Abstracts

RATIONALE: Inter-patient differences in absorption, protein binding, and the activity of metabolizing CYP450 enzymes contribute to the large variability seen in PHT concentration-dose relationships, but their respective effects are not well characterized. Our goal was to determine the association between the presence of CYP2C9 and 2C19 variants and PHT pharmacokinetics (PK) using a parenteral, stable labeled PHT formulation.
METHODS: Patients on maintenance PHT therapy, but no interacting drugs, were given an intravenous injection of 100 mg [2-13C,1,3,-15N2] 5,5- diphenylhydantoin (SL-PHT) with the remainder of their daily dose given orally. Blood samples were collected just prior to and up to 192 hours after the SL-PHT dose. Both unlabeled and SL-PHT were measured by GC-MS. Unbound drug was separated from total PHT by ultrafiltration. Non-compartmental PK analysis was performed with WinNonlin 3.0. Genotyping of the 2C9*2/*3 and 2C19*2/*3 variants was done using published PCR-RFLP assays, with some modifications. Statistical comparisons among groups were done by ANOVA.
RESULTS: Dose and genotype data are available for twenty-eight adult Caucasian patients, and PK data are available for sixteen of these patients. The allelic frequencies are 0.125 for 2C9*2, 0.054 for 2C9*3, 0.125 for 2C19*2 and 0 for 2C19*3. Patients were assigned to one of three groups based on genotype (Table 1). Preliminary comparisons of mg/kg doses and unbound PHT clearance (CLu) normalized to weight, showed a significant difference between group 1 and group 2 (p= 0.0112 and 0.03, respectively) and no difference between group 1 and group 3 (p= 0.3 and 0.4 respectively).[table1]Group 1: none of the screened polymorphisms present.
Group 2: heterozygous for either 2C9*2 or 2C9*3 polymorphisms, no 2C19 polymorphisms.
Group 3: heterozygous for 2C19*2, no 2C9 polymorphisms.
CONCLUSIONS: The allelic frequencies of the variants observed in our patients agree with reported values for Caucasians. Patients with the 2C9 polymorphisms, but not those with the 2C19 polymorphism, appear to require lower mg/kg doses than patients without any of the screened polymorphisms. When unbound PHT plasma concentrations are comparable, patients with the 2C9 polymorphisms have lower CLu/wt, while patients with the 2C19*2 polymorphism do not have a lower CLu/wt, than group 1. These preliminary results offer evidence that genotyping patients who are being considered for PHT therapy may reduce the risk of toxicity.
[Supported by: NIH-NINDS P50-NS16308 and M01-RR00400.]