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Tamoxifen is the generic name for the most widely used of the SERMs in the treatment of breast cancer; it is usually sold under the trade name Nolvadex. Since the expiration of it's patent in 2002, tamoxifen is now widely available in generic form.Wikipedia

Up to 7-10% of women with breast cancer may not receive the full medical benefit from taking tamoxifen, because of their unique genetic make-up. These women have less functional versions of the CYP2D6 gene, which may reduce the effectiveness of tamoxifen and increase their chance of breast cancer recurrence.

A medical genetic summary (last updated 2016; [1]), summarizes the Dutch PGx Working Group therapeutic recommendations based on CYP2D6 phenotypes as follows:

CYP2D6 Phenotype CYP2D6 Genotype Therapeutic Recommendation re Tamoxifen
Ultrarapid metabolizer More than two functional alleles None
Intermediate metabolizer Either one inactive allele, or two decreased activity alleles Increased risk for relapse. Avoid concomitant use of CYP2D6 inhibitors. Consider aromatase inhibitor for postmenopausal women.
Poor metabolizer Two inactive alleles Increased risk for relapse of breast cancer. Consider aromatase inhibitor for postmenopausal women.

Note that this same reference ([2]) summarizes activity levels for various CYP2D6 alleles as follows:

  • Active alleles: *1, *2, *33, *35
  • Decreased activity alleles: *9, *10, *17, *29, *36, *41
  • Inactive alleles: *3, *4, *5, *6, *7, *8, *11-*16, *19-*21, *38, *40, *42

On Oct 18, 2006 the Subcommittee for Clinical Pharmacology recommended relabeling tamoxifen to include information about this gene in the package insert.

A 2009 review of several studies since then mostly confirms worse clinical outcome in patients with decreased CYP2D6 metabolism and supports the notion that CYP2D6 genotype may well become a clinically relevant predictive marker.[PMID 19118028]

An additional retrospective study published after this review extended the earlier findings by specifically studying primarily post-menopausal women with early (I-III) stage, ER+/PR+ breast cancer. Based on a median follow-up time of 6 years, this report found that the presence of 2 functional CYP2D6 alleles was associated with better clinical outcomes and the presence of nonfunctional or reduced-function alleles with worse outcomes.[PMID 19809024]

However, an argument has also been made that making conclusions based on such studies is still premature, primarily because there are also over ten studies with "widely heterogeneous results with relative-risk estimates outside the range of reasonable bounds".[PMID 19647203]

A 2010 study has extended the CYP2D6 risk by also pointing to risk associated with SNP rs3740065 in the ABCC2 gene; patients taking tamoxifen who had all 4 risk alleles at these two genes were said to have a 45 fold higher risk of breast cancer recurrence compared to patients with one or less risk alleles. For CYP2D6 alone, carriers of one or two risk alleles had adjusted hazard ratios for recurrence of ~4 fold and ~9 fold, respectively, compared to women with two fully functioning alleles.[PMID 20124171]

Another 2010 study finds a significantly increased risk for thromboembolic events in women who carry a rs6025(A) SNP and who are taking tamoxifen as part of their treatment for breast cancer.[PMID 20554945]

A 2018 review, including clinical practice guidelines, acknowledging that conflicting literature exists, still concluded that the majority of current evidence indicates that less active CYP2D6 alleles have worse survival outcomes after tamoxifen treatment. This review concluded that there is value in using clinically validated CYP2D6 genotyping panels to guide treatment decisions for non-metastatic ER-positive breast cancer patients, and that alternatives to tamoxifen be considered in CYP2D6 poor or intermediate metabolizers.[PMID 30411242]