Tiazofurin enhances the anabolism and toxicity of 5-fluorouracil.

Tiazofurin, a clinically active anticancer agent, is undergoing additional clinical testing in combination with other agents. We found that tiazofurin is an effective biochemical modulator of 5-fluorouracil anabolism. Pretreatment of cultured L1210 cells with tiazofurin at concentrations of 1-100 microM results in an increase in the rate of conversion of 5-fluorouracil to phosphorylated metabolites. Concentrations of tiazofurin effective in increasing 5-fluorouracil anabolism cause a corresponding increase in the 5-phosphoribosyl-1-pyrophosphate pool. Studies in mice show that tiazofurin increases the lethal toxicity of 5-fluorouracil and increases the antitumor effectiveness of low doses of 5-fluorouracil: however, the combination is not more effective than an optimal dose of 5-fluorouracil given alone. These results indicate that caution should be exercised in the concurrent use of tiazofurin with other drugs, particularly 5-fluorouracil, that require 5-phosphoribosyl-1-pyrophosphate for activation or that are affected by a decrease in pyrimidine nucleotide synthesis.