1 Department of Pharmacokinetics and Pharmacodynamics, Gause Institute of New Antibiotics, Russian Academy of Medical Sciences, 11 Bolshaya Pirogovskaya Street, Moscow, 119992 Russia; 2 Department of Medicine, Mount Auburn Hospital, Harvard Medical School, Cambridge, MA, USA
Received 22 January 2002; returned 15 May 2002; revised 11 July 2002; accepted 15 July 2002
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Abstract |
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Introduction |
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These findings were reported in single-dose studies, where the impact of truncated areas (ABBC, AAC and AUBC) on the determination of antimicrobial effect is more dramatic than in multiple-dose simulations, where the effects of preceding doses might mask differences in the measured effect of the final dose. The present study was designed to examine whether the fundamental differences between IE and the -related endpoints influence AUC/MICresponse relationships determined in multiple-dose simulations. Once-daily administration of moxifloxacin or levofloxacin for 3 days was simulated using a wide range of AUC/MIC ratios, and the quinolone pharmacodynamics observed with Staphylococcus aureus were compared in terms of the AUC/MIC relationships of IE, ABBC, AAC and AUBC.
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Materials and methods |
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Moxifloxacin and levofloxacin powders were kindly provided by Bayer Corporation (West Haven, CT, USA) and Ortho-McNeill Pharmaceuticals (Raritan, NJ, USA), respectively. A clinical isolate of methicillin-resistant S. aureus (S. aureus 916) for which the MICs were comparable to the MIC50s of moxifloxacin (0.15 mg/L10) and levofloxacin (0.7 mg/L10), was selected for the study. Susceptibility testing was performed in triplicate by broth microdilution techniques at 24 h post-exposure, with the organism grown in Ca2+- and Mg2+-supplemented MuellerHinton broth at an inoculum size of 106 cfu/mL. To obtain more precise values, MICs were determined with starting concentrations of 5, 6, 7 and 8 mg/L as described earlier.11 The true MICs for S. aureus 916 determined by these multiple serial dilutions were 0.37 mg/L of moxifloxacin and 0.6 mg/L of levofloxacin.
To reveal possible changes in susceptibility during treatment, the true fluoroquinolone MICs for bacterial cultures sampled from the model were determined daily for 4 days.
In vitro dynamic model and simulated pharmacokinetic profiles
A dynamic model described previously8 was used in the study. The operation procedures, reliability of simulations of quinolone pharmacokinetic profiles and the high reproducibility of the timekill curves provided by the model have been reported elsewhere.12
A series of monoexponential profiles that simulate once-daily administration of moxifloxacin and levofloxacin for 3 days was modelled (Figure 1). The simulated half-lives (12.1 h for moxifloxacin and 6.8 h for levofloxacin) represented weighted means of values reported in humans (9.113.4 h13,14 and 6.07.4 h,1519 respectively). The rates of fresh nutrient medium influxed into the 60 mL central compartment and the antibiotic- and bacteria-containing medium effluxed from this compartment were 3.5 mL/h and 6.1 mL/h, respectively. Daily doses of the quinolones were designed to correspond to comparable mean values of the AUC/MIC ratio (average of values reached after the first, second and third doses): 14444 h with moxifloxacin and 15484 h with levofloxacin, where
is a 24 h dosing interval.
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In each experiment multiple sampling of the bacteria-containing medium from the central compartment was performed throughout the observation period. The duration of the experiments, in each case, was defined as the time until the numbers of antibiotic-exposed bacteria, after the third dose, reached the maximum observed in the absence of antibiotic (109 cfu/mL). The lower limit of accurate detection was 2 x 102 cfu/mL.
Based on timekill data, ABBC, AAC and AUBC were determined for each dosing interval, i.e. from time zero to 24 h, from 24 to 48 h and from 48 to 72 h. The cumulative antimicrobial effect (072 h) was defined as the sum of these partial ABBC, AAC or AUBC (ABBC, AAC
and AUBC
, respectively). Unlike these
-related endpoints of antimicrobial effect, IE was determined from time zero to the time when the effect could no longer be detected, i.e. the time after the third quinolone dose at which the number of antibiotic-exposed bacteria reached 109 cfu/mL. The upper limit of bacterial numbers, i.e. the cut-off level on the regrowth and control growth curves used to determine IE and ABBC, or on the regrowth curve only for AAC and AUBC determination, was 109 cfu/mL. If counts were lower, they were extrapolated to the cut-off level using a logistic function.10 The computation of IE, ABBC, AAC and AUBC at two simulated AUC
/MIC ratios of moxifloxacin is depicted graphically in Figure 2.
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For both quinolones, the endpoints of antimicrobial effect were fitted to the log AUC/MIC ratios using an Emax model:
Y = Emax + (EminEmax)/{1 + exp [(xx50)/dx]} (1)
where x is the log AUC/MIC ratio, Y is IE, ABBC, AAC or AUBC, Emax and Emin are the maximal and minimal values of the antimicrobial effect, x50 is x corresponding to Emax/2 and dx is the parameter reflecting width.9
Quasi-linear portions of the IElog AUC/MIC ratio curves, i.e. IEs at AUC
/MIC ratios > 100 h, were fitted by the linear equation
IE = a + b log (AUC/MIC) (2)
where a and b are intercept and slope parameters, respectively.
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Results |
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The time courses of viable count, which reflect killing and regrowth of S. aureus exposed to moxifloxacin and levofloxacin administered once daily, are shown in Figure 3. At lower AUC/MIC ratios, from 14 to 28 h with moxifloxacin and 15 to 61 h with levofloxacin, regrowth occurred during each dosing interval following either a negligible (at the lowest AUC
/MIC ratio of moxifloxacin) or a considerable reduction in bacterial numbers (at a moxifloxacin AUC
/MIC ratio of 28 h and levofloxacin AUC
/MIC ratios of 1561 h). At an AUC
/MIC ratio of 56 h, moxifloxacin gave more pronounced reductions in bacterial count after the first and second doses, with regrowth occurring during the third dosing interval. No regrowth occurred during the three dosing intervals when higher AUC
/MIC ratios were used for moxifloxacin (111444 h) and levofloxacin (121484 h). However, at all these AUC
/MIC ratios except for the highest for moxifloxacin (444 h), bacterial regrowth was observed on the fourth day.
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Relationships of the antimicrobial effect to AUC/MIC ratio
The AUC/MIC ratio relationships of the cumulative antimicrobial effect, expressed by IE, ABBC, AAC and AUBC and fitted by equation (1), are shown in Figure 5 and the respective best-fit estimates of Emax, Emin, x50 and dx are presented in Table 1. As seen in Figure 5, the IElog AUC
/MIC ratio curves display less sigmoidicity than the ABBC, AAC and AUBClog AUC
/MIC ratio curves. Moreover, the same 32-fold range in the simulated AUC
/MIC ratios is associated with larger ranges of IE, from 0 to
600 (log cfu/mL) x h for moxifloxacin and 0550 (log cfu/mL) x h for levofloxacin, than of ABBC [0425 (log cfu/mL) x h], AAC [160210 to 200220 (log cfu/mL) x h] or AUBC [600640 to 220230 (log cfu/mL) x h]. Based on the IElog AUC
/MIC ratio relationships, the different effects of moxifloxacin and levofloxacin are seen distinctly over most of the range of simulated AUC
/MIC ratios (40400 h). However, with the
-related endpoints, the AUC
/MIC ratio relationships display differences only over a relatively narrow range (40100 h; ABBC and AUBC) or they do not distinguish between the quinolones at all (AAC).
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Discussion |
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Similar differences in the AUC/MICresponse curves were seen when using different endpoints for antimicrobial effect (Figure 5) in single-dose simulations with gemifloxacin.9 The largest differences in effect areas, corresponding to the simulated AUC
/MIC ratios, were seen when using IE as the endpoint. This appears to be the most descriptive outcome measure. Using IE, the effects of moxifloxacin and levofloxacin could be distinguished over a wider range of AUC
/MIC ratio than with ABBC and AUBC endpoints, whereas no differences in effect could be seen between the two agents with the AACAUC
/MIC ratio curves. Although ABBC and AUBC endpoints were more descriptive than AAC, the former two endpoints could distinguish the fluoroquinolone effects only over a narrow AUC
/MIC ratio range (
40100 h). Given the difference in therapeutic AUC
/MIC ratios seen with 500 mg levofloxacin and 400 mg moxifloxacin against staphylococci (AUC/MIC50 ratios of 65 and 205 h20), these values fall in the therapeutic range for levofloxacin but not for moxifloxacin. This might also be a limitation when making comparisons between the other new fluoroquinolones that have larger therapeutic AUC/MIC50 ratios, such as 130 h for grepafloxacin, 140 h for gatifloxacin, 190 h for trovafloxacin and 310 h for gemifloxacin.20 The fact that these values all fall in the plateau portion of the ABBC and AUBCAUC
/MIC curves (Figure 5) precludes accurate fluoroquinolone comparisons and clinically relevant predictions. As shown previously,9 this plateau results from the underestimation of the true antimicrobial effect with the use of
-related endpoints when the effect actually persists longer than
, in this case, 3 x
. ABBC, AAC and AUBC all consider the truncated areas after the third dose and ignore substantial portions of the actual effect (Figure 2). However, as mentioned above, the
-related endpoints may be useful to compare the effects provided by each subsequent dose of quinolone. Although the systematic decrease in antimicrobial effect over the course of treatment with moxifloxacin (AUC
/MIC ratio 28 h) and levofloxacin (AUC
/MIC ratios of 31 and 61 h) was demonstrated using ABBC
, a similar conclusion might be drawn using AUBC
.
As reported previously,1 IE analysis provides predictions of fluoroquinolone effects that may be relevant clinically. Recently, in single-dose simulations with moxifloxacin and levofloxacin,10 AUC/MIC ratio values of 80 and 130 h, respectively, were predicted as equivalent to the reported ciprofloxacin AUC/MIC ratio breakpoint of 125 h.21 To verify these predictions, a similar analysis was performed in this multiple-dose study. Although the Emax model was used to fit the IE versus log AUC/MIC ratio over the entire AUC
/MIC ratio range, at AUC
/MIC ratios >50 h, the IElog AUC
/MIC ratio data may be approximated by a linear function (Figure 6). A levofloxacin AUC
/MIC ratio of 120 h, which corresponds to an AUC/MIC ratio of 130 h, provides the same IE as a moxifloxacin AUC
/MIC ratio of 60 h, which in turn, corresponds to an AUC/MIC ratio of 80 h. Thus, the AUC/MIC ratio breakpoints predicted in single and multiple-dose studies using IElog AUC/MIC ratio or IElog AUC
/MIC ratio relationships are very similar.
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Acknowledgements |
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Footnotes |
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References |
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