Risk of Toxicity From Topical 5‐Fluorouracil Treatment in Patients Carrying DPYD Variant Alleles

Patients carrying DPYD variant alleles have increased risk of severe toxicity from systemic fluoropyrimidine chemotherapy. There is a paucity of data regarding risk of toxicity from topical 5‐fluorouracil (5‐FU) treatment in these patients, leading to inconsistent guideline recommendations for pretreatment testing and topical 5‐FU dosing. The objective of this retrospective cohort study was to investigate whether DPYD variant allele carriers have increased risk of toxicity from topical 5‐FU. Treatment and toxicity data were retrospectively ed from the electronic medical records. Genotypes for the five DPYD variants that are associated with increased toxicity from systemic fluoropyrimidine chemotherapy (DPYD*2A, DPYD*13, DPYD p.D949V, DPYD HapB3, and DPYD p.Y186C) were collected from a genetic data repository. Incidence of grade 3+ (primary end point) and 1+ (secondary end point) toxicity was compared between DPYD variant carriers vs. wild‐type patients using Fisher's exact tests. The analysis included 201 patients, 7% (14/201) of whom carried a single DPYD variant allele. No patients carried two variant alleles or experienced grade 3+ toxicity. DPYD variant allele carriers did not have a significantly higher risk of grade 1+ toxicity (21.4% vs. 10.2%, odds ratio = 2.40, 95% confidence interval: 0.10–2.53, P = 0.19). Given the low toxicity risk in patients carrying a single DPYD variant allele, there is limited potential clinical benefit of DPYD genetic testing prior to topical 5‐FU. However, the risk of severe toxicity in patients with complete DPD deficiency remains unknown and topical 5‐FU treatment should be avoided in these patients.