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Antimicrobial Agents and Chemotherapy, May 1997, 950-955, Vol 41, No. 5
L Licata, CE Smith, RM Goldschmidt, JF Barrett and M Frosco
The postantibiotic subminimum inhibitory concentration effect (PA SME) may
simulate in vivo drug exposure more accurately than the postantibiotic
effect (PAE) since subinhibitory concentrations of drug persist between
antibiotic dosings. In this study, we compared the PAEs and PA SMEs of
levofloxacin and ciprofloxacin for clinical isolates of
fluoroquinolone-susceptible Staphylococcus aureus and Streptococcus
pneumoniae. At two times the MIC, PAEs of levofloxacin were an average of
0.6 h longer than the PAEs obtained for ciprofloxacin for
methicillin-susceptible and methicillin-resistant S. aureus strains. The
PAEs of levofloxacin and ciprofloxacin ranged from 1.8 to 3.1 and 1.1 to
2.4 h, respectively. Continued exposure of the methicillin- resistant
strain to 1/16, 1/8, and 1/4 the MIC resulted in PA SMEs of 6.5, 15.3, and
>22.3 h, respectively, for levofloxacin and 3.8, 8.0, and 12.3 h,
respectively, for ciprofloxacin. For isolates of S. pneumoniae, at two
times the MIC of both fluoroquinolones, the average PAEs of levofloxacin
and ciprofloxacin were equivalent: 1.3 h for the penicillin-susceptible
isolate and 0.6 h for the penicillin-resistant isolate. Continued exposure
of the penicillin-susceptible S. pneumoniae strain to 1/16, 1/8, and 1/4
the MIC resulted in average PA SMEs of 1.0, 1.4, and 2.8 h, respectively,
for levofloxacin and 1.8, 2.0, and 2.5 h, respectively, for ciprofloxacin.
Continued exposure of penicillin-resistant S. pneumoniae to 1/16, 1/8, and
1/4 the MIC of the same fluoroquinolones resulted in average PA SMEs of
0.6, 1.1, and 2.9 h, respectively, for levofloxacin and 0.6, 1.1, and 1.5
h, respectively, for ciprofloxacin. The PA SMEs observed demonstrate the
superior activity of levofloxacin against methicillin-susceptible or
methicillin-resistant S. aureus. Although PAEs were similar for the
penicillin-susceptible and penicillin-resistant S. pneumoniae strains, the
PA SME of levofloxacin at one-fourth the MIC was longer for
penicillin-resistant S. pneumoniae.
Copyright © 1997 by the American Society for Microbiology. All rights reserved.
Comparison of the postantibiotic and postantibiotic sub-MIC effects of levofloxacin and ciprofloxacin on Staphylococcus aureus and Streptococcus pneumoniae
Department of Microbiology, The R. W. Johnson Pharmaceutical Research Institute, Raritan, New Jersey 08869, USA.
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