This clinical trial focuses on patients with gastrointestinal cancers, which affect the digestive system. The study is particularly interested in patients who lack a specific liver enzyme called dihydropyrimidine dehydrogenase (DPD). This enzyme is important because it helps break down certain chemotherapy drugs. The trial will use chemotherapy treatments that include fluorouracil and capecitabine, which are types of drugs known as fluoropyrimidines. These drugs are often used in combination with another drug called oxaliplatin. The purpose of the study is to find the best way to adjust the doses of these drugs for patients who have a deficiency in the DPD enzyme, to reduce the risk of severe side effects.

Participants in the study will receive chemotherapy treatments that are adjusted based on their levels of a substance called uracilemia, which is measured before treatment begins. The study will compare different dosing strategies to see which one is most effective in preventing early severe side effects during the first two cycles of treatment. These side effects can include issues with blood cells and the digestive system. The study aims to develop guidelines for safely reducing doses in patients with DPD deficiency.

The trial will involve treatments over a period of several weeks, with regular monitoring to assess how patients respond to the adjusted doses. The study will also look at how effective the treatments are in terms of disease progression and overall survival in patients with stage III and stage IV colorectal cancer. By the end of the study, researchers hope to provide clear recommendations for dosing adjustments in patients with gastrointestinal cancers who have a DPD deficiency.