Fluorouracil Toxicity and DPYD

5-Fluorouracil (5FU) is a fluorinated pyrimidine analogue commonly used in combination chemotherapy regimens for patients with breast, colorectal, lung, and other malignancies. Dihydropyrimidine dehydrogenase (DPD), an enzyme encoded by the DPYD gene, catalyzes the first catabolic step of the 5FU degradation pathway, converting 80% of 5FU to its inactive metabolite. ^[1]  

5-Fluorouracil is frequently used for treatment of solid cancers. A deficiency in the enzyme that catabolizes 5FU (DPD) leads to severe toxicity. The gene responsible for this enzyme, DPYD, is located on chromosome 1q22. The most prevalent alteration described is DPYD*2A, which leads to a splicing defect and thus skipping of translation of an entire exon. A retrospective, single-center study found that despite previous reports, the most prevalent variation in patients with severe adverse events was DPYD*9A. As this variant was previously reported to be benign, study authors suggest that screening for DPD deficiency should be extended across multiple exons of the DPYD gene. ^[2]

True deficiency of DPD affects approximately 5% of the overall population. In these patients, lack of enzymatic activity increases the half-life of the drug, resulting in excessive drug accumulation and toxicity. ^[3] In addition, 3-5% of the population has a partial DPD deficiency due to sequence variations in the DPYD gene, which potentially limits their ability to fully metabolize the drug, thereby resulting in toxicity. ^{[4, 5, 6, 7]}  Approximately 0.3% of the population demonstrates complete DPD deficiency, translating to extreme toxicity of 5FU. ^[1]

Chemotherapy toxicity causes a significant clinical problem due to decreased quality of life, prolongation of treatment, and reinforcement of negative emotions associated with therapy. ^[8]  Individuals with DPD deficiency are typically asymptomatic until exposed to 5FU or capecitabine (which forms 5FU) for treatment of gastrointestinal or breast cancer. These individuals are then at considerably increased risk of severe to life-threatening adverse events. ^[9]

Four well-established risk variants within the DPYD gene are known to encode DPD. Although consensus guidelines for genotype-guided dosing of 5FU and capecitabine have existed for several years, this type of personalized medicine has not been widely implemented. ^[9]  5-Fluorouracil is one of the most commonly used cytostatic drugs for systemic treatment of cancer. Treatment with 5FU may cause severe or life-threatening side effects; the treatment-related mortality rate is 0.2-1.0%. ^[10]

The IVS14+1G>A mutation in intron 14 coupled with the exon 14 deletion (known as DPYD*2A) is the best known variant resulting in partial DPD deficiency and 5FU toxicity. ^[3] Other recognized variants associated with toxicity include 496A>G in exon 6; 2846A>T in exon 22 ^{[11, 12]} ; and T1679G (DPYD*13) in exon 13, ^[13] although multiple other mutations have been detected in individual families and via full gene sequences.

Patients with DPD deficiency who are treated with 5FU or capecitabine are at significantly increased risk of developing severe (grade III/IV) and potentially fatal neutropenia, mucositis, and diarrhea. ^{[4, 11, 12, 14, 15]} As noted in their respective product labels, both 5FU and capecitabine are therefore contraindicated in patients with known DPD deficiency.

By contrast, the clinical effects of DPYD variants and partial DPD deficiency are unclear. Different series have demonstrated increased toxicity to varying degrees, ^{[11, 12]} but mutations in DPYD have, for the most part, been unable to account for the magnitude of toxicity seen in the general population. Some groups have begun to evaluate the contributions of mutations in other candidate genes, ^[12] but effects of these and other genetic and nongenetic factors will remain unknown until all pathways involved in 5FU/capecitabine metabolism have been clearly elucidated. ^[16]

The German Society for Hematology and Medical Oncology in cooperation with 13 medical associations from Austria, Germany, and Switzerland has organized a consensus statement providing the following recommendations for genetic testing and tailoring of treatment with 5FU derivatives. ^[10]

• Patients should be tested for the 4 most common genetic DPYD variants before receiving treatment with drugs containing FU.
• Testing forms the basis for a differentiated, risk-adapted algorithm with recommendations for treatment with FU-containing drugs.
• Testing may be supplemented by therapeutic drug monitoring.

The US Food and Drug Administration (FDA) approved uridine triacetate (Vistogard), a pyrimidine analog, for emergency treatment following a fluorouracil or capecitabine overdose in patients with early-onset, severe or life-threatening toxicity affecting the heart or the central nervous system, as well as after early-onset, unusually severe adverse reactions (GI toxicity and/or neutropenia) within 96 hours after fluorouracil or capecitabine administration. Following oral administration, uridine triacetate is deacetylated by nonspecific esterases, yielding uridine in the circulation, which competitively inhibits cell damage and cell death caused by fluorouracil. Among participants in 2 trials who were treated with uridine triacetate for overdose, 97% were still alive at 30 days. Of those treated with uridine triacetate for early-onset, severe or life-threatening toxicity, 89% were alive at 30 days. ^[17]

Enzymatic activity in patients with suspected DPD deficiency can be determined via RNA extracted from peripheral blood mononuclear cells and by measurement of DPD mRNA copy number. High-throughput genetic analysis using denaturing high-performance liquid chromatography (DHPLC) can be performed if the patient is severely neutropenic. ^[18]

Testing for DPD deficiency and the IVS14+1G>A DPYD variant (DPYD*2A) is available; researchers are exploring ways of testing for other variants.

In a study conducted to identify candidate risk variants for severe toxicity to 5FU chemotherapy, Hamzic and colleagues found that DPYD-c.496A>G is a strong candidate risk allele for 5FU toxicity. Study data suggest that DPYD-c.85T>C might be protective; however, the deleterious impact of the linked alleles c.496A>G and c.1129-5923C>G likely limits this effect in patients. Study authors concluded that the possible protective effects of c.85T>C and linkage disequilibrium with c.496A>G and c.1129-5923C>G may have hampered prior association studies and should be considered in future clinical studies. ^[19]

In a study including patients with breast cancer, Tecza and associates found the strongest associations between concurrent presence of clinical factors—overall and recurrent anemia, nephrotoxicity, and early nausea—and genetic polymorphisms in genes responsible for DNA repair, drug metabolism, and transport pathways. These findings suggest the possibility of selecting patients with expected high tolerance to fluorouracil, doxorubicin, and cyclophosphamide (FAC) treatment and consequently with a high chance of chemotherapy completion without dose reduction, treatment delays, and declining quality of life. ^[8]

Recommendations by agencies and professional societies, both in Europe and in the United States, highlight the need to begin an informed discussion about whether now is an appropriate time to advocate for routine access to testing for this enzyme deficiency in cancer patients. ^[9]

Botticelli and coworkers have structured a nomogram to derive a score that predicts the likelihood of severe toxicity based on metabolic parameters and patient characteristics. Compared with available pharmacogenetic tests, they suggest that this approach can be applied to the whole population, predicting risk for severe toxicity through an easy, low-cost, noninvasive technique. ^[20]

Adverse drug reactions can be unpredictable. However, pharmacogenomic testing can help identify patients who may be more susceptible to the toxic effects of certain drugs. Genetic variations in DPD and in thymidylate synthase genes have been shown to increase the risk of 5FU toxicity. 5-Fluorouracil toxicity can be life threatening. Fortunately, uridine triacetate is available for treatment of 5FU toxicity. Indications for its use limit administration to within 96 hours of receipt of 5FU: this has important implications for pharmacogenomic testing. ^[21]

Preemptive testing of DPD enzyme activity or of genetic variation in its gene, DPYD, is recommended and has been calculated to be cost saving and thus beneficial from a healthcare economy perspective. ^[22]