Fluoropyrimidine Toxicity: the Hidden Secrets of DPYD

Fluoropyrimidine-induced toxicity is a main limitation of therapy. Currently, polymorphisms in the DPYD gene, which encodes the 5-FU activation enzyme dihydropyrimidine dehydrogenase (DPD), are used to adjust the dosage and prevent toxicity. Despite the predictive value of DPYD genotyping, a great proportion of fluoropyrimidine toxicity cannot be solely explained by DPYD variations.

We herein summarize additional sources of DPD enzyme activity variability, spanning from epigenetic regulation of DPYD expression, factors potentially inducing protein modifications, as well as drug-enzyme interactions that contribute to fluoropyrimidine toxicity.

While seminal in vitro studies provided evidence that DPYD promoter methylation downregulates DPD expression, the association of DPYD methylation with fluoropyrimidine toxicity was not replicated in clinical studies. Different non-coding RNA molecules, such as microRNA, piwi-RNAs, circular-RNAs and long non-coding RNAs, are involved in post-transcriptional DPYD regulation. DPD protein modifications and environmental factors affecting enzyme activity may also add a proportion to the pooled variability of DPD enzyme activity. Lastly, DPD-drug interactions are common in therapeutics, with the most well-characterized paradigm the withdrawal of sorivudine due to fluoropyrimidine toxicity deaths in 5-FU treated cancer patients; a mechanism involving DPD severe inhibition.

DPYD polymorphisms are the main source of DPD variability. A study on DPYD epigenetics (both transcriptionally and post-transcriptionally) holds promise to provide insights into molecular pathways of fluoropyrimidine toxicity. Additional post-translational DPD modifications, as well as DPD inhibition by other drugs, may explain a proportion of enzyme activity variability. Therefore, there is still a lot we can learn about the DPYD/DPD fluoropyrimidine-induced toxicity machinery.