Carbamazepine, a drug used to treat epilepsy and bipolar disorder, will now come with new warnings that Asian patients should undergo genetic testing before using the drug. According to the Food & Drug Administration (FDA), a human leukocyte antigen called HLA-B*1502 that is exclusive to people with Asian ancestry makes it more likely that carbamazepine will cause the serious skin disorders Stevens-Johnson Syndrome and toxic epidermal necrolysis.
Carbamazepine is an anticonvulsant and mood stabilizing drug, used primarily in the treatment of epilepsy and bipolar disorder. It is also used to treat other disorders, including Attention Deficit Hyperactivity Disorder and schizophrenia. It is sold under the brand names Carbatrol, Equetro and Tegretol.
According to the FDA, the HLA-B*1502 leukocyte puts carbamazepine patients at a higher risk of developing Stevens- Johnson Syndrome and toxic epidermal necrolysis. Stevens-Johnson Syndrome is a hypersensitivity complex affecting the skin and the mucous membrane. The disorder causes blistering of mucous membranes, typically in the mouth, eyes, and vagina, and patchy areas of rash. Toxic epidermal necrolysis presents with a similar blistering of mucous membranes. However, in addition to blistering, the entire epidermis peels off in sheets from large areas of the body. Both disorders can be life threatening.
The FDA says that the HLA-B* 1502 leukocyte occurs in varying frequency in persons of Asian decent. Roughly 15% or more of patients may carry the leukocyte in parts of China, Thailand, Malaysia, Indonesia, the Philippines, and Taiwan. South Asians, including Indians, appear to have intermediate prevalence of HLA-B*1502, averaging 2 to 4%, but higher in some groups. HLA-B*1502 appears to be present at a low frequency, (less than 1%) in Japan and Korea. However, as the freqiemcu of HLA-B*1502 in Asia has not been subjected to a great deal of study, the FDA says that doctors should only use these statistics as a guideline when considering the use of carbamazepine. A simple genetic blood test is the only way to determine if a patient carries this leukocyte.
The new carbamazepine warning, carried in the label’s boxed warnings section, advises physicians to have patients of Asian decent undergo the blood test. Those who test positive for HLA-B*1502 should not be prescribed carbamazepine. The warning also cautions that such patients might also be at risk from Stevens-Johnson Syndrome and toxic epidermal necrolysis from other drugs for which these disorders are a potential side effect. Tested patients who are found to be negative for HLA-B*1502 have a low risk of Stevens-Johnson Syndrome and toxic epidermal necrolysis from carbamazepine, but these disorders can still rarely occur, so healthcare professionals should watch for symptoms in these patients.
The FDA also says that 90% of Stevens-Johnson Syndrome and toxic epidermal necrolysis occurs within the first few months of treatment with carbamazepine. Thus, patients of any ethnicity or genotype (including HLA-B*1502 positive) who have been taking carbamazepine for more than a few months are at low risk of developing Stevens-Johnson Syndrome and toxic epidermal necrolysis.
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