Cytochrome P450 2D6 Genotyping

CYP2D6 (Cytochrome P450 2D6) acts on 25% of all prescription drugs. 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, which especially elevates the risk of adverse drug reactions when these individuals are taking multiple drugs. Drugs that CYP2D6 metabolizes include selective serotonin reuptake inhibitors (SSRI), tricylic antidepressants (TCA), beta-blockers, opiates, neuroleptics, antiarrhythmics and a variety of toxic plant substances. Specific notable drugs 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 also 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 P450 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.

Specimen Information
Please call Client Services at 800-523-6487 to obtain specimen kits.

  • Buccal Swabs: 4 sterile buccal swabs
  • Blood: 5-10 cc whole blood lavender-top EDTA or Yellow-top ACD-A tubes
  • Turnaround Time:  5 days, 4 day STAT

CPT Codes
CYP2D6 Mutation DNA Analysis (provided for guidance only)
81226

Clinical Significance 

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 P450 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), ultra rapid 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. Ultra rapid 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. 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.

Individualized drug therapy based on knowledge of genetic polymorphisms can be used to improve drug therapy and decrease the incidence of adverse drug effects.

Laboratory Test Interpretation

PCR based assays detect listed alleles, including all common and most rare variants with known clinical significance at analytical sensitivity and specificity >99%.  CYP2D6: active *1, *2, *2A,*35; inactive *3,*4, *5, *6, *7, *8, *11, *12, *14, *15, *19,*20; partially active *9, *10, *17, *29,*41; gene duplications *1, *2, *4, *10, *41. Rare variants may not have been observed at Genelex. Assays developed and performance characteristics determined by Genelex.

Testing places individuals in one of four categories:

  • Normal metabolizers (NM) represent the norm for metabolic capacity. In general normal metabolizers can be administered drugs which are substrates of the CYP2D6 enzyme following standard dosing practices. Genotypes consistent with the normal metabolizer phenotype include two active CYP2D6 alleles or one active and one partially active CYP2D6 allele. Increased caution may be appropriate for individuals having one partially active allele.
  • Intermediate metabolizers (IM) may require lower than average drug dose for optimal therapeutic response to medications with the exception of prodrugs. For the majority of drugs consider decreased dosage. For prodrugs, like tamoxifen, that require activation by CYP2D6, an alternative treatment or increased dose should be considered. Genotypes consistent with the intermediate metabolizer phenotype are those with one active and one inactive CYP2D6 allele, one inactive and one partially active CYP2D6 allele, or two partially active CYP2D6 alleles.
  • Poor metabolizers (PM) are at increased risk of drug-induced side effects due to diminished drug elimination or for prodrugs, like tamoxifen, lack of therapeutic effect resulting from failure to generate the active form of the drug. Alternative treatment should be considered. Genotypes consistent with the poor metabolizer phenotype are those with no active CYP2D6 alleles.
  • Ultra rapid metabolizers (UM) exhibit higher than average rates of metabolism. Ultra rapid metabolizers are at increased risk of therapeutic failure due to increased drug elimination and thus may require an increased dose of drugs that are inactivated by CYP2D6. For prodrugs, ultra rapid metabolizers may also be at increased risk of drug-induced side effects due to increased exposure to active drug metabolites, in which case they may require lower than average doses. Genotypes consistent with ultra rapid metabolizer phenotype include three or more active CYP2D6 alleles due to duplication of an active allele.

Co-administration of other drugs. Genotype results should be interpreted in context of the individual clinical situation. In all cases monitor for co-administration of CYP2D6 inhibitors which may convert patients to poor metabolizer status. Potential adverse outcomes included overdose toxicity or treatment failure particularly for prodrugs. For more information see YouScript drug-drug and drug-gene interaction software and Cytochrome P450 Metabolism Inhibitor/Inducer Tables. Access YouScript via the patient access code provided at YouScript.net/DNAlogin.

Dosage Recommendations

Therapeutic drug monitoring is recommended in patients with metabolic variations. Keep in mind that subjects with metabolic deficiency will have decreased drug clearance and require additional time to achieve steady-state. In contrast, subjects with increased metabolic activity (UMs) have increased drug clearance and will achieve steady-state sooner that extensive metabolizers.

A complicating factor in correlating CYP2D6 genotype with phenotype is that many drugs may reduce or increase CYP2D6 catalytic activity. Consequently, an individual may require a dosing decrease greater than predicted based upon genotype alone. It is important to interpret the results of testing in the context of other co-administered drugs.

Patients with an extensive or intermediate metabolizer genotype may have CYP2D6 enzyme activity inhibited by a variety of medications or their metabolites, including many TCAs, SSRIs, many histamine H1 receptor antagonists, amiodarone, celecoxib, cimetidine, cocaine, methadone, quinidine, and ritonavir, as well as several other drugs. Treatment with drugs that are inhibitors of CYP2D6, or produce inhibitors through metabolism, may generate a poor metabolizer phenotype in an individual who has an extensive or intermediate metabolizer genotype.

CYP2D6 activity may appear to be altered because of age-associated changes in hepatic blood flow or a decrease in renal elimination of metabolites. It is important to interpret the results of testing and dose adjustments in the context of renal and hepatic function and age.

Changes in metabolic capacity for an individual do not change the pharmacologic action of the medication. Therefore standard therapeutic drug concentration target intervals can be used to optimize dosage titration. The advantage of knowing the subject's genotype is in predicting the general dosage range for initiation and recognizing changes in time to achieve steady-state for interpretation of blood concentration monitoring.

For specific dosages see charts and tables adapted from Julia Kirchheiner, et al Molecular Psychiatry Feature Review, 9 442-473 (2004), "Pharmacogenetics of antidepressants and antipsychotics: the contribution of allelic variations to the phenotype of drug response," a meta analysis of published research from 1970-2003 on the relevance of pharmacogenetic effects of CYP 2D6 and CYP 2C19 on 36 antidepressants and 38 antipsychotics.

Drug Metabolism Guide

This list is not all inclusive and is for your guidance only.

Substrates Metabolized through Cytochrome P450 2D6
Substrates refers to drugs that are either activated or deactivated by the pathway.

italics = brand name
{brackets} = minor or less potent
bold = potent
(p) = pro-drug 

alprenolol

Detrol

lidocaine

propranolol

amitriptyline

dextromethorphan

MDMA

Reglan

amphetamines

diphenhydramine

metoclopramideprolol

risperidone

aripiprazole

donepezil

metoprolol

Rythmol

atomoxetine

doxepin

mexiletine

{sertraline}

benztropine

duloxetine

mirtazapine

tamoxifen (p)

Bystolic

Ecstasy

nebivolol

thioridazine

carvedilol

encainide

nortripyline

timolol

chlorpheniramine

flecainide

ondansetron

tolterodine

chlorpromazine

fluoxetine

{oxycodone}

tramadol

clomipramine

fluvoxamine

paroxetine

venlafaxine

clonidine

haloperidol perphenazine zuclopenthixol

codeine(p)

hydrocodone (p)

promethazine

 

desipramine

imipramine

propafenone

 

 

Inhibitors Of Cytochrome P450 2D6

Inhibitors refers to drugs that reduce the ability of the pathway to process drugs. 
Co-administration will decrease the rate of metabolism of drugs through the metabolic pathway listed, increasing the possibility of toxicity.

amiodarone

clomipramine

halofantrine

pimozide

amitriptyline

cocaine

haloperidol

propafenone

bupropion

desipramine

hydroxyzine

quinidine/quinine

celecoxib

diphenhydramine

idarubicin

ranitidine

chlorpheniramine

doxepin

levomepromazine

ritonavir

chlorpromazine

duloxetine

methadone

Sensipar

cimetidine

escitalopram

metoclopramide

sertraline

cinacalcet

fluoxetine

moclobemide

terbinafine

cisapride

fluvoxamine

paroxetine

thioridazine

citalopram

goldenseal

perphenazine

ticlopidine

 

Inducers of Cytochrome P450 2D6
Inducers refers to drugs that increase the activity of a pathway.
Co-administration increases the rate of excretion for drugs metabolized through the pathway indicated, reducing the drug's effectiveness.

There are no inducers of CYP2D6.

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 PCR based technologies that detect 17 nucleotide variants and two gene rearrangements. The performance of this assay was validated by Genelex Corporation. 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, qualified to perform high complexity clinical testing.

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.

View References >

 

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