Biochemical modulation of 5-fluorouracil with or without leucovorin by a low dose of brequinar in MGH-U1 cells

T. L. Chen, C. Erlichman

Research output: Contribution to journalArticle

3 Citations (Scopus)

Abstract

Combination of low doses of de novo pyrimidine biosynthesis inhibitors with 5-fluorouracil (FU) has been proposed to increase the antitumor activity of FU. Brequinar is such an inhibitor that has little clinical antitumor effect when used alone. We determined the clonogenic survival of MGH-U1 cells treated with FU ± leucovorin (LV) ± brequinar and examined the effects of these treatments on thymidylate synthase (TS). After 24 h exposure, the concentrations resulting in 50% inhibition of cell growth (IC 50) for brequinar, FU, and FU + LV (100 μM) were 0.4, 20, and 10 μM, respectively. Both 24 h pretreatment and 48 h continuous treatment with the IC 10 (0.1 μM) of brequinar increased the cytotoxicity of FU but did not enhance that of FU + LV. Simultaneous 24 h exposure to 0.1 μM brequinar and FU ± LV did not increase the cytotoxicity of FU ± LV. Intracellular cytidine triphosphate (CTP) and uridine triphosphate (UTP) pools, free TS binding sites, and levels of free fluorodeoxyuridine monophosphate (FdUMP) and deoxyuridine monophosphate (dUMP) were measured in cells pretreated with 0.1 μM brequinar for 24 h alone or followed by a 2-h exposure to FU (25 μM) ± LV (100 μM). In brequinar-treated cells, CTP and UTP pools amounted to 68% and 46% of control values, respectively. The free TS binding sites remaining amounted to 70% of control values in cells treated with FU and 9% of control levels in those treated with FU + brequinar. Free FdUMP levels increased 5-fold in cells pretreated with brequinar as compared with those treated with FU alone. The increased formation of FdUMP was inhibited by simultaneous exposure to 100 μM hypoxanthine and 25 μM FU. Intracellular dUMP levels were not affected by brequinar. We conclude that a low dose of brequinar increases the cytotoxicity of FU but does not enhance that of FU + LV when exposure to brequinar precedes FU treatment. This potentiation appears to be mediated by the increased formation of FdUMP as a consequence of an increase in the cosubstrate phosphoribosyl pyrophosphate (PRPP).

Original languageEnglish (US)
Pages (from-to)370-376
Number of pages7
JournalCancer Chemotherapy and Pharmacology
Volume30
Issue number5
StatePublished - 1992
Externally publishedYes

Fingerprint

brequinar
Leucovorin
Fluorouracil
Modulation
Floxuridine
Thymidylate Synthase
Cytotoxicity
Cytidine Triphosphate
Deoxyuridine
Uridine Triphosphate

ASJC Scopus subject areas

  • Cancer Research
  • Oncology
  • Pharmacology

Cite this

Biochemical modulation of 5-fluorouracil with or without leucovorin by a low dose of brequinar in MGH-U1 cells. / Chen, T. L.; Erlichman, C.

In: Cancer Chemotherapy and Pharmacology, Vol. 30, No. 5, 1992, p. 370-376.

Research output: Contribution to journalArticle

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abstract = "Combination of low doses of de novo pyrimidine biosynthesis inhibitors with 5-fluorouracil (FU) has been proposed to increase the antitumor activity of FU. Brequinar is such an inhibitor that has little clinical antitumor effect when used alone. We determined the clonogenic survival of MGH-U1 cells treated with FU ± leucovorin (LV) ± brequinar and examined the effects of these treatments on thymidylate synthase (TS). After 24 h exposure, the concentrations resulting in 50{\%} inhibition of cell growth (IC 50) for brequinar, FU, and FU + LV (100 μM) were 0.4, 20, and 10 μM, respectively. Both 24 h pretreatment and 48 h continuous treatment with the IC 10 (0.1 μM) of brequinar increased the cytotoxicity of FU but did not enhance that of FU + LV. Simultaneous 24 h exposure to 0.1 μM brequinar and FU ± LV did not increase the cytotoxicity of FU ± LV. Intracellular cytidine triphosphate (CTP) and uridine triphosphate (UTP) pools, free TS binding sites, and levels of free fluorodeoxyuridine monophosphate (FdUMP) and deoxyuridine monophosphate (dUMP) were measured in cells pretreated with 0.1 μM brequinar for 24 h alone or followed by a 2-h exposure to FU (25 μM) ± LV (100 μM). In brequinar-treated cells, CTP and UTP pools amounted to 68{\%} and 46{\%} of control values, respectively. The free TS binding sites remaining amounted to 70{\%} of control values in cells treated with FU and 9{\%} of control levels in those treated with FU + brequinar. Free FdUMP levels increased 5-fold in cells pretreated with brequinar as compared with those treated with FU alone. The increased formation of FdUMP was inhibited by simultaneous exposure to 100 μM hypoxanthine and 25 μM FU. Intracellular dUMP levels were not affected by brequinar. We conclude that a low dose of brequinar increases the cytotoxicity of FU but does not enhance that of FU + LV when exposure to brequinar precedes FU treatment. This potentiation appears to be mediated by the increased formation of FdUMP as a consequence of an increase in the cosubstrate phosphoribosyl pyrophosphate (PRPP).",
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N2 - Combination of low doses of de novo pyrimidine biosynthesis inhibitors with 5-fluorouracil (FU) has been proposed to increase the antitumor activity of FU. Brequinar is such an inhibitor that has little clinical antitumor effect when used alone. We determined the clonogenic survival of MGH-U1 cells treated with FU ± leucovorin (LV) ± brequinar and examined the effects of these treatments on thymidylate synthase (TS). After 24 h exposure, the concentrations resulting in 50% inhibition of cell growth (IC 50) for brequinar, FU, and FU + LV (100 μM) were 0.4, 20, and 10 μM, respectively. Both 24 h pretreatment and 48 h continuous treatment with the IC 10 (0.1 μM) of brequinar increased the cytotoxicity of FU but did not enhance that of FU + LV. Simultaneous 24 h exposure to 0.1 μM brequinar and FU ± LV did not increase the cytotoxicity of FU ± LV. Intracellular cytidine triphosphate (CTP) and uridine triphosphate (UTP) pools, free TS binding sites, and levels of free fluorodeoxyuridine monophosphate (FdUMP) and deoxyuridine monophosphate (dUMP) were measured in cells pretreated with 0.1 μM brequinar for 24 h alone or followed by a 2-h exposure to FU (25 μM) ± LV (100 μM). In brequinar-treated cells, CTP and UTP pools amounted to 68% and 46% of control values, respectively. The free TS binding sites remaining amounted to 70% of control values in cells treated with FU and 9% of control levels in those treated with FU + brequinar. Free FdUMP levels increased 5-fold in cells pretreated with brequinar as compared with those treated with FU alone. The increased formation of FdUMP was inhibited by simultaneous exposure to 100 μM hypoxanthine and 25 μM FU. Intracellular dUMP levels were not affected by brequinar. We conclude that a low dose of brequinar increases the cytotoxicity of FU but does not enhance that of FU + LV when exposure to brequinar precedes FU treatment. This potentiation appears to be mediated by the increased formation of FdUMP as a consequence of an increase in the cosubstrate phosphoribosyl pyrophosphate (PRPP).

AB - Combination of low doses of de novo pyrimidine biosynthesis inhibitors with 5-fluorouracil (FU) has been proposed to increase the antitumor activity of FU. Brequinar is such an inhibitor that has little clinical antitumor effect when used alone. We determined the clonogenic survival of MGH-U1 cells treated with FU ± leucovorin (LV) ± brequinar and examined the effects of these treatments on thymidylate synthase (TS). After 24 h exposure, the concentrations resulting in 50% inhibition of cell growth (IC 50) for brequinar, FU, and FU + LV (100 μM) were 0.4, 20, and 10 μM, respectively. Both 24 h pretreatment and 48 h continuous treatment with the IC 10 (0.1 μM) of brequinar increased the cytotoxicity of FU but did not enhance that of FU + LV. Simultaneous 24 h exposure to 0.1 μM brequinar and FU ± LV did not increase the cytotoxicity of FU ± LV. Intracellular cytidine triphosphate (CTP) and uridine triphosphate (UTP) pools, free TS binding sites, and levels of free fluorodeoxyuridine monophosphate (FdUMP) and deoxyuridine monophosphate (dUMP) were measured in cells pretreated with 0.1 μM brequinar for 24 h alone or followed by a 2-h exposure to FU (25 μM) ± LV (100 μM). In brequinar-treated cells, CTP and UTP pools amounted to 68% and 46% of control values, respectively. The free TS binding sites remaining amounted to 70% of control values in cells treated with FU and 9% of control levels in those treated with FU + brequinar. Free FdUMP levels increased 5-fold in cells pretreated with brequinar as compared with those treated with FU alone. The increased formation of FdUMP was inhibited by simultaneous exposure to 100 μM hypoxanthine and 25 μM FU. Intracellular dUMP levels were not affected by brequinar. We conclude that a low dose of brequinar increases the cytotoxicity of FU but does not enhance that of FU + LV when exposure to brequinar precedes FU treatment. This potentiation appears to be mediated by the increased formation of FdUMP as a consequence of an increase in the cosubstrate phosphoribosyl pyrophosphate (PRPP).

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