Is Tamoxifen Pharmacogenetics Clinically Relevant?

Tamoxifen is a well-established adjuvant therapy for estrogen and/or progesterone receptor (HR)–positive breast cancer, and is the only hormonal treatment available to premenopausal patients. Tamoxifen itself is considered a relatively clinically inactive prodrug. Its most abundant metabolite, endoxifen, has much greater affinity for the estrogen receptor than tamoxifen, and is thought to be the primary compound responsible for clinical benefit.

Bioactivation of tamoxifen to endoxifen depends in large part upon the function of the P450 enzyme CYP2D6. This enzyme is known to have significant genetic variability, and can also be inhibited by several medications, especially fluoxetine, paroxetine and bupropion. There is ample evidence that diminished activity of CYP2D6 (whether caused by pharmacogenetic variation or use of an inhibiting drug) correlates with reduced circulating levels of endoxifen in humans treated with tamoxifen.

In theory, patients with normal CYP2D6 activity (extensive metabolizers, EM) should have a better clinical response to tamoxifen than those with moderately reduced (intermediate metabolizers, IM) or severely reduced/absent CYP2D6 activity (poor metabolizers, PM). In addition, strong inhibitors of CYP2D6 would also reduce the efficacy of tamoxifen.

However, clinical outcome studies have been mixed, with some confirming a reduced recurrence-free interval in IM and PM patients, while others showed no correlation with CYP2D6 phenotype. Most of these are small, retrospective studies that suffer from multiple limitations. The result is uncertainty as to whether pharmacogenetic testing of CYP2D6 prior to prescribing tamoxifen, and/or drug or dose selection based upon CYP2D6 phenotype, is clinically appropriate.

A review of 17 independent retrospective studies of CYP2D6 and tamoxifen response evaluated several possible factors that might explain some of this discrepancy (Oncologist 2012;17:620-30). Some of the more significant cofounders were misclassification of CYP2D6 activity and failure to account for tamoxifen adherence or use of concurrent treatments.

• Hormone receptor classification. Some of the studies included patients whose cancers were HR negative. Since tamoxifen is only effective in HR-positive cancers, inclusion of HR-negative patients could easily confound the data. However, there was no obvious correlation between inclusion of HR-negative cases and finding or not finding a CYP2D6 effect on outcome.

• Menopausal status. In postmenopausal women, tamoxifen and one of its many other non-endoxifen metabolites saturate more than 99.9% of estrogen receptors, as opposed to only 90%–95% saturation by these two molecules in premenopausal women. This leads to speculation that endoxifen availability (and thus CYP2D6 activity) is only relevant in premenopausal women. Again, however, this review found no obvious influence of menopausal status on the results of these studies of CYP2D6 and tamoxifen response.

• Tamoxifen combination therapy. Coadministration of chemotherapy or an aromatase inhibitor with tamoxifen could mask the variability in tamoxifen response attributable to CYP2D6 variation. Indeed, there was a clear trend toward not finding a CYP2D6 effect in studies with fewer patients receiving tamoxifen monotherapy and positive CYP2D6 effects in studies with more monotherapy patients.

• Genotyping comprehensiveness. The normal (EM) version of the CYP2D6 gene cannot actually be positively identified. Rather, it is assumed to be present when a known variant conferring IM or PM status is not found. There are several dozen known genetic variants in CYP2D6, but most of the published studies only looked at one or a few of them. The result can be misclassification of IM or PM patients as EM, thus masking possible pharmacogenetic effects. This review showed a trend toward positive CYP2D6 effects in studies that used more extensive genotyping.

• CYP2D6 inhibitor coadministration. Studies that accounted for inhibitor use were more likely to indicate CYP2D6 effects than those that did not. Furthermore, two studies that did not support a CYP2D6 association despite accounting for inhibitor use appeared to overestimate the effects of some relatively weak or noninhibitory drugs, which might have confounded the results.

• Tamoxifen adherence. Tamoxifen can be a difficult drug to take, especially because of hot flashes, and discontinuation is more common in CYP2D6 EM patients (who have higher circulating endoxifen levels). On average, just under 80% of patients are adherent at 1 year, with further declines to as low as 50% by the fourth year. Only one of the studies adjusted their analysis for estimated tamoxifen adherence, which resulted in a stronger correlation between CYP2D6 activity and tamoxifen benefit.

Two recent reports from large randomized trials – ATAC (Arimidex, Tamoxifen, Alone or in Combination) and BIG (Breast International Group 1-98) – failed to identify any significant difference in clinical outcome among EM, IM, and PM patients treated with tamoxifen (J. Natl. Cancer Inst. 2012;104:452-60; J. Natl. Cancer Inst. 2012;104:441-51). The accompanying editorial asserts that the question of CYP2D6 clinical utility for tamoxifen therapy "has likely been laid to rest" (J. Natl. Cancer Inst. 2012;104:427-8).

However, a more thorough consideration of the details of these two studies still leaves open the possibility that CYP2D6 genotyping is clinically relevant.

Both studies involved the retrospective analysis of a subgroup from a larger prospective randomized trial. Their strengths include large sample sizes and analysis restricted to tamoxifen monotherapy. But there are multiple limiting factors.

In the ATAC study, only 92.5% of the tumors were HR positive. All tumors in BIG were HR positive.

In both studies, the genetic analyses were performed on a convenience sample of tumor blocks from which adequate DNA could be extracted, and both had statistical anomalies that might suggest sampling bias. In ATAC, there were significant differences between the genotyped subgroup and the overall study population with respect to frequency of surgical, radiation, and chemotherapy treatment prior to adjuvant tamoxifen therapy. In BIG, the frequencies of genotyped subjects with 0, 1, or 2 copies of one of the most common variants (CYP2D6*4) were inconsistent with the expected proportions in a random population sample.

The genetic analyses looked at only six or nine known CYP2D6 variants.

Only ATAC adjusted for potent and intermediate CYP2D6 inhibitors, although those drugs did not appear to affect the results.

Neither study included significant numbers of premenopausal patients, so the results may apply only to postmenopausal women.

Despite the relatively large populations studied, modest effects of CYP2D6 variation may have been missed. ATAC was estimated to be able to identify a 35% or greater variation associated with PM status; BIG was powered to detect only a 55% or greater effect size.

Neither study measured actual endoxifen levels or had any other way of assessing adherence to therapy.

In summary, it is certainly possible that CYP2D6 variation is not valuable in predicting clinical response to tamoxifen therapy. Perhaps endoxifen is not the most biologically relevant metabolite, or perhaps the effect of reduced CYP2D6 activity is small enough that it does not account for a significant proportion of tamoxifen failures. But it is also possible that CYP2D6 activity is clinically important for at least a subset of tamoxifen-treated patients. A more definitive answer awaits results from truly prospective trials that control for multiple confounding factors and measure actual endoxifen levels.

For now, in the absence of better data to address this issue, it does not seem appropriate to order CYP2D6 genotyping prior to starting tamoxifen therapy. Likewise, even if the CYP2D6 genotype is known, it should not influence the choice between tamoxifen and an aromatase inhibitor, or the dosing of tamoxifen. However, if a patient receiving tamoxifen requires medication for depression and/or hot flashes, it certainly seems reasonable to start with something other than fluoxetine, paroxetine, or bupropion, just in case those potent CYP2D6 inhibitors do turn out to reduce clinical efficacy.

Dr. Levy is at the division of general internal medicine and the McKusick-Nathans Institute of Genetic Medicine of Johns Hopkins University, Baltimore. He reports having no conflicts of interest.

Tamoxifen is a well-established adjuvant therapy for estrogen and/or progesterone receptor (HR)–positive breast cancer, and is the only hormonal treatment available to premenopausal patients. Tamoxifen itself is considered a relatively clinically inactive prodrug. Its most abundant metabolite, endoxifen, has much greater affinity for the estrogen receptor than tamoxifen, and is thought to be the primary compound responsible for clinical benefit.

Bioactivation of tamoxifen to endoxifen depends in large part upon the function of the P450 enzyme CYP2D6. This enzyme is known to have significant genetic variability, and can also be inhibited by several medications, especially fluoxetine, paroxetine and bupropion. There is ample evidence that diminished activity of CYP2D6 (whether caused by pharmacogenetic variation or use of an inhibiting drug) correlates with reduced circulating levels of endoxifen in humans treated with tamoxifen.

In theory, patients with normal CYP2D6 activity (extensive metabolizers, EM) should have a better clinical response to tamoxifen than those with moderately reduced (intermediate metabolizers, IM) or severely reduced/absent CYP2D6 activity (poor metabolizers, PM). In addition, strong inhibitors of CYP2D6 would also reduce the efficacy of tamoxifen.

However, clinical outcome studies have been mixed, with some confirming a reduced recurrence-free interval in IM and PM patients, while others showed no correlation with CYP2D6 phenotype. Most of these are small, retrospective studies that suffer from multiple limitations. The result is uncertainty as to whether pharmacogenetic testing of CYP2D6 prior to prescribing tamoxifen, and/or drug or dose selection based upon CYP2D6 phenotype, is clinically appropriate.

A review of 17 independent retrospective studies of CYP2D6 and tamoxifen response evaluated several possible factors that might explain some of this discrepancy (Oncologist 2012;17:620-30). Some of the more significant cofounders were misclassification of CYP2D6 activity and failure to account for tamoxifen adherence or use of concurrent treatments.

• Hormone receptor classification. Some of the studies included patients whose cancers were HR negative. Since tamoxifen is only effective in HR-positive cancers, inclusion of HR-negative patients could easily confound the data. However, there was no obvious correlation between inclusion of HR-negative cases and finding or not finding a CYP2D6 effect on outcome.

• Menopausal status. In postmenopausal women, tamoxifen and one of its many other non-endoxifen metabolites saturate more than 99.9% of estrogen receptors, as opposed to only 90%–95% saturation by these two molecules in premenopausal women. This leads to speculation that endoxifen availability (and thus CYP2D6 activity) is only relevant in premenopausal women. Again, however, this review found no obvious influence of menopausal status on the results of these studies of CYP2D6 and tamoxifen response.

• Tamoxifen combination therapy. Coadministration of chemotherapy or an aromatase inhibitor with tamoxifen could mask the variability in tamoxifen response attributable to CYP2D6 variation. Indeed, there was a clear trend toward not finding a CYP2D6 effect in studies with fewer patients receiving tamoxifen monotherapy and positive CYP2D6 effects in studies with more monotherapy patients.

• Genotyping comprehensiveness. The normal (EM) version of the CYP2D6 gene cannot actually be positively identified. Rather, it is assumed to be present when a known variant conferring IM or PM status is not found. There are several dozen known genetic variants in CYP2D6, but most of the published studies only looked at one or a few of them. The result can be misclassification of IM or PM patients as EM, thus masking possible pharmacogenetic effects. This review showed a trend toward positive CYP2D6 effects in studies that used more extensive genotyping.

• CYP2D6 inhibitor coadministration. Studies that accounted for inhibitor use were more likely to indicate CYP2D6 effects than those that did not. Furthermore, two studies that did not support a CYP2D6 association despite accounting for inhibitor use appeared to overestimate the effects of some relatively weak or noninhibitory drugs, which might have confounded the results.

• Tamoxifen adherence. Tamoxifen can be a difficult drug to take, especially because of hot flashes, and discontinuation is more common in CYP2D6 EM patients (who have higher circulating endoxifen levels). On average, just under 80% of patients are adherent at 1 year, with further declines to as low as 50% by the fourth year. Only one of the studies adjusted their analysis for estimated tamoxifen adherence, which resulted in a stronger correlation between CYP2D6 activity and tamoxifen benefit.

Two recent reports from large randomized trials – ATAC (Arimidex, Tamoxifen, Alone or in Combination) and BIG (Breast International Group 1-98) – failed to identify any significant difference in clinical outcome among EM, IM, and PM patients treated with tamoxifen (J. Natl. Cancer Inst. 2012;104:452-60; J. Natl. Cancer Inst. 2012;104:441-51). The accompanying editorial asserts that the question of CYP2D6 clinical utility for tamoxifen therapy "has likely been laid to rest" (J. Natl. Cancer Inst. 2012;104:427-8).

However, a more thorough consideration of the details of these two studies still leaves open the possibility that CYP2D6 genotyping is clinically relevant.

Both studies involved the retrospective analysis of a subgroup from a larger prospective randomized trial. Their strengths include large sample sizes and analysis restricted to tamoxifen monotherapy. But there are multiple limiting factors.

In the ATAC study, only 92.5% of the tumors were HR positive. All tumors in BIG were HR positive.

In both studies, the genetic analyses were performed on a convenience sample of tumor blocks from which adequate DNA could be extracted, and both had statistical anomalies that might suggest sampling bias. In ATAC, there were significant differences between the genotyped subgroup and the overall study population with respect to frequency of surgical, radiation, and chemotherapy treatment prior to adjuvant tamoxifen therapy. In BIG, the frequencies of genotyped subjects with 0, 1, or 2 copies of one of the most common variants (CYP2D6*4) were inconsistent with the expected proportions in a random population sample.

The genetic analyses looked at only six or nine known CYP2D6 variants.

Only ATAC adjusted for potent and intermediate CYP2D6 inhibitors, although those drugs did not appear to affect the results.

Neither study included significant numbers of premenopausal patients, so the results may apply only to postmenopausal women.

Despite the relatively large populations studied, modest effects of CYP2D6 variation may have been missed. ATAC was estimated to be able to identify a 35% or greater variation associated with PM status; BIG was powered to detect only a 55% or greater effect size.

Neither study measured actual endoxifen levels or had any other way of assessing adherence to therapy.

In summary, it is certainly possible that CYP2D6 variation is not valuable in predicting clinical response to tamoxifen therapy. Perhaps endoxifen is not the most biologically relevant metabolite, or perhaps the effect of reduced CYP2D6 activity is small enough that it does not account for a significant proportion of tamoxifen failures. But it is also possible that CYP2D6 activity is clinically important for at least a subset of tamoxifen-treated patients. A more definitive answer awaits results from truly prospective trials that control for multiple confounding factors and measure actual endoxifen levels.

For now, in the absence of better data to address this issue, it does not seem appropriate to order CYP2D6 genotyping prior to starting tamoxifen therapy. Likewise, even if the CYP2D6 genotype is known, it should not influence the choice between tamoxifen and an aromatase inhibitor, or the dosing of tamoxifen. However, if a patient receiving tamoxifen requires medication for depression and/or hot flashes, it certainly seems reasonable to start with something other than fluoxetine, paroxetine, or bupropion, just in case those potent CYP2D6 inhibitors do turn out to reduce clinical efficacy.

Dr. Levy is at the division of general internal medicine and the McKusick-Nathans Institute of Genetic Medicine of Johns Hopkins University, Baltimore. He reports having no conflicts of interest.

Tamoxifen is a well-established adjuvant therapy for estrogen and/or progesterone receptor (HR)–positive breast cancer, and is the only hormonal treatment available to premenopausal patients. Tamoxifen itself is considered a relatively clinically inactive prodrug. Its most abundant metabolite, endoxifen, has much greater affinity for the estrogen receptor than tamoxifen, and is thought to be the primary compound responsible for clinical benefit.

Bioactivation of tamoxifen to endoxifen depends in large part upon the function of the P450 enzyme CYP2D6. This enzyme is known to have significant genetic variability, and can also be inhibited by several medications, especially fluoxetine, paroxetine and bupropion. There is ample evidence that diminished activity of CYP2D6 (whether caused by pharmacogenetic variation or use of an inhibiting drug) correlates with reduced circulating levels of endoxifen in humans treated with tamoxifen.

In theory, patients with normal CYP2D6 activity (extensive metabolizers, EM) should have a better clinical response to tamoxifen than those with moderately reduced (intermediate metabolizers, IM) or severely reduced/absent CYP2D6 activity (poor metabolizers, PM). In addition, strong inhibitors of CYP2D6 would also reduce the efficacy of tamoxifen.

However, clinical outcome studies have been mixed, with some confirming a reduced recurrence-free interval in IM and PM patients, while others showed no correlation with CYP2D6 phenotype. Most of these are small, retrospective studies that suffer from multiple limitations. The result is uncertainty as to whether pharmacogenetic testing of CYP2D6 prior to prescribing tamoxifen, and/or drug or dose selection based upon CYP2D6 phenotype, is clinically appropriate.

A review of 17 independent retrospective studies of CYP2D6 and tamoxifen response evaluated several possible factors that might explain some of this discrepancy (Oncologist 2012;17:620-30). Some of the more significant cofounders were misclassification of CYP2D6 activity and failure to account for tamoxifen adherence or use of concurrent treatments.

• Hormone receptor classification. Some of the studies included patients whose cancers were HR negative. Since tamoxifen is only effective in HR-positive cancers, inclusion of HR-negative patients could easily confound the data. However, there was no obvious correlation between inclusion of HR-negative cases and finding or not finding a CYP2D6 effect on outcome.

• Menopausal status. In postmenopausal women, tamoxifen and one of its many other non-endoxifen metabolites saturate more than 99.9% of estrogen receptors, as opposed to only 90%–95% saturation by these two molecules in premenopausal women. This leads to speculation that endoxifen availability (and thus CYP2D6 activity) is only relevant in premenopausal women. Again, however, this review found no obvious influence of menopausal status on the results of these studies of CYP2D6 and tamoxifen response.

• Tamoxifen combination therapy. Coadministration of chemotherapy or an aromatase inhibitor with tamoxifen could mask the variability in tamoxifen response attributable to CYP2D6 variation. Indeed, there was a clear trend toward not finding a CYP2D6 effect in studies with fewer patients receiving tamoxifen monotherapy and positive CYP2D6 effects in studies with more monotherapy patients.

• Genotyping comprehensiveness. The normal (EM) version of the CYP2D6 gene cannot actually be positively identified. Rather, it is assumed to be present when a known variant conferring IM or PM status is not found. There are several dozen known genetic variants in CYP2D6, but most of the published studies only looked at one or a few of them. The result can be misclassification of IM or PM patients as EM, thus masking possible pharmacogenetic effects. This review showed a trend toward positive CYP2D6 effects in studies that used more extensive genotyping.

• CYP2D6 inhibitor coadministration. Studies that accounted for inhibitor use were more likely to indicate CYP2D6 effects than those that did not. Furthermore, two studies that did not support a CYP2D6 association despite accounting for inhibitor use appeared to overestimate the effects of some relatively weak or noninhibitory drugs, which might have confounded the results.

• Tamoxifen adherence. Tamoxifen can be a difficult drug to take, especially because of hot flashes, and discontinuation is more common in CYP2D6 EM patients (who have higher circulating endoxifen levels). On average, just under 80% of patients are adherent at 1 year, with further declines to as low as 50% by the fourth year. Only one of the studies adjusted their analysis for estimated tamoxifen adherence, which resulted in a stronger correlation between CYP2D6 activity and tamoxifen benefit.

Two recent reports from large randomized trials – ATAC (Arimidex, Tamoxifen, Alone or in Combination) and BIG (Breast International Group 1-98) – failed to identify any significant difference in clinical outcome among EM, IM, and PM patients treated with tamoxifen (J. Natl. Cancer Inst. 2012;104:452-60; J. Natl. Cancer Inst. 2012;104:441-51). The accompanying editorial asserts that the question of CYP2D6 clinical utility for tamoxifen therapy "has likely been laid to rest" (J. Natl. Cancer Inst. 2012;104:427-8).

However, a more thorough consideration of the details of these two studies still leaves open the possibility that CYP2D6 genotyping is clinically relevant.

Both studies involved the retrospective analysis of a subgroup from a larger prospective randomized trial. Their strengths include large sample sizes and analysis restricted to tamoxifen monotherapy. But there are multiple limiting factors.

In the ATAC study, only 92.5% of the tumors were HR positive. All tumors in BIG were HR positive.

In both studies, the genetic analyses were performed on a convenience sample of tumor blocks from which adequate DNA could be extracted, and both had statistical anomalies that might suggest sampling bias. In ATAC, there were significant differences between the genotyped subgroup and the overall study population with respect to frequency of surgical, radiation, and chemotherapy treatment prior to adjuvant tamoxifen therapy. In BIG, the frequencies of genotyped subjects with 0, 1, or 2 copies of one of the most common variants (CYP2D6*4) were inconsistent with the expected proportions in a random population sample.

The genetic analyses looked at only six or nine known CYP2D6 variants.

Only ATAC adjusted for potent and intermediate CYP2D6 inhibitors, although those drugs did not appear to affect the results.

Neither study included significant numbers of premenopausal patients, so the results may apply only to postmenopausal women.

Despite the relatively large populations studied, modest effects of CYP2D6 variation may have been missed. ATAC was estimated to be able to identify a 35% or greater variation associated with PM status; BIG was powered to detect only a 55% or greater effect size.

Neither study measured actual endoxifen levels or had any other way of assessing adherence to therapy.

In summary, it is certainly possible that CYP2D6 variation is not valuable in predicting clinical response to tamoxifen therapy. Perhaps endoxifen is not the most biologically relevant metabolite, or perhaps the effect of reduced CYP2D6 activity is small enough that it does not account for a significant proportion of tamoxifen failures. But it is also possible that CYP2D6 activity is clinically important for at least a subset of tamoxifen-treated patients. A more definitive answer awaits results from truly prospective trials that control for multiple confounding factors and measure actual endoxifen levels.

For now, in the absence of better data to address this issue, it does not seem appropriate to order CYP2D6 genotyping prior to starting tamoxifen therapy. Likewise, even if the CYP2D6 genotype is known, it should not influence the choice between tamoxifen and an aromatase inhibitor, or the dosing of tamoxifen. However, if a patient receiving tamoxifen requires medication for depression and/or hot flashes, it certainly seems reasonable to start with something other than fluoxetine, paroxetine, or bupropion, just in case those potent CYP2D6 inhibitors do turn out to reduce clinical efficacy.

Dr. Levy is at the division of general internal medicine and the McKusick-Nathans Institute of Genetic Medicine of Johns Hopkins University, Baltimore. He reports having no conflicts of interest.