Recently, new polymorphisms were described in connection with intermediate and ultrarapid CYP2D6 metabolism. These may allow a much desired prediction of metabolic activity within the extensive metabolizer group. The functional consequences are still being discussed with few data available for clinical patients. Methods: We conducted a prospective, blinded twocenter study seeking correlations between CYP2C19 (*2,*3, and *4; conventional PCR) and CYP2D6 genotypes (*1 to *10, *35, and *41;

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... me and multiplex PCR) and drug concentrations (Emit ® and HPLC) in 50 Caucasians receiving amitriptyline (AT; 75 mg twice a day). Results: Eighteen CYP2C19 heterozygotes (*1/*2) had higher AT (P ‫؍‬ 0.033) and lower nortriptyline (NT; P ‫؍‬ 0.059) concentrations than 30 homozygotes (*1/*1). For CYP2D6, we calculated two new indices, i.e., the allelespecific change of concentration on identical background (ASCOC) and a quantitative functional gene dose. The ASCOC describes the change in NT concentration attributable to a mutant allele compared with the wild type. We found significantly higher concentrations for alleles *4 (95.6%; P <0.0001), *10 (63.3%; P <0.001), and *41 (39.8%; P <0.0001) but not for *2 and *35. Assigning of semiquantitative gene doses of 0, 0.5, or 1 to each allele instead of applying the current classification system (predicted phenotypes: 3 intermediate metabolizers, 46 extensive metabolizers, and 1 ultrarapid metabolizer) produced significant NT concentration differences: gene doses of 0.5 (n ‫,)3؍‬ 1 (n ‫؍‬ 14), 1.5 (n ‫؍‬ 11), 2 (n ‫؍‬ 21) and 3 (n ‫؍‬ 1; P <0.00001). Conclusions: AT and NT concentrations can be predicted within the group of CYP2D6 extensive metabolizers. The ASCOC provides substantial advantages compared with current methods of analysis. CYP2D6 but not CYP2C19 correlates with the sum of both concentrations used to guide AT therapy. Tricyclic antidepressants (TCAs) 4 have been the cornerstones of antidepressive therapy for over three decades, and more than 1 million patients received TCAs in the US in 2000 (1 ).