Pharmacokinetics of nortriptyline and its 10-hydroxy metabolite in Chinese subjects of different CYP2D6 genotypes*

OBJECTIVES: To study the impact of the CYP2D6*10 allele on the disposition of nortriptyline in Chinese subjects. METHODS: A single dose of 25 mg nortriptyline was given orally to 15 healthy Chinese volunteers who were classified as extensive metabolizers after phenotyping with debrisoquin (INN, debrisoquine) and who were genotyped by allele-specific polymerase chain reaction. Five subjects were homozygous for CYP2D6*1, 5 subjects were homozygous for CYP2D6*10, and 5 subjects were heterozygous for these 2 alleles. Plasma concentrations of nortriptyline and its main metabolite 10-hydroxynortriptyline were measured by liquid chromatography-mass spectrometry, and the pharmacokinetics were studied during 168 hours after the dose. RESULTS: Subjects who were homozygous for CYP2D6*10 had significantly higher total areas under the plasma concentration-time curve (AUC), lower apparent oral clearances, and longer mean plasma half-life of nortriptyline than subjects in the CYP2D6*1/*1 and the heterozygous groups. For 10-hydroxynortriptyline, the AUC was lower and the plasma half-life was longer in subjects who were homozygous for CYP2D6*10 than in subjects in the other 2 groups. CONCLUSION: The CYP2D6*10 allele in Chinese subjects was associated with significantly higher plasma levels of nortriptyline compared with the CYP2D6*1 allele because of an impaired metabolism of nortriptyline to 10-hydroxynortriptyline, particularly in the subjects with the CYP2D6*10/*10 genotype. The results suggest that genotyping of CYP2D6 may be a useful tool in predicting the pharmacokinetics of nortriptyline.