In vitro enhancement of fluoropyrimidine-induced cytotoxicity by leucovorin in colorectal and gastric carcinoma cell lines but not in non-small-cell lung carcinoma cell lines

Levoleucovorin as replacement for leucovorin in cancer treatment

OBJECTIVE

To comprehensively review the literature regarding the efficacy, safety, and costs associated with the use of levoleucovorin in cancer treatment and to assess whether levoleucovorin would be a reasonable alternative to the use of racemic leucovorin.

DATA SOURCES

A MEDLINE search was conducted for English-language human studies published between January 1980 and April 2012 using the terms l-LV, levoleucovorin, d,l-LV, leucovorin, folinic acid, folinate, 5-formyltetrahydrofolate, folic acid, folates, methotrexate, 5-fluorouracil, and clinical trials.

STUDY SELECTION AND DATA EXTRACTION

Articles pertinent to clinical trials (Phase 1, 2, 3) related to evaluating the efficacy, interchangeability, and safety of levoleucovorin were collected and their contents reviewed.

DATA SYNTHESIS

From these pharmacokinetics and clinical studies, information on the use of levoleucovorin as a modulator of fluorouracil as well as when combined with other antitumor agents were scrutinized and extracted for comparison with leucovorin whenever possible. Two randomized Phase 3 clinical studies comparing the efficacy and adverse effect profiles of leucovorin and levoleucovorin demonstrated that levoleucovorin is as effective as leucovorin in terms of response, toxicity, and survival. Six randomized Phase 3 clinical studies demonstrated the safety and efficacy of levoleucovorin as a modulator of fluorouracil in combination with/without other antitumor agents in colorectal cancer patients. Levoleucovorin has been studied in other cancers. These clinical Phase 1/2/3 studies demonstrated efficacy and safety of levoleucovorin in combination chemotherapeutic regimens comprising fluorouracil and other antitumor agents.

CONCLUSIONS

The results of the clinical studies suggest that levoleucovorin is efficacious and can be used safely in combination with fluorouracil and other antitumor agents. Levoleucovorin can be used interchangeably with leucovorin for modulating fluorouracil. The current shortage of the supply of leucovorin centered in North America renders levoleucovorin a reasonable alternative in terms of efficacy and toxicity profile, but from the perspective of cost, leucovorin remains the drug of choice.

Comparison of 5-fluorouracil and 5-fluoro-2'-deoxyuridine as an effector in radiation-activated prodrugs

The purpose of this study was to clarify whether 5-fluoro-2'-deoxyuridine (FdUrd) is superior to 5-fluorouracil (5-FU) as an effector in the radiation-activated prodrugs which we have been developing. The in vitro cytotoxicity of 5-FU and FdUrd was compared in two murine tumor and four human pancreatic cancer cell lines using a colony assay and in vivo efficacy was compared with SCCVII tumor using a growth delay time assay. FdUrd was slightly more hydrophilic than 5-FU. In vitro, FdUrd was more efficient than 5-FU in two lines, whereas 5-FU was more efficient in two lines and the two drugs were almost equal in efficacy in the remaining two. The concentration to reduce tumor cell survival to 50% after 24-h drug exposure was 5-32 microM for both 5-FU and FdUrd in murine lines, while it was 30-210 microM in human pancreatic cancer cell lines. The difference in relative efficacy of the two drugs among these cell lines could not be attributed to the rate of intracellular uptake of the compounds. FdUrd was less toxic than 5-FU in C3H/He mice, and FdUrd was less efficient than 5-FU in SCCVII tumors in vivo. These results suggest that FdUrd is not necessarily more potent than 5-FU, and development of the FdUrd prodrugs may not necessarily turn out to be fruitful.

Mismatch repair proficiency and in vitro response to 5-fluorouracil

BACKGROUND & AIMS

The DNA mismatch repair (MMR) system recognizes certain DNA adducts caused by alkylation damage in addition to its role in recognizing and directing repair of interstrand nucleotide mismatches and slippage mistakes at microsatellite sequences. Because defects in the MMR system can confer tolerance to acquired DNA damage and, by inference, the toxic effects of certain chemotherapeutic agents, we investigated the effect of 5-fluorouracil (5-FU) on colon cancer cell lines.

METHODS

We determined growth selection by cell enrichment assay and cloning efficiency after treatment with 5 micromol/L 5-FU, assayed nucleic 3H-5-FU incorporation, and analyzed the cell cycle by flow cytometry.

RESULTS

5-FU treatment provided a growth advantage for MMR-deficient cell lines, indicating a relative degree of tolerance to 5-FU by the MMR-deficient cell lines. Enhanced survival was statistically significant after 5 days of growth, and a 28-fold reduction in survival was noted in the MMR-proficient cells by clonagenic assays after 10 days of growth. Differences in nucleotide uptake of 5-FU did not account for the observed growth differences, and specific cell cycle checkpoint arrest was not detected.

CONCLUSIONS

Intact DNA MMR seems to recognize 5-FU incorporated into DNA but may do so in a different manner than other types of alkylation damage. Defective DNA MMR might be one mechanism for tumor resistance to 5-FU.

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.

Mechanisms of acquired resistance to modulation of 5-fluorouracil by leucovorin in HCT-8 human ileocecal carcinoma cells

Repeated (10x) exposure of HCT-8 human ileocecal carcinoma cells to 5-fluorouracil (5-FU) for 2 or 72 hr, both incubations in the continuous presence of 20 microM leucovorin (LV), yielded two stable modulation-resistant sublines, FL2h and FL72h. Although LV potentiated growth inhibition by 5-FU 2-fold in parental HCT-8 cells, it did not potentiate the effect of 5-FU in the FL2h or FL72h sublines. LV modulation of 5-fluorodeoxyuridine (5-FdUrd) was also reduced (FL72h) or eliminated (FL2h). In the FL2h and FL72h sublines, the level of thymidylate synthase (TS) protein and TS activity in cell extracts, TS activity in situ, the rate of cellular uptake and metabolism of LV, and the level of 5-FU incorporation into total cellular RNA were similar to those in parental HCT-8 cells. However, LV significantly (P < 0.01) potentiated the inhibition of TS activity in situ in HCT-8 cells at 24 hr after a 2-hr treatment with either 5-FU or 5-FdUrd, but had no such activity in the FL2h and FL72h sublines (P > 0.1). Resistance to modulation of 5-FU by LV was associated with the inability of LV to increase the formation of intracellular TS-FdUMP-methylenetetrahydrofolate ternary complexes, and these complexes dissociated more rapidly (T1/2 > 1.5- to 3-fold faster) in the presence of different concentrations of 5,10-methylenetetrahydropteroylpentaglutamate. Thus, decreased stability of ternary complexes appears to be the mechanism of acquired resistance to the LV modulation of fluoropyrimidine cytotoxicity, possibly due to mutation(s) of TS in these two modulation-resistant HCT-8 sublines.

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

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