16α-Bromoepiandrosterone, an Antimalarial Analogue of the Hormone Dehydroepiandrosterone, Enhances Phagocytosis of Ring Stage Parasitized Erythrocytes: a Novel Mechanism for Antimalarial Activity

Dehydroepiandrosterone (DHEA) and DHEA-sulfate (DHEA-S), which are the most abundant hormones secreted by the adrenal cortex and are present in plasma at approximately 6 μM, as well as their analogue, 16α-bromoepiandrosterone (EPI), exerted antimalarial activities against two chloroquine-sensitive Plasmodium falciparum strains (Palo Alto, 50% inhibitory concentration [IC(50)] of EPI, 4.8 ± 0.68 μM; T996/86, IC(50) of EPI, 7.5 ± 0.91 μM, and IC(50) of DHEA-S, 19 ± 2.6 μM) and one mildly chloroquine-resistant strain (FCR-3, IC(50) of EPI, 6.5 ± 1.01 μM). Both EPI and DHEA/DHEA-S are potent inhibitors of glucose-6-phosphate dehydrogenase (G6PD), and G6PD deficiency is known to exert antimalaria protection via enhanced opsonization and phagocytosis of rings, the early forms of the parasite. Plasma-compatible antimalarial EPI concentrations did not inhibit G6PD activity and did not induce ring opsonization by immunoglobulin G and complement fragments, as observed in G6PD deficiency, but nevertheless remarkably stimulated ring phagocytosis. Plasma-compatible, low-micromolar concentrations of EPI induced exposure on the ring surface of phosphatidylserine, a signal for phagocytic removal independent of opsonization. We propose that enhanced ring phagocytosis due to exposure of negatively charged membrane phospholipids may explain the antimalarial activity of EPI.