Fluorouracil: active in ZD1694 (tomudex)-resistant cell lines with markedly elevated thymidylate synthase levels

The Potential Role of Tomudex in the Treatment of Advanced Colorectal Cancer

Aims and Background

The quinazoline folate analog thymidylate synthase inhibitor, Tomudex, is about to enter the Italian pharmaceutical market. Its place among the therapeutic options for advanced colorectal cancer is discussed.

Methods

The pros and cons of currently available chemotherapeutic regimens are briefly described with special attention to patient's and tumor's determinants of treatment outcome. The mechanism of action and the results of phase I, II and III studies of Tomudex are reviewed.

Results

Not all patients need to be treated. Guidelines are given in this respect. Tomudex at the dose of 3 mg/m2 given i.v. every three weeks has antitumor activity similar to that of currently available regimens, with a favorable toxicity profile.

Conclusions

Current research approaches are unlikely to dramatically improve the treatment outcome of this disease in the near future. What can reasonably be expected is less toxicity and more convenient routes and schedules of drug administration that may translate into better quality of life for our patients. Tomudex has been devised along these lines.

Mechanisms of acquired chemoresistance to 5-fluorouracil and tomudex: thymidylate synthase dependent and independent networks

PURPOSE

Thymidylate synthase (TS) over-expression is widely accepted as a major molecular mechanism responsible for 5-fluorouracil (5-FU) and tomudex (TDX) resistance. In this study, the importance of TS in 5-FU and TDX resistance was evaluated.

METHODS

The sensitivity of TS-over-expressing 5-FU (3) and TDX (3) resistant cell lines to 5-FU and TDX was analysed. The cross-resistance between 5-FU and TDX resistant cell lines was determined. The relationship between p53 and NF-kappaB status and the sensitivity to 5-FU and TDX was evaluated.

RESULTS

Compared to relevant parental sensitive cell lines, the 5-FU resistant cell lines were highly cross-resistant to TDX (over 20,000-fold). In contrast, over-expression of TS did not significantly confer 5-FU resistance on the TDX resistant cell lines (0.8- to 1.3-fold). Thymidine (20 microM) rescue induced TDX resistance in TDX sensitive cell lines (over 10,000-fold) but only moderately influenced 5-FU sensitivity in 5-FU sensitive cell lines (1.1- to 2.4-fold). Uridine moderately protected one cancer cell line (RKO) from 5-FU-induced, but not TDX-induced, cytotoxicity. NF-kappaB transfected MCF-7 and p53 knockout HCT116 cells were resistant to 5-FU (4.4- and 2.4-fold, respectively) but not to TDX. TS protein expression in NF-kappaB transfected and p53 knockout cell lines was comparable to the relevant parental cell lines.

CONCLUSION

In some cancer cell lines, TS-independent molecular events may play a key role in 5-FU resistance. Loss of p53 function and NF-kappaB over-expression may be involved in TS-independent 5-FU chemoresistance in some cancer cell lines.

Future potential of thymidylate synthase inhibitors in cancer therapy

Thymidylate synthase (TS) catalyses the de novo synthesis of deoxythymidylate and is a key rate-limiting enzyme of DNA synthesis. The primary site of action of the classic antifolate methotrexate is direct inhibition of dihydrofolate reductase, but it also inhibits TS indirectly by diminishing levels of the TS cosubstrate 5,10-methylenetetrahydrofolate. Polyglutamated metabolites of methotrexate also directly bind and inhibit TS. The prototype fluoropyrimidine fluorouracil is metabolised to an irreversible inhibitor of TS and is the standard chemotherapy for gastrointestinal carcinomas. It is also frequently used in combination with other anticancer drugs against breast cancer and head and neck cancers. The clinical efficacy of fluorouracil is routinely increased by concomitant administration of the biomodulating compound leucovorin (folinic acid). Both the success and limitations of these early drugs led to a search for new, more efficacious TS inhibitors active against a broader range of neoplasms. Raltitrexed (ZD1694, Tomudex) is an antifolate TS inhibitor developed over the last decade that is similarly effective, yet better tolerated, than fluorouracil against colorectal cancer. Additional antifolate and fluoropyrimidine-based TS inhibitors continue to be developed. Many of these experimental drugs have been designed to exploit or thwart selective metabolism in neoplasms, including specific mechanisms of resistance. As the curative potential of relatively non-selective antiproliferative drugs like TS inhibitors is limited against most neoplasms, the future role of TS inhibitors will likely continue to be adjunctive in surgically resectable tumours and palliative in combination with other agents for non-resectable disease. Although TS inhibitors will eventually be supplanted by yet to be discovered agents targeting more tumour-specific cellular signalling pathways, they will probably remain important for the above uses for some time. Future advances in the effective use of TS inhibitors may be forthcoming in the form of improved dosing, fewer untoward effects and increased tumour selectivity with novel fluorouracil prodrug formulations. Furthermore, there is emerging evidence that some novel antifolate TS inhibitors are active against a broader range of neoplams, including lung carcinomas and mesothelioma, compared to classical TS inhibitors. Other possible advances to come include effective biomodulation of antifolate TS inhibitors with nucleoside transport inhibitors and individualised patient therapy based on tumour gene expression and resistance patterns (pharmacogenetics).

Effect of 2'-O-methyl antisense ORNs on expression of thymidylate synthase in human colon cancer RKO cells

Translation of thymidylate synthase (TS) mRNA is controlled by its own protein end-product TS in a negative autoregulatory manner. Disruption of this regulation results in increased synthesis of TS and may lead to the development of cellular drug resistance to TS-directed anticancer agents. As a strategy to inhibit TS expression, antisense 2'-O-methyl RNA oligoribonucleotides (ORNs) were designed to directly target the 5' upstream cis-acting regulatory element (nucleotides 80-109) of TS mRNA. A 30 nt ORN, HYB0432, inhibited TS expression in human colon cancer RKO cells in a dose-dependent manner but had no effect on the expression of beta-actin, alpha-tubulin or topoisomerase I. TS expression was unaffected by treatment with control sense or mismatched ORNs. HYB0504, an 18 nt ORN targeting the same core sequence, also repressed expression of TS protein. However, further reduction in oligo size resulted in loss of antisense activity. Following HYB0432 treatment, TS protein levels were reduced by 60% within 6 h and were maximally reduced by 24 h. Expression of p53 protein was inversely related to that of TS, suggesting that p53 expression may be directly linked to intracellular levels of TS. Northern blot analysis demonstrated that TS mRNA was unaffected by HYB0432 treatment. The half-life of TS protein was unchanged after antisense treatment suggesting that the mechanism of action of antisense ORNs is mediated through a process of translational arrest. These findings demonstrate that an antisense ORN targeted at a critical cis-acting element on TS mRNA can specifically inhibit expression of TS protein in RKO cells.

A phase II study of Tomudex alternated with methotrexate, 5-fluorouracil, leucovorin in first-line chemotherapy of metastatic colorectal cancer

PURPOSE

This multicenter phase II study was designed to assess the efficacy of the alternating schedule of tomudex with methotrexate (MTX)/5-fluorouracil (5-FU)/leucovorin (LV) in first-line chemotherapy for metastatic colorectal cancer.

PATIENTS AND METHODS

Patients with histologically proven metastatic colorectal cancer and at least one bidimensionally measurable lesion, aged 18-70, with performance status < or = 2, normal baseline biological values, and no prior chemotherapy, were selected. Treatment was tomudex 3 mg/m2 and, after two weeks, MTX, 200 mg/m2 by 30' infusion after hydration with 1500 ml saline solution, followed on day 2 by 5-FU, 600 mg/m2 and leucovorin, orally, 15 mg for six times every 6 hours, beginning 24 hours after MTX. Cycles were repeated every four weeks. Tumor response assessment was performed after three cycles.

RESULTS

Thirty-four patients were enrolled in this study, of whom twenty-four had liver metastases, nine local relapse, five lymph node involvement, four lung metastases, and three peritoneal carcinomatosis. Four patients achieved objective responses (one complete and three partial), for an overall response rate of 12% (95% CI: 0%-22%). Twelve patients had stable disease and 18 progressed on therapy. Median survival for all patients was 13 months. Two patients experienced grade 3 WHO neutropenia while hepatotoxicity was reported in 13 patients (6 grade 1, 3 grade 2, 3 grade 3, 1 grade 4), suggesting that this combination could increase hepatic toxicity in comparison to tomudex or MTX/5-FU alone.

CONCLUSIONS

Our results suggest that this regimen does not warrant further investigation in advanced colorectal cancer patients, at least not with this schedule and doses.

Single agent infusional 5-fluorouracil is not effective second-line therapy after raltitrexed (Tomudex) in advanced colorectal cancer

Raltitrexed (Tomudex) is currently licensed for first-line treatment of advanced colorectal cancer. We evaluated 101 patients treated with raltitrexed whose data were collected prospectively, in order to study the outcome of second-line treatments used after this drug. Of 98 evaluable patients, 50 received second-line treatments, the commonest being 5-fluorouracil (5-FU)-based therapy (22 patients with 20 evaluable) and mitomycin-c (MMC) (22 patients with 18 evaluable). Only 1 response was seen in a patient treated with 5-FU and MMC and none following other treatments. This patient was not evaluable for outcome of raltitrexed treatment, having stopped after two courses. Patients who had responded to raltitrexed and later progressed off treatment were more likely to be offered second-line 5-FU, but despite the earlier sensitivity to thymidylate synthase inhibition, response rates were minimal. Underlying mechanisms for this lack of activity and proposals for future studies are discussed.