Inhibition of in vitro growth of Plasmodium falciparum by a brief exposure to the cationic rhodamine dyes.

The effects of eight permeant fluorescent dyes on the in vitro growth of Plasmodium falciparum was investigated. First, P. falciparum-infected human erythrocytes were synchronized with D-sorbitol and treated with the cationic fluorescent dye rhodamine 123 at 37 degrees C for 30 minutes, and the growth of the treated parasites monitored by examining daily parasitaemias. Rhodamine 123 inhibited the parasite growth at more than 5 microM, the 50% effective concentration being 6 microM. Ring forms and trophozoites were more susceptible to the dye than schizonts. The development of dye-treated ring forms and trophozoites to schizonts was greatly inhibited, and so few new ring forms were produced. In contrast, the dye-treated schizonts produced a large number of new ring forms, though to a slightly lesser extent than untreated schizonts. The rhodamine 123-induced growth inhibition was partially reversed by treating the dye-pre-exposed infected erythrocytes with the proton ionophore carbonyl-cyanide m-chlorophenylhydrazone, which dissipates transmembrane proton gradients. A survey of seven other fluorescent dyes demonstrated that the cationic dyes, including rhodamine 123, rhodamine 6G, rhodamine 6G perchlorate and rhodamine 3B perchlorate, exerted the growth inhibitory effect, whereas the neutral dyes rhodamine B, rhodamine 110, and rhodamine 19 perchlorate, and the anionic dye fluorescein, did not. Fluorescent microscopy revealed that P. falciparum accumulated the cationic dyes but not the neutral and anionic dyes. These results indicate that the cationic rhodamine dyes, which accumulated in the parasite, inhibit the growth of P. falciparum.