a Department of Medicine, Mount Auburn Hospital, Harvard Medical School, 330 Mount Auburn Street, Cambridge, MA 02138, USA; b Department of Pharmacokinetics & Pharmacodynamics, Gause Institute of New Antibiotics, 11 Bolshaya Pirogovskaya Street, Moscow 119992, Russia; c Roger Williams Medical Center, 825 Chalkstone Avenue, Providence, RI 02908, USA
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Abstract |
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Introduction |
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Materials and methods |
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Gatifloxacin lactate and ciprofloxacin hydrochloride were kindly provided by Bristol-Myers Squibb (New Brunswick, NJ, USA) and Bayer Corporation (West Haven, CT, USA), respectively. Two clinical isolates of S. pneumoniae, a relatively more susceptible S. pneumoniae 6691 and a less susceptible S. pneumoniae 18032 were selected for the study. The MICs for S. pneumoniae 6691 and 18032 determined by broth microdilution were 0.125 and 2 mg/L, respectively, of gatifloxacin, and 1 and 32 mg/L, respectively, of ciprofloxacin.
In vitro dynamic model and simulated pharmacokinetic profiles
The in vitro dynamic model used in this study has been described previously in detail.5 Briefly, this two-compartment model consists of a central compartment and six bioreactors, artificial capillary chambers (Unisyn Fibertech Corporation, San Diego, CA, USA) connected in series, which model the peripheral compartments. For all experiments the bacterial inoculum was prepared from frozen inocula by thawing, diluting with an equal part of fresh MuellerHinton broth (MHB) supplemented with lysed horse blood (LHB; 2% v/v) and incubating for 90 min at 37°C to bring the organisms into growth phase. This mixture was then inoculated into each peripheral compartment, which also contained MHBLHB-2% via an entry port and incubated until a density of c. 106 cfu/mL was achieved, at which time the antibiotic was introduced into the central compartment (time zero). Antibiotic-free, sterile MHB with 10% pooled horse serum was infused and eluted at flow rates selected to mimic t½ values of 6.5 h (gatifloxacin) and 4.5 h (ciprofloxacin), typical of those reported in humans: 6.08.4 h68 and 3.25.0 h,911 respectively. Control experiments without antimicrobial agent were performed with each isolate to characterize growth kinetics. All model experiments were performed in triplicate.
A series of monoexponential profiles that mimic single dose administration of gatifloxacin and two 12 hourly doses of ciprofloxacin were simulated. At the end of a 60 min infusion, the drug concentration was analogous to the peak concentrations reached after oral administration. The AUC/MIC ratios and the respective AUCs simulated in the central compartment of the model are presented in the Table. As the antimicrobial effect depends on quinolone concentration in peripheral compartments (where the organisms contact the antibiotic), peripheral compartments were sampled for 48 h to determine quinolone concentrations by bioassay using well plates seeded with E. coli ATCC 25922 for ciprofloxacin and Bacillus subtilis ATCC 6633 spore suspension for gatifloxacin. The overall range of the AUC/MIC ratios that reflected different doses was 1578 h for gatifloxacin and 1148 h for ciprofloxacin. Typical pharmacokinetic profiles observed in the peripheral compartments at comparable AUC/MIC ratios are shown in Figure 1
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In each experiment the peripheral compartments were sampled for 48 h to determine bacterial counts. The numbers of surviving organisms were determined by serial dilution of sample in cold sterile saline and inoculation of 20 µL in triplicate on to MuellerHinton agar supplemented with 5% sheep blood (MHA5% SB). Small numbers of bacteria were counted by placing 100 µL of sample into 10 mL of cold sterile saline and filtering this mixture through a 0.45 µm filter. The filter was then placed on to MHA5% SB. After overnight incubation at 37°C the resulting bacterial colonies were counted, and the cfu/mL were calculated. Ten cfu/mL was the theoretically achievable limit of detection.
The total antimicrobial effect was expressed as the area between the control growth and the antibiotic-induced timekill/regrowth curves (IE)12,13 (Figure 2). The cut-off level was chosen as 108 cfu/mL, the lowest level of maximal counts observed with or without antibiotic. To quantify pharmacodynamic data reported in another study,2 the published timekill curves were scanned and digitalized using Grafula II, v. 1.90 software (WESik Soft Haus, Moscow, Russia). Then, the 24 h areas under the time kill curves (AUBC) were calculated as described elsewhere.14,15
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The IE versus log AUC/MIC data sets combined for both strains of S. pneumoniae exposed to each quinolone were fitted by the equation
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This equation is specific for a particular quinolone but it is strain independent. However, using equation (1), the antimicrobial effect against any bacterial strain with its specific MIC can be predicted. This also applies to a hypothetical strain whose MIC is equal to the MIC50. Placing MIC = MIC50 in equation (1)
, the respective quinolone- and strain-specific relationship of IE to log AUC may be derived:
![]() | (2) |
To express the antimicrobial effects as a function of quinolone dose (D), the AUC in the MIC50-specific relationship (2) for a given quinolone was substituted by D according to the polynomial equation
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The values of c, d and e for gatifloxacin (0, 7.0 x 102 and 3.6 x 105, respectively) and for ciprofloxacin (0.54, 1.26 x 102 and 1.6 x 105, respectively) were calculated by considering the curvilinear pattern of the AUC versus D plots constructed from pharmacokinetic data on gatifloxacin (AUCs at Ds from 100 to 600 mg)7,8 and ciprofloxacin (AUCs at Ds from 100 to 1000 mg)9 reported in humans.
To predict dose-dependent antimicrobial effects on a hypothetical strain of S. pneumoniae with MICs of gatifloxacin and ciprofloxacin equal to the respective MIC50S, weighted geometric means of MIC50S of gatifloxacin1620 and ciprofloxacin1620 reported in studies using the broth dilution method were calculated. The weighted geometric mean of the MIC50 of gatifloxacin for S. pneumoniae was estimated as 0.4 mg/L and that of ciprofloxacin as 1 mg/L.
Correlation and regression analyses of the relationships between IE and log AUC/MIC for each quinolone were performed at a level of significance of P = 0.05. To examine the impact of AUC/MIC ratios and t½ of gatifloxacin on its antimicrobial effect, multiple regression analysis of the reciprocal values of AUBC (1/AUBC) versus the simulated AUC/MIC ratios and t½S was performed based on reported timekill curves of S. pneumoniae.2
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Results |
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Discussion |
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For most of the simulated AUC/MIC ratios (2050 h), the effects of a given AUC/MIC ratio were slightly greater with ciprofloxacin than with gatifloxacin (Figure 4). At first glance, this contrasts with previously reported findings that ciprofloxacin was less efficient than those fluoroquinolones with longer t½ values.3,2325 This apparent discrepancy may be attributed to the different AUC/MIC ranges used in these different studies (1148 h with ciprofloxacin and 1578 h with gatifloxacin versus 60920 h in the previous studies). The slightly larger IES observed with ciprofloxacin in the present study should not be overinterpreted, because the IE versus AUC/MIC data sets obtained with gatifloxacin and ciprofloxacin could be combined resulting in a correlation coefficient (r2 = 0.76) that is comparable to those for the individual fluoroquinolones (r2 = 0.730.94). This is consistent with our recent findings with other quinolones26 that showed no significant differences between the pharmacokinetically differing quinolones, gemifloxacin and ciprofloxacin, when examined at AUC/MICs of <60 h.
Based on the strain-independent patterns of the IElog AUC/MIC relationships that were established in this study, the respective doseresponse curves were derived for a hypothetical strain of S. pneumoniae whose MICs were equal to the MIC50S of the two fluoroquinolones. This allowed the prediction of equiefficient doses of the two drugs. According to our analysis, a 400 mg daily dose of gatifloxacin would be equiefficient to a 1400 mg daily dose of ciprofloxacin (Figure 5).
Overall, this in vitro study demonstrates advantages of gatifloxacin relative to ciprofloxacin in terms of the dose-dependent total antimicrobial effect.
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Acknowledgements |
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Notes |
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References |
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Firsov, A. A., Lubenko, I. Y., Vostrov, S. N., Kononenko, O. V., Zinner, S. H. & Portnoy, Y. A. (2000). Comparative pharmacodynamics of moxifloxacin and levofloxacin in an in vitro dynamic model: prediction of the equivalent AUC/MIC breakpoints and equiefficient doses. Journal of Antimicrobial Chemotherapy 46, 72532.
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Firsov, A. A., Vostrov, S. N., Shevchenko, A. A., Zinner, S. H. & Portnoy, Y. A. (1998). A new approach to in vitro comparisons of antibiotics in dynamic models: equivalent area under the curve/MIC breakpoints and equiefficient doses of trovafloxacin and ciprofloxacin against bacteria of similar susceptibilities. Antimicrobial Agents and Chemotherapy 42, 28417.
25 . Firsov, A. A., Zinner, S. H., Lubenko, I. Y. & Vostrov, S. N. (2000). Gemifloxacin and ciprofloxacin pharmacodynamics in an in-vitro dynamic model: prediction of the equivalent AUC/MIC breakpoints and doses. International Journal of Antimicrobial Agents 16, 40714.[ISI][Medline]
26 . Firsov, A. A., Vostrov, S. N., Lubenko, I. Yu., Kononenko, O. V., Zinner S. H. & Cornaglia, G. (1999). A comparison of the AUC/MIC-response plots of gemifloxacin and ciprofloxacin: critical value of the AUC/MIC ranges simulated in an in vitro dynamic model. Journal of Antimicrobial Chemotherapy 44, Suppl. A, 130.
Received 13 February 2001; returned 4 June 2001; revised 20 July 2001; accepted 29 August 2001