Management of patients with reduced dihydropyrimidine dehydrogenase activity receiving combined 5-fluoruracil-/capecitabine-based chemoradiotherapy.

Background: 5‑Fluoruracil (5-FU) and its oral prodrug capecitabine are mainstays in combined chemoradiotherapy regimens. They are metabolized by dihydropyrimidine dehydrogenase (DPYD). Pathogenic variants of the DPYD gene cause a reduction in DPYD activity, leading to possibly severe toxicities. Therefore, patients receiving 5‑FU-/capecitabine-based chemoradiotherapy should be tested for DPYD variants. However, there are limited clinical data on treatment adjustments and tolerability in patients with decreased DPYP activity receiving combined chemoradiotherapy. Therefore, a retrospective analysis of the toxicity profiles of patients with decreased DPYD activity treated at our center was conducted.

Materials and methods: For all patients receiving 5‑FU-/capecitabine-based chemo(radio)therapy at our department, DPYD activity was routinely tested. Genotyping of four DPYD variants (DPYD*2A, DPYD*13, c.2846A > T, and haplotype B3) was conducted according to the recommendation of the German Society for Hematooncology (DGHO) using TaqMan hydrolysis polymerase chain reaction (PCR; QuantStudy 3, Thermo FisherScientific, Darmstadt). DPYD variants and activity score as well as clinical data (tumor entity, treatment protocol, dose adjustments, and toxicity according to the Common Terminology Criteria for Adverse Events [CTCAE]) were assessed and reported.

Results: Of 261 tested patients, 21 exhibited DPYD variants, 18 of whom received chemoradiotherapy. All but one patient was treated for rectal or anal carcinoma. The observed rate of DPYD variants was 8.0%, and heterozygous haplotype B3 was the most common (5.75%). One patient exhibited a homozygous DPYD variant. DPYD activity score was at least 0.5 in heterozygous patients; chemotherapy dose was adjusted accordingly, with an applied dose of 50-75%. CTCAE grade 2 skin toxicity (50%) and grade 3 leukopenia (33.3%) were most common. One patient experienced a transient grade 4 increase in transaminases. All high-grade toxicities were manageable with supportive treatment and transient. No CTCAE grade 5 toxicities related to 5‑FU administration were observed.

Conclusion: With dose reduction in heterozygous patients, toxicity was within the range of patients without DPYD variants. Our clinical data suggest that dose-adapted 5‑FU-/capecitabine-chemoradiotherapy regimens can be safely considered in patients with heterozygous clinically relevant DPYD variants, but that the optimal dosage still needs to be determined to avoid both increased toxicity and undertreatment in a curative setting.