The activity of IMP dehydrogenase (EC 1.2.1.14), the key enzyme of de novo guanylate biosynthesis, was shown to be increased in tumor cells. Tiazofurin (TR), a potent and specific inhibitor of this enzyme, proved to be effective in the treatment of refractory granulocytic leukemia in blast crisis. We examined the effects of tiazofurin as a single agent and in combination with hypoxanthine and allopurinol in six different neuroectodermal tumor cell lines, the STA-BT-3 and 146-18 human glioblastoma cell lines, the SK-N-SH, LA-N-1 and LA-N-5 human neuroblastoma cell lines, and the STA-ET-1 Ewing tumor cell line. Tiazofurin inhibited tumor cell growth with IC50 values between 2.2 microM (LA-N-1 cell line) and 550 microM (LA-N-5 cells) and caused a significant decrease of intracellular GTP pools (GTP concentrations decreased to 39-79% of control). Incorporation of [8-14C]guanine into GTP pools was determined as a measure of guanylate salvage activity; incubation with 100 microM hypoxanthine caused a 62-96% inhibition of the salvage pathway. Incubation with tiazofurin (100 microM) and hypoxanthine (100 microM) synergistically inhibited tumor cell growth, and the addition of allopurinol (100 microM) strengthened these effects. Therefore, this drug combination, inhibiting guanylate de novo and salvage pathways, may prove useful in the treatment of human neuroectodermal tumors.