College of Pharmacy, Washington State University Spokane, 310 North Riverpoint Boulevard, P.O. Box 1495, Spokane, WA 99210-1495, USA
Received 14 March 2003; returned 30 April 2003; revised 17 June 2003; accepted 17 June 2003
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Abstract |
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Materials and methods: An in vitro pharmacodynamic modelling apparatus (PDMA) characterized specific bacterial kill profiles for simulated regimens of levofloxacin and ciprofloxacin against four strains of S. pneumoniae. Total log reduction, time for 3-log reduction and AUC/MIC were determined.
Results: Ciprofloxacin was less effective than the levofloxacin regimens against all four study isolates. Ciprofloxacin produced 3-log reduction in only one isolate compared with all four isolates with the levofloxacin regimens. Bacterial regrowth did not occur over 12 h with levofloxacin; however, three of four isolates demonstrated bacterial regrowth with ciprofloxacin. None of the isolates were cleared from the PDMA by ciprofloxacin. The 500 mg levofloxacin regimen cleared two of four isolates and the 750 mg dose of levofloxacin cleared all study isolates. Respective AUC/MIC values for levofloxacin (500 and 750 mg) and ciprofloxacin were 4489, 63126 and 13, which correlated well with bacterial kill data.
Conclusions: Both levofloxacin regimens were more effective than ciprofloxacin against the study isolates tested. The 750 mg levofloxacin regimen generated more favourable bacterial killing compared with the 500 mg levofloxacin regimen. In addition to using the 750 mg levofloxacin dose for nosocomial infections, this dose may also prove useful for the management of resistant pneumococcal infections.
Keywords: levofloxacin, resistant S. pneumoniae, quinolones, pharmacodynamics
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Introduction |
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The purpose of this study was to comparatively evaluate bacterial kill profiles of simulated regimens of levofloxacin (500 and 750 mg) and ciprofloxacin (500 mg) against penicillin susceptible and non-susceptible strains of S. pneumoniae. Specific bacterial kill profiles associated with levofloxacin and ciprofloxacin were determined and differences in antimicrobial activity were assessed by comparing total logarithmic reduction in bacterial counts and time required to achieve a 3-log reduction in the initial inoculum for each quinolone. Common pharmacodynamic parameters for the quinolone regimens were also determined and compared.
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Materials and methods |
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Four clinical isolates of S. pneumoniae from different patients were evaluated in the study. SP-52, a penicillin-susceptible strain (MIC 0.016 mg/L), was cultured from the cerebrospinal fluid; SP-82, a penicillin-intermediate strain (MIC 0.125 mg/L), was cultured from the blood; and the remaining two strains were penicillin-resistant isolates, SP-140 (MIC 1.5 mg/L) and SP-123 (MIC 8.0 mg/L), both obtained from sputum cultures. To arrive at a consistent starting inoculum of approximately 1 x 106 cfu/mL of exponentially growing bacteria, 13 isolated colonies were placed in 50 mL of sterile Todd Hewitt broth, incubated overnight at 35°C and 5% CO2, and resuspended in fresh broth to a turbidity equivalent to that of a 1.0 McFarland standard. The broth culture was incubated for an additional 23 h and 15 mL was inoculated into the pharmacodynamic modelling apparatus (PDMA) just before the start of the experiment. Direct plating was used to confirm actual starting inoculum for each experiment.
Antimicrobials and susceptibility testing
Levofloxacin powder was provided by RW Johnson Pharmaceutical Research Institute (Raritan, NJ, USA) and ciprofloxacin powder was obtained from Bayer Corporation (West Haven, CT, USA). The MICs of levofloxacin, ciprofloxacin and penicillin were determined in duplicate using Etest strips (AB Biodisk, Solna, Sweden) according to procedures outlined by the National Committee for Clinical Laboratory Standards (NCCLS).4 MICs were determined before antibiotic exposure and were repeated following exposure to the study quinolones.
Pharmacodynamic modelling apparatus and bacterial kill profiles
An in vitro pharmacodynamic modelling apparatus (PDMA)6 simulated single dose regimens of levofloxacin 500 mg (Cmax = 7.0 mg/L), levofloxacin 750 mg (Cmax = 10.0 mg/L) and ciprofloxacin 500 mg (Cmax = 2.4 mg/L). Simulated concentrations represented total drug concentrations. Elimination half-lives of 7 h and 3.5 h were used for levofloxacin and ciprofloxacin, respectively. Nine samples were withdrawn from the PDMA to determine viable bacterial counts over time and to confirm the desired pharmacokinetic profile of each quinolone. Samples were serially diluted (1:10) in sterile Todd Hewitt broth, plated on trypticase soy agar supplemented with 5% sheep blood (BBL, Cockeysville, MD, USA) and incubated overnight at 35°C in a CO2 enriched environment. The lower limit for accurately detecting bacterial counts was 2.5 log10 cfu/mL. Duplicate experiments were conducted for each regimen against the four study isolates and control growth experiments were carried out to assure growth of the study isolates within the PDMA in the absence of antibiotics.
An agar diffusion bioassay was used to determine quinolone concentrations in samples withdrawn from the PDMA. Details of the assay have been previously described.7 The standards and PDMA samples were assayed in duplicate. The range of linearity for the bioassays was 0.2510.0 mg/L and coefficients of variation (intra- and inter-day) were <6% for all assays. The trapezoidal rule was used to define the area under the concentrationtime curve extrapolated out to 24 h (AUC024); this value was divided by the MIC of the study isolates to determine the AUC024/MIC ratio (AUC/MIC).
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Results |
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Discussion |
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Previous studies have evaluated the in vitro activity of levofloxacin and ciprofloxacin against S. pneumoniae. Ciprofloxacin consistently demonstrated a slower rate and extent of pneumococcal killing when compared to levofloxacin,8,10,13 even in studies evaluating a higher 750 mg dose of ciprofloxacin.12 An in vitro pharmacodynamic study similar to the one reported here, evaluated the activity of levofloxacin against four strains of S. pneumoniae with reduced susceptibility to ciprofloxacin. Consistent with our findings, the 750 mg regimen of levofloxacin demonstrated rapid killing and complete eradication of all S. pneumoniae strains from the in vitro modelling apparatus. The 500 mg regimen also had pronounced bacterial killing; however, activity was reduced in strains with higher levofloxacin MICs (2.6 and 3.3 mg/L).13
It is necessary to note the limitations associated with our study. First, our experiments were conducted over a 12 h period. Extending experiments to 24 h would have provided a more thorough assessment of quinolone activity, particularly in reference to bacterial regrowth. In addition, only four strains of S. pneumoniae were evaluated in the study. Subsequently, our findings may not accurately reflect the overall activity of the study regimens against S. pneumoniae. These shortcomings must be considered when interpreting the results of our study.
The primary purpose of this study was to evaluate the activity of common dosage regimens of levofloxacin (500 mg) and ciprofloxacin (500 mg), as well as a higher 750 mg regimen of levofloxacin against a variety of S. pneumoniae strains. Based on our findings, both levofloxacin regimens demonstrated better activity against the study isolates relative to ciprofloxacin. The 750 mg levofloxacin regimen generated more favourable bacterial kill profiles relative to the 500 mg regimen of levofloxacin. Although approved indications for the 750 mg dosage regimen are currently limited to nosocomial and complicated skin and skin structure infections, the 750 mg levofloxacin dose may prove a desirable approach for the management of pneumococcal infections in a setting where resistance is a significant and growing concern.
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Acknowledgements |
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Footnotes |
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References |
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2 . Schmitz, F. J., Verhoef, J., Fluit, A. C. et al. (1999). Comparative activity of 27 antimicrobial compounds against 698 Streptococcus pneumoniae isolates originating in 20 European university hospitals. European Journal of Clinical Microbiology and Infectious Diseases 18, 4503.[CrossRef][ISI][Medline]
3
.
Jones, R. N. & Pfaller, M. A. (2000). Macrolide and fluoroquinolone (levofloxacin) resistances among Streptococcus pneumoniae strains: significant trends from the SENTRY Antimicrobial Surveillance Program (North America 19971999). Journal of Clinical Microbiology 38, 42989.
4 . National Committee for Clinical Laboratory Standards. (2002). Performance Standards for Antimicrobial Susceptibility TestingTwelfth Informational Supplement: Document M100-S12. NCCLS, Wayne, PA, USA.
5 . West, M., Boulanger, B. R., Fogarty, C. et al. (2003). Levofloxacin compared with imipenem/cilastatin followed by ciprofloxacin in adult patients with nosocomial pneumonia: a multicenter, prospective, randomized, open-label study. Clinical Therapeutics 25, 485506.[CrossRef][ISI][Medline]
6 . Garrison, M. W., Malone, C. L., Eiland, J. et al. (1997). Influence of pH on the antimicrobial activity of clarithromycin and 14-hydroxyclarithromycin against Haemophilus influenzae using an in vitro pharmacodynamic model. Diagnostic Microbiology and Infectious Disease 27, 13945.[CrossRef][ISI][Medline]
7 . Chapin-Robertson, K. & Edberg, S. C. (1991). Measurement of antibiotics in human body fluids: techniques and significance. In Antibiotics in Laboratory Medicine, 3rd edn (Lorian, V., Ed), p. 313. Williams and Wilkins, Baltimore, MD, USA.
8
.
Lacy, M. K., Lu, W., Xu, X. et al. (1999). Pharmacodynamic comparisons of levofloxacin, ciprofloxacin and ampicillin against Streptococcus pneumoniae in an in vitro model of infection. Antimicrobial Agents and Chemotherapy 43, 6727.
9
.
Lister, P. D. (2002). Pharmacodynamics of gatifloxacin against Streptococcus pneumoniae in an in vitro pharmacokinetic model: impact of area under the curve/MIC ratios on eradication. Antimicrobial Agents and Chemotherapy 46, 6974.
10
.
Zhanel, G. G., Walters, M., Laing, N. et al. (2001). In vitro pharmacodynamic modelling simulating free serum concentrations of fluoroquinolones against multi-drug resistant Streptococcus pneumoniae. Journal of Antimicrobial Chemotherapy 47, 43540.
11
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Zhanel, G. G., Roberts, D., Waltky, A. et al. (2002). Pharmacodynamic activity of fluoroquinolones against ciprofloxacin-resistant Streptococcus pneumoniae. Journal of Antimicrobial Chemotherapy 49, 80712.
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Lister, P. D. & Sanders, C. C. (1999). Pharmacodynamics of levofloxacin and ciprofloxacin against Streptococcus pneumoniae. Journal of Antimicrobial Chemotherapy 43, 7986.
13 . Lister, P. D. (2002). Pharmacodynamics of 750 mg and 500 mg doses of levofloxacin against ciprofloxacin-resistant strains of Streptococcus pneumoniae. Diagnostic Microbiology and Infectious Disease 44, 439.[CrossRef][ISI][Medline]
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