Bioflavonoid effects on in vitro cultures of Plasmodium falciparum. Inhibition of permeation pathways induced in the host cell membrane by the intraerythrocytic parasite

Abstract

A series of polyhydroxyphenol glycosides including bioflavonoid-glycosides structurally-related to phlorizin (phloretin-2-beta-glucoside) have been tested for their capacity to inhibit permeation pathways induced in red cell membranes by intraerythrocytic Plasmodium falciparum parasites. The permeation through these pathways has been assessed on trophozoites by sorbitol-mediated hemolysis based on a novel technique of high sensitivity and time resolution which has been adapted for handling relatively large number of samples in microtitration plates. Changes in the number of phenolic groups and to a lesser extent changes in the relative position of these groups had a substantial effect on the inhibitory efficacy of the phlorizin derivatives. Diglycoside derivatives were completely ineffective while various monoglycoside derivatives had comparable effects. Structure-activity relationship (SAR) studies of 3-monosubstituted phlorizin derivatives indicate that the inhibitory potency varied considerably with the chemical nature of the group substituted in the 3 position. Inhibition correlated best (r = 0.90) with Hammett's constant, underscoring the role of the electron withdrawing capacity of the chemical groups substituted on the hydroxydihydrochalcone moiety. On the other hand, substitution with lipophilic groups had either minimal effects or reduced the inhibitory power of the derivatives. Inhibition of transport correlated with the inhibition of intraerythrocytic parasite growth and provides a basis for new therapeutic approaches of malaria. Based on the SAR studies, a 3-isothiocyano analog of phlorizin was synthesized and shown to block irreversibly the above permeation pathways (20 microM, 10 min reaction at ambient temperature) as well as the intraerythrocytic growth of the parasite. The present study provides proof for the involvement of amino groups in red cell membrane components as controlling elements of the permeation pathways induced by the intraerythrocytic parasite. The putative groups could serve as targets for affinity labeling of the membrane components associated with the permeation function.