DPYD Pharmacogenomics and Fluoropyrimidine (FP) Dose-Adjustment

Fluropyrimidines (FPs), such as 5-Fluorouracil (5-FU) and Capecitabine, are cornerstone chemotherapy agents widely used in the treatment of various cancers, including gastrointestinal, head and neck, and breast cancers. Dosing of FP, specifically 5-FU, has been a challenge since this drug was first synthesized in 1957 by Charles Heidelberger1. 5FU had been given on numerous schedules and doses, including weekly 24 hr infusion, 48-hour infusion every two weeks, and daily five-times-a-day infusion with leucovorin (Mayo Regimen) and weekly bolus with leucovorin infusion (Roswell, Park Regimen). In addition, oral fluropyrimidines, including Capecitabine, Tegafur, and S-1, are widely used throughout the world and have more prolonged dosing schedules, typically consisting of 2- to 3-week courses given twice daily. Despite their efficacy, the administration of these therapies has historically been challenging due to significant inter-patient variability in toxicity. A small but critical proportion of patients experience severe, potentially fatal toxicities, primarily attributed to a deficiency in Dihydropyrimidine Dehydrogenase (DPD) enzyme. DPD is the key enzyme in the catabolism of 5-FU, metabolizing it into less toxic metabolites, such as FBAL. DPD activity may also be involved in decreasing the absorption of 5FU from the oral GI tract and accounts for its variability in absorption. When using irreversible DPD inhibitors, such as ethinyl-uracil, the daily oral dose of 5FU was less than 2 mg/m² compared to more than 1000 mg/m² when given orally. Genetic variants in the DPYD gene, which encodes the DPD enzyme, are responsible for reduced or absent enzyme function. These variants are catalogued by the Clinical Pharmacogenetics Implementation Consortium (CPIC) and range from no function to reduced or normal function. While the prevalence of any DPYD reduced activity variant is estimated at 2-4% in the American population (heterozygous abnormality), homozygous abnormalities are much rarer (\<0.0025%). Historically, routine DPYD gene testing has not been widely recommended in the United States due to concerns about cost, treatment delays, and inconvenience2. However, the European community, with a higher incidence of DPYD deficiency and more affordable testing, has recommended routine testing for some time. However, the FDA only recently added a black box warning against using 5-FU in patients with DPD enzyme deficiency. On January 24, 2025, the FDA, as part of Project Renewal, recommended that clinicians consider testing all patients and discuss the issue with them. The FDA notes that for complete DPD deficiency, no dose is considered safe; however, for "partial deficiency," they state, "there is insufficient data to recommend a specific dose in patients with partial DPD deficiency."4 Given that the new warnings are now part of the package insert for 5-FU and Capecitabine, it is more incumbent on the practitioner to test for this abnormality for all patients beginning fluoropyrimidine therapy. CPIC has proposed reduction guidelines for patients with one gene abnormality (heterozygotes), which include a 50% dose reduction or possibly a 30% dose reduction for those with less severe DPD activity.³ However, these guidelines lack widespread clinical validation, raising questions about their necessity for toxicity control versus potential underdosing of such patients, affecting therapeutic outcomes. This prospective interventional treatment study aims to validate these DPYD-guided dosing strategies in a real-world clinical setting