CYP2D6 genotype predicts tamoxifen discontinuation and drug response: a secondary analysis of the KARISMA trial

Genetic variants in tamoxifen metabolism and early treatment discontinuation among premenopausal breast cancer patients

PURPOSE

Premenopausal, estrogen receptor (ER)-positive breast cancer patients should receive tamoxifen for at least 5 years, but many prematurely discontinue. Activation, transport, and deactivation of tamoxifen and its metabolites are controlled by proteins encoded by genes with functional variations. We examined the impact of genetic polymorphisms in the tamoxifen pathway on early treatment discontinuation.

METHODS

We included premenopausal women diagnosed with ER-positive breast cancer (2002-2011) in Denmark who initiated tamoxifen. We genotyped 26 genetic variants in 15 enzymes involved in tamoxifen metabolism. Early discontinuation was defined as tamoxifen use for < 5 years. We estimated individual and combined effects of genetic variants using a Bayesian pathway approach. We report Bayes Factors (BF), wherein values > 1 indicate support of an effect of the genetic pathway on discontinuation (compared with no effect).

RESULTS

Among 3,729 patients, 536 (14%) discontinued tamoxifen within 5 years. Genetic variants involved in tamoxifen activation impacted early discontinuation (BF = 7.5), in a manner driven almost entirely by CYP2D6 activity (BF = 22.6). Several variants in CYP2D6 and transporter genes synergistically increased the hazard of early discontinuation (e.g., CYP2D6*2 and ABCC2; BF = 138).

CONCLUSIONS

Variants in enzymes responsible for activating tamoxifen metabolites-particularly within CYP2D6-influence early tamoxifen discontinuation. CYP2D6 variants synergistically interact with transporter gene variants, namely ABCC2, to further raise the risk of discontinuation.

Influence of endoxifen on mammographic density: results from the KARISMA-Tam trial

BACKGROUND

Monitoring metabolites of tamoxifen, such as endoxifen, has been suggested as a strategy to ascertain therapeutic effect of tamoxifen therapy, but clinical guidelines are missing. Herein, we aim to investigate the outcome of endoxifen concentrations of low-dose tamoxifen, using change in mammographic breast density as a proxy for therapy response.

METHODS

In the randomized KARISMA-Tam (Karolinska Mammography project for risk prediction of breast cancer -Intervention Study with Tamoxifen) trial, including 5 doses of tamoxifen, measurements of plasma endoxifen concentrations, determination of CYP2D6 metabolizer status, and mammographic breast density change over the trial period were carried out. Association between endoxifen concentrations and relative mammographic breast density change after 6 months treatment was analyzed using linear regression in a spline model.

RESULTS

A total of 824 women (335 premenopausal, 489 postmenopausal) were included. In analyses of premenopausal women, a spline model described a mammographic breast density decrease, equivalent to the mean (-18.5%) seen in women exposed to 20 mg tamoxifen, at endoxifen concentrations of 2-3 ng/mL. The mammographic breast density decrease reached a nadir at endoxifen levels of 3 ng/mL and did not decrease further at higher endoxifen concentrations. Most intermediate and normal tamoxifen metabolizers (about 90% of all participants) reached an endoxifen concentration of more than 2 ng/mL at tamoxifen doses of 5 and 10 mg. No mammographic breast density decrease was seen in the postmenopausal group.

CONCLUSIONS

We have identified a possible window of effect on mammographic breast density at endoxifen concentrations of 2-3 ng/mL in premenopausal women, which corresponds to the doses of 5 and 10 mg tamoxifen. Because mammographic breast density change was used as a surrogate marker for therapy response, results should be confirmed using clinically established outcomes measures.

CYP2D6 polymorphisms and endoxifen concentration in Chinese patients with breast cancer

BACKGROUND

The plasma concentration of endoxifen, the active metabolite of tamoxifen, might be affected by different CYP2D6 genotypes in patients with breast cancer, but solid evidence is still lacking in Asian patients. This prospective study aimed to investigate the relationship between CYP2D6 genetic polymorphisms and endoxifen plasma concentrations among Chinese patients with breast cancer treated with tamoxifen.

METHODS

From August 2015 to June 2018, 110 patients with breast cancer were enrolled. CYP2D6 variant alleles and endoxifen plasma concentration were determined using Sanger sequencing and high-performance liquid chromatography-tandem mass spectrometry, respectively.

RESULTS

The most frequent allele of CYP2D6 was *10 (56.4%). The most frequent genotype of CYP2D6 was *10/*10 (33%), *1/*10(28.2%) and *2/*10(14.5%). Sixty-four patients (58.2%) were Normal Metabolizers (NM), while 46 (41.8%) were Intermediate Metabolizers (IM). All patients except two had endoxifen concentrations above the threshold of 5.9ng/ml. The median endoxifen plasma concentrations for patients with CYP2D6 genotypes *1/*2 and *1/10 were higher compared to other genotypes (p = 0.012). The median endoxifen plasma concentration was higher in NM than in IM (18ng/ml vs. 13ng/ml, p = 0.0077). Patients with CYP2D6*10(T/T) had lower endoxifen concentrations than those with *10(C/T) and *10(C/C) but the difference was not significant. There were no significant differences in adverse events between patients in the NM and IM groups or between patients with the CYP2D6*10 (T/T) genotype and non-*10 (T/T) genotype.

CONCLUSION

Only CYP2D6 IMs and NMs were identified in this study. Almost all patients had the endoxifen concentrations above the threshold. The endoxifen plasma concentration was lower in CYP2D6 IMs than in NMs, but these variants did not compromise the adverse effects of tamoxifen in Asian patients with breast cancer.

TRIAL REGISTRATION

The study protocol was approved by the institutional review boards of Sun YatSen University Cancer Center (Ethics approval number, B201506501,20160115).

Low-dose tamoxifen treatment reduces collagen organisation indicative of tissue stiffness in the normal breast: results from the KARISMA randomised controlled trial

BACKGROUND

Tissue stiffness, dictated by organisation of interstitial fibrillar collagens, increases breast cancer risk and contributes to cancer progression. Tamoxifen is a standard treatment for receptor-positive breast cancer and is also aproved for primary prevention. We investigated the effect of tamoxifen and its main metabolites on the breast tissue collagen organisation as a proxy for stiffness and explored the relationship between mammographic density (MD) and collagen organisation.

MATERIAL AND METHODS

This sub-study of the double-blinded dose-determination trial, KARISMA, included 83 healthy women randomised to 6 months of 20, 10, 5, 2.5, and 1 mg of tamoxifen or placebo. Ultrasound-guided core-needle breast biopsies collected before and after treatment were evaluated for collagen organisation by polarised light microscopy.

RESULTS

Tamoxifen reduced the amount of organised collagen and overall organisation, reflected by a shift from heavily crosslinked thick fibres to thinner, less crosslinked fibres. Collagen remodelling correlated with plasma concentrations of tamoxifen metabolites. MD change was not associated with changes in amount of organised collagen but was correlated with less crosslinking in premenopausal women.

CONCLUSIONS

In this study of healthy women, tamoxifen decreased the overall organisation of fibrillar collagens, and consequently, the breast tissue stiffness. These stromal alterations may play a role in the well-established preventive and therapeutic effects of tamoxifen. Trial registration ClinicalTrials.gov ID: NCT03346200. Registered November 1st, 2017. Retrospectively registered.

The Role of Pharmacogenetic-Based Pharmacokinetic Analysis in Precise Breast Cancer Treatment

Given the high prevalence of breast cancer and the diverse genetic backgrounds of patients, a growing body of research emphasizes the importance of pharmacogenetic-based pharmacokinetic analysis in optimizing treatment outcomes. The treatment of breast cancer involves multiple drugs whose metabolism and efficacy are influenced by individual genetic variations. Genetic polymorphisms in drug-metabolizing enzymes and transport proteins are crucial in the regulation of pharmacokinetics. Our review aims to investigate the opportunities and challenges of pharmacogenomic-based pharmacokinetic analysis as a precision medicine tool in breast cancer management.

Distribution of CYP2D6 and CYP2C19 gene polymorphisms in Han and Uygur populations with breast cancer in Xinjiang, China

The aim of this study was to investigate the frequency distribution of the cytochrome P450 (CYP450) enzymes, CYP2D6 and CYP2C19, and the form of tamoxifen metabolisation in premenopausal patients with breast cancer in the Han and Uygur ethnic groups of Xinjiang to guide rational clinical drug use. A total of 125 Han patients and 121 Uygur patients with premenopausal hormone-receptor-positive breast cancer treated at the Xinjiang Uygur Autonomous Region Cancer Hospital between 1 June 2011 and 1 December 2013 were selected. The common mutation sites in CYP450 were analysed using TaqMan® minor groove binder technology. Genetic testing was performed to determine other metabolic types of tamoxifen, and the genotypes and metabolic types were compared using a Chi-squared test. Between the Han and Uygur groups, there were significant differences in the frequencies of the CYP2D6 (*10/*10) and CYP2C19 (*1/*1) genotypes, with P-values of 0.002 and 0.015, respectively. Genotypes of CYP2D6 (*1/*1), CYP2D6 (*1/*5), CYP2D6 (*5/*5), CYP2D6 (*5/*10) and CYP2C19 (*3/*3) were expressed in the two patient groups, and the difference was not statistically significant (P > 0.05). In the Han patients, the proportions of extensive, intermediate and poor metabolisers of tamoxifen were 72, 24 and 4%, respectively, whereas those in the Uygur patients were 76.9, 17.4 and 5.7%, respectively, with no significant difference (P > 0.05). In conclusion, There were partial differences in the CYP2D6 and CYP2C19 gene polymorphisms of CYP450 between the Han and Uygur patients with premenopausal breast cancer, but there was no significant difference between the CYP2D6 and CYP2C19 phenotypes. Further research is needed to determine the relationship between the enzyme genetic differences of CYP450 and the pharmacokinetics and efficacy of tamoxifen. Although there were some differences in genotypes, these did not result in differences in the predicted tamoxifen metabolisation phenotype between the Han and Uygur patients with breast cancer. Therefore, the doses should be adjusted according to the individual genotype data.

Genetic Testing Enhances the Precision Diagnosis and Treatment of Breast Cancer

The contemporary comprehension of breast cancer has progressed to the molecular level. As a heterogeneous malignancy, conventional pathological diagnosis and histological classification could no longer meet the needs of precisely managing breast cancer. Genetic testing based on gene expression profiles and gene mutations has emerged and substantially contributed to the precise diagnosis and treatment of breast cancer. Multigene assays (MGAs) are explored for early-stage breast cancer patients, aiding the selection of adjuvant therapy and predicting prognosis. For metastatic breast cancer patients, testing specific genes indicates potentially effective antitumor agents. In this review, genetic testing in early-stage and metastatic breast cancer is summarized, as well as the advantages and challenges of genetic testing in breast cancer.

Pharmacogenetics and pharmacokinetics of tamoxifen in a Zimbabwean breast cancer cohort

Tamoxifen is the most used hormonal therapy for oestrogen receptor-positive breast cancer. CYP2D6 is the main enzyme in the metabolic pathway of tamoxifen to endoxifen. Variations in endoxifen plasma concentrations are associated with CYP2D6 polymorphisms. This study aimed to determine the association between the CYP2D6 polymorphisms and endoxifen plasma concentrations in a cohort of Zimbabwean breast cancer patients (n = 40). TaqMan genotyping and copy number assays were done to determine CYP2D6 genotypes. Tamoxifen and metabolites were quantitated using LC-MS/MS. The population had high frequencies of the CYP2D6 reduced function alleles, *17 (15%) and *29 (18%). The median endoxifen concentration was 4.78 ng/mL, and in 55% of the patients, mostly intermediate metabolizers were below the endoxifen therapeutic threshold of 5.97 ng/mL. The CYP2D6 phenotypes and activity scores were significantly associated with endoxifen plasma concentrations (P = 0.0151) and with endoxifen to N-desmethyl-tamoxifen ratios (P = 0.0006).

Adherence to endocrine therapy in early breast cancer in relation to Cytochrome P450 2D6 genotype: a comparison between pharmacy dispensation data and medical records

PURPOSE

Suboptimal adherence to adjuvant endocrine treatment (AET) is an important clinical concern. A correlation between CYP2D6 activity and tamoxifen discontinuation has been described. The main aim of this study was to investigate the consistency between pharmacy dispensation data and medical records on adherence to AET.

METHODS

Adherence was calculated for patients with at least 4.5 years of follow up and was defined as Medical Possession Rate ≥ 80%. Subgroup analyses were performed based on menopausal status, recurrence risk and CYP2D6 activity.

RESULTS

In 86% of the 1235 included patients the consistency between the two sources of information was within 80-125%. Poor consistency, < 80%, was most frequent in the premenopausal/ high-risk group and CYP2D6 Poor Metabolizers (PMs). Among 899 patients with at least 4.5 years follow up, 72% were adherent to tamoxifen based on pharmacy dispensation data, compared with 77% as reported by medical records. When including patients who switched to aromatase inhibitors after tamoxifen, adherence increased to 82% and 88%, respectively. Adherence did not differ by menopausal status or risk for recurrence. CYP2D6 PMs had poorer adherence (54%) to tamoxifen compared to patients with the highest CYP2D6 activity (83%).

CONCLUSIONS

There was a good consistency between medical records and pharmacy dispensing data on the use of AET. Adherence to AET was adequate, especially when including switch to aromatase inhibitors. Surprisingly, CYP2D6 PMs had low adherence to tamoxifen, despite a likely reduced risk of side effects according to previous data.

Short-term changes in ultrasound tomography measures of breast density and treatment-associated endocrine symptoms after tamoxifen therapy

Although breast density decline with tamoxifen therapy is associated with greater therapeutic benefit, limited data suggest that endocrine symptoms may also be associated with improved breast cancer outcomes. However, it is unknown whether endocrine symptoms are associated with reductions in breast density after tamoxifen initiation. We evaluated treatment-associated endocrine symptoms and breast density change among 74 women prescribed tamoxifen in a 12-month longitudinal study. Treatment-associated endocrine symptoms and sound speed measures of breast density, assessed via novel whole breast ultrasound tomography (m/s), were ascertained before tamoxifen (T0) and at 1-3 (T1), 4-6 (T2), and 12 months (T3) after initiation. CYP2D6 status was genotyped, and tamoxifen metabolites were measured at T3. Using multivariable linear regression, we estimated mean change in breast density by treatment-associated endocrine symptoms adjusting for age, race, menopausal status, body mass index, and baseline density. Significant breast density declines were observed in women with treatment-associated endocrine symptoms (mean change (95% confidence interval) at T1:-0.26 m/s (-2.17,1.65); T2:-2.12 m/s (-4.02,-0.22); T3:-3.73 m/s (-5.82,-1.63); p-trend = 0.004), but not among women without symptoms (p-trend = 0.18) (p-interaction = 0.02). Similar declines were observed with increasing symptom frequency (p-trends for no symptoms = 0.91; low/moderate symptoms = 0.03; high symptoms = 0.004). Density declines remained among women with detectable tamoxifen metabolites or intermediate/efficient CYP2D6 metabolizer status. Emergent/worsening endocrine symptoms are associated with significant, early declines in breast density after tamoxifen initiation. Further studies are needed to assess whether these observations predict clinical outcomes. If confirmed, endocrine symptoms may be a proxy for tamoxifen response and useful for patients and providers to encourage adherence.

Design, synthesis, and metabolite identification of Tamoxifen esterase-activatable prodrugs

Tamoxifen (TAM) is used in treatment of hormonal dependent breast cancer, both in premenopausal and postmenopausal women. TAM is intrinsically metabolized by CYP450 enzymes to more active metabolites. Recent reports identified CYP2D6, an enzyme involved in the conversion of TAM to the more potent 4-OH-TAM, is encoded by theCYP2D6gene, which is highly polymorphic. Women with inactive alleles are poor metabolizers; in many cases they suffer acquired TAM resistance. Herein we report synthesis and biological evaluation of novel TAM analogues. The novel analogues are designed to elude CYP2D6 metabolism. Hydrolysis of the carbamate moiety on ring C is mediated via carboxylesterases. Compound 3d [E/Z Benzyl-carbamic acid4-{2-benzyl-1-[4-(2-pyrrolidin-1-yl-ethoxy)-phenyl]-but-1-enyl}-phenyl ester] showed GI₅₀ = 0.09 µM on MCF-7 and GI₅₀ = 1.84 µM on MDA-MB231 cell lines. To further validate our hypothesis, metabolites of selected novel analogues were determined in vitro under different incubation conditions. The hydroxylated analogues were obtained under non CYP2D6 dependent conditions. Compound 8d, a benzyl carbamate derivative, was the least-stable analog and showed the highest rate of metabolism among all tested analogues. Our in silico model showed the novel flexible analogues can still adopt an antiestrogenic binding profile occupying the same pocket as 4-OH-TAM.

Role of Genetic Polymorphisms in Drug-Metabolizing Enzyme-Mediated Toxicity and Pharmacokinetic Resistance to Anti-Cancer Agents: A Review on the Pharmacogenomics Aspect

The inter-individual differences in cancer susceptibility are somehow correlated with the genetic differences that are caused by the polymorphisms. These genetic variations in drug-metabolizing enzymes/drug-inactivating enzymes may negatively or positively affect the pharmacokinetic profile of chemotherapeutic agents that eventually lead to pharmacokinetic resistance and toxicity against anti-cancer drugs. For instance, the CYP1B1*3 allele is associated with CYP1B1 overexpression and consequent resistance to a variety of taxanes and platins, while 496T>G is associated with lower levels of dihydropyrimidine dehydrogenase, which results in severe toxicities related to 5-fluorouracil. In this context, a pharmacogenomics approach can be applied to ascertain the role of the genetic make-up in a person's response to any drug. This approach collectively utilizes pharmacology and genomics to develop effective and safe medications that are devoid of resistance problems. In addition, recently reported genomics studies revealed the impact of many single nucleotide polymorphisms in tumors. These studies emphasized the importance of single nucleotide polymorphisms in drug-metabolizing enzymes on the effect of anti-tumor drugs. In this review, we discuss the pharmacogenomics aspect of polymorphisms in detail to provide an insight into the genetic manipulations in drug-metabolizing enzymes that are responsible for pharmacokinetic resistance or toxicity against well-known anti-cancer drugs. Special emphasis is placed on different deleterious single nucleotide polymorphisms and their effect on pharmacokinetic resistance. The information provided in this report may be beneficial to researchers, especially those who are working in the field of biotechnology and human genetics, in rationally manipulating the genetic information of patients with cancer who are undergoing chemotherapy to avoid the problem of pharmacokinetic resistance/toxicity associated with drug-metabolizing enzymes.

E-Cadherin-Deficient Cells Are Sensitive to the Multikinase Inhibitor Dasatinib

The CDH1 gene, encoding the cell adhesion protein E-cadherin, is one of the most frequently mutated genes in gastric cancer and inactivating germline CDH1 mutations are responsible for the cancer syndrome hereditary diffuse gastric cancer (HDGC). CDH1-deficient gastric cancers exhibit high AKT serine/threonine kinase 3 (AKT3) expression, but specific drugs against this AKT isoform are not available. We therefore used two publicly available datasets to identify AKT3-associated genes which could be used to indirectly target AKT3. Reactome analysis identified an enrichment of extracellular matrix remodelling genes in AKT3-high gastric cancers. Of the 51 genes that were significantly correlated with AKT3 (but not AKT1), discoidin domain receptor tyrosine kinase 2 (DDR2) showed the strongest positive association. Treatment of isogenic human cells and mouse gastric and mammary organoids with dasatinib, a small molecule inhibitor of multiple kinases including SRC, BCR-ABL and DDR2, preferentially slowed the growth and induced apoptosis of E-cadherin-deficient cells. Dasatinib treatment also preferentially slowed the growth of gastric and mammary organoids harbouring both Cdh1 and Tp53 mutations. In organoid models, dasatinib treatment was associated with decreased phosphorylation of total AKT, with a stronger effect seen in Cdh1-deficient organoids. Treatment with combinations of dasatinib and an inhibitor of AKT, MK2206, enhanced the effect of dasatinib in breast MCF10A cells. In conclusion, targeting the DDR2-SRC-AKT3 axis with dasatinib represents a promising approach for the chemoprevention and chemotherapy of gastric and breast cancers lacking E-cadherin.

Untangling SNP Variations within CYP2D6 Gene in Croatian Roma

CYP2D6 is a highly polymorphic gene whose variations affect its enzyme activity. To assess whether the specific population history of Roma, characterized by constant migrations and endogamy, influenced the distribution of alleles and thus phenotypes, the CYP2D6 gene was sequenced using NGS (Next Generation Sequencing) method-targeted sequencing in three groups of Croatian Roma (N = 323) and results were compared to European and Asian populations. Identified single nucleotide polymorphisms (SNPs) were used to reconstruct haplotypes, which were translated into the star-allele nomenclature and later into phenotypes. A total of 43 polymorphic SNPs were identified. The three Roma groups differed significantly in the frequency of alleles of polymorphisms 6769 A > G, 6089 G > A, and 5264 A > G (p < 0.01), as well as in the prevalence of the five most represented star alleles: *1, *2, *4, *10, and *41 (p < 0.0001). Croatian Roma differ from the European and Asian populations in the accumulation of globally rare SNPs (6089 G > A, 4589 C > T, 4622 G > C, 7490 T > C). Our results also show that demographic history influences SNP variations in the Roma population. The three socio-culturally different Roma groups studied differ significantly in the distribution of star alleles, which confirms the importance of a separate study of different Roma groups.

Rapid Reductions in Breast Density following Tamoxifen Therapy as Evaluated by Whole-Breast Ultrasound Tomography

Purpose: Women whose mammographic breast density declines within 12–18 months of initiating tamoxifen for chemoprevention or adjuvant treatment show improved therapeutic responses compared with those whose density is unchanged. We tested whether measuring changes in sound speed (a surrogate of breast density) using ultrasound tomography (UST) could enable rapid identification of favorable responses to tamoxifen. Methods: We evaluated serial density measures at baseline and at 1 to 3, 4 to 6, and 12+ months among 74 women (aged 30–70 years) following initiation of tamoxifen for clinical indications, including an elevated risk of breast cancer (20%) and diagnoses of in situ (39%) or invasive (40%) breast carcinoma, enrolled at Karmanos Cancer Institute and Henry Ford Health System (Detroit, MI, USA). For comparison, we evaluated an untreated group with screen negative mammography and frequency-matched on age, race, and menopausal status (n = 150), at baseline and 12 months. Paired t-tests were used to assess differences in UST sound speed over time and between tamoxifen-treated and untreated patients. Results: Sound speed declined steadily over the 12 month period among patients receiving tamoxifen (mean (SD): −3.0 (8.2) m/s; p = 0.001), whereas density remained unchanged in the untreated group (mean (SD): 0.4 (7.1) m/s; p = 0.75 (relative change between groups: p = 0.0009)). In the tamoxifen group, we observed significant sound speed reductions as early as 4–6 months after tamoxifen initiation (mean (SD): −2.1 (6.8) m/s; p = 0.008). Sound speed reductions were greatest among premenopausal patients (P-interaction = 0.0002) and those in the middle and upper tertiles of baseline sound speed (P-interaction = 0.002). Conclusions: UST can image rapid declines in sound speed following initiation of tamoxifen. Given that sound speed and mammographic density are correlated, we propose that UST breast imaging may capture early responses to tamoxifen, which in turn may have utility in predicting therapeutic efficacy.

Comparison of adverse drug reactions between tamoxifen and toremifene in breast cancer patients with different CYP2D6 genotypes: a propensity-score matched cohort study

CYP2D6 gene polymorphism had a profound impact upon the effect of tamoxifen as adjuvant endocrine therapy in breast cancers. However, it had never been reported whether the adverse drug reactions vary by CYP2D6 metabolic status for patients treated with tamoxifen or toremifene. We conducted an retrospective study in breast cancer patients to investigate the impact of CYP2D6 metabolizers on liver dysfunction events, gynecological events, and dyslipidemia events. According to CYP2D6*10 (100C → T) genotype, the enrolled patients were further categorized into four cohorts (extensive metabolizers taking tamoxifen [EM + TAM], extensive metabolizers taking toremifene [EM + TOR], intermediate metabolizers taking tamoxifen [IM + TAM], intermediate metabolizers taking toremifene cohort [IM + TOR]). A total of 192 patients were included into the study, with a median follow-up time of 26.2 months. In EM + TAM cohort, the risks of liver dysfunction events (P = 0.004) and gynecological events (P = 0.004) were significantly higher compared with EM + TOR cohort. In IM + TAM cohort, the risks of liver dysfunction events (P = 0.14) and gynecological events (P = 0.99) were not significantly different from IM + TOR cohort. Significant decrease of total cholesterol was observed in EM + TAM cohort around 1 year after taking tamoxifen (P < 0.001). Significant interactions between CYP2D6 metabolic status and endocrine agents were observed in terms of liver dysfunction events (p-interaction = 0.007) and gynecological events (p-interaction = 0.026). These findings suggested that CYP2D6 gene polymorphism played a significant role in predicting liver dysfunction, gynecological diseases and lipid metabolism changes among patients taking tamoxifen or toremifene. This article is protected by copyright. All rights reserved.