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

Abstract

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.

DPYD genotyping and dihydropyrimidine dehydrogenase (DPD) phenotyping in clinical oncology. A clinically focused minireview

“…Three studies determined DPD phenotype and the four clinically relevant DPYD variants 19,27,28 . However, the correlation between [U] in wild‐type patients and DPYD variants carriers is still uncertain.…”

“…The [U] concentration is also affected by kidney function with higher values observed in patients with end stage renal desease 18 . A recent study reported significant between‐centre differences in the [U], underlining that measurement of uracil is sensitive to preanalytical conditions 19 …”

Testing for Dihydropyrimidine Dehydrogenase Deficiency to Individualize 5-Fluorouracil Therapy

“…While phenotypic tests have been instrumental as research tools, they have not been as widely accepted in clinical decision making as genetic tests. This may be in part due to the high degree of variability noted within and between phenotypic DPD tests [28,66], particularly when specimens are collected and analyzed at more than one site [38,67]. In addition, while clear correlations with clinical 5-FU toxicity have been established at the individual level for genetic risk factors, the same level of evidence for toxicity association has not been demonstrated for phenotypic tests.…”

“…Threshold levels of plasma U have been proposed as indicative of DPD deficiency [79][80][81]. However, these cutoff levels have not been clinically validated as predictive of severe toxicity [67], and no prospective clinical trials have demonstrated that pre-treatment metabolite levels or ratios can be used to improve patient safety. In addition, extreme center-to-center differences have been reported for metabolite measures [38,66,67], and both circadian variation and food intake have been shown to affect plasma metabolite levels [76,82].…”

“…However, these cutoff levels have not been clinically validated as predictive of severe toxicity [67], and no prospective clinical trials have demonstrated that pre-treatment metabolite levels or ratios can be used to improve patient safety. In addition, extreme center-to-center differences have been reported for metabolite measures [38,66,67], and both circadian variation and food intake have been shown to affect plasma metabolite levels [76,82]. While the high variability inherent in these assays limits the interpretation of results at the individual level, the method has been highly useful as a research tool to identify DNA biomarkers of DPD deficiency.…”