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Antimicrobial Agents and Chemotherapy, June 1999, p. 1406-1411, Vol. 43, No. 6
Clinique de Réanimation des Maladies
Infectieuses,
Received 28 September 1998/Returned for modification 3 February
1999/Accepted 7 April 1999
The effects of various regimens containing combinations of
Acinetobacter baumannii
is recognized as an increasingly resistant nosocomial pathogen,
responsible for pneumonia especially in mechanically ventilated
patients (7). Recent isolates of A. baumannii
have exhibited antibiotic resistance, making them extremely difficult
to treat (13). The majority of clinical isolates of A. baumannii overproduce cephalosporinase and are resistant to
aminoglycosides. In addition, strains resistant to virtually all
antibiotics, including imipenem, were recently responsible for
outbreaks in intensive care unit patients (9). Thus, since there is no "gold standard" for the treatment of nosocomial
pneumonia due to multiresistant A. baumannii, new
potentially active regimens must be urgently evaluated. We previously
demonstrated the enhanced in vitro killing of A. baumannii
by (This study was presented in part at the 36th Interscience Conference
on Antimicrobial Agents and Chemotherapy, New Orleans, La., 15 to 18 September 1996, and the 37th Interscience Conference on Antimicrobial
Agents and Chemotherapy, Toronto, Canada, 28 September to 1 October
1997.)
Drugs used.
The antimicrobial agents used in this study were
obtained from laboratory standard powders and were used immediately
after being diluted. The agents and their suppliers were ticarcillin and ticarcillin-clavulanate at ratios of 25/1, 15/1, and 10/1 (SmithKline Beecham, Nanterre, France); sulbactam (Pfizer, Orsay, France); imipenem (Merck Sharp & Dohme, Paris, France); and rifampin (Marion Merrell SA, Puteaux, France).
Bacterial strains.
Two different strains were used.
SAN-94040 is a cephalosporinase-overproducing strain resistant to
aminoglycosides and fluoroquinolones but susceptible to imipenem,
ticarcillin, and sulbactam. It was isolated from the blood culture of a
patient with nosocomial pneumonia. RCH-69 is a multiresistant strain
with low susceptibility to imipenem and susceptibility only to
rifampin. It was isolated from the peritoneal fluid of a patient with
postoperative peritonitis.
In vitro tests. (i) MICs and MBCs.
MICs and MBCs were
determined by agar dilution and broth dilution methods with geometric
twofold serial dilutions in Mueller-Hinton broth (MHB). The final
inoculum was 106 CFU/ml. The MIC was determined after
incubation at 30°C for 18 h. MBC endpoints were determined by
subculturing 100 µl from the first cloudy tube on MHB agar. The MBC
was defined as the antibiotic concentration inducing a 99.9% reduction
in CFU/ml (<10 CFU/plate) (20). All controls and test
samples were run in triplicate.
(ii) In vitro bactericidal effects of Animal experiments. (i) Experimental infection.
Six-week-old, specific-pathogen-free, C3H/HeN female mice (20 g) were
used. Animals were rendered transiently neutropenic by injecting
cyclophosphamide (Mead Johnson Pharmaceuticals, Evansville, Ind.)
intraperitoneally (i.p.) (150 mg/kg of body weight) in a volume of 0.2 ml 4 and 3 days before A. baumannii inoculation (day 0). The
mice were anesthetized by i.p. injection of 0.2 ml of 0.65% sodium
pentobarbital given before bacterial inoculation. Animals were infected
by intratracheal instillation via the mouth as previously described
(15). Briefly, the trachea was cannulated with a blunt
needle, and 50 µl of a bacterial suspension containing 108 CFU/ml (spectrophotometrically controlled) was
instilled. The size of inoculum was confirmed by quantitative cultures.
The efficacy of inoculation was systematically tested by quantitation
of viable organisms in the lungs removed from two control untreated
infected animals, immediately after bacterial inoculation and 3 h later.
(ii) In vivo bactericidal effect of therapy.
In these sets
of experiments, the treatment was initiated 3 h after inoculation.
At that time, the log CFU (per gram of lung tissue) were 7.6 ± 0.49 for animals infected with SAN-94040 and 7.25 ± 0.71 for
animals infected with RCH-69. Regimens tested against SAN-94040.
Four i.p. injections of
the following regimens were given every 3 h: ticarcillin (500 mg/kg), imipenem (50 mg/kg), sulbactam (100 mg/kg),
ticarcillin-clavulanate at a ratio of 25/1 (500/20 mg/kg), ticarcillin
(500 mg/kg)-sulbactam (100 mg/kg), ticarcillin-clavulanate (500/20
mg/kg)-sulbactam (100 mg/kg), ticarcillin-clavulanate at a ratio of
15/1 (500/33 mg/kg)-sulbactam (100 mg/kg), and ticarcillin-clavulanate at a ratio of 10/1 (500/50 mg/kg)-sulbactam (100 mg/kg). A single i.p.
dose of rifampin (25 mg/kg) was administered alone or combined with
imipenem, sulbactam, or ticarcillin-clavulanate (25/1 ratio)-sulbactam. These doses were chosen according to previously published experimental models which have taken into account human kinetics (2, 3, 15,
23).
Regimens tested against RCH-69.
Four i.p. injections of the
following regimens were given every 3 h: imipenem (50 mg/kg),
sulbactam (100 mg/kg), imipenem (50 mg/kg)-sulbactam (100 mg/kg), and
ticarcillin-clavulanate at a ratio of 25/1 (500/20 mg/kg)-sulbactam
(100 mg/kg). A single i.p. dose of rifampin (25 mg/kg) (23)
was administered alone or combined with imipenem or
ticarcillin-clavulanate (25/1)-sulbactam.
Effect of therapy on survival rate.
In our previously
described model, mice were neutropenic only during the first 2 days of
infection. Transient leukocytosis was observed on day 3 (12,000/mm3). After day 4, surviving animals cleared
bacteria. In these experiments, treatment was initiated 8 h after
inoculation, when histological patterns of pneumonia were present
(15). At that time, the log CFU (per gram of lung tissue)
were 9.3 ± 0.44 for animals infected with the SAN-94040 strain
and 8.7 ± 0.7 for animals infected with RCH-69. The same regimens
as those administered in the in vivo bactericidal experiments were
given for the survival study. Pharmacokinetic parameters.
Pharmacokinetic parameters were
evaluated for infected mice. Concentrations of antibiotics in lungs and
sera were measured after administration of single doses of ticarcillin
(500 mg/kg), ticarcillin-clavulanate at a 25/1 ratio, sulbactam (100 mg/kg), and rifampin (25 mg/kg) given 3 h after infection. Animals
were killed by exposure to CO2 and exsanguinated by cardiac
puncture. Serum was separated and immediately stored at Drug assays.
Imipenem concentrations were determined by an
agar well microbiological assay with Bacillus subtilis
spores (Difco, Detroit, Mich.) as the indicator organism and antibiotic
medium 2 (Diagnostics Pasteur, Marnes-la-Coquette, France). Standard
samples were prepared in sulfonate buffer, pH 6 (15). The
calibration curve was linear from 0.06 to 64 mg/liter, and the limit of
sensitivity was 0.06 mg/liter. Variation within replicates was <5%.
The method for measuring ticarcillin concentrations was derived from
the technique described by Itoh and Yamada (11). Plasma and
lung concentrations were determined by reversed-phase high-performance
liquid chromatography (RP-HPLC) with UV detection at 205 nm.
Chromatographic separation was performed by using a C18
column (Ultrasphere; 250 by 4.6 mm; Beckman) with a mobile phase
consisting of PicA-orthophosphoric acid-water-acetonitrile
(0.3/0.1/79.5/20%). Proteins were precipitated from the samples with
acetonitrile. Limits of quantification were 2 mg/liter and 2 mg/kg for
plasma and lung samples, respectively.
Statistical analyses.
All bacterial counts and
pharmacokinetic data are presented as means ± SD. Analysis of
variance was used to compare intergroup differences between bacterial
counts. Survival rate data were analyzed by Student's t
test; P In vitro studies. (i) MICs and MBCs.
MICs and MBCs (milligrams
per liter) for SAN-94040 were as follows: ticarcillin alone, 32 and 64;
ticarcillin-clavulanate (ratio, 25/1), 32 and 64; imipenem, 0.5 and
0.5; and sulbactam, 0.5 and 0.5, respectively. The strain exhibited
lower susceptibility to rifampin, which had a MIC and an MBC of 8 and 8 mg/liter, respectively. MICs and MBCs (mg/liter) for RCH-69 were as
follows: ticarcillin alone, 256 and >256; ticarcillin-clavulanate
(ratio, 25/1), 512 and >512; imipenem, 8 and 32; sulbactam, 8 and 8;
and rifampin, 4 and 4, respectively.
(ii) In vitro bactericidal studies.
Against the two strains,
only two regimens, namely, ticarcillin-clavulanate-sulbactam and
rifampin alone, showed a true bactericidal effect (
0066-4804/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
In Vivo Efficacies of Combinations of
-Lactams,
-Lactamase Inhibitors, and Rifampin against Acinetobacter
baumannii in a Mouse Pneumonia Model
ABSTRACT
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
-lactams, -lactam inhibitor(s), and rifampin were assessed in a
recently described mouse model of Acinetobacter baumannii
pneumonia (M. L. Joly-Guillou, M. Wolff, J. J. Pocidalo, F. Walker, and C. Carbon, Antimicrob. Agents Chemother. 41:345-351,
1997). Two aspects of the therapeutic response were studied: the
kinetics of the bactericidal effect (treatment was initiated 3 h
after intratracheal inoculation, and bacterial counts were determined over a 24-h period) and survival (treatment was initiated 8 h after inoculation, and the cumulative mortality rate was assessed on
day 5). Two clinical strains were used: a cephalosporinase-producing strain (SAN-94040) and a multiresistant strain (RCH-69). For SAN-94040 and RCH-69, MICs and MBCs (milligrams per liter) were as follows: ticarcillin, 32, 64, 256, and >256, respectively;
ticarcillin-clavulanate, 32, 64, and 512, and >512, respectively;
imipenem, 0.5, 0.5, 8, and 32, respectively; sulbactam, 0.5, 0.5, 8, and 8, respectively; and rifampin, 8, 8, 4, and 4, respectively.
Against SAN-94040, four regimens, i.e., imipenem, sulbactam,
imipenem-rifampin, and ticarcillin-clavulanate (at a 25/1
ratio)-sulbactam produced a true bactericidal effect
(
3-log10 reduction of CFU/g of lung). The best survival
rate (i.e., 93%) was obtained with the combination of
ticarcillin-clavulanate-sulbactam, and regimens containing rifampin
provided a survival rate of 65%. Against RCH-69, only regimens
containing rifampin and the combination of imipenem-sulbactam had a
true bactericidal effect. The best survival rates (80%) were
obtained with regimens containing rifampin and sulbactam. These results
suggest that nonclassical combinations of -lactams, -lactamase
inhibitors, and rifampin should be considered for the treatment of
nosocomial pneumonia due to A. baumannii.
INTRODUCTION
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
-lactamase inhibitors combined with -lactams, particularly
ticarcillin-clavulanate and sulbactam (14). When we assessed
the in vitro activities of rifampin against 30 strains of A. baumannii, the median MIC was 3 mg/liter and was independent of
-lactam resistance (unpublished data). We recently described a new
mouse model of A. baumannii pneumonia which offers a
reproducible acute course of pneumonia and provides a rigorous test of
therapeutic drug efficacy (15). The current study was
designed to evaluate the efficacies of various monotherapies and
combined regimens including -lactams, -lactamase inhibitors, and/or rifampin in treatment of experimental pneumonia caused by
A. baumannii.
MATERIALS AND METHODS
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
-lactams, -lactams
and -lactamase inhibitors, and rifampin.
Tubes containing fresh
MHB and appropriate amounts of antibiotics were inoculated with an
aliquot from a 6-h culture to give a final density of 107
CFU/ml to simulate in vivo conditions at the start of therapy. Shaking
cultures of SAN-94040 and RCH-69 strains were incubated at 37°C for
18 h. Aliquots were sampled after 0, 3, 5, 7, and 24 h of
incubation and immediately diluted with 10 ml of sterile saline (0.9%)
to prevent any drug carryover. CFU were counted on agar plates
(20). All controls and test samples were run in duplicate in
a single experiment. Bactericidal studies were performed with
antibiotics at twice the MIC when tested alone and at the MIC when
tested in combination. When MICs were too high to have any clinical
relevance, antibiotics were tested at the breakpoint. In order to
detect resistant mutants, rifampin was also tested at the MIC.
-Lactams and -lactamase inhibitors
were administered in four i.p. doses, and rifampin was administered as
a single dose. Bacterial counts in lungs were determined every 3 h, over a 12-h period from the start of treatment; 15 animals/regimen
were used (three animals/data point). For quantitative bacteriological
studies, lungs were removed, weighed, and homogenized in 10 ml of
saline. Serial 10-fold dilutions of the homogenates were plated onto
Trypticase soy agar (0.1 ml; 9-cm-diameter plates). Results are
expressed as the means ± standard deviations (SD) of
log10 CFU/gram of lung tissue. The lower limit of detection was 102 CFU/g of lung. The 
log10 was defined
for all regimens as the change in bacterial counts from the onset of
treatment to 3 h after the last -lactam dose.
-Lactams and -lactamase inhibitors
were administered every 3 h as five i.p. injections, and rifampin
was administered as a single i.p. dose. The observation period was 5 days, a time at which no further deaths occurred. Cumulative survival
rates were recorded daily and compared. Controls consisted of infected,
untreated animals. Experiments were repeated twice with 15 to 18 animals in each treatment group and 22 to 27 control animals.
80°C. The
lungs were removed from exsanguinated mice, briefly washed in sterile water, weighed, and cryohomogenized. Sera and lungs were collected from
groups of three mice at the following times postinjection: 10, 15, 30, 45, and 60 min and 2 h for ticarcillin, clavulanate, and sulbactam
and 10, 30, and 60 min and 2, 6, 12, 24, and 48 h for rifampin.
Pharmacokinetic parameters were evaluated by standard methods
(10). The parameters were maximal concentration observed (Cmax), time to Cmax, and
the elimination half-life calculated by using linear least-squares
regression. The inhibitory quotient (IQ) was calculated as follows:
IQ = Cmax/MIC. t MIC is the
time at which the antibiotic level exceeded the MIC in serum or lungs.
0.05 for either test was considered statistically significant.
RESULTS
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
3-log decrease at
T0 + 7 h). A low bactericidal effect was
observed with all other regimens (Table
1). Resistant strains (MIC, 64 mg/liter)
were detected after 24 h of SAN-94040 culture containing a
rifampin concentration equal to the MIC. With the RCH-69 strain, a
regrowth was observed without the emergence of a resistant strain.
TABLE 1.
In vitro bactericidal activities of -lactams,
-lactamase inhibitors, and rifampin against A. baumannii
SAN-94040 and RCH-69a
Pharmacokinetic parameters.
The main pharmacodynamic
parameters of each antibiotic are reported in Table
2. For rifampin, high peak concentrations
and prolonged half-lives were observed in sera and lungs. Due to
differences in MICs, the Cmax/MIC ratios (IQs)
and the times above the MICs for the three -lactams in sera and
lungs were much higher against SAN-94040 than against RCH-69.
Clavulanate (20 mg/kg) produced peak concentrations in sera and lungs
of 35 mg/ml and 17 mg/kg, respectively. The elimination half-lives in
sera of the different -lactams were very short (<0.5 h) compared to
that of rifampin (12 h). Although the t MICs in sera of
the -lactams differed, they were all much higher for SAN-94040
than for RCH-69. In the lung, the sulbactam t MIC was
longer than that of imipenem for SAN-94040, and it was the only
-lactam to reach concentrations above the MIC for RCH-69.
|
In vivo efficacy (i) In vivo bactericidal effects against
SAN-94040.
Table 3 shows the in vivo
bactericidal effects of various regimens against SAN-94040. When the
bactericidal effect was assessed at 12 h as the
log10, all monotherapies administered, except ticarcillin, significantly reduced the bacterial counts in lungs compared to controls 3 h postinstillation (P < 0.05). However, only two monotherapies, i.e., sulbactam and
imipenem, provided true bactericidal effects (3-log10
decrease of bacterial count). No significant differences were observed
between regimens containing various ratios of clavulanate (data not
shown). Only two combinations provided true bactericidal effects,
i.e., rifampin-imipenem and ticarcillin-clavulanate (25/1
ratio)-sulbactam, although there was essentially no effect of
combinations beyond that of the single most active component.
|
(ii) Survival in mice infected with SAN-94060. Compared to controls, sulbactam, rifampin, or imipenem alone significantly (P < 0.001) prolonged survival, as did ticarcillin-clavulanate (25/1 ratio) (P = 0.02) (Fig. 1a). Among the regimens administered as combinations, the best survival rate (93%) was obtained with ticarcillin-clavulanate (25/1 ratio)-sulbactam. All combinations containing rifampin yielded a survival rate of at least 65%. Ticarcillin-sulbactam, although less effective, significantly prolonged survival compared to controls (P = 0.02) (Fig. 1b).
|
, controls; 
, imipenem; 
, sulbactam;
, ticarcillin; 
, ticarcillin-clavulanate; 
, rifampin. Symbols
for combinations (b and d): (iii) In vivo bactericidal effects against RCH-69.
Table 3
also shows the in vivo bactericidal effects of the various regimens
against RCH-69. All regimens except sulbactam and
ticarcillin-clavulanate-sulbactam significantly reduced the bacterial
counts in lungs compared to controls 3 h postinstillation (P < 0.05). When the bactericidal effect was assessed
at 12 h as the log10, rifampin and imipenem, but
not sulbactam, provided true bactericidal effects (3-log decrease of
bacterial count). Compared to controls 3 h postinstillation, all
combined regimens except ticarcillin-clavulanate (25/1 ratio)-sulbactam
provided true bactericidal effects. Again, there was essentially no
effect of combinations beyond that of the single most active component.
(iv) Survival in mice infected with RCH-69.
Among agents
administered as monotherapy (Fig. 1c), the best survival rate was
obtained with rifampin (94%). Moreover, rifampin alone at the given
dose significantly prolonged survival compared to that for animals
treated with imipenem alone (P = 0.01). Although sulbactam was bacteriostatic only in vivo, the survival rate of mice
treated with this compound alone reached 76% (Fig. 1c). All combined
regimens (Fig. 1d) significantly prolonged survival compared to control
values (P < 0.01). The best effect was obtained with those containing rifampin (80%). These regimens at the given doses
significantly prolonged survival compared to those in animals treated
with imipenem-sulbactam (P = 0.03) or
ticarcillin-clavulanate-sulbactam (P = 0.05).
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DISCUSSION |
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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
|
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Faced with the increasing role of A. baumannii in
nosocomial pneumonia associated with mechanical ventilation, a mouse
model of pneumonia caused by A. baumannii that resembles the
human disease (1) was developed (15).
Experimental models of acute systemic infection and urinary tract
infection with Acinetobacter spp. were first developed in
mice in 1985 to examine the virulence factors and the efficacy of
therapy with tetracyclines and aminoglycosides (18) or
sulbactam (19). These models demonstrated a good correlation between antibiotics that were active in vitro and their 50% effective doses. However, nearly 50% of the strains isolated in France are resistant to all available antibiotics except imipenem and sulbactam. Therefore, treatment of nosocomial infections due to A. baumannii spp. has become extremely complicated, and imipenem is
often the only effective treatment that can be prescribed except when
imipenem-resistant strains emerge. During the last few years, several
outbreaks of imipenem-resistant strains have been described for
different intensive care units in Europe and the United States (9,
25). Recently, the emergence of imipenem resistance during
treatment was described (5). Imipenem, which is considered
to be the gold standard therapy for A. baumannii infections,
had a bactericidal effect in the lung against both of our tested
strains. However, the magnitude of this effect was relatively low,
being around 3 log10 CFU, even after administration of four
i.p. doses. The in vivo bactericidal effect observed against RCH-69 may
be explained by the high concentrations of imipenem obtained in the
lungs and the presence of a postantibiotic effect of long duration
(15). This suboptimal effect was associated with a moderate
efficacy of mouse survival. It is tempting to draw a parallel between
these experimental results and the high mortality rate in patients with
ventilator-acquired pneumonia caused by A. baumannii and
usually treated with imipenem (7). These results prompted us
to evaluate therapeutic alternatives to imipenem in this model. An in
vitro study with a killing curve showed a synergistic or additive
effect of nonclassical combinations with -lactamase inhibitors
(clavulanate, sulbactam, or tazobactam) and ticarcillin or piperacillin
(14). That study pointed out the intrinsic activity of
sulbactam, which acts as a true -lactam against A. baumannii. The in vivo antibacterial activity of sulbactam had
already been tested against Acinetobacter calcoaceticus in mouse peritonitis (19). This drug was administered alone or in the ampicillin-sulbactam formulation to treat patients with nosocomial infections caused by imipenem-resistant
Acinetobacter strains (4, 26). In the present
study, sulbactam produced a bactericidal effect in mice inoculated with
the susceptible SAN-94040 strain but not in mice inoculated with
RCH-69. The dose used in our experiments was the same as that
previously given to mice (3). Three hours after i.p.
injection, concentrations in serum were very similar to those obtained
in humans after intravenous administration of 1 g (27).
In addition, the maximum pulmonary concentrations of sulbactam in mice
were also comparable to those measured in the alveolar lining fluid of
patients with respiratory infections (27). The
t MIC appeared to be an important pharmacodynamic determinant of the bactericidal activity of sulbactam in mouse lungs.
Indeed, pulmonary sulbactam concentrations were above the MIC for
SAN-94040 throughout the entire interval between two doses but during
only 43% of the interdose interval in animals challenged with RCH-69.
This finding is in accordance with previous experimental data
(6). Nonetheless, sulbactam achieved a satisfactory survival rate in animals inoculated with SAN-94040 or RCH-69, despite the differences observed in their in vivo bactericidal effects. These experimental results suggest that sulbactam administration to treat
severe pulmonary infections caused by A. baumannii should ensure concentrations in lung tissue above the MIC throughout the
entire interval between doses. Thus, short intervals and high doses
(e.g., 4 to 6 g), such as those used with a true -lactam, are
recommended. Ticarcillin was not effective against SAN-94040, although
its level in the lungs was above the MIC for the strain during 57% of
the dosing interval. This result is in accordance with a previous in
vitro study in which ticarcillin was less bactericidal than sulbactam
against a cephalosporinase-producing strain (14). We used a
ticarcillin-clavulanate dosage which provided concentrations in serum
within the ranges of those previously reported for mice (2)
and humans (12). The ticarcillin-clavulanic acid combination at a 25/1 ratio was moderately active, providing a weak bactericidal effect in lungs (<2 log10 CFU) and a mortality rate of
60% in animals infected with SAN-94040. In addition, no further
benefit was obtained by increasing the clavulanic acid dose within the combination. The absence of clavulanic acid activity against
cephalosporinase-producing strains could explain the poor results
observed in our model. At present, we do not know why the
ticarcillin-clavulanate-sulbactam combination yielded better activity,
as previously observed in vitro (14), and why the
combination of ticarcillin and sulbactam appeared to be antagonistic
compared to sulbactam alone. Given the limited number of available
antibiotics active against multiresistant strains, it is crucial to
evaluate new potentially effective regimens. As a matter of fact, the
in vitro bactericidal effect of rifampin observed against both strains
led us to test this antibiotic alone and in combination in our model.
Rifampin was very effective, providing a strong bactericidal effect in
lungs against RCH-69 and a high survival rate in animals inoculated
with either strain. These findings may reflect the very long time
during which rifampin concentrations in both sera and lungs are above
the MICs for these strains. The dose of 25 mg/kg was selected on the
basis of a previously published model of experimental murine
brucellosis (23) in which the maximal concentration of
rifampin in serum was 28 mg/liter. In another study,
Cmax was 11 mg/liter after a 10-mg/kg dose
(17). Thus, although the concentrations observed in our
study are very similar to those reported in previous experimental
models, they are somewhat higher than those usually observed in humans.
However, the relationship between the peak levels in serum and the size of the dose is nonlinear, since larger doses resulted in
greater-than-proportional peak levels. Thus, after a 1,200-mg dose, the
peak level in serum in patients is usually >30 mg/liter
(16). Considering the rifampin MIC against susceptible
A. baumannii strains, rifampin should certainly be
administered in high doses, like those given for severe staphylococcal
infections. Moreover, because of the high risk of development of
resistant mutants, rifampin should always be administered in
combination therapy. Indeed, in our model, all combinations containing
rifampin were effective. Although we did not observe the emergence of
resistant mutants in vivo, this phenomenon was detected in vitro after
24 h of culture containing a rifampin concentration equal to the
MIC for SAN-94040 (8 mg/liter).
A. baumannii is an opportunistic bacterium with a high degree of resistance to various antibiotics. As a consequence, the antibiotic effect is more often bacteriostatic than bactericidal. These characteristics could explain the discrepancies sometimes observed between a drug's bactericidal effect and its efficacy in survival, as we observed in the present study for sulbactam and RCH-69. Interference with some other host parameters was probably responsible for these discrepancies. Therefore, both the survival rate and in vivo bactericidal effects in lungs must be taken into consideration for the analysis and interpretation of the global antibiotic activity in this model. This approach has also been applied in a mouse model of pneumonia due to Streptococcus pneumoniae (21).
In conclusion, this study pointed out (i) the relatively low in vivo
bactericidal effect of imipenem on A. baumannii isolates, even against a susceptible strain, and (ii) the provision of a good
bactericidal effect against a sulbactam-susceptible strain by this
drug, which acts as a true -lactam. However, the best in vivo effect
was obtained with the triple combination of
ticarcillin-clavulanate-sulbactam. Furthermore, the high efficacy of
rifampin suggested that this antibiotic could be used in combination to
treat pneumonia caused by strains with reduced susceptibility to
imipenem and when the MIC is not >4 mg/liter.
Nonclassical combinations, containing antibiotics such as sulbactam, rifampin, and ticarcillin-clavulanate, could be effective alternative treatments for A. baumannii infections and warrant clinical evaluation.
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ACKNOWLEDGMENTS |
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This work was supported by a grant from SmithKline Beecham, France.
We are indebted to Janet Jacobson for technical assistance in the preparation of the manuscript.
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FOOTNOTES |
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* Corresponding author. Mailing address: Service de Microbiologie, Hôpital Louis Mourier, 178 rue des Renouillers, 92701 Colombes, France. Phone: 33 1 47 60 60 13. Fax: 33 1 47 60 60 48. E-mail: marie-laure.joly-guillou{at}lmr.ap-hop-paris.fr.
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REFERENCES |
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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
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