Enhanced therapeutic efficacy of 5'deoxy-5-fluorouridine in 5-fluorouracil resistant head and neck tumours in relation to 5-fluorouracil metabolising enzymes

New perspectives on the roles of pyrimidines in the central nervous system

Since 1956, when exogenous uridine and cytidine were found to be necessary for the maintenance of perfused rat brain function, the co-existence of de novo synthesis, salvage pathways and removal of pyrimidine bases in the CNS has been a controversial subject. Here, we review studies on metabolites and enzymes of pyrimidine metabolism through more than 60 years. In view of known and newly-described inherited pyrimidine and purine disorders - some with complex clinical profiles of neurological impairments - we underline the necessity to investigate how the different pathways work together in the developing brain and then sustain plasticity, regeneration and neuro-transmission in the adult CNS. Experimentally, early incorporation studies in animal brain slices and homogenates with radio-labelled nucleosides or precursors demonstrated salvage activity or de novo synthesis. Later, the nucleoside transporters and organic anionic transporters underlying uptake of metabolites and anti-pyrimidine drugs in the CNS were identified. Recently, the expression of de novo enzymes in glial cells and neurons was verified using (immuno) histochemical and in-situ-hybridization techniques. Adult brain was shown to take up or produce all pyrimidine (deoxy) ribonucleosides or, after uptake and phosphorolysis of nucleosides, to make use of ribose for different purposes, including energy. More recently, non-canonical pyrimidine bases (5mC, 5hmC) have been found most notably in brain, pointing to considerable postreplicative DNA metabolism, with the need for pyrimidine-specific enzymes. Even more perspectives are emerging, with advances in genome analysis and in the manipulation of expression from the gene.

Effects of smoking and alcohol consumption on 5-fluorouracil-related metabolic enzymes in oral squamous cell carcinoma

Lifestyle, particularly smoking and alcohol consumption, may induce and/or inhibit drug metabolism. In order to reveal the effects of smoking and alcohol consumption on the 5-fluorouracil (5-FU)-related metabolic enzymes, namely thymidylate synthase, dihydropyrimidine dehydrogenase (DPD; a sole catabolic enzyme of 5-FU), orotate phosphoribosyl transferase (OPRT) and thymidine phosphorylase, in oral squamous cell carcinomas, the mRNA expression of these enzymes was investigated in 29 surgical specimens and compared by the Brinkman index and drinking years. The surgical specimens were divided into normal and tumor regions and were independently analyzed using quantitative reverse transcription-polymerase chain reaction. There was a significantly positive correlation between DPD mRNA expression in these tissues and Brinkman index/drinking years, with OPRT mRNA expression being significantly correlated to the Brinkman index in tumor tissues. These results revealed that lifestyle habits, including smoking and alcohol consumption, may vary the activity of the 5-FU-related metabolic enzymes. DPD is the initial and rate-limiting enzyme in the catabolic pathway of 5-FU. Therefore, smoking and alcohol consumption may reduce the anticancer activity of 5-FU, possibly through the induction of DPD activity.

Novel mRNA isoforms and mutations of uridine monophosphate synthetase and 5-fluorouracil resistance in colorectal cancer

The drug fluorouracil (5-FU) is a widely used antimetabolite chemotherapy in the treatment of colorectal cancer. The gene uridine monophosphate synthetase (UMPS) is thought to be primarily responsible for conversion of 5-FU to active anticancer metabolites in tumor cells. Mutation or aberrant expression of UMPS may contribute to 5-FU resistance during treatment. We undertook a characterization of UMPS mRNA isoform expression and sequence variation in 5-FU-resistant cell lines and drug-naive or -exposed primary and metastatic tumors. We observed reciprocal differential expression of two UMPS isoforms in a colorectal cancer cell line with acquired 5-FU resistance relative to the 5-FU-sensitive cell line from which it was derived. A novel isoform arising as a consequence of exon skipping was increased in abundance in resistant cells. The underlying mechanism responsible for this shift in isoform expression was determined to be a heterozygous splice site mutation acquired in the resistant cell line. We developed sequencing and expression assays to specifically detect alternative UMPS isoforms and used these to determine that UMPS was recurrently disrupted by mutations and aberrant splicing in additional 5-FU-resistant colorectal cancer cell lines and colorectal tumors. The observed mutations, aberrant splicing and downregulation of UMPS represent novel mechanisms for acquired 5-FU resistance in colorectal cancer.

Efficacy of laser capture microdissection plus RT-PCR technique in analyzing gene expression levels in human gastric cancer and colon cancer

Background

Thymidylate synthase, dihydropyrimidine dehydrogenase, thymidine phosphorylase, and orotate phosphoribosyltransferase gene expressions are reported to be valid predictive markers for 5-fluorouracil sensitivity to gastrointestinal cancer. For more reliable predictability, their expressions in cancer cells and stromal cells in the cancerous tissue (cancerous stroma) have been separately investigated using laser capture microdissection.

Methods

Thymidylate synthase, dihydropyrimidine dehydrogenase, thymidine phosphorylase, and orotate phosphoribosyltransferase mRNA in cancer cells and cancerous stroma from samples of 47 gastric and 43 colon cancers were separately quantified by reverse transcription polymerase chain reaction after laser capture microdissection.

Results

In both gastric and colon cancers, thymidylate synthase and orotate phosphoribosyltransferase mRNA expressions were higher (p < 0.0001, p <0.0001 respectively in gastric cancer and P = 0.0002, p < 0.0001 respectively in colon cancer) and dihydropyrimidine dehydrogenase mRNA expressions were lower in cancer cells than in cancerous stroma (P = 0.0136 in gastric cancer and p < 0.0001 in colon cancer). In contrast, thymidine phosphorylase mRNA was higher in cancer cells than in cancerous stroma in gastric cancer (p < 0.0001) and lower in cancer cells than in cancerous stroma in colon cancer (P = 0.0055).

Conclusion

By using this method, we could estimate gene expressions separately in cancer cells and stromal cells from colon and gastric cancers, in spite of the amount of stromal tissue. Our method is thought to be useful for accurately evaluating intratumoral gene expressions.

Activity and substrate specificity of pyrimidine phosphorylases and their role in fluoropyrimidine sensitivity in colon cancer cell lines

Thymidine phosphorylase (TP) and uridine phosphorylase (UP) are often upregulated in solid tumors and catalyze the phosphorolysis of natural (deoxy)nucleosides and a wide variety of fluorinated pyrimidine nucleosides. Because the relative contribution of each of the two enzymes to these reactions is still largely unknown, we investigated the substrate specificity of TP and UP in colon cancer cells for the (fluoro)pyrimidine nucleosides thymidine (TdR), uridine (Urd), 5'-deoxy-5-fluorouridine (5'DFUR), and 5FU. Specific inhibitors of TP (TPI) and UP (BAU) were used to determine the contribution of each enzyme in relation to their cytotoxic effect. The high TP expressing Colo320TP1 cells were most sensitive to 5'DFUR and 5FU, with IC50 values of 1.4 and 0.2 microM, respectively, while SW948 and SW1398 were insensitive to 5'DFUR (IC50>150 microM for 5'DFUR). TPI and BAU only moderately affected sensitivity of Colo320, SW948, and SW1398, whereas TPI significantly increased IC(50) for 5'DFUR (50-fold) and 5FU (11-fold) in Colo320TP1 and BAU that in C26A (9-fold for 5'DFUR; p<0.01). In the epithelial skin cell line HaCaT both inhibitors were able to decrease sensitivity to 5'DFUR and 5FU separately. HaCaT might be a model for 5'DFUR toxicity. In the colon cancer cells 5'DFUR degradation varied from 0.4 to 50 nmol 5FU/h/10(6)cells, that of TdR from 0.3 to 103 nmol thymine/h/10(6)cells, that of Urd from 0.8 to 79 nmol uracil/h/10(6)cells, while conversion of 5FU to FUrd was from 0.3 to 46 nmol/h/10(6)cells. SW948 and SW1398 were about equally sensitive to 5'DFUR and 5FU, but SW1398 had higher phosphorylase activity (>65-fold) compared to SW948. In SW948 and HaCaT TPI and BAU inhibited TdR and Urd phosphorolysis (>80%), respectively. Both TP and UP contributed to the phosphorolysis of 5'DFUR and 5FU. In the presence of both inhibitors, still phosphorolysis of 5FU (>40%) was detected in the tumor and HaCaT cell lines, and remarkably, that of all four substrates in SW1398 cells. 5'DFUR phosphorolysis was also measured in situ, where Colo320TP1, SW1398, and HaCaT cells produced significant amounts 5FU from 5'DFUR (>10 nmol/24h/10(6)cells). In Colo320TP1 and in HaCaT cells TPI completely prevented 5FU production, but not in SW1398 cells, where BAU decreased this by 67% (p<0.01). High uracil and dUrd levels were detected in the medium. Uracil accumulation was heavily reduced in the presence of TPI for Colo320TP1 and HaCaT cells, whereas 5FU-induced dUrd production by these cell lines increased (p<0.01). In contrast, for SW1398 cells only BAU was able to reduce uracil levels, and dUrd production remained unchanged. In conclusion, overlapping substrate specificity was found for TP and UP in the cell lines, in which both enzymes were responsible for converting TdR and Urd, and 5'DFUR. 5'DFUR and 5FU were converted to their products in both the colon cancer cells and keratinocytes.

The role of thymidine phosphorylase and uridine phosphorylase in (fluoro)pyrimidine metabolism in peripheral blood mononuclear cells

Thymidine phosphorylase (TP) and uridine phosphorylase (UP) catalyze the (in)activation of several fluoropyrimidines, depending on their catalytic activity and substrate specificity. Blood cells are the first compartment exposed to most anticancer agents. The role of white blood cells in causing toxic side effects and catalyzing drug metabolism is generally underestimated. Therefore we determined the contribution of the white blood cell compartment to drug metabolism, and we investigated the activity and substrate specificity of TP and UP for the (fluoro)pyrimidines thymidine (dThd), uridine (Urd), 5'-deoxy-5-fluorouridine (5' dFUrd) and 5-fluorouracil (5FU) in peripheral blood mononuclear cells (PBMC) and undifferentiated monocytes and differentiated monocytes: macrophages and dendritic cells. PBMC had an IC50 of 742 microM exposed to 5'dFUrd, increasing to > 2000 microM when both TP and UP activities were inhibited. Total phosphorolytic activity was higher with dThd than with Urd, 5'dFUrd or 5FU. Using a specific TP inhibitor (TPI) and UP inhibitor (BAU) we concluded that dThd and Urd were preferentially converted by TP and UP, respectively, while 5'dFUrd and 5FU were mainly converted by TP (about 80%) into 5FU and FUrd, respectively. 5FU was effectively incorporated into RNA. dThd conversion into thymine was highest in dendritic cells (52.6 nmol thymine/h/10(6) cells), followed by macrophages (two-fold) and undifferentiated monocytes (eight-fold). TPI prevented dThd conversion almost completely. In conclusion, PBMC were relatively insensitive to 5'dFUrd, and the natural substrates dThd and Urd were preferentially converted by TP and UP, respectively. TP and UP were both responsible for converting 5'dFUrd/5FU into 5FU/FUrd, respectively.

Influence of chemotherapeutic agents and cytokines on the expression of 5-fluorouracil-associated enzymes in human colon cancer cell lines

BACKGROUND

Several studies have demonstrated that intratumoral expression of catabolizing and anabolizing enzymes for 5-fluorouracil (5-FU) is important in the response of cancers to 5-FU-based chemotherapy. We investigated the influence of other chemotherapeutic agents or cytokines, which are often administered for enhancing the efficacy of 5-FU, on the tumoral expression of 5-FU-associated enzymes, i.e., dihydropyrimidine dehydrogenase (DPD), thymidylate synthase (TS), orotate phosphoribosyl transferase (OPRT), and thymidine phosphorylase (TP).

METHODS

Human colon cancer cell lines (HT-29, Caco-2, and DLD-1) were incubated with 5-FU and with 5-FU combined with cisplatin, camptothecin, paclitaxel, mitomycin C, interferon, or TNF-related apoptosis-inducing ligand. mRNA expression of 5-FU-associated enzymes was assessed by real-time PCR. Activity of each enzyme and intracellular 5-FU accumulation after incubation with such agents were also evaluated.

RESULTS

Each agent had a synergistic effect on the cytotoxicity of 5-FU. All chemotherapeutic agents other than cytokines induced marked alteration of the mRNA expression profile of 5-FU-associated enzymes; depression of DPD, elevation of TS, and slight suppression of OPRT and TP. In accordance with mRNA expression, enzyme activity of DPD was significantly depressed by such agents. Furthermore, although 5-FU itself increased DPD mRNA expression, a mechanism considered to be related to the acquisition of 5-FU resistance, the addition of cisplatin or camptothecin significantly inhibited the 5-FU-induced elevation of DPD.

CONCLUSIONS

5-FU-associated enzymes in colon cancer cells were greatly influenced by various chemotherapeutic agents; in particular, DPD expression was depressed. These results appear important in planning chemotherapy and also in understanding the development of adverse effects of 5-FU.

Enzyme-catalyzed activation of anticancer prodrugs

The rationale fo the development of prodrugs relies upon delivery of higher concentrations of a drug to target cells compared to administration of the drug itself. In the last decades, numerous prodrugs that are enzymatically activated into anti-cancer agents have been developed. This review describes the most important enzymes involved in prodrug activation notably with respect to tissue distribution, up-regulation in tumor cells and turnover rates. The following endogenous enzymes are discussed: aldehyde oxidase, amino acid oxidase, cytochrome P450 reductase, DT-diaphorase, cytochrome P450, tyrosinase, thymidylate synthase, thymidine phosphorylase, glutathione S-transferase, deoxycytidine kinase, carboxylesterase, alkaline phosphatase, beta-glucuronidase and cysteine conjugate beta-lyase. In relation to each of these enzymes, several prodrugs are discussed regarding organ- or tumor-selective activation of clinically relevant prodrugs of 5-fluorouracil, axazaphosphorines (cyclophosphamide, ifosfamide, and trofosfamide), paclitaxel, etoposide, anthracyclines (doxorubicin, daunorubicin, epirubicin), mercaptopurine, thioguanine, cisplatin, melphalan, and other important prodrugs such as menadione, mitomycin C, tirapazamine, 5-(aziridin-1-yl)-2,4-dinitrobenzamide, ganciclovir, irinotecan, dacarbazine, and amifostine. In addition to endogenous enzymes, a number of nonendogenous enzymes, used in antibody-, gene-, and virus-directed enzyme prodrug therapies, are described. It is concluded that the development of prodrugs has been relatively successful; however, all prodrugs lack a complete selectivity. Therefore, more work is needed to explore the differences between tumor and nontumor cells and to develop optimal substrates in terms of substrate affinity and enzyme turnover rates fo prodrug-activating enzymes resulting in more rapid and selective cleavage of the prodrug inside the tumor cells.

Role of platelet-derived endothelial cell growth factor/thymidine phosphorylase in fluoropyrimidine sensitivity

Platelet-derived endothelial cell growth factor (PD-ECGF)/thymidine phosphorylase (TP) catalyses the reversible phosphorolysis of thymidine to thymine and 2-deoxyribose-1-phosphate and is involved in the metabolism of fluoropyrimidines. It can also activate 5'-deoxyfluorouridine (5'DFUR) and possibly 5-fluorouracil (5FU) and Ftorafur (Ft), but inactivates trifluorothymidine (TFT). We studied the contribution of TP activity to the sensitivity for these fluoropyrimidines by modulating its activity and/or expression level in colon and lung cancer cells using a specific inhibitor of TP (TPI) or by overproduction of TP via stable transfection of human TP. Expression was analysed using competitive template-RT-PCR (CT-RT-PCR), Western blot and an activity assay. TP activity ranged from nondetectable to 70678 pmol h(-1) 10(-6) cells, in Colo320 and a TP overexpressing clone Colo320TP1, respectively. We found a good correlation between TP activity and mRNA expression (r=0.964, P&<0.01) in our cell panel. To determine the role of TP in the sensitivity to 5FU, 5'DFUR, Ft and TFT, cells were cultured with the various fluoropyrimidines with or without TPI and differences in IC(50)'s were established. TPI modified 5'DFUR, increasing the IC(50)'s 2.5- to 1396-fold in WiDR and Colo320TP1, respectively. 5-Fluorouracil could be modified by inhibiting TP but to a lesser extent than 5'DFUR: IC(50)'s increased 1.9- to 14.7-fold for WiDR and Colo320TP1, respectively. There was no effect on TFT or Ft. There appears to be a threshold level of TP activity to influence the 5'DFUR and 5FU sensitivity, which is higher for 5FU. Even high levels of TP overexpression only had a moderate effect on 5FU sensitivity.

Overexpression of pyrimidine nucleoside phosphorylase enhances the sensitivity to 5'-deoxy-5-fluorouridine in tumour cells in vitro and in vivo

5-Fluorouracil (5-FU) and 5'-deoxy-5-fluorouridine (5'-DFUR), a prodrug of 5-FU, are representative of the chemotherapeutic agents for colorectal adenocarcinomas. Pyrimidine nucleoside phosphorylase (PyNPase) catalyses the conversion of 5'-DFUR to 5-FU, the activated form. Murine adenocarcinoma CT26 cells were transfected with human PyNPase cDNA. The engineered transfectants producing PyNPase augmented the response to 5'-DFUR in vitro and in vivo. Animals were administered by means of intraperitoneal (i.p.) injection, and not orally, in order to obtain a better efficiency of absorption. The tumours of the transfected cells nearly all disappeared, even following treatment with quite a small amount of the anticancer agent. The animals injected with the tranfected cells were protected against subsequent challenge with the parental tumour cell line. These findings demonstrate that PyNPase gene transfection increases the sensitivity to 5'-DFUR, and thereby decreases the toxicity of the agent.

Thymidine phosphorylase: its role in sensitivity and resistance to anticancer drugs

Thymidine phosphorylase (TP) is an angiogenic enzyme present in normal tissues. Increased levels are found in many tumors, in stromal cells, tumor cells or both. High tumor TP levels may confer a poor prognosis. Cytokines (including interferons), tissue hypoxia and low pH increase TP levels. The influence of tumor TP on fluoropyrimidine toxicity is variable, but capecitabine is a prodrug of fluorouracil that requires activation by TP and hence may have a higher therapeutic index than other fluoropyrimidines. Folate-based thymidylate synthase inhibitors may also be more effective in tumors with a high TP because of increased degradation of endogenous thymidine. Copyright 1999 Harcourt Publishers Ltd.

Antitumor effects of 5'-deoxy-5-fluorouridine in combination with recombinant human interleukin 2 on murine colon carcinoma 26

The antitumor activity of recombinant human interleukin 2 (rIL-2) in combination with 5'-deoxy-5-fluorouridine (doxifluridine; 5'-DFUR) against marine colon carcinoma 26 (Colon 26) was studied. BALB/c mice were treated daily for 15 days with 5'-DFUR, rIL-2 or both, beginning on day 7 after subcutaneous transplantation of Colon 26. While mice treated with 5'-DFUR or rIL-2 alone died of tumor growth with pulmonary metastases within 9 weeks posttransplantation, the survival time was significantly prolonged in mice treated with both 5'-DFUR and rIL-2. Most of the combination-treated animals showed the regression of local tumors and the inhibition of pulmonary metastasis. Histopathologically, many tumor cells were degenerated and necrotized, with marked infiltration of mononuclear cells including large granular lymphocytes (LGLs) with periodic acid-Schiff-positive cytoplasmic granules. The cells were positive for CD3 epsilon, asialo GM1 and NK1.1. Spleen cells from the combination-treated mice showed high activities of natural killer (NK) cytotoxicity as well as growth inhibition of Colon 26 and Meth A fibrosarcoma in mice. The results suggest that the combination therapy of 5'-DFUR plus rIL-2 enhanced non-specific cytotoxicity of LGL/NK cells for Colon 26 in tumor-bearing mice and was effective in the inhibition of tumor growth.

New targets for pyrimidine antimetabolites in the treatment of solid tumours. 1: Thymidylate synthase

Thymidylate synthase forms the target for anticancer therapy with fluoropyrimidines. Anticancer activity can be increased by the use of different modulators of fluoropyrimidine metabolism, which lead to an enhanced inhibition of thymidylate synthase. In vitro and in vivo studies with fluoropyrimidines and two of these modulators, folinic acid (leucovorin) and interferon, are summarized. The promise of these preclinical results is reflected by the response data of several clinical trials. The biochemical effects of these modulators are described and illustrated by the fluoropyrimidine-mediated inhibition of thymidylate synthase in tumour samples, which is clearly enhanced by folinic acid. The regulation of thymidylate synthase synthesis may also be crucial for total blockade of thymidylate synthase activity. This regulation may be influenced by interferon-gamma. Although the addition of modulators increases the activity of fluoropyrimidines at the level of thymidylate synthase, most solid tumours, especially colorectal carcinomas, are resistant to these combinations. For this reason, new, more potent inhibitors of thymidylate synthase have been developed, the antifolates. Preclinical data show that some of these compounds have good antitumour activity, but they still have to prove their value in the clinic. These two approaches, the use of modulators and new compounds, have shown activity preclinically and the extension of these findings to clinical studies stresses the importance of thymidylate synthase as a target in fluoropyrimidine therapy of solid tumours.

Interferon alpha and 5'-deoxy-5-fluorouridine in colon cancer: effects as single agents and in combination on growth of xenograft tumours

Interferon-alpha (IFN-alpha) enhances the activity of the 5-fluorouracil (5-FU) prodrug 5'-deoxy-5-fluorouridine (5'-dFUrd) in colorectal cancer cells in vitro by upregulating the enzyme pyrimidine nucleoside phosphorylase (PNPase), which is responsible for converting 5'-dFUrd to 5-FU. We examined whether such enhancement also occurs in vivo using human colorectal xenografts in nude mice. The Co-115 line has high basal levels of PNPase and the enzyme level was increased in tumours from mice treated for 3 weeks with 50,000 IU/day (5 days/week) of IFN-alpha A/D. The prodrug 5'-dFUrd (200 mg/day, 5 days/week) had a much greater antitumour activity than 5-FU had when it was used at an approximately equitoxic dose (20 mg/day, 5 days/week). However, because of the high activity of 5'/dFUrd as a single agent, no enhancement by IFN-alpha A/D was observed. Studies on xenografts of WiDr cells indicated that this line is much less sensitive to 5'-dFUrd. However, treatment of animals with IFN-alpha A/D at doses of 75,000 IU/day or 150,000 IU/day resulted in significant inhibition of WiDr tumour growth. Combination treatment with 75 mg/kg/day or 150 mg/kg/day of 5'-dFUrd resulted in enhanced antitumour activity, particularly at the higher dose of IFN-alpha A/D. Synergy of this drug combination was confirmed by isobologram analysis. Analysis of PNPase levels in WiDr tumours, excised from mice treated with IFN-alpha A/D, demonstrated that the enzyme activity was increased by IFN-alpha in a dose-dependent manner. Slight increases were also seen in normal liver and intestine from the same animals. Our results indicate that modulation of converting enzymes for anticancer prodrugs by cytokines could be a novel therapeutic strategy for combination therapy of colorectal cancer.

Transformation of mouse fibroblasts with the oncogenes H-ras OR trk is associated with pronounced changes in drug sensitivity and metabolism

Malignant activation of oncogenes ras or trk is implicated in a number of solid tumors and leukemias. We determined the chemosensitivity profile of wild-type mouse NIH-3T3 fibroblasts, and that of NIH-3T3 lines transformed by the H-ras (S2-721) and trk (106-632) oncogenes, against 11 different drugs from various classes. Differences in sensitivity were related to drug accumulation and metabolism. Both ras- and trk-transformed cell lines were less sensitive to cisplatin (CDDP) and doxorubicin (DXR) than the wild type. NIH-3T3 transformants expressing H-ras were less sensitive than those expressing trk or the wild type to the indoloquinone EO9, methotrexate and arabino-furanosylcytosine. No clear difference in sensitivity was observed for vincristine, VP-16, or the new cytidine analog 2',2'-difluoro-deoxycytidine. In both ras- and trk-transformed cell lines sensitivity to 5FU was increased moderately, but sensitivity to 5'deoxy-5-fluorouridine (5'dFUR) was increased markedly. Only the trk-transformed line NIH-3T3 was more sensitive to 2'deoxy-5-fluorouridine. Expression of P-glycoprotein was not different between the 3 cell lines but DXR accumulation in both mutants was decreased, indicating a non-P-glycoprotein-associated difference in sensitivity. Conversion of 5'dFUR to 5FU (catalyzed by pyrimidine nucleoside phosphorylases) was 5-10 times higher in both mutants than in the wild type. The activity of the phosphoribosyl-transferase (direct conversion of 5FU to FUMP) was comparable, but the rate of conversion of 5FU to fluorouridine (FUR) was lower in the wild type, as well as that of 5FU to FUMP via FUR. In contrast, the activity of thymidylate synthase, the target enzyme for fluoropyrimidines, was higher in the wild-type cells. The concentrations of both purine and pyrimidine nucleotides were lower in cells expressing trk. In conclusion, transformation of cells with the H-ras or trk oncogenes can markedly influence sensitivity to several drugs and affect normal metabolism and that of several anti-cancer agents.

Deoxycytidine kinase and deoxycytidine deaminase activities in human tumour xenografts

Deoxycytidine kinase (dCK) and deaminase (dCDA) are both key enzymes in the activation and inactivation, respectively, of several deoxycytidine antimetabolites. We determined the total dCK and dCDA activities using standard assays, in 28 human solid tumours grown as xenografts in nude mice, and four corresponding cell lines. dCK activities in colon tumours varied from 11 to 12 nmol/h/mg protein, in ovarian tumours from 3 to 10 nmol/h/mg protein, in soft tissue sarcomas from 2 to 7 nmol/h/mg protein and in squamous cell carcinomas of the head and neck about 45-fold, between 0.4 and 18 nmol/h/mg protein. The dCDA activities showed a larger variation, from 243 to 483, 14 to 1231, 3 to 7 and 1 to 222 nmol/h/mg protein, respectively. The ratios of dCK vs. dCDA activities in these tumours varied from 0.025 to 0.046, 0.004 to 0.240, 0.581 to 1.123 and from 0.012 to 4.227, respectively. In four cell lines (A2780, OVCAR-3, WiDr and UM-SCC-14C), sources for some of the above mentioned tumours, a different pattern in dCK and dCDA was observed than in the corresponding tumours. The variation in dCDA activities was in a smaller range (20-fold) than in the tumours (40-fold). In all cell lines dCK activity was higher than dCDA activity, in contrast to the corresponding tumours, in which the reverse pattern was observed. Previously, some of the tumours were tested for sensitivity to the deoxycytidine analogues 5-aza-deoxycytidine and 2',2'-difluorodeoxycytidine. In the sensitive tumours, both the highest and lowest dCK activity was observed, indicating that dCK activity in solid tumours is high enough to activate deoxycytidine analogues.

A comparison of 5-fluorouracil metabolism in human colorectal cancer and colon mucosa

The metabolism of 5-fluorouracil (5-FU) was studied in biopsy specimens of primary colorectal cancer and healthy colonic mucosa obtained from previously untreated patients immediately after surgical removal. The conversion of 5-FU to anabolites was measured under saturating substrate (5-FU) and cosubstrate concentrations. For all enzymes, the activity was about threefold higher in tumor tissue compared with healthy mucosa of the same patient. The activity of pyrimidine nucleoside phosphorylase with deoxyribose-1-phosphate (dRib-1-P) was about tenfold higher (about 130 and 1200 nmol/hr/mg protein in tumors) than with ribose-1-phosphate (Rib-1-P), both in tumor and mucosa. Synthesis of the active nucleotides (5-fluoro-uridine-5'-monophosphate [FUMP] and 5-fluoro-2'-deoxyuridine-5'-monophosphate [FdUMP]) was studied by adding physiologic concentrations of adenosine triphosphate (ATP) to the reaction mixture; the rate of FdUMP synthesis was 50% of that of FUMP (about 4 and 7 nmol/hr/mg protein in tumors). Direct synthesis of FUMP from 5-FU in the presence of 5-phosphoribosyl-1-pyrophosphate (PRPP) was about 2 nmol/hr/mg protein. With the natural substrate for this reaction, orotic acid, the activity was about 14-fold higher. To obtain insight into the recruitment of precursors for these cosubstrates, the authors also tested the enzyme activity of pyrimidine nucleoside phosphorylase with inosine and ribose-5-phosphate (Rib-5-P, as precursors for Rib-1-P) and deoxyinosine (as a precursor for dRib-1-P); enzyme activities were approximately 7%, 7%, and 3%, respectively, of that with the normal substrates, both in tumors and mucosa. However, when ATP and Rib-5-P were combined, the synthesis of FUMP was about 70% of that with PRPP, but only in tumors. In normal tissues no activity was detectable. These data suggest a preference of colon tumor over colon mucosa for the conversion of 5-FU to active nucleotides by a direct pathway; a selective antitumor effect of 5-FU may be related to this difference.

In vitro antiproliferative and metabolic activity of eight novel 5-fluorinated uracil nucleosides

The in vitro growth inhibitory activity of eight novel 5-fluorinated uracil nucleosides was assessed in four human tumour cell lines, one of colon and three of head and neck squamous cell origin. These compounds are ribose or deoxyribose sugars with an acetoxy or an hydroxyl-group at the 6-position in the uracil part of the molecule, and their respective diastereoisomers. Antiproliferative effects were tested in an automated microculture assay based on the reduction of a tetrazolium dye, the MTT assay. Using a continuous drug exposure for four days, all novel nucleosides were more potent inhibitors of cell growth than 5-fluorouracil (5-FU). Most drugs were very active, having an IC50 value at least 10 fold lower than that of 5-FU, and this was consistently found for all cell lines. The 6-acetoxy compounds were generally more active than the compounds with a hydroxyl-group at the 6-position, while diastereoisomerism did not seem to influence the antiproliferative effect. Their capacity to inhibit the incorporation of tritiated deoxyuridine into DNA, which reflects the inhibition of thymidylate synthase, was measured in a short term assay. When tested at a concentration of 10(-6) mol/l, most of the compounds were found to block this incorporation more efficiently than 5-FU.

Biochemical pharmacology and analysis of fluoropyrimidines alone and in combination with modulators

After more than three decades since their introduction, fluoropyrimidines, especially FUra, are still a mainstay in the treatment of various solid malignancies. The antitumor effects of fluoropyrimidines are dependent upon metabolic activation. FdUMP, FUTP and FdUTP were identified as the key cytotoxic metabolites that interfere with the proper function of thymidylate synthase and nucleic acids. The relevance of these metabolites is cell-type specific. Recently, fluorouridine diphospho sugars have been detected, but the precise function of this class of metabolites is currently unknown. In mammalian systems fluoropyrimidines and their natural counterparts share the same metabolic pathways since the substrate properties in enzyme-catalyzed reactions are frequently comparable. Ongoing studies indicate that the metabolism and action of fluoropyrimidines exhibit circadian rhythms, which appear to be due to variations in the activity of metabolizing enzymes. Essential for the expanding knowledge of the pathways and effects of fluoropyrimidines has been the constant improvement of analytical methods. These include ligand binding techniques, numerous dedicated HPLC systems and 19F-NMR. Because the overall response rates achieved with fluoropyrimidines are modest, strategies based on biochemical modulation have been devised to enhance their therapeutic index. Biochemical modulators include a wide range of various compounds with different modes of action. In recently completed clinical trials, combinations of FUra with leucovorin, a precursor for 5,10-methylene tetrahydrofolate, or with levamisole, an anthelminthic with immunomodulatory activity, appeared to be superior to FUra alone. At the preclinical level combinations of fluoropyrimidines with, e.g. interferons or L-histidinol were demonstrated to be interesting candidates for further testing. The future therapeutic utility of fluoropyrimidines will depend on both the improvement of combination regimens currently used in the treatment of cancer patients and the judicious clinical implementation of promising experimental modulation strategies. Moreover, novel fluoropyrimidines with superior pharmacological properties may become important as part of or instead of modulation approaches.

5'-Deoxy-5-fluorouridine improves cachexia by a mechanism independent of its antiproliferative action in colon 26 adenocarcinoma-bearing mice

The cytostatic agent 5'-deoxy-5-fluorouridine (5'-dFUrd) improves cachexia and prolongs survival, suppressing tumor growth in mice bearing large burdens of colon 26 adenocarcinoma. To investigate the mode of this anticachectic action, we isolated colon 26 variants that were resistant to the anticachectic activity in vivo in tumor-bearing mice that initially responded to 5'-dFUrd in terms of tumor growth and cachexia but again became cachectic and refractory to the drug after prolonged treatment. The original line and variants were equally susceptible to the antiproliferative action of 5'-dFUrd, and their growth was stopped. However, 5'-dFUrd given to cachectic mice exhibiting large burdens of these variants could not reverse wasting and only slightly prolonged the survival period. These results indicate that the anticachectic activity of 5'-dFUrd is independent of its antiproliferative action and that the survival of colon 26-bearing mice is shorter when the size of the tumors is not reduced to levels below those that cause cachexia.

Retention of in vivo antipyrimidine effects of Brequinar sodium (DUP-785; NSC 368390) in murine liver, bone marrow and colon cancer

Brequinar sodium (DUP-785) is a potent inhibitor of the pyrimidine de novo enzyme, dihydroorotic acid dehydrogenase (DHO-DH). In order to determine whether in vitro data could be extrapolated to the in vivo situation we investigated antipyrimidine effects of DUP-785 in mice bearing colon cancer. Two tumor models were used, Colon 26 and Colon 38, resistant and moderately sensitive to DUP-785, respectively. DUP-785 at 50 mg/kg caused a depletion of plasma uridine in mice, and depleted tissue uridine levels in Colon 38 down to 10%, which was retained for several days; in Colon 26 the decrease was less and tissue uridine levels recovered rapidly. In livers of these mice no significant effect on uridine was observed. DUP-785 depleted UTP in bone marrow cells within 2 hr to 25% of control levels, after 4 days normal levels were found. In livers of both Balb-c mice (bearing Colon 26) and C57Bl/6 mice (bearing Colon 38) a small decrease of uridine nucleotide pools was found. In Colon 26 DUP-785 increased uridine nucleotide pools to 170% after 2 hr, at 1 day normal levels were observed, but after 2 days again an increase was found. In Colon 38 DUP-785 decreased the uridine nucleotide pool by 50% after 1 and 2 days. DUP-785 did not affect cytidine nucleotide pools of livers and of Colon 26 and Colon 38. The ratio between uridine nucleotides and cytidine nucleotides decreased from 2.2 to 0.90 in Colon 38, in the other tissues the decrease was less. DHO-DH was measured in bone marrow cells and Colon 26 and 38 before and after treatment. Basal levels of DHO-DH were 3 times higher in Colon 26 than in Colon 38. In treated tumors DHO-DH was initially inhibited by more than 90%, after 7 days enzyme activity in Colon 26 was 50% and in Colon 38 about 200% of basal levels. In bone marrow cells DHO-DH was also rapidly inhibited but recovered within 4 days. It is concluded that the retention of antipyrimidine effects of DUP-785 in Colon 38 were more pronounced than in Colon 26, which is in agreement with the better antitumor effect of DUP-785 in Colon 38.