Relaxometric studies of erythrocyte suspensions infected by Plasmodium falciparum: a tool for staging infection and testing anti-malarial drugs.

Purpose: Malaria is a global health problem with the most malignant form caused by Plasmodium falciparum (P. falciparum). Parasite maturation in red blood cells (RBCs) is accompanied by changes including the formation of paramagnetic hemozoin (HZ) nanocrystals, and increased metabolism and variation in membrane lipid composition. Herein, MR relaxometry (MRR) was applied to investigate water exchange across RBCs' membrane and HZ formation in parasitized RBCs.

Methods: Transverse water protons relaxation rate constants (R = 1/T ) were measured for assessing HZ formation in P. falciparum-parasitized human RBCs. Moreover, water exchange lifetimes across the RBC membrane (τ ) were assessed by measuring longitudinal relaxation rate constants (R = 1/T ) at 21.5 MHz in the presence of a gadolinium complex dissolved in the suspension medium.

Results: τ increased after invasion of parasites (ring stage, mean τ / = 1.234 ± 0.022) and decreased during maturation to late trophozoite (mean τ / = 0.960 ± 0.075) and schizont stages (mean τ / = 1.019 ± 0.065). The HZ accumulation in advanced stages was revealed by T -shortening. The curves reporting R (1/T ) vs. magnetic field showed different slopes for non-parasitized RBCs (npRBCs) and parasitized RBCs (pRBCs), namely 0.003 ± 0.001 for npRBCs, 0.009 ± 0.002, 0.028 ± 0.004 and 0.055 ± 0.002 for pRBCs at ring-, early trophozoite-, and late trophozoite stage, respectively. Antimalarial molecules dihydroartemisinin and chloroquine elicited measurable changes in parasitized RBCs, namely dihydroartemisinin modified τ , whereas the interference of chloroquine with HZ formation was detectable by a significant T increase.

Conclusions: MRR can be considered a useful tool for reporting on P. falciparum blood stages and for screening potential antimalarial molecules.