Genetic Testing
This article was originally published in RAJ Devices
Executive Summary
Pre-treatment testing recommended in US for carbamazepine users of Asian ancestry
Pre-treatment testing recommended in US for carbamazepine users of Asian ancestry
Manufacturers of products containing carbamazepine as an active ingredient in the US have agreed to add to the prescribing information for their products a recommendation that patients with Asian ancestry, before starting therapy, should undergo a genetic blood test to ascertain the risk of developing a rare, but serious, skin reaction.
Carbamazepine is used for treating epilepsy, bipolar disorder and neuropathic pain. It is sold under the brand names Carbatrol, by Shire Pharmaceuticals, Equetro, by Validus Pharmaceuticals Inc and Tegretol, by Novartis. Generic versions are also available.
The prescribing information for these drugs already includes a warning for all patients starting carbamazepine therapy, regardless of ethnicity, that rare but severe and sometimes life-threatening skin reactions can occur. These conditions include toxic epidermal necrolysis and Stevens-Johnson syndrome, characterised by multiple skin lesions, blisters, fever, itching and other symptoms. The risk of these reactions is estimated to be about 1 to 6 per 10,000 new users of the drug in countries with mainly white populations. However, the risk is estimated to be about 10 times higher in some Asian countries.
“When being considered for treatment with carbamazepine, genetically high-risk patients can be given a test that will help their health care providers make personalised drug treatment decisions and help avoid potentially serious skin reactions,” said Food and Drug Administration deputy commissioner Janet Woodcock.
The FDA estimates that about 5% of patients being considered for treatment with carbamazepine are of Asian ancestry and will need to have the test.
Warfarin labelling
The carbamazepine labelling change follows the addition earlier in 2007 of genetic information to the labelling of the blood-thinning drug warfarin. This was the first time the FDA had added such information for such a widely used drug. The labelling suggested that variations of two specific genes may affect patients' response to the drug, enabling doctors to improve their initial dosing estimates of the anticoagulant for individual patients.
References
1. Food and Drug Administration press release, 12 December 2007, www.fda.gov/bbs/topics/NEWS/2007/NEW01755.html
2. The Regulatory Affairs Journal - Pharma, 2007, 18(9), 653