DPD Guided 5-FU Precision Treatment in Gastro-intestinal Cancer Patients

"Patients without common DPYD variant will be used for this study as a control group. Patients will receive the first 5-FU-dose based on their body surface area. 5-FU plasma concentrations will be measured during cycle 1. If patients experience CTC grade 3-4 toxicity, DPYD sequencing will be performed. Dose reduction or discontinuation of 5-FU therapy will be considered according to the physician's discretion.~Patients with a DPYD variant will receive the first dose based on their body surface area and gene activity score according to Clinical Pharmacogenetics Implementation Consortium (CPIC) guideline or according to the physician's descretion. Patients' blood sample will be collected and the 5-FU dose for the next cycle will be calculated according to a modified algorithm of Goirand or according to the pharmacist's discretion. 5-FU plasma concentrations will be monitored until a steady state AUC of 20-30 mg.h/L is reached."