Dihydrofluorouracil, a fluorouracil catabolite with antitumor activity in murine and human cells

Dihydrofluorouracil (FUH₂), the initial catabolite of 5-fluorouracil (FUra), was examined to determine whether this derivative had antitumor activity or host cell (bone marrow) toxicity. Studies were undertaken with Ehrlich ascites tumor and bone marrow cells isolated from CF-1 mice. Cells were exposed for 1 h either to no drug (control) or to varying concentrations, ranging from 1 to 250 μ M, of either FUra, FUH₂, or α-fluoro-β -alanine. Cells were then cultured and colony formation was assessed after 10 to 14 days. Ehrlich ascites tumor cells were more sensitive to FUra [50% lethal dose (LD₅₀) = 18 μM] than to FUH₂ [LD50 = 50 μM], with no sensitivity to α-fluoro-β -alanine even at 250 μM. Bone marrow cells had a toxicity profile similar to that of FUra (LD50 = 10 μM) but were relatively insensitive to FUH₂ (LD50 > 250 MM), with no sensitivity to α-fluoro-β -alanine. Subsequent studies examined colony formation of the human breast carcinoma cell line MCF-7 following 1 h exposure to varying concentrations of FUra and FUH₂. These cells were less sensitive to both FUra (LD₅₀ ~ 80 μM) and FUH₂ (LD₅₀ ~ 350 MM). Initial studies on the mechanism of toxicity of FUH₂ demonstrated that this FUra catabolite could produce inhibition of thymidylate synthase activity in Ehrlich ascites tumor cells with a pattern similar to that resulting from exposure to FUra. This is the first study to demonstrate that FUH₂ (a quantitatively important catabolite of FUra) is cytotoxic, and it suggests that FUH₂ may contribute to the toxicity of FUra in viVo, possibly by being anabolized to FUra.