Background: Fluoropyrimidines are chemotherapeutic agents used in various types of cancer such as colorectal, breast, gastric, etc. Approximately 30% of patients treated with fluoropyrimidines present severe toxicity, which can lead to death in around 1% of them. Reduced activity of the enzyme dihydropyrimidine dehydrogenase (DPD) can cause severe fluoropyrimidine-related toxicity. The objective of this study was to identify the frequency of fluoropyrimidine-related toxicity in patients treated at an oncology center from Northeast Mexico. Methods: Retrospective and observational study of patients treated at the Centro Universitario contra el Cáncer (CUCC) in Monterrey, Mexico. Patients older than 18 years with a diagnosis of colon cancer who were treated during 2021 with the diagnosis of colon and considered candidates for fluoropyrimidine-based chemotherapy (5-FU, capecitabine) were included. Pregnant patients were excluded. None of them had a DPYD testing performed. Results: We analyzed 88 patients, of whom 51 (58%) were men, 6 (7%) in stage I, 16 (18%) in stage II, 31 (35%) in stage III, and 35 (40%) in stage IV. The mean age was 59 years, 15 (17%) of them had a PS>2, overall, 94% of patients received fluoropyrimidine-based CT, presenting up to 47 (57%) of toxicities, 26 (32%) in G1, 11 (13%) in G2, and 10 (12%) in G3; no G > 4 toxicities were documented. There were 7 patients (9%) that required CT discontinuation due to toxicity at some point during their treatment (see Table 1). In the subgroup of patients in stage IV, 8 (24%) had PS>2, and 6 (18%) of them had some mutational panel. Conclusions: This is one of the few descriptive studies of toxicities in a population from northeastern Mexico undergoing treatment with fluoropyrimidine-based CT, in which up to 9% of the patients discontinue treatment due to toxicity. Our findings evidence a high percentage of patients with fluoropyrimidine-related toxicity. DPYD testing has not been widely adopted in most oncology centers in Mexico, mostly because of the limited access to genome sequencing. Genetic studies in this population could identify poor metabolizers patients with a high risk of toxicity before starting treatment.

* Excluding stage I