In vitro activity of natural and synthetic naphthoquinones against erythrocytic stages of Plasmodium falciparum.

In an attempt to identify new antimalarial compounds we studied the blood schizonticide effect of chemically defined natural products which were isolated from plants (Bignoniaceae) or synthesized. Different concentrations of these drugs (up to 20 microM) were incubated in vitro with blood forms of Plasmodium falciparum for 72 h. A total of 12 drugs of the naphthoquinones group were tested. Eight of them were isolated from plants (NP) and 4 synthesized (S). Three of the drugs (2 NP and one S) were very active and completely inhibited parasite growth at the higher concentrations used (20 microM); 5 drugs were partially active (3S and 2NP) and 3 (NP) were totally inactive. Lapachol was among the drugs tested. Although it has been considered a potential antimalarial agent, it exhibited very low activity (20% inhibition of schizogony). The antimalarial activity of our naphthoquinones against drug-resistant strains was superior to that of chloroquine and quinine which were used as controls. Two of the P. falciparum strains used for the tests were strongly chloroquine resistant. If these naphthoquinones prove to be active in vivo and are of low toxicity, they will be promising candidates for treatment of human malaria particularly since they are easily synthesized.