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Antimicrobial Agents and Chemotherapy, August 2000, p. 2068-2076, Vol. 44, No. 8
0066-4804/00/$04.00+0
Dose-Dependent Pharmacokinetics of Amphotericin
B Lipid Complex in Rabbits
Thomas J.
Walsh,1,*
Andre J.
Jackson,2
James W.
Lee,1
Michael
Amantea,1
Tin
Sein,1
John
Bacher,3 and
Loren
Zech4,
Mycology Unit and Immunocompromised Host
Section1 and Laboratory of Mathematical
Biology,4 National Cancer Institute, and
Veterinary Resource Program,3 National
Institutes of Health, Bethesda, Maryland, and Division of
Bioequivalence, Center for Drug Evaluation and Research, Rockville,
Maryland2
Received 7 October 1998/Returned for modification 13 October
1999/Accepted 8 May 2000
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ABSTRACT |
Amphotericin B lipid complex (ABLC) was recently approved by the
Food and Drug Administration for treatment of patients with invasive
fungal infections who are intolerant of or refractory to conventional
amphotericin B therapy. Little is known, however, about the
pharmacokinetics of this new antifungal compound. We therefore
investigated the pharmacokinetics of ABLC in comparison with those of
conventional desoxycholate amphotericin B (DAmB) in rabbits. The
pharmacokinetics of DAmB in a rabbit model were similar to those
previously reported in humans. The pharmacokinetics of ABLC differed
substantially from those of DAmB. Plasma amphotericin B levels
following ABLC administration were 10 times lower than those following
administration of an equal dosage of DAmB. The levels of ABLC in whole
blood were approximately 40 times greater than those in plasma. The
ABLC model differed from the DAmB model by (i) a dose- and
time-dependent uptake and return between the plasma compartment and
apparent cellular components of the blood-sediment compartment and (ii)
time-dependent tissue uptake and return to plasma from serially
connected compartments. Following infusion of ABLC, there was a
nonlinear uptake into the apparent cellular components of the
blood-sediment compartment. This uptake was related to the reciprocal
of the integral of the total amount of drug infused (i.e., the more
drug infused the greater the fractional uptake between 0.5 and 5 mg/kg
of body weight for ABLC). The transfer of drug from plasma to the
cellular components of the blood-sediment compartment resulted in
initial uptake followed by rapid redistribution back to the plasma. The
study describes a detailed model of the pharmacokinetics of ABLC and
characterizes a potential role of the cellular components of the
blood-sediment compartment in the distribution of this new antifungal
compound in tissue.
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INTRODUCTION |
Invasive fungal infections have
emerged as important causes of morbidity and mortality in
immunocompromised hosts (14, 15). Desoxycholate amphotericin
B (DAmB) remains the treatment of choice for treatment of many
life-threatening opportunistic mycoses. However, dose-limiting
nephrotoxicity often compromises the ability to administer DAmB,
resulting in therapeutic failures. Certain lipid formulations of
amphotericin B can be administered at higher dosages with reduced
nephrotoxicity (6, 9, 13, 17).
Amphotericin B lipid complex (ABLC or Abelcet; The Liposome Company,
Princeton, N.J.) was developed to reduce the toxicity and maximize the
therapeutic utility of amphotericin B in the treatment of invasive
fungal infections (10). The preparation of ABLC consists of
amphotericin B complexed with two lipids in a 1:1 drug-to-lipid molar
ratio. These lipids, dimyristoylphosphatidylcholine (DMPC) and
dimyristoylphosphatidylglycerol (DMPG), are present in a 7:3 molar ratio.
Amphotericin B lipid complex was recently approved by the Food and Drug
Administration for treatment of patients with invasive fungal
infections who are intolerant of or refractory to conventional amphotericin B therapy. Despite its clinical availability, little is
known about the pharmacokinetics of ABLC. As ABLC becomes more widely
used, increased understanding of its optimal modes of administration, distribution, and clearance mechanisms becomes more imperative. We
therefore investigated the pharmacokinetics of ABLC in comparison with
those of conventional amphotericin B in rabbits in order to more
clearly define the relationships of dosage, distribution, and clearance
of this new agent.
A series of studies with conventional DAmB and ABLC were conducted with
rabbits. The studies with DAmB were used to develop a compartmental
model that with appropriate modifications would also describe the
kinetics of DAmB after administration as the lipid complex (ABLC). As
ABLC may be used over a range of dosages, its pharmacokinetics were
studied at doses of 0.5 to 10 mg/kg of body weight.
When preliminary analysis of these single-dose data revealed a
dose-related rapid disappearance of ABLC from plasma, we sought to
identify a readily sampled tissue(s) responsible for this initial distribution. Therefore, cellular components of blood-sediment which
may equilibrate with ABLC (3) were studied by sampling whole blood.
Finally, because the drug is most often chronically administered, a
study with repeated dosing for 27 days was performed to evaluate the
performance of the single-dose model in predicting the results of
chronic administration. The data from each study with ABLC were fitted
by using a nonlinear pharmacokinetic model constructed to explain the
study data.
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MATERIALS AND METHODS |
Animals.
Female New Zealand White rabbits (weight, 2.5 to
3.5 kg) were used throughout these experiments. Animals were
individually housed and were provided food and water ad libitum,
following National Institutes of Health guidelines on the care and use
of laboratory animals. Silastic venous catheters were surgically placed
under sterile operative conditions for nontraumatic venous access, as
described previously (16). The central silastic venous catheter facilitated parenteral administration of medications and
withdrawal of blood for pharmacokinetics.
Antifungal compounds.
DAmB (Fungisone; Bristol-Myers Squibb,
Princeton, N.J.) was reconstituted with distilled water, maintained at
4°C, and diluted 1:5 (by volume) with sterile 5% dextrose in water
immediately prior to use. ABLC (The Liposome Company) was provided as a
5-mg/ml solution. The stock solution of ABLC was diluted 1:5 (by
volume) with sterile 5% dextrose in water to yield an infusion
solution of 1.0 mg/ml for ABLC.
Administration of antifungal compounds and plasma
sampling.
DAmB (0.5 to 1.5 mg/kg) was administered by constant
infusion over 5 min with subsequent sampling of plasma. Sampling times were at 0, 5, 10, 15, 30, and 60 min and 2, 3, 4, 6, 8, 12, 24, 48, and
72 h postinfusion. Infusion of DAmB at dosages of 0.5 to 1.0 mg/kg
is well tolerated by rabbits. However, DAmB at 1.5 mg/kg is poorly
tolerated by intravenous infusion over times that range from 5 to 15 min. Approximately one-half of the rabbits that receive DAmB at 1.5 mg/kg will sustain ventricular fibrillation, presumably due to the
extracellular efflux of potassium due to amphotericin B-induced
myocardial membrane injury during infusion through the central silastic
venous catheter. Thus, the ability to study DAmB at 1.5 mg/kg was
limited by the mortality induced by the drug at this dosage.
ABLC (0.5, 1.0, 2.5, 5.0, and 10.0 mg/kg) was administered by constant
infusion over 5 min, with subsequent sampling of plasma. Four
additional animals were given an infusion of the 5.0-mg/kg dose over 5 min, with whole blood then sampled. There was no intolerance of
infusion of ABLC. The sampling times were the same as those for DAmB.
Analytical methods.
Amphotericin B levels in plasma were
assayed by high-pressure liquid chromatography based upon a
modification of the method developed by Granich et al. (8).
Calibration curves were prepared with plasma with low (25 to 1,000 ng/ml) and high (1 to 20 µg/ml) concentration ranges. The resulting
curves were linear, with correlation coefficients of 0.998. Assay
sensitivity was 25 ng/ml, with intraday variabilities of 10.2 and 5.4%
at concentrations of 75 and 1,000 ng/ml, respectively. Interday
variabilities at these respective concentrations were 12.4 and 6.2%.
Assay accuracy was 92%.
Whole-blood samples that contained ABLC were analyzed by extraction
with dimethyl sulfoxide-methanol, after an aliquot had
been removed for
analysis of plasma, with the resulting supernatant
being quantitatively
injected onto a high-pressure liquid chromatograph
for analysis by UV
detection. The assay did not use an internal
standard. Standard
calibration curves prepared with whole blood
were constructed in the
ranges of 50 to 100 ng/ml and 1 to 30
µg/ml. Intraday variability at
concentrations of 50 ng/ml and
20 µg/ml were 6.5 and 3.2%,
respectively, with interday variabilities
at the same concentrations
being 7 and 5.1%, respectively. Assay
accuracy was 95%, and drug
recovery was 90%.
Data analysis.
Plasma and cellular components of
blood-sediment data were fitted simultaneously by using CONSAM. (The
CONSAM software is available at no cost from Peter Greif at the
National Cancer Institute, Building 10, Room 6B13, Bethesda, MD 20892 [(301) 496-8915], or by e-mail
[Greif{at}SAAM.NCI.NIH.GOV] or anonymous FTP
[ftp@SAAM.NCI.NIH.GOV].) When the final model was developed, the
parameters were adjusted individually for each rabbit by nonlinear
least-squares techniques to obtain a best fit of the data, as judged
visually both by the sum of squared deviations of the model-calculated
values from the observed data and by the magnitude of the percent
coefficient of variation for the estimated parameters. Clearance from
plasma was estimated as the quotient of the administered dose and the fitted area under the plasma concentration-time curve.
Model validation.
Model validation was done by external
validation (4), with the single-dose data for the 5-mg/kg
dose with sampling of whole blood used as the training data set and the
data from the chronically infused rabbit, from which only plasma was
sampled, used as the validation data set. Both sets of data were
analyzed by use of the full model.
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RESULTS |
DAmB model.
The reported pharmacokinetic properties of DAmB
include multicompartmental kinetics, a half-life of 24 to 48 h,
excretion via urine and bile, partitioning into hydrophilic
environments, and binding to cellular components of blood-sediment and
plasma proteins. In order to account for these properties and the
present observations, a previously proposed model for DAmB
(2) which included separate equilibrium distributions of
drug in plasma with fast compartments (interstitial fluid of tissues
with discontinuous capillaries [liver, spleen, and intestine]) with
small distribution volumes, slowly equilibrating tissues (interstitial
fluid of muscles and blood cells), and elimination from plasma was used
to describe the data (Fig. 1).
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FIG. 1.
Model for DAmB showing all linear transfers with
equilibrium between compartment 2 and compartments 3 and 6.
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The rate constants
L(3,2) and
L(2,3) (for
intercompartmental transfer between fast equilibrating tissues and
plasma, respectively),
L(2,6) and
L(6,2) (for
intercompartmental transfer between slowly
equilibrating tissues and
plasma, respectively), and
L(0,2) (elimination
of drug from
plasma) were estimated from the best fit of the data.
All kinetic
transfers were first order, and the volume of plasma
in the rabbit was
estimated to be 38.8 ml/kg (T. J. Walsh, T.
Whitcomb, and P. A. Pizzo, Abstr. Am. Soc. Microbiol. Conf. Candida
and Candidiasis.
Biology, Pathogenesis, and Management, abstr.
1341, 1996), while the
volume of the cellular components of blood-sediment
was 16.8 ml/kg
(
18). The differential equation used to describe
the
concentration of drug in the central compartment is given
in the
Appendix.
The fitted data for individual rabbits that received doses of 0.5, 1.0, and 1.5 mg/kg are presented in Fig.
2.
Model parameters
based upon the fitted data are given in Table
1. The fitted parameters
for
equilibration between compartments 2 and 6 were similar for
all three
doses. However, the rate constants
L(3,2) and
L(2,3)
for equilibration with compartment 3 were larger for
the 1.5-mg/kg
dose but smaller for the 1.0- and 0.5-mg/kg doses. The
area under
the plasma concentration-time curve as a function of dose
presented
in Table
1 exhibits an apparent loss in dose proportionality
after administration of the 1.0-mg/kg dose.
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FIG. 2.
Best-fit curves for amphotericin B in rabbits that
received 0.5 mg (n = 3) (A), 1.0 mg (n = 3) (B), and 1.5 mg (n = 2) (C) of DAmB per kg via
a 5-min infusion.
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TABLE 1.
Parameter values for the rabbits dosed with DAmB at 1.5, 1.0, and 0.5 mg/kg via a 5-min infusion based on the best fit of
the curves for concentrations determined
in plasmaa
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ABLC model.
The a priori model used to begin the development
of a suitable model to describe the pharmacokinetics of ABLC in rabbits
was based upon several changes in that for DAmB. The DAmB model was modified to account for observations at doses between 2 and 10 mg/kg.
It was apparent from preliminary examination of the data for the
amphotericin B concentration in plasma (Fig. 2) following DAmB
administration that they were much higher following the 5-min infusion
than following administration of the equivalent dose of ABLC (Fig.
3). This apparent drug formulation effect
was investigated with an additional study with four rabbits that
received 5 mg/kg, with sampling of whole blood to determine if
significant levels of ABLC were associated with blood compartments.
From this supplemental study it was apparent that the uptake
(association with) of ABLC by cellular components of blood-sediment
occurred during the infusion period, resulting in levels in cellular
components of blood-sediments 40-fold higher than the levels in plasma
at the end of the infusion (Fig. 4). This
information was used to modify the DAmB model to describe ABLC by
including the following factors: (i) dose- and time-dependent uptake by
(association with) and return from cellular components of
blood-sediment and (ii) time-dependent tissue uptake and delayed return
to plasma.
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FIG. 3.
Best-fit curves for amphotericin B with the ABLC model
for rabbits following administration of ABLC at doses of 0.5 mg/kg
(n = 4) (A), 1.0 mg/kg (n = 4) (B), 2.5 mg/kg (n = 4) (C), 5.0 mg/kg (n = 4)
(D), and 10.0 mg/kg (n = 3) (E) via a 5-min infusion.
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FIG. 4.
Best-fit of amphotericin B levels in red cell-sediment
(A) and plasma (B) for rabbit B, which received 5 mg of ABLC per kg,
with the ABLC model.
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The transfer of drug from cellular components of blood-sediment to
plasma,
L(2,6), was described by a function (see equation
A3
in the
Appendix) whose sign at the beginning is negative, indicating
that initially the net transfer of drug is only from plasma to
cellular
components of blood-sediment. At some point during infusion,
L(2,6) switches to a positive value. The values for
L(2,6) during
infusion are related to the reciprocal of the
cumulative amount
of drug infused. Thus, the larger the amount of drug
infused per
unit of time, the higher the total dose, the slower the
increase
of
L(2,6) toward zero, and the smaller the positive
value of
L(2,6)
prior to the end of the infusion at 5 min.
During the infusion
period, the larger the infusion rate, the greater
the amount of
drug accumulated by (associated with) the cellular
components
of blood-sediment at the end of the infusion period. This
function
was turned off at 5 min (i.e., the end of infusion) via a step
function,
u(
t 
a) (equation 1).
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(1)
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where
t is time elapsed since initiation of drug input,
a is the length of infusion,
u(
t a) is equal to 1 for
t less than
a, and
u(t
a) is equal to 0 for
t greater
than
a, and
R0 is
the amount infused
over 5
min.
In other words, the amount of ABLC within the apparent cellular
components of the blood-sediment compartment is dependent
on the
balance between the rates of association and disassociation
with the
cellular components of blood-sediment. As these rates
cannot be
independently determined from this study design, we
chose to make the
fractional rate of accumulation constant and
build the nonlinearity and
time dependence into the rate of dissociation
of ABLC from the cellular
components of blood-sediment. However,
during infusion, prior to
collection of data, we modeled
L(2,6)
so that it was
negative (i.e., double arrow in Fig.
5).
This not
only allowed control of efflux from the cellular components of
the blood-sediment but also facilitated a very rapid influx of
drug to
the cellular components of the blood-sediment compartment
to describe
the observed high levels in blood at the end of the
infusion as a
function of the ABLC dose (Fig.
3).
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FIG. 5.
Model for amphotericin B in rabbits following ABLC
administration. The model consists of red blood cells (compartment 6)
and plasma (compartment 2). The model has a nonlinear return from red
blood cells to plasma, a time-dependent uptake of drug into compartment
7 with a delayed return to plasma, an equilibrium with compartment 3, and nonlinear elimination from the central compartment.
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The final model was determined from the sampling of whole blood after
administration of the 5-mg/kg dosage to rabbits. This
dosage is that
which is currently approved by the Food and Drug
Administration for
treatment of invasive aspergillosis in humans.
The amphotericin B
levels in the cellular components of blood-sediment
were estimated from
the concentrations in whole blood on the basis
of the rabbits'
hematocrits and the concentrations in
plasma.
Similar to DAmB, there was an equilibrium of ABLC with a peripheral
compartment. However, unlike DAmB, ABLC required a slow
time-dependent
uptake into peripheral compartment 7, with a delayed
return to plasma.
The return of drug from compartment 7 to plasma
was first order. The
rate constants for the delayed uptake into
the peripheral compartments,
L(14,7),
L(15,14),
L(16,15),
L(17,16),
and
L(2,17), were all set equal to the
same value to allow each
to contribute to drug delay
equally.
Elimination of drug from the central compartment,
L(0,2),
was dependent upon the quotient of the concentration in plasma and
the
cumulative area under the curve to 5 min (see equation A2
in the
Appendix), which resulted in the elimination of a decreasing
fraction
of drug with time. The final model is presented in Fig.
5.
The results of the fits for the rabbits from which plasma and whole
blood were sampled are given in Table
2.
The results
reveal very large rate constants for transfer of drug from
plasma
to compartment 3 and also from plasma to the cellular components
of blood-sediment.
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TABLE 2.
Parameter values for four rabbits dosed with 5 mg of ABLC
per kg via a 5-min infusion based on best-fit curves from
concentrations determined in whole blood and plasma
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Although most of the values were similar for the four rabbits that
received 5 mg/kg and from which whole blood was sampled,
the parameters
related to tissue uptake and delayed return to
plasma, i.e.,
P(3),
P(4) [the exponential and intercept in
equation
A3 in the
Appendix that define
L(7,2)], and
P(10) (
P values are
unassociated nonlinear
estimated parameters in SAAM [simulation,
analysis, and modeling]),
did exhibit large interanimal
differences.
This was a result of the fact that the values for these tissue
parameters were estimated solely from concentrations in plasma.
Therefore,
P(10),
P(3), and
P(4) were
highly variable, as indicated
by their high coefficients of
variation.
The mean blood-sediment parameters for the rabbits from which whole
blood was sampled was used to define the cellular components
of the
blood-sediment compartment for those animals from which
only plasma was
sampled. Parameters
L(2,3),
L(3,2),
P(10),
P(4),
and
P(3), related to
uptake by other tissue, were allowed to vary
to fit the data for the
rabbits from which only plasma was sampled.
Although the same mean
cellular components of the blood-sediment
parameters were used for each
dose, the dose effect was reflected
in
L(2,6) by the
magnitude of
R0 (i.e., the amount of drug
infused),
as defined by equation A3 in the
Appendix, during the 5-min
infusion.
The dose effect seen for the rabbits on the basis of the analysis of
plasma is presented in Fig.
3, which gives representative
individual
fits for curves for concentrations in plasma after
administration of
0.5, 1.0, 2.5, 5.0, and 10.0 mg/kg. A nonlinear
response to dose is
indicated by the 10-fold increase in initial
levels in plasma following
a 20-fold increase in the dose. The
values of the parameters from the
fits are given in Table
3.
The parameters
for plasma did not exhibit a trend related to dose.
However, there was
some indication of nonlinearity in the area
under the plasma
concentration-time curve versus dose (Fig.
6)
above 5 mg/kg. This was supported by
the 95% confidence interval
(i.e., 4.8 to 2.4) for the mean slope
calculated by using a power
model (
7).
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TABLE 3.
Parameter values for rabbits dosed with ABLC at 10, 5, 2.5, 1.0, and 0.5 mg/kg on the basis of the best-fit curves
determined from concentrations in plasmaa
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FIG. 6.
Plot of area under the plasma concentration-time curve
(AUC) (± standard deviation) for amphotericin B versus dose based upon
the best-fit curves for plasma for the rabbits that received ABLC at
0.5 to 10 mg/kg.
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The maximum capacity for an association between the cellular components
of blood-sediment and ABLC was achieved with doses
of between 5 and 10 mg/kg, as indicated by the unexpected increase
in the initial
postinfusion concentration to 4 µg/ml for a dose
of 10 mg/kg compared
to the low initial postinfusion values (1
µg/ml) for doses of between
0.5 and 5 mg/kg. The change in the
rate constant for the elimination of
ABLC from the central compartment
with respect to dose and time,
L(0,2), is given in Fig.
7.
Figure
7 shows that as time increases, there is a decrease in the rate
constant for the elimination of amphotericin B following ABLC
dosing,
L(0,2).
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FIG. 7.
Rate constant for elimination from the central
compartment, L(0,2), plotted as a function of dose versus
time. ABLC was administered at 0.5 mg/kg
(.....), 1.0 mg/kg (---),
2.5 kg/kg (-.-.-), 5.0 mg/kg (-----), and 10.0 mg/kg ( ) (curves
from top to bottom, respectively) via a 5-min infusion.
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Model validation.
The mean values for the variable parameters
L(3,2), L(2,3), P(10), and
P(4) from the data obtained after administration of a single
dose of 5 mg/kg in Table 2 were used to fit the data obtained after
administration of multiple doses by allowing these parameters to
deviate by no more than 10% from the mean. The resulting fit indicated
that the parameters related to the recycling of ABLC from the
single-dose model would have to be quite different to adequately
predict the multiple-dose data.
Further testing and application of the model was done by fitting the
data from the multiple-dose study by allowing the parameters
L(3,2),
L(2,3),
P(10),
P(3), and
P(4) to vary as required to best
fit
the data. The best fit of the validation data following chronic
administration of 5 mg/kg for 27 days is given in Fig.
8. Visual
inspection of the data
indicates that the single-dose model adequately
predicts the effects of
administration of multiple doses of ABLC
to rabbits but requires a
different set of parameter values related
to tissue recycling of drug.
On the other hand, the nonlinear
functions related to the cellular
components of blood-sediment
and elimination were predictive for the
multiple-dose data. Best-fit
parameters (and their respective
coefficients of variation) were
as follows:
L(3,2), 3,997 h
1 (11.8 h
1);
L(2,3), 8.42 h
1 (1.69 h
1);
P(10), 0.073 h
1 (60 h
1);
P(3), 1.6 × 10
3 h
1 (86 h
1); and
P(4), 1.29 × 10
3 h
1 (112 h
1).
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FIG. 8.
Data obtained following the chronic administration of
ABLC at 5 mg/kg/day for 27 days via a daily 5-min infusion to a rabbit.
The levels in plasma following chronic administration were fit by using
the model presented in Fig. 5.
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DISCUSSION |
The data obtained in this study demonstrate differences in the
pharmacokinetics of amphotericin B when it is infused as DAmB (a mixed
micellar dispersion) or as ABLC (a ribbon-like lipid structure which is
not a true liposome). Pharmacokinetic data for DAmB appeared to be
linear to 1 mg/kg, as has been reported previously (2). As
illustrated in Fig. 1, the model developed by Atkinson and Bennett
(2) to describe the data for the concentration in plasma
included equilibrium with a fast and a slowly equilibrating tissue. The
pharmacokinetics of DAmB in a rabbit model was similar to that in
humans (2). All transfers and elimination from the central
compartment were linear. The equilibrium with compartment 6 was
speculated to represent interaction with cellular components of
blood-sediment on the basis of the large uptake of AmB when it is given
as the lipid complex (ABLC). The previously reported nonlinearity above
1 mg/kg was not further investigated, but it is possible that it may
also be related to an interaction with cellular components of
blood-sediment (2, 3).
Administration of amphotericin B as ABLC resulted in significant
changes in the pharmacokinetics of the parent compound. The most
notable changes in pharmacokinetics were the substantially smaller
plasma amphotericin B concentrations after the administration of
10-mg/kg doses of ABLC compared to those seen after administration of
the largest DAmB dose (1.5 mg/kg). This difference appears to be the
result of the nonlinear uptake of ABLC by (association with) the
cellular components of blood-sediment and/or cell membranes. This
greater proportional amount of drug delivered to tissues may have
important implications for the greater efficacy of higher dosages
against severe infections.
The nonlinear uptake of ABLC by tissue was related to the reciprocal of
the integral of the total amount of drug infused over 5 min, as
described in equation 1. Consequently, the greater the amount infused
the smaller the rate constant L(2,6) for the transport of
amphotericin B from cellular components of blood-sediment to plasma
during the 5-min infusion. During the 5-min infusion, the value for
L(2,6) is initially negative and becomes positive, which would not be considered physiologically possible. Although alternative models without negative rate constants that describe the data during
infusion exist, it would be difficult to definitively establish an
appropriate model during the infusion since samples were not collected
during the 5-min infusion. The change in sign is a technique that
allows the nonlinear process to be attributed to the transfer of drug
from cellular components of blood-sediment to plasma. However, this
does not preclude the possibility that the transfer from plasma to
cellular components of blood-sediment is also nonlinear. The procedure
that we chose allows the appropriate accumulation of drug by
(association with) the cellular components of the blood-sediment compartment to simulate the observed datum points for cellular components of blood-sediment and plasma at the conclusion of the 5-min
infusion. Conversely, the smaller the amount of ABLC infused over time,
the greater the amount of drug delivered to plasma, thus permitting the
drug to reach other tissues. Thus, a slow rate of infusion may permit
improved delivery to noncellular components of blood-sediment tissue
compartments (i.e., interstitial fluid of tissues with discontinuous
capillaries [liver, spleen, and intestine]).
Figure 6 shows the nonlinear relationship between the area under the
concentration-time curve and dose for plasma following dosing with
ABLC. Initially, little drug escapes the blood-sediment, resulting in
low levels in plasma which increase with dose and result in an apparent
nonlinearity between 0.5 and 1 mg/kg. The derived area under the curve
data for blood-sediment in Table 3 shows nonlinearity for doses between
5 and 10 mg/kg, although it is not clearly seen for the concentration
in plasma since the concentration in plasma is in equilibrium with
those in several other tissues and plasma is not the tissue of origin
for the nonlinearity. On the other hand, the area under curve data for
plasma in Table 1 show that some possible nonlinearity for DAmB occurs
at a dose of 1.5 mg/kg, although the small sample size precludes a
definitive conclusion.
Approximately 2 to 5% of the administered DAmB has been reported to be
excreted unchanged in urine and bile each (1, 2, 4,5). No
metabolites have yet been identified in plasma or urine. The model used
to describe the kinetics of amphotericin B after the administration of
DAmB has linear elimination. However, the elimination rate constant
L(0,2) becomes nonlinear when AmB is administered as ABLC.
This change is dependent on both the time following infusion and the
total dose infused and suggests diminished renal or hepatic function
with time during infusion.
As ABLC is used over a number of dosages, understanding of its
pharmacokinetic parameters may further facilitate improved approaches
to administering this novel agent. ABLC is approved for use for the
treatment of invasive aspergillosis at 5.0 mg/kg/day. However, higher
dosages (7.5 to 10 mg/kg/day) are sometimes used to treat profoundly
immunocompromised patients whose infections fail to respond to 5.0 mg/kg/day (9). A lower dosage of 2.5 mg/kg/day was
administered for 6 weeks for the successful treatment of hepatosplenic
candidiasis in children (Walsh et al., Abstr. Am. Soc. Microbiol. Conf.
Candida and Candidiasis).
The role of cellular components of blood-sediment in serving as a major
compartment carries several pathophysiological implications. This
interaction occurs in the apparent absence of hemolysis. First, the
improved therapeutic index afforded by the DMPG-DMPC (3:7) lipid
complex protects cellular components of blood-sediment from high
concentrations of AmB, in contrast to the effect observed with DAmB, in
which hemolysis occurs at relatively low concentrations (11). Second, immunocompromised patients receiving ABLC for treatment of life-threatening mycoses are often anemic, with hematocrit values 25 to 50% below normal baseline values. The implications of
anemia on the pharmacokinetics of ABLC have yet to be explored; however, anemic states may allow transfer of ABLC to noncellular components of blood-sediment membrane sites that otherwise would not be
involved in patients with normal hematocrit values. Third, the impact
of acquired or inherited membrane defects in cellular components of
blood-sediment that result in altered binding or interaction sites
could further influence the distribution of ABLC (12).
The transfer of ABLC into noncellular components of blood-sediment
tissue compartments occurs predominantly in reticuloendothelial system
(RES) tissues, particularly the liver and spleen. The slow time-dependent uptake into peripheral compartment 7 with a delayed return to plasma may reflect this uptake by RES tissues. This distribution into RES tissues provides a compelling pharmacodynamic rationale for treatment of hepatosplenic (chronic disseminated) candidiasis. These properties have led to the use of ABLC in a protocol
for children with hepatosplenic candidiasis (17;
Walsh et al., Abstr. Am. Soc. Microbiol. Conf. Candida and
Candidiasis). All patients in that study responded to ABLC with
complete or partial resolution of physical findings and lesions of
hepatosplenic candidiasis. During the course of ABLC infusions and
monitoring, there was no progression of hepatosplenic candidiasis,
breakthrough fungemia, or posttherapy recurrence. Hepatic lesions
continued to resolve after completion of administration of ABLC,
reflecting the high concentrations in the liver and spleen.
The physiological entities which comprise the cellular components of
blood-sediment compartment are red blood cells, leukocytes, platelets,
lipoproteins, and nonlipoprotein plasma proteins. This was presented as
the rapidly equilibrating compartment since clear definition of the
component which interacts with ABLC and which results in high blood
ABLC levels would require the analysis of washed red blood cells to be
certain that the reported levels are a result of drug that has
partioned into the red blood cell and not merely the result of an
association with the cellular components of blood-sediment.
In summary, this study is the first to our knowledge to describe a
detailed model of the pharmacokinetics of ABLC and the first to
characterize a possible role of the cellular components of the
blood-sediment compartment in the distribution of this novel antifungal
compound in tissue.
| |
APPENDIX |
Model equations for amphotericin B in the central compartment
(CP):
![dC<SUB>p</SUB>/dt=−[L(0,2)+L(3,2)+L(6,2)]·C<SUB>p</SUB>](/content/vol44/issue8/fulltext/2068/img002.gif)
|
(A1)
|
where
C6 and
C3 are
amounts in compartments 6 and 3,
respectively.
Model equations for ABLC in the central compartment:
![dC<SUB>p</SUB>/dt=−[L(0,2)+L(3,2)+L(6,2)+L(7,2)]·C<SUB>p</SUB>](/content/vol44/issue8/fulltext/2068/img006.gif)
|
(A2)
|
where
Crbc is the red blood cell sediment
compartment.
Model equation for cellular components of blood-sediments:
|
(A3)
|
where
where E is exponential function for natural log,
P(11)
is 1.0 h
1, and
T is
time.
| |
FOOTNOTES |
*
Corresponding author. Mailing address: Pediatric
Oncology Branch, Bldg. 10, Room BN240, National Institutes of Health,
Bethesda, MD 20892. Phone (301) 402-0023. Fax: (301) 402-0575. E-mail:
Walsht{at}mail.NIH.GOV.
Deceased.
| |
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Top
Abstract
Introduction
![L(2,6)={P(12)·[(1−E<SUP>−P(13)·T</SUP>)−40]}](/content/vol44/issue8/fulltext/2068/img014.gif)
