Important Role of the Dihydrouracil/Uracil Ratio in Marked Interpatient Variations of Fluoropyrimidine Pharmacokinetics and Pharmacodynamics

Integrating rare genetic variants into DPYD pharmacogenetic testing may help preventing fluoropyrimidine-induced toxicity

Variability in genes involved in drug pharmacokinetics or drug response can be responsible for suboptimal treatment efficacy or predispose to adverse drug reactions. In addition to common genetic variations, large-scale sequencing studies have uncovered multiple rare genetic variants predicted to cause functional alterations in genes encoding proteins implicated in drug metabolism, transport and response. To understand the functional importance of rare genetic variants in DPYD, a pharmacogene whose alterations can cause severe toxicity in patients exposed to fluoropyrimidine-based regimens, massively parallel sequencing of the exonic regions and flanking splice junctions of the DPYD gene was performed in a series of nearly 3000 patients categorized according to pre-emptive DPD enzyme activity using the dihydrouracil/uracil ([UH2]/[U]) plasma ratio as a surrogate marker of DPD activity. Our results underscore the importance of integrating next-generation sequencing-based pharmacogenomic interpretation into clinical decision making to minimize fluoropyrimidine-based chemotherapy toxicity without altering treatment efficacy.

Identification of DPYD variants and estimation of uracil and dihydrouracil in a healthy Indian population

Aim: This study aimed to identify DPYD variants and the related but previously unexplored phenotype (plasma uracil, dihydrouracil [DHU], and the DHU-to-uracil ratio) in a healthy adult Indian population. Methods: Healthy adult volunteers (n = 100) had their uracil and DHU levels measured and were genotyped for selected variants. Results: Among the nine variants studied, c.1906-14763G>A and c.85T>C were the most prevalent. Participants with any of the variants except for c.85T>C and c.1627A>G had a significantly lower DHU-to-uracil ratio and those with c.1905+1G>A variant had significantly increased uracil concentration compared with wild type. Conclusion: Participants with five variants were identified as having altered phenotypic measures, and 40% of the intermediate metabolizers had their phenotype in the terminal population percentiles.

Individualized Dosing of Fluoropyrimidine-Based Chemotherapy to Prevent Severe Fluoropyrimidine-Related Toxicity: What Are the Options?

Fluoropyrimidines are widely used in the treatment of several types of solid tumors. Although most often well tolerated, severe toxicity is encountered in ~ 20-30% of the patients. Individualized dosing for these patients can reduce the incidence of severe fluoropyrimidine-related toxicity. However, no consensus has been achieved on which dosing strategy is preferred. The most established strategy for individualized dosing of fluoropyrimidines is upfront genotyping of the DPYD gene. Prospective research has shown that DPYD-guided dose-individualization significantly reduces the incidence of severe toxicity and can be easily applied in routine daily practice. Furthermore, the measurement of the dihydropyrimidine dehydrogenase (DPD) enzyme activity has shown to accurately detect patients with a DPD deficiency. Yet, because this assay is time-consuming and expensive, it is not widely implemented in routine clinical care. Other methods include the measurement of pretreatment endogenous serum uracil concentrations, the uracil/dihydrouracil-ratio, and the 5-fluorouracil (5-FU) degradation rate. These methods have shown mixed results. Next to these methods to detect DPD deficiency, pharmacokinetically guided follow-up of 5-FU could potentially be used as an addition to dosing strategies to further improve the safety of fluoropyrimidines. Furthermore, baseline characteristics, such as sex, age, body composition, and renal function have shown to have a relationship with the development of severe toxicity. Therefore, these baseline characteristics should be considered as a dose-individualization strategy. We present an overview of the current dose-individualization strategies and provide perspectives for a future multiparametric approach.

Food-effect study on uracil and dihydrouracil plasma levels as marker for dihydropyrimidine dehydrogenase activity in human volunteers

AIMS

This study aimed to determine the effect of food intake on uracil and dihydrouracil plasma levels. These levels are a promising marker for dihydropyrimidine dehydrogenase activity and for individualizing fluoropyrimidine anticancer therapy.

METHODS

A randomized, cross-over study in 16 healthy volunteers was performed, in which subjects were examined in fasted and fed state on two separate days. In fed condition, a high-fat, high-caloric breakfast was consumed between 8:00 h and 8:30 h. Whole blood for determination of uracil, dihydrouracil and uridine plasma levels was drawn on both test days at predefined time points between 8:00 h and 13:00 h.

RESULTS

Uracil levels were statistically significantly different between fasting and fed state. At 13:00 h, the mean uracil level in fasting state was 12.6 ± 3.7 ng ml-1 and after a test meal 9.4 ± 2.6 ng ml-1 (P < 0.001). Dihydrouracil levels were influenced by food intake as well (mean dihydrouracil level at 13:00 h in fasting state 147.0 ± 36.4 ng ml-1 and in fed state 85.7 ± 22.1 ng ml-1 , P < 0.001). Uridine plasma levels showed curves with similar patterns as for uracil.

CONCLUSIONS

It was shown that both uracil and dihydrouracil levels were higher in fasting state than in fed state. This is hypothesized to be an direct effect of uridine plasma levels, which were previously shown to be elevated in fasting state and reduced after intake of food. These findings show that, when assessing plasma uracil and dihydrouracil levels for adaptive fluoropyrimidine dosing in clinical practice, sampling should be done between 8:00 h and 9:00 h after overnight fasting to avoid bias caused by circadian rhythm and food effects.

The impact of liver resection on the dihydrouracil:uracil plasma ratio in patients with colorectal liver metastases

PURPOSE

The dihydrouracil (DHU):uracil (U) plasma ratio is a promising marker for identification of dihydropyrimidine dehydrogenase (DPD)-deficient patients. The objective of this study was to determine the effect of liver resection on the DHU:U plasma ratio in patients with colorectal liver metastases (CRLM).

METHODS

An observational study was performed in which DHU:U plasma ratios in patients with CRLM were analyzed prior to and 1 day after liver resection. In addition, the DHU:U plasma ratio was quantified in six additional patients 4-8 weeks after liver resection to explore long-term effects on the DHU:U plasma ratio. Quantification of U and DHU plasma levels was performed using a validated ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) assay.

RESULTS

The median (range) DHU:U plasma ratio in 15 patients prior to liver resection was 10.7 (2.6-14.4) and was significantly reduced to 5.5 (< quantification limit (LLOQ-10.5) 1 day after resection (p = 0.0026). This reduction was caused by a decrease in DHU plasma levels from 112.0 (79.8-153) ng/mL to 41.2 (< LLOQ-160) ng/mL 1 day after resection (p = 0.0004). Recovery of the DHU:U plasma ratio occurred 4-8 weeks after liver resection, which was shown by a median (range) DHU:U plasma ratio in six patients of 9.1 (6.9-14.5).

CONCLUSION

Liver resection leads to very low DHU:U plasma ratios 1 day after liver resection, which is possibly caused by a reduction in DPD activity. Quantification of the DHU:U plasma ratios directly after liver resection could lead to false-positive identification of DPD deficiency and is therefore not advised.

A new sample preparation and separation combination for the precise, accurate, and simultaneous determination of uracil and dihydrouracil in human plasma by reversed-phase HPLC

We have developed an efficient procedure and detection method using reversed-phase high-performance liquid chromatography for the simultaneous measurement of uracil and dihydrouracil in human plasma. The procedure, including chromatographic conditions and sample preparation, was optimized and validated. Optimization of the sample preparation included deproteinization, extraction, and cleanup. A new sample preparation method which resulted in an improved extraction yield of analytes and significantly reduced interference at low-wavelength UV detection was developed. The developed method was validated for specificity, linearity, limits of detection and quantitation, precision, and accuracy. All calibration curves showed excellent linear regression (R²  > 0.9990) within the testing range. The limit of detection for uracil and dihydrouracil was 2.5 and 5.0 ng/mL, respectively. The extraction yields were >94% for uracil and 91% for dihydrouracil. Intra- and interassay precision and accuracy for uracil and dihydrouracil were lower than 8% at all tested concentrations. The proposed method was successfully applied to measure plasma concentrations of uracil and dihydrouracil in colorectal cancer patients scheduled to receive fluoropyrimidine-based chemotherapy.

Pretreatment serum uracil concentration as a predictor of severe and fatal fluoropyrimidine-associated toxicity. Br J Cancer. 2017;116:1415-1424. [PMID: 28427087 PMCID: PMC5520099 DOI: 10.1038/bjc.2017.94]

Background:

We investigated the predictive value of dihydropyrimidine dehydrogenase (DPD) phenotype, measured as pretreatment serum uracil and dihydrouracil concentrations, for severe as well as fatal fluoropyrimidine-associated toxicity in 550 patients treated previously with fluoropyrimidines during a prospective multicenter study.

Methods:

Pretreatment serum concentrations of uracil and dihydrouracil were measured using a validated LC-MS/MS method. The primary endpoint of this analysis was global (any) severe fluoropyrimidine-associated toxicity, that is, grade ⩾3 toxicity according to the NCI CTC-AE v3.0, occurring during the first cycle of treatment. The predictive value of uracil and the uracil/dihydrouracil ratio for early severe fluoropyrimidine-associated toxicity were compared. Pharmacogenetic variants in DPYD (c.2846A>T, c.1679T>G, c.1129-5923C>G, and c.1601G>A) and TYMS (TYMS 5′-UTR VNTR and TYMS 3′-UTR 6-bp ins/del) were measured and tested for associations with severe fluoropyrimidine-associated toxicity to compare predictive value with DPD phenotype. The Benjamini-Hochberg false discovery rate method was used to control for type I errors at level q<0.050 (corresponding to P<0.010).

Results:

Uracil was superior to the dihydrouracil/uracil ratio as a predictor of severe toxicity. High pretreatment uracil concentrations (>16 ng ml⁻¹) were strongly associated with global severe toxicity (OR 5.3, P=0.009), severe gastrointestinal toxicity (OR 33.7, P<0.0001), toxicity-related hospitalisation (OR 16.9, P<0.0001), as well as fatal treatment-related toxicity (OR 44.8, P=0.001). None of the DPYD variants alone, or TYMS variants alone, were associated with severe toxicity.

Conclusions:

High pretreatment uracil concentration was strongly predictive of severe, including fatal, fluoropyrimidine-associated toxicity, and is a highly promising phenotypic marker to identify patients at risk of severe fluoropyrimidine-associated toxicity.

Improving safety of fluoropyrimidine chemotherapy by individualizing treatment based on dihydropyrimidine dehydrogenase activity - Ready for clinical practice?

Fluoropyrimidines remain the cornerstone of treatment for different types of cancer, and are used by an estimated two million patients annually. The toxicity associated with fluoropyrimidine therapy is substantial, however, and affects around 30% of the patients, with 0.5-1% suffering fatal toxicity. Activity of the main 5-fluorouracil (5-FU) metabolic enzyme, dihydropyrimidine dehydrogenase (DPD), is the key determinant of 5-FU pharmacology, and accounts for around 80% of 5-FU catabolism. There is a consistent relationship between DPD activity and 5-FU exposure on the one hand, and risk of severe and potentially lethal fluoropyrimidine-associated toxicity on the other hand. Therefore, there is a sound rationale for individualizing treatment with fluoropyrimidines based on DPD status in order to improve patient safety. The field of individualized treatment with fluoropyrimidines is now rapidly developing. The main strategies that are available, are based on genotyping of the gene encoding DPD (DPYD) and measuring of pretreatment DPD phenotype. Clinical validity of additional approaches, including genotyping of MIR27A has also recently been demonstrated. Here, we critically review the evidence on clinical validity and utility of strategies available to clinicians to identify patients at risk of developing severe and potentially fatal toxicity as a result of DPD deficiency. We evaluate the advantages and limitations of these methods when used in clinical practice, and discuss for which strategies clinical implementation is currently justified based on the available evidence and, in addition, which additional data will be required before implementing other, as yet less developed strategies.

Development and validation of a rapid and sensitive UPLC-MS/MS method for determination of uracil and dihydrouracil in human plasma

Quantification of the endogenous dihydropyrimidine dehydrogenase (DPD) substrate uracil (U) and the reaction product dihydrouracil (UH2) in plasma might be suitable for identification of patients at risk of fluoropyrimidine-induced toxicity as a result of DPD deficiency. In this paper, we describe the development and validation of a rapid and sensitive ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) assay for quantification of U and UH2 in human plasma. Analytes were extracted by protein precipitation, chromatographically separated on an Acquity UPLC(®) HSS T3 column with gradient elution and analyzed with a tandem mass spectrometer equipped with an electrospray ionization source. U was quantified in the negative ion mode and UH2 in the positive ion mode. Stable isotopes for U and UH2 were used as internal standards. Total chromatographic run time was 5min. Validated concentration ranges for U and UH2 were from 1 to 100ng/mL and 10 to 1000ng/mL, respectively. Inter-assay bias and inter-assay precision for U were within ±2.8% and ≤12.4%. For UH2, inter-assay bias and inter-assay precision were within ±2.9% and ≤7.2%. Adequate stability of U and UH2 in dry extract, final extract, stock solution and plasma was demonstrated. Stability of U and UH2 in whole blood was only satisfactory when stored up to 4hours at 2-8°C, but not at ambient temperatures. An accurate, precise and sensitive UPLC-MS/MS assay for quantification of U and UH2 in plasma was developed. This assay is now applied to support clinical studies with fluoropyrimidine drugs.

Translating DPYD genotype into DPD phenotype: using the DPYD gene activity score

The dihydropyrimidine dehydrogenase enzyme (DPD, encoded by the gene DPYD) plays a key role in the metabolism of fluoropyrimidines. DPD deficiency occurs in 4-5% of the population and is associated with severe fluoropyrimidine-related toxicity. Several SNPs in DPYD have been described that lead to absent or reduced enzyme activity, including DPYD*2A, DPYD*13, c.2846A>T and c.1236G>A/haplotype B3. Since these SNPs differ in their effect on DPD enzyme activity, a differentiated dose adaption is recommended. We propose the gene activity score for translating DPYD genotype into phenotype, accounting for differences in functionality of SNPs. This method can be used to standardize individualized fluoropyrimidine dose adjustments, resulting in optimal safety and effectiveness.

Pretherapeutic uracil and dihydrouracil levels of colorectal cancer patients are associated with sex and toxic side effects during adjuvant 5-fluorouracil-based chemotherapy

BACKGROUND

In Nordic countries, the standard treatment of colorectal cancer (CRC) in the adjuvant setting is bolus 5-fluorouracil (5-FU) plus leucovorin alone or in combination with oxaliplatin. 5-FU competes with the natural occurring pyrimidine uracil (Ura) as a substrate for dihydropyrimidine dehydrogenase (DPD; enzyme commission number 1.3.1.2). Low DPD activity is associated with toxicity during treatment. Pretherapeutic detection of DPD deficiency could prevent severe toxicity otherwise limiting drug administration. Assays showing that DPD deficiency impairs breakdown of Ura to dihydrouracil (UH(2)) seem promising for clinical use.

METHODS

Urine was collected from 56 untreated volunteers and 143 patients with CRC before adjuvant treatment. Ura and UH(2) were analyzed using a column-switching high-performance liquid chromatography method that incorporates reversed-phase and cation-exchange columns. Ura, UH(2), and UH(2)/Ura levels were related to toxicity.

RESULTS

Ura and UH(2) in patients were not different from controls. UH(2) was significantly higher in women compared with men. The UH(2)/Ura ratio, however, did not differ according to sex. Low UH(2) and UH(2)/Ura levels were associated with diarrhea in men. Women experiencing thrombocytopenia had significantly higher Ura compared with women with no thrombocytopenia. The UH(2)/Ura ratio correlated negatively with total toxicity score in men (r = -0.39, P = .020).

CONCLUSION

Pretherapeutic Ura and UH(2) levels per se may be related to risk of side effects during adjuvant 5-FU-based treatment, whereas the UH(2)/Ura ratio may not always reveal such a risk. Sex is a strong risk factor for toxicity, showing the importance of evaluating male and female patients separately.

Clinical, laboratory and molecular factors predicting chemotherapy efficacy and toxicity in colorectal cancer

Colorectal cancer (CRC) treatment has evolved significantly over the last ten years with the use of active chemotherapeutic agents including fluoropyrimidines, oxaliplatin and irinotecan plus targeted monoclonal antibodies bevacizumab, cetuximab and panitumumab. The addition of newer chemotherapeutic agents and targeted therapies has improved patient outcomes at the cost of increased toxicity with not all patients benefiting from these treatments. It is necessary for clinicians to more accurately predict clinical outcomes particularly in the predominantly elderly CRC patient population. This review aims to summarise existing data regarding the use of clinical and laboratory variables plus molecular markers in predicting response, survival and toxicity to chemotherapy agents and targeted monoclonal antibodies currently used in the treatment of CRC.

Dihydropyrimidine dehydrogenase polymorphisms and fluoropyrimidine toxicity: ready for routine clinical application within personalized medicine?

Fluoropyrimidines, including 5-fluorouracil (5-FU), are widely used in the treatment of solid tumors and remain the backbone of many combination regimens. Despite their clinical benefit, fluoropyrimidines are associated with gastrointestinal and hematologic toxicities, which often lead to treatment discontinuation. 5-FU undergoes complex metabolism, dihydropyrimidine dehydrogenase (DPD) being the rate-limiting enzyme of inactivation of 5-FU and its prodrugs. Several studies have demonstrated significant associations between severe toxicities by fluoropyrimidines and germline polymorphisms of DPD gene. To date, more than 30 SNPs and deletions have been identified within DPD, the majority of these variants having no functional consequences on enzymatic activity. However, the identification of deficient DPD genotypes may help identify poor-metabolizer patients at risk of developing potentially life-threatening toxicities after standard doses of fluoropyrimidines.

HPLC with UV or mass spectrometric detection for quantifying endogenous uracil and dihydrouracil in human plasma

BACKGROUND

We developed and compared 2 different methods for quantifying uracil (U) and dihydrouracil (UH(2)) in BSA and human plasma. Special attention was paid to the selectivity/specificity and the absence of a matrix effect. The UH(2)/U ratio is intended as a biomarker to identify patients with deficiency in 5-fluorouracil metabolism.

METHODS

We quantified U and UH(2) with 2 liquid chromatography methods after solid-phase extraction, one with UV detection (LC-UV) and the other with mass spectrometric detection (LC-MS). We selected 2 internal standards to prevent the risk of interferences. Separation was achieved with a Waters Atlantis dC18 column (LC-MS) or a Waters SymmetryShield RP18 column connected with an Atlantis dC18 (LC-UV). Mass spectrometric data were acquired in single-ion monitoring mode.

RESULTS

Assay imprecision in BSA solution was <15% (LC-UV) and <12% (LC-MS); in plasma, assay imprecision was <9.5% and <9.0%, respectively. Recoveries were 88.2%-110% (LC-UV) and 94.8%-107% (LC-MS). Extraction efficiencies were >or=89.0%. In BSA, the lower limits of quantification for U and UH(2) were 2.5 microg/L and 6.25 microg/L, respectively, for the LC-UV method and 2.5 microg/L and 3.1 microg/L for LC-MS. The corresponding values in plasma were 11.6 microg/L and 21.5 microg/L, and 4.1 microg/L and 12.1 microg/L.

CONCLUSIONS

To estimate endogenous U and UH(2) concentrations and their ratio, we recommend the use of a drug-free human plasma pool in which baseline U and UH(2) concentrations have previously been measured with the standard-addition method. Our LC-MS method, which has the better test performance and is useful for measuring UH(2)/U ratios in cancer patients, is preferred when this equipment is available.

Concentrations of the DNA methyltransferase inhibitor 5-fluoro-2'-deoxycytidine (FdCyd) and its cytotoxic metabolites in plasma of patients treated with FdCyd and tetrahydrouridine (THU)

PURPOSE

Although the DNA methyltransferase inhibitor 5-fluoro-2'-deoxycytidine (FdCyd), is being evaluated clinically, it must be combined with the cytidine deaminase inhibitor tetrahydrouridine (THU) to prevent rapid metabolism of FdCyd to the pharmacologically active, yet unwanted, metabolites 5-fluoro-2'-deoxyuridine (FdUrd), 5-fluorouracil (FU), and 5-fluorouridine (FUrd). We assessed plasma concentrations of FdCyd and metabolites in patients receiving FdCyd and THU.

METHODS

We validated an LC-MS/MS assay, developed for a preclinical study, to quantitate FdCyd and metabolites in human plasma. Patients were treated with five daily, 3-h infusions of FdCyd at doses of 5-80 mg/m(2) with 350 mg/m(2) THU. Plasma was obtained during, and before the end of infusions on days 1 and 5.

RESULTS

The lower limits of quantitation for FU, FdUrd, FUrd, FC and FdCyd were 1, 1.5, 10, 3, and 10 ng/ml, respectively. Plasma FdCyd increased with dose, from 19-96 ng/ml at 5 mg/m(2) to 1,600-1,728 ng/ml at 80 mg/m(2). FdUrd was undetectable in patients treated with FdCyd doses <20 mg/m(2), and increased from 2.3 ng/ml at 20 mg/m(2) to 3.5-5.7 ng/ml at 80 mg/m(2). FU increased from 1.2-5.5 ng/ml at 5 mg/m(2) to 6.0-12 ng/ml at 80 mg/m(2).

CONCLUSIONS

By co-administering FdCyd with THU, FdCyd plasma concentrations were achieved that are known to inhibit DNA methylation in vitro. The accompanying plasma FU and FdUrd concentrations are <10% those observed after therapeutic infusions of FU or FdUrd, while FdCyd levels are well above those required to inhibit methylation in vitro. Therefore, inhibition of DNA methylation with FdCyd and THU appears feasible.

Should DPD analysis be required prior to prescribing fluoropyrimidines?

Dihydropyrimidine dehydrogenase (DPD) is a key enzyme in the metabolic catabolism of chemotherapeutic agent 5-fluorouracil (5FU) and its derivatives, including capecitabine. Numerous genetic mutations have been identified in the DPD gene locus (DPYD), with a few key variants having functional consequences on enzymatic activity. Deficiencies in DPD activity have been shown to cause 5FU-treated cancer patients to experience severe drug-related toxicities, often requiring extensive medical intervention. We review the performance of assays that assess DPD and DPYD status, with an emphasis on the robustness for routine clinical applications. None of the current strategies are adequate to mandate routine DPD testing prior to starting a fluoropyrimidine-based therapy. However, further research and technological improvements will hopefully allow prospective identification of potentially toxic patients, in order to reduce the number of patients with severe, life-threatening side effects to 5FU treatment.

Profiling dihydropyrimidine dehydrogenase deficiency in patients with cancer undergoing 5-fluorouracil/capecitabine therapy

Fluoropyrimidine drugs such as 5-fluorouracil (5-FU) and capecitabine are a mainstay in the treatment of numerous solid tumors, including colorectal cancers, alone or as part of combination therapies. Cytotoxic drugs such as 5-FU and oral capecitabine display narrow therapeutic indexes combined with high interpatient pharmacokinetic variability. As a result, severe toxicities often limit or delay the administration of successive, optimal chemotherapeutic courses, leading to unfavorable clinical outcome in patients with cancer. Catabolism and deactivation of fluoropyrimidine drugs depend on a single and exclusive enzymatic step driven by dihydropyrimidine dehydrogenase (DPD). Dihydropyrimidine dehydrogenase is prone to marked circadian rhythms, drug-drug interactions, and genetic polymorphisms; influence of its erratic activity on 5-FU pharmacokinetics and toxicity profile has been extensively investigated, and it is now well known that DPD deficiency leads to severe toxicities with 5-FU or possibly capecitabine exposure. With the ever-increasing number of patients with cancer likely to be treated with fluoropyrimidines, predicting and preventing the occurrence of such toxicities is now a major issue in clinical oncology. Early determination of DPD status in patients with cancer would allow identification of those at risk and help in subsequent dose adjustment or selection of other treatment modalities. Numerous methods, either genotypic or phenotypic, have been proposed to achieve this goal. This review covers a wide range of techniques available to establish DPD status in patients with cancer.

A rapid and inexpensive method for anticipating severe toxicity to fluorouracil and fluorouracil-based chemotherapy

Dihydropyrimidine dehydrogenase (DPD) deficiency leads to dramatic overexposure to fluorouracil (5-FU), resulting in a potentially lethal outcome in patients treated with standard doses. The aim of this study was to validate, in a routine clinical setting, a simple and rapid method to determine the DPD status in a subset of cancer patients, all presenting with life-threatening toxicities following 5-FU or capecitabine intake. In this study, 80 out of 615 patients (13%) suffered severe toxicities, including 5 lethal ones (0.8%), during or after chemotherapy with a fluoropyrimidine drug. Patients with severe toxicities were treated with 5-FU (76 patients) or capecitabine-containing protocols (4 patients). Simplified uracil to di-hydrouracil (U/UH2) ratio determination in plasma was retrospectively performed in these 80 patients, as a surrogate marker of DPD activity. When possible, 5-FU Css determination was performed, and screenings for the canonical IVS14+1G>A mutation were systematically carried out. Comparison of the U/UH2 ratios with a reference, non-toxic population, showed abnormal values suggesting impaired DPD activity in 57 out of the 80 toxic patients (71%) included in this study, and in 4 out of 5 patients (80%) with a fatal outcome. Similarly, drug exposures up to 15 times higher than the range observed in the non-toxic population were also observed. Importantly, no IVS14+1G>A mutation was found in these patients, including those displaying the most severe or lethal toxicities. These data warrant systematic detection of DPD-deficient patients prior to fluoropyrimidine administration, including when oral capecitabine (Xeloda) is scheduled. Finally, the simplified methodology presented here proved to be a low cost and rapid way to identify routinely patients at risk of toxicity with 5-FU or capecitabine.

How may anticancer chemotherapy with fluorouracil be individualised?

Fluorouracil is used clinically against various solid tumours. Both fluorouracil toxicity and pharmacokinetics vary highly within and between individuals. The reasons why doses are not individualised routinely are difficulties in defining, predicting and achieving an optimal fluorouracil exposure or dose because of a narrow therapeutic index, nonlinear pharmacokinetics, variabilities in administration rates and metabolism, and in targets like thymidylate synthase. To individualise fluorouracil administration before the first dose, assessment of the individual dihydropyrimidine dehydrogenase (DPD) activity may be useful, because this genetically highly polymorphic enzyme controls approximately 80% of fluorouracil elimination. A complete or partial loss of DPD activity in 0.1 and 3-5% of Caucasians, respectively, leads to increased fluorouracil exposure and toxicity. Several methods to assess DPD activity in patients have been proposed (genotyping, various phenotyping methods), but each of them has limitations, as has the fluorouracil test dose approach. To adapt exposure towards fluorouracil a priori, a combination of genotyping and phenotyping may yield better prediction of toxicity than one method alone. A prerequisite for dose adaptation is the definition of fluorouracil exposure ranges with sufficient therapeutic activity, but without serious toxicity. While an increased risk of leukopenia, diarrhoea, stomatitis, and hand-foot syndrome during continuous 5-day infusions was related to fluorouracil exposures above an area under the plasma concentration-time curve (AUC) threshold of 25-30 mg.h/L, tumour response was higher when an AUC of approximately 30 mg.h/L was achieved, illustrating the extremely narrow therapeutic window of fluorouracil. Pharmacokinetic target values are less clear for other regimens, including chronomodulated regimens, which yielded a superior clinically efficacy and tolerability in several trials. However, the monitoring of fluorouracil plasma concentrations seems principally useful for individual a posteriori dose adjustment. Whether an adaptation of the fluorouracil starting dose to the results of two DPD activity tests before fluorouracil administration a priori, and the adaptation of doses to fluorouracil exposure a posteriori is a reasonable approach to better prevent toxicity and increase efficacy, remains to be evaluated in randomised clinical studies comparing these strategies to routine clinical safety monitoring.

Liquid chromatography–tandem mass spectrometric assay for the analysis of uracil, 5,6-dihydrouracil and β-ureidopropionic acid in urine for the measurement of the activities of the pyrimidine catabolic enzymes

A liquid chromatography-tandem mass spectrometric assay for the determination of uracil, 5,6-dihydrouracil and beta-ureidopropionic acid in urine was developed to measure the activities of enzymes involved in pyrimidine breakdown. The assay was required to investigate the relation between the uracil-dihydrouracil ratio and toxicities observed after treatment with fluoropyrimidines drugs. After addition of stable isotopically labelled internal standards, the analytes were isolated from a 100-microl urine sample using liquid-liquid extraction with ethyl acetate-2-propanol. Compounds were separated on an Atlantis dC18 column, using ammonium acetate-formic acid in water as the eluent. The eluate was totally led into an electrospray interface with positive ionisation and the analytes were quantified using triple quadrupole mass spectrometry. The assay was validated in the range 1.6-1600 microM, using both, artificial urine and pooled urine as matrices. Intra-day precisions were < or = 8% and inter-day precisions were < or = 10%. Accuracies between 91 and 108% were found. The analytes were chemically stable under all relevant conditions and the assay was successfully applied in two clinical studies of cancer patients treated with 5-fluorouracil or capecitabine.

High-performance liquid chromatographic assay with UV detection for measurement of dihydrouracil/uracil ratio in plasma

A rapid, robust and sensitive HPLC method for analysis of uracil (U) and dihydrouracil (UH2) in plasma was developed using solid phase extraction and ultraviolet detection. Separation was achieved with a SymmetryShield RP18 column and an Atlantis dC18 column using a 10 mM potassium phosphate buffer as mobile phase. Compounds were eluted within 15 min without interference. Recovery was 80.4 and 80.6% for U and UH2. Calibration curves were linear from 2.5 to 80 ng/mL for U and 6.75 to 200 ng/mL for UH2. The LLQ was, respectively, 2.5 ng/mL for U, and 6.75 ng/mL for UH2. Within-run and between-run precision were less than 5.94% and inaccuracy did not exceed 7.80%. The overall procedure has been applied to correlate UH2/U ratio with dihydropyrimidine dehydrogenase activity in 165 cancer patients.

The uracil breath test in the assessment of dihydropyrimidine dehydrogenase activity: pharmacokinetic relationship between expired 13CO2 and plasma [2-13C]dihydrouracil

PURPOSE

Dihydropyrimidine dehydrogenase (DPD) deficiency is critical in the predisposition to 5-fluorouracil dose-related toxicity. We recently characterized the phenotypic [2-(13)C]uracil breath test (UraBT) with 96% specificity and 100% sensitivity for identification of DPD deficiency. In the present study, we characterize the relationships among UraBT-associated breath (13)CO(2) metabolite formation, plasma [2-(13)C]dihydrouracil formation, [2-(13)C]uracil clearance, and DPD activity.

EXPERIMENTAL DESIGN

An aqueous solution of [2-(13)C]uracil (6 mg/kg) was orally administered to 23 healthy volunteers and 8 cancer patients. Subsequently, breath (13)CO(2) concentrations and plasma [2-(13)C]dihydrouracil and [2-(13)C]uracil concentrations were determined over 180 minutes using IR spectroscopy and liquid chromatography-tandem mass spectrometry, respectively. Pharmacokinetic variables were determined using noncompartmental methods. Peripheral blood mononuclear cell (PBMC) DPD activity was measured using the DPD radioassay.

RESULTS

The UraBT identified 19 subjects with normal activity, 11 subjects with partial DPD deficiency, and 1 subject with profound DPD deficiency with PBMC DPD activity within the corresponding previously established ranges. UraBT breath (13)CO(2) DOB(50) significantly correlated with PBMC DPD activity (r(p) = 0.78), plasma [2-(13)C]uracil area under the curve (r(p) = -0.73), [2-(13)C]dihydrouracil appearance rate (r(p) = 0.76), and proportion of [2-(13)C]uracil metabolized to [2-(13)C]dihydrouracil (r(p) = 0.77; all Ps < 0.05).

CONCLUSIONS

UraBT breath (13)CO(2) pharmacokinetics parallel plasma [2-(13)C]uracil and [2-(13)C]dihydrouracil pharmacokinetics and are an accurate measure of interindividual variation in DPD activity. These pharmacokinetic data further support the future use of the UraBT as a screening test to identify DPD deficiency before 5-fluorouracil-based therapy.

Toxic death-case after capecitabine + oxaliplatin (XELOX) administration: probable implication of dihydropyrimidine deshydrogenase deficiency

This report here is the case of a 52-year-old male patient who suffered from extremely severe haematological toxicities (G4 neutropenia, G4 thrombocytopenia) while undergoing Xelox (Xeloda + Oxaliplatin) treatment for his multifocal hepatocarcinoma. Despite appropriate supportive treatment, his condition quickly deteriorated and led to death. It was hypothesized that dihydropyrimidine deshydrogenase (DPD) gene polymorphism could be, at least in part, responsible for this fatal outcome. To test this hypothesis, both phenotypic and genotypic studies were undertaken, and fully confirmed the DPD-deficient status of this patient. Uracil to dihydrouracil ratio in plasma was evaluated as a surrogate marker for DPD deficiency, and showed values out of the range previously recorded from a reference, non-toxic population. Interestingly, the canonical IVS14+1G>A single nucleotide polymorphism, usually associated with the most severe toxicities reported with 5-fluorouracil (5-FU), was not found in this patient, but further investigations showed instead a heterozygosity for the 1896C>T mutation located in the exon 14 of the DPYD gene. Taken together, the data strongly suggest for the first time that a toxic-death case after capecitabine-containing protocol could be, at least in part, linked with a DPD-deficiency syndrome. The case reported here warrants therefore systematic detection of patients at risk, including when oral capecitabine is scheduled.

Feasibility of measuring 5-fluorouracil catabolic potential by oral loading

The efficacy of 5-fluorouracil (5-FU) treatment and the incidence of adverse events differ among patients and depend to some extent on individual variations in drug catabolism. This feasibility study aimed to determine the optimum conditions for a 5-FU oral load test, which would allow the simple evaluation of individual differences in 5-FU catabolism. Patients with colon cancer were given oral 5-FU (200 mg/day) for 3 days (n = 36) or a single 100 mg dose (n = 14). Serum concentrations of uracil, dihydrouracil, 5-FU and 5-fluoro-5,6-dihydrouracil were measured before and after 5-FU administration. The results suggested that a decline in 5-FU metabolism was associated with continuous administration and increasing age. We conclude that a continuous load of 5-FU is necessary in order to predict the efficacy and side-effects of the drug. The 3-day regimen, with its ease of administration, merits further study to assess its possible clinical application.