DPYD and hyperinsulinemic hypoglycemia, familial, 4: While 5FU is cytotoxic, when catabolized in the liver by the dihydropyrimidine dehydrogenase (DPD) enzyme, it is excreted in the urine as nontoxic α-fluoro-β-alanine.13 However, patients with DPD deficiency have been demonstrated to experience severe toxicities with a prolonged 5FU half-life.14