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Antimicrobial Agents and Chemotherapy, May 2001, p. 1487-1492, Vol. 45, No. 5
0066-4804/01/$04.00+0 DOI: 10.1128/AAC.45.5.1487-1492.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Improved Efficacy of Ciprofloxacin Administered in Polyethylene
Glycol-Coated Liposomes for Treatment of Klebsiella
pneumoniae Pneumonia in Rats
Irma A. J. M.
Bakker-Woudenberg,1,*
Marian T.
ten Kate,1
Luke
Guo,2
Peter
Working,2 and
Johan W.
Mouton3
Department of Medical Microbiology & Infectious Diseases, Erasmus University Medical Center
Rotterdam, 3000 DR Rotterdam,1 and
Department of Medical Microbiology, Canisius Wilhelmina
Hospital, Nijmegen,3 The Netherlands, and
ALZA Corporation, Mountain View, California2
Received 12 July 2000/Returned for modification 12 November
2000/Accepted 7 February 2001
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ABSTRACT |
Animal and clinical data show that high ratios of the area under
the concentration-time curve and the peak concentration in blood to the
MIC of fluoroquinolones for a given pathogen are associated with a
favorable outcome. The present study investigated whether improvement
of the therapeutic potential of ciprofloxacin could be achieved by
encapsulation in polyethylene glycol (PEG)-coated long-circulating
sustained-release liposomes. In a rat model of unilateral
Klebsiella pneumoniae pneumonia (MIC = 0.1 µg/ml), antibiotic was administered at 12- or 24-h intervals at
twofold-increasing doses. A treatment period of 3 days was started
24 h after inoculation of the left lung, when the bacterial count
had increased 1,000-fold and some rats had positive blood cultures. The
infection was fatal within 5 days in untreated rats. Administration of
ciprofloxacin in the liposomal form resulted in delayed ciprofloxacin
clearance and increased and prolonged ciprofloxacin concentrations in
blood and tissues. The ED50 (dosage that results in 50%
survival) of liposomal ciprofloxacin was 3.3 mg/kg of body weight/day
given once daily, and that of free ciprofloxacin was 18.9 mg/kg/day once daily or 5.1 mg/kg/day twice daily. The ED90 of
liposomal ciprofloxacin was 15.0 mg/kg/day once daily compared with
36.0 mg/kg/day twice daily for free ciprofloxacin; 90% survival could not be achieved with free ciprofloxacin given once daily. In summary, the therapeutic efficacy of liposomal ciprofloxacin was superior to
that of ciprofloxacin in the free form. PEG-coated liposomal ciprofloxacin was well tolerated in relatively high doses, permitting once daily administration with relatively low ciprofloxacin clearance and without compromising therapeutic efficacy.
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INTRODUCTION |
The pharmacodynamic and
pharmacokinetic properties of antimicrobial agents have been
intensively investigated in the recent past, years resulting in the
optimization of dosage regimens. In vitro studies and dose-effect
studies of experimental infections in animals have been performed with
various classes of antibiotics to investigate which
pharmacodynamic parameters determine therapeutic efficacy (9, 24,
26, 28, 42). Comparative studies of humans with different doses
and dosage regimens have also been utilized to optimize antimicrobial
treatment (9, 24).
The fluoroquinolones are concentration dependent in their rates of
bacterial killing. The area under the concentration-time curve
(AUC)/MIC ratio is the parameter that best correlates with the
therapeutic efficacy of these agents. In addition, a sufficiently high
peak concentration in serum/MIC ratio seems necessary to suppress the
emergence of resistant mutants during treatment. This can be concluded
from in vitro models that simulate human pharmacokinetics to study the
effects of changing the concentrations of fluoroquinolones on a number
of pathogens (6, 12, 13, 15, 23, 29, 32). For
fluoroquinolones, a 24-h AUC/MIC ratio of >125 is needed for rapid
bacterial eradication, whereas a peak serum drug concentration/MIC
ratio of >8 is needed to prevent the selection of resistant organisms.
The effect of dose or dose interval on the therapeutic efficacy of
fluoroquinolones has been evaluated in experimental infections such as
pneumonia, sepsis, peritonitis, and thigh infection caused by
gram-negative pathogens in mice, rats, guinea pigs, and rabbits (11, 19, 20, 27, 33, 41). Dosage schedules resulting in
high AUC/MIC ratios and high peak serum drug concentration/MIC ratios
of 10 or 20 effected higher therapeutic efficacy than did regimens in
which a more fractionated schedule was used at the same daily dose
(24, 28).
In clinical studies with fluoroquinolones, the AUC/MIC ratio and the
peak serum drug concentration/MIC ratio are important predictors of
both clinical and microbiological cure (16, 24, 28, 36).
Forrest et al. investigated the pharmacodynamics of intravenous
ciprofloxacin in seriously ill patients and found that a 24-h AUC/MIC
ratio of 
125 was significant for a satisfactory clinical and
microbiological outcome (16). The data from this study
show that most treatment failures with ciprofloxacin are consequences
of high MIC, low AUC, or both. More recent clinical studies with
grepafloxacin and levofloxacin showed that a peak serum drug
concentration/MIC ratio of 10 or greater was associated with successful
outcome; when the ratio was less than 10 the AUC/MIC ratio was most
closely linked to outcome (17, 38).
Based on the pharmacodynamic properties of the fluoroquinolones, as
shown in in vitro studies and in dose-response studies in animals and
patients, regimens of high doses at infrequent intervals might be most
efficacious in terms of eradication time, killing bacteria, and
reducing the selection of drug-resistant bacteria. Relatively
infrequent dosing (possibly once daily) may be superior. However, very
high doses of some quinolones might prove to be toxic when given once daily.
The present study was undertaken to investigate whether improvement of
the therapeutic potential of fluoroquinolones could be obtained by
liposomal encapsulation of the drugs. In a rat model of left-sided
Klebsiella pneumoniae pneumonia, the therapeutic efficacy of
liposome-encapsulated ciprofloxacin versus free ciprofloxacin was
determined. The liposome type used has shown relatively long blood
circulation times, as a result of coating of the liposome surface with
polyethylene glycol (PEG), and the PEG-coated liposomes are engineered
to release ciprofloxacin slowly. Liposomal encapsulation results in a
decrease in toxic side effects of the drug, permitting the use of
relatively high doses. The aim of using liposomes as carriers of
ciprofloxacin in the present study was, first, to delay ciprofloxacin
clearance and achieve sustained liposomal release of ciprofloxacin in
the blood over time, thus extending ciprofloxacin activity in the blood
(increased AUC) and tissues. The second objective was to permit
relatively high doses and decreased dose frequency and, as a result, a
once-daily treatment schedule.
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MATERIALS AND METHODS |
Animals.
Specific-pathogen-free female RP/AEur/RijHsd albino
rats (Harlan, Horst, The Netherlands) were used in all experiments.
Animals were 18 to 25 weeks old and weighed 185 to 225 g.
Bacteria.
K. pneumoniae ATCC 43816, capsular
serotype 2, was used to infect the rats. The minimum bactericidal
concentration of ciprofloxacin for this strain was 0.1 µg/ml as
determined by the tube dilution test.
Infection model.
A left-sided pneumonia was produced as
previously described (2). In brief, rats were anesthetized
with fluanisone and fentanyl citrate (Hypnorm) (Janssen, Animal Health,
Saunderton, United Kingdom), followed by pentobarbital (Nembutal)
(Sanofi Santé b.v., Maassluis, The Netherlands). The left primary
bronchus was intubated, and the left lung was inoculated with 0.02 ml
of a saline suspension containing 106 viable K. pneumoniae bacteria in the logarithmic phase of growth. After
bacterial inoculation, the narcotic antagonist nalorphine bromide
(Onderlinge Pharmaceutische Groothandel, Utrecht, The Netherlands) was
injected. Inoculation of the lung resulted in an acute unilateral
pneumonia. The course of the infection was assessed by measuring the
number of viable bacteria in the infected left lung, the right lung,
and the blood. Animals were sacrificed by CO2 inhalation, a
blood sample was taken, and the left and right lungs were removed and
homogenized (VirtTis, Gardiner, N.Y.) in 20 ml of phosphate-buffered
saline for 30 s at 10,000 rpm. Lung homogenate suspensions and
blood were serially diluted and plated on tryptone soy agar (Unipath
Ltd., Basingstoke, United Kingdom). At 24 h after bacterial
inoculation, the bacterial numbers in the left lung had increased
103-fold, up to 109 (range, 4 × 108 to 5 × 109; n = 10).
Untreated rats developed septicemia and pleuritis; at 24 h after
inoculation some rats had positive blood cultures. Untreated rats died
between day 3 and day 6 after bacterial inoculation.
Liposomes.
PEG-coated liposome preparations of ciprofloxacin
(PL Cipro) consisted of the PEG 2000 derivative of
distearoylphosphatidylethanolamine, hydrogenated soybean
phosphatidylcholine, and cholesterol in a molar ratio of 5:50:45.
Ciprofloxacin-containing liposomes and placebo liposomes were kindly
supplied by ALZA Corporation (Mountain View, Calif.). Mean particle
size was determined by dynamic light scattering (4700 system; Malvern
Instruments, Malvern, United Kingdom). The ciprofloxacin-containing
liposomes had a mean particle size of 107 ± 7 nm and contained
256 ± 51 µg of ciprofloxacin/µmol of total lipid (mean ± standard deviation [SD] of 10 preparations). The mean particle
size of the placebo liposomes was 105 ± 6 nm (mean ± SD of
two preparations).
Radiolabeling of liposomes.
Pharmacokinetics and
biodistribution of intact liposomes were determined by the use of a
high-affinity 67Ga-deferoxamine-mesylate
(67Ga-DF) complex as an aqueous liposomal marker
(44). As shown by Gabizon et al. (18), this
complex is appropriate for in vivo tracing of intact liposomes because
of the advantages of minimal translocation of radioactive labels to
plasma proteins and the rapid renal clearance rate when the label is
released from the liposomes. 67Ga was obtained as
67Ga-citrate from Mallinckrodt Medical b.v., Petten, The
Netherlands. The labeling was performed as described by Gabizon et al.
(18). The radiolabeling resulted in the formation of a
67Ga-DF complex in the aqueous interior of the liposomes.
Nonentrapped DF and radiolabels were removed by gel filtration on a
Sephadex G-50 column eluted with HEPES buffer. The circulation times of liposomes in the blood and localization in the infected left lung, right lung, liver, spleen, and kidneys were determined using the 67Ga-DF complex as a marker for intact liposomes.
Pharmacokinetics and biodistribution.
The pharmacokinetics
and biodistribution of antimicrobially active ciprofloxacin after
administration in the free or liposome-encapsulated form were
determined in rats at 24 h after bacterial inoculation. At
different intervals after intravenous (i.v.) administration, blood
samples were obtained by retro-orbital bleeding under CO2 anesthesia. Then the rats were sacrificed, and the left lung, right
lung, spleen, liver, and kidneys were removed. 67Ga served
as a marker for intact liposomes and was quantitated in a Minaxi
Autogamma 5000 gamma counter (Packard Instrument Company, Meriden,
Conn.). Correction for the blood content of the tissues was done using
111In-oxine-labeled syngeneic erythrocytes, injected i.v.
10 min before dissection. Erythrocytes were labeled as previously
described (25). Ciprofloxacin concentrations in the blood
and tissues were determined as previously described (5).
In short, with diagnostic sensitivity test agar (Oxoid, Basingstoke,
United Kingdom) and an Escherichia coli test strain
susceptible to 0.025 µg of ciprofloxacin per ml, all tests were done
by a standard large-plate agar diffusion procedure. Samples of 100 µl
were assayed. Twofold-increasing standard concentrations ranging from
0.1 to 1.6 µg of ciprofloxacin per ml were used. The assay system was
sensitive to 0.1 µg of ciprofloxacin per ml. The coefficient of
variation of 15 determinations of solutions containing 0.1 to 1.6 µg
of ciprofloxacin per ml was 1 to 3%.
Blood and tissue homogenates from rats that received PL Cipro were
incubated in 0.1% Triton X-100 for 30 min at 25°C to disintegrate intact liposomes before determination of the presence of
antimicrobially active ciprofloxacin. After centrifugation of the
samples for 5 min at 12,000 × g, ciprofloxacin
concentrations in the supernatant were determined. To exclude the
possibility that the presence of Triton X-100 in the unknown samples
may have had an effect on the bacterial growth inhibition zone on the
agar plates, the standard concentrations were also incubated with
Triton X-100.
Toxicity.
The maximum tolerated dose (MTD) was assessed
using various parameters. Acute toxicity was characterized in terms of
seizures, irritability, and an apparent dazed state. Long-term toxicity was assessed in terms of a significant change in renal or hepatic function. Renal function abnormalities were determined by measuring blood urea nitrogen and serum creatinine; hepatic function
abnormalities were detected by measuring the serum aspartate
aminotransferase and alanine aminotransferase by established tests
(Merck Diagnostica, Darmstadt, Germany).
Antimicrobial treatment.
Ciprofloxacin in the free form
(CIP) or PL Cipro was administered at 24 h after bacterial
inoculation of the left lung. The doses were escalated by twofold
increases (n, 8 to 10 per dosage), ranging from 0.3 to 80 mg/kg of body weight/day. The injection frequency was 12 or 24 h
for CIP and 24 h for PL Cipro. The duration of treatment was 3 days in all cases. Therapeutic efficacy was assessed by the survival of
rats at day 21 after bacterial inoculation. Estimates of
ED50 (dosage that effects 50% survival of rats) and ED90 were obtained using the PROBIT procedure from the SAS
program (SAS user's guide, SAS Institute Inc., Cary, N.C.), assuming a logistic distribution of data. Postmortem cultures of the left lung and
blood from rats were performed to check for the presence of K. pneumoniae only, as well as for susceptibility to ciprofloxacin. Cultures were also done for rats that survived at day 21 postinoculation.
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RESULTS |
Blood circulation time of liposomes and ciprofloxacin.
After
i.v. administration, intact liposomes demonstrated a relatively long
blood residence time, with a half-life of approximately 18 h (Fig.
1). This long-term circulation in blood
was not altered when the liposomes contained encapsulated
ciprofloxacin. Figure 2 shows that
ciprofloxacin is slowly released from the intact liposomes in blood.
Within 6 h after i.v. administration about 90% of the encapsulated
ciprofloxacin is released, while the liposomes remain intact (Fig. 2).
The concentrations of ciprofloxacin indicated in Fig. 2 represent
primarily PL Cipro, since it is known that CIP rapidly diffuses from
intravascular to extravascular space.
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FIG. 1.
Levels of 67Ga-DF-labeled liposomes
containing ciprofloxacin or placebo liposomes in blood. Liposomes were
injected (inj.) i.v. as a single dose (ciprofloxacin, 20 mg/kg; total
lipid, 83 µmol/kg) in rats at 24 h after inoculation of K. pneumoniae in the left lung. Levels of 67Ga-DF label
in the blood after injection of ciprofloxacin-containing liposomes
( ) or placebo liposomes ( ) were determined. Data are expressed as
mean ± SD for six rats.
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FIG. 2.
Levels of 67Ga-DF-labeled liposomes
containing ciprofloxacin in blood. Liposomes were injected (inj.) i.v.
as a single dose (ciprofloxacin, 20 mg/kg; total lipid, 83 µmol/kg)
into rats at 24 h after inoculation of K. pneumoniae in
the lung. Levels of 67Ga-DF label and antimicrobially
active ciprofloxacin in the blood were determined. Data are expressed
as mean ± SD for six rats.
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Concentration of ciprofloxacin in blood after administration in the
free or liposome-encapsulated form.
Concentrations of
ciprofloxacin in the blood were determined at various intervals after
i.v. administration of 20 mg/kg as a single dose. Total (CIP plus PL
Cipro) concentrations of ciprofloxacin are presented in Table
1. At 3 min after administration of CIP only 5% of the injected dose was present in blood, and at 30 min only
1% was present, corresponding with 5 µg of ciprofloxacin/ml. In
contrast, after treatment with PL Cipro, 77% of the injected dose was
still circulating in the blood 30 min after administration. Administration of ciprofloxacin in the liposome-encapsulated form resulted in a substantial increase in AUC0-6 h, 800 µg · h/ml for PL Cipro compared to 15.8 µg · h/ml for
CIP.
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TABLE 1.
Concentrations of total ciprofloxacin in blood at various
intervals after i.v. administration of PL Cipro or CIP in rats with
K. pneumoniae lung infectiona
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Toxic side effects.
Toxicity was assessed using various
parameters in a treatment schedule at twofold-increasing doses for a
period of 3 days, with PL Cipro administered once daily and CIP
administered once or twice daily. The MTD for CIP was 40 mg/kg/dose. At
higher doses, acute toxicity was observed (e.g., seizures, irritability
followed by an apparent dazed state) shortly after the first dose.
After administration of PL Cipro, no signs of acute toxicity were
observed at dosages up to 160 mg/kg/dose. Long-term toxicity was
investigated at dosage schedules yielding 100% survival of rats: 40 mg
of CIP/kg/dose twice daily and 20 mg of PL Cipro/kg/day once daily. At
12 or 24 h after the last dose of CIP or PL Cipro, respectively,
significant abnormalities in renal or hepatic functions were not
observed. Thus, PL Cipro was well tolerated at doses above the MTD of CIP.
Therapeutic efficacy of PL Cipro versus CIP.
In untreated
rats, after inoculation of the left lung with 106 CFU of
K. pneumoniae, the infection in the left lung developed progressively, whereas no infection developed in the right lung.
Antimicrobial treatment was started at 24 h after bacterial
inoculation, when the bacterial count in the left lung had increased
approximately 10
3-fold, to 3 × 10
9 CFU
(range, 5 × 10
8 to 8 × 10
9;
n = 10), and 7 out of 10 rats had developed positive blood cultures.
All untreated rats died between day 3 and day 6 after bacterial
inoculation. The parameter for therapeutic efficacy of antimicrobial
treatment was the survival of rats assessed for a period of 21
days
after bacterial inoculation. Treatment with CIP twice daily
was
effective in a dose-dependent manner and resulted in 100%
survival of
rats at doses that were well tolerated (Table
2).
When CIP was administered once daily,
100% survival of rats could
not be achieved at doses below the MTD. In
contrast, the administration
of PL Cipro once daily was fully effective
at a dosage of 20 mg/kg/day.
ED
50s and ED
90s calculated from the survival
data demonstrate that the therapeutic efficacy of PL Cipro was superior
to that
of CIP (Table
2). PL Cipro was significantly (
P < 0.05) more
effective than CIP in achieving 50% survival of rats
with once-daily
administration and slightly, but not significantly,
more active
than CIP given twice daily. The difference between PL Cipro
and
CIP with once-daily administration is even more striking at the
ED
90. Survival of 90% of rats could not be achieved with
once-daily
treatment with CIP; twice-daily treatment was required. In
contrast,
PL Cipro given once daily was effective. The ED
90
daily dose for
CIP given twice daily was slightly higher than the
ED
90 daily
dose for PL Cipro given once
daily.
As shown in Fig.
3, the administration of
placebo liposomes had no effect on the mortality rate. In this figure,
the mortality
rates of rats treated with PL Cipro at doses of 20 mg/kg,
2.5
mg/kg, or 0.3 mg/kg for 3 days are also represented.
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FIG. 3.
Survival of rats with K. pneumoniae lung
infection after administration of PL Cipro at 20 mg/kg ()
(n = 8), 2.5 mg/kg ( ) (n = 8), or 0.3 mg/kg ( ) (n = 5); placebo
liposomes ( ) (n = 10); or buffer
() (n = 10) at 24, 48, and 72 h
after bacterial inoculation of the lung. Total lipid doses were 81 µmol/kg/dose.
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K. pneumoniae was present only in the infected left lung of
rats that had died. In addition, the susceptibility to ciprofloxacin
of
the bacteria that were recovered from the infected left lung
tissue of
treated rats appeared to be
unchanged.
Biodistribution of PL Cipro versus CIP.
Concentrations of
ciprofloxacin (CIP plus PL Cipro) in different organs and the blood of
infected rats were determined at 1- and 6-h intervals after
administration of a single dose of CIP or PL Cipro at 20 mg/kg (Table
3). When rats were sacrificed 1 h
after injection of CIP, about 1% of the injected dose was still
present in the blood. Rats treated with PL Cipro were sacrificed at 1 or 6 h after administration. One hour after the administration of
PL Cipro, the total recovery of ciprofloxacin based on its levels in
the blood, liver, spleen, kidney, and lung was 75% of the injected
dose. In contrast, when CIP was administered, recovery was only 3% of
the injected dose at the same time point. Concentrations of
ciprofloxacin in tissue were substantially increased following administration in the liposomal form compared with injected CIP. The
degrees of localization of ciprofloxacin in the infected left lung and
in the uninfected right lung were not different and appeared to
decrease with time.
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TABLE 3.
Biodistribution of total ciprofloxacin at various
intervals after i.v. administration of PL Cipro or CIP in rats with
K. pneumoniae lung infectiona
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DISCUSSION |
A possible approach for intensifying antibiotic treatment may be
the use of an appropriate delivery system, such as liposomes, which may
enhance antibiotic pharmacokinetics or penetration to infected sites.
Liposomes are considered to be versatile delivery systems. Depending on
the liposomal size and physicochemical characteristics, which can be
manipulated by changing the lipid composition (surface charge, surface
coating, bilayer rigidity), liposomes can be used in various ways. One
rationale for using liposomes as carriers of antibiotics is to reduce
the toxicity of potentially toxic antibiotics such as amphotericin B
(21, 22). Liposomes can also be exploited to achieve high
and prolonged intracellular antibiotic concentrations in infected cells
(3, 37). Another application of liposomes targets the
delivery of antibiotics to infected tissues. In this respect, previous
studies in this experimental model of K. pneumoniae
pneumonia in rats have demonstrated a substantial increase in
therapeutic efficacy for gentamicin or ceftazidime resulting from
administration in long-circulating liposomes (4, 40).
Finally, liposomes carrying antibiotics may also be used as
microreservoirs of antibiotics during circulation. Again,
long-circulating liposomes are needed for this purpose. Experimental
evidence to support the application of liposomes in this way is
provided in the present study.
In the current study, liposomes were used first to influence the
pharmacokinetics of ciprofloxacin, thus extending ciprofloxacin activity in the blood and tissues, and secondly to protect encapsulated ciprofloxacin, which facilitates the use of relatively high doses and
possibly once-daily dosing. The liposomes exhibited sustained liposomal
release of ciprofloxacin in the blood over time while remaining intact.
In earlier studies with PEG-coated liposomes containing gentamicin or
ceftazidime, the liposomes retained their content during circulation
(4), and as a consequence, substantial targeting of
liposomal antibiotic to the infected left lung tissue was observed
(40). In the present study, as expected, targeting of
liposomal ciprofloxacin to the infected tissue was not achieved, because of the relatively rapid release of the antibiotic from the
liposomes. Ciprofloxacin concentrations in the infected left lung and
uninfected right lung were similar. However, the administration of PL
Cipro resulted in relatively low ciprofloxacin clearance, prolonged
ciprofloxacin concentrations in the blood, and increased ciprofloxacin
concentrations in both infected and uninfected tissues. Probably as a
result of this, the therapeutic efficacy of PL Cipro was superior to
that of CIP. It was also observed that liposomal ciprofloxacin was well
tolerated in relatively high doses and could be administered once daily
without compromising its therapeutic efficacy.
Our observation that prolonged residence of ciprofloxacin in the blood
is important for therapeutic efficacy agrees with the findings of
others investigating the therapeutic efficacy of fluoroquinolones in
the free form at various dose schedules in animal infection models. In
models of pneumonitis caused by K. pneumoniae and thigh infection caused by K. pneumoniae or Pseudomonas
aeruginosa in mice, Leggett et al. investigated dose-effect
relations for ciprofloxacin (27). It was shown that the
AUC/MIC ratio was the variable most closely linked to outcome, even
though the peak serum drug concentration/MIC ratio was >10. The dosing
interval had little impact. In contrast, Drusano et al. emphasized the
role of the dosing interval for therapeutic efficacy. They examined the
impact of dose fractionation and altered MICs in a neutropenic rat
model of P. aeruginosa sepsis using lomefloxacin
(11). Once-daily administration of the drug, produced a
peak serum drug concentration/MIC ratio of 20 and resulted in better
efficacy than a more fractionated treatment schedule at the same daily
dose. At lower doses producing peak serum drug concentration/MIC ratios
of <10, the AUC/MIC ratio appeared to be most closely linked to
outcome. The relative importance of the peak serum drug
concentration/MIC ratio for therapeutic efficacy has also been
demonstrated in a model of P. aeruginosa pneumonia in
neutropenic guinea pigs. Gordin et al. showed that ciprofloxacin and
pefloxacin had the same rate of bacterial killing when the peak serum
drug concentration/MIC ratio was the same for each agent
(19). Similar conclusions can be drawn from studies by Hackbarth et al. and Shibl et al. that examined the efficacy of ciprofloxacin and pefloxacin in experimental meningitis caused by
P. aeruginosa or E. coli, respectively, in
rabbits (20, 41). Both studies demonstrated that the peak
serum drug concentration/MIC ratio was predictive for bacterial killing
in cerebrospinal fluid.
The PEG-coated liposomes used in the present study show a relatively
long blood residence time due to decreased uptake by the mononuclear
phagocyte system (1). Long-term circulation of liposomes
is needed to achieve prolonged ciprofloxacin activity in blood. Other
investigators using liposomes containing fluoroquinolones focused on
the treatment of intracellular infections, which are difficult to treat
due to poor penetration of antibiotics into the infected cells or
decreased intracellular activity. In these studies, classical
non-PEG-coated liposomes were used, which rapidly accumulate in cells
of the mononuclear phagocyte system after i.v. administration,
resulting in increased intracellular concentrations. Enhanced efficacy
of ciprofloxacin in the protection and treatment of mice with
intracellular Francisella tularensis infection was demonstrated when it was administered in the liposomal form i.v. intranasally (10), or by aerosol delivery
(8). Liposomal ciprofloxacin also appeared effective in
the i.v. treatment of intracellular infections caused by
Salmonella enterica serovar Dublin (30) or
S. enterica serovar Typhimurium (43) in mice. In vitro studies in which monocytes or macrophages in culture were
infected with Mycobacterium avium-M. intracellulare complex and exposed to ciprofloxacin (31, 34) or ofloxacin
(35) in the free or liposome-encapsulated form also showed
the superiority of the liposome-encapsulated drugs. In contrast, free
and liposomal sparfloxacin had similar effects on the growth of
intracellular M. avium-M. intracellulare complex
(14).
Sustained drug release of fluoroquinolones from liposomes has also been
demonstrated for enrofloxacin encapsulated in non-PEG-coated liposomes
composed of phosphatidylcholine and cholesterol. This formulation was
administered intramuscularly in rabbits and provided therapeutic and
prolonged plasma concentrations (7). In addition, ciprofloxacin liposomes have been used for external coating of catheters to prevent catheter-associated urinary tract infections in a
rabbit model (39).
For the fluoroquinolones, animal data showing that high ratios of the
AUC and the peak concentration in blood to the MIC for the pathogen are
associated with favorable outcomes are in agreement with the clinical
data. For infections caused by highly susceptible bacteria, the optimal
AUC/MIC ratio and peak serum drug concentration/MIC ratio criteria can
easily be reached. The difficulty lies in infections caused by bacteria
such as Staphylococcus and Pseudomonas species that are only marginally susceptible to fluoroquinolones (MICs 0.5 µg/ml). In future studies the efficacy of PL Cipro will be
investigated in a model of P. aeruginosa pneumonia and
septicemia in rats.
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ACKNOWLEDGMENT |
The financial support of ALZA Corporation (Mountain View, Calif.)
is gratefully acknowledged.
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FOOTNOTES |
*
Corresponding author. Mailing address: Department of
Medical Microbiology & Infectious Diseases, Erasmus University Medical Center Rotterdam, P.O. Box 1738, 3000 DR Rotterdam, The Netherlands. Phone: 31 10 4087666. Fax: 31 10 4089454. E-mail:
bakker{at}kmic.fgg.eur.nl.
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Antimicrobial Agents and Chemotherapy, May 2001, p. 1487-1492, Vol. 45, No. 5
0066-4804/01/$04.00+0 DOI: 10.1128/AAC.45.5.1487-1492.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
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Abstract
Introduction
