Mutant Prevention Concentrations of Doripenem and Meropenem Alone and in Combination with Colistin (Polymyxin E), Levofloxacin and Tobramycin in Pseudomonas aeruginosa
Authors: Zhanel George G , Vishisht Vibhu , Tam Ed , Hoban Daryl J , Karlowsky James A 
Published: May 2016

BACKGROUND: With a limited number of available antimicrobial
agents to treat Pseudomonas aeruginosa infections, the prevention of
emergence of antimicrobial resistance and its subsequent spread is
critical. In the present study, the mutant prevention concentration
(MPC) of doripenem was examined and compared with meropenem
for its ability to prevent resistant mutant selection for P aeruginosa
when used alone and in combination with the other antipseudomonal
agents colistin (polymyxin E), levofloxacin and tobramycin.
OBJECTIVE: To determine if two antimicrobial agents that possessed
different mechanisms of action and separate demonstrated activities
against P aeruginosa would produce a reduced MPC in combination
compared with the MPC of each agent alone.
METHODS: Twelve clinical isolates of P aeruginosa were plated on
Mueller‐Hinton agar containing 1×, 2×, 4×, 8×, 16× and 32× the
doripenem, imipenem or meropenem minimum inhibitory concentration
(MIC), and also on agar containing doripenem or meropenem in
combination with either tobramycin (6 μg/mL), colistin (2 μg/mL or
8 μg/mL), levofloxacin (8 μg/mL) or azithromycin (0.4 μg/mL). The
MPC for each antimicrobial agent‐isolate combination was defined as
the lowest antibiotic concentration that prevented the visible growth
of mutant colonies at 48 h of incubation. The MPC/MIC (μg/mL)
ratio was defined as the ratio of the MPC obtained to the original
MIC.
RESULTS: The MPC/MIC ratios of doripenem alone ranged from 8 to
32 for the twelve isolates tested compared with 32 for two isolates and
greater than 32 for 10 isolates with imipenem, and 32 for three isolates
and greater than 32 for nine isolates with meropenem. All antimicrobials
tested exhibited markedly elevated MPCs compared with their
original MICs with MPC/MIC ratios of 8 to 32 for doripenem, 32 to
greater 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 levofloxacin
and 8 to 32 for tobramycin. When a second antimicrobial was
used in combination with doripenem, the MPC/MIC ratio decreased
up 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 with
levofloxacin, and decreased four‐ to 16‐fold for doripenem combined
with tobramycin. Adding a second antimicrobial in combination with
meropenem resulted in the following decreases in MPC/MIC: no
decrease for meropenem combined with colistin (2 μg/mL), four to
greater than eightfold decrease for meropenem combined with colistin
(8 μg/mL), four‐ to 16‐fold decrease for meropenem combined with
levofloxacin, and two‐ to 16‐fold decrease for meropenem combined
with tobramycin. For all antimicrobial combinations tested, doripenem
yielded greater decreases in MPC/MIC ratios than did meropenem.
CONCLUSION: The present study found that individual antipseudomonal
antimicrobial agents tested against 12 clinical isolates of
P aeruginosa had eight‐ to greater than 32‐fold higher MPCs than
MICs, that combining doripenem or meropenem with a second active
antipseudomonal agent with a different mechanism of action was more
effective at preventing resistance selection than the two agents used
individually, and finally, that doripenem was less likely than both imipenem
and meropenem to select for spontaneous resistance mutants of
P aeruginosa.