(Chest. 2001;120:118S-123S.)
© 2001
American College of Chest Physicians
Microbiological and Immunologic Considerations With Aerosolized Drug Delivery*
John J. LiPuma, MD
*
From the Department of Pediatrics and Communicable Diseases, University of Michigan Medical School, Ann Arbor, MI.
Correspondence to: John J. LiPuma, MD, University of Michigan, 1150 W Medical Center Dr, 8323 MSRB III, Box 0646, Ann Arbor, MI 48109; e-mail: jlipuma{at}umich.edu
 |
Abstract
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The development of drug resistance is a major theoretical
concern with the long-term delivery of aerosolized antibiotics via
inhalation. A randomized, placebo-controlled, double-blind study, which
compared inhaled tobramycin plus standard cystic fibrosis (CF) care to
placebo plus standard CF care, examined the following microbiological
parameters: percentage of patients with at least one Pseudomonas
aeruginosa (PA) strain with a minimal inhibitory concentration
(MIC) > 16 µg/mL (ie, the breakpoint for tobramycin
resistance delivered by the parenteral route); changes in the levels of
the lowest concentration required to inhibit the growth of 50% of
strains tested (MIC50) and 90% of strains tested
(MIC90); the percentage of patients with an increase,
decrease, or change in the MIC of the most resistant and most prevalent
PA strains; and the percentage of patients in whom the PA strain with
the highest MIC also was the most prevalent. During the first 6 months,
which included three on-drug and off-drug cycles of 4 weeks’ duration
each, the percentage of tobramycin-treated patients with at least one
PA isolate and with an MIC > 16 µg/mL was 13% at baseline, 26% at
20 weeks, and 23% at 24 weeks vs 10%, 17%, and 8%, respectively,
for placebo-treated patients. No significant change was observed in
MIC50 at 20 and 24 weeks. The increase in MIC90
was not statistically significant. At 24 weeks, there was no increase
in the percentage of patients in either group in whom the PA strain
with the highest MIC became most the prevalent strain. After the third
on-drug cycle, 33% of the tobramycin group showed an increase in the
MIC of the strain with the highest MIC. This decreased to 26% after 1
month off drug therapy. A preliminary analysis of the 12-month and
18-month data showed a decrease in the proportion of resistant PA
isolates after each off-drug cycle. This return to susceptibility
following an off-drug cycle was not observed at 24 months. The
mechanism of resistance in this setting is believed to be increased
impermeability to drug. At all time points, pulmonary function improved
even in patients with MICs of 
128 µg/mL. At 6 months, no increase
was seen in the rates of superinfection with tobramycin-resistant,
Gram-negative pathogens. Increases in Stenotrophomonas
maltophilia were detected in patients after 18 and 24 months of
tobramycin therapy and were similar to those rates in patients
receiving placebo. These rates may be independent of inhalation
therapy.
Key Words: aerosolized • breakpoint • cystic fibrosis • Gram-negative pathogens • minimal inhibitory concentration • Pseudomonas aeruginosa • superinfection • tobramycin
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Introduction
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The
traditional paradigm in the treatment of infectious diseases is to
treat the patient with the antimicrobial agent that is the most
effective and covers the narrowest spectrum for the shortest time to
effect a cure and to sterilize the site of infection. These guidelines
are aimed at reducing the opportunity for the pathogen to develop
antibiotic resistance. However, this model is less relevant for
treating the lung infections of patients with cystic fibrosis (CF).
CF lung infections are chronic and polymicrobial. Sterilization of the
site of infection is not usually an achievable therapeutic end point.
The lung is relatively inaccessible to therapeutic agents, and
antibiotic therapy is further impeded by the nature of CF sputum, which
contains glycoproteins and other agents that bind aminoglycosides (Fig 1
).1
Accordingly, a relatively high concentration of
aminoglycosides is needed to reduce the density of Pseudomonas
aeruginosa (PA) in sputum.
|
Rationale for Aerosolized Antibiotic Therapy in CF
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The primary rationale for the use of aerosolized antibiotics,
rather than parenteral antibiotics, in the treatment of PA in CF
patients is improved delivery to the site of infection. Table 1
shows the differences in mean peak sputum and serum concentrations of
the drug when delivered IV and as an aerosol by means of three
different nebulizers.1
2
3
While the highest serum
concentration is achieved with IV dosing, sputum concentration, which
reflects drug delivery to the site of bacterial infection, is highest
with the breath-enhanced nebulizer (PARI-LC Plus; PARI Respiratory
Equipment; Richmond, VA).
|
Concerns and Questions
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A major concern of the long-term delivery of an antibiotic to the
lung is the development of resistant forms of PA, the principal
organism infecting the airway in CF patients. The following two
questions arise from this concern: (1) what is the significance of this
resistance for inhaled therapy?; and (2) what are the implications of
this resistance for subsequent parenteral therapy using the same
antimicrobial?
To address these questions, it is important to understand the concept
of minimal inhibitory concentration (MIC). The MIC is the lowest
concentration of an antibiotic that is needed to inhibit growth of a
bacterial isolate in vitro. The MIC50
is the concentration of an antibiotic that is required to inhibit the
growth of 50% of strains tested (ie, within a given
bacterial species), not the concentration required to inhibit 50% of
the bacteria within an individual patient. Similarly, the
MIC90 is the lowest concentration required to
inhibit the growth of 90% of strains tested.
Another concept that is significant in this setting is that of
breakpoint. The breakpoints are the concentrations of a particular
antibiotic that are used to classify organisms as resistant and
susceptible. The breakpoint is a function of the MIC of the infecting
organism and the achievable nontoxic serum levels of an antibiotic.
When administering an antimicrobial agent whereby one can achieve high
serum levels without toxicity, the susceptibility breakpoint can be
calculated using a higher MIC. Conversely, if only low serum levels of
a given drug can be reached before toxicity develops, a much lower MIC
must be used in this calculation. To assure consistency, antibiotic
breakpoints for specific organisms are established by the National
Committee for Clinical Laboratory Standards. Parenteral tobramycin MIC
breakpoints for PA are shown in Table 2
.
The concept of breakpoint assumes that the infection is in the
bloodstream or a pharmacokinetically comparable compartment, that the
population of bacteria is homogeneous, and that there is a clear end
point to define successful therapy. However, the establishment of
breakpoints for inhaled therapy is problematic because these
assumptions are not operative in the case of airway infection in CF
patients. Rather than being restricted to the bloodstream or other
similar compartments, the site of infection is an inflamed lung where
drug delivery is inconsistent. The infection is polymicrobial and
consists of phenotypically diverse populations of PA isolates, usually
containing more than one strain or clone. The multiple strains, all
present simultaneously, may have different levels of susceptibility.
Furthermore, there is no clear microbiological end point to define
successful therapy in the setting of pseudomonal lung infections in CF
patients.
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Mechanisms of Resistance
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Microorganisms can acquire drug resistance by a number of
different mechanisms (Table 3
). Some have inactivating enzymes such as aminoglycoside transferases or
ß-lactamases. Aminoglycoside transferases are enzymes that are
generally encoded by plasmids, while ß-lactamases can be carried
either on plasmids or on a chromosome. Microorganisms also can alter
the target for a specific antibiotic. For example, organisms can become
resistant to ß-lactam antibiotics by altering their
penicillin-binding proteins; eg, resistance to trimethoprim
can result from altered dihydrofolate reductase. In addition, bacteria
can limit drug access by means of an antibiotic efflux pump, which
pumps an antibiotic from the cell as soon as it is taken up.
Finally, organisms may decrease drug uptake by becoming relatively less
permeable to a particular antibiotic.
Antimicrobial resistance mechanisms can be intrinsic. For instance,
Burkholderia cepacia has a lipopolysaccharide structure that
renders it constitutively resistant to polycationic agents. Certain
species also may be capable of inducing so-called adaptive (or
transient) resistance by up-regulating relevant genes when the
antibiotic is present and down-regulating these genes when the
antibiotic is absent.
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Chronic Intermittent Inhaled Tobramycin Trial: Microbiological
Outcomes at 6 Months
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A randomized, placebo-controlled, double-blind study compared
300-mg bid dosing of tobramycin solution for inhalation (TSI) (TOBI;
PathoGenesis Corporation; Seattle, WA) and standard CF care to placebo
and standard CF care over a 6-month period.3
4
The
microbiological parameters studied during the first 6 months of the
trial included the following:
- the percentage of patients with at least one PA isolate with an
MIC 16 µg/mL (the resistance breakpoint for parenteral
tobramycin);
- changes in MIC50 and
MIC90;
- the percentage of patients with an increase in the MIC of the strain
with the highest MIC (ie, the most resistant);
- the percentage of patients with an increase in the MIC of the most
prevalent strain; and
- the percentage of patients in whom the strain with the highest MIC
also was the most prevalent strain.
The first 6 months of the study included three 28-day-on-drug
treatment cycles each followed by a 28-day-off-drug treatment cycle.
During this period, there was an increase in the number of patients
with at least one Pseudomonas isolate with a tobramycin MIC 16
µg/mL (ie, the parenteral breakpoint for resistance). In
the TSI group, this percentage rose from 13% at baseline to 26% at 20
weeks (ie, the end of the third on-drug interval) and to
23% at 24 weeks (ie, the end of the third off-drug
interval) vs 10%, 17%, and 8%, respectively, in the placebo group.
In the TSI group, no change in the tobramycin
MIC50 and only a small increase in
MIC90 (from 8 µg/mL at baseline to 16 µg/mL
at 20 and 24 weeks) was observed.
At 24 weeks, there was no increase in the percentage of patients in
either the placebo or the TSI groups in whom the strain with the
highest MIC became the most prevalent strain. Thus, after 6 months of
intermittent therapy, the more highly resistant strains did not replace
the predominating Pseudomonas strains.
After three on-drug periods (week 20), 33% of the patients treated
with TSI showed an increase in the MIC of the strain with the highest
MIC. However, after 1 month off drug therapy (week 24), this percentage
had decreased to 26%, suggesting that some strains of Pseudomonas were
regaining some susceptibility to tobramycin during the off-drug cycle.
Another possibility is that strains with high tobramycin MICs had
significantly decreased in density in vivo after the
off-drug cycle and were no longer detected by culturing. At the
completion of the 6-month trial (week 24), 26% of patients treated
with TSI showed an increase in the MIC of the most prevalent strain.
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Unpublished Data at 18 and 24 Months
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Following the 6-month randomized study, patients were allowed to
enroll in three consecutive 6-month open-label extension studies,
resulting in up to 24 months of total exposure to TSI, during which
microbiology continued to be monitored.
Unpublished data (PathoGenesis Corporation) show an incremental
increase in the percentage of patients with an isolate with the highest
MIC that would be considered resistant at the parenteral breakpoint
(ie, 16 µg/mL) with each successive 6 months of
exposure (Fig 2 ). To put this into context, studies by other investigators have shown
that changes in susceptibility following short courses of both oral
ciprofloxacin and IV tobramycin were more rapid (Fig 2)
.
Off-drug intervals appeared to result in some return toward increased
susceptibility following 6, 12, and 18 months of treatment (Fig 3
). Following each of the 1-month off-drug intervals at these time
points, the percentage of patients with resistant isolates decreased.
This pattern of increased susceptibility following 1-month off of drug
therapy was not observed at the 24-month time point.
The finding of a return toward susceptibility following an off-drug
interval has been observed in a previous trial by Smith et
al5
This is depicted at the right side of Figure 3
. In
this trial, patients received inhaled tobramycin therapy (600 mg tid)
continuously for 3 months. The percentage of resistant isolates
increased from approximately 30% at baseline to nearly 80% after 3
months of continuous therapy and then decreased to nearly 0% at 1 year
off of drug therapy. These data raise the question of whether an
off-drug interval that is longer than 1 month during TSI treatment
will result in a more significant return to drug susceptibility than
was observed in the present study.
Recently published data6
indicate that the increasing
resistance demonstrated in this study results from increased bacterial
impermeability to the drug. Although this type of resistance is quite
stable in vitro, it is not clear whether organisms
expressing increased impermeability may be at a competitive
disadvantage with more susceptible organisms in vivo. A
further evaluation of the stability of this resistance in the setting
of inhaled tobramycin is needed.
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Improvement in Pulmonary Function
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While changes in bacterial susceptibility to tobramycin
did occur over time, the clinical significance of this effect in terms
of pulmonary function did not appear to be important. Figure 4
shows the percentage of patients with improvement in
FEV1 as a function of MIC. At all time points,
even patients with an MIC 128 µg/mL, which would be considered a
resistant value using parenteral breakpoint values, continued to show a
clinical response to TSI therapy. This clinical improvement, despite
colonization by resistant Pseudomonas strains, indicates
that the parenteral breakpoint is not applicable to TSI treatment.
The precise reasons for this therapeutic effect are not known; however,
it is clear that very high sputum concentrations of tobramycin are
achieved via the inhalational route, while systemic concentration
remains low.
There was a slight increase in the percentage of patients with
multidrug-resistant Pseudomonas strains from baseline to 18 months (Fig 5
); data regarding antibiotics other than tobramycin were not collected
between 18 and 24 months. There was no increase in the rate of
colonization with B cepacia with up to 24 months of TSI
treatment. Isolation of Stenotrophomonas maltophilia
increased over time but appeared to be independent of TSI treatment.
The percentage of patients colonized with this organism increased
from 7% at baseline to 12% following 6 months of receiving placebo,
and from 10% at baseline to 17% following 24 months of TSI treatment.
Similar findings were noted for Alcaligenes xylosoxidans.
Although a significant increase in Aspergillus colonization was
observed at 24 months, there was not an increase in the reporting of
allergic bronchopulmonary aspergillosis for TSI-treated patients.
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Immunologic Considerations of Inhaled Therapies
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In addition to the concern of altered drug susceptibility
in the setting of long-term intermittent therapy with inhaled
antibiotics, there is also a concern that inhaling any therapeutic
agent may result in host immune reactions to that agent. The concern
that such reactions may occur is based on responses to such allergens
as grain dust, fungi, and molds, all of which can trigger
profound allergic reactions when inhaled.7
While the
studies of inhaled tobramycin did not measure these parameters, there
are some data indicating a lack of immune response to inhaled
proteins from the clinical trials8
used to support the
approval of recombinant human DNase (rhDNase) (Pulmozyme; Genentech,
Inc; South San Francisco, CA).
In these 6-month placebo-controlled phase III studies, CF patients
either inhaled 2.5 mg rhDNase once or twice daily or received
intermittent doses of 10 mg. Specific antibodies to rhDNase were
detected in 2.5% of those who received once-daily dosing, in 3.7% of
those who received the drug two times per day, in 4.4% of those who
received the 10-mg dose intermittently, and in 0.3% of those receiving
placebo. All patients receiving 10 mg were challenged while under
clinical observation, and none developed bronchoconstriction or rhDNase
allergy. Thus, it would seem that antibody formation did not compromise
drug safety or efficacy. This experience is particularly relevant in
view of the extensive database compiled during the 6 years that this
drug has been marketed.
It has been suggested that drugs delivered by inhalation may actually
have a lower potential for immunogenicity than those delivered
subcutaneously or IV because of increased immune tolerance to
aeroallergens entering the lungs.7
9
This mechanism may
exist because of the greater exposure of the airways to airborne
antigens in comparison to the blood, where minimal exposure to external
allergens occurs.7
This hypothesis is supported by studies
that have shown that lower levels of antibodies to human growth hormone
develop when it is delivered by inhalation rather than
subcutaneously.10
The data regarding potential adverse immune reactions to inhaled
therapeutic proteins have been encouraging. Further study of these
parameters with other inhaled therapies, including antibiotics, is
needed, particularly as the volume of antibiotics administered begins
to exceed other therapeutic agents delivered by inhalation.
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Conclusions: Microbiology
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Lung infection in patients with CF cannot be eradicated with
current therapeutic modalities. Rather, repeated courses of
antimicrobial therapy are aimed at slowing the lung damage that results
from such infection. The experience to date with TSI indicates that
long-term, intermittent treatment results in the emergence of PA
with increased tobramycin MIC values. This finding is not surprising.
Repeated courses of antibiotics, regardless of class or administration
route, typically result in the emergence of microorganisms that are
relatively more resistant to those agents. However, studies with TSI
also demonstrate that the definition of tobramycin resistance as
defined for parenteral therapy does not apply to inhaled tobramycin.
Nevertheless, an important question remains to be answered. Will
patients harboring Pseudomonas strains with tobramycin MICs 16
µg/mL continue to respond to parenteral tobramycin treatment? Further
investigation of the impact of long-term inhaled antimicrobial therapy
on microbial ecology and patient management in patients with CF will
help to address this and other important questions.
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Footnotes
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Abbreviations:
CF = cystic fibrosis; MIC = minimal inhibitory concentration;
MIC50 = the lowest concentration required to inhibit the
growth of 50% of strains tested; MIC90 = the lowest
concentration required to inhibit the growth of 90% of strains tested;
PA = Pseudomonas aeruginosa; rhDNase = recombinant
human DNase; TSI = tobramycin solution for inhalation
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References
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-
Mendelman, PM, Smith, AL, Levy, J, et al (1985) Aminoglycoside penetration, inactivation and efficacy in cystic fibrosis sputum. Am Rev Respir Dis 132,761-765[ISI][Medline]
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Ramsey, BW, Pepe, MS, Quan, JM, et al (1999) Intermittent administration of inhaled tobramycin in patients with cystic fibrosis. N Engl J Med 340,23-30[Abstract/Free Full Text]
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Holt, PG, Britten, D, Sedgwick, JD (1987) Suppression of IgE responses by antigen inhalation: studies on the role of genetic and environmental factors. Immunology 60,97-102[ISI][Medline]
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TOP
Abstract
Introduction
