CIENCIASMEDICASNEWS: CYP2D6 Pharmacogenomics of Tamoxifen Treatment

CYP2D6 Pharmacogenomics of Tamoxifen Treatment

Executive Summary

Background

Tamoxifen is prescribed as a component of adjuvant endocrine therapy to prevent hormone receptor-positive breast cancer recurrence, as treatment of metastatic breast cancer, and to prevent disease in high-risk populations and in women with ductal carcinoma in situ. The cytochrome P450 2D6 (CYP2D6) metabolic enzyme has a major role in tamoxifen metabolism. The CYP2D6 gene is polymorphic; variant DNA gene sequences resulting in proteins with markedly reduced or absent enzyme function may be associated with lower plasma levels of active tamoxifen metabolites, particularly endoxifen, which is predominantly dependent on CYP2D6 for its production, and are thus hypothesized to have an impact on tamoxifen treatment efficacy. Patients who have little or no CYP2D6 enzyme function are called poor metabolizers (PMs) compared with patients with 2 fully functional alleles, termed wild-type or extensive metabolizers (EMs). Those with enzyme activity in between are called intermediate metabolizers.

Objective

This Assessment evaluates the evidence for CYP2D6 genotyping, compared with no testing, to direct treatment regimen choices for patients at high risk for primary breast cancer or breast cancer recurrence, and to improve survival outcomes.

Search Strategy

MEDLINE® was searched (via PubMed) using the search string (“Breast Neoplasms”[MeSH®] AND “Tamoxifen”[MeSH®]) AND “Cytochrome P-450 Enzyme System”[MeSH®] through November 2013. Clinical trials, recent reviews (2008-2013), editorials, and letters related to the pharmacogenomics of tamoxifen were retrieved. Text and reference lists of retrieved articles were examined for additional relevant articles.

Selection Criteria

Full-length, peer-reviewed publications reporting studies of postmenopausal women undergoing endocrine therapy whose treatment regimen selection is based on CYP2D6 genotyping versus usual selection methods or studies of the association of CYP2D6 genotype with intermediate (e.g., tamoxifen- active metabolite levels) or final outcomes (e.g., time to recurrence, survival) were selected for review. Studies were assessed for potential bias, such as survival bias (genotyping whole blood from surviving participants of retrospective studies); misclassification of metabolizer phenotype; and adjustment for variables not considered confounders of the genotype-outcome association.

Main Results

One U.S. Food and Drug Administration-cleared test for CYP2D6 genotyping has consistent evidence of analytic validity (i.e., technical accuracy and reliability). There is a chain of evidence and direct evidence for clinical validity (i.e., association of CYP2D6 genotype with drug efficacy). The chain of evidence associates an intermediate outcome, tamoxifen metabolite plasma levels, with both CYP2D6 genotype and health outcomes (recurrence, survival) in women treated with tamoxifen. Direct evidence compares outcomes in women treated with tamoxifen or other endocrine therapy stratified by metabolizer status.

Chain of Evidence

* Association of genotype with plasma levels of active tamoxifen metabolites

Five prospective cohort studies of adjuvant tamoxifen treatment provide consistent evidence that CYP2D6 nonfunctional variant alleles are associated with significantly reduced plasma endoxifen levels. However, endoxifen levels overlap across all genotypes, indicating that CYP2D6 genetic variability only partly explains endoxifen level variability. Although generation of 4-hydroxy tamoxifen (4-OH tamoxifen), another active tamoxifen metabolite, does not depend solely on CYP2D6, 3 of 4 studies showed that low CYP2D6 function was associated with reduced plasma 4-OH tamoxifen levels. Coadministration of potent CYP2D6 inhibitors to CYP2D6 homozygous wild-type patients is associated with endoxifen levels near those of patients who are PMs.

* Association of in vivo endoxifen levels with clinical outcomes

Two studies examined the relationship between CYP2D6 genotype and active tamoxifen metabolites, and between genotype and clinical outcomes in the same patient population. Both studies enrolled patients from Asian populations, focusing almost exclusively on the prevalent reduced-function CYP2D6*10 variant in this population. Both studies reported reduced endoxifen and/or 4-OH tamoxifen concentrations in patients homozygous or heterozygous for variant alleles, and in conjunction reported decreased disease- or recurrence-free survival. One or both studies have design flaws likely resulting in selection bias. In addition both studies are small, resulting in estimates of association with wide confidence intervals. Thus, the relationship between endoxifen (or 4-OH tamoxifen) plasma concentrations and clinical outcomes has not been established in Asian populations, nor has it been studied in white populations with null function CYP2D6 genotypes.

Direct Evidence

Association of genotype with clinical outcomes

One groupa directly compared tamoxifen-treated women with those clinically eligible for but not receiving tamoxifen, stratified by CYP2D6 genotype. This retrospective study used archived tumor samples from a randomized controlled trial of tamoxifen treatment in the adjuvant setting. Investigators found that tamoxifen-treated EMs obtained no significant clinical benefit compared with EMs not treated with tamoxifen, and, paradoxically, that carriers of a CYP2D6*4 nonfunctional variant allele benefited from tamoxifen treatment. There were several limitations to this study such that results are questionable.

Most included studies examined tamoxifen use in the adjuvant setting in postmenopausal women. Most enrolled only tamoxifen-treated women and evaluated outcomes by CYP2D6 genotype. A few separately evaluated a non-tamoxifen-treated control population but without direct comparison. Seven small studies in Asian populations focused on the CYP2D6*10 reduced function allele; 4 reported significant results for the association of CYP2D6 genotype with outcomes of tamoxifen treatment, but may be affected in unpredictable ways by different types of bias (survival bias and adjustment for variables that are not confounders). Two of the 3 studies that reported no association may have less potential for bias. Sixteen studies evaluated samples from primarily white patients. Of the 5 largest studies, 3 reported no significant association between CYP2D6 reduced or null activity genotype and time to breast cancer recurrence. Two of the negative studies were retrospective analyses of clinical trial samples and one was a matched case-control study nested within a well-documented breast cancer registry. All 3 were designed to minimize the potential for bias; their sizes allowed comparison of homozygous nonfunctional CYP2D6 genotypes (PMs) with fully functional wild-type genotypes (EMs), that is, the most extreme comparison and most likely to reveal a true association. Two studies reported significant positive results; 1 study combined samples from different sources, some of which had already been analyzed for this hypothesis. In addition, it is unclear from the report whether nearly half of the samples were obtained from patients who had survived and were available at a time distant from their diagnosis and surgery, a survivorship selection bias that can unpredictably affect results. The other positive study matched samples from a randomized controlled trial on several breast cancer prognostic variables, neither causally related to CYP2D6 genotype nor surrogates for genotype, potentially biasing selection of cases and controls. The remaining 9 studies report a variety of significant and nonsignificant results; no pattern of bias, genotyping or group scheme, or accounting for CYP2D6 inhibitor use (among possibilities) explains the differences in results. Heterogeneity of results across all studies, and clear results of no genotype-tamoxifen treatment outcome in 2 large trials with the least apparent potential for bias, suggest lack of support for clinical validity.

There is no direct evidence of clinical utility (whether use of CYP2D6 genotype testing for endocrine therapy regimen selection improves recurrence and survival outcomes). Without demonstrable clinical validity, there is no basis for changing management in patients with specific genotypes to improve outcomes (clinical utility).

Author’s Conclusions and Comments

The question examined in this Assessment is whether patients with CYP2D6 gene variants that result in markedly reduced or absent enzyme function have reduced tamoxifen metabolism and lower endoxifen levels compared with genotypic wild-type extensive metabolizers, and as a result have poorer clinical outcomes. This question rests on the assumption, not yet supported by evidence, that some level of endoxifen is sufficient and necessary for tamoxifen efficacy, and that this level is not achieved in patients with markedly reduced or no CYP2D6 enzymatic function. However, because tamoxifen metabolism is complex and CYP2D6 does not appear to account for all variability in endoxifen levels, it is conceivable that polymorphisms in other tamoxifen metabolic pathway enzymes may affect active metabolite levels, and in theory direct measurement of the metabolite(s) itself might be the better predictor of benefit from tamoxifen treatment. However, measuring metabolite levels is not practical for clinical applications.

Whether lower endoxifen levels can affect the pharmacodynamics of tamoxifen, the interaction of tamoxifen metabolites with estrogen receptors, and ultimately tamoxifen efficacy, is unclear. Dissociation constants of even the more weakly binding molecules, including tamoxifen itself, are reportedly still sufficient to effectively block estrogen binding. Moreover, it is estimated that at doses used for adjuvant treatment, which are intended to saturate the estrogen receptor, more than 99% of estrogen receptors are bound by tamoxifen and its metabolites.

Lacking the appropriate mechanistic evidence, it remains to examine the clinical evidence, the bulk of which addresses clinical validity, the CYP2D6 genotype-tamoxifen treatment outcome association. As noted, heterogeneous results are observed across studies. Heterogeneity in effect estimates, both in magnitude and significance, is likely due to the lack of power in most studies and potential biases. The analysis of archived samples from 2 large completed clinical trials was undertaken to achieve adequate power, to more fully evaluate CYP2D6 genotype, to evaluate aromatase inhibitor-treated control populations in tandem, and to avoid potential sources of bias. That the results of these studies discovered no evidence of association between CYP2D6 genotype and either tamoxifen- or aromatase inhibitor-treated patient outcomes has suggested that using results of CYP2D6 genetic testing to influence decisions about tamoxifen treatment is not currently warranted.

Based on the available evidence, the Blue Cross and Blue Shield Association Medical Advisory Panel made the following judgments about whether CYP2D6 genotyping for directing endocrine therapy regimen selection for women at high risk for primary breast cancer or breast cancer recurrence meets the Blue Cross and Blue Shield Association Technology Evaluation Center (TEC) criteria.

1. The technology must have final approval from the appropriate governmental regulatory bodies.

The Roche AmpliChip CYP450 Test is cleared by the U.S. Food and Drug Administration (FDA) and is “intended to identify a patient’s CYP2D6 and CYP2C19 genotype from genomic DNA extracted from a whole blood specimen. Information about CYP2D6 and CYP2C19 genotype may be used as an aid to clinicians in determining therapeutic strategy and treatment dose for therapeutics that are metabolized by the CYP2D6 or CYP2C19 gene product” (http://molecular.roche.com/assays/ Pages/AmpliChipCYP450Test.aspx).

CYP2D6 genotyping assays are also available as laboratory-developed tests (LDT). Clinical laboratories may develop and validate tests in-house and market them as a laboratory service; laboratories offering LDTs as a clinical service must meet the general regulatory standards of the Clinical Laboratory Improvement Act (CLIA) and must be licensed by CLIA for high-complexity testing.

FDA has considered updating the label for tamoxifen (brand and generics) with information or recommendations regarding CYP2D6 genotyping and impact on tamoxifen efficacy. On October 18, 2006, FDA held an Advisory Committee meeting to answer specific questions regarding the evidence and recommendations for the label update. Since that Advisory Committee meeting, AstraZeneca, the brand name (Nolvadex®) manufacturer, has ceased producing tamoxifen and is no longer maintaining the prescribing information. As of the date of this Assessment, no direction has come from FDA regarding revised labeling of generic versions of tamoxifen to include CYP2D6 genotyping information.

2. The scientific evidence must permit conclusions concerning the effect of the technology on health outcomes.

There are several limitations to the overall body of evidence, but the largest, most well-designed studies do not support clinical validity of the test. In the absence of evidence for clinical validity, evidence to support clinical utility is lacking.

3. The technology must improve the net health outcome.

Evidence for clinical utility is currently lacking.

4. The technology must be as beneficial as any established alternatives.

Because the available evidence does not clearly support a significant association between CYP2D6 genotype and tamoxifen treatment outcome, a chain of evidence supporting the clinical utility of CYP2D6 genotyping for directing endocrine therapy regimen selection for women at high risk for or with breast cancer cannot be constructed.

5. The improvement must be attainable outside the investigational settings.

The use of CYP2D6 genotyping for directing endocrine therapy regimen selection for women at high risk for or with breast cancer to improve health outcomes has not been demonstrated in the investigational setting.

Based on the above, CYP2D6 genotyping does not meet the TEC criteria for directing endocrine therapy regimen selection for women at high risk for primary breast cancer or breast cancer recurrence.

a Wegman P, Vainikka L, Stal O et al. (2005). Genotype of metabolic enzymes and the benefit of tamoxifen in postmenopausal breast cancer patients. Breast Cancer Res, 7(3):R284-90.