Up Close and Personal

Researchers have found that when patients with certain SNP variants of cytochrome P450 (CYP) take warfarin, metabolism and patient sensitivity are affected. Once this was discovered, the Food and Drug Administration (FDA) subsequently approved prescribing label changes for warfarin that incorporated pharmacogenomics information.3 This change included information on increased bleeding risk for patients carrying either the CYP2C9*2 or CYP2C9*3 alleles.4

More recently, Eckman et al published a cost-effectiveness analysis evaluating pharmacogenetic information related to warfarin dosing in patients with nonvalvular atrial fibrillation.5 In the study, the authors concluded that currently limited data exist utilizing pharmacogenomics for dosing of warfarin and its effects on major bleeding. They did note, however, that genotyping may be beneficial and cost-effective in patients who are at high risk of hemorrhage.

Additionally, there has been recent media coverage of the genetic variations of CYP2C19 affecting clopidogrel metabolism and efficacy.6,7 Both Mega et al and Collet et al noted that patients carrying at least one genetic variation of CYP2C19 had diminished platelet inhibition. Published earlier this year, both studies noted that patients carrying the genetic variation of CYP2C19 exhibited a higher rate of major cardiovascular events—including death, stent thrombosis, myocardial infarction, or stroke—than did noncarriers.

The FDA warned healthcare providers in November 2008 about using phenytoin or fosphenytoin in patients with the HLA-B*1502 allele due to a potentially increased risk of Stevens-Johnson syndrome and toxic epidermal necrolysis. A similar warning exists for carbamazepine in the same patient population. This is another example of pharmacogenomics information at work.8

Patient-specific variables have been identified that can help determine how an individual will respond to certain medications. Ultimately, this could decrease healthcare costs. It is a slow process and might be the wave of the future, but we are not there yet. TH

Michele B. Kaufman, PharmD, BSc, RPh, is a freelance medical writer based in New York City.

References

1. Just the facts: a basic introduction to the science underlying NCBI resources. National Center for Biotechnology Information Web site. Available at: www.ncbi.nlm.nih.gov/About/primer/pharm.html. Accessed Jan. 31, 2009.
2. Gulseth MP, Grice GR, Dager WE. Pharmacogenomics of warfarin: uncovering a piece of the warfarin mystery. Am J Health Syst Pharm. 2009;66:123-133.
3. FDA letter on approval of pharmacogenomics information for Coumadin label. Food and Drug Administration Web site. Available at: www.fda.gov/cder/foi/appletter/2007/009218s105ltr.pdf. Accessed Feb. 4, 2009.
4. Coumadin label updated. Food and Drug Administration Web site. Available at: www.fda.gov/cder/foi/label/2007/009218s105lblv2.pdf. Accessed Feb. 4, 2009.
5. Eckman MH, Rosand J, Greenberg SM, Gage BF. Cost-effectiveness of using pharmacogenetic information in warfarin dosing for patients with nonvalvular atrial fibrillation. Ann Intern Med. 2009;150:73-83.
6. Mega JL, Close SL, Wiviott SD, et al. Cytochrome p-450 polymorphisms and response to clopidogrel. N Engl J Med. 2009;360:354-362.
7. Collet JP, Hulot JS, Pena A, et al. Cytochrome P450) 2C19 polymorphism in young patients treated with clopidogrel after myocardial infarction: a cohort study. Lancet. 2009;373:309-317.
8. Phenytoin and fosphenytoin information. Food and Drug Administration Web site. Available at: www.fda.gov/cder/drug/infopage/phenytoin_fosphenytoin/default.htm. Accessed Feb. 4, 2009.
9. Mylan receives final approval for first-to-file generic version of antiepileptic Keppra and launches immediately. Mylan Web site. Available at: investor.mylan.com/phoenix.zhtml?c=66563&p=irol-newsArticle&ID=1221778. Accessed Feb. 4, 2009.
10. Prism pharmaceuticals receives FDA approval of Nexterone for life-threatening ventricular fibrillation and ventricular tachycardia. Prism Pharmaceuticals Web site. Available at: www.prismpharma.com/news.html. Accessed Feb. 4, 2009.
11. Now in double concentration. E-Pharm/alert Web site. Available at: whatcounts.jobson.com/dm?id=0CCEED6291EC025CFED437DC0261D862. Accessed Feb. 4, 2009.
12. Meeting of the Cardiovascular and Renal Drugs Advisory Committee. Food and Drug Administration Web site. Available at: www.fda.gov/cder/audiences/acspage/meetings/crdac_meeting_20090203.htm. Accessed Feb. 4, 2009.
13. Bratulic, A. FDA panel recommends Eli Lilly’s, Daiichi Sankyo’s Effient. FirstWord Web site. Available at: www.firstwordplus.com/Fws.do?articleid=F4B502BDC684486986AE56DED7B532F3&logRowId=281890. Accessed Feb. 4, 2009.
14. Now available 12-mg tablets. E-Pharm/alert Web site. Available at: whatcounts.jobson.com/dm?id=0CCEED6291EC025C1D5D3BD762CE3ECB. Accessed Feb. 4, 2009.
15. Tom WC. New venlafaxine extended-release formulation. Pharmacist’s Letter/Prescriber’s Letter. 2009;25(1)250108.