5-Fluorouracil (5-FU) has long been used for the treatment of gastrointestinal tumors harboring interindividual variability in both the pharmacokinetic and the pharmacogenetic profiles, which in turn may lead to life-threatening toxicities. We carried out a prospective cohort study of adult patients initiating treatment with 5-FU between 2013 and 2015. Primary exposures of interest were the methylenetetrahydrofolate reductase single nucleotide polymorphism in exons 4 and 7 and 5'-untranslated region-thymidylate synthase VNTR genotypes, in addition to baseline clinical and demographic variables. The primary outcome was the time to the occurrence of severe toxicity. We used a Cox regression model to evaluate patients' survival and toxicity experience and its association with baseline characteristics and a priori determined genetic polymorphisms. A total of 197 patients were included, 40.1% developed severe toxicity during follow-up. Variables that were significantly associated with developing severe toxicity were the European Organization for Research and Treatment of Cancer functional score [hazard ratio (HR): 0.98; 95% confidence interval (CI): 0.97-0.99]; type of tumor [anus (HR: 2.50; 95% CI: 1.07-5.82), head and neck/esophagus/stomach (HR: 2.95; 95% CI: 1.64-5.33)] and 5-FU continuous infusion regimens over 4-5 days (HR: 9.35; 95% CI: 2.68-32.59). We found a significant association between baseline functional status, type of tumor and continuous infusion regimens and the occurrence of severe toxicity during the follow-up of patients receiving 5-FU. No association was found with the genotypic variants evaluated. Future validation and modeling of an everyday easy-to-use score to predict toxicity among these subgroup of patients remains warranted.

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Microsatellite instability (MSI) is a hallmark tool for Lynch syndrome (LS) screening and a prognostic marker for sporadic colorectal cancer (CRC). In regions with limited resources and scarce CRC molecular characterization as South America, the implementation of universal MSI screening is under debate for both its purposes. We sought to estimate the frequency of BAT26 in colorectal adenocarcinomas and to determine associated clinical and histological features. Consecutive patients from a CRC registry were included. BAT26 determination was performed in all cases; if instability was found, immunohistochemistry (IHC) and BRAF mutation analyses were done, as appropriate. Differences were assessed by chi-squared or Fisher's exact test, or by T test or Mann-Whitney. Multiple logistic regression was used to identify factors independently associated with BAT26-unstable tumors. We included 155 patients; mean age was 65.6 (SD 14.4) and 56.1% were male. The frequency of BAT26-unstable tumors was 22% (95% CI 15.7-29.3). Factors independently associated with BAT26-unstable tumors were right colon localization (OR 3.4, 95% CI 1.3-8.7), histological MSI features (OR 5.1, 95% CI 1.9-13.6) and Amsterdam criteria (OR 23.2, 95% CI 1.9-286.7). IHC was altered in 85.3% BAT26-unstable tumors and 70.6% lacked MLH1 expression; 47.8% of these harbored BRAF V600E mutation. We provide evidence to link the frequency of BAT26 to an increased diagnostic yield (up to 1.4-folds) of suspected LS cases in comparison to the revised Bethesda guidelines alone. In regions with limited resources, clinical and histological features associated with BAT26-unstable status could be useful to direct MSI screening in sporadic CRCs and may help guide clinical care and future research.

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