FDA Patient Safety News: Show #84, March 2009

Serious Skin Reactions in Asian Patients on Phenytoin and Fosphenytoin

FDA is investigating new preliminary data suggesting an increased risk of serious skin reactions from the anti-epileptic drugs phenytoin and fosphenytoin if they are taken by Asian patients who are positive for the human leukocyte antigen allele HLA-B*1502. These reactions include Stevens Johnson Syndrome (SJS) and toxic epidermal necrolysis (TEN). Phenytoin is marketed as Dilantin, Phenytek and generics. Fosphenytoin sodium is marketed as Cerebyx and generics.

It is estimated that in parts of China, Taiwan, Thailand, Malaysia, Indonesia and the Philippines, 15 percent or more of the population may carry the HLA-B*-1502 allele. The frequency in South Asia, including India, is somewhat lower, and in Japan and Korea it is under one percent.

It was previously established that another anti-epileptic drug, carbamazepine, increases the risk of these skin reactions in Asian patients with the HLA-B*1502 allele. FDA has recommended screening patients of Asian ancestry for this allele before prescribing carbamazepine and not starting the drug unless the expected benefit clearly outweighs the risk.

In the case of phenytoin and fosphenytoin, the possible risk is still being studied, and there is not enough information at this point to recommend testing for the allele before starting therapy. However, FDA is recommending that physicians avoid using phenytoin as a substitute for carbamazepine in patients who test positive for the HLA-B*1502 allele.

With carbamazepine, over 90 percent of serious skin reactions occur during the first few months of treatment. Patients who have taken the drug for longer than this without having a skin reaction are at low risk for developing one. Likewise, the risk for serious skin reactions with phenytoin seems to be greatest during the first few months of treatment.

Patients on phenytoin, fosphenytoin or carbamazepine should be instructed to contact their physicians immediately if they develop a skin rash and ask whether they should take another dose. At the same time, they should understand the risk of discontinuing their medication without medical consultation.
Additional Information:
FDA MedWatch Safety Alert. Phenytoin (marketed as Dilantin, Phenytek and generics) and Fosphenytoin Sodium (marketed as Cerebyx and generics). November 24, 2008.

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