Development, validation, and use of a novel [2-carbon-13] uracil breath test to detect dihydropyrimidine dehydrogenase deficiency

Dihydropyrimidine dehydrogenase (DPD) deficiency is an autosomal codominantly inherited pharmacogenetic syndrome associated with severe, unanticipated, 5-fluorouracil (5-FU) toxicity in cancer patients. As the initial and rate-limiting enzyme of the pyrimidine catabolic pathway, DPD degrades more than 80% of an administered dose of 5-FU. When DPD deficient cancer patients are administered 5-FU, dangerously increased 5-FU AUC, exposure, and toxicity may occur. DPD deficiency is present in 3-5% of the general population.; Given the frequency of DPD deficiency and the severity of DPD-mediated 5-FU toxicity, development of a diagnostic test to rapidly screen cancer patients for DPD deficiency prior to 5-FU administration is critical. Unfortunately, due to a lack of characterization of the majority of the 30 reported DPYD (gene that encodes DPD) sequence variations, development of a clinically useful genotypic test is not feasible. Therefore the development of a rapid phenotypic test, an oral [2-13C] uracil breath test (UraBT), was the primary objective of this dissertation research. The major findings of this research are the following: (1) the amino acid sequence of the DPD enzyme is conserved among mammals and invertebrates, suggesting that a comparative genetic approach may be used to prioritize DPYD sequence variations for future functional characterization; (2) DPD deficient individuals may be identified by reduced breath 13CO 2 concentrations and altered breath 13CO2 kinetics following oral administration of [2-13C] uracil in UraBT; (3) UraBT sensitivity and specificity is 100 and 96%; (4) DPD deficient subjects have altered plasma [2-13C] uracil and [2- 13C] dihydrouracil pharmacokinetics; (5) PBMC DPD activity, [2- 13C] uracil and [2-13C] dihydrouracil pharmacokinetics significantly correlate with the UraBT DOB50; (6) the UraBT is a rapid assay which may be used to screen previously unexamined populations, such as Indians, for DPD deficiency; and (7) the African American population, particularly African American women, have may have significantly reduced pyrimidine catabolism as demonstrated by reduced UraBT DOB50 values and PBMC DPD activity, which may increase the risk of 5-FU toxicity. These data support the future evaluation and use of the UraBT as a screening assay to identify DPD deficiency in cancer patients prior to 5-FU administration.