Recently, cancer has become an obvious public health problem worldwide. The Fluoropyrimidine-based regimen has been the most widely used chemotherapy regimen worldwide in several types of solid tumors, including gastric and colorectal cancer. Of the total number of patients treated with 5-Fluorouracil (5-FU), 10-40% have severe toxicities, which usually result in prolonged and costly hospitalizations. The principle of personalized medicine is to study responses to medications based on individual genomic information. The high degree of miscegenation is a challenge for the worldwide implementation of personalized medicine in clinical practice. Many studies in the specialized literature have reported the influence of pharmacogenomic markers in mixed populations such as the Brazilian population. The aim of this study was to investigate the pharmacogenomic variability of different biomarkers in pharmacogenes involved in the metabolism pathway of Fluoropyrimidines in patients with gastric cancer or colorectal cancer, which are sub-strutured according to response and toxicity to treatment. To perform the research we used 216 patients with colorectal or gastric cancer who received 5-FU chemotherapy treatment. We investigated 33 genetic polymorphisms in 17 pharmacogens (ABCB1, ABCC2, ABCC4, ABCG2, CYP2A6, DPYD, FPSG, ITGB5, MTHFR, SLC22A7, SLC29A1, TP53, TYMS, UMPS, GGH, RRM1, TYMP) involved in the metabolism pathway of fluoropyrimidines. Our results showed that 77.3% of the patients presented some type of toxicity related to 5-FU treatment, of which 22% presented severe toxicities classified in grade 3 and 4. Death occurred in 23 patients, where three cases were related to toxicity and four cases with tumor progression and chemotherapeutic toxicity. Population substructuration was not influential in the association results for pharmacogenetic polymorphisms with the use of 5-FU. The FPGS gene (rs4451422) was shown to be significant in association with overall toxicity (p = 0.0052; OR 0.32) and toxicity events (p = 0.0004; OR 0.22). The ABCC4 gene (rs148551) had a significant association with the clinical response (p = 0.0056; OR 0.28). The SLC29A1 gene (rs760370) was shown to be significant for grade 3 and 4 toxicities (p = 0.0033; OR 4.73). In conclusion, due to the high degree of miscegenation in the Brazilian population, and particularly in the North of Brazil, the generated 5-FU pharmacogenomics data are particularly unique when compared to the homogenous populations investigated to date. The ABCC4, FPGS and SLC29A1 genes have been shown to be important biomarkers predictive of personalized medicine therapy using 5-FU.
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