Implementation of Pre-emptive Geno- and Phenotyping in 5-Fluorouracil- or Capecitabine-treated Patients

5-FU (5-fluorouracil) and its oral prodrug capecitabine are commonly used drugs in various chemotherapy regimens. According to the literature, approximately 30% of the patients experience at least a grade three toxicity during treatment, mainly characterized by diarrhea, mucositis, hematological toxicity or hand-foot syndrome. This toxicity can lead to discontinuation or interruption of the therapy, hospitalization and in 1% of the cases even to mortality. This leads to an increase in health care costs, mainly driven by the cost of hospitalization. The metabolism of 5-FU and its prodrug is primarily determined by the dehydropyrimidine dehydrogenase (DPD) enzyme. The gene encoding for this enzyme, the DPYD, is known for his genetic polymorphism. Five percent of the population has a partial DPYD deficiency and 0.01 to 0.1 percent has a full DPYP deficiency. Partial deficiency leads to a reduced activity of DPD and therefore to a reduced degradation of 5-FU or capecitabine to its inactive metabolites. This leads to an increased toxicity. The four DPYD variants considered most clinically relevant are DPYD \* 2A, DPYD \* 13, c.1236G\> A and c.2846A\> T. Patients with polymorphisms DPYD \* 2A and DPYD \* 13 have no residual enzyme activity, while in patients with polymorphisms 1236G\> A and c.2846A\> T, there is still partial enzyme activity present. In addition to genotyping, partial DPYD deficiency can also be detected by phenotyping. In the case of reduced enzyme activity of DPYD, the degradation of uracil is disturbed, causing an increase in uracil and a decrease in UH2 in plasma. There is a strong correlation between the UH2 / U ratio in plasma and the halflife, clearance and plasma levels of 5-FU. French guidelines, HAS (La Haute Autorité de santé), recommend to adjust the dose of these drugs at the start of treatment based on the results of both the genetic and phenotypic studies. The aim of this study is to investigate if the systematic implementation of pre-emptive geno- and phenotyping, and therefore a dose reduction based on the French guidelines and the literature during the first month of treatment, reduces grade 3 or greater toxicity in patients treated with 5-FU or capecitabine. Therefore, a monocentric, partial prospective and partial retrospective trail was designed.