346 F.C.W. Van Nieuwerburgh et al. disorder or patients with a total score of 15 or more on the 17-item HAM-D on admission were excluded. Other exclusion criteria were diagnoses of major depression, bipolar disorder, schizophrenia, other psychotic conditions, substance related disorder within the last 6 months, primary anxiety disorders, personality disorders, use of a concomitant psycho- tropic drug, organic mental disorders, epilepsy and any structural central nervous system disorder. Furthermore, clinically significant cardiovascular, gastrointestinal, pulmonary, renal, hepatic, endo- crine or hematological conditions resulted in exclu- sion from this study. Of these 91 patients, 17 dropped out of this study because of lack of motivation, uncertain medication use or failure to collect blood for determination of drug plasma concentrations. The 74 remaining patients have following character- istics: 39.2% male; positive family history, 36.1%; age at onset, 19.599.0; age at admission to study, 36.3911.6; baseline YBOCS score, 25.095.7. The patients were randomly assigned in a 12-week, double-blind trial to receive dosages titrated upward to 300 mg/day of venlafaxine or 60 mg/day of parox- etine, using a fixed dosing schedule. (see Denys et al. [4] for details on the dosing schedule). The patients received no additional drugs affecting the P450 enzyme system and were treatment free at baseline for at least 1 month. Primary efficacy was assessed by the change from baseline on the YBOCS, and response was defined as a ]25% reduction on the YBOCS. Blood samples were collected from each subject and frozen at 808C. Blood levels of paroxetine, venlafaxine and O-desmethylvenlafaxine were deter- mined at weeks 0, 1, 3, 5, 8, 10 and 12 by high- performance liquid chromatography with fluorescence detection [5]. Blood for the drug level determinations was collected at the response assessment in those weeks, at a random time point after ingestion of the medicine. DNA was extracted from 10 ml of peripheral blood according to standard procedures. The samples were genotyped using TaqMan† Drug Metabolism Gen- otyping Assays from Applied Biosystems for the NCBI dbSNP identification numbers rs3892097 Paroxetine treated patients (N 35) Poor metabolizer (N 17) 177982 Normal metabolizer (N 18) 125961 Venlafaxine treated patients (N 39) Poor metabolizer (N 22) 3909387 Normal metabolizer (N 17) 1519113 Table I. Plasma levels and response in patients with at least one reduced activity CYP2D6 allele (poor metabolizers) versus patients with only normal alleles. Plasma levels (ng/ml) Response (YBOCS decrease 25%) Response (YBOCS decrease 35%) (1846G A polymorphism, resulting in the inactive CYP2D6 allele 4 [6]), rs5030655 (1707T Del polymorphism, resulting in the inactive CYP2D6 allele 6 [6]), rs1065852 (100C T polymorphism, resulting in the reduced activity CYP2D6 allele 10 when not in combination with the 1846G A polymorphism [7]) and rs28371725 (2988G A polymorphism, resulting in the reduced activity CYP2D6 allele 41 [8]. In a Caucasian population, the frequency of all inactive and all reduced activity alleles is about 35%, of which 80 90% are caused by the four analyzed SNPs [9,10]. Results The measured minor allele frequencies for rs3892097, rs5030655, rs1065852, and rs28371725 are 0.24, 0.02, 0.26 and 0.08, respectively. This is in accordance with the minor allele frequencies reported by Applied Biosystems: 0.20, 0.00, 0.21 and 0.11, respectively. Fifty-seven percent of the patients have at least one reduced activity CYP2D6 allele while 13% have two reduced activity CYP2D6 alleles. To assess the effect of the polymorphisms on the medication plasma levels, all 6 measurements were averaged and two-tailed T-test statistics was per- formed on this average (see Table I). Paroxetine- treated patients with at least one reduced activity CYP2D6 allele show significantly higher paroxetine levels (P 0.037) compared to patients with normal alleles. Venlafaxine-treated patients with at least one reduced activity CYP2D6 allele show significantly higher venlafaxine levels (P 0.018) compared to patients with normal alleles. When considering the venlafaxine metabolites, insignificant differences are found with lower O-desmethylvenlafaxine levels (P 0.397) and higher combined venlafaxine O-desmethylvenlafaxine levels (P 0.065) compared to patients with normal alleles. In patients with two reduced activity CYP2D6 alleles, the same results are found but with a higher statistical significance level. The plasma metabo- lite levels also differ significantly. Patients with two reduced activity CYP2D6 alleles show higher Two-tailed t-test P 0.037 x2-test df 1 P 1.00 x2-test df 1 P 1.00 Two-tailed t-test P 0.018 x2-test df 1 P 0.77 x2-test df 1 P 0.76 75% 80% 52% 58% 55% 50% 44% 53%