The anticancer drug ifosfamide is a prodrug requiring activation
through 4-hydroxyifosfamide to ifosforamide mustard, to exert cytotoxicity. Deactivation of ifosfamide leads to 2- and
3-dechloroethylifosfamide and the release of potentially neurotoxic
chloracetaldehyde. The aim of this study was to quantify and to compare
the pharmacokinetics of ifosfamide, 2- and 3-dechloroethylifosfamide,
4-hydroxyifosfamide, and ifosforamide mustard in short (1-4 h), medium
(24-72 h), and long infusion durations (96-240 h) of ifosfamide. An
integrated population pharmacokinetic model was used to describe the
autoinducible pharmacokinetics of ifosfamide and its four metabolites
in 56 patients. The rate by which autoinduction of the metabolism of ifosfamide developed was found to be significantly dependent on the
infusion schedule. The rate was 52% lower with long infusion durations
compared with short infusion durations. This difference was, however,
comparable with its interindividual variability (22%) and was,
therefore, considered to be of minor clinical importance. Autoinduction
caused a less than proportional increase in the area under the
ifosfamide plasma concentration-time curve (AUC) and more than
proportional increase in metabolite exposure with increasing ifosfamide
dose. During long infusion durations dose-corrected exposures (AUC/D)
were significantly decreased for ifosfamide and increased for
3-dechloroethylifosfamide compared with short infusion durations. No
differences in dose-normalized exposure to ifosfamide and metabolites
were observed between short and medium infusion durations. This study
demonstrates that the duration of ifosfamide infusion influences the
exposure to the parent and its metabolite 3-dechloroethylifosfamide.
The observed dose and infusion duration dependence should be taken into
account when modeling ifosfamide metabolism.
 |
Introduction |
Ifosfamide (Holoxan, Ifex) is an alkylating agent, which has been
proven to be active against a number of solid tumors and hematological
malignancies in adults and children. Currently, ifosfamide is mostly
used in combination with other anticancer drugs, but also as single
agent. Many different infusion schedules are in use, e.g., continuous
infusion over either 24, 72, or 240 h with doses of 5, 9, or
7.5-15 g/m2 ifosfamide, respectively (Kaijser et
al., 1996
; Nielsen et al., 2000
).
Ifosfamide is a prodrug, which needs activation by cytochrome P450-3A4
(CYP3A41)
to 4-hydroxyifosfamide, as depicted in Fig.
1. Spontaneous decomposition of
4-hydroxyifosfamide yields the ultimate alkylating metabolite ifosforamide mustard and acrolein (Kerbusch et al., 2001
). The alkylating activity of ifosforamide mustard is responsible for both the
antitumor activity and hematological toxicity. Acrolein causes
hemorrhagic cystitis, which can generally be prevented by mesna
coadministration. Ifosfamide is deactivated to the noncytotoxic metabolites 2- and 3-dechloroethylifosfamide. Each dechloroethylation reaction yields an equimolar amount of chloracetaldehyde, which is held
responsible for neurotoxicity observed in about 10% of all patients
receiving conventional single-agent dosing of ifosfamide (Cerny and
Küpfer, 1992
). In addition, observed renal tubular abnormalities
may also be correlated with the formation of chloracetaldehyde (Kerbusch et al., 2001
). Chloracetaldehyde is very unstable. Therefore, assessment of 2- and 3-dechloroethylifosfamide exposure is preferred when quantifying the relationship between the pharmacokinetics and
neurotoxicity after ifosfamide infusion.
Ifosfamide is subject to autoinduction, which leads to an
increase in metabolism of ifosfamide over time (Kerbusch et al., 2001
).
Several comparative studies observed that fractionation of the
ifosfamide dose leads to increased autoinduction (Lewis et al., 1990
;
Kerbusch et al., 2000b
). Pharmacokinetic studies can provide further
insight into the effect of the infusion schedule on the autoinduction
of ifosfamide metabolism. Aim of this study is to quantify and to
compare the pharmacokinetics of ifosfamide, 2- and
3-dechloroethylifosfamide, 4-hydroxyifosfamide, and ifosforamide mustard in short (1-4 h), medium (24-72 h), and long infusion durations (96-240 h) of ifosfamide. Assessment of pharmacokinetics will indicate whether the development of autoinduction is dependent on
the dose and/or infusion duration, and may further aid in the design of
ifosfamide infusion schedules.
 |
Patients and Methods |
Eligibility Criteria.
Thirty-three patients received ifosfamide (Holoxan, ASTA Medica,
Diemen, The Netherlands) as i.v. infusion as part of the regular
therapy with ifosfamide as single agent or as combination chemotherapy
with cisplatin, carboplatin, etoposide, Adriamycin, vincristine, or
methotrexate, during the period of March 1997 and April 2000. None of
these anticancer agents are known to induce CYP-mediated metabolism or
are known to pharmacokinetically interact with ifosfamide. Typical
infusion schedules used for ifosfamide were 1.2 g/m2 once daily over 1, 3, or 4 h for three
consecutive days, 1.5 g/m2 once daily in
22.5 h for two consecutive days, 5 g/m2 in
24 h once, and 9 g/m2 by means of a 72-h
continuous infusion. The study protocol was approved by the Ethics
Board of the Antoni van Leeuwenhoek Hospital/The Netherlands Cancer
Institute. Another group of 23 patients received a 10-day continuous
infusion of single-agent ifosfamide using an ambulatory pump. This
group was treated at the Leiden University Medical Center, in a study
that was approved by the Ethics Board of the hospital (Kaijser et al.,
1996
). Supportive care consisted of mesna and extensive hydration to
prevent hemorrhagic cystitis and bicarbonate to prevent acidosis.
Standard 5-hydroxytryptamine3 blockers as
antiemetics and methylene blue (a neurotoxicity antidote) were given
when indicated (Cerny and Küpfer, 1992
). Pharmacokinetic sampling
was performed after written informed consent was obtained from the patients.
Pharmacokinetic Sampling.
Blood samples were drawn before the start of, during, and after the end
of the infusion at selected time points, depending on the infusion
schedule. Ten milliliters of whole blood was collected in a lithium
heparin-coated Vacutainer (Becton Dickinson, Plymouth, UK) and placed
in ice water. The plasma was immediately separated by centrifugation at
1000g for 5 min at 4°C. The plasma was aliquoted in 4 volumes of which three were precisely 1-ml volumes. To two 1-ml
volumes, 100 µl of 2 M semicarbazide solution, pH 7.4, was added to
stabilize 4-hydroxyifosfamide. To another 1-ml volume, 100 µl of 1 M
sodium chloride, 2 M semicarbazide solution, pH 8.0, was added to
prevent 4-hydroxyifosfamide degradation to ifosforamide mustard and to
stabilize the latter. The remaining plasma was used for ifosfamide, 2- and 3-dechloroethylifosfamide analysis. Urine was collected from the
start of the (first) infusion until 24 h after the end of the
(last) ifosfamide infusion and was analyzed for ifosfamide, 2- and
3-dechloroethylifosfamide. Both plasma and urine samples were stored at
70°C, pending analysis.
Bioanalysis.
Gas chromatography with selective nitrogen-phosphorus detection was
used for the determination of ifosfamide, 2- and
3-dechloroethylifosfamide in urine and plasma (Kerbusch et al., 2000c
).
Sample pretreatment consisted of alkalinized liquid-liquid extraction
with ethyl acetate, transfer of the organic extract to another tube,
evaporation to dryness, and subsequent reconstitution in ethyl acetate.
This method was validated and proved to be specific, sensitive,
accurate (93.3-104.5%), and precise (within and between day <5.5%)
within the concentration range of 0.192 to 383 µM, with a lower limit of quantification (LLQ) of 0.192 µM for ifosfamide, 2- and
3-dechloroethylifosfamide.
High-performance liquid chromatography (HPLC) was used for
determination of 4-hydroxyifosfamide plasma levels (Kerbusch et al.,
1998
). In brief, this method determined the
4-hydroxyifosfamide-semicarbazone derivative in plasma. Sample
pretreatment consisted of liquid-liquid extraction with ethyl acetate.
The HPLC column used was reversed phase C8 with acetonitrile-0.025 M
potassium dihydrogenphosphate (32:68 v/v) as mobile phase. Detection
was performed at 230 nm. This method was specific, sensitive, accurate
(94.1-107.9%), and precise (within and between day <7.2%) in the
concentration range of 0.361 to 361 µM, with a LLQ of 0.361 µM
4-hydroxyifosfamide (measured as semicarbazone derivative).
Plasma concentrations of ifosforamide mustard were determined using a
validated HPLC method (Kerbusch et al., 2000a
). In brief, this method
determined the ifosforamide mustard-diethyldithiocarbamate derivative
in plasma. Sample pretreatment consisted of derivatization followed by
liquid-liquid extraction with acetonitrile. Derivatization was achieved
by incubating 500 µl of plasma with 350 µl of 1 M sodium chloride,
0.1 M potassium dihydrogenphosphate buffer, pH 8.0, and 50 µl of
diethyldithiocarbamate for 30 min at 70°C. The HPLC column used was a
reversed phase C8 column with acetonitrile-0.025 M potassium
dihydrogenphosphate, pH 8.0 (32:68 v/v), as mobile phase. Detection was
performed at 276 nm. This method was specific, sensitive, accurate
(101.4-110.0%), and precise (<15.8%) in the concentration range of
0.452 to 226 µM, with an LLQ of 0.452 µM ifosforamide mustard
(measured as diethyldithiocarbamate derivative).
Data Evaluation.
Pharmacokinetic models were fitted to the plasma concentration data
from the individuals using the population pharmacokinetic program
NONMEM (Nonlinear Mixed Effects Modeling, version V 1.1, double
precision, first order estimation) (Beal et al., 1992
; Boeckman et al.,
1994
). The population pharmacokinetic models resulted in model
predictions and Bayesian individual predictions for the concentrations
of ifosfamide and metabolites (Sheiner and Grasela, 1991
). The
nonlinear pharmacokinetics of ifosfamide were described using a
recently developed model, which incorporated the development of
autoinduction (Kerbusch et al., 2000b
). According to this recent study
the time-dependent pharmacokinetics of ifosfamide can be described by a
one-compartment model (Fig. 2). The
change of the amount of ifosfamide (Aifo, µmol)
in the central compartment over time is described by eq. 1:
|
(1)
|
in which Tinf (h) is the infusion
duration, R (µmol · h
1) is the
infusion rate of ifosfamide, CL (liters · h
1) the ifosfamide clearance, which changes
over time, and Vifo (liters) the volume of
distribution. The predicted concentration of ifosfamide
Cifo (µM) is the ratio of
Aifo and Vifo. The
development of autoinduction is described using a hypothetical enzyme
compartment. CL is expressed as initial clearance
(CLini,ifo, liters · h
1) multiplied by the relative amount of enzyme
(Aenz) in the hypothetical enzyme compartment, as
given in eq. 2:
|
(2)
|
The change of Aenz (a relative measure
with no dimension) over time in the enzyme compartment is dependent on
Cifo as follows:
|
(3)
|
in which Kenz,out
(h
1) is the first order rate constant for
enzyme degradation/inactivation and IC50 (µM)
is the ifosfamide concentration at 50% of the maximum inhibition of
enzyme degradation. At t = 0 and
Aenz = 1, the enzyme formation rate
(Kenz,in) is equal to
Kenz,out (Fig. 2). The induction half-life of the
enzyme (t1/2enz, h) was calculated by the
ratio of ln(2) and Kenz,out.

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Fig. 2.
Pharmacokinetic model for ifosfamide
metabolism describing the autoinducible pharmacokinetics of ifosfamide
(IFO), 2-dechloroethylifosfamide (2DCE), 3-dechloroethylifosfamide
(3DCE), 4-hydroxyifosfamide (4OHIF), and ifosforamide mustard (IFM).
Autoinduction is modeled with a hypothetical enzyme compartment
described by an enzyme formation rate (Kenz,in) and
elimination rate (Kenz,out). The amount of enzyme increases
the clearance of IFO (CLIFO). The amount of IFO, described
by the IFO concentration and IFO volume of distribution
(VIFO), inhibits Kenz,out. Metabolite
compartments are described by metabolite formation (F) and elimination
rates (K).
|
|
The pharmacokinetics of ifosfamide and metabolites were described
sequentially: Bayes estimations (post hoc estimations) of the
pharmacokinetic parameters of ifosfamide were used during the
assessment of the pharmacokinetics of the metabolites. The change in
the amount of a metabolite (Am) over time could
be described by eq. 4:
|
(4)
|
in which Km
(h
1) is the elimination rate constant of the
metabolite and Fm is the fraction of ifosfamide
metabolized to the metabolite. The values for Fm
and the volume of distribution of the metabolite
(Vm, liters) cannot be estimated separately in this model. Therefore, the ratio of Fm over
Vm was estimated: F*
(liters
1).
The change in the amount of ifosforamide mustard
(Aifm) over time could be described by eq. 5:
|
(5)
|
in which Kifm (h
1)
is the elimination rate constant of ifosforamide mustard and
Fifm is the fraction of 4-hydroxyifosfamide metabolized to ifosforamide mustard. Equation 5 can be rearranged to
eq. 6:
|
(6)
|
in which V4ohif (liters) is the volume of
distribution of 4-hydroxyifosfamide and C4ohif
(µM) is the 4-hydroxyifosfamide concentration. The values for
Fifm and the volumes of distribution of
4-hydroxyifosfamide and ifosforamide mustard
(Vifm, liters) cannot be estimated separately in
this model and were, therefore, replaced by one parameter:
F**ifm, which has no dimension.
The residual or intraindividual variability of the pharmacokinetics of
ifosfamide and metabolites was described separately with a combined
proportional and additive term. The interindividual variability of each
pharmacokinetic parameter was estimated using a proportional error model.
The post hoc estimations of the areas under the concentration-time
curves (AUCs) of ifosfamide and its metabolites were obtained by
describing the cumulative concentrations of these compounds over a
given time. AUCs were obtained with extrapolation of ifosfamide and
metabolite concentration-time profiles up to 72 h after the end of
the (last) ifosfamide infusion, where concentrations were below the
LLQ. The effect of the infusion duration on the autoinduction of
ifosfamide metabolism and its effect on the metabolite exposures were
investigated by calculating dose-corrected AUCs. The dose-corrected AUC
was obtained by dividing the AUC by the administered ifosfamide dose. A
nonparametric test was conducted to determine significant differences
in dose-corrected AUCs between the infusion schedules. The
pharmacokinetics of each group of infusion schedules was compared with
the other two groups using the Kolmogorov-Smirnov test for two
independent samples.
The influence of the independent variable infusion duration was tested
on the estimation of each population parameter of ifosfamide, 2- and
3-dechloroethylifosfamide according to eq. 7:
|
(7)
|
in which Ppop is a population parameter,
1 is the typical value of that parameter with
short duration infusions,
2 is the change in
1 with medium infusion duration, and
3 is the change in
1
with long infusion duration. If the 95% confidence interval of the
estimate of
2 and
3
incorporated 1, no significant difference between the short infusion
durations with medium or long infusion durations was observed. The
urinary excretions for ifosfamide and 2- and 3-dechloroethylifosfamide
were calculated as the equimolar amount of the administered dose
recovered in urine. Differences between the urinary excretions of the
different infusion durations were tested for significance using the
nonparametric Mann-Whitney test. Statistical analysis was performed
using SPSS software (version 6.1 for Windows; SPSS, Inc., Chicago, IL).
The level of significance (p) was set at 0.05. All
tests for significance were two-tailed.
 |
Results |
A total of 56 patients was divided into three subpopulations with
12 patients receiving short (1-4 h), 21 patients receiving medium
(24-72 h), and 23 patients receiving long (96-240 h) infusion durations of ifosfamide (Table 1). The
number of patients in which 4-hydroxyifosfamide and ifosforamide
mustard were analyzed was insufficient for the separate estimation of
the pharmacokinetics of these metabolites for each infusion schedule.
The pharmacokinetic population parameters with their standard error and
interindividual variability and the residual variability of ifosfamide
and 2- and 3-dechloroethylifosfamide are presented in Table
2. The corresponding estimates of
4-hydroxyifosfamide and ifosforamide mustard are listed in Table
3.
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TABLE 1
Number of patients pharmacokinetically assessed for ifosfamide (IFO),
2-dechloroethylifosfamide (2DCE), 3-dechloroethylifosfamide (3DCE),
4-hydroxyifosfamide (4OHIF), and ifosforamide mustard (IFM) for each
infusion schedule
|
|
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TABLE 2
Estimates of population pharmacokinetic parameters of ifosfamide (IFO)
2-dechloroethylifosfamide (2DCE), and 3-dechloroethylifosfamide (3DCE)
with their relative standard error of the mean, interindividual
variability, and residual variability, including all statistically
significant (p < 0.05) change in the parameter estimate due
to infusion duration
|
|
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TABLE 3
Estimates of pharmacokinetic population parameters of
4-hydroxyifosfamide (4OHIF) and ifosforamide mustard (IFM) with their
relative standard error of the mean, interindividual variability, and
residual variability
|
|
The estimates of most pharmacokinetic parameters were not influenced by
the duration of infusion. Only minor differences were found for
Kenz,out, F* of 2-dechloroethylifosfamide
(F*2dce), and 3-dechloroethylifosfamide
(F*3dce). Kenz,out during
long infusion durations was 52% (
long = 0.48)
lower in comparison with short and medium infusion durations. F*2dce and F*3dce decreased
with 51% (
medium = 0.487) and 33%
(
medium = 0.666), respectively, after medium
infusion durations when compared with short and long infusion
durations. No outliers or trends could be observed in the plots of the
population predictions and individual Bayesian predictions of the
concentrations versus the observed concentrations (plots not shown).
The observed and individual Bayesian estimated pharmacokinetic profiles
of ifosfamide of all patients are presented in Fig.
3.

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Fig. 3.
Bayesian individual predicted plasma
concentration-time profiles of ifosfamide (IFO) for all patients during
short (1-4 h), medium (24-72 h), and long (96-240 h) infusion
durations.
The observations are represented by the dots.
|
|
The urinary excretion of unchanged ifosfamide and its dechloroethylated
metabolites are presented in Table 4.
Increase of infusion duration resulted in a significant decrease of
urinary recovery of unchanged ifosfamide and the sum of ifosfamide and metabolites. For unchanged ifosfamide the urinary recovery after short
and long infusion durations was 21.2 ± 6.6 and 8.6 ± 2.6%, respectively. No clear trend was observed for 2- and
3-dechloroethylifosfamide, although urine recovery of
2-dechloroethylifosfamide during long infusions was slightly lower than
during medium infusion duration.
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TABLE 4
Percentage of dose excreted in urine either unchanged as ifosfamide
(IFO) or as metabolite 2-dechloroethylifosfamide (2DCE) and
3-dechloroethylifosfamide (3DCE) in 40 patients
Values are means ± S.D.
|
|
The individual profiles of the ifosfamide clearances over time of each
infusion schedule are depicted in Fig. 4.
Modest interindividual variability was observed. The clearance of
ifosfamide increased during the infusion and decreased after cessation.
Notably, during the first 24 h for all patients the model
predicted an average doubling of the clearance.

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Fig. 4.
Individual time profiles of the clearance of
ifosfamide (IFO) for all patients.
The profiles were obtained on the basis of the individual Bayesian
parameter estimates based on the total population.
|
|
Figure 5 depicts the relationships
between the AUCs of ifosfamide and its metabolites and the ifosfamide
dose for the total population. The AUC of ifosfamide increased less
than proportional with increase in absolute ifosfamide dose. In
contrast, the AUCs of the metabolites increased more than proportional
with increase in absolute ifosfamide dose.

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Fig. 5.
From top left to bottom right: relationships
(with Loess-smooth) between the ifosfamide dose (mmol) and ifosfamide
(IFO), 4-hydroxyifosfamide (4OHIF), ifosforamide mustard (IFM),
2-dechloroethylifosfamide (2DCE), and 3-dechloroethylifosfamide (3DCE)
exposures as calculated by the AUC of each patient.
|
|
The dose-corrected AUCs are depicted in Fig.
6. The dose-corrected exposure to
ifosfamide during long infusions was decreased 26%
(p < 0.001) and 25%
(p < 0.05) compared with short and medium infusion duration, respectively. In comparison with medium infusion durations dose-corrected exposure to 3-dechloroethylifosfamide during
long infusion durations was increased with 45%
(p < 0.01).

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Fig. 6.
From top left to bottom right: relationships
between the ifosfamide infusion duration (h) and ifosfamide (IFO),
4-hydroxyifosfamide (4OHIF), ifosforamide mustard (IFM),
2-dechloroethylifosfamide (2DCE), and 3-dechloroethylifosfamide (3DCE)
dose-standardized exposures as calculated by the ratio of the AUC and
the ifosfamide dose of each patient.
The bar indicates the mean. *, significantly decreased 26%
(p < 0.001) and 25% (p < 0.05), respectively, compared with 1-4- and 24-72-h infusions. **,
significantly increased 45% (p < 0.01) compared
with 24-72-h infusions (Kolmogorov-Smirnov).
|
|
 |
Discussion |
This study investigated the effect of the duration of the
ifosfamide infusion on the pharmacokinetics of ifosfamide and its metabolites. In a review on the clinical pharmacokinetics of ifosfamide in 1994 it was proposed that fractionation of the ifosfamide dose increased the metabolism of ifosfamide (Wagner, 1994
). However, more
recent studies have indicated that the ifosfamide infusion schedule
does not influence the degree of metabolism. In a comparative study in
pediatric patients receiving 3 g/m2 ifosfamide
administered as a 1-h infusion for three consecutive days or 9 g/m2 ifosfamide administered as a 72-h continuous
infusion, no major differences in exposure were observed, except a
minor decrease in exposure to deactivated metabolites with the bolus
infusion (Boddy et al., 1995
). In addition, no differences in metabolic exposures were found in a similar study in adults (Singer et al., 1998
). Singer and coworkers concluded that there is no identifiable pharmacokinetic basis for insistence on either bolus or infusional methods of ifosfamide administration.
In our study a group of 56 patients was divided into three
subpopulations receiving either short (1-4 h), medium (24-72 h), or
long (96-240 h) infusion durations with ifosfamide. All patients studied received ifosfamide as part of standard therapy for the treatment of various malignancies. A previously developed autoinduction model was applied to assess the population pharmacokinetics of ifosfamide and metabolites (Kerbusch et al., 2000b
). This model describes the development of autoinduction with a relationship between
the ifosfamide plasma concentration and a hypothetical, dynamic enzyme
compartment, which is responsible for ifosfamide metabolism. Thus,
clearance of ifosfamide over time changes with ifosfamide
concentrations. From some of the patients included in the study only a
limited number of blood samples were available for pharmacokinetic
analysis. In such cases conventional noncompartmental analysis of the
concentration-time profiles does not allow accurate estimation of the
pharmacokinetic parameters, due to insufficient data. The applied
population approach with Bayesian estimation, on the other hand, does
not have these drawbacks.
The influence of the ifosfamide concentration on the enzyme amount was
modeled as an inhibiting influence on the degradation of the enzyme.
Ifosfamide has been shown to reduce protein synthesis and could have a
similar effect on the enzymes responsible for CYP3A4
degradation/inactivation, thus increasing ifosfamide clearance (Lewis
et al., 1990
). A similar mechanism of inhibition was proposed for
troleandomycin, which did not increase protein synthesis but inhibited
catalytic enzymes responsible for CYP degradation (Watkins et al.,
1986
). However, increase of enzyme synthesis through stabilization of
mRNA was suggested for cyclophosphamide (Hassan et al. 1999
) and
ifosfamide (Chang et al., 1997
). Whatever the mechanism, all modeling
approaches will result in an increased amount of enzyme (Hassan et al.
1999
). We also included the interaction term on the synthesis of the
enzyme and an identical fit to the concentration-time data of
ifosfamide was achieved. Thus, modeling autoinduction as increased
enzyme translation/transcription or as decreased enzyme degradation
will yield similar conclusions.
Statistically significant differences were found for
Kenz,out between long and short infusion
durations and for F*2dce and F*3dce between medium and short infusion
durations. These differences in parameter estimates were considered to
be minor (33-52%) and not clinically relevant, since they were in the
same order of magnitude as the interindividual variability of the
respective parameters (22-43%). The observed differences may be
explained by different amounts of data (information) within the
concentration-time profiles of the various infusion schedules, which
are necessary for the estimation of these pharmacokinetic parameters.
Estimation of Kenz,out in long infusion durations
is more feasible than in short infusion durations, because the effect
of autoinduction can be estimated over a longer period of time (Fig.
4). Nevertheless, the derived half-life of the enzyme
(t1/2enz) increased from 8.5 h with
short duration infusions to 17.7 with long duration infusions. Thus, in
the latter group maximum induction would be reached after approximately
88 h (5 × t1/2enz,long), whereas
maximum clearance in the short duration infusions would already be
reached after 42 h. This difference in enzymatic behavior can also
be observed in Fig. 4. The relatively lower number of samples (on
average three per patient) drawn during the first 24 h of medium
infusion durations, compared with short infusion durations (on average seven per patient), may have resulted in different estimations of the
formation rates of the deactivated metabolites. Sampling frequency
during the first 24 h with long infusions was more similar to that
of short infusion durations (on average five samples were collected per
patient). Hence, no difference was observed in metabolite formation
with long infusion durations compared with short.
All other estimates of the population parameters were not significantly
influenced by infusion duration. This indicates that no clinically
relevant differences were observed in the pharmacokinetics between the
different infusion durations.
The population pharmacokinetic parameters were in accordance with a
recent study on the pharmacokinetics of ifosfamide after administration
of 9 to 12 g/m2 ifosfamide in a 72-h continuous
intravenous infusion (Kerbusch et al., 2000b
). In another study in 13 cancer patients receiving 1.5 g/m2 for 5 days
ifosfamide clearance and autoinduction were described using a
population pharmacokinetic model for the enantiomers of ifosfamide and
its metabolites (Di Marco et al., 2000
). In that study clearance
increased from 4 to 7 and 5 to 10 l/h from day 1 to 5 for
R-ifosfamide and S-ifosfamide, respectively (Di
Marco et al., 2000
). This is in accordance with our findings as
presented in Fig. 4. Individual ifosfamide clearances increase from 4 to 7 l/h (range 5-15).
Initially, K values of the metabolites (K2DCE = 0.385, K3DCE = 0.124, K4OHIF = 73.8) were greater than the value
calculated for ifosfamide (CLini,
ifo/Vifo = 0.0656 h
1). This indicates that the observed decay in
plasma of the metabolites was formation rate limited. Maximum
autoinduction was reached after 42, 42, and 88 h in short, medium,
and long duration infusions, respectively. The K value of ifosfamide
ranged then approximately between 0.109 and 0.327 h
1. In other words, after autoinduction
ifosfamide elimination half-life decreased from 10.5 to 2.1 to 6.4 h and is now slower than the elimination half-life of 2- and
3-dechloroethylifosfamide (2 and 6 h, respectively). Thus, no
formation rate limited elimination was observed with the
dechloroethylated metabolites after development of autoinduction.
Visual inspection of the terminal slopes confirmed this (data not
shown). The observed lack of formation rate-limited elimination
warranted robust and uncorrelated estimates for these metabolite
parameters. In contrast, 4-hydroxyifosfamide still exhibited formation
rate-limited elimination. Unfortunately, no long infusion duration data
were available for this metabolite, preventing robust parameter estimates.
These findings are in accordance with previous findings. Di Marco et
al. (2000)
observed the same "flip-flop" phenomenon with formation
rate-limited dechloroethylated metabolite elimination at day 1, and
elimination rate-limited elimination after the development of
autoinduction on day 5.
Previously, Kaijser et al. (1996)
calculated metabolite half-lives of
47 and 19 h for 2- and 3-dechloroethylifosfamide, respectively. This corresponds with lower K values of 0.015 and 0.036 h
1, respectively. However, these values were
based on the assumption that these metabolites are not metabolized
further. This assumption may not be valid because a
didechloroethylifosfamide metabolite has been detected in human plasma
(Gilard et al., 1993
).
Table 4 indicates that considerable amounts of unchanged ifosfamide are
excreted in the urine. Separate estimation of renal clearance, however,
did not improve the goodness-of-fit of the model. The urinary excretion
data found in this study were similar to previous observations. Gilard
et al. (1993)
reported urinary excretion of 18, 16, and 7% for
ifosfamide, 2- and 3-dechloroethylifosfamide, respectively, in patients
receiving 3 g/m2/3 h ifosfamide for 3 days. The
decrease in excretion of ifosfamide with increasing infusion duration
can be explained by the development of the autoinduction. The decrease
in excretion of ifosfamide with increasing infusion durations can be
explained by decreasing AUCs due to autoinduction. For 2- and
3-dechloroethylifosfamide increased AUCs would theoretically produce
increased urine recovery. This was however not observed. The mechanism
of this discrepancy remains to be elucidated. Nevertheless, the latter
observation is in accordance with the previously reported finding that
the urinary excretions of 2- and 3-dechloroethylifosfamide were
superimposable for the different infusion schedules in pediatric
patients (Boos et al. 1995
).
The AUC of ifosfamide increased less than proportional with the
absolute dose and the AUCs of the metabolites increased more than
proportional with the absolute ifosfamide dose. This can be explained
by a greater extent of autoinduction due to higher ifosfamide plasma
concentrations (eqs. 2-4). These findings are in accordance with a
previous report in which the pharmacokinetic model for ifosfamide was
developed (Kerbusch et al., 2000b
). The dose-corrected exposures to
ifosfamide and its metabolites changed with increasing infusion
duration. In comparison with short infusions, a decrease of 25% in
dose-corrected ifosfamide exposure and an increase of 45% in
3-dechloroethylifosfamide exposure were observed with long infusion
durations. The decrease in ifosfamide exposure can be explained by an
increased average clearance during the long infusions (Fig. 4).
As a result of the increased average clearance of ifosfamide during
long infusions dose-corrected exposure to the dechloroethylated metabolites was expected to increase. Both isoenzymes CYP2B6 and 3A4
have been indicated in the dechloroethylation of ifosfamide (Roy et
al., 1999
). Equal changes for both deactivated metabolites were
expected. However, the increased average clearance observed with
3-dechloroethylifosfamide was not observed with
2-dechloroethylifosfamide. This contrasting finding may be explained by
the observed large interindividual variability that could obscure the
expected effect of long infusion durations on the dose-corrected
exposure of 2-dechloroethylifosfamide (Fig. 6). Unfortunately, the
effect of infusion duration on the dose-corrected exposure to
4-hydroxyifosfamide and ifosforamide mustard could not be evaluated
since no samples were available for the long infusions. Nevertheless,
the observed increased exposure to the dechloroethylated metabolites
during long infusion durations demonstrates the importance of the
assessment of the pharmacokinetics when comparing the pharmacodynamics
during different infusion durations.
In conclusion, a population pharmacokinetic model for ifosfamide and
its metabolites was used to investigate any differences in ifosfamide
metabolism due to the duration of the ifosfamide infusion. Estimates of
Kenz,out, F*2dce, and
F*3dce were found to depend on the infusion
schedule. The clinical relevancy of the observed differences was,
however, considered to be of minor importance. Concentration-dependent
development of autoinduction produced a less than proportional increase
in ifosfamide exposure and more than proportional increase in
metabolite exposure with increasing ifosfamide dose. Schedule
dependence could be demonstrated for the dose-corrected exposures to
ifosfamide and 3-dechloroethylifosfamide between short and long
infusion durations, which could be explained by the increased average
clearance during infusion. No change in dose-corrected exposure to
ifosfamide and metabolites was observed between short and medium
infusion durations in this population. The observed dependence on dose
and infusion duration should be taken into account when the
pharmacodynamics of different infusion schedules of ifosfamide is evaluated.
Received November 15, 2000; accepted March 14, 2001.
Thomas Kerbusch, Ph.D.,
Division of Pharmacokinetics and Drug Therapy, Department of
Pharmaceutical Biosciences, Faculty of Pharmacy, University of Uppsala,
BMC Box 591, S-751 24, Uppsala, Sweden. E-mail:
thomas.kerbusch{at}biof.uu.se
Abbreviations used are:
CYP, cytochrome P450;
LLQ, lower limit of quantification;
HPLC, high-performance liquid
chromatography;
CL, clearance;
AUC, area under the plasma
concentration-time curve.