Evaluation of cytochrome P4502C9 metabolic activity with tolbutamide in CYP2C9*1 heterozygotes

Objectives: Multiple single-nucleotide polymorphisms in the gene encoding cytochrome P450 (CY-P) 2C9 have been identified, but the functional significance of the various putative defective genotypes in humans merits further study. Methods. Using tolbutamide as a probe of CYP2C9 activity, we evaluated CYP2C9 phenotype in 15 healthy individuals expressing the CYP2C9 *1/*1,*11*2, and *1/*3 genotypes (n = 5 per group). CYP2C9 genotype was determined by polymerase chain reaction-restriction fragment length polymorphism methods. Subjects received 500 mg of tolbutamide, with plasma and urine collected over a 24-hour period. Plasma tolbutamide and urinary tolbutamide, 4'-hydroxytolbutamide, and carboxytolbutamide concentrations were determined by an HPLC method. Results. Tolbutamide area under the plasma concentration-time curve from time zero to infinity [AUC(0-infinity)] significantly increased by 1.5-fold and 1.9-fold, respectively, in subjects expressing the CYP2C9*1/2 and *1/*3 genotypes compared with *1/*1 subjects. Statistically significant reductions in tolbutamide oral clearance (29% and 48%) and formation clearance (38% and 56%) were detected in the *11*2 and *1/*3 individuals, respectively, compared with *1/*1 subjects. The increases in AUC(0-infinity) and decreases in oral clearance observed in the *1/*3 individuals were also significantly greater than those expressing the *11*2 genotype (P < .05). The amount of urinary 4'-hydroxytolbutamide and carboxytolbutamide excreted in the 0- to 12-hour and 6- to 12-hour collection intervals was significantly less in *1/*2 and *1/*3 individuals compared with *1/*1 subjects. With tolbutamide used as a CYP2C9 probe, CYP2C9 genotype was the major determinant of CYP2C9 phenotype (r(2) = 0.77) Conclusions.- CYP2C9 activity was significantly reduced in *1 heterozygotes; compared with *1 homozygotes, and metabolism was more severely impaired in *1/*3 individuals compared with those expressing *11*2.