Treatment optimization of fluoropyrimidine-based chemotherapy
Authors:   , Knikman Jonathan Emanuel 
Published: January 2024

This thesis is focused on a group of anticancer agents known as fluoropyrimidines, vital in the treatment of several solid tumors. Although the extensive experience with fluoropyrimidines severe toxicity remains a major clinical problem. Only recently has attention turned to the influence of inter-individual variability in activity of the DPD enzyme – encoded by the DPYD gene – on the safety of fluoropyrimidines. Pre-therapeutic screening of the DPYD gene for relevant single nucleotide polymorphisms (DPYD*2A, c.1236G>A, c.2846A>T, and c.1679T>G) and subsequent dose-individualizations have shown to significantly reduced severe toxicity. However, despite the reproducible link between the four DPYD variants and toxicity, ~23% of patients who do not carry any of these variants still experience severe toxicity.

The first part of this thesis focuses on the dose-individualization strategies enhance fluoropyrimidine safety. We have sought to identify potential biomarkers inside of severe toxicity in- and outside of the DPYD gene to explain the remaining toxicity. We have shown that it is unlikely that at a population level, testing for single markers in addition to the four established DPYD variants, currently has limited value in improving fluoropyrimidine toxicity prediction. Measurement of uracil as a DPD phenotyping method as an alternative to DPYD-genotyping was studied, revealed that uracil is highly susceptible to pre-analytical factors. In a large prospective clinical trial, we demonstrated that uracil-based dose-individualization reduces severe fluoropyrimidine-related toxicity. However, this approach led to inadequate exposure to the primary metabolite 5-fluorouracil. Therefore, this strategy is currently not recommended for dose-individualization of fluoropyrimidines. Additionally, we developed a predictive model based on patient-related and treatment-related factors aimed at estimating the risk of developing severe capecitabine-related toxicity. This model includes readily available parameters and demonstrates a good discriminative ability of prediction of severe toxicity and may be a helpful tool for clinicians to assess the risk of developing severe toxicity.

In the second part we explored the clinical outcomes of DPYD variant carriers treated with a reduced dose. Our findings indicate that DPYD-guided dosing does not negatively impact PFS and OS in DPYD variant carriers. However, in the individual group of c.1236G>A carriers a shorter PFS was found when treated with a 25% reduced dose. Therefore, close monitoring with early dose-modifications based on toxicity is recommended, especially for c.1236G>A carriers receiving a reduced starting dose. Furthermore, predictors of severe toxicity in elderly treated with fluoropyrimidines were studied and showed that polychemotherapy, reduced starting dose of polychemotherapy and low BMI trended towards increased risk of severe toxicity. In addition, multiple deficits across multiple geriatric domains and combination chemotherapy were predictors of poor treatment tolerability.

In the third part, the focus is on bioanalysis supporting fluoropyrimidine-based chemotherapy. We developed a bioanalytical assay for the quantification of capecitabine and its metabolites in a single assay. Additionally, we studied the stability of uracil and dihydrouracil. This study revealed that uracil is highly instable at room temperature and should be processed within 1 hour after blood sampling, when stored at room temperature, to ensure stable and reliable results.