Thymidylate synthase from untreated human colorectal cancer and colonic mucosa: enzyme activity and inhibition by 5-fluoro-2'-deoxy-uridine-5'-monophosphate

Novel mass spectrometry-based assay for thymidylate synthase activity

Thymidylate synthase (TS) is an enzyme responsible for the conversion of deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP), with the co-substrate 5,10-methylenetetrahydrofolate (5,10-CH2-THF) as the methyl donor. TS is the only enzyme capable of de novo biosynthesis of dTMP in humans, a nucleotide crucial for DNA synthesis and therefore cell proliferation and survival. As such, TS is a major drug target in chemotherapy by compounds such as 5-fluorouracil. Due to the clinical and physiological importance of TS, the ability to accurately assay its activity is crucial. Several assays have been developed for this purpose, relying on spectrophotometry or radioisotope labeling methods. In this study, we have developed a liquid chromatography - mass spectrometry-based method for assessing TS activity by direct and specific measurement of the reaction product, dTMP. The assay was tested on mouse liver homogenates. We noted that excessive 5,10-CH2-THF concentration (400 µM) led to substrate inhibition and therefore 200 µM was used. The activity assayed at 1 µM dUMP was linear with protein content and time (up to 60 min) and was 0.56 ± 0.12 pmol/mg protein/min, in line with previously reported values. Additionally, by using a high mass resolution Orbitrap instrument side reactions were monitored, revealing major changes in folate pools and nucleotide metabolism. These findings highlight the value of the developed TS assay for routine TS activity monitoring in complex matrixes such as clinical material.

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

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

Cytotoxic effects of pemetrexed in gastric cancer cells

Pemetrexed is a newly developed multitargeted antifolate with promising clinical activity in many solid tumors including gastric cancer. The aim of the present study was to evaluate the cytotoxicity of pemetrexed and its mode of interaction with cisplatin in gastric cancer cell lines, and to identify genes associated with sensitivity to pemetrexed. The cytotoxic activity of pemetrexed was assessed by tetrazolium-based colorimetric assay (MTT assay) and the interaction between pemetrexed and cisplatin was evaluated by the isobologram method. Western immunoblotting and real time RT-PCR analysis of thymidylate synthase (TS), folylpoly-gamma-glutamate synthetase (FPGS) and reduced folate carrier (RFC1) were performed in order to determine whether sensitivity to pemetrexed would be predictable by protein or mRNA expression levels. Pemetrexed was more cytotoxic than 5-fluorouracil, with IC50 between 17 and 310 nM in most of the gastric cancer cell lines examined and the pemetrexed/cisplatin combination resulted in additive or synergistic interaction. The protein expressions of TS, FPGS, and RFC1 were significantly associated with IC50 for 5-fluorouracil, but no such association was found for pemetrexed chemosensitivity. The mRNA expressions of RFC1, FPGS and other target and resistance related genes revealed no significant association with pemetrexed sensitivity. In conclusion, pemetrexed is active against gastric cancer cell lines and the pemetrexed/cisplatin combination showed a synergistic or additive interaction, supporting its clinical use in gastric cancer. Drug sensitivity toward pemetrexed could not be predicted by the expressions of TS, RFC1, or FPGS and we suggest that it is determined by interactions between multiple genes.

Induction of resistance to the multitargeted antifolate Pemetrexed (ALIMTA) in WiDr human colon cancer cells is associated with thymidylate synthase overexpression

Pemetrexed (ALIMTA, MTA) is a novel thymidylate synthase (TS) inhibitor and has shown activity against colon cancer, mesothelioma and nonsmall-cell lung cancer. We induced resistance to Pemetrexed in the human colon cancer cell line WiDr by using a continuous exposure to stepwise increasing Pemetrexed concentrations (up to 20 microM) as well as a more clinically relevant schedule with intermittent exposure (up to 50 microM) for 4 hr every 7 days, resulting in WiDr variants WiDr-cPEM and WiDr-4PEM, respectively. However, using the same conditions, it was not possible to induce resistance in the WiDr/F cell line, a variant adapted to growth under low folate conditions. Mechanisms of resistance to Pemetrexed were determined at the level of TS, folylpolyglutamate synthetase (FPGS) and reduced folate carrier (RFC). WiDr-4PEM and WiDr-cPEM showed cross-resistance to the polyglutamatable TS inhibitor Raltitrexed (6- and 19-fold, respectively) and the nonpolyglutamatable TS-inhibitor Thymitaq (6- and 42-fold, respectively) but not to 5-fluorouracil. The ratios of TS mRNA:beta actin mRNA in WiDr-4PEM and WiDr-cPEM were 5-fold (P=0.01) and 18-fold (P=0.04) higher, respectively, compared to WiDr (ratio: 0.012). In addition, TS protein expression in the resistant WiDr variants was elevated 3-fold compared to WiDr, while the catalytic activity of TS with 1 microM dUMP increased from 30 pmol/hr/10(6) cells in WiDr cells to 2201 and 7663 pmol/hr/10(6) cells in WiDr-4PEM and WiDr-cPEM, respectively. The activity of FPGS was moderately decreased, but not significantly different in all WiDr variants. Finally, no evidence was found that decreased catalytic activity of RFC was responsible for the obtained Pemetrexed resistance. Altogether, these results indicate that resistance to Pemetrexed in the colon cancer cell line WiDr was solely due to upregulation of TS of which all related parameters (mRNA and protein expression and TS activity) were increased, rather than alterations in FPGS or RFC activity.

Induction of thymidylate synthase as a 5-fluorouracil resistance mechanism

Thymidylate synthase (TS) is a key enzyme in the de novo synthesis of 2'-deoxythymidine-5'-monophosphate (dTMP) from 2'-deoxyuridine-5'-monophosphate (dUMP), for which 5,10-methylene-tetrahydrofolate (CH(2)-THF) is the methyl donor. TS is an important target for chemotherapy; it is inhibited by folate and nucleotide analogs, such as by 5-fluoro-dUMP (FdUMP), the active metabolite of 5-fluorouracil (5FU). FdUMP forms a relatively stable ternary complex with TS and CH(2)THF, which is further stabilized by leucovorin (LV). 5FU treatment can induce TS expression, which might bypass dTMP depletion. An improved efficacy of 5FU might be achieved by increasing and prolonging TS inhibition, a prevention of dissociation of the ternary complex, and prevention of TS induction. In a panel of 17 colon cancer cells, including several variants with acquired resistance to 5FU, sensitivity was related to TS levels, but exclusion of the resistant variants abolished this relation. For antifolates, polyglutamylation was more important than the intrinsic TS level. Cells with low p53 levels were more sensitive to 5FU and the antifolate raltitrexed (RTX) than cells with high, mutated p53. Free TS protein down-regulates its own translation, but its transcription is regulated by E2F, a cell cycle checkpoint regulator. Together, this results in low TS levels in stationary phase cells. Although cells with a low TS might theoretically be more sensitive to 5FU, the low proliferation rate prevents induction of DNA damage and 5FU toxicity. TS levels were not related to polymorphisms of the TS promoter. Treatment with 5FU or RTX rapidly induced TS levels two- to five-fold. In animal models, 5FU treatment resulted in TS inhibition followed by a two- to three-fold TS induction. Both LV and a high dose of 5FU not only enhanced TS inhibition, but also prevented TS induction and increased the antitumor effect. In patients, TS levels as determined by enzyme activity assays, immunohistochemistry and mRNA expression, were related to a response to 5FU. 5FU treatment initially decreased TS levels, but this was followed by an induction, as seen with an increased ratio of TS protein over TS-mRNA. The clear retrospective relation between TS levels and response now forms the basis for a prospective study, in which TS levels are measured before treatment in order to determine the treatment protocol.

Diagnostic and prognostic tumor markers in the gastrointestinal tract

The gastrointestinal tract is the most common site of malignancies of any anatomic system in the body. An early detection of primary tumors of the bowel, pancreas, liver, stomach, and esophagus is often difficult in asymptomatic patients and for this reason these tumors are often detected at a relatively advanced stage, when symptoms lead to a diagnostic evaluation. Furthermore, gastrointestinal tract tumors have an extremely variable prognosis; thus, the identification of new prognostic parameters may be useful for selecting patients to more tailored therapies. In this work, the main molecular, genetic, tissular, and circulating tumor markers proposed for diagnosis and prognosis of gastrointestinal malignancies are reviewed and discussed.

5-Fluorouracil induced Fas upregulation associated with apoptosis in liver metastases of colorectal cancer patients

BACKGROUND

In vitro, thymidylate synthase (TS) inhibition by 5-fluorouracil (5-FU) induces thymineless apoptosis possibly via Fas receptor Fas ligand interactions and cell-cycle arrest. In colorectal cancer patients we evaluated whether 5-FU administration also resulted in apoptosis and cell-cycle arrest and which proteins might be involved.

PATIENTS AND METHODS

Biopsy specimens were taken from 36 patients 2, 22 or 46 hours after administration of 500 mg/m2 5-FU, and from 12 control patients who did not receive 5-FU. In frozen tissue-sections from liver metastases immunohistochemistry was performed with antibodies directed against p53, p21, E2F2, Rb, Ki67 and TS (cell-cycle related) and bax, BCL-2, BCL-x, mcl-1, PARP, caspase-3, Fas receptor and Fas ligand (apoptosis related). Apoptosis was determined by M30 immunostaining, which recognises a cleavage product of cytokeratin 18.

RESULTS

Fas receptor expression was 50% higher (P = 0.036) 46 hours after 5-FU administration compared to the control group. This was associated with a 12% increase (P < 0.02) in M30 positive tumour cells and with elevation of caspase-3 and PARP expression. The expression of Ki67 and E2F2 was 30% lower after 46 hours compared to the control group, whereas TS was 56% lower after 2 hours and 32% higher again after 46 hours. No differences in the expression of the other proteins were found.

CONCLUSIONS

These results suggest that 5-FU decreases proliferation status and induces apoptosis possibly via the Fas pathway. Since Fas mediated cell killing is important for cytotoxic T cells this indicates that clinical studies combining immunotherapy for activation of T cells and chemotherapy using 5-FU might be very effective.

Thymidylate synthase expression in patients with colorectal carcinoma using a polyclonal thymidylate synthase antibody in comparison to the TS 106 monoclonal antibody

Colorectal cancer is one of the most common human cancers, for which 5-fluorouracil (5FU) is usually part of the treatment. Thymidylate synthase (TS), the target enzyme for 5FU, can be predictive for the outcome of 5FU-based therapy. TS levels in tumor samples can be determined with radiochemical enzyme assays, RT-PCR, and immunohistochemical staining. We validated TS immunohistochemistry with a polyclonal rabbit anti-human TS antibody using the avidin-biotin method. This antibody can be used on paraffin-embedded, formalin-fixed material using an antigen retrieval method with citrate buffer and microwave treatment. The antibody shows a granular cytosolic staining pattern. The reproducibility in cross-sections from colorectal tumors from 50 patients was 90% and the interobserver variability was acceptable with a kappa of 0.45. On Western blotting it detects purified TS at 36 kD, while in 5FU-treated cells the ternary complex between FdUMP, TS, and 5, 10-methylene-tetrahydrofolate is clearly visible at 38 kD, with no other interfering bands. In a separate set of tumors, immunostaining was compared with enzyme levels; Western blots correlated with enzyme levels. Because both this polyclonal antibody and the monoclonal antibody TS-106 are being used for large-scale studies, we also determined whether they could be used interchangeably. No differences were observed. This polyclonal antibody is specific and gives reproducible results. A study on a larger scale is ongoing to determine the role of TS as a predictive parameter in patients with colorectal cancer treated either with postoperative adjuvant 5FU/levamisole or with surgery only.

Sequence-dependent growth inhibition and DNA damage formation by the irinotecan-5-fluorouracil combination in human colon carcinoma cell lines

We evaluated irinotecan (CPT-11) together with 5-fluorouracil (5-FU) for improved cell growth inhibition with respect to that by either agent alone in the human colon carcinoma cell lines SW620, HT-29 and SNU-C4. Cells were exposed for 24 h to each drug, as well as to various combinations and sequences of low, fixed doses of one drug with higher varying doses of the other, cultured for two more days in drug-free medium and then assessed for growth response with the sulphorhodamine B assay. Multiple drug effect analysis was used to evaluate the data, which were then related to the amount of DNA damage occurring in the cells which was determined by a fluorescence-enhancement assay for DNA unwinding. Cellular responses were also related to thymidylate synthase topoisomerase I and carboxyl esterase activities, which were assessed by a ligand-binding and a 3H-release assay; a DNA decatenation assay; and a spectrophotometric method, respectively. IC50 values for 5-FU alone in the SW620, HT29 and SNU-C4 cells were 15.3 +/- 0.8, 8.2 +/- 1.3 and 2.2 +/- 0.7 microM, respectively, and for CPT-11 2.0 +/- 0.9, 2.5 +/- 0.5 and 3.8 +/- 0.3 microM, respectively. The differential responses to 5-FU alone were possibly determined by differences in substrate affinity and conversion rate of thymidylate synthase (K(m) of approximately 7.5, 5.0 and 2.5 microM and V0 of approximately 800, 200 and 2400 microM/h, respectively). The comparable cellular responses to CPT-11 alone might be accounted for by the counterbalancing effects of differences in topoisomerase I (1, 1, and 1.5 arbitrary units, respectively) and carboxyl esterase activities (5055 +/- 1789, 4080 +/- 752, 1713 +/- 522 mU/mg, respectively). IC20 CPT-11 prior to 5-FU was additive to synergistic in SW620, HT-29 and SNU-C4 cells (CIs of 0.7 +/- 0.1). By contrast, pre-treatment with IC20 5-FU antagonised the CPT-11-mediated growth inhibition (CIs of 1.9 +/- 0.4, 1.7 +/- 1.1, 2.5 +/- 0.9, respectively). Simultaneous drug treatment did not produce more cell growth inhibition than either drug alone in the SW620 and the HT-29 cells, but was additive or antagonistic in the SNU-C4 cells (CIs of 1.1 +/- 0.3 and 2.2 +/- 1.4), depending on the ratio of the drugs. Increased DNA damage in the SW620 and HT-29 cells was only seen when IC20 CPT-11 preceded IC50 5-FU, resulting in approximately 40 and 25%, respectively, more lesions than for IC50 5-FU alone. In the SNU-C4 cells, not only such a treatment, but also simultaneous drug treatment produced (30 to 60%) more DNA damage than either drug alone. Our results show clear sequence-dependent antiproliferative effects and DNA damage formation by CPT-11 and 5-FU at combinations of low, fixed doses with higher, varying doses in cultured human colon carcinoma cells, and may be of relevance to the design of improved chemotherapeutic regimens in this disease.

Comparison of 5-fluoro-2'-deoxyuridine with 5-fluorouracil and their role in the treatment of colorectal cancer

Despite more than 30 years of intensive studies on new drugs against advanced colorectal cancer, the fluoropyrimidines remain the drugs of choice for systemic treatment and for hepatic artery infusion (HAI). This overview describes new developments in advanced colorectal cancer chemotherapy, providing a rationale for more effective use of the fluoropyrimidines, with biochemical modulation, scheduling or by revealing biochemical mechanisms of action that correlate with antitumour activity. In human colorectal cancer cell lines and various animal tumour model systems 5-fluoro-2'-deoxyuridine (FdUrd) is more effective than 5-fluorouracil (5-FU). Comparably, FdUrd's modulation by leucovorin (LV) is more potent than 5-FU. In animal studies it is shown that intermittent high-bolus administration of FdUrd generates better antitumour activity, compared with equal toxic doses or any other schedule of 5-FU. These effects are related to prolonged-thymidylate synthase (TS) inhibition and the prevention of TS induction, rather than RNA incorporation. Preclinical studies with modulators such as N-phosphonacetyl-L-aspartate (PALA), WR-2721, mitomycin C and platinum derivatives provide a rationale for clinical use in the future. The first choice systemic chemotherapy of patients with advanced colorectal cancer remains 5-FU combined with LV. Some improvement in therapeutic efficacy has been achieved with locoregional HAI. In randomised studies HAI FdUrd improves the quality of life and survival as compared with optimal systemic therapy. Chronomodulation decreases toxicity, allowing dose intensification, while modulators such as LV or dexamethasone increase survival of patients treated with HAI FdUrd to 86% after 1 year. In conclusion, the clinical use of FdUrd has not been fully explored. Intermittent high-dose FdUrd, chronomodulation together with the use of modulators or drugs focused on prolonged TS inhibition, should be studied in large randomised studies.

Cross-resistance to antifolates in multidrug resistant cell lines with P-glycoprotein or multidrug resistance protein expression

Resistance to some (lipophilic) antifolates has been associated with P-glycoprotein (P-gp)-mediated multidrug resistance (MDR). A possible relationship with non-P-gp MDR has not been established. We studied resistance to antifolates in SW-1573 human lung carcinoma cells, a P-gp overexpressing variant SW-1573/2R160 and a multidrug resistance protein (MRP) overexpressing variant SW-1573/2R120. In this study, thymidylate synthase (TS) inhibitors with different properties concerning the efficiency of membrane transport and the efficiency of polyglutamylation were tested for cross-resistance in SW-1573/2R120 and SW-1573/2R160 cells. Growth inhibition patterns in this cell line panel were measured by the Sulforhodamine B (SRB) assay. Resistance factors for TS inhibitors were: 2.4 and 0.4 for 5-fluorouracil (5FU), 18.8 and 8.8 for ZD1694, 17 and 0.7 for AG337, and 40 and 8.3 for BW1843U89 in SW-1573/2R160 and SW-1573/2R120, respectively. This study showed changes in the TS enzyme kinetics during the induction of doxorubicin resistance in both SW-1573 variants, resulting in 2-fold lower Km values for 2'-deoxyuridine-5'-monophosphate (dUMP) in both resistant variants compared to the parental cell line. TS activity, TS protein induction and TS mRNA expression all had 2-fold increased in the SW-1573/2R120 compared to the SW-1573/2R160. 3H-MTX influx was 2-fold lower in SW-1573/2R160 cells compared to SW-1573/2R120 and SW-1573 cells. In the SW-1573/2R160 cell line, an aberrant intracellular trafficking towards the target TS was observed, compared to SW-1573/2R120 and SW-1573 cells as measured by the TS in situ assay. The rate of TS inhibition by the TS inhibitors used in this study was similar in all cell lines. In conclusion, collateral sensitivity to 5FU and the lipophilic AG337 and cross-resistance to other antifolates were observed in non-P-gp MDR SW-1573/2R120 cells, as well as resistance to all antifolates in P-gp SW-1573/2R160 cells. The mechanism of resistance in SW-1573/2R160 cells possibly involves reduced influx and changes in intracellular trafficking routes. For the SW-1573/2R120 cell line, several changes related to the TS enzyme possibly play a role in the observed cross-resistance and collateral sensitivity pattern.

Lack of correlation between thymidylate synthase levels in primary colorectal tumours and subsequent response to chemotherapy

The increasing interest in 5-fluorouracil (5-FU) modulation and the development of new antifolates has focused attention in recent studies on the expression of the target enzyme thymidylate synthase (TS) as a determinant of drug sensitivity and resistance. Resistance to TS-directed drugs has been shown to occur in vitro and in vivo with increased expression of the enzyme (determined by enzymatic assays as well as protein and gene expression assays). Several studies have evaluated the role of TS as a prognostic indicator of clinical response to chemotherapy containing TS-directed drugs. We have used a polyclonal antibody to recombinant human TS to establish a silver-enhanced immunogold staining method to localize TS in human tumours. Human tumour cell lines with acquired resistance to TS inhibitors owing to increased levels of TS were used to confirm the specificity of immunostaining. Stained sections were evaluated by image analysis. Immunostaining in tumour sections was greatly reduced (>80%) by preabsorption of the antiserum with recombinant TS. The method was used to determine the extent of TS immunostaining in 134 primary human colorectal tumours. The results were then compared with the clinical outcome and response to chemotherapy for the treatment of subsequent metastatic disease. A wide range (approximately 100-fold) of TS immunostaining was observed in these primary tumour sections. Normal mucosal tissue levels were 5-10 times lower than those observed in the adjacent tumour tissue. The values for TS immunostaining did not correlate with clinical endpoints, such as time from diagnosis to relapse, response to chemotherapy for disseminated disease, nor with Dukes' staging. This lack of correlation may be because this group of patients was selected on the basis of their need for palliative chemotherapy and did not include patients who were cured of their disease. Also, primary tumour TS expression may not give a good indication of the TS expression in metastatic lesions. The prognostic significance of TS protein expression in primary and metastatic lesions requires further evaluation.

The relationship between intrinsic thymidylate synthase expression and sensitivity to THYMITAQ in human leukaemia and colorectal carcinoma cell lines

Thymidylate synthase (TS) expression has been characterized for a panel of eight human colorectal carcinoma and five human leukaemia cell lines, to relate differences in intrinsic TS activity, protein and mRNA levels to growth inhibition caused by continuous exposure to THYMITAQ, a specific non-classical antifolate TS inhibitor. Although a 20-fold variation in sensitivity to THYMITAQ was found within the colorectal cell line panel (IC50 0.12-2.7 microM), sensitivity was not related to TS activity, TS protein or TS mRNA levels. For the leukaemic cell lines, only a twofold range in sensitivity to THYMITAQ was observed (IC50 0.87-2.3 microM), and this did not correlate with TS activity, TS protein or TS mRNA levels. Across all of the cell lines, TS activity was linearly related to TS protein levels (r2 = 0.87, P < 0.0001). However, for both the colorectal and leukaemia cell line panels, no relationship was found between TS mRNA/18S rRNA ratios and either TS activity or TS protein, consistent with the importance of post-transcriptional mechanisms in regulating TS activity. Two of the colorectal cell lines (BE and HCT116) and one of the human leukaemic cell lines (HL60), were intrinsically resistant to THYMITAQ (IC50 > 2 microM) in the absence of TS overexpression, suggesting that, subsequent to TS inhibition, events such as DNA repair and tolerance to apoptotic stimuli are also important determinants of sensitivity to THYMITAQ.

Thymidylate synthase and drug resistance

Thymidylate synthase is an important target for both fluorinated pyrimidines and for new folate analogues. Resistance to 5-fluorouracil (5FU) can be related to insufficient inhibition of thymidylate synthase. The 5FU-nucleotide FdUMP induces inhibition of thymidylate synthase which is enhanced and retained for longer in the presence of increased folate pools, for which leucovorin is a precursor. In a murine model system, 5FU treatment caused a 4-fold induction of thymidylate synthase levels which may have contributed to resistance. Addition of leucovorin to this treatment prevented this induction and increased the antitumour effect 2-3-fold. In the clinical setting, 5FU administration to patients resulted in approximately 50% inhibition of TS after 48 h. The combination with leucovorin resulted in a more pronounced inhibition after 48 h (approximately 70%). A significant relationship was observed with outcome of treatment; when thymidylate synthase levels were high and inhibition was low, no response was observed. A separate study showed that low thymidylate synthase levels appeared to be an independent prognostic factor for adjuvant therapy.

Quantitation of intratumoral thymidylate synthase expression predicts for resistance to protracted infusion of 5-fluorouracil and weekly leucovorin in disseminated colorectal cancers: preliminary report from an ongoing trial

A clinical trial for patients with measurable, disseminated colorectal cancer is being conducted to determine: (1) if intratumoral expression of thymidylate synthase (TS) affects response to protracted-infusion 5-fluorouracil (5FU); and (2) whether intratumoral expression of TS increases when clinical resistance is found after response to 5-FU. Polymerase chain reaction technology is employed to determine TS expression. Using beta-actin as an internal standard, TS expressions for 26 patients range from 0.5 x 10(-3) to 22.6 x 10(-3). Currently, 22 patients are evaluable for response and TS quantitation of their measurable tumour. 8 patients (36%) have had partial responses; 3 responding patients had been previously treated with 5-FU. A strong statistical association between TS expression and resistance to therapy has been found (P = 0.004). No patient with TS expression of 4.0 x 10(-3) or greater has responded. On average, patients previously treated with 5-FU have slightly higher levels of TS expression in their measurable tumours (P = 0.4). Whether responding patients will develop increased expressions of TS upon clinical progression of their cancer remains to be determined. Confirmation of these results in a larger cohort could lead to a scientific rationale for deciding upon specific therapy for patients with disseminated colorectal cancers.

Phase II trial of 5-fluorouracil and the natural l isomer of folinic acid in the treatment of advanced colorectal carcinoma

Between February 1991 and July 1992, 79 previously untreated patients with metastatic colorectal carcinoma were enrolled in a phase II study of combined 5-fluorouracil (5-FU) and l-folinic acid (FA). 5-FU 370 mg/m2/day was administered for 5 consecutive days as an intravenous (i.v.) bolus injection preceded by l-FA 100 mg/m2/day with the same administration modality. Treatment was given every 4 weeks until progression. 79 patients were evaluable for toxicity and 64 for response. 2 patients (3%) achieved a complete remission and 8 (12.5%) a partial remission, 33 (52%) had stable disease and 21 patients (33%) had progressive disease. Median duration of remission was 32.5 weeks and median survival for all evaluable patients was 64.5 weeks. Substantial to severe side-effects occurred in 39% of patients. Dose-limiting toxicity (grade 3-4) was mainly diarrhoea (18%) and mucositis (15%). Nausea/vomiting, cutaneous toxicity, leucopenia, alopecia and conjunctivitis of grade 3-4 occurred respectively in 6, 4, 2.5, 1 and 1% of cases. Toxicity appeared to be substantially similar to that characteristic of combined 5-FU and the chiral mixture of d,l-FA. Efficacy was within the range of that observed with the 5-FU/d,l-FA combination, although at the lower level.

In vitro antitumour activity of cis- and trans-5-fluoro-5,6-dihydro-6-alkoxy-uracils; effects on thymidylate synthesis

A class of new 5-fluorouracil (FU) analogues, the 5-fluoro-5,6-dihydro-6- alkoxy-uracils was synthesised with a modification at the 6-position of the pyrimidine ring. At this position the analogues have a hydroxy or alkoxy group of different chain lengths either in the cis- or trans-configuration. The antiproliferative effect of these compounds was tested on five cell lines of different origin. Generally, the analogues with a cis-configuration had a higher activity than those with a trans-configuration. The growth inhibitory effect of the compounds decreased with increasing alkoxy chain length, but the compound with a hydroxy group had the lowest growth inhibitory effect. One analogue, cis-5-F-5,6-dihydro-6-methoxy-uracil had a higher antiproliferative effect than FU in one of the cell lines. Effects on thymidylate synthase (TS), the possible target of these analogues, were evaluated by thymidine rescue of growth inhibition and incorporation of tritiated deoxyuridine (3H-UdR) into DNA. In solid tumour cell lines addition of TdR reversed the antiproliferative effect. Inhibition of TS in intact cells was determined by measuring 3H-UdR incorporation in two cell lines. The effect of cis-5-F-5,6-dihydro-6-methoxy-uracil on incorporation of 3H-UdR was 2- to 5-fold stronger than that of FU in both cell lines. All other compounds produced a higher 3H-UdR incorporation than FU both at equimolar and equi-toxic concentration. Concluding from these results we regard cis-5-F-5,6-dihydro-6-methoxy-uracil as the most promising FU analogue of this series, because of its higher antiproliferative activity than FU and marked inhibition of TS in intact cells.

Effect of folate diastereoisomers on the binding of 5-fluoro-2'-deoxyuridine-5'-monophosphate to thymidylate synthase

A series of natural and unnatural stereoisomers of reduced folate coenzymes have been studied for their capacity to facilitate binding of 5-fluoro-2'-dUMP (FdUMP) to bacterial thymidylate synthase (TS). The natural cosubstrate for the enzyme, (6R)-5,10-methylene-tetrahydrofolate (CH2-H4-folate), was 4-fold more potent than the unnatural 6S-form in promoting FdUMP binding to TS, but in a racemic mixture the effect of the 6R-form was not affected by the 6S-form. FdUMP binding to TS was also stimulated by tetrahydrofolate and dihydrofolate (85% and 30% as compared to (6RS)-CH2-H4-folate, respectively), but not by the stereoisomers of 5-methyl-tetrahydrofolate and 5-formyl-tetrahydrofolate (leucovorin). These results suggest that folates, which are not a natural cosubstrate for TS, have an additional role in facilitating FdUMP binding to TS.

Biochemical modification of the toxicity and the anti-tumour effect of 5-fluorouracil and cis-platinum by WR-2721 in mice

WR-2721 (ethiofos) was tested on Balb/c mice for its chemoprotective capacity against 5-fluorouracil (5FU) monotherapy. In this combination WR-2721 was not active, but WR-2721 pretreatment allowed an elevation of the cisplatin (CDDP) dose in 5FU/CDDP combination therapy in these mice. Thrombocytopenia caused by the 5FU/CDDP (100 and 7 mg/kg, respectively) therapy was prevented by WR-2721 (200 mg/kg) and a partial protection against leukopenia was observed in C57Bl/6 mice. Various WR-2721/CDDP/5FU combinations were tested on two murine colon tumour models. The best antiproliferative effect against Colon 26 (in Balb/c mice) and the lowest toxicity were found with 5FU (100 mg/kg) and CDDP (5.5 mg/kg) delivered together 30 min after WR-2721 (200 mg/kg). The increased efficacy of WR-2721/CDDP/5FU both in Colon 26 and Colon 38 (in C57Bl/6 mice) compared to single 5FU or 5FU/CDDP treatment at the same dose could not be explained by enhanced inhibition of thymidylate synthase (TS), the 5FU target enzyme. The protection by WR-2721 against toxicity of CDDP/5FU might enable the use of high doses of CDDP in this combination.

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.

Clinical relevance of biochemical modulation of 5-fluorouracil

Biochemical modulation is a special type of combination chemotherapy which aims to selectively improve the therapeutic index by increasing the antitumor effect and protecting against toxic side effects. Biochemical modulation seems to be an attractive way to circumvent quantitative and qualitative heterogeneity of tumors. In the past decade a number of biochemical modulation approaches have been tested to improve the activity of 5-fluorouracil (5FU). 5FU itself has only modest anticancer activity but has been shown to be a very attractive target for biochemical modulation. A number of the combinations have been ineffective in the clinic despite extensive testing in a number of schedules. Some other combinations were initially tested in an inappropriate schedule, but were active when applied in another schedule. The latter was made possible by a systematic preclinical development of combinations with a proper translation to the clinic accompanied by pharmacodynamic evaluation. This review describes a number of biochemical modulation combinations, both inactive and active. The main conclusion is that properly applied biochemical modulation schedules may lead to successful use in the clinic.