Quantitative Contribution of rs75017182 to Dihydropyrimidine Dehydrogenase mRNA Splicing and Enzyme Activity

Dihydropyrimidine dehydrogenase (DPD; DPYD gene) variants have emerged as reliable predictors of adverse toxicity to the chemotherapy agent 5-fluorouracil (5-FU). The intronic DPYD variant rs75017182 has been recently suggested to promote alternative splicing of DPYD. However, both the extent of alternative splicing and the true contribution of rs75017182 to DPD function remain unclear. In the present study we quantified alternative splicing and DPD enzyme activity in rs75017182 carriers utilizing healthy volunteer specimens from the Mayo Clinic Biobank. Although the alternatively spliced transcript was uniquely detected in rs75017182 carriers, canonically spliced DPYD levels were only reduced by 30% (P = 2.8 × 10^-6 ) relative to controls. Similarly, DPD enzyme function was reduced by 35% (P = 0.025). Carriers of the well-studied toxicity-associated variant rs67376798 displayed similar reductions in DPD activity (31% reduction). The modest effects on splicing and function suggest that rs75017182 may have clinical utility as a predictor of 5-FU toxicity similar to rs67376798.

Novel Deleterious Dihydropyrimidine Dehydrogenase Variants May Contribute to 5-Fluorouracil Sensitivity in an East African Population

Clinical studies have identified specific genetic variants in dihydropyrimidine dehydrogenase (DPD; DPYD gene) as predictors of severe adverse toxicity to the commonly used chemotherapeutic 5-fluorouracil (5-FU); however, these studies have focused on European and European-American populations. Our laboratory recently demonstrated that additional variants in non-European haplotypes are predictive of 5-FU toxicity. The objective of this study was to identify potential risk variants in an understudied East African population relevant to our institution's catchment area. The DPYD protein-coding region was sequenced in 588 individuals of Somali or Kenyan ancestry living in central/southeast Minnesota. Twelve novel nonsynonymous variants were identified, seven of which significantly decreased DPD activity in vitro. The commonly reported toxicity-associated variants, *2A, D949V, and I560S, were not detected in any individuals. Overall, this study demonstrates a critical limitation in our knowledge of pharmacogenetic predictors of 5-FU toxicity, which has been based on clinical studies conducted in populations of limited diversity.

Clinical Pharmacogenetics Implementation Consortium guidelines for dihydropyrimidine dehydrogenase genotype and fluoropyrimidine dosing

The fluoropyrimidines are the mainstay chemotherapeutic agents for the treatment of many types of cancers. Detoxifying metabolism of fluoropyrimidines requires dihydropyrimidine dehydrogenase (DPD, encoded by the DPYD gene), and reduced or absent activity of this enzyme can result in severe, and sometimes fatal, toxicity. We summarize evidence from the published literature supporting this association and provide dosing recommendations for fluoropyrimidines based on DPYD genotype (updates at http://www.pharmgkb.org).

Epigenetic mechanisms of protein tyrosine phosphatase 6 suppression in diffuse large B-cell lymphoma: implications for epigenetic therapy

Protein tyrosine phosphatases such as PTPN6 can be downregulated in various neoplasms. PTPN6 expression by immunohistochemistry in 40 diffuse large B-cell lymphoma (DLBCL) tumors was lost or suppressed in 53% (21/40). To elucidate the molecular mechanisms of PTPN6 suppression, we performed a comprehensive epigenetic analysis of PTPN6 promoter 2 (P2). None of the DLBCL primary tumors (0/37) had PTPN6 hypermethylation on the CpG1 island using methylation-specific PCR, pyrosequencing, and high-resolution melting assays. However, hypermethylation in 57% (21/37) of cases was found in a novel CpG island (CpG2) in P2. PTPN6 gene suppression was reversed by 5-aza-deoxycytidine (5-Aza), a DNA methyltransferase inhibitor, and the histone deacetylase inhibitor (HDACi) LBH589. LBH589 and 5-Aza in combination inhibited DLBCL survival and PTPN6 hypermethylation at CpG2. The role of histone modifications was investigated with a chromatin-immunoprecipitation assay demonstrating that PTPN6 P2 is associated with silencing histone marks H3K27me3 and H3K9me3 in DLBCL cells but not normal B cells. 3-Deazaneplanocin A, a histone methyltransferase inhibitor, decreased the H3K27me3 mark, whereas HDACi LBH589 increased the H3K9Ac mark within P2 resulting in re-expression of PTPN6. These studies have uncovered novel epigenetic mechanisms of PTPN6 suppression and suggest that PTPN6 may be a potential target of epigenetic therapy in DLBCL.

Prognostic significance of thymidylate synthase, dihydropyrimidine dehydrogenase and thymidine phosphorylase protein expression in colorectal cancer patients treated with or without 5-fluorouracil-based chemotherapy

BACKGROUND

Low tumour expression levels of thymidylate synthase (TS), dihydropyrimidine dehydrogenase (DPD) and thymidine phosphorylase (TP) have been linked with improved outcome for colorectal cancer (CRC) patients treated with 5-fluorouracil (5-FU). It is unclear whether this occurs because such tumours have better prognosis or they are more sensitive to 5-FU treatment.

PATIENTS AND METHODS

Associations between TS, DPD and TP levels, determined by tissue microarrays and immunohistochemistry, and survival was evaluated in 945 CRC patients according to treatment status.

RESULTS

Low TS and DPD expression associated with worse prognosis in stage II [hazard ratio (HR) = 1.69, 95% confidence interval (CI) (1.09-2.63) and HR = 1.92 (95% CI 1.23-2.94), respectively] and stage III CRC patients treated by surgery alone [HR = 1.39 (95% CI 0.92-2.13) and HR = 1.49 (95% CI 1.02-2.17), respectively]. Low TS, DPD and TP associated with trends for better outcome in stage III patients treated with 5-FU [HR = 0.81 (95% CI 0.49-1.33), HR = 0.70 (95% CI 0.42-1.15) and HR = 0.66 (95% CI 0.39-1.12), respectively].

CONCLUSION

Low TS and DPD expression are prognostic for worse outcome in CRC patients treated by surgery alone, whereas low TS, DPD and TP expression are prognostic for better outcome in patients treated with 5-FU chemotherapy. These results provide indirect evidence that low TS, DPD and TP protein expression are predictive of good response to 5-FU chemotherapy.

Prognostication of pancreatic adenocarcinoma by expression of thymidine phosphorylase (TP) and its correlation with survival

4606

Background: Preclinical studies have indicated that TP is one of the most useful markers of tumor response to capecitabine; with elevated TP expression resulting in higher intratumor levels of 5-FU. TP level in the tumor may be associated directly with survival and may be up-regulated by XRT. Methods: Twenty patients (pts) who had newly diagnosed locally advanced pancreatic cancer, age ≥ 19 years, ECOG PS 0–2, and adequate organ function received 50.4 Gy XRT with capecitabine 1,600 mg/m2 M-F x 6 wks determined from our phase I study (JCO, Dec 2005). Following capecitabine-XRT, stable and responding pts received capecitabine 2,000mg/m2 x 14 days every 3 wks till progression. Restaging was performed every 9 wks. Tumor specimens were procured with EUS-FNA prior and week 2 after starting capecitabine-XRT to evaluate TP mRNA levels by RT-PCR. Results: TP levels were elevated post-XRT when compared to pre-XRT TP (p= 0.01). Each of the 20 pts had at least one observation of TP. Mean TP level of each pt was obtained by averaging the pre-XRT and post-XRT TP levels. If one value was missing, then the other value was used for the mean TP level. Mean TP level was significantly associated with survival using the Cox proportional hazards model (p= 0.0072). Pts were separated into two groups - those with high baseline TP level and those with lower TP level with a cut off value of 213.85 using median of mean TP level. It was observed that group with higher TP levels had better survival as compared to the group with lower TP levels. However, the survival curves of these two groups crossed over after 18 months. Conclusions: Our study suggests that the higher the TP level in tumor post-XRT, the better the survival. This is most probably due to a higher accumulation of 5-FU in tumor due to up-regulation of TP. Possible explanation for the two groups crossing over after 18 months may include heterogeneity in TP due to a difference in pharmacogenomics or idiosyncratic drug toxicity profile of capecitabine. Further studies of long-term effects of treatment with a larger number of pts are needed to study exact effects of capecitabine and XRT, and to evaluate the differential role of TP with survival.

[Table: see text]

Elucidating the mechanisms responsible for the previous failure of phase III clinical trials with eniluracil (EU) and development of a novel scheduling approach to optimize the efficacy of EU/5-fluorouracil (5-FU) combination therapy

2557

Background: Irreversible inhibition of dihydropyrimidine dehydrogenase (DPD) by EU blocks 5-FU catabolism allowing for oral 5-FU administration with complete bioavailability. Unfortunately, phase III trials with co-administered EU/5-FU showed inferiority vs. 5-FU/leucovorin, and were discontinued. We recently reported that competitive inhibition of human uridine phosphorylase (UP) and thymidine phosphorylase (TP) 5-FU-anabolic enzymes by EU is an important mechanism potentially responsible for clinical failure of the combined EU/5- FU regimen. We hypothesize that EU inhibition of UP and TP is transient, while that of DPD is prolonged, allowing for novel schedule dependent optimization of EU/5-FU dosing regimens with improved efficacy. Methods: In this phase I study, five patients received a single oral dose (2 mg, 5 mg or 10 mg) of EU 12–14 hours prior to scheduled resection of primary/metastatic colorectal cancer. Dosage was as follows: Two patients received the 2 mg dose, one patient received the 5 mg dose and two patients received the 10 mg dose. Matched normal and tumor tissue biopsies were immediately snap frozen and subsequently UP, TP and DPD activity was measured in vitro via HPLC detection of [6- 14C]-5-FU catabolites/anabolites. Peripheral blood mononuclear cell (PBMC) DPD activity was determined at baseline prior to EU administration, 30 min prior to surgery (Day 1), and on Days 2, 5 and 14 following EU administration. Results: At 12–14 hours following EU administration, there was an absence of inhibition of UP and TP, while DPD was significantly inhibited in matched tumor and normal tissue. Importantly, PBMC DPD activity was significantly inhibited by EU on Day 1 (12–14 hours after EU administration) and Day 2 (36 hours after EU administration) at 0 ± 0% and 17 ± 11% (mean ± SD) of baseline, respectively. Conclusions: These data demonstrate a differential recovery time of EU mediated inhibition of UP and TP compared to DPD, which permits future schedule dependent optimization of EU/5-FU therapy.

No significant financial relationships to disclose.

The 2-13C-5-fluorouracil breath test (FUBT) as a novel, rapid method for assessment of dihydropyrimidine dehydrogenase (DPD) activity in cancer patients: Initial characterization and comparison to the 2-13C-uracil breath test (UraBT)

2551

Background: The UraBT is currently in development as a phenotypic test to screen for DPD deficiency. Following an oral dose of 2-13C-uracil, the UraBT shows a significant relationship between breath 13CO2 metabolite formation and plasma 2-13C-uracil and 2-13C-dihydrouracil pharmacokinetics. We herein describe a novel, potentially more clinically relevant test in which a small oral dose of 2-13C-5-fluorouracil (5-FU) is administered, followed by assessment of breath 13CO2 metabolite formation as previously described for the UraBT. We hypothesize that the FUBT can rapidly assess interindividual variability in 5-FU catabolism and predisposition to 5-FU toxicity. Methods: Over two sessions separated by a seven day washout, a single dose (6mg/kg, p.o.) of 2-13C-uracil or 2-13C-5-FU was administered to patients with stage III-IV colorectal cancer (n = 4). Subsequent to drug administration, in each session, 13CO2 catabolite formation was quantified in the breath over eight hours. In a separate investigation over two sessions separated by a seven day washout, a single dose (3mg/kg, p.o.) of 2-13C-uracil or 213C-5-FU was administered to colorectal cancer patients with previously documented severe (n=2) or moderate (n=2) 5-FU dose-related hematological/gastrointestinal toxicity. Following drug administration 13CO2 catabolite formation was quantified over eight hours. 13CO2 concentration was expressed as Delta Over Baseline (DOB) in all sessions. Results: Compared to the UraBT, the FUBT showed an increased Cmax (50.7 ± 6.6 DOB/mg vs. 36.8 ± 7.8 DOB/mg; mean ± SD) and decreased Tmax (25 ± 4 min vs. 45 ± 6 min) for 13CO2 formation (p<0.05). The FUBT was able to distinguish patients with previously reported severe and moderate 5- FU toxicity, with 13CO2 Cmax values of 35.5 ± 9.5 DOB/mg (mean ± SD) and 59.8 ± 7.3 DOB/mg, respectively. Importantly, FUBT Cmax values positively correlated with DPD activity (rs=1.00, p<0.01). Conclusions: These data lend support to further development of the FUBT as a rapid and informative test to assess DPD activity and to predict susceptibility to severe dose-related 5-FU toxicity. [CA116964]

No significant financial relationships to disclose.

Dose dependent inhibition of uridine phosphorylase (UP) by eniluracil (EU): Was the clinical inferiority of the EU/5-fluorouracil (5-FU) phase III trials due to an unrecognized inhibition of 5-FU anabolism?

2058

Background: Eniluracil (EU) is an irreversible inactivator of dihydropyrimidine dehydrogenase (DPD). EU inhibits DPD dependent 5-fluorouracil (5-FU) catabolism, allowing for oral 5-FU administration with essentially complete bioavailability. Although earlier murine studies suggested an increased antitumor effect when EU was administered before 5-FU, clinical studies used co-administered EU and 5-FU (ratio: 10 EU:1 5-FU) (b.i.d.). These phase III trials demonstrated inferiority compared to a regimen of 5-FU/leukovorin, leading to discontinuation of EU development. We hypothesize that the clinical failure of this EU/5-FU schedule may have resulted from competitive inhibition of 5-FU anabolic (ANA) activation by EU. In this study we examined whether EU could competitively inhibit uridine phosphorylase (UP) or orotate phosphoribosyl transferase (OPRT), the primary ANA enzymes of 5-FU. Methods: The cytoplasmic fraction of human embryonic kidney cells (HEK-293) was used as the source of UP and OPRT. Reverse phase HPLC with radioactivity detection was used to quantify [2-14C]-uracil formation from [2-14C]-uridine (UP activity) and to quantify [6-14C]-FUMP formation from [6-14C]-5-FU (OPRT activity). For UP activity, reaction mixtures consisted of increasing concentrations of EU in phosphate buffer containing 150 μM [2-14C]-uridine. For OPRT activity, reaction mixtures consisted of increasing concentrations of EU in phosphate buffer containing 5 μM [6-14C]-FU and 100 μM benzylacyclouridine (UP inhibitor). Reactions were initiated with addition of UP/OPRT enzyme source and allowed to proceed for 30 min at 37°C. Results: EU displayed dose-dependent competitive inhibition of UP activity (IC50 = 0.375 mM). In particular, the EU/5-FU ratio of 2.5:1 produced approximately 50% inhibition of UP activity. In contrast, EU did not inhibit OPRT activity. Conclusions: This study demonstrates that EU competitively inhibits UP, an important enzyme in 5-FU ANA activation, and suggests that changing the dose ratio and/or increasing the interval between EU and 5-FU administration may be useful in optimizing 5-FU antitumor efficacy.

[Table: see text]

Prognostic and predictive significance of 5-fluorouracil metabolic enzymes in colorectal cancer

2021

Background: Low levels of the 5-Fluorouracil (5-FU) metabolic enzymes, thymidylate synthase (TS), dihydropyrimdine dehydrogenase (DPD) and thymidine phosphorylase (TP) are linked to improved survival in colorectal cancer (CRC) patients receiving 5-FU chemotherapy. However, whether they are prognostic (related to tumor biology) or predictive (drug sensitivity) indicators is not clarified. This study aimed to (1) discriminate the prognostic and predictive significance of TS, DPD and TP and (2) ascertain the clinical and molecular subtype associations of the proteins in a large sample series. Methods: Tissue arrays containing sections from 956 stage 2/3 CRC cases were stained immunohistochemically for TS, DPD and TP. Associations with clinical and molecular characteristics and survival according to treatment status were assessed by Kruskall-Wallis and Kaplan-Meier analysis. Results: Low TS levels were associated with late stage, proximal tumor location and absence of microsatellite instability, low DPD with younger age and late stage and low TP with proximal tumor location. Low levels of all three enzymes associated with absence of tumor infiltrating lymphocytes. There were no associations with gender, grade or ras and p53 mutation. In stage 2 patients treated by surgery alone, those with low DPD levels had a worse survival than those with high levels (p<0.01). Stage 3 patients treated with chemotherapy had a better survival than those without in subgroups of patients with low TS (p=0.05), DPD (p<0.01) and TP (p=0.03) but not high levels of the proteins. Conclusions: Our results suggest the improved outcome of 5FU-treated patients with low tumor TS, DPD and TP levels may be due primarily to a benefit from adjuvant treatment rather than a favorable prognosis. TS, DPD and TP levels may be useful indicators for identifying CRC patients likely to benefit from 5-FU treatment.

No significant financial relationships to disclose.

Dihydropyrimidine dehydrogenase deficiency (DPD) in GI malignancies: Experience of 4 years

2056

Background: 5-Fluorouracil (5-FU) is an integral part of treatment of GI malignancies. While normal DPD enzyme activity is rate limiting in 5-FU catabolism, its deficiency could increase concentrations of bioavailable 5-FU anabolic products leading to 5-FU related toxicity syndrome. With DPD deficiency, 5-FU is discontinued. Data regarding safety of capecitabine (CAP) in this population is scarce. Methods: Patients were tested for DPD deficiency after excessive toxicities from 5-FU and CAP at UAB between 2001 and 2005. DPD activity was evaluated by PBMC radio assay, genotyping of DPYD gene by DHPLC, or 2-13C uracil breath test (UraBT). Results: Of 23 patients with GI malignancies (small intestine, gastric, pancreatic, HCC, and colorectal) evaluated, 7 (30%) were DPD deficient. Among these 7 patients, DPD activity ranged from 0.064 - 0.18 nmol/min/mg. Age ranged from 51–75 years, M:F ratio = 1.3:1, and ethnicities included Caucasian (71%), African-American (14%) and South-Asian (14%). Four were treated with 5-FU/LV (2 Roswell; 2 Mayo); 2 CAP (1800mg/m2); and 2 high dose bolus 5-FU (1400mg/m2) + PN401 (tri-acetyluridine). Toxicities included mucositis (71%), diarrhea (43%), nausea (29%), memory loss/altered mental status (43%), cytopenias (43%), hypotension (14%), respiratory distress (14%), acute renal failure (14%), and severe skin rashes (43%). Re-challenge with CAP in 1 patient after the Mayo regimen caused grade 3 HFS only on dorsal surfaces of hands. One patient on PN401 had a grade 3 facial rash as the worst toxicity. Genotypic analysis of the DPYD gene in the second on PN401, who had severe leucopenia, demonstrated a heterozygous mutation (IVS14+1 G>A, DPYP*2A). UraBT in 2 patients revealed 1 to be DPD-deficient (DOB50 of 112.8; PDR of 49.4%) and borderline normal values (DOB50 of 130.9; PDR of 52.5%) in a second patient. There were 2 toxicity-related deaths (28%): 1 on CAP and 1 on 5-FU + PN401. Conclusions: DPD deficiency was observed in several ethnicities. Patients with CAP toxicities should also be tested for DPD deficiency. Role of PN401 in rescuing 5-FU toxicity in DPD deficiency is not clear. Screening patients for DPD deficiency prior to administration of 5-FU or CAP, using UraBT, could potentially lower risk of toxicity. Future studies should validate this technique.

[Table: see text]

Molecular basis of altered uracil catabolism in individuals with 5-FU toxicity and normal DPD enzyme activity

3070

Background: Dihydropyrimidine dehydrogenase (DPD) deficiency accounts for approximately 43% of grade 3–4 toxicity to 5-Fluorouracil (5-FU). However, a significant number of patients with normal DPD enzyme activity remain with unexplained molecular basis of 5-FU toxicity. It has been suggested by the few cases previously reported that deficiency of dihydropyrimidinase (DHP)enzyme encoded by the DPYS gene and/or beta-ureidopropionase enzyme, encoded by the BUP-1 gene, could also be implicated in 5-FU toxicity. Methods: This study included 40 volunteers with known 13C-UraBT and DPD enzyme activity, 25 cancer patients with 5-FU toxicity despite normal DPD enzyme activity, and 25 liver biopsies from cancer patients with different grades of toxicity. All samples were analyzed for molecular defects in the DPYS and BUP-1 genes, using DHPLC and RT-PCR techniques. Results: Molecular analysis of the DPYS gene revealed the presence of two non-conservative amino acid changes, one frame-shift mutation that leads to a stop codon and premature termination of the DHP protein, five silent mutations, nine intronic sequence variations, two sequence variations in the 5’UTR and one in the non-coding region of exon 10. Molecular analysis of the BUP-1 gene revealed the presence of two non-conservative amino acid changes, one of which (314C>A: A85E) has already been reported to abolish enzyme activity; six silent mutations, four intronic sequence variations, one polymorphism in the 5’upstream sequence, and one sequence variation in each of the 5’UTR and the 3’UTR. Conclusions: The molecular basis of 5-FU toxicity is not limited to DPD deficiency; since molecular defects in genes downstream of DPD can potentially also impair 5-FU catabolism. Genetic testing for molecular defects in DPYS and BUP-1 may predict patients at risk of developing 5-FU toxicity despite having normal DPD enzyme activity. Assessing the integrity of the entire uracil catabolic pathway might be crucial to avoid toxicity in a significant group of patients receiving 5-FU or a related drug (CA62164).

No significant financial relationships to disclose.

Upregulation of thymidine phosphorylase (TP) by radiation (XRT): Phase II study of capecitabine (CAP) with XRT in pts with locally advanced (LA) pancreatic cancer

14001

Background: Preliminary studies have shown that XRT unregulates TP, the main enzyme responsible for activity of CAP. The objectives of this phase II study were to further evaluate effect of XRT on TP, DPD, and TNF-alpha as well as response and efficacy of CAP with concurrent XRT in pts with LA pancreatic cancer. Methods: Pts received 50.4 Gy XRT with CAP at 1600 mg/m2 M-F × 6 wks determined from our phase I study (JCO, Dec 2005). Following CAP-XRT, stable and responding pts on CT scan were treated with CAP 2000mg/m2 × 14 days q 3 wks till progression. Restaging was performed every 9 wks. Tumor specimens were procured with EUS-FNA 1 wk prior and 2 wks after CAP-XRT to evaluate TP, DPD, and TNF-alpha mRNA levels by RT-PCR. A sample size of 20 was selected (mean 1 vs. S with mean 0=13.5; alpha = 0.05; power = .99; t-test). Results: 19 pts (median age: 67; M/F: 7/12) were enrolled at UAB between March 2004 and June 2005. 4 pts (21%) had confirmed partial responses and 13 (68%) had stable disease. 2 pts underwent surgery (Ro in 1; extensive fibrosis in 1). Six-month survival rate was 89%. We have not met enough deaths to estimate median survival. Grade 3 and 4 toxicities included: nausea/vomiting (5%), thrombosis (5%), hyperbilirubinemia (5%), and grade 3 GI bleeding (5%). No hematological toxicities except grade 1 thrombocytopenia (5%) and grade 1 anemia (10%). All pts completed full CAP-XRT with 3 dose reductions (by 20%). Pts received mean of 5.4 cycles (range: 3–15) of CAP alone with a total of 104 cycles with dose reductions in 7 cycles (by 25%). TP was elevated during wk 2 when compared to pre-XRT TP (P = .005) but DPD was not (P = .13) nor was TNF-alpha (P = .37). No correlation between TP and TNF-alpha was noticed. No association between TP/DPD ratio and efficacy of CAP was identified. Tumor TP expression was higher (183.16) in pt with GI bleeding. Conclusions: This Phase II study further confirms our Phase I results that CAP-XRT is an effective, tolerable, and an easy alternative to infusional 5-FU regimen for pts with LA pancreatic cancer. Also TP upregulation by XRT was statistically significant. While these results support use of CAP-XRT in pancreatic cancer, there appears to be additional genes (other than TP, DPD) associated with response to CAP and CAP-XRT.

[Table: see text]

Retroviral transduction of human dihydropyrimidine dehydrogenase cDNA confers resistance to 5-fluorouracil in murine hematopoietic progenitor cells and human CD34+-enriched peripheral blood progenitor cells

Severe 5-fluorouracil (5-FU) toxicity has been reported among patients lacking dihydropyrimidine dehydrogenase (DPD) enzymatic activity. DPD is the principal enzyme involved in the degradation of 5-FU to 5'-6'-dihydrofluorouracil, which is further metabolized to fluoro-beta-alanine. We demonstrate here that overexpression of human DPD confers resistance to 5-FU in NIH3T3 cells, mouse bone marrow cells, and in human CD34+-enriched hematopoietic progenitor cells. An SFG-based dicistronic retroviral vector containing human DPD cDNA, an internal ribosomal entry site (IRES), and the neomycin phosphotransferase (Neo) gene was constructed (SFG-DPD-IRES-Neo). Transduced NIH3T3 cells demonstrated a 2-fold (ED50) increase in resistance to a 4-hour exposure of 5-FU in comparison to nontransduced cells. Expression of DPD was confirmed by Northern and Western blot analyses, and DPD enzyme activity was detectable only in transduced cells. Infection of mouse bone marrow cells with this retroviral construct resulted in an increased number of 5-FU-resistant CFU-GM colonies, compared to mock-transduced bone marrow in both 4-hour and 12- to 14-day exposures. Infection of human CD34+-enriched cells with this construct and incubation with 5-FU (10(-6) M) for 14 days also resulted in an increased number of 5-FU-resistant colonies. Retroviral transduction of human hematopoietic progenitor cells with a cDNA-expressing human DPD conferred resistance to 5-FU in NIH3T3 cells, mouse bone marrow cells, and human CD34+-enriched cells. These results encourage the use of this gene as a method to protect patients from 5-FU myelotoxicity.

Role of matrix metalloproteinases in colorectal carcinogenesis

OBJECTIVE

To measure coexpression of matrix metalloproteinase (MMP)-2, MMP-7, and MMP-9 genes by real-time quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR) in benign and malignant phases of colorectal carcinogenesis.

SUMMARY BACKGROUND DATA

Matrix metalloproteinases degrade and remodel the extracellular matrix and have been implicated in facilitating carcinoma cells to invade and metastasize. MMP-2, MMP-7, and MMP-9 have been shown to be overexpressed in various carcinomas; however, simultaneous examination of these enzymes in human normal mucosa, adenoma, and carcinoma has not been performed to date.

METHODS

Between January 1, 1998, and June 15, 2000, 40 patients underwent colectomy and harvest and snap-freezing of normal mucosa, adenoma, and carcinoma. Five patients had adenoma and carcinoma in the same specimen; 35 had either adenoma (n = 6) or carcinoma (n = 29). Taqman qRT-PCR methodology was used to measure MMP gene copy number and normalized to beta-actin RNA expression.

RESULTS

The mean age was 62 +/- 4 years, with 22 men and 18 women. One fifth of the adenomas exhibited severe dysplasia. MMP-7 gene expression was significantly increased in adenomas (43 times normal mucosa) but did not increase further in carcinomas (50 times normal mucosa). MMP-2 and MMP-9 were not different in adenomas (1.8 and 1.4 times normal mucosa, respectively) but were elevated in carcinomas (2.2 and 1.8 times normal mucosa, respectively). There was no correlation between size or dysplasia in adenomas or AJCC stage in carcinomas and MMP gene expression.

CONCLUSIONS

Overexpression of MMP-7 is an early event in the adenoma-to-carcinoma pathway, and expression does not appear to increase further in carcinomas. MMP-2 and MMP-9 appear to be primarily overexpressed in carcinomas. This may be one mechanism by which adenoma cells gain the ability to invade and carcinoma cells to metastasize.

Current status of oral chemotherapy for colorectal cancer

The treatment of advanced colorectal cancer over the past 4 decades has required the use of intravenous chemotherapy, most typically fluorouracil (5-FU). The possibility of providing an alternative to intravenous delivery while at the same time improving the quality of life of patients who require fluorouracil for advanced or adjuvant therapy has provided the stimulus for the development of oral fluoropyrimidine drugs. Five oral fluoropyrimidine drugs have recently entered clinical trials in the United States. These include capecitabine (Xeloda), UFT (uracil and tegafur) or UFT/leucovorin (Orzel), eniluracil (ethynyluracil), S-1, and BOF A-2. At least two of these drugs have demonstrated survival equivalent to the standard intravenous fluorouracil and leucovorin regimens used to treat advanced colorectal cancer. This, together with less severe toxicity and potential increased quality of life, should lead to approval of one or more of these oral agents in the near future. Based on both patient and physician acceptance of oral fluoropyrimidines, other oral drugs from classes other than fluoropyrimidines will likely be developed in the near future.

Clinical implications of dihydropyrimidine dehydrogenase on 5-FU pharmacology

Dihydropyrimidine dehydrogenase (DPD) is the initial rate-limiting enzyme in the catabolism of 5-fluorouracil (5-FU), accounting for catabolism of over 85% of an administered dose of 5-FU. DPD plays an important role in regulating the availability of 5-FU for anabolism. DPD also accounts for much of the variability observed with the therapeutic use of 5-FU. This includes variable 5-FU levels over 24 hours during a continuous infusion; the widely reported variability in the pharmacokinetics of 5-FU; the observed variable bioavailability that led to the recommendation that 5-FU not be administered as an oral agent; and lastly, the observed variability in both toxicity and drug response (resistance) after identical 5-FU doses. Knowledge of the DPD level, as well as the levels of other potentially important molecular markers (e.g., thymidylate synthase), may permit adjustments or modulation of the 5-FU dose that can result in an increase in the therapeutic efficacy of 5-FU.

Molecular cloning and characterization of the human dihydropyrimidine dehydrogenase promoter

Several studies have demonstrated that dihydropyrimidine dehydrogenase (EC 1.3.1.2) has a critical role in the pharmacokinetics of the anticancer agent 5-fluorouracil. We previously reported the structural organization of the human DPYD gene. In this article, we describe the molecular cloning and functional characterization of 1.2 kb of the 5' flanking region of the DPYD gene. Sequence analysis demonstrated that this region of the DPYD gene lacks the typical TATA or CCAAT boxes with several GC-rich regions containing potential cis-regulatory elements. Progressive 5' deletions of the 5' flanking region were fused to the luciferase reporter gene and transient expression measured following transfection into HeLa and 293 cells. Comparative analysis of luciferase activity revealed that a 208 bp region of the DPYD gene (-121/+86) contained equivalent transcriptional activity to the complete 1.2 kb 5' flanking region of the DPYD gene. Site-directed mutagenesis of the luciferase reporter constructs demonstrated that the -72/-23 sequence contained two regulatory regions (designated elements I and II) essential for promoter activity. Gel shift experiments demonstrated that both regulatory elements specifically bind with protein(s) from nuclear extracts of 293 cells. Competitive binding experiments with 293 nuclear extracts and radiolabeled oligonucleotides (corresponding to elements I and II) suggest that the same protein(s) bind to both regulatory elements. We conclude that constitutive expression of the DPYD gene involves a limited GC-rich region of the 5' flanking sequence of the DPYD gene which contains two regulatory elements.

Activation of protein kinase C induces nuclear translocation of RFX1 and down-regulates c-myc via an intron 1 X box in undifferentiated leukemia HL-60 cells

Treatment of human promyelocytic leukemia cells (HL-60) with phorbol 12-myristate 13-acetate (PMA) is known to decrease c-myc mRNA by blocking transcription elongation at sites near the first exon/intron border. Treatment of HL-60 cells with either PMA or bryostatin 1, which acutely activates protein kinase C (PKC), decreased the levels of myc mRNA and Myc protein. The inhibition of Myc synthesis accounted for the drop in Myc protein, because PMA treatment had no effect on Myc turnover. Treatment with PMA or bryostatin 1 increased nuclear protein binding to MIE1, a c-myc intron 1 element that defines an RFX1-binding X box. RFX1 antiserum supershifted MIE1-protein complexes. Increased MIE1 binding was independent of protein synthesis and abolished by a selective PKC inhibitor, which also prevented the effect of PMA on myc mRNA and protein levels and Myc synthesis. PMA treatment increased RFX1 in the nuclear fraction and decreased it in the cytosol without affecting total RFX1. Transfection of HL-60 cells with myc reporter gene constructs showed that the RFX1-binding X box was required for the down-regulation of reporter gene expression by PMA. These findings suggest that nuclear translocation and binding of RFX1 to the X box cause the down-regulation of myc expression, which follows acute PKC activation in undifferentiated HL-60 cells.

Oral DPD-inhibitory fluoropyrimidine drugs

Over the past decade, increasing data have emphasized both the importance of dihydropyrimidine dehydrogenase (DPD), the initial, rate-limiting enzyme in the catabolism of fluorouracil (5-FU), and its role as a control step in 5-FU metabolism, regulating the availability of 5-FU for anabolism. It is now clear that DPD also accounts for much of the variability observed with therapeutic use of 5-FU, including variabilities in 5-FU levels over a 24-hour infusion, interindividual pharmacokinetics, bioavailability, toxicity, and drug response (resistance). This variability makes effective dosing of 5-FU and related drugs difficult. In order to lessen this variability, and potentially improve 5-FU pharmacology, the pharmaceutical industry has made an effort to develop DPD inhibitors to modulate 5-FU metabolism, which has resulted in the creation of a new subclass of orally administered fluoropyrimidines, known as DPD-inhibiting fluoropyrimidines (DIF). Four drugs--uracil and tegafur (UFT) or the combination of UFT and leucovorin, ethynyluracil (eniluracil), S-1, and BOF-A2--have recently undergone clinical evaluation in the United States. The biochemical basis for using these drugs is reviewed.

The role of pharmacogenetics and pharmacogenomics in cancer chemotherapy with 5-fluorouracil

There is increasing evidence supporting the important role of genetics in determining the effect (response and toxicity) to cancer chemotherapy. This has included both pharmacogenetics, where the alteration of a gene coding for an important drug metabolizing enzyme results in increased toxicity (and occasionally altered efficacy), and pharmacogenomics, where knowledge of the expression of genes critical to the action of the cancer chemotherapy drug can be used to individualize therapy. This manuscript focuses on the widely used cancer chemotherapy drug 5-fluorouracil (5-FU) to illustrate the following concepts: (1) The effect of the pharmacogenetic syndrome known as dihydropyrimidine dehydrogenase (DPD) deficiency on 5-FU pharmacology; (2) the role of pharmacogenomics in individualizing 5-FU therapy, and (3) the potential value of pharmacogenomics in designing new drugs.

Improving fluorouracil chemotherapy with novel orally administered fluoropyrimidines

During the 40 years since the initial synthesis of fluorouracil, there have been many attempts to improve fluoropyrimidine chemotherapy. These have included the utilisation of different schedules of fluorouracil administration, modulation of the metabolism of fluorouracil with other drugs to increase its therapeutic benefit, and the synthesis of prodrugs of fluorouracil that are potentially more effective and less toxic. Of particular interest at present is the clinical evaluation of several new fluoropyrimidine drugs that can be orally administered. These include capecitabine, tegafur/uracil (UFT), eniluracil (GW-776C85; 5-ethynyluracil), S-1, and emitefur (BOF-A2). The pharmacological principles that have influenced the development of these new drugs are initially presented. This is followed by a review of our current knowledge of the clinical pharmacology of each of these new agents, focusing on antitumour activity and toxicity from studies conducted in the US. Studies of capecitabine, tegafur/uracil, and early studies with eniluracil indicate that these drugs have at least similar activity to protracted fluorouracil infusion but with additional quality-of-life and economic benefits.

Colorectal tumors responding to 5-fluorouracil have low gene expression levels of dihydropyrimidine dehydrogenase, thymidylate synthase, and thymidine phosphorylase

We had previously shown that high gene expressions (mRNA levels) of thymidylate synthase (TS; Leichman et al., J. Clin. Oncol., 15: 3223-3229, 1997) and thymidine phosphorylase (TP; Metzger et al., Clin. Cancer Res., 4: 2371-2376, 1998) in pretreatment tumor biopsies could identify tumors that would be nonresponsive to 5-fluorouracil (5-FU)-based therapy. In this study, we investigated the association between intratumoral gene expression of the pyrimidine catabolism enzyme dihydropyrimidine dehydrogenase (DPD) and the response of colorectal tumors to the same 5-FU-based protocol. DPD expressions were measured by quantitative reverse transcription-PCR in 33 pretreatment biopsies of colorectal tumors from patients who went on to receive treatment with 5-FU and leucovorin (LV). The range of DPD gene expression in those tumors that were nonresponsive to 5-FU was much broader than that of the responding tumors. None of the tumors with basal-level DPD expressions above a DPD:beta-actin ratio of 2.5 x 10(-3) (14 of 33) were responders to 5-FU/LV therapy, whereas those tumors with DPD gene expressions below DPD: beta-actin ratio of 2.5 x 10(-3) had a response rate of 50%. There was no correlation among DPD, TS, and TP expression values in this set of colorectal tumors, which indicated that these gene expressions are independent variables. All of the tumors that responded to 5-FU therapy (11 of 33) had expression values of all three of the genes, TS, TP, and DPD, below their respective nonresponse cutoff values, whereas, in each of the nonresponding tumors, at least one of these gene expressions was high. The patients with low expression of all three of the genes had significantly longer survival than patients with a high value of any one of the gene expressions. The results of this study show that intratumoral gene expression level of DPD is associated with tumor response to 5-FU and that the use of more than one independent determinant of response permits the identification of a high percentage of responding patients.

Quantitation of dihydropyrimidine dehydrogenase expression by real-time reverse transcription polymerase chain reaction

Several recent studies have reported a correlation between intratumor dihydropyrimidine dehydrogenase (DPD) messenger RNA (mRNA) levels and sensitivity to 5-fluorouracil (5-FU). However, significant tissue requirements and labor-intensive methodology have limited the large-scale studies necessary for statistical validation. In addition, the semiquantitative results obtained by these methods further limit their application. We have developed a real-time reverse transcription-PCR (RT-PCR) assay, based on TaqMan fluorescence methodology, capable of rapid and accurate quantitation of DPD mRNA levels in biopsy-sized tissue samples. Results obtained with this approach indicate a linear dynamic range of 10(8)-10(3) DPD mRNA copies, with an intra-assay variation of <5%. We evaluated the data using three different methods (absolute standard curve, relative standard curve, and comparative C(T)) and show them to be equivalent. This RT-PCR assay was validated by quantitative comparison to Northern blot analysis in five tissues. In addition, analysis of 18 colorectal tumor and liver tissue specimens demonstrated a significant correlation (r(2) = 0.90) between DPD enzyme activity and mRNA levels. This method provides the first high-throughput, reproducible, and sensitive technique capable of determining DPD mRNA expression levels in nanogram amounts of total RNA.

Phase I and pharmacologic study of oral fluorouracil on a chronic daily schedule in combination with the dihydropyrimidine dehydrogenase inactivator eniluracil

PURPOSE

To determine the maximum-tolerated dose (MTD), toxicities, and pharmacokinetics of oral fluorouracil (5-FU) administered twice daily in combination with oral eniluracil, an inactivator of dihydropyrimidine dehydrogenase, administered for 28 days every 35 days.

PATIENTS AND METHODS

Oral 5-FU 1.35 mg/m(2) twice daily was administered with oral eniluracil 10 mg daily for 14 to 28 days, followed by a 1-week rest period. Eniluracil was started 1 day before 5-FU. Patients then received escalated doses of oral 5-FU 1. 35 to 1.8 mg/m(2) twice daily with an increased dose of eniluracil 10 mg twice daily for 28 days. A reduced dose of 5-FU 1.0 mg/m(2) with eniluracil 20 mg twice daily was evaluated.

RESULTS

Thirty-six patients with solid malignancies were enrolled onto the study. Diarrhea was the principal dose-limiting toxicity of oral 5-FU and eniluracil given on this chronic schedule. The recommended phase II dose is 5-FU 1.0 mg/m(2) twice daily with eniluracil 20 mg twice daily. Mean (SD) values for terminal half-life, apparent volume of distribution, and systemic clearance of 4.5 hours (0.83 hours), 19 L/m(2) (3.0 L/m(2)), and 51 mL/min/m(2) (13 mL/min/m(2)), respectively. An average of 77% of 5-FU was excreted unchanged in urine after 28 days of treatment. The mean (range) 5-FU C(SS,min) values achieved at the 1.0 mg/m(2) dose level were 22 ng/mL (8 to 38 ng/mL).

CONCLUSION

Chronic oral administration of 5-FU with oral eniluracil is tolerable and produces 5-FU steady-state concentrations similar to those achieved with protracted intravenous administration of 5-FU on clinically relevant dose schedules. Eniluracil provides an attractive means of administering 5-FU on protracted schedules.

Life-threatening toxicity in a dihydropyrimidine dehydrogenase-deficient patient after treatment with topical 5-fluorouracil

In humans, 80-90% of an administered dose of 5-fluorouracil (5-FU) is degraded by dihydropyrimidine dehydrogenase (DPD; EC 1.3.1.2), the initial rate-limiting enzyme in pyrimidine catabolism. Cancer patients with decreased DPD activity are at increased risk for severe toxicity including diarrhea, stomatitis, mucositis, myelosuppression, neurotoxicity, and, in some cases, death. We now report the first known cancer patient who developed life-threatening complications after treatment with topical 5-FU and was shown subsequently to have profound DPD deficiency. RT-PCR and genomic PCR methodologies were used to identify a G to A mutation in the GT 5' splicing recognition sequence of intron 14, resulting in a 165-bp deletion (corresponding to exon 14) in this patient's DPD mRNA. Immunoprecipitation and Western blot analysis were then used to demonstrate that the aberrant DPD mRNA is translated into a nonfunctional DPD protein that is ubiquitinated. We conclude that the presence of this metabolic defect combined with topical 5-FU (a drug demonstrating a narrow therapeutic index) results in the unusual presentation of life-threatening toxicity after treatment with a topical drug. These data further suggest that degradation by the ubiquitin-proteosome-mediated system plays a role in the elimination of the DPD protein.

Clinical implications of dihydropyrimidine dehydrogenase inhibition

Dihydropyrimidine dehydrogenase (DPD) is the initial, rate-limiting enzyme in the catabolism of 5-fluorouracil (5-FU). DPD has an important role in regulating the availability of 5-FU for anabolism. It is now clear that DPD also accounts for much of the variability observed with the therapeutic use of 5-FU, including variable drug levels during 24-hour infusion, erratic pharmacokinetics, variable bioavaialability, inconsistent toxicity, and variability in drug response (resistance). The use of DPD inhibitors has been explored as a means to improve 5-FU pharmacology. This article describes how drugs that modulate DPD activity have been used to develop a new class of orally administered fluoropyrimidines, now referred to as DPD-inhibiting fluoropyrimidine (DIF) drugs. The biochemical basis for using four DIF drugs--uracil and tegafur (UFT), ethynyluracil, S-1, and BOF-A2--currently in clinical evaluation in the United States, is hereby reviewed. Early clinical data suggest that these drugs may achieve antitumor efficacy equivalent to that of conventional intravenously administered 5-FU therapy, with the additional advantages of reduced toxicity, less expense, and improved quality of life.

Nomenclature for human DPYD alleles

To standardize DPYD allele nomenclature and to conform with international human gene nomenclature guidelines, an alternative to the current arbitrary system is described. Based on recommendations for human genome nomenclature, we propose that each distinct allele be designed by DPYD followed by an asterisk and an Arabic numeral. The number specifies the key mutation and, where appropriate, a letter following the number indicates an additional mutation on the mutant allele. Criteria for classification as a distinct allele are also presented.

The role of dihydropyrimidine dehydrogenase (DPD) modulation in 5-FU pharmacology

Over the past several years, the pyrimidine catabolic pathway and, in particular, the first enzymatic step involving dihydropyrimidine dehydrogenase (DPD) have been recognized as being critical in determining the ultimate metabolism and, in turn, the pharmacology of the antimetabolite drug 5-fluorouracil (5-FU). Variability in DPD activity in the normal population accounts for observed differences in the pharmacokinetics and oral bioavailability of 5-FU with an additional smaller percentage (< 5%) of the population having a relatively profound deficiency in DPD activity. Diurnal variation of DPD activity is responsible for the observed variation in 5-FU levels during continuous or protracted 5-FU infusions. Relatively elevated levels of DPD in tumor tissue may also be partially responsible for observed 5-FU tumor resistance. Finally, the pyrimidine catabolic pathway may have a role for at least some of the observed 5-FU clinical toxicities, including cardiotoxicity, hand-foot syndrome, and at least some types of neurotoxicity. In order to reduce DPD variation and potentially some of the 5-FU toxicities, there have been attempts to synthesize new fluoropyrimidine drugs used together with drugs that inhibit DPD activity. In this paper, several new types of DPD inhibitors recently introduced into the clinic will be discussed.

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.

Improving 5-FU with a novel dihydropyrimidine dehydrogenase inactivator

GW776C85 is a new drug that has been shown to be an effective inactivator of dihydropyrimidine dehydrogenase (DPD). Preclinical studies demonstrated that administration of GW776C85 with 5-fluorouracil (5-FU) resulted in several desirable pharmacologic effects. Initial clinical data on 5-FU combined with GW776C85 suggest potentially increased antitumor activity in at least some malignancies with tolerable toxicity, as well as several distinct economic and quality-of-life advantages including the following: (1) The possibility of administering 5-FU as an oral drug due to excellent bioavailability of 5-FU following inactivation of DPD; (2) a cost-effective alternative to continuous or protracted infusion of 5-FU without the need for hospitalization or surgical placement of an intravenous access and availability of an ambulatory pump; and (3) potential for less interpatient variation of 5-FU toxicity (e.g., in host tissues, such as bone marrow and gastrointestinal mucosa cells) due to inactivation of DPD in essentially all patients treated, permitting better 5-FU dosing guidelines. Finally, because tumors may theoretically become resistant to 5-FU by increased levels of DPD, the use of GW776C85 to inactivate DPD may provide a potential means by which tumor resistance can be reversed.

Decreased dihydropyrimidine dehydrogenase activity in a population of patients with breast cancer: implication for 5-fluorouracil-based chemotherapy

Dihydropyrimidine dehydrogenase (DPD) is the initial, rate-limiting enzyme in the catabolism of 5-fluorouracil (5-FU), one of the most widely used chemotherapeutic agents in the treatment of breast cancer. The objective of this study was to determine the population characteristics of DPD activity in patients with breast cancer as well as the frequency of DPD deficiency in this population. DPD activity in peripheral blood mononuclear cells (PBM-DPD) was determined in 360 patients with breast cancer, with the mean PBM-DPD (0.26 +/- 0.01 nmol/min/mg protein) being significantly lower than that observed in female controls (0.44 +/- 0.02 nmol/min/mg protein; P < 0.01). ANOVA analysis examining the significance of differences in DPD activity among various groups indicated that only disease difference (breast cancer versus normal subjects) was significant after adjustments for race and age. In the present study, 21 (5.8%) patients were considered to be DPD deficient, indicating that this pharmacogenetic syndrome may be more common than anticipated (no DPD-deficient individual was found in the controls). Significantly lower DPD activity in patients with breast cancer may predispose to 5-FU-associated toxicity. These results provide further rationale for individualizing the 5-FU dose, thus reducing the risk of toxicity and/or improving therapeutic efficacy in patients with breast cancer.

Semi-automated radioassay for determination of dihydropyrimidine dehydrogenase (DPD) activity. Screening cancer patients for DPD deficiency, a condition associated with 5-fluorouracil toxicity

Dihydropyrimidine dehydrogenase (DPD) catalyzes the reduction of the naturally occurring pyrimidines, uracil and thymine, and the fluoropyrimidine anticancer drug, 5-fluorouracil (FUra) to 5,6-dihydropyrimidines. Previous studies have demonstrated that cancer patients who are DPD deficient exhibit severe toxicity (including death) following treatment with FUra. To date, the direct measurement of DPD enzyme activity has been the only reliable method to identify DPD deficient cancer patients. We now report a semi-automated radioassay for measuring DPD activity in human peripheral lymphocytes. Following incubation of lymphocyte cytosol (at a fixed protein concentration of 200 micrograms) with [6-14C]FUra at timepoints ranging from 0 to 30 min, samples are ethanol precipitated, filtered and analyzed by HPLC. Determination of radioactivity is accomplished using an in-line flow scintillation analyzer with automatic quantitation of peaks. This method provides the first specific assay for DPD enzyme activity which is rapid, reproducible and sensitive enough to be used in the routine screening of cancer patients for DPD deficiency prior to treatment with FUra.

Structural organization of the human dihydropyrimidine dehydrogenase gene

Deficiency of the pyrimidine catabolic enzyme, dihydropyrimidine dehydrogenase (DPD), has been shown to be responsible for a pharmacogenetic syndrome in which administration of 5-fluorouracil is associated with severe and potentially life-threatening toxicity. Following the recent availability of the cDNA for DPD, there were initial reports of several molecular defects (point mutations, deletions due to exon skipping) that were suggested as a potential molecular basis for DPD deficiency, even before the complete physical structure of the DPD gene was known. To understand the mechanism responsible for DPD deficiency, we have determined the genomic structure and organization of the human DPD gene. The gene is approximately 150 kb in length, and it consists of 23 exons, ranging in size from 69 to 1404 bp. The sequences of intronic regions flanking the exon boundaries have been determined. The physical map of the DPD gene should permit development of rapid assays to detect point mutations or small deletions in the DPD gene associated with 5-fluorouracil toxicity.

The effect of sorivudine on dihydropyrimidine dehydrogenase activity in patients with acute herpes zoster

OBJECTIVE

Bromovinyl-uracil (BVU) is the principal metabolite of sorivudine, a potent anti-zoster nucleoside. BVU binds to, and irreversibly inhibits, the enzyme dihydropyrimidine dehydrogenase (DPD). The objective of this study was to assess the time course of recovery of DPD activity after oral administration of sorivudine in patients with herpes zoster and to correlate restoration of DPD activity and levels of uracil with the elimination of sorivudine and its metabolite BVU from the circulation.

METHODS

Sorivudine was given orally as 40 mg once-daily doses for 10 consecutive days to a total of 19 patients with herpes zoster. Serum sorivudine, BVU, and circulating uracil and DPD activity in peripheral blood mononuclear cells (PBMCs) were determined before, during, and after administration of sorivudine.

RESULTS

BVU was eliminated from the circulation within 7 days after the last sorivudine dose. DPD activity in PBMCs, which was completely suppressed in 18 of the 19 subjects and markedly suppressed in the remaining subject during administration of sorivudine, recovered to baseline levels within 19 days after the last dose of sorivudine in all subjects and within 14 days in all but one of the subjects. The restoration of DPD activity was temporally associated with elimination of BVU from the circulation. The elevated uracil concentrations produced by inhibition of DPD activity fell rapidly after cessation of sorivudine administration and also were temporally associated with elimination of BVU from the circulation. The time course of recovery of DPD activity in three patients with renal impairment was similar to that of the other subjects.

CONCLUSIONS

This study indicates that sorivudine therapy is associated with a profound depression of DPD activity. Recovery of DPD activity occurred within 4 weeks of the completion of sorivudine therapy, which indicates that fluorinated pyrimidines may be safely administered 4 weeks after completion of sorivudine therapy.

High-performance liquid chromatographic determination of 9-(3-pyridylmethyl)-9-deazaguanine (BCX-34) in biological fluids

9-(3-Pyridylmethyl)-9-deazaguanine (BCX-34), a new purine nucleoside phosphorylase inhibitor, has selective immunosuppressive activity with potential therapeutic value in T-cell-mediated disease. We now report a sensitive, specific and reproducible method for measurement of 9-(3-pyridylmethyl)-9-deazaguanine in biological fluids using high-performance liquid chromatography (HPLC). 9-(3-Pyridylmethyl)-9-deazaguanine was extracted from plasma using perchloric acid precipitation followed by passage through Sep-Pak C18 cartridges (average extraction efficiency, 64.6%). Standard curves were linear over the range of interest (28-1120 ng/ml in plasma and 200-4000 ng/ml in urine, r2 > 0.999). Within-day and between-day coefficients of variation were less than 8%. The limit of quantitation was 28 ng/ml in plasma and 200 ng/ml in urine. This HPLC method should be useful in future clinical studies with this drug.

Dihydropyrimidine dehydrogenase activity in hepatocellular carcinoma: implication in 5-fluorouracil-based chemotherapy

Dihydropyrimidine dehydrogenase (DPD) is the initial, rate-limiting enzyme in the catabolism of 5-fluorouracil, one of the most widely used cancer chemotherapeutic agents. Previous studies have demonstrated the clinical importance of determination of DPD in cancer patients, suggesting that the efficacy and toxicity of 5-fluorouracil may directly relate to the DPD activity in both tumor and host tissues. In the present study, DPD activity was determined in 50 pairs of tumor and uninvolved liver specimens in Chinese cancer patients with hepatocellular carcinoma. Mean enzyme activity in uninvolved liver tissues (0.45 +/- 0.02 nmol/min/mg protein) was significantly higher than that in tumor specimens (0.34 +/- 0.03 nmol/min/mg protein). Statistical analysis revealed no significant differences in DPD activity of tumor and uninvolved liver specimens among different age and gender groups. Compared to previously reported tumor studies, hepatomas were found to have relatively high DPD activity. Since high levels of DPD would be expected to metabolize 5-fluorouracil, these findings may provide an explanation for the relative 5-fluorouracil resistance of hepatoma and may have implications for designing a new therapeutic strategy such as modulation of 5-fluorouracil chemotherapy by DPD inhibitors.

Dihydropyrimidine dehydrogenase deficiency: a pharmacogenetic defect causing severe adverse reactions to 5-fluorouracil-based chemotherapy

PURPOSE/OBJECTIVES

To describe the pharmacogenetic syndrome of dihydropyrimidine dehydrogenase (DPD) deficiency, which predisposes patients with cancer to potentially lethal adverse reactions following 5-fluorouracil (5-FU)-based chemotherapy.

DATA SOURCES

Published articles, abstracts, and conference proceedings.

DATA SYNTHESIS

Genetic deficiencies in DPD, the rate-limiting enzyme responsible for 5-FU catabolism, may occur in 3% or more of patients with cancer putting them at increased risk for unusually severe adverse reactions (e.g., diarrhea, stomatitis, mucositis, myelosuppression, neurotoxicity) to standard doses of 5-FU. Diagnosis of DPD deficiency must be confirmed by specialized laboratory tests. The principle treatment for DPD-deficient patients with severe acute 5-FU reactions is supportive care; however, the administration of thymidine potentially may reverse severe 5-FU-induced neurologic symptoms such as encephalopathy and coma.

CONCLUSIONS

Early recognition of this serious pharmacogenetic syndrome may allow for the modification of future chemotherapy, thus avoiding further life-threatening toxicities.

IMPLICATIONS FOR NURSING PRACTICE

Nurses must understand the pharmacology, mechanism of action, clinical presentation, potentially lethal risks, and traumatic psychosocial stresses experienced by DPD-deficient patients with cancer receiving 5-FU therapy in order to develop timely interventions and alternative plans of care.

Severe neurotoxicity following 5-fluorouracil-based chemotherapy in a patient with dihydropyrimidine dehydrogenase deficiency

Patients with decreased dihydropyrimidine dehydrogenase (DPD) activity are at increased risk for experiencing serious adverse reactions following 5-fluorouracil (5-FU)-based chemotherapy. Symptoms include severe and potentially life-threatening gastrointestinal toxicity, myelosuppression, and neurological toxicity. In the present study, we describe a 50-year-old Caucasian man who developed severe encephalopathy during his second cycle of 5-FU chemotherapy. The patient remained in a comatose state for 4 days but then showed dramatic improvement in his neurological status following continuous i.v. infusion of thymidine at 8 g/m2/day. Laboratory studies revealed the patient to be severely DPD deficient, as demonstrated by DPD enzyme activity from peripheral blood mononuclear cells being below the lower limit of the 95th percentile of a control population and by Western immunoblot analysis showing undetectable levels of DPD protein. Additional studies revealed a significant defect in pyrimidine catabolism with a 3.3- and 365-fold increase in the levels of uracil in plasma and urine, respectively, compared to normal subjects. Family studies suggest that the inheritance pattern of this syndrome is complex and most consistent with an autosomal recessive trait. This study demonstrates that cancer patients with DPD deficiency are at increased risk for developing severe neurological toxicity secondary to 5-FU chemotherapy, and that infusional thymidine should be considered as a potential rescue agent against this particular host toxicity.