Abstract:
The objective of this study was to determine the in vitro effects of triple
combination; amikacin, ceftazidime, and fosfomycin on multidrug-resistant
Pseudomona aeruginosa (MDR-PA) clinical isolates and to determine the optimal
dosage regimen of this combination by using pharmacokinetic/pharmacodynamic
(PK/PD) approach. This study, designed as an experimental study, was performed
at the Department of Microbiology, Faculty of Pharmacy, Mahidol University
during July 2005 to March 2006. The study comprised 34 MDR-PA clinical
isolates from patients admitted at Chonburi Hospital. They showed high level
resistance with the minimum inhibitory concentration at 50% (MIC50) and MIC90
of amikacin, ceftazidime, and fosfomycin, at > 256 μg/mL, > 256 μg/mL, and >
1024 μg/mL, respectively. The combination of amikacin and ceftazidme can
decrease the MIC of either amikacin or ceftazidime within 14 isolates (41.18%).
According to the fractional inhibitory concentration (FIC) index, there was 1
synergism (2.94%) and 33 additivities (97.06%) with the mean FIC index
1.61±0.53. More importantly, none of them showed antagonistic activities.
The agar well method demonstrated that the addition of fosfomycin to the
combination of amikacin and ceftazidime could increase the number of susceptible
isolates. For instance, the higher concentration of fosfomycin provided the trends
of greater activities of the triple combination. In addition, concentration of
fosfomycin which was used in combination with amikacin and ceftazidime should
be at least its MIC to optimize antimicrobial interaction. Throughout, the
calculated dosage regimen based on PK/PD data resulted in extremely huge
dosage. However, if the strain had MIC against amikacin, ceftazidime, and
fosfomycin equal to or less than 80 μg/mL, 50 μg/mL, and 200 μg/mL,
respectively, this combination would be a candidate for treatment of MDR-PA
infections. In conclusion, the study demonstrated that this triple combination
might be of benefit to overcome the organism.