Download Pharmacogenomics of Tamoxifen Therapy Review

Review
Clinical Chemistry 55:10
1770–1782 (2009)
Pharmacogenomics of Tamoxifen Therapy
Hiltrud Brauch,1,2* Thomas E. Mürdter,1,2 Michel Eichelbaum,1,2 and Matthias Schwab1,2,3
BACKGROUND: Tamoxifen is a standard endocrine therapy for the prevention and treatment of steroid hormone receptor–positive breast cancer.
CONTENT:
Tamoxifen requires enzymatic activation by
cytochrome P450 (CYP) enzymes for the formation of
active metabolites 4-hydroxytamoxifen and endoxifen.
As compared with the parent drug, both metabolites
have an approximately 100-fold greater affinity for the
estrogen receptor and the ability to inhibit cell proliferation. The polymorphic CYP2D6 is the key enzyme in
this biotransformation, and recent mechanistic, pharmacologic, and clinical evidence suggests that genetic variants and drug interaction by CYP2D6 inhibitors influence
the plasma concentrations of active tamoxifen metabolites and the outcomes of tamoxifen-treated patients. In
particular, nonfunctional (poor metabolizer) and severely impaired (intermediate metabolizer) CYP2D6 alleles are associated with higher recurrence rates.
SUMMARY: Accordingly, CYP2D6 (cytochrome P450,
family 2, subfamily D, polypeptide 6) genotyping before treatment to predict metabolizer status may open
new avenues for individualizing endocrine treatment,
with the maximum benefit being expected for extensive metabolizers. Moreover, strong CYP2D6 inhibitors such as the selective serotonin reuptake inhibitors
paroxetine and fluoxetine, which are used to treat hot
flashes, should be avoided because they severely impair
formation of the active metabolites.
© 2009 American Association for Clinical Chemistry
The pharmacogenomics of drug-metabolizing enzymes involved in the biotransformation of tamoxifen
has become a major area of interest, owing to its potential to predict a breast cancer patient’s treatment outcome before the initiation of treatment. If the tamoxifen pharmacogenomic paradigm were to be borne out
in proof of principle, patients eligible for endocrine
1
Dr. Margarete Fischer-Bosch Institute of Clinical Pharmacology, Stuttgart, Germany; 2 University Tübingen, Tübingen, Germany; 3 Department of Clinical
Pharmacology, University Hospital Tübingen, Tübingen, Germany.
* Address correspondence to this author at: Dr. Margarete Fischer-Bosch Institute
of Clinical Pharmacology, Auerbachstrasse 112, 70376 Stuttgart, Germany. Fax
⫹49-(0)711-859295; e-mail [email protected].
Received November 28, 2008; accepted June 8, 2009.
Previously published online at DOI: 10.1373/clinchem.2008.121756
1770
treatment would be able to exploit it by opting for their
personally most powerful medication. Most breast
cancers, particularly those of postmenopausal women,
are hormone receptor positive; therefore, hundreds of
thousands of women worldwide initiate endocrine
treatment each year. On the basis of results of the Early
Breast Cancer Trialist Collaborative Group, the standard recommendation has been 5 years of therapy with
the selective estrogen receptor (ER)4 modulator tamoxifen (1 ). Tamoxifen is currently prescribed in
⬎120 countries worldwide as a component of standard
adjuvant therapy in early breast cancer and in the metastatic setting for patients with steroid hormone
receptor–positive breast tumors. In primary breast
cancer, adjuvant tamoxifen significantly decreases relapse rates and mortality in pre- and postmenopausal
patients, and the therapy benefit from 5 years of adjuvant tamoxifen is maintained, even ⬎10 years after primary diagnosis (1 ). In postmenopausal women with
endocrine-responsive disease, tamoxifen is a valid
therapy option, along with aromatase inhibitors (AIs)
(2 ), and is considered the standard care for the prevention of invasive breast cancer in premenopausal
women at high risk, including those who have had ductal carcinoma in situ (3 ), and for the treatment of male
breast cancer (4 ). Tamoxifen is generally well tolerated, and menopausal symptoms, including hot
flashes, are the most common side effects. Severe side
effects, such as thromboembolic events or endometrial
carcinoma, are rare (1 ). The clinical benefit of tamoxifen has been evident for more than 3 decades; however, up to 50% of patients who receive adjuvant
tamoxifen relapse or die from tumor-specific resistance or host genome–associated factors.
The field of tamoxifen pharmacogenomics gained
substantial impetus from the elucidation of tamoxifen
metabolism and metabolite pharmacology through
studies that identified major active metabolites formed
by cytochrome P450 (CYP) enzymes, particularly
CYP2D6, which exhibit substantial genetic and phenotypic polymorphism. Several clinical studies have reported on the relationship of genotype and/or phenotype variants with the clinical outcome of tamoxifen
4
Nonstandard abbreviations: ER, estrogen receptor; CYP, cytochrome P450; UGT,
UDP-glucuronosyltransferase; SULT, sulfotransferase; EM, extensive metabolizer; PM, poor metabolizer; IM, intermediate metabolizer; UM, ultrarapid
metabolizer; SSRI, serotonin reuptake inhibitor; AI, aromatase inhibitor.
Tamoxifen Pharmacogenomics
therapy, and international efforts are currently under
way to clarify this relationship.
In light of the potential for a future translation of
tamoxifen pharmacogenomics into clinical practice,
this review seeks to impart the underlying pharmacologic, genetic, and phenotypic principles for a mechanistic explanation of tamoxifen efficacy. It highlights
the biotransformation of tamoxifen into primary and
secondary metabolites with an emphasis on the clinically active metabolites 4-hydroxytamoxifen and
4-hydroxy-N-desmethyltamoxifen (endoxifen). Owing to the key role of CYP2D6, this review focuses on
the relationships between the CYP2D65 (cytochrome
P450, family 2, subfamily D, polypeptide 6) genotype
and phenotype. This discussion also includes the phenocopying effect of CYP2D6 inhibitors, which are frequently coadministered to alleviate hot flashes in postmenopausal women treated with tamoxifen. These
basic research findings provide the scientific background for a thorough discussion of the currently
available literature on tamoxifen pharmacogenetic
studies. Finally, there is a possibility that other drugmetabolizing enzymes and even nonmetabolic factors
can influence tamoxifen efficacy. In considering these
topics, this review provides an overview of the principles of the emerging practice of personalized medicine
for the improvement of the outcomes of endocrine
drug treatment in breast cancer.
Tamoxifen Metabolism and Active Metabolites
trans-Tamoxifen {(Z)-2-[4-(1,2-diphenylbut-1-enyl)phenoxy]-N,N-dimethyl-ethanamine} undergoes extensive phase I and phase II metabolism in the human liver (Fig. 1). The bioconversion of tamoxifen
involves N-oxidation, N-demethylation, and hydroxylation. Formation of the major metabolite
N-desmethyltamoxifen is primarily catalyzed by
CYP3A4 and 3A5, with minor contributions by
CYP2D6, 1A1, 1A2, 2C19, and 2B6 (5–7 ). The steadystate plasma concentration of N-desmethyltamoxifen
after 20 mg tamoxifen is administered daily for at least
3 months is approximately twice as high as that of the
5
Human genes: CYP2D6, cytochrome P450, family 2, subfamily D, polypeptide 6;
CYP2D7P1, cytochrome P450, family 2, subfamily D, polypeptide 7 pseudogene
1; CYP2D7P2, cytochrome P450, family 2, subfamily D, polypeptide 7 pseudogene 2; CYP2D8P1, cytochrome P450, family 2, subfamily D, polypeptide 8
pseudogene 1; CYP2D8P2, cytochrome P450, family 2, subfamily D, polypeptide
8 pseudogene 2; CYP2C9, cytochrome P450, family 2, subfamily C, polypeptide
9; CYP2C19, cytochrome P450, family 2, subfamily C, polypeptide 19; CYP2B6,
cytochrome P450, family 2, subfamily B, polypeptide 6; CYP3A4, cytochrome
P450, family 2, subfamily A, polypeptide 4; CYP3A5, cytochrome P450, family
2, subfamily A, polypeptide 5; SULT1A1, sulfotransferase family, cytosolic, 1A,
phenol-preferring, member 1; BRCA1, breast cancer 1, early onset; BRCA2,
breast cancer 2, early onset.
Review
parent drug (100 –290 ␮g/L vs 72–160 ␮g/L) (8 –14 ).
This fact is of utmost clinical importance because
N-desmethyltamoxifen is subject to hydroxylation,
predominantly at the para position, to produce the major clinically active metabolite endoxifen. Importantly,
the conversion of N-desmethyltamoxifen into endoxifen is catalyzed almost exclusively by CYP2D6
(15, 16 ). Plasma concentrations of endoxifen have
been observed to range from a mean of 8.1 ␮g/L (n ⫽ 51)
for patients with 2 variant CYP2D6 alleles to 20.7 ␮g/L
(n ⫽ 55) for patients with 2 wild-type alleles (17 ). In addition, N-desmethyltamoxifen can also be desmethylated
by CYP3A4 to form N,N-didesmethyltamoxifen.
Another clinically active metabolite is 4hydroxytamoxifen, which is formed by 4-hydroxylation,
also at the para position of the phenyl ring of the parent
drug. This conversion is catalyzed by a number of CYPs,
including CYP2D6, 3A4, 2C9, 2B6, and 2C19 (7, 18 –
21 ). Compared with endoxifen, the steady-state concentrations of 4-hydroxytamoxifen are lower, ranging
from 1.15 ␮g/L to 6.4 ␮g/L (11, 14, 22 ). With the exception of endoxifen and 4-hydroxytamoxifen, no
other highly active metabolites have been described
thus far.
Further hydroxylation also occurs at the 4⬘ position of the other phenyl ring system, leading to 4⬘hydroxytamoxifen, which is mainly mediated by
CYP2B6 and 2D6 (7 ), and to 4⬘-hydroxy-Ndesmethyltamoxifen. Another hydroxylated metabolite, ␣-hydroxytamoxifen, is produced mainly by
CYP3A4 (5, 6, 23, 24 ).
4-Hydroxylated metabolites undergo in vitro
chemical isomerization into the respective E or cis isomers (25 ), which are weak ER antagonists. In addition,
isomerization of 4-hydroxytamoxifen is catalyzed by
CYP1B1, 2B6, and 2C19 (7 ). Of note, an accumulation
of cis-4-hydroxytamoxifen was observed in tumor tissues of patients whose tumors showed resistance to tamoxifen treatment (26 ); however, because data on the
plasma concentrations of cis isomers are sparse, this
observation may be regarded as preliminary. Additional hydroxylation of 4-hydroxytamoxifen by
CYP3A4 and 2D6 at the phenyl moiety leads to 3,4dihydroxytamoxifen (27 ), a compound that is capable
of binding covalently to protein and to DNA, thereby
contributing to the reported toxic and carcinogenic effects associated with tamoxifen treatment (28, 29 ).
Another route of tamoxifen metabolism is the formation of tamoxifen-N-oxide by flavin-containing
monooxygenases 1 and 3, with a chance for tamoxifenN-oxide to be reduced back to tamoxifen by a number
of different CYPs, including CYP2A6, 1A1, 3A4, and
others (30, 31 ). From an analytical point of view, however, this metabolite cannot be ignored because of the
likelihood of chemical reduction of the N-oxide during
Clinical Chemistry 55:10 (2009) 1771
Review
Fig. 1. Metabolic transformation of tamoxifen in humans.
Major metabolic pathways are highlighted with bold arrows. Enzymes preferentially catalyzing a distinct metabolic step are
indicated in bold. Hb, hemoglobin; FMO1, flavin-containing monooxygenase 1.
1772 Clinical Chemistry 55:10 (2009)
Review
Tamoxifen Pharmacogenomics
Table 1. Tamoxifen and metabolites.
Compounds
Mean plasma
concentrations,
a
nmol/L
Effect on ER/affinity for ER
(estradiol ⴝ 100%)
Tamoxifen
190–420
Weak antagonist/2%b
N-Desmethyltamoxifen
280–800
Weak antagonist/1%b
Involvement of
CYP2D6
Minor
N,N-Didesmethyltamoxifen
90–120
Weak antagonist
No
Endoxifen
14–130
Strong antagonist/equal to 4-hydroxytamoxifen
Almost exclusively
Strong antagonist/188%b
Among others
None
No
3–17c
4-Hydroxytamoxifen
␣-Hydroxytamoxifen
1
3,4-Dihydroxytamoxifen
Weak antagonist/high affinity
Together with CYP3A4
Weak antagonistd
No
No data available
No antagoniste
See 4-hydroxy-tamoxifen
4-Hydroxytamoxifen-N -glucuronide
No data available
No antagoniste
See 4-hydroxy-tamoxifen
Endoxifen-O-glucuronide
No data available
No antagoniste
See endoxifen
␣-Hydroxytamoxifen sulfate
No data available
No data available
No
Tamoxifen-N-oxide
No data available
15–24
4-Hydroxytamoxifen-O-glucuronide
⫹
a
Range of mean plasma concentrations according to different investigators [Dowsett et al. (9 ), Hutson et al. (10 ), Jin et al. (11 ), Lee et al. (12 ), Sheth et al. (14 ),
Lim et al. (17 ), Stearns et al. (22 ), Gjerde et al. (32 ), Langan-Fahey et al. (108 )].
b
According to Wakeling and Slater (109 ).
c
MacCallum et al. (13 ) reported much higher concentrations (67 nmol/L).
d
Might be due to reduction to tamoxifen.
e
According to Lazarus et al. (110 ).
sample preparation, a reason why the quantification of
tamoxifen-N-oxide may be regarded as a problem in
the analysis of tamoxifen metabolites. Thus far, few
data on this issue are available, suggesting that the
N-oxide in a patient’s plasma accounts for ⬍15% of
tamoxifen (32 ).
At the level of phase II tamoxifen metabolism, sulfation and glucuronidation are major mechanisms.
O-glucuronidation of 4-hydroxytamoxifen is mainly
mediated by UDP-glucuronosyltransferases (UGTs)
UGT1A4, 2B15, 2B7, 1A8, and various others to produce
4-hydroxytamoxifen-O-glucuronide (33–35 ). Endoxifen is predominantly glucuronidated by UGT1A10 and
1A8 to the corresponding O-glucuronide. Of note is
that in addition to hydroxylated metabolites that undergo phase II metabolism at the hydroxyl moiety,
tamoxifen itself is conjugated by UGT1A4 to the corresponding N⫹-glucuronide (36, 37 ). In contrast to
endoxifen, which does not form any N⫹-glucuronide,
4-hydroxytamoxifen is glucuronidated by UGT1A4 at
the amino group to produce 4-hydroxytamoxifenN⫹-glucuronide (34, 37 ). The formation of sulfates
of 4-hydroxytamoxifen and endoxifen is catalyzed
by sulfotransferases (SULTs) SULT1E1, 1A1, and 2A1
(32, 38 ). E isomers of both 4-hydroxytamoxifen and
endoxifen are also substrates for these conjugation
reactions but seem to have different affinities for different isoenzymes (33 ). ␣-Hydroxytamoxifen is sulfatized
by SULT2A1 (39 ); the resulting ␣-hydroxytamoxifen
sulfate is suspected to exert carcinogenic effects after
covalently binding to DNA (40, 41 ).
Although the number of tamoxifen metabolites
that have been identified in vitro is large (Fig. 1), in
vivo analytical measurements of plasma samples
from tamoxifen-treated patients have quantified few
metabolites, including N-desmethyltamoxifen, endoxifen, 4-hydroxytamoxifen, N,N-didesmethyltamoxifen,
␣-hydroxytamoxifen, and tamoxifen-N-oxide (Table 1).
Therefore, there may be other, yet-unidentified tamoxifen metabolites present at relevant concentrations in patients’ plasma.
CYP2D6 Biochemistry and Genetics
CYP2D6 is a member of CYP enzyme family 2, which in
humans constitutes one third of all CYPs and is one of the
largest and best studied of isoenzyme families. Human
CYPs are heme-containing monooxygenases, and the human genome contains 57 CYP genes and about the same
number of pseudogenes grouped into 18 families and
44 subfamilies according to sequence similarities (http://
drnelson.utmem.edu/CytochromeP450.html). CYP2D6
is involved in the metabolism of many clinically important drugs, including ␤-blockers, antiarrhythmics,
antihypertensives, antipsychotics, antidepressants,
opioids, and others. A recent analysis of the routes of
Clinical Chemistry 55:10 (2009) 1773
Review
elimination for the “top 200 drugs” in the US (http://
www.rxlist.com; most frequently prescribed 200 drugs,
April 2008) showed that 15% were drugs that are
CYP2D6 substrates, compared with subfamilies
CYP3A (37%) and CYP2C (33%) (42 ).
The human CYP2D6 locus on chromosome 22 includes the CYP2D6 gene and pseudogenes CYP2D7P1
(cytochrome P450, family 2, subfamily D, polypeptide 7 pseudogene 1), CYP2D7P2 (cytochrome P450,
family 2, subfamily D, polypeptide 7 pseudogene 2),
CYP2D8P1 (cytochrome P450, family 2, subfamily D,
polypeptide 8 pseudogene 1), and CYP2D8P2 (cytochrome P450, family 2, subfamily D, polypeptide 8
pseudogene 2) originally described as pseudogenes
CYP2D7 and CYP2D8 (43 ). The CYP2D6 gene consists
of 9 exons and 8 introns, and the sequence is highly
polymorphic. By way of clinical observation (i.e., administration of the antiarrhythmic and oxocytic drug
sparteine (44 ) and the antihypertensive agent debrisoquine (45 )), the first CYP2D6 phenotypic variant
(sparteine/debrisoquine polymorphism) distinct from
an extensive metabolizer (EM) phenotype was identified more than 30 years ago and was termed a “poor
metabolizer” (PM) phenotype. Currently, 4 CYP2D6
phenotypes are commonly observed in Caucasian populations on the basis of their drug-oxidation capacities:
EM, intermediate metabolizer (IM), PM, and ultrarapid metabolizer (UM) (46 – 48 ). Among Caucasians,
about 7%–10% of individuals are PMs, 10%–15% are
IMs, and, at the opposite end of the activity spectrum,
up to 10%–15% are UMs.
The PM status can be deduced with ⬎99% certainty from the presence of 2 nonfunctional alleles,
with ⬎20 null alleles having been identified (43 ).
Therefore, it is possible to exactly predict the CYP2D6
PM phenotype (i.e., lack of catalytic function of the
enzyme) by genotyping the patient’s DNA without the
need to phenotype (42, 46, 48, 49 ). The EM phenotype
is due to the presence of 1 or 2 allelic variants with
wild-type function, such as *1 or *2. This phenotype
can be separated by genotype into homozygous or
heterozygous EMs, depending on whether they carry
1 or 2 functional alleles. Because heterozygous EMs
who carry one *1 or *2 allele in combination with an
IM or PM allele have somewhat impaired enzyme
production and function, they have been classified as
IMs, assuming a gene-dosage effect such that heterozygous EMs would have only 50% of the enzyme
amount and catalytic activity of homozygous EMs.
This assumption is not correct, however, and there is
substantial overlap between homozygous and heterozygous EMs in both enzyme content and activity.
Consequently, the genotype has a rather poor predictive value. Of note is that the IM has a phenotype
and genotype distinct from the heterozygous EM
1774 Clinical Chemistry 55:10 (2009)
(47, 50 –52 ) that involves impaired gene expression
and enzyme function (these variants include *9, *10,
and *41) and/or nonfunctional variants (47, 52 ).
Within the German population, 2%–3% are carriers
of a duplicated/multiplied CYP2D6 gene and therefore have very high enzyme activity (UM). These differences in enzyme activity can have profound consequences on the plasma concentrations of drug
metabolites, as has been observed for the tricyclic
antidepressant nortriptyline. A ⬎30-fold difference
between PMs and UMs in steady-state plasma concentrations of nortriptyline was observed when nortriptyline was prescribed as a standard daily dose of
100 –150 mg (53, 54 ). With respect to UM phenotype, however, only 20%–30% of UM phenotypes
observed in the Caucasian population are identifiable through genotyping (46, 48, 55 ).
Thus far, systematic genetic analyses of large
numbers of individuals have led to the discovery of
⬎100 different alleles [http://www.cypalleles.ki.se,
(56 )]. At least 15 of these alleles encode nonfunctional gene products caused by aberrant splicing,
nonsense codons, mutations of single base pairs,
small insertions/deletions, larger chromosomal deletions of the entire CYP2D6 gene, CYP2D6/CYP2D7
hybrid genes, or mutations that cause lack of heme
incorporation or otherwise produce nonfunctional
full-length proteins.
There are significant ethnic differences with respect to PM, IM, and UM frequencies, heralding the
possibility that different ethnic groups vary with respect to the clinical outcomes of drug therapy with
CYP2D6 substrates. Within this context it is important
to appreciate that the frequency of gene duplication is
much higher in northeastern African populations
[e.g., 29% in Ethiopia (57 )] and in Saudi Arabia
[21% (58 )] compared with populations of European
descent (59, 60 ). In Asian populations, however, the
CYP2D6*10-associated IM is prevalent (61 ), with
the frequency in Han Chinese being 57% and the PM
playing a minor role (59 ).
Overall, an awareness of the CYP2D6 genotype–
phenotype relationship may influence treatment decisions, particularly in cases for which an effective alternative drug is available. As in the case of orally
administered codeine, which in the 10% of Caucasians
who are PMs is not metabolized efficiently to morphine and therefore provides little analgesic effect,
there is a chance that women with a CYP2D6 PM or IM
genotype/phenotype also will not benefit from the antiestrogenic effects of tamoxifen, owing to insufficient
production of active metabolites. With respect to UMs,
who in cases of codeine treatment develop severe opioid side effects due to rapid morphine formation
(62, 63 ), it is important to note that such women pa-
Tamoxifen Pharmacogenomics
tients may be more susceptible to hot flashes during
tamoxifen therapy.
CYP2C9, 2C19, 2B6, 3A4, and 3A5 Genetics
Other important CYP isoenzymes of subfamily 2 that
are involved in the bioactivation of tamoxifen are
CYP2C9, 2C19, and 2B6 (15, 18 ); these enzymes are
also polymorphic. Of the ⬎30 variant alleles of
CYP2C9 (cytochrome P450, family 2, subfamily C,
polypeptide 9), the *2 and *3 alleles have been thoroughly investigated and found to be associated with
significant but highly variable reductions in intrinsic
clearance, depending on the substrate (64 ). The *3 allele is more strongly affected than *2, with a reduction
in enzyme activity of up to 90% for some specific drugs
(65 ). Both alleles are present in approximately 35% of
Caucasians but are less prevalent in black and Asian
populations (42, 66 ). About 2% and 24% of individuals in the Caucasian population are homozygous and
heterozygous for the variants, respectively (67 ). Numerous clinical studies have demonstrated the clinical
significance of CYP2C9 genetics with respect to an association with higher incidences of adverse drug reactions. The most prominent example is warfarin, an anticoagulant, and several retrospective and prospective
studies have confirmed that CYP2C9 genetics is clinically useful for adjusting warfarin dosage to reduce serious warfarin-related bleeding events (68, 69 ). The
anticoagulant response also depends on the genetics of
vitamin K epoxide reductase (68 ). Moreover, gastrointestinal bleeding from nonsteroidal antiinflammatory
drugs (70 ) and such side effects as hypoglycemia
caused by sulfonylureas (71 ) have also been attributed
to CYP2C9 polymorphisms.
For the CYP2C19 gene (cytochrome P450, family
2, subfamily C, polypeptide 19), the known null alleles
(CYP2C19*2, *3, *4, *5, *6, *7, and *8) have no
CYP2C19 enzyme activity (PM); the *2 allele is prevalent in Caucasians. These null alleles are due to a splice
defect (*2), a premature stop codon (*3), or an alteration in CYP2C19 structure and/or stability (72 )
(http://www.cypalleles.ki.se/). Recently, several new
CYP2C19 alleles have been identified (*9–*25) in individuals from different racial groups; however, whether
these mutations produce significant alterations in enzyme activity in vivo is not clear. CYP2C19*2 and *3
are the most frequent variants. According to genotyping and phenotyping results and in analogy to
CYP2D6, the distribution of PMs shows wide interethnic differences. In Caucasian Europeans, the mean
frequency of PM individuals is 3%, whereas PM frequencies as high as 23% have been identified in Asian/
Oceanian populations (72, 73 ). Carriers of heterozygous variants constitute 32% of Caucasians, however
Review
(74 ). A promoter variant of CYP2C19*17 has recently
been identified and shown to be associated with increased CYP2C19 activity in vivo (UM) with the
CYP2C19 substrate omeprazole [a proton pump inhibitor (75 )] and the antidepressant escitalopram
(76 ). Differences in CYP2C19*17 allele frequency have
been reported: 18% in both a Swedish and an Ethiopian
population (75 ), 25% in a German population (77 ),
and 27% in a Polish population (78 ). A lower frequency (4%) has been reported for Chinese individuals
(75 ). Given these genotype/phenotype relationships,
there is a possibility that the CYP2C19 UM may play a
role in tamoxifen metabolism and clinical outcome, as
we have reported for our breast cancer tamoxifen pharmacogenetic study (79 ).
With respect to CYP2B6 (cytochrome P450, family
2, subfamily B, polypeptide 6), the most common variant allele, *6, occurs at frequencies of 15%– 60% across
different populations (80 ). Genotyping of CYP2B6*6
predicted increased plasma concentrations of efavirenz
and nevirapine and efavirenz-related neurotoxicity in
HIV-infected individuals (81, 82 ), and the results suggested reducing the dose by 35% in African patients
who were homozygous for CYP2B6*6 (83 ). These findings are in agreement with the lower activities of
CYP2B6*6 isoenzyme, which may be substrate dependent, however. At present, any contribution of CYP2B6
variants to tamoxifen outcome is unknown.
The most important CYP isoenzyme subfamilies
involved in human drug metabolism are CYP3A4 and
3A5, which participate in the metabolism of 40% of the
drugs that are most frequently prescribed (42 ). There is
little evidence for a relevant contribution of CYP3A4
(cytochrome P450, family 2, subfamily A, polypeptide
4) gene expression and enzyme function, although defective CYP3A4 mutants may account for toxicity in
very rare cases (84 ). In contrast, genetic polymorphisms define much of the variation in CYP3A5 (cytochrome P450, family 2, subfamily A, polypeptide 5)
expression. The higher incidence of the inactive
CYP3A5*3 variant in Caucasians (85%–95%) vs African Americans (30%–50%) causes the lower CYP3A5
protein level seen in Caucasians compared with African
Americans (⬎30% vs 50%). CYP3A5*6 and *7 lack any
functional activity and occur solely in individuals of
African origin. Apart from a clear effect on the immunosuppressant tacrolimus (85 ), the contribution of the
polymorphic CYP3A5 enzyme to CYP3A-mediated
metabolism remains controversial. It is difficult to delineate the relative contributions of CYP3A4 and
CYP3A5 because their protein structures, functions,
and substrates are so similar. In fact, one of these enzymes may functionally compensate for the lack of the
other. Whether CYP3A4 and/or CYP3A5 variants contribute to tamoxifen outcome is unknown.
Clinical Chemistry 55:10 (2009) 1775
Review
Tamoxifen Pharmacogenomics
Effects of Tamoxifen Metabolite Concentrations
The rationale underlying the tamoxifen pharmacogenomic principle is that variant DNA sequences of
drug-metabolizing enzymes that encode proteins
with reduced or absent enzyme function may be associated with lower plasma concentrations of active
tamoxifen metabolites, which could have an impact
on the efficacy of tamoxifen treatment. About 30
years ago, Jordan et al. characterized the first potent
antiestrogen metabolite, 4-hydroxytamoxifen, and
reported a 100-fold greater affinity for the ER than
the parent drug (86 ). This metabolite was later
shown to be 30- to 100-fold more potent than tamoxifen in suppressing estrogen-dependent cell
proliferation (86 – 89 ). Despite its potency as an antiestrogen, the contribution of this metabolite to the
overall clinical effect of tamoxifen has remained unclear, because its plasma concentrations are relatively low compared with those of tamoxifen and
other metabolites (86 ). Our knowledge of the link
between tamoxifen metabolism and treatment response rapidly expanded after the characterization
of endoxifen (16, 22 ), which, although it had been
identified in the late 1980s, initially remained obscure with respect to its biological activity. Finally, a
series of laboratory studies for the characterization
of its pharmacology established that endoxifen has a
potency equivalent to 4-hydroxytamoxifen in terms
of its binding affinity for ERs (16 ), suppression of
estrogen-dependent proliferation of breast cancer
cells (16, 89, 90 ), and modulation of estrogenmediated global gene expression (91 ). A detailed in
vitro analysis showed that endoxifen is formed
mainly by 4-hydroxylation of the primary metabolite N-desmethyltamoxifen, with the CYP2D6 enzyme catalyzing this rate-limiting step (15 ). Owing to
the dominant role of CYP2D6 in the formation of endoxifen, variation in the CYP2D6 genotype and phenotype is at the heart of tamoxifen pharmacogenetics. The
currently available evidence for this notion is based on
findings obtained at 2 levels of clinical investigations,
which addressed (a) the association between the concentrations of active tamoxifen metabolites either with
CYP2D6 genotype or by clinical outcome, and (b) the
association between CYP2D6 genotype and clinical
outcome. The latter approach has shown that patients
with 2 functional CYP2D6 alleles benefited the most
from tamoxifen treatment. Further elucidation of the
relationship between plasma concentrations of endoxifen in vivo and clinical outcomes will require
additional detailed investigations with large patient
cohorts.
Prospective cohort studies of adjuvant tamoxifen treatment have shown wide interindividual variation in the
formation of tamoxifen metabolites and substantial reductions in the steady-state plasma concentrations of
endoxifen during tamoxifen treatment in women carrying CYP2D6 gene variants (8, 11, 22 ). Moreover,
convincing evidence have shown that selective serotonin reuptake inhibitors (SSRIs) such as paroxetine and
fluoxetine, which are known to be strong CYP2D6 inhibitors, reduce plasma endoxifen concentrations. In
particular, the phenocopy of a significant reduction in
endoxifen plasma concentrations induced by SSRIs
was observed in breast cancer patients homozygous for
the wild-type CYP2D6 genotype, whereas the concentrations of other metabolites remained unaffected by
the CYP2D6 genotype/phenotype. Although the relationship between CYP2D6 variants and plasma endoxifen concentrations was first shown for patients with
the PM CYP2D6*4 genotype (11 ), a quantitative approach that included PM, IM, and UM genotypes substantiated this relationship (8 ); however, endoxifen
concentrations overlap across genotypes. It follows
that other factors may modify plasma endoxifen
concentrations.
A relationship between CYP2D6 variants and
higher concentrations of N-desmethyltamoxifen (i.e.,
the endoxifen precursor) has been reported at the level
of chemoprevention. Significantly higher plasma concentrations of N-desmethyltamoxifen were reported
for mutation carriers after 1 year of tamoxifen therapy,
indicating that the conversion to clinically active endoxifen may be impaired (92 ).
A more recent study addressed the relationship between CYP2D6 and SULT1A1 (sulfotransferase family,
cytosolic, 1A, phenol-preferring, member 1) genotypes, including the effect of SULT1A1 copy number
on the pharmacokinetics of tamoxifen during steadystate treatment (32 ). Whereas both CYP2D6 and
SULT1A1 genotypes influenced the pharmacokinetics of tamoxifen metabolites, SULT1A1 copy number did not. Lower metabolic ratios with respect to
the formation of endoxifen and 4-hydroxytamoxifen
but higher metabolic ratios for the formation of
N-desmethyltamoxifen (endoxifen precursor) were
observed in carriers of CYP2D6 variant genotypes, a
result consistent with a gene-dosage effect. In contrast,
patients carrying CYP2D6 alleles with high predicted
enzymatic activity showed higher metabolic ratios for
both active metabolites. Whether such metabolic ratios
are of clinical relevance remains to be determined.
Similarly, a study of a prospective cohort of Korean patients with early or metastatic breast cancer
found an association between the IM CYP2D6*10 ho-
1776 Clinical Chemistry 55:10 (2009)
Tamoxifen Pharmacogenomics
Review
Fig. 2. Kaplan–Meier probabilities of relapse-free time (RFT) of breast cancer patients for CYP2D6-metabolizer
phenotypes predicted from genotypes.
(A), Patients treated with adjuvant tamoxifen (TAM). EMs had a significantly more favorable RFT than patients with impaired
phenotypes (PMs or IMs). (B), Patients without TAM showed no differences with respect to a relationship between the CYP2D6predicted phenotype and RFT [Schroth et al. (79)]. hetEM, heterozygous EM. Originally published in Schroth, W et al.: J Clin Oncol 25
(33), 2007: 5187–93. Reprinted with permission. © 2008 American Society of Clinical Oncology. All rights reserved.
mozygous variant and lower steady-state plasma concentrations of 4-hydroxytamoxifen and endoxifen
(17 ), and a Chinese study found that patients homozygous for CYP2D6*10 had lower serum concentrations
of 4-hydroxytamoxifen (93 ). The high prevalence of
the CYP2D6*10 allele in East Asia, together with the IM
association of impaired formation of an active metabolite,
confirms the CYP2D6 PM findings in Caucasians.
Clinical Outcome of Tamoxifen Therapy and
Prediction
The first evidence linking CYP2D6 variants with treatment response was obtained from a prospective randomized phase III trial of postmenopausal women
with ER-positive breast cancer (North Central Cancer
Treatment Group adjuvant breast cancer trial) for the
investigation of the effect of adding the androgen fluoxymesterone for 1 year to the standard regimen of 5
years of adjuvant tamoxifen. The pharmacogenetic investigation of patients from the tamoxifen-only arm
showed that after a median follow-up of 11.4 years, the
CYP2D6*4 variant allele was an independent predictor
of a higher risk of relapse and a lower incidence of hot
flashes (94 ). A follow-up study found that in addition
to CYP2D6 genetics, the phenocopying due to the
coprescription of CYP2D6 inhibitors (SSRIs) was an
independent predictor of breast cancer outcome in
postmenopausal women taking tamoxifen (95 ). Recently, a robust association between CYP2D6 genotype
and treatment outcome was obtained from a nonrandomized retrospective cohort of ER-positive postmenopausal breast cancer patients undergoing adjuvant tamoxifen therapy (79 ). At a median follow-up of
71 months, carriers of PM and IM genotypes (i.e., carriers of CYP2D6*4, *5, *10, and *41 alleles) had significantly more breast cancer recurrences, shorter relapsefree times, and worse event-free survival than carriers
of functional alleles (Fig. 2). This association was not
observed in postmenopausal ER-positive patients not
treated with tamoxifen. Interestingly, the UM
CYP2C19*17 variant also had a favorable effect on tamoxifen treatment outcome. Patients with the homozygous *17 genotype had significantly fewer breast
cancer recurrences, longer relapse-free times, and better event-free survival than non-*17 carriers. Overall,
this study suggested that genotyping for CYP2D6*4, *5,
*10, and *41 could identify patients who would derive
little benefit from adjuvant tamoxifen therapy. Although the CYP2D6 EM phenotype will identify the
patients likely to benefit from tamoxifen, accounting
for about 50% of all patients, the benefit will be maximal for individuals with the combination of fully functional CYP2D6 alleles and the CYP2C19 UM. The latter will apply to one third of all patients, indicating that
the tamoxifen pharmacogenetics issue will be relevant
Clinical Chemistry 55:10 (2009) 1777
Review
for a substantial fraction of breast cancer patients receiving endocrine treatment.
Clinical studies from Korea, China, and Japan also
have linked poor clinical outcome with CYP2D6 genetics. As expected for populations with a high prevalence
of the IM CYP2D6*10 allele, the *10 homozygote genotype was associated with a poor clinical outcome in a
Korean cohort of metastatic breast cancer patients,
whereas the *10 heterozygote and wild-type homozygote genotypes were not (17 ). Likewise, patients from
China who were homozygous for the CYP2D6*10 allele
(93 ) showed an association with unfavorable diseasefree survival. The latter result was substantiated
through comparison with a control patient group
without tamoxifen treatment, in which no association
between clinical outcome and the CYP2D6*10 variant
was observed. Moreover, patients homozygous for
CYP2D6*10 from a Japanese breast cancer cohort that
underwent adjuvant tamoxifen monotherapy showed
a significantly higher incidence of recurrence within 10
years of follow-up, compared with patients with wildtype CYP2D6 (96 ). Although some of the sample sizes
were low in the Asian studies demonstrating the genotype– efficacy correlation, the findings of the clinical
implications of CYP2D6 genotypes predictive for tamoxifen efficacy are in line with the findings of others.
On the other hand, a study from the US reported
no association between CYP2D6 genetics and tamoxifen outcome (97 ), and contradictory results for this
relationship were reported in a study from Sweden,
which found the CYP2D6*4 variant to be associated
with a better clinical outcome in tamoxifen-treated patients (98 ). An extended study showed favorable
disease-free survival in CYP2D6*4 carriers compared
with patients homozygous or heterozygous for the
functional CYP2D6 allele (99 ).
The issue of the role of CYP2D6 in tamoxifen therapy for breast cancer has also been addressed within the
context of breast cancer prevention. For example, data
from the Italian Tamoxifen Trial suggest that women
with a CYP2D6*4/*4 genotype may be less likely to benefit from tamoxifen as a chemopreventive agent. This
finding supports the notion of CYP2D6 playing an important role in tamoxifen’s metabolic activation and
efficacy (100 ). Moreover, the “a priori” hypothesis that
hot flashes may be an independent predictor of tamoxifen efficacy has been addressed in the Women’s
Healthy Eating and Living randomized trial (101 ). Of
the 864 patients taking tamoxifen, 674 (78%) reported
hot flashes, and 12.9% of these patients had experienced recurrent breast cancer after 7.3 years of followup, whereas 21% of the patients who did not have hot
flashes had recurrent breast cancer during this period.
Because hot flashes were a stronger predictor of a breast
cancer–specific outcome than age, hormone receptor
1778 Clinical Chemistry 55:10 (2009)
status, or tumor stage at diagnosis, the authors suggested an association between side effects, tamoxifen
metabolism, and efficacy. Finally, a small study of familial breast cancer patients who were carriers of either
BRCA1 (breast cancer 1, early onset) or BRCA2 (breast
cancer 2, early onset) mutations and treated with tamoxifen suggested a relationship between CYP2D6 PM
status and a worse survival in familial breast cancer
(102 ); however, because of the small numbers of patients as well as the inclusion of ER-positive and ERnegative patients in this investigation, clarification
provided by further studies will be needed to distinguish a pharmacogenetic effect from a poor prognostic
effect in carriers of these BRCA mutations.
Given the current treatment practice of long-term
estrogen deprivation in ER-positive postmenopausal
breast cancer patients with the use of AIs as a valid
option, the question of the impact of pharmacogenetic
variation on the optimal choice for adjuvant endocrine
therapy has been addressed in a modeling analysis
(103 ). A Markov model was created to examine
whether the optimal treatment strategy for patients
with the wild-type CYP2D6 gene differs from that for
carriers of the CYP2D6*4 mutation. The study used
patients from the BIG1–98 trial, information from this
trial on relapse risk, and the corresponding genotype
data of Goetz et al. (94 ). Under the assumption that AI
metabolism is independent from CYP2D6, the model
suggests that the 5-year benefit of adjuvant tamoxifen
therapy may exceed even that of up-front AI treatment
in postmenopausal CYP2D6 EM patients.
Conclusion: Clinical Relevance of CYP2D6 in Breast
Cancer
Strong mechanistic, pharmacologic, and clinical evidence, as well as modeling data, now indicate that tamoxifen efficacy and clinical outcome depend on
CYP2D6 metabolism controlled by CYP2D6 enzyme
polymorphisms and on pharmacologic interactions.
Data from international studies have consistently demonstrated that plasma concentrations of active tamoxifen metabolites are linked with genetically determined
CYP2D6 metabolizer status, phenocopying by strong
CYP2D6 inhibitors, and clinical outcome. The few
conflicting data may be explained by variation in the
studies with respect to patient-inclusion criteria, tamoxifen doses, length of treatment, additional chemotherapy regimens, or a lack of consistent ER testing.
Importantly, most authors agree that CYP2D6 gene
variants, as well as inhibition of CYP2D6 by prescribed
comedications such as SSRIs, may decrease tamoxifen
metabolism and thus negatively affect tamoxifen efficacy and treatment outcome.
Tamoxifen Pharmacogenomics
There are a number of potential clinical consequences from these emerging data on CYP2D6 and the
outcomes of tamoxifen treatment. First, potent SSRIs
such as paroxetine or fluoxetine should not be used to
relieve hot flashes in breast cancer patients receiving tamoxifen. Although SSRIs are one of the few evidencebased therapy options for menopausal vasomotor symptoms (104 ), convincing data now indicate that these
drugs may compromise tamoxifen efficacy via a phenocopying effect due to interference with CYP2D6dependent tamoxifen metabolism. Yet, differences in the
plasma concentrations of tamoxifen metabolites have
been observed, depending on the strength of the CYP2D6
inhibitor (11, 105 ). If treatment of hot flashes is indicated,
an SSRI such as citalopram or escitalopram or a selective
norepinephrine reuptake inhibitor such as venlafaxine
should be used, because these compounds have shown no
appreciable inhibition of CYP2D6.
Second, the relationship between CYP2D6 genotype, phenotype, and treatment outcome points to a
possible benefit of up-front CYP2D6 genotyping prior
to a decision on an adjuvant endocrine treatment. A
comprehensive robust, standardized, and qualitycontrolled CYP2D6-genotyping assay will have to test
for genetic variants that could affect tamoxifen metabolism. According to the data of Goetz et al. (94 ) and
Schroth et al. (79 ), such assays should include testing
for common PM alleles (CYP2D6*3, *4, and *5) and for
IM alleles, depending on the individual’s ethnic origin.
Of note, *41 is the most frequent IM allele in Europeans, *17 is the principal IM allele in Africans, and *10
dominates in Asians (*9 should also be considered)
(59 ). Other areas of interest with respect to clinical
application are the measurement of plasma endoxifen
concentrations as a surrogate of CYP2D6 phenotype.
Given alternative treatment options (i.e., tamoxifen vs AI), and considering the available scientific and
clinical evidence, an individualized approach for endocrine treatment of postmenopausal breast cancer
patients is desirable. One may speculate that tamoxifen alone is adequate for CYP2D6 EMs and EM carriers, whereas postmenopausal patients with variant
CYP2D6 alleles may fare better with up-front AI therapy. Although this approach may be regarded as
straightforward for PM patients, the best treatment
may be less clear for IM patients. IM is a common phenotype among many ethnic groups, including Caucasians, African Americans, and Asians, so data on linking IM genotypes with therapeutic threshold and
efficacy are in demand to adequately address the clinically important question of tamoxifen dose adjustment. Similarly, any impact of UM phenotypes on
metabolite concentrations, treatment efficacy, and toxicity that have potential implications for dosing requires further investigations. Formal recommenda-
Review
tions on the integration of CYP2D6 genotypes into
treatment decisions still must await validation of these
genotypes in larger retrospective studies, as is being
attempted by the International Tamoxifen Pharmacogenetics Consortium (http://www.pharmgkb.org/do/
serve?objId⫽63&objCls⫽Project), or prospective
clinical trials. Thus far, no study has addressed the
question of whether genetically predisposed differences in 4-hydroxytamoxifen and endoxifen concentrations are associated with treatment response or disease progression and with side effects such as hot
flashes, including phenocopying effects; therefore,
therapeutic drug monitoring as a useful surrogate is
currently not available in the case of tamoxifen.
Whether determination of the CYP2D6 genotype will
become a diagnostic tool for selecting the appropriate
adjuvant endocrine therapy for ER-positive postmenopausal breast cancer patients awaits validation in prospective clinical trials that randomize tamoxifen vs AI
treatment according to CYP2D6 genotypes. Such prospective clinical trials are currently being planned.
Other open questions may address the clinical relevance of other drug-metabolizing enzymes and mutations, as well as ethnic variation, in the prevalence of
their treatment outcome–relevant genotypes. Finally,
there is the possibility that pharmacokinetic genes will
only partly explain the pharmacogenomics of tamoxifen. It will therefore be important to also explore the
contribution of pharmacodynamic genes in evaluating
antiestrogen resistance as a feature of the tumor cell
and in addressing the role of genes associated with
estrogen-mediated cell proliferation. Within this context, it will be interesting to learn whether genes encoding the ER, its coactivators, or its corepressors (106 ), as
well as antiestrogen resistance genes (107 ) and their
variants, will affect the response to tamoxifen. Such
results may increase the overall potential of tamoxifen
pharmacogenomics.
To this end, it is important to appreciate that most
cancer therapies in current use have been established
empirically. The recent progress in our understanding
of the pharmacology and pharmacogenetics of tamoxifen, however, holds promise for the improvement of
treatments through personalized medicine. Because
the genome-based approach uses CYP2D6 genotyping
to predict a patient’s metabolizer phenotype, ethical
issues need to be sufficiently addressed. In the light of
acceptable alternatives, an informed choice about adjuvant endocrine treatment and, most importantly,
avoiding a therapy that may lack efficacy must be of
prime interest. It will therefore be important to make
patients and their caregivers aware of these issues and
to initiate discussions with regulatory authorities.
Clinical Chemistry 55:10 (2009) 1779
Review
Author Contributions: All authors confirmed they have contributed to
the intellectual content of this paper and have met the following 3 requirements: (a) significant contributions to the conception and design,
acquisition of data, or analysis and interpretation of data; (b) drafting
or revising the article for intellectual content; and (c) final approval of
the published article.
Employment or Leadership: None declared.
Consultant or Advisory Role: None declared.
Stock Ownership: None declared.
Honoraria: None declared.
Research Funding: Robert Bosch Foundation, Stuttgart, Germany,
and Bundesministerium für Bildung und Forschung Grant No.
01ZP0502.
Expert Testimony: None declared.
Authors’ Disclosures of Potential Conflicts of Interest: Upon
manuscript submission, all authors completed the Disclosures of Potential Conflict of Interest form. Potential conflicts of interest:
Role of Sponsor: The funding organizations played no role in the
design of study, choice of enrolled patients, review and interpretation
of data, or preparation or approval of manuscript.
References
1. Early Breast Cancer Trialists’ Collaborative
Group (EBCTCG). Effects of chemotherapy and
hormonal therapy for early breast cancer on
recurrence and 15-year survival: an overview of
the randomised trials. Lancet 2005;365:1687–
717.
2. Goldhirsch A, Wood WC, Gelber RD, Coates AS,
Thürlimann B, Senn HJ. Progress and promise:
highlights of the international expert consensus
on the primary therapy of early breast cancer
2007. Ann Oncol 2007;18:1133– 44.
3. Fisher B, Costantino JP, Wickerham DL, Cecchini
RS, Cronin WM, Robidoux A, et al. Tamoxifen
for the prevention of breast cancer: current status of the National Surgical Adjuvant Breast and
Bowel Project P-1 study. J Natl Cancer Inst
2005;97:1652– 62.
4. Fentiman IS, Fourquet A, Hortobagyi GN. Male
breast cancer. Lancet 2006;367:595– 604.
5. Boocock DJ, Brown K, Gibbs AH, Sanchez E,
Turteltaub KW, White IN. Identification of human CYP forms involved in the activation of
tamoxifen and irreversible binding to DNA. Carcinogenesis 2002;23:1897–901.
6. Coller JK, Krebsfaenger N, Klein K, Wolbold R,
Nussler A, Neuhaus P, et al. Large interindividual variability in the in vitro formation of
tamoxifen metabolites related to the development of genotoxicity. Br J Clin Pharmacol 2004;
57:105–11.
7. Crewe HK, Notley LM, Wunsch RM, Lennard MS,
Gillam EM. Metabolism of tamoxifen by recombinant human cytochrome P450 enzymes:
formation of the 4-hydroxy, 4⬘-hydroxy and
N-desmethyl metabolites and isomerization of
trans-4-hydroxytamoxifen. Drug Metab Dispos
2002;30:869 –74.
8. Borges S, Desta Z, Li L, Skaar TC, Ward BA,
Nguyen A, et al. Quantitative effect of CYP2D6
genotype and inhibitors on tamoxifen
metabolism: implication for optimization of
breast cancer treatment. Clin Pharmacol Ther
2006;80:61–74.
9. Dowsett M, Cuzick J, Howell A, Jackson I, and
the ATAC Trialists’ Group. Pharmacokinetics of
anastrozole and tamoxifen alone, and in combination, during adjuvant endocrine therapy for
early breast cancer in postmenopausal women:
a sub-protocol of the ‘Arimidex™ and Tamoxifen Alone or in Combination’ (ATAC) trial. Br J
Cancer 2001;85:317–24.
10. Hutson PR, Love RR, Havighurst TC, Rogers E,
Cleary JF. Effect of exemestane on tamoxifen
pharmacokinetics in postmenopausal women
1780 Clinical Chemistry 55:10 (2009)
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
treated for breast cancer. Clin Cancer Res 2005;
11:8722–7.
Jin Y, Desta Z, Stearns V, Ward B, Ho H, Lee KH,
et al. CYP2D6 genotype, antidepressant use,
and tamoxifen metabolism during adjuvant
breast cancer treatment. J Natl Cancer Inst
2005;97:30 –9.
Lee KH, Ward BA, Desta Z, Flockhart DA, Jones
DR. Quantification of tamoxifen and three metabolites in plasma by high-performance liquid
chromatography with fluorescence detection: application to a clinical trial. J Chromatogr B Analyt
Technol Biomed Life Sci 2003;791:245–53.
MacCallum J, Cummings J, Dixon JM, Miller WR.
Concentrations of tamoxifen and its major metabolites in hormone responsive and resistant
breast tumours. Br J Cancer 2000;82:1629 –35.
Sheth HR, Lord G, Tkaczuk K, Danton M, Lewis
LM, Langenberg P, et al. Aging may be associated with concentrations of tamoxifen and its
metabolites in breast cancer patients. J Womens
Health (Larchmt) 2003;12:799 – 808.
Desta Z, Ward BA, Soukhova NV, Flockhart DA.
Comprehensive evaluation of tamoxifen sequential biotransformation by the human cytochrome
P450 system in vitro: prominent roles for CYP3A
and CYP2D6. J Pharmacol Exp Ther 2004;310:
1062–75.
Johnson MD, Zuo H, Lee KH, Trebley JP, Rae JM,
Weatherman RV, et al. Pharmacological characterization of 4-hydroxy-N-desmethyl tamoxifen,
a novel active metabolite of tamoxifen. Breast
Cancer Res Treat 2004;85:151–9.
Lim HS, Ju LH, Seok LK, Sook LE, Jang IJ, Ro
J. Clinical implications of CYP2D6 genotypes
predictive of tamoxifen pharmacokinetics in
metastatic breast cancer. J Clin Oncol 2007;25:
3837– 45.
Coller JK, Krebsfaenger N, Klein K, Endrizzi K,
Wolbold R, Lang T, et al. The influence of
CYP2B6, CYP2C9 and CYP2D6 genotypes on the
formation of the potent antioestrogen Z-4hydroxy-tamoxifen in human liver. Br J Clin
Pharmacol 2002;54:157– 67.
Crewe HK, Ellis SW, Lennard MS, Tucker GT.
Variable contribution of cytochromes P4502D6,
2C9 and 3A4 to the 4-hydroxylation of tamoxifen by human liver microsomes. Biochem Pharmacol 1997;53:171– 8.
Dehal SS, Kupfer D. CYP2D6 catalyzes tamoxifen
4-hydroxylation in human liver. Cancer Res
1997;57:3402– 6.
Mani C, Gelboin HV, Park SS, Pearce R, Parkinson A, Kupfer D. Metabolism of the antimam-
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
mary cancer antiestrogenic agent tamoxifen. I.
Cytochrome P-450-catalyzed N-demethylation
and 4-hydroxylation. Drug Metab Dispos 1993;
21:645–56.
Stearns V, Johnson MD, Rae JM, Morocho A,
Novielli A, Bhargava P, et al. Active tamoxifen
metabolite plasma concentrations after coadministration of tamoxifen and the selective serotonin reuptake inhibitor paroxetine. J Natl
Cancer Inst 2003;95:1758 – 64.
Kim SY, Suzuki N, Santosh Laxmi YR, Rieger R,
Shibutani S. Alpha-hydroxylation of tamoxifen
and toremifene by human and rat cytochrome
P450 3A subfamily enzymes. Chem Res Toxicol
2003;16:1138 – 44.
Notley LM, Crewe KH, Taylor PJ, Lennard MS,
Gillam EM. Characterization of the human cytochrome P450 forms involved in metabolism of
tamoxifen to its ␣-hydroxy and ␣,4-dihydroxy
derivatives. Chem Res Toxicol 2005;18:1611– 8.
Katzenellenbogen JA, Carlson KE, Katzenellenbogen BS. Facile geometric isomerization of phenolic
non-steroidal estrogens and antiestrogens: limitations to the interpretation of experiments characterizing the activity of individual isomers. J Steroid
Biochem 1985;22:589 –96.
Osborne CK, Wiebe VJ, McGuire WL, Ciocca DR,
DeGregorio MW. Tamoxifen and the isomers of
4-hydroxytamoxifen in tamoxifen-resistant tumors from breast cancer patients. J Clin Oncol
1992;10:304 –10.
Dehal SS, Brodie AMH, Kupfer D. The aromatase
inactivator 4-hydroxyandrostenedione (4-OH-A)
inhibits tamoxifen metabolism by rat hepatic
cytochrome P-450 3A: potential for drug-drug
interaction of tamoxifen and 4-OH-A in combined anti-breast cancer therapy. Drug Metab
Dispos 1999;27:389 –94.
Dehal SS, Kupfer D. Evidence that the catechol
3,4-dihydroxytamoxifen is a proximate intermediate to the reactive species binding covalently
to proteins. Cancer Res 1995;56:1283–90.
Liu X, Pisha E, Tonetti DA, Yao D, Li Y, Yao J, et
al. Antiestrogenic and DNA damaging effects
induced by tamoxifen and toremifene metabolites. Chem Res Toxicol 2003;16:832–7.
Hodgson E, Rose RL, Cao Y, Dehal SS, Kupfer D.
Flavin-containing monooxygenase isoform specificity for the N-oxidation of tamoxifen determined by product measurement and NADPH
oxidation. J Biochem Mol Toxicol 2000;14:118 –
20.
Parte P, Kupfer D. Oxidation of tamoxifen by
human flavin-containing monooxygenase (FMO)
Review
Tamoxifen Pharmacogenomics
32.
33.
34.
35.
36.
37.
38.
39.
40.
41.
42.
43.
44.
1 and FMO3 to tamoxifen-N-oxide and its novel
reduction back to tamoxifen by human cytochromes P450 and hemoglobin. Drug Metab
Dispos 2005;33:1446 –52.
Gjerde J, Hauglid M, Breilid H, Lundgren S,
Varhaug JE, Kisanga ER, et al. Effects of CYP2D6
and SULT1A1 genotypes including SULT1A1
gene copy number on tamoxifen metabolism.
Ann Oncol 2008;19:56 – 61.
Nishiyama T, Ogura K, Nakano H, Ohnuma T,
Kaku T, Hiratsuka A, et al. Reverse geometrical
selectivity in glucuronidation and sulfation of
cis- and trans-4-hydroxytamoxifens by human
liver UDP-glucuronosyltransferases and sulfotransferases. Biochem Pharmacol 2002;63:
1817–30.
Ogura K, Ishikawa Y, Kaku T, Nishiyama T,
Ohnuma T, Muro K, Hiratsuka A. Quaternary
ammonium-linked glucuronidation of trans-4hydroxytamoxifen, an active metabolite of tamoxifen, by human liver microsomes and UDPglucuronosyltransferase 1A4. Biochem Pharmacol
2006;71:1358 – 69.
Sun D, Sharma AK, Dellinger RW, BlevinsPrimeau AS, Balliet RM, Chen G, et al. Glucuronidation of active tamoxifen metabolites by
the human UDP glucuronosyltransferases. Drug
Metab Dispos 2007;35:2006 –14.
Kaku T, Ogura K, Nishiyama T, Ohnuma T, Muro
K, Hiratsuka A. Quaternary ammonium-linked
glucuronidation of tamoxifen by human liver
microsomes and UDP-glucuronosyltransferase
1A4. Biochem Pharmacol 2004;67:2093–102.
Sun D, Chen G, Dellinger RW, Duncan K, Fang
JL, Lazarus P. Characterization of tamoxifen and
4-hydroxytamoxifen glucuronidation by human
UGT1A4 variants. Breast Cancer Res 2006;8:
R50.
Falany JL, Pilloff DE, Leyh TS, Falany CN. Sulfation of raloxifene and 4-hydroxytamoxifen by
human cytosolic sulfotransferases. Drug Metab
Dispos 2006;34:361– 8.
Apak TI, Duffel MW. Interactions of the stereoisomers of alpha-hydroxytamoxifen with human
hydroxysteroid sulfotransferase SULT2A1 and
rat hydroxysteroid sulfotransferase STa. Drug
Metab Dispos 2004;32:1501– 8.
Kim SY, Laxmi YR, Suzuki N, Ogura K, Watabe T,
Duffel MW, Shibutani S. Formation of
tamoxifen-DNA adducts via O-sulfonation, not
O-acetylation, of alpha-hydroxytamoxifen in rat
and human livers. Drug Metab Dispos 2005;33:
1673– 8.
Osborne MR, Hewer A, Phillips DH. Resolution
of alpha-hydroxytamoxifen; R-isomer forms
more DNA adducts in rat liver cells. Chem Res
Toxicol 2001;14:888 –93.
Zanger UM, Turpeinen M, Klein K, Schwab M.
Functional pharmacogenetics/genomics of human cytochromes P450 involved in drug biotransformation. Anal Bioanal Chem 2008;392:
1093–108.
Zanger UM. The CYP2D subfamily. In: Ioannides
C, ed. Cytochromes P450: role in the metabolism and toxicity of drugs and other xenobiotics.
London: Royal Chemical Society; 2008. p 241–
75.
Eichelbaum M, Spannbrucker N, Dengler HJ.
Proceedings: N-oxidation of sparteine in man
and its interindividual differences. Naunyn
45.
46.
47.
48.
49.
50.
51.
52.
53.
54.
55.
56.
57.
Schmiedebergs Arch Pharmacol 1975;287
(Suppl):R94.
Mahgoub A, Idle JR, Dring LG, Lancaster R,
Smith RL. Polymorphic hydroxylation of debrisoquine in man. Lancet 1977;2:584 – 6.
Griese EU, Zanger UM, Brudermanns U, Gaedigk
A, Mikus G, Morike K, et al. Assessment of the
predictive power of genotypes for the in-vivo
catalytic function of CYP2D6 in a German population. Pharmacogenetics 1998;8:15–26.
Raimundo S, Toscano C, Klein K, Fischer J,
Griese EU, Eichelbaum M, et al. A novel intronic
mutation, 2988G⬎A, with high predictivity for
impaired function of cytochrome P450 2D6 in
white subjects. Clin Pharmacol Ther 2004;76:
128 –38.
Sachse C, Brockmoller J, Bauer S, Roots I. Cytochrome P450 2D6 variants in a Caucasian
population: allele frequencies and phenotypic
consequences. Am J Hum Genet 1997;60:284 –
95.
Marez D, Legrand M, Sabbagh N, Guidice JM,
Spire C, Lafitte JJ, et al. Polymorphism of the
cytochrome P450 CYP2D6 gene in a European
population: characterization of 48 mutations
and 53 alleles, their frequencies and evolution.
Pharmacogenetics 1997;7:193–202.
Raimundo S, Fischer J, Eichelbaum M, Griese
EU, Schwab M, Zanger UM. Elucidation of the
genetic basis of the common ‘intermediate metabolizer’ phenotype for drug oxidation by
CYP2D6. Pharmacogenetics 2000;10:577– 81.
Toscano C, Klein K, Blievernicht J, Schaeffeler E,
Saussele T, Raimundo S, et al. Impaired expression of CYP2D6 in intermediate metabolizers
carrying the *41 allele caused by the intronic
SNP 2988G⬎A: evidence for modulation of
splicing events. Pharmacogenet Genomics 2006;
16:755– 66.
Zanger UM, Fischer J, Raimundo S, Stuven T,
Evert BO, Schwab M, Eichelbaum M. Comprehensive analysis of the genetic factors determining expression and function of hepatic CYP2D6.
Pharmacogenetics 2001;11:573– 85.
Bertilsson L, Dahl ML, Sjöqvist F, Aberg-Wistedt
A, Humble M, Johansson I, et al. Molecular basis
for rational megaprescribing in ultrarapid hydroxylators of debrisoquine. Lancet 1993;341:
63.
Dalen P, Dahl ML, Bernal Ruiz ML, Nordin J,
Bertilsson L. 10-Hydroxylation of nortriptyline in
white persons with 0, 1, 2, 3, and 13 functional
CYP2D6 genes. Clin Pharmacol Ther 1998;63:
444 –52.
Lovlie R, Daly AK, Matre GE, Molven A, Steen
VM. Polymorphisms in CYP2D6 duplicationnegative individuals with the ultrarapid metabolizer phenotype: a role for the CYP2D6*35 allele
in ultrarapid metabolism? Pharmacogenetics
2001;11:45–55.
Algeciras-Schimnich A, O’Kane DJ, Snozek CL.
Pharmacogenomics of tamoxifen and irinotecan
therapies. Clin Lab Med 2008;28:553– 67.
Aklillu E, Persson I, Bertilsson L, Johansson I,
Rodrigues F, Ingelman-Sundberg M. Frequent
distribution of ultrarapid metabolizers of debrisoquine in an Ethiopian population carrying
duplicated and multiduplicated functional
CYP2D6 alleles. J Pharmacol Exp Ther 1996;278:
441– 6.
58. McLellan RA, Oscarson M, Seidegard J, Evans
DA, Ingelman-Sundberg M. Frequent occurrence
of CYP2D6 gene duplication in Saudi Arabians.
Pharmacogenetics 1997;7:187–91.
59. Sistonen J, Sajantila A, Lao O, Corander J, Barbujani G, Fuselli S. CYP2D6 worldwide genetic
variation shows high frequency of altered activity variants and no continental structure. Pharmacogenet Genomics 2007;17:93–101.
60. Ingelman-Sundberg M. Genetic polymorphisms
of cytochrome P450 2D6 (CYP2D6): clinical consequences, evolutionary aspects and functional
diversity. Pharmacogenomics J 2005;5:6 –13.
61. Bradford LD. CYP2D6 allele frequency in European Caucasians, Asians, Africans and their descendants. Pharmacogenomics 2002;3:229 – 43.
62. Gasche Y, Daali Y, Fathi M, Chiappe A, Cottini S,
Dayer P, Desmeules J. Codeine intoxication associated with ultrarapid CYP2D6 metabolism.
N Engl J Med 2004;351:2827–31.
63. Koren G, Cairns J, Chitayat D, Gaedigk A, Leeder
SJ. Pharmacogenetics of morphine poisoning in
a breastfed neonate of a codeine-prescribed
mother. Lancet 2006;368:704.
64. Lee CR, Goldstein JA, Pieper JA. Cytochrome
P450 2C9 polymorphisms: a comprehensive review of the in-vitro and human data. Pharmacogenetics 2002;12:251– 63.
65. King BP, Khan TI, Aithal GP, Kamali F, Daly
AK. Upstream and coding region CYP2C9
polymorphisms: correlation with warfarin dose
and metabolism. Pharmacogenetics 2004;14:
813–22.
66. Garcia-Martin E, Martinez C, Ladero JM, Agundez JA. Interethnic and intraethnic variability of
CYP2C8 and CYP2C9 polymorphisms in healthy
individuals. Mol Diagn Ther 2006;10:29 – 40.
67. Xie HG, Kim RB, Wood AJ, Stein CM. Molecular
basis of ethnic differences in drug disposition
and response. Annu Rev Pharmacol Toxicol
2001;41:815–50.
68. Flockhart DA, O’Kane D, Williams MS, Watson
MS, Flockhart DA, Gage B, et al. Pharmacogenetic testing of CYP2C9 and VKORC1 alleles for
warfarin. Genet Med 2008;10:139 –50.
69. Limdi NA, Veenstra DL. Warfarin pharmacogenetics. Pharmacotherapy 2008;28:1084 –97.
70. Pilotto A, Seripa D, Franceschi M, Scarcelli C, Colaizzo D, Grandone E, et al. Genetic susceptibility
to nonsteroidal anti-inflammatory drug-related
gastroduodenal bleeding: role of cytochrome P450
2C9 polymorphisms. Gastroenterology 2007;133:
465–71.
71. Holstein A, Plaschke A, Ptak M, Egberts EH,
El-Din J, Brockmoller J, Kirchheiner J. Association between CYP2C9 slow metabolizer genotypes and severe hypoglycaemia on medication
with sulphonylurea hypoglycaemic agents. Br J
Clin Pharmacol 2005;60:103– 6.
72. Desta Z, Zhao X, Shin JG, Flockhart DA. Clinical
significance of the cytochrome P450 2C19 genetic polymorphism. Clin Pharmacokinet 2002;
41:913–58.
73. Xie HG, Stein CM, Kim RB, Wilkinson GR, Flockhart DA, Wood AJ. Allelic, genotypic and phenotypic distributions of S-mephenytoin 4⬘hydroxylase (CYP2C19) in healthy Caucasian
populations of European descent throughout
the world. Pharmacogenetics 1999;9:539 – 49.
74. Wedlund PJ. The CYP2C19 enzyme polymor-
Clinical Chemistry 55:10 (2009) 1781
Review
phism. Pharmacology 2000;61:174 – 83.
75. Sim SC, Risinger C, Dahl ML, Aklillu E, Christensen M, Bertilsson L, Ingelman-Sundberg M. A
common novel CYP2C19 gene variant causes
ultrarapid drug metabolism relevant for the drug
response to proton pump inhibitors and antidepressants. Clin Pharmacol Ther 2006;79:103–
13.
76. Rudberg I, Mohebi B, Hermann M, Refsum H,
Molden E. Impact of the ultrarapid CYP2C19*17
allele on serum concentration of escitalopram in
psychiatric patients. Clin Pharmacol Ther 2008;
83:322–7.
77. Justenhoven C, Hamann U, Pierl CB, Baisch C,
Harth V, Rabstein S, et al. CYP2C19*17 is associated with decreased breast cancer risk.
Breast Cancer Res Treat 2008;115:391– 6.
78. Kurzawski M, Gawronska-Szklarz B, Wrzesniewska J, Siuda A, Starzynska T, Drozdzik M.
Effect of CYP2C19*17 gene variant on Helicobacter pylori eradication in peptic ulcer patients.
Eur J Clin Pharmacol 2006;62:877– 80.
79. Schroth W, Antoniadou L, Fritz P, Schwab M,
Muerdter T, Zanger UM, et al. Breast cancer
treatment outcome with adjuvant tamoxifen relative to patient CYP2D6 and CYP2C19 genotypes. J Clin Oncol 2007;25:5187–93.
80. Zanger UM, Klein K, Saussele T, Blievernicht J,
Hofmann MH, Schwab M. Polymorphic CYP2B6:
molecular mechanisms and emerging clinical
significance. Pharmacogenomics 2007;8:743–
59.
81. Rotger M, Colombo S, Furrer H, Bleiber G, Buclin
T, Lee BL, et al. Influence of CYP2B6 polymorphism on plasma and intracellular concentrations and toxicity of efavirenz and nevirapine in
HIV-infected patients. Pharmacogenet Genomics 2005;15:1–5.
82. Rotger M, Tegude H, Colombo S, Cavassini M,
Furrer H, Decosterd L, et al. Predictive value of
known and novel alleles of CYP2B6 for efavirenz
plasma concentrations in HIV-infected individuals. Clin Pharmacol Ther 2007;81:557– 66.
83. Nyakutira C, Röshammar D, Chigutsa E, Chonzi
P, Ashton M, Nhachi C, Masimirembwa C. High
prevalence of the CYP2B6 516G3 T(*6) variant
and effect on the population pharmacokinetics
of efavirenz in HIV/AIDS outpatients in Zimbabwe. Eur J Clin Pharmacol 2008;64:357– 65.
84. Westlind-Johnsson A, Hermann R, Huennemeyer
A, Hauns B, Lahu G, Nassr N, et al. Identification
and characterization of CYP3A4*20, a novel
rare CYP3A4 allele without functional activity.
Clin Pharmacol Ther 2006;79:339 – 49.
85. Anglicheau D, Legendre C, Beaune P, Thervet E.
Cytochrome P450 3A polymorphisms and immunosuppressive drugs: an update. Pharmacogenomics 2007;8:835– 49.
86. Jordan VC. Metabolites of tamoxifen in animals
and man: identification, pharmacology, and sig-
1782 Clinical Chemistry 55:10 (2009)
87.
88.
89.
90.
91.
92.
93.
94.
95.
96.
97.
98.
nificance. Breast Cancer Res Treat 1982;2:123–
38.
Coezy E, Borgna JL, Rochefort H. Tamoxifen and
metabolites in MCF7 cells: correlation between
binding to estrogen receptor and inhibition of
cell growth. Cancer Res 1982;42:317–23.
Robertson DW, Katzenellenbogen JA, Long DJ,
Rorke EA, Katzenellenbogen BS. Tamoxifen antiestrogens. A comparison of the activity, pharmacokinetics, and metabolic activation of the
cis and trans isomers of tamoxifen. J Steroid
Biochem 1982;16:1–13.
Buck MB, Coller JK, Mürdter TE, Eichelbaum M,
Knabbe C. TGF␤2 and T␤RII are valid molecular
biomarkers for the antiproliferative effects of
tamoxifen and tamoxifen metabolites in breast
cancer cells. Breast Cancer Res Treat 2008;107:
15–24.
Lim YC, Desta Z, Flockhart DA, Skaar TC. Endoxifen (4-hydroxy-N-desmethyl-tamoxifen) has
anti-estrogenic effects in breast cancer cells
with potency similar to 4-hydroxy-tamoxifen.
Cancer Chemother Pharmacol 2005;55:471– 8.
Lim YC, Li L, Desta Z, Zhao Q, Rae JM, Flockhart
DA, Skaar TC. Endoxifen, a secondary metabolite of tamoxifen, and 4-OH-tamoxifen induce
similar changes in global gene expression patterns in MCF-7 breast cancer cells. J Pharmacol
Exp Ther 2006;318:503–12.
Decensi A, Gandini S, Serrano D, Cazzaniga M,
Pizzamiglio M, Maffini F, et al. Randomized
dose-ranging trial of tamoxifen at low doses in
hormone replacement therapy users. J Clin Oncol 2007;25:4201–9.
Xu Y, Sun Y, Yao L, Shi L, Wu Y, Ouyang T, et al.
Association between CYP2D6 *10 genotype and
survival of breast cancer patients receiving tamoxifen treatment. Ann Oncol 2008;19:1423–9.
Goetz MP, Rae JM, Suman VJ, Safgren SL, Ames
MM, Visscher DW, et al. Pharmacogenetics of
tamoxifen biotransformation is associated with
clinical outcomes of efficacy and hot flashes.
J Clin Oncol 2005;23:9312– 8.
Goetz MP, Kamal A, Ames MM. Tamoxifen
pharmacogenomics: the role of CYP2D6 as a
predictor of drug response. Clin Pharmacol Ther
2008;83:160 – 6.
Kiyotani K, Mushiroda T, Sasa M, Bando Y,
Sumitomo I, Hosono N, et al. Impact of
CYP2D6*10 on recurrence-free survival in breast
cancer patients receiving adjuvant tamoxifen
therapy. Cancer Sci 2008;99:995–9.
Nowell SA, Ahn J, Rae JM, Scheys JO, Trovato A,
Sweeney C, et al. Association of genetic variation in tamoxifen-metabolizing enzymes with
overall survival and recurrence of disease in
breast cancer patients. Breast Cancer Res Treat
2005;91:249 –58.
Wegman P, Vainikka L, Stal O, Nordenskjold B,
Skoog L, Rutqvist LE, Wingren S. Genotype of
99.
100.
101.
102.
103.
104.
105.
106.
107.
108.
109.
110.
metabolic enzymes and the benefit of tamoxifen
in postmenopausal breast cancer patients.
Breast Cancer Res 2005;7:R284 –90.
Wegman P, Elingarami S, Carstensen J, Stal O,
Nordenskjold B, Wingren S. Genetic variants of
CYP3A5, CYP2D6, SULT1A1, UGT2B15 and tamoxifen response in postmenopausal patients
with breast cancer. Breast Cancer Res 2007;9:
R7.
Bonanni B, Macis D, Maisonneuve P, Johansson
HA, Gucciardo G, Oliviero P, et al. Polymorphism in the CYP2D6 tamoxifen-metabolizing
gene influences clinical effect but not hot
flashes: data from the Italian Tamoxifen Trial.
J Clin Oncol 2006;24:3708 –9.
Mortimer JE, Flatt SW, Parker BA, Gold EB,
Wasserman L, Natarajan L, Pierce JP. Tamoxifen, hot flashes and recurrence in breast cancer.
Breast Cancer Res Treat 2008;108:421– 6.
Newman WG, Hadfield KD, Latif A, Roberts SA,
Shenton A, McHague C, et al. Impaired tamoxifen metabolism reduces survival in familial
breast cancer patients. Clin Cancer Res 2008;
14:5913– 8.
Punglia RS, Burstein HJ, Winer EP, Weeks JC.
Pharmacogenomic variation of CYP2D6 and the
choice of optimal adjuvant endocrine therapy
for postmenopausal breast cancer: a modeling
analysis. J Natl Cancer Inst 2008;100:642– 8.
Carlson RW, Hudis CA, Pritchard KI. Adjuvant
endocrine therapy in hormone receptor-positive
postmenopausal breast cancer: evolution of
NCCN, ASCO, and St Gallen recommendations.
J Natl Compr Canc Netw 2006;4:971–9.
Lien EA, Solheim E, Kvinnsland S, Ueland PM.
Identification of 4-hydroxy-N-desmethyltamoxifen
as a metabolite of tamoxifen in human bile. Cancer Res 1988;48:2304 – 8.
Jordan VC, O’Malley BW. Selective estrogenreceptor modulators and antihormonal resistance in breast cancer. J Clin Oncol 2007;25:
5815–24.
van Agthoven T, Sieuwerts AM, Meijer-van
Gelder ME, Look MP, Smid M, Veldscholte J, et
al. Relevance of breast cancer antiestrogen resistance genes in human breast cancer progression and tamoxifen resistance. J Clin Oncol
2009;27:542–9.
Langan-Fahey SM, Tormey DC, Jordan VC. Tamoxifen metabolites in patients on long-term
adjuvant therapy for breast cancer. Eur J Cancer
1990;26:883– 8.
Wakeling AE, Slater SR. Estrogen-receptor binding and biologic activity of tamoxifen and its
metabolites. Cancer Treat Rep 1980;64:741– 4.
Lazarus P, Blevins-Primeau AS, Zheng Y, Sun D.
Potential role of UGT pharmacogenetics in cancer treatment and prevention: focus on tamoxifen. Ann N Y Acad Sci 2009;1155:99 –111.