Severe phenytoin toxicity in a CYP2C9*3*3 homozygous mutant from India.

The authors report an Indian adult female patient with a history of generalized tonic clonic seizures who developed severe features of phenytoin (DPH) toxicity on therapeutic dosage of this antiepileptic drug. Administration of 300 mg/day of DPH in this patient resulted in toxic symptoms associated with an excessive serum DPH concentration of 33 microg/ml. The PCR-RFLP analysis revealed a homozygosity involving CYP2C9*3*3. This mutation results in a marked decrease in the enzymatic activity (CYP2C9) and leads to a decreased clearance of the drug which can lead to severe acute and chronic toxicity. On switching the antiepileptic therapy from DPH to sodium valproate, there was reversal of both.