Science Behind the Assay

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This gene encodes a member of the cytochrome P450 super-family of enzymes. The cytochrome P450 proteins are monooxygenases that catalyze many reactions involved in drug metabolism and synthesis of cholesterol, steroids and other lipids. The gene is located within a cluster of cytochrome P450 genes on chromosome no.10 arm q24.

In 2009 the Federal Drug Administration approved wording on the drug label encouraging genetic testing for DNA variants based on the following risks:

• Compared to other patients, individuals with these variants are noted to have an approximate 30% decrease in active metabolite in plasma.
• Approximately 30% of the US population will carry at least one variant allele at the CYP2C19 locus.
• Patients who have at least one CYP2C19 variant can experience adverse reactions including serious or life-threatening vascular events when given the usual Plavix dose.
• Compared to other individuals, patients with Cytochrome P450 2C19 Drug Metabolism Analysis variants have an approximate 50% increase in risk of stroke, myocardial infarction or death.
• Compared to other individuals with stent placements, patients with these variants have an approximate 3-fold increase risk of stent thrombosis.

The *1 (“star 1″) allele is the normal copy that has full enzymatic activity. The *2 (“star 2″) and *3 (“star 3″) alleles are the most common variants and result in complete loss of enzymatic activity. Consequently, carriers of the *2 and *3 alleles have reduced formation of the drug’s active metabolite and demonstrate reduced drug-induced platelet inhibition.

The prevalence of the *2 and *3 alleles vary by ethnicity. In Caucasians, Blacks, and Asians, the proportion of patients who carry at least one copy of *2 is 25%, 30%, and 40-50% respectively, while the proportion for *3 is <1%, <1%, and 7%, respectively. Additional variants, *4, *5 *6, *7 and *8, also result in no enzymatic activity in all ethnicities (< 1%). Finally, the variant *17 is present in nearly 40% of Caucasians, Blacks, and Asians, and results in increased CYP2C19 activity, higher production of active metabolite, and improved clopidogrel-induced platelet inhibition.

Collection Types and Turn-Around-Time

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• FTA Buccal Collection Kits: Contains supplies for oral buccal cell collection.
• Blood: 5-10 cc whole blood lavender-top EDTA
• Turnaround Time: 10 days

CPT Code Guidance

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CYP2C19 Mutation DNA Analysis:
1 x 83891, 2 x 83892, 1 x 83900, 7 x 83901,
9 x 83914, 2 x 83909, 1 x 83912, 1 x 83912-26

The Cytochrome P450 super family is involved with the metabolism and elimination of 5-10% commonly prescribed drugs. Genetic polymorphisms in CYP2C19 are common. Enzyme activity is expressed at highly variable levels and three phenotypes are identified on a simple one-page report: Poor Metabolizer, Intermediate Metabolizer and Normal Metabolizer.

World Class Testing Methodology

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SNP Genotyping using the Sequenom MALDI-TOF Mass Spectrometer The MassARRAY® is the leading technology for SNP genotyping for sub-whole genome study applications. Assays can be multiplexed from 2-36 SNPs per individual reaction. The assay design is automated and straight forward.

How it works:

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• PCR amplifies ~100bp fragment containing SNP of interest
• SAP treatment cleans up left over primers and dNTPs
• Single-base extension reaction resulting in fragments of different masses
• Resin clean up to get rid of extra salt
• Nanodispensing on SpectroChip
• Data acquisition with MALDI-TOF mass spectrometer provides detection and ratio analysis

What is MALDI-TOF?

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• Matrix-assisted laser desorption/ionization – time of flight mass spectrometry.
• Sequenom’s SpectroChip is coated with a matrix which allows crystallization of the PCR product on its surface. A laser is then fired at the crystal which ionizes the molecules. These ions fly through a vacuum tube to an ion detector based on their mass. Smaller molecules travel faster than larger ones. Time of flight measures the difference in time be molecules hit the detector and the software calculates the mass of the fragments. MALDI-TOF can resolve mass differences of 16 Daltons.

Dosage Information

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A complicating factor in correlating CYP2C19 genotype with phenotype is that many drugs may reduce or increase CYP2C19 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. CYP2C19 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 – Individuals qualified as PM may have a significantly reduced metabolic efficacy and are likely to require dosage adjustment/or and drug interaction evaluation.
• Intermediate Metabolizers – Individuals qualified as IM may have a reduced metabolic efficacy and are likely to require dosage adjustment and/or drug interaction evaluation.
• Normal Metabolizers – Individuals qualified as NM have typical metabolic efficacy.
• Extensive Metabolizers – Individuals qualified as EM may have an increased metabolic efficacy and are likely to require dosage adjustment and/or drug interaction evaluation.
• Ultra-Rapid Metabolizers – Individuals qualified as UM may have a significantly increased metabolic efficacy and are likely to require dosage adjustment and/or drug interaction evaluation.

Drug Metabolism Guide – This list is not all inclusive and is for your guidance only.

Substrates Metabolized through Cytochrome P-450 2C19…Substrates refer to drugs that are either activated or deactivated by the pathway. Note=italics indicated minor pathway

Inhibitors of Cytochrome P-450 2C19…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.

Inducers of Cytochrome P-450 2C19…Inducers refers to drugs that increase the activity of a   pathway. Coadministration increases the rate of excretion for drugs metabolized through the pathway indicated, reducing the drug’s effectiveness.

ginko biloba

rifampin

St John’s Wort

**PLEASE NOTE**  THIS PRODUCT CANNOT BE PURCHASED ONLINE BY THE GENERAL PUBLIC.  THIS TEST CAN ONLY BE ORDERED BY A MEDICAL PHYSICIAN. Cytochrome P450 2C19 (abbreviated CYP2C19), a member of the cytochrome P450 mixed-function oxidase system, is involved in the metabolism of xenobiotics in the body. It is involved in the metabolism of several important groups of drugs including many proton pump inhibitors and antiepileptics. In humans, the CYP2C19 protein is encoded by the CYP2C19 gene. Plavix is clearly at the forefront of benefical dosing through genetic analysis.

References

1. Romkes M, Faletto MB, Blaisdell JA, Raucy JL, Goldstein JA (April 1991). “Cloning and expression of complementary DNAs for multiple members of the human cytochrome P450IIC subfamily”. Biochemistry 30 (13): 3247–55. doi:10.1021/bi00227a012. PMID 2009263.

2. Gray IC, Nobile C, Muresu R, Ford S, Spurr NK (July 1995). “A 2.4-megabase physical map spanning the CYP2C gene cluster on chromosome 10q24″. Genomics 28 (2): 328–32. doi:10.1006/geno.1995.1149. PMID 8530044.

3. “Entrez Gene: CYP2C19 cytochrome P450, family 2, subfamily C, polypeptide 19″. http://www.ncbi.nlm.nih.gov/sites/entrez?Db=gene&Cmd=ShowDetailView&TermToSearch=1557.

4. Bertilsson L (September 1995). “Geographical/interracial differences in polymorphic drug oxidation. Current state of knowledge of cytochromes P450 (CYP) 2D6 and 2C19″. Clin Pharmacokinet 29 (3): 192–209. doi:10.2165/00003088-199529030-00005. PMID 8521680.

5. Desta Z, Zhao X, Shin JG, Flockhart DA (2002). “Clinical significance of the cytochrome P450 2C19 genetic polymorphism”. Clin Pharmacokinet 41 (12): 913–58. doi:10.2165/00003088-200241120-00002. PMID 12222994.

6. De Morais SM, Wilkinson GR, Blaisdell J, Nakamura K, Meyer UA, Goldstein JA, The major genetic defect responsible for the polymorphism of S-mephenytoin metabolism in humans. J Biol Chem. 1994 Jun 3;269(22):15419-22

7. Goldstein JA, Ishizaki T, Chiba K, de Morais SM, Bell D, Krahn PM, Evans DA. Frequencies of the defective CYP2C19 alleles responsible for the mephenytoin poor metabolizer phenotype in various Oriental, Caucasian, Saudi Arabian and American black populations. Pharmacogenetics 1997, 7: 59-64

8. Ferguson RJ, De Morais SM, Benhamou S, Bouchardy C, Blaisdell J, Ibeanu G, Wilkinson GR, Sarich TC, Wright JM, Dayer P, Goldstein JA. A new genetic defect in human CYP2C19: mutation of the initiation codon is responsible for poor metabolism of S-mephenytoin. J Pharmacol Exp Ther. 1998 Jan;284(1):356-61.

9. Xiao ZS, Goldstein JA, Xie HG, Blaisdell J, Wang W, Jiang CH, Yan FX, He N, Huang SL, Xu ZH, Zhou HH. Differences in the incidence of the CYP2C19 polymorphism affecting the S-mephenytoin phenotype in Chinese Han and Bai populations and identification of a new rare CYP2C19 mutant allele. J Pharmacol Exp Ther. 1997 Apr;281(1):604-9.

10. Blaisdell J, Mohrenweiser H, Jackson J, Ferguson S, Coulter S, Chanas B, Xi T, Ghanayem B, Goldstein JA.. Identification and functional characterization of new potentially defective alleles of human CYP2C19. Pharmacogenetics. 2002 Dec;12(9):703-11.

11. Brockmoller J et.al. Pharmacogenetic diagnosis of cytochrome P450 polymorphisms in clinical drug development and in drug treatment. Pharmacogenetics. 2000:1:125-51.

12. Kirchheiner J, Brosen K, Dahl ML, et al.: CYP2D6 and CYPSC19 genotype-based dose recommendations for antidepressants: a first step towards subpopulation-specific dosages. Acta Psych Scand 2001 Sept;104(3):173192

13. Ibeanu GC, Blaisdell J, Ghanayem BI, Beyeler C, Benhamou S, Bouchardy C, Wilkinson GR, Dayer P, Daly AK, Goldstein JA. An additional defective allele, CYP2C19*5, contributes to the S-mephenytoin poor metabolizer phenotype in Caucasians. Pharmacogenetics. 1998 Apr;8(2):129-35

14. Ibeanu GC, Goldstein JA, Meyer U, Benhamou S, Bouchardy C, Dayer P, Ghanayem BI, Blaisdell J. Identification of new human CYP2C19 alleles (CYP2C19*6 and CYP2C19*2B) in a Caucasian poor metabolizer of mephenytoin. J Pharmacol Exp Ther. 1998 Sep;286(3):1490-5

15. Cozza KL, Armstrong SC, Oesterheld JR (2003) Drug Interaction principles for Medical Practice. American Psychiatric Publishing Inc

16. Linder MW and Valdes RJr. Pharmacogenetics in the Practice of Laboratory Medicine. Molecular Diagnosis. 1999;4:365-79. Mol Pharmacol. 1994 Oct;46(4):594-8

17. De Morais SM, Wilkinson GR, Blaisdell J, Meyer UA, Nakamura K, Goldstein JA. Identification of a new genetic defect responsible for the polymorphism of (S)-mephenytoin metabolism in Japanese. Mol Pharmacol. 1994 Oct;46(4):594-8

18. Brandt JT, Kirkwood S, Mukopadhay N. CYP2C19*2 polymorphism contributes to a diminished pharmacodynamic response to clopidogrel. J Am Coll Cardiol. 2006;47:380A.

19. Mega JL, Close SL, Wiviott SD, Shen L, Hockett RD, Brandt JT. Cytochrome p-450 polymorphisms and response to clopidogrel. N Engl J Med. Jan 22 2009;360(4):354-62. [Medline].

20. Frére C, Cuisset T, Gaborit B, Alessi MC, Hulot JS. The CYP2C19*17 allele is associated with better platelet response to clopidogrel in patients admitted for non-ST acute coronary syndrome. J Thromb Haemost. Aug 2009;7(8):1409-11. [Medline].