Cytochrome P450 2C9 Genotyping

Web Seminar: Pharmacogenetics in the Practice of Medicine

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Cytochrome P450 2C9 (CYP2C9) acts on 16% of drugs in current clinical use. About 35% of Caucasians have a slow acting form of this enzyme. CYP2C9 is an important drug-metabolizing enzyme that catalyzes the biotransformation of many other clinically useful drugs including angiotensin II blockers, nonsteroidal anti-inflammatory drugs, the alkylating anticancer prodrugs, sulfonylureas, some antidepressants, tamoxifen and many others. Of special interest are those drugs with narrow therapeutic index, such as S-warfarin, tolbutamide and phenytoin, where impairment in CYP2C9 metabolic activity might cause difficulties in dose adjustment as well as toxicity. Indications for testing include lack of therapeutic effect or difficulties with side effects to any of the drugs metabolized by CYP2C9.

Genelex offers improved detection rates using an extended Cytochrome P-450 CYP2C9 DNA mutation panel. This test identifies 5 of the most common variants of CYP2C9 (2C9*2 - 2C9*6) and is the most extensive on the market. Most other laboratories only test for CYP2C9*2 and CYP2C9*3. 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 CYP2C9. Confirm presence of genotypes that affect the metabolism of any drugs that are metabolized by cytochrome CYP2C9.

Specimen Types

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: 6 days

CPT Codes

CYP2C9 Mutation DNA Analysis (provided for your guidance only)
1 X 83891, 2 X 83892, 1 X 83900, 5 X 83914, 1 X 83909, 1 X 83912, 1 X 83912-26

Clinical Significance

CYP2C9 phenotype prevalence is 2-4% Poor Metabolizer, >35% Intermediate Metabolizer for CYP2C9.
Drugs metabolized by this enzyme approximately 5-10%.

Incidence of CYP2C9*2: Caucasians 8-13%, Asians 2-6%, African Americans less than 1%
Incidence of CYP2C9*3: Caucasians 6-10%, Asians less than 1%, African Americans 1-4%

CYP2C9 is a highly polymorphic liver enzyme of the cytochrome P450 super family involved with the metabolism and elimination of many commonly prescribed drugs. Genetic polymorphisms in CYP2C9 are common and can affect therapeutic response to drugs. The enzyme activity is expressed at highly variable levels. Three phenotypes are identified: poor metabolizers (PM), intermediate metabolizers (IM) and normal metabolizers (NM).

This assay detects all common and most rare CYP2C9 variants with known clinical significance. The five CYP2C9 allelic variants detected in this CYP2C9 genotyping test provide greater than 98% coverage of the variant alleles found for this gene. The wildtype allele of the CYP2C9 gene is designated CYP2C9*1. Homozygous wild-type individuals have a normal metabolizer phenotype (NM). The most common poor metabolizer phenotypes have been identified as CYP2C9*2 and CYP2C9*3. CYP2C9*2 (C430T) and CYP2C9*3 (A1075C) each differ from the normal CYP2C9*1 by a single nucleotide substitution, which leads to impaired enzyme activity. Lee et al (2002) determined that these two poor metabolizer types CYP2C9*2 and CYP2C9*3 were found in up to 35% of Caucasians (42% Croatians). Among different white populations CYP2C9*2 and CYP2C9*3 are of significance with allelic frequencies of 8-19% and 4-16% respectively. In Africans and Asians both variants are much less frequent (0.5-4%). CYP2C9*4 has been exclusively identified in Japanese, and CYP2C9*5 and CYP2C9*6 found in African Americans with low allelic frequency (>2%). Homozygosity for the CYP2C9*3 or CYP2C9*2 genotype is relatively rare (~1-2 %) in Caucasians.

Detecting genetic variations in drug-metabolizing enzymes is useful for identifying individuals who may experience adverse drug reactions (ADRs) with conventional doses of certain medications. Individuals who possess CYP2C9 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 CYP2C9 for biotranformation. Conversely, medications that do not require CYP2C9 biotranformation may be preferentially selected for patients with potentially impaired CYP2C9 metabolic capacity to avoid ADRs.

Laboratory Test Interpretation

Genelex offers improved detection rates using an extended Cytochrome P-450 CYP2C9 and VKORC1 DNA mutation panel. This test identifies 5 of the most common variants of CYP2C9 (2C9*2 - 2C9*6).

Cytochrome P-240 2C9 Mutations Detected
CYP2C9 allele	Nucleotide change	Effect on Enzyme Metabolism
*1	None (wildtype)	Normal
*2	430C>T	Inactive
*3	1075A>C	Inactive
*4	1076T>C	Inactive
*5	1080C>G	Inactive
*6	818delA	Inactive

For additional information see the CYP2C9 allele nomenclature database at http://www.cypalleles.ki.se/cyp2c9.htm

Testing places individuals in one of three categories:

• Normal metabolizers (NM) represent the norm for metabolic capacity. In general normal metabolizers can be administered drugs which are substrates of the CYP2C9 enzyme following standard dosing practices. Genotypes consistent with the normal metabolizer phenotype include two active CYP2C9 alleles.
  
  
• 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, such as Plavix, that require activation by CYP2C9, an alternative treatment or increased dose should be considered. Genotypes consistent with the intermediate metabolizer phenotype are those with one active and one inactive CYP2C9 allele.
  
  
• Poor metabolizers (PM) are at increased risk of drug-induced side effects due to diminished drug elimination or for prodrugs, such as Plavix, 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 CYP2C9 alleles.
  
  

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 CYP2C9 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 GeneMedRx drug-drug and drug-gene interaction software and Cytochrome P450 Metabolism Inhibitor/Inducer Tables. Access GeneMedRx via the patient access code provided at www.GeneMedRx.com/DNAlogin.

Dosage Recommendations

A complicating factor in correlating CYP2C9 genotype with phenotype is that many drugs may reduce or increase CYP2C9 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.
CYP2C9 activity also is dependent upon hepatic and renal function status, as well as age. Patients also may develop toxicity if hepatic or renal function is decreased. Consider the results of testing and dose adjustments in the context of renal and hepatic function and age.

• Poor Metabolizers
  Reduce dose to 20-60% of standard dosage.
  
  
• Intermediate Metabolizers
  Start IM’s at lowest efficacious dose, avoid multiple drug therapy that inhibits or activated through the same pathway.
  
  
• Therapeutic drug monitoring in PM and IM subjects is highly recommended. Again standard measures of efficacy (INR for Warfarin or therapeutic target interval for Phenytoin, for example) can be applied to ensure optimal therapy.
  
  

For specific genotype-specific clearance rates see charts from Julia Kirchheiner, et al: The CYP2C9 polymorphism: from enzyme kinetics to clinical dose recommendations Personalized Med 2004 1(1) 63-84.

TEST Methodology and limitations

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

This assay detects all common and many rare CYP2C9 variants with known clinical significance. Laboratory specimens were analyzed using PCR based technologies that detect the 5 most common CYP2C9 variants. The performance of this assay was validated by Genelex Corporation. Rare CYP2C9 variants may not yet have been observed at Genelex. 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 CYP2C9. 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.

Drug Metabolism Guide

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

Substrates Metabolized through Cytochrome P-450 2C9

Substrates refers to drugs that are either activated or deactivated by the pathway.

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  

{Avandia}	fluvastatin	{naproxen}	{tamoxifen}
Amaryl	glimepiride	nateglinide	tenoxicam
Atacand	glipizide	Orinase	tetrahydro-cannabinol (marijauna)
bosentan	Glucotrol	phenobarbital	Tracleer
candesartan	glyburide	phenytoin	tolbutamide
celecoxib	ibuprofen	piroxicam	Tomide
chlorpropamide	indomethacin	{rosiglitazone	torsemide
Cozaar (p)	irbesartan	{sertraline}	valproic acid
DiaBeta	Lescol	Starlix	zafirlukast
diclofenac	lornoxicam	sulfa drugs
fluoxetine	losartan	suprofen
flurbiprofen	meloxicam	S-warfarin

 

Inhibitors of Cytochrome P-450 2C9

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.

Accolate	delavirdine	fluvoxamine	teniposide
Arimidex	efavirenz	isoniazid	valproic acid
Tricor	fenofibrate	phenylbutazone	voriconazole
Vfend	fluconazole	sertraline	zafirlukast

Inducers of Cytochrome P-450 2C9

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.

aprepitant-long term	bosentan	lopinavir/ritonavir	St John’s Wort-long term
barbituates	carbamazepine	rifampin-chronic

References

1. Kirchheiner J et al. Pharmacogenetics of antidepressants and antipsychotics: the contribution of allelic variations to the phenotype of drug response. Molecular Psychiatry 2004;9:442-473.
2. Julia Kirchheiner, Martina Tsahuridu, Wafaa Jabrane, Ivar Roots, Jürgen Brockmöller The CYP2C9 polymorphism: from enzyme kinetics to clinical dose recommendations. Future Medicine. 2004:1, 63-84
3. Schwarz UI. Clinical relevance of genetic polymorphisms in the human CYP2C9 gene. Eur J Clin Invest. 2003;33 Suppl 2:23-30.
4. Kirchheiner J, Brockmöller J. Clinical consequences of cytochrome P450 2C9 polymorphisms. Clin Pharmacol Ther. 2005 Jan;77(1):1-16.
5. Brockmoller J et al. Pharmacogenetic diagnosis of cytochrome P450 polymorphisms in clinical drug development and in drug treatment. Pharmacogenetics 2000;1:125-51.
6. Human Cytochrome P450 (CYP) Allele Nomenclature Committee; CYP2C9 allele nomenclature database at http://www.cypalleles.ki.se/cyp2c9.htm
7. Cozza KL, Armstrong SC, Oesterheld JR (2003) Drug Interaction principles for Medical Practice. American Psychiatric Publishing Inc
8. Aynacioglu AS, et al. Frequency of cytochrome P450 CYP2C9 variants in a Turkish population and functional relevance for phenytoin. Br J Clin Pharmacol 1999; 48(3):409-415
9. Brockmoller J et.al. Pharmacogenetic diagnosis of cytochrome P450 polymorphisms in clinical drug development and in drug treatment. Pharmacogenetics. 2000:1:125-51.
10. Chang TK, et al. Enhanced cyclophosphamide and ifosfamide activation in primary human hepatocyte cultures: response to cytochrome P-450 inducers and autoinduction by oxazaphosphorines. Cancer Res 1997; 57(10):1946-54.
11. Hamman MA, Thompson GA, Hall SD. Regioselective and stereoselective metabolism of ibuprofen by human cytochrome P450 2C. Biochem Pharmacol 1997; 54(1):33-41.
12. Higashi MK, Veenstra DL, Kondo LM, Wittkowsky AK, Srinouanprachanh SL, Farin FM, Rettie AE. Association between CYP2C9 genetic variants and anticoagulation-related outcomes during warfarin therapy. JAMA. 2002 Apr 3;287(13):1690-8.
13. Ho PC, et al. Influence of CYP2C9 genotypes on the formation of a hepatotoxic metabolite of valproic acid in human liver microsomes. Pharmacogenomics J 2003; 3(6):335-42.
14. Kirchheiner J, Brockmoller J. Clinical consequences of cytochrome P450 2C9 polymorphisms. Clin Pharmacol Ther. 2005 Jan;77(1):1-16.
15. Miners J. CYP2C9 polymorphism: impact on tolbutamide pharmacokinetics and response. Pharmacogenetics 2002; 12(2):91-2.
16. Peyvandi F, Spreafico M, Siboni SM, Moia M and Mannucci PM.  Cyp2C9 genotypes and dose requirements during the induction phase of oral anticoagulation therapy. Clinical Pharmacology and Therapeutics 2004; 75(3):198-203
17. Scordo MG, et al. Genetic polymorphism of cytochrome P450 2C9 in a Caucasian and a black African population. Br J Clin Pharmacol 2001; 52(4):447-450.

 

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