Artemisinin-Based Drugs Target the Plasmodium falciparum Heme Detoxification Pathway

Abstract

Artemisinin – based antimalarial drugs are believed to exert lethal effects on malarial parasites by alkylating a variety of intracellular molecular targets. Recent work with live parasites has shown that one of the alkylated targets is free heme within the parasite digestive vacuole, which is liberated upon hemoglobin catabolism by the intraerythrocytic parasite, and that reduced levels of heme alkylation occur in artemisinin resistant parasites. One implication of heme alkylation is that these drugs may inhibit parasite detoxification of free heme via inhibition of heme to hemozoin crystallization; however, previous reports that have investigated this hypothesis present conflicting data. By controlling reducing conditions and hence the availability of ferrous vs ferric forms of free heme, we modify a previously reported hemozoin inhibition assay to quantify the ability of ART – based drugs to target the heme detoxification pathway under reduced vs oxidizing conditions. Contrary to some previous reports, we find that artemisinins are potent inhibitors of hemozoin crystallization, with effective half maximal concentrations approximately an order of magnitude lower than those for most quinoline – based antimalarial drugs. We also examine hemozoin and in vitro parasite growth inhibition for drug pairs found in the most commonly used ART – based combination therapies (ACTs). All ACTs examined inhibit hemozoin crystallization in an additive fashion, and all but one inhibit parasite growth in additive fashion.

Phytochemical Analysis and Anti-Inflammatory Activity of Different Ethanolic Phyto-Extracts of Artemisia annua L.

“…Initially, the pharmacological studies of ART were focused on its antimalarial action. In particular, ART and its derivatives were found to be selectively absorbed by erythrocytes infected by the malarial plasmodium and subsequently localized in the membranes and mitochondria of the parasite [7]. The endoperoxide bridge present in the chemical structure of ART and its derivatives is responsible for the generation of free radicals that damage the plasmodium membrane and kill it.…”

“…The endoperoxide bridge present in the chemical structure of ART and its derivatives is responsible for the generation of free radicals that damage the plasmodium membrane and kill it. ART and its derivatives have been found to be effective against the various severities of malaria, especially those resistant to conventional therapies [7].…”

Studies of Potency and Efficacy of an Optimized Artemisinin-Quinoline Hybrid against Multiple Stages of the Plasmodium Life Cycle

“…The 163A hybrid was as potent as chloroquine in inhibiting β-hematin formation under ferrous heme and it was only slightly less potent than artemisinin, which is a strong inhibitor assayed in the presence of ferrous heme. The inhibitory activity on the β-hematin formation was higher when assayed for ferrous heme than ferric hematin, which is also a behavior observed for 7-chloroquinoline [40]. Other hybrid-based drugs have displayed potency equal or superior to parental drug and antimalarial quinolines of reference, such as amodiaquine and chloroquine [13,26].…”

“…The 163A hybrid has been shown to inhibit β-hematin formation and to suppress the in vivo parasite heme detoxification [29]; however, this heterobivalency property was not previously examined in terms of 163A targeting ferrous heme and its underlying redox chemistry reactivity. To examine this heterobivalency property (Figure 5), we employed a recent methodology that allows the simultaneous examination of β-hematin inhibitory activity (BHIA) under an oxidizing condition (ferric hematin) and under a reducing condition (ferrous heme) [40], in addition to determining the association constant of drugs for ferric hematin (log K) [41].…”

Recent advances in identification of potential drug targets and development of novel drugs in parasitic diseases. Part I: Drug resistance

“…Potent inhibitory activity on hemozoin formation was suggested as another mechanism of ART action. In other words, there is FPIX abundance, and heam accumulation in sensitive strains [62] .…”