Selective toxicity to malaria parasites by non-intercalating DNA-binding ligands

The DNA of malarial parasites is significantly richer in A and T than that of mammalian cells. Antibiotics which bind to the minor grove of B-DNA with a preference for AT-rich sequences, such as distamycin A, netropsin, 4′-6-diamidino-2-phenylindole (DAPI) and bis-benzimide (Hoechst 33258) were found to inhibit the growth and propagation of Plasmodium falciparum in culture. Distamycin A readily inhibited nucleic acid and protein synthesis and was more toxic to the ring stage than to the trophozoite stage in various parasite strains, irrespective of their susceptibility to chloroquine. Distamycin A, netropsin, DAPI and Hoechst 33258 were considerably more toxic to parasites than to mammalian cells, while chromomycin A₃ and mithramycin A, which bind preferentially to GC-rich sequences, were either equally toxic or more harmful to mammalian cells. These results suggest that the mere difference in DNA base composition of parasites and host cells may account for the selective toxicity of minor groove ligands. Distamycin A, DAPI and Hoechst 33258 were also found to be more toxic to Saccharomyces cerevisiae grown on glycerol than to yeast cells grown on glucose, consistent with the preferential binding of these ligands to the relatively AT-rich mitochondrial DNA of yeast cell. These results underscore the generality of selective toxicity of minor groove binders endowed by the DNA base composition.