The Uracil Breath Test in the Assessment of Dihydropyrimidine Dehydrogenase Activity: Pharmacokinetic Relationship between Expired 13CO2 and Plasma [2-13C]Dihydrouracil

Abstract

Purpose: Dihydropyrimidine dehydrogenase (DPD) deficiency is critical in the predisposition to 5-fluorouracil dose-related toxicity. We recently characterized the phenotypic [2-13 C]uracil breath test (UraBT) with 96% specificity and 100% sensitivity for identification of DPD deficiency. In the present study, we characterize the relationships among UraBT-associated breath C]uracil concentrations were determined over 180 minutes using IR spectroscopy and liquid chromatography-tandem mass spectrometry, respectively. Pharmacokinetic variables were determined using noncompartmental methods. Peripheral blood mononuclear cell (PBMC) DPD activity was measured using the DPD radioassay. Results: The UraBT identified 19 subjects with normal activity, 11 subjects with partial DPD deficiency, and 1 subject with profound DPD deficiency with PBMC DPD activity within the corresponding previously established ranges. UraBT breath Dihydropyrimidine dehydrogenase (EC 1.3.1.2, DPD) is the rate-limiting enzyme in uracil and 5-fluorouracil (5-FU) catabolism, converting >80% of an administered dose of 5-FU to inactive metabolites (1, 2). The initial step of catabolism is mediated by DPD converting 5-FU to 5-dihydrofluorouracil, with subsequent catabolism by dihydropyrimidinase and h-ureidopropionase enzymes to ultimately produce fluoroh-alanine, ammonia, and CO 2 . The latter final metabolic endproducts are excreted in the urine and breath (3).The pharmacogenetic syndrome of complete and partial DPD deficiency is prevalent in f0.1% and 3% to 5% of the general population, respectively (4). DPD deficiency is a significant pharmacogenetic factor in the predisposition of cancer patients to increased risk of altered 5-FU pharmacokinetics and associated toxicity. Specifically, 60% of patients presenting with severe 5-FU-related hematologic toxicity showed reduced DPD activity (5).Recent studies have investigated the predictive value of the ratio of plasma dihydrouracil area under the curve (AUC) to uracil AUC (DUUR) for the assessment of DPD activity and potential individualization of 5-FU therapy. Specifically, 5-FU dose optimization may be based on the plasma DUUR observed before 5-FU administration (6). Jiang et al. have also showed that the pre-5-FU treatment DUUR may be a good index of DPD activity (7,8).Our laboratory recently reported the rapid noninvasive phenotypic [2-13 C]uracil breath test (UraBT) for assessment of DPD activity with 96% specificity and 100% sensitivity (9). Application of the UraBT to a large population of cancer-free subjects (n = 255) showed an observed 86% sensitivity (with 12 of 14 DPD-deficient subjects identified as DPD deficient) and

Increased Prevalence of Dihydropyrimidine Dehydrogenase Deficiency in African-Americans Compared with Caucasians

“…More recently, we validated the UraBT by evaluating plasma [2-13 C]uracil, [2-13 C]dihydrouracil, and breath 13 CO 2 pharmacokinetics in subjects with normal DPD enzyme activity as well as DPD deficiency. We showed that UraBT 13 CO 2 concentrations are significantly correlated with DPD enzyme activity and plasma [2-13 C]uracil clearance and [2-13 C]dihydrouracil formation (19). These results suggest that the UraBT may be used to rapidly assess variability in in vivo pyrimidine catabolism.…”

“…Recently, we developed and optimized the UraBT to rapidly (<1 hour) screen cancer patients for reduced DPD enzyme activity (18). Subsequently, we did a pharmacokinetic validation of the UraBT by characterizing relationships present among UraBT-associated breath 13 CO 2 metabolite formation, plasma [2-13 C]dihydrouracil formation, plasma [2-13 C]uracil clearance, and PBMC DPD enzyme activity in normal and DPD-deficient subjects (19). More recently, we showed that the UraBT is a rapid method suitable for population studies by screening 13 East Indian subjects for DPD deficiency subsequent to our initial identification and characterization of DPD deficiency in an East Indian cancer patient with severe 5-FU toxicity (1).…”

“…Recently, we did a pharmacokinetic evaluation of the UraBT and showed that the UraBT DOB 50 is significantly related to markers of [2-13 C] uracil degradation (i.e., clearance, half-life, and area under the curve) and [2-13 C]dihydrouracil formation (C max , T max , and rate of appearance; ref. 19). In the current study, we observed that African-Americans had significantly lower UraBT DOB 50 values compared with Caucasians.…”

Influence of dihydropyrimidine dehydrogenase gene (DPYD) coding sequence variants on the development of fluoropyrimidine-related toxicity in patients with high-grade toxicity and patients with excellent tolerance of fluoropyrimidine-based chemotherapy

“…Since eighties, the numerous methods based on determination of DPD status were advised to predict serious 5-FU-related toxicity. The function tests have been used for measurement of DPD enzyme activity by 2-[ 13 C]-uracil breath test, or determining uracil/dihydrouracil or [ 14 C]-thymine/dihydrothymine plasma ratio [32][33][34]. Substantial drawbacks of function tests are a need for specialized technology approaches or the use of radiolabeled chemicals not available worldwide [34].…”

Applications of stable isotopes in clinical pharmacology

“…Van Staveren et al [59] showed that a plasma-based uracil screening test can differentiate between patients with deficient or normal DPD activity based on the uracil/dihydrouracil ratio in plasma. Mattison et al [60,61] showed that breath 13 CO2 appearance after administration of 2-13 C-uracil parallels plasma 2-13 C-uracil and 2-13 C-dihydrouracil pharmacokinetics and is an accurate measure of interindividual variation in DPD activity. Currently, none of the approaches is preferred.…”