Page 50 - Personalised medicine of fluoropyrimidines using DPYD pharmacogenetics Carin Lunenburg

Chapter 3
for DPYD*2A and DPYD*13, but deviate in the dose advice for c.2846A>T and include a dose advice for c.1236G>A/HapB3. We have summarized in vitro, ex vivo and in vivo studies to determine the appropriate dose recommendation for these SNPs. In addition, we have shown our own experimental data. Our data are in agreement with previous data and show a 50% reduced DPD enzyme activity in patients heterozygous for DPYD*2A and DPYD*13 and an ∼25% decreased activity for heterozygous patients with c.2846A>T. Unfortunately, our data on c.1236G>A do not correspond and additional data containing DPD enzyme activity measurements in patients with c.1236G>A/HapB3 are scarce and not in agreement. Including our study, three out of four studies suggest that c.1236G>A results in an enzyme activity close to normal levels. However, Sistonen et al. showed a significant reduction in DHU/U ratio in patients carrying this variant50 and associations with the development of severe toxic side effects have also been described. The toxicity data point out that a dose reduction for c.1236G>A/HapB3 is required, but a dose reduction of 50% would be too large considering the measured enzyme activities. Therefore a dose reduction to 75% of the normal dose for heterozygous patients seems appropriate in order to prevent toxicity as well as to prevent underdosing. After the initial dose reduction the patient should be closely monitored and the dose can be adjusted according to occurring toxicity.
Currently only four SNPs in DPYD are allocated a gene activity score, since we consider these variants are the most relevant polymorphisms. It has been described before that 13 to 19 variants are expected to result in DPD deficiency61,62. However, more research is necessary on the effect of these other SNPs on DPD enzyme activity before they can be included in the gene activity score. With the gene activity score approach it is possible to continuously keep adding variant alleles or updating the values of the gene activity score that are assigned to variant alleles. When new information on effects on enzyme activity is published, this can be included, while the currently proposed gene activity score can already be used in clinical practice. In addition, more research is needed with regard to compound heterozygous patients (patients who carry two different SNPs) and homozygous patients. These patients would benefit from an additional phenotyping test to measure the DPD enzyme activity as to determine the optimal dose adjustment or decide to treat with an alternative drug.
Both genotyping and phenotypic biomarkers have been proposed in order to predict and reduce toxicity in patients. However, the gold standard of phenotyping (measuring DPD enzyme activity in PBMCs) is not easy to implement as a routine test and other phenotyping methods, such as uracil test dose, endogenous DHU/U ratio and 2-13C-uracil breath test, have not yet been fully validated or standardized.63 Compared with phenotyping methods, genotyping methods are faster, easier and less expensive, so it is expected that it will be implemented more often as standard of care for patients undergoing fluoropyrimidine treatment.
The dose recommendations described in this article will be implemented in an upcoming large prospective clinical trial (NCT02324452) in the Netherlands where upfront genotypic assessment of DPYD will be performed for around 1250 patients treated with capecitabine or 5-FU. Simultaneously, our work was recently implemented by the Dutch Pharmacogenetics Working Group by using the gene activity score for translating DPYD genotype into DPD phenotype.21
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