Genetic variants of CYP3A5, CYP2D6, SULT1A1, UGT2B15 and tamoxifen response in postmenopausal patients with breast cancer

Induction of UDP-glucuronosyltransferase 2B15 gene expression by the major active metabolites of tamoxifen, 4-hydroxytamoxifen and endoxifen, in breast cancer cells

We previously reported upregulation of UGT2B15 by 17β-estradiol in breast cancer MCF7 cells via binding of the estrogen receptor α (ERα) to an estrogen response unit (ERU) in the proximal UGT2B15 promoter. In the present study, we show that this ERα-mediated upregulation was significantly reduced by two ER antagonists (fulvestrant and raloxifene) but was not affected by a third ER antagonist, 4-hydroxytamoxifen (4-OHTAM), a major active tamoxifen (TAM) metabolite. Furthermore, we found that, similar to 17β-estradiol, 4-OHTAM and endoxifen (another major active TAM metabolite) elevated UGT2B15 mRNA levels, and that this stimulation was significantly abrogated by fulvestrant. Further experiments using 4-OHTAM revealed a critical role for ERα in this regulation. Specifically; knockdown of ERα expression by anti-ERα small interfering RNA reduced the 4-OHTAM-mediated induction of UGT2B15 expression; 4-OHTAM activated the wild-type but not the ERU-mutated UGT2B15 promoter; and chromatin immunoprecipitation assays showed increased ERα occupancy at the UGT2B15 ERU in MCF7 cells upon exposure to 4-OHTAM. Together, these data indicate that both 17β-estradiol and the antiestrogen 4-OHTAM upregulate UGT2B15 in MCF7 cells via the same ERα-signaling pathway. This is consistent with previous observations that both 17β-estradiol and TAM upregulate a common set of genes in MCF7 cells via the ER-signaling pathway. As 4-OHTAM is a UGT2B15 substrate, the upregulation of UGT2B15 by 4-OHTAM in target breast cancer cells is likely to enhance local metabolism and inactivation of 4-OHTAM within the tumor. This represents a potential mechanism that may reduce TAM therapeutic efficacy or even contribute to the development of acquired TAM resistance.

Pharmacogenomics toward personalized tamoxifen therapy for breast cancer

Tamoxifen has been used not only for the treatment or prevention of recurrence in patients with estrogen receptor positive breast cancers but also for recurrent breast cancer. Because CYP2D6 is known to be an important enzyme responsible for the generation of the potent tamoxifen metabolite, ‘endoxifen’, lots of studies reported that genetic variation which reduced its enzyme activity were associated with poor clinical outcome of breast cancer patients treated with tamoxifen. However, there are some discrepant reports questioning the association between CYP2D6 genotype and clinical outcome after tamoxifen therapy. Dose-adjustment study of tamoxifen based on CYP2D6 genotypes provides the evidence that dose adjustment is useful for the patients carrying reduced or null allele of CYP2D6 to maintain the effective endoxifen level. This review describes critical issues in pharmacogenomic studies as well as summarizes the results of the association of CYP2D6 genotype with tamoxifen efficacy.

Pharmacogenetics and breast cancer management: current status and perspectives

INTRODUCTION

Breast cancer has benefited from a number of innovative therapeutics over the last decade. Cytotoxics, hormone therapy, targeted therapies and biologics can now be given to ensure optimal management of patients. As life expectancy of breast cancer patients has been significantly stretched and that several lines of treatment are now made available, determining the best drug or drug combinations to be primarily given and the best dosing and scheduling for each patient is critical for ensuring an optimal toxicity/efficacy balance.

AREAS COVERED

Defining patient's characteristics at the tumor level (pharmacogenomics) and the constitutional level (pharmacogenetics) is a rising trend in oncology. This review covers the latest strategies based upon the search of relevant biomarkers for efficacy, resistance and toxicity to be undertaken at the bedside to shift towards precision medicine in breast cancer patients.

EXPERT OPINION

In the expanding era of bioguided medicine, identifying relevant and clinically validated biomarkers from the plethora of published material remains an uneasy task. Sorting the variety of genetic and molecular markers that have been investigated over the last decade on their level of evidence and addressing the issue of drug exposure should help to improve the management of breast cancer therapy.

Association of SULT1A1 Arg²¹³His polymorphism with male breast cancer risk: results from a multicenter study in Italy

Male breast cancer (MBC) is rare and poorly understood. Like female breast cancer (FBC), MBCs are highly sensitive to hormonal changes, and hyperestrogenism, specifically, represents a major risk factor for MBC. MBC is considered similar to late-onset, post-menopausal estrogen/progesteron receptors positive FBC (ER+/PR+). Sulfotransferase 1A1 (SULT1A1) is an enzyme involved in the metabolism of estrogens. Recently, SULT1A1 common functional polymorphism Arg(213)His (638G>A) variant has been found to be associated with increased breast cancer (BC) risk, particularly in post-menopausal women. For this reason, we decided to explore whether SULT1A1 Arg(213)His could exert an effect on MBC development. The primary aim of this study was to evaluate the influence of the SULT1A1 Arg(213)His polymorphism on MBC risk. The secondary aim was to investigate possible associations with relevant clinical-pathologic features of MBC. A total of 394 MBC cases and 786 healthy male controls were genotyped for SULT1A1 Arg(213)His polymorphism by PCR-RFLP and high-resolution melting analysis. All MBC cases were characterized for relevant clinical-pathologic features. A significant difference in the distribution of SULT1A1 Arg(213)His genotypes was found between MBC cases and controls (P < 0.0001). The analysis of genotype-specific risk showed a significant increased MBC risk in individuals with G/A (OR 1.97, 95% CI 1.50-2.59; P < 0.0001) and A/A (OR 3.09, 95% CI 1.83-5.23; P < 0.0001) genotypes in comparison to wild-type genotype, under co-dominant model. A significant association between SULT1A1 risk genotypes and HER2 status emerged. Results indicate that SULT1A1 Arg(213)His may act as a low-penetrance risk allele for developing MBC and could be associated with a specific tumor subtype associated with HER2 overexpression.

Pharmacogenetics of SULT1A1

Cytosolic SULT1A1 participates in the bioconversion of a plethora of endogenous and xenobiotic substances. Genetic variation in this important enzyme such as SNPs can vary by ethnicity and have functional consequences on its activity. Most SULT1A1 genetic variability studies have been centered on the SULT1A1*1/2 SNP. Highlighted here are not only this SNP, but other genetic variants associated with SULT1A1 that could modify drug efficacy and xenobiotic metabolism. Some studies have investigated how differential metabolism of xenobiotic substances influences susceptibility to or protection from cancer in multiple sites. This review will focus primarily on the impact of SULT1A1 genetic variation on the response to anticancer therapeutic agents and subsequently how it relates to environmental and dietary exposure to both cancer-causing and cancer-preventative compounds.

Impact of CYP2D*6 in the adjuvant treatment of breast cancer patients with tamoxifen

Biotransformation of tamoxifen to the potent antiestrogen endoxifen is performed by cytochrome P450 (CYP) enzymes, in particular the CYP2D6 isoform. CYP2D6*4 is one of the most frequent alleles associated with loss of enzymatic activity. The incidence of CYP2D6*4 among Caucasians is estimated up to 27%, while it is present in up to 90% of all poor metabolizers within the Caucasian population. The hypothesis under question is whether the presence of one or two non-functioning (null) alleles predicts an inferior outcome in postmenopausal women with breast cancer receiving adjuvant treatment with tamoxifen. The numerous existing studies investigating the association of CYP2D6 with treatment failure in breast cancer are inconsistent and give rather conflicting results. Currently, routine CYP2D6 testing among women with breast cancer is not recommended and the significance of CYP2D6 phenotype in decision making regarding the administration of tamoxifen is unclear. The present study summarizes current literature regarding clinical studies on CYP2D6*4, particularly in terms of response to tamoxifen therapy and breast cancer outcome.

Tamoxifen resistance in breast cancer

Tamoxifen is a central component of the treatment of estrogen receptor (ER)-positive breast cancer as a partial agonist of ER. It has been clinically used for the last 30 years and is currently available as a chemopreventive agent in women with high risk for breast cancer. The most challenging issue with tamoxifen use is the development of resistance in an initially responsive breast tumor. This review summarizes the roles of ER as the therapeutic target of tamoxifen in cancer treatment, clinical values and issues of tamoxifen use, and molecular mechanisms of tamoxifen resistance. Emerging knowledge on the molecular mechanisms of tamoxifen resistance will provide insight into the design of regimens to overcome tamoxifen resistance and discovery of novel therapeutic agents with a decreased chance of developing resistance as well as establishing more efficient treatment strategies.

The effects of a novel hormonal breast cancer therapy, endoxifen, on the mouse skeleton

Endoxifen has recently been identified as the predominant active metabolite of tamoxifen and is currently being developed as a novel hormonal therapy for the treatment of endocrine sensitive breast cancer. Based on past studies in breast cancer cells and model systems, endoxifen classically functions as an anti-estrogenic compound. Since estrogen and estrogen receptors play critical roles in mediating bone homeostasis, and endoxifen is currently being implemented as a novel breast cancer therapy, we sought to comprehensively characterize the in vivo effects of endoxifen on the mouse skeleton. Two month old ovariectomized C57BL/6 mice were treated with vehicle or 50 mg/kg/day endoxifen hydrochloride via oral gavage for 45 days. Animals were analyzed by dual-energy x-ray absorptiometry, peripheral quantitative computed tomography, micro-computed tomography and histomorphometry. Serum from control and endoxifen treated mice was evaluated for bone resorption and bone formation markers. Gene expression changes were monitored in osteoblasts, osteoclasts and the cortical shells of long bones from endoxifen treated mice and in a human fetal osteoblast cell line. Endoxifen treatment led to significantly higher bone mineral density and bone mineral content throughout the skeleton relative to control animals. Endoxifen treatment also resulted in increased numbers of osteoblasts and osteoclasts per tissue area, which was corroborated by increased serum levels of bone formation and resorption markers. Finally, endoxifen induced the expression of osteoblast, osteoclast and osteocyte marker genes. These studies are the first to examine the in vivo and in vitro impacts of endoxifen on bone and our results demonstrate that endoxifen increases cancellous as well as cortical bone mass in ovariectomized mice, effects that may have implications for postmenopausal breast cancer patients.

Functional characterization of wild-type and 49 CYP2D6 allelic variants for N-desmethyltamoxifen 4-hydroxylation activity

Genetic variations in cytochrome P450 2D6 (CYP2D6) contribute to interindividual variability in the metabolism of clinically used drugs, e.g., tamoxifen. CYP2D6 is genetically polymorphic and is associated with large interindividual variations in therapeutic efficacy and drug toxicity. In this study, we performed an in vitro analysis of 50 allelic variants of CYP2D6 proteins. Wild-type CYP2D6.1 and 49 variants were transiently expressed in COS-7 cells, and the enzymatic activities of the CYP2D6 variants were characterized using N-desmethyltamoxifen as a substrate. The kinetic parameters K(m), V(max), and intrinsic clearance (V(max)/K(m)) of N-desmethyltamoxifen 4-hydroxylation were determined. Among the 50 CYP2D6 variants, the kinetic parameters for N-desmethyltamoxifen 4-hydroxylation were determined for 20 CYP2D6 variants. On the other hand, the kinetic parameters of 30 CYP2D6 variants could not be determined because the amount of metabolite produced was at or below the detection limit at the lower substrate concentrations. Among them, 8 variants, i.e., CYP2D6.2, .9, .26, .28, .32, .43, .45, and .70, showed decreased intrinsic clearance at <50% of CYP2D6.1. The comprehensive in vitro assessment of CYP2D6 variants provides novel insights into allele-specific activity towards tamoxifen and may be valuable when interpreting in vivo studies.

No association found between CYP2D6 genotype and early breast cancer events in tamoxifen-treated patients

BACKGROUND

CYP2D6 is considered the key enzyme in tamoxifen metabolism. Several studies have investigated the relationship between the CYP2D6 genotype and tamoxifen treatment outcome, with discrepant results. CYP2D6 inhibitor use, aromatase inhibitor use, and chemotherapy may account for some of the discrepancies. We examined the association between CYP2D6 genotype and early breast cancer events in tamoxifen-treated breast cancer patients, in relation to CYP2D6 inhibitor use, aromatase inhibitor use, and chemotherapy.

MATERIAL AND METHODS

Pre- and postoperative questionnaires on lifestyle and concomitant medications were completed by 634 primary breast cancer patients between 2002 and 2008, among whom 333 patients had ER-positive tumors and received tamoxifen. CYP2D6*3, *4, *6, *10 and *41 were genotyped. Information on clinical data, breast cancer events, and tumor characteristics was obtained from patients' charts, population registries, the Regional Tumor Registry, and pathology reports.

RESULTS

Median follow-up was 4.9 years. Neither poor metabolizers (adjusted HR 0.50; 95% CI 0.07-3.82) nor intermediate metabolizers (adjusted HR 1.00; 95% CI 0.47-2.11) had an increased risk of early breast cancer events when compared with extensive metabolizers. CYP2D6 activity score (taking into account genotype and CYP2D6 inhibitor use) was not associated with early breast cancer events (LogRank, Ptrend = 0.44).

CONCLUSIONS

CYP2D6 genotype was not associated with tamoxifen treatment outcome, even when CYP2D6 inhibitor use, aromatase inhibitor use, or chemotherapy was taken into account. CYP2D6 genotype may be of minor importance for tamoxifen-treated patients in Scandinavia.

Metabolism and transport of tamoxifen in relation to its effectiveness: new perspectives on an ongoing controversy

Tamoxifen reduces the rate of breast cancer recurrence by approximately a half. Tamoxifen is metabolized to more active metabolites by enzymes encoded by polymorphic genes, including cytochrome P450 2D6 (CYP2D6). Tamoxifen is a substrate for ATP-binding cassette transporter proteins. We review tamoxifen's clinical pharmacology and use meta-analyses to evaluate the clinical epidemiology studies conducted to date on the association between CYP2D6 inhibition and tamoxifen effectiveness. Our findings indicate that the effect of both drug-induced and/or gene-induced inhibition of CYP2D6 activity is likely to be null or small, or at most moderate in subjects carrying two reduced function alleles. Future research should examine the effect of polymorphisms in genes encoding enzymes in tamoxifen's complete metabolic pathway, should comprehensively evaluate other biomarkers that affect tamoxifen effectiveness, such as the transport enzymes, and focus on subgroups of patients, such as premenopausal breast cancer patients, for whom tamoxifen is the only guideline endocrine therapy.

Association between CYP2D6 genotypes and the clinical outcomes of adjuvant tamoxifen for breast cancer: a meta-analysis

Aim: Tamoxifen is one of the most commonly used endocrine therapeutic agents for breast cancer. Although many studies have examined whether the treatment outcomes of tamoxifen for breast cancer differ according to CYP2D6 genotype, the study results have been inconsistent, and the role of CYP2D6 in the prediction of patient outcomes from tamoxifen therapy remains controversial. This study evaluated the association between CYP2D6 genotypes and postoperative tamoxifen treatment outcome in patients with breast cancer, using the available previous study results. Materials & methods: We performed a meta-analysis of ten previous clinical reports (n = 5183) to evaluate the association between CYP2D6 genotype and hazard ratios for the recurrence risk of breast cancer after postoperative tamoxifen treatment. Pooled estimates of hazard ratios were computed using R and NONMEM® software. Results: A significantly increased risk of breast cancer recurrence in patients carrying variant CYP2D6 genotypes was found in this investigation. The mean hazard ratios and 95% CI were 1.60 (1.04–2.47) in the random effect model implemented in R and 1.63 (1.01–2.62) in the random effect model in NONMEM. The bootstrap result (2000 replicates) of NONMEM was 1.64 (1.07–2.79). Conclusion: Our present findings suggest that genetic polymorphisms of CYP2D6 may be important predictors of the clinical outcomes of adjuvant tamoxifen treatment for the patients with breast cancer. A large-scale, prospective, randomized, well-controlled trial is warranted to confirm our findings.

Original submitted 23 July 2013; Revision submitted 30 September 2013

Impact of metabolizing enzymes on drug response of endocrine therapy in breast cancer

Estrogen-receptor positive breast cancer accounts for 75% of diagnosed breast cancers worldwide. There are currently two major options for adjuvant treatment: tamoxifen and aromatase inhibitors. Variability in metabolizing enzymes determines their pharmacokinetic profile, possibly affecting treatment response. Therefore, prediction of therapy outcome based on genotypes would enable a more personalized medicine approach, providing optimal therapy for each patient. In this review, the authors will discuss the current evidence on the most important metabolizing enzymes in endocrine therapy, with a special focus on CYP2D6 and its role in tamoxifen metabolism.

CYP2D6 genotype and tamoxifen response for breast cancer: a systematic review and meta-analysis

OBJECTIVE

To evaluate evidence on the association between CYP2D6 genotype and tamoxifen response through.

DESIGN

Systematic review and meta-analysis of prospective, cross-sectional and case-control studies published to 2012. For each study, relative risks and 95% confidence intervals were extracted and pooled with a fixed and random effects model. Heterogeneity, publication bias, subgroup, and meta-regression analyses were performed.

DATA SOURCES

PubMed (inception-2012) and EMBASE (inception-2012).

ELIGIBILITY CRITERIA FOR SELECTING STUDIES

Criteria for inclusion were studies reporting breast cancer outcomes in patients treated with tamoxifen and genotyped for polymorphisms in the CYP2D6 gene.

RESULTS

Twenty-five studies of 13,629 individuals were identified, of which 22 investigated the association of CYP2D6 genotype with outcomes in breast cancer women all receiving tamoxifen treatment ("treatment-only" design). Three randomized trials evaluated the effect of CYP2D6 genotype on tamoxifen response ("effect modification" design). In analysis of treatment-only studies, the relative risk (RR) of all-cause mortality (>307 events in 4,936 patients) for carriers of a CYP2D6 reduced function allele was 1.11 (95% confidence interval (CI): 0.94 to 1.31) compared to individuals with normal/increased function CYP2D6 alleles. When we investigated a composite outcome including all-cause mortality and surrogate endpoints for overall survival (>307 events in 6,721 patients), carriers of a CYP2D6 reduced function allele had a RR of 1.27 (95% CI: 1.11 to 1.45). From two randomized trials that permitted effect-modification analysis, one had only 154 patients and showed evidence of effect modification of tamoxifen by CYP2D6 genotype for distant recurrence but was directionally opposite to that predicted, whereas a larger trial of 2,537 patients failed to show evidence of effect modification for breast cancer-free interval (P values for interaction 0.02 and 0.44, respectively).

CONCLUSIONS

Based on these findings, there is insufficient evidence to recommend CYP2D6 genotyping to guide tamoxifen treatment.

Sulfotransferase genetic variation: from cancer risk to treatment response

Cytosolic sulfotransferases (SULTs) are phase II detoxification enzymes that are involved in the biotransformation of a wide variety of structurally diverse endo- and xenobiotics. Single-nucleotide polymorphisms (SNPs) in SULTs can alter the phenotype of the translated proteins. SNPs in some SULTs are fairly uncommon in the population, but some, most notably for SULT isoform 1A1, are commonly found and have been associated with cancer risk for a variety of tumor sites and also with response to therapeutic agents. SNPs in many SULTs vary by ethnicity, another factor that could influence SULT-associated disease risk and pharmacogenetics. This review surveys the current knowledge of SULT genetic variability in relation to cancer risk and response to therapy, focusing primarily on SULT1A1.

Phase two steroid metabolism and its roles in breast and prostate cancer patients

Breast and prostate cancer are diseases in which steroids and steroid metabolism could markedly influence clinical outcomes for patients. In both malignancies the modification of ketone and hydroxyl groups attached to the steroid backbone (phase one metabolism) has been examined in detail but the conjugation reactions (phase two metabolism) have not been extensively studied. Therefore, in this review we aim to summarize phase two metabolism in breast and prostate cancers from a number of perspectives, including the impact of variation in serum levels of conjugated steroids, tissue, and pathology specific expression of phase two enzymes, and consequences of genetic variations of these conjugation enzymes. In addition to this biological perspective, we will also address current pharmacological efforts to manipulate phase two metabolism as a potential therapy for hormone dependent cancers, including clinical trials of STS inhibitors and preclinical STS inhibitor development. While this review is not intended to cover any one particular area in great technical depth, it is intended as an introduction to and/or update on the importance of variance in phase two metabolic pathways in breast and prostate cancers.

Modification of the anticancer drug tamoxifen to avoid CYP2D6 polymorphism

The prodrug tamoxifen (TAM) is the most widely used drug to treat breast cancer, and is metabolised to the active 4-hydroxy derivatives dominantly by hepatic CYP2D6. However, the application to patients with different polymorphic CYP2D6 has been under debate, because the efficacy of TAM is suspected to be suppressed in patients who have diminished CYP2D6 activity, resulting in inadequate active metabolites. We here propose modified structures, such as 4-methylTAM, which is highly possible to be activated by CYP3A, the most abundant CYP isoforms in the liver, whereby the genetic polymorphism of CYP2D6 is avoided. The diversity of CYP catalyzed metabolic paths for TAM and its derivatives are studied by quantum chemistry calculations on the reaction energies of the initial H atom abstraction steps. The ability of forming DNA adducts is compared through the formation enthalpy of the carbocation intermediate. The results suggest that the modified structures are safe with regard to forming DNA adducts and may be used as prodrugs in a wide range of patients, due to CYP3A, rather than CYP2D6, mediated activation.

CYP2D6 polymorphisms influence tamoxifen treatment outcomes in breast cancer patients: a meta-analysis

PURPOSE

To evaluate whether breast cancer (BC) patients with CYP2D6 gene variation have different clinical tamoxifen (TAM) treatment outcomes to those with normal function of CYP2D6.

METHODS

Systematic searches of the PubMed up to February 21, 2013, were retrieved. The study end points were disease-free survival (DFS) and overall survival (OS). Fixed or random-effects meta-analytical models were used to calculate summary hazard ratio (HR) and corresponding 95 % confidence intervals (CIs). Meta-regression, Galbraith plots, subgroup analysis, and sensitivity analysis were also performed.

RESULTS

A total of 11,701 BC patients from 20 trials were included. Compared with reduced CYP2D6 function, normal function was associated with a trend toward improved DFS (HR = 1.37, 95 % CI 1.12-1.69, P = 0.002) and OS (HR = 1.25, 95 % CI 1.03-1.50, P = 0.021). We found significant heterogeneity between studies. When the analysis was stratified into subgroups, significantly worse DFS was found in the groups of intermediate metabolizer versus extensive metabolizer (HR = 1.65, 95 % CI 1.04-2.64, P = 0.035), Asian population (HR = 3.29, 95 % CI 1.64-6.63, P = 0.001), 5 years TAM treatment duration (HR = 1.59; 95 % CI 1.14-2.22, P = 0.006), concomitant chemotherapy (HR = 1.35, 95 % CI 1.04-1.76, P = 0.025), and TAM alone (HR = 1.44, 95 % CI 1.44-2.06, P = 0.045). With respect to OS, no significant association was demonstrated in stratified analyses.

CONCLUSIONS

We concluded that CYP2D6 polymorphisms may influence tamoxifen treatment outcomes of DFS in BC patients.

Association of CYP2D6 and CYP2C19 polymorphisms and disease-free survival of Thai post-menopausal breast cancer patients who received adjuvant tamoxifen

PURPOSE

To investigate the impact of CYP2D6 and CYP2C19 polymorphisms in predicting tamoxifen efficacy and clinical outcomes in Thai breast cancer patients.

METHODS

Polymorphisms of CYP2D6 and CYP2C19 were genotyped by the AmpliChip™ CYP450 Test (Roche Molecular Diagnostics, Branchburg, NJ, USA) for 57 patients, who were matched as recurrent versus non-recurrent breast cancers (n = 33 versus n = 24, respectively, with a 5-year follow-up).

RESULTS

Based on the genotype data, five CYP2D6 predicted phenotype groups were identified in this study including homozygous extensive metabolizer (13 of 57, 22.80%), extensive/intermediate metabolizer (23 of 57, 40.40%), extensive/poor metabolizer (3 of 57, 5.30%), homozygous intermediate metabolizer (14 of 57, 24.50%), and intermediate/poor metabolizer (4 of 57, 7.00%), and three CYP2C19 genotype groups including homozygous extensive metabolizer (27 of 57, 47.40%), extensive/intermediate metabolizer (27 of 57, 47.40%), and homozygous poor metabolizer (3 of 57, 5.30%). The CYP2D6 variant alleles were *10 (52 of 114, 45.60%), *5 (5 of 114, 4.40%), *41 (2 of 114, 1.80%), *4 (1 of 114, 0.90%), and *36 (1 of 114, 0.90%); the CYP2C19 variant alleles were *2 (27 of 114, 23.70%) and *3 (6 of 114, 5.30%). Kaplan-Meier estimates showed significantly shorter disease-free survival in patients with homozygous TT when compared to those with heterozygous CT or homozygous CC at nucleotides 100C>T and 1039C>T (CYP2D6*10) post-menopausal (log-rank test; P = 0.046). They also had increased risk of recurrence, but no statistically significant association was observed (hazard ratio 3.48; 95% confidence interval 0.86-14.07; P = 0.080).

CONCLUSION

The CYP2D6 and CYP2C19 polymorphisms were not involved in tamoxifen efficacy. However, in the subgroup of post-menopausal women, the polymorphisms in CYP2D6 and CYP2C19 might be useful in predicting tamoxifen efficacy and clinical outcomes in breast cancer patients receiving adjuvant tamoxifen treatment. As the number of breast cancer patients was relatively small in this study, results should be confirmed in a larger group of prospective patients.

Important and critical scientific aspects in pharmacogenomics analysis: lessons from controversial results of tamoxifen and CYP2D6 studies

Tamoxifen contributes to decreased recurrence and mortality of patients with hormone receptor-positive breast cancer. As this drug is metabolized by phase I and phase II enzymes, the interindividual variations of their enzymatic activity are thought to be associated with individual responses to tamoxifen. Among these enzymes, CYP2D6 is considered to be a rate-limiting enzyme in the generation of endoxifen, a principal active metabolite of tamoxifen, and the genetic polymorphisms of CYP2D6 have been extensively investigated in association with the plasma endoxifen concentrations and clinical outcome of tamoxifen therapy. In addition to CYP2D6, other genetic factors including polymorphisms in various drug-metabolizing enzymes and drug transporters have been implicated to their relations to clinical outcome of tamoxifen therapy, but their effects would be small. Although the results of association studies are controversial, accumulation of the evidence has revealed us the important and critical issues in the tamoxifen pharmacogenomics study, namely the quality of genotyping, the coverage of genetic variations, the criteria for sample collection and the source of DNAs, which are considered to be common problematic issues in pharmacogenomics studies. This review points out common critical issues in pharmacogenomics studies through the lessons we have learned from tamoxifen pharmacogenomics, as well as summarizes the results of pharmacogenomics studies for tamoxifen treatment.

CYP3A4 and seasonal variation in vitamin D status in addition to CYP2D6 contribute to therapeutic endoxifen level during tamoxifen therapy

Tamoxifen is a widely utilized adjuvant anti-estrogen agent for hormone receptor-positive breast cancer, known to undergo CYP2D6-mediated bioactivation to endoxifen. However, little is known regarding additional genetic and non-genetic determinants of optimal endoxifen plasma concentration. Therefore, 196 breast cancer patients on tamoxifen were enrolled in this prospective study over a 24-month period. Blood samples were collected for pharmacogenetic and drug-level analysis of tamoxifen and metabolites. Regression analysis indicated that besides CYP2D6, the recently described CYP3A4*22 genotype, seasonal variation, and concomitant use of CYP2D6-inhibiting antidepressants were significant predictors of endoxifen concentration. Of note, genetic variation explained 33 % of the variability while non-genetic variables accounted for 13 %. Given the proposed notion of a sub-therapeutic endoxifen concentration for predicting breast cancer recurrence, we set the therapeutic threshold at 18 nM, the 20th percentile for endoxifen level among enrolled patients in this cohort. Nearly 70 % of CYP2D6 poor metabolizers as well as extensive metabolizers on potent CYP2D6-inhibiting antidepressants exhibited endoxifen levels below 18 nM, while carriers of CYP3A4*22 were twofold less likely to be in sub-therapeutic range. Unexpectedly, endoxifen levels were 20 % lower during winter months than mean levels across seasons, which was also associated with lower vitamin D levels. CYP3A4*22 genotype along with sunshine exposure and vitamin D status may be unappreciated contributors of tamoxifen efficacy. The identified covariates along with demographic variables were integrated to create an endoxifen concentration prediction algorithm to pre-emptively evaluate the likelihood of individual patients falling below the optimal endoxifen concentration.

Variants in tamoxifen metabolizing genes: a case-control study of contralateral breast cancer risk in the WECARE study

Tamoxifen has been shown to greatly reduce risk of recurrence and contralateral breast cancer (CBC). Still, second primary contralateral breast cancer is the most common malignancy to follow a first primary breast cancer. Genetic variants in CYP2D6 and other drug-metabolizing enzymes that alter the metabolism of tamoxifen may be associated with CBC risk in women who receive the drug. This is the first study to investigate the impact of this variation on risk of CBC in women who receive tamoxifen. From the population-based Women's Environment Cancer and Radiation Epidemiology (WECARE) Study, we included 624 Caucasian women with CBC (cases) and 1,199 women with unilateral breast cancer (controls) with complete information on tumor characteristics and treatment. Conditional logistic regression was used to assess the risk of CBC associated with 112 single nucleotide polymorphisms (SNPs) in 8 genes involved in the metabolism of tamoxifen among tamoxifen users and non-users. After adjustment for multiple testing, no significant association was observed between any of the genotyped variants and CBC risk in either tamoxifen users or non-users. These results suggest that when using a tagSNP approach, common variants in selected genes involved in the metabolism of tamoxifen are not associated with risk of CBC among women treated with the drug.

Functional polymorphisms in xenobiotic metabolizing enzymes and their impact on the therapy of breast cancer

Breast cancer is the top cancer among women, and its incidence is increasing worldwide. Although the mortality tends to decrease due to early detection and treatment, there is great variability in the rates of clinical response and survival, which makes breast cancer one of the most appealing targets for pharmacogenomic studies. The recognition that functional CYP2D6 polymorphisms affect tamoxifen pharmacokinetics has motivated the attempts of using CYP2D6 genotyping for predicting breast cancer outcomes. In addition to tamoxifen, the chemotherapy of breast cancer includes combinations of cytotoxic drugs, which are substrates for various xenobiotic metabolizing enzymes. Because of these drugs’ narrow therapeutic window, it has been postulated that impaired biotransformation could lead to increased toxicity. In the present review, we performed a systematic search of all published data exploring associations between polymorphisms in xenobiotic metabolizing enzymes and clinical outcomes of breast cancer. We retrieved 43 original articles involving either tamoxifen or other chemotherapeutic protocols, and compiled all information regarding response or toxicity. The data indicate that, although CYP2D6 polymorphisms can indeed modify tamoxifen pharmacokinetics, CYP2D6 genotyping alone is not enough for predicting breast cancer outcomes. The studies involving other chemotherapeutic protocols explored a great diversity of pharmacogenetic targets, but the number of studies for each functional polymorphism is still very limited, with usually no confirmation of positive associations. In conclusion, the application of pharmacogenetics to predict breast cancer outcomes and to select one individual’s chemotherapeutic protocol is still far from clinical routine. Although some very interesting results have been produced, no clear practical recommendations are recognized yet.

Frequencies of Functional Polymorphisms in Three Pharmacokinetic Genes of Clinical Interest within the Admixed Puerto Rican Population

OBJECTIVE

This cross-sectional study was aimed at determining the allele frequencies for the CYP2C19*2, CYP2C19*3, CYP2D6*10 and PON1 (rs662) polymorphisms in the Puerto Rican population. The CYP2C19, CYP2D6 and PON1 genes are known to be associated with functional changes in drug metabolism and activation. Individuals carrying the aforementioned polymorphisms are at a higher risk of suffering from drug-induced adverse events and/ or unresponsiveness from a variety of drugs that includes antidepressants, atypical antipsychotics and antiplatelet compounds. Information on the frequency of these polymorphisms is more commonly found on homogeneous populations, but is scarce in highly heterogeneous populations like Hispanics, as in the case of Puerto Ricans.

METHOD

Genotyping was carried out in 100 genomic DNA samples from dried blood spots supplied by the Puerto Rican Newborn Screening program using Taqman® Genotyping Assays.

RESULTS

The Minor Allele Frequencies (MAF) obtained were 9% for CYP2C19*2 and CYP2D6*10, 50% for PON1 (rs662), while the CYP2C19*3 variant was not detected in our study. Furthermore, Hardy Weinberg equilibrium analysis was assessed as well as a comparison between Puerto Rico and other reference populations using a Z-test for proportions.

CONCLUSION

The observed allele and genotype frequencies on these relevant pharmacogenes in Puerto Ricans were more closely related to those early reported in two other reference populations of Americans (Mexicans and Colombians).

The use of the 13C-dextromethorphan breath test for phenotyping CYP2D6 in breast cancer patients using tamoxifen: association with CYP2D6 genotype and serum endoxifen levels

PURPOSE

Adjuvant therapy with tamoxifen significantly reduces breast cancer recurrence and mortality in estrogen receptor positive disease. CYP2D6 is the main enzyme involved in the activation of the prodrug tamoxifen into the anti-estrogen endoxifen. Endoxifen is thought to be a main determinant for clinical efficacy in breast cancer patients using tamoxifen. As the large interindividual variation in endoxifen levels is only partly explained by CYP2D6 genotype, we explored the use of the (13)C-dextromethorphan breath test (DM-BT) for phenotyping CYP2D6 and to predict serum steady-state endoxifen levels as a marker for clinical outcome in breast cancer patients using tamoxifen.

METHODS

In 65 patients with early breast cancer using tamoxifen, CYP2D6 phenotype was assessed by DM-BT. CYP2D6 genotype using Amplichip and serum steady-state levels of endoxifen were determined. Genotype was translated into the gene activity score and into ultrarapid, extensive, heterozygous extensive, intermediate or poor metabolizer CYP2D6 predicted phenotype.

RESULTS

CYP2D6 phenotype determined by the DM-BT explained variation in serum steady-state endoxifen levels for 47.5% (R(2) = 0.475, p < 0.001). Positive and negative predictive values for a recently suggested threshold serum level of endoxifen (5.97 ng/mL) for breast cancer recurrence rate were 100 and 90%, respectively, for both CYP2D6 phenotype by DM-BT (delta-over-baseline at t = 50 min (DOB(50)) values of 0.7-0.9) and genotype (CYP2D6 gene activity score of 1.0).

CONCLUSION

DM-BT might be, along with CYP2D6 genotyping, of value in selection of individualized endocrine therapy in patients with early breast cancer, especially when concomitant use of CYP2D6 inhibiting medication alters the phenotype.

CYP2D6 phenotype indicative for optimized antiestrogen efficacy associates with outcome in early breast cancer patients

BACKGROUND

Endoxifen serum concentrations seem to correlate with outcome in breast cancer (BC) patients. Concurrently, cytochrome P450 2D6 (CYP2D6) enzyme activity and dextromethorphan (DM) metabolism are deemed a surrogate marker for the formation of endoxifen. Here, we conducted a matched cohort study to determine the impact of an extensive CYP2D6 phenotype on relapse in patients with early-stage estrogen receptor (ER)-positive BC and adjuvant tamoxifen intake.

METHODS

CYP2D6 extensive metabolism was determined upon appropriate dextromethorphan/dextrorphan (DM/DX) urinary excretion ratios (≤0.30). Fifty-nine BC patients were identified as extensive phenotype metabolizers, while for 148 matched controls, CYP2D6 was not determined. Patients and controls did not differ with respect to age, stage, hormone receptor status, HER2, grade, menopausal status, chemotherapy and antihormonal therapy. Survival analysis was performed according to clinical follow-up.

RESULTS

Disease-free survival (DFS) of patients identified as extensive CYP2D6 metabolizers did not differ significantly from controls (p = 0.10). However, when patients with ER expression of ≤ 20 % were excluded from the analysis, DFS was associated with a more favorable outcome (p = 0.06).

CONCLUSIONS

This study suggests a positive association between extensive CYP2D6 metabolism and outcome in early-stage ER-positive BC patients using tamoxifen and in particular, when a sufficient number ERs are represented on the primary tumor.

SULT1A1 rs9282861 polymorphism-a potential modifier of efficacy of the systemic adjuvant therapy in breast cancer?

Background

Sulfotransferase 1A1 (SULT1A1) participates in the elimination of 4-hydroxy-tamoxifen (4-OH-TAM), which is one of the major active metabolites of tamoxifen (TAM). Homozygous SULT1A1 variant allele genotype has been associated with lower catalytic activity and thermostability of the enzyme. Previous clinical studies suggest that the SULT1A1 rs9282861 polymorphism may influence the survival of breast cancer patients treated with TAM in the adjuvant setting. We investigated the effect of rs9282861 genotypes on the survival of Finnish breast cancer patients treated with adjuvant chemotherapy or TAM.

Methods

The rs9282861 genotypes of 412 Finnish breast cancer patients with early breast cancer were identified by using PCR-RFLP method. Seventy six patients were treated with adjuvant cyclophosphamide based chemotherapy only, 65 patients received adjuvant TAM, and four patients were treated with both adjuvant chemotherapy and TAM. Overall long-term survival (OS), breast cancer specific survival (BCSS), and relapse-free survival (RFS) by rs9282861 genotypes were evaluated by the Kaplan-Meier method and Cox regression analysis.

Results

The multivariate analysis of 145 patients receiving either adjuvant TAM or chemotherapy showed a statistically significantly improved OS in patients with the rs9282861 homozygous variant AA genotype (hazard ratio [HR] = 0.50, 95% confidence interval [CI] = 0.29-0.88, P = 0.015). In the separate analyses of patients receiving only chemotherapy or adjuvant TAM, there were no statistically significant differences in survival.

Conclusions

In this prospective study, we observed a previously unreported association between the SULT1A1 rs9282861 genotype and OS of breast cancer patients treated with adjuvant chemotherapy or TAM. This novel finding suggests that the rs9282861 polymorphism modifies the long-term clinical outcome of patients receiving adjuvant TAM or chemotherapy.

Impact of CYP2D6 polymorphism on tamoxifen therapy: where are we?

Tamoxifen is a mainstay in the treatment of hormone-receptor sensitive breast cancer. To be effective, it needs conversion into 4-hydroxy-tamoxifen and endoxifen. The key enzyme involved is encoded by the gene CYP2D6 of which several, sometimes population-specific alleles are known. Corresponding enzyme variants may result in poor, intermediate, and extensive metabolization and therefore different steady-state plasma levels of active metabolites. Those are hypothesized to be linked to clinical outcomes of tamoxifen therapy. However, a wealth of mostly retrospective cohort studies came up with conflicting results. Appraisal of these studies is difficult and a metaanalysis impossible due to heterogeneity of patient populations, disease factors, treatment modalities, and measured outcomes. As standardization would not overcome intrinsic limitations of retrospective analyses, prospective trials comparing genotype-guided versus unsighted tamoxifen treatment are required to prove whether routine CYP2D6 genotyping is clinically effective and cost-effective.

Tamoxifen and CYP2D6: a contradiction of data

Tamoxifen is an effective antiestrogen used in the treatment of hormone receptor-positive breast cancer. Bioconversion of tamoxifen to endoxifen, its most abundant active metabolite, is primarily dependent on the activity of cytochrome P450 2D6 (CYP2D6), which is highly polymorphic. Over 20 published studies have reported on the potential association between CYP2D6 polymorphism and tamoxifen treatment outcome, with highly inconsistent results. The purpose of this review is to explore differences among 17 independent studies to identify factors that may have contributed to the discrepant findings. This report discusses six putative factors that are grouped into two categories: (a) clinical management criteria: hormone receptor classification, menopausal status, and tamoxifen combination therapy; (b) pharmacologic criteria: genotyping comprehensiveness, CYP2D6 inhibitor coadministration, and tamoxifen adherence. Comparison of these factors between the positive and negative studies suggests that tamoxifen combination therapy, genotyping comprehensiveness, and CYP2D6 inhibitor coadministration may account for some of the contradictory results. Future association studies on the link between CYP2D6 genotype and tamoxifen treatment efficacy should account for combination therapy and CYP2D6 inhibition, and interrogate as many CYP2D6 alleles as possible.

CYP2D6 and UGT2B7 genotype and risk of recurrence in tamoxifen-treated breast cancer patients

BACKGROUND

Adjuvant tamoxifen therapy substantially decreases the risk of recurrence and mortality in women with hormone (estrogen and/or progesterone) receptor-positive breast cancer. Previous studies have suggested that metabolic conversion of tamoxifen to endoxifen by cytochrome P450 2D6 (CYP2D6) is required for patient benefit from tamoxifen therapy.

METHODS

Tumor specimens from a subset of postmenopausal patients with hormone receptor-positive early-stage (stages I, II, and IIIA) breast cancer, who were enrolled in the randomized double-blind Arimidex, Tamoxifen, Alone or in Combination (ATAC) clinical trial, were genotyped for variants in CYP2D6 (N = 1203 patients: anastrozole [trade name: Arimidex] group, n = 615 patients; tamoxifen group, n = 588 patients) and UDP-glucuronosyltransferase-2B7 (UGT2B7), whose gene product inactivates endoxifen (N = 1209 patients; anastrozole group, n = 606 patients; tamoxifen group, n = 603 patients). Genotyping was performed using polymerase chain reaction-based TaqMan assays. Based on the genotypes for CYP2D6, patients were classified as poor metabolizer (PM), intermediate metabolizer (IM), or extensive metabolizer (EM) phenotypes. We evaluated the association of CYP2D6 and UGT2B7 genotype with distant recurrence (primary endpoint) and any recurrence (secondary endpoint) by estimating the hazard ratios (HRs) and corresponding 95% confidence intervals (CIs) using Cox proportional hazards models. All statistical tests were two-sided.

RESULTS

After a median follow-up of 10 years, no statistically significant associations were observed between CYP2D6 genotype and recurrence in tamoxifen-treated patients (PM vs EM: HR for distant recurrence = 1.25, 95% CI = 0.55 to 3.15, P = .64; HR for any recurrence = 0.99, 95% CI = 0.48 to 2.08, P = .99). A near-null association was observed between UGT2B7 genotype and recurrence in tamoxifen-treated patients. No associations were observed between CYP2D6 and UGT2B7 genotypes and recurrence in anastrozole-treated patients.

CONCLUSION

The results do not support the hypothesis that CYP2D6 genotype predicts clinical benefit of adjuvant tamoxifen treatment among postmenopausal breast cancer patients.

The Discriminatory Value of CYP2D6 Genotyping in Predicting the Dextromethorphan/Dextrorphan Phenotype in Women with Breast Cancer

BACKGROUND: The growth inhibitory effect of tamoxifen is used for the treatment of breast cancer. Tamoxifen efficacy is mediated by its biotransformation, predominantly via the cytochrome P450 2D6 (CYP2D6) isoenzyme, to the active metabolite endoxifen. We investigated the relationship of CYP2D6 genotypes to the metabolism of dextromethorphan (DM), which is frequently used as a surrogate marker for the formation of endoxifen. METHODS: The CYP2D6 genotype was determined by polymerase chain reaction (PCR) in previously untreated patients with hormone receptor-positive invasive breast cancer considered to receive antihormonal therapy. The DM/dextrorphan (DX) urinary excretion ratios were obtained in a subset of patients by high-pressure liquid chromatography (HPLC)-mediated urine analysis after intake of 25 mg DM. The relationships of genotype and corresponding phenotype were statistically analyzed for association. RESULTS: From 151 patients predicted based on their genotype data for the 'traditional' CYP2D6 phenotype classes poor, intermediate, extensive and ultrarapid, 83 patients were examined for their DM/DX urinary ratios. The genotype-based poor metabolizer status correlated with the DM/DX ratios, whereas the intermediate, extensive and ultrarapid genotypes could not be distinguished based on their phenotype. Citalopram intake did not significantly influence the phenotype. CONCLUSIONS: The DM metabolism can be reliably used to assess the CYP2D6 enzyme activity. The correlation with the genotype can be incomplete and the metabolic ratios do not seem to be compromised by citalopram. DM phenotyping may provide a standardized tool to better assess the CYP2D6 metabolic capacity.

Therapeutic drug monitoring of tamoxifen using LC-MS/MS

Tamoxifen is a selective estrogen receptor modulator (SERM) that is used widely in the treatment of estrogen receptor positive breast cancer (ER+). Therapeutic monitoring of tamoxifen, and its metabolites N-desmethyltamoxifen (NDTam) and 4-hydroxy-N-desmethyltamoxifen (endoxifen), may be clinically useful for guiding treatment decisions. Two significant barriers to tamoxifen efficacy are: (1) variability in conversion of tamoxifen into the potent antiestrogenic metabolite, endoxifen, and (2) poor compliance and adherence to tamoxifen therapy. Therapeutic monitoring can be used to address both of these issues. Low levels of endoxifen indicate either poor compliance or poor metabolism of tamoxifen. Low tamoxifen levels would suggest poor compliance while a low ratio of endoxifen to NDTam would be indicative of poor metabolism. Solid phase extraction of patient serum followed by liquid chromatography tandem mass spectrometry (LC-MS/MS) detection enables rapid, accurate, detection of tamoxifen, N-desmethyltamoxifen, and endoxifen.

Prioritizing pharmacogenetic research: a value of information analysis of CYP2D6 testing to guide breast cancer treatment

OBJECTIVES

To demonstrate how value of information (VOI) analysis can be used to establish research priorities regarding the use of pharmacogenetic tests using CYP2D6 testing to select adjuvant hormonal therapy in early stage breast cancer as a case study.

METHODS

The following four treatment pathways are compared in a Markov model: tamoxifen treatment; CYP2D6 test and treat homozygous and heterozygous wild type patients (wt/wt; wt/*4) with tamoxifen and *4/*4 patients with anastrozole (HetTam); CYP2D6 test and treat homozygous wild type patients with tamoxifen and others with anastrozole (HomTam); and anastrozole treatment. Pharmacogenetic testing efficacy is estimated by synthesizing randomized controlled trial data comparing tamoxifen to anastrozole with observational data linking CYP2D6 genotype to tamoxifen outcomes.

RESULTS

In order of increasing effectiveness the comparators are tamoxifen, HetTam, HomTam, anastrozole. Health outcomes for test and treatment strategies are highly uncertain. Differences in comparator costs depend on assumptions made regarding anastrozole patent expiry. The expected value of a decision taken with perfect information is £69 to £106 million (pound sterling) for the United Kingdom depending on patent expiry assumptions and the acceptable cost-effectiveness threshold. The most valuable research (VOI £53-£82 million) elucidates the relationship between CYP2D6 genotype and tamoxifen effectiveness. It is uncertain whether values of other research designs would exceed their costs.

CONCLUSIONS

Retrospective analysis of one of the large adjuvant aromatase inhibitor trials is warranted to better understand any association between CYP2D6 genotype and tamoxifen outcomes. VOI approaches may be helpful for prioritising evidence needs and structuring coverage with evidence development agreements for pharmacogenetics.

Population differences in major functional polymorphisms of pharmacokinetics/pharmacodynamics-related genes in Eastern Asians and Europeans: implications in the clinical trials for novel drug development

Drug lag, recently discussed extensively in Japan, can be divided into two phases: clinical development time and application review time. The former factor is still an important problem that might be improved by promoting multi-regional clinical trials and considering the results from other similar populations with Japanese, such as Koreans and Chinese. In this review, we compare the allelic or genotype frequencies of 30 relatively common functional alleles mainly between Eastern Asians and Europeans as well as among 3 major populations in Eastern Asian countries, Japan, Korea, and China, in 12 pharmacokinetics (PK)/pharmacodynamics (PD)-related genes; CYP2C9 (*2 and *3), CYP2C19 (*2, *3 and *17), 13 CYP2D6 haplotypes including *4, *5 and *10, CYP3A5 (*3), UGT1A1 (*28 and *6), NAT2 (*5, *6 and *7), GSTM1 and GSTT1 null genotypes, SLCO1B1 521T>C, ABCG2 421C>A, and HLA-A*31:01 and HLA-B*58:01. In this review, differences in allele frequencies (AFs) or genotype frequencies (GFs) less than 0.1 (in the cases of highest AF (GF) ≥0.1) or less than 0.05 (in the cases of lowest AF (GF) <0.1) were regarded as similar. Between Eastern Asians and Europeans, AFs (or GFs) are regarded as being different for many alleles such as CYP2C9 (*2), CYP2C19 (*2, *3 and *17), CYP2D6 (*4 and *10), CYP3A5 (*3), UGT1A1 (*28 and *6), NAT2 (*5*7), GSTT1 null and ABCG2 421C>A. Among the 3 Eastern Asian populations, however, only AFs of CYP2C19*3, CYP2D6*10, HLA-A*31:01 and HLA-B*58:01 are regarded as dissimilar. For CYP2C19*3, the total functional impact on CYP2C19 could be small if the frequencies of the two null alleles CYP2C19*2 and *3 are combined. Regarding CYP2D6*10, frequency difference over 0.1 is observed only between Japanese and Chinese (0.147). Although environmental factors should be considered for PK/PD differences, we could propose that among Japan, Korea, and China, genetic differences are very small for the analyzed common PK-related gene polymorphisms. On the other hand, AFs of the two HLA alleles important for cutaneous adverse drug reactions are diverse even among Eastern Asians and thus should be taken into account.

Assessment of the impact of CYP3A polymorphisms on the formation of α-hydroxytamoxifen and N-desmethyltamoxifen in human liver microsomes

Tamoxifen, an antiestrogen used in the prevention and treatment of breast cancer, is extensively metabolized by cytochrome P450 enzymes. Its biotransformation to α-hydroxytamoxifen (α-OHT), which may be genotoxic, and to N-desmethyltamoxifen (N-DMT), which is partially hydroxylated to 4-hydroxy-N-DMT (endoxifen), a potent antiestrogen, is mediated by CYP3A enzymes. However, the potential contribution of CYP3A5 and the impact of its low-expression variants on the formation of these metabolites are not clear. Therefore, we assessed the contributions of CYP3A4 and CYP3A5 and examined the impact of CYP3A5 genotypes on the formation of α-OHT and N-DMT, by using recombinant CYP3A4 and CYP3A5 and human liver microsomes (HLM) genotyped for CYP3A5 variants. We observed that the catalytic efficiency [intrinsic clearance (CL(int))] for α-OHT formation with recombinant CYP3A4 was 5-fold higher than that with recombinant CYP3A5 (0.81 versus 0.16 nl · min⁻¹ · pmol cytochrome P450⁻¹). There was no significant difference in CL(int) values between the three CYP3A5-genotyped HLM (*1/*1, *1/*3, and *3/*3). For N-DMT formation, the CL(int) with recombinant CYP3A4 was only 1.7-fold higher, relative to that with recombinant CYP3A5. In addition, the CL(int) for N-DMT formation by HLM with CYP3A5*3/*3 alleles was approximately 3-fold lower than that for HLM expressing CYP3A5*1/*1. Regression analyses of tamoxifen metabolism with respect to testosterone 6β-hydroxylation facilitated assessment of CYP3A5 contributions to the formation of the two metabolites. The CYP3A5 contributions to α-OHT formation were negligible, whereas the contributions to N-DMT formation ranged from 51 to 61%. Our findings suggest that polymorphic CYP3A5 expression may affect the formation of N-DMT but not that of α-OHT.

Pharmacogenetic testing affects choice of therapy among women considering tamoxifen treatment

Background

Pharmacogenetic testing holds major promise in allowing physicians to tailor therapy to patients based on genotype. However, there is little data on the impact of pharmacogenetic test results on patient and clinician choice of therapy. CYP2D6 testing among tamoxifen users offers a potential test case of the use of pharmacogenetic testing in the clinic. We evaluated the effect of CYP2D6 testing in clinical practice to determine whether genotype results affected choice of hormone therapy in a prospective cohort study.

Methods

Women planning to take or currently taking tamoxifen were considered eligible. Participants were enrolled in an informational session that reviewed the results of studies of CYP2D6 genotype on breast cancer recurrence. CYP2D6 genotyping was offered to participants using the AmpliChip CYP450 Test. Women were classified as either poor, intermediate, extensive or ultra-rapid metabolizers. Results were provided to clinicians without specific treatment recommendations. Follow-up was performed with a structured phone interview 3 to 6 months after testing to evaluate changes in medication.

Results

A total of 245 women were tested and 235 completed the follow-up survey. Six of 13 (46%) women classified as poor metabolizers reported changing treatment compared with 11 of 218 (5%) classified as intermediate, extensive or ultra-rapid metabolizers (P < 0.001). There was no difference in treatment choices between women classified as intermediate and extensive metabolizers. In multi-variate models that adjusted for age, race/ethnicity, educational status, method of referral into the study, prior knowledge of CYP2D6 testing, the patients' CYP2D6 genotype was the only significant factor that predicted a change in therapy (odds ratio 22.8; 95% confidence interval 5.2 to 98.8). Genetic testing did not affect use of co-medications that interact with CYP2D6.

Conclusions

CYP2D6 genotype testing led to changes in therapy among poor metabolizers, even in the absence of definitive data that an alternative medicine improved outcomes. Pharmacogenetic testing can affect choice of therapy, even in the absence of definitive data on clinical impact.

SULT1A1, CYP2C19 and disease-free survival in early breast cancer patients receiving tamoxifen

AIM

Tamoxifen biotransformation to endoxifen, a potent antiestrogen, is catalyzed by CYP2D6. In addition, CYP2C19 and SULT1A1 have also been implicated in the metabolism of tamoxifen. We sought to evaluate the importance of SULT1A1 copy number and CYP2C19*17 on disease-free survival (DFS) in postmenopausal women randomized to tamoxifen monotherapy in North Central Cancer Treatment Group 89-30-52 from January 1991 to April 1995.

MATERIALS & METHODS

We extracted DNA from paraffin-embedded tumors and determined tumor SULT1A1 copy number and CYP2C19*17 genotype. The association of genotype with DFS was determined using the log-rank test. Multivariate cox modeling was performed using traditional prognostic factors, as well as CYP2D6 genotype. SULT1A1 copy number and CYP2C19*17 genotype was determined in 190 out of 256 patients (95% Caucasian).

RESULTS

The median follow-up for living patients was 14 years. DFS did not differ according to SULT1A1 copy number (p = 0.482) or CYP2C19*17 genotype (p = 0.667). Neither SULT1A1 copy number or CYP2C19*17 genotype was associated with disease recurrence in this cohort.

CONCLUSION

Future studies are needed to identify whether other genetic and environmental factors which affect tamoxifen metabolism are associated with tamoxifen clinical outcomes.

CYP2C19*2 polymorphism is associated with increased survival in breast cancer patients using tamoxifen

AIMS

Variant alleles of the CYP2C19 gene were recently associated with survival in breast cancer patients on tamoxifen therapy. CYP2C19 is one of the enzymes involved in the metabolism of tamoxifen into active metabolites. We investigated the hypothesis that CYP2C19*2 and *3 variants, known for their lack of enzyme activity, are associated with an increased breast cancer mortality rate in patients using tamoxifen.

MATERIALS & METHODS

In the prospective population based Rotterdam study, the association between CYP2C19*2 carriers and breast cancer mortality was studied among 80 incident users of tamoxifen. Survival was analyzed with life tables and Cox regression analysis, with drug exposure as a time-dependent variable. Adjustments were made for calendar time, average tamoxifen dose, age, the indication for tamoxifen, CYP2D6 genotype and concomitant use of CYP2C19 inhibitors or inducers.

RESULTS

In patients on tamoxifen, CYP2C19*2 carriers were associated with a significantly longer breast cancer survival rate than patients with the wild-type (hazard ratio 0.26, 95%CI: 0.08-0.87).

CONCLUSION

This study suggests that CYP2C19 genotype may possibly be a predictive factor for survival in breast cancer patients using tamoxifen.