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Vol. 26, Issue 9, 875-882, September 1998
Department of Drug Metabolism, Pfizer Central Research
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Abstract |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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The substrate structure-activity relationships described for the major human drug-metabolizing cytochrome P450 (P450 or CYP) enzymes suggest that the H1 receptor antagonist terfenadine could interact with CYP2D6 either as a substrate or as an inhibitor, in addition to its known ability to act as a substrate for CYP3A4. Based on this substrate structure-activity relationship, computer modeling studies were undertaken to explore the likely interactions of terfenadine with CYP2D6. An overlay of terfenadine and dextromethorphan, a known substrate of CYP2D6, showed that it was possible to superimpose the site of hydroxylation (t-butyl group) and the nitrogen atom of terfenadine with similar regions in dextromethorphan. These observations were substantiated by the ease of docking of terfenadine into a protein model of CYP2D6. Experimentally, terfenadine inhibited CYP2D6 activity in human liver microsomes with an IC50 of 14-27 µM, depending on the CYP2D6 substrate used. The inhibition of CYP2D6 was further defined by determining the Ki for terfenadine against bufuralol 1'-hydroxylase activity in four human livers. Terfenadine inhibited bufuralol 1'-hydroxylase activity with a Ki of approximately 3.6 µM. The formation of the hydroxylated metabolite (hydroxyterfenadine) in microsomes prepared from human liver and specific P450 cDNA-transfected B lymphoblastoid cells indicated that only CYP2D6 and CYP3A4 were involved in this transformation. As expected, the rate of formation was greatest with CYP3A4 (Vmax = 1257 pmol/min/nmol of P450), with CYP2D6 forming the metabolite at a 6-fold lower rate (Vmax = 206 pmol/min/nmol of P450). The two enzymes had similar KM values (9 and 13 µM, respectively). These data indicate that, as predicted from modeling studies, terfenadine has the structural features necessary for interaction with CYP2D6.
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Introduction |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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Terfenadine,
a nonsedating H1 receptor
antagonist, is used in the treatment of histamine-mediated allergic
conditions such as allergic rhinitis (Woodward and Munro, 1982
). In
humans, terfenadine is well absorbed but undergoes extensive first-pass
metabolism. It is metabolized by two routes, i.e.
N-dealkylation to azacyclonol and hydroxylation of the
t-butyl group to form hydroxyterfenadine (Garteiz et
al., 1982). Hydroxyterfenadine undergoes subsequent oxidation to
the corresponding carboxylic acid, which is thought to be the
biologically active antihistamine (von Moltke et al., 1994).
CYP3A41 has been shown to be the principal P450 enzyme
involved in the metabolism of terfenadine to both azacyclonol and
hydroxyterfenadine (Yun et al., 1993) (fig.
1).
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Terfenadine exhibits adverse clinical drug interactions, which have
been related to inhibition of its CYP3A4-mediated metabolism (Jurima-Romet et al., 1994). These adverse interactions are
manifested as torsade de pointes, brought about by QT
prolongation and ventricular arrhythmias in susceptible individuals
(Kivisto et al., 1994). This toxicity has been related to an
increase in terfenadine plasma levels as a result of administration of
known CYP3A4 inhibitors such as ketoconazole (Honig et al.,
1993).
In terms of chemical structure, terfenadine can be considered to be a
lipophilic arylalkylamine. The SSARs described for the major human
drug-metabolizing P450s suggest that terfenadine should be a CYP3A4
substrate, because it is lipophilic and contains a basic nitrogen atom
(Smith and Jones, 1992). However, these same physicochemical features
also suggest that terfenadine could interact with CYP2D6, either as a
substrate or as an inhibitor (Smith and Jones, 1992). The important
feature that determines whether a compound is a CYP2D6
substrate is the distance between the site of metabolism and the basic
nitrogen atom. Template models developed for CYP2D6, using
substrate/inhibitor overlays, have determined this distance to be 5-7
Å (Koymans et al., 1992; Strobl et al., 1993).
The butyl chain in the center of the terfenadine molecule has
sufficient flexibility to allow terfenadine to exist in conformations that could satisfy the CYP2D6 substrate/inhibitor criteria. The compound therefore is a useful probe, because it is not a member of the
recognized families of CYP2D6 inhibitors, such as cyclic antidepressants.
We are engaged in an ongoing program of research to validate the use of P450 models in the prediction of drug metabolism. The CYP2D6 models are most advanced in their development, and it should be possible in the near future to use them either for drug designing or for screening to select compounds that interact with this isoform. The purpose of this study was therefore to investigate whether, as the SSAR predicts, terfenadine and its metabolites interact with CYP2D6 as either substrates or inhibitors.
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Materials and Methods |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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Chemicals and Reagents. Furafylline, 4'-hydroxydiclofenac, (S)-mephenytoin, 4-hydroxymephenytoin, bufuralol, 1'-hydroxybufuralol, hydroxyterfenadine, and sulfaphenazole were obtained from Salford Ultrafine Chemicals and Research Ltd. (Manchester, UK). Specific P450 cDNA-transfected human B lymphoblastoid cell-derived microsomes were obtained from Gentest Corp. (Woburn MA). All other reagents were obtained from Sigma Chemical Co. (Dorset UK) and were of the highest grade available.
Computer Modeling.
Computer modeling studies were undertaken using Quanta (Molecular
Simulations, Burlington, MA) and Sybyl (Tripos Associates, St. Louis,
MO) software. Protein homology models of CYP2D6 were obtained from the
report of Modi et al. (1996) and were used without modification. Dextromethorphan was used in its X-ray crystal structure conformation, whereas reasonable low-energy conformers of terfenadine were modeled and energy-minimized using Sybyl. Terfenadine was manually
docked into the CYP2D6 active site using Quanta.
Preparation of Microsomes.
Transplant-quality human liver tissue was obtained from the
International Institute for the Advancement of Medicine (Exton, PA).
Hepatic microsomes were prepared from individual human livers by the
process of differential centrifugation. The liver tissue was
homogenized in 50 mM Tris-HCl (pH 7.4) containing 250 mM sucrose and
was then centrifuged at 9000g for 20 min, to remove cell
debris and the nuclear fraction. The supernatant was removed and
further centrifuged at 105,000g for 60 min, to pellet the
microsomal fraction. This pellet was washed with 100 mM Tris-HCl (pH
7.4) and centrifuged at 105,000g for 60 min, to remove any
contaminating hemoglobin. The final pellet was resuspended in 100 mM
potassium phosphate (pH 7.4) and stored at 
80°C until use. For the
chemical inhibition experiments, a separate batch of microsomes was
produced from a combination of equal amounts of four human livers
(HM-3, HM-6, HM-14, and HM-16). P450 contents were determined using the
method of Omura and Sato (1964), and protein concentrations were
determined using the method of Lowry et al. (1951), with
bovine serum albumin as the protein standard.
Inhibition of Metabolism of P450-Selective Substrates by Terfenadine and Its Metabolites. The inhibition of the metabolism of P450-selective substrates by terfenadine and its metabolites was investigated using human liver microsomes prepared from a pool of equal amounts of four human livers. Terfenadine and its metabolites were incubated in a concentration range of 0-1000 µM, in the presence of P450-selective substrates at concentrations equal to their previously determined KM values. The inhibitory effects of terfenadine and its metabolites were assessed by determining IC50 values.
The methods for determining the P450-selective enzyme activities were as described previously. Phenacetin O-deethylase activity (CYP1A2) was determined at a substrate concentration of 10 µM (Jones et al., 1997), diclofenac 4'-hydroxylase activity (CYP2C9) was determined at a substrate concentration of 40 µM (Jones et al., 1997), (S)-mephenytoin 4-hydroxylase activity
(CYP2C19) was determined at a substrate concentration of 130 µM
(Meier et al., 1985), bufuralol 1'-hydroxylase activity
(CYP2D6) was determined at a substrate concentration of 10 µM
(Kronbach et al., 1987), debrisoquine 4'-hydroxylase
activity (CYP2D6) was determined at a substrate concentration of 100 µM (Kronbach et al., 1987), dextromethorphan O-demethylase activity (CYP2D6) was determined at a
substrate concentration of 10 µM (Kronbach et al., 1987),
and testosterone 6
-hydroxylase activity (CYP3A4) was determined at a
substrate concentration of 150 µM (Funae and Imaoka, 1987).
Chlorzoxazone 6-hydroxylase activity (CYP2E1) was determined with 33 µM chlorzoxazone and 0.5 mg/ml microsomal protein for 15 min, in a
total incubation volume of 120 µl. The incubations were terminated by
the addition of 10 µl of perchloric acid (600 mM), followed by 10 µl of the internal standard (zoxazolamine, 10 µg/ml). The
precipitated microsomal protein was pelleted by centrifugation at 3000 rpm for 5 min. The supernatant was removed, and 80 µl was injected
into the HPLC system. The samples were chromatographed on a 15-cm
Spherisorb-5-ODS2 column with a mobile phase of 20 mM sodium
perchlorate (pH 2.5)/acetonitrile (70:30, v/v), at a flow rate of 1 ml/min. Detection was by UV absorbance (Shimadzu SPD-6A) at 295 nm.
Under these conditions, 6-hydroxychlorzoxazone had a retention time of
3 min, zoxazolamine had a retention time of 5 min, and chlorzoxazone
had a retention time of 9 min.
The Ki for terfenadine against CYP2D6 was
determined in hepatic microsomes prepared from four individual human
livers, using bufuralol 1'-hydroxylase as the CYP2D6-selective
reaction. The Ki was estimated from the
IC50 value using the following equation (Segel,
1993): Ki = IC50/[1 + ([S]/KM)], where [S] is the
bufuralol concentration (10 µM) and KM
is the KM for bufuralol in human liver
microsomes (10 µM). Based on this estimated
Ki value, the incubations were carried out
at terfenadine concentrations equal to
1/2Ki,
Ki, 2Ki, and
4Ki and bufuralol concentrations equal to 1/2KM,
KM, 2KM, and
4KM.
Assay for Terfenadine and Hydroxyterfenadine Metabolism. The metabolism of terfenadine and hydroxyterfenadine by human liver microsomes or expressed recombinant P450s was assessed according to the following method. Each incubation (final volume, 120 µl) was composed of microsomal protein, 50 mM Tris-HCl (pH 7.4), 5 mM MgCl2, and 5 µM MnCl2. Reducing equivalents required for P450 metabolism were provided by NADPH, which was regenerated in situ by an isocitric acid/isocitric acid dehydrogenase system. The incubation mixture was preincubated at 37°C, in the presence of substrate, before the addition of NADPH to initiate the reaction.
At the end of each incubation, the reaction was terminated by the addition of 10 µl of 600 mM perchloric acid to precipitate the microsomal protein. The precipitated microsomal protein was pelleted by centrifugation at 3000 rpm for 5 min. The supernatant was removed, and 80 µl was subjected to HPLC analysis.HPLC Analysis.
The formation of hydroxyterfenadine was determined using the method of
Yun et al. (1993). Samples were chromatographed on a
Spherisorb-5-CN column (150 × 4.6 mm i.d.; Hichrom), with elution with 10 mM ammonium acetate (pH 4)/acetonitrile/methanol (55:22.5:22.5, v/v), at a flow rate of 1.3 ml/min. Quantitation was achieved using a
fluorescence detector (Merck-Hitachi F1080) operating with an
excitation wavelength of 230 nm and an emission wavelength of 270 nm.
Under these conditions, azacyclonol had a retention time of 4 min,
hydroxyterfenadine had a retention time of 6 min, and terfenadine had a
retention time of 8 min. Quantitation of hydroxyterfenadine was
achieved by interpolation from standard curves constructed with
microsomes using authentic hydroxyterfenadine (1-1000 ng).
Metabolism by Expressed Recombinant P450s. The incubations with microsomes derived from specific P450 cDNA-transfected human B lymphoblastoid cells were conducted as described above, with terfenadine and hydroxyterfenadine concentrations of 5 µM, incubation times of 30 min, and protein concentrations of 1 mg/ml. The kinetic constants (Vmax and KM) for the formation of hydroxyterfenadine from terfenadine in microsomes derived from B lymphoblastoid cells expressing only CYP2D6 or CYP3A4 were determined under conditions of linearity with respect to time and protein concentration (1 mg/ml microsomal protein, for 60 min), with terfenadine concentrations of 0.5-100 µM.
Metabolism by Human Liver Microsomes. The kinetic constants (Vmax and KM) for hydroxyterfenadine formation from terfenadine in human liver microsomes were determined under conditions of linearity with respect to protein concentration and time (0.5 mg/ml microsomal protein, for 30 min), with terfenadine concentrations of 1-100 µM.
Chemical Inhibition of Terfenadine Metabolism by Specific P450 Inhibitors. In the chemical inhibition experiments, methanolic stock solutions of each inhibitor were added to the incubation mixtures before initiation of the reaction. In all cases, the methanol concentration did not exceed 0.1% (v/v). This resulted in a minimal inhibitory effect (<10%) on the rate of terfenadine hydroxylation. The concentration of terfenadine used in the chemical inhibition experiments was 50 µM. Each of the specific inhibitors was used at two concentrations, which were found to yield 50 and 90% inhibition of the relevant P450 enzyme activity, as determined using an appropriate probe substrate.
Analysis of Results.
All results are presented as the mean ± SD. Determination of
apparent KM,
Vmax, Ki, and
IC50 values were carried out using GraFit version
3 (Leatherbarrow, 1992).
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Results |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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Computer Modeling.
Computer modeling studies were undertaken to explore the likely binding
conformation and interactions of terfenadine with CYP2D6. An overlay
between terfenadine (fig. 2,
magenta) and dextromethorphan (fig. 2,
yellow), a known substrate of CYP2D6 (Koymans et
al., 1992; Guengerich, 1995), showed that it was possible to
overlay the site of oxidation, the phenyl group, and the nitrogen atom with similar regions in dextromethorphan (fig. 2), suggesting that the
compound was a potential CYP2D6 substrate and/or inhibitor.
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), which had been developed by a direct study of the interaction of codeine with the enzyme using paramagnetic relaxation effects on the NMR spectrum of the substrate (fig.
3). Positioning of the t-butyl
group of terfenadine 4.5 Å from the catalytic surface of the heme
group allowed placement of the protonated nitrogen in the region of
Asp-301 (fig. 3), the amino acid responsible for binding (Ellis
et al., 1995). Consequently, the bulky
diphenyl-4-piperidinomethanol group was orientated in a lipophilic
pocket bounded by sections of the F (residues 204-212), G (residues
244-248), and I (residues 296-304) helices and a loop region adjacent
to the B' helix (residues 112-115). Amino acids that appeared to be in
direct contact with the diphenyl-4-piperidinomethanol group in this
model included Ala-300, Leu-248, Phe-247, Leu-208, Gly-113, and
Pro-114.
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Effect of Terfenadine on P450 Enzyme-Selective Activities.
The ability of terfenadine to inhibit the metabolism of P450
enzyme-selective substrate probes was investigated with microsomes prepared from equal amounts of four different human livers. In the
concentration range used in this study, terfenadine did not significantly inhibit the activity of substrate probes for CYP1A2, CYP2C9, CYP2C19, or CYP2E1 (IC50 > 100 µM).
Terfenadine was an inhibitor of testosterone 6-hydroxylase activity
(a substrate probe activity for CYP3A4), with an
IC50 of 62 µM. Terfenadine was previously shown
to be a substrate for CYP3A4 (Yun et al., 1993); hence, the
inhibition of testosterone 6-hydroxylase activity was expected. In
addition, terfenadine was also found to inhibit bufuralol
1'-hydroxylase activity (an assay that represents CYP2D6 activity),
with an IC50 of 15 µM.
), albeit to a lesser extent than CYP2D6. However, the
lack of inhibition of phenacetin O-deethylase activity by terfenadine suggests that the inhibition of bufuralol 1'-hydroxylase is
the result of inhibition of CYP2D6. To further investigate the
inhibition of CYP2D6 by terfenadine, two additional CYP2D6-selective substrates, i.e. debrisoquine and dextromethorphan, were
used. Terfenadine inhibited the CYP2D6-mediated 4'-hydroxylation of debrisoquine and O-demethylation of dextromethorphan with
IC50 values of 27 and 14 µM, respectively,
supporting the contention that terfenadine inhibited CYP2D6 activity.
The inhibition of CYP2D6 activity by terfenadine was further
characterized by the determination of Ki
values against bufuralol 1'-hydroxylase activity in human liver
microsomes prepared from four separate individuals, possessing a range
of CYP2D6 activities. Terfenadine was a competitive inhibitor of
bufuralol 1'-hydroxylase activity in all four livers, with a mean
Ki of 4.6 µM (table
1). A Dixon plot illustrating the results
obtained from the liver designated HM-3 is shown in fig.
4.
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100 and >300 µM,
respectively), whereas hydroxyterfenadine inhibited bufuralol
1'-hydroxylase activity with an IC50 of 18 µM
(fig. 5). The inhibition by
hydroxyterfenadine was comparable to that observed with terfenadine
(IC50 = 15 µM).
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Metabolism by Expressed Recombinant P450s. The oxidation of terfenadine (5 µM) to hydroxyterfenadine was investigated in microsomes derived from B lymphoblastoid cells expressing human CYP1A2, CYP2A6, CYP2C9, CYP2C19, CYP2D6, CYP2E1, or CYP3A4 (fig. 6). Under these conditions, only CYP2D6 and CYP3A4 formed hydroxyterfenadine. In addition to hydroxyterfenadine, the CYP3A4-expressing cell line also formed significant amounts of azacyclonol.
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Enzyme Kinetics in Human Liver Microsomes. Terfenadine is metabolized to hydroxyterfenadine in human liver microsomes. The hydroxylation was linear with time up to 45 min and with protein concentration up to 1 mg/ml. The apparent kinetic constants for this conversion were estimated using terfenadine concentrations up to 100 µM. The conversion followed Michaelis-Menten kinetics in the four human livers investigated. This is illustrated for HM-16 in fig. 8, with the corresponding Eadie-Hofstee plot (fig. 8, inset), which suggests either that a single enzyme is responsible for this transformation or that multiple enzymes with similar KM values mediate the transformation. The results obtained with microsomes from B lymphoblastoid cells expressing CYP2D6 and CYP3A4 suggest that there are two enzymes involved, with similar KM values (KM for CYP2D6, 13 µM; KM for CYP3A4, 9 µM).
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Inhibition of Terfenadine Hydroxylase Activity by P450
Enzyme-Selective Inhibitors/Substrates.
The effect of specific P450 inhibitors on the rate of terfenadine
hydroxylation was investigated using furafylline (CYP1A2), sulfaphenazole (CYP2C9), quinidine (CYP2D6), and ketoconazole (CYP3A4)
as inhibitors. For enzymes for which there are no identified specific
inhibitors, specific substrates were used as competitive inhibitors.
These were coumarin (CYP2A6), (S)-mephenytoin
(CYP2C19), and chlorzoxazone (CYP2E1). Compounds found to inhibit
terfenadine hydroxylation by >10% were quinidine (which inhibited the
activity by 17 and 33% at 2.5 and 25 µM, respectively),
chlorzoxazone (which inhibited the activity by 22% at 500 µM), and
ketoconazole (which inhibited the activity by 52 and 78% at 2.5 and 25 µM, respectively). The remaining compounds had no effect on
terfenadine hydroxylation. Recent studies have shown that chlorzoxazone
is metabolized by CYP3A4 in addition to CYP2E1 (Gorski et
al., 1997). Therefore, the inhibition of terfenadine hydroxylase
activity observed at the highest chlorzoxazone concentration might be a
result of chlorzoxazone inhibition of CYP3A4.
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). However, because quinidine has also been shown to be a substrate for CYP3A4 (Guengerich et al., 1986), this
inhibitory effect on terfenadine hydroxylation might have arisen as a
result of the two compounds competing for CYP3A4. At low quinidine
concentrations (1 µM), terfenadine hydroxylase activity was inhibited
to a small degree (10%). This concentration of quinidine had no
effect on CYP3A4, as measured by testosterone 6-hydroxylase
activity, but had a significant inhibitory effect on CYP2D6 activity,
as measured by bufuralol 1'-hydroxylase activity (>80% inhibition)
(data not shown). This suggests that there is a small component of
terfenadine hydroxylase activity in human liver microsomes that is
mediated by CYP2D6.
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Discussion |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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The SSARs for the major human drug-metabolizing P450s predict that
the H1 receptor antagonist terfenadine should
interact with both CYP3A4 and CYP2D6 (Smith and Jones, 1992). A number of reports have detailed the CYP3A4-related metabolism of terfenadine (Yun et al., 1993; Jurima-Romet et al., 1994;
Rodrigues et al., 1995), but the role of CYP2D6 in this
metabolism, if any, has not been determined.
The CYP2D6 SSAR requires a site of metabolism 5-7 Å from a basic
nitrogen (Smith and Jones, 1992). In the case of terfenadine, these
requirements are met by the nitrogen atom of the piperidine moiety and
the t-butyl group, the site of hydroxylation. Indeed, substrate overlays indicate that terfenadine has the basic nitrogen atom and the site of metabolism in a spatial orientation that would
facilitate binding to CYP2D6, by comparison with the prototypical CYP2D6 substrate dextromethorphan. When terfenadine is docked into a
protein model of CYP2D6 with the t-butyl group orientated close to the heme to allow metabolism, the basic nitrogen is able to
interact with Asp-301, the amino acid residue identified as being key
in the ion-pair interaction that is characteristic of this enzyme
(Ellis et al., 1995). Furthermore, this binding orientation identified other potential interactions between terfenadine and CYP2D6.
These findings confirm the SSAR-based observations that terfenadine
should interact with CYP2D6. However, it is not clear whether
terfenadine would interact as a substrate or as an inhibitor. The
inhibition of CYP2D6 by terfenadine is competitive and is characterized
by a relatively low mean Ki of 4.6 µM
against bufuralol 1'-hydroxylase in four individual human livers. The
only other P450 enzyme-selective reaction to be inhibited by
terfenadine was testosterone 6-hydroxylase activity, which is
selective for CYP3A4. This was expected, because terfenadine was
previously shown to be a CYP3A4 substrate (Yun et al., 1993;
Jurima-Romet et al., 1994; Rodrigues et al.,
1995).
All of the P450-derived metabolites of terfenadine retain the basic center (piperidine nitrogen). In terms of interaction with CYP2D6, only hydroxyterfenadine inhibited the enzyme with a potency approximately equal to that of terfenadine; however, there was no detectable metabolism of hydroxyterfenadine to carboxyterfenadine by CYP2D6. This was not the case for CYP3A4, which metabolized hydroxyterfenadine to carboxyterfenadine and azacyclonal.
These studies show that terfenadine is one of a number of
H1 antagonists, including loratidine (Yumibe
et al., 1996) and promethazine (Nakamura et al.,
1996), that interact with CYP2D6. The SSARs of the
H1 receptor and CYP2D6 show a degree of
commonality, in that both proteins bind molecules via an
aspartic acid residue (ter Laak et al., 1995). This residue
is proposed to be Asp-301 in the case of CYP2D6 (Ellis et
al., 1995), whereas it is Asp-116 in the H1
receptor (ter Laak et al., 1995). In terms of other interactions with the proteins, it has been demonstrated that the
diphenyl-4-piperidinomethanol group is critical for interaction with
the H1 receptor (Zhang et al., 1993).
This may explain why carboxyterfenadine is still an
H1 antagonist, because the area in which the
molecule undergoes metabolism appears to be less important, in terms of
binding to the receptor. This is not the case for CYP2D6, where the
change from terfenadine to carboxyterfenadine results in a >20-fold
reduction in apparent affinity for the enzyme.
Terfenadine is metabolized to hydroxyterfenadine and azacyclonol in human liver microsomes. It was shown previously that the hydroxylation is the major route of metabolism. The kinetics of the hydroxylation reaction in four human livers suggest that only one enzyme mediates the process, with a mean apparent KM of 14 µM. The hydroxylation of terfenadine is inhibited by the CYP3A4-selective inhibitor ketoconazole and to a lesser extent by the CYP2D6-selective inhibitor quinidine.
By comparison, terfenadine metabolism by microsomes from B lymphoblastoid cells expressing single P450 enzymes showed the formation of hydroxyterfenadine to be mediated by CYP3A4 and to a lesser extent by CYP2D6, with little or no metabolism being observed with the other enzymes investigated. Additional studies indicated that CYP2D6 and CYP3A4 have similar KM values (13 µM for CYP2D6 and 9 µM for CYP3A4) with respect to the formation of hydroxyterfenadine, but the Vmax value for CYP3A4 is approximately 6-fold greater than that for CYP2D6.
These observations group terfenadine with compounds such as the
antiarrhythmic agent quinidine and the antimalarial drug halofantrine, which are both inhibitors of, but poor substrates for, CYP2D6 (Halliday
et al., 1995). The Ki value for
terfenadine against CYP2D6 is comparable to that of halofantrine, but
terfenadine is approximately 100-fold less potent as an inhibitor of
CYP2D6, compared with quinidine (Halliday et al., 1995).
Clinically, it has been shown that small doses of quinidine can produce
phenocopying. That is, quinidine can alter the apparent phenotype of an
extensive metabolizer for CYP2D6 to that of a poor metabolizer (Brosen
et al., 1987), with obvious implications. This is unlikely
to be the case with terfenadine, because it is rapidly converted to hydroxyterfenadine, which is in turn rapidly converted to
carboxyterfenadine, a much less potent inhibitor of CYP2D6.
In conclusion, SSAR and modeling studies predicted that terfenadine would interact with CYP2D6. Further in vitro studies using human liver microsomes and expressed recombinant P450s demonstrated that terfenadine is a relatively potent inhibitor of CYP2D6 but a poor substrate for this enzyme. Terfenadine is metabolized to hydroxyterfenadine, carboxyterfenadine, and azacyclonol by human liver microsomes. Of these metabolites, only hydroxyterfenadine produced significant inhibition of CYP2D6 (approximately equal to that by terfenadine). However, CYP2D6 did not further metabolize hydroxyterfenadine to either carboxyterfenadine or azacyclonol.
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Footnotes |
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Received January 30, 1998; accepted May 8, 1998.
Send reprint requests to: Barry C. Jones, Ph.D., Department of Drug Metabolism, Pfizer Central Research, Sandwich, Kent, CT13 9NJ, UK.
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Abbreviations |
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Abbreviations used are: CYP or P450, cytochrome P450; SSAR, substrate structure-activity relationship.
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References |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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