Otsuka Pharmaceutical was developing URACIL C-13, 2- ([2-13C]uracil) breath test for diagnosis of cancer and gastric emptying disorders. Dihydropyrimidine dehydrogenase (DPD) deficiency is critical in the predisposition to 5-fluorouracil dose-related toxicity. The phenotypic [2-(13)C]uracil breath test (UraBT) demonstrated 96% specificity and 100% sensitivity for identification of DPD deficiency. Phase II development of the breath test was ongoing. As phase II clinical study didn't exploit performance as diagnostic medicines, the development of [2-13C]uracil was discontinued.

1-(2-DEOXY-2-FLUORO-β-D-ARABINOFURANOSYL)URACIL (FAU) is a thymidine analog. In several cancer cell lines, FAU was phosphorylated intracellularly to its monophosphate, 1-(2-deoxy-2-fluoro--Darabinofuranosyl) uracil monophosphate (FAUMP), by thymidine kinase and methylated in the 5-position by thymidylate synthase to form the product, 1-(2-deoxy-2-fluoro- -D-arabinofuranosyl) 5-methyluracil monophosphate (FMAUMP). FAU strongly inhibits the growth of tumor cells with high thymidylate synthase activity. FAU had been in phase I clinical trial for the treatment of advanced solid tumors.