By Turna Ray
Two recently published pharmacogenetic analyses looking at the impact of CYP2D6 alleles on tamoxifen response found no evidence that patients who harbor these mutations fail to respond to the hormone therapy.
The studies represent the latest data in a history of conflicting evidence regarding the effectiveness of CYP2D6 genotyping to predict tamoxifen response. The resulting uncertainty among researchers and industry players about whether to adopt CYP2D6 genotyping to personalize treatment with tamoxifen illustrates the challenges associated with validating PGx markers and the pitfalls of commercializing diagnostics in a rapidly evolving space.
The clinical trials, published online March 6 in the Journal of the National Cancer Institute, are accompanied by an editorial that takes issue with commercial labs offering CYP2D6 PGx testing for tamoxifen response despite insufficient clinical evidence supporting the validity and utility of these markers in predicting which patients are likely to benefit from the hormone therapy.
"Although no consensus was reached regarding routine CYP2D6 genotype testing [in previously published studies], many laboratories began testing for CYP2D6 allelic variants," Catherine Kelly of Sunnybrook Health Sciences Center in Toronto and Kathleen Pritchard of the Mater Misericordiae University Hospital in Dublin wrote in the editorial. "Additional tests and charges were administered to patients, and therapies were presumably adjusted accordingly, all prematurely."
Noting that the two new studies reported in JNCI also found no association between CYP2D6 variants and poor response to tamoxifen, the uncertainty about genetic testing in this setting "has likely been laid to rest," Kelly and Pritchard concluded.
Meanwhile, Howard Coleman, CEO of genomic testing firm Genelex, blames the waning adoption of the company's CYP2D6 genotyping service, called Tamoxitest, on contradictory clinical evidence from studies that are plagued by a host of design issues. "It seems like it only takes one study, no matter how shaky, to knock out a series of studies in favor of PGx testing," Coleman told PGx Reporter. "Academic medicine seems to be looking for any excuse not to do DNA testing and is asking for unreasonable standards of proof to endorse its implementation."
One of the first studies to identify a possible association between CYP2D6 genotype and tamoxifen was reported in 2005, when Matthew Goetz of the Mayo Clinic and colleagues published a retrospective genomic analysis in the Journal of Clinical Oncology that found that tamoxifen-treated breast cancer patients with the CYP2D6 *4/*4 genotype, or poor metabolizers, relapsed sooner than intermediate or extensive metabolizers. However, CYP2D6 genotypes were not associated with whether patients lived longer. This study, which evaluated 223 samples, also found that patients with the CYP2D6 *4/*4 genotype had a lower incidence of tamoxifen-induced hot flushes.
Subsequent studies have both confirmed and contradicted these findings, but most of these trials have involved small cohorts of patients or been retrospective in nature. The JNCI studies published last week represent the largest trials so far to explore the connection between CYP2D6 genotype and tamoxifen response.
An advisory panel in 2006 recommended that the US Food and Drug Administration update the drug's label to inform doctors and patients that individuals with certain CYP2D6 alleles may not benefit from tamoxifen therapy (PGx Reporter 11/15/2006). Although the FDA usually takes the advice of its advisory committees, in the case of tamoxifen, the agency did not update the label of the drug with genetic testing information, likely due to the conflicting PGx conclusions in the published literature.
In 2009, Timothy Lash of Boston University and colleagues reviewed data reported in 10 epidemiology studies investigating the association between CYP2D6 genotype and breast cancer recurrence and found "widely heterogeneous" results.
"The studies reporting a positive association might receive the most attention, because they report a result consistent with the profile of metabolite concentrations; not because they are more reliable by design," Lash and his colleagues wrote in their review, published in Lancet Oncology. "We argue that a recommendation for CYP2D6 genotyping of candidates for tamoxifen therapy, and its implicit conclusion regarding the association between genotype and recurrence risk, is premature."
The Studies
Tamoxifen is the standard hormone therapy doctors prescribe to reduce the recurrence of breast cancer in women who have early-stage disease driven by the estrogen hormone. Treatment with adjuvant tamoxifen following tumor resection has been shown to decrease cancer recurrence by 50 percent in patients in whom the cancer has spread to the lymph nodes and by more than 30 percent in patients whose disease do not involve the lymph nodes.
However, previously published data suggest that more than 20 percent of node-negative women may still see their cancer recur within 15 years, even with tamoxifen treatment. The National Comprehensive Cancer Network recommends treatment with aromatase inhibitors as an alternative to tamoxifen for postmenopausal women with hormone-sensitive breast cancer.
In one of the JNCI studies published last week, led by Meredith Regan of the Dana-Farber Cancer Institute, researchers assessed tumor tissue from more than 4,800 postmenopausal women with hormone receptor-positive breast cancer who were enrolled in the Phase III Breast International Group 1-98 trial from 1998 to 2003, and were randomized to receive adjuvant tamoxifen alone or in combination with letrozole. In their analysis, Regan et al. found no evidence that patients with reduced CYP2D6 enzyme activity due to certain CYP2D6 genotypes had limited response to tamoxifen.
Furthermore, the researchers found that both poor and intermediate metabolizers of CYP2D6 were at greater risk of experiencing tamoxifen-induced hot flushes. This finding contradicts the earlier hypothesis from Goetz et al. that patients who have diminished ability to convert tamoxifen into a more active metabolite, called endoxifen, are more likely to experience such hot flushes.
"CYP2D6 phenotypes of reduced enzyme activity were not associated with worse disease control but were associated with increased hot flushes, contrary to the [previous] hypothesis," Regan et al. concluded. "The results of this study do not support using the presence or absence of hot flushes or the pharmacogenetic testing of CYP2D6 to determine whether to treat postmenopausal breast cancer patients with tamoxifen."
Importantly, the researchers suggest in their paper that there are better tamoxifen metabolites than endoxifen to use in gauging treatment benefit. Although endoxifen is more effective than tamoxifen at blocking ER function and repressing proliferation of breast cancer cells, plasma concentrations of tamoxifen and another primary metabolite, N-desmethyltamoxifen, are higher than both endoxifen and 4-hydroxytamoxifen, Regan et al. noted. Additionally, "tamoxifen and metabolites N-desmethyltamoxifen, didesmethyltamoxifen, and 4-hydroxytamoxifen have been estimated to nearly saturate ER with 99.94 percent occupancy," they wrote.
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In the second study published in JNCI, James Rae of the University of Michigan Medical Center and colleagues genotyped archival tumor samples for polymorphisms in the CYP2D6 and UGT287 genes from more than 1,300 postmenopausal women with hormone receptor-positive breast cancer. These patients were enrolled in the ATAC (Arimidex, Tamoxifen, Alone or in Combination) trial from 1996 to 2000 in the UK, randomized to receive anastrozole alone, tamoxifen alone, or a combination of the two treatments, and followed for about a decade.
Rae et al. observed no statistically significant associations between CYP2D6 extensive metabolizer or poor metabolizer status and disease recurrence in tamoxifen-treated patients. The researchers also found nearly no associations between UGT2B7 genotypes and cancer recurrence in patients treated with tamoxifen.
"Although variants in CYP2D6 and the use of CYP2D6 inhibitors correlate with decreased plasma concentrations of endoxifen, these differences do not appear to influence benefit from tamoxifen," Rae et al. wrote. "Furthermore, our results do not suggest an association between UGT2B7 genotype, which is also expected to influence plasma endoxifen concentrations and clinical outcomes on tamoxifen."
The strength of these studies is that both are retrospective genomic analyses of large, prospectively designed, randomized trials. Since prospectively proving that a molecular marker is predictive of treatment response often requires a prohibitively large cohort and lengthy follow up, the so-called retrospective-prospective genotyping substudy is increasingly performed by researchers as a more manageable and cost-effective way of investigating the validity of PGx tests.
However, this study design has flaws that often raise questions. For example, although Kelly and Pritchard laud Regan et al. and Rae et al. for conducting retrospective, blinded genomic analysis of thousands of patients from randomized, controlled studies, they acknowledge that the "statistical power [in these studies] may still be insufficient to show a positive association between CYP2D6 activity and outcome in patients taking tamoxifen."
Furthermore, studies have shown that CYP2D6 genotype may influence endoxifen levels in patients treated with tamoxifen alongside other CYP2D6-inhibiting drugs, such as certain selective serotonin re-uptake inhibitors prescribed as anti-depressants. However, in the BIG 1-98 trial, study investigators did not collect information on concomitant medications the patients may have been taking. The suggestion that patients treated with tamoxifen should avoid other CYP2D6 inhibitors "is controversial, and our study cannot contribute to the clinical decision about whether to avoid CYP2D6 inhibitors with tamoxifen treatment," Regan et al. wrote.
In the ATAC genomic substudy, meantime, the number of patients on concomitant CYP2D6 inhibitors was low. Rae et al. highlight this point to note that this observation "strengthens" their findings "since [CYP2D6] inhibitors could not be a major confounding factor in our genotype-alone analyses."
Premature Adoption?
Although the editorial accompanying the two JNCI studies bemoans the premature commercialization of PGx testing for tamoxifen, industry observers PGx Reporter spoke to say there hasn't been much of a market for CYP2D6 in this setting for more than a year now.
According to Genelex's Coleman, Tamoxitest was "gaining traction" until early data from the BIG 1-98 and ATAC genomic substudies were presented at the 2010 San Antonio Breast Cancer Symposium. "As a result, what testing volume we were getting has pretty well died off," Coleman said.
Similarly, Brian Leyland-Jones, director of Emory University's Winship Cancer Institute and one of the lead authors of the BIG 1-98 genomic substudy, said that among his "extensive network of colleagues, no one has ordered CYP2D6 testing since our presentations at SABCS 2010."
Given the publication of these studies in JNCI, it is unclear the extent to which CYP2D6 testing will continue to be available at all.
Genelex markets Tamoxitest as a laboratory-developed test and has no immediate plans to discontinue the service based on the JNCI papers. The website for the test states that Tamoxitest "analyzes the CYP2D6 gene and looks for variations. This can predict your body's ability to process tamoxifen in order to produce the desired effect." The company advises patients to print out information about the test from Genelex's website and discuss it with their doctors.
Coleman highlighted the need to adjust for other clinical factors, particularly the concomitant use of CYP2D6 inhibitors, when assessing the validity of CYP2D6 genotyping to predict tamoxifen benefit. The fact that the BIG 1-98 substudy did not account for the potential impact of other CYP2D6 inhibitors is a black mark against it, in his view.
"It is unlikely that these women were not taking other meds that would interfere with 2D6 activity. Patients thus phenoconverted to poor metabolizer status could easily confound the results … to the extent that the conclusions are erroneous," Coleman said.
"This is an unfortunately common problem in many, if not the majority of PGx studies," he noted, adding that the CYP2D6 enzyme metabolizes approximately 25 percent of the most commonly prescribed drugs, including SSRIs, beta blockers, and opioid analgesics, many of which in turn also inhibit the CYP2D6 enzyme.
In contrast, Leyland-Jones felt that although the BIG 1-98 substudy didn't control for co-medications, it's unlikely that this confounded the findings in a substantive way. "It is true that we did not control for co-medications, but no one prescribes strong CYP2D6 inhibitors or modulators anymore (nor have done for the past several years)," he said over e-mail. "Hence, it is extremely unlikely that co-medications could have greatly compromised the analyses and our definitively negative results."
In addition to Tamoxitest, Genelex also markets a genetic testing service to predict adverse drug reactions called GeneMedRx. As part of the service, physicians and patients have access to an online tool that lists all the prescription, over-the-counter, and herbal medications the patient is on, alongside genetic test results. CYP2D6 testing is offered as part of this service alongside a host of other genes associated with ADRs.
"We plan to continue offering [Tamoxitest] in case interest revives," Coleman said, adding that the company's customers generally order several tests that gauge polymorphisms in the Cytochrome P450 pathway. "Our sweet spot is polypharmacy management," he said. "In other words, our focus is not specific gene drug combinations, but comprehensive medication management."
Other than Genelex, pharmacy-benefit manager Medco also includes CYP2D6 testing to gauge tamoxifen response among its genetic testing offerings to clients enrolled in its personalized medicine programs. "Testing patients for CYP2D6 can help identify patients who are likely to benefit from tamoxifen therapy," Medco states on its website. CVS Caremark also offers clients PGx testing for tamoxifen through a similar program.
AutoGenomics markets a CYP2D6 test labeled "research use only," and as a result, in line with FDA's guidance on RUO and investigational-use only in vitro diagnostics, this test is not allowed to be marketed for clinical decision making.
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