Methylation of the DPYD Promoter: An Alternative Mechanism for Dihydropyrimidine Dehydrogenase Deficiency in Cancer Patients

Dihydropyrimidine dehydrogenase (DPD) deficiency is prevalent in 3-5% of the Caucasian population; however, the frequency of this pharmacogenetic syndrome in the Indian population and other racial and ethnic groups remains to be elucidated. We describe an Indian patient who presented to clinic for the treatment of gastric adenocarcinoma with 5-flurouracil (5-FU) therapy who subsequently was diagnosed with DPD deficiency by using the peripheral blood mononuclear cell (PBMC) DPD radioassay. This observation prompted us to examine the data generated from healthy (cancer-free) Indian subjects who were enrolled in a large population study to determine the sensitivity and specificity of the uracil breath test (UraBT) in the detection of DPD deficiency. Thirteen Indian subjects performed the UraBT. UraBT results were confirmed by PBMC DPD radioassay. The Indian cancer patient demonstrated reduced DPD activity (0.11 nmol/min/mg protein) and severe 5-FU toxicities commonly associated with DPD deficiency. Of the 13 Indian subjects [ten men and three women; mean age, 26 years (range: 21-31 years)] enrolled in the UraBT, 12 Indian subjects demonstrated UraBT breath profiles and PBMC DPD activity within the normal range; one Indian subject demonstrated a reduced breath profile and partial DPD deficiency. DPD deficiency is a pharmacogenetic syndrome which is also present in the Indian population. If undiagnosed, the DPD deficiency can lead to death. Future epidemiological studies would be helpful to determine the prevalence of DPD deficiency among racial and ethnic groups, allowing for the optimization of 5-FU chemotherapy.

To the editor, With interest we read the article by Dr Cubero and colleagues, in which they evaluated the safety of tegafur-uracil (UFT®) in five cases with partial dihydropyrimidine dehydrogenase (DPD) deficiency [Cubero et al. 2012]. Based on our previous experience [Deenen et al. 2010], however, we would like to express our concern about their conclusion that UFT is a safe alternative for the treatment of patients with partial DPD deficiency. Cubero and colleagues make the erroneous and unproven statement that the presence of uracil in UFT creates an artificial DPD deficiency, and that the DPD activity in patients with normal DPD activity would then be similarly as low as in DPD-deficient patients. This assumption, however, is incorrect. As uracil is a competitive inhibitor of DPD, it competes with 5-fluorouracil (5-FU) for DPD-mediated metabolism. This does not mean that the activity of DPD is depleted, as suggested by Cubero and colleagues, in contrast, its activity is fully utilized, as well as for the metabolism of uracil, as for the metabolism of 5-FU. We would like to caution that treating patients with partial DPD deficiency with the standard dose of UFT may unnecessarily lead to severe, potentially lethal toxicity. Unlike the cases described by Cubero and colleagues, we could previously describe four cases presenting with comparable severe toxicity profiles upon treatment with UFT as had previously occurred during treatment with capecitabine or 5-FU. In all subjects an underlying partial DPD deficiency was identified by genotype and phenotype analyses [Deenen et al. 2010]. Furthermore, there are several pharmacological lines of argument that support our clinical observation, i.e. that the standard dose of UFT is not safe in (partial) DPD-deficient patients. First, pharmacokinetic studies have shown that DPD remains essential for the metabolism of UFT, with significantly longer half-lives of 5-FU after administration of UFT compared with 5-FU administered intravenously [Ho et al. 1998]. This is due to the presence of uracil in UFT. Since DPD-deficient patients already have longer half-lives of 5-FU than other patients [Mattison et al. 2006], presence of uracil increases its half-life even further. This in turn leads to prolonged and elevated circulating levels of 5-FU, with a subsequently increased risk of 5-FU-induced severe toxicity. Another argument underscoring the importance of normal DPD function in the safe application of UFT, is the experience with S-1. S-1 is another drug combination of tegafur, consisting of tegafur, 5-chloro-2,4-dihydroxypyridine (CDHP) and potassium oxonate in a molar ratio of 1:0.4:1. CDHP inhibits DPD 200-fold more potently than does uracil [Shirasaka et al. 1996a, 1996b]. Even after administration of S-1, the primary 5-FU metabolite formed by DPD is observed in significant concentrations in plasma [Kim et al. 2007]. Thus, DPD remains an essential detoxification enzyme of 5-FU, even when its activity is strongly inhibited. The ultimate proof of theory is the occurrence of 18 treatment-related deaths in patients with cancer and herpes zoster given UFT plus the antiviral drug sorivudine [Pharmaceutical Affairs Bureau, 1994]. Subsequent studies in rats showed that a metabolite of sorivudine, (E)-5-(2-bromovinyl)uracil, instantly and irreversibly inactivates DPD by covalent binding, which has been identified as the underlying mechanism of these toxic deaths [Ogura et al. 1998; Okuda et al. 1998]. It is for these arguments that the Summary of Product Characteristics of UFT notes a known DPD deficiency as a contra-indication [Merck Serono, 2011]. The fact that the patients described by Cubero and colleagues did not develop significant toxicity might be due to patient selection, the slightly decreased dose intensity of 90%, or despite their DPYD*2A genotype a DPD enzyme activity within the (lower) range of normal. We are not aware of this, because DPD enzyme activity was not determined in these patients. In summary, we would like to state that standard-dose UFT is not a safe treatment in (partial) DPD-deficient patients. Instead, dose reductions of on average 50% of either capecitabine, 5-FU or UFT with careful monitoring of safety and further dose titration are proposed as the standard of care [Deenen et al. 2011].

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 (13)CO(2) metabolite formation, plasma [2-(13)C]dihydrouracil formation, [2-(13)C]uracil clearance, and DPD activity. An aqueous solution of [2-(13)C]uracil (6 mg/kg) was orally administered to 23 healthy volunteers and 8 cancer patients. Subsequently, breath (13)CO(2) concentrations and plasma [2-(13)C]dihydrouracil and [2-(13)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. 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 (13)CO(2) DOB(50) significantly correlated with PBMC DPD activity (r(p) = 0.78), plasma [2-(13)C]uracil area under the curve (r(p) = -0.73), [2-(13)C]dihydrouracil appearance rate (r(p) = 0.76), and proportion of [2-(13)C]uracil metabolized to [2-(13)C]dihydrouracil (r(p) = 0.77; all Ps < 0.05). UraBT breath (13)CO(2) pharmacokinetics parallel plasma [2-(13)C]uracil and [2-(13)C]dihydrouracil pharmacokinetics and are an accurate measure of interindividual variation in DPD activity. These pharmacokinetic data further support the future use of the UraBT as a screening test to identify DPD deficiency before 5-fluorouracil-based therapy.

BackgroundPretherapeutic screening for dihydropyrimidine dehydrogenase (DPD) deficiency is recommended or required prior to the administration of fluoropyrimidine-based chemotherapy. However, the best strategy to identify DPD-deficient patients remains elusive.MethodsAmong a nationwide cohort of 5886 phenotyped patients with cancer who were screened for DPD deficiency over a 3 years period, we assessed the characteristics of both DPD phenotypes and DPYD genotypes in a subgroup of 3680 patients who had completed the two tests. The extent to which defective allelic variants of DPYD predict DPD activity as estimated by the plasma concentrations of uracil [U] and its product dihydrouracil [UH2] was evaluated.ResultsWhen [U] was used to monitor DPD activity, 6.8% of the patients were classified as having DPD deficiency ([U] > 16 ng/ml), while the [UH2]:[U] ratio identified 11.5% of the patients as having DPD deficiency (UH2]:[U] < 10). [U] classified two patients (0.05%) with complete DPD deficiency (> 150 ng/ml), and [UH2]:[U] < 1 identified three patients (0.08%) with a complete DPD deficiency. A defective DPYD variant was present in 4.5% of the patients, and two patients (0.05%) carrying 2 defective variants of DPYD were predicted to have low metabolism. The mutation status of DPYD displayed a very low positive predictive value in identifying individuals with DPD deficiency, although a higher predictive value was observed when [UH2]:[U] was used to measure DPD activity. Whole exon sequencing of the DPYD gene in 111 patients with DPD deficiency and a “wild-type” genotype (based on the four most common variants) identified seven heterozygous carriers of a defective allelic variant.ConclusionsFrequent genetic DPYD variants have low performances in predicting partial DPD deficiency when evaluated by [U] alone, and [UH2]:[U] might better reflect the impact of genetic variants on DPD activity. A clinical trial comparing toxicity rates after dose adjustment according to the results of genotyping or phenotyping testing to detect DPD deficiency will provide critical information on the best strategy to identify DPD deficiency.

Dihydropyrimidine dehydrogenase (DPD) deficiency (McKusick 274270) is an autosomal recessive disease characterized by thymine-uraciluria in homozygous-deficient patients and associated with a variable clinical phenotype. Cancer patients with this defect should not be treated with the usual dose of 5-fluorouracil because of the expected lethal toxicity. In addition, heterozygosity for mutations in the DPD gene increases the risk of toxicity in cancer patients treated with this drug. Sequence analysis in a patient with complete DPD deficiency, previously shown to be heterozygous for the delta C1897 frame-shift mutation, revealed the presence of a novel missense mutation, R235W. Expression of this novel mutation and previously identified missense mutations C29R and R886H in Escherichia coli showed that both C29R and R235W lead to a mutant DPD protein without significant residual enzymatic activity. The R886H mutation, however, resulted in about 25% residual enzymatic activity and is unlikely to be responsible for the DPD-deficient phenotype. We show that the E. coli expression system is a valuable tool for examining DPD enzymatic variants. In addition, two new patients who were both heterozygous for the C29R mutation and the common splice donor site mutation were identified. Only one of these patients showed convulsive disorders during childhood, whereas the other showed no clinical phenotype, further illustrating the lack of correlation between genotype and phenotype in DPD deficiency.

Is Capecitabine Safe in Patients with Gastrointestinal Cancer and Dihydropyrimidine Dehydrogenase Deficiency?

521 Background: DPD deficiency is a pharmacogenetic syndrome associated with dose-limiting toxicity to fluoropyrimidines. Oncologists are expected to recognize and diagnose this syndrome, as toxicities could be fatal. Over 40 single nucleotide polymorphisms (SNPs) and deletions have been identified within the DPYD gene. IVS14+1G&gt;A (DPYD*2A) is the most common (40-50%) and best studied of these SNPs. Yet, it showed a median sensitivity of 30% and is absent in Japanese, Korean and African Americans. Overall, the data on DPYD testing is insufficient to provide enough guidance to diagnose DPD deficiency. Herein we describe our experience with germline pharmacogenomics in patients with DPD deficiency. Methods: Between 2011 and 2015, 35 patients with gastrointestinal malignancies were tested for DPYD mutations; 17 were tested after developing toxicities to treatment and 18 were tested prior to treatment. IVS14+1G&gt;A (DPYD*2A) was tested in all patients. DPYD c.85T&gt;C (DPYD*9A), DPYD c.1679T&gt;G (DPYD*13A), DPYD c.-1590T&gt;C, and DPYD c.2846A&gt;T were tested in 24 patients (69%) only. We explored the association between DPYD mutations and fluoropyrimidine-related toxicity using Fisher’s exact test. Results: Median age was 60 years, 43% were male, 80% were Caucasian and 20% were African American. Capecitabine-based regimens (71%) and 5-Fluorouracil-based regimens (29%). 14 out of 35 patients (40%) had DPYD mutations. Grade 3 toxicities were encountered in 64% of patients with DPYD mutation and 48% of patients with no DPYD mutation. In patients who received full dose fluoropyrimidines (57% of patients with DPYD mutation and 81% of patients with no DPYD mutation), DPYD mutations were associated with a significantly higher rate of grade 3 diarrhea (p=0.026). In patients with DPYD mutation, 2 (14%) had DPYD*A2 mutation and 12 (86%) had DPYD*9A mutation. Conclusions: In patients treated with fluoropyrimidines, the rate of grade 3 diarrhea was significantly higher in patients with mutated DPYD gene. Testing for DPYD*2A alone to diagnose DPD deficiency is suboptimal. Testing for other DPYD mutation variants including DPYD*9A provides a more comprehensive approach. These data should further be validated in prospective clinical trials.

AimsTreatment with dihydropyrimidines poses a significant risk of serious adverse reactions for patients with dihydropyrimidine dehydrogenase (DPD) deficiency. This study seeks to analyse the correlation between DPD deficiency and plasmatic...

The cytotoxic effect of 5-fluorouracil (5-FU) is mediated by the inhibition of thymidylate synthase (TS), however, at the same time 5-FU is catabolized by dihydropyrimidine dehydrogenase (DPD). Efficacy of 5-FU may therefore depend on the TS and DPD activity and on pharmacogenetic factors influencing these enzymes. Our aims were (1) to determine the distribution of DPD activity, the frequency of DPD deficiency and the DPD (IVS14+1G>A) mutation in the peripheral blood mononuclear cells of colorectal cancer (CRC) patients, and study the relationship between DPD deficiency and toxicity of 5-FU; (2) to investigate the influence of TS polymorphisms and DPD activity on the survival of CRC patients receiving 5-FU-based adjuvant therapy. The frequency of DPD deficiency was determined by radiochemical methods in the peripheral blood mononuclear cells (PBMCs) of 764 CRC patients treated with 5-FU. The relationship between the TS polymorphisms, DPD activity and the disease-free and overall survival was studied in 166 CRC patients receiving 5-FU-based adjuvant therapy. TS polymorphisms were determined in the DNA samples separated from the PBMCs, by PCR-PAGE and PCR-RFLP-PAGE (restriction fragment length polymorphism) methods. Low DPD values (<10 pmol/min/106 PBMCs) were demonstrated in 160/764 patients (20.9%), and of those DPD deficiency (<5 pmol/min/106 PBMCs) was verified in 38 patients (4.9%). In the latter group severe (>Gr 3) toxicity was found in 87%. The prevalence of the DPD IVS14+1G>A mutation among the 38 DPD-deficient patients was 7.8% (3/38) and was accompanied by severe Gr 4 toxic symptoms (neutropenia, mucositis, diarrhea). TS polymorphisms showed a relationship with the survival of CRC patients. It is important to mention that by combining the 3-3 genotypes of 5'-TSER and 3'-TSUTR polymorphisms the obtained 8 genotype combinations showed significantly different Kaplan-Meier survival curves. The evaluation of these curves with Cox regression analysis resulted in two prognostically different groups: "A" good prognosis (RR<1) and "B" bad prognosis (RR>1). The disease-free- and overall survival of these two groups were significantly different. DPD activity also showed correlation with the survival; patients with DPD activity <10 pmol/min/106 PBMCs showed significantly longer disease-free and overall survival. The determination of DPD activity proved to be a more valuable parameter in the evaluation of serious 5-FU-related toxicity compared to the IVS14+1G>A mutation analysis. According to the Cox multivariate analysis the combination of germline TS polymorphisms and DPD activity is/an independent prognostic marker of survival in CRC patients treated with adjuvant 5-FU therapy.

Dihydropyrimidine dehydrogenase (DPD) deficiency can lead to severe toxicity in patients treated with a standard dose of a fluoropyrimidine such as 5-fluorouracil or capecitabine (CAP). Administration of oral uracil and subsequent measurement of uracil and dihydrouracil (DHU) plasma concentrations has been used to identify patients with DPD deficiency. Liver metastasis might influence systemic DPD activity. The aim of the study was to investigate the effect of metastatic disease on the pharmacokinetics of uracil and DHU after oral administration of uracil. 500mg/m(2) uracil was administered orally to 12 subjects with stages II-III colorectal cancer (CRC) who were treated in the adjuvant setting and to 12 subjects with stage IV metastasized CRC, all treated with CAP containing therapy. All subjects had a normal DPD activity defined as >6nmol/mg/h determined in peripheral blood mononuclear cells. The mean uracil clearance [CL 51.7 (SD 6.4) vs. 46.7 (SD 13.0) l/h], area under the curve [AUC0-220min 20.6 (SD 6.4) vs. 21.0 (SD 5.7) hmg/l], elimination half-life [t 1/2 21 (SD 7) vs. 21 (SD 8) min], maximum concentration time [T max 27 (SD 9) vs. 25 (SD 9) min], volume of distribution [V 26.58 (SD 10.11) vs. 21.10 (SD 8.48) l] and the elimination constant [k el 2.01 (SD 0.56) vs. 2.41 (SD 0.72) h(-1)] did not differ significantly (p>0.05) non-metastatic CRD versus metastatic CRC. Metastasis does not alter uracil pharmacokinetics andis similar in CRC patients with and without metastasis. Therefore, the uracil test dose could be used as a DPD phenotype test in both adjuvantly treated and metastatic CRC patients using similar cutoff criteria to identify patients with DPD deficiency.

Dihydropyrimidine dehydrogenase (DPD) deficiency is a rare autosomal recessive disorder of the pyrimidine degradation pathway and can lead to intellectual disability, motor retardation, and seizures. Genetic variations in DPYD have also emerged as predictive risk factors for severe toxicity in cancer patients treated with fluoropyrimidines. We recently observed a child born to non-consanguineous parents, who demonstrated seizures, cognitive impairment, language delay, and MRI abnormalities and was found to have marked thymine-uraciluria. No residual DPD activity could be detected in peripheral blood mononuclear cells. Molecular analysis showed that the child was homozygous for the very rare c.257C>T (p.Pro86Leu) variant in DPYD. Functional analysis of the recombinantly expressed DPD mutant showed that the DPD mutant carrying the p.Pro86Leu did not possess any residual DPD activity. Carrier testing in parents revealed that the father was heterozygous for the variant but unexpectedly the mother did not carry the variant. Microsatellite repeat testing with markers covering chromosome 1 showed that the DPD deficiency in the child is due to paternal uniparental isodisomy. Our report thus extends the genetic spectrum underlying DPYD deficiency.

Dihydropyrimidine dehydrogenase (DPD) enzyme is responsible for the elimination of approximately 80% of administered dose of 5-FU. DPD deficiency has been associated with severe 5-FU toxicity. Syndrome of DPD deficiency manifests as diarrhea, stomatitis, mucositis, and neurotoxicity and in some cases death. This is a true pharmacogenetic syndrome, with symptoms being unrecognizable until exposure to the drug. A 75-year-old patient with metastatic pancreatic adenocarcinoma developed grade 4 thrombocytopenia, grade 3 coagulopathy, and grade 3 neurologic toxicity with a fatal outcome following administration of 5-FU. Due to pancytopenia, DPD activity could not be determined in peripheral blood mononuclear cells (PBMC) using a previously described radioassay. Therefore, screening and genotypic analysis of homozygous and heterozygous, known and unknown sequence variants, in the DPYD gene were performed using DHPLC as previously described. All DPYD sequence variants identified by DHPLC were confirmed by DNA sequencing using a dideoxynucleotide chain termination method and capillary electrophoresis on an ABI 310 Automated DNA Sequencer. Genotyping analysis of the DPYD gene revealed the presence of the heterozygous mutation, IVS14 + 1 G > A, DPYD*2A. Genotypic analysis using DHPLC can be employed to screen DPD deficiency in a patient with severe neutropenia. The mutation IVS14 + 1 G > A, DPYD*2A, is the most common mutation associated with DPD deficiency. A G > A base change at the splice recognition sequence of intron 14, leads to exon skipping and results in a 165-bp deletion in the DPD mRNA. We have previously demonstrated that a homozygote DPYD*2A genotype results in complete deficiency while the heterozygous DPYD*2A genotype results in partial deficiency of DPD.

e13505 Background: The enzyme dihydropyrimidine dehydrogenase (DPD) catalyzes the first step in degradation of 5-fluorouracil (5FU). Patients with complete or partial DPD deficiency (about 0.1% and 3% of the population, respectively) can experience severe or lethal toxicities after receiving standard doses of 5FU, and impaired clearance may underlie a large fraction of the ∼1,300 deaths per year due to 5FU toxicity in the US. Orally-administered uridine triacetate (vistonuridine) prevents or diminishes toxicities when administered after 5FU overexposure, and has been used successfully as an antidote in more than 28 patients to date who had received accidental overdoses of 5FU. Because DPD deficiency may alter 5FU clearance kinetics (versus 5FU overdoses in the setting of normal DPD activity), studies were conducted on reversal of 5FU toxicity in mice pretreated with the potent DPD inhibitor ethynyluracil (eniluracil; EU) to model DPD deficiency. Methods: Balb/C mice received 2 mg/kg EU i.p., followed by 100 mg/kg 5FU (weekly bolus MTD in mice). Groups of 10 mice each then received either vehicle or oral uridine triacetate (2,000 mg/kg t.id. × 5) beginning at a range of times after 5FU. Survival was monitored. Results: 100 mg/kg 5FU was lethal in mice pretreated with EU. Uridine triacetate administration beginning within 24 hours after EU + 5FU resulted in survival of all the mice in the treatment groups. Fewer mice survived if treated with uridine triacetate at later time points after EU plus 5FU. Conclusions: Timely treatment with uridine triacetate reduced 5FU toxicity and mortality in DPD-inhibited mice. Its benefit has previously been demonstrated in patients (and mice) overdosed with 5FU. Therefore, DPD-deficient patients who have received 5FU should also benefit from treatment with uridine triacetate if the deficiency is identified soon enough after 5FU dosing. Therapeutic monitoring of 5FU during or after infusions could permit rapid detection of 5FU overexposure due to DPD-deficiency, enabling the use of uridine triacetate as an antidote in DPD-deficient patients. Author Disclosure Employment or Leadership Position Consultant or Advisory Role Stock Ownership Honoraria Research Funding Expert Testimony Other Remuneration Wellstat Therapeutics Corporation

2056 Background: 5-Fluorouracil (5-FU) is an integral part of treatment of GI malignancies. While normal DPD enzyme activity is rate limiting in 5-FU catabolism, its deficiency could increase concentrations of bioavailable 5-FU anabolic products leading to 5-FU related toxicity syndrome. With DPD deficiency, 5-FU is discontinued. Data regarding safety of capecitabine (CAP) in this population is scarce. Methods: Patients were tested for DPD deficiency after excessive toxicities from 5-FU and CAP at UAB between 2001 and 2005. DPD activity was evaluated by PBMC radio assay, genotyping of DPYD gene by DHPLC, or 2-13C uracil breath test (UraBT). Results: Of 23 patients with GI malignancies (small intestine, gastric, pancreatic, HCC, and colorectal) evaluated, 7 (30%) were DPD deficient. Among these 7 patients, DPD activity ranged from 0.064 - 0.18 nmol/min/mg. Age ranged from 51–75 years, M:F ratio = 1.3:1, and ethnicities included Caucasian (71%), African-American (14%) and South-Asian (14%). Four were treated with 5-FU/LV (2 Roswell; 2 Mayo); 2 CAP (1800mg/m2); and 2 high dose bolus 5-FU (1400mg/m2) + PN401 (tri-acetyluridine). Toxicities included mucositis (71%), diarrhea (43%), nausea (29%), memory loss/altered mental status (43%), cytopenias (43%), hypotension (14%), respiratory distress (14%), acute renal failure (14%), and severe skin rashes (43%). Re-challenge with CAP in 1 patient after the Mayo regimen caused grade 3 HFS only on dorsal surfaces of hands. One patient on PN401 had a grade 3 facial rash as the worst toxicity. Genotypic analysis of the DPYD gene in the second on PN401, who had severe leucopenia, demonstrated a heterozygous mutation (IVS14+1 G&gt;A, DPYP*2A). UraBT in 2 patients revealed 1 to be DPD-deficient (DOB50 of 112.8; PDR of 49.4%) and borderline normal values (DOB50 of 130.9; PDR of 52.5%) in a second patient. There were 2 toxicity-related deaths (28%): 1 on CAP and 1 on 5-FU + PN401. Conclusions: DPD deficiency was observed in several ethnicities. Patients with CAP toxicities should also be tested for DPD deficiency. Role of PN401 in rescuing 5-FU toxicity in DPD deficiency is not clear. Screening patients for DPD deficiency prior to administration of 5-FU or CAP, using UraBT, could potentially lower risk of toxicity. Future studies should validate this technique. [Table: see text]

Approximately 30-40% of grade III-IV toxicity to 5-FU has been associated with partial or profound deficiency in dihydropyrimidine dehydrogenase (DPD), the first of three enzymes in the catabolic pathway of fluoropyrimidines. There remains, however, a subset of patients presenting with 5-FU-associated toxicity despite normal DPD activity, suggesting possible deficiencies in enzymes downstream of DPD: dihydropyrimidinase (DHP), encoded by the DPYS gene, and/or beta-ureidopropionase (BUP-1), encoded by the UPB1 gene. Previously, we reported the identification of inactivating mutations in the DPYS gene that could potentially alter the uracil catabolic pathway in healthy individuals with normal DPD enzyme activity. This study investigates the possible role of UPB1 genetic variations in the regulation of the uracil catabolic pathway in individuals presenting with a deficient uracil breath test (13C-UraBT) despite normal DPD enzyme activity. This study included 219 healthy asymptomatic volunteers with known DPD enzyme activity and [2-(13)C]-uracil breath test (UraBT). All samples were genotyped for sequence variations in the UPB1 gene using denaturing high performance liquid chromatography (DHPLC) and Surveyor enzyme digestion with confirmation of detected sequence variants by direct sequencing. Seven novel and six previously reported sequence variations were identified, including one nonconservative mutation, which demonstrated 97.3% reduction in BUP-1 activity when expressed in the RKO cell line. Data presented in this study demonstrate that alterations of uracil catabolism are not limited to DPD and/or DHP deficiency and that inactivating mutations in the UPB1 gene might impair uracil catabolism.