IRREVERSIBLE CYTOTOXIC EFFECT OF A NOVEL LOWLY IMMUNOSUPPRESSIVE ANTITUMOR FLUOROURIDINE DERIVATIVE, UK-21

Previously, we reported that the 5-fluorouridine derivative, -[N-(2-n- propyl-n-pentanoylglycyl]-5-fluorouridine dine (UK-21), is a newly syn thesized lowly immunosuppressive and potent antitumor drug in comparis on with other fluorouridine derivatives such as B-fluorouracil (5-FU), 5-fluorouridine (5-FUR) and 5-fluorodeoxyuridine (5-FUDR). In order t o elucidate the molecular mechanism of antitumor activity of UK-21, we compared the effect of the four drugs on cell proliferation, cell cyc le progression and macromolecular syntheses. When KB cells were subjec ted to a colony-forming inhibition assay designed to expose the cells to the drugs for 4-96 h and wash out, UK-21 and 5-FUR inhibited the co lony formation at concentrations ranging from 0.01 to 0.1 mu M, wherea s 1-100 mu M was needed for the cytotoxicity of 5-FU and 5-FUDR. By ex posure for 24-48 h, all these drugs inhibited cell growth and caused a ccumulation of the cells in S or G(2) phase at almost the same concent rations of 0.32-8 mu M. These results suggest that the cytotoxic effec ts of UK-21 and 5-FUR are irreversible, while those of 5-FU and 5-FUDR are reversible. To confirm this, KB cells were treated with UK-21 and /or 5-FU for 1 h, and continued to be cultured for 1-7 days, resulting in the inhibition of the cell growth by UK-21 in a dose-dependent man ner at concentrations of 10-100 mu M, but not by 5-FU even at 100 mu M . UK-21, 5-FUR and 5-FU showed a linear relationship between exposure time and IC50 in the colony formation assay with a slope of almost -1, but 5-FUDR did not, suggesting that UK-21, 5-FUR and 5-FU are cell cy cle non-specific inhibitors, while 5-FUDR is a cell cycle-specific inh ibitor. UK-21 and 5-FUR, but not 5-FU and 5-FUDR inhibited the incorpo ration of [H-3]uridine into the acid insoluble fraction, while UK-21 a nd 5-FUDR, but not inhibited the incorporation of results suggest that irreversible cytotoxic effects of UK-21 like 5-FUR are exerted throug h inhibition of RNA synthesis.