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Antimicrobial Agents and Chemotherapy, November 1998, p. 2841-2847, Vol. 42, No. 11
0066-4804/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
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
Alexander A.
Firsov,1,*
Sergey N.
Vostrov,1
Alexander A.
Shevchenko,1
Yury A.
Portnoy,1 and
Stephen
H.
Zinner2
Department of Pharmacokinetics, Centre of
Science & Technology LekBioTech, Moscow 117246, Russia,1 and
Division of Infectious Diseases, Roger Williams Medical Center,
Rhode Island Hospital, Brown University, Providence, Rhode
Island2
Received 30 January 1998/Returned for modification 19 May
1998/Accepted 10 August 1998
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ABSTRACT |
Time-kill studies, even those performed with in vitro dynamic
models, often do not provide definitive comparisons of different antimicrobial agents. Also, they do not allow determinations of equiefficient doses or predictions of area under the
concentration-time curve (AUC)/MIC breakpoints that might be related to
antimicrobial effects (AMEs). In the present study, a wide range of
single doses of trovafloxacin (TR) and twice-daily doses of
ciprofloxacin (CI) were mimicked in an in vitro dynamic model. The AMEs
of TR and CI against gram-negative bacteria with similar
susceptibilities to both drugs were related to AUC/MICs that varied
over similar eight-fold ranges [from 54 to 432 and from 59 to 473 (µg · h/ml)/(µg/ml), respectively]. The observation periods
were designed to include complete bacterial regrowth, and the AME was
expressed by its intensity (the area between the control growth in the
absence of antibiotics and the antibiotic-induced time-kill and
regrowth curves up to the point where viable counts of regrowing
bacteria equal those achieved in the absence of drug
[IE]). In each experiment monoexponential
pharmacokinetic profiles of TR and CI were simulated with
half-lives of 9.2 and 4.0 h, respectively. Linear
relationships between IE and log AUC/MIC were
established for TR and CI against three bacteria: Escherichia
coli (MIC of TR [MICTR] = 0.25 µg/ml; MIC of CI
[MICCI] = 0.12 µg/ml), Pseudomonas
aeruginosa (MICTR = 0.3 µg/ml; MICCI = 0.15 µg/ml), and Klebsiella pneumoniae
(MICTR = 0.25 µg/ml; MICCI = 0.12 µg/ml). The slopes and intercepts of these relationships differed for
TR and CI, and the IE-log AUC/MIC plots were
not superimposed, although they were similar for all bacteria with a
given antibiotic. By using the relationships between IE and log AUC/MIC, TR was more efficient than
CI. The predicted value of the AUC/MIC breakpoint for TR [mean for all
three bacteria, 63 (µg · h/ml)/(µg/ml)] was approximately
twofold lower than that for CI. Based on the
IE-log AUC/MIC relationships, the respective dose (D)-response relationships were reconstructed.
Like the IE-log AUC/MIC relationships, the
IE-log D plots showed TR to be more efficient than CI. Single doses of TR that are as efficient as two
500-mg doses of CI (500 mg given every 12 h) were similar for the
three strains (199, 226, and 203 mg). This study suggests that in vitro
evaluation of the relationships between IE and
AUC/MIC or D might be a reliable basis for comparing
different fluoroquinolones and that the results of such comparative
studies may be highly dependent on their experimental design and datum quantitation.
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INTRODUCTION |
Over the past few decades
development of new antimicrobial agents with improved pharmacokinetics
has been a major focus of the pharmaceutical industry. The actual
antimicrobial potential as well as the possible advantages of a newly
developed drug over its precursors cannot be demonstrated by
traditional methods, especially if the comparators have similar
intrinsic activities. In fact, the usual in vitro estimates of
antimicrobial activity (MIC, minimum bactericidal concentration,
results of time-kill studies) determined at constant drug
concentrations (static conditions) do not consider pharmacokinetic
parameters. Also, data obtained with animal models of infection have
limited relevance to clinical practice because of different dosing
considerations associated with the scaling up to the doses used for humans.
The true therapeutic potential of pharmacokinetically different
antimicrobial agents can be revealed by using in vitro dynamic models that simultaneously consider both intrinsic activity and the
pharmacokinetics of antibiotics. These models have been applied in
comparative studies with many antimicrobial agents (3, 10, 21). However, all too often, the full potential of this
approach is not realized because of inappropriate experimental design
and/or suboptimal quantitation of bacterial killing and regrowth curves or the antimicrobial effect itself. More specifically, most of these
studies have simulated human pharmacokinetics over a narrow dosage range. Although this approach is reasonable for
predicting the antimicrobial effect of a drug on a given organism, it
might not be optimal for an accurate comparison of different
drugs, especially if the antimicrobial effects are close to either
minimum or maximum values (8). Moreover, the one-dose nature
of such studies does not provide evaluation of the equiefficient dose of a new drug (the dose of the new drug that produces the same effect
observed with a reference drug at its usual dose). Finally, most
studies do not include a sufficient duration of observation to cover
the entire regrowth phase in time-kill curve studies and therefore do
not provide an accurate quantitation of the total antimicrobial effect.
The purpose of this investigation was (i) to compare the kinetics of
bacterial killing and regrowth for gram-negative pathogens with similar
susceptibilities to trovafloxacin and ciprofloxacin by in vitro
simulation of their human pharmacokinetics over a wide range of area
under the concentration-time curve (AUC)/MIC ratios; (ii) to establish
the relationships between the AUC/MIC ratio and the antimicrobial
effect as expressed by its intensity (the area between the control
growth and time-kill and regrowth curves estimated up to the point
where viable counts on the regrowth curve are close to maximum values
observed without drug [IE]
[9]) for each drug; (iii) to propose the AUC/MIC
breakpoint for predicting an acceptable antimicrobial effect; and (iv)
to predict the single dose of trovafloxacin which would be as efficient
as two 500-mg doses of ciprofloxacin.
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MATERIALS AND METHODS |
Antimicrobial agents and bacterial strains.
Trovafloxacin
mesylate and ciprofloxacin lactate powders (kindly provided by Roerig,
a division of Pfizer, and by Bayer AG, respectively) were used in the
study. Stock solutions of the quinolones were prepared in sterile
distilled water.
The clinical isolates Escherichia coli 224, Pseudomonas aeruginosa 48, and Klebsiella
pneumoniae 121 were used in the study. Susceptibility testing was
performed in duplicate in Ca2+- and
Mg2+-supplemented Mueller-Hinton broth at an inoculum size
of 106 CFU/ml after 24 h of quinolone exposure. The
MICs of trovafloxacin for these organisms, 0.25, 0.3, and 0.25 µg/ml,
respectively, were comparable to those of ciprofloxacin (0.12, 0.15, and 0.12 µg/ml, respectively).
Simulated pharmacokinetic profiles.
Preliminary examination
of the absorption and distribution phases of the pharmacokinetic
profiles of trovafloxacin and ciprofloxacin in humans (31,
32) showed that the respective areas contribute less than 10% to
the total AUCs (1 and 7%, respectively). Since the contribution of
these two phases is relatively small, the comparison of the
antimicrobial effects of trovafloxacin and ciprofloxacin was performed
by using simulations of simple monoexponential profiles. The simulated
half-lives (9.25 h for trovafloxacin and 4.0 h for ciprofloxacin)
were consistent with the values reported in humans: 7.2 to 9.9 h
(27, 32) and 3.2 to 5.0 h (2, 17, 31), respectively.
In all experiments single doses of trovafloxacin and two doses of
ciprofloxacin administered every 12 h were mimicked. The
simulated
values of the AUC and the respective amounts (
A) of
the
drugs administered in the model were chosen with respect to
the MICs
for the three organisms to provide similar eightfold
ranges of the
AUC/MIC ratios. These ratios averaged from 54 to
432 (µg · h/ml)/(µg/ml) for trovafloxacin and from 59 to 473 (µg
· h/ml)/(µg/ml) for ciprofloxacin (Table
1). For ciprofloxacin,
the AUC values
presented in Table
1 reflect the sum of two AUCs
provided by two doses
of the quinolone administered at a 12-h
interval taking into account
the residual concentrations at the
end of the first interval.
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TABLE 1.
Simulated AUCs and the respective values of total
amounts of trovafloxacin and ciprofloxacin administered in
the model
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To provide similar AUC/MIC ratios of trovafloxacin and ciprofloxacin,
the latter of which has a shorter half-life, the peak
concentration/MIC
ratios of ciprofloxacin were higher than those
of trovafloxacin. This
is illustrated by the time (
t) courses
of trovafloxacin and
ciprofloxacin concentrations (
C) related
to the MIC at one
of the AUC/MIC ratios (Fig.
1; natural
C/MIC
scale) and a series of the pharmacokinetic profiles
simulating
different AUC/MIC ratios (Fig.
2; logarithmic
C/MIC scale).
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FIG. 1.
In vitro simulated pharmacokinetic profiles of
trovafloxacin ( ) and ciprofloxacin (---) at
AUC/MIC ratios of 59 and 54 (µg · h/ml)/(µg/ml),
respectively.
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FIG. 2.
In vitro simulated pharmacokinetic profiles of
trovafloxacin () and ciprofloxacin (---). The
averaged value of the simulated AUC/MIC ratios [in (µg · h/ml)/(µg/ml)] is indicated by the number at each plot.
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In vitro dynamic model and operating procedure.
The in vitro
dynamic model described previously (13) was used in the
study. Briefly, the model consisted of two connected flasks, one of
them containing fresh Ca2+- and
Mg2+-supplemented Mueller-Hinton broth and the other, the
central unit, containing the same broth and either a bacterial culture alone (control growth experiments) or a bacterial culture plus antibiotic (killing and regrowth experiments). The central unit was
incubated at 37°C in a shaking water bath. Peristaltic pumps (Minipuls 2; Gilson) circulated fresh nutrient medium to the bacteria or the bacteria and antibiotic mixture and from the central 40-ml unit
at a flow rate of 3 or 7 ml/h when simulating trovafloxacin or
ciprofloxacin pharmacokinetics, respectively. Hence, the clearances provided by the designed flow rates plus the volume of the central unit
ensured monoexponential elimination of the quinolones and bacteria from
the system with elimination rate constants of 0.075 h
1
(half-life = 9.25 h) and 0.170 h1
(half-life = 4.0 h), respectively. Accurate simulations of
the desired pharmacokinetic profiles were provided by maintaining a
constant volume of the central unit and constant flow rates. Validation
of the model by determination of ciprofloxacin and trovafloxacin
concentrations by high-pressure liquid chromatography showed no
systematic deviation of the observed values from the expected values as
reported previously (12).
The system was filled with sterile Mueller-Hinton broth and was placed
in a temperature-regulated incubator at 37°C. The central
unit was
inoculated with 18-h cultures of
E. coli,
P. aeruginosa,
or
K. pneumoniae, and after a further 2-h
incubation, trovafloxacin
or ciprofloxacin was injected into the
central unit. The resulting
exponentially growing cultures approached
approximately 10
6 CFU/ml. Exact values (standard
deviations) of the starting inocula
of
E. coli,
P. aeruginosa, and
K. pneumoniae were 5.93 (0.10),
5.99 (0.08), and 5.97 (0.05) log CFU/ml,
respectively.
The duration of the experiments was defined in each case as the time
until bacteria exposed to antibiotics (
NA)
reached the
maximum numbers observed without antibiotic (control growth
[
Nc]),
i.e., the time when
NA becomes equal to
NC.
In all cases the experiments
were stopped when
NA reached
10
11 CFU/ml. Since the
experiments that simulated low AUC/MIC ratios
met this requirement
earlier than those that simulated high AUC/MIC
ratios, the duration of
the former experiments was shorter than
that of the latter: the lower
the AUC/MIC ratio, the shorter the
observation period (Fig.
2).
Quantitation of bacterial growth and killing.
In each
experiment 0.1-ml samples were withdrawn from bacterium-containing
media removed from the central unit throughout the observation period,
at first every 30 min, later hourly, then every 3 h, and, during
the last 6 to 7 h, again hourly. These samples were subjected to
serial 10-fold dilutions with chilled, sterile 0.9% NaCl and were
plated in duplicate on Mueller-Hinton agar. Antibiotic carryover at low
counts was avoided by washing the bacteria with 0.9% NaCl and
resuspending them in saline prior to plating. After overnight
incubation at 37°C the resulting bacterial colonies were counted, and
the numbers of CFU per milliliter were calculated. The limit of
detection was 2 × 102 CFU/ml. High within- and
interday reproducibilities of the results have been reported previously
(12).
To reveal possible changes in susceptibility, the quinolone
concentrations (
Cregrowth) corresponding to the
time when the
numbers of surviving organisms in the regrowth curves
reached
the level of the initial inoculum were determined in each run
(
10). No AUC/MIC-induced systematic differences in the
Cregrowth values were documented with any of the
regimens; moreover, the
appearance of bacterial regrowth was associated
with quinolone
concentration-to-MIC ratios of
unity.
Quantitative evaluation of the antimicrobial effect.
The
antimicrobial effect (E) was determined as the difference
log NC 
log NA. For
each pair of bacterial growth-regrowth and control growth curves,
IE was estimated as the area between these
curves which is equivalent to the area under the E-t curve from the zero point (the moment of drug input into the model) up to the
time when viable counts on the regrowth curve are close to the maximum
values observed without drug (9). The upper limit of
bacterial numbers in the regrowth and control growth curves and the
lower limit in the time-kill curve used to determine the
IE were 1 × 1011
(13) and 2 × 102 CFU/ml, respectively
(Fig. 3).
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FIG. 3.
Schematic presentation of the IE
determination applied to the kinetics of killing and regrowth of
K. pneumoniae exposed to a single dose of trovafloxacin
[AUC/MIC = 55 (µg · h/ml)/(µg/ml)] and two doses of
ciprofloxacin [AUC/MIC = 61 (µg · h/ml)/(µg/ml)].
IE describes the dashed area between the control
growth (empty symbols) and killing and regrowth (filled symbols) curves
limited from above by a level of 1011 CFU/ml.
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Quantitative relationships between the effect and the AUC/MIC or
dose.
The IE versus log AUC/MIC data sets
obtained with each quinolone against E. coli, K. pneumoniae, and P. aeruginosa were fitted by the
equation IE = a + b
log AUC/MIC (equation 1).
To express the antimicrobial effects as a function of quinolone dose
(
D), the AUC in the linear relationship between
IE and
log AUC that corresponds to equation 1 written for a given quinolone-pathogen
pair was substituted by
D according to the polynomial equation
AUC =
c +
dD +
eD2
(equation
2).
The values of
c,
d, and
e for
trovafloxacin (
0.01, 7.5 × 10
2, and 9.6 × 10
5, respectively) and for ciprofloxacin (0.10, 1.4 × 10
2, and 7.5 × 10
6, respectively)
were calculated by considering the curvilinear
pattern of the
AUC-
D plots (Fig.
4)
reconstructed from reported
data (
2,
27).
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FIG. 4.
Dose-dependent changes in the quinolone AUCs fitted by
polynomial equations of the second order. The observed AUCs are shown
by open symbols, and the respective theoretical values are indicated by
the lines.
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When predicting the AUC/MIC breakpoint for trovafloxacin, the reported
breakpoint value for ciprofloxacin, 125 (µg · h/ml)/(µg/ml),
that correlated with bacterial eradication in patients with respiratory
tract infections (
14) was used. This reference breakpoint
reflects
the critical value of the area under the inhibitory curve
(AUIC)
that is very similar to the AUC/MIC, since AUIC is defined by
the AUC/MIC measured for the time period where
C is greater
than
the MIC (
24).
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RESULTS |
The time courses of viable counts that reflect killing and
regrowth of E. coli, P. aeruginosa, and K. pneumoniae exposed to monoexponentially decreasing concentrations
of trovafloxacin and ciprofloxacin as well as the respective control
growth curves are shown in Fig. 5. As
seen in Fig. 5, the time-kill curves observed with both quinolones
against these three bacterial species yielded similar patterns. At the
AUC/MIC ratios studied, the regrowth followed a remarkable reduction in
bacterial numbers. Unlike the parameter of minimal bacterial
numbers after antibiotic exposure, the shift of the regrowth
phase to the right along the time axis was distinctly dependent
on the simulated AUC/MIC: the higher the AUC/MIC, the later
the regrowth. For all three bacterial species exposed to trovafloxacin
and at every AUC/MIC ratio simulated, regrowth of bacteria with
trovafloxacin was observed later than regrowth of bacteria with
ciprofloxacin.
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FIG. 5.
The kinetics of killing and regrowth of gram-negative
bacteria exposed to trovafloxacin () and ciprofloxacin
(---). The simulated AUC/MIC ratio [in (µg · h/ml)/(µg/ml)] is indicated by the number at each curve. The control
growth curves are indicated by thin lines.
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The inherent difference in trovafloxacin and ciprofloxacin efficacies
becomes even more evident when the relationships between IE and log AUC/MIC are examined (Fig.
6). As seen in Fig. 6, the antimicrobial
effect of trovafloxacin was more pronounced than that of
ciprofloxacin against these three microorganisms over the entire
AUC/MIC range studied. Regardless of the microorganism, a specific relationship was established for each of the quinolones. The
slopes of the IE-log AUC/MIC plots differed in
shape and were 1.5- to 2.5-fold higher with trovafloxacin than with
ciprofloxacin, i.e., 240 versus 160 (log CFU/ml) · h for
E. coli, 309 versus 127 (log CFU/ml) · h for
P. aeruginosa, and 282 versus 166 (log CFU/ml) · h for K. pneumoniae. These differences resulted in more striking contrasts between the antimicrobial effects produced by
trovafloxacin and ciprofloxacin at high AUC/MIC ratios. For example, at
an AUC/MIC ratio of 125 (µg · h/ml)/(µg/ml), which is
considered to be a significant breakpoint for predicting acceptable clinical outcome (14), the IEs of
trovafloxacin for E. coli, P. aeruginosa,
and K. pneumoniae were 1.41-, 1.41-, and 1.42-fold higher, respectively, than those of ciprofloxacin. The respective differences in the antimicrobial effect at an AUC/MIC ratio of 250 (µg · h/ml)/(µg/ml) were more pronounced, with
IE values being 1.43-, 1.57-, and 1.47-fold
higher, respectively, for trovafloxacin. Thus, despite similar or even
lower intrinsic activities, at a given AUC trovafloxacin is more
efficient than ciprofloxacin.
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FIG. 6.
AUC/MIC-dependent antimicrobial effects of trovafloxacin
and ciprofloxacin on gram-negative bacteria as expressed by the
IE parameter. The equivalent value of the
breakpoint (BP) for trovafloxacin is indicated by the boxed number.
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Based on the IE-AUC/MIC relationships, the
AUC/MIC breakpoints for trovafloxacin which correspond to the AUC/MIC
breakpoints for ciprofloxacin were predicted. Taking the
IE produced by the AUC/MIC of 125 (µg · h/ml)/(µg/ml) of ciprofloxacin as acceptable, the AUC/MIC of
trovafloxacin which produces the same IE was
estimated. The predicted breakpoint values of the
trovafloxacin AUC/MIC for E. coli, P. aeruginosa, and K. pneumoniae were close: 58.9, 68.5, and 61.5 (µg · h/ml)/(µg/ml), respectively. Thus, an
average AUC/MIC ratio of 63 (µg · h/ml)/(µg/ml) for
trovafloxacin administered as a single dose might be equivalent to that
corresponding to two doses of ciprofloxacin.
Based on the relationship (equation 1) between
IE and log AUC/MIC and the relationship
(equation 2) between AUC and D, the respective relationships
between IE and log D were
reconstructed for each organism. Due to the higher AUCs produced by a
given dose of trovafloxacin, the IE-log
D curves differ from those for ciprofloxacin more than the
respective linear IE-log AUC/MIC plots. As seen
in Fig. 7, both the slopes and the
positions of the IE-log D plots are
quite different for the two quinolones. So, the antimicrobial effect of
trovafloxacin is more dose dependent than that of ciprofloxacin. As
seen in Fig. 7, the effects produced by the originally proposed dose of
trovafloxacin (300 mg) against E. coli, P. aeruginosa, and K. pneumoniae are 20 to 30%
greater than those produced by two 500-mg doses of ciprofloxacin. Based
on the IE-log D curves, the
single dose of trovafloxacin which is as efficient as two 500-mg doses
of ciprofloxacin (500 mg given twice daily) was established. Like the
predicted AUC/MIC breakpoints, the estimates of the equiefficient dose of trovafloxacin for E. coli, P. aeruginosa, and K. pneumoniae were close to each
other: 199, 226, and 203 mg, respectively. Thus, acceptable
antimicrobial effects might be provided by a single trovafloxacin dose
of approximately 209 mg.
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FIG. 7.
Dose-dependent antimicrobial effects of trovafloxacin
and ciprofloxacin. TD is the therapeutic dose of ciprofloxacin (500 mg
given twice daily). The equiefficient doses of trovafloxacin are
indicated by the boxed numbers.
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DISCUSSION |
Although comparison of pharmacokinetically different antimicrobial
agents is frequently cited as a reason to introduce in vitro dynamic
models, these sophisticated time-kill studies do not often allow such
quantitative comparisons because of inappropriate experimental design
and/or suboptimal quantitation of the data. Such shortcomings are also
inherent in specific time-kill studies with fluoroquinolones in dynamic
models. In most of these studies the antimicrobial effects were
compared only when one dose level (4, 30, 33) or a narrow
dose range (5, 18, 19) of the quinolones was mimicked.
Moreover, the designed doses or the respective simulated AUCs were
usually chosen at the "clinically relevant" level without respect
to the MICs, even though the AUC/MIC ratio but not the AUC itself or
the dose has been suggested as a predictor of a quinolone's
antimicrobial effect (23-25). The design-associated
differences in the AUC/MIC ratios for the drugs often resulted in
efficacies whose differences were either excessive or negligible
(4, 5, 18, 19, 30, 33) for meaningful quantitative
comparisons. These factors really prevent accurate interpretation of
the observed effects and preclude comparison of quinolones in terms of
the AUC/MIC-response curve. On the other hand, such comparisons were
not performed even in the most comprehensive study of two
fluoroquinolones (ciprofloxacin and ofloxacin) which simulated a
10-fold range of AUC/MIC ratios (20). Because of the
investigators' a priori assumption about the quinolone-independent nature of relationships between the antimicrobial effect and AUC/MIC, only combined data for ciprofloxacin and ofloxacin were analyzed in their report.
Also, in many of the cited studies, an adequate comparison of
fluoroquinolones was not possible due to an insufficient duration of
the observation period (usually 24 h), which might or might not
include the entire regrowth phase. The importance of the full evaluation of this regrowth phase has recently been emphasized in the
accurate determination of the antimicrobial effect of ciprofloxacin (13), as assessed by its intensity. In the present
comparative study the antimicrobial effects of trovafloxacin and
ciprofloxacin were related to a wide range of AUC/MIC ratios for each
drug. This approach made possible an accurate comparison of the
fluoroquinolones in terms of the relationships between the
antimicrobial effect and logarithms of AUC/MIC or dose.
With each bacterial strain studied, a specific linear relationship
between the antimicrobial effect (as expressed by its intensity, IE) and log AUC/MIC was inherently associated
with a given quinolone (Fig. 6). The IE-log
AUC/MIC plots for trovafloxacin and ciprofloxacin differed
substantially and were not superimposed. Comparison of these plots
showed distinct advantages for trovafloxacin over ciprofloxacin: at
each of the AUC/MIC ratios simulated, trovafloxacin produced a greater
antimicrobial effect against the three gram-negative bacteria than
ciprofloxacin, despite similar intrinsic activities (MICs). These data
suggest that a given AUC of trovafloxacin might be "more
productive" than the same AUC of ciprofloxacin with respect to the
antimicrobial effects which are pharmacokinetic profile dependent in
their nature.
Since the slopes of the IE-log AUC/MIC plots
were different for the two quinolones, the difference between their
effects depends on the AUC/MIC: the higher the AUC/MIC, the
greater the difference. To make definitive quantitative comparisons
of the quinolones, a certain reference value of AUC/MIC is
needed. In this study an AUC/MIC value of 125 (µg · h/ml)/(µg/ml), which has been reported to be a significant breakpoint
in an in vivo study with ciprofloxacin (14), was used as the
reference value. The respective equivalent values of AUC/MIC of
trovafloxacin, i.e., AUC/MICs that provide the same
IEs as ciprofloxacin, were twofold lower:
58.9, 68.5, and 61.5 (µg · h/ml)/ (µg/ml) for
E. coli, P. aeruginosa, and K. pneumoniae, respectively.
Of course, the average AUC/MIC ratio of 63 (µg · h/ml)/(µg/ml) of trovafloxacin as predicted in this study might or
might not correspond to the AUC/MIC breakpoint that remains to be
established in an in vivo setting. Indeed, the AUC/MIC breakpoint
reported by Forrest et al. (14) was based on clinical data
with multiple ciprofloxacin dosing regimens, whereas only single doses
of trovafloxacin and two doses of ciprofloxacin were mimicked in our in
vitro study. Therefore, further correlations between the AUC/MIC
breakpoints based on in vitro and on in vivo data are necessary.
However, the described indirect approach to predicting the AUC/MIC
breakpoint for a new quinolone might be more useful than providing
a direct in vitro evaluation of the AUC/MIC breakpoint without any
reference to in vivo data (20). Unlike the cited study
(20), our approach provides the ability to operate with both
in vitro IE-log AUC/MIC relationships for the
two drugs and an in vivo-established AUC/MIC breakpoint for the
reference drug. Such an indirect prediction of the AUC/MIC
breakpoint does not require knowledge of the correspondence of
antimicrobial effects observed in vitro and in vivo or of the critical
value of the effect in vitro that corresponds to an acceptable outcome
in vivo.
The AUC/MIC analysis of the antimicrobial effect presented in this
report reveals differences between drugs that are related to inherent
pharmacokinetic properties (half-life, among others). To compare the
quinolones in terms of the dose-response relationships, the
IE-log AUC/MIC relationships were converted
into the respective relationships between IE and
log D, taking into account the fact that the
organisms studied are representative in terms of the MICs
(1, 6, 15). For all three gram-negative bacteria, the
antimicrobial effect of trovafloxacin was more dose dependent than that
of ciprofloxacin (Fig. 7). Based on the IE-log
D curves, the single doses of trovafloxacin which are as
efficient as two 500-mg doses of ciprofloxacin (the reference point)
were established. As with the predicted equivalent AUC/MICs,
estimates of the equiefficient dose of trovafloxacin were bacterial
species independent (199 to 226 mg).
Like the predicted AUC/MIC breakpoint, the single dose of trovafloxacin
which is as efficient as two doses of ciprofloxacin may or may
not be numerically identical to the same equiefficient daily dose. To
verify the correspondence between the equiefficient single dose
established in this study and the equiefficient daily dose of
trovafloxacin, clinical correlations are necessary. In this
light, the close correspondence between the equiefficient single dose
of trovafloxacin (209 mg) and its clinically proven daily dose
(200 mg [16, 22, 26]) in reality might only be encouraging.
It should be noted that in this model neither the
IE-log AUC/MIC nor
IE-log D relationships could have
been established were the observations discontinued at 24 h, as is
usual for most reported time-kill studies. Indeed, for three of the
four AUC/MIC ratios studied, discrimination between the two quinolones
would not have been possible because regrowth did not occur until
24 h (Fig. 5). Therefore, obvious differences between the
effects of trovafloxacin and ciprofloxacin could not have been
reflected by the numbers of surviving bacteria at 24 h, areas
under (area under the bacterial count curve [28]) or
above (area above the curve [29]) the log
NA-t curve or between this curve
and the respective control growth curve (area between the bacterial
count curves [11]) measured from 0 to 24 h, or
log Nmin, or the times to 100- and 1,000-fold reductions in the initial inoculum
(T99% and T99.9%,
respectively). As might be expected, no reasonable correlations between
dose and T99.9% were reported in a comparative
time-kill study with three quinolones (18). Unlike these
endpoints, the use of IE allowed the
differentiation of the effects of the two quinolones; moreover, the
determination of IE by its very definition
includes the entire killing and regrowth curve from the onset to the
end of the drug's effect.
In conclusion, the data presented here support the application of the
relationships between the antimicrobial effect and the AUC/MIC for
comparisons of antimicrobial agents (7). At first glance,
the use of a wide range of AUCs (or doses) as reported here might
appear to be a contradiction given the small range of the usual
clinical doses. However, since a clinical dose given to different
patients would be presented to bacteria of different susceptibilities,
the AUC/MIC ratios actually do vary widely in vivo. As shown in this
study, this situation may be simulated in vitro by varying the AUC with
a given organism since the relationships between the antimicrobial
effect and AUC/MIC are bacterial species independent
(12). Similar time-kill studies in in vitro
dynamic models might be applicable to other antibiotic classes.
| |
ACKNOWLEDGMENTS |
This study was supported by Roerig, a division of Pfizer.
We are grateful to H. Mattie for useful comments.
| |
FOOTNOTES |
*
Corresponding author. Mailing address: Department of
Pharmacokinetics, Centre of Science & Technology LekBioTech,
8 Nauchny proezd, Moscow 117246, Russia. Phone: 7 (095) 332-3435. Fax:
7 (095) 331-0101. E-mail: Biotec{at}glas.apc.org.
| |
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Abstract
Introduction
