Dihydropyrimidine dehydrogenase activity and fluorouracil chemotherapy

Severe toxicity related to the 5-fluorouracil/leucovorin combination (the Mayo Clinic regimen): a prospective study in colorectal cancer patients

The Mayo Clinic regimen of leucovorin 20 mg/m followed immediately by 5-fluorouracil 425 mg/m administered for 5 consecutive days every 4 weeks is commonly used in the treatment of colorectal cancer. This study was aimed at prospectively determining the incidence and pattern of severe toxicity associated with this regimen. We evaluated prospectively 243 patients with colorectal cancer treated in our department with the Mayo Clinic regimen for the incidence of severe toxicity (defined as toxicity requiring hospitalization). Of the 243 patients, 32 (13%) were hospitalized for chemotherapy-related toxicity. Major toxicities included neutropenic fever in 21 (9%), grade III/IV mucositis in 25 (10%) and grade III/IV diarrhea in 20 (8%). There were five (2%) treatment-related deaths. Female patients exhibited a higher incidence of severe toxicity (18%) and toxic death (4/105) than did male patients (9% and 1/138, respectively). Elderly patients (> or =70 years) had a higher incidence of severe toxicity than younger patients did (24% versus 7%, < 0.001). Toxic death occurred in 4 of 89 patients aged 70 years or more compared to 1 of 154 in younger patients. Most episodes of severe toxicity (56%) and toxic deaths (4/5) were observed after the first cycle. We conclude that the Mayo Clinic regimen can be associated with severe toxicity, usually occurring after the first cycle. Female gender and advanced age predict severe toxicity; therefore, dose reduction in high-risk patients should be considered, especially during the first cycle.

The Contribution of Cytochrome P450 to the Metabolism of Tegafur in Human Liver

Fluorouracil (5-FU) prodrug tegafur (FT) is used widely for treating cancer patients. It has been reported that CYP2A6 and thymidine phosphorylase (TP) are involved in the formation of 5-FU from FT. In this study, the relative contribution of cytochrome P450 (P450) to the formation of 5-FU from FT was assessed using human liver 9000 x g supernatant (S9) and hepatocytes, which contain both enzymes. Intrinsic clearances of 5-FU formation from FT by P450 (NADPH dependent) and TP (NADPH independent) in human liver S9 were 1.36 and 0.169 microL/min/mg protein, respectively. The formation of 5-FU from FT in human liver S9 was inhibited over 82% by 8-methoxypsoralen, a CYP2A6-selective inhibitor. The formation of 5-FU from FT was also evaluated in human hepatocytes, cells that not only exhibit P450 and TP activity but also have anabolic capacity. The results indicated that CYP2A6 played a major role in 5-FU formation, which was consistent with the results using human liver S9. Factors that can affect the level of CYP2A6 activity in patients, e.g., genetic polymorphism, should be considered when using FT for chemotherapy.

Development and characterization of a monoclonal antibody with cross-reactivity towards uracil and thymine, and its potential use in screening patients treated with 5-fluorouracil for possible risks

BACKGROUND

Dihydropyrimidine dehydrogenase (DPD) is the initial and rate-limiting enzyme in catabolism of pyrimidines including 5-fluorouracil. There have been efforts to isolate a monoclonal antibody that will bind selectively to pyrimidine and can be used to measure the concentration of pyrimidine in blood and/or in urine that may reflect the activity of dihydropyrimidine dehydrogenase. However, the monoclonal antibodies selective to pyrimidine have not been available.

METHODS

Using 1-carboxymethyl-uracil as a hapten, in which steric conformation of uracil is thought to be well maintained, extensive screening was done to isolate a monoclonal antibody specific to uracil.

RESULTS

We established the first monoclonal antibody that reacted with uracil and thymine but not with pseudouridine, dihydrouracil, dihydrothymine, cytosine, uridine, or N-carbamyl-beta-alanine at the concentration of 100 microg/ml.

CONCLUSIONS

The monoclonal antibody can be used to develop a simple screening assay for patients with dihydropyrimidine dehydrogenase deficiency. This may increase the safety of 5-fluorouracil treatment.

Fluoropyrimidine resistance in colon cancer

A significant obstacle for the management of patients with colorectal cancer is intrinsic drug resistance or in patients that respond to chemotherapy, acquired drug resistance. Drug resistance can occur through a variety of mechanisms. These include alterations in drug influx, drug efflux, intracellular metabolic activation, intracellular catabolism, through alterations in the drug's target or through numerous changes downstream of the target including alterations in genes involved in the regulation of the cell cycle, apoptosis or in DNA damage repair. In this article, the mechanisms of action and the mechanisms of resistance to the fluoropyrimidines are reviewed focusing on newer studies using tumor samples obtained from patients. Clinical trials that can potentially overcome the relevant mechanisms of resistance are described.

Novel aspects of resistance to drugs targeted to dihydrofolate reductase and thymidylate synthase

Drug resistance is often a limiting factor in successful chemotherapy. Our laboratory has been interested in studying mechanisms of resistance to drugs that are targeted to the thymidylate biosynthesis pathway especially those that target thymidylate synthase (TS) and dihydrofolate reductase (DHFR). We have used leukemia as a model system to study resistance to methotrexate (MTX) and colorectal cancer as the model system to study 5-fluorouracil (5-FU) resistance. In leukemias, we and others have shown that transport, efflux, polyglutamylation and hydrolase activities are major determinants of MTX resistance. We have further reported that some leukemic cells have an increase in DHFR gene copy number possibly contributing to the resistant phenotype. Recently, we have begun to study in detail the molecular mechanisms that govern translational regulation of DHFR in response to MTX as an additional resistance mechanism. Studies thus far involving colorectal tumors obtained from patients have focused predominantly on the predictive value of levels of TS expression and p53 mutations in determining response to 5-FU. Although the predictive value of these two measures appears to be significant, given the variety of resistance to 5-FU observed in cell lines, it is not likely that these are the only measures predictive of response or responsible for acquired resistance to this drug. The enzyme uridine-cytidine monophosphate kinase (UMPK) is an essential and rate-limiting enzyme in 5-FU activation while dihydropyrimidine dehydrogenase (DPD) is a catabolic enzyme that inactivates 5-FU. Alterations in UMPK and DPD may therefore explain failure of 5-FU response in the absence of alterations in TS or p53. Transcription factors that regulate TS may also influence drug sensitivity. We have found that mRNA levels of the E2F family of transcription factors correlates with TS message levels and are higher in lung metastases than in liver metastases of colorectal cancers. Moreover, gene copy number of the E2F-1 gene appears to be increased in a significant number of samples obtained from metastases of colorectal cancer. We have also generated mutants of both DHFR and TS that confer resistance to MTX as well as 5-FU by random as well as site-directed mutagenesis. These mutants used alone or as fusion cDNAs of the mutants have proven to be useful in transplant studies where transfer of these mutant cDNAs to bone marrow cells have been shown to confer drug resistance to recipients. The fusion cDNAs of DHFR such as the DHFR-herpes simplex virus type 1 thymidine kinase (HSVTK) are also useful for regulation of gene expression in vivo using MTX as the small molecule regulator that can be monitored by positron emission tomography (PET) scanning or by optical imaging using a fusion construct such as DHFR-EGFP.

Dihydropyrimidine dehydrogenase enzyme deficiency: clinical and genetic assessment of prevalence in Turkish cancer patients

BACKGROUND

Fluorouracil has been reported to induce severe side-effects in particular subjects who have deficiency in dehydropyrimidine dehyrogenase activity, the major enzyme in the catabolism of fluorouracil.

PATIENTS AND METHODS

In this study, we aimed to analyze the heterozygote and homozygote frequencies of dehydropyrimidine dehyrogenase gene mutation in 200 patients receiving fluorouracil based chemotherapy together with the assessment of the toxicity profile of these chemotherapy regimens.

RESULTS

According to the results of clinical toxicity assessments, grade 3-4 hematologic toxicity was noted in 12% of the patients. Grade 3 gastrointestinal toxicity was present in 5% of the subjects with no grade 4 side effects. The heterozygote (2q(1-q)) and homozygote (q2) frequencies of dehydropyrimidine dehyrogenase gene mutation were calculated as 1.5% (3/200) and 0.000055% (1/18,043) in the analyzed samples.

CONCLUSION

In this report, for the first time we documented the frequency of dehydropyrimidine dehydrogenase gene mutation in Turkish cancer patients. The determination of enzyme activity in suspected individuals and analysis of other mutations on a population basis would be the next steps for our country.

A comparative analysis of translated dihydropyrimidine dehydrogenase cDNA; conservation of functional domains and relevance to genetic polymorphisms

A pharmacogenetic syndrome caused by molecular defects in the dihydropyrimidine dehydrogenase gene (DPYD ) results in partial to complete loss of dihydropyrimidine dehydrogenase (DPD) enzyme activity with patients exhibiting life-threatening toxicity following administration of routine doses of 5-fluorouracil. To date, more than 19 reported mutations have been putatively associated with DPD deficiency with 16 occurring within the open reading frame of the cDNA. The purpose of this study was to examine the conservation of functional domains (including the uracil, flavine adenine dinucleotide and NADPH binding sites) across three phyla (Chordata, Arthropoda and Nematoda) and the conservation of regions corresponding to the previously reported mutations. Comparative analysis of the uracil and NADPH binding sites in mammals and invertebrates demonstrated 100% amino acid identity between mammals and Drosophila melanogaster. Caenorhabditis elegans demonstrated 89% and 88% identity in these domains, respectively. The mammalian sequences demonstrated 100% identity in two iron sulphur motifs (amino acids 953-964 and 986-997) with significant conservation in D. melanogaster (92% and 92% identity, respectively) and C. elegans (100% and 92% identity, respectively). Comparative amino acid analysis revealed non-conservation in the loci of four DPYD mutations [DPYD*12 (R21Q), DPYD*5 (I543V), DPYD*6 (V732I), DPYD*9A (C29R)]. Seven mutations occurred in highly conserved regions [M166V, DPYD*8 (R235W), DPYD*11 (V335l), DPYD*4 (S534N), DPYD*9B (R886H), D949V, DPYD*10 (V995F)]. In summary, this comparative analysis identified conserved regions which may be critical to enzyme structure and/or function. The conservation of loci where DPYD mutations occur was also examined to evaluate their functional significance on DPD enzyme activity. These data should prove useful in the evaluation of newly discovered mutations in the DPYD gene.

High-performance liquid chromatographic assay with ultraviolet detection for quantification of dihydrofluorouracil in human lymphocytes: application to measurement of dihydropyrimidine dehydrogenase activity

The anticancer drug 5-fluorouracil (5FU) undergoes extensive biotransformation to 5-dihydrofluorouracil (5FUH2) by the enzyme dihydropyrimidine deshydrogenase (DPD). A new HPLC method with direct UV detection for the determination of 5FUH2 in peripheral lymphocytes has been developed to detect DPD deficiency in patients treated with 5FU-based therapy. The method has been shown to be valid over the 5FUH2 concentration range of 1.14-37.88 nmol/ml. Optimal enzymatic conditions for DPD activity measurement were studied: incubation time, protein and 5FU concentrations. The assay was successfully cross-validated with the existing method using HPLC with radiochemical detection.

Prognostic significance of thymidylate synthase activity in bladder carcinoma

BACKGROUND

5-fluorouracil (5-FU) is an anticancer agent clinically used against various cancers including bladder carcinoma. 5-FU inhibits thymidylate synthase (TS) and blocks DNA synthesis. TS is the key enzyme in the catalysis of the methylation from deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate. Little is known about the significance of TS in bladder carcinoma. The authors investigated the activity of TS in 82 bladder cancers and prognostic significance of the levels of TS and/or activities of dihydropyrimidine dehydrogenase (DPD), an important enzyme in the degradation of 5-FU.

METHODS

The levels of TS and DPD activities in nonfixed, fresh, frozen, bladder carcinoma and normal bladder specimens were determined biochemically by the FdUMP binding assay and the 5-FU degradation assay, respectively.

RESULTS

The activity of TS was 10-fold higher in bladder carcinoma compared with normal bladder. TS activity in muscle-invasive bladder carcinoma was threefold higher than that in Ta and T1 cancer. In addition, the activity of TS in T1 bladder carcinoma was threefold higher than that in Ta cancer. The level of TS activity in Grade 3 bladder carcinoma was 4.5-fold and 3.5-fold higher than that in Grade 1 and Grade 2 cancers, respectively. Patients with Ta and T1 bladder carcinoma with low TS activity had a longer postoperative tumor-free period than those with high activity in the 2-year follow-up. Patients with Ta and T1 bladder carcinoma with high or low TS activity were divided into four subgroups: high or low DPD activity subgroups. Patients with low TS activity and high DPD activity had the longest postoperative disease-free period among the 4 subgroups during the 2-year follow-up.

CONCLUSIONS

To the authors' knowledge, the current study is the first study that demonstrated that the level of TS activity correlates with both the progression of the stage and the increase of the grade of bladder carcinoma and that elevated TS activity predicts early recurrence in Ta and T1 bladder carcinoma. These results suggested that elevated TS activity might have been associated with a higher chance of progression and recurrence of bladder carcinoma in the patients who participated in this study.

Oral fluoropyrimidine treatment of colorectal cancer

5-Fluorouracil (5-FU) has been utilized as part of standard chemotherapy for treatment of early-stage and metastatic colorectal cancer for more than 4 decades. The oral fluoropyrimidines have been studied extensively as an alternative to intravenous 5-FU. The goal of such an approach is to simplify drug administration and to improve the toxicity profile while maintaining efficacy that is at least equivalent to intravenous therapy. The goal of this article is to review the features of the main oral 5-FU prodrugs, which include capecitabine, uracil and tegafur (UFT)/leucovorin, S-1, and BOF-A2 and to describe their potential efficacy in treating colorectal cancer.

Determinants of prognosis and response to therapy in colorectal cancer

Identification of the molecular determinants of 5-fluorouracil (5-FU) and irinotecan (CPT-11) efficacy and toxicity is critically important for the development of more efficient and less toxic treatment strategies for patients with colon cancer. We have identified molecular predictors of response to chemotherapy with 5-FU and survival in patients with advanced colorectal cancer. Low gene expression levels of thymidylate synthase (TS), dihydropyrimidine dehydrogenase (DPD), and thymidine phosphorylase (TP) are associated with response and survival. Preliminary data suggest that gene expression levels of topoisomerase I, p21, bcl-2, and ICE may be predictive of response to therapy with CPT-11. Increased toxicity seen in patients treated with CPT-11 may be explained by polymorphism in the UGT1A1 gene, which is responsible for glucuronidation of the active metabolite of CPT-11.

The significance of dihydropyrimidine dehydrogenase (DPD) activity in bladder cancer

Dihydropyrimidine dehydrogenase (DPD) is the rate-limiting enzyme in the pathway of uracil and thymine catabolism. DPD is also the principal enzyme involved in the degradation of 5-fluorouracil (5-FU), which is one of the anticancer chemotherapeutic agents currently used in the treatment of bladder cancer. Little is known about the significance of DPD activity in human cancers. We investigated the activity of DPD in 74 bladder cancers and the relationship between the DPD activity and the sensitivity to 5-FU. The levels of DPD activity in bladder cancer and normal bladder tissues were determined by the 5-FU degradation assay. The sensitivity to 5-FU was assessed by the microculture tetrazolium dye (dimethylthiazolyl-2-5-diphenyltetrazolium bromide; MTT) assay. The activity of DPD was approximately 2-fold higher in bladder cancer tissues compared with normal bladder tissues. DPD activity in invasive bladder cancers was approximately 2-fold higher than that in superficial cancers. In addition, the levels of DPD activity in grade 2 and grade 3 bladder cancers were approximately 3-fold and 4-fold higher than that in grade 1 cancers, respectively. Patients with superficial bladder cancer with a low DPD activity had a slightly longer postoperative tumour-free period than those with a high DPD activity over a 2-year follow-up period, but this was not significant. There was an inverse correlation between DPD activity in bladder cancer cells and their sensitivity to 5-FU. Furthermore, 5-chloro-2,4-dihydroxypyridine (CDHP), a potent DPD inhibitor, enhanced the sensitivity to 5-FU. The present study has demonstrated that the level of DPD activity correlated with the progression of the stage and an increase in the grade of the bladder cancer. These results suggest that an elevated DPD activity might be associated with the malignant potential of the bladder cancer. In addition, it might be possible to overcome 5-FU insensitivity by using DPD inhibitors in the treatment protocols of 5-FU-based chemotherapy for bladder cancers.

5-fluorouracil steady state pharmacokinetics and outcome in patients receiving protracted venous infusion for advanced colorectal cancer

PVI 5FU gives increased response rates and reduced toxicity when compared to bolus 5FU (J Clin Oncol 1989, 425-432). PVI 5FU administration was reported to give highly variable (>1000-fold) plasma 5FU concentrations at steady state (FU Css) which correlated with toxicity (Ann Oncol 1996, 47-53); but only 19 patients were studied. Therefore, we performed a study of PVI 5FU in 61 patients with advanced colorectal cancer to assess the variability (inter- and intra-subject) in 5FU Css associated with PVI 5FU (300 mg m(-2)day(-1)) and to attempt to correlate pharmacodynamic end-points (anti-tumour activity, toxicity) with 5FU Css as a prelude to 'exposure-guided' 5FU administration. All 5FU sampling was performed between 10 am and noon. PVI 5FU administration continued to 26 weeks in patients with disease improvement or stabilization. The response rate was 26% (33% stable disease) and median survival was 11 months. Hand-foot syndrome was the most common dose limiting toxicity. Variability in 5FU(300)Css was considerably less than previously reported; 94 +/- 25 ng ml(-1)(CV = 27%). No relationships were demonstrated between subject mean 5FU(300)Css and PD end-points such as response, mucositis, diarrhoea and hand-foot syndrome. The lack of correlation suggests that measurement of 5FU concentrations should not be used to individualize dosing in patients receiving PVI 5FU for advanced colorectal cancer.

Fluorouracil and the new oral fluorinated pyrimidines

OBJECTIVE

To briefly review the biotransformation and bioavailability of fluorouracil (5-FU); discuss the effects of dihydropyrimidine dehydrogenase (DpD) on the efficacy and toxicity profiles of 5-FU; and review a new class of drugs known collectively as the oral fluorinated pyrimidines, which inhibit or circumvent DpD activity and, when administered with 5-FU, alter its pharmacokinetic and pharmacodynamic properties.

DATA SOURCES

A MEDLINE literature search was conducted (1966-March 1999) using the search terms fluoropyrimidines, fluorouracil, 5-FU, fluorinated pyrimidines, capecitabine, eniluracil, uracil-tegafur, uracil-ftorafur, UFT, S1, BMS-247616, and BOF-A2. Reference lists, bibliographies of pertinent articles, and abstracts from the American Society of Clinical Oncology and the San Antonio Breast Cancer Symposium annual meetings were also identified and reviewed. Both preclinical and clinical literature were reviewed and analyzed.

DATA SYNTHESIS

The new oral fluorinated pyrimidines appear to produce antitumor activity equivalent or superior to that of intravenously administered 5-FU by achieving higher intratumoral 5-FU concentrations or sustained 5-FU exposure. These agents are generally associated with manageable and non-life-threatening toxicities. The oral route of administration facilitates ease of administration and may reduce total healthcare costs associated with 5-FU-sensitive tumors. More studies are needed to assess the therapeutic and economic benefits of the oral fluorinated pyrimidines.

CONCLUSIONS

The bioavailability, efficacy, and toxicity of 5-FU depend on its catabolic rate-limiting enzyme, The new oral fluorinated pyrimidines inhibit or circumvent DpD activity and, when combined with 5-FU, increase 5-FU's bioavailability and cytotoxic effects and decrease its toxicities. Results of Phase I and II studies in patients with a variety of malignancies suggest positive outcomes, including greater efficacy, less drug-related toxicity, lower costs related to drug administration, and greater patient convenience.

Eniluracil: an irreversible inhibitor of dihydropyrimidine dehydrogenase

One of the most widely used drugs in cancer chemotherapy is 5-fluorouracil (5-FU). 5-FU is optimally delivered via continuous iv. infusion, which is both cumbersome and expensive. Prolonged oral dosing of 5-FU could mimic continuous infusion with less inconvenience and cost. However, oral administration of 5-FU has been hampered by incomplete and erratic bioavailability due to substantial variability in the activity of dihydropyrimidine dehydrogenase (DPD), the rate-limiting enzyme in 5-FU catabolism. Eniluracil (ethynyluracil, GlaxoWellcome, USA), a uracil analogue, which irreversibly inhibits DPD, increases the oral bioavailability of 5-FU to 100%, facilitating uniform absorption and predictable toxicity. Cytotoxicity is enhanced one- to five-fold in cell lines treated with eniluracil plus 5-FU compared with 5-FU alone. Though eniluracil is neither toxic nor active as a single agent in animals, it improves the antitumour efficacy and therapeutic index of 5-FU. In Phase I trials, eniluracil markedly reduced the maximum tolerated dose of oral 5-FU, increased the half-life 20-fold and decreased the clearance 22-fold. DPD is completely inactivated within 1 h of eniluracil administration. Two dosing schedules have been evaluated in combination with oral 5-FU: a 5-day schedule every 28 days and a 28-day schedule every 35 days. The dose-limiting toxicity on the first schedule is myelosuppression with diarrhoea being dose-limiting on the 28-day schedule. Phase II trials employing the 28-day schedule have been completed in cancers of the colon, breast, liver and pancreas. Phase III trials in colorectal and pancreatic carcinoma have been completed and await analysis. Eniluracil is a promising drug, which permits reliable and safe administration of oral 5-FU and has the potential to overcome 5-FU resistance mediated by overexpression of DPD.

Characterization of dihydropyrimidine dehydrogenase on immunohistochemistry in colon carcinoma, and correlation between immunohistochemical score and protein level or messenger RNA expression

BACKGROUND

Dihydropyrimidine dehydrogenase (DPD) is the first enzyme that metabolizes 5-fluorouracil (5-FU). Until now, enzymatic activity or mRNA expression of DPD has been investigated. However, there are no papers on immunohistochemical evaluation of DPD. We investigated DPD staining on immunohistochemistry, and examined the relationship among immunohistochemical score, protein level and mRNA expression of DPD.

MATERIALS AND METHODS

Forty-seven resected colon cancer specimens, four colon cancer cell lines, two xenografts by colon cancer cell lines, and human mononuclear cells were used. Immunohistochemistry was performed using DPD monoclonal antibody. Protein levels were determined by Western blot analysis. And mRNA levels were calculated by semi-quantitative reverse transcription polymerase chain reaction (RT-PCR).

RESULTS

DPD was strongly expressed in the cytoplasm of cancer cells, and in the cytoplasm of macrophage and plasma cells. The immunohistochemical score was more correlated with protein levels (P = 0.0054) than mRNA expression (P = 0.9028).

CONCLUSIONS

We investigated the characterization of DPD immunohistochemically, and showed that immunohistochemical expression of DPD can be used to predict the sensitivity of colorectal carcinomas to 5-FU.

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.

Oral fluoropyrimidines in the treatment of colorectal cancer

5-Fluorouracil (5-FU) has been the mainstay of systemic therapy for colorectal cancer since its initial development 40 years ago. Efforts to improve the therapeutic index of 5-FU have included alteration of schedule and addition of biochemical modulators. An understanding of 5-FU mechanisms of action has resulted in major therapeutic advances in the past 10 years; however, a plateau has been reached in the efficacy of 5-FU, mandating a paradigm shift for those involved in colorectal cancer drug development. One direction vigorously pursued is the development of orally administered fluoropyrimidines that maintain or improve upon the effectiveness of intravenous 5-FU. In this paper the preclinical and clinical development of oral fluoropyrimidines and their modulators is reviewed, including UFT, capecitabine, ethynyluracil and S-1.

Characterization of the human dihydropyrimidine dehydrogenase gene

Dihydropyrimidine dehydrogenase (DPD) catabolizes endogenous pyrimidines and pyrimidine-based antimetabolite drugs. A deficiency in human DPD is associated with congenital thymine-uraciluria in pediatric patients and severe 5-fluorouracil toxicity in cancer patients. The dihydropyrimidine dehydrogenase gene (DPYD) was isolated, and its physical map and exon-intron organization were determined by analysis of P1, PAC, BAC, and YAC clones. The DPYD gene was found to contain 23 exons ranging in size from 69 bp (exon 15) to 961 bp (exon 23). A physical map derived from a YAC clone indicated that DPYD is at least 950 kb in length with 3 kb of coding sequence and an average intron size of about 43 kb. The previously reported 5' donor splice site mutation present in pediatric thymine-uraciluria and cancer patients can now be assigned to exon 14. All 23 exons were sequenced from a series of human DNA samples, and three point mutations were identified in three racial groups as G1601A (exon 13, Ser534Asn), A1627G (exon 13, Ile543Val), and G2194A (exon 18, Val732Ile). These studies, which have established that the DPYD gene is unusually large, lay a framework for uncovering new mutations that are responsible for thymine-uraciluria and toxicity to fluoropyrimidine drugs.

Life-threatening toxicity of oral tegafur-uracil (UFT) plus leucovorin

UFT. a a combination of tegafur and uracil, is increasingly used in the treatment of gastrointestinal malignancies. We report on a life-threatening toxicity observed in a 74-year-old female while undergoing adjuvant treatment with oral UFT plus leucovorm for adenocarcinoma of the rectum. The patient developed grade 4 thrombocytopenia, grade 4 neutropenia, grade 3 infection, grade 3 neurotoxicity and grade 2 cutaneous toxicity. This oral double-agent regimen should be used with caution in the adjuvant treatment of elderly patients with gastro-intestinal malignancies.

Experimental chemotherapeutic agents for the treatment of colorectal carcinoma

Colorectal cancer is the third leading cause of cancer mortality for men and women in the United States. For the past 40 years fluorouracil has been the only agent with significant activity in this disease. More recently, advances in our understanding of the molecular biology of cancer have permitted the development of effective new agents for this disease. This article examines the current and future status of these new agents.

Simultaneous determination of dihydrofluorouracil and 5-fluorouracil in plasma by high-performance liquid chromatography

Dihydrofluorouracil (FUH2) is the product of the first rate-limiting step in catabolism of 5-fluorouracil (5-FU), catalyzed by the enzyme dihydropyrimidine dehydrogenase. In humans, more than 80% of administered 5-FU is degraded through this catabolic pathway. The ability to measure FUH2 and 5-FU simultaneously may provide an index of the extent to which 5-FU is catabolized. A sensitive and efficient extraction and HPLC method has been developed for simultaneous measurement of FUH2 and 5-FU in patients' plasma. Trichloroacetic acid precipitation of plasma proteins was followed by extraction into ethyl acetate, evaporation under nitrogen, and reconstitution in phosphate buffer. The extract was analyzed by isocratic chromatography using a C18 reversed-phase column with uv detection at 268 nm (5-FU) and 220 nm (FUH2). The detection limit is 0.005 nmol on column for aqueous standards or 0.20 microM in 1 ml of plasma standards for both compounds. This method can be applied to pharmacokinetic studies of 5-FU in patients and may be useful as a means of assessing the activity of dihydropyrimidine dehydrogenase.