SUSCEPTIBILITY OF PLASMODIUM-FALCIPARUM TO A COMBINATION OF THYMIDINEAND ICI D1694, A QUINAZOLINE ANTIFOLATE DIRECTED AT THYMIDYLATE SYNTHASE

Unlike mammalian cells, malarial parasites lack the enzymes to salvage preformed pyrimidines. For this reason, a combination of a thymidylat e synthase inhibitor and the nucleoside thymidine should provide selec tive antimalarial activity even in the absence of any known active sit e differences between malarial and mammalian thymidylate synthases. To test this hypothesis, we evaluated the in vitro antimalarial activity of ICI D1694, a quinazoline antifolate that inhibits thymidylate synt hase in mammalian cells. ICI D1694 inhibited the in vitro proliferatio n of Plasmodium falciparum With a 50% inhibitory concentration of 20 m u M. AS predicted, this antimalarial activity was not affected by the presence of 10 mu M thymidine in the culture medium. In contrast, five different mammalian cells, several of which were susceptible to nanom olar levels of ICI D1694 in the absence of thymidine, were rescued by thymidine. At doses of 100 mu M ICI D1694 and 10 mu M thymidine, the p roliferation of parasites was completely inhibited, but the proliferat ion of all mammalian cells remained unaffected. A test of susceptibili ty patterns among five different isolates of P. falciparum revealed th at strains resistant to pyrimethamine, cycloguanil, or chloroquine had susceptibilities to ICI D1694 essentially the same as those of wild-t ype parasites. These findings are consistent,vith the hypothesis that, intracellularly, ICI D1694 inhibits P. falciparum thymidylate synthas e. Overall, it is clear that even with an inhibitor of malarial thymid ylate synthase that is not particularly effective in itself, one can o btain selective inhibition of parasites if the antimalarial agent is u sed in combination with thymidine. More effective inhibitors of malari al thymidylate synthase will undoubtedly lead to selective chemotherap y in vivo.