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Background The relationship between phenotype and genotype of the polymorphic cytochrome P450 2D6 enzyme (CYP2D6) has been intensively studied, however few studies are conducted among older persons. In a study among 900 relatively young venlafaxine users (mean age 45 years), 83% were genotyped as an extensive metabolizer (EM), but 21% were observed to have a poor metabolizer (PM) phenotype (1). This so-called 'phenoconversion' is one of the reasons for clinicians to mainly rely on therapeutic drug monitoring (TDM) as a sufficient indicator for genotype and to adjust the drug dosage if indicated. We determined associations between blood drug levels, genotype and phenotype in an old age sample of patients treated for depression with nortriptyline (NTP) or venlafaxine (VFX). Methods We analyzed post-hoc data from a clinical trial among older starters of NTP or VFX (2). The study population was monitored by TDM for twelve weeks. The drug levels of NTP and VFX as well as the main CYP2D6 metabolites, OH-nortriptyline and O-desmethylvenlafaxine, were measured. In addition, the genotypes for the CYP2D6 ∗3 and ∗4 alleles were available. We sub-grouped the data into phenotypes according to the metabolite/mother compound ratio. Next, we compared the phenotype with the genotype results and translated the blood levels into clinical outcomes being below, above, or within drug-specific therapeutic windows according to guidelines and tested for significant (p

Background: Venlafaxine (VEN) is an antidepressant which exerts both serotonin and norepinephrine reuptake inhibition. The drug is mainly metabolized by CYP2D6 to its active metabolite O-desmetylvenlafaxine (ODMV). Due to large interindividual differences in clearance in which genetic variance in CYP2D6 activity plays a major role, therapeutic drug monitoring (TDM) is considered useful in patients treated with VEN. We developed an assay for combined determination of VEN and ODMV in dried blood spots (DBS). Methods: A fast and robust LC-MS/MS method was developed and fully validated for simultaneous determination of VEN and ODMV in DBS. 9 mm circles were punched out from DBS collected on Whatman DMPK-C paper and mixed with acetonitril:methanol 1:3 containing the internal standard. The extract was analyzed by LC-MS/MS. Results: The assay was found to be linear in the range 20-1000 mcg/L for both VEN and ODMV. Inter-assay variability at LLQ (20 mcg/L), Low (L, 80 mcg/L), Medium (M, 300 mcg/L) and High (H, 750 mcg/L) levels was 12.0%, 6.1%, 7.0%, and 4.9% for VEN and 10.8%, 6.8%, 5.4%, and 6.3% respectively for ODMV. Bias at LLQ, L, M and H was 7.2%, 1.3%, 0.2%, and 20.4% for VEN and 5.6%, 1.7%, 0.4%, and 0% for ODMV. The influence of punch position and blood volume per spot did not influence the results, but a low hematocrit (10% negative bias. Comparison of venous blood samples with DBS samples revealed a linear correlation with a R2 of >0.98 and a DBS/plasma ratio of 1.11 for VEN and 1.28 for ODMV. Conclusions: This fully validated method is suitable for determination of VEN and its metabolite ODMV in DBS and applicable for TDM. The method will be used in the Dutch CYSCE multicenter trial (ClincialTrail.gov Identifier NCT01778907) in which the effect of CYP2D6 genotyping combined with TDM on time to reach adequate blood drug levels will be investigated in depressed elderly patients.

Background: Therapeutic Drug Monitoring (TDM) of Tricyclic Antidepressants (TADs) is considered useful in patients with major depressive disorders, since these drugs display large individual differences in clearance and therapeutic windows of these drugs are relatively small. We developed an assay for determination of amitriptyline (AMI), nortriptyline (NOR), imipramine (IMI) desipramine (DESI), clomipramine (CLOM) and desmethylclomipramine (DES-CLOM) in dried blood spots (DBS). Methods: A fast and robust LC-MS/MS method was developed and fully validated for simultaneous determination of AMI/NOR, IMI/DESI, or CLOM/ DES-CLOM in DBS. Nine millimetre circles were punched out from DBS collected on Whatman DMK-C paper and mixed with acetonitril:methanol 1:3 containing the internal standard. The extract was analyzed by LC-MS/MS Results: The assay was found to be linear in the range 20-500 mcg/L for all compounds. The Inter-assay variability (VC) and bias values are displayed below: The influence of punch position and blood volume per spot did not influence the results, but a low hematocrit (10% negative bias for all compounds. Comparison of venous blood samples with DBS samples revealed linear correlations and DBS/plasma ratio's ranging from 1.15 to 1.25. Conclusions: This fully validated method is suitable for determination of TADs in DBS and applicable for TDM. The method will be used to determine NOR in the Dutch CYSCE multicenter trial (ClincialTrail.gov Identifier NCT01778907) in which the effect of CYP2D6 genotyping combined with TDM on time to reach adequate blood drug levels will be investigated in depressed elderly patients.