Purpose: Around 20%-30% of patients treated with fluoropyrimidines develop severe treatment-related adverse events (AEs). These are mainly caused by deficiency of dihydropyrimidine dehydrogenase, its main metabolizing enzyme. The DPYD*7 variant allele contains a frameshift mutation that leads to absence of dihydropyrimidine dehydrogenase. Clinical studies on this variant in patients treated with fluoropyrimidines are lacking because of its low minor allelic frequency. However, the DPYD*7 minor allelic frequency is 56-times higher in the Dutch compared with the global population. This allowed us to evaluate fluoropyrimidine tolerability in DPYD*7 variant allele carriers.
Materials and methods: Patients treated with standard-of-care fluoropyrimidine who were pretreatment DPYD genotyped for DPYD*2A, *13, 2846A>T, and 1236G>A single-nucleotide polymorphisms were included for analyses. Patients were additionally screened for the DPYD*7 allele (rs72549309, 295-298delTCAT). AEs were graded if they worsened from baseline, according to Common Terminology Criteria for Adverse Events version 5.0. AEs ≥ grade 3 were considered severe.
Results: From 3,748 patients, we found 13 patients carrying heterozygous DPYD*7. Relevant clinical data were available for 11 patients. All patients developed fluoropyrimidine-related AEs, of which five patients developed severe AEs (46%). From these five patients, three patients were started with 65% or 50% of standard dose, but apparently still developed severe toxicity. Because of severe AEs, three patients discontinued treatment prematurely (one patient already started with 50% of standard dose) and one patient who started with 50% of standard dose was further reduced to 35% of standard dose.
Conclusion: In this study, the clinical consequences of carrying the DPYD*7 variant allele were confirmed as 46% of the patients developed severe AEs, even in the presence of initial dose reductions. This underlines the need for prospective studies investigating the required fluoropyrimidine dose for DPYD*7 carriers.