Insights into platelet factor 4-derived peptide macrocycles; the mechanistic basis of their rapid and selective antiplasmodial actions.

The malarial parasite Plasmodium can acquire resistance to most mainstay antimalarial drugs, necessitating the development of new antiplasmodial agents with different modes of action. The innate defense protein, human platelet factor 4 (PF4), has a unique antiplasmodial action that involves selective entry into Plasmodium-infected red blood cells (RBC) and subsequent destruction of the parasite's digestive vacuole (DV). This activity is recapitulated in PF4-derived internalization peptides (PDIPs). Here, we characterized the actions of PDIP analogs and PF4 in live P. falciparum-infected human RBC to understand their kinetics, effects on cell and parasite viability, and molecular requirements for antiplasmodial activity. The entry and accumulation of PDIP, and peptide-induced DV destruction, were distinguishable as ordered and rapidly occurring events that were equivalent to PF4. Both host cell and parasite plasma membranes remained intact and undamaged following destruction of the DV, although modest changes in phosphatidylserine (PS) exposure on the surface of the host cells (indicative of changes to its phospholipid organization) and swelling (but not lysis) of the intracellular parasite were observed. PDIP retained its macrocyclic structure, and its activity depended on elevated levels of PS on the surface of infected versus uninfected cells. Neither the intramolecular disulfide bond of PDIP, nor the parasite's nutrient and ion transporter functions were required. These actions on the parasite DV were not detected for other antiplasmodial drugs and compounds. In conclusion, this study reveals the unique, rapid, and distinct antiplasmodial actions of PDIP, highlighting its potential for future antimalarial development.