Dihydropyrimidine Dehydrogenase Phenotyping Using Pretreatment Uracil: A Note of Caution Based on a Large Prospective Clinical Study.

In clinical practice, 25-30% of the patients treated with fluoropyrimidines experience severe fluoropyrimidine-related toxicity. Extensively clinically validated DPYD genotyping tests are available to identify patients at risk of severe toxicity due to decreased activity of dihydropyrimidine dehydrogenase (DPD), the rate limiting enzyme in fluoropyrimidine metabolism. In April 2020, the European Medicines Agency recommended that, as an alternative for DPYD genotype-based testing for DPD deficiency, also phenotype testing based on pretreatment plasma uracil levels is a suitable method to identify patients with DPD deficiency. Although the evidence for genotype-directed dosing of fluoropyrimidines is substantial, the level of evidence supporting plasma uracil levels to predict DPD activity in clinical practice is limited. Notwithstanding this, uracil-based phenotyping is now used in clinical practice in various countries in Europe. We aimed to determine the value of pretreatment uracil levels in predicting DPD deficiency and severe treatment-related toxicity. To this end, we determined pretreatment uracil levels in 955 patients with cancer, and assessed the correlation with DPD activity in peripheral blood mononuclear cells (PBMCs) and fluoropyrimidine-related severe toxicity. We identified substantial issues concerning the use of pretreatment uracil in clinical practice, including large between-center study differences in measured pretreatment uracil levels, most likely as a result of pre-analytical factors. Importantly, we were not able to correlate pretreatment uracil levels with DPD activity nor were uracil levels predictive of severe treatment-related toxicity. We urge that robust clinical validation should first be performed before pretreatment plasma uracil levels are used in clinical practice as part of a dosing strategy for fluoropyrimidines.

Effects of Pharmacogenetic Screening for CYP2D6 Among Elderly Starting Therapy With Nortriptyline or Venlafaxine: A Pragmatic Randomized Controlled Trial (CYSCE Trial).

PURPOSE/BACKGROUND: The duration of untreated depression is a predictor for poor future prognosis, making rapid dose finding essential. Genetic variation of the CYP2D6 isoenzyme can influence the optimal dosage needed for individual patients. The aim of this study was to determine the effectiveness of CYP2D6 pharmacogenetic screening to accelerate drug dosing in older patients with depression initiating nortriptyline or venlafaxine. METHODS/PROCEDURES: In this randomized controlled trial, patients were randomly allocated to one of the study arms. In the intervention arm (DG-I), the specific genotype accompanied by a standardized dosing recommendation based on the patients' genotype and the prescribed drug was directly communicated to the physician of the participant. In both the deviating genotype control arm (DG-C) and the nonrandomized control arm, the physician of the participants was not informed about the genotype and the associated dosing advise. The primary outcome was the time needed to reach adequate drug levels: (1) blood levels within the therapeutic range and (2) no dose adjustments within the previous 3 weeks. FINDINGS/RESULTS: No significant difference was observed in mean time to reach adequate dose or time to adequate dose between DG-I and DG-C. Compared with the nonrandomized control arm group, adequate drug levels were reached significantly faster in the DG-I group (log-rank test; P = 0.004), and there was a similar nonsignificant trend for the DG-C group (log-rank test; P = 0.087). IMPLICATIONS/CONCLUSIONS: The results of this study do not support pharmacogenetic CYP2D6 screening to accelerate dose adjustment for nortriptyline and venlafaxine in older patients with depression.

A clinical validation study for application of DBS in therapeutic drug monitoring of antidepressants.

BACKGROUND: A bridging study of plasma and DBS concentrations for therapeutic drug monitoring of antidepressants was performed. RESULTS & METHODOLOGY: Potassium-based hematocrit analysis was included. In addition, we defined acceptance criteria based on the differences between individual data points of plasma and DBS concentrations. These criteria were applied to test acceptability of error found in predicted nortriptyline plasma concentrations. Potassium-based hematocrit predicted a negative bias for DBS concentrations of amitriptyline, but not for the other compounds. To predict plasma concentrations of antidepressants based on DBS concentrations, a factor of 0.8, 0.65, 0.84 and 0.78 was found for nortriptyline, desmethylclomipramine, venlafaxine and desmethylvenlafaxine, respectively. DISCUSSION & CONCLUSION: Application of the factor and newly formulated acceptance criteria demonstrated prediction of nortriptyline plasma concentrations based on DBS concentrations.

Effects and cost-effectiveness of pharmacogenetic screening for CYP2D6 among older adults starting therapy with nortriptyline or venlafaxine: study protocol for a pragmatic randomized controlled trial (CYSCEtrial).

BACKGROUND: Nortriptyline and venlafaxine are commonly used antidepressants for treatment of depression in older patients. Both drugs are metabolized by the polymorphic cytochrome P450-2D6 (CYP2D6) enzyme and guidelines for dose adaptations based on the CYP2D6 genotype have been developed. The CYP2D6 Screening Among Elderly (CYSCE) trial is designed to address the potential health and economic value of genotyping for CYP2D6 in optimizing dose-finding of nortriptyline and venlafaxine. METHODS/DESIGN: In a pragmatic randomized controlled trial, patients diagnosed with a major depressive disorder according to the DSM-IV and aged 60 years or older will be recruited from psychiatric centers across the Netherlands. After CYP2D6 genotyping determined in peripheral blood obtained by finger-prick, patients will be grouped into poor, intermediate, extensive, or ultrarapid metabolizers. Patients with deviant genotype (that is poor, intermediate or ultrarapid genotype) will be randomly allocated to an intervention group in which the genotype and dosing advice is communicated to the treating physician, or to a control group in which patients receive care as usual. Additionally, an external reference group of patients with the extensive metabolizer genotype is included. Primary outcome in all groups is time needed to obtain an adequate blood level of the antidepressant drug. Secondary outcomes include adverse drug reactions measured by a shortened Antidepressant Side-Effects Checklist (ASEC), and cost-effectiveness of the screening. DISCUSSION: Results of this trial will guide policy-making with regard to pharmacogenetic screening prior to treatment with nortriptyline or venlafaxine among older patients with depression. TRIAL REGISTRATION: ClinicalTrials.gov: NCT01778907 ; registration date: 22 January 2013.

Selective targeting of homologous DNA recombination repair by gemcitabine.

PURPOSE: Gemcitabine (2',2'-difluoro-2'-deoxycytidine, dFdC) is a potent radiosensitizer. The mechanism of dFdC-mediated radiosensitization is yet poorly understood. We recently excluded inhibition of DNA double-strand break (DSB) repair by nonhomologous end-joining (NHEJ) as a means of radiosensitization. In the current study, we addressed the possibility that dFdC might affect homologous recombination (HR)-mediated DSB repair or base excision repair (BER). METHODS AND MATERIALS: DFdC-mediated radiosensitization in cell lines deficient in BER and in HR was compared with that in their BER-proficient and HR-proficient parental counterparts. Sensitization to mitomycin C (MMC) was also investigated in cell lines deficient and proficient in HR. Additionally, the effect of dFdC on Rad51 foci formation after irradiation was studied. RESULTS: DFdC did induce radiosensitization in BER-deficient cells; however, the respective mutant cells deficient in HR did not show dFdC-mediated radiosensitization. In HR-proficient, but not in HR-deficient, cells dFdC also induced substantial enhancement of the cytotoxic effect of MMC. Finally, we found that dFdC interferes with Rad51 foci formation after irradiation. CONCLUSION: DFdC causes radiosensitization by specific interference with HR.