Relationship between 5-fluoro-2'-deoxyuridylate, 2'-deoxyuridylate, and thymidylate synthase activity subsequent to 5-fluorouracil administration, in xenografts of human colon adenocarcinomas

Removal of uracil by uracil DNA glycosylase limits pemetrexed cytotoxicity: overriding the limit with methoxyamine to inhibit base excision repair

Uracil DNA glycosylase (UDG) specifically removes uracil bases from DNA, and its repair activity determines the sensitivity of the cell to anticancer agents that are capable of introducing uracil into DNA. In the present study, the participation of UDG in the response to pemetrexed-induced incorporation of uracil into DNA was studied using isogenic human tumor cell lines with or without UDG (UDG(+/+)/UDG(-/-)). UDG(-/-) cells were very sensitive to pemetrexed. Cell killing by pemetrexed was associated with genomic uracil accumulation, stalled DNA replication, and catastrophic DNA strand breaks. By contrast, UDG(+/+) cells were >10 times more resistant to pemetrexed due to the rapid removal of uracil from DNA by UDG and subsequent repair of the resultant AP sites (abasic sites) via the base excision repair (BER). The resistance to pemetrexed in UDG(+/+) cells could be reversed by the addition of methoxyamine (MX), which binds to AP sites and interrupts BER pathway. Furthermore, MX-bound AP sites induced cell death was related to their cytotoxic effect of dual inactivation of UDG and topoisomerase IIα, two genes that are highly expressed in lung cancer cells in comparison with normal cells. Thus, targeting BER-based therapy exhibits more selective cytotoxicity on cancer cells through a synthetic lethal mechanism.

Enhancement of chemosensitivity toward peplomycin by calpastatin-stabilized NF-kappaB p65 in esophageal carcinoma cells: possible involvement of Fas/Fas-L synergism

Chemosensitivity to anticancer drugs was compared between two human esophageal carcinoma cell lines, T.Tn and YES-6 cells. T.Tn cells were more resistant than YES-6 cells to peplomycin (PEP) but not to the other anticancer drugs such as camptothecin, mitomycin C and cytosine arabinoside. Western blot analysis showed higher expression levels of m-calpain and activated mu-calpain in T.Tn cells than in YES-6 cells. On the other hand, YES-6 cells showed a high expression level of calpastatin, which is a calpain-specific endogenous inhibitor. To investigate whether calpain activity was involved in the chemosensitivity, T.Tn cells were transfected with calpastatin cDNA in an inducible expression vector. The induction of calpastatin was accompanied by increased chemosensitivity to PEP. The increases in calpastatin levels were followed by serial increases in the expression levels of NF-kappaB p65 and Fas. Since purified m- or mu-calpain degraded NF-kappaB p65 in vitro, it is possible that calpastatin suppressed calpain-mediated degradation of NF-kappaB p65. Fas ligand (Fas-L) protein levels increased after treatment of the parental T.Tn and calpastatin-transfected cells with PEP, suggesting the synergism between calpastatin-induced Fas and PEP-induced Fas-L. These results suggest that calpain/calpastatin expression levels are effective markers for predicting the sensitivity of human esophageal carcinoma cells to PEP.

Translational up-regulation of antifolate drug targets in the human malaria parasite Plasmodium falciparum upon challenge with inhibitors

The thymidylate cycle in Plasmodium falciparum is essential for cell growth and replication, and dihydrofolate reductase (DHFR), a key enzyme in this cycle, is the target of important antimalarial drugs such as pyrimethamine and cycloguanil. Following previous work, where we found no evidence of upregulation of the dhfr-ts gene upon challenge with pyrimethamine, we investigated the expression at the protein level of the bifunctional gene product, which also carries thymidylate synthase (TS) activity. Challenge of parasite cultures with fluoro-substituted bases that are specific TS inhibitors at levels close to the IC(50) resulted in five to seven-fold increases in enzyme level, as monitored by both DHFR and TS activities, while pyrimethamine and another DHFR-binding inhibitor, WR99210, induced smaller but still significant increases of approximately three-fold. However, when parasites were challenged with tetracycline, an antimalarial not directed at the folate pathway, although an increase was consistently seen above untreated controls, this was at a level of approximately 1.8-fold. These increases reflect enhanced synthesis of the DHFR-TS enzyme, rather than liberation of a latent activity, as they were completely abolished if cultures were pre-incubated with cycloheximide to block de novo protein synthesis. Moreover, none of the above antimalarial drugs was found to significantly alter absolute levels of the dhfr-ts mRNA under the conditions of challenge used. We conclude that, in common with mammalian systems, where a similar phenomenon has been reported, malaria parasites are able to significantly relieve translational constraint when faced with antifolate drug challenge. The data indicate that there is a specific component in addition to a low-level non-specific increment, and that binding to the TS domain of the DHFR-TS protein appears to be better able to relieve this constraint than binding to the DHFR domain.

Overexpression of thymidylate synthase mediates desensitization for 5-fluorouracil of tumor cells

The prognosis of cancers of various organs overexpressing thymidylate synthase (TS) has been reported to be poor. It has been suggested that the poor prognosis is partly due to a low sensitivity of TS-overexpressing tumors to TS-targeting 5-fluorouracil (5-FU). To investigate the relationship between TS expression and sensitivity to 5-FU, we used the TS-overexpressing cervical cancer cell line SKG-II/TS and SKG-I/TS that had been established by TS gene transfer. The 50% growth inhibitory concentration (IC(50)) of 5-FU for SKG-II/TS was 24 +/- 6.0 microM, which was 6 times as high as that for the control (4.0 +/- 1.1 microM), showing significantly decreased sensitivity to 5-FU (p < 0.01). The IC(50) of 5-FU for SKG-I/TS was 90 +/- 15 microM, which was over 2 times as high as that for the control (40 +/- 0.6 microM), showing significantly decreased sensitivity to 5-FU (p < 0.05). Thus, TS-overexpressing tumors have decreased sensitivity to 5-FU, which may be one of the factors that determine the prognosis of these tumors.

Enhanced expression of thymidylate synthase mediates resistance of uterine cervical cancer cells to radiation

It has been shown that there is an inverse relationship between the level of thymidylate synthase (TS) and therapeutic outcomes in patients with malignancies including cervical cancer. To clarify the mechanism of the poor prognosis of cervical cancer with high TS expression, we introduced TS cDNA to the human uterine cervical cancer cell line SKG-II and evaluated the effect of TS expression on its radiosensitivity. After selection, stable transformants of SKG-II cells expressing high level of TS (SKG-II/TS) and control cells (SKG-II/luciferase) were obtained. The level of TS measured by the FdUMP-TS binding assay was significantly higher (p < 0.05) in SKG-II/TS than in control (2.0 +/- 0.1 and 1.3 +/- 0.1 pmol/mg, respectively). No difference was observed in in vitro cell growth or in vivo tumor growth. On evaluation of in vitro radiosensitivity, the 50% growth inhibitory dose (ID(50)) was 3.1 +/- 0.1 Gy in SKG-II/TS and was significantly higher (p < 0.01) than that in control (2.2 +/- 0.1 Gy). From these results, it is suggested that one of the reasons of poor outcome of cervical cancer to radiation is high TS expression.

Facilitation of adenoviral wild-type p53-induced apoptotic cell death by overexpression of p33(ING1) in T.Tn human esophageal carcinoma cells

To investigate the effect of p33(ING1) on wild-type p53 gene therapy, T.Tn human esophageal carcinoma cells were stably transfected with p33(ING1) cDNA. Infection with Ad-p53 (recombinant adenovirus containing wild-type p53) into p33-transfected cells reduced cell viability, while infection with empty vector had little effect. This reduced viability was shown to be due to apoptotic cell death by the TUNEL (terminal deoxynucleotidyl transferase-mediated nick end-labeling) assay. Following infection with Ad-p53, levels of p53 were similar in p33-expressing cells and in the parental line. However, levels of p21 and Mdm2 were elevated in p33-transfected cells. Nonetheless, this enhanced expression of Mdm2 appeared to be ineffective in downregulating p53. Transient transfection with mutant Mdm2 prior to Ad-p53 infection provided a significant protection as compared with cells transfected with wild-type Mdm2. These results imply a synergistic effect between p33 and p53 in the induction of apoptosis of human esophageal carcinoma cells. A role for Mdm2 in this synergism is suggested.

Thymidylate Synthase expression as a predictor of clinical response to fluoropyrimidine-based chemotherapy in advanced colorectal cancer

Thymidylate Synthase (TS) is a rate-limiting enzyme in the DNA synthetic pathway and represents the cellular target of the antimetabolite drug 5-fluorouracil (FUra). Both preclinical and clinical studies have shown that the level of expression of this enzyme and the ability to achieve its inhibition are the major determinants of sensitivity and resistance to fluoropyrimidines (FP). In particular, five recent studies have consistently demonstrated an inverse correlation between the level of TS gene or protein expression measured in colorectal cancer metastases and the clinical response to either FUra or 5-fluorodeoxyuridine (FUdR). Patients with low levels of TS expression in their metastases have indeed shown response rates that are three to ten times higher compared to those obtained in patients with high TS levels. The independent predictive value demonstrated in a logistic regression model, the longer survival shown by patients with low TS levels in three of five studies and the consistency of the results obtained by independent groups using different techniques to quantitate TS expression, strengthen the predictive role of TS. Targeted treatment of colorectal cancer based on TS quantitation has thus been hypothesized similar to the use of hormone receptor in breast cancer. In this review preclinical and clinical data supporting the use of TS quantitation to predict for the clinical response to FUra will be described and unresolved problems including assays standardization, response prediction based on TS levels measured in primary tumors, intrapatient variations in TS levels and biological/biochemical limitations of this strategy will be discussed.

Enhanced growth suppression in esophageal carcinoma cells using adenovirus-mediated fusion gene transfer (uracil phosphoribosyl transferase and herpes simplex virus thymidine kinase)

Advanced esophageal cancers are highly malignant and frequently resistant to 5-fluorouracil (5-FU). Escherichia coli uracil phosphoribosyltransferase (UP) is a pyrimidine salvage enzyme that alters 5-FU metabolism and sensitivity. A recombinant adenovirus encoding the UP gene (AxCA.UP) has been applied in gastric cancer gene therapy to sensitize cancer cells to lower concentrations of 5-FU. We have generated a recombinant adenovirus (AxCA.UT) encoding UP and herpes simplex virus thymidine kinase fusion protein (UT) to examine whether it would enhance the antitumor activity of AxCA.UP treatment. AxCA.UT treatment significantly enhanced the sensitivity of human esophageal cancer cells to and significantly enhanced the growth inhibition effects of UP gene therapy in vitro. Moreover, both 5-FU and ganciclovir showed bystander effects on growth inhibition. In an in vivo study, the therapeutic outcome of AxCA.UT treatment significantly enhanced the antitumor activity of AxCA.UP treatment. These observations suggest that AxCA.UT may be useful in esophageal cancer gene therapy.

Positive interaction between 5-FU and FdUMP[10] in the inhibition of human colorectal tumor cell proliferation

Interaction between 5-fluorouracil (5-FU) and FdUMP[10], a novel pro-drug formulation of the thymidylate synthase (TS) inhibitory nucleotide 5-fluoro-2'-deoxyuridine-5'-O-monophosphate (FdUMP), was investigated to evaluate the feasibility of using these two forms of fluorinated pyrimidine in combination chemotherapy regimens. 5-FU and FdUMP[10] are expected to differ in their relative intracellular distribution of active metabolites, and their combined administration may result in either a positive or a negative interactive effect. The dose-response behaviors of 5-FU and FdUMP[10] toward H630 and H630-10 (human colorectal tumor) cells were first investigated separately. Effects on cell viability were measured using an assay for 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), while cytotoxicity and apoptosis were investigated using clonogenic and TUNEL assays, respectively. Exposure of H630 cells to concentrations of FdUMP[10] insufficient to inhibit cell proliferation as a single agent markedly increased the cytotoxicity of 5-FU. The results indicate that 5-FU and FdUMP[10] interact in a positive manner, and that combining these two forms of fluorinated pyrimidine may be clinically beneficial.

Increased cytotoxicity and decreased in vivo toxicity of FdUMP[10] relative to 5-FU

The efficacy of treatment with 5-Fluorouracil (5-FU) is limited, in part, by its inefficient conversion to 5-Fluoro-2'-deoxyuridine-5'-O-monophosphate (FdUMP). We present data indicating that FdUMP[10], designed as a pro-drug for intracellular release of FdUMP, is cytotoxic as a consequence of uptake of the multimeric form. FdUMP[10] is stable in cell culture medium, with more than one-half of the material persisting as multimers of at least six nucleotides after a 48 h incubation at 37 degrees C. FdUMP[10] is more than 400 times more cytotoxic than 5-FU towards human colorectal tumor cells (H630). FdUMP[10] also has decreased toxicity in vivo, with doses as high as 200 mg/kg/day (qdx3) administered to Balb/c mice without morbidity, compared to a maximum tolerated dose of 45 mg/kg/day for 5-FU using the same protocol. FdUMP[10] shows reduced sensitivity to OPRTase- and TK-mediated drug resistance, relative to 5-FU and FdU, respectively, and is much more cytotoxic than 5-FU towards cells that overexpress thymidylate synthase. Thus, FdUMP[10] is less susceptible to resistance mechanisms that limit the clinical utility of 5-FU. The increased cytotoxicity, decreased toxicity in vivo, and reduced sensitivity to drug resistance of FdUMP[10], relative to 5-FU, indicates multimeric FdUMP is potentially valuable as an anti-neoplastic agent, either as a single agent, or in combination with 5-FU.

Enhanced expression of thymidylate synthase may be of prognostic importance in advanced cervical cancer

The enhanced expression of thymidylate synthase (TS) has been associated with a poor prognosis in patients with several types of epithelial tumors. To determine the association between TS expression and the prognosis of patients with advanced cervical cancer after radiation therapy, we immunohistochemically assayed TS levels in paraffin-embedded tissue sections from 66 patients with stage IIIb cervical cancer using a polyclonal antibody to recombinant human TS. In the 30 patients with high TS expression, the cumulative 5- and 8-year survival rates were 36.8% (95% CI: 17. 4-56.2) and 31.6% (95% CI: 12.4-50.7), respectively. In contrast, the 36 patients with low TS expression showed a significantly (p < 0. 001) better prognosis, with cumulative 5- and 8-year survival rates of 87.2% (95% CI: 75.5-99.0) and 69.2% (95% CI: 50.7-87.7), respectively. These results suggest that TS expression may be useful in determining the prognosis of patients with advanced cervical cancer.

Immunohistochemical investigation of thymidylate synthase in cervical cancer

We prepared a polyclonal antibody against recombinant human thymidylate synthase (TS) to evaluate its use in immunohistochemical analysis. Formalin-fixed, paraffin-embedded sections of specimens of cervical tissue were prepared from 35 patients with cervical cancer. The TS immunoreactivity was studied by employing the peroxidase-labeled antibody method with our prepared antibody. At the same time, the TS activity was determined with the 5-fluoro-2'-deoxyuridylate (FdUMP)-TS-binding assay in fresh-frozen specimens of these cervical cancers. We observed a positive correlation (r = 0.658) between the degree of TS immunoreactivity and the TS activity in the binding assay of 9 patients with grade 4 (strongly positive) TS immunoreactivity, while 7 patients (78%) exhibited a TS activity of 5 pmol/g tissue or greater. Nine patients with a TS immunoreactivity of grade 1 (negative to weakly positive) showed a TS activity <5 pmol/g tissue. The difference between the two groups was statistically significant (p < 0.01). TS expression was easily detected by the immunohistochemical technique which will allow the assessment of the relationship between TS activity and patient outcome.

Reduced activity of anabolizing enzymes in 5-fluorouracil-resistant human stomach cancer cells

The mechanism of resistance to 5-fluorourcil (5-FU) was studied with NUGC-3/5FU/L, a human stomach cancer cell line which had acquired resistance as a consequence of repeated 5-day exposures to stepwise-increasing concentrations of 5-FU in vitro. NUGC-3/5FU/L was 200-fold and over 16-fold resistant to 96-h and 1-h exposures to 5-FU, respectively. NUGC-3/5FU/L incorporated less 5-FU into RNA, indicating resistance to the RNA-directed action of 5-FU. On the other hand, NUGC-3/5/5FU/L also showed resistance to in situ thymidylate synthase (TS) inhibition by 5-FU. Polymerase chain reaction-single-strand conformation polymorphism analysis of TS cDNA and a FdUMP ligand binding assay showed that quantitative and qualitative alterations of TS are not responsible for this resistance. In contrast, the ability to metabolize 5-FU to its active metabolites, FUTP and FdUMP, was reduced in NUGC-3/5FU/L. We found that not only the activities of uridine phosphorylase/kinase and orotate phosphoribosyl-transferase (OPRT), but also the level of phosphoribosyl pyrophosphate, a cosubstrate for OPRT, were significantly lower in NUGC-3/5FU/L than in the parent NUGC-3. These results indicated that resistance to 5-FU in NUGC-3/5FU/L is due to reduced activities of 5-FU-anabolizing enzymes, but not to an alteration of TS. 2'-Deoxyinosine effectively enhanced TS inhibition by 5-FU in the resistant cells, thus markedly sensitizing them to 5-FU.

Antifolates: current developments

In summary, the problem of MTX resistance has been approached in a mechanistic fashion, based on the wealth of information generated over the years. To date, these strategies have produced several new classes of anticancer drugs, with a variety of anticipated and unanticipated mechanisms of action. Several of these have shown promising preclinical activity, and these are moving into more stringent testing in the clinic.

Thymidylate synthase gene amplification in human colon cancer cell lines resistant to 5-fluorouracil

A series of 5-fluorouracil (5-FU)-resistant human colon H630 cancer cell lines were established by continuous exposure of cells to 5-FU. The concentration of 5-FU required to inhibit cell proliferation by 50% (IC50) in the parent colon line (H630) was 5.5 microM. The 5-FU IC50 values for the resistant H630-R1, H630-R10, and H630-R cell lines were 11-, 29-, and 27-fold higher than that for the parent H630 cell line. Using both the radioenzymatic 5-fluoro-2'-deoxyuridine-5'-monophosphate (FdUMP) binding and catalytic assays for measurement of thymidylate synthase (TS) enzyme activity, there was significantly increased TS activity in resistant H630-R1 (13- and 23-fold), H630-R10 (37- and 40-fold), and H630-R (24- and 34-fold) lines, for binding and catalytic assays, respectively, compared with the parent H630 line. The level of TS protein, as determined by western immunoblot analysis, was increased markedly in resistant H630-R1 (23-fold), H630-R10 (33-fold), and H630-R (26-fold) cells. Northern analysis revealed elevations in TS mRNA levels in H630-R1 (18-fold), H630-R10 (39-fold), and H630-R (36-fold) cells relative to parent H630 cells. Although no major rearrangements of the TS gene were noted by Southern analysis, there was significant amplification of the TS gene in 5-FU-resistant cells, which was confirmed by DNA slot blot analysis. These studies demonstrate that continuous exposure of human colon cancer cells to 5-FU leads to TS gene amplification and overexpression of TS protein with resultant development of fluoropyrimidine resistance.

Phase II study of 5-fluorouracil/leucovorin for pediatric patients with malignant solid tumors

BACKGROUND

5-Fluorouracil (5-FU) activity for various carcinomas of adults has been enhanced through the synergistic effect of leucovorin. Few pediatric studies of 5-FU in pediatric patients have been previously reported.

METHODS

Fifty-eight patients were treated with a 4-hour infusion of leucovorin, 400 mg/m2, administered daily for 5 days every 3-4 weeks. 5-Fluorouracil was administered by bolus injection 1 hour into each leucovorin infusion. Eleven adolescent patients with colorectal carcinoma, Stage 3 or 4, were treated with therapeutic intent, and other patients with a variety of drug-resistant pediatric solid neoplasms received similar treatment.

RESULTS

Patients with measurable disease of colorectal carcinoma responded favorably to 5-FU/leucovorin. Stable disease activity was seen with other tumor types. Specifically, there were no objective responses in 12 patients with Ewing's Sarcoma or 11 with osteosarcoma. There were 4 deaths in this study from causes related to toxicity. Nonfatal grade 3/4 toxicities included mucositis, rash, myelosuppression, nausea, vomiting, diarrhea, and infection.

CONCLUSION

The authors do not plan further evaluation of 5-FU/leucovorin in additional pediatric patients with colon cancer or other heavily pretreated malignant solid tumors and are presently treating their patients with colon carcinoma with 5-FU/leucovorin/interferon-alpha 2a.

Combination of reduced folates with methotrexate or 5-fluorouracil. Comparison between 5-formyltetrahydrofolate (folinic acid) and 5-methyltetrahydrofolate in vitro activities

Folinic acid (dlFA) is increasingly used in clinical oncology. The active isomer lFA is intensively metabolized into l5-methyltetrahydrofolate (l5MTHF), the relative proportions of lFA, dFA and l5MTHF in blood varying considerably between oral and i.v. FA administration. The purpose of the study was to compare the in vitro activities of pure lFA and pure l5MTHF at equivalent drug exposure [area under curve (AUC)], taking into account their respective chemical stability in the culture medium. The in vitro growth inhibition [3-(4,5-dimethylthiazol-2-yl)-2,5 diphenyltetrazolium bromide (MTT) test] was evaluated on five human tumor cell lines after methotrexate (MTX)-folate or 5-fluorouracil (5FU)-folate exposures. Not only were the activities of lFA and l5MTHF compared, but also clinically relevant mixtures of lFA + dFA + l5MTHF corresponding to the proportions found at steady state during oral (PO mixture, 4, 39 and 57%, respectively) and i.v. administrations (i.v. mixture, 7, 81 and 12%, respectively). Measurement of folates demonstrated the marked lability of l5MTHF (65.8% loss over 5 days in the culture medium) as compared to lFA (2.6% loss). Whatever the pharmacological model tested (MTX-folate or 5FU-folate), comparison of the folate effects at equivalent drug exposure taking into account their relative stability showed that l5MTHF was never more potent than lFA. Moreover, a higher efficiency of lFA was demonstrated for the cell line most sensitive to 5FU; in this case, as expected, the i.v. mixture was more potent than the PO mixture. This study shows that depending on the tumor, lFA can be more potent than its main circulating metabolite l5MTHF. Along with the limited capacity of oral absorption, the choice between oral and i.v. route for FA administration in patients should take into consideration the different pharmacological activities between lFA and l5MTHF which suggest that the oral route is potentially detrimental to the optimal activity of the 5FU-FA combination as compared to i.v. administration.

Thymidylate synthase activity in normal and malignant gynecologic tissues

We assayed thymidylate synthase (TS) activity in normal and malignant gynecologic tissues. TS activity in the normal cervix, secretory endometrium, and ovaries from a total of 61 patients was 1.00 +/- 0.52 pmol/g tissue (mean +/- SD). Thus the upper limit (mean + 2SD) of TS activity in normal gynecologic tissue was 2.00 pmol/g tissue. TS activity was generally higher in malignant tissue than in normal tissue, and particularly high activity was detected in cervical cancer. TS activity in endometrial cancer was relatively low, but activity in poorly and moderately differentiated tumors was significantly elevated compared to well differentiated tumors. These findings suggest that cervical cancer is a highly proliferative tumor and that in endometrial cancer, cell proliferation is more rapid in poorly differentiated tumors than in well differentiated ones.

Schedule dependent inhibition of thymidylate synthase and tumor growth by 5-fluorouracil in Yoshida sarcoma bearing rats

Schedule dependent inhibition of thymidylate synthase (TS) and tumor growth by 5-fluorouracil (FUra) was examined in Yoshida sarcoma (YS) bearing Donryu rats. After implantation of YS cells (1 x 10(4), FUra (20 mg/kg/day) was continuously (group C) or daily bolus injection (group B) administered for 6 days. On day 7, tumor weight was 1.57 +/- 0.58 g in group C and 0.45 +/- 0.10 g in group B (P less than 0.01), free TS was 2.23 +/- 83 fmol/mg protein in group B and 96 +/- 55 fmol/mg protein in group C (P less than 0.05), and inhibition rate of TS was 88.3 +/- 5.3% in group C and 94.7 +/- 3.0% in group B (P less than 0.05). A significant correlationship was found between free TS and tumor weight (P less than 0.05). As the next step, continuous infusion (group C) or daily bolus injection (group B) for 6 days was started on day 5 after implantation of YS cells. The relative increase of tumor on day 9 was 256 +/- 111% in group C and 112 +/- 22.1% in group B (P less than 0.05). On day 11, total TS of the resected tumor was 650 +/- 153 fmol/mg protein in group C and 391 +/- 124 fmol/mg protein in group B (P less than 0.05), and inhibition rate of TS was 78.8 +/- 12.4% in group C and 84.4 +/- 8.6% in group B. Daily bolus injection of FUra causes a superior antitumor and antimetabolic effect. The schedule dependent cytotoxicity of FUra should be taken into account when a chemotherapeutic protocol is designed.

Mechanisms of collateral sensitivity to fluorouracil of a cis-diamminedichloroplatinum(II)-resistant human non-small lung cancer cell line

A cisplatin(CDDP)-resistant subline of a human lung cancer cell line, PC-7/CDDP, was 4.7-fold more resistant to CDDP than the parent line in a colony-forming assay. The sensitivity of this cell line to anthracyclines, vinca-alkaloid, etoposide, mitomycin C, and bleomycin was similar to that of the parental line, PC-7. However, PC-7/CDDP exhibited 4-fold higher sensitivity to fluorouracil (FUra). Possible mechanisms associated with the collateral sensitivity to FUra were studied in PC-7/CDDP cells. The sensitivity of both cell lines to FUra did not correlate with the effect of FUra on RNA. On the other hand, FUra induced a greater reduction in dTTP pools and more single strand breaks in PC-7/CDDP than in PC-7 cells. These results suggest that the pathway for de novo deoxyribonucleotide synthesis may be a target for FUra in PC-7/CDDP cells. However, inhibition of thymidylate synthase after FUra treatment did not correlate with the DNA-directed activity of FUra. Based on the above findings, the decreased salvage synthesis of dTTP was considered a possible mechanism of the greater reduction of dTTP pools in PC-7/CDDP cells. However, the activity of dThd kinase was the same in both cell lines. In the presence of physiological concentrations of exogenous dThd in the serum, uptake of dThd was less in PC-7/CDDP cells than that in PC-7 cells. Our data suggest that FUra-induced cytotoxicity in PC-7/CDDP cells is associated with the inhibition of dTTP synthesis and that the decreased uptake of dThd is a possible mechanism of the collateral sensitivity to FUra in PC-7/CDDP cells.

Molecular targets of 5-fluoroorotate in the human malaria parasite, Plasmodium falciparum

5-Fluoroorotate is known to have potent antimalarial activity against chloroquine-susceptible as well as chloroquine-resistant clones of Plasmodium falciparum. It was hypothesized that this activity was mediated through synthesis of 5-fluoro-2'-deoxyuridylate, an inactivator of thymidylate synthase, or through incorporation of 5-fluoropyrimidine residues into nucleic acids. Treatment of P. falciparum in culture with 100 nM 5-fluoroorotate resulted in rapid inactivation of malarial thymidylate synthase activity. A 50% loss of thymidylate synthase activity as well as a 50% decrease in parasite proliferation were seen with 5 nM 5-fluoroorotate. Dihydrofolate reductase activity, which resides on the same bifunctional protein as thymidylate synthase, was not affected by 5-fluoroorotate treatment. Incubation of malarial parasites with 3 to 10 microM radioactive 5-fluoroorotic acid for 48 h resulted in significant incorporation of radioactivity into the RNA fraction of P. falciparum; approximately 9% of the uridine residues were substituted with 5-fluorouridine. However, compared with the 50% inhibitory concentrations of 5-fluoroorotate, a 1,000-fold higher concentration of the pyrimidine analog was required to see significant modification of RNA molecules. Results of these studies are consistent with the hypothesis that thymidylate synthase is the primary target of 5-fluoroorotate in malarial parasites.

Loss of murine tumor thymidine kinase activity in vivo following 5-fluorouracil (FUra) treatment by incorporation of FUra into RNA

The effects of 5-fluorouracil (FUra) treatment on thymidine kinase (TKase) activity were examined in vivo in CD8F1 mice bearing first generation CD8F1 mouse mammary tumors. TKase activity was not affected by low dose FUra25 (25 mg/kg), a dose which substantially inhibited thymidylate synthase (TSase), but was severely inhibited 24 hr following treatment with FUra100, a weekly maximally tolerated dose, as judged by activity measurements and labeling of DNA with [3H]thymidine. The amount of (FU)RNA was increased markedly with increasing FUra dose from 0.4 nmol/mg DNA at FUra25 to 2.2 nmol/mg DNA at FUra100. At FUra100, TKase activity gradually declined over 24 hr to less than 10% of the control value, remained low for a further 48 hr, and then was gradually restored to control levels by 168 hr. The loss of TKase activity followed the incorporation of FUra into RNA which peaked at 4-5 hr. TKase activity was not restored by removal of endogenous inhibitors but was restored by treatment with uridine. TKase activity was not inhibited by therapeutic levels of methotrexate (300 mg/kg). TKase from murine colon 38 carcinoma was also severely inhibited, but the activity from colon 26 was only partially (50%) inhibited. Ornithine decarboxylase was also inhibited by FUra100 treatment in the CD8F1 tumor. These results demonstrate that certain short-lived, proliferation-related enzymes are affected by FUra doses higher than those required for TSase inhibition, and this effect appears to correlate with incorporation of FUra into RNA. Thus, in some tumors high doses of FUra can inhibit salvage as well as de novo synthesis of thymidylate providing an increased block of DNA synthesis and increased therapeutic advantage.

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.

Kinetic analysis of 5-fluorouracil action against various cancer cells

Based on our recent kinetic analysis, which made it possible to distinguish between the cell-killing actions of cell cycle phase-specific and non-specific agents, we attempted to elucidate the actions of 5-fluorouracil (FUra) on three different cancer cell lines. By colony-forming assay, the concentrations of fluorouridine (FUrd), fluorodeoxyuridine (FdUrd) or FUra giving 90% cell kill (IC90) at various exposure times (texps) were obtained. With P388 cells, the curve of texps-IC90 for FUrd on a log-log scale was linear with a slope of -1, which is typical for cell cycle phase-nonspecific agents. In contrast, the curve for FdUrd showed a much steeper slope than -1, which is characteristic for cell cycle phase-specific agents. We found that the curve for FUra was exactly the same as that for FUrd, indicating that the mode of FUra action on P388 leukemia is analogous to that of FUrd. Similar results were observed with human colon and renal cancer cell lines, HT-29 and KU-2, although when the cells were exposed to relatively low concentrations of FUra for a long time, a cell cycle phase-specific action became evident.

Stereochemical mechanism of action for thymidylate synthase based on the X-ray structure of the covalent inhibitory ternary complex with 5-fluoro-2'-deoxyuridylate and 5,10-methylenetetrahydrofolate

The structure of the Escherichia coli thymidylate synthase (TS) covalent inhibitory ternary complex consisting of enzyme, 5-fluoro-2'-deoxyuridylate (FdUMP) and 5,10-methylene tetrahydrofolate (CH2-H4PteGlu) has been determined at 2.5 A resolution using difference Fourier methods. This complex is believed to be a stable structural analog of a true catalytic intermediate. Knowledge of its three-dimensional structure and that for the apo enzyme, also reported here, suggests for the first time how TS may activate dUMP and CH2-H4PteGlu leading to formation of the intermediate and offers additional support for the hypothesis that the substrate and cofactor are linked by a methylene bridge between C-5 of the substrate nucleotide and N-5 of the cofactor. By correlating these structural results with the known stereospecificity of the TS-catalyzed reaction it can be inferred that the catalytic intermediate, once formed, must undergo a conformational isomerization before eliminating across the bond linking C-5 of dUMP to C-11 of the cofactor. The elimination itself may be catalyzed by proton transfer to the cofactor's 5 nitrogen from invariant Asp169 buried deep in the TS active site. The juxtaposition of Asp169 and bound tetrahydrofolate in TS is remarkably reminiscent of binding geometry found in dihydrofolate reductase where a similarly conserved carboxyl group serves as a general acid for protonating the corresponding pyrazine ring nitrogen of dihydrofolate.

Cellular pharmacology of 5-fluorouracil in a human colon adenocarcinoma cell line selected for thymidine kinase deficiency

A human colon adenocarcinoma cell line (GC3TK-) was selected for thymidine kinase (TK) deficiency from cloned parental cells (GC3C1) by exposure to 5-bromodeoxyuridine (BrdUrd). The cellular pharmacology of 5-fluorouracil (FUra) and the influence of physiological concentrations of thymidine (dThd; 0.1 to 1 microM) on FUra cytotoxicity during brief exposure in both cell lines were examined. The uptake of FUra during a 1-hr drug exposure, its metabolism to ribo- and deoxyribonucleotides, incorporation into RNA, and inhibition of thymidylate synthase were similar in GC3C1 and GC3TK- cells as were the IC50 values for FUra (26 and 23 microM respectively). TK deficiency did not reduce the intracellular concentrations of FdUMP generated from FUra. In GC3C1, at FUra concentrations up to 100 microM, cytotoxicity was prevented by co-administration of dThd (0.1 to 20 microM). The relationship between cell survival and thymidylate synthase inhibition was close under these conditions. At higher drug concentrations, less dThd protection was observed, and none was detected in GC3TK- cells. Thus, the metabolism of FUra did not appear to be altered substantially in GC3C1 cells selected for TK deficiency. Also in these cells, at concentrations of FUra less than 100 microM, FUra cytotoxicity appeared to be mediated via the inhibition of thymidylate synthase.

Biochemical modulation of fluoropyrimidines by antifolates and folates in an in vitro model of human leukemia

Although 5-fluorouracil (FUra) is one of the most effective cytotoxic agents in the treatment of various solid tumors (carcinomas of the gastro-intestinal tract, breast, head and neck), remissions occur in only 20 to 30% of cases and usually are of short duration. Recently, preclinical studies have shown that the antitumor activity of FUra can be potentiated by modulating the metabolism of this drug by using other substances, in particular antifolates of folates. Pretreatment with antifolates may, by blocking de novo purine biosynthesis and consequently increasing phosphoribosyl pyrophosphate (PRPP) pools, enhance the conversion of FUra to active fluoronucleotide pools via orotate phosphoribosyltransferase. Methotrexate (MTX) pretreatment may also enhance binding of the fluoropyrimidine inhibitor, 5-fluodeoxyuridylate (FdUMP), to the target enzyme, thymidylate synthase (TS), indirectly by increasing dihydrofolate polyglutamates or directly, as MTX polyglutamates, by enhancing the formation of ternary complexes with FdUMP and TS. Exogenous folates, in particular 5-formyltetrahydrofolate (folinate, leucovorin, LV), can, by raising the intracellular levels of 5, 10-methylenetetrahydrofolate, lead to increased formation and stabilization of the ternary complex formed by TS, the folate coenzyme, and FdUMP. In vitro studies have also shown potentiation of FUra cytotoxicity by antifolates and folates against human lymphoblastic leukemia cell lines. Thus, while FUra may have little or no single agent activity in leukemias and lymphomas, it may be converted to an active drug in these neoplasms by appropriate modulation. Clinical studies of sequential MTX-FUra or combined LV-FUra based upon experimental tumor results reviewed herein, are warranted.

Enhancement of the antitumor effects of 5-fluorouracil by folinic acid

Although 5-fluorouracil (FUra) is one of the most effective cytotoxic agents in the treatment of various solid tumors (carcinomas of the gastro-intestinal tract, breast, head and neck), remissions occur in only 20-30% of cases and usually are of short duration. Recently, preclinical studies have shown that the antitumor activity of FUra can be potentiated by modulating the metabolism of this drug by using other substances, in particular 5-formyltetrahydrofolate (folinate, LV). Reduced folates (LV and 5-methyltetrahydrofolate) at concentrations greater than or equal to 1 microM can, by raising the intracellular levels of 5,10-methylenetetrahydrofolate, increase and prolong the inhibition of the target enzyme, thymidylate synthase, with formation of a stable ternary complex formed by the enzyme, the folate coenzyme and the fluoropyrimidine inhibitor (5-fluorodeoxyuridylate). After phase II clinical trials reported encouraging results with the combination LV-FUra in the treatment of patients with various solid tumors, randomized controlled studies in patients with colorectal carcinoma have documented an increase in the response rate of the combination compared to treatment with FUra alone. The integration of the LV-FUra combination into multidrug regimens is now under investigation for the treatment of carcinomas of the breast, stomach, and head and neck.

Deoxyuridylate effects on thymidylate synthase-5-fluorodeoxyuridylate-folate ternary complex formation

The competitive basis and specificity of deoxyuridylate (dUMP)-mediated decreases in thymidylate synthase-5'-fluorodeoxyuridylate-folate (TS-FdUMP-folate) ternary complex formation at low concentrations of folates were investigated using charcoal isolation of protein-bound [3H]FuUMP ligand. Reaction conditions used 0.02 microM TS (Lactobacillus casei) and 0.10 microM [3H]FdUMP incubated for 10 min at 37 degrees and pH 7.4. Decreases in counts below control (C) values in dUMP-added samples (S) were expressed as C/S ratios. At CH2--H4PteGlu1 or H4PteGlu1 concentrations below 10 microM, highly linear relationships were found to exist between C/S value and dUMP concentrations, expressed as dUMP/FdUMP ratios. For H4PteGlu1, maximal C/S values for dUMP interference occurred at the lowest H4PteGlu1 concentrations, approaching the value of the TS-FdUMP binary complex. The efficiency of ternary complex formation by H4PteGlu1 was 28 +/- 5% of CH2--H4PteGlu1 values at concentrations below 1.0 microM. The protective effect of increasing H4PteGlu1 against dUMP interference resulted in a linear relationship between the logarithm of H4PteGlu1 concentration and the slope of dUMP interference (C/S vs dUMP/FdUMP). In contrast, the results with CH2--H4PteGlu1 were biphasic. At concentrations of CH2--H4PteGlu1 lower than 0.5 microM, C/S values were greater than those for binary complex alone, a result related to CH2--H4PteGlu1 consumption based on [5-3H]dUMP tritium-release studies. At concentrations of CH2--H4PteGlu1 above 1.0 microM, however, dUMP interference was nearly abolished. Kinetic analysis of the data suggests that this effect of the 5,10-methylene moiety may result in part from positive allosteric effects of first site TS-FdUMP-CH2--H4PteGlu1 ternary complex binding on acceleration of second site binding, in addition to slowed rates of dissociation. Other folylmonoglutamates showed relatively poor TS-[3H]FdUMP-folate complex formation: at 500 microM folate, as a percentage of CH2--H4PteGlu1 values, these were 29.6% for dihydrofolate, 7.5% for 5-CH3--H4PteGlu1, 3.0% for CH = H4PteGlu1, 1.6% for folic acid, 1.1% for 5-CHO--H4PteGlu1 (leucovorin) and 0.9% for 10-CHO--H4PteGlu1. Inhibitory effects by dUMP were consistent with binary complex effects alone for these folates. Study of methotrexate, as the monoglutamate and the hexaglutamate, suggested that ternary complexes with dUMP are favored over those with FdUMP at high concentrations of the antifolate. Our results indicate that activation of leucovorin to over 0.5 microM in intracellular CH2--H4PteGlu1 equivalents may be a requirement for achieving complete TS inhibition by FdUMP in the presence of excess conce

Mutants of human colon adenocarcinoma, selected for thymidylate synthase deficiency

GC3/c1 human colon adenocarcinoma cells were treated with the mutagen ethyl methanesulfonate, and three clones deficient in thymidylate synthase (5,10-methylenetetrahydrofolate:dUMP C-methyltransferase, EC 2.1.1. 45) activity were selected and characterized. Growth in medium deficient in thymidine caused cell death in two clones (TS- c1 and TS- c3), whereas one clone (TS- c2) showed limited growth. Growth correlated with thymidine synthase activity and 5-fluoro-2'-deoxyuridine 5'-monophosphate-binding capacity and with incorporation of 2'-deoxy[6-3H]uridine into DNA. In the presence of optimal thymidine, growth rates were only 5-18% that of the parental clone (GC3/c1), which grew equally well in thymidine-deficient or -replete medium. Analysis of poly(A)+ RNA showed normal levels of a 1.6-kilobase transcript in TS- c1 and TS- c2 but decreased levels (approximately 6% control) in TS- c3. Clone TS- c3 was 32-, 750-, and greater than 100,000-fold more resistant than the parental clone to 5-fluorouracil, 5-fluoro-2'-deoxyuridine, and methotrexate, respectively. When inoculated into athymic nude mice, each TS- clone produced tumors, demonstrating continued ability to proliferate in vivo.

Effect of polyglutamylation of 5,10-methylenetetrahydrofolate on the binding of 5-fluoro-2'-deoxyuridylate to thymidylate synthase purified from a human colon adenocarcinoma xenograft

CH2-H4PteGlu and H4PteGlu exist in human colon adenocarcinoma xenografts predominantly in the form of polyglutamate species at concentrations of less than 3 microM. The interaction of polyglutamates of [6R]CH2-H4PteGlu in the formation and stability of [6-3H]FdUMP-thymidylate synthase-CH2-H4PteGlun ternary complexes has therefore been examined using enzyme purified from a human colon adenocarcinoma xenograft. Dissociation of these complexes was first-order and was dependent upon the concentration of folate. [6R]CH2-H4PteGlu3-6 (0.9 to 1.6 microM) were greater than 200-fold and [6R]CH2-H4PteGlu2 (18.2 microM) was 18-fold more effective than [6R]CH2-H4PteGlu1 (335 microM) at stabilizing ternary complexes for a T1/2 for dissociation of 100 min. Polyglutamylation of CH2-H4PteGlu also increased the affinity of binding of [6-3H]FdUMP to thymidylate synthase as determined by Scatchard analysis at folate concentrations of 10 microM, where the Kd in the presence of [6R]CH2-H4PteGlu1 was in the order of 4.0 x 10(-8) M, and for [6R]CH2-H4PteGlu3-5 was between 3.7 and 5.5 x 10(-9) M. To examine whether this effect was due to differences in the rates at which [6-3H]FdUMP was bound (kon) or dissociated (koff) from the enzyme, the apparent rate of [6-3H]FdUMP binding was determined in the presence of [6R]CH2H4PteGlu1, [6R]CH2-H4PteGlu3 and [6R]CH2-H4PteGlu4. The kon values were similar and were in the range of 1.7 to 2.3 x 10(6) M-1 min-1 for 10 or 20 microM folate concentrations. Differences in binding affinity determined for [6R]CH2-H4PteGlu1 and longer polyglutamate forms of [6R]CH2-H4PteGlu were thus due to differences in koff. The Vmax for the initial velocity of [6-3H]FdUMP binding was achieved at 10 microM folate. Consequently, at concentrations of CH2-H4PteGlu polyglutamates present in tumors, inhibition of thymidylate synthase by FdUMP in vivo would be expected to be transient, based upon the concentration of [6R]CH2-H4PteGlun required for maximal formation and stability of the covalent ternary complex. It would be advantageous for modulation of CH2-H4PteGlun pools to increase the concentrations of the longer polyglutamate species (n greater than or equal to 3) to maximize the interaction between FdUMP, thymidylate synthase and CH2-H4PteGlu.

Characteristics of thymidylate synthase purified from a human colon adenocarcinoma

Thymidylate synthase has been purified greater than 4000-fold from a human colon adenocarcinoma maintained as a xenograft in immune-deprived mice. In this disease, the enzyme is an important target for the cytotoxic action of 5-fluorouracil, which is influenced by the reduced folate substrate CH2-H4PteGlu. Due to the importance of this interaction, and the existence in cells of folate species as polyglutamyl forms, the interaction of folylpolyglutamates with thymidylate synthase was examined. Polyglutamates of PteGlu were used as inhibitors, and the interaction of CH2-H4PteGlu polyglutamates as substrates or in an inhibitory ternary complex were also examined. Using PteGlu1-7, Ki values were determined. A maximal 125-fold decrease in Ki was observed between PteGlu1 and PteGlu4; further addition of up to three glutamyl residues did not result in an additional decrease in Ki. Despite the increased binding affinity of folypolyglutamates for this enzyme, no change in the Km values for either dUMP (3.6 microM) or CH2-H4PteGlu (4.3 microM) were detected when polyglutamates of [6R]CH2-H4PteGlu were used as substrates. Product inhibition studies demonstrated competitive inhibition between dTMP and dUMP in the presence of CH2-H4PteGlu5. In addition, CH2-H4PteGlu4 stabilized an inhibitory ternary complex formed between FdUMP, thymidylate synthase, and CH2-H4PteGlu4. Thus the data do not support a change in the order of substrate binding and product release upon polyglutamylation of CH2-H4PteGlu reported for non-human mammalian enzyme. This is the first study to characterize kinetically thymidylate synthase from a human colon adenocarcinoma.

Methods for thymidylate synthase pharmacodynamics: serial biopsy, free and total TS, FdUMP and dUMP, and H4PteGlu and CH2-H4PteGlu assays

This report details our methods for performance of the major parameters related to quantitation of TS inhibition resulting from fluoropyrimidine administration to patients, methods equally applicable to preclinical studies. Sampling of tumors before and after drug treatment is done by 4 mm disposable punch biopsy or forceps biopsy via subcutaneous tunneling. Homogenates are prepared using N2 or polytron-mincing. Cytosolic free TS is measured by either the tritium-release method for small biopsies or by [3H]FdUMP ligand-binding. FdUMP and dUMP are separated by DEAE-cellulose column and measured by competitive binding and [14C]dTMP synthesis by the Moran methods. Total, post-FUra TS is measured by pre-incubation dissociation of FdUMP-bound TS after neutral charcoal removal of cytosolic ligands. H4PteGlu and CH2-H4PteGlu are measured by the Priest method using L. Casei TS. The materials and methods are described in sufficient detail to permit wide application of this approach.

Purine nucleosides as cell-specific modulators of 5-fluorouracil metabolism and cytotoxicity

Purine nucleosides and ribose-5-phosphate (Rib-5-P) were used to modulate the metabolism and cytotoxicity of 5-fluorouracil (5-FU) in order to get a better understanding of the mechanism of action of 5-FU. In extracts from five different cell lines both Rib-5-P and inosine were relatively good precursors for Rib-1-P, but deoxyinosine was a moderate to poor precursor for deoxyRib-1-P. In the human colon carcinoma WiDr and the human epithelial intestinal Intestine 407 inosine enhanced Rib-1-P concentrations 3-6-fold. Incubation with deoxyinosine resulted in the appearance of deoxyRib-1-P in both cell lines in levels comparable to those of Rib-1-P. dIMP had the same effect as deoxyinosine in Intestine 407 cells, but not in WiDr cells. Both inosine and deoxyinosine caused a depletion of phosphoribosyl-pyrophosphate. In WiDr cells deoxyinosine (0.1-1.0 mM) clearly potentiated the growth inhibition by 0.1-0.5 microM 5-FU after 24 h of culture, but growth between 24 and 48 h was normal. In Intestine 407 cells the potentiation of 5-FU cytotoxicity by deoxyinosine was even more pronounced at 48 h than at 24 h. In WiDr cells dIMP did not potentiate 5-FU cytotoxicity, but in Intestine 407 cells the effect was comparable to that of deoxyinosine. The lack of potentiation in WiDr was accompanied by a low metabolism of dIMP. Growth inhibition by 5-FU and deoxyinosine could be reversed by thymidine in Intestine 407 cells but not completely in WiDr cells. Since the predominant target of the deoxyinosine-5-FU combination was thymidylate synthase, we analyzed the inhibition of this enzyme by FdUMP and the retention of the inhibition in cell culture. In both cell lines FdUMP was a potent competitive inhibitor of thymidylate synthase with a Ki of between 0.5 and 2 nM. Culture of cells in the presence of 5-FU and deoxyinosine resulted in an almost complete inhibition of thymidylate synthase activity after 24 h but after 48 h the activity was partly recovered. In Intestine 407 cells replenishment of the culture medium at 24 h even enhanced the recovery. Analysis of 5-FU anabolism into nucleic acids demonstrated that deoxyinosine inhibited the incorporation of 5-FU into RNA. It is concluded that in Intestine 407 cells addition of deoxyinosine enhanced the effects of 5-FU on growth inhibition due to increased formation of FdUMP leading to enhanced inhibition of thymidylate synthase.(ABSTRACT TRUNCATED AT 400 WORDS)

Determination of thymidylate synthase activity in colon tumor tissues after treatment with 5-fluorouracil

The formation and isolation of [6-3H]FdUMP-thymidylate synthase-5,10-methylenetetrahydrofolate covalent complex have been examined in tumor cytosols incubated with albumin-dextran coated charcoal used to remove endogenous nucleotide. Charcoal suspension (10% charcoal, 0.5% albumin, 0.05% dextran) absorbed greater than 98% of dUMP added to cytosols, but it reduced by 42-87% covalent complex isolated from subsequent incubation with [6-3H]FdUMP and cofactor using cytosols from different tumors. Initial treatment of ternary complex with charcoal suspension did not cause a decrease in stability of covalent complex during subsequent incubation (37 degrees), but complex separated from free ligand by 10% charcoal suspension was not stable to further treatment with 4% charcoal suspension. Treatment of tumor cytosols with 10% charcoal suspension, to remove nucleotide, did not decrease the rate at which enzyme catalyzed the release of 3H2O from [5-3H]dUMP, or release active enzyme from the ternary complex. Based on these observations, a sensitive procedure for determining thymidylate synthase activity has been developed in which unbound nucleotides (dUMP, FdUMP) are removed prior to assay of enzyme activity. The procedure is suitable for assay of small samples of tissue or of tissues with a low (or inhibited) level of thymidylate synthase activity.

Binding of 5-fluorodeoxyuridylate to thymidylate synthase in human colon adenocarcinoma xenografts

The formation and stability of the covalent ternary complex formed between thymidylate synthase (E.C. 2.1.1.45), 5-fluoro 2'-deoxyuridylate (FdUMP) and 5,10-methylenetetrahydrofolate (CH2-H4PteGlu) has been examined in cytosols derived from xenografts of human colon adenocarcinomas. The rate of association (ka) for FdUMP was low being between 3.4 +/- 0.9 and 10.2 +/- 2.6 X 10(6) M-1 min-1, with the lowest ka value being determined in cytosols from a tumor (HxELC2) which has demonstrated some sensitivity to 5-fluoropyrimidines. Relative to reported ka values for human leukemic cells, the rate of association of FdUMP was 20- to 59-fold lower. This difference is not a consequence of FdUMP catabolism, or metabolism of CH2-H4PteGlu. In cytosols the apparent Km values for dUMP (3.6-4.2 microM) and and [6RS]- CH2-H4PteGlu (25-26.7 microM) were similar to reported values for human enzyme. Data derived from cytosols were similar to those derived using affinity purified enzyme from HxVRC5 colon adenocarcinoma xenografts. The net dissociation of [6-3H] FdUMP from the covalent ternary complex was 31-33 min in the absence of added CH2-H4PteGlu, and the rate of dissociation was dependent upon the concentration of cofactor. The concentration of [6RS]-CH2-H4PteGlu required to stabilize ternary complex derived from HxELC2 cytosols was slightly lower than that required for the same degree of stabilization of complex formed in cytosols from resistant tumors (HxGC3,HxVRC5). Addition of 5-CHO-H4PteGlu, 5-CH3-H4PteGlu, H2PteGlu, and PteGlu did not stabilize the covalent complex, but H4PteGlu substituted for CH2-H4PteGlu.