Comparative antimicrobial activity of levofloxacin and ciprofloxacin against Streptococcus pneumoniae

Mark W. Garrison*

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


    Abstract
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Objectives: Levofloxacin has good coverage against both Gram-positive and Gram-negative pathogens. Recent reports demonstrate enhanced activity associated with a higher 750 mg dosage of levofloxacin. The objective of this study was to comparatively evaluate the activity of common regimens of levofloxacin (500 mg) and ciprofloxacin (500 mg), and a higher 750 mg levofloxacin regimen against penicillin susceptible and non-susceptible strains of S. pneumoniae.

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 44–89, 63–126 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


    Introduction
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Ciprofloxacin was introduced roughly two decades ago and became widely used as a result of its impressive activity against Gram-negative aerobes. The introduction of levofloxacin followed ciprofloxacin and provided enhanced activity against Streptococcus pneumoniae. With the increasing incidence of infections involving penicillin-resistant S. pneumoniae (PRSP), which are also frequently resistant to macrolides and other traditional agents,1,2 it is important to accurately characterize the activity of quinolones against resistant strains of S. pneumoniae.3 Newer anti-pneumococcal quinolones such as gatifloxacin are now available; however, activity of gatifloxacin against Gram-negative aerobic infections outside of the urinary tract is limited.4 Levofloxacin remains an effective agent against both Gram-negative and Gram-positive aerobes and recent reports have demonstrated enhanced activity associated with larger 750 mg doses of levofloxacin.5

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.


    Materials and methods
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Bacterial strains

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, 1–3 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 2–3 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 0.25–10.0 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 concentration–time curve extrapolated out to 24 h (AUC0–24); this value was divided by the MIC of the study isolates to determine the AUC0–24/MIC ratio (AUC/MIC).


    Results
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Susceptibility and AUC/MIC data for the study isolates are displayed in Table 1. MICs following quinolone exposure remained constant and did not vary from values determined at the onset by more than one subsequent concentration on the gradient Etest scale. Quinolone concentrations achieved throughout the experiments fell within 4% of desired pharmacokinetic parameters (data not shown). Control growth experiments demonstrated appropriate growth of each study isolate within the PDMA.


View this table:
[in this window]
[in a new window]
 
Table 1. Susceptibility and 24 h AUC/MIC data for the study isolates
 
Specific bacterial kill profiles for each isolate are described in Figure 1. For SP-52 (Figure 1a), ciprofloxacin produced an initial 1.8 log reduction in the first 6 h followed by bacterial regrowth that approached starting inoculum; a 3-log reduction was not achieved by ciprofloxacin (Table 2). In contrast, both levofloxacin regimens cleared SP-52 from the PDMA after 6 h with no evidence of regrowth by 12 h. Total logarithmic reduction of SP-52 exceeded 5.5 logs in both levofloxacin regimens (Table 2). SP-82 was most susceptible to both quinolones and represented the only study isolate for which ciprofloxacin was able to successfully achieve a 3-log reduction (Figure 1b). Ciprofloxacin produced a maximum reduction of 4.5 logs over 6 h; however, bacterial regrowth occurred after this point. The levofloxacin regimens achieved a 3-log reduction within 2–3 h and cleared SP-82 from the PDMA within 6 h. Total log reduction of SP-82 was over 5.8 logs for both regimens (Table 2). The rate and extent of bacterial killing were less pronounced for SP-140 (Figure 1c). Ciprofloxacin produced a maximum reduction of 1.5 logs over the course of 6 h. Kill profiles for the two levofloxacin regimens were similar over the initial 6 h; however, the 750 mg dose of levofloxacin was the only regimen able to clear SP-140 from the PDMA (Table 2). Kill profiles for SP-123 paralleled the SP-140 data (Figure 1d).



View larger version (17K):
[in this window]
[in a new window]
 
Figure 1. Bacterial kill profiles of 500 mg ciprofloxacin (diamonds), 500 mg levofloxacin (squares) and 750 mg levofloxacin (triangles) versus S. pneumoniae strains: SP-52 (a); SP-82 (b); SP-140 (c); and SP-123 (d). Each data point represents the mean cfu/mL value from the duplicate experiments. Control growth curves in the absence of antibiotics are shown for each strain (circles).

 

View this table:
[in this window]
[in a new window]
 
Table 2. Approximate time (h) required for 3-log reduction of the initial inoculum (T3-log) and total logarithmic reduction in inoculum after 12 h (LR12) for each study isolate
 

    Discussion
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Overall, the kill curve profiles illustrated in Figure 1 clearly demonstrated that the two regimens of levofloxacin were more effective than ciprofloxacin in both rate and extent of logarithmic reduction for all four study strains of S. pneumoniae. Both levofloxacin regimens successfully achieved 3-log reductions in starting inoculum for all study isolates, whereas ciprofloxacin only accomplished this with SP-82. Furthermore, regrowth within 12 h was absent for all experiments involving levofloxacin, but occurred with ciprofloxacin in three of the four study isolates. As anticipated, inspection of the two levofloxacin regimens showed greater activity when the larger 750 mg regimen was compared to the 500 mg regimen. This was particularly apparent against SP-140 and SP-123 (Figure 1c and d), in which the 750 mg dose was able to clear both isolates from the PDMA. These kill curve findings are consistent with AUC/MIC data reported in Table 1. Lacy et al.8 reported favourable bacterial killing of S. pneumoniae when quinolone AUC/MIC values were 30 or greater. The levofloxacin regimens in our study had AUC/MIC values ranging from 44 to 126, whereas the values for ciprofloxacin were all less than 30. In a separate study, quinolone AUC/MIC values that fell below 30, were less likely to result in bacterial eradication.9 Additional studies have evaluated the role of quinolone AUC/MIC as a reliable parameter for activity against S. pneumoniae.1012 Data from our study lend further support to the importance of maintaining quinolone AUC/MIC values at or above 30 to optimize bacterial killing of S. pneumoniae.

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.


    Acknowledgements
 
I would like to express thanks to Ms Connie Malone and Ms Brenda Edmunds for their valuable technical assistance. This work was supported by a grant from R. W. Johnson Pharmaceutical Research Institute, Raritan, NJ, USA.


    Footnotes
 
* Tel: +1-509-358-7658; Fax: +1-509-358-7744; E-mail: garrism{at}wsu.edu Back


    References
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
1 . Doern, G. V., Pfaller, M. A., Kugler, K. et al. (1998). Prevalence of antimicrobial resistance among respiratory tract isolates of Streptococcus pneumoniae in North America: 1997 results from the SENTRY antimicrobial surveillance program. Antimicrobial Agents and Chemotherapy 27, 764–70.

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, 450–3.[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 1997–1999). Journal of Clinical Microbiology 38, 4298–9.[Free Full Text]

4 . National Committee for Clinical Laboratory Standards. (2002). Performance Standards for Antimicrobial Susceptibility Testing–Twelfth 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, 485–506.[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, 139–45.[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, 672–7.[Abstract/Free Full Text]

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, 69–74.[Abstract/Free Full Text]

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, 435–40.[Abstract/Free Full Text]

11 . Zhanel, G. G., Roberts, D., Waltky, A. et al. (2002). Pharmacodynamic activity of fluoroquinolones against ciprofloxacin-resistant Streptococcus pneumoniae. Journal of Antimicrobial Chemotherapy 49, 807–12.[Abstract/Free Full Text]

12 . Lister, P. D. & Sanders, C. C. (1999). Pharmacodynamics of levofloxacin and ciprofloxacin against Streptococcus pneumoniae. Journal of Antimicrobial Chemotherapy 43, 79–86.[Abstract/Free Full Text]

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, 43–9.[CrossRef][ISI][Medline]





This Article
Abstract
FREE Full Text (PDF)
All Versions of this Article:
52/3/503    most recent
dkg380v1
Alert me when this article is cited
Alert me if a correction is posted
Services
Email this article to a friend
Similar articles in this journal
Similar articles in ISI Web of Science
Similar articles in PubMed
Alert me to new issues of the journal
Add to My Personal Archive
Download to citation manager
Disclaimer
Request Permissions
Google Scholar
Articles by Garrison, M. W.
PubMed
PubMed Citation
Articles by Garrison, M. W.