Page 256 - Personalised medicine of fluoropyrimidines using DPYD pharmacogenetics Carin Lunenburg

Chapter 10
This assay uses a personalized dose of 2-13C uracil, a stable isotope of uracil, and is based on the conversion of 2-13C uracil into 13CO2 which can be measured in exhaled breath.23
Several of these assays have been evaluated in head-to-head comparisons to DPD enzyme activity measurements in PBMCs in healthy volunteers, or patients selected after experiencing severe toxicity.11,21,23-27 However, when using an enriched patient cohort for severe toxicity, calculated assay characteristics such as sensitivity or specificity will be biased and not representative for routine clinical care. Strengths and weaknesses of these assays have been reviewed independently of each other;28,29 however, a head-to-head comparison in clinical practice has never been investigated. Therefore, in this prospective study, we evaluated four DPD phenotyping assays in patients prior to treatment with fluoropyrimidines in order to determine the association with the onset of severe fluoropyrimidine-induced toxicity and detecting DPD deficiency, defined as DPD enzyme activity in PBMCs below the cut-off value.
Materials and methods
Study design and patients
This study was a pre-planned analysis in a large prospective multi-centre clinical trial (clinicaltrials.gov identifier NCT02324452, here referred to as main study cohort or clinical trial) improving the safety of fluoropyrimidines by prospective DPYD genotyping.7 Two out of four phenotyping assays (endogenous DHU/U ratio and endogenous uracil levels) were executed in patients recruited in the seventeen Dutch hospitals participating in the clinical trial. All four DPD phenotyping assays were executed in three hospitals, and three out of four assays were executed in another hospital (excluding the 2-13C-uracil breath test). Patient recruitment for this study was open from 30 April, 2015, until 21 December, 2017. Ethical approval of this study was granted by the medical ethical committee of The Netherlands Cancer Institute, Amsterdam, the Netherlands. All patients provided written informed consent before enrollment in this study.
All assays were executed before start of fluoropyrimidine therapy. 92 patients were asked to participate in all four phenotyping assays, which made intra-patient comparisons possible. Results of the DPD phenotyping assays were determined after start of treatment and were not used for dose individualization. Dose adjustments of the fluoropyrimidine drug were done based on DPYD genotype only as per protocol of the clinical trial.
Toxicity was graded according to the National Cancer Institute common terminology criteria for adverse events (CTCAE; version 4.03)30 and severe toxicity was defined as CTCAE grade ≥3. Only toxicity defined by the treating physician as definitely, probably and possibly related to fluoropyrimidine treatment was taken into account. Patients were followed for toxicity during the entire treatment period and were evaluated for toxicity if they received at least one fluoropyrimidine drug administration. The endpoints of this study were the association of each DPD phenotyping assay with the onset of severe fluoropyrimidine- induced toxicity and DPD deficiency, defined as low DPD activity levels in PBMCs (≤5.9 nmol/ [mg*h]).6
Inclusion and exclusion criteria for this study were the same as in the clinical trial; eligibility to start with fluoropyrimidine-based therapy, 18 years or older, an adequate performance
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