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Hydroxyurea inhibited the growth and DNA synthesis of Toxoplasma gondii growing in human fibroblast cells. A concentration of 18 micrograms/ml totally suppressed plaque formation. The synthesis of T. gondii RNA was not acutely inhibited. The parasite was equally sensitive to hydroxyurea when grown in wild type or hydroxyurea resistant host cells. With the aid of chemical mutagenesis, we isolated a stable hydroxyurea resistant mutant of T. gondii. This mutant showed no increased ability to incorporate [3H]uracil into its pyrimidine deoxynucleotide pool. Hydroxyurea depressed the [3H]uracil labeling of the pyrimidine deoxynucleotide pool in the wild type parasite but not in the mutant, suggesting that the mutant ribonucleotide reductase was resistant to the inhibitory effect of the drug.

The effect of human recombinant interferon-gamma (IFN-gamma) on Toxoplasma gondii in cultured human fibroblasts is predominantly parasitostatic. This effect is dependent upon the induction in the host cell of a potent indoleamine 2,3-dioxygenase that converts tryptophan to N-formylkynurenine. This product is, in turn, degraded to kynurenine by a formamidase. Within 24 h of treatment with IFN-gamma most of the tryptophan originally present in the medium is converted to these products together with some minor metabolites. When added to the medium of infected cultures at concentrations equimolar to the tryptophan content, neither N-formylkynurenine nor kynurenine suppresses the growth of T. gondii, although at higher concentrations they are effective. The medium of uninfected cultures treated with IFN-gamma for 24 h has no effect on the growth of T. gondii, when transferred to fresh cultures provided that the residual IFN-gamma is first removed by ultrafiltration or neutralized with a specific monoclonal antibody. Thus minor metabolites produced from tryptophan in response to IFN-gamma and excreted into the medium are not parasitostatic. When cultures treated with IFN-gamma for 24 h are incubated with medium that contains [3H]tryptophan, the radioactive amino acid is converted to N-formylkynurenine and kynurenine as rapidly as it enters the cell. This degradation not only results in a very low intracellular concentration of tryptophan but also produces intracellular concentrations of tryptophan metabolites that are significantly higher than the tryptophan concentration in control cells. However, it is unlikely that either metabolite reaches intracellular concentrations that are sufficient to suppress the growth of the parasite. The parasitostatic effect of IFN-gamma is most likely to result from the starvation of T. gondii for tryptophan.