Germline Pharmacogenetics of Tamoxifen Response: Have We Learned Enough?

The selective estrogen receptor modulator tamoxifen is effective for the treatment of estrogen receptor–positive breast cancer and for the prevention of breast cancer in women at high risk.1,2 However, women taking tamoxifen are at increased risk for rare, yet potentially life-threatening adverse effects such as endometrial cancer and deep venous thrombosis.1 Tamoxifen may also reduce quality of life and compliance through non–life-threatening common adverse effects such as hot flashes.1,3 Since both the beneficial and adverse effects of tamoxifen are largely unpredictable for individual patients, it is important to attempt to develop predictors that will allow clinicians to anticipate risks and benefits. In this issue of the Journal, Schroth et al4 tested the ability of germline genetic variants in five tamoxifen-metabolizing genes to predict breast cancer events during tamoxifen treatment. Sixteen genetic variants were analyzed in DNA samples obtained from 206 patients on adjuvant tamoxifen monotherapy and 280 patients without tamoxifen therapy in a nonrandomized cohort study. With a median follow-up of 71 months, the authors found that women with the CYP2D6 variant alleles experienced significantly more recurrence of their disease, shorter relapse-free time, and shorter event-free survival (hazard ratio, 2.24; P = .02).No data are presented on whether CYP2D6 variants were associated with any of the adverse effects of tamoxifen. Patients who carried the CYP2C19*17 allele had significantly fewer breast cancer events during tamoxifen therapy. No impact of the reduced-function CYP2C19*2 and *3 genotypes was observed with tamoxifen response, so the mechanism by which CYP2C19*17 influences tamoxifen response deserves further study. The authors conclude that genotyping for CYP2D6 variants and the CYP2C19*17 allele may identify patients who are most likely to derive benefit from tamoxifen therapy. The role of metabolic activation in tamoxifen activity was recognized during the 1970s by Jordan et al, who also characterized the first potent antiestrogen metabolite, 4-hydroxytamoxifen, which has 100-fold greater affinity than tamoxifen toward estrogen receptors.5 This metabolite was later shown to be 30- to 100-fold more potent than tamoxifen in suppressing estrogen-dependent cell proliferation.6–8 As a result, 4-hydroxytamoxifen has been a focus of research for the past three decades, and is now a standard laboratory tool used as a substitute for the parent drug in vitro. Despite its high 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 small compared with tamoxifen or some of its metabolites.6 Our knowledge of the link between tamoxifen metabolism and response expanded rapidly subsequent to the characterization of another active metabolite, N-desmethyl 4-hydroxy-tamoxifen (endoxifen), by our group some 5 years ago.9,10 Endoxifen was identified in the late 1980s,11 but until recently, its biologic activity remained obscure. A series of laboratory studies carried out to characterize its pharmacology have now established that endoxifen has equivalent potency to 4-hydroxytamoxifen in terms of binding affinity to estrogen receptors,10 suppression of estrogen-dependent proliferation of breast cancer cells,10,12 and modulation of estrogen-mediated global gene expression.13 In vitro, we have shown that endoxifen is formed primarily via 4-hydroxylation of the primary tamoxifen metabolite, N-desmethyltamoxifen, by the CYP2D6 enzyme.14 Consistent with these in vitro findings, the steady-state endoxifen plasma concentrations during tamoxifen treatment were substantially reduced in women that carry CYP2D6 genetic variants or are coprescribed CYP2D6 inhibitors,9,15,16 while the concentrations of tamoxifen or other metabolites remained unaffected by CYP2D6 metabolic status. More importantly, the plasma concentrations of endoxifen were on average 10-fold higher than those of 4-hydroxytamoxifen, with a large degree of inter-patient variability (up to 100-fold greater endoxifen exposure relative to 4-hydroxytamoxifen in some patients).9,16 The first clinical evidence linking endoxifen to tamoxifen response was provided by Goetz et al,17 who analyzed a prospective, randomized phase III trial, and reported that the CYP2D6*4 variant allele was an independent predictor of a higher risk of relapse and a lower incidence of hot flashes in postmenopausal women. A subsequent study18 reported that coprescription of CYP2D6 inhibitors, in addition to CYP2D6 genetic variation, was an independent predictor of breast cancer outcome in postmenopausal women receiving tamoxifen. These new clinical data and the mechanistic information described above prompted the US Food and Drug Administration Pharmaceutical Science Advisory Committee (Clinical Pharmacology Subcommittee) to consider relabeling of tamoxifen. During its October 18, 2006, meeting, the committee endorsed the need to incorporate in the label the fact that CYP2D6 is an important pathway in the formation of endoxifen and that this metabolite contributes importantly to the in vitro activity of tamoxifen. However, two studies have reported contradictory results. In a retrospective study of banked samples by Nowell et al, the CYP2D6*4 allele did not predict tamoxifen response or breast cancer prognosis.19 Wegman et al, reported that patients who were treated with tamoxifen and who carried the CYP2D6*4 variant allele had a decreased recurrence rate compared with those not treated.20 Differences in chemotherapeutic regimens, length of tamoxifen treatment, and tamoxifen doses in the studies by Wegman et al, as well as the lack of consistent, central testing for estrogen receptor status, and the different inclusion criteria of these trials,19,20 make comparison with the data presented by Goetz et al17,18 difficult. Given the apparent disparity in study results, the findings of Schroth et al,4 showing a robust association between CYP2D6 genetic variation and tamoxifen response in a large cohort, are important. By genotyping an extended number of variants of the CYP2D6 gene for the first time, including those associated with reduced function (eg, *41 and *10), the authors reported an improved predictive value of CYP2D6 genotype toward tamoxifen response. The conclusions derived from their results are essentially similar to those of Goetz et al17,18 and of a recent study in the metastatic setting in Korean patients.21 In all these studies,4,17,18 a gene-dose effect of CYP2D6 genotypes on tamoxifen response has been suggested, consistent with a similar relationship observed between CYP2D6 metabolic status and endoxifen exposure in patients. The predictive value of CYP2D6 genotype does not seem to be limited to treatment of breast cancer with tamoxifen. Recently, Bonanni et al, provided evidence that suggests that tamoxifen chemoprevention may be compromised in carriers of the CYP2D6*4/*4 genotype. 22 Together, the results from the studies by all of these authors advance our understanding of the pharmacogenetics of tamoxifen metabolism, and the data are lent credence by the mechanistic data linking endoxifen to tamoxifen activity and to CYP2D6 metabolic status. Despite this progress, a large number of important questions remain: What is the relationship between endoxifen and 4-hydroxytamoxifen serum concentrations and outcome? Could these be used as biomarkers of effective tamoxifen therapy? Since the current data are nearly all from white populations, what might be the result of CYP2D6 testing in African or Asian populations in the adjuvant settings? Since the absolute differences in outcomes between tamoxifen and aromatase inhibitor therapy in the majority of trials is in the 2% to 4% range, are there subpopulations of patients who would achieve better responses with tamoxifen? Could biomarker-driven therapeutic approaches using the pharmacogenetics of CYP2D6, polymorphic transporters, or estrogen receptors identify these patients, or patients who are more or less likely to experience the adverse effects that limit tamoxifen treatment? The data available at present to address these questions represent retrospective analyses of banked samples and of a single prospective trial. No prospective trial has been conducted to test the hypothesis that CYP2D6 pharmacogenetic testing can predict outcomes. In the absence of primary evidence from prospective trials, it seems reasonable at present to use clinical judgment to address this question and to confine CYP2D6 testing to situations where tamoxifen is being seriously considered in postmenopausal women, and where an alternative such as an aromatase inhibitor is available. For example, if the risks of an aromatase inhibitor seem significant because of a high risk of osteoporosis, it would be reasonable to perform CYP2D6 testing, and to prescribe tamoxifen if the genotype predicts an extensive metabolizer, but otherwise, to use an aromatase inhibitor with a bisphosphonate. Formal recommendations that address the use of germline pharmacogenetic testing for CYP2D6 and other genetically polymorphic genes must await further data. Clearly, the journey of deciphering germline polymorphisms that predict tamoxifen response has begun, but much work remains before we are able to effectively translate this progress to clinical outcomes of safety and efficacy.