Population characteristics of hepatic dihydropyrimidine dehydrogenase activity, a key metabolic enzyme in 5-fluorouracil chemotherapy

Lack of carcinogenicity and increased survival in F344 rats treated with 5-fluorouracil for two years

The carcinogenicity of 5-fluorouracil (5-FU), a compound employed as an antineoplastic drug, was investigated in F344 rats of both sexes. 5-FU was administered to groups of 50 male and 50 female rats ad lib. for 104 weeks, added to drinking water at concentrations of 0 (control), 62 and 125 ppm, these dose levels being selected on the basis of results of a 13-week subchronic toxicity study. Body weight gains were slightly depressed in the 125 ppm group of both sexes. While not statistically significant in females, final survival rates at week 111 in the 125 ppm group of both sexes were higher than those in the control group, suggesting an ability of 5-FU to prolong the lifespan. Histopathologically, a decreased incidence of islet cell adenomas in males and increased incidences of pituitary gland adenomas and pheochromocytomas in females were observed in the 62 ppm group without dose dependence. There was no significant induction of any other neoplastic or non-neoplastic lesions. These results indicate a lack of carcinogenicity of 5-FU under the present experimental conditions using rats.

Simultaneous determination of 5-fluorouracil and uracil by high-performance liquid chromatography using four serial columns

A sensitive assay was developed for the quantitation of 5-fluorouracil (5-FU) and uracil using liquid-liquid extraction (LLE) and HPLC with UV detection. Analyses were performed with four microBondapak C18 columns connected in series using 20 mM acetic acid with 1% ACN as mobile phase. The calibration curves were linear across the range of 26-1000 ng ml(-1) (0.21-7.8 microM) for 5-FU and 1.0-14.0 microg ml(-1) (0.01-110 microM) for uracil. This assay has been implemented to determine the plasma concentrations for pharmacokinetic studies for 5-FU and uracil in conjunction with clinical trials.

Pharmacogenetics and cancer chemotherapy

Cancer chemotherapy is limited by significant inter-individual variations in responses and toxicities. Such variations are often due to genetic alterations in drug metabolising enzymes (pharmacokinetic polymorphisms) or receptor expression (pharmacodynamic polymorphisms). Pharmacogenetic screening prior to anticancer drug administration may lead to identification of specific populations predisposed to drug toxicity or poor drug responses. The role of polymorphisms in specific enzymes, such as thiopurine S-methyltransferases (TPMT), dihydropyrimidine dehydrogenase (DPD), aldehyde dehydrogenases (ALDH), glutathione S-transferases (GST), uridine diphosphate glucuronosyl-transferases (UGTs) and cytochrome P450 (CYP 450) enzymes in cancer therapy are discussed in this review.

Dihydropyrimidine dehydrogenase pharmacogenetics in Caucasian subjects

AIMS

Dihydropyrimidine dehydrogenase (DPD) catalyses the reduction of pyrimidines, including the anticancer agent 5-fluorouracil (5FU). Impaired 5FU degradation, through low DPD activity, has led to severe, life-threatening or fatal toxicity after administration of 5FU. Complete DPD deficiency is associated with the inherited metabolic disease thymine uraciluria. Several mutations in the gene encoding DPD have recently been identified, but the phenotype-genotype concordance of these alterations in the general population has not been reported.

METHODS

Mononuclear cells were isolated from whole blood and DPD activity was determined after ex vivo incubation with 14C-5FU followed by h.p.1.c. analysis of 5FU metabolites. Analysis of mutations in the DPD gene at an exon splice site, codons 534, 543, and 732, and a deletion at base 1897 (deltaC1897) were performed in 30 subjects with the lowest and 30 subjects with the highest enzyme activity using PCR-RFLP.

RESULTS

DPD activity was measured in 226 Caucasian subjects and was highly variable (range 19.1-401.4 pmol min(-1)mg(-1) protein). Mutations were frequently observed at codons 543 (allele frequency 28%), 732 (allele frequency 5.8%), and 534 (allele frequency 0.8%), but were not associated with low DPD activity. There were no splice site or deltaC1897 mutations found in this population.

CONCLUSIONS

The five mutations analysed in this study are insufficient for identification of patients at risk for 5FU toxicity or thymine uraciluria. Both the splice site mutation and deltaC1897 are relatively rare in the general Caucasian population. Therefore, identification of further molecular alterations is required to facilitate the use of DPD analysis in genetic diagnosis and cancer therapeutics.

Sorivudine and 5-fluorouracil; a clinically significant drug-drug interaction due to inhibition of dihydropyrimidine dehydrogenase

Sorivudine (1-beta-D-arabinofuranosyl-E-5-[2-bromovinyl] uracil; BV-araU; SQ32,756) is an antimetabolite which is a synthetic analogue of thymidine. This drug has demonstrated antiviral activity against varicella zoster virus, herpes simplex type 1 virus, and Epstein-Barr virus. Clinical studies in Japan and subsequently worldwide showed this drug to be a potent agent for treating varicella zoster infections. Although in general well tolerated, a fatal drug interaction with fluoropyrimidine drugs was subsequently observed. While three deaths resulting from this interaction were recognized to have occurred during the initial clinical evaluation in Japan, the full impact of the interaction was not recognized in Japan until post-marketing when an additional 23 cases of severe toxicity were reported including 16 patients who subsequently died from fluoro-pyrimidine toxicity. Worldwide recognition of this potentially fatal drug-drug interaction led to subsequent disapproval in the US and elsewhere. The interaction has been shown to be due to suppression of 5-fluorouracil (5-FU) catabolism, resulting in higher levels of 5-FU than would normally be observed. The mechanism of this interaction is mediated through inhibition of the 5-FU rate-limiting catabolizing enzyme dihydropyrmidine dehydrogenase (DPD) by the BV-araU metabolite BVU. This drug-drug interaction of sorivudine and 5-FU further emphasizes the critical importance of DPD on the clinical pharmacology of 5-FU.

Characterization of dihydropyrimidine dehydrogenase in human colorectal tumours

Dihydropyrimidine dehydrogenase (DPD) is the rate-limiting enzyme for degradation of 5-fluorouracil (5-FU). DPD activity is highly variable in liver and peripheral mononuclear cells (PMNCs) and it has not been well studied in human tumours. Characterization of DPD in colorectal cancer is of clinical interest through its role in the regulation of 5-FU, the main chemotherapeutic agent used in this disease. Therefore, DPD activity was analysed in colorectal tumour and adjacent normal tissue from 63 patients, including three liver metastasis. DPD activity was highly variable in all tissues studied (coefficient of variation 43-61%) and was higher in normal tissue than in tumour. The tumour-normal activity ratio ranged from 0.19 to 3.32 (median 0.76). PMNC DPD activity was available for 57 patients and was correlated with tumour activity (r(s) = 0.29, P < 0.001). A higher correlation was observed between PMNCs and tumour samples that were both obtained in the morning (r(s) = 0.49), consistent with circadian variation in DPD activity. Normal tissue DPD activity was not correlated with either tumour (r(s) = 0.11) or PMNC activity (r(s) = -0.06). This study provides the first analysis of DPD activity in colorectal cancer and illustrates the large degree of variation in tumour activity. The tumour-normal activity ratio results suggest that elevated tumour DPD can play a role in 5-FU resistance through increased inactivation in tumour cells, but is an uncommon event in colorectal tumours. The results support the use of PMNCs for monitoring tumour DPD activity, particularly when circadian variation is taken into account. As a large degree of the variation in tumour DPD activity is not explained by PMNC activity, more accurate alternatives are needed before DPD activity can be used for targeting 5-FU therapy.

Co-variables influencing 5-fluorouracil clearance during continuous venous infusion. A NONMEM analysis

The objective of this study was to attempt to identify patient co-variables which could influence interpatient variability in 5-fluorouracil (5-FU) clearance during a 5-day continuous venous infusion (CVI, cisplatin 100 mg/m2 day 1 then 5-FU 1 g/m2/day days 2-6). The analysis was performed using a NONMEM program according to a linear one-compartment model. A total of 186 cycles (2 samples per day, 8 a.m. and 5 p.m.) were analysed from 104 patients with various cancers (the majority of head and neck and oesophagus). The study co-variables were age, sex, body surface area, hepatic metastases, peripheral mononuclear cell dihydropyrimidine dehydrogenase activity (PMNC-DPD), liver enzymes, clock-time (8 a.m. versus 5 p.m.), elapsed time during CVI. The data showed that 5-FU clearance was significantly reduced by increased age, low PMNC-DPD, high serum alkaline phosphatase and elapsed time during infusion. These data may be useful for improving knowledge of predictive factors which can influence 5-FU exposure and thus predispose to toxic manifestations.

Sex- or age-related differences were not detected in the activity of dihydropyrimidine dehydrogenase from rat liver

We examined the influence of ageing and gender on the activity of dihydropyrimidine dehydrogenase (DPD) in male and female Sprague-Dawley rats at 3, 8, 40 and older than 60 weeks of age. The body and liver weights of male rats at 8 weeks and older of age were heavier than those of female rats at the same age and the ratio of liver to body weight was significantly reduced with ageing. The amount of produced 5FU metabolites, dihydrofluorouracil and fluoroureidopropionic acid, remained unchanged in the reaction mixture with the cytosol from any group of the rats. The formation rates of 5FU metabolite were 1.03+/-0.08/1.12+/-0.06, 0.98+/-0.11/1.16+/-0.10, 1.13+/-0.09/1.13+/-0.11 and 1.17+/-0.16/1.24+/-0.08 nmol min(-1) mg protein(-1) (mean+/-SD, male/female) in rats at 3, 8, 40 and older than 60 weeks of age, respectively. The results suggest that the DPD activity in the rat liver may not be affected by ageing or gender.

Molecular basis of the human dihydropyrimidine dehydrogenase deficiency and 5-fluorouracil toxicity

Dihydropyrimidine dehydrogenase (DPD) deficiency constitutes an inborn error in pyrimidine metabolism associated with thymine-uraciluria in pediatric patients and an increased risk of toxicity in cancer patients receiving 5-fluorouracil (5-FU) treatment. The molecular basis for DPD deficiency in a British family having a cancer patient that exhibited grade IV toxicity 10 d after 5-FU treatment was analyzed. A 165-bp deletion spanning a complete exon of the DPYD gene was found in some members of the pedigree having low DPD catalytic activity. Direct sequencing of lymphocyte DNA from these subjects revealed the presence of a G to A point mutation at the 5'-splicing site consensus sequence (GT to AT) that leads to skipping of the entire exon preceding the mutation during pre-RNA transcription and processing. A PCR-based diagnostic method was developed to determine that the mutation is found in Caucasian and Asian populations. This mutation was also detected in a Dutch patient with thymine-uraciluria and completely lacking DPD activity. A genotyping test for the G to A splicing point mutation could be useful in predicting cancer patients prone to toxicity upon administration of potentially toxic 5-FU and for genetic screening of heterozygous carriers and homozygous deficient subjects.