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Vol. 27, Issue 4, 533-541, April 1999
Département de Pharmacologie, Université de Montréal, Montréal, Québec, Canada (C.P.G., M.S.); Department of Pharmacology, Georgetown University Medical Center, Washington, DC (C.P.G., I.W.W.); and Department of Pharmacology and Toxicology, Center for Environmental and Occupational Health, University of Kansas Medical Center, Kansas City, Kansas (A.M., A.P.)
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Abstract |
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 Top
 Abstract
 Introduction
Materials and Methods
Results
Discussion
References
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The central nervous system toxicity of ifosfamide (IFF), a chiral
antineoplastic agent, is thought to be dependent on its N-dechloroethylation by hepatic cytochrome P-450 (CYP)
enzymes. The purpose of this study was to identify the human CYPs
responsible for IFF-N-dechloroethylation and their
corresponding regio- and enantioselectivities. IFF exists in two
enantiomeric forms, (R) - and (S)-IFF,
which can be dechloroethylated at either the N2 or N3 positions,
producing the corresponding
(R,S)-2-dechloroethyl-IFF [(R,S)-2-DCE-IFF] and
(R,S)-3-dechloroethyl-IFF
[(R,S)-3-DCE-IFF]. The results of the
present study suggest that the production of (R)-2-DCE-IFF and (S)-3-DCE-IFF from
(R)-IFF is catalyzed by different CYPs as is the
production of (S)-2-DCE-IFF and
(R)-3-DCE-IFF from (S)-IFF. In vitro
studies with a bank of human liver microsomes revealed that the
sample-to-sample variation in the production of
(S)-3-DCE-IFF from (R)-IFF and
(S)-2-DCE-IFF from (S)-IFF was highly
correlated with the levels of (S)-mephenytoin
N-demethylation (CYP2B6), whereas
(R)-2-DCE-IFF production from (R)-IFF and
(R)-3-DCE-IFF production from (S)-IFF
were both correlated with the activity of testosterone
6
-hydroxylation (CYP3A4/5). Experiments with cDNA-expressed P-450
and antibody and chemical inhibition studies supported the conclusion
that the formation of (S)-3-DCE-IFF and (S)-2-DCE-IFF is catalyzed primarily by CYP2B6, whereas
(R)-2-DCE-IFF and (R)-3-DCE-IFF are
primarily the result of CYP3A4/5 activity.
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Introduction |
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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
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Ifosfamide
(IFF;1 Fig. 1) is
an extensively utitlized anticancer agent whose clinical use has been
questioned recently on the basis of adverse side effects, such as renal
toxicity and neurotoxicity (Kamen et al., 1995
). Neurotoxicity is
perhaps the key barrier to the safe and effective use of IFF and has
been observed with high-dose (Lewis and Meanwell, 1990), multiple-dose
(Lewis et al., 1990), and oral (Lind et al., 1990) treatment
regimes. IFF is a prodrug whose pharmacological activity results from
its biotransformation to active metabolites. The antitumor effect of
IFF is dependent on hydroxylation at the C4 position on the
oxazaphosphorine ring followed by rearrangement to isophosphoramide
mustard, a DNA cross-linking agent (Brade et al., 1985). The enzymes
responsible for IFF activation have been identified as CYP3A, CYP2B1/2,
and CYP2C6/11 in rats (Weber and Waxman, 1993) and
CYP3A4 in humans (Walker et al., 1994).
In addition to C4-hydroxylation of the oxazaphosphorine ring, IFF is
metabolized by -oxidation of exocyclic chloroethyl alkyl moieties.
This transformation takes place at either the N2 or N3 position on the
oxazaphosphorine ring, producing N-2-dechloroethyl-IFF (2-DCE-IFF) or N-3-dechloroethyl-IFF (3-DCE-IFF) and
chloroacetaldehyde as shown in Fig. 1
(Brade et al., 1985). The chloroacetaldehyde produced by this pathway
has been proposed as the underlying cause of IFF-induced neurotoxicity
(Goren et al., 1989). However, this was questioned recently, and a link
between neurotoxicity toxicity and a 3-DCE-IFF metabolite has been
suggested (Wainer et al., 1994a).
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Although neurotoxicity is treatment-limiting, few reports have
addressed the source of the DCE-IFF metabolites. One study demonstrated
that hepatic microsomes from rats were capable of catalyzing IFF
N-dechloroethylation and that this activity was induced by
phenobarbital pretreatment and inhibited by SKF 525A and tamoxifen
(Ruzicka and Ruenitz, 1992). A second in vitro study with human liver
microsomes identified CYP3A4 as the key enzyme involved in the
N-dechloroethylation of IFF (Walker et al., 1994).
However, both of these studies, as well as those concerning the 4-hydroxylation pathway, have ignored a key aspect of IFF, namely, that this agent is a chiral molecule. IFF contains an asymmetrically substituted phosphorous atom and exists in two enantiomeric forms, (R)-IFF and (S)-IFF. In clinical practice, IFF is administered as a racemic, i.e., a 50:50 mixture of the two enantiomers.
The 2- and 3-DCE-IFF metabolites are also chiral and exist as R- and S-enantiomers (Fig. 1). The N2-dechloroethylation of (R)- and (S)-IFF produces (R)-2-DCE-IFF and (S)-2-DCE-IFF, respectively. However, the N3-dechloroethylation of (R)- and (S)-IFF yields (S)-3-DCE-IFF and (R)-3-DCE-IFF, respectively. This apparent inversion in stereochemical configuration is an artifact produced by the Cahn-Ingold-Prelog chemical nomenclature system.
In both humans and rats, the pharmacokinetics and metabolism of IFF are
enantioselective where (S)-IFF is more extensively cleared
by the N-dechloroethylation pathway than (R)-IFF
(Misiura et al., 1983; Boos et al., 1990; Crom et al., 1991; Prasad et al., 1994; Corlett et al., 1995; Granvil et al., 1996; Wainer et al.,
1996). This difference may be clinically relevant because there appears
to be a relationship between (S)-IFF N3-dechloroethylation [as determined the urinary excretion of (R)-3-DCE-IFF] and
the occurrence of IFF-induced neurotoxicity (Wainer et al.,
1994a).
A recent study also has identified two distinct urinary excretion
patterns (Wainer et al., 1996). In 11 patients, the cumulative excretion of (R)-3-DCE-IFF [from (S)-IFF] was
significantly correlated with (R)-2-DCE-IFF [from
(R)-IFF] and the cumulative excretion of
(S)-3-DCE-IFF [from (R)-IFF] was significantly
correlated with that of (S)-2-DCE-IFF [from
(S)-IFF]. These results suggest that formation of
(R)-2-DCE-IFF and (R)-3-DCE-IFF is catalyzed by
the same cytochrome P-450 (CYP) isoform and that formation of
(S)-2-DCE-IFF and (S)-3-DCE-IFF is catalyzed by a
second CYP isoform. A similar relationship has been observed in rats
(Granvil et al., 1994; Lu et al., 1998). In these studies, pretreatment
of the rats with phenobarbital selectively induced the formation of
(R)-2-DCE-IFF and (R)-3-DCE-IFF.
The identification of the CYP isoforms responsible for IFF N-dechloroethylation would assist greatly in the effective clinical management of the drug, especially because this pathway is associated with treatment-limiting neurotoxicity. Thus, this information will aid in the prediction of metabolic drug interactions and their effects on IFF efficacy and toxicity. In addition, the establishment of the regio- and enantioselectivities of the enzymes could help determine whether single-enantiomer IFF would be a better clinical agent than the racemate.
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Materials and Methods |
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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
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Chemicals.
(R,S)-IFF was purchased from Bristol-Myers Canada
(Belleville, Ontario), and individual IFF enantiomers were prepared by
enantioselective HPLC using a previously described method (Masurel and
Wainer, 1989). The purity of each enantiomer was greater than 99.5%.
Microsomes prepared from a cell line expressing a recombinant human
P-450 enzyme were obtained from Gentest Corporation (Woburn, MA). These microsomes were prepared from the human lymphoblastoid cell line, AHH-1
(originally derived from RPMI 1788 cell line), that was transfected
with cDNA encoding a human CYP enzyme. Sulfaphenazole was obtained from
Ciba-Giegy Ltd. (Basel, Switzerland). All other chemicals were obtained
from Sigma Chemical Co. (St. Louis, MO). Human liver microsomes used in
this study have been described elsewhere (Pearce et al., 1996).
Antibodies against CYP2B and CYP3A.
The antibodies against rat CYP2B1 and CYP3A1 enzymes were raised in
male New Zealand White rabbits as described by Thomas et al. (1979,
1981). The purification and immunoabsorption of these antibodies were
carried out by previously described methods (Dutton and Parkinson,
1989; Halvorson et al., 1990).
Metabolism of IFF Enantiomers. (R)- and (S)-IFF metabolism was carried out in 0.5-ml incubation mixtures containing potassium phosphate buffer (100 mM, pH 7.4), EDTA (1 mM), MgCl2 (3 mM), microsomes (0.5 mg/ml protein), NADP+ (1 mM), glucose 6-phosphate (5 mM), glucose 6-phosphate dehydrogenase (1 U/ml), and (R)- and (S)-IFF (200 µM) with or without other additions as described in Results. The reactions were initiated by the addition of an NADPH-generating system. Reactions were carried out in a shaking water bath and were stopped after 0 to 60 min by sequential addition of 0.4 ml of 5.5% ZnSO4, 0.4 ml of saturated Ba(OH)2, and 0.2 ml of 0.01 M HCl.
IFF enantiomers and the DCE-IFF metabolites were quantified by an enantioselective gas chromatographic method with mass spectrometric detection (GC/MS), as described previously (Granvil et al., 1993). Briefly, the supernatant from the microsomal incubations was mixed with
10 µl of 100 µg/ml trofosfamide in methanol (internal standard) and
3 ml of chloroform. The resulting mixture was mixed on a vortexer for 1 min and then centrifuged at 1000g for 15 min. The aqueous layer was discarded and an aliquot of the organic phase was evaporated to dryness. The residue was reconstituted in 100 µl of toluene, and 1 µl was analyzed.
The GC was a Varian 3400 GC equipped with a Finnigan A 200S GC
autosampler operating in the splitless mode containing a capillary column (8 m × 0.25 mm I.D., 0.25-mm film thickness) coated with a
chiral stationary phase based on heptakis
[(2,6-di-o-methyl-3-o-pentyl)--cyclodextrin]. The conditions for the GC were: injection port temperature, 210°C; the GC column temperature was programmed from 110°C at 4°C/min to
180°C; helium pressure, 55.2 kPa. The total analysis time was 18 min.
The MS was a Finnigan Mat Model Incos 50 operating in the
electron-impact and selective ion-monitoring (SIM) mode. Temperatures were set as follows: ion source temperature, 180°C; transfer line temperature, 250°C; electron energy, 70 eV; and emission current, 300 mA. The detection and quantification of the analytes were accomplished
by selected-ion monitoring at: m/z 211 (IFF), m/z 149 (2- and 3-DCE-IFF), m/z 273 (trofosfamide). The method was linear
for IFF concentrations ranging from 0.48 to 268 nmol/ml of each IFF
enantiomer and 0.15 to 101 nmol/ml for each enantiomer of 2-DCE-IFF and
3-DCE-IFF. The intraday and interday coefficients of variance for
precision and accuracy were less than 8%.
Kinetic Analyses. Kinetic parameters were then determined by nonlinear regression using Grafit V.3.09b Software (Limited, London, UK). Eadie-Hofstee plots (V/S versus V) were constructed to assess whether one or two enzymes were involved in the N-dechloroethylation of (R)- and (S)-IFF.
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Results |
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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
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Kinetic Analyses of N-Dechloroethylation of (R)- and (S)-IFF in Human Liver Microsomes. The rate of formation of (R)-2-DCE-IFF and (S)-3-DCE-IFF from (R)-IFF and (S)-2-DCE-IFF and that of (R)-3-DCE-IFF from (S)-IFF was linear with incubation times of up to 120 min and human liver microsome protein concentration of up to 1.0 mg/ml (data not shown). Unless otherwise noted, an incubation time of 60 min and a concentration of 0.5 mg/ml microsomal protein were used to ensure initial rate conditions for all experiments.
An attempt was made to estimate the kinetic constants for the dechloroethylation of (R)- and (S)-IFF using a pool of seven human liver microsomes, and the data are presented in Fig. 2. For the following discussion, KM [apparent KM or KM(app)] is defined as the substrate concentration that results in a reaction rate that is half the maximum velocity. The multiphasic nature of the Eadie-Hofstee plots (Fig. 2) suggests that at least two kinetically distinct processes, possibly multiple enzymes, are involved in the dechloroethylation of IFF. The rates of IFF dechloroethylation at substrate concentrations of 10, 16.7, 33.3, and 125 µM were used to estimate the KM for the high-affinity enzyme(s), and the results are presented in Table 1. The data were analyzed with the underlying assumption that the low-affinity enzyme(s) do not contribute to the reaction at these concentrations.
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). All
subsequent experiments were carried out at an IFF concentration equal
to 200 µM, which is also the pharmacologically relevant
concentration. It is possible that, at this concentration, the enzyme
reaction may involve contribution from both low and high
KM enzymes.
Biotransformation of N-Dechloroethylation of (R)- and (S)-IFF in Human Liver Microsomes. All N-dechloroethylated IFF metabolites were detected in the incubates from a bank of human liver microsomes (N = 16). There was a large sample-to-sample variation in the rate of formation of (R)-2-DCE-IFF and (S)-3-DCE-IFF from (R)-IFF (7-25 and 13-309 pmol/min/mg protein, respectively) and (S)-2-DCE-IFF and (R)-3-DCE-IFF from (S)-IFF (8-163 and 47-688 pmol/min/mg protein, respectively).
The sample-to-sample variations in the formation of (R)-2-DCE-IFF from (R)-IFF were correlated significantly with the sample-to-sample variations in the formation of (R)-3-DCE-IFF from (S)-IFF (r2 = 0.53; p < .001). The sample-to-sample variations in the formation of (S)-3-DCE-IFF from (R)-IFF were also highly correlated with the sample-to-sample variations in the formation of (S)-2-DCE-IFF from (S)-IFF (r2 = 0.92; p < .0001), Fig. 3.
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-hydroxylation (Fig. 4),
a reaction catalyzed by CYP3A4/5 that suggests the possible contribution of other enzymes. The formation of
(S)-2-DCE-IFF and (S)-3-DCE-IFF was highly
correlated with (S)-mephenytoin N-demethylation, a reaction catalyzed by CYP2B6 (Heyn et al., 1996) (Fig. 5), suggesting that this pathway is mediated primarily by CYP2B6. It should be noted
that the correlation data shown in Figs. 3B, 4B, and 5 appear to be
influenced by an apparent outlying data point. Therefore, correlation
analyses also were performed by deleting the apparent outlying data
point. The correlation coefficients, albeit lower than those reported,
were highly significant even when the outlying data points were removed
(data not shown). (R)-IFF and (S)-IFF metabolism
was not significantly correlated with marker activities of CYP1A2,
CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP4A9/11 (Table 2).
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).
N-Dechloroethylation of (R)- and (S)-IFF by cDNA-Expressed P-450 Enzymes. (R)- and (S)-IFF (200 µM) were incubated independently with microsomes from cells transfected with cDNAs encoding CYP2B6 or CYP3A4, and the results are shown in Fig. 6. The formation of group 1 metabolites was catalyzed preferentially by CYP3A4, and the formation of group 2 metabolites was catalyzed preferentially by CYP2B6. These data are consistent with those obtained from the correlation analysis of the sample-to-sample variation in IFF metabolism by human liver microsome.
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Inhibition of (R)- and (S)-IFF N-Dechloroethylation. The effect of various chemical inhibitors and inhibitory antibodies on the N-dechloroethylation of (R)- and (S)-IFF was investigated in human liver microsomes. The results are shown in Figs. 7 and 8. Coumarin (50 µM), sulfaphenazole (50 µM), hexobarbital (500 µM), quinidine (10 µM), and methyl pyrazole (10 µM), which are inhibitors of CYP2A6, CYP2C9, CYP2C19, CYP2D6, and CYP2E1, respectively, had little or no effect on any of the pathways of ifosfamide metabolism (results not shown).
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), was found to have a strong inhibitory effect on the formation of
group 1 metabolites at 0.2, 0.5, 2.0, and 10 µM concentrations (Fig.
7, A and B). It also markedly inhibited the formation of group 2 metabolites (mediated by CYP2B6), and high concentrations of
ketoconazole produced a strong inhibition of (S)-2-DCE-IFF
formation. However, high concentrations of ketoconazole produce
nonselective CYP inhibition (Baldwin et al., 1995).
Orphenadrine, which is an inhibitor of both CYP2B6 and CYP3A4 (Reidy et
al., 1989; Chang et al., 1993; Royer et al., 1996) and possibly CYP2C9
(Ren et al., 1997), inhibited the formation of group 2 metabolites (Fig. 7, C and D). At orphenadrine concentrations of 250, 500, and 1000 µM, the generation of (S)-2-DCE-IFF was decreased by 35, 45, and 55% whereas the formation of
(S)-3-DCE-IFF was reduced by 19, 31, and 51%. The addition
of 250, 500, and 1000 µM orphenadrine to the incubation also produced
a 19 to 50% inhibition of (R)-3-DCE-IFF formation (Fig.
7B). These results suggest that CYP3A4/5 and CYP2B6 contribute
significantly to the N-dechloroethylation of IFF by human
liver microsomes.
The metabolism of (R)-IFF and (S)-IFF was studied
using human liver microsomes that had been incubated with
affinity-purified inhibitory antibodies. The results from these studies
are presented in Fig. 8. The antibody concentrations of anti-CYP2B1 IgG
and anti-CYP3A1 IgG and experimental conditions used in these studies were determined in independent experiments examining the inhibition of
the 7-ethoxy 4-trifluoromethylcoumarin O-deethylase activity of cDNA-expressed CYP2B6 and the testosterone 6-hydroxylase activity of cDNA-expressed CYP3A4 (data not shown). The experimental conditions used in this study are described in the legend of Fig. 8.
The antibodies against CYP3A significantly inhibited (75-91%) the
formation of group 1 metabolites whereas the antibodies against CYP2B
predominately inhibited (40-67%) the formation of group 2 metabolites. A weak inhibitory effect of anti-CYP2B1 on (R)-2-DCE-IFF production was also observed. The data from
these experiments are consistent with the hypothesis that CYP3A4 and CYP2B6 are responsible for the formation of group 1 and group 2 metabolites, respectively.
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Discussion |
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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
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IFF-induced neurotoxicity is a serious and poorly understood side
effect that has been linked to N-dechloroethylation (Goren et al., 1989; Wainer et al., 1994a). Therefore, it is clinically important to determine the metabolic pathway(s) involved in the formation of the N-dechloroethylated metabolites of IFF.
Previously reported in vitro studies carried out with rat and human
liver microsomes (Weber and Waxman, 1993) have suggested that CYP3A is
responsible for this metabolism. In contrast, a recent study with liver
microsomes from phenobarbital-treated rats indicated that IFF
N-dechloroethylation is catalyzed primarily by CYP2B1 and
that this pathway was distinct from CYP3A activation of IFF via
4-hydroxylation (Yu et al., 1996). However, these metabolic studies
measured the formation of chloroacetaldehyde. Thus, the data could not
reflect the chirality of IFF nor the fact that IFF
N-dechloroethylation is both enantioselective and
regioselective (Fig. 1). It is difficult to obtain a clear picture of
the N-dechloroethylation pathway without an understanding of
the stereochemical consequences of IFF metabolism.
Using a pharmacologically relevant concentration of (R)-IFF
and (S)-IFF (200 µM), this report presents the first study
to address the stereoselective aspects of CYP-mediated IFF
N-dechloroethylation. The results indicate that in human
liver microsomes, CYP3A4/5 is primarily responsible for the formation
of (R)-2-DCE-IFF from (R)-IFF and of
(R)-3-DCE-IFF from (S)-IFF and that CYP2B6 is
primarily responsible for the formation of (S)-3-DCE-IFF
from (R)-IFF and of (S)-2-DCE-IFF from
(S)-IFF. The results also demonstrate a unique
regioselectivity/enantioselectivity in IFF
N-dechloroethylation, i.e., (R)-IFF is
N-dechloroethylated at the N2 position by CYP2B6 and at the
N3 position by CYP3A4/5, whereas (S)-IFF is metabolized at
the N2 moiety by CYP3A4/5 and at the N3 moiety by CYP2B6. These results
are in agreement with Bullock et al. (1997), which suggested that total
N-dechloroethylation of (R)-IFF and
(S)-IFF is enantioselective and that it is catalyzed quite
differently by CYP2B6 and CYP3A4 in a concentration-dependent manner.
The multiphasic Eadie-Hofstee plots (Fig. 2) suggest the involvement of
both high- and low-affinity enzymes in the biotransformation of
(R)- and (S)-IFF. Although the data obtained in
the present study did not produce an accurate determination of
Km and Vmax for
either the low- or high-affinity enzymes, it could be used to estimate
the Km and Vmax
values for the high-affinity enzymes (Table 1 and Fig. 2). In turn,
these results can be used to estimate the relative importance of the
individual metabolic pathways as (R)-3-DCE-IFF > (S)-2-DCE-IFF · (S)-3-DCE-IFF > (R)-2-DCE-IFF. Although this study cannot definitively
establish this relationship, the suggested order is consistent with
previously published data obtained in cancer patients wherein the
relative cumulative urinary excretion was (R)-3-DCE-IFF > (S)-3-DCE-IFF 
(S)-2-DCE-IFF > (R)-2-DCE-IFF (Wainer et al., 1996).
The in vitro and in vivo data indicate that the CYP3A4/5-catalyzed N-dechloroethylation of (S)-IFF at the N3 position occurs to a greater extent than the CYP3A4/5-catalyzed N-dechloroethylation of (R)-IFF at the N2 position. Thus, there is a high degree of enantioselectively and regioselectively in the CYP3A4/5 catalyzed N-dechloroethylation of (S)-IFF and (R)-IFF. In contrast, the CYP2B6-mediated N-dechloroethylation of (R)- and (S)-IFF is regioselective, N3 versus N2, but proceeds with little or no enantioselectivity.
The clinical relevance of these observations can be extrapolated from
the urinary excretion data. In a recent case study, one patient who
experienced IFF-related neurotoxicity excreted increased amounts of
(R)-3-DCE-IFF and (R)-2-DCE-IFF (Wainer et al.,
1994a, 1996). This suggests that a high level of CYP3A activity was the underlying metabolic cause of the observed neurotoxicity. It
should be noted that two additional patients in the study excreted increased amounts of dechloroethylated metabolites but did not experience neurotoxicity. These metabolites arose from
N3-dechloroethylation of (R)-IFF and N2-dechloroethylation
of (S)-IFF, which may be due to increased levels of CYP2B6.
This suggests that an overall increase in IFF
N-dechloroethylation was not the underlying cause of
neurotoxicity and that chloroacetaldehyde, a product common to all IFF
N-dechloroethylation pathways, was not the neurotoxic agent.
The recent study with liver microsomes from phenobarbitalinduced
rats (Yu et al., 1996) suggested that oxazaphosphorine activation and
deactivation/neurotoxicity are catalyzed by distinct CYPs in a manner
that may allow for improvements in the therapeutic index of IFF by
using CYP isoform-selective modulators. The data from our study
underscore the potential merits of this approach. Our results suggest
that when (R,S)-IFF is used, the concomitant administration
of agents that induce CYP3A activity, e.g., dexamethasone, rifampin,
phenobarbital, phenylbutazone, etc., should be contraindicated.
The pathway leading to the formation of the putative neurotoxic
metabolite, (R)-3-DCE-IFF, is the CYP3A-mediated
N3-dechloroethylation of (S)-IFF. Thus, an alternative
approach would be to administer (R)-IFF rather than
(R,S)-IFF. Previous studies in tumor-bearing rats have
demonstrated that (R)-IFF and (S)-IFF have
equivalent antitumor efficacy and that elimination of
(S)-IFF does not produce an increase in (R)-IFF
N-dechloroethylation (Wainer et al., 1994b). Perhaps
the optimization of IFF therapy lies in the use of (R)-IFF and CYP3A- or CYP2B6-selective modulators.
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Acknowledgments |
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We thank Jason D. Latham, Phil Carrot, Brian Ogilvie (XenoTech, LLC), and Nicole Arial for technical assistance.
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Footnotes |
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Received July 31, 1998; accepted January 4, 1999.
This work was supported in part by grants from the Cancer Research Society, Inc. and ChiroScience Ltd. and by the National Institute of Environmental Health and Sciences (Grant ES03765).
Send reprint requests to: Irving W. Wainer, Ph.D., Georgetown University, Department of Pharmacology, Room C-305, Medical-Dental Building, Washington, DC 20007-2197. E-mail: waineri{at}gunet.georgetown.edu
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Abbreviations |
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Abbreviations used are: IFF, ifosfamide; (R)-IFF and (S)-IFF, (R)- and (S)-ifosfamide; (R,S)-2-DCE-IFF and (R,S)-3-DCE-IFF, (R,S)-2-dechloroethyl-IFF and (R,S)-3-dechloroethyl-IFF; CYP or P-450, cytochrome P-450.
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References |
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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
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Pharmacology, safety and therapeutic potential.
Cancer Treat Rev
12:
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 C.-S. Chen, J. T. Lin, K. A. Goss, Y.-a. He, J. R. Halpert, and D. J. Waxman Activation of the Anticancer Prodrugs Cyclophosphamide and Ifosfamide: Identification of Cytochrome P450 2B Enzymes and Site-Specific Mutants with Improved Enzyme Kinetics Mol. Pharmacol., May 1, 2004; 65(5): 1278 - 1285. [Abstract] [Full Text]
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 T. Richter, T. E. Murdter, G. Heinkele, J. Pleiss, S. Tatzel, M. Schwab, M. Eichelbaum, and U. M. Zanger Potent Mechanism-Based Inhibition of Human CYP2B6 by Clopidogrel and Ticlopidine J. Pharmacol. Exp. Ther., January 1, 2004; 308(1): 189 - 197. [Abstract] [Full Text] [PDF]
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 M. S H Lam and R. J Ignoffo A guide to clinically relevant drug interactions in oncology Journal of Oncology Pharmacy Practice, June 1, 2003; 9(2-3): 45 - 85. [Abstract] [PDF]
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A. Madan, R. A. Graham, K. M. Carroll, D. R. Mudra, L. A. Burton, L. A. Krueger, A. D. Downey, M. Czerwinski, J. Forster, M. D. Ribadeneira, et al. Effects of Prototypical Microsomal Enzyme Inducers on Cytochrome P450 Expression in Cultured Human Hepatocytes Drug Metab. Dispos., April 1, 2003; 31(4): 421 - 431. [Abstract] [Full Text] [PDF]
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H. Chen, W. Chen, L.-S. Gan, and A. E. Mutlib Metabolism of (S)-5,6-Difluoro-4-cyclopropylethynyl-4-trifluoromethyl-3, 4-dihydro-2(1H)-quinazolinone, a Non-Nucleoside Reverse Transcriptase Inhibitor, in Human Liver Microsomes. Metabolic Activation and Enzyme Kinetics Drug Metab. Dispos., January 1, 2003; 31(1): 122 - 132. [Abstract] [Full Text] [PDF]
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S. R. Faucette, R. L. Hawke, E. L. Lecluyse, S. S. Shord, B. Yan, R. M. Laethem, and C. M. Lindley Validation of Bupropion Hydroxylation as a Selective Marker of Human Cytochrome P450 2B6 Catalytic Activity Drug Metab. Dispos., October 1, 2000; 28(10): 1222 - 1230. [Abstract] [Full Text] [PDF]
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K. Kobayashi, S. Abe, M. Nakajima, N. Shimada, M. Tani, K. Chiba, and T. Yamamoto Role of Human CYP2B6 in S-Mephobarbital N-Demethylation Drug Metab. Dispos., December 1, 1999; 27(12): 1429 - 1433. [Abstract] [Full Text]
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P. Roy, O. Tretyakov, J. Wright, and D. J. Waxman Stereoselective Metabolism of Ifosfamide by Human P-450s 3A4 and 2B6. Favorable Metabolic Properties of R-Enantiomer Drug Metab. Dispos., November 1, 1999; 27(11): 1309 - 1318. [Abstract] [Full Text]
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