title: Levamisole potentiation of fluorouracil antiproliferative activity mimicked by orthovanadate, an inhibitor of tyrosine phosphatase.
abstract: BACKGROUND
  Levamisole is an effective antihelminthic drug with immunomodulatory and anticancer activities in model systems. Combined with fluorouracil (5-FU) as adjuvant treatment following resection of Dukes' stage C colon carcinomas, levamisole significantly reduces mortality. However, neither 5-FU nor levamisole alone has a significant effect on survival in this patient group. Previously, we noted that in vitro levamisole potentiated the antiproliferative activity of 5-FU.
  
  
  OBJECTIVE
  Because levamisole is known to inhibit alkaline phosphatases and has been reported to inhibit dephosphorylation of some membrane phosphoproteins, we studied the effects of levamisole analogues and of chemically unrelated inhibitors of phosphatases for their ability to potentiate 5-FU inhibition of tumor cell line proliferation in vitro.
  
  
  METHODS
  Human cancer cell lines were exposed to drugs alone or in combination with 5-FU. Antiproliferative activity was measured by determining the extent of reduction of colony formation by the cell lines in test plates compared with control plates.
  
  
  RESULTS
  We found that potentiation of 5-FU cytotoxicity by levamisole and by p-hydroxytetramisole, a metabolite of levamisole, is mimicked by orthovanadate, an inhibitor of tyrosine phosphatases, but not by okadaic acid, an inhibitor of serine and threonine phosphatases, Furthermore, l-p-bromotetramisole, a synthetic analogue of levamisole that is 10-fold more potent in inhibition of alkaline phosphatase than levamisole, potentiates the antiproliferative activity of 5-FU to a greater extent than d-p-bromotetramisole, a stereoisomer of l-p-bromotetramisole with little antiphosphatase activity.
  
  
  CONCLUSIONS
  Inhibition of tyrosine phosphatases may be responsible for the potentiation by levamisole of the inhibitory activity of 5-FU in vitro.
  
  
  CONCLUSIONS
  Inhibition of dephosphorylation of regulatory phosphoproteins may be related to the therapeutic efficacy of the combination of levamisole and 5-FU in the adjuvant treatment of colon carcinoma and may underlie at least some of the multiple effects of levamisole on immune parameters.
authors.affiliation: Department of Oncology, Mayo Foundation, Rochester, Minn.
authors.forename: J S
authors.initials: JS
authors.lastname: Kovach
authors.forename: P A
authors.initials: PA
authors.lastname: Svingen
authors.forename: D J
authors.initials: DJ
authors.lastname: Schaid
mesh_terms: D001943:Breast Neoplasms / Q000188:drug therapy; D003110:Colonic Neoplasms / Q000188:drug therapy; D004357:Drug Synergism; D004988:Ethers, Cyclic / Q000494:pharmacology; D005472:Fluorouracil / Q000494:pharmacology*; D006801:Humans; D007978:Levamisole / Q000494:pharmacology*; D008545:Melanoma / Q000188:drug therapy; D019319:Okadaic Acid; D010744:Phosphoric Monoester Hydrolases / Q000037:antagonists & inhibitors*; D012878:Skin Neoplasms / Q000188:drug therapy; D014407:Tumor Cells, Cultured; D014410:Tumor Stem Cell Assay; D014638:Vanadates / Q000494:pharmacology*
chemical_list: D004988:Ethers, Cyclic; D019319:Okadaic Acid; D007978:Levamisole; D014638:Vanadates; D010744:Phosphoric Monoester Hydrolases; D005472:Fluorouracil