The role of polysaccharides in plasmodium falciparum malaria.

Malarial infections involving P. falciparum are becoming increasingly difficult to treat with conventional antirnalarials due to the emergence of drug resistant strains and the high level of toxicity associated with compounds currently in use. Novel antimalarial therapies are therefore in great demand. Two 13-1,3-D-glucans, lentinan and curdlan sulphate were investigated as potential therapeutic candidates . Lentinan is an immunomodulating polysaccharide which is able to augment the body’s natural response to cancer as well as various kinds of parasitic, bacterial and viral infections including AIDS. Lentinan’s immunologic effects are host-dependant, mediated indirectly through both monocytes and cytotoxic T cells. The direct effect of lentinan on P. falciparum and the effect of monocyte supernatants from monocytes stimulated with parasite culture medium, parasites and Lentinan at varying concentrations, were assessed using the tritiated hypoxanthine uptake method. Monocytes were isolated from human peripheral blood according to the -modified method of Boyum. Lentinan was found to exert no direct effect on P. falciparum in vitro and had no stimulatory effect on monocytes in vitro. The next phase of experimentation i.e. the bridge between in vitro and in vivo work, determined the effect on P. falciparum of supernatants from rabbit monocytes stimulated in vivo with Img/kg lentinan. There was no difference in percent parasite survival in vitro when supernatants from the rabbit monocytes stimulated in vivo with lentinan compared to the control group were tested. The final step in experimentation dealt with the in vivo effect of lentinan at lOmg/kg/day on the clinical course and outcome of Plasmodium berghei infection in B.dtVc mice. Lentinan was ineffective in clearing parasitaemia, and had no effect on body mass, spleeii index or haematocrit when compared to the control animals. Since Lentinan is ineffective both in vitro and in vivo, it does not appear to be an effective candidate for treatment in acute Plasmodium infection. Curdlan Sulphate (CRDS), has exhibited blocking effects on the binding of HTV-1 virions to the surfaces of target cells in vitro without the toxicity exhibited by other srlphated polysaccharides. It is proposed that this compound may similarly inhibit the binding to, and subsequent invasion of erythrocytes by P. falciparum merozoites. Drug sensitivity tests were carried out on P. falcipanim in vitro, using the uptake of radiolabelled tritiated hypoxanthine [3 H], to determine the IC 50 values of CRDS over one and two cycles of parasite growth. Combination experiments with the aminoquinolme antimalaria'ls: quinine, chloroquine and mefloquine, were then carried out on both the chloroquine-sensitive (3D7) and chloroquine-resistant (FCR3) strains of P. falciparum. No IC50 was obtained for the first cycle of growth since merczoite reinvasion of erythrocytes occvrs only in the second cycle. The IC 50 value here was found to be 4.5 ± 1 . 2 |. ig/m] , which is very similar to the concentration of CRDS which inhibits HTV-1 infection in vitro. All the combination experiments indicated a synergistic effect between the CRDS and the quinoline compounds tested.The in vivo effect of CRDS on the clinical course and outcome of P.bergkei infected Balb/c mice was then determined. A dose response curve was developed using a wide range of CRDS concentrations administered subcutaneously to P.berghei infected mice. CRDS was successful in inhibiting parasite multiplication in vivo. The IC50 in vivo was estimated at 25mg/kg/day. These positive results both in vitro and in vivo, indicate that CRDS may be a potentially excellent candidate for adjunct therapy with the classical antimalarials.