Fluoropyrimidines (5-Fluorouracil, Capecitabine and Tegafur), widely used in treatment of solid tumors, produce side effects ranging from very mild to very serious (including lethal events) and preemptive prediction of severe toxicity is a main goal for oncology. 5-Fluorouracil (5FU), the active form of fluoropyrimidines, is mainly metabolized in the inactive metabolite 5,6-dihydro-5-fluorouracil by the dihydropyrimidine dehydrogenase enzyme (DPD, encoded by the DPYD gene). The remaining drug is catabolized by various enzymes in pharmacologically active metabolites mediating cytotoxicity. Since 5FU toxicity is mainly mediated by impaired DPD activity (causing increased level of bioactive metabolites), biochemical DPD activity assays or pharmacogenetic tests screening for impairing polymorphisms are used to characterized patients before drug administration. We previously described a pre-treatment ex-vivo assay (5FUDR, e.g. 5FU Degradation Rate) to determine the rate at which the peripheral blood mononuclear cells metabolize 5FU. The 5FUDR allows stratification of patients in poor-, normal- and ultrarapid-metabolizers (PM, NM, UM). Interestingly, our data showed that ultrarapid metabolism was significantly associated with development of severe toxicity but also with improved clinical outcome. We hypothesized that the ultrarapid metabolism can be determined by either an increased rate of 5FU biotransformation in cytotoxic metabolites or by 5FU inactivation by increased rate of DPD activity. The latter mechanism could explain the fraction of 5FU UM subjects who do not develop severe toxicity. In this study, we start testing this hypothesis by sequencing the DPYD gene, including putative regulative regions, in a cohort of UM subjects, to identify polymorphisms or mutations possible involved in a basally increased DPD activity. We will take advantage by the availability of a large DNA collection from poor, normal and ultrarapid 5FU metabolizers from previous studies.
The proposed project intends to explore a scarcely investigated class of subjects which are supposed to have consistently increased DPD activity. These subjects may be at high risk of inefficacy when treated with standard doses of fluoropyrimidines, and pre-emptive identification could improve clinical outcomes allowing early dose/therapy adjustment.
The identification of genetic factors associated with increased DPD activity could have different impacts on research and diagnostics: i) novel genetic markers of the inter-individual variability in 5FU response could be identified; ii) among these, non-rare genetic markers (polymorphisms) could be investigated for their association with fluoropyrimidine response and considerate as novel pharmacogenetic markers; iii) the identification of rare genetic markers of increased DPD activity could enhance the development of guidelines considering the opportunity to make a more diffuse use for biochemical assays of DPD activity.