Click here to download an information package

Tamoxifen is a SERM (Selective Estrogen Receptor Modulators) used to treat treat breast cancer by preventing the binding of estrogen to breast cells. Tamoxifen is metabolized by the liver enzyme CYP2D6, which converts it into the active metabolite called endoxifen. Endoxifen is responsible for the estrogen blocking activity of tamoxifen. The CYP2D6 enzyme is translated from the CYP2D6 gene in the DNA. Patients taking Tamoxifen experience side effects of which 'Hot Flashes' are most common. This is treated by the administration of Serotonin Selective reuptake Inhibitors (SSRIs), commonly known an antidepressants. These medications interfere with the ability of the gene to undergo transcription and translation to produce the enzyme. Genetic variations in the CYP2D6 gene also play an important role in determining the amount of enzyme available to activate tamoxifen.

Interactions between the drug and the gene as well as interactions between multiple drugs prescribed to the patient play a strong role in determining the efficacy of the drug. GeneMedRx is a software that predicts such harmful interactions and will help you alter the dosing regimen of the patient to eliminate drugs that can lead to adverse drug reactions. Genelex provides physicians with a free 90 day accress to the software after every patient referral to enable analysis of test results at a higher level. For more information on GeneMedRx, please visit www.genemedrx.com

CYP2D6 (cytochrome P450 2D6) acts on one-fourth of all prescription drugs, including the selective serotonin reuptake inhibitors (SSRI), tricylic antidepressants (TCA), betablockers, opiates, neuroleptics, antiarrhythmics and a variety of toxic plant substances. Some 7-14% of the population has a slow acting form of this enzyme and 7% a super-fast acting form. Thirty-five percent are carriers of a non-functional CYP2D6 allele, especially elevating the risk of adverse drug reactions when these individuals are taking multiple drugs. Drugs that CYP2D6 metabolizes include Prozac, Zoloft, Paxil, Effexor, Hydrocodone, Amitriptyline, Claritin, Cyclobenzaprine, Haldol, Metoprolol, Rythmol, Tagamet, Tamoxifen, and the over-the-counter diphenylhydramine drugs, Allegra, Dytuss, and Tusstat. CYP2D6 is responsible for activating the prodrugs codeine and other opioids into their active forms. The analgesic activity of the drugs is therefore reduced or absent in CYP2D6 poor metabolizers. Refer to list for substrates, inhibitors and inducers of CYP2D6.

Genelex offers improved detection rates using an extended Cytochrome P-450 2D6 DNA mutation panel. This test identifies 17 small nucleotide variants and two gene rearrangements in PCR-multiplex format, providing increased sensitivity and quality performance. This CYP2D6 Mutation Detection Panel is the most extensive on the market and covers over 93-97% of poor metabolizer phenotypes. Analytical specificity and sensitivity for detection of these mutations are >99%.

Indication for Testing
For individuals with a personal or family history of adverse drug reactions to medications metabolized by CYP2D6. Confirm presence of genotypes that affect the metabolism of drugs such as Tamoxifen that are metabolized by CYP2D6.

Clinical Significance

Mayo Research
Researchers at the Mayo Clinic discovered that one of the active metabolites Endoxifen, is primarily responsibe for the patients response to tamoxifen treatment for breast cancer. The other active metabolite 4-Hydroxytamoxifen is found to be produced in very small quantities by the liver and is unable to inhit the production of estrogen. Tamoxifen is converted to Endoxifen by the CYP2D6 enzyme and the researchers found that tamoxifen is less effective in women who have a deficiency in the enzyme CYP2D6. A team of Mayo Clinic investigators, including Matthew Goetz, M.D., Matthew Ames, Ph.D., and James Ingle, M.D., have shown that women with certain variations in the CYP2D6 gene — so-called "poor metabolizers" — have a significantly higher risk of relapse when they take tamoxifen.

This study was presented at the Cancer Therapy & Research Center-American Association for Cancer Research (CTRC-AACR) 31st Annual San Antonio Breast Care Symposium.

Clinical evidence on endoxifen activity presented by Mayo Research

CYP2D6 Metabolic Activity
Phenotype prevalence is approximately 10 % PM, 7% UM, and 35% IM.
Drugs metabolized by this enzyme - approximately 25%.
Low-capacity, high-affinity enzyme.

Cytochrome P450 2D6 (CYP2D6) is a highly polymorphic liver enzyme of the cytochrome P450 super family involved with the metabolism and elimination of many commonly prescribed drugs. Genetic polymorphism in CYP2D6 is common and can affect therapeutic response to these drugs. The enzyme activity is expressed at highly variable levels.

Detecting genetic variations in drug-metabolizing enzymes is useful for identifying individuals who may experience adverse drug reactions with conventional doses of certain medications. Individuals who possess CYP2D6 poor metabolizer variants may exhibit different pharmacokinetics (drug levels) than normal individuals. As a result, such individuals may require non-conventional doses of medications that require CYP2D6 activity for biotranformation. Conversely, medications that do not require CYP2D6 biotranformation may be preferentially selected for patients with potentially impaired CYP2D6 metabolic capacity to avoid adverse drug reactions.

CYP2D6 is considered a low-capacity, high-affinity enzyme and CYP2D6 will preferentially metabolize drugs at lower concentrations. As the concentration of a drug increases, the metabolism spills over to CYP3A4 and CYP1A2, which are high-capacity, low-affinity enzymes. Thus if a drug that has several metabolic pathways but relies on CYP2D6 as its major pathway is given to a patient with poor CYP2D6 activity, the other P-450 enzymes that are high capacity, low affinity will clear the drug, but clearance will be slower and less efficient, and drug levels will increase, increasing the risk for adverse drug reactions.
Four phenotypes are identified: poor metabolizers (PM), ultrarapid metablizers (UM), intermediate metabolizers (IM) and normal metabolizers (NM).

Normal functional activity alleles of the CYP2D6 gene are designated CYP2D6*1 and CYP2D6*2. Homozygous normal allele individuals have a normal metabolizer phenotype (NM). The alteration of alleles from normal fall into six mutant allele categories: one amino acid change or deletion, frameshift, splicing defect, stop codon, insertion and entire gene deletion. Ultrarapid metabolizers (UM) have duplicate or multiple copies of the complete CYP2D6 gene.

Some 7-14% of Caucasians are poor metabolizers (PM) and lack functional CYP2D6. The genetic basis for poor metabolizers is now well defined. The four most common mutant alleles are CYP2D6*3, CYP2D6*4, CYP2D6*5, and CYP2D6*6 and account for 93-97% of the PM phenotypes in the Caucasian population. The most common of these alleles, CYP2D6*4 has a reported frequency of 21.5-28.6% and involves a base substitution from G to A at position 1846, which causes a splicing defect in exon 3. CYP2D6*3 has a reported frequency of 2.7% and involves a deletion of at position 2549, causing a frameshift in exon 5. In the relatively common PM allele CYP2D6*5 (2.6%) the entire CYP2D6 gene is deleted. Individuals who are homozygous for PM alleles do not display CYP2D6 enzyme activity, nor do any those who carry combinations of these alleles. Additional alleles CYP2D6*6-8, *11-16, *19-20 and *38 are also associated with lack of enzyme activity identified with bufarulol, dexromethorphan, debrisoquine or sparteine. However, these alleles are rare. Some 35% of Caucasians are intermediate metabolizers (IM) with a combination of one functional CYP2D6 and one mutant CYP2D6 allele.

There are ethnic differences in distribution of PMs, IMs and UMs. PMs are reported to make up 7-14% of populations of European origin (Caucasian) with CYP2D6*4 being the predominant non-functional allele observed. A recent review indicated that Asians, Pacific Inlanders, African and African Americans have higher percentages of reduced functional or non-functional CYP2D6 alleles (between 40% and 50%) than do Europeans (26%). Therefore the percentages of PMs in the former groups are most likely higher. Pacific Islanders have a high frequency (41%) of a reduced functional allele CYP2D6*10, indicating slower metabolism. Non-functional PMs and reduced function IMs represent about 50% of African  populations (non functional CYP2D6*17 represents 35% of allele variation). African Americans show twice the allele frequency of PMs compared with Africans (14.5% vs 6.3%).

Not anticipating that patients may be PMs is potentially dangerous. Any drug that is primarily metabolized by CYP2D6 and ingested by a PM will have a delayed metabolism. A drug may also be less effective for a PM at CYP2D6 if the drug needs to be activated by CYP2D6. PMs usually require lower doses to achieve desired effects. The PM will accumulate the parent drug and incur the risk of enhanced side effects. The drug may be secondarily metabolized by another P450 enzyme that is higher in capacity but that has a lower affinity for the drug or substrate. Often the alternative is CYP3A4. This shifting to a less efficient enzyme leads PMs to have higher drug levels of the parent compound. Evidence suggests that PMs at CYP2D6 have a poorer tolerance of side effects and higher drug levels, even though the CYP3A4 route is available.

Specimen Information

Please call Client Services at 800-523-3080 to obtain specimen kits.
• Buccal Swabs: 4 sterile Buccal Swabs
• Turnaround Time: 10 days, 5 Day STAT

CPT Codes

CYP2D6 Mutation DNA Analysis (provided for your guidance only)
1 X 83891, 2 X 83892, 1 X 83900, 14 X 83914, 1 X 83909, 1 X 83912, 1 X 83912-26

Test Methodology and limitations

DNA extraction / Polymerase Chain Reaction (PCR) / Bead Hybridization.

This assay detects all common and most rare CYP2D6 variants with known clinical significance. Laboratory specimens were analyzed using the xTAGTM Mutation Detection system for P450-2D6 (Luminex Molecular Diagnostics) which detects 17 nucleotide variants and two gene rearrangements in a multiplex polymerase chain reaction and allele-specific primer extension format. The performance of the xTAGTM Mutation Detection system for P450-2D6 for use with the Luminex 100 xMAP IS System was validated by Genelex Corporation. Rare CYP2D6 variants may not yet have been observed at Genelex (<1% of the population). As with all laboratory testing there is a possibility of error. Genelex Corporation is certified by the Clinical Laboratory Improvement Amendments (CLIA No. 50D0980559) and as Washington State Medical Test Site No. MTS-3919 is qualified to perform high complexity clinical testing. Genetic counseling is recommended.

DNA testing will not detect all the known mutations that result in decreased or inactive CYP2D6. Absence of a detectable gene mutation or polymorphism does not rule out the possibility that a patient has an intermediate or poor metabolizer phenotype. This test does not detect polymorphisms other than those listed. Other polymorphisms in the primer binding regions can affect the testing, and ultimately, the genotyping assessments made. Rare diagnostic errors may occur due to primer site mutations. Mutations in other genes associated with drug metabolism will not be detected. Drug metabolism may be affected by non-genetic factors. DNA testing does not replace the need for clinical and therapeutic drug monitoring.

3000 First Ave., Suite One, Seattle, WA 98121

800 523-3080