 for colistin (tested at 2 μg/mL), 8 to 16 for levofloxacinand 8 to 32 for tobramycin. When a second antimicrobial wasused in combination with doripenem, the MPC/MIC ratio decreasedup to twofold for doripenem combined with colistin (2 μg/mL),decreased four- to 16-fold for doripenem combined with colistin(8 μg/mL), decreased eight- to 32-fold for doripenem combined withlevofloxacin, and decreased four- to 16-fold for doripenem combinedwith tobramycin. Adding a second antimicrobial in combination withmeropenem resulted in the following decreases in MPC/MIC: nodecrease for meropenem combined with colistin (2 μg/mL), four togreater than eightfold decrease for meropenem combined with colistin(8 μg/mL), four- to 16-fold decrease for meropenem combined withlevofloxacin, and two- to 16-fold decrease for meropenem combinedwith tobramycin. For all antimicrobial combinations tested, doripenemyielded greater decreases in MPC/MIC ratios than did meropenem.CONCLUSION: The present study found that individual antipseudomonalantimicrobial agents tested against 12 clinical isolates ofP aeruginosa had eight- to greater than 32-fold higher MPCs thanMICs, that combining doripenem or meropenem with a second activeantipseudomonal agent with a different mechanism of action was moreeffective at preventing resistance selection than the two agents usedindividually, and finally, that doripenem was less likely than both imipenemand meropenem to select for spontaneous resistance mutants ofP aeruginosa