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

Background: A lack of vaccine and rampant drug resistance demands new anti-malarials. Methods: 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. Results: Unlike the monomeric decapeptides {(Ac-GXRKXHKXWA-NH2) (X = F,Delta F) (Fm, Delta Fm IC50 > 100 mu M)}, the lysine-branched, dimeric versions showed far greater potency {IC50 (mu M) Fd 1.5, Delta Fd 1.39}. The more helical and proteolytically stable Delta Fd was studied for mechanistic details. Delta Fq, a K-K-2 dendrimer of Delta Fm and (Delta Fm)(2) a linear dimer of Delta Fm showed IC50 (mu M) of 0.25 and 2.4 respectively. The healthy/infected red cell selectivity indices were >35 (Delta Fd), >20 (Delta Fm)(2) and 10 (Delta Fq). FITC-Delta Fd showed rapid and selective accumulation in parasitized red cells. Overlaying DAPI and FITC florescence suggested that Delta Fd binds DNA. Trophozoites and schizonts incubated with Delta Fd (2.5 mu M) 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. Conclusion: Good selectivity (>35), good resistance index (1.1) and low cytotoxicity indicate the promise of Delta Fd against malaria.