Anti-plasmodial action of de novo-designed, cationic, lysine-branched, amphipathic, helical peptides

A lack of vaccine and rampant drug resistance demands new anti-malarials.

In vitro blood stage anti-plasmodial properties of several de novo-designed, chemically synthesized, cationic, amphipathic, helical, antibiotic peptides were examined against Plasmodium falciparum using SYBR Green assay. Mechanistic details of anti-plasmodial action were examined by optical/fluorescence microscopy and FACS analysis.

Unlike the monomeric decapeptides {(Ac-GXRKXHKXWA-NH₂) (X = F,ΔF) (Fm_, ΔFm IC₅₀ >100 μM)}, the lysine-branched,dimeric versions showed far greater potency {IC₅₀ (μM) Fd 1.5 , ΔFd 1.39}. The more helical and proteolytically stable ΔFd was studied for mechanistic details. ΔFq, a K-K₂ dendrimer of ΔFm and (ΔFm)₂ a linear dimer of ΔFm showed IC₅₀ (μM) of 0.25 and 2.4 respectively. The healthy/infected red cell selectivity indices were >35 (ΔFd), >20 (ΔFm)₂ and 10 (ΔFq). FITC-ΔFd showed rapid and selective accumulation in parasitized red cells. Overlaying DAPI and FITC florescence suggested that ΔFd binds DNA. Trophozoites and schizonts incubated with ΔFd (2.5 μM) egressed anomalously and Band-3 immunostaining revealed them not to be associated with RBC membrane. Prematurely egressed merozoites from peptide-treated cultures were found to be invasion incompetent.

Good selectivity (>35), good resistance index (1.1) and low cytotoxicity indicate the promise of ΔFd against malaria.