Familial occurrence of hypersensitivity to phenytoin

Therapy with anticonvulsants such as phenytoin, phenobarbital, and carbamazepine can be complicated by severe hypersensitivity reactions. Previous work has suggested that the predisposition to such reactions is based on an inherited abnormality in the detoxification of reactive metabolites of the drugs. However, there are no reports of familial occurrence of the reactions in the literature. In the current study, we examined a family in which three siblings developed hypersensitivity reactions to phenytoin, confirming the inheritance of a predisposition to the reactions. Detoxification of reactive metabolites of the anticonvulsants was studied in cells from the patients and their siblings.

Three siblings from a family of 12 siblings developed hypersensitivity reactions to phenytoin characterized by fever, rash, lymphadenopathy, and anicteric hepatitis. All recovered completely after discontinuation of treatment. One sibling tolerated phenobarbital without toxic sequelae. Peripheral blood mononuclear cells from the three patients and five additional siblings who had never taken anticonvulsants were exposed to oxidative metabolites of phenytoin, phenobarbital, and carbamazepine generated by a hepatic microsomal drug-metabolizing system in vitro. The toxicity of metabolites in the cells from the siblings was compared with that in cells from control subjects.

Cells from each of the patients who had experienced a hypersensitivity reaction exhibited increased toxicity from metabolites of phenytoin and carbamazepine, while the cellular response to metabolites of phenobarbital was within normal limits. Cells from four of the other siblings showed an abnormal response to phenytoin metabolites, while cells from the final sibling detoxified phenytoin metabolites normally.

Our observations on the patients confirm the inherited nature of phenytoin hypersensitivity reactions in vivo. In vitro studies demonstrated abnormal metabolite detoxification in the patients and several of their siblings. The detoxification defect included metabolites of phenytoin and carbamazepine but not of phenobarbital. A family history of a drug hypersensitivity reaction should alert physicians to the probability of a markedly increased risk of an adverse reaction in family members. In vitro assays to confirm adverse reaction risks may ultimately be able to provide individualized risk assessment for patients who must take anticonvulsants.