DPYD Genotyping in Fluoropyrimidine Therapy: Reducing Toxicity, Improving Outcome

Fluoropyrimidines such as 5-fluorouracil (5-FU), capecitabine, and tegafur are widely used in oncology, treating over 3.5 million patients annually—with 600,000 in Europe alone. Despite their effectiveness, these drugs carry a significant risk of toxicity: 10–40% of patients develop severe side effects (Grade 3–4) requiring hospitalisation and occasionally intensive care admission. From 0.2% to 0.8% die from treatment-related complications.

Up to 50% of these toxicities are linked to deficiency of dihydropyrimidine dehydrogenase (DPD), the key enzyme responsible for 80% of fluoropyrimidine metabolism. When DPD activity is reduced, drug accumulation can lead to life-threatening adverse effects. The primary cause of DPD deficiency is inherited variants in the DPYD gene. Four well-characterized variants—DPYD*2A, c.2846A>T, DPYD*13, and the HapB3 haplotype—, over the 1600 identified, are widely recognized for their established associations with fluoropyrimidine-induced toxicity. Individuals carrying these variants are classified as intermediate or poor metabolizers, depending on the number and type of mutations.

Two large prospective trials—PREPARE and PACIFIC-PGx—have shown that pre-treatment DPYD genotyping followed by dose adjustment reduces both the frequency and severity of toxicities without compromising treatment efficacy.

Two large prospective multicentre studies have shown that pre-emptive DPYD genotyping identifies patients at risk of severe fluoropyrimidine toxicity, and that dose adjustment reduces both the frequency and severity of adverse effects without compromising efficacy. [6-10] These results were recently confirmed by the prospective, multi-site PACIFIC-PGx clinical trial and by the single-country secondary analysis stemming from the international, prospective, multi-centre PREPARE clinical trial.

Based upon the expanded evidence from published literature and clinical trials supporting the association between some DPYD variants and fluoropyrimidine toxicity, leading pharmacogenetics experts have developed a series of guidelines and recommendations.

In 2013, Clinical Pharmacogenetics Implementation Consortium (CPIC) issued its first guideline regarding fluoropyrimidines treatment and DPYD testing. These recommendations were later updated four times, between 2014 and 2024.

Similarly, the Dutch Pharmacogenetics Working Group (DPWG) recommended pre-emptive DPYD genotyping with genotype-guided dosing strategies or avoidance of fluoropyrimidines in patients carrying certain high-risk DPYD variants.

In 2024, the Pharmacogenomics Working Group of the Clinical Practice Committee, Association for Molecular Pathology (AMP) has provided a guideline that recommends a minimum set and an extended list of DPYD variant alleles to be analysed.

Since April 2020 The European Medicines Agency (EMA) has recommended, that all patients starting fluoropyrimidines undergo testing for DPD deficiency. Patients with complete deficiency should avoid these drugs entirely, while intermediate metabolizers should start at a reduced dose.

In the United States, on March 2024 the FDA updated its labels for 5-FU and capecitabine, advising clinicians to “consider DPYD testing prior to treatment” and to “inform patients of the potential for severe reactions due to DPD deficiency”.

Most recently, the NCCN guidelines (March 2025) for colorectal and related cancers recommend discussing DPYD genotyping with all patients before starting fluoropyrimidine-based therapies.

A guidance for DPYD and fluoropyrimidines was provided also by regulatory agencies of United States, Canada European Union, UK, Switzerland and Japan.

In conclusion, DPYD genotyping is a clinically validated, cost-effective tool that reduces preventable toxicity, improves patient safety, and optimizes cancer therapy. With broad support from regulators, clinicians, and health economists alike, its implementation represents a concrete step toward safer, more precise oncology.

References

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Irinotecan and UGT1A1: How Genetic Testing Can Improve Chemotherapy Safety and Efficacy

Since its clinical introduction in 1998, irinotecan has become a cornerstone in the treatment of solid tumors. Each year, approximately 33,000 patients with colorectal carcinoma in the United States receive this drug.

Despite its therapeutic benefits, irinotecan is associated with significant toxicities. Nearly 40% of patients experience severe delayed diarrhea and up to 50% develop grade ?3 neutropenia, often requiring hospitalization. These adverse effects are primarily due to the accumulation of SN-38, the active metabolite of irinotecan, which inhibits topoisomerase-I, an enzyme critical for DNA replication. Under normal circumstances, SN-38 is inactivated through glucuronidation by the enzyme UGT1A1, allowing its elimination via bile.

However, genetic variations in the UGT1A1 gene can impair this detoxification process, leading to higher systemic exposure to SN-38 and increasing the risk of toxicity. The most significant polymorphisms are UGT1A1*28 and UGT1A1*6. The *28 allele, more common in Caucasians and Africans, results in reduced gene expression due to an extra TA repeat in the promoter region. Meanwhile, *6, more prevalent in Asian populations, corresponds to the nucleotide substitution c.211G>A, that also reduces enzyme activity.

Individuals carrying homozygous variants of either allele (*28/*28 or *6/*6) are classified as poor metabolizers (PM) and are at significantly higher risk for irinotecan-related toxicities whereas Heterozygous carriers of UGT1A1*28 or UGT1A1*6 are considered intermediate metabolizers (IM). Genetic testing for UGT1A1 polymorphisms has become a crucial tool in personalized medicine to reduce adverse effects and optimize dosing.

Numerous studies and meta-analyses have confirmed that genotype-guided dosing of irinotecan can help prevent severe toxicities without compromising treatment efficacy. For example, clinical simulations have demonstrated that reducing the irinotecan dose by 25% in patients with the *28/*28 genotype can reduce the incidence of severe neutropenia from 45% to 18% and severe diarrhea from 19% to 9%. A prospective multicenter study further validated these findings, showing that 30% dose reduction in poor metabolizers decreased the incidence of febrile neutropenia from 24% to 6.5% while maintaining the drug’s therapeutic effectiveness.

In addition to clinical safety, UGT1A1 testing also offers substantial economic benefits. A study conducted within the PREPARE trial (with 563 patients) revealed that genetic-guided dosing led to lower toxicity rates and reduce costs due to fewer hospitalizations and interventions.

While genetic testing for UGT1A1 polymorphisms is increasingly recommended in clinical guidelines, there are still variations in the specific recommendations across different agencies. The Dutch Pharmacogenetics Working Group (DPWG), for example, recommends pre-treatment UGT1A1 genotyping for all patients who will receive irinotecan. In cases where patients are identified as poor metabolizers, they should start with 70% of the standard irinotecan dose, followed by careful titration. Similarly, the French National Network of Pharmacogenetics (RNPGx) supports pre-treatment testing and suggests a 25–30% dose reduction in homozygous *28/*28 patients, particularly when irinotecan doses exceed 180 mg/m².

Both the FDA and EMA recognize the increased risk of toxicity in patients with the *28/*28 genotype but only the irinotecan product labels provide specific guidance on lowering the initial dose in these patients.

The integration of UGT1A1 genetic testing in chemotherapy protocols for irinotecan is an essential advancement in personalized cancer treatment. By identifying patients at higher risk of severe toxicity, clinicians can adjust dosing strategies to optimize treatment outcomes while minimizing adverse events.

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