The Proteasome Inhibitor Epoxomicin Has Potent Plasmodium falciparum Gametocytocidal Activity

Abstract

Malaria continues to be a major global health problem, but only a limited arsenal of effective drugs is available. None of the antimalarial compounds commonly used clinically kill mature gametocytes, which is the form of the parasite that is responsible for malaria transmission. The parasite that causes the most virulent human malaria, Plasmodium falciparum, has a 48-h asexual cycle, while complete sexual differentiation takes 10 to 12 days. Once mature, stage V gametocytes circulate in the peripheral blood and can be transmitted for more than a week. Consequently, if chemotherapy does not eliminate gametocytes, an individual continues to be infectious for several weeks after the clearance of asexual parasites. The work reported here demonstrates that nanomolar concentrations of the proteasome inhibitor epoxomicin effectively kill all stages of intraerythrocytic parasites but do not affect the viability of human or mouse cell lines. Twenty-four hours after treatment with 100 nM epoxomicin, the total parasitemia decreased by 78%, asexual parasites decreased by 86%, and gametocytes decreased by 77%. Seventy-two hours after treatment, no viable parasites remained in the 100 or 10 nM treatment group. Epoxomicin also blocked oocyst production in the mosquito midgut. In contrast, the cysteine protease inhibitors epoxysuccinyl-L-leucylamido-3-methyl-butane ethyl ester and N-acetyl-L-leucyl-Lleucyl-L-methioninal blocked hemoglobin digestion in early gametocytes but had no effect on later stages. Moreover, once the cysteine protease inhibitor was removed, sexual differentiation resumed. These findings provide strong support for the further development of proteasome inhibitors as antimalaria agents that are effective against asexual, sexual, and mosquito midgut stages of P. falciparum.The current recommended treatments for malaria caused by Plasmodium falciparum, including artemisinin combination therapy, eliminate intraerythrocytic asexual parasites that are responsible for the clinical symptoms. However, these treatments do not kill mature intraerythrocytic gametocytes that are required for the transmission of the parasite (24). In contrast to the 2-day asexual cycle of P. falciparum, the production of a mature stage V gametocyte takes 10 to 12 days. Once mature gametocytes are taken up by a mosquito during a blood meal, fertilization is stimulated. The resulting zygotes develop into oocysts where thousands of sporozoites are produced that can be transmitted to humans during a subsequent blood meal. The prolonged period required for P. falciparum gametocyte maturation in the human host suggests that malaria can be transmitted for several weeks after asexual parasites are eliminated (23). Thus, the development of drugs that are effective against both asexual-stage parasites and gametocytes may directly decrease malaria morbidity and mortality and reduce the spread of the disease.Cysteine protease and proteasome inhibitors have been found to affect asexual intraerythrocytic parasites and are being evaluated as possible an...

Plasmodium Dipeptidyl Aminopeptidases as Malaria Transmission-Blocking Drug Targets

“…Hemoglobin digestion is complete by stage III of gametocytogenesis, as demonstrated by the lack of an effect of cysteine protease inhibitor E64d on food vacuole morphology in stage III to V gametocytes (17,18), and yet the parasite continues to increase in size, suggesting continued metabolism. Additional support for a metabolic shift during sexual development is provided by a report showing that oocyst production, but not asexual growth, is arrested in type II NADH:ubiquinone dehydrogenase and succinate-ubiquinone oxidoreductase knockout parasites (36,37).…”

“…dpap2 is transcribed only in gametocytes (16), and its role remains unknown since the gametocyte and mosquito stages were not included in the initial inhibitor analysis (15). Since hemoglobin digestion is essentially complete by stage III of gametocytogenesis (17,18). DPAP2 might have a role in alternative metabolic pathways in late-stage gametocytes and during sporogonic development in the mosquito.…”

Maduramicin Rapidly Eliminates Malaria Parasites and Potentiates the Gametocytocidal Activity of the Pyrazoleamide PA21A050

“…Maduramicin-induced rounding is distinct from the response of late-stage gametocytes to the proteasome inhibitor epoxomicin. Following epoxomicin treatment, gametocytes retain their elongated shape but shrink in diameter, ultimately resembling a needle before disintegrating (39). Hliscs et al have recently shown that tubulin and actin remain in complexes in stage V gametocytes (40), but the changes in the cytoskeleton during rounding were not evaluated.…”

New Chemotherapeutic Strategies Against Malaria, Leishmaniasis and Trypanosomiases

“…Therefore, proteasome inhibitors are considered promising antimalarial agents. An essential role of the Plasmodium proteasome has been demonstrated for the liver, blood and gametocyte stages [83,[85][86][87][88][89][90][91]. is often referred to as the parent compound of the thiopeptide antibiotics (for review, see [92]).…”